Ionic Liquid-Impregnated\nMetal–Organic Frameworks\nfor CO₂/CH₄ Separation

Abstract

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