Extractive Desulfurization of Fuel Oil Using Alkylimidazole and Its

Extractive Desulfurization of Fuel Oil Using Alkylimidazole and Its Mixture with. Dialkylphosphate Ionic Liquids. Yi Nie, Chun-Xi Li,* and Zi-Hao Wang...
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Ind. Eng. Chem. Res. 2007, 46, 5108-5112

Extractive Desulfurization of Fuel Oil Using Alkylimidazole and Its Mixture with Dialkylphosphate Ionic Liquids Yi Nie, Chun-Xi Li,* and Zi-Hao Wang State Key Laboratory of Chemical Resource Engineering and College of Chemical Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China

The sulfur partition coefficient KN is usually used to characterize the ability of extractive desulfurization (EDS). In this work, several KN values are measured at 298.15 K, which are between straight-run fuel oil and N-ethylimidazole (EIM), N-methylimidazole (MIM), and its mixture with a dialkylphosphate ionic liquid (IL), viz. N-ethyl-N-methylimidazolium diethylphosphate ([EMIM][DEP]) or N-butyl-N-methylimidazolium dibutylphosphate ([BMIM][DBP]). KN values between the oil and solvent at varying water content are also measured at 298.15 K. The results indicate that both EIM and MIM have excellent EDS performance with KN above 3.1 for dibenzothiophene. They have some solubility in fuel oil, but their solubility decreases with addition of IL. The EDS ability of the solvents for a specified sulfur compound followed the order alkylimidazole (EIM > MIM) > mixed solvent (MIM + IL) > IL ([BMIM][DBP] > [EMIM][DEP]); the extractive selectivity of sulfur for a specified solvent followed the order dibenzothiophene (DBT) > benzothiophene (BT) > 3-methylthiophene (3-MT). The used sulfur-containing solvent can be regenerated via a water diluting process followed by a simple distillation process; therefore, the solvent can be reclaimed in a cost-effective way. This work shows that the alkylimidazole solvent and/or its mixture with an IL can be used as a potential extractant for the EDS of fuel oils. Introduction Fuel combustion may cause pollution to the environment due to the sulfur in fuel oils, which forms SOx during combustion. In order to minimize SOx emission, increasingly stringent regulations are being imposed on oil refineries to reduce the sulfur content (S content) to a very low limit, around 10-20 ppm.1 However, the present hydrodesulfurization (HDS) process encounters a great challenge to meet this requirement cost effectively as hydrogenation of aromatic sulfur compounds (S compounds), e.g., benzothiophene (BT), dibenzothiophene (DBT), and their alkyl derivatives, is quite difficult by the available catalyst. In addition to the HDS process, alternative desulfurization processes based on adsorption, extraction, oxidation, alkylation, and complexation of aromatic S compounds have been studied extensively. Among them, extractive desulfurization (EDS) is an attractive one, which can be performed under mild conditions at low energy consumption and without hydrogen consumption. Regarding the EDS process some molecular solvents,2-3 e.g., polyalkylene glycol, imidazolidinone, pyrimidinone, and dimethyl sulfoxide, have been patented. However, their abilities of EDS are not sufficient and solubility in fuel is noticeable, which may cause cross contamination; more efficient molecular extractants are expected. Compared to molecular solvents, some ionic liquids (ILs), e.g., [EMIM][DEP], [BMIM][DBP],4 [BMIM][Cl/AlCl3], and [BMIM][BF4],5-8 show high extractability for sulfur component (S component), which indicates that ILs might be a novel and competitive extractive solvent if their shortcomings on high cost and viscosity can be somehow overcome. The objective of this paper is to explore some new and efficient molecular solvents for the EDS of fuel oil and use them together with ionic liquid so as to (1) adjust the viscosity of the ILs to a suitable range for extraction operation and mass * To whom correspondence should be addressed. Tel.: 86-1064444911, Fax: 86-10-64410308. E-mail: [email protected].

