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

C-H activation

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 conversion to acrolein

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

Propene Oxidation Mechanism

Pudar et al. 2007

Ammonia Activation

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 of propene to formacrylonitrile

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.