The Mo−V−Te−Nb−O mixed metal oxide catalyst possessing the M1 phase structure is uniquely capable of directly converting propane into acrylonitrile. However, the mechanism of this complex eight-electron transformation, which includes a series of oxidative H-abstraction and N-insertion steps, remains poorly understood. We have conducted a density functional theory study of cluster models of the proposed active and selective site for propane ammoxidation, including the adsorption of propane, isopropyl (CH3CHCH3), and H which are involved in the first step of this transformation, that is, the methylene CH bond scission in propane, on these active site models. Among the surface oxygen species, the telluryl oxo (TeO) is found to be the most nucleophilic. Whereas the adsorption of propane is weak regardless of the MO
x
species involved, isopropyl and H adsorption exhibits strong preference in the order of TeO > VO > bridging oxygens > empty Mo apical site, suggesting the importance of TeO
x
species for H abstraction. The adsorption energies of isopropyl and H and consequently the reaction energy of the initial dehydrogenation of propane are strongly dependent on the number of ab planes included in the cluster, which points to the need to employ multilayer cluster models to correctly capture the energetics of surface chemistry on this mixed metal oxide catalyst.
Density functional theory calculations (GGA-PBE) have been performed to investigate the adsorption of C 3 (propane, isopropyl, propene, and allyl) and H species on the proposed active center present in the surface ab planes of the bulk Mo-V-Te-Nb-O M1 phase in order to better understand the roles of the different surface cations in propane ammoxidation. Modified cluster models were employed to isolate the closely spaced V=O and Te=O from each other and to vary the oxidation state of the V cation. While propane and propene adsorb with nearly zero adsorption energy, the isopropyl and allyl radicals bind strongly to V=O and Te=O with adsorption energies, DE, being B-1.75 eV, but appreciably more weakly on other sites, such as Mo=O, bridging oxygen (Mo-O-V and Mo-O-Mo), and empty metal apical sites (DE [ -1 eV). Atomic H binds more strongly to Te=O (DE B -3 eV) than to all the other sites, including V=O (DE = -2.59 eV). The reduction of surface oxo groups by dissociated H and their removal as water are thermodynamically favorable except when both H atoms are bonded to the same Te=O. Consistent with the strong binding of H, Te=O is markedly more active at abstracting the methylene H from propane (E a B 1.01 eV) than V=O (E a = 1.70 eV on V 5? =O and 2.13 eV on V 4? =O). The higher-thanobserved activity and the loose binding of Te=O moieties to the mixed metal oxide lattice of M1 raise the question of whether active Te=O groups are in fact present in the surface ab planes of the M1 phase under propane ammoxidation conditions.
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 O 2 activation, and NH x insertion into C 3 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, oxodepleted 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 NH 2 , and is as low as 0.28 eV when NH 2 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 p-allyl gas intermediate was also found to have a low activation energy barrier of 0.03 eV on the reduced Mo site.
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