Acid and Alkaline Sol-Gel Synthesis of Amorphous Aluminosilicates, Dry Gel Properties, and Their Use in Probing Sol Phase Reactions

Abstract Amorphous aluminosilicates with narrow pore size distributions were prepared by two sol-gel synthesis procedures, starting from alkaline mixtures of Na4SiO4and NaAlO2(pH > 12) and acid solutions of Si(CH3CH2O)4and Al(NO3)3(pH 2). Gel formation was induced by titration with HCl and NH3, respectively. The gel texture and the surface charge, as inferred from N2adsorption and cation-exchange capacity measurements, were investigated as a function of the Al content and the final pH. It is shown that the properties of the dry gels reflect some well-known features of aluminosilicate solutions (enhanced nucleation compared to silicate solutions, high and low sensitivity toward, respectively, acid- and base-catalyzed depolymerization). The interpretation of dry gel properties also leads to new insights in the sol-gel chemistry of aluminosilicates. As such, a consistent model for the variation of the porous structure of aluminosilicates with the Al content is proposed, based mainly on the degree of incorporation of Al in the tetrahedral network and the subsequent changes in the condensation reactivity.