11760
Ind. Eng. Chem. Res. 2010, 49, 11760–11763
Oxidative Desulfurization of Dibenzothiophene Catalyzed by Ionic Liquid [BMIm]HSO4 Wei Zhang,†,‡ Ke Xu,†,‡ Qian Zhang,†,‡ Daliang Liu,†,‡ Shuyao Wu,†,‡ Francis Verpoort,†,‡,§ and Xi-Ming Song*,†,‡ Liaoning ProVincial Key Laboratory for Green Synthesis and PreparatiVe Chemistry of AdVanced Materials, Liaoning UniVersity, Shenyang 110036, People’s Republic of China, College of Chemistry, Liaoning UniVersity, Shenyang 110036, People’s Republic of China, and Department of Inorganic and Physical Chemistry, Laboratory of Organometallic Chemistry and Catalysis, Ghent UniVersity, Krijgslaan 281 (S-3), 9000 Ghent, Belgium
An acidic ionic liquid N-butyl-N-methylimidazolium hydrogen sulfate ([BMIm]HSO4) was applied as extractant and catalyst for the oxidative desulfurization of dibenzothiophenes (DBT) in the presence of H2O2 in model oil. Several parameters, e.g., catalyst amount, hydrogen peroxide quantity, reaction time, and temperature, were investigated in detail. The catalytic oxidative desulfurization rate can reach 100% for DBT in model oil. The ionic liquid [BMIm]HSO4 can be recycled 5 times with only a slight reduction in activity. Introduction In view of stringent environmental regulations, utilization of sulfur-containing fuel oils has severe limitations regarding emission of sulfur dioxide. The Environmental Protection Agency (EPA) regulations in the United States1,2 require a reduction of the sulfur content in fuel oils. By December 1, 2010, the petroleum industry will produce ultra low sulfur diesel (ULSD) fuel containing a maximum of 15 ppm sulfur. In the past few decades, oxidative desulfurization (ODS) has attracted much attention. In ODS process aromatic sulfurcontaining compounds, which are difficult to remove by conventional hydrodesulfurization (HDS) process, can be oxidized efficiently to their corresponding sulfoxides and sulfones. The oxidized products are subsequently removed by either solvent extraction or adsorption from oil phase.3-11 However, a large amount of flammable and volatile organic compounds are consumed as an extractant in this process resulting in additional environmental and safety concerns. Ionic liquids (ILs) have recently gained recognition as environmentally benign alternative solvents applicable in separations, synthesis, electrochemistry, and catalysis.12-17 Various kinds of ionic liquids have been used as extractants for the removal of sulfur-containing compounds from fuels.18-22 Nevertheless, the efficiencies of sulfur removal are not favorable and multiple extractions are required to obtain deep desulfurization. In 2003, Lo et al.23 first reported chemical oxidation in conjunction with IL extraction for oxidative desulfurization. Using the IL [BMIm]PF6 (Figure 1) as extractant, acetic acid as catalyst, and H2O2 as oxidant, the sulfur removal of DBT in model oil was increased significantly to 85%. The oxidation of organosulfur compounds to their corresponding sulfones catalyzed by polyoxometallic acids and their salts in ILs was also reported.24,25 Furthermore, it is well-known that homogeneous catalysts are difficult to separate from their reaction products limiting their recyclability. Currently, more efficient ODS systems solely containing H2O2 as oxidant, acidic ionic liquid * To whom correspondence should be addressed. Tel: +86-2462202378. Fax: +86-24-62202380. E-mail:
[email protected]. † Liaoning Provincial Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University. ‡ College of Chemistry, Liaoning University. § Ghent University.
