INVESTIGATING THE ACTIVITY OF ZIRCONIA AS A
CATALYST AND AS A SUPPORT FOR NOBLE METALS IN
ORGANIC OXIDATION REACTIONS
Presented by
MOHAMMAD SADIQ
NATIONAL CENTRE OF EXCELLENCE IN PHYSICAL
CHEMISTRY UNIVERSITY OF PESHAWAR
2009
INVESTIGATING THE ACTIVITY OF ZIRCONIA AS A
CATALYST AND AS A SUPPORT FOR NOBLE METALS
IN ORGANIC OXIDATION REACTIONS
A dissertation submitted to the University of Peshawar in partial
fulfillment for the degree of
DOCTOR OF PHILOSOPHY IN PHYSICAL CHEMISTRY
Presented by
MOHAMMAD SADIQ
NATIONAL CENTRE OF EXCELLENCE IN PHYSICAL
CHEMISTRY UNIVERSITY OF PESHAWAR
2009
i
ii
Acknowledgment
I would like to express my thanks to all those who have supported me and encouraged
me to pursue the study of Physical Chemistry particularly during my doctoral studies
First I would like to thank my supervisor Prof Dr Mohammad Ilyas for giving me the
opportunity to complete doctoral studies in his laboratory under his kind supervision
During the last three years he fulfilled all of my wishes with regard to giving me
scientific freedom broadening the research topic providing instrumentation and
interesting courses The atmosphere in his laboratory was pleasant and stress-free I am
grateful to him for the very fast review of my work his helpful remarks his generosity
and his confidence in me
I wish to thank Prof Dr Syed Mustafa Director NCE in Physical chemistry
University of Peshawar for providing me all the available facilities during the study
I would like to acknowledge the work and support from the glassblowing staff
who have made every possible effort to designed and constructed different Pyrex glass
reactors for experimental setup
Further I appreciate the staff of Centralized Resources Laboratory at Physics
Department and NCE in Geology for helping me in characterization of the catalysts
I am thankful from the core of my heart to my junior brother Mohammad Ali for
his support through out my study I also say a big ldquothank yourdquo to Saima my cute wife for
all her care her understanding her love and spiritual support
During the last three years of my PhD study I have met many nice colleagues
most of them deserve to be thanked for some reasons Heartfelt thanks to my Lab fellows
Mr Mohammad Taufiq Mr Imdad Khan Mr Mohammad Saeed Rahmat Ali and
Mohammad Hamayun for their sincere cooperation and friendly behavior throughout the
time I spent with them
And at last
Dear family members thank you very much for standing with me through thick and thin
Mr Mohammad Sadiq
iii
Abstract
Alcohols and cyclic alkanes oxidation in an environment friendly protocol was carried
out in a typical batch reactor These reactions were carried out in solvent free conditions
andor in eco-friendly solvents using molecular oxygen as the only oxidant and ZrO2
andor ZrO2 supported noble metals (Pt Pd) as catalysts The influence of different
reaction parameters (speed of agitation reaction time and temperature) catalyst
parameters (calcination temperature and loading) and oxygen partial pressure on the
catalyst performance was studied Different modern techniques such as (FT-IR XRD
SEM EDX surface and pores size analyzer and particle size analyzer) were used for the
characterization of catalyst ZrO2 calcined at 1223 K was found to be more active as a
single catalyst than the one calcined at 723 K for alcohol oxidation to the corresponding
carbonyl products under solvent free conditions and in ecofriendly solvent as well
Platinum supported on zirconia was highly active and selective for oxidation of benzyl
alcohol to benzaldehyde in n- heptane and toluene to benzoic acid in both solvent free
conditions and in aqueous medium Similarly zirconia supported Pt or Pd catalysts were
tested for cyclohexane oxidation in solvent free conditions and for phenol oxidation in
aqueous medium Both catalysts have shown magnificent catalytic activity Bismuth was
added as a promoter to these catalysts Bismuth promoted PtZrO2 has shown outstanding
catalytic performance These catalysts are insoluble in the reaction mixture and can be
easily separated by simple filtration and reused Typical batch reactorrsquos kinetic data were
obtained and fitted to the classical LangmuirndashHinshelwood Marsndashvan Krevelen and as
well as to the Eley-Rideal model of heterogeneously catalyzed reactions In alcohol
oxidation reactions the Langmuir-Hinshelwood model was found to give a better fit The
rate-determining step was proposed to involve direct interaction of an adsorbed oxidizing
species with the adsorbed reactant or an intermediate product of the reactant While in
toluene oxidation the Eley-Rideal model was found to give a better fit Eley-Rideal
mechanism envisages reaction between adsorbed oxygen with hydrocarbon molecules
from the fluid phase The calculated apparent activation energy and agitation effect have
shown the absence of mass transfer effect
Keywords Catalysis solvent free eco-friendly solvents organic oxidation reactions mild conditions
iv
List of Publications
Thesis includes the following papers which were published in different international
journals and presented at various conferences
I Ilyas M Sadiq M Imdad K Chin J Catal 2007 28 413
II Ilyas M Sadiq M Chem Eng Technol 2007 30 1391-1397
III Ilyas M Sadiq M Chin J Chem 2008 26 146
IV Ilyas M Sadiq M Catal Lett 2009 128 337
V Ilyas M Sadiq M ldquoInvestigating the activity of zirconia as a catalyst
and a support for noble metals in green oxidation of cyclohexanerdquo J
Iran Chem Soc Submitted
VI M Ilyas M Sadiq ldquoA model catalyst for aerobic oxidation of toluene in
aqueous solutionrdquo presented in 12th International Conference of the
Pacific Basin Consortium for Environment amp Health Sciences at Beijing
University China 26-29 October 2007 (Submitted to Catalysis Letter)
VII M Ilyas M Sadiq ldquoOxidation of benzyl alcohol in aqueous medium by
zirconia catalyst at mild conditionsrdquo presented in 18th National Chemistry
Conference in Institute of Chemistry University of Punjab Lahore
Pakistan 25-27 February 2008
VIII M Ilyas M Sadiq ldquoComparative study of commercially available ZrO2
and laboratory prepared ZrO2 for liquid phase solvent free oxidation of
cyclohexanolrdquo presented in 18th National Chemistry Conference Institute
of Chemistry University of Punjab Lahore Pakistan 25-27 February
2008
IX M Ilyas M Sadiq ldquoZirconia-supported noble metals catalyst for
oxidation of phenol in artificially contaminated water at milder
conditionsrdquo presented in 1st National Symposium on Analytical
Environmental and Applied Chemistry in Shah Abdul Latif University
Khairpur Sindh Pakistan 24-25 October 2008
v
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Acknowledgment ii
Abstract iii
List of Publications iv
Chapter 1 Introduction
11 Aims and objective 01
12 Zirconia in Catalysis 02
13 Oxidation of alcohols 03
14 Oxidation of toluene 06
15 Oxidation of cyclohexane 09
16 Oxidation of phenol 09
17 Characterization of catalyst 11
171 Surface area Measurements 11
172 Particle size measurement 11
173 X-ray differactometry 12
174 Infrared Spectroscopy 12
175 Scanning Electron Microscopy 13
Chapter 2 Literature review 14
References 20
vi
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 3 Experimental
31 Material 30
32 Preparation of catalyst 30
321 Laboratory prepared ZrO2 30
322 Optimal conditions 32
323 Commercial ZrO2 32
324 Supported catalyst 32
33 Characterization of catalysts 32
34 Experimental setups for different reaction 33
35 Liquid-phase oxidation in solvent free conditions 37
351 Design of reactor for liquid phase oxidation in
solvent free condition 37
36 Liquid-phase oxidation in eco-friendly solvents 38
361 Design of reactor for liquid phase oxidation in
eco-friendly solvents 38
37 Analysis of reaction mixture 39
38 Heterogeneous nature of the catalyst 41
References 42
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
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40 Christoskova St Final Report Contract X-123 National Science Fund
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41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
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54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
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57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
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62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
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63 Pintar and Levec J J Catal 1992 135 345
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65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
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66 Hussain ST Sayari A Larachi F J Catal 2001 201153
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68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
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69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
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71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
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74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
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77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
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78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
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24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
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81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
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89 Dow Chemical Corp US Patent 2 727 926 1955
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91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
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93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
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94 Pennington J in C A Heaton (eds) An Introduction to Industrial
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95 US Environmental Protection Agency Integrated Risk Information
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96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
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97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
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98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
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102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
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108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
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109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
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114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
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115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
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116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
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117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
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26
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119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
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120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
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123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
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125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
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126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
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129 Birkby I Stevens R Key Eng Mater 1996 122 527
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132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
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133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
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137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
INVESTIGATING THE ACTIVITY OF ZIRCONIA AS A
CATALYST AND AS A SUPPORT FOR NOBLE METALS
IN ORGANIC OXIDATION REACTIONS
A dissertation submitted to the University of Peshawar in partial
fulfillment for the degree of
DOCTOR OF PHILOSOPHY IN PHYSICAL CHEMISTRY
Presented by
MOHAMMAD SADIQ
NATIONAL CENTRE OF EXCELLENCE IN PHYSICAL
CHEMISTRY UNIVERSITY OF PESHAWAR
2009
i
ii
Acknowledgment
I would like to express my thanks to all those who have supported me and encouraged
me to pursue the study of Physical Chemistry particularly during my doctoral studies
First I would like to thank my supervisor Prof Dr Mohammad Ilyas for giving me the
opportunity to complete doctoral studies in his laboratory under his kind supervision
During the last three years he fulfilled all of my wishes with regard to giving me
scientific freedom broadening the research topic providing instrumentation and
interesting courses The atmosphere in his laboratory was pleasant and stress-free I am
grateful to him for the very fast review of my work his helpful remarks his generosity
and his confidence in me
I wish to thank Prof Dr Syed Mustafa Director NCE in Physical chemistry
University of Peshawar for providing me all the available facilities during the study
I would like to acknowledge the work and support from the glassblowing staff
who have made every possible effort to designed and constructed different Pyrex glass
reactors for experimental setup
Further I appreciate the staff of Centralized Resources Laboratory at Physics
Department and NCE in Geology for helping me in characterization of the catalysts
I am thankful from the core of my heart to my junior brother Mohammad Ali for
his support through out my study I also say a big ldquothank yourdquo to Saima my cute wife for
all her care her understanding her love and spiritual support
During the last three years of my PhD study I have met many nice colleagues
most of them deserve to be thanked for some reasons Heartfelt thanks to my Lab fellows
Mr Mohammad Taufiq Mr Imdad Khan Mr Mohammad Saeed Rahmat Ali and
Mohammad Hamayun for their sincere cooperation and friendly behavior throughout the
time I spent with them
And at last
Dear family members thank you very much for standing with me through thick and thin
Mr Mohammad Sadiq
iii
Abstract
Alcohols and cyclic alkanes oxidation in an environment friendly protocol was carried
out in a typical batch reactor These reactions were carried out in solvent free conditions
andor in eco-friendly solvents using molecular oxygen as the only oxidant and ZrO2
andor ZrO2 supported noble metals (Pt Pd) as catalysts The influence of different
reaction parameters (speed of agitation reaction time and temperature) catalyst
parameters (calcination temperature and loading) and oxygen partial pressure on the
catalyst performance was studied Different modern techniques such as (FT-IR XRD
SEM EDX surface and pores size analyzer and particle size analyzer) were used for the
characterization of catalyst ZrO2 calcined at 1223 K was found to be more active as a
single catalyst than the one calcined at 723 K for alcohol oxidation to the corresponding
carbonyl products under solvent free conditions and in ecofriendly solvent as well
Platinum supported on zirconia was highly active and selective for oxidation of benzyl
alcohol to benzaldehyde in n- heptane and toluene to benzoic acid in both solvent free
conditions and in aqueous medium Similarly zirconia supported Pt or Pd catalysts were
tested for cyclohexane oxidation in solvent free conditions and for phenol oxidation in
aqueous medium Both catalysts have shown magnificent catalytic activity Bismuth was
added as a promoter to these catalysts Bismuth promoted PtZrO2 has shown outstanding
catalytic performance These catalysts are insoluble in the reaction mixture and can be
easily separated by simple filtration and reused Typical batch reactorrsquos kinetic data were
obtained and fitted to the classical LangmuirndashHinshelwood Marsndashvan Krevelen and as
well as to the Eley-Rideal model of heterogeneously catalyzed reactions In alcohol
oxidation reactions the Langmuir-Hinshelwood model was found to give a better fit The
rate-determining step was proposed to involve direct interaction of an adsorbed oxidizing
species with the adsorbed reactant or an intermediate product of the reactant While in
toluene oxidation the Eley-Rideal model was found to give a better fit Eley-Rideal
mechanism envisages reaction between adsorbed oxygen with hydrocarbon molecules
from the fluid phase The calculated apparent activation energy and agitation effect have
shown the absence of mass transfer effect
Keywords Catalysis solvent free eco-friendly solvents organic oxidation reactions mild conditions
iv
List of Publications
Thesis includes the following papers which were published in different international
journals and presented at various conferences
I Ilyas M Sadiq M Imdad K Chin J Catal 2007 28 413
II Ilyas M Sadiq M Chem Eng Technol 2007 30 1391-1397
III Ilyas M Sadiq M Chin J Chem 2008 26 146
IV Ilyas M Sadiq M Catal Lett 2009 128 337
V Ilyas M Sadiq M ldquoInvestigating the activity of zirconia as a catalyst
and a support for noble metals in green oxidation of cyclohexanerdquo J
Iran Chem Soc Submitted
VI M Ilyas M Sadiq ldquoA model catalyst for aerobic oxidation of toluene in
aqueous solutionrdquo presented in 12th International Conference of the
Pacific Basin Consortium for Environment amp Health Sciences at Beijing
University China 26-29 October 2007 (Submitted to Catalysis Letter)
VII M Ilyas M Sadiq ldquoOxidation of benzyl alcohol in aqueous medium by
zirconia catalyst at mild conditionsrdquo presented in 18th National Chemistry
Conference in Institute of Chemistry University of Punjab Lahore
Pakistan 25-27 February 2008
VIII M Ilyas M Sadiq ldquoComparative study of commercially available ZrO2
and laboratory prepared ZrO2 for liquid phase solvent free oxidation of
cyclohexanolrdquo presented in 18th National Chemistry Conference Institute
of Chemistry University of Punjab Lahore Pakistan 25-27 February
2008
IX M Ilyas M Sadiq ldquoZirconia-supported noble metals catalyst for
oxidation of phenol in artificially contaminated water at milder
conditionsrdquo presented in 1st National Symposium on Analytical
Environmental and Applied Chemistry in Shah Abdul Latif University
Khairpur Sindh Pakistan 24-25 October 2008
v
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Acknowledgment ii
Abstract iii
List of Publications iv
Chapter 1 Introduction
11 Aims and objective 01
12 Zirconia in Catalysis 02
13 Oxidation of alcohols 03
14 Oxidation of toluene 06
15 Oxidation of cyclohexane 09
16 Oxidation of phenol 09
17 Characterization of catalyst 11
171 Surface area Measurements 11
172 Particle size measurement 11
173 X-ray differactometry 12
174 Infrared Spectroscopy 12
175 Scanning Electron Microscopy 13
Chapter 2 Literature review 14
References 20
vi
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 3 Experimental
31 Material 30
32 Preparation of catalyst 30
321 Laboratory prepared ZrO2 30
322 Optimal conditions 32
323 Commercial ZrO2 32
324 Supported catalyst 32
33 Characterization of catalysts 32
34 Experimental setups for different reaction 33
35 Liquid-phase oxidation in solvent free conditions 37
351 Design of reactor for liquid phase oxidation in
solvent free condition 37
36 Liquid-phase oxidation in eco-friendly solvents 38
361 Design of reactor for liquid phase oxidation in
eco-friendly solvents 38
37 Analysis of reaction mixture 39
38 Heterogeneous nature of the catalyst 41
References 42
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
1 Homogeneous Catalysis Parshall GW Ittel SD 2Ed John Wiley amp Sons
Inc Nova Iorque 1992
2 Cornils B Herrmann W Eds Applied Homogeneous Catalysis with
Organometallic Compounds Vol 1 VCH 1996 Chapter 24
3 Anastas PT Warner JC Green Chemistry Theory and Practice Oxford
University Press Oxford 1998
4 Puzari A Jubaraj B J Mol Catal A Chem 2002 187 149
5 Gates B C Catalytic Chemistry John Wiley and Sons New York 1992
6 Yamaguchi T Catal Today 1994 20 199
7 Ozawa M Kimura M J Mater Sci Lett 1990 9 446
8 Inoue M Kominami H Inui T Appl Catal A 1993 97 L25-30
9 Aiken B Hsu W P Matijevid E J Mater Sci1990 25 1886
10 Garg A Matijevid E J Colloid Interface Sci1988 126 243
11 Mercera P D L Van Ommen J G Doesburg E B M Burggraaf AJ
Ross JRH Appl Catal1990 57127
12 Mercera PDL Van Ommen JG Doesburg EBM Burggraaf AJ Ross
JRH Appl Catal1991 78 79
13 Srinivasan R Taulbee D Davis BH Catal Lett 1991 9 1
14 Norman C J Goulding PA McAlpine I Catal Today1994 20 313
15 Mallat T Baiker A Chem Rev 2004 104 3037
16 Muzart J Tetrahedron 2003 59 5789
17 Rafelt J S Clark J H Catal Today 2000 57 33
18 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 57 143
19 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432
21
20 Hutchings G J Carrettin S Landon P Edwards JK Enache D
Knight DW Xu Y CarleyAF Top Catal 2006 38 223-230
21 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
22 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
23 Opre Z Grunwaldt JD Mallat T BaikerA J Mol Catal A Chem 2005
242 224-232
24 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241
287-293
25 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
26 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
27 Ji H B Song J He B Qian Y React Kinet Catal Lett 2004 82 97
28 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-
52
29 Choudhary V R Dhar A Jana P Jha R de Upha B S Green Chem
2005 7 768
30 Choudhary V R Jha R Jana P Green Chem 2007 9 267
31 Enache D I Edwards J K Landon P Espiru B S Carley A F
Herzing A H Watanabe M Kiely C J Knight D W Hutchings G J
Science 2006 311 362
32 Li G Enache D I Edwards J K Carley A F Knight D W Hutchings
G J Catal Lett 2006 110 7
33 Ilyas M Abdullah M N U Phys Chem 2003 14 19
34 Ilyas M Ikramullah Catal Commun 2004 5 1
35 Rache A Kumari V Rao P K In Gupta N M Chakrabarty D K eds
Catalysis Modern Trends New Delhi Narosa 1995 346
36 Li X Xu J Wang F Gao J Zhou L Yang G Catalysis Letters
2006 108 137
37 Heyns K Blazejewicz L Tetrahedron 1960 9 67
22
38 Heyns K Paulsen H in ldquo Newer Methods of Preparative Organic
Chemistryrdquo W Forest Eds Academic Press New York 1963 Vol 2 pp
303-335
39 Christoskova St Stoyanova M Water Res 2002 36 2297-2303
40 Christoskova St Final Report Contract X-123 National Science Fund
Ministry of Education and Science Republic of Bulgaria 1993
41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
Appl Catal A General 1995 128 219ndash229
43 Munter R Proc Estonian Sci Chem 2001 50 59-804
44 Mishra V S Mahajani VV Joshi JB Ind Eng Chem Res 1995 34 2
45 Imamura S Ind Eng Chem Res 1999 38 1743
46 Pintar Catal Today 2003 77 451
47 Matatov-Meytal Y I Sheintuch M Ind Eng Chem Res 1998 37 309
48 Luck F Catal Today 1999 53 81
49 Kolaczkowski S T Plucinski P Beltran FJ Rivas F Lurgh DB Chem
Eng J 1999 73 143
50 Iliuta Larachi F Chem Eng Proc 2001 40175
51 Fortuny C Ferrer C Bengoa J Font and Fabregat A Catal Today 1995
24 79
52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
B Environ 1998 16 53
53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
2153
54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
55 Santos A Yustos P Durban B Garcia-Ochoa F Environ Sci Technol
2001 35 2828
56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
181
57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
2001 32 151
60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
Technol Biotechnol 2001 76 743
61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
1998 37 3561
63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
1999 188 311
65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
JE Appl Catal B Environ 2001 30 195
69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
i
ii
Acknowledgment
I would like to express my thanks to all those who have supported me and encouraged
me to pursue the study of Physical Chemistry particularly during my doctoral studies
First I would like to thank my supervisor Prof Dr Mohammad Ilyas for giving me the
opportunity to complete doctoral studies in his laboratory under his kind supervision
During the last three years he fulfilled all of my wishes with regard to giving me
scientific freedom broadening the research topic providing instrumentation and
interesting courses The atmosphere in his laboratory was pleasant and stress-free I am
grateful to him for the very fast review of my work his helpful remarks his generosity
and his confidence in me
I wish to thank Prof Dr Syed Mustafa Director NCE in Physical chemistry
University of Peshawar for providing me all the available facilities during the study
I would like to acknowledge the work and support from the glassblowing staff
who have made every possible effort to designed and constructed different Pyrex glass
reactors for experimental setup
Further I appreciate the staff of Centralized Resources Laboratory at Physics
Department and NCE in Geology for helping me in characterization of the catalysts
I am thankful from the core of my heart to my junior brother Mohammad Ali for
his support through out my study I also say a big ldquothank yourdquo to Saima my cute wife for
all her care her understanding her love and spiritual support
During the last three years of my PhD study I have met many nice colleagues
most of them deserve to be thanked for some reasons Heartfelt thanks to my Lab fellows
Mr Mohammad Taufiq Mr Imdad Khan Mr Mohammad Saeed Rahmat Ali and
Mohammad Hamayun for their sincere cooperation and friendly behavior throughout the
time I spent with them
And at last
Dear family members thank you very much for standing with me through thick and thin
Mr Mohammad Sadiq
iii
Abstract
Alcohols and cyclic alkanes oxidation in an environment friendly protocol was carried
out in a typical batch reactor These reactions were carried out in solvent free conditions
andor in eco-friendly solvents using molecular oxygen as the only oxidant and ZrO2
andor ZrO2 supported noble metals (Pt Pd) as catalysts The influence of different
reaction parameters (speed of agitation reaction time and temperature) catalyst
parameters (calcination temperature and loading) and oxygen partial pressure on the
catalyst performance was studied Different modern techniques such as (FT-IR XRD
SEM EDX surface and pores size analyzer and particle size analyzer) were used for the
characterization of catalyst ZrO2 calcined at 1223 K was found to be more active as a
single catalyst than the one calcined at 723 K for alcohol oxidation to the corresponding
carbonyl products under solvent free conditions and in ecofriendly solvent as well
Platinum supported on zirconia was highly active and selective for oxidation of benzyl
alcohol to benzaldehyde in n- heptane and toluene to benzoic acid in both solvent free
conditions and in aqueous medium Similarly zirconia supported Pt or Pd catalysts were
tested for cyclohexane oxidation in solvent free conditions and for phenol oxidation in
aqueous medium Both catalysts have shown magnificent catalytic activity Bismuth was
added as a promoter to these catalysts Bismuth promoted PtZrO2 has shown outstanding
catalytic performance These catalysts are insoluble in the reaction mixture and can be
easily separated by simple filtration and reused Typical batch reactorrsquos kinetic data were
obtained and fitted to the classical LangmuirndashHinshelwood Marsndashvan Krevelen and as
well as to the Eley-Rideal model of heterogeneously catalyzed reactions In alcohol
oxidation reactions the Langmuir-Hinshelwood model was found to give a better fit The
rate-determining step was proposed to involve direct interaction of an adsorbed oxidizing
species with the adsorbed reactant or an intermediate product of the reactant While in
toluene oxidation the Eley-Rideal model was found to give a better fit Eley-Rideal
mechanism envisages reaction between adsorbed oxygen with hydrocarbon molecules
from the fluid phase The calculated apparent activation energy and agitation effect have
shown the absence of mass transfer effect
Keywords Catalysis solvent free eco-friendly solvents organic oxidation reactions mild conditions
iv
List of Publications
Thesis includes the following papers which were published in different international
journals and presented at various conferences
I Ilyas M Sadiq M Imdad K Chin J Catal 2007 28 413
II Ilyas M Sadiq M Chem Eng Technol 2007 30 1391-1397
III Ilyas M Sadiq M Chin J Chem 2008 26 146
IV Ilyas M Sadiq M Catal Lett 2009 128 337
V Ilyas M Sadiq M ldquoInvestigating the activity of zirconia as a catalyst
and a support for noble metals in green oxidation of cyclohexanerdquo J
Iran Chem Soc Submitted
VI M Ilyas M Sadiq ldquoA model catalyst for aerobic oxidation of toluene in
aqueous solutionrdquo presented in 12th International Conference of the
Pacific Basin Consortium for Environment amp Health Sciences at Beijing
University China 26-29 October 2007 (Submitted to Catalysis Letter)
