Case Study of Quantification of Aromatic Ring Structures in Lignite

Feb 1, 2016 - By the sequential oxidation of a sample of lignite, the filtrates and residues were analyzed and quantified by high-performance liquid ...
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Case Study of Quantification of Aromatic Ring Structures in Lignite Using Sequential Oxidation Hong-Xi Zhang,*,†,‡ Zhen-Yu Liu,† and Qing-Ya Liu† †

State Key Laboratory of Chemical Resource Engineering, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ Department of Chemical and Chemical Engineering, College of Life Science, Tarim University, Alar, Xinjiang 843300, People’s Republic of China ABSTRACT: The quantification of aromatic ring structures in lignite was studied. By the sequential oxidation of a sample of lignite, the filtrates and residues were analyzed and quantified by high-performance liquid chromatography, gel permeation chromatography, and ultimate analysis. The types and amount of aromatic ring structures in lignite were determined. The results indicated the presence of three types of aromatic ring structures based on the oxidation characteristics of lignite: (1) type I structure, which can be oxidized into benzene poly(carboxylic acid)s, (2) type II structure, in which the aromatic ring is appended by containing O/N/S groups, such as benzofuran, indol, benzothiophene, phenol, and anisole, and which can be overoxidized into small-molecule fatty acids, and (3) type III structure, in which the aromatic organic matter cannot be oxidized. The concentration of type I aromatic ring structures is 0.71 mmol/g of lignite (on a dry and ash-free basis). For type II structures, the concentrations of aromatic heterocycles containing nitrogen and sulfur are 0.07 and 0.14 mmol/g, respectively, assuming that these aromatic heterocycles contain an average of one nitrogen or sulfur atom per heterocycle. For type III structures, the concentration of tetrahydrofuran-soluble substances, which are primarily 2,5-cyclohexadiene-1,4-dione, 2,6-bis(1,1dimethylethyl), dibutyl phthalate, di-n-octyl phthalate, and butylated hydroxytoluene, is 0.035 mmol/g. The results of this study enhance our quantitative understanding of the aromatic structures of lignite.

1. INTRODUCTION A better quantitative understanding of the structure of lignite will promote effective utilization of lignite. Coal consists of two major structural elements, aromatic clusters (condensed aromatic rings) and intercluster bridges (short alkyl bridges, ether linkages, and thioether linkages). In addition, aliphatic structures are also present in coal.1−3 Aromatic ring structures are an integral part of coal. In combination with previous knowledge on the coal structure,1,3 there are three types of aromatic ring structures in lignite. First, aromatic hydrocarbons in the lignite structure are important chemical products. Polycyclic aromatic hydrocarbons (PAHs, two or more aromatic rings fused together) are a class of environmental organic contaminants.4 PAHs may affect humans, especially in the case of indoor combustion of coal.5 Second, the aromatic ring structures appended by containing O/N/S groups (such as benzofuran and phenol) in lignite are also an essential part.6,7 These aromatic rings are more reactive and easier to be substituted and oxidized than aromatic hydrocarbons. Third, a few aromatic ring structures in lignite (such as benzene and dibenzofuran) are inert and unoxidizable to some chemical agents, such as alkaline permanganate.8,9 Various chemical methods, such as oxidation, hydrogenation, depolymerization, pyrolysis, alkylation, and halogenation, have been used to study the structure of coal.10−15 As a result of its superior selectivity on the structure unit of coal, oxidation is a valuable method in elucidating the coal structure.16−21 From oxidation of brown coal in alkaline solution (Na2CO3, O2, 85 °C, and atmosphere), Hayashi et al.22 found that the amount of carboxyl groups formed peripherally on the neighboring © XXXX American Chemical Society

