Metal cation-exchanged LTA zeolites for CO₂/N₂ and CO₂/CH₄ separation: The roles of gas-framework and gas-cation interactions

Selective CO₂ adsorption for efficient carbon capture from flue gas (CO₂/N₂ - 15/85, v/v) and biogas (CO₂/CH₄ - 50/50, v/v) is important for achieving global energy and climate goals but remains a challenge due to the lack of effective adsorbents. To address this issue, we attempted to develop zeolite adsorbents by systematically investigating the effect of different extra-framework metal cations (i.e., Na⁺, Ca²⁺, Mn²⁺, and Ce³⁺-exchanged in LTA zeolites) on selective CO₂ adsorption from CO₂/N₂ and CO₂/CH₄. Analyzing the isosteric heat of adsorption results showed that CO₂ adsorption at moderate pressure (e.g., 15 and 50 kPa relevant to the compositions of flue gas and biogas, respectively) is governed by the gas-framework interaction. On the other hand, the adsorption of N₂ and CH₄ was found to be dominated by the gas-cation interaction. Therefore, we concluded that metal cations with a small charge-to-size ratio are beneficial for selective CO₂ adsorption from flue gas and biogas because they tend to induce strong CO₂-framework interaction (due to the enhanced charge induction to zeolite framework O) and weak N₂ or CH₄-cation interaction (due to the weakened gas-cation electrostatic interaction). Specifically, LTA zeolites in the form of Na⁺, with the smallest charge-to-size ratio of cation in this study, exhibit the highest CO₂ uptake and separation factor of CO₂/N₂ and CO₂/CH₄, as demonstrated by both static single-component adsorption and dynamic binary adsorption results. In addition, the potential of metal cation-exchanged LTA zeolites for VSA and PSA processes was evaluated. Our study provides valuable insights for designing small-pore zeolites as adsorbents for carbon capture.