Removal of Thiophenic Sulfurs Using an Extractive ... - ACS Publications

Apr 27, 2012 - Xiaoqing Du , Jiao Liu , Hong Chen , and Zhao Zhang ... Yong Chen , Hong-yan Song , Ying-zhou Lu , Hong Meng , Chun-xi Li , Zhi-gang Le...
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Removal of Thiophenic Sulfurs Using an Extractive Oxidative Desulfurization Process with Three New Phosphotungstate Catalysts Hongxing Zhang,†,‡ Jiajun Gao,†,‡ Hong Meng,‡ and Chun-Xi Li*,†,‡ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China

‡

S Supporting Information *

ABSTRACT: Three Keggin-type phosphotungstates, i.e. [C5H5NH]3PW12O40, [C4H6N2H]3PW12O40·3C4H6N2 and [(C4H9)4N]3PW12O40, were synthesized and characterized by elemental analysis, X-ray diffraction, and infrared spectra, meanwhile their catalysis in an extractive catalytic oxidative desulfurization process was studied with ionic liquid (IL) as extractant and H2O2 as oxidant. The main factors affecting the desulfurization process were investigated, including temperature, hydrophobicity of IL, and variety of S-compounds, as well as the amount of catalyst, IL, and H2O2. Under the optimal conditions, the S-content of DBT oil can be decreased from 1000 to 2 ppm. A new interpretation is proposed for the current process, in which IL is assumed as a reaction phase, and the amount of the extracted S-compound and the peroxidized catalyst wherein greatly affect the desulfurization rate. Besides, the IL with the dissolved catalyst can be reused many times and regenerated easily.

1. INTRODUCTION Desulfurization of diesel oils and gasoline has long been a major concern in the refinery industry, and the impetus or pressure comes either from the ever-stringent sulfur (S- hereinafter) limit of fuels, say 10 ppm, to meet the requirements of various clean air acts, or from the potential use of ultraclean oil in fuel cells.1,2 Till now, hydrodesulfurization (HDS) is the most widely practiced process, by which most of the S-compounds, e.g. thiols, sulfides, and disulfides, are converted to H2S under the catalysis of Co/Mo/Al2O3 or Ni/Mo/Al2O3 at high temperature (300−350 °C) and pressure (50−100 atm). However, deep desulfurization is still a challenging task for the conventional HDS process since some benzothiophenic Scompounds present in diesel oils like dibenzothiophene (DBT) and its derivatives are hardly removed due to their extremely low hydrogenation activity.3 Therefore, it is necessary to develop alternative approaches for the production of low or even ultralow S-content diesel oils by virtue of some other Sremoval mechanisms. Toward deep desulfurization, some alternative processes have been reported, e.g. selective adsorption desulfurization (ADS),4 extraction desulfurization (EDS),5,6 oxidative desulfurization (ODS),7,8 complexing desulfurization,9 and specific combinations thereof. Among them, extraction combined with catalytic ODS (ECODS), is regarded as one of the most promising processes for its high S-removal efficiency and selectivity under mild operating conditions, and simple, safe, reproducible, and environment friendly post-treatment.10 Regarding the ODS process, different oxidants have been studied, e.g. molecular oxygen,11 hydrogen peroxide,12 nitric lacid/NO2,13 ozone,14 tert-butyl hydroperoxide (t-BuOOH),15 and superoxides,16 arising from different aspects of consideration of the researchers. Among them, H2O2 is attractive because besides water as the sole byproduct, it provides a high content of active oxygen and is much cheaper and safer than organic peroxides or peracids.17 Moreover, it is of high © 2012 American Chemical Society

