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Energy & Fuels 2009, 23, 1354–1357
Deep Oxidative Desulfurization of Fuels Catalyzed by Phosphotungstic Acid in Ionic Liquids at Room Temperature Huaming Li,*,†,‡ Lining He,§ Jidong Lu,*,† Wenshuai Zhu,‡ Xue Jiang,‡ Yan Wang,§ and Yongsheng Yan‡ State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology, Wuhan 430074, P. R. China, College of Chemistry and Chemical Engineering and College of the EnVironment, Jiangsu UniVersity, Zhenjiang 212013, P. R. China ReceiVed September 20, 2008. ReVised Manuscript ReceiVed December 23, 2008
A simplified extraction and catalytic oxidative desulfurization (ECODS) system composed of phosphotungstic acid (H3PW12O40 · 14H2O), 30% H2O2, and [bmim]BF4 was found suitable for the deep removal of sulfur in model oil. By this desulfurization system dibenzothiophene (DBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and benzothiophene (BT) could be effectively removed. Removal of DBT could reach 98.2% at room temperature (30 °C) for 1 h, which was remarkably superior to mere solvent extraction with IL (14.2%) or catalytic oxidation without IL (15.9%). When the reaction temperature increased to 70 °C, treatment of BT, DBT, and 4,6DMDBT with our ECODS system showed 100% removal of sulfur compounds in 3 h. This desulfurization system could be recycled five times with slight decrease in activity.
Introduction Sulfur compounds in transportation fuels are converted by combustion to SOx, which is a major source of acid rain and air pollution. Thus, environmental legislations have been implanted to reduce the sulfur content in gasoline and diesel to less than 10 ppm of sulfur of by 2010.1 Removal of sulfur compounds from fuels is carried out industrially by a catalytic hydrodesulfurization (HDS) method. However, it is difficult to achieve ultralow sulfur levels because of some refractory sulfur compounds such as dibenzothiophene (DBT) and especially 4,6alkyl-substituted DBTs. Attributing to their stereo hindrance, severe operating conditions and large capital cost are required for HDS to achieve ultralow sulfur level of fuels.2 From an environmental and economic viewpoint, it is extremely desirable to develop more energy-efficient desulfurization alternatives for production of virtually sulfur-free fuel. More recently, oxidative desulfurization (ODS),2-5 extraction * Corresponding author phone: + 86 511 88791800; fax: + 86 511 88791708; e-mail:
[email protected]. † Huazhong University of Science and Technology. ‡ College of Chemistry and Chemical Engineering, Jiangsu University. § College of the Environment, Jiangsu University. (1) Zhu, W. S.; Li, H. M.; Jiang, X.; Yan, Y. S.; Lu, J. D.; Xia, J. X. Energy Fuels 2007, 21, 2514–2516. (2) Lu¨, H. Y.; Gao, J. B.; Jiang, Z. X.; Jing, F.; Yang, Y. X.; Wang, G.; Li, C. J. Catal. 2006, 239, 369–375. (3) Campos-Martin, J. M.; Capel-Sanchez, M. C.; Fierro, J. L. G. Green Chem. 2004, 6, 557–562. (4) Komintarachat, C.; Trakarnpruk, W. Ind. Eng. Chem. Res. 2006, 45, 1853–1856. (5) Cheng, S. S.; Yen, T. F. Energy Fuels 2008, 22, 1400–1401. (6) Bösmann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 2494–2495. (7) Zhang, S. G.; Zhang, Z. C. Green Chem. 2002, 4, 376–379. (8) Nie, Y.; Li, C. X.; Sun, A. J.; Meng, H.; Wang, Z. H. Energy Fuels 2006, 20, 2083–2087. (9) Huang, C. P.; Chen, B. H.; Zhang, J.; Liu, Z. C.; Li, Y. X. Energy Fuels 2004, 18, 1862–1864. (10) Zhang, S. G.; Zhang, Q. L.; Zhang, Z. C. Ind. Eng. Chem. Res. 2004, 43, 614–622.
