Deep Desulfurization of Fuels by Extraction with 4 ... - ACS Publications

10.1021/ef401015c. Publication Date (Web): July 1, 2013. Copyright © 2013 American Chemical Society. *Phone: +86 57187952525. Fax: +86 5718795252...
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Deep Desulfurization of Fuels by Extraction with 4‑Dimethylaminopyridinium-Based Ionic Liquids Qianli Wang, Lecheng Lei, Jingke Zhu, Bin Yang, and Zhongjian Li* Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Yuquan Campus, Hangzhou 310027, China ABSTRACT: In this work, three 4-dimethylaminopyridinium-based ionic liquids (ILs), N-ethyl-4-dimethylaminopyridinium dicyanamide ([C24DMAPy][N(CN)2]), N-butyl-4-dimethylaminopyridinium dicyanamide ([C44DMAPy][N(CN)2]) and Nhexyl-4-dimethylaminopyridinium dicyanamide ([C64DMAPy][N(CN)2]), were synthesized and then demonstrated to be efficient for aromatic sulfur compounds extraction from fuels. The mutual solubility evaluating results indicated that 4dimethylaminopyridinium-based ILs hardly dissolved in the fuels, while the solubility of 97# gasoline in ILs varies from 7.0 wt % for [C24DMAPy][N(CN)2] to 12.4 wt % for [C64DMAPy][N(CN)2] and the solubility of 0# diesel in ILs varies from 6.7 wt % for [C24DMAPy][N(CN)2] to 9.7 wt % for [C64DMAPy][N(CN)2]. Also, the sulfur partition coefficient was evaluated. In the case of model gasoline, the thiophene (TS) partition coefficient in model gasoline varies from 0.885 for [C24DMAPy][N(CN)2] to 1.218 for [C64DMAPy][N(CN)2]. In the case of model diesel, 4-dimethylaminopyridinium-based ILs exhibit a relatively higher sulfur partition coefficient, compared to alkyl modified pyridinium-based ILs. 1H NMR results confirmed that the high aromatic π-electron density of the dimethylaminopyridinium cation was the main reason for the good extraction performance. 1H NMR results also confirmed the KN sequence for each IL ([C24DMAPy][N(CN)2] < [C44DMAPy][N(CN)2] < [C64DMAPy][N(CN)2]) is mainly due to the sulfur compounds and ILs structures. Furthermore, the extractive selectivity results indicated a more preferable extraction of TS than toluene with 4-dimethylaminopyridinium-based ILs. To have a better evaluation of the overall sulfur extraction performance, the three 4-dimethylaminopyridinium-based ILs were compared to other typical ILs (e.g., pyridinium-based ILs and imidazolium-based ILs), which suggested our synthesized 4-dimethylaminopyridinium-based ILs exhibited good balance between mutual solubility and the sulfur partition coefficient. ILs after use were regenerated by a water dilution process. Based on these results, 4-dimethylaminopyridinium-based ILs can be used as potential extractants for EDS processes.

1. INTRODUCTION In the past decade, great attention has been paid to deep desulfurization of transportation fuels due to the strict regulations on sulfur content in fuel (99%), iodoethane, 1-iodobutane, and 1-iodohexane were purchased from TCI Chemical Reagent Co. Ltd. Sodium dicyanamide was purchased from Aladdin Chemical Reagent Co. Ltd. TS, BT, DBT, and 4,6-DMDBT were purchased from Acros Organics, USA. Acetone, n-octane, 1hexene, toluene, and n-dodecane were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents are analytically pure. Commercial 97# gasoline and 0# diesel were purchased from a gas station of Sinopec Corporation. 2.2. Preparation of ILs. The IL [Cn4DMAPy][N(CN)2] was synthesized by two-step reactions (see Figure 2). The procedure is presented as follows: The first step, [Cn4DMAPy]I, was prepared according to the paper published by Kupetic et al.24 The second step, the synthesis of [Cn4DMAPy][N(CN)2], was according to the paper published by Crosthwaite et al.25 The 4-dimethylaminopyridinium-based ILs have been identified by 1H NMR in D2O (Brulcer DMX 500 MHz, Switzerland). Various molar ratios of TS/[C24DMAPy][N(CN)2] 4618

