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Desulfurization of Diesel Fuel by Extraction with [BF4]−-based Ionic Liquids
Chinese Journal of Chemical Engineering, 16(6) 881ü884 (2008)
Desulfurization of Diesel Fuel by Extraction with [BF4] -based Ionic Liquids* ˉ
CHU Xuemei (ᬖ༲ਜ), HU Yufeng (ܑံ**)ע, LI Jiguang (ह)ڜݽ, LIANG Qianqing (ॣሯு), LIU Yansheng (ঞཧಖ), ZHANG Xianming (ຕੜ), PENG Xiaoming (໒ੜ) and YUE Wenjia (ၣำ)ޤ State Key Laboratory of Heavy Oil, China University of Petroleum, Beijing 102249, China
Abstract The extractive removal of sulfur compounds (S-compounds) from Dongying and Liaohe diesel fuels with [BF4]ˉ-based ionic liquids were systematically investigated. The results show that the absorption capacity of an ionic liquid for the S-compounds in diesel fuels relies on its structure and its size. In the case of the two examined diesel fuels, both elongating the cation tail length and increasing the mass ratio of ionic liquid/diesel fuel promote the desulfurization ability of the examined ionic liquids. The results also show that imidazolium-based ionic liquids display higher extraction efficiencies than pyridinium-based ionic liquids, presumably owing to the fact that the rings of the S-compounds are similar to the imidazolium head ring. With the 1Ή1 mass ratio of ionic liquid/diesel fuel, the rates of the first desulfurization of Dongying and Liaohe diesel fuels using [C8mim][BF4] amount to 29.96% and 39.76%, suggesting that [C8mim][BF4] is a promising extractant for desulfurization of these diesel fuels. Keywords desulfurization, diesel fuel, ionic liquid
1
INTRODUCTION
With the fast development of petrochemistry and the automobile industry, air pollution caused by diesel sulfur oxide (SOx) emissions has become one of the increasingly serious problems in the world. In recent years, considerable attention has been paid to deep desulfurization of gasoline and diesel fuels as the environment regulations on the sulfur limits of fuels are increasingly stringent. In the petroleum industry, desulfurization of diesel is carried out by hydrocracking or hydrotreating processes [1]. Hydrotreating processes are highly effective for reduction of sulfur levels; however, a further improvement of the hydrodesulfurization efficiency is limited to increasingly severe operational conditions at escalated cost [2]. To reduce the sulfur mass content down to 0.001%0.002% [3], the traditional hydrodesulfurization (HDS) process will encounter a great challenge, as hydrogenation of aromatic sulfur compounds (S-compounds) such as benzothiophene (BT) and dibenzothiophene (DBT) requires not only considerably increased energy and hydrogen consumptions but also the substantially improved reactivity and selectivity of the used catalyst. In addition, undesired side reactions will also be induced to result in the decrease of the octane number of gasoline. Therefore, it is critically important to develop new desulfurization technologies to minimize the negative health and environmental pollutions from diesel exhaust gas. Ionic liquids (IL) have been recognized as novel designable solvents (i.e., their properties can be tuned by altering their ionic structures to meet specific demands), which are liquids over a wide temperature range including room temperature. One of the most important advantages is that ionic liquids can be
tuned/controlled by tailoring their cationic and anionic structures to optimize their physicochemical properties [47]. As a kind of extraction solvent, IL does not remain in the organic phase, which can be greatly convenient for separation [8], and therefore, the desulfurization using ionic liquids has received growing ˉ attention [812]. [BF4] -based ionic liquids are promising extractants in future industrial applications because of their low viscosities and wide liquid ranges [1320]. Therefore, in the present article, we focused on the extraction of sulfur compounds from China diesel fuels using this type of ionic liquids. 2
EXPERIMENTAL
All chemicals used in this study were of reagent grade. N-methylimidazole was purchased from Aldrich. n-C4H11Cl, n-C8H17Cl, and NaBF4 were supplied by Shanghai Jiachen Chemical Ltd. Pyridine, benzylchloride, acetone, and ethyl acetate were purchased from Beijing Yili Chemical Ltd. Diesel fuels with different sulfur contents were provided by Sinopec Corporation station. All the present ionic liquids were prepared using well-established procedures [1321]. For illustration, the procedures for preparing the ionic liquids are summarized as follows. The chloride salts were prepared by reacting N-methylimidazole or pyridine with RCl, where, R benzyl, n-C4H11 or n-C8H17. The products were purified by repeated extractions of the remaining starting materials with ethyl acetate. After the last extraction, the remaining ethyl acetate was removed at 343.15 K under vacuum. The resulting [1-alkyl-py][Cl], [1-benzyl-py][Cl], and [1-alkyl-mim][Cl] were dried at 343.15 K under vacuum for 6 d. Equimolar amounts of [1-alkyl-mim][Cl] and NaBF4 were dissolved in acetone separately. The two solutions were gradually mixed together with stirring.
