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Oxidation desulfurization of fuel using pyridinium-based ionic liquids as phase-transfer catalysts
Fuel Processing Technology 91 (2010) 1803–1806
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Oxidation desulfurization of fuel using pyridinium-based ionic liquids as phase-transfer catalysts Dishun Zhao a,⁎, Yanan Wang b, Erhong Duan c, Juan Zhang a a b c
School of Chemistry and Pharmaceutical Engineering, Hebei University of Science and Technology, Shi jiazhuang 050018, China School of Textile and Garment, Hebei University of Science and Technology, Shijiazhuang 050018, China School of Environmental Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China
a r t i c l e
i n f o
Article history: Received 20 February 2010 Received in revised form 22 July 2010 Accepted 1 August 2010 Keywords: Desulfurization Phase-transfer catalysts Oxidation Ionic liquids
a b s t r a c t In this work, several ionic liquids based on pyridinium cations are prepared. The ionic liquids are employed as phase-transfer catalysts (PTCs) for phase-transfer catalytic oxidation of dibenzothiophene (DBT) dissolved in n-octane. The partition coefficients of DBT between ionic liquids and n-octane are investigated. Then H2O2–formic acid is used as an oxidant and ionic liquids are used as PTCs. The reaction turns to be heterogeneous and desulfurization rate of DBT increased apparently. When IL ([BPy]HSO4) is used as PTC, and the condition are: temperature is 60 °C, time is 60 min, H2O2/sulfur molar ratio (O/S) is 4, the desulfurization rate reaches the maximum (93.3%), and the desulfurization of the real gasoline is also investigated, 87.7% of sulfur contents are removed under optima reaction conditions. The PTC [BPy]HSO4 can be recycled for five times without significant decrease in activity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Through combustion, the sulphocompound existing in fuel are converted to SOx which causes air pollution and is also one of the major sources of acid rain [1]. Therefore, the desulfurization of fuel has received worldwide attention. The traditional desulfurization method is catalytic hydrodesulphurization (HDS) using CoMo or NiMo as catalysts [2], which requires both high temperature and high pressure. HDS was more effective to remove aliphatic sulfur structures such as thiols, thioethers and disulfides, etc. than to remove sulfur-containing aromatic compounds. Alternative desulfurization technologies have been developed due to this reason, including adsorption [3,4], extraction [5], and selective oxidation, etc. [6,7] In recent years, a promising green solvent, room-temperature ionic liquids (RTILs) have been widely employed in the process of catalysis, chemical synthesis and separations. Using ionic liquids to remove sulfur content in fuel has been reported in the literature because ionic liquids with aromatic ring cations have better extractive effect to remove aromatic sulfur compounds in fuel, [8–21]. Bosmann et al. [9] have studied the desulfurization with 1,3-dialkylimidazolium cation ionic liquids. Lo and co-workers [10] combined the methods of oxidation and extraction, using H2O2–acetic acid as the oxidant, [BMIM]BF4 and [BMIM]PF6 as the extraction. That process increased the desulfurization rate by about an order of magnitude relative to
⁎ Corresponding author. Tel./fax: +86 311 88632009. E-mail address: [email protected] (D. Zhao). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.08.001
that of merely extracting with RTILs. Cheng et al. [21] first reported that imidazolium-based ionic liquid was employed as phase-transfer catalysis to remove sulfur content in navy diesel. Most of the reported literature is related to desulfurization by ionic liquids with imidazolium cations, while other types of cations of ionic liquids are rarely reported. Hence, in this work, several pyridiniumbased ionic liquids were designed as the phase-transfer catalysts (PTC) because of their extractive ability with aromatic sulfur compounds. Then H2O2–formic acid system was used as the oxidant. The desulfurization of dibenzothiophene (DBT) in model oil was studied as well as in real gasoline. 2. Experiment 2.1. Preparation of model oil Model oil (1000 μg/mL S) was prepared by dissolving dibenzothiophene (2.879 g) in n-octane (500 mL). 2.2. Preparation of ionic liquids Intermediates of ionic liquids (alkyl pyridinium salt) were prepared by equal molars of pyridine and the corresponding butyl bromide at 333.2 K for 12 h, then the white crystalloid intermediate [BPy]Br was filtrated off and evaporated in a vacuum drying oven, then ionic liquids [BPy]BF4, [BPy]SCN were prepared by equal molars of intermediate and sodium tetrafluoroborate, sodium rhodanate in cyclohexane at room temperature for 12 h, the viscous liquids had been washed three times with diethyl ether at room temperature
