Biocompatible ammonium-based ionic liquids/ZIF-8 composites for CO2/CH4 and CO2/N2 separations

The development of sorbent materials with high carbon dioxide (CO₂) selectivity is of vital importance to minimize the impact of global warming by separating and capturing this major greenhouse gas. In this work, for the first time five ammonium-based ionic liquids (ILs) were impregnated into the metal-organic framework (MOF) ZIF-8 for gas sorption applications. Characterization results showed that the produced IL@ZIF-8 composites are thermally stable and of microporous nature. IL impregnation was successfully accomplished and did not affect the crystalline structure of ZIF-8. Sorption-desorption equilibrium isotherms of different gases, including CO₂, methane (CH₄) and nitrogen (N₂) were measured at 303 K for the pristine MOF and the composites. Given the careful choice of the ILs, the anion and cation effects were studied in terms of their impact on the sorption capacity and selectivity performance of each composite. Acetate-based composites showed high CO₂ selectivity when compared to ZIF-8; specifically, the composite [N_{2 1 1 4}][Ac]@ZIF-8 showed 51% increase, at 1 bar and for flue gas conditions. Because of the good CO₂/N₂ selectivity of the acetate-based composites, sorption-desorption equilibrium isotherms of the above-mentioned gases were also measured at 323 K, along with ZIF-8. Ideal selectivities were thus calculated to study the temperature impact on the selectivity performance of the materials. It can be observed that the higher the temperature, the less selective the materials are. Nevertheless, at 323 K and when compared to ZIF-8, the composite [N_{2 1 1 4}][Ac]@ZIF-8 showed 56% increase, at 1 bar and for flue gas conditions. Finally, the isosteric heats of adsorption of ZIF-8 and the acetate-based ILs were calculated revealing that all gases are physisorbed by the three materials.