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Efficient Extraction of Neutral Heterocyclic Nitrogen Compounds from Coal Tar via Ionic Liquids and Its Mechanism Analysis Lianzheng Zhang, Mi Zhang, Jun Gao, Dongmei Xu, Shixue Zhou, and Yinglong Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02297 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on September 3, 2018

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Efficient Extraction of Neutral Heterocyclic Nitrogen Compounds from Coal Tar via Ionic Liquids and Its Mechanism Analysis Lianzheng Zhang a, Mi Zhang a, Jun Gao* a, Dongmei Xu* a, Shixue Zhou a, Yinglong Wang b

a

College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China

b

College of Chemical and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China

Abstract: As a valuable chemical materials or energy fuels, the reasonable and efficiency utilization of coal tar is important. 4 different ionic liquids (ILs), 1-Ethyl-3-methyl

imidazolium

tosylate

([emim][TOS]),

1-Butyl-1-methyl

pyrrolidinium chloride ([bmpyrr][Cl]), N-butyl pyridinium chloride ([bpy][Cl]), 1-Ethyl-3-methyl imidazolium trifluoroacetate ([emim][TFA]), were investigated as extractants to extract the value-added neutral heterocyclic nitrogen compounds, indole and carbazole, from model coal tar in the present work. And the separation performance was valued by calculating the experimental extraction efficiency and

1

ACS Paragon Plus Environment

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distribution coefficient. For neutral nitrogen compounds, indole and carbazole, the adopted 4 ILs have higher extraction efficiency and selectivity than basic nitrogen compounds, pyridine and quinoline. Then, the key experiment factors, extraction temperature, extraction time, mass ratio of ILs to model oil, and initial N-content were explored and optimized. Meanwhile, the extraction mechanism was studied by analyzing the intermolecular interactions that formed between the anions of the adopted ILs and neutral nitrogen compounds through molecular simulation. And the investigated 4 ILs were ascertained to be the potential extractants with high separation capacity for indole and carbazole. In addition, the reusability of the IL extractant was confirmed.

Keywords: Nitrogen Heterocyclic Compounds, Ionic Liquids, Coal Tar, Extraction, COSMO-SAC

*Corresponding author

Jun Gao, E-mail address: [email protected], Tel: +86-532-86057103

Dongmei Xu, E-mail address: [email protected], Tel: +86-532-86057798

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1. Introduction In today’s society, the issue of environmental protection is receiving more and more attention and focus from people and governments. And most of these issues are centered around the use of fossil fuels, which could produce large amounts of harmful gases when burnt, pollute the environment, damage the ozone layer and caused global warming. Coal plays an important role in all fossil fuels, especially in countries rich in coal and less in oil. Among them, china ranks the first in coal production and reserves from all over the world, and coal has always played a very important role in its energy structure. But unreasonable utilization of coal not only pollutes the environment, but also wastes the non-renewable energy resources. Thus, as the main source of energy and chemical raw materials, it is more important to clean, reasonably and efficiently use coal in a comprehensive manner. And many methods have already been developed, such as coal gasification, coal liquefaction, coke and its chemical products recycling and so on, to make a deep utilization of coal. Then, a lot of high value-added materials can be produced, from which more than 500 compounds are identified from coal tar, and many of the compounds are irreplaceable in petroleum industry

1, 2

. The typical with high content

aromatic compositions are shown in Table S1 according to its ring numbers and hetero

atom

types.

Among

them,

the

heterocyclic

(N-compounds), indole, carbazole, pyridine and quinoline

nitrogen

compounds

3, 4

, studied in this work

have been widely used in producing medicine, pesticides, spice, dyestuff, and plastics 3


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etc. Thence, it is of great significance to investigate the efficient utilization and separation method of high value-added materials produced from the deep processing of coal. However, the separation technology has limited its deep development and utilization. In the present literatures, there have already existed some ways to separate different kinds of compounds from coal tar, for instance sulfuric acid washing, alkali fusion, ion-exchange resin separation

5, 6

, solvent extraction

7, 8

and so on. In general, the

liquid-liquid extraction (LLE) is considered as a more economical and efficient method to separate N-compounds from different sources. Since the advantage properties of ionic liquids (ILs), such as non-volatile, good thermostability, strong dissolving capacity, designability, selectivity, and environment friendly, ILs are rapidly studied and utilized in variety fields by many researchers

9-18

. To our

knowledge, unlike the extensive investigations for separating phenolic compounds from coal tar

