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

Abstract

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 VdO group substituted for a Si-H group in the corner of silsesquioxane, Si 8 O 12 H 8 . 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-OCH 3 species to the VdO 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 O 2 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.

Density functional theory study of the oxidation of methanol to formaldehyde on a hydrated vanadia cluster

“…Broken-symmetry has also been used in previous works where oxidation processes have been studied in gas phase models. 32,[56][57][58][59] In those studies, the results are in good agreement with the experimental data confirming the appropriate use of such approach applied to catalytic systems.…”

“…8,9,28,30,32,56,57,[68][69][70][71][72][73] Sauer and co-workers have studied the oxidation of methanol to formaldehyde by means of a O¼ ¼V(OCH 3 ) 3 model and the corresponding activation energy for the CÀ ÀH bond breaking process is 35.1 kcal/mol. 32 In the case of the oxidation of methane to methanol which involves similar steps, the activation energy of CÀ ÀH bond breaking is calculated to be of 36.6 kcal/mol with the same O¼ ¼V(OH) 3 model cluster.…”

Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

“…This would result in the formation of a hydroxyl on the ceria support and a vanadylmethoxy complex (i.e., O=V -OCH 3 ), as previously suggested by DFT studies of methanol ODH on vanadia monomers supported by SiO 2 or TiO 2 [39,[74][75][76] (structure II). Another possibility is that isolated vanadyl species are not as sterically hindered as vanadyl groups in vanadia thin films, and both the hydroxyl and the methoxy are bound to the vanadia forming a HO-V-OCH3 species (structure III).…”

Mechanism and Site Requirements for Ethanol Oxidation on Vanadium Oxide Domains

“…42,47,48 However, the rate of methanol oxidation is proportional to methanol and independent of O 2 pressure, whereas the rate of ethanol oxidation is independent of both ethanol and O 2 pressure. This difference merely reflects the lower coverages of methoxides prevalent at reaction conditions relative to those of ethoxide.…”