Vanadia-Silica Catalysts Prepared by Sol–Gel Method: Application for Epoxidation Reaction

Abstract

Vanadia-silica xerogel and aerogel catalysts, with various V/Si molar ratios (0.05, 0.1, and 0.2), were prepared by sol–gel processing. Gels were obtained under acid catalysed conditions (HNO₃) from vanadium (III) acetylacetonate (V(acac)₃) and tetraethoxysilane (TEOS). Under these conditions, it was found that the gelation time increased with V/Si molar ratio. The structure and catalytic properties of the mixed oxides were found to be mainly influenced by the V/Si molar ratio and drying mode. The catalytic behaviour of the xerogels and aerogels was tested in the epoxidation reaction of trans-2-hexen-1-ol. The results showed that the V/Si molar ratio did not significantly influence the catalytic activity, although the effect of drying (xerogel vs. aerogel) was remarkable. Using a molar ratio TBHP/alcohol = 4/7, the epoxide yield was about 50% in the presence of xerogels, with ca. 8 h required to achieve stationary-conversion conditions in this heterogeneous-catalysis system. In contrast, the aerogel catalysts required only 2 h to reach stationary conversion conditions, due to partial leaching of the vanadium species from the solid and the associated contribution of a relatively rapid homogeneous catalysis mechanism to the overall conversion. These differences in catalytic behaviour were related to the vanadium speciation and the texture of the catalysts. To explore the origin of these differences, the catalysts were characterised by nitrogen physisorption, XPS, Raman spectroscopy, TGA/DTA and XRD.