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