Received 2008-01-30, accepted 2008-06-13. * Supported by the National Natural Science Foundation of China (40673043, 20576073), CNPC Innovation Fund (04E7031), and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-06-0088). ** To whom correspondence should be addressed. E-mail: [email protected]
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Chin. J. Chem. Eng., Vol. 16, No. 6, December 2008
The precipitated sodium chloride was separated from the liquid by filtration. The excess acetone was removed by evaporation, and crude product was dried in vacuum for 48 h. Similar synthesis methods were ˉ adopted for other [BF4] -based ionic liquids. The water contents after drying, measured by Karl Fisher titration, were 0.05%0.1% (by mass). The desulfurization experiments were carried out in glass vials placed in a water bath controlled at (298.15f0.1) K. The mass ratios of ionic liquids to diesel fuel were conducted at 1Ή5, 1Ή2, and 1Ή1, respectively. The preliminary measurement results presented in Fig. 1 show that the liquid-liquid equilibrium (LLE) can be achieved after the biphasic mixtures were stirred for 10 min. Therefore, the real equilibration time was fixed at 2030 min and the organic phase at absorption equilibrium was separated from the ionic liquid phase for analysis. The sulfur contents in diesel fuels before and after each extraction experiment were analyzed using the quantitative elemental analysis conducted on an NS analytical apparatus (ANTEK 7000NS). 3 3.1
RESULTS AND DISCUSSION Determination of the equilibrium time
In the present study, Liaohe diesel fuel was treated with [C8mim][BF4] to establish the time required to reach the absorption equilibrium. The single extractions were conducted for 2, 5, 10, and 20 min at 298.15 K with a fixed mass ratio of 1Ή2 (the ratio of ionic liquid to diesel fuel). The sample in the upper phase was carefully separated with a pipet from the ionic liquid phase for analysis. The results, in Fig. 1 show that 10 min of contact between the diesel fuel phase and the ionic liquid phase is more than sufficient to establish the equilibrium.
Table 1
Sulfur extraction ability of different ILs for diesel fuel from Dongying at 298.15 K
Ionic liquid
Mass ratios of ionic liquid to diesel fuel
Sulfur content treated/%
Sulfur removal/%
[C8Py][BF4]
1Ή5
0.0653
8.16
[BePy][BF4]
1Ή5
0.0673
5.34
[C4Py][BF4]
1Ή5
0.0681
4.22
[C8mim][BF4]
1Ή5
0.0639
10.13
[C4mim][BF4]
1Ή5
0.0670
5.77
[C8Py][BF4]
1Ή2
0.0629
11.53
[C8mim][BF4]
1Ή2
0.0582
18.14
[C8Py][BF4]
1Ή1
0.0596
16.17
[C8mim][BF4]
1Ή1
0.0498
29.96
Note: General conditions: extraction time 20 min, initial sulfur mass content 0.0711%.
