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.