19-24

, few investigations in the separation of N-compounds with ILs has

been employed, which imidazole phosphate and imidazole dihydrogen phosphate ILs with different alkyl carbon chain lengths were synthesized by using organophosphate and dihydrogen phosphate as the anions respectively. The best extraction efficiency can reach 92.3% using 1-butyl-3-methylimidazolium dihydrogen phosphate ILs as the extraction agent at 40℃ and mass ratio of ILs to oil is 0.2 for 30 min 25. In addition, indole was separated by the imidazolium-based ILs as extractants from wash oil and

4


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the extraction efficiencies was larger than 90% 9. But used as an extractant in separating coal tar, there has been less interest in developing and synthesizing new ILs, and of course, the effect of the ILs’ structure on its separation process is unclear. Though the relevant investigations on separating different kinds of N-compounds from coal tar are less, but the intervention of ILs in denitrogenation from diesel and gasoline

26-30

is not new. The details of some representative ILs applied in the

separation or removal of neutral N-compounds are reviewed in Table 1. According to those previous works, it is feasible that ILs can be adopted as potential extractants for N-compounds from coal tar. And it is obvious that the imidazolium, pyridinium cation with different types of anions have a relatively high extraction capacity for the heterocyclic neutral N-compounds. Most of the extraction process in those works are in room temperature with mass ratio of 1: 1 (IL: oil). But the temperatures for the ILs containing halide anion are little higher due to there higher melting point. Table 1 Comparison of the reported ILs in the separation of neutral N-compounds.

It is contemplated that ILs can be adjusted to have targeted physicochemical properties by altering their combination of cations and anions. Many works have been successfully applied in the denitrogenation process, which confirmed that ILs could be act as potential extractants in the separation of coal tar. As a result, the aims of the present work are mainly focus on filling the defects found in separating heterocyclic neutral N-compounds in coal tar by ILs, and obtaining one or more usable IL 5


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extractants with high single stage extraction efficiency and low dosage. Based on our previous work analysis, three kinds of cations with different types of anions are screened and confirmed to extract neutral heterocyclic N-compounds from model coal tar based on the COSMO-SAC model. After the selection of one or more appropriate ILs as the extractants, the main extraction factors that may affect the extraction effects, extraction temperature, extraction time, mass ratio of the extractant to model oil, and initial N-content, are explored and systematically optimized. As far as we know, the selected IL extractants based on the above have not been investigated in separating coal tar. Then, the extraction mechanism needs to be verified, which the presence of the strong polar atoms in the anion could favor better interactions with neutral N-compounds. Therefore, a computational study is developed to explore the mechanism of the extraction process in molecular level. And the intermolecular interactions that formed between the anions of the adopted ILs and neutral N-compounds are analyzed. At last, the re-using and recycling of the extractant are investigated, and the investigated 4 IL extractants were ascertained to be the potential extractants with high separation capacity for indole and carbazole.

2. Experimental 2.1 Chemicals and Preparation of the Model Oil Since coal tar is an involved compound of various heteroatom compounds, such as different types phenol compounds, basic N-compounds, neutral N-compounds, neutral aliphatic and aromatic compounds etc., thus, many kinds of analytical pure reagents 6


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were used in this work. All of the common organic reagents were obtained commercially and their purities are the mass fractions reported by the suppliers. In addition, the IL extractants used in this work were also purchased directly, and all of them were made a pretreatment before the extraction experiment (Dried at 150℃ in vacuum conditions by a rotary evaporator, Shanghai Shensheng Tech. Co., Ltd, R205D). And the information, such as CAS number, formulas etc., for the used chemicals are supplied in Table 2. Table 2 CAS number, formula, providers and mass content of the used chemicals.

Considering the complication of coal tar, the experimental model oil was prepared by selecting various of heteroatom compounds, such as indole, carbazole, pyridine, quinoline, naphthalene, and acenaphthene, in real coal tar. Meanwhile, methylbenzene was adopted to be the alternative solvent in the model oil. Then, the model coal tar was weighed accurately and mixed according to the concrete composition of coal tar fractions. For the convenience of experiment, two kinds of model oils were confirmed. One was the mass ratio of 1: 1: 1: 10 for indole, pyridine, quinoline, and naphthalene respectively in model oil I, and the other one was 1: 1: 10: 5 for carbazole, pyridine, naphthalene, and acenaphthene in model oil II. This way to prepare the model oils can avoid the mutual influence by the similarity N-compounds during the extraction process and accurately know the separation ability of each extractants. Meanwhile, considering the dissolving capacity of acenaphthene and 7