sel fuel increase with increasing cation tail length. Besides, it is seen from Table 1 that the order for the desulfurization rate, [C8Py][BF4] ˘ [C8mim][BF4], remains unchanged as the mass ratio decreases from 1Ή5 to 1Ή2 or 1Ή1, implying that the used mass ratio has a little influence on the relative absorption capacities of the examined ionic liquids. Imidazoliumbased ionic liquids show higher extraction efficiencies than pyridinium-based ionic liquids, presumably owing to the fact that the rings of the S-compounds are similar to the imidazolium head ring. It is also seen in Table 1 that the extraction efficiency increases with increasing the mass ratio. 3.3
Desulfurization of Liaohe diesel fuel
Table 2 shows the results for the desulfurization of Liaohe diesel fuel using different ionic liquids at 298.15 K. All ionic liquids under investigation show good abilities to extract S-compounds from the examined diesel fuel. The desulfurization rate measured at a fixed mass ratio of 1 Ή 5 increases in the order [BePy][BF4] ˘ [C4Py][BF4] ˘ [C4mim][BF4] ˘ [C8Py][BF4]§[C8mim][BF4]. An inspection of the Table 2
Figure 1
3.2
Sulfur content versus contact time
Desulfurization of Dongying diesel fuel
The desulfurization results of Dongying diesel fuel with 0.0711% initial sulfur content using different ionic liquids at 298.15 K are shown in Table 1. The used mass ratio of ionic liquid to diesel fuel is 1Ή5. It can be seen that the desulfurization rate follows the order [C4Py][BF4] ˘[BePy][BF4] ˘[C4mim][BF4] ˘ [C8Py][BF4] ˘ [C8mim][BF4]. The conclusion thus derived is that the absorption capacities of the examined ionic liquids for S-compounds in Dongying die-
Sulfur removal from Liaohe diesel fuel using various ionic liquids at 298.15 K
Ionic liquid
Mass ratio of ionic liquid to diesel fuel
Sulfur mass Sulfur content treated/ % removal/%
[C8Py][BF4]
1Ή5
0.1323
15.84
[C4Py][BF4]
1Ή5
0.1462
7.00
[BePy][BF4]
1Ή5
0.1492
5.09
[C4mim][BF4]
1Ή5
0.1485
8.23
[C8mim][BF4]
1Ή5
0.1319
16.09
[C8Py][BF4]
1Ή2
0.1172
25.45
[C8mim][BF4]
1Ή2
0.1183
24.75
[C4mim][BF4]
1Ή2
0.1374
12.60
[C8Py][BF4]
1Ή1
0.0949
39.63
[C8mim][BF4]
1Ή1
0.0947
39.76
Note: General conditions: extraction time 20 min, initial mass sulfur content 0.1572%.
Chin. J. Chem. Eng., Vol. 16, No. 6, December 2008
results of the 1Ή2 mass ratio also reveals that the desulfurization rate enhances in the order [C4mim][BF4]˘ [C8Py][BF4] § [C8mim][BF4]. These comparisons indicate that these comparisons indicate that the longer the alkyl chain length of cation in the ILs, the better is the absorption capacity of S-compounds. Another conclusion that can be drawn from Table 2 is that the extraction efficiency increases with increasing the mass ratio. Nonetheless, varying the mass ratio from 1Ή1 to 1Ή2 to 1Ή5 does not produce any appreciable change in the desulfurization ability of [C8mim][BF4] relative to that of [C8Py][BF4]. In the case of the influence of diesel fuels, a comparison of the orders obtained from the extraction results for Dongying and Liaohe diesel fuels shows that a variable diesel fuel also produces a changeable order for the extraction efficiency of [BePy][BF4] compared to that of [C4Py][BF4], suggesting that the selectivity of an ionic liquid is necessarily regarded as relative rather than absolute. This accentuates that the structure of the ionic liquid extractant should be specifically optimized to achieve the best desulfurization results for a given diesel fuel. It is reported that AlCl3-based ionic liquids are effective for the removal of S-compounds [10]. However, these ionic liquids are water- and moisture-sensitive; they often form dark precipitates when they contact thiols-containing compounds and thereby cause darkened color in processed fuels [8]. Therefore, this type of ionic liquids may be unlikely to be accepted by refiners [10]. The most commonly used water-insensitive ionic liquids are ˉ ˉ ˉ those involving [BF4] , [PF6] , and [Tf2N] . It was seen that [C6mim][PF6] was slightly less effective on the extraction of S-compounds from gasoline and diesel fuel than [C8mim][BF4] [8]. Our preliminary experimental results of [C4mim][PF6] and [C4mim][BF4] for sulfur removal from Liaohe-diesel fuel also show that the desulfurization capacities of the two ionic liquids are nearly the same. On the other hand, the visˉ cosities and the melting points of the [PF6] -based ionic liquids are higher in comparison with those