Synthesis of Ionic-Liquid-Based Deep Eutectic Solvents for Extractive

Sep 12, 2016 - Up to present, hydrodesulfurization (HDS) by catalysis is one of the most common technique to remove sulfur from fuels in industry. How...
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Synthesis of Ionic-Liquid-Based Deep Eutectic Solvents for Extractive Desulfurization of Fuel Wei Jiang,† Hongping Li,‡ Chao Wang,‡ Wei Liu,‡ Tao Guo,‡ Hui Liu,‡ Wenshuai Zhu,*,‡ and Huaming Li*,† †

Institute for Energy Research and ‡School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang, Jiangsu 212013, People’s Republic of China S Supporting Information *

ABSTRACT: Deep eutectic solvents (DESs) have been widely applied in organic synthesis, extraction processes, electrochemistry, enzymatic reactions, and many others. However, the research of DESs on extractive desulfurization of fuel is only in its infancy. Here, a new class of DESs, ionic liquid-based DESs, was synthesized and characterized, where ionic liquid was produced in situ. Among the three DESs prepared from 1-methylimidazole (MIM) and diethanolamine as hydrogen bond acceptors and propanoic acid (PA) and nitric acid as hydrogen bond donors, MIM/PA showed the highest extractive efficiency and the sulfur partition coefficient (KN) reached 2.31. The viscosity of MIM/PA is much lower than other DESs and the lowviscosity ionic liquids in the literature, which is of benefit to mass transfer. The extraction mechanism was discussed by 1H nuclear magnetic resonance, and the activity differences among aromatic sulfur compounds were explained by density functional theory calculations. The addition of p-xylene and cyclohexene in model oil has no obvious effect on extraction of dibenzothiophene, meaning that the extractive desulfurization with DESs may also be suitable for the actual fuel. Finally, the freesulfur fuel can be obtained after 5 times extraction.

1. INTRODUCTION Deep desulfurization of transportation fuels is still an urgent issue in recent years, although more and more electric vehicles have emerged. Up to present, hydrodesulfurization (HDS) by catalysis is one of the most common technique to remove sulfur from fuels in industry. However, the harsh operation conditions of HDS make it a high cost to eliminate aromatic sulfur compounds, such as dibenzothiophene (DBT) and its derivatives [such as 4,6-dimethyldibenzothiophene (4,6DMDBT)].1−3 Liquid−liquid extraction has been proposed for sulfur removal at ambient temperature and pressure.4−7 The conventional organic extractants, such as N,N-dimethylformamide, dimethyl sulfoxide, imidazolidinone, and N-methylpyrrolidone, are inefficient and environmentally unfriendly. Therefore, it is of great significance to explore new extractants that are highly efficient, environmentally friendly, and economical.8 Ionic liquids (ILs) are low-temperature molten salts with very low vapor pressures. They are used as green solvents to replace traditional volatile organic solvents.9−13 Thus far, many efforts have focused on the design of complex ILs to increase the desulfurization efficiency (DE) of fuels.14−16 Most of the reported ILs used for extractive desulfurization (EDS) are imidazolium- or pyridinium-based ILs.17−19 The common anions are bis(trifluoromethylsulfonyl)imide ([NTf2]−),6 tetrachloroaluminate ([AlCl4]−),17 tetrafluoroborate ([BF4]−), hexafluorophosphate ([PF6]−), etc.20 Although ILs seem to be an ideal extractant for a extractive process as a result of their unique physicochemical properties, they also have some drawbacks, including difficulties in product purification, high cost, high viscosity, etc., which hamper their industrial applications.21−23 © 2016 American Chemical Society

