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Oxidative desulfurization of model diesel using [(C4H9)4N]6Mo7O24 as a catalyst in ionic liquids
Fuel Processing Technology 119 (2014) 87–91
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Oxidative desulfurization of model diesel using [(C4H9)4N]6Mo7O24 as a catalyst in ionic liquids Hongying Lü a,b, Changliang Deng a, Wangzhong Ren b, Xin Yang a a
Green Chemistry Centre, College of Chemistry and Chemical Engineering, Yantai University, 32 Qingquan Road, Yantai 264005, China Shandong Provincial Engineering Research Center for Light Hydrocarbon Comprehensive Utilization, College of Chemistry and Chemical Engineering, Yantai University, 32 Qingquan Road, Yantai 264005, China b
a r t i c l e
i n f o
Article history: Received 8 July 2013 Received in revised form 24 October 2013 Accepted 30 October 2013 Available online 22 November 2013 Keywords: Ionic liquid Oxidative desulfurization Dibenzothiophene Polyoxometalate
a b s t r a c t An extraction and catalytic oxidation desulfurization (ECODS) system, composed of an Anderson-type catalyst [(C4H9)4N]6Mo7O24, model diesel, 30% H2O2 and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim] PF6), was conducted under mild conditions. The sulfur-containing compounds, such as benzothiophene (BT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6DMDBT), were extracted into ionic liquid (IL) from the model oil and oxidized to corresponding sulfones using H2O2 as oxidant in the presence of the [(C4H9)4N]6Mo7O24. In the case of ECODS, the sulfur removal of DBT can reach 99.0%, which was superior to that of the simple extraction with IL or the chemical oxidation. The amounts of ILs and oxidant dosage play vital roles in ECODS. The reactivity of sulfur-containing compounds in the ECODS was followed: DBT N 4-MDBT N 4,6-DMDBT N BT. This ECODS system could be recycled six times without a significant decrease in activity. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Organosulfurs are undesirable in fuels, which are the major source of air pollution. The U.S. environmental regulations limited the sulfur level in diesel to b 15 ppm by the year 2006. It is extremely difficult and costly to achieve with the current technology, which requires catalytic reactors operated at high pressure and temperature. Selectively catalytic oxidative desulfurization of diesel, as one of the most promising alternative desulfurization method, has attracted intensive attention in the past two decades [1,2]. Oxidative desulfurization (ODS) process is involved in the oxidation of sulfur-containing compounds to corresponding sulfones and the removal of sulfones from diesel by extraction or adsorption [3–6]. Organic solvents are usually utilized as extraction solvents, which would cause further environmental and security problems as these solvents are generally flammable and volatile organic compound [7–11]. Ionic liquids (ILs) have several advantages over organic solvents, such as low melting point, negligible vapor pressure and excellent thermally stability. It can effectively avoid further environmental and security problems. Therefore, extraction desulfurization (EDS) with ILs and oxidative desulfurization in the IL are attracting wide interest in past decade [12–20]. Herein, an efficient extraction and catalytic oxidation desulfurization (ECODS) system for sulfur compounds in a model diesel was developed. It was found that an Anderson-type polyoxometalate
E-mail address: [email protected] (H. Lü).