of ˉ the [BF4] -produced ionic liquids, and the ionic liqˉ uids of [Tf2N] are apparently more expensive than ˉ the ionic liquids involving [BF4] . Therefore, the ionic liquids composed of the latter two anions are less attractive to serve as extractive mediums. [C8mim][BF4] is water-insensitive; it was regenerated by wetting with water after equilibrium absorption. The greater polarity of water empowers its molecules to display stronger interactions with [C8mim][BF4], so that the absorbed S-compounds are quantitatively repelled from the [C8mim][BF4] liquid. The absorbed water was then vaporized from the ionic liquid phase under vacuum at 373.15 K for about 10 h to regenerate the ionic liquid (a typical experiment showed that 95% of ionic liquid was recovered). Thus, the multistage desulfurization of Liaohe diesel fuel with [C8mim][BF4] at 298.15 K was assessed. The contents of sulfur after various cycles of treatments are illustrated in Fig. 2. It is seen from Fig. 2 that the ˉ forth extraction reduces the sulfur level to 701×10 6. Since the examined sample is the “real”, high-sulfur diesel oil, this result indicates that the extraction power of [C8mim][BF4] is remarkable. However, fur-
883
Figure 2 Sulfur content in Liaohe diesel fuel treated with [C8mim][BF4] for different cycles at 298.15 K (initial sulfur content 0.1572%; mass ratio of ionic liquid to oil 1Ή2)
ther investigations are required to reach future technical sulfur content specifications (˘0.0050%). 4
CONCLUSIONS
Ionic liquids can be used as novel extractive solvents for deep desulfurization of diesel fuels, especially for the removal of the sulfur compounds intractable with common hydrodesulfurization techniques. The sulfur-removal ability of an ionic liquid depends on its structure and size. For the two examined diesel fuels, the cation chain length exhibits a pronounced influence on the absorption of S-compounds, with promoted absorption capacity at elongated chain length. The desulfurization ability of the examined ionic liquids also increases with increasing the mass ratio of ionic liquid/diesel fuel. However, the relative ability of an ionic liquid to remove the S-compounds from a given diesel fuel is related to its cation head group. Moreover, with the mass ratio of IL/diesel fuel as 1 Ή 1, the rates of the first desulfurization of Dongying diesel fuel using [C8Py][BF4] and [C8mim][BF4] can reach up to 16.17% and 29.96%, respectively. The rates of the first desulfurization of Liaohe diesel fuel using [C8Py][BF4] and [C8mim][BF4] can reach up to 39.63% and 39.76%, respectively, implying that these ionic liquids are indeed promising extractants for desulfurization. REFERENCES 1
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Kwak, C., Lee, J.J., Bae, J.S., Choi, K., Moonm, S.H., “Hydrodesulfurization of DBT, 4-MDBT, and 4,6-DMDBT on fluorinated CoMoS/Al2O3 catalysts”, Appl. Catal. A, 200, 233242 (2000). Huang, C.P., Chen, B.H., Zhang, J., Liu, Z.C., Li, Y.X., “Desulfurization of gasoline by extraction with new ionic liquids”, Energy & Fuels, 18, 18621864 (2004). Song, C.S., “An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel”, Catal. Today, 86, 211263 (2003). Fukumoto, K., Yoshizawa, M., Ohno, H., “Room temperature ionic liquids from 20 natural amino acids”, J. Am. Chem. Soc., 127, 23982399 (2002). Visser, A.E., Rogers, R.D., “Room-temperature ionic liquids: new solvents for f-element separations and associated solution chemistry”, J. Solid State Chem., 171, 109113 (2003). Fadeev, A.G., Meagher, M.M., “Opportunities for ionic liquids in recovery of biofuels”, Chem. Commun., 295296 (2001). Freemantle, M., “New horizons for ionic liquids”, Chem. Eng. News,
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Chin. J. Chem. Eng., Vol. 16, No. 6, December 2008 79, 2125 (2001). Zhang, S.G., Zhang, Q.L., Zhang, Z.C., “Extractive desulfurization and denitrogenation of fuels using ionic liquids”, Ind. Eng. Chem. Res., 43, 614622 (2004). Zhang, S.G., Zhang, Z.C., “Novel properties of ionic liquids in selective sulfur removal from fuels at room temperature”, Green Chem., 4, 376379 (2002). Bösmann, A., Datsevich, L., Jess, A., Lauter, A., Schmitz, C., Wasserscheid, P., “Deep desulfurization of diesel fuel by extraction with ionic liquids”, Chem. Commun., 24942495 (2001). Esser, J., Wasserscheid, P., Jess, A., “Deep desulfurization of oil refinery streams by extraction with ionic liquids”, Green. Chem., 6, 316322 (2004). Alonso, L., Arce, A., Francisco, M., Soto, A., “Solvent extraction of thiophene from n-alkanes (C7, C12, and C16) using the ionic liquid [C8mim][BF4]”, J. Chem. Thermodyn., 40, 966972 (2008). Fuller, J., Carlin, R.T., Osteryoung, R.A., “The room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate: electrochemical couples and physical properties”, J. Electrochem. Soc., 144, 38813885 (1997). McEwen, A.B., Ngo, H.L., LeCompte, K., Joldman, J.L., “Electrochemical properties of imidazolium salt electrolytes for electrochemical
