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 (H DMS) and a remarkable increase in the catalytic performances in the cyclohe xene epoxidation with tert-butylhydroperoxide obtained. We report on a comb ined FTIR, diffuse reflectance UV-Vis-NIR, photoluminescence and Si-29 MAS NMR study, augmented by TGA and XRD analysis, of the local environment of T i sites in both calcined and silylated Ti-MCM-48 catalysts. Spectroscopic s tudies were also performed on the as-synthesised material and showed that b oth silanols and titanols are formed during the decomposition of the cetylt rimethylammonium hydroxide surfactant used to template the mesoporous struc ture. Unequivocal evidences that a further silica condensation of the MCM-4 8 framework structure occurs both during thermal treatments in vacuum and a fter calcination at 540 degreesC are also reported. Upon silylation, a more hydrophobic surface is obtained where trimethylsilyl groups (Si(CH3)(3)) a re anchored on to the silica surface by reaction of the silylating agent, H DMS, with silanols, and in addition. (SiO)(3)TiOH sites transformed into (S iO)(3)TiOSi(CH3)(3). It is proposed that the more hydrophobic character of the silylated material prevents the deactivation of the catalyst by decreas ing the amount of glycols which may be formed by reaction of the epoxide wi th water adsorbed on the surface of the Ti-MCM-48, and that poison the tita nium catalytic centres. (C) 2001 Elsevier Science B.V. All rights reserved.