Decoding Aluminum Distribution in SSZ-13 Zeolites: Effects of Synthesis Parameters and Post-Treatment Protocols

Abstract

SSZ-13 zeolites with chabazite (CHA) topology are pivotal in industrial catalysis, particularly in methanol-to-olefins (MTO) conversion and selective catalytic reduction (SCR) of nitrogen oxides. The catalytic efficiency of these materials is governed by the spatial distribution of aluminum (Al) atoms within the silica framework, which dictates the nature and availability of active sites. This study investigates the impact of synthesis parameters and post-synthesis treatments on Al distribution in SSZ-13 zeolites, with a focus on paired versus isolated Al sites. By systematically varying the ratios of organic (TMAda⁺) and inorganic (Na⁺) structure-directing agents during synthesis, the critical role of these agents in modulating Al arrangements is demonstrated. Low Na⁺/TMAda⁺ ratios favored isolated Al sites, enhancing Brønsted acidity, whereas intermediate ratios promoted Al pairing, facilitating divalent cation stabilization, such as Co²⁺. Beyond a threshold ratio, phase transitions to mordenite (MOR) were observed, underscoring the delicate balance required for CHA formation. Post-synthesis treatments, including alternative drying protocols, further influenced Al distribution, with deviations from standardized methods predominantly yielding isolated Al sites and reducing Co²⁺ ion-exchange capacity. Cation-exchange experiments and spectroscopic analyses provided quantitative insights into Al arrangements, highlighting a strong correlation between synthesis conditions, framework stability, and catalytic performance. The findings emphasize the need for precise control of synthesis and post-treatment protocols to tailor SSZ-13 properties for specific applications. This work offers a framework for optimizing SSZ-13 zeolites by linking Al distribution to catalytic functionality, paving the way for advancements in industrial catalysis and the rational design of zeolitic materials. Future research should explore standardized post-synthesis treatments and expand topological studies to enhance reproducibility and generalizability