Fuel Processing Technology 120 (2014) 8–15

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Fuel Processing Technology

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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 HuSchool of Chemistry and Chemical Engineering, Anhui Key Laboratory of Clean Coal Conversion & Utilization, Anhui University of Technology, 243002 Ma'anshan, China

⁎ Corresponding author. Tel.: +86 13955530691; fax:E-mail address: zhicaiw@ahut.edu.cn (Z. Wang).

0378-3820/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.fuproc.2013.11.017

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 March 2013Received in revised form 14 November 2013Accepted 27 November 2013Available online 18 December 2013

Keywords:Xianfeng ligniteThermal extractionNonspecific solventExtract characterization

Thermal extraction (TE)with nonspecific solvent at high temperature is a potential technology to separate organ-ic materials from coal, especially low-rank coal such as lignite. In this paper, thermal extract (TES) of Xianfenglignite (XL) in toluene/methanol (3:1, volume) mixed solvent at 300 °C was separated into different sub-fractions 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 compoundsincluding C12–30 higher aliphatic hydrocarbons (HAHs), aromatic hydrocarbons (AHs), C17–27 fatty acid methylesters (FAMEs) and other heteroatomic compoundswere identified from the TES. As two groups of predominantcomponents, HAHs and FAMEs are the intrinsic components inXL except for small amount of FAMEs produced byesterification and transesterification reactions in the TE process. Further, the mechanism of TE was alsospeculated by the characterization results of TES. As a result, the TEwith nonspecific solvent at high temperaturecan not only improve the extract yield of organic materials, but also obtain chemicals such as HAHs and FAMEsfrom lignite.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Coal is not only an important energy resource, but also an indispens-able organic carbon resource. With the rapid growth of crude oilconsumption, the conversion technologies of coal to produce transportfuel and fine chemicals have recently been paid much more attentionin China. As a low quality coal, lignite and/or brown coal has abundantrecoverable reserve, but it is mainly used to generate electricity nearmine at present. So the high efficiency utilization technology of ligniteneeds to be developed to make up for the shortage of petroleumresource.

In general, lignite is regarded as a suitable liquefaction feed becauseof its high H/C and reactivity, so that direct liquefaction of lignite hasbeen investigated extensively [1–4]. However, high hydrogenconsumption resulting from high oxygen content and moisture limitsthe liquefaction economics of lignite [4]. Since Iino et al. [5] found thatCS2/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent could give40–65% extraction yield for many bituminous coals at room tempera-ture, the solvent extraction has become an important technique toinvestigate the structure of coal and realize the cleaning conversion ofcoal [6–11].

As well known, coal consists of complex macromolecular networkand some dissolved organic materials (guest). Nishioka and Larsen[12] considered that these organic materials are bonded in the networkby non-covalent interactions, which are also one of the important cross-

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linking interactions of the network. In order to separate out the originalguestmaterials asmuch as possible, the extraction usually carried out atsuitable temperature or in specific solvent so that these non-covalentinteractions can be broken. For example, the specific solvent, such aspyridine [13], amines [14], CS2/pyridine [15], CS2/NMP [16], etc., hadshown higher extraction yield than the non-specific solvent under therefluxing temperature of solvent, even at room temperature. The specif-ic solvent can break the non-covalent bonds in coal to increase theextraction yield, but it cannot break the covalent bonds of coal macro-molecules [13–16]. Meanwhile, the specific solvent is mainly used toinvestigate the structure of coal, and is not very suitable for thecommercial application due to its expensive cost. Since the non-specific solvent has the poor solubility and cannot break the non-covalent bonds in coal at refluxing temperature, the extraction of coalin non-specific solvent, such as toluene and lower alcohols, is generallycarried out under supercritical conditions. In the process of supercriticalextraction, the pyrolytic reaction can also improve the extraction yieldto varying degrees [17–19]. However, significant solvent loss wasobserved in supercritical extraction because the pyrolytic reactions ofsolvent could also occur at higher than 350 °C [17]. Although the extrac-tion yields of some low rank coals such as Illinois No. 6 coal were below20 wt.% with supercritical toluene [17,18], Yuan et al. [19] found thatthe extraction yield of Leping coal (a Chinese lignite) at 380 °C reached64 wt.%. Obviously, the coal characteristics also have an important influ-ence on the extraction besides temperature and solvent.

