Article pubs.acs.org/IECR
Extractive Desulfurization of Liquid Fuels by Energy Efficient Green Thiazolium based Ionic Liquids Swapnil A. Dharaskar,† Kailas L. Wasewar,*,† Mahesh N. Varma,*,† Diwakar Z. Shende,† and Chang Kyoo Yoo*,‡ †
Advance Separation and Analytical Laboratory (ASAL), Department of Chemical Engineering, Visvesvaraya National Institute of Technology (VNIT), Nagpur, MS 440010, India ‡ Environmental Management & Systems Engineering Lab (EMSEL), Dept. of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do 446-701, Korea ABSTRACT: In the present paper, experimental data on extractive desulfurization of liquid fuel using 3-butyl-4methylthiazolium thiocyanate [BMTH]SCN is presented. The Fourier transform infrared (FTIR), 1H NMR, and 13C NMR analyses have been discussed for the molecular confirmation of synthesized [BMTH]SCN. Further, conductivity, solubility, and viscosity analyses of [BMTH]SCN were performed. The effects of reaction time, reaction temperature, sulfur compounds, ultrasonication, and recycling of [BMTH]SCN without regeneration on removal of dibenzothiophene from liquid fuel were also investigated. In the extractive desulfurization process, the removal of dibenzothiophene in octane was 81.2% for mass ratio of 1:1 in 30 min at 30 °C under the mild reaction conditions. Thiazolium ionic liquids could be reused five times without a significant decrease in activity. Also, the desulfurization of real fuels, multistage extraction was examined. The data and results provided in present paper explore the significant insights of thiazolium based ionic liquids as novel extractant for extractive desulfurization of liquid fuels. desulfurization ability. The first studies focused on ILs containing the [PF6]− and [BF4]− anions.14,15 However, these ILs are prone to hydrolyze even in the presence of humidity, generating hazardous hydrolysis products.16,17 ILs are almost insoluble in liquid fuel and favorably disposed toward the extraction of thiophenic S compounds.18 Many ILs used in extractive desulfurization are mostly imidazolium and pyridinium based, coupled with anions such as tetrafluoroborate [BF4], hexafluorophosphate [PF6], chloride-based [Cl/ AlCl3], and [Cl/FeCl3], alkylsulfate, and alkylphosphate.14 Some of these ILs pose undesirable features during extractive desulfurization, for example, [Cl/AlCl3] based ILs form dark precipitates and is sensitive to water, generate HCl and become unstable in air, and also, corrosive hydrogen fluoride (HF) is easily produced from the decomposition of fluorinated anions.19 As a result, the use of other ILs incorporating anions such as thiocyanate or bis (trifluoromethylsulfonyl) imide became more powerful because of their lower viscosities.20−22 Regarding the selection of cations, a trend to use pyridinium cations acquired relevance in recent years, since Holbrey et al. established the following ranking of ILs for desulfurization depending on the cations: alkylpyridinium ≥ pyridinium ≈ imidazolium ≈ pyrrolidinium.23−26
1. INTRODUCTION Over the years, global environmental policies have gradually decreased the permissible limits of sulfur content in automotive fuels, in order to diminish atmospheric pollution and derived effects such as acid rain.1 The classical hydrodesulfurization (HDS) process uses high pressure and temperature and has a inadequate capability to proficiently eliminate the refractory sulfur compounds in the fuel. In order to meet the more restrictive regulations, novel alternative techniques to attain ultralow sulfur fuels have been required.2−4 Among these, extractive desulfurization (EDS) is an eye-catching technology, which may be carried out at ambient temperature and pressure and without