Thermal stability of some imidazolium [NTf2] ionic liquids: Isothermal and dynamic kinetic study through thermogravimetric procedures

Abstract

Thermal stability is an essential property to select the most suitable compounds for applications as high temperature lubricants, thermal fluids and solvents for high-temperature organic reactions. The degradation processes were determined by dynamic thermogravimetric analysis under dry air atmosphere for four ionic liquids (ILs). The ILs are based in the anion bis(trifluoromethylsulfonyl)imide and four cations with imidazolium ring. These cations are 1-ethyl-3-methylimidazolium, 1-propyl-3-methylimidazolium, 1-butyl-3-methylimidazolium and 1-butyl-2,3-dimethylimidazolium.For the four ILs dynamic and isothermal thermogravimetric analyses under air atmosphere were performed using a thermogravimetric analyser. Weak differences in the isothermal and dynamic degradation of the ILs have been found according to the alkyl chain length.Four methods were applied to the dynamic results to determine thermal degradation kinetic parameters: two differential methods due to Kissinger and Friedman and two integral ones proposed by Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose. Additionally, kinetic analysis was also made using data from the isothermal measurements. Small differences have been found for the activation energies obtained from the application of these different kinetic methods to the selected ILs.The highest activation energy values for all the kinetic methods chosen in this study were obtained for the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide IL.