Ultralow sulfur diesel production with defective 12-molybdophosphoric acid polyoxometalate

Abstract

A number of phosphomolybdic acid dispersed on SBA-15 catalysts with structural defects and strong acidity were synthesized and applied for the production of ultralow sulfur diesel. The oxygen defect concentration in the catalysts was quantitatively determined by the Rietveld refinement method. These catalysts contained many Brønsted (B) and Lewis (L) acid sites (648 to 1479 μmol g⁻¹) depending on the heteropolyacid content and the thermal treatment. The Mo⁵⁺/Mo⁶⁺ ratio calculated by XPS technique varied from 0.21 to 0.27%, indicating the creation of oxygen defects in the Keggin unit. The area under the IR absorption band at 980 cm⁻¹ (characteristic of the MoO bond) in the FTIR spectra of different catalysts was found to be inversely proportional to the variation of L acidity as a function of temperature, indicating that oxygen defects are the origin of the L acidity. In the oxidation reaction for removing dibenzothiophene, DBT, from a model diesel, the DBT conversion correlated well with the oxygen defect concentration and total surface acidity, confirming that the surface acidity and oxygen defects played key roles in DBT adsorption and oxidation. Almost 100% DBT conversion was achieved on the best 30 wt% H₃PMo₁₂O₄₀/SBA-15 catalyst under the optimal reaction conditions (reaction time 60 min, reaction temperature 70 °C, H₂O₂/DBT molar ratio 6–8, catalyst concentration 2–2.5 mg ml⁻¹, and hydrogen peroxide to formic acid molar ratio 1.5). A novel mechanism of the DBT oxidative removal in a biphasic system involving the participation of the neighboring L and B acid sites, oxygen defects, and the formation of peroxometallic and superoxometallic species was proposed.