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Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature
FUPROC-03591; No of Pages 5 Fuel Processing Technology xxx (2012) xxx–xxx
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Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature Weidong Liang ⁎, Shuo Zhang, Haifeng Li, Guodong Zhang School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China
a r t i c l e
i n f o
Article history: Received 17 October 2011 Received in revised form 25 March 2012 Accepted 18 September 2012 Available online xxxx Keywords: Benzoimidazole Ionic liquid Synthesis Oxidative desulfurization Model oil
a b s t r a c t A series of acetic acid-based ionic liquids (ILs) were synthesized and used for oxidative desulfurization as both catalyst and extractant. The practical and orthogonal experiments showed that [Otbim] +CH3COO − containing eight-carbon side chain has of best catalytic activity. 87.5% of thiophene in the model oil was removed under the optimal conditions of oxidation temperature at 70 °C, oxidation time of 180 min, simulated oil dosage of 10 ml, and ILs/H2O2 volume ratio of 1:1.1. Meanwhile, the ILs could be recycled 5 times without any apparent loss of the catalytic activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a result of the dramatic environmental impact of sulfur-containing engine exhaust emissions has been recognized in the past few decades, specifications regarding sulfur content in transportation fuels are becoming more and more stringent worldwide [1–3]. To minimize the emissions of sulfur oxides, various desulfurization methods of fuels have been rapidly developed. Catalytic hydrodesulfurization (HDS), a traditional method using CoMo or NiMo as catalysts for more than 60 years, usually requires high temperatures and the presence of H2 which involves the reactions of paraffinic sulfur-containing compounds such as thiols, thioethers and disulfides to readily produce H2S and corresponding hydrocarbons [4]. Moreover, aromatic sulfur compounds such as benzothiophenes (BTs) and dibenzothiophenes (DBTs) are hardly desulfurized by HDS because of the sterically hindered adsorption of these compounds on the catalyst surface [5]. Oxidative desulfurization (ODS), one of the alternative technologies including extraction [6,7], adsorption [8,9] and biodesulfurization [10] with the advantages of mild reaction conditions, is generally considered the most promising process that consists of two steps. The first step is selective oxidation of organic sulfur compounds, and the second one is the removal of sulfoxides or sulfones by polar extractant [11]. However, the extractant is usually volatile organic compounds which would lead to further environmental and safety concerns. Recently, ODS combined with H2O2 as the oxidant and with ionic liquids (ILs) working as both solvent and extractant were reported to be effective if the shortcomings of ILs on high cost and viscosity can be ⁎ Corresponding author. E-mail address: [email protected] (W. Liang).
somehow overcome. Fenton-like ILs [(C8H17)3CH3N]Cl/FeCl3–H2O2 system [12] showed that the sulfur removal of DBT-containing model oil reached 97.9% and that the sulfur level of FCC gasoline could be reduced from 360 ppm to 110 ppm. Acidic ILs [13] containing \COOH groups in the cations were oxidized by H2O2 for ODS process, and the computational investigation by density functional theory (DFT) found that catalytic properties of the acidic ILs were closely related to the structures, acidities and extraction capabilities. Dishun Zhao [14] reported that the sulfur contents of DBT dissolved in n-octane could be dramatically reduced using H2O2–formic acid as an oxidant and pyridinium-based ILs as phase-transfer catalysts (PTCs). In this study, several acetic acid-based ILs were synthesized from acetic acid and benzoimidazole by alkylation and acid–base neutralization (Fig. 1). The acetic acid-based ILs were used as both phase-transfer catalysts and extractants, which showed good oxidative ability. Meanwhile, the optimization of oxidative desulfurization of model fuel was investigated by orthogonal experiments (L16(4)5) in detail. To the best of our knowledge, it is the first report on oxidative desulfurization with the acetic acid-based ionic liquids without adding an additional catalyst. 2. Experimental Chemicals o-phenylenediamine, 1-bromopentane, 1-bromohexane, 1-bromoheptane, 1-bromooctane and solvents were commercially available and used as supplied. Each IL sample used for the oxidization and adsorption experiments was further purified by silica gel column and gel chromatography using petroleum ether and ethyl acetate (volume ratio 1:1) as the elution, followed by removal of the volatile components under high vacuum at room temperature for 5 h [15].
0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2012.09.034
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
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W. Liang et al. / Fuel Processing Technology xxx (2012) xxx–xxx
H N
R N
RBr
N
R N
CH3COOH
N
CH3COO-
N H
3
3 3 3 3
Fig. 1. Synthesis of the acetic acid-based ionic liquids.
1 H and 13C NMR spectra were recorded on a Varian INOVA-400 Spectrometer. Chemical shifts were reported in ppm from a TMS reference. FT-IR spectra were recorded on a Nicolet Nexus-670 spectrometer by dispersing the sample in a KBr pellet. Mass spectra were recorded on a VG ZAB-HS spectrometer. In order to remove the residual water mixed in IL, the sample was dissolved in distilled acetonitrile, and then dried by anhydrous sodium sulfate. After desiccant filtration and rotary evaporation, the IL was dried in vacuum to constant weight. The viscosity was measured with a spindle viscometer equipped with a thermostated water bath at 25 °C. The measurements have a precision of ±2%. The quantity of thiophene absorbed onto the ILs was measured by Gas Chromatography model GC 7890II. A standard addition method was used for the accuracy of the results.
