While molar volume-based models for gas solubility in ionic liquids (ILs) have been proposed, free volume within the IL can be shown to be the underlying property driving gas solubility and selecitivity. Previously published observations as to the distinct differences in solubility trends for gases such as CH4 and N2 relative to CO2 in systematically varied ILs can be attributed to positive and negative effects arising from increasing free volume with increasing alkyl chain length. Through the use of COSMOtherm as a powerful and rapid tool to calculate free volumes in 165 existing and theoretical 1-n-alkyl-3-methylimidazolium ([Cnmim]­[X]) ILs, a previously unreported, yet speculated, critical underlying relationship between gas solubility in ILs is herein described. These results build upon previous assertions that Regular Solution Theory is applicable to imidazolium-based ILs, which appeared to indicate that a global maximum had already been observed for CO2 solubility in imidazolium-based ILs. However, the findings of this computational study suggest that the perceived maximum in CO2 solubility might be exceeded through rational design of ILs. We observe that although Henry’s constants in ILs are dependent on the inverse of molar volume and free volume, the volume-normalized solubility of CH4 and N2 are proportional to free volume, while CO2 is inversely proportional to the square root of free volume. Our free volume model is compared to experimental data for CO2/CH4 and CO2/N2 selectivity, and a nearly identical plot of selectivity relative to IL molar volume can be generated from the computational method alone. The overall implication is that large, highly delocalized anions paired with imidazolium cations that have minimally sized alkyl chains may hold the key to achieving greater CO2 solubility and selectivity in ILs.

Density, viscosity, and CO2 solubility of a series of 10 1-n-alkylimidazoles with chain lengths ranging from methyl (C1) to tetradecyl (C14) were characterized. Density and viscosity values were comparable to many common organic solvents over a temperature range of 20–80 °C. The measured data were utilized to develop empirical models for these physical properties with respect to temperature and the contribution of the n-alkyl chain. Solubility of CO2 in 1-n-alkylimidazoles at ambient temperature (25 °C) and low pressures (3–7 atm) was found to be less than most common organic solvents, even though the 1-n-alkylimidazole motif offers opportunities to tune the molecule’s solubility parameter. This effect was attributed to lower fractional free volume (FFV) that is available in molecules with systematically variable side chains. 1-n-Alkylimidazoles were less dense than most 1-n-alkyl-3-methylimidazolium-based ionic liquids (ILs), with differences ranging from 10 to 50%, based on contributions to increased density from the anion. However, much larger differences in viscosities were observed between the two classes of compounds, with ILs at least an order of magnitude more viscous than their neutral 1-n-alkylimidazole counterparts. Solubility levels of CO2 were similar in both types of solvents, indicating that no advantage (or disadvantage) in CO2 uptake is achieved by transforming a neutral 1-n-alkylimidazole to a charged IL solvent. While CO2 solubility levels in each are insufficient to provide a viable solvent for postcombustion CO2 capture applications, 1-n-alkylimidazoles can be used in combination with amines as a low-volatility, high-capacity solvent to capture CO2, using only inexpensive and readily available components. The viscosity of a highly CO2-rich liquid phase formed between monoethanolamine (MEA) and 1-butylimidazole was found to be 85–100 cP at 298 K, which is less viscous than many neat ILs. Initial data indicate 1-butylimidazole has a synergistic effect on CO2 capture as the solution can easily exceed the stoichiometric limitations of 2 mol of MEA/(1 mol of CO2). Thus, while certainly more volatile than IL-based analogues, 1-n-alkylimidazole solvents may offer some unique capabilities and advantages in CO2 capture processes.

