Evolution of Framework Al Arrangements in CHA Zeolites during Crystallization in the Presence of Organic and Inorganic Structure-Directing Agents

The arrangement of Al heteroatoms in zeolite frameworks influences turnover rates of Brønsted acid-catalyzed reactions and the speciation of exchanged metal cations and complexes that are active sites for redox catalysis. The substitution of Al for Si in zeolite frameworks generates anionic lattice charges and is thus influenced by the structure and charge density of the cationic structure directing agents (SDAs) that guide zeolite crystallization. Here, we investigate how framework Al structure and arrangements in chabazite (CHA) zeolites evolve as a function of hydrothermal treatment time as amorphous Al and Si precursors convert into partially and fully crystalline aluminosilicate phases in the presence of organic and inorganic SDAs. With N,N,N-trimethyl-1-adamantylammonium (TMAda+) as the sole SDA, an amorphous aluminosilicate network initially forms that contains a large fraction of proximal Al sites, as quantified by Co2+ titration, which evolves into CHA crystallites that contain a high fraction of proximal Al sites in six-membered rings (6-MR). After bulk crystallization has been completed (433 K, 36 h), continued hydrothermal treatment causes rearrangement of framework Al to become more site-isolated, eventually resulting in CHA crystallites with undetectable numbers of 6-MR paired Al sites (433 K, 144 h). These temporal changes in framework Al arrangement indicate that Si–O–Al linkages remain labile and undergo restructuring within crystalline domains under hydrothermal conditions, allowing for atomic rearrangement toward thermodynamically preferred framework Al distributions (e.g., 6-MR isolated Al sites in the presence of TMAda+ only). Ab initio molecular dynamics (AIMD) simulations report isolated Al configurations to be lower in energy than 6-MR Al pair configurations in a field of TMAda+, supporting a thermodynamic driving force for Al sites to isolate when TMAda+ is the sole SDA. In contrast, using both Na+ and TMAda+ as inorganic and organic co-SDAs did not result in any further changes to the framework Al arrangement after CHA crystallization was complete (433 K, 96 h), suggesting that the co-occlusion of Na+ and TMAda+ suppresses the lability of framework Si–O–Al bonds and Al rearrangement in crystalline phases. AIMD simulations report 6-MR paired Al sites to become lower in energy than isolated Al configurations when charge-balanced by both Na+ and TMAda+. These findings indicate that Al arrangements in zeolite frameworks can evolve upo