Mechanism of seeding in hydrothermal synthesis of zeolite Beta with organic structure-directing agent-free gel

Abstract

The organic structure-directing agent-free synthesis of zeolite Beta was carried out using several zeolite Beta seeds that differed in SiO2/Al2O3 ratio and crystal size. The synthesis was studied using X-ray diffraction, X-ray fluorescence, scanning electron microscopy, transmission electron microscopy, ultraviolet-Raman spectroscopy, infrared spectroscopy, and N2 physisorption. Synthesis was successful using different zeolite Beta seeds including pure silica seeds. During the induction period, the seeds underwent dissolution. The SiO2/Al2O3 ratio and crystal size, pretreatment (calcination), and seed addition time had a significant influence on seed dissolution behavior, crystallization process, and product. Morphological studies revealed that the seed residues produced by dissolution (except for pure silica) resulted in the formation of “immobilized” surface nuclei, which allowed for the dense growth of fresh small zeolite Beta crystals. The dissolved small seed fragments yielded dispersed nuclei, which formed relatively scattered small zeolite Beta crystals thought to be the main nuclei source of the pure silica seed. It is suggested that the use of an appropriately high SiO2/Al2O3 ratio, small size, and precalcined zeolite Beta seed is helpful for the synthesis of highly crystalline and pure zeolite Beta from the organic structure-directing agent-free gel.