Zeolites, a series of crystalline microporous aluminosilicate, are of great importance in industry because of their unique framework structures and molecular-sized pores. Since crystallization of zeolites involves a complex sequence of chemical reactions under hydrothermal conditions, it is a great challenge to control their structures and properties. Therefore, understanding the crystallization mechanism of zeolites leads to better control over their structures and properties by controlling the sequence of hydrothermal reactions involved. Many zeolites with unconventional compositions and/or new structures have been synthesized by introducing organic structure-directing agents (OSDAs). Even though OSDAs are typically occluded in zeolites as a single molecule or cation, it has been suggested that molecular aggregate of tetraethylammonium cations (TEA+) takes part in the crystallization of zeolite beta. However, its crystallization mechanism has not been fully understood yet at an atomic level.
In this study, zeolite beta is synthesized under hydrothermal condition in the absence of alkali cation in order to focus on the structure-directing effect of TEA+. Changes in the aluminosilicate structure and TEA+-aluminosilicate interaction during the crystallization of zeolite beta are investigated, employing several characterization tools such as XRD, SEM, TEM, solid-state MAS NMR and Raman spectroscopy, TG-DTA, and gas adsorption). Entire crystallization procedure can be subdivided into following steps: formation of TEA+–amorphous aluminosilicate composite, induction period, nucleation and crystal growth. It is revealed that the formation of TEA+–amorphous aluminosilicate composite and its structural evolution toward zeolite beta-like structure during the induction period are essential in the crystallization of zeolite beta. Based on the results, a comprehensive scheme of the formation of zeolite beta is proposed with paying attention to the aggregation behavior of TEA+ cations.
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