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SYNTHESIS, CHARACTERIZATION AND APPLICATION OF
MESOPOROUS MATERIALS
Submitted To
The Maharaja Sayajirao University of Baroda
For
The Degree of
DDOOCCTTOORR OOFF PPHHIILLOOSSOOPPHHYY
in
Applied Chemistry
by
Rajeshkumar M. Patel
Applied Chemistry Department Faculty of Technology and Engineering
The Maharaja Sayajirao University of Baroda Vadodara - 390001, Gujarat, India
August 2012
Applied Chemistry Department, Faculty of Technology & Engineering, The Maharaja Sayajirao University of Baroda, Post Box No. 51, Kalabhavan, Vadodara - 390001, Gujarat, India.
Prof. Uma V. Chudasama Research Guide
Tel: +265-2434188 Extn. 415, 212
Date: 27/08/2012
CCEERRTTIIFFIICCAATTEE
The thesis entitled “SYNTHESIS, CHARACTERIZATION AND APPLICATION
OF MESOPOROUS MATERIALS” incorporates the original research work
carried out by MR. RAJESHKUMAR M. PATEL under my supervision.
Head,
Applied Chemistry Department
Dean,
Faculty of Technology and Engineering
With profound sense of indebtedness and
reverence, I wish to express my sincere feelings of
gratitude to my research guide Dr. Uma V.
Chudasama, Professor, Applied Chemistry
Department, Faculty of Technology and
Engineering, The M. S. University of Baroda,
Vadodara, for her inspiring and enthusiastic
guidance throughout the research work. Special
thanks and recognition go to her, whose drive and
commitment to research has inspired, motivated
and enabled me to complete this endeavour. She
has played a fundamental role in defining my
future.
(Rajeshkumar Patel)
*** ACKNOWLEDGEMENTS ***
Primarily, I would like to express my profound gratitude to
Dr. (Mrs.) Uma V. Chudasama, Professor, Applied Chemistry Department,
Faculty of Technology and Engineering, The M. S. University of Baroda,
who constantly encouraged, inspired and guided me right from the
beginning of this work. Her receptive attitude, untiring enthusiasm and
positive approach will always remain a source of inspiration. Her deep
passion for perfection of work has not only taken care of my present study,
but shall always be a guide to me in future.
I wish to express my gratitude to the Dean, Faculty of Technology
and Engineering and to the Head, Applied Chemistry Department for
providing me the required facilities to carry out my research work.
My sincere thanks to the Applied Chemistry Department and
Department of Metallurgical and Material Engineering for providing
Thermal analysis, FTIR and SEM, EDX facilities respectively.
I wish to express my gratitude to the management of Sud-Chemie
India Pvt. Limited for providing me instrumentation and other Laboratory
facilities for conducting my research work. My special thanks to Mr. T. A.
Siddiquie (Asstt. Vice president-Operation), Mr. V. Sreedharan (Asstt. Vice
president-Works), Mr. R. M. Cursetji (Head- R & D(Tech. Development)),
Dr Arun Basarur (GM- R & D), Dr. D. P. Sabde (Manager- R & D), Mr. S.
A. Shaikh (Manager- Per.& HRD), and my research staff at Sud-Chemie
for helping me in many ways.My special thanks to Dr. Rikesh Joshi (Asstt.
Manager- R & D) for his help, support and useful suggestions at various
stages of my research work.
I extend my sincere thanks to all the teaching staff members of the
Applied Chemistry Department who have helped me in many ways during
my research work.
I had the pleasure of working with an excellent group of lab mates,
Dr. Rikesh Joshi, Dr. Rakesh Thakkar, Dr. Parimal Patel, Mr. Tarun
Parangi, Mr. Brijesh Shah and Mr. Shrinivas Ghodke for making my work
an enjoyable one. I would like to thank my lab mates for all the help.
I cannot forget the cooperation extended to me by the Research
students working in the Department and non-teaching staff members.
I thank my family members for their love and encouragement, who
above all have taught me that patience, determination, persistence and
faith are the keys to success.
There were so many people who did so many things to help me out
at critical times. In this short space, it is impossible to acknowledge all of
them individually. I thank all of you, my friends, colleagues and well
wishers, who have always been with me, in times of need.
(Rajeshkumar Patel)
CONTENTS
CHAPTER 1 INTRODUCTION 1-37 1.1 Introduction 1 1.2 Towards a workable definition 3 1.3 Catalyst development and commercialization 5 1.4 Catalysis concepts and terminologies 7 1.5 Catalyst characterization 13 1.6 Present trends in catalysis 16 1.7 Solid acid catalysts-An alternate approach to
liquid acid catalysts 18
1.8 Pores and porosity 23 1.9 From micropores to mesopores 25
1.10 Ordered mesoporous materials 26 1.11 Summarising porous materials 28 1.12 Aim and scope of the present work 30
References 33 CHAPTER 2 Synthesis and Characterization of MCM-41
Based Catalysts and their Applications as Solid Acid Catalysts in Esterification and Friedel-Crafts Acylation and Alkylation
38-130
2.1 Introduction 38 2.2 Sol-gel process 38 2.3 Sol-gel process as applied to silica 44 2.4 Sol-gel synthesis as applied to Zeolites 46 2.5 Sol-gel process using templates 49 2.6 The formation of mesoporous structures 53 2.7 Generalized LCT mechanism 56 2.8 Removal of template 58 2.9 Synthesis strategies and characterization
methodologies- A literature survey 60
2.10 Experimental 71 2.11 Materials characterization 75 2.12 Application of Al-MCM-41 and 12TPA-MCM-41
as solid acid catalysts 83
2.13 Esterification 84 2.14 Literature survey in the current area of study 87 2.15 Experimental 91 2.16 Results and discussion 93
2.17 Friedel-Crafts acylation and alkylation 101 2.18 Literature survey in the current area of study 106 2.19 Experimental 112 2.20 Results and Discussion 113 2.21 Conclusions 118
References 119 CHAPTER 3 Synthesis and Characterization of Zr-
MCM-41 and Ti-MCM-41 and their Application as Oxidation Catalysts
131-160
3.1 Introduction 131 3.2 Literature survey in the current area of study 131 3.3 Synthesis of Zr-MCM-41 and Ti-MCM-41 134 3.4 Material characterization 137 3.5 Application 143 3.6 Experimental 148 3.7 Results and discussion 149 3.8 Conclusions 153
References 155 CHAPTER 4 Synthesis and Characterization of a
Palladium Loaded Perovskite MCM-41 material and its Application as an Automotive Catalyst
161-181
4.1 Introduction 161 4.2 Automotive exhaust purification catalysis –
Terms and concepts 162
4.3 Materials used as automotive exhaust purification catalysts
168
4.4 Synthesis strategies for automotive catalysts- A literature survey
171
4.5 Aim and scope of the present work 172 4.6 Experimental 173 4.7 Results and discussion 175 4.8 Conclusions 179
References 180 SUMMARY PUBLICATIONS
Chapter 1 - Introduction
1
1.1 INTRODUCTION Catalysts have been successfully used in the chemical industry for
more than 100 years, examples being the synthesis of sulfuric acid, the
conversion of ammonia to nitric acid, and catalytic hydrogenation. Later
developments include new highly selective multi component oxide and
metallic catalysts, zeolites, and the introduction of homogeneous transition
metal complexes in the chemical industry. This was supplemented by new
high-performance techniques for probing catalysts and elucidating the
mechanisms of heterogeneous and homogenous catalysis [1]. A brief
historical survey given in Table 1.1 shows how closely the development of
catalysis is linked to the history of industrial chemistry.
Catalysis provides the chemist and the technologist with a valuable tool
for developing existing industries on a more economic and sound footing,
besides discovering new industrial processes. It is estimated that over 80% of
the existing chemical processes are catalytic, and of the newly developed
processes, 90% are catalytic based. Technological advances in chemical,
petrochemical, oil processing, food and many other industries, involve the
application of catalysts. Catalysts not only reduce the cost of production but
are directed primarily at improving the quality of products. The answer to
some problems of atmospheric pollution is also sought through catalysis.
Catalysts play an important role in purifying waste gases and reducing
pollution. Obviously, the importance of catalysis, both fundamental and
applied, in the economic and industrial growth of a country cannot be
overestimated [2].
From a commercial point of view, a catalyst is supposed to lower the
raw material consumption or the energy requirement of a chemical reaction.
The former can be achieved, by increasing the yield or the selectivity towards
a particular product, whereas for the latter, a catalyst must bring down the
activation energy and thereby reaction temperature. Thus, a major target of
the catalyst is to obtain chemical products in high purity and high yields with
excellent selectivity at a considerably lower temperature. Further, the catalyst
also plays a significant role in pharmaceutical and fine chemicals by reducing
the number of steps involved in producing a desired chemical. It thereby
Chapter 1 - Introduction
2
reduces potential environmental hazards by reducing the large amount of
waste in each individual step [3]. Table 1.1 History of Industrial catalytic processes [1]
Catalytic reaction Catalyst Discoverer or company/ year
Sulphuric acid (lead- chamber process) NOx Désormes, Clement, 1806 Chlorine production by HCl oxidation CuSO4 Deacon, 1867
Sulfuric acid (contact process) Pt, V2O5 Winkler, 1875; Knietsch, 1888 (BASF)
Nitric acid by NH3 oxidation Pt/Rh nets Ostwald, 1906 Fat hardening Ni Normann, 1907
Ammonia synthesis from N2, H2 Fe
Mittasch, Haber, Bosch, 1908;Production,1913 (BASF)
Hydrogenation of coal to hydrocarbons Fe, Mo, Sn Bergius, 1913; Pier, 1927 Oxidation of benzene, naphthalene
to MSA or PSA V2O5
Weiss, Downs, 1920
Methanol synthesis from CO/H2 ZnO/Cr2O3 Mittasch, 1923
Hydrocarbons from CO/H2 (motor fuels) Fe, Co, Ni Fischer, Tropsch, 1925 Oxidation of ethylene to ethylene oxide Ag Lefort, 1930
Alkylation of olefins with isobutane to gasoline
AlCl3
Ipatieff, Pines, 1932
Cracking of hydrocarbons Al2O3/SiO2 Houdry, 1937 Hydroformylation of ethylene to
Propanal Co Roelen, 1938 (Ruhrchemie)
Cracking in a fluidized bed aluminosilicates Lewis, Gilliland, 1939
(Standard Oil) Ethylene polymerization,
low-pressure Ti compounds
Ziegler, Natta, 1954
Oxidation of ethylene to acetaldehyde Pd/Cu chlorides Hafner, Smidt (Wacker)
Ammoxidation of propene to Acrylonitrile
Bi/Mo
Idol, 1959 (SOHIO process)
Olefin metathesis Re, W, Mo Banks, Bailey, 1964 Hydrogenation, isomerization,
Hydroformylation Rh-, Ru
complexes
Wilkinson, 1964
Asymmetric hydrogenation Rh/chiral phosphine
Knowles, 1974; l-Dopa (Monsanto)
Three-way catalyst Pt, Rh/monolith General Motors, Ford, 1974 Methanol conversion to
Hydrocarbons Zeolites
Mobil Chemical Co., 1975
α- olefines from ethylene Ni/chelate
Phosphine Shell (SHOP process) 1977
Sharpless oxidation, epoxidation Ti/ROOH/tartrate May & Baker, Upjohn, ARCO,
1981 Selective oxidations with H2O2 titanium zeolite
(TS-1) Enichem, 1983
Hydroformylation Rh/phosphine/ aqueous
Rhône-Poulenc/Ruhrchemie, 1984
Polymerization of olefines zirconocene/MAO Sinn, Kaminsky, 1985 Selective catalytic reduction
SCR (power plants) V, W, Ti oxides/
Monolith ~1986
Acetic acid Ir / I– / Ru “Cativa”-process, BP Chemicals, 1996
Chapter 1 - Introduction
3
Studying catalysis is an academically valuable exerise and clearly
demonstrates the essential unity of science and technology. An understanding
of the phenomenon of catalysis, requires some familiarity with all three
classical branches of chemistry as well as the fundamental concepts of
chemical engineering. The catalysts used are mostly inorganic or
organometallic compounds while the reactions carried out using the
developed catalyst are organic transformations. Hence a catalyst chemist
must be familiar with inorganic chemistry, coordination chemistry, and organic
chemistry. The overall performance of a catalyst depends on various factors
such as turn over number, turn over frequency, conversion, selectivity and
rate of reaction linked to the kinetics of the reactions. Therefore a sound
knowledge of physical chemistry is required to understand the kinetics of
reaction and hence reaction mechanism, reactor design and its effect on the
performance of a catalyst. In most industrial applications, it is a requirement to
predict the overall rate of a chemical process with respect to entire reactor,
which depends on several factors such as interface mass transfer, mixing of
the reactants, temperature profile and intrinsic kinetics of the reaction.
Therefore, the large scale application of a developed process requires the
skills and knowledge of a chemical engineer [4].
1.2 TOWARDS A WORKABLE DEFINITION Several substances have the ability to exercise a force on other
substances and are able to bring about a transformation in the reactants
giving rise to new products without themselves undergoing a chemical
change. This force was termed as catalytic force and the transformation
brought about by this force was termed “Catalysis” which was introduced by
Berzelius as early as 1836.
Ostwald in 1895 applied the principles of thermodynamics to show that
a catalyst just modifies the rate at which the chemical transformation takes
place (provided it is thermodynamically feasible). A catalyst does not alter the
equilibrium concentration of the various species present in a chemical
transformation. Catalysis therefore emerges as a kinetic phenomenon, and
one can define a catalyst as a substance that alters the rate of a reaction
Chapter 1 - Introduction
4
without itself being consumed in it and also without modifying its equilibrium
constant.
While it was formerly assumed that the catalyst remained unchanged
during the course of the reaction, it is now known that the catalyst is involved
in chemical bonding with the reactants during the catalytic process, termed as
intermediate which in most cases are highly reactive and difficult to detect. In
theory, an ideal catalyst would not be consumed, but this is not the case in
practice. Owing to competing reactions, the catalyst undergoes chemical
changes, and its activity becomes lower (catalyst deactivation). Thus catalysts
must be regenerated or eventually replaced.
The basic concept is that a catalyzed reaction involves the transitory
adsorption of one or more of the reactants on the surface of the catalyst,
rearrangement of bonding and desorption of the products. The catalyst does
not remain as it is during the reaction. It participates in the reaction at some
stage, but this occurs in a cycle and the catalyst is regenerated at the end of
every cycle. Often the catalyst undergoes a structural change, or change in
the stoichiometry in course of the reaction. But the total quantity of the
catalyst remains more or less unchanged.
Catalysis is a kinetic phenomenon which occurs due to a catalyst in
action. Catalyst is a substance that aids in the attainment of chemical
equilibrium by reducing the potential energy barriers in the reaction path. The
catalyst can neither force a reaction to occur, nor can it alter the equilibrium
concentration of various species present in the reaction mixture. It can be said
to alter the rate of a reaction, without itself being consumed in the process.
Catalyzed reactions usually involve a reaction intermediate formed by the
reaction of a catalyst with one or more of the reactants. This transitory
intermediate then leads to product formation. Thus, it is clear that it
participates in the reaction at some stage and is regenerated at the end of the
reaction, may be in a different physical form but the mass essentially remains
the same. In some cases, an additional substance is added to the active
catalyst, termed as “Promoter” or a “Co-catalyst”. It not only enhances the
activity of the catalyst but in some cases also helps in combating catalyst
sintering and poisoning [4].
Chapter 1 - Introduction
5
Apart from accelerating reactions, catalysts have another important
property; they can influence the selectivity of chemical reactions. This means
that completely different products can be obtained from a given starting
material by using different catalyst systems. Industrially, this targeted reaction
control is often even more important than the catalytic activity. A good catalyst
is one which not only produces, selective products but also does not undergo
deactivation quickly. There are various reasons for deactivation, for example,
the catalyst may restructure as a consequence of selective adsorption of
impurities from the reactant stream, carbon may be deposited and thus
suitable additives must be incorporated to resist these changes on the
catalyst surface.
1.3 CATALYST DEVELOPMENT AND COMMERCIALIZATION For the successful development of a practical catalyst, the following
infrastructural facilities are essential:
Basic Research provides a thorough understanding of the catalyst science
through morden scientific tools and techniques. Basic Research generates
data leading to the understanding of the catalyst phenomena, kinetics and
mechanism of catalytic reactions and other fundamental aspects. The
literature available in the relevant fields provide the starting point, as also the
guidelines for planning the working program. The data generated in the
laboratory are used in identifying the catalyst for a particular reaction
mechanism, studying the phenomena associated with its deactivation, which
help to predict the performance, life and selectivity of the catalyst in actual
use.
Applied Research leads to the development of the “real” catalyst and
optimization of the production through pilot scale trial production and
evaluation facilities. Applied research in catalysis is based on two distinct
types of activities: the first ,related to developmental research aimed at the
formulation of practical catalysts, and the other related to operational research
involving catalyst application techniques in the industry.
Developmental Research aims at the formulation of a practical catalyst on a
bench scale level, and its commercial rationalisation through pilot scale
investigation and standardisation. The developmental research is carried out
Chapter 1 - Introduction
6
with close interaction and cooperation of various disciplines of science,
technology and engineering. The first step in this direction is catalyst
preparation after ascertaining the probable combination of active components,
carriers, promoters etc. Catalyst preparation involves ways and means of
compounding the formulation and its activation. There may be several
possible routes to arrive at the desired chemical composition of the catalyst,
but its physical and physico-chemical properties and hence the catalytic
activities may differ widely. Judicious selection of the method of formulation of
a catalyst through bench scale experimentation constitutes the major activity
in the first step of developmental research.
Efficient functioning of mordern chemical plants depends, to a great
extent, on performance, life and stability of the catalyst used. High activity,
selectivity, good mechanical strength, good thermal stability and resistance to
poisons are the prime requisites of an industrial catalyst. In addition, it should
withstand plant instabilities, and abnormal operating conditions. It is also
essential to avoid a sudden failure of the catalyst in a plant which can cause a
heavy loss of production. So, a commercial catalyst formulation should have
the following essential features:
Optimum activity: to achieve a close approach to equilibrium with minimum
catalyst volume and under economic operating conditions.
Sufficient mechanical strength and Good thermal stability : to withstand
handling and charging operations as well as the cycle of stress and strain
involved in plant operation including shut-downs and start-ups.
Stable activity: to sustain satisfactory performance for a reasonable period of
time.
Sufficient resistance: to normal level of poisons present in the feed.
The above qualities of a catalyst depend on certain essential factors
like surface chemical composition, concentration and distribution of active
sites, and structure and textural stability of the catalyst support or matrix. A
catalyst cannot work in isolation in an industrial reactor as its efficiency
depends,to a great extent, on the environment in which it is operating. This, in
turn, is greatly influenced by reaction conditions, reactor design and mode of
Chapter 1 - Introduction
7
operation. Optimization of all these factors is essential for the efficient
performance of a catalyst [5]. Commercial Production of a catalyst recipe is taken up on the basis of the
procedures standardized during pilot trial production. During commercial
production of a catalyst, the active involvement of the following disciplines is
essential.
• Process design and engineering: to establish efficient facilities for catalyst
production and catalyst improvements.
• Plant operation and maintenance: to ensure the regular production of
catalyst.
• Quality control of catalyst at different stages of production.
• Commercial strategies to establish linkages with customers.
1.4 CATALYSIS CONCEPTS AND TERMINOLOGIES The suitability of a catalyst for an industrial process depends mainly on
the following properties such as Activity, Selectivity, Stability (deactivation
behavior) and Environment compability [1].
Activity It can be defined as the rate at which the catalyst causes the reaction
to proceed to equilibrium. Active sites are the specific sites of importance,
present on the catalyst surface which induces a catalytic action. The rate of a
catalytic reaction is site dependant and hence can be increased by increasing
the surface area of a catalyst. It is generally believed that higher the surface
area of a catalyst, higher will be the activity. It is notable, that only a fraction of
the whole catalyst surface (active site) is active during the reaction. However,
the nature of activity of a catalyst differs under different reaction conditions.
Hence it is important to distinguish the active behavior of a catalyst under
different conditions. Activity is a measure of how fast one or more reactions
proceed in the presence of the catalyst. Activity can be defined in terms of
kinetics or from a more practically oriented viewpoint. In a formal kinetic
treatment, it is appropriate to measure reaction rates in the temperature and
concentration ranges that will be present in the reactor. There are three ways
Chapter 1 - Introduction
8
of expressing catalyst activity: Reaction rate (r), Rate constant (k) and
Activation energy (Ea).
Reaction rate (r)
The reaction rate (r is calculated as the rate of change of the amount
of reactant molecules nA of reactant A with time relative to the reaction volume
or the mass of catalyst:
h h (1.1)
Rate constant (k)
Kinetic activities are derived from the fundamental rate laws. For a
simple irreversible reaction A P:
(1.2)
Where, k = rate constant, f cA = concentration term that can exhibit a first or
higher order dependence on adsorption equilibria.
Activation energy (Ea)
The effect of a catalyst is always to reduce the activation energy
(enthalpy) of a reaction. If the temperature dependence of rate constant (k) is
given by the Arrhenius equation (1.3), then the implication is that is always
reduced and A remains more or less unchanged.
(1.3)
where k = rate constant, Ea = activation energy, k0 = pre-exponential factor
and R = gas constant.
Empirical rate equations are obtained by measuring reaction rates at
various concentrations and temperatures. If, however, different catalysts are
to be compared for a given reaction, the use of constant concentration and
temperature conditions is often difficult because each catalyst requires its own
optimal conditions. In this case it is appropriate to use the initial reaction rates
ro obtained by extrapolation to the start of the reaction. For comparative measurements following activity measures are used:
A given constant conversion is expressed by space velocity.
(1.4)
Where V0 = volume flow rate and mcat = relative to the catalyst mass. If we
replace the catalyst mass in Equation (1.4) with the catalyst volume, then we
Chapter 1 - Introduction
9
see that the space velocity is proportional to the reciprocal of the residence
time. For conversion under constant reaction conditions, since catalysts are
often investigated in continuously operated test reactors, in which the
conversions attained at constant space velocity are compared, the conversion
XA is the ratio of the amount of reactant A n1A0 that has reacted to the
amount that was introduced into the reactor. For a batch reactor:
, / , / % (1.5)
Often the performance of a reactor is given relative to the catalyst
mass or volume, so that reactors of different size or construction can be
compared with one another. This quantity is known as the space time yield
(STY):
(1.6)
Temperature required for a given conversion is another method of
comparing catalysts. The best catalyst is the one that gives the desired
conversion at a lower temperature. This method cannot however, be
recommended since the kinetics are often different at higher temperature,
making misinterpretations likely. This method is better suited for carrying out
deactivation measurements on catalysts in pilot plants.
Catalysis is a kinetic phenomenon. The speed of a catalyzed reaction
is often designated by the parameter called “turn over number” which denotes
the number of reactant molecules that are converted on an active site or on a
unit catalyst surface area per second at a given temperature, pressure and
concentration of reactants and products. Since the catalytic reaction occurs at
specific sites on solid surfaces, the rate of catalytic reaction can be increased
by using catalysts with very high surface area. This could be achieved by
dispersing the active species and therefore a parameter termed ‘catalyst dispersion’ defined as the number of surface atoms per total number of
atoms is important which can assume any value from 1 when all of the
surface area active sites to approximately 0.01. The catalyst particle sizes
may vary between 10-500Å. This dispersed system must maintain structural
and chemical stability for thousands of hours under the conditions of high
temperature (400-900K) and high pressure (1-102 atm). The design of new
Chapter 1 - Introduction
10
and stable, catalysts which resist chemical attrition and sintering of the small
particles is a constant concern of the catalyst scientist.
The common terminologies used to describe the efficiency of a catalyst
in terms of rate of conversion are described as follows:
• Turn Over Rate (TOR): The speed of a catalyzed reaction is often
described in terms of TOR, defined as the conversion of the number of
reactant molecules to products, per unit surface area of the catalyst at a
given temperature, pressure and concentration.
• Specific Rate: It indicates the number of reactant molecules reacting or
product molecules produced per unit catalyst area per second at a given
temperature, pressure and concentration. Its value can be used to judge a
suitable catalyst by comparing the activity of different catalysts for the same
reaction under similar conditions.
• Turn Over Number (TON): The effectiveness of a catalyst can be
expressed in terms of its “turn over number” – which is the number of
molecules of the substrate, reacting per minute due to the intervention of
one molecule of the catalyst. This depends on temperature, concentration
of substrate and the number of active sites on the catalyst. TON can also
be defined as the number of moles of substrate converted to product by a
mole of catalyst (metal or active compound in case of supported catalysts).
TON can also be defined as, mass/volume of substrate converted to the
product per unit mass/volume of the catalyst.
• Reaction Probability (RP): It is defined as the number of product
molecules formed per number of reactant molecules, incident on the
catalytic surface. It is readily obtained by dividing the specific rate of
product formation by the rate of incident reactants (on catalytic surface) in a
flow reactor. Rp reveals the overall efficiency of the catalyst and it is often
quoted in place of turnover rate.
Selectivity It is the efficiency with which the catalyst causes the reaction to
proceed in a direction to give the desired product. A chemical reaction leads
to the formation of several different thermodynamically feasible products. A
selective catalyst will facilitate the formation of one product molecule, while
Chapter 1 - Introduction
11
inhibiting the formation of other molecules, even though formation of other
products is thermodynamically feasible.
The selectivity Sp of a reaction is the fraction of the starting material that is
converted to the desired product P. It is expressed by the ratio of the amount
of desired product to the reacted quantity of a reaction partner A and therefore
gives information about the course of the reaction. In addition to the desired
reaction, parallel and sequential reactions can also occur (Fig. 1.2).
Fig. 1.2. Parallel and sequential reactions
Since this quantity compares starting materials and products, the
stoichiometric coefficients i of the reactants must be taken into account,
which gives rise to the following equation:
υ⁄
, | |⁄|υ |
, ⁄ % (1.7)
In comparative selectivity studies, the reaction conditions of
temperature and conversion or space velocity must, of course, be kept
constant. If the reaction is independent of the stoichiometry, then the
selectivity SP = 1. The selectivity is of great importance in industrial catalysis.
Stability The chemical, thermal, and mechanical stability of a catalyst
determines its lifetime in industrial reactors. Catalyst stability is influenced by
numerous factors, including decomposition, coking, and poisoning. Catalyst
deactivation can be followed by measuring activity or selectivity as a function
of time. Catalysts that lose activity during a process can often be regenerated
before they ultimately have to be replaced. The total catalyst lifetime is of
crucial importance for the economics of a process. Today the efficient use of
raw materials and energy is of major importance, and it is preferable to
Chapter 1 - Introduction
12
optimize existing processes than to develop new ones. For various reasons,
the target quantities should be given the following order of priority:
Selectivity >Stability > Activity
The following are the terminologies recognized for catalyst deactivation [4]. Poisoning – It is a chemical effect and occurs on the catalyst surface due to
the chemisorption of impurities, reactants, products or byproducts. Catalyst
activity is affected due to blocking of the active centers. This blocking occurs
either due to the permanent chemisorption of species at the active centre or
due to the blockage of the pathway of adsorbed reactive species towards the
active centers. Fouling – It occurs when a carbonaceous material is deposited on the
catalyst. Carbonaceous materials, either coke or carbon is the major cause of
deactivation in most cases. Coke or carbon affects catalyst activity by
adsorbing strongly on the active centers or by plugging the micro and
mesopores of the catalyst. Thermal degradation – It is common in case of supported metal catalyst
systems or with oxides of high surface area. It comes into effect either due to
loss of metal surface area due to crystallite growth (Metal Sintering) or due to
loss of support surface area due to pore collapse (Support Sintering). Attrition – It is due to the inherent low mechanical strength of the catalyst.
The catalyst when subjected to shear and stress, collapses, thereby losing its
activity.
Environmental compatibility Catalytic processes should produce zero or minimum emission with
potential environmental hazards Some popular terminologies used in the
current scenario, to define process environmental compatibility [3] include:
E-factor: Indicates the amount of waste produced per kilogram of the product.
E-factor for various segments of chemical industry is given as. Industry Product tonnage kg byproduct/kg product (E-factor)
Petroleum 106–108 <0.1
Bulk chemicals 104–106 <1–5
Fine chemicals 102–104 5–>50
Pharmaceuticals 10–103 25–>100
Chapter 1 - Introduction
13
Environmental Quotient (EQ): Indicates the environmental impact of the
waste produced, during a particular process.
_ , (1.8)
(where, Q = 1 for NaCl and 100 -1000 for heavy metal salts)
Atom efficiency: Refers to the effectiveness with which a desired product is
obtained in a particular process.
∑ ⁄ (1.9)
Zero Emission Technology is the emission of waste products that do not pollute the environment or disrupt the climate. It is focused towards achieving
high selectivity towards desired product and recycle/reuse of reactants. A lot
of attention is focused on development of zero emission technologies for
major industrial processes. To meet these challenges, the industry requires
innovative catalytic technologies that offer high space-time yield, improved
selectivity, higher atom efficiency, as well as low solvent requirement.
1.5 CATALYST CHARACTERIZATION Catalyst characterization is an important aspect in catalysis as it gives
an idea about the physico-chemical properties associated with a material.
Generally used characterization techniques are:
Elemental analysis Elemental analysis gives us an idea about the composition of the
catalyst. It is important to know the composition of a catalyst before use,
during use and after being used for a number of cycles. Conventional
methods involve gravimetric/volumetric analysis. Instrumental methods used
for elemental analysis are Flame photometry, Atomic absorption spectroscopy
(AAS) and Inductively coupled plasma-Atomic emission spectroscopy (ICP-
AES) which are both popular as well as accurate.
Chemical resistivity The chemical resistivity of the catalyst in various media (e.g. acids,
bases and organic solvents etc) or in the media/environment where catalyst
would operate, gives us an idea about the stability/resistivity of the material in
these environments.
Chapter 1 - Introduction
14
Thermal analysis (TGA) It gives an idea about the thermal stability of the catalyst and the
possible phase changes that occur during the thermal treatment of the
catalyst. An understanding of the thermal behavior is of basic importance for
utilizing the catalyst in various temperature ranges where it is thermally stable.
FTIR spectroscopy It is routinely used to derive information regarding the various chemical
bonds, functional groups and the interactions among them. In case of solid
acid catalyst, the catalyst material is adsorbed with ammonia or pyridine. The
IR spectrum provides a direct evidence for the existence of Bronsted and
Lewis acid sites on the surface of catalysts, a technique very useful for solid
acid catalysts.
X-ray Diffraction It indicates the amorphous or crystalline nature of the material. In case
of crystalline materials, distinct peaks at characteristic 2θ values are obtained,
whereas absence of peaks indicates amorphous nature of material. Besides,
if any impurity is present, can be detected by the characteristic 2θ value of the
peak in the X-ray diffractogram.
Microscopy Scanning electron microscopy (SEM) and Transmission electron
microscopy (TEM) are used to study the morphology of the material. Besides
it also gives an idea about the changes in shape, size and surface that occur
in a used catalyst.
Energy-dispersive X-ray spectroscopy (EDX) This analytical technique is used for both identification of an element as
well as to have a rough estimate of the composition of the materials. EDX is
used in coordination with and as supportive analysis with ICP-AES which is
more accurate compared to EDX.
Surface area (BET method) BET surface area can be obtained by N2 adsorption under liquid
nitrogen atmosphere. The activity of any catalyst is linked to its surface area
and hence the number of active sites present. Further, pore size can also be
Chapter 1 - Introduction
15
determined from the adsorption desorption curve obtained, during the
measurement process.
Temperature programmed desorption It involves Temperature programmed desorption of ammonia
(NH3TPD), Temperature programmed reduction (TPR) and Temperature
programmed oxidation (TPO). NH3TPD gives an idea about the nature of acid
sites present in the material through an adsorption desorption programme.
TPR and TPO give an idea about the active metal surface area of the
material.
Electron spin resonance (ESR) spectroscopy ESR provides information about the symmetry of the catalyst sites, the
oxidation state and coordination environment of the metal as well as
interaction of the adsorbed species with the active sites.
X-ray photoelectron spectroscopy (XPS) XPS is a quantitative spectroscopic technique that measures the
elemental composition, empirical formula, chemical state and electronic state
of the elements that exist within a material. XPS is also known as ESCA, an
abbreviation for Electron Spectroscopy for Chemical Analysis. XPS is a
surface chemical analysis technique that can be used to analyze the surface
chemistry of a catalyst in its as synthesized state, or after catalytic run.
Diffuse Reflectance UV-visible spectroscopy This technique measures the scattered light reflected from the surface
of samples in the UV-visible range (200-800 nm). For most of the
isomorphously substituted molecular sieves, transitions in the UV region (200-
400) nm are of prime interest. This spectroscopic technique is used to
determine the coordination state of transition metal ions substituted in the
matrix of the molecular sieves, involving ligand-to- metal charge transfer
transitions at ~ 200- 220 nm.
Mechanical properties Mechanical properties of a catalyst are important, as it gives an idea
about the utility of the catalyst in a reactor. The properties to be monitored are
abrasion and attrition resistance, crushing strength etc.
Chapter 1 - Introduction
16
The field of catalyst characterization is so widespread and important that
ASTM has developed few standard test procedures of catalyst
characterization which is practiced universally. A list of such procedures is
summarized in table 1.2. Table 1.2 ASTM procedures for catalyst characterization [6]:
Properties ASTM NO. Surface area of catalyst D3663 Pore volume distribution by mercury intrusion porosimetry D 4284 Pore distribution of catalysts from N2 desorption isotherms D 4641 Hydrogen chemisorption for platinum on alumina catalyst D 3908 Catalyst acidity by ammonia chemisorption D 4824 Particle size determination by laser light scattering D 4464 Attrition and abrasion of catalysts and catalyst carriers D 4058
1.6 PRESENT TRENDS IN CATALYSIS Over the past few years, there has been an increasing concern for
pollution prevention and the approach to solve this problem has been towards
the development of processes and technologies that produce minimum or
zero waste. This new approach is popularly known as Green Chemistry and
involves the synthesis, processing and use of chemicals so as to reduce the
potential risks for human health and the environment. This new approach is
also popular by the names like environmentally benign chemistry, clean
chemistry, Sustainable chemistry, Atom economy and Benign by design
chemistry. Today, green chemistry is a frontier area of research and is
receiving considerable attention.
Green chemistry Green chemistry focuses on the design, manufacture, and use of
chemicals and chemical processes that have little or no pollution potential or
environmental risk, and processes that are economically and technologically
feasible, thus providing the best opportunity for chemists, manufacturers, and
processors to use chemicals safely and to carry out their work under safe
conditions. The basic idea of green chemistry is to increase production
efficiency through atom economy, and at the same time eliminate or at least
minimize wastes and emissions at their source, rather than treat them at the
end of the process, after they have been generated.
Twelve principles [7] outlined below, provide a significant guideline in
dealing with the concept of green chemistry.
Chapter 1 - Introduction
17
• It is better to prevent waste than to treat or clean up waste after it is
generated.
• Synthetic methods should be designed to maximize the utility of all
materials used in the process, in the conversion of final product.
• Whenever practicable, synthetic methodologies should be designed to use
and generate substances that possess little or no toxicity to human health
and the environment.
• Chemical products should be designed to preserve efficacy of function
while reducing toxicity.
• The use of auxiliary substances (solvents, separation agents etc.) should
be made unnecessary whenever possible and when used, innocuous.
• Energy requirements should be recognized for their environmental and
economic impacts and should be minimized. Synthetic methods should be
conducted at ambient temperature and pressure.
• A raw material or feedstock should be renewable, rather than depleting
whenever technically and economically practical.
• Unnecessary derivatization (blocking groups, protection/deprotection, and
temporary modification of physical/chemical processes) should be avoided
whenever possible.
• Catalysts (as selective as possible) are superior to stoichiometric reagents.
• Chemical products should be designed, so that at the end of their function
they do not persist in the environment and instead break down into
innocuous degradation products.
• Analytical methodologies need to be further developed, to allow for real
time in-process monitoring and control prior to the formation of hazardous
substances.
• Substances and the form of a substance, used in a chemical process
should be chosen such, so as to minimize the potential for chemical
accidents, including releases, explosions and fires.
The principles of green chemistry can be applied to all areas of
chemistry including synthesis, reaction conditions, separations, analysis,
monitoring and catalysis.
Chapter 1 - Introduction
18
Green chemistry and catalysis Catalysis provides an important opportunity to achieve the goals of
green chemistry. Both, greener catalytic processes and catalytic processes for
greener products, must play key roles in green chemistry. Asymmetric
catalysis, biocatalysis, heterogeneous catalysis, environmental catalysis,
shape selective catalysis, phase transfer catalysis and solid acid catalysis are
just a few examples that have a direct and significant impact on
accomplishing the goals of green chemistry. Amongst the various catalytic
systems used, solid acid catalysts are making a huge impact.
1.7 SOLID ACID CATALYSTS - AN ALTERNATE APPROACH TO LIQUID ACID CATALYSTS
Liquid acids such as H2SO4, HF, H3PO4 have been extensively used as
catalysts in a variety of organic transformations for long. Though they are very
effective, liquid acid catalysts are cited as potential environmentally
hazardous chemicals and are becoming a major area of concern mainly due
to operational difficulties such as toxicity, corrosiveness, effluent disposal,
product separation, storage and handling. Owing to increasing environmental
awareness and a quest for zero emission technologies, much attention is
focused on developing alternatives to these existing acids. Solid acids are
safe alternatives for conventional liquid acid catalysts, used in synthetic
organic chemistry in petroleum refineries, fine chemical synthesis,
pharmaceuticals etc [8].
In general terms, a solid acid can be described as a solid on which the
color of a basic indicator changes, or as a solid on which a base is chemically
adsorbed. More strictly, following both the Bronsted and Lewis definitions, a
solid acid shows a tendency to donate a proton or to accept an electron pair.
Though they differ in structure from liquid acids, solid acid catalysts work on
the same principles.
