Energy & Fuels 2008, 22, 1687–1690
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Extractive Desulfurization Using Fe-Containing Ionic Liquids Nan Hee Ko,† Je Seung Lee,† Eun Soo Huh,† Hyunjoo Lee, Kwang Deog Jung,‡ Hoon Sik Kim,*,† and Minserk Cheong*,† Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee UniVersity, Seoul 130-701, Korea, and EnVironment and Process DiVision, Korea Institute of Science and Technology, 39-1, Hawolgokdong, Seongbukgu, Seoul 136-791, Korea ReceiVed December 5, 2007. ReVised Manuscript ReceiVed March 3, 2008
FeIII-containing ionic liquids (ILs), prepared from the reaction of anhydrous FeCl3 and imidazolium chloride ([imidazolium]Cl), were used as effective extractants for the desulfurization of a model oil containing dibenzothiophene (DBT). The amount of DBT extracted increased with an increasing molar ratio of FeCl3/ [imidazolium]Cl. The ability of the ILs to extract DBT seems to be attributed to the combined effects of Lewis acidity and fluidity of ILs.
Introduction Deep desulfurization (DDS) of fuel oils, such as gasoline and diesel, has attracted increasing interest because of the enforcing regulation on the environmental pollution caused by exhaust gases, such as SOx and NOx to the atmosphere. To reduce the emission by SOx, many developed countries are planning to reduce the allowable sulfur level in fuels down to 10 ppm by 2009.1 Traditionally, the removal of sulfur compounds from fuel oils has been conducted by a catalytic hydrodesulfurization (HDS) using catalysts based on CoMo or NiMo, which involves the reactions of sulfur compounds with H2 to produce H2S and corresponding hydrocarbons.2–5 Paraffinic sulfur-containing compounds, including thiols, thioethers, and disulfides, are readily desulfurized by the conventional HDS process, but aromatic sulfur compounds, such as benzothiophenes (BTs) and dibenzothiophenes (DBTs), are hardly desulfurized by the process.6 Much effort has been devoted to improve the performance of current HDS catalysts for the effective desulfurization of BTs and DBTs, but no significant advancement has yet been made. For this reason, many alternative processes have been developed, including adsorption,7 extraction,8,9 and selective oxidation.10,11 * To whom correspondence should be addressed. E-mail: khs2004@ khu.ac.kr (H.S.K.);
[email protected] (M.C.). † Kyung Hee University. ‡ Korea Institute of Science and Technology. (1) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607–631. (2) Ferrari, M.; Maggi, R.; Delmon, B.; Grange, P. J. Catal. 2001, 198, 47–55. (3) Dumeignil, F.; Sato, K.; Imamura, M.; Matsubayashi, N.; Payen, E.; Shimada, H. Appl. Catal., A 2006, 315, 18–28. (4) Pawelec, B.; Mariscal, R.; Fierro, J. L. G.; Greenwood, A.; Vasudevan, P. T. Appl. Catal., A 2001, 206, 295–307. (5) Lee, J. J.; Han, S.; Kim, H.; Koh, J. H.; Hyeon, T.; Moon, S. H. Catal. Today 2003, 86, 141–149. (6) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021– 2058. (7) Salem, A. B. S. H.; Hamid, H. S. Chem. Eng. Technol. 1997, 20, 342–347. (8) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 4538–4544. (9) Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2001, 40, 293–303.
Figure 1. FTIR spectra of FeCl3-[BMIm]Cl melts. Molar ratio of FeCl3/[BMIm]Cl: (a) 0, (b) 0.5, (c) 0.75, (d) 1, (e) 1.5, and (f) 2. Table 1. Frequency Calculation of [EMIm]Cl and [EMIm]FeCl4 normal mode of vibration
[EMIm]Cl (cm-1)
[BMIm]FeCl4 (cm-1)
C(2)-H stretching ethyl C-H symmetric stretching methyl C-H symmetric stretching
2749.42 3043.24 3078.28
3273.98 3092.32 3088.34
Among these, extractive desulfurization has been most extensively studied because of its facile operation. A number of organic solvents, such as polyalkyleneglycol and polyalkyleneglycol ether, were tested as extractants for the removal of sulfur-containing compounds from the fuel oils, but their performances were never satisfactory. Recently, various types of ionic liquids (ILs) have been employed in the DDS of fuel oils because of their unique properties, such as the immiscibility with fuel oils, high affinity to sulfur-containing compounds, nonvolatility, and high thermal stability.12–19 In particular, Lewis acidic ILs containing metal halide anions, such as AlCl4- and CuCl2-, showed promising (10) Yazu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukegawa, K. Energy Fuels 2001, 15, 1535–1536. (11) Te, M.; Fairbridge, C.; Ring, Z. Appl. Catal., A 2001, 219, 267– 280. (12) Bossmann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 2494–2495.
