- Email: [email protected]
ionic liquids
Chinese Chemical Letters 24 (2013) 755–758
Contents lists available at SciVerse ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
Original article
Deep extractive desulfurization of diesel fuels by FeCl3/ionic liquids Li-Li Ban, Ping Liu, Cun-Hua Ma, Bin Dai * School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 January 2013 Received in revised form 1 April 2013 Accepted 12 April 2013 Available online 30 May 2013
The extractive desulfurization of dibenzothiophene (DBT), benzothiophene (BT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in model oil was carried out using anhydrous FeCl3 and 1-methyl-3octylimidazolium chloride system ([Omim]Cl2FeCl3). This new system exhibited high extractive efficiency and the sulfur removal of DBT in model oil (VIL/Voil = 1/20) could reach 99.4% at room temperature for 30 min, which was obviously superior to single [Omim]Cl as extractant (22.9%). When the [Omim]Cl2FeCl3 was used, the S-removal of 4,6-DMDBT and BT could also be up to 99.3% and 96.2%, respectively. Moreover, the ionic liquid could be recycled five times without a significant decrease in extractive ability. ß 2013 Bin Dai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Extractive desulfurization FeCl3/[OMIM]Cl Diesel fuels
1. Introduction The development of efficient deep desulfurization processes for light oils has been a challenging work for chemists, since pollution caused by exhaust emissions (especially SOX) is one of the most serious environmental problems in the world [1]. The environmental regulations for transportation fuels are becoming more and more severe, and the S-limit of the gasoline and diesel fuels should be reduced to about 10 ppm in many countries [2]. In the petroleum refining industry, hydrodesulfurization (HDS) is a conventional method for the removal of sulfur compounds [3]. However, high temperature, high-pressure, and the associated high-energy costs are required in the catalytic process. Therefore, alternative desulfurization technologies for diesel fuels have been developed during the last few decades, such as oxidation, extraction, adsorption, biodesulfrization and others. Among these methods, extractive desulfurization (EDS) attracts much more attentions because of its facile operation. Many organic solvents, such as dimethyl sulfoxide (DMSO), acetonitrile, 1-methyl-2-pyrrolidinone (NMP), and dimethylforamide (DMF) have been used as extractive solvents in deep desulfurization. However, the further environmental and safety concerns caused by the flammable and volatile organic compounds, such as wastewater emission and fire hazards, become one of the prime concerns [4]. On the contrary, room temperature ionic liquids (RTILs) are non-volatile, non-flammable, and have high thermal stability. In addition, RTILs are easily regenerated and
* Corresponding author. E-mail address: [email protected] (B. Dai).
recycled, which can effectively avoid further environmental concerns [5]. The extractions of fuels merely using ILs to remove sulfur compounds have been recently reported, but the efficiencies of sulfur removal are rather low. The multistage direct extraction with a large amount of [Omim][BF4] ILs was also studied by Alonso [6], after three stages, the concentration of thiophene is reduced by 79 wt% and that of dibenzothiophene is reduced by 87 wt%. The combination of ionic liquids extraction with chemical oxidation showed improved extractive efficiency. Zhu et al. [7] reported that in the oxidative desulfurization (ODS) of DBT using Et3NHClFeCl3 as the extractive solvent in the presence of H2O2, the sulfur removal could reach 98.3%. The Lewis acidic ILs containing metal halide anions, such as AlCl4 , FeCl4 , and CuCl2 were also found to show promising results on the removal of sulfur-containing compounds [8–10]. To improve the EDS system of model diesel at mild and simple operating conditions and to achieve one-pot extraction desulfurization in a short time with a small amount of ILs, we prepared a 1methyl-3-octylimidazolium chloride ([Omim]Cl) and anhydrous FeCl3 ([Omim]ClxFeCl3, x = 1–2) system easily from cheap starting materials [Omim]Cl and FeCl3, and investigated its properties using it as extractant in EDS of model oil. 2. Experimental ILs [Omim]ClxFeCl3 were prepared by reacting [Omim]Cl (0.1 mol) with anhydrous FeCl3 (x mol, x = 0.5, 1, 1.5, 2) with stirring for 30 min at room temperature. When x = 1.5 and 2, the IL is a reddish brown liquid. ILs [Omim]Cl2CuCl2, [Omim]Cl2BiCl3, [Omim]Cl2CoCl2, [Omim]Cl2MnCl2, [Omim]Cl2ZnCl2 and [Omim]Cl2NiCl2 were synthesized by the same method.
