Guide to CO2 Separations in Imidazolium-Based Room-Temperature Ionic Liquids

Abstract

Room-temperature ionic liquids (RTILs) are nonvolatile, tunable solvents that have generated significant interest across a wide variety of engineering applications. The use of RTILs as media for CO 2 separations appears especially promising, with imidazolium-based salts at the center of this research effort. The solubilities of gases, particularly CO 2, N 2, and CH 4, have been studied in a number of RTILs. Process temperature and the chemical structures of the cation and anion have significant impacts on gas solubility and gas pair selectivity. Models based on regular solution theory and group contributions are useful to predict and explain CO 2 solubility and selectivity in imidazolium-based RTILs. In addition to their role as a physical solvent, RTILs might also be used in supported ionic liquid membranes (SILMs) as a highly permeable and selective transport medium. Performance data for SILMs indicates that they exhibit large permeabilities as well as CO 2 /N 2 selectivities that outperform many polymer membranes. Furthermore, the greatest potential of RTILs for CO 2 separations might lie in their ability to chemically capture CO 2 when used in combination with amines. Amines can be tethered to the cation or the anion, or dissolved in RTILs, providing a wide range of chemical solvents for CO 2 capture. However, despite all of their promising features, RTILs do have drawbacks to use in CO 2 separations, which have been overlooked as appropriate comparisons of RTILs to common organic solvents and polymers have not been reported. A thorough summary of the capabilities—and limitations—of imidazolium-based RTILs in CO 2 -based separations with respect to a variety of materials is thus provided.