Deep oxidative desulfurization of fuels using peroxophosphomolybdate catalysts in ionic liquids

A combination of catalytic oxidation and extraction in ionic liquid (IL) was used for the removal of benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from the model oil. Three peroxophosphomolybdates Q(3){PO4[MoO(O-2)(2)](4)} (Q = [(C4H9)(4)N](+), [C14H29N(CH3)(3)](+) and [C16H33NC5H5](+)) were synthesized and characterized. In the catalytic oxidation desulfurization (CODS) system containing the peroxophosphomolybdate with short alkyl chain ([(C4H9)(4)N](3)[PO4[MoO(O-2)(2)](4)}) and H2O2, the process exhibited low sulfur removal (16.8%). However, with addition of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4), the extraction and catalytic oxidative desulfurization (ECODS) system remarkably increased the removal of sulfur to 97.3% (with stoichiometric amounts of H2O2). The process was superior to the simple extraction with IL (16.3%). The results demonstrated that the ECODS system could deeply remove DBT from the model oil, and this desulfurization system could be recycled 4 times with slight decrease in activity. We also found that the catalysts with short alkyl chains exhibited higher catalytic activity than that with long alkyl chain in the ECODS system. Moreover, the reactivity of sulfur compounds decreased in the order of DBT > 4,6-DMDBT > BT.