Catalytic Oxidative Desulfurization of Gasoline Using Ionic Liquid

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Catalytic Oxidative Desulfurization of Gasoline Using Ionic Liquid Emulsion System Jianhua Ge,† Yuming Zhou,*,† Yong Yang,† and Mengwei Xue§ †

School of Chemistry and Chemical Engineering, Southeast University, 211189 Jiang Ning Region, Nangjing, P. R. China Biochemical and Environmental Engineering College, Nanjing Xiaozhuang University, Nanjing 211171, P. R. China

§

ABSTRACT: With the aim of deep desulfurization of the gasoline, an amphiphilic catalyst, which is composed of lacunary anion [PW11O39]7‑ and quaternary ammonium cation [C18H37(CH3)3]N+, assembled in hydrophobic ionic liquid emulsions, can oxidize the sulfur compounds present in oil into their corresponding sulfones under ambient reaction conditions. In this process, catalytic oxidation of sulfur-containing molecules in model oil was investigated in detail under different reaction conditions (including different desulfurization systems, H2O2/DBT molar ratio, temperature, and various sulfur compounds). Moreover; this ionic liquid emulsion system could be recycled five times with an unnoticeable decrease in catalytic activity, and, from the kinetics study, it can be shown that the catalytic oxidative reaction is a pseudofirst-order reaction and the half-life is 30.4 min. Furthermore, the mechanism of catalytic oxidation desulfurization was elaborated, and the total sulfur level of real gasoline can be decreased from 1236 to 65 ppm after catalytic oxidation using an ionic liquid emulsion system.

1. INTRODUCTION Sulfur-containing compounds in transportation fuels are converted by combustion to SOx, which is a major source of acid rain and air pollution.1 For environmental protection purposes,2 many countries have mandated a reduction in fuels sulfur level down to 10 ppm by 2009,3 and with more and more stringent regulatory constraints, it is a trend to achieve little-to-no sulfur fuels in the next several years.4 At present, the removal of sulfurcontaining compounds is carried out industrially by a catalytic hydrodesulfurization (HDS) method.5 It requires severe conditions including high temperature and high hydrogen pressure to produce light oil having low levels of sulfur compounds.5 The efficiency of HDS is limited, however, to treat dibenzothiophene (DBT) and its derivatives, owing to their stereo hindrance on the sulfur atom.6 Hence, several new technologies such as extraction desulfurization,7
11 selective adsorption desulfurization,12
16 catalytic oxidation desulfurization,4,6,17,18,45 and biodesulfurization19,20 were proposed. Oxidation combined with extraction is considered as one of the most promising desulfurization processes,19 since they display superior desulfurization properties. Recently, a desulfurization system has been reported by the Li group that amphiphilic catalysts composed of polyoxometalate anions and quaternary ammonium cations, assembled in emulsions in diesel, could selectively oxidize DBT and its derivatives into their corresponding sulfones using H2O2 as an oxidant. Then, the sulfones are removed by volatile organic compounds (VOCs) in the following process,21
25 which raises further environmental and safety concerns. Compared with VOCs, ionic liquids (ILs) are regarded as “green solvents” because of their low melting point, wide liquid range, negligible vapor pressure, and good solubility characteristics etc.26 Currently, Li and co-workers have studied a combination of extraction with ionic liquid and catalytic oxidative desulfurization system (ECODS),27
31 using the phosphotungstic acid,1 peroxotungsten complex and polymolybdates as the r 2011 American Chemical Society

catalysts,32 and conventional ILs as the extractant in the presence of H2O2. In this process, the sulfur-containing compounds in the light oils were first extracted into ILs and oxidized to their corresponding sulfones simultaneously by catalysts.2 Especially, the water-immiscible ionic liquid [Bmim]PF6 was found to be a more-effective,5,29 the sulfur compounds could be almost completely removed from oil. However, owing to reactions with H2O2 always involved two phases, which more or less inhibited the reactivity of catalytic oxidative desulfurization. Polyoxometalates are very active and selective oxidation catalysts. For example, keggin-type heteropolyanions have been used by several groups for mild and selective oxidations with H2O2.33
37 However, there seems be to have been few studies on lacunary anion [PW11O39]7‑ with long alkyl chain is used for deep desulfurization of gasoline. Based on the above summarizations, we develop a new catalytic oxidation ionic liquid emulsion desulfurization system, which composed of water-immiscible ionic liquid ([Bmim]PF6), 30 wt % H2O2, and an amphiphilic catalyst ([C18H37N(CH3)3]7[PW11O39]). This kind of catalyst not only maintains the emulsion droplets stable but also provides higher interfacial surface area where the oxidation of organic sulfur molecules takes place.24 During the reaction, the ionic liquid emulsion functions as highly dispersed microreactors. The sulfur-containing compounds in the model oil were first extracted into the ionic liquid emulsion phase and then oxidized to their corresponding sulfones by [C18H37N(CH3)3]7[PW11O39] in the H2O2/[Bmim]PF6 interface. The sulfones accumulated in the [Bmim]PF6 phase. After reaction, the desulfurization system quickly divided into two layers; the deep desulfurization can be achieved in this way. Received: June 21, 2011 Accepted: October 28, 2011 Revised: October 14, 2011 Published: October 28, 2011 13686

