Formation of acid sites in amorphous silica-alumina

A suite of amorphous silica-aluminas (ASAs) was prepared by homogeneous deposition–precipitation (HDP) of aluminium on a silica surface followed by calcination. The HDP process was investigated in detail by 27Al NMR spectroscopy of solid and liquid aliquots of the synthesis mixture. Deposition occurs predominantly via a hydrolytic adsorption of aluminium onto the hydroxyl groups of the silica surface. Precipitation becomes more significant at higher aluminium concentration. Depending on the aluminium loading, the surface contains four- and six-coordinated aluminium as well as patches of aluminium hydroxides. Calcination results in two competing process, that is the diffusion of aluminium into the silica network and sintering of aluminium into separate patches of a phase which mainly consists of octahedral Al. These ASAs exhibit Brønsted acidity similar to industrial amorphous silica-aluminas prepared by the grafting of aluminium on very reactive silica gels. Their acidity does not vary systematically with the aluminium concentration, except below 5 wt% Al2O3. The acidity increases with the calcination temperature. The active sites form due to the diffusion of aluminium into the silica network at high temperatures, leading to Al substitutions of Si atoms. This is expected as the acidity does not correlate with anything else, viz., the amount of four-coordinated aluminium nor the presence of segregated Al or five-coordinated aluminium at the interface of these domains and the mixed silica-alumina phase. The surface of an amorphous silica-alumina consists of isolated aluminium grafted onto the silica surface (pure silica-alumina phase) with a very small amount of aluminium in the silica network, which brings about the Brønsted acidity, and small patches of aluminium oxides.