Design and application of dual sulfonic acid-based ternary deep eutectic solvents for facile extraction combined with oxidative desulfurization of fuel

Green and efficient deep desulfurization of fuels is in significant demand. Current deep eutectic solvent (DES)-based extraction combined with oxidative desulfurization (EODS) faces challenges: the roles of DES components remain unclear, and diluents are often necessary for viscosity reduction, complicating solvent recovery. Here, acidic DES-based EODS was investigated to remove recalcitrant aromatic S compounds (namely, dibenzothiophene and its derivatives) from fuels. Eight binary acidic DESs with different acidities and viscosities were prepared by combining two H-bond acceptors (HBAs)–tetraethylammonium chloride and tetrabutylammonium chloride (TBAC)–and four H-bond donors (HBDs), namely, acetic acid, oxalic acid, methanesulfonic acid (MSA), and p-toluenesulfonic acid (PTSA). DESs comprising sulfonic acids and HBAs with longer alkyl chains showed superior desulfurization performance without concern for viscosity increase. Accordingly, a dual sulfonic acid-based ternary DES, termed TD-2 (TBAC/2PTSA/2MSA) was designed, achieving ≥ 99.97 % S removal at 60 °C within 15 min. Experimental and computational studies revealed that PTSA and MSA catalyzed the oxidation of thiophenes via a peroxysulfonic acid-based cycle powered by H₂O₂, with PTSA being preferred over MSA. TBAC acted as a phase-transfer catalyst, while the mass transfer was maintained by low DES viscosity, which was rather attributed to MSA. TD-2 demonstrated excellent regenerability over seven cycles, with minimal environment, health, and safety impacts. These results indicate that the proposed method is facile, highly efficient, and eco-friendly.