Experimental and Molecular Insights for Separating Phenol and Pyridine from Coal Tar Using ChCl-SUF Deep Eutectic Solvent

Efficient separation of phenol and pyridine from coal tar is essential to improve fuel quality and achieve effective resource utilization. In this study, a choline chloride-sulfolane deep eutectic solvent (ChCl-SUF) was identified as a suitable solvent by the COSMO-RS model. The solvent shows extraction efficiencies above 98.6% and 70.8% for phenol and pyridine, respectively, after single-stage extraction. The effects of the solvent and the number of stages were evaluated to determine the optimal operating parameters. The competitive relationship between phenol and pyridine during the extraction process was also clarified. Even under competitive conditions, ChCl–SUF maintained high extraction performance, with the decrease in extraction efficiency for both solutes remaining below 1.0%. Quantum chemical calculations and molecular dynamics simulations were used to elucidate the separation mechanism. We found that ChCl plays a more important role than SUF and that dual hydrogen bonding between ChCl and phenol/pyridine is the fundamental driving force that endows selected with excellent extraction performance. The difference in extraction efficiencies between phenol and pyridine can be attributed to the relative strengths of the hydrogen-bonding interactions.