1001-8417/$ – see front matter ß 2013 Bin Dai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2013.04.031
756
L.-L. Ban et al. / Chinese Chemical Letters 24 (2013) 755–758 Table 1 Effects of molar ratios of FeCl3 to [Omim]Cl in the desulfurization system.a
Model diesel was prepared by dissolving DBT, BT or 4,6-DMDBT in n-octane with 200 mg/mL of sulfur content. Desulfurization experiments were conducted in a 50 mL two-necked flask. The required amounts of IL [Omim]Cl2FeCl3 and the model oil containing 200 ppm of DBT were added into the flask in turn, and the mixture was stirred vigorously for 30 min at room temperature. After the extractive solution was rested for 5 min, the upper layer was analyzed. With the same experimental method, the extractive desulfurization experiment was carried out at other temperatures. After the reaction was over, the upper clear solution was collected and determined by WK-2D Microcoulometric detector (Jiangfen Electroanalysis Co., Ltd).
3. Results and discussion The desulfurization results of the model oil extracted by different ILs are shown in Fig. 1. The sulfur removal only got to 17%–30% in the absence of any metal salts. Among the different metal salts systems, the IL [Omim]Cl2FeCl3 exhibited high extraction efficiency, and the sulfur removal of DBT could reach 99.7%. Comparatively, the different metal salts containing various anions showed different desulfurization efficiencies, such as CoCl2 (46.7%), ZnCl2 (60.8%), NiCl2 (48%), MnCl2 (46.7%), and BiCl3 (17.8%). It is clear that [Omim]Cl2FeCl3 had a better extraction ability than the others, and the anion played an important role in the desulfurization system. As such, further research was focused on studying desulfurization systems containing anhydrous FeCl3, and [Omim]Cl. As shown in Table 1, the amount of DBT extracted increased with the molar ratio of FeCl3 and [Omim]Cl increased when the molar ratios of FeCl3 and [Omim]Cl was increased from 0.5:1 to 2:1. DBT was almost completely extracted from the model oil when the molar ratios of FeCl3 and [Omim]Cl was 2:1. This result showed the important role of the molar ratios of [Omim]Cl and FeCl3. For the industrial application of IL, the dosages of IL are of vital importance. The amount of [Omim]Cl2FeCl3 was an important factor in EDS. To investigate the effects of the volume ratios of model oil and IL [Omim]Cl2FeCl3, various ratios of model oil and IL on the removal of DBT are given in Fig. 2. The experimental results exhibited the removal of DBT from the model oil reduced from 99.7% to 90.2% with the ratios increased from 5:1 to 40:1. It is clear that the sulfur removal reached a plateau of 99.4% when the model oil/IL ratio got to 20:1. Fig. 3 shows the effect of reaction time and temperature on the [(Fig._1)TD$IG]DBT removal percentage. Sulfur removal of DBT increased with the
Fig. 1. Effects of ionic liquids on sulfur removal of DBT. Experimental conditions: T = 25 8C, t = 10 min, model oil = 2 mL and IL = 1 mL.