2.1. Preparation of ionic liquids The ILs were prepared by two step synthesis [16] through equal molars of benzoimidazole combined firstly with the corresponding alkyl bromide to form the compounds I (a–d), and then the compounds II (IL1–IL4) were synthesized according to an experimental procedure based on a neutralization reaction [17]. 2.1.1. General procedure for the synthesis of the compounds I (a–d) A mixture of benzoimidazole (0.1 mol), tetrabutyl ammonium bromide and 30% aqueous solution of sodium hydroxide was placed in a three-necked flask, which was provided with a magnetic stirrer and reflux condenser. Then equal mole of freshly distilled alkyl bromine was added drop-wise. The mixture was heated at 45 °C for 12 h until two phases formed. The organic layer was extracted with ethyl acetate, washed with deionized water and dried with anhydrous magnesium sulfate. The solvent was removed under reduced pressure at 50 °C for 3 h, giving the compounds I (a–d) as light yellow liquids. 2.1.2. General procedure for the synthesis of the compounds II (IL1–IL4) The compounds I (a–d) and acetonitrile were charged into a 100 ml round-bottom flask, and the mixture was stirred at 0 °C for 1 min. An equal amount of acetic acid was added drop wise with a magnetic stirrer for 1 h at 0 °C and then for 2 h at room temperature. The solvent was removed under reduced pressure, and the raw IL1–IL4 was finally obtained. The acetic acid-based ionic liquids were washed repeatedly with toluene and diethyl ether to remove nonionic residues, and dried in vacuum at room temperature for 6 h. The products were formed quantitatively and in high purity as assessed by NMR spectroscopy. IL1: viscosity 20.7 mPa s. IR (KBr), v, cm − 1; 3392, 3154, 3078, 2927, 2858, 1834, 1649, 1621, 1574, 1455, 1409, 1381, 746. 1H NMR (DMSO), δ; 9.97 (s, 1H, im-H), 8.16 (d, 1H, J = 7.2 Hz, 2.8 Hz, Ar\H), 8.08 (d, 1H, J = 7.6 Hz, 2.4 Hz, Ar\H), 7.73 (dd, 2H, J = 7.6 Hz,
2.0 Hz, Ar\H), 4.61 (t, 2H, J = 6.4 Hz, N\CH2\), 2.49 (s, 3H, CH3C_O), 1.95 (m, 2H, \CH2\), 1.37 (m, 4H, \CH2\CH2\), 0.88 (t, 3H, J = 7.2 Hz, CH3\). 13C NMR (DMSO), δ: 167.8, 143.4, 131.6, 130.7, 126.8, 113.9, 47.8, 46.8, 28.3, 27.8, 21.6, 13.8. MS, m/z: 189.2 (M+, 100%). IL2: viscosity 147.8 mPa s. IR (KBr), v, cm−1; 3392, 3154, 3078, 2927, 2858, 1834, 1649, 1621, 1574, 1455, 1409, 1381, 746. 1H NMR (DMSO), δ; 9.97 (s, 1H, im-H), 8.16 (m, 2H, J = 7.2 Hz, 2.8 Hz, Ar\H), 8.08 (d, 1H, J = 7.6 Hz, 2.4 Hz, Ar\H), 7.73 (dd, 2H, J = 7.6 Hz, 2.0 Hz, Ar\H), 4.61 (t, 2H, J = 6.4 Hz, N\CH2\), 2.49 (s, 3H, CH3C_O), 1.95 (m, 2H, \CH2\), 1.37 (m, 6H, \CH2\CH2\CH2\), 0.88 (t, 3H, J = 7.2 Hz, CH3\). 13C NMR (DMSO), δ: 167.8, 143.4, 131.6, 130.8, 126.8, 113.9, 47.8, 46.8, 31.5, 28.3, 27.8, 21.6, 13.8. MS, m/z: 203.2 (M+, 100%). IL3: viscosity 173.2 mPa s. IR (KBr), v, cm −1; 3419, 3147, 3087, 2958, 2828, 1842, 1649, 1624, 1576, 1453, 1401, 1376, 764. 1H-NMR (DMSO), δ; 9.89 (s, 1H, im-H), 8.15 (d, 1H, J = 7.2 Hz, 2.4 Hz, Ar\H), 8.07 (d, 1H, J=7.2 Hz, 2.8 Hz, Ar\H), 7.73 (dd, 2H, J=7.2 Hz, 2.4 Hz, Ar\H), 4.59 (t, 2H, J=7.2 Hz, N\CH2\), 2.49 (s, 3H, CH3C_O), 1.92 (m, 2H, \CH2\), 1.32 (m, 8H, \CH2\CH2\CH2\CH2\), 0.86 (t, 3H, J = 6.8 Hz, CH3\). 13C NMR (DMSO), δ: 167.9, 143.3, 131.6, 130.6, 126.8, 113.9, 47.6, 46.8, 31.1, 28.5, 28.1, 25.6, 21.9, 13.9. MS, m/z: 217.2 (M +, 100%). IL4: viscosity 210.5 mPa s. IR (KBr), v, cm−1; 3419, 3147, 3088, 2959, 2829, 1842, 1649, 1625, 1576, 1453, 1401, 1376, 764. 1H-NMR (DMSO), δ; 9.89 (s, 1H, im-H), 8.15 (d, 1H, J = 7.2 Hz, 2.4 Hz, Ar\H), 8.07 (d, 1H, J = 7.2 Hz, 2.8 Hz, Ar\H), 7.73 (dd, 2H, J = 7.2 Hz, 2.4 Hz, Ar\H), 4.59 (t, 2H, J = 7.2 Hz, N\CH2), 2.49 (s, 3H, CH3C_O), 1.92 (m, 2H, \CH2\), 1.32 (m, 10H, \CH2\CH2\CH2\CH2\CH2\), 0.86 (t, 3H, J =6.8 Hz, CH3\). 13C NMR (DMSO), δ: 167.9, 143.3, 131.6, 130.6, 126.8, 113.9, 47.6, 46.8, 31.1, 29.5, 28.5, 28.1, 25.6, 21.9, 13.9. MS, m/z: 231.2 (M+, 100%). 2.2. Catalytic oxidative desulfurization Model oil was prepared by dissolving thiophene in n-heptane to form solutions with sulfur content of 1500 ppm. The oxidative desulfurization experiments were carried out in a 100 ml round-bottom flask. A Table 1 Influence of different reaction systems. Entry
1 2 3
Desulfurization system
ILs + oil H2O2 + oil ILs + H2O2 + oil
Sulfur removal (%) IL1
IL2
IL3
IL4
No IL
40.7 – 68.3
50.1 – 77.7
55.6 – 83.2
59.9 – 87.5
– 32.6 –
Experimental condition: model oil=10 ml, volume ratio VILs/VH2O2 =1/1.1 (VILs +VH2O2 = 20 ml), T=70 °C, t=3 h.
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
W. Liang et al. / Fuel Processing Technology xxx (2012) xxx–xxx
250 200 150
Vp / mV
typical run was as follows: a certain amount of model oil was added into the reactor and then a certain amount of [Abim]CH3COO was added. The mixture was stirred and heated to the desired temperature in a water bath. Then, a certain amount of hydrogen peroxide (30%) was added to the mixture and stirred vigorously. Upon standing, the upper phase (model oil) was separated easily from the IL phase by decantation, and the sulfur content was analyzed according to gas chromatography external standard method.
