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 ([hmim][Tf2N]), 2,3-dimethyl-1-hexylimidazolium bis(trifluoromethylsulfonyl)imide ([hmmim][Tf2N]), and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([omim][Tf2N]). 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.

Previously we showed that CO2 could be used to extract organic molecules from ionic liquids without contamination of the ionic liquid. Consequently a number of other groups demonstrated that ionic liquid/CO2 biphasic systems could be used for homogeneously catalyzed reactions. Large differences in the solubility of various gases in ionic liquids present the possibility of using them for gas separations. More recently we and others have shown that the presence of CO2 increases the solubility of other gases that are poorly soluble in the ionic liquid phase. Therefore, a knowledge and understanding of the phase behavior of these ionic liquid/CO2 systems is important. With the aim of finding ionic liquids that improve CO2 solubility and gaining more information to help us understand how to design CO2-philic ionic liquids, we present the low- and high-pressure measurements of CO2 solubility in a range of ionic liquids possessing structures likely to increase the solubility of CO2. We examined the CO2 solubility in a number of ionic liquids with systematic increases in fluorination. We also studied nonfluorinated ionic liquids that have structural features known to improve CO2 solubility in other compounds such as polymers, for example, carbonyl groups and long alkyl chains with branching or ether linkages. Results show that ionic liquids containing increased fluoroalkyl chains on either the cation or anion do improve CO2 solubility when compared to less fluorinated ionic liquids previously studied. It was also found that it was possible to obtain similar, high levels of CO2 solubility in nonfluorous ionic liquids. In agreement with our previous results, we found that the anion frequently plays a key role in determining CO2 solubility in ionic liquids.

Ionic liquids have been suggested as replacement solvents in reactions and separations since they have negligible vapor pressure; thus, they would reduce fugitive emissions that are common when organic solvents are used in these applications. To fully utilize ionic liquids in reactions and separations, a fundamental understanding of the factors that govern the phase behavior of ionic liquids with other common liquids is necessary. In this work, we present a systematic study of the impact of different factors on the phase behavior of imidazolium-based ionic liquids with alcohols. All systems examined showed upper critical solution temperature (UCST) behavior, with low solubility of the ionic liquid in the alcohol and high solubility of the alcohol in the ionic liquid. An increase in the alkyl chain length of the alcohol resulted in an increase in the UCST. Branching of the alcohol resulted in a higher solubility of the alcohol in the ionic-liquid-rich phase. By increasing the alkyl chain length on the cation, the UCST decreased, while replacement of the hydrogen at the C2 position of the ring with a methyl group resulted in an increase in the UCST. The choice of anion was shown to have a large impact on the UCST of the system. The relative alcohol affinity for the different anions observed was (CN)2N > CF3SO3 > (CF3SO2)2N > BF4 > PF6.

The solvent strength and polarity of four imidazolium and pyridinium based ionic liquids, as measured using two different fluorescent probes, indicate that these liquids are more polar than acetonitrile but less polar than methanol.

A novel technique to separate ionic liquids from organic compounds is introduced which uses carbon dioxide to induce the formation of an ionic liquid-rich phase and an organic-rich liquid phase in mixtures of methanol and 3-butyl-1-methyl-imidazolium hexafluorophosphate ([C4mim][PF6]). If the temperature is above the critical temperature of CO2 then the methanol-rich phase can become completely miscible with the CO2-rich phase, and this new phase is completely ionic liquid-free. Since CO2 is nonpolar, it is not equipped to solvate ions. As the CO2 dissolves in the methanol/[C4mim][PF6] mixture, the solvent power of the CO2-expanded liquid is significantly reduced, inducing the formation of the second liquid phase that is rich in ionic liquid. This presents a new way to recover products from ionic liquid mixtures and purify organic phases that have been contaminated with ionic liquid. Moreover, these results have important implications for reactions done in CO2/ionic liquid biphasic mixtures.