Selective CO2 adsorption over alkali metal cation-exchanged UZM-9 zeolites

Abstract

Adsorption on zeolites reduces CO₂ emissions and cuts the energy costs of processing gas mixtures, such as natural gas, biogas, and landfill gas (CO₂/CH₄ of various concentrations). Among zeolite frameworks, LTA stands out for its CO₂ adsorption and/or separation potential, particularly the Na-LTA zeolite with a Si/Al ratio of ∼5. However, the impact of different cations on the separation efficiency of this system remains unknown. In this study, we tested various alkali-metal-exchanged UZM-9 zeolites (Si/Al = 4.5) for their selective adsorption of CO₂ over CH₄. K⁺-exchanged UZM-9 reached the highest CO₂ affinity, isosteric heat of adsorption, and selectivity, outperforming more commonly used Na⁺ forms. This enhanced performance likely stems from the predominant location of K⁺ in the 8-ring window, which fosters strong CO₂ interactions, potentially via bridging CO₂ species. Due to partial pore blocking, the total uptake may decrease slightly, but the K-UZM-9 system effectively balances CO₂/CH₄ selectivity and adsorption capacity. Therefore, K-UZM-9 emerges as a promising adsorbent for energy-efficient gas separation and carbon capture applications.