3
before
100 50 0 15 10
3. Results and discussion
after
5
3.1. Influence of different desulfurization systems of model oil
0
As shown in Table 1, the effects of ILs and hydrogen peroxide on desulfurization of model oil were investigated by the three different sulfur removal systems. The entry 1 indicated that the oxidative desulfurization system containing four kinds of ILs was used as extractant in the absence of H2O2, and the removal of sulfur compounds only reached 40.7%, 50.1%, 55.6% and 59.9% respectively. The entry 2 showed that the oxidative desulfurization system containing H2O2 was used as the oxidative agent in the absence of ILs. The oxidation of model oil in the H2O2 system under H2O2/sulfur molar ratio of 4 was carried out at 70 °C. After 3 h, the sulfur content in the model oil was reduced from 1500 to 1017 ppm which equaled to the desulfurization of 32.6% by single oxidation using H2O2 as the oxidant. The last entry illuminated that sulfur content decreased remarkably within the system containing ILs and H2O2, and the removal of sulfur reached 68.3%, 77.7%, 83.2% and 87.5% at 70 °C for 3 h respectively. The results demonstrated that ILs played a vital role in the removal of sulfur content in the desulfurization system, and that the combination of ILs and H2O2 was rather helpful to deep desulfurization. The evidence was that the desulfurization of model oil was increased after adding H2O2 to IL, and the desulfurization efficiency was up from 40.7% to 68.3% (IL1). The higher the volume ratio of ILs/H2O2, the easier the deep desulfurization by oxidation was, which was in accordance with the paper reported [18]. The carboxylate group (\COO−) was oxidized by the H2O2 to the corresponding hyperoxide. So, it should be better with the ILs/H2O2 volume ratio 1:1.1 considering the economical factor. 3.2. Process of extraction–oxidation of thiophene using acetic acid-based ionic liquids [Abim]CH3COO as catalytic system The possible reaction mechanism was proposed to experience three steps (see Fig. 2) [13,19]. The carboxylate group (\COO −) existing in the anion reacted with H2O2, resulted in a hyperoxide (\COOO −); and then, owing to the phase transfer catalysis character of ILs, the sulfur compounds dissolved in the ILs were oxidized by the hyperoxide (\COOO−) to the corresponding sulfones; finally, ILs were reoxidized to peroxide state, and returned the water phase. The sulfone has higher polarity compared to sulfide, so it was easy to dissolve into the mixture of ILs and water. The comparison of GC-FPD of thiophene in n-heptane
1
2
3
4
5
6
t / min Fig. 3. Comparison of GC-FPD of thiophene in n-heptane before and after reaction.
before and after reaction, showed that the oxidation products of thiophene did not exist in the oil phase because there was no new peak in GC-FPD analysis of oil layer after oxidation (Fig. 3). 3.3. Optimization of the catalytic oxidative desulfurization parameters in ILs–H2O2 system by orthogonal experiment The catalytic oxidative desulfurization of thiophene in n-heptane with the acetic acid-based ILs [Abim]CH3COO as the catalytic solvent and H2O2 as the oxidant was optimized by orthogonal experiment. Get the optimum conditions by using the orthogonal experiments. The results indicate that orthogonal test can economize trial-manufactured expenditure, time and work load, and get the highest desulfurization efficiency with the least experiments. An orthogonal L16(4)5 test designed in the catalytic oxidative desulfurization process was used for the optimization of the catalytic oxidative desulfurization conditions. Factors and levels for orthogonal L16(4)5 test were presented in Table 2. The results of the orthogonal test and the extreme difference analysis were presented in Table 3. K represented the arithmetic mean of desulfurization rate with different factors in the same level, and the biggest parameter of K represented the optimum process conditions. For example, in the temperature factor, there are 4 parallel experiments for the same 40 °C level (A1): K1 = (30.7 + 66.2 + 45.7 + 69.1) / 4 = 52.93. R represented the difference of maximum and minimum of K in different levels in the same factor. We could get the influence of the different factors for desulfurization rate from the worst value of R, i.e., in the same factor of temperature, R = Kmax − Kmin = K4 − K1 = 83 − 52.93 = 30.07. As seen in Table 3, optimization of the catalytic oxidative desulfurization parameters, A4, B4, C2, D4, and E3, was acquired from the orthogonal L16(4) 5 test. In other words, the optimized conditions for catalytic oxidative desulfurization were oxidation temperature at 70 °C, oxidation time of 180 min, simulated oil dosage of 10 ml, 8 carbons, and
H 2O 2
[Abim]CH3COOO
[Abim]CH3COO 1
water phase phase interface
3
S
organic phase
+ [Abim]CH3 COO
2
+ [Abim]CH3COOO S
O A = pentyl, hexyl, heptyl, octyl Fig. 2. Mechanism extraction–oxidation of thiophene.
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
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W. Liang et al. / Fuel Processing Technology xxx (2012) xxx–xxx
Table 2 Factor and levels for orthogonal test. Level
1 2 3 4
Table 4 The desulfurization of model oil by regenerated ILs.
Factor A
B
C
D
E
40 50 60 70
30 80 130 180
5 10 15 20
5 6 7 8
0.9:1 1:1 1:1.1 1:1.2
A: temperature T/°C; B: time t/min; C: simulated oil dosage V/ml; D: number of carbon; E: volume ratio VILs/VH2O2 (VILs + VH2O2 = 20 ml).
ILs/H2O2 volume ratio of 1:1.1 respectively. The influence of some factors on the catalytic oxidative desulfurization decreased in the order A >D >E > B >C according to the extreme difference analysis (R values). We could find from the orthogonal experiments that the efficiency of desulfurization was different according to the ILs which had the different carbon numbers of side chain. The ILs containing 8 carbons in side chain had the best efficiency of desulfurization. This was mainly because the alkyl having more carbons and the lipotropism was better, and the chances of entering the oil phase was consequently better [20]. And then the oxidative agent of peroxycarboxylic acid could be moved to oil phase that will be conducive to the oxidation reaction in the oil phase. As a result, the ILs containing 8 carbons of side chain had the best efficiency of desulfurization. Furthermore, the efficiency of desulfurization with benzoimidazole-based ILs was better than imidazole-based ILs [21,22]. The main reason was that the cation of ILs and thiophene has aromatic rings. The aromatic nucleuses in the chain of imidazole make the benzoimidazole have higher polarizable aromatic π-electron density than imidazole. The interaction between the cation of ILs, which had an aromatic ring structure with a conjugated system, and the thiophene sulfur compounds leads to the formation of liquid-clathrate compounds and the π–π complexation [23–25] between unsaturated bonds of aromatic sulfur-compounds and the benzoimidazole cation. The orthogonal experiment also indicated that the value of K decreased from 70.9% to 61.4% because the polarity of the ILs was changed with the adding of H2O2. But as the oxidization proceeded, thiophene was converted to CO, CO2 and SO2, extraction equilibrium was broken [1], and K increased to 75.3% at 180 min which was more than the
Cycle
0
1
2
3
4
5
6
Desulfurization efficiency (%)
87.5
87.2
86.6
86.5
86.2
85.6
79.9
Conditions: T = 70 °C, t = 180 min, V= 10 ml, N = 8, VILs/VH2O2 = 1:1.1 (VILs + VH2O2 = 20 ml).
