Ch120 Lecture: The BiMoO x Story

Transcription

1 Ch120 Lecture: The Bi x Story Heterogeneous selective (amm)oxidation of propene. Kimberly Chenoweth November 28, 2007

2 What do these processes have in common? Late 1800s Early 1900s 1950s 1960s 1970s 1980s Beer brewing by malting procedure Vinegar by aerobic fermentation of ethanol Yogurt formation from milk by lactose to lactic acid conversion Methane from syngas Coal liquefaction Ammonia synthesis Polypropylene (Ziegler-Natta) Naphthalene oxidation to phthalic anhydride Acrylonitrile via ammoxidation of propene (SHI) Propene oxidation to acrolein/acrylic acid Auto exhaust gas catalysts (catalytic converter) Gasoline from methanol process (bil) Malt enzymes Acetobacter Lactobacillus Ni Fe Fe/K Ti V, oxides Bi, oxides Bi, oxides Pt/Rh Zeolite

3 Aim: Functionalization of Hydrocarbons Convert hydrocarbon (i.e. propene, propane) to more useful products Use catalysts to do this selectively ur focus is on Bi x Extensively studied yet not atomistically understood Results from DFT calculations to provide insight Connections between theoretical and experiment results

4 20 most important organic chemicals are produced by catalytic processes 85% of industrial organic chemicals are produced from petroleum and natural gas 21% are produced by heterogenous catalysis: allylic oxidation (acrolein, H 2 C=CHCH) and ammoxidation (acrylonitirle, H 2 C=CHCN), epoxidation, aromatic oxidation. RKG, Boston ACS, 2007

5 verview of Typical Process Diagram rganic Hydrocarbon Fixed Bed Reactor Re-oxidant Functionalized Hydrocarbon xidized Product

6 Heterogeneous Catalysis General reaction steps: 1. Diffusion of reactant to catalyst 2. Adsorption of reactant on catalyst surface 3. Reaction to convert reactant to product 4. Desorption of product from catalyst surface 5. Re-oxidation of active site(s)

7 Products of Conversion Acrylonitrile: World production: 6x10 6 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 Acrolein: World production: 3x10 6 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

8 Industrial Process: Ammoxidation of Propene to Acrylonitrile Current Process (SHI/BP) C 3 H 6 + NH 3 + 3/2 2 (air) cat. CH 2 =CH-CN + 3 H 2 Catalyst: (K,Cs) (Ni,Co,Mn,Mg) (Fe,Cr) Bi x Si 2 AN Yield: 80+ % (fluid bed) (MCM) Future Process C 3 H 8 + NH (air) cat. CH 2 =CH-CN + 4 H 2 Advantages: Propane feedstock cheaper & more abundant than propylene Save one process step Catalyst: SbVAlWSnTe V(Nb,Ta)(Te,Sb) VAlN (SHI/BP) (Mitsubishi + thers) (Prada Silvy) Conversion 77% Selectivity 49% AN Yield 39% 89% 70% 62% 55% 66% 36% Best so far, but need >70% RKG, Boston ACS, 2007

9 Multi-metal oxides (MM) General synthesis procedure: 1. Slurry of metal salts 2. Dry material in air (120 C) 3. Grind into powder 4. Calcination in air (250 C) ensures metal salts are converted to metal oxides Primary components Secondary components

10 Activity of Bi and oxides Propylene Allyl alcohol Catalyst (320 C) % selectivity % conversion % conversion M 2+ a M 3+ b Bi x y z α-bi β-bi γ-bi Bi (hexadiene) C 3 H 6 Reactivity: M.C. > β, α > γ > 3, Bi 2 3 AA Reactivity: M.C. > γ > α > 3, Bi 2 3 Bi 2 3 most effective in generating allyl radicals Bi 2 3 cannot convert allyl radicals to acrolein 3 cannot activate propene 3 + allyl radicals give acrolein at 50% yield 3 + allyl alcohol give acrylonitrile at 70% yield Bi (α phase) and Bi (β phase) show better performance Functions of Bi 2 3 and 3 are not additive Grasselli et al

11 Catalyst Structure: α-bi Exposed Catalyts Surface (010) SEM images (Fansuri 2005) Bulk characteristics: Scheelite structures with ordered cation vacancies - distorted tetrahedra with at 1.72Å and 1.78Å Additional at 2.2Å from tetrahedra Bi - 8 neighbors and the Bi- distances can be divided into two distinct groups ( Å and Å)

12 Proposed Mechanism C-H activation Reactants Allyl adsorption C-H activation Ammoxidation NH 3 Act. xidation 2 nd H abstraction 4 th H abstraction 3 rd H abstraction Regeneration of Catalyst Allyl adsorption Re-oxidation of active site Products 2 nd H abstraction

13 Experimental Results Reaction temperature: oxidation 320 C or ammoxidation C Rate determining step is activation of propene Rate determining step for conversion of allyl to product is 2 nd H abstraction NH 3 decreases propene conversion to acrolein at oxidation temp. Require higher temperature because NH 3 blocks active site at 320 ; at higher temp. =NH forms and allows propene chemisorption Activation energy for NH 3 higher compared to propene activation Allylic N insertion more favorable than allylic insertion ne NH 3 involved Two NH 3 involved HN HN HN HN NH HN NH Increase partial pressure of NH 3 Increase conversion of C 3 H 6

14 C-H activation

15 Cluster model for α-bi 2 3 crystal α-bi 2 3 Bi 4 6 Crystal structure - Bi connected to 3 nearest neighbor Vaporization experiments show presence of closed-shell (Bi 2 3 ) n clusters Bi 4 6 cluster can mimic chemistry and retains stoichiometry, neutrality, and coordination of bulk

