Deep Oxidative Desulfurization of Fuel Oils Catalyzed by

Sep 2, 2009 - results reported previously.30 There are four linear W-O-W bridge bonds in the [W10O32]4- anion. The absorption at 320 nm in the low-ene...
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Ind. Eng. Chem. Res. 2009, 48, 9034–9039

Deep Oxidative Desulfurization of Fuel Oils Catalyzed by Decatungstates in the Ionic Liquid of [Bmim]PF6 Huaming Li,*,†,‡ Xue Jiang,† Wenshuai Zhu,† Jidong Lu,‡ Huoming Shu,† and Yongsheng Yan† College of Chemistry and Chemical Engineering, Jiangsu UniVersity, 301 Xuefu Road, Zhenjiang 212013, People’s Republic of China, and State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology, Wuhan 430074, People’s Republic of China

Three decatungstates with short carbon chains as the cations, such as tetrabutylammonium decatungstate ([(C4H9)4N]4W10O32), tetramethylammonium decatungstate ([(CH3)4N]4W10O32), and benzyltriethylammonium decatungstate ([(C2H5)3NC7H7]4W10O32), were synthesized and then used as a catalyst in the extractive catalytic oxidative desulfurization (ECODS) system in the ionic liquid (IL) of [Bmim]PF6, and hydrogen dioxide (H2O2) was used as an oxidant. During the optimized process, the sulfur level in the model oil (1000 ppm S) can be reduced to 8 ppm, which is consistent with the standards of deep desulfurization. The temperature, the reaction time, and the amount of H2O2 and catalyst, as well as the type of the cations of decatungstates, all played vital roles in desulfurization efficiency, which were studied in detail to optimize the reaction conditions. The system could be recycled five times before the sulfur removal decreased sharply. 1. Introduction The desulfurization of fuels has received much attention recently in items of environmental issues. Sulfur that is present in fuels leads to SOx pollution generated by the combustion of fuels in vehicle engines. To reduce this pollution, many countries have determined to restrict the sulfur level in fuels to 10 ppm, even the zero level of sulfur in the near future.1 The traditional route to desulfurization is hydrodesulfurization (HDS), which can remove most sulfur compounds, such as thiols, sulfides, disulfides, and other sulfur compounds, by converting them to hydrogen sulfide.2 Some sulfur compounds in fuels such as aromatic thiophenes, especially dibenzothiophenes (DBTs) and its derivatives with alkyl substitutions at the 4 and 6 positions, are difficult to remove via HDS. Therefore, the sulfur level of fuels after the HDS process is still far from the standard of environmental legislations. To achieve deep desulfurization through typical HDS, severe conditions are required, such as higher temperature, higher hydrogen pressure, and more-active catalysts, which inevitably leads to the high capital. Therefore, to get fuels with low sulfur content, it is stringently necessary to explore the alternative desulfurization approaches, among which oxidative desulfurization (ODS) is quite an effective way to remove those refractory sulfur compounds. In the ODS process, these sulfur compounds are oxidized into more polar sulfones, which can be removed in the later extraction process. The electron density of sulfur compounds increased in the order of DBT < 4-MDBT < 4,6-DMDBT, which is the same as the order of the reactivity of these compounds in ODS, but opposite that in HDS.3 Dibenzothiophene, and its derivatives, then can be oxidized more easily than thiophenes, which is very convenient for deep desulfurization. So far, much attention has been focused on the ODS and its successive extraction step. The extractants used are usually N,Ndimethylformamide (DMF), acetonitrile (ACN), methanol, dimethyl sulfoxide (DMSO), and ionic liquids (ILs).3-5 * To whom correspondence should be addressed. Tel.: 86-051188791800. Fax: 86-0511-88791708. E-mail: [email protected]. † College of Chemistry and Chemical Engineering, Jiangsu University. ‡ State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology.

