Investigation of the microstructure of amorphous aluminosilicate / silicate precursors (gel) during its hydrotermal transformation

Atomic force microscopy (AFM) in combination with other well established techniques such as transmission electron microscopy (TEM), infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Differential thermal gravimetry (DTG), electron diffraction (ED), scanning electrom microscopy (SEM), were used to investigate the very early stages of nucleation and crystallization on the surface of gel precursor in the synthesis of zeolite A and silicalite-1. Analysis of the precipitated sodium aluminosilicate /silicate precursor (gel) has shown that predominantly true amorphous phase of the gel contains small proportions of partially crystalline (“quasi-crystalline”) or even fully crystalline phase. In the case of the gel precursor for the synthesis of zeolite A is also found that the freshly prepared gel is mainly composed of disc-shaped primary particles, but also partially or even fully crystalline entities were observed by AFM analysis. AFM analysis of the solids separated from the hydrogel at various stages of its hydrothermal treatment (heating at 80o C) indicates that the particles of the partially and/or fully crystalline phase are nuclei for further crystallization of zeolite [1]. In this study is also monitored the behavior of the silicate species in relation with the structure directing agent TPABr during the formation of nuclei for the crystallization of silicalite 1. It is found that TPA+ cations linked together with silicate species by Vand der Waals forces and form short range ordered units having size below 10 nm that correspod to primary building units. The units agglomerate and form particles having size about 10 nm that was confirmed by AFM and TEM. These entities acts as the nucleation centers for the crystallization of silicalite-1 [2]. [1] Kosanović, C., Bosnar, S., Subotić, B., Svetličić, V., Mišić, T., Drazić, G. Havancsak, K., Microporous and Mesoporous Materials, 110 (2008)177-185. [2] De Moor, P-P. E. A., Beleen, TPM, Komanschek, B. U. , Beck, L. W., Wagner, P, Davis, M. E., Van Santen, R. A., Chem Eur. 1999, 5, 2083-2088.