Abstract: 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 pseudoboehmite 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.