Structural features of liquefaction residue from Shenmu-Fugu subbituminous coal

Fuel 242 (2019) 819–827

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Structural features of liquefaction residue from Shenmu-Fugu subbituminous coal

T

Peng Lia,b, Zhi-Min Zonga, , Xian-Yong Weia,c, Yu-Gao Wangd, Gui-Xia Fanb ⁎

a

Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou 221116, China School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China c State Key Laboratory of High-efficiency Utilization and Green Chemical Engineering, Ningxia University, Yinchuan 750021, Ningxia, China d College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China b

ARTICLE INFO

ABSTRACT

Keywords: Coal liquefaction residue Solvent extraction Structural features Association and dissociation Capsule

Liquefaction residue (LRSFSBC) from Shenmu-Fugu subbituminous coal was ultrasonically extracted with petroleum ether, methanol, cyclohexane, ethyl acetate, acetone, carbon disulfide (CDS) and isometric CDS/acetone mixture (IMCDSAM), respectively. A number of alkanes, arenes, arenols, alkanols, alkenones, esters, and nitrogen-containing organic compounds were identified according to the analyses with Fourier transform infrared spectrometer, gas chromatograph/mass spectrometer, direct analysis in real-time ionization source coupled with ion trap mass spectrometer, and solid-state 13C nuclear magnetic resonance (SS 13C NMR). The tetra- and pentasubstituted anthracene or phenanthrene unit could be the average structures in LRSFSBC by SS 13C NMR analysis. The main intermolecular interactions among the organic species in LRSFSBC are π–hydrogen bonds and π–π interactions. These interactions are also the cause for “capsule” formation. Stronger interactions between LRSFSBC and solvents, especially CDS and IMCDSAM, destroyed the interactions and thereby led to high extract yields.

1. Introduction Direct coal liquefaction (DCL) is performed at 400–470 °C and 19 MPa of H2 pressure with a catalyst to convert coals into clean liquid fuels and value-added chemicals (VACs). After flash distillation, the resulting oil was separated from the reaction mixture, remaining coal liquefaction residue (CLR), which accounts for ca. 30 wt% of the raw coals [1,2]. CLR can be converted to VACs by oxidation [3,4], hydroconversion [5], and pyrolysis [6–8]. It generally contains ca. 30% of oil, 25% of asphaltene and preasphaltene, and 45% of tetrahydrofuran-insoluble portion, including tetrahydrofuran-insoluble organic species, mineral matter inherently existing in the coals [9]. The main gas chromatograph/mass spectrometer (GC/MS)-detectable compounds in CLR are condensed arenes. As the dominated components in CLR, the aromatics were extracted by normal solvents [10–12], ionic liquids [13,14], and subcritical fluids [15–17] to afford an excellent precursor for preparing value-added carbon materials [18–21]. Understanding the structural and compositional features of CLR at molecular level is essential for efficient utilization of CLR and beneficial for investigating coal structures and liquefaction mechanism [22]. Multi-substituted aromatic rings (ARs) proved to be the normal structural feature of CLR [23–25].

⁎

A number of analytical tools, especially GC/MS, were applied in investigating molecular compositions of soluble species (SSs) from coals and their derivates [26–29]. However, GC/MS is not proper for analyzing less volatile and/or strongly polar components in CLR. Fortunately, as an ambient ionization technique undergoing rapid development, direct analysis in real-time ionization source coupled to iontrap mass spectrometer (DARTIS/ITMS) has the advantages of allowing for rapid mass spectrometry analysis of gaseous, liquid, and solid samples without chromatography and only with minimal sample preparation [30,31]. Thereby, it has been used for characterizing SSs from coals and their derivates [32–34]. In our previous work [35], a structure model in the liquefaction residue from Shenmu-Fugu subbituminous coal (LRSFSBC) was reported as “capsule”, which consists of “capsule dressing” and “capsule core”. Some polar and/or condensed aromatics associated each other by intermolecular interactions (IMIAs), including hydrogen bonds (HBs), including hydrogen bonds, π–π interactions, and π–HBs, forming a molecular cage which was named as “capsule dressing”. During the “capsule dressing” formation process, some small molecules would be enwrapped into the “capsule”, forming the “capsule core”. Similar structures would also exist in LRSFSBC and play important roles in extracting LRSFSBC. Therefore, in this study, we used several solvents to