transfer, (2) reduce the cost of the extractant since the molecular solvent is generally cheaper than ILs, and (3) decrease the solubility of the molecular solvent in the fuel oil. In this paper, the excellent EDS ability of N-ethylimidazole (EIM) and N-methylimidazole (MIM) from fuel is reported for the first time. Meanwhile, the sulfur partition coefficients for 3-MT, BT, and DBT between fuel oil and EIM, MIM, or its mixture with an IL ([EMIM][DEP] or [BMIM][DBP]) are measured, and the reclaiming method for the used extractant is studied briefly. Experimental Section Chemical Materials. MIM and EIM of A.R grade used in this work are from the Zhejiang Kaiyue Chemical plant. The straight-run gasoline is provided by the Qilu refinery of Sinopec, and its composition is analyzed by GC-MS (Shimadzu QP2010). Commercial 97# gasoline was purchased from a gas station of Sinopec Corporation. Ionic liquids [EMIM][DEP] and [BMIM][DBP] are prepared by reacting equal moles of MIM and the corresponding trialkyl phosphate at 423.2 K for 10 h with a yield of 97%.9 The resulting yellowish viscous liquid has been washed three times with diethyl ether at room temperature followed by rotary evaporation under reduced pressure of 1 kPa for 12 h to remove all volatile residues. The purity and structure of these ILs have been analyzed by 1H NMR and electronic spray mass spectroscopy. Mutual Solubility of Solvent and Fuel Oil. Mutual solubility is an important factor to be considered in choosing an extractant because a noticeable solubility of a nitrogen-bearing solvent in fuel may contaminate the fuel and lead to NOx pollution and a noticeable solubility of oil in solvents can increase the separation cost. The solubility of fuel oil in solvent and solubility of solvent in fuel oil are measured using gravimetric methods, gas chromatography (Shimadzu GC-2010 equipped with a FID detector; DB-1 column, 30 m × 0.25 mm i.d. × 5 µm; carrier gas N2; temperature program 50-5 °C/min-100-20 °C/min-

10.1021/ie070385v CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007

Ind. Eng. Chem. Res., Vol. 46, No. 15, 2007 5109 Table 1. Equilibrium Distribution of DBT, BT, and 3-MT between MIM and Straight-Run Gasoline and the Corresponding Sulfur Partition Coefficients KN at 298.15 K DBT S content in oil (ppm), x 151.4 332.6 457.4 619.1 684.0 755.0 820.5 869.0 923.1 977.4

S content in MIM (ppm), y

512.2 1030.9 1387.5 1866.8 2130.8 2324.6 2529.9 2723.9 2916.8 3005.3 y ) 3.10x (R2 ) 0.999)

BT KN

S content in oil (ppm), x

3.38 3.10 3.03 3.02 3.12 3.08 3.08 3.13 3.16 3.07

81.3 137.0 196.0 246.1 317.3 363.9 418.2 458.7 526.7 545.4

3-MT

S content in MIM (ppm), y

151.8 372.8 542.5 678.4 832.0 923.8 1039.4 1169.2 1308.7 1397.2 y ) 2.55x (R2 ) 0.995)