It has now become a trend to use ILs as extractants in extractive desulfurization, because of their unique properties, such as high thermal stability, nonvolatility, immiscibility with many nonpolar organic solvents and good solubility characteristics, etc.6-11 In addition, as a green solvent, used ILs can be easily regenerated in many ways.9,12 However, sulfur removal as the sole extraction of fuels with ILs varied over a large range and was not satisfactory.7,13-16 That may be because the polarity of aromatic compounds is not high enough to be extracted sufficiently by ILs. To enhance the polarity of the aromatic compounds, as well as improve the sulfur removal, extraction desulfurization was used, in combination with the ODS, which has also been studied extensively. Lu et al. used the IL of [Hmim]BF4 as a catalyst and solvent in a desulfurization system that was comprised of oil, hydrogen dioxide (H2O2) as an oxidant, and IL. The sulfur removal could reach >90%, which was superior to the simple extraction with IL.17 Lo and his partners combined the chemical oxidation with solvent extraction by room-temperature ionic liquid toward desulfurization.18 With ILs of [Bmim]BF4 and [Bmim]PF6, they also compared the results of ILs with those of normal organic solvents such as DMF and CH3CN, which indicated that ILs, especially the water-immiscible ionic liquid, showed good ability of extraction to sulfur compounds and the sulfur removal can reach ∼100%, much better than mere solvent extraction with ILs, although the reaction time was somewhat long.18 Herein, we develop a new desulfurization system: extractive catalytic oxidative desulfurization (ECODS), which has been reported in our previous investigations.19,20 It is known that many types of polyoxometalates usually have good selectivity for the oxidation of sulfur compounds.5,19-22 The catalyst used in our work herein was decatungstate. Decatungstates were first taken as photocatalytic catalysts23,24 and then were introduced into chemical oxidation such as the selective oxidation of alcohols25 and cyclohexene26 by Guo as catalysts, in which this type of catalyst showed high selectivity and catalytic activity. In addition, decatungstates that have quaternary ammonium with a long carbon chain as the cation have also been investigated, and their performance in desulfurization is good.27 However, decatungstates that have quaternary ammonium with a short carbon chain as the cation did not show good catalytic activity

10.1021/ie900754f CCC: $40.75  2009 American Chemical Society Published on Web 09/02/2009

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Figure 1. TG/DSC of three catalysts (a) [(C4H9)4N]4W10O32, (b) [(C2H5)3NC7H7]4W10O32, and (c) [(CH3)4N]4W10O32.

in the ODS system, which can be concluded from our prepared experiments. Based on the previous study, this type of decatungstste was considered as a catalyst to combine with ILs to form a new catalytic system in the ECODS system. In this work, three decatungstatess[(C4H9)4N]4W10O32, [(CH3)4N]4W10O32, and [(C2H5)3NC7H7]4W10O32sthat have quaternary ammonium with short carbon chains as the cation were synthesized and used as a catalyst while H2O2 was used as an oxidant. The IL of [Bmim]PF6 in the system was not only served as the extractant and the media, but also was used to stabilize the active peroxo species in the process of the reaction to make high efficiency. DBT was oxidized into the corresponding sulfone, and then extracted into the IL phase continuously, leading to the low sulfur content in the oil.

model oil were added into the two-necked kettle in turn. Later, the mixture was stirred for 0.5 h at a temperature of 60 °C under atmospheric pressure. After reaction, the kettle was cooled to room temperature. The model oil was removed and prepared for further analysis via gas chromatography-flame ionization detection (GC-FID) (Agilent 7890A, HP-5 column, 30 m long × 0.32 mm inner diameter (id) × 0.25 µm film thickness) with the method of adding an internal standard. The GC process started at 100 °C and the temperature was increased to 200 °C at a rate of 15 °C/min. 2.4. Recycle of the ECODS System. After reaction, the model oil was transferred from the kettle and was heated in 120 °C to remove excess H2O2. Fresh H2O2 and model oil then were added into the original reaction kettle for the next run.

