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Structural characterization of the thermal extracts of lignite
Fuel Processing Technology 120 (2014) 8–15
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Structural characterization of the thermal extracts of lignite Zhicai Wang ⁎, Hengfu Shui, Chunxiu Pan, Liang Li, Shibiao Ren, Zhiping Lei, Shigang Kang, Cheng Wei, Jingchen Hu School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Clean Coal Conversion & Utilization, Anhui University of Technology, 243002 Ma'anshan, China
a r t i c l e
i n f o
Article history: Received 2 March 2013 Received in revised form 14 November 2013 Accepted 27 November 2013 Available online 18 December 2013 Keywords: Xianfeng lignite Thermal extraction Nonspecific solvent Extract characterization
a b s t r a c t Thermal extraction (TE) with nonspecific solvent at high temperature is a potential technology to separate organic materials from coal, especially low-rank coal such as lignite. In this paper, thermal extract (TES) of Xianfeng lignite (XL) in toluene/methanol (3:1, volume) mixed solvent at 300 °C was separated into different subfractions by the method of column chromatography combined with Soxhlet extraction in tetrahydrofuran (THF). These sub-fractions were characterized by element analysis, FTIR, 1H NMR and GC/MS. 78 compounds including C12–30 higher aliphatic hydrocarbons (HAHs), aromatic hydrocarbons (AHs), C17–27 fatty acid methyl esters (FAMEs) and other heteroatomic compounds were identified from the TES. As two groups of predominant components, HAHs and FAMEs are the intrinsic components in XL except for small amount of FAMEs produced by esterification and transesterification reactions in the TE process. Further, the mechanism of TE was also speculated by the characterization results of TES. As a result, the TE with nonspecific solvent at high temperature can not only improve the extract yield of organic materials, but also obtain chemicals such as HAHs and FAMEs from lignite. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Coal is not only an important energy resource, but also an indispensable organic carbon resource. With the rapid growth of crude oil consumption, the conversion technologies of coal to produce transport fuel and fine chemicals have recently been paid much more attention in China. As a low quality coal, lignite and/or brown coal has abundant recoverable reserve, but it is mainly used to generate electricity near mine at present. So the high efficiency utilization technology of lignite needs to be developed to make up for the shortage of petroleum resource. In general, lignite is regarded as a suitable liquefaction feed because of its high H/C and reactivity, so that direct liquefaction of lignite has been investigated extensively [1–4]. However, high hydrogen consumption resulting from high oxygen content and moisture limits the liquefaction economics of lignite [4]. Since Iino et al. [5] found that CS2/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent could give 40–65% extraction yield for many bituminous coals at room temperature, the solvent extraction has become an important technique to investigate the structure of coal and realize the cleaning conversion of coal [6–11]. As well known, coal consists of complex macromolecular network and some dissolved organic materials (guest). Nishioka and Larsen [12] considered that these organic materials are bonded in the network by non-covalent interactions, which are also one of the important cross⁎ Corresponding author. Tel.: +86 13955530691; fax: +86 555 2311552. E-mail address: [email protected] (Z. Wang). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.11.017
linking interactions of the network. In order to separate out the original guest materials as much as possible, the extraction usually carried out at suitable temperature or in specific solvent so that these non-covalent interactions can be broken. For example, the specific solvent, such as pyridine [13], amines [14], CS2/pyridine [15], CS2/NMP [16], etc., had shown higher extraction yield than the non-specific solvent under the refluxing temperature of solvent, even at room temperature. The specific solvent can break the non-covalent bonds in coal to increase the extraction yield, but it cannot break the covalent bonds of coal macromolecules [13–16]. Meanwhile, the specific solvent is mainly used to investigate the structure of coal, and is not very suitable for the commercial application due to its expensive cost. Since the nonspecific solvent has the poor solubility and cannot break the noncovalent bonds in coal at refluxing temperature, the extraction of coal in non-specific solvent, such as toluene and lower alcohols, is generally carried out under supercritical conditions. In the process of supercritical extraction, the pyrolytic reaction can also improve the extraction yield to varying degrees [17–19]. However, significant solvent loss was observed in supercritical extraction because the pyrolytic reactions of solvent could also occur at higher than 350 °C [17]. Although the extraction yields of some low rank coals such as Illinois No. 6 coal were below 20 wt.% with supercritical toluene [17,18], Yuan et al. [19] found that the extraction yield of Leping coal (a Chinese lignite) at 380 °C reached 64 wt.%. Obviously, the coal characteristics also have an important influence on the extraction besides temperature and solvent. Recently, the TE with high boiling point solvent by thermal filtration, which can promote the separation of extracts from the residue, has been extensively investigated to produce valuable chemicals and Hypercoal
Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15
[20–26]. Yoshida et al. [11,21] carried out the TE of various bituminous coals at less than 380 °C in order to produce ashless coal, and found that the TE yield of Illinois No. 6 coal at 360 °C was 38% in light cycle oil (LCO) by filtration at ambient temperature, but those were respectively 60% and 80% in LCO and crude methylnaphthalene oil (CMO) by hot filtration. Miura et al. [22] successfully separated coal into different molecule size fractions by the TE in a flowing stream of tetralin (THN), methylnaphthalene (MN) or derived coal liquids under 10 MPa at 200–400 °C. Although the TE property varied with bituminous coals, the ultimate analysis, the structure and the molecular weight distribution were little different between different extracted fractions from the same coal. Here, the stronger solubility of high boiling point solvent may be responsible for higher TE yields. Therefore, the TE is a potential technology of coal conversion to produce the cleaning fuels and the chemical raw materials [22–25]. In order to overcome the difficulty in the recovery of high boiling solvent, Lu et al. [26] carried out the sequential TE of Huolinguole lignite with methanol and ethanol as solvent, respectively. Since low-carbon alkanol can act as hydrogen donor and alkylation reagent during TE of coal [27], some thermal degradations of lignite that included dissociation of intermolecular interactions, esterification and alkanolysis were observed in the TEs with methanol and ethanol as solvents [26]. The TE with low boiling point solvent not only is an effective method for understanding thermal degradation of lignite, but also is easy to know the structure and composition of dissolved organic materials in coal. In our previous work [28], the TE of XL by hot filtration had been investigated in low boiling non-specific solvents, such as methanol, toluene and their mixture, respectively. The results showed that the TE significantly improved the extraction yield, and there was no obvious pyrolysis reaction to be observed at 300 °C. To increase the TE temperature can further improve the extract yield, but obvious pyrolysis reaction occurred in the TE process at 380 °C. In this paper, the TES obtained by toluene/methanol mixed solvent at 300 °C was separated through column chromatography into different sub-fractions, and their structures were further characterized by FTIR, 1H NMR, GC/MS, etc. in order to analyze the major components of TES and understand the mechanism of TE. 2. Experimental 2.1. Preparation of TES In the present work, the TES was prepared with toluene/methanol (3:1 volume) mixed solvent in a 1 L autoclave extractor with a stainless steel filter (0.5 μm). 30.0 g dried XL and 300 mL mixed solvent were charged into autoclave. The extractor was purged with 99.99% N2 three times, and finally pressurized to 0.1 MPa at room temperature. Then the XL was extracted at 300 °C for 3 h. The TE mixtures were separated by in-situ hot filtration. Subsequently, the TE residue was washed with above mixed solvent and filtered at 300 °C for three times. All filtrate was incorporated to remove the mixed solvents by rotary evaporation. Then the TES was obtained by desiccation in a vacuum at 80 °C for 48 h. A detailed description can be found elsewhere [28]. The ultimate and proximate analyses results of XL and its TES were shown in Table 1. All solvents used in the present work were commercially purchased as analytical reagents. 2.2. Separation of TES Firstly, above dried TES was exhaustively extracted with THF solvent in a Soxhlet extractor to afford the THF insoluble fraction (sub-fraction VII) and THF soluble (THFS). Then, THFS was separated by column chromatography. 1.0 g dried THFS was dissolved in THF and mixed with 10 g of neutral silica gel under ultrasonic irradiation for 30 min. Subsequently, THF solvent was removed by rotary evaporation, and the silica gel sample obtained was dried in a vacuum at 80 °C for 24 h. The silica
9
Table 1 Element analyses of the XL, TES and its sub-fractions. Sample
XL TES I II III IV V VI VII a
Yield/%
– – 21.1 43.4 12.5 0.5 3.4 10.5 8.8
Ultimate analysis, wdaf/%
H/C
C
N
S
H
Oa
63.1 77.7 82.6 77.9 77.4 76.9 50.8 82.7 61.8
1.8 0.9 0.6 0.9 1.1 0.7 1.6 0.8 1.8
0.4 0.8 0.6 0.6 0.4 0.2 0.5 0.6 0.5
6.0 10.6 12.3 11.4 10.8 10.0 7.4 12.2 4.6
28.7 10.0 3.9 9.2 10.3 12.2 39.7 3.7 31.3
1.14 1.64 1.79 1.76 1.67 1.56 1.76 1.78 0.90
By difference.
