Step-like crystallization of BEA zeolite: Accumulation of oligomeric species in liquid phase followed by the conversion of solid phase to crystals

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In this study, the hydrothermal reactions of the synthesis mixtures of BEA zeolite and the synthesis cakes prepared by the filtration of the synthesis mixtures were investigated. The crystallization curves constructed based on the XRD patterns and the SEM images of the solid phases recovered at different reaction times exhibited step-like curves characterized by very long induction periods (70 h for the synthesis mixtures and 37 h for the synthesis cakes) and a short crystal growth time (4 h for both) at 150 °C. The higher concentration of the reactants in the synthesis cakes than in the synthesis mixture quickly generated more nuclei, consequently forming smaller crystals. In order to elucidate the changes occurring in the course of the hydrothermal reaction, a total of seven additional analysis techniques including 29Si MAS NMR spectra, TEM, IR, and UV–Vis spectra were applied to the solid and liquid phases recovered at various times. No significant changes were observed in the solid phases during the induction period in terms of the crystallinity, chemical environment of silicon atoms, porous properties, capture state of structure-directing agent, and the IR bands of the hydroxyl groups, whereas the contents of oligomeric aluminosilicates increased in the liquid phases. Based on these analyses, the long induction periods were required for sufficient accumulation of the building blocks, and the blocks rapidly assembled to form BEA zeolite crystals in the subsequent growth stage, resulting in the step-like crystallization curves.