Corresponding author. E-mail address: [email protected] (Z.-M. Zong).

https://doi.org/10.1016/j.fuel.2019.01.004 Received 12 January 2018; Received in revised form 11 December 2018; Accepted 1 January 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclatures

FTIRS Fourier transform infrared spectrometer GC/MS gas chromatograph/mass spectrometer HBs hydrogen bonds HCAs highly condensed arenes IMCDSAM isometric CDS/acetone mixture IMCDSMPM isometric CDS/N-pyrrolidinone mixture IMIA intermolecular interaction LCAs less condensed arenes LRSFSBC liquefaction residue from Shenmu-Fugu subbituminous coal MM molecular mass NCOCs nitrogen-containing organic compounds NSAs non-substituted aromatics OCOCs oxygen-containing organic compounds PE petroleum ether QMIs quasi-molecular ions RA relative abundance RCs relative contents SFSBC Shenmu-Fugu subbituminous coal SS 13C NMR solid-state 13C nuclear magnetic resonance SSs soluble species USSs unsaturated species VACs value-added chemicals VDWFs van der Waals forces molar content of aromatic bridgehead carbons b

AAs alkylarenes AMs aliphatic moieties AMCSs AMs-containing species ARCAs AR-containing amines ARs aromatic rings σ average substituted degree of ARs CDS carbon disulfide CH cyclohexane CGs carbonyl groups CGCSs CG-containing species CLR coal liquefaction residue Cn average carbon number of methylene chain DARTIS/ITMS direct analysis in real-time ionization source coupled to ion-trap mass spectrometer DCL direct coal liquefaction EA ethyl acetate fa aromaticity fal aliphaticity fal2 aromatic methylene carbons f ala aromatic methyl carbons fab aromatic bridgehead carbons faH aromatic protonated carbons FIs fragmental ions extract LRSFSBC in order to understand the IMIAs in LRSFSBC.

mode [30,36] using He as the discharge gas and N2 as an alternative gas at a flow rate 2 L·min−1 and 450 °C. The details about the methology of the analysis with DARTIS/ITMS have been reported [33]. In the case of FTIR analysis, the LRSFSBC/KBr tablet was made by mixing and pressing LRSFSBC and KBr powder in a mould. Each extract was completely dissolved in corresponding solvent. The solution was dropped into a KBr tablet and then irradiated with a mercury lamp to remove the solvent. The FTIR spectra were recorded by collecting 64 scans at a resolution of 4 cm−1 operating in the reflectance mode with measuring regions of 4000–400 cm−1. The extracts were also analyzed with a HewlettPackard 6890/5793 GC/MS, which is equipped with a capillary column coated with HP-5MS (cross-link 5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, and 0.25 μm film thickness) and a quadrupole analyzer with a m/z range from 33 to 500 and operated in electron impact (70 eV) mode.

2. Experimental 2.1. LRSFSBc LRSFSBC was obtained by liquefying SFSBC at 455 °C under 19 MPa of H2 in a pilot-scale DCL unit. It was pulverized to < 74 μm followed by desiccation in a vacuum at 80 °C for 24 h. The proximate and ultimate analyses of LRSFSBC are listed in Table 1. 2.2. Solid-state LRSFSBC

13

C nuclear magnetic resonance (SS

13

C NMR) analysis of

A Bruker Avance III SS 13C NMR spectrometer was equipped with a 4 mm cross-polarization magic angle spinning double-resonance probhead and operated at a 13C frequency of 100.63 MHz at room temperature. About 200 mg of LRSFSBC was packed into a 5 mm diameter zirconia rotor with a spin rate at 14 kHz. The recycle delay time, spectral width, and contact time are 0.5 s, 10 kHz, and 1 ms, respectively. A total accumulation of over 12,000 transients was made and 2000 data points were collected with 10 ms. Curve-fitting of the SS 13C NMR spectrum was performed using PeakFit v4.12 software to obtain the distributions of different carbon types in LRSFSBC.

