Supercritical CO₂ Makes Green Solvents Greener

Contents
Introduction
Ionic Liquids Slow Catching On
Inexpensive Means of Controlling Reactions
Ionic Liquid/CO₂ in Action

Introduction (Back to Top)
One of the newest ideas in organic chemistry is ionic liquids—room-temperature organic fluids whose macro behavior resembles high-class polar aprotic solvents but that look and behave electronically like salt melts. What sets these fluids apart from dimethylformamide (DMF) and other polar aprotics is that ionics remain fluid over temperature ranges as wide as 300°C. And unlike more conventional solvents, ionic fluids have very low vapor pressures, which makes them safe to handle. Chemical versatility and safety make ionic fluids ideal for investigating new bench-scale organic syntheses at the discovery and scaleup/development stages, as well as for processing pharmaceuticals and fine chemicals. Except for one problem: Getting high-boiling products out of ionic liquids requires extraction, which means you're back to using potentially hazardous solvents that must be recycled or disposed of.

Now, a group at the University of Notre Dame (Notre Dame, IN) has discovered that supercritical CO₂ efficiently extracts organics from ionic liquids, eliminating the need to use conventional organic solvents for product isolation. With supercritical fluids easier to use than ever (even at the bench scale), this development should improve the prospects for ionic liquids in a wide range of experimental, development, and processing settings.

"We have discovered a way to make ionic liquids more useful" said Joan Brennecke, a professor of chemical engineering at Notre Dame and lead author of a report in Nature describing the new extraction process. She points out that supercritical CO₂ extraction will allow more chemists and more processes to exploit this class of "terrific, environmentally benign solvents." "Using conventional solvents to extract green solvents defeats the whole point of using a green solvent in the first place," she adds. "We showed that you could use another green solvent— CO₂—to extract the reaction products out of the ionic liquid."

Industrial interest in using CO₂ as an environmentally benign solvent has been picking up. General Foods has been decaffeinating coffee with CO₂ for more than a decade. Dry-cleaning of clothes with CO₂ went commercial this year. Union Carbide has a spray painting process called UniCarb CO₂. But industrial interest in ionic liquids is still embryonic stage.

Ionic Liquids Slow Catching On (Back to Top)
Low vapor pressure is what makes traditional solvents hazardous in labs and plants. Because many solvents evaporate easily, workers inhale them. If the gases escape they eventually oxidize, creating carbon dioxide (a greenhouse gas). In recent years, ionic liquids have emerged as possible "green" solvents largely because they have no measurable vapor pressure.

"People are showing that you can do all kinds of reactions in ionic liquids," Brennecke said, "but getting the product out of the ionic liquid has not been straightforward." Water-soluable compounds can be extracted with water, and distillation can be used to remove chemicals with high vapor pressures. But distillation is out of the question for many fine chemicals and most pharmaceutical products and intermediates.

In an experiment described in May 6 issue of Nature, Brennecke and colleague Eric J. Beckman of the University of Pittsburgh forced supercritical carbon dioxide through a solution of naphthelene dissolved in an ionic liquid, 1-butyl-3-methylimidazolium hexaflourophosphate. When droplets of supercritical CO₂ were forced through the ionic liquid/naphthalene solution, the CO₂ completely pulled the naphthalene out with it, yet left all of the ionic liquid behind. After depressurization the supercritical CO₂ returns it its gaseous form, leaving behind pure solid naphthalene. While significant amounts of CO₂ remained dissolved in the ionic liquid, no measurable ionic liquid dissolved into the CO₂.

Inexpensive Means of Controlling Reactions (Back to Top)
Ionic liquids dissolve a wide range of inorganic and organic molecules in very high concentrations. Because ionic liquids are fluid from about –100° to about 200°, chemists can choose the reaction temperature most suitable to their transformation: Low-temperature reactions result in less dissociation, disproportionation, and degradation while stabilizing transient reactive species; wide temperature ranges may be used to obtain kinetic (low-temperature) or thermodynamic (high-temperature) products.

Because ionic liquids suppress conventional solvation and solvolysis, side reactions may be cut down or eliminated. Thus, they could provide an attractive alternative to conventional solvents and solvent-catalyst combinations. Ionic liquids are excellent solvents for a wide variety of industrially important reactions, including Friedel-Crafts alkylations and acylations, isomerizations, hydrogenations, and Diels-alder reactions.

Since they are truly ionic, the constituents of ionic liquids are constrained by high coulombic forces and thus have practically no vapor pressure. This property allows development of recovery schemes for organic products that compliment traditional liquid/liquid extraction, distillation, and pervaporation.

Ionic Liquid/CO₂ in Action (Back to Top)

Imidazole-based ionic liquids (Figure 1) are relatively inexpensive to manufacture. The "R" group in Brennecke's experiment was butyl, and her choice of anion was hexafluorophosphate, so the compound is 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆). Other anions work, e.g. trifluoromethanesulfonate, tetrafluoroborate, and lactate.

CO₂ is highly soluble in BMIM-PF₆, achieving a mole fraction of 0.6 at 8MPa, yet the two phases are not completely miscible, which makes for fairly good extraction with the supercritical fluid. What was encouraging about this result was that even with high solubility no detectable BMIM-PF₆ wound up in the supercritical extract. By contrast, you could expect significant quantities of conventional organic solvents to pull through with the supercritical extract.

A mixture of 0.12 mole fraction of naphthalene was extracted with supercritical CO₂ with recoveries in the 94–96% range. Near-quantitative recovery with no ionic liquid contamination in the final product suggests this combination of solvent and extractant may be suitable for other organic reactions. Dissolution of CO₂ in the ionic liquid is completely reversible on depressurization.

For more information: Joan F. Brennecke, Professor, Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556. Tel: 219-631-5847. Fax: 219-631-8366. Email: joan.brennecke.1@nd.edu.

By Angelo DePalma