Reaction Mechanism for Ammonia Activation in the Selective Ammoxidation of Propene on Bismuth Molybdates

In this paper, we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for various pathways of ammonia activation on bismuth molybdates, a process required for ammoxidation of propene to acrylonitrile. Using a Mo₃O₉ cluster to model the bulk surface, we examined the activation of ammonia by both fully oxidized (Mo^IV) and reduced (Mo^IV) molybdenum sites. Our results show that ammonia activation does not take place on the fully oxidized Mo(VI) sites. Here the net barriers for the first hydrogen transfer (ΔE^‡ = 44.6 kcal/mol, ΔG^‡_673K = 44.2 kcal/mol) and the second hydrogen transfer (ΔE^‡ = 54.5 kcal/mol, ΔG^‡_673K = 51.7 kcal/mol) are prohibitively high for the reaction temperature of 400 °C. Instead, our calculations show that the reduced Mo(IV) surface sites are far more suitable for this process. Here, the calculated barrier for the first hydrogen transfer from a Mo(IV)-NH₃ to an adjacent Mo(VI)=O is 18.2 kcal/mol (ΔG^‡_673K = 15.4 kcal/mol). For the second hydrogen transfer step, we explored three pathways, and found that the H transfer from a Mo-NH₂ to an adjacent Mo(V)-OH to form water is more favorable (ΔE^‡ = 26.2 kcal/mol (ΔG^‡_673K = 24.0 kcal/mol) than transfer to an adjacent Mo(VI)=O or Mo(V)=O group. These studies complement previous studies for activation and reaction of propene on these surfaces, completing the QM study into the fundamental mechanism.