Recently, the TEwith high boiling point solvent by thermal filtration,which can promote the separation of extracts from the residue, has beenextensively investigated to produce valuable chemicals and Hypercoal

Table 1Element analyses of the XL, TES and its sub-fractions.

Sample Yield/% Ultimate analysis, wdaf/% H/C

C N S H Oa

XL – 63.1 1.8 0.4 6.0 28.7 1.14TES – 77.7 0.9 0.8 10.6 10.0 1.64I 21.1 82.6 0.6 0.6 12.3 3.9 1.79II 43.4 77.9 0.9 0.6 11.4 9.2 1.76III 12.5 77.4 1.1 0.4 10.8 10.3 1.67IV 0.5 76.9 0.7 0.2 10.0 12.2 1.56V 3.4 50.8 1.6 0.5 7.4 39.7 1.76VI 10.5 82.7 0.8 0.6 12.2 3.7 1.78VII 8.8 61.8 1.8 0.5 4.6 31.3 0.90

a By difference.

9Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15

[20–26]. Yoshida et al. [11,21] carried out the TE of various bituminouscoals at less than 380 °C in order to produce ashless coal, and foundthat the TE yield of Illinois No. 6 coal at 360 °C was 38% in light cycleoil (LCO) by filtration at ambient temperature, but those were respec-tively 60% and 80% in LCO and crude methylnaphthalene oil (CMO) byhot filtration. Miura et al. [22] successfully separated coal into differentmolecule size fractions by the TE in a flowing stream of tetralin (THN),methylnaphthalene (MN) or derived coal liquids under 10 MPa at200–400 °C. Although the TE property varied with bituminous coals,the ultimate analysis, the structure and the molecular weight distribu-tion were little different between different extracted fractions fromthe same coal. Here, the stronger solubility of high boiling point solventmay be responsible for higher TE yields. Therefore, the TE is a potentialtechnology of coal conversion to produce the cleaning fuels and thechemical raw materials [22–25]. In order to overcome the difficulty inthe recovery of high boiling solvent, Lu et al. [26] carried out the sequen-tial TE of Huolinguole lignite with methanol and ethanol as solvent, re-spectively. Since low-carbon alkanol can act as hydrogen donor andalkylation reagent during TE of coal [27], some thermal degradationsof lignite that included dissociation of intermolecular interactions, es-terification and alkanolysis were observed in the TEs with methanoland ethanol as solvents [26]. The TE with low boiling point solvent notonly is an effective method for understanding thermal degradation oflignite, but also is easy to know the structure and composition of dis-solved organic materials in coal.

In our previous work [28], the TE of XL by hot filtration had beeninvestigated in low boiling non-specific solvents, such as methanol,toluene and their mixture, respectively. The results showed that theTE significantly improved the extraction yield, and therewas noobviouspyrolysis reaction to be observed at 300 °C. To increase the TE temper-ature can further improve the extract yield, but obvious pyrolysis reac-tion occurred in the TE process at 380 °C. In this paper, the TES obtainedby toluene/methanol mixed solvent at 300 °C was separated throughcolumn chromatography into different sub-fractions, and theirstructures were further characterized by FTIR, 1H NMR, GC/MS, etc. inorder to analyze the major components of TES and understand themechanism 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 stainlesssteel filter (0.5 μm). 30.0 g dried XL and 300 mL mixed solvent werecharged 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 TEmixtures were sep-arated by in-situ hotfiltration. Subsequently, the TE residuewaswashedwith above mixed solvent and filtered at 300 °C for three times. Allfiltrate was incorporated to remove the mixed solvents by rotary evap-oration. Then the TES was obtained by desiccation in a vacuum at 80 °Cfor 48 h. A detailed description can be found elsewhere [28]. The ulti-mate and proximate analyses results of XL and its TES were shown inTable 1. All solvents used in the present work were commerciallypurchased as analytical reagents.

