Thermal stability and decomposition mechanism of dicationic imidazolium-based ionic liquids with carboxylate anions

Abstract

This work presents the synthesis, thermal stability, and decomposition mechanisms of the dicationic ionic liquids (DILs) [Bis(CnMIM)][CmCOO]2 (in which n = 4 and 10, and m = 3 and 6). Short-term thermal stability was determined using conventional thermogravimetric analysis, while long-term thermal stability was studied through the kinetic analysis of thermal decomposition, using isothermal and isoconversional methodologies to determine the kinetic triplet (activation energy[Ea], pre-exponential factor [A], and kinetic model [f(α)]). For the isothermal methodology, several kinetic models were applied to determine which best fit the experimental data. In the isoconversional approach, the methods of Friedman, Osawa-Flynn-Wall and Kissinger-Akahira-Sunose were used, together with the Ea values. When the Ea values did not vary with the variation in α, the compensation effect was used to calculate lnA and f(α). [Bis(C4MIM)][C6COO]2 was found to be the stablest of the DILs analyzed, and there was a relationship between thermal stability and molar mass in both the isothermal and isoconversional methodologies, in which an increase in the cation's spacer chain led to a decrease in stability. Additionally, the results showed that there was more than one reaction occurring in the decomposition process. The thermal decomposition mechanism of the DILs was determined by analyzing the thermal decomposition residue, using electrospray ionization mass spectroscopy, nuclear magnetic resonance, and thermogravimetric analysis coupled to Fourier-transform infrared spectroscopy. The results indicated that [Bis(C4MIM)][C3COO]2 and [Bis(C10MIM)][C3COO]2 decompose mainly via nucleophilic substitution reactions, but also through formation of N-heterocyclic carbenes and elimination reactions.