clusters was equal to the amount of bridges bound to the eliminated aromatic clusters; the average amount of bridges per eliminated aromatic cluster was 1.3−2.9. This work employed a oxidation method of quantifying the bridges in brown coal. The quantification of aromatic ring structures in lignite has not yet been reported by an experimental approach. Aromatic rings are an important component in the structure of coal; thus, quantifying the aromatic rings would further promote our knowledge of the coal structure. When lignite is oxidized, partially oxidized products are derived from the oxidation of the aromatic rings contained in the lignite structure. The type and amount of oxidized products of benzene poly(carboxylic acid)s (BPCAs) could indicate the types and quantities of the aromatic ring structures in lignite. Wang et al.23 found that the alkali−oxygen oxidation of Xiaolongtan lignite (NaOH, O2, 240 °C, and 5 MPa) produced BPCAs; the sources of these BPCAs were aromatic hydrocarbons, such as naphthalene, fluorene, and phenanthrene. Randall et al.9 oxidized aromatic hydrocarbons using alkaline permanganate (KOH, KMnO4, 100 °C, and atmosphere) and found that the main aromatic products were BPCAs; the amount of BPCAs was nearly equal to the amount of oxidized aromatic hydrocarbons. According to the oxidation mechanism, the present study aimed to quantify type I (the definition of type I is provided in the Nomenclature) structures via the sequential oxidation of lignite in alkaline permanganate. Received: November 6, 2015 Revised: January 9, 2016

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DOI: 10.1021/acs.energyfuels.5b02617 Energy Fuels XXXX, XXX, XXX−XXX

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time period (100 °C, atmosphere, 600 revolutions/min, and 12 h). The solvent and residue were separated and collected. In the following step, the residue, the same amount of KMnO4/NaOH (m/m = 7:1), and deionized water as mentioned above were added to a roundbottom flask and then the reaction mixture was refluxed for a certain time period (100 °C, atmosphere, 600 revolutions/min, and 24 h). After the sequential oxidation [NaOH, KMnO4, 100 °C, atmosphere, and 36 (12 + 24) h], solvent (filtrate 1) and residue (residue 1) were separated and analyzed. The processing procedure of filtrate 1 is shown in Figure 2. Residue 1 was extracted by tetrahydrofuran (THF;

It is important to investigate type II (the definition of type II is provided in the Nomenclature) aromatics in the lignite structure. Two aromatic structures, phenol and benzofuran, were easily overoxidized into small-molecule fatty acids (SMFAs), carbon dioxide, and water.9,22 Some aromatic ring types in the lignite structure can be overoxidized into SMFAs, while others are oxidized into BPCAs. The present study tried to investigate type II aromatics. In addition, the investigation of type III (the definition of type III is provided in the Nomenclature) aromatics was also necessary. In the current study, we address the key issue of protecting aromatic ring from excessive oxidation or incomplete oxidation in alkaline permanganate. The sequential oxidation of lignite in alkaline permanganate is expected to adequately quantify the aromatic ring structures. Such a quantitative knowledge about the aromatic ring structures would promote more effective utilization of lignite.

Figure 2. Sample processing procedure. 2 g of residue 1 was extracted by 10 mL of THF), and filtrate 2 (the THF-soluble part of the type III aromatics) was analyzed by gel permeation chromatography (GPC) and gas chromatography/mass spectrometry (GC/MS). Oxidation of representative model compounds in the lignte structure was an effective method to study the mechanism of lignite oxidation. Benzene was unoxidizable in sequential oxidation of aromatic hydrocarbons (NaOH, KMnO4, 100 °C, 12 + 24 h, and atmosphere), and decarboxylation of BPCAs did not occur. The ratio of the number of BPCAs (NBPCAs) to the number of model compounds oxidized (Nmodel compounds oxidized) (mol/mol) reached 0.9−1.0, as shown in eqs 1−5. The total amount of these BPCAs was nearly equal to the total amount of the aromatic hydrocarbons oxidized, in accordance with the results reported by Randall et al.9 In the reaction system, the incomplete conversion of some model compounds, such as naphthalene, phenanthrene, and anthracene, resulted from sublimation. However, the sublimation did not occur in the sequential oxidation of lignite.

