On the Structure–Property Relationships of Cation‐Exchanged ZK‐5 Zeolites for CO2 Adsorption

Abstract The CO2 adsorption properties of cation‐exchanged Li‐, Na‐, K‐, and Mg‐ZK‐5 zeolites were correlated to the molecular structures determined by Rietveld refinements of synchrotron powder X‐ray diffraction patterns. Li‐, K‐, and Na‐ZK‐5 all exhibited high isosteric heats of adsorption (Qst) at low CO2 coverage, with Na‐ZK‐5 having the highest Qst (ca. 49 kJ mol−1). Mg2+ was located at the center of the zeolite hexagonal prism with the cation inaccessible to CO2, leading to a much lower Qst (ca. 30 kJ mol−1) and lower overall uptake capacity. Multiple CO2 adsorption sites were identified at a given CO2 loading amount for all four cation‐exchanged ZK‐5 adsorbents. Site A at the flat eight‐membered ring windows and site B/B* in the γ‐cages were the primary adsorption sites in Li‐ and Na‐ZK‐5 zeolites. Relatively strong dual‐cation adsorption sites contributed significantly to an enhanced electrostatic interaction for CO2 in all ZK‐5 samples. This interaction gives rise to a migration of Li+ and Mg2+ cations from their original locations at the center of the hexagonal prisms toward the α‐cages, in which they interact more strongly with the adsorbed CO2.