Relationship Between Electronegativity of the Extra-Framework Cations and Adsorption Capacity for CO₂ Gas on Mordenite Framework

Synthetic mordenite is widely used as a molecular sieve, adsorbent, and catalyst. To enhance these functionalities, it is crucial to understand the ion-exchange properties and cation-exchange sites of the zeolite. In this study, we analyzed the structural changes in fully Cs-, Sr-, Cd-, and Pb-exchanged mordenite by using synchrotron X-ray powder diffraction under ambient conditions. Rietveld structure refinement revealed that the Cs⁺ cation is predominantly located near the 8-membered ring (8MR) due to its low electronegativity and hydration energy. In contrast, divalent cations such as Sr²⁺ and Cd²⁺ cations, with higher hydration energies compared to monovalent cations, are present as hydrated ions at the center of the 12-membered ring along the <i>c</i>-axis (12MRc). Pb²⁺ ions, due to their higher electronegativity than the framework atoms, exhibit a strong affinity for the electron cloud of framework oxygen atoms, which positions them close to the wall of the 12MRc. The observed differences in the locations of the extra-framework cations are attributed to electrostatic and hydration effects. Furthermore, the CO₂ adsorption capacity was assessed based on the type and site of exchangeable cations. The findings indicate that an increase in the CO₂ adsorption capacity correlates with the number of cations that can effectively interact with CO₂.