Article pubs.acs.org/EF
Structure Characterization and Model Construction of Indonesian Brown Coal Hua-lin Lin,*,†,‡ Ke-jian Li,‡ Xuwen Zhang,‡ and Hongxue Wang‡ †
Shanghai Institute of Technology, Shanghai 201418, People’s Republic of China Shanghai Research Institute, China Shenhua Coal to Liquid and Chemical Corporation, Limited, Shanghai 201108, People’s Republic of China
‡
ABSTRACT: Indonesian brown coal is analyzed using 13C cross-polarization/magic angle spinning nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction (XRD) to obtain the information and parameters of the coal structural unit. A macromolecular structural model of Indonesian brown coal was constructed on the basis of the structural parameters and elemental analysis results. The 13C chemical shift of this model was calculated using the ACD/13C NMR predictor. The results indicate that the aromaticity of Indonesian brown coal is 0.3412, and the structures of aromatic carbon are mainly types of naphthalene and benzene. The ratio of bridge carbon/surrounding carbon is 0.0696. Oxygen in the structural model mainly exists in the form of phenolic hydroxyl oxygen, carboxyl oxygen, and ether oxygen, of which phenolic hydroxyl oxygen and carboxyl oxygen are the most prominent. Nitrogen atom exists in the form of pyridine and pyrrole. The peak of XRD spectra at 20−30° is broad with a strong γ width. The calculated chemical shift spectrogram of the model is highly consistent with that of the experimental results. The structural formula calculated for Indonesian brown coal is C190H170O50N2. This information will be the basis of making use of Indonesian brown coal.
1. INTRODUCTION Lignite or brown coal is a type of coal characterized with a low coalification degree, high moisture and oxygen contents, and low bulk density. Characterized with its low ash and sulfur contents, Indonesian brown coal from Muara Enim, Palembang, Sumatera Selatan, Indonesia, is used as a kind of green energy resource for power generation in Indonesia by the Shenhua Company.1 In addition, with the development of coal chemical processing technologies, brown coal has been selected more and more as a feedstock in the coal chemical industry application.2 It is a high-efficient and economic technical route using some brown coals to produce liquid fuels. Therefore, a deep understanding of the brown coal structure is required to predict the reaction properties during liquefaction and to maximize the use of brown coal. The coal molecular structural model reflects detailed information on the coal physical and chemical structures. Several studies have focused on the structural features of coal macromolecules,3−6 but few structural models of brown coal have been established. In the Indonesian brown coal construction model study, structural unit information, alicyclic structure, and nitrogen and oxygen distribution types and parameters were determined through 13C cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Then, the 13C NMR chemical shift was calculated using the ACD/13C NMR predictor. The 13C NMR chemical shift calculated was compared to the experimental spectra to obtain a similar structural model.
The sample was obtained from the MuYin open-pit coal mine in a south Sudanese province. The proximate, ultimate, and petrographical analyses of the Indonesian brown coal sample are shown in Tables 1 and 2. Tables 1 and 2 show that Indonesian brown coal contains after deashing, which could more typically describe the structure of the coal, approximately 54.63% organic matter, high up to 24.49% oxygen, up to 0.89 of the atomic ratio of hydrogen/carbon, and up to 89.66% vitrinite. All of these parameters indicate that Indonesian brown coal is suitable for direct coal liquefaction. 2.2. 13C NMR Spectra Analysis and Characterization. NMR analysis was carried out on a Bruker Avance 400 superconducting NMR spectrometer (resonant frequency of 13C nucleus of 100.13 MHz) equipped with a double-resonance solid-state NMR probe and a ZrO2 rotor (outer diameter of 6 mm). Data acquisition parameters included a MAS speed of 8 kHz, 0.05 s sampling time, 4 μs pulse width, 5 s cycle delay time, and 7000 scans. The CP technique with total sideband suppression (TOSS) was employed using a contact time of 5 ms and a spectral width of 30 000 Hz. Peak fitting was carried out on 13C CP/MAS NMR spectra using NUTS98 software. The peak position of each functional group and its relative percentage thus obtained were used to calculate the structural parameters of Indonesian brown coal.7 2.3. FTIR. FTIR spectra were recorded using a Thermo Nicolet 6700 FTIR spectrometer. The wavelength requirement is less than or equal to 0.1 cm−1, whereas the transmittance is less than or equal to 0.1. A total of 32 scans were recorded at a resolution of 4 cm−1 within 4000−400 cm−1. 2.4. XPS. XPS patterns were collected using an X-ray diffractometer (Thermo ESCALAB 250 diffractometer with Al Kα radiation) at 200 W. Full-scan transmission energy was 150 eV in a 0.5 eV step, and narrow transmission energy was 60 eV in a 0.05 eV step. The vacuum degree was set at 10−7 Pa, with C 1s (284.6 eV) as a calibration standard for correction. In the XPS spectra of carbon (1s), oxygen (1s), and
2. EXPERIMENTAL SECTION
Received: January 5, 2016 Revised: April 5, 2016
2.1. Coal Sample. The Indonesian coal sample is young brownish−dark brown, tarnish, soft, plastic, and visible from plant roots. © XXXX American Chemical Society
A
DOI: 10.1021/acs.energyfuels.5b02696 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Proximate and Ultimate Analyses of the Indonesian Brown Coal Samplea proximate analysis (wt %)
ultimate analysis (wt %, daf)
Mad
Ad
Vdaf
C
H
N
S
Ob
H/C
8.12
15.73
54.63
69.20
5.14
0.89
0.28
24.49
0.8913
a
Mad, moisture on an air-dried basis; Ad, ash on a dry basis; Vdaf, volatile matter on a dry and ash-free basis; and wt %, daf, weight percentage of various elements on a dry and ash-free basis. bBy difference.
