Direct air capture (DAC) of CO₂ is a promising carbon-negative technology for achieving net-zero emissions by 2050, but its success depends on scalable, low-cost, and energy-efficient materials. Zeolites, known for their structural stability and lower cost compared to MOFs, are strong candidates for DAC adsorbents. The CO₂ adsorption performance of zeolites is primarily influenced by cations, framework structure and pore size, among which cations play the most significant role. This study examines the effects of alkali and alkaline earth metal cations (Na⁺, K⁺, Mg²⁺, Ca²⁺ and Ba²⁺) on CO₂ capture performance and behavior in ion-exchanged ZSM-5. NH₄-ZSM-5 (Si/Al = 11.5) was ion-exchanged and was tested for CO₂ adsorption performance by directly injecting real atmospheric air. To identify the effect of different metal cations on the DAC performance, CO₂ adsorption capacities were plotted against cation charge density (valence/ionic radius³) which influences adsorption through electrostatic interactions. Moderate charge density cations in ZSM-5 enhance CO₂ adsorption via electrostatic interactions, while excessively high charge density (e.g., Mg²⁺) resulted in stronger binding and higher heat of adsorption, making adsorption less efficient at room temperature. In addition, H₂O-TPD revealed that high charge density cations such as Ca²⁺ were more susceptible to H₂O contamination, leading to decreased CO₂ uptake under humidity. Under humid air, Ba-ZSM-5 showed distinctive behavior where initially adsorbed CO₂ was gradually desorbed as H₂O displaced CO₂ over time through dynamic adsorption equilibrium similar to moisture swing adsorption moisture swing adsorption, as confirmed by DRIFTS. The findings on how cation type and humidity affect CO₂ adsorption and behavior in ZSM-5 highlight its potential for practical use as a DAC adsorbent under real atmospheric conditions.

Min Bum Park

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