Table 1. Sulfur Removal of Different Desulfurization Systemsa sulfur removal (%) entry
different IL
IL
IL + H2O2
IL + H2O2 + HPW
1 2 3 4 5
[bmim]BF4 [bmim]PF6 [omim]BF4 [omim]PF6
14.2 12.1 17.6 18.3
26.0 27.3 26.9 34.7
98.2 98.0 64.7 63.5
H2O2 + HPW
15.9
DBT dissolved in n-octane as model oil (S-Content ) 1000 ppm). Conditions: T ) 30 °C, t ) 1 h, IL ) 1 mL, model oil ) 5 mL, [n(H2O2)/n(DBT)/n(HPW))300:100:1]. HPW: H3PW12O40 · 14H2O. a
with ionic liquids,6-13 selective adsorption,14 and biodesulfurization15 have been described. Due to the short reaction time at ambient conditions, high efficiency, and selectivity, ODS combined with extraction is considered to be one of the most promising processes. Sulfur compounds are extracted from fuels and then oxidized in extractant. Oxidation of sulfur compounds present in fuels is a key process to the desulfurization. Various studies on the ODS processes have used different oxidants such as molecular oxygen,16 H2O2,2-5 nitric acid/NO2,17 ozone,18 tertbutyl hydroperoxide (t-BuOOH),19 and potassium superoxide.20 (11) Ko, N. H.; Lee, J. S.; Huh, E. S.; Lee, H.; Jung, K. D.; Kim, H. S.; Cheong, M. Energy Fuels 2008, 22, 1687–1690. (12) Schmidt, R. Energy Fuels 2008, 22, 1774–1778. (13) Gao, H. S.; Luo, M. F.; Xing, J. M.; Wu, Y.; Li, Y. G.; Li, W. L.; Liu, Q. F.; Liu, H. Z. Ind. Eng. Chem. Res. 2008, 47, 8384–8388. (14) Mckinley, S. G.; Angelici, R. J. Chem. Commun. 2003, 2620–2621. (15) Sipma, J.; Henstra, A. M.; Parshina, S. N.; Lens, P. N. L.; Lettinga, G.; Stams, A. J. M. Crit. ReV. Biotechol. 2006, 26, 41–65. (16) Lu¨, H. Y.; Gao, J. B.; Jiang, Z. X.; Yang, Y. X.; Song, B.; Li, C. Chem. Commun. 2007, 150–152. (17) Tam, P. S.; Kittrell, J. R.; Eldridge, J. W. Ind. Eng. Chem. Res. 1990, 29, 321–324. (18) Zaykina, R. F.; Zaykin, Y. A.; Yagudin, S. G.; Fahruddinov, I. M. Radiat. Phys. Chem. 2004, 71, 467–470. (19) Ishihara, A.; Wang, D. H.; Dumeignil, F.; Amano, H.; Qian, E. W.; Kabe, T. Appl. Catal. A: Gen. 2005, 279, 279–287. (20) Chan, N. Y.; Lin, T. Y.; Yen, T. F. Energy Fuels 2008, 22, 3326– 3328.
