Application of Iron-Containing Magnetic Ionic Liquids in Extraction

Article pubs.acs.org/IECR

Application of Iron-Containing Magnetic Ionic Liquids in Extraction Process of Coal Direct Liquefaction Residues Jieli Wang,†,‡ Hongwei Yao,† Yi Nie,† Lu Bai,† Xiangping Zhang,*,† and Jianwei Li‡ †

Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ College of Chemical Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Three kinds of iron-containing magnetic ionic liquids (ILs), including imidazole-based, pyridine-based, and pyrrolidine-based ILs, were synthesized respectively, and were used to dissolve coal direct liquefaction residues (CDLR) to obtain asphaltene fractions under the conditions of given time, temperature, and mass ratio of ILs to CDLR. The extracts from CDLR were characterized by ultimate analysis, proximate analysis, FT-IR, and 13C NMR. The results show that physicochemical properties of the extracts obtained with different magnetic ILs consisting of different cationic rings are different, and pyridinebased magnetic IL is an effective extractant to extract asphaltenes from CDLR among the three magnetic ILs. The extracts might be good precursors for preparing high-value-added carbon materials because of their higher carbon content, lower H/C, and ash content.

1. INTRODUCTION Coal direct liquefaction residues (CDLR) come from the coal direct liquefaction process.1 CDLR as byproduct usually account for about 30 wt % of coal,2 containing about 30 wt % heavy oils, 25 wt % asphaltenes, and 45 wt % other substances.3,4 Along with the understanding on CDLR components and progressing technology of liquefaction process, more and more attention has been paid to the residues as feedstocks to utilize. At present, the main measures used to deal with the residues are gasification for hydrogen production with the Texaco treatment process,5,6 combustion providing heat,7 and carbonization.8 These approaches have low-added value and also cause environmental problems. CDLR contain a mass of polycyclic aromatics of asphaltenes, which are important precursors for preparing high value-added carbon materials. Therefore, it is necessary to look for an effective separation method to extract asphaltene fractions from CDLR, which makes an effective use of the residues for taking account of the economic and environmental factors. Usually, traditional organic solvents such as benzene, tetrahydrofuran, pyridine, and benzopyridine, have the ability to dissolve CDLR due to their acceptor groups that may form strong hydrogen bonds, similar to how these solvents are used to swell and dissolve coal.9,10 As traditional solvents are volatile, toxic, and environmental pollutants, it is necessary to exploit new solvents as replacements. Compared with traditional solvents, ionic liquids (ILs) are used as green solvents or excellent media due to their unique physicochemical properties, including wide liquid range, higher ionic conductivity, excellent solubility, thermal stability,11 and designability by appropriate modifications of cationic or anionic structures. Meanwhile, many studies show that the cations and anions connect to each other to form hydrogen-bonded networks in ILs, and the bonds have a significant influence on the physical properties of the ILs.12 Magnetic ILs not only © 2012 American Chemical Society

have above excellent properties but also exhibit an unexpectedly strong response to an additional magnetic field. These properties give magnetic ILs more advantages and potential application prospects than conventional solvents in catalytic reactions,13 solvent effects,14 and separation processes.15 In consideration of the aromatic structures of asphaltene fractions in CDLR and the rule of similarity in extraction, three kinds of room temperature magnetic ILs with different cationic rings were synthesized by two steps.16 They were used to dissolve CDLR under the given experimental conditions. The extracts from CDLR were characterized by ultimate analysis, proximate analysis, FT-IR, and 13C NMR spectral analysis. The studies indicate that the cationic structures and sizes of magnetic ILs probably have the major influence on the compositions and structures of the extracts, and pyridinebased magnetic IL is more feasible and preponderant to extract asphaltene fractions from CDLR than the other two magnetic ILs.

