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Application of Gas

By Evelyn Williams,2014-11-14 12:22
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Application of Gas

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    Application of Gas Chromatography/Mass

    Spectrometry in Studies on Separation and

    Identification of Organic Species in Coals Abstract: The use of gas chromatography/mass spectrometry (GC/MS) in the investigation of coal structure and identification for composition of small molecules was reviewed. Macromolecules and small molecules coexist in coals. Understanding the role of small molecules is the fundamental for effective, clean and value-added utilization of coals. For coals after treatments (extraction, oxidation and liquefaction), small molecules can be extracted from the macromolecular network and analyzed using GC/MS. Various kinds of organic species were identified using GC/MS, which gave explanation for coal structure, mechanism of coal liquefaction and oxidation. Keywords: GC/MS, extraction, liquefaction, oxidation, organic species, coal 1. Introduction

    As a minable and non-renewable natural resource, coal has extremely complex

    1-3structures and consists of a wide range of organic species. Coals at different ranks

    have various molecular compositions and structures. Even within the same rank, the structures of coal may be heterogeneous depending on the environmental conditions

    4of coalification. Understanding the structure and composition of molecules in coals is

    5important for effective, clean and value-added utilization of coals. Studies have been

    undertaken with the goal of understanding the coal structure and molecular 地址;杭州市西湖科技园西园八路11 联系电话;0571-81953317 http://www.zgny17.com/

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    6-8composition. Although the structures of coals are under debating, it is widely agreed that low molecular weight compounds (small molecules) and macromolecules

    7, 9coexist in coals. Macromolecules consist of a skeletal network with small

    molecules scattered inside, which can be separated from the macromolecular skeleton

    101112with extraction, liquefaction and oxidation etc.

    Lots of efforts have been contributed to separation and characterization of organic

    10, 13compounds in coals. For example, knowledge of the molecular composition of the organic heteroatomic species in coals leads to effective removal of harmful species

    14-16such as organic nitrogen, sulfur and halogens in coals. For raw coals and coals

    after treatments (pyrolysis, oxidation or liquefaction), small molecules can be extracted from the macromolecular network and analyzed using different methods to

    16-18reveal the coal structure. It was found that organic species extracted by solvents were mainly compounds with low molecular weight and high molecular mass

    17molecules left in the residual coals. Moreover, the agglomerating and plasticizing

    19capacities of residual coals changed after extraction. Therefore it is particularly

    important to understand the role of components with small molecular weight in coals, which is the fundamental for effective, clean and value-added utilization of coals. The separation and identification of organic species in coals are still hard and challenging work due to the complexity in structure and composition of coals. Chromatography coupled with mass spectrometry has been developed for characterizing components in coals and proved a very powerful tool for unknown

    20-21species identification. Unlike Liquid chromatography/mass spectrometry

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    (LC/MS), gas chromatography/mass spectrometry (GC/MS) is especially suitable for small, volatile and thermally stable molecules whatever the molecular polarity is high

    22-23or low. Molecules in coals mainly are straight alkyl groups, branched alkyl groups

    6, 13, 24and aromatic species, which have a low molecular polarity. Although the

    molecular mass measured by GC/MS is limited to 400-500 Da, it plays a crucial role in the investigation of coal structure and identification for composition of small

    17, 25-27molecules because of the low molecular polarity of organic species in coals.

    LC/MS can not realize the same function as well as GC/MS due to the “soft”

    28-29ionization design of corresponding ion source. Different methods such as solvent

    101112extraction, liquefaction and oxidation etc., were used to obtain small molecules

    in coals. GC/MS were applied to investigate the composition structure and molecular weight distribution of these small molecules, and finally identify the small molecules occurring in the process of solvent extraction, liquefaction and oxidation. Previous studies on the characterization of small molecules in coals using GC/MS accomplished by our group were addressed in the present article for better understanding coal structure and effective utilization of coals in future. 2. Extraction

    Nondestructive solvent extraction of coal is the extraction of soluble organic species from coal under mild conditions where thermal decomposition does not occur.

    3031Although destructive and inseparable analyses have been applied for the

    investigation of coal structure, they cannot provide an effective approach to the understanding of the molecular composition of the organic species in coals as 地址;杭州市西湖科技园西园八路11 联系电话;0571-81953317 http://www.zgny17.com/

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    nondestructive solvent extraction does. The extraction solvent is well mixed with coal

    powders to allow soluble organic compounds to transfer to the solvent. The solvent properties, extraction conditions and coal type determine the constituents of extract

    32liquor and residual coal. Undestroyed structure and molecular composition of

    soluble small organic molecules were identified in the following GC/MS detection process.

    For the first time, Wei and co-workers used a separable and nondestructive method, including fractional extraction and GC/MS analysis, to obtain the molecular-level

    20determination for organohalogens in coals. Four bituminous coals and seven

    extraction solvents were used in the work, resulting 28 extracts. Two organobromines (OBs) and six organochlorines (OCs) were identified in eight extracts from the coals and listed in Table 1. The mass range of the mass spectrometer used in the work was from 30 to 500 Da.

    There is an argument about the real source of dialkyl phthalates (plasticizers and additives) detected in coal extracts. Liu et al. isolated and identified these species

    33from Lingwu coals. By the extraction with methanol and subsequent column

    chromatography separation as well as GC/MS, FTIR and NMR analysis with great care, dialkyl benzenedicarboxylates, including a dialkyl phthalate and a dialkyl

    10terephthalate were identified. In another work of Liu et al., methyl alkanoates (MAs)

    extracted from Lingwu coal were identified using GC/MS. It was the first reported work on isolation of MAs from any coals. The MAs may originate from some plants 地址;杭州市西湖科技园西园八路11 联系电话;0571-81953317 http://www.zgny17.com/