[(C4H9)4N]6Mo7O24 in [bmin]PF6 exhibited so high activity that the removal of DBT could reach 99.0% in 120 min at 50 °C using hydrogen peroxide as oxidant. The effect of several important factors (reaction temperature, the amount of ILs and oxidant dosage, and the recycling of ILs and catalyst) on desulfurization efficiency was studied. 2. Experimental section 2.1. Synthesis and characterization of catalysts All chemicals were used as received. The [(C4H9)4N]6Mo7O24 was prepared as following: an ethanol solution of tetrabutyl ammonium bromide (6 mmol) was dropwise into an aqueous solution of (NH4)6Mo7O24 (1 mmol) under stirring at room temperature. A snowwhite precipitate was immediately formed. After continuous stirring for 4 h, the resulting mixture was filtered, washed and dried at 60 °C in a vacuum for 24 h to obtain the required catalysts. The infrared spectrum (IR) of the catalyst, diluted with KBr and pressed into a pellet, was recorded on a Nicolet 470 FTIR spectrometer. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) was recorded on a TU-1901 (Beijing General Analytical Instrument Ltd. Co., China) with BaSO4 as the internal standard. The scanning patterns were recorded at 200–800 nm in a step-scan mode with a step of 5 nm. 2.2. Oxidation and desulfurization of the model diesel In a typical run, a water bath was heated to a desired temperature. The sulfur concentration was kept at 500 ppm by dissolving one of the
0378-3820/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.10.023
H. Lü et al. / Fuel Processing Technology 119 (2014) 87–91
model sulfur-containing compound (BT, DBT, 4-MDBT or 4,6-DMDBT) in 500 mL n-octane. The mixture, containing 10 mL of model diesel and 1 mL of IL was stirred vigorously using a stir bar at reaction temperature for 15 min before it was sampled and analyzed by microcoulometry. Then 30 wt.% H2O2 (H2O2/S = 5, molar ratio) and catalyst (S/catalyst = 100, molar ratio) were added and stirred vigorously for 3 h. The ratio of reactants was adjusted according to the different experimental conditions. The mixture was periodically sampled and analyzed. The total sulfur content of the samples was determined by microcoulometry (detection limit, 0.1 ppm) [5]. 3. Results and discussion
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3.1. Characterization of the catalyst 0
The IR spectrum of [(C4H9)4N]6Mo7O24 is shown in Fig. 1a. The catalyst reveals the characteristic bands of bridged Mo\O\Mo bonds in the molecule of polyoxometalate around 559 and 665 cm−1. The bands at 904, 912 and 924 cm−1 are attributed to the terminal group of Mo_O bond [21,22]. In addition, the other peaks at 1479, 2876 and 2976 cm−1 belong to the vibrations of the quaternary ammonium cations. UV–vis spectrum of [(C4H9)4N]6Mo7O24 reveals very intense absorption in the range of 210–350 nm spectral region (Fig. 1b), which is the characteristic of polymer Mo\O\Mo structures originated by the charge transfer processes from O2− to Mo6+ in octahedral coordination [23,24]. 3.2. Effect of different desulfurization systems on DBT removal
Transmittanc/ %
Four different desulfurization systems of extraction, extraction coupled with chemical oxidation, catalytic oxidation and extraction coupled with catalytic oxidation were shown in Fig. 2. Three ILs, including [bmim]BF4, [bmim]PF6 and [omim]PF6 are immiscible with the model diesel. The solubility of [(C4H9)4N]6[Mo7O24] in ILs is far greater than the solubility in the model diesel [25]. The aqueous hydrogen peroxide could only dissolve in [bmim]BF4 but not in [bmim]PF6 or [omim] PF6. The reaction system with [bmim]BF4 was a biphasic system (IL/ Diesel), in which the model diesel was the upper layer and IL phase, along with catalyst and aqueous hydrogen peroxide was in the lower layer. Besides, the reaction system with [bmim]PF6 or [omim]PF6 was triphasic system (IL/H2O2/Diesel), in which the model diesel was the upper layer, aqueous hydrogen peroxide was in the middle layer and IL phase, along with catalyst, was in the lowest layer [26].
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Fig. 2. Effect of different desulfurization systems on DBT removal. Conditions: T, 50 °C, t = 3 h, S/H2O2 = 1/5 (molar ratio), DBT (S: 500 ppm) in n-octane.