2.2. Separation of TES

Firstly, above dried TESwas exhaustively extractedwith THF solventin a Soxhlet extractor to afford the THF insoluble fraction (sub-fractionVII) and THF soluble (THFS). Then, THFSwas separated by column chro-matography. 1.0 g dried THFS was dissolved in THF and mixed with10 g of neutral silica gel under ultrasonic irradiation for 30 min. Subse-quently, THF solvent was removed by rotary evaporation, and the silicagel sample obtained was dried in a vacuum at 80 °C for 24 h. The silica

gel adsorbed THFS was loaded onto a 1.0 cm × 30 cm neutral silica gelcolumn activated at 260 °C. According to the separation of asphalteneand preasphaltene of coal, the THFS was eluted sequentially with thefollowing solvents: toluene, toluene/THF (4:1, volume), toluene/THF(1:2, volume), THF and methanol, so that the components of TEScould gradually be eluted by their polarity as much as possible. Aftereluting the column with a particular solvent, the solvent was removedby rotary evaporation and the eluent was dried in a vacuum at 80 °Cfor 24 h. All separated elutes, which were defined sequentially assub-fractions I, II, III, IV and V, were weighted and analyzed. Finally,the silica gel on the top of column was exhaustively extracted by THFin 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 ambienttemperature by Nicolet 6700 FTIR spectrometer. An approximately1 mg sample was mixed with 0.1 g KBr, and pressed into a pellet. Thenumber of scans was 32 in the scanning range of 400–4000 cm−1. Theelement analysis was carried out at the mode of CHNS by the Vario ELIII elementary analyzer. Sub-fractions I–VI were respectively dissolvedin THF, and then were analyzed by GCMS-QP2010 Plus with a RestekRtx-5MS capillary column (i.d. 0.25 mm, length 30 m, thickness0.25 μm). The temperature program used was from 130 °C, held for2 min, raised at 15 °C min−1 to a final temperature of 250 °C, andheld for 20 min. The carrier gas was helium (99.999%), with a1 mL min−1

flow. The injector temperature was 250 °C, with theGC–MS interface at 250 °C. The data were acquired and processedusing ChemStation software and the compounds were determined bycomparing their mass spectra to NIST05 library data. 1H NMR spectraof 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 ofXL in toluene/methanol (3:1, volume) mixed solvent at 300 °C and380 °Cwere 15.7 wdaf%, 28.5 wdaf%. For comparison, the Soxhlet extrac-tions (at refluxing temperature of solvent) of XL in toluene, methanoland THFwere carried out in the presentwork. However, the Soxhlet ex-traction 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 mixedsolvent at room temperature. Above results show that the extractionyields at lower temperature are obviously less than the TE yield for XLthough CS2/NMP mixed solvent could give high extraction yield(40–60 wdaf%) for many bituminous coals [5]. So we speculated thatthere are a lot of non-covalent bond cross-linking interactions in thenetwork structure of XL so that small amount of small guest moleculeswas trapped inside. For the extraction at lower temperature, the extrac-tion 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 thenetwork structure to release the guest molecules (extracts). However,for the TE at higher temperature, it is still unknown whether otherfactors are also responsible for thehigh TEyield except thermal cleavageof hydrogen bond cross-linking interactions. In our previous work [28],it had been found that there was no obvious pyrolysis of XL occurred inthe process of TE at 300 °C. In order to investigate the TE of XL, the TESwere further separated and analyzed as follows.

Table 1 shows the yields and the ultimate analyses results of XL, TESand all sub-fractions separated from the TES. Yields of sub-fractions dis-play 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 andtoluene/THF solvents are major fractions, in which sub-fraction IIshows the highest yield (43.4 wt.%). In addition, there is also10.5 wt.% of residue on column not to be eluted (sub-fraction VI). Yieldsof sub-fractions IV and V, which were respectively eluted by THF andmethanol, are very little in TES. It suggested that the TESmainly consistsof the weak polar compounds and somemacromolecules or aggregates.Table 1 further lists the element analysis results of all sub-fractions. It isobserved that, with the increase of the eluent polarity, C % and H % ofsub-fraction obtained decrease and O % increases. Sub-fraction VIremained in the chromatography column shows higher C % and H %,but only 3.7% of oxygen. Fraction VII shows the second lowest C % andthe lowest H %, but the second highest O % in all sub-fractions. H/C ofsub-fractions from I to IV decrease gradually from 1.79 to 1.56, andH/C of sub-fractions V and VI are respectively 1.76 and 1.78, near tothat of sub-fraction II. However, H/C of sub-fraction VII is only 0.90. Itsuggested thatmost of components in TES consist of aliphatic structureswith various amounts of oxygen-containing groups except sub-fractionVII.

