Energy & Fuels 2008, 22, 3065–3069
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Oxidative Desulfurization of Thiophene Catalyzed by (C4H9)4NBr · 2C6H11NO Coordinated Ionic Liquid Dishun Zhao,*,†,‡ Zhimin Sun,†,§ Fatang Li,§ Ran Liu,‡ and Haidan Shan§ School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, China, School of Chemical and Pharmaceutical Engineering, Hebei UniVersity of Science and Technology, Shijiazhuang Hebei 050018, China, and College of Sciences, Hebei UniVersity of Science and Technology, Shijiazhuang Hebei 050018, China ReceiVed March 4, 2008. ReVised Manuscript ReceiVed June 25, 2008
Oxidative desulfurization (ODS) of thiophene with hydrogen peroxide/acetic acid using a quaternary ammonium coordinated ionic liquid (IL) (C4H9)4NBr · 2C6H11NO as an effective catalyst has been studied. The catalyst of coordinated IL (C4H9)4NBr · 2C6H11NO without fluorous anions is environmentally benign and easy prepare from cheap amines. Thiophene can be oxidized in the oil phase to greater polar sulfoxide, sulfone, and SO42-. Most of the oxidation products transfer to the water phase for their higher polar property, resulting in the removal of thiophene from the n-octane phase without additional extraction by solvent. The effect of the amount of catalyst, hydrogen peroxide, and acetic acid, reaction time, and temperature were investigated in detail. Optimal amounts of catalyst, hydrogen peroxide, and acetic acid were obtained at amounts near 0.20 g, 2, and 4 mL/18 mL of model oil, respectively. The catalytic oxidative desulfurization, 98.8% removal rate thiophene in n-octane, and 95.3% sulfur removal rate of actual fluidized catalytic cracking (FCC) gasoline were achieved after 30 min under optimal experimental conditions. Kinetics parameters of the catalytic oxidation of thiophene were measured and calculated. The results show that the kinetics of catalytic oxidation of thiophene is pseudo-first-order, with an apparent rate constant of 1.05 × 10-3 s-1 and half-life of 660 s.
Introduction Exhaust gases containing SOx are not only one of the major sources of air pollution but also poison noble metal catalysts irreversibly in automobiles. Therefore, desulfurization of gasoline has been investigated extensively in recent years because of the reasons above and stringent environmental regulations. For example, the European Union proposed to reduce the sulfur content in gasoline to a maximum of 50 mg/L in 2005, down to 10 mg/L in 2009, and sulfur-free gasoline in 2011.1–3 The S content in both gasoline and diesel will probably have reached an average value of about 10 mg/L in many countries by 2010.4 The conventional sulfur compound removing method is catalytic hydrodesulfurization (HDS), which requires both a high temperature and a high pressure of hydrogen gas. The purpose of HDS is to convert the sulfur compounds into hydrogen sulfide and hydrocarbons. Although the process is efficient with aliphatic sulfur structures, some sulfur-containing aromatic compounds (notably thiophene compounds and other multiple alkylated derivatives) are highly resistant to hydrotreatment and difficult to remove by HDS.5 Therefore, alternative desulfur* To whom correspondence should be addressed. Telephone: +86-031188632009. Fax: +86-0311-88632009. E-mail:
[email protected]. † Tianjin University. ‡ School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology. § College of Sciences, Hebei University of Science and Technology. (1) Huang, D.; Wang, Y. J.; Yang, L. M.; Luo, G. S. Ind. Eng. Chem. Res. 2006, 45, 1880–1885. (2) Liu, B. S.; Xu, D. F.; Chu, J. X.; Liu, W.; Au, C. T. Energy Fuels 2007, 21, 250–255. (3) Huang, D.; Zhai, Z.; Lu, Y. C.; Yang, L. M.; Luo, G. S Ind. Eng. Chem. Res. 2007, 46, 1447–1451. (4) Zhu, W. S.; Li, H. M.; Jiang, X.; Yan, Y. S.; Lu, J. D.; Xia, J. X. Energy Fuels 2007, 21, 2514–2516. (5) Wan, M. W.; Yen, T. F. Appl. Catal., A 2007, 319, 237–245.
