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

SixN-morpholinium dicationic ionic liquids (DILs) with the following general formula [C4H8NO]2(CH2)n[X]2,beingn= 4, 6 and 8 and X = Br, NTf2(where NTf2= bis(trifluoromethane)sulfonimide) have been synthesized.Their thermal behavior was studied by thermogravimetry (TG) usingfive different heating rates (2, 4, 8, 10 and12 K min−1) aiming at assessing their relative thermal stability. All the DILs investigated undergo a single step ofmass loss in the temperature range between 200 and 520 °C, except in the case of both the C4 DILs (with bromideand NTf2imide anion), where two partially overlapping steps occur. NTf2DILs are more stable than bromide onesaccording to the thermal stability scale based on the onset decomposition temperature. Taking into account thekinetic analysis of the thermal decomposition, the estimated decomposition time at given degree of conversionfor the same temperature (250 °C) was evaluated as an alternative stability parameter. A substantial agreementbetween the stability trends assessed with the two different approaches was found, and the effect of the length ofthe linker between the two morpholinium rings of the dicationic structures was also considered. The reactionmechanism beneath the decomposition of the shortest member (C4) of the bromide series was investigated byDFT. A double retro-SN2 pathway was found to be the reasonable two-step mechanism for thefirst thermal deg-radation event. This hypothesis was further corroborated by analyzing the samples obtained from TG afterheating the bromide and NTf2C4 DILs up to thefirst degradation temperatures as well as the thermal behaviorof the assumed spiro intermediate ILs.