Oxidative Desulfurization of Fuel Catalyzed by

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Oxidative Desulfurization of Fuel Catalyzed by Amphiphilic Peroxomolybdate a

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F. Zou , X. Y. Wu , W. S. Zhu , H. M. Li , D. Xu & H. Xu

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College of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, P.R. China Available online: 12 Apr 2011

To cite this article: F. Zou, X. Y. Wu, W. S. Zhu, H. M. Li, D. Xu & H. Xu (2011): Oxidative Desulfurization of Fuel Catalyzed by Amphiphilic Peroxomolybdate, Petroleum Science and Technology, 29:11, 1113-1121 To link to this article: http://dx.doi.org/10.1080/10916460903530499

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Petroleum Science and Technology, 29:1113–1121, 2011 Copyright © Taylor & Francis Group, LLC ISSN: 1091-6466 print/1532-2459 online DOI: 10.1080/10916460903530499

Oxidative Desulfurization of Fuel Catalyzed by Amphiphilic Peroxomolybdate F. ZOU,1 X. Y. WU,1 W. S. ZHU,1 H. M. LI,1 D. XU,1 AND H. XU1

Downloaded by [Southeast University] at 07:42 19 December 2011

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College of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, P.R. China Abstract The oxidation of sulfur-containing compounds (benzothiophene [BT], dibenzothiophene [DBT], and 4,6-dimethyldibenzothiophene [4,6-DMDBT]) was studied in an emulsion system composed of model oil, hydrogen peroxide, and an amphiphilic catalyst [C7 H7 C12 H25 (CH3 )2 N]2 Mo2 O3 (O2 )4 . The most suitable conditions were suggested: n (DBT): n (catalyst): n (H2 O2 ) D 1:0.1:10, at 60ıC for 2 hr. Under optimized experimental conditions, the removal of DBT, BT, and 4,6-DMDBT could reach 98.0, 94.0, and 62.7%, respectively. The oxidation product sulfones could be readily separated by extraction. The catalyst could be recycled five times without a significant decrease in catalytic activity. Keywords amphiphilic catalyst, catalytic oxidation, hydrogen peroxide, oxidative desulfurization, peroxomolybdate

1. Introduction Currently, ultra-deep desulfurization of fuel oil has become extremely urgent due to environmental crisis. In fact, zero emission and zero levels of sulfur are being called for worldwide in the coming 5–10 years. The latest environmental regulations limit the sulfur levels in diesel fuels to less than 10 ppm by the year 2010 (Esser et al., 2004). However, in terms of current development, using conventional hydrodesulfurization (HDS) it is difficult to remove some refractory sulfur compounds such as benzothiophene (BT), dibenzothiophene (DBT), and especially 4,6-alkyl-substituted DBTs due to their stereo hindrance (Garcia-Gutierrez et al., 2006). From an environmental and economic viewpoint, more energy-efficient desulfurization alternatives for production of virtually sulfurfree fuel emerged. Among various sorts of techniques, oxidative desulfurization (ODS) has been a promising process under mild conditions in recent decades (Selvavathi et al., 2008; Zhao et al., 2008). The oxidant in ODS plays an important role considering its benign relationship to the environment. Various oxidants are efficient, such as molecular oxygen (Lu et al., 2007; Zhou et al., 2009), H2 O2 (Komintarachat and Trakarnpruk, 2006; Lu et al., 2006), nitric acid/NO2 (Tam et al., 1990), ozone (Zaykina et al., 2004), tert-butyl hydroperoxide (t-BuOOH; Ishihara et al., 2005), and potassium superoxide (Chan et al., 2008). Among these, H2 O2 has been widely used as an oxidant because it is environmentally compatible, affordable, commercially available, and not strongly corrosive (Li et al., 2009). It is key to choosing a catalyst for desulfurization, such as organic peracetic acids, heteropolyacids Address correspondence to Huaming Li, Jiangsu University of College of Chemistry and Chemical Engineering, 301 Xuefu Road, Zhenjiang 212013, P.R. China. E-mail: [email protected]

