Synthesis and application of SBA-15 mesoporous silica by silatrane route via sol-gel process

Abstract

In this work, the novel room temperature sol-gel synthesis of well-ordered and stable two dimensional mesoporous silica SBA-15 is described using a silatrane, an inexpensive and conveniently prepared silica, as silica precursor and a non-ionic triblock copolymer (EO20PO70EO20) as the structure directing agent. Using a combination of small angle X-ray scattering (SAXS), transmission electronmicroscopy (TEM) and field emission scanning electron microscopy (FESEM), and nitrogen gas absorption-desorption isotherms, the room temperature product was found to be equivalent to SBA-15 prepared using more elaborative microwaveassisted hydrothermal methods. Both synthesis routes yielded large surface areas (486-613 m2/g), pore diameters (45-67 A.) and channel volumes (0.6-0. 8 cm3/g). All materials condensed as sinuous bundles of silica tubes, in a morphology that approximates p6mm symmetry. Functionalized M-SBA-15 (M = Ti, Fe, and Mo) mesoporous silica catalysts are also synthesized through this simple room temperature sol-gel process. Catalyst performance of Ti-SBA-15 was optimized by systematically investigating the influence of acidity, reaction time and temperature, and titanium loading. SAXS, TEM, and FESEM confirmed well-ordered 2D mesoporous hexagonal structures of M-SBA-15 (M= Ti, Fe and Mo). Diffuse reflectance UV-visible spectroscopy (DRUV) showed that Ti4+, Fe3+ and Mo6+ were incorporated into the framework through displacement of Si4+ to loadings of 7 mol%Ti, 10 mol% Fe and 1 mol% Mo, respectively, without perturbation of the ordered mesoporous structure of the parent SBA-15. N2 adsorption/desorption measurements yielded high surface areas (670-729 m2/g), with large pore diameters (5.4—6.8 nm) and volumes (0.83-1.04 cm3/g). The catalytic activity of metal loaded SBA-15 (Ti-,Fe- and Mo-SBA-15) towards the epoxidation of styrene monomer with H2O2 employed as the oxidant was investigated as a function of temperature, time, catalyst loading, ratio of Styrene/H202, and amount of catalyst used. The only products of this reaction were styrene oxide, and benzaldehyde. The selectivity of styrene oxide are 34.2%, 34.5% and 39.9%, at a styrene conversion of 25.8%, 10.3% and 7.7% over the 7 mol% Ti, 10 mol% Fe, and 1 mol% Mo loaded in SBA-15 catalyst, respectively. The catalytic performance of the sol-gel catalysts is shown to be superior to conventional materials produced by incipient wetness impregnation method under identical reaction conditions.