Generation of Superoxide Ion in Pyridinium, Morpholinium, Ammonium, and Sulfonium-Based Ionic Liquids and the Application in the Destruction of Toxic Chlorinated Phenols

Abstract

Generation of superoxide ion (O₂^•–) was carried out in four ionic liquids (ILs) having the same anion, bis­(trifluoromethylsulfonyl)­imide [N­(Tf)₂]⁻, and different cations, N-hexylpyridinium [HPy]⁺, N-methoxyethyl-N-methylmorpholinium [MO1,1O2]⁺, N-ethyl-N,N-dimethyl-2-methoxyethylammonium [N112,1O2]⁺, and triethylsulfonium [S222]⁺. Cyclic voltammetry (CV) and chronoamperometry (CA) electrochemical techniques were used in this investigation. It was found that O₂^•– is not stable in the [HPy]⁺-based IL. On the other hand, CV showed that the electrochemically generated O₂^•– is stable in [MO1,1O2]⁺-, [N112,1O2]⁺-, and [S222]⁺-based ILs for the time duration of the experiment. The long-term stability of the generated O₂^•– was then investigated by dissolving potassium superoxide (KO₂) in dimethyl sulfoxide (DMSO) in the presence of the corresponding IL. It was found that ILs containing [MO1,1O2]⁺ and [N112,1O2]⁺ offer a promising long-term stability of O₂^•– for various reactions to be used for several applications. However, it was found that after 2 h, about 92.5% of the generated O₂^•– in [S222]⁺ based IL was consumed. The diffusion coefficient and solubility of O₂ in the studied ILs were then determined using CV and CA techniques simultaneously. It was found that diffusion coefficients and CA steady-state currents increase with temperature increases, while the solubility of O₂ decreased. To our best knowledge, this is the first time that morpholinium and sulfoniumbased ILs were utilized as media for chemical and electrochemical generation of O₂^•–. Additionally, the chemically generated O₂^•–, by dissolving KO₂, was then used for the destruction of 2,4-dichlorophenol (DCP) in [MO1,1O2]­[N­(Tf)₂] under ambient conditions. The destruction percentage was higher than 98%. This work represents a novel application of the chemically generated O₂^•– for the destruction of toxic chlorinated phenols in ILs media