carbon part is higher and sharper, which can explain that there are more fat structures and less aromatic structures, having a peak in 23 ppm at the fat carbon area. Aromaticity ( fa′) is an important parameter of the coal structure. Table 3 shows the structural parameters of the coal sample. The aromaticity of Indonesian brown coal is 0.3412, indicating the presence of saturated hydrocarbon in the coal structure. The contents of carbonyl carbon (fac) and aromatic carbon ( faH) in Indonesian brown coal are 3.84 and 16.71%, respectively. Aromatic carbon represents the relative content of quaternary carbon, primary carbon, and secondary carbon in the coal structure. This result indicates that the non-protonated aromatic carbon is high, while alkylated aromatic ( faS) and phenolic or phenolic ether ( faP) are relatively low. Furthermore, these functional groups have higher contents, and the content of aromatic bridgehead (faB) is only 2.22%. The content of aliphatic carbon in Indonesian coal is 62.04%, with methyl carbon being the highest at 35.59%. This result indicates that the aliphatic carbon in the coal macromolecular structure is mainly connected with the aromatic ring structure and relatively few side chains. The ratio of aromatic bridge carbon to aromatic peripheral carbon, Xb = faB/( faH + faP + faS), is an important parameter that can be used to calculate the aromatic cluster size.15 According to the above formula combined with the data in Table 3, the value of Xb for this sample of Indonesian coal was 0.0696. 3.2. FTIR Analysis. Figure 2 shows the FTIR spectrogram of Indonesian brown coal. The bands of the sample are strong and sharp. The band at 1610 cm−1 is associated with the C−C
Table 2. Petrographical Analysis vitrinite
inertinite
exinite
89.66
9.56
0.78
nitrogen (1s), the ordinate and abscissa represent the electronic counting and binding energy, respectively. The fitted binding energies are outlined in the literature.8−10 The identity and content of different atoms were obtained, and spectral peaks were fitted using the Origin software. 2.5. Powder XRD. XRD patterns were collected using an X-ray diffractometer (Rigaku D/max-rB diffractometer with Cu Kα radiation; λ = 1.540 56 Å) at 40 kV and 70 mA and were recorded as a function of 2θ in the range of 10−70°. At 2θ = 10−25°, SD = 0.5, SR = 0.16, and SS = 1. At 2θ = 15−70°, SD = 1, SR = 0.16, and SS = 1. 2.6. Chemical Shift Prediction of 13C NMR. The ACD/13C NMR predictor software was applied to calculate the 13C NMR chemical shift. This software can only be used for chemical shift prediction involving no more than 256 atoms (excluding H). This software has been widely used in the chemical displacement prediction of the coal structure.11,12
3. RESULTS AND DISCUSSION 3.1. 13C CP/MAS NMR Spectra. Figure 1 and Table 3 show the 13C CP/MAS NMR peak fitting spectra and structural
Figure 1. 13C CP/MAS NMR spectrum of Indonesian coal.
parameters of Indonesian brown coal, and the fitted nuclear magnetic peaks are outlined in the literature.13,14 As shown in Figure 1, the solid 13C NMR spectra of the Indonesian brown coal mainly consist of two parts: the aliphatic carbon part (0−90 × 10−6) and the aromatic carbon part (90−165 × 10−6). In comparison to the aromatic carbon area, the peak of the fat
Figure 2. FTIR of Indonesian coal.
