Effect of calcination on the structure and catalytic activities of titanium incorporated SBA-15

Abstract

Well-ordered Ti-incorporated SBA-15 materials (shortly termed Ti–SBA-15) with Ti loading up to 10 mol% of Si were prepared by a one pot co-condensation method using tetraethyl orthosilicate (TEOS) as the silica source and titanium oxychloride as the Ti source in an acidic environment close to the isoelectric point of silanol groups (pH = 1.27–2.37). The Ti–SBA-15 materials with Ti/Si ratios smaller than 0.05 were curved solid tubes, while those with the Ti/Si ratios between 0.05–0.1 possessed platelet morphology and short mesochannels (ca. 150–500 nm). Titanium was homogeneously distributed in the silica framework of the as-made Ti–SBA-15 materials as isolated Ti species when the Ti/Si ratios were lower than 0.03. It formed small TiO₂ nanocrystallites when the Ti loading was further increased or after the materials were calcined at temperatures higher than 500 °C. It is noteworthy that tetrahedrally coordinated Ti(IV) species were generated on the superficial areas of SBA-15 silica framework upon calcination at 600–1000 °C. These tetrahedrally coordinated Ti(IV) species not only stabilized the mesopore ordering up to 950–1000 °C, but also acted as effective catalytic sites in cyclohexene epoxidation.