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followed by rotary evaporation under reduced pressure of 1 kPa for 1 h to remove all volatile residues. Ionic liquids [BPy]HSO4 and [BPy]H2PO4 were prepared by dropping equal molars of sulfuric acid and phosphoric acid in intermediate with cyclohexane, and viscous liquids had been distilled by rotary evaporation to remove HBr and volatile residues. 2.3. Measuring of partition coefficients (KN) Extraction experiments were performed as follows: equal volumes (5 mL) of model oil (sulfur content is 1000 μg/mL) and ionic liquids were mixed and stirred in a conical beaker for 60 min at 40 °C by a magnetic stirring apparatus, followed by stewing for 15 min. Then the sample of upper n-octane was taken and analyzed by gas chromatograph with a flame photometric detector (GC-FPD), the partition coefficients of DBT to the ionic liquids and octane was calculated by the formula as follows after analyzing the initial sample without extraction. KN = ½DBTðinitial−octaneÞ = ½DBToctane 2.4. The desulfurization procedure Desulfurization of the model light oil was carried out as follows: equal molars of formic acid and H2O2 (30%) were mixed together to form peroxyformic acid, then certain amount of peroxyformic acid was added to IL. The mixture was stirred and heated to a certain temperature after adding the model oil (the same volumes of IL). Then the sulfur content was determined by microcoulometric detector (WK-2D). 2.5. Regeneration of used ionic liquid The oil phase was separated by separatory funnel from the ionic liquids. Used ionic liquids were evaporated at 110 °C to remove H2O2, water and formic acid entirely, and then IL was regenerated by reextraction to remove DBTO with an equal volume of DMF for three times. 3. Results and discussion 3.1. Sulfur partition coefficient KN in different ionic liquids The extraction of model oil consistent of n-octane containing DBT as typical sulphide with a sulfur content of 1000 μg/mL was studied with ionic liquids. The partition coefficients KN for DBT between model oil and ionic liquids were calculated according to the experiment process described above. The partition coefficients KN can reflect the extraction ability to sulfur compound of ionic liquids intuitively. Table 1 shows the partition coefficient KN value for DBT in different pyridinium-based ionic liquids. The ability of the ionic liquids to extract DBT from oil phase follows the order below: [BPy] HSO4 > [BPy]H2PO4 > [BPy]SCN > [BPy]BF4. The ionic liquid [BPy]HSO4 has the highest KN value 1.42, followed by the ionic liquid [BPy]H2PO4, the KN value is 1.24.
Scheme 1. Oxidization mechanism of DBT using ionic liquid as phase-transfer catalyst.
3.2. Oxidation desulfurization of model oil with ionic liquids as PTC As we know that ionic liquids were found to be effective to extract DBT from the oil phase to ionic phase [18]. In this work, H2O2–formic acid was used as an oxidation system, since H2O2 can be decomposed to form hydroxyl radicals with stronger oxidizing property under acidic conditions. The system of H2O2–formic acid was immiscible in model oil, and all the ionic liquids prepared were water-soluble, therefore oxidation system could be dissolved in ionic liquids to make the reaction homogeneous. The reaction rate could be increased apparently. Here the ionic liquids played a role of phase-transfer catalysts. The oxidization mechanism of DBT was shown as Scheme 1. Fig. 1 shows the desulfurization rate of DBT with ionic liquids as PTC. The sulfur removal did not change after 60 min. The results indicated that desulfurization rate increased after ionic liquids were added as PTC, the oxidation desulfurization rate followed the order [BPy]HSO4 > [BPy]H2PO4 > [BPy]SCN > [BPy]BF4, which was the same order as the partition coefficient KN above. It also indicated that the leading effect factor of desulfurization was oxidation reaction but not the extraction process. Ionic liquid [BPy]HSO4 showed the best capability both in extraction and in catalytic oxidation, so using [BPy]HSO4 as the PTC was studied in further work. 3.3. Effect of O/S molar ratio on ODS process The H2O2–formic acid system was used as an oxidant in the oxidation process, DBT was extracted to ionic liquid phase and then was oxidized to corresponding sulfones by hydrogen peroxide. Theoretically, 2 molar of hydrogen peroxide could oxidize 1 molar of sulfur-containing compound to corresponding sulfones. To investigate the effect of O/S molar ratio on the oxidation desulfurization, the extraction-oxidation of DBT in ionic-oxidation system was carried out at 40 °C for 60 min. The influence of O/S molar ratio on oxidation
Table 1 DBT partition coefficient KN in different ionic liquids. IL
DBT in model oil
DBT in IL
Percent extracted
KN
[BPy]BF4 [BPy]SCN [BPy]HSO4 [BPy]H2PO4
546 468 414 446
454 532 586 554
45.4% 53.2% 58.6% 55.4%
0.83 1.14 1.42 1.24
Condition: T = 40 °C, V model oil/V IL = 1:1, t = 60 min, S = 1000 μg/mL.