Entry
IL
Sulfur removal (%)
1 2 3 4 5 6b
[Omim]Cl0.5FeCl3 [Omim]Cl1.0FeCl3 [Omim]Cl1.5FeCl3 [Omim]Cl2FeCl3 [Omim]Cl2.5FeCl3 Anhydrous FeCl3
24.4 36.4 80.2 99.4 99.5 43.6
a b
Experimental conditions: T = 25 8C, model oil = 20 mL, IL = 1 mL and t = 30 min. 8.6 mmol FeCl3 was used.
increasing temperatures. The results also demonstrated that, at 25 8C sulfur removal of DBT increased from 95.0% to 99.4% when extractive time rose from 2 min to 20 min. After 20 min, this value tended to be stable, and the sulfur removal of DBT is up to 99.4%. Such an insensitivity of S-removal to extractive temperature is desired in industrial application for ensuring extractive desulfurization carried out at ambient conditions and reducing energy consumption required. To study the performance of [Omim]Cl2FeCl3 on the different sulfur compounds, three model sulfur compounds such as benzothiophene (BT), DBT and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out under the same conditions. As shown in Fig. 4, with the same reaction time, the S-removal through EDS decreased in the order DBT > 4,6-DMDBT > BT. The removal of BT, DBT and 4,6-DMDBT were 78.5%, 95% and 79% after 2 min, respectively. In the case of DBT, the sulfur removal could reach 99.4% in 10 min. However, the result hardly continued improving after 10 min. For BT and 4,6-DMDBT, 96.2% and 99.3% Sremovals were achieved in 30 min. From an industrial point of view, it is vitally important to recycle the IL after the extraction of S-compounds. So, the recycling of EDS system for IL [Omim]Cl2FeCl3 was investigated in removal of DBT in model oil. After the reaction, the system was still a biphasic system, with the IL phase (under-layer) and oil phase (upper-layer). Therefore, the oil phase could be separated from the system easily, the ionic liquid phase was recycled again, then fresh DBT-containing model oil was directly added in for the next run. The data in Fig. 5 indicated that IL could be recycled five times with an unnoticeable decrease in extractive efficiency at room temperature. However, when six cycles finished, the desulfurization yield decreased slightly. The reason was that as the recycle times increased, more and more DBT was produced in the IL, which
[(Fig._2)TD$IG]
Fig. 2. Effects of the volume ratios of model oil and [Omim]Cl2FeCl3 on sulfur removal of DBT. Experimental conditions: T = 25 8C and t = 30 min.
[(Fig._3)TD$IG]
[(Fig._5)TD$IG]
L.-L. Ban et al. / Chinese Chemical Letters 24 (2013) 755–758
Fig. 3. Effects of the time and temperature on sulfur removal of DBT. Experimental conditions: model oil = 20 mL and IL ([Omim]Cl2FeCl3) = 1 mL.
led to the decrease in the extraction performance of IL and the desulfurization activity. So at the end of each cycle, the IL phase was re-extracted with tetrachloromethane twice in an oil bath at 50 8C in order to remove DBT from the IL. The regenerated IL was then recycled into the desulfurization. [Omim]Cl2FeCl3 shows the highest sulfur removal efficiency among the investigated ILs. Unlike AlCl3-based ionic liquids, [Omim]Cl2FeCl3 is air and moisture stable. From the above experiments, such a difference indicates that the desulfurization mechanism for [Omim]Cl2FeCl3 is different from that of other ionic liquids. The electronic configuration of Fe3+ is 1s22s22p63s23p63d54s0. According to p-complexation mechanism the cations with empty s-orbitals and the electron density can form the usual s bonds with their s-orbitals, in addition, their d-orbitals can back-donate electron density to the anti-bonding p-orbitals of the sulfur rings. Fe3+ can form p-complexation bonding with aromatic sulfur compounds. Our results support the theory proposed in the literature [11]: the p-complexation of thiophene and Fe2Cl7 is one of the important desulfurization mechanisms for FeCl3-based ionic liquid. All our above theoretical results just explain the high EDS
[(Fig._4)TD$IG]
757
Fig. 5. Recycling of ionic liquid [Omim]Cl2FeCl3. Experimental conditions: T = 25 8C, t = 30 min, model oil = 20 mL and IL ([Omim]Cl2FeCl3) = 1 mL.
