Development of Low Melting Ionic Liquid Compositions Using Mixtures of Imidazolium and Pyrazolium Ionic Liquids

Development and applications of room temperature ionic liquids have largely focused on those based on 1,3dialkylimidazolium cation. In order to achieve low melting points the imidazolium cation was made asymmetric by the introduction of a large alkyl group on one nitrogen, while keeping a small alkyl group (e.g. methyl) on the other nitrogen. In the case of 1,3dialkylimidazolium tetrachloroaluminate ionic liquids the melting point decreased from 75 C for 1,3dimethylimidazolium tetrachloroaluminate, to 7 C for 1ethyl-3-methylimidazolium tetrachloroaluminate (1). For the corresponding tetrafluoroborate ionic liquids the melting point decreased from 103.4 C for 1,3dimethylimidazolium tetrafluoroborate (2) to about 13 C for 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4). One of the commonly used ionic liquids (1butyl-3-methylimidazolium hexafluorophosphate, m. pt. 10 C) contains a four carbon n-butyl group on one of the nitrogens. However, the presence of a long chain alkyl group on the cation increases the viscosity and decreases the density of the ionic liquid. Further, the ionic liquid undergoes oxidation at lower potentials and becomes less thermally stable.