Reactive and Reversible Ionic Liquids for CO2 Capture and Acid Gas Removal

The use of ionic liquids (ILs) for CO2 capture and the removal of acid gases from natural gas and other industrial processes has been one of the foremost research applications for this unique class of non-volatile solvents. However, most of the most broadly studied ILs lack sufficient capacities for CO2 and other acid gases such as H2S, SO2, etc. to be viewed as viable replacements for aqueous amine technologies which have been used industrially for acid gas removal for nearly a century. Furthermore, many of the most well-known ILs are too viscous to be used within conventional process equipment and are likely too costly for use at large scales. As the negligible vapor pressure of ILs is an attractive property for gas separations, it is desirable to find new ILs with improved properties that can be synthesized from lower cost starting materials and/or natural products. Recently, new reactive and reversible IL solvents have emerged in efforts to improve upon the CO2 capacity, physical properties and costs of IL-based gas separation technologies. In this review, we detail the differences between these novel approaches and the standard crop of ILs that have been reported in the literature. The various strategies that have been employed to develop these materials for energy-related separation applications will be examined, with an emphasis on how chemistry and physical properties relate to the demands of efficient chemical process engineering. Where applicable, comparisons to conventional (i.e., aqueous amine) solvents will be made so as provide baselines to commercial technologies. Finally, we introduce the concept of imidazoles and imidazole-amine hybrid solvents as another tunable platform for the removal of CO2, SO2, and H2S.