The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo-V-Te-Nb-O M1 phase catalyst received considerable attention because of the low cost and high abundance of propane and its environmentally friendly nature. This process has been proposed to consist of a series of elementary steps including propane oxidative dehydrogenation (ODH), ammonia and O2 activation, and N-insertion into C3surface intermediates. However, the limited fundamental understanding of the reaction mechanism and the roles of the constituent metal cations have hindered the progress in further improving the reactivity of the M1 catalyst required for the commercial process.In this talk, we discuss the results of density functional theory (DFT) calculations that we performed to investigate the propane ammoxidation pathway on the cluster models of the proposed selective and active sites present in the surface ab plane of the M1 phase. The adsorption of H, C3 species (propane, isopropyl, propene, and p-allyl), and NHx (x=0-3) species, as well as the activation energies for the sequential ODH of propane up to p-allyl and the formation of acrylonitrile, were calculated on different cation sites. The activation barrier for propane ODH on V5+=O was found to be 1.3 eV, which is consistent with this being the rate-limiting step according to the proposed propane ammoxidation mechanism over the M1 phase and the current understanding of V5+ as the active site for alkane activation present in V-based mixed oxides. NHx adsorption was found to be preferred on reduced Mo sites. The activation of NH3and the coupling of C-N bonds on the reduced Mo sites were both facile compared to propane activation. The unselective pathway initiated by the oxygen insertion into the allyl intermediate and leading to combustion products was also investigated.Furthermore, a microkinetic model was developed for the selective ammoxidation of propane into acrylonitrile based on the results of DFT calculations for the key elementary reaction steps, both selective that lead to acrylonitrile and unselective, leading to precursors of combustion products. The predictions of this model are compared to experimentally measured initial reaction rates of propane consumption and product selectivities (propene, acrylonitrile, and COx) as a function of environmental conditions.

Propane ammoxidation pathway over Mo-V-Te-Nb-O M1 phase catalyst probed by density functional theory calculations

248th National Meeting (2014)

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