Structural Defects, Lewis Acidity, and Catalysis Properties of Mesostructured WO₃/SBA-15 Nanocatalysts

For the first time, creation of oxygen defects in the crystalline structure of WO₃/SBA-15 catalysts and its correlation with Lewis acidity and catalytic activity in the oxidation of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in a model diesel were reported. All the WO₃/SBA-15 catalysts predominantly contained Lewis acid sites and some oxygen defects in the crystalline structure of WO₃. The oxygen defect concentration increased from 1.67% to 16.66% per lattice cell unit, and the number of Lewis acid sites varied from 92 to 458 μmol/g as the WO₃ content increased from 5 to 25 wt %. The 4,6-DMDBT conversion was almost proportional to both the number of Lewis acid sites and oxygen defects. Formic acid addition led to formation of peroxyformic acid which coordinated with surface W⁶⁺ to generate reactive oxygen species like peroxometallic complex, improving oxidant stability and 4,6-DMDBT oxidation efficiency. More than 99% of 4,6-DMDBT was removed with the best 25 wt % WO₃/SBA-15 catalyst within 15 min of reaction under the optimal condition. A reaction mechanism involving peroxometallic complexes formation, 4,6-DMDBT molecules adsorption, and surface oxidation reaction on structural defects and vanadia nanoparticles was proposed. This biphasic reaction system consisting of a catalyst bearing Lewis acid site, a green oxidant, an oxidant promoter, and a polar solvent would simultaneously perform the oxidation and separation of polyaromatic sulfur compounds in one operation which was very practical for ultralow sulfur diesel production.