Formation mechanism of microwave synthesized LTA zeolite membranes

In order to optimize the synthesis conditions for a specific zeolite membrane with high efficiency, and generalize this method to other types of zeolite membranes, a fundamental understanding of the membrane formation mechanism is of great importance. In present paper, gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FTIR), and gas permeation were carried out to characterize the whole formation process of LTA zeolite membrane which was synthesized by "in situ aging-microwave heating" method (AM method), and the formation mechanism of LTA zeolite membrane was proposed. With this mechanism, a gel layer is first formed on the support after in situ aging, which contains plenty of pre-nuclei. During the following microwave assistant crystallization, these pre-nuclei rapidly and simultaneously develop into crystal nuclei, and then crystal growth by propagation through the amorphous primary particles (with size of ca. 50 nm) goes on, and finally, the amorphous particles transform into LTA crystal particles with the same size. This propagation growth process is followed by or parallel with the agglomeration and densification of the primary particles. In this way, compact LTA zeolite membrane consisting of sphere grains with undefined crystal facets is obtained. (c) 2006 Elsevier B.V. All rights reserved.