Mesoporous silica-aluminas derived from precipitation: a study of the acidity, textural properties and catalytic performance

Silica-alumina covering broad range of compositions have been obtained by combined hydrolysis precipitation. These materials were characterized mainly by solid-state NMR spectroscopy. Textural properties were investigated by measurements of nitrogen sorption, and the acidity of Brønsted sites was studied by TPD and FTIR spectroscopy using ammonia as probe molecule. The catalytic performance of these materials was studied by the Brønsted acid-catalyzed acetalization reaction. New insights into the nature of the silicate compartment and the acidity of Brønsted sites of amorphous silica-alumina have been obtained. Silica-aluminas of different silica content do not represent a homologous row. They differ mainly in the connectivity of the silicate compartment. The results show that silica-alumina contain a quantitative amount of Brønsted sites. The appearance of tetrahedral Al is closely related to silica. The concentration of tetrahedral Al, and hence Brønsted acid sites, follows a volcano shape. After increasing with SiO₂ content, the site concentration reaches a maximum at 20 wt% of silica and decreases again due to the marked decrease in the total alumina content in high silica samples. Surprisingly, the catalytic activity does not follow this trend. It increases especially with high silica samples due to the interplay of acid site concentration and the strength of acid sites. The aluminosilicate compartment of high silica samples shows a high Si/Al ratio. The improved acid strength of the sites overcompensates the lower site concentration, leading to a distinctly enhanced catalytic activity.