ARTICLE pubs.acs.org/IECR
Deep Desulfurization of Fuel Oils Using Low-Viscosity 1-Ethyl-3-methylimidazolium Dicyanamide Ionic Liquid Guangren Yu, Xi Li, Xinxing Liu, Charles Asumana, and Xiaochun Chen* College of Chemical Engineering, Beijing University of Chemical Technology, 100029 Beijing, People's Republic of China ABSTRACT: In this work, the extractive and oxidative deep desulfurizations of model fuel oils using a low-viscosity ionic liquid, i.e., 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][N(CN)2]), are investigated. [C2mim][N(CN)2] is capable of effectively extracting thiophene (TS) and dibenzothiophene (DBT) from oils. The sulfur content in the raffinate phases is only ∼10 ppm after a few extraction steps. A short extraction equilibrium time of 300 C) and high pressure (3-10 MPa); it is also less effective for removing some condensed heterocyclic S-compounds such as thiophene (TS) and dibenzothiophene (DBT) or their derivatives because of the sterically hindered adsorption of these compounds on the catalyst surface.9 Therefore, some alternative deep desulfurization methods are desired. Recently, studies on deep desulfurization of fuel oils are being focused on ionic liquid (IL) technology, where S-compounds are removed through direct extraction by ILs10-32 (extractive desulfurization, EDS) or oxidization of S-compounds after their extraction into IL phase (oxidative desulfurization, ODS).33-43 ILs are a new class of organic salts that are entirely composed of organic cations and organic/inorganic anions, and have been studied intensively as green solvents in the past few decades.44-48 With some desirable properties such as nonvolatility, excellent solubility for organic/inorganic compounds, good thermal/ chemical stability, nonflammability, recyclability, and environmental friendliness,44,46,47 ILs are suitable for deep-desulfurization processes. The performance in EDS of some ILs that are composed of anions such as [BF4]-, [PF6]-, [AlCl4]-, and [EtSO4]- and popular cations of imidazolium and pyridinium has been investigated, and the results are good (e.g., an undetectable solubility r 2010 American Chemical Society
of ILs in fuel oils, ILs regeneration through a simple distillation, high partitioning for S-compounds in ILs, reusability of ILs, etc.).10-32 However, some ILs employed in previous studies, such as those with fluorinated anions and sensitive to air/ moisture, are not desirable, and to make matters worse, they generally have very high viscosities at room temperature (as shown in Table 1). Obviously, high viscosity gives rise to some problems in practical applications such as high-power requirement and difficult handling in transportation, dispersion, decantation, filtration, etc. Moreover, it reduces the mass transfer rate in liquid-liquid extraction systems and prolongs the equilibrium time. Similarly, high viscosity is not desired in the ODS process.37-40 In this work, a fluorine-free, air/moisture stable, and low-viscosity dicyanamide-based IL, 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][N(CN)2]), is investigated to directly extract TS/DBT and oxidatively remove DBT from model fuel oils. The viscosity of [C2mim][N(CN)2] is only 21 cP at 25 C.49 The chemical structure is shown in Figure 1. In section 3.3.2, ab initio calculations are incorporated to shed light on the desulfurization mechanism from the molecular viewpoint.