gel adsorbed THFS was loaded onto a 1.0 cm × 30 cm neutral silica gel column activated at 260 °C. According to the separation of asphaltene and preasphaltene of coal, the THFS was eluted sequentially with the following solvents: toluene, toluene/THF (4:1, volume), toluene/THF (1:2, volume), THF and methanol, so that the components of TES could gradually be eluted by their polarity as much as possible. After eluting the column with a particular solvent, the solvent was removed by rotary evaporation and the eluent was dried in a vacuum at 80 °C for 24 h. All separated elutes, which were defined sequentially as sub-fractions I, II, III, IV and V, were weighted and analyzed. Finally, the silica gel on the top of column was exhaustively extracted by THF in a Soxhlet extractor to afford the residue fraction on eluted column (as sub-fraction VI). 2.3. Characterization of sub-fractions FTIR spectra of all sub-fractions were determined at ambient temperature by Nicolet 6700 FTIR spectrometer. An approximately 1 mg sample was mixed with 0.1 g KBr, and pressed into a pellet. The number of scans was 32 in the scanning range of 400–4000 cm−1. The element analysis was carried out at the mode of CHNS by the Vario EL III elementary analyzer. Sub-fractions I–VI were respectively dissolved in THF, and then were analyzed by GCMS-QP2010 Plus with a Restek Rtx-5MS capillary column (i.d. 0.25 mm, length 30 m, thickness 0.25 μm). The temperature program used was from 130 °C, held for 2 min, raised at 15 °C min−1 to a final temperature of 250 °C, and held for 20 min. The carrier gas was helium (99.999%), with a 1 mL min− 1 flow. The injector temperature was 250 °C, with the GC–MS interface at 250 °C. The data were acquired and processed using ChemStation software and the compounds were determined by comparing their mass spectra to NIST05 library data. 1H NMR spectra of sub-fractions I–III, V and VI were obtained using a Brucker AM500 (500 MHz) in d3-chloroform solvent. 3. Results and discussion In our previous work [28], it had been reported that the TE yields of XL in toluene/methanol (3:1, volume) mixed solvent at 300 °C and 380 °C were 15.7 wdaf%, 28.5 wdaf%. For comparison, the Soxhlet extractions (at refluxing temperature of solvent) of XL in toluene, methanol and THF were carried out in the present work. However, the Soxhlet extraction yields were only 2.5, 4.0 and 8.9 wdaf%, respectively. Further, the extraction yield of 12.9 wdaf% was obtained in CS2/NMP mixed solvent at room temperature. Above results show that the extraction yields at lower temperature are obviously less than the TE yield for XL though CS2/NMP mixed solvent could give high extraction yield (40–60 wdaf%) for many bituminous coals [5]. So we speculated that there are a lot of non-covalent bond cross-linking interactions in the network structure of XL so that small amount of small guest molecules was trapped inside. For the extraction at lower temperature, the extraction yields are related to the hydrogen bonding abilities of solvents,
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which can disrupt the hydrogen bond cross-linking interactions in the network structure to release the guest molecules (extracts). However, for the TE at higher temperature, it is still unknown whether other factors are also responsible for the high TE yield except thermal cleavage of hydrogen bond cross-linking interactions. In our previous work [28], it had been found that there was no obvious pyrolysis of XL occurred in the process of TE at 300 °C. In order to investigate the TE of XL, the TES were further separated and analyzed as follows. Table 1 shows the yields and the ultimate analyses results of XL, TES and all sub-fractions separated from the TES. Yields of sub-fractions display that the TES consists of 8.8 wt.% THF insoluble (sub-fraction VII) and 91.2 wt.% THFS. Sub-fractions I, II and III eluted by toluene and toluene/THF solvents are major fractions, in which sub-fraction II shows the highest yield (43.4 wt.%). In addition, there is also 10.5 wt.% of residue on column not to be eluted (sub-fraction VI). Yields of sub-fractions IV and V, which were respectively eluted by THF and methanol, are very little in TES. It suggested that the TES mainly consists of the weak polar compounds and some macromolecules or aggregates. Table 1 further lists the element analysis results of all sub-fractions. It is observed that, with the increase of the eluent polarity, C % and H % of sub-fraction obtained decrease and O % increases. Sub-fraction VI remained in the chromatography column shows higher C % and H %, but only 3.7% of oxygen. Fraction VII shows the second lowest C % and the lowest H %, but the second highest O % in all sub-fractions. H/C of sub-fractions from I to IV decrease gradually from 1.79 to 1.56, and H/C of sub-fractions V and VI are respectively 1.76 and 1.78, near to that of sub-fraction II. However, H/C of sub-fraction VII is only 0.90. It suggested that most of components in TES consist of aliphatic structures with various amounts of oxygen-containing groups except sub-fraction VII. Fig. 1 shows FTIR spectra of all sub-fractions. For the absorption peaks of OH and Cal\H stretching vibrations (3440 cm−1 and 2952– 2850 cm−1, respectively), their intensities are in agreement with the O % and H/C listed in Table 1, except that sub-fraction I only shows a very weak peak of OH. Meanwhile, sub-fractions I–IV and VI also display a obvious rocking vibration peak of (CH2)n (n ≥ 4) at 722 cm− 1. It suggested that major sub-fractions of TES contain a lot of long chain aliphatic groups and various amounts of OH. Further, a strong absorption peak of ester group (attributed to the stretching vibration of C_O in aliphatic ester [29,30]) can be observed at 1742 cm−1 from the spectra of sub-fractions I–VI shown in Fig. 1. The absorption peak of aromatic ring stretching vibration in the range of 1626–1590 cm−1, gradually strengthens from sub-fractions II to VII except of sub-fraction VI, but it is hardly observed in sub-fraction I. However, the spectrum of subfraction VII is significantly different from other spectra, and shows a
3440
2952
16261461 1034
I II III
T/%
IV V VI VII 722 1742 1590 1168
2921 4000
3500
3000
2500
2000
1500
1000
Wavenumber / cm-1 Fig. 1. FTIR spectra of sub-fractions separated from TES.