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(Yazu et al., 2003), inorganic solid acids (Garcia-Gutierrez et al., 2006), cobalt catalysts (Murata et al., 2004), and polyoxometalates catalysts (Zhu et al., 2008; Lai and Luo, 2009; Trakarnpruk and Rujiraworawut, 2009). Amphiphilic catalysts have been regarded as high active catalysts in water–oil biphase reaction systems (Lu et al., 2006, 2007; Jiang et al., 2009). In principle, the amphiphilic catalyst may act as an emulsifying agent to stabilize the emulsion droplets instead of only simple surfactant. The amphiphilic catalysts were favorable to transfer active oxygen species of hydrogen peroxide into the organic phase and are called functional phase transfer catalysts (FPTCs). The amphiphilic catalyst could deposit between the water and oil phase after demulsification. In this work, the ŒC7 H7 C12 H25 .CH3 /2 N2 Mo2 O3 .O2 /4 catalyst catalyzed sulfurcontaining molecules into sulfones with H2 O2 as oxidant. The sulfur level of the model oil could be lowered from 500 ppm to below 10 ppm as analyzed by gas chromatography, and deep desulfurization of fuel could be achieved.

2. Experimental 2.1.

Preparation of Catalyst

The catalyst was synthesized according to the literature (Bailey et al., 1995): 5 mmol Na2 MoO4 2H2 O was dissolved in 10 mL H2 O followed by the addition of 6 mL 30% H2 O2 in an ice bath while stirring continuously. Then the pH of the solution was adjusted to 4.3 with diluted hydrochloric acid under stirring. Meanwhile, 10 mmol ŒC7 H7 C12 H25 .CH3 /2 NCl was dissolved in 15 mL 95% ethanol to obtain the solution which was added into the molybdate solution. After 10 min the white precipitate was filtered, washed with water and ether, respectively, and finally dried in a vacuum dring oven at 50ı C. 2.2.

Preparation of Model Oil

The model oil was obtained as follows: DBT was dissolved in n-octane to obtain the model oil with sulfur concentration of 500 ppm (g/mL) and tetradecane was dissolved as an internal standard substance. A mixture of the catalyst, H2 O2 , and model oil was added into a two-necked kettle. Then the reaction system was stirred for 2 hr at 60ıC under atmospheric pressure. After the reaction, the kettle was cooled down to room temperature. The sulfur content was analyzed by gas chromatography-flame ionization detection (GC-FID) with an internal standard (Agilent 7890A equipped with a capillary column, HP-5, 30 m 0.32 mm i.d. 0.25 m; FID: Agilent).

3. Results and Discussion 3.1.

Characterization of Catalyst

Infrared (IR) and ultraviolet-visible (UV-Vis) characterization results were as follows: IR (selected bands, KBr disc; cm 1 ) 955 (s; (MoDO)), 853 (m; (O-O)), 637 (s; sym [Mo(O2 )]), 586 (m, asym [Mo(O2 )]). UV-Vis (CH3 CN; nm) 207. The elemental analyses and contents of molybdenum and peroxo species of complex are listed in Table 1. Compared with the calculated values, the results were satisfactory. Results of thermogravimetric-differential scanning calorimetry (TG-DSC) analysis of ŒC7 H7 C12 H25 .CH3 /2 N2 Mo2 O3 .O2 /4 is shown in Figure 1, which indicates that the

Oxidation of Sulfur-Containing Compounds

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Table 1 Data from the catalyst elementary analysis, gravimetry, and peroxo species titration Complex

Data

C%

H%

N%

Mo%

(O22 )%

ŒC7 H7 C12H25 .CH3 /2 N2 Mo2 O3 .O2 /4

Anal. Calc.

52.00 51.64

8.08 7.84

2.43 2.87

19.30 19.64

12.43 13.10

catalyst had no crystalline water because there was no mass loss and endothermic peak around 100ıC. The peroxide decomposition temperature of catalyst was 105ıC. In contrast with the DSC curve exothermic peak at the time of the peroxide decomposition, a wide and flat peak was observed. After 230ı C, the corresponding TG curve manifested mass loss, which was caused by decomposition of the ammonium cation. Until 400ıC, the sample tended to constant weight, and the final residue was metal oxide. In this work, DBT was selected as a sulfur compound representative of those in fuel because it is one of the main refractory sulfur-containing compounds in the HDS treatment. The conversion of DBT in model oil was used to study the removal of sulfur. 3.2.

