The distillation and volatility of ionic liquids

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Abstract
Interest in ionic liquids has grown in line with their use as solvents in ‘green’ chemistry, which aims to avoid the use or production of hazardous substances during chemical processes. Ionic liquids are salts that are liquid at or close to room temperature. It has been widely assumed — but not proven — that they will decompose before they boil, so they are generally considered undistillable, and hence very difficult to purify. This now turns out to be incorrect: at low pressures and moderate temperatures, certain classes of ionic liquids have now been distilled without decomposition. As well as the intrinsic importance of this observation, it serves as a reminder that a ‘widely held assumption’ does not constitute proof: always obtain experimental verification. It is widely believed that a defining characteristic of ionic liquids (or low-temperature molten salts) is that they exert no measurable vapour pressure, and hence cannot be distilled1,2. Here we demonstrate that this is unfounded, and that many ionic liquids can be distilled at low pressure without decomposition. Ionic liquids represent matter solely composed of ions, and so are perceived as non-volatile substances. During the last decade, interest in the field of ionic liquids has burgeoned3, producing a wealth of intellectual and technological challenges and opportunities for the production of new chemical and extractive processes4,5,6, fuel cells and batteries7, and new composite materials8,9. Much of this potential is underpinned by their presumed involatility. This characteristic, however, can severely restrict the attainability of high purity levels for ionic liquids (when they contain poorly volatile components) in recycling schemes, as well as excluding their use in gas-phase processes. We anticipate that our demonstration that some selected families of commonly used aprotic ionic liquids can be distilled at 200–300 °C and low pressure, with concomitant recovery of significant amounts of pure substance, will permit these currently excluded applications to be realized.