Quantitatively Understanding the Insights into CO₂ Adsorption on Faujasite from the Heterogeneity and Occupancy Sequence of Adsorption Sites

The heterogeneity in adsorption site is a fundamental characteristic of adsorption that is difficult to quantify. Herein, we quantify the site heterogeneity for CO₂ adsorption on NaX by recognizing the characteristics, types, and site occupancies at different loadings. We find an adsorption site geometry of Na···OCO⊥ O_z (where O_z denotes an oxygen atom in the zeolite) and that the distance between C and O_z is an indicator of heterogeneity. This distance is controlled by the interacting cations on SIII (Na_III), where the energies (numbers) of the dual-, single-, and zero-Na_III adsorption sites are −57.2, −43.1, and −29.5 kJ/mol (17, 57, and 26 uc, (per unit cell)), respectively. The adsorption heats and isotherms calculated using heterogeneity-based models agree reasonably with experimental results for both NaX and NaY. An abundance of mechanisms underlying the trends in the adsorption heats and isotherms are elucidated based on the heterogeneity in the adsorption sites. Applying the proposed method to an LTA-4A zeolite shows that the adsorption site geometries do not change and that the distance between C and O_z remains an indicator of heterogeneity. These consistencies support the transferability of the proposed method to other zeolites.