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A Theoretical Investigation of the Selective Oxidation of Methanol to Formaldehyde on Isolated Vanadate Species Supported on Silica
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posted on 2007-10-11, 00:00 authored by Anthony Goodrow, Alexis T. BellA 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 VO 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 VO 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.
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peroxide speciesrate theoryRapid reoxidationequilibrium constantsbondrate coefficientsTheoretical Investigationvanadium atomvanadate speciesmethanol adsorptiontransition statesformaldehydeO 2O atomsSi 8O Calculationshydrogen atomsiterate coefficientIsolated Vanadate SpeciesSelective Oxidationcenteractivation energyelectron reduction
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