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Table 1. Extractive Performance with Different Pyridinium-Based ILsg KN, mg(s) kg(IL)‑1/mg(s) kg(Oil)‑1 IL [C4Py][BF4] [C6Py][BF4] [C8Py][BF4] [C43MPy][BF4] [C63MPy][BF4] [C83MPy][BF4] [C43,5DMPy][BF4] [C63,5DMPy][BF4] [C83,5DMPy][BF4] [C24DMAPy][N(CN)2] [C44DMAPy][N(CN)2] [C64DMAPy][N(CN)2]

TS a

0.53 0.70a 0.79a 0.85a 1.00a 1.07a 0.90b 1.09b 1.15b 0.885d 1.028d 1.218d

BT a

0.74 1.28a 1.40a 1.75a 2.08a 2.16a 1.93b 2.45b 2.58b 2.93e 3.18e 3.68e

DBT

4,6-DMDBT

a

a

0.77 1.42a 1.78a 2.08a 2.89a 3.11a 2.54b 3.59b 5.18b 4.36e 5.60e 6.45e

0.16 0.48a 0.83a 0.56a 0.93a 1.29a 0.87b 1.32b 1.38b 1.90e 1.97e 2.75e

solubility, mg(diesel)/g(IL) c

4.9 13c 19.7c 61c 76c 95c 132b,c 167b,c 196b,c 67f 74f 97f

ref 31 31 31 22 22 22 24 24 24 this work this work this work

a

Room temperature. b333 K. cReal diesel. dModel gasoline: 500 ppm sulfur as sulfur compound in 45% n-octane, 30% hexene, 25% toluene; mass ratio 1:1; mixture 30 min; room temperature. eModel diesel: 500 ppm sulfur as sulfur compound in n-dodecane; mass ratio 1:1; mixture 30 min; room temperature. f0# diesel. gModel oil: 160 ppm sulfur as sulfur compound in n-dodecane; mass ratio 1:1, mixture 15 min.

Figure 3. 1H NMR chemical shifts of protons on pyridinium-based ILs in D2O. The solubility of fuel in ILs was measured by the gravimetric method. The detail steps are as follows: Remove upper oil phase by syringe and then weigh the mass of ILs phase. The mass of fuel dissolved in ILs was determined by calculating the mass difference between ILs mass before and after extractive experiment. 2.5. Evaluation of Extractive Desulfurization. The sulfur partition coefficient (KN) and extractive selectivity (S) are used to evaluate extractive desulfurization performance. KN is based on gravity, which is defined as the ratio of sulfur content in IL to sulfur content in fuels. KN measurements were carried out in both model gasoline and model diesel. S is defined as the ratio of the TS partition coefficient to the toluene partition coefficient. This is because most sulfur compounds exist in the form of TS in gasoline. Also, due to the π−π-interaction between pyridinium-based ILs and aromatic compounds, aromatic sulfur compounds and aromatic hydrocarbon could be a competitor for the TS extraction process with pyridinium-based ILs.26−28 According to most published papers in this area,8,12 toluene was chosen as the representative aromatic hydrocarbon. The S measurement was carried out only in model gasoline.

