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

Abstract

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. © 2024 Elsevier B.V.