Propane ammoxidation over Mo-V-Te-Nb-O M1 phase: Density functional theory study of propane oxidative dehydrogenation steps

Propane ammoxidation to acrylonitrile catalyzed by the bulk Mo-V-Te-Nb oxides has received considerable attention because it is more environmentally benign than the current process of propylene ammoxidation and relies on a more abundant feedstock. This process is proposed to consist of a series of elementary steps including propane oxidative dehydrogenation (ODH), ammonia and O-2 activation, NHx insertion into C-3 surface intermediates, etc. Density functional theory calculations were performed hereto investigate the three sequential H abstraction steps that successively convert propane into isopropyl, propene, and pi-allyl on cation sites in the proposed selective and active center present in the ab plane of the Mo-V-Te-Nb-O M1 phase. The initial H abstraction from propane was found to be the rate-limiting step of this process, consistent with both the proposed reaction mechanism for propane ammoxidation on the Mo-V-Te-Nb oxides and current understanding of V5+ as the active site for alkane activation on V-based oxides. Te=O was found to be significantly more active than V5+=O for the H abstraction from propane, which suggests that the surface and bulk Te species may be different. The role of Mo=O is most likely limited to being an H acceptor from isopropyl to form propene under ammoxidation conditions. (C) 2014 Elsevier B.V. All rights reserved.