Relationship Between Electronegativity of the Extra-Framework Cations and Adsorption Capacity for CO 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 Sr and Cd cations, with higher hydration energies compared to monovalent cations, are present as hydrated ions at the center of the 12-membered ring along the -axis (12MRc). Pb 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 CO adsorption capacity was assessed based on the type and site of exchangeable cations. The findings indicate that an increase in the CO adsorption capacity correlates with the number of cations that can effectively interact with CO.