Influence of Desulfurization Systems on Removal of DBT

Table 2 shows the effect of different catalysts in the desulfurization system. Firstly, the removal of DBT was merely 4.0% without the catalyst, and it was 3.4% when the catalyst was Na2 MoO4 . Judging from the experimental phenomena, Na2 MoO4 contributed to the decomposition of most hydrogen peroxide under 60ıC in the system, leading to the lower sulfur removal. After the catalyst was modified to an amphiphilic one, the sulfur removal increased sharply, reaching 98.0%. Consequently, in this desulfurization system, the amphiphilic catalyst served not only as phase transfer reagent in favor of forming the emulsion droplets but also stabilized and activated hydrogen peroxide. The utilization rate of the hydrogen peroxide increased, which led to the higher sulfur removal. In addition, it was equally effective for different substrates. The removal of BT and 4,6-DMDBT could

Figure 1. TG-DSC of ŒC7 H7 C12 H25 .CH3 /2 N2 Mo2 O3 .O2 /4 .

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F. Zou et al. Table 2 Effect of different desulfurization systems Entry

Substrate

Catalyst

t/h

Sulfur removal, %

1 2 3 4 5

DBT DBT DBT BT 4,6-DMDBT

— Na2 MoO4 Catalysta Catalysta Catalysta

2 2 2 2 4

4.0 3.4 98.0 94.0 62.7


Experimental conditions: model oil D 5 mL, T D 60ıC, n (substrate):n (catalyst):n (H2 O2 ) D 1:0.1:10. a Catalyst: ŒC H C H .CH / N Mo O .O / . 7 7 12 25 3 2 2 2 3 2 4

reach 94.0 and 62.7%, respectively. These results indicated the remarkable advantage of this amphiphilic catalyst over the simple catalyst in the desulfurization of model oil. 3.3.

Influence of Reaction Time and Temperature on Removal of DBT

The removal of DBT with different times at different temperatures is shown in Figure 2. The results indicated that the removal of DBT could reach 98.0% at 60ıC with the reaction time of 2 hr. No significant change was shown when the reaction time was prolonged. However, the removal of DBT was merely 75.6% when the reaction time was 2 hr at 40ı C. Therefore, 60ı C was taken as the optimum reaction temperature with the reaction time of 2 hr. This experiment showed that the reaction temperature and time were the main factors that influence the desulfurization reaction activity. The higher temperature and the longer time, the easier it was to obtained deep desulfurization.

Figure 2. DBT removal versus the reaction time at 40ı C, 50ı C, and 60ıC. Experimental conditions: n (DBT):n (catalyst):n (H2 O2 ) D 1:0.1:10.



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Figure 3. Influence of the H2 O2 /sulfur molar ratio (O/S) on the reaction. Experimental conditions: model oil D 5 mL, t D 2 hr, T D 60ıC, n (DBT):n (catalyst) D 1:0.1.

3.4.

Influence of H2 O2 /Sulfur Molar Ratio (O/S) on Removal of DBT

Figure 3 presents the molar ratio relation between H2 O2 and sulfur. The removal of DBT was merely 38.0% (residual S: 310 ppm) when the H2 O2 /sulfur (O/S) molar ratio was 6:1. The removal of DBT grew to 85.2% (residual S: 74 ppm) rapidly when the O/S molar ratio was 8:1, and it reached 98.0% as the ratio of H2 O2 increased continually. The residual DBT was about 10 ppm. The results indicate that an appropriate amount of H2 O2 could achieve deep desulfurization. 3.5.

Influence of Amount of Catalyst on Removal of DBT

Table 3 shows that the removal of DBT increased with the molar ratio of the catalyst. The molar ratio of catalyst and DBT changed from 1:40 to 1:10. In the desulfurization system, the catalyst exhibited the emulsion droplets, which were a stable, transparent, and isotropic micro-heterogeneous system. These experiments showed that the removal of DBT was 98.0% when the catalyst/DBT molar ratio was 1:10, and the removal of DBT was only 55.6% when it was 1:40. Obviously, the blank experiment without catalyst

Table 3 Influence of n (catalyst):n (DBT) on the reaction Entry

1

2

3

4

5

n (Catalyst):n (DBT) Sulfur removal (%)

No catalyst 4.0

1:40 55.6

1:30 84.6

1:20 92.7

1:10 98.0

Experimental conditions: model oil D 5 mL, t D 2 hr, T D 60ı C, n (DBT):n (H2 O2 ) D 1:10.


