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 (Bi_4O_6/Bi_4O_7) and molybdenum oxide (Mo_3O_9) cluster models. For CH activation, we find a low-energy pathway on a Bi^V 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 Bi_2O_3 condensed phase. We find this process to be not feasible on Bi^(III) (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) Bi^V sites on the Bi_2O_3 surface. The expected low concentration of Bi^V could explain the 3 kcal/mol discrepancy between our calculated barrier and experiment. We present in detail the allyl oxidation mechanism over Mo_3O_9, 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 O_2 suggest that the reoxidation significantly weakens the coordination of acrolein to the reduced MoIV site, helping drive desorption of acrolein from the surface.

Group Members

William A. Goddard III

Charles and Mary Ferkel Professor