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Dissolution of lignite in ionic liquid 1-ethyl-3-methylimidazolium acetate
Fuel Processing Technology 135 (2015) 47–51
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Dissolution of lignite in ionic liquid 1-ethyl-3-methylimidazolium acetate Zhi-Ping Lei ⁎, Lu-Lu Cheng, Su-Fang Zhang, Heng-Fu Shui ⁎, Shi-Biao Ren, Shi-Gang Kang, Chun-Xiu Pan, Zhi-Cai Wang School of Chemistry & Chemical Engineering, Anhui Key laboratory of Coal Clean Conversion & Utilization, Anhui University of Technology, Ma'anshan, 243002 Anhui Province, China
a r t i c l e
i n f o
Article history: Received 24 July 2014 Received in revised form 3 October 2014 Accepted 12 October 2014 Available online 5 November 2014 Keywords: Lignite Extraction EMIA
a b s t r a c t The extraction behaviors of three lignites with ionic liquid (IL)-1-ethyl-3-methylimidazolium acetate (EMIA) were studied in this work. The effects of extraction temperature and lignite type on extract yield and product compositions were primarily investigated. Also the relationship between the extract yield of lignites and the quantities of oxygen-containing functional groups in lignites was investigated. It was found that EMIA is a more efficient solvent for the dissolution of lignites compared to the performance of 1-butyl-3-methylimidazolium chloride (BMIC). Extraction temperature and lignite type significantly affect extraction behaviors of lignite with EMIA. The extract yield of lignites with EMIA increases with the quantity of carboxylic groups in lignite increases. Only a small amount of aliphatic alkyl structure components are extracted at low temperature. More aromatic structure components are extracted by EMIA at high extraction temperature. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Solvent extraction is usually used to probe the basic structure of coal and to produce ash-free coal (HyperCoal) or value-added products [1–3]. From the view of elucidation of the chemical structure of coals, the extract yield of coals must be high at mild extraction conditions. Then, the analyses of extracts can provide the information of the main structure of coal. Also the extract yield of coal must exceed 60% for the production of HyperCoal. But the extract yield of low-rank coals with many conventional organic solvents is very low at mild extraction conditions. In order to obtain high extract yield, the extraction used organic solvents is usually carried at higher temperature (about 350 °C). At these temperatures coal has thermal pyrolyzed. Then the thermal dissolution soluble fraction obtained at higher extraction temperature could not provide the real structure information of coals and hard to be hydroliquefied to oil. Then the new extraction method at mild condition must be developed. In 2010, Painter et al. reported that certain ILs (such as BMIC) could be used to fragment, swell, partially solubilize, and disperse some coals [4]. Also we had found that BMIC has high performance on the dissolution of Xianfeng lignite (XL) [5,6]. The effect of the chemical structure of anions and cations of ILs on their dissolving capability for lignite had also been studied [7]. Among the ILs studied, BMIC exhibited the best dissolving capability that up to 80% extract yield of XL can be obtained by heating at 200 °C [5]. However, the
⁎ Corresponding authors. Tel.: +86 5552311552; fax: +86 5552311822. E-mail addresses: [email protected] (Z.-P. Lei), [email protected] (H.-F. Shui).
http://dx.doi.org/10.1016/j.fuproc.2014.10.010 0378-3820/© 2014 Elsevier B.V. All rights reserved.
relatively high melting points and viscosities of ILs containing chloride anion possibly limit their practical application in lignite processing. Relatively high dissolution temperatures (often above 200 °C) are often required for dissolving lignite, which possibly results in thermal decomposition of BMIC. From aforementioned point of view, it will be of great interest to seek for powerful, halogen-free ILs with low melt point and low viscosity. During the study of cellulose dissolution, it was found that EMIA showed higher capability for cellulose and wood dissolution [8–12]. Moreover, EMIA has low melt point (b−20 °C), low toxicity (LD50 N 2000 mg kg - 1 ) and viscosity (∼ 140 cPs at 25 °C) and was considered to be less toxic and corrosive than comparable chlorides and even biodegradable. Wu et al. have recently shown higher solvating power of 1-butyl-3-methylimidazolium acetate for chitin than 1-alkyl3-methylimidazolium chloride or BMIC [13]. In particular, BASF, one of the pioneers in industrial usage of ILs, has turned to EMIA for the dissolution of cellulose. These features make EMIA a promising solvent for the processing of lignites. These findings lead us to consider if such IL can be used for the extraction of lignites. However, little information about these is available in literature based on our knowledge. In the present paper, the extraction behaviors of three lignites with EMIA at mild condition were investigated and the results were compared the results with the previously studied of BMIC. The extracts obtained from the extraction of XL with EMIA were characterized with Fourier transform infrared (FTIR) spectra spectrometer. The relationship between the nature of different lignites and the extract yields with EMIA was correlated with the nature of different lignites (especially the number of oxygen-containing functional groups in lignites). The investigation facilitates the better understanding the dissolution performance of lignites in ILs.
