Zn-Containing Ionic Liquids for the Extractive Denitrogenation of a

The interaction of EtSO4− and ZnCl2(EtSO4)− with a heterocyclic N compound was theoretically investigated. The zinc-containing IL, [EMIm]ZnCl2(EtS...
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Energy & Fuels 2009, 23, 3032–3038

Zn-Containing Ionic Liquids for the Extractive Denitrogenation of a Model Oil: A Mechanistic Consideration Eun Soo Huh,† Alexey Zazybin,† Jelliarko Palgunadi,†,‡ Sungho Ahn,§ Jongki Hong,§ Hoon Sik Kim,*,† Minserk Cheong,*,† and Byoung Sung Ahn*,‡ Department of Chemistry and Research Institute of Basic Sciences, and College of Pharmacy, Kyung Hee UniVersity, 1 Hoegi-dong, Dongdaemoon-gu, Seoul 130-701, Korea, Clean Energy Research Centre, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Korea ReceiVed January 25, 2009. ReVised Manuscript ReceiVed April 13, 2009

Imidazolium-based zinc-containing ionic liquids (ILs), [1-R-3-R′-imidazolium]alkylsulfate-ZnCl2 (R and R′ ) H or alkyl), were highly effective for the denitrogenation of a model oil containing quinoline, indole, or acridine in n-heptane. Fast atom bombardment (FAB)-mass spectra and a computational study imply that the interaction of 1-ethyl-3-methylimidazolium ethylsulfate ([EMIm]EtSO4) with ZnCl2 produces Zn-containing ILs, presumably [EMIm]ZnCl2(EtSO4) and [EMIm]ZnCl(EtSO4)2 as the major ionic species. The interaction of EtSO4- and ZnCl2(EtSO4)- with a heterocyclic N compound was theoretically investigated. The zinccontaining IL, [EMIm]ZnCl2(EtSO4), used for the extraction of quinoline was successfully regenerated by employing diethyl ether as a back extractant.

Introduction In recent years, desulfurization and denitrogenation of transportation fuels have attracted increasing interest because of the stringent regulation on the environmental pollution caused by exhaust gases, such as SOx and NOx, to the atmosphere. To protect the environment against contamination, many developed countries, including countries in Europe and Japan, are enforcing the regulation to reduce the allowable sulfur level in diesel fuel down to 10 parts per million by weight (ppmw) by 2010.1-3 Industrially, the removal of organosulfur and organonitrogen compounds in fuel oils is being carried out by means of a simultaneous hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) process at around 350 °C using catalysts based on CoMo or NiMo, which involves the C-S and C-N bond cleavage to produce H2S and NH3, respectively.4-7 However, current hydrotreating catalysts used for this purpose are not very effective for the removal of aromatic sulfur compounds, such as benzothiophenes (BTs) and dibenzothiophenes (DBTs), and aromatic nitrogen compounds. Furthermore, a HDS catalyst is * To whom correspondence should be addressed. Telephone: +82-2961-0432 (H.S.K.); +82-2-961-0239 (M.C.); +82-2-958-5854 (B.S.A.). Fax: +82-2-965-4408. E-mail: [email protected] (H.S.K.); [email protected] (M.C.); [email protected] (B.S.A.). † Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University. ‡ Korea Institute of Science and Technology. § College of Pharmacy, Kyung Hee University. (1) Zhou, A.; Ma, X.; Song, C. J. Phys. Chem. B 2006, 110, 4699– 4707. (2) Kim, J. H.; Ma, X.; Zhou, A.; Song, C. Catal. Today 2006, 111, 74–83. (3) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607–631. (4) Ferrari, M.; Maggi, R.; Delmon, B.; Grange, P. J. Catal. 2001, 198, 47–55. (5) Dumeignil, F.; Sato, K.; Imamura, M.; Matsubayashi, N.; Payen, E.; Shimada, H. Appl. Catal., A 2006, 315, 18–28. (6) Pawelec, B.; Mariscal, R.; Fierro, J. L. G.; Greenwood, A.; Vasudevan, P. T. Appl. Catal., A 2001, 206, 295–307. (7) Caeiro, G.; Costa, A. F.; Cerqueira, H. S.; Magnoux, P.; Lopes, J. M.; Matias, P.; Ribeiro, F. R. Appl. Catal., A 2007, 320, 8–15.