10.1021/ef7007369 CCC: $40.75 2008 American Chemical Society Published on Web 04/22/2008
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Figure 2. Structures of (a) [EMIm]Cl and (b) [EMIm]FeCl4 showing the interactions between Cl and H atoms.
results on the selective removal of aromatic sulfur compounds even though the extraction was carried out at relatively high temperatures around 70 °C because of their inherent low fluidity.12–14 During the course of our work on the DDS of fuel oils, we have found that Lewis acidic ILs prepared from 3-butyl-1methylimidazolium chloride ([BMIm]Cl) and FeCl3 are highly effective for the extraction of sulfur compounds present in the hydrocarbon mixtures at an ambient temperature. In this paper, we report our results on the use of imidazoliumbased ILs with FeIII species for the DDS of model oils containing dibenzothiophene (DBT), as well as the spectroscopic and theoretical investigation of these ILs. Experimental Section FeIII-containing
ILs, FeCl3-[imidazolium]Cl, were prepared using the methods described in the literature,20–24 by reacting FeCl3 with 1-butyl-2,3-dimethylimidazolium chloride ([BDMIm]Cl), [BMIm]Cl, 1,2-dimethylimidazole hydrochloride ([HDMIm]Cl), 1-methylimidazole hydrochloride ([HMIm]Cl), or imidazole hydrochloride ([HHIm]Cl) at room temperature. FeCl3-[BMIm]Cl and FeCl3[BDMIm]Cl existed as liquids, whereas all of the other ILs existed as solids at room temperature. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 380 spectrophotometer (Thermo Electron Co.). To avoid contact with water and air, FTIR measurements were performed using a specially designed IR cell equipped with two KRS-5 windows.25 Electrospray ionization tandem mass (ESI-MS) spectra of LiCl-[imidazolium]Cl melts were collected on a Finnigan TSQ Quantum Ultra triple quadrupole mass spectrometer. Analytes were (13) Zhang, S.; Zhang, Q.; Zhang, Z. C. Ind. Eng. Chem. Res. 2004, 43, 614–622. (14) Huang, C.; Chen, B.; Zhang, J.; Liu, Z.; Li, Y. Energy Fuels 2004, 18, 1862–1864. (15) Wu, W.; Han, B.; Gao, H.; Liu, Z.; Jiang, T.; Huang, J. Angew. Chem., Int. Ed. 2004, 43, 2415–2417. (16) Nie, Y.; Li, C.; Sun, A.; Meng, H.; Wang, Z. Energy Fuels 2006, 20, 2083–2087. (17) Planeta, J.; Karasek, P.; Roth, M. Green Chem. 2006, 8, 70–77. (18) Lu, L.; Cheng, S.; Gao, J.; Gao, G.; He, M.-Y. Energy Fuels 2007, 21, 383–384. (19) Cassol, C. C.; Umpierre, A. P.; Ebeling, G.; Ferrera, B.; Chiaro, S. S. X.; Dupont, J. Int. J. Mol. Sci. 2007, 8, 593–605. (20) Sitze, M. S.; Schreiter, E. R.; Patterson, E. V.; Freeman, R. G. Inorg. Chem. 2001, 40, 2298–2304. (21) Kölle, P.; Dronskowski, R. Inorg. Chem. 2004, 43, 2803–2809. (22) Hayashi, S.; Hamaguchi, H. Chem. Lett. 2004, 33, 1590–1591. (23) Lee, S. H.; Ha, S. H.; Ha, S.-S.; Jin, H.-B.; You, C.-Y.; Koo, Y.M. J. Appl. Phys. 2007, 101, 09J102. (24) Lee, S. H.; Ha, S. H.; You, C. Y.; Koo, Y. M. Korean J. Chem. Eng. 2007, 24, 436–437. (25) Jones, W. O. J. Chem. Soc. 1954, 1808–1813.
