Green and facile sol-gel synthesis of the mesoporous SiO 2 -TiO 2 catalyst by four different activation modes.

The most environmentally friendly protocol for obtaining mesoporous SiO 2 -TiO 2 catalysts has been sought. Water has been employed as a green solvent, the energy input has been minimized, and three further principles (1, 3, and 12) of Green Chemistry have been considered. Four different modes for promoting the reaction have been comparatively evaluated, namely near-infrared and microwave electromagnetic irradiations, ultrasound, and traditional mantle heating. Brunauer-Emmett-Teller (BET) analyses of the catalysts produced revealed that the non-conventional activation modes afforded both large surface areas (335-441 m 2 g -1 ) and smaller crystal sizes (7.2-15.3 nm) than the mantle heating process. These modes also generated the catalysts in shorter reaction times than traditional mantle heating, 10-30 min versus 3 h, with anatase as the sole crystalline phase. The photocatalytic degradation of 4-chlorophenol has been carried out to assess the catalytic efficiencies of the hybrid materials. The catalyst synthesized with microwave assistance showed the best mineralization activity (97%), followed by those prepared with ultrasound, near-infrared, and mantle heating. The materials have been extensively characterized by FTIR, XRD, DRS-UV/Vis, SEM, 29 Si MAS NMR, and BET analyses. To the best of our knowledge, this is the first such comparative assessment of green energetic alternatives in developing a sol-gel process.