To overcome disadvantages of ILs, a new generation of solvents, called deep eutectic solvents (DESs), has emerged at the beginning of this century.24−29 They are systems formed from a eutectic mixture of two or more components, which are cheap, biodegradable, and without purification.30 DESs are largely divided into four categories by Smith et al. depending upon the nature of the complexing agent.25 Li et al. reported one type of the DESs formed from quaternary ammonium salt as hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) for EDS of fuels.4 After 5 cycles, deep desulfurization can be obtained. Except for the four types of DESs, a mixture formed from Lewis or Brönsted acids and bases has also been deemed as DESs.31−33 They are used in extraction of bioactive natural products, starch dissolution, and soil washing. Therefore, this type of DESs may also be promising extractants in desulfurization of fuels. Herein, we report a new class of DESs that exhibit high DE. The prepared three DESs employ 1-methylimidazole (MIM) and diethanolamine (DEA) as HBAs mixing with propanoic acid (PA) and nitric acid (NA) as HBDs. Among the three DESs, MIM/PA, DEA/PA, and MIM/NA, MIM/PA shows the lowest viscosity and the highest extractive capacity. Furthermore, the viscosity of MIM/PA is lower than most of the reported ILs, which is beneficial to practical industrial applications, such as transportation, dispersion, and dissolution, as well as the decrease of the equilibrium times. Received: August 7, 2016 Revised: September 11, 2016 Published: September 12, 2016 8164

DOI: 10.1021/acs.energyfuels.6b01976 Energy Fuels 2016, 30, 8164−8170

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Energy & Fuels

diameter (id) × 0.25 μm film thickness; injector port temperature, 250 °C; detector temperature, 300 °C; and oven temperature, starting at 100 °C and rising to 200 °C at 15 °C/min for RSH, DBT, and BT and starting at 100 °C and rising to 160 °C at 20 °C/min and then rising to 230 °C at 25 °C/min for 4,6-DMDBT]. The DE was calculated as shown in eq 1, where C0 (ppm) is the initial sulfur concentration in the model oil and Cf (ppm) is the final sulfur concentration in the model oil after extraction. The sulfur partition coefficient (KN) based on the mass concentration was calculated by eq 2.

2. EXPERIMENTAL SECTION 2.1. Preparation of the DESs and Model Oil. The typical procedure for preparation of DESs is as follows. The reaction was carried out under an inert atmosphere (Ar). For MIM/PA, MIM (30 mL) was loaded into a three-necked flask equipped with a magnetic stirrer and cooled using an ice bath. Then, an equimolar of propionic acid was added dropwise to the flask with rapid stirring. A colorless liquid can be obtained after the reaction mixture lasted for 24 h at 50 °C. In this process, IL was generated and mixed with raw materials to form a DES. The other two DESs were synthesized with the same method. DEA/PA was obtained as a yellow liquid, and MIM/NA was obtained as a white solid. The structure of the DESs is listed in Scheme 1.

Table 1. Viscosities, Densities, and Water Contents of DESs or ILs at 25 °C and Atmospheric Pressure

MIM/PA DEA/PA

9.1 1254

MIM/NA [EMI][N(CN)2] [BMI][N(CN)2] [EtMe2S][N(CN)2] [S2][N(CN)2] a

16.1 29.3 25.3 34.2

ρ (g cm−3)

water content (%)

1.042

0.44

1.102

1.1

1.232a

0.59

1.06 1.06 n/a 1.135

KN = mg (sulfur) g −1 (IL)/mg (sulfur) g −1 (oil)

(2)

