Extraction–oxidation desulfurization by pyridinium-based task-specific ionic liquids

Extraction–oxidation desulfurization by pyridinium-based task-specific ionic liquids

Fuel 102 (2012) 580–584 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Extraction–oxidation...

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Fuel 102 (2012) 580–584

Contents lists available at SciVerse ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Extraction–oxidation desulfurization by pyridinium-based task-specific ionic liquids Cun Zhang a,⇑, Xiaoyu Pan a, Feng Wang a, Xiaoqin Liu b a b

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" The extraction–oxidation

desulfurization system by TSIL [CH2COOHPy][HSO4] without adding additional catalyst. " The TSIL which was halogen-free and had high catalytic oxidative activity was synthesized and used as both extractant and catalyst. " The TSIL showed the good recycling performance.

a r t i c l e

i n f o

Article history: Received 9 September 2011 Received in revised form 13 July 2012 Accepted 16 July 2012 Available online 28 July 2012 Keywords: Extraction–oxidation Desulfurization Task-specific ionic liquids

a b s t r a c t The Brönsted acidic ionic liquids N-carboxymethylpyridine hydrosulphate ([CH2COOHPy][HSO4]) and N-carboxyethylpyridine hydrosulphate ([(CH2)2COOHPy][HSO4]) were synthesized and used as the extractant and catalyst for the extraction–oxidation desulfurization of model oil. The structures of the ILs were confirmed by 1H NMR and 13C NMR. The density, viscosity and acid strength of the ILs were also investigated. The acid strength was in order of H2SO4 (98%) > [CH2COOHPy][HSO4] > [(CH2)2COOHPy][HSO4]. The [CH2COOHPy][HSO4] showed better activity during the removal of dibenzothiophene (DBT) in n-octane by a combination of extraction and oxidation, and the sulfur removal reached 99.9% under the conditions of Vmodel oil = 20 mL, VIL = 1.2 mL, T = 30 °C and H2O2/S molar ratio (O/S) = 6. In the same conditions, the removal of benzothiophene (BT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) by using [CH2COOHPy][HSO4] reached 82.5% and 89.1%, respectively. Ionic liquid [CH2COOHPy][HSO4] can be recycled 9 times without a significant decrease in the sulfur removal. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction In the last few decades, the environmental pollution caused by the exhaust gases containing SOx from cars has become more and more serious. When the diesel fuel has a high content of sulfur, the combustion products will be rich in SOx which not only leads to air pollution but also do harm to human health [1–3]. Therefore, much attention has been transferred to the obtaining of the ultralow sulfur fuel due to the more and more stringent sulfur content specifications in fuels [4,5]. ⇑ Corresponding author. Tel.: +86 514 879755909105; fax: +86 514 879755 908410. E-mail address: [email protected] (C. Zhang). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.07.040

The conventional method for effectively removing the sulfur compounds including thiols, thioethers and disulfides from fuels in industry is catalytic hydrodesulfurization (HDS), but the aromatic sulfur compounds which account for more than 55% of the total sulfur content, such as benzothiophene (BT), dibenzothiophene (DBT) and their derivatives, especially 4,6-dimethyldibenzothiophene (4,6-DMDBT), are hardly eliminated by HDS because of their steric hindrance. The severe operating conditions and large captical cost are required for HDS to obtain the ultra-low sulfur fuels [6,7]. Thus, the alternative ultra-deep desulfurization methods that are carried out at moderate conditions without requiring H2, such as oxidation desulfurization (ODS), bio-desulfurization, extraction desulfurization and adsorption desulfurization, have been extensively investigated [8–10].


