Zn-Containing Ionic Liquids for the Extractive Denitrogenation of aModel 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 HeeUniVersity, 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 andR ) H or alkyl), were highly effective for the denitrogenation of a model oil containing quinoline, indole, oracridine in n-heptane. Fast atom bombardment (FAB)-mass spectra and a computational study imply that theinteraction of 1-ethyl-3-methylimidazolium ethylsulfate ([EMIm]EtSO4) with ZnCl2 produces Zn-containingILs, presumably [EMIm]ZnCl2(EtSO4) and [EMIm]ZnCl(EtSO4)2 as the major ionic species. The interactionof EtSO4- and ZnCl2(EtSO4)- with a heterocyclic N compound was theoretically investigated. The zinc-containing IL, [EMIm]ZnCl2(EtSO4), used for the extraction of quinoline was successfully regenerated byemploying diethyl ether as a back extractant.

Introduction

In recent years, desulfurization and denitrogenation oftransportation fuels have attracted increasing interest becauseof the stringent regulation on the environmental pollution causedby exhaust gases, such as SOx and NOx, to the atmosphere. Toprotect the environment against contamination, many developedcountries, including countries in Europe and Japan, are enforcingthe regulation to reduce the allowable sulfur level in diesel fueldown to 10 parts per million by weight (ppmw) by 2010.1-3

Industrially, the removal of organosulfur and organonitrogencompounds in fuel oils is being carried out by means of asimultaneous hydrodesulfurization (HDS) and hydrodenitroge-nation (HDN) process at around 350 C using catalysts basedon CoMo or NiMo, which involves the C-S and C-N bondcleavage to produce H2S and NH3, respectively.4-7 However,current hydrotreating catalysts used for this purpose are not veryeffective for the removal of aromatic sulfur compounds, suchas benzothiophenes (BTs) and dibenzothiophenes (DBTs), andaromatic nitrogen compounds. Furthermore, a HDS catalyst is

easily deactivated by small amounts of aromatic nitrogencompounds present in the fuel, which is preferably adsorbedon the surfaces of the catalyst. Although the allowable nitrogencontent is not strictly specified,2,7 the development of newapproaches to drastically reduce the nitrogen content in trans-portation fuel oils presumably below 10 ppmw is urgentlydemanded to meet the need of ultra-clean fuel for environmentalprotection.

Several alternative processes, including extraction,8,9 mem-brane,10 and selective oxidation,11 were proposed for the removalof nitrogen compounds, which can be classified into two groups:basic (pyridine, quinoline, and acridine) and neutral (indole andcarbazole). Adsorption by ion-exchange resins12 and liquid-liquidextraction with carboxylic acids8 as well as ionic liquids(ILs)13,14 have been employed to remove basic nitrogen com-pounds. A much more restricted set of methods includingadsorption2,16 and extraction with highly polar organic solvents15

and ILs13,14 has been reported for the removal of neutral nitrogencompounds. Among these, denitrogenation based on solventextraction has been most extensively studied because of its facileoperation, less energy consumption, and the retention of the

* To whom correspondence should be addressed. Telephone: +82-2-961-0432 (H.S.K.); +82-2-961-0239 (M.C.); +82-2-958-5854 (B.S.A.). Fax:+82-2-965-4408. E-mail: khs2004@khu.ac.kr (H.S.K.); mcheong@khu.ac.kr(M.C.); bsahn@kist.re.kr (B.S.A.).

Department of Chemistry and Research Institute of Basic Sciences,Kyung Hee University.

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Energy & Fuels 2009, 23, 303230383032

10.1021/ef900073a CCC: $40.75 2009 American Chemical SocietyPublished on Web 05/06/2009

chemical structures of the fuels. Considering the energyconsumption, ILs seem to be more attractive than organicsolvents17 because of their negligible volatility, immiscibilitywith fuel oils, and higher affinity to sulfur- and nitrogen-containing compounds, and therefore, target compounds can beeasily removed from the fuel oils through a simple layerseparation.18-20 Several papers were published describing theuse of imidazolium-based ILs as extractants in the denitroge-nation process,13,18,19 but the interaction between ILs and Ncompounds was never a subject of detailed investigation.