500
shoulder peak at 1706 cm−1, which may be attributed to the stretching vibration of C_O in aromatic carboxyl. Therefore, the TES mainly consists of a lot of HAHs and higher aliphatic esters, and some aromatic compounds. Fig. 2 shows 1H NMR spectra of sub-fractions I–III, and V–VI. Here, the spectra of all sub-fractions show the largest peak at δ 1.25 ppm resulting from the aliphatic methylene (CH2) groups, and a small multipeak at δ 0.9 ppm from the terminal methyl group of the aliphatic chain. Meanwhile, the characteristic peak of methoxyl protons (O\CH3) can be observed as a singlet at δ 3.65 ppm and a triplet of α-CH2 protons at δ 2.32 ppm in all spectra. Further, the proton peak near to δ 1.56 ppm, which could be attributed to the β-carbonyl methylene proton [31,32], is gradually broadening from sub-fractions I to VI. Based on above results, it can be speculated that these sub-fractions should mainly consist of HAHs and FAMEs. However, no signal of aromatic proton (Car\H) was observed in all spectra shown in Fig. 1, except of the residual proton peak of deuterated chloroform solvent at δ 7.28 ppm. In addition, some very weak signals can also be observed in the range of δ 2.0–4.5 ppm, suggesting that these sub-fractions are more complicated in components. Fig. 3 shows the total ion chromatographs (TICs) of the sub-fractions I–VI, respectively. In the present work, major compounds were identified by their MS spectra, and GC/MS characterization results were listed in Table 2. The results show that there are 30, 30, 25, 26, 21 and 25 compounds to be identified from sub-fractions I to VI, respectively. 78 different compounds in total are found in the sub-fractions from the TES. These compounds mainly consist of HAHs (C12–30), FAMEs (C16–26), and other compounds such as aromatic compounds, other oxygen-containing compounds (OCCs), etc. All HAHs identified in the TES are the normal paraffins except 2,6,10,14-tetramethylheptadecene (Pristane), 2,6,10,14-tetramethyl-2-heptadecene (Pristene) and 2,6,10,14tetramethylhexadecane (Phytane). FAMEs include 10 kinds of saturated normal fatty acids ester and 4 kinds of unsaturated normal fatty acids esters. However, two kinds of saturated normal fatty acids were only determined. Meanwhile, Table 2 also shows that 8 AHs, such as tetralin, 5-methyltetralin, naphthalene, 2-methyl naphthalene, 1-methyl naphthalene, anthracene, 1,3-diphenyl-1-butene and 1,3,5triisopropyl benzene, are identified in the TES. Although a lot of OCCs, which exist mainly in sub-fractions II–IV, are identified in the TES, their relative contents are lower than those of HAHs and FAMEs. These OCCs include phthalates, other carboxylic esters, aliphatic ketones, phenolic compounds, tetrahydrofuran derivatives, ethers, aromatic aldehyde and quinine. In addition, 6 types of nitrogencontaining compounds, in which quinoline, indole and carbazole are all found, are identified in the TES. Ethyl (1,3-benzothiazol-2-ylsulfanyl) acetate and imino(triphenyl)phosphorane are respectively the only compound containing sulphur and phosphorus identified from the TES. Further, Table 2 shows that sub-fraction I mainly consists of HAHs and FAMEs, and other sub-fractions show varying amounts of OCCs besides HAHs and FAMEs. Therefore, the GC/MS results are basically consistent with the characterization results of elementary analyses, FTIR and 1H NMR, suggesting that main components of sub-fractions had been determined by GC/MS analysis. Based on above resolution results, these identified compounds can be grouped into HAHs, AHs, FAMEs, OCCs and OACs (compounds containing sulphur and phosphorus). Their relative contents (RCs) in sub-fractions obtained by the normalization method are shown in Fig. 4. It can be seen that the RCs of HAHs, which is the highest in all fractions except fraction IV, are in the range of 34.8–83.2%. HFAMEs, as the second major component, mainly consist in sub-fraction I (RC 45.9%) and II (RC 33.4%). Meanwhile, its RC in sub-factions VI, V and III also arrives to 18.8%, 11.7% and 9.7%, respectively, but only 1.7% in fraction IV. After taking the yields of sub-fractions into account, the contents of HAHs and HFAMs in the TES are estimated respectively about 47 wt.% and 28 wt.%, on the assumption that unidentified micro-components are ignored in fractions I–VI. Therefore, the TES is rich in HAHs and
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VI 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
V 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
III 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
II 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
I 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
δ / ppm Fig. 2. 1H NMR spectra of sub-fractions separated from TES.
HFAMEs, which are potential chemicals and important chemical raw material. Further, taking the yields of sub-fractions into consideration, it was suggested that HAHs and FAMEs should be the main components of TES, which are present in all sub-fractions. These HAHs identified in the TES are in agreement with those in the extracts of brown coal [33] and lignite tar [34,35], but the range of carbon atom numbers in FAMEs is similar to that in normal fatty acids reported in previous publications [36–38]. In organic geochemistry, the normal fatty acids and the normal alkanes in sediments are always used to study the biological source and the geochemical environment [39]. The decarboxylation of normal fatty acids was supposed as an important resource of normal alkanes [40]. However, the saturated fatty acids have greater thermal stability [41], so that many fatty acids can still be determined in coal [42]. In addition, there were also reports about the noncatalytic esterification and transesterification of fatty acids and their esters in subcritical and supercritical methanol to produce the FAMEs [43]. In order to investigate the origin of FAMEs, we compared the FTIR
spectra of TESs obtained by different solvents, which includes toluene, THF, methanol and toluene/methanol mixed solvents (3:1 and 9:1, volume). A strong absorption peak of carbonyl attributed to aliphatic ester can be observed at 1740 cm− 1 in the spectra of all TESs as shown in Fig. 5. Meanwhile, a shoulder peak of carbonyl in carboxylic acid can also be seen in the range of 1712–1720 cm−1. It is strong in TESs obtained by toluene and THF solvents, but very weak in TESs obtained by methanol solvent and toluene/methanol mixed solvent (3:1, volume). It is in agreement with the results of GC/MS analysis of subfractions. Therefore, we thought that most of FAMEs in TES determined by GC/MS were intrinsic in XL, and a little of FAMEs was only produced by esterification of carboxylic acid and methanol in the TE process. In addition, the pyrolysis of aliphatic moiety of alkyl substituted compounds should also be no significant at 300 °C [44,45], because no any alkene was identified from the sub-fractions of TES except for the pristene. So HAHs were intrinsic small molecular compounds “dissolved” in lignite. In the TE process, the relaxation of the network structure of lignite by the rupture of non-covalent bonds at high
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22
I
25
18
1
24
20
3
2
8 15 17 9-11 7 1214 16 5 6
4
27
26
23
28
19 21
29
30
1
II 8
29
15 9
26
12 16 19 1314 23 18 20 21
10
7
3 5 24
22
17
11
24
6
28
25
30
27
21 19
III
23 25
18
Relative intensity
17 1 5-6 9 7 248 3
16 14 15
12 13
20
10 11
15
45
IV 20
18
23
21
17 1
24
22
3 2
11-1214 10 6-9 13
22
16
24
19
14
15
26
25
V
17
9 18
6
20 2 1
5
7
3
8
4
11 1012 13
16
19
21
22
4
VI 5
14
1
910 18 7-8 17 20 6 15 22 13 16 12 21 11 19
2 3
5
10
15
23 24
20
25
25
Retention time / min Fig. 3. Total ion chromatograph (TIC) of sub-fractions I–VI separated from TES(▲ HAHs,● FAMEs).
