English

• 

  Coal pyrolysis char (CPC) mainly refers to the solid products which were obtained by destructive distillation of the Jurassic non-stick coal and weakly caking coal at 500-700 ℃^[1]. In China, the annual output of CPC is approximately 50 Mt, 10% of which is char powder (CP)^{[2, 3]}. CP is currently either directly placed in the open air or simply treated as fuel for combustion because of the difficulty in using CP to produce iron alloy, calcium carbide, silicon iron, silicomanganese, and fertilizer, leading to serious environmental pollution and huge waste of energy^[3-5]. Hence, growing attention was paid to the efficient utilization of CP^[3-7]. Meanwhile, it is very necessary to understand the properties of CP on molecular level.

  Extraction is a separable and nondestructive way to isolate organic species from coals and their derivates^{[8, 9]}. Especially, fractional extraction was widely used to separate group components^{[10, 11]} and trace species^{[12, 13]}. Soxhlet extraction was also generally applied in extracting organic matter (OM) from coals or their derivates^{[14, 15]} and vegetable oil from oleaginous materials^{[16, 17]}. The extracts can be identified with various analytical instruments, such as gas chromatograph/mass spectrometer (GC/MS), which is most commonly used to detect organic species in coals^{[8-14, 18, 19]}. Nevertheless, GC/MS can only analyze thermally stable and relatively volatile molecules.

  Atmospheric solid analysis probe/time-of-flight mass spectrometer (ASAP/TOF-MS) can broaden detection range because of its good response to larger molecules^{[6, 18]} and provide in-situ analysis of samples with mid-polarity in various phases without complex pretreatments^[20]. ASAP has been successfully applied for identifying steroids^{[20, 21]}, contaminants^[22], drugs^[23], crude oils^[24], and polymers^{[25, 26]}. However, few reports focused on the compositional features of extractable species from chars by analysis with ASAP/TOF-MS.

  As a soft ionization technology, electrospray ionization (ESI) is suitable for detecting strongly polar and thermally labile molecules without decomposition during ionization process. ESI equipped with a liquid chromatograph and time-of-flight mass spectrometer has been successfully used to identify phenolic compounds from olive oils^[27-29], drugs^[30], and metabolites^{[31, 32]}. However, almost no reports were issued on analyzing coals and their derivates with ESI/TOF-MS.

  In the present work, we tried to understand compositional features of extractable species in Shenmu char powder (SCP) by analyzing the extracts from SCP with GC/MS, ASAP/TOF-MS, and ESI/TOF-MS.

  1   Experimental

  1.1   Materials

  SCP was collected from Shenmu County, Shaanxi, China. It was pulverized to pass through a 200-mesh sieve (particle size of < 74 μm) followed by desiccation in a vacuum oven at 80 ℃ for 24 h prior to use. Table 1 lists the proximate and ultimate analyses of SCP. Petroleum ether (PE), carbon disulfide (CDS), dichloromethane (DCM), acetone, and methanol used in the experiments are analytical reagents and were distilled before use.

  Table 1.  Proximate and ultimate analyses of SCP

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  Proximate analysis w/%				Ultimate analysis wdaf/%
  Mad	Ad	Vdaf		C	H	N	S	Oa
  5.11	21.23	12.15		78.29	1.22	0.87	0.56	19.06
  a: by difference

  Table 1.  Proximate and ultimate analyses of SCP

  1.2   Experimental procedure

  As Figure 1 displays, 4 g SCP was sequentially and exhaustively extracted with 100 mL of PE, CDS, DCM, acetone, and methanol in a Soxhlet extractor to obtain extracts 1-5 (E_1-5) and final residue. Each extraction was conducted at a temperature near the solvent boiling point for 10 days. Each extract yield (Y) was calculated as the mass ratio of the extract (m_E) to the SCP sample on dry and ash-free base (m_{SCP, daf}).