The ability to lend protons makes solid acids valuable as catalysts.
Protons are often released from ionisable hydroxyl groups in which the bond
between hydrogen and oxygen is severed to give H+ and O-. Protons may
also be released in the form of hydrated ions such as H3O+. When a reactant
receives and incorporates a proton from an acid, it forms a reactive
Chapter 1 - Introduction
19
intermediate. This positively charged intermediate may change shape and
configuration. It may then undergo either isomerization or rearrangement by
shedding the proton or may undergo some organic transformation, leading to
the formation of a new molecule. In any case the proton is returned to the
catalyst [9].
Solid acid catalysts are appealing, since the nature of the acid sites are
known and it is possible to modify the acidic properties of these materials by
adopting various synthetic and post synthetic treatments. The main
characteristic of solid acids, as compared to liquid acids, is that solid acids
encompasses different population of sites, differing in their nature and
strength (weak acid and strong acid sites) and hence depending on the
reaction conditions, the same catalyst can be active for one reaction and
inactive for another [10]. The effectiveness of a particular solid acid catalyst
for a given reaction, depends on various factors including surface area,
porosity, acidity, crystallinity and nature of acid sites. Several review articles
have been published, dealing with the use of solid acid catalysts for the
preparation of speciality and fine chemicals [11-13].
Advantages of solid acid catalysts • Though environmental benefits have been the major reasons for the
introduction of solid acids in many chemical processes, these catalysts
have proved to be more economical and often produce better quality
products.
• They are very effective and some of them are known to exceed the acidity
of concentrated H2SO4. Besides, they hold their acidity internally and thus
easy to handle and also reaction vessels or reactors are not corroded.
• They can allay concerns about safety and environmentally hazardous
emissions as they are nontoxic and nonvolatile.
• They possess high catalytic activity and selectivity.
• Being heterogeneous in nature, separation from reaction mixture is easy
and the catalyst can be regenerated and reused.
• Problems associated with the disposal of used solid acids is less compared
to the disposal of liquid acids that require much money and efforts, for post
use treatment and effluent neutralization.
Chapter 1 - Introduction
20
Some important solid acid catalysts A drive for clean technology associated with the problems encountered
while using the liquid acids has led to the development of a variety of solid
acids. Several inorganic materials tested as solid acid catalysts include silica-
alumina gels, zeolites, oxides and hydrous oxides, heteropolyacids, clays,
solid superacids and TMA salts.
• Silica Alumina Gels: Silica-alumina gels have no well defined structures
and contain many pores, which range in diameter from few to few hundred
angstroms. Further, their amorphous nature make them less than ideal
catalysts. These compounds tend to lose their activity due to clogging of
pores. In addition, because of their irregular structure, protons are unevenly
distributed and are therefore difficult to control catalysis precisely. Silica
gels were replaced by zeolites in mid 1960's.
• Zeolites: Zeolites are crystalline solids with three dimensional frame work,
containing micropores of uniform size throughout the structure. They are
formed by the corner sharing of (SiO4)4- and (AlO4)5- tetrahedra. Due to the
excess negative charge on the tetrahedron, a counter ion is required to
neutralize the charge. When this counter ion used is a proton, the material
behaves as a solid acid (Bronsted acidity). If the zeolite is heated, water
may then be eliminated from the Bronsted sites leaving aluminum atoms
coordinated to only three oxygen atoms. These will act as Lewis acids [14].
Zeolites have found widespread application as solid acid catalysts in
petroleum industries [15]. The acid strength of the protons in some zeolites
can be very high, quite often being 100 % stronger than H2SO4 and hence
the H-zeolite makes excellent solid acid catalysts [16]. However, zeolites
are unstable in acid media, have low aberration resistance and undergo
rapid deactivation due to plugging of the pores.
• Oxides and Hydrous Oxides: In oxide based solid acids, protons balance
net negative charges produced by the replacement of a high valent cation
with one of lower valence or by the attachment of an anion to the surface of
a neutral oxide support. These protons are responsible for acidity. They
include oxide and hydrous oxides of Zr4+, Ti4+, Fe3+, Al3+, Nb5+, Cr3+, Th4+,
etc. Of the hydrous oxides, zirconia has received much attention as a solid
Chapter 1 - Introduction
21
acid catalyst [17]. A mechanism for the generation of acid sites by mixing
two oxides has been proposed by Tanabe [18]. They suggest that the
acidity generation is caused by excess of a negative or positive charge in
model structure of a binary oxide related to the coordination number of a
positive and negative element. Zr(OH)4 and Ti(OH)4 is synthesized by
traditional sol gel method. Obtaining high acid strength comparable to
sulfuric acid, halides, or oxy halides in these catalysts remains a challenge
due to the smaller electronegativity difference in metal-oxygen bond
compared to the metal halide bonds [19].
• Solid superacids: Solid acids with low surface acidity could be converted
to solid acids with high surface acidity. Such catalysts are known as solid
superacids. Solid super acids of oxides and hydrous oxides are prepared
by introducing sulphate ion (sulphation). Sulphation is carried out by
washing hydrous ZrO2 at room temperature thoroughly with
H2SO4/(NH4)2SO4 or passing H2SO4 (SO42-) through a packed column of
ZrO2 or immersing ZrO2 into H2SO4, stirring and filtering. Though different
methods are used, enough H2SO4 should be added to form a monolayer.
These processes lead to adsorption of sulphate ions on ZrO2. Finally, the
resultant material is calcined at > 500 °C to remove excess H2SO4. Some
get firmly grafted on surface- introducing acidity. The solids consist mainly
of the metal dioxides with sulphate ions coordinated to the metal ions on
the surface. They have the general formula S042-/MxOy (M = Zr, Ti)
exhibiting acidity higher than 100 % H2SO4. They can be easily prepared,
are stable at elevated temperatures and can be easily regenerated. They
have been extensively used for a range of important organic
transformations such as isomerization, alkylation, acylation, esterification,
etherification, oligomerization, oxidation [20-26] etc. The main limitation of
these catalysts is, they get easily deactivated by loosing the sulfate ions.
• Heteropolyacids: Heteropoly acids offer appealing characteristics as solid
acid catalysts in many acid-catalyzed reactions [27]. Various types of
heteropoly compounds [28] are known, but the most popular is based on
Keggin structure corresponding to the formula [XM12O40]n-, M being a
transition element (usually Mo or W) and X a hetero atom (P, Si, As, Ge
Chapter 1 - Introduction
22
etc.). It displays a tetrahedral symmetry based on a central XO4
tetrahedron, surrounded by twelve MO6 octahedra arranged in four groups
of M3O13 of three edge-bridged octahedra. The central atom is the
important factor in determining the acid strength and the acidity is related to
the total charge on the anion.
• Clays: Clays are some of the most abundant, porous and benign materials
on the earth. Clays are complex layered oxides essentially comprising of
parallel tetrahedral silicate and octahedral aluminate sheets [29]. For
charge compensation, various cations (especially Na+ and Ca2+), may
occupy the interlayer gallery. When these cations are replaced by hydrated
protons in the form of H3O+, acidity is introduced. They have been used in a
variety of reactions including Friedel-Crafts alkylation, acylation and
production of methyl tertiary butyl ether (MTBE) [30].
• Tetravalent Metal Acid salts: Abbreviated as TMA salts possess the
general formula M(IV)(HXO4)2.nH2O [31]. TMA salts possessing structural
hydroxyl groups (the H+ of the –OH being the exchangeable sites) indicates
good potential for application as solid acid catalysts due to presence of
surface protons/acidity. TMA salts can be obtained in both amorphous and
crystalline forms. It is observed that both surface area and surface acidity
decreases with increasing crystallinity of the material [32, 33]. Hence, their
acidity can be tailored for a specific application by controlling the
crystallinity of the material. The preparation procedure thus affects the
structural hydroxyl groups, which is reflected in the performance of TMA
salts as solid acid catalysts. Besides they possess excellent thermal
stability and chemical resistivity. TMA salts have been used as catalysts in
various organic transformations by various groups - Dr. A. Clearfield (USA),
Dr. G. Alberti (Italy), Dr. W. Holderich (Germany), Dr. D. Whittaker (UK)
and Dr. U. V. Chudasama (The M. S. University of Baroda, India).
TMA salts have been investigated for cyclohexanol dehydration to
cyclohexene [32,34,35], oxidative dehydrogenation of cyclohexene to
benzene[36,37], conversion of ethylbenzene to styrene[38,] amination
reactions [39-41] hydrogenation of alkenes [42-44], reverse Prins reaction
[45], dehydration of cyclohexanol and methylcyclohexanols [46], terpene
Chapter 1 - Introduction
23
rearrangements [33], ionic and radical rearrangements of α and β-pinene [47]
and Friedel-Crafts alkylation of anisole with alcohols [48]. The catalytic
aspects of TMA salts have been extensively studied from our laboratory that
include esterification [49-57], dehydration of alcohols [58,59], hydration of
nitriles to amides [60], ketalization of ketones [61] and Pechmann
condensation reactions [62].
1.8 PORES AND POROSITY Porous materials have attracted the interest of scientists and industry
due to various applications for instance in molecular separation,
heterogeneous catalysis, adsorption technology as well as new challenges in
the fundamental materials research, owing to their high surface area, tunable
pore size, adjustable framework and surface properties [63]. The behavior
and performance of such materials can be determined by many
characteristics such as surface area, porosity and pore size distribution.
Physical properties such as density, thermal conductivity and strength are
dependent on the pore structure of the solid .Porosity influences the chemical
reactivity of a solid and the physical interaction of solids with gases and
liquids.
In general, the surface area of a porous material is higher than the
surface of an analogous non-porous material. Thereby the internal surface
area is usually much higher than the one contributed by the external surface.
Due to the fact that heterogeneous catalyzed chemical reactions basically
occur on surfaces or at phase boundaries, a higher surface area would,
theoretically, directly yield to an improved reactivity. Apart from the surface
area, other important characteristics of porous solids are the crystallinity or
regularity if present, the distribution of pore sizes and the chemistry of the
walls. In an ideal porous material these attributes should be tailored exactly to
the needs of the application.
The science and technology of porous materials has progressed
steadily and is expanding in many new directions with respect to processing
methods and applications. Many synthetic pathways have been reported for
the synthesis of porous materials, either with a disordered pore system or
ordered with various structures, which can meet the demands of the target
Chapter 1 - Introduction
24
application. To be commercially interesting, such a material should be
inexpensive and highly stable for regeneration. Materials based on silicate
show up such kind of flexibility and have hence found wide fields of industrial
applications.
A solid material that contains cavities, channels or interstices can be
regarded as porous.The voids between linked atoms in any material arranged
in an ordered manner are called pores.
Based on the accessibility of an external fluid, pores can be classified
into closed pores and open pores. Closed pores are totally isolated from their
neighbours and surface of the particle. They influence macroscopic properties
such as bulk density, mechanical strength and thermal conductivity, but are
inactive in processes such as fluid flow and adsorption of gases. Pores which
have a continuous channel of communications with external surface of the
body are called open pores. In Fig. 1.3 region ‘a’ represents the closed pores,
and regions such as b, c, d, e and f represent open pores, ‘b’ and ‘f’ are also
described as blind or saccate pores.
Fig.1.3. Classification of pores
The international Union of Pure and Applied Chemistry (IUPAC)
classifies three categories for pore sizes in solids. Pore size distributions
larger than 500 Å are macroporous, materials having pores between 20 Å to
500 Å represent mesoporous materials, and materials with pore size
distribution less than 20 Å represent microporous materials. Mesoporous
materials belong to a new family of material with sizes intermediate to those
usually studied by chemists and material scientist, and therefore mesoporous
materials pose new challenge in their synthesis and characterization.
Chapter 1 - Introduction
25
1.9 FROM MICROPORES TO MESOPORES It is possible to say that zeolites are the most widely used catalysts in
industry. They are crystalline microporous materials which have become
extremely successful as catalysts for oil refining, petrochemistry, and organic
synthesis in the production of fine and speciality chemicals, particularly when
dealing with molecules having kinetic diameters below 10 Å. The reason for
their success in catalysis is related to the following specific features of these
materials:[64] (i) They have very high surface area and adsorption capacity.
(ii) The adsorption properties of the zeolites can be controlled, and they can
be varied from hydrophobic to hydrophilic type materials. (iii) Active sites,
such as acid sites for instance, can be generated in the framework and their
strength and concentration can be tailored for a particular application. (iv) The
sizes of their channels and cavities are in the range typical for many
molecules of interest (5-12 Å), and the strong electric fields [65] existing in
these micropores together with an electronic confinement of the guest
molecules [66] are responsible for a preactivation of the reactants. (v) Their
intricate channel structure allows the zeolites to present different types of
shape selectivity, i.e., product, reactant, and transition state, which can be
used to direct a given catalytic reaction towards the desired product avoiding
undesired side reactions. (vi) All of these properties of zeolites, which are of
paramount importance in catalysis and make them attractive choices for the
types of processes listed above, are ultimately dependent on the thermal and
hydrothermal stability of these materials. In the case of zeolites, they can be
activated to produce very stable materials not just resistant to heat and steam
but also to chemical attacks.
Despite these catalytically desirable properties of zeolites they become
inadequate when reactants with sizes above the dimensions of the pores
have to be processed. In this case the rational approach to overcome such a
limitation would be to maintain the porous structure, which is responsible for
the benefits described above, but to increase their diameter to bring them into
the mesoporous region. The strategy used by the scientist to do this was
based on the fact that most of the organic templates used to synthesize
zeolites affect the gel chemistry and act as void fillers in the growing porous
Chapter 1 - Introduction
26
solids. Consequently, attempts were made that employed larger organic
templates that would result in larger voids in the synthesized material. This
approach did not give positive results in the case of zeolites, but in contrast
was quite successful when using Al and P or Ga and P as framework
elements [67-77]. A 14-member ring (MR) unidirectional zeolite (UTD-1) could
be synthesized using a Co organometallics complex as the template [78,79].
The template can be removed, and the thermal stability of the framework of
the organometallic-free material is high, resisting calcination temperatures up
to 1000 °C. The presence of framework tetrahedral Al generates Brønsted
acidity which is strong enough to carry out the cracking of paraffins. In a
general way, the different strategies directed towards the synthesis of
ultralarge pore zeolites has been summarized [80].
However, when the zeolite and zeotypes with the largest known
diameters were considered for their possible use as catalysts, it was observed
that cacoxenite,[81] which is a naturally occurring mineral with a 15 Å pore
system, is thermally unstable and thus cannot be used as a catalyst. Though
Cloverite, has potentially large pores, the diffusion of large molecules is
restricted, owing to the unusual shape of the pore openings which are altered
due to protruding hydroxyl groups. Likewise in VPI-5, stacking disorder or
deformation of some of the 18-membered rings during dehydration results in a
decrease in the pore size from 12 Å to about 8 Å. In the case of the new
zeolite UTD-1, the fact that it has to be synthesized with an organometallic
Cobalt complex, which has then to be destroyed, and the Cobalt left has to be
acid leached raises strong questions concerning its practical application,
which remains in doubt unless a more suitable template and activation
procedure can be found.
In conclusion, it can be said that despite the outstanding progress
made in producing large pore molecular sieves, the materials synthesized
were not suitable to be used in catalytic processes.
1.10. ORDERED MESOPOROUS MATERIALS It is true to say that one of the most exciting discoveries in the field of
materials synthesis over the last years is the formation of mesoporous silicate
and aluminosilicate molecular sieves using liquid crystal templates. In 1992,
Chapter 1 - Introduction
27
researchers at the Mobil Oil Company reported a novel family of materials
called M41S [82-84]. The breakthrough assured a bright future due to their
properties with a well-defined pore size between 15-100Å. With the discovery
of this new type of material, the pore size constraint of microporous materials,
with pore diameter smaller than 15Å, was overcome. The family of
mesoporous M41S material consists of three types, as summarized below in
the Fig 1.4.
Fig.1.4 The mesoporous M41S family [82]
About MCM-41 • MCM-41 is one of the most studied and promising member of the M41S
family.
• MCM-41 is the abbreviation for Mobil Crystalline Material or Mobil
composition of matter.
• The mesoporous material presents regular arrays of uniform channels,
which has a honeycomb structure as a result of hexagonal packing of uni-
dimensional cylindrical pores.
• By choosing adequate reactants and reaction conditions, it is possible to
tailor the channel dimension in the range of 15-100Å or even larger.
• The BET surface area is typically over 1000 m2/g [83,84]. The pore is
usually between 0.7 and 1.2 cm3/g with long-range order.
• With increasing pore size, the regularity of the structure is affected.
• The adsorption capacity is exceptionally high (more than 50 wt% for
cyclohexane at 40 Torr, 67 wt% for benzene at 50 Torr).
• MCM-41 possesses excellent thermal and hydrothermal stability (up to
800°C).
• It is relatively stable in acidic medium (pH 2) [85]. However, it is destroyed
in a basic medium (pH12).
Chapter 1 - Introduction
28
• MCM-41 is composed of silica framework, which is almost catalytically
inactive.
• The isomorphous substitution of silicon by a variety of metals (Al, Ga, Fe)
gives rise to acidic properties [86].
• The possibility of using the pore channels of MCM-41 as a support for
existing catalysts has also been considered [80, 87].
There is no doubt that the synthesis of these materials opens definitive
new possibilities for preparing catalysts with uniform pores in the mesoporous
region. Obviously when a new type of material such as these is discovered,
an explosion of scientific and commercial development swiftly follows, and
new investigations on every conceivable aspect of their nature, the synthesis
procedures and synthesis mechanisms, heteroatom insertion,
characterization, adsorption, and catalytic properties, rapidly occurs.
1.11. SUMMARISING POROUS MATERIALS Well known microporous materials are zeolites [88] and
aluminophosphate molecular sieves [89] which are inorganic composites
having a crystalline three-dimensional framework woven with tetrahedral
atoms (T-atoms) like aluminium, silicon, phosphorous etc. bridged by oxygen
atoms. These materials possess uniform channels or cavities circumscribed
by rings of a definite number of T-atoms. The exploitation of the architectural
features of zeolites resulting in different acid sites and acid strengths,
exchangeable ions, shape and size selective channels and pores etc. has
been well established by now. Modification of the framework and extra-
framework composition makes these materials useful for catalyzing organic
reactions.
Even though, zeolites, having pore dimensions of 5 to 7 Å, have served
the purpose for most of the industrial reactions by providing high surface area,
the pore dimensions are not sufficient enough to accommodate broad
spectrum of larger molecules. The performance of the zeolitic systems is
limited by diffusional constraint associated with smaller pores. To a certain
extent, it is possible to overcome this problem with aluminophosphates, with
pore dimensions up to 13 Å. However, these materials suffer from limited
thermal stability as well as negligible catalytic activity due to framework
Chapter 1 - Introduction
29
neutrality. Moreover, there is a need for present day catalytic studies dealing
with processing of hydrocarbons with high molecular weights. These factors
led to the discovery of mesoporous solids and there has been an ever-
growing interest in expanding the pore size of the zeotype materials from the
micropore region to mesopore region.
Materials with super large pores possessing catalytic properties are
attractive candidates as catalysts for non-shape selective conversion of large
molecules, especially for the cracking of heavier petroleum feed stock. The
large void volumes of such materials make them potential adsorbents with
high adsorption capacity. The large pore can also act as host for various
guest species so that the alteration of redox properties can also be achieved.
With the first successful report on the mesoporous materials (M41S) by
Mobil researchers, with well defined pore sizes of 20 – 500 Å, the pore size
constraint (15 Å) of microporous zeolites observed a breakthrough. The high
surface area (> 1000 m2/g) and the precise tuning of the pores are among the
desirable properties of these materials. Mainly, these materials ushered in a
new synthetic approach where, instead of a single molecule as a templating
agent as in the case of zeolites, self-assembly of molecular aggregates or
supra-molecular assemblies are employed as templating agents. The basic
difference in the synthesis of microporous and mesoporous molecular sieves
can be shown pictorially.
Fig. 1.5 Microporous materials using single molecule as template
Fig. 1.6 Mesoporous materials using molecular aggregates or supra-molecular assemblies as
template [90]
Chapter 1 - Introduction
30
1.12. AIM AND SCOPE OF THE PRESENT WORK Though it is possible to overcome the existing pore size constraints of
microporous solids as mentioned earlier, the MCM-41 based materials have
negligible catalytic activity due to framework neutrality, however with
advantageous properties like mesoporous nature of the material, good
thermal stability, high surface area and retention of surface area at high
temperatures. Thus, the main aim of the present study was to encash the
advantageous properties of MCM-41 and enhance its practicability in the area
of catalysis using Green Chemistry principles. There are a number of ways by
which catalytic activity can be generated into the MCM-41 neutral framework.
(i)Substitution of an M3+ cation e.g. Al3+ in the Si4+ framework, leading to
negatively charged framework, followed by balancing these charges by H+
ions to create Bronsted acid sites (via NH4+ ion exchange and subsequent
thermal decomposition to give H+ and NH3) to result in a material with inherent
acidity.
(ii)Immobilization/anchoring/impregnation of homogenous acid catalyst e.g.
heteropoly acids (HPAs) onto MCM-41 to result in a material with induced
acidity.
(iii)Isomorphous replacement of Zr4+and Ti4+ in the siliceous MCM-41
framework to induce redox properties.
Chapter II of the thesis includes the synthesis of mesoporous (i)
Siliceous MCM-41, (ii) Al-MCM-41 and (iii) 12TPA-MCM-41, (where 12-TPA =
12-Tungstophosphoric acid a HPA). Materials (i) and (ii) have been
synthesized by sol-gel method, using templates varying several parameters
such as silica source, templating agent/types, reaction conditions such as pH,
time of reaction, aging, temperature etc. and these parameters optimized,
using surface area as an indicative tool. In case of Al-MCM-41 SiO2:Al2O3
ratios have been varied in order to obtain material with maximum surface
acidity and hence in case of Al-MCM-41 surface acidity has been used as an
indicative tool. The salient feature is the synthesis of MCM-41 and Al-MCM-41
at room temperature. 12-TPA supported MCM-41 was prepared by a process
of anchoring and calcination, with varying 12-TPA loading (10-40 wt.%) in
Chapter 1 - Introduction
31
order to obtain material with maximum surface acidity and hence, here also
surface acidity has been used as an indicative tool.
All synthesized materials were characterized for Elemental analysis by
ICP-AES, X-ray diffraction (XRD), Transmission electron microscopy (TEM),
Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy
(EDX), Surface area (BET method), Pore volume and pore distribution(BJH
method), surface acidity by temperature programmed desorption (TPD) of
ammonia, Diffuse reflectance spectroscopy (UV-DRS), Fourier transform
infrared spectroscopy (FT-IR) and Thermogravimetric analysis (TGA).
The potential use of Al-MCM-41 and 12-TPA-MCM-41 as solid acid
catalysts was explored studying (i) Esterification and (ii) Friedel-Crafts
alkylation and acylation as model reactions. In case of esterification
monoesters such as ethyl acetate (EA), propyl acetate(PA), butyl acetate (BA)
and benzyl acetate (BzA) and diesters such as diethyl malonate (DEM),
dioctyl phthalate (DOP) and dibutyl phthalate (DBP) have been synthesized.
Friedel-Crafts acylation of anisole and veratrole with acetic anhydride
and alkylation of toluene with benzyl chloride have been performed to obtain
4-methoxy acetophenone (4MA), 3,4-dimethoxy acetophenone (3,4DMA) and
parabenzyltoluene (PBT) under solvent free condition.
In the above reactions, parameters such as catalyst amount, reaction
time and reaction temperature, mole ratio of reagents etc. have been
optimized including catalyst regeneration capacity. The catalytic activity of
both catalysts have been compared and the results have been correlated with
surface properties of the materials.
Chapter III aims at synthesizing oxidation catalysts Zr-MCM-41 and Ti-
MCM-41 by a sol-gel method using templates towards achieving
mesoporosity, with high surface area, good thermal stability and maximum
M4+ incorporation. For incorporation of maximum Zr4+ and Ti4+, SiO2:ZrO2 and
SiO2:TiO2 ratios have been varied. The salient feature is that the material is
synthesized at room temperature.
All synthesized materials were characterized for Elemental analysis by ICP-
AES, X-ray diffraction (XRD), Transmission electron microscopy (TEM),
Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy
Chapter 1 - Introduction
32
(EDX), Surface area (BET method), Pore volume and pore distribution(BJH
method), surface acidity by temperature programmed desorption (TPD) of
ammonia, Diffuse reflectance spectroscopy (UV DRS), Fourier transform
infrared spectroscopy (FT-IR) and Thermogravimetric analysis (TGA).
Further, the catalytic potential of the materials Zr-MCM-41 and Ti-
MCM-41 has been explored by studying Epoxidation as a model reaction
using H2O2 as an oxidant in the conversion of allyl chloride to selectively
produce Epichlorohydrin in a single step at room temperature.
In the above reaction, parameters such as catalyst amount, reaction
time and reaction temperature, mole ratio of reagents etc. have been
optimized including catalyst regeneration capacity. The catalytic activity of
both catalysts have been compared.
Chapter IV involves the use of mesoporous MCM-41 as a support in
the synthesis of automobile catalyst. Automobile emissions form an important
source of atmospheric pollution. Automobile catalysts are now widely
recognized for the conversion of mainly CO → CO2, oxides of nitrogen → N2
and unburnt hydrocarbons → CO2 and H2O. At present, noble metals
particularly Rh and Pt catalysts are used for these purposes which are
expensive. There is thus a search for relatively cheaper materials that must
operate efficiently under a wide variety of conditions. Perovskite type oxides
have attracted much attention recently in environment pollution control.
In the present endeavour, LaCoO3 (LC) a Perovskite has been
synthesized on the surface of mesoporous MCM-41 (LCM) by citrate solution
combustion route. Pd has been incorporated in the Perovskite lattice as well
as on the MCM-41 surface and the materials characterized for XRD, surface
area (BET method) and temperature programmed reduction (TPR). In order to
check the thermal stability/durability, the materials have been aged at 1000oC
for 3h and again characterized. Catalytic activity of fresh and aged materials
have been explored for oxidation of CO, hydrocarbons and reduction of NOx
in a tubular down flow test reactor under simulated exhaust conditions and
known air/fuel ratios and the results correlated with surface and redox
properties of the materials.
Chapter 1 - Introduction
33
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CCHHAAPPTTEERR 22
Synthesis and Characterization of MCM-41 based Catalysts and their Applications as Solid Acid Catalyst in Esterification and Friedel- Crafts Acylation and Alkylation. _____________________________________________
Chapter 2. Synthesis and Characterization of MCM-41 based materials
38
2.1 INTRODUCTION
Advanced materials are the enablers of new technologies. The present
demands are better materials, new materials and cheaper materials prepared
by ecofriendly routes. An important goal of materials research is the ability to
design and synthesize in high yields, materials whose structures and
properties can be predicted, varied and controlled. This is the challenge for
the synthetic chemist by the demands of material technology. Traditional
ceramic processes use high temperatures. The present demands are making
use of soft chemistry routes (low temperatures) which is popularly known as
“Chemie Douce” by the French. Sol-gel method of synthesis is a soft
chemistry route. Advantages of materials prepared by sol-gel synthesis is high
homogeneity, high purity, low temperature processing, structural control of
materials formed, materials with improved or desired properties and
preparation of porous materials by use of templates.A great deal of interest
has been shown in the application of sol-gel chemistry in various fields of
technology. A majority of the materials prepared using the sol-gel method are
ceramics, refractories and glasses. Attempts to apply the sol-gel technique for
the preparation of catalysts is a relatively new venture, made since the last
decade.
2.2 SOL-GEL PROCESS
Concepts and Terminologies
The sol-gel process is a wet-chemical technique for the fabrication of
materials, employing low temperature, starting either from a chemical solution
or colloidal particles (sol for solution or nanoscale particle) to produce an
integrated network (gel). In general, sol-gel process can be regarded as the
preparation of the sol, gelation of the sol and removal of the solvent. The
overall sol-gel process can be represented by the following sequence of
transformations [1]:
Precursor Sol Gel Product
Precursors are starting materials, in which the essential basic entities for
further network formation are present in the correct stoichiometry. Typical
precursors are metal alkoxides and metal chlorides.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
39
Sol is a colloidal suspension of particles in a liquid, the particles typically
ranging from 1-100 nm in diameter.The solid particles in the colloidal phase
are stable due to short-range forces such as Van der Waals attraction and
surface charges.
Gel is a semi–rigid solid, in which solvent is contained in a network/framework
of the material, which is either colloidal (essentially a concentrated sol) or
polymeric.Gel is defined as a substance that contains a continuous solid
skeleton enclosing a continuous liquid / fluid phases of colloidal dimensions.
In sol-gel processing, a sol of a given precursor is prepared, which
involves the dissolution of the required metal ions either as alkoxides or other
metallo-organic salts in a suitable solvent (alcohol) or as inorganic salts in
water, which undergo hydrolysis,followed by condensation and polymerisation
reactions to produce highly condensed and branched network polymers, the
gel. The networking depends on the functionality of the metal. Silicon with
coordination number four, forms highly branched networks [2] (Fig. 2.1a). In
the gelation step,the fluid sol is transformed to a semirigid solid gel. Two types
of gels are usually formed, colloidal and polymeric. Colloidal gels are formed
from metal salt solutions, oxides and hydroxide sols, while polymeric gels are
formed from metal alkoxide based sols. The name “sol-gel” is thus given to
the process, because of the distinctive viscosity increase that occurs at a
particular point in the sequence of steps. A sudden increase in viscosity is the
common feature in sol-gel processing, indicating the onset of gel formation.
Sol-gel process can be distinguished from precipitation by its specific property
to stabilize a finely dispersed (mostly colloidal) phase in solution.
Typically, formation of a metal oxide via sol-gel route involves
connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges,
generating metal-oxo or metal-hydroxo polymers in solution.The
transformation of sol to gel takes place via hydrolysis and condensation
reactions of the precursors. The hydrolysis reaction is represented taking
silicon as an example:
Si (OR) 4 + n H2O (HO) n-Si (OR) 4-n + ROH (2.1)
In case of metal alkoxide precursors, R represents an alkyl group. The metal
is totally hydrolyzed when n = 4. For any other value of n, partial hydrolysis
Chapter 2. Synthesis and Characterization of MCM-41 based materials
40
takes place. In the condensation reaction, the two partially hydrolyzed
molecules link together and liberate a small molecule such as H2O or ROH.
The general reaction is represented as:
(OR)4-nSi(OH)n+(HO)nSi (OR)4-n(OR)4-n(OH)n-1 Si-O-Si (HO)n-1 (OR) 4-n+ H2O(2.2)
The condensation takes place in such a way so as to maximize the
number of M-O-M bonds and minimize terminal hydroxyl groups through
internal condensation. Initially monomers add to form rings, creating 3-D
structures. These compact structures are formed by leaving the hydroxyl
groups outside, (Fig.2.1b) that serve as nuclei for further particle growth[3],
that proceeds by Ostwald ripening mechanism, a process by which small
particles precipitate on relatively larger insoluble particles, indicated by arrow
heads in (Fig.2.1c). As particles grow in size the number of particles
decrease. The polymerization behaviour of aqueous silica via sol-gel process
at different pH is presented in (Fig.2.1d).
(a) (b)
(c) (d)
Fig. 2.1(a) Highly branched networks of silicon, (b)Condensation reactions leading to closed
ring 3D structure (c) SEM showing Ostwald ripening mechanism[3].(d)Polymerization
behavior of aqueous silica. A = in presence of salts / acidic medium, B = alkaline medium.
Sequential steps involved in sol-gel synthesis
Chapter 2. Synthesis and Characterization of MCM-41 based materials
41
Hydrolysis:It involves reaction of inorganic or organometallic precursor with
water or a solvent, at ambient or slightly elevated temperature. Acid or base
catalysts are added to speed up the reaction.
Polymerization:This step involves condensation of adjacent molecules
wherein water/alcohol are eliminated and metal oxide linkages are formed.
Polymeric networks grow to colloidal dimensions in the liquid (sol) state.
Gelation: It leads to the formation of a three dimensional network throughout
the liquid, by the linking up of polymeric networks.
Ageing:A continuous change in structure and properties of a completely
immersed gel in liquid is called ageing and represents the time between
formation of the gel and removal of solvent.Aggregation of smaller polymeric
units to the main network, progressively continues on ageing the gel. Solvent
molecules however, remain inside the pores of the gel. Extensive ageing
however, causes shrinkage of gel. Factors that affect ageing processes
include temperature, time and pH of the pore liquid.
Drying:Here, solvent is removed at moderate temperatures (<200 °C) leaving
the residue behind. During drying, the gel initially shrinks due to loss of pore
fluid maintaining the liquid-vapour interface at the exterior surface of the gel.
At the final stage of drying, liquid-vapour menisci recede into the gel
interior[4]. The magnitude of the capillary pressure, Pc, exerted on the
network, depends on the surface tension of the liquid, γ, the constant angle θ,
and the pore size, r: given by Pc = 2 γ Cosθ/r.If the pore size is very small, the
capillary pressure will be large. The original gel network collapses due to this
pressure[5]. Aging may be used to reduce the extent of collapse of the gel
structure during drying.The resulting materials are identified based on drying
conditions. Conventional evaporative drying such as heating a gel in an oven
induces capillary pressure associated with the liquid-vapour interface within a
pore, resulting in the collapse of the porous network. The sample thus
obtained is called a xerogel, which has a relatively low surface area and pore
volume.In supercritical drying, on the other hand, these deleterious effects are
minimized due to differential capillary pressure and the resultant materials are
known as aerogels. Consequently, they have high pore volumes, surface
areas and low bulk densities.A third method of drying involves the freeze
Chapter 2. Synthesis and Characterization of MCM-41 based materials
42
Hydrolysis Condensation
Polymerization
drying of the solvents at low temperature under reduced pressure. This
method is similar to the lyophilization technique adopted in pharmaceutical
industries and the product is called cryogel.Another method of drying is
subjecting the gel to ultrasonic vibration at room temperature to remove the
solvent. The gel thus obtained is called a sonogel.
Dehydration:This step is carried out between 400 °C and 800 °C to drive off
the organic residues and chemically bound water. A thermal treatment
firing/calcination may be performed in order to favour further
polycondensation and enhanced mechanical property, when following
changes, such as loss of solvents, pyrolysis of the organics, structural
rearrangement and densification or crystallization are observed.
Densification:Heating the porous gel at high temperatures, leads to
formation of a dense oxide product. The densification temperature depends
considerably on the dimensions of the pore network, the connectivity of the
pores, and surface area. Sequential steps involved in sol-gel process is
presented in Fig.2.2
Fig.2.2. Sequential steps involved in sol-gel process
Conditions for sol-gel synthesis
Precursor Solvent Catalyst
Inorganic Salt
Metal alkoxides
Water
Organic
Optional
Sol
Gel
Ageing
Washing
Drying
Xerogel Aerogel
Sonogel
Cryogel
Calcination
Structural rearrangement
Pyrolysis of the organic
Crystallization/ Densification
Improved mechanical property
Final Material
Chapter 2. Synthesis and Characterization of MCM-41 based materials
43
It is possible to tailor specific properties in a material by tuning the
various conditions of the sol-gel process, outlined as follows:
pH of the hydrolysis : The pH during hydrolysis mainly decides the nature of
the pores, surface area and density of the materials. In general, acid
catalyzed hydrolysis gives a microporous network, while base catalyzed
hydrolysis leads to the formation of a mesoporous network. The preparation of
microor mesoporous materials in a neutral medium has been reported [6].
Rate of addition of water: The rate of addition of water during the
preparation of the sol affects the rate of hydrolysis and condensation which in
turn influence the texture and morphology of the gel.
Temperature of gelation: The rate of gelation depends on the temperature at
which the sol is aged or heated for the removal of the solvent or for the
facilitation of hydrolysis. If the temperature is lower, the rate of hydrolysis is
slower and the particle size is relatively smaller. This results in reduced pore
collapse and yields a well defined porous network.
Aging of Gels:Aging is the process of keeping the gels in various solutions
for a period of time in order to increase the strength of the gel network, so that
cracking of gels during drying can be prevented. The chemical reactions that
cause gelation, continue long after gel point strengthening, stiffening and
shrinkage of the network [4,7]. The composition, structure and properties of
gel change during aging. The changes that occur during aging are
categorized as,
Polymerization: Increase in connectivity of the gel network by condensation
reactions.
Coarsening: Process of dissolution and reprecipitation driven by differences
in solubility between surfaces with different radii of curvature.
Syneresis: Shrinkage of the gel and the resulting expulsion of liquid from
the pores.
Drying control chemical agents (DCCAs):The presence of DCCAs has a
significant influence on the particle texture and morphology. Various DCCAs
useful in controlling the porosity and bulk density are formamide, glycerol and
oxalic acid.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
44
Calcination temperature: the calcination temperature is also important in
controlling the pore size and density of the materials.
Conclusions, Advantages and Disadvantages of Sol-Gel process
The sol-gel method, thus offers the possibility to prepare solids with
pre-determined structure, by varying the experimental conditions such as the
choice of reagents, concentration, mode and rate of mixing, temperature, pH,
ageing and drying conditions. Variation in any of these parameters yields
materials with different characteristics. The preparation procedure thus affects
the composition and structure, which is further reflected in the
properties/performance such as porosity, surface polarity and crystallinity. The
various steps involved in the sol-gel technique described above may or may
not be followed. In practice, however, a modified sol-gel route is followed.
Advantages of sol-gel process include increased homogeneity, high
purity, low processing temperature and high surface area of the gels or
powders obtained. The inherent usefulness of this approach is largely due to
the ease with which sol-gel derived materials can be prepared, modified, and
processed. The mild reaction conditions afford an opportunity to incorporate
various organic moieties into inorganic compounds. Furthermore, the average
pore size, pore size distribution, surface area, refractive index and polarity of
the resultant matrix can also be controlled and tailored by manipulations in the
sol-gel processing conditions.Disadvantages of the Sol-Gel Process include,
large shrinkage during processing, creation of fine pores, presences of
hydroxyl groups when hydroxides are used, residual carbon in final material
originating from templates, health hazards of organic solvents, and finally long
processing times.