320 °C; area unitary method and calibration curve), or liquid chromatography.10 It should be pointed out that the solubility of the mixed solvent, MIM + IL, in fuel oil refers to that of MIM since the IL component is insoluble in fuel oil.4-5 Sulfur Partition Coefficients Measurement for Fuel Oil/ Solvent System. The sulfur partition coefficient (KN), which is defined as the ratio of S concentration (on weight basis) in solvent to S concentration in gasoline, is an important parameter for an EDS process, and the higher the KN the better the desulfurization performance of a solvent. In this work, KN is measured as described below. First, an extracting solvent with known S content in ppm, i.e., mg(S)‚kg(IL)-1, is prepared by gravimetric method. A definite amount of 3-MT, BT, or DBT is dissolved in known quantities of solvent, which is used as a S source in the following extraction experiment. For example, by dissolving 0.714 g of DBT in 19.994 g of MIM, a 5996 ppm S content MIM sample is obtained. Second, a known weight of S-free solvent and fuel oil is mixed in a 100 mL conical flask under vigorous magnetic stirring for pre-equilibrium so as to minimize the effect of oil dissolution in solvent phase on calculation of the sulfur partition coefficient. Third, a known amount of S-concentrated solvent is added to the above biphasic mixture, magnetically stirred for 15 min at room temperature to reach thermodynamic equilibrium,6-7 and then laid aside for 10 min for phase splitting and settling. The S content in the oil phase is measured by liquid chromatography using the external standard method (Shimadzu10AVP equipped with UV-vis detector and a C-18 column; mobile phase methanol/water ) 9/1 for DBT, methanol/water ) 8/2 for BT and 3-MT; wavelength 310 nm for DBT, 251 nm for BT, 242 nm for 3-MT; flow rate 1.0 mL/min), and the S content in the solvent phase is calculated via mass balance as such the sulfur partition coefficient is calculated. The third step is repeated several times until the equilibrium S content in oil falls into the S concentration range expected. In order to investigate the effect of water content in solvent on the EDS performance, the KN values for DBT, BT, and 3-MT between MIM + 40%(wt %)[EMIM][DEP] aqueous solution and fuel oil at room temperature are also measured. Recovery of Used Solvents. The oil-saturated S-containing solvents are regenerated by successive dissolution with water followed by a simple distillation for the aqueous solution at 373.15 K under reduced pressure of about 1 kPa for 12 h.

KN 1.87 2.72 2.77 2.76 2.62 2.54 2.49 2.55 2.48 2.56

S content in oil (ppm), x 133.9 259.7 332.5 411.4 490.3 573.5 653.6 728.1 854.3

S content in MIM (ppm), y

KN

111.6 229.1 290.5 358.0 427.4 546.0 659.7 730.5 832.9

0.83 0.88 0.87 0.87 0.87 0.95 1.01 1.00 0.97

y ) 0.96x (R2 ) 0.990)

component DBT, BT, and 3-MT between fuel and solvent according to the experimental procedure described above. Table 1 shows the equilibrium S content in MIM and oil phases and the corresponding KN values for DBT, BT, and 3-MT at 298.15 K. As shown in Table 1, the KN value for a specified S component (namely, DBT, BT, and 3-MT) is virtually constant regardless of the S content in fuel oil, which means the S content in solvent is linearly proportional to the S content in fuel oil at equilibrium in the S concentration range studied (see the second line from the top in Figure 1). A similar behavior is observed for EIM, phosphate ILs, and the mixtures thereof. The independence of KN on S concentration is mainly attributed to the thermodynamic behavior of the dilute solution since the mole fractions of S component in both phases are extremely low, and hence, the nonideality of S component is negligible. Although EIM and MIM have high partition coefficients for S compounds, their mutual solubility with fuel oil is quite high, as shown in Table 2, which may cause fuel contamination. Compared to the molecular solvents, IL is virtually insoluble in fuel oil and shows good EDS ability but has the drawback of high viscosity. Considering these factors a ‘solvent cocktail’, i.e., a mixture of MIM and a dialkylphosphate IL, combining the merits of both is prepared, and its EDS performance is measured in terms of sulfur partition coefficients. The presence of ionic liquid can slightly decrease the solubility of solvent in fuel oil, while the solubility of fuel oil in a mixed solvent depends on the solvent composition and solubility of fuel in the corresponding pure solvents. As shown in Table 3 the sulfur partition coefficient for a specified S compound follows the order EIM > MIM > MIM + 20%(wt %) IL > MIM + 40%(wt %) IL > IL, the EDS

Results and Discussion EDS Performance of EIM, MIM, and Its Mixture with a Dialkylphosphate IL for Straight-Run Gasoline. The EDS performance of MIM and EIM is investigated for the first time in terms of the sulfur partition coefficients KN for typical S

Figure 1. Sulfur partition between different solvent and straight-run gasoline at 298.15 K.

