Intermolecular Interactions Governing Extraction Efficiency of Polycyclic Thiophenes by Deep Eutectic Solvents

Abstract

Deep eutectic solvents (DESs) are a promising low-cost, low-toxicity medium for extractive desulfurization of petroleum, but the relationship between extraction performance and solvent properties requires further investigation. In this study, polycyclic thiophenes were extracted from a petroleum fuel analog (n-heptane) using different DESs to probe the effect of intermolecular interactions in DES systems on the extraction efficiency. Six DESs were prepared by combining quaternary ammonium salts [tetrabutylammonium chloride (TBAC), tetrabutylammonium bromide (TBAB), choline chloride (ChCl), benzyltriethylammonium chloride (BTAC), and benzyltributylammonium chloride (BTBAC)] with hydrogen bond donors [ethylene glycol (EG) and glycerol (Gly)]. Fourier-transform infrared (FTIR) and 1H NMR analysis was carried out to investigate the hydrogen bonding interactions and molar ratios. Model sulfur-containing aromatic impurities benzothiophene, dibenzothiophene, and 4,6-dimethyldibenzothiophene were extracted from n-heptane using the prepared DESs, and the extraction performance was quantified by the partition coefficient, defined as the ratio of solute concentrations between the DES and organic phase. Interestingly, the highest partition coefficient was achieved with tetrabutylammonium rather than benzyltributylammonium or benzyltriethylammonium ions, suggesting that possible π–π interactions are not sufficient to increase the partition coefficient in these systems. The interactions between DES components and polycyclic thiophenes were further investigated by electronic structure calculations, which indicate a preference for CH−π rather than π–π stacking interactions arising from the highly polarized C–H bonds of the quaternary ammonium ions.