The desulfurization efficiency of extraction, extraction coupled with chemical oxidation of ILs and catalytic oxidation was very poor. However, the removal of DBT increased sharply as [(C4H9)4N]6[Mo7O24] and H2O2 were employed together in the presence of all three ILs. The removal of DBT can reach to 99.0% with the addition of [(C4H9)4N]6 [Mo7O24] and H2O2 in [bmim]PF6. Similar result was founded in [omim]PF6, which obtained a sulfur removal of 97.3%. However, BmimBF4 showed low reaction activity, and the removal of DBT from model diesel was only 73.0% in 3 h. This result indicated that the nature of IL plays a vital role in desulfurization system. The efficiency of desulfurization was higher in water-immiscible [bmim]PF6 and [omim]PF6 than that in water-miscible [bmim]BF4 probably owing to the effects of water. As determined in previous work, there is a strong interaction between the Anderson-type catalyst and water, which may occupy lots of the active sites of this polyoxometalate, leading to a decreased removal of DBT[27]. On the basis of some previous works and the forgoing experiment [13,28], a probable pathway for ECODS of DBT in the [bmim]PF6 system is suggested as shown in Scheme 1. First, the DBT was extracted from the oil phase to ionic phase. Meanwhile, the H2O2 can continuously supply active oxygen to the heteropolyanions in the interface of IL phase and aqueous phase at the same time. Then, the heteropolyanions oxidize the DBT to dibenzothiophene sulfones (DBTO2) in [bmim]PF6. Therefore, a continuous decrease of DBT concentration in model diesel was observed. 3.3. Effect of reaction temperature on DBT removal The effect of temperature on the ECODS system was also investigated. As shown in Fig. 3, the removal of DBT reached 61.0%, 75.5%, 94.1% and 99.3% in 90 min, when the temperature was at 30 °C, 40 °C, 50 °C,
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Absorbance/ %
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H 2O2
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Wavelength/ nm Fig. 1. Spectroscopic characterization of the [(C4H9)4N]6Mo7O24 catalyst. (a) FTIR spectrum; (b) UV–vis DRS.
[(C4H9)4N]6Mo7O24
water phase
ionic liquid phase
Scheme 1. A probable pathway for ODS in [bmim]PF6.
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H. Lü et al. / Fuel Processing Technology 119 (2014) 87–91
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60 O/S=7 O/S=6 O/S=5 O/S=4 O/S=3
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Reaction time/ min Fig. 3. Removal of DBT versus the reaction time using at different reaction temperatures. Conditions: S/H2O2 = 1/5 (molar ratio), DBT (S: 500 ppm) in n-octane.
and 60 °C, respectively. The removal of DBT reached 99.0% at 50 C in 120 min. The result indicated that a lower reaction temperature was unfit for oxidation of DBT and the removal of DBT was increased with increasing temperature. However, the high temperature is conducive to nonproductive decomposition of hydrogen peroxide [29]. Therefore, reaction temperature was set to 50 °C in most cases in the present study. 3.4. Effect of the IL/oil volume ratio and the H2O2/DBT molar ratios on removal of DBT A similar set of ECODS reactions involving different volume ratio of [bmim]PF6 and model diesel at 50 °C was investigated as shown in Fig. 4. The results imply that the amount of IL was an important factor in ECODS system. The sulfur removal of model diesel increases with the increase of IL/oil volume ratio. The removal of DBT reached 87.8%, 94.1% and 99.0% in 90 min, when the volume ratio was 1/20, 1/10 and 1/5, respectively. The removal of DBT can reach 99.0% with the volume ratio 1/10 in 120 min. However, the sulfur removal still remained 99.0% as further increasing the volume ratio to 1/5. Although the extraction ability of [bmim]PF6 to extract DBT from model diesel increased with the increasing amount of IL, the concentration of the catalyst in [bmim]PF6 was reduced. This experimental phenomenon was caused by these two reasons simultaneously [30]. So volume ratio of [bmim]PF6
Fig. 5. Removal of DBT versus the reaction time at different H2O2/DBT molar ratios Conditions: T, 50 °C, DBT (S: 500 ppm) in n-octane.