efficiency in the presence of an appropriate catalyst or in acidic media.18 With respect to the EDS process, ionic liquids (ILs) have attracted much attention and been studied extensively due to their good extraction performance for thiophenic S-compounds, clear phase separation, and negligible solubility in fuel oils. But the viability of an EDS process with ILs is often restricted by its low partition coefficient and selectivity for S-compounds and the costly regeneration of the extractant.19 However, these limitations can be overcome by an ECODS process. In such a process, S-compounds are extracted from oil to IL phase and then oxidized to their corresponding sulfone by oxidants, which can break the S-partition equilibrium between oil and IL phase, resulting in a further extraction of Scompounds and a further oxidation of the extracted Scompounds until a complete S-removal is achieved. Moreover, the IL with dissolved catalyst can be reused many times and regenerated easily, which can decrease the desulfurization cost.19,20 With respect to the catalysts of the ECODS process, many types of polyoxometalates were used since their selectivity, acidity, and redox property can be designed and tuned at a molecular or atomic level,21 e.g. phosphotungstic acid,22 molybdate,23 decatungstates,24 and peroxophosphomolybdate.20 Although the catalysis mainly depends on the unique anionic structure of the polyoxometalates, the cationic structure may also play an important role in tuning their acidity, hydrophobicity, and affinity to the S-compounds. This paper is aimed to investigate the performance of three organic phosphotungstates for the catalytic oxidation of thiophenic S-compounds in model oil via an ECODS process using H2O2 as oxidant and ILs as extractant. The features of the catalysts are as follows. First, they can be prepared easily Received: Revised: Accepted: Published: 6658

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polyoxometalates catalysts in an ECODS process (Table 1S in the Supporting Information). The mechanism of the triphase ECODS process, as shown in Figure 1, is assumed to include the following: (1) the formation of the metastable emulsion of H2O2 aqueous solution in IL with the help of the catalyst as a surfactant and the vigorous stirring (Figure 1S); (2) the extraction of DBT from oil to IL phase; (3) the degradation of [PW12O40]3‑ (denoted as PW12) to its peroxide form [PO4{WO(O2)2}4]3‑ (denoted as PW4) and free tungsten species (denoted as FTS) in the presence of excess H2O2;27 (4) the oxidation of the extracted DBT to DBT sulfoxide (DBTO) and DBT sulfone (DBTO2) by PW4, and PW4 is reduced to [PO4{WO(O2)}4]3‑;28 (5) the regeneration of PW4 via oxidizing its reduced form [PO4{WO(O2)}4]3‑ by H2O2;27 (6) the precipitation of the oxidation products of DBT, which can promote a further extraction of DBT and its oxidation until a complete desulfurization; (7) the transformation of PW4 into PW12 species with the free tungsten species, after the desulfurization.27

through neutralization or metathesis reaction between phosphotungstic acid and the companion species, viz. Nmethylimidazole, pyridine, and tetrabutyl ammonium bromide. Compared with other reported catalysts,20,24 the synthetic method of these catalysts is very simple. Second, they are hydrophobic but soluble in ILs and tend to accumulate on the water/ILs interphase, being similar to a cationic surfactant. Third, the catalysts with protonated cations like pyridinium and N-methylimidazolium show higher acidity than quaternary ammonium, which is helpful for the catalytic oxidation of Scompounds. Fourth, the heterocyclic cations used here may promote the concentration of S-compounds in the vicinity of phosphotungstates due to the favorable π−π interaction between the two aromatic rings, i.e. the cation and thiophenic S-compound.25 Finally, the catalytic performance of the above three catalysts has not yet been studied to our knowledge. Two different ILs were used here, viz. a hydrophobic 1-butyl-3methylimidazolium hexafluorophosphate ([Bmim]PF6) and a hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) (their chemical structures are shown in Chart 1), and [Bmim]PF6 can form a biphase or triphase ECODS

2. EXPERIMENTAL SECTION 2.1. Materials. Phosphotungstic acid (AR grade, crystallization water excluded) was purchased from Tianjin Jinke Fine Chemical Co., Ltd. Tetrabutyl ammonium bromide and pyridine both with AR grade were purchased from Tianjin Fuchen Chemical Reagent Co., Ltd. N-Methylimidazole (purity >99%) was purchased from Linhai Kaile Chemical Factory (Zhejiang, China). Dibenzothiophene (DBT, 99%), benzothiophene (BT, 97%), and 3-methylthiophene (3-MT, 99%) were purchased from Acros Organics, USA. Hydrogen peroxide (AR grade, 30%) was purchased from Beijing Chemical Work. NOctane (AR grade) was purchased from Tianjin Guangfu Fine Chemical Engineering Research Institute. Trichloromethane (AR grade) was purchased from Beijng Yili Fine Chemical Co., Ltd. Ionic liquids [Bmim]BF4 and [Bmim]PF6 both with AR grade and purity >99% were purchased from Henan Lihua Pharmaceutical Co., Ltd. All reagents were used as received. 2.2. Preparation of Catalysts. Tetrabutyl ammonium phosphotungstate, [(C4H9)4N]3PW12O40, was synthesized according to the procedure described previously.27 A solution

