Structural Organization in Amorphous Silico-Aluminas

A structure model for amorphous hydrated or dehydrated silico-aluminas with composition varying between 0 < Al: Al + Si < 1 is presented. A central core made from a tetrahedral network in which silicon is partially substituted by aluminium carries a net negative electrical charge. This charge is balanced by more or less polymerized hydroxyaluminium cations forming a coating around the core. As Al: Al + Si increases, the number of substitutions in the core increases as well as the complexity of the hydroxyaluminium cations in the coating. For Al: Al + Si ≳ 0·8, a demixing is observed, leading to the formation of a crystalline pseudo-boehmite and bayerite. Upon heating, the coating as well as the demixed phases are transformed into a spinel structure containing tetrahedral aluminium, while the core structure remains unaffected. This model could explain the solubility features, the phosphate reaction and the catalytic properties of amorphous silico-aluminas.