16 Propene Activation (1 st H abstraction) on Bi x Many experiments suggest that C-H activation occurs on oxygen associated with bismuth Bi(III) is widely accepted as the active site Bi Bi Bi III Bi Bi Bi Bi Bi Bi Bi V Reaction ΔG 673 (kcal/mol) Bi propene Bi 4 6 H + allyl 41.6 Bi Bi Bi propene Bi 4 7 H + allyl 2.5 Bi 4 7 H + propene Bi H 2 + allyl Bi III ΔG 673 far too high to play a role. Bi V ΔG 673 ok by barrier? No experimental evidence. Small amounts of Bi V 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 -Bi-Co-Fe-) Jang & Goddard 2002

17 Calculated Kinetics for C-H Activation By Bi V Transition State (9.4) (0.0) (4.9) (-1.6) C-H activation of propene (singlet surface) Transition state mode: H transfer between C and u=665i cm-1 Delocalization stabilizes allyl Calculated ΔH barrier = 11.0 kcal/mol Experimental ΔH barrier (on Bi 2 3 at K) = 14 kcal/mol Difference may arise from strain in the finite cluster Pudar et al. 2007

18 Cluster model for 3 crystal Stabilized oxo oxygen (2.3Å) Terminal oxo oxygen (1.68Å) Bridging ether xygen (2.02Å) has similar reactivity, stoichiometry and coordination to that found in both pure 3 and α-bi catalysts

19 C-H Propene Activation over H 3 C H C CH (0.0) -5.1 (-4.4) H 2 C H C CH (23.9) H 2 C H 21.6 (20.4) H C CH (8.8) H H 2 C ΔH barrier = 28.3 kcal/mol. Thus, 3 inactive for propene oxidation, in agreement with experiment. Pudar et al. 2007

20 Allyl conversion to acrolein

21 Allyl trapping over 3 Trapping of allyl radical on 3 is favorable (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

22 2nd H-abstraction to convert bound allyl to acrolein TS Dashed Line: Absence of 2 Solid Line: 2 assisted acrolein desorption Re-oxidation of reduce sites significantly improves acrolein desorption process Net barrier (35.5 kcal/mol) suggests that 3 is capable of allyl oxidation but with lower activity than Bi x, in agreement with experiment Pudar et al. 2007

23 2nd H-abstraction to convert bound allyl to acrolein TS Spectator oxo effect Spectator group free to use 2 dπ orbitals to form super double bond, whereas the 2nd = bond requires one of these dπ orbitals 7 1 Allison & Goddard 1985

24 Propene xidation Mechanism Pudar et al. 2007

25 Ammonia Activation

26 Ammonia Activation on VI 2 nd H abs. 1 st H abs. Coordination is quite exothermic: explains the rapid decrease in conversion upon NH 3 addition Hydrogen abstraction barriers 1 st H abs. barrier: ΔE = 41 kcal/mol 2 nd H abs. barrier: ΔE = 30 kcal/mol This suggests that ammonia activation occurs on reduced sites (i.e. IV )

27 Ammonia Activation on IV Net energy cost is roughly the same on VI compared to IV but reduced at each step Highest barrier is ΔE = 21.8 kcal/mol NH 3 activation much easier on reduce sites

28 Ammonia Activation on IV Dashed Line: No barrier for NH 3 -assisted H 2 desorption (similar to 2 -assisted desorption) Solid Line: ΔE=29 kcal/mol for desorption of H 2 After initiating oxidation and ammoxidation, ammonia is activated much more rapidly

29 Ammoxidation of propene to form acrylonitrile

30 Ammoxidation Kinetics HN HN HN HN NH HN NH Low partial pressure of NH 3 /C 3 H 6 ne NH 3 involved Low conversion of C 3 H 6 Intermediate partial pressure of NH 3 /C 3 H 6 Two NH 3 involved Intermediate conversion of C 3 H 6 High partial pressure of NH 3 /C 3 H 6 Two NH 3 involved High conversion of C 3 H 6 Rate determining step for conversion of allyl radical to acrylonitrile is 2 nd allylic H abstraction Calculate barriers to explain different reactivity under different pressures of ammonia

31 Ammoxidation of Allyl over 3 : Low Partial Pressure 2 nd H abstraction barrier: ΔE = 33.0 kcal/mol Reduce barriers by re-oxidizing surface

32 Ammoxidation of Allyl over 3 : Intermediate Partial Pressure 2 nd H transfer to imido barrier: ΔE = 22.8 kcal/mol 2 nd H transfer to oxo barrier: ΔE = 33.7 kcal/mol (8.2 kcal/mol higher than 2 nd H abstraction by imido)

33 Ammoxidation of Allyl over 3 : High Partial Pressure 2 nd H transfer to NH barrier: ΔE = 18.6 kcal/mol Pink line provides alternate pathway to the same product

34 Number of NH Groups vs. 2nd Allylic H Abstraction Barrier Number NH groups HN Barrier 2 nd Allylic H abstraction Conversion of C Low HN HN 22.8 Medium HN NH HN NH 18.6 High Higher partial pressures of feed (more NH groups) give rise to higher conversion of propene to acrylonitrile (in agreement with experiment)

35 Key References Pudar, S., xgaard, J., van Duin, A.C.T., Chenoweth, K., Goddard III, W.A. Journal of Physical Chemistry C, 2007, 111, (and references within) Spectator xo Effect: Allison, J. N.; Goddard, W. A. In Active Sites on lybdenum Surfaces, Mechanistic Considerations for Selective xidation and Ammoxidation of Propene; Grasselli, R. K., Bradzil, J. F., Eds.; American Chemical Society: Washington, DC, 1985; Vol. 279, p 23. Rappe, A. K.; Goddard, W. A. J. Am. Chem. Soc. 1982, 104, 3287.