SCIENCE & TECHNOLOGY
SWITCHABLE SOLVENTS Using CO2 as a trigger, reversible solvent systems aim to facilitate greener reactions and PRODUCT SEPARATIONS SOLVENTS THAT reversibly switch from be-
Jessop explains. But under the influence of CO2, the solvent converts to a bicarbonate species that is miscible with water. Bubbling nitrogen or air through the solution or heating it flushes CO2 out of the mixture and re-forms the biphasic system. This “switchable hydrophilicity solvent” is ideal for extracting low-polarity organic compounds, such as vegetable oils, Jessop notes. For example, his team has used it to mimic the industrial extraction of soybean oil from soybean flakes. The current process involves NR extracting soybeans with NR2 R + HNR HCO3– hexane, followed CO2 by removing the NR2 R Hydrophobic hexane via distilHydrophilic –CO2 solvent solvent and lation to leave water Water the pure oil. The hydrophilicity R = butyl solvent makes it possible to exO tract the oil and Tetrahydrofuran then separate the O + oil and solvent by NR Tetrahydrofuran 2 R2N adding CO2 and + Water + water. Once the + NR 2 R2N H oil is removed, H CO2 + the solvent is – 2 HCO Nonsalty 3 + separated from Salty –CO2 Water the water by reR = methyl moving CO2 and is then reused— BACK AND FORTH no hexane and Conceptual scheme for a switchable hydrophilicity solvent (top) no distillation and switchable water (bottom). required. GreenCentre Canada, vents to improve air quality and eliminate an institute that develops technologies the need for energy-intensive distillations. from Canadian universities, is working to Jessop and colleagues devised the first scale up the model process, Jessop says. switchable solvent in 2005, opening the way for a budding list of CO2-switchable solIN A SEPARATE STUDY, Jessop and gradvents, surfactants, and catalysts. In one new uate student Sean M. Mercer figured out study, Jessop and coworkers describe using a way to reversibly switch between salty CO2 to reversibly switch the hydrophobic and nonsalty water (ChemSusChem, DOI: solvent N,N,Nʹ-tributylpentanamidine into 10.1002/cssc.201000001). “Salting out” is a hydrophilic solvent (Green Chem., DOI: an effective method for separating water10.1039/b926885e). soluble organic compounds from water, The amidine isn’t normally miscible but it requires adding a large amount of sowith water and forms a biphasic system, dium chloride to the solution and results in ing hydrophobic to hydrophilic or between extremes in ionic strength simply by the addition or removal of carbon dioxide are being reported by Philip G. Jessop and coworkers of Queen’s University, in Kingston, Ontario. The difference between the two states of the solvents is large enough that many compounds are soluble in only one form or the other. Harnessing these switchable solvents for chemical processing could make it easy in some cases to replace volatile organic sol-
WWW.CEN-ONLINE.ORG
47
M ARC H 2 2 , 20 10
leftover salty water for disposal. Jessop and Mercer instead added a neutral diamine to water, forming an aqueous solution with essentially zero ionic strength. After CO2 is introduced, the diamine turns into a diammonium bicarbonate salt, significantly raising the ionic strength, Jessop says. To illustrate how this “switchable water” system might be useful, the researchers added tetrahydrofuran, which is miscible with the nonsalty diamine solution. When exposed to CO2, the diammonium salt forms and forces tetrahydrofuran out of solution. The tetrahydrofuran layer can be removed and the aqueous layer recycled to the nonsalty form by purging the CO2. “The notion of reversible salting out is pretty neat—I’ve never seen that before,” says University of Pittsburgh chemical engineer Eric J. Beckman, an expert on using CO2 as a solvent and chemical feedstock. It’s hard to be any more environmentally benign or inexpensive than by using CO2 and water, Beckman adds. These model solvent systems created by Jessop’s group have potential if they can be optimized and put to work, Beckman says.—STEVE RITTER