Chart 1. Chemical Structure of [Bmim]BF4 and [Bmim]PF6

system depending on the relative amount of ILs to water, while [Bmim]BF4 only forms a biphase ECODS system. The triphase ECODS is found to be superior to the biphase one. Compared with other reported literature using Mo- or W-based catalysts,10,19,23 to obtain a similar S-removal rate, the amount of IL, H2O2, and catalyst has been decreased sharply, the reaction temperature is lowered too, and the time is also shortened. Besides, our results are comparable with Zhu et al.26 using [MIMPS]3PW12O40·2H2O (MIMPS = 1-(3-sulfonic group) propyl-3-methyl imidazolium) as catalyst, implying the superiority of the phosphotungstates over Mo- or W-based

Figure 1. Supposed mechanism of the ECODS in [Bmim]PF6. 6659

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of phosphotungstic acid (2.883 g, 1.0 mmol) in deionized water (15 mL) was added dropwise to a solution of tetrabutyl ammonium bromide (3.0 mmol) in deionized water (50 mL) while stirring vigorously. The white precipitate of tetrabutyl ammonium phosphotungstate appeared immediately, and the suspension continued stirring for 2 h. The white precipitate was filtered, washed with deionized water twice, and then dried at 50 °C in vacuum. Calcd for [(C4H9)4N]3PW12O40: H, 3.02; C, 15.99; N, 1.17; O, 17.76; P, 0.86; W, 61.20. Found: H, 3.14; C, 16.10; N, 1.08; O, 17.52; P, 0.85; W, 60.38. The preparation process for the other two compounds, viz. [C5H5NH]3PW12O40 and [C4H6N2H]3PW12O40·3C4H6N2, was similar to [(C4H9)4N]3PW12O40 except tetrabutyl ammonium bromide was replaced by pyridine and N-methylimidazole. Calcd for [C5H5NH]3PW12O40: H, 0.58; C, 5.78; N, 1.35; O, 20.53; P, 0.99; W, 70.77. Found: H, 0.64; C, 5.90; N, 1.36; O, 20.45; P, 0.99; W, 70.48. Calcd for [C4H6N2H]3PW12O40·3C4H6N2: H, 1.17; C, 8.55; N, 4.98; O, 18.98; P, 0.92; W, 65.41. Found: H, 1.47; C, 8.66; N, 5.08; O, 18.83; P, 0.91; W, 64.92. 2.3. Catalyst Characterization. The C, H, and N elemental analysis were performed on an Elementar Vario EL elemental analyzer. X-ray fluorescence (XRF) for O, P, and W elemental analysis was performed with a Panalytical Magix spectrometer. X-ray diffraction (XRD) powder patterns were recorded by a Rigaku 2500 VBZ+/PC diffractometer using monochromatized Cu-Kα radiation under 40 kV and 200 mA in the scan range of 2θ from 5° to 90° with a scan step of 0.02°. The infrared spectra (IR) of the catalysts were recorded on a Nicolet Nexus 8700 IR spectrometer with samples prepared as KBr disks on the 400−4000 cm−1 range. 2.4. ECODS of Model Oil with Phosphotungstate Catalysts. Model oils were prepared by dissolving DBT, BT, or 3-MT in n-octane, respectively, with their initial S-content all being 1000 ppm (μg/g). The catalyst, 30% H2O2 aqueous solution, IL, and model oil were added in turn to a 100 mL three-necked flask in experimental proportion (the specified amounts are referred to the related sections and noted in the figures), which was placed in a thermostatic water bath at specific temperature (from 30 to 70 °C) and stirred vigorously with magnetic stirrer. After the reaction, the upper clear phase (model oil) was withdrawn and analyzed for S-content by gas chromatography-flame ionization detector (GC-FID) (SHIMADZU, GC-2010) equipped with a SE-54 capillary column (30 m × 0.25 mm i.d. × 0.25 μm film thickness) for DBT and BT, and an AT.FFAP capillary column (30 m × 0.53 mm i.d. × 1.0 μm film thickness) for 3-MT. The GC operating conditions were as follows: the temperature for injector, detector, and oven were 340 °C, 250 °C, and 250 °C, respectively, for DBT, and 250 °C, 250 °C, and 200 °C, respectively, for BT. For 3MT, the temperature for injector and detector were 180 and 200 °C, and the oven temperature was programmed from 60 °C (hold for 3 min) to 80 °C (hold for 2 min) at 2.5 °C·min−1. The injection volume was 0.4 μL for all samples. The S-removal of model S-compound is calculated simply by the concentration decrease of DBT, BT, or 3-MT in the respective model oils.

