Article pubs.acs.org/EF
Deep Extraction Desulfurization with a Novel Guanidinium-Based Strong Magnetic Room-Temperature Ionic Liquid Tian Yao, Shun Yao, Caihui Pan, Xuezhi Dai, and Hang Song* School of Chemical Engineering, Sichuan University, Chengdu 610065, China S Supporting Information *
ABSTRACT: A series of novel guanidinium-based magnetic ionic liquids were synthesized. Among them, [TMG]Cl/1.5FeCl3 exhibited a very strong paramagnetic strength with the value of magnetic susceptibility as great as 59.1 × 10−6 emu/g, which exceeded any other reported magnetic IL so far. By virtue of good physical and chemical properties, it was representatively selected to extract dibenzothiophene (DBT) and thiophene (T) from model oils with sulfur content of 900−1000 μg/g, which was compared with commonly reported imidazolium magnetic ionic liquid [BMIM]Br/1.5FeCl3. The results revealed that [TMG]Cl/1.5FeCl3 had perfect desulfurization efficiency (nearly 100%) with mass ratio of oil to MIL (g/g) of 1−4 within only 5 min at room temperature, and the method could easily meet the latest European sulfur emission standard (Euro 5). It showed significant selectivity for sulfur over toluene, as well as little pollution of the model oils, which were both better than [BMIM] Br/ 1.5FeCl3. Satisfactory desulfurization performance for 93 gasoline was further achieved by [TMG]Cl/1.5FeCl3. Furthermore, it could be recycled at least 7 times without any noticeable decrease in desulfurization efficiency. The novel MIL demonstrates promise in practical application in the future.
1. INTRODUCTION Deep desulfurization (DDS) of fuel oils, such as diesel and gasoline, has attracted increasing interest because of the enforcing of regulations on the environmental pollution caused by exhaust gases, such as SOx and NOx, to the atmosphere.1 Japan has already reduced the allowable sulfur level in gasoline or diesel down to 10 μg/g by 2008, followed by Europe renewing its standard to 10 μg/g by 2010.2 Zero-emission and zero-level of sulfur content are previewed in the near future.3 Hydrodesulfurization (HDS) by catalysis is a conventional method for the removal of sulfur compounds.4 However, the catalytic process requires high temperature, high pressure, and high energy cost and has a limited capacity to efficiently eliminate the refractory sulfur compounds in the fuel.5 Therefore, alternative desulfurization technologies for fuel oils have been developed during the last few decades, such as oxidation, extraction, adsorption, biodesulfurization, and others.6 Oxidative desulfurization (ODS) is promising due to its high desulfurization efficiency,7 but the oxidants used can also oxidize the hydrocarbon parts, as well as the organosulfur compounds, in fuel oils, which drastically reduces the quality of fuel oils.2,8 Extractive desulfurization (EDS) is attracting more attention due to its mild and simple operating conditions.9 Besides, it could not alter the chemical structure of the compounds in the fuel oils and thus has little effect on the quality of fuel oils.10 However, some organic solvents used in EDS can cause further environmental and safety concerns for its flammability and volatility.11 Recently, some ionic liquids (IL) with many desirable properties such as high thermal stability, nonvolatility, and perfect solubility characteristics have been used in EDS of fuel oils. The first ionic liquid for the selective extraction of sulfur compounds from fuel oils was reported by Wassercheid and coworkers12 in 2001. Since then more ILs incorporating anions © XXXX American Chemical Society