time reported by Cao [22]. In view of the main factor A, K increased smoothly from 52.93% to 83% as the desulfurization temperature increased from 40 to 70 °C and accelerated the reaction rate. However, H2O2 was decomposed dramatically when the desulfurization temperature was more than 70 °C. 3.4. Recycle of used ILs The acetic acid-based ILs could be used as a catalyst for desulfurization of model oil when H2O2 was used as the oxidant. The catalytic oxidative desulfurization of thiophene in n-heptane could reach 87.5% under the optimal reaction conditions. The used ILs were regenerated by re-extraction in dichloromethane, then dichloromethane was removed under vacuum in a rotary evaporator. The results of desulfurization on model oil by regenerated ILs were shown in Table 4, which indicated that the recycled ILs could be recycled five times without an obvious decrease in activity. 4. Conclusions In conclusion, the acetic acid-based ILs showed high activity in the oxidative desulfurization of model oil with hydrogen peroxide as the oxidant, which not only served as an extractant and reaction medium but also acted as a phase transfer catalyst. It was found to be effective in the extraction of sulfur components in model oil. The desulfurization process was achieved after H2O2 was added to the extraction system. The environmental benign oxidizability of H2O2 was utilized for oxidizing the sulfur components to the corresponding sulfones followed by extraction by the ILs. The catalytic oxidative desulfurization of thiophene in n-heptane was optimized by orthogonal experiments. [Otbim]+CH3COO− containing eight-carbon side chain has of best catalytic activity. 87.5% of thiophene in the model oil was removed
Table 3 Results and analysis of orthogonal test. Experiment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Arithmetic mean
Extreme difference Optimization level Order of factors
Factor
K1 K2 K3 K4 R
Desulfurization efficiency (%)
Desulfurization temperature, A (°C)
Reaction time, B (min)
Simulated oil dosage, C (ml)
Number of carbon, D
Volume ratio, E (VILs/VH2O2)
40 40 40 40 50 50 50 50 60 60 60 60 70 70 70 70 52.93 67.18 73.35 83 30.07 A4 1
30 80 130 180 30 80 130 180 30 80 130 180 30 80 130 180 68.85 70.9 61.4 75.3 13.9 B4 4
5 10 15 20 10 5 20 15 15 20 5 10 20 15 10 5 67.68 77.63 67.05 64.1 13.53 C2 5
5 6 7 8 7 8 5 6 8 7 6 5 6 5 8 7 60.33 68.95 65.33 81.85 21.52 D4 2
0.9:1 1:1 1:1.1 1:1.2 1:1.2 1:1.1 1:1 0.9:1 1:1 0.9:1 1:1.2 1:1.1 1:1.1 1:1.2 0.9:1 1:1 57.38 72.25 75.45 71.38 18.07 E3 3
30.7 66.2 45.7 69.1 73.9 86.6 48.8 59.4 86.9 54.6 66.3 85.6 83.9 76.2 84.8 87.1 – – – – – – –
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
W. Liang et al. / Fuel Processing Technology xxx (2012) xxx–xxx
under the optimized conditions (T=70 °C, t=180 min, V=10 ml, N= 8, VILs/VH2O2 =1:1.1 (VILs +VH2O2 =20 ml)). The used ILs were recovered through re-extraction in dichloromethane, and could be recycled five times without any obvious decrease in activity. We anticipated that this oxidative desulfurization system would be a simple, mild and environmentally benign method in industry in the future. References [1] I.V. Babich, J.A. Moulijn, Science and technology of novel processes for deep desulfurization of oil refinery streams: a review, Fuel 82 (2003) 607–631. [2] J. Eßer, P. Wasserscheid, A. Jess, Deep desulfurization of oil refinery streams by extraction with ionic liquids, Green Chemistry 6 (2004) 316–322. [3] C.S. Song, An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel, Catalysis Today 86 (2003) 211–263. [4] F. Dumeignil, K. Sato, M. Imamura, Characterization and hydrodesulfurization activity of CoMo catalysts supported on boron-doped sol–gel alumina, Applied Catalysis A: General 315 (2006) 18–28. [5] R. Shafi, G.J. Hutchings, Hydrodesulfurization of hindered dibenzothiophenes: an overview, Catalysis Today 59 (2000) 423–442. [6] Y. Nie, C.X. Li, A.J. Sun, H. Meng, Extractive desulfurization of gasoline using imidazolium-based phosphoric ionic liquids, Energy & Fuels 20 (2006) 2083–2087. [7] W.H. Lo, H.Y. Yang, G.T. Wei, One-pot desulfurization of light oils by chemical oxidation and solvent extraction with room temperature ionic liquids, Green Chemistry 5 (2003) 639–642. [8] W. Wu, B. Han, H. Gao, Desulfurization of flue gas: SO2 absorption by an ionic liquid, Angewandte Chemie 43 (2004) 2415–2417. [9] J.H. Kim, X.L. Ma, A.N. Zhou, Ultra-deep desulfurization and denitrogenation of diesel fuel by selective adsorption over three different adsorbents: a study on adsorptive selectivity and mechanism, Catalysis Today 111 (2006) 74–83. [10] J. Sipma, A.M. Henstra, S.N. Parshina, Microbial CO conversions with applications in synthesis gas purification and bio-desulfurization, Critical Reviews in Biotechnology 26 (2006) 41–65. [11] C. Komintarachat, W. Trakarnpruk, Oxidative desulfurization using polyoxometalates, Industrial and Engineering Chemistry Research 45 (2006) 1853–1856. [12] Y.Q. Jiang, W.S. Zhu, H.M. Li, Oxidative desulfurization of fuels catalyzed by Fenton-like ionic liquids at room temperature, ChemSusChem 4 (2011) 399–403. [13] J.Z. Gui, D. Liu, Z.L. Sun, Deep oxidative desulfurization with task-specific ionic liquids: an experimental and computational study, Journal of Molecular Catalysis A: Chemical 331 (2010) 64–70.