performance of the Lewis-acid ILs, since then it is derived by the [Omim]Cl2FeCl3, [Omim]Cl2FeCl3 might be used as a promising solvent for the desulfurization of diesel by an extractive desulfurization process. 4. Conclusion A series of ionic liquids based on metal salts were synthesized and employed in extractive desulfurization systems, the IL containing anhydrous FeCl3 and [Omim]Cl showed the highest extractive efficiency. The sulfur removal of DBT-containing model oil could reach 99.4% with [Omim]Cl2FeCl3 as extractant at room temperature, even when the model oil/IL ratio got to 20:1. For BT and 4,6-DMDBT, 96.2% and 99.3% S-removals were also achieved in 30 min. The extractive efficiency of sulfur-containing compounds decreased in the order of DBT > BT > 4,6-DMDBT. Moreover, this IL [Omim]Cl2FeCl3 could be recycled five times without a significant decrease in activity. With the efficient production of ionic liquids in the chemical industry, this method of extractive desulfurization may be a simple, promising and environmentally friendly process. Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 21063012), Changjiang Scholars and Innovative Research Team in University (No. IRT1161) and Innovation Funds for distinguished young Scientists of Xinjiang Bingtuan (No. 2011CD001). References
Fig. 4. Effects of the substrate nature on sulfur removal. Experimental conditions: T = 25 8C, t = 10 min, model oil = 20 mL and IL ([Omim]Cl2FeCl3) = 1 mL.
[1] Y.W. Hu, Q.H. He, Z. Zhang, et al., Oxidative desulfurization of dibenzothiophene with hydrogen peroxide catalyzed by selenium(IV)-containing peroxotungstate, Chem. Commun. 47 (2011) 12194–12196. [2] V.C. Srivastava, An evaluation of desulfurization technologies for sulfur removal from liquid fuels, RSC Adv. 2 (2012) 759–783. [3] C. Sepulveda, V. Bellie`re, D. Laurenti, et al., Supported rhenium sulfide catalysts in thiophene and 4,6-dimethyldibenzothiophene hydrodesulfurization: effect of acidity of the support over activities, Appl. Catal. A: Gen. 393 (2011) 288–293. [4] F.T. Li, R.H. Liu, J.H. Wen, et al., Desulfurization of dibenzothiophene by chemical oxidation and solvent extraction with Me3NCH2C6H5Cl32ZnCl2 ionic liquid, Green Chem. 11 (2009) 883–892. [5] A. Seeberger, A. Jess, Desulfurization of diesel oil by selective oxidation and extraction of sulfur compounds by ionic liquids – a contribution to a competitive process design, Green Chem. 12 (2010) 602–608.
758
L.-L. Ban et al. / Chinese Chemical Letters 24 (2013) 755–758
[6] L. Alonso, A. Arce, M. Francisco, et al., Measurement and correlation of liquid– liquid equilibria of two imidazolium ionic liquids with thiophene and methylcyclohexane, J. Chem. Eng. Data 52 (2007) 2409–3241. [7] W.S. Zhu, J.T. Zhang, H.M. Li, et al., Fenton-like ionic liquids/H2O2 system: one-pot extraction combined with oxidation desulfurization of fuel, RSC Adv. 2 (2012) 658–664. [8] C. Huang, B. Chen, J. Zhang, et al., Desulfurization of gasoline by extraction with new ionic liquids, Energy Fuels 18 (2004) 1862–1864.
[9] A. Bosmann, L. Datsevich, A. Jess, et al., Deep desulfurization of diesel fuel by extraction with ionic liquids, Chem. Commun. 23 (2001) 2494–2495. [10] M. Wang, B. Li, C.J. Zhao, J. Korean, Recovery of [BMIM]FeCl4 from homogeneous mixture using a simple chemical method, Korean J. Chem. Eng. 27 (2010) 1275–1277. [11] N.H. Ko, J.S. Lee, E.S. Huh, et al., Extractive desulfurization using Fe-containing ionic liquids, Energy Fuels 22 (2008) 1687–1690.