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capacitor applications”, J. Electrochem. Soc., 146, 16871695 (1999). Wasserscheid, P., Bösmann, A., “1-n-Butyl-3-methylimidazolium ([bmim]) octylsulfate—an even ‘greener’ ionic liquid”, Green Chem., 4, 400404 (2002). Hussey, L., “Density, electric conductivity and viscosity of several N-alkylpyridium halides and their mixtures with aluminum chloride”, J. Electrochem. Soc: Electrochem. Science Tech., 22, 10291031 (1979). Swatloski, R.P., Spear, S.K., Holbrey, J.D., Rogers, R.D., “Dissolution of cellose with ionic liquids”, J. Am. Chem. Soc., 124, 49744976 (2002). Visser, A.E., Holbrey, J.D., Rogers, R.D., “Hydrophobic ionic liquids incorporating N-alkylisoquinolinium cations and their utilization in liquid-liquid separations”, Chem. Comm., 24842485 (2001). Gu, Z.Y., Brennecke, J.F., “Volume expansivities and isothermal compressibilities of imidazolium and pyridinium-based ionic liquids”, J. Chem. Eng. Data, 47, 339346 (2002). Wasserscheid, P., Welton, T., Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, Germany (2003). Jain, N., Kumar, A., Chauhan, S., Chauhan, S.M.S., “Chemical and biochemical transformations in ionic liquids”, Tetrahedron, 61, 10151060 (2005).
Desulfurization of Diesel Fuel by Extraction with [BF4] -based Ionic Liquids* ˉ
CHU Xuemei (ᬖ༲ਜ), HU Yufeng (ܑံ**)ע, LI Jiguang (ह)ڜݽ, LIANG Qianqing (ॣሯு), LIU Yansheng (ঞཧಖ), ZHANG Xianming (ຕੜ), PENG Xiaoming (໒ੜ) and YUE Wenjia (ၣำ)ޤ State Key Laboratory of Heavy Oil, China University of Petroleum, Beijing 102249, China
Abstract The extractive removal of sulfur compounds (S-compounds) from Dongying and Liaohe diesel fuels with [BF4]ˉ-based ionic liquids were systematically investigated. The results show that the absorption capacity of an ionic liquid for the S-compounds in diesel fuels relies on its structure and its size. In the case of the two examined diesel fuels, both elongating the cation tail length and increasing the mass ratio of ionic liquid/diesel fuel promote the desulfurization ability of the examined ionic liquids. The results also show that imidazolium-based ionic liquids display higher extraction efficiencies than pyridinium-based ionic liquids, presumably owing to the fact that the rings of the S-compounds are similar to the imidazolium head ring. With the 1Ή1 mass ratio of ionic liquid/diesel fuel, the rates of the first desulfurization of Dongying and Liaohe diesel fuels using [C8mim][BF4] amount to 29.96% and 39.76%, suggesting that [C8mim][BF4] is a promising extractant for desulfurization of these diesel fuels. Keywords desulfurization, diesel fuel, ionic liquid
1
INTRODUCTION
With the fast development of petrochemistry and the automobile industry, air pollution caused by diesel sulfur oxide (SOx) emissions has become one of the increasingly serious problems in the world. In recent years, considerable attention has been paid to deep desulfurization of gasoline and diesel fuels as the environment regulations on the sulfur limits of fuels are increasingly stringent. In the petroleum industry, desulfurization of diesel is carried out by hydrocracking or hydrotreating processes [1]. Hydrotreating processes are highly effective for reduction of sulfur levels; however, a further improvement of the hydrodesulfurization efficiency is limited to increasingly severe operational conditions at escalated cost [2]. To reduce the sulfur mass content down to 0.001%0.002% [3], the traditional hydrodesulfurization (HDS) process will encounter a great challenge, as hydrogenation of aromatic sulfur compounds (S-compounds) such as benzothiophene (BT) and dibenzothiophene (DBT) requires not only considerably increased energy and hydrogen consumptions but also the substantially improved reactivity and selectivity of the used catalyst. In addition, undesired side reactions will also be induced to result in the decrease of the octane number of gasoline. Therefore, it is critically important to develop new desulfurization technologies to minimize the negative health and environmental pollutions from diesel exhaust gas. Ionic liquids (IL) have been recognized as novel designable solvents (i.e., their properties can be tuned by altering their ionic structures to meet specific demands), which are liquids over a wide temperature range including room temperature. One of the most important advantages is that ionic liquids can be