3. Results 3.1. SS

13

C NMR analysis of LRSFSBC

As shown in Fig. 2, using a curve-fitting software, the SS 13C NMR spectrum of the LRSFSBC was clearly separated into 14 peaks, which represent different types of aliphatic (10–60 ppm), aromatic (100–160 ppm), and carbonyl (160–180 ppm) carbons. The chemical shifts for different carbon types in the LRSFSBC and their assignments [37] are listed in Table 2. The structural parameters summarized in Table 3 on the basis of data in Table 2 better elaborate carbon skeletons

2.3. Solvent extraction and subsequent analyses All the solvents used, as listed in Table S1 (Supplementary material), are analytical reagents and were distilled with a Büchi R-210 rotary evaporator before use. As shown in Fig. 1, 0.1 g of LRSFSBC was ultrasonically extracted with 20 mL of a solvent for 15 min. The extracts obtained with solvents petroleum ether (PE), methanol, cyclohexane (CH), ethyl acetate (EA), acetone, carbon disulfide (CDS), and isometric CDS/acetone mixture (IMCDSAM) are denoted as E1–E7, respectively, for convienence in description. Both LRSFSBC and its extracts were analyzed with a Nicolet Magna IR-560 Fourier transform infrared spectrometer (FTIRS) and a SVP-100 ion source coupled with an Agilent XCT ITMS in the positive ionization

Table 1 Proximate and ultimate analyses (wt%) of LRSFSBC. Proximate analysis

Ultimate analysis (daf)

Mad

Ad

VMdaf

C

H

N

Odiff

0.19

22.82

38.93

85.86

4.61

1.01

4.55

St,d

H/C ratio

3.39

0.6398

Mad: moisture content on air dried base; Ad: ash yield on dry base; VMdaf: volatile matter yield on dry and ash-free base; St,d: total sulfur content on dry base; Odiff: oxygen content calculated by difference. 820

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analysis with SS

LRSFSBC

13

PE

E1

methanol

E2

CH

E3

EA

E4

acetone

E5

CDS

E6

IMCDSAM

E7

C NMR

extraction with

analyses with DARTIS/ITMS & FTIRS analysis with GC/MS

Table 3 Structural parameters in of LRSFSBC determined by SS fa (%)

fal (%)

75.51

24.17

b

(%)

28

13

C NMR analysis.

Cn

σ (%)

1.31

34

fa = f ao1+ f ao2 + faH + fab + faS + faN + f ao3 ; fal = fal1 + f ala + fal2 + f al3 + f al4 + f alO ; fab / fa ; Cn = fal2 / faS ; = (f ao1+ f ao2 + faS + faN + f ao3 )/fa .

b=

Fig. 1. Extraction procedure for LRSFSBC.

Fig. 3. Yields of the extracts from LRSFSBC.

Table 4 Ultimate analysis (wt%, daf) of E6 and E7.

Fig. 2. SS

13

C NMR spectrum and its fitted curves of LRSFSBC.