easily deactivated by small amounts of aromatic nitrogen compounds present in the fuel, which is preferably adsorbed on the surfaces of the catalyst. Although the allowable nitrogen content is not strictly specified,2,7 the development of new approaches to drastically reduce the nitrogen content in transportation fuel oils presumably below 10 ppmw is urgently demanded to meet the need of ultra-clean fuel for environmental protection. Several alternative processes, including extraction,8,9 membrane,10 and selective oxidation,11 were proposed for the removal of nitrogen compounds, which can be classified into two groups: basic (pyridine, quinoline, and acridine) and neutral (indole and carbazole). Adsorption by ion-exchange resins12 and liquid-liquid extraction with carboxylic acids8 as well as ionic liquids (ILs)13,14 have been employed to remove basic nitrogen compounds. A much more restricted set of methods including adsorption2,16 and extraction with highly polar organic solvents15 and ILs13,14 has been reported for the removal of neutral nitrogen compounds. Among these, denitrogenation based on solvent extraction has been most extensively studied because of its facile operation, less energy consumption, and the retention of the (8) Qi, J.; Yan, Y.; Su, Y.; Qu, F.; Dai, Y. Energy Fuels 1998, 12, 788– 791. (9) Gao, P.; Cao, Z.; Zhao, D.; Li, D.; Zhang, S. Pet. Sci. Technol. 2005, 23, 1023–1031. (10) Matsumoto, M.; Mikami, M.; Kondo, K. J. Jpn. Pet. Inst. 2006, 49, 256–261. (11) da Conceicao, L.; de Almeida, C. L.; Egues, S.; Dallago, R. M. Energy Fuels 2005, 19, 960–963. (12) Prudich, M. E.; Cronauer, D. C.; Vogel, R. F.; Solash, J. Ind. Eng. Chem. Proc. Des. DeV. 1986, 25, 742–746. (13) Eber, J.; Wasserscheid, P.; Jess, A. Green Chem. 2004, 6, 316– 322. (14) Xie, L. L.; Favre-Reguillon, A.; Wang, X. X.; Fu, X.; PelletRostaing, S.; Toussaint, G.; Geantet, C.; Vrinat, M.; Lemaire, M. Green Chem. 2008, 10, 524–531. (15) Merdrignac, I.; Behar, F.; Albrecht, P.; Briot, P.; Vandenbroucke, M. Energy Fuels 1998, 12, 1342–1355. (16) Liu, D.; Gui, J.; Sun, Z. J. Mol. Catal. A: Chem. 2008, 291, 17– 21.

10.1021/ef900073a CCC: $40.75  2009 American Chemical Society Published on Web 05/06/2009

Zn-Containing Ionic Liquids

chemical structures of the fuels. Considering the energy consumption, ILs seem to be more attractive than organic solvents17 because of their negligible volatility, immiscibility with fuel oils, and higher affinity to sulfur- and nitrogencontaining compounds, and therefore, target compounds can be easily removed from the fuel oils through a simple layer separation.18-20 Several papers were published describing the use of imidazolium-based ILs as extractants in the denitrogenation process,13,18,19 but the interaction between ILs and N compounds was never a subject of detailed investigation. Recently, Lewis acidic ILs bearing metal halide anions based on AlCl3,19,20 FeCl3,21 and CuCl22 were shown to exhibit promising performances on the selective removal of aromatic sulfur compounds. Being motivated by these results, we have tested metal-containing Lewis acidic ILs as extractants for the selective removal of aromatic nitrogen compounds from hydrocarbon mixtures. During the course of our study on the denitrogenation of fuel oils, we have found that Zn-containing imidazolium-based ILs have strong affinities to aromatic nitrogen compounds. We now report on the use of Zn-containing imidazoliumbased ILs for the extraction of nitrogen compounds present in hydrocarbon mixtures at an ambient temperature as well as on the theoretical investigation on the interaction between ILs and heterocyclic N compounds. Experimental Section General. All of the chemicals used for the synthesis of RTILs were purchased from Aldrich Chemicals Co. and used as received. 1 H nuclear magnetic resonance (NMR) spectra were recorded on a 400 MHz Bruker NMR spectrometer using DMSO-d6 as the solvent. Fourier transform infrared (FTIR) spectra were recorded in an inert atmosphere on a Nicolet 380 spectrophotometer equipped with a smart MIRacle ATR accessory (Thermo Electron Co.). Fast atom bombardment (FAB)-mass spectra for the characterization of Zncontaining ILs were recorded with a JMS-700 Mstation doublefocusing mass spectrometer (JEOL, Tokyo, Japan) using a MSMP9020D data system. The ion source was operated at 10 kV accelerating voltage, with a mass resolution of 1500 (10% valley). Fast atoms were produced by FAB using a xenon atom gun operating at 6 keV. Samples were dissolved in methanol and mixed with 1 µL of 3-nitrobenzyl alcohol (NBA, Sigma, St. Louis, MO) on a FAB probe tip. Calibration was performed with an Ultramark 1621 (PCR, Gainesville, FL) in the positive-ion mode as a standard compound. Analysis of the upper heptane layers was conducted using an Agilent 6890 gas chromatograph equipped with a flameionized detector and a DB-wax capillary column (30 m × 0.32 mm × 0.25 µm), and an Agilent 6890-5973 GC-MSD mass spectrometer equipped with a HP-5MS capillary column (30 m × 0.32 mm × 0.5 µm). Synthesis of Room Temperature Ionic Liquids (RTILs). RTILs, 1-alkyl-3-methylimidazolium alkylsulfate,23 1-alkyl-3methylimidazolium alkylphosphite,24 and 1-alkyl-3-methylimida(17) Funakoshi, I.; Aida, T. U.S. Patent 5,753,102, 1998. (18) Zhao, H.; Xia, S.; Ma, P. J. Chem. Technol. Biotechnol. 2005, 80, 1089–1096. (19) Zhang, S.; Zhang, Q.; Zhang, Z. C. Ind. Eng. Chem. Res. 2004, 43, 614–622. (20) Gao, Z. R.; Liao, K. J.; Liu, D. S.; Dai, Y. L. Pet. Sci. Technol. 2005, 27, 1001–1008. (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) Huang, C.; Chen, B.; Zhang, J.; Liu, Z.; Li, Y. Energy Fuels 2004, 18, 1862–1864. (23) Holbrey, J. D.; Reichert, W. M.; Swatloski, R. P.; Broker, G. A.; Pitner, W. R.; Seddon, K. R.; Rogers, R. D. Green Chem. 2002, 4, 407– 413. (24) Nguyen, H.-P.; Baboulene, M. PCT Int. Appl., WO 2008101881A2, 2007.

