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. 2022 Dec 5;7(50):46384–46390. doi: 10.1021/acsomega.2c05051

Insight into the Compositional Features of Organic Matter in Xilinguole Lignite through Two Mass Spectrometers

Sheng Li †,*, Fen-Fen Du †,‡, Zhi-Hao Ma †, Li-Hong Zhang †, Zhong-Qiu Liu ‡, Tian-Sheng Zhao §, Xian-Yong Wei §, Mei-Ling Xu ∥, Xing-shun Cong †,*
PMCID: PMC9773933  PMID: 36570262

Abstract

graphic file with name ao2c05051_0008.jpg

Ethanolysis of lignite is an effective approach for converting organic matter of lignite to liquid coal derivatives. Xilinguole lignite (XL) was reacted with ethanol at 320 °C. Then ethanol and isometric carbon disulfide/acetone mixture were used to extract the reaction mixture in a modified Soxhlet extractor to afford extractable portion 1 (EP1) and extractable portion 2 (EP2), respectively. According to analysis of EP1 with a gas chromatography/mass spectrometer, phenolic compounds made up more than 33% of the compounds detected. This could be ascribed to the ethanolysis of XL; that is, ethanol could selectively break the Calkyl–O bonds in lignite, producing more phenolic compounds. Furthermore, a quadrupole Orbitrap mass spectrometer equipped with an atmospheric pressure chemical ionization source was used for comprehensive analysis of the compositional features of EP1. The analysis indicated that O1–3, N1O0–2, and N2S1O3–6 were predominant class species in EP1. Nitrogen atoms in NO-containing organic compounds may exist in the form of pyridine or amidogen, while oxygen atoms primarily exist in furan, alkoxy, carbonyl, and ester groups. In addition, possible chemical structures of NO-containing organic compounds were speculated.

1. Introduction

Lignites are mainly used for nearby combustion and power generation due to the disadvantages of high moisture content, high ash content, easy weathering, and poor thermal stability.1,2 However, lignite retains partial structures derived from coal-forming plants, resulting in its high oxygen content. Oxygen atoms in organic matter of lignite exist in the forms of COO–, C=O, C–O–, and −OH.3−6 This natural advantage makes lignite having potential to obtain chemicals. Condensed aromatics containing hetero-atoms are value-added organic chemicals which were used for medicine, pesticides, dyes, and functional materials.7,8 Thus, it is of great significance to explore new ways for efficient utilization of lignite.

Supercritical alkanolysis of lignite is an important method to obtain liquid coal derivatives.9,10 Low carbon alkanol can effectively destroy the non-covalent association in lignite, which has a good swelling of lignite. Furthermore, alkanol can nucleophilic attack the >C–O–C< in lignite.11 Mondragon12 speculated that the attack of active free radicals generated by ethanol at high temperature may be the cause of the greater yield of coal derivatives. Lu13 investigated the thermal dissolution behaviors of Holingol lignite in methanol and ethanol at various temperatures, which indicated that 270 and 240 °C were the critical temperatures for methanolysis and ethanolysis of lignite, respectively. Since the nucleophilicity of ethyl is stronger than that of methyl, ethanol can cleave more >C–O–, resulting in higher yields of soluble portions from ethanolysis of lignite than that from methanolysis. Compared with coal pyrolysis, the reaction temperature of ethanolysis of lignite is lower, and most of the products retain the aromatic ring structure.

Supercritical alkanolysis of lignite is an important method to obtain soluble portions. Due to the complex composition of soluble portions, the identifications of the molecular component of its components were facing great challenges. A mass spectrometer is an important instrument for analyzing the molecular weight of organic compounds.14 The combination of a gas chromatograph and mass spectrometer can realize the separation and analysis of organic compounds in coal derivatives with a relative molecular weight less than 500 u. Our previous study has successfully demonstrated selective destruction of coal and identified the compositional feature of coal derivatives by gas chromatography/mass spectrometry (GC/MS).15−17 However, GC/MS was only applicable to analyze volatile and thermally stable organic compounds, which cannot parse out all the compounds in the coal derivatives.

In recent years, development of ionization technologies and high-resolution MS could greatly make up for the shortcomings of GC/MS. New MS techniques have emerged in obtaining more information on the molecular weight of coal derivatives.18−20 Quadrupole-Orbitrap MS can accurately determine the molecular formula of organic compounds without chromatographic separation, which possess resolution up to 140,000 at m/z of 200 and a mass accuracy less than 5 ppm. Combined with an atmospheric pressure chemical ionization (APCI) source and Orbitrap MS, the molecular formula of compounds can be well detected, especially those containing O, N, and S atoms.21,22 In addition, lignite derivatives contain a large number of heteroatomic compounds. The study of these heteroatomic compounds by high-resolution MS will promote the understanding and utilization of compounds obtained from the lignite soluble portion.

