Mechanism of selective oxidation of propene to acrolein on bismuth molybdates from quantum mechanical calculations

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) Biv 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 Mo ^IV site, helping drive desorption of acrolein from the surface.