engleski

On the "memory" effect of the aluminosilicate gel precursors

Two different aluminosilicate gels, G1 and G2, having the molar oxide compositions: 3.31Na2OxAl2O3x2 SiO2x333.33 H2O, (G1) and 6Na2OxAl2O3x2 SiO2x333.33 H2O, (G2) were prepared by mixing sodium silicate and sodium aluminate solutions of appropriate concentrations preheated at 80o C, and thereafter the gels were treated in different ways, before crystallization: (i) Gel G1 was diluted with 50 ml of 2.14 M NaOH solution preheated at 80o C and gel G2 was diluted with 50 ml of distilled water preheated at 80o C. Thereafter the systems were stirred at 80o C. (ii) Solid phases of the gels were separated from the liquid phase by centrifugation. Solid phase was redispersed in distilled water and centrifuged repeatedly. The procedure was repeated until the pH value of the liquid phase above the sediment was about 10. Washed gels were redispersed in 100 ml of distilled water and centrifuged repeatedly. The procedure was repeated until the pH value of the liquid phase above sediment was about 10. Washed gels were redispersed in 100 ml of distilled water preheated at 80o C and put in stirred 4 M NaOH solution preheated at 80o C. (iii) The gels washed by the procedure (ii) were dried at 105o C for 24 h, pulverized and heated with stirring in 2 M NaOH solution at 80o C. Rates of nucleation, crystal growth and crystallization considerably depend on the gel pretreatment and crystallization conditions, but these factors did not affect particulate properties (particle number, particle sizedistribution, particle shape) of the crystalline end products. This indicates that the essential part of nuclei was formed in the gel phase during its precipitation, and that the number and distribution of the nuclei in the gel matrix is not influenced neither by the way of gel treatment before crystallization nor by the crystallization conditions.

alumosilicate gel; precursor; "memory" efect; zeolite; nucleation; crystal growth; crystallization

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