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Extractive Desulfurization of Fuel Using 3-Methylpyridinium-Based Ionic Liquids Hongshuai Gao,†,‡ Yuguang Li,†,‡ Yong Wu,†,‡ Mingfang Luo,‡ Qiang Li,†,‡ Jianmin Xing,*,† and Huizhou Liu*,† Key Laboratory of Green Process and Engineering, State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed January 4, 2009. ReVised Manuscript ReceiVed February 16, 2009
3-Methylpyridinium-based ionic liquids (ILs) were demonstrated to be effective for the selective removal of aromatic heterocyclic sulfur compounds from diesel at room temperature. The results indicated that the extractive performance using 3-methylpyridinium-based ILs followed the order of 1-octyl-3-methylpyridinium tetrafluoroborate ([C83MPy][BF4]) > 1-hexyl-3-methylpyridinium tetrafluoroborate ([C63MPy][BF4]) > 1-butyl3-methylpyridinium tetrafluoroborate ([C43MPy][BF4]). For a given IL, the sulfur removal selectivity of sulfur compounds followed the order of dibenzothiophene (DBT) > benzothiophene (BT) > thiophene (TS) > 4,6dibenzothiophene (4,6-DMDBT) under the same conditions, except for [C83MPy][BF4] IL, which followed the order of DBT > BT > 4,6-DMDBT > TS. The 3-methylpyridinium-based ILs are insoluble in diesel, while diesel has a certain solubility in 3-methylpyridinium-based ILs, with the content varying from 6.1 wt % for [C43MPy][BF4] to 9.5 wt % for [C83MPy][BF4]. The spent IL saturated sulfur compounds could be regenerated by a water dilution process. Considering these results, ILs studied in this work are more competitive and feasible for extractive desulfurization applications. Moreover, the extractive desulfurization using 3-methylpyridinium-based ILs could be used at least as a complementary process to hydrodesulfurization (HDS).
1. Introduction In the past decade, air pollution has been a major public concern. The combustion of sulfur compounds in gasoline and diesel fuel results in emission of SOx and sulfate particulate matter, which is a major cause of air pollution and acid rain. To reduce atmospheric pollution by sulfur oxides, governments around the world have established stringent environmental regulations.1 Hydrodesulfurization (HDS) is a conventional method to remove sulfur compounds for industrial purposes. However, the main drawbacks of HDS include high-temperature (>300 °C), high-pressure (>4 MPa), high-energy cost, and difficult-to-remove aromatic heterocyclic sulfur compounds, such as dibenzothiophene (DBT) and its derivatives. To meet the new environmental regulations, various alternative deep desulfurization processes have been extensively investigated in the past few years, including biodesulfurization,2,3 adsorption,4,5 * To whom correspondence should be addressed. Telephone: +86-1062550913. Fax: +86-10-62550913. E-mail:
[email protected] (J.X.); Telephone: +86-10-62554264. Fax: +86-10-62554264. E-mail: hzliu@ home.ipe.ac.cn (H.L.). † Institute of Process Engineering, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. (1) Song, C. S. Catal. Today 2003, 86, 211–263. (2) Shan, G. B.; Xing, J. M.; Zhang, H. Y.; Liu, H. Z. Appl. EnViron. Microbiol. 2005, 71, 4497–4502. (3) Xu, P.; Yu, B.; Li, F. L.; Cai, X. F.; Ma, C. Q. Trends Microbiol. 2006, 14, 398–405. (4) Li, W. L.; Liu, Q. F.; Xing, J. M.; Gao, H. S.; Xiong, X. C.; Li, Y. G.; Li, X.; Liu, H. Z. AIChE J. 2007, 53, 3263–3268. (5) Herna´ndez-Maldonado, A. J.; Yang, R. T. J. Am. Chem. Soc. 2004, 126, 992–993. (6) Otsuki, S.; Nonaka, T.; Taksshima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232–1239.