H2 as a catalyst. Moreover, conventional volatile organic solvents, the use of ionic liquids (ILs) to extract sulfur from gasoline and diesel, have been the aim of energetic research in recent era due to their fascinating properties.5−10 ILs are organic salts with melting points around or below ambient temperature, termed as “green solvents” in a range of basic research and applications, including synthesis, separations, and catalysis, owing to their unique and valuable properties.11,12 Many ILs are chemically and thermally stable, and these ILs are in liquid state over a wide range of temperatures. Due to the ionic character of ILs, they have an almost negligible vapor pressure under normal process operating conditions, thus being easily recoverable from molecular volatile compounds and avoiding losses to the atmosphere. Moreover, a careful selection of their constituent ions can tune the properties of the ILs to a significant extent, therefore allowing the “design” of a specific IL to meet the requirements for a particular target. Selective extraction of sulfur compounds from liquid fuels using ILs was first reported in 2001 by Wassercheid and coworkers.13 Since, many ILs has been tested to find out the © 2014 American Chemical Society
Special Issue: Energy System Modeling and Optimization Conference 2013 Received: Revised: Accepted: Published: 19845
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flask containing acetone (30 mL) to obtain [BMTH]SCN. After reacting at 40 °C for 12 h, a precipitate is formed which is then filtered, off, leaving behind a liquid solvent that undergoes rotary evaporation. The resulting product is washed with dichloromethane, filtered, and again made to undergo rotary evaporation. The [BMTH]SCN is finally obtained after traces of volatile impurities were removed by heating at 60 °C under vacuum for 2 h.33 The synthesis route of [BMTH]SCN is shown in Figure 1.
Many research articles are based on the ability of ILs to extract sulfur compounds based on a category of experiments carried out (mainly thiophene and/or dibenzothiophene) from their mixtures with only a hydrocarbon.5,27 Other studies focus on the liquid−liquid equilibrium of systems composed by an IL, a sulfur containing compound, and an aliphatic hydrocarbon.28,29 The high selectivities obtained in some of these systems carry the extraction with ILs a promising substitute in desulfurization processes. However, few studies are found in literature that spotlight on the desulfurization related equilibria while including other compounds such as aromatic hydrocarbons or nitrogen-containing compounds.30,31 The chemistry of thiocyanate cations is well recognized, and some thiazolium salts are commercially available; these salts are amazingly represented in the improvement and use of ILs.32 The synthesis and characterization of 3-butyl-4-methylthiazolium thiocyanate [BMTH]SCN by NMR was reported by Chen et al., and this thiazolium IL has been used in extraction of cobalt from nickel salts, and also in liquid membrane. [BMTH]SCN was also reported as a solvent used in organic reactions.33 In the present work, a novel kind of thiazolium 3-butyl-4methylthiazolium thiocyanate was synthesized and employed as an extractant to investigate its extractive efficiency toward fuel containing S compounds; here, dibenzothiophene was selected as the S compound. [BMTH]SCN is insensitive to air or moisture, has high thermal or chemical stability, and its anions are fluorine-free. The effects of reaction time, reaction temperature, sulfur compounds, reusability of [BMTH]SCN, desulfurization of real fuels, and multistage extraction are systematically investigated.
Figure 1. Synthesis route of [BMTH]SCN.
