Ind. Eng. Chem. Res. 2004, 43, 3049-3054
3049
Gas Solubilities in Room-Temperature Ionic Liquids Dean Camper,*,† Paul Scovazzo,‡ Carl Koval,§ and Richard Noble† Departments of Chemical and Biological Engineering and Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, and Department of Chemical Engineering, University of Mississippi, University, Mississippi 38677
The ability to predict the solubility of gases in room-temperature ionic liquids, RTILs, would be very useful in determining the most efficient RTIL to use in an industrial process. This work uses data from CO2 and C2H4 solubility measurements to show that regular solution theory can be used to model gas solubilities in RTILs at low pressures. This work further discusses how changes in pressure and temperature affect the solubility of gases in RTILs. 1. Introduction
2. Experimental Methods
Room-temperature ionic liquids (RTILs) are organic salts that are liquids at 298 K.1,2 Most of the research dealing with RTILs has been applied toward using RTILs to replace solvents in reactions such as hydrogenations and oxidations.1 Common solvents such as volatile organic compounds (VOCs) are a concern because of the harmful effects they have on air quality.1,3 In contrast, RTILs have a negligible vapor pressure and, therefore, would limit the release of toxic gases into the atmosphere.1-4 RTILs are also nonflammable, and most are considered nontoxic.3 These properties make RTILs a promising alternative to solvents as a medium for both reactions and gas separations. To choose the most efficient RTIL for use as a reaction medium or in gas separations, it is necessary to know the solubilities of the gases and liquids involved. If the gas in a chemical reaction has a low solubility in an RTIL, then that RTIL could be a poor candidate as a solvent replacement. A good solubility match is also necessary for an RTIL to be used as a medium in gas separations because permeability is often linearly related to solubility. Although some experimental work has related the difference in solubility of carbon dioxide in various RTILs to the properties of the anions of the RTILs, no comprehensive model has been developed to model gas solubility in RTILs.4 Little information is available in the literature on the solubility properties of RTILs, so a model for predicting gas solubility in RTILs would be very helpful.1,3 In this work, C2H4 is used to further develop and test a model that employs regular solution theory to account for enthalpic effects seen in solubility measurements of CO2 in RTILs at low pressure.5 C2H4 was chosen because it has a solubility parameter similar to that of CO2. This work also discusses temperature and pressure effects on the solubility of CO2. The temperature effects are modeled by varying the solubility parameter of the gas, and the pressure effects are modeled using the twosuffix Margules equation.
2.1. Data Used from the Literature. The effects of temperature on CO2 solubility were modeled using data found in Husson-Borg et al.1 for 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). The effects of pressure on CO2 solubility in 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-n-octyl-3-methylimidazolium hexafluorophosphate ([C8mim][PF6]), 1-n-octyl-3-methylimidazolium tetrafluoroborate ([C8mim][BF4]), 1-n-butyl-3-methylimidazolium nitrate ([bmim][NO3]), N-butylpyridinium tetrafluoroborate ([N-buyp][BF4]), and 1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO4]) were modeled using data found in Blanchard et al.4 2.2. Materials. The RTILs used in this study are listed in Table 1. These RTILs were chosen because all, with the exception of [thtdp][Cl], have similar cations. The residual halide content of each [emim][Tf2N], [bmim][PF6], and [emim][CF3SO3] was