and model diesel was set to 1/10 in this paper, taking into account the efficiency and economic point of view. According to the stoichiometric reaction, 2 mol of H2O2 are consumed for the oxidation of 1 mol of sulfur-containing compound to corresponding sulfones. To investigate the effect of the dosage of oxidation agent on the oxidative properties, the oxidation of DBT in [bmim]PF6 system under various H2O2/sulfur (O/S) molar ratios was carried out at 50 °C. As depicted in Fig. 5, the O/S molar ratio has a strong influence on the reaction rate. The removal of DBT increased from 96.2% at O/ S = 3 to 97.8% at O/S = 4 in 2 h. When the O/S ratio reached 5, the time for 99.0% removal of DBT can be reach in 2 h. Further increasing the O/S ratio to 6 and 7 decreased the desulfurization efficiency and the removal of DBT can only reach to 96.2% and 94.2% in 2 h, respectively. It agrees well with our previous work that the large amount of water may lead to significant changes of reaction system and the reduction of reactivity [28]. 3.5. Effect of the nature of sulfur-containing compounds To study the effect of the [(C4H9)4N]6[Mo7O24] in [bmim]PF6 on the different sulfur compounds, the oxidation of four model sulfur compounds such as BT, DBT, 4-MDBT and 4,6-DMDBT was carried out under the same conditions. The catalytic oxidation reactivity of sulfur-containing compounds can be listed as DBT N 4MDBT N 4,6-DMDBT N BT as shown in Fig. 6. The reactivity of
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Reaction time/ min Fig. 4. Removal of DBT versus the reaction time with the different volume ratio of [bmim] PF6 and model diesel. Conditions: T, 50 °C, S/H2O2 = 1/5 (molar ratio), DBT (S: 500 ppm) in n-octane.
Fig. 6. Removal of sulfur-containing compounds versus reaction time. Conditions: T, 50 °C, S/H2O2 = 1/5 (molar ratio), sulfur-containing compounds (S: 500 ppm) in n-octane.
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of Shandong Province Higher Educational Science and Technology Program (J11LB16). This work was also partly supported by the Research Award Fund for outstanding young scientists of Shandong Province in China (BS2010NJ017) and the SKLC cooperation project (N-08-08).
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Recycling runs Fig. 7. Recycling of IL on removal of DBT in model diesel. Conditions: T, 50 °C, t = 3 h, S/H2O2 = 1/5 (molar ratio), DBT (S: 500 ppm) in n-octane.
those sulfur compounds was influenced mainly by two factors, that is, electron density on the sulfur atom and steric hindrance of sulfur compounds. The lowest electron density on the sulfur atom of BT (5.696) leads to a lowest reactivity [31]. The electron density differences of DBT, 4-MDBT and 4,6-DMDBT (5.758, 5.759 and 5.760 for DBT, 4MDBT and 4,6-DMDBT respectively) is so small that they can be ignored [31,32]. Therefore, reactivity was mainly affected by the steric hindrance of the methyl groups, which became an obstacle for the approach of the sulfur atom to the catalytic active species in IL. It indicated that the reaction rates of these sulfur-containing compounds are sensitive to the electron density on sulfur atoms and the steric hindrance of the substituted groups of sulfur containing compounds [28]. 3.6. Effect of recycling of IL on removal of DBT in model diesel The effect of recycling [bmim]PF6 containing [(C4H9)4N]6[Mo7O24] was studied in the ECODS system (Fig. 7). At the end of each run of reaction, The IL phase (under-layer) was separated from the model diesel phase by decantation. The residue H2O2 and water were removed under reduced pressure by rotary evaporation equipment. Then the fresh H2O2 and the model diesel were introduced for the next reaction under the same conditions. The catalytic systems could be recycled six times with an unnoticeable decrease in activity. After first recycle, trace white precipitation was found in the reaction system and more and more precipitation was generated in following recycles. It was removed by filtration after the sixth cycle. The specific infrared absorption of these precipitations at 1166 and 1288 cm−1 is attributed to sulfone groups [31]. 