Fig. 1 shows FTIR spectra of all sub-fractions. For the absorptionpeaks of OH and Cal\H stretching vibrations (3440 cm−1 and 2952–2850 cm−1, respectively), their intensities are in agreement with theO % and H/C listed in Table 1, except that sub-fraction I only shows averyweak peak of OH.Meanwhile, sub-fractions I–IV and VI also displaya obvious rocking vibration peak of (CH2)n (n ≥ 4) at 722 cm−1. Itsuggested that major sub-fractions of TES contain a lot of long chain al-iphatic groups and various amounts of OH. Further, a strong absorptionpeak of ester group (attributed to the stretching vibration of C_O inaliphatic ester [29,30]) can be observed at 1742 cm−1 from the spectraof sub-fractions I–VI shown in Fig. 1. The absorption peak of aromaticring stretching vibration in the range of 1626–1590 cm−1, graduallystrengthens from sub-fractions II to VII except of sub-fraction VI, but itis hardly observed in sub-fraction I. However, the spectrum of sub-fraction VII is significantly different from other spectra, and shows a

4000 3500 3000 2500 2000 1500 1000 500

V

VII

VI

IV

III

II

T /

%

Wavenumber / cm-1

I3440 2952

2921 1742

1626

1590

1461

1168

1034

722

Fig. 1. FTIR spectra of sub-fractions separated from TES.

shoulder peak at 1706 cm−1, whichmay be attributed to the stretchingvibration of C_O in aromatic carboxyl. Therefore, the TES mainly con-sists of a lot of HAHs and higher aliphatic esters, and some aromaticcompounds.

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 ppmresulting from the aliphatic methylene (CH2) groups, and a smallmultipeak at δ 0.9 ppm from the terminal methyl group of the aliphaticchain. Meanwhile, the characteristic peak of methoxyl protons(O\CH3) can be observed as a singlet at δ 3.65 ppm and a triplet ofα-CH2 protons at δ 2.32 ppm in all spectra. Further, the proton peaknear to δ 1.56 ppm, which could be attributed to the β-carbonyl meth-ylene proton [31,32], is gradually broadening from sub-fractions I to VI.Based on above results, it can be speculated that these sub-fractionsshould mainly consist of HAHs and FAMEs. However, no signal ofaromatic proton (Car\H)was observed in all spectra shown in Fig. 1, ex-cept of the residual proton peak of deuterated chloroform solvent at δ7.28 ppm. In addition, some very weak signals can also be observed inthe range of δ 2.0–4.5 ppm, suggesting that these sub-fractions aremore complicated in components.