VII M Ilyas M Sadiq ldquoOxidation of benzyl alcohol in aqueous medium by
zirconia catalyst at mild conditionsrdquo presented in 18th National Chemistry
Conference in Institute of Chemistry University of Punjab Lahore
Pakistan 25-27 February 2008
VIII M Ilyas M Sadiq ldquoComparative study of commercially available ZrO2
and laboratory prepared ZrO2 for liquid phase solvent free oxidation of
cyclohexanolrdquo presented in 18th National Chemistry Conference Institute
of Chemistry University of Punjab Lahore Pakistan 25-27 February
2008
IX M Ilyas M Sadiq ldquoZirconia-supported noble metals catalyst for
oxidation of phenol in artificially contaminated water at milder
conditionsrdquo presented in 1st National Symposium on Analytical
Environmental and Applied Chemistry in Shah Abdul Latif University
Khairpur Sindh Pakistan 24-25 October 2008
v
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Acknowledgment ii
Abstract iii
List of Publications iv
Chapter 1 Introduction
11 Aims and objective 01
12 Zirconia in Catalysis 02
13 Oxidation of alcohols 03
14 Oxidation of toluene 06
15 Oxidation of cyclohexane 09
16 Oxidation of phenol 09
17 Characterization of catalyst 11
171 Surface area Measurements 11
172 Particle size measurement 11
173 X-ray differactometry 12
174 Infrared Spectroscopy 12
175 Scanning Electron Microscopy 13
Chapter 2 Literature review 14
References 20
vi
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 3 Experimental
31 Material 30
32 Preparation of catalyst 30
321 Laboratory prepared ZrO2 30
322 Optimal conditions 32
323 Commercial ZrO2 32
324 Supported catalyst 32
33 Characterization of catalysts 32
34 Experimental setups for different reaction 33
35 Liquid-phase oxidation in solvent free conditions 37
351 Design of reactor for liquid phase oxidation in
solvent free condition 37
36 Liquid-phase oxidation in eco-friendly solvents 38
361 Design of reactor for liquid phase oxidation in
eco-friendly solvents 38
37 Analysis of reaction mixture 39
38 Heterogeneous nature of the catalyst 41
References 42
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
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41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
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52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
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53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
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54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
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56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
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57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
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60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
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61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
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63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
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65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
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69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
ii
Acknowledgment
I would like to express my thanks to all those who have supported me and encouraged
me to pursue the study of Physical Chemistry particularly during my doctoral studies
First I would like to thank my supervisor Prof Dr Mohammad Ilyas for giving me the
opportunity to complete doctoral studies in his laboratory under his kind supervision
During the last three years he fulfilled all of my wishes with regard to giving me
scientific freedom broadening the research topic providing instrumentation and
interesting courses The atmosphere in his laboratory was pleasant and stress-free I am
grateful to him for the very fast review of my work his helpful remarks his generosity
and his confidence in me
I wish to thank Prof Dr Syed Mustafa Director NCE in Physical chemistry
University of Peshawar for providing me all the available facilities during the study
I would like to acknowledge the work and support from the glassblowing staff
who have made every possible effort to designed and constructed different Pyrex glass
reactors for experimental setup
Further I appreciate the staff of Centralized Resources Laboratory at Physics
Department and NCE in Geology for helping me in characterization of the catalysts
I am thankful from the core of my heart to my junior brother Mohammad Ali for
his support through out my study I also say a big ldquothank yourdquo to Saima my cute wife for
all her care her understanding her love and spiritual support
During the last three years of my PhD study I have met many nice colleagues
most of them deserve to be thanked for some reasons Heartfelt thanks to my Lab fellows
Mr Mohammad Taufiq Mr Imdad Khan Mr Mohammad Saeed Rahmat Ali and
Mohammad Hamayun for their sincere cooperation and friendly behavior throughout the
time I spent with them
And at last
Dear family members thank you very much for standing with me through thick and thin
Mr Mohammad Sadiq
iii
Abstract
Alcohols and cyclic alkanes oxidation in an environment friendly protocol was carried
out in a typical batch reactor These reactions were carried out in solvent free conditions
andor in eco-friendly solvents using molecular oxygen as the only oxidant and ZrO2
andor ZrO2 supported noble metals (Pt Pd) as catalysts The influence of different
reaction parameters (speed of agitation reaction time and temperature) catalyst
parameters (calcination temperature and loading) and oxygen partial pressure on the
catalyst performance was studied Different modern techniques such as (FT-IR XRD
SEM EDX surface and pores size analyzer and particle size analyzer) were used for the
characterization of catalyst ZrO2 calcined at 1223 K was found to be more active as a
single catalyst than the one calcined at 723 K for alcohol oxidation to the corresponding
carbonyl products under solvent free conditions and in ecofriendly solvent as well
Platinum supported on zirconia was highly active and selective for oxidation of benzyl
alcohol to benzaldehyde in n- heptane and toluene to benzoic acid in both solvent free
conditions and in aqueous medium Similarly zirconia supported Pt or Pd catalysts were
tested for cyclohexane oxidation in solvent free conditions and for phenol oxidation in
aqueous medium Both catalysts have shown magnificent catalytic activity Bismuth was
added as a promoter to these catalysts Bismuth promoted PtZrO2 has shown outstanding
catalytic performance These catalysts are insoluble in the reaction mixture and can be
easily separated by simple filtration and reused Typical batch reactorrsquos kinetic data were
obtained and fitted to the classical LangmuirndashHinshelwood Marsndashvan Krevelen and as
well as to the Eley-Rideal model of heterogeneously catalyzed reactions In alcohol
oxidation reactions the Langmuir-Hinshelwood model was found to give a better fit The
rate-determining step was proposed to involve direct interaction of an adsorbed oxidizing
species with the adsorbed reactant or an intermediate product of the reactant While in
toluene oxidation the Eley-Rideal model was found to give a better fit Eley-Rideal
mechanism envisages reaction between adsorbed oxygen with hydrocarbon molecules
from the fluid phase The calculated apparent activation energy and agitation effect have
shown the absence of mass transfer effect
Keywords Catalysis solvent free eco-friendly solvents organic oxidation reactions mild conditions
iv
List of Publications
Thesis includes the following papers which were published in different international
journals and presented at various conferences
I Ilyas M Sadiq M Imdad K Chin J Catal 2007 28 413
II Ilyas M Sadiq M Chem Eng Technol 2007 30 1391-1397
III Ilyas M Sadiq M Chin J Chem 2008 26 146
IV Ilyas M Sadiq M Catal Lett 2009 128 337
V Ilyas M Sadiq M ldquoInvestigating the activity of zirconia as a catalyst
and a support for noble metals in green oxidation of cyclohexanerdquo J
Iran Chem Soc Submitted
VI M Ilyas M Sadiq ldquoA model catalyst for aerobic oxidation of toluene in
aqueous solutionrdquo presented in 12th International Conference of the
Pacific Basin Consortium for Environment amp Health Sciences at Beijing
University China 26-29 October 2007 (Submitted to Catalysis Letter)
VII M Ilyas M Sadiq ldquoOxidation of benzyl alcohol in aqueous medium by
zirconia catalyst at mild conditionsrdquo presented in 18th National Chemistry
Conference in Institute of Chemistry University of Punjab Lahore
Pakistan 25-27 February 2008
VIII M Ilyas M Sadiq ldquoComparative study of commercially available ZrO2
and laboratory prepared ZrO2 for liquid phase solvent free oxidation of
cyclohexanolrdquo presented in 18th National Chemistry Conference Institute
of Chemistry University of Punjab Lahore Pakistan 25-27 February
2008
IX M Ilyas M Sadiq ldquoZirconia-supported noble metals catalyst for
oxidation of phenol in artificially contaminated water at milder
conditionsrdquo presented in 1st National Symposium on Analytical
Environmental and Applied Chemistry in Shah Abdul Latif University
Khairpur Sindh Pakistan 24-25 October 2008
v
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Acknowledgment ii
Abstract iii
List of Publications iv
Chapter 1 Introduction
11 Aims and objective 01
12 Zirconia in Catalysis 02
13 Oxidation of alcohols 03
14 Oxidation of toluene 06
15 Oxidation of cyclohexane 09
16 Oxidation of phenol 09
17 Characterization of catalyst 11
171 Surface area Measurements 11
172 Particle size measurement 11
173 X-ray differactometry 12
174 Infrared Spectroscopy 12
175 Scanning Electron Microscopy 13
Chapter 2 Literature review 14
References 20
vi
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 3 Experimental
31 Material 30
32 Preparation of catalyst 30
321 Laboratory prepared ZrO2 30
322 Optimal conditions 32
323 Commercial ZrO2 32
324 Supported catalyst 32
33 Characterization of catalysts 32
34 Experimental setups for different reaction 33
35 Liquid-phase oxidation in solvent free conditions 37
351 Design of reactor for liquid phase oxidation in
solvent free condition 37
36 Liquid-phase oxidation in eco-friendly solvents 38
361 Design of reactor for liquid phase oxidation in
eco-friendly solvents 38
37 Analysis of reaction mixture 39
38 Heterogeneous nature of the catalyst 41
References 42
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
1 Homogeneous Catalysis Parshall GW Ittel SD 2Ed John Wiley amp Sons
Inc Nova Iorque 1992
2 Cornils B Herrmann W Eds Applied Homogeneous Catalysis with
Organometallic Compounds Vol 1 VCH 1996 Chapter 24
3 Anastas PT Warner JC Green Chemistry Theory and Practice Oxford
University Press Oxford 1998
4 Puzari A Jubaraj B J Mol Catal A Chem 2002 187 149
5 Gates B C Catalytic Chemistry John Wiley and Sons New York 1992
6 Yamaguchi T Catal Today 1994 20 199
7 Ozawa M Kimura M J Mater Sci Lett 1990 9 446
8 Inoue M Kominami H Inui T Appl Catal A 1993 97 L25-30
9 Aiken B Hsu W P Matijevid E J Mater Sci1990 25 1886
10 Garg A Matijevid E J Colloid Interface Sci1988 126 243
11 Mercera P D L Van Ommen J G Doesburg E B M Burggraaf AJ
Ross JRH Appl Catal1990 57127
12 Mercera PDL Van Ommen JG Doesburg EBM Burggraaf AJ Ross
JRH Appl Catal1991 78 79
13 Srinivasan R Taulbee D Davis BH Catal Lett 1991 9 1
14 Norman C J Goulding PA McAlpine I Catal Today1994 20 313
15 Mallat T Baiker A Chem Rev 2004 104 3037
16 Muzart J Tetrahedron 2003 59 5789
17 Rafelt J S Clark J H Catal Today 2000 57 33
18 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 57 143
19 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432
21
20 Hutchings G J Carrettin S Landon P Edwards JK Enache D
Knight DW Xu Y CarleyAF Top Catal 2006 38 223-230
21 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
22 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
23 Opre Z Grunwaldt JD Mallat T BaikerA J Mol Catal A Chem 2005
242 224-232
24 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241
287-293
25 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
26 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
27 Ji H B Song J He B Qian Y React Kinet Catal Lett 2004 82 97
28 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-
52
29 Choudhary V R Dhar A Jana P Jha R de Upha B S Green Chem
2005 7 768
30 Choudhary V R Jha R Jana P Green Chem 2007 9 267
31 Enache D I Edwards J K Landon P Espiru B S Carley A F
Herzing A H Watanabe M Kiely C J Knight D W Hutchings G J
Science 2006 311 362
32 Li G Enache D I Edwards J K Carley A F Knight D W Hutchings
G J Catal Lett 2006 110 7
33 Ilyas M Abdullah M N U Phys Chem 2003 14 19
34 Ilyas M Ikramullah Catal Commun 2004 5 1
35 Rache A Kumari V Rao P K In Gupta N M Chakrabarty D K eds
Catalysis Modern Trends New Delhi Narosa 1995 346
36 Li X Xu J Wang F Gao J Zhou L Yang G Catalysis Letters
2006 108 137
37 Heyns K Blazejewicz L Tetrahedron 1960 9 67
22
38 Heyns K Paulsen H in ldquo Newer Methods of Preparative Organic
Chemistryrdquo W Forest Eds Academic Press New York 1963 Vol 2 pp
303-335
39 Christoskova St Stoyanova M Water Res 2002 36 2297-2303
40 Christoskova St Final Report Contract X-123 National Science Fund
Ministry of Education and Science Republic of Bulgaria 1993
41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
Appl Catal A General 1995 128 219ndash229
43 Munter R Proc Estonian Sci Chem 2001 50 59-804
44 Mishra V S Mahajani VV Joshi JB Ind Eng Chem Res 1995 34 2
45 Imamura S Ind Eng Chem Res 1999 38 1743
46 Pintar Catal Today 2003 77 451
47 Matatov-Meytal Y I Sheintuch M Ind Eng Chem Res 1998 37 309
48 Luck F Catal Today 1999 53 81
49 Kolaczkowski S T Plucinski P Beltran FJ Rivas F Lurgh DB Chem
Eng J 1999 73 143
50 Iliuta Larachi F Chem Eng Proc 2001 40175
51 Fortuny C Ferrer C Bengoa J Font and Fabregat A Catal Today 1995
24 79
52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
B Environ 1998 16 53
53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
2153
54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
55 Santos A Yustos P Durban B Garcia-Ochoa F Environ Sci Technol
2001 35 2828
56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
181
57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
2001 32 151
60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
Technol Biotechnol 2001 76 743
61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
1998 37 3561
63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
1999 188 311
65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
JE Appl Catal B Environ 2001 30 195
69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
iii
Abstract
Alcohols and cyclic alkanes oxidation in an environment friendly protocol was carried
out in a typical batch reactor These reactions were carried out in solvent free conditions
andor in eco-friendly solvents using molecular oxygen as the only oxidant and ZrO2
andor ZrO2 supported noble metals (Pt Pd) as catalysts The influence of different
reaction parameters (speed of agitation reaction time and temperature) catalyst
parameters (calcination temperature and loading) and oxygen partial pressure on the
catalyst performance was studied Different modern techniques such as (FT-IR XRD
SEM EDX surface and pores size analyzer and particle size analyzer) were used for the
characterization of catalyst ZrO2 calcined at 1223 K was found to be more active as a
single catalyst than the one calcined at 723 K for alcohol oxidation to the corresponding
carbonyl products under solvent free conditions and in ecofriendly solvent as well
Platinum supported on zirconia was highly active and selective for oxidation of benzyl
alcohol to benzaldehyde in n- heptane and toluene to benzoic acid in both solvent free
conditions and in aqueous medium Similarly zirconia supported Pt or Pd catalysts were
tested for cyclohexane oxidation in solvent free conditions and for phenol oxidation in
aqueous medium Both catalysts have shown magnificent catalytic activity Bismuth was
added as a promoter to these catalysts Bismuth promoted PtZrO2 has shown outstanding
catalytic performance These catalysts are insoluble in the reaction mixture and can be
easily separated by simple filtration and reused Typical batch reactorrsquos kinetic data were
obtained and fitted to the classical LangmuirndashHinshelwood Marsndashvan Krevelen and as
well as to the Eley-Rideal model of heterogeneously catalyzed reactions In alcohol
oxidation reactions the Langmuir-Hinshelwood model was found to give a better fit The
rate-determining step was proposed to involve direct interaction of an adsorbed oxidizing
species with the adsorbed reactant or an intermediate product of the reactant While in
toluene oxidation the Eley-Rideal model was found to give a better fit Eley-Rideal
mechanism envisages reaction between adsorbed oxygen with hydrocarbon molecules
from the fluid phase The calculated apparent activation energy and agitation effect have
shown the absence of mass transfer effect
Keywords Catalysis solvent free eco-friendly solvents organic oxidation reactions mild conditions
iv
List of Publications
Thesis includes the following papers which were published in different international
journals and presented at various conferences
I Ilyas M Sadiq M Imdad K Chin J Catal 2007 28 413
II Ilyas M Sadiq M Chem Eng Technol 2007 30 1391-1397
III Ilyas M Sadiq M Chin J Chem 2008 26 146
IV Ilyas M Sadiq M Catal Lett 2009 128 337
V Ilyas M Sadiq M ldquoInvestigating the activity of zirconia as a catalyst
and a support for noble metals in green oxidation of cyclohexanerdquo J
Iran Chem Soc Submitted
VI M Ilyas M Sadiq ldquoA model catalyst for aerobic oxidation of toluene in
aqueous solutionrdquo presented in 12th International Conference of the
Pacific Basin Consortium for Environment amp Health Sciences at Beijing
University China 26-29 October 2007 (Submitted to Catalysis Letter)
VII M Ilyas M Sadiq ldquoOxidation of benzyl alcohol in aqueous medium by
zirconia catalyst at mild conditionsrdquo presented in 18th National Chemistry
Conference in Institute of Chemistry University of Punjab Lahore
Pakistan 25-27 February 2008
VIII M Ilyas M Sadiq ldquoComparative study of commercially available ZrO2
and laboratory prepared ZrO2 for liquid phase solvent free oxidation of
cyclohexanolrdquo presented in 18th National Chemistry Conference Institute
of Chemistry University of Punjab Lahore Pakistan 25-27 February
2008
IX M Ilyas M Sadiq ldquoZirconia-supported noble metals catalyst for
oxidation of phenol in artificially contaminated water at milder
conditionsrdquo presented in 1st National Symposium on Analytical
Environmental and Applied Chemistry in Shah Abdul Latif University
Khairpur Sindh Pakistan 24-25 October 2008
v
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Acknowledgment ii
Abstract iii
List of Publications iv
Chapter 1 Introduction
11 Aims and objective 01
12 Zirconia in Catalysis 02
13 Oxidation of alcohols 03
14 Oxidation of toluene 06
15 Oxidation of cyclohexane 09
16 Oxidation of phenol 09
17 Characterization of catalyst 11
171 Surface area Measurements 11
172 Particle size measurement 11
173 X-ray differactometry 12
174 Infrared Spectroscopy 12
175 Scanning Electron Microscopy 13
Chapter 2 Literature review 14
References 20
vi
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 3 Experimental
31 Material 30
32 Preparation of catalyst 30
321 Laboratory prepared ZrO2 30
322 Optimal conditions 32
323 Commercial ZrO2 32
324 Supported catalyst 32
33 Characterization of catalysts 32
34 Experimental setups for different reaction 33
35 Liquid-phase oxidation in solvent free conditions 37
351 Design of reactor for liquid phase oxidation in
solvent free condition 37
36 Liquid-phase oxidation in eco-friendly solvents 38
361 Design of reactor for liquid phase oxidation in
eco-friendly solvents 38
37 Analysis of reaction mixture 39
38 Heterogeneous nature of the catalyst 41
References 42
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
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Catal A 2005 288 175-184
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2004 126 10657-10666
27 Ji H B Song J He B Qian Y React Kinet Catal Lett 2004 82 97
28 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-
52
29 Choudhary V R Dhar A Jana P Jha R de Upha B S Green Chem
2005 7 768
30 Choudhary V R Jha R Jana P Green Chem 2007 9 267
31 Enache D I Edwards J K Landon P Espiru B S Carley A F
Herzing A H Watanabe M Kiely C J Knight D W Hutchings G J
Science 2006 311 362
32 Li G Enache D I Edwards J K Carley A F Knight D W Hutchings
G J Catal Lett 2006 110 7
33 Ilyas M Abdullah M N U Phys Chem 2003 14 19
34 Ilyas M Ikramullah Catal Commun 2004 5 1
35 Rache A Kumari V Rao P K In Gupta N M Chakrabarty D K eds
Catalysis Modern Trends New Delhi Narosa 1995 346
36 Li X Xu J Wang F Gao J Zhou L Yang G Catalysis Letters
2006 108 137
37 Heyns K Blazejewicz L Tetrahedron 1960 9 67
22
38 Heyns K Paulsen H in ldquo Newer Methods of Preparative Organic
Chemistryrdquo W Forest Eds Academic Press New York 1963 Vol 2 pp
303-335
39 Christoskova St Stoyanova M Water Res 2002 36 2297-2303
40 Christoskova St Final Report Contract X-123 National Science Fund
Ministry of Education and Science Republic of Bulgaria 1993
41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
Appl Catal A General 1995 128 219ndash229
43 Munter R Proc Estonian Sci Chem 2001 50 59-804
44 Mishra V S Mahajani VV Joshi JB Ind Eng Chem Res 1995 34 2
45 Imamura S Ind Eng Chem Res 1999 38 1743
46 Pintar Catal Today 2003 77 451
47 Matatov-Meytal Y I Sheintuch M Ind Eng Chem Res 1998 37 309
48 Luck F Catal Today 1999 53 81
49 Kolaczkowski S T Plucinski P Beltran FJ Rivas F Lurgh DB Chem
Eng J 1999 73 143
50 Iliuta Larachi F Chem Eng Proc 2001 40175
51 Fortuny C Ferrer C Bengoa J Font and Fabregat A Catal Today 1995
24 79
52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
B Environ 1998 16 53
53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
2153
54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
55 Santos A Yustos P Durban B Garcia-Ochoa F Environ Sci Technol
2001 35 2828
56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
181
57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
2001 32 151
60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
Technol Biotechnol 2001 76 743
61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
1998 37 3561
63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
1999 188 311
65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
JE Appl Catal B Environ 2001 30 195
69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
iv
List of Publications
Thesis includes the following papers which were published in different international
journals and presented at various conferences
I Ilyas M Sadiq M Imdad K Chin J Catal 2007 28 413
II Ilyas M Sadiq M Chem Eng Technol 2007 30 1391-1397
III Ilyas M Sadiq M Chin J Chem 2008 26 146
IV Ilyas M Sadiq M Catal Lett 2009 128 337
V Ilyas M Sadiq M ldquoInvestigating the activity of zirconia as a catalyst
and a support for noble metals in green oxidation of cyclohexanerdquo J
Iran Chem Soc Submitted
VI M Ilyas M Sadiq ldquoA model catalyst for aerobic oxidation of toluene in
aqueous solutionrdquo presented in 12th International Conference of the
Pacific Basin Consortium for Environment amp Health Sciences at Beijing
University China 26-29 October 2007 (Submitted to Catalysis Letter)
VII M Ilyas M Sadiq ldquoOxidation of benzyl alcohol in aqueous medium by
zirconia catalyst at mild conditionsrdquo presented in 18th National Chemistry
Conference in Institute of Chemistry University of Punjab Lahore
Pakistan 25-27 February 2008
VIII M Ilyas M Sadiq ldquoComparative study of commercially available ZrO2
and laboratory prepared ZrO2 for liquid phase solvent free oxidation of
cyclohexanolrdquo presented in 18th National Chemistry Conference Institute
of Chemistry University of Punjab Lahore Pakistan 25-27 February
2008
IX M Ilyas M Sadiq ldquoZirconia-supported noble metals catalyst for
oxidation of phenol in artificially contaminated water at milder
conditionsrdquo presented in 1st National Symposium on Analytical
Environmental and Applied Chemistry in Shah Abdul Latif University
Khairpur Sindh Pakistan 24-25 October 2008
v
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Acknowledgment ii
Abstract iii
List of Publications iv
Chapter 1 Introduction
11 Aims and objective 01
12 Zirconia in Catalysis 02
13 Oxidation of alcohols 03
14 Oxidation of toluene 06
15 Oxidation of cyclohexane 09
16 Oxidation of phenol 09
17 Characterization of catalyst 11
171 Surface area Measurements 11
172 Particle size measurement 11
173 X-ray differactometry 12
174 Infrared Spectroscopy 12
175 Scanning Electron Microscopy 13
Chapter 2 Literature review 14
References 20
vi
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 3 Experimental
31 Material 30
32 Preparation of catalyst 30
321 Laboratory prepared ZrO2 30
322 Optimal conditions 32
323 Commercial ZrO2 32
324 Supported catalyst 32
33 Characterization of catalysts 32
34 Experimental setups for different reaction 33
35 Liquid-phase oxidation in solvent free conditions 37
351 Design of reactor for liquid phase oxidation in
solvent free condition 37
36 Liquid-phase oxidation in eco-friendly solvents 38
361 Design of reactor for liquid phase oxidation in
eco-friendly solvents 38
37 Analysis of reaction mixture 39
38 Heterogeneous nature of the catalyst 41
References 42
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
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Inc Nova Iorque 1992
2 Cornils B Herrmann W Eds Applied Homogeneous Catalysis with
Organometallic Compounds Vol 1 VCH 1996 Chapter 24
3 Anastas PT Warner JC Green Chemistry Theory and Practice Oxford
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4 Puzari A Jubaraj B J Mol Catal A Chem 2002 187 149
5 Gates B C Catalytic Chemistry John Wiley and Sons New York 1992
6 Yamaguchi T Catal Today 1994 20 199
7 Ozawa M Kimura M J Mater Sci Lett 1990 9 446
8 Inoue M Kominami H Inui T Appl Catal A 1993 97 L25-30
9 Aiken B Hsu W P Matijevid E J Mater Sci1990 25 1886
10 Garg A Matijevid E J Colloid Interface Sci1988 126 243
11 Mercera P D L Van Ommen J G Doesburg E B M Burggraaf AJ
Ross JRH Appl Catal1990 57127
12 Mercera PDL Van Ommen JG Doesburg EBM Burggraaf AJ Ross
JRH Appl Catal1991 78 79
13 Srinivasan R Taulbee D Davis BH Catal Lett 1991 9 1
14 Norman C J Goulding PA McAlpine I Catal Today1994 20 313
15 Mallat T Baiker A Chem Rev 2004 104 3037
16 Muzart J Tetrahedron 2003 59 5789
17 Rafelt J S Clark J H Catal Today 2000 57 33
18 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 57 143
19 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432
21
20 Hutchings G J Carrettin S Landon P Edwards JK Enache D
Knight DW Xu Y CarleyAF Top Catal 2006 38 223-230
21 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
22 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
23 Opre Z Grunwaldt JD Mallat T BaikerA J Mol Catal A Chem 2005
242 224-232
24 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241
287-293
25 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
26 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
27 Ji H B Song J He B Qian Y React Kinet Catal Lett 2004 82 97
28 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-
52
29 Choudhary V R Dhar A Jana P Jha R de Upha B S Green Chem
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30 Choudhary V R Jha R Jana P Green Chem 2007 9 267
31 Enache D I Edwards J K Landon P Espiru B S Carley A F
Herzing A H Watanabe M Kiely C J Knight D W Hutchings G J
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32 Li G Enache D I Edwards J K Carley A F Knight D W Hutchings
G J Catal Lett 2006 110 7
33 Ilyas M Abdullah M N U Phys Chem 2003 14 19
34 Ilyas M Ikramullah Catal Commun 2004 5 1
35 Rache A Kumari V Rao P K In Gupta N M Chakrabarty D K eds
Catalysis Modern Trends New Delhi Narosa 1995 346
36 Li X Xu J Wang F Gao J Zhou L Yang G Catalysis Letters
2006 108 137
37 Heyns K Blazejewicz L Tetrahedron 1960 9 67
22
38 Heyns K Paulsen H in ldquo Newer Methods of Preparative Organic
Chemistryrdquo W Forest Eds Academic Press New York 1963 Vol 2 pp
303-335
39 Christoskova St Stoyanova M Water Res 2002 36 2297-2303
40 Christoskova St Final Report Contract X-123 National Science Fund
Ministry of Education and Science Republic of Bulgaria 1993
41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
Appl Catal A General 1995 128 219ndash229
43 Munter R Proc Estonian Sci Chem 2001 50 59-804
44 Mishra V S Mahajani VV Joshi JB Ind Eng Chem Res 1995 34 2
45 Imamura S Ind Eng Chem Res 1999 38 1743
46 Pintar Catal Today 2003 77 451
47 Matatov-Meytal Y I Sheintuch M Ind Eng Chem Res 1998 37 309
48 Luck F Catal Today 1999 53 81
49 Kolaczkowski S T Plucinski P Beltran FJ Rivas F Lurgh DB Chem
Eng J 1999 73 143
50 Iliuta Larachi F Chem Eng Proc 2001 40175
51 Fortuny C Ferrer C Bengoa J Font and Fabregat A Catal Today 1995
24 79
52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
B Environ 1998 16 53
53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
2153
54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
