(147d) Density Functional Theory Study of Propane Ammoxidation Over Mo-V-Te-Nb-O M1 Catalyst

The selective ammoxidation of propane into acrylonitrile catalyzed by the Mo-V-Te-Nb-O mixed metal oxide received considerable attention because of its environmentally friendly nature.  This process consists of a series of steps involving propane oxidative dehydrogenation (ODH), ammonia and O₂ activation, and N-insertion into C₃ surface intermediates.  However, the limited fundamental understanding of the catalyst has hindered the progress in improving their activity and selectivity required for commercial applications.  In this study, density functional theory (DFT) calculations were performed to investigate the mechanism of propane ammoxidation over cluster models of the proposed selective and active sites present in the surface ab plane of the M1 phase, in order to understand the roles of the different metal cation species in the reaction.  We have calculated the activation energy barriers for the oxidative dehydrogenation (ODH) of propane on vanadyl, molybdyl, and telluryl oxo groups (V=O, Mo=O, and Te=O).  V⁵⁺=O is more active than V⁴⁺=O, in line with the current proposed reaction mechanism.  However, Te=O is found to be the strongest adsorption site for H atoms and a significantly more active site for propane ODH than V=O regardless of the oxidation state of V.  Radical C₃ species (isopropyl and allyl) energetically prefer to adsorb on V=O and Te=O.  We also explored the adsorption of NH_x (x=0-3) species and the N-insertion steps into C₃ intermediates over proposed active sites present in the surface ab plane of Mo-V-Te-Nb-O M1 phase.  The reactivity trends established for the constituent metal cations and the information about elementary reaction steps for propane ODH found in this study are highly useful for further improvement of the M1 catalyst.