2.3SOL-GEL PROCESS AS APPLIED TO SILICA
Silica gels are most often synthesized by hydrolysing monomeric,
tetrafunctional alkoxide precursors employing a mineral acid or base as a
catalyst. Most common tetraalkoxysilanes used in the sol-gel process are
tetraethoxysilane [TEOS, (SiOC2H5)4] and tetramethoxysilane [TMOS,
Si(OCH3)4]. The sol-gel process involving silica can be described as follows,
Chapter 2. Synthesis and Characterization of MCM-41 based materials
45
The hydrolysis reaction replaces alkoxide groups with hydroxyl groups.
Subsequent condensation reactions involving the silanol groups produce
siloxane bonds and the by-products, alcohol or water. Under most conditions,
condensation commences before hydrolysis is complete. Since water and
alkoxysilanes are immiscible, a mutual solvent such as alcohol is normally
used as a homogenising agent. In order to reduce the reactivity of the
alkoxide precursor, organic functional groups are introduced into the siloxane
network. This type of alkoxide precursors are known as organoalkoxysilanes.
The surface and structural characteristics of the silica gels are affected
by various parameters that control the rate of hydrolysis and condensation
and these can be summarized into three categories[8]:
Composition: Type of starting materials, quantity of water, catalysts and
solvents.
Reactions (up to gel formation): Rate of mixing, reaction temperature and
gelation schemes.
Process variables (after gel formation): Type of dehydration, drying
temperature and heating.
Factors affecting silica gel synthesis are mole ratio of H2O to alkoxides
and pH values that play a significant role in determining reaction rates,
morphology and composition. Changes in pH values during the sol
preparation stage significantly affects the appearance of sample, porosity,
density, viscosity, gelation time, activation energy, surface area, pore volume,
and pore size distribution. Increase in the gelation temperature could make
the gelation process shorter,while increase in drying temperature would cause
the pore diameter to become bigger. Intermediate conditions produce
structures intermediate to these extremes.
Si OR H2OHydrolysis
Esterification
Si OH ROH
Si OR SiHO
+
+
Alcohol Condensation
Alcoholysis
Si O Si ROH +
+
Si OH + SiHO Si O Si +
Hydrolysis
WaterCondensation
H2O
Chapter 2. Synthesis and Characterization of MCM-41 based materials
46
2.4SOL-GEL SYNTHESIS AS APPLIED TO ZEOLITES
The factors that influence the synthesis of crystalline zeolite are summarized
as follows
Composition of the reaction mixture.
Nature of reactants
Initial and final pH of the system.
Temperature of the process and its variation with time (if any).
Ambient- 25 to 60 °C
Low –90 to 120 °C
Moderate- 120 to 200 °C
High- 250 °C or higher
Time allowed for the reaction to take place, including the calcination time.
Mixture, whether homogeneous or heterogeneous.
Seeding.
Template molecules (if any).
Other factors
Aging
Stirring (fast/slow) (time)
Nature of mixing
Order of mixing
Composition of the Reaction Mixture:
The composition of the reaction mixture is one of the most important factor,
governing the product properties,which includes :
Silica to alumina ratio.
Hydroxyl, ion concentration.
Inorganic cations.
SiO2 / Al2O3in the gel phase decides the framework composition of the
zeolite. The hydrophobic / hydrophilic nature of zeolite is also affected by this
ratio as aluminium is hydrophilic and silicon is hydrophobic. Also, high
aluminium contents give higher acidic sites, which are useful for many
applications. Zeolites with higher silica / alumina ratio are used for catalytic
applications in cracking and isomerization. As the aluminium content is
Chapter 2. Synthesis and Characterization of MCM-41 based materials
47
increased the acid resistance & thermal stability of the zeolite reduces. These
effects can be summarized as follows:
Increasing the silica / alumina ratio affects following physical properties of the
zeolite:
Increases acid resistance.
Increases thermal stability.
Increases hydrophobicity.
Decreases affinity for polar adsorbents.
Decreases cation content.
Decreasing the silica / alumina ratio affects following physical properties of the
zeolite:
Increases hydrophilicity.
Increases cation exchange properties.
Decreases the pore size for same numbered ring, as aluminium has lower
atomic radius than silicon.
Hydroxide ion concentration:It functions as structure director through
control of the degree of polymerization of silicates in solution. OH¯ ion
modifies the nucleation time by influencing transport of silicates from the solid
phase to solution. It enhances the crystal growth and controls the phase
purity. In a study[9] it was found that the OH¯/Si ratio influences the pore size.
i.e. higher the ratio, wider were the pores.
Role of Inorganic Cations: Inorganic cations are added to the reaction
mixture, to induce crystallization of specific zeolite structures, that could not
have been formed in their absence. These cations are used in the zeolite
synthesis for following reasons:
They act as structure directing agents.
They balance the framework charge.
They govern the morphology of the zeolite.
They affect the crystal purity
They also affect the product yield.
Due to their charge and their orientation in the reaction mixture,
inorganic cations alter the pore sizes of the zeolites. Most commonly used
cations are alkyl ammonium ions such as tetramethyl ammonium (TMA),
Chapter 2. Synthesis and Characterization of MCM-41 based materials
48
tetraethyl ammonium (TEA), etc. Apart from silica to alumina ratio and pH, the
amine concentration is also a very important variable [10].
Zeolite synthesis is described in two steps: nucleation and
crystallization. Nucleation is a process where small aggregates of precursors
give rise to germ nuclei (embryos), which become larger with time. The rate of
nucleation of a new phase from a melt increase by decreasing the
temperature of the system.Crystallization starts by involving nuclei and
ingredients from the solution mixture. The deposition on a seed or stable
nucleus increases with the extent of stirring and temperature. The yield of the
crystals increases at a rate proportional to total free external surfaces of the
crystal.
Crystallization can be split into four stages:
Formation of water icebergs. These icebergs are ring like structures of
water molecules, interconnected by hydrogen bonding.
Depolymerizationof the condensed large molecules of the precursors then
takes place.
These depolymerized molecules start orienting around the water icebergs,
thus forming thenuclei.
These nuclei form terminated tetrahedral. resulting into crystalline
structures.
pH of the Reaction Mixture
The zeolite synthesis via sol-gel process is carried out in alkaline pH (>
10). Crystal formation is accompanied by increase in pH indicating that SiOH
in colloidal state is incorporated in the framework in the form of SiO2. Since
SiOH is acidic in nature, the crystallization is accompanied by increase in pH.
Further, the pH of the system is also very important for stabilizing the sol and
control of the particle size.
Role of additives
Addition of colloidal particles e.g. Polystyrenecauses organization of the
pores. Polystyrene acts as a template as well as nucleating agent.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
49
2.5SOL-GEL PROCESS USING TEMPLATES
Introduction
One of the important modified routes in sol-gel process involves use of
templates. Templates are structure directing agents, that find applications in
synthesis of porous materials with tailor made dimensions. They are organic
molecules (singular or assembly) around which the main structure is built up –
a process very similar to a casting process. Template synthesis began from
using quaternary alkyl ammonium ions - with alkyl chain length containing 10-
20 carbons.
Templates, when used at optimum concentration, referred to as Critical
Micelle Concentration (CMC), orient themselves to form an assembly with the
polar head groups pointing outside, around which the anions orient to form a
network. The layers of inorganic materials seem to distort and crosslink
around the polar head groups to form a new mesoporous structure. The
driving force for this layer folding, is most likely the ion pairing between the
positively charged head groups and the negatively charged inorganic
components. The template can subsequently be removed from the system,
either by solvent extraction method or by calcination, to obtain finished
product with predetermined pore size and structure. Excellent up-to-date
reviews on the use of various organic templates and the mechanism of
structure directing agents are available in the literature [11-13].
Concepts of templating
The main concept of obtaining well defined mesostructures is to use a
surfactant templated polymerization instead of an uncontrolled reaction. In
general, the Lyotropic (i.e. amphiphilic) molecules of the surfactant form a
liquid crystal by aggregation in aqueous solution[14,15]. Formation of the
liquid crystal matrix is strongly dependent on the conditions in the solution and
the structure of the liquid crystal is the so called mesostructure. Important
parameters for the mesophase formation are, the temperature, concentration
and pH- value of the solution. Depending on these conditions, the structure of
the mesophase can be for example ordered with spherical, cylindrical,
lamellar or cubic phases or disordered.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
50
In the broadest sense, a template may be defined as a central structure
about which a network forms in such a way, that removal of the template
creates a cavity with morphological and/or stereochemical features related to
those of the template[16]. A general template approach is illustrated in Fig.
2.3, where primary structural units are organized around a molecular template
and solidified to form a matrix.
Fig.2.3Schematic of the organic template approach showing the incorporation and removal of
the template.
The fidelity of the imprint created by template removal depends on several
factors:
The nature of the interaction between the template and the embedding
matrix.
The ability of the matrix to conform to the template.
The relative sizes of the template and the primary units used to construct
the matrix. Examples of single molecules used as templates are
ethylamine(EA), isopropylamine(IPA), ethylmethylamine (EMA),
diethylamine (DEA), n-propylamine(nPA), ethylenediamine (en)
tetramethyl ammonium(TMA), and tetraethyl
ammonium(TEA).Surfactants/assemblies used as templates are cetyl
trimethyl ammonium bromide (CTABr), cetyl trimethyl ammonium chloride
(CTACl), cetyl pyridinium bromide (CPBr),and cetyl pyridinium
chloride(CPCl).
Surfactants are amphiphilic molecules which consist of a hydrophilic, polar
head group and a hydrophobic, non-polar tail. Due to their amphiphilic nature,
surfactant molecules have a high affinity towards surfaces and interfaces,
thereby the term “surfactant” emerges which is an abbreviation for “surface
active agent”. Surfactants may be classified into four different groups,
depending on the nature of the polar head group: anionic surfactants, cationic
surfactants, zwitterionic surfactants and non-ionic surfactants[17].In aqueous
solutions, the surfactants associate into aggregates called micelles, if
Chapter 2. Synthesis and Characterization of MCM-41 based materials
51
theconcentration is above the critical micelle concentration (CMC). If the
concentration is increased even further, or a polar additive is added, the
surfactants self-assemble into liquid crystalline mesophases.
The geometry of the surfactant aggregates formed in solution is
dependent upon the shape of surfactant and its concentration, which is
expressed by the term surfactant packing parameter:[18,19]v / a lwhere v is
the volume of the surfactant tail, a is the effective head group area and l is the
length of the extended surfactant tail, Fig. 2.4a
Due to variations in size of different types of surfactant tails and head
groups, this ratio willvary for different types of surfactants. The relative sizes
of the tail and head group thereforegovern the optimal way of packing the
surfactants together into aggregates of differentgeometry,[17,19] as shown in
Fig. 2.4b. If the packing parameter is below 1/3, only spherical micelles exist
in the solution. An increase in concentration causes these spherical micelles
toorganize themselves in the solution, which creates a three-dimensional,
cubic ordering. A v/alratio above 1/3 creates aggregates with a rod-like shape.
If the concentration of the surfactantsis sufficiently high, these rod-shaped
micelles assemble into a hexagonal array, therebycreating a hexagonal liquid
crystal.
(a)
(b) (C)
Chapter 2. Synthesis and Characterization of MCM-41 based materials
52
Fig. 2.4(a) Visualization of the geometrical considerations for the surfactant packing Parameter [19].(b) Different liquid-crystalline phases as a function of the surfactant packing Parameter [17]. (c). Curvatures of different types of surfactant surfaces.
For v/al = 1 there is a balance between sizes of the head group and
tail, which causes the surfactants to form planar aggregates with a sheet-like,
bilayer structure. With sufficient concentration, three-dimensional liquid
crystals with lamellar or bicontinuous cubic structures are created. If v/al is
increased above 1, the surfactants form reversed “water-in-oil” systems.The
value of the packing parameter may be influenced by the addition of co-
solutes in the surfactant solution[17,19]. Hydrophobic moleculesdissolve in
theinterior of the micelles, causing an increase of ‘v’. Short-chained alcohols
reside in the palisade layer in the micelles, thereby decreasing
‘a’andincreasing ‘v’. Electrolytes in the solution adsorb on the micelle surface
(of ionic surfactants), whichdecrease a due to screening of the repulsion
between the head groups.For non-ionic surfactants, the polarity and thereby
the solubility of the head group isdependent upon temperature. An increase in
temperature decreases this polarity, whichdecreases ‘a’.Introduction of other
types of surfactants into the solution also affects the packingparameter. The
surfactants interact with each other and form mixed micelle systems,
whichhas an average packing parameter.
The surface area of surfactant mesophases is highly dependent upon
the curvature of the surface, as visualized in Fig.2.4c. A convex surface has a
higher surface area than a concave surface, with the planar surface in
between.For ionic surfactants, the surface of the aggregates is charged. The
charge density of a charged surface is defined [20] as: σ = Q / A where σ is
the charge density, Q is the surface charge and A is the surface area. Fig.2.4c
shows that concave surfaces have higher charge densities than convex
surfaces, due to the lower surface area. The different liquid-crystalline
structures shown in Fig.2.4b have different surface curvatures and thereby
different surface charge densities. Itcan be seen that the surface charge
density of the surfactant mesophases increases withincreasing packing
parameter, due to the closer packing of the charged head groups.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
53
2.6THE FORMATION OF MESOPOROUS STRUCTURES
Introduction
A number of models have been proposed to explain the formation of
mesoporous materials and to provide a rational basis for the various synthetic
routes followed. All these models are proposed on the basis of structure
directing ability of surfactants or templates in solution. Surfactants with
hydrophilic head groups and hydrophobic tail within the same molecule get
self-organized so as to minimize the contact with incompatible ends. The
principal difference amongst various synthetic routes is the way in which the
surfactants interact with inorganic species. Earlier it was thought that the
formation of these materials is a result of electrostatic complementary
between charged surfactant and inorganic species. But later the
mesostructured materials have been prepared by exploring other possible
interactions other than electrostatic pathways. The formation of these
materials can be viewed alternatively as the interface chemistry between the
surfactant and inorganic species. The mesoporous materials can be prepared
by exploiting ionic, hydrogen bonding as well as covalent bonding
interactions.Different synthesis mechanisms have been proposed to explain
theformation of mesoporous materials. A few of them are described below:
Liquid Crystal Templating (LCT) Mechanism
There are threemain liquid crystalline phases with hexagonal, cubic
and lamellar structures. Because of the similarity between the different M41S
phases MCM-41 hexagonal, MCM-48-cubic, MCM-50-lamellar and known
liquid crystalphases, the first mechanism proposed for the synthesis of these
materials was the liquidcrystal templating mechanism [14,21-23]. In aqueous
solution, surfactant molecules exist asrandomly dispersed monomolecules at
low concentrations. With increasing concentration,the surfactant molecules
aggregate with their hydrophobic tails together exposing their polarheads to
the aqueous solution to reach a minimum energy configuration and thus
formspherical micelles decreasing the system entropy. The lowest
concentration at which monomolecules aggregate to form spherical isotropic
micelles is called critical micellie concentration (CMC1). There exists a
Chapter 2. Synthesis and Characterization of MCM-41 based materials
54
second critical concentration(CMC2) corresponding tothe further aggregration
of spherical into cylindrical or rod-like micelles. The hexagonalphase is the
result of hexagonal packing of cylindrical micelles, the lamellar
phasecorresponds to the formation of surfactant bilayers and the cubic phase
may be regarded as abicontinuous structure. The structure of the mesophase
depends on the composition of themixture, the pH and the temperature [24].
Two possible pathways have been proposed [14] for the LCT
mechanism which are schematically shown in Fig. 2.5. Pathway-1 is a
surfactant controlled pathway, in which the surfactant arrays, prior to the
condensation of framework materials. In this pathway, it is considered that
first there is aformation of the surfactant hexagonal liquid-crystal phase
around which the growth of theinorganic materials is directed. Thesurfactant
micelles aggregate to formhexagonal arrays of rods. Silicate anions present in
the reaction mixture interact with thesurfactant cationic head groups.
Condensation of the silicate species leads to the formation ofan inorganic
polymer.
Fig. 2.5 Possible mechanistic pathways for the formation of MCM-41: (1) liquid crystal phase
initiated and (2) silicate anion initiated [14].
Pathway-2 is a silicate controlled mechanism,in which the silicate
species condense continuously around micelles, as they form rods and pack
into a hexagonal structure.This is thought of as a cooperative self assembly
pathway.In this pathway, it has been proposed that the randomly ordered
rodlike micelles interact with silicate species by coulombic interactions in the
reaction mixture to produce approximately two or three monolayers of silicate
around the external surfaces of the micelles. These randomly ordered
Chapter 2. Synthesis and Characterization of MCM-41 based materials
55
composite species spontaneously pack into a highly ordered mesoporous
phase with an energetically favourable hexagonal arrangement, accompanied
by silicate condensation. With the increase in heating time, the inorganic wall
continues to condense. The absence of hexagonal liquid crystalline
mesophases, either in the synthesis gel or in the surfactant solution (used as
template) it was concluded that formation of MCM-41 phase is possibly via
pathway 2 rather than pathway 1.
Transformation mechanism from lamellar to hexagonal phase
It has been proposed that[24-26] in a surfactant/silicate aqueous
mixture with relatively low pH, low degree of polymerization of silica species,
and low temperatures, small silica oligomers (three to eight silicon atoms)
interact with surfactant cations by coulombic interactions at the interfaces
forming multidentate binding between them. These subsequently polymerize
to form larger ligands, enhancing the binding between the surfactant and
silicate species. These surfactant silicate multidentate ligands lead to a
lamellar biphase governed by the optimal surfactant average head group area
(A). As the polymerization of silicate species proceeds, the average
headgroup area of surfactant assembly increases due to the decrease in the
charge density of larger silicate layers and ultimately results in the hexagonal
mesophase precipitation(Fig.2.6).
Fig. 2.6 Transformation mechanism from lamellar to hexagonal phase [25].
Folded Sheet Mechanism
Yanagisawa et al. [27] and Inagaki et al[28,29] synthesized crystalline
mesoporous silicate andaluminosilicate materials designated as FSM-16
(Folded Sheet Mesoporous Materials). Theyproposed a folded sheet
mechanism (Fig. 2.7) for the formation of mesostructures derivedfrom
kanemite (layered silicate). The surfactant cations intercalate into the bilayers
ofkanemite by ion-exchange process. The transformation to the hexagonal
Chapter 2. Synthesis and Characterization of MCM-41 based materials
56
phase occurs duringhydrothermal treatment by condensation of silanol
groups. MCM-41 and FSM-16 are similarbut show slightly different properties
in adsorption[22]and surface chemistry[30].
Fig. 2.7 Folded sheet mechanism [29]
2.7GENERALIZED LCT MECHANISM
Based on the specific type of electrostatic interaction between a given
inorganic precursor “I” and surfactant head group “S” various synthesis routes
have been evolved as presented in scheme 2.1.
Scheme 2.1 Possible synthetic routes for mesoporous materials
Direct pathways:
In the case of direct pathway, surfactants are directly bonded to
inorganic precursors through electrostatic interactions. So far, most of the
materials have been prepared with cationic surfactants like
CTABr/CTACl.Based on the original LCT mechanism, which involves the
Chapter 2. Synthesis and Characterization of MCM-41 based materials
57
anionic silicate species and cationic quaternary ammonium surfactant, it could
be categorized as the S+I– pathway. However, a variety of surfactants can
serve the purpose. In all these cases, control of pH is critical. Various
materials have been prepared with S+I- and S- I+ pathways [14,31-35].
Mediated Pathways:
Here charge interaction pathways are S-I+,S+X-I+ (where x is a counter
anion) and S- M+ I- (where M is metal cation). By operating well below the
isoelectric point of silica under acidic conditions (pH ~2), the silicate species
are cationic. In this case the halide ions (X-) act as mediators.In charge
reversed situation, where the surfactants and inorganic species are negatively
charged the metal ion M+ (where M = Na+ or K+) act as mediators. By
maintaining higher pH conditions, it is possible to prepare mesoporous
materials by S- M+ I- pathway through metal ion mediation. Materials prepared
by S+X- I+ and S- M+ I- path ways are reported [33-35].
Neutral Pathways:
Tanev and Pinnavaia [36] proposed a neutral templating mechanism
based on hydrogen bonding interactions (S°-I°) betweenneutral primary amine
(S°) which acts as template and neutral inorganic precursors (I°). Hydrolysis
of tetraethylorthosilicate in an aqueous solution of primary amine yields the
neutral Si(OC2H5)4-X (OH)X species which then binds through H-bonding to the
surfactant head group. This leads to the formation of rod like micelles.
Further, hydrolysis followed by condensation, leads to Hexagonal
Mesoporous Silica (HMS). The neutral templating route provides several
advantages over materials prepared by electrostatic pathways, mainly the
synthesis can be carried out at room temperature and the surfactant can be
removed by extraction with ethanol.
Even though this process, S°I° offers the practical advantage of facile
template recovery by non- corrosive solvent extraction or evaporation
methods, these surfactants have some limitations. Neutral amines are costly
and toxic and not ideally suitable for the industrial scale preparation of
materials. So there exists a need to think of a process with low cost and
environmentally compatible neutral templating route. Polymeric polyethylene
oxide (PEO) surfactants have been used to prepare these materials which are
Chapter 2. Synthesis and Characterization of MCM-41 based materials
58
relatively inexpensive and biodegradable. The main advantage of using
polymeric surfactants is the requirement of the lower concentration of the
surfactant. These surfactants form spherical to flexible rod or worm like
micelles at critical concentrations approximately one hundredth of those
required for ionic surfactants [37]. It is also observed that polymeric
polyethylene oxide tri-block copolymer is a promising surfactant. In the
presence of suitable solvents or combination of solvents these
PEO/PPO/PEO tri-block copolymeric surfactants arrange into different
lyotropic phases.
Ligand Assisted interactions
By a different synthetic approach [38], it is possible to prepare
mesoporous materials through covalent interactions. Instead of relying on
charge interaction, the surfactants were pretreated with the metal alkoxides in
the absence of water to form metal-ligand covalent bonded complexes. High
quality materials are formed by the use of amine surfactants, due to the strong
affinity for nitrogen-metal bond formation between surfactant head group and
the inorganic precursor. In this ligand assisted templating approach, the
control of the mesostructure was found possible by adjusting the
metal/surfactant ratio, and it has been established that the M41S family of
mesoporous materials can be prepared through this approach.Mesoporous
materials prepared by various methods are presented in Table2.1.
2.8 REMOVAL OF TEMPLATE
Once the framework condenses around the micellar rods, a nonporous solid is
formed. To produce the porous species, the template must be removed from
the framework. The method of template removal depends on the desired
morphology and the thermal stability of the synthesized compound. MCM-41
shows reasonable thermal stability. As such, the surfactant template is
removed by calcination. In this context, calcination refers to simply heating the
sample sufficiently to burn out the organic phase and leave behind the porous
framework [38,39]. The calcination has to be done in the flow of inert gases at
the initial stages followed by the flow of air. This is to maintain the crystallanity
of the material. The template can also be removed by washing the as
synthesized material with extracting solvents.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
59
Table 2.1Summary of Mesoporous materials prepared by various methods [14,31-38]
Class of Materials
Type Preparation Method
Observed phases
M41S MCM-41 S+ I
– Hexagonal
MCM-48 “ Cubic
MCM-50 “ Lamellar
T-M41S (T=Transition metals)
“
---------
SBA SBA-1 S+ X
– I
+ Cubic
SBA-2 “ 3D-Hexagonal
SBA-3/APM “ MCM-41 Like Hexagonal
SBA-11 “ Cubic
SBA-12 “ 3D-Hexagonal Material
SBA-14 “ Lamellar
SBA-15 “ 2D-Hexagonal
SBA-16 “ 3D-Cubic cage structure
T-SBA (T=Ti,V,Mn,Mo,Cr,Zr)
“ -----------
MSU MSU-1 Silica N◦ I
◦ Worm-like
disordered
MSU-2 Silica “ Worm-like disordered
MSU-3 Silica “ Worm-like disordered
MSU-V “ Lamellar
Ti-MSU- Silica “ Worm-like disordered
Zr-MSU- Silica “ Worm-like disordered
Nb-TMS1
Ta-TMS1 Hexagonal
TMS Nb-TMS2 Hexagonal
Nb-TMS3 Hexagonal
Nb-TMS4 Cubic Layered
PHTS PHTS S+ X
– I
+ Analogue to SBA-15
MCF MCF Swelling agent added to
Synthesis of SBA-15
Sponge-like foam with 3D-structure with large uniform
spherical cell
HMS T-HMS (T=Si,V,Al,Ga,Fe,Cr,Mo)
S◦ I
◦ Hexagonal
MSU- Michigan State University HMS- Hexagonal Mesoporous Silica PHTS- Plugged Hexagonal Templated Silica, MCF - Meso Cellular Form SBA - Santa Barbara Amorphos or Santa Barbara Acid Material, TMS - Transition Metal Oxide Mesoporous Molecular Sieves, M41S- Mobil Composition of Material or Mobil Composition of Matter
Chapter 2. Synthesis and Characterization of MCM-41 based materials
60
Some of the more effective solvents are ethanol and supercritical CO2.
When using supercritical extraction (SCE), polar modifiers are added to
enhance the extraction capability of the supercritical fluid. Commonly used
modifiers are dichloromethane (DCM) and methanol [40]. In the case of
materials prepared through hydrogen bonding interactions, the removal of the
template can be achieved either by repeated extraction with ethanol or by
calcination.
Each method of template removal has advantages and disadvantages.
Standard calcination programs are effective at removing all of the template,
but the high temperatures generally cause some loss of long range order and
shrinkage of pores within the MCM structure. Extractions with ethanol are
easy to perform at room temperature but are not as effective at completely
removing the template. Supercritical extractions are extremely efficient at
removing surfactants. It is a more environmentally benign extraction method
as the surfactant can be recovered for use in subsequent synthesis as
opposed to being burned to oxidation products in typical calcinations. Due to
the interesting properties of supercritical fluids, SCE does not induce
structural degradation whereas pore shrinkage and some loss of long-range
order arise in thermal calcinations [40]. Unfortunately, supercritical extraction
is expensive and difficult to perform.
2.9SYNTHESIS STRATEGIES AND CHARACTERIZATION
METHODOLOGIES - A LITERATURE SURVEY
Synthesis strategies of Siliceous MCM-41- A Literature Survey
The discovery of MCM-41, has attracted considerable attention due to
large and uniform pore size distribution, high surface area (>800 m2/g) and
distinct adsorption properties [21].Siliceous MCM-41 when synthesized is
chemically inert in which catalytic activity has to be imparted. It is essential
that the MCM-41 thus synthesized should be hydrothermally stable for
functionalizing, after which it can be held at relatively higher temperatures and
even under harsher conditions, which is generally prevalent when used in
industries. In addition, if one needs to scale up for producing larger quantities,
the synthesis has to be simple, lesser energy demanding, cost effective and
Chapter 2. Synthesis and Characterization of MCM-41 based materials
61
environment friendly, like handling and usage of appropriate concentration of
chemicals without much of secondary wastes.
Several synthesis methods have been proposed and successfully used
to synthesize mesoporous MCM-41 molecular sieves [24,41-44].These
materials are normally synthesized by hydrothermal procedures and their
structures are obtained from amorphous inorganic silica walls around
surfactant molecules. It is now well documented in literature that the formation
of mesostructures is influenced by surfactant concentration, pH, presence of
co-surfactant, and its concentration and temperature [45-52].
Generally siliceous MCM-41 materials are synthesized from gels with
surfactant/silica molar ratio of more than 0.12 [45-48, 53-59] and involves
hydrothermal treatment of precursor gel in the temperature range between
60–150°C for a long time (1–6 days) in presence of quaternary ammonium
surfactants, CnH2n+1 (CH3)3N+, with different alkyl chain lengths (n = 8–18),
using sodium silicate or tetraalkylorthosilicate as sources of silica
[15,48,50,53-56]. Siliceous MCM-41has also been synthesized under
refluxing [53,57] and microwave irradiation [58]. Attempts have been made to
synthesize siliceous MCM-41 materials under ambient conditions [60-68].
Voegtlin et al. [64] have prepared highly ordered MCM-41 at room
temperature in 1 h; however, stability above 873 K has not been reported.
Different strategies have been employed to improve thermal and hydrothermal
stability of these materials, such as synthesis of materials with thicker pore
wall under hydrothermal condition by using low surfactant to silica molar ratio
in the range of 0.06 to 0.1 [56], by addition of salts in the synthesis gel before
or during hydrothermal crystallization [69-72] and/or intermittent pH
adjustment with acid during hydrothermal treatment [71,73] and using highly
condensed silica source such as fumed silica [74,75] and calcined MCM-41
silica [76]. Cheng et al [77] have synthesized MCM-41 using fumed silica at
438 K in 48 h with pore wall thickness of 2.68 nm and showed improved
thermal stability. Kumar et al [78] have synthesized MCM-41 analogue at
room temperature using hexadecylamine as templating agent that exhibited
improved stability. However, most of the approaches suffer process difficulties
Chapter 2. Synthesis and Characterization of MCM-41 based materials
62
in large scale preparation, as the synthesis involves longer crystallization time
at higher temperatures (100°C-150°C).
A highly ordered mesoporous siliceous MCM-41 has been synthesized
in, using a simple synthesis methodology at room temperature with surfactant
to silica ratio 0.1, using sodium silicate as silica source and CTAB as structure
directing agent, adding silicate solution to the surfactant solution at a
controlled rate, resulting in synthesis of a well crystalline MCM-41, with
improved thermal and hydrothermal stability at room temperature in less than
3 h [79].
Modified MCM-41 materials
As already mentioned in chapter I, MCM-41 based materials have
negligible catalytic activity due to framework neutrality. MCM-41 can be used
as solid acid catalyst by generation of acid properties in MCM-41 that
enhances its practicability and exhibit remarkable catalytic performance.
A purely siliceous framework is electronically neutral.When lattice Si4+
cations are replaced by Al3+cations,puresiliceous MCM-41 loses neutrality.
The negatively charged framework is balanced by Na+ ions present in the
system. In order to form acidic mesoporous materials, ion exchange with
ammonium nitrate is carried out, followed by thermal decomposition of the
NH4+ cations into protons and ammonia. The Brønsted acid sites are protons,
loosely attached to lattice oxygen atoms in the vicinity of aluminium. With
increased Al3+ incorporation in the MCM-41lattice the acidity increases. Thus,
by incorporation of Al3+ in the MCM-41 framework, material with inherent
acidity is obtained, generally entitled as Al-MCM-41.
Heteropoly acids (HPAs) have proved to be the alternative to traditional
mineral acid catalysts due to both strong acidity and appropriate redox
properties. However, limitations for HPAs to be used as solid acid catalysts
are low thermal stability, low surface area (1-10 m2/g) and difficulty in
separation from reaction mixture due to their high solubility in polar solvents.
For HPAs to be effective as catalysts, they should be supported on a carrier
with a large surface area. Owing to a very large surface area and a uniform
large pore size, the MCM-41 materials can act as excellent supports that
provide an opportunity for HPAs to be dispersed over a large surface area
Chapter 2. Synthesis and Characterization of MCM-41 based materials
63
and hence increased catalytic activity. Further, such mesoporous materials,
which have relatively small diffusion hindrance, can aid the easy diffusion of
bulky organic molecules in and out of their mesopores.Thus, heteropoly acids
(HPAs) supported onto MCM-41 by process of anchoring and calcination
yields material with induced acidity [80,81].
Characterization Methodologies - A Literature Survey
Characterization of MCM-41
As the most investigated member of the M41S family, MCM-41
provides an excellent example in characterizing mesoporous materials. MCM-
41 has a honeycomb structure that is the result of hexagonal packing of
unidimensional cylindrical pores. Reliable characterization of the porous
hexagonal structure requires the use of three independent techniques [82]: X-
ray diffraction (XRD), transmission electron microscopy (TEM) and adsorption
analysis.
The XRD pattern of MCM-41 shows typically three to five reflections
between 2=2° and 5° (Fig. 2.8), although samples with more reflections have
also been reported [83,84]. The reflections are due to the ordered hexagonal
array of parallel silica tubes and can be indexed, assuming a hexagonal unit
cell as (100), (110), (200), (210) and (300). Out of the XRD peaks exhibited in
the low angle region for mesoporous phase, the most intense peak is the
(100) reflection. The powder pattern is the finger print of the molecular sieve
structure and can be ascertained by comparing with the standard pattern for
the molecular sieves under investigation. Since the materials are not
crystalline at the atomiclevel, no reflections at higher angles are observed.
Moreover, these reflections would only bevery weak in any case, owing to the
strong decrease of the structure factor at high angles. By means of XRD it is
not possible to quantify the purity of the material. Samples with only one
distinct reflection have also been found to contain substantial amounts of
MCM-41. It is reported that even when the hexagonal pore structure contains
a large number of defects, a hexagonally indexable three-reflection pattern
can be calculated [85].
Transmission Electron Microscopy (TEM)is used to elucidate the
pore structure of mesoporous molecular sieves [14,22,35] It provides
Chapter 2. Synthesis and Characterization of MCM-41 based materials
64
topographic information of materials at near atomic resolution. However, the
exact analysis of pore sizes and thickness of the pore walls is very difficult
and not possible without additional simulations because of the ‘focus’
problem.Chen et al [86] have reported that the thickness of MCM-41 depends
strongly on the focus conditions, and careful modelling is necessary for
precise analysis.More than one model with a hexagonal array of large
cylindrical pores with thin walls gives a similar XRD pattern, but TEM gives a
direct, precise and simultaneous measurement of the pore diameter and pore
thickness. HRTEM can be successfully used to examine the microstructural
feature of mesoporous molecular sieves [87,88]. In addition to structural
characterization, it can also be used to detect the location of metal clusters
and heavy cations in the framework [88].Fig. 2.9 shows a TEM image of the
hexagonal arrangement of uniform, 4 nm sized pores in a sample of MCM-41.
Most MCM-41 samples not only show ordered regions but also disordered
regions, lamellar and fingerprint-like structures [87]. The existence of a
lamellar phase after calcination is unlikely, because silicate layers are too
distant from one another to preserve the spacing in the silicate organic phase
and collapse without additional post-treatments.
Scanning electron microscopy (SEM) is also used to study the
morphology of the material. It also gives an idea about the changes in shape,
size and surface that occur in a used material.
Adsorption of probe molecules has been widely used to determine the
surface area and to characterizethepore size and pore-size distribution of
MCM-41 type materials.
The Braunauer-Emmett-Teller (BET) volumetric gas adsorption
technique using nitrogen, argon, etc. is a standard method for the
determination of the surface areas and pore size/pore size distribution of
finely divided porous samples [89]. The relation between the amount
adsorbed and the equilibrium pressure of the gas at constant temperature is
defined by the adsorption isotherm.The physisorptionof gases such as N2, O2
and Ar has been studiedto characterize the porosity [90-93]. The nitrogen
adsorption isotherm for MCM-41 with pores of around 4.0 nm, which is type IV
in the IUPAC classification [94], shows two distinct features: a sharp capillary
Chapter 2. Synthesis and Characterization of MCM-41 based materials
65
condensation step at a relative pressure of 0.4 and no hysteresis between the
adsorption and desorption branches (Fig.2.10).The adsorption at very low
relative pressure, p/p0, is due to monolayer adsorption of N2 on the wallsof the
mesopores and does not represent the presence of any micropores
[95,96].The steep increase in N2 adsorption (within the p/p0 range between
0.2 to 0.4) corresponds to capillary condensation within uniform pores. The
sharpness and the height of this step reflects the uniformity of the pore size
and the pore volume respectively. However, in thecase of materials with pores
larger than 4.0 nm [97] or using O2 or Ar as adsorbate [91], the isotherm is still
type IV but also exhibits welldefined hysteresis loops. The presence and size
of the hysteresis loops depend on the adsorbate [91], pore size [97] and
temperature [98].
The traditional method for analyzing pore-size distributions in the
mesopore range is the Barrett–Joyner–Halenda (BJH) method [99,100] which
is based on the Kelvin equation and, thus, has a thermodynamic origin.
However, compared with new methods that rely on more localized
descriptions such as density functional theory (DFT) [96,101] and Monte Carlo
(MC) simulation [102], the thermodynamically based methods over estimate
the relative pressure at desorption and therefore underestimate the calculated
pore diameters by 1.0 nm. Moreover, the theoretical basis for the BJH
analysis becomes fairly weak if the step at 77 K lies below p/p0=0.42, because
this is considered to be the stability limit of the meniscus. Pore sizes
calculated in such cases are still probably in the right range, but a sound
theoretical foundation for such values is missing.
Fourier transform infrared spectroscopy (FTIR) yields information
concerning the structural details of a siliceous inorganic material [103,104]. In
addition, it can be used to confirm surface characteristics (such as acidity)
and isomorphous substitution by other elements in the material. The
technique allows to relate different materials by their common structural
features, such as a classification of zeolite structures.
Broad bands in the region ~3400 cm-1are assigned to –OH stretching
vibration of MCM-41 which could be associated to Si-OH and water vibrations,
confirming the presence of the silanol groups [80,105] or bridged hydroxyl
Chapter 2. Synthesis and Characterization of MCM-41 based materials
66
groups. Bands ~1650 cm-1are attributed to H-O-H bending vibration. A broad
band ~1300-1000 cm-1is assigned Si-O-Si asymmetric stretching mode.
Bands at 800cm-1and 458 cm-1are attributed to symmetric stretching vibration
and bending vibration (rocking mode) of Si-O-Si. The band at 960 cm-1is
assigned to the presence of Si-OH stretching vibration.
Fig.2.8X-ray diffraction pattern of high-quality calcined MCM-41 made by Huo and Margolese [46].