Contents lists available at SciVerse ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
Original article
Deep extractive desulfurization of diesel fuels by FeCl3/ionic liquids Li-Li Ban, Ping Liu, Cun-Hua Ma, Bin Dai * School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 January 2013 Received in revised form 1 April 2013 Accepted 12 April 2013 Available online 30 May 2013
The extractive desulfurization of dibenzothiophene (DBT), benzothiophene (BT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in model oil was carried out using anhydrous FeCl3 and 1-methyl-3octylimidazolium chloride system ([Omim]Cl2FeCl3). This new system exhibited high extractive efficiency and the sulfur removal of DBT in model oil (VIL/Voil = 1/20) could reach 99.4% at room temperature for 30 min, which was obviously superior to single [Omim]Cl as extractant (22.9%). When the [Omim]Cl2FeCl3 was used, the S-removal of 4,6-DMDBT and BT could also be up to 99.3% and 96.2%, respectively. Moreover, the ionic liquid could be recycled five times without a significant decrease in extractive ability. ß 2013 Bin Dai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Extractive desulfurization FeCl3/[OMIM]Cl Diesel fuels
1. Introduction The development of efficient deep desulfurization processes for light oils has been a challenging work for chemists, since pollution caused by exhaust emissions (especially SOX) is one of the most serious environmental problems in the world [1]. The environmental regulations for transportation fuels are becoming more and more severe, and the S-limit of the gasoline and diesel fuels should be reduced to about 10 ppm in many countries [2]. In the petroleum refining industry, hydrodesulfurization (HDS) is a conventional method for the removal of sulfur compounds [3]. However, high temperature, high-pressure, and the associated high-energy costs are required in the catalytic process. Therefore, alternative desulfurization technologies for diesel fuels have been developed during the last few decades, such as oxidation, extraction, adsorption, biodesulfrization and others. Among these methods, extractive desulfurization (EDS) attracts much more attentions because of its facile operation. Many organic solvents, such as dimethyl sulfoxide (DMSO), acetonitrile, 1-methyl-2-pyrrolidinone (NMP), and dimethylforamide (DMF) have been used as extractive solvents in deep desulfurization. However, the further environmental and safety concerns caused by the flammable and volatile organic compounds, such as wastewater emission and fire hazards, become one of the prime concerns [4]. On the contrary, room temperature ionic liquids (RTILs) are non-volatile, non-flammable, and have high thermal stability. In addition, RTILs are easily regenerated and
* Corresponding author. E-mail address: [email protected] (B. Dai).
recycled, which can effectively avoid further environmental concerns [5]. The extractions of fuels merely using ILs to remove sulfur compounds have been recently reported, but the efficiencies of sulfur removal are rather low. The multistage direct extraction with a large amount of [Omim][BF4] ILs was also studied by Alonso [6], after three stages, the concentration of thiophene is reduced by 79 wt% and that of dibenzothiophene is reduced by 87 wt%. The combination of ionic liquids extraction with chemical oxidation showed improved extractive efficiency. Zhu et al. [7] reported that in the oxidative desulfurization (ODS) of DBT using Et3NHClFeCl3 as the extractive solvent in the presence of H2O2, the sulfur removal could reach 98.3%. The Lewis acidic ILs containing metal halide anions, such as AlCl4 , FeCl4 , and CuCl2 were also found to show promising results on the removal of sulfur-containing compounds [8–10]. To improve the EDS system of model diesel at mild and simple operating conditions and to achieve one-pot extraction desulfurization in a short time with a small amount of ILs, we prepared a 1methyl-3-octylimidazolium chloride ([Omim]Cl) and anhydrous FeCl3 ([Omim]ClxFeCl3, x = 1–2) system easily from cheap starting materials [Omim]Cl and FeCl3, and investigated its properties using it as extractant in EDS of model oil. 2. Experimental ILs [Omim]ClxFeCl3 were prepared by reacting [Omim]Cl (0.1 mol) with anhydrous FeCl3 (x mol, x = 0.5, 1, 1.5, 2) with stirring for 30 min at room temperature. When x = 1.5 and 2, the IL is a reddish brown liquid. ILs [Omim]Cl2CuCl2, [Omim]Cl2BiCl3, [Omim]Cl2CoCl2, [Omim]Cl2MnCl2, [Omim]Cl2ZnCl2 and [Omim]Cl2NiCl2 were synthesized by the same method.
