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

Abstract

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 Co 2+ 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.

Influence of an N,N,N-Trimethyl-1-adamantyl Ammonium (TMAda+) Structure Directing Agent on Al Distributions and Pair Features in Chabazite Zeolite

“…44 Computational comparisons of Al configurations in which TMAda + -alone are occluded within CHA are consistent with an energy bias against 6MR Al−Al pairs. 38 Calculations in which TMAda + and Na + or K + are co-occluded within the same CHA cage are consistent, with the former pair resulting in an enrichment in 6MR Al−Al pairs, 36,45,46 while the latter promotes Al−Al pairs at greater separation. 36 The available evidence indicates that computed energies do report on experimentally observed Al siting, independent of the inclusion of H 2 O in those models.…”

“…Al pair distributions are thus sensitive to TMAda + relative ordering, suggesting a strategy for controlling that distribution. CHA zeolites crystallized with TMAda + as the sole SDA, with no additional Na + , are observed to be poor in 6MR Al pairs, 11,34,38 consistent with a large manifold of low-energy AAA configurations and the lowest-energy AAB configurations.…”

“…CHA zeolites crystallized solely with TMAda + and at short crystallization times (2 days) are observed to uptake Co 2+ , but materials prepared at longer crystallization times exhibit no Co 2+ uptake capacity and to exchange Cu 2+ only in its monovalent, CuOH + form. , These observations indicate that the framework evolves away from six-membered rings (6MRs) containing two Al 3+ with increasing crystallization time, consistent with a thermodynamic preference against 6MR Al pairs. In contrast, synthesis with Na + as a secondary, inorganic SDA results in an enrichment in these Al–Al pair features in the 6MRs. , Vibrational spectroscopy provides independent verification of these differences . Synthesis with the larger K + cation as the secondary SDA again results in CHA zeolites that lack the 6MR Al–Al pair feature …”

“…In contrast, synthesis with Na + as a secondary, inorganic SDA results in an enrichment in these Al−Al pair features in the 6MRs. 34,38 Vibrational spectroscopy provides independent verification of these differences. 11 Synthesis with the larger K + cation as the secondary SDA again results in CHA zeolites that lack the 6MR Al−Al pair feature.…”

“…On frameworks constructed from a single symmetry-distinct T-site, such as CHA, these influences are manifested in differences in the proximity of Al 3+ sites. − CHA is formed from ABC stackings of double-six-ring (d6r) secondary building units and can be crystallized with N , N , N -trimethyl-1-adamantyl ammonium (TMAda + ) OSDA . CHA zeolites crystallized solely with TMAda + and at short crystallization times (2 days) are observed to uptake Co 2+ , but materials prepared at longer crystallization times exhibit no Co 2+ uptake capacity and to exchange Cu 2+ only in its monovalent, CuOH + form. , These observations indicate that the framework evolves away from six-membered rings (6MRs) containing two Al 3+ with increasing crystallization time, consistent with a thermodynamic preference against 6MR Al pairs. In contrast, synthesis with Na + as a secondary, inorganic SDA results in an enrichment in these Al–Al pair features in the 6MRs. , Vibrational spectroscopy provides independent verification of these differences .…”