  Figure 1.  Procedure for fractional extraction of SCP and subsequent analyses

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  1.3   Analytical methods

  All of the extracts were analyzed with a Hewlett-Packard 7890/5975 GC/MS, IonSense/Agilent 6210 ASAP/TOF-MS, and Agilent 6210 ESI/TOF-MS. The GC/MS is equipped with a capillary column coated with HP-5MS (cross-link 5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, 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. The capillary column was heated at 5 ℃/min from 60 to 300 ℃ and held for 5 min. Data acquired were processed using Chemstation software. Compounds were identified by comparing mass spectra with NIST11 library data. Mass resolution of both ASAP/TOF-MS and ESI/TOF-MS is up to 40 000 and its mass accuracy is 3×10^-6. Mass spectral data were processed using MassHunter software for analyses with both ASAP/TOF-MS and ESI/TOF-MS. For analysis with ASAP/TOF-MS, the corona discharge current and capillary voltage were set to 4 μA and 4 kV, respectively. The hot nitrogen stream and drying gas temperatures were set to 250 and 350 ℃, respectively. The operation was run in positive mode and the m/z range was set from 100 to 1 000. For analysis with ESI/TOF-MS, the source voltage, drying gas temperature, drying gas flow rate, spray gas pressure, and evaporator temperature were set to 4.0 kV, 350 ℃, 10 L/min, 30 psig, and 350 ℃, respectively. The samples were analyzed in positive ion mode.

  2   Results and discussion

  2.1   Extract yields

  Chars are usually regarded to be nonvolatile, so the extractable species (ESs) in chars were paid little attention^[33]. As Figure 2 shows, the yields of E_1-5 are 0.28%, 0.25%, 0.08%, 0.56%, and 0.59%, respectively, and in total, only 1.76% of OM was extracted from SCP. Although the amount of ESs is low, as a part of chars, ESs also affect the quality of chars. Therefore, understanding the compositional features of the ESs is necessary.

  Figure 2.  Yields of the extracts from SCP

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  2.2   Analysis of the extracts with GC/MS

  As shown in Figures S1-S5 (omitted) and Tables S1-S5 (omitted) in the supporting information, 127, 173, 46, 155, and 108 organic compounds were identified in E_1-5, respectively. They can be grouped into alkanes, arenes, alkenes, and heteroatom-containing organic species (HACOSs).

  As Figure 3 displays, alkanes are predominant in E₁ and arenes are the main compounds detected in E₂, while most of detected species in E_3-5 are HACOSs. The twist of PE with the alkanes in E₁ could be responsible for effectively extracting the alkanes. The effectiveness of CDS for extracting the arenes in E₂ could be ascribed the π-π interactions between CDS and the arenes, while hydrogen bonds and hydrogen bond/π-π complex interactions among other solvents (DCM, acetone, and methanol) and HACOSs in E_3-5 could be the main reason for enriching the HACOSs into the solvents.

  Figure 3.  Distribution of group components in the extracts according to GC/MS analysis

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  In total, 18 normal alkanes (C₉ and C_14-31) were detected in E₁ and most of them are C_15-24 (Figure 4). The arenes with 3 rings are the most abundant in E₂ and carbon number of the longest alkyl substituents of arenes decreases with increasing ring number and the maximum carbon number is 10 (Figure 5).

  Figure 4.  Distribution of normal alkanes in E₁ according to analysis with GC/MS

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  Figure 5.  RC and carbon number of the longest alkyl substituents of arenes in E₂

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  The HACOSs detected in the extracts can be classified into organooxygen, organonitrogen, and organosulfur compounds. As Figure 6 demonstrates, organooxygen compounds (OOCs) are predominant in HACOSs, which corresponds to the high oxygen content of SCP shown in Table 1. The OOCs include alcohols, ketones, carboxylic acids, esters, alkoxybenzenes, furans, dioxolanes, and other OOCs. Among them the first four are the main existence forms. The RCs of both organonitrogen compounds (ONCs) and organosulfur compounds (OSCs) are very low in each extract except E₃, in which the RC of ONCs is near 40%.