1001-8417/$ – see front matter ß 2013 Bin Dai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2013.04.031
756
L.-L. Ban et al. / Chinese Chemical Letters 24 (2013) 755–758 Table 1 Effects of molar ratios of FeCl3 to [Omim]Cl in the desulfurization system.a
Model diesel was prepared by dissolving DBT, BT or 4,6-DMDBT in n-octane with 200 mg/mL of sulfur content. Desulfurization experiments were conducted in a 50 mL two-necked flask. The required amounts of IL [Omim]Cl2FeCl3 and the model oil containing 200 ppm of DBT were added into the flask in turn, and the mixture was stirred vigorously for 30 min at room temperature. After the extractive solution was rested for 5 min, the upper layer was analyzed. With the same experimental method, the extractive desulfurization experiment was carried out at other temperatures. After the reaction was over, the upper clear solution was collected and determined by WK-2D Microcoulometric detector (Jiangfen Electroanalysis Co., Ltd).
3. Results and discussion The desulfurization results of the model oil extracted by different ILs are shown in Fig. 1. The sulfur removal only got to 17%–30% in the absence of any metal salts. Among the different metal salts systems, the IL [Omim]Cl2FeCl3 exhibited high extraction efficiency, and the sulfur removal of DBT could reach 99.7%. Comparatively, the different metal salts containing various anions showed different desulfurization efficiencies, such as CoCl2 (46.7%), ZnCl2 (60.8%), NiCl2 (48%), MnCl2 (46.7%), and BiCl3 (17.8%). It is clear that [Omim]Cl2FeCl3 had a better extraction ability than the others, and the anion played an important role in the desulfurization system. As such, further research was focused on studying desulfurization systems containing anhydrous FeCl3, and [Omim]Cl. As shown in Table 1, the amount of DBT extracted increased with the molar ratio of FeCl3 and [Omim]Cl increased when the molar ratios of FeCl3 and [Omim]Cl was increased from 0.5:1 to 2:1. DBT was almost completely extracted from the model oil when the molar ratios of FeCl3 and [Omim]Cl was 2:1. This result showed the important role of the molar ratios of [Omim]Cl and FeCl3. For the industrial application of IL, the dosages of IL are of vital importance. The amount of [Omim]Cl2FeCl3 was an important factor in EDS. To investigate the effects of the volume ratios of model oil and IL [Omim]Cl2FeCl3, various ratios of model oil and IL on the removal of DBT are given in Fig. 2. The experimental results exhibited the removal of DBT from the model oil reduced from 99.7% to 90.2% with the ratios increased from 5:1 to 40:1. It is clear that the sulfur removal reached a plateau of 99.4% when the model oil/IL ratio got to 20:1. Fig. 3 shows the effect of reaction time and temperature on the [(Fig._1)TD$IG]DBT removal percentage. Sulfur removal of DBT increased with the
Fig. 1. Effects of ionic liquids on sulfur removal of DBT. Experimental conditions: T = 25 8C, t = 10 min, model oil = 2 mL and IL = 1 mL.
