Comprehensive investigation of two environmentally-friendly imidazolium nitrate ionic liquids: from calorimetry to thermal risk evaluation

Due to their low volatility, ionic liquids (ILs) are widely used in many industries. However, some ILs are reported to be explosive under unstable conditions in the literature, and therefore a comprehensive assessment of the fire safety risks should be conducted before industrial applications. This study aims to investigate the thermal stability of two environmentally friendly used ILs (1-butyl-3-methylimidazolium nitrate ([C(4)mim]NO3) and 1-butyl-2,3-dimethylimidazolium nitrate ([C(4)mmim]NO3) using simultaneous thermogravimetric analyzer, differential scanning calorimetry, accelerating rate calorimeter, and thermogravimetry coupled to Fourier transform infrared spectroscopy. Based on the results of dynamic and isothermal thermogravimetric experiments, [C(4)mmim]NO3 shows higher thermal stability compared with [C(4)mim]NO3, indicating that the methyl substitution at the imidazolium C2 has a positive effect on the thermal stability of imidazolium nitrate. Additionally, the increased methyl substitution leads to more significant thermal decomposition of [C(4)mmim]NO3 and enhanced gas production under adiabatic conditions, which means that the potential thermal hazards are increased. Furthermore, [C(4)mim]NO3 and [C(4)mmim]NO3 may have different thermal decomposition pathways because the possible thermal decomposition products are not identical. The produced flammable gases also indicate that the operating temperatures of [C(4)mim]NO3 and [C(4)mmim]NO3 must be strictly controlled to prevent the occurrence of accidents.