Energy & Fuels 2008, 22, 3303–3307
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Removal of Organic Sulfur from Hydrocarbon Resources Using Ionic Liquids Yuuki Mochizuki and Katsuyasu Sugawara* Faculty of Engineering and Resources Science, Akita UniVersity, 1-1 Tegata Gakuen-cho, Akita City, Akita Prefecture 010-8502, Japan ReceiVed May 27, 2008. ReVised Manuscript ReceiVed July 23, 2008
To develop an advanced desulfurization process that can be carried out under mild conditions without pressurized hydrogen or catalysis that has been evaluated for the extraction of thiophenic sulfur from a model fuel using the ionic liquids, 1-alkyl-3-alkyl imidazolium alkyl sulfate at room temperature was investigated. Six types of halogen-free ionic liquids with different alkyl chain lengths were prepared. The extraction yield of dibenzothiophene was higher than that of diphenylsulfide and diphenyldisulfide. The extraction yield of dibenzothiophene increased linearly with an increase in the length of alkyl chains and the mass ratio of the ionic liquid to the model fuel. The effect because of the change in the type of solvent was not appreciable, and dibenzothiophene was efficiently removed regardless of whether tetralin, benzene, or n-dodecane was used as the solvent.
Introduction Because of limited reserves and environmental concerns, an advanced process for the conversion and cleaning of low-rank coal needs to be developed. Numerous studies on solvent extraction as one method of converting coal to liquids have been reported. Recently, it has been shown that the hyper coal process can be used to obtain clean coal with less than 200 ppm of ash content.1-8 This process is effective for sub-bituminous and bituminous coals but not for brown coals. The distribution of sulfur forms in the products of the hyper coal process has not been determined. Miura et al. have carried out solvent extraction with different types of coal, including brown coal, using organic solvents and reaction conditions of 350 °C and 10 MPa.9-11 The solute obtained in this process showed low ash content and a high content of thiophenic sulfur at higher extraction yield. Thus, solvent extraction does not remove organic sulfur efficiently. Clean coal with less than 1000 ppm of sulfur content is desirable. Hydrodesulfurization is widely used in oil refining. It is carried out at high temperature with high-pressurized hydrogen * To whom correspondence should be addressed. Telephone/Fax: +8118-889-2750. E-mail:
[email protected]. (1) Yoshida, T.; Takanohashi, T.; Sakanishi, K.; Saito, I.; Fujita, M.; Mashimo, K. Fuel 2002, 81, 1463. (2) Yoshida, T.; Li, C.; Takanohashi, T.; Matsumura, A.; Sato, S.; Saito, I. Fuel Process. Technol. 2004, 86, 61. (3) Okuyama, N.; Komatsu, N.; Shigehisa, T.; Kaneko, T.; Tsuruya, S. Fuel Process. Technol. 2004, 85, 947. (4) Masaki, K.; Yoshida, T.; Li, C.; Takanohashi, T.; Saito, I. Energy Fuels 2004, 18, 995. (5) Okuyama, N.; Furuya, A.; Komatsu, N.; Shigehisa, T. Research and DeVelopment Kobe Steel Engineering Reports 2006, 56, 15. (6) Kashimura, N.; Takanohashi, T.; Saito, I. Energy Fuels 2006, 20, 2063. (7) Kashimura, N.; Takanohashi, T.; Masaki, K.; Shishido, T.; Sato, S.; Matsumura, A.; Saito, I. Energy Fuels 2006, 20, 2088. (8) Masaki, K.; Kashimura, N.; Takanohashi, T.; Sato, S.; Matsumura, A.; Saito, I. Energy Fuels 2005, 19, 2021. (9) Miura, K.; Nakagawa, H.; Ashida, R.; Ihara, T. Fuel 2004, 83, 733. (10) Miura, K.; Shimada, M.; Mae, H.; Sock, Y. H. Fuel 2001, 80, 1573. (11) Miura, K.; Mae, K.; Shimada, H.; Ashida, R.; Ihara, T. J. Chem. Eng. Jpn. 2003, 36, 742.