55 Santos A Yustos P Durban B Garcia-Ochoa F Environ Sci Technol
2001 35 2828
56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
181
57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
2001 32 151
60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
Technol Biotechnol 2001 76 743
61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
1998 37 3561
63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
1999 188 311
65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
JE Appl Catal B Environ 2001 30 195
69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
v
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Acknowledgment ii
Abstract iii
List of Publications iv
Chapter 1 Introduction
11 Aims and objective 01
12 Zirconia in Catalysis 02
13 Oxidation of alcohols 03
14 Oxidation of toluene 06
15 Oxidation of cyclohexane 09
16 Oxidation of phenol 09
17 Characterization of catalyst 11
171 Surface area Measurements 11
172 Particle size measurement 11
173 X-ray differactometry 12
174 Infrared Spectroscopy 12
175 Scanning Electron Microscopy 13
Chapter 2 Literature review 14
References 20
vi
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 3 Experimental
31 Material 30
32 Preparation of catalyst 30
321 Laboratory prepared ZrO2 30
322 Optimal conditions 32
323 Commercial ZrO2 32
324 Supported catalyst 32
33 Characterization of catalysts 32
34 Experimental setups for different reaction 33
35 Liquid-phase oxidation in solvent free conditions 37
351 Design of reactor for liquid phase oxidation in
solvent free condition 37
36 Liquid-phase oxidation in eco-friendly solvents 38
361 Design of reactor for liquid phase oxidation in
eco-friendly solvents 38
37 Analysis of reaction mixture 39
38 Heterogeneous nature of the catalyst 41
References 42
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
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Inc Nova Iorque 1992
2 Cornils B Herrmann W Eds Applied Homogeneous Catalysis with
Organometallic Compounds Vol 1 VCH 1996 Chapter 24
3 Anastas PT Warner JC Green Chemistry Theory and Practice Oxford
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4 Puzari A Jubaraj B J Mol Catal A Chem 2002 187 149
5 Gates B C Catalytic Chemistry John Wiley and Sons New York 1992
6 Yamaguchi T Catal Today 1994 20 199
7 Ozawa M Kimura M J Mater Sci Lett 1990 9 446
8 Inoue M Kominami H Inui T Appl Catal A 1993 97 L25-30
9 Aiken B Hsu W P Matijevid E J Mater Sci1990 25 1886
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12 Mercera PDL Van Ommen JG Doesburg EBM Burggraaf AJ Ross
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20 Hutchings G J Carrettin S Landon P Edwards JK Enache D
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27 Ji H B Song J He B Qian Y React Kinet Catal Lett 2004 82 97
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29 Choudhary V R Dhar A Jana P Jha R de Upha B S Green Chem
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Herzing A H Watanabe M Kiely C J Knight D W Hutchings G J
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33 Ilyas M Abdullah M N U Phys Chem 2003 14 19
34 Ilyas M Ikramullah Catal Commun 2004 5 1
35 Rache A Kumari V Rao P K In Gupta N M Chakrabarty D K eds
Catalysis Modern Trends New Delhi Narosa 1995 346
36 Li X Xu J Wang F Gao J Zhou L Yang G Catalysis Letters
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37 Heyns K Blazejewicz L Tetrahedron 1960 9 67
22
38 Heyns K Paulsen H in ldquo Newer Methods of Preparative Organic
Chemistryrdquo W Forest Eds Academic Press New York 1963 Vol 2 pp
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39 Christoskova St Stoyanova M Water Res 2002 36 2297-2303
40 Christoskova St Final Report Contract X-123 National Science Fund
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41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
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43 Munter R Proc Estonian Sci Chem 2001 50 59-804
44 Mishra V S Mahajani VV Joshi JB Ind Eng Chem Res 1995 34 2
45 Imamura S Ind Eng Chem Res 1999 38 1743
46 Pintar Catal Today 2003 77 451
47 Matatov-Meytal Y I Sheintuch M Ind Eng Chem Res 1998 37 309
48 Luck F Catal Today 1999 53 81
49 Kolaczkowski S T Plucinski P Beltran FJ Rivas F Lurgh DB Chem
Eng J 1999 73 143
50 Iliuta Larachi F Chem Eng Proc 2001 40175
51 Fortuny C Ferrer C Bengoa J Font and Fabregat A Catal Today 1995
24 79
52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
B Environ 1998 16 53
53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
2153
54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
55 Santos A Yustos P Durban B Garcia-Ochoa F Environ Sci Technol
2001 35 2828
56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
181
57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
2001 32 151
60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
Technol Biotechnol 2001 76 743
61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
1998 37 3561
63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
1999 188 311
65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
JE Appl Catal B Environ 2001 30 195
69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
vi
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 3 Experimental
31 Material 30
32 Preparation of catalyst 30
321 Laboratory prepared ZrO2 30
322 Optimal conditions 32
323 Commercial ZrO2 32
324 Supported catalyst 32
33 Characterization of catalysts 32
34 Experimental setups for different reaction 33
35 Liquid-phase oxidation in solvent free conditions 37
351 Design of reactor for liquid phase oxidation in
solvent free condition 37
36 Liquid-phase oxidation in eco-friendly solvents 38
361 Design of reactor for liquid phase oxidation in
eco-friendly solvents 38
37 Analysis of reaction mixture 39
38 Heterogeneous nature of the catalyst 41
References 42
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
1 Homogeneous Catalysis Parshall GW Ittel SD 2Ed John Wiley amp Sons
Inc Nova Iorque 1992
2 Cornils B Herrmann W Eds Applied Homogeneous Catalysis with
Organometallic Compounds Vol 1 VCH 1996 Chapter 24
3 Anastas PT Warner JC Green Chemistry Theory and Practice Oxford
University Press Oxford 1998
4 Puzari A Jubaraj B J Mol Catal A Chem 2002 187 149
5 Gates B C Catalytic Chemistry John Wiley and Sons New York 1992
6 Yamaguchi T Catal Today 1994 20 199
7 Ozawa M Kimura M J Mater Sci Lett 1990 9 446
8 Inoue M Kominami H Inui T Appl Catal A 1993 97 L25-30
9 Aiken B Hsu W P Matijevid E J Mater Sci1990 25 1886
10 Garg A Matijevid E J Colloid Interface Sci1988 126 243
11 Mercera P D L Van Ommen J G Doesburg E B M Burggraaf AJ
Ross JRH Appl Catal1990 57127
12 Mercera PDL Van Ommen JG Doesburg EBM Burggraaf AJ Ross
JRH Appl Catal1991 78 79
13 Srinivasan R Taulbee D Davis BH Catal Lett 1991 9 1
14 Norman C J Goulding PA McAlpine I Catal Today1994 20 313
15 Mallat T Baiker A Chem Rev 2004 104 3037
16 Muzart J Tetrahedron 2003 59 5789
17 Rafelt J S Clark J H Catal Today 2000 57 33
18 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 57 143
19 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432
21
20 Hutchings G J Carrettin S Landon P Edwards JK Enache D
Knight DW Xu Y CarleyAF Top Catal 2006 38 223-230
21 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
22 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
23 Opre Z Grunwaldt JD Mallat T BaikerA J Mol Catal A Chem 2005
242 224-232
24 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241
287-293
25 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
26 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
27 Ji H B Song J He B Qian Y React Kinet Catal Lett 2004 82 97
28 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-
52
29 Choudhary V R Dhar A Jana P Jha R de Upha B S Green Chem
2005 7 768
30 Choudhary V R Jha R Jana P Green Chem 2007 9 267
31 Enache D I Edwards J K Landon P Espiru B S Carley A F
Herzing A H Watanabe M Kiely C J Knight D W Hutchings G J
Science 2006 311 362
32 Li G Enache D I Edwards J K Carley A F Knight D W Hutchings
G J Catal Lett 2006 110 7
33 Ilyas M Abdullah M N U Phys Chem 2003 14 19
34 Ilyas M Ikramullah Catal Commun 2004 5 1
35 Rache A Kumari V Rao P K In Gupta N M Chakrabarty D K eds
Catalysis Modern Trends New Delhi Narosa 1995 346
36 Li X Xu J Wang F Gao J Zhou L Yang G Catalysis Letters
2006 108 137
37 Heyns K Blazejewicz L Tetrahedron 1960 9 67
22
38 Heyns K Paulsen H in ldquo Newer Methods of Preparative Organic
Chemistryrdquo W Forest Eds Academic Press New York 1963 Vol 2 pp
303-335
39 Christoskova St Stoyanova M Water Res 2002 36 2297-2303
40 Christoskova St Final Report Contract X-123 National Science Fund
Ministry of Education and Science Republic of Bulgaria 1993
41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
Appl Catal A General 1995 128 219ndash229
43 Munter R Proc Estonian Sci Chem 2001 50 59-804
44 Mishra V S Mahajani VV Joshi JB Ind Eng Chem Res 1995 34 2
45 Imamura S Ind Eng Chem Res 1999 38 1743
46 Pintar Catal Today 2003 77 451
47 Matatov-Meytal Y I Sheintuch M Ind Eng Chem Res 1998 37 309
48 Luck F Catal Today 1999 53 81
49 Kolaczkowski S T Plucinski P Beltran FJ Rivas F Lurgh DB Chem
Eng J 1999 73 143
50 Iliuta Larachi F Chem Eng Proc 2001 40175
51 Fortuny C Ferrer C Bengoa J Font and Fabregat A Catal Today 1995
24 79
52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
B Environ 1998 16 53
53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
2153
54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
55 Santos A Yustos P Durban B Garcia-Ochoa F Environ Sci Technol
2001 35 2828
56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
181
57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
2001 32 151
60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
Technol Biotechnol 2001 76 743
61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
1998 37 3561
63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
1999 188 311
65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
JE Appl Catal B Environ 2001 30 195
69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
vii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4A Results and discussion
Oxidation of alcohols in solvent free
conditions by zirconia catalyst 43
4A 1 Characterization of catalyst 43
4A 2 Brunauer-Emmet-Teller method (BET) 43
4A 3 X-ray diffraction (XRD) 43
4A 4 Scanning electron microscopy 43
4A 5 Effect of mass transfer 45
4A 6 Effect of calcination temperature 46
4A 7 Effect of reaction time 46
4A 8 Effect of oxygen partial pressure 48
4A 9 Kinetic analysis 48
426 Mechanism of reaction 49
427 Role of oxygen 52
References 54
Chapter 4B Results and discussion
Oxidation of alcohols in aqueous medium by
zirconia catalyst 56
4B 1 Characterization of catalyst 56
4B 2 Oxidation of benzyl alcohols in Aqueous Medium 56
4B 3 Effect of Different Parameters 59
References 62
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
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40 Christoskova St Final Report Contract X-123 National Science Fund
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41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
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57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
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59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
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62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
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65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
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66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
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69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
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72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
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76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
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77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
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78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
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24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
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82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
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86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
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97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
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98 Bastock T E Clark J H Martin K Trentbirth B W Green
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99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
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106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
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115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
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116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
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123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
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125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
viii
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4C Results and discussion
Oxidation of toluene in solvent free
conditions by PtZrO2 63
4C 1 Catalyst characterization 63
4C 2 Catalytic activity 63
4C 3 Time profile study 65
4C 4 Effect of oxygen flow rate 67
4C 5 Appearance of trans-stilbene and
methyl biphenyl carboxylic acid 67
References 70
Chapter 4D Results and discussion
Oxidation of benzyl alcohol by zirconia supported
platinum catalyst 71
4D1 Characterization catalyst 71
4D2 Oxidation of benzyl alcohol 71
4D21 Leaching of the catalyst 72
4D22 Effect of Mass Transfer 74
4D23 Temperature Effect 74
4D24 Solvent Effect 74
4D25 Time course of the reaction 75
4D26 Reaction Kinetics Analysis 75
4D27 Effect of Oxygen Partial Pressure 80
4D 28 Mechanistic proposal 83
References 84
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
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Inc Nova Iorque 1992
2 Cornils B Herrmann W Eds Applied Homogeneous Catalysis with
Organometallic Compounds Vol 1 VCH 1996 Chapter 24
3 Anastas PT Warner JC Green Chemistry Theory and Practice Oxford
University Press Oxford 1998
4 Puzari A Jubaraj B J Mol Catal A Chem 2002 187 149
5 Gates B C Catalytic Chemistry John Wiley and Sons New York 1992
6 Yamaguchi T Catal Today 1994 20 199
7 Ozawa M Kimura M J Mater Sci Lett 1990 9 446
8 Inoue M Kominami H Inui T Appl Catal A 1993 97 L25-30
9 Aiken B Hsu W P Matijevid E J Mater Sci1990 25 1886
10 Garg A Matijevid E J Colloid Interface Sci1988 126 243
11 Mercera P D L Van Ommen J G Doesburg E B M Burggraaf AJ
Ross JRH Appl Catal1990 57127
12 Mercera PDL Van Ommen JG Doesburg EBM Burggraaf AJ Ross
JRH Appl Catal1991 78 79
13 Srinivasan R Taulbee D Davis BH Catal Lett 1991 9 1
14 Norman C J Goulding PA McAlpine I Catal Today1994 20 313
15 Mallat T Baiker A Chem Rev 2004 104 3037
16 Muzart J Tetrahedron 2003 59 5789
17 Rafelt J S Clark J H Catal Today 2000 57 33
18 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 57 143
19 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432
21
20 Hutchings G J Carrettin S Landon P Edwards JK Enache D
Knight DW Xu Y CarleyAF Top Catal 2006 38 223-230
21 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
22 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
23 Opre Z Grunwaldt JD Mallat T BaikerA J Mol Catal A Chem 2005
242 224-232
24 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241
287-293
25 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
26 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
27 Ji H B Song J He B Qian Y React Kinet Catal Lett 2004 82 97
28 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-
52
29 Choudhary V R Dhar A Jana P Jha R de Upha B S Green Chem
2005 7 768
30 Choudhary V R Jha R Jana P Green Chem 2007 9 267
31 Enache D I Edwards J K Landon P Espiru B S Carley A F
Herzing A H Watanabe M Kiely C J Knight D W Hutchings G J
Science 2006 311 362
32 Li G Enache D I Edwards J K Carley A F Knight D W Hutchings
G J Catal Lett 2006 110 7
33 Ilyas M Abdullah M N U Phys Chem 2003 14 19
34 Ilyas M Ikramullah Catal Commun 2004 5 1
35 Rache A Kumari V Rao P K In Gupta N M Chakrabarty D K eds
Catalysis Modern Trends New Delhi Narosa 1995 346
36 Li X Xu J Wang F Gao J Zhou L Yang G Catalysis Letters
2006 108 137
37 Heyns K Blazejewicz L Tetrahedron 1960 9 67
22
38 Heyns K Paulsen H in ldquo Newer Methods of Preparative Organic
Chemistryrdquo W Forest Eds Academic Press New York 1963 Vol 2 pp
303-335
39 Christoskova St Stoyanova M Water Res 2002 36 2297-2303
40 Christoskova St Final Report Contract X-123 National Science Fund
Ministry of Education and Science Republic of Bulgaria 1993
41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
Appl Catal A General 1995 128 219ndash229
43 Munter R Proc Estonian Sci Chem 2001 50 59-804
44 Mishra V S Mahajani VV Joshi JB Ind Eng Chem Res 1995 34 2
45 Imamura S Ind Eng Chem Res 1999 38 1743
46 Pintar Catal Today 2003 77 451
47 Matatov-Meytal Y I Sheintuch M Ind Eng Chem Res 1998 37 309
48 Luck F Catal Today 1999 53 81
49 Kolaczkowski S T Plucinski P Beltran FJ Rivas F Lurgh DB Chem
Eng J 1999 73 143
50 Iliuta Larachi F Chem Eng Proc 2001 40175
51 Fortuny C Ferrer C Bengoa J Font and Fabregat A Catal Today 1995
24 79
52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
B Environ 1998 16 53
53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
2153
54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
55 Santos A Yustos P Durban B Garcia-Ochoa F Environ Sci Technol
2001 35 2828
56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
181
57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
2001 32 151
60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
Technol Biotechnol 2001 76 743
61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
1998 37 3561
63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
1999 188 311
65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
JE Appl Catal B Environ 2001 30 195
69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX
ix
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
Chapter 4E Results and discussion
Oxidation of toluene in aqueous medium
by PtZrO2 86
4E 1 Characterization of catalyst 86
4E 2 Effect of substrate concentration 86
4E 3 Effect of temperature 88
4E 4 Agitation effect 88
4E 5 Effect of catalyst loading 88
4E 6 Time profile study 90
4E 7 Effect of oxygen partial pressure 90
4E 8 Reaction kinetics analysis 90
4E 9 Comparison of different catalysts 94
References 95
Chapter 4F Results and discussion
Oxidation of cyclohexane in solvent free
by zirconia supported noble metals 96
4F1 Characterization of catalyst 96
4F2 Oxidation of cyclohexane 98
4F3 Optimal conditions for better catalytic activity 100
References 102
Chapter 4G Results and discussion
Oxidation of phenol in aqueous medium
by zirconia-supported noble metals 103
4G1 Characterization of catalyst 103
4G2 Catalytic oxidation of phenol 108
x
TABLE OF CONTENTS
CHAPTER No PARTICULARS PAGE No
4G3 Effect of different parameters 108
4G4 Time profile study 108
4G5 Comparison of different catalysts 108
4G6 Effect of Pd and Pt loading on catalytic activity 110
4G 7 Effect of bismuth addition on catalytic activity 110
4G 8 Influence of reduction on catalytic activity 110
4G 9 Effect of temperature 110
References 112
Chapter 5 Concluding review 113
1
Chapter 1
Introduction
Oxidation of organic compounds is well established reaction for the synthesis of
fine chemicals on industrial scale [1 2] Different reagents and methods are used in
laboratory as well as in industries for organic oxidation reactions Commonly oxidation
reactions are performed with stoichiometric amounts of oxidants such as peroxides or
high oxidation state metal oxides Most of them share common disadvantages such as
expensive and toxic oxidants [3] On industrial scale the use of stoichiometric oxidants
is not a striking choice For these kinds of reactions an alternative and environmentally
benign oxidant is welcome For industrial scale oxidation molecular oxygen is an ideal
oxidant because it is easily accessible cheap and non-toxic [4] Currently molecular
oxygen is used in several large-scale oxidation reactions catalyzed by inorganic
heterogeneous catalysts carried out at high temperatures and pressures often in the gas
phase [5] The most promising solution to replace these toxic oxidants and harsh
conditions of temperature and pressure is supported noble metals catalysts which are
able to catalyze selective oxidation reactions under mild conditions by using molecular
oxygen The aim of this work was to investigate the activity of zirconia as a catalyst and a
support for noble metals in organic oxidation reactions at milder conditions of
temperature and pressure using molecular oxygen as oxidizing agent in solvent free
condition andor using ecofriendly solvents like water
11 Aims and objectives
The present-day research requirements put pressure on the chemist to divert their
research in a way that preserves the environment and to develop procedures that are
acceptable both economically and environmentally Therefore keeping in mind the above
requirements the present study is launched to achieve the following aims and objectives
i To search a catalyst that could work under mild conditions for the oxidation of
alkanes and alcohols
2
ii Free of solvents system is an ideal system therefore to develop a reaction
system that could be run without using a solvent in the liquid phase
iii To develop a reaction system according to the principles of green chemistry
using environment acceptable solvents like water
iv A reaction that uses many raw materials especially expensive materials is
economically unfavorable therefore this study reduces the use of raw
materials for this reaction system
v A reaction system with more undesirable side products especially
environmentally hazard products is rather unacceptable in the modern
research Therefore it is aimed to develop a reaction system that produces less
undesirable side product in low amounts that could not damage the
environment
vi This study is aimed to run a reaction system that would use simple process of
separation to recover the reaction materials easily
vii In this study solid ZrO2 and or ZrO2 supported noble metals are used as a
catalyst with the aim to recover the catalyst by simple filtration and to reuse
the catalyst for a longer time
viii To minimize the cost of the reaction it is aimed to carry out the reaction at
lower temperature
To sum up major objectives of the present study is to simplify the reaction with the
aim to minimize the pollution effect to gather with reduction in energy and raw materials
to economize the system
12 Zirconia in catalysis
Over the years zirconia has been largely used as a catalytic material because of
its unique chemical and physical characteristics such as thermal stability mechanical
stability excellent chemical resistance acidic basic reducing and oxidizing surface
properties polymorphism and different precursors Zirconia is increasingly used in
catalysis as both a catalyst and a catalyst support [6] A particular benefit of using
zirconia as a catalyst or as a support over other well-established supportscatalyst systems
is its enhanced thermal and chemical stability However one drawback in the use of
3
zirconia is its rather low surface area Alumina supports with surface area of ~200 m2g
are produced commercially whereas less than 50 m2g are reported for most available
zirconia But it is known that activity and surface area of the zirconia catalysts
significantly depends on precursorrsquos material and preparation procedure therefore
extensive research efforts have been made to produce zirconia with high surface area
using novel preparation methods or by incorporation of other components [7-14]
However for many catalytic purposes the incorporation of some of these oxides or
dopants may not be desired as they may lead to side reactions or reduced activity
The value of zirconia in catalysis is being increasingly recognized and this work
focuses on a number of applications where zirconia (as a catalyst and a support) gaining
academic and commercial acceptance
13 Oxidation of alcohols
Oxidation of organic substrates leads to the production of many functionalized
molecules that are of great commercial and synthetic importance In this regard selective
oxidation of alcohols to carbonyl compounds is a fundamental transformation in organic
chemistry as carbonyl compounds are widely used as intermediates for fine chemicals
[15-17] The traditional inorganic oxidants such as permanganate and dichromate
however are toxic and produce a large amount of waste The separation and disposal of
this waste increases steps in chemical processes Therefore from both economic and
environmental viewpoints there is an urgent need for greener and more efficient methods
that replace these toxic oxidants with clean oxidants such as O2 and H2O2 and a
(preferably separable and reusable) catalyst Many researchers have reported the use of
molecular oxygen as an oxidant for alcohol oxidation using different catalysts [17-28]
and a variety of solvents
The oxidation of alcohols can be carried out in the following three conditions
i Alcohol oxidation in solvent free conditions
ii Alcohol oxidation in organic solvents
iii Alcohol oxidation in water
4
To make the liquid-phase oxidation of alcohols more selective toward carbonyl
products it should be carried out in the absence of any solvent There are a few methods
reported in the published reports for solvent free oxidation of alcohols using O2 as the
only oxidant [29-32] Choudhary et al [32] reported the use of a supported nano-size gold
catalyst (3ndash8) for the liquid-phase solvent free oxidation of benzyl alcohol with
molecular oxygen (152 kPa) at 413 K U3O8 MgO Al2O3 and ZrO2 were found to be
better support materials than a range of other metal oxides including ZnO CuO Fe2O3
and NiO Benzyl alcohol was oxidized selectively to benzaldehyde with high yield and a
relatively small amount of benzyl benzoate as a co-product In a recent study of benzyl
alcohol oxidation catalyzed by AuU3O8 [30] it was found that the catalyst containing
higher gold concentration and smaller gold particle size showed better process
performance with respect to conversion and selectivity for benzaldehyde The increase in
temperature and reaction duration resulted in higher conversion of alcohol with a slightly
reduced selectivity for benzaldehyde Enache and Li et al [31 32] also reported the
solvent free oxidation of benzyl alcohol to benzaldehyde by O2 with supported Au and
Au-Pd catalysts TiO2 [31] and zeolites [32] were used as support materials The
supported Au-Pd catalyst was found to be an effective catalyst for the solvent free
oxidation of alcohols including benzyl alcohol and 1-octanol The catalysts used in the
above-mentioned studies are more expensive Furthermore these reactions are mostly
carried out at high pressure Replacement of these expensive catalysts with a cheaper
catalyst for alcohol oxidation at ambient pressure is desirable In this regard the focus is
on the use of ZrO2 as the catalyst and catalyst support for alcohol oxidation in the liquid
phase using molecular oxygen as an oxidant at ambient pressure ZrO2 is used as both the
catalyst and catalyst support for a large variety of reactions including the gas-phase
cyclohexanol oxidationdehydrogenation in our laboratory and elsewhere [33- 35]
Different types of solvent can be used for oxidation of alcohols Water is the most
preferred solvent [17- 22] However to avoid over-oxidation of aldehydes to the
corresponding carboxylic acids dry conditions are required which can be achieved in the
presence of organic solvents at a relatively high temperature [15] Among the organic
solvents toluene is more frequently used in alcohol oxidation [15- 23] The present work
is concerned with the selective catalytic oxidation of benzyl alcohol (BzOH) to
5
benzaldehyde (BzH) Conversion of benzyl alcohol to benzaldehyde is used as a model
reaction for oxidation of aromatic alcohols [23 24] Furthermore benzaldehyde by itself
is an important chemical due to its usage as a raw material for a large number of products
in organic synthesis including perfumery beverage and pharmaceutical industries
However there is a report that manganese oxide can catalyze the conversion of toluene to
benzoic acid benzaldehyde benzyl alcohol and benzyl benzoate [36] in solvent free
conditions We have also observed conversion of toluene to benzaldehyde in the presence
of molecular oxygen using Nickel Oxide as catalyst at 90 ˚C Therefore the use of
toluene as a solvent for benzyl alcohol oxidation could be considered as inappropriate
Another solvent having boiling point (98 ˚C) in the same range as toluene (110 ˚C) is n-
heptane Heynes and Blazejewicz [37 38] have reported 78 yield of benzaldehyde in