Energy & Fuels, Vol. 23, 2009 3033 Scheme 1. Structures of ILs Tested for the Denitrogenation of the Model Oil

zolium dialkylphosphate,25 were prepared using the methods described in the literature. RTILs containing ZnCl2 were prepared by dissolving anhydrous ZnCl2 in a RTIL at 60 °C with vigorous stirring. The weight ratio of RTIL/ZnCl2 was varied from 1 to 20. All of the Zn-containing ILs obtained were liquids at room temperature with a moderate viscosity. Computational Details. The formation of [EMIm]ZnCl2(EtSO4) and [EMIm]ZnCl(EtSO4)2 and the interactions of [EMIm]ZnCl2(EtSO4) and [EMIm]EtSO4 with quinoline and indole were theoretically investigated using Gaussian 03.26 The geometry optimizations and thermodynamic corrections were performed with a hybrid Becke 3-Lee-Yang-Parr (B3LYP) exchange-correlation functional with the 6-31+G* basis sets for C, H, N, and O and LanL2DZ(ECP) basis sets for S, Cl, and Zn. To investigate the structures of the complexes, all kinds of possible interaction patterns were optimized, giving rise to the most stable final geometries. No restrictions on symmetries were imposed on the initial structures. All stationary points were verified as minima by full calculation of the Hessian and a harmonic frequency analysis. Denitrogenation Experiments. Unless otherwise stated, all of the denitrogenation experiments were conducted under a nitrogen atmosphere. A typical denitrogenation experiment is as follows: In a 50 mL Schlenk tube, an IL (1 g) was mixed with 5 g of model oil containing 5000 ppm of quinoline, acridine, or indole and 20 000 ppm of n-octane as an internal standard in n-heptane. The resulting mixture was stirred for 20 min and then allowed to stand for 10 min at room temperature. After the extraction was completed, the upper heptane layer was analyzed by GC and GC-mass. The bottom IL layer was characterized by 1H NMR. Back Extraction and Regeneration of RTIL. Recycle experiments were performed using [EMIm]EtSO4-ZnCl2 (1 g, [EMIm]EtSO4/ZnCl2 ) 2) as an extractant and diethyl ether as a back extractant. In a 25 mL Schlenk tube, 5 g of model oil containing 5000 ppm of quinoline and 20 000 ppm of n-octane as an internal standard in n-heptane was mixed with 1 g of IL with vigorous stirring for 10 min and then allowed to stand for 5 min. After the extraction, the upper layer was removed by layer separation and the remaining IL layer was washed with 3 g of diethyl ether with vigorous shaking for 5 min, followed by standing for 5 min. The upper diethyl ether layer was then separated, evaporated, and analyzed by 1H NMR spectroscopy. The bottom IL layer was evaporated under a vacuum to remove diethyl ether and used for further extraction with a fresh charge of the model oil.

Results and Discussion Denitrogenation with Neat RTILs. The denitrogenation of the model oil containing 5000 ppm of quinoline and 20 000 ppm of n-octane as an internal standard in n-heptane was conducted at room temperature using RTILs shown in Scheme 1. The Nernst partition coefficient KN,13,27 defined as the mass ratio of the nitrogen content in a RTIL to the nitrogen content in the model oil [mg (N) g (IL)-1/mg (N) g (oil)-1], was measured for each extraction. As listed in Table 1, all of the (25) Kuhlmann, E.; Himmler, S.; Giebelhaus, H.; Wasserscheid, P. Green Chem. 2007, 9, 233–242. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; et al. Gaussian 03, revision C.02, Gaussian, Inc., Pittsburgh, PA, 2004. (27) Nie, Y.; Li, C.-X.; Wang, Z. H. Ind. Eng. Chem. Res. 2007, 46, 5108–5112.

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Table 1. Denitrogenation of the Model Oil Containing Quinoline Using Various Types of ILsa IL

DEb (%)

KNc

[DMIm]MeSO4 [EMIm]MeSO4 [BMIm]MeSO4 [OMIm]MeSO4 [HEIm]EtSO4 [EMIm]EtSO4 [BEIm]EtSO4 [EMIm]EtPHO3 [BMIm]BuPHO3 [EMIm]Et2PO4 [BMIm]Bu2PO4 [EMIm]Cl [EMIm]2ZnCl2Br2d

36.2 47.9 52.7 57.3 40.8 40.6 55.3 40.7 45.7 54.0 52.7 29.0 49.3

2.84 4.60 5.57 6.71 3.45 3.42 6.19 3.43 4.21 5.89 5.57 2.04 4.86

a The extraction of quinoline was conducted at room temperature with the model oil containing 5000 ppm of quinoline 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 DE ) degree of extraction (%). c Nernst partition coefficient ) [mg (N) g (IL)-1/mg (N) g (oil)-1]. d A total of 20 wt % in [BEIm]EtSO4.