2. EXPERIMENTAL SECTION 2.1. Materials. Crystalline 1-butyl-3-methylimidazolium chloride ([bmim]Cl), N-butylpyridium chloride ([bPy]Cl) and 1-butyl-1-methylpyrrolium chloride ([bmP]Cl) with purity 99% were purchased from Henan Lihua Pharmaceutical Co., Ltd. FeCl3·6H2O, diethyl ether and deionized water were of analytical grade (>99%) and were used without further purification. CDLR (mean: 14.6 μm) were supplied by Shenhua Group Corporation Limited. 2.2. Preparation of Magnetic ILs. 1-Butyl-3-methylimidazolium tetrachloroferrate ([bmim]FeCl4) was synthesized by mixing equimolar amounts of [bmim]Cl and FeCl3·6H2O in a Received: Revised: Accepted: Published: 3776

December 15, 2011 February 8, 2012 February 9, 2012 February 9, 2012 dx.doi.org/10.1021/ie202940k | Ind. Eng. Chem. Res. 2012, 51, 3776−3782

Industrial & Engineering Chemistry Research

Article

Figure 1. Molecular structures of iron-containing magnetic ionic liquids.

performed using a Bruker Daltonics APEX-II (America, Bruker Daltonics Inc.). FT-IR spectra were registered with a Nicolet Magna-IR-560 spectrometer infrared spectrophotometer (America, Nicolet). The magnetic susceptibilities of magnetic ionic liquids were measured using a MPMS (SQUID) (America, Quantum Design) in Peking University. Proximate analysis was measured using thermogravimetric differential thermal integrated thermal analyzer TG/DTA-7300 (Japan, SIINT). 13C NMR spectra were measured by use of BRUKER AVANCE-III 400 M solid state spectrometer (Germany, Bruker). Proximate Analysis. Moisture (M), volatile matter (VM), fixed carbon (FC), and ash content were measured under the following temperature program: (1) purging the TG/DTA system with ultrahigh-purity nitrogen at 30 °C for 10 min; (2) weighing a sample of 16−20 mg; (3) raising the temperature to 110 °C at a heating rate of 20 °C·min−1 and holding for 5 min, the weight loss is defined as M; (4) Ramping the temperature to 950 at 40 °C·min−1 and holding for 10 min, the weight loss is defined as VM; (5) Introducing air at 950 °C holding for 30− 60 min upon reaching normal baseline levels, the weight loss is defined as FC, and the remained weight is ash. 13 C NMR Analysis. 13C NMR experiments were measured by using cross-polarization (CP), magic-angle-spinning (MAS), total suppression of sideband (TOSS). and dipolar-dephasing (DD). The measurement conditions were as follows: solid double resonance detector; rotor outer diameter, 6 mm ZrO2; MAS rotary speed, 6 kHz; resonant frequency, 75.43 MHz; sampling time, 0.05 s; contact time, 2 ms; pulse repetition time, 4 s; scanning 2000−4000 times. To eliminate the rotating

dry flask under N2 atmosphere with a mechanical stirrer for 24 h. The coarse product was washed several times with diethyl ether and deionized water. Then the product was dried in vacuum, and a dark brown IL was obtained at last. N-Butylpyridium tetrachloroferrate ([bPy]FeCl4) and 1butyl-1-methylpyrrolium tetrachloroferrate ([bmP]FeCl4) were synthesized by a similar process at 50 and 30 °C, respectively. The molecular structures of three kinds of ironcontaining magnetic ionic liquids were shown in Figure 1. 2.3. Extraction Process. CDLR were treated with a grinder so as to meet the experimental requirement and measured using the laser particle size analyzer. A certain amount of [bmim]FeCl4 (6.0 g), [bPy]FeCl4 (6.0 g), and [bmP]FeCl4 (6.0 g) were put in three dry flasks and respectively mixed with CDLR (1.0 g) at 45 °C for 30 min with magnetic stirring. The insoluble matter was separated by vacuum filter. Soluble materials were precipitated from the IL solutions using water as stripping agent. Then the three extracts, MZ-extracts from CDLR with [bmim]FeCl4, BD-extracts from CDLR with [bPy]FeCl4, and BL-extracts from CDLR with [bmP]FeCl4 were obtained. The extraction process was repeated three times for each magnetic IL to extract CDLR. 2.4. Characterization. The average particle size of ground CDLR was evaluated by laser particle size analyzer LS-230 (America, Beckman Coulter). Nitrogen, carbon, and hydrogen element contents of the synthesized magnetic ionic liquids and extracts obtained from CDLR were determined by combustion analysis in a Vario ELIII ultimate analyzer (Germany, Elementar). The ESI-MS analysis of the synthesized magnetic ionic liquids was 3777

