Effect of aluminium incorporation by the “pH-adjusting” method on the structural, acidic and catalytic properties of mesoporous SBA-15

A series of Al-SBA-15 catalysts in a wide compositional range (Si/Al = 130-5.2) was prepared by “pH-adjusting” aiming to evaluate the effect of aluminium incorporation on their structural, acidic and catalytic properties. The calcined materials were characterized by ICP, XRD, N2 adsorption/desorption, TEM, 27Al MAS NMR, microcalorimetry of ammonia adsorption and FTIR of adsorbed pyridine. It was found that the progressive incorporation of aluminium in SBA-15 has a positive effect on the structural and textural characteristics of catalysts as well as on the number of Brönsted and Lewis acid sites, whereas no net effect was observed on the relative distribution of acid sites according to their nature or strength. The catalytic properties of mesoporous SBA-15 aluminosilicates were finally evaluated by test-reactions in the liquid phase tert-butylation of phenol and isomerization of styrene oxide, under mild conditions. The catalytic results showed that the progressive incorporation of aluminium results in higher reaction rates in the O- and C-alkylation of phenol as well as isomerization of styrene oxide, providing evidence that the number of acid sites controls the catalytic activity of Al-SBA-15. On the other hand, the incorporation of aluminium had no effect on the selectivity to the reaction products, indicating that selectivity levels are controlled by the relative distribution of acid sites. It was also reported that Al-SBA-15 prepared by “pH-adjusting” are highly active catalysts in the isomerization of styrene oxide and very selective to phenylacetaldehyde.

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► The properties of Al-SBA-15 catalysts prepared by “pH-adjusting” were investigated.
► The progressive Al incorporation had a positive effect on the structural properties.
► The number of acid sites increases with the Al content.
► The relative distribution of acid sites does not depend on the Si/Al ratio.
► The catalytic activity and selectivity depend on the surface acidity.