Prehydrolysis approach to direct synthesis of Fe, Al, Cr-incorporated SBA-15 with good hydrothermal stability and enhanced acidity

• Pre-hydrolysis approach to direct synthesis of Fe, Al-, and Cr-incorporated SBA-15.
• No need in mineral acids addition and in situ incorporation of cations in host framework.
• Flexible metal content with M/Si molar ratio up to 0.05 (Fe, Cr)–0.08 (Al).
• Metal content can also be readily controlled by varying the pre-hydrolysis period.
• As-synthesised Fe-SBA-15 shows good (hydro)thermal stability and enhanced B/L acidity.

The Fe-incorporated mesoporous SBA-15 materials with the Fe content up to a Fe/Si molar ratio = 0.05 were synthesised via pre-hydrolysis of tetraethyl orthosilicate (TEOS) employing P123 triblock copolymer as the template. The synthetic strategy is the hydrolysis of Fe salt precursor can provide acidic medium essential for the pre-hydrolysis of TEOS. Different Fe/Si ratios and TEOS pre-hydrolysis periods were employed to modify the structure and texture of the Fe-incorporated SBA-15. The impact of Fe sources (ferric nitrate, ferric chloride, and ferric sulfate) on the characteristics of Fe-SBA-15 has also been investigated. Ferric nitrate showed the best effect among the three Fe sources. UV–Vis study reveals that besides the four-coordinated iron species, the iron oxide domains may also exist in the samples. Pyridine adsorption IR study demonstrates enhancement in Brønsted and Lewis acidity of as-synthesised Fe-SBA-15 materials, potential as solid acid catalyst or catalyst support. Moreover, the employed treatments (700 °C for 10 h in air and 600 °C for 3 h in 100% steam) verified good thermal and hydrothermal stability of the derived Fe-SBA-15. For comparison, Al- and Cr-incorporated SBA-15, with the Al/Si and Cr/Si molar ratio of 0.08 and 0.05 respectively, were also synthesised through the same strategy.

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