Simple Molecular Synthetic Approach to Dinuclear Titanium Sites in Ti-Containing Silica-Based Catalysts

Ti-containing silica-based catalysts are widely used for various organic oxidation reactions. Recently, increasing attention has been paid to the catalytic role of dinuclear (SiO)₃Ti-O-Ti(OSi)₃ sites in titanosilicate zeolite TS-1. A promising experimental approach for elucidating the active sites is to use model compounds with dinuclear Ti sites. In this study, we report the synthesis and catalytic properties of a μ-oxo-bridged dinuclear titanosiloxane compound with a (SiO)₃Ti-O-Ti(OSi)₃ site. A Ti-containing cage siloxane compound (ⁱBu₇Si₇O₁₂)TiOⁱPr (mononuclear Ti-polyhedral oligomeric silsesquioxane (POSS)) is prepared by the reaction of incompletely condensed POSS with titanium tetraisopropoxide. Then, it is converted to the dinuclear species (ⁱBu₇Si₇O₁₂)₂Ti₂(μ-O) (dinuclear Ti-POSS) by the simple hydrolytic condensation of the TiOⁱPr groups. The addition of H₂O₂ to dinuclear Ti-POSS leads to the formation of a bishydroperoxo complex, as revealed by Ti K-edge X-ray absorption fine structure analysis. Dinuclear Ti-POSS possesses high catalytic activity in the cyclohexene epoxidation reaction, indicating that the (SiO)₃Ti-O-Ti(OSi)₃ sites can function as efficient catalytically active sites. These findings contribute to the comprehension of the catalytic properties exhibited by dinuclear titanium sites within titanosilicate zeolite catalysts.