2.3. Preparation of Model Fuel. A model fuel with 500 ppmw (parts per million by weight) sulfur (DBT as sulfur source) was prepared in octane. Similarly, the model fuels were prepared by dissolving BT, 4-MDBT, 4,6-DMDBT, T, and 3MT individually in octane, respectively. Actual diesel and gasoline with total S content of 600 and 90 ppmw, respectively, were used. 2.4. Extractive Deep-Desulfurization. 100 mL two necked flask were used for the extractive desulfurization experiments where 10 mL model liquid fuel and defined amount of [BMTH]SCN with various mass ratio (model fuel to [BMTH]SCN as 5:1, 3:1, and 1:1) were mixed by vigorous stirring for time range between 5 and 30 min at 30 °C in a water bath. The upper phase (model fuel) was separated after completion of the reaction and settling of the reaction mixture. The upper phase was analyzed for sulfur content. The extraction efficiency is presented in terms of the sulfur removal based on the initial and final sulfur content in the fuel. 2.5. Instrumentation. A structure of [BMTH]SCN was analyzed by Fourier transform infrared (FTIR) Shimadzu IRAffinity 1 Spectrometer (Japan), using the method of KBr pellet. [BMTH]SCN was characterized by 1H NMR and 13C NMR using CDCl3 as solvent on a (Varian, U.S.A., Mercury plus 299.95 MHz for 1H NMR, and 75.43 MHz for 13C NMR) were used for determination of molecular structures and conformations of the compounds. Conductivity of [BMTH]SCN was measured by PICO+ (Lab India) pH/conductivity meter. Viscosity of [BMTH]SCN was measured using an ARG2 Rheometer (TA Instruments, U.S.A.). Solubility of [BMTH]SCN and various solvents is tested by placing them into a 50 mL round-bottom flask, magnetically stirring, and allowing them to settle. After phase equilibrium and splitting, the top layer was analyzed by high performance liquid chromatography (Agilent Technologies, 1200 series equipped with a UV−vis detector under 237 nm wavelength, column (C-18), mobile phase, methanol/water = 80/20, flow rate, 1.0 mL/min). S content in model fuel and real fuels before and after extraction was analyzed by X-ray Fluorescence Spectrometer (XRF), Model PW 2404, Phillips (PANAlytical, Spectris Technology, Netherlands), Centre of Sophisticated Analytical Instrumental Facility, Indian Institute of Technology, Mumbai (MS) India. The contact time of 30 min between the model
2. EXPERIMENTAL SECTION 2.1. Chemical and Materials. The IL used in the experiment was synthesized using analytical grade chemicals. The details of source, and grades of the chemicals used are as follows: 4-methylthiazole (CAS 822-26-2, Acros 98%), 1chlorobutane (CAS 109-69-3, Acros 99%), potassium thiocyanate (CAS 333-20-0, Sigma 99%), ethyl acetate (CAS 20108-L25, SDFCL 99.5%), octane (CAS 111-65-9, Acros 99%), dibenzothiophene (DBT) (CAS 132-65-0, Acros 98%), thiophene (T) (CAS 110-02-1, Sigma-Aldrich 99%), 3methylthiophene (3-MT) (CAS 616-44-4, Sigma-Aldrich 98%), benzothiophene (BT) (CAS 95-15-8, Sigma-Aldrich, 99%), 4-methyldibenzothiophene (4-MDBT) (CAS 7372-88-5, Sigma-Aldrich, 96%), 4,6-dimethyldibenzothiophene (4,6DMDBT) (CAS 1207-12-1, Sigma-Aldrich, 97%). All chemicals were used without any further purification. Real fuels were purchased from Local Petroleum Pump House, Nagpur, Maharashtra, India. 2.2. Synthesis of IL. 2.2.1. Synthesis of [BMTH]Cl. [BMTH]Cl was prepared by the reaction of 4-methylthiazolium (10.497 g) and 1-chlorobutane (17.408 g) are added to a slurry of acetone (20 mL) in a round-bottom flask fixed with a reflux condenser and magnetically stirred under the nitrogen gas protection at 60 °C for 48 h, and the resulting mixture is allowed to settled for phase splitting. Then, the product was washed thrice with ethyl acetate. The remaining ethyl acetate was removed by heating at 60 °C under vacuum for 2 h. The solution was filtered, and the remaining resulting product was obtained [BMTH]Cl.21,33 2.2.2. Synthesis of [BMTH]SCN. [BMTH]Cl (7.345 g) and potassium thiocyanate (2.522 g) are place in a round-bottom 19846
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fuel and [BMTH]SCN phase is more than enough to achieve the equilibrium. So, the optimum time required for the desulfurization of model fuel was 30 min. However, [BMTH]SCN was best suitable to remove DBT reflected at 30 °C, which was taken as the optimal reaction temperature. 2.6. Characterization of [BMTH]SCN. FTIR, 1H NMR, and 13C NMR analyses were carried out for the characterization of 3-butyl-4-methylthiazolium thiocyanate. 2.6.1. FTIR Analysis. A structure of [BMTH]SCN was analyzed by Fourier transform infrared (FTIR) Shimadzu IRAffinity 1 spectrometer (Japan), using the method of KBr pellet. The results are shown in Figure 2.
