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 applicationin desulfurization of diesel fuel. In this work, a series of caprolactam-based DESs were prepared using caprolactam as a hydrogen bonddonor (HBD) and tetrabutylammonium halides as hydrogen bondacceptors (HBAs). By screening different halide anions and molarratios of HBD and HBA, the DES tetrabutylammonium bromide/caprolactam (TBAB/CPL = 1:2) showed the highest desulfurizationefficiency with the Nernst partition coefficient of 5.04, which wasmuch higher than those of most metal-free DESs and ILs. Thedesulfurization efficiency could reach as high as 96.6% after a two-stage extraction. Finally, the extraction mechanism was investigatedsystematically using1H nuclear magnetic resonance and Fourier transform infrared spectroscopies. Then, a density functional theorystudy suggested that hydrogen bonding and C-Hmiddotmiddotmiddot pi interaction between DES and dibenzothiophene accounted for the higher desulfurization efficiency, enabling DES to effectively remove dibenzothiophene from the model fuels