In this investigation, Xilinguole lignite (XL) was subjected to ethanolysis at 320 °C for obtaining soluble portions which were subsequently analyzed with two mass spectrometers. The compositional features of lignite derivatives will provide a theoretical basis for the subsequent separation of value-added coal-based chemicals.

2. Experimental Section

2.1. Coal Samples and Reagents

XL was acquired from Xilinguole, Inner Mongolia, China. The proximate and ultimate analyses of XL are reported in the previous literature.10 The XL sample was dried at 80 °C for 24 h in a vacuum oven after crushing into a powder below 200-mesh before reaction with ethanol. All the reagents, such as ethanol, acetone, and carbon disulfide (CDS), employed in the experiment were distilled before use.

2.2. Ethanolysis of XL

A 100 mL magnetically stirred autoclave was used for the ethanolysis of XL, and 2 g of XL was placed into it with 20 mL of ethanol. The equipment was heated to 320 °C and remained for 2 h after swapping out air with nitrogen three times. Then the reaction mixture was transferred to a modified Soxhlet extractor after cooling to room temperature. 150 mL of ethanol and isometric CDS/acetone were added into the extractor for extracting organic soluble components. Each extraction was proceeded 48 h at reflux temperatures to provide extractable portion 1 (EP1) and extractable portion 2 (EP2). The yield of each EP was computed as Y = mEP/mXL,daf, where mEP is the mass of the EP and mXL,daf is the mass of the XL on a dry and ash-free basis. The ethanolysis of lignite and subsequent extractions were repeated three times. The yields of EPs were the average of three experiments, and the errors for the yields are less than 3%.

2.3. Analyzing EP with GC/MS and Quadrupole-Orbitrap MS

The EPs were analyzed with an Agilent 7890/5975 GC/MS and a Thermo Scientific Q Exactive quadrupole-Orbitrap MS coupled with an APCI source. The instrument parameters were described previously.10 The temperature program of GC/MS was set as 3 °C·min–1 from 60 to 300 °C and maintained for 6 min. The main parameters for the APCI source including discharge current, capillary temperature, and vaporizer temperature were set at 4.0 μA, 275, and 300 °C, respectively. The GC/MS data analysis was performed on ChemStation software with NIST11 library. TraceFinder 3.0 was applied for identification, confirmation, and quantification analysis of quadrupole-Orbitrap MS data cooperated with Microsoft Excel 2010. Relative intensity, m/z, errors, double bond equivalent (DBE), and composition were exported in an Excel table. The data with the errors less than 3 ppm were selected. Then a self-programmed program was executed by macro instruction for sorting the compositions.

3. Results and Discussion

3.1. Yields of EPs

A large amount of organic matter would be released in thermal dissolution of coal. The yields of EPs obtained from XL ethanolysis at 320 °C are 37.7 and 20.7 wt % (daf), respectively, which are much higher than that from thermal dissociation with methanol.10 After the two-step extraction of the ethanolysis reaction mixture, 58.4% organic matter in XL was released to EPs. This means that ethanol is an effective solvent to obtain organic matter from XL under supercritical conditions.

3.2. Molecular Compositions of the EP1 Analyzed with GC/MS

Analyzing with GC/MS, 129 organic compounds in EP1 were detected, which are listed in Tables S1–S12. All the compounds were systematized into normal alkanes, alkenes, arenes, phenolic compounds, alkanols, aldehydes, ketones, carboxylic acids, esters, alkoxy compounds, nitrogen-containing organic compounds (NCOCs), and sulfur-containing organic compounds (SCOCs).

As shown in Figure 1, phenolic compounds make up 33.30% of the compounds detected in EP1, and 20.17% phenolic compounds were phenol substituted with two alkyls. A possible reason for high content of phenolic compounds could be the ethanolysis of XL; that is, ethanol can selectively cleave the Calkyl–O, Cacyl–O, and/or Caryl–O bridged bonds, producing more phenolic compounds.13 Furthermore, the dissociation of methoxy group in guaiacols23 and alkylation reaction of phenols24,25 produce the higher relative content of alkyl-substituted phenols. It will be an important strategy to produce phenolic compounds from ethanolysis of lignite since phenols are an important chemical raw material.

Figure 1.

Figure 1

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Distribution of group components in EP1 by GC/MS analysis.