of fuel in ILs complies with the following sequence: [C64DMAPy][N(CN)2] (9.7 wt %) > [C44DMAPy][N(CN)2] (7.4 wt %) >[C24DMAPy][N(CN)2] (6.7 wt %). It can be seen from the sequences that the solubility of both 97# gasoline and 0# diesel in ILs increases with the length of the alkyl substituents in the 4-dimethylpyridinium cation, which is because the solubility of fuels in 4-dimethylpyridinium-based ILs is strongly influenced by the size and nonpolarity of the 4dimethylpyridinium cation. Since size and nonpolarity increase from [C24DMAPy][N(CN)2] to [C64DMAPy][N(CN)2], the mutual solubility followed the above-mentioned sequence.12,21 3.2. KN of 4-Dimethlaminopyridinium-Based ILs for Model Fuels. In this section, we examined the KN for 4dimethlaminopyridinium-based ILs in both model gasoline and model diesel. For model gasoline, TS was chosen as the typical sulfur compound. While for model diesel, BT, DBT, and 4,6DMDBT were chosen as the typical sulfur compounds. The KN for TS in model gasoline are as follows: 0.885 for [C24DMAPy][N(CN)2], 1.028 for [C44DMAPy][N(CN)2], and 1.218 for [C64DMAPy][N(CN)2]. The KN for BT, DBT, and 4,6-DMDBT in model diesel are listed in Table 1, which revealed that 1) for each type of sulfur compound, KN for different ILs species in model gasoline or model diesel complied with the following sequence: [C24DMAPy][N(CN)2] < [C44DMAPy][N(CN)2] < [C64DMAPy][N(CN)2] and 2) for each type of IL, the KN for different aromatic compounds complied with the following sequence: TS < 4,6-DMDBT < BT < DBT.

3. RESULTS AND DISCUSSION 3.1. Mutual Solubility of ILs and Fuels. The mutual solubility is an important index for evaluating an extractant. HPLC results indicated the solubility of 4-dimethylaminopyridinium-based ILs in fuels was lower than the level of detection. However, fuels could be partly dissolved in 4-dimethylaminopyridinium-based ILs. For 97# gasoline, the solubility of fuel in ILs complies with the following sequence: [C64DMAPy][N(CN)2] (12.4 wt %) > [C44DMAPy][N(CN)2] (9.3 wt %) > [C24DMAPy][N(CN)2] (7.0 wt %). For 0# diesel, the solubility 4619

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shown in Table 2, H6 and H7).18 The result indicates that the concentration of TS has a tiny influence on aromatic π-electron density on [C24DMAPy][N(CN)2]. Additionally, chemical shifts of ring protons of TS are not affected by the concentration of [C24DMAPy][N(CN)2] (as shown in Table 2, H1 and H2). Based on the above results, the [C24DMAPy]+ cation is proved to have high aromaticity. These are reasons for the high KN archived by the 4-dimethylaminopyridinium-based ILs. Our synthesized ILs and aromatic sulfur compounds were all electron-donating compounds, and thus the interaction mechanism between them were π−π-interaction between aromatic sulfur compounds and aromatic cations of ILs. Different from our desulfurization mechanism, Holbrey et al.33 used an electron-withdrawing group cyanogen to modify pyridinium cation and synthesized 1-alkyl-4-cyanopyridinium ILs. These synthesized ILs were electron-accepting, which can interact with electron-donating compound, such as 1methylnaphthalene. The interaction mechanism between electron-accepting and electron-donating compounds was proved to form a charge transfer complex. Therefore, the two interaction mechanisms are different. The π−π-interaction between aromatic structures of sulfur compounds and cations of ILs is affected not only by aromatic π-electron density on cation of ILs but also by aromatic πelectron density on aromatic sulfur compounds. Previous published papers have demonstrated that the aromatic sulfur compounds with a higher aromatic π-electron density can be extracted more easily by ILs.29,34 The densities of aromatic πelectron on sulfur compounds are 5.696 for TS, 5.739 for BT, 5.758 for DBT, and 5.760 for 4,6-DMDBT.35 The π-electron density difference explains the KN sequence for TS, BT, and DBT, while, for 4,6-DMDBT, the π−π-interaction is affected not only by aromatic π-electron density but also by the steric hinder.29,34 The existence of a methyl substituent in 4,6DMDBT significantly decreases KN. Thus, for the 4dimethylpyridinium-based ILs, KN for different sulfur compounds follow the order TS < 4,6-DMDBT < BT < DBT. The KN sequence for each IL ([C24DMAPy][N(CN)2] < [C44DMAPy][N(CN)2] < [C64DMAPy][N(CN)2]) is mainly due to the sulfur compounds and ILs structures, besides aromatic π-electron on them. A previously published paper demonstrated that ILs with longer alkyl N-substituents have a better extractive performance, owing to the higher aromatic πelectron density and larger size.12,22 In our work, 1H NMR analysis results indicated the chemical shifts of the ring protons of these three 4-dimethylaminopyridinium-based ILs is 0.03, as the length of alkyl N-substituent increases from ethyl to hexyl. This proved that the polarizable aromatic π-electron densities of these three 4-dimethylaminopyridinium-based ILs are almost the same. Thus, in our case, larger size is the dominating factor for the KN increase. A longer alkyl N-substituent could increase the cation size. As the cation size increases, the Coulombic interaction between cation and anion decreases. This results in the increase of π−π-interaction between sulfur compounds and ILs.12,22 3.3. S of 4-Dimethlaminopyridinium-Based ILs for Model Gasoline. S is another important index for the EDS process. The results indicated that S decreases with the increase of the length of alkyl N-substituent in 4-dimethylaminopyridinium cation and increases as the increase of the toluene content in model gasoline. For all three 4-dimethylaminopyridinium-based ILs, S are higher than 2. Especially for [C24DMAPy][N(CN)2], S varies from 3.X to 4.X. This