2. EXPERIMENTAL SECTION 2.1. Raw Coal and Reagents. Lignite from the Huolinhe coal plant in Inner Mongolia, China, was used in this study. Table 1 lists

Table 1. Proximate and Ultimate Analyses (wt %) of a Sample of Lignite proximate analysis (wt %)

a

ultimate analysis (wt %, daf)

Mad

Aad

Vad

C

H

Oa

N

S

11.82

17.97

36.48

65.83

4.87

26.03

1.85

1.42

By difference.

the proximate and ultimate analyses of the coal sample. The model compounds (99.9%, including aromatic hydrocarbons, such as toluene, naphthalene, tetrahydronaphthalene, phenanthrene, anthracene, pyrene, and biphenyl; aromatic heterocycles, such as benzofuran, dibenzofuran, indol, carbazole, thiophenol, and benzothiophene; phenol; anisole; diphenyl ether; BPCAs; and SMFAs) were purchased from Tokyo Chemical Industry. Sodium hydroxide [analytical reagent (AR) grade] and potassium permanganate (AR grade) were purchased from Beijing Chemical Company. 2.2. Sequential Oxidation. The scheme of sequential oxidation of the sample is shown in Figure 1. In the early stage of oxidation of

Figure 1. Sequential oxidation of a sample of lignite. phenanthrene with different time scales (NaOH, KMnO4, 100 °C, atmosphere, and 1−36 h), the yield of BPCAs rapidly increased with increasing time. After 12 h of the reaction, the BPCA yield slowly increased with time. After 24 h, the BPCA yield began to gradually decline. On the basis of the oxidizable step of phenanthrene, to avoid incomplete or excessive oxidation of lignite, sequential oxidation of a sample of lignite was schemed as follows. In the first step of oxidation (NaOH, KMnO4, 100 °C, atmosphere, and 12 h), the main type I strucutres in lignite were oxidized into BPCAs, carbon dioxide, and water. The solvent was separated and collected. In the second step of oxidation (NaOH, KMnO4, 100 °C, atmosphere, and 24 h), the other type I aromatics in the residue were further oxidized. The sample of lignite or model compound, a certain amount of KMnO4/NaOH (m/m = 7:1), and deionized water were added to a round-bottom flask, and the reaction mixture was refluxed for a certain

2.3. High-Performance Liquid Chromatography (HPLC) Analysis. A binary gradient elution procedure was used for HPLC (Waters 2695, Milford, MA) analysis of BPCAs. The mobile phase was an aqueous solution of acetonitrile and 0.1% (volume fraction) phosphoric acid, and the stationary phase was C18 bonded by silica gel. A photodiode array detector (235 nm) was used to quantify the products. The flow rate of the mobile phase was 1 cm3/min, and the column temperature was 35 °C. The gradient elution procedure was as follows: first, the volume ratio of acetonitrile/phosphoric acid aqueous was 5:95; then the ratio was increased linearly up to 20:80 in 10 min; and then the ratio was kept for 2 min. The ratio was decreased to 5:95 within 2 min. The test time was 18 min, and the dalay time was 5 min. HPLC was also used for analysis of SMFAs. The mobile phase was 0.01 mol dm−1 sulfuric acid, and the stationary phase was SHODEX SH 1011. A differential refractive index detector (410 nm) was used to B