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carbazole, acetone was used to avoid the phase splitting in the model oil II. Then, model oil I and II were used to the following experiments for extraction of indole and carbazole respectively. The concrete compositions of the two model oils are presented in Table 3. Table 3 Concentrations of two model coal tar. 2.2 Experiments and Analysis The detailed equipment and procedures of the extraction experiments were described in detail in our previous work 31, 41-43. After the extraction experiments, two new layers were formed and separated, and the mass of the two layers were weighted accurately. Then, the samples drawn from the raffinate phase by a syringe were quantitative analyzed by the gas chromatography (Lunan SP-7820). The instrument configuration of GC was listed as follows: nitrogen phosphorus detector (NPD), capillary column (KB-FFAP, 30m×0.32mm×0.5µm, Kromat Technologies for the model oil I and DB-WAX, 30 m×0.53 mm×1.00 µm, Agilent Technologies for the model oil II). For more accurate determination, the programmed temperature procedure was adopted to analysis different samples and good peak shape was confirmed which could avoid large analytical errors. The detailed GC conditions for the two model oils are shown in Table 4. And N-methyl pyrrolidone (NMP) was selected as an internal standard substance to make a quantitative analysis. The content analysis of all

8


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samples was confirmed by the N2000 workstation. To reduce the measurement deviation, each sample were analyzed at least three times and then the mean values were adopted. Table 4 The detailed gas chromatography analysis conditions for N-compounds.

2.3 Experimental Extraction Efficiency and Distribution Coefficient After the extraction experiments, the experimental performance of indole and carbazole by different kinds of ILs were obtained. And the extraction capability of the selected ILs was evaluated by calculating the extraction efficiency (EE) and distribution coefficient (D), which were presented by Eq. 1 and 2. Both of the two equations are presented as follows: = [( − )⁄ ] × 100

(1)

= ⁄

(2)

where is the concentration of N-compounds before extraction, is the concentration of N-compounds after extraction, and is the concentration of N-compounds in IL phase. Then, the concentration of the lower layer can be obtained by the conservation of mass. 2.4 Computational Details The interactions between the IL extractants and N-compounds were investigated by quantum chemical calculations, which explored the mechanism of the extraction process in a micro level. The bond length, the interaction energy, total density maps and deformation charge density maps calculated by quantum chemical calculations 9


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were adopted to illuminate the extraction mechanism. In general, density functional theory (DFT) is increasingly used in chemistry and materials science to explain and predict complex system behaviors and interactions at the molecular level. And the most common software or program were Dmol3, Turbomole and Gaussian etc. Thus, the Dmol3 module incorporated the DFT theory in Accelrys Material Studio was adopted here

42

because of it was convenient and available to use. The computational

details are mainly two steps which are expressed as follows: Firstly, it was necessary to draw the selected ILs and N-compounds in three dimensions accurately. Therefore, to avoid the potential mistakes about the studied structures, all the structures of ILs and N-compounds used in this work were downloaded

from

the

online

databases,

such

as

the

NIST

database

(http://webbook.nist.gov/ chemistry/), Scifinder (http://www.cas.org/SCIFINDER/ SCHOLAR/index.html),

and

the

ChemSpider

(http://www.chemspider.com/

Search.aspx). After drawing the confirmed molecular structures in Materials Studio, the “clean” tool was used to adjust the initially bond lengths and angles, because this tool can arrange the positions of all atoms and make a rough geometry optimization to decrease the computational time of the following steps. Afterwards, the appropriate geometry structures in ideal gas circumstance could be confirmed by running a “Geometry Optimization” task. During the progress of geometry optimization, the GGA/VWN-BP functional settings with a real space

10


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cutoff of 5.5 Å were used with the DNP 4.4 basis set. Then, to accomplish a rapid convergence, 0.005 Ha was adopted for the smearing energy. In addition, the convergence criteria of the SCF tolerance was set at 1.0*10-6 Ha, and the convergence tolerances were set as follows: energy tolerance, 1.0*10-5, maximum force, 0.002 Ha/Å, maximum displacement, 0.005 Å, all the detailed information are presented in our previous work. Then, a global energy minimum structure for each molecule should be obtained instead of a local minimum one. In order to obtain the global energy minimum structure of the studied molecule, three or more different initial geometries were tested and confirmed, and the structure with the lowest energy was used for subsequent calculations. In addition, several different randomized conformations were compared to verify that the conformation can be restored to a low energy state. Then the bond length between the ILs and N-compounds could be obtained. Secondly, the interaction energy between the IL extractants and indole/ carbazole can be obtained by calculating the energy difference between the monomer and the complex with the same parameter settings of the “Geometry Optimization” procedure. The counterpoise correction method

44

was used here to correct the Basis Set

Superposition Error (BSSE), which could make the calculated results more accurate. The interaction energy calculation formulas are presented in the following: ∆E"#$%&'($")# = *+ − * − + + +--.