Entry
IL
Sulfur removal (%)
1 2 3 4 5 6b
[Omim]Cl0.5FeCl3 [Omim]Cl1.0FeCl3 [Omim]Cl1.5FeCl3 [Omim]Cl2FeCl3 [Omim]Cl2.5FeCl3 Anhydrous FeCl3
24.4 36.4 80.2 99.4 99.5 43.6
a b
Experimental conditions: T = 25 8C, model oil = 20 mL, IL = 1 mL and t = 30 min. 8.6 mmol FeCl3 was used.
increasing temperatures. The results also demonstrated that, at 25 8C sulfur removal of DBT increased from 95.0% to 99.4% when extractive time rose from 2 min to 20 min. After 20 min, this value tended to be stable, and the sulfur removal of DBT is up to 99.4%. Such an insensitivity of S-removal to extractive temperature is desired in industrial application for ensuring extractive desulfurization carried out at ambient conditions and reducing energy consumption required. To study the performance of [Omim]Cl2FeCl3 on the different sulfur compounds, three model sulfur compounds such as benzothiophene (BT), DBT and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out under the same conditions. As shown in Fig. 4, with the same reaction time, the S-removal through EDS decreased in the order DBT > 4,6-DMDBT > BT. The removal of BT, DBT and 4,6-DMDBT were 78.5%, 95% and 79% after 2 min, respectively. In the case of DBT, the sulfur removal could reach 99.4% in 10 min. However, the result hardly continued improving after 10 min. For BT and 4,6-DMDBT, 96.2% and 99.3% Sremovals were achieved in 30 min. From an industrial point of view, it is vitally important to recycle the IL after the extraction of S-compounds. So, the recycling of EDS system for IL [Omim]Cl2FeCl3 was investigated in removal of DBT in model oil. After the reaction, the system was still a biphasic system, with the IL phase (under-layer) and oil phase (upper-layer). Therefore, the oil phase could be separated from the system easily, the ionic liquid phase was recycled again, then fresh DBT-containing model oil was directly added in for the next run. The data in Fig. 5 indicated that IL could be recycled five times with an unnoticeable decrease in extractive efficiency at room temperature. However, when six cycles finished, the desulfurization yield decreased slightly. The reason was that as the recycle times increased, more and more DBT was produced in the IL, which
[(Fig._2)TD$IG]
Fig. 2. Effects of the volume ratios of model oil and [Omim]Cl2FeCl3 on sulfur removal of DBT. Experimental conditions: T = 25 8C and t = 30 min.
[(Fig._3)TD$IG]
[(Fig._5)TD$IG]
L.-L. Ban et al. / Chinese Chemical Letters 24 (2013) 755–758
Fig. 3. Effects of the time and temperature on sulfur removal of DBT. Experimental conditions: model oil = 20 mL and IL ([Omim]Cl2FeCl3) = 1 mL.
led to the decrease in the extraction performance of IL and the desulfurization activity. So at the end of each cycle, the IL phase was re-extracted with tetrachloromethane twice in an oil bath at 50 8C in order to remove DBT from the IL. The regenerated IL was then recycled into the desulfurization. [Omim]Cl2FeCl3 shows the highest sulfur removal efficiency among the investigated ILs. Unlike AlCl3-based ionic liquids, [Omim]Cl2FeCl3 is air and moisture stable. From the above experiments, such a difference indicates that the desulfurization mechanism for [Omim]Cl2FeCl3 is different from that of other ionic liquids. The electronic configuration of Fe3+ is 1s22s22p63s23p63d54s0. According to p-complexation mechanism the cations with empty s-orbitals and the electron density can form the usual s bonds with their s-orbitals, in addition, their d-orbitals can back-donate electron density to the anti-bonding p-orbitals of the sulfur rings. Fe3+ can form p-complexation bonding with aromatic sulfur compounds. Our results support the theory proposed in the literature [11]: the p-complexation of thiophene and Fe2Cl7 is one of the important desulfurization mechanisms for FeCl3-based ionic liquid. All our above theoretical results just explain the high EDS
[(Fig._4)TD$IG]
757
Fig. 5. Recycling of ionic liquid [Omim]Cl2FeCl3. Experimental conditions: T = 25 8C, t = 30 min, model oil = 20 mL and IL ([Omim]Cl2FeCl3) = 1 mL.