Fig. 1. Oxidative desulfurization of model oil with ionic liquids as PTCs. Conditions: T = 40 °C, t = 70 min and S = 1000 μg/mL.
D. Zhao et al. / Fuel Processing Technology 91 (2010) 1803–1806
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the hydrogen peroxide was easily decomposed to H2O and O2 at a high temperature, so the temperature 60 °C was optimal in this study. 3.5. Phase-transfer catalytic oxidation of gasoline by ionic liquid
Fig. 2. Effect of the H2O2/S (O/S) molar ratio on sulfur removal. Conditions: T = 40 °C, [BPy]HSO4 as PTC, t = 60 min and S = 1000 μg/mL.
desulfurization was shown in Fig. 2. When [BPy]HSO4 was used as PTC, the variation of O/S molar ratio influenced desulfurization obviously. The desulfurization of DBT increased from 73% at O/S = 2 to 92% at O/S = 4, indicating that hydrogen peroxide was not only decomposed to hydroxyl radicals but also decomposed to H2O and O2. Therefore, when the O/S = 2, the hydroxyl radicals that decomposed were not sufficient to oxidize all the DBT. The desulfurization of DBT was reduced from O/S = 4 to O/S = 8. The reason for this may be that the existence of excess H2O2–formic acid will dilute the ionic liquids, which influence the extraction of DBT from n-octane. Then the peak value of desulfurization was O/S = 4. 3.4. Effect of temperature on oxidation desulfurization The influence of temperature on partition coefficient KN for DBT between model oil and ionic liquids [BPy]HSO4 was researched firstly. The temperature 20 °C, 40 °C, 60 °C, and 80 °C were tested, the initial concentration of DBT was 1000 μg/mL, the model oil/IL volume ratio was 1:1, and the corresponding values of KN were 1.37, 1.42, 1.47, and 1.53, respectively. The value of KN increased with increasing temperature. The extraction ability of ionic liquids increased because the viscosity of ionic liquid was reduced while the temperature increased. Then, the effect of reaction temperature (from 20 °C to 80 °C) on the oxidation desulfurization of model oil is shown in Table 2, O/S molar ratio is 4. The reaction rate increased obviously when the temperature increased from 20 °C to 60 °C; when the temperature increased to 60 °C, the desulfurization reached the maximum 93.3%. When the temperature was higher than 60 °C, the desulfurization rate decreased appreciably. The reason was that
All the experiments above were about the desulfurization of “simulated oil”, which was prepared by dissolving some typical sulfur compound such as thiophene, benzothiophenes (BT), or dibenzothiophene (DBT) in solvent. To investigate the catalytic oxidation effect on real fuel, an experiment was conducted using commercial 93# gasoline which was purchased from a gas station. The sulfur concentration of gasoline was 390 μg/mL which was detected by a microcoulometric detector (WK-2D). Extraction ability of ionic liquids to sulfur content in gasoline was studied first. The reaction was carried out as follows: gasoline (10 mL) was mixed with ionic liquid [BPy] HSO4 (10 mL) at 60 °C, sulfur contents in both phases were not changed after stirring for 60 min. The sulfur partition coefficient (for S-concentration) dropped to 1.2 (177 μg/mL of S in gasoline and 213 μg/mL of S in ionic liquid). Then ODS process was conducted in gasoline, with O/S molar ratio 4, 60 °C. The S-concentration in gasoline reduced from 390 μg/mL to 48 μg/mL, sulfur removal was 87.7%. Comparing with the desulfurization data of model oil, sulfur removal efficiency for the gasoline was slightly lower. The reason was that gasoline contains many non-aromatic sulfur components such as thiol, sulfide which cannot be extracted by IL effectively. On the other hand, the aromatic hydrocarbon and olefin in gasoline had influence on the sulfur extraction as well [22]. 