one hour when pure PtO2 was used as catalyst for benzyl alcohol oxidation using n-
heptane as solvent at 60 ˚C in the presence of molecular oxygen They obtained benzoic
acid (97 yield 10 hours) when PtC was used as catalyst in reflux conditions with the
same solvent In the present work we have reinvestigated the use of n-heptane as solvent
using zirconia supported platinum catalysts in the presence of molecular oxygen
In relation to strict environment legislation the complete degradation of alcohols
or conversion of alcohols to nontoxic compound in industrial wastewater becomes a
debatable issue Diverse industrial effluents contained benzyl alcohol in wide
concentration ranges from (05 to 10 g dmminus3) [39] The presence of benzyl alcohol in
these effluents is challenging the traditional treatments including physical separation
incineration or biological abatement In this framework catalytic oxidation or catalytic
oxidation couple with a biological or physical-chemical treatment offers a good
opportunity to prevent and remedy pollution problems due to the discharge of industrial
wastewater The degradation of organic pollutants aldehydes phenols and alcohols has
attracted considerable attention due to their high toxicity [40- 42]
To overcome environmental restrictions researchers switch to newer methods for
wastewater treatment such as advance oxidation processes [43] and catalytic oxidation
[39- 42] AOPs suffer from the use of expensive oxidants (O3 or H2O2) and the source of
energy On other hand catalytic oxidation yielded satisfactory results in laboratory studies
[44- 50] The lack of stable catalysts has prevented catalytic oxidation from being widely
6
employed as industrial wastewater treatment The most prominent supported catalysts
prone to metal leaching in the hot acidic reaction environment are Cu based metal oxides
[51- 55] and mixed metal oxides (CuO ZnO CoO) [56 57] Supported noble metal
catalyst which appear much more stable although leaching was occasionally observed
eg during the catalytic oxidation of pulp mill effluents over Pd and Pt supported
catalysts [58 59] Another well-known drawback of catalytic oxidation is deactivation of
catalyst due to formation and strong adsorption of carbonaceous deposits on catalytic
surface [60- 62] During the recent decade considerable efforts were focused on
developing stable supported catalysts with high activity toward organic pollutants [63-
76] Unfortunately these catalysts are expensive Search for cheap and stable catalyst for
oxidation of organic contaminants continues Many groups have reviewed the potential
applications of ZrO2 in organic transformations [77- 86] The advantages derived from
the use of ZrO2 as a catalyst ease of separation of products from reaction mixture by
simple filtration recovery and recycling of catalysts etc [87]
14 Oxidation of toluene
Selective catalytic oxidation of toluene to corresponding alcohol aldehyde and
carboxylic acid by molecular oxygen is of great economical and industrial importance
Industrially the oxidation of toluene to benzoic acid (BzOOH) with molecular oxygen is
a key step for phenol synthesis in the Dow Phenol process and for ɛ-caprolactam
formation in Snia-Viscosia process [88- 94] Toluene is also a representative of aromatic
hydrocarbons categorized as hazardous material [95] Thus development of methods for
the oxidation of aromatic compounds such as toluene is also important for environmental
reasons The commercial production of benzoic acid via the catalytic oxidation of toluene
is achieved by heating a solution of the substrate cobalt acetate and bromide promoter in
acetic acid to 250 ordmC with molecular oxygen at several atmosphere of pressure
Although complete conversion is achieved however the use of acidic solvents and
bromide promoter results in difficult separation of product and catalyst large volume of
toxic waste and equipment corrosion The system requires very expensive specialized
equipment fitted with extensive safety features Operating under such extreme conditions
consumes large amount of energy Therefore attempts are being made to make this
7
oxidation more environmentally benign by performing the reaction in the vapor phase
using a variety of solid catalysts [96 97] However liquid-phase oxidation is easy to
operate and achieve high selectivity under relatively mild reaction conditions Many
efforts have been made to improve the efficiency of toluene oxidation in the liquid phase
however most investigation still focus on homogeneous systems using volatile organic
solvents Toluene oxidation can be carried out in
i Solvent free conditions
ii In solvent
Employing heterogeneous catalysts in liquid-phase oxidation of toluene without
solvent would make the process more environmentally friendly Bastock and coworkers
have reported [98] the oxidation of toluene to benzoic acid in solvent free conditions
using a commercial heterogeneous catalyst Envirocat EPAC in the presence of catalytic
amount of carboxylic acid as promoter at atmospheric pressure The reaction was
performed at 110-150 ordmC with oxygen flow rate of 400 mlmin The isolated yield of
benzoic acid was 85 in 22 hours Subrahmanyan et al [99] have performed toluene
oxidation in solvent free conditions using vanadium substituted aluminophosphate or
aluminosilictaes as catalyst Benzaldehyde (BzH) and benzoic acid were the main
products when tert-butyl hydro peroxide was used as the oxidizing agent while cresols
were formed when H2O2 was used as oxidizing agent Raja et al [100101] have also
reported the solvent free oxidation of toluene using zeolite encapsulated metal complexes
as catalysts Air was used as oxidant (35 MPa) The highest conversion (451 ) was
achieved with manganese substituted aluminum phosphate with high benzoic acid
selectivity (834 ) at 150 ordm C in 16 hours Li and coworkers [36-102] have also reported
manganese oxide and copper manganese oxide to be active catalyst for toluene oxidation
to benzoic acid in solvent free conditions with molecular oxygen (10 MPa) at 190-195
ordmC Recently it was observed in this laboratory [103] that when toluene was used as a
solvent for benzyl alcohol (BzOH) oxidation by molecular oxygen at 90 ordmC in the
presence of PtZrO2 as catalyst benzoic acid was obtained with 100 selectivity The
mass balance of the reaction showed that some of the benzoic acid was obtained from
toluene oxidation This observation is the basis of the present study for investigation of
the solvent free oxidation of toluene using PtZrO2 as catalyst
8
The treatment of hazardous wastewater containing organic pollutants in
environmentally acceptable and at a reasonable cost is a topic of great universal
importance Wastewaters from different industries (pharmacy perfumery organic
synthesis dyes cosmetics manufacturing of resin and colors etc) contain toluene
formaldehyde and benzyl alcohol Toluene concentration in the industrial wastewaters
varies between 0007- 0753 g L-1 [104] Toluene is one of the most water-soluble
aromatic hydrocarbons belonging to the BTEX group of hazardous volatile organic
compounds (VOC) which includes benzene ethyl benzene and xylene It is mainly used
as solvent in the production of paints thinners adhesives fingernail polish and in some
printing and leather tanning processes It is a frequently discharged hazardous substance
and has a taste in water at concentration of 004 ndash 1 ppm [105] The maximum
contaminant level goal (MCLG) for toluene has been set at 1 ppm for drinking water by
EPA [106] Several treatment methods including chemical oxidation activated carbon
adsorption and biological stabilization may be used for the conversion of toluene to a
non-toxic substance [107-109 39- 42] Biological treatment is favored because of the
capability of microorganisms to degrade low concentrations of toluene in large volumes
of aqueous wastes economically [110] But efficiency of biological processes decreases
as the concentration of pollutant increases furthermore some organic compounds are
resistant to biological clean up as well [111] Catalytic oxidation to maintain high
removal efficiency of organic contaminant from wastewater in friendly environmental
protocol is a promising alternative Ilyas et al [112] have reported the use of ZrO2 catalyst
for the liquid phase solvent free benzyl alcohol oxidation with molecular oxygen (1atm)
at 373-413 K and concluded that monoclinic ZrO2 is more active than tetragonal ZrO2 for
alcohol oxidation Recently it was reported that Pt ZrO2 is an efficient catalyst for the
oxidation of benzyl alcohol in solvent like n-heptane 1 PtZrO2 was also found to be an
efficient catalyst for toluene oxidation in solvent free conditions [103113] However
some conversion of benzoic acid to phenol was observed in the solvent free conditions
The objective of this work was to investigate a model catalyst (PtZrO2) for the oxidation
of toluene in aqueous solution at low temperature There are to the best of our
knowledge no reports concerning heterogeneous catalytic oxidation of toluene in
aqueous solution
9
15 Oxidation of cyclohexane
Poorly reactive and low-cost cyclohexane is interesting starting materials in the
production of cyclohexanone and cyclohexanol which is a valuable product for
manufacturing nylon-6 and nylon- 6 6 [114 115] More than 106 tons of cyclohexanone
and cyclohexanol (KA oil) are produced worldwide per year [116] Synthesis routes
often include oxidation steps that are traditionally performed using stoichiometric
quantities of oxidants such as permanganate chromic acid and hypochlorite creating a
toxic waste stream On the other hand this process is one of the least efficient of all
major industrial chemical processes as large-scale reactors operate at low conversions
These inefficiencies as well as increasing environmental concerns have been the main
driving forces for extensive research Using platinum or palladium as a catalyst the
selective oxidation of cyclohexane can be performed with air or oxygen as an oxidant In
order to obtain a large active surface the noble metal is usually supported by supports
like silica alumina carbon and zirconia The selectivity and stability of the catalyst can
be improved by adding a promoter (an inactive metal) such as bismuth lead or tin In the
present paper we studied the activity of zirconia as a catalyst and a support for platinum
or palladium using liquid phase oxidation of cyclohexane in solvent free condition at low
temperature as a model reaction
16 Oxidation of phenol
Undesirable phenol wastes are produced by many industries including the
chemical plastics and resins coke steel and petroleum industries Phenol is one of the
EPArsquos Priority Pollutants Under Section 313 of the Emergency Planning and
Community Right to Know Act of 1986 (EPCRA) releases of more than one pound of
phenol into the air water and land must be reported annually and entered into the Toxic
Release Inventory (TRI) Phenol has a high oxygen demand and can readily deplete
oxygen in the receiving water with detrimental effects on those organisms that abstract
dissolved oxygen for their metabolism It is also well known that even low phenol levels
in the parts per billion ranges impart disagreeable taste and odor to water Therefore it is
necessary to eliminate as much of the phenol from the wastewater before discharging
10
Phenols may be treated by chemical oxidation bio-oxidation or adsorption Chemical
oxidation such as with hydrogen peroxide or chlorine dioxide has a low capital cost but
a high operating cost Bio-oxidation has a high capital cost and a low operating cost
Adsorption has a high capital cost and a high operating cost The appropriateness of any
one of these methods depends on a combination of factors the most important of which
are the phenol concentration and any other chemical pollutants that may be present in the
wastewater Depending on these variables a single or a combination of treatments is be
used Currently phenol removal is accomplished with chemical oxidants the most
commonly used being chlorine dioxide hydrogen peroxide and potassium permanganate
Heterogeneous catalytic oxidation of dissolved organic compounds is a potential
means for remediation of contaminated ground and surface waters industrial effluents
and other wastewater streams The ability for operation at substantially milder conditions
of temperature and pressure in comparison to supercritical water oxidation and wet air
oxidation is achieved through the use of an extremely active supported noble metal
catalyst Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising
process but at elevated conditions of pressure and temperature in the presence of metal
oxide and supported metal oxide [45] Although homogeneous copper catalysts are
effective for the wet oxidation of industrial effluents but the removal of toxic catalyst
made the process debatable [117] Recently Leitenburg et al have reported that the
activities of mixed-metal oxides such as ZrO2 MnO2 or CuO for acetic acid oxidation
can be enhanced by adding ceria as a promoter [118] Imamura et al also studied the
catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the
other model pollutant compounds Ruthenium platinum and rhodium supported on CeO2
were found to be more active than a homogeneous copper catalyst [45] Atwater et al
have shown that several classes of aqueous organic contaminants can be deeply oxidized
using dissolved oxygen over supported noble metal catalysts (5 Ru-20 PtC) at
temperatures 393-433 K and pressures between 23 and 6 atm [119] Carlo et al [120]
reported that lanthanum strontium manganites are very active catalyst for the catalytic
wet oxidation of phenol In the present work we explored the effectiveness of zirconia-
supported noble metals (Pt Pd) and bismuth promoted zirconia supported noble metals
for oxidation of phenol in aqueous solution
11
17 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization
of catalysts The field of surface science of catalysis is helpful to examine the structure
and composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime Because heterogeneous
catalytic activity is so strongly influence surface structure on an atomic scale the
chemical bonding of adsorbates and the composition and oxidation states of surface
atoms Surface science offers a number of modern techniques that are employed to obtain
information on the morphological and textural properties of the prepared catalyst These
include surface area measurements particle size measurements x-ray diffractions SEM
EDX and FTIR which are the most common used techniques
171 Surface Area Measurements
Surface area measurements of a catalyst play an important role in the field of
surface chemistry and catalysis The technique of selective adsorption and interpretation
of the adsorption isotherm had to be developed in order to determine the surface areas
and the chemical nature of adsorption From the knowledge of the amount adsorbed and
area occupied per molecule (162 degA for N2) the total surface area covered by the
adsorbed gas can be calculated [121]
172 Particle size measurement
The size of particles in a sample can be measured by visual estimation or by the
use of a set of sieves A representative sample of known weight of particles is passed
through a set of sieves of known mesh sizes The sieves are arranged in downward
decreasing mesh diameters The sieves are mechanically vibrated for a fixed period of
time The weight of particles retained on each sieve is measured and converted into a
percentage of the total sample This method is quick and sufficiently accurate for most
purposes Essentially it measures the maximum diameter of each particle In our
laboratory we used sieves as well as (analystte 22) particle size measuring instrument
12
173 X-ray differactometry
X-ray powder diffractometry makes use of the fact that a specimen in the form of
a single-phase microcrystalline powder will give a characteristic diffraction pattern A
diffraction pattern is typically in the form of diffraction angle Vs diffraction line
intensity A pattern of a mixture of phases make up of a series of superimposed
diffractogramms one for each unique phase in the specimen The powder pattern can be
used as a unique fingerprint for a phase Analytical methods based on manual and
computer search techniques are now available for unscrambling patterns of multiphase
identification Special techniques are also available for the study of stress texture
topography particle size low and high temperature phase transformations etc
X-ray diffraction technique is used to follow the changes in amorphous structure
that occurs during pretreatments heat treatments and reactions The diffraction pattern
consists of broad and discrete peaks Changes in surface chemical composition induced
by catalytic transformations are also detected by XRD X-ray line broadening is used to
determine the mean crystalline size [122]
174 Infrared Spectroscopy
The strength and the number of acid sites on a solid can be obtained by
determining quantitatively the adsorption of a base such as ammonia quinoline
pyridine trimethyleamine In this method experiments are to be carried out under
conditions similar to the reactions and IR spectra of the surface is to be obtained The
IR method is a powerful tool for studying both Bronsted and Lewis acidities of surfaces
For example ammonia is adsorbed on the solid surface physically as NH3 it can be
bonded to a Lewis acid site bonding coordinatively or it can be adsorbed on a Bronsted
acid site as ammonium ion Each of the species is independently identifiable from its
characteristic infrared adsorption bands Pyridine similarly adsorbs on Lewis acid sites as
coordinatively bonded as pyridine and on Bronsted acid site as pyridinium ion These
species can be distinguished by their IR spectra allowing the number of Lewis and
Bronsted acid sites On a surface to be determined quantitatively IR spectra can monitor
the adsorbed states of the molecules and the surface defects produced during the sample
pretreatment Daturi et al [124] studied the effects of two different thermal chemical
13
pretreatments on high surface areas of Zirconia sample using FTIR spectroscopy This
sample shows a significant concentration of small pores and cavities with size ranging 1-
2 nm The detection and identification of the surface intermediate is important for the
understanding of reaction mechanism so IR spectroscopy is successfully employed to
answer these problems The reactivity of surface intermediates in the photo reduction of
CO2 with H2 over ZrO2 was investigated by Kohno and co-workers [125] stable surface
species arises under the photo reduction of CO2 on ZrO2 and is identified as surface
format by IR spectroscopy Adsorbed CO2 is converted to formate by photoelectron with
hydrogen The surface format is a true reaction intermediate since carbon mono oxide is
formed by the photo reaction of formate and carbon dioxide Surface format works as a
reductant of carbon dioxide to yield carbon mono oxide The dependence on the wave
length of irradiated light shows that bulk ZrO2 is not the photoactive specie When ZrO2
adsorbs CO2 a new bank appears in the photo luminescence spectrum The photo species
in the reaction between CO2 and H2 which yields HCOO is presumably formed by the
adsorption of CO2 on the ZrO2 surface
175 Scanning Electron Microscopy
Scanning electron microscopy is employed to determine the surface morphology
of the catalyst This technique allows qualitative characterization of the catalyst surface
and helps to interpret the phenomena occurring during calcinations and pretreatment The
most important advantage of electron microscopy is that the effectiveness of preparation
method can directly be observed by looking to the metal particles From SEM the particle
size distribution can be obtained This technique also gives information whether the
particles are evenly distributed are packed up in large aggregates If the particles are
sufficiently large their shape can be distinguished and their crystal structure is then
determining [126]
14
Chapter 2
Literature review
Zirconia is a technologically important material due to its superior hardness high
refractive index optical transparency chemical stability photothermal stability high
thermal expansion coefficient low thermal conductivity high thermomechanical
resistance and high corrosion resistance [127] These unique properties of ZrO2 have led
to their widespread applications in the fields of optical [128] structural materials solid-
state electrolytes gas-sensing thermal barriers coatings [129] corrosion-resistant
catalytic [130] and photonic [131 132] The elemental zirconium occurs as the free oxide
baddeleyite and as the compound oxide with silica zircon (ZrO2SiO2) [133] Zircon is
the most common and widely distributed of the commercial mineral Its large deposits are
found in beach sands Baddeleyite ZrO2 is less widely distributed than zircon and is
usually found associated with 1-15 each of silica and iron oxides Dressing of the ore
can produce zirconia of 97-99 purity Zirconia exhibit three well known crystalline
forms the monoclinic form is stable up to 1200 C the tetragonal is stable up to 1900 C
and the cubic form is stable above 1900C In addition to this a meta-stable tetragonal
form is also known which is stable up to 650C and its transformation is complete at
around 650-700 C Phase transformation between the monoclinic and tetragonal forms
takes place above 700C accompanied with a volume change Hence its mechanical and
thermal stability is not satisfactory for the use of ceramics Zirconia can be prepared from
different precursors such as ZrOCl2 8H2O [134 135] ZrO(NO3)22H2O[136 137] Zr
isopropoxide [137 139] and ZrCl4 [140 141] in order to attained desirable zirconia
Though synthesizing of zirconia is a primary task of chemists the real challenge lies in
preparing high surface area zirconia and maintaining the same HSA after high
temperature calcination
Chuah et al [142] have studied that high-surface-area zirconia can be prepared by
precipitation from zirconium salts The initial product from precipitation is a hydrous
zirconia of composition ZrO(OH)2 The properties of the final product zirconia are
affected by digestion of the hydrous zirconia Similarly Chuah et al [143] have reported
15
that high surface area zirconia was produced by digestion of the hydrous oxide at 100degC
for various lengths of time Precipitation of the hydrous zirconia was effected by
potassium hydroxide and sodium hydroxide the pH during precipitation being
maintained at 14 The zirconia obtained after calcination of the undigested hydrous
precursors at 500degC for 12 h had a surface area of 40ndash50 m2g With digestion surface
areas as high as 250 m2g could be obtained Chuah [144] has reported that the pH of the
digestion medium affects the solubility of the hydrous zirconia and the uptake of cations
Both factors in turn influence the surface area and crystal phase of the resulting zirconia
Between pH 8 and 11 the surface area increased with pH At pH 12 longer-digested
samples suffered a decrease in surface area This is due to the formation of the
thermodynamically stable monoclinic phase with bigger crystallite size The decrease in
the surface area with digestion time is even more pronounced at pH 137 Calafat [145]
has studied that zirconia was obtained by precipitation from aqueous solutions of
zirconium nitrate with ammonium hydroxide Small modifications in the preparation
greatly affected the surface area and phase formation of zirconia Time of digestion is the
key parameter to obtain zirconia with surface area in excess of 200 m2g after calcination
at 600degC A zirconia that maintained a surface area of 198 m2g after calcination at 900degC
has been obtained with 72 h of digestion at 80degC Recently Chane-Ching et al [146] have
reported a general method to prepare large surface area materials through the self-
assembly of functionalized nanoparticles This process involves functionalizing the oxide
nanoparticles with bifunctional organic anchors like aminocaproic acid and taurine After
the addition of a copolymer surfactant the functionalized nanoparticles will slowly self-
assemble on the copolymer chain through a second anchor site Using this approach the
authors could prepare several metal oxides like CeO2 ZrO2 and CeO2ndashAl(OH)3
composites The method yielded ZrO2 of surface area 180 m2g after calcining at 500 degC
125 m2g for CeO2 and 180 m2g for CeO2-Al (OH)3 composites Marban et al [147]
have been described a general route for obtaining high surface area (100ndash300 m2g)
inorganic materials made up by nanosized particles (2ndash8 nm) They illustrate that the
methodology applicable for the preparation of single and mixed metallic oxides
(ferrihydrite CuO2CeO2 CoFe2O4 and CuMn2O4) The simplicity of technique makes it
suitable for the mass scale production of complex nanoparticle-based materials
16
On the other hand it has been found that amorphous zirconia undergoes
crystallization at around 450 degC and hence its surface area decreases dramatically at that
temperature At room temperature the stable crystalline phase of zirconia is monoclinic
while the tetragonal phase forms upon heating to 1100ndash1200 degC Under basic conditions
monoclinic crystallites have been found to be larger in size than tetragonal [144] Many
researchers have tried to maintain the HSA of zirconia by several means Fuertes et al
[148] have found that an ordered and defect free material maintains HSA even after
calcination He developed a method to synthesize ordered metal oxides by impregnation
of a metal salt into siliceous material and hydrolyzing it inside the pores and then
removal of siliceous material by etching leaving highly ordered metal oxide structures
While other workers stabilized tetragonal phase ZrO2 by mixing with CaO MgO Y2O3
Cr2O3 or La2O3 at low temperature Zirconia and mixed oxide zirconia have been widely
studied by many methods including solndashgel process [149- 156] reverse micelle method
[157] coprecipitation [158142] and hydrothermal synthesis [159] functionalization of
oxide nanoparticles and their self-assembly [146] and templating [160]
The real challenge for chemists arises when applying this HSA zirconia as
heterogeneous catalysts or support for catalyst For this many propose researchers
investigate acidic basic oxidizing and or reducing properties of metal oxide ZrO2
exhibits both acidic and basic properties at its surface however the strength is rather
weak ZrO2 also exhibits both oxidizing and reducing properties The acidic and basic
sites on the surface of oxide both independently and collectively An example of
showing both the sites to be active is evidenced by the adsorption of CO2 and NH3 SiO2-
Al2O3 adsorbs NH3 (a basic molecule) but not CO2 (an acid molecule) Thus SiO2-Al2O3
is a typical solid acid On the other hand MgO adsorb CO2 and NH3 and hence possess
both acidic and basic properties ZrO2 is a typical acid-base bifunctional oxide ZrO2
calcined at 600 C exhibits 04μ molm2 of acidic sites and 4μ molm2 of basic sites
Infrared studies of the adsorbed Pyridine revealed the presence of Lewis type acid sites
but not Broansted acid sites [161] Acidic and basic properties of ZrO2 can be modified
by the addition of cationic or anionic substances Acidic property may be suppressed by
the addition of alkali cations or it can be promoted by the addition of anions such as
halogen ions Improvement of acidic properties can be achieved by the addition of sulfate
17
ion to produce the solid super acid [162 163] This super acid is used to catalyze the
isomerrization of alkanes Friedal-Crafts acylation and alkylation etc However this
supper acid catalyst deactivates during alkane isomerization This deactivation is due to
the removal of sulphur reduction of sulphur and fermentation of carbonaceous polymers
This deactivation may be overcome by the addition of Platinum and using the hydrogen
in the reaction atmosphere
Owing to its unique characteristics ZrO2 displays important catalytic properties
ZrO2 has been used as a catalyst for various reactions both as a single oxide and
combined oxides with interesting results have been reported [164] The catalytic activity
of ZrO2 has been indicated in the hydrogenation reaction [165] aldol addition of acetone
[166] and butane isomerization [167] ZrO2 as a support has also been used
successively Copper supported zirconia is an active catalyst for methanation of CO2
[168] Methanol is converted to gasoline using ZrO2 treated with sulfuric acid
Skeletal isomerization of hydrocarbon over ZrO2 promoted by platinum and
sulfate ions are the most promising reactions for the use of ZrO2 based catalyst Bolis et
al [169] have studied chemical and structural heterogeneity of supper acid SO4 ZrO2
system by adsorbing CO at 303K Both the Bronsted and Lewis sites were confirmed to
be present at the surface Gomez et al [170] have studied ZirconiaSilica-gel catalysts for
the decomposition of isopropanol Selectivity to propene or acetone was found to be a
function of the preparation methods of the catalysts Preparation of the catalyst in acid
developed acid sites and selective to propene whereas preparation in base is selective to