oxidation,6,7 and extractive desulfurization.8 Among these new processes, extractive desulfurization appears to be particularly promising and is currently receiving growing attention. A lot of organic solvents, such as polyalkyleneglycol, were used as extractants for the removal of sulfur compounds from the fuel, but the results were not satisfactory. Recently, various ionic liquids (ILs) have been used as extractants for the removal of sulfur compounds from the fuel because of their unique chemical and physical properties, such as negligible vapor pressure, high chemical and thermal stabilities, and ability to dissolve a wide range of organic and inorganic compounds.9,10 The extractive desulfurization process using ILs (7) Lu¨, H. Y.; Gao, J. B.; Jiang, Z. X.; Yang, Y. X.; Song, B.; Li, C. Chem. Commun. 2007, 2, 150–152. (8) Shiraishi, Y.; Hirai, Y.; Komasawa, I. Ind. Eng. Chem. Res. 2001, 40, 293–303. (9) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772– 3789. (10) Welton, T. Chem. ReV. 1999, 99, 2071–2084. (11) Bo¨smann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 23, 2494–2495. (12) Zhang, S. G.; Zhang, Z. C. Green Chem. 2002, 4, 376–379. (13) Huang, C. P.; Chen, B. H.; Zhang, J.; Liu, Z. C.; Li, Y. X. Energy Fuels 2004, 18, 1862–1864. (14) Zhang, S. G.; Zhang, Q. L.; Zhang, Z. C. Ind. Eng. Chem. Res. 2004, 43, 614–622. (15) Esser, J.; Wasserscheid, P.; Jess, A. Green Chem. 2004, 6, 316– 322. (16) Nie, Y.; Li, C. X.; Sun, A. J.; Meng, H.; Wang, Z. H. Energy Fuels 2006, 20, 2083–2087. (17) Planeta, J.; Kara´sek, P.; Roth, M. Green Chem. 2006, 8, 70–77. (18) Nie, Y.; Li, C. X.; Wang, Z. H. Ind. Eng. Chem. Res. 2007, 46, 5108–5112. (19) Alonso, L.; Arce, A.; Franciso, M.; Rodrg´uez, O.; Soto, A. AIChE J. 2007, 53, 3108–3115.
10.1021/ef900009g CCC: $40.75 2009 American Chemical Society Published on Web 04/14/2009
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Figure 1. Chemical structures of the 3-methylpyridinium-based ILs: 1-butyl-3-methylpyridinium tetrafluoroborate ([C43MPy][BF4]), 1-hexyl-3methyl pyridinium tetrafluoroborate ([C63MPy][BF4]), and 1-octyl-3-methylpyridinium tetrafluoroborate ([C83MPy][BF4]).
can be a complementary technology for the HDS process.11-28 These studies11-23 indicated that the ILs have high extraction ratios and greater selectivity compared to molecular solvents because of the unique solvent characteristics of ILs. The sulfur compounds mainly studied are aromatic sulfur compounds, such as BT and DBT, dissolved in dodecane as a model oil system. All of those studied focused only on N-methylimidzole-based ILs. For the IL extractive desulfurization process, the attention is focused on finding an effective IL that is thermally stable, is nonsensitive to moisture and air, has a low cost for commercial application, and has a high extractive performance. In our previous study,22 pyridinium-based ILs, N-butylpyridinium tetrafluoroborate ([C4Py][BF4]), N-hexylpyridinium tetrafluoroborate ([C6Py][BF4]), and N-octylpyridinium tetrafluoroborate ([C8Py][BF4]), were synthesized and used for treating model diesel and real diesel and they all showed good extractive desulfurization performance for fuel. To have a better insight into the structure-property relationship of this series of ILs and design a promising extractant, the structure of the cation of pyridinium-based ILs was altered. Therefore, another three 3-methylpyridinium-based ILs, [C43MPy][BF4], [C63MPy][BF4], and [C83MPy][BF4], were investigated for the feasibility to desulfurize diesel according to the Nernst partition coefficient (KN) values. 2. Experimental Section 2.1. Materials. Benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) were purchased from Acros Organics, Morris Plains, NJ. All other reagents, including 3-picoline (purity, >99%), 1-bromobutane, 1-bromohexane, 1-bromooctane, ethyl acetate, dichloromethane, sodium tetrafluoroborate, n-dodecane, naphthalene, and thiophene (TS), were purchased from Beijing Chemical Company and were of analytical grade. Diesel fuel with 16% aromatics was kindly provided by the Research Institute of Petroleum Processing. 