Propane Ammoxidation Over the Mo–V–Te–Nb–O M1 Phase: Reactivity of Surface Cations in Hydrogen Abstraction Steps

Density functional theory calculations (GGA-PBE) have been performed to investigate the adsorption of C₃ (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, ΔE, being ≤−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 (ΔE > −1 eV). Atomic H binds more strongly to Te=O (ΔE ≤ −3 eV) than to all the other sites, including V=O (ΔE = −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  ≤ 1.01 eV) than V=O (E _a  = 1.70 eV on V⁵⁺=O and 2.13 eV on V⁴⁺=O). The higher-than-observed 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.