Synthesis, thermal behavior and kinetic study of N-morpholinium dicationic ionic liquids by thermogravimetry

Six N-morpholinium dicationic ionic liquids (DILs) with the following general formula [C4H8NO]2(CH2)n[X]2, being n = 4, 6 and 8 and X = Br, NTf2 (where NTf2 = bis(trifluoromethane)sulfonimide) have been synthesized. Their thermal behavior was studied by thermogravimetry (TG) using five different heating rates (2, 4, 8, 10 and 12 K min−1) aiming at assessing their relative thermal stability. All the DILs investigated undergo a single step of mass loss in the temperature range between 200 and 520 °C, except in the case of both the C4 DILs (with bromide and NTf2 imide anion), where two partially overlapping steps occur. NTf2 DILs are more stable than bromide ones according to the thermal stability scale based on the onset decomposition temperature. Taking into account the kinetic analysis of the thermal decomposition, the estimated decomposition time at given degree of conversion for the same temperature (250 °C) was evaluated as an alternative stability parameter. A substantial agreement between the stability trends assessed with the two different approaches was found, and the effect of the length of the linker between the two morpholinium rings of the dicationic structures was also considered. The reaction mechanism beneath the decomposition of the shortest member (C4) of the bromide series was investigated by DFT. A double retro-SN2 pathway was found to be the reasonable two-step mechanism for the first thermal degradation event. This hypothesis was further corroborated by analyzing the samples obtained from TG after heating the bromide and NTf2 C4 DILs up to the first degradation temperatures as well as the thermal behavior of the assumed spiro intermediate ILs.