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Table 2. Mutual Solubility of Solvent and Straight-Run Gasoline at 298.15 K system

solubility, g of solvent/100 g of oil

solubility, g of oil/100 g of solvent

EIM/fuel oil MIM/fuel oil MIM+20%[EMIM][DEP]/fuel oil MIM+40%[EMIM][DEP]/fuel oil MIM+20%[BMIM][DBP]/fuel oil MIM+40%[BMIM][DBP]/fuel oil [EMIM][DEP]/fuel oil4 [BMIM][DBP]/fuel oil4

1.71a

20.6a 15.22a 11.87a 9.76a 18.95a 20.2a 4.25c 20.6c

1.51a 1.21a,d 0.99a,d 1.26a,d 1.21a,d 0b 0b

a The solubility data was measured using gas chromatography. b The solubility data was measured using liquid chromatography. c The solubility data was measured using the gravimetric method. d Solubility of MIM in fuel oil.

Table 3. Sulfur Partition Coefficients KN at 298.15K in Different Solvent/Straight-Run Gasoline Systemsa DBT solvent

KN

R2

EIM MIM [BMIM][DBP]4 [EMIM][DEP] 4 MIM + 20 %[BMIM][DBP] MIM + 40 %[BMIM][DBP] MIM + 20 %[EMIM][DEP] MIM + 40 %[EMIM][DEP]

3.28 3.10 1.59 1.27 2.63 2.28 2.28 1.73

0.997 0.999 0.998 0.999 0.999 0.999 0.997 0.999

BT

3-MT

KN

R2

KN

R2

2.67 2.55 1.37 0.94 2.20 2.03 2.02 1.47

0.996 0.995 0.994 0.992 0.987 0.995 0.992 0.981

1.17 1.03 0.59 0.47 0.95 0.92 0.85 0.69

0.980 0.987 0.977 0.984 0.985 0.994 0.991 0.988

a K was obtained by fitting the experimental S content in both phases N with a least-square method using the equation y ) KNx, where y and x represent S content (ppm) in solvent and fuel, respectively. R2 is the squared correlation coefficient.

ability of MIM + [EMIM][DEP] is always between MIM and IL and depends on the mass fraction of MIM. This trend is also valid for other solvent mixtures studied herein. For a specified solvent, the sulfur partition coefficient always follows the order DBT > BT > 3-MT. The extractive sulfur removal ability of MIM and EIM is superior to any extractant reported except ionic liquid [BMIM][Cl/AlCl3],2,4,5,8 as can be seen from the KN values for DBT listed in Table 4. Considering the fact that [BMIM][Cl/AlCl3] is very viscous and sensitive to water and thus incompatible with a water environment, the solvent EIM, MIM, and its mixture with a phosphate IL is more competitive and feasible for EDS applications. The most likely mechanism for extraction of S compounds with EIM, MIM, and its mixture with an imidazolium-based phosphate IL is formation of liquid clathrate due to the π-π interaction between aromatic structures of the extraction target sulfur compounds and the imidazole ring system.11 EDS Performance of MIM for Commercial 97# Gasoline. In all experiments the “simulated oil” is prepared by dissolving a specified S component, e.g., 3-MT, BT, or DBT, to straightrun oil obtained from an oil refinery of Sinopec. In order to show the difference of EDS performance of MIM between straight-run oil and real oil, a complementary experiment is conducted using commercial 97# gasoline purchased from a gas station of Sinopec Corporation. The sulfur (as DBT) partition coefficient between MIM and 97# gasoline is measured and listed in Table 5. As shown in Table 5, the sulfur partition coefficient drops from 3.10 between MIM and straight-run gasoline to 2.61 between MIM and 97# gasoline. This suggests that sulfur removal from real gasoline is a little harder than desulfurization of straight-run gasoline. Influence of EDS on the Composition of Fuel Oil. The EDS process has two potential influences on fuel. On one hand, the solubility of EIM, MIM, and the imidazolium-based phosphate ILs in fuel can cause NOx pollution; on the other hand, the fuel composition is likely to be changed due to the selective

