The distillation and volatility of ionic liquids

Abstract

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 distilled. 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 burgeoned, producing a wealth of intellectual and technological challenges and opportunities for the production of new chemical and extractive processes, fuel cells and batteries, and new composite materials. 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 degrees C and low pressure, with concomitant recovery of significant amounts of pure substance, will permit these currently excluded applications to be realized.

Protic Ionic Liquids: Preparation, Characterization, and Proton Free Energy Level Representation

“…They, and some unusual, strongly ion-paired aprotic IL cousins, have been called "distillable" ionic liquids. 29,30 On the other hand, protic ILs formed by proton transfers from superacids to strong bases behave like aprotic ILs.…”

“…The volatility of low-gap PILs has been noted in other recent publications. 29,30 The present article provides a basis for predicting the volatility in individual cases.…”

Novel 2-alkyl-1-ethylpyridinium ionic liquids: synthesis, dissociation energies and volatility

“…[17][18][19][20][21][22][23][24][25] Due to the large number of possible ionic liquids, predictive models have been proposed to overcome this limitation. [26][27][28][29][30] Until recently, ionic liquids were regarded as non-volatile, and thus did not exhibit measurable vapor pressures, even at higher temperatures. However, Earle et al 29 have shown that some ionic liquids can be distilled at low pressures without decomposition.…”

“…[26][27][28][29][30] Until recently, ionic liquids were regarded as non-volatile, and thus did not exhibit measurable vapor pressures, even at higher temperatures. However, Earle et al 29 have shown that some ionic liquids can be distilled at low pressures without decomposition. Later on, Paulechka et al 31 have evaluated the possibility of measuring the vapor pressure and enthalpy of vaporization of [C 4 C 1 im][NTf 2 ] in vacuum.…”

Tunable Wavelength Soft Photoionization of Ionic Liquid Vapors

“…Since ILs are generally perceived as "green solvents", their decomposition pathways can lead to a variety of products that need to be considered in specific applications. Given recent works 19 demonstrating the distillation of selected families of ILs, we subsequently prepared molecular beams of IL vapor, providing a gas phase target of isolated ion pair species to obtain the photoelectron (PE) spectrum of 20 Other recent studies 3 aimed at investigating the properties of IL vapor also include line of sight mass spectroscopy, 23,24 IR spectroscopy, 25 and matrix isolated FTIR spectroscopy. 26 Moreover, mass spectra of cation and anion substituted ILs 20 showed that the ionization of these isolated IL ion-pairs by absorption of a single 23.2 eV photon, which is more than 14 eV above the ionization threshold, results in fragmentation with a predominant product of the intact cations.…”