2. EXPERIMENTS AND CALCULATIONS 2.1. Chemicals. The chemicals and suppliers are the following: N-methylimidazole (>99%) and silver nitrate (>99%), Shanghai SenHao Fine Chemical; iodoethane (>98%), Alfa Aesar; sodium dicyanamide (>95%), TNT Chemical; TS (>99%) and DBT (>98%), J&K Chemical; analytical-grade 1,1,1-trichloroethane, ethyl acetate, acetonitrile, dichloromethane, n-hexane, toluene, and acetic acid (CH3COOH), along Received: June 18, 2010 Accepted: December 14, 2010 Revised: December 13, 2010 Published: December 30, 2010 2236
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Table 1. Some Ionic Liquids Used in Extractive Desulfurization or Oxidative Desulfurization and Their Viscosities ionic liquid
abbreviation
temp (K)
viscosity (cP)
ref
1-ethyl-3-methylimidazolium ethyl sulfate
[C2mim][EtSO4]
298.15
100.4
10
1-butyl-3-methylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium tetrafluoroborate
[C4mim][ PF6] [C4mim][BF4]
298.15 298.15
273.0 103.5
21, 33 10, 33, 41, 42 27
1-butyl-3-methylimidazolium ethanoate
[C4mim][CH3CO2]
298.15
139.7
1-butyl-3-methylimidazolium chloride
[C4mim][Cl]
298.15
3950.0
10
l-butyl-3-methylimidazolium methyl sulfate
[C4mim][MeSO4]
298.15
167.2
10
l-butyl-3-methylimidazolium methyl sulfate
[C4mim][OcSO4]
298.15
874.5
10
1-octylimidazolium tetrafluoroborate
[C8mim][BF4]
298.15
222.8
10
1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide
[C8mim][NTf2]
298.15
95.0
1-butyl-pyridinium tetrafluoroborate 1-butyl-3-methylpyridinium tetrafluoroborate
[C4py][BF4] [C43mpy][BF4]
298.15 298.15
145.2 128.1
1-butyl-4-methylpyridinium tetrafluoroborate
[C44mpy][BF4]
298.15
200.7
27
N-octylpyridinium tetrafluoroborate
[C8py][BF4]
298.15
233.5
24
28 27, 29 27
1-benzylpyridinium tetrafluoroborate
[BePy][BF4]
298.15
240.9
17
1-octyl-3-methylpyridinium tetrafluoroborate
[C83mPy][BF4]
298.15
505.5
30
1-hexyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide
[C6mmpy][NTf2]
298.15
104.0
43
Figure 1. 1-Ethyl-3-methylimidazolium [N(CN)2].
dicyanamide,
[C2mim]-
with hydrogen peroxide (H2O2, aqueous solution, 30%), Beijing Chemical Plant. N-Methylimidazole and acetonitrile are further purified by distillation, and the other chemicals are used as received without further purification. 2.2. Preparation of [C2mim][N(CN)2]. [C2mim][N(CN)2] is prepared by the following route, which has been described in detail in the literature.49
2.3. Extractive and Oxidative Desulfurizations. Model gasoline is prepared by dissolving a certain amount of TS in a solution of n-hexane (85 wt %) and toluene (15 wt %), while model diesel is prepared by dissolving a certain amount of DBT in n-hexane; the S-content in both model oils is ∼500 ppm (here, ppm refers to weight ratio). In a typical experiment, IL and model oil are added into a 50 mL round-bottomed flask, and the mixture is magnetically stirred at a fixed time and temperature. After the extraction equilibrium, the mixture is settled for 5 min to obtain phase splitting; the upper oil phase is measured by highperformance liquid chromatography (HPLC) to determine sulfur or toluene content. Here, a settling time of 5 min is enough for a complete separation of oil and IL, as will be discussed below. In oxidative desulfurization, after attainment
of extraction equilibrium, CH3COOH and H2O2 are separately placed into the system, and after a desired time for oxidation, the S-content is analyzed by HPLC. 2.4. IL Regeneration. IL is recovered by successive dilutions with water, followed by a simple distillation. The regenerated IL is finally dried under vacuum at 60 C for 24 h to remove trace quantities of water. The regenerated IL is identified by Fourier transform infrared (FT-IR) and 1H NMR characterization. 2.5. Mutual Solubility between IL and Model Oils. IL and model oils (the mixture of n-hexane and toluene) are placed into a 50 mL round-bottomed flask, magnetically stirred, and allowed to settle. After phase equilibrium and splitting, the top oil phase is analyzed by HPLC to determine the solubility of IL in oils, and the bottom phase is measured by using the gravimetric method to determine the solubility of oils in IL. 2.6. Analysis. The prepared IL is characterized by 1H NMR (Bruker AV600 MHz, Germany) and FT-IR (Thermo Electron, NEXUS8700, USA). The results are as follows. 