A size-matched POM@MOF composite catalyst for highly efficient and recyclable ultra-deep oxidative fuel desulfurization

Abstract

A composite of Keggin-type phosphotungstic acid (H₃PW₁₂O₄₀) encapsulated in sized-matched metal–organic framework UiO-67 (PW₁₂@UiO-67) was prepared as a heterogeneous catalyst for extractive and catalytic oxidative desulfurization systems (ECODS). The appropriate cage size and narrow windows of UiO-67 serve to avoid the leaching of phosphotungstic acid, and also allow most sulfur compounds commonly found in fuels to approach the catalyst. The as-synthesized composite catalyst was characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), energy dispersive spectroscopy (EDS), and N₂ adsorption–desorption isotherms to confirm that PW₁₂ was successfully encapsulated in UiO-67 and the structure of UiO-67 was retained after immobilization. The obtained composite was examined as a heterogeneous catalyst in ECODS and showed remarkably high catalytic activity in the ultra-deep oxidative desulfurization of three sulfur compounds (benzothiophene, dibenzothiophene, and 4,6-dimethyldibenzothiophene) under mild conditions. The sulfur removal from 1000 ppm model oil reached 99.5% within 1 h at 70 °C for dibenzothiophene. The composite catalyst PW₁₂@UiO-67 could be easily regenerated and recycled for eight consecutive cycles without significant loss in catalytic activity.