A large set of distinct ionic liquid (IL)-impregnated metal–organic framework (MOF) composites were produced by a direct-contact method to study their performance as sorbents for gas separation applications. The IL anion/cation impact on the sorption capacity and ideal CO2/CH4 selectivity were fully detailed. A reproducible methodology and rigorous characterization were defined to evaluate the IL impact on the IL@ZIF-8 performance. Results show that the IL impregnation was successful, the ZIF-8 structure is conserved after IL incorporation, and the microporous composites are thermally stable at the working temperatures. CO2 and CH4 adsorption–desorption equilibria in the composites were measured at the temperature of 303 K and up to 16 bar of pressure. The respective data were then compared with that obtained for pristine ZIF-8. At high pressure, all composites show reversible, although inferior, gas uptake (total pore volume loss due to IL pore occupation/blockage). At low pressure, because of synergistic effects arising from IL–MOF interactions, one composite displays superior CO2 uptake compared to ZIF-8. Four IL@ZIF-8 composites show distinct low-pressure trends from ZIF-8, due to their IL structure/size, with an increase in the selectivity that can be above 40% at 0.5 bar. An IL-free basis analysis was also assessed considering a normalization of the gas uptake per gram of ZIF-8 in the composites. This shows that ILs do have an impact on the adsorption capacity of the composites. A new approach, based on the materials’ pore volume as a key factor, is discussed toward the sorption data of the IL@ZIF-8 composites. Through mapping of the composites data, it is possible to understand the effect of the IL for high- and low-pressure applications. The results obtained herein indicate that IL@MOF composites are potential alternative materials for low-pressure gas separation.

No abstract

Global warming is arguably the biggest scientific challenge of the twenty-first century and its environmental consequences are already noticeable. To mitigate the emissions of greenhouse gases, particularly of CO 2 , there is an urgent need to design materials with improved adsorbent properties. Five different magnetic ionic liquids were impregnated into the metal–organic framework ZIF-8. The composites were produced by a direct-contact method, and their performance as sorbents for gas separation applications was studied. The impact of the ionic liquid anion on the sorption capacity and ideal CO 2 /CH 4 and CO 2 /N 2 selectivities were studied, focusing on understanding the influence of metal atom and ligand on the adsorbent properties. Reproducible methodology, along with rigorous characterization, were established to assess the impact of the ionic liquid on the performance of the composite materials. Results show that the ionic liquid was well-impregnated, and the ZIF-8 structure was maintained after ionic liquid impregnation. The produced composites were of microporous nature and were thermally stable. CO 2 , CH 4 , and N 2 adsorption–desorption isotherms were obtained at 303 K and between 0 and 16 bar. The adsorption-desorption data of the composites were compared with that obtained for original ZIF-8. The general trend in composites is that the increased gas uptake per available pore volume compensates the pore volume loss. Adsorption data per unit mass showed that composites have reversible sorption, but inferior gas uptake at all pressure ranges. This is due to the observed total pore volume loss by the ionic liquid pore occupation/blockage. In most cases, composites showed superior selectivity performance at all pressure range. In particular, the composite [C 4 MIM] 2 [MnCl 4 ]@ZIF-8 shows a different low-pressure selectivity trend from the original MOF, with a 33% increase in the CO 2 /N 2 selectivity at 1 bar and 19% increase in the CO 2 /CH 4 selectivity at 10 bar. This material shows potential for use in a post-combustion CO 2 capture application that can contribute to greenhouse gas mitigation.