Quantitative determination of oxygen defects, surface lewis acidity, and catalytic properties of mesoporous MoO₃/SBA-15 catalysts

A set of MoO₃/SBA–15 mesoporous catalysts were characterized with a variety of spectroscopic techniques and their crystalline structures were refined with Rietveld method. Oxygen defect concentration crystallite size, phase composition, surface acidity, mesoporous regularity, and textural properties were reported. Both α–MoO₃ and β–MoO₃ phases coexisted but α–MoO₃ was predominated. Oxygen defects were created in the orthorhombic structure and its concentration decreased from 3.08% for the 20 wt%MoO₃/SBA-15 to 0.55% for the 25 wt%MoO₃/SBA-15. All the MoO₃/SBA–15 catalysts chiefly contained a big number of Lewis acid sites originating from oxygen defects in MoO₃ crystals. In the absence of formic acid, the oxidation of 4,6-dibenzothiophene (4,6–DMDBT) in a model diesel was almost proportional to the number of Lewis acid sites. In the presence of formic acid, 4,6–DMDBT oxidation was significantly affected by the formation of surface peroxometallic complex and Lewis acidity. Formic acid addition could improve the ODS efficiency by promoting peroxometallic complex formation and enhancing oxidant stability. Under the optimal reaction condition using the best 15 and 20 wt%MoO₃/SBA-15 catalysts, more than 99% 4,6–DMDBT could be removed at 70 °C within 30 min. This work confirmed that 4,6–DMDBT oxidation is a texture and particle size sensitive and Lewis acidity dependent reaction. This work also shows that crystalline structure refinement combination with experiments can gain new insights in the design of heterogeneous nanocatalysts and help to better understand the catalytic behavior in the oxidative desulfurization reactions.