tuned/controlled by tailoring their cationic and anionic structures to optimize their physicochemical properties [47]. As a kind of extraction solvent, IL does not remain in the organic phase, which can be greatly convenient for separation [8], and therefore, the desulfurization using ionic liquids has received growing ˉ attention [812]. [BF4] -based ionic liquids are promising extractants in future industrial applications because of their low viscosities and wide liquid ranges [1320]. Therefore, in the present article, we focused on the extraction of sulfur compounds from China diesel fuels using this type of ionic liquids. 2
EXPERIMENTAL
All chemicals used in this study were of reagent grade. N-methylimidazole was purchased from Aldrich. n-C4H11Cl, n-C8H17Cl, and NaBF4 were supplied by Shanghai Jiachen Chemical Ltd. Pyridine, benzylchloride, acetone, and ethyl acetate were purchased from Beijing Yili Chemical Ltd. Diesel fuels with different sulfur contents were provided by Sinopec Corporation station. All the present ionic liquids were prepared using well-established procedures [1321]. For illustration, the procedures for preparing the ionic liquids are summarized as follows. The chloride salts were prepared by reacting N-methylimidazole or pyridine with RCl, where, R benzyl, n-C4H11 or n-C8H17. The products were purified by repeated extractions of the remaining starting materials with ethyl acetate. After the last extraction, the remaining ethyl acetate was removed at 343.15 K under vacuum. The resulting [1-alkyl-py][Cl], [1-benzyl-py][Cl], and [1-alkyl-mim][Cl] were dried at 343.15 K under vacuum for 6 d. Equimolar amounts of [1-alkyl-mim][Cl] and NaBF4 were dissolved in acetone separately. The two solutions were gradually mixed together with stirring.
Received 2008-01-30, accepted 2008-06-13. * Supported by the National Natural Science Foundation of China (40673043, 20576073), CNPC Innovation Fund (04E7031), and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-06-0088). ** To whom correspondence should be addressed. E-mail: [email protected]
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Chin. J. Chem. Eng., Vol. 16, No. 6, December 2008
The precipitated sodium chloride was separated from the liquid by filtration. The excess acetone was removed by evaporation, and crude product was dried in vacuum for 48 h. Similar synthesis methods were ˉ adopted for other [BF4] -based ionic liquids. The water contents after drying, measured by Karl Fisher titration, were 0.05%0.1% (by mass). The desulfurization experiments were carried out in glass vials placed in a water bath controlled at (298.15f0.1) K. The mass ratios of ionic liquids to diesel fuel were conducted at 1Ή5, 1Ή2, and 1Ή1, respectively. The preliminary measurement results presented in Fig. 1 show that the liquid-liquid equilibrium (LLE) can be achieved after the biphasic mixtures were stirred for 10 min. Therefore, the real equilibration time was fixed at 2030 min and the organic phase at absorption equilibrium was separated from the ionic liquid phase for analysis. The sulfur contents in diesel fuels before and after each extraction experiment were analyzed using the quantitative elemental analysis conducted on an NS analytical apparatus (ANTEK 7000NS). 3 3.1
RESULTS AND DISCUSSION Determination of the equilibrium time
In the present study, Liaohe diesel fuel was treated with [C8mim][BF4] to establish the time required to reach the absorption equilibrium. The single extractions were conducted for 2, 5, 10, and 20 min at 298.15 K with a fixed mass ratio of 1Ή2 (the ratio of ionic liquid to diesel fuel). The sample in the upper phase was carefully separated with a pipet from the ionic liquid phase for analysis. The results, in Fig. 1 show that 10 min of contact between the diesel fuel phase and the ionic liquid phase is more than sufficient to establish the equilibrium.