Chemical shift (ppm)

Carbon type

Mole content (%)

Aliphatic 1

14.74

1 fal

4.45

2

22.63

f ala

6.93

3

30.76

fal2

7.20

4

37.06

fal3

3.02

5

42.55

f al4

1.51

6

50.31

f alO

1.06

7

111.37

f ao1

5.08

8

118.49

f ao2

8.83

9

125.16

faH

28.52

10

129.83

fab

21.45

11

139.32

faS

5.49

12

144.25

faN

3.93

13

154.71

f ao3

2.21

Carbonyl 14

169.35

f aC = O

0.32

Aromatic

C

H

N

Odiff

S

H/C ratio

E6 E7

90.61 89.24

6.21 6.09

0.94 1.04

1.62 3.00

0.62 0.63

0.8167 0.8132

daf: dry and ash-free base; diff: by difference.

of LRSFSBC. As expected, aromatic carbons are dominant in LRSFSBC. The aromaticity ( fa ) and aliphaticity ( fal ) indexes are 75.51 and 24.17%, respectively, i.e., there are ca. 75 aromatic carbons and 24 aliphatic carbons per 100 carbon atoms in LRSFSBC. In the aliphatic carbon region, the bimodal peak centered at 22 and 31 ppm originates from aromatic methyl ( f ala ) and methylene ( fal2 ) carbons, respectively. Among the aromatic carbons, aromatic protonated carbons ( faH , 125.16 ppm) and aromatic bridgehead carbons ( fab , 129.83 ppm) are the most abundant with their molar contents of 28.52 and 21.45%, respectively. The molar content ( b ) of fab can be used to calculate the aromatic cluster rings [38,39]. The b calculated for LRSFSBC is 28%, which is the same as the b (28%) of anthracene or phenanthrene, suggesting that the average number of ARs per cluster in LRSFSBC is 3. The average carbon number (Cn) of methylene chain in LRSFSBC is 1.31. The average substituted degree (σ) of AR is 34%, implying that the number of substituents on each AR is 2 or 3, averagely. Therefore, tetra-substituted and penta-substituted anthracene or phenanthrene could be the average structure of LRSFSBC. As Table 3 shows, both fa (75.51%) and b (28%) of LRSFSBC are higher than those of SFSBC ( fa = 66.20%, b = 20% ) [40], since condensed aromatics are less volatile than aliphatic species with the same carbon number. The resonance signals related to oxygen and nitrogen function carbons were also detected.

Table 2 Distributions of different carbon types in LRSFSBC. Peak

Sample

3.2. Extract yields with different solvents As illustrated in Fig. 3, the extract yields from LRSFSBC increase in the order: E1 < E2 < E3 < E4 < E5 < E6 < E7, indicating that the solubility of LRSFSBC in the solvents increases following: PE < methanol < CH < EA < acetone < CDS < IMCDSAM. Taking E6 and E7 as example, the extracts have significantly higher carbon and

fal1 : aliphatic methyl; f al3 : α-CH2 and β-CH in hydroaromatic rings; f al4 : methine, non-protonated; f alO : CH3O; f ao1: ortho oxy-aromatic protonated; f ao2 : ortho oxyaromatic branched; faS : aromatic branched; faN : nitrogen-substituted aromatic; f ao3 : oxygen-aromatic; fa > C=O: carbonyl esters or > N–C=O.

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Fig. 4. FTIR spectra of LRSFSBC and its extracts.

hydrogen contents, obviously higher H/C ratio but much lower oxygen and sulfur contents than LRSFSBC, as listed in Tables 1 and 4. 3.3. FTIR analysis of LRSFSBC and its extracts Aromatic C–H, carbonyl groups (CGs), non-substituted ARs, and multi-substituted ARs [41] can be seen from their characteristic absorbances around 3440, 3037, 1705, 1599, 1542, 875, 835, and 753 cm−1, respectively, while aliphatic moieties (AMs) appear in the absorbances around 2922, 2855, 1449, and 1338 cm−1 in LRSFSBC and its extracts, as exhibited in Fig. 4. The absorbances of ARs largely increase from E1 to E7. Noteworthily, the absorbance of CGs around 1705 cm−1 in the extracts, especially in E2–E4 and E7, is significantly stronger than that in LRSFSBC. 3.4. Group components identified in the extracts As shown in Fig. S1 and Tables S2–S8, in total 55 compounds were detected in the extracts with GC/MS. They can be classified into alkanes, arenes, arenols, alkanals, alkenones, esters, and nitrogen-containing organic compounds (NCOCs). Among them, arenes are the most abundant (Fig. 5) with the relative contents (RCs) of 96.04, 78.16, 94.28, 87.23, 96.67, 95.71, and 97.36% in E1–E7, respectively. Noteworthily, some condensed arenes were isolated from LRSFSBC by solvent extraction and subsequent column chromatography [42]. Oxygen-containing organic compounds (OCOCs), i.e. arenols, alkanals, ketones, esters, and a NCOC (i.e., 2,6-dimethyl-6-nitro-2-hepten-4-one) are the secondarily dominated species in the extracts. Esters, i.e., 2-ethylhexyl acrylate (peak 10) and 5 methyl alkanoates (peaks 20, 24, 26, 28, and 29) listed in Table S7, are dominated in the OCOCs.