[HMIm]BF4 or [Hnmp]BF4 (Figure 1) as extractant and catalyst have been reported by Lu et al.26 and Zhao et al.27 respectively. Although high DBT removal was obtained in these two ODS systems, fluoroboric acid, a strong acid with extreme corrosion and high toxicity, was required in the preparation of these two acidic ionic liquids. Besides, it has been proved that fluorous containing materials are potentially harmful to the environment.28 Clearly, it is significant to explore more green acidic ionic liquids for ODS. [BMIm]HSO4 (Figure 1) is an ionic liquid containing an acidic counterion, which is prepared easily. It is “more green” compared with [HMIm]BF4 and [Hnmp]BF4 because it does not contain any fluorous element. In this study [BMIm]HSO4 was used as extractant and catalyst for the ODS of DBT in model oil in combination with H2O2. The effects of the amount of IL and oxidant, reaction time, and temperature on the DBT removal were investigated in detail. Experimental Section Preparation of [BMIm]HSO4. [BMIm]HSO4 was prepared according to a literature procedure.29 A mixture of equimolar amounts of 1-methylimidazole and 1-chlorobutane was stirred at 70 °C for 72 h. The resulting viscous liquid was cooled to room temperature and washed with ethyl acetate until no raw materials were detected by thin-layer chromatography. The residue was dried in vacuum to give 1-butyl-3-methylimidazolium chloride ([BMIm]Cl) as a white solid. Equivalent amounts of [BMIm]Cl and sodium hydrogen sulfate were added into dry acetonitrile (300 mL) and the mixture was stirred vigorously at 25 °C under nitrogen atmosphere for 4 days. After removal of the precipitated salt (NaCl) by filtration, the solvent was removed using rotary evaporation to give a pale yellow liquid, see note.30
Figure 1. Structures of ILs [BMIm]PF6, [HMIm]BF4, [Hnmp]BF4, and [BMIm]HSO4.
10.1021/ie100957k 2010 American Chemical Society Published on Web 10/08/2010
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010
11761
Scheme 1. Desulfurization Process of DBT in Model Oil Applying the H2O2-[BMIm]HSO4 System
Figure 2. HPLC chromatograms of DBT in model oil before extraction, after extraction and before oxidation, during oxidation, and after oxidation. Conditions: VOil ) 5 mL; mIL ) 3 g; O/S ) 4; T ) 60 °C; t ) 30 min.
Oxidative Desulfurization of Model Oil. Model oil (Scontent 1000 µg/mL) was prepared by dissolving DBT (1.4368 g) in n-octane (250 mL). A typical oxidative desulfurization reaction was executed as follows: model oil (5 mL) and ionic liquid (3 g) were stirred vigorously at 60 °C for 30 min in the reactor. The DBT removal determined at this time represent the extractive ability of [BMIm]HSO4. The oxidative desulfurization reaction starts from the moment a certain amount of H2O2 (37%) is added carefully to the mixture. Subsequently, samples are withdrawn periodically from the upper phase (octane) and analyzed for their DBT amount. Recovery/Regeneration of Used Ionic Liquid. The oil phase was separated by decantation from the IL after reaction. The IL phase was reused after being dried in vacuum at room temperature for 6 h. Analytical Methods. HPLC was used for the quantitative assay of DBT in the n-octane phase. HPLC was performed applying an Agilent 1100 (HP 1100, Agilent, USA) liquid chromatograph equipped with an auto sampler, a reversed-phase Zorbax SB-C18 column (4.6 mm ×150 mm; 3.6 µm), and a diode array detector. The mobile phase was 90% methanol in water (v/v, %) with a f1ow rate of 1.0 mL/min. For the quantification of DBT, the external standard method was used at 280 nm. Results and Discussion Process of Extraction/Oxidation Desulfurization by [BMIm]HSO4. Figure 2 shows the HPLC analyses of DBT in n-octane before extraction, at extraction equilibrium, during the oxidation, and after the oxidation. By comparing the DBT peaks in the two upper chromatograms, it can be found that merely a small part of DBT in oil phase was removed when the extraction equilibrium was reached after addition of [BMIm]HSO4. However, when H2O2 was added, the DBT content reduced rapidly. The bottom curve shows that the DBT peak disappeared completely, indicating a full removal of DBT from the model oil applying this H2O2-[BMIm]HSO4 system. Moreover, no products were found in the oil phase because of the high polarity of the ionic liquid. From the above it follows that the desulfurization process in this H2O2-[BMIm]HSO4 system can be deduced, just like the process H2O2-AcOH-IL system studied by Lo et al.,23 as shown in Scheme 1. In a combination of extraction and oxidation, DBT was oxidized in the ionic liquid phase as it was extracted from the oil phase, so a continuous decrease in the concentration of DBT in n-octane was observed during the oxidation process.
