Experimental investigation on the influence of anion structure on the thermal decomposition properties of EMIM-based ionic liquids

To reveal the relationship between anion structure and thermal decomposition property, an experimental system integrating comprehensive thermal analysis and Fourier-transform infrared spectroscopy is constructed to investigate the thermal decomposition mechanisms of four 1-ethyl-3-methyl imidazolium-based ([EMIM] + ) ionic liquids with anion structures of thiocyanate ([SCN] - ), dicyanamide ([DCA] - ), tetrafluoroborate ([BF 4 ] - ), and bis(trifluoromethylsulfonyl)imine ([NTF 2 ] - ). Results show that the thermal stability follows the order of [EMIM][NTF 2 ] > [EMIM][BF 4 ] > [EMIM][DCA] > [EMIM][SCN] by decomposition temperature. Spectrum analyses reveal that the anion structures are thoroughly disrupted, forming different gas products, while the cation structure only undergoes substitution and elimination reactions on the imidazole ring. Due to the differences in anion structure, methyl isothiocyanate, trifluoromethanesulfonamide and sulfur dioxide, ammonia, and boron trifluoride are found as key products during the decomposition of [EMIM][SCN], [EMIM][NTF 2 ], [EMIM][DCA], and [EMIM][BF 4 ], respectively. Following designs of anion structure for propellant may start from [SCN] - and [DCA] - to obtain better decomposition property.