Recently, Lewis acidic ILs bearing metal halide anions basedon AlCl3,19,20 FeCl3,21 and CuCl22 were shown to exhibitpromising performances on the selective removal of aromaticsulfur compounds. Being motivated by these results, we havetested metal-containing Lewis acidic ILs as extractants for theselective removal of aromatic nitrogen compounds from hy-drocarbon mixtures. During the course of our study on thedenitrogenation of fuel oils, we have found that Zn-containingimidazolium-based ILs have strong affinities to aromaticnitrogen compounds.

We now report on the use of Zn-containing imidazolium-based ILs for the extraction of nitrogen compounds present inhydrocarbon mixtures at an ambient temperature as well as onthe theoretical investigation on the interaction between ILs andheterocyclic N compounds.

Experimental Section

General. All of the chemicals used for the synthesis of RTILswere purchased from Aldrich Chemicals Co. and used as received.1H nuclear magnetic resonance (NMR) spectra were recorded on a400 MHz Bruker NMR spectrometer using DMSO-d6 as the solvent.Fourier transform infrared (FTIR) spectra were recorded in an inertatmosphere on a Nicolet 380 spectrophotometer equipped with asmart MIRacle ATR accessory (Thermo Electron Co.). Fast atombombardment (FAB)-mass spectra for the characterization of Zn-containing ILs were recorded with a JMS-700 Mstation double-focusing mass spectrometer (JEOL, Tokyo, Japan) using a MS-MP9020D data system. The ion source was operated at 10 kVaccelerating voltage, with a mass resolution of 1500 (10% valley).Fast atoms were produced by FAB using a xenon atom gunoperating at 6 keV. Samples were dissolved in methanol and mixedwith 1 L of 3-nitrobenzyl alcohol (NBA, Sigma, St. Louis, MO)on a FAB probe tip. Calibration was performed with an Ultramark1621 (PCR, Gainesville, FL) in the positive-ion mode as a standardcompound. Analysis of the upper heptane layers was conductedusing an Agilent 6890 gas chromatograph equipped with a flame-ionized detector and a DB-wax capillary column (30 m 0.32mm 0.25 m), and an Agilent 6890-5973 GC-MSD massspectrometer equipped with a HP-5MS capillary column (30 m 0.32 mm 0.5 m).

Synthesis of Room Temperature Ionic Liquids (RTILs).RTILs, 1-alkyl-3-methylimidazolium alkylsulfate,23 1-alkyl-3-methylimidazolium alkylphosphite,24 and 1-alkyl-3-methylimida-

zolium dialkylphosphate,25 were prepared using the methodsdescribed in the literature. RTILs containing ZnCl2 were preparedby dissolving anhydrous ZnCl2 in a RTIL at 60 C with vigorousstirring. The weight ratio of RTIL/ZnCl2 was varied from 1 to 20.All of the Zn-containing ILs obtained were liquids at roomtemperature 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 theoreti-cally investigated using Gaussian 03.26 The geometry optimizationsand thermodynamic corrections were performed with a hybridBecke 3-Lee-Yang-Parr (B3LYP) exchange-correlation functionalwith 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 thecomplexes, all kinds of possible interaction patterns were optimized,giving rise to the most stable final geometries. No restrictions onsymmetries were imposed on the initial structures. All stationarypoints were verified as minima by full calculation of the Hessianand a harmonic frequency analysis.

Denitrogenation Experiments. Unless otherwise stated, all ofthe denitrogenation experiments were conducted under a nitrogenatmosphere. A typical denitrogenation experiment is as follows:In a 50 mL Schlenk tube, an IL (1 g) was mixed with 5 g of modeloil containing 5000 ppm of quinoline, acridine, or indole and 20 000ppm of n-octane as an internal standard in n-heptane. The resultingmixture was stirred for 20 min and then allowed to stand for 10min at room temperature. After the extraction was completed, theupper heptane layer was analyzed by GC and GC-mass. Thebottom IL layer was characterized by 1H NMR.