Table 2. Desulfurization of the Model Oil by FeCl3-[BMIm]Cl at Various Molar Ratios of FeCl3/[BMIm]Cla molar ratio (FeCl3/[BMIm]Cl)
degree of desulfurization (%)
[BMIm]Clb 0.5 0.7 1 1.5 2 2.5 3 5 FeCl3c
17.2 34.1 38.9 42.2 77.4 100 100 100 100 33.9
a The extraction of DBT was conducted at room temperature with the model oil containing 5000 ppm of DBT and 20 000 ppm of n-octane as an internal standard in n-heptane. The weight ratio of model oil/IL was set at 5. b [BMIm]Cl only. c FeCl3 only.
Table 3. Desulfurization of the Model Oil by Various Types of ILsa ILb
degree of desulfurization (%)
FeCl3-[HHIm]Cl FeCl3-[HMIm]Cl FeCl3-[HDMIm]Cl FeCl3-[BMIm]Cl FeCl3-[BMIm]Clc FeCl3-[BDMIm]Cl FeCl3-[BDMIm]Clc AlCl3-[BMIm]Clc CuCl-[BMIm]Clc FeCl3-[BMIm]Clc,d
9.6 11.7 18.5 42.2 100 44.8 100 23.5 17.2 100
a The extraction of DBT was conducted at room temperature with the model oil containing 5000 ppm of DBT and 20 000 ppm of n-octane as an internal standard in n-heptane. The weight ratio of model oil/IL was set at 5. b Molar ratio of FeCl3/IL ) 1. c Molar ratio of metal chloride/ [BMIm]Cl ) 2. d Diesel oil with a sulfur content of 1180 ppm was used instead of the model oil.
detected in the negative mode at 4200 V. Full-scan mode was used in the range of m/z 30-600, and data were processed using Xcalibur version 1.4. Desulfurization experiments were conducted in a 50 mL Schlenk tube. An IL was added into the model oil containing 5000 ppm of DBT and 20 000 ppm of n-octane as an internal standard in n-heptane and stirred for 30 min at room temperature. The weight ratio of IL/model oil was set at 1:5. After the extraction, the upper layer was analyzed using an Agilent 6890 gas chromatograph equipped with a flame-ionized detector and a DB-wax column (30 m × 0.32 mm × 0.25 µm) and an Agilent 6890-5973 GC-MSD spectrometer. Diesel oil with a sulfur content of 1180 ppm was purchased from a station of Hyundai Oil Co., and the sulfur contents before and
Desulfurization Using Fe-Containing ILs
Figure 3. Negative ions ESI-MS spectra of FeCl3-[BMIm]Cl. Molar ratio of FeCl3/[BMIm]Cl: (a) 0.5, (b) 0.75, (c) 1, (d) 1.5, and (e) 2.
Figure 4. 1H NMR spectra of (a) [BMIm]Cl, (b) FeCl3-[BMIm]Cl, (c) DBT, and (d) FeCl3-[BMIm]Cl with DBT in DMSO-d6.
after the desulfurization were determined using an Agilent 355 sulfur chemiluminescence detector.
Results and Discussion The formation of imidazolium-based ILs containing anionic FeIII species was investigated using FTIR and 1H nuclear magnetic resonance (NMR) spectroscopies. As shown in Figure 1, all of the Fe-containing ILs prepared from FeCl3 and [BMIm]Cl at various molar ratios of FeCl3/[BMIm]Cl displayed FTIR spectra that were different from those of FeCl3 and [BMIm]Cl, indicating the strong interaction of FeCl3 with [BMIm]Cl and, consequently, the formation of new species. The broad peaks that appeared in the range of 3000–3100 cm-1 can be assigned to the interaction of aromatic C-H with Cl-.26,27 It is noteworthy that the band corresponding to the C(2)-H (26) Tait, S.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 4352–4360. (27) Dieter, K. M.; Dymek, C. J., Jr.; Heimer, N. E.; Rovang, J. W.; Wilkes, J. S. J. Am. Chem. Soc. 1988, 110, 2722–2726.
