J. Phys. Chem. B 2008, 112, 2335-2339
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Ideal Gas Solubilities and Solubility Selectivities in a Binary Mixture of Room-Temperature Ionic Liquids Alexia Finotello, Jason E. Bara, Suguna Narayan, Dean Camper, and Richard D. Noble* Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309-0424 ReceiVed: July 16, 2007; In Final Form: NoVember 1, 2007
This study focuses on the solubility behaviors of CO2, CH4, and N2 gases in binary mixtures of imidazoliumbased room-temperature ionic liquids (RTILs) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][Tf2N]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]) at 40 °C and low pressures (∼1 atm). The mixtures tested were 0, 25, 50, 75, 90, 95, and 100 mol % [C2mim][BF4] in [C2mim][Tf2N]. 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 [C2mim][BF4] can be enhanced by adding 5 mol % [C2mim][Tf2N].
1. Introduction Room-temperature ionic liquids (RTILs) are “green” materials with great potential to replace the volatile organic solvents used throughout industrial and laboratory settings.1-4 RTILs possess obvious advantages over traditional solvents when considering user safety and environmental impact. Under many conditions, RTILs have negligible vapor pressures, are largely inflammable, and exhibit thermal and chemical stability.5,6 However, it is the ability to tailor the chemistry and properties of an RTIL solvent in a variety of ways that may prove to be their most useful feature. RTILs can be synthesized as custom or “task-specific” solvents with functional groups that enhance physical properties, provide improved interaction with solutes, or are themselves chemically reactive.7-9 Multiple points are available for tailoring within the imidazolium platform, presenting a seemingly infinite number of opportunities to design RTIL solvents matched to individual solutes of interest. Furthermore, many imidazoliumbased RTILs are miscible with one another or with other solvents; thus, mixtures of RTILs serve to multiply the possibilities for creating the ideal solvent for any particular application. Separations involving liquids or gases are just one area where the design of selective RTIL solvents is of great utility and interest.10-13 Improved and highly efficient separations involving “light” gases (CO2, O2, N2, CH4, H2, and hydrocarbons) are of critical importance as fuel use, demand, and costs rise. Research into gas separation and storage with RTILs is growing in academia and industry.10,11,14-18 RTIL solvents are under investigation to displace materials used in energy-intensive technologies, such as amine scrubbing, for the capture of “acid” gases (CO2, H2S, SO2, etc.). The presence of acid gases in many natural gas fields around the world negatively impacts the quality and viability of those sources.19 RTILs show promising separation properties for these gases, but more research is needed to make them feasible candidates to replace current materials for these types of applications.16,20,21 * To whom correspondence should be addressed. E-mail: nobler@ colorado.edu.
Our group has previously obtained gas solubility data for a wide variety of common RTILs as well as several novel RTILs synthesized in our labs.10,14,15,22 We employed regular solution theory (RST) to model the experimentally determined solubility and ideal solubility selectivity of CO2, N2, and CH4 in a variety of 1-n-alkyl-3-methylimidazolium RTILs.10,15,23,24 RST models indicate an exponential increase in CO2/N2, and CO2/CH4 ideal solubility selectivity should occur as the molar volume of the RTIL falls below 150 cm3/mol.10 However, it does not appear that imidazolium-based RTILs with molar volumes below this value can be synthesized. Among this subset of RTILs, [C2mim][BF4] possesses the smallest molar volume (155 cm3/ mol) and thus represents the lower limit for which previous RST models for gas solubility and solubility selectivity can be applied. [C2mim][BF4] has received little attention in the gas separation literature for RTILs, even though it can be produced in a straightforward method and in large quantities.25,26 Furthermore, our prior work with [C2mim][BF4] and [C1mim][MeSO4] has illustrated that RST models do not accurately predict the solubility of CO2 and behavior of CO2-based separations near this minimum.25 CO2 solubility in these RTILs is lower than RST predicts and is the lowest for any imidazolium-based RTIL