Figure 2. XRD pattern of the catalysts (a) H3PW12O40; (b) [C5H5NH]3PW12O40; (c) [C4H6N2H]3PW12O40·3C4H6N2; and (d) [(C4H9)4N]3PW12O40.

range of 7°−10°, 16°−22°, and 25°−30° (2θ) are observed, especially the strongest peak in the first band, i.e. 7°−10°. According to the literature29 the three diffraction patterns arise from the Keggin structure of the phosphotungstates, while the difference between the phosphotungstic acid and its organic salts closely related to the cationic structure of the organic phosphotungstates. The Keggin structure of the catalysts was further confirmed by IR spectroscopy. As shown in Figure 3, these samples

Figure 3. IR spectra of the catalysts (a) H3 PW 12 O40; (b) [C5H5NH]3PW12O40; (c) [C4H6N2H]3PW12O40·3C4H6N2; and (d) [(C4H9)4N]3PW12O40.

displayed four peaks located between 700 and 1110 cm−1 which were attributed to the absorption modes of the Keggin primary structure of [PW12O40]3‑. The peaks belonging to the W=Ot bonds shift from 982 to 981 cm−1, and the W-Ob and W-Oc bonds shift from 890, 810 to 895, 815 cm−1 (Ot = terminal oxygen, Ob = bridged oxygen between corner-sharing octahedra, Oc = bridged oxygen between edge-sharing octahedra); the P−O bonds appear at nearly identical frequencies (ν(P−O) = 1079 cm−1) to that found for H3PW12O40.30 It is shown that the polyanion retains its basic Keggin structure accompanying a little distortion due to the coordination influence which is consistent with XRD results. 3.2. Catalytic Activity of the Phosphotungstates. The S-removal performance of the three catalysts in the ECODS system is displayed in Figure 4. As seen from the figure, [C4H6N2H]3PW12O40·3C4H6N2 shows the best catalysis followed by [(C4H9)4N]3PW12O40 and [C5H5NH]3PW12O40 with their S-removal rate being 96.9%, 80.8%, and 76.5%, respectively, in 30 min. Meanwhile, a high desulfurization rate

3. RESULTS AND DISCUSSION 3.1. Characterization of the Catalysts. The XRD patterns of the three phosphotungstates and the parent H3PW12O40 are exhibited in Figure 2. The results show that the positions (2θ) of the XRD peaks for these compounds are of a little difference, and the main characteristic peaks in the 6660

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Figure 4. Influence of catalysts on the ECODS of DBT. Conditions: DBT oil = 10 g, T = 40 °C, mass ratio of m(oil):m([Bmim]PF6) = 4:1, mole ratio of n(H2O2):n(S):n(catalyst) = 300:100:1.

Figure 5. Influence of ILs on the ECODS of DBT with [C5H5NH]3PW12O40 as catalyst. Conditions: DBT oil = 10 g, T = 40 °C, mass ratio of m(oil):m(IL) = 4:1, mole ratio of n(H2O2):n(S):n(catalyst) = 300:100:1.

