selective oxidation of hydrocarbons part-1 dr.k.r.krishnamurthy national centre for catalysis...
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Selective Oxidation of Selective Oxidation of hydrocarbonshydrocarbons
Part-1Part-1
Dr.K.R.KrishnamurthyDr.K.R.Krishnamurthy
National Centre for Catalysis Research (NCCR)National Centre for Catalysis Research (NCCR)
Indian Institute of TechnologyIndian Institute of Technology
Chennai-600036Chennai-600036
INDIAINDIA
10 th Orientation Course in Catalysis for Research Scholars28 th November to 16 th December,2009
Selective oxidation of Hydrocarbons- Part-1
Oxidation /ammoxidation of Propylene Epoxidation of Ethylene Oxychlorination of Ethylene
Chemical Industry- Products pattern
Chemicals- Intricately woven with our day to day life
Petrochemicals-37%Petrochemicals-37%
Major catalytic processes for Petrochemicals
RK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980
Important heterogeneous oxidation processes
RK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980
Scenario in feedstock for petrochemicals
RK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980
Current scenario reflects the predictions
1
2
3
4
5
Processes for manufacture of Acrylonitrile
JL.Callahan, RK.Grasselli, EC.Millberger & HA Strecker. Ind.Eng.Chem.,Proc.Res & Dev.9, 134 (1970)
Acrylonitrile- Fact fileAcrylonitrile- Fact file
Global production & Consumption 2008- 5.2 MMT Growth rate - 3% /yr Versatile chemical SOHIO’s Ammoxidation process Significant Landmark in History
of chemical industry
Allylic Oxidation processes
Oxidation/ Ammoxidation of Propylene – Key Process
RK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980
Selective oxidation /Ammoxidation of Propylene
Proceed through Mars- Krevelen mechanism Cyclic reduction- re-oxidation of the catalyst Catalyst systems contain binary/multi compoent metal oxides Bismuth molybdates (α-β-γ- phases ) most active & selective Facile reduction- re-oxidation capability Hydrocarbon gets activated and not oxygen
Redox Cycle for the catalyst
Surface reactions inselective oxidation/ Ammoxidation of propylene
Mechanism of Oxidation/Ammoxidation of Propylene
Experiments labeled with 14C
Labeling in 1-or 3- position results in acrolein with 14C scrambled in both positions Oxidation with 2- 14C Propylene did not lead to scrambling
Formation of allylic species from adsorbed propylene proposed as the first step
Sachtler WH & de Boer, NH, Proc.Inetrn Congr.Catal.3rd 1964,252(1965)
Mechanism of oxidation/ammoxidation of Propylene
α-Hydrogen abstraction leading to allylic species- rate determining step
CR Adams & JT Jennings,J.Catal.3,549,1964 HH.Voge, CD.Wagner & DP.Stevenson,J.Catalysis, 2, 58,1963
Role of Bi & Mo
Bi2O3 - Highly active but not selective
MoO3 - Highly selective but not that active
Bismuth molybdates- Active & Selective
On Bi2O3 propylene forms 1,5 Hexadiene / Benzene via allyl radical
On MoO3 Allyl iodide gets converted to acrolein
Bi-O sites – Abstraction of alpha Hydrogen & formation of allyl radical
Mo-O sites- Selective insertion of oxygen/nitrogen in allylic moiety
* Grzybowska B & Haber J & Janas J., J.catalysis, 49, 150 (1977)
Role of gas phase/lattice oxygen
Oxidation of propylene in the absence of gas phase oxygenParticipation of lattice oxygen in oxidation/ ammoxidationOxidation with 18O2 in gas phase & on 18O2 exchanged Bi-Mo