Table 1
Sulfur extraction ability of different ILs for diesel fuel from Dongying at 298.15 K
Ionic liquid
Mass ratios of ionic liquid to diesel fuel
Sulfur content treated/%
Sulfur removal/%
[C8Py][BF4]
1Ή5
0.0653
8.16
[BePy][BF4]
1Ή5
0.0673
5.34
[C4Py][BF4]
1Ή5
0.0681
4.22
[C8mim][BF4]
1Ή5
0.0639
10.13
[C4mim][BF4]
1Ή5
0.0670
5.77
[C8Py][BF4]
1Ή2
0.0629
11.53
[C8mim][BF4]
1Ή2
0.0582
18.14
[C8Py][BF4]
1Ή1
0.0596
16.17
[C8mim][BF4]
1Ή1
0.0498
29.96
Note: General conditions: extraction time 20 min, initial sulfur mass content 0.0711%.
sel fuel increase with increasing cation tail length. Besides, it is seen from Table 1 that the order for the desulfurization rate, [C8Py][BF4] ˘ [C8mim][BF4], remains unchanged as the mass ratio decreases from 1Ή5 to 1Ή2 or 1Ή1, implying that the used mass ratio has a little influence on the relative absorption capacities of the examined ionic liquids. Imidazoliumbased ionic liquids show higher extraction efficiencies than pyridinium-based ionic liquids, presumably owing to the fact that the rings of the S-compounds are similar to the imidazolium head ring. It is also seen in Table 1 that the extraction efficiency increases with increasing the mass ratio. 3.3
Desulfurization of Liaohe diesel fuel
Table 2 shows the results for the desulfurization of Liaohe diesel fuel using different ionic liquids at 298.15 K. All ionic liquids under investigation show good abilities to extract S-compounds from the examined diesel fuel. The desulfurization rate measured at a fixed mass ratio of 1 Ή 5 increases in the order [BePy][BF4] ˘ [C4Py][BF4] ˘ [C4mim][BF4] ˘ [C8Py][BF4]§[C8mim][BF4]. An inspection of the Table 2
Figure 1
3.2
Sulfur content versus contact time
Desulfurization of Dongying diesel fuel
The desulfurization results of Dongying diesel fuel with 0.0711% initial sulfur content using different ionic liquids at 298.15 K are shown in Table 1. The used mass ratio of ionic liquid to diesel fuel is 1Ή5. It can be seen that the desulfurization rate follows the order [C4Py][BF4] ˘[BePy][BF4] ˘[C4mim][BF4] ˘ [C8Py][BF4] ˘ [C8mim][BF4]. The conclusion thus derived is that the absorption capacities of the examined ionic liquids for S-compounds in Dongying die-
Sulfur removal from Liaohe diesel fuel using various ionic liquids at 298.15 K
Ionic liquid
Mass ratio of ionic liquid to diesel fuel
Sulfur mass Sulfur content treated/ % removal/%
[C8Py][BF4]
1Ή5
0.1323
15.84
[C4Py][BF4]
1Ή5
0.1462
7.00
[BePy][BF4]
1Ή5
0.1492
5.09
[C4mim][BF4]
1Ή5
0.1485
8.23
[C8mim][BF4]
1Ή5
0.1319
16.09
[C8Py][BF4]
1Ή2
0.1172
25.45
[C8mim][BF4]
1Ή2
0.1183
24.75
[C4mim][BF4]
1Ή2
0.1374
12.60
[C8Py][BF4]
1Ή1
0.0949
39.63
[C8mim][BF4]
1Ή1
0.0947
39.76
Note: General conditions: extraction time 20 min, initial mass sulfur content 0.1572%.