(3)

+--. = * − (*,*+) + + − +,*+

(4)

11


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where *+ is the energy of the complex of AB in the A, B basis set, * is the energy of A in the A basis set, + is the energy of B in the B basis set, (*,*+) is the energy of A in the A, B basis set and +,*+ is the energy of B in the A, B basis set. In addition, the total density maps and deformation charge density maps of the ILs and N-compounds could be obtained by the electron density analysis.

3. Results and Discussion Considering the extensive applications of ILs in the extraction process in the presented literatures. There are some potential mechanisms proposed by different researchers, which are the formation of the clathrate between cations of the ILs and the solvents 45, the sandwich-like structure 11, 46 caused by the existed π-π interactions between the ILs and aromatic hydrocarbons, the formation of the hydrogen bond formed by the polar groups of the anions of the ILs extractants for some basicity compounds

47, 48

, or the acidity of the IL

49

. All the proposed extraction mechanism

may explain the good separation ability of the studied IL extractants. Moreover, hydrogen bonds formed between the IL extractants and the targeted compounds belongs to an intermolecular force, which is reversible and can be destroyed by the back-extraction regents or heated. Thus, it is worthy of designing a series of IL extractants to extract N-compounds through the hydrogen bond. Due to the strong polarity of the ILs, it is of great possibility to generate the intermolecular interactions with the targeted compound from many kinds of potential 12


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IL extractants developed theoretically. For separating or removing neutral N-compounds from coal tar or diesel fuels, the verified feasible IL structures for the extraction process in the open literatures are summarized in Table 1. It is noticeable that the extraction ability for indole and carbazole were mostly influenced by the anionic structure. Subsequently, the COSMO-SAC model was adopted to select the appropriate ILs for the separation of N-compounds in this work. The details of the selection and analysis are described in part 1 in the supplementary material. Afterwards, two kinds of imidazolium based ILs with functional anions, [emim][TOS], [emim][TFA], and three imidazolium, pyridinium, and pyrrolidinium cations with the strong polar halogen anion, [bmim][Cl], [bpy][Cl], and [bmpyrr][Cl] were confirmed to extract the neutral N-compounds. Thereinto, [bmim][Cl] have already been verified in the previous work

29

, here only used to compare the effects

caused by different kinds of cations. 3.1 σ-profile Analysis The σ-profile of the selected ILs were obtained by the COSMO calculations and more detailed information are listed in Table S4 in the supplementary material and our previous work

50

. The σ-profile of the above five ILs, [Emim][Tos],

[emim][TFA], [bmim][Cl], [bpy][Cl], and [bmpyrr][Cl] are presented in Fig. 1. Meanwhile, the screening charge distribution for the five ILs are [-0.013, +0.016] e/ Å2, [-0.013, +0.017] e/ Å2, and [-0.013, +0.018] e/ Å2, [-0.013, +0.019] e/ Å2, and [-0.011, +0.019] e/ Å2, respectively. 13


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Fig. 1 The σ-profile comparison of [emim][TOS], [emim][TFA], [bmim][Cl], [bpy][Cl], and [bmpyrr][Cl].

The screening charge distributions of the five ILs are all extended to the polar region with a strong charge distribution in the positive region. The peak of O atom and Cl atom in the anions are distributed in the polar region, respectively, and can serve as a hydrogen bond acceptor. On the contrary, the screening charge distributions of indole and carbazole have a wide distribution in the negative charge region and can be served as hydrogen bond donor, which are expressed in Fig. S2 in the supplementary material. Therefore, an intermolecular force can be formed between both of the five selected IL extractants and the neutral N-compounds, which could complete the separation process. 3.2 Optimization of Extraction Conditions 3.2.1 Effect of extraction temperature Single factor experiments were adopted to explore the influence of the extraction temperature. According to the melting points of the selected ILs, [bpy][Cl] and [bmpyrr][Cl] which are solid at room temperature are investigated in the temperature range of 30-80℃. Moreover, [emim][TFA] and [emim][TOS] which are in liquid form at room temperature are investigated in the temperature range of 20-55℃, and the extraction experiment was conducted with the mass ratio of 1: 2 for 30min. The single-factor experiment results are shown in Fig. 2 and listed in Table S5.

