Titanium catalysts supported on silica. X-ray absorption investigation on their structures and comparison of their catalytic activities in Diels-Alder and epoxidation reactions

The correlation between the catalytic activity and the local structure of different silica-titanium supported catalysts prepared from different promoters was studied by means of X-ray absorption spectroscopy (XAS). The treatment of silica with TiCl4 gives rise to a very active catalyst of Diels-Alder cycloaddition and epoxidation of cyclohexene with TBHP, but the selectivity in the latter reaction is not very high. The solid obtained by treatment of silica with Ti((OPr)-Pr-i)(4) is very active in epoxidation but not in Diels-Alder. The catalyst obtained by treatment of silica with TiCl2((OPr)-Pr-i)(2) displays an intermediate catalytic activity in both reactions, although it is better in epoxidation than in Diels-Alder. The XAS results show how the Ti environment, and hence the catalytic activity of the solid, depends on the Ti compound used as the precursor. Titanium is bonded to four oxygens in the case of Ti((OPr)-Pr-i)(4), while chlorine is present in the first coordination sphere when TiCl4 is used. The catalyst prepared from TiCl2((OPr)-Pr-i)(2) gives rise instead to the formation of small size anatase particles.