- Lattice oxygen gets incorporated in the product [CR.Adams, Proc.Intern Congr.Catal.3rd 1964,1,240 (1965) WH.Sachtler & NH deBoer, Proc.Itern Congr.Catal.3rd 1964,1,252 (1965)]
Lattice oxygen vacancies replenished by gas phase oxygen Facile internal diffusion of oxygen leads to oxygen insertion / replenishment
[GW.Kelks J.Cat.19, 232,(1970); T.Otsubo et.al J.Catal.36,240,1975]
Terminal Mo-O bond with double bond character responsible for selective oxidation- IR absorption band at 990-1000 cm-1
[F.Trifiro et.al J Catal.19,21(1970)]
Two types of lattice oxygen in Bi-Mo-O- Selective & Non selective [RK.Grasselli & DD.Suresh, J Catal.25, 273,(1972)]
Loss of selectivity related to disappearance of terminal Mo-O bond- IR study
(TSR Prasada Rao,KR Krishnamurthy & PG.Menon, Proc.Intrn Conf “ Chemistry & uses of Molybdenum, Michigan, p.132,1979)
Shear structure of Bismuth molybdate
Mo-O- Corner shared Oh On loss of oxygen
edge shared Oh formed Shear structure imparts
Structural stability Amenable to redox cycles Partial reduction tempers
M-O bond strength - Criterion for selectivity
Features of selective oxidation catalysts
Selection of appropriate redox-couple- redox potentialSuitable electronic configuration - Partially filled orbitals - Alpha H abstraction - Full orbitals - Olefin adsn. , O/N insertion
Typical commercial catalyst formulations
Desirable catalyst characteristics
Hydrogen abstraction
Labile lattice oxygen
O/N insertion
Redox stability
Layered structure/Shear structure
Matrix stabilization
Typical redox process – Phase stability is the key
Model for multi-component molybdate catalysts
Role of different phasesBi-Mo - Activity & SelectivityFe-Mo - Facilitate re-oxidation of Bi & MoCo,Ni-Mo - Hold excess MoO3 in bulk molybdate phase - Ensure structural stabilityK,Cs - Moderate Mo-O bond strength, acidity,
Fe3+phase
Fe2+ phase
Seven principles/Seven pillars for selective oxidation
Lattice oxygen,
Metal–oxygen bond strength,
Host structure,
Redox characteristics
Multi-functionality of active sites,
Site isolation,
Phase co-operation
RK Grasselli, Topics in Catalysis, 21,79,2002
Epoxidation of ethylene - Fact file
First patented in 1931 Process developed by Union Carbide in1938 Currently 3 major processes - DOW, SHELL & Scientific Design Catalyst- Ag/α-alumina with alkali promoters Temperature 200-280°C; Pressure - ~ 15- 20 bar Organic chlorides (ppm level) as moderators Reactions
C2H4 + 1/2O2 -> C2H4O C2H4O + 2 1/2O2 -> 2CO2 + 2H2O
C2H4 + 3O2 -> 2CO2 + 2H2O Per pass conversion -10-20 % EO Selectivity 80- 90 % Global production -19 Mill.MTA
(SRI Report- 2008)
Best example of Specificity - catalyst (Ag) & reactant ( Ethylene)
Utilization of Ethylene Oxide
71%
7%
9%
5%8%
MEG
Higher glycols
Ethoxylates
Ethanolamine
Others
Epoxidation of ethylene - EO selectivity
6 C2H4 + 6O2- → 6 C2H4O + 6 O-
C2H4 + 6O- → 2 CO2 + 2H2OMaximum theoretical selectivity- 6/7 = 85.7 %
AssumptionsO2
- Selective oxidationO- - Non selective oxidation - No recombinationCl- - Retards O- formationAlkali/Alkaline earth - Form Peroxy linkages - Retard Ag sintering Selective oxidation
Non- selective oxidation
WMH Sachtler et. al.,Catal. Rev. Sci. Eng, 10,1,(1974)&23,127(1981); Proc. Int. CongrCatal.5 th, 929 (1973)
EO selectivity > 86 % realizedin lab & commercial scale !!!
