Metal-organic frameworks (MOFs) have received tremendous interest for gas storage and separation. Recently, ionic MOFs with extra-framework ions have been synthesized such as rho-ZMOF, soc-MOF, rht-MOF, and Li-MOF. Most current studies on gas separation in MOFs have been conducted in neutral frameworks. We have reported the molecular simulation study for the separation of industrially important gas mixtures (CO2/CH4, CO2/H2, and CO2/N2) in ionic MOFs towards CO2 capture.
The static and dynamic properties of the extra-framework ions in rho-ZMOF, soc-MOF, rht-MOF, and Li-MOF were first characterized by Monte Carlo and molecular dynamics simulations in canonical ensembles. The presence of extraframework ions in porous materials increases the interaction strength between host and guest molecules and in turn enhances the storage, separation and ion-exchange capability. A wealth of studies on zeolitic materials has showed that the microscopic properties of extraframework ions are crucial to understanding the fundamental chemistry of these materials and in tailoring their properties. The adsorptive separation of CO2 from binary mixtures was subsequently predicted by grand canonical Monte Carlo simulations. CO2 is highly selectively adsorbed over other gases in binary mixtures. The ionic framework and the presence of extraframework ions create an electric field and enhance the adsorption, particularly the quadrapolar CO2 molecules, whereas the adsorption of other gases is substantially small. The predicted selectivity of CO2 over H2 and N2 in these ionic MOFs shows different behavior as a function of pressure. The selectivity in these ionic MOFs decreases with increasing channel dimension and increases with increasing charge density. Furthermore, charge density plays a more dominant role in determining the selectivity.
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