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DOI: 10.1039/C5RA27266A (Paper) RSC Adv., 2016, 6, 30345-30352

A simple and cost-effective extractive desulfurization process with novel deep eutectic solvents†

Xin Wanga, Wei Jiangb, Wenshuai Zhu*a, Hongping Lia, Sheng Yina, Yonghui Changac and Huaming Li*b
aSchool of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China. E-mail: zhuws@ujs.edu.cn; Fax: +86-511-88791708; Tel: +86-511-88791800
bInstitute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, P. R. China. E-mail: lihm@ujs.edu.cn
cSchool of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, P. R. China

Received 20th December 2015 , Accepted 10th March 2016

First published on 14th March 2016

Abstract

Deep eutectic solvents (DESs), a new class of ionic liquid (IL) analogues, are easily produced through mixing Lewis or Brønsted acid and base. In this study, a class of DESs was prepared by mixing low cost triethylamine and organic acid with different molar ratios. It was found that the base/acid molar ratio (B/A) played an outstanding role in the solubility and extractive ability of DESs. When B/A was 1[thin space (1/6-em)]:[thin space (1/6-em)]3 and 1[thin space (1/6-em)]:[thin space (1/6-em)]5, the loss of DESs in model oil was even less than 0.003%. Take the DES with B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]3 for example, the extraction ability of DESs showed the following order [TEtA][Pr] > [TEtA][Ac] > [TEtA][Fo], which could be explained by 1H NMR analysis. The extractive mechanism was also discussed by density functional theory (DFT) calculations. The sulfur partition coefficient (KN) was 2.14 using [TEtA][Pr] as the extractant, and the sulfur content could be reduced from 500 ppm to 10 ppm after four times extraction.

1. Introduction

Air pollution has been a major worldwide concern. As one of the main components of air pollution, SOx can be released during fuel combustion. In this regard, new regulations and stricter sulfur level requirements are continuously being promulgated to limit the sulfur content in diesel fuels by most countries. Hydrodesulfurization (HDS) is a traditional method that is widely used for the removal of sulfur compounds in oil refineries around the world, where high temperature (>573 K) and high pressure (>3 MPa) are required.1,2 Besides some aromatic heterocyclic sulfur compounds, such as dibenzothiophene (DBT) and its derivatives, are tough to be removed by HDS due to their high steric hindrance. Thus, the development of non-HDS process, such as adsorption, extraction, oxidation and biodesulfurization, has become a hot topic.3–10 Among those selective desulfurization techniques, extractive desulfurization (EDS) seems to be the most attractive method because of its moderate and convenient operation conditions.11–15 However, the traditional extractants used for the desulfurization process are conventional molecular solvents, which will make a great compact on the environment because of their volatile feature and high toxicity. Therefore, it is necessary to seek new environmentally friendly extractants for the desulfurization process.

Ionic liquids (ILs) have attracted increasing interest over the last few decades.16–18 They are usually regarded as promising replacements for traditional volatile organic solvents due to their unique properties, such as non-flammability, high thermal stability, negligible vapor pressures, and excellent solubility.19–23 Extractive desulfurization with ILs was first reported by Bösmann et al. in 2001.1,24 Following this initial report, many reports about extractive desulfurization by new ILs have been emerged and reviewed recently.25–27 As the review described, ILs could be categorized according to their types of cation, anion or acidic. However, most of the reported ILs for extractive desulfurization are imidazolium- or pyridinium-based ILs,28,29 such as [Bmim]Cl/2FeCl3,11 [BPy][BF4],12 [MMIM][DMP],30,31 [BMI][N(CN)2],4 and [EMIM][SCN].32 Their complex synthetic procedures and high cost are the main problems in practical industrial applications.

Deep eutectic solvents (DESs), a new class of ILs analogues, may solve the problems. They are usually obtained through combination of a hydrogen-bond donor (HBD) and a hydrogen-bond acceptor (HBA) in principle.33–36 They present similar physical and chemical properties to ILs and have the advantage of being cost-effective and easily prepared without residual by-products. Up to now, DESs have been used as efficient green solvents in a variety of applications such as catalysis, extraction, electrochemistry and biochemistry.37–39 In this work, a class of DESs has been prepared by mixing triethylamine and organic acid with different molar ratios in order to extract DBT from model oils. Interestingly, the solubility of DESs in model oil was tuned by changing the acid and base molar ratio and was studied in detail to found an ideal extractant. Then some important factors, such as initial sulfur concentration, mass ratio of DESs/fuel, temperature, and multiple extractions, which influence the extractive desulfurization performance were investigated. Moreover, the extraction mechanism was also discussed by 1H NMR and density functional theory (DFT) calculations.