1 H NMR, (DMSO-d6, ppm): δ 1.416 (t, CH3), 3.842 (s, N-CH3), 4.192 (q, N-CH2), 7.661 (s, CH), 7.746 (s, CH), 9.074 (s, N-CH-N). IR (KBr, cm-1): 3151.1 (s), 3107.1 (s), 2989.1 (s), 2235.5 (vs), 2196.0 (vs), 2134.8 (vs), 1637.3 (w), 1573.0 (s), 1452.6 (m), 1427.1 (w), 1388.5 (w), 1312.2 (s), 1170.3 (s), 1089.6 (w), 1029.8 (w), 958.4 (w), 904.9 (w), 844.7 (w), 802.2 (w), 754.0 (m), 701.8 (w), 648.1 (m), 622.5 (s). The content of TS, DBT, and toluene are analyzed by HPLC (WUFENG LC-100, China, reversed phase Ultimate XB-C18 column, 4.6 mm 150 mm, 3.6 μm, UV detector) with external standard method. The selected wavelength is 230 nm for TS, 262 nm for toluene, and 280 nm for DBT. For TS and toluene, the mobile phase is 65% methanol in water (v/v, %) with a flow rate of 0.9 mL/min; for DBT, it is 90% methanol in water (v/v, %) with a flow rate of 1.0 mL/min. The solubility of IL in model oils is analyzed using HPLC at 295 nm. 2.7. Calculations. All the quantum mechanical calculations are performed with the Gaussian 03 software package.50 The density functional theory method B3LYP and frozen-core second-order perturbation approximation Møller-Plesset (MP2) are used to optimize geometric structure and do a potential 2237
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Figure 2. Solubility of model oils in [C2mim][N(CN)2] at different toluene contents.
ARTICLE
Figure 4. Sulfur content in raffinate oils versus settling time (temperature, 25 C; mass ratio of IL:oil = 1:1; extraction time, 20 min; initial S-content, 525 ppm for TS and 566 ppm for DBT).
Figure 3. Photograph of model oils and [C2mim][N(CN)2] after settling for 5 min.
energy surface (PES) scan, combined with the 6-31g* basis set.51-54 The zero point energy correction (ZPE) and frequency calculation are performed at the respective theoretical level, where the latter is used to specify the nature of each obtained structure, i.e., a true minimum or a transition state.
3. RESULTS AND DISCUSSION 3.1. Mutual Solubility between IL and Model Oils. Mutual
solubility is an important factor in assessing the applicability of IL extractants;10,12-15,18,20,24,30 e.g., solubility of ILs in fuel oils may lead to contamination of the fuel oils and give rise to extractant loss. The solubilities of [C4Py][BF4], [C6Py][BF4], [C83MPy][BF4],24,30 [C4mim][OcSO4], [C2mim][EtSO4],10 [C2mim][DEP], [C4mim][DBP], and [C2mim][DMP]13,15 in fuel oils have been demonstrated to be negligible. The HPLC result indicates the solubility of [C2mim][N(CN)2] in model oils is undetectable. The solubility of model oils in IL is shown in Figure 2. As indicated in Figure 2, model oil is slightly soluble in IL, especially in the case of low-content toluene (which almost perfectly simulates real fuel oil where the content of aromatics is generally less than 30 wt %); the solubility increases with the mass fraction of toluene in model oils. 3.2. EDS Process of Model Gasoline. In this section, the EDS performance, along with factors affecting extraction such as temperature, mass ratio of IL:oil, selectivity, multiple extraction, and reusability, is discussed. 3.2.1. Settling Time. Settling time is an important factor to see the technical relevance of this extraction medium. A photograph for the mixture of oils and IL at a settling time of 5 min, in which the two phases are clearly formed without any emulsion, can be seen in Figure 3. The extraction data at different settling times are presented in Figure 4, depicting that the S-content in the raffinate phases of both oils attained a constant value after only 2 or 3 min settling time; hence, a settling time of 5 min is enough and accepted in all the extraction experiments.
Figure 5. Sulfur content in model gasoline versus extraction time.
