High-performance dicationic ionic liquids based on imidazolium and quaternary ammonium for phenol extraction

Phenolic compounds serve as crucial chemical raw materials abundant in coal tar, so it is of great economic value to extract phenolic compounds from coal tar. Ionic liquids (ILs) are regarded as desirable extractants in this separation field due to considerable advantages over traditional methods. Compared to traditional mono-cationic ILs, dicationic ionic liquids (DILs) exhibit outstanding thermal stability and a wider operating temperature range. Our research focused on developing DILs based on typical imidazolium and quaternary ammonium structures for phenol separation, examining the impact of the methylene chain length between the cations on the extraction efficiency. Hydrogen bonding and π-π interactions between DILs and phenol were also investigated as driving forces in the extraction process. 1-methyl-3-(3-(trimethylammonio)propyl)-1H-imidazol-3-ium bromide ([MIMC₃N₁₁₁][Br]₂) with a moderate chain length was selected as the optimal extractant due to its weaker coulomb force inside the DIL, facilitating stronger hydrogen bonding with phenol compared to the other two DILs. Under optimized conditions, [MIMC₃N₁₁₁][Br]₂ exhibited the highest extraction efficiency of 96.03 % in model oil (toluene and phenol) with a phenol content of 200 g dm⁻³ and the DIL to phenol mole ratio was only around 0.3. [MIMC₃N₁₁₁][Br]₂ also featured excellent recyclability, maintaining high extraction efficiency through multiple cycles. Importantly, its lower solubility in oil-phenol mixtures compared to mono-cationic ILs significantly reduced contamination of the oil phase. The DILs in this work can exhibit significant potential for separating phenolic compounds with certain advantages.