3. RESULTS AND DISCUSSION 3.1. Viscosity, Density, and Water Content of DESs. The viscosity, density, and water content of DESs were measured at 25 °C, except MIM/NA. The density of MIM/NA was tested at 60 °C. Generally speaking, the viscosity of extractants is crucial to mass transfer and the design of the extraction process. ILs usually have a high viscosity, which has been one of the main obstacles to the practical application of ILs.34 As shown in Table 1, the viscosity of MIM/PA is only 9.1, which is greatly lower than that of DEA/PA. The extraction equilibrium time of MIM/PA is also much lower than that of DEA/PA, which is just in accordance with the viscosity of DESs. Chen et al. have even reported some low-viscosity dicyanamide-based ILs, and the lowest viscosity is still 16.1 cP.22 Hence, the DES MIM/PA may be a good extractant with respect to mass transfer. The density and water content as important physical properties are also listed in Table 1. 3.2. Comparison of Different DESs and ILs for Extraction of DBT. Three DESs composed of Brönsted acid and base are used as extractants for extraction of DBT in this work. The results are listed in Table 2. MIM/PA shows the best extractive performance, and the sulfur partition coefficient (KN) increases to 2.31. The value is much higher than that of DEA/ PA or MIM/NA. The possible reason is attributed to the different components of DESs. As expected, the components of the three DESs consist of acid/base molecules and their formed ILs, but their individual content in different DESs displays great difference. The amount of ILs formed is dependent upon the relative difference in aqueous pKa between the acid and base. Investigations by Angell et al. indicate ΔpKa [pKa(base) − pKa(acid)] > 8 is suggestive of good proton transfer;35 that is, the reaction between the acid and base has been performed nearly completely. The pKa values searched in the PubChem Compound Database are 4.87, −1.38, 6.95, and 8.96 for PA, NA, MIM, and DEA, respectively. Herein, the calculated ΔpKa values of MIM/PA, DEA/PA, and MIM/NA are 2.08, 4.09, and 8.33, respectively. It is indicated that the components of MIM/ PA consist of a lot of neutral molecules and a little ILs, MIM/ NA is composed of nearly ILs, and the status of DEA/PA may stand between MIM/PA and MIM/NA. Therefore, the amount of neutral molecules among the three DESs follows the order MIM/PA > DEA/PA > MIM/NA, which is in accordance with their extractive capacities.

The model oil was prepared according to our previous reported procedure. The initial sulfur contents of DBT, benzothiophene (BT), 4,6-DMDBT, and 1-dodecanethiol (RSH) in n-octane were 500, 250, 250, and 250 ppm, respectively, where tetradecane was added as an internal standard. 2.2. Characterization. The structure of the DESs was characterized by nuclear magnetic resonance (NMR) spectra and positive electrospray ionization mass spectrometry (ESI−MS) in Figures S1−S9 of the Supporting Information. Viscosities were measured in a DV-2 digital display viscometer with an accuracy of 0.1%. Densities were determined by a 25 mL pycnometer. Water contents of DESs were obtained by a Karl Fisher method (Metrohm 870) with an accuracy of less than 0.2%. The detailed data of the three DESs are listed in Table 1.

η (cP)

(1)

2.4. Solubility of DESs in Model Oil. The dissolution of DESs in fuel will cause a loss of the DESs and contamination of the fuel. DESs are a mixture including the molecular acid and base that are likely to dissolve in model oil. Through the gas chromatography (GC) analysis, the solubility of MIM/PA in oil was about 1.8% and DEA/PA and MIM/NA cannot dissolve in the model oil.

Scheme 1. Structure of the Synthesized DESs

DESs or ILs

DE (%) = (1 − Cf /C0) × 100

reference this work this work this work 22 22 22 22

Measured at 60 °C.

2.3. EDS. The EDS was carried out in a self-made flask with 1.75 g of DES and 5 mL of model oil. Then, the extraction system was stirred at 30 °C until the extraction equilibrium was reached. The extraction temperature for MIM/NA was set at 60 °C as a result of its melting point of about 60 °C. The extraction time of MIM/PA and MIM/NA was set at 10 min, while the extraction time of DEA/PA was set at 20 min because of its high viscosity. The sulfur content of the model oil was detected by gas chromatography−flame ionization detector [GC− FID, Agilent 7890A, HP-5 column, 30 m long × 0.32 mm inner 8165

DOI: 10.1021/acs.energyfuels.6b01976 Energy Fuels 2016, 30, 8164−8170

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Energy & Fuels Table 2. Extraction of DBT from Model Oil with Different DESs or ILs entry

