Propane Ammoxidation over Mo–V–Te–Nb–O M1 Phase Investigated by DFT: Elementary Steps of Ammonia Adsorption, Activation and NH Insertion into π-Allyl Intermediate

Abstract

The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo–V–Te–Nb–O system has received significant attention because it is more environmentally benign than the current process of propene ammoxidation and relies on more abundant propane feedstock. The reaction mechanism is proposed to consist of a series of elementary steps including propane oxidative dehydrogenation, ammonia and O2 activation, and NHx insertion into C3 intermediates. In this study density functional theory calculations have been performed to investigate the energetics of ammonia adsorption and activation in the proposed active center in the ab plane of the M1 phase. The formation of NH x (x = 0, 1, 2, 3) species is found to be highly favored on reduced, oxo-depleted metal sites. The reduced Mo site is determined to be the most favorable site for ammonia activation by comparing the reaction energy profiles for the sequential dehydrogenation of ammonia on the various metal sites. The activation barrier for the initial H abstraction from ammonia was found to depend strongly on the surface sites that stabilize H and NH2, and is as low as 0.28 eV when NH2 is stabilized by the reduced Mo site and H is abstracted by the telluryl oxo group. The subsequent step of surface NH insertion into a π-allyl gas intermediate was also found to have a low activation energy barrier of 0.03 eV on the reduced Mo site.