Local Environmental Modification of Ti-SiO2 via Terminal Alcohols Grafting for Liquid-Phase Cyclohexene Epoxidation...

Selective oxidation of hydrocarbons requires highly selective catalysts toward desired reaction products. In this study, SiO2-supported Ti catalysts were synthesized featuring isolated Ti oxide species (0.2 Ti atoms/nm2) that serve as active Lewis acid sites for the selective oxidation of cyclohexene with H2O2. To investigate the effect of the surface microenvironment on catalytic performance, post-modification was performed using alcohols with different terminal groups (octyl (O), triglycol (TG), and fluorous (F)). FT-IR spectroscopy confirms that the deposited organic groups were successfully installed on the silica support. Furthermore, the change in the silanol peak (3730 cm-1) confirms that the introduced organic groups effectively minimize moisture contact on the catalyst surface, creating a hydrophobic/hydrophilic environment near the Ti active sites. In the cyclohexene epoxidation reaction (60 °C), the surface-modified catalysts increase the epoxide yield by an average of 25% compared to the unmodified catalyst. Notably, Ti-SiO2-O exhibits the highest direct pathway selectivity of 93.1%, representing a relative increase of 12% compared to the Ti-SiO2 catalyst (83.4%). Raman and XPS results confirm that the reaction proceeds via a titanium hydroperoxide (Ti-OOH) intermediate. Furthermore, density functional theory (DFT) calculations elucidate that the hydrophobic octyl groups provide an optimal balance between cyclohexene adsorption and product desorption. Conversely, the hydrophilic nature of the TG surface and the electron-rich characteristics of the fluorous chains result in the homolytic activation of H2O2. These findings demonstrate that tailoring the surface microenvironment via post-synthetic modification serves as a vital strategy for maximizing the efficiency of catalysts for selective oxidation reactions.