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Figure 4. 4,6-DMDBT and BT (insert) sulfur removal versus reaction time. Experimental conditions: T D 60ı C, n (S):n (catalyst):n (H2 O2 ) D 1:0.1:10 and sulfur content 250 ppm in n-octane.

revealed that the application of the amphiphilic catalyst could effectively promote the reaction. The catalyst dosage could influence the removal of DBT to a certain extent. 3.6.

Influence of Different Substrates on Sulfur Removal

To study the selectivity of this catalyst on different model oils, BT and 4,6-DMDBT were also chosen as substrates. From Figure 4, it is clear that this catalyst showed the same high activity as BT. The removal of BT could reach 94.0% at 2 hr and nearly 100% at 2.5 hr. However, the removal of 4,6-DMDBT could reach merely 62.7% at 4 hr due to its low sulfur electron density and high steric hindrance. It was obvious that this catalyst showed high activity to the refractory sulfur-containing compounds such as DBT, 4,6-DMDBT, and especially BT. 3.7.

Influence of the Recycle of the Catalyst

The recycle of the catalyst ŒC7 H7 C12 H25 .CH3 /2 N2 Mo2 O3 .O2 /4 was investigated in ODS of DBT-containing model oil. After the reaction, the kettle was cooled down. The upper model oil was decanted slowly. Then fresh H2 O2 and model oil were added into the original reaction kettle for the next run. The data in Figure 5 showed that the removal of DBT did not significantly decrease after five recycling reactions. It can be seen from the above experiments that the catalyst showed high catalytic activity and could be recycled for reuse.



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Figure 5. Recycle of ŒC7 H7 C12 H25 .CH3 /2 N2 Mo2 O3 .O2 /4 in the desulfurization system.

Figure 6. Suggested catalytic circle of the oxidation.

3.8.

Mechanism of Reaction System

Based on the results of this experiment and our former study (Jiang et al., 2009), a possible catalytic cycle was determined as shown in Figure 6. An emulsion system composed of a nonpolar solvent (model oil), H2 O2 , and amphiphilic peroxomolybdate catalyst formed with vigorous stirring. The lipophilic quaternary ammonium cations ŒC7 H7 C12H25 .CH3 /2 NC and the hydrophilic ŒMo2 O3 .O2 /4 2 played different roles in this emulsion system, and ŒMo2 O3 .O2 /4 2 formed a new oxoperoxo species with the active oxygen from H2 O2 . Finally, DBT was oxidized to DBTO2 , by which desulfurization was obtained.

4. Conclusions In summary, the amphiphilic catalyst ŒC7 H7 C12 H25 .CH3 /2 N2 Mo2 O3 .O2 /4 as a phase transfer agent exhibited high catalytic activity in desulfurization with H2 O2 as the oxidant. The catalyst was easily prepared and reused. Refractory sulfur-containing compounds like

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DBT and BT could be oxidized into sulfones, and model oil with low sulfur content could be achieved. The catalyst could be recycled five times without any significant decrease in catalytic activity. Under the optimized experimental conditions, the removal of BT and 4,6-DMDBT was 94.0% in 2 hr and 62.7% in 4 hr.

Acknowledgment


This work was financially supported by the National Nature Science Foundation of China (Nos. 21076099, 20876071), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20103227110016) Postdoctoral Foundation of China and Jiangsu Province (Nos. 20090461067, 201003556, 1001025C), and Advanced Talents of Jiangsu University (09JDG063).