Effect of H2O2/Sulfur Molar Ratio (O/S) on DBT Removal. According to the stoichiometric reaction, 2 mol of H2O2 are consumed for the oxidation of 1 mol of sulfur-containing compound producing the corresponding sulfones (Scheme 1). To investigate the influence of the amount of oxidant, the oxidation of DBT applying the H2O2-[BMIm]HSO4 system under various O/S molar ratios was executed first. As depicted in Figure 3, the O/S molar ratio has a strong influence on the reaction rate. Removal of DBT increased from 86% at O/S ) 2 to 97% at O/S ) 3 in 30 min. When O/S ) 4, removal of DBT reached 99% in 30 min. This effect can be assigned to the increasing concentration of H2O2 resulting in a higher possibility to oxidize sulfur compounds. Since it is not economically meaningful using larger amounts of H2O2, an O/S molar ratio of 4 is taken as the optimal ratio in this study. Effect of the Amount of [BMIm]HSO4 on the Removal of DBT. The minimum amount of the ionic liquid for deep oxidative desulfurization at a fixed H2O2/sulfur molar ratio (O/S) was investigated. Figure 4 shows that the extractive desulfurization (%) before the addition of H2O2 is relative to the amount of [BMIm]HSO4. After addition of the oxidant, removal of DBT increased rapidly. A removal of 72% was obtained applying 2 g of ionic liquid whereas 3 g of [BMIm]HSO4 resulted in a removal of 97% in 30 min. When the mass of [BMIm]HSO4 was raised to 4 g, DBT removal decreased to 90%. Although one would expect a higher DBT removal using a larger amount of [BMIm]HSO4 this is not the case. A reasonable explanation can be given after drawing the conclusion from Figure 3. It is clear from Figure 3 that with an increasing O/S ratio the removal of DBT will increase too. Important to
Figure 3. Effect of H2O2/sulfur molar ratio (O/S) on the removal of DBT. Conditions: T ) 70 °C; VOil ) 5 mL; mIL ) 3 g.
11762
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 Table 1. Recycling Results of [BMIm]HSO4 in the Desulfurization of DBT-Containing Model Oila cycle
1
2
3
4
5
6
DBT removal (%)
100
100
99
98
97
95
a
Conditions: T ) 60 °C; VOil ) 5 mL; mIL ) 3 g; O/S ) 4; t ) 60
min.
Figure 4. Effect of the amount of [BMIm]HSO4 on DBT removal. Conditions: T ) 70 °C; VOil ) 5 mL; O/S ) 3.
vacuum at room temperature for 6 h after the reaction. Subsequently, fresh H2O2 and model oil were introduced for the successive reaction. The data listed in Table 1 indicate that [BMIm]HSO4 can be recycled five times with a slight loss in activity, e.g., DBT removal dropped from 100% to 95% under the same experimental conditions. After the first recycling of the IL, some white crystalline product was generated at the bottom of the reactor. After each recycling step the amount of the white crystalline product increased. From the IR-spectrum of this crystalline product two absorption bands at 1125 and 1289 cm-1 were observed which can be assigned to the two sulfone groups respectively.7 Conclusions The ionic liquid [BMIm]HSO4 represents a new green recyclable catalyst with a high activity in the oxidative desulfurization of DBT in model oil. No addition of acid and no fluorous containing materials are necessary. DBT removal can reach 100% under optimized reaction conditions, which is superior to the simple extraction with ionic liquids. The IL can be recycled five times with a slight decrease in activity. Acknowledgment
Figure 5. Effect of the reaction temperature on DBT removal. Conditions: VOil ) 5 mL; mIL ) 3 g; O/S ) 4.