48
Z.-P. Lei et al. / Fuel Processing Technology 135 (2015) 47–51
2. Experimental
EMIA
2.1. Properties of samples
lignite thermal dissolution
reaction mixture
Three Chinese lignites, XL, Shengli lignite (SL), and Xilinggele NO.6 lignite (XL6), as received were ground to b 200 mesh, stored under nitrogen atmosphere, and dried under vacuum at 80 °C overnight before use. The ultimate and proximate analyses of samples were shown in Table 1. The EMIA used in this study was purchased from Shanghai Cheng Jie Chemical Co. LTD. and dried at 100 °C in a vacuum oven 48 h prior to the experiments.
extraction with NMP filtration
F1
FC
solvent evaporation
EM
washed with H2O
R
precipitation
2.2. Extraction of lignites with EMIA The thermal dissolution experiments of lignites with EMIA were carried out in a microwave apparatus with temperature measurement, time controlling and stirring. As shown in Fig. 1, 1.0 g lignite was extracted with 10.0 g EMIA in Teflon tube at different temperature for 30 min. After the thermal dissolution, the reaction mixture was cooled to room temperature and extracted with fresh N-methyl-2pyrrolidinone (NMP). Then the reaction mixture was separated to filtrate 1 (F1) and filter cake (FC) by filtration through a membrane filter (pore size of 0.45 μm). The residue (R) was obtained by washing FC with water many times and dried in a vacuum at 353 K overnight. The extract yield (on a dry basis) was determined from the weight of the residues. NMP in F1 were removed through rotary evaporation to afford the extraction mixture (EM). EM was then precipitated by the addition of water and acetone (4:1 v:v) and filtered to afford the extracts (E) and filtrate 2 (F2). It is worth noting that there are traces of the extracts in F2. Finally acetone and water in F2 were removed by rotary evaporation to afford EMIA.
2.3. Sample characterization FTIR spectra of the lignites, extracts and residues were measured on a Nicolet 6700 spectrometer using the KBr-pellet technique. Quantitative measurement of oxygen-containing group in lignites: total acid and carboxyl in lignites were measured by titrimetric method based on the following reactions:
E
FTIR analysis
F2 solvent evaporation
EMIA Fig. 1. Procedure for extraction of lignite with EMIA and subsequent treatment.
3. Results and discussion 3.1. Extraction behavior of XL with EMIA As exhibited in Fig. 2, the extract yield of XL with EMIA significantly increases with the increase of extraction temperature from 100 to 200 °C. At 100 °C or 150 °C the extract yield of XL with EMIA is very low (about 3.2% or 13.4%), suggesting that only small portion of the fraction of XL can be released by EMIA at these temperatures. When the extractions with EMIA were carried out at 180 °C and 200 °C, extract yields of XL with EMIA reach to about 71.6% and 92.9%. The above result clearly shows that extraction temperature significantly affects the dissolution of lignite with EMIA. This phenomenon is consistent with the results of XL dissolution in BMIC [5]. Our previous studies showed that the extract yield of XL at this temperature without the addition of ILs is very low [5]. The above results clearly show that ILs have high extraction ability for lignite. Compared to the dissolution performance of BMIC (extract yield of XL in BMIC is about 80% at 200 °C), EMIA is an effective solvent for the extraction of lignite.
Ba(OH)2 + 2HA = BaA2 + 2H2O (total acid) 100
2(\COOH) + Ca(CH3COO)2 = \COO\Ca\OOC\ + 2CH3COOH
CH3COOH + NaOH = CH3COONa + H2O (carboxyl). Phenolic hydroxyl was obtained by subtraction of total acid and carboxyl.
Table 1 Proximate and ultimate analyses (wt%) of three lignites. Sample
XL SL XL6
Extract yield (wt%, d)
80
60
40
20
Proximate analysis
Ultimate analysis (daf)
Ad
Mad
Vdaf
C
H
Oa
N
St,d
14.4 15.1 16.2
33.6 15.1 16.2
60.6 33.3 51.1
64.75 68.25 68.45
5.81 5.23 6.35
N27.08 N24.34 N22.65
1.84 1.12 1.26
0.52 1.05 1.30
a By difference; daf: dry and ash-free base; Mad: moisture (air dried base); Ad: ash (dry base, i.e., moisture-free base); Vdaf: volatile matter (dry and ash-free base); St,d: total sulfur (dry base).
0 100
150
180
200
o Temperature ( C) Fig. 2. Effect of extraction temperatures on extract yield of XL with EMIA (ratio of XL/EMIA: 1:10 by weight).