2. Experimental

2.1. Preparation of the DESs and model oil

The DESs were made from the following chemicals: formic acid (>98%), acetic acid (99.5%), propionic acid (>99.5%), n-butyric acid (99%), n-pentanoic acid (>99%), and triethylamine (99%). And the typical procedure was as follows. The reaction was carried out under an inert atmosphere (Ar). Triethylamine (30 mL) was loaded into a three-necked flask equipped with a magnetic stirrer and cooled using an ice bath. Organic acid with different base/acid molar ratios (B/A) (1[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]2, 1[thin space (1/6-em)]:[thin space (1/6-em)]3, 1[thin space (1/6-em)]:[thin space (1/6-em)]5, 1[thin space (1/6-em)]:[thin space (1/6-em)]7, and 1[thin space (1/6-em)]:[thin space (1/6-em)]9) was then added dropwise to the three-necked flask while stirring rapidly. The DESs could be obtained after the reaction mixture lasted for 24 h at room temperature. For formic acid and acetic acid with B/A = 1, no homogeneous products can be obtained, leading to unsuccessful preparation of DESs at B/A = 1. Other DESs with different B/A molar ratio could be obtained. Then all of the DESs were stored in a dry nitrogen atmosphere.

The model oil was prepared by dissolving dibenzothiophene (DBT), benzothiophene (BT), 4,6-dimethyldibenzothiophene (4,6-DMDBT) and 1-dodecanethiol (RSH) in n-octane and the initial sulfur content were 500, 250, 250 and 250 ppm, respectively, where tetradecane was added as internal standard.

2.2. Characterization

The structure of DESs was characterized by positive electrospray ionization mass spectra (ESI-MS) and 1H nuclear magnetic resonance (NMR) spectra (see the ESI†). Viscosities were measured in a DV-S digital display viscosimeter with an accuracy of 1%. Densities were determined by EDM 4000/5000 with an estimated precision of 5× ±10−5 g cm−3. Water contents of DESs were obtained by Karl Fisher method (Metrohm 870) with an accuracy of less than 0.2%.

2.3. Extractive desulfurization

The extraction experiments were carried out in a self-made equipment with a certain amount of DES and 5 mL model oil. Then the mixture was stirred at 30 °C for 10 min until the extractive equilibrium was achieved. All of the experiments in this study were carried out at least in triplicate to determine reproducibility. The sulfur content of the model oil was detected by the gas chromatograph (GC-FID) (Agilent 7890A, HP-5 column, 30 m long × 0.32 mm inner diameter (id) × 0.25 μm film thickness). The sulfur partition coefficient (KN) based on mass concentration is used to evaluate the extractive desulfurization performance.

Different DESs were abbreviated as triethylammonium formate ([TEtA][Fo]), triethylammonium acetate ([TEtA][Ac]), triethylammonium propionate ([TEtA][Pr]), triethylammonium butanoate ([TEtA][Bu]) and triethylammonium pentanoate ([TEtA][Pe]). The abbreviation represented a kind of DESs but did not mean the equal molar ratio of base and acid.

2.4. Solubility of DESs in model oil

The loss of DESs after mixing with model oil was measured by a tube in a graduate to the nearest 0.2 mL. But when the solubility was less than 10 wt%, the sample of the model oil after extraction was analyzed by gas chromatograph.

3. Result and discussion

3.1. Characterization of the DESs

1H NMR and ESI-MS are usually used to characterize the structure of DESs (base/acid molar ratio was 1[thin space (1/6-em)]:[thin space (1/6-em)]3) and listed in ESI (Fig. S1–S6†). Positive ESI-MS analysis showed the same cation of DESs, indicating the existence of ionic species.