Table 3. Structural Parameters of the Samplea sample
fa (%)
fac (%)
fa′ (%)
faN (%)
faH (%)
faP (%)
faS (%)
faB (%)
fal (%)
fal* (%)
Indonesian coal
37.96
3.84
34.12
17.41
16.71
5.49
9.69
2.22
62.04
19.68
−6
falH (%)
falO (%)
35.59
6.77
fa, total aromatic carbon; fal, total aliphatic carbon; carbonyl δ (chemical shift) > 165 × 10 ; fa′, in an aromatic ring; protonated and aromatic; faN, non-protonated and aromatic; faP, phenolic or phenolic ether; faS, alkylated aromatic; faB, aromatic bridgehead; fal*, CH3 or nonprotonated; falH, CH or CH2; and falO, bonded to oxygen. a
fac,
B
faH,
DOI: 10.1021/acs.energyfuels.5b02696 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels group of the phenolic group of aromatic carbon, indicating the abundance of this structure. The band at 2800−3000 cm−1 corresponds to the stretching vibration of saturated hydrocarbon, showing several saturated rings or side chains, whereas the bands at 1450−1460 and 1380−1375 cm−1 are sensitive to the flexural vibration of Indonesian coal. The strong band at 547 cm−1 indicates several mineral substances in the coal. The bands at 750, 800, and 908 cm−1 are stronger than those of hulunbuir coal, a Chinese brown coal, which can contribute to the structure of benzene on two replace (including four hydrogen atoms), three replace (including three hydrogen atoms), and five replace (including one hydrogen atom) structures. The band of Indonesian coal is obviously strong at 1000−1310 cm−1, which corresponds to the stretching vibration of aryl ether, phenoxy, ether, or alcohol C−O in the coal macromolecular structure. Similar to the conclusion of Sun,16 a sharp peak about 1600 cm−1 should be the skeletal vibration of the aromatic ring and CO would be in about 1650 cm−1 in Figure 2. Aliphatic hydrogen (Hal) and aromatic hydrogen (Har) are important parameters of the coal structure. The proportions of Hal and Har are generally indicated by the stretching vibration of the absorption area of aliphatic CH at 2800−3000 cm−1 and outside the plane bending vibration of the absorption area of aromatic CH at 700−900 cm−1.17 Table 4 illustrates the related
Table 5. XPS (C 1s) Data of the Coal Sample from Indonesian Coal BE (eV)
carbon form
content (wt %mol)
284.7 286.53 287.6 289.0
C−C and C−H C−O CO COO−
53.49 30.36 1.85 14.3
Table 5 demonstrates that the major content of the carbon structure of Indonesian coal is aromatic graphitized carbon and C−H, with a content of approximately 53.49%. The contents of phenolic carbon and ether carbon are 30.36%, and the rest is minimal. 3.4. XPS Analysis of the Combined Form of Nitrogen. Four forms of nitrogen exist in the coal surface structure. The binding energies at 399.5, 400.46, 401.7, and 403 eV correspond to pyridine nitrogen, pyrrolic nitrogen, seasonal nitrogen, and nitrogen oxides, respectively. The N1s spectra of Indonesian coal and its peak fitting are shown in Figure 4,
Table 4. Relative Indexes of Indonesian Coal sample
calculation
Indonesian coal
Hal/Har aliphatics/aromatics ν(CH2)/ν(CH3)
A2800−3000/A700−900 A2800−3000/A1600 A2926 + 2854/A2956 + 2875
1.19 0.11 0.78
parameters of the three samples, wherein Hal/Har is 1.19, which indicates the presence of several saturated hydrocarbon chains, whereas ν(CH2)/ν(CH3) is 0.78. These data may be used to construct a macromolecular structural model of coal. 3.3. XPS Analysis of the Combined Form of Carbon. There are five forms of carbon that exist in the surface structure of coal: aromatic graphitized carbon (284.48 eV, C−C), aliphatic carbon (285.22 eV, C−H), phenolic or ether carbon (286.53 eV, C−O), carboxide (287.61 eV, CO), and carboxyl (289.0 eV, COOH), as shown in Figure 3 and Table 5.
Figure 4. XPS N 1s spectra of Indonesian coal.
