(228h) 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 structure of exchanged metal cations and complexes that are active sites for redox catalysis. Here, we investigate how framework Al arrangements evolve as a function of crystallization time, by characterizing the Al structure and arrangement as amorphous phases become fully crystalline CHA zeolites in the presence of organic and inorganic structure-directing agents (SDAs). Sample crystallinity was determined from powder XRD, Ar adsorption-desorption (87 K) isotherms, SEM, and ²⁷Al and ²⁹Si MAS NMR spectra, and the fraction of paired Al sites in six-membered rings (6-MR) was quantified by Co²⁺ titration. Using N,N,N-trimethyl-1-adamantylammonium (TMAda⁺) as the sole SDA initially forms an amorphous aluminosilicate network that contains a large fraction of 6-MR paired Al sites (433 K, 8 h); yet, continued hydrothermal treatment causes bulk CHA crystallization (433 K, 36 h), after which gradual rearrangement of framework Al results in predominantly 6-MR isolated Al sites (433 K, 144 h). When using both Na⁺ and TMAda⁺ as co-SDAs, by contrast, the initially formed amorphous aluminosilicate contains negligible amounts of paired Al sites (433 K, 80 h), which converts into crystalline CHA zeolite phases with high fractions of 6-MR paired Al sites (433 K, 96 h) that do not rearrange with prolonged hydrothermal treatment. These findings indicate that framework Al atoms can rearrange within CHA crystallites even after bulk crystallization is complete, reflecting a combination of both thermodynamic and kinetic factors. Rearrangement of lattice Al requires reversible cleavage and formation of Si-O-Al bonds through diffusion of framework Al and lattice vacancy defects via kinetically-accessible routes. This synthesis strategy can be extended to alter Al arrangements in other zeolite frameworks synthesized with organic or inorganic SDAs.