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.

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.

Novel imidazolium-based room-temperature ionic liquids (RTILs) with one, two, or three oligo(ethylene glycol) substituents were synthesized. Solubilities and ideal solubility selectivities of CO 2 , N 2 , and CH 4 at low pressure (1 atm) in these RTILs were determined using a pressure decay technique. Comparison to corresponding alkyl analogues of these RTILs reveals similar levels of CO 2 solubility but lower solubilities of N 2 and CH 4 . As a consequence, RTILs with oligo(ethylene glycol) substituents were observed to have 30-75% higher ideal solubility selectivities for CO 2 /N 2 and CO 2 /CH 4 .

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In this work, tuning the solubility parameter of room-temperature ionic liquids (RTILs) with appended functional groups was explored using a combination of experiment and theory. By predictably altering the solubility parameters of several RTIL solvents, their gas solubility and separation performance were tailored. This concept was demonstrated by synthesizing and characterizing imidazolium-based RTILs that incorporate nitrile and alkyne functional substituents. The ideal solubility and selectivity values of CO 2 , N 2 , and CH 4 at near ambient temperature and pressure were measured for these RTILs. These functionalized RTIL solvents exhibited lower CO 2 , N 2 , and CH 4 solubility values but improved CO 2 /N 2 and CO 2 /CH 4 solubility selectivity when compared to analogous nonfunctionalized, n-alkyl-substituted RTILs. A group contribution method was used to predict the solubility parameters of the functionalized RTILs, and these values were used with regular solution theory to predict the solubility and selectivity of the three gases. These predicted gas solubility values were found to be in good agreement with those measured experimentally. Furthermore, the predictions from the group contribution method indicated that inclusion of the nitrile and alkyne functional groups increased the solubility parameter relative to the analogous, n-alkyl-substituted RTILs. These initial results show that the group contribution method offers a valuable guide for systematically designing functionalized RTILs with specific gas solubility and selectivity performance.

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