As shown in Table 1, 21 ethyl alkanoates were found in EP1, accounting for 21.48%. It was found that all the mass spectrograms of ethyl alkanoates contained base peaks m/z = 88. As displayed in Figure 2, the selective ion chromatogram of m/z = 88 was analyzed, in which the carbon number of ethyl alkanoates was mainly concentrated in C5–C27. According to literature reports, there is a large amount of methyl alkanoates in the EPs from methanolysis of lignite,11 which is attributed to esterification or transesterification reaction. On one hand, it can be inferred that the ethyl alkanates are derived from the esterification reaction between alkanic acid in lignite and ethanol. On the other hand, alkanoates are linked to the large skeleton structure of lignite. The oxygen atom in ethanol is relatively electronegative, preferring to attack the carbon atom linked to the oxygen atom, which is highly electronegative. As a result, the C–O bond is activated, resulting in further fracture, and the straight chain alkanate compound is generated.

Table 1. Ethyl Esters Detected in EP1 with GC/MS.

peak compound RA (%)
8 ethyl 3-methylpentanoate 0.198
11 ethyl hexanoate 2.913
12 ethyl hex-4-enoate 0.870
13 ethyl hex-3-enoate 2.285
16 ethyl hex-2-enoate 5.772
24 ethyl heptanoate 0.568
25 ethyl 2-ethylhexanoate 0.481
28 ethyl octanoate 0.331
32 ethyl 3-cyclohexylpropanoate 0.288
34 ethyl benzoate 0.308
37 ethyl octanoate 1.531
38 ethyl oct-3-enoate 0.729
44 ethyl oct-2-enoate 1.391
45 ethyl 2-methylbenzoate 0.222
50 ethyl nonanoate 0.375
62 ethyl decanoate 0.790
94, 119 & 122 ethyl tridecanoate 0.261, 0.067 & 0.081
101 ethyl 1,1-dimethyl-2,3-dihydro-indene-4-carboxylate 0.612
107 & 124 ethyl palmitate 0.126 & 0.073
113 & 128 ethyl stearate 0.147 & 0.219
127 & 129 ethyl tetracosanoate 0.305 & 0.532
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Figure 2.

Figure 2

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Selective ion chromatogram of ethyl alkanoates in EP1.

3.3. Molecular Mass and Class Distribution of Compounds Detected with Orbitrap MS in EP1

Orbitrap MS equipped with an APCI source has proved to be a practical instrument for determining the molecular weight distribution of soluble coal derivatives.26,27 An APCI source is suitable for the detection of medium-polarity and volatile compounds. As shown in Figure 3, although the molecular masses of EP1 exhibit nearly normal distribution in both positive ion mode and negative ion mode APCI sources, compounds were easily ionized in the positive ion mode APCI source. Moreover, the molecular mass distribution of EP1 analyzed in the positive ion mode APCI source is different from that in the negative ion mode APCI source. As shown in Figure 3, the molecular masses of compounds analyzed in EP1 mainly concentrate on 150 to 450 u in the positive ion mode APCI source, while the range is from 200 to 500 u in the negative ion mode APCI source.

Figure 3.

Figure 3

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Broadband mass spectra of EP1 from analysis with Orbitrap MS in both positive ion mode and negative ion mode APCI sources.

The DBE can be calculated from the molecular formula, which reflects the condensation degree of compounds.28 As clarified in Figure S4, DBE values of the compound detected in EP1 with the positive ion mode APCI source were concentrated in 0–7, while the values were widely scattered under the negative ion mode APCI source. EP1 was analyzed with a Q-OMS under positive ion mode APCI and negative ion mode APCI sources and classed into O0–6N0–2S0–2. As exhibited in Figure 4, O1–3, N1O0–2, and N2S1O3–6 group components recognized under the positive ion mode APCI source are prominent in EP1, while S2O2 is the primary component identified with the negative ion mode APCI source.

Figure 4.

Figure 4

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Distributions of OxNySz group components in EP1 from analysis with Orbitrap MS in both positive ion mode and negative ion mode APCI sources.

3.4. Ox and N1Ox Components Detected with Orbitrap MS in EP1

As shown in Figure 5, the distributions of O1–3 components were similar, while O4–6 components have a scattered distribution. The O1–3 species mainly concentrated in carbon numbers of 7–24 and DBEs of 3–7 under the positive ion mode APCI source, implying that most compounds contain two aromatic rings. Previous studies29 have shown that oxygen atoms in soluble coal derivatives may emerge in phenols, alkoxy groups, furans, acids, esters, and ketones. Therefore, the O1 component with DBE values of 3 and 6 could be attributed to alkyl furan and alkyl benzofuran, respectively. The O1 components with DBE = 4 are most likely to be alkoxy benzene since alkoxy groups widely existed in lignite. Aromatic ketones could be the primary O1 component with DBE = 5. Meanwhile, most of the O2–6 components contain one or more ester groups. The O2–3 components with DBE = 5 and O4–5 components were speculated to alkyl benzoate and alkyl phthalate, respectively. The analysis result is consistent with that from GC/MS.