2. Experimental Section

3. Results and Discussion

2.1. Preparation of Three Decatungstates. Tetrabutylammonium decatungstate [(C4H9)4N]4W10O32 was synthesized according to the published procedure.28,29 In this methodology, 0.0194 mol (6.4 g) Na2WO4 · 2H2O was mixed with 40 mL of H2O in a three-necked flask (with a volume of 250 mL), followed by the addition of 13.4 mL of 3 mol/L HCl. After boiling for a few minutes, the clear yellow solution was precipitated by adding the aqueous solution of tetrabutylammonium bromide (C4H9)4NBr (2.50 g/4 mL). During the procedure, the temperature was maintained at 100 °C under continuous stirring. The white precipitate was filtered, washed with water (50 mL × 2), and then dried at 50 °C under vacuum. Two other catalystsstetramethylammonium decatungstate ([(CH3)4N]4W10O32) and benzyltriethylammonium decatungstate ([(C2H5)3NC7H7]4W10O32)swere prepared using the same method. 2.2. Catalyst Characterization. Infrared (IR) spectra of all the catalysts (KBr pellets) were recorded on Nicolet Model Nexus 470 FT-IR equipment, while ultraviolet-visible light (UV-Vis) spectra were obtained using a Model UV-2450 spectrophotometer (Shimadzu Corporation, Japan) in ACN. Thermogravimetric and differential scanning (TG/DSC) analysis was done on a Model STA-449C Jupiter apparatus (Netzsch Corporation, Selb, Germany). The testing process was 30-800 °C at a heating rate of 15 °C/min and holding at 800 °C for 40 min. The content of tungsten was measured by gravimetric determination (GD) in a muffle furnace burning at 800 °C for 40 min. 2.3. ECODS of Model Oil with Decatungstates. The model sulfur compound (DBT) was dissolved in n-octane to obtain the model oil with a sulfur concentration of 1000 ppm (BT and 4,6-DMDBT were dissolved in n-octane to make solutions of 250 ppm, respectively) with the tetradecane as the internal standard (4000 ppm). The catalyst, 30% H2O2, [Bmim]PF6, and

3.1. Characterizations of Catalysts. 3.1.1. TG/DSC Curves of Catalysts. Thermogravimetric and differential scanning analysis of [(C4H9)4N]4W10O32 (Figure 1a) showed that this compound had no crystalline water, because there was no mass loss and no exothermic peak at ∼140 °C, which was in good accordance with the DSC curve. There are two exothermic peaks (262.6 and 406.5 °C) on the DSC curve, which may be due to the decomposition of the [W10O32]4- anion and the quaternary ammonium cation, respectively. The final remainder was tungsten oxide. The compound of [(CH3)4N]4W10O32 was given with a similar decomposition process as [(C4H9)4N]4W10O32. Its exothermic peaks appear at 273.2 and 424.4 °C (see Figure 1c). As for [(C2H5)3NC7H7]4W10O32, there are three exothermic peaks on its corresponding DSC curve. The peak at 280.1 °C may be due to the decomposition of the [W10O32]4- anion, which is consistent with the other two compounds (262.6 °C for [(C4H9)4N]4W10O32 and 273.2 °C for [(CH3)4N]4W10O32). The loss of the C7H7 group may cause the mass loss as well as the peak of 360.3 °C on the DSC curve. The peak at 405.8 °C may be caused by the decomposition of the quaternary ammonium cation in [(C2H5)3NC7H7]4W10O32, whereas for the two other compounds, the decomposition of the quaternary ammonium cation caused the peaks at 406.5 °C for [(C4H9)4N]4W10O32 and 424.4 °C for [(CH3)4N]4W10O32. 3.1.2. Infrared Spectra of Catalysts. The IR spectra of catalysts are shown in Figure 2. Three characteristic frequencies, at ∼954, 893, and 800 cm-1 from 1100 to 400 cm-1 respectively belonged to the vibrations of W-Ot, W-Ob-W, and W-Oc-W, demonstrating energy transition caused by stretching and bending vibrations of the bond of W-O in the isopolyacid [W10O32]4- anion. The similarity of the IR spectra of catalysts

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Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009 Table 1. Sulfur Removal of Four Catalysts in Different Desulfurization Systema Sulfur Removal (%)b entry

catalyst

1 2 3 4 5

[(C4H9)4N]4W10O32 [(CH3)4N]4W10O32 [(C2H5)3NC7H7]4W10O32 Na4W10O32

Catal + H2O2 Catal + IL + H2O2 IL + H2O2 30.6 38.6 38.6 8.5

98.0 96.9 66.0 94.6 38.3

Experimental conditions: 5 mL of model oil, t ) 0.5 h, T ) 60 °C, n(H2O2)/n(sulfur) ) 3.0, n(catalyst)/n(sulfur) ) 1/100, and 1 mL of IL. b Catal ) catalyst; IL ) ionic liquid. a

Figure 2. Infrared (IR) spectra of the three catalysts.

