Direct air capture of CO₂ by metal cation-exchanged LTA zeolites : Effect of the charge-to-size ratio of cations

Direct air capture of CO₂ (DAC) has been increasingly recognized as a promising carbon-negative technology. The challenge in deploying energy-efficient DAC lies in effective sorbent materials. In this research, we comprehensively investigated the DAC behavior of LTA zeolites exchanged with different metal cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Y³⁺, La³⁺, Ce³⁺, Eu³⁺, Tb³⁺, and Yb³⁺) by both static single-component gas adsorption and dynamic mixture gas adsorptive separation tests. We found that a large charge-to-size ratio of cations is critical to imparting a high DAC capacity of LTA zeolites, which is ascribed to the enhanced electrostatic interaction and/or π-back bonding toward CO₂. Meanwhile, a detrimental effect is associated with an excessively large charge-to-size ratio, that is, a significant “shielding effect” of (pre-) adsorbed contaminants (e.g., H₂O and CO₂) on cations (e.g., Mn²⁺ and Mg²⁺) reduce the accessible CO₂ capacity. Ca-LTA featuring Ca²⁺ with an appropriate charge-to-size ratio exhibits the highest DAC capacity (350 ppm CO₂ in the air, 1.20 mmol/g) with fast kinetics and good reusability. These results provide valuable insights for the design of zeolites-based physisorbents for DAC. © 2023 American Institute of Chemical Engineers.