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Energy & Fuels quantify the products. The flow rate of the mobile phase was 0.5 cm3/ min, and the column temperature was 55 °C. The test time was 30 min, and the dalay time was 5 min. 2.4. GPC Analysis. A Waters 2695 was used for the GPC analysis of the THF-soluble substances in residue 1. The mobile phase was THF, and the stationary phase was Styragel (Waters Styragel HR series: Styragels HR0.5, 1, 2, and 3). A differential refractive index detector (410 nm) was used to quantify the products. The flow rate of the mobile phase was 1 cm3/min, and the column temperature was 35 °C. The test time was 50 min, and the delay time was 5 min. During GPC testing, the peak time for the aromatic ring in filtrate 2 was in the range from 29.13 min (peak time of anthracene) to 29.68 min (peak time of naphthalene). The THF-soluble aromatic ring structures were approximately quantified on the basis of the standard curve of naphthalene. 2.5. GC/MS Analysis. Agilent 7890B/5977A was used for GC/MS analysis of filtrate 2. The system was equipped with a HP-5MS capillary column (cross-link 5% PH ME siloxane, 30 m × 250 μm inner diameter, with a 0.25 μm film thickness) and a quadrupole analyzer. Mass spectra were obtained at an electron impact potential of 70 eV. Helium was used as the carrier gas. The column was heated at a rate of 15 °C/min from 50 °C (held for 1 min) to 300 °C (held for 3 min). Injector and detector temperatures were set at 280 and 300 °C, respectively. The mass range scanned was from m/z 35 to 550. 2.6. 13C Nuclear Magnetic Resonance (NMR) Analysis. A Bruker AV300 NMR spectrometer equipped with a 4 mm crosspolarization/magic angle spinning (CP/MAS) probe head with a z gradient was used for aromatic carbon analysis. The 13C resonance frequency was 75.5 MHz, and the amount of scans was 1000−3000. A pulse width of 2.5 us, a pulse repetition time of 5 s, a MAS spinning rate of 12 kHz, and a contact time of 3 ms were used. The chemical shifts of 13C spectra were reported in parts per million relative to tetramethylsilane (TMS) using solid glycine as an external reference. 2.7. Synchronous Spectra Analysis. A Varian Cany Eclipse spectrofluorometer (RF-5301) was used for the fluorescence spectra analysis. The excitation and emission slit widths were 5 nm. A data pitch of 0.2 nm, scanning speed of 240 nm/min, and time response of 0.002 s were used. The excitation wavelength was 254 nm, and synchronous spectra were ranged at 240−700 nm. All measurements were performed at high sensitivity. We performed three time-parallel experiments at each set of conditions, and the results are reported as mean values. The reproducibility of experimental results was estimated less than an average relative deviation of 5%.

Figure 3. HPLC spectrogram of filtrate 1.

Figure 4. Amount of BPCAs in filtrate 1.

BPCAs were not counted (Figures 3 and 4). These results demonstrated that the effect of decarboxylation was not considered in the sequential oxidation of lignite. In the reaction system, the alkaline circumstance was protecting BPCAs from excessive oxidation. The sample of lignite was effectively converted during the sequential oxidation. As shown in Figure 4, the total concentration of BPCAs in filtrate 1 was 0.71 mmol/g. The main BPCAs were mellitic acid, pentacarboxylic acid, pyromellitic acid, prehnitic acid, and phthalic acid. As mentioned, the total amount of these BPCAs was nearly equal to the total amount of type I structures (aromatic hydrocarbons oxidized). Correspondingly, the concentration of type I structures in lignite was 0.71 mmol/g. Considering that the main size of the aromatic ring structures in lignite was 1−2 rings,6,24 the main structures of type I based on the oxidizable pathway of model compounds shown in eqs 1−5 are given in Figure 5. The concentrations of aromatic ring structures (a−k) based on the concentrations of BPCAs shown in Figure 4 are given in Table 2. R1−R6 consist of methylene, methine, carboxyl, and carbonyl groups, as shown in Figure 9. 3.2. Analysis of the Aromatic Heterocycle. The quantification of type II structures in lignite was difficult because the relationship between the amount of type II structures (Ntype II) and amount of SMFAs (NSMFAs) was uncertain. Ultimate analysis may be a feasible way to partially quantify type II aromatic structures. On the basis of the difference between raw coal and residue 1, the mass losses of nitrogen and sulfur were 0.7 and 3.2 mg, respectively. Assuming

3. RESULTS AND DISCUSSION 3.1. Analysis of Aromatic Hydrocarbons and Derivatives. The percent conversions of carbon η1 and η2 (definitions of η1 and η2 are provided in the Nomenclature) during the sequential oxidation of lignite reached 93.1 and 93.4, respectively. The sample of lignite was nearly completely converted during the sequential oxidation. The main aromatic products were BPCAs (Figure 3). The BPCAs resulted from the oxidation of the aromatic ring structures in lignite. To analyze the decarboxylation, which can be attributed to the overoxidation of BPCAs, some BPCA compounds (mellitic acid, pentacarboxylic acid, benzene tetracarboxylic acids, benzene tricarboxylic acids, and benzenedicarboxylic acids) were oxidized (NaOH, KMnO 4 , 100 °C, 36 h, and atmosphere). During the oxidation, BPCAs were mainly converted into terephthalic acid. The loss of BPCAs was less than 10% of the total amount of BPCAs. However, the yield of terephthalic acid during the sequential oxidation of lignite was negligible compared to those of the other BPCAs, as shown in Figure 4, and the conversion that BPCAs can be converted into terephthalic acid was little during the sequential oxidation. In addition, benzene was unoxidizable in the system. The losses of C