Fig. 5. RCs of arenes (upper) and other group components (lower) identified in E1–E7.

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Fig. 6. Distribution on the MM of LRSFSBC and its extracts.

3.5. NCOCs identified in LRSFSBC and its extracts with DARTIS/ITMS

The presence of complex π–π interactions, HBs, π–HBs interactions, and van der Waals forces (VDWFs) in LRSFSBC can be imaged since most of the compounds detected in LRSFSBC are aromatics, including arenes (Fig. 5) and ARCAs (Fig. 8) with small amounts of alkanes and OCOCs. In the arenes, π–π interactions among highly condensed arenes (HCAs) such as benzo[ghi]perylene, naphtho[7,8,1,2,3-nopqr]tetraphene, and coronene should be stronger than those among less condensed arenes (LCAs) due to higher electron cloud density of HCAs than that of LCAs. Although steric hindrance of alkyl groups on alkylaromatics (AAs) is unfavorable for the close contact among the AAs, electron-donating effect of the alkyl groups increases electron cloud density in ARs of the AAs and thereby enhances π–π interactions among the AAs and other unsatured species (USSs), especially non-substitured aromatcs (NSAs). In addition, the presence of VDWFs in the AAs further increases IMIAs among the AAs. Other VDWFs result from AMs in other species except for NSAs and chrysen-6-ol, while HBs mainly come from IMIAs among the heteroatom-containing organic species (HACOSs) in LRSFSBC, especially among amines and arenols due to their base-acid interactions [44]. Much weaker absorbance of CGs around 1705 cm−1 in LRSFSBC than that in the extracts (Fig. 4) may result from the strong interactions among CG-containing species (CGCSs) and some insoluble macromolecular species in LRSFSBC. Our further work will focus on clarifying such interactions. Hence, π–π interactions among the USSs especially among various aromatics, are the main asssociated features of organic species in LRSFSBC, while VDWFs among the AM-containing species (AMCSs) and HBs among the HACOSs also play important roles in the associations among the organic species in LRSFSBC and also for the “capsule dressing”.

