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Desulfurization of Gasoline by Extraction with New Ionic Liquids Chongpin Huang,† Biaohua Chen,*,† Jie Zhang,† Zhichang Liu,‡ and Yingxia Li† The Key Laboratory of Science and Technology of Controllable Chemical Reactions, Bejing University of Chemical Technology, Ministry of Education, Box 35, Bejing 100029, People’s Republic of China, and State Key Laboratory of Heavy Oil Processing, University of Petroleum, Beijing 102200, People’s Republic of China Received May 17, 2004. Revised Manuscript Received August 12, 2004
CuCl-based ionic liquid was synthesized by mixing 1-butyl-3-methylimidazolium chloride with purified anhydrous CuCl, and its structures were studied using fast atom bombardment mass spectrometry (FAB-MS). It was determined that Cu(I) anionic species such as CuCl2-, Cu2Cl3-, and Cu3Cl4- existed in the ionic liquid, and these anions were moisture-insensitive and stable in air. CuCl-based ionic liquid exhibited remarkable desulfurization ability in the desulfurization of gasoline when used as an extraction absorbent. The effectiveness of sulfur removal may be attributed to the π-complexation of Cu(I) with thiophene, which shows promise as an approach for the deep desulfurization of motor fuel.
Introduction Sulfur that is present in transportation fuels leads to sulfur oxide (SOx) emissions into the air and inhibits the performance of pollution control equipment on vehicles; therefore, to minimize the negative health and environmental effects from automobile exhaust, many countries recently have mandated a reduction in the sulfur content in motor fuel. For example, in 2005, the maximum sulfur content will be limited to 10-50 ppm, compared to today’s permitted value of 500 ppm in most western countries.1 Consequently, the deep desulfurization of motor fuels has attracted increased attention in the research community worldwide. In the petroleum industry, low-sulfur fuels are often obtained from hydrocracking processes or hydrotreating processes.2 Although hydrotreating processes have been highly effective for the reduction of sulfur levels, further improvement of the hydrodesulfurization efficiency is limited to increasingly severe operational conditions at escalated cost. Moreover, when the deep hydrodesulfurization of motor fuels is needed, not only the energy and hydrogen consumption will be evidently increased, but undesired side reactions (such as the saturation of more olefins) also will be induced. Such side reactions result in a decrease in the octane number of the gasoline. Selective adsorption of sulfur compounds from gasoline has been regarded as an effective supplement to the hydrotreating process for deep desulfurization.3-5 * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † Beijing University of Chemical Technology. ‡ University of Petroleum. (1) Parkinson, G. Chem. Eng. 2001, 37. (2) Kwak, C.; Lee, J. J.; Bae, J. S.; Choi, K.; Moonm, S. H. Appl. Catal., A 2000, 200, 233. (3) Salem, A. S. H.; Hamid, H. S. Chem. Eng. Technol. 1997, 20, 342.
However, its utility is dependent entirely on the availability of separation sorbents. Excellent sorbents often contain Cu(I) or Ag(I), because of their stronger π-complexation interaction with thiophene, relative to that with benzene.6 The problem involved with the preparation of Cu(I) sorbents is the sensitivity of Cu(I) to air and moisture. The conventional preparation of Cu(I) sorbent is greatly complicated by the autoreduction of Cu(II) to Cu(I) in helium at 450 °C before application.5 We have devised another way to prepare an absorbent with a stable Cu(I) species for desulfurization, using a mixture of CuCl and dialkylimidazolium chlorides, which is liquid at room temperature. This liquid has been called an ionic liquid and has been receiving increased attention in separation processes for its nonvolatility.7,8 Ionic liquid systems of 1-butyl-3-methylimidazolium tetrachloroaluminate (BMImAlCl4) and 1-butyl3-methylimidazolium tetrafluoroborate (BMImBF4) have been investigated for the desulfurization of motor fuel.9,10 Excellent desulfurization results have been observed for AlCl3-based ionic liquids. However, because of the sensitivity of AlCl3-based ionic liquids to moisture and air and side reaction such as polymerization of olefin in gasoline induced by the strong Lewis acidity of Al2Cl7-, the use of AlCl3-free ionic liquids has been regarded as a particularly promising approach for the (4) Weitkamp, J.; Schwark, M.; Ernst, S. Chem. Commun. 1991, 1133-1134. (5) Takahashi, A.; Yang, F. H.; Yang, R. T. Ind. Eng. Chem. Res. 2002, 41, 2487-2496. (6) Hernandez, A. J.; Yang, R. T. Ind. Eng. Chem. Res. 2003, 42, 123-129. (7) Willauer, D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765-1766. (8) Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. Chem. Commun. 2003, 572-573. (9) Bo¨smann, A.; Datsevich, L.; Jess, A. Chem. Commun. 2001, 2494-2495. (10) Zhang, S.; Zhang, Q.; Zhang, Z. C. Ind. Eng. Chem. Prod. Res. 2004, 43, 614-622.
