Article pubs.acs.org/EF
Structural Characterization of Typical Organic Species in Jincheng No. 15 Anthracite Yu-Gao Wang, Xian-Yong Wei,* Rui-Lun Xie, Fang-Jing Liu, Peng Li, and Zhi-Min Zong Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, People’s Republic of China S Supporting Information *
ABSTRACT: The structures of typical organic species in Jincheng No. 15 anthracite (J15A) were characterized by solid-state 13 C nuclear magnetic resonance, X-ray photoelectron spectrometry, X-ray diffraction, and Fourier transform infrared spectrometry in combination with gas chromatography/mass spectrometry and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry analyses of the resulting soluble organic species from ruthenium-ion-catalyzed oxidation (RICO) of J15A. The results show that the typical organic species in J15A are condensed aromatics, along with small amounts of methyl group as the dominant side chain on the condensed aromatic rings (CARs) and methylene linkage connecting the CARs. Every aromatic cluster contains five rings on average, and the substituted degree of each aromatic ring is very low. In addition, J15A is rich in peri-condensed aromatics but poor in cata-condensed aromatics and polyaryls. The oxygen functional groups in J15A include C−O and >CO groups. Pyrrolic nitrogen species and arylthiophenes are the main organic nitrogen and sulfur species in J15A, respectively.
1. INTRODUCTION The reserve of Jincheng No. 15 anthracite (J15A) is significantly abundant, accounting for ca. 33% of the total reserves of anthracites in Jincheng region, which is known as a major anthracite base in China.1,2 However, J15A cannot be used for traditional gasification and electric power generation because of its high ash yield, high sulfur content, and high ash fusion temperature.2,3 Therefore, developing new technologies is desirable to efficiently use J15A. The ash-agglomerating fluidized-bed gasification was already developed to gasify J15A to afford syngas for producing methanol.2 The feasibility of preparing the vehicle friction material using J15A was also investigated.3 Given the condensed aromatic structures of an anthracite, we managed to oxidize J15A under mild conditions using cheap and easily available oxidants to obtain benzenepolycarboxylic acids (BPCAs).4 A deep insight into organic structures of coals is significantly important for efficient use of coals, especially for obtaining value-added chemicals from coals.5−9 Solid-state 13C nuclear magnetic resonance (SS 13C NMR),10−13 X-ray photoelectron spectrometry (XPS),13−16 X-ray diffraction (XRD),17−20 and Fourier transform infrared (FTIR) spectrometry21−23 were used to investigate organic structures of coals and other fossil resources, with the advantages that whole, unaltered, and solid samples could be directly and conveniently analyzed. Another useful approach for characterizing organic moieties in coals is selective chemical degradation. Ruthenium-ioncatalyzed oxidation (RICO) can selectively convert aromatic carbons in coals to carboxylic groups and thereby has been widely used to provide particularly useful information on aromatic rings, alkylene bridges, and alkyl side chains in coals in combination with subsequent analyses of the resulting soluble organic species (SOSs).6,24−26 Gas chromatography/mass spectrometry (GC/MS) succeeds in analyzing volatile and © XXXX American Chemical Society
less polar species but fails to detect polar and less volatile species. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been paid increasing attention in recent years because of its extremely high mass resolution (exceeding 200 000) and mass accuracy (CO ( faCC) groups, whereas the −COOH group was not detected, which is consistent with the OFG distributions in B
DOI: 10.1021/ef502373p Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. Carbon Structural Parameters in DMJ15A Determined by the SS 13C NMR Analysis structural parameter
symbol
definition
value
aromaticity ratio of aliphatic carbon ratio of carbonyl carbon mole percent of aromatic bridgehead carbon substituted degree of aromatic ring average methylene chain length
fa fal faCC xb σ Cn
fa = faH + faB + faS fal = fal1 + fala + fal2 + fal3 + fal4 + fal5 + falO faCC = faCC xb = faB/fa σ = faS/fa Cn = ( fal2 + fal3)/faS
89.8% 9.7% 0.5% 0.4 0.02 1.4
faS/fa of the aromatic ring is 0.02, which implies that the aromatic ring is lowly substituted. In XPS analysis, the C 1s signal is often used to characterize the OFG distributions because the C 1s signal is sensitive toward the change of the oxidation state around oxygen atoms.45,46 As exhibited in Figure 2, the XPS C 1s spectrum of DMJ15A is separated into three peaks, corresponding to C−C or C−H, C−O− or C−OH, and >CO, respectively. The C−