3.2.2. EDS Performance. The EDS result for the extraction of TS is shown in Figure 5, where it can be seen that [C2 mim][N(CN)2] presents a good extractive performance for the removal of TS from model gasoline, and the S-content is significantly reduced from an initial 527 ppm to 319 ppm in a single extraction, a S-removal efficiency of 41.7%. The S-removal efficiency of [C2mim][N(CN)2] is compared with those of other ILs (Table 2). An interesting observation is that an extraction equilibrium is almost completely reached after only 5 min. Such a short equilibrium time is very necessary to generate high production yield or meet the requirement of small-volume equipment in industrial applications. The short extraction time can be attributed to the low viscosity, which facilitates better dispersion of IL in oils and accelerates the rate of mass transfer. In the following discussion, the desulfurization time is fixed at 20 min, a more than sufficient contact time to establish extraction equilibrium. 3.2.3. Temperature. Five sets of parallel experiments at varying temperatures (15, 25, 35, 45, and 55 C) are performed to investigate the effect of extraction temperature on S-removal efficiency. The results in Figure 6 indicate that the S-removal efficiency is not sensitive to temperature, which can be ascribed to the low viscosity of the IL. Therefore, extraction desulfurization can be performed at mild conditions, e.g., at or around room temperature. The S-removal efficiency is slightly decreased with an increase in temperature, e.g., the S-removal efficiencies are 41.5 and 36.2% at 15 and 55 C, respectively. A high efficiency of 40.7% is obtained at room temperature (25 C) 2238
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Table 2. TS Removal from Model Oils by Extraction with Ionic Liquids S-removal (%) for IL:model oil IL
model oil a
[C4Py][BF4]
[C2Py][BF4]a
temp (K)
extraction time (min)
1:1
1:3
n-heptane, xylol
room temp
30-40
45.5
16.9
n-heptane, xylol
room temp
30-40
17.9
4.9
323
30-40
21.8
8.9
41.7
16.2
[C2mim][N(CN)2]b
n-hexane, toluene
298
5
[C4mim][PF6]c
n-octane, toluene
298
30
[C2mim][BF4]d
n-octane, toluene
303
10
29.3
[C4mim][BF4]d
n-octane, toluene
313
50
38.8
[C6mim][BF4]d [C8mim][BF4]d
n-octane, toluene n-octane, toluene
323 333
40 30
38.6 39.5
[C10mim][BF4]d
n-octane, toluene
343
20
42.5
1:5
8.9 5
a
From ref 25; initial S-content is 498 ng/μL, IL:model oil is volume ratio. b In this work; initial S-content is 579 ppm, IL:model oil is mass ratio. c From ref 31; initial S-content is 1500 ppm, IL:model oil is volume ratio. d From ref 32; initial S-content is 1500 ppm, IL:model oil is volume ratio.
Figure 6. Sulfur-removal efficiency versus extraction temperature. Figure 8. Multiple extraction desulfurization of model gasoline by [C2mim][N(CN)2].
Figure 7. Sulfur content in model gasoline versus mass ratio of [C2mim][N(CN)2]:model gasoline.
for [C2mim][N(CN)2], while the S-removal efficiency of highviscosity ILs (e.g., [C2Py][BF4],25 [C4mim][PF6]24,31) showed a sensitivity to temperature. 3.2.4. Mass Ratio of IL:Oil. The effect of mass ratio of IL:oil is shown in Figure 7. As shown in Figure 7, S-removal efficiency decreases with decreasing mass ratio of IL:oil; e.g., the S-content in the raffinate phase is 519 ppm at 1:5 (w/w) IL:oil, while it is 347 ppm (41.7% S-removal efficiency) at 1:1 (w/w) IL:oil. When Chu et al.17 used a series of [BF4]--based imidazolium and pyridinium ILs to investigate the influence of mass ratio at 20 min extraction time and 1:1 (w/w) IL:oil, a lower S-removal efficiency of less than 40% was realized.
3.2.5. Multiple Extraction. As indicated above, it is difficult to reduce the S-content to a permitted level (e.g.,
