Article pubs.acs.org/IECR
Deep Desulfurization of 4,6-Dimethyldienzothiophene by an Ionic Liquids Extraction Coupled with Catalytic Oxidation with a Molybdic Compound Bo-bo Shao, Li Shi, and Xuan Meng* The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: A series of the imidazolium-based phosphoric ionic liquids, N-methyl-N-methylimidazolium dimethyl phosphate ([Mmim]DMP), N-ethyl-N-methylimidazolium diethyl phosphate ([Emim]DEP), and N-butyl-N-methylimidazolium dibutyl phosphate ([Bmim]DBP), were synthesized and employed in the extraction and catalytic oxidation desulfurization system (ECODS) for the removal of 4,6-dimethyldienzothiophene (4,6-DMDBT) from a model oil, with hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) as catalyst and 30 wt % hydrogen peroxide (H2O2) solution as oxidant. The effects of the type of ionic liquid, amount of catalyst and H2O2, ILs/oil mass ratio, reaction time, reaction temperature, and regeneration of ionic liquid on the 4,6-DMDBT removal of a model oil were investigated in detail. The results indicate that a sulfur system containing H2O2, (NH4)6Mo7O24·4H2O, and [Bmim]DBP, exhibited such a high catalytic activity that the removal of 4,6-DMDBT could reach 89.2% at 50 °C in 3 h, which was markedly superior to mere solvent extraction with IL (9.29%) or catalytic oxidation without IL (5.34%). The desulfurization system for a model oil could be recycled six times with an unnoticeable decrease in activity. Meanwhile, (NH4)6Mo7O24·4H2O not only possessed good catalytic activity but hardly contaminated the oil. This work shows that ECODS containing (NH4)6Mo7O24 ·4H2O, [Bmim]DBP, and H2O2 may be a new option for producing cleaning fuel.
1. INTRODUCTION Ultradeep desulfurization of fuel oil has attracted wide interest because of more stringent legislation and an increasing need for environmental protection. Sulfur is converted to SOx during combustion, which not only pollutes air greatly, but also irreversibly poisons the noble metal catalysts used in the chemical industries. Because of these reasons the removal of sulfur has become a world-wide subject. Since 2009, the sulfur content of gasoline in Europe must be less than 10 ppm.1 In the conventional desulfurization process, hydrodesulfurization (HDS) is used to remove thiols sulfides and disulfides. This process, however, does not easily remove sulfur compounds such as dibenzothiophene (DBT) and its derivatives, especially 4,6-dimethyldienzothiophene (4,6DMDBT). To remove these compounds, severe operating conditions (T > 350 °C, 30−100 bar),2 and especially highly active catalysts3,4 are required. These operating conditions result in large hydrogen consumption and a significant increase in the operating expenses. Consequently, many alternative desulfurization technologies have been extensively studied, for example, adsorption,5−7 oxidation,8,9 and extraction,10,11 bioprocesses. Among all these processes, oxidative desulfurization (ODS) combined with extraction is considered to be a promising process. Organic sulfides, such as DBT and its derivatives, can be oxidized to their corresponding sulfoxides and sulfones easily, which are then removed by organic solvents. Room-temperature ionic liquids (ILs), as being environmental-friendly solvents for organic chemical reaction media separation,12,13 and for electrochemical applications, have attracted extensive research interest because of their particular © 2014 American Chemical Society
properties, such as high thermal stability, undetectable vapor pressure, excellent solubility, and tunable acidity or basicity by varying the alkyl substituents and the anions.14,15 Owing to such advantageous properties, ionic liquids as extractant have been used to remove sulfur compounds from diesel fuels. Many types of ILs have been reported, such as [Bmim]BF4, [Bmim]PF6, [Mmim]DMP, [Bmim]DBP, [Emim]DEP; however, the efficiency of sulfur removal is very low, only in the range of 15%−40%. Recently, Zhao et al.16 studied Bronsted acid ionic liquid N-methyl-pyrrolidonium tetrafluoroborate ([Hnmp]BF4) as extractant and catalyst for the ODS of a model oil in the presence of H2O2. The results show that the oxidative/extraction is far superior to simple extraction with ionic liquid; the removal of DBT-containing model oil sulfur is more than 90%. So far, most of the published papers were focus on the removal of sulfur compounds by extraction or oxidation−extraction, and little attention has been paid to the catalytic oxidation desulfurization. In this study, we employed the extraction and catalytic oxidation desulfurization system (ECODS) for evaluating the removal of 4,6-dimethyldienzothiophene (4,6-DMDBT) from the model oil. This desulfurization system is composed of (NH4)6Mo7O24·4H2O (used as catalyst), 30 wt % hydrogen peroxide (H2O2) solution (used as oxidant), and imidazoliumbased phosphoric ionic liquids ([Mmim]DMP, [Emim]DEP, and [Bmim]DBP). Three different ILs were immiscible with Received: Revised: Accepted: Published: 6655