Scheme 1. NMR Analysis of [BMTH]SCN
3. RESULTS AND DISCUSSION 3.1. Physical Properties of [BMTH]SCN. The physical properties such as conductivity, solubility, and viscosity analyses of [BMTH]SCN were studied. The production of pure [BMTH]SCN is very important since impurities have a strong influence on their physical properties and stability. [BMTH]SCN conductivity mostly depends on the mobility of its cation, as the diffusion coefficients of ILs cations are higher than that of anions. Thiazolium cation based ILs have the highest ionic conductivity. The conductivity and thermal stability are required in a separation or extraction process, thiazolium based ILs with stable thiocyanate anions.34 [BMTH]SCN shows conductivity value of 75 (μs·cm−1), which is comparatively small. After exchanging of anion, the conductivity of [BMTH]SCN could be increases. Consequently, [BMTH]SCN has great advantages as compared to conventional organic solvents.35,36 Viscosity is a noteworthy property of ILs. It is the internal friction or resistance to flow caused by intermolecular interactions.37 The viscosities of most ILs are 1−3 orders of magnitude greater than those of traditional organic liquids, which affect the rate of mass transfer.38 Factors that affect the IL viscosity are poorly unwritten; however, the chemical structure of the anion is known to have a particularly strong influence. Lowest viscosity of ILs is formed from small anions that have a diffuse negative charge and are unlikely to take part in any hydrogen bonding.39 The high viscosity of a few thiazolium ILs may be decreased by an increase in temperature and/or addition of diluents.40 It has been pointed out that the viscosity of [BMTH]SCN is governed by van der Waals interactions. Consequently, the structure and basicity of the anion affects the viscosity. The decrease of size of anion decreases the van der Waals interaction but increases the electrostatic interaction through hydrogen bonding.41 Viscosity is also a function of temperature. The viscosity of synthesized [BMTH]SCN was measured using ARG2 Rheometer. Viscosity of [BMTH]SCN varies inversely with respect to shear rate; its well recognized that larger thiazolium cations make IL more viscous because of the increased intermolecular van der Waals interactions. Figure 5 indicates that as shear rate increases from 0 to 250 (s−1) viscosity of [BMTH]SCN decreases from 0.1878 to 0.1690 (Pa s), respectively. Also, it was observed that optimum viscosity found to be at shear rate 250 (s−1). Similar trends were observed in the available literature.11,12,42 Viscosity of [BMTH]SCN decreases with respect to time at constant temperature (298.2 K), From Figure 6, it was observed that the optimum value of viscosity was obtained at 340 s.42 For thiazolium based ionic liquids cationic structures plays a key role in the viscosity. The extraction of organic product from aqueous media requires ILs with low solubility of water, since the higher the water solubility in ILs the lower their extractive potential. This is due to the intrinsic equilibrium competition between water and organic compounds under study for IL, which is directly
Figure 2. FTIR spectra of [BMTH]SCN.