Z.-P. Lei et al. / Fuel Processing Technology 135 (2015) 47–51
Oxygen content (wt%, daf)
27
XL6 SL XL
26
25
24
23
22 0.95
Carboxyl content (mmol/g)
Ordinarily, fragmentation breaks the coal network, allowing the dissolution of trapped materials. Low rank coals are thought to form mainly aggregated structure through interaction among functional groups containing heteroatom and hydrogen bond [14]. The high extraction of XL with ILs may be the result of ILs-induced structural relaxation followed by dissolution of coal components in the solvent. With the raising of extraction temperature, some non-covalent bonds are released by ILs-induce, resulting in the increase of extract yield of lignite. The above clearly indicates that the mainly aggregated structure of XL can be fragmented at higher temperature in EMIA. Up to now, it is considered that extract yield of coal at low temperature mainly depends on the cleavage degree of aggregated structure of coal. Then the higher extract yield of XL with EMIA may be attributed to that more aggregated structure of XL can be dissociated in EMIA than that in BMIC at same extraction condition. It has been reported that ILs containing carboxylic acid anions have stronger coordination ability or hydrogenbond acceptability (basicity) compared with those anion derived from strong acid [13]. Then ILs with stronger coordination anion than chloride can disrupt the more complex hydrogen-bond formed in lignite.
3.2. Effect of lignite type on the extract yield of lignites with EMIA
XL6 SL XL
0.90 0.85 0.80 0.75 0.70 0.65 0.60
20
The above result clearly shows that EMIA has an outstanding performance on the dissolution of lignite. Our previous study showed that the extract yields of coals with BMIC vary with the ranks of coals [5]. For different lignites, the extract yield of lignites with BMIC is different. In order to investigate if this phenomenon is the same at that with BMIC, three lignites with almost the same ultimate analysis results were used to determine the dependence of extract yield of lignites with EMIA on lignite type. As shown in Fig. 3, the extract yield of lignites with EMIA is significantly different from each other. Among the three lignites studied, XL gives the highest extract yield of about 93.0%. However, XL6 gives the lowest extract yield of 30.7%. The difference of extract yield of lignites with EMIA is probably related to the crosslinking structure of the coal network. It has been postulated that low-rank coals form crosslinking structures through interaction among functional groups containing heteroatomics and hydrogen bond. Then the relationship between oxygen content of lignites and the extract yield of lignites with EMIA is investigated. As shown in Fig. 4, the extract yield of lignites with EMIA increases with the increase of the oxygen content up from 22% C to 27% O. This result may strongly demonstrate that the interaction among the ILs and the oxygen-
100
Extract yield (wt%, d)
80
60
40
49
30
40
50
60
70
80
90
100
Extract yield (wt%, d) Fig. 4. Relationship between oxygen content and carboxyl content of lignites and extract yield of lignites with EMIA (filled symbols) and BMIC (open symbols).
containing functional groups in lignites can release the interaction among the oxygen-containing functional groups. Then it can be concluded that the high extract yield of lignites with IL may result from the ILs can break the noncovalent interaction such as hydrogen bond among carboxyls and phenolic hydroxyls [14]. Lignite contains substantial amounts of oxygen in the form of carboxyls and phenolic hydroxyls. Then the carboxyls and phenolic hydroxyls in three lignites were determined by chemical titration. As listed in Table 2, the difference of the total acid and hydroxyl content of three lignites is not big, but their carboxyl content significantly varied. Then the correlation between carboxyl and extract yield of the three lignites with EMIA was studied. As exhibited in Fig. 4, the extract yield of lignites with EMIA increases with the increase of carboxyl content of up to 0.949 mmol/g. This result suggests that the interaction between carboxyl group of lignites and ILs significantly affect extraction behaviors of lignites with EMIA. Also the extract yield of three lignites with BMIC were correlated with oxygen content and carboxyl content in three lignites, the same rule can be found. Pulati et al. [4] proposed that the ability of a solvent to dissolve a portion of a particular coal depends upon the chemical structure of that coal and the ability of the solvent to interact with polar functional groups, particularly those that hydrogen bond. Also our previous studies on dissolving coal by ILs have suggested that the dissolution mainly involved the interaction of the ILs with hydrogen bond in coal [7]. Then it can be concluded that extract yield of lignites with ILs mainly depends on the oxygen content and carboxyl content of lignites. The ability of different ILs on the dissolution of lignite structure (especially polar functional groups such as carboxyl et al.) determines the extract yield of lignite
20 Table 2 Total acid, carboxyl, and phenolic hydroxyl (–OH) content (mmol/g) in three lignites.
0 XL6
SL
XL
Lignite type Fig. 3. Effect of the lignite type on extract yield of lignites with EMIA: 200 °C, 30 min, and ratio of lignite to EMIA: 1:10.
