Oxidative Desulfurization Of Fuels By Novel Fenton-like Catalysts In Ionic Liquids

Production of ultra-low-sulfur diesel has become a major task of refineries all over the world. The presence of sulfur compounds in diesel fuel has shown an adverse impact on the environment and hence it is getting ample attention from the media and scientific community. It is because sulfur-bearing compounds are converted to SOx during the combustion in car engines. This conversion not only results in acid rain, but also high contents of sulfur oxides in exhaust fumes lowers the efficiency of catalytic converters in cars. Sulfur oxides also poison catalysts in catalytic converters used for reducing CO and NOx emissions and this severely affects environment. The hydrodesulfurization (HDS) process has been used for the past several years to eliminate sulfur compounds for industrial purposes. It is efficient for the removal of thiols, sulfides and thiophenes, but less effective in removing the refractory sulfur compounds such as dibenzothiophene and their alkyl derivatives. Sulfur removal by oxidative process aims to promote a reaction in the opposite direction to HDS, i.e. by forming oxidized sulfur species.Three quaternary ammonium Fenton-like materials Q[FeCl4][Q=(CH3)4N, C14H29N(CH3)3, and C18H37N(CH3)3] were synthesized and characterized by IR, UV-vis, Raman spectrum and elemental analysis. Then, they were used as effective catalysts in oxidative desulfurization combining ionic liquids (ILs) extraction and H2O2oxidation. The reaction conditions were optimized by investigating reaction temperature, time, the type of ILs, the amount of H2O2and catalysts in detail. The removal of dibenzothiophene (DBT) in model oil could get up to97.0%at30℃for1h. In addition, the utilization rate of H2O2could be improved by adding it in batches. The catalytic system, consisting of IL and catalysts, could be easily separated by applying an external magnetic field and regenerated by washing with water, which could be recycled for six times with a slight decrease in desulfurization efficiencies. However, the desulfurization efficiency of this catalyst was poor in hydrophobic IL.In order to further enlarge the application field of Fenton-like catalysts, various choline Fenton-like catalysts prepared by choline chloride (ChCl) and metal salts (FeCl3, ZnCl2, SnCl2) were characterized by FT-IR, UV-vis, Raman, ESI-MS and elemental analysis. High desulfurization efficiency of dibenzothiophene (DBT) could be obtained not only in hydrophilic ionic liquid (IL)[C4mim]BF4(94.9%) but also in hydrophobic IL [C8mim]BF4(97.2%) by using ChFeCl4as a catalyst under mild conditions. ESR measurements could give the evidence that the active oxygen species generated by ChFeCl4and H2O2in IL were involved in the catalytic oxidation of DBT. The aromatic sulfur compounds, benzothiophene (BT), DBT, and4,6-dimethyldibenzothiophene (4,6-DMDBT), could be oxidized to the corresponding sulfones, which were detected by GC-MS. Moreover, the catalytic system containing IL and catalyst could be easily separated from oil and could be recycled at least five times without a significant decrease on removal of DBT.A new family of dialkylpiperidinium tetrachloroferrate catalysts, such as [C2OHmpip]FeCl4,[C4mpipJFeCl4,[Csmpip]FeCl4, and [C12mpip]FeCl4, were synthesized and characterized by FT-IR, Raman, and ESI-MS spectra. Their catalytic activities for removal of dibenzothiophene (DBT) in extraction and catalytic oxidative desulfurization system (ECODS) were evaluated under different reaction conditions. Results indicated that the97.1%removal of DBT was obtained with [C4mpip]FeCl4as catalyst in ionic liquid [C8mim]BF4at30℃in60min. The optimal H2O2/sulfur molar ratio was only3.5:1, which suggested that the catalyst was one of the most efficient catalysts reported so far. UV-vis spectra provided an evidence that the lower desulfurization reactivity in PF6--containing ionic liquids was attributed to the strong interaction between ionic liquid and DBT. Through the gas chromatography-mass spectrometer (GC-MS) analysis, dibenzothoiphene sulfone was proved to be the only product of oxidation of DBT. Furthermore, the process of ECODS system was confirmed by GC-MS.Though H2O2is one of the best candidates as an oxidant, high concentration of H2O2is potentially explosive issue during transportation and may do harm to human health. In order to obtain deep-desulfurization fuel with low concentration Of H2O2, a series of organic hexacyanoferrates were synthesized and employed as catalysts in ionic liquids (ILs) for catalytic oxidation of dibenzothiophene (DBT) and benzothiophene (BT) and4,6-dimethyldibenzothiophene (4,6-DMDBT). High activity was achieved by using1-butyl-3-methylimidazolium hexacyanoferrate ([C4mim]3Fe(CN)6) as catalyst and1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4) as extractant in the presence of H2O2under mild conditions. It was interesting to find that the concentration of H2O2had a significant influence on desulfurization efficiency. The sulfur removal was76.3%with30wt%H2O2as oxidant while it could reach97.9%with7.5wt%H2O2. Electron spin-resonance (ESR) spectroscopy measurements gave the evidence that the active oxygen species O2·-was generated in the catalytic oxidative desulfurization process and gas chromatography-mass spectrometer (GC-MS) analysis indicated that the sulfur compounds were oxidized to the corresponding sulfones. The kinetic investigations showed that oxidation of sulfur compounds presented a pseudo first-order kinetic.Magnetic ionic liquids (MILs)1-n-butyric acid-3-methylimidazolium chloride/xFeCl3(x=0.5,1,1.5,2)([C3H6COOHmim]Cl/xFeCl3) were synthesized and characterized. In desulfurization peocess, the IL acted not only as catalyst but also extractant. The acidities of these MILs were characterized by using pyridine and acetonitrile as IR spectroscopy probes. It found that [C3H6COOHmim]Cl/2FeCl3was both Bransted and Lewis acidic. The magnetic susceptibilities of [C3H6COOHmim]Cl/2FeCl3were measured using a Quantum Design superconducting quantum interference device (SQUID). The MIL [C3H6COOHmim]Cl/2FeCl3was found to be highly active for extraction and catalytic oxidative desulfurization of model oil under mild conditions. Of note, the removal of benzothiophene (BT), which has been regarded as a refractory aromatic sulfur compound, could be achieved up to100%in10min. After reaction, the MIL and model oil could be easily separated by applying an external magnetic field due to its paramagnetic property.