Fig. 3 shows the total ion chromatographs (TICs) of the sub-fractionsI–VI, respectively. In the present work, major compounds were identi-fied by their MS spectra, and GC/MS characterization results were listedin Table 2. The results show that there are 30, 30, 25, 26, 21 and 25compounds to be identified from sub-fractions I to VI, respectively. 78different compounds in total are found in the sub-fractions fromthe TES. These compounds mainly consist of HAHs (C12–30), FAMEs(C16–26), and other compounds such as aromatic compounds, otheroxygen-containing compounds (OCCs), etc. All HAHs identified in theTES are the normal paraffins except 2,6,10,14-tetramethylheptadecene(Pristane), 2,6,10,14-tetramethyl-2-heptadecene (Pristene) and 2,6,10,14-tetramethylhexadecane (Phytane). FAMEs include 10 kinds of saturatednormal fatty acids ester and 4 kinds of unsaturated normal fattyacids esters. However, two kinds of saturated normal fatty acidswere 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 1,3,5-triisopropyl benzene, are identified in the TES. Although a lot ofOCCs, which exist mainly in sub-fractions II–IV, are identified in theTES, their relative contents are lower than those of HAHs andFAMEs. These OCCs include phthalates, other carboxylic esters,aliphatic ketones, phenolic compounds, tetrahydrofuran derivatives,ethers, aromatic aldehyde and quinine. In addition, 6 types of nitrogen-containing compounds, in which quinoline, indole and carbazole are allfound, are identified in the TES. Ethyl (1,3-benzothiazol-2-ylsulfanyl)acetate and imino(triphenyl)phosphorane are respectively the onlycompound containing sulphur and phosphorus identified from the TES.Further, Table 2 shows that sub-fraction I mainly consists of HAHs andFAMEs, and other sub-fractions show varying amounts of OCCs besidesHAHs and FAMEs. Therefore, the GC/MS results are basically consistentwith the characterization results of elementary analyses, FTIR and 1HNMR, suggesting that main components of sub-fractions had beendetermined by GC/MS analysis.

Based on above resolution results, these identified compounds canbe grouped into HAHs, AHs, FAMEs, OCCs and OACs (compoundscontaining sulphur and phosphorus). Their relative contents (RCs) insub-fractions obtained by the normalization method are shown inFig. 4. It can be seen that the RCs of HAHs,which is the highest in all frac-tions except fraction IV, are in the range of 34.8–83.2%. HFAMEs, as thesecond 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 alsoarrives to 18.8%, 11.7% and 9.7%, respectively, but only 1.7% in fractionIV. After taking the yields of sub-fractions into account, the contents ofHAHs and HFAMs in the TES are estimated respectively about 47 wt.%and 28 wt.%, on the assumption that unidentified micro-componentsare ignored in fractions I–VI. Therefore, the TES is rich in HAHs and

VI

V

III

II

I0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

/ ppmδ

Fig. 2. 1H NMR spectra of sub-fractions separated from TES.

11Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15

HFAMEs, which are potential chemicals and important chemical rawmaterial.

Further, taking the yields of sub-fractions into consideration, it wassuggested that HAHs and FAMEs should be the main components ofTES, which are present in all sub-fractions. These HAHs identified inthe 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 inFAMEs is similar to that in normal fatty acids reported in previouspublications [36–38]. In organic geochemistry, the normal fatty acidsand the normal alkanes in sediments are always used to study the bio-logical source and the geochemical environment [39]. The decarboxyl-ation of normal fatty acids was supposed as an important resource ofnormal alkanes [40]. However, the saturated fatty acids have greaterthermal stability [41], so that many fatty acids can still be determinedin coal [42]. In addition, there were also reports about the non-catalytic esterification and transesterification of fatty acids and their es-ters in subcritical and supercriticalmethanol 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 aliphaticester can be observed at 1740 cm−1 in the spectra of all TESs asshown in Fig. 5. Meanwhile, a shoulder peak of carbonyl in carboxylicacid can also be seen in the range of 1712–1720 cm−1. It is strong inTESs obtained by toluene and THF solvents, but very weak in TESs ob-tained by methanol solvent and toluene/methanol mixed solvent (3:1,volume). It is in agreement with the results of GC/MS analysis of sub-fractions. Therefore, we thought that most of FAMEs in TES determinedby GC/MS were intrinsic in XL, and a little of FAMEs was only producedby esterification of carboxylic acid and methanol in the TE process. Inaddition, the pyrolysis of aliphatic moiety of alkyl substituted com-pounds should also be no significant at 300 °C [44,45], because no anyalkene was identified from the sub-fractions of TES except for thepristene. So HAHs were intrinsic small molecular compounds“dissolved” in lignite. In the TE process, the relaxation of the networkstructure of lignite by the rupture of non-covalent bonds at high

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Fig. 3. Total ion chromatograph (TIC) of sub-fractions I–VI separated from TES(▲ HAHs,● FAMEs).

12 Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15

Table 2Result of GC/MS analyses sub-fractions from the TES.