Sample
Total acid
Carboxyl
Phenolic hydroxyl
XL SL XL6
2.321 2.032 2.119
0.949 0.705 0.643
1.372 1.327 1.476
3.3. Characteristics of extraction products of XL
4. Conclusions
1260 1220 1160
2913 2845
3420
1600
The results of extraction of lignites with EMIA show that EMIA has higher performance on the dissolution of lignites than that of BMIC. The extraction temperature significantly affects extraction behaviors of lignites with EMIA. The extract yield of XL with EMIA reaches to 93.0% at 200 °C for 30 min, when the ratio of EMIA/XL is 10. The interaction between carboxyl group of lignites and ILs significantly affects extraction behaviors of lignite with ILs. The extract yield of lignites
1030
R200
Absorbance
As shown in Fig. 5, the intensity of 2920 cm−1 (assigned to aliphatic C–H stretching) for E100 and E150 obtained from 100 °C and 150 °C is significantly higher than that of XL. This suggests that aliphatic alkyl structure components are mainly extracted at low temperature by EMIA. With the increase of extraction temperature, the intensity of 1600 cm− 1 (assigned to aromatic ring stretching vibration modes), 1260 cm− 1, 1220 cm− 1 and 1160 cm− 1 (assigned to C-O (phenol), Car–O–Car, C–O (alcohol), and Car–O–Cal structures respectively) for E200 significantly increase, suggesting more and more aromatics structures, phenol and/or ether groups components are extracted by EMIA. Also the intensity of band near 3400 cm−1 (assigned to hydrogen bond) for E increases with the increase of extraction temperature. This result indicates that hydrogen bond in extracts results from the presence of oxygen-containing functional group in aromatic structure components. On the contrary, as shown in Fig. 6, the intensity of 2920 cm−1 for R increases with the increase of extraction temperature. This result suggests that the content of aliphatic alkyl structure components in R increases with the increase of extraction temperature. This indicates that aromatic fractions in lignite can be extracted by EMIA. Also as shown in Fig. 6, the intensity of 1030 cm−1 (assigned to minerals) for R200 obtained from extraction at 200 °C significantly increases compared to that for the raw coal. This suggests that the EMIA mainly extracts the organic material in the raw coal.
1260
2845
3420
in ILs. The further investigation of interaction of carboxyl and hydroxyl groups with ILs is now under investigation in our laboratory.
1600
Z.-P. Lei et al. / Fuel Processing Technology 135 (2015) 47–51
2913
50
R150
R100
XL
3500
3000
2500
2000
1500
1000
-1 Wavenumbers (cm ) Fig. 6. FTIR spectra of the residues obtained from the extraction of XL with EMIA at different extraction temperatures (RTemperature) and XL.
with EMIA increases with the quantity of carboxylic groups in lignite increases. A small amount of aliphatic alkyl structure components are extracted at low temperature. More aromatics structure components are extracted by EMIA at high extraction temperature. Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, Grant 2011CB201302), the Key Project of Coal Joint Fund from the Natural Science Foundation of China and Shenhua Group Corporation Limited (Grant U1261208), the Natural Scientific Foundation of China (Grants 21476002, 21476003, 21476004, 21176001, U1361125, 21076001, 51174254, 20936007). This work was subsidized by the Strategic Chinese–Japanese Joint Research Program (Grant 2013DFG60060). Authors are also appreciative for the financial supports from the Anhui Provincial Innovative Group for Processing & Clean Utilization of Coal Resource and Program for Innovative Research Team in Anhui University of Technology. References
Absorbance
E200
E150 E100
XL
3500
3000
2500
2000
1500
1000
-1
Wavenumbers (cm ) Fig. 5. FTIR spectra of the extracts obtained from the extraction of XL with EMIA at different extraction temperatures (ETemperature) and XL.
[1] N. Okuyama, N. Komatsu, T. Shigehisa, T. Kaneko, S. Tsutuya, Hyper-coal process to produce the ash-free coal, Fuel Processing Technology 85 (2004) 947–967. [2] K. Koyano, T. Takanohashi, I. Saito, Catalytic hydrogenation of HyperCoal (ashless coal) and reusability of catalyst, Energy & Fuels 23 (2009) 3652–3657. [3] H.F. Shui, Y. Zhou, H.H. Li, Z.C. Wang, Z.P. Lei, S.B. Ren, C.X. Pan, W.W. Wang, Thermal dissolution of Shenfu coal in different solvents, Fuel 108 (2013) 385–390. [4] P. Painter, N. Pulati, R. Cetiner, M. Sobkowiak, G. Mitchell, J. Mathews, Dissolution and dispersion of coal in ionic liquids, Energy & Fuels 24 (2010) 1848–1853. [5] Z.P. Lei, L. Wu, Y.Q. Zhang, H.F. Shui, Z.C. Wang, C.X. Pan, Microwave-assisted extraction of Xianfeng lignite in 1-butyl-3-methyl-imidazolium chloride, Fuel 95 (2012) 630–633. [6] Z.P. Lei, L. Wu, Y.Q. Zhang, H.F. Shui, Z.C. Wang, S.B. Ren, Effect of noncovalent bonds on the successive sequential extraction of Xianfeng lignite, Fuel Processing Technology 111 (2013) 118–122. [7] Z.P. Lei, Y.Q. Zhang, L. Wu, H.F. Shui, Z.C. Wang, S.B. Ren, The dissolution of lignite in ionic liquids, RSC Advances 3 (2013) 2385–2389. [8] J. Wu, J. Zhang, H. Zhang, J. He, Q. Ren, M. Guo, Homogeneous acetylation of cellulose in a new ionic liquid, Macromolecules 5 (2004) 266–268. [9] N. Sun, M. Rahman, Y. Qin, M.L. Maxim, H. Rodríguez, R.D. Rogers, Complete dissolution and partial delignification of wood in the ionic liquid 1-ethyl-3methylimidazolium acetate, Green Chemistry 11 (2009) 646–655. [10] J. Zhang, H. Zhang, J. Wu, J. Zhang, J. He, J. Xiang, NMR spectroscopic studies of cellobiose solvation in EmimAc aimed to understand the dissolution
Z.-P. Lei et al. / Fuel Processing Technology 135 (2015) 47–51 mechanism of cellulose in ionic liquids, Physical Chemistry Chemical Physics 12 (2010) 1941–1947. [11] S. Singh, B.A. Simmons, K.P. Vogel, Visualization of biomass solubilization and cellulose regeneration during ionic liquid pretreatment of switchgrass, Biotechnology and Bioengineering 104 (2009) 68–75. [12] Y. Cao, J. Wu, J. Zhang, H.Q. Li, Y. Zhang, J.S. He, Room temperature ionic liquids (RTILs): a new and versatile platform for cellulose processing and derivatization, Chemical Engineering Journal 147 (2009) 13–21.
