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

Abstract

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.

Propane Ammoxidation over Mo–V–Te–Nb–O M1 Phase Investigated by DFT: Elementary Steps of Ammonia Adsorption, Activation and NH Insertion into π-Allyl Intermediate

“…The high reactivity of the Te center toward NH x is consistent with our previous findings that the radical C 3 and H species adsorb most strongly at Te=O [15]. However, the high reactivity of the Te center toward C 3 radical and H adsorption and propane activation [15,17] is not consistent with the experimentally measured apparent activation barrier for propane conversion on the M1 phase (*1.3 eV [18]) or with the fact that tellurium oxides are not known to activate C-H bonds in alkanes [33]. We speculate that the state or location of the Te species on the surface of a working M1 catalyst is different from those in the proposed bulk structure of the M1 phase.…”

“…1) [9,21]. We have shown previously that three truncated ab planes were sufficient to allow the bulk energy per layer and surface reactivity to converge [15,22]. Accordingly, the cluster models used in this study each consisted of three truncated ab planes.…”

“…Model A (with a formal composition of Mo 12 Te 6 V 3 O 63 H 24 ) contained the proposed active centers in the bulk ab plane of the M1 phase, where the vanadyl and telluryl oxo (V=O and Te=O) groups were present on one side of the ab plane, while the molybdyl oxo (Mo=O) groups were present on the opposite side. This model was used in our previous studies [15][16][17]. For the NH 3 activation and NH insertion studies, the direction of the Te=O groups was reversed so that the Te=O and Mo=O groups were present on the same side of the ab plane and adjacent to each other, as suggested by the reaction mechanism proposed by Grasselli [8,9,19].…”

“…However, the first N-H bond in NH 3 is stronger than the secondary C-H bond in propane (calculated values are 4.88 vs. 4.32 eV). If we assume that E a for the gas-phase NH 3 activation mechanism is equal to the N-H bond energy minus the H adsorption energy to the acceptor site, as is shown to be the case for propane activation on the M1 ab plane [15,17], then the E a for initial hydrogen abstraction from NH 3 would be equal to 2.22 eV on V 5? =O (where DE H is -2.66 eV).…”

“…So far, however, only a handful of theoretical studies have been performed to better understand the catalytic behavior of the Mo-V-Te-Nb-O M1 and M2 phases [10][11][12][13][14]. Previously, we have performed DFT calculations using cluster models of the proposed active surface center of the M1 phase to study the hydrogen abstraction steps from propane to p-allyl intermediate on the various metal sites [15][16][17]. We found that the ODH of propane is the most difficult step in the formation of the p-allyl (C 3 H 5 ) intermediate from propane.…”