AGING OF ALUMINOSILICATE AND SILICA-GELS IN AQUEOUS-SOLUTIONS OF VARIOUS PH AND AL CONTENT, A TEXTURAL AND STRUCTURAL EVALUATION

Silicas and aluminosilicates are prepared from tetraethylorthosilicate , Al(NO3)(3) and alternatively formamide or dimethylformamide. Gel bod ies cast under acid conditions (pH=2-3) are subsequently aged in aqueo us solutions of higher pH. Changes in the resulting dry gel texture, s tructure and composition are inferred from N-2 adsorption, Al-27 MAS-N MR, cation exchange capacity measurements, thermogravimetric and quant itative Al analyses. Aging has a pronounced effect on the gel porosity and/or the surface charge. For silicas, a strong increase of the meso pore volume is observed, whereas for gels with 25% Al, a shift towards a higher degree of microporosity takes place. At the same time, the s urface charge increases, rather irrespective of the pH of aging. In a second set of experiments, wet silica gel bodies are impregnated with a diluted Al(NO3)(3) solution. Incorporation of mainly tetrahedral Al (up to Si/Al=13) gives rise to a reduction of the pore volume and/or t he pore size. The results indicate that particularly formamide strongl y influences the Al coordination, allowing for high surface charges at low synthesis pH.