Back Extraction and Regeneration of RTIL. Recycle experi-ments were performed using [EMIm]EtSO4-ZnCl2 (1 g, [EMIm]-EtSO4/ZnCl2 ) 2) as an extractant and diethyl ether as a backextractant. In a 25 mL Schlenk tube, 5 g of model oil containing5000 ppm of quinoline and 20 000 ppm of n-octane as an internalstandard in n-heptane was mixed with 1 g of IL with vigorousstirring for 10 min and then allowed to stand for 5 min. After theextraction, the upper layer was removed by layer separation andthe remaining IL layer was washed with 3 g of diethyl ether withvigorous shaking for 5 min, followed by standing for 5 min. Theupper diethyl ether layer was then separated, evaporated, andanalyzed by 1H NMR spectroscopy. The bottom IL layer wasevaporated under a vacuum to remove diethyl ether and used forfurther extraction with a fresh charge of the model oil.

Results and Discussion

Denitrogenation with Neat RTILs. The denitrogenation ofthe model oil containing 5000 ppm of quinoline and 20 000ppm of n-octane as an internal standard in n-heptane wasconducted at room temperature using RTILs shown in Scheme1. The Nernst partition coefficient KN,13,27 defined as the massratio of the nitrogen content in a RTIL to the nitrogen contentin the model oil [mg (N) g (IL)-1/mg (N) g (oil)-1], wasmeasured for each extraction. As listed in Table 1, all of the

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Scheme 1. Structures of ILs Tested for the Denitrogenation ofthe Model Oil

Zn-Containing Ionic Liquids Energy & Fuels, Vol. 23, 2009 3033

RTILs showed KN values in the range of 2.8-6.7, which areequivalent to 36-58% quinoline extraction. The quinolineextraction ability of IL or the KN value increased with anincreasing chain length of the alkyl group on the cation andanion for the same series of RTILs. For methylsulfate series,the KN value was found in the order: 1,3-dimethylimidazo-lium methylsulfate ([DMIm]MeSO4) < 1-ethyl-3-methylimida-zolium methylsulfate ([EMIm]MeSO4) < 1-butyl-3-methylimi-dazolium methylsulfate ([BMIm]MeSO4) < 1-octyl-3-methylim-idazolium methylsulfate ([OMIm]MeSO4). The reason for thisis not clear at the moment, but the extraction ability of IL seemsto be related to the free volume in an IL because the molarvolume increases going from [DMIm]MeSO4 to [OMIm]-MeSO4. The interrelation between the length of the alkyl chainof the IL and free volume was well-described in the literature.28,29

On the contrary, the extraction ability of RTIL was notaffected by the type of anion, as long as the RTIL possessesthe equivalent number of the same alkyl groups: the degree ofextraction with [EMIm]EtSO4 and 1-ethyl-3-methylimidazoliumethylphosphite ([EMIm]EtPHO3) was found to be 40.6 (KN )3.42) and 40.7% (KN ) 3.43), respectively. However, whenphosphate and phosphite series ILs were compared, phosphateseries ILs showed better extraction ability than the phosphiteseries ILs. For example, the extraction abilities of 1-ethyl-3-methylimidazolium diethylphosphate ([EMIm]Et2PO4) and[EMIm]EtPHO3 were determined as 54.0 (KN ) 5.89) and40.7% (KN ) 3.43), respectively. The presence of one moreethyl group in [EMIm]Et2PO4 than in [EMIm]EtPHO3 seemsto provide a larger free volume in the IL, thereby resulting inhigher extraction ability. On the other hand, stronger basicityof phosphite anion is likely to result in a stronger interactionbetween the imidazolium cation and phosphite anion, therebyreducing the interaction of quinoline with the C(2)-H of theimidazolium cation.

As expected from the physical state, [EMIm]Cl, existing asa solid at room temperature, showed a greatly reduced extractionability compared to sulfate, phosphite, and phosphate seriesRTILs.

