Surface acidity of sulfated TiO2-SiO2 sol-gels

Sulfated titania-silica gels have been obtained from tetraethoxysilane and titanium isopropoxide gelled either in acid medium (pH 3, HCl) or basic medium (pH 9, NH4OH). These gels were sulfated using sulfuric acid or ammonium sulfate. Three different methods have been used to study the acidity of these solids: formation of H-bonds with benzene, selective H/D exchange with deuterobenzene, and adsorption of pyridine. The nonsulfated gels show only weak Lewis acidity, the strength of which increases with sulfation. The solid obtained by in situ sulfation, i.e. gellation using sulfuric acid, shows a strong Bronsted acidity. Their acid strength evaluated by the shift of the infrared bands of the hydroxyls when contacting the solid with benzene is comparable to that of HY zeolites. H-D exchange at low temperature can be observed between deuterated benzene and the gels obtained with sulfuric acid, which confirms the high acid strength of sulfated TiO2-SiO2 samples. Titanium is at least partly incorporated in the lattice of silica and can easily be extracted by self-steaming.