acetone Tetragonal Zirconia has been investigated [171] for its surface reactivity and
was found to exhibits differences with respect to the better-known monoclinic phase
Yttria-stabilized t-ZrO2 and a commercial powder ceramic material of similar chemical
composition were investigated by means of Infrared spectroscopy and adsorption
microcalarometry using CO as a probe molecule to test the surface acidic properties of
the solids The surface acidic properties of t-ZrO2 were found to depend primarily on the
degree of sintering the preparation procedure and the amount of Y2 O3 added
Yori et al [172] have studied the n-butane isomerization on tungsten oxide
supported on Zirconia Using different routes of preparation of the catalyst from
ammonium metal tungstate and after calcinations at 800C the better WO3 ZrO2 catalyst
18
showed performance similar to sulfated Zirconia calcined at 620 C The effects of
hydrogen treated Zirconia and Pt ZrO2 were investigated by Hoang et al [173] The
catalysts were characterized by using techniques TPR hydrogen chemisorptions TPDH
and in the conversion of n-hexane at high temperature (650 C) ZrO2 takes up hydrogen
In n-hexane conversions high temperature hydrogen treatment is pre-condition of
the catalytic activity Possibly catalytically active sites are generated by this hydrogen
treatment The high temperature hydrogen treatment induces a strong PtZrO2 interaction
Hoang and Co-Workers in another study [174] have investigated the hydrogen spillover
phenomena on PtZrO2 catalyst by temperature programmed reduction and adsorption of
hydrogen At about 550C hydrogen spilled over from Pt on to the ZrO2 surface Of this
hydrogen spill over one part is consumed by a partial reduction of ZrO2 and the other part
is adsorbed on the surface and desorbed at about 650 C This desorption a reversible
process can be followed by renewed uptake of spillover hydrogen No connection
between dehydroxylable OH groups and spillover hydrogen adsorption has been
observed The adsorption sites for the reversibly bound spillover hydrogen were possibly
formed during the reducing hydrogen treatment
Kondo et al [175] have studied the adsorption and reaction of H2 CO and CO2 over
ZrO2 using IR spectroscopy Hydrogen is dissociatively adsorbed to form OH and Zr-H
species and CO is weakly adsorbed as the molecular form The IR spectrum of adsorbed
specie of CO2 over ZrO2 show three main bands at Ca 1550 1310 and 1060 cm-1 which
can be assigned to bidentate carbonate species when hydrogen was introduced over CO2
preadsorbed ZrO2 formate and methoxide species also appears It is inferred that the
formation of the format and methoxide species result from the hydrogenation of bidentate
carbonate species
Miyata etal [176] have studied the properties of vanadium oxide supported on ZrO2
for the oxidation of butane V-Zr catalyst show high selectivity to furan and butadiene
while high vanadium loadings show high selectivity to acetaldehyde and acetic acid
Schild et al [177] have studied the hydrogenation reaction of CO and CO2 over
Zirconia supported palladium catalysts using diffused reflectance FTIR spectroscopy
Rapid formation of surface format was observed upon exposure to CO2 H2 Similarly
CO was rapidly transformed to formate upon initial adsorption on to the surfaces of the
19
activated catalysts The disappearance of formate as observed in the FTIR spectrum
could be correlated with the appearance of gas phase methane
Recently D Souza et al [178] have reported the preparation of thermally stable
HSA zirconia having 160 m2g by a ldquocolloidal digestingrdquo route using
tetramethylammonium chloride as a stabilizer for zirconia nanoparticles and deposited
preformed Pd nanoparticles on it and screened the catalyst for 1-hexene hydrogenation
They have further extended their studies for the efficient preparation of mesoporous
tetragonal zirconia and to form a heterogeneous catalyst by immobilizing a Pt colloid
upon this material for hydrogenation of 1- hexene [179]
20
Chapter 1amp 2
References
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Inc Nova Iorque 1992
2 Cornils B Herrmann W Eds Applied Homogeneous Catalysis with
Organometallic Compounds Vol 1 VCH 1996 Chapter 24
3 Anastas PT Warner JC Green Chemistry Theory and Practice Oxford
University Press Oxford 1998
4 Puzari A Jubaraj B J Mol Catal A Chem 2002 187 149
5 Gates B C Catalytic Chemistry John Wiley and Sons New York 1992
6 Yamaguchi T Catal Today 1994 20 199
7 Ozawa M Kimura M J Mater Sci Lett 1990 9 446
8 Inoue M Kominami H Inui T Appl Catal A 1993 97 L25-30
9 Aiken B Hsu W P Matijevid E J Mater Sci1990 25 1886
10 Garg A Matijevid E J Colloid Interface Sci1988 126 243
11 Mercera P D L Van Ommen J G Doesburg E B M Burggraaf AJ
Ross JRH Appl Catal1990 57127
12 Mercera PDL Van Ommen JG Doesburg EBM Burggraaf AJ Ross
JRH Appl Catal1991 78 79
13 Srinivasan R Taulbee D Davis BH Catal Lett 1991 9 1
14 Norman C J Goulding PA McAlpine I Catal Today1994 20 313
15 Mallat T Baiker A Chem Rev 2004 104 3037
16 Muzart J Tetrahedron 2003 59 5789
17 Rafelt J S Clark J H Catal Today 2000 57 33
18 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 57 143
19 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432
21
20 Hutchings G J Carrettin S Landon P Edwards JK Enache D
Knight DW Xu Y CarleyAF Top Catal 2006 38 223-230
21 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
22 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
23 Opre Z Grunwaldt JD Mallat T BaikerA J Mol Catal A Chem 2005
242 224-232
24 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241
287-293
25 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
26 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
27 Ji H B Song J He B Qian Y React Kinet Catal Lett 2004 82 97
28 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-
52
29 Choudhary V R Dhar A Jana P Jha R de Upha B S Green Chem
2005 7 768
30 Choudhary V R Jha R Jana P Green Chem 2007 9 267
31 Enache D I Edwards J K Landon P Espiru B S Carley A F
Herzing A H Watanabe M Kiely C J Knight D W Hutchings G J
Science 2006 311 362
32 Li G Enache D I Edwards J K Carley A F Knight D W Hutchings
G J Catal Lett 2006 110 7
33 Ilyas M Abdullah M N U Phys Chem 2003 14 19
34 Ilyas M Ikramullah Catal Commun 2004 5 1
35 Rache A Kumari V Rao P K In Gupta N M Chakrabarty D K eds
Catalysis Modern Trends New Delhi Narosa 1995 346
36 Li X Xu J Wang F Gao J Zhou L Yang G Catalysis Letters
2006 108 137
37 Heyns K Blazejewicz L Tetrahedron 1960 9 67
22
38 Heyns K Paulsen H in ldquo Newer Methods of Preparative Organic
Chemistryrdquo W Forest Eds Academic Press New York 1963 Vol 2 pp
303-335
39 Christoskova St Stoyanova M Water Res 2002 36 2297-2303
40 Christoskova St Final Report Contract X-123 National Science Fund
Ministry of Education and Science Republic of Bulgaria 1993
41 Christoskova St Stoyanova M Water Res 2000 3096 1ndash5
42 Christoskova St Danova N Georgieva M Argirov O Mehandjiev D
Appl Catal A General 1995 128 219ndash229
43 Munter R Proc Estonian Sci Chem 2001 50 59-804
44 Mishra V S Mahajani VV Joshi JB Ind Eng Chem Res 1995 34 2
45 Imamura S Ind Eng Chem Res 1999 38 1743
46 Pintar Catal Today 2003 77 451
47 Matatov-Meytal Y I Sheintuch M Ind Eng Chem Res 1998 37 309
48 Luck F Catal Today 1999 53 81
49 Kolaczkowski S T Plucinski P Beltran FJ Rivas F Lurgh DB Chem
Eng J 1999 73 143
50 Iliuta Larachi F Chem Eng Proc 2001 40175
51 Fortuny C Ferrer C Bengoa J Font and Fabregat A Catal Today 1995
24 79
52 Alejandre F Medina A Fortuny P Salagre and Suerias JE Appl Catal
B Environ 1998 16 53
53 Alvarez PM McLurgh D Plucinsky P Ind Eng Chem Res 2002 41
2153
54 Hu X Lei L Chu HP Yue PL Carbon 1999 37 631
55 Santos A Yustos P Durban B Garcia-Ochoa F Environ Sci Technol
2001 35 2828
56 Fortuny A Bengoa C Font J Fabregat A J Hazard Mater 1999 64
181
57 Zhang Q Chuang KT Environ Sci Technol1999 33 3641
58 Zhang Q Chuang KT Can J Chem Eng1999 77 399
23
59 Wu Q Hu X Yue PL Zhao XS Lu GQ Appl Catal B Environ
2001 32 151
60 Stuber F Polaert I Delmas H Font J Fortuny A Fabregat A J Chem
Technol Biotechnol 2001 76 743
61 Hamoudi S Larachi F Sayari A J Catal 1998 77 247
62 Hamoudi S Larachi F Cerrella G Casssanello M Ind Eng Chem Res
1998 37 3561
63 Pintar and Levec J J Catal 1992 135 345
64 Alejandre A Medina F Rodriguez X Salagre P Suerias JE J Catal
1999 188 311
65 Hamoudi S Sayari A Belkacemi K Bonneviot L Larachi F Catal
Today 2000 62 379
66 Hussain ST Sayari A Larachi F J Catal 2001 201153
67 Hussain ST Sayari A Larachi F Appl Catal B Environ 2001 34 1
68 Alejandre A Medina F Rodriguez X Salagre P CesterosYSuerias
JE Appl Catal B Environ 2001 30 195
69 Gallezot P Laurain N Isnard P Appl Catal B Environ 1996 9 L11
70 Beziat JC Besson M Gallezot P Durecu S Ind Eng Chem Res 1999
381310
71 Pintar Besson M Gallezot P Appl Catal B Environ 2001 30 123
72 Pintar Besson M Gallezot P Appl Catal B Environ 2001 31 275
73 Duprez S Delano F Barbier J Isnard P Blanchard G Catal Today
1996 29 317
74 An W Zhang Q Ma Y Chuang KT Catal Today 2001 64 289
75 Hocevar S Batista J Levec J J Catal 1999 184 39
76 Hocevar S Krasovec UO Orel B Arico A S Kim H Appl Catal B
Environ 2000 28113
77 Reddy M Thrimurthulu G Saikia P Bharali P J Mole Catal A
Chemical 2007 275 167-173
78 Solinas V Rombi E Ferino I Cutrufello M G Coloacuten G Naviacuteo J
A J Mole Catal A Chemical 2003 204 629-635
24
79 Sun YH Sermon PAJ Chem Soc Chem Commu 1993 16 1242
80 Ma Z Yang C Wei W Li W Sun Y J Mole Catal A Chemical 2005
231 75ndash81
81 Zong H Hattori H Tanabe K J Catal 1998 36 139
82 Vijay S Wolf EE Appl Catal A Gen 2004 264 117-124
83 Hwanga H C Chena X R Wonga ST Chenc CL Mou CY Appl
Catal A General 2007 323 9-17
84 Wong S Li T Cheng S Lee J Mou C J Catal 2003 215 45ndash56
85 Mamedov EA Corberfin V C Appl Catal A General 1995 127 1-40
86 Tomishig K Ikeda Y Sakaihori T Fujimoto K J Catal 2000 192 355-
362
87 Ilyas M Sadiq M Chin J Chem2008 26 941
88 Collinn D E Richery F A in J A Kent (Eds) Reigle Handbook of
Industrial Chemistry C B S New Delhi 1987 Chap 22 p 800
89 Dow Chemical Corp US Patent 2 727 926 1955
90 California Research Corp US Patent 2 762 838 1956
91 Bujis W J Molecular Catal A 1999146 237
92 Dubreuil JF Serna JG Verdugo EG Dudda L M Aird G R
Thomas W B Poliakoff M J Supercritical Fluids 2006 39 220
93 Bujjs W Frijns L H B Offermanns M R J US Patent 5 210 331
1993
94 Pennington J in C A Heaton (eds) An Introduction to Industrial
Chemistry Leonard Hill London 1984 Chap 9 p 323
95 US Environmental Protection Agency Integrated Risk Information
System (IRIS) on Toluene National Center for Environmental Assistance
Office of Research and Development Washington DC 1999
96 Bulushev D A Rainone F Minsker L K Catalysis Today 2004 96
195
97 Worayingyong A Nitharach A Poo-arporn Y Science Asia 2004
30 341
98 Bastock T E Clark J H Martin K Trentbirth B W Green
25
Chemistry 2002 4 615
99 Subrahmanyama Ch Louisb B Viswanathana B Renkenb A
Varadarajan TK Applied Catalysis A General 2005 282 67
100 Raja R Thomas J M Dreyerd V Catalysis Letters 2006110 179
101 Thomas J M Raja R Catalysis Today 2006 117 22
102 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H
Catalysis Letters 2006 110 255
103 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
104 Enright A M Collins G FlahertyVO Water Res 2007 411465
105 httpwwweco-usanettoxicstolueneshtml
106 httpwwwfreedrinkingwatercomwater-contaminanttoluene-
contaminantsremoval-waterhtm
107 Langwaldt J H Puhakka J A Environ Pollut 2000 107 197
108 De Nardi IR Varesche MB Zaiat M Foresti E Water Sci Technol
2002 45 180
109 De Nardi I R Ribeiro R Zaiat M ForestiE Process Biochem 2005
40 587
110 Stenstrom M K Cardinal L Libra J Environ Prog 19898 107
111 Mantzavinos D Sahibzada M Livingston A Metcalfe I Hellgardt
K Catal Today 1999 53 93
112 Ilyas M Sadiq M KhanI Chin J Catal 2007 28 413
113 Ilyas M Sadiq M Catal Lett (Online first) DOI 101007s10562-008-
9750-8
114 Chandalia SB Oxidation of Hydrocarbons 1st Ed Sevak Bombay
1977
115 Musser MT inW Gerhartz (Ed) Encyclopedia of Industrial Chemistry
VCH Weinheim 1987 p 217
116 Suresh AK Sharma MM Sridhar T Ind Eng Chem Res 2000 39
3958
117 Wang R Qi Y Shen Z Wu Z Huadong Huagong Xueyuan Xue
1982 4 411-18
26
118 Leitenburg C Goi D Primavera A Trovarelli A Dolcetti G Appl
Catal B 1996 11 L29-L35
119 Atwater J E Akse J R Mckinnis J A Thompson J O Appl Catal
B 1996 11 L11-L18
120 Carlo R Federico C Silvia B Ombretta P Guido B Appl Catal B
Environ 2008 84 678-683
121 Adomson AW ldquoPhysical Chemistry of Surfacesrdquo 4th ed John Wiley and
sons Newyork 1982
122 Packertand M Baikev A JChem Soc Faraday Trans 1 1985 81
2797
123 Yamashita H Yoschikawas M Fanahiki T Yoshida S J Chem Soc
Faraday Trans1 1986 82 1771
124 Daturi M Binet C Berneal S Omil J A P Larvalley J C J Chem
Soc Faraday Trans 1998 94 1143
125 Kohno Y Tanaka T Funaziki T YoshidaS J Chem Soc Faraday
Trans 1998 94 1875
126 Che and Bennet CO ldquoAdvances in Catalysisrdquo Academic Press Inc
1998 36 55-97
127 Harrison HDE McLamed NT Subbarao EC J Electrochem Soc
1963 110 23
128 Kourouklis GA Liarokapis E J Am Ceram Soc1991 74 52
129 Birkby I Stevens R Key Eng Mater 1996 122 527
130 Murase Y Kato E J Am Ceram Soc1982 66196
131 Sorek Y Zevin M Reisfeld R Hurvita T RuschinS Chem Mater
1997 9 670
132 Salas P Rosa-Cruz E D Mendoza D Gonzales P Rodryguez R
Castano VM Mater Lett 2000 45 241
133 Stevens R ldquoAn Introduction to Zirconiardquo Magnesium Elecktron Ltd
Publication no113 Litho 2000 Twickenhom UK July (1986)
134 Arata K Hino H in ldquoProceeding 9th International Congress on
27
Catalysis Calgary 1088rdquo (MJPhillips and M ternan Eds) Vol 4 p
1727 Chem Institute of Canada Ottawa 1988
135 Sohn JR Jang HJ J Mol Catal 1991 64 349
136 Garvie RC J Phy Chem 1965 69 1238
137 Yamaguchi T Tanabe K Kung Y C Matter Chem Phys 1986 16
67
138 Bensitel M Saur O Lavalley J C Mabilon G Matter Chem Phys
1987 17 249
139 Morterra C Cerrato G Emanuel C Bolis V J Catal 1993 142 349
140 Srinivasan R Davis B H Catal Lett 1992 14 165
141 Ardizzone S Bassi G Matter Chem Phys 1990 25 417
142 Chuah G K Jaenicke S Pong B K J Catal1998 175 80-92
143 Chuah G K Jaenicke S Appl Catal A General 1997 163 261-273
144 Chuah G K Catal Today 1999 49 131
145 Calafat A Studies Surf Sci Catal 1998 118 837-843
146 Chane-Ching JY Cobo F Aubert D Harvey HG Airiau M
Corma A Chem Eur J 2005 11 979
147 G Marbaacuten A B Fuertes T V Soliacutes Micropor Mesopor Mater
2008112 291-298
148 Fuertes AB J Phys Chem Solids 2005 66 741
149 Parvulescu V Coman NS Grange P Parvulescu VI Appl Catal
A1999 176 27
150 Parvulescu VI Parvulescu V Endruschat U Lehmann CW
Grange P Poncelet G Bonnemann H Micropor Mesopor Mater
2001 44 221
151 Parvulescu VI Bonnemann H Parvulescu V Endruschat U
Rufinska A Lehmann CW Tesche B Poncelet G Appl Catal
A2001 214 273
152 Ward DA Ko EI J Catal 1995 157 321
153 Mamak M Coombs N Ozin GA Chem Mater 2001 13 3564
154 Li Y He D YuanY Cheng Z Zhu Q Energy Fuels 2001 151434
28
155 Xu W Luo Q Wang H Francesconi LC Stark RE Akins DL
J Phys Chem B 2003 107 497
156 Navio JA Hidalgo MC Colon G Botta SG Litter MI
Langmuir 2001 17 202
157 Sun W Xu L Chu Y Shi W J Colloid Interface Sci 2003 266
99
158 Stichert W Schuth F J Catal 1998 174 242
159 Tani E Yoshimura M Somiya S J Am Ceram Soc 1983 6611
160 Kristof C Thierry L Katrien A Pegie C Oleg L Gustaaf VG
Rene VG Etienne FV J Mater Chem 2003 13 3033
161 Nakano Y Izuka T Hattori H Taanabe K J Catal 1978 51 1
162 Zarkalis A S Hsu C Y Gates B C Catal Lett 1996 37 5
163 Rezgui S Gates B C Catal Lett 1996 37 5
164 Tanabe K YamaguchiT Catal Today 1994 20 185
165 Nakano Y Yamaguchi K Tanabe K J Catal 1983 80 307
166 Zong H Hattori H Tanabe K J Catal 198836139
167 Pajonk G M Tanany A E React Kinet Catal Lett1992 47 167
168 DeniseB SneedenRPA Beguim B Cherifi O Appl Catal
198730353
169 Bolis V Cerrate G Morterra C Langmuir 1997 13 888
170 Gomez R LopezT Tzompantzi F Garciafigueroa E Acosta D W
Novaro O Langmuir 1997 13 970
171 Morterra Cerrato G Bolis V Lamberti C Ferroni L Montanaro
LJ Chem Soc Faraday Trans 1995 91 113
172 Yori J C Vera C R Peraro J M Appl CatalA Gen 1997 163 165
173 Hoang D L Lieske H Catal Lett 1994 27 33
174 Hoang DL Berndt H LieskeH Catal Lett 1995 31165
175 Kondo J Abe H Sakata Y Maruya K Domen K Onishi T
JChem Soc Faraday TransI 1988 84 511
176 Miyata H Kohna M Ono I Ohno T Hatayana F J Chem Soc
Faraday Trans I 1989 85 3663
29
177 Schild C Wokeun A Baiker A J Mol Catal 1990 63 223
178 Souza L D Subaie J S Richards R M J Colloid Interface Sci 2005
292 476ndash485
179 Souza L D Suchopar A Zhu K Balyozova D Devadas M
Richards R M Micropor Mesopor Mater 2006 88 22ndash30
30
Chapter 3
Experimental
31 Material
ZrOCl28H2O (Merck 8917) commercial ZrO2 ( Merk 108920) NH4OH (BDH
27140) AgNO3 (Merck 1512) PtCl4 (Acros 19540) Palladium (II) chloride (Scharlau
Pa 0025) benzyl alcohol (Merck 9626) cyclohexane (Acros 61029-1000) cyclohexanol
(Acros 27870) cyclohexanone (BDH 10380) benzaldehyde (Scharlu BE0160) toluene
(BDH 10284) phenol (Acros 41717) benzoic acid (Merck 100136) alizarin
(Acros 400480250) Potassium Iodide (BDH102123B) 24-Dinitro phenyl hydrazine
(BDH100099) and trans-stilbene (Aldrich 13993-9) were used as received H2
(99999) was prepared using hydrogen generator (GCD-300 BAIF) Nitrogen and
Oxygen were supplied by BOC Pakistan Ltd and were further purified by passing
through traps (CRSInc202268) to remove traces of water and oil Traces of oxygen
from nitrogen gas were removed by using specific oxygen traps (CRSInc202223)
32 Preparation of catalyst
Two types of ZrO2 were used in this study
i Laboratory prepared ZrO2
ii Commercial ZrO2
321 Laboratory prepared ZrO2
Zirconia was prepared using an aqueous solution of zirconyl chloride [1-4] with
the drop wise addition of NH4OH for 4 hours (pH 10-12) with continuous stirring The
precipitate was washed with triply distilled water using a Soxhletrsquos apparatus for 24 hrs
until the Cl- test with AgNO3 was found to be negative Precipitate was dried at 110 degC
for 24 hrs After drying it was calcined with programmable heating at a rate of 05
degCminute to reach 950 degC and was kept at that temperature for 4 hrs Nabertherm C-19
programmed control furnace was used for calcinations
31
Figure 1
Modified Soxhletrsquos apparatus
32
322 Optimal conditions for preparation of ZrO2
Optimal conditions were set for obtaining predictable results i concentration ~
005M ii pH ~12 iii Mixing time of NH3 ~12 hours iv Aging ~ 48 hours v Washing
~24h in modified Soxhletrsquos apparatus vi Drying temperature~110 0C for 24 hours in
temperature control oven
323 Commercial ZrO2
Commercially supplied ZrO2 was grounded to powder and was passed through
different US standard test sieves mesh 80 100 300 to get reduced particle size of the
catalyst The grounded catalyst was calcined as above
324 Supported catalyst
Supported Catalysts were prepared by incipient wetness technique For this
purpose calculated amount (wt ) of the precursor compound (PdCl4 or PtCl4) was taken
in a crucible and triply distilled water was added to make a paste Then the required
amount of the support (ZrO2) was mixed with it to make a paste The paste was
thoroughly mixed and dried in an oven at 110 oC for 24 hours and then grounded The
catalyst was sieved and 80-100 mesh portions were used for further treatment The
grounded catalyst was calcined again at the rate of 05 0C min to reach 950 0C and was
kept at 950 0C for 4 hours after which it was reduced in H2 flow at 280 ordmC for 4 hours
The supported multi component catalysts were prepared by successive incipient wetness
impregnation of the support with bismuth and precious metals followed by drying and
calcination Bismuth was added first on zirconia support by the incipient wetness
impregnation procedure After drying and calcination Bizirconia was then impregnated
with the active metals such as Pd or Pt The final sample then underwent the same drying
and calcination procedure The metal loading of the catalyst was calculated from the
weight of chemicals used for impregnation
33 Characterization of catalysts
33
XRD analyses were performed using a JEOL (JDX-3532) diffractometer with
CuKa radiation (k = 15406 A˚) operated at 40 kV and 20 mA BET surface area of the
catalyst was determined using a Quanta chrome (Nova 2200e) surface area and pore size
analyzer The samples of ZrO2 was heat-treated at a rate of 05 ˚ Cmin to 950 ˚ C and
maintained at that temperature for 4 h in air and then allowed to cool to room
temperature Thus pre-treated samples were used for surface area and isotherm
measurements N2 was used as an adsorbate For surface area measurements seven-point
isotherm data were considered (PP0 between 0 and 03) Particle size was measured by
analysette 22 compact (Fritsch Germany) FTIR spectra were recorded with Prestige 21
Shimadzu Japan in the range 500-4000cm-1 Furthermore SEM and EDX measurements
were performed using scanning electron microscope of Joel 50 H super prob 733
34 Experimental setups for different reaction
In the present study we use three types of experimental set ups as shown in
(Figures 2 3 4) The gases O2 or N2 or a mixture of O2 and N2 was passed through the
reactor containing liquid (reactant) and solid catalyst dispersed in it The partial pressures
of the gases passed through the reactor were varied for various experiments All the pipes
used in the systemrsquos assembly were of Teflon tubes (quarter inch) with Pyrex glass
connections and stopcocks The gases flow was regulated by stainless steel and Teflon
needle valves The reactor was heated by heating tapes connected to a temperature
controller or by hot water circulation The reactor was connected to a condenser with
cold-water circulation supply in order to avoid evaporation of products reactant The
desired partial pressure of the gases was controlled by mixing O2 and N2 (in a particular
proportion) having a constant desired flow rate of 40 cm3 min-1 The flow was measured
by flow meter After a desired period of time the reaction was stopped and the reaction
mixture was filtered to remove the solid catalyst The filtered reaction mixture was kept
in sealed bottle and was used for further analysis
34
Figure 2
Experimental setup for oxidation reactions in
solvent free conditions
35
Figure 3
Experimental setup for oxidation reactions in
ecofriendly solvents
36
Figure 4
Experimental setup for solvent free oxidation of
toluene in dry conditions
37
35 Liquid-phase oxidation in solvent free conditions
The liquid-phase oxidation in solvent free conditions was carried out in a
magnetically stirred Pyrex glass single walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using heating tapes A
predetermined quantity (10 ml) was taken in the reactor and 02 g of catalyst was then
added O2 and N2 gases at atmospheric pressure were allowed to pass through the reaction
mixture at a flow rate of 40 mlmin at a fixed temperature All the reactants were heated
to the reaction temperature before adding to the reactor Samples were withdrawn from
the reaction mixture at predetermined time intervals
351 Design of reactor for liquid phase oxidation in solvent free condition
Figure 5
Reactor used for solvent free reactions
38
36 Liquid-phase oxidation in ecofriendly solvents
The liquid-phase oxidation in ecofriendly solvent was carried out in a
magnetically stirred Pyrex glass double walled flat bottom three-necked batch reactor
equipped with a reflux condenser and a mercury thermometer for measuring the reaction
temperature The reaction temperature was maintained by using water circulator
(WiseCircu Fuzzy control system) A predetermined quantity of substrate solution was
taken in the reactor and a desirable amount of catalyst was then added The reaction
during heating period was negligible since no direct contact existed between oxygen and
catalyst O2 and N2 gases at atmospheric pressure were allowed to pass through the
reaction mixture at a flow rate of 40 mlmin at a fixed temperature When the temperature
and pressure reached the designated values the stirrer was turned on at 900 rpm
361 Design of reactor for liquid phase oxidation in ecofriendly solvents
Figure 6
Reactor used for liquid phase oxidation in
ecofriendly solvents
39
37 Analysis of reaction mixture
The reaction mixture was filtered and analyzed for products by [4-9]
i chemical methods
This method adopted for the determination of ketone aldehydes in a reaction
mixture 5 cm3 of the filtered reaction mixture was added to 250cm3 conical
flask containing 50cm3 of a saturated solution of pure 2 4 ndash dinitro phenyl
hydrazine in 2N HCl (containing 4 mgcm3) and was placed in ice to achieve 0
degC Precipitate (hydrazone) formed after an hour was filtered thoroughly
washed with 2N HCl and distilled water respectively and dried at 110 degC in
oven Then weigh the dried precipitate
ii Thin layer chromatography
Thin layer chromatographic analysis was carried out using standard
chromatographic plates (Merck) with silica gel 60 F254 support (Merck TLC
105554 and PLC 113793) Ethyl acetate (10 ) in cyclohexane was used as
eluent
iii FTIR (Shimadzu IRPrestigue- 21)
Diffuse reflectance spectra of solids (trans-Stilbene) were recorded on
Shimadzu IRPrestigue- 21 FTIR-8400S using diffuse reflectance accessory
[DRS- 8000A] Solid samples were diluted with KBr before measurement
The spectra were recorded with resolution of 4 cm-1 with 50 accumulations
iv UV spectrophotometer (UV-160 SHAMIDZO JAPAN)
For UV spectrophotometic analysis standard addition method was adopted In
this method the matrix (medium in which the analyte exists) of standard and
unknown match exactly Known amount of spikes was added to known
volume of reaction mixture A calibration plot is obtained that is offset from
zero A linear regression should generate a straight-line equation of (y = mx +
b) where m is the slope and b is intercept The concentration of the unknown
is equal to the value of x and is determined by solving the straight-line
equation for y = 0 yields x = b m as shown in figure 7 The samples were
scanned for λ max The increase in absorbance for added spikes was noted
The calibration plot was obtained by plotting standard solution verses
40
Figure 7 Plot for spiked and normalized absorbance
Figure 8 Plot of Abs Vs COD concentrations (mgL)
41
absorbance Subtracting the absorbance of unknown (amount of product) from
the standard added solution absorbance can normalize absorbance The offset
shows the unknown concentration of the product
v GC (Clarus 500 Perkin Elmer)
The GC was equipped with (FID) and capillary column (Elite-5 L 30m ID
025 DF 025) Nitrogen was used as the carrier gas For injecting samples 10
microl gas tight injection was used Same standard addition method was adopted
The conversion was measured as follows
Ci and Cf are the initial concentration and final concentration respectively
vi Determination of COD
COD was determined by closed reflux colorimetric method according to
which the organic substances are oxidized (digested) by potassium dichromate
K2Cr2O7 at 160degC in a sealed tube When orange colored Cr2O2minus
7 is reduced
green colored Cr3+ is formed which can be detected in a spectrophotometer at
λ = 600 nm The relation between absorbance and COD concentration is
established by calibration with standard solutions of potassium hydrogen
phthalate in the range of COD values between 200 and 1200 mgL as shown
in Fig 8
38 Heterogeneous nature of the catalyst
The heterogeneity of catalytic reaction was confirmed with Alizarin test for Zr+4
ions and potassium iodide test for Pt+4 and Pd+2 ions in the reaction mixture For Zr+4 test
5 ml of reaction mixture was mixed with 5 ml of Alizarin reagent and made the total
volume up to 100 ml by adding 01 N HCl solution No change in color (which was
expected to be red in case of Zr+4 presence) and no absorbance at λ max = 513 nm was
observed For Pt+4 and Pd+2 test 1 ml of 5 KI and 2 ml of reaction mixture was mixed
and made the total volume to 50 ml by adding 01N HCL solution No change in color
(which was to be brownish pink color of PtI6-2 in case of Pt+4 ions presence) and no
absorbance at λ max = 496nm was observed
100() minus
=Ci
CfCiX
42
Chapter 3
References
1 Ilyas M Sadiq M Chem Eng Technol 2007 30 1391
2 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
5 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
6 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006 108 137
7 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
8 Zhao Y Wang G Li W Zhu Z Chemom Intell Lab Sys 2006 82 193
9 Christoskova ST Stoyanova M Water Res 2002 36 2297
43
Chapter 4A
Results and discussion
Reactant Cyclohexanol octanol benzyl alcohol
Catalyst ZrO2
Oxidation of alcohols in solvent free conditions by zirconia catalyst
4A 1 Characterization of catalyst
An important step in the field of heterogeneous catalysis is the characterization of
catalysts The field of surface science of catalysis is helpful to examine the structure and
composition of the catalytically active surface and to correlate this information with
catalytic reaction rates selectivity activity and catalyst lifetime
4A 2 Brunauer-Emmet-Teller method (BET)
Surface area of ZrO2 was dependent on preparation procedure digestion time pH
agitation and concentration of precursor solution and calcination time During this study
