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

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 (Q(st)) at low CO2 coverage, with Na-ZK-5 having the highest Qst (ca. 49 kJmol(-1)). Mg2+ was located at the center of the zeolite hexagonal prism with the cation inaccessible to CO2, leading to a much lower Q(st) (ca. 30 kJmol(-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 gamma-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 alpha-cages, in which they interact more strongly with the adsorbed CO2.