Extraction desulfurization process of fuels with ammonium-based deep eutectic solvents

Organic sulfides in fuels have become one of the main sources of serious pollution. The desulfurization of fuels has become a frontier scientific topic demanding prompt solutions. Research shows that the emergence of ionic liquid analogues-deep eutectic solvents (DESs)-has provided a new opportunity for the deep extraction desulfurization of fuels because of their cheap and easily obtained raw materials, higher extraction desulfurization efficiencies, and simple and environmentally friendly synthesis process. In this study, some DESs were designed and synthesized. Choline chloride (ChCl), tetramethyl ammonium chloride (TMAC), and tetrabutyl ammonium chloride (TBAC) were chosen as typical hydrogen bond acceptors (HBA), and malonic acid (MA), glycerol (Gl), tetraethylene glycerol (TEG), ethylene glycol (EG), polyethylene glycol (PEG), and propionate (Pr) were chosen as hydrogen bond donors (HBD), from which a series of deep eutectic solvents were synthesized. Research shows that the above DESs can be used for the desulfurization of fuels successfully. Tetrabutyl ammonium chloride-based DESs possess the highest extraction efficiency. In optimal conditions, the extraction efficiency of TBAC/PEG can reach as high as 82.83% for one cycle, which is much higher than the traditional and functionalized ionic liquids (ILs). After five cycles, the extraction efficiency can reach up to 99.48%. In addition, sulfur content in fuels can be reduced to less than 8.5 ppm and deep desulfurization was realized. Finally, the extraction mechanism was investigated systematically with quantitative super(1)H NMR and FT-IR. The hydrogen bonds formed between DESs and benzothiophene (BT) account for the higher desulfurization efficiency. This study will provide new green solvents for the extraction desulfurization process of fuels.