Synthesis and crystallization mechanism of EUO zeolite

Abstract

Nucleation and growth of zeolites are closely related to the state of amorphous aluminosilicate precursors which may lead to different crystallization pathways. This article provided a deep understanding of the formation mechanisms of EUO from different precursors generated through hydrolysis of tetraethoxysilane (TEOS) at different temperatures as well as by introducing the aluminum source at different time. A combination of diffraction, NMR and microscopy techniques was applied to characterize the assembly and evolution of amorphous species during the synthesis of EUO zeolites. Hydrolysis temperature of TEOS was closely associated with the nucleation rate of EUO. The evolution of worm-like precursors to bulk precursors was accelerated when the gel was hydrolyzed at low temperature. Moreover, the introduction of sodium aluminate could also promote the nucleation when a homogeneous gel with uniform Si–Al distribution was formed. When the gel environment was non-uniform, amorphous-to-crystalline transformation involved a two-step pathway, beginning with the appearance of *MRE due to the existence of Si-rich regions, followed by inter-crystalline transformation to EUO. Such crystallization pathway led to the formation of EUO single crystal around 1.5 μm. Given that numerous zeolite syntheses involve the formation of metastable precursors with heterogeneous composition, the choice of gel preparation procedure in this study may be generalized to other related materials.