Solvent Extraction of Sr²⁺ and Cs⁺ using Protic Amide-Based Ionic Liquids

Abstract

Sixteen protic amide-based ionic liquids (ILs) derived from N,N-dimethylformamide and other protophilic amide derivatives with bis(trifluoromethanesulfonyl)imide or bis(perfluoroethylsulfonyl)imide as conjugated anions were synthesized in a one-pot reaction with very high yields. All sixteen of these protic ionic liquids (PILs) were characterized by NMR spectra, thermogravimetric analysis, and differential scanning calorimetry. These protic amide-based ionic liquids were tested as extraction solvents using dicyclohexano-18-crown-6 (DCH18C6) as an extractant for separation of Sr²⁺ and Cs⁺ from aqueous solutions. The extraction efficiencies were studied in comparison with those derived from both imidazolium-based and ammonium-based IL extraction systems. Excellent extraction efficiencies were found for a number of these ILs using DCH18C6 as an extractant. Unlike findings for imidazolium-based and ammonium-based ILs, the observed enhancement trend for the extraction efficiency associated with our amide-based ILs is not directly correlated with the enhanced hydrophilicity of the corresponding cations of the PIL system. The effects on extraction efficiencies of solution acidities, anions, and alkyl chain lengths in the cations of ILs were also investigated and reported.

Keywords:

• Ionic liquids
• protic amide-based ionic liquids
• solvent extraction

ACKNOWLEDGEMENTS

This research was supported by the Basic Energy Science Program of the Office of Science, U.S. Department of Energy, under Contract DE-AC05-0096OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC.

Notes

^a T_onset: Onset decomposition temperature based on TGA measurement. The estimated errors are ±5°C.

^b NM: not measured.

^a NM: not measured.