Roles of the structural defects and the combined acidity of H3PW12O40/Zr-MCM-41 catalysts in ultralow sulfur diesel production

Abstract

Oxidative desulfurization (ODS) has been become an important technical route for the production of ultralow-sulfur fuel. This work focuses on a deep understanding of the roles of structural defects, surface acidity and catalytic properties of phosphotungstic acid grafted on Zr-doped MCM-41 catalysts in the oxidation of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in a model diesel. Two kinds of Zr-MCM-41 mesoporous solids (termed ZrM-I and ZrM-II) were synthesized with different methods and their structural defects and surface acidity were investigated. When H₃PW₁₂O₄₀ (termed HPW) was grafted on the ZrM solids, their dispersion and structural deformation were largely governed by the interaction with the ZrM support. Three types of HPW nanoparticles, confirmed by ³¹P-NMR-MAS spectra, were formed, which were related to the unchanged, deformed and defective Keggin units. The amount of total surface acidity was 227 μmol g⁻¹ for HPW/ZrM-I and 307 μmol g⁻¹ for the HPW/ZrM-II catalyst. For 4,6-DMDBT oxidation, the pore regularity and structural defects of the ZrM supports and the surface acidity of the HPW/ZrM catalysts played the key roles. The HPW/ZrM-II catalyst showed higher catalytic activity, due to its larger number of surface acid sites resulting from the greater amount of deformed and defective Keggin-type nanoclusters. The 4,6-DMDBT conversion reached nearly 100% over the HPW/ZrM-II catalyst under optimal condition (reaction temperature: 70 °C; reaction time: 60 min; catalyst concentration: 0.3 g L⁻¹; H₂O₂/4,6-DMDBT molar ratio: 8; HCOOH/H₂O₂ molar ratio: 1.5). The 4,6-DMDBT oxidation was sterically favored on the combined Lewis (L) and Brønsted (B) acid sites in the catalysts. L acid sites were the centers for 4,6-DMDBT adsorption and the neighboring B acid sites participated in the transfer of protons and oxygen atoms between adsorbed sulfur compounds and the surface peroxophosphotungstate complex, benefiting the 4,6-DMDBT oxidation.