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Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15
13
Table 2 Result of GC/MS analyses sub-fractions from the TES. Name
Molecular formula
Number of peak Ia
II
III
IV
V
VI
–
5
–
–
–
– – – – – – – 15 17 22 24 26 28 30 –
– – – – – – 16 17 18 19 21 23 25 – –
– – – – – – 16 17 18 20 21 22 24 26 –
– 3 – 6 – 9 – 14 15 17 18 20 22 – –
– – 7 11 – 15 16 17 18 20 22 24 25 – –
–
–
–
–
–
Aliphatic hydrocarbons (HAHs) n-Dodecane n-Tetradecane n-Hexadecane n-Heptadecane n-Octadecane n-Nonadecane n-Eicosane n-Heneicosane n-Docosane n-Tricosane n-Tetracosane n-Pentacosane n-Hexacosane n-Heptacosane n-Octacosane n-Nonacosane n-Triacontane 2,6,10,14-Tetramethylpentadecane (Pristane) 2,6,10,14-Tetramethyl-2-pentadecane (Pristene) 2,6,10,14-Tetramethylhexadecane (Phytane)
C12H26 C14H30 C16H34 C17H36 C18H38 C19H40 C20H42 C21H44 C22H46 C23H48 C24H50 C25H52 C26H54 C27H56 C28H58 C29H60 C30H62 C19H40 C19H38 C20H42
– 4 5 7 9 12 14 15 17 18 20 22 24 25 27 29 – 8 10 11
Aromatic hydrocarbons (AHs) Tetralin Naphthalene 5-Methyltetralin 2-Methyl naphthalene 1-Methyl naphthalene Anthracene 1,3-Diphenyl-1-butene 1,3,5-Triisopropyl benzene
C10H12 C10H8 C11H14 C11H10 C11H10 C14H10 C16H36 C15H24
1 6 – – – 3 – –
1 2 – 5 – – – –
1 – 4 7 8 – – –
– – – 1 – – 7 9
– – – – – – – –
1 2 – 4 5 – – –
Higher fatty acids & higher fatty acid methyl esters(HAs and FAMEs) n-Hexadecanoic acid n-Octadecanoic acid Methyl n-hexadecanoate Methyl n-octadecanoate Methyl n-nonadecanoate Methyl n-eicosanoate Methyl n-heneicosanoate Methyl n-docosanoate Methyl n-tricosnoate Methyl n-tetracosanoate Methyl n-pentacosanoate Methyl n-hexacosanoate Methyl 9-octadecenoate Methyl 16-octadecenoate Methyl 13-docosenoate Methyl 9-tetracosenoate
C16H32O2 C18H36O2 C17H34O2 C19H38O2 C20H40O2 C21H42O2 C22H44O2 C23H46O2 C24H48O2 C25H50O2 C26H52O2 C27H54O2 C19H36O2 C19H36O2 C23H44O2 C25H48O2
– – 13 16 – 19 21 23 – 26 28 29 – – – –
– – 9 11 14 – 19 – 25 27 29 10 12 16 23
13 14 12 – 15 – – – 20 22 24 – – – – –
– – 12 – – – – – – – – 25 – – – –
– – 5 8 11 – – 16 – 19 21 – 7 – – –
10 14 8 – – – – – 21 23 – – 12 – – –
Other compounds containing oxygen (OCCs) Allyl n-butyrate Cyclohexyl formate n-Butyl n-butyrate Dimethyl azelate 4-pentadecyl n-butyrate Diethyl-o-phthalate Dibutyl phthalate 3,5-Di-tert-butyl-4-hydroxybenzyl acrylate 3,5-Di-tert-butyl-4-hydroxybenzyl butyrate 2-Ethylhexyl-hexyl-o-phthalate Diisooctyl phthalate Dioctyl phthalate 3,5-di-tert-butyl-4-hydroxybenzaldehyde 2,6-Di-tert-butyl-para-benzoquinone 2,6-Di-tert-butyl-4-methylene-2,5-cyclohexadiene-1-one 6,10,14-Trimethyl-2-pentadecanone 2-Nonadecanone Docosa-2,21-dione Cyclohexyl ethyl ether Benzyl 2-methoxy-4-propenylphenyl ether 2-Butyltetrahydrofuran Butoxyl tetrahydrofuran 2,3′-Bi-tetrahydrofuran
C7H12O2 C7H12O2 C8H16O2 C11H20O4 C19H38O2 C12H14O4 C16H22O4 C18H26O3 C19H30O3 C22H34O4 C24H38O4 C24H38O4 C15H22O2 C14H20O2 C15H22O C18H36O C19H38O C22H42O2 C8H16O C17H18O2 C8H16O C8H16O2 C8H14O2
– – – – – 3 – – – – – – – – – – – – – – – – –
3 4 – 6 21 – 8 – – – – 20 – – – 7 – 18 – – – 13 –
– – 9 – – 10 11 – – – – – – – – – – – – – 2 – –
– – – – – – – 14 15 – 19 – 6 3 4 – – – – 13 – – –
– – – – – 2 4 – – 12 – – – – – – 10 – 1 – – – 13
– – – – – – 6 – – – – – – – – – – – – – – – –
(continued on next page)
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Table 2 (continued) Name
Molecular formula
Number of peak Ia
II
III
IV
V
VI
2,4-Ditert-butyl-5-methylphenol Other compounds containing oxygen (OCCs) 2,6-Di-tert-butyl-p-cresol 2,6-Di-tert-para-ethylphenol 4,4′-Ethylene-bis(2,6-di-tert-butylphenol)
C15H24O
–
–
–
2
–
–
C15H24O C16H26O C30H44O2
– – –
– – –
– – –
5 8 23
– – –
– – –
Compounds containing other heteroatoms Quinoline Indole Carbazole 3-Ethylcarbazole N,N-dibutyl oxamide amyl ester Ethyl (1,3-benzothiazol-2-ylsulfanyl) acetate Imino(triphenyl)phosphorane
C9H7N C8H7N C12H9N C14H13N C15H29NO3 C11H11NO2 S2 C18H16NP
– – – – – – –
– – – – – – –
3 6 – – – – –
– – 10 11 – – –
– – – – – – –
3 – – – 9 13 19
a
The number of peak in the TIC of corresponding sub-fraction.
temperature improved the diffusion of solvent and small molecular compounds such as HAHs, FAMEs, fatty acids, etc. In addition, a relatively few aromatic compounds were identified in TES, suggesting that most of aromatic moieties should be bonded in the network structure of XL by covalent bond, and no obvious pyrolysis of C\C bond occurs in the TE at 300 °C.
HAH
AH
FAME
OCC
OAC
100
Relative content / %
80
60
40
0 I
II
III
IV
V
VI
Fractions Fig. 4. Relative contents of different types of compounds identified in sub-fractions I–VI.
1.0
Normalized absorbance
At higher temperature, TE in non-specific solvent can disrupt the noncovalent bonding interactions to relax the macromolecular network, so that more dissolved organic compounds are extracted from coal without obvious pyrolysis of C\C bond. The TE yield of XL in toluene/methanol (3:1, volume) mixed solvent at 300 °C is 15.7 wdaf%. By isolation and structural characterization, there are 78 compounds identified in THF solubles from TES, which including HAHs, FAMEs, AHs, and other compounds containing O, N, S and P. HAHs and FAMEs are predominant components of TES. The HAHs, which mainly consist of C12–30 normal HAHs, should be intrinsic small molecular compounds “dissolved” in XL. Most of FAMEs (C17–27) are also the intrinsic components in XL, but there is also small amount of FAMEs produced by esterification and transesterification in the TE process. Therefore, the TE with methanol as solvent at certain temperature is a potential separation technique of lignite to produce the chemicals, and these structural characterizations of TES are advantageous to understand the structure of XL. Acknowledgment
20
Methanol (M) T/M=3 T/M=9 Toluene (T) THF
0.8
0.4
0.2
1800
1750
1700
1650
1600
1550
Wavenumber /cm-1 Fig. 5. FTIR spectra of TESs obtained by different solvents.