January 19, 2014 March 24, 2014 March 31, 2014 March 31, 2014 dx.doi.org/10.1021/ie500236b | Ind. Eng. Chem. Res. 2014, 53, 6655−6663
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Scheme 1. Synthesis of the Ionic Liquids
electrospray ionization mass spectra (ESI-MS). The FT-IR was carried out on a Nicolet 6700 FT-IR spectrometer in the range of 400−4000 cm−1. 1H NMR and 13C NMR spectra were obtained in d6-DMSO on the AVANCE III 400 nuclear magnetic resonance spectrometer, with tetramethyl silane (TMS) as the internal standard. Chemical shifts were reported in parts per million (ppm, δ). ESI-MS spectra were recorded on a Shimadzu LC-MS 2010A instrument. ESI-MS spectra were recorded on a Shimadzu LC-MS 2010A instrument. The thermal stability of ionic liquids was characterized by thermogravimetric analysis (TGA). TGA curves were obtained using a TA Instruments thermal analyzer. The samples were exposed to an increase in temperature of 10 °C/min up to 500 °C while the nitrogen flow rate was held constant at 100 mL/ min. 2.4. Viscosity Measurements. The measurements of ionic liquid viscosity were carried out with calibrated Pinkevitch glass capillary kinematic viscometers purchased from Liang jing glass instrument factory of Shanghai. The viscometer was mounted on a vertical platform submerged in a well-stirred oil bath set at a certain temperature. The passage of the liquid meniscus past two arrow marks above the capillary was timed with a stopwatch. The kinematic viscosity was calculated using ASTM methods D 445-97. 2.5. Extractive Desulfurization Procedure and Analysis. The extraction and catalytic oxidation desulfurization experiments were carried out in a 50 mL Erlenmeyer flask. The model oil (5 g) and a certain amount of ionic liquids, H2O2, catalyst were added to flask. The mixture was vigorously stirred for a period of time at a specified temperature for extraction. Then the mixture was put aside for 2 h for phase splitting and settling at room temperature and the S-content in the upper oil phase was measured. The sulfur concentration of oil separated from the ionic liquid was analyzed by Antek 9000s total sulfur analyzer. The sulfur removal (Y) was obtained by the following equation:
the model oil, which not only served as extractants and reaction media, but also enhanced the catalytic performance. 4,6DMDBT was extracted from the model oil and oxidized in the ionic liquid. The synthesized ionic liquids were verified by Fourier transform infrared spectrometry (FT-IR), nuclear magnetic resonance (NMR), and thermogravimetry (TG). Furthermore, to optimize the reaction conditions, the influences of various extraction parameters, such as the amount of catalyst and H2O2, type of ionic liquid, reaction temperature, reaction time, and the reusability of the ionic liquid, were extensively investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. N-Methylimidazole (AR) and (NH4)6Mo7O24.4H2O (AR) were purchased from the Sinopharm Chemical Reagent Co., Ltd. Trimethyl phosphate (CP), triethyl phosphate (CP), tributyl phosphate (AR), n-octane (CP), H2O2 aqueous solution (30 wt %) and ethyl ether absolute (AR) were purchased from Shanghai Ling Feng Chemical Reagent Company. 4,6-Dimethyldibenzothiophene (AR) was purchased from Aldrich. The model oil, which was used as the experimental raw material, was prepared by dissolving 4,6-DMDBT in n-octane; 4,6-DMDBT (0.4655 g) was dissolved in 1000 mL of n-octane to form the model oil, the sulfur concentration in the model oil is 100 ppm (ppm refers to weight ratio). 2.2. Preparation of Ionic Liquids. The imidazolium-based phosphoric ionic liquids were prepared according to previous reports,17−19 and the procedures are shown in Scheme 1. Imidazolium-based phosphoric ionic liquid was synthesized by a reaction of N-methylimidazole (0.15 mol) and the corresponding trialkyphosphate (0.15 mol). First, the mixture were added into a 250 mL round-bottom flask fitted with a reflux condenser and equipped with a magnetic agitator at 90 °C. Then the mixture was slowly heated up to 150 °C and was stirred for 10 h. The resulting viscous liquid was cooled to room temperature and then washed three times with ethyl ether absolute. Finally, ionic liquids were obtained by rotary evaporation under reduced pressure. 2.3. Characterization of the Ionic Liquids. The structures of the imidazolium-based phosphoric ionic liquids were analyzed by Fourier transform infrared spectrometry (FTIR), nuclear magnetic resonance spectrometer (NMR), and
Y (%) =
(C 0 − C ) 100 C0
(1)
where C0 is the initial number of 4,6-DMDBT content of the model oil (g/g), C is the final number of 4,6-DMDBT content of the model oil (g/g). 6656
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Figure 1. The FT-IR spectra of the synthesized ionic liquids: (a) [Mmim]DMP; (b) [Emim]DEP; (c) [Bmim]DBP; (d) [Bmim]DBP after recovery.