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[14] D.S. Zhao, Y.N. Wang, E.H. Duan, Oxidation desulfurization of fuel using pyridinium-based ionic liquids as phase-transfer catalysts, Fuel Processing Technology 91 (2010) 1803–1806. [15] P.S. Kulkarni, L.C. Branco, J.G. Crespo, A comparative study absorption and selectivity of organic vapors by using ionic liquids based on imidazolium, quaternary ammonium, and guanidinium cations, Chemistry A European Journal 13 (2007) 8470–8477. [16] Y.Y. Wang, D. Jiang, L.Y. Dai, Novel bronsted acidic ionic liquids based on benzimidazolium cation: synthesis and catalyzed acetalization of aromatic aldehydes with diols, Catalysis Communications 9 (2008) 2475–2480. [17] P. Luis, L.A. Neves, C.A.M. Afonso, I.M. Coelhoso, J.G. Crespo, A. Garea, A. Irabien, Facilitated transport of CO2 and SO2 through supported ionic liquid membranes, Desalination 245 (2009) 485–493. [18] D.S. Zhao, H.W. Ren, J.L. Wang, Y. Yang, Y. Zhao, Kinetics and mechanism of quaternary ammonium salts as phase-transfer catalysts in the liquid–liquid phase for oxidation of thiophene, Energy & Fuels 21 (2007) 2543–5475. [19] J.M.C. Martin, M.C.C. Sanchez, P.P. Presas, J.L.G. Fierro, Oxidative processes desulfurization of liquid fuels, Journal of Chemical Technology and Biotechnology 85 (2010) 879–890. [20] L. Alonso, A. Arce, M. Francisco, A. Soto, Phase behavior of 1-methyl-3octylimidazolium bis[trifluoromethylsulfonyl]imide with thiophene and aliphatic hydrocarbons: the influence of n-alkane chain length, AICHE Journal 53 (2007) 3108–3115. [21] D. Liu, J. Gui, L. Song, X. Zhang, Z. Sun Deep, Desulfurization of diesel fuel by extraction with task-specific ionic liquids, Petroleum Science and Technology 26 (2008) 973–982. [22] Q. Cao, H.L. Cheng, R.X. Zhao, X.P. Li, Study on oxidative desulfurization of simulated oil with the acetic acid-based ionic liquid phase transfer catalyst, Contemporary Chemical Industry 39 (2010) 245–247. [23] J.D. Holbrey, W.M. Reichert, M. Nieuwenhuyzen, O. Sheppard, C. Hardacre, R.D. Rogers, Liquid clathrate formation in ionic liquid–aromatic mixtures, Chemical Communications 4 (2003) 467–477. [24] S. Zhang, Q. Zhang, Z. Zhang, Extractive desulfurization and denitrogenation of fuels using ionic liquids, Industrial and Engineering Chemistry Research 43 (2004) 614–622. [25] S. Otsuki, T. Nonaka, N. Takashima, Oxidative desulfurization of light gas oil and vacuum gas oil by oxidation and solvent extraction, Energy & Fuels 14 (2000) 1232–1239.
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
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Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature Weidong Liang ⁎, Shuo Zhang, Haifeng Li, Guodong Zhang School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China
a r t i c l e
i n f o
Article history: Received 17 October 2011 Received in revised form 25 March 2012 Accepted 18 September 2012 Available online xxxx Keywords: Benzoimidazole Ionic liquid Synthesis Oxidative desulfurization Model oil
a b s t r a c t A series of acetic acid-based ionic liquids (ILs) were synthesized and used for oxidative desulfurization as both catalyst and extractant. The practical and orthogonal experiments showed that [Otbim] +CH3COO − containing eight-carbon side chain has of best catalytic activity. 87.5% of thiophene in the model oil was removed under the optimal conditions of oxidation temperature at 70 °C, oxidation time of 180 min, simulated oil dosage of 10 ml, and ILs/H2O2 volume ratio of 1:1.1. Meanwhile, the ILs could be recycled 5 times without any apparent loss of the catalytic activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a result of the dramatic environmental impact of sulfur-containing engine exhaust emissions has been recognized in the past few decades, specifications regarding sulfur content in transportation fuels are becoming more and more stringent worldwide [1–3]. To minimize the emissions of sulfur oxides, various desulfurization methods of fuels have been rapidly developed. Catalytic hydrodesulfurization (HDS), a traditional method using CoMo or NiMo as catalysts for more than 60 years, usually requires high temperatures and the presence of H2 which involves the reactions of paraffinic sulfur-containing compounds such as thiols, thioethers and disulfides to readily produce H2S and corresponding hydrocarbons [4]. Moreover, aromatic sulfur compounds such as benzothiophenes (BTs) and dibenzothiophenes (DBTs) are hardly desulfurized by HDS because of the sterically hindered adsorption of these compounds on the catalyst surface [5]. Oxidative desulfurization (ODS), one of the alternative technologies including extraction [6,7], adsorption [8,9] and biodesulfurization [10] with the advantages of mild reaction conditions, is generally considered the most promising process that consists of two steps. The first step is selective oxidation of organic sulfur compounds, and the second one is the removal of sulfoxides or sulfones by polar extractant [11]. However, the extractant is usually volatile organic compounds which would lead to further environmental and safety concerns. Recently, ODS combined with H2O2 as the oxidant and with ionic liquids (ILs) working as both solvent and extractant were reported to be effective if the shortcomings of ILs on high cost and viscosity can be ⁎ Corresponding author. E-mail address: [email protected] (W. Liang).
somehow overcome. Fenton-like ILs [(C8H17)3CH3N]Cl/FeCl3–H2O2 system [12] showed that the sulfur removal of DBT-containing model oil reached 97.9% and that the sulfur level of FCC gasoline could be reduced from 360 ppm to 110 ppm. Acidic ILs [13] containing \COOH groups in the cations were oxidized by H2O2 for ODS process, and the computational investigation by density functional theory (DFT) found that catalytic properties of the acidic ILs were closely related to the structures, acidities and extraction capabilities. Dishun Zhao [14] reported that the sulfur contents of DBT dissolved in n-octane could be dramatically reduced using H2O2–formic acid as an oxidant and pyridinium-based ILs as phase-transfer catalysts (PTCs). In this study, several acetic acid-based ILs were synthesized from acetic acid and benzoimidazole by alkylation and acid–base neutralization (Fig. 1). The acetic acid-based ILs were used as both phase-transfer catalysts and extractants, which showed good oxidative ability. Meanwhile, the optimization of oxidative desulfurization of model fuel was investigated by orthogonal experiments (L16(4)5) in detail. To the best of our knowledge, it is the first report on oxidative desulfurization with the acetic acid-based ionic liquids without adding an additional catalyst. 2. Experimental Chemicals o-phenylenediamine, 1-bromopentane, 1-bromohexane, 1-bromoheptane, 1-bromooctane and solvents were commercially available and used as supplied. Each IL sample used for the oxidization and adsorption experiments was further purified by silica gel column and gel chromatography using petroleum ether and ethyl acetate (volume ratio 1:1) as the elution, followed by removal of the volatile components under high vacuum at room temperature for 5 h [15].
