(116a) Propane Ammoxidation Pathway Over Mo-V-Te-Nb-O M1 Catalyst Investigated By Density Functional Theory

The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo-V-Te-Nb-O system 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 consists of a series of elementary steps including propane oxidative dehydrogenation (ODH), ammonia and O₂ activation, and N-insertion into C₃ surface intermediates.  However, the limited fundamental understanding of the reaction mechanism and the roles of the different cations has hindered the progress in further improving its activity and selectivity required for the commercial application.  

In this talk, we discuss the results of the density functional theory (DFT) calculations performed to investigate overall 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 activation energies for the oxidative dehydrogenation (ODH) of propane and sequential intermediates (isopropyl, propene, and allyl) were calculated on different cation sites. Propane activation on V⁵⁺=O was found to be the rate-limiting step (E_a = 1.2 eV), consistent with the current proposed reaction mechanism for propane activation on the Mo-V-Te-Nb oxides and the current understanding of V⁵⁺ as the active site for alkane activation present in V-based mixed oxides.  The unselective pathway leading to C3 combustion starting from propene activation on Mo=O site was also explored.

Furthermore, a linear relationship was established between the H adsorption energy and the activation energy for H abstraction from various C3 intermediates, which is highly useful for predicting the energy barriers of H abstraction from C3 species based solely on H adsorption energy. The energy barriers of ammonia activation on different surface sites and NH insertion into the allyl species were investigated and compared with the hypothetical pathway reported in the literature. These elementary reaction steps were indicated to be energetically barrier-less. The distribution of NH_x and OH_x species over surface sites were then explored using micro-kinetic models to study the impact on selectivity of propane ammoxidation to acrylonitrile on the Mo-V-Te-Nb-O M1 phase.