A comprehensive study of titanium-substituted SBA-15 mesoporous materials prepared by direct synthesis

The influence of different synthesis parameters on the physicochemical properties of titanium-substituted SBA-15 silicas prepared by direct one-step synthesis was systematically studied by a combination of N(2) physisorption at - 196 degrees C, X-ray diffraction (XRD), diffuse reflectance UV-vis, and elemental analysis. The results showed that for low titanium precursor concentrations used in the initial. synthesis gel, incorporation of titanium in the final material is not complete. Under these synthesis conditions, Ti atoms are well-dispersed in the silica framework and mainly exist in tetrahedral coordination. When the isolated titanium species present in the gel reach a critical concentration, an increase in incorporation ratio is observed due to the formation of anatase TiO(2) clusters on the materials surface. It was also shown that this critical titanium loading is mainly influenced by synthesis temperature, hydrothermal treatment time, and silicon precursor concentration and does not depend on acid concentration. Moreover, it was shown that when the formation of anatase TiO(2) takes place, the amorphous titanium species dispersed into the silica materials are resolubilized in the acidic solution increasing the fraction of bulk anatase into the mesoporous silica.