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 upon extended hydrothermal treatment
after bulk crystallization has been completed from amorphous Si and
Al precursors and that such evolution is influenced by thermodynamic
factors, extending prior reports of such phenomena occurring during
interzeolite conversion routes.