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bond interacting with Cl- at 3067 cm-1 shifted to a higher frequency upon interaction with FeCl3. The degree of the shift increased with an increasing Fe content in the ILs. The disappearance of the broad peak upon mixing with FeCl3 can be attributed to the formation of ionic liquids, such as [BMIm]FeCl4 and [BMIm]Fe2Cl7, because the interaction between C(2)-H and FeCl4- or Fe2Cl7- is much weaker than the interaction between C(2)-H and Cl-. This behavior is supported by the theoretical frequency calculation with FeCl3 and (1-ethyl3-methylimidazolium)FeCl4 ([EMIm]FeCl4) (see Table 1 and Figure 2). For simplicity, the calculation was made with [EMIm]FeCl4 instead of [BMIm]FeCl4. A doublet type peak seen above 3100 can be ascribed to the presence of more than one Fe-containing IL species when there is excess of FeCl3. The extraction ability of FeCl3-[imidazolium]Cl was evaluated for the removal of DBT from the model oil. As shown in Table 2, the amount of DBT extracted increased with an increasing molar ratio of FeCl3/[BMIm]Cl. This result can be attributed to the increased Lewis acidity of the resulting IL at higher molar ratios of FeCl3/[BMIm]Cl.28 Surprisingly, DBT was completely extracted from the model oil at the FeCl3/ [BMIm]Cl molar ratios of 2 and higher. In general, FeCl3 alone exhibited lower extraction ability than the Fe-containing ILs, suggesting the important role of [BMIm]Cl. The effect of the substituent on the imidazolium ring was also investigated for the removal of DBT using various ILs (FeCl3/[imidazolium]Cl ) 1). It is hoped that the presence of acidic H atom or atoms on the imidazolium ring could promote the interaction between FeCl3-[imidazolium]Cl with DBT through a hydrogen bond. Contrary to our expectation, the presence of the H atom was not helpful to improve the performance of ILs to extract DBT. As listed in Table 3, the IL with one or two acidic hydrogen atoms on the imidazolium ring exhibited much lower extraction ability than the ILs containing two or three alkyl groups. The highest DBT extraction was obtained with FeCl3-[BDMIm]Cl and FeCl3-[BMIm]Cl that existed as liquids at room temperature. These results may imply that the ability of FeIII-based ILs to remove DBT is also affected by the fluidity of the IL. Fe-containing ILs were also applied to the desulfurization of commercial diesel oil containing sulfur content of 1180 ppm. Desulfurization with FeCl3-[BMIm]Cl (FeCl3/[BMIm]Cl ) 2) clearly demonstrated that all of the sulfur compounds were completely extracted from the diesel oil (see Table 3). For a comparison, the performances of ILs prepared from [BMIm]Cl and CuCl or AlCl3 (CuCl/[BMIm]Cl ) 2, and AlCl3/ [BMIm]Cl ) 2) were tested for the removal of DBT from the model oil (see Table 3). However, the extraction ability of these ILs were significantly lower than that of the corresponding FeCl3-based IL (FeCl3/[BMIm]Cl ) 2). The reason for the higher extraction ability of FeCl3-[BMIm]Cl is not clear at the moment, but it is likely that the fluidity of FeCl3-[BMIm]Cl and affinity of FeCl3 to DBT are more important than the Lewis acidity because FeCl3-[BMIm]Cl is less Lewis acidic and less viscose than AlCl3-[BMIm]Cl (see the Supporting Information). To have a better understanding of the FeIII species and their interaction with DBT, ESI-MS and 1H NMR spectral analyses were carried out with the ILs of various molar ratios of FeCl3/ [BMIm]Cl. ESI-MS spectra in Figure 3 showed that the major anionic FeIII species observed in the molar ratio of FeCl3/ [BMIm]Cl from 0.5 to 1 was the mononuclear FeCl4- along with very small amounts of high nuclear Fe species. However, at the molar ratios above 1, the formation of high nuclear FeIII (28) Yang, Y.; Kou, Y. Chem. Commun. 2004, 226–227.