RTILs showed KN values in the range of 2.8-6.7, which are equivalent to 36-58% quinoline extraction. The quinoline extraction ability of IL or the KN value increased with an increasing chain length of the alkyl group on the cation and anion for the same series of RTILs. For methylsulfate series, the KN value was found in the order: 1,3-dimethylimidazolium methylsulfate ([DMIm]MeSO4) < 1-ethyl-3-methylimidazolium methylsulfate ([EMIm]MeSO4) < 1-butyl-3-methylimidazolium methylsulfate ([BMIm]MeSO4) < 1-octyl-3-methylimidazolium methylsulfate ([OMIm]MeSO4). The reason for this is not clear at the moment, but the extraction ability of IL seems to be related to the free volume in an IL because the molar volume increases going from [DMIm]MeSO4 to [OMIm]MeSO4. The interrelation between the length of the alkyl chain of the IL and free volume was well-described in the literature.28,29 On the contrary, the extraction ability of RTIL was not affected by the type of anion, as long as the RTIL possesses the equivalent number of the same alkyl groups: the degree of extraction with [EMIm]EtSO4 and 1-ethyl-3-methylimidazolium ethylphosphite ([EMIm]EtPHO3) was found to be 40.6 (KN ) 3.42) and 40.7% (KN ) 3.43), respectively. However, when phosphate and phosphite series ILs were compared, phosphate series ILs showed better extraction ability than the phosphite series ILs. For example, the extraction abilities of 1-ethyl-3methylimidazolium diethylphosphate ([EMIm]Et2PO4) and [EMIm]EtPHO3 were determined as 54.0 (KN ) 5.89) and 40.7% (KN ) 3.43), respectively. The presence of one more ethyl group in [EMIm]Et2PO4 than in [EMIm]EtPHO3 seems to provide a larger free volume in the IL, thereby resulting in higher extraction ability. On the other hand, stronger basicity of phosphite anion is likely to result in a stronger interaction between the imidazolium cation and phosphite anion, thereby reducing the interaction of quinoline with the C(2)-H of the imidazolium cation. As expected from the physical state, [EMIm]Cl, existing as a solid at room temperature, showed a greatly reduced extraction ability compared to sulfate, phosphite, and phosphate series RTILs. Characterization of Zn-Containing RTILs: FTIR. Even though the neat RTILs exhibited some ability to extract quinoline (28) Jacquenmin, J.; Husson, P.; Padua, A. A. H.; Majer, V. Green Chem. 2006, 8, 172–180. (29) Blanchard, L. A.; Gu, Z.; Brennecke, J. F. J. Phys. Chem. B 2001, 105, 2437–2444.

Figure 1. FTIR spectra of (a) [EMIm]EtSO4, (b) [EMIm]EtSO4-ZnCl2 (molar ratio of IL/ZnCl2 ) 2), and (c) ZnCl2.

from the model oil, their performances are far from being commercialized. For the complete extraction of Lewis basic heterocyclic nitrogen compounds, the complexation with Lewis acidic metal halide, such as ZnCl2 and AlCl3, would be more desirable but the following recovery of the nitrogen compound and metal halide from the resulting metal complexes would be much more difficult. It is well-known that amines interact with Lewis acidic zinc halide to form stable bisamine zinc halide complexes, (amine)2ZnX2.30-33 Therefore, to facilitate the recovery of amines, the bond strength between amine and ZnX2 should be reduced. One way to weaken the bond strength would be the lowering of the Lewis acidity on the Zn center, by transforming zinc halide into zincate anion (ZnX42-, X ) halide) through a reaction with an imidazolium halide. It is reported that the reaction of zinc halide, ZnX2 (X ) Cl and Br) with 2 equiv of [BMIm]Cl produces an IL, [BMIm]2ZnCl2X2, which possesses both Lewis acidity and basicity.33 We hoped that these imidazolium zinc tetrahalides would exhibit high quinoline extraction ability from the model oil. However, contrary to our expectation, the addition of 20 wt % [EMIm]2ZnCl2Br2 did not improve the extraction ability of [BEIm]EtSO4. It is likely that the Zn center is fully occupied by strongly coordinating halide ligands, and thus, no vacant site is generated for the coordination of a nitrogen compound. This result may suggest that the Zncontaining IL should possess at least one labile ligand to provide a vacant site for the interaction with a nitrogen compound. In this regard, an attempt was made to replace two halide ions (X2) in [EMIm]2ZnCl2X2 by a weakly coordinating ligand or ligands such as EtSO4-. For this purpose, ZnCl2 was treated with 2 equiv of [EMIm]EtSO4 and the resulting viscous liquid was investigated by FTIR spectroscopy. As can be seen in Figure 1, the FTIR spectrum of the viscous liquid is quite different from those of ZnCl2 and [EMIm]EtSO4, indicating the formation of a new ionic species. The broad peak centered at 3103 cm-1, associated with the interaction of aromatic C(2)-H with EtSO4in [EMIm]EtSO4,34-37 shifted to a higher frequency at 3109 (30) Jain, B.; Singh, J. M.; Goyal, R. N.; Tandon, S. N. Can. J. Chem. 1980, 58, 1558–1561. (31) Wang, W.; Zhang, X.; Huang, D.; Zhu, H.; Chen, C.; Liu, Q. Acta Crystallogr., Sect. E: Struct. Rep. Online 2001, 57, m561–m563. (32) Mahan, R. I.; Bailey, J. R. J. Am. Chem. Soc. 1937, 59, 2449– 2450. (33) Palgunadi, J.; Kwon, O.-S.; Lee, H.; Bae, J. Y.; Ahn, B. S.; Min, N.-Y.; Kim, H. S. Catal. Today 2004, 98, 511–514. (34) Tait, S.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 4352–4360. (35) Dieter, K. M.; Dymek, C. J., Jr.; Heimer, N. E.; Rovang, J. W.; Wilkes, J. S. J. Am. Chem. Soc. 1988, 110, 2722–2726. (36) Shiraishi, Y.; Tachibana, K.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2001, 40, 4919–4924. (37) Kim, Y. J.; Varma, R. S. J. Org. Chem. 2005, 70, 7882–7891.