We are grateful for financial support from Science Foundation of Educational Department of Liaoning Province(2006T070, 2009S042, 2005L158) and Science foundation of Liaoning University (2008LDQN09). Literature Cited
mention is that the amount of IL is kept constant. The opposite is also true, meaning that with a declining O/S ratio the removal of DBT will decrease when keeping the amount of IL constant. Studying the effect of the amount of IL, the O/S ratio is kept constant and the amount of IL will be raised. However, applying higher amounts of IL means that higher amounts of DBT can be extracted (see Figure 4 at t ) 0) and eventually results in a diminishment of the O/S ratio. Since the O/S ratio is reduced using more IL this will result in a lower removal of DBT. This reasoning clearly explains the results. Therefore, as a consequence of these results, 3 g of [BMIm]HSO4 will be used in the continuation of this study. Effect of Reaction Temperature on DBT Removal. From Figure 5 it follows that the extractive desulfurization (19-21%) before addition of H2O2 slightly modified with the increasing of the temperature. Raising the reaction temperature from 50 to 60 °C, a remarkable increase in the reaction rate was observed. The complete conversion at 60 °C was realized in 30 min. If the temperature was raised to 70 or 80 °C, the desulfurization decreased to 99% and 96%, respectively. This may be caused by the decomposition of H2O2 at higher temperature.26,31 Therefore, 60 °C was taken as the optimal reaction temperature in the present study. Regeneration/Recycling of the Ionic Liquid. In the last part of this study the recycling of [BMIm]HSO4 in the oxidative desulfurization of DBT in model oil was also investigated. Therefore, the ionic liquid phase (under-layer) was dried in
(1) US Environmental Protection Agency. Federal Register February 10, 2000 (65 FR 6698). (2) http://www.epa.gov/cleandiesel/comphelp.htm. (3) Tam, P. S.; Kittrell, J. R.; Eldridge, J. W. Desulfurization of Fuel Oil by Oxidation and Extraction: 1. Enhancement of extraction oil yield. Ind. Eng. Chem. Res. 1990, 29, 321. (4) Zannikos, F.; Lois, E.; Stournas, S. Desulfurization of Petroleum Fractions by Oxidation and Solvent Extraction. Fuel Process. Technol. 1995, 42, 35. (5) Collins, F. M.; Lucy, A. R.; Sharp, C. Oxidative Desulfurization of Oils via Hydrogen Peroxide and Heteropolyanion Catalysis. J. Mol. Catal. A: Chem. 1997, 117, 397. (6) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W. H.; Ishihara, A.; Imai, T.; Kabe, T. Oxidative Desulfurization of Light Gas Oil and Vacuum Gas Oil by Oxidation and Solvent Extraction. Energy Fuels 2000, 14, 1232. (7) Shiraishi, Y.; Tachibana, K.; Hirai, T.; Komasawa, I. Desulfurization and Denitrogenation Process for Light Oils Based on Chemical Oxidation Followed by Liquid-Liquid Extraction. Ind. Eng. Chem. Res. 2002, 41, 4362. (8) Wang, Y.; Li, G.; Wang, X. S.; Jin, C. Z. Oxidative desulphurization of 4,6-dimethyl-dibenzothiophene with hydrogen peroxide over Ti-HMS. Energy Fuels 2007, 21, 1415. (9) Filippis, P. D.; Scarsella, M. Functionalized Hexagonal Mesoporous Silica as an Oxidizing Agent for the Oxidative Desulfurization of Organosulfur Compounds. Ind. Eng. Chem. Res. 2008, 47, 973. (10) Kong, L. Y.; Li, G.; Wang, X. S.; Wu, B. Oxidative Desulfurization of Organic Sulfur in Gasoline over Ag/TS-1. Energy Fuels 2006, 20, 896. (11) Rao, T. V.; Sain, B.; Kafola, S.; Nautiyal, B. R.; Sharma, Y. K.; Nanoti, S. M.; Garg, M. O. Oxidative Desulfurization of HDS Diesel Using the Aldehyde/Molecular Oxygen Oxidation System. Energy Fuels 2007, 21, 3420. (12) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Green Processing Using Ionic Liquids and CO2. Nature 1999, 399, 28.
