Ch120 Lecture: The BiMoOx Story
Kimberly Chenoweth
November 28, 2007
Heterogeneous selective (amm)oxidation of propene.
What do these processes have in common?
ZeoliteGasoline from methanol process (Mobil)1980s
Pt/RhAuto exhaust gas catalysts (catalytic converter)1970s
Bi, Mo oxides
Bi, Mo oxides
Acrylonitrile via ammoxidation of propene (SOHIO)
Propene oxidation to acrolein/acrylic acid
1960s
Ti
V, Mo oxides
Polypropylene (Ziegler-Natta)
Naphthalene oxidation to phthalic anhydride
1950s
Ni
Fe
Fe/K
Methane from syngas
Coal liquefaction
Ammonia synthesis
Early1900s
LactobacillusYogurt formation from milk by lactose to lactic acidconversion
Late1800s
AcetobacterVinegar by aerobic fermentation of ethanol
Malt enzymesBeer brewing by malting procedure
Aim: Functionalization of Hydrocarbons
• Convert hydrocarbon (i.e. propene, propane) to more useful
products
• Use catalysts to do this selectively
• Our focus is on BiMoOx
• Extensively studied yet not atomistically understood
• Results from DFT calculations to provide insight
• Connections between theoretical and experiment results
20 most important organic chemicals areproduced by catalytic processes
• 85% of industrial organic chemicals are produced from petroleum and naturalgas• 21% are produced by heterogenous catalysis: allylic oxidation (acrolein,H2C=CHCHO) and ammoxidation (acrylonitirle, H2C=CHCN), epoxidation,aromatic oxidation.
RKG, Boston ACS, 2007
Overview of Typical Process Diagram
Re-oxidant
Oxidized Product
OrganicHydrocarbon
Fixed BedReactor
FunctionalizedHydrocarbon
Heterogeneous Catalysis
General reaction steps:1. Diffusion of reactant to catalyst2. Adsorption of reactant on catalyst
surface3. Reaction to convert reactant to
product4. Desorption of product from
catalyst surface5. Re-oxidation of active site(s)
Products of Conversion
Acrolein:
• World production: 3x106 tonnes
• Simplest unsaturated aldehyde
• Acrid smell similar to that of burning fat
• Highly reactive, most often immediately
reacted to form other products such as
acrylic acid and methionine
• Important monomer for the
manufacture of useful plastics, acrylic
and carbon fiber, and synthetic rubber
Acrylonitrile:
• World production: 6x106 tonnes
• Pungent-smelling colorless liquid
• Acrylonitrile is highly flammable and
toxic
• Undergoes explosive polymerization
• Used in the preparation of polyester
resin, polyurethane, propylene glycol,
and glycerol
Industrial Process: Ammoxidation of Propene to Acrylonitrile
C3H6 + NH3 + 3/2 O2 (air)
Catalyst: (K,Cs)0.1-0.2(Ni,Co,Mn,Mg)7.5-9.5(Fe,Cr)2.3-2.5Bi0.5-1.0Mo12Ox – SiO2 (MCM)
AN Yield: 80+ % (fluid bed)
cat. CH2=CH-CN + 3 H2O
C3H8 + NH3 + 2 O2 (air) CH2=CH-CN + 4 H2O
Propane feedstock cheaper & more abundant than propylene Save one process step
Advantages:
Current Process (SOHIO/BP)
Future Process
RKG, Boston ACS, 2007
Catalyst: (SOHIO/BP) (Mitsubishi + Others)
Conversion 77%49%39%
89%70%62%
SelectivityAN Yield
SbVAlWSnTeO MoV(Nb,Ta)(Te,Sb)O VAlON(Prada Silvy)
55%66%36%
cat.
Best so far, but need >70%
Multi-metal oxides (MMO)
Primary components
Secondary componentsGeneral synthesis procedure:1. Slurry of metal salts2. Dry material in air (120°C)3. Grind into powder4. Calcination in air (250°C) ensures metal
salts are converted to metal oxides
Activity of Bi and Mo oxides
• Bi2O3 most effective in generating allyl radicals
• Bi2O3 cannot convert allyl radicals to acrolein
• MoO3 cannot activate propene
• MoO3 + allyl radicals give acrolein at 50% yield
• MoO3 + allyl alcohol give acrylonitrile at 70% yield
• Bi2Mo3O12 (α phase) and Bi2Mo2O9 (β phase) show better performance
• Functions of Bi2O3 and MoO3 are not additive
Grasselli et al. 1984.
