Elucidating the local environment of Ti(IV) active sites in Ti-MCM-48: a comparison between silylated and calcined catalysts

A mesoporous Ti-MCM-48 material was silylated using hexamethyldisilazane (HDMS) and a remarkable increase in the catalytic performances in the cyclohexene epoxidation with tert-butylhydroperoxide obtained. We report on a combined FTIR, diffuse reflectance UV–Vis–NIR, photoluminescence and 29Si MAS NMR study, augmented by TGA and XRD analysis, of the local environment of Ti sites in both calcined and silylated Ti-MCM-48 catalysts. Spectroscopic studies were also performed on the as-synthesised material and showed that both silanols and titanols are formed during the decomposition of the cetyltrimethylammonium hydroxide surfactant used to template the mesoporous structure. Unequivocal evidences that a further silica condensation of the MCM-48 framework structure occurs both during thermal treatments in vacuum and after calcination at 540°C are also reported. Upon silylation, a more hydrophobic surface is obtained where trimethylsilyl groups (Si(CH3)3) are anchored on to the silica surface by reaction of the silylating agent, HDMS, with silanols, and in addition, (SiO)3TiOH sites transformed into (SiO)3TiOSi(CH3)3. It is proposed that the more hydrophobic character of the silylated material prevents the deactivation of the catalyst by decreasing the amount of glycols which may be formed by reaction of the epoxide with water adsorbed on the surface of the Ti-MCM-48, and that poison the titanium catalytic centres.