The FTIR spectrum of the synthesized 3-butyl-4-methylthiazolium thiocyanate is shown in Figure 2. The aliphatic asymmetric and symmetric C−H stretching vibrations were observed 2950 and 3050 cm−1, respectively, and in-plane bending vibrations at 1380 cm−1 are due to C−H in methyl group. Strong bands at 2064 and 1425 cm−1 are due to C−N and CN stretching, respectively. The weak band at 810 cm−1 corresponds to C−S vibration stretching. The IR spectrum of [BMTH]SCN after extraction of sulfur shows similar vibrational bands. From Figure 2, it was observed that a synthesized IL is [BMTH]SCN. 2.6.2. 1H NMR and 13C NMR analyses of [BMTH]SCN. [BMTH]SCN was characterized by 1H NMR and 13C NMR using CDCl3 as solvent on a (Varian, U.S.A., Mercury plus 300 MHz for 1H NMR spectrometer) for the determination of molecular structures and conformations. The 1H NMR data was reported as follows in ppm (δ) from the internal standard (TMS, 0.0 ppm), and chemical shift (multiplicity, integration).The schematic details of NMR analysis are shown in Scheme 1. The results of 1H NMR and 13CNMR of [BMTH]SCN are as follows. 1 HNMR (300 MHz, CDCl3). δ (ppm) 9.257 (C2, 1H, t), 7.510 (C4, 1H, t), 7.289 (C5, 1H, t), 4.232 (C6, 2H, t), 2.48 (C7, 2H, m), 1.897 (C8, 2H, m), 1.394 (C9, 2H, m), 0.977 (C10, 3H, s). Spectra are shown in Figure 3. 13 CNMR (76 MHz, CDCl3). δ (ppm) 134.5 (NCS), 121.26, 126.5 (C5, d), 49.57 (C6, d), 31.86 (C7), 19.25 (C8), 13.27 (C9), 36.04 (C10). Spectra are shown in Figure 4. 19847
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Figure 3. 1H NMR spectra of [BMTH]SCN.
Figure 4. 13C NMR spectra of [BMTH]SCN.
anions. Novel two-phase system can be created and used for various applications such as synthesis and extraction.44 For the application of [BMTH]SCN extractant the solubility mechanism of IL is needed. [BMTH]SCN solubility in liquid fuel may give rise to extractant loss and liquid fuel contamination. These results suggest that the solubility of [BMTH]SCN in liquid fuel has to be optimized for future applications. 3.2. Effect of Reaction Time on Sulfur Removal. The extractions of model fuel (DBT in octane) with [BMTH]SCN were carried out for 5, 10, 20, 30, and 35 min at 30 °C with mass ratios of 5:1, 3:1, and 1:1 (Mass ratio of model liquid fuel to [BMTH]SCN as shown in Figure 7. The desulfurization process went quite quickly, and the S concentration in the
related to the IL hydrophobic nature. Hence, the knowledge of mutual solubilities between water and ILs is of major significance prior to their consideration for extractive applications. [BMTH]SCN comes up as a new alternative of extremely hydrophobic ILs, especially when large alkyl chain length is present, and thus, their use can be valuable for the extraction processes.43 Thus, solubility of [BMTH]SCN with six conventional solvents was studied. [BMTH]SCN may be dissolved in some conventional organic solvents such as methanol, acetonitrile, ethanol, acetone, and water, but not all the organic solvents (e.g., [BMTH]SCN not dissolved in ethyl acetate). The [BMTH]SCN solubility might be changed by changing the 19848
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Figure 5. Effect of shear rate on viscosity of [BMTH]SCN.
Figure 7. Removal of sulfur from model fuel [BMTH]SCN at different extraction time. (Temperature = 30 °C, extraction time = 5−35 min, initial S concentration = 500 ppmw)
also reported in other ILs that extraction process for [BMTH]SCN was attributed for higher polarizable π-electron density of DBT, which tends to insert the molecular structure of ILs.46 DBT extraction with [BMTH]SCN is recognized to the π−π interaction between the aromatic ring of thiazolium and thiophenic ring of DBT.47 The sulfur partition coefficient (KN) is used to evaluate the extractive desulfurization performance. KN is based on gravity, which is defined as ratio of S content in IL to S content in model fuels. KN measurements were carried out in both model fuel and real fuel (gasoline and diesel). The DBT concentration in model fuel and [BMTH]SCN was directly determined using X-ray fluorescence spectrophotometer. Partition coefficient values of [BMTH]SCN with good DBT removal are shown in Figure 8. 3.3. Effect of Reaction Temperature on Sulfur Removal. Reaction temperature plays vital role in the extractive desulfurization process, Figure 9 shows the effect of
Figure 6. Effect of time on viscosity of [BMTH]SCN at constant temperature (298.2 K).
