ACID AND ALKALINE SOL-GEL SYNTHESIS OF AMORPHOUS ALUMINOSILICATES, DRY GEL PROPERTIES, AND THEIR USE IN PROBING SOL PHASE REACTIONS

Amorphous aluminosilicates with narrow pore size distributions were pr epared by two sol-gel synthesis procedures, starting from alkaline mix tures of Na4SiO4 and NaAlO2 (pH > 12) and acid solutions of Si(CH3CH2O )(4) and Al(NO3)(3) (pH 2). Gel formation was induced by titration wit h HCl and NH3, respectively. The gel texture and the surface charge, a s inferred from N-2 adsorption and cation-exchange capacity measuremen ts, 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-kn own features of aluminosilicate solutions (enhanced nucleation compare d to silicate solutions, high and low sensitivity toward, respectively , acid- and base-catalyzed depolymerization). The interpretation of dr y gel properties also leads to new insights in the sol-gel chemistry o f aluminosilicates. As such, a consistent model for the variation of t he porous structure of aluminosilicates with the Al content is propose d, based mainly on the degree of incorporation of Al in the tetrahedra l network and the subsequent changes in the condensation reactivity. ( C) 1996 Academic Press, Inc.