Structure-Dependent Fenton Reactivity and Degradation Pathway of Methylimidazolium Ionic Liquids

Short-chain imidazolium-based ionic liquids (IBILs) substituted by methyl groups are highly water-soluble and have the potential risk of entering the water environment with wastewater. In this work, the structure-dependent Fenton reactivity and degradation pathway of methylimidazolium ionic liquids were investigated, including the influence of the proton activity and structural symmetry of IBILs. The chemical reactivity of specific sites (atoms) was quantitatively predicted by a condensed Fukui function (f_A^–). Aprotic IBILs ([DMIM]­[BF₄] and [TMIM]­[BF₄]) were more resistant to the ^•OH attacks than protic IBILs ([HHIM]­[BF₄] and [HMIM]­[BF₄]) because deprotonation reduced the f_A^– value of the attack sites. Structural symmetry increased the initial rate of the total organic carbon (TOC) reduction (k_0,TOC) due to the existence of symmetric Fenton reaction sites. Trimethyl-substituted [TMIM]­[BF₄] showed the lowest TOC removal efficiency and k_0,TOC presumably because the C2-methyl substitution greatly weakened the f_A^– value of C2. The plausible mineralization pathways were proposed on the basis of gas chromatography coupled with mass spectrometry analyses. The more methyl substitution resulted in weaker Fenton reactivity but more oxidation pathways. For [TMIM]­[BF₄], C2-methyl reduced the Fenton reactivity of the imidazolium skeleton but increased the oxidation opportunities of dual N-methyls.