extraction of solvent for some specified components of fuel. In effect the loss of fuel and variation of fuel composition can be reflected by the fuel solubility in the solvent, and the lower the solubility, the smaller the variation of fuel quality should be. As an extreme case, the fuel quality and composition will remain unchanged when it is immiscible with the solvent. Hence, the cross contamination of the EDS process is closely related to the mutual solubility of fuel and solvent involved. As shown in Table 2, the solubility of fuel oil in pure solvent follows the order [BMIM][DBP] ) EIM > MIM > [EMIM][DEP]; the solubility of molecular solvents EIM and MIM in fuel oil is relatively low, while ILs are insoluble in fuel oil. The mutual solubility of a solvent mixture and fuel oil is basically proportional to the solvent composition and mutual solubility of the corresponding pure solvent/fuel systems, e.g., the solubility of fuel oil in solvent follows the order of [BMIM][DBP] > MIM + 40%[BMIM][DBP] > MIM + 20%[BMIM][DBP] > MIM. The solubility of molecular solvent in fuel oil decreases slightly by addition of IL component. In order to investigate the influence of the EDS process on the composition of fuel oil, GC analysis is conducted for the following four samples: straight-run gasoline (sample A); a mixture of sample A and DBT with S content of 558 ppm (sample B); sample B extracted three times by solvent MIM + 40%[EMIM][DEP] with a mass ratio of solvent/oil being 2:1 and the resulting S content being 61 ppm (sample C); sample B extracted three times by solvent MIM with a mass ratio of solvent/oil being 2:1 and the resulting S content being ca. 11 ppm (sample D). On the basis of the GC spectrum measured, the area percentage for peaks 1-10 is listed in Table 6, and the peaks are identified by GC-MS (Shimadzu QP2010; ion energy 70 eV; scan range 20-600 m/z; other conditions are the same as the conditions for gas chromatography) as shown in Figure 2. The results show that 2,3-dimethylbutane and methylcyclopentane among the components of fuel oil are selectively extracted by the solvent used, which can be seen by the decrease of normalized area percentage as shown in Table 6. However, the overall influence of an EDS process on the composition of fuel is limited. Regeneration of the Used Extractant. Considering the fact that all extractants used here are hydrophilic while all S components are hydrophobic, separation of solvent and S components can be carried out by diluting the solvent with water,6-7 as the S component in the vicinity of solvent molecules is repelled by water. For this purpose, sulfur partition coefficients between MIM aqueous solution and oil are measured and graphically shown in Figure 3. When about 50% water (mass ratio, water:MIM ) 1:1) is added into the oil-saturated MIM aqueous solution, the sulfur partition coefficient reaches null. This indicates that 50% water is enough to make sulfur transfer from MIM aqueous solution phase into the oil phase completely.

Ind. Eng. Chem. Res., Vol. 46, No. 15, 2007 5111 Table 4. Sulfur Partition Coefficients KN for Extraction of DBT with Different Solvent solvent

KN, mg(S) kg(IL)p-1/mg(s) kg(oil)-1

solvent

KN, mg(S) kg(IL)p-1/mg(s) kg(oil)-1

4.0 3.22 3.10 1.9 1.73 1.59 1.59 1.50 1.27 1.1

[BMIM][MeSO3]b [BMIM][PF6]b [EMIM][EtSO4]a [BMIM][CF3SO3]b DMFd

1.1 0.9 0.8 0.8 0.72 0.7 0.7 0.46 0.38 0.26

[BMIM][AlCl4]a EIMc MIMc [BMIM][OcSO4]a MIM + 40%[EMIM][DEP]c [BMIM][DBP]c NMPd DMId [EMIM][DEP]c [BMIM][MeSO4]b

[MMIM][Me2PO4]a [BMIM][BF4]a [MMIM][DMP]c TMPId TMPBd

a Model oil:5 500 ppm sulfur as DBT in N-dodecane, room temperature. b Model oil:5 500 ppm sulfur as DBT in N-dodecane, 60 °C. c Straight-run gasoline:4 sulfur as DBT, 298.15 K. d Light oil:2 450 ppm sulfur as 4-MeDBT, 4, 6-Me2DBT.

