Niobia–silica aerogel mixed oxide catalysts: Effects of the niobium content, the calcination temperature and the surface hydrophilicity on the epoxidation of olefins with hydrogen peroxide

Abstract

Niobia–silica mixed oxides can be synthesized under acidic conditions and evaporated under supercritical conditions to yield meso-macroporous materials. The amount of active metal in the catalyst changes the acidity of the final material and in the epoxidation of allylic alcohols (geraniol) a high amount of niobium improves the production of glycols. Surface modification with methyl groups is possible by addition of methyltriethoxysilane without any change in the morphological properties of the final materials. The extent of methylation does not change significantly the surface area and the porosity but does change the catalytic activity of the materials in the epoxidation of the cyclooctene and geraniol. Methylation improves the activity of the catalysts and the use of the oxidant in the epoxidation of unsubstitued olefins, whereas only small differences were observed in the epoxidation of geraniol.