(101e) Mapping Interactions between Cationic Structure-Directing Agents and Framework Anions in Zeolites Using Computational Tools

Structure directing agents (SDA) provide a template around which zeolite frameworks grow during synthesis and influence cationic charge density, which together help determine the locations of anionic framework Al. The final framework depends sensitively on the presence and structure of any organic SDA (OSDA) used and the computed interaction energy between OSDA and the zeolite frameworks can identify the OSDA’s selectivity toward certain frameworks [1]. The interplay between organic and inorganic SDAs can also alter Al distributions. For example, increasing the ratio between inorganic Na⁺ SDA and organic N,N,N-trimethyl-1-adamantyl ammonium (TMAda⁺) SDA during CHA synthesis systematically increases the amount of ‘paired’ Al that share a 6-member ring [2]. Na⁺ and TMAda⁺ co-occlude within the CHA cage, while using K⁺ and TMAda⁺ does not produce paired sites because K⁺ displaces TMAda⁺ in CHA [3]. Our work in MFI shows that including Na⁺ during synthesis with tetrapropyl ammonium (TPA⁺) can also increase the fraction of paired sites, albeit less systematically than in CHA, and that such paired sites can form in MFI with only TPA⁺. Moreover, density functional theory (DFT) calculations indicate that MFI has 43 unique Al pairs that Co²⁺ can titrate—compared to two in CHA—owing to MFI’s many small rings and low symmetry [4]. Finally, similar synthesis and DFT approaches in MEL with tetrabutyl ammonium (TBA⁺) indicate that Na⁺ can partly displace TBA⁺, like K⁺ with TMAda⁺ in CHA, and that there is a limit to cation loading in zeolites during synthesis. Machine learning has recently permitted us to expand this approach using faster screening techniques to evaluate SDA behavior in thousands of existing and hypothetical zeolites.

References

[1] Science, 2021, 374, 308–315.

[2] Chem. Mater. 2016, 28, 2236–2247.

[3] J. Am. Chem. Soc. 2020, 142, 4807–4819.

[4] Chem. Mater. 2020, 32, 9277–9298.