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 CO2 separations appears especially promising, with imidazolium-based salts at the center of this research effort. The solubilities of gases, particularly CO2, N2, and CH4, 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 CO2 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 CO2/N2 selectivities that outperform many polymer membranes. Furthermore, the greatest potential of RTILs for CO2 separations might lie in their ability to chemically capture CO2 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 CO2 capture. However, despite all of their promising features, RTILs do have drawbacks to use in CO2 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 CO2-based separations with respect to a variety of materials is thus provided.

Clean energy production has become one of the most prominent global issues of the early 21st century, prompting social, economic, and scientific debates regarding energy usage, energy sources, and sustainable energy strategies. The reduction of greenhouse gas emissions, specifically carbon dioxide (CO(2)), figures prominently in the discussions on the future of global energy policy. Billions of tons of annual CO(2) emissions are the direct result of fossil fuel combustion to generate electricity. Producing clean energy from abundant sources such as coal will require a massive infrastructure and highly efficient capture technologies to curb CO(2) emissions. Current technologies for CO(2) removal from other gases, such as those used in natural gas sweetening, are also capable of capturing CO(2) from power plant emissions. Aqueous amine processes are found in the vast majority of natural gas sweetening operations in the United States. However, conventional aqueous amine processes are highly energy intensive; their implementation for postcombustion CO(2) capture from power plant emissions would drastically cut plant output and efficiency. Membranes, another technology used in natural gas sweetening, have been proposed as an alternative mechanism for CO(2) capture from flue gas. Although membranes offer a potentially less energy-intensive approach, their development and industrial implementation lags far behind that of amine processes. Thus, to minimize the impact of postcombustion CO(2) capture on the economics of energy production, advances are needed in both of these areas. In this Account, we review our recent research devoted to absorptive processes and membranes. Specifically, we have explored the use of room-temperature ionic liquids (RTILs) in absorptive and membrane technologies for CO(2) capture. RTILs present a highly versatile and tunable platform for the development of new processes and materials aimed at the capture of CO(2) from power plant flue gas and in natural gas sweetening. The desirable properties of RTIL solvents, such as negligible vapor pressures, thermal stability, and a large liquid range, make them interesting candidates as new materials in well-known CO(2) capture processes. Here, we focus on the use of RTILs (1) as absorbents, including in combination with amines, and (2) in the design of polymer membranes. RTIL amine solvents have many potential advantages over aqueous amines, and the versatile chemistry of imidazolium-based RTILs also allows for the generation of new types of CO(2)-selective polymer membranes. RTIL and RTIL-based composites can compete with, or improve upon, current technologies. Moreover, owing to our experience in this area, we are developing new imidazolium-based polymer architectures and thermotropic and lyotropic liquid crystals as highly tailorable materials based on and capable of interacting with RTILs.

Room-temperature ionic liquids (RTILs) with polymerizable groups can be readily converted into solid, dense poly(RTILs) for use as gas separation membranes. A series of RTIL monomers with varying length n-alkyl substituents were synthesized and converted into polymer films. These membranes were tested for their performance in separations involving CO 2 , N 2 , and CH 4 . CO 2 permeability was observed to increase in a nonlinear fashion as the n-alkyl substituent was lengthened. CO 2 /N 2 separation performance was relatively unaffected as CO 2 permeability increased. Plotting the performance of these membranes on a "Robeson plot" for CO 2 /N 2 shows that first-generation poly(RTILs) "hug" the "upper bound" of the chart, indicating that they perform as well or better than many other polymers for this separation. The CO 2 /CH 4 separation is less impressive when compared to other polymer membranes on a "Robeson plot", but poly(RTILs) perform as well or better than molten RTILs do in bulk fluid gas absorptions for that gas pair. Furthermore, poly(RTILs) were determined to be able to absorb about twice as much CO 2 as their liquid analogues, an important factor which may give them potential use as gas and vapor sorbents.

Solutions of room-temperature ionic liquids (RTILs) and commercially available amines were found to be effective for the capture of CO2 as carbamate salts. RTIL solutions containing 50 mol % (16% v/v) monoethanolamine (MEA) are capable of rapid and reversible capture of 1 mol of CO2 per 2 moles MEA to give an insoluble MEA−carbamate precipitate that helps to drive the capture reaction (as opposed to aqueous amine systems). Diethanolamine (DEA) can also be used in the same manner for CO2 capture in RTILs containing a pendant hydroxyl group. The captured CO2 in the resulting RTIL−carbamate salt mixtures can be readily released by either heating and/or subjecting them to reduced pressure. Using this unprecedented and industrially attractive mixing approach, the desirable properties of RTILs (i.e., nonvolatility, enhanced CO2 solubility, lower heat capacities) can be combined with the performance of amines for CO2 capture without the use of specially designed, functionalized “task-specific” ionic liquids. By mixing RTILs with commercial amines, reactive solvents with a wide range of amine loading levels can be tailored to capture CO2 in a variety of conditions and processes. These RTIL−amine solutions behave similarly to their water-based counterparts but may offer many advantages, including increased energy efficiency, compared to current aqueous amine technologies.

This study focuses on bulk fluid solubility of carbon dioxide (CO 2 ), methane (CH 4 ), hydrogen (H 2 ), and nitrogen (N 2 ) gases in the imidazolium-based RTILs: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim] and 1,3-dimethylimidazolium methyl sulfate ([mmim][MeSO 4 ]) as a function of temperature (25, 40, 55, and 70 °C) at near-atmospheric pressures. The experimental behaviors are explained in terms of thermodynamic relationships that account for the negligible vapor pressure of the RTIL as well as the low solubilities of the gases. Results show that, as temperature increases, the solubility of CO 2 decreases in all RTILs, the solubility of CH 4 remains constant in [emim][Tf 2 N] and [hmim][Tf 2 N] but increases in [mmim][MeSO 4 ] and [emim][BF 4 ], and the solubility of N 2 and H 2 increases. Also, the ideal solubility selectivity (ratio of pure-component solubilities) increases as temperature decreases for CO 2 /N 2 , CO 2 /CH 4 , and CO 2 /H 2 systems. Experimental values for the enthalpy and entropy of solvation are reported.