Active metal cation exchanged in ZSM-5 for enhanced direct air capture of CO₂

Zeolites have proved their potential as cost-effective adsorbents for CO₂ capture; further development is worth optimizing their performance for large-scale direct air capture (DAC) applications. In this study, ZSM-5 zeolites were prepared and exchanged with alkali cations (Na and K) and alkaline earth cations (Mg, Ca and Ba) to investigate their performances for the CO₂ capture from atmospheric air in the DAC system. We found that the cation charge density is critical to determining the DAC capacity of ZSM-5 zeolites. In detail, ZSM-5 with a low cation charge density (e.g., K⁺ with a charge density of 0.39) struggles to effectively capture CO₂ at low concentrations since CO₂ adsorption relies on electrostatic interactions with quadrupole CO₂ by cation charge density. Conversely, an excessively high cation charge density has a detrimental effect as adsorption sites become shielded by H₂O and CO₂ on cations (e.g., Ca²⁺ and Mg²⁺ with charge densities of 2.06 and 7.28, respectively), reducing the accessible CO₂ capacity. Consequently, Ba-ZSM-5, featuring Ba²⁺ with a moderate charge cation density of 0.81, exhibits the highest DAC capacity (500 ppm CO₂ in the air at RH 13%, 0.4 mmol g⁻¹), with fast kinetics and stable reproducibility, appealing that appropriate cation charge density is critical to imparting the high DAC capacity of ZSM-5 zeolites. In addition, DRIFTS results confirmed the moisture swing adsorption behavior, in which the adsorbed CO₂ is desorbed directly by water over Ba-ZSM-5. These results provide valuable insights for the design of zeolites-based DAC systems.