Viscosity, density and water content of DESs were measured by the corresponding instruments. As shown in Table 1, the viscosities of DESs ranged from 25.5 to 44.5 cp, which were much lower than the ILs in the literatures.4,15,40,41 The low viscosity will be beneficial to the practical industry application, such as transportation, dispersion and liquid–liquid equilibrium time. The densities decreased a little with increasing alkyl chain length of acids. The water contents of all the DESs were less than 0.5 wt%, which was acceptable due to the lack of dehydration process during preparation of DESs.

Table 1 Viscosities, densities and water contents of DESs at 298.15 K and atmospheric pressure
DESs Base/acid molar ratio η (cp) ρ (g cm−3) Water content (%)
[TEtA][Fo] 1[thin space (1/6-em)]:[thin space (1/6-em)]3 25.5 1.0405 0.46
1[thin space (1/6-em)]:[thin space (1/6-em)]5 26.0 1.1007 0.47
[TEtA][Ac] 1[thin space (1/6-em)]:[thin space (1/6-em)]3 39.5 1.0038 0.47
1[thin space (1/6-em)]:[thin space (1/6-em)]5 28.0 1.0366 0.45
[TEtA][Pr] 1[thin space (1/6-em)]:[thin space (1/6-em)]3 44.5 0.9790 0.48
1[thin space (1/6-em)]:[thin space (1/6-em)]5 30.0 0.9922 0.50

3.2. Solubility of DESs in model oil

The solubility of DESs in model oil is considered as an important factor in choosing an extractant because conspicuous dissolution of DESs in fuel oil causes loss of the DESs and contamination of the fuel oils. DESs are generally mixture including the molecular acid and base that were likely to dissolve in model oil. It is interesting to found the solubility of the prepared DESs in model oil varied with the different base/acid (B/A) molar ratios. As shown in Table 2, [TEtA][Fo], [TEtA][Ac], and [TEtA][Pr] with B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]3 or 1[thin space (1/6-em)]:[thin space (1/6-em)]5 and [TEtA][Fo] (B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]2) could be used as extractants due to their low solubility. The solubility of the DESs [TEtA][Bu], [TEtA][Pe] in model oil was always so great that the two DESs were not suitable to be used as extractants in extractive desulfurization. [TEtA][Fo] and [TEtA][Ac] with B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]1 could not be obtained as described in Experimental section. In addition, the solubility of [TEtA][Pr] (B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]1), [TEtA][Pr] (B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]7) and [TEtA][Pr] (B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]9) in model oil were also great and reached 42.9%, 25.0% and 33.5%, respectively. To sum up, the [TEtA][Fo], [TEtA][Ac], and [TEtA][Pr] (B/A = 3) are selected as extractants to evaluate extractive performance due to their high extractive ability and relatively low solubility. In addition, the extractive performance of DES [TEtA][Pr] (B/A = 5) was also tested in ESI† due to its ignorable solubility in model oil.
Table 2 Extractive desulfurization and solubility of model oil and DESsa
DESs Base/acid molar ratio The content dissolved in oil (%) Sulfur removal (%) Standard error (%)
a Experimental conditions: DES = 1.75 g, model oil = 5 mL, T = 30 °C, t = 10 min.b The DESs were not be obtained when base/acid molar ratio was 1[thin space (1/6-em)]:[thin space (1/6-em)]1.c The data were not given as the solubility of DESs in model oil was too great.d The DESs completely dissolved in model oil.
[TEtA][Fo] 1[thin space (1/6-em)]:[thin space (1/6-em)]1b — — —
1[thin space (1/6-em)]:[thin space (1/6-em)]2 0.85 48.1 0.8
1[thin space (1/6-em)]:[thin space (1/6-em)]3 0.007 37.5 0.5
1[thin space (1/6-em)]:[thin space (1/6-em)]5 0.007 24.2 0.2
[TEtA][Ac] 1[thin space (1/6-em)]:[thin space (1/6-em)]1b — — —
1[thin space (1/6-em)]:[thin space (1/6-em)]2 8.2 48.9 0.6
1[thin space (1/6-em)]:[thin space (1/6-em)]3 0.90 49.4 0.2
1[thin space (1/6-em)]:[thin space (1/6-em)]5 0.006 37.2 0.2
[TEtA][Pr] 1[thin space (1/6-em)]:[thin space (1/6-em)]1 42.9 — —
1[thin space (1/6-em)]:[thin space (1/6-em)]2 10.8 52.3 0.4
1[thin space (1/6-em)]:[thin space (1/6-em)]3 0.92 51.6 0.2
1[thin space (1/6-em)]:[thin space (1/6-em)]5 0.003 37.6 0.4
1[thin space (1/6-em)]:[thin space (1/6-em)]7c 25.0 — —
1[thin space (1/6-em)]:[thin space (1/6-em)]9c 33.5 — —
[TEtA][Bu] 1[thin space (1/6-em)]:[thin space (1/6-em)]1c 70.9 — —
1[thin space (1/6-em)]:[thin space (1/6-em)]2c 14.9    
1[thin space (1/6-em)]:[thin space (1/6-em)]3 10.1 52.3 0.4
[TEtA][Pe] 1[thin space (1/6-em)]:[thin space (1/6-em)]1d — — —
1[thin space (1/6-em)]:[thin space (1/6-em)]2d — — —
1[thin space (1/6-em)]:[thin space (1/6-em)]3d — — —

