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Mechanism and Catalytic Behavior of Quaternary Ammonium Salts in Oxidative Desulfurization

Abstract For the insolubility of binary phase system of oxidative desulfurization, quaternary ammonium salts as phase-transfer catalysts have been applied to enhance the reaction. To investigate the mechanism, nine catalytic systems were carried out. The results showed that the reaction was of pseudo-first-order in phase-transfer catalyst and substrate of organosulfur, and slightly influenced by the stirring rate documented the reaction was chemical controlled. In the oxidant anion [RCOOO−] transfer process, there was an ion-pair [Q+ROOO−] formed. To clarify the catalytic behavior of phase-transfer catalyst in the liquid-liquid phase system, oxidant anion transfer, interfacial tension and the activity of anion were exploited. It is shown that ion-pair [(C4H9)4N+RCOOO−] could be transferred from aqueous to organic phase, and catalyze the oxidative reaction best. Dibenzothiophene (with larger electrical density) was easier to be removed than thiophene, and the desulfurization rate of dibenzothiophene by tetrabutyl ammonium bisulfate was 97.45%. Some kinds of phase-transfer catalysts can reduce the interfacial tension to a proper value, therefore increasing the interfacial area and improving the rate of reaction. With greater distance separating cation from anion (0.317 nm) [(C4H9)4N+RCOOO−] was much easier to react with organosulfur than [RCOOOH].