Thermochemical study of some thiocyanate protic ionic liquids for evaluation of their temperature operating window

We studied the thermochemical properties of eight thiocyanate (SCN) protic ionic liquids or molten salts with diverse cation structures: 1-methylimidazolium ([MIm]), 1-ethylimidazolium ([EIm]), n-butylammonium ([BA]), triethylammonium ([TEA]), diisopropylethylammonium ([DIPEA]), guanidinium ([Gu]), 1,1,3,3-tetramethylguanidinium ([TMG]), and thiazolium ([Tz]) thiocyanates. The temperature and enthalpy of phase transitions of the ionic liquids were determined by differential scanning calorimetry. The thermal stability was investigated by thermogravimetric analysis (TGA) in dynamic (heating from 30 to 500 °C) and isothermal (keeping at 120 °C) modes. In dynamic experiments, the stability was shown to increase in the order [Tz][SCN] < [BA][SCN] < [TEA][SCN] ≈ [DIPEA][SCN] < [MIm][SCN] ≈ [EIm][SCN] < [TMG][SCN] ≈ [Gu][SCN]. [Tz][SCN] and [DIPEA][SCN] were found to start decomposing below their melting temperature. Isothermal experiments have shown long-term thermal stability of [MIm][SCN], [EIm][SCN], [TEA][SCN], [TMG][SCN], and [Gu][SCN] in a wide range of temperatures above the melting point, which makes them promising for extraction, separation, catalytic reactions, and other industrial or laboratory applications. The obtained results can be used to select the optimum conditions for these processes and for the regeneration of ionic liquids. © 2025 Elsevier B.V., All rights reserved.