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 kPa 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.