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[13] Y.S. Wu, T. Sasaki, S. Irie, K. Sakurai, A novel biomass-ionic liquid platform for the utilization of native chitin, Polymer 49 (2008) 2321–2327. [14] N. Kashimura, T. Takanohashi, I. Saito, Effect of noncovalent bonds on the thermal extraction of subbituminous Coals, Energy & Fuels 20 (2006) 1605–1608.
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Dissolution of lignite in ionic liquid 1-ethyl-3-methylimidazolium acetate Zhi-Ping Lei ⁎, Lu-Lu Cheng, Su-Fang Zhang, Heng-Fu Shui ⁎, Shi-Biao Ren, Shi-Gang Kang, Chun-Xiu Pan, Zhi-Cai Wang School of Chemistry & Chemical Engineering, Anhui Key laboratory of Coal Clean Conversion & Utilization, Anhui University of Technology, Ma'anshan, 243002 Anhui Province, China
a r t i c l e
i n f o
Article history: Received 24 July 2014 Received in revised form 3 October 2014 Accepted 12 October 2014 Available online 5 November 2014 Keywords: Lignite Extraction EMIA
a b s t r a c t The extraction behaviors of three lignites with ionic liquid (IL)-1-ethyl-3-methylimidazolium acetate (EMIA) were studied in this work. The effects of extraction temperature and lignite type on extract yield and product compositions were primarily investigated. Also the relationship between the extract yield of lignites and the quantities of oxygen-containing functional groups in lignites was investigated. It was found that EMIA is a more efficient solvent for the dissolution of lignites compared to the performance of 1-butyl-3-methylimidazolium chloride (BMIC). Extraction temperature and lignite type significantly affect extraction behaviors of lignite with EMIA. The extract yield of lignites with EMIA increases with the quantity of carboxylic groups in lignite increases. Only a small amount of aliphatic alkyl structure components are extracted at low temperature. More aromatic structure components are extracted by EMIA at high extraction temperature. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Solvent extraction is usually used to probe the basic structure of coal and to produce ash-free coal (HyperCoal) or value-added products [1–3]. From the view of elucidation of the chemical structure of coals, the extract yield of coals must be high at mild extraction conditions. Then, the analyses of extracts can provide the information of the main structure of coal. Also the extract yield of coal must exceed 60% for the production of HyperCoal. But the extract yield of low-rank coals with many conventional organic solvents is very low at mild extraction conditions. In order to obtain high extract yield, the extraction used organic solvents is usually carried at higher temperature (about 350 °C). At these temperatures coal has thermal pyrolyzed. Then the thermal dissolution soluble fraction obtained at higher extraction temperature could not provide the real structure information of coals and hard to be hydroliquefied to oil. Then the new extraction method at mild condition must be developed. In 2010, Painter et al. reported that certain ILs (such as BMIC) could be used to fragment, swell, partially solubilize, and disperse some coals [4]. Also we had found that BMIC has high performance on the dissolution of Xianfeng lignite (XL) [5,6]. The effect of the chemical structure of anions and cations of ILs on their dissolving capability for lignite had also been studied [7]. Among the ILs studied, BMIC exhibited the best dissolving capability that up to 80% extract yield of XL can be obtained by heating at 200 °C [5]. However, the
⁎ Corresponding authors. Tel.: +86 5552311552; fax: +86 5552311822. E-mail addresses: [email protected] (Z.-P. Lei), [email protected] (H.-F. Shui).
http://dx.doi.org/10.1016/j.fuproc.2014.10.010 0378-3820/© 2014 Elsevier B.V. All rights reserved.