Characterization of Zn-Containing RTILs: FTIR. Eventhough the neat RTILs exhibited some ability to extract quinoline

from the model oil, their performances are far from beingcommercialized. For the complete extraction of Lewis basicheterocyclic nitrogen compounds, the complexation with Lewisacidic metal halide, such as ZnCl2 and AlCl3, would be moredesirable but the following recovery of the nitrogen compoundand metal halide from the resulting metal complexes would bemuch more difficult. It is well-known that amines interact withLewis acidic zinc halide to form stable bisamine zinc halidecomplexes, (amine)2ZnX2.30-33 Therefore, to facilitate therecovery of amines, the bond strength between amine and ZnX2should be reduced. One way to weaken the bond strength wouldbe the lowering of the Lewis acidity on the Zn center, bytransforming zinc halide into zincate anion (ZnX42-, X ) halide)through a reaction with an imidazolium halide. It is reportedthat the reaction of zinc halide, ZnX2 (X ) Cl and Br) with 2equiv of [BMIm]Cl produces an IL, [BMIm]2ZnCl2X2, whichpossesses both Lewis acidity and basicity.33 We hoped that theseimidazolium zinc tetrahalides would exhibit high quinolineextraction ability from the model oil. However, contrary to ourexpectation, the addition of 20 wt % [EMIm]2ZnCl2Br2 did notimprove the extraction ability of [BEIm]EtSO4. It is likely thatthe Zn center is fully occupied by strongly coordinating halideligands, and thus, no vacant site is generated for the coordinationof a nitrogen compound. This result may suggest that the Zn-containing IL should possess at least one labile ligand to providea vacant site for the interaction with a nitrogen compound. Inthis regard, an attempt was made to replace two halide ions(X2) in [EMIm]2ZnCl2X2 by a weakly coordinating ligand orligands such as EtSO4-. For this purpose, ZnCl2 was treatedwith 2 equiv of [EMIm]EtSO4 and the resulting viscous liquidwas investigated by FTIR spectroscopy. As can be seen in Figure1, the FTIR spectrum of the viscous liquid is quite differentfrom those of ZnCl2 and [EMIm]EtSO4, indicating the formationof a new ionic species. The broad peak centered at 3103 cm-1,associated with the interaction of aromatic C(2)-H with EtSO4-in [EMIm]EtSO4,34-37 shifted to a higher frequency at 3109

(28) Jacquenmin, J.; Husson, P.; Padua, A. A. H.; Majer, V. Green Chem.2006, 8, 172180.

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(31) Wang, W.; Zhang, X.; Huang, D.; Zhu, H.; Chen, C.; Liu, Q. ActaCrystallogr., Sect. E: Struct. Rep. Online 2001, 57, m561m563.

(32) Mahan, R. I.; Bailey, J. R. J. Am. Chem. Soc. 1937, 59, 24492450.

(33) Palgunadi, J.; Kwon, O.-S.; Lee, H.; Bae, J. Y.; Ahn, B. S.; Min,N.-Y.; Kim, H. S. Catal. Today 2004, 98, 511514.

(34) Tait, S.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 43524360.(35) Dieter, K. M.; Dymek, C. J., Jr.; Heimer, N. E.; Rovang, J. W.;

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Table 1. Denitrogenation of the Model Oil Containing QuinolineUsing Various Types of ILsa

IL DEb (%) KNc

[DMIm]MeSO4 36.2 2.84[EMIm]MeSO4 47.9 4.60[BMIm]MeSO4 52.7 5.57[OMIm]MeSO4 57.3 6.71[HEIm]EtSO4 40.8 3.45[EMIm]EtSO4 40.6 3.42[BEIm]EtSO4 55.3 6.19[EMIm]EtPHO3 40.7 3.43[BMIm]BuPHO3 45.7 4.21[EMIm]Et2PO4 54.0 5.89[BMIm]Bu2PO4 52.7 5.57[EMIm]Cl 29.0 2.04[EMIm]2ZnCl2Br2d 49.3 4.86

a The extraction of quinoline was conducted at room temperature withthe model oil containing 5000 ppm of quinoline and 20 000 ppm ofn-octane as an internal standard in n-heptane. The weight ratio of modeloil/IL was set at 5. b DE ) degree of extraction (%). c Nernst partitioncoefficient ) [mg (N) g (IL)-1/mg (N) g (oil)-1]. d A total of 20 wt %in [BEIm]EtSO4.