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Figure 5. (a−k) Structures of type I in lignite.

Table 2. Quantification of Type I Structures in Lignite concentrations of aromatic ring structures (mmol/g) Ntype 1 (mmol/g)

η1 (%)

η2 (%)

a + b source of mellitic acid

c + d source of pentacarboxylic acid

e + f + j + k source of pyromellitic and phthalic acids

g + h + i source of prehnitic acid

0.71

93.1

93.4

0.20

0.12

0.13

0.08

that each aromatic ring containing nitrogen or sulfur only has one nitrogen or sulfur atom, the concentrations of type II aromatic structures containing nitrogen and sulfur atoms are 0.07 and 0.14 mmol/g, respectively. The oxygen-containing aromatic ring was complex, and the mass loss of oxygen was difficult to quantify. The concentration of oxygen-containing aromatic ring structures will be further investigated in the future. It is necessary to identify type II structures in lignite. In this study, the sequential oxidation of representative aromatic heterocycle structures in lignite6,24 (benzofuran, dibenzofuran, indol, carbazole, thiophenol, and benzothiophene; appending phenol, anisole, biphenyl, and diphenyl ether; NaOH, KMnO4, 100 °C, 12 + 24 h, and atmosphere) containing aromatic ring structures, including phenol, anisole, benzofuran, indole, carbazole, methyl thiophenol, and benzothiophene, was overoxidized into oxalic acid, carbon dioxide, and water and not BPCAs (panels a and b of Figure 6). Under the same test conditions used for the HPLC analysis of the BPCAs, the BPCAs were also not detected in filtrates from the sequential oxidation of aromatic heterocycles mentioned above. Type II structures were considered to be those consisting of phenol, anisole, benzofuran, indole, carbazole, methyl thiophenol, and benzothiophene. Phenol, anisole, and benzofuran were indicated in Figure 9. In addition, a few unknown aromatic ring structures in type II, such as oxygen-containing groups, will be further identified in the future. 3.3. Analysis of Residue. After sequential oxidation, the aromatic ring structures in residue 1 were referred to as type III structures. Type III structures were separated into a THFsoluble part and THF-insoluble part. For the type III THF-soluble part, the amount of the type III THF-soluble part was little, complex, and difficult to analyze. Thus, a approximate quantification was taken using the GPC analysis. In comparison of the peak times of filtrate 2 and model compounds (Figure 7), the molecular weight of type III THF-

Figure 6. (a and b) HPLC spectrogram of filtrates from the sequential oxidation of lignite and aromatic heterocycles.

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tribenzoylenebenzene, rubicene, and truxene.9 The aromatic ring structures in residue 2 were difficult to be completely dissected and will be furthur investigated in the future. 3.4. Distribution of Aromatic Ring Structures. To further analyze the distribution of aromatic ring structures in lignite, the theoretical calculation based on the ratio of the total amount of these aromatic carbon atoms to the average amount of the aromatic carbon atoms per aromatic ring is presented, as follows: Nt =

1Cdaf fa McarbonNa

× 1000

(mmol/g)

(6)

where Mcarbon = 12 (g/mol), Cdaf = 65.83%, Nt is the total number of aromatic ring structures per gram of lignite (on a dry and ash-free basis), Na is the average number of aromatic carbon atoms per aromatic ring, and fa = Aaromatic carbon/ (Aaromatic carbon + Aaliphatic carbon) (aliphatic carbon, 4−70 ppm; aromatic carbon, 90−180 ppm). From analysis of the 13C NMR data of the raw coal (Figure 9), the value of fa was found to be 0.66.

