Synthesis and oxidative desulfurization of novel lactam-based Brønsted-Lewis acidic ionic liquids

•A novel kind of Brønsted-Lewis acidic ILs was developed.•The ILs were investigated in ODS process and exhibited highly efficient catalytic performance.•S-content in diesel was reduced to less than 10 ppm through a one-step ODS.•Multi-phase systems consisted of water, oil and ILs were investigated.

Novel Brønsted-Lewis acidic ionic liquids having a protonated N-octylcaprolactam-based cation and Cl/nZnCl2 anion (n = 1, 2 and 3) were synthesized and investigated as catalysts in the oxidative desulfurization (ODS) of both model oil and real diesel fuel, with hydrogen peroxide (H2O2, 30 wt%) as oxidant. It is observed that the ODS performance increases significantly with the increase of ZnCl2 proportion. Then detailed experiments for the ODS of model oil were carried out with [Hnoc]Cl/3ZnCl2 to investigate the influence of some important factors, including reaction temperature, molar ratio of H2O2/S, ILs dosage, initial S-content and sulfide species. It is worth noting that, with the mass ratio of ILs/oil of only 1:20, satisfactory conversion rate can also be achieved for the removal of dibenzothiophene (DBT), and the ILs were used for six cycles without a noticeable decrease in activity. What is more important, the ILs can reduce the S-content of a real diesel from 559.7 ppm to 8.2 ppm with 98.5% sulfur removal rate and 96.3% diesel recovery through a one-step ODS process. Deduced from the GC-PFPD spectra, almost all the original S-compounds and their oxidized products were completely converted and extracted, respectively. The outstanding desulfurization efficiency can be attributed to the good phase transfer property of the ILs.