The influence of Ti-loading on the acid behavior and on the catalytic efficiency of mesoporous Ti-MCM-41 molecular sieves

Ti-containing mesoporous catalysts with MCM-41 structure and high surface area have been successfully prepared by direct hydrothermal synthesis. Various techniques including XRD, DRUV-vis, ICP, N(2) adsorption and absorption of pyridine coupled to IR spectroscopy are employed. A detailed study about the relationships existing between the Ti loading in the catalyst, the acid properties and the catalytic efficacy is presented in this contribution. The incorporation of titanium into the silica framework provoked mainly a proportional increase of Lewis acid sites. However, Bronsted acid sites were also generated at high Ti loadings. Whereas the Lewis sites are related with presence of tetrahedral Ti species in the framework, the Bronsted sites would be generated by the vicinity of these Ti species to the structure SiOH groups. The oxidation of cyclohexene with aqueous hydrogen peroxide was employed as a test reaction to evaluate the activity and effectiveness of the active sites. We could corroborate that the Ti sites were active and available even at high Ti loadings. The reaction presented a high velocity during the 1st hours and afterwards it could not continue evolving due to the almost total peroxide consumption. Cyclohexene epoxide was the main product from reaction. (C) 2007 Elsevier B.V. All rights reserved.