reported thus far. Hence, more information is needed to better define the ranges of molar volumes where RST models are most accurate for imidazolium-based RTILs. The solubility behavior of light gases (with perhaps the exception of CO2, which has been studied in detail) in RTILs with molar volumes between 155 and 261 cm3/mol is not completely defined.10 This molar volume range encompasses the apparent minimum and maximum CO2 solubility (end points) in imidazolium-based RTILs, respectively, observed in the solvents [C2mim][BF4] and [C2mim][Tf2N].10,25 The use of mixtures allows for exploration of CO2 solubility as a function of molar volumes from minimum to maximum solubility. There are few choices of imidazolium-based RTILs available within this molar volume range that can be easily and cost-effectively synthesized. Synthesis of a family of [C2mim][X] salts where X is a variety of small, molecular anions would be required to thoroughly characterize gas solubility and solubility selectivity based on
10.1021/jp075572l CCC: $40.75 © 2008 American Chemical Society Published on Web 02/02/2008
2336 J. Phys. Chem. B, Vol. 112, No. 8, 2008 molar volume in this range. Such a task would require a large number of chemical reagents, various purification techniques, and a great deal of time. However, as many imidazolium-based RTILs are miscible with one another, this range of molar volumes can be explored through the use of binary mixtures. This method provides a controllable approach to exploring this range of molar volumes and requires the synthesis of only the two RTILs at the endpoints, [C2mim][BF4] and [C2mim][Tf2N]. A systematic study of gas solubility and solubility selectivity in binary mixtures of RTILs has not yet been reported, though there has been interest in using mixtures to lower the viscosities of RTILs with perfluoroalkyl components.17 The use of mixed RTIL solvents not only allows for control over molar volume but also presents the opportunity to tune the Hildebrand solubility parameter (δi) of the mixture.27 The squared solubility parameter of the mixture and molar volume (V14/3) are inversely proportional to each other (details described in section 3.3.1), and linearly related to gas solubility (natural log of the Henry’s constant).10 Matching the solubility parameter of the solvent (RTIL mixture) to that of the gas of interest (CO2) will ideally result in maximum gas solubility and high solubility selectivity.27 Conversely, the larger the difference between the solubility parameter of the solvent to that of the other gases (N2 and CH4 from which CO2 separation is desired) will result in a lower solubility for N2 and CH4 in the RTILs and a higher CO2 solubility selectivity. Commonly used imidazolium-based RTILs appear to all lie above the value of the Hildebrand solubility parameter for CO2 (δi ) 21.8 MPa1/2) with reported Hildebrand solubility parameters between 25 and 30 MPa1/2.27,28 Thus, these RTILs will have even better performance as selective solvents for gases or vapors with solubility parameters in this range. While the performance of current pure RTILs as bulk fluids for CO2-based separations are well described by RST, understanding the properties of binary mixtures of RTILs will yield insight into the behavior of these solutions in separation applications. Our recent work focuses on the ideal solubility and solubility selectivity of several light gases, CO2, N2, and CH4, in mixtures of pure RTILs, [C2mim][BF4], and [C2mim][Tf2N], comprising a range of molar volumes, at 40 °C. The solubility of a single gas in each RTIL is expressed as a Henry’s constant (H (atm)). The ideal solubility selectivity of the gas pairs in the RTIL mixtures was calculated to determine if mixtures can be used to enhance ideal gas separation performance. The experimental results show that RST can be used to describe the gas solubility behaviors in RTILs mixtures using either an average mixture solubility parameter or a measured mixture molar volume. Interestingly, the solubility selectivity of CO2 with N2 or CH4 in [C2mim][BF4] can be enhanced by adding 5 mol % [C2mim][Tf2N] to the liquid phase. 2. Experimental Methods 2.1. Materials. All gases were of at least 99.99% purity and purchased from AirGas. 1-Ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide ([C2mim][Tf2N]) were synthesized in our laboratory. Physical constants of the RTILs (pure and mixtures) are shown in Table 1. The densities of [C2mim][BF4] and [C2mim] [Tf2N] were measured and agree with previously published values.29-31 The average densities of the RTIL mixtures were measured in our lab. These RTILs were chosen because they are readily miscible in each other when mixed, and represent a range of molar volumes. Additionally, experimental observations and RST have shown that all gases