Name Molecular formula Number of peak

Ia II III IV V VI

Aliphatic hydrocarbons (HAHs)n-Dodecane C12H26 – – 5 – – –

n-Tetradecane C14H30 4n-Hexadecane C16H34 5n-Heptadecane C17H36 7 – – – – –

n-Octadecane C18H38 9 – – – 3 –

n-Nonadecane C19H40 12 – – – – 7n-Eicosane C20H42 14 – – – 6 11n-Heneicosane C21H44 15 – – – – –

n-Docosane C22H46 17 – – – 9 15n-Tricosane C23H48 18 – 16 16 – 16n-Tetracosane C24H50 20 15 17 17 14 17n-Pentacosane C25H52 22 17 18 18 15 18n-Hexacosane C26H54 24 22 19 20 17 20n-Heptacosane C27H56 25 24 21 21 18 22n-Octacosane C28H58 27 26 23 22 20 24n-Nonacosane C29H60 29 28 25 24 22 25n-Triacontane C30H62 – 30 – 26 – –

2,6,10,14-Tetramethylpentadecane (Pristane) C19H40 8 – – – – –

2,6,10,14-Tetramethyl-2-pentadecane (Pristene) C19H38 102,6,10,14-Tetramethylhexadecane (Phytane) C20H42 11 – – – – –

Aromatic hydrocarbons (AHs)Tetralin C10H12 1 1 1 – – 1Naphthalene C10H8 6 2 – – – 25-Methyltetralin C11H14 – – 4 – – –

2-Methyl naphthalene C11H10 – 5 7 1 – 41-Methyl naphthalene C11H10 – – 8 – – 5Anthracene C14H10 3 – – – – –

1,3-Diphenyl-1-butene C16H36 – – – 7 – –

1,3,5-Triisopropyl benzene C15H24 – – – 9 – –

Higher fatty acids & higher fatty acid methyl esters(HAs and FAMEs)n-Hexadecanoic acid C16H32O2 – – 13 – – 10n-Octadecanoic acid C18H36O2 – – 14 – – 14Methyl n-hexadecanoate C17H34O2 13 9 12 12 5 8Methyl n-octadecanoate C19H38O2 16 11 – – 8 –

Methyl n-nonadecanoate C20H40O2 – 15 – 11 –

Methyl n-eicosanoate C21H42O2 19 14 – – – –

Methyl n-heneicosanoate C22H44O2 21 – – – – –

Methyl n-docosanoate C23H46O2 23 19 – – 16 –

Methyl n-tricosnoate C24H48O2 – – 20 – – 21Methyl n-tetracosanoate C25H50O2 26 25 22 – 19 23Methyl n-pentacosanoate C26H52O2 28 27 24 – 21 –

Methyl n-hexacosanoate C27H54O2 29 29 – 25 – –

Methyl 9-octadecenoate C19H36O2 – 10 – – 7 12Methyl 16-octadecenoate C19H36O2 – 12 – – – –

Methyl 13-docosenoate C23H44O2 – 16 – – – –

Methyl 9-tetracosenoate C25H48O2 – 23 – – – –

Other compounds containing oxygen (OCCs)Allyl n-butyrate C7H12O2 – 3 – – – –

Cyclohexyl formate C7H12O2 – 4 – – – –

n-Butyl n-butyrate C8H16O2 – – 9 – – –

Dimethyl azelate C11H20O4 – 6 – – – –

4-pentadecyl n-butyrate C19H38O2 – 21 – – – –

Diethyl-o-phthalate C12H14O4 3 – 10 – 2 –

Dibutyl phthalate C16H22O4 – 8 11 – 4 63,5-Di-tert-butyl-4-hydroxybenzyl acrylate C18H26O3 – – – 14 – –