DESs or ILs

KN (mgS gIL−1/mgS goil−1)

reference

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

MIM/PA DEA/PA MIM/NA polyether-based ILs [TBCMP][Br] [TBEP][Br] [Bmim]PF6 [Omim]BF4 [Bmim][CF3SO3] [Bmim][MeSO4] [Bmim][N(CN)2] [Emim][N(CN)2] [S2][N(CN)2] [EtMe2S][N(CN)2] [OPy]BF4 [C4mim][SCN] [C1C1pi][Lac]2 [TMG][Lac] [Emim][DEP] [Bmim][DBP] [Mmim][DMP]

2.31 0.43 0.21a 1.37 1.44 0.48 0.68 1.58 0.81 1.10 2.28 1.3 1.08 0.84 1.79 2.01 0.48 1.31 1.59 1.27 0.46

this work this work this work 8 15 15 17 17 17 17 22 22 22 22 37 38 39 39 40 40 40

a

Figure 1. 1H NMR of MIM/PA and the mixtures of MIM/PA and DBT.

3.4. EDS of Fuel with Different DESs. In the previous paragraph, the melting point of DES MIM/NA is above room temperature. Then, the other two DESs, MIM/PA and DEA/ PA, are selected as the extractants to study the effect of desulfurization at 30 °C. Various model oils were prepared by dissolving different sulfur compounds in n-octane. As seen, four sulfur compounds, mercaptan (RSH in this work), BT, DBT, and 4,6-DMDBT, are chosen as representative substrates. The extractive results are shown in Figure 2. It is found that the

Measured at 60 °C.

The same trend cannot be merely coincidental. Nie et al. have reported a kind of extractant that was a mixture of molecular solvents and ILs.36 The results showed that MIM had high partition coefficients for sulfur compounds, but its mutual solubility in fuel oil is quite high. As a result, the neutral molecules in DESs may play an important positive effect on extraction of DBT but a negative effect on solubility of DESs in fuel. Therefore, MIM/PA presents the highest extractive performance compared to DEA/PA probably as a result of the larger amount of neutral molecules existing in DESs. It is instructive to compare the EDS of other extractants, especially ILs, reported in the literature.8,15,17,22,37−40 As shown in Table 2, most of the ILs exhibiting good extractive performance are based on the imidazolium cation. However, the highest KN value was obtained as 2.28 using [N(CN)2]− as the anion. Thus, DES MIM/PA prepared in this work with 2.31 of the KN value is one of the most efficient extractants. The difference between them is that, in comparison to ILs, the synthesis of DESs is simpler and cheaper, which will be conductive to industrial application. 3.3. Discussion of the Mechanism for EDS with DESs. The general extractive mechanism is the π−π interaction, CH−π interaction, hydrogen-bonding interaction, etc.4,38,41 1H NMR analysis is used for the mechanistic study. As shown in Figure 1, the active hydrogen signal of MIM/PA appeared at 11.99 ppm. After DBT was extracted to DES, this hydrogen signal moved upfield to 12.11 ppm, leading to a increase of 0.12; meanwhile, the other hydrogen signal did not move. Thus, the change may result from the interaction between DES and DBT. In combination of our previous work,42 the main EDS mechanism of MIM/PA may be the π−π interaction between the imidazole ring and aromatic sulfur compounds and the hydrogen-bond interaction between DESs and sulfur compounds. For DEA/PA, the main mechanism may be the CH−π interaction and hydrogen-bond interaction.

