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.
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This work was supported by a grant from FC&T, Portugal (to J.M.S.S.E. and L.P.N.R.). The Belfast team wishes to thank the industrial members of QUILL for support and Al Robertson (Cytec) for supplying the phosphonium ionic liquids. For J.W.M. and J.A.W. this work represents an official contribution of the National Institute of Standards and Technology and is not subject to copyright in the USA.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Describes in detail the methodologies of most of the distillations performed as well as those of the NMR Spectra. (DOC 43 kb)
Plots of the 1H, 13C and 19F NMR spectra of distilled and undistilled ionic liquids corresponding to the presentation of 30 figures, labelled from Supplementary Figure 1a to Supplementary Figure 12b. (DOC 243 kb)
Supplementary Figure 13: [C6mim][NTf2] in the sublimation apparatus: photographs of atmospheric pressure distillation; and Supplementary Figure 14: < 0.001 mbar distillation compared. (DOC 1143 kb)
This movie (56 seconds) shows the Kugelrohr distillation process of [C2mim][NTf2] in action – 100 times speeded up. (AVI 4359 kb)
This movie (50 seconds) shows the Kugelrohr distillation process of [C10mim][NTf2] in action – 150 times speeded up. (AVI 6062 kb)
Video 1. Timelapse video of the distillation of [C2mim][NTf2]. Video 2. Timelapse video of the distillation of [C10mim][NTf2]. (DOC 27 kb)
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Earle, M., Esperança, J., Gilea, M. et al. The distillation and volatility of ionic liquids. Nature 439, 831–834 (2006). https://doi.org/10.1038/nature04451
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