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As environmentally friendly alternative solvents, room temperature ionic liquids (RTILs) have been widely applied to separation, synthesis, electrochemistry and catalysis [11]. IL-based technology has various advantages, such as wide liquid range, negligible vapor pressure [12] and high stability [13]. ILs also show good extraction ability for aromatic sulfur-containing compounds, and are immiscible with aliphatic liquids such as fuel oil [14]. Therefore, the new approach for desulfurization of diesel fuels by extraction with ILs has been reported recently [15–18]. Nevertheless, the efficiency of sulfur removal by extraction is rather low, only in the range from 10% to 40%, and multiple extractions are required to obtain deep desulfurization [19]. In order to increase the efficiency of sulfur removal, the extraction using ILs is combined with oxidation desulfurization process [20–23]. In these processes, sulfur removal could reach more than 90.0% with H2O2 as an oxidant, and ILs, such as [Bmim][PF6], [Bmim][BF4], served as an extractant. Furthermore, an additional catalyst, such as CH3COOH, Na2MoO42H2O, was needed [3]. Currently, more efficient ODS systems solely containing H2O2 as oxidant, Brönsted acid ionic liquid [Hmim]BF4 or [Hnmp]BF4 as extractant and catalyst have been reported by Lu et al. and Zhao et al. respectively [19], and the sulfur removal of the model oil also could reach over 90% [14,24]. Zhao et al. proposed that the catalytic role of the acidic ILs is to decompose H2O2 to form hydroxyl radicals that are strong oxidizing agents for DBT. DBT which was extracted into the ionic liquid phase is oxidized to its corresponding sulfoxides and sulfones with high polarity by the hydroxyl radicals [24]. However, the drawback of this method is that the experiments have to be carried out in the conditions of a high temperature and large amount of ILs. At the same time, the anion of these ILs is BF4 , while fluoroboric acid required in the preparation of these two acidic ionic liquids is a strong acid with extreme corrosion and high toxicity. Besides, it has been proven that materials containing fluorine are potentially harmful to the environment [19]. So there is still much room for the development of more effective and greener halogen-free acidic ionic liquids for ODS. Recent advance in ILs research provided another route for achieving Task-Specific Ionic Liquids (TSILs) in which a functional group is covalently tethered to the cation or anion of the ILs. It was expected that these TSILs may further enlarge the application scope and improve the efficiency. In order to design TSILs for oxidation desulfurization, i.e. ILs can work as both extractant and catalyst, the ILs should have a good extraction capacity to sulfurcontaining compounds and the functional group for catalytic oxidation should be introduced [11]. Gao and his coworkers used the Brönsted acidic ionic liquids 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]) and N-butylpyridinium hydrogen sulfate ([C4Py][HSO4]) as extractant and catalyst for desulfurization of diesel fuels, and concluded that pyridinium cation had better extractive performance than imidazolium cation [3,7]. In this paper, the carboxylic acid group ( COOH) with catalytic oxidative function was introduced into the cations of pyridinium-based Brönsted acidic ILs, the halogen-free and greener TSILs [CH2COOHPy][HSO4] and [(CH2)2COOHPy][HSO4] were synthesized and used as both extractant and catalyst for the extraction–oxidation desulfurization of model oil. The physical properties of the two TSILs and the efficiencies of sulfur removal by the two TSILs were investigated, and the possible oxidative desulfurization mechanism of DBT was proposed.