ization technologies are desired. The oxidative desulfurization (ODS) process has been considered to be one of the most promising among various processes because it avoids the use of hydrogen and allows the process to be conducted at ambient conditions.6–10 Although the ODS process is effective, using volatile organic compounds (VOCs) as an extractant leads to further environmental and safety concerns. Ionic liquids (ILs) have recently gained recognition as environmentally benign alternative solvents for separations, chemical synthesis, electrochemistry, and catalysis. The ODS/ extraction using ILs as extractants/catalysts to remove sulfur compounds have been reported recently.4,11–21 For example, Lu et al.11 have described oxidative desulfurization of fuels (6) Wang, Y.; Li, G.; Wang, X. S.; Jin, C. Z. Energy Fuels 2007, 21, 1415–1419. (7) Filippis, P. D.; Scarsella, M. Ind. Eng. Chem. Res. 2008, 47, 973– 975. (8) Shiraishi, Y.; Naito, T.; Hirai, T. Ind. Eng. Chem. Res. 2003, 42, 6034–6039. (9) Kong, L. Y.; Li, G.; Wang, X. S.; Wu, B. Energy Fuels 2006, 20, 896–902. (10) Rao, T. V.; Sain, B.; Kafola, S.; Nautiyal, B. R.; Sharma, Y. K.; Nanoti, S. M.; Garg, M. O. Energy Fuels 2007, 21, 3420–3424. (11) Lu, L.; Cheng, S.; Gao, J.; Gao, G.; He, M. Y. Energy Fuels 2007, 21, 383–384. (12) Zhao, D. S.; Liu, R.; Wang, J. L.; Liu, B. Y. Energy Fuels 2008, 22, 1100–1103. (13) Eβer, J.; Wasserscheid, P.; Jess, A. Green Chem. 2004, 6, 316– 322. (14) Nie, Y.; Li, C. X.; Wang, Z. H. Ind. Eng. Chem. Res. 2007, 46, 5108–5112. (15) Planeta, J.; Karasek, P.; Roth, M. Green Chem. 2006, 8, 70–77. (16) Nie, Y.; Li, C. X.; Sun, A. J.; Meng, H.; Wang, Z. H. Energy Fuels 2006, 20, 2083–2087. (17) Jiang, X. C.; Nie, Y.; Li, C. X.; Wang, Z. H. Fuel 2008, 87, 79– 84. (18) Wang, J. L.; Zhao, D. S.; Zhou, E. P.; Dong, Z. J. Fuel Chem. Technol. 2007, 35, 293–296.
10.1021/ef800162w CCC: $40.75 2008 American Chemical Society Published on Web 07/29/2008
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catalyzed by the acidic IL [Hmim]BF4 in the presence of H2O2. In these desulfurization processes, sulfur-containing compounds can be oxidized to greater polar sulfoxides and sulfones, which can be extracted easily by IL. The oxidation of sulfur-containing compounds in [Bmim]PF6 in the presence of H2O2 and UV irradiation has been studied by Zhao et al.,12 who found that IL [Bmim]PF6 was used to be the extractant and photochemical reaction medium to promote the oxidation of sulfur-containing compounds. The results of both experiments showed that the sulfur removal of model oil could reach about 90%, which was superior to the simple extraction with IL.13–20 However, most of the reported ILs is imidazolium cation,13–17,19,20 with counteranions, such as tetrafluoroborate18 or hexafluorophosphate.22–24 Swatlowski et al.25 reported ILs with fluorous anions can do harm to the environment. Moreover, the cost for most of reported ILs is still greatly concerned for developing a commercially feasible ODS process. In this paper, we attempt to use a kind of quaternary ammonium coordinated IL [(C4H9)4NBr · 2C6H11NO] without fluorous anions, which is environmentally benign and easy to be prepared from cheap amines, as the catalyst for the ODS process of fluidized catalytic cracking (FCC) gasoline in the presence of hydrogen peroxide and acetic acid. The possible oxidative desulfurization mechanism of thiophene by coordinated IL [(C4H9)4NBr · 2C6H11NO] catalysts was proposed. The effects of the amount of catalyst, oxidant, reactive time, and temperature on the removal ratio of thiophene were investigated in detail. Kinetics parameters of the catalytic oxidation of thiophene were measured and calculated. Experimental Section Experimental Procedure. The preparations of [(C4H9)4NBr · 2C6H11NO] coordinated IL was similar to those used in the previous literature,26 and the detailed preparations were as follows: 0.05 mol of tetrabutylammonium bromide (TBAB) and 0.1 mol of caprolactam were stirred in a three-necked flask, and then the mixture was stirred at 90 °C