we observe fluctuations in the surface area of ZrO2 by applying various conditions
Surface area of ZrO2 was found to depend on calcination temperature Fig 1 shows that at
a higher temperature (1223 K) ZrO2 have a monoclinic geometry and a lower surface area
of 8860m2g while at a lower temperature (723 K) ZrO2 was dominated by a tetragonal
geometry with a high surface area of 17111 m2g
4A 3 X-ray diffraction (XRD)
From powder XRD we obtained diffraction patterns for 723K 1223K-calcined
neat ZrO2 samples which are shown in Fig 2 ZrO2 calcined at 723K is tetragonal while
ZrO2 calcined at1223K is monoclinic Monoclinic ZrO2 shows better activity towards
alcohol oxidation then the tetragonal ZrO2
4A 4 Scanning electron microscopy
The SEM pictures with two different resolutions of the vacuum dried neat ZrO2 material
calcined at 1223 K and 723 K are shown in Fig 3 The morphology shows that both these
44
Figure 1
Brunauer-Emmet-Teller method (BET)
plot for ZrO2 calcined at 1223 and 723 K
Figure 2
XRD for ZrO2 calcined at 1223 and 723 K
Figure 3
SEM for ZrO2 calcined at 1223 K (a1 a2) and
723 K (b1 b2) Resolution for a1 b1 1000 and
a2 b2 2000 at 25 kV
Figure 4
EDX for ZrO2 calcined at before use and
after use
45
samples have the same particle size and shape The difference in the surface area could be
due to the difference in the pore volume of the two samples The total pore volume
calculated from nitrogen adsorption at 77 K is 026 cm3g for the sample calcined at 1223
K and 033 cm3g for the sample calcined at 723 K Elemental analysis results were
obtained for laboratory prepared ZrO2 calcined at 723 and 1223 K which indicate the
presence of a small amount of hafnium (Hf) 2503 wt oxygen and 7070 wt zirconia
reported in Fig4 The test also found trace amounts of chlorine present indicating a
small percentage from starting material is present Elemental analysis for used ZrO2
indicates a small percentage of carbon deposit on the surface which is responsible for
deactivation of catalytic activity of ZrO2
4A 5 Effect of mass transfer
Preliminary experiments were performed using ZrO2 as catalyst for alcohol
oxidation under the solvent free conditions at a high agitation speed of 900 rpm for 24 h
with O2 bubbling through the reaction mixture Analysis of the reaction mixture shows
that benzaldehyde (yield 39) was the only product detected by FID The presence of
oxygen was necessary for the benzyl alcohol oxidation to benzaldehyde No reaction was
observed when no oxygen was bubbled through the reaction mixture or when oxygen was
replaced by nitrogen Similarly no reaction was observed when oxygen was passed
through the reactor above the surface of the reaction mixture This would support the
conclusion of Kluytmans et al [1] that direct contact of gaseous oxygen with catalyst
particles is necessary for the alcohol oxidation over supported platinum catalysts A
similar result was obtained for n-octanol Only cyclohexanol shows some conversion
(~15) in a deoxygenated atmosphere after 24 h For the effective use of the catalyst it
is necessary that the reaction should be carried out in the absence of mass transfer
limitations The effect of the mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation from 150 to 1200 rpm
Fig 5 shows that the conversion of alcohol increases with the increase in the speed of
agitation from 150 to 900 rpm The increase in the agitation speed above 900 rpm has no
effect on the conversion indicating a minimum effect of mass transfer resistance at above
900 rpm All the subsequent experiments were performed at 1200 rpm
46
4A 6 Effect of calcination temperature
Table 1 shows the effect of the calcination temperature on the catalytic activity of
ZrO2 The catalytic activity of ZrO2 calcined at 1223 K is higher than ZrO2 calcined at
723 K for the oxidation of alcohols This could be due to the change in the crystal
structure [2 3] Ferino et al [4] also reported that ZrO2 calcined at temperatures above
773 K was dominated by the monoclinic phase whereas that calcined at lower
temperatures was dominated by the tetragonal phase The difference in the catalytic
activity of the tetragonal and monoclinic zirconia-supported catalysts was also reported
by Yori et al [5] Yamasaki et al [6] and Li et al [7]
4A 7 Effect of reaction time
The effect of the reaction time was investigated at 413 K (Fig 6) The conversion
of all the alcohols increases linearly with the reaction time reaches a maximum value
and then remains constant for the remaining period The maximum attainable conversion
of benzyl alcohol (~50) is higher than cyclohexanol (~39) and n-octanol (~38)
Similarly the time required to reach the maximum conversion for benzyl alcohol (~30 h)
is shorter than the time required for cyclohexanol and n-octanol (~40 h) Considering the
establishment of equilibrium between alcohols and their oxidation products the
experimental value of the maximum attainable conversion for benzyl alcohol is much
different from the theoretical values obtained using the standard free energy of formation
(∆Gordmf) values [8] for benzyl alcohol benzaldehyde and H2O or H2O2
Table 1 Effect of calcination temperature on the catalytic
performance of ZrO2 for the liquid-phase oxidation of alcohols
Reaction condition 1200 rpm ZrO2 02 g alcohols 10 ml p(O2) =
101 kPa O2 flow rate 40 mlmin 413 K 24 h ZrO2 was calcined at
1223 K
47
Figure 5
Effect of agitation speed on the catalytic
performance of ZrO2 for the liquid-phase
oxidation of alcohols (1) Benzyl
alcohol (2) Cyclohexanol (3) n-Octanol
(Reaction conditions ZrO2 02 g
alcohols 10 ml p(O2) = 101 kPa O2
flow rate 40 mlmin 413 K 24 h ZrO2
was calcined at 1223 K
Figure 6
Effect of reaction time on the catalytic
performance of ZrO2 for the liquid-
phase oxidation of alcohols
(1) Benzyl alcohol (2) Cyclohexanol
(3) n-Octanol
Figure 7
Effect of O2 partial pressure on the
catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at
different temperatures (1) 373 K (2) 383
K (3) 393 K (4) 403 K (5) 413 K
(Reaction condition total flow rate (O2 +
N2) = 40 mlmin)
Figure 8
Plots of 1r vs1pO2 according to LH
kinetic equation for moderate
adsorption
48
4A 8 Effect of oxygen partial pressure
The effect of oxygen partial pressure on the catalytic performance of ZrO2 for the
liquid-phase oxidation of cyclohexanol at different temperatures was investigated Fig 7
shows that the average rate of the cyclohexanol conversion increases with the increase in
the partial pressure of oxygen and temperature Higher conversions are however
accompanied by a small decline (~2) in the selectivity for cyclohexanone The major
side products for cyclohexanol detected at high temperatures are cyclohexene benzene
and phenol Eanche et al [9] observed that the reaction was of zero order at p(O2) ge 100
kPa for benzyl alcohol oxidation to benzaldehyde under solvent free conditions They
used higher oxygen partial pressures (p(O2) ge 100 kPa) This study has been performed in
a lower range of oxygen partial pressure (p(O2) le 101 kPa) Fig7 also shows a zero order
dependence of the rate on oxygen partial pressure at p(O2) ge 76 kPa and 413 K
confirming the observation of Eanche et al [9] The average rates of the oxidation of
alcohols have been calculated from the total conversion achieved in 24 h Comparison of
these average rates with the average rate data for the oxidation of cyclohexanol tabulated
by Mallat et al [10] shows that ZrO2 has a reasonably good catalytic activity for the
alcohol oxidation in the liquid phase
4A 9 Kinetic analysis
The kinetics of a solvent-free liquid phase heterogeneous reaction can be studied
when the mass transfer resistance is eliminated Therefore the effect of agitation was
investigated first Fig 5 shows that the conversion of alcohol increases with increase in
speed of agitation from 150mdash900 rpm which was kept constant after this range till 1200
rpm This means that beyond 900 rpm mass transfer effect is minimum Both the effect of
stirring and the apparent activation energy (ca 654 kJmol-1) show that the reaction is in
the kinetically controlling regime This is a typical slurry reaction having the catalyst in
the solid state and the reactants in liquid phase During the development of mechanistic
interpretations of the catalytic reactions using macroscopic rate equations that find
general acceptance are the Langmuir-Hinshelwood (LH) [11] Eley Rideal mechanism
[12] and Mars-Van Krevelen mechanism [13]
Most of the reactions by heterogeneous
49
catalysis are found to obey the Langmuir Hinshelwood mechanism The data were fitted
to different LH kinetic equations (1)mdash(4)
Non-dissociative adsorption
2
21
O
O
kKpr
Kp=
+ (1)
Dissociative Adsorption
( )
( )
2
2
1
2
1
21
O
O
k Kpr
Kp
=
+
(2)
Where ldquorrdquo is rate of reaction ldquokrdquo is the rate constant and ldquoKrdquo is the adsorption
equilibrium constant
The linear form of equation (1)
2
1 1 1
Or kKp k= + (3)
The data fitted to equation (3) for non-dissociative adsorption shows sharp linearity as
indicated in figure 8 All other forms weak adsorption of oxygen (2Or kKp= ) or the
linear form of equation (2)
( )2
1
2
1 1 1
O
r kk Kp
= + (4)
were not applicable to the data
426 Mechanism of reaction
In the present research work the major products of the dehydrogenation of
alcohols over ZrO2 are ketones aldehydes Increase in rate of formation of desirable
products with increase in pO2 proves that oxidative dehydrogenation is the major
pathway of the reaction as indicated in Fig 7 The formation of cyclohexene in the
cyclohexanol dehydrogenation particularly at lower temperatures supports the
dehydration pathway The formation of phenol and other unknown products particularly
at higher temperatures may be due to inter-conversion among the reaction components
50
The formation of cyclohexene is due to the slight use of the acidic sites of ZrO2 via acid
catalyzed E2 mechanism which is supported by the work reported [14-17]
To check the mechanism of oxidative dehydrogenation of alcohol to corresponding
carbonyl compounds in which the oxygen acts as a receptor for hydrogen methylene blue
was introduced in the reaction mixture and the reaction was run in the absence of oxygen
After 14 h of the reaction duration the blue color of the reaction mixture (due to
methylene blue) disappeared It means that the dye goes over into colorless liquor due to
the extraction of hydrogen from alcohol by the methylene blue This is in excellent
agreement with the work reported [18-20] Methylene blue as a hydrogen receptor was
also verified by Nicoletti et al [21] Fabiana et al[22] have investigated dehydrogenation
of cyclohexanol over bi-metallic RhmdashCu and proposed two different reaction pathways
Dehydration of cyclohexanol to cyclohexene proceeds at the acid sites and then
cyclohexanol moves toward the RhmdashCu sites being dehydrogenated to benzene
simultaneously dehydrogenation occurs over these sites to cyclohexanone or phenol
At a very early stage Heyns et al [23 24] suggested that liquid phase oxidation of
alcohols on metal surfaces proceed via a dehydrogenation mechanism followed by the
oxidation of the adsorbed hydrogen atom with dissociatively adsorbed oxygen This was
supported by kinetic modeling of oxidation experiments [25] and by direct observation of
hydrogen evolving from aldose aqueous solutions in the presence of platinum or rhodium
catalysts [26] A number of different formulae have been proposed to describe the surface
chemistry of the oxidative dehydrogenation mechanism Thus in a study based on the
kinetic modeling of the ethanol oxidation on platinum van den Tillaart et al [27]
proposed that following the first step of abstraction of the hydroxyl hydrogen of ethanol
the ethoxide species CH3CH2Oads
did not dehydrogenate further but reacted with
dissociatively adsorbed oxygen
CH3CH
2OHrarr CH
3CH
2O
ads+ H
ads (1)
CH3CH
2O
ads+ O
adsrarrCH
3CHO + OH
ads (2)
Hads
+ OHads
rarrH2O (3)
51
In this research work we propose the same mechanism of reaction for the oxidative
dehydrogenation of alcohol to aldehydes ketones over ZrO2
C6H
11OHrarrC
6H
11O
ads+ H
ads (4)
C6H
11O
ads + O
adsrarrC
6H
10O + OH
ads (5)
Hads
+ OHads
rarrH2O (6)
In the inert atmosphere we propose the following mechanism for dehydrogenation of
cyclohexanol to cyclohexanone which probably follows the dehydrogenation pathway
C6H
11OHrarrC
6H
11O
ads + H
ads (7)
C6H
11O
adsrarrC
6H
10O + H
ads (8)
Hads
+ Hads
rarrH2
(9)
The above mechanism proposed in the present research work is in agreement with the
mechanism proposed by Ahmad et al [28] who studied the dehydrogenation and
dehydration of cyclohexanol over CuCrFeO4 and CuCr2O4
We also identified cyclohexene as the side product of the reaction which is less than 1
The mechanism of cyclohexene formation from cyclohexanol also follows the
dehydration pathway
C6H
11OHrarrC
6H
10OH
ads+ H
ads (10)
C6H
10OH
adsrarrC
6H
10 + OH
ads (11)
Hads
+ OHads
rarrH2O (12)
In the formation of cyclohexene it was observed that with the increase in partial pressure
of oxygen no increase in the formation of cyclohexene occurred This clearly indicates
that oxygen has no effect on the formation of cyclohexene
52
427 Role of oxygen
Oxygen plays an important role in the oxidation of organic compounds which
was believed to be dissociatively adsorbed on transition metal surfaces [29] Various
forms of oxygen may exist on the surface and in the bulk of oxide catalyst which include
(a) chemisorbed surface oxygen species uncharged and charged (mono-atomic O- andor
molecular) (b) lattice oxygen of the formal charge O2-
According to Haber [30] O2
- and O- being strongly electrophilic reactants attack
the organic molecule in the regions of its high electron density and peroxy and epoxy
complexes formed as a result of such attack are in the unstable conditions of a
heterogeneous catalytic reaction and represent intermediates in the degradation of the
organic molecule letting Haber propose a classification of oxidation reactions into two
groups ldquoelectronic oxidation proceeding through the activation of oxygen and
nucleophilic oxidation in which activation of the organic molecule is the first step
followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction
[31] The simplest view of a metal oxide is that it will have two distinct types of lattice
points a positively charged site associated with the metal cation and a negatively charged
site associated with the oxygen anion However many of the oxides of major importance
as redox catalysts have metal ions with anionic oxygen bound to them through bonds of a
coordinative nature Oxygen chemisorption is of most interest to consider that how the
bond rupturing occurs in O2 with electron acquisition to produce O2- As a gas phase
molecule oxygen ldquoO2rdquo has three pairs of electrons in the bonding outer orbital and two
unpaired electrons in two anti-bonding π-orbitals producing a net double bond In the
process of its chemisorption on an oxide surface the O2 molecule is initially attached to a
reduced metal site by coordinative bonding As a result there is a transfer of electron
density towards O2 which enters the π-orbital and thus weakens the OmdashO bond
Cooperative action [32] involving more than one reduction site may then affect the
overall dissociative conversion for which the lowest energy pathway is thought to
involve a succession of steps as
O2rarr O
2(ads) rarr O2
2- (ads)-2e-rarr 2O
2-(lattice)
53
This gives the basic description of the effective chemisorption mechanism of oxygen as
involved in many selective oxidation processes It depends upon the relatively easy
release of electrons associated with the increase of oxidation state of the associated metal
center Two general mechanisms can be investigated for the oxidation of molecule ldquoXrdquo
on the oxide surface
X(ads) + O(lattice) rarr Product + Lattice vacancy
12O2(g) + Lattice vacancy rarr O (lattice)
ie X(ads) reacts with oxygen from the oxide lattice and the resultant vacancy is occupied
afterward using gas phase oxygen The general action represented by this mechanism is
referred to as Mars-Van Krevelen mechanism [33-35] Some catalytic processes at solid
surface sites which are governed by the rates of reactant adsorption or less commonly on
product desorption Hence the initial rate law took the form of Rate = k (Po2)12 which
suggests that the limiting role is played by the dissociative chemisorption of the oxygen
on the sites which are independent of those on which the reactant adsorbs As
represented earlier that
12 O2 (gas) rarr O (lattice)
The rate of this adsorption process would be expected to depend upon (pO2)12
on the
basis of mass action principle In Mar-van Krevelen mechanism the organic molecule
Xads reacts with the oxygen from an oxide lattice preceding the rate determining
replenishment of the resultant vacancy with oxygen derived from the gas phase The final
step in the overall mechanism is the oxidation of the partially reduced surface by O2 as
obvious in the oxygen chemisorption that both reductive and oxidative actions take place
on the solid surfaces The kinetic expression outlined was derived as
p k op k
p op k k Rate
redred2
n
ox
red2
n
redox
+=
where kox and kred
represent the rate constants for oxidation of the oxide catalysts and
n =1 represents associative and n =12 as dissociative oxygen adsorption
54
Chapter 4A
References
1 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten J
C Catal Today 2000 57 143
2 Chuah G K Catal Today 1999 49 131
3 Liu H Feng L Zhang X Xue Q J Phys Chem 1995 99 332
4 Ferino I Casula M F Corrias A Cutrufello M Monaci G R
Paschina G Phys Chem Chem Phys 2000 2 1847
5 Yori J C Parera J M Catal Lett 2000 65 205
6 Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K Catal
Commun 2006 7 24
7 Li X Nagaoka K Simon L J Olindo R Lercher J A Catal Lett 2007
113 34
8 Dean A J Langersquos Handbook of Chemistry 13th Ed New York McGraw Hill
1987 9ndash72
9 Enache D I Edwards J K Landon P Espiru B S Carley A F Herzing
A H Watanabe M Kiely C J Knight D W Hutchings G J Science 2006
311 362
10 Mallat T Baiker A Chem Rev 2004 104 3037
11 Bonzel H P Ku R Surf Sci 1972 33 91
12 Somorjai G A Chemistry in Two Dimensions Cornell University Press Ithaca
New York 1981
13 Xu X De Almeida C P Antal M J Jr Ind Eng Chem Res 1991 30 1448
14 Narayan R Antal M J Jr J Am Chem Soc 1990 112 1927
15 Xu X De Almedia C Antal J J Jr J Supercrit Fluids 1990 3 228
16 West M A B Gray M R Can J Chem Eng 1987 65 645
17 Wieland H A Ber Deut Chem Ges 1912 45 2606
18 Wieland H A Ber Duet Chem Ges 1913 46 3327
19 Wieland H A Ber Duet Chem Ges 1921 54 2353
20 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759
55
21 Fabiana M T Appl Catal A General 1997 163 153
22 Heyns K Paulsen H Angew Chem 1957 69 600
23 Heyns K Paulsen H Ruediger G Weyer J F Chem Forsch 1969 11 285
24 de Wilt H G J Van der Baan H S Ind Eng Chem Prod Res Dev 1972 11
374
25 de Wit G de Vlieger J J Kock-van Dalen A C Heus R Laroy R van
Hengstum A J Kieboom A P G Van Bekkum H Carbohydr Res 1981 91
125
26 Van Den Tillaart J A A Kuster B F M Marin G B Appl Catal A General
1994 120 127
27 Ahmad A Oak S C Darshane V S Bull Chem Soc Jpn 1995 68 3651
28 Gates B C Catalytic Chemistry John Wiley and Sons Inc 1992 p 117
29 Bielanski A Haber J Oxygen in Catalysis Marcel Dekker New York 1991 p
132
30 Haber J Z Chem 1973 13 241
31 Brazdil J F In Characterization of Catalytic Materials Ed Wachs I E Butter
Worth-Heinmann Inc USA 1992 96 p 10353
32 Mars P Krevelen D W Chem Eng Sci 1954 3 (Supp) 41
33 Sivakumar T Shanthi K Sivasankar B Hung J Ind Chem 1998 26 97
34 Saito Y Yamashita M Ichinohe Y In Catalytic Science amp Technology Vol
1 Eds Yashida S Takezawa N Ono T Kodansha Tokyo 1991 p 102
35 Sing KSW Pure Appl Chem 1982 54 2201
56
Chapter 4B
Results and discussion
Reactant Alcohol in aqueous medium
Catalyst ZrO2
Oxidation of alcohols in aqueous medium by zirconia catalyst
4B 1 Characterization of catalyst
ZrO2 was well characterized by using different modern techniques like FT-IR
SEM and EDX FT-IR spectra of fresh and used ZrO2 are reported in Fig 1 FT-IR
spectra for fresh ZrO2 show a small peak at 2345 cm-1 as we used this ZrO2 for further
reactions the peak become sharper and sharper as shown in the Fig1 This peak is
probably due to asymmetric stretching of CO2 This was predicted at 2640 cm-1 but
observed at 2345 cm-1 Davies et al [1] have reported that the sample derived from
alkoxide precursors FT-IR spectra always showed a very intense and sharp band at 2340
cm-1 This band was assigned to CO2 trapped inside the bulk structure of the oxide which
is in rough agreement with our results Similar results were obtained from the EDX
elemental analysis The carbon content increases as the use of ZrO2 increases as reported
in Fig 2 These two findings are pointing to complete oxidation of alcohol SEM images
of ZrO2 at different resolution were recoded shown in Fig3 SEM image show that ZrO2
has smooth morphology
4B 2 Oxidation of benzyl alcohols in Aqueous Medium
57
Figure 1
FT-IR spectra for (Fresh 1st time used 2nd
time used 3rd time used and 4th time used
ZrO2)
Figure 2
EDX for (Fresh 1st time used 2nd time used
3rd time used and 4th time used ZrO2)
58
Figure 3
SEM images of ZrO2 at different resolutions (1000 2000 3000 and 6000)
59
Overall oxidation reaction of benzyl alcohol shows that the major products are
benzaldehyde and benzoic acid The kinetic curve illustrating changes in the substrate
and oxidation products during the reaction are shown in Fig4 This reveals that the
oxidation of benzyl alcohol proceeds as a consecutive reaction reported widely [2] which
are also supported by UV spectra represented in Fig 5 An isobestic point is evident
which points out to the formation of a benzaldehyde which is later oxidized to benzoic
acid Calculation based on these data indicates that an oxidation of benzyl alcohol
proceeds as a first order reaction with respect to the benzyl alcohol oxidation
4B 3 Effect of Different Parameters
Data concerning the impact of different reaction parameters on rate of reaction
were discuss in detail Fig 6a and 6b presents the effect of concentration studies at
different temperature (303-333K) Figures 6a 6b and 7 reveals that the conversion is
dependent on concentration and temperature as well The rate decreases with increase in
concentration (because availability of active sites decreases with increase in
concentration of the substrate solution) while rate of reaction increases with increase in
temperature Activation energy was calculated (~ 86 kJ mole-1) by applying Arrhenius
equation [3] Activation energy and agitation effect supports the absence of mass transfer
resistance Bavykin et al [4] have reported a value of 79 kJ mole-1 for apparent activation
energy in a purely kinetic regime for ruthenium catalyzed oxidation of benzyl alcohol
They have reported a value of 61 kJ mole-1 for a combination of kinetic and mass transfer
regime The partial pressure of oxygen dramatically affects the rate of reaction Fig 8
shows that the conversion increases linearly with increase of partial pressure of
oxygen The selectivity to required product increases with increase in the partial pressure
of oxygen Fig 9 shows that the increase in the agitation above the 900 rpm did not affect
the rate of reaction The rate increases from 150-900 rpm linearly but after that became
flat which is the region of interest where the mass transfer resistance is minimum or
absent [5] The catalyst reused several time after simple drying in oven It was observed
that the activity of catalyst remained unchanged after many times used as shown in Fig
10
60
Figure 6a and 6b
Plot of Concentration Vs Conversion
Figure 4
Concentration change of benzyl alcohol
and reaction products during oxidation
process at lower concentration 5gL Reaction conditions catalyst (02 g) substrate solution (10 mL) pO2 (101 kPa) flow rate (40
mLmin) temperature (333K) stirring (900 rpm)
time 6 hours
Figure 5
UV spectrum i to v (225nm)
corresponding to benzoic acid and
a to e (244) corresponding to
benzaldehyde Reaction conditions catalyst (02 g)
substrate solution (5gL 10 mL) pO2 (101
kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm)
61
Figure 7
Plot of temperature Vs Conversion Reaction conditions catalyst (02 g) substrate solution (20gL 10 mL) pO2 (101 kPa) stirring (900 rpm) time
(6 hrs)
Figure 11 Plot of agitation Vs
Conversion
Figure 9
Effect of agitation speed on benzyl
alcohol oxidation catalyzed by ZrO2 at
333K Reaction conditions catalyst (02 g) substrate
solution (20gL 10 mL) pO2 (101 kPa) time (6
hrs)
Figure 8
Plot of pO2 Vs Conversion Reaction conditions catalyst (02 g) substrate solution (10gL 10 mL) temperature (333K)
stirring (900 rpm) time (6 hrs)
Figure 10
Reuse of catalyst several times Reaction conditions catalyst (02 g) substrate solution
(10gL 10 mL) pO2 (101 kPa) flow rate (40 mLmin) temperature (333K) stirring (900 rpm) time (6 hrs)
62
Chapter 4B
References
1 Davies L E Bonini N A Locatelli S Gonzo EE Latin American Applied
Research 2005 35 23-28
2 Christoskova St Stoyanova Water Res 2002 36 2297-2303
3 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
4 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
5 Ilyas M Sadiq M Chin J Chem 2008 26 941
63
Chapter 4C
Results and discussion
Reactant Toluene
Catalyst PtZrO2
Oxidation of toluene in solvent free conditions by PtZrO2
4C 1 Catalyst characterization
BET surface area was 65 and 183 m2 g-1 for ZrO2 and PtZrO2 respectively Fig 1
shows SEM images which reveal that the PtZrO2 has smaller particle size than that of
ZrO2 which may be due to further temperature treatment or reduction process The high
surface area of PtZrO2 in comparison to ZrO2 could be due to its smaller particle size
Fig 2a b shows the diffraction pattern for uncalcined ZrO2 and ZrO2 calcined at 950 degC
Diffraction pattern for ZrO2 calcined at 950 degC was dominated by monoclinic phase
(major peaks appear at 2θ = 2818deg and 3138deg) [1ndash3] Fig 2c d shows XRD patterns for
a PtZrO2 calcined at 750 degC both before and after reduction in H2 The figure revealed
that PtZrO2 calcined at 750 degC exhibited both the tetragonal phase (major peak appears
at 2θ = 3094deg) and monoclinic phase (major peaks appears 2θ = 2818deg and 3138deg) The
reflection was observed for Pt at 2θ = 3979deg which was not fully resolved due to small
content of Pt (~1 wt) as also concluded by Perez- Hernandez et al [4] The reduction
processing of PtZrO2 affects crystallization and phase transition resulting in certain
fraction of tetragonal ZrO2 transferred to monoclinic ZrO2 as also reported elsewhere [5]
However the XRD pattern of PtZrO2 calcined at 950 degC (Fig 2e f) did not show any
change before and after reduction in H2 and were fully dominated by monoclinic phase
However a fraction of tetragonal zirconia was present as reported by Liu et al [6]
4C 2 Catalytic activity
In this work we first studied toluene oxidation at various temperatures (60ndash90degC)
with oxygen or air passing through the reaction mixture (10 mL of toluene and 200 mg of
64
Figure 1
SEM images of ZrO2 (calcined at 950 degC) and PtZrO2 (calcined at 950 degC and reduced in H2)
Figure 2
XRD pattern of ZrO2 and PtZrO2 (a) ZrO2 (uncalcined) (b) ZrO2 (calcined at 950 degC) (c) PtZrO2
(unreduced calcined at 750 degC) and (d) PtZrO2 (calcined at 750 degC and reduced in H2) (e) PtZrO2
(unreduced calcined at 950 degC) and (f) PtZrO2 (calcined at 950 degC and reduced in H2)
65
1(wt) PtZrO2) with continuous stirring (900 rpm) The flow rate of oxygen and air
was kept constant at 40 mLmin Table 1 present these results The known products of the
reaction were benzyl alcohol benzaldehyde and benzoic acid The mass balance of the
reaction showed some loss of toluene (~1) Conversion rises with temperature from
96 to 372 The selectivity for benzyl alcohol is higher than benzoic acid at 60 degC At
70 degC and above the reaction is more selective for benzoic acid formation 70 degC and
above The reaction is highly selective for benzoic acid formation (gt70) at 90degC
Reaction can also be performed in air where 188 conversion is achieved at 90 degC with
25 selectivity for benzyl alcohol 165 for benzaldehyde and 516 for benzoic acid
Comparison of these results with other solvent free systems shows that PtZrO2 is very
effective catalyst for toluene oxidation Higher conversions are achieved at considerably
lower temperatures and pressure than other solvent free systems [7-12] The catalyst is
used without any additive or promoter The commercial catalyst (Envirocat EPAC)
requires trimethylacetic acid as promoter with a 11 ratio of catalyst and promoter [7]
The turnover frequency (TOF) was calculated as the molar ratio of toluene converted to
the platinum content of the catalyst per unit time (h-1) TOF values are very high even at
the lowest temperature of 60degC
4C 3 Time profile study
The time profile of the reaction is shown in Fig 3 where a linear increase in
conversion is observed with the passage of time An induction period of 30 min is
required for the products to appear At the lowest conversion (lt2) the reaction is 100
selective for benzyl alcohol (Fig 4) Benzyl alcohol is the main product until the
conversion reaches ~14 Increase in conversion is accompanied by increase in the