  Figure 6.  Distribution of HACOCs in the extracts

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  Few reports paid attention on elemental sulfur in coals and their derivates^[34-36]. However, in total 9 sulfur-containing species were detected in the extracts, including 7 OSCs (peaks 1, 2, 4, and 5-9) and 2 elemental sulfurs (peaks 3 and 5, detected in E₂ and E₅), as displayed in Figure S6 (omitted) and Table 2. Some of the OSCs contain oxygen (peaks 1, 7, and 9) and nitrogen (peaks 8 and 9).

  Table 2.  Sulfur-containing species detected in the extracts with GC/MS

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  Peak	Name	Detected in
  		E1	E2	E3	E4	E5
  1	2, 6-dimethyl-4H-pyran-4-thione				×
  2	3-methyl-2, 3-dihydrobenzo[ b]thiophene			×
  3	hexathiane		×			×
  4	dibenzo[ b, d]thiophene		×
  5	octathiocane					×
  6	octahydro-2 H, 2' H-4, 4'-bithiopyran			×
  7	1, 3, 2-benzodioxathiole 2-oxide	×
  8	1, 2, 3, 4-tetrahydropyrimido[4', 5':4, 5]thieno[2, 3-c]isoquinolin-8-amine		×
  9	(Z)-2-(3, 6-dimethylbenzo[ d]thiazol-2(3 H)-ylideneamino) ethanol	×

  Table 2.  Sulfur-containing species detected in the extracts with GC/MS

  2.3   Analysis of E_1-5 with ASAP/TOF-MS

  The ionization process of ASAP is similar to that of atmospheric pressure chemical ionization proposed by Petucci et al^[37]. The nitrogen cation radical (N₂^+·) is formed via the corona discharge, then N₂^+·reacts with other N₂ to generate N₄^+·. The N^+· ₄can react with trace water vapor to generate H₃O^{+[18, 38]}. Then the samples suffer from proton transfer with either H₃O⁺ or H⁺(H₂O)_n to create [M+H]⁺ or charge transfer with N₂^+·/N₄^+·to obtain M^+·. As Figure S7 (omitted) exhibits, the molecular masses of the extracts are largely up to 540 u. The detected species can be classified into hydrocarbons and HACOSs, including oxygen-, nitrogen-, sulfur-, and/or chlorine-containing species, which are predominant in the detected species (Figure 7) because ionizing HACOSs is much easier than ionizing hydrocarbons. Most of the HACOSs were not detected with GC/MS, implying that ASAP/TOF-MS is much more effective than GC/MS for analyzing HACOSs. The molecular formulae of OSCs can be determined as C₁₃H₁₄S, C₁₆H₃₄S, and C₂₆H₅₀S according to their m/z values of 202.081 10, 258.238 15, and 395.370 53, respectively. They could be methyl-1, 2, 3, 4-tetrahydrodibenzo[b, d] thiophene (C₁₃H₁₄S), diotylsulfane or hexadecanethiol (C₁₆H₃₄S), and hexacosadienethiol or ditridecenylsulfane (C₂₆H₅₀S).

  Figure 7.  Distribution of group components detected in the extracts according to analysis with ASAP-TOF/MS.

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  2.4   ESI/TOF-MS analysis of E_1-5

  ESI has also a good response to polar molecules^[39]. As Figure S8 (omitted) and Table S6 (omitted) exhibit, few species were detected in E₁, since most of the species in E₁ are alkanes and their ionization with ESI is difficult. O₁ class species are the most abundant in other extracts. Species with m/z 389+14n (n=0-2) and m/z 447+14n(n=0-2) could be assigned to alkylnaphthalenetetraols, dioxoalkylbenzoates, or alkyltetralindicarboxylates with formula C₂₄H₃₆O₄(CH₂)_m (m=0-2) and alkoxycarbonylphenyoxoalkanoic acids or dioxoalkylbenzoates with formula C₂₁H₃₁O₂(CH₂)_m(COOH) (m=0-2), respectively. Such species were not detected with GC/MS because of their less volatility and stronger polarity.

