Publication costs assisted by the Institute of Physical Chemistry of the University of HamburgIn the hydrothermal system Na2O/A1203/SiO2/H20 we have studied the growth of faujasite crystals with varying Si/A1 ratios using seed crystals of the zeolite NaX and a water glass solution as the silica source. The kinetics of the process of growth of the faujasite from amorphous alumosilicate gel has been investigated and procedures for the synthesis of faujasites with varying but definite silicon contents are described. It can be shown that the growing of faujasite crystals from nuclei is possible in concentration ranges where nucleation of this or other species is negligible. In a wide range of crystallization, the growth obeys a kinetic law of the order 213.
Starting from the results of a previous paper, in which the validity of a simple kinetic equation for the growth of zeolites of the faujasite type has been shown, the influences of various parameters on the rate of crystallization are reported. In the considered range of concentration the linear crystallization rate proves to be proportional to the concentration of the silicate in the solution phase for final products of equal Si/Al ratios. The crystallization rate varies for the growth of zeolite X to that of zeolite Y of an Si/Al ratio of 3.4 over three orders of magnitude. Simultaneously, the apparent activation energy of this growth is increased considerably. Based on the experimental results, a model for the explanation of the crystallization process is developed. The rate-determining step seems to be given by the connection of silicate species, specific for the special structure, to the surface of the crystal.
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