along with catalysis. Oxydesulfurization, in which organic sulfur is converted into sulfone using acetic acid and hydroperoxide by acid catalysis, causes the oxidation of organic and sulfur compounds.12-15 Alkylation-precipitation and photo-oxidation methods have been reported to be used for desulfurization, despite disadvantages, such as the use of expensive alkylation reagents and slow reaction rates.16,17 The objective of this study is to develop an advanced desulfurization process that can be carried out under mild conditions, such as atmospheric pressure and room temperature. The process is expected to be used for clean fuel production. The authors have been investigating the selective removal of organic sulfur, taking into consideration the different forms of sulfur present in model fuels.18 Several studies have been carried out on the application of ionic liquids, and some studies have indicated the potential of ionic liquids containing fluoride and chloride ions for sulfur removal.19-23 However, hydrogen fluoride and hydrochloric acid are generated from these ionic liquids during hydrolysis.24-32 In this study, six types of halogen(12) Aida, T. J. Jpn. Inst. Energy 1999, 78, 396. (13) Tam, S. P.; Kittrell, R. J.; Eldridge, W. J. Ind. Eng. Chem. Res. 1990, 29, 321. (14) Yu, G. ; Lu, S.; Chen, H.; Zhu, Z. Energy Fuels 2005, 19, 447. (15) De, F. P.; Scarsella, M. Energy Fuels 2003, 17, 1452. (16) Shiraishi, Y.; Hirai, T.; Komasawa, I. Kgaku Kougaku Ronnbunnsyuu 2002, 28, 231. (17) Shiraishi, Y.; Tachibana, K.; Hirai, T.; Komasawa, I. Energy Fuels 2003, 17, 95. (18) Holbrey, D. J.; Matthew, W. R.; Swatloski, P. R.; Broker, A. G.; Pinter, R. W.; Seddon, R. K.; Rogers, D. R. Green Chem. 2002, 4, 407. (19) Mochizuki, Y.; Sugawara, K.; Sugawara, T. Am. Chem. Soc., DiV. Pet. Chem. 2006, Sept 10-14, Technical Program 148. (20) Scammells, J. P.; Scott, L. J.; Singer, D. R. Aust. J. Chem. 2005, 58, 155. (21) Wassersheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (22) Wassersheid, P.; Hal, V. R.; Bosmann, A. Green Chem. 2002, 4, 400. (23) Ionic Liquid in Synthesis; Wassersheid, P.; Welton, T., Eds.; WileyVCH: New York, 2006; p 7. (24) Nie, Y.; Li, C.; Sun, A.; Meng, H.; Wang, Z. Energy Fuels 2006, 20, 2083. (25) Zhang, S.; Zhang, C. Z. Green Chem. 2002, 4, 376.