14


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As shown in Fig. 2 (a), for indole, the extraction efficiencies (EE) and distribution coefficients (D) of [bpy][Cl] and [bmpyrr][Cl] are greatly influenced by the temperature. As the extraction temperature increases, EE and D values of indole increased first and then decreased. [bpy][Cl] reached the maximum at 60℃, and [bmpyrr][Cl] was at 40℃. In contrast, the extraction temperature has less effect on [emim][TFA] and [emim][TOS]. As the temperature increases, EE and D values generally show a decreasing trend, but the decreasing degree is limited. The higher melting point of [bpy][Cl] and [bmpyrr][Cl] can explain the experimental phenomena. With the extraction temperature rising, both of [bpy][Cl] and [bmpyrr][Cl] are gradually changes from the solid state to the liquid state, which could make the extraction process more efficiency due to the larger contact area with the model oil during the thorough stirring extraction process. Meanwhile, the solubility of indole in the model oil solvent would increase with the increasing of the extraction temperature, which caused the decrease of the extraction efficiency. Meanwhile, the extraction temperature has little effect on [emim][TFA] and [emim][TOS], which is caused that both of the two ILs exist in liquid form at room temperature and there is no phase transition during the extraction process. However, as the temperature increases, the solubility of indole increases caused the removal efficiency decreases. For carbazole, Fig 2 (b), the same trends are presented as indole, but the extraction capacity of the selected 4 ILs are generally less due to its complex structure, leading

15


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to a decrease in EE and D values. And this is caused by a large steric hindrance of carbazole than indole, which reduces the interaction between the IL extractants and carbazole. Fig. 2 Influence of extraction temperature for EE and D, indole (a), carbazole (b) at the mass ratio of 1: 2 for 30min.

Thus, the optimized extraction temperature, EE and D values for indole and carbazole are presented as follows: [bpy][Cl], 60℃, 90.12%, 45.63 for indole and 80.30%, 21.26 for carbazole, [bmpyrr][Cl], 40℃, 90.87%, 49.79 for indole and 50℃, 75.56%, 14.64 for carbazole, [emim][TFA], 20℃, 87.36%, 26.33 for indole and 73.36%, 22.11 for carbazole, [emim][TOS], 20℃, 89.68%, 28.38 for indole and 73.68%, 23.32 for carbazole, respectively. Therefore, the extraction temperature was confirmed for the selected ILs and would be used in the work behind. 3.2.2 Effect of mass ratio of IL to model oil It is well known that the amount of extraction solvent used in the extraction process determines the economy of the extraction operation. Therefore, the reasonable extractant dosage is studied experimentally. The experiment conditions are at 60℃ for 1 h with the mass ratio of IL extractants to model oil 1: 10, 1: 5, 1: 2, 1: 1 and 2: 1, and the EE and D results are presented in Fig. 3 and listed in Table S6. The extraction efficiency of the selected IL extractants for indole (a) and carbazole (b) increased with the increase of IL dosage. For indole, the EE values of the four ILs were exceeded

16


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93% at mass ratio of 2: 1, while that of carbazole was weaker than indole. And when the mass ratio is less than 1: 2, the extraction efficiency of indole and carbazole increased rapidly with the increase of mass ratio, then the extraction efficiency increases slowly. On the contrary, the distribution coefficient for both the two neutral N-compounds are different with the regular increase of extraction efficiency. Fig. 3 Influence of mass ratio for EE and D, indole (a), carbazole (b) at 60℃ for 1 h.

In order to determine the best dosage of IL extractants, the economic efficiency and separation effect of the extraction process should be considered comprehensively, and the D values should be as high as possible while ensuring the high EE values. Therefore, for indole, although [emim][TFA] and [emim][TOS] achieved the maximum D value at the mass ratio of 2: 1. Considering the economic efficiency of the extraction process, the mass ratio of 1: 5 was adopted, and the experimental D values were 29.27 and 27.79, respectively, which its corresponding EE values were 85.41 and 84.75. And the experimental D values for [bpy][Cl] and [bmpyrr][Cl] decreased with the increase of mass ratio, so 1:10 was selected as the best mass ratio, and the experimental D values were 19.06 and 38.95, respectively, which its corresponding EE values were 81.24 and 79.95.For carbazole, the changes of [bpy][Cl], [emim][TFA] and [emim][TOS] were similar and reached the maximum D values at the mass ratio of 1: 5, which were 16.06, 18.96 and 21.03, respectively, and its corresponding EE values were 76.26, 61.84 and 68.30. The D values of 17


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[bmpyrr][Cl] decreases with the increase of the mass ratio, so 1:10 was selected as the best mass ratio, which is 17.27. 3.2.3 Effect of extraction time Extraction time was experimental investigated from 5 to 90 min, which were 5, 10, 30, 60, 90 min. All the extraction temperature and mass ratio were referenced to the above optimum conditions. During the experiment, it was found that indole and carbazole in the organic phase were immediately migrate to the ILs phase after the IL extractant was added to the model oil. And the experimental results are shown in Fig. 4 and listed in Table S7. Fig. 4 Influence of extraction time for EE and D, indole (a), carbazole (b) with the mass ratio of 1: 5 at optimized extraction temperature.

