Solvatochromic Studies on the CO₂ Antisolvent Ability in Ionic Liquid/Organic Mixtures

            Ionic Liquids (ILs) have proven to be apt solvents for a variety of reactions.  However, separations of products and impurities, such as spent catalyst, can be difficult to remove.  Easy separations are needed to ensure the ability to recycle the IL, an important aspect of economic feasibility.  CO₂ can be used as a separation aid, such as for supercritical CO₂.  However, this method only works for solutes that are soluble in CO₂.  In this paper, we discuss the use of CO₂ to remove solutes that are not soluble in CO₂ by using CO₂ as an antisolvent.  This behavior can be explained in terms of solvent strength using solvatochromic probes.

            CO₂ can be used as a separation aid by acting as an antisolvent.  As CO₂ is added to a liquid with a dissolved solute, the pressure increases and CO₂ dissolves into the liquid mixture.  This alters the solvation ability of the solvent, and essentially creates a supersaturated solution.  At this point, for a specific temperature and pressure, a phase separation occurs.  For a liquid solute, this is the Lower Critical End Point (LCEP).  For a solid solute, it is the nucleation pressure.  This behavior has been documented for a variety of organic-based systems.  The accepted explanation for this behavior  is that the CO₂ causes the organic to expand greatly, which lowers the solvent strength and thus induces a phase split.  This behavior has also been seen in IL systems, as well as for liquid organic solvents in ILs[1-3] and solid solutes in ILs[4, 5].  However, ILs do not expand greatly.  This behavior is not well understood in IL systems

            In order to understand the mechanisms behind this behavior, a variety of ILs and organics were studied.  The main IL studied was [hmim][Tf₂N], the IUPAC standard.  The organics chosen for this study were acetonitrile, which is polar and aprotic, 2-butanone, a hydrogen bond acceptor, and 2,2,2-trifluoroethanol, a strong hydrogen bond donor.  In order to study the effect of the acidic hydrogen on the C2 position of the imidazolium ring, we studied [hmmim][Tf₂N]/2-butanone mixtures.  For the anion, we studied [hmim][TfO], where [TfO] is a stronger hydrogen bond acceptor, with 2,2,2-trifluoroethanol.  As another type of IL, we looked at [N₂₁₁₃][Tf₂N] with each of the organics.  The solubility of CO₂ in the IL/organic mixtures and the volume expansion were also measured.  It was found that less CO₂ was required with mixtures that have less interactions between the IL and organic.  The ease of separation from [hmim][Tf₂N] is as follows: 2-butanone > acetonitrile > 2,2,2-trifluoroethanol.  In addition, it was found that the ease of separation depends on the CO₂ solubility rather than volume expansion.  The higher the CO₂ solubility in the IL/organic mixture, the easier it is to induce a phase split.

            CO₂ can also be used to induce a phase split for solid solutes in ILs.  A copper compound, which acts as a model catalyst, can be removed from IL/organic mixtures using CO₂ pressure.  We did not see a nucleation pressure for the copper compound dissolved in pure IL.  In general, as the concentration of the copper compound increases, the nucleation pressure lowers.

            We will explain the ability of CO₂ to act as an antisolvent by determining the solvent strength of these mixtures: IL/organic, IL/CO₂ and organic/CO₂ binary mixtures and IL/organic/CO₂ ternary mixtures.  Solvatochromic probes can be used as a measure of solvent strength.  We use the Kamlet-Taft parameters, which divide solvent strength into three categories: dipolarity/polarizability π*, hydrogen bond donating α, and hydrogen bond accepting β.  The organics do not have a strong effect on the solvent strength of the ILs until large mole fractions are present.  The exceptions are the interaction between the ILs and 2,2,2-trifluorothanol, which dominates the hydrogen bond donating interactions.  The CO₂ does not have a significant effect on the solvent strength of the ILs, but does have a large effect on the solvent strength, especially the general polarity/polarizability π*, for the organic solvents.  The IL/organic mixtures with CO₂ behave somewhat between the IL/CO₂ and organic/CO₂ binaries, although the CO₂ seems to alter the solvent strength of the mixture less, it may be because the IL preferentially solvates the probe in this situation.

            The CO₂ definitely affects the solvation ability of the IL/organic mixtures enough to induce a phase split.  The degree to which either the organic or CO₂ effects the solvent strength of the IL can be used to predict this behavior.  

1.         Aki, S.N.V.K., A.M. Scurto, and J.F. Brennecke, Ternary Phase Behavior of Ionic Liquid (IL)-Organic-CO₂ Systems. Industrial & Engineering Chemistry Research, 2006. 45(16): p. 5574-5585.

2.         Scurto, A.M., S.N.V.K. Aki, and J.F. Brennecke, CO₂ as a Separation  Switch for Ionic Liquid/Organic Mixtures. Journal of the American Chemical Society, 2002. 124(35): p. 10276-10277.

3.         Mellein, B.R. and J.F. Brennecke, Characterization of the Ability of CO₂ to Act as an Antisolvent for Ionic Liquid/Organic Mixtures. Journal of Physical Chemistry B, 2007. 108(52): p. 20355-20365.

4.         Saurer, E.M., S.N.V.K. Aki, and J.F. Brennecke, Removal of Ammonium Bromide, Ammonium Chloride, and Zinc Acetate from Ionic Liquid/Organic Mixtures Using Carbon Dioxide. Green Chemistry, 2006. 2: p. 141-143.

5.         Kroon, M.C., et al., Recovery of pure products from ionic liquids using supercritical carbon dioxide as a co-solvent in extractions or as an anti-solvent in precipitations. Green Chemistry, 2006(8): p. 246 - 249.