2.2. Preparation of ILs. Preparation of [C43MPy][BF4]. The IL [C43MPy][BF4] was prepared according to the published procedure.29 Equimolar amounts of 3-picoline (1.0 mol) and 1-bromobutane (1.0 mol) were added to a round-bottomed flask (500 mL) fitted with a reflux condenser and reacted at 70 °C for 24 h. The white precipitates were filtered off and washed 3 times with ethyl acetate. Then, it was dried under vacuum at 60 °C for 12 h to remove the remaining ethyl acetate. The residues were recovered, (20) Wang, J. L.; Zhao, D. S.; Zhou, E. P.; Dong, Z. J. Fuel Chem. Technol. 2007, 35, 293–296. (21) 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. (22) 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. (23) Nie, Y.; Li, C. X.; Meng, H.; Wang, Z. H. Fuel Process. Technol. 2008, 89, 978–983. (24) Lo, W. H.; Yang, H. Y.; Wei, G. T. Green Chem. 2003, 5, 639– 642. (25) Lu, L.; Cheng, S. F.; Gao, J. B.; Gao, G. H.; He, M. Y. Energy Fuels 2007, 21, 383–384. (26) Zhu, W. S.; Li, H. M.; Jiang, X.; Yan, Y. S.; Lu, J. D.; Xia, J. X. Energy Fuels 2007, 21, 2514–2516. (27) Zhao, D. S.; Wang, J. L.; Zhou, E. P. Green Chem. 2007, 9, 1219– 1222.
and an equimolar amount of NaBF4 and some deionized water (100 mL) was added while stirring at room temperature for 24 h. The resulting mixture was extracted with dichloromethane (30 mL × 3). The combined organic layer was washed with deionized water until no Br- salt remained. Br- ion was tested by the addition of a saturated AgNO3 solution into the washed aqueous phase.30 Solvent was removed by rotary evaporation to leave a yellowish liquid [C43MPy][BF4], which was dried under vacuum at 60 °C for 24 h to remove trace quantities of dichloromethane and water. Preparation of [C63MPy][BF4] and [C83MPy][BF4]. [C63MPy][BF4] and [C83MPy][BF4] were prepared according to a procedure similar to that used for [C43MPy][BF4]. The structure of these ILs, as shown in Figure 1, has been identified by 1H nuclear magnetic resonance (NMR), and the 1H NMR data are as follows. 1H NMR, δH (600 MHz, D2O) [C43MPy][BF4]: 8.683 (s, 1H), 8.634 (d, 1H), 8.354 (d, 1H), 7.924 (t, 1H), 4.555 (t, 2H), 2.543 (s, 3H), 1.981 (m, 2H), 1.352 (m, 2H), 0.940 (t, 3H). [C63MPy][BF4]: 8.686 (s, 1H), 8.635 (d, 1H), 8.361 (d, 1H), 7.930 (t, 1H), 4.556 (t, 2H), 2.548 (s, 3H), 2.003 (m, 2H), 1.312 (m, 6H), 0.853 (t, 3H). [C83MPy][BF4]: 8.690 (s, 1H), 8.636 (d, 1H), 8.363 (d, 1H), 7.932 (t, 1H), 4.556 (t, 2H), 2.549 (s, 3H), 2.005 (m, 2H), 1.267 (m, 10H), 0.852 (t, 3H). 2.3. Extractive Desulfurization Process. All of the extractive desulfurization experiments were conducted in a 50 mL flask. The mass ratios of ILs to model diesel or diesel fuel were 1:1 or 1:3. The ILs were added to the model diesel or diesel fuel, magnetically stirred for 15 min at room temperature to reach thermodynamic equilibrium, and then allowed to settle for 5 min to obtain phase splitting and settling. 2.4. Analytical Methods. High-performance liquid chromatography (HPLC) was used for the quantitative assay of TS, BT, DBT, and 4,6-DMDBT in the n-dodecane phase. HPLC was performed on a Agilent 1100 (HP1100, Agilent, Santa Clara, CA) liquid chromatograph equipped with an autosampler, a reversed-phase Zorbax SB-C18 column (4.6 × 150 mm; 3.6 µm), and a diode array detector. The mobile phase was 90% of methanol in water (v/v, %), with a flow rate of 1.0 mL/min. For the quantification of TS, the external standard method was used at 230 nm, and for the quantification of BT, DBT and 4,6-DMDBT, the external standard method was used at 280 nm. The method to analyze the IL-saturated diesel sample using HPLC was as follows. First, the ILs and diesel were mixed together and then set aside for 10 min for splitting and settling. Second, the diesel sample in the upper phase was analyzed by HPLC. The mobile phase with a flow rate of 1.0 mL/min was 75% (v/v, %) of methanol in water, which was buffered with an appropriate content of potassium phosphate KH2PO4/H3PO4 to reach 30 mM and pH 3 in the final mobile phase. For 3-methylpyridinium-based ILs, the external standard method was used at 259 nm. The gravimetric method is as follows. First, a known weight of IL and diesel was mixed together in a known weight test tube and then set aside for 10 min for splitting and settling. Second, the diesel in the upper phase was carefully separated with a pipet from the IL phase; then the test tube and IL was weighed by balance with 0.0001 accuracy; and the solubility of diesel in ILs in mass percentage was calculated. The next step was repeated 2 times, and the result was the average of three experiments. The total sulfur content (by weight) in diesel fuel was measured in triplicate for each sample by combustion of samples and
