Mechanism of Selective Oxidation of Propene to Acrolein on Bismuth Molybdates from Quantum Mechanical Calculations

Abstract

In order to provide a basis for understanding the fundamental chemical mechanisms underlying the selective oxidation of propene to acrolein by bismuth molybdates, we report quantum mechanical studies (at the DFT/B3LYP/LACVP** level) of various reaction steps on bismuth oxide (Bi4O6/Bi4O7) and molybdenum oxide (Mo3O9) cluster models. For CH activation, we find a low-energy pathway on a BiV site with a calculated barrier of ΔH ⧧ = 11.0 kcal/mol (ΔG ⧧ = 30.4 kcal/mol), which is ∼3 kcal/mol lower than the experimentally measured barrier on a pure Bi2O3 condensed phase. We find this process to be not feasible on BiIII (it is highly endothermic, ΔE = 50.9 kcal/mol, ΔG = 41.6 kcal/mol) or on pure molybdenum oxide (prohibitively high barriers, ΔE ⧧ = 32.5 kcal/mol, ΔG ⧧ = 48.1 kcal/mol), suggesting that the CH activation event occurs on (rare) BiV sites on the Bi2O3 surface. The expected low concentration of BiV could explain the 3 kcal/mol discrepancy between our calculated barrier and experiment. We present in detail the allyl oxidation mechanism over Mo3O9, which includes the adsorption of allyl to form the π-allyl and σ-allyl species, the second hydrogen abstraction to form acrolein, and acrolein desorption. The formation of σ-allyl intermediate is reversible, with forward ΔE ⧧ (ΔG ⧧) barriers of 2.7 (9.0 with respect to the π-allyl intermediate) kcal/mol and reverse barriers of 21.6 (23.7) kcal/mol. The second hydrogen abstraction is the rate-determining step for allyl conversion, with a calculated ΔE ⧧ = 35.6 kcal/mol (ΔG ⧧ = 37.5 kcal/mol). Finally, studies of acrolein desorption in presence of gaseous O2 suggest that the reoxidation significantly weakens the coordination of acrolein to the reduced MoIV site, helping drive desorption of acrolein from the surface.

Structures, Mechanisms, and Kinetics of Selective Ammoxidation and Oxidation of Propane over Multi-metal Oxide Catalysts

“…In addition to earlier work [7,8], which explored the thermodynamics of prospective reaction pathways, we have now also examined the relevant kinetics for this chemistry [27]. The reaction barrier for conversion of propene into allyl on a Bi V site was explored using a Bi V Bi III 3 O 4 cluster model, and was determined to be 12.8 kcal/mol (TS1, Fig.…”

“…In addition to the general oxidation scheme studied above, we have investigated the mechanism of ammoxidation of propene [28]. This process involves activation of ammonia by terminal Mo=O groups to form imido species (Mo=NH), followed by propene activation, N insertion, and three subsequent hydrogen abstractions to form acrylonitrile as a final product.…”

Active Sites for Propane Oxidation: Some Generic Considerations

“…Surface-science approaches work only with activated molecules such as methanol [5,6] and theory [5,7,8] is confined to simplified yet highly valuable model systems [6,9,10]. It is still a great challenge to construct concepts of active sites [11,12] for selective oxidation that are based upon experimental observations [13] of reacting surfaces.…”

“…For this reason the exact crystallographic analysis [40,45] of the basal plane structure not only in terms of its connectivity but also in terms of its cation ordering statistics [45][46][47] is of particular relevance. The high level of structural definition has inspired theoretical work [11,32,41,48] aiming at a detailed quantum chemical explanation [11,12,41] of the mode of operation; such work is only possible with a reasonable knowledge about the local chemistry and with the atom coordinates available now in high precision from structure determination. Transmission electron microscopy, a technique since long of great value [2,42,47,49,50] in the analysis of the complex oxide catalysts, helped [51] also here to elucidate the surface termination [52] of M1.…”

Chemistry in the Center for Catalytic Hydrocarbon Functionalization: An Energy Frontier Research Center

“…Oxidation of propene by bismuth molybdates (Bi x Mo y O z ) accounts for the majority of the eight billion pounds of acrolein produced annually [46,47]. Bi x Mo y O z operates by a two site mechanism where the Bi(V) site activates the propene and the remainder of the chemistry occurs on the MoO z [48,49]. A number of multi-metal oxides that have shown activity for selective oxidation or ammoxidation of propane and selective oxidations of butane are known but the mechanism by which they work is not.…”