Long term stability of superoxide ion in piperidinium, pyrrolidinium and phosphonium cations-based ionic liquids and its utilization in the destruction of chlorobenzenes

Abstract

Investigation of the chemical generation and long term stability of superoxide ion, O 2 · - , in three bis (trifluoromethylsulfonyl) imide [TFSI −] anion based ionic liquids, ILs, with cations of 1-(3-methoxypropyl)-1-methylpiperidinium [MOPMPip +], 1-hexyl-1-methyl-pyrrolidinium [HMPyrr +] and trihexyl (tetradecyl) phosphonium [P14,666 +] has been conducted. The chemical generation of O 2 · - in ILs was carried out by dissolving potassium superoxide, KO 2, in the corresponding IL, while electrochemical generation was carried out by reduction of O 2 to O 2 · - in ILs followed by analysis using cyclic voltammetry technique. It was found the solubility of KO 2 in the studied ILs depends on the structure of ILs in addition to the temperature. However, in all cases the solubility was high enough for many applications. The Long term stability of the generated O 2 · - was carried out by the application of KO 2 in aprotic solvent, dimethyl sulfoxide, in the presence of the corresponding IL. UV–visible spectrophotometry at an absorbance range of 190–400 nm was used to determine the stability of O 2 · - . It was found that ILs containing [MOPMPip +] and [HMPyrr +] cations offer a promising long term stability of O 2 · - for various reactions and applications, while consumption of O 2 · - in [P14,666 +][TFSI −] with a rate constant of 16.2 × 10 −5 s −1 indicates that the generated O 2 · - in this IL is not stable enough for practical applications. To our best knowledge this is the first time a piperidinium and pyrrolidinium based ILs were used as media for the chemical generation of O 2 · - . The chemically generated O 2 · - by dissolving KO 2 was then used for the destruction of chlorobenzenes in [MOPMPip +][TFSI −] and [HMPyrr +][TFSI −] at ambient conditions. This work presents the first attempt to use KO 2 for the destruction of chlorinated aromatic hydrocarbons in ILs media.