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Table 1. Solubility of Diesel in ILs at 298.15 K in Mass Percentage ILs
[C43MPy] [BF4]
[C63MPy] [BF4]
[C83MPy] [BF4]
oil solubility in IL (wt %)
6.1
7.6
9.5
measurement of the released sulfur dioxide with a microcoulomb analyzer (RPA-200, JiangHuan Electroanalysis, China).
3. Results and Discussion 3.1. Cross-Solubility of the ILs and Diesel. The crosssolubility is an important factor in evaluating the applicability of an extractant, because the variation of diesel composition and quality can be affected by the cross-solubility of the ILs and diesel. If the ILs have negligible solubility in the diesel, the diesel was never contaminated with ILs. If 3-methylpyridinium-based ILs in diesel have noticeable solubility, they may on one hand contaminate the fuel and further lead to a NOx pollution, as well as increase the cost of recycling ILs. By analyzing the IL-saturated diesel sample using HPLC, no IL peak was found. Therefore, the 3-methylpyridinium-based ILs studied here have negligible solubility in the diesel. The solubility of diesel in ILs was measured using a gravimetric method by weighing the mass difference of a given amount of ILs and the ILs saturated with diesel. As seen in Table 1, the diesel has a certain solubility in 3-methylpyridinium-based ILs and its solubility in 3-methylpyridinium-based ILs varies to a large extent for different IL species and follows the order [C83MPy][BF4] > [C63MPy][BF4] > [C43MPy][BF4], with the maximum solubility being 9.5 wt % for [C83MPy][BF4], which is smaller than that reported earlier in the literature.16,23 This diesel solubility order could be mainly attributed to the hydrophobic increase from [C43MPy][BF4] to [C83MPy][BF4] as the alkyl substituents in the ILs increase from butyl to octyl. The results also indicate that the diesel solubility in 3-methylpyridinium-based ILs studied in this paper is larger than our previous study.22 The reason may be that hydrophobicity of these three 3-methylpyridiniu-based ILs is stronger than those mentioned in our previous reports because of the alkyl substituent in the ring of pyridine. 3.2. Effect of Sulfur Species and Mass Ratio of ILs to Model Diesel on Extractive Performance with 3-Methylpyridinium-Based ILs. To study the effect of sulfur species on the extraction properties of ILs, the extraction of different sulfur compounds, namely, TS, BT, DBT, and 4,6-DMDBT in n-dodecane by ILs, was carried out at room temperature (Figure 2). Table 2 shows the KN values for selected sulfur compound typically found in diesel. It also indicates that, for each sulfur compound studied, the extractive performance using 3-methylpyridinium-based ILs followed the order of [C83MPy][BF4] > [C63MPy][BF4] > [C43MPy][BF4]. For a given IL, the sulfur removal selectivity of sulfur compounds followed the order of DBT > BT > TS > 4,6-DMDBT under the same conditions, except for the [C83MPy][BF4] IL that followed the order of DBT > BT > 4,6-DMDBT > TS. The mechanism for the extraction of sulfur compounds with 3-methylpyridinium-based ILs can be explained as the formation of liquid clathrates and the π-π interaction between aromatic structures of sulfur compounds and the pyridinium ring system. The formation of liquid clathrates because of the interaction between the ILs and aromatics through the π-π interaction has been confirmed by Holbrey.31 Su32 also indicated that the cation and anion sizes (28) 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.
