Ionic liquid - carbon dioxide systems: Phase behavior, solubilities and extraction

Supercritical carbon dioxide (SCCO₂) and ionic liquids (ILs) hold great potential as replacements for traditional volatile organic solvents. ILs are excellent solvents and have been shown to be a viable medium for numerous types of reactions. ILs are nonvolatile, which makes their use as solvents attractive by effectively eliminating the hazard of fugitive emissions typical of traditional organic solvents. However, the recovery of reaction products from ILs, while maintaining the overall “greenness” of the process, remains a challenge. This work investigates the feasibility of using supercritical carbon dioxide, another environmentally benign solvent, for the separation of organics from ILs.; The ionic liquid—CO₂ biphasic system, a combination of two environmentally benign entities, is a versatile system for reaction and subsequent separation. An essential aspect of these biphasic systems is the knowledge of the phase behavior of ILs with CO₂. This work presents the high-pressure phase behavior of CO₂ with six ionic liquids: 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmin][PF₆]), 1-n-octyl-3-methylimidazolium hexafluorophosphate ([C₈-mim][PF₆]), 1-n-octyl-3-methylimidazolium tetrafluoroborate ([C₈-mim][BF₄]), 1-n-butyl-3-methylimidazolium nitrate ([bmim][NO₃]), 1-ethyl-3-methylimidazolium ethylsulfate ([emim][EtSO₄]) and N-butylpyridinium tetrafluoroborate ([N-bupy][BF ₄]). The effect of systematically changing the anionic and cationic components of the ionic liquid on the CO₂—ionic liquid phase behavior was explored. For all of the ionic liquids tested, large quantities of CO₂ dissolved in the ionic liquid phase but no appreciable amount of ionic liquid solubilized in the CO₂ phase. Also, the introduction of large amounts of CO₂ into the IL phase resulted in a negligible liquid phase volume expansion, a stark contrast to the large volume expansion observed for typical organic liquids. The results are related to the nature of the IL anion and cation, as well as the liquid molar volume of the IL.; The solubility of twenty-one organic solutes in [bmim][PF₆], and subsequent recovery rate with SCCO₂ extraction, are reported This work shows that supercritical fluid extraction of organics from an IL is a viable separation technique with the additional benefits of environmental sustainability and pure product recovery.