Mesostructured SiO2-doped TiO2 with enhanced thermal stability prepared by a soft-templating sol-gel route

Abstract

Mesostructured SiO2-TiO2 mixed oxides have been prepared by a soft-templating sol-gel route, using a non-ionic triblock copolymer as structure directing agent. Tetraethylorthosilicate (TEOS) and titanium tetraisopropoxide (TTIP) have been employed as Si and Ti sources, respectively. Using a prehydrolysis TEOS step allows mixed oxides to be produced with a homogeneous porosity and with no phase segregation, in a wide range of Si/Ti compositions. Both the hydrolysis molar ratio and the silicon content have been found to be important factors determining the final properties of these materials. For instance, mixed oxides containing low silicon concentrations exhibit N2 physisorption isotherms typical of mesoporous materials, although with an important contribution of microporosity. On the other hand, increasing the hydrolysis molar ratio makes more difficult to reach a total dispersion of SiO2 through the TiO2 matrix. Even with low SiO2 loadings, the thermal stability is effectively enhanced, when compared to the equivalent pure TiO2 materials, as a consequence of a delay in the titania crystallization to anatase. Thus, after calcination at 300 ºC for 3 h, mixed oxides containing low Si/Ti ratios (¿ 20) show BET surface area in the range 290-346 m2/g, while pure TiO2 materials largely collapse under the same treatment and their BET surface area drop strongly to values around 125 m2/g. This synthesis route, therefore, provides mesoporous TiO2-rich materials with enhanced stability and textural properties, which is of high interest for applications as catalysts and supports.