Rational Design of Caprolactam-Based Deep Eutectic Solvents for Extractive Desulfurization of Diesel Fuel and Mechanism Study

Deep eutectic solvents (DESs) are considered novel alternatives compared to organic solvents and ionic liquids. However, the low extractive efficiency of fuel oil has limited its wide application in desulfurization of diesel fuel. In this work, a series of caprolactam-based DESs were prepared using caprolactam as a hydrogen bond donor (HBD) and tetrabutylammonium halides as hydrogen bond acceptors (HBAs). By screening different halide anions and molar ratios of HBD and HBA, the DES tetrabutylammonium bromide/caprolactam (TBAB/CPL = 1:2) showed the highest desulfurization efficiency with the Nernst partition coefficient of 5.04, which was much higher than those of most metal-free DESs and ILs. The desulfurization efficiency could reach as high as 96.6% after a two-stage extraction. Finally, the extraction mechanism was investigated systematically using 1H nuclear magnetic resonance and Fourier transform infrared spectroscopies. Then, a density functional theory study suggested that hydrogen bonding and C–H···π interaction between DES and dibenzothiophene accounted for the higher desulfurization efficiency, enabling DES to effectively remove dibenzothiophene from the model fuels.