Activation and properties of Mo=O bonds in Mo/SiO2 catalysts

Abstract

The existence of the charge transfer excited triplet state [Mo5+-O-] produced by UV-irradiation of Mo/SiO2 catalysts, and its reactivity are evidenced by experiments of photoluminescence, photoinduced metathesis, and photoreduction of CO. Mo5+ ions can be produced separately by thermal activation and O- ions by further adsorption of N2O on those Mo5+ ions. The latter of which are adsorbed on Mo6+ ions are found to be more reactive than O2- of [Mo6+ =O2-] bond. They are able either to add a molecule such as CO or C2H4, or to abstract hydrogen from H2, CH4 or trans-dicyanoethylene, or a CN group form tetracyanoethylene (TCNE). The Mo5+ ions are able to coordinate gas phase ligands when their coordination sphere possesses vacant sites. This is the case for tetracoordinated Mo5+ 4c ions arising from reduction of tetrahedral Mo6+ ions (Eq. (7)). These Mo5+ 4c ions are similar to those produced by UV-irradiaiion (Eq. (2)). In addition, if the adsorbed molecule has a sufficiently large electron affinity, such as TCNE or O2, an electron transfer can occur (Eq. (9) and (17)). The [Mo5+-O-] bond obtained by thermal activation is more difficult to evidence than that obtained with UV-activation because it is not detectable by EPR. However, the EPR results obtained at low temperature show that the O- ions adsorbed on Mo/SiO2 catalysts as well as the [Mo5+-O-] excited triplet state obtained by UV-irradiation of 1Mo6+=O2] interact with methanol (Eq. (16)). They are consistent with the mechanism of methanol oxidation occurring at high temperature (Eq. (4)).