such as alkylsulfates, thiocyanate, or even acetate or dialkylphosphate and incorporating cations such as pyridinium, imidazolium, or pyrrolidinium have been used in the desulfurization process.13−17 However, desulfurization efficiency is rather low, with a value of less than 80% for most of these kinds of ILs. To solve this problem, Gao and coworkers18 synthesized a carbonium pseudo-IL to get an excellent extractive desulfurization performance of 99% but failed to regenerate and recycle the IL, which may pollute the environment. Nan and co-workers1 introduced Fe-containing ILs with an increasing molar ratio of FeCl3 to [imidazolium]Cl up to 2 and achieved a nearly 100% of desulfurization. But the recycling of IL was not investigated either. Many aromatic ILs including the most reported imidazolium ILs show low selectivity between aromatic sulfur components and toluene because of their similar π−π interaction; however more than 15% of fuel oils are toluene-series constituents. Moreover, these ILs could partly get dissolved in fuel oils, which will alter the oil composition and pollute the fuel oils. Therefore, there is an urgent need to find a novel room temperature IL with significant desulfurization efficiency, high selectivity, less pollution to the oil, and desirable recyclability. Magnetic ILs (MILs) not only possess the excellent properties of conventional ILs but also display an unexpectedly strong response to an additional magnet allowing us to easily separate and reuse them.19 Zhu and co-workers20 applied pyridinium-based magnetic ILs for ODS of fuels and realized simple recovery operation. However, there have not been any reported guanidinium-based magnetic ionic liquids applied in the oil desulfurization field. Received: March 25, 2016 Revised: May 21, 2016
A
DOI: 10.1021/acs.energyfuels.6b00684 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels In this work, a series of novel guanidinium-based magnetic ILs were synthesized and applied in the extraction desulfurization process. After a preliminary screening, two kinds of room temperature magnetic ionic liquids, 1,1,3,3-tetramethylguanidinium-based ILs [TMG]Cl/xFeCl3 and 3-butyl-1-methylimidazolium-based ILs [BMIM]Br/xFeCl3, were selected for a more detailed investigation with respect of magnetic susceptibility, desulfurization efficiency, selectivity, and recovery performance.
2. EXPERIMENTAL SECTION 2.1. Materials. Dibenzothiophene (DBT, 99%), 1,1,3,3-tetramethylguanidine (99%), benzothiophene (BT, 99%), thiophene (T, 99%), and 2-methylpyrazine (99%) were purchased from Chengdu Best Co., Ltd. N-Methylimidazole (AR grade) was purchased from Shanghai Mr Ren Chemical Technology Co., Ltd. Other chemical reagents were purchased from Chengdu Kelon Chemical Reagent Co., Ltd., with AR grade. The 93 gasoline was supplied by Sinopec Chengdu. 2.2. Synthesis and Characterization of Magnetic Ionic Liquids. 1,1,3,3-Tetramethylguanidinium (0.1 mol) was added into 25 mL of acetonitrile under violent stirring. A slight molar excess of hydrochloric acid was then added dropwise into the solution within 2 h, and the mixture was continuously stirred for another 4 h. After reaction, the solvent was removed, and the intermediate product, [TMG]Cl, was dried under low pressure for 2 h at 60 °C. Second, an equimolar ratio of anhydrous FeCl3 and [TMG]Cl were mixed to form a liquid mixture under N2 atmosphere. The mixture was filtered and dried under vacuum to obtain [TMG][FeCl4]. Finally, extra amount of FeCl3 was simply added into [TMG][FeCl4] to get a series of magnetic ILs [TMG]Cl/xFeCl3 (x = 1, 1.25, 1.33, 1.5, 1.67, 1.75, and 2.00) according to a literature method.9 Other intermediate products [BMIM]Br, [BTMG]Br, [HTMG]Br, [HBpy]Cl, [BPZ]Br, and [HPZ]Cl were synthesized by reacting equimolar amounts of halohydrocarbons or hydrochloric acid and raw material at 70 °C with a magnetic stirring for 24 h. The intermediates were repeatedly recrystallized in acetonitrile and dried under vacuum for a period of time. Final products [BMIM]Br/xFeCl3 (x = 1, 1.25, 1.33, 1.5, 1.67, 1.75, or 2), [C12TMG][FeCl3Br], [HBpy][FeCl4], [HBth][FeCl4], and [HPZ][FeCl4] were obtained by the same procedure. [TMG][FeCl4], [TMG]Cl/FeCl3, and [BMIM][FeCl3Br] were characterized by FT-IR