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.