of ILs played an important role in determining the interaction of absorbed thiophene and ILs. This work further confirms these results. The 3-methylpyridinium-based ILs studied in this work have the same anions, and their cations, with the substitution of a longer alkyl group, have better extractive performance. It is known that the alkyl group on the pyridine ring is an electrondonting group, which makes the cation N-alkylpyridinium highly polarizable aromatic π-electron densities compared to pyridine. Therefore, the interaction of sulfur compounds and the ILs would be expected to be strong. The polarizable aromatic π-electron densities increase from [C43MPy][BF4] to [C83MPy][BF4] as the alkyl substituents in the ILs increase from butyl to octyl. The results also suggest that these 3-methylpyridinium-based ILs have better extractive desulfurization performance than our previous research,22 which could be mainly attributed to the cations size of ILs becoming larger and higher polarizable aromatic π-electron densities because of two methyl groups on the pyridine ring. On one hand, as the cation size increases, the Coulombic interaction between cation and anion decreases and the π-π interaction between aromatic structures of sulfur compounds and the pyridinium ring system increases. On the other hand, alkyl is an electron-donating group and, as the size of cation increases, the polarizable aromatic electron densities increase and the π-π interaction between aromatic structures of sulfur compounds and the pyridinium ring system would be strong.14 Su32 further proved that molecules with highly polarizable π-electron density preferably insert into the molecular structure of the ILs. Zhang also stated that the sulfur compounds with a higher density of aromatic π electron are favorably absorbed by ILs.12,14 In our work, the relatively large differences in the partition coefficients of the individual sulfur compounds may be due to the difference of aromatic π-electron density of sulfur compounds. As calculated by Otsuki et al.,6 the electron density on the sulfur atoms is 5.760 for 4,6DMDBT, 5.758 for DBT, 5.739 for BT, and 5.696 for TS and this difference may further cause the relatively large differences of the sulfur compound partition coefficients. For sulfur compounds DBT, BT, and TS, these results indicate that the extractive performance becomes better with the increase of the aromatic π-electron density. However, for 4,6-DMDBT, the methyl substitution at the 4 and 6 positions of DBT remarkably retards the extractive performance of 3-methylpyridinium-based ILs. As shown in Figure 2, the mass ratio of ILs to model diesel has a strong influence on the sulfur content after extraction by ILs. The sulfur content (Figure 2) at zero of the mass ratio of ILs to model diesel reflects the original sulfur content in n-dodecane. The sulfur content treated by ILs decreased greatly as the mass ratio of ILs to model diesel increased from 1:5 to 1:1. Although the higher mass ratio of ILs to model diesel, the higher the cost of ILs, and the problem may be resolved by the ILs property that is recycling. The sulfur removal performance of 3-methylpyridinium-based ILs [C43MPy][BF4], [C63MPy][BF4], and [C83MPy][BF4] was superior to most ILs reported in the literature except IL [C4MIM][Cl/AlCl3],15,16 as can be seen from the KN values for DBT listed in Table 3. Considering the fact that [C4MIM][Cl/ AlCl3] is very sensitive to water and thus incompatible with a (29) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Gratzel, M. Inorg. Chem. 1996, 35, 1168–1178. (30) Wang, J. J.; Pei, Y. C.; Zhao, Y.; Hu, Z. G. Green Chem. 2005, 7, 196–202. (31) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Chem. Commun. 2003, 4, 476–477. (32) Su, B. M.; Zhang, S. G.; Zhang, Z. C. J. Phys. Chem. B 2004, 108, 19510–19517.