spectra, Raman spectra, and ESI-MS as demonstrated in Figures 2−4. [C12TMG][FeCl3Br], [HBpy][FeCl4], [HBth][FeCl4], and [HPZ][FeCl4] were characterized by FT-IR spectra and Raman spectra as shown in Figures S1−S4 available in Supporting Information. Structure of these MILs are listed in Figure 1. Fourier transform infrared (FT-IR) spectra of ILs were recorded on a Spectrum Two, L1600300, FT-IR instrument purchased from PerkinElmer, America. Raman spectra were recorded on a LabRAM HR Laser Raman spectrometer purchased from HORIBA Co., France. Electrospray ionization mass spectra (ESI-MS) were collected on an amaZon SL mass spectrometer, Germany, and data were processed by Bruker Compass Data Analysis 4.0. UV−vis spectra were measured on a TU-1810 UV spectrophotometer (wavelength range of 190−900 nm, Pgeneral Co., Ltd., China). Thermogravimetric (TG) analysis was performed on a HTG-2 microcomputer differential thermal balance, China. The magnetic measurements were conducted with a vibrating sample magnetometer (Lake Shore 7410 VSM, USA). Model oils were marked as oils 1, 2, 3, and 4. As representative main components in gasoline or diesel, DBT (dibenzothiophene), BT (benzothiophene), and T (thiophene) were dissolved in n-octane to prepare model oils 1 and 3 with sulfur contents of 1000 or 900 ppm, respectively. Toluene with mass fraction of 0.15 was added into noctane together with sulfur-containing aromatic compounds to give model oils 2 and 4. The sulfur concentrations and compositions of these model oils and no. 93 gasoline are listed in Table 1. 2.3. Density and Viscosity Measurement. Each IL (40 ± 0.1 mL) was added dropwise into a 50 mL precision measuring cylinder and kept in a thermostatic water bath with an uncertainty of ±0.1 °C for 30 min at different temperatures. Then the densities of the
Figure 1. Structures of synthesized MILs.
Table 1. Compositions and Initial Sulfur Contents of Model Oils and No. 93 Gasoline model oils oil oil oil oil 93
1 2 3 4 gasoline
initial sulfur content (μg/ g)
oil composition DBT + n-octane DBT + toluene + n-octane T + n-octane DBT, BT, T + toluene + n-octane
1000 1000 1000 900 47.8a
a
Initial sulfur content of 93 gasoline was determined by an ultraviolet fluorescence sulfur analyzer.
prepared ILs were calculated with mass value determined by a FA/JA type electronic analytical balance (uncertainty of ±0.0001 g). Densities were measured 3 times and determined with average value. Viscosities of them were measured by a 0.4−0.5 mm type Ubbelohde viscosimeter with deionized water as reference. All these measurements were repeated 3 times, and the average values were measured results. The density and viscosity values are presented in Table 2.
Table 2. Magnetic Property Comparison of Different Magnetic Ionic Liquids MIL [BPy][FeCl4] [BMIM][FeCl4] [C3H6COOHmim]Cl/2FeCl3 [bmP][FeCl4] pDaDmAm+FeCl4− pViEtIm+FeBrCl3− pViBuIm+Fe2Cl7− [TMG]Cl/1.5FeCl3
magnetic susceptibility (emu/ g) 43.1 40.6 45.2 9.5 29.6 35.3 45.1 59.1
× × × × × × × ×
−6
10 10−6 10−6 10−6 10−6 10−6 10−6 10−6
ref 22 24 7 19 21 21 21 this work
2.4. Evaluation of IL Solubilities in Model Oils. For observing probable oil contamination by ILs, solubilities of ILs in a few solvents were evaluated with conventional gravimetric method. The composition of model oil 3 and oil 4 were similar to oil 1 and oil 2. So experimental solvents were determined as model oil 1, model oil 2, and pure toluene. A slight excess of IL was accurately weighed and added into 400 g of solvent in the thermostatic stirrer to form an oversaturated solution. The solution was vigorously stirred at constant B
