Optimization of composition of a directly combined catalyst in dibenzothiophene oxidation for deep desulfurization

This paper describes the conceptual mechanism in dibenzothiophene (DBT) oxidation with hydrogen peroxide using a directly combined catalyst of quaternary ammonium bromide and phosphotungstic acid. The effect of the catalyst composition on DBT oxidation was investigated in detail and a series of different amphiphilic quaternary ammonium salts was tested, including octadecyltrimethylammonium bromide (STAB), cetyltrimethylammonium bromide (CTAB), (1-tetradecyl)trimethylammonium bromide (TTAB), and dodecyltrimethylammonium bromide (DTAB). The catalyst with a proper cation is required to achieve high activity toward the sulfur compounds and to be easily separated from the system. The exact position of the amphiphilic catalyst in the interface will directly affect the resistance in the DBT mass transfer step and therefore the rate of the overall catalysis process. Further, to better understand the effect of interface properties on DBT oxidation, a comparative study was carried out in different alkanes. The reaction rates increase when the carbon number of the alkane decreases. Finally, a conceptual model was established in the DBT oxidation process based on the interaction between phosphotungstic acid (TPA) and STAB. It is suggested that the mass transfer of DBT from the organic media toward the interface, rather than the oxidized TPA phase transfer step, may be rate limiting.