Finotello et al. TABLE 1: Physical Properties of Room-Temperature Ionic Liquids Used in This Study ionic liquid
mol. weight (g/mol)
density (g/cm3)
molar volume (cm3/mol)
[C2mim][Tf2N] 25 mol % [C2mim][BF4] 50 mol % [C2mim][BF4] 75 mol % [C2mim][BF4] 90 mol % [C2mim][BF4] 95 mol % [C2mim][BF4] [C2mim][BF4]
391 343 295 246 217 208 198
1.50a 1.52 1.48 1.42 1.35 1.30 1.28b
261 226 199 174 161 159 155
a
Reference 31.31
b
Reference 30.30
TABLE 2: Gas Solubility Trends in RTIL Mixtures CO2
N2
CH4
ionic liquid
H (atm)
H (atm)
H (atm)
[C2mim][Tf2N] 25 mol %[C2mim][BF4] 50 mol %[C2mim][BF4] 75 mol %[C2mim][BF4] 90 mol %[C2mim][BF4] 95 mol %[C2mim][BF4] [C2mim][BF4]
50 ( 1 58 ( 3 65 ( 1 85 ( 5 91 ( 1 94 ( 1 100 ( 2
1200 ( 60 1700 ( 60 2400 ( 100 4000 ( 600 4500 ( 350 5000 ( 300 3800 ( 100
560 ( 10 740 ( 10 980 ( 20 1600 ( 20 1800 ( 60 1900 ( 20 2000 ( 200
of interest have higher solubility in [C2mim][Tf2N] and lower solubility in [C2mim][BF4]. However, the solubility selectivity for CO2 with respect to N2 and CH4 is higher in [C2mim][BF4] than in [C2mim][Tf2N].25 These experiments will examine how the combination of the two RTILs properties affect gas solubility behaviors and how to extend RST to describe these behaviors in RTIL mixtures. 2.2. Synthesis. All chemicals were used as purchased from Sigma-Aldrich. Synthesis details for [C2mim][BF4] and [C2mim][Tf2N] can be found in previous works from our group and others.25,32 2.3. Solubility Apparatus and Measurement. A diagram and detailed information about the pressure decay solubility apparatus and experimental procedure have been previously reported.23,25,33 The temperature was held constant at 40 oC. The major source of error in the calculations is due to the uncertainty of the pressure gauge (Omega PX303-015, 0.25% error). All data shown in the tables and figures are the average measurement from numerous trials. The error bars illustrate one standard deviation obtained from at least three replicates. 3. Results and Discussion 3.1. Data Analysis. To determine if the gas-liquid system equilibrium had been reached, the pressure in the cell volume was plotted as a function of time (one measurement per min). After 30 min of constant pressure readings, it was assumed that equilibrium had been reached. All trials displayed similar pressure change behaviors. For each trial, the Henry’s constant was determined from the ideal gas law using the difference between Pt)0 and Pequil at each temperature.15,23,24 3.2. Solubility Trends in RTIL Mixtures. Table 2 shows the experimental Henry’s constants for each gas/RTIL mixture combination. The Henry’s constant for CO2 and CH4 increases with increasing [C2mim][BF4] content. The Henry’s constant for N2 increases with increasing [C2mim][BF4] content, except in pure [C2mim][BF4], where the Henry’s constant decreased. The decrease in Henry’s constant for N2 in pure [C2mim][BF4] is not completely understood. 3.3. Regular Solution Theory. 3.3.1. Extension of Regular Solution Theory to RTIL Mixtures. RST dictates that for lowpressure systems, where Henry’s law is applicable, gas solubili-
Binary Mixture of Room-Temperature Ionic Liquids
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ties (Henry’s constant, H1) can be described by solubility parameters using eq 1 for both the solute and the pure solvent (1 ) RTIL, 2 ) gas) and where a and b are empirically determined constants (depending on gas being used and temperature).34
ln[H2,1] ) a + b(δ1 - δ2)2
(1)
The solubility parameter (δ1) for pure imidazolium-based RTILs can be estimated using the Kapustinskii equation for lattice energy density and the definition of a solubility parameter. This substitution results in a solubility parameter that is a function of pure RTIL molar volume (eq 2).10
δ1 ∝
( ) 1
1/2
4/3
V1
(2)
Specifically for mixtures, RST states that a volume fraction averaged solubility parameter (δ h 1), and related volume fraction averaged molar volume (V h 1) for the solvent be used in theoretical calculations (eqs 3 and 4), where is φi the volume fraction and Vi of each pure solvent.35
δ h1 )
∑i φiδi
(3)