Zn-Containing Ionic Liquids

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Figure 2. FAB-MS spectra of [EMIM]EtSO4-ZnCl2 in negative-ion mode (A ) EtSO4-). Molar ratio of [EMIM]EtSO4/ZnCl2: (a) 1, (b) 1.5, (c) 2, and (d) 5.

cm-1 upon treating with ZnCl2. The SdO asymmetric stretching frequency centered at 1214 cm-1 also moved to a higher frequency at 1289 cm-1, implying that EtSO4- is bonded to ZnCl2. Characterization of Zn-Containing RTILs: FAB-Mass. To confirm the formation of the new Zn-containing IL, FAB-mass spectral analyses were carried out with ILs obtained from [EMIm]EtSO4 and ZnCl2 (molar ratio of [EMIm]EtSO4/ ZnCl2 ) 1-5). FAB-mass spectra in Figure 2 showed that the major anionic ZnII species observed were 1:1 and 2:1 adduct species, ZnCl2(EtSO4)- (MW ) 261) and ZnCl(EtSO4)2- (MW ) 349), irrespective of the molar ratio of [EMIm]EtSO4/ZnCl2. The absence of the 2:1 dianionic species, ZnCl2(EtSO4)22-, similar to ZnCl2Br22-, can be attributed to the chelating property of EtSO4- through oxygen atoms. Computational Studies on the Structures of Zn-Containing RTILs. The formations of [EMIm]ZnCl2(EtSO4) and [EMIm]ZnCl(EtSO4)2 were also supported by the theoretical investigation at the B3LYP level of the theory (6-31+G* for C, H, and O and LanL2DZ(ECP) for S, Cl, and Zn) using Gaussian 03.26 The optimized structures of [EMIm]ZnCl2(EtSO4) and [EMIm]ZnCl2(EtSO4) are shown in Figure 3. As can be deduced from the bidentate character of ethylsulfate, ZnCl2(EtSO4)shows a stable tetrahedral arrangement of ligands around Zn, with the Gibbs free energy of formation of -35.8 kcal/mol for the reaction between ZnCl2 and [EMIm]EtSO4. Coordination of one more ethylsulfate ligand with a concomitant loss of a strongly coordinating Cl- ligand seems to give a less stable square pyramidal complex (∆G ) -27.1 kcal/mol) for the reaction between ZnCl2 and 2[EMIm]EtSO4. The computational results on the formation of two major species, [EMIm]ZnCl2(EtSO4) and [EMIm]ZnCl(EtSO4)2, are in good agreement with those from the FAB-mass spectral analysis. Denitrogenation with Zn-Containing RTILs. The effect of ZnCl2 content in [EMIm]EtSO4 was evaluated for the denitrogenation of the model oil containing 5000 ppm of quinoline and 20 000 ppm of n-octane as an internal standard in n-heptane. ZnCl2 was completely dissolved first in [EMIm]EtSO4 at 60 °C with vigorous stirring to form Zn-containing ILs, [EMIm]EtSO4-ZnCl2. As shown in Table 2, the extraction

Figure 3. Optimized structures of the species formed from the interaction of ZnCl2 with ethylsulfate: (a) ZnCl2 + [EMIm]EtSO4 f [EMIm][ZnCl2(EtSO4)] (∆G ) -35.8 kcal/mol) and (b) ZnCl2 + 2[EMIm]EtSO4 f [EMIm]Cl + [EMIm][ZnCl(EtSO4)2] (∆G ) -27.1 kcal/mol). Table 2. Effect of the [EMIm]EtSO4/ZnCl2 Molar Ratio on the Extraction of Quinoline Present in the Model Oila molar ratio ([EMIm]EtSO4/ZnCl2)

DEb (%)

KNc

d 20 10 5 4 3 2 1 2e 2f ZnCl2

40.6 81.6 84.6 85.3 86.8 88.8 89.2 90.2 68.8 59.2 42.3

3.42 22.2 27.5 29.0 32.9 39.6 41.3 46.0 44.1 43.5 5.48

a The extraction of quinoline was conducted at room temperature with the model oil containing 5000 ppm of quinoline and 20 000 ppm of n-octane as an internal standard in n-heptane. The weight ratio of model oil/[EMIm]EtSO4 was set at 5. b DE ) degree of extraction (%). c Nernst partition coefficient ) [mg (N) g (IL)-1/mg (N) g (oil)-1]. d [EMIm]EtSO only. e Weight ratio of model oil/[EMIm]EtSO ) 20. 4 4 f Weight ratio of model oil/[EMIm]EtSO ) 30. 4

ability of [EMIm]EtSO4 expressed in the KN value increased from 8.1 to 46.0 when an equimolar amount of ZnCl2 was dissolved in [EMIm]EtSO4. Although the KN value decreased

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Table 3. Effect of the Alkyl Substituent on the Extraction Ability of [RR′Im]RSO4-ZnCl2a IL

DEb (%)