3.3. Extraction performance of DESs

[TEtA][Fo], [TEtA][Ac], and [TEtA][Pr] referred to the DESs with B/A = 3 in the following text. As shown in Table 2, the extraction capability order of the DESs was [TEtA][Pr] > [TEtA][Ac] > [TEtA][Fo]. To explain the difference among three DESs, 1H NMR analysis was used for the mechanism study. As shown in Fig. 1, the active hydrogen signal of [TEtA][Fo] appeared at 12.77 ppm moved upfield to 12.80 ppm, leading to a decrease of 0.03, meanwhile, the other hydrogen signal did not move. Thus, the change may result from the interaction between DES and DBT. For [TEtA][Ac] and [TEtA][Pr], the NMR chemical shift of active hydrogen decreased 0.09 and 0.12, respectively. Thus, the order of this change for the DESs was [TEtA][Pr] > [TEtA][Ac] > [TEtA][Fo]. It can be concluded that the greater change of the chemical shift of active hydrogen meant the stronger interaction between DESs and DBT and the better desulfurization performance of DESs.
image file: c5ra27266a-f1.tif
Fig. 1 1H NMR of DESs, DBT and their mixtures.

In order to reach a deep understanding of the thermodynamic property and extractive mechanism for the current system, density functional theory (DFT) calculations have been carried out at M06-2X/6-31++G** level of theory. This level of theory is suggested to be an optimal method to study ionic compounds.42,43 Here one of the ionic pairs was chose as representative model to study the interaction between DES and DBT. In addition, solvent effects on the extraction reactions are also considered by using SMD solvation model.44

Firstly, reaction heat has been calculated, which is based on the difference between the solvation energy of DBT in octane (model oil) and [TEtA][Pr]. It should be noted that following results are obtained from the most stable configurations. The solvation energy of DBT in octane is calculated to be −4.8 kcal mol−1 while the corresponding energy in DES is −6.9 kcal mol−1. Hence, the reaction heat for the extraction is −2.1 kcal mol−1. This result shows that the extractive process is an exothermic reaction. Second, the extractive mechanism is analyzed by reduced density gradient analysis (RDG).45 This method can visualize and categorize the non-covalent interactions by analyze the reduced density (Fig. 2). It can be seen that there are hydrogen bonding interactions existed between O25⋯H50 (red region). It can be also found that attractive interactions exist over the plane of DBT molecule. This type of interaction can be called CH–π interactions (e.g. C3–H7⋯π). Hence, both hydrogen bonding interaction and CH–π interaction contributed to the extractive desulfurization.


image file: c5ra27266a-f2.tif
Fig. 2 Gradient isosurfaces (s = 0.35 au) for the most stable configuration during extractive desulfurization. The surfaces are colored on a red-green-blue scale according to values of sin(λ)ρ, ranging from −0.01 to 0.02 au. Red indicates strong attractive interactions, and blue indicates strong nonbonded overlap.