This work was supported by the National Basic Research Program of China (973 Program, Grant 2011CB201302), the Key Project of Coal Joint Fund from Natural Science Foundation of China and Shenhua Group Corporation Limited (Grant U1261208) and the Natural Scientific Foundation of China (Grants 51174254, 21076001, 21176001, 20936007). The authors are also appreciative for the financial support from the Provincial Innovative Group for Processing & Clean Utilization of Coal Resource and Program for Innovative Research Team in Anhui University of Technology. References
0.6
0.0 1850
4. Conclusion
1500
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Structural characterization of the thermal extracts of lignite Zhicai Wang ⁎, Hengfu Shui, Chunxiu Pan, Liang Li, Shibiao Ren, Zhiping Lei, Shigang Kang, Cheng Wei, Jingchen Hu School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Clean Coal Conversion & Utilization, Anhui University of Technology, 243002 Ma'anshan, China
a r t i c l e
i n f o
Article history: Received 2 March 2013 Received in revised form 14 November 2013 Accepted 27 November 2013 Available online 18 December 2013 Keywords: Xianfeng lignite Thermal extraction Nonspecific solvent Extract characterization
a b s t r a c t Thermal extraction (TE) with nonspecific solvent at high temperature is a potential technology to separate organic materials from coal, especially low-rank coal such as lignite. In this paper, thermal extract (TES) of Xianfeng lignite (XL) in toluene/methanol (3:1, volume) mixed solvent at 300 °C was separated into different subfractions by the method of column chromatography combined with Soxhlet extraction in tetrahydrofuran (THF). These sub-fractions were characterized by element analysis, FTIR, 1H NMR and GC/MS. 78 compounds including C12–30 higher aliphatic hydrocarbons (HAHs), aromatic hydrocarbons (AHs), C17–27 fatty acid methyl esters (FAMEs) and other heteroatomic compounds were identified from the TES. As two groups of predominant components, HAHs and FAMEs are the intrinsic components in XL except for small amount of FAMEs produced by esterification and transesterification reactions in the TE process. Further, the mechanism of TE was also speculated by the characterization results of TES. As a result, the TE with nonspecific solvent at high temperature can not only improve the extract yield of organic materials, but also obtain chemicals such as HAHs and FAMEs from lignite. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Coal is not only an important energy resource, but also an indispensable organic carbon resource. With the rapid growth of crude oil consumption, the conversion technologies of coal to produce transport fuel and fine chemicals have recently been paid much more attention in China. As a low quality coal, lignite and/or brown coal has abundant recoverable reserve, but it is mainly used to generate electricity near mine at present. So the high efficiency utilization technology of lignite needs to be developed to make up for the shortage of petroleum resource. In general, lignite is regarded as a suitable liquefaction feed because of its high H/C and reactivity, so that direct liquefaction of lignite has been investigated extensively [1–4]. However, high hydrogen consumption resulting from high oxygen content and moisture limits the liquefaction economics of lignite [4]. Since Iino et al. [5] found that CS2/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent could give 40–65% extraction yield for many bituminous coals at room temperature, the solvent extraction has become an important technique to investigate the structure of coal and realize the cleaning conversion of coal [6–11]. As well known, coal consists of complex macromolecular network and some dissolved organic materials (guest). Nishioka and Larsen [12] considered that these organic materials are bonded in the network by non-covalent interactions, which are also one of the important cross⁎ Corresponding author. Tel.: +86 13955530691; fax: +86 555 2311552. E-mail address: [email protected] (Z. Wang). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.11.017
linking interactions of the network. In order to separate out the original guest materials as much as possible, the extraction usually carried out at suitable temperature or in specific solvent so that these non-covalent interactions can be broken. For example, the specific solvent, such as pyridine [13], amines [14], CS2/pyridine [15], CS2/NMP [16], etc., had shown higher extraction yield than the non-specific solvent under the refluxing temperature of solvent, even at room temperature. The specific solvent can break the non-covalent bonds in coal to increase the extraction yield, but it cannot break the covalent bonds of coal macromolecules [13–16]. Meanwhile, the specific solvent is mainly used to investigate the structure of coal, and is not very suitable for the commercial application due to its expensive cost. Since the nonspecific solvent has the poor solubility and cannot break the noncovalent bonds in coal at refluxing temperature, the extraction of coal in non-specific solvent, such as toluene and lower alcohols, is generally carried out under supercritical conditions. In the process of supercritical extraction, the pyrolytic reaction can also improve the extraction yield to varying degrees [17–19]. However, significant solvent loss was observed in supercritical extraction because the pyrolytic reactions of solvent could also occur at higher than 350 °C [17]. Although the extraction yields of some low rank coals such as Illinois No. 6 coal were below 20 wt.% with supercritical toluene [17,18], Yuan et al. [19] found that the extraction yield of Leping coal (a Chinese lignite) at 380 °C reached 64 wt.%. Obviously, the coal characteristics also have an important influence on the extraction besides temperature and solvent. Recently, the TE with high boiling point solvent by thermal filtration, which can promote the separation of extracts from the residue, has been extensively investigated to produce valuable chemicals and Hypercoal
Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15
[20–26]. Yoshida et al. [11,21] carried out the TE of various bituminous coals at less than 380 °C in order to produce ashless coal, and found that the TE yield of Illinois No. 6 coal at 360 °C was 38% in light cycle oil (LCO) by filtration at ambient temperature, but those were respectively 60% and 80% in LCO and crude methylnaphthalene oil (CMO) by hot filtration. Miura et al. [22] successfully separated coal into different molecule size fractions by the TE in a flowing stream of tetralin (THN), methylnaphthalene (MN) or derived coal liquids under 10 MPa at 200–400 °C. Although the TE property varied with bituminous coals, the ultimate analysis, the structure and the molecular weight distribution were little different between different extracted fractions from the same coal. Here, the stronger solubility of high boiling point solvent may be responsible for higher TE yields. Therefore, the TE is a potential technology of coal conversion to produce the cleaning fuels and the chemical raw materials [22–25]. In order to overcome the difficulty in the recovery of high boiling solvent, Lu et al. [26] carried out the sequential TE of Huolinguole lignite with methanol and ethanol as solvent, respectively. Since low-carbon alkanol can act as hydrogen donor and alkylation reagent during TE of coal [27], some thermal degradations of lignite that included dissociation of intermolecular interactions, esterification and alkanolysis were observed in the TEs with methanol and ethanol as solvents [26]. The TE with low boiling point solvent not only is an effective method for understanding thermal degradation of lignite, but also is easy to know the structure and composition of dissolved organic materials in coal. In our previous work [28], the TE of XL by hot filtration had been investigated in low boiling non-specific solvents, such as methanol, toluene and their mixture, respectively. The results showed that the TE significantly improved the extraction yield, and there was no obvious pyrolysis reaction to be observed at 300 °C. To increase the TE temperature can further improve the extract yield, but obvious pyrolysis reaction occurred in the TE process at 380 °C. In this paper, the TES obtained by toluene/methanol mixed solvent at 300 °C was separated through column chromatography into different sub-fractions, and their structures were further characterized by FTIR, 1H NMR, GC/MS, etc. in order to analyze the major components of TES and understand the mechanism of TE. 2. Experimental 2.1. Preparation of TES In the present work, the TES was prepared with toluene/methanol (3:1 volume) mixed solvent in a 1 L autoclave extractor with a stainless steel filter (0.5 μm). 30.0 g dried XL and 300 mL mixed solvent were charged into autoclave. The extractor was purged with 99.99% N2 three times, and finally pressurized to 0.1 MPa at room temperature. Then the XL was extracted at 300 °C for 3 h. The TE mixtures were separated by in-situ hot filtration. Subsequently, the TE residue was washed with above mixed solvent and filtered at 300 °C for three times. All filtrate was incorporated to remove the mixed solvents by rotary evaporation. Then the TES was obtained by desiccation in a vacuum at 80 °C for 48 h. A detailed description can be found elsewhere [28]. The ultimate and proximate analyses results of XL and its TES were shown in Table 1. All solvents used in the present work were commercially purchased as analytical reagents. 2.2. Separation of TES Firstly, above dried TES was exhaustively extracted with THF solvent in a Soxhlet extractor to afford the THF insoluble fraction (sub-fraction VII) and THF soluble (THFS). Then, THFS was separated by column chromatography. 1.0 g dried THFS was dissolved in THF and mixed with 10 g of neutral silica gel under ultrasonic irradiation for 30 min. Subsequently, THF solvent was removed by rotary evaporation, and the silica gel sample obtained was dried in a vacuum at 80 °C for 24 h. The silica
9
Table 1 Element analyses of the XL, TES and its sub-fractions. Sample
XL TES I II III IV V VI VII a
Yield/%
– – 21.1 43.4 12.5 0.5 3.4 10.5 8.8
Ultimate analysis, wdaf/%
H/C
C
N
S
H
Oa
63.1 77.7 82.6 77.9 77.4 76.9 50.8 82.7 61.8
1.8 0.9 0.6 0.9 1.1 0.7 1.6 0.8 1.8
0.4 0.8 0.6 0.6 0.4 0.2 0.5 0.6 0.5
6.0 10.6 12.3 11.4 10.8 10.0 7.4 12.2 4.6
28.7 10.0 3.9 9.2 10.3 12.2 39.7 3.7 31.3
1.14 1.64 1.79 1.76 1.67 1.56 1.76 1.78 0.90
By difference.