10.1021/ef800400k CCC: $40.75 2008 American Chemical Society Published on Web 08/28/2008
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Table 1. Ionic Liquids 1-Alkyl(R1)-3-alkyl(R2) Imidazolium Alkyl(R3) Sulfate Prepared ionic liquid
abbreviation
R1
R2
R3
1,3-dimethylimidazolium methyl sulfate l-ethyl-3-methylimidazolium ethyl sulfate l-ethyl-3-methylimidazolium methyl sulfate l-ethyl-3-ethylimidazolium ethyl sulfate l-butyl-3-methylimidazolium methyl sulfate l-butyl-3-ethylimidazolium ethyl sulfate
MMIMMeSO4
methyl
methyl
methyl
EMIMEtSO4
ethyl
methyl
ethyl
EMMMeSO4
ethyl
methyl
methyl
EEIMEtSO4
ethyl
ethyl
ethyl
BMIMMeSO4
butyl
methyl
methyl
BEIMEtSO4
butyl
ethyl
ethyl
free ionic liquids are prepared from alkylimidazoles and dialkylsulfates. The extraction behavior of organic sulfur from a model fuel using these ionic liquids is investigated. The effects of the type of ionic liquid, length of alkyl chains, and co-existing organic solvents on the extraction yield have been studied. Dibenzothiophene is known to be thermally stable and difficult to remove by conventional desulfurization processes. The present study focuses on the selective removal of dibenzothiophene using ionic liquids. Experimental Section Preparation of Ionic Liquids. Six types of ionic liquids were prepared by adding dialkyl sulfates to alkyl imidazoles according to the method devised by Holbrey et al.18 Dialkyl sulfates (Nacalai tesque, EP) were added to alkylimidazoles (Tokyo Kasei, GR) in toluene and placed in an ice bath under a nitrogen atmosphere. The mixture was then stirred at room temperature for 1 h. After the reaction, the solution was found to have separated into two phases. The upper phase, consisting of organic solvents, was recovered by decantation, and the lower phase, consisting of the ionic liquid, was washed several times with toluene. The ionic liquid phase was heated at 75 °C, and aspiration was carried out to remove unreacted organic solvents. Characterization of Ionic Liquids. The samples for thermogravimetric analysis were prepared by mixing the ionic liquids with alumina powder. Using a differential thermogravimetric analyzer (Rigaku TAS-200), the samples were heated at a rate of 20 °C/min and observed in an air stream at a terminal temperature of 500 °C. Fourier transform infrared (FTIR) measurement was also carried out on the ionic liquids immersed in KBr pellets using a FTIR spectrometer (Perkin-Elmer Spectrum 2000). Extraction. The model fuel was prepared from dibenzothiophene and n-dodecane (Nacalai tesque, GR). The ionic liquids were mixed with the model fuel in a certain ratio and stirred for a certain time interval at room temperature. A gas chromatograph-mass spectrometer (Shimadzu GCMS-QP 2010) was used to determine the sulfur content in the model fuel.
Results and Discussion Ionic Liquids. Table 1 shows the ionic liquids, 1-alkyl(R1)3-alkyl(R2) imidazolium alkyl(R3) sulfate, prepared in this study. Figure 1 shows the thermogravimetric analysis results for the (26) Zhang, S.; Zhang, Q.; Zhang, C. Z. Ind. Eng. Chem. Res. 2004, 43, 614. (27) Bosmann, A.; Datsevich, L.; Jees, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 2494. (28) Huang, C.; Chen, B.; Zhang, J.; Liu, Z.; Li, Y. Energy Fuels 2004, 18, 1862. (29) Lu, L.; Cheng, S.; Gao, J.; Gao, G.; He, Y. Energy Fuels 2007, 21, 383. (30) Eβer, J.; Wassersheid, P.; Jess, A. Green Chem. 2004, 6, 316. (31) Zhang, S.; Zhang, C. Z. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 2002, 47, 449. (32) Swatloski, P. R.; Holbrey, D. J.; Rogers, D. R. Green Chem. 2003, 5, 361.
Figure 1. Thermogravimetric changes of starting materials and ionic liquids prepared.