The KN sequence for different ILs can be explained based on π−π-interaction between aromatic structures of sulfur compounds and aromatic cations of ILs. The extraction performance of aromatic sulfur compounds using ILs with different aromatic cations of ILs is positively correlated to the π−πinteraction,18−21 which is influenced by two factors: 1) Aromatic π-electron density on sulfur compounds and aromatic π-electron density on cations of ILs. The higher π-electron densities on both parts would lead to a stronger π−πinteraction between them. 2) The structure of sulfur compounds and ILs. Among all other reported pyridiniumbased ILs,19,21,22,29−31 pyridinium-based ILs with metallic anions containing halogen has the highest KN, owing to strong π-complexation between aromatic sulfur compounds and metallic anions of ILs.16 However, metallic anions containing halogen not only are sensitive to moisture13 but also coextract the aromatic hydrocarbon compounds.14 Due to these mentioned problems for anion modifications, cation modification provides an alternative method for increasing the sulfur partition coefficient. Compared to other reported alkyl modified pyridinium-based ILs,21,22,29 the 4-dimethylaminopyridinium-based ILs have higher KN. This is mainly attributed to the higher electron donating capacity of the dimethylamino substituent than the alkyl substituent. This hypothesis was confirmed by the 1H NMR chemical shifts of the ring protons in neat ILs (Figure 3). It can be concluded that the 1H NMR chemical shifts of the same position ring protons on pyridinium-based ILs21,22,32 in D2O complies with the sequence [C4Py]+ > [C43MPy]+ > [C43,5DMPy]+ > [C44DMAPy]+. It is known that the lower 1H NMR chemical shifts of the same position ring protons on pyridinium-based ILs are, the aromatic π-electron density and aromaticity of pyridinium cation are higher. Therefore, the aromatic π-electron density on pyridinium cations follows the order [C4Py]+ < [C43MPy]+ < [C43,5DMPy]+ < [C44DMAPy]+. The hypothesis was further confirmed by the 1H NMR chemical shifts of the ring protons in TS and [C24DMAPy][N(CN)2] at various molar ratios of TS/[C24DMAPy][N(CN)2] (Figure 4 and Table 2). 1H NMR

Figure 4. Structure and 1H NMR signal assignments of TS and [C24DMAPy][N(CN)2].

