Low-Pressure Solubility of Carbon Dioxide in Room-Temperature Ionic Liquids Measured with a Quartz Crystal Microbalance

Abstract

The solubility of carbon dioxide in a series of imidazolium-based room-temperature ionic liquids has been determined using a quartz crystal microbalance. Henry's constants were obtained for CO 2 in nine different ionic liquids: 1-methyl-3-propylimidazolium bis[trifluoromethylsulfonyl]amide (C 3 mimTf 2 N), 1- n -butyl-3-methylimidazolium bis[trifluoromethylsulfonyl]amide (C 4 mimTf 2 N), C 4 mimTf 2 N with polyethylenimine, 1- n -hexyl-3-methylimidazolium bis[trifluoromethylsulfonyl]amide (C 6 mimTf 2 N), 1-methyl-3- n -octylimidazolium bis[trifluoromethylsulfonyl]amide (C 8 mimTf 2 N), 1-methyl-3-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)imidazolium bis[trifluoromethylsulfonyl]amide (C 8 F 13 mimTf 2 N), 1,4-dibutyl-3-phenylimidazolium bis[trifluoromethylsulfonyl]amide, 1-butyl-3-phenylimidazolium bis[trifluoromethylsulfonyl]amide, and 1-methyl-3-propylimidazolium hexafluorophosphate (C 3 mimPF 6 ). All results were obtained at 25 °C with CO 2 pressures at or less than 1 bar. A comparison of results helps in understanding the role of chemical structure on the separation capabilities of these materials. Notable among the results is a significantly greater measured CO 2 solubility in the ionic liquid having the fluorine-substituted cation as compared to the corresponding ionic liquid with a nonfluorinated cation. CO 2 solubility was found to be lower in the ionic liquid containing PF 6 - than for the corresponding liquid with Tf 2 N - anion. Addition of an imine polymer to the ionic liquid did not significantly change CO 2 solubility characteristics. The presence of water had a minor effect on CO 2 solubility for C 8 mimTf 2 N for environments with a relative humidity of 40% or less. Henry's constant for CO 2 in C 3 mimPF 6 obtained in this study is in good agreement with previously reported Henry's constant values for CO 2 in C 4 mimPF 6 that were obtained by other means. This study not only provides some important thermodynamic property information for these unique materials but also illustrates the utility of using a quartz crystal microbalance to obtain this information.