Degradation of ionic liquids by Fenton reaction; the effect of anions as counter and background ions

The influence of counter ions (Cl−, C(CN)3− and CF3SO3−) and background ions (C6F11O2−, C8F15O2− and C10F19O2−) on the degradation rates of 1-butyl-3-methylimidazolium cation (bmim+) in a Fenton-like system were investigated. The degradation rate of bmim+ was influenced by the particular anions as follows: Cl− > C(CN)3− > CF3SO3−. The inhibition of imidazolium moiety oxidation was observed in the case of counter anions in ionic liquid which entered the competition for hydroxyl radicals with bmim+ or which formed complexes with ferric and ferrous ions. The presence of perfluoric acids as a background in solution also had significance for the bmim+ degradation process. These organic anions can inhibit bmim+ oxidation through strong interactions between positively charged bmim+ and negatively charged perfluocarboxylic anions (PFCA). Additionally, bmim+ seemed to be more resistant to oxidation in the presence of a longer n-perfluoric alkyl chain anions. However, it was observed that the inhibition of bmim+ oxidation decreased when the concentration of PFCA in solution exceeded a critical micellar concentration. This fact can be explained by weaker interactions between bmim+ and perfluoric anions, stronger associated with aggregated PFCA in these experimental conditions (pH 3). Our investigations also indicated the possibility of PFCA interactions with ferric and ferrous ions.