gel adsorbed THFS was loaded onto a 1.0 cm × 30 cm neutral silica gel column activated at 260 °C. According to the separation of asphaltene and preasphaltene of coal, the THFS was eluted sequentially with the following solvents: toluene, toluene/THF (4:1, volume), toluene/THF (1:2, volume), THF and methanol, so that the components of TES could gradually be eluted by their polarity as much as possible. After eluting the column with a particular solvent, the solvent was removed by rotary evaporation and the eluent was dried in a vacuum at 80 °C for 24 h. All separated elutes, which were defined sequentially as sub-fractions I, II, III, IV and V, were weighted and analyzed. Finally, the silica gel on the top of column was exhaustively extracted by THF in a Soxhlet extractor to afford the residue fraction on eluted column (as sub-fraction VI). 2.3. Characterization of sub-fractions FTIR spectra of all sub-fractions were determined at ambient temperature by Nicolet 6700 FTIR spectrometer. An approximately 1 mg sample was mixed with 0.1 g KBr, and pressed into a pellet. The number of scans was 32 in the scanning range of 400–4000 cm−1. The element analysis was carried out at the mode of CHNS by the Vario EL III elementary analyzer. Sub-fractions I–VI were respectively dissolved in THF, and then were analyzed by GCMS-QP2010 Plus with a Restek Rtx-5MS capillary column (i.d. 0.25 mm, length 30 m, thickness 0.25 μm). The temperature program used was from 130 °C, held for 2 min, raised at 15 °C min−1 to a final temperature of 250 °C, and held for 20 min. The carrier gas was helium (99.999%), with a 1 mL min− 1 flow. The injector temperature was 250 °C, with the GC–MS interface at 250 °C. The data were acquired and processed using ChemStation software and the compounds were determined by comparing their mass spectra to NIST05 library data. 1H NMR spectra of sub-fractions I–III, V and VI were obtained using a Brucker AM500 (500 MHz) in d3-chloroform solvent. 3. Results and discussion In our previous work [28], it had been reported that the TE yields of XL in toluene/methanol (3:1, volume) mixed solvent at 300 °C and 380 °C were 15.7 wdaf%, 28.5 wdaf%. For comparison, the Soxhlet extractions (at refluxing temperature of solvent) of XL in toluene, methanol and THF were carried out in the present work. However, the Soxhlet extraction yields were only 2.5, 4.0 and 8.9 wdaf%, respectively. Further, the extraction yield of 12.9 wdaf% was obtained in CS2/NMP mixed solvent at room temperature. Above results show that the extraction yields at lower temperature are obviously less than the TE yield for XL though CS2/NMP mixed solvent could give high extraction yield (40–60 wdaf%) for many bituminous coals [5]. So we speculated that there are a lot of non-covalent bond cross-linking interactions in the network structure of XL so that small amount of small guest molecules was trapped inside. For the extraction at lower temperature, the extraction yields are related to the hydrogen bonding abilities of solvents,
10
Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15
which can disrupt the hydrogen bond cross-linking interactions in the network structure to release the guest molecules (extracts). However, for the TE at higher temperature, it is still unknown whether other factors are also responsible for the high TE yield except thermal cleavage of hydrogen bond cross-linking interactions. In our previous work [28], it had been found that there was no obvious pyrolysis of XL occurred in the process of TE at 300 °C. In order to investigate the TE of XL, the TES were further separated and analyzed as follows. Table 1 shows the yields and the ultimate analyses results of XL, TES and all sub-fractions separated from the TES. Yields of sub-fractions display that the TES consists of 8.8 wt.% THF insoluble (sub-fraction VII) and 91.2 wt.% THFS. Sub-fractions I, II and III eluted by toluene and toluene/THF solvents are major fractions, in which sub-fraction II shows the highest yield (43.4 wt.%). In addition, there is also 10.5 wt.% of residue on column not to be eluted (sub-fraction VI). Yields of sub-fractions IV and V, which were respectively eluted by THF and methanol, are very little in TES. It suggested that the TES mainly consists of the weak polar compounds and some macromolecules or aggregates. Table 1 further lists the element analysis results of all sub-fractions. It is observed that, with the increase of the eluent polarity, C % and H % of sub-fraction obtained decrease and O % increases. Sub-fraction VI remained in the chromatography column shows higher C % and H %, but only 3.7% of oxygen. Fraction VII shows the second lowest C % and the lowest H %, but the second highest O % in all sub-fractions. H/C of sub-fractions from I to IV decrease gradually from 1.79 to 1.56, and H/C of sub-fractions V and VI are respectively 1.76 and 1.78, near to that of sub-fraction II. However, H/C of sub-fraction VII is only 0.90. It suggested that most of components in TES consist of aliphatic structures with various amounts of oxygen-containing groups except sub-fraction VII. Fig. 1 shows FTIR spectra of all sub-fractions. For the absorption peaks of OH and Cal\H stretching vibrations (3440 cm−1 and 2952– 2850 cm−1, respectively), their intensities are in agreement with the O % and H/C listed in Table 1, except that sub-fraction I only shows a very weak peak of OH. Meanwhile, sub-fractions I–IV and VI also display a obvious rocking vibration peak of (CH2)n (n ≥ 4) at 722 cm− 1. It suggested that major sub-fractions of TES contain a lot of long chain aliphatic groups and various amounts of OH. Further, a strong absorption peak of ester group (attributed to the stretching vibration of C_O in aliphatic ester [29,30]) can be observed at 1742 cm−1 from the spectra of sub-fractions I–VI shown in Fig. 1. The absorption peak of aromatic ring stretching vibration in the range of 1626–1590 cm−1, gradually strengthens from sub-fractions II to VII except of sub-fraction VI, but it is hardly observed in sub-fraction I. However, the spectrum of subfraction VII is significantly different from other spectra, and shows a
3440
2952
16261461 1034
I II III
T/%
IV V VI VII 722 1742 1590 1168
2921 4000
3500
3000
2500
2000
1500
1000
Wavenumber / cm-1 Fig. 1. FTIR spectra of sub-fractions separated from TES.