Molecular Vs Atomic adsorbed Oxygen – Key for selectivity
Epoxidation of ethylene- Surface species & reactivity
No adsorption of ethylene on clean Ag surface
Ethylene adsorbs on Ag surface with
pre-sorbed Oxygen
O2- unstable beyond 170 K
EO formed with atomic O- - in-situ IR & TPRS studies
( EL Force & AT Bell, J.Catal,44,175, (1976)
Sub-surface Oss oxygen essential for EO formation
Oss influences the nature of Oads
Cl- decreases Oads but weakens its binding to Ag
Alkali facilitates adsorption of O2 & ethylene
[ RA.van Santen et.al, J.Catal. 98, 530,(1986);
AW.Czanderna, J. Vac.Sci.Technolgy, 14,408,(1977)] Surface species identified
Comprehensive picture of surface species
Epoxidation of ethylene - Reaction pathways
Strength & nature of adsorbed oxygen holds the key 2 different Oads species besides subsurface oxygen Reactivity of oxygen species governs the selectivity
Elelctrophillic attack /insertion of Oxygen → Selective oxidation
Nucleophillic attack of Oxygen → Non selective oxidation
RA.van Santen &PCE Kuipers, Adv.Catal. 35, 265,1987
Reaction paths in line with observed higher selectivity
Epoxidation of ethylene - Transition state
RA. Van Santen & HPCE Kuipers, Adv.Catalysis, 35,265,1987
Ethylene adsorbed on oxygenated Ag surface
Electrophillic attack by Oads on Ethylene leads to EO ( Case a)
Cl- weakens Ag-O bond & helps in Formation of EO (Case c)
Strongly bound bridged Oads attacks C-H bond leading to non-selective Oxidation ( Case b)
Non-selective oxidation proceeds via isomerization of EO to acetaldehyde which further undergoes oxidation to CO2 & H2O
Epoxidation of ethylene- Surface transformations
J.Greeley & M Mavrikakis, J.Pys.Chem. C, 111, 7992,2007S.Linic & MA.Barteau, JACS,124,310,2002; 125,4034,2003S.Linic, H.Piao,K.Adib & MA.Barteau, Angew.Chem.Intl.Ed.,43,2918,2004
Based on DFT , TPD & HREELS studiesSimilar intermediates in epoxidation of butadiene
A new approach to surface transformations
Ethyene epoxidation- Reactivity of Surface species
Reactivity of oxametallacycle governs selectivity
Epoxidation of Ethylene- Why only Silver & Ethylene?
Bond strength & nature of adsorbed oxygen
Governed by Oss & Clads
No stable oxide under reaction conditions
Inability to activate C-H bond
Other noble metals activate C-H bond
Oxametallacycles on other metals are more stable
Butadiene forms epoxide- 3,4 epoxy 1-butene
Propylene does not form epoxide due to
- facile formation of allylic species
- its high reactivity for further oxidation
Ethylene Oxychlorination- Major route for VCM
Alternative routes for VCM
VCM Production-Feedstocks
82%
18%
Ethylene Acetylene
Global VCM capacity- 42.7 MMTA (2008) ( Nexant Report)
C2H4 + Cl2 → C2H4Cl2
C2H4Cl2 → C2H3Cl + HCl
C2H4 + 2HCl + ½O2 → C2H4Cl2 + H2O
C2H4 + Cl2 → C2H4Cl22 C2H4Cl2 → 2 C2H3Cl + 2 HCl
C2H4 + 2HCl + ½O2 → C2H4Cl2 + H2Ooverall,
2 C2H4 + Cl2 + ½O2 → 2 C2H3Cl + H2O
Ethylene Oxychlorination –Relevance to VCMProcess steps for VCM
Direct chlorination to EDC
Thermal cracking of EDC
