Solubility of Gases in Ionic Liquids

Published studies on the solubilities of gases in ionic liquids (ILs) have increased dramatically since 2000. These studies include common industrial gases such as CO2, reaction gases such as O2, as well as other gases of interest, such as light hydrocarbons. The number of ILs synthesized and analyzed has also increased, though the majority of these studies stiU focus on l-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) as a benchmark IL. 

Ellegaard, M. D., J. Abildskov, and J. P. O Connell. 2011. Solubilities of gases in ionic liquids using a corresponding-states approach to Kirkwood-Buff solution theory. Fluid Phase Equilibria. 302, 93. 

Research efforts aimed at quantifying the solubility of CO2 in ionic liquids revealed a significant influence of the ionic liquid s water content on the CO2 solubility. While water-saturated [BMIM][PF6] (up to 2.3 wt.% water) has a CO2 solubility of only 0.13 mol fraction, 0.54 mol fraction CO2 dissolves in dry [BMIM][PF6] (about 0.15 wt.% water) at 57 bar and 40 °C [7]. Kazarian et al. used ATR-IR to determine the CO2 solubility in [BMIMJfPFs] and [BMIM] [BF4]. They reported a solubility of 0.6 mol fraction CO2 in [BMIMJfPFs] at 68 bar and 40 °C [8]. More recent research activities by Brennecke et al. have been directed towards the elucidation of the solvent strength [9] and the phase behavior of SCCO2/IL mixtures [10] and the determination of the solubility of various gases in ionic liquids. These studies have revealed that the solubility of CO2 in ionic liquids is mainly dependent on the anion structure. While [PFfi] and [BF4]-based ILs show similar solubility, an increased solubility can... 

Green WG, Field PE (1980) Interaction of gases in ionic liquid. 2. The solubility of argon and nitrogen in molten lithium nitrate and potassiiun nitrate. J Phys Chem 84 3111-3114... 

Field PE, Green WG (1971) Interactions of gases in ionic liquids. 1. Solubility of nonpolar gases in molten sodium nitrate. J Phys Chem 75 821-825... 

A wide variety of physical properties are important in the evaluation of ionic liquids (ILs) for potential use in industrial processes. These include pure component properties such as density, isothermal compressibility, volume expansivity, viscosity, heat capacity, and thermal conductivity. However, a wide variety of mixture properties are also important, the most vital of these being the phase behavior of ionic liquids with other compounds. Knowledge of the phase behavior of ionic liquids with gases, liquids, and solids is necessary to assess the feasibility of their use for reactions, separations, and materials processing. Even from the limited data currently available, it is clear that the cation, the substituents on the cation, and the anion can be chosen to enhance or suppress the solubility of ionic liquids in other compounds and the solubility of other compounds in the ionic liquids. For instance, an increase in allcyl chain length decreases the mutual solubility with water, but some anions ([BFJ , for example) can increase mutual solubility with water (compared to [PFg] , for instance) [1-3]. While many mixture properties and many types of phase behavior are important, we focus here on the solubility of gases in room temperature IFs. 

The solubility of various gases in ionic liquids is extremely important in evaluating ILs as solvents for reactions, separations, and materials processing. There are a number of viable techniques for measuring gas solubilities in ILs, including the... 

Although the solubilities of gases in ILs are extremely important, at the time of this writing the number of published studies are limited. Some measurements were presented in oral and poster presentations at a five day symposium dedicated to ionic liquid research at the American Chemical Society national meeting in San Diego in April, 2001. Scovazzo et al. [14], for instance, presented preliminary results for CO2 and N2 solubility in [BMIM][PF6], and Rooney et aL [15] presented the solubihty of several gases in several different ILs as determined by the stoichiometric technique. A recent manuscript [16] presented Henry s law constants for H2 in two ILs. Given the lack of availability of other data, we concentrate below on the data collected in our laboratories. 

Zhao Y, Zhang X, Dong H, et al Solubilities of gases in novel alcatnines ionic liquid 2,2-hydroxyethyl (methyl) amino ethanol chloride, Fluid Phase Equilib 302 60-64, 2011. 

Vega, L.F. Vilaseca, O. Llovell, F. Andreu, J.S. (2010). Modeling ionic liquids and the solubility of gases in them Recent advances and perspectives. Fluid Phase Equilib. 294,15-30. 

Ionic liquids display good solubility for some organic compounds, typically aromatics, but poor solubility for many saturated hydrocarbons, and solubilities of gases also depend on their properties. It has therefore been possible to run chemical reactions with reactants that are more soluble in the ionic liquids than products. 

Anthony, J. L., Maginn, E. J., and Brennecke, J. F, Solubilities and thermodynamic properties of gases in the ionic liquid l-n-butyl-3-methyl imidazolium hexafluorophosphate, /. Phys. Chem. B, 106, 7315, 2002. 

Kim YS, Jang JH, Lim DB et al. (2007) Solubility of mixed gases containing carbon dioxide in ionic liquids measurements and predictions. Fluid Phase Equilib 228 70-74... 

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