Thermal stability of imidazolium-based ionic liquids investigated by TG and FTIR techniques

Abstract

Thermal stability and thermal decomposition kinetics of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) were investigated using isothermal and non-isothermal thermo-gravimetric analysis. The results indicated that isothermal test in the air showed that after storage at 180 and 300 °C both for 10 h, the conversion rate of [bmim]BF4 was 1.28 and 2.57 %, respectively. When stored at less than 300 °C, all bands of IR spectra are nearly unchanged. It suggests that [bmim]BF4 basically is stable. When isothermal stored at 350 °C for 2 h, the ATR–FTIR spectra show that the relative intensity of BF 4 −1 anion near band of 1058 cm−1 fell by 36.7 %. Due to the intensity of the absorption band declined significantly in the range of 300–350 °C, the anion of BF 4 −1 was possibly dissociated by the action of air and heat; thus, at the time [bmim]BF4 is not stable. The average apparent activation energy of thermal degradation obtained by multiple scanning methods is 105.6 kJ mol−1. Moreover, mass spectrometric analysis also shows that the z/m of [bmim+] is 139.1; in the thermal degradation process, the possibility of generating a dimer between imidazolium cation exists.