"A Simple Molecular Synthetic Approach to Dinuclear Titanium Sites in Ti-containing Silica-based Catalysts"
T. Hikino, Y. Kawahkubo, T. Matsuno, S. Yamazoe, K. Kuroda, A. Shimojima
Chem. Mater., Accepted.
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.