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 Q_st (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 Q_st (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 .