A Sol−Gel Method Using Tetraethoxysilane and Acetic Anhydride:  Immobilization of Cubic μ-Oxo Si−Ti Complex in a Silica Matrix

A sol−gel procedure using acetic anhydride as a substitute for water using a condensing reagent for the tetraethoxysilane (TEOS) to silica conversion is reported. The mixed solution obtained from the reaction of TEOS and acetic anhydride at 110 °C was allowed to stand in air for several weeks, resulting in gel formation of the solution. This prepared silica was mesoporous based on the nitrogen adsorption−desorption isotherms. Even when the gelation was performed in the presence of a large amount of benzene, silica was also obtained. The immobilization of a cubic μ-oxo Si−Ti complex, which is a good homogeneous catalyst for the epoxidation of olefins, in a silica matrix was achieved by using this technique. Although the complex readily afforded TiO 2 anatase on treatment with water, no free TiO 2 phase was found in the solid obtained by this procedure according to XRD, DR−UV−vis, and Raman spectra. The titanium was highly dispersed in the silica matrix. On the other hand, a solid prepared by the same procedure using titanium tetra-isopropoxide as the titanium source contained the TiO 2 anatase phase. The obtained SiO 2 −TiO 2 mixed oxide solid was an effective heterogeneous catalyst for the highly efficient epoxidation of cyclohexene in the liquid phase by tert -butylhydroperoxide.