relatively high melting points and viscosities of ILs containing chloride anion possibly limit their practical application in lignite processing. Relatively high dissolution temperatures (often above 200 °C) are often required for dissolving lignite, which possibly results in thermal decomposition of BMIC. From aforementioned point of view, it will be of great interest to seek for powerful, halogen-free ILs with low melt point and low viscosity. During the study of cellulose dissolution, it was found that EMIA showed higher capability for cellulose and wood dissolution [8–12]. Moreover, EMIA has low melt point (b−20 °C), low toxicity (LD50 N 2000 mg kg - 1 ) and viscosity (∼ 140 cPs at 25 °C) and was considered to be less toxic and corrosive than comparable chlorides and even biodegradable. Wu et al. have recently shown higher solvating power of 1-butyl-3-methylimidazolium acetate for chitin than 1-alkyl3-methylimidazolium chloride or BMIC [13]. In particular, BASF, one of the pioneers in industrial usage of ILs, has turned to EMIA for the dissolution of cellulose. These features make EMIA a promising solvent for the processing of lignites. These findings lead us to consider if such IL can be used for the extraction of lignites. However, little information about these is available in literature based on our knowledge. In the present paper, the extraction behaviors of three lignites with EMIA at mild condition were investigated and the results were compared the results with the previously studied of BMIC. The extracts obtained from the extraction of XL with EMIA were characterized with Fourier transform infrared (FTIR) spectra spectrometer. The relationship between the nature of different lignites and the extract yields with EMIA was correlated with the nature of different lignites (especially the number of oxygen-containing functional groups in lignites). The investigation facilitates the better understanding the dissolution performance of lignites in ILs.
48
Z.-P. Lei et al. / Fuel Processing Technology 135 (2015) 47–51
2. Experimental
EMIA
2.1. Properties of samples
lignite thermal dissolution
reaction mixture
Three Chinese lignites, XL, Shengli lignite (SL), and Xilinggele NO.6 lignite (XL6), as received were ground to b 200 mesh, stored under nitrogen atmosphere, and dried under vacuum at 80 °C overnight before use. The ultimate and proximate analyses of samples were shown in Table 1. The EMIA used in this study was purchased from Shanghai Cheng Jie Chemical Co. LTD. and dried at 100 °C in a vacuum oven 48 h prior to the experiments.
extraction with NMP filtration
F1
FC
solvent evaporation
EM
washed with H2O
R
precipitation
2.2. Extraction of lignites with EMIA The thermal dissolution experiments of lignites with EMIA were carried out in a microwave apparatus with temperature measurement, time controlling and stirring. As shown in Fig. 1, 1.0 g lignite was extracted with 10.0 g EMIA in Teflon tube at different temperature for 30 min. After the thermal dissolution, the reaction mixture was cooled to room temperature and extracted with fresh N-methyl-2pyrrolidinone (NMP). Then the reaction mixture was separated to filtrate 1 (F1) and filter cake (FC) by filtration through a membrane filter (pore size of 0.45 μm). The residue (R) was obtained by washing FC with water many times and dried in a vacuum at 353 K overnight. The extract yield (on a dry basis) was determined from the weight of the residues. NMP in F1 were removed through rotary evaporation to afford the extraction mixture (EM). EM was then precipitated by the addition of water and acetone (4:1 v:v) and filtered to afford the extracts (E) and filtrate 2 (F2). It is worth noting that there are traces of the extracts in F2. Finally acetone and water in F2 were removed by rotary evaporation to afford EMIA.
2.3. Sample characterization FTIR spectra of the lignites, extracts and residues were measured on a Nicolet 6700 spectrometer using the KBr-pellet technique. Quantitative measurement of oxygen-containing group in lignites: total acid and carboxyl in lignites were measured by titrimetric method based on the following reactions:
E
FTIR analysis
F2 solvent evaporation
EMIA Fig. 1. Procedure for extraction of lignite with EMIA and subsequent treatment.
3. Results and discussion 3.1. Extraction behavior of XL with EMIA As exhibited in Fig. 2, the extract yield of XL with EMIA significantly increases with the increase of extraction temperature from 100 to 200 °C. At 100 °C or 150 °C the extract yield of XL with EMIA is very low (about 3.2% or 13.4%), suggesting that only small portion of the fraction of XL can be released by EMIA at these temperatures. When the extractions with EMIA were carried out at 180 °C and 200 °C, extract yields of XL with EMIA reach to about 71.6% and 92.9%. The above result clearly shows that extraction temperature significantly affects the dissolution of lignite with EMIA. This phenomenon is consistent with the results of XL dissolution in BMIC [5]. Our previous studies showed that the extract yield of XL at this temperature without the addition of ILs is very low [5]. The above results clearly show that ILs have high extraction ability for lignite. Compared to the dissolution performance of BMIC (extract yield of XL in BMIC is about 80% at 200 °C), EMIA is an effective solvent for the extraction of lignite.