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

3034 Energy & Fuels, Vol. 23, 2009 Huh et al.

cm-1 upon treating with ZnCl2. The SdO asymmetric stretchingfrequency centered at 1214 cm-1 also moved to a higherfrequency at 1289 cm-1, implying that EtSO4- is bonded toZnCl2.

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 obtainedfrom [EMIm]EtSO4 and ZnCl2 (molar ratio of [EMIm]EtSO4/ZnCl2 ) 1-5). FAB-mass spectra in Figure 2 showed that themajor anionic ZnII species observed were 1:1 and 2:1 adductspecies, 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 propertyof EtSO4- through oxygen atoms.

Computational Studies on the Structures of Zn-ContainingRTILs. The formations of [EMIm]ZnCl2(EtSO4) and [EMIm]-ZnCl(EtSO4)2 were also supported by the theoretical investiga-tion at the B3LYP level of the theory (6-31+G* for C, H, andO 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 deducedfrom 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 forthe reaction between ZnCl2 and [EMIm]EtSO4. Coordinationof one more ethylsulfate ligand with a concomitant loss of astrongly coordinating Cl- ligand seems to give a less stablesquare pyramidal complex (G ) -27.1 kcal/mol) for thereaction between ZnCl2 and 2[EMIm]EtSO4. The computationalresults on the formation of two major species, [EMIm]-ZnCl2(EtSO4) and [EMIm]ZnCl(EtSO4)2, are in good agreementwith those from the FAB-mass spectral analysis.

Denitrogenation with Zn-Containing RTILs. The effect ofZnCl2 content in [EMIm]EtSO4 was evaluated for the denitro-genation of the model oil containing 5000 ppm of quinolineand 20 000 ppm of n-octane as an internal standard in n-heptane.ZnCl2 was completely dissolved first in [EMIm]EtSO4 at 60C with vigorous stirring to form Zn-containing ILs,[EMIm]EtSO4-ZnCl2. As shown in Table 2, the extraction

ability of [EMIm]EtSO4 expressed in the KN value increasedfrom 8.1 to 46.0 when an equimolar amount of ZnCl2 wasdissolved in [EMIm]EtSO4. Although the KN value decreased

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

Figure 3. Optimized structures of the species formed from theinteraction 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 ) -27.1kcal/mol).

Table 2. Effect of the [EMIm]EtSO4/ZnCl2 Molar Ratio on theExtraction of Quinoline Present in the Model Oila

molar ratio ([EMIm]EtSO4/ZnCl2) DEb (%) KNc

d 40.6 3.4220 81.6 22.210 84.6 27.55 85.3 29.04 86.8 32.93 88.8 39.62 89.2 41.31 90.2 46.02e 68.8 44.12f 59.2 43.5ZnCl2 42.3 5.48

a The extraction of quinoline was conducted at room temperature withthe model oil containing 5000 ppm of quinoline and 20 000 ppm ofn-octane as an internal standard in n-heptane. The weight ratio of modeloil/[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]EtSO4 only. e Weight ratio of model oil/[EMIm]EtSO4 ) 20.f Weight ratio of model oil/[EMIm]EtSO4 ) 30.

Zn-Containing Ionic Liquids Energy & Fuels, Vol. 23, 2009 3035

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

ZnCl2 and consequently forming an active species for theextraction of N compounds. The quinoline extraction was alsoconducted at much higher mass ratios of model/IL ) 20 and30 (IL ) [EMIm]EtSO4-ZnCl2, [EMIm]EtSO4/ZnCl2 ) 2) toobtain more meaningful results in terms of an economic pointof view. High KN values of 44.1 and 43.5 were maintained evenat higher molar ratios of 20 and 30, respectively, suggestingthat the Zn-containing RTILs could be used as promising Nextractants for the practical application.