DOI: 10.1021/acs.energyfuels.6b00684 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 2. FT-IR spectra of [TMG][FeCl4], [TMG]Cl/1.5FeCl3, and [BMIM][FeCl3Br]. temperature (±0.1 °C) for 0.5 h and allowed to settle for at least 0.5 h until the dark IL droplet was observed. Then another accurately weighed 5 g of the solvent was added into the oversaturated solution, and the solution was stirred for another 0.5 h. The operation was repeated until the final solution was just saturated and steady for at least 4 h. The solubility could be determined. 2.5. Extractive Desulfurization Procedures. ILs [HTMG][FeCl3Br], [HBpy][FeCl4], [BPZ][FeCl3Br], [HPZ][FeCl4], and [TMG]Cl/xFeCl3 and [BMIM]Br/xFeCl3 (x = 1.00, 1.25, 1.33, 1.50, 1.67, 1.75, and 2.00) were applied for extractive desulfurization. Based on various mass ratios of sulfur-containing model oil to IL (1:1, 2:1, 3:1, 4:1, 5:1, g/g), a certain amount of accurately measured IL was added into 50 mL of the model oil and kept stirring at a constant temperature for IL extracting sulfur-containing organics from the oil. The same operation was performed at different temperatures. During the extraction, about 0.1 mL of sample was taken at each required time interval and centrifuged for 4 min at 4000 rpm. The clear upper phase (model oil) was analyzed by high performance liquid chromatography (HPLC, purchased from Yilite Co., Ltd. Dalian, China). Limit of detection was 0.2 μg/g for DBT, 0.14 μg/g for T) with an external standard method. All these standard curves of DBT, T, and toluene were fitted with correlation coefficient higher than 0.9998. The chromatographic conditions were as follows: chromatographic column, Welchrom C18 (4.6 mm × 250 mm,5 μm, Welch Materials, Inc.); column temperature, 30 °C; UV−vis detector under 281 nm wavelength for oils 1 and 2 and 230 nm for oil 3; mobile phase, methanol/water = 80/20; flow rate, 1.1 mL/min. The total sulfur concentration in the tested model oil 4 and no. 93 gasoline was analyzed with a KDS-2000 ultraviolet fluorescence sulfur analyzer by combustion of the sample and measurement of the released gaseous sulfur compounds. Desulfurization rate (R) was calculated as shown in eq 1.
R(%) =
(C0 − C1) × l00 C0
S=
MIL Csulfur oil Csulfur
,
KAr =
3. RESULTS AND DISCUSSION 3.1. Characterization of [TMG]Cl/1.5FeCl3, [TMG][FeCl4], and [BMIM] [FeCl3Br]. The cationic structures of [TMG]Cl/1.5FeCl3, [TMG][FeCl4], and [BMIM][FeCl3Br] were clearly confirmed with FT-IR spectra. As shown in Figure 2a ([TMG]Cl/1.5FeCl3 and [TMG][FeCl4] nearly the same), the bands at 3473 and 3375 cm−1 resulted from the primary amine stretching vibration. Additional weak features near 2967 and 2814 cm−1 were assigned to the methyl stretching vibration. The prominent peak at 1615 cm−1 was attributed to the CN stretching vibration, while the band at 1556 cm−1 came from the bending vibration of NH. Two bands at 1410 and 1454 cm−1 were observed due to the methyl bending vibration. In the spectrum of [BMIM][FeCl3Br] (Figure 2b), the bands at 3116 and 3147 cm−1 came from the streching vibration of aromatic protons. The bands at 2961, 2935, and 2874 cm−1 were related to the fatty C−H stretching vibration. Absorption bands near 1591 and 1567 cm−1 came from the stretching vibration of CN in the imidazole ring. Two bands at 1463 and 1384 cm−1 were interpreted as methyl bending vibrations. Absorption at 1164 cm−1 was from the C−Br stretching vibration, and the bands near 830 and 741 cm−1 were due to the bending vibration of C−H in the imidazole ring. The anion structures of the three MILs were confirmed through ESI-MS with negative ion mode. Main peaks were clearly attributed in Figure 3. Lack of the (quasi-) molecular ion peak was due to the easy ionization of the MILs. Figure 3a revealed the existence of the FeCl4− anion (197.9) for [TMG][FeCl4]. In Figure 3b, the formation of Fe1.5Cl5.5− was observed. The excellent sulfur removal was probably attributable to the presence of the more Lewis acidic high nuclear Fe species (Fe1.5Cl5.5−) according to the literature.1 In Figure 3c, anion peak of FeCl3Br− was observed with 241.9, while the presence of FeCl4− and FeCl2Br2− were probably because of reversible halogen atom exchange among anions.