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Figure 2. Effect of sulfur species and mass ratio of ILs to model diesel on the extractive performance using 3-methylpyridinium-based ILs. Table 2. Sulfur Partition Coefficients for Extraction of Sulfur Compounds with Different 3-Methylpyridinium-Based ILsa KN in mg of S (kg of IL)-1/mg of S (kg of oil)-1 IL
TS
BT
DBT
4,6-DMDBT
[C43MPy][BF4] [C63MPy][BF4] [C83MPy][BF4]
0.85 1.00 1.07
1.75 2.08 2.16
2.08 2.89 3.11
0.56 0.93 1.29
a Model oil, 160 ppm S as the sulfur compound in n-dodecane; mass ratio, 1:1, mixing time, 15 min; room temperature.
Table 3. Sulfur Partition Coefficients for Extraction of DBT with Different ILs
IL
KN in mg of S (kg of IL)-1/ mg of S (kg of oil)-1
[C4MIM][Cl/AlCl3]a [C4MIM][BF4]a [C4MIM][PF6]d [C4MIM][OcSO4]a [C2MIM][EtSO4]a [CMIM][Me2PO4]a [C4MIM][DBP]b [C2MIM][DEP]b
4.0 0.7 0.9 1.9 0.8 0.7 1.59 1.27
IL
KN in mg of S (kg of IL)-1/ mg of S (kg of oil)-1
[CMIM][DMP]b [C4Py][BF4]c [C6Py][BF4]c [C8Py][BF4]c [C43MPy][BF4]c [C63MPy][BF4]c [C83MPy][BF4]c
0.46 0.77 1.42 1.79 2.08 2.89 3.11
a Model oil,15 500 ppm S as DBT in n-dodecane; mass ratio, 1:1; mixing time, 15 min; room temperature. b Real gasoline,16 sulfur as DBT; 298.15 K. c Model oil, 160 ppm S as DBT in n-dodecane; mass ratio, 1:1; mixing time, 15 min; room temperature. d At 60 °C.
water environment, the 3-methylpyridinium-based ILs studied in this work are more competitive and feasible for extractive
desulfurization applications. Moreover, the extractive desulfurization using 3-methylpyridinium-based ILs could be used at least as a complementary process to the HDS. 3.3. Influence of Aromatic Naphthalene on Extractive Desulfurization of ILs. Removing sulfur compounds without removing aromatics is desired in the extractive desulfurization process. Considering this factor, the extractive selectivity of 3-methylpyridinium-based ILs for DBT to naphthalene was determined. Model diesel, with 5.0 mmol/L DBT and 5.0 mmol/L naphthalene, was treated with 3-methylpyridiniumbased ILs [C43MPy][BF4], [C63MPy][BF4], and [C83MPy][BF4]. As shown in Figure 3, ILs showed extraction of a small amount of naphthalene and a relatively larger amount of DBT. It is evident that the ILs showed remarkable selectivity for removal of DBT over naphthalene. Because of the interaction of the aromatic ring with the cation of the ILs, naphthalene can also be extracted by ILs. The ILs favorably extracted molecules with a higher density of aromatic π electron, while the density of aromatic π electron of DBT was higher than naphthalene. For the DBT and naphthalene, [C83MPy][BF4] exhibits the highest extraction capacity among the three ILs. The results also suggest that the extraction capacity was greatly influenced by the structural properties of ILs cation. 3.4. Extractive Desulfurization of Sulfur Compounds from Diesel Fuel. The 3-methylpyridinium-based ILs were also applied to the desulfurization of diesel fuel containing sulfur content of 97 ppm. Table 4 shows the sulfur removal and sulfur content from the diesel by 3-methylpyridinium-based ILs. The
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Figure 3. Influence of aromatic naphthalene on extractive desulfurization of ILs. Table 4. Sulfur Removal from Diesel Fuel by ILs at Room Temperature IL [C43MPy][BF4] [C63MPy][BF4] [C83MPy][BF4]
step
sulfur content after treatment (ppm)
sulfur removal (%)
1 2 3 1 2 3 1 2 3
73.1 63.4 54.4 62.6 53.4 45.0 56.1 47.2 38.4
24.6 34.6 43.9 35.5 44.9 53.6 42.2 51.3 60.4
