Improvement of the Structural Model for the M1 Phase Mo-V-Nb-Te-O Propane (Amm)oxidation Catalyst

An improved structural model for the M1 phase in the Mo-V-Nb-Te-O propane (amm)oxidation catalyst has been refined after accounting for a molybdenum-substituted-V2O5 impurity and by making adjustments based on aberration-corrected imaging results. The newly refined unit cell has Pba2 symmetry with a = 21.134(1) angstrom, b = 26.647(1) angstrom, c = 4.0140(2) angstrom, and Z = 4, in good agreement with our earlier findings (DeSanto et al. Top Catal 23: 23 [20], DeSanto et al. Z Kristallogr 219: 152 [22]). From the newly refined occupancies, the formula unit is {TeO}(0.86(1))center dot Mo7.48(6)V1.52(6)NbO28. As in the earlier models, V is concentrated in sites that link the pentagonal rings of M1. Careful analysis of bond valences, in combination with the electroneutrality constraint, suggest that the linking sites S3, S4, and S7 all have mixed Mo/V occupancies and valences (d(1)/d(0)). Furthermore, these sites may contain a mix of Mo5+ and V5+, which is consistent with the proposed catalytic mechanism in which V5+ plays an important role in propane activation.