3. RESULTS AND DISCUSSION 3.1. Characterization of the Ionic Liquid. 3.1.1. FT-IR Analyses of the Ionic Liquids. FT-IR spectra of the imidazolium-based phosphoric ionic liquids used in the experiments are shown in Figure 1. The presence of a band near 3403 cm−1 was an indication that a little water existed in the synthesized ionic liquids. The bands around 3153, 3099, 1667, and 1573 cm−1 were assigned to the NH, CH, C N, and CC ring stretching vibrations of imidazole, respectively. For the ionic liquid [Mmim]DMP, the stretching and bending vibrations of the −OCH3 group were observed at 2846 and 1468 cm−1, respectively. For [Emim]DEP and [Bmim]DBP, the C−H stretching vibration of CH3 was observed around 2970 cm−1, and the band near 1460 cm−1 was caused by bending vibration of CH2. The band around 1235 cm−1 was assigned to the stretching vibration of PO, and the band near 1050 cm−1 was ascribe to the stretching vibration of P−O. Therefore, the FT-IR data were consistent with the structures of the imidazolium-based phosphoric ionic liquids shown in Scheme 1. A comparison of Figure 1 panels c and d reveals that IL after the recovery showed a similar pattern to that of the fresh IL. In other words, the properties of IL did not change. 3.1.2. 1H NMR and 13C NMR Analyses of the Ionic Liquids. The synthesized ionic liquids were also analyzed by 1H NMR and 13C NMR spectroscopy. The 1H NMR and 13C NMR spectral data are as follows. [Mmim]DMP: 1H NMR (400 MHz, DMSO): δ = 9.331 (s, 1H), 7.743 (s, 1H), 3.857 (s, 1H), 3.272 (s, 1H), 3.246 (s, 1H). 13 C NMR (100 MHz, DMSO): δ = 137.934, 123.900, 51.724, 51.665, 36.039. [Emim]DEP: 1H NMR (400 MHz, DMSO): δ = 9.486 (s, 1H), 7.838 (s, 1H), 7.748 (s, 1H), 4.201 (q, 2H, J = 7.2 Hz), 3.866 (s, 3H), 3.613 (m, 4H), 1.414 (t, 3H, J = 7.2 Hz), 1.065 (t, 6H, J = 6.8 Hz). 13C NMR (100 MHz, DMSO): δ =
137.258, 124.014, 122.438, 59.525, 59.468, 44.496, 36.073, 17.213, 17.144, 15.643. [Bmim]DBP: 1H NMR (400 MHz, DMSO): δ = 9.478 (s, 1H), 7.825 (s, 1H), 7.755 (s, 1H), 4.182 (t, 2H, J = 7.2 Hz), 3.867 (s, 1H), 3.568 (m, 4H), 1.767 (m, 2H), 1.428 (m, 4H), 1.266 (m, 6H), 0.877 (m, 9H). 13C NMR (100 MHz, DMSO): δ = 137.921, 123.992, 122.734, 63.726, 63.667, 48.753, 35.929, 33.166, 33.095, 31.944, 19.205, 19.112, 18.585, 14.070, 13.741, 13.640. The 1H NMR and 13C NMR spectral data of the ionic liquids agreed with their designed structures (Scheme 1). 3.1.3. ESI-MS Analyses of the Ionic Liquids. Figure 2 shows the electrospray ionization mass spectra of the imidazoliumbased phosphoric ionic liquids. (The left spectra are the positive ion spectra and the right spectra are the negative ion spectra). As can be seen in Figure 2, in the negative ion spectra the peaks at m/z 125.0, 153.1, and 209.1 corresponded to the negative ions of [Mmim]DMP, [Emim]DEP, and [Bmim]DBP, respectively. In the positive ion spectra, for the [Mmim]DMP, the peak at m/z 319.1 was the complex of the positive ion and molecular of the [Mmim]DMP. For the [Emim]DEP and [Bmim]DBP, the intense peaks at m/z 111.1 and 139.2 were assigned to the positive ions produced from [Emim]DEP and [Bmim]DBP, respectively. The peaks at m/z 375.2 and 487.4 corresponded to the complex of the positive ions and molecules of the [Emim]DEP and [Bmim]DBP, respectively. 3.1.3. TG Analyses of the Ionic Liquid. The thermal stability of the [Bmim]DBP and [Emim]DEP were investigated by differential scanning calorimetry−thermogravimetric analysis (DSC−TGA). As shown in Figure 3, the TG curves of the [Bmim]DBP ionic liquid show a total weight loss of 10% from room temperature to 250 °C due to the removal of water and organic solvent. According to the DTG curves of ionic liquid, the largest peak, which was centered between 250 and 320 °C, was caused by the decomposition of [Bmim]DBP ionic liquid. For the [Emim]DEP ionic liquid, a similar change in regularity 6657
