Catalytical studies on trimethylsilylated Ti-MCM-41 and Ti-MCM-48 materials

Titanium-incorporated MCM-41 and MCM-48 materials (Ti-MCM-41 and Ti-MCM-48, respectively) have been trimethylsilylated by employing various silylating agents (i.e., hexamethyldisilazane (HMDZ) and trimethylchlorosilane (TMCS) in toluene, and a mixture of TMCS and hexametyldisiloxane (HMDS)). Their catalytic activities in various oxidation reactions as well as their physical properties are compared. The highly reactive HMDZ has achieved the maximum degree of silylation to introduce 2.35 and 2.06 trimethylsilyl groups per 1 nm2 on the surface of Ti-MCM-41 and -48, respectively. IR spectra have confirmed that HMDZ can react with hydrogen-bonded silanol groups as well as isolated silanol groups, generating a new band at 3700 cm−1, while TMCS does not. These silylated materials show remarkably high catalytic activity in the oxidation of substrates with various molecular sizes (from C6 to C12) with H2O2 and tert-butyl hydroperoxide (TBHP) compared to non-silylated samples. Introducing a small amount of H2O so as to double the H2O content in a reaction system for the oxidation of cyclododecene using TBHP has approximately doubled the catalytic activity. Such unexpected results were only observed with highly hydrophobic samples, Furthermore, by conducting the reaction at 343 K for 6 h over highly silylated Ti-MCM-48, high conversion of 98% has been attained.