chemical shifts of the ring protons in neat TS (H1, 7.568 and H2, 7.151) and the chemical shifts in neat [C24DMAPy][N(CN)2] (H6, 7.046 and H7, 8.324) are listed in Table 2, which suggested aromatic π-electron density in neat [C24DMAPy][N(CN)2] is close to that in neat TS. Also, because TS is πexcessive, we can conclude that aromatic π-electron density in [C24DMAPy][N(CN)2] is also high. Meanwhile, 1H NMR chemical shifts values of ring protons in [C24DMAPy]+ cation decreased approximately 0.02, when the molar ration of TS/ [C24DMAPy][N(CN)2] was increased from 0 to 3.35 (as 4620

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Table 2. 1H NMR Chemical Shifts of Protons on Thiophene and N-Ethyl-4-dimethylaminopyridinium Dicyanamide in Each TS/[C24DMAPy][N(CN)2] Solutiona R thiophene 3.35 2.58 1.29 0.61 [C24DMAPy][N(CN)2]

H1 7.558 7.565 7.653 7.654 7.565

(0.007) (0.005) (0.006) (0.007)

H2 7.151 7.154 7.153 7.155 7.155

(0.003) (0.002) (0.004) (0.004)

H3 1.380 1.383 1.386 1.386 1.391

H4

(−0.011) (−0.008) (−0.005) (−0.005)

3.179 3.180 3.184 3.186 3.192

(−0.013) (−0.012) (−0.008) (−0.006)

H5 4.189 (−0.006) 4.190 (−0.005) 4.194 (−0.001) 4.195 (0) 4.1995

H6 7.028 7.027 7.031 7.032 7.046

(−0.018) (−0.019) (−0.015) (−0.0014)

H7 8.313 8.310 8.312 8.312 8.324

(−0.011) (−0.014) (−0.012) (−0.012)

a

The 1H NMR signals corresponding to the protons on TS and to those on the pyridinium cation of [C24DMAPy][N(CN)2] for the system is assigned in Figure 3. (): δ0 - δR, where δ0 is the 1H chemical shift of neat TS or of neat IL and δR is the 1H chemical shift of TS and of the IL for a molar ratio R.

relatively high S indicated a preferable extraction of TS than toluene, which is desired in a practical EDS process. Since the toluene content could influence both TS and toluene partition coefficients, the effect of the toluene content on S was investigated. As shown in Figure 5, when the toluene

coefficients increased with the length of alkyl increase. The possible reason is the longer alkyl in the pyridinium cation expands the size of the pyridinium cation and then decreases the Coulombic interaction of cation and anion.12,21,22 This eventually increased the TS and toluene partition coefficients. 3.4. Overall Evaluation of Extractive Performance of 4-Dimethylaminopyridinium-Based ILs by Comparison with Other Typical ILs. In this section, we evaluated the extractive performance of our 4-dimethylaminopyridiniumbased ILs for deep desulfurization based on mutual solubility and KN. BT, DBT, and 4,6-DMDBT were chosen as representative sulfur compounds in model diesel. Nine pyridinium-based ILs with similar structure (alkyl group substituted pyridinium-based ILs) were chosen as comparisons. The mutual solubility and KN data are listed in Table 1. Compared to [CnPy][BF4], [Cn4DMAPy][N(CN)2] have higher mutual solubility and significantly higher KN; compared to [Cn3MPy][BF4], [Cn4DMAPy][N(CN)2] have the same level mutual solubility and higher KN; and compared to [Cn3,5DMPy][BF4], [Cn4DMAPy][N(CN)2] have lower mutual solubility and higher KN. The main reasons for the differences in mutual solubility are the differences in the size and polarity of pyridinium cations.12,21 Compared to the methyl substituent on the pyridinium cation, the dimethylamino group substituent has a bigger size and higher polarity. The size of the pyridinium cation with different substituents complied with the following sequence: [Cn4DMAPy]+ > [Cn3,5DMPy]+ > [Cn3MPy]+ > [CnPy]+, and the polarity complied with the following sequence: [C n Py] + < [C n 3,5 DMPy] + < [C n 3 MPy] + < [Cn4DMAPy]+. These resulted in the following mutual solubility sequence: [C 6Py][BF4 ] < [C 63MPy][BF4 ] ≈ [C64DMAPy][N(CN)2] < [C63,5DMPy][BF4]. As for the KN, aromatic sulfur compounds extraction with pyridinium-based ILs is related to π−π-interaction between aromatic sulfur compounds and ILs, which is strongly influenced by aromatic π-electron density of pyridinium cation.18−21 Substituents having electron donating capacity (e.g., dimethylamino and methyl) could increase the aromatic π-electron density of the pyridinium cation. Due to the higher electron donating capacity of the dimethylamino substituent than the methyl substituent, π-electron densities of the pyridinium cation with different substituents follow the order: [Cn4DMAPy]+ > [Cn3,5DMPy]+ > [Cn3MPy]+ > [CnPy]+. This eventually leads to the KN order: [Cn4DMAPy][N(CN)2] > [Cn3,5DMPy][BF4] > [Cn3MPy][BF4] > [CnPy][BF4]. Based on the above results, our synthesized 4-dimethylamino modified pyridinium-based ILs can achieve a better balance between mutual solubility and KN than alkyl modified pyridinium-based ILs. For further evaluating the sulfur extraction performance, the 4-dimethyla-