4. Conclusion In conclusion, a new oxidative desulfurization for model diesel, based on extraction using [Bmim]PF6 and catalytic oxidation with an Anderson-type (C4H9)4 N]6[Mo7O24] catalysts, was developed. The sulfur removal of DBT can reach 99.0% under mild conditions. Both extraction efficiency and the oxidative desulfurization efficiency increased with the increasing of temperature. The reactivity of sulfur compounds in the ECODS system decreased in the order of DBT N 4-MDBT N 4,6DMDBT N BT. The [(C4H9)4N]6[Mo7O24] and [Bmim]PF6 can be recycled six times with an unnoticeable decrease in activity. Acknowledgments The authors are grateful for the financial support of the National Nature Science Foundation of China (no. 21373177) and the Project
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Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Oxidative desulfurization of model diesel using [(C4H9)4N]6Mo7O24 as a catalyst in ionic liquids Hongying Lü a,b, Changliang Deng a, Wangzhong Ren b, Xin Yang a a
Green Chemistry Centre, College of Chemistry and Chemical Engineering, Yantai University, 32 Qingquan Road, Yantai 264005, China Shandong Provincial Engineering Research Center for Light Hydrocarbon Comprehensive Utilization, College of Chemistry and Chemical Engineering, Yantai University, 32 Qingquan Road, Yantai 264005, China b
a r t i c l e
i n f o
Article history: Received 8 July 2013 Received in revised form 24 October 2013 Accepted 30 October 2013 Available online 22 November 2013 Keywords: Ionic liquid Oxidative desulfurization Dibenzothiophene Polyoxometalate
a b s t r a c t An extraction and catalytic oxidation desulfurization (ECODS) system, composed of an Anderson-type catalyst [(C4H9)4N]6Mo7O24, model diesel, 30% H2O2 and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim] PF6), was conducted under mild conditions. The sulfur-containing compounds, such as benzothiophene (BT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6DMDBT), were extracted into ionic liquid (IL) from the model oil and oxidized to corresponding sulfones using H2O2 as oxidant in the presence of the [(C4H9)4N]6Mo7O24. In the case of ECODS, the sulfur removal of DBT can reach 99.0%, which was superior to that of the simple extraction with IL or the chemical oxidation. The amounts of ILs and oxidant dosage play vital roles in ECODS. The reactivity of sulfur-containing compounds in the ECODS was followed: DBT N 4-MDBT N 4,6-DMDBT N BT. This ECODS system could be recycled six times without a significant decrease in activity. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Organosulfurs are undesirable in fuels, which are the major source of air pollution. The U.S. environmental regulations limited the sulfur level in diesel to b 15 ppm by the year 2006. It is extremely difficult and costly to achieve with the current technology, which requires catalytic reactors operated at high pressure and temperature. Selectively catalytic oxidative desulfurization of diesel, as one of the most promising alternative desulfurization method, has attracted intensive attention in the past two decades [1,2]. Oxidative desulfurization (ODS) process is involved in the oxidation of sulfur-containing compounds to corresponding sulfones and the removal of sulfones from diesel by extraction or adsorption [3–6]. Organic solvents are usually utilized as extraction solvents, which would cause further environmental and security problems as these solvents are generally flammable and volatile organic compound [7–11]. Ionic liquids (ILs) have several advantages over organic solvents, such as low melting point, negligible vapor pressure and excellent thermally stability. It can effectively avoid further environmental and security problems. Therefore, extraction desulfurization (EDS) with ILs and oxidative desulfurization in the IL are attracting wide interest in past decade [12–20]. Herein, an efficient extraction and catalytic oxidation desulfurization (ECODS) system for sulfur compounds in a model diesel was developed. It was found that an Anderson-type polyoxometalate
E-mail address: [email protected] (H. Lü).