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Figure 1. FT-IR spectra of various samples (A: K7PW11O39; B: [(C18H37)N(CH3)3]7[PW11O39]).

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Figure 2. UV
vis spectrum of [[(C18H37)N(CH3)3]7[PW11O39].

2. EXPERIMENTAL SECTION 2.1. Preparation of Ionic Liquids and Model Oil. The ionic liquid [Bmim]PF6 was prepared as mentioned in the literature.38 DBT (1.4655 g, 7.7944 mmol, 98%) was dissolved in a solvent of n-octane (250 mL). The sulfur content of model oil containing DBT was 1000 mg/L. With the same method, the sulfur content of model oil containing BT and 4,6-DMDBT was 1000 mg/L, respectively. 2.2. Preparation of Catalyst [C18H37N(CH3)3]7[PW11O39]. The K7PW11O39 was synthesized as mentioned in the literature procedure.39Then 5.184 g of K7PW11O39 obtained was dissolved in 40 mL of deionized water under stirring at 60 °C, followed by the addition of 5.8875 g of octadecyltrimethyl ammonium bromide in alcohol to the colorless solution. A white precipitate was immediately formed. After continuously stirring for 1 h, the white precipitate was filtered, washed with water and ethanol, and dried in vacuum for 12 h. Calcd for C147H322N7PW11O39: H, 6.67; C, 36.29; N, 2.02; P, 0.64; W, 41.56; Found: H, 6.58; C, 36.32; N, 1.97; P, 0.65; W, 41.33. 2.3. Instrumentation. Infrared (FT-IR) spectra of all the catalysts (KBr pellets) were recorded on Tensor 27 (BrukerT27) equipment, while ultraviolet
visible light (UV
vis) spectra were obtained using a Model Shimadzu UV-2201 spectrophotometer (Shimadzu Corporation, Japan). Elementary (C, H, and N) analyses were performed using Elementar Vario Micro (Germany); X-ray fluorescence (XRF) was performed with a Panalytical Magix spectrometer. Solid-state 31P MAS NMR spectra were collected in a Bruker DSX-400 spectrometer. 2.4. Catalytic Oxidation of Model Sulfur-Containing Molecules. In a typical experiment, the required amounts of ionic liquid, catalyst, 30 wt % H2O2, and model oil were added to the flask in turns and stirred vigorously in oil bath at reaction temperature. After the reaction, the model oil was withdrawn and analyzed by gas chromatography with tetradecane as internal standard, coupled with a flame ionization detector (GC-FID). AC5 capillary column (30 m  0.53 mm inner diameter  1.0 μm film thickness) was used for separation.

3. RESULTS AND DISCUSSION 3.1. Characterization of the [(C18H37)N(CH3)3]7[PW11O39]. As shown in Figure 1, the FT-IR spectra of K7PW11O39 (Figure 1A) exhibited characteristic peaks at 1085
1034, 951, and 900
737 cm
1 which were attributed to v(P
O), v(W = O),

Figure 3. TG curves of [[(C18H37)N(CH3)3]7[PW11O39].