Figure 7. GPC spectrograms of filtrate 2 and model compounds.

soluble substances ranged from 128 (molecular weight of naphthalene) to 178 (molecular weight of anthracene). Approximate quantification based on the standard curve of naphthalene indicated the concentration of type III THFsoluble substances to be 0.035 mmol/g.

Figure 9. CP/MAS 13C NMR spectrum of the raw coal.

The size of the aromatic ring structures in lignite has been investigated. Hayashi et al. assumed the average size of the aromatic ring strucutres per cluster in Morwell brown coal to be 1 ring.22 Spiro et al. suggested that the average size of the aromatic ring strucutres per cluster in low-rank coal (C %, daf, 74.8) was 1.44 rings.24 By dichloromethane Soxhlet extractions of lignites (C %, daf, 70.3−74.3), Wang et al. found that the lignites were dominated by 4−6-ring PAHs structures, which contribute 67.7−77.9% of the total in lignites.5 Kidena et al. found the average size of the aromatic ring strucutres in coals generally increased with the heat treatment temperature; the average amount of aromatic carbon atoms per cluster in Illinois No. 6 coal (C %, daf, 77.7) ranged from 10 to 14 with the heat

Figure 8. Total ion chromatogram of filtrate 2 by GC/MS analysis.

As Figure 8 and Table 3 demonstrate, in total, six products were detected in filtrate 2 with GC/MS. The type III THFsoluble substances are considered to be 2,5-cyclohexadiene-1,4dione, 2,6-bis(1,1-dimethylethyl), dibutyl phthalate, di-n-octyl phthalate, and butylated hydroxytoluene. In addition, little unknown aromatic ring structures, such as a fused aromatic ring, may not be detected. Type III THF-insoluble substances may consist of heterocyclic aromatic compounds and PAHs, such as Table 3. Products Detected in Filtrate 2 peak

product

formula

m/z

1 2 3 4 5 6

acetamide, 2,2,2-trifluoro-N-methyl-N-(trimethylsilyl)2,5-cyclohexadiene-1,4-dione, 2,6-bis(1,1-dimethylethyl)1,2,3,4,5,6-hexahydro-1,1,5,5-tetramethyl-2,4a-methanonaphthalen-7(4aH)-one butylated hydroxytoluene dibutyl phthalate di-n-octyl phthalate

C6H12F3NOSi C14H20O2 C15H22O C15H24O C16H22O4 C24H38O4

73 177 161 205 149 148.9

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Energy & Fuels treatment temperature increased from 25 to 367 °C.25 We found that the size of aromatic ring strucutres in THF extracts of the raw Huolinhe coal at 25 °C ranged from 1 to 3 rings, as shown in Figure 10. The total number of aromatic ring

understanding of the aromatic structures in lignite to characteristic reaction26 and quantification on a molecular level.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-10-64421077. E-mail: zhanghongxi3@ 163.com. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work was funded by the National Basic Research Program of China (2011CB201306), the National Natural Science Foundation of China (21306122), and the State Key Laboratory of Chemical Resource Engineering Fund Project (CRE-2015-C-306).

■ Figure 10. Synchronous spectra of the sample of THF extracts of the raw Huolinhe coal.

NOMENCLATURE type I = aromatic ring that can be oxidized into BPCAs type II = aromatic ring that can be overoxidized into SMFAs type III = aromatic ring that is unoxidizable conversion = η1 (%) = (1Craw coal − mresidue 1Cresidue 1) × 100/Craw coal

structures (Nt) in the lignite with the assumed average size of 1−6 condensed ring per cluster was calculated by eq 6, as shown in Table 4. In fact, the average size of condensed ring per cluster in the lignite was difficult to identify, and the value of Nt in Table 4 was still significant for a referance. Table 4 illustrated that type I and II structures were the main types of lignite. The aromatic ring appended by oxygencontaining groups was a main component in type II structures. The oxygen content was much higher than the contents of nitrogen and sulfur in lignite (Table 1). Figure 9 identifies the oxygen-containing aromatic groups as hydroxyl, ether, carboxyl, and carbonyl groups. The main type II structures consisted of an oxygen-containing group, such as phenol, anisole, and benzofuran, which corresponded to the model coal structure.3,24