As can be seen from Fig. 4, the EE and D values for indole and carbazole increase rapidly with the increase of the time. And the EE and D values increased rapidly at the beginning 5-10 min. Then, reached the maximum value at 10-30 min. After more than 30 min, its curves flattened as the time increased to 90 min. And the maximum EE and D values for [emim][TFA], [emim][TOS], [bpy][Cl], and [bmpyrr][Cl] are 83.23 and 24.19, 85.35 and 24.36, 88.63 and 38.99, 86.42 and 31.81, respectively, for indole, 69.34 and 21.44, 79.24 and 21.75, 82.63 and 18.98, 80.96 and 21.26, respectively, for carbazole. Through the above analysis we can see that, it was needed

18


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at least 10 min to achieve the higher EE values. Therefore, 30 min was adopted in the presented work. 3.2.4 Effect of initial neutral N-content Taking into account the composition difference of the coal tar obtained under different pyrolysis conditions or different fractions. 4-5 kinds of model oils were adopted to investigate the influence of the initial N-content by the 4 selected IL extractants, and its initial neutral N-content was changed from 2% to 16%. And the extraction performance of the IL extractants were experimentally studied at the mass ratio of 1: 5 under the optimal extraction temperatures obtained from the above experiments. The experimental results are presented in Fig. 5 and listed in Table S8. Fig. 5 Influence of N-initial for EE and D, indole (a), carbazole (b) with the mass ratio of 1: 5 at optimized extraction temperatures.

As can be seen from Fig. 5, the EE and D values of the four ILs are all decreased with the increase of the initial N-content. Both EE and D values for the ILs are all have the same trend. For indole, less influence was caused by the increase of the initial neutral N-content for [emim][TFA] and [emim][TOS]. The extraction efficiency was decreased from 86.79% to 79.53%, and 89.26% % to 80.71%, respectively, indicating that these two ILs have excellent separation performance for indole even though the initial N-content was increased from 2% to 10%. However, the separation efficiency of [bpy][Cl] and [bmpyrr][Cl] showed a relatively significant

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decrease when the N-content of indole changed from 2% to 10%, and the decline range was generally about 8-10%, indicating that these two ILs were affected by the increase of the N-content and its separation ability was relatively weaker. Compared with indole, the extraction efficiency of carbazole for the four ILs were affected significantly, which was decreased about 10% when the content of carbazole verified from 2% to 10%. Fig. 6 The EE and D of different N-compounds by the screened ILs.

The optimal conditions were obtained by the above detailed extraction experiments. And good results were shown, which could be explained by existing interactions between the screened functional groups of the IL extractants and the target N-compounds. However, during those experiment, it was found that the extraction capability of the adopted extractants for the separation of basic N-compounds (pyridine and quinoline) is weak. Therefore, to compare the selectivity for different N-compounds, the experimental EE and D values of [emim][TFA], [emim][TOS] and [bpy][Cl] were obtained at the optimal temperatures with mass ratio of 1: 1 for 30 min, and the results are shown in Fig. 6 and listed in Table S9. The adopted IL extractants have good selectivity and extraction separation capacity for neutral N-component (indole, carbazole), but poor separation capacity for basic N-component (pyridine, quinoline), which can effectively avoid the low separation efficiency caused by the complexity compositions in the coal pyrolysis oil. 20