dx.doi.org/10.1021/ie202940k | Ind. Eng. Chem. Res. 2012, 51, 3776−3782


Article

Figure 2. The CDLR extraction process with magnetic ionic liquids.

Table 1. Ultimate and Proximate Analysis of CDLR and the Extractsa ultimate analysis (wt %, daf base)

a

proximate analysis (wt %, dry base)

sample

C

H

N

O

H/C (atomic ratio)

VM

FC

ash

CDLR MZ-extracts BD-extracts BL-extracts

69.01 86.51 88.71 82.62

3.61 5.46 5.20 5.35

0.67 1.87 1.28 1.79

10.64 5.51 4.29 5.79

0.63 0.76 0.70 0.78

39.27 79.74 88.24 94.55

30.71 18.72 10.51 1.39

30.02 1.54 1.25 4.06

Conditions: 45 °C, 30 min, magnetic stirring.

sideband of aromatic carbon signals, the four vary interval π pulses were added to constitute TOSS sequences before collecting data.

3. RESULTS AND EDISCUSSION 3.1. Characterization of Magnetic ILs. The structure, purity, and magnetic susceptibility of three magnetic ILs were

Figure 4. The DTA of CDLR (a), MZ-extracts (b), BD-extracts (c), and BL-extracts (d).

[bPy]FeCl4. Yield ca. 83%. Anal. Calcd for C9H14NCl4Fe (333.87): C, 32.38; H, 4.23; N, 4.20. Found: C, 32.37; H, 4.24; N, 4.19. IR (KBr, v, cm−1): 3027, 3019, 2961, 2872, 1632, 1487, 1466. Electrospray MS+m/z: 136.1[bPy]+. MS−m/z: 198.8[FeCl4]−. MPMS (SQUID) χg: 4.11 × 10−5 emu·g−1. [bmP]FeCl4. Yield ca. 70%. Anal. Calcd for C9H20NCl4Fe (339.92): C, 31.80; H, 5.93; N, 4.12. Found: C, 31.78; H, 5.95; N, 4.11. IR (KBr, v, cm−1): 2963, 2875, 1634, 1467. Electrospray MS+m/z: 142.1[bmP]+. MS−m/z: 198.8[FeCl4]−. MPMS (SQUID) χg: 0.95 × 10−5 emu·g−1. 3.2. Characterization of the Extracts. The CDLR extraction process with magnetic ILs is shown in Figure 2. It can be seen from Figure 2 that magnetic ILs and water may be recycled. It is more environmental friendly in extraction and the separation process if magnetic ILs and water could be separated completely by magnetic separator or adding a kind of inorganic

Figure 3. The DTG of CDLR(a), MZ-extracts(b), BD-extracts(c), and BL-extracts(d).

characterized. The results are shown in the following and indicate that the three magnetic ILs were synthesized and have similar magnetic susceptibilities: [bmim]FeCl4. Yield ca. 86%. Anal. Calcd for C8H15N2Cl4Fe (336.88): C, 28.52; H, 4.49; N, 8.32. Found: C, 28.51; H, 4.51; N, 8.31. IR (KBr, v, cm−1): 3148, 3116, 2961, 2935, 2847, 1464, 1570, 1592. Electrospray MS+m/z: 139.1[bmim]+. MS−m/z: 198.8[FeCl4]−. MPMS (SQUID) χg: 4.04 × 10−5 emu·g−1. 3778