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Figure 2. Electrospray ionization mass spectra (ESI-MS) of three ionic liquids.
was obtained, and the thermal decomposition temperature was 243 °C. The results revealed that the ionic liquids possessed high thermal stability and wide liquid range. 3.2. TG Analyses of the Catalyst. The stability of the catalysts was investigated by differential scanning calorimetry−
thermogravimetric analysis (DSC−TGA). It was shown in Figure 4 that crystalline water in ammonium molybdate was lost in the first step from 123 to 152 °C. Then, the ammonium ion of molybdate decomposed in two steps at 230 and 330 °C, respectively. Li20 has reported that the crystal water is 6658
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Figure 3. DSC−TGA curves in nitrogen for the [Emim]DEP and [Bmim]DBP.
Figure 4. DSC−TGA curves in nitrogen for the catalyst.
completely removed up to 300 °C and the MoO3 phase was derived before the complete decomposition of ammonium in ammonium molybdate. With a further increase in the temperature, the final remainder was MoO3. The results revealed that the catalyst possessed high thermal stability at the present temperature. 3.3. Viscosities of Ionic Liquids. Ionic liquids are much more viscous compared to traditional organic solvents, and the viscosity is at a range of 10−1000 cP at room temperature.21 The viscosities of the most ionic liquids have influence on important parameters including rates of mixing and separation of the liquid−liquid phase and mass transfer, thus, viscosity is an important physical property. The kinematic viscosities of the imidazolium-based phosphoric ionic liquids were measured at temperatures between 25 and 65 °C, and these values are displayed in Figure 5. With an increase of the alkyl chain of the ILs, the viscosity of the ionic liquid increased, which is shown by comparing [Bmim]DBP with [Emim]DEP and [Mmim][DMP]. This occurs mainly because strong intermolecular
forces between the ionic liquids, for example, van der Waals interactions and hydrogen bonding,22,23 influence the viscosities of ionic liquids. Reducing van der Waals interactions by decreasing the alkyl chain can slightly lower the viscosity.24 Furthermore, the viscosity of the ionic liquid decreases with increasing temperature, and this was in agreement with the Arrhenius equation.25 3.4. Effect of Different ILs on the Removal of 4,6DMDBT. ILs were used in three different desulfurization systems of extraction, extraction coupled with oxidation, and extraction combined with catalytic oxidation. The effect of different desulfurization systems on the desulfurization performance was investigated by [Mmim]DMP, [Emim]DEP, and [Bmim]DBP. The results were shown in Table 1. Three ILs were immiscible with the model oil, and the catalyst could dissolve in these ILs, but hardly dissolved in the model oil. Consequently, the ILs, H2O2, oil, and (NH4)6Mo7O40·4H2O formed biphasic systems, in which the oil phase was the upper layer, and ILs along with catalyst and oxidizing agent were the 6659
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DMDBT is extracted from the oil phase into the ionic liquid. The second step is the oxidation of 4,6-DMDBT. The extractive equilibrium of ionic liquid with a longer alkyl group can be achieved rapidly, and then 4,6-DMDBT was oxidized by H 2 O 2 . The decrease of the 4,6-DMDBT concentration in ionic liquid promotes the extraction process, and the sulfur content in the oil phase decreases continuously. Therefore, [Bmim]DBP ionic liquid was chosen as a reaction media in the subsequent experiments. 3.5. Effect of Amount of Catalyst. The desulfurization system containing (NH4)6Mo7O24·4H2O, H2O2 and [Bmim]DBP was used to study the effect of the mass ratio of catalyst to oil (0.0025−0.03) on the removal of 4,6-DMDBT. It could be observed from Figure 6 that the sulfur removal was found to
Figure 5. Kinematic viscosities of the ionic liquids at various temperatures.