Numerous species were detected in LRSFSBC and its extracts with molecular masses (MMs) up to 600 u with DARTIS/ITMS (Fig. S2). The MMs are distributed mainly in 300–400 u with largely normal distribution. As Fig. 6 shows, a number of species are NCOCs according to their odd mass numbers of protonized ions (i.e., [M+H]+), which are called quasi-molecular ions (QMIs) [43], whereas only 2 NCOCs (peaks 7 and 8 in Fig. S1 and Tables S6 and S8) were identified with GC/MS. Three QMIs with m/z = 292.3, 334.4, and 348.4 were selected to examine their tandem mass spectra (Fig. 7) and to infer possible structures of corresponding molecules and fragmentation mechanisms (Fig. 8). The NCOCs were also identified in the products from coal liquefaction with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry [25]. 4. Discussion 4.1. Associated features of the LRSFSBC The absorbances of AMs from both LRSFSBC and its extracts are strong (Fig. 4) but only small amounts (RCs < 8%) of alkanes were detected in the extracts (Fig. 5). In fact, all the substitured arenes contain AMs. Aromatic methyl and alkylated aromatics were also identified with SS 13C NMR. As listed in Table 3, b and σ of LRSFSBC are 28% and 34%, respectively, implying disubstituted naphthalene and trisubstituted anthracene (and/or phenanthrene) units are the average structural features of the macromolecules in LRSFSBC. Besides, in Table S3, the substituted arenes mainly consist of mono- and di-substituted arenes with 4–7 rings by the analysis with GC/MS. All the 3 QMIs selected may lose %NH2 in their tandem mass spectra as indicated by the peaks at [M+H − 16]+ (i.e., at m/z 276.2, 318.4, and 332.3 in Fig. 7). In addition, the QMI at 334.4 may loss %NH=CH2 and %CH(CH3)2 to form fragmental ions (FIs) at m/z 305.3 and 291.3, respectively, and the FIs at m/z 319.3, 305.3, and 291.3 may result from the loss of %NH=CH2, %CH(CH3)2, and %CH2(CH2)2CH3, respectively, from the QMI at m/z 348.4, as depicted in Figs. 7 and 8. The higher resonance stability of the leaving group %CH(CH3)2 than that of either %NH=CH2 or %CH2(CH2)2CH3 leads to the higher relative abundance (RA) of FI at m/z 291.3 compared to RAs of FIs at m/z 319.3, 305.3, and 291.3. These results suggest the possible exsiting of ARcontaining amines (ARCAs), in which –CH3 and/or > CH2 are present. Evidences for the exsiting of naphthenic > CH2 and hydroaromatic > CH2 and > CH were supported by SS 13C NMR analysis shown in Table 2.

4.2. Dissociation of LRSFSBC by solvent extraction Normally, the dissolution behavior can be divided in several stages. First, a cavity must be created in the solvent to accommodate the solute. Then the solute is placed in the cavity and permitted to interact with its nearest solvent molecules, eventually forming a new entity. Finally, this entity may interact further with its surroundings, by orienting solvent molecules, by the formation or disruption of HBs, or by other interactions. Apparently, as shown in Fig. 3, CDS is more effective for extracting LRSFSBC than other single solvents used. CDS proved to be an effective solvent for extracting alkanes, arenes [45], and aliphatic amides [46] in coals due to the VDWFs between CDS and alkanes, π–π interactions between CDS and arenes, and between CDS and aliphatic amides. Therefore, the important role of CDS in dissociating VDWFs and π–π interactions in LRSFSBC can be considered. 823

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Fig. 7. DART mass spectrum of LRSFSBC and tandem mass spectra of QMIs with [M+H]+/z = 292.3, 334.4, and 348.4.

HBs between the solute and neighboring solvent molecules may be formed if one or both of them are protic. The solute may also undergo changes in the dissolution process, being affected by the strengthening or weakening certain bonds, by redistributing the partial electrical charges on its atoms, or by favoring a certain conformation of a flexible solute molecule. Acetone is a good acceptor for forming HBs. In other words, acetone is effective for dissociating HBs in LRSFSBC. Similar to isometric CDS/N-methyl-2-pyrrolidinone mixture (IMCDSMPM) [47,48], synergic effect of CDS with acetone in IMCDSAM on extracting organic species can also be considered. The C=S/C=O π–π system in ICDSAM could be more effective for dissociating π–π interactions among the USSs in LRSFSBC. Thus, higher extract yield of LRSFSBC with IMCDSAM than that with CDS can be ascribed to the role of acetone in dissociating HBs in LRSFSBC and synergic effect of CDS with acetone on

dissociating π–π interactions among the USSs in LRSFSBC. IMCDSAM is more practical as a solvent than IMCDSMPM for extracting coals and their derivates (e.g., LRSFSBC) since much lower boiling point of acetone than that of N-methyl-2-pyrrolidinone facilitates the solvent separation and recovery. 4.3. Modified “capsule” structure in LRSFSBC As shown in Fig. 9, the main interactions between the small molecules and macromolecular aromatic skeleton in the “capsule” are HBs, π–HBs, and π–π interaction, forming the “capsule dressing”. In this work, some structural portions were modified by solvent extraction. The interactions between solvents and the structural portions are presented in the picture.

