Alkali aluminosilica gels

Alkali aluminosilica gels from which zeolites usually are crystallized represent heterogenous colloidal systems consisting of liquid and solid phases which strongly differ in their chemical compositions. In those cases when aluminosilica gels are obtained under equal conditions and from the same initial materials—for example, from solutions of the silicates and aluminates—the results of their crystallization reproduce well,... 

Thus, studies on the chemical structure of alkali aluminosilica gels utilized in zeolite synthesis reveal a complicated dependence in the distribution of components between the solid and liquid phases. At the same time, in the case of gels prepared from the homogenous solutions, the structural elements of disordered (Si,Al,0)-network in the gel skeleton and those of the regular (Si,Al,0) -frameworks in zeolites probably are... 

Low-alkali aluminosilica gels do not crystallize at all, even at 90 °C. This indicates the existence of some minimum alkali concentrations at which crystallization at a given temperature becomes possible. 

Effect of Alkalinity on the Rate of Crystallization. The causes of the effect of alkalinity on the rate of crystallization have not been discussed in the literature. The authors (22) only reported that crystallization is catalyzed by excess alkali. Usually, an increase in alkali concentration in aluminosilica gels and aluminosilicate mixtures leads to a decrease in both the duration of crystallization and crystal sizes (32, 38). This can be explained by the growth of the rate of nucleation with increasing alkali concentration. Therefore, a greater number of nuclei should form during crystallization in a more alkaline medium. 

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