Electrochemical oxidation of imidazolium-based ionic liquids: The influence of anions

The electrochemical oxidation of ionic liquids (ILs) in aqueous solutions on boron-doped diamond was investigated with the aim of explaining the influence of the IL composition Cl-,Br-,BF4-,PF6-,CF3SO3-,CH3C6H4SO3-. By keeping the cation (1-butyl-3-methylimidazolium) constant and varying the anion, a comparative study of their oxidizability could be carried out. The experimental data suggest that hydroxyl radicals were the dominant oxidant reacting with the IL ionic pair. The chloride ion enhanced imidazolium cation degradation, whereas CH3C6H4SO3- competed with the cation for anodically produced hydroxyl radicals. The nature of the background anions (Na2SO4, NaCl, NaBr) also influenced the efficiency of the electrochemical process. The addition of trace halides further decreased chemical oxygen demand (COD). The results suggest that different oxidation mechanisms prevail at different halide concentrations. However, peroxodisulphate generated at the BDD anode in a Na2SO4 supporting electrolyte did not play a significant role in imidazolium salt oxidation. The intermediates found in the single compartment reactor after 3 h of electrolysis suggest that O2- also occured and governed in part the electrochemical oxidation of IL.

► The explanation influence of IL composition on electrochemical oxidation at BDD anode.
► Identification of IL oxidants in different electrolyte.
► Two pathways of ILs transformation at BDD anode.
► Influence of halide ions on ILs electrooxidation at BDD.
► Identification of by-products for imidazolium salts with the different anions.