for 15 min. When the mixture was clear, it was kept at 90 °C and continuously stirred for 15 min. After the reaction, the IL was dried in vacuum at 120 °C for 2 h, yielding 98% of a colorless liquid product. The IL was analyzed by 1H nuclear magnetic resonance (NMR), 13C NMR, and Fourier transform infrared (FTIR) spectroscopy. Model oil (1000 µg/mL S) was prepared by dissolving thiophene (1.3125 g) in n-octane (500 mL). A typical oxidative desulfurization reaction run was as follows: 18 mL of the model oil solutions was added to the reactor, heated in a water bath, and stabilized to the desired reaction temperature of 40 °C. Hydrogen peroxide (30 wt %, 2 mL), acetic acid (4 mL), and 0.20 g of the coordinated IL [(C4H9)4NBr · 2C6H11NO] were added into the reactor. The resulting mixture was stirred, reacted for 30 min at the reaction temperature, and was analyzed periodically. The concentration of sulfur in model (19) Huang, C. P.; Chen, B. H.; Zhang, J.; Liu, Z. C.; Li, Y. X. Energy Fuels 2004, 18, 1862–1864. (20) Bo¨smann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 2494–2495. (21) Okrasa, K.; Guibe-Jampel, E.; Therisod, M. Tetrahedron: Asymmetry 2003, 14, 2487–2490. (22) Zhao, D. S.; Wang, J. L.; Zhou, E. P. Green Chem. 2007, 9, 1219– 1222. (23) Lo, W. H.; Yang, H. Y.; Wei, G. T. Green Chem. 2003, 5, 639– 642. (24) Chauhan, S. M. S.; Kumar, A.; Srinivas, K. A. Chem. Commun. 2003, 2348–2349. (25) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Green Chem. 2003, 5, 361–363. (26) Zhao, D. S.; Wang, N.; Li, X. G. J. Chem. Ind. Eng. 2007, 58, 1457–1460.
Zhao et al.
Figure 1. GC-FPD of thiophene in n-octane.
oil and FCC gasoline were detected by a microcoulometric detector (WK-2D) and gas chromatograph with a flame photometric detector (GC-FPD).
Results and Discussion 1. Oxidization Mechanism of Thiophene Catalyzed by Coordinated IL [(C4H9)4NBr · 2C6H11NO]. Figure 1 shows the gas chromatograph of thiophene in n-octane before and after the oxidation reaction. We detected the specific infrared absorptions of 1378 and 1080 cm-1 of the bending vibration absorption peaks of SdO27 with FTIR in the examination of extraction oxidation products by N,N-dimethylformamide (DMF) from the water phase, which indicated the existence of sulfoxide and sulfone of thiophene. Further examination of mass spectrometry (MS) also confirmed the formation of thiophene sulfoxide and sulfone in the water phase. When BaCl2 solution was dropped into the water phase, a white deposit obviously appeared and was not dissolved by dropping hydrochloric acid again. When there was no thiophene in n-octane, the above phenomenon was not observed. Figure 1 proves that thiophene containing in model oil can be effectively removed by a H2O2/ CH3COOH coordinated IL catalysis system. The chromatogram also indicates that the oxidation products do not exist in the oil phase. On the basis of the above analysis, we conjectured the possible mechanism as follow: when using this coordinated IL as an effective catalyst for the removal of thiophene with a hydrogen peroxide/acetic acid system, a biphasic reaction system [(the oil layer as the upper phase (containing thiophene in n-octane), and an aqueous solution as the bottom phase (containing hydrogen peroxide, acetic acid, and coordinated IL)] is formed. The oxidative agent CH3COOOH was easily formed in the water phase. The CH3COOO- anion of oxidative agents can transfer to the oil phase with the help of the lipophilic carbon chains C4H9 from quaternary ammonium salt27 of a coordinated IL catalyst. Moreover, the ability of the water phase to extract thiophene from n-octane is very low (5.0-7.8%) (the value of y axis in Figure 3 at time zero); therefore, the most oxidative process of thiophene occurs in the oil phase. Thiophene can be oxidized to greater polar sulfoxide, sulfone, and SO42-, which can be extracted easily by the water phase. The oxidative desulfurization mechanism of thiophene using coordinated IL (27) Zhao, D. S.; Ren, H. W.; Wang, J. L.; Yang, Y.; Zhao, Y. Energy Fuels 2007, 21, 2543–5475.