Ba(OH)2 + 2HA = BaA2 + 2H2O (total acid) 100
2(\COOH) + Ca(CH3COO)2 = \COO\Ca\OOC\ + 2CH3COOH
CH3COOH + NaOH = CH3COONa + H2O (carboxyl). Phenolic hydroxyl was obtained by subtraction of total acid and carboxyl.
Table 1 Proximate and ultimate analyses (wt%) of three lignites. Sample
XL SL XL6
Extract yield (wt%, d)
80
60
40
20
Proximate analysis
Ultimate analysis (daf)
Ad
Mad
Vdaf
C
H
Oa
N
St,d
14.4 15.1 16.2
33.6 15.1 16.2
60.6 33.3 51.1
64.75 68.25 68.45
5.81 5.23 6.35
N27.08 N24.34 N22.65
1.84 1.12 1.26
0.52 1.05 1.30
a By difference; daf: dry and ash-free base; Mad: moisture (air dried base); Ad: ash (dry base, i.e., moisture-free base); Vdaf: volatile matter (dry and ash-free base); St,d: total sulfur (dry base).
0 100
150
180
200
o Temperature ( C) Fig. 2. Effect of extraction temperatures on extract yield of XL with EMIA (ratio of XL/EMIA: 1:10 by weight).
Z.-P. Lei et al. / Fuel Processing Technology 135 (2015) 47–51
Oxygen content (wt%, daf)
27
XL6 SL XL
26
25
24
23
22 0.95
Carboxyl content (mmol/g)
Ordinarily, fragmentation breaks the coal network, allowing the dissolution of trapped materials. Low rank coals are thought to form mainly aggregated structure through interaction among functional groups containing heteroatom and hydrogen bond [14]. The high extraction of XL with ILs may be the result of ILs-induced structural relaxation followed by dissolution of coal components in the solvent. With the raising of extraction temperature, some non-covalent bonds are released by ILs-induce, resulting in the increase of extract yield of lignite. The above clearly indicates that the mainly aggregated structure of XL can be fragmented at higher temperature in EMIA. Up to now, it is considered that extract yield of coal at low temperature mainly depends on the cleavage degree of aggregated structure of coal. Then the higher extract yield of XL with EMIA may be attributed to that more aggregated structure of XL can be dissociated in EMIA than that in BMIC at same extraction condition. It has been reported that ILs containing carboxylic acid anions have stronger coordination ability or hydrogenbond acceptability (basicity) compared with those anion derived from strong acid [13]. Then ILs with stronger coordination anion than chloride can disrupt the more complex hydrogen-bond formed in lignite.
3.2. Effect of lignite type on the extract yield of lignites with EMIA
XL6 SL XL
0.90 0.85 0.80 0.75 0.70 0.65 0.60
20
The above result clearly shows that EMIA has an outstanding performance on the dissolution of lignite. Our previous study showed that the extract yields of coals with BMIC vary with the ranks of coals [5]. For different lignites, the extract yield of lignites with BMIC is different. In order to investigate if this phenomenon is the same at that with BMIC, three lignites with almost the same ultimate analysis results were used to determine the dependence of extract yield of lignites with EMIA on lignite type. As shown in Fig. 3, the extract yield of lignites with EMIA is significantly different from each other. Among the three lignites studied, XL gives the highest extract yield of about 93.0%. However, XL6 gives the lowest extract yield of 30.7%. The difference of extract yield of lignites with EMIA is probably related to the crosslinking structure of the coal network. It has been postulated that low-rank coals form crosslinking structures through interaction among functional groups containing heteroatomics and hydrogen bond. Then the relationship between oxygen content of lignites and the extract yield of lignites with EMIA is investigated. As shown in Fig. 4, the extract yield of lignites with EMIA increases with the increase of the oxygen content up from 22% C to 27% O. This result may strongly demonstrate that the interaction among the ILs and the oxygen-
100
Extract yield (wt%, d)
80
60
40
49
30
40
50
60
70
80
90
100
Extract yield (wt%, d) Fig. 4. Relationship between oxygen content and carboxyl content of lignites and extract yield of lignites with EMIA (filled symbols) and BMIC (open symbols).