η2 (%) = (1Craw coal − mresidue 2Cresidue 2) × 100/Craw coal

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REFERENCES

(1) Larsen, J. W.; Green, T. K.; Kovac, J. The nature of the macromolecular network structure of bituminous coals. J. Org. Chem. 1985, 50, 4729−4735. (2) Xue, J.; Liu, G. J.; Niu, Z. Y.; Chou, C. L.; Qi, C. C.; Zheng, L. G.; Zhang, H. Y. Factors that influence the extraction of polycyclic aromatic hydrocarbons from coal. Energy Fuels 2007, 21, 881−890. (3) Huttinger, K. J.; Michenfelder, A. W. Molecular structure of brown coal. Fuel 1987, 66, 1164−1165. (4) Wang, R. W.; Liu, G. J. Variations of concentration and composition of polycyclic aromatic hydrocarbons in coals in response to dike intrusion in the Huainan coalfield in eastern China. Org. Geochem. 2015, 83−84, 202−214. (5) Wang, R. W.; Liu, G. J.; Zhang, J. M.; Chou, C. L.; Liu, J. J. Abundances of polycyclic aromatic hydrocarbons (PAHs) in 14 Chinese and American coals and their relation to coal rank and weathering. Energy Fuels 2010, 24, 6061−6066. (6) Tromp, P. J. J.; Moulijn, J. Slow and rapid pyrolysis of coal. In New Trends in Coal Science; Yuda, Y., Ed.; Springer: Dordrecht, Netherlands, 1988; Series C: Mathematical and Physical Sciences, Vol. 244, pp 305−338, DOI: 10.1007/978-94-009-3045-2_14. (7) Wender, I. Catalytic synthesis of chemicals from coal. Catal. Rev.: Sci. Eng. 1976, 14 (1), 97−129. (8) Philip, C. V.; Anthony, R. G; Cui, Z. D. Structure and liquefaction reaction of Texas lignite. In The Chemistry of Low-Rank Coals; Schobert, H. H., Ed.; American Chemical Society (ACS): Washington, D.C., 1984; ACS Symposium Series, Vol. 264, Chapter 19, pp 287− 302, DOI: 10.1021/bk-1984-0264.ch019. (9) Randall, R. B.; Benger, M. B.; Groocock, C. M. The alkaline permanganate oxidation of organic substances selected for their

4. CONCLUSION The percentage of carbon conversion during the sequential oxidation of a sample of lignite reached 93.1%. Three types of aromatic rings, types I, II, and III, are present according to the oxidation characteristic of lignite. Type I structures are aromatic hydrocarbons and their derivatives, such as the structures a−k. The main type II structures are phenol, anisole, benzofuran, indol, carbazole, and benzothiophene. The main THF-soluble type III substances are 2,5-cyclohexadiene-1,4-dione, 2,6bis(1,1-dimethylethyl), dibutyl phthalate, di-n-octyl phthalate, and butylated hydroxytoluene. In lignite, the concentration of type I structures is 0.71 mmol/g. The concentration of THFsoluble type III structures is 0.035 mmol/g. In type II structures, the concentrations of aromatic heterocycles containing nitrogen and sulfur are 0.07 and 0.14 mmol/g, respectively. The results of this work promote our

Table 4. Distribution of Aromatic Ring Structures in Lignite (mmol/g) Nt

Ntype II

Ntype I

1 ring (Na = 6)

2 ring (Na = 10)

3 ring (Na = 14)

4 ring (Na = 18)

5 ring (Na = 22)

6 ring (Na = 26)

0.71

6.0

3.6

2.6

2.0

1.6

1.4

F

NO

Ntype III

NN

NS

NTHF soluble

0.07

0.14

0.035

NTHF insoluble

DOI: 10.1021/acs.energyfuels.5b02617 Energy Fuels XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.energyfuels.5b02617 Energy Fuels XXXX, XXX, XXX−XXX