Probing the effect of acidic NH and C(2)-H protons of imidazolium cation on ionic liquids thermal decomposition

Knowledge of thermal stability and understanding how it depends on the structure are crucial for practical application of ionic liquids (ILs). Most studies dedicated to structure–thermal stability relationships separately consider two subclasses of ILs – aprotic and protic ones, which differ from each other by the presence of an acidic proton in the structure. In contrast, comparative studies of thermal stability of both ILs classes are much less common in the literature. The effect of acidic protons on the kinetics of thermal decomposition of imidazolium-based ILs has been aimed to study within the framework of current research. For this purpose, we have selected four ILs that are composed of the same anion but a cation with changing amounts of NH and C(2)-H acidic protons. The treatment of thermogravimetric analysis (TGA) data in terms of isoconversional methodology has allowed us to evaluate relative reactivities of ILs under consideration. TGA coupled with infrared spectroscopy (IR) of gaseous products has been performed in order to gain mechanistic insights into the acceleration effect of acidic protons. Taking into account both kinetic and mechanistic data, we have suggested the explanation of the effect of NH and C(2)-H acidic protons on the thermal stability of imidazolium ionic liquids.