MOLECULAR-ORBITAL STUDY OF C-H BOND BREAKING DURING THE OXIDATIVE DEHYDROGENATION OF METHANOL CATALYZED BY METAL-OXIDE SURFACES

Extended Huckel calculations were performed to study the rate-limiting step in the oxidative dehydrogenation of methanol as catalyzed by transition-metal oxides and polyoxometalates. The reaction was modeled as the transfer of hydrogen from a methyl adduct to the surface of a transition-metal oxide fragment whose valence electronic structure was shown to resemble those of the Keggin ions which are known to catalyze the reaction. The results indicate that the reaction requires just a single, reducible transition-metal cation; that hydrogen transfers as a hydride which initially approaches the metal cation, not its oxygen ligands; and that adjacent metal cations do not affect strongly the energy of the activated complex but may serve to increase the density of accessible electronic states, thereby enhancing the reaction rate by increasing the entropy of the activated complex. The calculations are not able to distinguish between the two current models for the bonding of a surface methoxy intermediate, but the results are in accord with experiments and previous calculations, which suggest that bridging oxygens are more labile than terminal oxygens once the metal oxide is reduced.