The effect of the alkyl substituent on the imidazolium ringwas also investigated for the removal of quinoline using variousZn-containing ILs, [1-R-3-R-imidazolium]RSO4-ZnCl2([RRIm]RSO4-ZnCl2), at the molar ratio of [1-R-3-R-imida-zolium]RSO4/ZnCl2 ) 2. It was expected that the presence ofan acidic H (N-H) atom on the imidazolium ring could promotethe interaction between [1-H-3-R-imidazolium]RSO4-ZnCl2with quinoline through a hydrogen bond. However, the presenceof the H atom was not helpful in improving the performance ofILs to extract quinoline. As listed in Table 3, the IL with anacidic H atom on the imidazolium ring, [ethylimidazo-lium]EtSO4-ZnCl2 ([HEIm]EtSO4-ZnCl2), exhibited a similar

Table 3. Effect of the Alkyl Substituent on the ExtractionAbility of [RRIm]RSO4-ZnCl2a

IL DEb (%) KNd

[HEIm]EtSO4-ZnCl2 86.5 32.0[EMIm]EtSO4-ZnCl2 89.2 41.3[BEIm]EtSO4-ZnCl2 91.2 51.8[EMIm]MeSO4-ZnCl2 88.4 38.1[BMIm]MeSO4-ZnCl2 89.8 44.0[OMIm]MeSO4-ZnCl2 93.9 77.0[EMIm]Et2PO4-ZnCl2 47.5 4.52[BMIm]Bu2PO4-ZnCl2 43.1 3.79[EMIm]EtPHO3-ZnCl2 38.3 3.10EMImCl-ZnCl2d 64.3 9.00EMImCl-ZnCl2e 68.9 11.1

a The extraction of quinoline was conducted at room temperature withthe model oil containing 5000 ppm of quinoline and 20 000 ppm ofn-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 partitioncoefficient ) [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).

Scheme 2. Structures of Aromatic Nitrogen Compounds

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

indole acridine

IL DEb (%) KNc DE (%) KN

[EMIm]EtSO4 98.7 380 38.7 3.16[EMIm]EtSO4 94.5d 344[EMIm]EtSO4-ZnCl2 100 84.7 27.7[EMIm]EtSO4-ZnCl2 92.7d 253[BEIm]EtSO4 100 40.2 3.36[BEIm]EtSO4-ZnCl2 100 86.5 32.0[DMIm]MeSO4 97.8 222 33.5 2.52[DMIm]MeSO4-ZnCl2 98.7 380 83.8 25.9

a The extraction of indole or acridine was conducted at roomtemperature with the model oil containing 5000 ppm of indole or 5000ppm of acridine and 20 000 ppm of n-octane as an internal standard inn-heptane. The weight ratio of model oil/IL and the molar ratio of neatIL/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 ContainingQuinoline, Acridine, and Indole with [EMIm]EtSO4 and

[EMIm]EtSO4-ZnCl2a

IL N compound DEb (%) KNc

[EMIm]EtSO4 indole 98.9 450quinoline 40.8 3.45acridine 39.2 3.22

[EMIm]EtSO4-ZnCl2 indole 100quinoline 85.8 30.2acridine 84.2 26.6

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

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

recycle number DEb (%) KNc

1 89.0 40.52 87.3 34.43 86.7 32.64 86.2 31.25 82.9 24.26 82.0 22.87 81.4 21.98 80.5 20.6

a The extraction of quinoline was conducted at room temperature withthe model oil containing 5000 ppm of quinoline and 20 000 ppm ofn-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 2and 5, respectively. Diethyl ether (3 g) was used as the back extractantto regenerate IL. b DE ) degree of extraction (%). c Nernst partitioncoefficient ) [mg (N) g (IL)-1/mg (N) g (oil)-1].

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

3036 Energy & Fuels, Vol. 23, 2009 Huh et al.

extraction ability to [EMIm]EtSO4-ZnCl2. Moreover, the varia-tion of the chain length of the alkyl group on the imidazoliumring 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 Zn-containing ILs, prepared from ZnCl2 and [EMIm]Cl, [EMIm]-Et2PO4, or [EMIm]EtPHO3, were tested for the removal ofquinoline from the model oil (see Table 3). However, theextraction ability of these ILs was significantly lower than thatof 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 theanionic species, such as [ZnCl2(EtSO4)]-, or the easier bondingmode change of the ligand, EtSO4-, from bidentate to mono-dentate to provide a vacant site for the coordination of quinoline.

