Relationship\nBetween Electronegativity of the Extra-Framework\nCations and Adsorption Capacity for CO<sub>2</sub> Gas on Mordenite\nFramework

Synthetic mordenite is widely used as a molecular sieve,\nadsorbent,\nand catalyst. To enhance these functionalities, it is crucial to understand\nthe ion-exchange properties and cation-exchange sites of the zeolite.\nIn this study, we analyzed the structural changes in fully Cs-, Sr-,\nCd-, and Pb-exchanged mordenite by using synchrotron X-ray powder\ndiffraction under ambient conditions. Rietveld structure refinement\nrevealed that the Cs<sup>+</sup> cation is predominantly located near\nthe 8-membered ring (8MR) due to its low electronegativity and hydration\nenergy. In contrast, divalent cations such as Sr<sup>2+</sup> and\nCd<sup>2+</sup> cations, with higher hydration energies compared to\nmonovalent cations, are present as hydrated ions at the center of\nthe 12-membered ring along the <i>c</i>-axis (12MRc). Pb<sup>2+</sup> ions, due to their higher electronegativity than the framework\natoms, exhibit a strong affinity for the electron cloud of framework\noxygen atoms, which positions them close to the wall of the 12MRc.\nThe observed differences in the locations of the extra-framework cations\nare attributed to electrostatic and hydration effects. Furthermore,\nthe CO<sub>2</sub> adsorption capacity was assessed based on the type\nand site of exchangeable cations. The findings indicate that an increase\nin the CO<sub>2</sub> adsorption capacity correlates with the number\nof cations that can effectively interact with CO<sub>2</sub>.