Energy & Fuels 2005, 19, 447-452
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Oxidative Desulfurization of Diesel Fuels with Hydrogen Peroxide in the Presence of Activated Carbon and Formic Acid Guoxian Yu, Shanxiang Lu,* Hui Chen, and Zhongnan Zhu UNILAB Research Center of Chemical Reaction Engineering, East China University of Science and Technology, Shanghai, 200237, People’s Republic of China Received September 17, 2004. Revised Manuscript Received November 10, 2004
Oxidative desulfurization (ODS) of diesel fuels with hydrogen peroxide was studied, using activated carbon as the catalyst. The adsorption and catalytic properties of activated carbons for dibenzothiophene (DBT), and the correlation between them, were investigated. The higher the adsorption capacity of the activated carbons, the higher the catalytic performance in the oxidation of DBT. DBT is oxidized by active oxygen species on the carbon surfaces when DBT is adsorbed on the carbon surfaces. The effect of the aqueous pH on the catalytic activities of the activated carbons was also investigated. It is observed that the oxidation rate of DBT on the carbons is enhanced when the aqueous pH is W269 > C830 > C30 > C15. Table 1 shows that the highest surface area and total pore volume and the biggest average pore diameter are observed in the case of the wood activated carbon W101. To understand the role of adsorption of DBT in the oxidation reaction on activated carbon, adsorption of DBT in n-octane was initially performed on the selected activated carbons. Adsorption capacity of the selected activated carbons for DBT in n-octane was also summarized in Table 1. The adsorption capacity of activated carbons of wood origin is higher than that of activated carbons of coal origin, because of the larger surface area and bigger average pore diameter. Interestingly, Table 1 shows that the bigger the average pore diameter of the activated carbon, the higher the adsorption capacity
Figure 1. Oxidative removal of DBT on the selected activated carbons of saturation adsorption: A, W101; B, W660; C, W269; D, W602; E, C30; F, C830; G, C15; H, without active carbon. Reaction conditions were as follows: temperature, 333 K; reaction time, 60 min; n-octane volume, 36 mL; [DBT]initial ) 0.625 mmol; [H2O2]initial ) 21.53 mmol; aqueous phase volume, 7 mL; pH ) 2.0; and activated carbon dosage, 0.3 g.
of the activated carbon. Because DBT is a relatively large molecule, its adsorption capacity is greatly affected by pore size.24 Catalytic Activity of Activated Carbons in nOctane Containing DBT. To obtain the true catalytic performance of the carbons in the oxidative removal of DBT by hydrogen peroxide, the adsorptive removal of DBT must be eliminated. Therefore, before the activated carbons were introduced into the reaction system, they were thoroughly saturated with DBT by immerging them into an n-octane solution that contained DBT for 12 h. Figure 1 depicts the oxidative removal of DBT on selected activated carbons under saturated adsorption. To understand the role of activated carbon, the blank test without activated carbon and only using hydrogen peroxide acts as a control experiment. Figure 1 shows that the carbons have good catalytic activities in the oxidative removal of DBT with hydrogen peroxide and the control experiment has very low oxidative removal of DBT. One of its catalytic roles is to decompose hydrogen peroxide to form hydroxyl radicals. Firth and Watson25 found that activated carbon can catalyze the decomposition of hydrogen peroxide to produce hydroxyl radicals that are strong oxidizing agents. Surface reactions on the carbon surfaces, which result from thermal influences, often undergo free radical mechanisms. These reactions do not involve the participation of ions, and electrons are transferred not by themselves, but along with one or more attached atoms.26 The radicals produced during the surface reactions are likely to be resonance-stabilized on the carbon surfaces.20,21 Thus, (24) Jiang, Z.; Liu, Y.; Sun, X.; Tian, F.; Sun, F.; Liang, C.; You, W.; Han, C.; Li, C. Langmuir 2003, 19, 731. (25) Firth, J. B.; Watson, F. S. J Phys. Chem. 1925, 29, 987.
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Figure 2. Effect of pH on oxidation of DBT on the selected activated carbons: A, W101; B, W660; C, W269; D, W602; E, C30; F, C830; and G, C15. Reaction conditions were as follows: temperature, 333 K; reaction time, 60 min; n-octane volume, 36 mL; [DBT]initial ) 0.625 mmol; [H2O2]initial ) 21.53 mmol; aqueous phase volume, 7 mL; and activated carbon dosage, 0.3 g.
the radicals have enough time to oxidize the sulfur compound on the carbons. The sulfur compound was thought to be finally converted to SO42-, to be removed into the aqueous phase as well. This was confirmed by the addition of BaCl2 into the resulting aqueous phase, which made a white precipitate with the SO42- ion. However, catalytic performances of the carbons are different: the wood carbons have better catalytic performance than the coal carbons. The catalytic activity of the carbons decreased in the following order: W101 > W660 > W602 > W269 > C830 > C30 > C15. It is observed that the higher the adsorption capacity of the activated carbons, the higher the catalytic performance, when one compares the results of Figure 1 with those of Table 1. By analyzing the correlation between the oxidative removal of DBT on the carbons and the adsorption of DBT on the carbons, one can postulate that DBT is oxidized by active oxygen species on the carbon surfaces when DBT is adsorbed on the carbon surfaces. One of catalytic roles of activated carbon is to increase the collision probability of DBT and the active oxygen species to accelerate the reaction. Therefore, it is advantageous to the catalytic removal of DBT on activated carbon when the adsorption performance of activated carbon is good. Effect of Aqueous pH on the Catalytic Activities of Activated Carbons. The decomposition rate of hydrogen peroxide into hydroxyl radicals was reported to be dependent on the aqueous phase pH when the reaction was catalyzed by activated carbon.27 To investigate the effect of the aqueous phase pH on the catalytic activity of activated carbon, oxidation reactions of DBT were performed under various aqueous phase pH values, using HCl and NaOH solutions. Figure 2 shows the effect of the aqueous phase pH on the oxidation of DBT with hydrogen peroxide catalyzed by the activated carbons. It can be clearly observed that the oxidative removal of DBT on the carbons reaches the lowest value when the pH value is 14. The oxidative removal of DBT (26) Le´on y Le´on, D. C. A.; Radovic, L. R. Interfacial Chemistry and Electrochemistry of Carbon Surfaces. In Chemistry and Physics of Carbon, Vol. 24. Thrower, P. A., Ed.; Marcel Dekker: New York, 1994; p 247. (27) Elmer, C. L.; James, H. W. J. Phys. Chem. 1940, 44, 70.
Yu et al.
Figure 3. Co-catalysis by formic acid. Reaction conditions were as follows: temperature, 333 K; reaction time, 60 min; n-octane volume, 36 mL; [DBT]initial ) 0.625 mmol; [H2O2]initial ) 21.53 mmol; aqueous phase volume, 7 mL; and activated carbon dosage, 0.3 g.
for the carbons increases as the aqueous phase pH decreases. The results in Figure 2 indicate that the catalytic activity of the activated carbons increases as the aqueous phase pH decreases. The decomposition rate of hydrogen peroxide was investigated and determined to be catalyzed by activated carbon over the pH range of 2.6-9.5.27 The results of the study showed that the decomposition rate of hydrogen peroxide increased as the pH decreased when the pH was