Table 5. Equilibrium Sulfur Partition between MIM and Commercial 97# Gasoline at 298.15 K S content in 97# gasoline (ppm), x

S content in MIM (ppm), y

108. 6 288.2 423.1 490.8 y ) 2.61x, R2 ) 0.992

371.5 749.5 1072.1 1286.4

Table 6. Peak Area Percentage of the Corresponding Component of Straight-run Gasoline before and after Extraction with MIM and MIM+40%(wt%)[EMIM][DEP] peak no.a sampleb

1

2

3

4

5

6

7

8

9

10

A B C D av. of A and B

16.58 15.70 13.38 14.23 16.14

2.38 2.49 2.75 2.64 2.44

7.94 7.86 8.25 7.84 7.90

26.73 26.52 25.18 25.92 26.62

10.89 10.86 10.78 10.83 10.87

2.18 2.18 2.51 2.27 2.18

6.45 6.61 7.02 6.87 6.53

3.50 3.57 4.50 3.82 3.53

8.04 8.25 8.49 8.60 8.14

3.37 3.46 3.72 3.61 3.41

Figure 3. Sulfur partition coefficients between MIM aqueous solution and straight-run gasoline at 298.15 K.

a Components identified: (1) 2, 3-dimethylbutane; (2) 3-methylpentane; (3) hexane; (4) methylcyclopentane; (5) cyclohexane; (6) 3,7-dimethyloctene; (7) 1,3-dimethylcyclopentane; (8) heptane; (9) methylcyclohexane; (10) ethylcyclopentane. b See the description in text.

Figure 4. Schematic flowchart for a combined EDS and solvent regeneration process. Figure 2. GC-MS chromatogram of straight-run gasoline: (1) 2,3dimethylbutane; (2) 3-methylpentane; (3) hexane; (4) methylcyclopentane; (5) cyclohexane; (6) 3,7-dimethyloctene; (7) 1,3-dimethylcyclopentane; (8) heptane; (9) methylcyclohexane; (10) ethylcyclopentane.

In the dilution process, when about 50% water is added into the used oil-saturated MIM, two phases are formed, viz. a MIM aqueous phase being free of S compounds and oil and a solventfree oil phase containing all the S compounds precipitated during water dilution process. The composition of MIM aqueous phase and oil phase is identified using GC and Karl Fischer-Titration (CBS-1A), respectively. The solvent aqueous phase is distilled at 373.15 K to remove the water component and get the extractant at the bottom, while the sulfur-rich oil phase could be collected for further treatment. An extractive desulfurization process is proposed as shown in Figure 4.

When the solvent is regenerated via distillation, water residue in the reclaimed extractant is inevitable. In order to assess the influence of water content on the EDS performance, the sulfur partition coefficients KN for DBT, BT, and 3-MT between fuel oil and MIM + 40%[EMIM][DEP] at varying water content are measured and listed in Table 7. It is seen that the KN values decrease dramatically with the increase of water content in solvent, and even 1.42% of water can give rise to about 20% lowering of the extractive ability of the solvent for DBT, suggesting that the water residue in the reclaimed solvent should be removed as low as possible. The operation cost can be evaluated as following steps. Assume that (1) initial S content as DBT in fuel is 2000 ppm, (2) the required S content of fuel product is 10 ppm, and (3) mass ratio of solvent/fuel in an extraction column is 1:2; then

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Table 7. Effect of Water Content in Solvent on the Sulfur Partition Coefficients KN between MIM + 40%(wt %)[EMIM][DEP] and Straight-Run Gasoline DBT water content in solvent (wt %) 0 1.42 3.10 5.15

BT KN

water content in solvent (wt %)

