A Theoretical Investigation of the Selective Oxidation of Methanol to Formaldehyde on Isolated Vanadate Species Supported on Silica

A theoretical analysis has been conducted on the selective oxidation of methanol to formaldehyde catalyzed by isolated vanadate species supported on silica. 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.