10.1021/ef800797n CCC: $40.75 2009 American Chemical Society Published on Web 02/12/2009
Deep OxidatiVe Desulfurization of Fuels
H2O2 has been widely used as oxidant due to its environmental compatibility, affordable cost, and commercially availability. H2O2 also shows high efficiency in the ODS process.21 Further, removal of oxidized sulfoxides and sulfones products from fuels plays a vital role in the desulfurization. Reported oxidative desulfurization processes include, but are not limited to, polyoxometalate/H2O2,21-24 formic acid/H2O2,25,26 acetic acid/ H2O2,27 etc. Although these ODS processes can effectively remove sulfur compounds, one of the prime concerns is that a large amount of flammable and volatile organic compounds (VOCs) are employed as extractants. Use of VOCs raises further environmental and safety concerns. Ionic liquids (ILs), as novel green solvents, have been used as extractants for liquid/liquid extraction, in which sulfur compounds can be removed from fuels. However, sulfur removal is not favorable because of the similar polarity among alkyls, aromatics, and the sulfur compounds. Different ILs have been reported, such as [bmim]BF4,6,7 [bmim]PF6,6,7 [omim][OcSO4],6 [bmim]Cl/AlCl3,6 [bmim][DBP],8 TMAC/AlCl3,10 and FeCl3[bmim]Cl.11 Extraction and chemical oxidation desulfurization (EODS) systems can obviously increase sulfur removal. A desulfurization system (H2O2-acetic acid/[bmim]PF6) has been reported by the Lo group28 that oxidation of DBT in waterimmiscible [bmim]PF6 resulted in a high oxidation rate. When acidic ionic liquid [hmim]BF429 and [hnmp]BF430 were used for desulfurization in the presence of H2O2, sulfur compounds can be deeply removed due to formation of hydroxyl radicals. The results of above experiments indicate that the EODS system can remove sulfur compounds of fuels efficiently in a superior way to simple extraction using IL. A combination of catalytic oxidation and extraction with ILs is regarded to the “green desulfurization system”. However, the synthetic process of reported catalysts1,31 has been complicated and is difficult to be commercialized. In previous papers it has been reported that tungsten catalysts are effective for oxidation of thioethers to sulfoxides and consequently to sulfones using H2O2 as the oxidant in a two liquid-liquid system with a phase-transfer catalyst (PTC).3,22 Here we employed a simplified, industrially available, and highly effective acid catalyst for oxidation of aromatic sulfur compounds such as benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) without the use of PTCs. The extraction and catalytic oxidation desulfurization system (ECODS) was evaluated for removal of sulfur compounds by combining with 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4), 1-n-octyl-3- methylimidazolium (21) Gao, J. B.; Wang, S. G.; Jiang, Z. X.; Lu, H. Y.; Yang, Y. X.; Jing, F.; Li, C. J. Mol. Catal. A: Chem. 2006, 258, 261–266. (22) Huang, D.; Zhai, Z.; Lu, Y. C.; Yang, L. M.; Luo, G. S. Ind. Eng. Chem. Res. 2007, 46, 1447–1451. (23) Al-Shahrani, F.; Xiao, T. C.; Llewellyn, S. A.; Barri, S.; Jiang, Z.; Shi, H. H.; Martinie, G.; Green, M. L. H. Appl. Catal. B: EnViron. 2007, 73, 311–316. (24) Te, M.; Fairbridge, C.; Ring, Z. Appl. Catal. A: Gen. 2001, 219, 267–280. (25) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W. H.; Ishihara, A.; Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232–1239. (26) Yu, G. X.; Lu, S. X.; Chen, H.; Zhu, Z. N. Energy Fuels 2005, 19, 447–452. (27) Yazu, K.; Makino, M.; Ukegawa, K. Chem. Lett. 2004, 33, 1306– 1307. (28) Lo, W. H.; Yang, H. Y.; Wei, G. T. Green Chem. 2003, 5, 639– 642. (29) Lu, L.; Cheng, S. F.; Gao, J. B.; Gao, G. H.; He, M. Y. Energy Fuels 2007, 21, 383–384. (30) Zhao, D. S.; Wang, J. L.; Zhou, E. P. Green Chem. 2007, 9, 1219– 1222. (31) He, L. N.; Li, H. M.; Zhu, W. S.; Guo, J. X.; Jiang, X.; Lu, J. D.; Yan, Y. S. Ind. Eng. Chem. Res. 2008, 47, 6890–6895.