10.1021/ef049879k CCC: $27.50 © 2004 American Chemical Society Published on Web 09/23/2004
Desulfurization of Gasoline with Ionic Liquids
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deep desulfurization of motor fuels.9 The key problem for AlCl3-free ionic liquids such as BMImBF4 used in desulfurization is the low sulfur removal. Until now, no study about the co-use of AlCl3-free ionic liquid and Cu(I) for desulfurization has been performed. In this paper, we investigate the structure of CuCl ionic liquids and the property of CuCl-based ionic liquid in extractive desulfurization of gasoline. Experiments 1. Preparation of Ionic Liquids. The room-temperature ionic liquid used in this study was synthesized by mixing 1-butyl-3-methylimidazolium chloride (BMIC) and purified anhydrous CuCl, according to a BMIC:CuCl molar ratio of 1:2, at a temperature of 80 °C in heptane. The BMIC was synthesized according to previous literature.11 2. Detection of the Structure of Anions in Ionic Liquids. Structure of anions in ionic liquid were detected via fast atom bombardment mass spectrometry (FAB-MS),12 which had been confirmed by Wicelinski and Gale13 to be an ideal analytical tool in characterizing the structure of ions in ionic liquids. Investigations were processed with a double-focusing, forward-geometry mass spectrometer that had been fitted with a post-acceleration detector (Kratos model MS 80RFA). Fast Ar atoms were produced with a saddle-field, fast-atom gun (Ion-Tech); the gun was operated at 9 keV, and the accelerating voltage was 5 keV. Negative ion mode operation of the mass spectrometer was performed with a scan rate of 30 s/decade and a resolution of 1000. A sample of the ionic liquid (1-2 µL) was deposited on a copper probe tip that was placed into the insertion lock. Calibration of the instrument was from m/z 0 to m/z 1000. Data acquisition and processing were accomplished with DS90 (Version 3.1) software. 3. Extraction Desulfurization of the Model Oil or the Gasoline. All the desulfurization experiments were conducted in a 100-mL glass vial. The ionic liquid was added into model oil or gasoline at room temperature. The mass ratios of ionic liquids to model oil or gasoline were 1:5. The obtained biphasic mixture was then stirred for 30 min. The model oil used in this experiment was prepared by adding 680 ppm of thiophene and 20% toluene into heptane. Gasoline with different sulfur contents were obtained by adding thiophene into commercial gasoline obtained from a Sinopec Corporation station. 4. Analysis of Sulfur Content in Fuel. Quantitative elemental analysis of sulfur was conducted using a wavelengthdispersive X-ray fluorescence spectrometer (Bruker model S4 Explore).
Results and Discussion 1. Structures of Anions in CuCl-Based Ionic Liquids. Typical FAB-MS spectra of the BMImCu2Cl3 ionic liquid is shown in Figure 1. The sample was analyzed using negative ion spectra, to identify the copper-containing ionic complexes. The intensive peaks are observed at m/z 134, 233, 332. Using the atomic weights of Cu, Cl, O, and H, these intensive peaks correspond to CuCl2-, Cu2Cl3-, and Cu3Cl4-, respectively, as denoted in Figure 1. Compared to the analysis results of Wicelinski13 for AlCl3-based ionic liquids, no obvious oxygen-containing (11) Adams, C. J.; Earle, M. J.; Roberts, G.; Seddon, K. R. Chem. Commun. 1998, 2097-2098. (12) Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N. Chem. Commun. 1981, 325-326. (13) Wicelinski, S. P.; Gale, R. J. Anal. Chem. 1998, 60, 2228-2232. (14) Yoo, K.; Namboodiri, V. V.; Varma, R. S.; Smirniotis, P. G. J. Catal. 2004, 222, 511-522.
Figure 1. Fast-atom bombardment mass spectroscopy (FABMS) of anions in BMImCu2Cl3 ionic liquids. Table 1. Desulfurization of Model Oil by Extraction with Different Ionic Liquids ionic liquid
oil to be treated
sulfur content in oil to be treated (ppmw)
sulfur removal (%)
BMImCu2Cl3 BMImAlCl4a BMImBF4b
model oil model oil model oil
680 500 764
23.4 16.0 11.0
a Results from Bo ¨ smann et al.9 The extraction was performed at a temperature of 80 °C, and model oil was prepared by adding dibenzothiophene to n-dodecane. b Results from Akzo Nobel Chemical, Inc.10 The extraction was performed at room temperature, and the model oil was prepared by adding dibenzothiophene and piperidine to n-dodecane.