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Fig. 8. Proposed structures of the compounds corresponding to selected QMIs in Fig. 7 and corresponding fragmentation mechanism during the analysis with DARTIS/ITMS.

5. Conclusions

IMCDSAM are effective for dissociating the associated structure of LRSFSBC to afford extracts with significantly higher carbon and hydrogen contents, obviously higher H/C ratio but much lower oxygen and sulfur contents than their parent LRSFSBC. Such extracts are good precursors for preparing advanced carbon materials. The main interactions for forming the “capsule” are HBs, π–HBs, π–π interactions, which are destroyed by different solvents, leading to high extract yields of LRSFSBC.

The main associated features of the “capsule” in LRSFSBC are π–π interactions among the USSs, including NSA, AAs, ARCAs, and CGCSs. The contribution of VDWFs among AMCSs and HBs among the HACOSs to the associated structure of the “capsule” is also noteworthy. The tetra- and penta-substituted anthracene or phenanthrene could be the average structures of the organic macromolecules in LRSFSBC. CDS and 825

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Fig. 9. Representation for associated structures of capsule and other structure models in LRSFSBC.

Acknowledgements This work was subsidized by National Natural Science Foundation of China (Grant 21606210), the Key Project of Joint Fund for the Research on Coal-Based Low Carbon Technology from National Nature Science Foundation of China and the Government of Shanxi Province (Grant U1610223), the Key Project of Joint Fund from National Nature Science Foundation of China and the Government of Xinjiang Uygur Autonomous Region (Grant U1503293), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.01.004. References [1] Vasireddy S, Morreale B, Cugini A, Song C, Spivey JJ. Clean liquid fuels from direct coal liquefaction: chemistry, catalysis, technological status and challenges. Energy Environ Sci 2011;4:311–45. [2] Khare S, Dell'Amico M. An overview of solid-liquid separation of residues from coal liquefaction processes. Can J Chem Eng 2013;91:324–31. [3] Huang YG, Zong ZM, Yao ZS, Zheng YX, Mou J, Liu GF, et al. Ruthenium ioncatalyzed oxidation of Shenfu coal and its residues. Energy Fuels 2008;22:1799–806. [4] Sugano M, Ikemizu R, Mashimo K. Effects of the oxidation pretreatment with hydrogen peroxide on the hydrogenolysis reactivity of coal liquefaction residue. Fuel Process Technol 2002;77–78:67–73. [5] Yue XM, Wu YJ, Zhang SQ, Yang XQ, Wei XY. Chemical compositional analysis of catalytic hydroconversion products of Heishan coal liquefaction residue. Int J Anal Chem 2017;2017:1–6. [6] Li XH, Li LL, Li BF, Feng J, Li WY. Product distribution and interactive mechanism during co-pyrolysis of a subbituminous coal and its direct liquefaction residue. Fuel 2017;199:372–9. [7] Xu J, Bai Z, Li Z, Guo Z, Hao P, Bai J, et al. Interactions during co-pyrolysis of direct coal liquefaction residue with lignite and the kinetic analysis. Fuel 2018;215:438–45. [8] Li BF, Li XH, Li WY, Feng J. Co-pyrolysis performance of coal and its direct coal liquefaction residue with solid heat carrier. Fuel Process Technol 2017;166:69–76. [9] Wu M, Yang J, Zhang Y. Comparison study of modified asphalt by different coal liquefaction residues and different preparation methods. Fuel 2012;100:66–72. [10] Li J, Yang J, Liu Z. Hydrogenation of heavy liquids from a direct coal liquefaction

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