Thermal stability and exothermic behaviour of imidazole ionic liquids with different anion types under oxidising and inert atmospheres

Abstract

Ionic liquids (ILs), as a type of salt that is liquid at room temperature, break the entrenched views on salt when it was proved that some of them could be ignited. In this research, an innovative method was used to evaluate the thermal stability of such special salts and speculated their ignition mechanism based upon gas analysis. Four typical representative imidazole ILs (BMIMBF4, BMIMDCN, BMIMNO3, and BMIMOAc) were selected, and their thermal decomposition characteristics and thermal effect were obtained by simultaneous thermogravimetric analyser (STA) and differential scanning calorimetry (DSC), respectively. Furthermore, their decomposition products were investigated by thermogravimetry coupled with Fourier-transform infrared spectroscopy (TG-FTIR). It is noteworthy that they behaved diversely in DSC experiments and exhibited a radically different thermal effect in their decomposition. In addition, there exists positive feedback in the mechanism of the exothermic phenomenon of selected ILs in the air. Nevertheless, TG curves of four selected ILs were almost the same under air or nitrogen conditions. Series TG-FTIR experiments confirmed that the initial decomposition products of BMIMBF4 and BMIMDCN under air or nitrogen are extremely different. The decomposition products of BMIMNO3 and BMIMOAc in the air atmosphere have more oxidative products than nitrogen. The results denoted that the thermal decomposition mechanism of the four selected ILs in the air were the same as under nitrogen. The reducibility of the decomposition products induced the peculiar thermal effects of these imidazole ILs under the different gas environments. This study was innovative in researching the thermal effect and mechanism of imidazole ILs’ decomposition and provided certain safety guidance for the process safety and loss prevention of imidazole ILs.