OxidatiVe Desulfurization of Thiophene
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Scheme 1. Possible Oxidization Mechanism of Thiophene Catalyzed by Coordinated IL [(C4H9)4NBr · 2C6H11NO]
Figure 3. Effect of the amount of H2O2 on desulfurization (%). Conditions: T ) 40 °C, and 0.20 g of [(C4H9)4NBr · 2C6H11NO] catalyst.
[(C4H9)4NBr · 2C6H11NO] as a catalyst was proposed here, which is shown as Scheme 1. 2. Effect of the Amount of Coordinated IL [(C4H9)4NBr · 2C6H11NO] Catalyst on Desulfurization of Model Oil. The influence of the amount of coordinated IL [(C4H9)4NBr · 2C6H11NO] catalyst on the desulfurization (%) is shown in Figure 2. It is obvious that the reaction without coordinated IL catalyst is diffusion-controlled and the desulfurization (%) was too low. With the presence of a coordinated IL catalyst, the desulfurization (%) increased remarkably. The desulfurization (%) was up to 97.9% when 0.20 g of catalyst was added into the reaction system. However, when IL catalyst was more than 0.20 g, the desulfurization (%) did not increase obviously. Maybe this is caused by C4H9, the carbon chains of quaternary ammonium salt, the lipophilic part of coordinated IL. With an increasing amount of catalyst, this accelerates the CH3COOOanion of oxidative agents to transfer to the oil phase.27 Oxidative agents have more opportunities to contact with thiophene; therefore, the oxidative rate of thiophene becomes fast. However, too much catalyst would become waste. Thus, the amount of coordinated IL catalyst has an optimal value of 0.20 g.
Figure 2. Effect of the amount of IL [(C4H9)4NBr · 2C6H11NO] catalyst on desulfurization (%). Conditions: T ) 40 °C, and t ) 30 min.
Figure 4. Effect of temperature on desulfurization. Conditions: VH2O2 ) 2.0 mL, and 0.20 g of [(C4H9)4NBr · 2C6H11NO] catalyst.
3. Effect of the Amount of H2O2 on Desulfurization of Model Oil. The oxidative agent CH3COOOH was easily formed in the system of hydrogen peroxide-acetic acid.27 The concentration of CH3COOOH depended upon the various amounts of H2O2 because of the excessive amount of acetic acid under the experimental condition. To investigate the effect of the amount of oxidative agent on the oxidative properties, the oxidation of thiophene in the system under various amounts of H2O2 was carried out at 40 °C. The desulfurization (%) (5.0-7.8%) in Figure 3 at time zero reflects the ability of the water phase to extract thiophene from n-octane. It is shown, as Figure 3, that the amount of H2O2 has a strong influence on desulfurization (%). With the increase of H2O2, the oxidant and sulfur compounds had more opportunity to react; therefore, the desulfurization (%) increased. With the amount of oxidant increasing continually, it was not economical using large amounts of H2O2. Therefore, the optimal amount of 30 wt % H2O2 is 2 mL in the present study. 4. Effect of Temperature on Desulfurization of Model Oil. Figure 4 shows the effect of the reactive temperature (20, 30, 40, and 50 °C) on the desulfurization (%) of model oil. The reaction activity increased remarkablely when the temperature increased from 20 to 40 °C. When the temperature was 40 °C, the desulfurization (%) was up to 98.8% for 30 min. However,
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Figure 5. Pseudo-first-order kinetics for oxidation of thiophene. Table 1. Apparent Rate Constants and Half-Lives for Oxidation of Thiophene substrate
catalyst
thiophene
coordinated IL (C4H9)4NBr · 2C6H11NO (C4H9)4NBr27
thiophene
rate constant (s-1)
half-life (s)
1.05 × 10-3
660
5.31 × 10-4
1305
Figure 6. Sulfur-specific GC of FCC gasoline before and after oxidation. Table 2. Sulfur Content of Actual FCC Gasoline and Desulfurization Efficiencies before and after Oxidative Reactiona entry
sulfur content (µg/mL)
desulfurization (%)
FCC gasoline oxidized gasoline
1048 49.6
95.3
Conditions: T ) 40 °C, VH2O2 ) 2.0 mL, 0.20 g of [(C4H9)4NBr · 2C6H11NO] catalyst, and t ) 30 min. a
when the temperature increased to 50 °C, the desulfurization (%) decreased to 85.7%. This may be caused by the large decomposition of H2O2 at a high temperature in the presence of an amide compound.28 5. Kinetics of Catalytic Oxidation of Thiophene. Reaction kinetics is of great importance in explaining the reaction mechanism. Experiments to obtain kinetics parameters of the oxidation of thiophene were performed under optimal conditions from 2-4. The rate constant for the apparent consumption of thiophene was obtained from the pseudo-first-order equation
