Experimental and Theoretical Investigations of Propene Ammoxidation to Acrylonitrile over Bismuth Molybdate

Propene ammoxidation over Bi₂Mo₃O₁₂ was investigated to elucidate product (acrylonitrile, acetonitrile, HCN, acrolein, N₂, etc.) formation pathways. Propene consumption rate is first order in propene and zero order in ammonia and oxygen partial pressures, has an activation energy (E_a = 22 kcal/mol) comparable to that for propene oxidation, suggesting that the rate-limiting step for both reactions is the same. Two N-containing species are relevant at ammoxidation conditions: adsorbed NH₃ on surface Bi³⁺ ions that reacts with vinyl alkoxide species to form products with C-N bonds, and metastable Mo-NH₂ groups that are responsible for NH₃ oxidation to N₂. The proposed reaction mechanism and model of the reaction kinetics captures the experimental trends in product distribution as a function of partial pressures and temperature. DFT calculations of the elementary steps comprising the reaction pathway support the proposed mechanism, as well as revealing what processes control product selectivity.