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 dibenzothiophene from a model fuel has been investigated. Therefore, n-tetradecane was chosen as a diesel fuel matrix, and an aqueous H8[PV5Mo7O40] (HPA-5) polyoxometalate catalyst solution was used for the oxidative treatment. The oxidant of choice was molecular oxygen, and the desulfurization was conducted under moderate reaction conditions (130 °C, 20 bar O2). Thiophene and benzothiophene were found to degrade so fast into water-soluble substances that neither sulfoxides nor sulfones could be detected analytically as reaction intermediates. However, the analytical data show that the oxidation of refractory sulfur molecules, like dibenzothiophene (DBT) and its alkylated derivatives, into water-soluble products 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 in ECODS of dibenzothiophenes. The sulfoxide being formed initially is almost directly converted to the corresponding sulfone that accumulates and is only slowly transferred to water-soluble products. In contrast to thiophene and benzothiophene, DBT was found to inhibit the formation of active catalyst species. This inhibition could be reversed by adding a sacrificial substrate such as oxalic acid yielding the active catalyst species by reduction of HPA-5 from VV to VIV. Finally, the influence of organic nitrogen and oxygen compounds, which are also present in real distillation cuts, on ECODS has been studied.