and v(W
O
W), respectively. It was very similar to that reported by Beer and Radkov.40 There was no obvious difference between the FT-IR spectra of the [(C18H37)N(CH3)3]7[PW11O39] and K7PW11O39. The UV
vis spectrum of the [[(C18H37)N(CH3)3]7[PW11O39] was shown in Figure 2. It was measured to have two absorption (in acetonitrile) bands at 215 and 258 nm, which was similar to the structure of keggin-type polyoxometalates.The absorption band at 215 nm was due to OfP transition, and the absorption band at 258 nm was mainly regarded as charge transfer absorption (O2‑fW6+) where W atoms were located in W
Oe
W intrabridges between edge-sharing WO6 octahedra in the keggin units.41 The TG curve of [[(C18H37)N(CH3)3]7[PW11O39] (Figure 3) under nitrogen showed that the catalyst had not coordinated water and crystalline water, because there was no mass loss under 100 °C. After 200 °C there were two mass losses on the TG curve, which may be contributed to the decomposition of organic cations of catalysts and [PW11O39]7‑. Until about 600 °C, there was no more mass loss. The final remainders were phosphor oxide and tungsten oxide. The solid-state 31P MAS NMR spectra of catalysts were shown in Figure 4, the chemical shift of K7PW11O39 appeared at
10.5 ppm, and a similar spectrum of [[(C18H37)N(CH3)3]7[PW11O39] was obtained; therefore the structure of [[(C18H37)N(CH3)3]7[PW11O39] is similar to that of K7PW11O39, which has a lacunar keggin-type structure.22 13687

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Figure 4. Solid-state 31P MAS NMR spectra of catalysts (A: K7PW11O39; B: [[(C18H37)N(CH3)3]7[PW11O39]).

Figure 5. Effect of the H2O2/DBT molar ratio (O/S) on the reaction. Conditions: IL = [Bmim]PF6 = 1 mL, model oil = 5 mL, n(DBT)/ n([(C18H37)N(CH3)3]7[PW11O39]) = 150:1, T = 30 °C.

Table 1. Influence of Different Desulfurization Systemsa entry

desulfurization system

sulfur removal (%)

1

[Bmim]PF6

14.8

2

[Bmim]PF6+H2O2

30.7

3

[(C18H37)N(CH3)3]7[PW11O39]+ H2O2

97.6

4

[Bmim]PF6+[(C18H37)N(CH3)3]7[PW11O39]

99.1

+ H2O2 a Conditions: IL = [Bmim]PF6 = 1 mL, model oil = 5 mL, T = 30 °C, n(H2O2)/n(DBT)/n([(C18H37)N(CH3)3]7[PW11O39]) = 600:150:1, t = 1 h.

3.2. Influence of Different Desulfurization Systems on Removal of DBT. Table 1 shows four kinds of different desulfur-

ization systems, such as the following: extraction, extraction combined with chemical oxidation, catalytic oxidation without ionic liquid extraction, and extraction coupled with catalytic oxidation in the conditions of hydrophobic ionic liquid. In an extraction system, when using ionic liquid [Bmim]PF6 solely as the extractant for removing DBT-containing in model oil, the sulfur removal only reached 14.8%, with addition of H2O2 in [Bmim]PF6 the sulfur removal increased to 30.7%, on the other hand, when catalyst, H2O2, and hydrophobic ionic liquid were employed together, the removal of DBT increased sharply (99.1%). However, in this experiment, the catalyst with long alkyl can selectively oxidize DBT into their corresponding sulfones without hydrophobic ionic liquid, then the sulfones could be removed by VOCs as an extractant in the following process. Based on the results gained herein, the catalytic oxidation desulfurization system, which composed of [Bmim]PF6, 30 wt % H2O2, and [C18H37N(CH3)3]7[PW11O39], was superior to the simple extraction with IL or the oxidation/extraction without catalyst. 3.3. Effect of the H2O2/DBT Molar Ratio (O/S) on the Reaction. To study the effect of the amount of oxidizing agent on the oxidative properties, reactions under different O/S molar ratios were carried out at 30 °C. According to the stoichiometric reaction, 2 mol of H2O2 are consumed for 1 mol of sulfurcontaining compounds to their corresponding sulfones. From the data in Figure 5, the molar ratio of H2O2 and DBT has a significant affect on the reaction rate. The sulfur removal increased from 94.6% at O/S = 2 to 99.1% at O/S = 3 in 3 h. Further increasing the O/S to 4, DBT can be completely

Figure 6. Effect of different reaction temperature on the reaction. Conditions: IL = [Bmim]PF6 = 1 mL, model oil = 5 mL, n(H2O2)/ n(DBT)/n([(C18H37)N(CH3)3]7[PW11O39]) = 600:150:1, T = 30 °C.

