High-Pressure Phase Behavior of Carbon Dioxide with Imidazolium-Based Ionic Liquids

Previously we have shown that supercritical carbon dioxide can be used to extract organics from ionic liquids (ILs). Subsequently, ionic liquids/carbon dioxide biphasic solutions have been used for a variety of homogeneously catalyzed reactions. Therefore, an understanding of the phase behavior of carbon dioxide with ionic liquids is needed to design extraction and reaction processes necessary for these applications. We present measurements of the solubility of carbon dioxide in 10 different imidazolium-based ionic liquids at 25, 40, and 60 °C and pressures to 150 bar. As expected, the solubility increases with increasing pressure and decreases with increasing temperature for all the ILs investigated. To investigate the influence of the anion, seven of the ILs studied have 1-butyl-3-methylimidazolium (bmim) as the cation. The anions are dicyanamide (DCA), nitrate (NO3), tetrafluoroborate (BF4), hexafluorophosphate (PF6), trifuoromethanesulfonate (TfO), bis(trifluoromethylsulfonyl)imide (Tf2N), and tris(trifluoromethylsulfonyl)methide (methide). The other ILs considered in the study, chosen to investigate the influence of varying number and length of alkyl chains on the cation, include 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (hmimTf2N), 2,3-dimethyl-1-hexylimidazolium bis(trifluoromethylsulfonyl)imide (hmmimTf2N), and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (omimTf2N). Results show that the solubility of carbon dioxide is strongly dependent on the choice of anion. In particular, CO2 solubility is greater in ILs with anions, such as Tf2N and methide, which contain fluoroalkyl groups. Also, we observe that an increase in the alkyl chain length on the cation increases the CO2 solubility marginally.