lower layer. When [Mmim]DMP, [Emim]DEP, [Bmim]DBP were used as extractant for the 4,6-DMDBT-containing model oil at 50 °C in 3 h, the sulfur removal only reached 2.53%, 4.51%, and 9.29%, respectively. With the addition of H2O2 in [Mmim]DMP, [Emim]DEP, and [Bmim]DBP, the removal of sulfur had a slight increase after chemical oxidation, reaching 7.71 %, 10.36 %, and 6.89 %, respectively. When (NH4)6Mo7O24·4H2O, H2O2 were employed together with ILs, the removal of 4,6-DMDBT increased sharply, especially for [Bmim]DBP with a sulfur removal of 89.2%. However, the oxidative desulfurization system containing (NH4)6Mo7O24· 4H2O and H2O2 without ILs only led to 5.42% sulfur removal. So we concluded that the IL played a significant role in the desulfurization system, it not only served as an extractant and reaction media but also enhanced the activity of catalyst. These results also clearly demonstrated that catalytic oxidation combined with extraction in IL could deeply remove 4,6DMDBT from the model oil, which was superior to the desulfurization system of H2O2/NaMoO4·2H2O/[Bmim]BF4.26 It was found that there is a remarkable advantage of this process over the desulfurization of model oil by mere solvent extraction with IL or mere catalytic oxidation without IL. The longer the carbon chain of IL was, the higher the desulfurization rate of ECODS got. The reason for this behavior might be that 4,6DMDBT was a nonpolar compound, and the polarity of the ionic liquids would generally decrease as the alkyl group chain length extended.27 According to the like dissolves like theory, the ionic liquid with a longer alkyl group exhibited better extraction ability. The extraction and catalytic oxidation desulfurization process contains two steps, that is, the extractive equilibrium and the oxidative reaction. In the first step, 4,6-
Figure 6. Effect of amount of catalyst on the removal of 4,6-DMDBT. Reaction conditions: model oil = 5 g, IL = 1 g, m(H2O2) = 0.2 g, t = 3 h, T = 50 °C.
increase when the mass ratio was from 0.0025 to 0.02; a further increase of the mass ratio of catalyst to oil to 0.03 made the desulfurization rate decrease unexpectedly. The reason for this behavior might be that the excessive catalyst was not completely dissolved in ionic liquid [Bmim]DBP, and then it lay in the model oil in the form of a turbid liquid, which has a quite harmful impact on reaction. To explain the effect of excessive catalyst on the desulfurization rate, we did an additional test: first, 5 g of model oil and 0.1 g of catalyst were added to a flask, and then the mixture was stirred vigorously at 50 °C for 3 h. Second, the model oil without catalyst was added into another flask under identical conditions. The experimental results showed that the sulfur concentration of the model oil with catalyst was higher than that of model oil
Table 1. Comparison of Different Desulfurization Systems of the Sulfur Removal Performance sulfur removal (%)a
a
desulfurization system
[Mmim]DMP
[Emim]DEP
[Bmim]DBP
IL IL+ H2O2 IL + (NH4)6Mo7O24·4H2O IL + H2O2 + (NH4)6Mo7O24·4H2O (NH4)6Mo7O24·4H2O + H2O2
2.53 2.54 4.42 15.48
4.51 7.71 5.46 84.46
9.29 10.36 8.61 89.2 5.42
Reactions conditions: T = 50 °C, t = 3 h, model oil = 5 g, IL = 1 g, m(H2O2) = 0.2 g, m(catalyst) = 0.1 g, P = atmospheric pressure. 6660
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without catalyst. Therefore, 0.02 was chosen as the appropriate mass ratio of catalyst/oil in the subsequent experiments. 3.6. Effect of the Mass Ratio ILs/Oil on the Removal of 4,6-DMDBT. The desulfurization system containing (NH4)6Mo7O24·4H2O, H2O2, and [Bmim]DBP was used to investigate the influence of the mass ratio of ionic liquid to oil on the removal of 4,6-DMDBT and the mass ratio of ionic liquid to oil increased from 0.1 to 0.5. As illustrated in Figure 7,
Figure 8. Effect of the amount of H2O2 on the removal of 4,6DMDBT. Reaction conditions: model oil = 5 g, ILs = 1 g, m(catalyst) = 0.1 g, t = 3 h, P = atmospheric pressure.