3.6. Regeneration of used ionic liquid The recycling of oxidation system for ionic liquid [BPy]HSO4 was investigated in desulfurization on model oil with DBT. The used ionic liquid was re-extracted by an equal volume of DMF for three times. When used ionic liquid was re-extracted by DMF to remove DBTO, the desulfurization decreased inconspicuously compared with the data from fresh ionic liquid. The desulfurization ratio dropped from 93.3% to 89.5% after five cycles finished (Table 3). 4. Conclusions In conclusion, the pyridinium-based ionic liquid can be used as a phase-transfer catalyst for desulfurization of model oil and actual gasoline when H2O2-formic acid is used as oxidant. The desulfurization of the DBT-containing model oil can reach 93.3% under the optima reaction conditions by ionic liquid [BPy]HSO4 as PTC, which is far superior to simple extraction with ionic liquids. The ionic liquid [BPy]HSO4 can be recycled five times without a significant decrease in activity. All these results indicated that ionic liquids have a potential application in desulfurization as a benign reactant. Acknowledgments
Table 2 Effects of the reaction temperature on sulfur removal. Temperature (°C) Sulfur removal (%)
20 90.6
40 92.0
The work was financially supported by National Natural Science Foundation of China (20576026). 60 93.3
80 90.2
Conditions: O/S = 4, S = 1000 μg/mL, V model oil/V IL = 1:1.
Table 3 The desulfurization of model oil by regenerated ionic liquid [BPy]HSO4. Cycle Sulfur removal (%)
0 93.3
1 92.6
2 91.0
3 91.2
4 90.0
5 89.5
6 87.1
Conditions: T = 60 °C, O/S = 4; S = 1000 μg/mL, t = 60 min, V gasoline/V IL = 1:1.
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[5] W. Li, M.D. Wang, H. Chen, J.M. Chen, Y. Shi, Biodesulfurization of dibenzothiophene by growing cells of Gordonia sp. in batch cultures, Biotechnol. Lett. 28 (2006) 1175–1179. [6] W.L. Li, H. Tang, Q.H. Liu, J.M. Xing, Q. Li, D. Wang, M.H. Yang, H.Z. Liu, Deep desulfurization of diesel by integrating adsorption and microbial method, Biochem. Eng. J. 44 (2009) 297–301. [7] A.M. Dehkordi, Z. Kiaei, M.A. Sobati, Oxidative desulfurization of simulated light fuel oil and untreated kerosene, Fuel Process. Technol. 90 (2009) 435–445. [8] J. Esser, P. Wasserscheid, A. Jess, Deep desulfurization of oil refinery streams by extraction with ionic liquids, Green Chem. 6 (2004) 316–322. [9] A. Bösmann, L. Datsevich, A. Jess, A. Lauter, C. Schmitz, P. Wasserscheid, Deep desulfurization of diesel fuel by extraction with ionic liquids, Chem. Commun. (2001) 2494–2495. [10] W.H. Lo, H.Y. Yang, G.T. Wei, One-pot desulfurization of light oils by chemical oxidation and solvent extraction with room temperature ionic liquids, Green Chem. 5 (2003) 639–642. [11] D. Zhao, Z.M. Sun, F.T. Li, H.D. Shan, Oxidative desulfurization of thiophene catalyzed by (C4H9)4NBr·2C6H11NO coordinated ionic liquid, Energy Fuels 22 (2008) 3065–3069. [12] J.H. Kim, X.L. Ma, A.N. Zhou, C.S. Song, Ultra-deep desulfurization and denitrogenation of diesel fuel by selective adsorption over three different adsorbents: a study on adsorptive selectivity and mechanism, Catal. Today 111 (2006) 74–83. [13] C.P. Huang, B.C. Chen, J. Zhang, Z.C. Liu, Y.X. Li, Desulfurization of gasoline by extraction with new ionic liquids, Energy Fuels 18 (2004) 1863–1864.