of 99.8% is achieved for each catalyst in 1.5 h. Obviously, the overall catalytic activity of the three phosphotungstates for the oxidation of DBT is promising. It should be pointed out that the catalysts under study are soluble in IL [Bmim]PF6 but insoluble in oil and water. Therefore, when the catalyst, 30 wt % H2O2 aqueous solution, [Bmim]PF6, and model oil were added into the flask, a triphase reaction system was formed, which is comprised of the upper oil phase, the aqueous intermediate phase, and the bottom phase of [Bmim]PF6 with dissolved catalyst. The reaction mechanism is assumed to include the extraction of DBT from oil to IL, oxidation of the catalyst in the IL phase by H2O2, and the oxidation of DBT to DBTO and DBTO2 by the oxidized phosphotungstate catalyst, as presented in Figure 1. Considering the S-removal of [C 4 H 6 N 2 H] 3 PW 12 O 40 · 3C4H6N2 is as high as 96.9% in 30 min reaction time, it is adverse to use this catalyst to investigate the influence of other factors on the S-removal in a longer reaction time. Meanwhile, the polyoxometalates with quaternary ammonium cations as catalysts in an ECODS system have been studied extensively,20,24 while the phosphotungstates with protonated heterocyclic cations have been studied sparsely.26 Thus, [C5H5NH]3PW12O40 was selected as the representative catalyst to further optimize other reaction conditions in the following experiments. 3.3. Influence of Hydrophobicity of ILs on the SRemoval. [Bmim]PF6 and [Bmim]BF4 have been deemed as the most representative hydrophobic and hydrophilic ILs, and their applicability as an efficient extracting agent for removing various aromatic thiophenic S-components from fuel oils has been studied in the past.25,31 Keeping this in mind, [Bmim]PF6 and [Bmim]BF4 were chosen here to study the influence of the hydrophobicity of ILs on the S-removal in the current ECODS process. The experimental result is exhibited in Figure 5. As can be seen, the hydrophobic IL [Bmim]PF6 is superior to the hydrophilic [Bmim]BF4 in terms of the desulfurization rate at any specified time and fixed other conditions, which is also reported by Lo.32 For example, the S-removal with [Bmim]PF6 increases rapidly with time, and the remaining S-content of the model oil can be decreased to about 2 ppm in 1.5 h, while the residue S-content with [Bmim]BF4 is much higher, being about 100 ppm under similar conditions. This result may be closely associated with their susceptibility to the water content in the ILs involved. In fact, the differences of extracting ability between [Bmim]PF6 and [Bmim]BF4 is very slight for DBT.25,33 However, the partition coefficient of DBT between

IL and oil phase decreases drastically with the water content of IL, for example, 3 wt % water content in IL 1-ethyl-3methylimidazolium diethyl phosphate ([Emim][DEP]) can decrease the partition coefficient of DBT from 1.28 to 0.75, corresponding to a 41% decrease in extractive ability.34 For the present experiment with [Bmim]BF4 as extractant, all the H2O2 aqueous solution added is dissolved in the IL phase, leading to a 4.07 wt % water content of IL in line with the mole ratio of H2O2/S being 3:1. In contrast, the equilibrated or the maximum concentration of H2O2 aqueous solution in the hydrophobic IL [Bmim]PF6 is only 3.18 wt %, irrespective of the added amount of H2O2 aqueous solution.35 Therefore, it can be concluded that hydrophobic ILs are advantageous to hydrophilic ones in the ECODS process due to their lower water solubility and accordingly less reduction of extracting ability for aromatic S-components from oils. 3.4. Influence of the Amount of H2O2 on the SRemoval. According to the stoichiometry of the reaction, the lowest mole ratio of H2O2/S is 2:1 for converting DBT to DBTO2. However, as the oxidant of the present process, the amount of H2O2 has an important effect on the overall Sremoval performance. In order to study its influence and find the optimal usage, some experiments were done at different mole ratios of H2O2/S, i.e. 2:1, 3:1, 4:1, 5:1, and 6:1, and the results are presented in Figure 6. As shown from the figure, the desulfurization rate first increases with the increasing mole ratio of H2O2/S from 2:1 to 3:1 and then decreases gradually from

Figure 6. Influence of the mole ratio of H2O2/S on the S-removal with [C5H5NH]3PW12O40 as catalyst. Conditions: DBT oil = 10 g, T = 40 °C, mass ratio of m(oil):m([Bmim]PF6) = 4:1, mole ratio of n(S):n(catalyst) = 300:100:1. 6661

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25:1. As shown in Figure 7, the S-removal rate increases notably with the increasing amount of catalyst in the lower ratio

