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 (Bi4O6/Bi4O7) and molybdenum oxide (Mo3O9) cluster models. For CH activation, we find a low-energy pathway on a Bi-V site with a calculated barrier of Delta H-double dagger = 11.0 kcal/mol (Delta G(double dagger) = 30.4 kcal/mol), which is similar to 3 kcal/mol lower than the experimentally measured barrier on a pure Bi2O3 condensed phase. We find this process to be not feasible on Bi-III (it is highly endothermic, Delta E = 50.9 kcal/mol, Delta G = 41.6 kcal/mol) or on pure molybdenum oxide (prohibitively high barriers, Delta E-double dagger = 32.5 kcal/mol, Delta G(double dagger) = 48.1 kcal/mol), suggesting that the CH activation event occurs on (rare) Bi-V sites on the Bi2O3 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 Mo3O9, which includes the adsorption of allyl to form the pi-allyl and sigma-allyl species, the second hydrogen abstraction to form acrolein, and acrolein desorption. The formation of sigma-allyl intermediate is reversible, with forward Delta E-double dagger (Delta G(double dagger)) barriers of 2.7 (9.0 with respect to the pi-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 Delta E-double dagger = 35.6 kcal/mol (Delta G(double dagger) = 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.