Insight into effective denitrification and desulfurization of liquid fuel with deep eutectic solvents: an innovative evaluation criterion to filtrate extractants using the compatibility index

Tremendous attention has been paid to removal of N-/S-containing compounds from liquid fuel due to the necessity of reducing NOx and SOx emissions. In this work, the compatibility index (CI or PCI) was developed as a new criterion to evaluate the removal efficiency (Re) of quinoline, carbazole and dibenzothiophene (DBT) in deep eutectic solvents (DESs). For alkaline quinoline, acid–base extraction was pronounced, and ChCl/mandelic acid was the optimal extractant to achieve a removal efficiency of 99.2 wt% (CI = 0.37). For near-neutral carbazole and DBT, polarity compatible extraction was the predominant mechanism. The highest Re value of 94.9 wt% for carbazole removal was obtained in the ChCl/mandelic acid extractant (PCI = −0.044). DBT was difficult to remove directly because of its low molecular polarity. After adding H2O2, the desulfurization efficiency was found to increase to 99.1 wt% in the ChCl/p-toluenesulfonic acid extractant (PCI = 0.014). Furthermore, experiments with real diesel confirmed that the highest desulfurization and denitrification efficiency was 67.7 wt% and 63.7 wt%, respectively, in ChCl/p-toluenesulfonic acid. The compatibility index developed in this study has the potential to simplify the filtration operation of extractants and to intensify the removal efficiency of N-/S-containing compounds in the category of “green extraction”.