Cyclooctene epoxidation with methylated titania-silica catalysts

Epoxides find their places in our everyday life. They are widely used in fine chemistry, polymer and pharmaceutical industries. Epoxides are commonly synthesized by heterogenous catalysis from olefins and peroxides. Therefore, it is interesting to develop new and more efficient catalysts to improve the synthesis of epoxides. In the present work, titania-silica (TiO2-SiO2) catalysts were investigated in order to improve their activity for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP). The first goal was to increase the amount of active titanium species, called framework titanium (FW-Ti) and decrease the amount of inactive titanium species, called extra-framework titanium (EFW-Ti). For this purpose, a low titanium loading of 1.5% molar was used to synthesize the catalyst. According to the literature, this promotes the formation of FW-Ti. The second goal was to increase the catalytic activity by modifying the hydrophobicity of the catalyst surface with methyl groups. This hydrophobization should both increase the affinity of the catalyst surface for the hydrophobic olefin and decrease it for the epoxide formed. Also, the TBHP which is an organic oxidant, is expected to have better interactions with the hydrophobized surface. In this study, catalysts were synthesized and methyl-functionalized by a one-pot sol-gel approach. The calcined solids were characterized by TGA, IR-ATR and 29Si NMR to confirm the presence of methyl groups and quantify them. Titanium species were analyzed thanks to DRUV and XPS. It was observed that FW-Ti was mainly formed and that the residual amount of EFW-Ti was not influenced by the methylation degree, contrarily to the case of higher titanium loaded samples. Moreover, N2 physisorption highlighted the fact that TiO2-SiO2 catalysts are mainly microporous. Finally, catalytic, recyclability and leaching tests were performed to study the impact of methylation on the catalytic activity and catalyst stability. The most efficient catalyst was obtained with an intermediate methylation degree and the absence of titanium leaching during reactions was noticed.