Figure 2. EDS of fuel with different DESs.

extractive efficiency with different DESs follows the same order DBT > BT > 4,6-DMDBT > RSH. The order is in agreement with the literature and our previous work.39,42,43 Herein, the lowest desulfurization of RSH may be ascribed to its lack of delocalized π electrons. Although 4,6-DMDBT has the highest π-electron density, the steric hindrance of its two methyl groups leads to low extractive activity. To further explain the different activities among aromatic sulfur compounds, the interaction model between DES/noctane and sulfides was studied at the M06-2X/6-31++G** level of the density functional theory (DFT) using the Gaussian 09 program. Figure 3 plots typical interaction models between DBT and DES/n-octane. From the structure information and our previous analysis,44 these interaction types can be clearly ascribed to the π−π interaction (Figure 3a) and the C−H···π interaction (Figure 3b). The interaction energy between MIM/ PA and DBT, BT, and 4,6-DMDBT was calculated to be −9.0, −7.3, and −9.4 kcal mol−1. The interaction energy follows the order 4,6-DMDBT > DBT > BT, which does not seem to be equal to the extraction efficiency. However, generally speaking, the extraction process should be an equilibrium of the sulfide distribution between DES and n-octane. Consequently, an interaction model between fuel and sulfides was also explored, and the interaction energy value is −6.2, −7.0, and −7.4 kcal mol−1, respectively. Then, the reaction heat for the extraction 8166

DOI: 10.1021/acs.energyfuels.6b01976 Energy Fuels 2016, 30, 8164−8170

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Figure 3. Optimized structures of DBT interacting with (a) MIM/PA or (b) n-octane.

process can be obtained by comparison to the interaction energy in MIM/PA and octane. The reaction heat of the two types of interaction was DBT > 4,6-DMDBT > BT (the value is −2.8, −2.0, and −0.3, respectively). It can been seen from this result that the model can reasonably explain part of the experimental results, except for the activity difference between BT and 4,6-DMDBT. The theoretical prediction of exact activity order between BT and 4,6-DMDBT is usually difficult, which has been discussed by our previous work.41 However, the above discussions show that two clear conclusions can be drawn: (a) the EDS activity for DBT is higher than that for BT because more aromatic rings in DBT should strengthen the interaction energy, and (b) the alkyl derivative of DBT (e.g., 4,6-DMDBT) takes on negative effects for the activity, which leads to the activity following the order: DBT > 4,6-DMDBT. 3.5. Effect of the Temperature on Extraction of DBT. It is known that the reaction temperature of HDS in industry is higher than 300 °C.45 The decrease of the reaction temperature may be valuable to a petroleum refinery. EDS may be a promising technique just from the view of operation temperature. As shown in Figure 4, four relatively low temperatures

Figure 5. Effect of the initial sulfur concentration in fuel.

removal decreased from 54.7 to 53.4% as the sulfur concentration increased. The drop was only 1.3% for MIM/ PA, indicating that the DES can be applied in various fuels with a wide sulfur concentration. It will be of great value for future industrial application. The same trend was found with DEA/PA as the extractant, although it had a lower DE. Therefore, it can be deduced that the extraction with the DESs confirms the Nernst distribution law. 3.7. Methods To Increase the DE. The increase of the amount of DESs may be a direct method to increase the DE. As shown in Figure 6, the sulfur removal was only 22.6% with MIM/PA as the extractant at the mass ratio of 0.1:1 DES/fuel. When the mass ratio increased to 2:1, the sulfur removal increased to 84.6%. The rise of the sulfur removal was quite great but not enough because the deep desulfurization was not

Figure 4. Effect of the temperature on extraction of DBT.

are set and the DE decreased with the rising temperature. The DE decreased from 54.7% at 20 °C to 53.6% at 30 °C with MIM/PA as the extractant. This indicated that a low temperature was suitable for the extractive process with DESs and room temperature may be a good choice. 3.6. Effect of the Initial Sulfur Concentration in Fuel. The initial sulfur concentration in fuel should be an important parameter to evaluate the extractive capacity of DESs. The sulfur removals with varied sulfur concentrations from 200 to 800 ppm are listed in Figure 5. It can be seen that the sulfur