References Bailey, A. J., Griffith, W. P., and Parkin, B. C. (1995). Heterpolyperoxo- and isopolyperooxotungstates and -molybdates as catalysts for the oxidation of tertiary amines, alkenes and alcohols. J. Chem. Soc. Dalton Trans. 11:1833–1837. Chan, N. Y., Lin, T. Y., and Yen, T. F. (2008). Superoxides: Alternative oxidants for the oxidative desulfurization process. Energ. Fuel. 22:3326–3328. Esser, J., Wasserscheid, P., and Jess, A. (2004). Deep desulfurization of oil refinery streams by extraction with ionic liquids. Green Chem. 6:316–322. Garcia-Gutierrez, J. L., Fuentes, G. A., Hernandez-Teran, M. E., Murrieta, F., Navarrete, J., and Jimenez-Cruz, F. (2006). Ultra-deep oxidative desulfurization of diesel fuel with H2 O2 catalyzed under mild conditions by polymolybdates supported on Al2 O3 . Appl. Catal. Gen. 305:15–20. Ishihara, A., Wang, D. H., Dumeignil, F., Amano, H., Qian, E. W., and Kabe, T. (2005). Oxidative desulfurization and denitrogenation of a light gas oil using an oxidation/adsorption continuous flow process. Appl. Catal. Gen. 279:279–287. Jiang, X., Li, H. M., Zhu, W. S., He, L. N., Shu, H. M., and Lu, J. D. (2009). Deep desulfurization of fuels catalyzed by surfactant-type decatungstates using H2 O2 as oxidant. Fuel 88:431– 436. Komintarachat, C., and Trakarnpruk, W. (2006). Oxidative desulfurization using polyoxometalates. Ind. Eng. Chem. Res. 45:1853–1856. Lai, J., and Luo, G. (2009). Zinc-substituted polyoxometalate for oxidative desulfurization of dibenzothiophene. Petrol. Sci. Tech. 27:781–787. Li, H. M., He, L. N., Zhu, W. S., Jiang, X., Wang, Y., and Yan, Y. S. (2009). Deep oxidative desulfurization of fuels catalyzed by phosphotungstic acid in ionic liquids at room temperature. Energ. Fuel. 23:1354–1357. Lu, H. Y., Gao, J. B., Jiang, Z. X., Jing, F., Yang, Y. X., Wang, G., and Li, C. (2006). Ultradeep desulfurization of diesel by selective oxidation with ŒC18 H37 N.CH3 /3 4 ŒH2 NaPW10 O36 catalyst assembled in emulsion droplets. J. Catal. 239:369–375. Lu, H. Y., Gao, J. B., Jiang, Z. X., Yang, Y. X., Song, B., and Li, C. (2007). Oxidative desulfurization of dibenzothiophene with molecular oxygen using emulsion catalysis Chem. Comm. 150– 152. Murata, S., Murata, K., Kidena, K., and Nomura, M. (2004). A novel oxidative desulfurization system for diesel fuels with molecular oxygen in the presence of cobalt catalysts and aldehydes. Energ. Fuel. 18:116–121. Selvavathi, V., Meenakshisundaram, A., Sairam, B., and Sivasankar, B. (2008). Kinetics of oxidative desulfurization of sulfur compounds in diesel fuel. Petrol. Sci. Tech. 26:208–216. Tam, P. S., Kittrell, J. R., and Eldridge, J. W. (1990). Desulfurization of fuel oil by oxidation and extraction. Enhancement of extraction yield. Ind. Eng. Chem. Res. 29:321–324.



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Trakarnpruk, W., and Rujiraworawut, K. (2009). Oxidative desulfurization of gas oil by polyoxometalates catalysts. Fuel Process Tech. 90:411–414. Yazu, K., Furuya, T., Miki, K., and Ukegawa, K. (2003). Tungstophosphoric acid-catalyzed oxidative desulfurization of light oil with hydrogen peroxide in a light oil/acetic acid biphasic system. Chem. Lett. 32:920. Zaykina, R. F., Zaykin, Y. A., Yagudin, S. G., and Fahruddinov, I. M. (2004). Specific approaches to radiation processing of high-sulfuric oil. Radiat. Phys. Chem. 71:467–470. Zhao, D. S., Zhou, E. P., Wang, J. L., Li, F. T., and Wang, N. (2008). Oxidation desulfurization of thiophene using phase transfer catalyst/H2 O2 systems. Petrol. Sci. Tech. 26:1099–1107. Zhou, X. R., Li, J., Wang, X. N., Jin, K., and Ma, W. (2009). Oxidative desulfurization of dibenzothiophene based on molecular oxygen and iron phthalocyanine. Fuel Process Tech. 90:317–323. Zhu, W. S., Li, H. M., Jiang, X., Yan, Y. S., Lu, J. D., He, L. N., and Xia, J. X. (2008). Commercially available molybdic compound-catalyzed ultra-deep desulfurization of fuels in ionic liquids. Green Chem. 10:641–646.