Transcript MS PowerPoint

Selective Oxidation of hydrocarbons
Part-1
10 th Orientation Course in Catalysis for Research Scholars
28 th November to 16 th December,2009
Dr.K.R.Krishnamurthy
National Centre for Catalysis Research (NCCR)
Indian Institute of Technology
Chennai-600036
INDIA
Selective oxidation of Hydrocarbons- Part-1
Oxidation /ammoxidation of Propylene
Epoxidation of Ethylene
Oxychlorination of Ethylene
Chemical Industry- Products pattern
Petrochemicals-37%
Chemicals- Intricately woven with our day to day life
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
Oxidation & ammoxidation of Propylene
Scenario in feedstock for petrochemicals
RK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980
Current scenario reflects the predictions
Processes for manufacture of Acrylonitrile
1
2
3
4
5
JL.Callahan, RK.Grasselli, EC.Millberger & HA Strecker.
Ind.Eng.Chem.,Proc.Res & Dev.9, 134 (1970)
Acrylonitrile- 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
Redox Cycle for the catalyst
Surface reactions in
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
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 oxygen
Participation of lattice oxygen in oxidation/ ammoxidation
Oxidation 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)
Crystal structure of Bismuth Molybdate
Layered structure
helps in facile
Oxygen diffusion
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 potential
Suitable 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
RK.Grasselli, Appl.Catal.15, 127,1985
TSR.Prasada Rao & KR.Krishnamurthy, J.Catalysis,95,209,1985
Model for multi-component molybdate catalysts
Fe3+phase
Fe2+ phase
Role of different phases
Bi-Mo
- Activity & Selectivity
Fe-Mo
- Facilitate re-oxidation of Bi & Mo
Co,Ni-Mo - Hold excess MoO3 in bulk molybdate phase
- Ensure structural stability
K,Cs
- Moderate Mo-O bond strength, acidity,
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
Burrington, JD, Kartisek,CT,& Grasselli,RK J.Catalysis, 63, 235,(1980)
Selective oxidation / ammoxidation of Propylene
Surface transformations
Selective oxidation of Propylene- Mechanism
Selective ammoxidation of Propylene -Mechanism
Selective Oxidation/ammoxidation of Propylene
Epoxidation of Ethylene
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
Utilization of Ethylene Oxide
C2H4 + 1/2O2 -> C2H4O
C2H4O + 2 1/2O2 -> 2CO2 + 2H2O
8%
5%
C2H4 + 3O2 -> 2CO2 + 2H2O
Per pass conversion -10-20 %
EO Selectivity 80- 90 %
Global production -19 Mill.MTA
(SRI Report- 2008)
9%
MEG
Higher glycols
Ethoxylates
7%
Ethanolamine
Others
71%
Best example of Specificity - catalyst (Ag) & reactant ( Ethylene)
Epoxidation of ethylene - Reaction Scheme
Selective Epoxidation – 100 % atom efficient reaction
Epoxidation of ethylene - EO selectivity
Selective oxidation
Assumptions
O2- Selective oxidation
O- - Non selective oxidation
- No recombination
Cl- - Retards O- formation
Alkali/Alkaline earth
- Form Peroxy linkages
- Retard Ag sintering
EO selectivity > 86 % realized
in lab & commercial scale !!!
Non- selective oxidation
6 C2H4 + 6O2- → 6 C2H4O + 6 OC2H4 + 6O→ 2 CO2 + 2H2O
Maximum theoretical selectivity- 6/7 = 85.7 %
WMH Sachtler et. al.,
Catal. Rev. Sci. Eng, 10,1,(1974)&
23,127(1981); Proc. Int. Congr
Catal.5 th, 929 (1973)
Molecular Vs Atomic adsorbed Oxygen – Key for selectivity
Continuous improvements in 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
RA.van Santen &
PCE Kuipers, Adv.
Catal. 35, 265,1987
Nucleophillic attack of Oxygen → Non selective oxidation
Reaction paths in line with observed higher selectivity
Epoxidation of ethylene - Transition state
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
RA. Van Santen & HPCE Kuipers, Adv.Catalysis, 35,265,1987
Epoxidation of ethylene- Surface transformations
Based on DFT , TPD & HREELS studies
Similar intermediates in epoxidation of butadiene
J.Greeley & M Mavrikakis, J.Pys.Chem. C, 111, 7992,2007
S.Linic & MA.Barteau, JACS,124,310,2002; 125,4034,2003
S.Linic, H.Piao,K.Adib & MA.Barteau, Angew.Chem.Intl.Ed.,43,2918,2004
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
Oxychlorination of Ethylene
Ethylene Oxychlorination
Production of Ethylene Di Chloride (EDC) for VCM
Ethylene Oxychlorination- VCM production
EDC- Precursor for VCM
Ethylene Oxychlorination- Source for EDC
Ethylene Oxychlorination
Ethylene Oxychlorination- Major route for VCM
VCM Production-Feedstocks
18%
82%
Ethylene
Acetylene
Global VCM capacity- 42.7 MMTA (2008)
( Nexant Report)
Alternative routes for VCM
Ethylene Oxychlorination –Relevance to VCM
Process steps for VCM
C2H4 + Cl2 → C2H4Cl2
Direct chlorination to EDC
C2H4Cl2 → C2H3Cl + HCl
Thermal cracking of EDC
C2H4 + 2HCl + ½O2 → C2H4Cl2 + H2O
C2H4 + Cl2 → C2H4Cl2
2 C2H4Cl2 → 2 C2H3Cl + 2 HCl
C2H4 + 2HCl + ½O2 → C2H4Cl2 + H2O
overall,
2 C2H4 + Cl2 + ½O2 → 2 C2H3Cl + H2O
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
2CuCl + ½ O2
Cu2OCl2+ 2HCl
C2H4Cl2 + 2CuCl
Cu2OCl2
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.,9,75,1987
G.Lambert,et.al., J.Catalysis,189, 91 &105 2000
KR.Krishnamurthy et.al, Ind J,Chem.,35A,331,1996
Phase transformations in Catalyst during oxychlorination
Characterization of Ethylene Oxychlorination
catalysts
GC.Pandey, KV.Rao, SK.Mehtha, K.R.Krishnamurthy,DT.Goakak &PK.Bhattacharya,
Ind.J.Chemistry, 35A, 331, 1996
Characterization of Ethylene Oxychlorination
catalysts
Crystalline phase identified in oxychlorination catalysts
of different compositions by X-ray powder diffractometry
Sample
From DRS
(x 103cm-1)
Wt / Wt, %
Cu
K
Cu/K
Ratio
Phases identified
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]
Ethylene Oxychlorination catalyst- XPS study
No Potassium in the core
Cu2+
Cu+ states
Fresh catalyst contains
and
Spent catalyst shell has Cu in both oxidation states
Spent catalyst core shows only Cu+ state
R.Vetrivel, K.Seshan,KR Krishnamurthy & TSR Prasada Rao, Bull.Mat.Sci.,9,75,1987
Structural & electronic changes across catalyst geometry
XPS data on Oxychlorination catalysts
Ethylene oxychlorination catalyst- TPR study
TPR profiles indicate presence of Cu 2+ & Cu+ states in fresh & spent shell
Catalyst & only Cu+ in spent core section- Confirms XPS data
R.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 catalyst
R.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-Powder
R.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