kinetics of the desulfurization reaction. Therefore, the effect of the desulfurization system containing (NH4)6Mo7O24·4H2O, H2O2, and [Bmim]DBP at various times and temperatures on the sulfur removal properties was carried out. Figure 9 displays Figure 7. Effect of the mass ratio ILs/oil on the removal of 4,6DMDBT. Reaction conditions: model oil = 5 g, m(catalyst) = 0.1 g, m(H2O2) = 0.2 g, t = 3 h, P = atmospheric pressure.
the sulfur removal increased with an increase in the mass ratio of ionic liquid to oil (0.1−0.2). The reason for this was mainly that the catalyst could completely dissolve in [Bmim]DBP ionic liquid with an increasing mass ratio of the IL/oil, resulting in a higher rate of extraction. However, when the mass ratio of IL to oil further increased from 0.2, the amount of ILs had a slight effect on the sulfur removal. Thus, considering the desulfurization rate and the cost of ILs, 0.2 was chosen as the suitable mass ratio of IL/oil in the present experiments. 3.7. Effect of the H2O2 on the Removal of 4,6-DMDBT. Figure 8 displays the effect of the reaction temperature and the amount of H2O2 on the content of the 4,6-DMDBT in the model oil. As shown in Figure 8, there was a significant enhancement in the sulfur removal with an increase of H2O2 from 0 to 0.2 g at different temperatures, whereas, the sulfur removal slightly increased when the amount of H2O2 continued to increase. According to the stoichiometric reaction, the oxidation of 1 mol of 4,6-DMDBT to 4,6-DMDBT sulfone consumed 2 mol of hydrogen peroxide. It is noteworthy that there was a competition between 4,6-DMDBT oxidation and the decomposition of H2O2. Results showed that when the H2O2 (0.00106 g) was added according to the molar ratio of H2O2 to 4,6-DMDBT, it was found that the desulfurization rate was very low. The reason for this was that a large amount of hydrogen peroxide was decomposed water and oxygen gas, thus, to oxidize 4,6-DMDBT completely, excessive hydrogen peroxide was added to this desulfurization system. Although some of the hydrogen peroxide self-decomposed, the remaining hydrogen peroxide could oxidize the 4,6-DMDBT. 3.8. Effect of Time and Temperature on the Sulfur Removal of 4,6-DMDBT. Generally, the reaction temperature and time are important parameters that can influence the
Figure 9. Sulfur-removal after oxidation with (NH4)6Mo7O24·4H2O as catalyst in [Bmim]DBP at different temperature and reaction time. Reaction conditions: model oil = 5 g, ILs = 1 g, m(catalyst) = 0.1 g, m(H2O2) = 0.2 g, P = atmospheric pressure.