anionic species are observed in Figure 1. Because of the strong Lewis acidity of the anions, hydroxychloro and oxychloro anionic species (such as AlCl4OH-, Al2Cl6OH-, and Al2OCl5-) appear in the FAB-MS spectra of AlCl3based ionic liquids, which are induced by moisture during the experiment. The lack of an oxygen-containing anionic species in Figure 1 indicates that the BMImCu2Cl3 ionic liquid is more stable to moisture than the AlCl3-based ionic liquids. Moreover, no obvious Cu(II)-containing anionic species appears in Figure 1, which furthermore shows the stability of Cu(I) in BMImCu2Cl3 ionic liquid, relative to air. 2. Desulfurization of Model Oil with Different Ionic Liquids. The results of desulfurization with CuCl-based ionic liquids are listed in Table 1. For comparison, some results for desulfurization with other ionic liquids from the literature also are listed. BMImCu2Cl3 ionic liquid shows remarkable ability for sulfur removal. After extraction with BMImCu2Cl3 ionic liquid, more than 23% of the sulfur compounds have been removed from model oil, whereas the sulfur removal for BMImBF4 ionic liquids is only 11.0%. Although BMImAlCl4 ionic liquid exhibits a sulfur removal of 16.0%, because of the strong Lewis acidity of Al2Cl7- anion traces in ionic liquids, the olefin in commercial gasoline will polymerize to heavy compounds and furthermore crack into light compounds such as C4 and C5, which will result in a loss of gasoline.11 In our experiments for the desulfurization of gasoline with BMImCu2Cl3 ionic liquid, no polymerization of the olefins has been observed.
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Table 2. Desulfurization of Gasoline by Extraction with Ionic Liquids ionic liquid
oil to be treated
sulfur content in oil to be treated (ppmw)
sulfur removal (%)
BMIMCu2Cl3 BMIMCu2Cl3 BMIMCu2Cl3 BMIMCu2Cl3
gasoline gasoline gasoline gasoline
950 680 410 196
16.2 21.6 29.1 37.4
3. Desulfurization of Gasoline with CuCl-Based Ionic Liquids. The desulfurization results of gasoline with different sulfur contents, using BMImCu2Cl3 ionic liquid, are listed in Table 2. A comparison of these results to those in Table 1 shows that sulfur removal by BMImCu2Cl3 ionic liquid from gasoline is lower than that from model oil. More-complex compounds such as olefins, nitrogen-containing compounds, and aromatic compounds existed in the gasoline, compared to the model oil, which indicates that these compounds can decrease the sulfur removal ability of BMImCu2Cl3 ionic liquid. More interestingly, the data in Table 2 show that the sulfur removal by BMImCu2Cl3 ionic liquid increases as the sulfur content in the gasoline decreases. When the sulfur content in the gasoline decreased from 950 ppm to 410 ppm, the sulfur removal increased from 16% to 29%. However, Zhang et al.,10 who used a BF4--based ionic liquid, showed decreasing sulfur removal with decreasing sulfur content in the gasoline. Such a difference indicates that the desulfurization mechanism for BMImCu2Cl3 ionic liquid is different from that for BMImBF4 ionic liquid. Hernandez and Yang6 determined that π-complexation is the main interaction of Cu(I) with thiophene when performing desulfurization with Cu(I)-Y zeolite sorbents. Our results support their
opinion: the π-complexation of thiophene and Cu(I) is one of the important desulfurization mechanisms for CuCl-based ionic liquid. During the experiments, when the mass ratio of ionic liquid to gasoline is not changed, the quantity of Cu(I) in ionic liquid is not changed, and the quantity of sulfur possibly adopted by Cu(I) through π-complexation also is not changed, which results in an increase in sulfur removal with decreasing sulfur content in the gasoline, despite the reduction induced by the distribute equilibrium of sulfur between the ionic liquid and the gasoline. Moreover, from the data listed in Table 1, it can be speculated that, after multiple extraction by CuCl-based ionic liquid, gasoline with an ultralow sulfur content can be obtained. Conclusion Cu(I) anionic species existed in the ionic liquid, which was synthesized by mixing 1-butyl-3-methylimidazolium chloride and purified anhydrous CuCl. These Cu(I) anionic species were CuCl2-, Cu2Cl3-, and Cu3Cl4-, which are moisture-insensitive and stable in air. CuCl-based ionic liquid exhibits remarkable extraction desulfurization ability from gasoline, which may be attributed to the π-complexing interaction of thiophene and Cu(I). In addition, the extraction desulfurization of gasoline with CuCl-based ionic liquid has the advantage of no polymerization reaction of the olefins in the gasoline. Acknowledgment. The authors are grateful for the financial support from National Natural Science Foundation of China (under No. 2002-235). EF049879K
