Aluminosilicate aging during crystallization

The analysis of the kinetics of crystallization of different types of zeolites from aluminosilicate gels points to the conclusion that the crystallization takes place by the simultaneous growth of the constant number N0 of nuclei-I present in the system at the very start of the crystallization process and the number Na of nuclei-II released from the gel disolved during the crystallization process. Some characteristics of the crystallization systems such as the duration of the "induction period", the shortening of the "induction period" and the increase of the crystallization rate, respectively, with the gel ageing and the bimodal size distributions in the specific cases have been discussed and explained in relation to the ratio Na/N0 of particles (nuclei)-II and particles (nuclei)-I present in the crystallizing systems. 

The presence of even traces of ethanol in the synthesis batch, stemming from an incomplete hydrolysis of Si(OEt)4 during the ageing period results in a drastical reduction of the ZSM-20 crystallization rate zeolite Beta is then readily formed and achieves a fast growth at the expense of the ZSM-20 "Figure 3".This goes in line with the higher yields of Si-richer Beta observed when this zeolite is intentionally prepared in presence of ethanol (311.This was attributed to the lower solubility of silica in EtOH (311. but it is also probable that the whole system (nature and solubility of ZSM-20 aluminosilicate precursors and intermediates) is perturbed (321. 

M. Ogura, Y. Kawazu, H. Takahashi, and T. Okubo, Aluminosilicate Species in the Hydrogel Phase formed during the Aging Process for the Crystallization of FAU Zeolite. Chem. Mater., 2003, 15, 2661-2667. 

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