Quantifying Gas Adsorption Variability and Optimal Si/Al Ratio for Rational Design of Aluminum-Substituted Zeolite Frameworks

Experimental measurements often show significant variations in gas adsorption on aluminosilicate zeolites, thereby inducing considerable uncertainty in the gas separation and storage performance. These variations are largely attributed to the distribution of aluminum (Al) atoms within the zeolite framework. It is challenging to experimentally control the distribution of Al atoms during zeolite synthesis. The vast number of plausible Al-substituted configurations also makes it difficult to estimate the overall range of adsorption. To resolve this, we deploy a new representation of crystallographic frameworks using single repeating units (SRU). An SRU consists of the smallest network of tetrahedral atoms that can be repeated as a single building block to represent an entire zeolite framework. SRUs enable a selective enumeration of unique Al-substituted configurations, thereby leading to an efficient computational framework for quantifying the variations in equilibrium gas adsorption on Al-substituted zeolites without an exhaustive search. We apply this technique to analyze CO₂ adsorption on chabazite (CHA) zeolite. Using molecular simulations of gas adsorption on the unique Al-substituted configurations, we observe as much as 12% variation in the CO₂ adsorption due to differences in the locations of Al atoms within the zeolite framework. Interestingly, our results indicate that variability in CO₂ adsorption in Al-substituted zeolites is significant only at moderate Si/Al ratios, primarily due to the nonuniform distribution of Al. At very high or very low Si/Al ratios, this variability appears to be negligible. Surprisingly, we also observe that the adsorption does not always increase with the number of Al sites, and there exists an inflection point beyond which additional Al substitution leads to a decrease in adsorption. This trade-off indicates an optimal Si/Al ratio that maximizes the equilibrium adsorption of CO₂ on Al-substituted CHA zeolites at some moderate values. We are able to systematically identify the optimal Si/Al ratio and the corresponding locations of Al sites in the CHA framework that maximizes CO₂ adsorption. On further investigation using the Al-Al radial distribution function (RDF), we find the locations of Al sites that lead to high CO₂ adsorption. This demonstrates that the SRU-based selective enumeration combined with RDF-based structural screening is an enabling method toward the rational design of zeolites with optimal distribution of Al sites to achieve desired properties.