Zn-containing ILs were also tested for the extraction of indoleand acridine (Scheme 2). As listed in Table 4, indole wascompletely extracted from the model oil containing 5000 ppmof indole in n-heptane when treated with [EMIm]EtSO4-ZnCl2([EMIm]EtSO4/ZnCl2 ) 5, weight of model oil/weight ofZnCl2-[EMIm]EtSO4 ) 5). The Zn-containing IL, [EMIm]-EtSO4-ZnCl2, was also effective for the denitrogenation of themodel oil containing 5000 ppm of acridine, but the degree ofacridine extraction was lower than those of indole and quinolineextractions. To find the ease of extraction for the nitrogencompounds, [EMIm]EtSO4-ZnCl2 was mixed with the modeloil containing three nitrogen compounds: quinoline, indole, andacridine. As expected from the extraction of a single Ncompound, the degree of extraction was found in the followingorder: indole > quinoline > acridine (see Table 5).

The same trend was also observed for the extraction with aneat IL, [EMIm]EtSO4, suggesting that the interaction of IL witha N compound is affected by the presence of the N-H bond

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 QuinolineExtraction ([EMIm] Moiety Was Omitted for Clarity)

Zn-Containing Ionic Liquids Energy & Fuels, Vol. 23, 2009 3037

and also by the steric crowding around the nitrogen atom orthe basicity of the nitrogen compound.38 As shown in Tables 4and 5, over 99% of indole was removed from the model oilusing the neat IL only at the molar ratio of oil/IL ) 5, mostlikely because of the strong interaction between the H atom ofN-H and the basic anion of IL. Such a high indole extractionfrom fuel oil was previously observed by Wasserscheid et al.using [BMIm]OcSO4.7 The extraction of indole was alsoconducted at the higher mass ratio of oil/IL ) 20 to see moreclearly the effect of ZnCl2. As shown in Table 4, [EMIm]EtSO4exhibited a slightly higher extraction ability than [EMIm]-EtSO4-ZnCl2. This result is a strong indication that the indoleextraction by an IL is mostly governed by the interaction ofN-H of indole with EtSO4-. The effect of the interactionbetween indole and the Zn center through the N atom seems tobe negligible.

Regeneration of RTIL and Back Extraction of TrappedQuinoline. The regeneration of [EMIm]EtSO4-ZnCl2 ([EMIm]-EtSO4/ZnCl2 ) 2) used for the extraction of quinoline wasperformed using diethyl ether as a back extractant. As listed inTable 6, the Zn-containing IL, [EMIm]EtSO4-ZnCl2, wasshown to be recyclable up to 8 cycles without a significant lossof initial performance, demonstrating that diethyl ether is highlyefficient in the regeneration of IL and in the recovery of trappedquinoline out of the IL layer. The excellent performance ofdiethyl ether as a back extractant was further verified by the 1HNMR experiments. Panels a and b of Figure 4 are the 1H NMRspectra of [EMIm]EtSO4-ZnCl2 and the model oil, respectively.The 1H NMR spectrum in Figure 4c clearly shows that quinolinewas extracted from the model oil to the bottom IL layer afterdenitrogenation. Quinoline trapped in the bottom IL layer wassuccessfully removed from the bottom IL layer using diethylether as a back extractant. The 1H NMR spectrum of the diethylether layer after evaporation in Figure 4d reveals that quinolinetrapped in the IL layer was transferred to the diethyl ether layer.The characteristic peak of the IL at 9.176 ppm was not observedin the diethyl ether layer. Figure 4e is the 1H NMR spectrumof the IL layer after back extraction with diethyl ether. It isnoteworthy that the peaks corresponding to diethyl ether canbe seen in the spectrum of the bottom IL layer, suggesting thatdiethyl ether is coordinated to the Zn center in place of quinolineafter the back extraction. This could be rationalized by consider-ing the stronger affinity of Zn to the oxygen atom than to thenitrogen atom. The coordinated and/or free diethyl ether presentin the IL layer can be easily removed under vacuum (spectrumis not shown here).