(1)
MIL CAr oil CAr
(3)
oil MIL oil where CMIL sulfur, Csulfur, CAr , and CAr represent the sulfur and toluene concentrations in MIL and model oil after sulfur removal, respectively.
where C0 (μg/g) and C1 (μg/g) are the mass fraction of sulfur in model oils before and after desulfurization, respectively. Distribution coefficient of sulfur (Ksulfur) and toluene (KAr) and selectivity (S) were introduced to evaluate the desulfurization performance of ILs as three important parameters and were calculated as shown in eqs 2 and 3.
K sulfur =
K sulfur KAr
(2) C
DOI: 10.1021/acs.energyfuels.6b00684 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
magnetization intensities show linear response to the magnetic field, which is in accordance with reported MILs.22,23 As an indicator for the paramagnetic strength of a substance, the magnetic susceptibility reaches as high as 59.1 × 10−6 emu/g, which is larger than that of any other reported MILs as summarized in Table 2. The strong response to an external magnet of novel [TMG]Cl/1.5FeCl3 is pictured in Figure 7. Its significant paramagnetic strength is probably due to higher mass ratio of Fe element in the whole molecule. 3.2. Extractive Performance of Different MILs. Extractive desulfurization experiments were conducted with [C12TMG][FeCl3Br], [HBpy][FeCl4], [HBth][FeCl4], [HPZ][FeCl4], [TMG][FeCl4], and [BMIM][FeCl3Br] in model oil 1. According to the pre-experiment, extractive temperature was set at room temperature (27 °C), the mass ratio of model oil to IL was 2, and the extraction time was 5 min. The results are shown in Figure 8. The desulfurization rates of solid MILs 1−3 were less than 30%, while the liquid MILs 4−6 exhibited much higher rates over those solid MILs of about 40%. This reveals the importance of fluidity of the MILs for sulfur removal. MILs 4−6 were further applied in the investigation of their sulfur removal efficiency in model oil 2. However, [C12TMG][FeCl3Br], and [BMIM][FeCl3Br] could severely pigment the model oil, while IL [TMG][FeCl4] revealed excellent extraction performance for its nonpollution behavior to model oil 2. Consequently, the novel strong magnetic room temperature ILs [TMG]Cl/xFeCl3 as well as traditional imidazolium-based ILs [BMIM]Br/xFeCl3 were selected for further investigation and comparison. 3.3. Densities and Viscosities of MILs. The density and viscosity at different temperatures were measured as fundamental data of physical properties to characterize the synthesized MILs and are listed in Table 3. The viscosity of [TMG][FeCl4] was higher than that of [BMIM][FeCl3Br]. However, [TMG]Cl/1.5FeCl3 showed lower viscosity than [BMIM]Br/1.5FeCl3 with an extra amount of FeCl3, which could result in a better fluidity and make the extraction reach equilibrium more quickly. This is in agreement with the later investigation of the effect of extraction time on desulfurization of model oils. Many reported ionic liquids had high viscosity values, such as 0.145 Pa·s for [C4py][BF4],25 3.95 Pa·s for [C4mim][BF4],26 0.46 Pa·s for [C2mim][PO2(C2O)2],27 and 0.71 Pa·s for [C8mim][PF6].26 The viscosity value of [TMG]Cl/1.5FeCl3 is very close to the reported “low-viscosity” IL [S2][N(CN)2] whose value is 0.034 Pa·s.28 Sufur removal efficiency was significantly enhanced for this superior property. 3.4. Solubility of MILs in a Few Solvents at Room Temperature. The dissolution of nitrogenous ILs in fuel oils can cause NOx pollution to the environment, and the solubility data are important for further application of the ILs.21 Thus, the solubility data were determined and presented in Table 4. All MILs showed undetectable solubility in model oil 1. However, both model oil 2 and pure toluene were obviously pigmented by [BMIM][FeCl3Br], [BMIM]Br/1.5FeCl3, and [HTMG][FeCl3Br], while oil 2 still remained colorless after extraction with [TMG][FeCl4] or [TMG]Cl/1.5FeCl3. Solubilities of imidazolium-based MILs were much higher than those of tetramethyl guanidinium (TMG) based MILs in model oil 2 and pure toluene. Obviously, solubility difference in oil 2 resulted from the toluene constituent. It could be explained that π−π interactions between toluene and the imidazole ring played a key role to strengthen the mutual solubility. This indicates that [TMG][FeCl4] and [TMG]Cl/1.5FeCl3 caused
Figure 3. ESI(−) MS (negative ion mode) spectrum of [TMG][FeCl4] (a), [TMG]/1.5FeCl3 (b), and [BMIM][FeCl3Br] (c).