starting materials and ionic liquids prepared. The weight-loss curves for the starting materials are found in the temperature range of 50-200 °C, and those for the ionic liquids are found in the temperature range of 340-400 °C. The thermogravimetric behavior is similar to the observed result reported by Holbrey et al.18 The FTIR spectra of the ionic liquids are shown in Figure 2. Peaks are observed around 3100-3200 and 2900 cm-1, which are attributable to aromatic C-H and aliphatic C-H bonds, respectively. The peaks around 1500 and 800 cm-1 represent the CdN bond of the imidazolium ring and SO42-, respectively. The results of thermogravimetric analysis and FTIR measurements indicate that the ionic liquids have been successfully synthesized by this procedure. Extraction Behavior. Figure 3 shows the extraction behavior of dibenzothiophene from the model fuel using the ionic liquids prepared. The ratio of the ionic liquid/model fuel is 1:5 by weight. The initial concentration of dibenzothiophene is 1000 ppmw S. The extraction yield increases rapidly within an extraction time of 5 min, and equilibrium values attained at 30 min are in the range of 8-22%, depending upon the type of ionic liquid. Hereafter, all extraction experiments were carried out for an extraction time of 30 min. The effect of the initial concentration of dibenzothiophene on the extraction yield is shown in Figure 4. Two types of ionic liquids, EMIMEtSO4 and EMIMMeSO4, were used to extract dibenzothiophene with different initial concentrations, i.e., 1, 10, 100, and 1000 ppmw S. The extraction yield was in the range of 12-14% without an appreciable dependence upon the initial concentration of dibenzothiophene. Su et al. have studied the extraction mechanism of thiophene by imidazolium-based ionic liquids, BMIMPF6, BMIMBF4, and EMIMBF4, by comparing their 1H nuclear magnetic resonance (NMR) chemical shifts.33 According to their NMR measurements, the sulfur atom (33) Su, B.; Zhang, S.; Zhang, C. Z. J. Phys. Chem. B 2004, 108, 19510.
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Figure 4. Effect of the initial concentration on the extraction of dibenzothiophene. Table 2. Effect of the Kind of Alkyl Group on the Extraction Extent ionic liquid R1R2IM-R3SO4 MMIMMeSO4 EMMMeSO4 EMTMEtSO4 BMIMMeSO4 EEMEtSO4 BEIMEtSO4
Figure 2. FTIR spectra of ionic liquids prepared.
Figure 3. Extraction behavior of dibenzothiohene from the model fuel by using ionic liquids (the weight ratio of ionic liquid/model fuel was fixed to be 1:5; the initial cone of dibenzothiophene was 1000 ppm).
of thiophene faces the anion of the ionic liquid. Thiophene exists outside the shielding space of the imidazolium ring. The imidazolium ring exists inside the shielding space of the thiophene ring. In this study, dibenzothiophene may be extracted using ionic liquids with a similar conformation of molecules. Effect of the Length of the Alkyl Chain on Extraction. Table 2 shows the changes in the extraction yield of dibenzothiophene with the length of the alkyl chain of anion and cation groups. The ionic liquids EMIMMeSO4 and EMIMEtSO4 have the same cation EMIM+ but different alkyl groups at R3 in the anion R3SO4-. A higher extraction yield was observed for the ethyl group as compared to the methyl group at R3. MMIMMeSO4, EMIMMeSO4, and BMIMMeSO4 have the same
R1
R2
R3
extraction extent of dibenzothiophene (%)
methyl ethyl ethyl butyl ethyl butyl
methyl methyl methyl methyl ethyl ethyl
methyl methyl ethyl methyl ethyl ethyl
9 12 14 16 20 22
anion MeSO4- but different alkyl groups at R1, i.e., methyl, ethyl, and butyl, respectively, in the cation. With these ionic liquids, the extraction yield of dibenzothiophene was 9, 12, and 14% for the methyl, ethyl, and butyl groups, respectively, at R1; that is, the extraction yield increases with the length of the alkyl chain of the group at R1. Similarly, for the ionic liquids having the same anion EtSO4- but different alkyl groups at R1, i.e., butyl and ethyl, BEIMEtSO4 shows a higher extraction yield as compared to EEIMEtSO4. For a fixed anion group to EtSO4, the effect of the length of the alkyl chain of the group at R2 in the alkyl imidazolium cation on the extraction yield was investigated for a fixed anion group attached to EtSO4. The extraction yield for EEIMEtSO4 having an ethyl group at R2 was higher than that having a methyl group at R2. The effect of the length of the alkyl chain of the group at R2 on the extraction yield was more significant than that of the alkyl group at R1 or R3. The length of the alkyl chain of the group at R2 of 1-alkyl(R1)-3-alkyl(R2) imidazolium alkyl(R3) sulfate had the most significant effect on the extraction yield of dibenzothiophene. Figure 5 shows the relation between the extraction yield of dibenzothiophene and the total carbon number of the alkyl groups for the ionic liquids. The extraction yield increases linearly with an increase in the total carbon number of the alkyl groups. Holbery et al. have observed that ionic liquids form a liquid cluster consisting of a zigzag stacking structure of anions and cations.18 The increase in the extraction yield shown in Figure 5 may result from the increase in the extraction space because of the increase in the length of the alkyl chain. Effect of Mass Ratio of Ionic Liquid/Fuel. Figure 6 shows that the dependence of the extraction yield of dibenzothiophene on the mass ratio of the ionic liquid/model fuel varies from 0.2 to 1.0. The initial concentration of sulfur was 1000 ppmw S, and EMIMEtSO4 and MMIMMeSO4 were used. The extraction yield increases linearly with an increase in the mass ratio of the ionic liquid/model fuel for both ionic liquids. This result
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Figure 5. Relation between the extraction yield and total carbon number of the alkyl group.