removed in 1.5 h. However, there was no evident change of sulfur removal to further increase the amount of H2O2. Compared with other reported literatures,42,43 the utilization rate of hydrogen peroxide has been increased sharply and decreased the economic costs. 3.4. Effect of Different Reaction Temperature on the Reaction. Figure 6 displayed the sulfur removal of DBT in n-octane vs reaction time under various reaction temperatures. The sulfur removal of DBT increased with increasing temperature, within the temperature increase from 20 to 30 °C, the desulfurization efficiency was visibly advanced after reaction for 0.5 h. When the temperature was 30 °C, the removal of DBT can reach 97.6% in 0.5 h and can be almost completely removed for 1.5 h, whereas at 20 °C, the desulfurization efficiency appeared to be unsatisfying; this pheomenon may be because the oxidant H2O2 and the catalyst cannot work efficiently under lower reaction temperature (30 °C, the ionic liquid emulsion system was destroyed, so all the reactions were carried out at room temperature in this work. Based on the above results, DBT could be completely removed under ambient reaction temperature in a short reaction time. 13688

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Figure 7. Influence of the nature of the substrate in the desulfurization. Conditions: IL = [Bmim]PF6 = 1 mL, model oil = 5 mL, n(H2O2)/n(substrates)/n([(C18H37)N(CH3)3]7[PW11O39]) = 300:150:1, T =30 °C.

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Figure 9. Removal of DBT and ln(Ct/C0) as functions of reaction time. Conditions: IL = [Bmim]PF6 = 1 mL, model oil = 5 mL, n(H2O2)/ n(DBT)/n([(C18H37)N(CH3)3]7[PW11O39]) = 300:150:1, T = 30 °C.

system could be recycled five times with an unnoticeable decrease in catalytic activity. 3.7. Kinetics. Assuming that the volume and mass of the reaction mixtures were constant, only small amounts of liquid samples were withdrawn during the reaction, and experiments were performed under the optimal conditions. The rate constant (k) and reaction time (t) can be described using the following equation


dCt ¼ kCt dt

ð1Þ

C0 ¼ kt Ct

ð2Þ

ln

t1=2 ¼ Figure 8. Influence of the recycle times on sulfur removal. Conditions: IL = [Bmim]PF6 = 1 mL, model oil = 5 mL, t = 1 h, n(H2O2)/n(DBT)/ n([(C18H37)N(CH3)3]7[PW11O39]) = 600:150:1, T = 30 °C.

3.5. Effect of the Nature of the Substrates. To study the effect of the ionic liquid emulsion system on the various sulfur compounds, DBT, BT, and 4,6-DMDBT were chosen as the sulfur compounds to compare their oxidation reactivities under the same reaction conditions. As shown in Figure 7, BT exhibited much lower reactivities than that of DBT. Compared with DBT, the electron density on the sulfur atom on BT is lower, leading to the lower reactivity.29 At the same time, steric hindrance of the methyl of the 4,6-DMDBT was also an obstacle for the approach of the sulfur atom to the catalytic active species in an ionic liquid emulsion system. Owing to these two factors, the reactivity of sulfur compounds in the ionic liquid emulsion desulfurization system decreased on the order of DBT > 4,6-DMDBT > BT. 3.6. Regeneration of Used Ionic Liquids. To account for the advantage of the desulfurization system better, recycling the system was an important factor as well and was necessary to investigate. After the reaction, the reaction system was still a biphasic system, so the oil could be separated by simple decantation from the biphasic system of the ionic liquid easily. Through the 31P NMR and 1H NMR analysis of the used ionic liquids, whose structure does not change and the recovered IL can be reused for further emulsion catalytic oxidation cycles, the data in Figure 8 indicate that the [Bmim]PF6 emulsion desulfurization

0:693 k

ð3Þ

where C0 and Ct were the sulfur concentrations at time zero and time t (min) As shown in Figure 9, the experiment showed DBT removal increased rapidly in the first 60 min, to exceed 85%. A plot of ln(Ct/C0) versus reaction time displayed a linear relationship that confirmed the pseudofirst-order reaction kinetics. The rate constant for DBT catalytic oxidation reaction was determined to be 0.02279 min
1 and half-life was 30.4 min. 3.8. Possible Process and Mechanism of Desulfurization System. To clarify the processes of the ionic liquid emulsion systems, different photos are listed in Figure 10. The milky-white ionic liquid emulsions (Figure 10 b) are readily formed when H2O2 and [C18H37N(CH3)3]7[PW11O39] were added into [Bmim]PF6 (Figure 10 a), which was confirmed by optical microscopy (Figure 10 e). In this emulsion reaction system, the catalyst molecule acts as an emulsifying agent, could be uniformly distributed in the interface of H2O2
ionic liquid, and forms a film around the dispersed ionic liquid droplets (Scheme 1). Consequently, the lipophilic quaternary ammonium cations of the amphiphilic catalyst would lie on the oil side and the hydrophilic heteropolyanions would lie on the H2O2 side (Scheme 1). It has been well documented in the previous work that the heteropolyanions ([PW11O39]7‑) depolymerized into several smaller active species including [(PO4){WO(O2)2}4]3‑(PW4), [(PO4){WO(O2)2}2{WO(O2)2
(H2O)}]3‑(PW3), and [(PO3(OH)){WO(O2)2}2]2‑(PW2) in the presence of H2O2,39,44 because the 13689