Raman spectra of these MILs were shown in Figure 4. The bands at 331, 332, or 334 cm−1 were assigned to the totally symmetric Fe−Cl stretching vibration. A new peak at 361 cm−1 was found for [TMG]Cl/1.5FeCl3; it is probably due to the existence of Fe1.5Cl5.5− according to the ESI-MS results. In Figure 4c, the appearance of new bands at 222, 248, and 265 cm−1 were attributed to the Fe−Br stretching vibration according to the literature.21 The thermogravimetric (TG) curve of novel [TMG][FeCl4] is shown in Figure 5a. This MIL was very stable when the temperature was lower than 300 °C. The main weight loss occurred while temperature was rising from 310 to 755 °C due to the loss of the cation and a halogen element after which the Fe element was reserved as the residue. The UV−vis spectra of [TMG]Cl, [TMG][FeCl4], and [TMG]Cl/1.5FeCl3, which were dissolved in methanol at concentrations of 0.038, 0.0369, and 0.0407g/L, respectively, are presented in Figure 5b. They shared a great absorption band before 250 nm, but a new peak was detected at 364 nm in the spectra of [TMG][FeCl4] and [TMG]Cl/1.5FeCl3, which could attribute to FeCl4−.20 Novel ionic liquid [TMG]Cl/1.5FeCl3 was simply prepared by adding half mole of FeCl3 to [TMG][FeCl4] by a literature method.9 Its magnetic behavior was studied at 300 K. The magnetic field range was set from −20000 to 20000 Oe, and the results are shown in Figure 6. It can be seen that D
DOI: 10.1021/acs.energyfuels.6b00684 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 4. Raman spectra of [TMG][FeCl4] (a), [TMG]Cl/1.5FeCl3 (b), and [BMIM][FeCl3Br] (c).
Figure 5. TG curve of [TMG][FeCl4] (a) and UV−vis spectra of [TMG]Cl, [TMG][FeCl4], and [TMG]Cl/1.5FeCl3 (b).
considerable sulfur removal growth was not observed when the molar ratio of FeCl3 to [TMG]Cl was higher than 1.5, meanwhile precipitation could easily occur and lead to the decrease of fluidity and extraction efficiency. Subsequent magnetic recycling trouble also could happen, because the MIL was hardly attracted by a magnet. Then extraction investigation of model oils 1and 3 with [TMG]Cl/1.5FeCl3 and [BMIM]Br/1.5FeCl3 was carried out at time intervals 1, 3, 5, 10, 20, 30, and 40 min. It indicated that [TMG]Cl/1.5FeCl3 exhibited a better performance than [BMIM]Br/1.5FeCl3.
much less pollution to fuel oils than imidazolium-based MILs and TMG-based MILs with a long carbon chain. 3.5. Desulfurization Performance in Toluene-Free Model Oils. Extraction desulfurization conditions including extraction time, temperature, molar ratio of FeCl3/[TMG]Cl and mass ratio of oil to MIL were investigated to explore the effects of these factors on the sulfur removal of DBT and T from model oils 1 and 3. The results showed that the higher the molar ratio of FeCl3 to [TMG]Cl is, the better the sulfur removal is, especially in toluene-containing oil. However, E
DOI: 10.1021/acs.energyfuels.6b00684 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Table 4. Solubility Value of ILs (μg solute/g solvent) in Different Solvents at Room Temperature Solvents model oil 1 model oil 2 toluene
[BMIM] [FeCl3Br]
[BMIM]Br/ 1.5FeCl3
[BTMG] [FeCl3Br]
[TMG] [FeCl4]
[TMG]Cl/ 1.5FeCl3