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3.3 Mechanism Analysis The extraction ability of the selected IL extractants was verified through the extraction experiments described in section 3.2. From the above experimental results, it was found that the interactions between the IL extractants and N-compound are the reason for the high extraction efficiency and selectivity of the indole and carbazole. To clarify the interactions, the selected five extractants are studied and classified. Thereinto, [bpy][Cl], [emim][TFA] and [emim][TOS] are selected as the representatives of different anions and investigated the extraction mechanisms by molecular simulation, which bond length, bond energy and electron densities are adopted here to explore the interactions between the IL extractants and N-compounds. Through the σ-profile analysis in section 3.1, the hydrogen bond donor and acceptor relationship between the IL extractants and N-compounds could be determined. And the hetero-atoms, in indole and carbazole, had the possibility to form hydrogen bond with the IL extractants. The intermolecular interactions that formed between the extractants and N-compounds are calculated to verify the above hypothesis. 3.3.1 Bond length between the IL extractants and N-compounds The geometry optimization of the N-compounds, IL extractants and their complexes were done based on the procedures described in section 2.4, and the optimized complexed geometry are shown in Fig. 7. Fig. 7 Bond length for [bpy][Cl] with indole (a) and carbazole (b), [emim][TFA] with indole (c) and carbazole (d), [emim][TOS] with indole (e) and carbazole (f). 21


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Through the analysis of Fig. 7 we can see that there may exist intermolecular interactions between the hydrogen atom attached to nitrogen atom in N-compounds and Cl or O atom of the anions. From the configurational results of the above six complexes, the interactions distance between indole and the anions of [bpy][Cl], [emim][TFA], and [emim][TOS] are 2.122 Å, 1.886 Å, and 1.829 Å, respectively, the interaction distance between carbazole and the anions of [bpy][Cl], [emim][TFA], and [emim][TOS] are 2.129 Å, 1.905 Å, and 1.836 Å, respectively. All the interaction distance of the complexes is less than the sum of the Van der Waal’s radii of the corresponding atoms, such as the Van der Waals radii of H, N, O and Cl are 1.30 Å, 1.83 Å, 1.72 Å and 2.05 Å, respectively, indicating that strong intermolecular interactions are existed between indole, carbazole and the anions of the ILs. And the strength of the effect can be compared by the size of the bond lengths, which are listed in Table 5. Therefore, we can infer that there may exist N-H…Cl and N-H…O two hydrogen bonds between the ILs and N-compounds. Table 5 Atomic radii for the ILs and neutral N-compounds. 3.3.2 Bond energy between the IL extractants and N-compounds Hydrogen bond is a slightly stronger interaction than the Van der Waals interaction, and much weaker than the covalent and ionic bonds. Usually, the energy of the hydrogen bond is mostly between 25-40kJ/mol. And it is generally considered that hydrogen bonds with bond energy 40 kJ/mol are regarded as stronger hydrogen bonds. The interaction energies of the above six complexes can be directly calculated from the BSSE-corrected interaction energies as described in the previous section, and the results are shown in Table 6. The interaction energies of [bpy][Cl] and [emim][TOS] with indole and carbazole are -55.127684 kJ/mol, -55.576688 kJ/mol and -53.074345 kJ/mol,-52.347012 kJ/mol, all of which are larger than 40 kJ/mol and belongs to the stronger hydrogen bonds. Moreover, the interaction energies of [emim][TFA] with indole and carbazole are -35.970190 kJ/mol and -36.967976 kJ/mol and belongs to a moderate strong hydrogen bond. The calculated results are agreed with the experimental one, indicating that strong intermolecular hydrogen bonding between the ILs and N-compounds can be formed. Table 6 The revised BSSE interaction energy of eight compounds. 3.3.3 Electron densities analysis between the IL extractants and N-compounds In addition to the bond length and bond energy, the electron densities are also adopted to confirm the formation of the hydrogen bond by calculating the distribution of the electron density. The total electron density (electron density isovalue) between the ILs and the indole/ carbazole are shown in Figs. 8 and 9. For indole and carbazole, the N-H···Cl and N-H···O hydrogen bonds between indole/ carbazole and the anions of [bpy][Cl] and [emim][TOS] are still evident, even if the isovalue value reaches 0.2.

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Whereas, [emim][TFA] is more evident when the isovalue reaches 0.15. This can explain the superior extraction ability of [bpy][Cl] and [emim][TOS] in the extraction experiments. Meanwhile, the similar results could be obtained by the analysis of deformation electron density, which is presented in Fig. 10. It is obvious that a certain degree of H···Cl and H···O bonding interactions could be seen from the slice of the deformation density, the received electron area surrounded the Cl atom or O atom are expressed in red and the betatopic area encircling around H atom is expressed in blue, which indicates there exists a certain bonding effect between indole/carbazole and the Cl or O atoms in the anions. Fig. 8 Total density maps of different isovalues for[bpy][Cl] (a), [emim][TFA] (b), and [emim][TOS] (c) with Indole

Fig. 9 Total density maps of different isovalues for[bpy][Cl] (a), [emim][TFA] (b), and [emim][TOS] (c) with carbazole