  As Figure 8 shows, ONCs are predominant in E_2-5, especially the N₁ class species with the RCs of 61.0%, 52.4%, 59.2%, and 39.4%, respectively. According to the possible ionization mechanism at positive mode^[39], species with m/z 136+14n (n=0, 3-7, 11-12, 15-18) in E₂ may be attributed to C_m-pyridine (m=4, 7-11, 15-16, 19-22) (Figure S8 (omitted)). Species with m/z 188+14n (n=0, 3-12) may belong to C_m-dienylpyridine or C_m-ynylpyridine (m=8, 11-20) or polycyclic aromatics (Figure S9-a (omitted)). Species with m/z 176+14n (n=0, 3-10, 12-13, 15) could be C_m-enlylpyridine (m=7, 10-17, 19-20, 22) or polycyclic aromatics (Figure S9-b (omitted)). In addition, species with m/z 214+14n (n=0, 2-6) in mass spectrum of E₃ may be assigned to C_m-quinoline or C_m-naphthylamine (m=6, 8-12), which were not detected with GC/MS for their stronger polarity.

  Figure 8.  Distribution of group components detected in the extracts according to analysis with ESI-TOF/MS.

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  3   Conclusions

  The compositional features of the extracts from SCP were analyzed by GC/MS, ASAP/TOF-MS, and ESI/TOF-MS. On the basis of GC/MS analysis, the extracts can be classified into alkanes, arenes, alkenes, and HACOSs. The extracts are rich in alkanes and HACOSs. The HACOSs include OOCs, ONCs, and OSCs, and OOCs are the most abundant. Only TOSCs and 2 elemental sulfurs (S₆ and S₈) were detected in the extracts. N, O, NO, NOS, and NOCl class species are predominant in E_1-5 according to ASAP/TOF-MS analysis which has good response to mid-polar compounds. ESI/TOF-MS analysis can provide an effective approach for characterizing species with medium and strong polarity. Therefore, a series of alkylnaphthalenetetraols or dioxoaklylbenzoates, dioxoalkylbenzoates, C_m-enlylpyridines, C_m-dienylpyridines or C_m-ynylpyridines, C_m-chloroenes or C_m-chlorocycloanes, and so on, were identified in E_1-5, which were not detected with GC/MS and ASAP-TOF/MS.

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• 

  Figure 1  Procedure for fractional extraction of SCP and subsequent analyses

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  Figure 2  Yields of the extracts from SCP

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  Figure 3  Distribution of group components in the extracts according to GC/MS analysis

  : HACOSs; : arenes; : alkenes; : alkanes

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  Figure 4  Distribution of normal alkanes in E₁ according to analysis with GC/MS

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  Figure 5  RC and carbon number of the longest alkyl substituents of arenes in E₂

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  Figure 6  Distribution of HACOCs in the extracts

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  Figure 7  Distribution of group components detected in the extracts according to analysis with ASAP-TOF/MS.

  
  
  

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  Figure 8  Distribution of group components detected in the extracts according to analysis with ESI-TOF/MS.

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  Table 1.  Proximate and ultimate analyses of SCP

  Proximate analysis w/%				Ultimate analysis wdaf/%
  Mad	Ad	Vdaf		C	H	N	S	Oa
  5.11	21.23	12.15		78.29	1.22	0.87	0.56	19.06
  a: by difference

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  Table 2.  Sulfur-containing species detected in the extracts with GC/MS

  Peak	Name	Detected in
  		E1	E2	E3	E4	E5
  1	2, 6-dimethyl-4H-pyran-4-thione				×
  2	3-methyl-2, 3-dihydrobenzo[ b]thiophene			×
  3	hexathiane		×			×
  4	dibenzo[ b, d]thiophene		×
  5	octathiocane					×
  6	octahydro-2 H, 2' H-4, 4'-bithiopyran			×
  7	1, 3, 2-benzodioxathiole 2-oxide	×
  8	1, 2, 3, 4-tetrahydropyrimido[4', 5':4, 5]thieno[2, 3-c]isoquinolin-8-amine		×
  9	(Z)-2-(3, 6-dimethylbenzo[ d]thiazol-2(3 H)-ylideneamino) ethanol	×

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