O− or C−OH groups may be present in aliphatic ethers or alkanols, while >CO groups may exist in esters and ketones, which is in accordance with the 13C NMR analysis. However, the amounts of the two OFGs obtained by XPS analysis are more than those obtained by the SS 13C NMR analysis (Table 4), because XPS analysis is limited to the sample surface, while SS 13C NMR analysis provides the structural information on the sample bulk.45 Pyrrolic species are the most abundant nitrogen form in J15A according to curve fitting of the N 1s spectrum. In high-rank coals, a certain amount of quaternary nitrogen species may be combined with condensed aromatic rings (CARs), such as 4H-quinolizine, 3,5-dihydropyrido[3,2,1ij]quinoline, and 1,3a-dihydropyrido[2,1,6-de]quinolizine.50 Pyridinic nitrogen species are the lowest in organic nitrogen species, which may result from the removal of the basic pyridinic nitrogen species during J15A demineralization. As Table 1 lists, J15A is rich in inorganic sulfur, which is related to the marine and brackish environment of J15A deposition.51 The inorganic sulfur could be significantly reduced but not be completely removed by demineralization using HCl and HF.46 The organic sulfur species in J15A are mercaptan or thiophenol, thiophene, sulfoxide, and sulfone based on curve fitting of the S 2p spectrum. Among them, thiophene is the richest, which may exist as dibenzothiophene in the high-rank coal.51 As Figure 3 exhibits, the predominant band around 25.5° and the weak band around 43.7° imply that graphite-like structures (crystalline carbons) exist in J15A.17,52 Meanwhile, the asymmetric feature of the band around 25.5° suggests the existence of another band in its left-hand side. By curve fitting, the band could be separated into 002 and γ bands. The aromaticity (fa) of J15A defined as fa = A002/(A002 + Aγ) in XRD analysis17,52 (A means the area of the band) is 74.5%, which is similar to the result from the SS 13C NMR analysis of J15A. As shown in Figure 4, the predominant absorbance band at 1037 cm−1 and its minor shoulder band at 1123 cm−1 are attributed to the >C−O− vibrations of various ethers and alcohols. The spectrum is also characterized by the obvious absorbance of aromatic ring vibration at 1597 cm−1,53 indicating the high aromaticity of J15A. 3.2. RICO of J15A and Subsequent GC/MS and ESI FTICR MS Analyses. In total, 28 CAs, i.e., 10 alkanoic acids (AAs), 5 alkanedioic acids (ADAs), 1 alkanetricarboxylic acid (ATCA), and 12 BPCAs, were detected in MEOP and MEAP from RICO of J15A with GC/MS, as summarized in Figure S1 and Table S1 of the Supporting Information. As Figure 5 shows, BPCAs dominate in the CAs because of the high aromaticity of J15A. The detected monomethyl-substituted BPCAs (peaks 13 and 18) in MEOP evidence the existence of methyl groups on CARs, whereas their trace amounts confirm that the CARs are lowly substituted in J15A, as demonstrated by the SS 13C NMR analysis. The diarylmethane moieties in J15A could be oxidized to malonic acid. However, malonic acid was not detected in either MEOP or MEAP because it is labile
Figure 2. XPS C 1s, N 1s, and S 2p spectra and their fitting curves of DMJ15A. C
DOI: 10.1021/ef502373p Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 4. XPS Analysis for the Forms of Organic Oxygen, Nitrogen, and Sulfur in DMJ15A elemental peak
functionality
binding energy (eV)
molar content (%)
aromatic and aliphatic carbons alcohol, phenol or ether (C−O− or C−OH) carbonyl (>CO)
284.8 286.4 287.5
91.6 6.3 2.1
pyridinic nitrogen pyrrolic nitrogen quaternary nitrogen nitrogen oxides
399.0 400.4 401.5 402.4
5.4 44.6 28.4 21.6
mercaptan or thiophenol thiophene sulfoxide sulfone inorganic sulfur
162.9 164.1 165.2 168.1 169.1
15.5 40.0 2.8 12.3 29.5
C 1s
N 1s
S 2p
Figure 5. Yields of different types of CAs from RICO of J15A.
toward RICO.54 Various BPCAs are mainly generated from the oxidation of different aromatic rings in coals. The macromolecular precursors of the BPCAs can be classified into catacondensed aromatics (CCAs), polyaryls, and PCAs.4,55,56 As Table 5 exhibits, J15A is rich in PCAs but poor in CCAs and polyaryls, which is similar to the result of the sequential oxidation of J15A with aqueous sodium hypochlorite.4 The detection of abundant benzenepentacarboxylic and mellitic acids indicates that J15A contains large amounts of highly condensed aromatics. In general, organic sulfur in coals is covalently bonded to the coal matrix and, therefore, not readily characterized at the molecular level. The organic sulfur compounds (OSCs) in J15A could be released by selectively breaking the macromolecular structures during RICO of J15A. In the oxidation, some OSCs, e.g., sulfides and thiophenes, are easily converted to sulfoxides and sulfones,57 which could be effectively ionized by ESI in the positive mode.30,58 Therefore, MEAP was analyzed by positiveion ESI FT-ICR MS to afford some useful information on OSCs in J15A. Thousands of mass peaks appear in the mass spectrum of MEAP (see Figure S2 of the Supporting Information). Most of the detected species could be assigned to a unique molecular formula on the basis of the accurate mass and isotopic mass ratio. The identified OSCs are mainly S1Ox (x = 1−4) class species with double bond equivalent (DBE, rings plus double bonds) values of 6−19 and 11−32 carbon atoms. As depicted in Figure S3 of the Supporting Information, S1O3 class species are the most abundant OSCs and the other S1Ox class species have similar contents.