Figure 5.

Figure 5

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Iso-abundance plots of DBE vs carbon number for the Ox group components from analysis with Orbitrap MS in the positive ion mode APCI source in EP1.

High-resolution MS30,31 is effective for studying the occurrence of nitrogen in soluble coal derivatives. The nitrogen atom in pyridine could be ionized in the positive ion mode APCI source. N1, N1O1, and N1O2 components were concluded, in which N1O1 component was the most abundant group component. As displayed in Figure 6, the N1 components with the DBE values of 4 and 7 are probably alkylpyridine and alkylquinoline, respectively. The N1O1 component widely distributes in carbon numbers of 7–29 and DBEs of 3–9. The N1O1 components with DBE values of 5 and 6 were speculated to be furopyridines and benzamides, respectively. Analyzing with GC/MS, phenols, alkanols, aldehydes, ketones, carboxylic acids, esters, and alkoxy compounds were identified in EPs. The oxygen atom in N1O2 component may appear in alkoxy, ester, or carbonyl group. Hence, the N1O2 component could be ascribed to alkylquinoline or alkylfuropyridine-substituted methoxy and/or ester group. The reason for generating high content of N1Ox components is the nucleophilic attack of ethanol resulting in the cleavage of C–O bond between nitrogen-containing rings and macromolecular groups in lignite.11

Figure 6.

Figure 6

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Iso-abundance plots of DBE vs carbon number for the N1O0–2 group components from analysis with Orbitrap MS in the positive ion mode APCI source in EP1.

4. Conclusions

In total, more than 58.4% organic matter in XL was released to SPs after ethanolysis of XL at 320 °C. According to GC/MS analysis, 33.3% phenols and 21.48% ethyl alkanoates were detected in EP1, which could be a potential strategy to produce oxygen-containing compounds from ethanolysis of lignite. Meanwhile, O1–3, N1O0–2, and N2S1O3–6 group components identified in the positive ion mode APCI source were predominant in EP1, and the chemical structures were speculated. The oxygen atoms in O1–3 group components mainly exist in furan, alkoxy, carbonyl, and ester groups, while nitrogen atoms primarily exist in pyridine and amidogen.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant 51674223), the Key Research and Development Program of Shandong Province (grant 2017GGX70103), the Research Award Fund for Outstanding Middle-Aged and Young Scientists of Shandong Province (grants ZR2019BB050, ZR2021QB211, and BS2015NJ001), the Fund of Molecular Coal Chemistry Youth Technical Innovation Team of Shandong Province, the Opening Fund of State Key Laboratory of Heavy Oil Processing (grant SKLOP202003003), the Qingtan Scholar Talent Project of Zaozhuang University, the Science and Technology Development Project of Zaozhuang (2021GX04), the Research Foundation of Zaozhuang University, the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (grant no. 2021-K09), the Foundation of Zaozhuang Key Laboratory of Green Catalysis and Application (2021-55), the Natural Science Foundation of Shandong Province (grant no. ZR2021MB011), the Shandong Postdoctoral Innovation Project Special Fund (grant no. 202103069), the Jiangsu Postdoctoral Research Support Program (2021K580C), and the China Postdoctoral Science Foundation (grant no. 2021M691983).

Glossary

Nomenclature

APCI

atmospheric pressure chemical ionization

CDS

carbon disulfide

DBE

double bond equivalent

EPs

extractable portions

EP1

extractable portion 1

EP2

extractable portion 2

GC/MS

gas chromatography/mass spectrometry

NCOCs

nitrogen-containing organic compounds

Orbitrap MS

Orbitrap mass spectrometer

RA

relative abundance

SCOCs

sulfur-containing organic compounds

XL

Xilinguole lignite

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c05051.

  • Instrument parameters and analysis conditions of analyzing EP with GC/MS and quadrupole-Orbitrap MS; total ion chromatogram of EP1; molecular mass and DBE distribution of the species identified in EP1 with Orbitrap MS in both positive ion mode and negative ion mode APCI sources; and list of compounds detected in EP1 with GC/MS (PDF)

Author Contributions

⊥ S.L. and F.-F.D. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao2c05051_si_001.pdf (239.6KB, pdf)

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Supplementary Materials

ao2c05051_si_001.pdf (239.6KB, pdf)

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