Fig. 2.9 TEM of MCM-41 featuring 4.0 nm sized pores, hexagonally arranged [85].
Fig. 2.10 Adsorption isotherm of nitrogen on MCM-41 with 4.0 nm pores at 77 K [91].
Thermal Analysisgives an idea about the thermal stability of the
material and the possible phase changes that occur during the thermal
treatment of the material. An understanding of the thermal behavior is of basic
importance for utilizing the material in various temperature ranges where it is
thermally stable.
Determining the quantity and strength of the acid sites is crucial to
understanding and predicting the performance of a catalyst.Temperature-
Chapter 2. Synthesis and Characterization of MCM-41 based materials
67
Programmed Desorption (TPD) is one of the most widely used and flexible
techniques for characterizing the acid sites on surfaces. Ammonia is a very
basic molecule which is capable of titrating weak acid sites, which may not
contribute to the activity of catalysts. The strongly polar adsorbed ammonia is
also capable of adsorbing additional ammonia from the gas phase.
The acidity of different materialscan be determined using the TPD of
ammonia. This method involves three steps. The sample is first degassed and
then saturated with a mixture of 5% NH3+He gas at 120 oC, when ammonia
gets chemisorbed on the acidic sites of the catalyst. After removal of any
physisorbed ammonia from the surface by purging He at 120oC for 30 min, the
temperature programmed desorption is carried out at a heating rate of 10 oC/
min. The desorbed gas concentration is continuously monitored and recorded
with temperature by a thermal conductivity detector (TCD). This
concentration-temperature plot is referred to as the TPD profile. The area
under the profile is proportional to the amount of gas desorbed. Acid sites with
varying acid strength differ in their heat of adsorption, which is reflected in the
TPD profile by way of a number of distinct peaks representing the acid sites of
the catalyst. The acidity is reported as ml/g.The area under the curve
indicates the amount of NH3 desorbed and hence the number of surface acid
sites. In general, amorphous materials exhibit broad desorption peaks
compared to crystalline ones. Though the crystalline materials show sharper
peaks indicating less number of acid sites, the desorption temperatures of
NH3 are high indicating strong acid sites. Since siliceous MCM-41 have a
neutral framework, sharp desorption bands are absent in the NH3-TPD
profiles indicative of negligible surface acidity.
Elemental analysis gives us an idea about the composition of the
catalyst.It is important to know the composition of a catalyst before
use.Instrumental methods used for elemental analysis are Flame photometry,
Atomic absorption spectroscopy (AAS) and Inductively coupled plasma-
atomic emission spectroscopy (ICP-AES) which are both popular as well as
accurate.ICP-AES is a widely used analytical technique for the determination
of elements present in a wide variety of samples. The technique is based on
atomic emission spectroscopy, and as the name suggests, plasma is used as
Chapter 2. Synthesis and Characterization of MCM-41 based materials
68
the source of excitation/ionization of atoms.The intensity obtained for each
sample is matched against a calibration plot prepared for the particular
element for quantitative estimation. The concentration of different elements is
actually measured at ppm level, which can be later converted into the %
weight of the element, by incorporating the dilution factor .These values are
then converted into moles of each element.
Energy dispersive X-ray analysis (EDX) is used for both identification
of an element as well as to have a rough estimate of the composition of the
materials. EDX is used in coordination with and as supportive analysis with
ICP-AES which is more accurate compared to EDX.
Diffuse Reflectance UV-visible spectroscopyis a technique that
measures the scattered light reflected from the surface of samples in the UV-
visible range (200-800 nm). For most of the isomorphously substituted
molecular sieves, transitions in the UV region (200-400) nm are of prime
interest. This spectroscopic technique is used to determine the coordination
state of transition metal ions substituted in the matrix of the molecular sieves,
involving ligand-to-metal charge transfer transitions at ~ 200- 220 nm.
Characterization of Al-MCM-41
Isomorphous substitution of a heteroatom in the framework of the
molecular sieves results in changes in the unit cell parameters and unit cell
volume. This is one of the ways to confirm isomorphous substitution. The
XRD diffraction patterns for Al-MCM-41 are shown in Fig.2.11. The patterns
illustrate the characteristics of a typical mesoporous MCM-41 structure. The
d100reflections of Al-MCM-41 have been shifted to higher values compared to
its as-synthesized analogue.This is in agreement with Borade and Clearfield
[106], suggesting the framework substitution of Al3+ in MCM-41 structure.
The FTIR spectrum of MCM-41 has already been discussed earlier in
the text. It is observed that there is no significant change in the FTIR bands of
Al-MCM-41 compared to silicon MCM-41. The band at 960 cm-1is assigned to
the presence of Si-OH stretching vibration as well as metal ion substituted
MCM-41. A slight shift in band positions is observed due to ismorphous
substitution of metal ion for Si4+.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
69
For the Al3+ incorporated MCM-41, sharp desorption bands are
observed in the NH3-TPD profiles indicating presence of surface acidity,
combined with increase in surface acidity, with increase in Al3+ content.
. Fig.2.11 XRD pattern of the materials: (a) Al-MCM-41(25), (b) Al-MCM-41(50), (c) Al-MCM-41(75) and (d) Al-MCM-41(100) [80]
Characterization of Heteropoly acid (HPA) supported MCM-41
Heteropoly acid supported onto MCM-41have been prepared and
characterized exhaustively [80,105]. The three main reliable techniques used
for characterization of these materials are XRD, FTIR and surface properties-
pore size, pore size distribution, surface area and surface acidity which
ensure the anchoring of HPA’s onto MCM-41, wherein comparisons are made
between pure HPA’s, pure MCM-41 and a combination of these.
The XRD patterns of a typical mesoporous MCM-41 structure have
been discussed earlier in the text, where importance of the d100 reflection has
been indicated. Fig.2.12 illustrates the effect of the HPA loading on the XRD
of MCM-41 samples. HPA has a striking effect on the width and intensity of
the main reflection at high d100 spacing and this line becomes broader and
weaker as the loading increases. This suggests that the long-range order of
Si-MCM-41is decreased noticeably by the presence of HPA. MCM-41
presentsthe highest surface area and pore volume, with all pores being in the
mesopore range. The pore size distribution of MCM-41 shows a unique peak
centered at about 25Å diameters as given in the literature [107]. With
increasing HPA loading, a reduction in surface area, pore volume and a
notable compression of the pore size distribution and increase in surface
Chapter 2. Synthesis and Characterization of MCM-41 based materials
70
acidity are observed compared to MCM-41 (Table 2.2). Table 2.2 presents
changes in textural properties with increase in HPA wt. %.
Table 2.2Textural properties of various wt.%. HPA loaded materials[105]
.Catalysts d100
(Å)
Unit cell
a0 (nm)
Surface
area
(m2/g)
Pore size
BJHAds
(nm)
Pore
volume
BJHAds
(cc/g)
Si-MCM-41 44.21 5.10 938 2.60 0.60
10wt.% 12-TPA-MCM-41 42.15 4.86 526 2.55 0.32
15wt.% 12-TPA-MCM-41 38.45 4.44 265 2.40 0.12
20wt.% 12-TPA-MCM-41 35.37 4.08 235 1.90 0.11
Fig.2.12 XRD pattern of the materials (a) Si-MCM-41, (b) 10wt.%12-TPA-MCM-41, (c) 15wt.%12-TPA-MCM-41, (d) 20wt.%12-TPA-MCM-41 [80].
Fig.2.13FTIR spectra of mesoporous materials: (a) Al-MCM-41(25), (b) Al-MCM-41(50), (c) Al-MCM-41(75), (d) Al-MCM-41(100), (e) 10wt.% 12-TPA-MCM-41, (f) 15wt.% 12-TPA -MCM-41, (g) 20wt.% 12-TPA-MCM-41 and (h) 12-TPA[80].
The FTIR spectra of HPA supported onto MCM-41 are given in Fig
2.13. A broad band due to –OH stretch of water in ~3400cm-1region and the
corresponding –OH2 bending mode around ~1637 cm-1 very well correlate
with the water adsorption property (hydrophilic property) of the catalysts. Pure
HPA spectra with a Keggin structure with four strong bands, at 1082 cm-1 (P-
O), 988 cm-1 (W=O) and 800 cm-1 (W-O-W), and a weak band at 525 cm-1 (W-
O-P) [108] are observed. The framework bands of Si-MCM-41 at 1236, 1090,
965, 800, 564 and 465 cm-1[109] easily overlap with those of HPA i.e. 12-
tungsto phosphoric acid (12-TPA). For 10 wt.%12-TPA-MCM-41, none of the
12-TPA bands are observed, except for a slight increase in the intensity of the
Chapter 2. Synthesis and Characterization of MCM-41 based materials
71
800 cm-1 band. For 15 wt.%12-TPA-MCM-41, the bands at 988 and 891 cm-1
become visible. With 20 wt.% of the 12-TPA loading, the 1082 cm-1 band
become sharper and the intensity of the two bands at 988 and 891 cm-1
increases. Furthermore, it is observed that the intensity of 800 cm-1 band is
almost proportional to the increase in the amount of 12-TPA.The final and
most important evidence is the increase in surface acidity with increased 12-
TPA loading.
2.10 EXPERIMENTAL
Materials: Commercial grade sodium silicate (Na2SiO3) with composition
28%SiO2 and 7.5% Na2O was procured from Sapna Chemicals, Vadodara.
CTABr, CPBr, sodium hydroxide flakes, aluminium sulfate, ammonium nitrate
and 12-tungstophosphoric acid (12-TPA) were purchased from Loba
Chemicals, Mumbai. Tetra ethylortho silicate (TEOS) and analytical grade
sulphuric acid were obtained from E.Merck, Mumbai. All other chemicals and
reagents used were of analytical grade. Double distilled water(DDW) was
used for all studies
Synthesis of MCM-41:In the present endeavor, the objective is to synthesize
mesoporous MCM-41 at room temperature with good thermal stability and
high surface area as well as retention of surface area at high temperature. A
sol–gel method has been used to synthesize MCM-41 to achieve this
objective. Several sets of materials were prepared varying conditions in each
case, using surface area as an indicative tool in all cases. Table 2.3 describes
the parameters that have been optimized for synthesis of MCM-41.
Synthesis of MCM-41 at optimized condition
As indicated in table-2.3 entry No.-8 indicates optimum condition. We
hereby describe synthesis of MCM-41 under optimized conditions. The molar
gel composition for MCM-41 is 1 SiO2:0.5 CTABr:0.25 Na2O:80 H2O. In a
typical 500g batch size experiment, 63.02 g of Na2SiO3was mixed with 183
gDDW under continuous stirring at room temperature for~15 min. in a
polypropylene container (A). An aqueous solution of CTABr was prepared by
dissolving 54.14 gm CTABr in 200 gm DDW under continuous stirring at room
temperature (B). Template solution B was added to a precursor solution A
dropwise with continuous stirring within ~15 min. and the solution further
Chapter 2. Synthesis and Characterization of MCM-41 based materials
72
stirred for 15 min. The pH of the resultant solution was adjusted to 10.5 using
1:1 H2SO4(diluted 1:1V/V). A gel was obtained at this stage which was stirred
further for 30 min. The polypropylene container was now closed and allowed
to age at room temperature without stirring for 24 h. The resultant gel was
filtered, washed with DDW to remove adhering ions and dried at 120oC,
followed by calcination at 550oC for 6 h at a heating rate of 2oC/min. The final
material obtained was used for all further studies.
Note:In the present synthetic endeavor Na2SiO3 as a silica source is
preferred to TEOS, reasons being higher cost of TEOS. Further, the thermal
stability of final material obtained is better when Na2SiO3 is used as silica
source compared to TEOS. While preparation it is preferable to add template
to silica source. The pH of template being almost neutral and pH of silica
being >12, if template is added to silica source, the pH variation window is
narrowed down due to which pH of gel formation is easily adjusted. The pH in
the synthesis was adjusted to 10.5 because gel viscosity is maximum at this
pH, which can also be stirred with ease for homogenization. Finally at the
optimized condition, surface area retention between 550 - 900 oC is fairly
good.
Synthesis of Al-MCM-41
In the present synthetic endeavor the objective is to synthesize
mesoporous Al-MCM-41 at room temperature with good thermal stability and
high surface acidity. A sol-gel method has been used to achieve this
objective. Several sets of materials were prepared varying silica to alumina
ratios, apart from other conditions, where surface acidity has been used as an
indicative tool in all cases. Table 2.4 describes parameters that have been
optimized for synthesis of Al-MCM-41.
Synthesis of Al-MCM-41 at optimized condition
As indicated in optimization table 2.4 entry No-4 indicates optimum
conditions. We hereby describe synthesis of Al-MCM-41 under optimized
conditions. The molar gel composition of Al-MCM-41 is
1SiO2:0.033Al2O3:0.4CTABr:0.25Na2O:90H2O. In a typical 500g batch
experiment, the first step was preparation of the precursor solution. 57.48 g of
Na2SiO3was mixed with 197.5 g DDW under continuous stirring at room
Chapter 2. Synthesis and Characterization of MCM-41 based materials
73
temperature for ~15 min, in a polypropylene container, to which was added an
aqueous solution of aluminium sulfate (prepared by dissolving 5.643 g
aluminium sulfate in 40 g DDW) dropwise and with constant stirring within ~15
min. This is the precursor solution (A). An aqueous solution of CTABr was
prepared by dissolving 39.5 g CTABr in 160 g DDW under continuous stirring
at room temperature (B). Template solution (B) was added to precursor
solution (A), dropwise and under constant stirring within ~15 min. The pH of
the resultant solution was adjusted to ~10.5 using 1:1 H2SO4 (diluted 1:1V/V).
A gel was formed which was further stirred for 30 min. The polypropylene
container was now closed and allowed to age at room temperature without
stirring for 24 hrs. The resultant gel was filtered, washed with DDW to remove
adhering ions and dried at 120°C followed by calcination at 550°C for 6h, at a
heating rate of 2°C/min. After thermal treatment, the samples with various
Silica : Alumina ratios Al-MCM-41-20,Al-MCM-41-30,Al-MCM-41-50, and Al-
MCM-41-100 were subjected to ion exchange by treating them with aqueous
1.0%NH4NO3 solution under continuous stirring for 3h, followed by
calcination at 550◦C for 3h at a heating rate of 2°C/min. in air flow.As indicated
in Table 2.5 the sample Al-MCM-41-30 exhibits maximum surface acidity and
was used for all further studies.
Note: In the present synthetic endeavor Na2SiO3 as a silica source is
preferred to TEOS, reasons being higher cost of TEOS, thermal stability of
final material obtained is better, and excess negative charge developed in the
framework due to substitution of Si4+ by Al3+, requires Na+ for charge balance.
While preparation, it is preferable to add template to precursor source. The pH
of template being almost neutral and pH of precursor being >12, if template is
added to precursor source, the pH variation window is narrowed down due to
which pH of gel formation is easily adjusted. The pH in the synthesis was
adjusted to 10.5 because gel viscosity is maximum at this pH, which can also
be stirred with ease for homogenization.
Further, lower the SiO2:Al2O3 ratio, higher will be Al3+ content and
hence better is the acidity generated in the resulting materials. At the
optimized condition surface acidity is high as well as surface area retention
between 550 - 900 oC is fairly good.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
74
Table 2.3MCM-41 Synthesis Strategies - Parameters optimized
Parameters No SiO2
source Template
source Template
mole H2O mole
Temp. (°C)
Aging Time (h)
pH
BET Surface area at different Temperature
( m2/g)
550°C 700°C 900°C
Aging time
1 Na2SiO3 CTABr 0.25 80 RT 1 10.5 640 490 370
2 Na2SiO3 CTABr 0.25 80 RT 3 10.5 770 510 342
3 Na2SiO3 CTABr 0.25 80 RT 6 10.5 820 550 446
4 Na2SiO3 CTABr 0.25 80 RT 18 10.5 900 570 428
5 Na2SiO3 CTABr 0.25 80 RT 24 10.5 917 612 433
Temperature 6 Na2SiO3 CTABr 0.25 80 70 24 10.5 864 735 460
7 Na2SiO3 CTABr 0.25 80 100 24 10.5 974 820 300
Template mole 8 Na2SiO3 CTABr 0.5 80 RT 24 10.5 1136 1121 805
9 TEOS CTABr 0.5 40 RT 24 9.5 1126 1116 706
Template source 10 Na2SiO3 CPBr 0.5 80 RT 24 10.5 736 593 483 SiO2 mole = 1 mole; RT= Room Temperature (30±3 °C); % SiO2 = 99.89 (ICP-AES).
Table 2.4Al-MCM-41 Synthesis Strategies - Parameters optimized
Parameters No SiO2 mole
Al2O3 mole
SiO2 /Al2O3 Input mole ratio
Template mole
Element Analysis (ICP-AES)
SiO2 /Al2O3 Output mole ratio
*Total Acidity (ml/g)
BET Surface area at different Temperature
(m2/g)
%SiO2 %Al2O3 550°C 700°C 900°C
Template mole
1 1 0.01 100 0.5 98.59 1.35 124.15 0.86 1191 1028 554
2 1 0.01 100 0.4 98.22 1.55 107.70 1.14 1220 1044 577
SiO2/Al2O3 Mole ratio
3 1 0.02 50 0.4 93.31 3.05 52.01 1.74 884 730 600
4 1 0.033 30 0.4 94.90 4.90 32.92 3.03 580 510 470
5 1 0.05 20 0.4 89.42 6.64 22.89 2.60 500 438 280 SiO2 Source = Na2SiO3 ; Al2O3 source = Al2(SO4)3 ; Template Source = CTABr; H2O mole = 90; RT = (30±3 °C); pH = 10.5; Aging Time = 24 h
*Details in Table 2.5.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
75
Synthesis of 12-TPA supported MCM-41
The aim was to load different wt. % of 12-TPA onto MCM-41 and
induce acidity into the material, using surface acidity as an indicative tool in all
cases. Four samples of 12-TPA supported MCM-41 catalysts were prepared
with varying 12-TPA loading (10-40 wt.%). In a typical setup (10% 12-TPA
loading) 1g of 12-TPA was dissolved in 100 ml DDW, to which was added 9 g
of MCM-41 synthesized as described earlier in the text, and the resultant
slurry stirred continuously for 24h at room temperature. The excess solution
was removed under vacuum, dried and subsequently calcined at 350 ◦C for 2h
at a heating rate of 2◦C/min. Table 2.6 shows that 20%12-TPA loaded sample
exhibits maximum surface acidity and used for all studies.
2.11 MATERIAL CHARACTERIZATION
The synthesized materials in the present study MCM-41, Al-MCM-41
and 12-TPA supported onto MCM-41 were subjected toinstrumental methods
of analysis/characterization. Al-MCM-41 with SiO2:Al2O3 = 30, abbreviated as
Al-MCM-41-30 and 12-TPA supported onto MCM-41 with 12-TPA loading =
20 wt.%, abbreviated as 12TPA-MCM-41-20 have been used for
characterization, as they exhibit highest surface acidity and used for all
catalytic studies.
Instrumental Methods of Analysis
Elemental analysis was performed on ICP-AES spectrometer (Thermo
Scientific iCAP 6000 series).X-ray diffractogram(2= 1° - 40°)was obtained on
X-ray diffractometer (Bruker D8 Focus) with Cu-Kα radiation with nickel
filter.FTIR spectra was recorded using KBr pellet on Shimadzu (Model
8400S). Thermal analysis (TGA) was performed on a Shimadzu (Model TGA
50) thermal analyzer at a heating rate of 10 ºC·min-1. SEM and EDX of the
sample were scanned on Jeol JSM-5610-SLV scanning electron microscope.
TEM was performed using Philips CM30 ST electron microscope operated at
300kv. Surface area measurements weredetermined using Micromeritics
Gemini at -196oC using nitrogen adsorption isotherms. Surface acidity was
determined by NH3-TPD method using Micromeritics Chemisorb 2720. UV-
VIS.-diffuse reflectance spectra was obtained using Shimadzu(Model UV-
DRS 2450).
Chapter 2. Synthesis and Characterization of MCM-41 based materials
76
Characterization of MCM-41
The XRD of MCM-41 is presented in (Fig. 2.14). A peak for 2θ between
2º and 3º is observed which is characteristic of the Bragg plane reflection
(100). This is evidence for MCM-41 structure [80,83,84,106].
A TEM image (Fig.2.15) shows hexagonal arrangement of uniform, ~3
nm sized pores indicative of MCM-41 structure.SEM (Fig.2.16) exhibits
irregular morphology indicating amorphous nature of material. Elemental
analysis performed by ICP-AES shows %SiO2 to be 99.89 (Table 2.3), which
is also well supported by EDX (Fig.2.17) which shows atomic % of Si = 33.33
and atomic % of O = 66.67.
Surface areas (ABET) determined by N2adsorption BET method, exhibits
isotherms of type IV, in accordance with the IUPAC classification for
mesoporous materials [94] (Fig.2.18 and Fig.2.19). Pore diameters (~3 nm)
confirm the mesoporous nature of the synthesized materials with pore size
distribution between 2-6 nm which are in the range usually observed for
MCM-41 samples [110]. Surface area of MCM-41 calculated by BET method
is 1136 m2/gat 550 ◦C,1121m2/g at 700◦C and805 m2/gat900°C.
The FTIR spectrum presented in Fig. 2.20 exhibits presence of broad
bands in the region ~3400 cm-1 assigned to –OH stretching vibration of MCM-
41 which could be associated to Si-OH and water vibrations. Bands ~1630
cm-1are attributed to H-O-H bending vibration. A broad band ~1260-1000 cm-
1is assigned Si-O-Si asymmetric stretching mode. Bands at 800 cm-1and 450
cm-1are attributed to symmetric stretching vibration and bending vibration
(rocking mode) of Si-O-Si. The band at 960 cm-1is assigned to the presence of
Si-O-H stretching vibration.
TGA thermogram (Fig. 2.21) exhibits an initial weight loss of ~10%in
the temperature range 30-100 ◦C due to loss of moisture and hydrated water.
Thereafter in the region 200-600◦C there is a marginal/negligible weight loss
which indicates fairly stable nature of the materials.
Absence of sharp desorption bands as well as negligible acidity in NH3-
TPD profiles (Fig.2.22) is indicative of a neutral MCM-41 framework.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
77
Fig. 2.14 XRD of MCM-41
Fig. 2.15TEM of MCM-41
Fig. 2.16SEM of MCM-41
Fig. 2.17EDX of MCM-41
Fig. 2.18Nitrogen adsorption isotherm of MCM-
41
Fig. 2.19Pore distribution of MCM-41
Element Wt.% At. % Comp. %
Si 46.74 33.33 100.00
O 53.26 66.67
Totals 100
Formula SiO2
Chapter 2. Synthesis and Characterization of MCM-41 based materials
78
Fig. 2.20FTIR of MCM-41
Fig. 2.21 TGA of MCM-41
Fig. 2.22Ammonia TPD of MCM-41
Characterization of Al-MCM-41-30
XRD of Al-MCM-41 is presented in Fig. 2.23. It is observed that there is
shift in position of the peak characteristic of the Bragg plane reflection (100)
for 2θ between 2º and 3º positionindicating incorporation of Al3+ in the
framework of siliceous MCM-41. Further, with increase in SiO2:Al2O3 ratio
there is further shift in band position towards higher 2θ value.
TEM image of Al-MCM-41-30 (Fig.2.24) exhibits a well-defined
hexagonal arrangement with a fairly uniform pore structure. SEM (Fig.2.25) of
Al-MCM-41-30 exhibits irregular morphology. Elemental analysis performed
by ICP-AES shows %SiO2and %Al2O3to be 94.90 and 4.90 respectively(Table
Chapter 2. Synthesis and Characterization of MCM-41 based materials
79
2.4), which is also well supported by EDX (Fig.2.26) which shows atomic % of
Al = 1.73, atomic % of Si = 31.89 and atomic % of O = 66.38.
Surface areas (ABET) determined by N2adsorption BET method exhibits
isotherms of type IV, in accordance with the IUPAC classification for
mesoporous materials [94] (Fig.2.27 and 2.28). Pore diameters (~3.5 nm)
confirm the mesoporous nature of the synthesized material with pore size
distribution between 2.2-5 nm which are in the range usually observed for Al-
MCM-41 [110]. Surface area of Al-MCM-41-30 calculated by BET method is
580 m2/gat 550◦C,510 m2/g at 700◦C and 470 m2/gat900◦C (Table 2.3).
The FTIR spectrum presented in Fig. 2.29 exhibits of broad bands in
the region ~3400 cm-1 assigned to –OH stretching vibration of MCM-41 which
could be attributed to Si-OH and water vibrations. Bands ~1650 cm-1are
attributed to H-O-H bending vibration. A broad band ~1260-1000 cm-1is
assigned Si-O-Si asymmetric stretching mode. Bands at 795 cm-1and 450 cm-
1are attributed to symmetric stretching vibration and bending vibration (rocking
mode) of Si-O-Si. The band at 960 cm-1is assigned to the presence of Si-O-Al
stretching vibration.
TGA thermogram (Fig. 2.30) exhibits an initial weight loss of ~20%in
the temperature range 30-100 ◦C due to loss of moisture and hydrated water.
Thereafter in the region 200-600◦C there is a marginal/negligible weight loss
which indicates fairly stable nature of the material.
NH3-TPD patterns of Al-MCM-41 with varying SiO2:Al2O3ratios are
presented in Fig.2.31. Al-MCM-41-30 exhibits highest surface acidity (table-
2.5). Surface acidity of samples is presented in Table 2.5.
Fig. 2.32 presents UV-VIS-DRS of Al-MCM-41 with varying SiO2:Al2O3
ratio. Rise in intensity of band ~ 210 nm region is observed with increasing
Al3+ incorporation. The incorporation of Al3+ in the silica framework is further
evident from surface acidity values and elemental analysis results (Table 2.5).
Chapter 2. Synthesis and Characterization of MCM-41 based materials
80
Fig.2.23 XRD of Al-MCM-41-30
Fig. 2.24TEM of Al-MCM-41-30
Fig.2.25 SEM of Al-MCM-41-30
Fig. 2.26EDX of Al-MCM-41-30
Fig.2.27Nitrogen adsorption isotherm of Al-
MCM-41-30
Fig.2.28 Pore distribution of Al-MCM-41-30
Element Wt.% At. % Comp. %
Al 2.33 1.73 4.41
Si 44.69 31.89 95.59
O 52.98 66.38
Totals 100
Formula SiO2 and Al2O3
Chapter 2. Synthesis and Characterization of MCM-41 based materials
81
Fig.2.29FTIR of Al-MCM-41-30
Fig.2.30 TGA of Al-MCM-41-30
Fig 2.31 Ammonia TPD of Al-MCM-41
Fig 2.32UV-DRS of Al-MCM-41
Characterization of 12-TPA-MCM-41
FTIR spectra of HPA supported onto MCM-41 have been detailed
earlier in the text. The FTIR spectra (Fig. 2.33)with different 12-TPA loading
coincide well with those observed in literature [105,106,108].The surface
acidity increases gradually with increasing 12-TPA loading from 10 to 20
wt.%, after which a decrease is observed (Table 2.6 ).
SEM image of 12TPA-MCM-41-20 (Fig.2.34) exhibits irregular
morphology. EDX of 12TPA-MCM-41-20, presented in Fig.2.35 shows the
presence of W as atomic % = 2.20, Si atomic % = 30.44 and O atomic % =
67.40. Absence of P in EDX is probably due very low % of P in the original
compound, which is probably not detected in EDX due to instrument
limitations.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
82
Fig. 2.33 FT-IR of 12-TPA-MCM-41
Fig. 2.34SEM of12-TPA-MCM-41-20
Fig. 2.35EDX of12-TPA-MCM-41-20
Fig. 2.36Ammonia TPD of 12-TPA-MCM-41
Fig. 2.37XRD of 12-TPA-MCM-41
A=MCM 41, b=10% TPA-MCM-41 c=20%, d=30%, e=40% , f=12-TPA
Element Wt.% At. % Comp. %
W 17.30 2.20 21.81
Si 36.55 30.44 78.19
O 46.15 67.40
Totals 100
Formula SiO2 and WO3
Chapter 2. Synthesis and Characterization of MCM-41 based materials
83
Comparative TPD pattern of MCM 41 samples with different 12-TPA
loading (Fig.2.36) exhibits two distinct peaks at 200oC and 600oC indicating
presence of medium and strong acid sites respectively in all the
samples.Surface acidity of samples is presented in Table 2.6.
XRD of different wt% loading of 12-TPA-MCM-41 are presented in Fig.
2.37. With reference to MCM-41 (a) and 12-TPA (f) b, c, d, e shows that 12-
TPA is loaded onto MCM-41. e with the highest loading shows that the 12-
TPA remains on the surface and the X-ray pattern is close towards (f). The
observed results coincide well with those observed in literature [105,106].
Table2.5.Surface acidity of Al-MCM-41 with various SiO2:Al2O3 ratios
Materials Pore
volume (cc/g)
Surface acidity (ml/g) Total
Acidity (ml/g)
Weak acid Strong acid
Temp. (0C)
Volume (ml/g)
Temp. (0C)
Volume (ml/g)
Al-MCM-41-20 0.44 203 1.59 287 1.02 2.60
Al-MCM-41-30 0.27 207 2.91 287 0.12 3.03
Al-MCM-41-50 0.60 204 1.65 304 0.09 1.74
Al-MCM-41-100 0.91 201 1.09 311 0.05 1.14
Table 2.6.Surface acidity of different wt.% 12TPA loaded onto MCM-41
Materials
Surface acidity (ml/g)
Total Acidity (ml/g)
Weak Acid Strong Acid
Temp. (°C)
Volume (ml/g)
Temp. (°C)
Volume (ml/g)
Siliceous MCM-41 171 0.15 470 0.06 0.21
10% 12-TPA-MCM-41 209 1.67 651 3.74 5.41
20% 12-TPA-MCM-41 201 1.60 662 5.78 7.39
30% 12-TPA-MCM-41 200 2.40 609 2.90 5.30
40% 12-TPA-MCM-41 188 2.50 620 2.50 5.00
2.12 APPLICATION OF Al-MCM-41 AND 12-TPA-MCM-41 AS
SOLID ACID CATALYSTS
In chapter I, the importance of Green Chemistry, 12 principles of Green
Chemistry, and how Green Chemistry goals can be achieved through
catalysis has been discussed in details. Further, solid acid catalyst as an
Chapter 2. Synthesis and Characterization of MCM-41 based materials
84
alternative approach to liquid acid catalyst and its advantages over liquid acid
catalyst and important materials used as solid acid catalysts for various
organic transformations has been discussed. Siliceous MCM-41 has
poor/negligible catalytic activity due to framework neutrality, however with
high thermal stability and surface area. Thus, the aim of the present work was
to induce catalytic properties into MCM-41 and encash the advantageous
properties such as thermal stability and surface area, and put the resulting
material to practical use. The answers to these questions are emergence of
inherent acidity in Al-MCM-41 and induced acidity in 12TPA-MCM-41. Theory,
synthesis and characterization of these materials are well described in the
preceding pages in the text of this chapter (Section 2.10 and 2.11). In the
present study the utility of Al-MCM-41 and 12TPA-MCM-41 as solid acid
catalysts using Esterification and Friedel-Crafts alkylation and acylation as
model reactions has been explored.
2.13 ESTERIFICATION
Esterification is a widely employed reaction in the organic process
industry. Esters fall under a very wide category, ranging from aliphatic to
aromatic with various substitutions and multifunctional groups,organic esters
being valuable intermediates in the chemical industry. Esters are mostly used
as plasticizers, solvents, perfumery and flavour chemicals, and also as
precursors to many pharmaceuticals, agrochemicals and fine chemicals.
Esters are carboxylic acid derivatives with the general formula R-
COOR’ (R,R’=H, alkyl or aryl). When a carboxylic acid is treated with a large
excess of an alcohol, in presence of an acid catalyst, an ester is formed. The
reaction is called acid catalyzed esterification or sometimes Fischer
esterification after the great German chemist Emil Fischer.The esterification
process introduced by E.Fischer and A. Speier(1895) consists in refluxing
acid and excess methanol or ethanol in presence of about 3% hydrogen
chloride. The reaction is represented as follows.
RCOOH + R'OHH+
RCOOR'+ H2O (2.3)
The conventional esterification is an equilibrium reaction. For the
stoichiometric mixture of acid and alcohol, equilibrium generally reaches
Chapter 2. Synthesis and Characterization of MCM-41 based materials
85
~68% [111] of conversion for the straight chain saturated alcohol. In order to
obtain maximum yields Le Chatlier’s principle, is followed and the reaction is
driven to the right hand side/forward direction, as follows:
Addition of one of the reactants in excess:The reaction usually reaches a
point of equilibrium at ~60% conversion, but in a small scale experiment a
conversion of 60-80% can be achieved by use of a large excess of either acid
or alcohol.
Removal of one of the products:Either ester or water formed is removed as
soon as it is formed. Generally, suitable organic solvents are employed to
remove the water formed during the reaction as a binary azeotrope or by
employment of dehydrating agent such as anhydrous magnesium sulfate or
molecular sieves [112].
Synthetic routes to monoesters and diesters
Monoesters are typically synthesized by [113,114] (1) Solvolytic
reactions (2) Condensation reactions (3) Free radical processes (4)
Miscellaneous processes. The synthesis of mono esters can be presented as
shown in equation 2.1.
Diesters are prepared in two stages (Scheme 2.2) [115] e.g. reaction
between phthalic anhydride with alcohol. The first stage is very rapid and can
be carried out in the absence of a catalyst. However, esterification of the
second carboxylic group is very slow and needs to be facilitated by an acid
catalyst, resulting in the formation of water as a byproduct. The reaction is an
equilibrium one and hence to facilitate it in the forward direction, the water
molecule must be removed by azeotrope formation. The current commercial
process is a batch method which is very efficient with respect to its feed
stocks. Conversion (based on phthalic anhydride) and selectivity can reach
99.2 and 99.8 %, respectively. To reach this high conversion, a 20 % excess
of alcohol is used. The excess is recovered after reaction by a steam stripping
process. Hardly any purification is required after synthesis because of the
high selectivity, usually only a decolorization is carried out. A typical byproduct
is the dialkyl ether formed by the condensation of two molecules of alcohol.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
86
First step
+ ROH
O
O
OHeat
C
C
O
O
OR
OH
Second step
C
C
O
O
OR
OH
+ ROH Heat
Solvent, Solid acid
C
C
O
O
OR
OR
+ H2O
Phthalic anhydrideMonoester
Monoester Diester
R = 2-ethyl hexanol
Scheme 2.2Schematic presentation of diester formation
Problems / limitations in esterification
For the preparation of perfumery and flavour grade esters, only a few
of the above mentioned routes can be considered, due to the stringent
specifications of the final product. Normally, liquid phase catalysts such as
sulphuric acid, p-toluene sulfonic acid, methanesulfonic acid, hydrochloric
acid, phosphoric acid etc. have been used, that are cited as potential
environmentally hazardous chemicals that pose problems such as difficulty in
handling, causing acidic waste water, difficulty of catalyst recovery etc.
[116,117]. These catalysts are known to colour the product and cannot be
reused. As already mentioned, these liquid acids have several disadvantages.
Due to these problems, accompanied by the increasing environmental
awareness, there is a global effort to replace the conventional liquid acids by
suitable solid acids. The most widely employed and supposedly cleaner
production technique for such esters, involves the reaction of the appropriate
carboxylic acid with an alcohol using a heterogeneous catalyst such as solid
acid catalyst under reflux conditions, followed by separation of the ester by
distillation.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
87
2.14 LITERATURE SURVEY IN THE CURRENT AREA OF
STUDY
Monoesters using various solid acid catalysts
Cation exchange resins Dowex 50W and Amberlite IR-120 [118,119]
have been used as solid acid catalysts in the esterification of acetic acid with
isobutanol. Esterification of acrylic and lactic acids with butanol using
Amberlyst-15 [120, 121] and lactic and salicylic acids with methanol using
Dowex 50W resin as solid acid catalysts [122,123] has been reported. Salmi
et al [124] have studied methyl acetate formation on new polyolefin supported
sulfonic acid catalysts. Meunier has reported esterification reactions using
Nafion as solid acid catalyst [125]. Kaolinite [126] as well as montmorillonite
[127] clay has been used as catalyst in the esterification of carboxylic acids.
Giovanni Sartori has written an excellent review on clay catalyst for
monoesterification reaction [128].Manohar et al [129] have reported
esterification of acetic acid and benzoic acid using ZrO2 and Mo-ZrO2 as solid
acid catalysts and found that Mo-ZrO2 exhibits better catalytic activity than
ZrO2.Vishwanathan et al.[130] have reported esterification by solid acid
catalysts including clays, zeolites, sulphated metal oxides and
heteropolyacids.Toor et al.[131] have reported kinetic study of esterification of
acetic acid with n-butanol and isobutanol catalyzed by ion exchange
resin.Silicotungstic acid supported zirconia is reported as an effective catalyst
for esterification reactions using formic, acetic, propionic, n-butyricacid and n-
butyl alcohol, isobutyl alcohol and sec-butyl alcohol [132]. Chu et al [133]
have reported the vapour phase synthesis of ethyl and butyl acetate by
immobilized dodecatungstosilicic acid on activated carbon. The rate of
esterification was found to be dependent on the partial pressure of the
reactants. Dupont et al [134] have reported heteropolyacids supported on
activated carbon as catalysts for the esterification of acrylic acid by butanol.