Actually, the DESs consisted of ionic species and the molecular acid and base species in the extractive process. Then these acid and base species may also play an important role on extraction of sulfur compounds because of the theory of similarity and intermiscibility. This point can be verified by the extractive efficiency order of DESs [TEtA][Pr](B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]2) > [TEtA][Pr] (B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]3) > [TEtA][Pr](B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]5), while the amount of propionic acid increased among the three DESs. The result showed the excess acid may do harm to the extractive desulfurization. In summary, the extractive process contained complex interaction between DESs and sulfur compounds, including CH–π interaction, hydrogen bond effect and so on.46,47

The extractive performance of DESs and ILs is summarized in Table 3, where comparison is made with other ILs relative to the extraction of DBT. As shown in the table, the Nernst partition coefficients (KN) of [TEtA][Pr] for extraction of DBT was 2.14. This value is higher than that of most of the ILs in extractive desulfurization. Moreover, the one-step synthetic procedures and the low cost would make the DESs more competitive. In comparison, two or more steps are compulsory to prepare the final ILs in Table 3. It is indicated that the DESs of this work will be highly potential candidates in extractive desulfurization of fuel.

Table 3 DBT removal from model oils by DESs or ILs extractiona
Entry DESs or ILs KN (mgS gIL−1/mgS goil−1) Ref.
a Bmim = C4mim = 1-butyl-3-methylimidazolium, Emim = 1-ethyl-3-methylimidazolium, C6mim = 1-hexyl-3-methylimidazolium, Omim = C8mim = 1-octyl-3-methylimidazolium, BPy = 1-butylpyridinium, HPy = 1-hexylpyridinium, OPy = 1-octylpyridinium, Mmim = 1,3-dimethylimidazolium, NTf2 = bis(trifluoromethylsulfonyl)imide, OTf = trifluoromethanesulfonate, N,N′-dimethylpiperazinium dilactate = [C1C1pi][Lac]2, [TMG][Lac] = tetramethylguanidinium lactate.
1 [TEtA][Fo] 1.08 This work
2 [TEtA][Ac] 1.72 This work
3 [TEtA][Pr] 2.14 This work
4 [Bmim]BF4 0.95 1
5 [Bmim]PF6 0.68 1
6 [Emim]BF4 0.56 1
7 [Omim]BF4 1.58 1
8 [Bmim][CF3SO3] 0.81 1
9 [Bmim][MeSO4] 1.10 1
10 [Bmim][MeSO3] 1.10 1
11 [Bmim][OcSO4] 2.14 1
12 [Bmim]Cl/AlCl3 4.09 1
13 [BPy]BF4 0.77 11
14 [HPy]BF4 1.42 11
15 [OPy]BF4 1.79 11
16 [Bmim][N(CN)2] 2.28 3
17 [Emim][N(CN)2] 1.3 3
18 [S2][N(CN)2] 1.08 3
19 [EtMe2S][N(CN)2] 0.84 3
20 [C4mim][SCN] 2.01 13 and 23
21 [C4mim][C8H17SO4] 1.60 13
22 [C4mim][CH3CO2] 1.30 13
23 [C6mim][NTf2] 1.09 13
24 [C4mim][OTf] 0.81 13
25 [C1C1pi][Lac]2 0.48 25
26 [TMG][Lac] 1.31 25
27 [Emim][DEP] 1.59 28
28 [Bmim][DBP] 1.27 28
29 [Mmim][DMP] 0.46 28

3.4. The initial sulfur concentration

It was of great value to deal with a wide range of sulfur concentration in future industrial application. Thus, the initial sulfur concentration is an important factor that influences the desulfurization process. In this experiment, the extraction of DBT was carried out with the initial sulfur concentration varied from 200 to 800 ppm. The results are shown in Fig. 3. KN value dropped from 2.58 to 2.07 for [TEtA][Pr], 1.89 to 1.63 for [TEtA][Ac], and 1.22 to 1.04 for [TEtA][Fo], respectively, as the sulfur concentration increased from 200 to 800 ppm. The results indicated the Nernst partition coefficient was not very sensitive to the initial sulfur concentration, that is to say, the extractive ability of the DESs was not be affected obviously by the initial concentration. Therefore, the synthesized DESs can be applied to desulfurization of more fuels.
image file: c5ra27266a-f3.tif
Fig. 3 Effect of the initial sulfur concentration on sulfur removal. Experimental conditions: DES = 1.75 g, model oil = 5 mL, T = 30 °C, t = 10 min.