Figure 5. Selectivity of ILs between TS and toluene at room temperature in model gasoline.

content increased from 5.6 wt % to 27.9 wt %, S for [C24DMAPy][N(CN)2] increased from 3.3 to 4.7; S for [C44DMAPy][N(CN)2] increased from 2.6 to 4.6; and S for [C64DMAPy][N(CN)2] increased from 2.4 to 2.7. This is likely due to the aromatic property of model gasoline increasing as the toluene content increases. A higher aromatic property results in a stronger competitive π−π-interaction between toluene and ILs. As a consequence, the TS selectivity decreased. Figure 6 also implied that both TS and toluene partition

Figure 6. Influence of toluene content in gasoline on TS partition coefficient and toluene partition coefficient. 4621

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sulfur content in fuels. The other important index, selectivity, equals the ratio of the sulfur partition coefficient to the toluene partition coefficient, which is also a constant irrespective of the sulfur content in fuels. Therefore, our synthesized ionic liquids extraction performance (capacity, sulfur partition coefficient, selectivity) would not be affected by various conditions of real fuels.

mino modified pyridinium-based ILs were also compared to other typical ILs with N(CN)2− as the anion using the DBT partition coefficient as the index. As shown in Table 3, 4dimethylamino modified pyridinium-based ILs exhibited better sulfur extraction performance than other typical ILs (e.g., pyridinium-based ILs and imidazolium-based ILs).23,31,36 Table 3. Sulfur Partition Coefficients for Extraction of DBT with Different ILs IL [EMIM] [N(CN)2]a [BMIM] [N(CN)2]a [EM2S] [N(CN)2]a [S2] [N(CN)2]a

KN, mg(s) kg(IL)−1/ mg(s) kg(Oil)−1 1.3 2.28 0.84 1.08

IL 3

[C4 MPy] [N(CN)2]b [C44MPy] [N(CN)2]b [BMTH] [N(CN)2]c [C44DMAPy] [N(CN)2]