Polyimides and ionic liquids (ILs) are two classes of materials that have been widely studied as gas separation membranes, each demonstrating respective advantages and limitations. Both polyimides and ILs are amenable to modification/functionalization based on selection of the requisite precursors. However, there have been but a handful of reports considering how polyimides and ILs could be integrated to obtain fundamentally new materials with synergistic properties. In this manuscript, we demonstrate a new and versatile way to synthesize polyimides with imidazolium cations directly located within the polymer backbone to form polyimide−ionene hybrids, or "ionic polyimides". Our strategy for synthesizing ionic polyimides does not require the use of amino-functionalized ILs. Instead, the imidization reaction occurs prior to polymerization in the formation of an imidazole-functionalized diimide monomer. This monomer is then reacted via step-growth (condensation) polymerization with p-dichloroxylene via Menshutkin reactions, simultaneously linking the monomers and creating the ionic components. The resultant ionic polyimide is amenable to thermal processing (e.g., extrusion, melt-pressing) and capable of forming thin films. Upon soaking thin films of the ionic polyimide in a widely used IL, 1butyl-3-methylimidazolium bistriflimide ([C 4 mim][Tf 2 N]), a stoichiometric absorption of the IL into the ionic polyimide was observed, forming an ionic polyimide + IL composite. The gas separation performances of ionic polyimide and ionic polyimide + IL composite membranes were studied with respect to CO 2 , N 2 , CH 4 , and H 2 . The neat ionic polyimide exhibits low permeability to CO 2 and H 2 (∼0.9 and ∼1.6 barrers, respectively) and very low permeability to N 2 and CH 4 (∼0.03 barrers for both). For the ionic polyimide + IL composite, the permeabilities of CO 2 , N 2 , and CH 4 increase by 1800−2700%, while H 2 permeability only increased by ∼200%. The large increases in permeability for CO 2 , N 2 , and CH 4 are due to greatly increased gas diffusivity through the material, with gas solubility essentially unchanged with the IL present. The ionic polyimide and ionic polyimide + IL composite were characterized using a number of techniques. Most interestingly, X-ray diffractometry (XRD) of the films reveals that the ionic polyimide + IL composite displays a sharp peak, indicating that the ionic polyimide may experience supramolecular assembly around the IL. Although the performances of these first ionic polyimide and ionic polyimide + IL composite membranes fall short of Robeson's Upper Bounds, this work provides a strong foundation on which ionic polyimide materials with more sophisticated structural elements can be developed to understand the structure−property relationships underlying the ionic polyimide platform and ultimately produce high-performance gas separation membranes.

1-n-Alkylimidazoles are a class of tunable solvents with low volatility and low viscosities. Although imidazoles have been known for some time in the pharmaceutical industry, and as convenient precursors for synthesizing imidazolium-based ionic liquids (ILs), only recently have they been given consideration in some of the same solvent-based separations applications that ILs have been studied for, such as post-combustion CO 2 capture and natural gas treating. "Sweetening", the removal of CO 2 , H 2 S, and other "acid" gases from natural gas (CH 4 ), is an existing industrial application where low volatility, low viscosity physical solvents are already applied successfully and economically at large scale. Physical solvents are also used for syngas cleanup and in the emerging application of pre-combustion CO 2 capture. Given the similarities in physical properties between 1-n-alkylimidazoles, and physical solvents currently used in industrial gas treating, the 1-n-alkylimidazole class of solvents warrants further investigation. Solubilities of CO 2 and CH 4 in a series of 1-n-alkylimidazoles were measured under conditions relevant to the use of physical solvents for natural gas treating: ∼5 atm partial pressure of CO 2 and temperatures of 30À75 °C. Solubilities of CO 2 and CH 4 were found to be strongly dependent on temperature, with the solubility of each gas in all solvents diminishing with increasing temperature, although CO 2 exhibited a stronger temperature dependence than CH 4 . Ideal CO 2 /CH 4 solubility selectivities were also more favorable at lower temperatures in 1-n-alkylimidazole solvents with shorter chain lengths. CO 2 solubility decreased with increasing chain length, while CH 4 solubility exhibited a maximum in 1-hexylimidazole. The solubility trends observed with temperature and chain length can be explained through calculation of solution enthalpies and solvent fractional free volume as approximated from van der Waals volumes as calculated via atomic contributions. Of the solvents examined, 1-methylimidazole displays the most favorable CO 2 solubility and CO 2 /CH 4 selectivity, and has the lowest viscosity. A comparison of 1-methylimidazole to commercially used solvents reveals similar physical properties and the potential for use in industrial gas processing. Imidazolium-based ILs are also compared, although they appear less favorable for use within established process schemes given their higher viscosities and reduced capacity for CO 2 .

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