Theoretical Consideration of the Extraction with RTILs. Theinteractions of ZnCl2(EtSO4)- and EtSO4- with quinoline andindole were theoretically investigated at the B3LYP level oftheory using Gaussian 03.26 As shown in Figure 5a, there is asubstantial interaction between [EMIm]ZnCl2(EtSO4) and quino-line, and the interaction enthalpy (H) was calculated as -5.7kcal/mol. A similar interaction can also be seen between[EMIm]ZnCl2(EtSO4) and diethyl ether (see Figure 5b). Thecalculated interaction enthalpy of -12.7 kcal/mol is larger by1.8 kcal/mol than that between [EMIm]ZnCl2(EtSO4) andquinoline. This is probably the reason why diethyl ether iscapable of back extracting trapped quinoline in the IL layer.Both of the structures in panels a and b of Figure 5 show thatthe coordination of a Lewis base, such as quinoline or diethyl

ether, changes the binding mode of the ethylsulfate inZnCl2(EtSO4)- from bidentate to monodentate, thereby retaininga tetrahedral environment around Zn. In contrast, as shown inpanels c and d of Figure 5, indole was found to interact morestrongly with an ethylsulfate anion via hydrogen bonding thanwith the Zn atom in ZnCl2(EtSO4)-. The enthalpy for theinteractions of indole with [EMIm]EtSO4 and [EMIm]ZnCl2-EtSO4 was calculated as -7.6 and -4.3 kcal/mol, respectively,suggesting that indole extraction is mainly influenced by theinteraction between the H atom of N-H and EtSO4- rather thanthat between the N atom and Zn center.

From the experimental, spectroscopic, and computationalresults, the plausible extraction mechanism of the isomerizationis suggested as in Scheme 3. The active species 1 is likely toform by the reaction of [EMIm]EtSO4 and ZnCl2. The interac-tion of 1 with quinoline would produce quinoline-coordinatedspecies 2, which in turn transforms into diethyl-ether-coordinatedspecies 3 upon treated with diethyl ether, a back extractant. Thecoordinated ether can be removed under vacuum to regeneratethe active species 1.

Conclusions

Zn-containing ILs, prepared from the interaction of ZnCl2with imidazolium-based IL bearing a alkylsulfate anion, wereproven to be effective extractants for the denitrogenation of amodel oil containing quinoline, acridine, and/or indole. Inparticular, the performance of dialkylimidazolium alkyl sulfateRTIL for the extraction of basic nitrogen compounds, such asquinoline and acridine, was significantly improved up to morethan 2 times by the co-presence of Lewis acidic ZnCl2.

Diethyl ether was found to be an efficient back extractantfor the regeneration of [EMIm]EtSO4-ZnCl2, used for thedenitrogenation of quinoline from the model oil, and to recovertrapped quinoline in the IL. The quinoline extraction ability of[EMIm]EtSO4-ZnCl2 was maintained up to 8 cycles without asignificant loss of the initial performance using diethyl ether asthe back extractant. Computational studies show that active Zn-containing anionic species, such as [EMIm]ZnCl2(EtSO4) and[EMIm]ZnCl(EtSO4)2, can be generated from the interaction ofZnCl2 with [EMIm]EtSO4, and thus, the extraction of quinolinecan be facilitated through the coordination of quinoline to theZn center. The bonding mode of ethylsulfate ligand inZnCl2(EtSO4)- is changed from bidentate to monodentate forthe coordination of quinoline, thereby retaining a tetrahedralenvironment around Zn. The larger calculated interactionenthalpy for the coordination diethyl ether suggests that thecoordinated quinole can be replaced by diethyl ether. Theformation of anionic Zn species, ZnCl2(EtSO4)- (MW ) 261)and ZnCl(MeSO4)2- (MW ) 349), was supported by FAB-MSspectroscopic results.

In the case of the denitrogenation of a neutral nitrogencompound, such as indole, the extraction by an IL seems to bemostly governed by the interaction between the anion of IL andthe H atom of N-H rather than by the coordination of indoleto the Zn center through the N atom, as supported bycomputational calculation.

Acknowledgment. This work has been performed as an EnergyTechnology Innovation (ETI) Project under the Energy ResourcesTechnology Development program.

EF900073A(38) Harper, J. B. Compr. Heterocycl. Chem. III 2008, 7, 140.

3038 Energy & Fuels, Vol. 23, 2009 Huh et al.