V h1 )
∑i φiVi
(4)
By combining eqs 1 and 2, the RST model results in eqs 5 and 6, where R and β or β* are empirically determined constants that are dependent on the temperature and gas being tested.10,35
h 1)2 ln[H2,1] ) R + β(δ
(5)
( )
(6)
ln[H2,1] ) R +
β* V h 14/3
Previous work has shown that lower molar volumes tend to have higher ideal solubility selectivities for CO2. However, the theory is less accurate in the low molar volume range.10,25 3.3.2. Discussion of Experimental Results. To determine if the mixtures can be described by RST, a plot is made of the Henry’s constant versus the volume fraction average molar volume of the RTIL mixtures, as dictated by RST (eqs 3 and 4). However, use of the volume fraction average mixture molar volume did not result in a quality linear fit for the RST model, which indicates that RST is not a perfect model. This is due to the physical volume change that results from mixing the two RTILs. The measured mixture molar volume is not the same as the volume fraction average mixture molar volume (2-6% difference between the measured and calculated values). The difference in the mixture molar volumes indicates that RST is not a robust model; however, using the measured mixture molar volume (empirical data) and the RST equations allows for the investigation of gas solubility trends in RTILs. Therefore, the average measured mixture molar volume is used in the following plots because it allows for a more accurate description of the experimentally observed behaviors while using the RST model. For the case of an unknown mixture molar volume, however, it would still be possible to use the volume fraction average mixture molar volume (as stated in the theory) from the known pure component molar volume to get an initial estimate for the gas solubility behavior being investigated. While RST is not exact, it can be used to obtain initial predictions for gas solubilities in new RTILs.
Figure 1. Average natural log of the Henry’s constant versus average measured mixture molar volume to the -4/3 power at 40 °C. The lines represent the RST models (eq 6) for each gas.
TABLE 3: Gas Loading at 1 atm and 40 °C for Various Pure RTILs10,15,33,38 gas loading at 1 atm (mol gas/L RTIL)
ionic liquid
molar volume (cm3/mol)
CO2
N2
CH4
[C1mim][MeSO4] [C2mim][dca] [C2mim][CF3SO3] [C4mim][BF4] [C4mim][PF6] [C4mim][Tf2N] [C6mim][Tf2N] [C10mim][Tf2N]
157 167 188 189 211 293 313 382
0.037 0.063 0.076 0.073 0.078 0.082 0.076 0.078
1.1E-03 1.2E-03 2.1E-03
2.1E-03 3.0E-03 4.4E-03
3.9E-03
9.3E-03
Figure 1 shows a linear trend for the natural log of the Henry’s constants for each gas with respect to average measured mixture molar volume at 40 °C. All data shown, including mixtures and pure components, are within the 95% confidence intervals (not shown) of the theoretical line. RST is thus valid for the gas/ RTIL mixtures combinations that were investigated. Since RST is valid for these systems, it was expected that lower mixture molar volumes would result in the higher solubility selectivity as shown in Figure 2.10 The mixture solubility selectivity agrees with the theoretical line, indicating that RST can be used to describe the behavior of RTIL mixtures using measured molar volumes. All data shown are within the 95% confidence intervals (not shown) of the model. The pure [C2mim][BF4] solubility selectivity for both CO2/N2 and CO2/CH4 does not as closely agree (as compared with the other mixtures and [C2mim][Tf2N]) with the theoretical prediction, whereas the 90 and 95 mol % [C2mim][BF4] mixtures, at the lower molar volume range of this study, possessed the higher solubility selectivity closer to the RST prediction. Huang et al. has shown that [C2mim][BF4] has significant hydrogen bonding between the cation and anion below 335 K which is why RST may not as accurately describe the behavior of this pure RTIL.36 Addition of a small amount of [C2mim][Tf2N] to [C2mim][BF4] results in an improved solubility selectivity behavior closer to the theoretical prediction due to what may be disruption of the hydrogen bonding in pure [C2mim][BF4].36 For each gas, the gas loading at 1 atm, or mole fraction of gas dissolved in the RTIL that is in equilibrium with vapor phase, was also examined. Figures 3 a-c show the results for each gas. These plots use the theoretical parameters that were used in our previous work to show that the pure component
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Finotello et al.