were purchased from Aladdin Chemistry Co. Ltd. Other reagents, including pyridine, chloroacetic acid, sulfuric acid (98%), dichloromethane, methanol, dimethyl yellow, carbon tetrachloride, n-octane, hydrogen peroxide (30 wt.%), tributyl phosphate were purchased from Sinopharm Chemical Reagent Co. Ltd. 2.2. Preparation of ionic liquids The task-specific ionic liquids [CH2COOHPy][HSO4] and [(CH2)2COOHPy][HSO4] were prepared according to the two-step synthesis method: at first, a mixture of pyridine (1.2 mol) and chloroacetic acid (1.0 mol) were reacted for 2 h under ultrasound. The white precipitate was filtered off and washed three times with dichloromethane to obtain [CH2COOHPy]Cl which was dried under vacuum. And then equimolar amounts of sulfuric acid (98%, 5.8 g) was slowly dropped to [CH2COOHPy]Cl (10.0 g) under the same ultrasound conditions in order to get [CH2COOHPy][HSO4]. The by-product HCl was absorbed by sodium hydroxide solution. When the generated HCl cannot be detected by ammonia solution, the object product [CH2COOHPy][HSO4] was dried under vacuum and stored in a desiccator. IL [(CH2)2COOHPy][HSO4] was prepared by the similar procedures. The structures of ILs have been identified by 1H NMR and 13C NMR (AVANCE600, Bruker, Germany). The densities of ILs were measured by pyknometer method. Rotational Rheometer (RS600, Thermo, USA) was used to determine their viscosities. The results were as follows. [CH2COOHPy][HSO4]: density: 1.678 g/mL (30 °C); viscosity: 0.303 Pas (30 °C). 1H NMR (600 MHz, D2O): 5.21 (s, 2H), 7.79 (t, 2H), 8.30 (t, 1H), 8.49 (d,2H); 13C NMR (600 MHz, D2O): 58.66, 126.22, 143.64,145.04, 166.13. [(CH2)2COOHPy][HSO4]: density: 1.531 g/mL (30 °C); viscosity: 0.546 Pas (30 °C). 1H NMR (600 MHz, D2O): 2.78 (t, 2H), 4.05 (t, 2H), 7.31 (t, 2H), 7.83 (t, 1H), 7.95 (d,2H); 13C NMR (600 MHz, D2O): 27.71, 45.79, 125.13, 142.40, 143.93, 171.15 (see Fig. 1). 2.3. Acid strength analysis In order to evaluate the Brönsted acid strength of ionic liquids, the sulfuric acid (98%) was used as the reference substance. The dimethyl yellow (22.5 mg/L) acted as a Hammett indicator was respectively added to the same concentration solutions (0.32 mmol/L) of ILs or sulfuric acid dissolving in methanol. And the Brönsted acidities were compared by ultraviolet–visible absorption of an indicator in UV–visible spectroscopy (UV-2550, Shimadzu, Japan) [25]. 2.4. Extraction–oxidation desulfurization The model oil was prepared by dissolving DBT, BT, or 4,6DMDBT in n-octane, respectively, to form the solutions with sulfur content of 1000 ppm. The extraction–oxidation desulfurization experiments were conducted in a 50 mL of a round-bottom flask. The mixture which contained 20 mL of model oil and 1.2 mL of ionic liquid was stirred vigorously at 30 °C for 20 min when the sulfur content of the mod-

2. Experimental 2.1. Materials Benzothiophene (BT), dibenzothiophene (DBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and 3-chloropropionic acid

Fig. 1. Structures of ionic liquids.


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el oil was constant. Then 0.27 mL of H2O2 (30 wt.%) was added to the mixture. After the reaction, the resulting mixture was placed in a static state to form two layers. The upper oil phase (n-octane) was separated easily from the IL phase by decantation and analyzed its sulfur content by gas chromatography. The lower IL phase was reused after regeneration. 2.5. Sulfur content analysis The sulfur content of the model oil was determined by gas chromatograph (6890 N, Agilent, USA) with a flame ionization detector (GC-FID) and DB-1 column (30 m  250 lm  0.25 lm). Tributyl phosphate was used as the internal standard. The analysis conditions were as follows: injector temperature, 300 °C; detector temperature, 290 °C; column temperature, 120 °C for 2 min, then heated to 250 °C in rate of 5 °C/min; the carrier gas was nitrogen. 2.6. Regeneration of used ionic liquid The oil phase was separated from the IL phase after the reaction. The water and oxidant in the ionic liquid were completely removed by rotary evaporation at 80 °C for 4 h. Then the ionic liquid was reextracted three times with equal volume of carbon tetrachloride. And the IL phase can be reused after being dried under vacuum. 3. Results and discussion 3.1. Comparison of acid strength In the first of the experiments, the determinand solution, i.e. same concentration solution (16 mmol/L) of ILs or H2SO4 in methanol, was prepared. And the indicator solution, i.e. 150 mg/L solution of dimethyl yellow, was also prepared. Then 0.2 mL of the determinand solution and 1.5 mL of the indicator solution were mixed and diluted to 10 mL. The indicator combined with ionized H+ from Brönsted acid to form the structure of protonation. The more the amount of protonated form is, the stronger the acidity of Brönsted acid is. So the acid strength of ILs was easily compared by the absorbance of protonated form of an indicator in UV-visible spectrum. As shown in Fig. 2, the absorptions of unprotonated form and protonated form of the indicator are observed at 410 nm and 550 nm. When no determinand is added, the indicator exists only as unprotonated form. [CH2COOHPy][HSO4] exhibits the stronger acid strength than [(CH2)2COOHPy][HSO4], but they are both weaker than H2SO4. And their H+ mainly comes from ionization of HSO4 and carboxyl. The decrease of the amount of H+ with increasing the branch length of IL may be due to the fact that when the branch length increases, the inductive effect of electron-withdrawing from pyridyl to carboxyl weakens. Then the H+ becomes harder to be released from the carboxyl. But the amount of H+ from HSO4 is not changed. Therefore, the acid strength of ILs is in the order of [CH2COOHPy][HSO4] > [(CH2)2COOHPy][HSO4].

