Mg-Exchanged Gismondine for Superior CO₂/N₂ and CO₂/CH₄ Separations

The CO 2 adsorption performance of a zeolite is related to the framework structure and extra-framework composition. In this work, parent Na-GIS and partially exchanged NaMg1-GIS and NaMg3-GIS, with 30 and 50% degree of Mg 2+ exchange, respectively, were prepared. Further, the series of zeolites were thoroughly characterized and studied for their applicability for CO 2 adsorption and CO 2 /N 2 and CO 2 /CH 4 separations by single-component adsorption and dynamic breakthrough curve analysis methods. The lower concentration of Mg 2+ cations in NaMg1-GIS resulted in enhanced CO 2 adsorption due to beneficial distortion of the framework pore structure, making the adsorption of N 2 and CH 4 more challenging. However, this benefit was lost for higher Mg 2+ concentration (NaMg3-GIS) due to a more pronounced framework distortion, impairing not only the adsorption of N 2 and CH 4 but also the CO 2 one, ultimately leading to a loss of selectivity. The molecular dynamics simulations and density functional theory (DFT) calculations, aligned with experimental data, reveal an elevated heat of adsorption for CO 2 in NaMg3-GIS due to framework deformation caused by Mg 2+ cations. Consistent with the adsorption equilibrium experiments, the ternary dynamic experiments of CO 2 /N 2 /He evaluated by breakthrough curve analysis show a higher CO 2 /N 2 selectivity of 1673 and 1248 at 25 and 50 °C, respectively, for the NaMg1-GIS sample. For the CO 2 /CH 4 /He experiments, the CO 2 /CH 4 selectivity tended toward extremely high values due to negligible CH 4 uptake on NaMg1-GIS. In addition, the better dynamic adsorption/separation of CO 2 on NaMg1-GIS is ascribed to the greater distortion of the pore aperture due to the presence of Mg 2+ cations, affecting the diffusion of small molecules. Overall, our results demonstrate the high potential of NaMg-GIS materials for critically important energy separation processes involving CO 2, N 2, and CH 4 .