Figure 6. Effect of the amount of DESs on the extraction of DBT. 8167

DOI: 10.1021/acs.energyfuels.6b01976 Energy Fuels 2016, 30, 8164−8170

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Energy & Fuels obtained. The result for DEA/PA was not satisfied, and the sulfur removal was only 37.4% when the mass ratio increased to 2:1. However, in comparison to ILs, it may be feasible as a result of the low cost of DESs and simple preparation process.28 Another way to increase the DE is multi-stage extraction of the desulfurized fuel. The desulfurization experiment with MIM/PA as the extractant was tested. After the first desulfurization step, the upper oil was collected for the next further desulfurization with fresh DES. The result is shown in Figure 7. The sulfur removal can reach 97.6% after 4 times

Table 3. Effect of Olefins and Aromatics on Sulfur Removal by MIM/PA sulfur removal (%) component

this work

ref 44

none 15 wt % p-xylene 15 wt % cyclohexene

53.6 50.4 52.7

88.2 59.2 7.0

that of cyclohexene but should be acceptable, with the biggest drop of 3.2%. In contrast, oxidative desulfurization (ODS) suffers the effect of the two components severely. The DE dropped from 88.2 to 59.2 and 7.0% with the addition of 15 wt % p-xylene and 15 wt % cyclohexene in the model fuel, respectively.46 Furthermore, the results from our previous work about ODS also indicated that cyclohexene hindered the removal of sulfur compounds. In this respect, EDS may have a better advantage compared to ODS.

4. CONCLUSION By tailoring the constitute of DESs, one of the DESs, MIM/PA, has been certified as a novel and efficient extractant. It has not only a low viscosity but also a high sulfur partition coefficient between DES and fuel. The extraction mechanism showed that the interaction between sulfides and DES was the π−π interaction, CH−π interaction, and hydrogen-bonding interaction. The existence of the neutral molecules in DESs can increase the extractive capacity. DFT calculations explained EDS activity difference, where DBT is higher than BT because of its more aromatic rings. However, the exact prediction of activity order by the theoretical method is still a hard task. In comparison to ODS, EDS is hardly affected by olefins and aromatics and is more promising.

Figure 7. Effect of the extraction times on sulfur removal by MIM/PA.

extraction, and sulfur-free fuel can be obtained after 5 times extraction. Therefore, EDS with DES as the extractant is quite a promising method to produce the low-sulfur fuel, which can meet the strict sulfur standard thus far. In addition, used MIM/ PA was regenerated by re-extraction with n-octane several times. The extractive capacity of DES can be recovered, and the sulfur removal slightly decreased from 53.6 to 52.4 to 48.5% after the fifth regeneration (Figure 8). The drop may result from the little loss of the DES in every regeneration step.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01976. 1 H NMR of MIM/PA (Figure S1), DEA/PA (Figure S2), and MIM/NA (Figure S3), 13C NMR of MIM/PA (Figure S4), DEA/PA (Figure S5), and MIM/NA (Figure S6), and positive ESI−MS of MIM/PA (Figure S7), DEA/PA (Figure S8), and MIM/NA (Figure S9) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-511-88791800. Fax: +86-511-88791708. Email: [email protected]. *Telephone: +86-511-88791800. Fax: +86-511-88791708. Email: [email protected].

Figure 8. Effect of the recycle times on sulfur removal by MIM/PA.

Notes

3.8. Effect of Olefins and Aromatics on Extraction of DBT. In fact, the composition of actual fuel is complicated, including alkanes, olefins, aromatics, etc.46−49 The model oil was only composed of alkane and the refractory aromatic sulfur compounds. To study the effect of other main components on DE, 15 wt % p-xylene and 15 wt % cyclohexene were added to the model oil. It can be seen from Table 3 that the sulfur removal decreased from 53.6 to 50.4% with the addition of 15 wt % p-xylene and decreased to 52.7% with addition of 15 wt % cyclohexene. Hence, the effect of p-xylene may be greater than

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21506080, 2157612, and 21376109), the Natural Science Foundation of Jiangsu Province (BK20150485), the China Postdoctoral Science Foundation (2015M570412), and the Advanced Talents of Jiangsu University (15JDG053). 8168

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