Table 2. Effect of the Amount of H2O2 and Catalyst on the Sulfur Removal of DBTa Sulfur Removal (%) n(H2O2)/ n(sulfur)

n(catalyst)/ n(sulfur) ) 1/50

n(catalyst)/ n(sulfur) ) 1/100

n(catalyst)/ n(sulfur) ) 1/150

2.0 2.5 3.0

81.5 98.2 99.2

79.7 85.3 98.0

33.9 66.8 80.4

a Experimental conditions: 5 mL of model oil, t ) 0.5 h, T ) 60 °C, 1 mL of IL.

Figure 3. Ultraviolet-visible light (UV-Vis) spectra of the catalysts.

in Figure 2 demonstrated that the catalysts synthesized had the same first class of [W10O32]4- structure as expected. 3.1.3. Ultraviolet-Visible Light (UV-Vis) Spectra of Catalyst. UV-Vis spectra of the catalysts is shown in Figure 3. All of catalysts were measured to have absorptions (in ACN) near the wavelengths of 320 and 260 nm, which is proven to be the [W10O32]4- structure and in good agreement with the results reported previously.30 There are four linear W-O-W bridge bonds in the [W10O32]4- anion. The absorption at 320 nm in the low-energy area may be assigned to oxygen-totungsten charge transition. 3.1.4. Gravimetric Determination. The data of gravimetric determination of the tungsten content are as follows. The found values of [(C4H9)4N]4W10O32, [(C2H5)3NC7H7]4W10O32, and [(CH3)4N]4W10O32 were detected as 55.81%, 59.50%, and 69.04%, respectively; the calculated values were 55.37%, 58.93%, and 69.45%, respectively. Compared with the calculated values, the results obtained were satisfactory. All the characterization results demonstrated that catalysts have been synthesized as expected. 3.2. Investigation of Different Catalysts on Desulfurization. Decatungstate, as a type of catalyst with high selectivity on organic compounds, can effectively catalyze sulfur-containing compounds such as DBT, which can be proven by its performance in the experiments. Herein, desulfurization in the ECODS system was tested with a series of different decatungstates with short carbon chains in their relative cations. The results listed in Table 1 showed that this type of catalyst had high catalytic activity except [(C2H5)3NC7H7]4W10O32, whose sulfur removal was only 66.0%. Meanwhile, the system that was comprised of the catalyst, H2O2, and IL showed better performance than the situation where there is an absence of catalyst or IL. During the reaction, the catalyst was miscible into the lower IL phase and was transformed to the corresponding peroxo species by reacting with H2O2. The sulfur-containing substrates were

extracted from the oil into IL, where the oxidation proceeded. Meanwhile, considering the higher viscosity of IL ([Bmim]PF6) than that of the oil phase, cations of [(C4H9)4N]4W10O32 and [(CH3)4N]4W10O32 had comparatively small volumes and could not become an obstacle for its function as a surfactant. The sulfur removal was 98.0% and 96.9%, respectively. As for [(C2H5)3NC7H7]4W10O32, the C7H7 group was withdrawing the electron effect and was larger than a short alkyl chain and led to the stereo hindrance, which may explain why its catalytic activity was not high. To compare with the different effect of the cations, Na4W10O32 was also synthesized according to the published procedure29 and used as a catalyst herein. In the ODS system, the sulfur removal was only 8.5%, whereas in the corresponding ECODS system, the sulfur removal can reach 94.6% with Na4W10O32 as a catalyst. In experiments described above, [(C4H9)4N]4W10O32 exhibited the best activity. Then it was chosen as the representative catalyst to further optimize reaction conditions. 3.3. Optimization of Reaction Conditions. 3.3.1. Effects of the Amount of H2O2 and Catalyst on the Desulfurization Efficiency. The added amount of H2O2 and the catalyst had a great influence on the reaction. As can be seen in Table 2, when the ratio of n(H2O2)/n(S) increased from 2.0 to 2.5, the sulfur removal increased sharply under each same ratio of n(S)/n(catalyst). As this ratio increases to 3.0, the sulfur removal increased in a comparatively smaller scale than previously observed, which reached the standards of deep desulfurization. It has been reported that there are competing reactions between the DBT oxidation by H2O2 and the nonproductive decomposition of H2O2 itself;31,32 as a result, the addition of a strictly stoichiometric amount of H2O2 cannot yield high sulfur removal. The 3.0 ratio of n(H2O2)/n(S) was suitable. The data also showed that the amount of catalyst made a positive contribution to the oxidation reactivity. When the ratio of n(S)/n(catalyst) was 150, the amount of catalyst was very small and the number of catalytic active centers were not sufficient to catalyze the reaction in high efficiency. When the amount was increased (n(S)/n(catalyst) ) 100), the sulfur removal can reach a value of 98.0%, which is much better than that observed under the condition of n(S)/n(catalyst) ) 150. With the ratio of 50, the sulfur removal increased more, although this increasing scale was not as large as that previously observed from n(S)/