performance of the Lewis-acid ILs, since then it is derived by the [Omim]Cl2FeCl3, [Omim]Cl2FeCl3 might be used as a promising solvent for the desulfurization of diesel by an extractive desulfurization process. 4. Conclusion A series of ionic liquids based on metal salts were synthesized and employed in extractive desulfurization systems, the IL containing anhydrous FeCl3 and [Omim]Cl showed the highest extractive efficiency. The sulfur removal of DBT-containing model oil could reach 99.4% with [Omim]Cl2FeCl3 as extractant at room temperature, even when the model oil/IL ratio got to 20:1. For BT and 4,6-DMDBT, 96.2% and 99.3% S-removals were also achieved in 30 min. The extractive efficiency of sulfur-containing compounds decreased in the order of DBT > BT > 4,6-DMDBT. Moreover, this IL [Omim]Cl2FeCl3 could be recycled five times without a significant decrease in activity. With the efficient production of ionic liquids in the chemical industry, this method of extractive desulfurization may be a simple, promising and environmentally friendly process. Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 21063012), Changjiang Scholars and Innovative Research Team in University (No. IRT1161) and Innovation Funds for distinguished young Scientists of Xinjiang Bingtuan (No. 2011CD001). References
Fig. 4. Effects of the substrate nature on sulfur removal. Experimental conditions: T = 25 8C, t = 10 min, model oil = 20 mL and IL ([Omim]Cl2FeCl3) = 1 mL.
[1] Y.W. Hu, Q.H. He, Z. Zhang, et al., Oxidative desulfurization of dibenzothiophene with hydrogen peroxide catalyzed by selenium(IV)-containing peroxotungstate, Chem. Commun. 47 (2011) 12194–12196. [2] V.C. Srivastava, An evaluation of desulfurization technologies for sulfur removal from liquid fuels, RSC Adv. 2 (2012) 759–783. [3] C. Sepulveda, V. Bellie`re, D. Laurenti, et al., Supported rhenium sulfide catalysts in thiophene and 4,6-dimethyldibenzothiophene hydrodesulfurization: effect of acidity of the support over activities, Appl. Catal. A: Gen. 393 (2011) 288–293. [4] F.T. Li, R.H. Liu, J.H. Wen, et al., Desulfurization of dibenzothiophene by chemical oxidation and solvent extraction with Me3NCH2C6H5Cl32ZnCl2 ionic liquid, Green Chem. 11 (2009) 883–892. [5] A. Seeberger, A. Jess, Desulfurization of diesel oil by selective oxidation and extraction of sulfur compounds by ionic liquids – a contribution to a competitive process design, Green Chem. 12 (2010) 602–608.
758
L.-L. Ban et al. / Chinese Chemical Letters 24 (2013) 755–758
[6] L. Alonso, A. Arce, M. Francisco, et al., Measurement and correlation of liquid– liquid equilibria of two imidazolium ionic liquids with thiophene and methylcyclohexane, J. Chem. Eng. Data 52 (2007) 2409–3241. [7] W.S. Zhu, J.T. Zhang, H.M. Li, et al., Fenton-like ionic liquids/H2O2 system: one-pot extraction combined with oxidation desulfurization of fuel, RSC Adv. 2 (2012) 658–664. [8] C. Huang, B. Chen, J. Zhang, et al., Desulfurization of gasoline by extraction with new ionic liquids, Energy Fuels 18 (2004) 1862–1864.
[9] A. Bosmann, L. Datsevich, A. Jess, et al., Deep desulfurization of diesel fuel by extraction with ionic liquids, Chem. Commun. 23 (2001) 2494–2495. [10] M. Wang, B. Li, C.J. Zhao, J. Korean, Recovery of [BMIM]FeCl4 from homogeneous mixture using a simple chemical method, Korean J. Chem. Eng. 27 (2010) 1275–1277. [11] N.H. Ko, J.S. Lee, E.S. Huh, et al., Extractive desulfurization using Fe-containing ionic liquids, Energy Fuels 22 (2008) 1687–1690.