Figure 3. XRD spectrum of DMJ15A and its partial fitting curve.
Figure 4. FTIR spectrum of DMJ15A. D
DOI: 10.1021/ef502373p Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 5. Possible Precursors of the BPCAs
thiophenes. Therefore, the S1O1−S1O4 class species should be arylthiophene sulfoxides or arylthiophene sulfones. A series of species with DBE of 9 can be observed in the S1O1 class, with carbon numbers of 15−17 being the most abundant. These series of species are most likely attributed to alkyldibenzothiophene sulfoxides. The species of the S1O2 class with DBE of 11 and carbon numbers of 17−20 are the richest, which could possibly be alkyldiphenylthiophene sulfones. For the S1O3 class species, the dominant species with DBE values of 11 and 12 could be monocarbomethoxy-substituted alkyltetrahydrobenzonaphthothiophene sulfoxides and alkyldiphenylthiophene sulf-
As demonstrated in XPS analysis, thiophenes are the most abundant organic sulfur species in J15A. They could be converted to thiophene sulfoxides or thiophene sulfones during RICO of J15A. Therefore, the S1O1 and S1O2 class species are mainly attributed to thiophene sulfoxides and thiophene sulfones, respectively, which could not be detected with GC/ MS because of their strong polarity58 and/or low contents. Correspondingly, the S1O3 and S1O4 class species could be monocarbomethoxy-substituted thiophene sulfoxides and thiophene sulfones, respectively. As depicted in Figure 6, the DBE of the S1O1−S1O4 class species is much higher than that of E
DOI: 10.1021/ef502373p Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 6. Iso-abundance plots of DBE versus carbon number distributions of S1Ox class species.
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oxides, respectively, while the most abundant S1O4 class species could be monocarbomethoxy-substituted alkyltetrahydrobenzonaphthothiophene sulfones with DBE of 11 and carbon numbers of 20−22.
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-516-83884399. E-mail: wei_xianyong@163. com. Notes
4. CONCLUSION
The authors declare no competing financial interest.
Organic matter in J15A mainly consists of condensed aromatics, especially PCAs, according to SS 13C NMR and XRD analyses of DMJ15A and GC/MS analysis of the resulting MEOP and MEAP. Every cluster has five aromatic rings on average, and the CARs are lowly substituted. XPS, 13C NMR, and FTIR analyses show that the C−O group is the dominant OFG, followed by the >CO group, in J15A. Pyrrolic species and thiophenes are the main organic nitrogen and sulfur species, respectively, in J15A according to XPS analysis. A series of S1Ox (x = 1−4) class species with DBE > 6 were identified with positive-ion ESI FT-ICR MS. They are assigned to arylthiophene sulfoxides or arylthiophene sulfones, indicating that thiophenes are fused to aromatic rings in J15A. The investigation provides a useful approach for characterizing organic structures in coals.
ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant 21276268), the Fund from the National Natural Science Foundation of China for Innovative Research Group (Grant 51221462), Strategic Chinese− Japanese Joint Research Program (Grant 2013DFG60060), the Research and Innovation Project for College Graduates of Jiangsu Province (Grant KYLX_1396), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors also greatly appreciate Dr. Quan Shi, Dr. Ya-He Zhang, and Dr. HongXing Ni (State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, China) for their help in ESI FT-ICR MS analysis and Dr. Gui-Yun Yu (Nanjing University, Nanjing, Jiangsu, China) for her help in solid-state 13C NMR analysis.
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ASSOCIATED CONTENT
* Supporting Information S
Total ion chromatograms of MEOP and MEAP from the RICO of J15A (Figure S1), the CAs detected in MEOP and MEAP (Table S1), positive-ion ESI FT-ICR mass spectrum of MEAP (Figure S2), and relative contents of S1O1−S1O4 in S1Ox class species (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. F
NOMENCLATURE AA = alkanoic acid ADA = alkanedioic acid ATCA = alkanetricarboxylic acid BPCA = benzene poly(carboxylic acid) CA = carboxylic acid CARs = condensed aromatic rings CCA = cata-condensed aromatic DMJ15A = demineralized J15A DOI: 10.1021/ef502373p Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
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ESI FT-ICR MS = electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry FTIR = Fourier transform infrared spectroscopy GC/MS = gas chromatography/mass spectrometry J15A = Jincheng No. 15 anthracite MEAP = methyl-esterified aqueous portion MEOP = methyl-esterified organic portion OFGs = oxygen functional groups OSC = organic sulfur compound PCA = peri-condensed aromatic RICO = ruthenium-ion-catalyzed oxidation SOS = soluble organic species SS 13C NMR = solid-state 13C nuclear magnetic resonance XPS = X-ray photoelectron spectroscopy XRD = X-ray diffraction
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