Elucidating the Si/Al Ratio Dependence of N ₂ , CO ₂ , and NO ₂ Adsorption in LTA Zeolites via a Synergistic Guest–Cation Gating Effect

Small-pore zeolites are promising adsorbents for gas capture and separation because their confined pore environments can accommodate densely populated adsorption sites (e.g., charge-balancing cations). Although substantial efforts have been devoted to elucidating how cation identity governs adsorption behavior, the influence of framework silicon-to-aluminum (Si/Al) ratio remains poorly understood. Here, we systematically examine the role of Si/Al ratio on the adsorption behavior of sodium (Na + -), calcium (Ca 2+ )-, and manganese (Mn 2+ )-exchanged small-pore LTA zeolites toward nitrogen (N 2 ), carbon dioxide (CO 2 ), and nitrogen dioxide (NO 2 ). Combined analysis of static adsorption isotherms, dynamic breakthrough measurements, and adsorption thermodynamic analysis reveals a bell-shaped dependence of both gas uptake and adsorption strength on the Si/Al ratio in most cases. Although a cation-gating effect can partially rationalize this behavior, it fails to account for all experimental observations in this work. We therefore propose a synergistic guest–cation gating effect, in which gas admission and practical adsorption strength are jointly governed by framework cation density (i.e., Si/Al ratio), guest–cation interaction strength, and the effective size of the adsorbed species. Application-oriented evaluation further shows that Si/Al ratios of c.a. 2–3 are generally the most favorable for direct air capture (DAC), postcombustion CO 2 /N 2 separation, and ambient NO 2 removal. Overall, this study clarifies the role of Si/Al ratio in gas adsorption on small-pore LTA zeolites and provides a useful basis for designing zeolitic adsorbents for carbon capture and air purification, thereby contributing to the mitigation of environmental challenges associated with increasing fuel combustion driven by growing energy demand.