Hydrophobic titania-silica mixed oxides for the catalytic epoxidation of cyclooctene

Titania-silica (TiO ₂ -SiO ₂ ) mixed oxides are known catalysts for the epoxidation of olefins. It has been demonstrated that their active sites consist in isolated framework Ti atoms. However, the latter species are only obtained when Ti loading is low, limiting the number of active sites and the catalytic performances. Thus, the need to search other approaches to boost them. We succeeded improving the catalytic activity of TiO ₂ -SiO ₂ mixed oxides in the epoxidation of cyclooctene with hydrogen peroxide by introducing hydrophobic organic moieties at their surface. Hydrophobic TiO ₂ -SiO ₂ mixed oxides were prepared through a one-pot sol-gel method. Titanium butoxide (TiBut) and tetraethoxy silane (TEOS) were used as the Ti and Si precursors respectively. Hydrophobization was achieved by substituting a fraction of TEOS in the synthesis by methyltriethoxy silane (MTES). The characterization of the Ti species was performed by FTIR, DRUV, and XPS, confirming the presence of significant amount of the active framework Ti. The methylation degree was evaluated by Solid State ²⁹ Si-NMR and TGA-MS. Hydrophobic TiO ₂ -SiO ₂ performed better in the epoxidation reaction than a fully inorganic more hydrophilic TiO ₂ -SiO ₂ prepared without MTES. A too high methylation degree of the catalysts is however detrimental. Our contribution allows elucidating the debate in literature about the consequence of hydrophobization of epoxidation catalysts on their performance.