model fuel decreased with increased extraction time and was reduced from 500 to 220 ppmw (S removal 56%), 158.5 ppmw (S removal 68.3%), and 124 ppmw (S removal 75.2%) with mass ratios of 5:1, 3:1, and 1:1, respectively, in 20 min. However, S concentration decreased continuously with increased in extraction time and reduced from 500 to 197.5 ppmw (S removal 60.5%), 115 ppmw (S removal 77%), and 94 ppmw (S removal 81.2%) with mass ratios of 5:1, 3:1, and 1:1, respectively, in 30 min. No further reduction was observed in the S removal as equilibrium achieved at 30 min.11,12 At the initial stage of the reaction, S content in the model fuel was very high hence the extraction rate becomes high with high S removal rate. As the reaction proceeds, the extraction rate becomes low, with S removal rate no longer distinctly increaseing. The results, in Figure 7, show that viscous ILs took 30 min contact time to achieve liquid−liquid equilibrium between the model liquid fuel and [BMTH]SCN phase. So, the optimum time required for the desulfurization of model fuel was 30 min. [BMTH]SCN is more capable of efficiently extracting DBT than other S containing compounds.45 This observation was
Figure 8. Partition coefficient (KN) of sulfur removal from model fuel using [BMTH]SCN at different extraction time. (Temperature = 30 °C, extraction time = 5−35 min, initial S concentration = 500 ppmw) 19849
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Figure 9. Removal of sulfur from model fuel using [BMTH]SCN at different extraction temperature (°C). (Temperature = 20−40 °C, extraction time = 30 min, initial S concentration = 500 ppmw)
Figure 10. Partition coefficient (KN) of sulfur removal from model fuel using [BMTH]SCN at different extraction temperature (°C). (Temperature = 20−40 °C, extraction time = 30 min, initial S concentration = 500 ppmw)
the reaction temperature (20 °C, 25 °C, 30 °C, 35 °C, 40 °C) on the removal of sulfur. As shown in Figure 9, with the increasing reaction temperature from 20 to 30 °C, the removal efficiency of sulfur increases initially and then decreased. This effect may be attributed to the fact that when the reaction temperature was less than 30 °C, as temperature increases the viscosity of [BMTH]SCN reduced and the flexibility of [BMTH]SCN was also improved, which may be from the viscous flow layer. Thus, DBT removal efficiency in the model fuel by IL increased. When temperature exceeds 30 °C, the flexibility of [BMTH]SCN is not noticeably improved. Moreover, S removal rate will no longer increase and will even, to some extent, decline.48 Therefore, [BMTH]SCN was best suitable to remove DBT reflected at 30 °C, which was taken as the optimal reaction temperature. The S content of the model fuel decreased from 500 to 230.5 ppmw (53.9% S removal), 165 ppmw (67% S removal), and 130 ppmw (74% S removal) with mass ratios of 5:1, 3:1, and 1:1, respectively, as shown in Figure 9. Significant drop in S removal was observed when reaction temperature reached 40 °C, and the S removal was only 28.1%, 40%, and 45.3% with mass ratios of 5:1, 3:1, and 1:1, respectively. Insensitivity to temperature was also observed in other extraction systems such as [BPy]BF4, [(CH2)4SO3HMIM]Tos, [BMIM]BF6, and [BMIM]PF6.35 Subsequently, sulfur extraction may be performed at room temperature, which is encouraging for less energy consumption. Partition coefficient values of [BMTH]SCN by varying temperatures with good DBT removal are shown in Figure 10. 