Fig. 10 Deformation charge density maps for[bpy][Cl] (a), [emim][TFA] (b), and [emim][TOS] (c) with indole and [bpy][Cl] (d), [emim][TFA] (e), and [emim][TOS] (f) with carbazole

The above-mentioned intermolecular interaction analysis shows that the hydrogen bond existed between the indole/ carbazole and the IL extractants were adopted in this work, and the strength of intermolecular interactions are consistent with the 24


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Energy & Fuels

experimental results. Also, to verify the selectivity of the selected ILs, the electron density analysis between the basic pyridine and quinoline and selected IL extractants were also studied. However, due to the lack of a H atom attached to the N atom, it cannot form any hydrogen bonds with the anion of the IL extractants. Therefore, based on the results of the molecular simulation obtained, the formation of hydrogen bonds between anions and heteroatoms presents the mechanism for the ILs extraction process. 3.4 Re-use of the ILs As we all know, ILs are expensive materials even it was used in 1:2 or 1:5 proportion. Thus, considering the potential applications for extracting neutral heterocyclic nitrogen compounds by IL extractants in the industry, it is very important to re-use and recycling the ILs. Until now, there are almost two methods that adopted in recycling the IL extractants from many works have already been reported. One is reported by Eßer et al.

32

that the chemicals with low boiling points can be separated

from the extract by distillation process. But for the compounds with high boiling points the back-extraction process was widely accepted by many researchers

9, 37, 51

and our previous work 31. And combined with our previous work, the re-use of the ILs containing Cl- anions have been investigated, which good results were shown. Meanwhile, considering its good extraction performance and properties, [Emin][Tos] was assessed in terms of its reusability. And the results are listed in Fig. 11 and Table S10. Thereinto, the reusability experiments of [Emin][Tos] were adopted at 25℃ for 25


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30 min with the mass ratio 1: 1. After a single extraction cycle of the IL and model oil, the IL phase was separated by a decantation. Then, the same amount of new model oil was added to the above-mentioned IL phase to do the next cycle. It needs to be emphasized that the IL phase was used directly in the next cycle without any treatment. Fig. 11. The reusability of [emim][TOS] for indole and carbazole extraction efficiency (extraction conditions: temperature 25℃, time 30 min, mass ratio 1: 1 for total N-content 4.5% model oil).

As shown in Fig. 11, the total N-compounds extraction capability of [emim][TOS] has a slight reduce (99%

1: 1 -

(volume

15

ratio)

(CH3CH2)3N(CH2)3SO3HHSO4

>99% >99% >99%

Description Imidazolium-based ILs were developed as new extraction agents to separate indole from wash oil. It was fpund that the extraction efficiency and selectivity for the neutral N-compounds, indole and carbazole, is larger than basic one, pyridine and quinoline, by [bmim][Cl]. Extraction of S-and N-compounds from gasoline and diesel oil by halogen-free ILs. Phosphate-based ILs were prepared and used to remove N-compounds in the coal tar diesel fraction.

Model fuel oil

Carbazole

298.15

1: 1

30

1: 5

90 35


76

99.1

9 9 31

31

32

25

26 26 26 27

The removal of N-compounds, basic and neutral species, from fuel oils because of their inhibiting effect on the hydrodesulfurization process.

27 27 27

84.3 Model oil

Ref

26

ILs investigated can extract sulfur as well as nitrogen-aromatics in preference to aromatic hydrocarbons. And nitrogen hetero-aromatics are significantly better extracted than sulfur hetero-aromatics.

Undetected

[EtMe2S][N(CN)2] [C2mim][Cl]

83.43

RT

[bmim][N(CN)2]

[S2][N(CN)2]

86.47

Indole

[bmim]C(CN)3

[emim][N(CN)2]

91.36

90.99

333.15

[3-mebuPy]N(CN)2 [4-mebuPy]N(CN)2

IL: oil

Removal of N- and S-compounds from a model oil (dodecane) was studied.

33

Functionalized acidic ILs were synthesized and used to remove non-basic nitrogen ( indole) in model oil．

28

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[bmim][Cl] [QcPy][Cl]

48 Straight-run feed

Carbazole

333.15

1:10

24 h

58

[bmim][Cl]

Straight-run feed

Indole

333.15

1:10

60

>50

[bmim][AlCl4]

FCC diesel

Basic nitrides

323.15

0.03

3

99

[bmim]Cl/ZnCl2 [bmim]HSO4 [bmim]Cl/2ZnCl2

Model oil

Carbazole

298.15

1: 1

Efficient Extraction of Neutral Heterocyclic Nitrogen ... - ACS Publications

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