Figure 6. Change in the desulfurization extent with the ratio of the ionic liquid/model fuel ratio.
implies that an extremely low sulfur level might be attained by multiple extractions or changes in the mass ratio of the ionic liquid/model fuel. Multiple extractions were carried out using BEIMEtSO4 and EEIMEtSO4, where the mass ratio of the ionic fluid/model fuel was fixed in the range of 1-5. These ionic liquids were chosen because of their high extraction yield, shown in Figure 5. Figure 7 indicates that the sulfur content in the model fuel decreased linearly from 1000 to 350 ppmw S, after five rounds of extraction. Even when the sulfur content is low, the ionic liquids maintain constant extraction yields; therefore, the multiple extractions should be highly effective for the production of lowsulfur fuel. Selectivity to Organic Sulfur. The selectivity to organic sulfur exhibited by the ionic liquids was investigated using dibenzothiophene, diphenylsulfide, and diphenyldisulfide and three types of ionic liquids, BEIMEtSO4, EEIMEtSO4, and BMIMMeSO4. While the extraction yield of dibenzothiophene exceeded 15%, that of diphenylsulfide and diphenyldisulfide remained in the range of 3-6%, as shown in Figure 8. It has been reported that the stronger selective extraction of dibenzothiophene results from the π-π interaction between the
Mochizuki and Sugawara
Figure 7. Change in the sulfur content with the number of extraction steps.
Figure 8. Selectivity of the ionic liquid to organic sulfurs.
imidazolium and thiophene rings.33 The ionic liquids, in general, show significant π-π interaction with the aromatic ring. Therefore, the small extraction yield of diphenylsufide and diphenyldisulfide may be due to the interaction of the ionic liquids with the phenyl groups. In Figure 8, all three ionic liquids exhibit strong selectivity to thiophenic sulfur in organic sulfur. Effect of Organic Solvents. The effect of different types of organic solvents should also be clarified for efficient sulfur removal from the extracted coal liquid. Tetralin is commonly used as a solvent in studies on solvent extraction of coal. Benzene (Nacalai tesque, GR), tetralin (Nacalai tesque, GR), and 1-methyl naphthalene (Tokyo Kasei, EP) were used as the solvents for the model fuel. Benzene is a typical low-molecularweight aromatic compound. 1-Methyl naphthalene is used as a solvent for the extraction of clean fuel from coal in the hyper coal process. Dibenzothiophene was dissolved in benzene, tetralin, and 1-methylnapthalene at a sulfur content of 1000 ppmw S. BEIMEtSO4 and EEMEtSO4 were mixed with these solutions at a mass ratio of the ionic liquid/model fuel of 1:5 and agitated for 30 min. Figure 9 shows the change in the extraction yield for various types of solvents. The extraction yield was in the range of 13-17% for benzene and tetralin, whereas the yield was slightly lower for n-dodecane. Benzene showed a lower extraction yield as compared to tetralin. One
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Figure 9. Effect of the kind of organic solvent on the extraction yield. Table 3. Effect of Organic Nitrogens on the Extracation of DBT extent of extraction (%) compound
BEPMEtSO4
EEIMEtSO4
dibenzothiophene pyridine quinoline pyridine coexisting with dibenzothiophene dibezothiophene coexisting with pyridine
22 87 71 85 10
20 68 37 60 6