3:1 to 6:1, indicating an optimal mole ratio of 3:1. It is also noted that the ultimate S-removal is only 95.4% at the lowest mole ratio of H2O2/S as a result of the insufficient H2O2 due to its accompanying thermal decomposition; however, a complete desulfurization can be achieved at any of the other higher mole ratios of H2O2/S within 2.5 h. The results can be explained as follows. From Figure 1, there are two main reactions in the present ECODS process which are given as eqs 1 and 2, respectively H 2O2 + [PW12O40 ]3 − → [PO4 {WO(O2 )2 }4 ]3 − + H 2O (1)

[PO4 {WO(O2 )2 }4 ]3 − + DBT → [PO4 {WO(O2 )}4 ]3 − + DBTO2

(2)

It should be noted that reaction 2 mainly occurs in the IL phase since the catalyst is only soluble in IL. Therefore, the concentration of [PO4{WO(O2)2}4]3‑ (PW4) and DBT in the IL IL phase (denoted as CIL PW4 and CDBT respectively) will greatly affect the desulfurization rate, and any mean which is helpful for IL increasing CIL PW4 and CDBT is advantageous for enhancing the desulfurization rate. Regarding the present ECODS process, the reaction system is biphasic at the mole ratio of H2O2/S of 2:1 with the water-content of IL being 2.76% since such amount of H2O2 aqueous solution can be dissolved completely in [Bmim]PF6, but the system is triphasic at all higher mole ratios of H2O2/S studied here with their water-content being 3.18%, i.e. the solubility of water in [Bmim]PF6 at 40 °C. For the triphasic system, the insoluble portion of H2O2 aqueous solution and IL can be emulsified with the help of vigorous stirring and the catalyst which is partially functioned as the emulsifying surfactant, forming tiny droplets of H2O2 aqueous solution in IL (denoted as W/IL), and some catalysts are adsorbed on the surface of the emulsion droplets of W/IL to reduce the interfacial tension and stabilize the emulsion state. Thus, the total concentration of all forms of catalyst in the IL phase (denoted as CIL cat) will decrease with the mole ratio of H2O2/S from 2:1 to 3:1. However, the decrease of CIL cat is very slight because the insoluble portion of H2O2 aqueous solution at the H2O2/S mole ratio = 3:1 is only 0.96% in terms of mass ratio of the IL amount, and the augment of the total H2O2 aqueous solution is as high as 50%, which can greatly increase the total concentration of PW4 in the reaction system according to eq 1. So the value of CIL PW4 is increased with the mole ratio of H2O2/S from 2:1 to 3:1. Meanwhile, the water-content in the IL phase with the mole ratio of H2O2/S from 2:1 to 3:1 only increases by 0.42%, which may decrease the value of CIL DBT very slightly. Therefore, the S-removal rate is sped up when the mole ratio of H2O2/S increases from 2:1 to 3:1. For the mole ratio of H2O2/S above 3:1, i.e. H2O2/S increases from 3:1 to 6:1, the increasing amount of the insoluble portion of the H2O2 aqueous solution will form more emulsified W/IL droplets, leading to a decreasing of CIL PW4 since more and more catalysts will be transferred from the IL phase to the interface of the W/ IL droplets. Thus, the decreasing S-removal rate is attributed to the decreasing CIL PW4 since the content of water and DBT in IL are the same at the mole ratio of H2O2/S above 3:1. 3.5. Influence of the Amount of Catalyst on the SRemoval. The influence of the amount of catalyst on the oxidative desulfurization process was studied by varying the mole ratio of sulfur to catalyst, n(S)/n(catalyst), from 200:1 to

Figure 7. Influence of the mole ratio of n(S)/n(catalyst) on the Sremoval rate of DBT oil. Conditions: DBT oil = 10 g, T = 40 °C, mass ratio of m(model oil):m([Bmim]PF 6) = 4:1, mole ratio of n(H2O2):n(S) = 3:1, catalyst = [C5H5NH]3PW12O40.

