Synthesis, characterization, and activity in the epoxidation of cyclohexene with aqueous H2O2 of catalysts prepared by reaction of TiF4 with silica

Ti-supported silica materials have been prepared by reaction between TiF4 and silica in the liquid phase. Samples prepared in an organic solvent contain titanium and fluorine, but the fluorine can easily be removed upon treatment with ammonia. Directly defluorinated samples can be synthesized in aqueous NH4C(4)H solutions and are similar to those obtained in an organic medium after basic treatment. For low Ti contents (typically <2 wt%), titanium is highly dispersed on the silica surface in the form of isolated tetrahedrally coordinated Ti(IV) species or TiO2 domains, whose size does not exceed a few nanometers. Most of these TiO2 particles can be removed upon washing with concentrated acid. These materials possess a Lewis acidity, due to both isolated Ti centers and TiO2 nanoparticles, and catalyze the epoxidation of cyclohexene with concentrated aqueous H2O2 solutions. The reaction mainly proceeds by a radical mechanism, leading to the formation of allylic oxidation products like cyclohexenyl hydroperoxide. This intermediate reacts further with cyclohexene to cyclohexene oxide and cyclohexenol in similar amounts. Nevertheless, part of the H2O2 initially introduced is also used for the direct nonradical epoxidation of cyclohexene to cyclohexene oxide. Both direct epoxidation and bimolecular reaction between cyclohexenyl hydroperoxide and cyclohexene are inhibited in the presence of water. However, water does not influence the formation of cyclohexenyl hydroperoxide that occurs in the absence, as well as in the presence, of water. On the contrary, dropwise addition of H2O2 favors the nomadical epoxidation of cyclohexene to cyclohexene oxide in relatively high yields. Maximum epoxide selectivities are obtained over catalysts containing less than 2 wt% Ti, whilst higher loading leads to the formation of cyclohexanediol, formed on the Bronsted sites of the materials. (C) 1998 Academic Press.