containing functional groups in lignites can release the interaction among the oxygen-containing functional groups. Then it can be concluded that the high extract yield of lignites with IL may result from the ILs can break the noncovalent interaction such as hydrogen bond among carboxyls and phenolic hydroxyls [14]. Lignite contains substantial amounts of oxygen in the form of carboxyls and phenolic hydroxyls. Then the carboxyls and phenolic hydroxyls in three lignites were determined by chemical titration. As listed in Table 2, the difference of the total acid and hydroxyl content of three lignites is not big, but their carboxyl content significantly varied. Then the correlation between carboxyl and extract yield of the three lignites with EMIA was studied. As exhibited in Fig. 4, the extract yield of lignites with EMIA increases with the increase of carboxyl content of up to 0.949 mmol/g. This result suggests that the interaction between carboxyl group of lignites and ILs significantly affect extraction behaviors of lignites with EMIA. Also the extract yield of three lignites with BMIC were correlated with oxygen content and carboxyl content in three lignites, the same rule can be found. Pulati et al. [4] proposed that the ability of a solvent to dissolve a portion of a particular coal depends upon the chemical structure of that coal and the ability of the solvent to interact with polar functional groups, particularly those that hydrogen bond. Also our previous studies on dissolving coal by ILs have suggested that the dissolution mainly involved the interaction of the ILs with hydrogen bond in coal [7]. Then it can be concluded that extract yield of lignites with ILs mainly depends on the oxygen content and carboxyl content of lignites. The ability of different ILs on the dissolution of lignite structure (especially polar functional groups such as carboxyl et al.) determines the extract yield of lignite
20 Table 2 Total acid, carboxyl, and phenolic hydroxyl (–OH) content (mmol/g) in three lignites.
0 XL6
SL
XL
Lignite type Fig. 3. Effect of the lignite type on extract yield of lignites with EMIA: 200 °C, 30 min, and ratio of lignite to EMIA: 1:10.
Sample
Total acid
Carboxyl
Phenolic hydroxyl
XL SL XL6
2.321 2.032 2.119
0.949 0.705 0.643
1.372 1.327 1.476
3.3. Characteristics of extraction products of XL
4. Conclusions
1260 1220 1160
2913 2845
3420
1600
The results of extraction of lignites with EMIA show that EMIA has higher performance on the dissolution of lignites than that of BMIC. The extraction temperature significantly affects extraction behaviors of lignites with EMIA. The extract yield of XL with EMIA reaches to 93.0% at 200 °C for 30 min, when the ratio of EMIA/XL is 10. The interaction between carboxyl group of lignites and ILs significantly affects extraction behaviors of lignite with ILs. The extract yield of lignites
1030
R200
Absorbance
As shown in Fig. 5, the intensity of 2920 cm−1 (assigned to aliphatic C–H stretching) for E100 and E150 obtained from 100 °C and 150 °C is significantly higher than that of XL. This suggests that aliphatic alkyl structure components are mainly extracted at low temperature by EMIA. With the increase of extraction temperature, the intensity of 1600 cm− 1 (assigned to aromatic ring stretching vibration modes), 1260 cm− 1, 1220 cm− 1 and 1160 cm− 1 (assigned to C-O (phenol), Car–O–Car, C–O (alcohol), and Car–O–Cal structures respectively) for E200 significantly increase, suggesting more and more aromatics structures, phenol and/or ether groups components are extracted by EMIA. Also the intensity of band near 3400 cm−1 (assigned to hydrogen bond) for E increases with the increase of extraction temperature. This result indicates that hydrogen bond in extracts results from the presence of oxygen-containing functional group in aromatic structure components. On the contrary, as shown in Fig. 6, the intensity of 2920 cm−1 for R increases with the increase of extraction temperature. This result suggests that the content of aliphatic alkyl structure components in R increases with the increase of extraction temperature. This indicates that aromatic fractions in lignite can be extracted by EMIA. Also as shown in Fig. 6, the intensity of 1030 cm−1 (assigned to minerals) for R200 obtained from extraction at 200 °C significantly increases compared to that for the raw coal. This suggests that the EMIA mainly extracts the organic material in the raw coal.
1260
2845
3420
in ILs. The further investigation of interaction of carboxyl and hydroxyl groups with ILs is now under investigation in our laboratory.
1600
Z.-P. Lei et al. / Fuel Processing Technology 135 (2015) 47–51
2913
50
R150
R100
XL
3500
3000
2500
2000
1500
1000
-1 Wavenumbers (cm ) Fig. 6. FTIR spectra of the residues obtained from the extraction of XL with EMIA at different extraction temperatures (RTemperature) and XL.
with EMIA increases with the quantity of carboxylic groups in lignite increases. A small amount of aliphatic alkyl structure components are extracted at low temperature. More aromatics structure components are extracted by EMIA at high extraction temperature. Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, Grant 2011CB201302), the Key Project of Coal Joint Fund from the Natural Science Foundation of China and Shenhua Group Corporation Limited (Grant U1261208), the Natural Scientific Foundation of China (Grants 21476002, 21476003, 21476004, 21176001, U1361125, 21076001, 51174254, 20936007). This work was subsidized by the Strategic Chinese–Japanese Joint Research Program (Grant 2013DFG60060). Authors are also appreciative for the financial supports from the Anhui Provincial Innovative Group for Processing & Clean Utilization of Coal Resource and Program for Innovative Research Team in Anhui University of Technology. References
Absorbance
E200
E150 E100
XL
3500
3000
2500
2000
1500
1000
-1
Wavenumbers (cm ) Fig. 5. FTIR spectra of the extracts obtained from the extraction of XL with EMIA at different extraction temperatures (ETemperature) and XL.
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