Deactivation of the catalyst was observed under flow conditions (from 43 to
32% conversion) and was due to the dissolution of the supported HPA in the
reaction medium (25 %). Timofeeva et al [135] have reported esterification of
acetic acid and n-butyl alcohol using Keggin and Dawson type HPAs and
found that the reaction rate depends on the acidity, as well as on the structure
Chapter 2. Synthesis and Characterization of MCM-41 based materials
88
and composition of HPAs. The Dawson type heteropoly acids exhibited higher
activity compared to the Keggin type HPAs. In the esterification reaction of
acetic acid and n-butyl alcohol, the catalytic activity of HPAs has a good
correlation with the dissociation constant of HPAs. 12-TPA supported on
hydrous zirconia was used as solid acid catalyst in esterification of primary
and secondary alcohols [136]. Sharath et al studied benzyl acetate formation
in the presence of zeolites and their ion exchanged forms. They reported
reasonably good yield with 100 % selectivity [137]. Ma et al have reported the
synthesis of ethyl, butyl and benzyl acetates with high yields using
zeolitecatalyst [138]. From our laboratory, TMA salts have been widely
investigated as solid acid catalysts for synthesis of monoesters such as ethyl
acetate (EA), propyl acetate (PA), butyl acetate (BA) and benzyl acetate
(BzA) [139-146].
Diestersusing various solid acid catalysts
Suter has reported a noncatalytic process for the manufacture of DOP,
at very high temperatures, at which autocatalysis occurs [147]. Bekkum and
Schwegler investigated the use of HPAs (homogeneous and carbon
supported) for DOP synthesis [148]. They obtained a superior activity at low
temperatures in both homogeneous and supported form. Yadav et al [149]
have reported the use of solid super acids (sulfated and HPA supported onto
oxides) for the synthesis of DOP. They have reported a selectivity > 99 % and
demonstrated that selection of optimum calcination temperature is a must for
the optimum yield. Yadav et al[150] also have reported esterification of maleic
acid with ethanol over cation-exchange resin catalysts.G Lu [151] also
investigated DOP synthesis over solid superacids SO42-/Ti-M-O (M = Al, Fe,
Sn). They obtained superior activity in case of SO42-/Ti-Al-Sn-O system and
found that acid strength, surface area and catalytic activity of the system is
affected by the preparation conditions. Ma et al [152] studied the synthesis of
DOP using ZSM-5 and HY zeolites. Z H Zhao [153] has also reported the use
of aluminophosphate and silicoaluminophosphate molecular sieves as solid
acid catalyst for the synthesis of DOP. Amini et al [115] have reported the use
of heteropoly acids for the production of DOP and DBP. DEM synthesis has
been reported by Reddy et al [154] using montmorillonite clay, but the yield is
Chapter 2. Synthesis and Characterization of MCM-41 based materials
89
low (41 %) and relatively high amount of catalyst (0.5 g) was used. In another
report, DEM was synthesized by Jiang et al [155] using the reaction of CO
with ClCH2COOC2H5. In this case high yield was observed but the reaction
was carried out at high pressure.Kolah et al[156] have reported esterification
of succinic acid with ethanol and also have reported esterification of tri-ethyl
citrate via mono and di-ethyl citrate catalyzed by macroporous Amberlyst-15
ion exchange resin. From our laboratory, TMA salts have been widely
investigated as solid acid catalysts for synthesis of diesters such as dioctyl
phthalate (DOP), dibutyl phthalate (DBP) anddiethyl malonate(DEM) [144-
146,157].
Monoesters and diesters using MCM-41 type materials
Jiang et al [158] have studied catalytic activity of mesoporous TiO2
solid super acid for esterification of iso-amyl alcohol and salicylic acid. Salmi
et al [159] have studied methyl acetate formation on polyolefin supported
sulfonic acid catalysts. Sugi et al [160] have reported 12-TPA supported onto
MCM-48 as an efficient catalyst for the esterification of long chain fatty acid
and alcohols in supercritical CO2.Yarmo et al[161] have reported 12-TPA
supported on MCM-41 for esterification of fatty acid under solvent free
condition. The workers have also reported synthesis, characterization and
catalytic performance of porous nafion resin/silica nanocomposites for
esterification of lauric acid and methanol [162].Nascimento et al.[163] have
reported catalytic esterification of oleic acid over SO42-/MCM-41
nanostructured materials.Helen et al[164] have reported use of mesoporous
silica supported diarylammonium catalysts for esterification of free fatty acid in
greases.Zhu et al.[165] have reported synthesis, characterization and
application of sulfated zirconia/hexagonal mesoporous silica (HMS) catalyst in
the esterification of gossypol.Pandurangan et al. [166] have reported vapour
phase esterification of butyric acid with 1-pentanol and tert-butylbenzene with
iso-propyl acetate over Al-MCM-41 mesoporous molecular sieves.Srinivas et
al. [167] have reported the kinetics of esterification of fatty acids over solid
acid catalysts including large pore zeolite-β (Hβ), micro-mesoporous Fe/Zn
double-metal cyanide (DMC) and mesoporous Al-MCM-41.Said et al[168]
have reported perspective catalytic performance of Brønsted acid sites during
Chapter 2. Synthesis and Characterization of MCM-41 based materials
90
esterification of acetic acid with ethyl alcohol over 12-TPA supported on silica.
Rhijn et al[169] have reported sulfonic acid functionalized ordered
mesoporous materials as catalysts for condensation and esterification
reactions.Zhang Yijun et al. [170] have reported synthesis, characterization
and catalytic application of HPA/MCM-48 in the esterification of methacrylic
acid with n-butyl alcohol.Lingaiah et al. [171] have reported 12-TPA with
varying contents on SnO2 as efficient solid acid catalysts for esterification of
free acids with methanol for the production of biodiesel.Valdeilson et al [172]
have studied esterification of acetic acid with alcohols using supported
niobium pentoxide on silica-alumina catalysts.These catalysts were found to
be highly stable and active in esterification reaction.Patel et al [173] have
reported synthesis and characterization of 12-TPA anchored to MCM-41 as
well as its use as environmentally benign catalyst for synthesis of succinate
and malonate Diesters.
Conclusions
As mentioned in the above quoted literature, esterification reactions have
been widely investigated using several solid acids such as sulfated metal
oxides, zeolites, ion exchange resins, HPA, metal oxides, pillared clays etc.
Though solid acid catalysts are emerging as alternatives to liquid acid
catalysts a literature survey reveals that there are several limitations e.g.
though sulfated metal oxide is a very good esterification catalyst, it gets easily
deactivated by losing the sulfate ions, thereby recycling of the catalyst is
restricted. In case of HPA, the separation is difficult and when supported on
carbon the activity decreases.Sulfonic acid based resin (Nafion-H) has also
been found to be unsatisfactory due to its low operating temperature. Hence,
new materials are continuously being synthesized and explored as solid acid
catalysts to overcome the above mentioned limitations.Thus, the endeavour –
“Global effort to replace conventional liquid acid catalysts by solid acid
catalysts” is on.
As already indicated earlier in the text siliceous MCM-41 has a
poor/negligible catalytic activity due to framework neutrality. Catalytic
properties have been induced into siliceous MCM-41 via incorporation of Al3+
into the MCM-41 lattice to result in a catalyst with inherent acidity. HPAs have
Chapter 2. Synthesis and Characterization of MCM-41 based materials
91
proved to be the alternative for traditional acid catalysts due to both strong
acidity and appropriate redox properties. The major disadvantage of HPAs, as
catalyst lies in their low thermal stability, low surface area (1-10m2/g) and
separation problems from reaction mixture due to its high solubility in polar
solvent. HPAs can be made eco-friendly, insoluble solid acids, with high
thermal stability and high surface area by supporting them onto suitable
supports. The support provides an opportunity for HPAs to be dispersed over
a large surface area which increases catalytic activity. Thus, catalytic
properties have been induced into MCM-41 by supporting a HPA onto MCM-
41 by process of anchoring and calcination to result in a catalyst with induced
acidity.
In the present endeavour 12TPA-MCM-41(Induced Acidity) and Al-
MCM-41(Inherent Acidity) have been used as solid acid catalysts using
esterification as a model reaction wherein monoesters such as Ethyl Acetate
(EA), Propyl acetate (PA), Butyl acetate (BA) and Benzyl acetate (BzA) and
diesters such as Dioctyl phthalate (DOP), Dibutyl phthalate (DBP) andDiethyl
malonate(DEM) have been synthesized.The catalytic activity of 12TPA-MCM-
41 and Al-MCM-41have been compared and correlated with surface
properties of the materials.
2.15EXPERIMENTAL
Materials: Acetic acid, ethanol, 1-propanol, 1-butanol, benzyl alcohol,
cyclohexane, toluene, xylene, phthalic anhydride, 2-ethyl 1-hexanol, malonic
acid were procured from Merck.
Catalyst Synthesis:The synthesis and characterization of 12-TPA-MCM-41
(with various wt.% of 12-TPA loading) and Al-MCM-41(with varying SiO2:Al2O3
ratios) have been discussed earlier in the text (Section 2.10 and 2.11). It is
observed that 12-TPA-MCM-41 with 20wt.% 12-TPA loading abbreviated as
12TPA-MCM-41-20 and Al-MCM-41 with SiO2:Al2O3 ratio 30 abbreviated as
Al-MCM-41-30 exhibit highest surface acidity and thus used for all catalytic
studies.
Synthesis of monoesters (EA, PA, BA and BzA):In a typical reaction, a 100
mL round bottomed flask equipped with a Dean and Stark apparatus,
attached to a reflux condenser was used and charged with acetic acid (0.05 -
Chapter 2. Synthesis and Characterization of MCM-41 based materials
92
0.10 M), alcohol (0.05 - 0.10 M), catalyst (0.10 - 0.20 g) and a suitable solvent
(15 mL). Cyclohexane was used as a solvent for the synthesis of ethyl
acetate and toluene for propyl acetate, butyl acetate and benzyl acetate. The
reactions were carried out varying several parameters such as amount of
catalyst, mole ratio of reactants, reaction time and these parameters
optimized. After completion of reaction, catalyst was separated by decantation
and reaction mixture was distilled to obtain the product.
Synthesis of diesters (DEM, DOP and DBP): The diesters were synthesized
in two steps. In the first step, equimolar proportion (0.025 mole) of acid and
alcohol (malonic acid and ethanol for DEM, phthalic anhydride and 2-ethyl 1-
hexanol for DOP, phthalic anhydride and 1-butanol for DBP) were taken in a
round bottomed flask and the reaction mixture stirred at ~80 °C for DEM,
~145 °C for DOP, and ~115 °C for DBP for about 10-15 min in absence of any
catalyst and solvent. The dicarboxylic acid and anhydride are completely
converted to the monoester, so that the acid concentration at this stage is
taken as the initial concentration. The obtained product (monoester) was then
subjected to esterification reaction by addition of a second mole (0.025 mole)
of respective alcohol, catalyst (0.10 - 0.20 g) and 15 mL solvent (toluene for
DEM and DBP and xylene for DOP). The reactions were carried out varying
several parameters such as amount of catalyst, mole ratio of reactants,
reaction time and these parameters optimized. In all cases the round
bottomed flask was fitted with Dean and Stark apparatus, with a condenser to
remove water formed during the reaction. After completion of reaction,
catalyst was separated by decantation and reaction mixture was distilled to
obtain the product.
Regeneration and recyclability of catalyst: During the course of the
reaction, many a time the catalyst colour changes. This is probably due to the
adsorption of reacting molecules coming onto the surface of the catalyst. After
separation of catalyst in reaction mixtureby decantation, it is first refluxed in
ethanol for 30 minutes, followed by drying at 120°C. This material was used
as recycled catalyst. This regeneration procedure was followed in subsequent
recycle reaction.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
93
Calculation of % yield of esters: For both mono and diesters, % yields were
determined by titrating the reaction mixture with 0.1 M alcoholic KOH solution.
The yields of the esters were calculated using the formula, % yield = [(A - B) /
A] M 100, where A and B are acid values of the sample withdrawn before
and after reaction and M is mole ratio of acid: alcohol. The yield of ester
formed was also determined using GC.
2.16RESULTS AND DISCUSSION
Monoesterification
Equilibrium constants of the esterification reactions are low. As in any
equilibrium reaction, the reaction may be driven to the product side by
controlling the concentration of one of the reactants (Le Chatlier’s Principle).
When concentration of one of the reactant relative to the other is increased,
the reaction is driven to the product side. In order to obtain higher yield of
esters,Le Chatlier’s Principle has been followed. Solvents cyclohexane and
toluene have been employed to remove the water formed during the reaction
as a binary azeotrope. Monoesters EA, PA, BA and BzA were synthesized as
described in experimental section.
Firstly, reaction conditions were optimized using 12TPA-MCM-41-20 as
solid acid catalyst for BzA synthesis by varying such parameters as catalyst
amount, initial mole ratio of the reactant (alcohol to acid) and reaction time,
and the results obtained presented in Table 2.7 and a graphical presentation
(Fig. 2.38 and 2.39).
During optimization of reaction time, it is observed that the conversion
rate was very high initially, indicating that the reaction obeys first order
kinetics. As reaction time increases, percentage yield increases. However,
there is not much gain in product after 8 h. Hence, the optimum reaction time
was selected as 8 h.With increasing amount of the catalyst, the % yield
increased which is due to proportional increase in the number of active sites.
The influence of reactant ratio (alcohol: acid) was studied by increasing from
1:1.5 to 1.5:1. The yield can be increased by increasing the concentration of
either alcohol or acid. As observed from Table 2.7, the % yield of ester
increases with increase in mole ratio of acid while decreases with increasing
mole ratio of alcohol. This may be attributed to preferential adsorption of
Chapter 2. Synthesis and Characterization of MCM-41 based materials
94
alcohol on the catalyst which results in blocking of active sites. For economic
reasons also, the reactant that is usually less expensive of the two is taken in
excess. In the present study, acids were used in excess. The temperature
parameter has not been varied, due to the fact that the reaction temperature
is sensitive to boiling points of reactants as well as solvents used as
azeotrope.
At the optimum condition (mole ratio of reactants, alcohol:acid = 1:1.5,
amount of catalyst = 0.15 g, time = 8 h) mono esters EA, PA and BA have
been synthesized and the yields of esters are presented in Table 2.8. Further,
for comparative study, Al-MCM-41-30 is used as solid acid catalyst for
synthesis of mono esters BzA, EA, PA and BA at the above mentioned
optimum condition.The results are presented in Table 2.8.
It is observed that, the order of % yield of ester formedfor both
catalysts is BzA > EA > PA > BA. Though the yields in case of mono esters
using both catalysts are comparable, higher yields are observed in case of
12TPA-MCM-41-20 which could be attributed to higher surface acidity. Turn
over number (TON) reflects the effectiveness of a catalyst and this also
follows the order of ester formation.
Esterification of monoesters EA, PA and BA has been reported [153] in
absence of catalyst and exhibit poor yields. Therefore catalyst is a must for
these reactions. In case of BzA however, it is observed that with an excess of
acetic acid and in the absence of any catalyst the yield is as high as 90.6 %
which is attributed to auto catalysis. In another report [137] high yields of BzA
were obtained with small amount of the catalyst but the reaction time was
relatively high. In the present study, yields of BzA are ~95% for both catalysts
compared to above literature reports. Higher yields in case of benzyl acetate
could be attributed to an enhanced nucleophilicity due to presence of aromatic
ring in benzyl alcohol. The order of % yield of ester formed is EA > PA >
BAcould be explained due to increase in carbon chain length in the respective
alcohols used for ester formation. During condensation of these alcohols with
acetic acid probably steric effects are responsible for explaining decreasing
yields from EA through PA to BA.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
95
% yields obtained in recycled catalyst and % decrease in yields in
subsequent cycles is presented in Table 2.8 and 2.9 respectively, and a
graphical presentation (Fig. 2.40). It is observed that in subsequent cycles
decrease in % yields is less for Al-MCM-41-30 compared to 12TPA-MCM-41-
20. This could probably be due to the leaching of 12-TPA from surface of
MCM-41. It is observed that the colour of the catalyst changes after each
catalytic run. This gives an indication that during the course of the reaction the
reacting molecules come onto the surface of the catalyst. Some of them enter
into reaction to give the product while a few of them get adsorbed on the
surface, which is marked by the change in the colour of the catalyst. The fact
that the reactant molecules are weakly adsorbed is evident from the catalyst
regaining its original colour, when treated with ethanol. The possibility of
molecules entering interstices cannot be ruled out. This is observed from the
fact that the yields go downafter every regeneration, leading to deactivation of
the catalyst.
Fig.2.38Reaction time variation for BzA synthesis
Fig.2.39Catalyst amount variation for BzA
synthesis
Fig.2.40Comparative catalyst performance in the formation of monoesters
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10
% Y
ield
Reaction Time (h)
40
50
60
70
80
90
100
0.05 0.1 0.15 0.2 0.25
% Y
ieild
Catalyst amount (g)
0 10 20 30 40 50 60 70 80 90
100
Fresh 1st Cycle
2nd Cycle
Fresh 1st Cycle
2nd Cycle
Fresh 1st Cycle
2nd Cycle
Fresh 1st Cycle
2nd Cycle
% Y
ield
BzA EA PA BA
12-TPA-MCM-41-20 Al-MCM-41-30
Chapter 2. Synthesis and Characterization of MCM-41 based materials
96
Table 2.7Optimization of reaction conditions for monoesters using 12-TPA-MCM-41-20.
Sr.
No.
Reactants with
their mole ratio Product
Catalyst
amount
(g)
Time
(h)
Temp.
(0C)
%Yield
12-TPA-MCM-
41-20
(A) Time variation
1 Bz + AA (1:1) BzA 0.1 1 115 31.54
3 Bz + AA (1:1) BzA 0.1 2 115 46.43
4 Bz + AA (1:1) BzA 0.1 3 115 55.35
5 Bz + AA (1:1) BzA 0.1 4 115 67.26
6 Bz + AA (1:1) BzA 0.1 5 115 69.04
7 Bz + AA (1:1) BzA 0.1 6 115 70.23
8 Bz + AA (1:1) BzA 0.1 7 115 71.43
9 Bz + AA (1:1) BzA 0.1 8 115 71.65
(B) Catalyst amount optimization
10 Bz + AA (1:1) BzA 0.05 8 115 63.49
11 Bz + AA (1:1) BzA 0.15 8 115 90.63
12 Bz + AA (1:1) BzA 0.2 8 115 91.41
(C) Mole ratio optimization
13 Bz + AA (1:1.5) BzA 0.15 8 115 95.23
14 Bz + AA (1.5:1) BzA 0.15 8 115 62.50
Bz= Benzyl alcohol; AA= Acetic acid; Entry No.13 is optimum condition.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
97
Table 2.8% yields of monoesters using 12-TPA-MCM-41-20 and Al-MCM-41-30 at
optimized condition
Sr.
No Reactants Product
12TPA-MCM-41-20 Al-MCM-41-30
%Yield *TON %Yield *TON
1 Bz + AA BzA 95.23 34.12 93.55 34.50
2 E + AA EA 92.31 33.10 90.35 33.10
3 P + AA PA 90.23 33.00 88.25 33.56
B + AA BA 74.13 18.50 73.16 17.59
A Reusability of catalyst
4 Bz +AA1st cycle BzA 87.15 31.35 89.73 31.70
5 Bz + AA2nd cycle BzA 77.45 20.82 83.80 27.80
6 E + AA 1st cycle EA 87.10 31.89 85.20 29.20
7 E + AA 2nd cycle EA 78.65 21.40 79.15 22.00
8 P + AA 1st cycle PA 84.42 28.51 85.41 29.15
9 P + AA 2nd cycle PA 78.35 21.14 80.25 23.00
10 B + AA 1st cycle BA 69.00 17.50 69.45 15.45
11 Bz +AA1st cycle BA 61.48 12.00 64.32 14.55
Bz= Benzyl alcohol; AA= Acetic acid; E=Ethanol; P= 1-Propanol; B=1-Butanol; Mole ration of the reactants = 1.5:1 (Acid:Alcohol);Reaction Time = 8 h. Catalyst amount = 0.15 g; Reaction temperature 80˚C for EA and 115˚C for PA, BA and BzA; *TON = Turn over number, gram of ester formed per gram of catalyst.
Table.2.9% Decrease in yields of monoesters using regenerated catalysts in
subsequent cycles
Monoesters
% Decrease in yields
12TPA-MCM-41-20 Al-MCM-41-30
1st Cycle 2nd Cycle 1st Cycle 2nd Cycle
BzA 8 10 4 6
EA 5 9 5 6
PA 6 8 4 5
BA 4 8 3 4
Chapter 2. Synthesis and Characterization of MCM-41 based materials
98
Diesterification
Diesters DEM, DOP and DBP were synthesized as described in
experimental section
Firstly, reaction condition was optimized using 12TPA-MCM-41-20as
solid acid catalyst for DEM synthesis by varying such parameters as catalyst
amount, reaction time, temperature and mole ratio of the acid and alcohol.The
results obtained are presented in Table 2.10 and a graphical presentation
2.42 and 2.43.
Diesters are prepared in two stages (Scheme 2.2). The first stage is
very rapid and can be carried out in the absence of a catalyst. In second
stage with increasing amount of the catalyst, the % yield increases which is
due to proportional increase in the number of active sites. In diester formation
the acid/anhydride taken as substrate possesses two attacking sites
responsible for ester formation. Thus, only concentration of alcohol was varied
following Le Chatlier’s Principle in the present study. The influence of reactant
ratio (alcohol: acid) was studied increasing from 2:1 to 2.4:1. It is observed
that the % yield ofdiester was maximum with 2:1 mole ratio.As reaction time
increases, percentage yield increases. However, there is not much gain in
product after 8 h. Hence, the optimum reaction time was selected as 8 h.
At optimum condition (mole ratio of reactants, alcohol: diacid/anhydride
= 2: 1, amount of catalyst = 0.15 g, time = 8 h) diesters DOP and DBP have
been synthesized. Results are presented in Table 2.11. Further, for
comparative study, at optimized condition Al-MCM-41-30 is used as solid acid
catalyst for synthesis of diesters DEM, DOP and DBP. The results are
presented in Table 2.11.
From Table 2.11, it is observed that, there is no marginal difference in
the yields of DOP and DBP and order of the % yields of diesters formation is
DEM > DOP ≈ DBP, which is probably due to less steric hindrance felt by
incoming ethanol from monoethyl malonate formed in the first step in case of
DEM.
The mechanism of diester formation over solid acid catalyst is similar to
that of conventional mechanism involving the formation of protonated
dicarboxylic acid, using proton donated by the catalyst, followed by
Chapter 2. Synthesis and Characterization of MCM-41 based materials
99
nucleophilic attack of alcoholic group to yield the respective monoester. The
second carboxylic group present in monoester gets further esterified by the
same mechanism in a repeat reaction, which ultimately results in the diester
formation [154].
Fig.2.41Reaction time variation for DEM
synthesis
Fig.2.42Catalyst amount variation for DEM
synthesis
Fig. 2.43Comparative catalyst performance in the formation of diesters
% yields obtained in recycled catalyst and % decrease in yields in
subsequent cycles is presented in Table 2.11 and 2.12 respectively, and a
graphical presentation (Fig. 2.43). It is observed that in subsequent cycles
decrease in % yields is less for Al-MCM-41-30 compared to 12TPA-MCM-41-
20. This is could probably be due to leaching of 12-TPA from surface of
catalyst. The change in colour of catalyst is also observed after each catalytic
run.The same explanation can be forwarded as discussed in case of
monoester formation.
0
10
20
30
40
50
60
0 2 4 6 8 10
% Y
ield
Reaction Time (h)
30
35
40
45
50
55
60
0 0.05 0.1 0.15 0.2
% Y
ield
Catalyst amount (g)
0
10
20
30
40
50
60
Fresh 1st Cycle
2nd Cycle
Fresh 1st Cycle
2nd Cycle
Fresh 1st Cycle
2nd Cycle
% Y
ield
DEM DOP DBP
12TPA-MCM-41-20 Al-MCM-41-30
Chapter 2. Synthesis and Characterization of MCM-41 based materials
100
Table 2.10Optimization of reaction conditions for DEM using 12TPA-MCM-41-20.
Sr.
No Reactants with
their mole ratio Product
Catalyst
amount
(g)
Time
(h)
Temp.
(0C)
%Yield
12-TPA-
MCM-41-20
A Time variation
1 E + MA (2:1) DEM 0.1 2 115 16.66
2 E + MA (2:1) DEM 0.1 4 115 41.66
3 E + MA (2:1) DEM 0.1 6 115 50.01
4 E + MA (2:1) DEM 0.1 8 115 53.33
B Catalyst amount variation
5 E + MA (2:1) DEM 0.05 8 115 44.21
6 E + MA (2:1) DEM 0.15 8 115 53.79
C Mole ratio variation
7 E + MA (2.2:1) DEM 0.15 8 115 44.44
8 E + MA (2.4:1) DEM 0.15 8 115 46.45
E= Ethanol, MA = Malonic Acid; Entry No. 6 is optimum condition.
Table 2.11% yield of diesters using 12TPA-MCM-41-20 and Al-MCM-41-30 at
optimized condition
Sr.
No Reactants Product
12TPA-MCM-41-20 Al-MCM-41-30
%Yield *TON %Yield *TON
1 E + MA DEM 53.79 20.18 48.40 23.79
2 PA + O DOP 53.36 19.89 42.32 16.25
3 PA + B DBP 52.92 19.00 43.05 18.00
A Reusability of catalyst
4 E + MA1st cycle DEM 48.41 18.80 45.10 18.08
5 E + MA2nd cycle DEM 42.01 14.41 35.54 13.75
6 PA + O1st cycle DOP 50.12 19.25 40.82 14.15
7 PA + O2nd cycle DOP 44.70 16.00 36.2 14.20
8 PA + B 1st cycle DBP 49.36 19.00 40.38 14.00
9 PA + B 2nd cycle DBP 44.00 15.86 34.58 12.60
PA = Phthalic anhydride; O = 2-ethyl-1-hexanol; B = 1-Butanol; E = Ethanol; MA = Malonic Acid; Mole ration of the reactants = 1:2 (Acid/Anhydride:Alcohol); Reaction Time = 8 h. Catalyst amount = 0.15 g; Reaction temperature 115˚C for DOP and 145˚C DEM and DOP; *TON = Turn over number, gram of ester formed per gram of catalyst.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
101
Table 2.12% Decrease in yields of diesters using regenerated catalysts in
subsequent cycles
Monoesters
% Decrease in yields
12TPA-MCM-41-20 Al-MCM-41-30
1st Cycle 2nd Cycle 1st Cycle 2nd Cycle
DEM 5 6 3 3
DOP 3 6 2 4
DBP 8 5 3 6
Conclusions
The work outlined herein reveals the promising use of 12TPA-MCM-41-
20 and Al-MCM-41-30 as solid acid catalysts in the synthesis of monoesters
and diesters, the advantages being operational simplicity, mild reaction
conditions and eco-friendly nature of catalyst. The monoesters and diesters
formed can be simply distilled over, there is no catalyst contamination in
products formed, no acid waste formation and products are colourless a
limitation in the conventional process. The catalysts can be regenerated and
reused. Since the reactions are driven by surface acidity of the catalyst, there
is scope of obtaining better/higher yield by synthesizing material with high
surface acidity by modifying synthesis procedure. Though yields of
monoesters are high, the diester yields are low however, with the only
advantage of the product having no colour contamination.
2.17 FRIEDEL-CRAFTS ACYLATION AND ALKYLATION
Charles Friedel and James Crafts in 1877 developed a set of reactions
popularly known today as Friedel-Crafts reactions, involving electrophilic
aromatic substitution of two types, acylation and alkylation.
Friedel-Crafts acylation
Friedel-Crafts acylation (Scheme 2.3) involves the reaction of an acyl
chloride or acid anhydride with aromatic compounds in presence of a strong
Lewis acid catalyst. Due to the electron-withdrawing effect of the carbonyl
group, the ketone product is always less reactive than the original molecule,
therefore multiple acylations do not occur, which is an advantage over the
alkylation reaction (described later in the text).Also, there are no
Chapter 2. Synthesis and Characterization of MCM-41 based materials
102
carbocationrearrangements, as the carbonium ion is stabilized by a
resonance structure in which the positive charge is on the oxygen, inhibiting
intra molecular reactions.
+ CH3COClAlCl3
+
COCH3
HCl
Scheme 2.3 Reaction scheme for Friedel Crafts acylation
Mechanism for Friedel Crafts Acylation:
As seen from Scheme 2.4, the first step consists of dissociation of a
chlorine atom to form an acyl cation. This is followed by nucleophilic attack of
the arene towards the acyl group. Finally, a chlorine atom reacts to form HCl,
and the AlCl3 catalyst is regenerated:
R
C
Cl
O
AlCl3
H COCH3
+COCH3
H COCH3
Al
Cl
Cl
Cl ClAlCl3
COCH3
+ + HCl
Scheme 2.4 Reaction mechanism for Friedel Crafts acylation
The viability of the Friedel-Crafts acylation depends on the stability of
the acyl chloride reagent. For example, in synthesis of benzaldehyde via the
Friedel-Crafts pathway using formyl chloride as an acylating agent, since
formyl chloride is too unstable to be isolated, formyl chloride has to be
Chapter 2. Synthesis and Characterization of MCM-41 based materials
103
synthesized in situ. This is accomplished via the Gattermann-Koch reaction,
accomplished by reacting benzene with carbon monoxide and hydrogen
chloride under high pressure, catalyzed by a mixture of aluminium chloride
and cuprous chloride.
Friedel–Crafts acylation of aromatic compounds and aromatic
heterocyclic compounds is a ubiquitous reaction in the production of aromatic
ketones, largely used as intermediates in the synthesis of pharmaceuticals,
naproxen, dextromethorphan, ibuprofen and dyes, fragrances, and
agrochemicals [174-179]. In particular, the synthesis of substituted
acetophenones employing acylation is an important step for the production of
a variety of precursors which find application in the production of
pharmaceuticals, paint additives, photo initiators, fragrances, plasticizers,
dyes and other commercial products [180-184].
Friedel-Crafts alkylation
Friedel-Crafts alkylation (Scheme 2.5) involves the alkylation of an
aromatic ring and an alkyl halide using a strong Lewis acid catalyst. With
anhydrous aluminium chloride as a catalyst, the alkyl group substitutes the
chloride ion.
+ R-Cl
R
AlCl3
+ HCl
Scheme 2.5 Reaction scheme for Friedel Crafts alkylation
Mechanism for Friedel Crafts Alkylation
Al
Cl
Cl
Cl Cl
R-Cl + AlCl3 R+ +AlCl4
-
R+HR
R
Scheme 2.6 Reaction mechanism for Friedel Crafts alkylation
Chapter 2. Synthesis and Characterization of MCM-41 based materials
104
As seen from Scheme 2.6, the first step consists of dissociation of a
chlorine atom to form an alkyl cation. This is followed by nucleophilic attack of
the arene towards the alkyl group. Finally, a chlorine atom reacts to form HCl,
and the AlCl3 catalyst is regenerated.
In this reaction, the product is more nucleophilic than the reactant due
to the electron donating effect of alkyl-chain, therefore, another hydrogen is
substituted with an alkyl-chain, which leads to overalkylation of the molecule.
Further, if the chlorine is not on a tertiary carbon, carbocationrearrangement
reaction occurs, attributed to the relative stability of the tertiary carbocation
over the secondary and primary carbocations.Steric hindrance can be
exploited to limit the number of alkylations, as in the tertiary butylation of 1,4-
dimethoxybenzene (Scheme 2.7)
Scheme 2.7 Reaction scheme for t-butylation of 1,4-dimethoxybenzene.
Scheme 2.8 Reaction scheme for Friedel Crafts alkylation using bromonium ion as
electrophile
Alkylations are not limited to alkyl halides. Friedel-Crafts alkylation is
possible with any carbocationic intermediate such as those derived from
alkenes and a protic acid or lewis acid, enones and epoxides. In one study,
the electrophile is a bromonium ion derived from an alkene and N-
Chapter 2. Synthesis and Characterization of MCM-41 based materials
105
bromosuccinimide(NBS). In this reaction samarium(III) triflate is believed to
activate the NBS halogen donor in halonium ion formation(Scheme 2.8).
The liquid phase benzylation of benzene and other aromatic
compounds by benzyl chloride is important for the production of
diphenylmethane and substituted diphenylmethanes which are industrially
important compounds used as pharmaceutical intermediates and fine
chemicals [185-189].
The use of acyl halides or anhydrides as acetylating agents and
soluble Lewis acids as catalysts is polluting, expensive and difficult to work
with. In normal practice, strong mineral acids, such as H2SO4, HF, or
supported Lewis-acid catalysts like anhydrous AlCl3/SiO2 and BF3/SiO2 are
used for such reactions. However, these Lewis acids are consumed in more
than stoichiometric amounts due to the formation of 1:1 molar adduct with
aromatic ketones and further, the subsequent separation of the product by
hydrolysis is cumbersome and generates a large amount of environmentally
hazardous and corrosive waste.
Friedel-Crafts alkylation reactions catalyzed by homogeneous Lewis
acid catalysts generally give complex reaction mixtures. The formation of
reactant (and product) catalyst complexes, the increased tendency of
alkylated products towards further alkylation and isomerization, coupled with
the long contact of the reactant with the catalyst, result in decreased product
selectivity.
As indicated earlier in the text owing to stringent and growing
environmental regulations worldwide, there is a global effort to replace the
conventional liquid acid catalysts by solid acids, which are less toxic, easily
regenerable from the product, easy to handle and reuseable. In this context,
the focus has been towards design of processes to replace homogeneous
Lewis acid catalysts with environmentally benign heterogeneous catalysts.
The acid sites in solid acids being milder than the conventional Lewis acids,
would also inhibit side reactions such as polyalkylation, isomerization,
transalkylation, dealkylation and polymerization that occur in traditional
procedure.There is, therefore, substantial interest to carry out alkylation
reactions with solid acid catalysts which decrease these side reactions.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
106
2.18LITERATURE SURVEY IN THE CURRENT AREA OF
STUDY
Acylation/alkylation of aromatic compounds have been reported using
several solid acid catalysts in recent years. Kantam et al [189]have reported
Friedel–Crafts acylation of aromatics and heteroaromatics using micro
crystalline zeolites with different acid anhydrides.The micronized -zeolite
shows manifold activity over normal zeolite in acylation reactions of aromatics.
Deutsch et al [190] have reported acylation and benzoylation of various
aromatics on sulfated zirconiaand observed that the rate of acylation reactions
is dependent on the nature of the respective aromatic compound. The
application of sulfated zirconia as a catalyst for the acetylation of aromatics
was only successful in case of anisole amongst various aromatic compounds
used. Kaur et al[191] have reported Friedel–Crafts acylation of anisole and
toluene with acetic anhydride using HPA supported on silica as catalyst as
well as H- Zeolite. In contrast to anisole, the acylation of toluene with HPA is
far less efficient than that with H-The inhibited activity of HPA for toluene
could be attributed to preferential adsorption of acetic anhydride on the
catalyst. Beers et al [192] have reported use of dealuminated zeolites as
solid acid catalyst for acylation of anisole with octanoic acid and proposed a
structure–activity relation for the same. After dealumination, increased activity
and selectivity were found in the acylation of anisolewith octanoic acid.The
enhanced activity is suggested to resultfrom higher accessibility of the active
sites associated with framework-connected aluminum atoms.Bachiller-Baeza
et al[193] have studied and compared the behaviours of HPA catalysts
supported on a commercial silica and on a silica–zirconia mixed oxide for the
acylation of anisole with acetic anhydride.The yields of p-
methoxyacetophenone were highest for HPA/SiO2. Castro et al [194] have
reported a mechanistic overview on the acylation reactions of anisole
usingunsaturated organic acids as acylating agents and solid acids as
catalysts.The mechanism of acylation of anisole with unsaturated acids,
i.e. acrylic, crotonic and methylcrotonic acid,have been investigated using 12-
PTA, supported on SiO2 and in the form of cesium saltsas catalysts. Since
Chapter 2. Synthesis and Characterization of MCM-41 based materials
107
unsaturated acid can either alkylate and/or acylate the aromatic
compound, the influence of the catalyston the selectivity for these two
competing reactions was studied. Analysis of products obtained on the
acylation of aromaticcompounds with unsaturated acids shows that all the
catalysts are more active for acylation than alkylation. Secondaryproducts
coming from intermolecular reactions of the acylated product with anisole as
well as tertiary products coming fromits further decomposition and
recombination with another anisole molecule were observed. Heteropolyacids
supported onsilica were found to be more active and selective towards
acylation reactions than zeolites HY and H.Melero et al [195] have reported
Friedel Crafts acylation of aromatic compounds overarenesulfonic acid
containing mesostructured SBA-15 materials.Arenesulfonic acidcenters
anchored on the pore surface of a mesostructured SBA-15 material show
greater activity (normalised to the concentration of sulfonic groups) as
compared to other homogeneous and heterogeneous sulfonatedcatalysts and
even in solventless conditions. This high activity is accompanied with a
remarkable thermal stability of the acidcenters, without leaching of sulfur
species during the reaction.
Cardoso et al [196] have reported silica supported HPA catalyst for
acylation of anisole using acetic anhydride as acylating agent. High
conversions and very high p-selectivity were attained in the temperature
range of 61–110 ◦C. However, deactivation was observed due to strong
adsorption of the products. Ma et al [197] have reported Friedel-Crafts
acylation of anisole over Y-zeolite catalystwith alkanoic acids, anhydrides and
substituted benzoic acids.When carboxylic acids were used as acylating
agents, the activity ofthe Y zeolite increased with its Lewis acidity, showing
that the Lewis acid sites were more active than the Bronsted acid
sites.Further, the reaction mechanism was found to be similar to the
homogeneous catalysis, that is, the electrophilicintermediate formed from the
acylating agent over zeolite acid sites attacked the aromatic ring of anisole.