Fig. 2. Absorption spectra of dimethyl yellow for ILs or H2SO4 in methanol.

their viscosities which is caused by different branch length of cations. As is well known, H2O2 is a strong oxidant in the acidic medium. After the oxidant H2O2 was added, the carboxyl group of the TSILs reacted with it to form the peroxycarboxyl group (–COOOH) with high oxidation potential. Meanwhile, the DBT which was extracted into the ionic liquid phase was oxidized to its corresponding sulfoxides and sulfones by –COOOH. Therefore, the desulfurization was carried out by extraction combining with catalytic oxidation, and the TSILs served as both extractant and catalyst. The results were shown in Fig. 3. The removal of DBT increased sharply with the time increasing at the first of the desulfurization process but increased negligibly after 40 min. Furthermore, the sulfur removal of model oil by [CH2COOHPy][HSO4] was higher than that by [(CH2)2COOHPy] [HSO4]. This may be assigned to the rate of forming peroxycarboxylic acid by the reaction of these TSILs with H2O2. The peroxycarboxylic acid generated in situ is expected to be the active species for DBT oxidation. The formation rate of peroxycarboxylic acid is not only close related to the oxidation rate of DBT but also depends on the acidity of the ILs. It can be suggested that the stronger the acidity of ILs is, the larger the rate of producing the peroxyacid groups (–COOOH) is, and the better the desulfurization performance is [11]. So the preferred TSIL is [CH2COOHPy][HSO4].

3.2. Effect of different ILs on sulfur removal Under the same conditions, the two ILs were respectively used as extractant and catalyst to compare their desulfurization efficiency by a combination of both extraction and oxidation. As shown in Fig. 3, before the oxidant H2O2 was added (time t = 0) and when the extraction equilibrium was reached, the desulfurization of model oil was only by extraction and merely a small part of DBT was extracted by ILs. Their capability of extraction desulfurization decreased in the following order: [CH2COOHPy][HSO4] > [(CH2)2COOHPy][HSO4]. This may be due to the difference between

Fig. 3. Effect of different ILs on sulfur removal. Conditions: Vmodel VIL = 1.2 mL; T = 30 °C; O/S = 6.

oil (DBT)

= 20 mL;


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It could be concluded that the desulfurization efficiency depends upon both the extraction ability and the acidity of TSILs. 3.3. Effect of sulfur species on sulfur removal In order to investigate the desulfurization performance of [CH2COOHPy][HSO4] on the different sulfur compounds, under the same reaction conditions the extraction–oxidation desulfurization of the model oils, in which BT, DBT and 4,6-DMDBT were respectively chosen as the substrates, were carried out. As shown in Fig. 4, the same trend can be found on the three sulfur compounds, the sulfur removal of model oil sharply increased at first, then changed slowly. Furthermore, the sulfur removal by extraction combining with catalytic oxidation decreased in the order of DBT > 4,6-DMDBT > BT. The removal of DBT reached 99.9% in 60 min. However, the removal of 4,6-DMDBT and BT were only 89.1% and 82.5% in the same time. The relatively large differences in sulfur removal may be due to the difference of aromatic p-electron density of sulfur compounds [3]. Otsuki et al. [26] have reported the electron densities on the sulfur atom of sulfur-containing compounds and given the corresponding trend for the oxidation reactivity of sulfur compound: 4,6-DMDBT (5.760) > DBT (5.758) > BT (5.739). Because of the lower electron density and oxidation reactivity, the removal of BT is the lowest. But for 4,6DMDBT, the steric hindrance of the methyl substitution at the 4 and 6 positions of DBT significantly retards the extractive performance of [CH2COOHPy][HSO4]. The experiment results showed that the extraction capability of [CH2COOHPy][HSO4] for DBT was better than 4,6-DMDBT. In addition, that the obstacle for the approach of the sulfur atom to the catalytic active species in IL may be formed by the steric hindrance of 4,6-DMDBT was proposed in another catalytic oxidative desulfurization system [27]. Therefore, the removal of DBT is higher than that of 4,6-DMDBT.

