(19d) A Comparative Theoretical Study Of Methanol Oxidation On Isolated Vanadate Species Supported On Silica And Titania

Experimental studies have demonstrated that VO_x/MO₂ (M=Si, Ti) is an active catalyst for the selective oxidation of methanol even when the vanadium is present as isolated vanadate species. This work has shown that the turnover frequency for VO_x/TiO₂ is up to a thousand times higher than for VO_x/SiO₂ at identical reaction conditions. Theoretical methods were used in this study to develop a comprehensive understanding of the reaction mechanism, as well as to understand the fundamental differences in reactivity on these two supports. In the case of SiO₂, the active site was represented by a V=O group substituted for a Si-H group in the corner of silsesquioxane, Si₈O₁₂H₈. Calculations of ground and transition states were carried out using density functional theory, whereas statistical mechanics and absolute rate theory were used to determine equilibrium constants and rate coefficients for each elementary step. The formation of formaldehyde was found to involve two key steps. The first is the reversible adsorption of methanol, which occurs by addition across one of the three V-O-Si bonds of the active site. The rate-limiting step is the transfer of a hydrogen atom from the resulting V-OCH₃ species to the V=O bond of the active center. The release of formaldehyde and water from the active center leads to a two electron reduction of the vanadium atom in the center. Rapid reoxidation of the reduced vanadium can occur via adsorption of O₂ to form a peroxide species and subsequent migration of one of the O atoms associated with the peroxide across the surface of the support. The predicted heat of adsorption and equilibrium constant for methanol adsorption are in good agreement with those found experimentally, as is the infrared spectrum of the adsorbed methanol. The apparent first order rate coefficient and the apparent activation energy are also in very good agreement with the values determined experimentally. Similar methods have been used to understand the oxidation of methanol catalyzed by isolated vanadate species on TiO₂. The active site was modeled by replacing the Si atoms with Ti atoms in the silsesquioxane structure. The bond lengths and vibrational frequencies of the resultant species show very good agreement with the anatase phase of TiO₂. The adsorption of methanol is slightly preferred on the TiO₂-supported species and the intramolecular transfer of a hydrogen atom is very close to that on the SiO₂-supported species. However, these differences are not large enough to account for the observed differences in activity. A mechanism involving the reaction of methoxide groups associated with Ti centers with V=O bonds of vanadate species shows a lower activation barrier. Details of this new reaction pathway will be discussed and interpreted in the light of recent experimental results.