Solvent-Free Desulfurization System to Produce Low-Sulfur Diesel Using Hybrid Monovacant Keggin-Type Catalyst

Two quaternary ammonium catalysts based on the monovacant polyoxotungstate ([PW₁₁O₃₉]^7-, abbreviated as PW₁₁) were prepared and characterized. The desulfurization performances of the PW₁₁-based hybrids (of tetrabutylammonium and trimethyloctadecylammonium, abbreviated as TBA[PW₁₁] and ODA[PW₁₁], respectively), the corresponding potassium salt (K₇PW₁₁O₃₉, abbreviated as KPW₁₁) and the peroxo-compound (TBA-PO₄[WO(O₂)₂], abbreviated as TBA[PW₄]) were compared as catalysts for the oxidative desulfurization of a multicomponent model diesel (2000 ppm S). The oxidative desulfurization studies (ODS) were performed using solvent-free systems and aqueous H₂O₂ as oxidant. The nature of the cation in the PW₁₁ catalyst showed to have an important influence on the catalytic performance. In fact, the PW₁₁-hybrid catalysts showed higher catalytic efficiency than the peroxo-compound TBA[PW₄], known as Venturello compound. TBA[PW₁₁] revealed a remarkable desulfurization performance with 96.5% of sulfur compounds removed in the first 130 min. The reusability and stability of the catalyst were also investigated for ten consecutive ODS cycles without loss of activity. A treated clean diesel could be recovered without sulfur compounds by performing a final liquid/liquid extraction diesel/EtOH:H₂O mixture (1:1) after the catalytic oxidative step.