500
shoulder peak at 1706 cm−1, which may be attributed to the stretching vibration of C_O in aromatic carboxyl. Therefore, the TES mainly consists of a lot of HAHs and higher aliphatic esters, and some aromatic compounds. Fig. 2 shows 1H NMR spectra of sub-fractions I–III, and V–VI. Here, the spectra of all sub-fractions show the largest peak at δ 1.25 ppm resulting from the aliphatic methylene (CH2) groups, and a small multipeak at δ 0.9 ppm from the terminal methyl group of the aliphatic chain. Meanwhile, the characteristic peak of methoxyl protons (O\CH3) can be observed as a singlet at δ 3.65 ppm and a triplet of α-CH2 protons at δ 2.32 ppm in all spectra. Further, the proton peak near to δ 1.56 ppm, which could be attributed to the β-carbonyl methylene proton [31,32], is gradually broadening from sub-fractions I to VI. Based on above results, it can be speculated that these sub-fractions should mainly consist of HAHs and FAMEs. However, no signal of aromatic proton (Car\H) was observed in all spectra shown in Fig. 1, except of the residual proton peak of deuterated chloroform solvent at δ 7.28 ppm. In addition, some very weak signals can also be observed in the range of δ 2.0–4.5 ppm, suggesting that these sub-fractions are more complicated in components. Fig. 3 shows the total ion chromatographs (TICs) of the sub-fractions I–VI, respectively. In the present work, major compounds were identified by their MS spectra, and GC/MS characterization results were listed in Table 2. The results show that there are 30, 30, 25, 26, 21 and 25 compounds to be identified from sub-fractions I to VI, respectively. 78 different compounds in total are found in the sub-fractions from the TES. These compounds mainly consist of HAHs (C12–30), FAMEs (C16–26), and other compounds such as aromatic compounds, other oxygen-containing compounds (OCCs), etc. All HAHs identified in the TES are the normal paraffins except 2,6,10,14-tetramethylheptadecene (Pristane), 2,6,10,14-tetramethyl-2-heptadecene (Pristene) and 2,6,10,14tetramethylhexadecane (Phytane). FAMEs include 10 kinds of saturated normal fatty acids ester and 4 kinds of unsaturated normal fatty acids esters. However, two kinds of saturated normal fatty acids were only determined. Meanwhile, Table 2 also shows that 8 AHs, such as tetralin, 5-methyltetralin, naphthalene, 2-methyl naphthalene, 1-methyl naphthalene, anthracene, 1,3-diphenyl-1-butene and 1,3,5triisopropyl benzene, are identified in the TES. Although a lot of OCCs, which exist mainly in sub-fractions II–IV, are identified in the TES, their relative contents are lower than those of HAHs and FAMEs. These OCCs include phthalates, other carboxylic esters, aliphatic ketones, phenolic compounds, tetrahydrofuran derivatives, ethers, aromatic aldehyde and quinine. In addition, 6 types of nitrogencontaining compounds, in which quinoline, indole and carbazole are all found, are identified in the TES. Ethyl (1,3-benzothiazol-2-ylsulfanyl) acetate and imino(triphenyl)phosphorane are respectively the only compound containing sulphur and phosphorus identified from the TES. Further, Table 2 shows that sub-fraction I mainly consists of HAHs and FAMEs, and other sub-fractions show varying amounts of OCCs besides HAHs and FAMEs. Therefore, the GC/MS results are basically consistent with the characterization results of elementary analyses, FTIR and 1H NMR, suggesting that main components of sub-fractions had been determined by GC/MS analysis. Based on above resolution results, these identified compounds can be grouped into HAHs, AHs, FAMEs, OCCs and OACs (compounds containing sulphur and phosphorus). Their relative contents (RCs) in sub-fractions obtained by the normalization method are shown in Fig. 4. It can be seen that the RCs of HAHs, which is the highest in all fractions except fraction IV, are in the range of 34.8–83.2%. HFAMEs, as the second major component, mainly consist in sub-fraction I (RC 45.9%) and II (RC 33.4%). Meanwhile, its RC in sub-factions VI, V and III also arrives to 18.8%, 11.7% and 9.7%, respectively, but only 1.7% in fraction IV. After taking the yields of sub-fractions into account, the contents of HAHs and HFAMs in the TES are estimated respectively about 47 wt.% and 28 wt.%, on the assumption that unidentified micro-components are ignored in fractions I–VI. Therefore, the TES is rich in HAHs and
Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15
11
VI 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
V 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
III 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
II 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
I 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
δ / ppm Fig. 2. 1H NMR spectra of sub-fractions separated from TES.
HFAMEs, which are potential chemicals and important chemical raw material. Further, taking the yields of sub-fractions into consideration, it was suggested that HAHs and FAMEs should be the main components of TES, which are present in all sub-fractions. These HAHs identified in the TES are in agreement with those in the extracts of brown coal [33] and lignite tar [34,35], but the range of carbon atom numbers in FAMEs is similar to that in normal fatty acids reported in previous publications [36–38]. In organic geochemistry, the normal fatty acids and the normal alkanes in sediments are always used to study the biological source and the geochemical environment [39]. The decarboxylation of normal fatty acids was supposed as an important resource of normal alkanes [40]. However, the saturated fatty acids have greater thermal stability [41], so that many fatty acids can still be determined in coal [42]. In addition, there were also reports about the noncatalytic esterification and transesterification of fatty acids and their esters in subcritical and supercritical methanol to produce the FAMEs [43]. In order to investigate the origin of FAMEs, we compared the FTIR
spectra of TESs obtained by different solvents, which includes toluene, THF, methanol and toluene/methanol mixed solvents (3:1 and 9:1, volume). A strong absorption peak of carbonyl attributed to aliphatic ester can be observed at 1740 cm− 1 in the spectra of all TESs as shown in Fig. 5. Meanwhile, a shoulder peak of carbonyl in carboxylic acid can also be seen in the range of 1712–1720 cm−1. It is strong in TESs obtained by toluene and THF solvents, but very weak in TESs obtained by methanol solvent and toluene/methanol mixed solvent (3:1, volume). It is in agreement with the results of GC/MS analysis of subfractions. Therefore, we thought that most of FAMEs in TES determined by GC/MS were intrinsic in XL, and a little of FAMEs was only produced by esterification of carboxylic acid and methanol in the TE process. In addition, the pyrolysis of aliphatic moiety of alkyl substituted compounds should also be no significant at 300 °C [44,45], because no any alkene was identified from the sub-fractions of TES except for the pristene. So HAHs were intrinsic small molecular compounds “dissolved” in lignite. In the TE process, the relaxation of the network structure of lignite by the rupture of non-covalent bonds at high
12
Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15
22
I
25
18
1
24
20
3
2
8 15 17 9-11 7 1214 16 5 6
4
27
26
23
28
19 21
29
30
1
II 8
29
15 9
26
12 16 19 1314 23 18 20 21
10
7
3 5 24
22
17
11
24
6
28
25
30
27
21 19
III
23 25
18
Relative intensity
17 1 5-6 9 7 248 3
16 14 15
12 13
20
10 11
15
45
IV 20
18
23
21
17 1
24
22
3 2
11-1214 10 6-9 13
22
16
24
19
14
15
26
25
V
17
9 18
6
20 2 1
5
7
3
8
4
11 1012 13
16
19
21
22
4
VI 5
14
1
910 18 7-8 17 20 6 15 22 13 16 12 21 11 19
2 3
5
10
15
23 24
20
25
25
Retention time / min Fig. 3. Total ion chromatograph (TIC) of sub-fractions I–VI separated from TES(▲ HAHs,● FAMEs).