Energy & Fuels, Vol. 23, 2009 1355
Figure 1. Removal of DBT (S-Content ) 1000 ppm) versus the reaction temperature. Conditions: [bmim]BF4 ) 1 mL, model oil ) 5 mL, n(H2O2)/n(DBT)/n(HPW) ) 300:100:1.
tetrafluoroborate ([omim]BF4), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), and 1-n-octyl-3-methylimidazolium hexafluorophosphate ([omim]PF6), respectively. Four different ILs were immiscible with model oil, which not only served as extractants and reaction media but also stabilized H2O2 in the process of the reaction. The simple acid catalyst possesses high catalytic activity and hardly contaminates fuel at room temperature. While phosphotungstic acid (H3PW12O40 · 14H2O, HPW) and H2O2 were introduced together in the absence of [bmim]BF4, removal of sulfur has been very low. However, when HPW, H2O2, and [bmim]BF4 were employed together the system exhibited high performance. The sulfur compounds could be almost completely removed from model oil. Experimental Section Desulfurization Procedure. The ionic liquids [bmim]BF4, [omim]BF4, [bmim]PF6, and [omim]PF6 were synthesized as mentioned in the literature procedure.32,33 Model oil was prepared by dissolving DBT, 4,6-DMDBT, and BT in n-octane, respectively, to give a corresponding sulfur content 1000, 500, and 1000 ppm. The extraction and catalytic oxidation desulfurization reaction was carried out in a homemade 40 mL two-necked flask containing 5 mL of model oil, 0.048 mL of 30 wt % H2O2 [n(H2O2)/n(DBT) ) 3], 1 mL of IL, and catalyst [n(DBT)/n(HPW) ) 100]. The resulting mixture was stirred at 30 °C for 1 h in oil bath. With the same experimental method the desulfurization reaction was carried out at other temperatures. Analysis of Sulfur Content. The upper phase (n-octane) was withdrawn and analyzed for sulfur content by GC-flame ionization detector (GC-FID), SE-54 capillary column (15 m × 0.32 mm i.d. × 1.0 µm film thickness). Analysis conditions were as follows: injection port temperature, 340 °C; detector temperature, 250 °C; oven temperature, 80 °C.
Results and Discussion Influence of Different Desulfurization Systems for Model Oil. Table 1 shows four different desulfurization systems of extraction, extraction coupled with chemical oxidation, extraction coupled with catalytic oxidation, and catalytic oxidation without extraction. Four ILs were immiscible with model oil. HPW can dissolve in H2O2, but it hardly dissolved in the model oil. Moreover, H2O2 was miscible with [bmim]BF4, and a biphasic system was formed. When H2O2 was introduced in [bmim]PF6, [omim]BF4, and [omim]PF6, respectively, a triphasic (32) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156–164. (33) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Gratzel, M. Inorg. Chem. 1996, 35, 1168–1178.
1356 Energy & Fuels, Vol. 23, 2009 Scheme
1. Presumed Process of the Interaction [bmim]BF4 and HPW
Li et al. between
Table 2. Influence of the H2O2/DBT Ratio on Removal of DBTa entry
1
2
3
4
5
n(H2O2)/n(DBT) sulfur removal (%)
2:1 83.2
3:1 98.2
5:1 99.5
8:1 100
10:1 100
a DBT dissolved in n-octane as model oil (S-Content ) 1000 ppm). Conditions: T ) 30 °C, t ) 1 h, [bmim]BF4 ) 1 mL, model oil ) 5 mL, n(DBT)/n(HPW) ) 100:1.