0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2012.09.034
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
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W. Liang et al. / Fuel Processing Technology xxx (2012) xxx–xxx
H N
R N
RBr
N
R N
CH3COOH
N
CH3COO-
N H
3
3 3 3 3
Fig. 1. Synthesis of the acetic acid-based ionic liquids.
1 H and 13C NMR spectra were recorded on a Varian INOVA-400 Spectrometer. Chemical shifts were reported in ppm from a TMS reference. FT-IR spectra were recorded on a Nicolet Nexus-670 spectrometer by dispersing the sample in a KBr pellet. Mass spectra were recorded on a VG ZAB-HS spectrometer. In order to remove the residual water mixed in IL, the sample was dissolved in distilled acetonitrile, and then dried by anhydrous sodium sulfate. After desiccant filtration and rotary evaporation, the IL was dried in vacuum to constant weight. The viscosity was measured with a spindle viscometer equipped with a thermostated water bath at 25 °C. The measurements have a precision of ±2%. The quantity of thiophene absorbed onto the ILs was measured by Gas Chromatography model GC 7890II. A standard addition method was used for the accuracy of the results.
2.1. Preparation of ionic liquids The ILs were prepared by two step synthesis [16] through equal molars of benzoimidazole combined firstly with the corresponding alkyl bromide to form the compounds I (a–d), and then the compounds II (IL1–IL4) were synthesized according to an experimental procedure based on a neutralization reaction [17]. 2.1.1. General procedure for the synthesis of the compounds I (a–d) A mixture of benzoimidazole (0.1 mol), tetrabutyl ammonium bromide and 30% aqueous solution of sodium hydroxide was placed in a three-necked flask, which was provided with a magnetic stirrer and reflux condenser. Then equal mole of freshly distilled alkyl bromine was added drop-wise. The mixture was heated at 45 °C for 12 h until two phases formed. The organic layer was extracted with ethyl acetate, washed with deionized water and dried with anhydrous magnesium sulfate. The solvent was removed under reduced pressure at 50 °C for 3 h, giving the compounds I (a–d) as light yellow liquids. 2.1.2. General procedure for the synthesis of the compounds II (IL1–IL4) The compounds I (a–d) and acetonitrile were charged into a 100 ml round-bottom flask, and the mixture was stirred at 0 °C for 1 min. An equal amount of acetic acid was added drop wise with a magnetic stirrer for 1 h at 0 °C and then for 2 h at room temperature. The solvent was removed under reduced pressure, and the raw IL1–IL4 was finally obtained. The acetic acid-based ionic liquids were washed repeatedly with toluene and diethyl ether to remove nonionic residues, and dried in vacuum at room temperature for 6 h. The products were formed quantitatively and in high purity as assessed by NMR spectroscopy. IL1: viscosity 20.7 mPa s. IR (KBr), v, cm − 1; 3392, 3154, 3078, 2927, 2858, 1834, 1649, 1621, 1574, 1455, 1409, 1381, 746. 1H NMR (DMSO), δ; 9.97 (s, 1H, im-H), 8.16 (d, 1H, J = 7.2 Hz, 2.8 Hz, Ar\H), 8.08 (d, 1H, J = 7.6 Hz, 2.4 Hz, Ar\H), 7.73 (dd, 2H, J = 7.6 Hz,
2.0 Hz, Ar\H), 4.61 (t, 2H, J = 6.4 Hz, N\CH2\), 2.49 (s, 3H, CH3C_O), 1.95 (m, 2H, \CH2\), 1.37 (m, 4H, \CH2\CH2\), 0.88 (t, 3H, J = 7.2 Hz, CH3\). 13C NMR (DMSO), δ: 167.8, 143.4, 131.6, 130.7, 126.8, 113.9, 47.8, 46.8, 28.3, 27.8, 21.6, 13.8. MS, m/z: 189.2 (M+, 100%). IL2: viscosity 147.8 mPa s. IR (KBr), v, cm−1; 3392, 3154, 3078, 2927, 2858, 1834, 1649, 1621, 1574, 1455, 1409, 1381, 746. 1H NMR (DMSO), δ; 9.97 (s, 1H, im-H), 8.16 (m, 2H, J = 7.2 Hz, 2.8 Hz, Ar\H), 8.08 (d, 1H, J = 7.6 Hz, 2.4 Hz, Ar\H), 7.73 (dd, 2H, J = 7.6 Hz, 2.0 Hz, Ar\H), 4.61 (t, 2H, J = 6.4 Hz, N\CH2\), 2.49 (s, 3H, CH3C_O), 1.95 (m, 2H, \CH2\), 1.37 (m, 6H, \CH2\CH2\CH2\), 0.88 (t, 3H, J = 7.2 Hz, CH3\). 13C NMR (DMSO), δ: 167.8, 143.4, 131.6, 130.8, 126.8, 113.9, 47.8, 46.8, 31.5, 28.3, 27.8, 21.6, 13.8. MS, m/z: 203.2 (M+, 100%). IL3: viscosity 173.2 mPa s. IR (KBr), v, cm −1; 3419, 3147, 3087, 2958, 2828, 1842, 1649, 1624, 1576, 1453, 1401, 1376, 764. 1H-NMR (DMSO), δ; 9.89 (s, 1H, im-H), 8.15 (d, 1H, J = 7.2 Hz, 2.4 Hz, Ar\H), 8.07 (d, 1H, J=7.2 Hz, 2.8 Hz, Ar\H), 7.73 (dd, 2H, J=7.2 Hz, 2.4 Hz, Ar\H), 4.59 (t, 2H, J=7.2 Hz, N\CH2\), 2.49 (s, 3H, CH3C_O), 1.92 (m, 2H, \CH2\), 1.32 (m, 8H, \CH2\CH2\CH2\CH2\), 0.86 (t, 3H, J = 6.8 Hz, CH3\). 13C NMR (DMSO), δ: 167.9, 143.3, 131.6, 130.6, 126.8, 113.9, 47.6, 46.8, 31.1, 28.5, 28.1, 25.6, 21.9, 13.9. MS, m/z: 217.2 (M +, 100%). IL4: viscosity 210.5 mPa s. IR (KBr), v, cm−1; 3419, 3147, 3088, 2959, 2829, 1842, 1649, 1625, 1576, 1453, 1401, 1376, 764. 1H-NMR (DMSO), δ; 9.89 (s, 1H, im-H), 8.15 (d, 1H, J = 7.2 Hz, 2.4 Hz, Ar\H), 8.07 (d, 1H, J = 7.2 Hz, 2.8 Hz, Ar\H), 7.73 (dd, 2H, J = 7.2 Hz, 2.4 Hz, Ar\H), 4.59 (t, 2H, J = 7.2 Hz, N\CH2), 2.49 (s, 3H, CH3C_O), 1.92 (m, 2H, \CH2\), 1.32 (m, 10H, \CH2\CH2\CH2\CH2\CH2\), 0.86 (t, 3H, J =6.8 Hz, CH3\). 13C NMR (DMSO), δ: 167.9, 143.3, 131.6, 130.6, 126.8, 113.9, 47.6, 46.8, 31.1, 29.5, 28.5, 28.1, 25.6, 21.9, 13.9. MS, m/z: 231.2 (M+, 100%). 2.2. Catalytic oxidative desulfurization Model oil was prepared by dissolving thiophene in n-heptane to form solutions with sulfur content of 1500 ppm. The oxidative desulfurization experiments were carried out in a 100 ml round-bottom flask. A Table 1 Influence of different reaction systems. Entry
1 2 3
Desulfurization system
ILs + oil H2O2 + oil ILs + H2O2 + oil
Sulfur removal (%) IL1
IL2
IL3
IL4
No IL
40.7 – 68.3
50.1 – 77.7
55.6 – 83.2
59.9 – 87.5
– 32.6 –
Experimental condition: model oil=10 ml, volume ratio VILs/VH2O2 =1/1.1 (VILs +VH2O2 = 20 ml), T=70 °C, t=3 h.
