Homogeneous oxidative desulfurization catalyzed by a recoverable reaction-controlled phase transfer catalyst based on trilacunary Keggin polyoxometalate

Abstract

A phosphotungstic reaction-controlled phase transfer catalyst [C₇H₇(CH₃)₃N]₉PW₉O₃₄ (Q₉PW₉) with a trilacunary Keggin structure was successfully synthesized and characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and thermo-gravimetric analysis (TGA). This catalyst showed high activity for removing the most refractory sulfur compounds such as dibenzothiophene (DBT), benzothiophene (BT), thiophene (TH) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) by an oxidative desulfurization process. A nearly 100% DBT removal rate was achieved under optimal conditions (temperature: 60 °C; catalyst dosage: 0.2015 g; O/S molar ratio: 10; pre-immersion time of the catalyst in H₂O₂: 10 min). And the catalyst could be easily separated from the oxidative desulfurization system at the end of the reaction when H₂O₂ was consumed. The IR and TG analyses of the recycled catalyst indicated that the structure of the polyoxometalate anions could be retained intact after reuse. The catalyst Q₉PW₉ could be recycled for three consecutive ODS cycles with only a slight decrease of desulfurization efficiency. Moreover, a very positive desulfurization result was achieved in the oxidative desulfurization of actual diesel catalyzed by Q₉PW₉. Finally, according to the discussion of the experimental and analytical data, a mechanism was proposed for the oxidation process of DBT.