system was formed. In the above desulfurization systems the oil phase was the upper layer, so the model oil was easily separated from the system. When using [bmim]BF4 solely as the extractant for removing DBT from model oil sulfur removal reached 14.2%, and the result showed that the extractive efficiency of [bmim]BF4 was poor. With addition of H2O2 in [bmim]BF4 sulfur removal increased to 26.0%. In the case of the catalytic oxidation desulfurization system containing HPW and H2O2 in the absence of IL, removal of DBT only reached 15.9%. HPW is water soluble without an emulsifying ability and sticks to the inner wall of the glass reactor during the reaction, leading to low sulfur removal. However, in the case of the system containing HPW, H2O2, and IL sulfur removal increased remarkablely. The results indicated that IL played an important role in removal of DBT of model oil in the desulfurization system. When HPW, H2O2, and [bmim]BF4 were employed together sulfur removal could reach 98.2% at 30 °C for 1 h. This result also demonstrated that the catalytic activity of HPW was improved in IL. The acids (lower pH) could make better use of H2O2 in ECODS system.34 Moreover, it might be due to the interaction of a small amount of IL with HPW. The presumed process is shown in Scheme 1. The fast exchange of ions between HPW and [bmim]BF4 may give [bmim]3[PW12O40] and a small amount of HBF4 in situ, which improved the Brønsted acidity of the catalytic system and consequently accelerated oxidation of DBT. We concluded that IL not only served as extractant and reaction media but also improved the catalytic activity of HPW. These experiments clearly demonstrated that a combination of extraction and catalytic oxidation could remove the sulfur compounds of model oil intensively. Similar trends were found in other ILs, which had a desulfurization efficiency from 63.5% to 98.0% (Table 1, entries 2-4). The nature of the IL played a significant role in the desulfurization system. The efficiency of desulfurization was better in the biphasic system (with [bmim]BF4 as extractant) than that in the triphasic system. Because catalyst and H2O2 were in IL phase, DBT was extracted into IL and then easily oxidized in IL. Moreover, the data of Table 1 show the cations
of ILs are important parameters affecting the desulfurization efficiency in the extraction and catalytic oxidation desulfurization system. The shorter the carbon chain of the cation of IL was, the higher the sulfur removal of the extraction and catalytic oxidation desulfurization system got. Influence of Time and Temperature on Removal of DBT. The desulfurization system containing HPW, H2O2, and [bmim]BF4 was used to investigate the influence of various reaction times and temperatures. Figure 1 shows the removal of DBT vs reaction time at different temperatures. The data at time zero reflected the ability of [bmim]BF4 to extract DBT from n-octane at room temperature. After a combination of extraction and catalytic oxidation, DBT was oxidized in IL phase after it was extracted from n-octane. Thus, a continuous increase in the removal of DBT in n-octane was observed at various temperatures. The higher the reaction temperature was, the faster the oxidation rate of DBT. Removal of DBT increased from 65.0% at 20 °C to 98.2% at 30 °C in 1 h. Compared with removal of DBT at 30 °C, the sulfur removal slightly increased at 50 °C (98.5%). When the reaction time reached 2 or 3 h, removal of DBT could reach 99.0% or 99.5% at 30 °C. The results indicated that DBT could be completely removed at room temperature in short reaction time. Influence of the H2O2/ DBT Ratio on Removal of DBT. To investigate the influence of the amount of oxidizing agent on the oxidative properties different H2O2/sulfur (O/S) molar ratios were carried out at 30 °C. According to the stoichiometric reaction, 2 mol of H2O2 are consumed for 1 mol of sulfur compound. As shown in Table 2 the O/S molar ratio has a strong influence on the reaction. It was noteworthy that there was a competition between the hydrogen peroxide decomposition side reaction and DBT oxidation reaction. With an O/S molar ratio ) 2, removal of DBT only reached 83.2% due to the hydrogen peroxide decomposition. With an O/S molar ratio > 3 the sulfur compound of model oil can be completely removed due to the increase in the concentration of hydrogen peroxide. With a O/S molar ratio ) 3 the desulfurization system also can deeply remove sulfur compound of model oil. Considering the commercial capital expenditure, O/S ) 3 was chosen in most cases in the present study.
Figure 2. Removal of sulfur compounds versus the reaction time at 30 (a) and 70 °C (b). Conditions: [bmim]BF4 ) 1 mL, model oil ) 5 mL, n(H2O2)/n(substrate)/n(HPW) ) 300:100:1.