end and then increases steadily after 100:1. For example, the Sremoval rate at 30 min increases from 41.7% to 76.5% as the ratio of n(S)/n(catalyst) decreases from 200:1 to 100:1; however, further decreasing the ratio of n(S)/n(catalyst) from 100:1 to 50:1 can only slightly enhance the S-removal rate from 76.45% to 79.94%. These results indicate that there exists a threshold for the mole ratio of sulfur to catalyst, after which its influence becomes less important. In this regard, the appropriate ratio of n(S)/n(catalyst) is determined as 100:1 for the present ECODS process, whereby a 99.8% S-removal rate can be reached in 1.5 h. The above results may be ascribed to the increasing CIL PW4 which always increases with the amount of the added catalyst according to eq 1, resulting in an increasing S-removal rate since the content of water and DBT in IL are the same at all n(S)/n(catalyst). 3.6. Influence of the Amount of ILs on the S-Removal. In the present ECODS process, ILs is deemed as a reacting phase wherein a series of reactions take place among the Scomponent, H2O2, and the catalyst, and thus its amount may have a significant influence on the S-removal performance. Figure 8 presents the experimental results of S-removal at varying usage of IL and fixed other conditions as noted in the legend. It is shown that the S-removal rate increases greatly

Figure 8. Influence of the mass ratio of oil/IL on the S-removal of DBT oil with [C5H5NH]3PW12O40 as catalyst. Conditions: DBT oil = 10 g, T = 40 °C, mole ratio of n(H2O2):n(S):n(catalyst) = 300:100:1, IL = [Bmim]PF6. 6662

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with the IL amount as the mass ratio of oil/IL, m(oil)/m(IL), decreasing from 5:1 to 4:1, and then increases mildly with m(oil)/m(IL) from 4:1 to 2:1. For example, the S-removal rate at 30 min increases from 46.3% to 76.5% as m(oil)/m(IL) decreases from 5:1 to 4:1, corresponding to a 25% increase in IL usage; however, further doubling the IL amount from 4:1 to 2:1 can only slightly increase the S-removal performance from 76.5% to 85.5%. Meanwhile, a complete desulfurization can be achieved in 1.5 h for all m(oil)/m(IL) above 4:1, and thus 4:1 was chosen in most cases of the present study since further increasing IL can only slowly enhance the extraction effect with a fixed amount of DBT in oil phase. 3.7. Influence of Reaction Temperature on the SRemoval. The effect of temperature on the S-removal kinetics was studied in the temperature range from 30 to 70 °C, and the results are presented in Figure 9. Obviously, the reaction

The experiment results are presented in Figure 10. As shown from the figure, the S-removal rate first increases slightly with

Figure 10. Influence of the recycle times of IL on the S-removal of DBT oil with [C5H5NH]3PW12O40 as catalyst. Conditions: DBT oil = 10 g, T = 40 °C, reaction time = 1.5 h, mass ratio of m(oil):m([Bmim]PF6) = 4:1, mole ratio of n(H2O2):n(S):n(catalyst) = 200:100:1. ■: Without treatment; □: Simple treatment after the 11th cycle.

the recycle times before the fourth reuse of IL and then declines slowly until about 90% S-removal rate at 10th reuse. The increasing S-removal from the first to fourth reuse may be ascribed to the increasing amount of H2O2 as a result of the accumulation of the left oxidant in the previous process, and the declining S-removal rate from the fourth reuse may be attributed to the increasing accumulative amount of the white precipitate of DBTOx in the system, leading to a higher resistance to the mass transfer process of the ECODS system. The used IL with dissolved catalyst and white precipitate of DBTOx was washed once with chloroform since only DBTOx can be extracted to the chloroform layer although chloroform is partially soluble in the IL phase. The resulting two phases were separated via a separating funnel. The IL phase was evaporated using a rotary evaporator under reduced pressure to eliminate all the volatiles and get the reclaimed IL for reuse. The chloroform phase was air-dried to obtain the white precipitate which was then characterized by IR. As shown in Figure 10, the desulfurization activity of the reclaimed IL and the catalyst can be well recovered through such a simple post-treatment. The IR spectra of the white precipitate and DBT are presented in Figure 11. From the figure, the biggest difference between them is that the IR spectrum of the white precipitate demonstrates the presence of the absorption bands for sulfone at 1288.22

Figure 9. Influence of temperature on the S-removal rate of DBT oil with [C5H5NH]3PW12O40 as catalyst. Conditions: DBT oil = 10 g, mass ratio of m(oil):m([Bmim]PF 6 ) = 4:1, mole ratio of n(H2O2):n(S):n(catalyst) = 300:100:1.