Gaare et al [198] have reported effect of lanthanum ion exchange and Si/Al
ratio of Y-zeolite on the Friedel-Crafts acylation of anisole by acetyl chloride
and aceticanhydride.For the rare-earth modified zeolites, the activity was
Chapter 2. Synthesis and Characterization of MCM-41 based materials
108
found to be dependent on the lanthanum content, and the yieldincreased with
the level of lanthanum, even up to 93% exchange. Dealunminated Y-zeolites
were also found to be very active,and an almost linear increase in the yield
with decreasing aluminium fraction was found attributed to theincreased
hydrophobicity of dealuminated zeolites.Heidekum et al [199] have reported
use of Nafion/Silica composite materials as solid acid catalysts for acylation
reactions and claimed that entrapping nanosized Nafion particles in a silica-
matrix, effectivelyenhances the accessibility of the acid sites in comparisonto
the original material, Nafion resin. Chaudhari et al [200] have reported AlClx-
grafted Si-MCM-41 prepared by reacting anhydrous AlCl3 with terminal Si–OH
groups as an active and a reusable (if not exposed to atmosphere)
mesoporous solid catalyst for the Friedel–Crafts benzylation and acylation
reactions. However, like anhydrous AlCl3, it is highly moisture sensitive and
loses its activity on exposure to moist atmosphere. The active species on the
catalyst are (–Si-O)nAlCl3-n (n = 1–3). Cseri et al [201] have reported alkylation
of benzene and toluene with benzyl chloride and benzyl alcohol over a series
of clays obtained by exchanging the original cations of K10 by Ti4+, Fe3+, Zr4+,
Cu2+, Zn2+, Ce3+, Cr3+ and Sn2+ cations. The acidity of these solids has been
determined by infra red spectrometry using pyridine as molecular probe. The
acidity of K1O clays can be changed to a great extent by cation exchange and
by the thermal treatments applied to the solids. The rate of alkylation is
roughlyrelated to acidity when the substrate is benzyl alcohol, but not when
benzyl chloride is used. In that case, the catalysts containingreducible cations
( Fe3+, Sn4+, Cu2+) exhibit high activities in spite of their low number of acid
sites. Bachari et al [202-204] have investigated benzylation of benzene and
substitutedbenzenes, employing benzyl chloride as the alkylating agent over
mesoporous silica with different Sn, Cu and Ga contents. The mechanism
involves aredox step at the reaction initiation. The large pores of the
mesoporous catalyst donot limit the size of the molecules that could be
reacted. Chaudhary et al [205] have investigated benzylation of benzene by
benzyl chloride to diphenyl methane over InCl3, GaCl3, FeCl3 and ZnCl2
supportedon commercial clays (viz. Montmorillonite-K10, Montmorillonite-KSF
and Kaolin) or on high silica mesoporous MCM-41. The redox function
Chapter 2. Synthesis and Characterization of MCM-41 based materials
109
created due to the impregnation of the clays or Si-MCM-41 by InCl3, GaCl3,
FeCl3 or ZnCl2seems to play a very important role in the benzylation process.
Kinetics of the benzene benzylation (using excess of benzene)over the
supported metal chloride catalysts has also been investigated and a plausible
reaction mechanism for thebenzylation over the supported metal chloride
catalysts is proposed. Silva et al [206] have evaluated catalytic activity of gel
and macroreticular ion-exchange resins (Lewatit and Amberlyst-15) for the
reaction of benzene with benzyl alcohol and benzyl chloride at 80°C in the
liquid phase.With benzylchloride, the monobenzylation product,
diphenylmethane, was obtained in low yield, both with the gel and
themacroreticular resins. Better results were obtained with benzyl alcohol as
benzylation agent and the most active resin was Amberlyst-15, the conversion
of benzyl alcohol being proportional to the concentration of acid sites on the
resin. Mantri et al [207] have investigated Friedel–Crafts alkylation of
aromatics with benzyl alcohol as alkylating agent over rare earth metal
triflates, Sc(OTf)3,Hf(OTf)4, La(OTf)3, and Yb(OTf)3 supported on MCM-
41.The catalytic activity of triflates, was enhanced after being loaded onto
MCM-41 due to increased dispersion, and gave the benzylated product in
high yield. The rate of the benzylation of benzene was accelerated by electron
donating groups and retarded by electronwithdrawing groups. Narender et al
[208] have studied benzylation of benzene and toluene with benzyl alcohol
over a series of zeolites and metal modified zeolites. A reaction mechanism
has been proposed for formation ofdiphenylmethane and benzyl ether. Benzyl
ether formationfrom benzyl alcohol is explained on the basis of
theintermolecular reaction pathway, involving Bronsted acidsites of the
zeolite. Bachari et al [209] have reported benzylation of benzene by benzyl
chloride to diphenylmethane over FeCl3, InCl3, GaCl3, ZnCl2, CuCl2 and NiCl2
supported on mesoporous SBA-15.Further it is claimed that the redox
property due to the impregnation of the SBA-15 by transition metal chloride,
seems to play a very important role in the benzene benzylation process. Zhou
et al [210] have reported silica-supportedpolytrifluoromethanesulfosiloxane
(SiO2–Si–SCF3) catalyzed Friedel–Craftsbenzylation of benzene and
substituted benzenes.It was found that SiO2–Si–SCF3could catalyze Friedel–
Chapter 2. Synthesis and Characterization of MCM-41 based materials
110
Crafts benzylation of benzene and substituted benzenes with benzyl alcohol
under relatively mild experimental conditions. Reactions are very clean and
water is the only by-product of the reaction. The yields amounted to 97–100%.
Vinu et al [211] have reported benzylation of benzene and other aromatics by
benzyl chlorideover mesoporous Al-SBA-15 catalysts. Okuhara et al [212]
have explained various HPAs, like H3[PW12O40] and H4[SiW12O40], were used
as solid acid catalysts for the alkylation of 1,3,5-trimethylbenzene with -
butyrolactone to form 4-(2,4,6-trimethylphenyl) butyric acid. The catalysts
could be reused. Sugi et al [213] have reported Friedel–Crafts benzylation of
aromatics with benzyl alcohols catalyzed by heteropoly acids such as 12-TPA
(H3PW12O40·xH2O)(HPW), molybdophosphoric acid (H3PMo12O40·xH2O)
(HPMo) and tungstosilicic acid (H4SiW12O40·xH2O) (HSiW) supported on
mesoporous silica such as MCM-41, FSM-16 and SBA-15 by the
impregnation method to enhance the catalytic activity of these solid acids by
their dispersion on the support with high surface area. They also have used
rare earth metal triflates supported on MCM-41 mesoporous silica. Donghao
et al[214] have successfully prepared mesoporous silica materials, SBA-15,
functionalized with strong (-SO3H), moderate (-PO3H2) and weak (-COOH)
acid groups and these mesoporous acid catalysts have been applied to the
alkylation of phenol with tert-butanol. Subramaniam et al [215] have reported
synthesis and characterization of HPA catalysts and their cesium salts,
catalysts have been evaluated for the alkylation of isobutane with 1-butene.
Angelis et al [216] have reported solid acid catalysts based on HPAs
supported on different oxides catalyze the alkylation of isobutane with n-
butenes to yield high-octane gasoline components. Ramos-Galvan et al [217]
have reported alkylation of benzene with propylene over 12-TPA supported on
MCM-41 and -48 type mesoporous materials. Chaudhariet al [218] have
reported highly active Si-MCM-41 supported Ga2O3 and In2O3catalysts for
Friedel-Crafts-type benzylation and acylation reactions in presence or
absence of moisture.Mohammed et al [219] have reported
benzylation of benzene over sulfated zirconia supported as MCM-41 using a
single source precursor.Kalabasi et al. have reported vapor phase alkylation
of toluene using various alcohols over H3PO4/MCM-41 catalyst: influence of
Chapter 2. Synthesis and Characterization of MCM-41 based materials
111
reaction parameters on selectivity and conversion.Selvaraj at al [220] have
reported synthesis of 2-acetyl-6-methoxynaphthalene using mesoporous
SO42- /Al-MCM-41 molecular sieves. Murugesan et al [221] have reported
synthesis, characterization and catalytic activity of Al-MCM-41, Fe,Al-MCM-41
and Zn,Al-MCM-41 in the vapor phasealkylation and acylation of
ethylbenzene with ethyl acetate in the temperature range between 250 and
400 °C. Endud et al [222] have reported cubic aluminated mesoporous
materials, Al-MCM-48 as highly effective catalysts for Friedel-Crafts acylation
of 2-methoxynaphthalene and 2-acetyl-6-methoxynaphthalene. Iwamoto et al
[223] have reported Friedel-Crafts acylation of anisole with carboxylic
anhydrides of large molecular sizes on mesoporous silica MCM-41 catalyst.
Halligudi et al [224] have reported 12-TPA supported over zirconia in
mesoporous channels of MCM-41 as catalyst in veratrole acetylation.Liquid-
phase Friedel–Crafts alkylation and acylation reactions have been reported
using aluminosilicate MCM-41 [225-229].
Conclusions
As mentioned in the above quoted literature, Friedel-Crafts acylation
and alkylation reactions have been widely investigated using various solid
acids, however with several limitations such as,Friedel-Crafts acylation and
alkylation was successful only with certain substrates [191], leaching in case
of supported catalysts, generation of secondary products [194], catalyst
deactivation when exposed to moisture [200] etc. Many of the reactions using
solid acid catalysts were also not economically viable. Such catalysts must be
designed that are both economically viable and able to withstand industrial
conditions. Thus, our endeavour as mentioned earlier in the text– “Global
effort to replace conventional liquid acid catalysts by solid acid
catalysts”is on. In continuation we herein report the catalytic activity of
12TPA-MCM-41-20(Induced Acidity) and Al-MCM-41-30 (Inherent Acidity) as
solid acid catalysts using Friedel-Crafts acylation and alkylation as model
reactions wherein 4-methoxy acetophenone (4MA), 3,4-dimethoxy
acetophenone (3,4DMA) and p-benzyl toluene (PBT) have been synthesized
under solvent free conditions. The activity of both catalysts have been
compared and correlated with surface properties of the materials.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
112
2.19 EXPERIMENTAL
Materials: Anisole, acetic anhydride, veratrole, benzyl chloride and toluene
were procured from Merck India.
Catalyst Synthesis:The synthesis and characterization of 12-TPA-MCM-41
(with various wt.% of 12-TPA loading) and Al-MCM-41(with varying SiO2:Al2O3
ratios) have been discussed earlier in the text (Section 2.10 and 2.11). It is
observed that 12-TPA-MCM-41 with 20wt.% 12-TPA loading abbreviated as
12TPA-MCM-41-20 and Al-MCM-41 with SiO2:Al2O3 ratio 30, abbreviated as
Al-MCM-41-30 exhibit highest surface acidity and thus used for all catalytic
studies.
Experimental setup for Friedel-Crafts acylation and alkylation:The
reactions were carried out in a two necked 50 ml round bottomed flask
equipped with a magnetic stirrer under heating in an oil bath. In a typical set
up, a mixture of anisole or veratrole (10 mmol) and acetic anhydride (15
mmol) for acylation and toluene (10 mmol) and benzyl chloride (15 mmol) for
alkylation, along with the catalyst (0.2 g) were taken in a round bottomed flask
and stirred at 110oC for three hours.In all the reactions the substrates were
used as solvents and hence the reaction temperature was kept according to
solvent used (reflux temperature) for all the studies. The reactions were
monitored by GC. After completion of reaction, the catalyst was separated by
decantation, and reaction mixture was distilled to obtain the products 4 MA,
3,4DMA and PBT, the boiling points being ~273oC,286oC and 300oC
respectively.
Regeneration and recyclability of catalyst: During the course of the
reaction, many a time the catalyst colour changes. This is probably due to the
adsorption of reacting molecules coming onto the surface of the catalyst. After
separation of catalyst in reaction mixtureby decantation, it is first refluxed in
ethanol for 30 minutes, followed by drying at 120°C. This material was used
as recycled catalyst. This regeneration procedure was followed in subsequent
recycle reaction.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
113
2.20 RESULTS AND DISCUSSION
Firstly, reaction condition was optimized using 12TPA-MCM-41-20as
solid acid catalyst for 3,4DMA synthesis by varying such parameters as
catalyst amount, initial mole ratio of the reactants, reaction time and
temperature. The results obtained have been presented in Table 2.13 and a
graphical presentation (Fig. 2.44 to 2.47).
Friedel crafts acylation of veratrole with acetic anhydride, gave
selectively 3,4 DMA. It is observed that yield increases with reaction time until
equilibrium is reached within 4 h. For the same reaction time, yield increases
with increasing catalyst amount, since the number of active sites per gm of
substrate increases. The influence of reactant ratio (veratrole:acetic
anhydride) was studied increasing from 1:0.75 to 1:2. It is observed that the %
yield of3,4DMAwas maximum with 1:1.5 mole ratio. (Table 2.13)
It has been reported earlier that there is no significant effect of solvents
in the acylation of anisole and veratrole and best results were obtained when
aromatic ethers were used as self solvents [189]. In the present study
therefore anisole and veratrole (aromatic ethers) have been used both as
substrates and solvent and for this reason while optimizing reaction condition,
concentration of only the acylating agent was varied. Thus, the Green
Chemistry principle 5 which states that the “use of solvents should be
made unnecessary whenever possible and when used, innocuous” is
implemented.Further, the boiling point of solvent was taken as reaction
temperature. However, when reaction temperature is varied (100 ˚C and 120
˚C), there is no significant change in % yield. Therefore 70˚C is optimized as
reaction temperature for 3,4DMA synthesis.
At optimum condition (mole ratio of reactants, veratrole:acetic
anhydride = 1: 1.5, amount of catalyst = 0.2 g, time = 4 h) 4MA and PBT have
been synthesized. Acylation of anisole with acetic anhydride gave selectively
4-methoxy acetophenone (4 MA) and alkylation of toluene with benzyl
chloride gave selectively p-benzyl toluene (PBT). Results are presented in
Table 2.14. Further, for comparative study, at this optimized condition Al-
MCM-41-30 is used as solid acid catalyst for synthesis of 3,4DMA, 4MA and
PBT. The results are presented in Table 2.14.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
114
When comparison is made between anisole and veratrole the product
yield and turn over number (TON) are higher for veratrole. The rate-
determining step of the Friedel-Crafts acylation is the formation of the
electrophilic intermediate. The presence of an additional electron donating
methoxy group in veratrole makes it a more active compound for electrophilic
substitution of acyl group in the para position than anisole due to an increased
electron density at para position and resultant increased susceptibility for
attack by the electrophile.
It is reported that the mechanism for Friedel-Crafts acylation and
alkylation over solid acid catalysts is the same as homogeneous Lewis acid
catalysts, [197,202,203]. The proposed mechanism for the acylation and
alkylation reaction on solid acid catalyst implies the formation of an adsorbed
species by interaction of the acylating/alkylating agent with a Brønsted acid
site [202,203] (acyl/alkyl cation). The Brønsted acid site generates an acyl
carbonium ion, which in turn affects the electrophilic substitution. A higher
density of acid sites increases number of acyl cations enhancing activity of the
reaction. Catalytic activity is a function of number as well as type of acid sites
present on the catalyst surface. The acylation and alkylation reactions are
thus driven by the surface acidity of the catalyst.Probably this is the reason
why 12TPA-MCM-41-20 gives higher yields compared to Al-MCM-41-30 due
to higher surface acidity observed in case of the former catalyst.In an earlier
study using TMA salts as solid acid catalyst in Friedel-Crafts acylation and
alkylation a probable mechanism [230,231] has been proposed by us, as
given in Scheme 2.9 and 2.10. The same mechanism is thought to be
operating in the present study.
% yields obtained in recycled catalyst and % decrease in yields in
subsequent cycles is presented in Table 2.14 and 2.15 respectively, and a
graphical presentation (Fig. 2.48). It is observed that in subsequent cycles
decrease in % yields is less for Al-MCM-41 compared to 12TPA-MCM-41-20.
This could be due to the leaching of 12TPA from surface of MCM-41. It is
observed that the colour of the catalyst changes after each catalytic run.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
115
+C+
O
CH3
OCH3H
H3CO
COCH3
H3CO
COCH3
CH3COO-H+
+ CH3COOH +H+
C
O
0H3C
C
O
CH3
CH3COO-H+
Catalyst
H+
+C+
O
CH3
+
-
Anisole
Acetic Anhydride
2-Methoxy Acetophenone (2-MA)
COCH3
OCH3
4-Methoxy Acetophenone (4-MA)
+
major product minor product
Catalyst
Catalyst
Catalyst
Scheme 2.9 Reaction mechanism of Friedel-Crafts acylation of anisole using solid acid
catalyst.
+
+Catalyst
H+
CH3
H3C
Cl-H+
Cl-H+
+ HCl + H+
CH2ClCH2
+
CH2+
H2C
H
H3C H2C
Catalyst
Catalyst
Catalyst
Scheme 2.10 Reaction mechanism of Friedel-Crafts alkylation of toluene using solid acid
catalyst.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
116
This gives an indication that during the course of the reaction the
reacting molecules come onto the surface of the catalyst. Some of them enter
into reaction to give the product while a few of them get adsorbed on the
surface, which is marked by the change in the colour of the catalyst. The fact
that the reactant molecules are weakly adsorbed is evident from the catalyst
regaining its original colour, when treated with ethanol. The possibility of
molecules entering interstices cannot be ruled out. This is observed from the
fact that the yields go down by 3-6% after every regeneration, leading to
deactivation of the catalyst.The deactivation of the catalyst might be due to
the strongly adsorbed acetic acid and the product on the acid sites. It is
known that acetic acid is generally strongly absorbed on the acidic sites. The
above two reasons are responsible for decrease in % yield.
Fig.2.44 Reaction time variation
Fig. 2.45Catalyst amount variation
Fig. 2.46Reaction temperature variation
Fig. 2.47Catalyst amount variation
20
30
40
50
60
70
80
0 2 4 6 8
% Y
ield
Reaction Time (h)
72
72.5
73
73.5
74
74.5
75
60 80 100 120 140
% Y
ield
Temperature (˚C)
1:0.75 1:1 1:1.5 1:20
10
20
30
40
50
60
70
80
% Y
ield
Mole ratio if the reactants
Chapter 2. Synthesis and Characterization of MCM-41 based materials
117
Fig. 2.48Comparative catalyst performance in the Friedel-Crafts alkylation and acylation
Table 2.13Optimization of reaction conditions for Friedel Crafts acylation and
alkylation using 12TPA-MCM-41-20
No Reactants with mole ratio
Catalyst
Amount
(g)
Time
(h)
Temp.
(°C)
% Yield of
3,4DMA
A Time variation
1 V: AA (1:1.5) 0.10 1 70 47.46
2 V: AA (1:1.5) 0.10 2 70 53.69
3 V: AA (1:1.5) 0.10 3 70 55.69
4 V: AA (1:1.5) 0.10 4 70 61.55
5 V: AA (1:1.5) 0.10 5 70 60.85
6 V: AA (1:1.5) 0.10 6 70 61.35
B Catalyst amount variation
7 V: AA (1:1.5) 0.15 4 70 69.42
8 V: AA (1:1.5) 0.20 4 70 74.33
9 V: AA (1:1.5) 0.25 4 70 74.47
C Mole ratio variation
10 V: AA (1:0.75) 0.20 4 70 47.80
11 V: AA (1:1) 0.20 4 70 53.54
12 V: AA (1:2) 0.20 4 70 74.31
D Temperature variation
13 V: AA (1:1.5) 0.20 4 100 73.45
14 V: AA (1:1.5) 0.20 4 120 72.49
A = Anisole; AA = Acetic Anhydride; V = Veratrole; T = Toluene; BzCl= Benzyl Chloride
0
10
20
30
40
50
60
70
80
Fresh 1st Cycle 2nd Cycle
Fresh 1st Cycle 2nd Cycle
Fresh 1st Cycle 2nd Cycle
% Y
ield
3,4DMA 4MA PBT
12-TPA-MCM-41-20 Al-MCM-41-30
Chapter 2. Synthesis and Characterization of MCM-41 based materials
118
Table 2.14Friedel Crafts acylation and alkylation using12TPA-MCM-41-20and Al-
MCM-41-30 at optimized condition
No Reactants Product Temp. (C°)
12TPA-MCM-41-20 Al-MCM-41-30
% Yields *TON % Yields *TON
1 V: AA 3,4DMA 70 74.33 10.81 62.14 9.02
2 A: AA 4MA 70 59.39 9.00 59.21 9.00
3 T: BzCl PBT 110 72.02 9.82 58.95 8.92
A Catalyst reusability
1 V: AA 1st Cycle 3,4DMA 70 72.89 10.62 59.43 9.00
2 V: AA 2nd Cycle 3,4DMA 70 68.37 10.05 56.17 8.08
3 A: AA 1st Cycle 4MA 70 56.40 8.20 56.00 8.00
4 A: AA 2nd Cycle 4MA 70 51.63 7.10 52.71 7.52
5 T: BzCl1st Cycle PBT 110 66.15 10.00 54.35 7.40
6 T: BzCl 2nd Cycle PBT 110 61.74 9.45 47.89 6.50
A = Anisole; AA = Acetic Anhydride; V = Veratrole; T = Toluene; BzCl= Benzyl Chloride;
Catalyst amount = 0.20g; reaction time = 4 h. mole ratio of the reactants = 1:1.5
(Veratrole/Anisole/Toluene:acylating/alkylating agent); *TON = Turn over number, gram of
product formed per gram of catalyst.
Table 2.15% Decrease in yields of 3,4DMA, 4MA and PBT using recycled catalysts
in subsequent cycles
Product
% Decrease in yields
12TPA-MCM-41-20 Al-MCM-41-30
1st Cycle 2nd Cycle 1st Cycle 2nd Cycle
3,4DMA 2 4 3 3
4MA 3 5 3 4
PBT 5 7 4 5
2.21 CONCLUSIONS
In the present study, Green Chemistry goals have been achieved by
using solid acid catalysts (replacing liquid acid catalysts used in conventional
reactions) and under solvent free conditions with high selectivity of the
products. The products formed can be simply distilled over, there is no
catalyst contamination in product and no acid waste formed. The catalyst can
be regenerated and reused. Further, since the studied reactions are driven by
surface acidity of the catalyst, there is scope to obtain higher/better yields by
synthesizing materials with higher surface acidity. Finally, the MCM-41 neutral
framework has been successfully modified and put to practical use.
Chapter 2. Synthesis and Characterization of MCM-41 based materials
119
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CCHHAAPPTTEERR 3 Synthesis and Characterization of Zr-MCM-41 and Ti-MCM-41 and their Application as Oxidation Catalysts. _________________________________________________________________________
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
131
3.1 INTRODUCTION As already discussed in Chapter II, since Si-MCM-41 possesses
poor/negligible catalytic activity, the properties were modified by inducing
acidity into the material via Al3+ substitution and anchoring of 12-TPA onto
MCM-41.The catalysts Al-MCM-41-30 and 12-TPA-MCM-41-20 with
maximum surface acidity were explored as solid acid catalysts using
esterification and Friedel-Crafts alkylation and acylation as model reactions.
Literature survey prompted us to incorporate Zr4+ and Ti4+ in the silica
framework and examine the redox properties created in the material by
carrying out catalytic test reactions.
3.2 LITERATURE SURVEY IN THE CURRENT AREA OF STUDY Recently, many efforts have been made worldwide by researchers for
the modification of siliceous silica MCM-41 mesoporous molecular sieve in
order to enhance its practicability, of which the incorporation of transition
metal ions into the siliceous MCM-41 framework for promoting redox
properties, thermal and hydrothermal stabilities has been found to be an
effective strategy. Consequently, the study of metal based mesoporous
composites has been a hot subject. Various transition metal ions such as Fe
[1], Nd, Cu, Mo [2] Co [3-5], Ni [6, 7], Al [8], Mn [9], V [1, 10], Ti [11-14], Zr
[15-17] and Ce [18] have been introduced into the framework of MCM-41
mesoporous molecular sieve to give it some redox sites, and these prepared
mesoporous materials exhibits remarkable catalytic performance.
Much attention has been focused towards Zirconium containing
mesoporous materials as potential candidates in catalytic applications, due to
their both moderate acidity and oxidizing capacity. The limitation of low
surface area (lower than 50m2/g) can be overcome by supporting Zirconia on
high surface oxides mainly silica. The resulting catalysts display strong acidity
and show satisfactory activity in a diversity of organic reactions (alcohol
dehydration, alkene isomerization, and cumene dealkylation) [19, 20].
To extend the Zirconia catalytic capabilities, to larger organic substrates,
the feasibility of incorporating Zr atoms into mesoporous silica has been
explored. Zr atoms have been introduced into the framework of ordered
(MCM-41 [15, 21-23] and MCM-48 [24]) and disordered (HMS) [16, 25, 26]
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
132
mesoporous silicas. These solids combine high surface area (usually higher
than 800m2/g) and size selectivity, but the synthetic procedures used severely
affect the catalytic performances of the resulting materials.
Since the reported synthesis of microporous TS-1 [27], titanium
substituted zeolites have been attracting great attention because of their
remarkable catalytic performance for selective oxidations of various organic
substrates [28-34]. However, they cannot effectively catalyze conversion of
bulky molecules, which have no access to the active sites located inside the
micropores (0.7 nm). Thus, attention has increasingly been directed towards
the study of metal-containing mesoporous M41S type molecular sieves with
large pores (20–100 Å diameter) suitable for the transformation of bulky
organic compounds [35-40]. Titanium-containing MCM-41 were reported for
the first time in 1994 [41, 42]. Corma and co-workers reported Ti-MCM-41 [42]
causing great interest on its structure and catalytic features. The discovery of
ordered mesoporous titanosilicates with wider pore sizes (2–10 nm) [41-49]
and mesoporous titanium-containing zeolite [50] offers an opportunity to use
titanosilicates as versatile catalysts in the oxidation of bulky reactant
molecules. The most important features of the ordered mesoporous
titanosilicates are the high surface area, which potentially allows an efficient
dispersion of active sites, and the large and uniform pore diameters in the
mesopore range, which favour the diffusion of bulky molecules. Ti-MCM-41
materials have been studied as catalysts for various reactions, such as the
epoxidation of olefins [42], unsaturated alcohols [51], plant oils [52] and the
oxidation of organic sulphides [53]. In the oxidation of small reactant
molecules, however, they show much lower catalytic activity than TS-1 and Ti-
beta, probably due to the lower hydrophobicity that promotes water
adsorption, which poisons the catalytically active centers [43]. Ti-MCM-41
also suffers from leaching of the titanium active species in the presence of
H2O, which causes the gradual deactivation of the catalyst. Therefore, an
enhancement of the hydrophobicity is considered important to improve the
activity of Ti-MCM-41 and retard the Ti leaching in liquid phase oxidation [54].
Literature reports that most of the mesoporous molecular sieves have
been synthesized by traditional hydrothermal methods. Usually, transition
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
133
metal doped MCM-41 materials are focused on the hydrothermal synthesis
under basic conditions, in which the desired metal precursors are added into
the mixture of silicate / surfactant / base / water [55]. The highly basic solution
(pH≥10) required for the synthesis of MCM-41 seems to limit the incorporation
of metal cation dopants. Under such conditions, the metal dopants tend to
form a second phase, because of incompatible condensation and precipitation
rates [56]. To avoid the problem, another doping method under acid
conditions was developed [57, 58]. In this route, large amount of mineral
acids, such as HCl, is often needed, which is not environment friendly. Wong
et al. attempted to replace HCl by salt, such as KCl, in the synthesis of Zr
doped mesoporous material, but only nonmeso structured precipitate was
obtained [56].
An important condition for having very active metal ion supported
catalysts, lies in site isolation. The metal ion must be isolated and well-
dispersed throughout the silica network, thus avoiding the formation of metal
oxide clusters [59]. In this sense, a major problem in the preparation of mixed
oxides from aqueous media by sol-gel related procedures is the unequal
hydrolysis and condensation rates of the metal-containing reagents.
Zirconium species usually hydrolyze faster than silicon precursors. In most
cases, this results in partial segregation of ZrO2 together with a decrease of
the Zr/Si molar ratio in the resulting mixed oxide with respect to the initial (gel)
composition. In such cases, the catalysts’ performance substantially depends
on their purity and chemical homogeneity, thus phase segregation must be
avoided.
Metal incorporated MCM-41, could be considered as mesoporous mixed
oxide materials, wherein metal oxides are highly dispersed and more active
sites can be available. Moreover, the mixed oxides generate some particular
properties, which are not possessed by the silica or transition metal oxide
alone. For example, it has been reported that zirconium incorporated
mesoporous silica exhibited strong acidity, although the acidities of both
zirconia and silica are weak [60-62].
In the present chapter, Zr4+ and Ti4+ have been incorporated into
mesoporous siliceous MCM-41 framework, varying SiO2:ZrO2 and SiO2:TiO2
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
134
ratio via sol-gel process. The materials have been characterized and the
catalytic property created in the material has been examined using
epoxidation as a model reaction wherein, allyl chloride is converted to
epichlorhydrin, an industrially important compound/reaction.
3.3 SYNTHESIS OF Zr-MCM-41 AND Ti-MCM-41 In the present endeavour, the main objective is to synthesize
mesoporous Zr-MCM-41 and Ti-MCM-41 by incorporating (isomorphous
substitution) maximum amount of Zr4+and Ti4+ at room temperature with good
thermal stability, high surface area as well as retention of surface area at high
temperature, along with creation of redox properties. A sol-gel method has
been used to achieve this objective. Several sets of materials were prepared
varying conditions in each case, using surface area as an indicative tool in all
cases. Table 3.1 and 3.2 describes the parameters that have been optimized
for synthesis of Zr-MCM-41 and Ti-MCM-41 respectively.
Materials Tetra ethyl-ortho silicate(TEOS) and analytical grade liquor ammonia
were obtained from Merck. Tetra butyl ortho titanate(TBOT) was procured
from Sigma Aldrich. Cetyl trimethyl ammonium bromide (CTABr), 20% tetra
propylammonium hydroxide (TPAOH), and zirconium oxychloride
(ZrOCl2.8H2O) were purchased from Loba chemicals. Double distilled water
(DDW) was used for all studies.
Synthesis of Zr-MCM-41 at optimized condition As indicated in optimization table-3.1 entry No-3 indicates optimum
conditions. We hereby describe synthesis of Zr-MCM-41 under optimized
conditions. The molar composition of Zr-MCM-41 is 1SiO2:0.2 ZrO2:0.6CTABr:
40H2O. In a typical 500g batch experiment the first step was preparation of
the precursor solution. TEOS 87.42 g was mixed with 100 g DDW under
continuous stirring at room temperature for ~15 min, in a polypropylene
container, to which was added an aqueous solution of ZrOCl2 (prepared by
dissolving 28.72 g in 60 g DDW) dropwise and with constant stirring within
~15 min. This is the precursor solution (A). An aqueous solution of CTABr was
prepared by dissolving 90.11 g CTABr in 133.74 g DDW under continuous
stirring at room temperature (B).Template solution B was added to precursor
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
135
solution A, dropwise and under constant stirring within ~15 min. The pH of the
resultant solution was adjusted to ~9.5 using 20% TPAOH. A gel was formed
which was further stirred for 30 min. The polypropylene container was now
closed and allowed to age at room temperature without stirring for 24 hrs. The
resultant gel was filtered, washed with DDW to remove adhering ions and
dried at 120°C followed by calcination at 550°C for 6h, at a heating rate of
2°C/min. This material was used for all further studies.
Note: In the present endeavour, TEOS as a silica source is preferred to
Na2SiO3, because Na2SiO3 causes hydrolysis and precipitation of Zirconium
component/source. Further, residual Na2O in the product may pose problems
for use of this material as selective oxidation catalyst. Entry no-3 does not
give maximum surface area, however it is important that maximum Zr4+ be
incorporated in the silicate matrix to give improved redox property. The pH in
the synthesis was adjusted to 9.5 because gel viscosity is maximum at this
pH, which can also be stirred with ease for homogenization.
Synthesis of Ti-MCM-41 at optimized condition As indicated in optimization table 3.2, entry no. 2 indicates optimum
conditions. We hereby describe synthesis of Ti-MCM-41 under optimized
conditions. The molar composition of Ti-MCM-41 is 1SiO2:0.033
TiO2:0.25CTABr: 40H2O. In a typical 500g batch experiment the first step was
preparation of the precursor solution. TEOS 103.25 g was mixed with 5.46 g
TBOT under continuous stirring at room temperature for ~15 min, in a
polypropylene container. This is the precursor solution (A). An aqueous
solution of CTABr was prepared by dissolving 44.36 g CTABr in 346.93 g
DDW under continuous stirring at room temperature (B). Template solution B
was added to precursor solution A, dropwise and under constant stirring
within ~15 min. The pH of the resultant solution was adjusted to~9.5 using
20% TPAOH. A gel was formed which was further stirred for 30 min. The
polypropylene container was now closed and allowed to age at room
temperature without stirring for 24 hrs. The resultant gel was filtered, washed
with DDW to remove adhering ions and dried at 120°C followed by calcination
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
136
Table 3.1 Zr-MCM-41Synthesis strategies-Parameters optimized:
Parameters No SiO2 Mole
ZrO2 Mole
SiO2/ZrO2 Input Mole ratio
Template Mole
Elemental Analysis (ICP-AES)
SiO2/ZrO2 output Mole
ratio
BET Surface area At different Temperature (m2/g)
%SiO2 %ZrO2 550°C 700°C 900°C
Template mole 1 1 0.2 5 0.25 68.10 39.98 4.50 550 536 464 2 1 0.2 5 0.4 69.00 30.70 4.60 550 526 417 3 1 0.2 5 0.6 70.01 29.51 4.86 832 700 372
SiO2/ZrO2 Mole ratio
4 1 0.05 20 0.6 89.07 10.80 16.90 841 743 731 5 1 0.025 40 0.6 92.80 5.45 34.90 905 849 748 6 1 0.016 60 0.6 96.31 3.75 52.64 948 918 857
SiO2 Source = TEOS ; ZrO2 source = ZrOCl2.8H2O ; Template Source = CTABr; H2O mole = 40; Temperature = (30±3 °C); pH = 9.5; Aging Time = 24 h Table 3.2 Ti-MCM-41Synthesis strategies-Parameters optimized:
Parameters No SiO2 mole
TiO2 mole
Template Mole
SiO2/TiO2 Input mole ratio
Element Analysis (ICP-AES)
SiO2/TiO2 Output mole
ratio
BET Surface area at different Temperature (m2/g)
%SiO2 %TiO2 550°C 700°C 900°C
SiO2/TiO2 mole ratio
8 1 0.05 0.25 20 86.51 5.95 24.86 1098 970 735 9 1 0.033 0.25 30 88.85 4.61 32.96 1617 1134 948
Template mole 10 1 0.033 0.4 30 89.13 4.55 33.50 1564 1095 912 SiO2 Source = TEOS ; TiO2 source = TBOT ; Template Source = CTABr; H2O mole = 40; Temperature = (30±3 °C); pH = 9.5; Aging Time = 24 h
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
137
at 550°C for 6h, at a heating rate of 2°C/min. This material was used for all
further studies.
Note: In the present synthetic endeavour TEOS as a silica source is preferred
to Na2SiO3, because Na2SiO3 causes hydrolysis and precipitation of Titanium
component/source. Further residual Na2O in the product may pose problems
for use of this material as selective oxidation catalyst. The pH in the synthesis
was adjusted to 9.5 because gel viscosity is maximum at this pH, which can
also be stirred with ease for homogenization. In the synthesis of mesoporous
Ti-MCM-41, the SiO2:TiO2 ratio was adjusted such that the resulting material
has high surface area with a maximum of Ti4+ incorporation to induce redox
properties and enhanced hydrophobicity in the material so as to avoid the
leaching of Ti4+ during use in an aqueous environment [54].
3.4 MATERIAL CHARACTERISATION: Instrumental Methods of Analysis:
Elemental analysis was performed on ICP-AES spectrometer (Thermo
Scientific iCAP 6000 series). X-ray diffractogram was obtained on X-ray
diffractometer (Bruker D8 Focus) with Cu-Kα radiation with nickel filter. FTIR
spectra was recorded using KBr pellet on Shimadzu (Model 8400S). Thermal
analysis (TGA) was carried out on a Shimadzu (Model TGA 50) thermal
analyzer at a heating rate of 10 ºC·min-1. SEM and EDX of the sample were
scanned on Jeol JSM-5610-SLV scanning electron microscope. TEM was
performed using Philips CM30 ST electron microscope operated at 300kv.
Surface area measurement was carried out on Micromeritics Gemini at -
196oC using nitrogen adsorption isotherms. UV-Visible-diffuse reflectance
spectra was obtained using UV-DRS, 2450 Shimadzu. Conversion of allyl
chloride to epichlorohydrin was determined by GC using Chemito ceres 800
plus equipped with flame-ionization detector (FID).
Characterisation of Zr-MCM-41: XRD of Zr-MCM-41 is presented in Fig. 3.1. A peak for 2θ between 2o
and 3o characteristic of the Bragg plane reflection (100) is observed, which is
sufficient evidence to indicate the incorporation of Zr4+ in the framework of
MCM-41. With increasing incorporation of Zr4+ the diffraction peak (100) of the
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
138
Zr-MCM-41 becomes broad and weak, accompanied by decrease in intensity
and long range ordering of Zr-MCM-41, which is in agreement with results
obtained from the literature [63, 64].
XRD high-angle pattern of Zr-MCM-41 is presented in Fig. 3.2. Bulk
ZrO2 phase exhibits characteristic peaks in the high-angle XRD pattern. In Zr-
MCM-41 synthesized in the present study, this peak is absent, which is further
evidence to incorporation of Zr4+ in the MCM-41 framework with good
dispersion and absence of ZrO2 cluster [65].
TEM image of Zr-MCM-41-5 (Fig. 3.3) shows hexagonal arrangement
of uniform pores in the sample. SEM image of Zr-MCM-41-5 (Fig. 3.4) exhibits
irregular morphology. Elemental analysis for Zr-MCM-41-5 performed by ICP-
AES shows % ZrO2 and % SiO2 to be 29.51 and 70.01 respectively (Table
3.1), which is also supported by EDX, which shows atomic % of Si = 27.56,
atomic % of Zr = 5.66 and atomic % of O= 66.78 (Fig. 3.5).