Deep OxidatiVe Desulfurization of Fuels
Influence of the Nature of the Substrate. To investigate the influence of the HPW in [bmim]BF4 on the different sulfur compounds the oxidation of three model sulfur compounds such as BT, DBT, and 4,6-DMDBT was carried out at 30 and 70 °C. As shown in Figure 2, with the different reaction temperature, sulfur removal through catalytic oxidation combined with extraction decreased in the order DBT > 4,6-DMDBT > BT. The electron density on the sulfur atom on BT is significantly lower, leading to the lowest reactivity of BT. For DBT and 4,6DMDBT, the electron density on the sulfur of the former is 5.758, as much as the latter (5.760).34 Therefore, reactivity was mainly influenced by the stereo hindrance of the methyl groups, which became an obstacle for approach of the sulfur atom to the catalytic active species in IL. Removal of DBT, 4,6DMDBT, and BT was 98.2%, 74.9%, and 65.7% at 30 °C for 1 h, respectively. However, removal of DBT inconspicuously increased after 1 h. To 4,6-DMDBT and BT the results continued increasing after 1 h, and 96.7% and 88.9% sulfur removal was achieved in 3 h. When the reaction temperature increased to 70 °C, removal of DBT, 4,6-DMDBT, and BT could reach 98.9%, 86.7%, and 78.4% for 30 min, respectively. Removal of DBT, 4,6-DMDBT, and BT was 99.3%, 99.0%, and 99.1% for 1 h, respectively. The results indicated that DBT, 4,6-DMDBT, and BT could be effectively removed by extraction and catalytic oxidative desulfurization system. The three different sulfur compounds could be completely removed at 70 °C in 3 h. Influence of the Recycle of Ionic Liquid. The performance of the desulfurization system for [bmim]BF4 containing HPW and H2O2 was investigated on removal of DBT in model oil. Because the IL was immiscible with model oil,6,28 after the reaction the oil phase was still the upper layer and the IL phase along with catalyst and oxidizing agant was still the lower layer. The model oil could be separated by decantation from the biphasic system. After the first cycle little model oil (upper layer) remained in the biphasic system after decantation. To speed the volatilization of the residual model oil, the IL phase (underlayer) was distilled in an oil bath at 70 °C for 10 h, and then, fresh model oil and H2O2 were introduced for the next recycle under the same conditions. The data in Figure 3 indicate that the catalytic system could be recycled five times with a slight decrease in activity. Sulfur removal dropped from 98.2% to (34) Zhu, W. S.; Li, H. M.; Jiang, X.; Yan, Y. S.; Lu, J. D.; He, L. N.; Xia, J. X. Green Chem. 2008, 10, 641–646.
Energy & Fuels, Vol. 23, 2009 1357
Figure 3. Influence of the recycle times on sulfur removal. Conditions: T ) 30 °C, t ) 1 h, [bmim]BF4 ) 1 mL, model oil ) 5 mL, n(H2O2)/ n(DBT)/n(HPW) ) 300:100:1.
96.1% under the same experimental conditions. When the system was recycled for the sixth time, sulfur removal dropped from 96.1% to 94.3%. According to our observation, as the recycle time increased, more and more precipitation was produced, which possibly led to the decrease in activity. Conclusion At room temperature and short reaction time a commercially available H3PW12O40 · 14H2O combined with H2O2 and ILs [bmim]BF4, [bmim]PF6, [bmim]BF4, and [omim]PF6 is effective for removing DBT, 4,6-DMDBT, and BT. Removal of DBT of model oil in [bmim]BF4 can reach 98.2% under 30 °C for 1 h, which is obviously an advantage of this process over desulfurization by mere solvent extraction with IL or catalytic oxidation without IL. Moreover, DBT, 4,6-DMDBT, and BT can be completely removed under 70 °C in 3 h. This desulfurization system could be recycled five times with a slight decrease in activity. The simple desulfurization system composed of HPW, H2O2, and green solvents (ILs) has the potential to give environmentally friendly fuels and meet future environmental legislations. Acknowledgment. This work was financially supported by the National Nature Science Foundation of China (Nos. 20676057, 20876071, 20777029) and Doctoral Innovation Fund of Jiangsu (CX08B-143Z). EF800797N