3,5-Di-tert-butyl-4-hydroxybenzyl butyrate C19H30O3 – – – 15 – –

2-Ethylhexyl-hexyl-o-phthalate C22H34O4 – – – – 12 –

Diisooctyl phthalate C24H38O4 – – – 19 – –

Dioctyl phthalate C24H38O4 – 20 – – – –

3,5-di-tert-butyl-4-hydroxybenzaldehyde C15H22O2 – – – 6 – –

2,6-Di-tert-butyl-para-benzoquinone C14H20O2 – – – 3 – –

2,6-Di-tert-butyl-4-methylene-2,5-cyclohexadiene-1-one C15H22O – – – 4 – –

6,10,14-Trimethyl-2-pentadecanone C18H36O – 7 – – – –

2-Nonadecanone C19H38O – – – – 10 –

Docosa-2,21-dione C22H42O2 – 18 – – – –

Cyclohexyl ethyl ether C8H16O – – – – 1 –

Benzyl 2-methoxy-4-propenylphenyl ether C17H18O2 – – – 13 – –

2-Butyltetrahydrofuran C8H16O – – 2 – – –

Butoxyl tetrahydrofuran C8H16O2 – 13 – – – –

2,3′-Bi-tetrahydrofuran C8H14O2 – – – – 13 –

(continued on next page)

13Z. 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 C15H24O – – – 2 – –

Other compounds containing oxygen (OCCs)2,6-Di-tert-butyl-p-cresol C15H24O – – – 5 – –

2,6-Di-tert-para-ethylphenol C16H26O – – – 8 – –

4,4′-Ethylene-bis(2,6-di-tert-butylphenol) C30H44O2 – – – 23 – –

Compounds containing other heteroatomsQuinoline C9H7N – – 3 – – 3Indole C8H7N – – 6 – – –

Carbazole C12H9N – – – 10 – –

3-Ethylcarbazole C14H13N – – – 11 – –

N,N-dibutyl oxamide amyl ester C15H29NO3 – – – – – 9Ethyl (1,3-benzothiazol-2-ylsulfanyl) acetate C11H11NO2 S2 – – – – – 13Imino(triphenyl)phosphorane C18H16NP – – – – – 19

a The number of peak in the TIC of corresponding sub-fraction.

14 Z. Wang et al. / Fuel Processing Technology 120 (2014) 8–15

temperature improved the diffusion of solvent and small molecularcompounds such as HAHs, FAMEs, fatty acids, etc. In addition, a relative-ly fewaromatic compoundswere identified in TES, suggesting thatmostof aromaticmoieties should be bonded in the network structure of XL bycovalent bond, and no obvious pyrolysis of C\C bond occurs in the TE at300 °C.

I II III IV V VI0

20

40

60

80

100

Rel

ativ

e co

nten

t / %

Fractions

HAH AH FAME OCC OAC

Fig. 4. Relative contents of different types of compounds identified in sub-fractions I–VI.

1850 1800 1750 1700 1650 1600 1550 15000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

abs

orba

nce

Wavenumber /cm-1

Methanol (M) T/M=3 T/M=9 Toluene (T) THF

Fig. 5. FTIR spectra of TESs obtained by different solvents.

4. Conclusion

At higher temperature, TE in non-specific solvent can disrupt the non-covalent bonding interactions to relax the macromolecular network, sothat more dissolved organic compounds are extracted from coal withoutobvious 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 andstructural characterization, there are 78 compounds identified in THFsolubles from TES, which including HAHs, FAMEs, AHs, and othercompounds containing O, N, S and P. HAHs and FAMEs are predominantcomponents of TES. The HAHs, which mainly consist of C12–30 normalHAHs, should be intrinsic small molecular compounds “dissolved” in XL.Most of FAMEs (C17–27) are also the intrinsic components in XL, butthere is also small amount of FAMEs produced by esterification andtransesterification in the TE process. Therefore, the TE with methanol assolvent at certain temperature is a potential separation technique oflignite to produce the chemicals, and these structural characterizationsof TES are advantageous to understand the structure of XL.

Acknowledgment

This work was supported by the National Basic Research Program ofChina (973 Program, Grant 2011CB201302), the Key Project of CoalJoint Fund from Natural Science Foundation of China and ShenhuaGroupCorporation Limited (Grant U1261208) and theNatural ScientificFoundation of China (Grants 51174254, 21076001, 21176001,20936007). The authors are also appreciative for the financial supportfrom the Provincial Innovative Group for Processing & Clean Utilizationof Coal Resource and Program for Innovative Research Team in AnhuiUniversity of Technology.

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