Oxychlorination of ethylene
Overall process for VCM
Oxychlorination ensures Complete utilization of Chlorine
Ethylene Oxychlorination- Reaction mechanism
Follows redox pathway – CuCl2 / Cu2Cl2
Elementary steps
C2H4 + 2CuCl2 C2H4Cl2 + 2CuCl2CuCl + ½ O2 Cu2OCl2Cu2OCl2+ 2HCl 2CuCl2 + H2O
Unique role of CuCl2 lattice & redox character
Ethylene oxychlorination- Catalyst characteristics
CuCl2- KCl/ Alumina- + Rare earth oxide promoters
Active phases identified – CuCl2, K CuCl3, Cu (OH) Cl, Cu aluminate
Cu hydroxy chlorides bound to alumina
R.Vetrivel, K.Seshan,KR Krishnamurthy & TSR Prasada Rao, Bull.Mat.Sci.,9,75,1987G.Lambert,et.al., J.Catalysis,189, 91 &105 2000KR.Krishnamurthy et.al, Ind J,Chem.,35A,331,1996
Phase transformations in Catalyst during oxychlorination
GC.Pandey, KV.Rao, SK.Mehtha, K.R.Krishnamurthy,DT.Goakak &PK.Bhattacharya, Ind.J.Chemistry, 35A, 331, 1996
Characterization of Ethylene Oxychlorination catalysts
Sample From DRS
(x 103cm-1)
Wt / Wt, % Cu/K Ratio
Phases identified
Cu K
CB-1 19.80 2.74 1.56 2.30 CuCl2 [3Cu(OH)2],
CuOHCI
CB-2 17.85 6.00 1.56 2.30 CuCl2 [3Cu(OH)2]
CB-3 17.54 8.66 0.98 5.45 CuCl2 [3Cu(OH)2],
KCI
CB-4 18.87 6.13 2.07 1.82 CuCl2 [3Cu(OH)2] CuOHCI
CB-5 17.54 8.76 0.90 6.00 CuCl2 [3Cu(OH)2]
Crystalline phase identified in oxychlorination catalysts of different compositions by X-ray powder diffractometry
Ethylene Oxychlorination catalyst- XPS study
Fresh catalyst contains Cu2+ and Cu+ statesSpent catalyst shell has Cu in both oxidation statesSpent catalyst core shows only Cu+ state
Structural & electronic changes across catalyst geometry
R.Vetrivel, K.Seshan,KR Krishnamurthy & TSR Prasada Rao, Bull.Mat.Sci.,9,75,1987
No Potassium in the core
Ethylene oxychlorination catalyst- TPR study
TPR profiles indicate presence of Cu 2+ & Cu+ states in fresh & spent shellCatalyst & only Cu+ in spent core section- Confirms XPS dataR.Vetrivel, KV.rao, K.Seshan,KR Krishnamurthy & TSR.Prasada Rao,Proc.9 th Intern. Congr. Catal. Calgery, Canada, 1766,1988
XPS & TPR indicate slow re-oxidation of Cu+ in core part
Ethylene oxychlorination catalyst- TPO study
TPO profiles indicate the presence of Cu+ in fresh catalystR.Vetrivel, KV.rao, K.Seshan,KR Krishnamurthy & TSR.Prasada Rao,Proc.9 th Intern. Congr. Catal. Calgery,Canada,1766,1988
Ethylene oxychlorination catalyst- TPO study
Difference in re-oxidation rates- Core-Sphere & Core-PowderR.Vetrivel, KV.rao, K.Seshan,KR Krishnamurthy & TSR.Prasada Rao,Proc.9 th Intern. Congr. Catal. Calgery, Canada,1766,1988
Spherical shape detrimental – Retards re-oxidation of Cu
Ethylene oxychlorination catalyst – Further developments
Studies indicate that re-oxidation of Cu+ to Cu2+ is the limiting step Observations supported by G.Lamberti et.al
(J.Catalysis, 189,91 & 105 (2000), 202,279(2001) 205,375 (2002) Angew.Chem.Intl Ed., 41,2341(2002)
All further commercial formulations changed the shape- -Spherical to Annular ring – Racsig ring
Developments are towards increasing catalyst life