Table 6. XPS N 1s Data of Samples BE (eV)
nitrogen form
content (%) (mol ratio)
399.5 400.46 401.7 403.0
pyridine nitrogen N-6 pyrrolic nitrogen N-5 quaternary nitrogen N-Q oxidized nitrogen-X
28.74 47.22 18.06 5.99
whereas its nitrogen contents are shown in Table 6. The table shows that the contents of pyrrolic nitrogen and pyridine nitrogen are 47.22 and 28.74%, respectively. However, the carbon content and total number of molecules should be combined to determine the number of nitrogen atoms.18 3.5. XPS Analysis of the Combined Form of Oxygen. Oxygen-containing functional groups in coal include carboxyl, phenolic hydroxyl, quinone, methoxyl, and ether.19 The O 1s XPS peak fitting spectra and oxygen contents are shown in Figure 5 and Table 7, respectively. Oxygen atoms primarily exist in the forms of ether bond (37.85%), carbonyl (22.23%), and carboxyl keys (34.42%). 3.6. XRD. XRD is an important tool to analyze the crystal structure and has been widely used to investigate the physical structure of amorphous materials.20,21 Coal is an orderly shortand long-range disordered amorphous material with a certain
Figure 3. XPS C (1s) spectrum of the coal sample from Indonesian coal. C
DOI: 10.1021/acs.energyfuels.5b02696 Energy Fuels XXXX, XXX, XXX−XXX
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The presence of the 002 peak at 20−30° is due to the spatial arrangement of the aromatic ring carbon network layer and the distance between the aromatic ring layers. The broadening peak of the low angle is mainly concentrated in the γ band. The strong peak shape of the γ band is related to the saturated structures of lipid and alicyclic chains, which illustrate the presence of many alicyclic structures in coal. The 100 peak at 40−50° is attributed to the condensation degree of the aromatic ring and the size of the aromatic carbon network layer. The XRD spectrogram shows that the strength of the peak is wide and weak; thus, the aromatic ring structure is less. The XRD spectrograms of the 002 and 100 peaks were fitted, and the crystallite structure parameters of the coal samples were calculated (Table 8). The aromatic layer diameter of Indonesian coal is 15.24 Å; the aromatic microcrystalline piled high on average is 9.19 Å; and the spacing of aromatic layers is 3.73. These results may be used to construct a molecular structural model of coal. 3.7. Construction of the Coal Structural Model. The ratios of aromatic bridge carbon/aromatic peripheral carbon were 0.0696 and 0.0884 of Indonesian brown coal, whereas there ratios were 0.25 of naphthalene and 0 of benzene, respectively. Therefore, benzene and naphthalene can be the main aromatic compounds used in modeling. The number of benzenes is more than that of naphthalenes, and nitrogen mainly exists in the form of pyrrole and pyridine. The existence of sulfur was not considered in establishing the structural model because of its negligible amount. As shown in Table 9, the types and numbers of aromatic structures for this model were adopted by constantly adjusting the number of aromatic rings to make the ratio of the aromatic bridge carbon/aromatic peripheral carbon close to the experimental value. Table 9 shows that 65 aromatic carbon atoms are present in the model and that the degree of aromaticity in Indonesian
Figure 5. XPS O 1s spectra of the sample.
Table 7. XPS O 1s Data of Samples BE (eV)
carbon form
content (%)
fwhm
530.6 531.6 532.5 533.6 535.3
inorganic oxygen CO C−O COO− absorbance oxygen
2.86 22.23 37.85 34.42 2.62
1.16 1.16 1.16 1.45 1.16
number of aromatic microcrystalline or similar graphitization of small aromatic nuclear “crystals”. XRD can identify the crystal form of tiny crystals.22,23 The relationship between reduced intensity and 2θ after the normalization and factor correction of Indonesian coal can be determined using an XRD spectrogram (Figure 6). The XRD patterns demonstrate that the broadening diffraction peaks correspond to 2θ values of 20−30° and 40−50°.
Table 9. Types of Aromatic Unit Structures and Numbers
coal is 34.12% (Table 3). Thus, the total number of carbon atoms and aliphatic carbon atoms in Indonesian coal was determined. Moreover, all types of carbon atoms were calculated (Table 3). Considering the number of aromatic and aliphatic carbon atoms as well as the elemental analysis results of oxygen and nitrogen, we adjusted the types of aliphatic carbon atoms during modeling to ensure a H/C atom ratio (Table 1).
Figure 6. XRD spectrogram of Indonesian coal.
