Mechanism of selective ammoxidation of propene to acrylonitrile on bismuth molybdates from quantum mechanical calculations

In order to understand the mechanism for selective ammoxidation of propene to acrylonitrile by bismuth molybdates, we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for the various steps involved in converting the allyl-activated intermediate to acrylonitrile over molybdenum oxide (using a Mo₃O₉ cluster model) under conditions adjusted to describe both high and low partial pressures of NH ₃ in the feed. We find that the rate-determining step in converting of allyl to acrylonitrile at all feed partial pressures is the second hydrogen abstraction from the nitrogen-bound allyl intermediate (Mo-NH-CH₂-CH - CH₂) to form Mo-NH - CH-CH - CH₂). We find that imido groups (Mo - NH) have two roles: (1) a direct effect on H abstraction barriers, H abstraction by an imido moiety is (∼8 kcal/mol) more favorable than abstraction by an oxo moiety (Mo - O), and (2) an indirect effect, the presence of spectator imido groups decreases the H abstraction barriers by an additional ∼15 kcal/mol. Therefore, at higher NH₃ pressures (which increases the number of Mo - NH groups), the second H abstraction barrier decreases significantly, in agreement with experimental observations that propene conversion is higher at higher partial pressures of NH₃. At high NH₃ pressures we find that the final hydrogen abstraction has a high barrier [ΔH^‡_fourth-ab = 31.6 kcal/mol compared to ΔH^‡_second-ab = 16.4 kcal/mol] due to formation of low Mo oxidation states in the final state. However, we find that reoxidizing the surface prior to the last hydrogen abstraction leads to a significant reduction of this barrier to ΔH^‡ _fourth-ab = 15.9 kcal/mol, so that this step is no longer rate determining. Therefore, we conclude that reoxidation during the reaction is necessary for facile conversion of allyl to acrylonitrile.