Catalytic oxidative desulfurization of diesel fuels using air in a two-step approach

This study presents a novel approach to fuel desulfurization in two steps where air is used as oxidant for the in situ generation of hydroperoxides from aromatics in diesel fuel and then the sulfur compounds are oxidized by the hydroperoxides to sulfones. Bulk and supported CuO based catalysts were examined for oxidation of surrogate and real diesel fuels for generation of hydroperoxides in situ using air. Among bulk CuO, CuO/Al₂O ₃, CuO/SiO₂, CuO/TiO₂, and CuO-ZnO/Al ₂O₃ tested, CuO/Al₂O₃ gave the highest conversion, but the highest yield and selectivity for hydroperoxide was obtained on bulk CuO catalyst for liquid phase air oxidation of isopropylbenzene. H₂-Temperature programmed reduction (H ₂-TPR) revealed the presence of dispersed CuO phase on supported catalysts with bulk CuO as the dominant phase on the unsupported catalyst. Three real diesel fuels were tested for liquid phase air oxidation at 120 C in the presence of bulk CuO catalyst which revealed different levels of hydroperoxides generation. This was consistent with model aromatic compound study which showed different levels of hydroperoxide formation in the order of isopropylbenzene > cyclohexylbenzene ≈ sec-butylbenzene ≠isopropylnaphthalene. As the second step of the process, the hydroperoxides formed in the real diesel fuel were then tested as oxidant for oxidation of sulfur compounds over Al₂O₃ and SiO₂ supported MoO₃ catalysts. MoO₃/SiO₂ catalyst was more effective with higher activity and selectivity for oxidation of 4,6-dimethyldibenzothiophene in diesel fuel with a total S concentration of 41 ppmw. At 40 C, 90% of sulfur compounds in diesel were oxidized at oxidant to sulfur molar ratio of 25. Selectivity ((% of sulfone produced)/(% of peroxide consumed)) measurement revealed 82% selectivity over MoO₃/SiO ₂ catalyst and only 43% over MoO₃/Al₂O ₃ catalyst under similar conditions. Catalysts with varying MoO ₃ loadings on both supports were characterized by XRD and H ₂-TPR methods. The results show that MoO₃ based catalysts which have a weak interaction with the support might be more preferred for sulfur oxidation.