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 capabilitiesand limitationsof imidazolium-based RTILs in CO2-based separations with respect to a variety of materials is thus provided.
This study focuses on bulk fluid solubility of carbon dioxide (CO2), methane (CH4), hydrogen (H2), and nitrogen (N2) gases in the imidazolium-based RTILs: 1-ethyl-3-methylimidazolium ...bis(trifluoromethylsulfonyl)imide (emimTf2N), 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF4), 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (hmimTf2N), and 1,3-dimethylimidazolium methyl sulfate (mmimMeSO4) 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 CO2 decreases in all RTILs, the solubility of CH4 remains constant in emimTf2N and hmimTf2N but increases in mmimMeSO4 and emimBF4, and the solubility of N2 and H2 increases. Also, the ideal solubility selectivity (ratio of pure-component solubilities) increases as temperature decreases for CO2/N2, CO2/CH4, and CO2/H2 systems. Experimental values for the enthalpy and entropy of solvation are reported.
This study focuses on the solubility behaviors of CO2, CH4, and N2 gases in binary mixtures of imidazolium-based room-temperature ionic liquids (RTILs) using 1-ethyl-3-methylimidazolium ...bis(trifluoromethylsulfonyl)imide (C2mimTf2N) and 1-ethyl-3-methylimidazolium tetrafluoroborate (C2mimBF4) at 40 °C and low pressures (∼1 atm). The mixtures tested were 0, 25, 50, 75, 90, 95, and 100 mol % C2mimBF4 in C2mimTf2N. Results show that regular solution theory (RST) can be used to describe the gas solubility and selectivity behaviors in RTIL mixtures using an average mixture solubility parameter or an average measured mixture molar volume. Interestingly, the solubility selectivity, defined as the ratio of gas mole fractions in the RTIL mixture, of CO2 with N2 or CH4 in pure C2mimBF4 can be enhanced by adding 5 mol % C2mimTf2N.
A series of four imidazolium salts containing increasing lengths of fluoroalkyl substituents was synthesized. The three that exist as molten salts at 296
K were tested for their gas separation ...properties relating to CO
2, O
2, N
2 and CH
4 using a supported ionic liquid membrane (SILM) configuration. These fluoroalkyl-functionalized room-temperature ionic liquids (RTILs) were found to exhibit ideal selectivities for CO
2/N
2 separation that were lower than their alkyl-functionalized analogues, but higher ideal selectivity for CO
2/CH
4 separation. The differences in performance of fluoroalkyl-functionalized RTILs relative to their alkyl-functionalized counterparts are explained through the use of solubility parameters, group contributions and in context of the classically observed deviations of fluorocarbons from “regular” solution behavior.
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 CO2, ...N2, and CH4 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 CO2 solubility but lower solubilities of N2 and CH4. As a consequence, RTILs with oligo(ethylene glycol) substituents were observed to have 30−75% higher ideal solubility selectivities for CO2/N2 and CO2/CH4.
Electrolytic conductivity measurements for a total of ten ionic liquids (ILs) (four N-alkyl-N-methyl pyrrolidinium cations Cnmpyr and six 1-(methyl)benzyl-3-alkyl imidazolium cations (Bnmim or ...MeBnCnmim), all paired with bistriflimide Tf2N anions) are reported at p = 0.1 MPa over T = 293.15 – 323.15 K. All samples were thoroughly dried under vacuum and their water content before and after measurements was determined by Karl Fischer titration. For the conductivity measurements, an impedance bridge technique was used in conjunction with a sealed cell equipped with platinum black electrodes. The cell constant (Kcell = 101 m − 1) was determined with standard aqueous KCl solutions before and after measurements. The results show that electrolytic conductivity increases with increasing temperature in both families of ILs, while conductivity decreases as the length of the IL side chain increases, which is associated with increase in IL viscosity. Conductivities of the pyrrolidinium ILs were consistent with other data for these compounds already reported in the literature. The conductivities of the 1-(methyl)benzyl-3-alkyl imidazolium ILs are reduced by ∼70-90% compared to the analogous 1-alkyl-3-methylimidazolium ILs
Reports of imidazoliumionic liquids (ILs) with at least one benzylic substituent bound to the imidazolium cation are far less common than the ubiquitous 1-alkyl-3-methylimidazolium ILs (C n mimX). ...Yet, there is significant motivation to determine structure–property relationships for imidazolium-based ILs with at least one benzylic substituent. Not only are these ILs just as straightforward to synthesize as C n mimX ILs, but ILs with benzylic substituents are also representative segments of poly(ILs) and ionenes where imidazolium cations with benzylic groups are commonly found. This report focuses on the effects of benzyl and methylbenzyl substituents on the density and viscosity of a series of imidazolium cations paired with bis(trifluoromethyl)sulfonylimide (more commonly known as bistriflimide) (Tf2N−) anions. Furthermore, the solubility of CO2 in 1-benzyl-3-methylimidazolium bistriflimide BnmimTf2N (CAS: 433337-24-7) was measured at 303.15, 318.15, 333.15, and 348.15 K at pressures in the range of 1–9 bar.
Reports of imidazoliumionic liquids (ILs) with at least one benzylic substituent bound to the imidazolium cation are far less common than the ubiquitous 1-alkyl-3-methylimidazolium ILs (CnmimX). ...Furthermore, there is significant motivation to determine structure–property relationships for imidazolium-based ILs with at least one benzylic substituent. Not only are these ILs just as straightforward to synthesize as CnmimX ILs, but ILs with benzylic substituents are also representative segments of poly(ILs) and ionenes where imidazolium cations with benzylic groups are commonly found. This report focuses on the effects of benzyl and methylbenzyl substituents on the density and viscosity of a series of imidazolium cations paired with bis(trifluoromethyl)sulfonylimide (more commonly known as bistriflimide) (Tf2N–) anions. Furthermore, the solubility of CO2 in 1-benzyl-3-methylimidazolium bistriflimide BnmimTf2N (CAS: 433337-24-7) was measured at 303.15, 318.15, 333.15, and 348.15 K at pressures in the range of 1–9 bar.
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