Room temperature synthesis of Ti-SBA-15 from silatrane and titanium-glycolate and its catalytic performance towards styrene epoxidation

摘要

A novel room temperature sol–gel synthesis of Ti-SBA-15 is described using moisture stable silatrane and titanium glycolate precursors, and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer (EO₂₀PO₇₀EO₂₀) as the structure directing agent. Catalyst performance was optimized by systematically investigating the influence of acidity, reaction time and temperature, and titanium loading. Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) showed well-ordered 2D mesoporous hexagonal structures, while N₂ adsorption/desorption measurements yielded high surface areas (up to 670 m2/g), with large pore diameters (5.79 nm) and volumes (0.83 cm3/g). Diffuse reflectance UV–visible spectroscopy (DRUV) was found that tetravalent titanium as Ti4+O₄ tetrahedra were incorporated in the framework through displacement of Si4+O₄ after calcination (550°C/6 h) to loadings of 7 mol% Ti without perturbation of the ordered mesoporous structure, or decoration by extra-framework anatase containing Ti4+O₆ octahedra. The catalytic activity and selectivity of styrene epoxidation using hydrogen peroxide (H₂O₂) showed that the conversion of styrene increases significantly at higher titanium contents. The only products of this reaction were styrene oxide and benzaldehyde, with selectivity of 34.2 and 65.8%, respectively, at a styrene conversion of 25.8% over the 7 mol% Ti-SBA-15 catalyst. Beyond this titanium loading, anatase is deposited on the framework and catalytic activity degrades. The performance of the new catalyst is also shown to be superior to conventional materials produced by incipient wetness impregnation where Ti resides on the surface of SBA-15, giving a styrene conversion of 11.9% under identical reaction conditions.