3.4. Effect of S Compounds on Sulfur Removal. Table 1 represents the molecular structures and properties of S compounds normally found in real fuels such as diesel and gasoline on extraction with pure hydrocarbons. It might be seen that results for thiols, sulfides, and related compounds are quite low.35 However, the results for DBT, 4-MDBT, 4,6-DMDBT, T, BT, and 3-MT are excellent. The most likely mechanism for the extraction of S compounds with [BMTH]SCN is the formation of liquid clathrates and π−π interactions between aromatic structures of the extraction target and the thiazolium ring system.35,12 In real fuels, many nitrogen, oxygen, and aromatic compounds exist, which decreases the extraction performance
of [BMTH]SCN for S containing compounds. In real diesel, there were different kinds of alkyl substituted DBTs present such as 4-MDBT, BT, 4,6-DMDBT, T, and 3-MT. The removal of DBT reached 77% in 30 min. However, the removal of 4MDBT, 4,6-DMDBT, BT, T, and 3-MT was only 70%, 67.9%, 66.3%, 63.9, and 60% within 30 min, respectively, as shown in Table 1. Compared with that of DBT, the electron density of the sulfur atom on 4-MDBT, 4,6-DMDBT, BT, T, and 3-MT is lower, which leads to the lower reactivity of S compounds. However, the reactivity of the DBT decreased with increasing methyl substitutes at the derivative substitute positions; the reactivity sequencing was DBT > 4-MDBT > 4,6-DMDBT > BT > T > 3-MT.49,50 3.5. Recycling of Spent [BMTH]SCN without Regeneration. In practical processes, considering the high cost of ILs recycling or regeneration process is needed. The S extraction performance of [BMTH]SCN without regeneration was investigated, and the results are shown in Figure 11. It is observed that the desulfurization efficiency of [BMTH]SCN without regeneration was reused up to five cycles. It was seen that the spent [BMTH]SCN was able to remove DBT from liquid fuel even without regeneration, although at a lower efficiency of 36.1% from 60.5%, 52% from 77%, and 55.7% from 81.2% with mass ratios of model fuel to IL as 5:1, 3:1, and 1:1, respectively, with spent [BMTH]SCN. Reduction of S removal might be attributed to DBT, which dissolved in [BMTH]SCN and decreased the extraction performance of [BMTH]SCN. The results indicated that after the [BMTH]SCN was recycled five times, the rate of S removal decreases slightly.50,33 3.6. Effect of Ultrasonication on Sulfur Removal Using [BMTH]SCN. In ultrasonication experiments, an ultrasonic bath is used as an ultrasonic source. The ultrasonic cleaner microcleaner-103 (OSCAR ultrasonic Pvt. Ltd. Mumbai, India) is a rectangular container (23.5 cm × 13.3 cm × 10.2 cm), to which 50 kHz transducers were annealed at the bottom. The bath power rating is 250 W on the scale of 0−100%. The extraction of DBT is performed by adding specific amount of [BMTH]SCN solutions in a 50 mL flask. The flask is then partially immersed into the ultrasonic bath, which contains 2.5 L water. The water in the ultrasonic bath is circulated and regulated at the desired temperature to maintain the water 19850
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Table 1. Effect of S Compounds on Extraction with [BMTH]SCNa
a
Temperature = 30 °C, mass ratio of model fuel/IL = 3:1, extraction time = 30 min, initial S concentration = 500 ppmw.
ultrasonic treatment and the extraction efficiency of S removal are systematically studied. Table 2 shows that the maximum S Table 2. Effect of Ultrasonication on Sulfur Removal Using [BMTH]SCNa run
ultrasonic power (W)
time (min)
S content (ppmw)
temp. (°C)
S removal (%)
1 2 3 4
250 250 250 250
5 10 20 30
69.1 75.3 79.5 88.5
25 30 35 40
67.2 77.1 64.5 58
Temperature = 25−40 °C, mass ratio of model fuel/IL = 1:1, extraction time = 5−30 min, initial S concentration = 500 ppmw.
