In situXANES study on TiO2–SiO2 aerogels and flame made materials

Abstract

The structures of TiO₂–SiO₂ based aerogels and flame made particles, used as epoxidation catalysts, are compared by X-ray absorption near edge structure (XANES) spectroscopy and in situ XANES during dehydration of the calcined catalyst samples. The results are interpreted in terms of the titanium species present in the catalyst and the catalytic performance in the test reaction, the epoxidation of 2-cyclohexene-1-ol by tert-butylhydroperoxide (THBP) at 90 °C. XANES shows that tetrahedral titanium sites are the favoured sites at low TiO₂ concentrations (1–3.2 wt.% in this study). The catalytic performance (turnover frequency, selectivity) of such aerogels or flame made materials with low TiO₂ content is much better than that of corresponding samples with high TiO₂ content. In aerogels, regardless of the TiO₂ content, the amount of titanium with tetrahedral coordination increases significantly after dehydration at elevated temperature. In contrast, the pre-edge height and its position in the XANES spectra of the flame made catalyst samples is comparable, even without dehydration, to that of TS-1, which contains titanium in tetrahedral coordination. Thus the extent of tetrahedral Ti is intrinsically high in flame made catalysts and increases only slightly during the drying process. This behaviour can be attributed to the hydrophobic character of the flame made materials and ameliorates the catalytic performance, i.e. the selectivity.