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Figure 4. Effect of temperature and reaction time on sulfur removal. Experimental conditions: 5 mL of model oil, n(H2O2)/n(sulfur) ) 3.0, n(catalyst)/n(sulfur) ) 1/100, 1 mL of IL. Figure 6. HPLC chromatograms of IL, n-octane, model oil: (a) IL in methanol, (b) n-octane in methanol, and (c) model oil after ECODS.

Figure 5. Recycle of the ECODS system. Experimental conditions: 5 mL of model oil, T ) 60 °C, n(H2O2)/n(sulfur) ) 3.0, n(catalyst)/n(sulfur) ) 1/100, 1 mL of IL. The first three recycles were performed with a reaction time of 0.5 h, whereas the latter three cycles were undertaken for 1 h.

n(catalyst) ) 150 to n(S)/n(catalyst) ) 100. Considering the economic factor, n(S)/n(catalyst) ) 100 was suitable here. 3.3.2. Effect of Temperature and Reaction Time on the Desulfurization Efficiency. Figure 4 shows the sulfur removal of DBT with reaction time at different temperatures. The results indicated that DBT removal can reach 98.0% in 30 min and can be almost completely removed for 40 min under a temperature of 60 °C, whereas at 50 °C, the reaction time was 60 min to obtain a sulfur removal of 98.2%. The desulfurization efficiency appeared to be unsatisfying at a temperature of 40 °C, which demonstrated that a high temperature was good for desulfurization. This pheomenon may be because the oxidant H2O2 and the catalyst cannot work efficiently under low reaction temperature. 3.4. Recycle of the ECODS System. The ECODS system can be recycled five times without any significant decrease on the catalytic performance of ∼98.0% sulfur removal (see Figure 5). After the first recycle, there was no sulfone visible in the IL

Figure 7. ECODS system of different sulfur substrates in oil. Experimental conditions: 5 mL of model oil, T ) 60 °C, n(H2O2)/n(sulfur) ) 3.0, n(catalyst)/n(sulfur) ) 1/100, 1 mL of IL.

phase. However, after three recycles, some white precipitates came into being in the reaction system, which led to the decrease of sulfur removal. Therefore, the reaction time of the forth and latter recycles was prolonged to 1 h, which was proved to be quite an effective measure to increase the sulfur removal and

Table 3. Tungsten Content in Model Oila entry

catalyst

tungsten content in model oil (ppm)

1 2 3

[(C4H9)4N]4W10O32 [(C2H5)3NC7H7]4W10O32 [(CH3)4N]4W10O32

7.4 2.1 8.4

Reaction conditions: 5 mL model oil, T ) 60 °C, t ) 0.5 h, n(H2O2)/n(sulfur) ) 3.0, n(catalyst)/n(sulfur) ) 1/100. The original sulfur content in the model oil was 1000 ppm. a

Figure 8. GC chromatograms of DBT and its corresponding sulfone: (a) the sample of treated oil after ECODS (30.6% sulfur removal), (b) the sample of treated oil after ECODS (99.2% sulfur removal), and (c) sulfone in DMF. GC temperature process involves increasing the temperature from 100 °C to 300 °C at the rate of 25 °C/min.

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Scheme 1. Supposed Mechanism of the ECODS System