Figure 2. Solubility selectivity versus average measured molar volume of the IL at 40 °C: (a) CO2 with N2 and (b) CO2 with CH4. The lines represent the RST model prediction.
Figure 3. Gas loading at 1 atm and 40 °C as a function of molar volume for (a) CO2, (b) N2, and (c) CH4. The lines represent the RST models for each gas developed from pure RTIL solubility data.10,15,33,38
theory could be extended to describe the mixture data.10 The pure component data for CO2 includes the following RTILs: 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C4mim][Tf2N]), 1,3-dimethylimidazolium methylsulfate ([C1mim][MeSO4]), 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide ([C6mim][Tf2N]), 1-ethyl3-methylimidazolium trifluoromethanesulfonate ([C2mim][CF3SO3]), 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][dca]), 1-decyl-3-methylimidazolium trifluoromethanesulfonate ([C10mim][Tf2N]), and replicate data (from different experiments) of [C2mim][BF4] and [C2mim][Tf2N].10,23,25,37 The pure
component data for N2 and CH4 includes the following RTILs: 1,3-dimethylimidazolium methylsulfate ([C1mim][MeSO4]), 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide ([C6mim][Tf2N]), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([C2mim][CF3SO3]), 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][dca]), and replicate data (from different experiments) of [C2mim][BF4] and [C2mim][Tf2N].10,25,33 A summary of the pure component data is shown in Table 3. All mixture data points agree well (within the 95% confidence intervals) with the theoretical predictions for pure RTILs, and each gas exhibits a maximum gas loading at different molar volumes. [C2mim][Tf2N] exhibits the highest experimentally observed gas solubility in this study and is consistent with
Binary Mixture of Room-Temperature Ionic Liquids previously published data.10 The observed trends again validate the extension of RST to RTIL mixtures. 4. Conclusions The experimental results indicate that the behavior of gases in RTIL mixtures at constant temperature and low pressure obey RST. Solubility selectivity for CO2 with N2 and CH4 is higher in the 90 and 95 mol % mixtures of [C2mim][BF4] in [C2mim][Tf2N] than in both pure components or the other mixtures studied. These two mixtures represent the RTIL mixtures with the smaller molar volumes in this study, and the solubility selectivity is higher than in pure [C2mim][BF4], which has an even lower molar volume. These data confirm that RST is valid for RTIL mixtures using the average measured molar volume of the mixture. The results show that RTIL mixtures can be used to enhance CO2 solubility selectivity due to the control over RTIL molar volume. CO2 was the most soluble gas in all RTIL mixtures tested. Each gas exhibited a maximum gas loading at 1 atm at a different molar volume, again consistent with the predictive theory. Acknowledgment. The authors would like to thank the following for supporting this research: the National Science Foundation Grant No. EEC 0437144, the NASA Graduate Student Research Program, and the Membrane Applied Science and Technology Center. J.E.B. also acknowledges the U.S. Department of Education GAANN Fellowship for financial support. Thanks to Professor Douglas Gin for providing laboratory space and chemicals for the synthesis of RTILs. We would also like to acknowledge Sonja Lessman for NMR acquisition and analysis. References and Notes (1) Imperato, G.; Konig, B.; Chiappe, C. Ionic green solvents from renewable resources. Eur. J. Org. Chem. 2007, 1049-1058. (2) Anjan, S. T. Ionic liquids for aromatic extraction: Are they ready? Chem. Eng. Prog. 2006, 102, 30-39. (3) Zhao, H.; Xia, S. Q.; Ma, P. S. Use of ionic liquids as ‘green’ solvents for extractions. J. Chem. Technol. Biotechnol. 