the removal of the 4,6-DMDBT at different times and temperatures. The data at time zero reflected the performance of [Bmim]DBP to extract 4,6-DMDBT from n-octane at room temperature. An increase in the reaction temperature from 20 °C to 50 °C led to a sharp increase in 4,6-DMDBT removal in the initial 2 h, and a slower increase when the reaction time was prolonged. When the reaction time was 3 h, the sulfur removal reached 89.2%, 86.3%, 85.2%, and 84.4% at 50, 40, 30, and 20 °C, respectively. It was found that temperature influenced desulfurization rate but had little effect on the ultimate desulfurization rate. The reason lies in the difference in heat and mass transfer, because the viscosity of the ionic liquid is higher at lower temperature (shown in Figure 5) and it inhibits heat and mass transfer of the reaction system.28 When the 6661
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reaction time is increased, water is produced during the extraction and catalytic oxidation desulfurization process, and the ionic liquid is diluted. Consequently, the effect of reaction temperature on sulfur removal becomes smaller with increasing temperature as long as the reaction time is prolonged. 3.9. Reusability of [Bmim]DBP Ionic Liquid. For the industrial application of ILs containing catalyst catalytic oxidation/extraction, the regeneration and subsequent recycling is very important. So the possibility of recycling the ionic liquids containing (NH4)6Mo7O24·4H2O was investigated in ECODS of 4,6-DMDBT-containing model oil, and the results are shown in Figure 10. Because the oil is immiscible with ILs,
Figure 11. The process of extraction coupled with catalytic oxidation of 4,6-DMDBT.
continuously. The 4,6-DMDBT was oxidized to its corresponding sulfone and the sulfone accumulated in the IL phase and could be easily separated by decantation from the model oil.29 In this way, ultradeep desulfurization can be achieved. 3.11. Determination of Mo-Content in Model Oil. To study the effect of the catalyst on purity of the model oil, the determination of Mo-content in oil has been carried out by atomic emission spectrometry with inductively coupled plasma (ICP-OES). The results revealed that the Mo-content was very low, only 0.05778 μg/g in the oil phase. Thus, (NH4)6Mo7O24· 4H2O not only possessed good catalytic activity but hardly contaminated the model oil.
Figure 10. The reusability of [Bmim]DBP containing (NH4)6Mo7O24· 4H2O. Reaction conditions: T = 50 °C, t = 3 h, model oil = 5 g, ILs = 1 g, m(catalyst) = 0.0988 g, m(H2O2) = 0.2 g, P = atmospheric pressure.
the ILs phase-containing catalyst and oxidizing agent were still the lower layer and oil was still the upper layer after the reaction. After the first recycle, in order to remove the residual compounds of the ILs, the ionic liquid phase was distilled at 90 °C for 3 h by rotary evaporation under reduced pressure, and then, fresh model oil and hydrogen peroxide were added for the next recycle under identical reaction conditions. The data in Figure 10 indicated that for the [Bmim]DBP ionic liquid coupled with catalyst, as the recycle times increased, there was scarcely a decrease in the sulfur removal performance of the ionic liquids. When the system was recycled six times, the sulfur removal dropped from 89.21% to 87.98% at the same experimental condition. The reason was that more and more oxidation products were produced after recycling and the extraction performance of the ILs decreased, and also some IL was lost after being separated from the oil and regenerating, which led to a decrease in activity in the next run. As is shown in Figure 1, the FT-IR of IL after the recovery showed a similar pattern to that of the fresh IL. In other words, the properties of IL did not change. 3.10. Process and Mechanism of ECODS. The process of extraction and catalytic oxidation of 4,6-DMDBT is shown in Figure 11. In the first step, 4,6-DMDBT was extracted from oil into ionic liquid, and then it was oxidized by H2O2 in the presence of (NH4)6Mo7O40·4H2O. The decrease of 4,6DMDBT concentration in the ionic liquid promoted the extraction process, thus, the sulfur content in the oil decreased
4. CONCLUSIONS The imidazolium-based phosphoric ionic liquids ([Mmim]DMP, [Emim]DEP, [Bmim]DBP), which were moistureinsensitive, thermally stable, and readily regenerated, could be use to dissolve the catalyst for oxidizing 4,6-DMDBT with hydrogen peroxide under mild conditions. The sulfur removal of 4,6-DMDBT-containing model oil in ECODS could reach 89.2% at 50 °C in 3 h. This process had the remarkable advantage over the traditional process, achieving six times the recycling without a significant decrease in activity. Moreover, the oil phase hardly required the catalyst. The desulfurization system composed of (NH4)6Mo7O24·4H2O, H2O2, and IL has the potential to develop into a simple, mild, and environmentally benign method for deep desulfurization.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.021-64252383. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21276086). 6662
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dx.doi.org/10.1021/ie500236b | Ind. Eng. Chem. Res. 2014, 53, 6655−6663