Surface area (ABET) determined by N2 adsorption BET method exhibits
isotherms of type IV, in accordance with the IUPAC classification for
mesoporous materials [66]. Pore diameter (~3.8 nm) confirms the
mesoporous nature of the synthesized material with pore size distribution
between ~2.5 – 8.0 nm, which is in range usually observed for Zr-MCM-41
(Fig. 3.6 and 3.7) [67]. Surface area of Zr-MCM-41 with various SiO2:ZrO2
ratio calculated by BET method are presented in Table 3.1.
The FTIR spectrum presented in Fig. 3.8 exhibits broad band in the
region ~3400 cm-1 assigned to –OH stretching vibration of mesoporous MCM-
41 structure. Band around ~ 1640 cm-1 is attributed to H-O-H bending
vibration. A broad band between 1250 cm-1 to 1050 cm-1 is attributed to
asymmetrical stretching of Si-O-Si. The band at ~960 cm-1 is attributed to Si-
O-Zr stretching. Bands around ~795 cm-1 and 450 cm-1 are attributed to
symmetric Si-O-Si stretching and bending respectively [68].
TGA thermogram of Zr-MCM-41-5 (Fig. 3.9) exhibits an initial weight
loss of ~ 14 % in the temperature range of 30-1500C due to loss of moisture
and hydrated water. Thereafter in the region 150-8000C, there is a
marginal/negligible weight loss in the material indicating good thermal stability
of the material.
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
139
UV-DRS spectrum of Zr-MCM-41 (with various SiO2:ZrO2 ratio) (Fig.
3.10) can offer additional information on the dispersion and environment of Zr
atoms in the MCM-41 framework. The intense absorption band ~ 210 nm is
attributed to a charge transfer transition from an oxygen atom to an isolated Zr
cation in a tetrahedral environment. Further, the absorbance of band at 210
nm increases with increasing ZrO2 content, indicating more isomorphous
substitution of Zr4+ [16,66].
Characterisation of Ti-MCM-41: XRD of Ti-MCM-41 is presented in Fig. 3.11. A peak for 2θ between 2o
and 3o characteristic of the Bragg plane reflection (100) is observed, which is
sufficient evidence to indicate the incorporation of Ti4+ in the framework of
MCM-41. With increasing incorporation of Ti4+ the diffraction peak (100) of the
Ti-MCM-41 becomes broad and weak, accompanied by decrease in intensity
and long range ordering of Ti-MCM-41, which is in agreement with results
obtained from the literature [54].
TEM image of Ti-MCM-41-30 (Fig. 3.12) shows hexagonal
arrangement of uniform pores in the sample. SEM image of Ti-MCM-41-30
(Fig. 3.13) exhibits irregular morphology. Elemental analysis for Ti-MCM-41-
30 performed by ICP-AES shows % TiO2 and % SiO2 to be 4.61 and 88.85
respectively (Table 3.2), which is also supported by EDX, which shows atomic
% of Si= 32.79, atomic % of Ti= 0.54 and atomic % of O= 66.67(Fig. 3.14).
Surface area (ABET) determined by N2 adsorption BET method exhibits
isotherms of type IV, in accordance with the IUPAC classification for
mesoporous materials [66]. Pore diameter (~3.2 nm) confirms the
mesoporous nature of the synthesized material with pore size distribution
between ~2.0 – 8.0 nm, which is in range usually observed for Ti-MCM-41-30
(Fig. 3.15 and 3.16). Surface area of Ti-MCM-41 with various SiO2:TiO2 ratios
calculated by BET method are presented in Table 3.2.
The FTIR spectrum presented in Fig. 3.17 exhibits broad band in the
region ~3426 cm-1 assigned to –OH stretching vibration of mesoporous MCM-
41 structure. Band around ~ 1640 cm-1 is attributed to H-O-H bending
vibration. A broad band between 1225 cm-1 to 1030 cm-1 is attributed to
asymmetrical stretching of Si-O-Si. The band at ~960 cm-1 is attributed to Si-
O
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Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
141
Fig 3.5. EDX of Zr-MCM-41-5
Fig. 3.6 N2 Adsorption Isotherm of Zr-MCM-41-5
Fig.3.7 Pore size distribution of Zr-MCM-41-5
Fig.3.8 FTIR of Zr-MCM-41-5
Fig.3.9 TGA of Zr-MCM-41-5
Fig.3.10 UV-DRS of Zr-MCM-41
Element Wt.% At. % Comp. % Si 32.71 27.56 69.98 Zr 21.80 5.66 29.45 O 45.49 66.78 Totals 100 Formula SiO2 and ZrO2
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
142
Fig.3.11 XRD of Ti-MCM-41
Fig.3.12 TEM of Ti-MCM-41-30
Fig 3.13 SEM of Ti-MCM-41-30
Fig 3.14 EDX of Ti-MCM-41-30
Fig-3.15 N2 Asorption Isotherm of Ti-MCM-41-
30
Fig-3.16 Pore size distribution of Ti-MCM-41-30
Element Wt.% At. % Comp. %
Si 45.74 32.79 97.85 Ti 1.29 0.54 2.15 O 52.97 66.67 Totals 100 Formula SiO2 and TiO2
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-MCM-41-30
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Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
144
consumption of ECH is used to make epoxy resins. It is also used in the
production of epichlorohydrin elastomers, polyamide-epichlorohydrin resins,
water treatment chemicals, synthetic glycerol, polyols and glycidyl derivatives
[72].
The traditional epoxidation processes are, (i) non-catalytic process using
chlorine, (ii) co-epoxidation processes and (iii) catalytic processes based on
organic peroxides and peracids [73]. These processes are very capital-
intensive. The use of chlorine has environmental disadvantages due to the
large output of chlorine-laden sewage. The employment of peracids is not a
clean method as an equivalent amount of acid waste is produced.
Furthermore, the homogeneously catalytic processes usually suffer from the
difficulty of product separation and catalyst recovery. As to the co-epoxidation
processes, the coupling product should be an equivalently commercial
desired one [74]. Therefore, enantioselectivity of the epoxidation reaction
plays an important role, since absolute configuration is essential in the
synthesis of many industrially important products as mentioned above.
Hydrogen peroxide (H2O2) is an attractive option of oxidants that can
epoxidize olefinic compounds in the presence of various transition metal-
containing catalysts such as Ti, V, Cr and Mo etc [74]. As the oxidant, it does
not cause environmental contamination, and the reaction with its contribution
produces water, besides the major product under mild reaction conditions. An
additional advantage of the application of H2O2 in various chemical processes
is associated with its relatively low price. Due to good oxidative properties,
H2O2 has found applications in many processes, displacing other chemical
oxidants which are associated with the formation of wastes [75].
Literature survey in the current area of study: The first truly heterogeneous epoxidation catalyst was presented by
Shell workers in 1970 [76]. Impregnating a silica surface with a TiCl4 precursor
lead to the so called TiO2-on-SiO2 catalysts. The Ti-precursors were attached
by free silanol groups on the surface of the silica carrier. In this way, Ti (IV)
site isolation was achieved and Lewis acidity created by electron withdrawal
through the Si-O-ligands [77]. When using alkyl hydroperoxides as oxidants,
this supported oxide showed high activity. Today, this catalyst is still applied in
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
145
half of the worldwide propene oxide production performed by continuous liquid
phase epoxidation. However, when using H2O2, the hydrophilic catalyst is
rapidly deactivated by leaching of Ti.
An important milestone in the history of heterogeneous epoxidation
catalysts is the discovery of the crystalline microporous TS-1 by Taramasso et
al. [27]. This titanium-substituted molecular sieve was found to be an active
and selective oxidation catalyst using H2O2 as oxidant and due to its
hydrophobic surface, leaching of Ti could be prevented. Epoxidation takes
place at low temperatures and is carried out in a polar solvent.
Currently, new methods for the preparation of epoxides are being
developed while previous ones are being improved. Considerable effort is
directed towards the use of catalysts and reactants that are environment
friendly. Another desirable thing is to use heterogeneous catalysts because of
their easy separation, regeneration and operational simplicity. Further, the
epoxidation reaction using heterogeneous solid catalysts with H2O2 as an
oxidant are environment friendly routes to produce extensively useful
epoxides which are traditionally obtained from capital-intensive or
environmentally polluted processes. Therefore, development of
heterogeneous catalytic processes for epoxidation using H2O2 as an oxidant
is very demanding [74].
Zhou et. al. [74] have reported various types of solid catalysts for the
epoxidation of olefins with H2O2 as an oxidant. In their review, they have
reported: (i) microporus and mesoporous molecular sieves such as
framework-substituted zeolites, aluminium phosphate molecular sieves and
transition metal substituted mesoporous materials, (ii) layered type materials
such as hydrotalcites, tungstates and tungstic acid, (iii) inorganic oxides and
supported catalysts such as mixed oxides (Al2O3-ZrO2, Al2O3-TiO2, SiO2-TiO2
and ZrO2-TiO2 etc.), Re2O3 supported catalysts, Al2O3 supported catalysts,
(iv) porous materials encapsulated metal complexes, (v) polyoxometalates
and (vi) supported porphyrins.
All above mentioned catalysts have shown potential in olefin
epoxidation, sometimes depending on the reaction conditions. Among these,
the catalyst systems with W, Ti and Mo have much better prospect of
industrial application from the economical view point, although these catalysts
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
146
and reaction conditions should be further optimized [74]. However, the
catalytic performance of most catalysts still cannot satisfy all requirements of
commercialization. Further improvements should still be sought to meet the
need of industrial processes with H2O2 as an oxidant [74].
In recent years, catalytic systems which selectively activate hydrogen
peroxide are used which includes titanium silicalites such as: TS-1, TS-2, and
Ti-Beta that have replaced more dangerous acid catalysts (HF, HCl, H2SO4).
Their additional advantage is associated with frequent elimination of the
intermediate stages, which contributes to the reduction of the waste amounts.
As a result of heterogeneity, they can be easily separated from the reaction
environment and subjected to regeneration. One of the advantages is the
possibility of multiple regeneration and reduced corrosion of apparatus. The
application of titanium silicalite catalyst in alkene epoxidation eliminates the
formation of by-products, which are usually formed in the conventional
epoxidation processes. The titanium silicalite zeolites are used not only during
the epoxidation of alkenes [78-83], but also for the oxidation of alcohols to
aldehydes and ketones [84-86], for monoepoxidation of dienes [87],
stereoselective epoxidation of cis- and trans-isomers [88], and for the
oxidation of vinylbenzenes to corresponding aldehydes [89].
Since the application of titanium silicalite catalysts and hydrogen
peroxide in the synthesis processes causes an improvement in the selectivity
of transformation and the yields, extensive research has been generated in
this area [90-92]. Further, the processes can be carried out under mild
conditions, which results in the reduction of operating costs of the installations
[93].
Adam et al. [94, 95] have proposed the mechanism of allyl alcohol
epoxidation using H2O2 over titanium silicalite catalysts that assume the
formation of the active adduct A which is a five-membered hydroperoxide
adduct, in which both the H2O2 molecule and alcohol molecule (protic solvent)
are associated with the active centre (the Ti4+ ion). In this adduct there is also
a hydrogen bond between oxygen of the alcohol molecule and hydrogen of
the hydroperoxide group. ROH is an additional solvent molecule or the water
molecule, which is coordinatively bonded to the active centre [96]. In the
structure B the allyl alcohol molecule is associated with the active adduct A
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
147
(3.1)
(3.2)
through the hydrogen bond formed between the —OH group of allyl alcohol
and the oxygen atom located close to the Ti atom in the active adduct A. This
hydrogen bond stabilises the entire system. The presented mechanism
reveals that the nature of the solvent plays a very important role in the course
of epoxidation [96].
Cheng-Hua et al. [97] have reported that, the framework Ti is the active center
with tetrahedral coordination. In this process, H2O2 is first adsorbed on the
surface of the molecular sieves and interacts with the framework Ti to form
titanium peroxocomplex Ti-OOH species, an active intermediate, which can
carry out the surface reaction with the substrates to form the products.
According to them [97] the epoxidation reaction can be expressed as follows:
+ +(SiO3)Ti-OH H2O2 (SiO3)Ti-OOH H2O(fast)
CH
H2C Cl+ H2C
O
CHH2C Cl
Allyl chloride Epichlorohydrin
H2C(SiO3)Ti-OOH + (SiO3)Ti-OH(slow)
Scheme 3.1 Mechanism of allyl alcohol epoxidation over the titanium silicate catalyst [96].
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
148
Literature survey reveals that much of the researches are focused
towards Ti substituted molecular sieves. However, not much has been
reported on catalytic / redox behavoiur of Zr incorporated MCM-41.
Importance of epichlorohydrin prompted us to carry out epoxidation reaction
using Zr-MCM-41 and Ti-MCM-41 as heterogeneous solid catalysts in the
catalytic conversion of allyl chloride to epichlorohydrin.
3.6 EXPERIMENTAL Materials: Methanol, allyl chloride, tertiary butyl hydrogen peroxide (TBHP,
17.8% active oxygen), NaOCl (21.6% active oxygen) and hydrogen peroxide
50% (47.0% active oxygen) were obtained from Merck India. Sodium
thiosulfate (Na2S2O3) was obtained from Loba Chemicals.
Catalyst Synthesis: Synthesis and characterization of mesoporous Zr-MCM-
41 and Ti-MCM-41 has been discussed in detail in section 3.3. Experimental set up for conversion of allyl chloride to epichlorohydrin: In a typical reaction, 2.5 g catalyst (Zr-MCM-41-5), 10 mmol methanol (MT)
(as a solvent), 10 mmol allyl chloride (AC) and 10 mmol H2O2 (50%) were
taken in a 50 ml three necked round bottomed flask, equipped with a water
condenser and placed on a magnetic stirrer. The reaction mixture was stirred
at room temperature for 5h after which sample was withdrawn and analyzed
by GC. H2O2 concentration before and after reaction was determined by
iodometric titration (Eq.3.3). All reactions were carried out varying several
parameters such as reaction time, amount of catalyst used, catalysts used
with varying SiO2:ZrO2 and SiO2:TiO2 ratio in Zr-MCM-41 and Ti-MCM-41
samples respectively, and amount of oxidant used and these parameters
were optimized using Zr-MCM-41-5 (Table 3.3). Using these optimized
conditions, the activity of the other synthesized catalysts with different Zr and
Ti content has been studied. Calculation of % conversion based on H2O2 concentration
3.3
Where, A= Initial concentration of H2O2, B= Final concentration of H2O2 after
completion of reaction (unreacted H2O2), C= Amount of sample withdrawn, N=
Normality of Sodium thiosulfate (Na2S2O3).
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
149
GC chromatogram showing conversion of allyl chloride to ECH using
Zr-MCM-41-5 and Ti-MCM-41-30 have been presented in fig. 3.20 and 3.21
respectively.
Fig. 3.20 Gas chromatograph of allyl chloride conversion to ECH using Zr-MCM-41-5.
Fig. 3.21 Gas chromatograph of allyl chloride conversion to ECH using Ti-MCM-41-30.
Three peaks are observed in GC chromatogram corresponding to
solvent, allyl chloride and ECH. Since no other peaks are observed in the GC
chromatogram it is concluded that % selectivity for ECH formed is ~100 %.
Further, the % H2O2 consumed can be directly correlated to ECH formed.
Regeneration and recyclability of catalyst During the course of the reaction, many a time the catalyst colour
changes. This is probably due to the adsorption of reacting molecules coming
onto the surface of the catalyst. After separation of catalyst in reaction mixture
by decantation, it is first refluxed in ethanol for 30 minutes, followed by drying
at 120oC. This material was used as recycled catalyst. This regeneration
procedure was followed in subsequent recycle reaction.
3.7 RESULTS AND DISCUSSION The epoxidation reaction of allyl chloride using H2O2 as oxidant to give
ECH is presented as follows:
CH
CH3OH H2O2Catalyst R.T.
H2C Cl + + H2C
O
CHH2C Cl
Allyl chloride Epichlorohydrin
H2C
Scheme 3.2 Epoxidation of allyl chloride using H2O2.
Firstly, reaction conditions for conversion of allyl chloride to ECH were
optimized using Zr-MCM-41-5 catalyst and results presented in table 3.3. All
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
150
the reactions were carried out at room temperature, to avoid thermal
decomposition of hydrogen peroxide using methanol as a solvent. It is
observed that yield increases with reaction time until equilibrium is reached
within 5h (Fig. 3.20). For the same reaction time yield increases with
increasing catalyst amount, since the number of active sites per gm of
substrate increases(Fig. 3.21). In all cases selectively only ECH was formed
with selectivity ~99%, indicating no by-product formed.
Better catalytic activity of H2O2 as oxidant, compared to NaOCl and
TBHP can be explained due to, formation of active adduct with H2O2
compared to NaOCl and TBHP (Table 3.3, Entry No. 13 and 14) (Fig. 3.22).
When concentration of H2O2 was decreased (Table. 3.3, Entry No. 12) the %
yield decreases. The presence of methanol in the reaction medium causes
that methanol participates in the formation of active adduct [96]. This role is
performed by water in the presence of an aprotic solvent. However, the
electrophilic properties of the active adduct with the participation of water are
weaker than those with methanol as a solvent [96]. For this reason the
epoxidation in the aprotic solvent proceeds slowly. Therefore, protic solvent
methanol was used in the present study.
When mole ratio of SiO2/ZrO2 was increased from 5 to 20, 40 and 60
catalytic activity decreased which could be attributed to decrease in Zr4+
content in the silica framework. Zr-MCM-41-5 thus exhibits highest catalytic
activity (Table 3.3, Entry No. 10) (Fig. 3.23). Amongst Ti-MCM-41-20 and Ti-
MCM-41-30, the later is catalytically more active (Fig. 3.24).
Based on ESR studies, Chaudhary et al. [54] have reported that Ti4+
undergoes easy reduction compared to Zr4+ and thus higher catalytic activity
for Ti containing catalyst is expected compared to Zr containing catalyst. In
the present study, there is not much difference in % yield using Ti-MCM-41-30
compared to Zr-MCM-41-5 where, Zr4+ content is greater than Ti4+ content.
However, the average % yield of ECH calculated from entry no. 15 and 16
(Table 3.3), with SiO2: ZrO2 ratio 20 and 40 respectively, is ~37.69 %
compared to % yield of ECH from entry no. 19 (Table 3.3) with SiO2:TiO2 ratio
30 which is 39.45 % which is in line with observation made in literature [54],
indicating that the Ti4+ incorporated catalyst is catalytically more active
compared to Zr4+ incorporated catalyst. The mechanism involved in the
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
151
formation of ECH could be proposed as suggested in literature reports [96]
(Scheme 3.1) shown earlier in the text where, methanol is used as solvent
using TS-1 and TS-2 as catalysts. Table 3.3 Optimization of reaction conditions for conversion of allyl chloride to ECH*. Sr. No.
Reactants with their mole ratio
Catalyst used Amount of
Catalyst (g)
ReactionTime (h)
Oxidant Used
(mole)
% Conversion
of H2O2
%Selectivity
Optimization of Reaction Time 1 MT:AC (1:1)
Zr-MCM-41-5 0.5 2 H2O2 (1) 18.84 -
2 MT:AC (1:1) Zr-MCM-41-5 0.5 3 H2O2 (1) 25.50 - 3 MT:AC (1:1) Zr-MCM-41-5 0.5 4 H2O2 (1) 28.40 - 4 MT:AC (1:1) Zr-MCM-41-5 0.5 5 H2O2 (1) 30.86 - 5 MT:AC (1:1) Zr-MCM-41-5 0.5 6 H2O2 (1) 31.01 - 6 MT:AC (1:1) Zr-MCM-41-5 0.5 8 H2O2 (1) 31.30 -
Optimization of Catalyst amount 7 MT:AC (1:1) Zr-MCM-41-5 1.0 5 H2O2 (1) 39.08 99.45 8 MT:AC (1:1) Zr-MCM-41-5 1.5 5 H2O2 (1) 40.58 99.50 9 MT:AC (1:1) Zr-MCM-41-5 2.0 5 H2O2 (1) 41.01 99.50 10 MT:AC (1:1) Zr-MCM-41-5 2.5 5 H2O2 (1) 42.85 99.5611 MT:AC (1:1) Zr-MCM-41-5 3.0 5 H2O2 (1) 43.20 98.80
Optimization of Oxidant amount 12 MT:AC (1:1)
Zr-MCM-41-5 2.5 5 H2O2 (0.5) 31.14 98.80
Optimization of various oxidant used 13 MT:AC (1:1) Zr-MCM-41-5 2.5 5 TBHP (1) 37.23 99.20 14 MT:AC (1:1) Zr-MCM-41-5 2.5 5 NaOCl (1) 31.50 99.10
Optimization of SiO2:ZrO2 ratio of catalyst used 15 MT:AC (1:1) Zr-MCM-41-20 2.5 5 H2O2 (1) 38.46 99.60 16 MT:AC (1:1) Zr-MCM-41-40 2.5 5 H2O2 (1) 36.92 99.40 17 MT:AC (1:1) Zr-MCM-41-60 2.5 5 H2O2 (1) 30.76 99.25
Optimization of SiO2:TiO2 ratio of catalyst used 18 MT:AC (1:1) Ti-MCM-41-20 2.5 5 H2O2 (1) 36.81 99.17 19 MT:AC (1:1) Ti-MCM-41-30 2.5 5 H2O2 (1) 39.45 99.78
Recyclability of Zr-MCM-41-520 MT:AC (1:1) Zr-MCM-41-5
(1st Cycle) 2.5 5 H2O2 (1) 39.59 99.35
21 MT:AC (1:1) Zr-MCM-41-5 (2nd Cycle)
2.5 5 H2O2 (1) 36.12 99.28
Recyclability of Ti-MCM-41-30 22 MT:AC (1:1) Ti-MCM-41-30
(1st Cycle)2.5 5 H2O2 (1) 37.57 99.57
23 MT:AC (1:1) Ti-MCM-41-30 (2nd Cycle)
2.5 5 H2O2 (1) 35.84 99.42
*All reactions were carried out at room temperature. MT: Methanol, AC: Allyl Chloride
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
152
Fig.3.22 Optimization of reaction time
Fig.3.23 Optimization of catalyst amount
Fig. 3.24 Optimization of oxidants used
Fig 3.25 Effect of SiO2:ZrO2 ratio on ECH conversion
Fig 3.26 Effect of SiO2:TiO2 ratio on ECH
conversion
Fig. 3.27 Comparative catalytic performance in the conversion of ECH
Zr-MCM-41-5 Ti-MCM-41-30
Zr-MCM-41-5Zr-MCM-41-5
0
5
10
15
20
25
30
35
0 2 4 6 8 10
% C
onve
rsio
n of
EC
H
Reaction time (h)
38.539
39.540
40.541
41.542
42.543
43.5
0 1 2 3 4
% C
onve
rsio
n of
EC
H
Catalyst amount (g)
0
5
10
15
20
25
30
35
40
45
H2O2 TBHP NaOCl
% C
onve
rsio
n of
EC
H
05
1015202530354045
% C
onve
rsio
n of
EC
H
32
34
36
38
40
42
44
1st Cycle
2nd Cycle
3rd Cycle
1st Cycle
2nd Cycle
3rd Cycle
% C
onve
rsio
n of
EC
H
3535.5
3636.5
3737.5
3838.5
3939.5
40
Ti-MCM-41-20 Ti-MCM-41-30
% C
onve
rsio
n of
EC
H
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
153
Cheng-Hua et al. have reported the synthesis of ECH using allyl chloride
with H2O2 using Ti-ZSM-5 as catalyst [97]. The maximum conversion reported
by them based on allyl chloride, is ~67%, however using 8 g/L of catalyst (Ti-
ZSM-5) with reaction time ~4.5 h and at 450C temperature. In the present
study, ECH is obtained at room temperature. % yield of ECH (based on %
conversion of H2O2) is ~39% using only 2.5 g/L of catalyst (Ti-MCM-41-30)
with reaction time ~5h.
Recyclability study of the catalysts: Entry no. 20 to 23 (Table 3.3) indicates that there is not much decrease
in % yield, which indicates that catalysts synthesized are stable and there is
no indication of metal ion leaching (Fig. 3.25). It was observed that the colour
of the catalyst changes after each catalytic run. This gives an indication that
during the course of the reaction the reacting molecules come onto the
surface of the catalyst. Some of them enter into reaction to give the product
while a few of them get adsorbed on the surface, which is marked by the
change in the colour of the catalyst. The fact that the reactant molecules are
weakly adsorbed is evident from the catalyst regaining its original colour,
when treated with ethanol. The possibility of molecules entering interstices
cannot be ruled out. This is observed from the fact that the yields go down
after every regeneration, leading to deactivation of the catalyst. The
deactivation of the catalyst might be due to the adsorption of allyl chloride and
ECH formed on the active sites of the catalyst.
3.8 CONCLUSIONS The study indicates the promising use of Zr-MCM-41 and Ti-MCM-41
as environment friendly heterogeneous solid acid catalysts for epoxidation of
allyl chloride to ECH with ~99% selectivity at room temperature. The
epoxidation reaction involves operational simplicity. The product formed can
be simply distilled over and the catalyst can be regenerated and reused. The
conversion of allyl chloride to ECH by using Zr-MCM-41-5 and Ti-MCM-41-30
as heterogeneous solid acid catalysts, along with H2O2 as an oxidant
eliminates the use of corrosive chlorine and peracids (used in conventional
process) as well as generation of chlorine-laden sewage and acid waste.
Further, the reaction with H2O2 has the advantage of mild reaction condition,
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
154
production of only water as by-product with added contribution to the
epoxidation reaction. Finally, the conventional method for production of ECH
is a non-catalytic process using chlorine. By using eco-friendly solid acid
catalysts such as Zr-MCM-41 and Ti-MCM-41 the green chemistry principle
no. 9, “Catalysts (as selective as possible) are superior to stoichiometric reagents” is implemented.
Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts
155
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CCHHAAPPTTEERR 44 Synthesis and Characterization of a Palladium Loaded Perovskite MCM-41 material and its Application as an Automotive Catalyst ________________________________________________________
Chapter 4. Automotive Catalysis
161
4.1 INTRODUCTION: Automotive catalysts designed to detoxify the exhaust were
implemented in production in US on vehicles of the model year 1975 and, as
we are reaching a full quarter century of their use, there is ample information
available to allow us to declare that these devices, which are the principal
emission control tools, have proved to be an unqualified success. Following
the positive experience in US, in short order Japan and thereafter Europe, in
1986, adopted the use of automotive catalysts. Less affluent developing
societies have come to the realization that emission control in heavily
populated areas is not a costly frill but a tangible benefit for the quality of life
and the use of automotive catalysts is rapidly spreading around the globe.
Even a subjective, casual visitor to urban centers where these devices have
not yet been widely implemented will quickly notice the difference in air
quality. The ubiquity of automobiles, and by extension of catalysts, has made
catalysts and their function much more familiar to the population at large.[1]
The significant environmental implications of vehicles cannot be
denied. The need to reduce vehicular pollution has led to emission control
through regulations in conjunction with increasingly environment-friendly
technologies. In India, it was only in 1991 that the first stage emission norms
came into force for petrol vehicles and in 1992 for diesel vehicles. The
following chart indicates, step wise enforcement of norms for emission of
pollutants from two wheelers.
Fig. 4.1 Progressive Reduction of Indian Emission Norms 2 Wheelers (Both 2 and 4 stroke):
Chapter 4. Automotive Catalysis
162
Table 4.1 Stepwise enforcement of norms for emission of pollutants from two wheelers
Year 1991 1996 2000 2005 CO g/km 12 4.5 2.0 1.5
HC+NOx g/km 8 3.6 2.0 1.5
From April 1995 mandatory fitment of catalytic converters in new petrol
passenger cars sold in the four metros of Delhi, Calcutta, Mumbai and
Chennai along with supply of Unleaded Petrol (ULP) was effected. Availability
of ULP was further extended to 42 major cities and now it is available
throughout the country. The emission reduction achieved from pre-89 levels is
over 85% for petrol driven and 61% for diesel vehicles to 1991 levels. Since
the year 2000, passenger cars and commercial vehicles have been meeting
Euro I equivalent India 2000 norms, while two wheelers have been meeting
one of the tightest emission norms in the world. Euro II equivalent Bharat
Stage II norms are in force from 2001 in 4 metros of Delhi, Mumbai, Chennai
and Kolkata. Since India embarked on a formal emission control regime only
in 1991, there is a gap in comparison with technologies available in the USA
or Europe. Currently, we are behind Euro norms by few years, however, a
beginning has been made, and emission norms are being alligned with Euro
standards and vehicular technology is being accordingly upgraded. Vehicle
manufactures are also working towards bridging the gap between Euro
standards and Indian emission norms.
4.2. AUTOMOTIVE EXHAUST PURIFICATION CATALYSIS-
TERMS AND CONCEPTS: Catalytic converter is an exhaust emission control device which
converts toxic chemicals in the exhaust of an internal combustion engine into
less toxic substances. Inside a catalytic converter, a catalyst stimulates a
chemical reaction in which toxic byproducts of combustion are converted to
less toxic substances by way of catalysed chemical reactions. The specific
reactions vary with the type of catalyst installed. Most present-day vehicles
that run on gasoline are fitted with a "three way" converter, so named because
it converts the three main pollutants in automobile exhaust: an oxidizing
reaction converts carbon monoxide (CO) and unburnt hydrocarbons (HC), and
Chapter 4. Automotive Catalysis
163
a reduction reaction converts oxides of nitrogen (NOx) to produce carbon
dioxide (CO2), nitrogen (N2), and water (H2O).
The catalytic converter consists of several components:
The catalyst core. For automotive catalytic converters, the core is usually a
ceramic monolith with a honeycomb structure. Metallic foil monoliths made of
FeCrAl are used in some applications. This is partially a cost issue. Ceramic
cores are inexpensive when manufactured in large quantities. Metallic cores
are less expensive to build in small production runs. Either material is
designed to provide a high surface area to support the catalyst washcoat, and
therefore is often called a "catalyst support". The cordierite ceramic substrate
used in most catalytic converters was invented by Rodney Bagley, Irwin
Lachman and Ronald Lewis at Corning Glass, for which they were inducted
into the National Inventors Hall of Fame in 2002.
Monolith The combined requirements of compactness, high volumetric flow rates
and low back pressure led to the adoption of a monolithic embodiment for
automotive catalysts, quite different from the packed-bed forms that prevailed
in almost all industrial and petroleum catalysis. The monoliths were multi-
channeled ceramic catalyst bodies (square, triangular or honeycomb channel
configurations), with the exhaust gas flowing through the channels on whose
walls there is a coated high surface area porous layer with finely dispersed
noble metal catalytic particles. (At the dawn of the implementation period,
some manufacturers stuck to the more familiar granular catalysts. In use,
these catalyst granules or beads suffered from attrition and ultimately were
abandoned in favour of the monoliths and also due to poorer warm-up
characteristics.) Presently, monolithic catalysts (mostly ceramic but also
metallic in some special instances) are universally used in automotive
catalysis. Moreover, the “monolith” catalytic technology is migrating into the
realm of industrial catalysis, most often in processes for treatment of industrial
effluents.
The washcoat. A washcoat is a carrier for the catalytic materials and is used
to disperse the materials over a high surface area. Aluminum oxide, Titanium
dioxide, Silicon dioxide, or a mixture of silica and alumina can be used. The
catalytic materials are suspended in the washcoat prior to applying to the
Chapter 4. Automotive Catalysis
164
core. Washcoat materials are selected to form a rough, irregular surface,
which greatly increases the surface area compared to the smooth surface of
the bare substrate. This in turn maximizes the catalytically active surface
available to react with the engine exhaust.
Precious metal. The catalyst itself is most often a precious metal. Platinum is
the most active catalyst and is widely used, but is not suitable for all
applications because of unwanted additional reactions and high cost.
Palladium and rhodium are two other precious metals used. Rhodium is used
as a reduction catalyst, palladium is used as an oxidation catalyst, and
platinum is used both for reduction and oxidation. Cerium, iron, manganese
and nickel are also used, although each has its own limitations.
Types of catalytic converters: Two-way A two-way (or "oxidation") catalytic converter has two simultaneous tasks:
1. Oxidation of carbon monoxide to carbon dioxide:
2CO + O2 → 2CO2 (4.1)
2. Oxidation of hydrocarbons (unburnt and partially burnt fuel) to carbon
dioxide and water:
CxH2x+2 + [(3x+1)/2] O2 → xCO2 + (x+1) H2O (a combustion reaction) (4.2)
This type of catalytic converter is widely used on diesel engines to
reduce hydrocarbon and carbon monoxide emissions. They were also used
on gasoline engines in American- and Canadian-market automobiles until
1981. Because of their inability to control oxides of nitrogen, they were
superseded by three-way converters.
Three-way Since 1981, "three-way" (oxidation-reduction) catalytic converters have
been used in vehicle emission control systems in the United States and
Canada; many other countries have also adopted stringent vehicle emission
regulations that in effect require three-way converters on gasoline-powered
vehicles. The reduction and oxidation catalysts are typically contained in a
common housing, however in some instances they may be housed
separately. A three-way catalytic converter has three simultaneous tasks:
Chapter 4. Automotive Catalysis
165
I. Reduction of nitrogen oxides to nitrogen and oxygen:
2NOx → xO2 + N2 (4.3)
II. Oxidation of carbon monoxide to carbon dioxide:
2CO + O2 → 2CO2 (4.4)
III. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water:
CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O (4.5)
These three reactions occur most efficiently when the catalytic
converter receives exhaust from an engine running slightly above the
stoichiometric point. This point is between 14.6 and 14.8 parts air to 1 part
fuel, by weight, for gasoline. The ratio for Autogas (or liquefied petroleum gas
(LPG)), natural gas and ethanol fuels is each slightly different, requiring
modified fuel system settings when using those fuels. In general, engines
fitted with 3-way catalytic converters are equipped with a computerized
closed-loop feedback fuel injection system using one or more oxygen
sensors, though early in the deployment of three-way converters, carburetors
equipped for feedback mixture control were used.
Three-way catalysts are effective when the engine is operated within a
narrow band of air-fuel ratios near stoichiometry, such that the exhaust gas
oscillates between rich (excess fuel) and lean (excess oxygen) conditions.
However, conversion efficiency falls very rapidly when the engine is operated
outside of that band of air-fuel ratios. Under lean engine operation, there is
excess oxygen and the reduction of NOx is not favored. Under rich conditions,
the excess fuel consumes all of the available oxygen prior to the catalyst, thus
only stored oxygen is available for the oxidation function. Closed-loop control
systems are necessary because of the conflicting requirements for effective
NOx reduction and HC oxidation. The control system must prevent the NOx
reduction catalyst from becoming fully oxidized, yet replenish the oxygen
storage material to maintain its function as an oxidation catalyst.
Light off Temperature: Car catalyst do not work at low temperature due to
kinetic considerations and if the temperature of the catalyst is increased,
increase in conversion from zero to high levels occurs over a short
temperature range, the inflexion point of this curve is often called the light off
temperature. It is important that this temperature is as low as possible for
Chapter 4. Automotive Catalysis
166
efficient conversion quickly after the cold start of the engine. This is especially
so since the testing cycle require the catalyst to achieve total averaged
conversion from a cold start and include cycles of low temperature operation.
Precious metals are highly active materials which encourage low temperature
light-off , but below this temperature the metals tend to be covered by carbon
monoxide and light off only occurs when this begins to desorb from the
surface and liberate surface sites for reaction. Of late, the light off temperature
has gained significance in defining the efficiency of automotive catalysts, and
the definition has been conveniently labeled for individual pollutants (CO,
Hydrocarbon and NOx) as the temperature at which 50 % conversion is
achieved for the respective pollutant gas.
Air fuel ratio (Lambda,λ): Air–fuel ratio (AFR) is the mass ratio of air to fuel present in an
internal combustion engine. If exactly enough air is provided to completely
burn all of the fuel, the ratio is known as the stoichiometric mixture. AFR is an
important measure for anti-pollution and performance-tuning reasons.
For gasoline fuel, the stoichiometric air–fuel mixture is approximately 14.7; i.e.
for every one molecule of fuel, 14.7 molecules of O2 are required (the fuel
oxidation reaction is: 25/2 O2 + C8H18 -> 8 CO2 + 9 H2O). Any mixture less
than 14.7 to 1 is considered to be a rich mixture; any more than 14.7 to 1 is a
lean mixture – given perfect (ideal) "test" fuel.
Lambda (λ) is the ratio of actual AFR to stoichiometry for a given
mixture. Lambda is 1.0 at stoichiometry, less than 1.0 for rich mixtures, and
greater than 1.0 for lean mixtures. There is a direct relationship between
lambda and AFR. To calculate AFR from a given lambda, multiply the
measured lambda by the stoichiometric AFR for that fuel. Alternatively, to
recover lambda from an AFR, divide AFR by the stoichiometric AFR for that
fuel. This last equation is often used as the definition of lambda.
(4.6)
Because the composition of common fuels varies seasonally, and because
many modern vehicles can handle different fuels, when tuning, it makes more
sense to talk about lambda values rather than AFR.
Chapter 4. Automotive Catalysis
167
Oxygen storage Three-way catalytic converters can store oxygen from the exhaust gas
stream, usually when the air-fuel ratio goes lean. When insufficient oxygen is
available from the exhaust stream, the stored oxygen is released and
consumed. A lack of sufficient oxygen occurs either when oxygen derived
from NOx reduction is unavailable or when certain maneuvers such as hard
acceleration enrich the mixture beyond the ability of the converter to supply
oxygen.
Thermal stability The catalyst can cycle from cold start to temperatures as high as
900oC, and this is a very demanding situation not usually experienced by
catalysts in chemical processing. Thus both mechanical and chemical stability
are required. Mechanical support and thermal shock resistance are provided
by monolith support, as stated earlier. Clearly, the chemically active
components of the catalyst must also be thermally stable and this eliminates
many possible elemental compositions, including low melting metals and
oxides which may be reducible to more volatile species during the engine
cycle. Platinum group metals (Pt, Pd and Rh) are relatively high melting point
metals, compared to other base metals like Cu and Ag.