30
Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15
13
Table 2 Result of GC/MS analyses sub-fractions from the TES. Name
Molecular formula
Number of peak Ia
II
III
IV
V
VI
–
5
–
–
–
– – – – – – – 15 17 22 24 26 28 30 –
– – – – – – 16 17 18 19 21 23 25 – –
– – – – – – 16 17 18 20 21 22 24 26 –
– 3 – 6 – 9 – 14 15 17 18 20 22 – –
– – 7 11 – 15 16 17 18 20 22 24 25 – –
–
–
–
–
–
Aliphatic hydrocarbons (HAHs) n-Dodecane n-Tetradecane n-Hexadecane n-Heptadecane n-Octadecane n-Nonadecane n-Eicosane n-Heneicosane n-Docosane n-Tricosane n-Tetracosane n-Pentacosane n-Hexacosane n-Heptacosane n-Octacosane n-Nonacosane n-Triacontane 2,6,10,14-Tetramethylpentadecane (Pristane) 2,6,10,14-Tetramethyl-2-pentadecane (Pristene) 2,6,10,14-Tetramethylhexadecane (Phytane)
C12H26 C14H30 C16H34 C17H36 C18H38 C19H40 C20H42 C21H44 C22H46 C23H48 C24H50 C25H52 C26H54 C27H56 C28H58 C29H60 C30H62 C19H40 C19H38 C20H42
– 4 5 7 9 12 14 15 17 18 20 22 24 25 27 29 – 8 10 11
Aromatic hydrocarbons (AHs) Tetralin Naphthalene 5-Methyltetralin 2-Methyl naphthalene 1-Methyl naphthalene Anthracene 1,3-Diphenyl-1-butene 1,3,5-Triisopropyl benzene
C10H12 C10H8 C11H14 C11H10 C11H10 C14H10 C16H36 C15H24
1 6 – – – 3 – –
1 2 – 5 – – – –
1 – 4 7 8 – – –
– – – 1 – – 7 9
– – – – – – – –
1 2 – 4 5 – – –
Higher fatty acids & higher fatty acid methyl esters(HAs and FAMEs) n-Hexadecanoic acid n-Octadecanoic acid Methyl n-hexadecanoate Methyl n-octadecanoate Methyl n-nonadecanoate Methyl n-eicosanoate Methyl n-heneicosanoate Methyl n-docosanoate Methyl n-tricosnoate Methyl n-tetracosanoate Methyl n-pentacosanoate Methyl n-hexacosanoate Methyl 9-octadecenoate Methyl 16-octadecenoate Methyl 13-docosenoate Methyl 9-tetracosenoate
C16H32O2 C18H36O2 C17H34O2 C19H38O2 C20H40O2 C21H42O2 C22H44O2 C23H46O2 C24H48O2 C25H50O2 C26H52O2 C27H54O2 C19H36O2 C19H36O2 C23H44O2 C25H48O2
– – 13 16 – 19 21 23 – 26 28 29 – – – –
– – 9 11 14 – 19 – 25 27 29 10 12 16 23
13 14 12 – 15 – – – 20 22 24 – – – – –
– – 12 – – – – – – – – 25 – – – –
– – 5 8 11 – – 16 – 19 21 – 7 – – –
10 14 8 – – – – – 21 23 – – 12 – – –
Other compounds containing oxygen (OCCs) Allyl n-butyrate Cyclohexyl formate n-Butyl n-butyrate Dimethyl azelate 4-pentadecyl n-butyrate Diethyl-o-phthalate Dibutyl phthalate 3,5-Di-tert-butyl-4-hydroxybenzyl acrylate 3,5-Di-tert-butyl-4-hydroxybenzyl butyrate 2-Ethylhexyl-hexyl-o-phthalate Diisooctyl phthalate Dioctyl phthalate 3,5-di-tert-butyl-4-hydroxybenzaldehyde 2,6-Di-tert-butyl-para-benzoquinone 2,6-Di-tert-butyl-4-methylene-2,5-cyclohexadiene-1-one 6,10,14-Trimethyl-2-pentadecanone 2-Nonadecanone Docosa-2,21-dione Cyclohexyl ethyl ether Benzyl 2-methoxy-4-propenylphenyl ether 2-Butyltetrahydrofuran Butoxyl tetrahydrofuran 2,3′-Bi-tetrahydrofuran
C7H12O2 C7H12O2 C8H16O2 C11H20O4 C19H38O2 C12H14O4 C16H22O4 C18H26O3 C19H30O3 C22H34O4 C24H38O4 C24H38O4 C15H22O2 C14H20O2 C15H22O C18H36O C19H38O C22H42O2 C8H16O C17H18O2 C8H16O C8H16O2 C8H14O2
– – – – – 3 – – – – – – – – – – – – – – – – –
3 4 – 6 21 – 8 – – – – 20 – – – 7 – 18 – – – 13 –
– – 9 – – 10 11 – – – – – – – – – – – – – 2 – –
– – – – – – – 14 15 – 19 – 6 3 4 – – – – 13 – – –
– – – – – 2 4 – – 12 – – – – – – 10 – 1 – – – 13
– – – – – – 6 – – – – – – – – – – – – – – – –
(continued on next page)
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Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15
Table 2 (continued) Name
Molecular formula
Number of peak Ia
II
III
IV
V
VI
2,4-Ditert-butyl-5-methylphenol Other compounds containing oxygen (OCCs) 2,6-Di-tert-butyl-p-cresol 2,6-Di-tert-para-ethylphenol 4,4′-Ethylene-bis(2,6-di-tert-butylphenol)
C15H24O
–
–
–
2
–
–
C15H24O C16H26O C30H44O2
– – –
– – –
– – –
5 8 23
– – –
– – –
Compounds containing other heteroatoms Quinoline Indole Carbazole 3-Ethylcarbazole N,N-dibutyl oxamide amyl ester Ethyl (1,3-benzothiazol-2-ylsulfanyl) acetate Imino(triphenyl)phosphorane
C9H7N C8H7N C12H9N C14H13N C15H29NO3 C11H11NO2 S2 C18H16NP
– – – – – – –
– – – – – – –
3 6 – – – – –
– – 10 11 – – –
– – – – – – –
3 – – – 9 13 19
a
The number of peak in the TIC of corresponding sub-fraction.
temperature improved the diffusion of solvent and small molecular compounds such as HAHs, FAMEs, fatty acids, etc. In addition, a relatively few aromatic compounds were identified in TES, suggesting that most of aromatic moieties should be bonded in the network structure of XL by covalent bond, and no obvious pyrolysis of C\C bond occurs in the TE at 300 °C.
HAH
AH
FAME
OCC
OAC
100
Relative content / %
80
60
40
0 I
II
III
IV
V
VI
Fractions Fig. 4. Relative contents of different types of compounds identified in sub-fractions I–VI.
1.0
Normalized absorbance
At higher temperature, TE in non-specific solvent can disrupt the noncovalent bonding interactions to relax the macromolecular network, so that more dissolved organic compounds are extracted from coal without obvious pyrolysis of C\C bond. The TE yield of XL in toluene/methanol (3:1, volume) mixed solvent at 300 °C is 15.7 wdaf%. By isolation and structural characterization, there are 78 compounds identified in THF solubles from TES, which including HAHs, FAMEs, AHs, and other compounds containing O, N, S and P. HAHs and FAMEs are predominant components of TES. The HAHs, which mainly consist of C12–30 normal HAHs, should be intrinsic small molecular compounds “dissolved” in XL. Most of FAMEs (C17–27) are also the intrinsic components in XL, but there is also small amount of FAMEs produced by esterification and transesterification in the TE process. Therefore, the TE with methanol as solvent at certain temperature is a potential separation technique of lignite to produce the chemicals, and these structural characterizations of TES are advantageous to understand the structure of XL. Acknowledgment
20
Methanol (M) T/M=3 T/M=9 Toluene (T) THF
0.8
0.4
0.2
1800
1750
1700
1650
1600
1550
Wavenumber /cm-1 Fig. 5. FTIR spectra of TESs obtained by different solvents.
This work was supported by the National Basic Research Program of China (973 Program, Grant 2011CB201302), the Key Project of Coal Joint Fund from Natural Science Foundation of China and Shenhua Group Corporation Limited (Grant U1261208) and the Natural Scientific Foundation of China (Grants 51174254, 21076001, 21176001, 20936007). The authors are also appreciative for the financial support from the Provincial Innovative Group for Processing & Clean Utilization of Coal Resource and Program for Innovative Research Team in Anhui University of Technology. References
0.6
0.0 1850
4. Conclusion
1500
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