[14] Y. Nie, C.X. Li, Z.H. Wang, Extractive desulfurization of fuel oil using alkylimidazole and its mixture with dialkylphosphate ionic liquids, Ind. Eng. Chem. Res. 46 (2007) 5108–5112. [15] Y. Nie, C.X. Li, A.J. Sun, H. Meng, Z.H. Wang, Extractive desulfurization of gasoline using imidazolium-based phosphoric ionic liquids, Energy Fuels 20 (2006) 2083–2087. [16] C. Komintarachat, W. Trakarnpruk, Oxidative desulfurization using polyoxometalates, Ind. Eng. Chem. Res. 45 (2006) 1853–1856. [17] K. Yazu, M. Makino, K. Ukegawa, Oxidative desulfurization of diesel oil with hydrogen peroxide in the presence of acid catalyst in diesel oil/acetic acid biphasic system, Chem. Lett. 33 (2004) 1306–1307. [18] S.G. Zhang, Z.C. Zhang, Novel properties of ionic liquids in selective sulfur removal from fuels at room temperature, Green Chem. 4 (2002) 376–379. [19] Y. Nie, C.X. Li, H. Meng, Z.H. Wang, N,N-dialkylimidazolium dialkylphosphate ionic liquids: their extractive performance for thiophene series compounds from fuel oils versus the length of alkyl group, Fuel Process. Technol. 89 (2008) 978–983. [20] J.L. Wang, D.S. Zhao, E.P. Zhou, Z. Dong, Desulfurization of gasoline by extraction with N-alkyl-pyridinium-based ionic liquids, J. Fuel Chem. Technol. 35 (2007) 293–296. [21] S.S. Cheng, T.F. Yen, Use of ionic liquids as phase-transfer catalysis for deep oxygenative desulfurization, Energy Fuels 22 (2008) 1400–1401. [22] B.M. Su, S.G. Zhang, Z.C. Zhang, Structural elucidation of thiophene interaction with ionic liquids by multinuclear NMR spectroscopy, J. Phys. Chem. B 108 (2004) 19510–19517.
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Oxidation desulfurization of fuel using pyridinium-based ionic liquids as phase-transfer catalysts Dishun Zhao a,⁎, Yanan Wang b, Erhong Duan c, Juan Zhang a a b c
School of Chemistry and Pharmaceutical Engineering, Hebei University of Science and Technology, Shi jiazhuang 050018, China School of Textile and Garment, Hebei University of Science and Technology, Shijiazhuang 050018, China School of Environmental Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China
a r t i c l e
i n f o
Article history: Received 20 February 2010 Received in revised form 22 July 2010 Accepted 1 August 2010 Keywords: Desulfurization Phase-transfer catalysts Oxidation Ionic liquids
a b s t r a c t In this work, several ionic liquids based on pyridinium cations are prepared. The ionic liquids are employed as phase-transfer catalysts (PTCs) for phase-transfer catalytic oxidation of dibenzothiophene (DBT) dissolved in n-octane. The partition coefficients of DBT between ionic liquids and n-octane are investigated. Then H2O2–formic acid is used as an oxidant and ionic liquids are used as PTCs. The reaction turns to be heterogeneous and desulfurization rate of DBT increased apparently. When IL ([BPy]HSO4) is used as PTC, and the condition are: temperature is 60 °C, time is 60 min, H2O2/sulfur molar ratio (O/S) is 4, the desulfurization rate reaches the maximum (93.3%), and the desulfurization of the real gasoline is also investigated, 87.7% of sulfur contents are removed under optima reaction conditions. The PTC [BPy]HSO4 can be recycled for five times without significant decrease in activity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Through combustion, the sulphocompound existing in fuel are converted to SOx which causes air pollution and is also one of the major sources of acid rain [1]. Therefore, the desulfurization of fuel has received worldwide attention. The traditional desulfurization method is catalytic hydrodesulphurization (HDS) using CoMo or NiMo as catalysts [2], which requires both high temperature and high pressure. HDS was more effective to remove aliphatic sulfur structures such as thiols, thioethers and disulfides, etc. than to remove sulfur-containing aromatic compounds. Alternative desulfurization technologies have been developed due to this reason, including adsorption [3,4], extraction [5], and selective oxidation, etc. [6,7] In recent years, a promising green solvent, room-temperature ionic liquids (RTILs) have been widely employed in the process of catalysis, chemical synthesis and separations. Using ionic liquids to remove sulfur content in fuel has been reported in the literature because ionic liquids with aromatic ring cations have better extractive effect to remove aromatic sulfur compounds in fuel, [8–21]. Bosmann et al. [9] have studied the desulfurization with 1,3-dialkylimidazolium cation ionic liquids. Lo and co-workers [10] combined the methods of oxidation and extraction, using H2O2–acetic acid as the oxidant, [BMIM]BF4 and [BMIM]PF6 as the extraction. That process increased the desulfurization rate by about an order of magnitude relative to