Chin. J. Chem. Eng., Vol. 16, No. 6, December 2008
results of the 1Ή2 mass ratio also reveals that the desulfurization rate enhances in the order [C4mim][BF4]˘ [C8Py][BF4] § [C8mim][BF4]. These comparisons indicate that these comparisons indicate that the longer the alkyl chain length of cation in the ILs, the better is the absorption capacity of S-compounds. Another conclusion that can be drawn from Table 2 is that the extraction efficiency increases with increasing the mass ratio. Nonetheless, varying the mass ratio from 1Ή1 to 1Ή2 to 1Ή5 does not produce any appreciable change in the desulfurization ability of [C8mim][BF4] relative to that of [C8Py][BF4]. In the case of the influence of diesel fuels, a comparison of the orders obtained from the extraction results for Dongying and Liaohe diesel fuels shows that a variable diesel fuel also produces a changeable order for the extraction efficiency of [BePy][BF4] compared to that of [C4Py][BF4], suggesting that the selectivity of an ionic liquid is necessarily regarded as relative rather than absolute. This accentuates that the structure of the ionic liquid extractant should be specifically optimized to achieve the best desulfurization results for a given diesel fuel. It is reported that AlCl3-based ionic liquids are effective for the removal of S-compounds [10]. However, these ionic liquids are water- and moisture-sensitive; they often form dark precipitates when they contact thiols-containing compounds and thereby cause darkened color in processed fuels [8]. Therefore, this type of ionic liquids may be unlikely to be accepted by refiners [10]. The most commonly used water-insensitive ionic liquids are ˉ ˉ ˉ those involving [BF4] , [PF6] , and [Tf2N] . It was seen that [C6mim][PF6] was slightly less effective on the extraction of S-compounds from gasoline and diesel fuel than [C8mim][BF4] [8]. Our preliminary experimental results of [C4mim][PF6] and [C4mim][BF4] for sulfur removal from Liaohe-diesel fuel also show that the desulfurization capacities of the two ionic liquids are nearly the same. On the other hand, the visˉ cosities and the melting points of the [PF6] -based ionic liquids are higher in comparison with those of ˉ the [BF4] -produced ionic liquids, and the ionic liqˉ uids of [Tf2N] are apparently more expensive than ˉ the ionic liquids involving [BF4] . Therefore, the ionic liquids composed of the latter two anions are less attractive to serve as extractive mediums. [C8mim][BF4] is water-insensitive; it was regenerated by wetting with water after equilibrium absorption. The greater polarity of water empowers its molecules to display stronger interactions with [C8mim][BF4], so that the absorbed S-compounds are quantitatively repelled from the [C8mim][BF4] liquid. The absorbed water was then vaporized from the ionic liquid phase under vacuum at 373.15 K for about 10 h to regenerate the ionic liquid (a typical experiment showed that 95% of ionic liquid was recovered). Thus, the multistage desulfurization of Liaohe diesel fuel with [C8mim][BF4] at 298.15 K was assessed. The contents of sulfur after various cycles of treatments are illustrated in Fig. 2. It is seen from Fig. 2 that the ˉ forth extraction reduces the sulfur level to 701×10 6. Since the examined sample is the “real”, high-sulfur diesel oil, this result indicates that the extraction power of [C8mim][BF4] is remarkable. However, fur-
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Figure 2 Sulfur content in Liaohe diesel fuel treated with [C8mim][BF4] for different cycles at 298.15 K (initial sulfur content 0.1572%; mass ratio of ionic liquid to oil 1Ή2)
ther investigations are required to reach future technical sulfur content specifications (˘0.0050%). 4
CONCLUSIONS
Ionic liquids can be used as novel extractive solvents for deep desulfurization of diesel fuels, especially for the removal of the sulfur compounds intractable with common hydrodesulfurization techniques. The sulfur-removal ability of an ionic liquid depends on its structure and size. For the two examined diesel fuels, the cation chain length exhibits a pronounced influence on the absorption of S-compounds, with promoted absorption capacity at elongated chain length. The desulfurization ability of the examined ionic liquids also increases with increasing the mass ratio of ionic liquid/diesel fuel. However, the relative ability of an ionic liquid to remove the S-compounds from a given diesel fuel is related to its cation head group. Moreover, with the mass ratio of IL/diesel fuel as 1 Ή 1, the rates of the first desulfurization of Dongying diesel fuel using [C8Py][BF4] and [C8mim][BF4] can reach up to 16.17% and 29.96%, respectively. The rates of the first desulfurization of Liaohe diesel fuel using [C8Py][BF4] and [C8mim][BF4] can reach up to 39.63% and 39.76%, respectively, implying that these ionic liquids are indeed promising extractants for desulfurization. REFERENCES 1
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