selectivity for benzoic acid Selectivity for benzaldehyde (~ 20) is almost unaffected by
increase in conversion This reaction was studied only for 3 h The reaction mixture
becomes saturated with benzoic acid which sublimes and sticks to the walls of the
reactor
66
Table 1
Oxidation of toluene at various temperatures
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa) flow rate of O2Air (40 mLmin) a Toluene lost (mole
()) not accounted for bTOF (turnover frequency) molar ratio of converted toluene to the platinum content
of the catalyst per unit time (h-1)
Figure 3
Time profile for the oxidation of toluene
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
Figure 4
Selectivity of toluene oxidation at various
conversions
Reaction conditions
Catalyst (02 g) toluene (10 mL) pO2 (101 kPa)
flow rate (40 mLmin) temperature (90 degC) stirring
(900 rpm)
67
4C 4 Effect of oxygen flow rate
Effect of the flow rate of oxygen on toluene conversion was also studied Fig 5
shows this effect It can be seen that with increase in the flow rate both toluene
conversion and selectivity for benzoic acid increases Selectivity for benzyl alcohol and
benzaldehyde decreases with increase in the flow rate At the oxygen flow rate of 70
mLmin the selectivity for benzyl alcohol becomes ~ 0 and for benzyldehyde ~ 4 This
shows that the rate of reaction and selectivity depends upon the rate of supply of oxygen
to the reaction system
4C 5 Appearance of trans-stilbene and methyl biphenyl carboxylic acid
Toluene oxidation was also studied for the longer time of 7 h In this case 20 mL
of toluene and 400 mg of catalyst (1 PtZrO2) was taken and the reaction was
conducted at 90 degC as described earlier After 7 h the reaction mixture was converted to a
solid apparently having no liquid and therefore the reaction was stopped The reaction
mixture was cooled to room temperature and more toluene was added to dissolve the
solid and then filtered to recover the catalyst Excess toluene was recovered by
distillation at lower temperature and pressure until a concentrated suspension was
obtained This was cooled down to room temperature filtered and washed with a little
toluene and sucked dry to recover the solid The solid thus obtained was 112 g
Preparative TLC analysis showed that the solid mixture was composed of five
substances These were identified as benzaldehyde (yield mol 22) benzoic acid
(296) benzyl benzoate (34) trans-stilbene (53) and 4-methyl-2-
biphenylcarboxylic acid (108) The rest (~ 4) could be identified as tar due to its
black color Fig 6 shows the conversion of toluene and the yield (mol ) of these
products Trans-stilbene and methyl biphenyl carboxylic acid were identified by their
melting point and UVndashVisible and IR spectra The Diffuse Reflectance FTIR spectra
(DRIFT) of trans-stilbene (both of the standard and experimental product) is given in Fig
7 The oxidative coupling of toluene to produce trans-stilbene has been reported widely
[13ndash17] Kai et al [17] have reported the formation of stilbene and bibenzyl from the
oxidative coupling of toluene catalyzed by PbO However the reaction was conducted at
68
Figure 7
Diffuse reflectance FTIR (DRIFT) spectra of trans-stilbene
(a) standard and (b) isolated product (mp = 122 degC)
Figure 5
Effect of flow rate of oxygen on the
oxidation of toluene
Reaction conditions
Catalyst (04 g) toluene (20 mL) pO2 (101
kPa) temperature (90degC) stirring (900
rpm) time (3 h)
Figure 6
Conversion of toluene after 7 h of reaction
TL toluene BzH benzaldehyde
BzOOH benzoic acid BzB benzyl
benzoate t-ST trans-stilbene MBPA
methyl biphenyl carboxylic acid reaction
Conditions toluene (20 mL) catalyst (400
mg) pO2 (101 kPa) flow rate (40 mLmin)
agitation (900 rpm) temperature (90degC)
69
a higher temperature (525ndash570 degC) in the vapor phase Daito et al [18] have patented a
process for the recovery of benzyl benzoate by distilling the residue remaining after
removal of un-reacted toluene and benzoic acid from a reaction mixture produced by the
oxidation of toluene by molecular oxygen in the presence of a metal catalyst Beside the
main product benzoic acid they have also given a list of [6] by products Most of these
byproducts are due to the oxidative couplingoxidative dehydrocoupling of toluene
Methyl biphenyl carboxylic acid (mp 144ndash146 degC) is one of these byproducts identified
in the present study Besides these by products they have also recovered the intermediate
products in toluene oxidation benzaldehyde and benzyl alcohol and esters formed by
esterification of benzyl alcohol with a variety of carboxylic acids inside the reactor The
absence of benzyl alcohol (Figs 3 6) could be due to its esterification with benzoic acid
to form benzyl benzoate
70
Chapter 4C
References
1 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Microporous Mesoporous Mater 2006 88 22
2 Ferino I Casula M F Corrias A Cutrufello M Monaci G R Paschina G
Phys Chem Chem Phys 2000 2 1847
3 Ding J Zhao N Shi C Du X Li J J Alloys Compd 2006 425 390
4 Perez-Hernandwz R Aguilar F Gomez-Cortes A Diaz G Catal Today
2005 107ndash108 175
5 Zhan Y Cai G Xiao Y Wei K Cen T Zhang H Zheng Q Guang Pu
Xue Yu Guang Pu Fen Xi 2004 24 914
6 Liu H Feng l Zhang X Xue Q J Phys Chem 1995 99 332
7 Bastock T E Clark J H Martin K Trentbirth B W Green Chem 2002 4
615
8 Subrahmanyama C H Louisb B Viswanathana B Renkenb A Varadarajan
T K Appl Catal A Gen 2005 282 67
9 Raja R Thomas J M Dreyerd V Catal Lett 2006 110 179
10 Thomas J M Raja R Catal Today 2006 117 22
11 Li X Xu J Wang F Gao J Zhou L Yang G Catal Lett 2006108 137
12 Li X Xu J Zhou L Wang F Gao J Chen C Ning J Ma H Catal Lett
2006 110 255
13 Montgomery P D Moore R N Knox W K US Patent 3965206 1976
14 Lee T P US Patent 4091044 1978
15 Williamson A N Tremont S J Solodar A J US Patent 4255604 4268704
4278824 1981
16 Hupp S S Swift H E Ind Eng Chem Prod Res Dev 1979 18117
17 Kai T Nomoto R Takahashi T Catal Lett 2002 84 75
18 Daito N Ueda S Akamine R Horibe K Sakura K US Patent 6491795
2002
71
Chapter 4D
Results and discussion
Reactant Benzyl alcohol in n- haptane
Catalyst ZrO2 Pt ZrO2
Oxidation of benzyl alcohol by zirconia supported platinum catalyst
4D1 Characterization catalyst
BET surface area of the catalyst was determined using a Quanta chrome (Nova
2200e) Surface area ampPore size analyzer Samples were degassed at 110 0C for 2 hours
prior to determination The BET surface area determined was 36 and 48 m2g-1 for ZrO2
and 1 wt PtZrO2 respectively XRD analyses were performed on a JEOL (JDX-3532)
X-Ray Diffractometer using CuKα radiation with a tube voltage of 40 KV and 20mA
current Diffractograms are given in figure 1 The diffraction pattern is dominated by
monoclinic phase [1] There is no difference in the diffraction pattern of ZrO2 and 1
PtZrO2 Similarly we did not find any difference in the diffraction pattern of fresh and
used catalysts
4D2 Oxidation of benzyl alcohol
Preliminary experiments were performed using ZrO2 and PtZrO2 as catalysts for
oxidation of benzyl alcohol in the presence of one atmosphere of oxygen at 90 ˚C using
n-heptane as solvent Table 1 shows these results Almost complete conversion (gt 99 )
was observed in 3 hours with 1 PtZrO2 catalyst followed by 05 PtZrO2 01
PtZrO2 and pure ZrO2 respectively The turn over frequency was calculated as molar
ratio of benzyl alcohol converted to the platinum content of catalyst [2] TOF values for
the enhancement and conversion are shown in (Table 1) The TOF values are 283h 74h
and 46h for 01 05 and 1 platinum content of the catalyst respectively A
comparison of the TOF values with those reported in the literature [2 11] for benzyl
alcohol shows that PtZrO2 is among the most active catalyst
72
All the catalysts produced only benzaldehyde with no further oxidation to benzoic
acid as detected by FID and UV-VIS spectroscopy Selectivity to benzaldehyde was
always 100 in all these catalytic systems Opre et al [10-11] Mori et al [13] and
Makwana et al [15] have also observed 100 selectivity for benzaldehyde using
RuHydroxyapatite Pd Hydroxyapatite and MnO2 as catalysts respectively in the
presence of one atmosphere of molecular oxygen in the same temperature range The
presence of oxygen was necessary for benzyl alcohol oxidation to benzaldehyde No
reaction was observed when oxygen was not bubbled through the reaction mixture or
when oxygen was replaced by nitrogen Similarly no reaction was observed in the
presence of oxygen above the surface of the reaction mixture This would support the
conclusion [5] that direct contact of gaseous oxygen with the catalyst particles is
necessary for the reaction
These preliminary investigations showed that
i PtZrO2 is an effective catalyst for the selective oxidation of benzyl alcohol to
benzaldehyde
ii Oxygen contact with the catalyst particles is required as no reaction takes place
without bubbling of O2 through the reaction mixture
4D21 Leaching of the catalyst
Leaching of the catalyst to the solvent is a major problem in the liquid phase
oxidation with solid catalyst To test leaching of catalyst the following experiment was
performed first the solvent (10 mL of n-heptane) and the catalyst (02 gram of PtZrO2)
were mixed and stirred for 3 hours at 90 ˚C with the reflux condenser to prevent loss of
solvent Secondly the catalyst was filtered and removed and the reactant (2 m mole of
benzyl alcohol) was added to the filtrate Finally oxygen at a flow rate of 40 mLminute
was introduced in the reaction system After 3 hours no product was detected by FID
Furthermore chemical tests [18] of the filtrate obtained do not show the presence of
platinum or zirconium ions
73
Figure 1
XRD spectra of ZrO2 and 1 PtZrO2
Figure 2
Effect of mass transfer on benzyl
alcohol oxidation catalyzed by
1PtZrO2 Catalyst (02g) benzyl
alcohol (2 mmole) n-heptane (10
mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring
rate (900rpm) time (1hr)
Figure 3
Arrhenius plot for benzyl alcohol
oxidation Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) n-heptane (10 mL)
temperature (90 ordmC) O2 (760 torr
flow rate 40 mLMin) stirring rate
(900rpm) time (1hr)
74
4D22 Effect of Mass Transfer
The process is a typical slurry-phase reaction having one liquid reactant a solid
catalyst and one gaseous reactant The effect of mass transfer on the rate of reaction was
determined by studying the change in conversion at various speeds of agitation (Figure 2)
the conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above showing that conversion is independent of stirring This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
4D23 Temperature Effect
Effect of temperature on the conversion was studied in the range of 60-90 ˚C
(figure 3) The Arrhenius equation was applied to conversion obtained after one hour
The apparent activation energy is ~ 778 kJ mole-1 Bavykin et al [12] have reported a
value of 79 kJmole-1 for apparent activation energy in a purely kinetic regime for
ruthenium-catalyzed oxidation of benzyl alcohol They have reported a value of 61
kJmole-1 for a combination of kinetic and mass transfer regime The value of activation
energy in the present case shows that in these conditions the reaction is free of mass
transfer limitation
4D24 Solvent Effect
Comparison of the activity of PtZrO2 for benzyl alcohol oxidation was made in
various other solvents (Table 2) The catalyst was active when toluene was used as
solvent However it was 100 selective for benzoic acid formation with a maximum
yield of 34 (based upon the initial concentration of benzyl alcohol) in 3 hours
However the mass balance of the reaction based upon the amount of benzyl alcohol and
benzaldehyde in the final reaction mixture shows that a considerable amount of benzoic
acid would have come from oxidation of the solvent Benzene and n-octane were also
used as solvent where a 17 and 43 yield of benzaldehyde was observed in 25 hours
75
4D25 Time course of the reaction
The time course study for the oxidation of the reaction was monitored
periodically This investigation was carried out at 90˚C by suspending 200 mg of catalyst
in 10 mL of n-heptane 2 m mole of benzyl alcohol and passing oxygen through the
reaction mixture with a flow rate of 40 mLmin-1 at one atmospheric pressure Figure 4
shows an induction period of about 30 minutes With the increase in reaction time
benzaldehyde formation increases linearly reaching a conversion of gt99 after 150
minutes Mori et al [13] have also observed an induction period of 10 minutes for the
oxidation of 1- phenyl ethanol catalyzed by supported Pd catalyst
The derivative at any point (after 30minutes) on the curve (figure 6) gives the
rate The design equation for an isothermal well-mixed batch reactor is [14]
Rate = -dCdt
where C is the concentration of the reactant at time t
4D26 Reaction Kinetics Analysis
Both the effect of stirring and the apparent activation energy show that the
reaction is taking place in the kinetically controlled regime This is a typical slurry
reaction having catalyst in the solid state and reactants in liquid and gas phase
Following the approach of Makwana et al [15] reaction kinetics analyses were
performed by fitting the experimental data to one of the three possible mechanisms of
heterogeneous catalytic oxidations
i The Eley-Rideal mechanism (E-R)
ii The Mars-van Krevelen mechanism (M-K) or
iii The Langmuir-Hinshelwood mechanism (L-H)
The E-R mechanism requires one of the reactants to be in the gas phase Makwana et al
[15] did not consider the application of this mechanism as they were convinced that the
gas phase oxygen is not the reactive species in the catalytic oxidation of benzyl alcohol to
benzaldehyde by (OMS-2) type manganese oxide in toluene
However in the present case no reaction takes place when oxygen is passed
through the reactor above the surface of the liquid reaction mixture The reaction takes
place only when oxygen is bubbled through the liquid phase It is an indication that more
76
Table 2 Catalytic oxidation of benzyl alcohol
with molecular oxygen effect of solvent
Figure 4
Time profile for the oxidation of
benzyl alcohol Reaction conditions
Catalyst (02g) benzyl alcohol (2
mmole) solvent (10 mL) temperature
(90 ordmC) O2 (760 torr flow rate 40
mLMin) stirring rate (900rpm)
Reaction conditions
Catalyst (02g) benzyl alcohol (2 mmole)
solvent (10 mL) temperature (90 ordmC) O2 (760
torr flow rate 40 mLMin) stirring rate
(900rpm)
Figure 5
Non Linear Least square fit for Eley-
Rideal Model according to equation (2)
Figure 6
Non Linear Least square fit for Mars-van
Krevelen Model according to equation (4)
77
probably dissolved oxygen is not an effective oxidant in this case Replacing oxygen by
nitrogen did not give any product Kluytmana et al [5] has reported similar observations
Therefore the applicability of E-R mechanism was also explored in the present case The
E-R rate law can be derived from the reaction of gas phase O2 with adsorbed benzyl
alcohol (BzOH) as
Rate =
05
2[ ][ ]
1 ]
gkK BzOH O
k BzOH+ [1]
Where k is the rate coefficient and K is the adsorption equilibrium constant for benzyl
alcohol
It is to be mentioned that for gas phase oxidation reactions the E-R
mechanism envisage reaction between adsorbed oxygen with hydrocarbon molecules
from the gas phase However in the present case since benzyl alcohol is in the liquid
phase in contact with the catalyst and therefore it is considered to be pre-adsorbed at the
surface
In the case of constant O2 pressure equation 1 can be transformed by lumping together all
the constants to yield
BzOHb
BzOHaRate
+=
1 (2)
The M-K mechanism envisages oxidation of the substrate molecules by the lattice
oxygen followed by the re-oxidation of the reduced catalyst by molecular oxygen
Following the approach of Makwana et al [15] the rate expression for M-K mechanism
can be given
ng
n
g
OkBzOHk
OkBzOHkRate
221
221
+=
(3)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure the equation was transformed to
BzOHcb
BzOHaRate
+= (4)
78
The Lndash H mechanism involves adsorption of the reacting species (benzyl alcohol and
oxygen) on active sites at the surface followed by an irreversible rate-determining
surface reaction to give products The Langmuir-Hinshelwood rate law can be given as
1 2 2
1 2 2
2
1n
g
nn
g
K BzOH K O
kK K BzOH ORate
+ +
=
(5)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
benzyl alcohol an O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively
Again for a constant O2 pressure it can be transformed to
2BzOHcb
BzOHaRate
+= (6)
The rate data obtained from the time course study (figure 4) was subjected to
kinetic analysis using a nonlinear regression analysis according to the above-mentioned
three models Figures 5 and 6 show the models fit as compared to actual experimental
data for E-R and M-K according to equation 2 and 4 respectively Both these models
show a similar pattern with a similar value (R2 =0827) for the regression coefficient In
comparison to this figure 7 show the L-H model fit to the experimental data The L-H
Model (R2 = 0986) has a better fit to the data when subjected to nonlinear least square
fitting Another way to test these models is the traditional linear forms of the above-
mentioned models The linear forms are given by using equation 24 and 6 respectively
as follow
BzOH
a
b
aRate
BzOH+=
1 (7) [E-R model]
BzOH
a
c
a
b
Rate
BzOH+= (8) [M-K model]
and
BzOH
a
c
a
b
Rate
BzOH+= (9) [L-H-model]
It is clear that the linear forms of E-R and M-K models are similar to each other Figure 8
shows the fit of the data according to equation 7 and 8 with R2 = 0967 The linear form
79
Figure 7
Non Linear Least square fit for Langmuir-
Hinshelwood Model according to equation
(6)
Figure 8
Linear fit for Eley-Rideasl and Mars van Krevelen
Model according to equation (7 and 8)
Figure 9
Linear Fit for Langmuir-Hinshelwood
Model according to equation (9)
Figure 10
Time profile for benzyl alcohol conversion at
various oxygen partial pressures Reaction
conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
80
of L-H model is shown in figure 9 It has a better fit (R2 = 0997) than the M-K and E-R
models Keeping aside the comparison of correlation coefficients a simple inspection
also shows that figure 8 is curved and forcing a straight line through these points is not
appropriate Therefore it is concluded that the Langmuir-Hinshelwood model has a much
better fit than the other two models Furthermore it is also obvious that these analyses are
unable to differentiate between Mars-van Kerevelen and Eley-Rideal mechanism (Eqs
7 8 and 10)
4D27 Effect of Oxygen Partial Pressure
The effect of oxygen partial pressure was studied in the lower range of 95-760 torr with a
constant initial concentration of 02 M benzyl alcohol concentration (figure 10)
Adsorption of oxygen is generally considered to be dissociative rather than molecular in
nature However figure 11 shows a linear dependence of the initial rates on oxygen
partial pressure with a regression coefficient (R2 = 0998) This could be due to the
molecular adsorption of oxygen according to equation 5
1 2 2
2
1 2 21
g
g
kK K BzOH ORate
K BzOH K O
=
+ +
(10)
Where due to the low pressure of O2 the term 22 OK could be neglected in the
denominator to transform equation (10)
1 2 2
2
11
gkK K BzOH O
RateK BzOH
=+
(11)
which at constant benzyl alcohol concentration is reduced to
2Rate a O= (12)
Where a is a new constant having lumped together all the constants
In contrast to this the rate equation according to L-H mechanism for dissociative
adsorption of oxygen could be represented by
81
22
2
Ocb
OaRate
+= (13)
and the linear form would be
2
42
Oa
c
a
b
Rate
O+= (14)
Fitting of the data obtained for the dependence of initial rates on oxygen partial pressure
according to equation obtained from the linear forms of E-R (equation similar to 7) M-K
(equation similar to 8) and L-H model (equation 14) was not successful Therefore the
molecular adsorption of oxygen is favored in comparison to dissociative adsorption of
oxygen According to Engel et al [19] the existence of adsorbed O2 molecules on Pt
surface has been established experimentally Furthermore they have argued that the
molecular species is the ldquoprecursorrdquo for chemisorbed atomic species ldquoOadrdquo which is
considered to be involved in the catalytic reaction Since the steady state concentration of
O2ads at reaction temperatures will be negligibly small and therefore proportional to the
O2 partial pressure the kinetics of the reaction sequence
can be formulated as
gads
ad OkOkdt
Od22 == minus
(15)
If the rate of benzyl alcohol conversion is directly proportional to [Oad] then equation
(15) is similar to equation (12)
From the above analysis it could concluded that
a) The Langmuir-Hinshelwood mechanism is favored as compared to Eley-Rideal
and Mars-van Krevelen mechanisms
b) Adsorption of oxygen is molecular rather than dissoiciative in nature However
molecular adsorption of oxygen could be a precursor for chemisorbed atomic
oxygen (dissociative adsorption of oxygen)
It has been suggested that H2O2 could be an intermediate in alcohol oxidation on
Pdhydroxyapatite [13] which is produced by the reaction of the Pd-hydride species with
82
Figure 11
Effect of oxygen partial pressure on the initial
rates for benzyl alcohol oxidation
Conditions Catalyst (04g) benzyl alcohol (4
mmole) n-heptane (20 mL) temperature (90
ordmC) O2 (flow rate 40 mLMin) stirring (900
rmp)
Figure 12
Decomposition of hydrogen peroxide on
PtZrO2
Conditions catalyst (20 mg) hydrogen
peroxide (0067 M) volume 20 mL
temperature (0 ordmC) stirring (900 rmp)
83
molecular oxygen Hydrogen peroxide is immediately decomposed to H2O and O2 on the
catalyst surface Production of H2O2 has also been suggested during alcohol oxidation
on MnO2 [15] and PtO2 [16] Both Platinum [9] and MnO2 [17] have been reported to be
very active catalysts for H2O2 decomposition The decomposition of H2O2 to H2O and O2
by PtZrO2 was also confirmed experimentally (figure 12) The procedure adapted for
H2O2 decomposition by Zhou et al [17] was followed
4D 28 Mechanistic proposal
Our kinetic analysis supports a mechanistic model which assumes that the rate-
determining step involves direct interaction of the adsorbed oxidizing species with the
adsorbed reactant or an intermediate product of the reactant The mechanism proposed by
Mori et al [13] for alcohol oxidation by Pdhydroxyapatite is compatible with the above-
mentioned model This model involves the following steps
(i) formation of a metal-alcoholate species
(ii) which undergoes a -hydride elimination to produce benzaldehyde and a metal-
hydride intermediate and
(iii) reaction of this hydride with an oxidizing species having a surface concentration
directly proportional to adsorbed molecular oxygen which leads to the
regeneration of active catalyst and formation of O2 and H2O
The reaction mixture was subjected to the qualitative test for H2O2 production [13]
The color of KI-containing starch changed slightly from yellow to blue thus suggesting
that H2O2 is more likely to be an intermediate
This mechanism is similar to what has been proposed earlier by Sheldon and
Kochi [16] for the liquid-phase selective oxidation of primary and secondary alcohols
with molecular oxygen over supported platinum or reduced PtO2 in n-heptane at lower
temperatures ZrO2 alone is also active for benzyl alcohol oxidation in the presence of
oxygen (figure 2) Therefore a similar mechanism is envisaged for ZrO2 in benzyl
alcohol oxidation
84
Chapter 4D
References
1 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
2 Mallat T Baiker A Chem Rev 2004 104 3037-3058
3 Muzart J Ttetrahedron 2003 59 5789-5816
4 Rafelt J S Clark JH Catal Today 2000 57 33-44
5 Kluytmans J H J Markusse A P Kuster B F M Marin G B Schouten
J C Catal Today 2000 37 143-155
6 Gangwal V R van der Schaaf J Kuster B M F Schouten J C J Catal
2005 232 432-443
7 Hutchings G J Carrettin S Landon P Edwards JK Enache D Knight
DW Xu Y CarleyAF Top Catal 2006 38 223-230
8 Brink G Arends I W C E Sheldon R A Science 2000 287 1636-1639
9 Nicoletti J W Whitesides G M J Phys Chem 1989 93 759-767
10 Opre Z Grunwaldt JD Mallat T BaikerA J Molec Catal A-Chem 2005
242 224-232
11 Opre Z Ferri D Krumeich F Mallat T Baiker A J Catal 2006 241 287-
293
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Mori K Hara T Mizugaki T Ebitani K Kaneda K J Am Chem Soc
2004 126 10657-10666
14 Hashemi M M KhaliliB Eftikharisis B J Chem Res 2005 (Aug) 484-485
15 Makwana VD Son YC Howell AR Suib SL J Catal 2002 210 46-52
16 Sheldon R A Kochi J K Metal Catalyzed Oxidations of Organic Reactions
Academic Press New York 1981 p 354-355
17 Zhou H Shen YF Wang YJ Chen X OrsquoYoung CL Suib SL J Catal
1998 176 321-328
85
18 Charlot G Colorimetric Determination of Elements Principles and Methods
Elsvier Amsterdam 1964 pp 346 347 (Pt) pp 439 (Zr)
19 Engel T ErtlG in ldquoThe Chemical Physics of Solid Surfaces and Heterogeneous
Catalysisrdquo King D A Woodruff DP Elsvier Amsterdam 1982 vol 4 pp
71-93
86
Chapter 4E
Results and discussion
Reactant Toluene in aqueous medium
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of toluene in aqueous medium by Pt and PdZrO2
4E 1 Characterization of catalyst
The characterization of zirconia and zirconia supported platinum described in the
previous papers [1-3] Although the characterization of zirconia supported palladium
catalyst was described Fig 1 2 shows the SEM images of the catalyst before used and
after used From the figures it is clear that there is little bit different in the SEM images of
the fresh catalyst and used catalyst Although we did not observe this in the previous
studies of zirconia and zirconia supported platinum EDX of fresh and used PdZrO2
were given in the Fig 3 EDX of fresh catalyst show the peaks of Pd Zr and O while
EDX of the used PdZrO2 show peaks for Pd Zr O and C The presence of carbon
pointing to total oxidation from where it come and accumulate on the surface of catalyst
In fact the carbon present on the surface of catalyst responsible for deactivation of
catalyst widely reported [4 5] Fig 4 shows the XRD of monoclinic ZrO2 PtZrO2 and
PdZrO2 For ZrO2 the spectra is dominated by the peaks centered at 2θ = 2818deg and
3138deg which are characteristic of the monoclinic structure suggesting that the sample is
present mainly in the monoclinic phase calcined at 950degC [6] The reflections were
observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg respectively
4E 2 Effect of substrate concentration
The study of amount of substrate is a subject of great importance Consequently
the concentration of toluene in water varied in the range 200- 1000 mg L-1 while other
parameters 1 wt PtZrO2 100 mg temperature 323 K partial pressure of oxygen ~
101 kPa agitation 900 rpm and time 30 min Fig 5 unveils the fact that toluene in the
lower concentration range (200- 400 mg L-1) was oxidized to benzoic acid only while at
higher concentration benzyl alcohol and benzaldehyde are also formed
87
a b
Figure 1
SEM image for fresh a (Pd ZrO2)
Figure 2
SEM image for Used b (Pd ZrO2)
Figure 3
EDX for fresh (a) and used (b) Pd ZrO2
Figure 4
XRD for ZrO2 Pt ZrO2 Pd ZrO2
88
4E 3 Effect of temperature
Effect of reaction temperature on the progress of toluene oxidation was studied in
the range of 303-333 K at a constant concentration of toluene (1000 mg L-1) while other
parameters were the same as in section 321 Fig 6 reveals that with increase in
temperature the conversion of toluene increases reaching maximum conversion at 333 K
The apparent activation energy is ~ 887 kJ mole-1 The value of activation energy in the
present case shows that in these conditions the reaction is most probably free of mass
transfer limitation [7]
4E 4 Agitation effect
The process is a liquid phase heterogeneous reaction having liquid reactants and a
solid catalyst The effect of mass transfer on the rate of reaction was determined by
studying the change in conversion at various speeds of agitation A PTFE coated stir bar
(L = 19 mm OD ~ 5 mm) was used for stirring For the oxidation of a toluene to proceed
the toluene and oxygen have to be present on the platinum or palladium catalyst surface
Oxygen has to be transferred from the gas phase to the liquid phase through the liquid to
the catalyst particle and finally has to diffuse to the catalytic site inside the particle The
toluene has to be transferred from the liquid bulk to the catalyst particle and to the
catalytic site inside the particle The reaction products have to be transferred in the
opposite direction Since the purpose of this study is to determine the intrinsic reaction
kinetics the absence of mass transfer limitations has to be verified Fig 7 shows that the
conversion increases in the initial stages and becomes constant at the stirring speed of
900 rpm and above Chaudhari et al [8 9] also reported similar results This is the region
of interest and all further studies were performed at a stirring rate of 900 rpm or above
The value activation energy and agitation study support the absence of mass transfer
effect
4E 5 Effect of catalyst loading
The effect of catalyst amount on the progress of oxidation of toluene was studied
in the range 20 ndash 100 mg while all other parameters were kept constant Fig 8 shows
89
Figure 7
Effect of agitation on the conversion of