of the reasons for this may be the lower molecular weight of benzene as compared to tetralin, and benzene is accompanied by dibenzothiophene in the ionic liquids. In a preliminary experiment, benzene has been extracted preferably using the ionic liquid obtained from a mixture of benzene and tetralin. When 1-methylnapthalene is used as the solvent, the extraction yield is only in the range of 5-8%. 1-Methylnapthalene is analyzed using an ion chromatograph (Japan Dionex DX-120) and an automatic quick furnace (Mitsubishi Chemicals, AQF100). It contains 0.6% sulfur, and the sulfur form is estimated to be benzothiophene. Effect of Organic Nitrogen Compounds. Coal extracts contain organic nitrogen compounds as well as organic sulfur compounds. The effect of coexisting organic nitrogen compounds on the extraction of dibenzothiophene was investigated using pyridine (Nacalai tesque, GR) and quinoline (Nacalai tesque, GR). Pyridine and quinoline were dissolved in ndodecane with a nitrogen concentration of 1000 ppmw N and mixed and agitated with the ionic liquids BEIMEtSO4 and EEIMEtSO4 for 30 min. The experimental results are sum-
marized in Table 3. The extraction yield of pyridine was 87 and 68% for BEIMEtSO4 and EEIMEtSO4, respectively, and that of quinoline was 71 and 37% for BEIMEtSO4 and EEIMEtSO4, respectively. The basicity of quinoline is weaker than that of pyridine, which may have resulted in the smaller extraction yield. A mixture of dibenzothiophene and pyridine was extracted by the ionic liquid, and the results are presented in Table 3. The extraction yield of dibenzothiophene was in the range of 6-10%, while that of pyridine was higher, in the range of 60-85%. Choi et al. have investigated pyridine removal, taking advantage of the affinity between basic nitrogen compounds, e.g., pyridine and quinoline, and metal salts. The basic nitrogen compounds easily form complexes with the salts.34 It has also been reported that the extraction yield of pyridine is higher than that of thiophene for BMIMBF4.25 Thiophene can be extracted efficiently by the pre-removal of organic nitrogen from coal extracts. In other words, ionic liquids may be suitable for both desulfurization and denitrification of coal extracts and heavy oils. From the viewpoint of practical use, the aromatic sulfurs, such as benzothiophene and dibenzothiophene in gasoline, diesel oil, and jet fuels can be extracted selectively by the ionic liquids because these fuels consist of mainly aliphatic hydrocarbons. The challenge is to control the selectivity of ionic liquids for other aromatics, except organic sulfur contained in the crude oil and various liquid fuels. Conclusions Dibenzothiophene was extracted from a model fuel at room temperature using six types of halogen-free ionic liquids: BEIMEtSO4, EEIMEtSO4, EMIMETSO4, EMIMMeSO4, BMIMMeSO4, and MMIMMeSO4. The extraction yield of dibenzothiophene increased linearly with the number of carbons of the alkyl group in the ionic liquids. BEIMEtSO4, having the longest alkyl chain, showed the highest extraction yield among the six ionic liquids. When the mass ratio of the ionic liquid/ model fuel was 1.0, dibenzothiophene was successfully extracted using EMIMMeSO4 and MMIMMeSO4 with yields of 40 and 70%, respectively, after one round of extraction. With repeated extractions, the sulfur content could be decreased considerably, for example, from 1000 to 350 ppmw S, after five rounds of extraction. The ionic liquids could remove dibenzothiopehene when tetralin used as a solvent for coal extraction. Acknowledgment. The authors are grateful to Professor Takuo Sugawara, Akita University, for his comments. EF800400K (34) Choi, W. H.; Dines, B. M. Fuel 1985, 64, 4.