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
W. Liang et al. / Fuel Processing Technology xxx (2012) xxx–xxx
250 200 150
Vp / mV
typical run was as follows: a certain amount of model oil was added into the reactor and then a certain amount of [Abim]CH3COO was added. The mixture was stirred and heated to the desired temperature in a water bath. Then, a certain amount of hydrogen peroxide (30%) was added to the mixture and stirred vigorously. Upon standing, the upper phase (model oil) was separated easily from the IL phase by decantation, and the sulfur content was analyzed according to gas chromatography external standard method.
3
before
100 50 0 15 10
3. Results and discussion
after
5
3.1. Influence of different desulfurization systems of model oil
0
As shown in Table 1, the effects of ILs and hydrogen peroxide on desulfurization of model oil were investigated by the three different sulfur removal systems. The entry 1 indicated that the oxidative desulfurization system containing four kinds of ILs was used as extractant in the absence of H2O2, and the removal of sulfur compounds only reached 40.7%, 50.1%, 55.6% and 59.9% respectively. The entry 2 showed that the oxidative desulfurization system containing H2O2 was used as the oxidative agent in the absence of ILs. The oxidation of model oil in the H2O2 system under H2O2/sulfur molar ratio of 4 was carried out at 70 °C. After 3 h, the sulfur content in the model oil was reduced from 1500 to 1017 ppm which equaled to the desulfurization of 32.6% by single oxidation using H2O2 as the oxidant. The last entry illuminated that sulfur content decreased remarkably within the system containing ILs and H2O2, and the removal of sulfur reached 68.3%, 77.7%, 83.2% and 87.5% at 70 °C for 3 h respectively. The results demonstrated that ILs played a vital role in the removal of sulfur content in the desulfurization system, and that the combination of ILs and H2O2 was rather helpful to deep desulfurization. The evidence was that the desulfurization of model oil was increased after adding H2O2 to IL, and the desulfurization efficiency was up from 40.7% to 68.3% (IL1). The higher the volume ratio of ILs/H2O2, the easier the deep desulfurization by oxidation was, which was in accordance with the paper reported [18]. The carboxylate group (\COO−) was oxidized by the H2O2 to the corresponding hyperoxide. So, it should be better with the ILs/H2O2 volume ratio 1:1.1 considering the economical factor. 3.2. Process of extraction–oxidation of thiophene using acetic acid-based ionic liquids [Abim]CH3COO as catalytic system The possible reaction mechanism was proposed to experience three steps (see Fig. 2) [13,19]. The carboxylate group (\COO −) existing in the anion reacted with H2O2, resulted in a hyperoxide (\COOO −); and then, owing to the phase transfer catalysis character of ILs, the sulfur compounds dissolved in the ILs were oxidized by the hyperoxide (\COOO−) to the corresponding sulfones; finally, ILs were reoxidized to peroxide state, and returned the water phase. The sulfone has higher polarity compared to sulfide, so it was easy to dissolve into the mixture of ILs and water. The comparison of GC-FPD of thiophene in n-heptane
1
2
3
4
5
6
t / min Fig. 3. Comparison of GC-FPD of thiophene in n-heptane before and after reaction.
before and after reaction, showed that the oxidation products of thiophene did not exist in the oil phase because there was no new peak in GC-FPD analysis of oil layer after oxidation (Fig. 3). 3.3. Optimization of the catalytic oxidative desulfurization parameters in ILs–H2O2 system by orthogonal experiment The catalytic oxidative desulfurization of thiophene in n-heptane with the acetic acid-based ILs [Abim]CH3COO as the catalytic solvent and H2O2 as the oxidant was optimized by orthogonal experiment. Get the optimum conditions by using the orthogonal experiments. The results indicate that orthogonal test can economize trial-manufactured expenditure, time and work load, and get the highest desulfurization efficiency with the least experiments. An orthogonal L16(4)5 test designed in the catalytic oxidative desulfurization process was used for the optimization of the catalytic oxidative desulfurization conditions. Factors and levels for orthogonal L16(4)5 test were presented in Table 2. The results of the orthogonal test and the extreme difference analysis were presented in Table 3. K represented the arithmetic mean of desulfurization rate with different factors in the same level, and the biggest parameter of K represented the optimum process conditions. For example, in the temperature factor, there are 4 parallel experiments for the same 40 °C level (A1): K1 = (30.7 + 66.2 + 45.7 + 69.1) / 4 = 52.93. R represented the difference of maximum and minimum of K in different levels in the same factor. We could get the influence of the different factors for desulfurization rate from the worst value of R, i.e., in the same factor of temperature, R = Kmax − Kmin = K4 − K1 = 83 − 52.93 = 30.07. As seen in Table 3, optimization of the catalytic oxidative desulfurization parameters, A4, B4, C2, D4, and E3, was acquired from the orthogonal L16(4) 5 test. In other words, the optimized conditions for catalytic oxidative desulfurization were oxidation temperature at 70 °C, oxidation time of 180 min, simulated oil dosage of 10 ml, 8 carbons, and
H 2O 2
[Abim]CH3COOO
[Abim]CH3COO 1
water phase phase interface
3
S
organic phase
+ [Abim]CH3 COO
2
+ [Abim]CH3COOO S
O A = pentyl, hexyl, heptyl, octyl Fig. 2. Mechanism extraction–oxidation of thiophene.