4. CONCLUSION [C 2 4 DMAPy][N(CN) 2 ], [C 4 4 DMAPy][N(CN) 2 ], and [C64DMAPy][N(CN)2] were synthesized in this paper, and they were demonstrated to be effective for the selective removal of aromatic sulfur compounds from fuels at room temperature. Mutual solubility, KN, and S were chosen to be the evaluating index. First, the mutual solubility evaluating results indicated that 4-dimethylaminopyridinium-based ILs hardly dissolved in the fuels. While, for 97# gasoline, the solubility of fuel in ILs complies with the following sequence: [C64DMAPy][N(CN)2] (12.4 wt %) > [C44DMAPy][N(CN)2] (9.3 wt %) > [C24DMAPy][N(CN)2] (7.0 wt %). For 0# diesel, the solubility of fuel in ILs complies with the following sequence: [C64DMAPy][N(CN)2] (9.7 wt %) > [C44DMAPy][N(CN)2] (7.4 wt %) >[C24DMAPy][N(CN)2] (6.7 wt %). Second, the KN was evaluated. In the case of model gasoline, the TS partition coefficients in model gasoline are as follows: 0.885 for [C24DMAPy][N(CN)2], 1.028 for [C44DMAPy][N(CN)2], and 1.218 for [C64DMAPy][N(CN)2]. In the case of model diesel, compared to alkyl modified pyridinium-based ILs, 1H NMR results confirmed that high aromatic π-electron density of a dimethylaminopyridinium cation was the main reason for the good extraction performance. 1H NMR results also confirmed the KN sequence for each IL ([C24DMAPy][N(CN)2] < [C44DMAPy][N(CN)2] < [C64DMAPy][N(CN)2]) is mainly due to the sulfur compounds and ILs structures. Third, the extractive selectivity was examined. All three 4-dimethylaminopyridinium-based ILs have a relatively high selectivity factor, especially for [C2 DMAPy][N(CN)2]. This indicated a preferable extraction of TS than toluene, which is desired in a practical EDS process. To have a better evaluation of the overall sulfur extraction performance, finally, the three 4dimethylaminopyridinium-based ILs were compared to other typical ILs (e.g., pyridinium-based ILs and imidazolium-based ILs) with the same anion. Comparison results indicated our synthesized 4-dimethylaminopyridinium-based ILs exhibited a good balance between mutual solubility and KN. The extracted ILs loaded sulfur compounds could be regenerated by a water dilution process. Based on these results, 4-dimethylaminopyridinium-based ILs can be used as potential extractants for EDS processes.

KN, mg(s) kg(IL)−1/ mg(s) kg(Oil)−1 1.78 1.69 1.88 5.60

a

Model oil:23 500 ppm sulfur as DBT in hexane; mass ratio 1:1; mixture 15 min; 298 K. bModel oil:31 12400 ppm sulfur as sulfur compound in 64% n-heptane, 28% tetralin, 5% toluene, 0.5% TS, 0.5% BT, 2% DBT; mass ratio 1:1; mixture 15 min; 298 K. cModel oil:36 500 ppm sulfur as DBT in hexane; mass ratio 1:1; mixture 15 min; 313 K.

3.5. Regeneration of ILs. The regeneration and following recycling of ILs are very important from the industrial viewpoint. In this work, the water dilution process was used as the regeneration method. As shown in Figure 7, the

Figure 7. KN for TS and DBT between [C24DMAPy][N(CN)2] and fuels at different water contents (at 20 °C).

extraction performance of used ILs can be retrieved by adding 80% water. 1H NMR analyses also indicated that the ILs maintained their original structures after regeneration. However, due to a great deal of energy consumed in this process, this regeneration method is not suitable for practical application in industry. From the aspect of industrial application, the sulfur content of fuels in a practical desulfurization process varies a great deal. Thus, an ideal ionic liquid extractant should have enough extraction capacity to guarantee the desulfurization performance. The maximum absorption capacity for TS per mole of [C24DMAPy][N(CN)2] is approximately 3.35, which is high enough for the practical desulfurization process (sulfur content is usually lower than 1000 ppm). The sulfur partition coefficient for aromatic sulfur compounds is a constant irrespective of the

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 57187952525. Fax: +86 57187952525. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciated the financial support to this study by National Natural Science Foundation of China (NSFC No. 20976158, No. 21076189). 4622

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(36) Chen, X. C.; Liu, G.; Yuan, S.; Asumana, C.; Wang, W. B.; Yu, G. R. Sep. Purif. Technol. 2012, 47, 819−826.

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