Table 1 Results of recycling of [CH2COOHPy][HSO4] in desulfurization. Cycle

Sulfur removal (%)


Sulfur removal (%)

1 2 3 4 5

99.9 99.8 99.8 99.7 99.5

6 7 8 9 10

99.5 99.4 99.2 98.9 97.7

Conditions: Vmodel

oil (DBT)

= 20 mL; VIL = 1.2 mL; T = 30 °C; O/S = 6; t = 60 min.

Fig. 5. Extraction–oxidation desulfurization by TSILs-H2O2 system.

had been investigated. The results showed that it was essentially consistent with the initial one before the extraction, that is, there was no change in the structure of the IL. Table 1 shows the effect of the recycling times of the regenerated ionic liquid on the sulfur removal. The results indicate that ionic liquid [CH2COOHPy][HSO4] can be recycled 9 times without a significant decrease in sulfur removal and shows the very good recycling performance in the desulfurization. 3.5. The process and mechanism of extraction–oxidation desulfurization by TSILs

3.4. Recycling of ionic liquid The regeneration and recycling of IL have great significance for the industrial application. After the extraction–oxidation desulfurization, the water and oxidant in the IL ([CH2COOHPy][HSO4]) which separated from the model oil phase were removed by rotary evaporation, and the oxidation products of sulfur compounds were re-extracted by carbon tetrachloride. After being dried under vacuum, the regenerated IL was reused to the next extraction– oxidation desulfurization process. The NMR analysis or measurement of the physical properties of the IL after the extraction cycles

As shown in Fig. 5, in the extraction–oxidation desulfurization process by the TSILs-H2O2 system, DBT selected as the representative of sulfur compounds was extracted into the IL phase, and simultaneously oxidized to its corresponding sulfoxide (DBTO) and sulfone (DBTO2) by –COOOH formed by the carboxyl group of the TSILs reacting with H2O2. The oxidation products DBTO and DBTO2, because of their high polarity, still stayed in the IL phase. As the concentration of DBT decreased in ionic liquid phase, and the extraction equilibrium was broken, DBT was continuously extracted from oil phase to IL phase until DBT completely disappeared in the model oil. Therefore, in the extraction combining with the catalytic oxidation process, the IL acted as both extractant and catalyst. 4. Conclusions

Fig. 4. Effect of sulfur species on sulfur removal. Conditions: Vmodel VIL = 1.2 mL; T = 30 °C; O/S = 6.


= 20 mL;

In this paper, the Task-Specific Ionic Liquids [CH2COOHPy][HSO4] and [(CH2)2COOHPy][HSO4] which were halogen-free and had high catalytic oxidative activity were synthesized and used as both extractant and catalyst for the extraction–oxidation desulfurization of model oil. Under appropriate reaction conditions, the sulfur removal of DBT in model oil could reach 99.9% by [CH2COOHPy][HSO4], which was remarkable superior to desulfurization by simple extraction with ionic liquids. Moreover, the IL [CH2COOHPy][HSO4] can be recycled 9 times without a significant change on reactivity. Therefore, the extraction–oxidation desulfurization system by TSIL [CH2COOHPy][HSO4] without adding an additional catalyst offers some evident advantages such as green and environmentally friendly extractant and catalyst, higher effi-


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