⁎ Corresponding author. Tel./fax: +86 311 88632009. E-mail address: [email protected] (D. Zhao). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.08.001
that of merely extracting with RTILs. Cheng et al. [21] first reported that imidazolium-based ionic liquid was employed as phase-transfer catalysis to remove sulfur content in navy diesel. Most of the reported literature is related to desulfurization by ionic liquids with imidazolium cations, while other types of cations of ionic liquids are rarely reported. Hence, in this work, several pyridiniumbased ionic liquids were designed as the phase-transfer catalysts (PTC) because of their extractive ability with aromatic sulfur compounds. Then H2O2–formic acid system was used as the oxidant. The desulfurization of dibenzothiophene (DBT) in model oil was studied as well as in real gasoline. 2. Experiment 2.1. Preparation of model oil Model oil (1000 μg/mL S) was prepared by dissolving dibenzothiophene (2.879 g) in n-octane (500 mL). 2.2. Preparation of ionic liquids Intermediates of ionic liquids (alkyl pyridinium salt) were prepared by equal molars of pyridine and the corresponding butyl bromide at 333.2 K for 12 h, then the white crystalloid intermediate [BPy]Br was filtrated off and evaporated in a vacuum drying oven, then ionic liquids [BPy]BF4, [BPy]SCN were prepared by equal molars of intermediate and sodium tetrafluoroborate, sodium rhodanate in cyclohexane at room temperature for 12 h, the viscous liquids had been washed three times with diethyl ether at room temperature
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followed by rotary evaporation under reduced pressure of 1 kPa for 1 h to remove all volatile residues. Ionic liquids [BPy]HSO4 and [BPy]H2PO4 were prepared by dropping equal molars of sulfuric acid and phosphoric acid in intermediate with cyclohexane, and viscous liquids had been distilled by rotary evaporation to remove HBr and volatile residues. 2.3. Measuring of partition coefficients (KN) Extraction experiments were performed as follows: equal volumes (5 mL) of model oil (sulfur content is 1000 μg/mL) and ionic liquids were mixed and stirred in a conical beaker for 60 min at 40 °C by a magnetic stirring apparatus, followed by stewing for 15 min. Then the sample of upper n-octane was taken and analyzed by gas chromatograph with a flame photometric detector (GC-FPD), the partition coefficients of DBT to the ionic liquids and octane was calculated by the formula as follows after analyzing the initial sample without extraction. KN = ½DBTðinitial−octaneÞ = ½DBToctane 2.4. The desulfurization procedure Desulfurization of the model light oil was carried out as follows: equal molars of formic acid and H2O2 (30%) were mixed together to form peroxyformic acid, then certain amount of peroxyformic acid was added to IL. The mixture was stirred and heated to a certain temperature after adding the model oil (the same volumes of IL). Then the sulfur content was determined by microcoulometric detector (WK-2D). 2.5. Regeneration of used ionic liquid The oil phase was separated by separatory funnel from the ionic liquids. Used ionic liquids were evaporated at 110 °C to remove H2O2, water and formic acid entirely, and then IL was regenerated by reextraction to remove DBTO with an equal volume of DMF for three times. 3. Results and discussion 3.1. Sulfur partition coefficient KN in different ionic liquids The extraction of model oil consistent of n-octane containing DBT as typical sulphide with a sulfur content of 1000 μg/mL was studied with ionic liquids. The partition coefficients KN for DBT between model oil and ionic liquids were calculated according to the experiment process described above. The partition coefficients KN can reflect the extraction ability to sulfur compound of ionic liquids intuitively. Table 1 shows the partition coefficient KN value for DBT in different pyridinium-based ionic liquids. The ability of the ionic liquids to extract DBT from oil phase follows the order below: [BPy] HSO4 > [BPy]H2PO4 > [BPy]SCN > [BPy]BF4. The ionic liquid [BPy]HSO4 has the highest KN value 1.42, followed by the ionic liquid [BPy]H2PO4, the KN value is 1.24.