36.00.0–MoO3
19.61.0100 (hexadiene)Bi2O3
66.81.450.0γ-Bi2MoO6
–6.790.5β-Bi2Mo2O9
57.77.490.3α-Bi2Mo3O12
80.769.795.7Ma2+Mb
3+BixMoyOz
% conversion% conversion% selectivityCatalyst (320°C)
Allyl alcoholPropylene
C3H6 Reactivity: M.C. > β, α > γ > MoO3, Bi2O3
AA Reactivity: M.C. > γ > α > MoO3, Bi2O3
Catalyst Structure: α-Bi2Mo3O12Exposed Catalyts Surface (010)
Bulk characteristics:
• Scheelite structures with ordered cation
vacancies
• Mo - distorted tetrahedra with O at
1.72Å and 1.78Å
• Additional O at 2.2Å from Mo
tetrahedra
• Bi - 8 O neighbors and the Bi-O
distances can be divided into two distinct
groups (2.12-2.35Å and 2.60-2.93Å)
SEM images (Fansuri 2005)
Proposed Mechanism
Reactants
Products
Ammoxidation
Oxidation
C-H activation
Allyl adsorption
2nd H abstraction
Regeneration of Catalyst
Re-oxidation of active site
C-H activation
Allyl adsorption
2nd H abstraction 4th H abstraction
NH3 Act.
3rd H abstraction
Experimental Results• Reaction temperature:
oxidation 320 °C or ammoxidation 400-460 °C
• Rate determining step is activation of propene
• Rate determining step for conversion of allyl to product is 2nd
H abstraction
• NH3 decreases propene conversion to acrolein at oxidationtemp.
• Require higher temperature because NH3 blocks active Mosite at 320°; at higher temp. Mo=NH forms and allowspropene chemisorption
• Activation energy for NH3 higher compared to propeneactivation
• Allylic N insertion more favorable than allylic O insertion
Mo
HN O
Mo
O O
Mo
HN O
Mo
HN O
Mo
HN NH
Mo
HN NH
One NH3 involved
Two NH3 involved
Increase partial pressure of NH3
Increase conversion of C3H6
Cluster model for α-Bi2O3 crystal
α-Bi2O3
Bi4O6
• Crystal structure - Bi connected to 3 nearest neighbor O• Vaporization experiments show presence of closed-shell (Bi2O3)n clusters• Bi4O6 cluster can mimic chemistry and retains stoichiometry, neutrality, andcoordination of bulk
Propene Activation (1st H abstraction) on BiOx
Bi
O
O
Bi
Bi
O
O
O
O
Bi
O
BiV
Reaction ΔG673 (kcal/mol)
Bi4O6 + propene → Bi4O6H + allyl 41.6Bi4O6 + 0.5O2 → Bi4O7 36.9Bi4O7 + propene → Bi4O7H + allyl 2.5Bi4O7H + propene → Bi4O6 + H2O + allyl -35.9
BiIII ΔG673 far too high to play a role.
BiV ΔG673 ok by barrier? No experimental evidence.
•Small amounts of BiV might be present in oxidizing environment•Fe(II) efficiently chemisorbs dioxygen to generate atomic lattice oxygen•Large improvement seen when Fe(II)/Fe(III) used in catalyst (MC Mo-Bi-Co-Fe-O)
•Many experiments suggest that C-H
activation occurs on oxygen associated
with bismuth
•Bi(III) is widely accepted as the active site
Jang & Goddard 2002
BiIII
Bi
O
O
Bi
Bi
O
O
O
O
Bi
O
Calculated Kinetics for C-H Activation By BiV
C-H activation of propene (singlet surface)
Transition State
Transition state mode: H transfer between C and O u=665i cm-1Delocalization stabilizes allyl
2.07
1.10
1.40
1.451.35
143.6
• Calculated ΔH barrier = 11.0 kcal/mol• Experimental ΔH barrier (on Bi2O3 at 523-723K) = 14 kcal/mol• Difference may arise from strain in the finite cluster
(9.4)
(-1.6)
(4.9)(0.0)
Pudar et al. 2007
Cluster model for MoO3 crystal
MoO3
Bridging etherOxygen (2.02Å)
Terminaloxo oxygen
(1.68Å)
Stabilizedoxo oxygen (2.3Å)
1.70
1.91 131.7
103.8
Mo3O9
• Mo3O9 has similar reactivity, stoichiometry and coordination to that found inboth pure MoO3 and α-Bi2Mo3O12 catalysts
C-H Propene Activation over MoO3
1.401
1.18
1.81
1.70
1.431.37
179.3
MoO
Mo
O OMo O
O
O
O
O
O
H3C
HC
CH2+
MoO
Mo
O OMo O
O
O
O
OO
H2C
HC
CH2
MoO
Mo
O OMo O
O
O
O
OHO
H2C
HC
CH2
MoO
Mo
O OMoHO O
O
O
O
OH2C
0.0(0.0)
-5.1(-4.4)
27.4(23.9)
21.6(20.4)
7.9(8.8)
• ΔH‡ barrier = 28.3 kcal/mol. Thus, MoO3 inactive for propene oxidation, inagreement with experiment.
