Elucidating the Si/Al Ratio Dependence of N2, CO2, and NO2 Adsorption in LTA Zeolites via a Synergistic Guest–Cation Gating Effect
Journal
Energy and Fuels
Journal Volume
40
Journal Issue
29
Start Page
15958
End Page
15967
ISSN
0887-0624
1520-5029
Date Issued
2026-06-29
Author(s)
Abstract
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 (Ca2+)-, and manganese (Mn2+)-exchanged small-pore LTA zeolites toward nitrogen (N2), carbon dioxide (CO2), and nitrogen dioxide (NO2). 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 CO2/N2 separation, and ambient NO2 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.
Publisher
American Chemical Society (ACS)
Type
journal article