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Figure 5. FT-IR spectra of CDLR and the extracts.

oxygen contents of the three extracts are lower than that of CDLR. In the three extracts, the oxygen contents of BDextracts are lower than that of MZ-extracts and BL-extracts. The results show that the extracts obtained from CDLR with [bPy]FeCl4 contain higher carbon content and lower atomic ratios of H/C, which are conducive to the progress of the next modification step.19 3.2.2. Proximate Analysis. CDLR and the three extracts were measured by TG/DTA under the procedure temperaturecontrolled system. Proximate analyses data of CDLR and the three extracts were shown in Table 1. It can be found that the VM contents of the three extracts are much greater than that of CDLR, and the FC and ash contents of the three extracts are far less than that of CDLR. The results indicate that magnetic ILs have good selectivity for organic matters over inorganic matters. As TG curves of all samples are similar from 110 to 950 °C, the significant changes of the weight loss are quite unnoticeable. Through the first differential of TG curves with respect to time, DTG curves were obtained and shown in Figure 3. DTG curves are more accurate to reflect the relation of quality changing with time. In Figure 3, the strongest peaks clearly appeared at 473, 488, 502, and 480 °C, respectively, which correspond to the fastest weightlessness positions of CDLR and extracts. Therefore, their decomposition temperatures at 451 °C (CDLR), 472 °C (MZ-extracts), 489 °C (BDextracts), and 468 °C (BL-extracts) are determined by looking for the corresponding inflection points. The thermal decomposition temperature of BD-extracts is the highest among the samples, which is the biggest in keeping with the average molecular weight of BD-extracts. In addition, judging by the

Figure 6. 13C NMR spectra of CDLR (a), MZ-extracts (b), BDextracts (c), and BL-extracts (d).

salts.15,17 Therefore, this separation process shows more superiority than the conventional ILs that have been used in the literature.18 3.2.1. Ultimate Analysis. Average extraction yields with [bmim]FeCl4, [bPy]FeCl4, and [bmP]FeCl4 are 22.8%, 24.5%, and 11.2%, respectively. The ultimate analysis of CDLR and the extracts was determined by elemental analyzer adopting the N, C, and H analytical model and O analytical model. From Table 1, it can be seen the carbon, hydrogen, and nitrogen contents of the three extracts are much higher than that of CDLR, and the

Table 2. Carbon Structural Parameter Distribution of CDLR and the Extractsa name

fa

fal

faC

fa′

faH

faN

faP

faS

faB

faH

fal*

falO


0.70 0.74 0.75 0.72

0.30 0.26 0.25 0.29

0.05 0.04 0.03 0.04

0.65 0.70 0.72 0.68

0.16 0.17 0.18 0.17

0.49 0.53 0.54 0.51

0.01 0.02 0.02 0.01

0.09 0.11 0.11 0.12

0.39 0.40 0.41 0.38

0.23 0.20 0.19 0.22

0.07 0.06 0.06 0.07

0.04 0.07 0.05 0.09

Fractions of sp2-hybridized carbon: fa, total aromatic carbons; fal, total aliphatic carbons; faC, carbonyl, δ > 165 ppm; fa′ , in an aromatic ring; faH, protonated and aromatic; faN, nonprotonated and aromatic; faP, phenolic or phenolic ether, δ = 150−165 ppm; faS, alkylated aromatic, δ = 135−150 ppm; faB, aromatic bridgehead; falH, CH or CH2; fal*, CH3 or nonprotonated; falO, bonded to oxygen, δ = 50−90 ppm. a

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Table 3. Average Aromatic Cluster Structural Parameters of CDLR and the Extractsa structure parameter samples

