Influence of alkali metal cations on the local structure of amorphous precursors during zeolite crystallization†

Structure-directing agents are commonly used to regulate the structure of zeolites. In addition to organic structure-directing agents, alkali metal cations have been found to exhibit strong structure-directing abilities. However, the specific influence of alkali metal cations on zeolite formation remains unclear. To address this issue, it is essential to analyze the local structures of the amorphous precursors from which zeolites are formed. In this study, FAU- and MER-type zeolites are synthesized via amorphous precursors containing Na⁺ and K⁺, respectively, whose Si/Al ratios are equal at the moment just before the appearance of diffraction peaks, and the structural analysis of the amorphous precursors is performed using UV-Raman spectroscopy, pair distribution function analysis, and a combination of Reverse Monte Carlo modeling and molecular dynamics. The results reveal that the ring distributions and the angles of the ring structures in both amorphous precursors differ depending on the type of alkali metal cations used. Furthermore, the comparisons of the structural features, including ring distributions and bond angles, reveal the structural similarities between amorphous precursors and zeolites, suggesting that alkali metal cations regulate the organization of precursors and facilitate the formation of specific zeolites. The results show that Na⁺ preferentially stabilizes 6-membered rings while K⁺ promotes the formation of 8-membered rings, serving as precursors to the corresponding framework topologies in the final zeolite products. This study deepens the understanding of the structural evolution during zeolite crystallization regulated by alkali metal cations, and is anticipated to play a role in the development of more efficient and rational zeolite synthesis strategies.