Pudar et al. 2007
Allyl trapping over MoO3
• Trapping of allyl radical on MoO3 isfavorable (2.7 kcal/mol barrier)
• π-allyl complex can reversibly form σ-allyl intermediate
• Forward barrier: ΔE‡ = 2.7 kcal/mol• Reverse barrier: ΔE‡ = 21.6 kcal/mol
Pudar et al. 2007
2nd H-abstraction to convert bound allyl to acrolein
• Dashed Line: Absence of O2
• Solid Line: O2 assisted acrolein
desorption• Re-oxidation of reduce sitessignificantly improves acroleindesorption process
•Net barrier (35.5 kcal/mol) suggests that MoO3 is capable of allyl oxidation butwith lower activity than BiMoOx, in agreement with experiment
TS3
1.1231.50
1.83
1.86
1.97
1.91
1.35
Pudar et al. 2007
2nd H-abstraction to convert bound allyl to acrolein
• Spectator oxo effect
• Spectator group free to use 2Mo dπ orbitals to form super
double bond, whereas the 2nd
Mo=O bond requires one ofthese dπ orbitals
TS3
1.1231.50
1.83
1.86
1.97
1.91
1.35
7
1Allison & Goddard 1985
Ammonia Activation on MoVI
• Hydrogen abstraction barriers• 1st H abs. barrier: ΔE‡ = 41 kcal/mol• 2nd H abs. barrier: ΔE‡ = 30 kcal/mol
• This suggests that ammonia activation occurs on reduced Mo sites (i.e. MoIV)
2nd H abs.
1st H abs.
Coordination is quite exothermic: explainsthe rapid decrease in conversion upon NH3
addition
Ammonia Activation on MoIV
• Net energy cost is roughly the same on MoVI compared to MoIV but reduced at each step• Highest barrier is ΔE‡ = 21.8 kcal/mol• NH3 activation much easier on reduce Mo sites
Ammonia Activation on MoIV
• Dashed Line: No barrier for NH3-assisted H2O desorption (similar to O2-assisted desorption)• Solid Line: ΔE=29 kcal/mol for desorption of H2O• After initiating oxidation and ammoxidation, ammonia is activated much more rapidly
Ammoxidation Kinetics
Mo
HN O
Mo
O O
Mo
HN O
Mo
HN O
Mo
HN NH
Mo
HN NH
• Low partial pressure ofNH3/C3H6
• One NH3 involved
• Low conversion ofC3H6
• Intermediate partialpressure of NH3/C3H6
• Two NH3 involved
• Intermediate conversionof C3H6
• High partial pressure ofNH3/C3H6
• Two NH3 involved
• High conversion of C3H6
• Rate determining step for conversion of allyl radical toacrylonitrile is 2nd allylic H abstraction
• Calculate barriers to explain different reactivity under differentpressures of ammonia
Ammoxidation of Allyl over MoO3:Low Partial Pressure
• 2nd H abstraction barrier: ΔE‡ = 33.0 kcal/mol
• Reduce barriers by re-oxidizing surface
Ammoxidation of Allyl over MoO3:Intermediate Partial Pressure
• 2nd H transfer to imido barrier: ΔE‡ = 22.8 kcal/mol
• 2nd H transfer to oxo barrier: ΔE‡ = 33.7 kcal/mol (8.2 kcal/mol higherthan 2nd H abstraction by imido)
Ammoxidation of Allyl over MoO3:High Partial Pressure
• 2nd H transfer to NH barrier: ΔE‡ = 18.6 kcal/mol
• Pink line provides alternate pathway to the same product
Number of NH Groupsvs.
2nd Allylic H Abstraction Barrier
Mo
HN O
Mo
O O
Mo
HN O
Mo
HN O
Mo
HN NH
Mo
HN NH
Number NH groupsBarrier 2nd Allylic
H abstraction Conversion of C3
33.0 Low
22.8 Medium
18.6 High
• Higher partial pressures of feed (more NH groups) give rise to higherconversion of propene to acrylonitrile (in agreement with experiment)
Key References
• Pudar, S., Oxgaard, J., van Duin, A.C.T., Chenoweth, K.,Goddard III, W.A. Journal of Physical Chemistry C, 2007, 111,16405. (and references within)
Spectator Oxo Effect:• Allison, J. N.; Goddard, W. A. In Active Sites on MolybdenumSurfaces, Mechanistic Considerations for Selective Oxidation andAmmoxidation of Propene; Grasselli, R. K., Bradzil, J. F., Eds.;American Chemical Society: Washington, DC, 1985; Vol. 279, p23.• Rappe, A. K.; Goddard, W. A. J. Am. Chem. Soc. 1982, 104,3287.
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