3.5. Extractive temperature

The temperature on extraction process always plays an important role in energy consumption. The effect of the extractive temperature on desulfurization with different DESs is shown in Fig. 4. For [TEtA][Pr], when the temperature was set at 20 °C, 30 °C, 40 °C and 50 °C, the corresponding the sulfur partition coefficient was 2.2, 2.14, 2.01 and 1.83. According to the experimental results, the extractive ability of [TEtA][Pr] has reverse trends with the increase of the temperature. For the other two DESs, [TEtA][Fo] and [TEtA][Ac], the same trend was obtained. Therefore, a relatively low temperature was suitable for the extractive process with DESs, which was also one of advantages in extractive desulfurization in comparison with HDS.
image file: c5ra27266a-f4.tif
Fig. 4 Effect of temperature on sulfur removal by different DESs. Experimental conditions: DESs = 1.75 g, model oil (DBT) = 5 mL, t = 10 min.

3.6. The amount of DESs

The increase of the amount of DESs may be a direct method to increase the desulfurization efficiency. Then extraction of DBT was carried out with the mass ratios of DESs and model oil varying from 0.1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 2[thin space (1/6-em)]:[thin space (1/6-em)]1. As shown in Fig. 5, the removal of DBT increased sharply from 9.2% at [TEtA][Pr]/oil = 0.1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 82.1% at [TEtA][Pr]/oil = 2[thin space (1/6-em)]:[thin space (1/6-em)]1. However, the corresponding Nernst partition coefficient increased notably from 1.02 at [TEtA][Pr]/oil = 0.1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 2.10 at [TEtA][Pr]/oil = 0.25[thin space (1/6-em)]:[thin space (1/6-em)]1 and slowly to 2.29 at [TEtA][Pr]/oil. Therefore, desulfurization efficiency was promoted by increasing the amount of DES but the Nernst partition coefficient was not, especially at high [TEtA][Pr]/oil mass ratio. The same trend has been found in the extraction process by other two DESs. It is indicated that the extraction efficiency was much more sensitive than the Nernst partition coefficient to the amount of DESs. From the results, it is not worthy to obtain deep desulfurization by increasing the amount of DESs.
image file: c5ra27266a-f5.tif
Fig. 5 Effect of the amount of DESs on sulfur removal. Experimental conditions: model oil (DBT) = 5 mL, T = 30 °C, t = 10 min.

3.7. Multistage extraction and regeneration of [TEtA][Pr]

It is of great importance to reach the deep desulfurization process, considering the industrial production. The multistage extraction was performed at 30 °C with 5 mL model oil and 1.75 g of [TEtA][Pr]. After the first extractive step, the upper oil was collected for next extractive desulfurization with fresh [TEtA][Pr]. Then the operation was repeated for three times. The results were shown in Fig. 6. After four times extraction, the sulfur content was reduced from 500 ppm to 10 ppm. Though the deep desulfurization has been obtained, a large amount of DES was wasted. Therefore, regeneration and recycling of the used DESs are necessary and cost-effective.
image file: c5ra27266a-f6.tif
Fig. 6 Effect of the extraction times on sulfur removal by [TEtA][Pr]. Experimental conditions: model oil (DBT) = 5 mL, T = 30 °C, t = 10 min.

In this study, the DESs may be regenerated by distillation due to the distillable reactants.48 In a typical run, the used DES was treated at 100 °C to get the distillate under reduced pressure. As expected, the distillate was the regenerated DES because the boiling point of DBT was as high as 332 °C. The structure of the regenerated [TEtA][Pr] was confirmed by 1H NMR. As shown in Fig. 7, the active hydrogen signal moved upfield, which indicated that a little triethylamine escaped in the regenerated process. The extractive performance of the regenerated [TEtA][Pr] after different cycles is presented in Fig. 8. It can be seen that the KN value decreased a little from 2.14 to 1.96 after recycling for five times. The decrease may be resulted from the loss of triethylamine in the regenerated process. However, the extractive ability of DES did not drop obviously. Therefore, deep desulfurization and efficient utility of DESs could be achieved by multistage extraction and regeneration of DESs.


image file: c5ra27266a-f7.tif
Fig. 7 1H NMR of the fresh and regenerated [TEtA][Pr].

image file: c5ra27266a-f8.tif
Fig. 8 Effect of the recycle times on sulfur removal by [TEtA][Pr]. Experimental conditions: DES = 1.75 g, model oil (DBT, 500 ppm) = 5 mL, T = 30 °C, t = 10 min.

