Efficient extraction of S-heterocyclic aromatics from fuel oil with oxalic acid-based deep eutectic solvent

The presence of thiophenes in fuel oil not only affects the quality of oil products, but also causes serious environmental problems. The efficient extraction of S-heterocyclic aromatics from fuel oil by π-electron donor-based (i.e., oxalic acid) deep eutectic solvents (DESs) is proposed for the first time. The distribution coefficient and separation factor of n-heptane + 2-methylthiophene/3-methylthiophene/thiophene in 100 DESs were predicted by COSMO-RS model, and the optimal DESs (i.e., Tetraethylammonium chloride-oxalic acid / Tetraethylammonium bromide-oxalic acid) was selected as the potential extractants. The selectivity and extraction capacity of the two DESs were measured and compared by liquid–liquid equilibrium experiments. The thermodynamic consistency of the experimental data was tested by Othmer-Tobias and Hand equations, which show that R2 is>0.95, and the experimental results have high reliability. The interaction mechanism between the extractant and the target systems was analyzed by molecular dynamics simulation and quantum chemical calculation. The results of the mean square displacement and fraction free volume show that thiophene has higher displacement and diffusion in DES2 (i.e., tetraethylammonium bromide and oxalic acid) compared to 2-methylthiophene and 3-methylthiophene molecules. Moreover, the interaction analysis show that electrostatic energy is the main effect between molecules. Finally, the weak interaction between different molecular pairs was identified by the molecular surface electrostatic potential and the independent gradient model. This work provides insight into the design and screening of novel DESs for the extraction and separation of S-heterocyclic aromatics.