[(C4H9)4N]6Mo7O24 in [bmin]PF6 exhibited so high activity that the removal of DBT could reach 99.0% in 120 min at 50 °C using hydrogen peroxide as oxidant. The effect of several important factors (reaction temperature, the amount of ILs and oxidant dosage, and the recycling of ILs and catalyst) on desulfurization efficiency was studied. 2. Experimental section 2.1. Synthesis and characterization of catalysts All chemicals were used as received. The [(C4H9)4N]6Mo7O24 was prepared as following: an ethanol solution of tetrabutyl ammonium bromide (6 mmol) was dropwise into an aqueous solution of (NH4)6Mo7O24 (1 mmol) under stirring at room temperature. A snowwhite precipitate was immediately formed. After continuous stirring for 4 h, the resulting mixture was filtered, washed and dried at 60 °C in a vacuum for 24 h to obtain the required catalysts. The infrared spectrum (IR) of the catalyst, diluted with KBr and pressed into a pellet, was recorded on a Nicolet 470 FTIR spectrometer. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) was recorded on a TU-1901 (Beijing General Analytical Instrument Ltd. Co., China) with BaSO4 as the internal standard. The scanning patterns were recorded at 200–800 nm in a step-scan mode with a step of 5 nm. 2.2. Oxidation and desulfurization of the model diesel In a typical run, a water bath was heated to a desired temperature. The sulfur concentration was kept at 500 ppm by dissolving one of the
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H. Lü et al. / Fuel Processing Technology 119 (2014) 87–91
model sulfur-containing compound (BT, DBT, 4-MDBT or 4,6-DMDBT) in 500 mL n-octane. The mixture, containing 10 mL of model diesel and 1 mL of IL was stirred vigorously using a stir bar at reaction temperature for 15 min before it was sampled and analyzed by microcoulometry. Then 30 wt.% H2O2 (H2O2/S = 5, molar ratio) and catalyst (S/catalyst = 100, molar ratio) were added and stirred vigorously for 3 h. The ratio of reactants was adjusted according to the different experimental conditions. The mixture was periodically sampled and analyzed. The total sulfur content of the samples was determined by microcoulometry (detection limit, 0.1 ppm) [5]. 3. Results and discussion
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The IR spectrum of [(C4H9)4N]6Mo7O24 is shown in Fig. 1a. The catalyst reveals the characteristic bands of bridged Mo\O\Mo bonds in the molecule of polyoxometalate around 559 and 665 cm−1. The bands at 904, 912 and 924 cm−1 are attributed to the terminal group of Mo_O bond [21,22]. In addition, the other peaks at 1479, 2876 and 2976 cm−1 belong to the vibrations of the quaternary ammonium cations. UV–vis spectrum of [(C4H9)4N]6Mo7O24 reveals very intense absorption in the range of 210–350 nm spectral region (Fig. 1b), which is the characteristic of polymer Mo\O\Mo structures originated by the charge transfer processes from O2− to Mo6+ in octahedral coordination [23,24]. 3.2. Effect of different desulfurization systems on DBT removal
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Four different desulfurization systems of extraction, extraction coupled with chemical oxidation, catalytic oxidation and extraction coupled with catalytic oxidation were shown in Fig. 2. Three ILs, including [bmim]BF4, [bmim]PF6 and [omim]PF6 are immiscible with the model diesel. The solubility of [(C4H9)4N]6[Mo7O24] in ILs is far greater than the solubility in the model diesel [25]. The aqueous hydrogen peroxide could only dissolve in [bmim]BF4 but not in [bmim]PF6 or [omim] PF6. The reaction system with [bmim]BF4 was a biphasic system (IL/ Diesel), in which the model diesel was the upper layer and IL phase, along with catalyst and aqueous hydrogen peroxide was in the lower layer. Besides, the reaction system with [bmim]PF6 or [omim]PF6 was triphasic system (IL/H2O2/Diesel), in which the model diesel was the upper layer, aqueous hydrogen peroxide was in the middle layer and IL phase, along with catalyst, was in the lowest layer [26].
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Fig. 2. Effect of different desulfurization systems on DBT removal. Conditions: T, 50 °C, t = 3 h, S/H2O2 = 1/5 (molar ratio), DBT (S: 500 ppm) in n-octane.