temperature can profoundly enhance the S-removal rate especially in the lower temperature end. For example, the short time S-removal rate at 30 min is increased rapidly from 26.4% at 30 °C to 76.5% at 40 °C and even to 99.7% at 70 °C. On the other hand, a 99% S-removal or even higher can be achieved within 1.5 h in the temperature range of 40 to 70 °C, while a similar degree of desulfurization needs 2.5 h at 30 °C. From the practical point of view, 40 °C can be chosen as an appropriate temperature for the present ECODS process, since a higher temperature can result in higher operation cost and thermal decomposition of H2O2. 3.8. Influence of Recycle Times of Used IL on the SRemoval. Considering that the hydrophobic IL is solely used as a reaction medium, while its chemical nature remains unchanged, it is hoped that the IL can be used repeatedly. To study the repeating usability of the IL in the ECODS process, the following experiments were done at 40 °C. In the experiments, the amount of IL and catalyst was fixed in the whole process, and a specific amount of 30% H2O2 and model oil was added to the flask, reacted for 1.5 h. After the reaction, the treated oil was transferred from the flask, and then fresh oil and 30% H2O2 were refilled into the flask for the next run. It should be pointed out that the theoretical mole ratio of H2O2/S of 2:1 was used here so as to avoid the interference of the increasing excessive H2O2 for the S-removal of the succeeding experiment, and the water amount increased gradually with the recycle times since only the treated oil was carefully taken out using a syringe.

Figure 11. IR spectra of DBT (A) and its oxidation products (B). 6663

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cm−1 and 1166.72 cm−1, and the absorption band for sulfoxide at 1047.16 cm−1,36 implying that DBT was oxidized to DBTO and DBTO2 in the ECODS process. 3.9. Oxidation of Different S-Compounds. In the previous section, it is shown that DBT in n-octane can be effectively removed by ECODS. To investigate the efficiency of the present process for other aromatic S-compounds coexisting with DBT in diesel fuels, the S-removal performance of BT and 3-MT was also tested and compared with DBT under similar conditions with their initial S-concentration all being 1000 ppm. It is obvious from Figure 12 that the reactivity of the S-

with only a slight decrease in activity, and the used IL can be regenerated easily.

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ASSOCIATED CONTENT

* Supporting Information S

The comparison of the reactivity between the present ECODS system and other literatures, and the photographs of formed water-in-IL emulsion system. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86 10 64410308. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the support from the Fundamental Research Foundation of Sinopec (Grant No. X505015).

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REFERENCES

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Figure 12. Comparison of the desulfurization rate of 3-MT, BT, and DBT with [C5H5NH]3PW12O40 as catalyst. Conditions: model oil = 10 g, T = 40 °C, mass ratio of m(oil):m([Bmim]PF6) = 4:1, mole ratio of n(H2O2):n(S):n(catalyst) = 300:100:1.

compounds being oxidized by H2O2 followed the order of DBT > BT > 3-MT, which is consistent with the electron density of the S-atom of different S-compounds, i.e. 5.758, 5.739, and 5.697, respectively, for DBT, BT, and 3-MT.37,38 Therefore, the present ECODS process is of high efficiency for the removal of condensed aromatic S-compounds like BT and DBT, which are the key S-components of diesel oils and the most difficult ones to be removed through the catalytic hydrogenation process. In contrast, the present process is less effective for 3-MT or its homologues due to its intrinsic attribute, i.e. the lower electron density on S-atom and thus lower activity to be oxidized. However, a facile oxidation of 3-MT or its homologues is still expectable if a more efficient catalyst can be found in the future for the ECODS process.

4. CONCLUSIONS The hydrophobicity of the phosphotungstates can be tuned easily by substituting the proton with specific organic cations with their Keggin-type anion structure remaining unchanged. The three catalysts prepared here are only soluble in ILs of [Bmim]PF6 and [Bmim]BF4 and exhibit high catalytic activity for the oxidation of BT and DBT. For the present ECODS process, the hydrophobic IL is superior to the hydrophilic one, besides, the S-removal rate increases monotonically with temperature as well as the amount of IL and catalysts, while an optimal amount of H2O2 exists. The experimental results can be well interpreted by the mechanism proposed here for the ECODS process, where IL is assumed as a reaction phase, and any of the means that is conducive to increase the concentration of catalysts and extracted S-compounds in the IL phase is advantageous for the oxidative S-removal. Besides, the IL with the dissolved catalyst can be reused many times 6664

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