χb

Ca

CP

CB

CS

Cal

Cn

Cm

CT

Ra


0.60 0.58 0.57 0.55

38 34 32 30

15.2 14.2 13.8 13.2

22.8 19.4 18.2 16.8

5.8 6.3 5.8 5.7

17.5 12.6 11.1 12.8

13.4 9.7 8.4 9.7

4.1 2.9 2.7 3.1

58.5 48.6 44.4 42.1

9.0 8.0 7.5 7.0

a Ca is aromatic carbon atoms gained according to χb. CP is circumambient carbon atoms gained on the basis of faP and faS. CB is bridgehead carbon atoms computed by faB. CS is substituent aromatic carbon atoms gained on the basis of faH, faP, and faS. Cal is aliphatic carbon atoms computed by fal. Cn is the carbon atoms of substituent group (CH, CH2) on the aliphatic chains computed by falH. Cm is the non-proton carbon atoms and substituent group (CH3) carbon atoms on the aliphatic chains computed by falC*. CT is aromatic cluster carbon atoms, CT = Ca/ fa′ . Ra is the number of aromatic nucleus, Ra = (Ca − 2)/4.

peak of ashes. Hence it follows that MZ-extracts and BDextracts may contain a spot of ashes extracted from CDLR. 3.2.3. FT-IR Spectra. It can be found that the absorption peaks appeared at three areas from Figure 5. The absorption peaks around 3040 and 1600 cm−1 are attributed to the stretching vibration absorption spectra of the CC and C−H bond on the benzene rings of aromatic organic compounds. The peak around 1449 cm−1 is assigned to the skeleton vibration absorption spectra from benzene rings or benzene rings with nitrogen. The results illustrate that polycyclic aromatic hydrocarbons exist in the extracts.20,21 Within the wavelength of 750−910 cm−1, there are many strong absorption peaks, which are due to the out-of-plane ring bend vibration absorption spectra of the C−H bond from the substituted benzene rings. It shows the substituted group of aromatic nucleus with manifold types.22 Nitrogen functional groups rarely exist in the extracts. A small amount of nitrogen atoms may exist in the form of heterocyclic compounds such as pyridine or desquamate (around 1160 and 1024 cm−1). As the skeleton vibration peaks of nitrogen heterocyclic compounds are the overlap of the range of aromatics skeleton vibration peaks (from 1660 to 1415 cm−1), it is difficult to identify the FT-IR absorption spectra of the heterocyclic compounds with nitrogen atoms. Amine-based materials are easy to hydrogenate to generate gaseous ammonia. Two strong absorption peaks around 2921 and 2855 cm−1 are attributed to the C−H bond from the out-of-plane ring bend vibration, which indicates that the samples contain a significant amount of various alkyl hydrocarbon with different length of carbon chains. The infrared spectra have no obvious absorption peaks above 3300 cm−1. This indicates that no hydroxyl group exists in the samples. A small amount of oxygen atoms may exist in the form of aryl ethers or ring ethers (around 1161 and 1024 cm−1).23 3.2.4. 13C NMR Spectra. From Figure 6, it can be seen that all the 13C NMR spectra of the extracts split in two parts belong to aromatic carbons and aliphatic carbons. The strength of aromatic carbons is much stronger than that of aliphatic carbons. CDLR and the extracts samples are measured by CP/ MAS/TOSS and DD technology to acquire the 12 structural parameters as shown in Table 2.24 From Table 1 and Table 2, it can be seen that the value of the aromaticity fa and the aromatic bridgehead carbons faB gradually increased with an increase of the content of organic carbon of CDLR and the extracts. However, the value of the aliphatic carbons fal gradually decreased with an increase of the content of carbon. On one hand, it provides the potential information about some characteristics of the composition and

Figure 7. Average aromatic cluster size and the compound model per aromatic cluster.

Table 4. Average Aromatic Cluster Structural Parameters of CDLR and the Extractsa structure parameter name

fa

σ+1

P0

B.L.

S.C.