3.8. Extraction of different sulfur compounds

Different sulfur compounds were also investigated to study the extractive ability of DESs. As shown in Fig. 9, the sulfur removal followed the order of DBT > BT > 4,6-DMDBT > RSH. It can be deduced that the aromatic ring may play a positive role in extraction, which could enhance the interaction between DES and sulfur compounds. The extractive performance of RSH is in accord with the DFT calculations, as no CH–π interaction may not be found between DESs and RSH. However for 4,6-DMDBT, the steric hindrance of for 4,6-DMDBT, the steric hindrance of the methyl groups became a main obstacle for extraction.
image file: c5ra27266a-f9.tif
Fig. 9 Investigation of different sulfur compounds. Experimental conditions: DES = 1.75 g, model oil = 5 mL, T = 30 °C, t = 10 min.

3.9. Effect of olefins and aromatics on sulfur removal by [TEtA][Pr]

As is well known, the composition of actual fuel is complicated, including olefins, aromatics and so on. Most of the perfect oxidative desulfurization systems showed low desulfurization performance in actual fuel.49–53 However, there are a few reports on extractive desulfurization about taking the fuel composition into account. Here, p-xylene and cyclohexene were selected as representative compounds. In the experiment, 15 wt% p-xylene and cyclohexene were added to the model oil. The results are listed in Table 4. The sulfur removal decreased from 51.6% with no inhibiting compounds in the model oil to 49.8% and 51.4% with p-xylene and cyclohexene in the model fuel, respectively. The extractive desulfurization decreased 1.8% and 0.2%, respectively, which indicated that the presence of olefins and aromatics in the model fuel did not have a greatly negative impact. In contrast, the oxidative desulfurization efficiency in the literature decreased sharply from 88.2% to 59.2 and 7.0% with p-xylene and cyclohexene in the model fuel, respectively.49 Therefore, it can be deduced that the extractive desulfurization will be more promising than the oxidative desulfurization in actual fuel.
Table 4 Effect of olefins and aromatics on sulfur removal by [TEtA][Pr]
Inhibiting compounds Sulfur removal/%
This worka Ref. 33
a Experimental conditions: DES = 1.75 g, model oil (DBT) = 5 mL, T = 30 °C, t = 10 min. Max error = ±1.2%.
None 51.6 88.2
15 wt% p-xylene 49.8 59.2
15 wt% cyclohexene 51.4 7.0

4. Conclusion

A simple and economic non-hydrodesulfurization has been found by extractive desulfurization with DESs due to the cheap materials and simple preparation of DESs. The extractive ability of the three DESs followed the order [TEtA][Pr] > [TEtA][Ac] > [TEtA][Fo]. The extraction of different sulfur compounds indicated the aromatic ring played a positive role, but the steric hindrance of the methyl groups of 4,6-DMDBT was a main obstacle for extraction. Through evaluation of the extractive condition, the deep desulfurization can be obtained by multistage extraction at relatively low temperature. Furthermore, the extractive mechanism contained complex interaction between DESs and sulfur compounds, including CH–π interaction, hydrogen bond effect, polarization of DESs that confirmed by experimental and theoretical method. Compared with oxidative desulfurization, extractive desulfurization may be more suitable for actual fuel.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21506080, 21376109, and 21266007), Natural Science Foundation of Jiangsu Province (No. BK20150485), Natural Science Foundation of Hainan Province (No. 214029), China Postdoctoral Foundation (No. 2015M570412).

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Footnote

† Electronic supplementary information (ESI) available: 1H NMR and ESI-MS analysis of DESs, desulfurization performance of DES [TEtA][Pr] (B/A = 1[thin space (1/6-em)]:[thin space (1/6-em)]5). See DOI: 10.1039/c5ra27266a
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