Synthesis of kaolin-based zeolite NaA and its crystallization mechanism

Abstract

The zeolite NaA was successfully synthesized by hydrothermal method using kaolin as a raw material. In order to study the crystallization mechanism, the products and intermediates of different crystallization stages were systematically characterized using x-ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM)-energy-dispersive spectrometry (EDS). The results show that the metakaolin is completely dissolved during the first hour of the crystallization process, forming a rich crystal nucleus. As the crystallization time increases, the nucleus grows rapidly to form zeolite NaA crystals. When the crystallization time reaches 3 h, the percentage crystallinity of zeolite NaA reaches a maximum. Therefore, it can be concluded that the optimum crystallization time of zeolite NaA is 3 h. Meanwhile, we can come to the conclusion that the synthesis of zeolite NaA from kaolin is a liquid phase transition mechanism. The microstructure of zeolite NaA synthesized at 3 h of crystallization was further investigated by transmission electron microscopy (TEM) technique. The SAED patterns indicate the polycrystallinity of the zeolite NaA structure. The electron diffraction pattern indicates that the crystal structure type of the zeolite NaA belongs to the body center cube (BCC). The textural properties of the zeolite NaA synthesized at 3 h of crystallization were studied by nitrogen adsorption technique. It can be concluded that the BET surface area of the zeolite NaA obtained by the BET method is 21.106 m2·g−1. Meanwhile, from the BJH method, it is found that the microporous surface area and micropore volume of the zeolite NaA are 13.929 m2·g−1 and 0.052 cm3·g−1, respectively.