KNd

[HEIm]EtSO4-ZnCl2 [EMIm]EtSO4-ZnCl2 [BEIm]EtSO4-ZnCl2 [EMIm]MeSO4-ZnCl2 [BMIm]MeSO4-ZnCl2 [OMIm]MeSO4-ZnCl2 [EMIm]Et2PO4-ZnCl2 [BMIm]Bu2PO4-ZnCl2 [EMIm]EtPHO3-ZnCl2 EMImCl-ZnCl2d EMImCl-ZnCl2e

86.5 89.2 91.2 88.4 89.8 93.9 47.5 43.1 38.3 64.3 68.9

32.0 41.3 51.8 38.1 44.0 77.0 4.52 3.79 3.10 9.00 11.1

a The extraction of quinoline was conducted at room temperature with the model oil containing 5000 ppm of quinoline and 20 000 ppm of n-octane as an internal standard in n-heptane. The molar ratio of IL/ ZnCl2 and the weight ratio of model oil/IL were set at 2 and 5, respectively. b DE ) degree of extraction (%). c Nernst partition coefficient ) [mg (N) g (IL)-1/mg (N) g (oil)-1]. d [EMIm]Cl-ZnCl2 ([EMIm]Cl/ZnCl2 ) 1). e [EMIm]Cl-ZnCl2 ([EMIm]Cl/ZnCl2 ) 2).

Table 6. Recycling Study with [EMIm]EtSO4-ZnCl2a recycle number

DEb (%)

KNc

1 2 3 4 5 6 7 8

89.0 87.3 86.7 86.2 82.9 82.0 81.4 80.5

40.5 34.4 32.6 31.2 24.2 22.8 21.9 20.6

a The extraction of quinoline was conducted at room temperature with the model oil containing 5000 ppm of quinoline and 20 000 ppm of n-octane as an internal standard in n-heptane. The molar ratio of [EMIm]EtSO4/ZnCl2 and the weight ratio of model oil/IL were set at 2 and 5, respectively. Diethyl ether (3 g) was used as the back extractant to regenerate IL. b DE ) degree of extraction (%). c Nernst partition coefficient ) [mg (N) g (IL)-1/mg (N) g (oil)-1].

Scheme 2. Structures of Aromatic Nitrogen Compounds

Table 4. Denitrogenation of the Model Oil Containing Indole or Acridine by Various Types of Neat ILs and Zn-Containing ILsa indole

acridine

IL

DEb (%)

KNc

DE (%)

KN

[EMIm]EtSO4 [EMIm]EtSO4 [EMIm]EtSO4-ZnCl2 [EMIm]EtSO4-ZnCl2 [BEIm]EtSO4 [BEIm]EtSO4-ZnCl2 [DMIm]MeSO4 [DMIm]MeSO4-ZnCl2

98.7 94.5d 100 92.7d 100 100 97.8 98.7

380 344

38.7

3.16

84.7

27.7

40.2 86.5 33.5 83.8

3.36 32.0 2.52 25.9

253 222 380

a The extraction of indole or acridine was conducted at room temperature with the model oil containing 5000 ppm of indole or 5000 ppm of acridine and 20 000 ppm of n-octane as an internal standard in n-heptane. The weight ratio of model oil/IL and the molar ratio of neat IL/ZnCl2 were set at 5, respectively. b DE ) degree of extraction (%). c Nernst partition coefficient ) [mg (N) g (IL)-1/mg (N) g (oil)-1]. d Weight ratio of model oil/IL ) 20.

Table 5. Denitrogenation of the Model Oil Containing Quinoline, Acridine, and Indole with [EMIm]EtSO4 and [EMIm]EtSO4-ZnCl2a IL [EMIm]EtSO4 [EMIm]EtSO4-ZnCl2

N compound

DEb (%)

KNc

indole quinoline acridine indole quinoline acridine

98.9 40.8 39.2 100 85.8 84.2

450 3.45 3.22 30.2 26.6

a The extraction of quinoline, acridine, and indole was conducted at room temperature with the model oil containing 5000 ppm of nitrogen compounds (1:1:1) and 20 000 ppm of n-octane as an internal standard in n-heptane. The molar ratio of [EMIm]EtSO4/ZnCl2 and the weight ratio of model oil/IL were set at 5, respectively. b DE ) degree of extraction (%). c Nernst partition coefficient ) [mg (N) g (IL)-1/mg (N) g (oil)-1].

with the increase of the molar ratio of [EMIm]EtSO4/ZnCl2, the quinoline extraction was maintained above 80% in the molar ratio range of 1-20. In contrast to [EMIm]EtSO4-ZnCl2, powdered ZnCl2 only exhibited a much lower quinoline extraction ability, supporting the role of [EMIm]EtSO4 in dissolving

Figure 4. 1H NMR spectra of the IL ([EMIm]EtSO4-ZnCl2, molar ratio of [EMIm]EtSO4/ZnCl2 ) 2) in DMSO-d6 before and after regeneration: (a) [EMIm]EtSO4-ZnCl2, (b) model oil (5000 ppm of quinoline), (c) bottom IL layer after denitrogenation, (d) upper ether layer recovered after washing, and (e) bottom IL layer after washing with diethyl ether. Characteristic 1H NMR peaks: (*) IL, (2) quinoline, and (b) diethyl ether.