Table 8. Parameter of XRDa sample
2θ002 (deg)
2θγ (deg)
θ100 (deg)
d002 (Å)
dγ (Å)
Lc (Å)
La (Å)
Nc
fa
Indonesian coal
23.82
16.76
42.80
3.73
5.29
9.19
15.24
2.45
0.27
a d002 (Å), inter-aromatic layer distance (nm); Lc, diameter of the aromatic clusters perpendicular to the plane of the sheet (nm); La, dimeter of aromatic sheet carbons of side chains from the diamond curve (10) band (nm); Nc, average number of aromatic sheets associated in a stacked cluster; and fa, CA/Ctotal = A002/(A002 + Aγ).
D
DOI: 10.1021/acs.energyfuels.5b02696 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 7. Chemical structure model of Indonesian brown coal.
3.8. Macromolecular Structural Model of Indonesian Brown Coal. The proposed model of the macromolecular structure of Indonesian brown coal was established on the basis of the proximate and ultimate analyses, the types of structural units in the model, as described above, and other structural parameters, such as the H/C and O/C ratios and the degree of aromaticity. The ACD/13C NMR predictor software was used to calculate the 13C NMR spectra of the macromolecular structure. A comparison between the calculated 13C NMR spectra and the experimental spectra is shown in Figure 8. The structural parameter constant was used to continuously modify the macromolecular structure to match it with the experimental NMR spectra. Details on the construction and modification of the structural model are outlined in the literature.19 The final structural model is shown in Figure 7, and its chemical formula is shown in Table 10. The calculated values agree with the experimentally determined results. Figure 7 shows that the aromatic units in the structural model of Indonesian brown coal are mainly connected through aliphatic key rings or bridged bonds, which exist in the form of methylene keys, oxygen bridge, methoxyl group, and hydrogenated aromatic ring. Oxygen mainly exists in the form of phenolic hydroxyl oxygen, carboxyl oxygen, and ether oxygen, of which phenolic hydroxyl and carboxyl groups are the most prominent. The bridged bond easily cracks and ruptures during heating and hydrogenation liquefaction. The number of structural units directly correlates with the molecular size of coal.
Figure 8. Experimental and calculated Indonesian brown coal.
13
C NMR spectrum of
Table 10. Parameters of the Chemical Structural Model sample
formula
molecular weight
coal
C190H170O50N2
3362.01
ultimate analysis C, 69.20%; H, 5.14%; N, 0.89%; and O, 24.49%
4. CONCLUSION On the basis of characterization of the Indonesian brown coal sample and the structural model, the conclusions can be drawn E
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as follows: (1) The aromaticity of Indonesian brown coal is 34.12%; the ratio of bridge carbon/aromatic peripheral carbon is 0.0696; and the ratio of alicyclic hydrogen/aromatic hydrogen is 1.19. Several aliphatic side chains and saturated cyclic chains were observed. Oxygen mainly exists in the form of the hydroxyl group and carboxyl in the structural model, whereas nitrogen atoms exist in the form of pyridine and pyrrole. The existence of sulfur was not considered in establishing the structural model because of its negligible amount. (2) Several hydrogenated aromatic rings and side chains were found in the structure in Indonesian brown coal. Oxygen in the structural model mainly exists in the form of phenolic hydroxyl oxygen, carboxyl oxygen, and ether oxygen, of which phenolic hydroxyl oxygen and carboxyl oxygen are the most prominent. The side chain contains several methyl groups. (3) On the basis of the structural parameters obtained from 13C CP/MAS NMR spectra and proximate and ultimate analysis results, ACD/ ChemSketch and ACD/13C NMR predictor software were employed to build a chemical structure model for Indonesian brown coal, from which the calculated 13C CP/MAS NMR spectra showed good agreement with the experimental results. The structural formula calculated for Indonesian coal is C190H170O50N2. (4) In the structure model of Indonesian brown coal, aromatic units were connected mainly through fat key ring or bridged bond, which exists mainly in the form of methylene keys, oxygen bridge, methoxyl group, etc. There is more in the hydrogenated aromatic ring form. Oxygen exists mainly in the form of phenolic hydroxyl and carboxyl oxygen, oxygen carbonyl, and ether oxygen key, with phenolic hydroxyl and carboxyl groups constituting the majority of them. The bridged bond cracked and ruptured easily in the process of the heating effect and hydrogenation liquefaction, which was the preferred coal for direct liquefaction. (5) Given carboxyl oxygen in its structure, Indonesian brown coal can serve as a raw material in preparing humic acid.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 086-13482272609. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the Scientific Research Foundation of Shanghai Institute of Technology (YJ2015-35), and the Shanghai Talent Development Funding (Project Number 201335).
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REFERENCES
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DOI: 10.1021/acs.energyfuels.5b02696 Energy Fuels XXXX, XXX, XXX−XXX