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
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W. Liang et al. / Fuel Processing Technology xxx (2012) xxx–xxx
Table 2 Factor and levels for orthogonal test. Level
1 2 3 4
Table 4 The desulfurization of model oil by regenerated ILs.
Factor A
B
C
D
E
40 50 60 70
30 80 130 180
5 10 15 20
5 6 7 8
0.9:1 1:1 1:1.1 1:1.2
A: temperature T/°C; B: time t/min; C: simulated oil dosage V/ml; D: number of carbon; E: volume ratio VILs/VH2O2 (VILs + VH2O2 = 20 ml).
ILs/H2O2 volume ratio of 1:1.1 respectively. The influence of some factors on the catalytic oxidative desulfurization decreased in the order A >D >E > B >C according to the extreme difference analysis (R values). We could find from the orthogonal experiments that the efficiency of desulfurization was different according to the ILs which had the different carbon numbers of side chain. The ILs containing 8 carbons in side chain had the best efficiency of desulfurization. This was mainly because the alkyl having more carbons and the lipotropism was better, and the chances of entering the oil phase was consequently better [20]. And then the oxidative agent of peroxycarboxylic acid could be moved to oil phase that will be conducive to the oxidation reaction in the oil phase. As a result, the ILs containing 8 carbons of side chain had the best efficiency of desulfurization. Furthermore, the efficiency of desulfurization with benzoimidazole-based ILs was better than imidazole-based ILs [21,22]. The main reason was that the cation of ILs and thiophene has aromatic rings. The aromatic nucleuses in the chain of imidazole make the benzoimidazole have higher polarizable aromatic π-electron density than imidazole. The interaction between the cation of ILs, which had an aromatic ring structure with a conjugated system, and the thiophene sulfur compounds leads to the formation of liquid-clathrate compounds and the π–π complexation [23–25] between unsaturated bonds of aromatic sulfur-compounds and the benzoimidazole cation. The orthogonal experiment also indicated that the value of K decreased from 70.9% to 61.4% because the polarity of the ILs was changed with the adding of H2O2. But as the oxidization proceeded, thiophene was converted to CO, CO2 and SO2, extraction equilibrium was broken [1], and K increased to 75.3% at 180 min which was more than the
Cycle
0
1
2
3
4
5
6
Desulfurization efficiency (%)
87.5
87.2
86.6
86.5
86.2
85.6
79.9
Conditions: T = 70 °C, t = 180 min, V= 10 ml, N = 8, VILs/VH2O2 = 1:1.1 (VILs + VH2O2 = 20 ml).
time reported by Cao [22]. In view of the main factor A, K increased smoothly from 52.93% to 83% as the desulfurization temperature increased from 40 to 70 °C and accelerated the reaction rate. However, H2O2 was decomposed dramatically when the desulfurization temperature was more than 70 °C. 3.4. Recycle of used ILs The acetic acid-based ILs could be used as a catalyst for desulfurization of model oil when H2O2 was used as the oxidant. The catalytic oxidative desulfurization of thiophene in n-heptane could reach 87.5% under the optimal reaction conditions. The used ILs were regenerated by re-extraction in dichloromethane, then dichloromethane was removed under vacuum in a rotary evaporator. The results of desulfurization on model oil by regenerated ILs were shown in Table 4, which indicated that the recycled ILs could be recycled five times without an obvious decrease in activity. 4. Conclusions In conclusion, the acetic acid-based ILs showed high activity in the oxidative desulfurization of model oil with hydrogen peroxide as the oxidant, which not only served as an extractant and reaction medium but also acted as a phase transfer catalyst. It was found to be effective in the extraction of sulfur components in model oil. The desulfurization process was achieved after H2O2 was added to the extraction system. The environmental benign oxidizability of H2O2 was utilized for oxidizing the sulfur components to the corresponding sulfones followed by extraction by the ILs. The catalytic oxidative desulfurization of thiophene in n-heptane was optimized by orthogonal experiments. [Otbim]+CH3COO− containing eight-carbon side chain has of best catalytic activity. 87.5% of thiophene in the model oil was removed
Table 3 Results and analysis of orthogonal test. Experiment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Arithmetic mean
Extreme difference Optimization level Order of factors
Factor
K1 K2 K3 K4 R
Desulfurization efficiency (%)
Desulfurization temperature, A (°C)
Reaction time, B (min)
Simulated oil dosage, C (ml)
Number of carbon, D
Volume ratio, E (VILs/VH2O2)
40 40 40 40 50 50 50 50 60 60 60 60 70 70 70 70 52.93 67.18 73.35 83 30.07 A4 1
30 80 130 180 30 80 130 180 30 80 130 180 30 80 130 180 68.85 70.9 61.4 75.3 13.9 B4 4
5 10 15 20 10 5 20 15 15 20 5 10 20 15 10 5 67.68 77.63 67.05 64.1 13.53 C2 5
5 6 7 8 7 8 5 6 8 7 6 5 6 5 8 7 60.33 68.95 65.33 81.85 21.52 D4 2
0.9:1 1:1 1:1.1 1:1.2 1:1.2 1:1.1 1:1 0.9:1 1:1 0.9:1 1:1.2 1:1.1 1:1.1 1:1.2 0.9:1 1:1 57.38 72.25 75.45 71.38 18.07 E3 3
30.7 66.2 45.7 69.1 73.9 86.6 48.8 59.4 86.9 54.6 66.3 85.6 83.9 76.2 84.8 87.1 – – – – – – –
Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034
W. Liang et al. / Fuel Processing Technology xxx (2012) xxx–xxx
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Please cite this article as: W. Liang, et al., Oxidative desulfurization of simulated gasoline catalyzed by acetic acid-based ionic liquids at room temperature, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.034