ASA exhibit a predominant amount of various Lewis acid sites (LAS) but also possess BAS in lower amounts (Figure ). The latter are created by doping the alumina with silica. With increasing silica content, the number of LAS decreases while the formation of BAS increases up to a maximum at 40 wt% silica. For silica-rich alumina of 90 wt% and above, no LAS are detected and the acidity is almost completely controlled by BAS. This is explained by the enrichment of the surface with silica which exceeds what would be expected from the bulk material composition. The majority of the material's surface at silica-loadings higher than 40 wt% is covered by silica and only contains small zones of the mixed aluminosilicate. In fact, Daniell et al. assessed the strength of the BAS and correlated it to the shift of the surface hydroxyl group Δv(OH) at ~3748 cm -1 on adsorption of CO on the Siralox materials by FTIR-spectroscopy. The aluminosilicate surface shows a lower number of BAS which are strongly enhanced in strength by the addition of silica. Based on their measurement, we found that the formation of mesitylene correlates well (R 2 = 0.78) with the measured Δv(OH) (Figure ). Accordingly, stronger BAS promote mesitylene formation under reaction conditions. For silica loadings higher 60 wt%, the total number of acid sites and the strength of BAS decrease which results in the lower activities observed for these materials. Therefore, materials with silica contents of 30-40 wt% are most beneficial for the formation of mesitylene.