The crystallization mechanism of zeolites remains incompletely understood due to the multi-scale assembly processes that involve a variety of intermediates with unknown structures and complex interactions between inorganic and organics species. Gaining deeper insight into this mechanism is crucial for optimizing zeolite synthesis with tailored properties and for discovering new zeolite frameworks. In this study, we investigated the crystallization pathway of siliceous LTA zeolite in the presence of two organic structure directing agents (OSDAs) — dimethyl-3-(4-methyl-benzyl) imidazolium hydroxide (denoted as “BULKY-OH”), and tetramethyl ammonium hydroxide (TMAOH) —along with ammonium fluoride (NH4F). We employed a suite of ex situ characterization techniques, including X-ray diffraction (XRD), 19F MAS NMR, 13C MAS NMR, 29Si MAS NMR, and Raman spectroscopy. The experimental results were further investigated by periodic density functional theory (DFT) calculation. We identified the formation of fluoride-filled double-four-membered rings (F-/D4Rs) in the amorphous gel during the induction period. Our findings further confirm the continuous evolution of F-/D4Rs in the amorphous gel throughout this crystallization process. Raman spectroscopy revealed environmental changes experienced by the primary OSDA, BULKY, in the amorphous phase. Two distinct environments for BULKY were observed in the amorphous phase: (1) BULKY molecules embedded within amorphous or defective silica, and (2) excess or free BULKY molecules uninfluenced by the silica environment. A third environment, corresponding to BULKY incorporated into the zeolite structure, emerges after the formation of crystals. Similarly, secondary OSDA, TMA, also exhibits multiple distinct environments and contributes to the formation of cage-like structures in the amorphous phase. Together, these results provide compelling evidence of atomic rearrangements occurring during the induction period that ultimately leading to zeolite nucleation.