improve the desulfurization performance of this system on the item of recycling, although the amount of the sulfone greatly increased. As for the sixth recycle, the sulfur removal decreased to 83.3%. 3.5. Determination of Tungsten Content in Model Oil. The tungsten content in model oil was detected by ICP-AES to investigate whether the catalyst was leached into the model oil. Results in Table 3 showed that, for [(C4H9)4N]4W10O32, the tungsten content in the oil phase was 7.4 ppm, which means that ∼0.7% of the catalyst was leached into the oil phase. For [(C2H5)3NC7H7]4W10O32, the content was only 2.1 ppm, whereas the tungsten content of [(CH3)4N]4W10O32 in the oil phase was 8.4 ppm. The leached amounts of the two catalysts were 0.2% and 0.9%, respectively. 3.6. Investigation of [Bmim]PF6 in Model Oil. The amount of IL ([Bmim]PF6) was analyzed via high-performance liquid chromatography (HPLC, Agilent 1200). The peak of [Bmim]PF6 in methanol and n-octane in methanol is shown in Figures 6a and 6b, respectively. As shown in Figure 6c, there was only the peak of n-octane and no peak of [Bmim]PF6 in the model oil, which can prove that no IL was leached into the oil phase. 3.7. ECODS System of Different Sulfur Substrates in Oil. BT and 4,6-DMDBT were used as different sulfur substrates in the model oil, and [(C4H9)4N]4W10O32 was used as catalyst. The results in Figure 7 showed that, in the ECODS system, to BT, the sulfur content can be reduced from 250 ppm to 120 ppm, whereas to 4,6-DMDBT, the sulfur content can be reduced from 250 ppm to 100.8 ppm. From the data, it also could be observed that 4,6-DMDBT was more easily removed than BT although its reactivity was lower than that of DBT, which may be correlated with the electronic density of the S atom and the stereo hindrance of substituted groups. Among the three substrates, BT has the lowest electronic density of S atoms,3 as well as the lowest reactivity. The difference between the density of 4,6-DMDBT and DBT was very slight, so stereo hindrance played a vital role in the activity of 4,6-DMDBT and DBT. As a result, in the ECODS system, the reactivity of three sulfur substrates were in the following order: DBT > 4,6-DMDBT > BT. 4. Supposed Mechanism Figure 8 shows that, under the same GC process, there was no appearance of the sulfone in both oil samples with low sulfur removal (Figure 8a) and the one with high sulfur removal (Figure 8b). The sulfone of DBT was dissolved in DMF to prepare the sample to be subjected to GC to clarify the sulfone (see Figure 8c). A comparison of Figures 8b and 8c reveals that the sulfone of DBT was nonexistent in the model oil, which

also indicated that sulfone was extracted into IL. Therefore, DBT was removed from the oil and the desulfurization was achieved. Decatungstates that were used as catalysts in our ECODS system was believed to be transformed to the corresponding peroxo species, which had excellent performance in many organic reactions, including the desulfurization.19,33,34 In this process, the function of the catalyst was vital, because it can provide such a direction to guide the oxidant to access the substrate, making high selectivity, as well as providing active centers for the oxidation reaction. During the reaction, decatungstates were soluble in the IL of [Bmim]PF6 with stirring. DBT was oxidized by the peroxo species into its corresponding sulfone,17-20,35 whose polarity was high enough to be extracted into IL continuously, by which deep desulfurization can be achieved. The supposed mechanism is shown in Scheme 1. 5. Conclusion (1) In this extractive catalytic oxidative desulfurization (ECODS) system, three decatungstates with short carbon chains as the cations were used as a catalyst, while hydrogen dioxide (H2O2) was used as an oxidant and the ionic liquid (IL) of [Bmim]PF6 was used as the medium. During the process, the sulfur level in the model oil (1000 ppm S) can be reduced to 8 ppm through the optimization of reaction conditions, which is according with the need of ultradeep desulfurization. Cations with different carbon chains in decatungstates showed different performance on their catalytic activity. (2) The temperature, reaction time, and the amount of H2O2 and catalyst, as well as the type of the cations of decatungstates, all played vital roles in desulfurization efficiency. The ECODS system can be recycled five times with a slight decrease in sulfur removal. (3) In the ECODS system, the reactivity of three sulfur substrates were in the following order: DBT > 4, 6-DMDBT > BT. Acknowledgment This work was financially supported by the National Nature Science Foundation of China (Nos. 20676057, 20876071, 20777029) and Doctoral Innovation Fund of Jiangsu Province (No. CX08B-143Z). Literature Cited (1) Babich, I. V.; Moulijn, J. A. Science and technology of novel processes for deep desulfurization of oil refinery streams: A review. Fuel 2003, 82, 607–631. (2) Shafi, R.; Hutchings, G. J. Hydrodesulfurization of hindered dibenzothiophenes: An overview. Catal. Today 2000, 59, 423–442.

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ReceiVed for reView January 18, 2009 ReVised manuscript receiVed August 9, 2009 Accepted August 12, 2009 IE900754F