Mw

Mδ


0.70 0.74 0.75 0.72

5.85 6.31 5.78 5.74

0.30 0.54 0.54 0.46

1.76 3.41 3.12 2.64

4.09 2.90 2.66 3.10

712 663 594 615

42 39 35 43

σ + 1 is the average number of attachments per cluster; P0 is the fraction of all possible bridges; B.L. is the number of bridges and loops per cluster; S.C. is the number of side chains per cluster; Mw is the average molecular weight of a cluster; Mδ is the average molecular weight of a side chain or half of a bridge mass. a

duration of the weightlessness corresponding to the peak width, it signifies that the dispersions of molecule chains of organic matters of the extracts are very smaller compared to that of CDLR. Figure 4 shows the DTA curves of the three extracts and CDLR. Before 950 °C, the heat curves are similar, three peaks of absorption heat appearing at around 95, 201, and 482 °C. These peaks may be considered as the absorption heat peak of the moisture-volatile, solid-melting, decomposition of small molecule organic matters. At 950 °C, the curve of BL-extracts levels and the curves of MZ-extracts, BD-extracts, and CDLR appear as one absorption heat peak that corresponds to the peak of decomposition absorption heat of macromolecular compounds. After introducing air at 950 °C, one broad peak of absorption heat appears corresponding to the absorption heat 3780



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structure of CDLR and the extracts. On the other hand, it also provides the evidence for the relevancy among the 12 structural parameters of CDLR and the extracts. Several other important parameters can be obtained through the above 12 parameters. The average number of carbon atoms per aromatic cluster Ca may be calculated as described in the literature.24 The aromatic carbon atoms and other parameters carbon atoms can be seen in Table 3. It can be clearly seen in Table 3 that the value of χb decreased with the lessening of the value for Ca, Cb, and Ra, and other structural parameters of an aromatic cluster size shared similar variation tendency as a whole. When Ca exceeds 24, the circular catenation model becomes the appropriate limiting law for coals.24 According to these parameters and ultimate analysis, the average molecular formulas of the average aromatic cluster were obtained (MZextracts, C48H36O2.3N0.9; BD-extracts, C44H31O2.1N0.7; BLextracts, C42H33O2.2N0.8) and the compound models for the aromatic carbons per cluster were shown in Figure 7. To further explain the structures of CDLR and the extracts, several other important parameters are introduced.25 The other six structural parameters of the average aromatic cluster can be gained, such as shown in Table 4. From Table 4, it may be seen that the aromaticity fa gradually increased with the dwindling of Mw and Mδ. It can also be seen that the parameter S.C. increased and parameter B.L. decreased with an increase of the aromaticity fa, and other parameters changed very little overall. The average molecular weight of the heavy oil components was reported as 339 in the literature,26 which was far less than the value of Mw. Therefore, it could be inferred that asphaltenes are the main ingredients of the three extracts.

REFERENCES

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4. SUMMARY AND CONCLUSION Three species of magnetic ILs were first synthesized in this paper and were applied for environmentally friendly separation asphaltenes from CDLR, which expands the application fields of magnetic ILs. The characterizations of the extracts show that BD-extracts have the highest carbon content, the least ash, the largest average molecular weight, and the highest aromaticity fa among the three extracts. Therefore, [bPy]FeCl4 might be an effective extractant for extracting asphaltene fractions from CDLR. Physicochemical properties of extracts obtained from CDLR can be regulated and controlled by ILs modifications of the appropriate cationic or anionic structures with different constituent groups or different magnetic metal ions.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-010-82544877. Fax: 86-010-62558174. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support for this work was provided under National Basic Research Program of China (973 Program) (Grant No. 2009CB219901), Key Program of National Natural Science Foundation of China (Grant No. 21036007) and General Program Youth of National Natural Science Foundation of China (Grant No. 21006107). We thank the Shenhua Group Corporation Limited for permission to publish this work. 3781



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(26) Gu, X. H.; Zhou, M.; Shi, S. D. The molecular structure of heavy oil fraction from the Shenhua coal direct liquefaction residue. J. Coal Sci. Eng. (China) 2006, 31, 76.

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Application of Iron-Containing Magnetic Ionic Liquids in Extraction

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