1.72 1.37 1.20 0.92

0 0.94 3.13 5.02

3-MT KN

water content in solvent (wt %)

KN

1.47 1.35 1.12 0.85

0 1.20 3.30 5.09

0.67 0.54 0.47 0.34

the extraction stage number required can be estimated as about 10 and the extracting solvent required per ton of fuel is 0.5 ton of MIM. To regenerate such an amount of solvent at least 0.5 ton of low-pressure steam is required, and the resulting steam cost is about USD5. Since the energy cost is the main expense of regeneration, the operation cost of such EDS process is expected within USD10 per ton of fuel. From this very preliminary assessment it is seen that the cost of the EDS process is relatively low in comparison with the commercial HDS process. Therefore, the EDS process with alkylimidazole or its mixture with a dialkylphosphate ionic liquid from fuel oils is quite promising for cost-efficient practical application. Possible NOx contamination of fuel due to the solubility of nitrogen-bearing solvent in the fuel oil can be overcome by back extraction with water,2 as shown in Figure 4. Since the solvent MIM is highly hydrophilic, its partition coefficient between water and fuel oil at 298.15 K is as high as 10.18; thus, MIM and its mixture MIM + [EMIM][DEP] dissolved in the fuel can be removed conveniently and easily by a back extraction process. In summary, the desulfurization ability of EIM and MIM or mixtures of MIM with a dialkylphosphate IL is very attractive. The used S-containing solvent can be regenerated by a water diluting process followed by simple distillation. The EDS process with alkylimidazole or its mixture with a dialkylphosphate ionic liquid may be used for the deep desulfurization of fuel oil efficiently.

Acknowledgment The work was supported by the National Natural Science Foundation of China (under Grant No. 20376004) and the Fundamental Research Foundation of Sinopec (Grant No. X505015). Literature Cited (1) Song, C. S. An overview of approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211. (2) Horii, Y.; Onuki, H.; Doi, S. Desulfurization and denitration of light oil by extraction. U.S. Patent 5494572, 1996. (3) Paulino, F.; Yonkers, N. Y. Process for the removal of sulfur from petroleum fractions. U.S. Patent 5582714, 1996. (4) Nie, Y.; Li, C. X.; Sun, A. J.; Meng, H.; Wang, Z. H. Extractive desulfurization of gasoline using imidazolium-based phosphoric ionic liquids. Energy Fuels 2006, 20, 2083. (5) Esser, J.; Wasserscheid, P.; Jess, A. Deep desulfurization of oil refinery streams by extraction with ionic liquids. Green Chem. 2004, 6, 316. (6) Zhang, S. G.; Zhang, Q. L.; Zhang, Z. C. Extractive desulfurization and denitrogenation of fuels using ionic liquids. Ind. Eng. Chem. Res. 2004, 43, 614. (7) Zhang, S.; Zhang, Z. C. Novel properties of ionic liquids in selective sulfur removal from fuels at room temperature. Green Chem. 2002, 4, 376. (8) Bo¨smann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Deep desulfurization of diesel fuel by extraction with ionic liquids. Chem. Commun. 2001, 23, 2494. (9) Jiang, X. C.; Yu, C. Y.; Li, C. X.; Wang, Z. H. Synthesis and application of ionic liquid 1-butyl-3-methyl imidazolium dibutyl phosphate. J. Beijing UniV. Chem. Technol. (Nat. Sci. Ed.) 2006, 33, 5. (10) Stepnowski, P.; Mu¨ller, A.; Behrend, P.; Ranke, J.; Hoffmann, J.; Jastorff, B. Reverse phase liquid chromatographic method for the determination of selected room temperature ionic liquids cations. J. Chromatogr. A 2003, 993, 173. (11) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Liquid clathrate formation in ionic liquidaromatic mixtures. Chem. Commun. 2003, 3, 476.

ReceiVed for reView March 14, 2007 ReVised manuscript receiVed April 19, 2007 Accepted April 23, 2007 IE070385V