Surface properties and epoxidation catalytic activity of Ti-SBA15 prepared by direct synthesis

The influence of hydrothermal treatment time on the physicochemical properties and the catalytic activity in cyclohexene epoxidation of titanium-substituted SBA15 silicas prepared by direct one-step synthesis was systematically studied using a combination of N₂ physisorption at −196 °C, X-ray diffraction, X-ray photoelectron spectroscopy, diffuse reflectance UV–Vis, and elemental analysis. The other synthesis parameters were chosen to illustrate the different chemical environments of the titanium species formed before, during, and after the precipitation of anatase TiO₂. At the beginning of hydrothermal treatment, results showed that the titanium species are homogeneously dispersed in the silica framework. When anatase TiO₂ clusters precipitate, they do so mainly on the external surface of the mesoporous material. At higher hydrothermal treatment times, the material showed a decreased catalytic activity even if essentially no variation in their specific surface area was then observed. This lower activity was shown to be due to a partial coverage of active tetrahedral Ti species by extraframework higher coordination TiO₂ deposit.