Ionic Liquid-Impregnated Metal–Organic Frameworks for CO₂/CH₄ Separation

Abstract

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.