Investigation of the crystallization stages of LTA-type zeolite by complementary characterization techniqnes

This paper reports on the physicochemical reorganizations, at 40 degreesC, of a template-free aluminosilicate system yielding an LTA-type zeolite. The events that take place in the system during the transformations of the amorphous material into a crystalline material were monitored by various characterization techniques which provide information from the molecular to the micrometer scale. High-resolution solid-state Si-29 and Al-27 MAS NMR spectroscopy, small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used in order to achieve a better understanding of the structural properties of the precursor gel and its transformation into a crystalline zeolite-type material. The experimental results show that the first crystallization stage proceeds by reorganization of the amorphous aluminosilicate units formed during the mixing of the precursors. This reorganization involves an ordering of the Si-O-Al bond angles and operates on aggregates of constant volume. The crystallization process takes place in the volume of the aggregates by propagation through the gel phase. This propagation through the gel continues to complete the transformation of the particles into crystalline zeolite Na-A nanocrystals with a rough surf ace. After this stage, the growth in the system is dominated by the solution-mediated transport, which leads to the formation of well-shaped zeolite A crystals. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.