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 (13) Wasserscheid, P.; Keim, W. Ionic Liquid-new “Solutions” for Transition Metal Catalysis. Angew. Chem., Int. Ed. 2000, 39, 3772. (14) Armstrong, D. W.; He, L. F.; Liu, Y. S. Examination of Ionic Liquids and Their Interaction with Molecules, When Used as Stationary Phases in Gas Chromatography. Anal. Chem. 1999, 71, 3873. (15) Li, Z. Y.; Liu, H. T.; Liu, Y.; He, P.; Li, J. H. A Room-Temperature Ionic-Liquid-Templated Proton-Conducting Gelatinous Electrolyte. J. Phys. Chem. B 2004, 108, 17512. (16) Wang, J. J.; Pei, Y. C.; Zhao, Y.; Hu, Z. G. Recovery of Amino Acids by Imidazolium Based Ionic Liquids from Aqueous Media. Green Chem. 2005, 7, 196. (17) Wang, Y.; Yang, H. Synthesis of CoPt Nanorods in Ionic Liquids. J. Am. Chem. Soc. 2005, 127, 5316. (18) Zhang, S. G.; Zhang, Z. C. Novel Properties of Ionic Liquids in Selective Sulfur Removal from Fuels at Room Temperature. Green Chem. 2002, 4, 376. (19) Eβer, J.; Wasserscheid, P.; Jess, A. Deep Desulfurization of Oil Refinery Streams by Extraction with Ionic Liquids. Green Chem. 2004, 6, 316. (20) Planeta, J.; Kara´sek, P.; Roth, M. Distribution of Sulfur-Containing Aromatics between [Hmim][Tf2N] and Supercritical CO2: A Case Study for Deep Desulfurization of Oil Refinery Streams by Extraction with Ionic Liquids. Green Chem. 2006, 8, 70. (21) Nie, Y.; Li, C. X.; Wang, Z. H. Extractive Desulfurization of Fuel Oil Using Alkylimidazole and Its Mixture with Dialkylphosphate Ionic Liquids. Ind. Eng. Chem. Res. 2007, 46, 5108. (22) Gao, H. S.; Luo, M. F.; Xing, J. M.; Wu, Y.; Li, Y. G.; Li, W. L.; Liu, Q. F.; Liu, H. Z. Desulfurization of Fuel by Extraction with PyridiniumBased Ionic Liquids. Ind. Eng. Chem. Res. 2008, 47, 8384. (23) Lo, W. H.; Yang, H. Y.; Wei, G. T. One-Pot Desulfurization of Light Oils by Chemical Oxidation and Solvent Extraction with Room Temperature Ionic Liquids. Green Chem. 2003, 5, 639.
11763
(24) Zhu, W. S.; Li, H. M.; Jiang, X.; Yan, Y. S.; Lu, J. D.; Xia, J. X. Oxidative Desulfurization of Fuels Catalyzed by Peroxotungsten and Peroxomolybdenum Complexes in Ionic Liquids. Energy Fuels 2007, 21, 2514. (25) He, L. N.; Li, H. M.; Zhu, W. S.; Guo, J. X.; Jiang, X.; Lu, J. D.; Yan., Y. S. Deep Oxidative Desulfurization of Fuels Using Peroxophosphomolybdate Catalysts in Ionic Liquids. Ind. Eng. Chem. Res. 2008, 47, 6890. (26) Lu, L.; Cheng, S. F.; Gao, J. B.; Gao, G. H.; He, M. Y. Deep Oxidative Desulfurization of Fuels Catalyzed by Ionic Liquid in the Presence of H2O2. Energy Fuels 2007, 21, 383. (27) Zhao, D. S.; Wang, J. L.; Zhou, E. P. Oxidative Desulfurization of Diesel Fuel Using a Brønsted Acid Room Temperature Ionic Liquid in the Presence of H2O2. Green Chem. 2007, 9, 1219. (28) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Ionic Liquids are not always Green: Hydrolysis of 1-Butyl-3-methylimidazolium Hexafluorophosphate. Green Chem. 2003, 5, 361. (29) Arfan, A.; Bazureau, J. P. Efficient Combination of Recyclable Task Specific Ionic Liquid and Microwave Dielectric Heating for the Synthesis of Lipophilic Esters. Org. Process Res. DeV. 2005, 9, 743. (30) 1H NMR (300 MHz, D2O): δ 8.68 (s, 1H), 7.41 (d, 2H), 4.16 (t, 2H), 3.85 (s, 3H), 1.81 (m, 2H), 1.26 (m, 2H), 0.88 (t, 3H). (31) Lissner, E.; de Souza, W. F.; Ferrera, B.; Dupont, J. Oxidative Desulfurization of Fuels with Task-Specific Ionic Liquids. ChemSusChem 2009, 2, 962.
ReceiVed for reView April 24, 2010 ReVised manuscript receiVed September 15, 2010 Accepted September 24, 2010 IE100957K
