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

Abstract

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 (MoIV) and reduced (MoIV) 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.

Quantum mechanics based mechanisms for selective activation of hydrocarbons by mixed metal oxide heterogeneous catalysts – A tribute to Robert Grasselli

“…Later, Sanya Pudar continued the QM investigations of the mechanism [7,8,9], Leading to much more detail on the reaction barriers and intermediates as shown in figures 4 and 5. Figure 3.…”

Estudo da ecoeficiência de processos de produção de acrilonitrila

“…A mistura do oxigênio aos demais reagentes é realizada em configurações adequadas diretamente no reator devido ao seu caráter explosivo (Jordan, 1989) e às características do mecanismo de reação, no qual a quimissorção da amônia é um fator fundamental. A ausência de NH 3 nos sítios ativos do catalisador favorece a ocorrência de reações indesejadas do propileno com oxigênio, reduzindo a conversão do propileno no produto desejado e dificultando a sua recuperação (Pudar et Goddard, 2015;Dimian et Bildea, 2008).…”