Non-hydrolytic synthesis of titanosilicate xerogels by acetamide elimination and their use as epoxidation catalysts

Abstract

Novel non-hydrolytic syntheses of titanosilicate xerogels by polycondensation of silicon acetate, Si(OAc)4, with titanium (IV) dimethylamide or diethylamide, Ti(NR2)4 (R = Me, Et), are presented. The reactions are based on acetamide elimination and yield gels with a high content of Si–O−Ti bonds in comparison with the ester elimination route. Although a ligand exchange was observed, it was interestingly not followed by homo-condensation and during the synthesis the phase separation to SiO2 and TiO2 was avoided. The degree of condensation reached up to 68 %. The xerogels prepared for a comparison by ester elimination from Si(OAc)4 and titanium (IV) isopropoxide featured a significantly lower content of the Si–O–Ti bonds. The initial tests in the epoxidation of cyclohexene by cumyl hydroperoxide (CHP) indicated a high selectivity and moderate activity of the xerogels. The catalytic properties were significantly improved by combining non-hydrolytic and hydrolytic methods yielding mesoporous and homogeneous Si/Ti mixed oxides. The catalysts prepared by these methods provided a complete epoxidation of cyclohexene in 2 h at 65 °C.