(662b) Separations Using Switchable Solvents

Switchable solvents, meaning liquids that can be reversibly switched from one form to another, were envisioned to be solutions to the practical conundrum called "Murphy's Law of Solvents", which states that "The best solvent for any one process step is bad for the next step."  We wanted to design solvents that would meet the needs of one process step and could then be modified, in situ, to meet the needs of the subsequent step.  This could have utility in solvent-based separation processes ranging from simple extractions to post-reaction separations.

Our original switchable solvents, developed in collaboration with Drs. Eckert and Liotta at Georgia Tech., were switchable-polarity solvents (SPS).^1-3  They switched back and forth between a low-polarity form and a high polarity form (Figure 1), with the triggers for the transformation being addition of 1 bar of CO₂ (to boost the polarity) and the removal of that CO₂ (to drop the polarity).  This polarity change facilitated post-reaction separations.  In collaboration with Heldebrant at PNNL, we showed that the SPS could also be used as non-aqueous CO₂-capturing liquids, with the advantage that the heat capacity was lower (and CO₂-capturing capacity higher) than traditional aqueous CO₂ capture solutions.^4-6  While the SPS were the first switchable solvents and therefore a demonstration of the potential of the concept, they had several limitations. Most importantly, they were, unfortunately, somewhat water-sensitive.  Also, while the polarity change induced by the CO₂ trigger was large enough to change the solubilizing properties significantly, a larger polarity change was desired.  Even an SPS that was zwitterionic in its polar form failed to have a larger polarity change.⁵

Figure 1. "Switchable-polarity solvents".  These solvents have low polarity until they are exposed to an atmosphere of CO₂, at which point they change into high-polarity ionic liquids.  The polarity difference is large enough that many solutes are soluble in only one form of the solvent.  The process is reversed by removal of the CO₂ from solution.^1-6

We have now developed two new kinds of switchable solvents specifically for use in the presence of water: a) switchable-hydrophilicity solvents and b) switchable water.  These new solvents address those two issues with the previous switchable solvents.

"Switchable-hydrophilicity solvents" (SHS) are liquid solvents that are normally so hydrophobic that they have very little miscibility with water and form a biphasic mixture when mixed with water.  However, when exposed to CO₂, these solvents become very hydrophilic and completely miscible with water (Figure 2).  Therefore these solvents can behave like hexane but be easily removed, by extraction with carbonated water, without distillation.  Our first switchable hydrophilicity solvent was an amidine,⁷ but we have now found that certain amines exhibit the same phase behaviour.  The use of such solvents for the separation of soy oil from soybean flakes will be described as an example application.^7,8

Figure 2.  "Switchable-hydrophilicity solvents" are liquid solvents that are normally so hydrophobic that they have very little miscibility with water and form a biphasic mixture when mixed with water.  However, when exposed to CO₂, these solvents become very hydrophilic and completely miscible with water.  Therefore these solvents can behave like hexane but be easily removed, by extraction with carbonated water, without distillation.⁷

"Switchable water" is an aqueous solution of switchable ionic strength.  An aqueous solution of an uncharged additive has an ionic strength of zero and is miscible with some organics such as THF. However, after CO₂ is introduced, the additive changes to a salt, raising the ionic strength, and forcing the THF out of solution (Figure 3).  This represents a reversible method for "salting-out" of organic compounds from water.⁹

Figure 3. "Switchable water". An aqueous solution of an uncharged additive has an ionic strength of zero and is miscible with THF. However, after CO₂ is introduced, the additive changes to a salt, raising the ionic strength, and forcing the THF out of solution.  This represents a reversible method for "salting-out" of organic compounds from water.⁹

Switchable solvents of various kinds have the potential to address important issues in chemical manufacturing and separations, such as the need for a replacement for distillation in solvent removal steps and the need for alternative methods for removing organic contaminants from water.

            (1)       Jessop, P. G.; Heldebrant, D. J.; Xiaowang, L.; Eckert, C. A.; Liotta, C. L. Nature 2005, 436, 1102.

            (2)       Phan, L.; Andreatta, J. R.; Horvey, L. K.; Edie, C. F.; Luco, A.-L.; Mirchandi, A.; Darensbourg, D. J.; Jessop, P. G. J. Org. Chem. 2008, 73, 127-132.

            (3)       Phan, L.; Li, X.; Heldebrant, D. J.; Wang, R.; Chiu, D.; John, E.; Huttenhower, H.; Pollet, P.; Eckert, C. A.; Liotta, C. L.; Jessop, P. G. Ind. Eng. Chem. Res. 2008, 47, 539-545.

            (4)       Heldebrant, D. J.; Yonker, C. R.; Jessop, P. G.; Phan, L. Energy Env. Sci. 2008, 1, 487-493.

            (5)       Heldebrant, D. J.; Koech, P. K.; Ang, T.; Liang, C.; Rainbolt, J. E.; Yonker, C. R.; Jessop, P. G. Green Chem. 2010, 12, 713.

            (6)       Heldebrant, D. J.; Yonker, C. R.; Jessop, P. G.; Phan, L. Chem. Eur. J. 2009, 15, 7619-7627.

            (7)       Jessop, P. G.; Phan, L.; Carrier, A.; Robinson, S.; D?rr, C. J.; Harjani, J. R. Green Chem. 2010, in press.

            (8)       Phan, L.; Brown, H.; Peterson, T.; White, J.; Hodgson, A.; Jessop, P. G. Green Chem. 2009, 11, 53-59.

            (9)       Mercer, S. M.; Jessop, P. G. ChemSusChem 2010, in press.