Extraction-Coupled Oxidative Desulfurization (ECODS) of n-Tetradecane as Model Oil under Moderate Conditions Using Molecular Oxygen

The capability of extraction-coupled oxidative desulfurization(ECODS) to remove refractory sulfur compounds like dibenzothiophenefrom a model fuel has been investigated. Therefore, n-tetradecane was chosen as a diesel fuel matrix, and an aqueous H-8[PV5Mo7O40] (HPA-5) polyoxometalatecatalyst solution was used for the oxidative treatment. The oxidantof choice was molecular oxygen, and the desulfurization was conductedunder moderate reaction conditions (130 degrees C, 20 bar O-2). Thiophene and benzothiophene were found to degrade so fast intowater-soluble substances that neither sulfoxides nor sulfones couldbe detected analytically as reaction intermediates. However, the analyticaldata show that the oxidation of refractory sulfur molecules, likedibenzothiophene (DBT) and its alkylated derivatives, into water-solubleproducts occurs via the sulfoxides and sulfones in a subsequent reaction.Furthermore, the kinetic data reveal that the degradation of the sulfone,i.e., the carbon skeleton, represents the rate-determining step inECODS of dibenzothiophenes. The sulfoxide being formed initially isalmost directly converted to the corresponding sulfone that accumulatesand is only slowly transferred to water-soluble products. In contrastto thiophene and benzothiophene, DBT was found to inhibit the formationof active catalyst species. This inhibition could be reversed by addinga sacrificial substrate such as oxalic acid yielding the active catalystspecies by reduction of HPA-5 from V-V to V-IV. Finally, the influence of organic nitrogen and oxygen compounds,which are also present in real distillation cuts, on ECODS has beenstudied.