In situ formation of deep eutectic solvents for dephenolization of coal tar model oil via zwitterionic extractants: ecotoxicity evaluation, experimental investigation and intermolecular interactions

Phenol and its derivatives serve as critical intermediates in the synthesis of fine chemicals and pharmaceuticals. Their recovery from coal tar is economically beneficial and helps reduce environmental pollution. In this study, three zwitterionic compounds, 4-(1-methyl-1H-imidazol-3-ium-3-yl)butane-1-sulfonate [MImC4S], 3-(1-methyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate [MImC3S], and 4-(triethylammonio)butane-1-sulfonate [TEtAC4S], were prepared and applied for the in situ formation of deep eutectic solvents (DESs) to extract phenol and its derivatives from the coal-derived model oil. The algal growth inhibition test showed that the zwitterionic extractants exhibited low acute algal toxicity towards Raphidocelis subcapitata . The proposed method enabled efficient phenol recovery while avoiding the use of traditional aqueous treatments that rely on strong acids and alkalis. The effects of zwitterion-to-phenolics molar ratio M Zwi : M Ph , initial phenolics concentration, extraction time, and temperature on the in situ extraction process were systematically investigated. The results demonstrated that all the zwitterionic extractants achieved phenol extraction efficiencies exceeding 95% at an initial phenolics concentration of 200 g · dm −3 , M Zwi : M Ph = 1:1, extraction time of 15 min, and temperature of 298 K. The results of quantum chemical calculations identified hydrogen bonding between phenolic hydroxyl groups and the zwitterionic sulfonate oxygen atoms as the primary driving force for the extraction process. Finally, the reusability of the zwitterionic extractants was evaluated, wherein the regenerated extractants exhibited negligible loss in extraction efficiency over five cycles.