Extraction-Coupled Oxidative Desulfurization (ECODS) and Denitrogenation (ECODN) of Real Fuels under Mild Conditions Using a Polyoxometalate Catalyst and Molecular Oxygen

In this contribution, the difficulties of removing sulfur from actual refinery products, i.e., kerosene and diesel fuel, using an aqueous solution of H8PV5Mo7O40 (HPA-5) catalyst and molecular oxygen as an oxidizing agent (ECODS) are examined. Regarding oxidation of the S-containing aromatics, the reactivity in the distillation cuts proceeded in the following order: thiophenes > benzothiophenes > dibenzothiophenes. Kerosene consists primarily of easily removable thiophenes and benzothiophenes, whereas diesel contains benzothiophenes and mostly refractory dibenzothiophenes that are hard to remove. Hence, desulfurization of kerosene is significantly easier compared to diesel. Furthermore, the existence of aromatic hydrocarbons in the organic matrices of the real fuels poses challenges to the desulfurization process described in this work. They prevent the formation of the active catalyst by inhibiting the partial reduction of HPA-5 from VV to VIV. The inhibition of aromatic hydrocarbons in ECODS increases in the following order: dibenzothiophenes > benzothiophenes > thiophenes. Moreover, the level of inhibition rises in proportion to the quantity of condensed aromatic rings: tri+-aromatics > diaromatics > monoaromatics. To address this issue, oxalic acid as a sacrificial agent mitigates the inhibition of aromatic compounds. This acid partially reduces the HPA-5 from VV to VIV, yielding the active catalyst species. Vanadyl sulfate, which acts as a source of VIV, is also capable of dealing with aromatic inhibition. Moreover, the process of removing sulfur is expedited as the length of the n-alkane chain increases.