results were not as good as for treating model diesel because of the complex chemical composition of diesel fuel. [C83MPy][BF4] exhibits the highest extraction capacity for diesel fuel among the three ILs. The results also suggest that the structural properties of cation of ILs greatly influence the extraction capacity. Because the ILs can be recycled, deep desulfurization could be obtained. 3.5. Regeneration of ILs. For the industrial application of an IL extraction process, the regeneration and subsequent recycling of IL are of vital importance. Removal of sulfur compounds from an IL can be performed by (a) heating the IL to remove the sulfur compounds,14 (b) re-extraction of sulfur compounds with the low-boiling hydrocarbons pentane or hexane,15 and (c) precipitating the sulfur compounds by a water dilution process.16,18,22 In this work, regeneration of ILs was obtained through a water dilution process. The data shown in Table 5 suggest that the water content in an IL plays an important role in determining the values of the sulfur partition coefficient KN between [C63MPy][BF4] and diesel; even 1%
BT
water content in IL (wt %)
KN
water content in IL (wt %)
KN
0 0.94 3.1 4.3 10.4
2.89 2.63 2.35 2.18 1.64
0 0.97 2.92 5.03 10.3
2.08 1.91 1.82 1.67 1.37
water content in an IL can result in about 9% lowering of the extractive ability of [C63MPy][BF4] for DBT from diesel. This implies that, with the increase of the water content in an IL, the solubility of sulfur compounds in IL decreases drastically. This also indicates that sulfur compounds dissolved in the used IL can be precipitated by a water dilution process. Therefore, the spent IL-saturated sulfur compounds could be regenerated by the water dilution process. In this work, the used ILs could be nearly recovered by adding about 80% water and 1H NMR analyses indicated that the ILs also maintained their original structures after regeneration. However, considering the high energy costs for water evaporation, this method for regeneration of ILs is only suitable for laboratory-scale experiments and not in industry. Some efficient techniques for separating water and ILs need further investigation. 4. Conclusions The 3-methylpyridinium-based ILs were found to be effective for the selective removal of aromatic heterocyclic sulfur compounds from diesel at room temperature. The results suggested that the structure and size of the cation greatly affect the extractive performance of ILs. The extractive performance using pyridinium-based ILs followed the order [C83MPy][BF4] > [C63MPy][BF4] > [C43MPy][BF4], and for the ILs, the sulfur removal selectivity of sulfur compounds followed the order DBT > BT > TS > 4,6-DMDBT under the same conditions, except for the [C83MPy][BF4] IL that followed the order of DBT > BT > 4,6-DMDBT > TS. The mechanism for the extraction of sulfur compounds with 3-methylpyridinium-based ILs could be explained as the formation of liquid clathrates and the π-π interaction between aromatic structures of sulfur compounds and the pyridinium ring system. According to the results, [C43MPy][BF4], [C63MPy][BF4], and [C83MPy][BF4] can be used as potential exractants for the extractive desulfurization of diesel. Acknowledgment. We are grateful to the National High Technology Research and Development Program of China (Grant 2006AA02Z209) and the State Major Basic Research Development Program of China (Grant 2006CB202507) for their partial financial support of this research. EF900009G
