Fenton-like photodegradation of imidazolium and pyridinium ionic liquids: experimental study with theoretical insight

The widespread deployment of ionic liquids (ILs), particularly those incorporating imidazolium and pyridinium cationic frameworks, has led to significant environmental concerns due to their inherent chemical stability and potential ecotoxicological effects. This study presents a comprehensive analysis integrating experimental methodologies and quantum chemical calculations to elucidate the degradation of 1-ethyl-3-methylimidazolium chloride (IE) and 1-ethyl-3-methylpyridinium chloride (mPyE) via a photo-Fenton-like process under UV irradiation. The experimental approach involved analyzing degradation efficiency and kinetics using spectrophotometry. It is observed that both imidazolium (IE) and pyridinium (mPyE) ILs underwent substantial degradation, achieving 84.78% and 89.49% removal, respectively, within 120 min. The corresponding first-order rate constants were 1.511 × 10−2 min−1 for IE and 1.89 × 10−2 min−1 for mPyE. The higher degradation percentage and rate constant for the pyridinium IL (mPyE) indicate its greater reactivity compared to the imidazolium IL (IE) under the tested conditions. Quenching experiments confirmed that hydroxyl radicals (•OH) are the primary reactive species responsible for the degradation, while superoxide anions (O2•¯) and singlet oxygen (1O2) contribute to a lesser extent. Complementary theoretical quantum chemical calculations using density functional theory provided crucial insights into the reactivity of the ILs based on global and local descriptors. These theoretical findings supported the experimental observations, revealing that the reaction pathway is initiated by the attack of the hydroxyl radical primarily on the alkyl group in imidazolium IL, whereas the attack is preferentially directed toward the ring structure in pyridinium IL. A plausible degradation pathway is suggested based on Fukui indices.