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Figure 11. IR spectrum of the reclaimed DBTO2.

Figure 10. The photographs of formed ionic liquid emulsion and (a) [Bmim]PF6, (b) the photographs of formed ionic liquid emulsion (c) before oxidation, upper phase: model oil containing sulfur compounds, lower phase: ionic liquid emulsion and (d) after oxidation upper phase: free sulfur model oil, lower phase: ionic liquid emulsion containing sulfones, (e) optical micrograph of the formed ionic liquid emulsion, and (f) optical micrograph of the sulfones.

Scheme 1. Catalytic Oxidation of DBT in Ionic Liquid Emulsion System

system is effective for real gasoline, the catalytic oxidation desulfurization of commercial diesel, which was obtained from the Nanjing alkyl benzene plant, was carried out. The sulfur-containing compounds in the gasoline could be oxidized with 0.5 mL of H2O2, 1 mL of [Bmim]PF6, and 0.0758 g of [(C18H37)N(CH3)3]7[PW11O39] at 30 °C for 3 h. The upper phase was withdrawn and analyzed by X-ray fluorescence. The total sulfur level of gas oil was decreased from 1236 to 65 ppm. This suggested that ionic liquid emulsions had high catalytic activity for all kinds of sulfur-containing compounds present in real gasoline.

’ CONCLUSIONS A deep desulfurization process for gasoline, based on catalytic oxidation using the amphiphilic catalyst [C18H37N(CH3)3]7[PW11O39] assembled in ionic liquid emulsion, was developed in this work, and the main findings can be summarized as follows: 1 [C18H37N(CH3)3]7[PW11O39], which behaves as both an emulsifying agent and a catalyst instead of only a surfactant. 2 The catalyst is distributed in the interface of H2O2 and water-immiscible ionic liquids and formed milky-white ionic liquid emulsion droplets, which exhibits very high intrinsic catalytic activity in the process of catalytic oxidation of sulfur-containing model oil and real gasoline. 3 After reaction, the desulfurization system quickly divided into two layers. In this way, the catalyst in the emulsion droplets can be readily separated from the oil and recycled, and this ionic liquid emulsion system could be recycled five times without an obvious decrease in activity. ’ AUTHOR INFORMATION

ionic liquid emulsion was immiscible with n-octane and formed a biphasic system (Figure 10 d). The sulfur-containing compounds in the model oil were first extracted from oil phase into IL phase and oxidized to their corresponding sulfones by the active species simultaneously; the sulfones accumulated in IL phase. After reaction, the desulfurization system quickly divided into two layers (Figure 10d), and the sulfur-free oil is obtained; after processing 5 times, the white crystal solid (Figure 10 f) began to crystallize in the IL phase and was characterized by FT-IR. The IR spectrum demonstrated two absorption bands at 1289 cm
1 and 1130 cm
1, which were determined to be sulfone groups (Figure 11), and the sulfur-free oil is obtained, and, in this way, deep desulfurization of fuels can be achieved. 3.9. Catalytic Oxidative Desulfurization of Actual Gas Oil. To investigate whether the catalytic oxidative desulfurization

Corresponding Author

*Phone: +8625-52090617. Fax: +8625-52090618. E-mail: ymzhou@ seu.edu.cn.

’ ACKNOWLEDGMENT This work was supported by the National Nature Science Foundation of China (Nos. 51077013, 50873026), production and research prospective joint research project of Jiangsu Province of China (BY2009153), the Key Program for the Scientific Research Guiding Found of Basic Scientific Research Operation Expenditure, Southeast University (3207040103), 333 high-level talent training project, Jiangsu Province of China (BRA2010033), and Student Research Training Program of Southeast University (No. 091028644). 13690

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