Nature of active vanadium nanospecies in MCM-41 type catalysts for olefins oxidation

A multi-technique physicochemical investigation including UV–Vis-DRS, Raman spectroscopy, XPS, ESR and FTIRS with pyridine adsorption was performed to analyze the nature of different vanadium nanospecies present on MCM-41 type catalysts. By employing a direct hydrothermal synthesis, vanadium species were incorporated into siliceous structure mainly as tetrahedrally coordinated isolated V^δ+ ions, which would be located inside the wall and on the wall surface of the mesoporous channels. The coexistence of both vanadium oxidation states V⁴⁺ and V⁵⁺ was also revealed. Acidity measurements permitted to infer about the majority presence of Lewis acid sites, which increase with vanadium content. The catalytic performance of these materials was evaluated in the reaction of α-pinene oxidation with H₂O₂. The highest intrinsic activity of the sample with lower V loading was attributed to the high dispersion and efficiency of the isolated V^δ+ species that actuate as active sites. A mixture of reaction products arising from competitive processes of epoxidation and allylic oxidation was found. - Highlights: • Nature of vanadium nanospecies in mesoporous silicates was investigated. • From hydrothermal sol–gel synthesis, isolated V^δ+ sites were mainly generated. • The coexistence of both vanadium oxidation states V⁴⁺ and V⁵⁺ was revealed. • The catalytic performance was evaluated in α-pinene oxidation with H₂O₂. • The high catalytic activity is attributed to high dispersion of isolated V^δ+ ions.