Lifetime The catalyst must have considerable longevity and maintain high
conversion over at least 50000 miles operation in Europe or 100000 miles in
the USA. Thus all the materials used are also designed for great time stability
and many of the features described above would aid in catalyst engineering.
Catalyst Architecture The automotive catalyst assembly is required to operate with very high
efficiency at high gas flows. As a result much care must be taken in design
and construction of the catalyst to ensure that these requirements are met. Of
particular interest are the pore size distribution in the catalyst, the metal
particle size and the distribution of active metal within the pores. Two main
factors influence the design: the requirement for effective mass transport so
that reactant gases can reach the catalyst and product can be efficiently
removed, and chemical considerations to ensure that catalysis is effectively
Chapter 4. Automotive Catalysis
168
carried out with minimum need for expensive resources, particularly precious
metals. The mass transport usually needs a network of both
macropores(d>500 Ao) which carry most of the gas load, and meso pores (20
Ao<d<500 Ao) where most of the precious metal component is located and
catalysis occurs.
4.3 MATERIALS USED AS AUTOMOTIVE EXHAUST PURIFICATION CATALYSTS:
Gasoline-powered passenger cars, which comprise a large majority of the US market, emit CO, unburnt HC and oxides of nitrogen (NOx). Starting in 1981, the automobile industry was mandated to sharply reduce the emissions of all three (previously only CO and HC had to be removed). This required "three-way-conversion" (TWC): the simultaneous oxidation of CO/HCs and reduction of NOx, a feat without precedent in the chemical industry. It can only be accomplished by keeping the exhaust gas composition extremely close to the stoichiometric point. Cerium oxide was soon recognized as indispensable to the success of the TWC catalyst, due to its ability to rapidly change oxidation states at the surface. This enables it to "store" and "release" oxygen in response to changes in the gas phase composition. The use of ceria was greatly expanded by the addition of zirconia; the ceria-zirconia was far more thermally stable than ceria itself and allowed the TWC catalyst to survive much higher temperatures
Diesel exhaust differs from gasoline exhaust in important respects. It
is always "lean" (i.e., net oxidizing) and three-way conversion is thus ruled
out. The chief concern is particulate matter, which includes dry soot and a
"soluble organic fraction" (SOF), comprised of mainly C20-C28. Prior to 2000,
attention was mainly focused on SOF conversion, as dry soot emissions
could be controlled within the standards by optimizing fuel delivery, air intake
systems, and the combustion process. To meet the new emission standards
proposed for 2007-2009 in the U.S, Europe and Japan, the diesel particulate
filter (DPF) was created, in which the channel wall filters out the soot
particles. The latter are burned off, at suitable intervals, by raising the
temperature. DPFs were originally limited to trucks and buses, but their
proven effectiveness has led to their planned use on passenger cars as well.
Chapter 4. Automotive Catalysis
169
A final topic in diesel after treatment is NOx removal. A successful approach,
already in place in Europe, is the use of urea, carried on-board as an
aqueous solution. Urea hydrolyzes to release NH3, which is a highly effective
agent for converting NOx to N2.
Zeolites: With the continued improvements in three-way catalyst (TWC)
technology, most of the hydrocarbon (HC) emissions from gasoline engines
occur during engine cold start before the TWC would reach its effective
operating temperature. Since the TWC is deemed to be at or near its lowest
feasible light-off temperature, alternate approaches to achieve the reduction
are necessary. One approach is to use a zeolite to adsorb HC at low
temperatures followed by desorption at temperatures where the TWC is
active. Several Japanese car manufacturers have reported the use of zeolites
as HC traps for gasoline powered passenger cars to meet these stringent
emission standards [2, 3]. The concept of using zeolites as hydrocarbon traps
can be traced back to the early 1970s [4,5], and various zeolites have since
been considered as adsorbents for trapping exhaust HCs [6–15]. For
example, silicalite (MFI) was studied because of its hydrophobic properties
that lead to the ability to preferentially adsorb HCs over water present in the
exhaust [7]. Other zeolites such as H-ZSM-5 [7, 12], Ag-ZSM-5 [8], Beta
zeolite [9, 12], mordenite [10], EUO [10], SSZ-33 [11] and USY [12] have also
been investigated as HC trapping materials.
In diesel automotive emission control systems, zeolites have been
incorporated into the diesel oxidation catalysts (DOC) to help reduce HC
emissions [13]. Zeolites Beta and ZSM-5 have been considered to be
effective for DOC [14, 15].
Perovskites: Perovskite-type oxides have general formula as ABO3. Fig. 4.2 shows
structure of perovskite where red spheres are oxygen atoms, the blue
spheres are B atoms (a smaller metal cation, such as Ti4+), and Green
spheres are the A-atoms (a larger metal cation, Ca2+). When A is rare earth
or alkaline earth metal and B is transition metal, the perovskite are typically
P-type semiconductors.
Chapter 4. Automotive Catalysis
170
Fig 4.2. Schematic Perovskite structure
Pictured is the undistorted cubic structure, however the symmetry is lowered
to orthorhombic, tetragonal or trigonal in many perovskites. Their
composition can be varied in a wide range by partial substitution of lower
valent cation in A or B site yielding additional mobile anion vacancies. Their
mixed conductivity by both ion and electron migration and their high
nonstoichiometric composition have resulted in the applications of this group
of materials in the areas such as electrochemistry, catalysis, solid oxide fuel
cells, oxygen separation membranes, chemical sensors for the detection of
humidity, alcohol and gases such as oxygen, hydrocarbon and nitric oxide
[16]. Earlier studies reported on perovskite oxide LaCoxFe1-xO3 mainly
involved methane oxidation catalysis. Recently, noble metals combined with perovskites have developed into
an emerging field. Addition of small amounts of noble metals to perovskites
could improve their catalytic activity. Incorporation of small amounts of
precious metals into a perovskite structure can prevent their sintering, reduce
losses due to volatalization at high operating temperatures and avoid
reactions with the support that slow down the catalyst. Further, attention has
been concentrated on the use of palladium based catalyst for TWC (three way
catalyst) formulation. Pd is well known to have a good resistance to thermal
sintering, is much cheaper than Pt and Rh and also has a good activity for
oxidation of CO and hydrocarbons [16].
Chapter 4. Automotive Catalysis
171
Oxygen Storage and release in Perovskites: Y. Zhang-Steenwinkel et al [17] have studied the oxygen storage/
releasing property by reduction of La0.8Ce0.2MnO3 type Perovskite, using
labeled 1 vol.% C18O and balance He(99%), as reducing gas mixture over the
catalyst and monitoring the production of C16O, C18O, C18O16O and C16O18O,
and have provided following reaction mechanism of CO oxidation on
Perovskite surface.
Fig.4.3 The reaction mechanism of the reduction of La1−xCexMnO3 by C18O at 473 K [17].
4.4 SYNTHESIS STRATEGIES FOR AUTOMOTIVE
CATALYSTS-A LITERATURE SURVEY LaMnO3 perovskites supported noble metal (Pt, Pd, Rh) catalysts
prepared by the citrate method are used for the total oxidation of methane and
it is observed that the oxidation activity is enhanced [18]. Tanaka [19]
prepared LaFe0.95Pd0.05O3 perovskite catalyst by the alkoxide method. ‘‘The
intelligent catalyst’’ has a self-regenerative function. Pd in this catalyst moves
back and forth between the B-site in the perovskite structure and the metal
lattice.
Furthermore, La(Fe, Co)PdO3 [20,21] (La0.6Sr0.4)(Co0.94 Pt0.03Ru0.03)O3
[22,23] and LaMn0.976Rh0.224O3.15 [22,24] perovskite composition were
explored for TWC catalysis for exhaust treatment from single cylinder engine
and compared with synthetic CO + NO + C3H6 mixture .
Co-precipitation [25], sol-gel methods [26, 27] and combustion
methods [28, 29, 30–33] are common synthetic approaches for synthesis of
Chapter 4. Automotive Catalysis
172
perovskites. Amongst these synthetic methods, the solution combustion is an
attractive route. It is very facile and energy efficient. It can produce high purity,
homogeneous crystalline product with high specific surface area [28, 29,30–
33]. However, not much attention has been focussed towards the preparation
of perovskite catalysts incorporating noble metal, using a solution combustion
method. There is very limited information available on developing new active
catalysts, especially the perovskite oxides substituted by Pd via solution
combustion method. Perovskite-type oxides have been used for simultaneous
catalytic removal of NOx and diesel soot [34]. LaCoO3 has excellent catalytic
activity for oxidation. Pd could modify its structure and the physical properties
and increase its catalytic activity [22].
4.5 AIM AND SCOPE OF THE PRESENT WORK Although perovskite materials have not yet found application as
commercial catalysts, their importance in efforts to correlate solid-state
chemistry with catalytic properties, dependence of their properties on the
preparation methods, and the fact that they can be tailored for specific
catalytic needs make these oxides, prototype models for heterogeneous
catalysts.
Perovskite produced via conventional synthesis methods are found to
exhibit relatively low specific surface areas and low catalytic activity in the
reactions, thus its commercial applications are limited. Siliceous MCM-41 a
material with a neutral framework exhibits negligible catalytic activity.
However, one can encash its advantageous properties such as high surface
area, chemical inertness, high thermal and chemical stability. Thus, one of the
approaches for overcoming the limitations of Perovskites is to support them
on to materials with high surface area inert materials like MCM 41. Supporting
Perovskites on a high surface area inert material can ensure uniform and high
dispersion throughout the surface of carrier along with thermal stability of
resultant material and hence durability of the catalyst and generating
structural defects due to interaction with support and thereby improvement of
redox functionality.
In the present endeavor LaCoO3 Perovskites(LC) have been
synthesized on the surface of siliceous MCM 41 materials by citric acid
Chapter 4. Automotive Catalysis
173
solution combustion method abbreviated as LCM. Different wt.% Pd was
loaded by equilibrium adsorption using excess solution of palladium nitrate..
The materials have been characterized for XRD, BET surface area and
temperature programmed reduction(TPR). Catalytic activity of these materials
have been evaluated in a down flow tubular micro reactor using a simulated
exhaust gas mixture, where in concentration of CO,HC and NOX in the gas
mixture passing through the catalyst bed has been monitored as a function of
catalyst bed temperature.
4.6 EXPERIMENTAL Materials:
Lanthanum nitrate, Cobaltous nitrate, Palladium nitrate and citric acid
were procured from Merck India Ltd. as AR grade reagents.
Synthesis of Perovskite based automotive catalysts The synthesis consists of three parts.
(i) Synthesis of MCM 41 The synthesis has been performed as described in Chapter 2 section 2.10.
(ii) Synthesis of LC onto siliceous MCM 41-(LCM)
In a typical set up for preparation of LCM, lanthanum nitrate and cobaltous
nitrate were dissolved in water in a 1:1 mole ratio. To this solution was added
an aqueous solution of 20% citric acid. (Amount of Citric acid solution was
taken such that weight of citric acid was equivalent to oxide weight of resultant
LaCoO3). To this resultant solution, MCM 41 powder was added and kept
under agitation for 0.5 h. The slurry was transferred to a ceramic crucible,
dried at 120oC for 6 h and then calcined at 550 oC for 3 hrs at the rate of 2
◦C/min. This sample was designated as LCM. For comparison, LaCoO3 was
prepared following the same procedure as described above except that MCM
41 was not added. This sample was designated as LC.
(iii) Incorporation of precious metal (Pd) onto LCM to give Pd-LCM:
Palladium impregnation on LCM sample was carried out using equilibrium
adsorption using excess solution of Pd(NO3)2. In a typical set up, 1 g LCM
powder was kept with 1 % solution of palladium nitrate for 12 hours. After 12
hours, powder was separated by filtration, dried at 120oC, followed by
Chapter 4. Automotive Catalysis
174
calcination at 550oC for 3 hrs at a heating rate of 2 ◦C/min. This sample was
designated as Pd-LCM.
In the present study following materials were prepared LC, LCM-15 wt.%-
LC loading, LCM-40wt%-LC loading, 0.2wt%Pd-LCM-15wt%-LC loading, 0.2
wt%Pd-LCM 40 wt%- LC loading. 1wt%Pd-LCM and 1wt%-Pd-MCM-41.
Instrumental methods of characterization X-ray diffractograms (2θ=5-90o) were obtained on X-ray diffractometer
(Bruker D8)with Cu-Kα radiation and Nickel filter. Surface area measurement
(BET method) was carried out on Micromeritics Gemini 2120 at -196oC using
nitrogen adsorption isotherms. TPR was carried out on Micromeritics 2720,
using 10 % H2+N2 gas up to 800oC at a heating rate of 10oC/ min.
Evaluation of Catalytic Activity Catalytic activity of the above synthesized materials was evaluated
using a simulated exhaust gas mixture in a down flow tubular micro reactor,
fitted with a programmable furnace and data recording device which logs the
data of gas concentration and temperature at every 10 seconds to the
computer attached. Catalyst pellets were placed in the middle of the reactor
tube on a sintered disc and a thermocouple placed near the catalyst bed
detects the temperature and sends temperature signals to computer every 10
seconds. In a typical test set up, 1 g of material was pelletized using a
hydraulic die press and placed inside the reactor on the disc. Gas mixture
containing 0.4 % CO, 0.13 % H2, 0.04 % Propylene, 12.5 % CO2, 0.06 %
NOx, 0.4 % O2 and balance N2 was passed through the reactor at the rate of
125 L/h to give gas hourly space velocity of 125 KL/h. Temperature ramp up
was given at the rate of 10oC/min. from room temperature to 450oC and
temperature and composition of gases flowing through the reactor were
recorded using a Horiba gas analyzer. Air to fuel ratio (λ) of the exhaust gas
composition was calculated as,
(4.7)
Catalytic activity of the materials tested has been reported as (i) Light
off temperature for CO,HC and NOx and (ii) conversion of CO,HC and NOx at
400 oC, the temperature at which activation energy is enough for conversion
Chapter 4. Automotive Catalysis
175
to take place on the active sites, and there are no thermodynamic limitations
for CO and HC conversion.
4.7 RESULTS AND DISCUSSION XRD of LC is presented in Fig. 4.4 and matches with LaCoO3 structure
with cubic morphology (JCPDS card no.-075-0279). XRD of LCM-40 wt%-LC
loading is presented in Fig. 4.5 and matches with LaCoO3 structure with cubic
morphology (JCPDS card no.-075-0279). The intensity of peaks is very less
for 40 wt% LCM sample compared to 100 % LC sample, indicating that
LaCoO3 is well dispersed on the surface of MCM-41.
Fig.4.4 XRD of LC
Fig.4.5 XRD of 40wt% LCM
Fig.4.6 TPR patterns of LCM and Pd LCM Fig.4.7 Conversion curves for 1 % Pd LCM
Chapter 4. Automotive Catalysis
176
Fig.4.8 Conversion curves for 0.2 % Pd LCM
Since the materials are proposed to be used as automotive exhaust
catalysts, thermal stability is an important aspect of the same, as stated
earlier in the introduction part. In order to check the thermal stability, the
materials were subjected to calcination at 1000 oC in a muffle furnace for 3 h.
and checked for BET surface area. BET surface area of MCM-41, MCM-
41(LCM) fresh and aged samples are presented in (Table-4.2). BET surface
area of MCM-41 decreases after impregnation with perovskite. This may be
attributed to pore blockage by loading of perovskite on MCM-41. Further drop
in surface area is observed after thermal ageing at 1000 oC. This may be
attributed to sintering of perovskite structure as well as collapse of MCM-41
pores after severe thermal treatment. Table 4.2: BET surface area of fresh and thermally aged samples
Sample BET surface area (m2/g) Fresh Aged
MCM-41 1126 700 LC 16 10
LCM-15 wt.%-LC loading 535 25 LCM-40wt%-LC loading 158 3
0.2wt%Pd-LCM-15wt%-LC loading 487 17
0.2 wt%Pd-LCM 40 wt%- LC loading 119 14
1% Pd-LCM 89 2
TPR patterns of LCM and Pd LCM (Fig. 4.6) shows peaks due to
hydrogen consumption for reduction of perovskite materials. As observed,
Chapter 4. Automotive Catalysis
177
reduction temperature of perovskite decreases post impregnation with Pd
which may be attributed to Pd catalyzing reduction of perovskite resulting in a
lower reduction temperature. Area under the curve in TPR pattern indicates
amount of metal being reduced. It is observed that area of TPR curve
increases after Pd metal loading and as a function of amount of Pd metal
loading. The decrease in reduction temperatures can be explained by the
activated hydrogen atoms on the Pd particles [35]. These H atoms could
reduce the Con+ species at lower temperature [18]. Similar results have been
reported for Pd doped LaFeCoO3 catalysts [36] and LaMnPdO3 catalysts [18].
Lowering in the reduction temperature may be attributed to Pd
catalyzing complete reduction of perovskite material. Reduction temperature
in TPR pattern can be correlated with flexibility of oxygen up take and release
(redox functionality) under transient conditions (rich and lean) in automotive
exhaust. Lower the reduction temperature, lower the temperature of oxygen
release in rich condition in exhaust. Hence, from the TPR pattern it can be
concluded that LCM indicates potential as oxygen storage material for
automotive exhaust catalyst. Pd impregnation of LCM enhances reducibility of
perovskite and oxygen storage capacity of the material.
To check the activity of Perovskite for conversion of CO and HC, 15 %
LCM without Pd was evaluated first. As seen from (Table 4.3) entry no 2, no
light off for either CO or HC was observed for this sample and conversion of
CO was 8% at 400 oC, while no HC conversion was observed. Thus 40 wt%
LCM entry no 3 was subjected to light off with no response to CO and HC
though active sites were increased, however conversion of CO and HC at 400
◦C improved marginally (Table-4.4) entry no 3.
Further it is observed that when Pd is introduced in samples to provide
active sites for CO and HC conversion (Table-4.3 entry no 4 & 5) while light
off was not observed, for 15 % LCM, CO and HC conversion were
dramatically improved and for 40 % LCM, CO and HC light off were observed
at ~ 400 oC. These observations indicate that, CO and HC conversion occur
on Pd metal surface, and Pd is the active site for CO and HC conversion. ~ 10
% conversion of CO and HC for samples without Pd may be attributed on
account of free radical oxygen being released from Perovskite and reacting
with CO and HC.
Chapter 4. Automotive Catalysis
178
Pd concentration was further increased to 1 wt. % on 40 % LCM
sample and as seen from table 4.3( entry no 6), CO and HC light off were
significantly improved to 220 oC. There was no unreacted CO and HC at 400 oC for 1 % Pd on 40 % LCM sample (table 4.4 entry no 6).
For all the samples tested, NOx light off is not observed, which is
expected since there is no Rhodium present, which is necessary for NOx
reduction to occur.
For comparison purpose a catalyst with 1 % Pd on MCM 41 (Table-4.3
entry no 7) was evaluated for catalytic activity for which no light off for CO and
HC was observed upto 400 oC. This indicates that, CO and HC are oxidized
only when Pd is present as an active site on catalyst. Since Perovskites
reduce the activation energy of oxidation of CO and HC, it may even help to
reduce precious metal content in commercial catalysts. Table 4.3: Light off temperatures for synthesized catalysts
No Sample CO Light off T, oC
HC light off T ,oC
NOX Light off T, oC
1 LC No light off No light off No light off 2 LCM-15 wt%-LC loading No light off No light off No light off 3 LCM – 40 wt% -LC
loading No light off No light off No light off
4 0.2wt% Pd-LCM-15 Wt% LC Loading
No light off No light off No light off
5 0.2wt% Pd-LCM -40 Wt% LC Loading
410 394.5 No light off
6 1% Pd-LCM 220 223.5 No light off 7 1% Pd-MCM41 No light off No light off No light off
Table 4.4: Conversion of CO,HC and NOx at 400 oC
No Sample CO Conversion,%
HC Conversion,%
NOX Conversion,%
1 LC 15.5 0 3 2 LCM-15 wt%-LC loading 8 0 3 3 LCM – 40 wt% -LC
loading 10 5 3
4 0.2wt% Pd-LCM-15 Wt% LC Loading
25 20 3
5 0.2wt% Pd-LCM -40 Wt% LC Loading
37.5 50 5
6 1% Pd-LCM 100 100 10 7 1% Pd-MCM41 10 25 3
Chapter 4. Automotive Catalysis
179
4.8 CONCLUSIONS In the present endeavour a humble attempt has been made towards
synthesis of materials that could be probably used for automotive catalysts.
Since it is evident that storage oxygen is used for oxidation, a material that
can store oxygen is highly essential for automotive catalyst, which is met with
by use of perovskite (LC) in the present study.
Oxidation of CO and HC by free radical oxygen, released from surface
of Perovskite(LCM) has been demonstrated. The conventional precious metal
used is Pt. Using Pd makes the catalyst cost effective. In view of the relatively
larger drop of surface area of the samples post thermal ageing, and due to
low conversion for NOx, in the synthesized materials, it could be useful for
diesel oxidation catalyst, wherein exhaust temperatures are low, and only
oxidation function of the catalyst is required, since NOx reduction is treated
separately in diesel exhaust treatment system. It is also proposed that further
work on LCM system with incorporation of Rh can be studied, for three way
catalysis wherein NOX reduction will also occur on the same catalyst.
Chapter 4. Automotive Catalysis
180
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182
One of the most exciting discoveries in the field of materials synthesis
over the last years is the formation of mesoporous silicate by researchers at
the Mobil Oil Company who reported a novel family of materials called M41S
[1-3]. The family of mesoporous M41S materials consists of three types, as
summarized below in the Fig 1.
Fig.1 The mesoporous M41S family [1]
About MCM-41 • MCM-41 is one of the most studied and promising member of the M41S
family.
• MCM-41 is the abbreviation for Mobil Crystalline Material or Mobil
composition of matter.
• The mesoporous material presents regular arrays of uniform channels,
which has a honeycomb structure as a result of hexagonal packing of uni-
dimensional cylindrical pores.
• By choosing adequate reactants and reaction conditions, it is possible to
tailor the channel dimension in the range of 15-100Å or even larger.
• The BET surface area is typically over 1000 m2/g. The pore is usually
between 0.7 and 1.2 cm3/g with long-range order.
• With increasing pore size, the regularity of the structure is affected.
• The adsorption capacity is exceptionally high (more than 50 wt% for
cyclohexane at 40 Torr, 67 wt% for benzene at 50 Torr).
• MCM-41 possesses excellent thermal and hydrothermal stability (up to
800°C).
• It is relatively stable in acidic medium (pH 2). However, it is destroyed in a
basic medium (pH12).
• MCM-41 is composed of silica framework, which is almost catalytically
inactive.
• The isomorphous substitution of silicon by a variety of metals (Al, Ga, Fe)
gives rise to acidic properties [4].
183
• The possibility of using the pore channels of MCM-41 as a support for
existing catalysts has also been considered [5,6].
There is no doubt that the synthesis of these materials opens definitive
new possibilities for preparing catalysts with uniform pores in the mesoporous
region. Obviously when a new type of material such as these is discovered,
an explosion of scientific and commercial development swiftly follows, and
new investigations on every conceivable aspect of their nature, the synthesis
procedures and synthesis mechanisms, heteroatom insertion,
characterization, adsorption, and catalytic properties, rapidly occurs.
With the first successful report on the mesoporous materials (M41S) by
Mobil researchers, with well defined pore sizes of 20 – 500 Å, the pore size
constraint (15 Å) of microporous zeolites observed a breakthrough. The high
surface area (> 1000 m2/g) and the precise tuning of the pores are among the
desirable properties of these materials. Mainly, these materials ushered in a
new synthetic approach where, instead of a single molecule as a templating
agent as in the case of zeolites, self-assembly of molecular aggregates or
supra-molecular assemblies are employed as templating agents. The basic
difference in the synthesis of microporous and mesoporous molecular sieves
can be shown pictorially.
Fig. 2 Microporous materials using single molecule as template
Fig. 3 Mesoporous materials using molecular aggregates or supra-molecular assemblies as
template.
Though it is possible to overcome the existing pore size constraints of
microporous solids as mentioned earlier, the MCM-41 based materials have
negligible catalytic activity due to framework neutrality, however with
advantageous properties like mesoporous nature of the material, good
184
thermal stability, high surface area and retention of surface area at high
temperatures. Thus, the main aim of the present study was to encash the
advantageous properties of MCM-41 and enhance its practicability in the area
of catalysis using Green Chemistry principles. There are a number of ways by
which catalytic activity can be generated into the MCM-41 neutral framework.
(i)Substitution of an M3+ cation e.g. Al3+ in the Si4+ framework, leading to
negatively charged framework, followed by balancing these charges by H+
ions to create Bronsted acid sites (via NH4+ ion exchange and subsequent
thermal decomposition to give H+ and NH3) to result in a material with inherent
acidity. (ii)Immobilization/anchoring/impregnation of homogenous acid catalyst
e.g. heteropoly acids (HPAs) onto MCM-41 to result in a material with induced
acidity. (iii)Isomorphous replacement of Zr4+and Ti4+ in the siliceous MCM-41
framework to induce redox properties.
Chapter II of the thesis includes the synthesis of mesoporous (i)
Siliceous MCM-41, (ii) Al-MCM-41 and (iii) 12TPA-MCM-41, (where 12-TPA =
12-Tungstophosphoric acid a HPA). Materials (i) and (ii) have been
synthesized by sol-gel method, using templates varying several parameters
such as silica source, templating agent/types, reaction conditions such as pH,
time of reaction, aging, temperature etc. and these parameters optimized,
using surface area as an indicative tool. In case of Al-MCM-41 SiO2:Al2O3
ratios have been varied in order to obtain material with maximum surface
acidity and hence in case of Al-MCM-41 surface acidity has been used as an
indicative tool. The salient feature is the synthesis of MCM-41 and Al-MCM-41
at room temperature. 12-TPA supported MCM-41 was prepared by a process
of anchoring and calcination, with varying 12-TPA loading (10-40 wt.%) in
order to obtain material with maximum surface acidity and hence, here also
surface acidity has been used as an indicative tool.
All synthesized materials were characterized for Elemental analysis by
ICP-AES, X-ray diffraction (XRD), Transmission electron microscopy (TEM),
Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy
(EDX), Surface area (BET method), Pore volume and pore distribution(BJH
method), surface acidity by temperature programmed desorption (TPD) of
ammonia, Diffuse reflectance spectroscopy (UV-DRS), Fourier transform
infrared spectroscopy (FT-IR) and Thermogravimetric analysis (TGA).
185
The importance of green chemistry, 12 principals of green chemistry
and how green chemistry goals can be achieved through catalysis is a well
established concept [7,8]. Further solid acid catalysts as an alternative
approach to liquid acid catalyst and its advantages over liquid acid catalyst
and important materials used as solid acid catalysts for various organic
transformations is well documented in literature. New materials are
continuously synthesized and explored as solid acid catalysts. The endeavour
“Global effort to replace conventional liquid acid catalysts by solid acid catalysts” is on. This prompted us to use Al-MCM-41 and 12-TPA-MCM-41
as solid acid catalysts by studying (i) Esterification (ii) Friedel- Crafts
alkylation and acylation as model reactions. Since 12-TPA-MCM-41-20 and
Al-MCM-41-30 exhibit highest surface acidity amongst the various samples
prepared, they have been used for all catalytic test reactions.
In case of esterification monoesters such as ethyl acetate (EA), propyl
acetate(PA), butyl acetate (BA) and benzyl acetate (BzA) and diesters such
as diethyl malonate (DEM), dioctyl phthalate (DOP) and dibutyl phthalate
(DBP) have been synthesized.
In the above reactions, parameters such as catalyst amount, reaction
time and reaction temperature, mole ratio of reagents etc. have been
optimized including catalyst regeneration capacity. The catalytic activities of
both catalysts have been compared and the results have been correlated with
surface properties of the materials.
It is observed that the order of % yield of ester formed for both
catalysts is BzA> EA> PA> BA. Though the yield in case of mono esters using
both catalysts are comparable, higher yields are observed in case of 12TPA-
MCM-41-20 which could be attributed to higher surface acidity. Turn over
number, reflects the effectiveness of a catalyst and this also follows the order
of ester formation.
In case of diesters there is no marginal difference in the yields of DOP
and DBP and order of the % yields of diesters formation is DEM> DOP ~
DBP., which is probably due to less steric hindrance felt by incoming ethanol
from monoethyl malonate formed in the fist step in case of DEM.
The above esterification reveals the promising use of 12TPA-MCM-41-
20 and Al-MCM-41-30 as solid acid catalysts in the synthesis of monoesters
186
and diesters, the advantages being operational simplicity, mild reaction
conditions and eco-friendly nature of catalyst. The monoesters and diesters
formed can be simply distilled over, there is no catalyst contamination in
products formed, no acid waste formation and products are colorless a
limitation in the conventional process. The catalysts can be regenerated and
reused. Since the reactions are driven by surface acidity of the catalyst, there
is scope of obtaining better/higher yield by synthesizing material with high
surface acidity by modifying synthesis procedure. Though yields of
monoesters are high, the diester yields are low however, with the only
advantage of the product having no colour contamination. Friedel-Crafts acylation of anisole and veratrole with acetic anhydride
and alkylation of toluene with benzyl chloride have been performed to obtain
4-methoxy acetophenone (4MA), 3,4-dimethoxy acetophenone (3,4DMA) and
parabenzyltoluene (PBT) under solvent free condition.
In the above reactions, parameters such as catalyst amount, reaction
time and reaction temperature, mole ratio of reagents etc. have been
optimized including catalyst regeneration capacity. The catalytic activity of
both catalysts have been compared and the results have been correlated with
surface properties of the materials.
Friedel-Crafts acylation of veratrole with acetic anhydride, gave
selectively 3,4-DMA. Acylation of anisole with acetic anhydride gave
selectively 4MA and aklyation of toluene with benzyl chloride gave selectively
PBT.
The acylation and alkylation reactions are driven by the surface acidity
of the catalyst. Probably this is the reason why 12-TPA-MCM-41-20 gives
higher yields compared to Al-MCM-41-30 due higher surface acidity observed
in case of former catalyst.
In the above synthesis, Green Chemistry goals have been achieved by
using solid acid catalysts (replacing liquid acid catalysts used in conventional
reactions) and under solvent free conditions with high selectivity of the
products. The products formed can be simply distilled over, there is no
catalyst contamination in product and no acid waste formed. The catalyst can
be regenerated and reused. Further, since the studied reactions are driven by
surface acidity of the catalyst, there is scope to obtain higher/better yields by
187
synthesizing materials with higher surface acidity. Finally, the MCM-41 neutral
framework has been successfully modified and put to practical use.
Further, Chapter III of the thesis aims at synthesizing oxidation
catalysts Zr-MCM-41 and Ti-MCM-41 by a sol-gel method using templates
towards achieving mesoporosity, with high surface area, good thermal stability
and maximum M4+ incorporation. For incorporation of maximum Zr4+ and Ti4+,
SiO2:ZrO2 and SiO2:TiO2 ratios have been varied. The salient feature is that
the material is synthesized at room temperature.
All synthesized materials were characterized for Elemental analysis by
ICP-AES, X-ray diffraction (XRD), Transmission electron microscopy (TEM),
Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy
(EDX), Surface area (BET method), Pore volume and pore distribution(BJH
method), surface acidity by temperature programmed desorption (TPD) of
ammonia, Diffuse reflectance spectroscopy (UV DRS), Fourier transform
infrared spectroscopy (FT-IR) and Thermogravimetric analysis (TGA).
Further, the catalytic potential of the materials Zr-MCM-41 and Ti-
MCM-41 has been explored by studying Epoxidation as a model reaction
using H2O2 as an oxidant in the conversion of allyl chloride to selectively
produce Epichlorohydrin in a single step at room temperature.
In the above reaction, parameters such as catalyst amount, reaction
time and reaction temperature, mole ratio of reagents etc. have been
optimized including catalyst regeneration capacity. The catalytic activity of
both catalysts has been compared.
In the present study ECH is obtained at room temperature with very
small amount of catalyst, compared to previous reports that use a large
amount of catalyst and reaction performed at higher temperature. Ti-MCM-41
exhibits higher catalytic activity compared to Zr-MCM-41.
The study indicates the promising use of Zr-MCM-41 and Ti-MCM-41
as environment friendly heterogeneous solid acid catalysts for epoxidation of
allyl chloride to ECH with ~99% selectivity at room temperature. The
epoxidation reaction involves operational simplicity. The product formed can
be simply distilled over and the catalyst can be regenerated and reused. The
conversion of allyl chloride to ECH by using Zr-MCM-41-5 and Ti-MCM-41-30
as heterogeneous solid acid catalysts, along with H2O2 as an oxidant
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eliminates the use of corrosive chlorine and peracids (used in conventional
process) as well as generation of chlorine-laden sewage and acid waste.
Further, the reaction with H2O2 has the advantage of mild reaction condition,
production of only water as by-product with added contribution to the
epoxidation reaction. Finally, the conventional method for production of ECH
is a non-catalytic process using chlorine. By using eco-friendly solid acid
catalysts such as Zr-MCM-41 and Ti-MCM-41 the green chemistry principle
no. 9, “Catalysts (as selective as possible) are superior to stoichiometric reagents” is implemented.
Chapter IV involves the use of mesoporous MCM-41 as a support in
the synthesis of automobile catalyst. Automobile emissions form an important
source of atmospheric pollution. Automobile catalysts are now widely
recognized for the conversion of mainly CO → CO2, oxides of nitrogen → N2
and unburnt hydrocarbons → CO2 and H2O. At present, noble metals
particularly Rh and Pt catalysts are used for these purposes which are
expensive. There is thus a search for relatively cheaper materials that must
operate efficiently under a wide variety of conditions. Perovskite type oxides
have attracted much attention recently in environment pollution control [9,10].
In the present endeavour, LaCoO3 (LC) a Perovskite has been
synthesized on the surface of mesoporous MCM-41 (LCM) by citrate solution
combustion route. Pd has been incorporated in the Perovskite lattice as well
as on the MCM-41 surface with various wt.% of Palladium and the materials
characterized for XRD, surface area (BET method) and temperature
programmed reduction (TPR). In order to check the thermal stability/durability,
the materials have been aged at 1000oC for 3h and again characterized.
Catalytic activity of fresh and aged materials have been explored for oxidation
of CO, hydrocarbons and reduction of NOx in a tubular down flow test reactor
under simulated exhaust conditions and known air/fuel ratios and the results
correlated with surface and redox properties of the materials.
Conversion occurs only in the presence of Pd, therefore Pd is the
active site for CO and HC conversion.1 wt% Pd LCM(40 wt% LC) exhibited
light off at 220 ◦C ,leaving no unreacted CO and HC.
In conclusion, since it is evident that storage oxygen is used for
oxidation, a material that can store oxygen is highly essential for automotive
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catalysts, which is met with by use of Perovskite LC in the present study.
Further, the conventional precious metal is Pt. Using Pd makes the catalyst
cost effective. Since no conversion of NOx is observed the Pd-LCM could be
useful for diesel oxidation catalyst, wherein exhaust temperature are low and
only oxidation function of the catalyst is required, since NOx reduction is
treated separately in diesel exhaust treatment system. It is proposed that
further work on LCM system with incorporation of Rh can be studied, for three
way catalysis wherein NOx reduction will also take place on the same
catalyst.
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PUBLICATIONS IN SCIENTIFIC JOURNALS [1] An Investigation into the Thermal and Surface Properties of MCM-41 type
Materials for Catalytic Application Proceedings of DAE-BRNS 17th National
Symposium on Thermal Analysis, P. C. Kalsi, Rajesh V. Pai, Mrinal R. Pai,S
Bharadwaj, V.Venugopal (Eds), THERMANS (2010) 254-256.
[2] Synthesis and characterization of mesoporous materials possessing induced
and inherent acidity and their application as solid acid catalysts in some esterification reactions.(communicated)
[3] Friedel- Crafts acylation and alkylation of aromatic compounds using
mesoporous solid acid catalysts possessing induced and inherent acidity.(communicated)
[4] Mesoporous Zr-MCM-41 and Ti-MCM-41 as solid oxidation catalysts in the synthesis of epichlorohydrin.(communicated)
[5] Synthesis and characterization of a Palladium loaded Pervoskite MCM-41
material and its application as an automotive catalyst.(communicated)
PAPERS PRESENTED AT NATIONAL/INTERNATIONAL CONFERENCES
[1] “Synthesis and Characterization of Mesoporous Zirconium Silicate
using Templates for Catalytic Applications”, 2nd International Symposium on
Materials Chemistry (ISMC-2008) held at Bhabha Atomic Research Centre, Mumbai
organized by Society for Materials Chemistry, 2nd to 6th December, 2008.
[2] “An Investigation into the Thermal and Surface Properties of MCM-41 type
Materials for Catalytic Application”, 17th National Symposium on Thermal Analysis
(THERMANS 2010) held at Kurukshetra University Kurukshetra, Haryana,India,
organized by Indian Thermal Analysis Society, 9th -11th March 2010.
[3] “Friedel- Crafts Alkylation and Acylation using Acid Induced mesoporous Si-
MCM-41 Molecular Sieves”, 3Rd International Symposium on Materials Chemistry
(ISMC-2010) held at Bhabha Atomic Research Centre, Mumbai organized by Society
for Materials Chemistry, 7th to 11th December, 2010.
[4] “Synthesis and Characterization of Perovskites on MCM 41 for Automotive Exhaust Gas Purification”, 15th National Workshop on The role of Materials in
Catalysis, organized by Catalysis Society of India (CSI), held at National Centre for
Catalysis Research (NCCR), IIT-Madras, December 11th – 13th, 2011.
[5] “Synthesis and Characterization of Ti-MCM-41 for Catalytic Applications”,
National Seminar on Catalysis for Sustainable Development, The M. S. University of
Baroda, Vadodara, organized by Catalysis Society of India(CSI), Baroda Chapter,
January 27th -28th , 2012.