()
ln
C0 ) kpt Ct
(1)
where C0 and Ct are the concentrations of substrate at time zero and time t (in min) and kp is the first-order rate constant (in min-1). Half-lives [t1/2 (in min)] were calculated using eq 2, which was derived from eq 1 by replacing Ct with C0/2. t1/2 )
0.693 kp
(2)
original FCC gasoline before and after the catalytic oxidative reaction. These chromatograms suggest that the most sulfurcontaining compounds present in the actual FCC gasoline are oxidized and extracted by the hydrogen peroxide/acetic acid system at 40 °C in 30 min. The sulfur content of FCC gasoline can be reduced from 1048 to 49.6 µg/mL; this corresponds to 95.3% desulfurization (Table 2). We found that ODS with [(C4H9)4NBr · 2C6H11NO] from actual FCC gasoline is lower than that from model oil, which is consistent with other ILs.19,23 The reason is that the actual FCC gasoline contains many kinds of alkene and diene, which would hinder the reaction of RCOOO- with organosulfur and, at the same time, exhaust oxidant.27 The actual FCC gasoline contains a wide range of sulfurcontaining compounds, including thiophene, benzothiophene (BT), dibenzothiophene (DBT), and their alkyl-substituted derivatives. The reactivity of sulfur compounds for oxidation increased with electron density on the sulfur atom. Otsuki et al.29 have reported that the thiophene and thiophene derivatives with lower electron densities on the sulfur atoms could not be oxidized at 50 °C. However, when using the present reaction system, the oxidation of thiopene can be carried successfully with the presence of coordinated IL [(C4H9)4NBr · 2C6H11NO] catalyst.
When ln(C0/Ct) was plotted against t, a straight line with slope kp was obtained (Figure 5), indicating that oxidation of thiophene catalyzed by IL follows first-order kinetics. The rate constants and half-lives obtained for oxidation of thiophene are also listed in Table 1. The rate constant (1.05 × 10-3) for coordinated IL system in this study is higher than that of the (C4H9)4NBr system.27 6. Catalytic Oxidation of FCC Gasoline. The catalytic oxidation system was applied to actual FCC gasoline containing 1048 µg/mL sulfur, which was obtained from Shijiazhuang refinery as follows: 18 mL of gasoline oil was mixed with 2 mL of 30 wt % H2O2, 4 mL of CH3COOH, 0.20 g of IL [(C4H9)4NBr · 2C6H11NO] under 40 °C, reacting for 30 min by stirring. Figure 6 shows the sulfur-specific GC analyses of the
In conclusion, this coordinated IL [(C4H9)4NBr · 2C6H11NO] can be used as an environmentally benign catalyst for ODS of FCC gasoline in the presence of hydrogen peroxide and acetic acid. Oxidation products of thiophene were tentatively identified. Most of the oxidation products transfer to the water phase for their higher polar property, resulting in the removal of thiophene from the n-octane phase without additional extraction by solvent. The possible oxidative desulfurization
(28) Cooper, M. S.; Heaney, H.; Newbold, A. J.; Sanderson, W. R. Synlett 1990, 533–535.
(29) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232–1239.
Conclusions
OxidatiVe Desulfurization of Thiophene
mechanism of thiophene by coordinated IL [(C4H9)4NBr · 2C6H11NO] catalysts was proposed. The desulfurization (%) of thiophene-containing model oil and actual FCC gasoline can reach 98.8 and 95.3%, respectively, in 30 min at 40 °C under optimal experimental conditions. The kinetics of catalytic oxidation of thiophene is pseudo-first-order, with an apparent rate constant of 1.05 × 10-3 s-1 and half-life of 660 s. The coordinated IL (C4H9)4NBr · 2C6H11NO catalyst for ODS of FCC gasoline without fluorous anions is environ-
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mentally benign and easy prepare from cheap amines. Consequently, the coordinated IL [(C4H9)4NBr · 2C6H11NO] is a promising catalyst in ODS technology. Acknowledgment. The work was financially supported by National Natural Science Foundation of China (20576026, 20276015) and Science Foundation of Educational Department of Hebei (2007440). EF800162W