a
removal was 88.5% when the ultrasonication time was 30 min. Hence, 30 min is the optimum ultrasonication time required to achieve highest sulfur removal. However, S removal was 77.1%, when the ultrasonication temperature reached at 30 °C. Hence, it was also noted that 30 °C is the optimum ultrasonication temperature to achieve highest S removal.51−53 3.7. Desulfurization of Real Fuels using [BMTH]SCN. Real fuels extraction such as diesel and gasoline is much more difficult due to its typical content of various S compounds and other impurities. The results of extractive desulfurization of diesel and gasoline with thiazolium ILs are also promising.
Figure 11. Recycling of spent [BMTH]SCN without regeneration. (Model fuel = (Octane + DBT), temperature = 30 °C, extraction time = 30 min, initial S concentration = 500 ppmw).
temperature at a constant value and prevent it from being influenced by the ultrasonic exposure. After ultrasonication process, the model fuel is then cooled to the room temperature. The optimum ultrasonic time for 19851
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multiple extractions are effective for the reduction of S content of liquid fuel to considerably negligible amount. Similar results were reported in the literature.36,48
[BMTH]SCN display high S removal capability from gasoline and diesel in single stage extraction in 30 min at 30 °C with mass ratios of 5:1, 3:1, and 1:1, as shown in Table 3.
■
CONCLUSION [BMTH]SCN can be used for extractive desulfurization of liquid fuels, mainly with regards to those S compounds that are very complicated to eliminate by common HDS process. [BMTH]SCN is the most efficient in the removal of DBT containing liquid fuels, and it can reach to 81.2% for single stage extraction at 30 °C in 30 min with mass ratio 1:1, which is the noteworthy progress of EDS over HDS process. The data and results of the presented work might be providing significant insights of thiazolium based ILs. Thus, the EDS method could be developed into a simple, mild, and environmentally benign method for deep desulfurization.
Table 3. Desulfurization of Diesel and Gasoline with [BMTH]SCNa diesel/IL (mass ratio)
S content (ppmw)
S removal (%)
gasoline/IL (mass ratio)
S content (ppmw)
S removal (%)
5:1 3:1 1:1
359.4 310.6 240
40.1 48.2 60
5:1 3:1 1:1
40.4 26.8 20.8
55.1 70.2 76.9
a Temperature = 30 °C, extraction time = 30 min, initial S concentration of diesel and gasoline = 600 and 90 ppmw.
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[BMTH]SCN exhibits the best sulfur extraction ability for S removal in gasoline which was reduced from initial sulfur of 90 to 40.4 ppmw (55.1% S removal), 26.8 ppmw (70.2% S removal), and 20.8 ppmw (76.9% S removal) with mass ratios of 5:1, 3:1, and 1:1 in single stage extraction, respectively. However, in diesel, it was reduced from initial sulfur of 600 to 359.4 ppmw (40.1% S removal), 310.6 ppmw (48.2% S removal), and 240 ppmw (60% S removal) with mass ratios of 5:1, 3:1, and 1:1 in single stage extraction, respectively. Diesel and gasoline contains more heteronuclear compounds than the model fuel, such as nitrogen and sulfur containing compounds (alkylthiophene, benzothiophene), which decrease the ability of [BMTH]SCN for S removal. Because of steric effect of alkyl group in the aromatic rings, methyl-thiophene, methylbenzothiophene, methyl-dibenzothiophene, etc., S containing compounds in gasoline and diesel are extracted less than DBT in the model fuel by ILs.36,48 3.8. Multistage Extraction Using [BMTH]SCN. Although a high S removal by [BMTH]SCN is obtained as shown in Table 3, the final S content in fuels cannot meet the definite requirement of low sulfur fuels (e.g.,