ZnCl2 and consequently forming an active species for the extraction of N compounds. The quinoline extraction was also conducted at much higher mass ratios of model/IL ) 20 and 30 (IL ) [EMIm]EtSO4-ZnCl2, [EMIm]EtSO4/ZnCl2 ) 2) to obtain more meaningful results in terms of an economic point of view. High KN values of 44.1 and 43.5 were maintained even at higher molar ratios of 20 and 30, respectively, suggesting that the Zn-containing RTILs could be used as promising N extractants for the practical application. The effect of the alkyl substituent on the imidazolium ring was also investigated for the removal of quinoline using various Zn-containing ILs, [1-R-3-R′-imidazolium]RSO4-ZnCl2 ([RR′Im]RSO4-ZnCl2), at the molar ratio of [1-R-3-R′-imidazolium]RSO4/ZnCl2 ) 2. It was expected that the presence of an acidic H (N-H) atom on the imidazolium ring could promote the interaction between [1-H-3-R-imidazolium]RSO4-ZnCl2 with quinoline through a hydrogen bond. However, the presence of the H atom was not helpful in improving the performance of ILs to extract quinoline. As listed in Table 3, the IL with an acidic H atom on the imidazolium ring, [ethylimidazolium]EtSO4-ZnCl2 ([HEIm]EtSO4-ZnCl2), exhibited a similar

Zn-Containing Ionic Liquids

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Figure 5. Optimized structures showing the interactions of [EMIm]ZnCl2(EtSO4) or [EMIm]EtSO4 with quinoline, diethyl ether, or indole to form (a) [EMIm]ZnCl2(EtSO4) + quinoline f [EMIm]Zn(quinoline)Cl2(EtSO4), (b) [EMIm]ZnCl2(EtSO4) + diethyl ether f [EMIm]Zn(Et2O)Cl2(EtSO4), (c) [EMIm]ZnCl2(EtSO4) + indole f [EMIm]Zn(indole)Cl2(EtSO4), and (d) [EMIm]EtSO4 + indole f [EMIm](indole)EtSO4. Scheme 3. Plausible Mechanism of the Reversible Quinoline Extraction ([EMIm] Moiety Was Omitted for Clarity)

extraction ability to [EMIm]EtSO4-ZnCl2. Moreover, the variation of the chain length of the alkyl group on the imidazolium ring did not exert any noticeable effect on the extraction ability, supporting the pivotal role of the Zn-containing anion.

For a comparison, the performances of other types of Zncontaining ILs, prepared from ZnCl2 and [EMIm]Cl, [EMIm]Et2PO4, or [EMIm]EtPHO3, were tested for the removal of quinoline from the model oil (see Table 3). However, the extraction ability of these ILs was significantly lower than that of the corresponding [EMIm]EtSO4-ZnCl2 ([EMIm]EtSO4/ ZnCl2 ) 2). The higher extraction ability of ZnCl2-[EMIm]EtSO4 seems to be attributed to the easier formation of the anionic species, such as [ZnCl2(EtSO4)]-, or the easier bonding mode change of the ligand, EtSO4-, from bidentate to monodentate to provide a vacant site for the coordination of quinoline. Zn-containing ILs were also tested for the extraction of indole and acridine (Scheme 2). As listed in Table 4, indole was completely extracted from the model oil containing 5000 ppm of indole in n-heptane when treated with [EMIm]EtSO4-ZnCl2 ([EMIm]EtSO4/ZnCl2 ) 5, weight of model oil/weight of ZnCl2-[EMIm]EtSO4 ) 5). The Zn-containing IL, [EMIm]EtSO4-ZnCl2, was also effective for the denitrogenation of the model oil containing 5000 ppm of acridine, but the degree of acridine extraction was lower than those of indole and quinoline extractions. To find the ease of extraction for the nitrogen compounds, [EMIm]EtSO4-ZnCl2 was mixed with the model oil containing three nitrogen compounds: quinoline, indole, and acridine. As expected from the extraction of a single N compound, the degree of extraction was found in the following order: indole > quinoline > acridine (see Table 5). The same trend was also observed for the extraction with a neat IL, [EMIm]EtSO4, suggesting that the interaction of IL with a N compound is affected by the presence of the N-H bond

