(379k) Catalytic Oxidation Desulfurization of Dibenzothiophene By Bronsted-Lewis Acid Ionic Liquids

Introduction: Sulfur oxides formed by burning S-compounds in transportation fuel led to serious pollution to the environment. [1] Ionic liquids used in the desulfurization of fuel performs excellent desulfurization capability and has many advantages, such as mild reaction conditions, simple operation, and a high desulfurization rate of multi-sulfide. [2-3] Four kinds Bronsted-Lewis ionic liquids with different metal chloride ([HO₃SC₃NEt₃]AlCl₄, [HO₃SC₃NEt₃]ZnCl₃, [HO₃SC₃NEt₃]FeCl₄ and [HO₃SC₃NEt₃]CuCl₃) were investigated as catalyst in the oxidative desulfurization.

Materials and Methods: 1,3-propanesultone, HCl, triethylamine, acetone, anhydrous ZnCl₂, anhydrous FeCl₃, anhydrous AlCl₃, anhydrous CuCl₂, H₂O₂, n-octane, dibenzothiophene, glacial acetic acid, isopropanol, 1,4-benzoquinone, 4-amino-2,2,6,6-tetramethylpiperidine and 5,5-dimethyl-1-pyrroline N-oxide. For the desulfurization experiment, 10 mL model oil or actual diesel mixed with certain amount of ILs, H₂O₂ and AcOH were added into a three-necked flask under vigorous stirring for reacting a certain time, and using oil bath to adjust the different reaction temperature. During the reaction, the sulfur content in upper phase was periodically separated and detected by sulfur analyzer. The sulfur content was measured by THA-2000S Ultraviolet Fluorescence Sulfur Analyzer. And the sulfur types were detected by GC-FPD.

Results and Discussion: The effect of DBT removal by four ILs was investigated. The results showed that the catalytic activity of the four ILs followed the order of [HO₃SC₃NEt₃]AlCl₄ > [HO₃SC₃NEt₃]ZnCl₃ >> [HO₃SC₃NEt₃]FeCl₄ > [HO₃SC₃NEt₃]CuCl₃. In the two-phase oxidation desulfurization system, a very small amount of [HO₃SC₃NEt₃]AlCl₄ showed a remarkable catalytic ability for the oxidation of dibenzothiophene (10 mL model oil with 1000 ppm sulphur content, 1mL H₂O₂, 2mL acetic acid, 55℃, 20min, 0.25g IL. According the analysis of the potential ROSs by quenching experiments and Electron Paramagnetic Resonance (EPR). There are three types of ROSs, ¹O₂，•OH, and •O₂^―, in the system were detected. The desulfurization system can be recycled 4 times without noticeable decrease in activity (Table 1). [HO₃SC₃NEt₃]AlCl₄ showed excellent recycling performance. This IL-ODS system was applied to actual diesel and achieved a good desulfurization effect (Table 2). In summary, Among these four ILs, [HO₃SC₃NEt₃]AlCl₄ shows a remarkable catalytic activity. The EPR detection and quenching experiments verify the ¹O₂ is in charge of the oxidation of DBT. IL is not only to promote the generation of ROSs in the reaction system, but also to act as a reaction medium between the oxidants and sulfides. The process and mechanism for catalytic oxidation of DBT in IL-ODS system was proposed in Fig. 1.