The desulfurization efficiency of extraction, extraction coupled with chemical oxidation of ILs and catalytic oxidation was very poor. However, the removal of DBT increased sharply as [(C4H9)4N]6[Mo7O24] and H2O2 were employed together in the presence of all three ILs. The removal of DBT can reach to 99.0% with the addition of [(C4H9)4N]6 [Mo7O24] and H2O2 in [bmim]PF6. Similar result was founded in [omim]PF6, which obtained a sulfur removal of 97.3%. However, BmimBF4 showed low reaction activity, and the removal of DBT from model diesel was only 73.0% in 3 h. This result indicated that the nature of IL plays a vital role in desulfurization system. The efficiency of desulfurization was higher in water-immiscible [bmim]PF6 and [omim]PF6 than that in water-miscible [bmim]BF4 probably owing to the effects of water. As determined in previous work, there is a strong interaction between the Anderson-type catalyst and water, which may occupy lots of the active sites of this polyoxometalate, leading to a decreased removal of DBT[27]. On the basis of some previous works and the forgoing experiment [13,28], a probable pathway for ECODS of DBT in the [bmim]PF6 system is suggested as shown in Scheme 1. First, the DBT was extracted from the oil phase to ionic phase. Meanwhile, the H2O2 can continuously supply active oxygen to the heteropolyanions in the interface of IL phase and aqueous phase at the same time. Then, the heteropolyanions oxidize the DBT to dibenzothiophene sulfones (DBTO2) in [bmim]PF6. Therefore, a continuous decrease of DBT concentration in model diesel was observed. 3.3. Effect of reaction temperature on DBT removal The effect of temperature on the ECODS system was also investigated. As shown in Fig. 3, the removal of DBT reached 61.0%, 75.5%, 94.1% and 99.3% in 90 min, when the temperature was at 30 °C, 40 °C, 50 °C,
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Wavelength/ nm Fig. 1. Spectroscopic characterization of the [(C4H9)4N]6Mo7O24 catalyst. (a) FTIR spectrum; (b) UV–vis DRS.
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Scheme 1. A probable pathway for ODS in [bmim]PF6.
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and 60 °C, respectively. The removal of DBT reached 99.0% at 50 C in 120 min. The result indicated that a lower reaction temperature was unfit for oxidation of DBT and the removal of DBT was increased with increasing temperature. However, the high temperature is conducive to nonproductive decomposition of hydrogen peroxide [29]. Therefore, reaction temperature was set to 50 °C in most cases in the present study. 3.4. Effect of the IL/oil volume ratio and the H2O2/DBT molar ratios on removal of DBT A similar set of ECODS reactions involving different volume ratio of [bmim]PF6 and model diesel at 50 °C was investigated as shown in Fig. 4. The results imply that the amount of IL was an important factor in ECODS system. The sulfur removal of model diesel increases with the increase of IL/oil volume ratio. The removal of DBT reached 87.8%, 94.1% and 99.0% in 90 min, when the volume ratio was 1/20, 1/10 and 1/5, respectively. The removal of DBT can reach 99.0% with the volume ratio 1/10 in 120 min. However, the sulfur removal still remained 99.0% as further increasing the volume ratio to 1/5. Although the extraction ability of [bmim]PF6 to extract DBT from model diesel increased with the increasing amount of IL, the concentration of the catalyst in [bmim]PF6 was reduced. This experimental phenomenon was caused by these two reasons simultaneously [30]. So volume ratio of [bmim]PF6
Fig. 5. Removal of DBT versus the reaction time at different H2O2/DBT molar ratios Conditions: T, 50 °C, DBT (S: 500 ppm) in n-octane.
