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

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.