Morphology and surface properties of titania-silica hydrophobic xerogels

High surface area titania-silica hydrophobic xerogels were prepared by the sol-gel method. In this procedure, titanium ions are well dispersed and the coordination sphere of the titanium ion is induced by the silica matrix. The reaction of hydroxyl groups of the gel with the drying chemical compound additive protects the texture toward collapse during drying. The amounts of titania and silylant agent were varied, and thermal treatment was studied on both silylated and nonsilylated samples. Textural properties were evaluated from nitrogen adsorption-desorption isotherms. The degree of incorporation and dispersion of titanium ions in the silica matrix was evaluated by FTIR spectroscopy of the framework vibrations. To elucidate the nature and quantity of exposed titanium ions on the surface of the catalyst, X-ray photoelectron spectroscopy and FTIR of adsorbed deuterated acetonitrile were used. Upon increasing the Ti loading, the BET area, the pore volume, and the mean pore diameter decreased whereas the amount of exposed Ti followed the opposite trend. In contrast, an increase in the amount of silylant agent decreased the proportion of exposed titanium and simultaneously improved the hydrophobic character and BET area. Thermal treatment of the silylated samples removed the hydrophobic character, although it did not change to a large extent their texture after drying. In short, silylation of the gel before drying leads to an increase in BET area and to partial blockage of exposed titanium sites, controlling moreover, the porosity and the hydrophobic character. Precise knowledge of these parameters and the influence of the thermal treatments on them will play a key role in the understanding of their performance in the liquid-phase partial oxidation of alkylaromatics.