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and also by the steric crowding around the nitrogen atom or the basicity of the nitrogen compound.38 As shown in Tables 4 and 5, over 99% of indole was removed from the model oil using the neat IL only at the molar ratio of oil/IL ) 5, most likely because of the strong interaction between the H atom of N-H and the basic anion of IL. Such a high indole extraction from fuel oil was previously observed by Wasserscheid et al. using [BMIm]OcSO4.7 The extraction of indole was also conducted at the higher mass ratio of oil/IL ) 20 to see more clearly the effect of ZnCl2. As shown in Table 4, [EMIm]EtSO4 exhibited a slightly higher extraction ability than [EMIm]EtSO4-ZnCl2. This result is a strong indication that the indole extraction by an IL is mostly governed by the interaction of N-H of indole with EtSO4-. The effect of the interaction between indole and the Zn center through the N atom seems to be negligible. Regeneration of RTIL and Back Extraction of Trapped Quinoline. The regeneration of [EMIm]EtSO4-ZnCl2 ([EMIm]EtSO4/ZnCl2 ) 2) used for the extraction of quinoline was performed using diethyl ether as a back extractant. As listed in Table 6, the Zn-containing IL, [EMIm]EtSO4-ZnCl2, was shown to be recyclable up to 8 cycles without a significant loss of initial performance, demonstrating that diethyl ether is highly efficient in the regeneration of IL and in the recovery of trapped quinoline out of the IL layer. The excellent performance of diethyl ether as a back extractant was further verified by the 1H NMR experiments. Panels a and b of Figure 4 are the 1H NMR spectra of [EMIm]EtSO4-ZnCl2 and the model oil, respectively. The 1H NMR spectrum in Figure 4c clearly shows that quinoline was extracted from the model oil to the bottom IL layer after denitrogenation. Quinoline trapped in the bottom IL layer was successfully removed from the bottom IL layer using diethyl ether as a back extractant. The 1H NMR spectrum of the diethyl ether layer after evaporation in Figure 4d reveals that quinoline trapped in the IL layer was transferred to the diethyl ether layer. The characteristic peak of the IL at 9.176 ppm was not observed in the diethyl ether layer. Figure 4e is the 1H NMR spectrum of the IL layer after back extraction with diethyl ether. It is noteworthy that the peaks corresponding to diethyl ether can be seen in the spectrum of the bottom IL layer, suggesting that diethyl ether is coordinated to the Zn center in place of quinoline after the back extraction. This could be rationalized by considering the stronger affinity of Zn to the oxygen atom than to the nitrogen atom. The coordinated and/or free diethyl ether present in the IL layer can be easily removed under vacuum (spectrum is not shown here). Theoretical Consideration of the Extraction with RTILs. The interactions of ZnCl2(EtSO4)- and EtSO4- with quinoline and indole were theoretically investigated at the B3LYP level of theory using Gaussian 03.26 As shown in Figure 5a, there is a substantial interaction between [EMIm]ZnCl2(EtSO4) and quinoline, and the interaction enthalpy (∆H) was calculated as -5.7 kcal/mol. A similar interaction can also be seen between [EMIm]ZnCl2(EtSO4) and diethyl ether (see Figure 5b). The calculated interaction enthalpy of -12.7 kcal/mol is larger by 1.8 kcal/mol than that between [EMIm]ZnCl2(EtSO4) and quinoline. This is probably the reason why diethyl ether is capable of back extracting trapped quinoline in the IL layer. Both of the structures in panels a and b of Figure 5 show that the coordination of a Lewis base, such as quinoline or diethyl (38) Harper, J. B. Compr. Heterocycl. Chem. III 2008, 7, 1–40.

Huh et al.

ether, changes the binding mode of the ethylsulfate in ZnCl2(EtSO4)- from bidentate to monodentate, thereby retaining a tetrahedral environment around Zn. In contrast, as shown in panels c and d of Figure 5, indole was found to interact more strongly with an ethylsulfate anion via hydrogen bonding than with the Zn atom in ZnCl2(EtSO4)-. The enthalpy for the interactions of indole with [EMIm]EtSO4 and [EMIm]ZnCl2EtSO4 was calculated as -7.6 and -4.3 kcal/mol, respectively, suggesting that indole extraction is mainly influenced by the interaction between the H atom of N-H and EtSO4- rather than that between the N atom and Zn center. From the experimental, spectroscopic, and computational results, the plausible extraction mechanism of the isomerization is suggested as in Scheme 3. The active species 1 is likely to form by the reaction of [EMIm]EtSO4 and ZnCl2. The interaction of 1 with quinoline would produce quinoline-coordinated species 2, which in turn transforms into diethyl-ether-coordinated species 3 upon treated with diethyl ether, a back extractant. The coordinated ether can be removed under vacuum to regenerate the active species 1. Conclusions Zn-containing ILs, prepared from the interaction of ZnCl2 with imidazolium-based IL bearing a alkylsulfate anion, were proven to be effective extractants for the denitrogenation of a model oil containing quinoline, acridine, and/or indole. In particular, the performance of dialkylimidazolium alkyl sulfate RTIL for the extraction of basic nitrogen compounds, such as quinoline and acridine, was significantly improved up to more than 2 times by the co-presence of Lewis acidic ZnCl2. Diethyl ether was found to be an efficient back extractant for the regeneration of [EMIm]EtSO4-ZnCl2, used for the denitrogenation of quinoline from the model oil, and to recover trapped quinoline in the IL. The quinoline extraction ability of [EMIm]EtSO4-ZnCl2 was maintained up to 8 cycles without a significant loss of the initial performance using diethyl ether as the back extractant. Computational studies show that active Zncontaining anionic species, such as [EMIm]ZnCl2(EtSO4) and [EMIm]ZnCl(EtSO4)2, can be generated from the interaction of ZnCl2 with [EMIm]EtSO4, and thus, the extraction of quinoline can be facilitated through the coordination of quinoline to the Zn center. The bonding mode of ethylsulfate ligand in ZnCl2(EtSO4)- is changed from bidentate to monodentate for the coordination of quinoline, thereby retaining a tetrahedral environment around Zn. The larger calculated interaction enthalpy for the coordination diethyl ether suggests that the coordinated quinole can be replaced by diethyl ether. The formation of anionic Zn species, ZnCl2(EtSO4)- (MW ) 261) and ZnCl(MeSO4)2- (MW ) 349), was supported by FAB-MS spectroscopic results. In the case of the denitrogenation of a neutral nitrogen compound, such as indole, the extraction by an IL seems to be mostly governed by the interaction between the anion of IL and the H atom of N-H rather than by the coordination of indole to the Zn center through the N atom, as supported by computational calculation. Acknowledgment. This work has been performed as an Energy Technology Innovation (ETI) Project under the Energy Resources Technology Development program. EF900073A