2005, 80, 10891096. (4) Marsh, K. N.; Boxall, J. A.; Lichtenthaler, R. Room temperature ionic liquids and their mixtures-a review. Fluid Phase Equilib. 2004, 219, 93-98. (5) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N. et al. The distillation and volatility of ionic liquids. Nature 2006, 439, 831-834. (6) Smiglak, M.; Reichert, W. M.; Holbrey, J. D.; Wilkes, J. S.; Sun, L. Y. et al. Combustible ionic liquids by design: is laboratory safety another ionic liquid myth? Chem. Commun. 2006, 2554-2556. (7) Baltazar, Q. Q.; Chandawall, J.; Sawyer, K.; Anderson, J. L. Interfaicial and micellar properties of imidazolium-based monocationic and dicationic ionic liquids. Colloids Surf., A 2007, 124, 926-927. (8) Lombardo, M.; Pasi, F.; Trombini, C.; Seddon, K. R.; Pitner, W. R. Task-specific ionic liquids as reaction media for the cobalt-catalysed cyclotrimerisation reaction of arylethynes. Green Chem. 2007, 9, 321322. (9) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H. CO2 capture by a task-specific ionic liquid. J. Am. Chem. Soc. 2002, 124, 926-927. (10) Camper, D.; Bara, J. E.; Koval, C.; Noble, R. D. Bulk-Fluid, Solubility and Membrane Feasibility of Rmim-based, Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2006, 45, 6279-6283. (11) Tang, J. B.; Sun, W. L.; Tang, H. D.; Radosz, M.; Shen, Y. Q. Enhanced CO2 absorption of poly(ionic liquid)s. Macromolecules 2005, 38, 2037-2039. (12) Arce, A.; Earle, M. J.; Rodriguez, H.; Seddon, K. R. Separation of benzene and hexene by solvent extraction with 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ionic liquids: effect of the alkylsubstituent length. J. Phys. Chem. B 2007, 111, 4732-4736. (13) Arce, A.; Earle, M. J.; Rodriguez, H.; Seddon, K. R. Separation of aromatic hydrocarbons from alkanes using the ionic liquid 1-ethyl-3methylimidazolium bis{(trifluoromethyl) sulfonyl} amide. Green Chem. 2007, 9, 70-74.
J. Phys. Chem. B, Vol. 112, No. 8, 2008 2339 (14) Scovazzo, P.; Kieft, J.; Finan, D. A.; Koval, C.; DuBois, D. et al. Gas separations using non-hexafluorophosphate PF6 (-) anion supported ionic liquid membranes. J. Membr. Sci. 2004, 238, 57-63. (15) Camper, D.; Scovazzo, P.; Koval, C.; Noble, R. Gas solubilities in room-temperature ionic liquids. Ind. Eng. Chem. Res. 2004, 43, 30493054. (16) Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; et al. Why is CO2 so soluble in imidazolium-based ionic liquids? J. Am. Chem. Soc. 2004, 126, 5300-5308. (17) Baltus, R. E.; Culbertson, B. H.; Dai, S.; Luo, H. M.; DePaoli, D. W. Low-pressure solubility of carbon dioxide in room-temperature ionic liquids measured with a quartz crystal microbalance. J. Phys. Chem. B 2004, 108, 721-727. (18) Holbrey, J. D.; Plechkova, N. V.; Seddon, K. R. Recalling COIL. Green Chem. 2006, 8, 411-414. (19) Hao, J.; Rice, P. A.; Stem, S. A. Upgrading low-quality natural gas with H2S- and CO2-selective polymer membranes Part I. Process design and economics of membrane stages without recycle streams. J. Membr. Sci. 2002, 209, 177-206. (20) Wu, W. Z.; Han, B. X.; Gao, H. X.; Liu, Z. M.; Jiang, T.; et al. Desulfurization