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Figure 5. Optimized structures showing the interactions with DBT: (a) [EMIm]FeCl4, (b) [EMIm]Fe2Cl7, and (c) FeCl3 plus [EMIm]FeCl4.
species, such as Fe1.5Cl5.5- and Fe2Cl7-, was pronounced. It is well-known that the Lewis acidity of ILs prepared from AlCl3 and [BMIm]Cl increases with an increasing molar ratio of AlCl3/ [BMIm]Cl, because of the formation of high nuclear AlIII anionic species, such as Al2Cl7-.12,13,28 Therefore, from the experimental and ESI-MS spectral results, the increased sulfur-removal ability of ILs with higher molar ratios of FeCl3/[BMIm]Cl can be largely attributed to the presence of more Lewis acidic high nuclear FeIII species. Unfortunately, anionic FeIII species containing DBT were not observed from the ESI-MS spectroscopic analysis, possibly because of the instability of the resulting Fe species under the analysis condition. However, the interaction of the DBT with [BMIm]Cl was somewhat supported from the 1H NMR spectra in Figure 4. When a solution of DBT in CH2Cl2 was mixed with an equimolar amount of [BMIm]FeCl4 in CH2Cl2, brownish precipitates were produced. 1H NMR spectrum of the precipitates displayed a series of broad peaks associated with imidazolium ring and DBT, implying that DBT is strongly associated with the IL. The interactions of [EMIm]FeCl4- and [EMIm]Fe2Cl7- with DBT were theoretically investigated at the B3LYP level of the theory (6-31+G* for C, H, and N and LanL2DZ for Cl and Fe) using the Gaussian 03 program. As shown in Figure 5a, there is a substantial interaction between DBT and FeCl4-, and the interaction enthalpy (∆H) was calculated as –4.5 kcal mol-1. A similar interaction can also be seen between Fe2Cl7- and DBT (see Figure 5b). The calculated interaction enthalpy is –2.4 kcal mol-1, which is smaller by 2.1 kcal mol-1 than the interaction between FeCl4- and DBT (see the Supporting Information). This is in contrast to the experimental result that the Fe species with higher nuclearity exhibits better performance in removing DBT. Therefore, it can not be ruled out that a Fe species other than Fe2Cl7- is playing a certain role in the extraction of DBT from the model oil. One possibility is that, upon interaction with DBT, a small portion of [BMIm]Fe2Cl7 decomposes into [BMIm]FeCl4
and FeCl3, which in turn interacts with DBT to form [BMIm]FeCl4-DBT-FeCl3 species, as can be expected from the structure of [EMIm]FeCl4-DBT-FeCl3 shown in Figure 5c. The enthalpy difference between DBT-Fe2Cl7- and DBT-[FeCl4- plus FeCl3] was calculated as –2.7 kcal mol-1, implying that the formation of [EMIm]FeCl4-DBT-FeCl3 species by breaking the bridging Fe-Cl bond is highly favorable because of the strong interaction between Fe and S (Fe-S bond length of 2.58 Å). The difference in extraction ability between pure FeCl3 and [BMIm]Fe2Cl7 can also be rationalized by considering that FeCl3 formed from [BMIm]Fe2Cl7 is in a solution state, whereas pure FeCl3 exists as a solid. Conclusion FeCl3-based ILs can be used as effective extractants for removing DBT from fuel oils, and the ability of ILs to extract DBT is largely attributed to the interaction of Lewis acidic FeIII species with Lewis basic DBT, as well as the fluidity of IL. Further theoretical calculations are in progress to have a deeper understanding of the interactions between FeCl3-based ILs and DBT. Acknowledgment. The authors thank the financial support from MOST of Korea and KAERI. This work has been carried out under the Nuclear Hydrogen Development and Demonstration (NHDD) project. This work was also supported by the Korea Research Foundation Grant funded by the Korean government (MOEHRD, Basic Research Promotion Fund KRF-2005-070-C0072). Supporting Information Available: Positive ions ESI-MS data, viscosities of ILs, and Cartesian coordinates of optimized geometries. This material is available free of charge via the Internet at http://pubs.acs.org. EF7007369
