The relationship between the electronic and redox properties of dispersed metal oxides and their turnover rates in oxidative dehydrogenation reactions

The mechanistic connections among propane oxidative dehydrogenation (ODH) rates, H-2 reduction rates, and the electronic transitions responsible for the absorption edge in the electronic spectra of dispersed metal oxides were explored for VOx, MoOx, WOx, and NbOx samples consisting predominately of two-dimensional oxide domains supported on Al2O3, ZrO2, and MgO. For a given active oxide, propane turnover rates increased in parallel with the reduction rate of the oxide catalyst using H-2, but propane ODH rates differed significantly among different metal oxide samples with similar H-2 reduction rates. For all catalysts, ODH turnover rates increased monotonically as the energy of the absorption edge in the UV-visible spectrum decreased. These results, taken together with the respective mechanisms for electron transfer during C-H bond activation and during the ligand-to-metal charge-transfer processes responsible for the UV-visible edge, suggest that the stability of activated complexes in C-H bond dissociation steps depends sensitively on the ability of the active oxide domains to transfer electrons from lattice oxygen atoms to metal centers. The electronic transitions responsible for the UV-visible absorption edge are mechanistically related to the redox cycles involving lattice oxygens responsible for oxidative dehydrogenation turnovers of alkanes. As a result, the details of near-edge electronic spectra provide useful guidance about intrinsic reaction rates on active oxides typically used for these reactions. (C) 2002 Elsevier Science (USA).