Scheme 1. Oxidization mechanism of DBT using ionic liquid as phase-transfer catalyst.
3.2. Oxidation desulfurization of model oil with ionic liquids as PTC As we know that ionic liquids were found to be effective to extract DBT from the oil phase to ionic phase [18]. In this work, H2O2–formic acid was used as an oxidation system, since H2O2 can be decomposed to form hydroxyl radicals with stronger oxidizing property under acidic conditions. The system of H2O2–formic acid was immiscible in model oil, and all the ionic liquids prepared were water-soluble, therefore oxidation system could be dissolved in ionic liquids to make the reaction homogeneous. The reaction rate could be increased apparently. Here the ionic liquids played a role of phase-transfer catalysts. The oxidization mechanism of DBT was shown as Scheme 1. Fig. 1 shows the desulfurization rate of DBT with ionic liquids as PTC. The sulfur removal did not change after 60 min. The results indicated that desulfurization rate increased after ionic liquids were added as PTC, the oxidation desulfurization rate followed the order [BPy]HSO4 > [BPy]H2PO4 > [BPy]SCN > [BPy]BF4, which was the same order as the partition coefficient KN above. It also indicated that the leading effect factor of desulfurization was oxidation reaction but not the extraction process. Ionic liquid [BPy]HSO4 showed the best capability both in extraction and in catalytic oxidation, so using [BPy]HSO4 as the PTC was studied in further work. 3.3. Effect of O/S molar ratio on ODS process The H2O2–formic acid system was used as an oxidant in the oxidation process, DBT was extracted to ionic liquid phase and then was oxidized to corresponding sulfones by hydrogen peroxide. Theoretically, 2 molar of hydrogen peroxide could oxidize 1 molar of sulfur-containing compound to corresponding sulfones. To investigate the effect of O/S molar ratio on the oxidation desulfurization, the extraction-oxidation of DBT in ionic-oxidation system was carried out at 40 °C for 60 min. The influence of O/S molar ratio on oxidation
Table 1 DBT partition coefficient KN in different ionic liquids. IL
DBT in model oil
DBT in IL
Percent extracted
KN
[BPy]BF4 [BPy]SCN [BPy]HSO4 [BPy]H2PO4
546 468 414 446
454 532 586 554
45.4% 53.2% 58.6% 55.4%
0.83 1.14 1.42 1.24
Condition: T = 40 °C, V model oil/V IL = 1:1, t = 60 min, S = 1000 μg/mL.
Fig. 1. Oxidative desulfurization of model oil with ionic liquids as PTCs. Conditions: T = 40 °C, t = 70 min and S = 1000 μg/mL.
D. Zhao et al. / Fuel Processing Technology 91 (2010) 1803–1806
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the hydrogen peroxide was easily decomposed to H2O and O2 at a high temperature, so the temperature 60 °C was optimal in this study. 3.5. Phase-transfer catalytic oxidation of gasoline by ionic liquid
Fig. 2. Effect of the H2O2/S (O/S) molar ratio on sulfur removal. Conditions: T = 40 °C, [BPy]HSO4 as PTC, t = 60 min and S = 1000 μg/mL.
