High vacuum distillation of ionic liquids and separation of ionic liquid mixtures

Abstract

The vaporisation of ionic liquids has been investigated using temperature programmed desorption (TPD) and ultra-high vacuum (UHV) distillation. 1-Alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [C_nC₁Im][Tf₂N] (n = 2, 8), have been distilled at UHV and T > 500 K in a specially designed still. The distillation process yielded spectroscopically pure ionic liquid distillates with complete removal of volatile impurities such as water, argon and 1-methylimidazole. Such UHV distillation offers a method of obtaining high purity ionic liquids for analytical applications. The vapour phase of the ionic liquid mixtures [C₂C₁Im]_0.05[C₈C₁Im]_0.95[Tf₂N] and [C₂C₁Im][C₈C₁Im][Tf₂N][EtSO₄] has been analysed by TPD using line-of-sight mass spectrometry (LOSMS). The vapour phase consisted of all possible combinations of neutral ion pairs (NIPs) from the liquid mixture. Neither mixture showed evidence of decomposition during TPD, and the [C₂C₁Im]_0.05[C₈C₁Im]_0.95[Tf₂N] mixture was shown to obey Raoult’s law. Based on the TPD results, fractional distillations were attempted for [C₂C₁Im][C₈C₁Im][Tf₂N]₂ and [C₂C₁Im][C₈C₁Im][Tf₂N][EtSO₄] mixtures. The distillate from [C₂C₁Im][C₈C₁Im][Tf₂N]₂ was enhanced in the more volatile [C₂C₁Im][Tf₂N] components, but the [C₂C₁Im][C₈C₁Im][Tf₂N][EtSO₄] mixture underwent significant decomposition. The similarities and differences between UHV TPD, and high vacuum distillation, of ionic liquids, are discussed. Design parameters are outlined for a high vacuum ionic liquid still that will minimise decomposition and maximise separation of ILs of differing volatility.