Catalytic oxidative desulfurization systems based on Keggin phosphotungstate and metal-organic framework MIL-101

The tetrabutylammonium salt of phosphotungstic acid, TBA3PW12O40 (PW12), has been encapsulated in the chromium terephthalate metal-organic framework MIL-101. Characterization of the composite PW12@MIL-101 by powder X-ray diffraction, FT-IR, FT-Raman and 31P magic-angle spinning (MAS) NMR spectroscopies confirmed that the structures of MIL-101 and the polyoxometalate anion were retained after immobilization. Both PW12 and the composite material were examined as catalysts in systems for the oxidative desulfurization (ODS) of model oils containing dibenzothiophene (DBT), 1-benzothiophene (1-BT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate was employed as both extractant and reaction medium. In a one-pot operation carried out at 50 °C, the BT derivatives in the model oils were extracted into the IL and then oxidized (using 30% H2O2 as oxidant) to form the corresponding sulfones, resulting in a continuous decrease in the sulfur content in the oil phase. The sulfur removal from the model oil with a sulfur content (for each BT derivative) of 500 ppm reached 100% within 1 h for DBT, 100% within 3 h for 1-BT, and 99% within 4 h for 4,6-DMDBT (sulfur content, 3–7 ppm). The ODS system containing the composite PW12@MIL-101 could be recycled three times with only a slight decrease in the catalytic performance.