desulfurization was shown in Fig. 2. When [BPy]HSO4 was used as PTC, the variation of O/S molar ratio influenced desulfurization obviously. The desulfurization of DBT increased from 73% at O/S = 2 to 92% at O/S = 4, indicating that hydrogen peroxide was not only decomposed to hydroxyl radicals but also decomposed to H2O and O2. Therefore, when the O/S = 2, the hydroxyl radicals that decomposed were not sufficient to oxidize all the DBT. The desulfurization of DBT was reduced from O/S = 4 to O/S = 8. The reason for this may be that the existence of excess H2O2–formic acid will dilute the ionic liquids, which influence the extraction of DBT from n-octane. Then the peak value of desulfurization was O/S = 4. 3.4. Effect of temperature on oxidation desulfurization The influence of temperature on partition coefficient KN for DBT between model oil and ionic liquids [BPy]HSO4 was researched firstly. The temperature 20 °C, 40 °C, 60 °C, and 80 °C were tested, the initial concentration of DBT was 1000 μg/mL, the model oil/IL volume ratio was 1:1, and the corresponding values of KN were 1.37, 1.42, 1.47, and 1.53, respectively. The value of KN increased with increasing temperature. The extraction ability of ionic liquids increased because the viscosity of ionic liquid was reduced while the temperature increased. Then, the effect of reaction temperature (from 20 °C to 80 °C) on the oxidation desulfurization of model oil is shown in Table 2, O/S molar ratio is 4. The reaction rate increased obviously when the temperature increased from 20 °C to 60 °C; when the temperature increased to 60 °C, the desulfurization reached the maximum 93.3%. When the temperature was higher than 60 °C, the desulfurization rate decreased appreciably. The reason was that
All the experiments above were about the desulfurization of “simulated oil”, which was prepared by dissolving some typical sulfur compound such as thiophene, benzothiophenes (BT), or dibenzothiophene (DBT) in solvent. To investigate the catalytic oxidation effect on real fuel, an experiment was conducted using commercial 93# gasoline which was purchased from a gas station. The sulfur concentration of gasoline was 390 μg/mL which was detected by a microcoulometric detector (WK-2D). Extraction ability of ionic liquids to sulfur content in gasoline was studied first. The reaction was carried out as follows: gasoline (10 mL) was mixed with ionic liquid [BPy] HSO4 (10 mL) at 60 °C, sulfur contents in both phases were not changed after stirring for 60 min. The sulfur partition coefficient (for S-concentration) dropped to 1.2 (177 μg/mL of S in gasoline and 213 μg/mL of S in ionic liquid). Then ODS process was conducted in gasoline, with O/S molar ratio 4, 60 °C. The S-concentration in gasoline reduced from 390 μg/mL to 48 μg/mL, sulfur removal was 87.7%. Comparing with the desulfurization data of model oil, sulfur removal efficiency for the gasoline was slightly lower. The reason was that gasoline contains many non-aromatic sulfur components such as thiol, sulfide which cannot be extracted by IL effectively. On the other hand, the aromatic hydrocarbon and olefin in gasoline had influence on the sulfur extraction as well [22]. 3.6. Regeneration of used ionic liquid The recycling of oxidation system for ionic liquid [BPy]HSO4 was investigated in desulfurization on model oil with DBT. The used ionic liquid was re-extracted by an equal volume of DMF for three times. When used ionic liquid was re-extracted by DMF to remove DBTO, the desulfurization decreased inconspicuously compared with the data from fresh ionic liquid. The desulfurization ratio dropped from 93.3% to 89.5% after five cycles finished (Table 3). 4. Conclusions In conclusion, the pyridinium-based ionic liquid can be used as a phase-transfer catalyst for desulfurization of model oil and actual gasoline when H2O2-formic acid is used as oxidant. The desulfurization of the DBT-containing model oil can reach 93.3% under the optima reaction conditions by ionic liquid [BPy]HSO4 as PTC, which is far superior to simple extraction with ionic liquids. The ionic liquid [BPy]HSO4 can be recycled five times without a significant decrease in activity. All these results indicated that ionic liquids have a potential application in desulfurization as a benign reactant. Acknowledgments
Table 2 Effects of the reaction temperature on sulfur removal. Temperature (°C) Sulfur removal (%)
20 90.6
40 92.0
The work was financially supported by National Natural Science Foundation of China (20576026). 60 93.3
80 90.2
Conditions: O/S = 4, S = 1000 μg/mL, V model oil/V IL = 1:1.
Table 3 The desulfurization of model oil by regenerated ionic liquid [BPy]HSO4. Cycle Sulfur removal (%)
0 93.3
1 92.6
2 91.0
3 91.2
4 90.0
5 89.5
6 87.1
Conditions: T = 60 °C, O/S = 4; S = 1000 μg/mL, t = 60 min, V gasoline/V IL = 1:1.
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