and model diesel was set to 1/10 in this paper, taking into account the efficiency and economic point of view. According to the stoichiometric reaction, 2 mol of H2O2 are consumed for the oxidation of 1 mol of sulfur-containing compound to corresponding sulfones. To investigate the effect of the dosage of oxidation agent on the oxidative properties, the oxidation of DBT in [bmim]PF6 system under various H2O2/sulfur (O/S) molar ratios was carried out at 50 °C. As depicted in Fig. 5, the O/S molar ratio has a strong influence on the reaction rate. The removal of DBT increased from 96.2% at O/ S = 3 to 97.8% at O/S = 4 in 2 h. When the O/S ratio reached 5, the time for 99.0% removal of DBT can be reach in 2 h. Further increasing the O/S ratio to 6 and 7 decreased the desulfurization efficiency and the removal of DBT can only reach to 96.2% and 94.2% in 2 h, respectively. It agrees well with our previous work that the large amount of water may lead to significant changes of reaction system and the reduction of reactivity [28]. 3.5. Effect of the nature of sulfur-containing compounds To study the effect of the [(C4H9)4N]6[Mo7O24] in [bmim]PF6 on the different sulfur compounds, the oxidation of four model sulfur compounds such as BT, DBT, 4-MDBT and 4,6-DMDBT was carried out under the same conditions. The catalytic oxidation reactivity of sulfur-containing compounds can be listed as DBT N 4MDBT N 4,6-DMDBT N BT as shown in Fig. 6. The reactivity of
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Fig. 6. Removal of sulfur-containing compounds versus reaction time. Conditions: T, 50 °C, S/H2O2 = 1/5 (molar ratio), sulfur-containing compounds (S: 500 ppm) in n-octane.
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of Shandong Province Higher Educational Science and Technology Program (J11LB16). This work was also partly supported by the Research Award Fund for outstanding young scientists of Shandong Province in China (BS2010NJ017) and the SKLC cooperation project (N-08-08).
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those sulfur compounds was influenced mainly by two factors, that is, electron density on the sulfur atom and steric hindrance of sulfur compounds. The lowest electron density on the sulfur atom of BT (5.696) leads to a lowest reactivity [31]. The electron density differences of DBT, 4-MDBT and 4,6-DMDBT (5.758, 5.759 and 5.760 for DBT, 4MDBT and 4,6-DMDBT respectively) is so small that they can be ignored [31,32]. Therefore, reactivity was mainly affected by the steric hindrance of the methyl groups, which became an obstacle for the approach of the sulfur atom to the catalytic active species in IL. It indicated that the reaction rates of these sulfur-containing compounds are sensitive to the electron density on sulfur atoms and the steric hindrance of the substituted groups of sulfur containing compounds [28]. 3.6. Effect of recycling of IL on removal of DBT in model diesel The effect of recycling [bmim]PF6 containing [(C4H9)4N]6[Mo7O24] was studied in the ECODS system (Fig. 7). At the end of each run of reaction, The IL phase (under-layer) was separated from the model diesel phase by decantation. The residue H2O2 and water were removed under reduced pressure by rotary evaporation equipment. Then the fresh H2O2 and the model diesel were introduced for the next reaction under the same conditions. The catalytic systems could be recycled six times with an unnoticeable decrease in activity. After first recycle, trace white precipitation was found in the reaction system and more and more precipitation was generated in following recycles. It was removed by filtration after the sixth cycle. The specific infrared absorption of these precipitations at 1166 and 1288 cm−1 is attributed to sulfone groups [31]. 4. Conclusion In conclusion, a new oxidative desulfurization for model diesel, based on extraction using [Bmim]PF6 and catalytic oxidation with an Anderson-type (C4H9)4 N]6[Mo7O24] catalysts, was developed. The sulfur removal of DBT can reach 99.0% under mild conditions. Both extraction efficiency and the oxidative desulfurization efficiency increased with the increasing of temperature. The reactivity of sulfur compounds in the ECODS system decreased in the order of DBT N 4-MDBT N 4,6DMDBT N BT. The [(C4H9)4N]6[Mo7O24] and [Bmim]PF6 can be recycled six times with an unnoticeable decrease in activity. Acknowledgments The authors are grateful for the financial support of the National Nature Science Foundation of China (no. 21373177) and the Project
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