toluene in aqueous medium catalyzed by
PtZrO2 at 333 K Catalyst (100 mg)
solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) time (30 min)
Figure 8
Effect of catalyst loading on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Solution
volume (10 mL) toluene concentration
(200-1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm) time (30 min)
Figure 5
Effect of substrate concentration on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst
(100 mg) solution volume (10 mL)
toluene concentration (200-1000 mgL-1)
pO2 (101 kPa) stirring (900 rpm)
time (30
min)
Figure 6
Arrhenius plot for toluene oxidation
Temperature (303-333 K) Catalyst (100
mg) solution volume (10 mL) toluene
concentration (1000 mgL-1) pO2 (101
kPa) stirring (900 rpm) time (30 min)
90
that the rate of reaction increases in the range 20-80 mg and becomes approximately
constant afterward
4E 6 Time profile study
The time course study for the oxidation of toluene was periodically monitored
This investigation was carried out at 333 K by suspending 100 mg of catalyst in 10mL
(1000 mgL-1) of toluene in water oxygen partial pressure ~101 kPa and agitation 900
rpm Fig 9 indicates that the conversion increases linearly with increases in reaction
time
4E 7 Effect of Oxygen partial pressure
The effect of oxygen partial pressure was also studied in the lower range of 12-
101 kPa with a constant initial concentration of (1000 mg L-1) toluene in water at 333 K
The oxygen pressure also proved to be a key factor in the oxidation of toluene Fig 10
shows that increase in oxygen partial pressure resulted in increase in the rate of reaction
100 conversion is achieved only at pO2 ~101 kPa
4E8 Reaction Kinetics Analysis
From the effect of stirring and the apparent activation energy it is concluded that the
oxidation of toluene is most probably taking place in the kinetically controlled regime
This is a typical slurry reaction having catalyst in the solid state and reactants in liquid
and gas phase
As discussed earlier [111 the reaction kinetic analyses were performed by fitting the
experimental data to one of the three possible mechanisms of heterogeneous catalytic
oxidations
iv The Langmuir-Hinshelwood mechanism (L-H)
v The Mars-van Krevelen mechanism (M-K) or
vi The Eley-Rideal mechanism (E-R)
The Lndash H mechanism involves adsorption of the reacting species (toluene and oxygen) on
active sites at the surface followed by an irreversible rate-determining surface reaction
to give products The Langmuir-Hinshelwood rate law can be given as
91
2221
221
1n
n
g
gOKTK
OTKkKRate
++= (1)
Where k is the rate coefficient and K1 and K2 are the adsorption equilibrium constants for
Toluene [T] and O2 respectively The value of n can be taken 1or 05 for molecular or
dissociative adsorption of oxygen respectively For constant O2 or constant toluene
concentration equation (1) will be transformed by lumping together all the constants as to
2Tcb
TaRate
+= (1a) or
22
2
Ocb
OaRate
+= (1b)
The rate expression for Mars-van Krevelen mechanism can be given
ng
n
g
OkTk
OkTkRate
221
221
+=
(2)
Where 1k and 2k are the rate constants for oxidation of the substrate and the surface
respectively and (= 05) is the stoichiometric coefficient for O2 For a constant O2
pressure or constant Toluene concentration the equation was transformed to
Tcb
TaRate
+= (2a) or
ng
n
g
Ocb
OaRate
2
2
+= (2b)
The E-R mechanism envisage reaction between adsorbed oxygen with hydrocarbon
molecules from the fluid phase
ng
n
g
OK
TOkKRate
2
2
1+= (3)
In case of constant O2 pressure or constant toluene concentration equation 3 can be
transformed by lumping together all the constants to yield
TaRate = (3a) or
ng
n
g
Ob
OaRate
2
2
1+= (3b)
The data obtained from the effect of substrate concentration (figure 5) and oxygen
partial pressure (figure 10) was subjected to kinetic analysis using a nonlinear regression
analysis according to the above-mentioned three models The rate data for toluene
conversion at different toluene concentration obtained at constant O2 pressure (from
figure 5) was subjected to kinetic analysis Equation (1a) and (2a) were not applicable to
92
the data It is obvious from (figure 11) that equation (3a) is applicable to the data with a
regression coefficient of ~0983 and excluding the data point for the highest
concentration (1000 mgL) the regression coefficient becomes more favorable (R2 ~
0999) Similarly the rate data for different O2 pressures at constant toluene
concentration (from figure 10) was analyzed using equations (1b) (2b) and (3b) using a
non- linear least analysis software (Curve Expert 13) Equation (1b) was not applicable
to the data The best fit (R2 = 0993) was obtained for equations (2b) and (3b) as shown in
(figure 12) It has been mentioned earlier [1] that the rate expression for Mars-van
Krevelen and Eley-Rideal mechanisms have similar forms at a constant concentration of
the reacting hydrocarbon species However as equation (2a) is not applicable the
possibility of Mars-van Krevelen mechanism can be excluded Only equation (3) is
applicable to the data for constant oxygen concentration (3a) as well as constant toluene
concentration (3b) Therefore it can be concluded that the conversion of toluene on
PtZrO2 is taking place by Eley-Rideal mechanism It is up to the best of our knowledge
the first observation of a liquid phase reaction to be taking place by the Eley-Rideal
mechanism Considering the polarity of toluene in comparison to the solvent (water) and
its low concentration a weak or no adsorption of toluene on the surface cannot be ruled
out Ordoacutentildeez et al [12] have reported the Mars-van Krevelen mechanism for the deep
oxidation of toluene benzene and n-hexane catalyzed by platinum on -alumina
However in that reaction was taking place in the gas phase at a higher temperature and
higher gas phase concentration of toluene We have observed earlier [1] that the
Langmuir-Hinshelwood mechanism was operative for benzyl alcohol oxidation in n-
heptane catalyzed by PtZrO2 at 90 degC Similarly Makwana et al [11] have observed
Mars-van Krevelen mechanism for benzyl alcohol oxidation in toluene catalyzed by
OMS-2 at 90 degC In both the above cases benzyl alcohol is more polar than the solvent n-
heptan or toluene Similarly OMS-2 can be easily oxidized or reduced at a relatively
lower temperature than ZrO2
93
Figure 9
Time profile study of toluene oxidation
in aqueous medium catalyzed by PtZrO2
at 333 K Catalyst (100 mg) solution
volume (10 mL) toluene concentration
(1000 mgL-1) pO2 (101 kPa) stirring
(900 rpm)
Figure 10
Effect of oxygen partial pressure on the
conversion of toluene in aqueous medium
catalyzed by PtZrO2 at 333 K Catalyst (100
mg) solution volume (10 mL) toluene
concentration (200-1000 mgL-1) stirring (900
rpm) time (30 min)
Figure 11
Rate of toluene conversion vs toluene
concentration Data for toluene
conversion from figure 1 was used
Figure 12
Plot of calculated conversion vs
experimental conversion Data from
figure 6 for the effect of oxygen partial
pressure effect on conversion of toluene
was analyzed according to E-R
mechanism using equation (3b)
94
4E 9 Comparison of different catalysts
Among the catalysts we studied as shown in table 1 both zirconia supported
platinum and palladium catalysts were shown to be active in the oxidation of toluene in
aqueous medium Monoclinic zirconia shows little activity (conversion ~17) while
tetragonal zirconia shows inertness toward the oxidation of toluene in aqueous medium
after a long (t=360 min) run Nevertheless zirconia supported platinum appeared as the
best High activities were measured even at low temperature (T ~ 333k) Zirconia
supported palladium catalyst was appear to be more selective for benzaldehyde in both
unreduced and reduced form Furthermore zirconia supported palladium catalyst in
reduced form show more activity than that of unreduced catalyst In contrast some very
good results were obtained with zirconia supported platinum catalysts in both reduced
and unreduced form Zirconia supported platinum catalyst after reduction was found as a
better catalyst for oxidation of toluene to benzoic in aqueous medium Furthermore as
we studied the Pt ZrO2 catalyst for several run we observed that the activity of the
catalyst was retained
Table 1
Comparison of different catalysts for toluene oxidation
in aqueous medium
95
Chapter 4E
References
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
9 Markusse AP Kuster BFM Koningsberger DC Marin GB Catal
Lett1998 55 141
10 Markusse AP Kuster BFM Schouten JC Stud Surf Sci Catal1999 126
273
11 Ferino I Casula F M Corrias A Cutrufello MG Monaci R Paschina G
Phys Chem Chem Phys 2002 2 1847-1854
12 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App Catal
A 2005 288 175-184
13 Choudhary V R Dhar A Jana P Jha R de Upha B S GreenChem 2005
7 768
14 Choudhary V R Jha R Jana P Green Chem 2007 9 267
15 Makwana V D Son Y C Howell A R Suib S L J Catal 2002 210 46-52
16 Ordoacutentildeez S Bello L Sastre H Rosal R Diez F V Appl Catal B 2002 38
139
96
Chapter 4F
Results and discussion
Reactant Cyclohexane
Catalyst ZrO2 Pt ZrO2 Pd ZrO2
Oxidation of cyclohexane in solvent free by zirconia supported noble metals
4F1 Characterization of catalyst
Fig1 shows X-ray diffraction patterns of tetragonal ZrO2 monoclinic ZrO2 Pd
monoclinic ZrO2 and Pt monoclinic ZrO2 respectively Freshly prepared sample was
almost amorphous The crystallinity of the sample begins to develop after calcining the
sample at 773 -1223K for 4 h as evidenced by sharper diffraction peaks with increased
calcination temperature The samples calcined at 773K for 4h exhibited only the
tetragonal phase (major peak appears at 2 = 3094deg) and there was no indication of
monoclinic phase For ZrO2 calcined at 950degC the spectra is dominated by the peaks
centered at 2 = 2818deg and 3138deg which are characteristic of the monoclinic structure
suggesting that the sample is present mainly in the monoclinic phase The reflections
were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and 4628deg
respectively The X-ray diffraction patterns of Pd supported on tetragonal ZrO2 and Pt
supported on tetragonal ZrO2 annealed at different temperatures is shown in Figs2 and 3
respectively No peaks appeared at 2θ = 2818deg and 3138deg despite the increase in
temperature (from 773 to 1223K) It seems that the metastable tetragonal structure was
stabilized at the high temperature as a function of the doped Pd or Pt which was
supported by the X-ray diffraction analysis of the Pd or Pt-free sample synthesized in the
same condition and annealed at high temperature Fig 4 shows the X-ray diffraction
pattern of the pure tetragonal ZrO2 annealed at different temperatures (773K 823K
1023K and1223K) The figure reveals tetragonal ZrO2 at 773K increasing temperature to
823K a fraction of monoclinic ZrO2 appears beside tetragonal ZrO2 An increase in the
fraction of monoclinic ZrO2 is observed at 1023K while 1223K whole of ZrO2 found to
be monoclinic It is clear from the above discussion that the presence of Pd or Pt
stabilized tetragonal ZrO2 and further phase change did not occur even at high
97
Figure 1
XRD patterns of ZrO2 (T) ZrO2 (m) PdZrO2 (m)
and Pt ZrO2 (m)
Figure 2
XRD patterns of PdZrO2 (T) annealed at
773K 823K 1023K and 1223K respectively
Figure 3
XRD patterns of PtZrO2 (T) annealed at 773K
823K 1023K and1223K respectively
Figure 4
XRD patterns of pure ZrO2 (T) annealed at
773K 823K 1023K and1223K respectively
98
temperature [1] Therefore to prepare a catalyst (noble metal supported on monoclinic
ZrO2) the sample must be calcined at higher temperature ge1223K to ensure monoclinic
phase before depositing noble metal The surface area of samples as a function of
calcination temperature is given in Table 1 The main trend reflected by these results is a
decrease of surface area as the calcination temperature increases Inspecting the table
reveals that Pd or Pt supported on ZrO2 shows no significant change on the particle size
The surface area of the 1 Pd or PtZrO2 (T) sample decreased after depositing Pd or Pt in
it which is probably due to the blockage of pores but may also be a result of the
increased density of the Pd or Pt
4F2 Oxidation of cyclohexane
The oxidation of cyclohexane was carried out at 353 K for 6 h at 1 atmospheric
pressure of O2 over either pure ZrO2 or Pd or Pt supported on ZrO2 catalyst The
experiment results are listed in Table 1 When no catalyst (as in the case of blank
reaction) was added the oxidation reaction did not proceed readily However on the
addition of pure ZrO2 (m) or Pd or Pt ZrO2 as a catalyst the oxidation reaction between
cyclohexane and molecular oxygen was initiated As shown in Table 1 the catalytic
activity of ZrO2 (T) and PdO or PtO supported on ZrO2 (T) was almost zero while Pd or Pt
supported on ZrO2 (T) shows some catalytic activity toward oxidation of cyclohexane The
reason for activity is most probably reduction of catalyst in H2 flow (40mlmin) which
convert a fraction of ZrO2 (T) to monoclinic phase The catalytic activity of ZrO2 (m)
gradually increases in the sequence of ZrO2 (m) lt PdOZrO2 (m) lt PtOZrO2 (m) lt PdZrO2
(m) lt PtZrO2 (m) The results were supported by arguments that PtZrO2ndashWOx catalysts
that include a large fraction of tetragonal ZrO2 show high n-butane isomerization activity
and low oxidation activity [2 3] As one can also observe from Table 1 that PtZrO2 (m)
was more selective and reactive than that of Pd ZrO2 (m) Fig 5 shows the stirring effect
on oxidation of cyclohexane At higher agitation speed the rate of reaction became
99
Table 1
Oxidation of cyclohexane to cyclohexanone and cyclohexanol
with molecular oxygen at 353K in 360 minutes
Figure 5
Effect of agitation on the conversion of cyclohexane
catalyzed by Pt ZrO2 (m) at temperature = 353K Catalyst
weight = 100mg volume of reactant = 20 ml partial pressure
of O2 = 760 Torr time = 360 min
100
constant which indicate that the rates are kinetic in nature and unaffected by transport
restrictions Ilyas et al [4] also reported similar results All further reactions were
conducted at higher agitation speed (900-1200rpm) Fig 6 shows dependence of rate on
temperature The rate of reaction linearly increases with increase in temperature The
apparent activation energy was 581kJmole-1 which supports the absence of mass transfer
resistance [5] The conversions of cyclohexane to cyclohexanol and cyclohexanone are
shown in Fig 7 as a function of time on PtZrO2 (m) at 353 K Cyclohexanol is the
predominant product during an initial induction period (~ 30 min) before cyclohexanone
become detectable The cyclohexanone selectivity increases with increase in contact time
4F3 Optimal conditions for better catalytic activity
The rate of the reaction was measured as a function of different parameters like
temperature partial pressure of oxygen amount of catalyst volume of reactants agitation
and reaction duration The rate of reaction also shows dependence on the morphology of
zirconia deposition of noble metal on zirconia and reduction of noble metal supported on
zirconia in the flow of H2 gas It was found that reduced Pd or Pt supported on ZrO2 (m) is
more reactive and selective toward the oxidation of cyclohexane at temperature 353K
agitation 900rpm pO2 ~ 760 Torr weight of catalyst 100mg volume of reactant 20ml
and time 360 minutes
101
Figure 6
Arrhenius Plot Ln conversion vs 1T (K)
Figure 7
Time profile study of cyclohexane oxidation catalyzed by Pt ZrO2 (m)
Reaction condition temperature = 353K Catalyst weight = 100mg
volume of reactant = 20 ml partial pressure of O2 = 760 Torr
agitation speed = 900rpm
102
Chapter 4F
References
1 Ilyas M Ikramullah Catal Commun 2004 5 1
2 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
3 Ilyas M Sadiq M Chin J Chem 2008 26 941
4 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-
008-9750-8
5 Ilyas M Sadiq M Khan I Chin J Catal 2007 28 413
103
Chapter 4G
Results and discussion
Reactant Phenol in aqueous medium
Catalyst PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2
Oxidation of phenol in aqueous medium by zirconia-supported noble metals
4G1 Characterization of catalyst
X-ray powder diffraction pattern of the sample reported in Fig 1 confirms the
monoclinic structure of zirconia The major peaks responsible for monoclinic structure
appears at 2 = 2818deg and 3138deg while no characteristic peak of tetragonal phase (2 =
3094deg) was appeared suggesting that the zirconia is present in purely monoclinic phase
The reflections were observed for Pd at 2θ = 404deg and 469deg and for Pt at 2θ =3979deg and
4628deg respectively [1] For Bi2O3 the peaks appear at 2θ = 277deg 305deg33deg 424deg and
472deg while for MnO2 major peaks observed at 2θ = 261deg 289deg In this all catalyst
zirconia maintains its monoclinic phase SEM micrographs of fresh samples reported in
Fig 2 show the homogeneity of the crystal size of monoclinic zirconia The micrographs
of PtZrO2 PdZrO2 and Pt-PdZrO2 revealed that the active metals are well dispersed on
support while the micrographs of Bi2O3ZrO2 and MnO2ZrO2 show that these are not
well dispersed on zirconia support Fig 3 shows the EDX analysis results for fresh and
used ZrO2 PtZrO2 PdZrO2 Pt-PdZrO2 Bi2O3ZrO2 and MnO2ZrO2 samples The
results show the presence of carbon in used samples Probably come from the total
oxidation of organic substrate Many researchers reported the presence of chlorine and
carbon in the EDX of freshly prepared samples [1 2] suggesting that chlorine come from
the matrix of zirconia and carbon from ethylene diamine In our case we did used
ethylene diamine and did observed the carbon in the EDX of fresh samples We also did
not observe the chlorine in our samples
104
Figure 1
XRD of different catalysts
105
Figure 2 SEM of different catalyst a ZrO2 b Pt ZrO2 c Pd ZrO2 d Pt-Pd ZrO2 e
Bi2O3 f Bi2O3 ZrO2 g MnO2 h MnO2 ZrO2
a b
c d
e f
h g
106
Fresh ZrO2 Used ZrO2
Fresh PtZrO2 Used PtZrO2
Fresh Pt-PdZrO2 Used Pt-Pd ZrO2
Fresh Bi-PtZrO2 Used Bi-PtZrO2
107
Fresh Bi-PdZrO2 Used Bi-Pd ZrO2
Fresh Bi2O3ZrO2 Fresh Bi2O3ZrO2
Fresh MnO2ZrO2 Used MnO2 ZrO2
Figure 3
EDX of different catalyst of fresh and used
108
4G2 Catalytic oxidation of phenol
Oxidation of phenol was significantly higher over PtZrO2 catalyst Combination
of 1 Pd and 1 Pt on ZrO2 gave an activity comparable to that of the Pd ZrO2 or
PtZrO2 catalysts Adding 05 Bismuth significantly increased the activity of the ZrO2
supported Pt shows promising activity for destructive oxidation of organic pollutants in
the effluent at 333 K and 101 kPa in the liquid phase 05 Bismuth inhibit the activity
of the ZrO2 supported Pd catalyst
4G3 Effect of different parameters
Different parameters of reaction have a prominent effect on the catalytic oxidation
of phenol in aqueous medium
4G4 Time profile study
The conversion of the phenol with time is reported in Fig 4 for Bi promoted
zirconia supported platinum catalyst and for the blank experiment In the absence of any
catalyst no conversion is obtained after 3 h while ~ total conversion can be achieved by
Bi-PtZrO2 in 3h Bismuth promoted zirconia-supported platinum catalyst show very
good specific activity for phenol conversion (Fig 4)
4G5 Comparison of different catalysts
The activity of different catalysts was found in the order Pt-PdZrO2gt Bi-
PtZrO2gt Bi-PdZrO2gt PtZrO2gt PdZrO2gt CuZrO2gt MnZrO2 gt BiZrO2 Bi-PtZrO2 is
the most active catalyst which suggests that Bi in contact with Pt particles promote metal
activity Conversion (C ) are reported in Fig 5 However though very high conversions
can be obtained (~ 91) a total mineralization of phenol is never observed Organic
intermediates still present in solution widely reported [3] Significant differences can be
observed between bi-PtZrO2 and other catalyst used
109
Figure 4
Time profile study Temp 333 K
Cat 02g substrate solution 20 ml
(10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
Figure 5
Comparison of different catalysts
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 6
Effect of Pd loading on conversion
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol
in water pO2 760 Torr and
agitation 900 rpm
Figure 7
Effect of Pt loading on conversion
Temp 333 K Cat 02g substrate solution
20 ml (10g dm-3) of phenol in water pO2
760 Torr and agitation 900 rpm
110
4G6 Effect of Pd and Pt loading on catalytic activity
The influence of platinum and palladium loading on the activity of zirconia-
supported Pd catalysts are reported in Fig 6 and 7 An increase in Pt loading improves
the activity significantly Phenol conversion increases linearly with increase in Pt loading
till 15wt In contrast to platinum an increase in Pd loading improve the activity
significantly till 10 wt Further increase in Pd loading to 15 wt does not result in
further improvement in the activity [4]
4G 7 Effect of bismuth addition on catalytic activity
The influence of bismuth on catalytic activities of PtZrO2 PdZrO2 catalysts is
reported in Fig 8 9 Adding 05 wt Bi on zirconia improves the activity of PtZrO2
catalyst with a 10 wt Pt loading In contrast to supported Pt catalyst the activity of
supported Pd catalyst with a 10 wt Pd loading was decreased by addition of Bi on
zirconia The profound inhibiting effect was observed with a Bi loading of 05 wt
4G 8 Influence of reduction on catalytic activity
High catalytic activity was obtained for reduce catalysts as shown in Fig 10
PtZrO2 was more reactive than PtOZrO2 similarly Pd ZrO2 was found more to be
reactive than unreduce Pd supported on zirconia Many researchers support the
phenomenon observed in the recent study [5]
4G 9 Effect of temperature
Fig 11 reveals that with increase in temperature the conversion of phenol
increases reaching maximum conversion at 333K The apparent activation energy is ~
683 kJ mole-1 The value of activation energy in the present case shows that in these
conditions the reaction is probably free of mass transfer limitation [6-8]
111
Figure 8
Effect of bismuth on catalytic activity
of PdZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 9
Effect of bismuth on catalytic activity
of PtZrO2 Temp 333 K Cat 02g
substrate solution 20 ml (10g dm-3) of
phenol in water pO2 760 Torr and
agitation 900 rpm
Figure 10
Effect of reduction on catalytic activity
Temp 333 K Cat 02g substrate
solution 20 ml (10g dm-3) of phenol in
water pO2 760 Torr and agitation 900
rpm
Figure 11
Effect of temp on the conversion of phenol
Temp 303-333 K Bi-1wtPtZrO2 02g
substrate 20 ml (10g dm-3) pO2 760 Torr and
agitation 900 rpm
112
Chapter 4G
References
1 Souza L D Subaie JS Richards R J Colloid Interface Sci 2005 292 476ndash
485
2 Souza L D Suchopar A Zhu K Balyozova D Devadas M Richards R
M Micropor Mesopor Mater 2006 88 22ndash30
3 Zhang Q Chuang KT Ind Eng Chem Res 1998 37 3343 -3349
4 Resini C Catania F Berardinelli S Paladino O Busca G Appl Catal B
Environ 2008 84 678-683
5 Ilyas M Sadiq M Catal Lett 2008 (online first) DOI 101007s10562-008-
9750-8
6 Ilyas M Sadiq M ChemEng Technol 2007 30 1391
7 Ilyas M Sadiq M Chin J Chem 2008 26 941
8 Bavykin D V Lapkin A A Kolaczkowski S T Plucinski P K App
Catal A 2005 288 175-184
113
Chapter 5
Conclusion review
bull ZrO2 is an effective catalyst for the selective oxidation of alcohols to ketones and
aldehydes under solvent free conditions with comparable activity to other
expensive catalysts ZrO2 calcined at 1223 K is more active than ZrO2 calcined at
723 K Moreover the oxidation of alcohols follows the principles of green
chemistry using molecular oxygen as the oxidant under solvent free conditions
From the study of the effect of oxygen partial pressure at pO2 le101 kPa it is
concluded that air can be used as the oxidant under these conditions Monoclinic
phase ZrO2 is an effective catalyst for synthesis of aldehydes ketone
Characterization of the catalyst shows that it is highly promising reusable and
easily separable catalyst for oxidation of alcohol in liquid phase solvent free
condition at atmospheric pressure The reaction shows first order dependence on
the concentration of alcohol and catalyst Kinetics of this reaction was found to
follow a Langmuir-Hinshelwood oxidation mechanism
bull Monoclinic ZrO2 is proved to be a better catalyst for oxidation of benzyl alcohol
in aqueous medium at very mild conditions The higher catalytic performance of
ZrO2 for the total oxidation of benzyl alcohol in aqueous solution attributed here
to a high temperature of calcinations and a remarkable monoclinic phase of
zirconia It can be used with out any base addition to achieve good results The
catalyst is free from any promoter or additive and can be separated from reaction
mixture by simple filtration This gives us the idea to conclude that catalyst can
be reused several times Optimal conditions for better catalytic activity were set as
time 6h temp 60˚C agitation 900rpm partial pressure of oxygen 760 Torr
catalyst amount 200mg It summarizes that ZrO2 is a promising catalytic material
for different alcohols oxidation in near future
bull PtZrO2 is an active catalyst for toluene partial oxidation to benzoic acid at 60-90
C in solvent free conditions The rate of reaction is limited by the supply of
oxygen to the catalyst surface Selectivity of the products depends upon the
114
reaction time on stream With a reaction time 3 hrs benzyl alcohol
benzaldehyde and benzoic acid are the only products After 3 hours of reaction
time benzyl benzoate trans-stilbene and methyl biphenyl carboxylic acid appear
along with benzoic acid and benzaldehyde In both the cases benzoic acid is the
main product (selectivity 60)
bull PtZrO2 is used as a catalyst for liquid-phase oxidation of benzyl alcohol in a
slurry reaction The alcohol conversion is almost complete (gt99) after 3 hours
with 100 selectivity to benzaldehyde making PtZrO2 an excellent catalyst for
this reaction It is free from additives promoters co-catalysts and easy to prepare
n-heptane was found to be a better solvent than toluene in this study Kinetics of
the reaction was investigated and the reaction was found to follow the classical
Langmuir-Hinshelwood model
bull The results of the present study uncovered the fact that PtZrO2 is also a better
catalyst for catalytic oxidation of toluene in aqueous medium This gives us
reasons to conclude that it is a possible alternative for the purification of
wastewater containing toluene under mild conditions Optimizing conditions for
complete oxidation of toluene to benzoic acid in the above-mentioned range are
time 30 min temperature 333 K agitation 900 rpm pO2 ~ 101 kPa catalyst
amount 100 mg The main advantage of the above optimal conditions allows the
treatment of wastewater at a lower temperature (333 K) Catalytic oxidation is a
significant method for cleaning of toxic organic compounds from industrial
wastewater
bull It has been demonstrated that pure ZrO2 (T) change to monoclinic phase at high
temperature (1223K) while Pd or Pt doped ZrO2 (T) shows stability even at high
temperature ge 1223K It was found that the degree of stability at high temperature
was a function of noble metal doping Pure ZrO2 (T) PdO ZrO2 (T)
and PtO ZrO2
(T) show no activity while Pd ZrO2 (T)
and Pt ZrO2 (T)
show some activity in
cyclohexane oxidation ZrO2 (m) and well dispersed Pd or Pt ZrO2 (m)
system is
very active towards oxidation and shows a high conversion Furthermore there
was no leaching of the Pd or Pt from the system observed Overall it is
115
demonstrated that reduced Pd or Pt supported on ZrO2 (m) can be prepared which is
very active towards oxidation of cyclohexane in solvent free conditions at 353K
bull Bismuth promoted PtZrO2 and PdZrO2 catalysts are each promising for the
destructive oxidation of the organic pollutants in the industrial effluents Addition
of Bi improves the activity of PtZrO2 catalysts but inhibits the activity of
PdZrO2 catalyst at high loading of Pd Optimal conditions for better catalytic
activity temp 333K wt of catalyst 02g agitation 900rpm pO2 101kPa and time
180min Among the emergent alternative processes the supported noble metals
catalytic oxidation was found to be effective for the treatment of several
pollutants like phenols at milder temperatures and pressures
bull To sum up from the above discussion and from the given table that ZrO2 may
prove to be a better catalyst for organic oxidation reaction as well as a superior
support for noble metals
116
116
Table Catalytic oxidation of different organic compounds by zirconia and zirconia supported noble metals
mohammad_sadiq26yahoocom
Catalyst Solvent Duration
(hours)
Reactant Product Conversion
()
Ref
ZrO2(t) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
236
152
115
I
III
ZrO2(m) - 24 Cyclohexanol
Benzyl alcohol
n-Octanol
Cyclohexanone
Benzaldehyde
Octanal
367
222
197
I
ZrO2(m) water 6 Benzyl alcohol Benzaldehyde
Benzoic acid
23
887
VII
Pt ZrO2
(used
without
reduction)
n-heptane 3 Benzyl alcohol Benzaldehyde
~100 II
Pt ZrO2
(reduce in
H2 flow)
-
-
3
7
Toluene
Toluene
Benzoic acid
Benzaldehyde
Benzoic acid
Benzyl benzoate
Trans-stelbene
4-methyl-2-
biphenylcarbxylic acid
372
22
296
34
53
108
IV
Pt ZrO2
(reduce in
H2 flow)
water 05 Toluene Benzoic acid ~100 VI
Pt ZrO2(m)
(reduce in
H2 flow)
- 6 Cyclohexane Cyclohexanol
cyclohexanone
14
401
V
Bi-Pt ZrO2
water 3 Phenol Complete oxidation IX