of flue gas: SO2 absorption by an ionic liquid. Angew. Chem., Int. Ed. 2004, 43, 2415-2417. (21) Lee, S. H.; Kim, B. S.; Lee, E. W.; Park, Y. I.; Lee, J. M. The removal of acid gases from crude natural gas by using novel supported liquid membranes. Desalination 2006, 200, 21-22. (22) Bara, J. E.; Gabriel, C. J.; Lessmann, S.; Carlisle, T. K.; Finotello, A.; Gin, D. G.; Noble, R. D. Enhanced CO2 separation selectivity in oligo(ethylene glycol) functionalized room-temperature ionic liquids. Ind. Eng. Chem. Res. 2007, 46, 5380. (23) Camper, D.; Becker, C.; Koval, C.; Noble, R. Low pressure hydrocarbon solubility in room temperature ionic liquids containing imidazolium rings interpreted using regular solution theory. Ind. Eng. Chem. Res. 2005, 44, 1928-1933. (24) Scovazzo, P.; Camper, D.; Kieft, J.; Poshusta, J.; Koval, C. et al. Regular solution theory and CO2 gas solubility in room-temperature ionic liquids. Ind. Eng. Chem. Res. 2004, 43, 6855-6860. (25) Finotello, A.; Bara, J. E.; Camper, D.; Noble, R. D. Room Temperature Ionic Liquids: Temperature Dependence of Gas Solubility Selectivity. Ind. Eng. Chem. Res. ASAP Article (DOI: 10.1021/ie0704142). (26) Kim, Y. S.; Choi, W. Y.; Jang, J. H.; Yoo, K. P.; Lee, C. S. Solubility measurement and prediction of carbon dioxide in ionic liquids. Fluid Phase Equilib. 2005, 228, 439-445. (27) Lin, H. Q.; Freeman, B. D. Materials selection guidelines for membranes that remove CO2 from gas mixtures. J. Mol. Struct. 2005, 739, 57-74. (28) Lee, S. H.; Lee, S. B. The Hildebrand solubility parameters, cohesive energy densities and internal energies of 1-alkyl-3-methylimidazolium-based room temperature ionic liquids. Chem. Commun. 2005, 34693471. (29) Millefiorini, S.; Tkaczyk, A. H.; Sedev, R.; Efthimiadis, J.; Ralston, J. Electrowetting of ionic liquids. J. Am. Chem. Soc. 2006, 128, 30983101. (30) Nishida, T.; Tashiro, Y.; Yamamoto, M. Physical and electrochemical properties of 1-alkyl-3-methylimidazolium tetrafluoroborate for electrolyte. J. Fluorine Chem. 2003, 120, 135-141. (31) Luo, H.; Dai, S.; Bonnesen, P. V.; Haverlock, T. J.; Moyer, B. A.; et al. A striking effect of ionic-liquid anions in the extraction of Sr2+ and Cs+ by dicyclohexano-18-crown-6. SolVent Extr. Ion Exch. 2006, 24, 1931. (32) Parnham, E. R.; Morris, R. E. 1-alkyl-3-methyl imidazolium bromide ionic liquids in the ionothermal synthesis of aluminium phosphate molecular sieves. Chem. Mater. 2006, 18, 4882-4887. (33) Camper, D.; Becker, C.; Koval, C.; Noble, R. Diffusion and solubility measurements in room temperature ionic liquids. Ind. Eng. Chem. Res. 2006, 45, 445-450. (34) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983. (35) Prausnitz, J. M.; Lichtenthaler, R. N.; Gomez de Azevedo, E. Molecular Thermodynamics of Fluid-Phase Equilibria, 3rd ed.; Prentice Hall PTR: Upper Saddle River, 1999. (36) Huang, J. F.; Chen, P. Y.; Sun, I. W.; Wang, S. P. NMR evidence of hydrogen bond in methyl-3-methylimidazolium-tetrafluoroborate room temperature ionic liquid. Spectrosc. Lett. 2001, 34, 591-603. (37) Anthony, J. L.; Anderson, J. L.; Maginn, E. J.; Brennecke, J. F. Anion effects on gas solubility in ionic liquids. J. Phys. Chem. B 2005, 109, 6366-6374. (38) Camper, D.; Becker, C.; Koval, C.; Noble, R. Low pressure hydrocarbon solubility in room temperature ionic liquids containing imidazolium rings interpreted using regular solution theory. Ind. Eng. Chem. Res 2005, 44, 1928-1933.
