Brønsted acidity of Al/SBA-15

To increase the content of well-dispersed aluminium in Al/SBA-15 and its Brønsted acidity, a water-free synthesis method involving a Si-O-Al precursor was employed to preparea series of Al/SBA-15 of widely varying composition. The materials were characterized with ICP, BET, TEM, 27Al MAS NMR and IR spectroscopy and compared to conventionally prepared Al/SBA-15. For low Al content (Si/Al >10),the conventional approach leads to highly dispersed, predominantly isolated Al with well-ordered mesopores.Athigher Al content, the order decreasedor was even completely lost.In general, this led to the agglomeration of Al species on the surface. Although in most cases a substantial part of Al atomsexhibit tetrahedral coordination indicating their high dispersion and interaction with the silica, the overall Brønsted acidity of these Al/SBA-15-type materials is very low. The overall acidity of all Al/SBA-15 is of the same order as amorphous silica-alumina. By IR spectroscopy of H/D exchange with perdeuterobenzene, it was found that the intrinsic acidity of the acid sites in Al/SBA-15 is similar to that in zeolites and amorphous silica-alumina, thus further confirming that acid site concentration dominates differences in acidity of aluminosilicas