Imidazolium-based protic ionic liquids with perfluorinated anions: Influence of chemical structure on thermal properties

Abstract

The influence of the cation chemical structure, namely its side chain, on the thermal properties of imidazolium-based protic ionic liquids (PILs) with different perfluorinated anions (trifluoromethanesulfonimide (TFSI), trifluoromethanesulfonic (TFS), and trifluoroacetic (TFA) acids) was studied. With that purpose, twenty-one PILs with various alkyl (methyl-, ethyl-, butyl- and vinyl-) and fluorinated (-CH2CF3, -CH2CHF2, and -CH2CH2F) side chains were successfully synthesized. Special attention was paid to an optimization of their synthesis conditions. The structure of synthesized fluorinated and alkyl-substituted PILs was confirmed by means of nuclear magnetic resonance (NMR) analysis (1D: 1H, 19F, 13C and 2D: 1H – 19F HOESY experiments) and Fourier transform infrared (FTIR) spectroscopy, while PILs thermal behavior was determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) in dynamic and isothermal modes. The correlation between the PILs chemical structure and their thermal stability was established. It has been found that an increase of the length of alkyl side chain (methyl- < vinyl- < ethyl- < butyl-) leads to a lower melting point temperature. Moreover, it is shown that an increase in the amount of fluorine atoms in the cation chemical structure (-CH2F < -CHF2 < -CF3) leads to a PIL with a higher crystallinity (a higher melting point temperature) and a lower thermal stability. Thus, the best performance in terms of the thermal stability was reached for monofluorinated- (-CH2F) and butyl-TFSI PILs in liquid state: 370 °C and 400 °C, respectively, while the lowest thermal stability was obtained for trifluorinated- (-CF3) and vinyl-TFA PILs in solid state: 145 °C and 129 °C, respectively.