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Application of Gas

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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/

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    which generate Lingwu coal through coalification. Alternatively, they are possible products from methyl esterification of alkanoic acids during the coal-forming process. Vitrinite and inertinite are two typical coal macerals. Vitrinite structure basically

    34represents structural feature of corresponding bituminous coal, whereas inertinite is

    35considered to be aromatic species-rich moiety in coals. Zhao et al. examined the

    difference in chemical composition of CS-extractable fraction between vitrinite and 2

    inertinite from Shenfu-Dongsheng (SD) and Pingshuo (PS) coals using a GC/MS

    3system. More aliphatic moiety, epoxide and ester moiety were found in vitrinite-rich samples (VRSs) than inertinite-rich samples (IRSs) for both coals, whereas the contents of free and associated hydroxyl groups in IRSs were higher than VRSs. The analysis results of GC/MS also indicated significant difference in chemical composition of CS-extractable fractions from two coals. For example, C-O (alcohols) 2

    moiety was identified in PS coal but almost no such moiety in SD coal. The number of normal alkanes in VRSs is much less than that in IRSs from SD coal, whereas that in VRSs and IRSs from PS coal is almost the same. Coal processing technologies, especially value-added utilization under mild conditions will be guided by such fundamental researches on chemical composition and structure of coals revealed by GC/MS.

    3. Liquefaction

    36Coal liquefaction is a promising process for clean and effective utilization of coal.

    It is necessary to understand the mechanism for coal liquefaction on the molecular level for developing economical coal liquefaction processes. The reactivity of coals 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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    4toward liquefaction is related to the chemical structure of coals. Macromolecules in

    coals contain polycyclic aromatic and hydroaromatic rings as structural units

    37-38connected by methylenic, ethylenic and ether linkages, thus cleaving these

    linkages lead to the macromolecules depolymerization and subsequently induce coal

    39144041-424243liquefaction. Catalysts such as Ni, Ni-S, Pd/C, Fe, activated carbon and

    44solid superacid applied in coal liquefaction process have received remarkable attention. Compounds released from coals matrix after catalysis are identified by subsequent fractional extraction and GC/MS analysis. Researches on the relationship between coal structure and reactivity are very helpful for explaining the mechanism of

    45coal liquefaction and developing effective and feasible coal liquefaction process.

    Activated carbon (AC), a metal free material, was studied in-depth by Wei and

    15, 43, 46-48coworkers as a catalyst employed to hydroconversions. It was found that AC

    selectively catalyzed the hydrogenation of some polycyclic arenes and the reactivity of the arenes toward hydrogenation was dependent on the hydrogen-accepting ability

    43and adsorption strength of the arenes on the catalyst surface. Sun et al. use GC/MS

    to investigate the molecular-level determination for AC catalyzed monatomic

    46hydrogen transfer to α,ω-diarylalkanes. It revealed that, under mild conditions,

    selective cleavage of C-C bridges could occur during a process of hydrogen transfer to coals over an activated carbon. From these results, AC may become a good candidate for synthesizing special chemicals as well as for obtaining valuable aromatics and hydroaromatics from coals.

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     Microwave irradiation is well-known as accelerating numerous organic reactions

    49at mild conditions. Such a technique was also applied to hydrogenation reactions inducing a much lower temperature and hydrogen pressure compared to reactions

    44, 50-51under conventional heating conditions. Fifty six organic species were detected

    in hydrotreated demineralized coal liquefaction residues by Wang et al. using GC/MS 51 and ten of them were listed in Table 2. Molecular compositions of the extractable fractions were analyzed with GC/MS. Microwave-assisted hydrogen transfer to

    50anthracene and phenanthrene over Pd/C was investigated by Ma et al. Under

    microwave irradiation, catalysis reactions involving both monatomic and biatomic hydrogen transfer can significantly proceed even at low temperatures and hydrogen pressures.

    4. Oxidation

    52Oxidation was used as a pretreatment for the investigation of coal structure and

    53obtaining organic acids from coals . A large amount of carbon dioxide was

    generated in the severe oxidation of coals, inducing obstacle for the study on coal structure and production of organic acids. Mild oxidation of coals followed by GC/MS detection was recently paid great attention for coal investigation and

    54-55utilization to provide molecular-level determination for organic compound.

    5512, 5612 HO, RuO and NaOCl were effective oxidants used by our group for 224

    selective degradation of macromolecular network in coals to figure out the composition of organic species in coals. Huang et al. examined the differences in structural features among Shenfu coal (SFC), its liquefaction residue and extraction 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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    56residue through ruthenium ion-catalyzed oxidation (RICO). Eight seven kinds of

    alkanoic acids and benzene polycarboxylic acids were produced from RICO of SFC and its residues and identified by GC/MS. Ten of them are listed in Table 3. Yao et al.

    12compared SFC oxidation using RuO and NaOCl. The products from the oxidation 4

    of Shenfu coal with NaOCl are more complex and contain much more

    chloro-substituted species than those with RuO. The soluble species obtained with 4

    RuO are rich in long-chain alkanoic acids and alkane-α,ω-dicarboxylic acids, 4

    whereas those with NaOCl consist of considerable amounts of short-chain hloro-substituted alkanoic acids and alkanes. c

    5. Conclusions

    Understanding the structure and composition of molecules in coals is important for effective, clean and value-added utilization of coals. Coal treatments such as extraction, liquefaction and oxidation can obtain small molecules in coals to be identified using GC/MS. The application of GC/MS to identify organic species in coals should be considered as a high impressive method, which is used to examine coal structure and distinguish the composition of small molecules scattered in the macromolecular network.

    Acknowledgments

    This work was supported by Natural Science Foundation of China (Projects 20676142 and 20936007), the Fund from the Natural Science Foundation of China for Innovative Research Group (Grant 50921002), the Special Fund for Major State Basic Research Project (Project 2011CB201302), the Program of the Universities in Jiangsu 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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    Province for Development of High-Tech Industries (Project JHB05-33), the Fundamental Research Funds for the Central Universities (China University of Mining and Technology; Grant 2011QNA22) and jointly financed by National Natural Science Foundation of China and Shenhua Group Corporation Limited (Grant 51134021).

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    农业仪器权威门户http://www.zgny17.com/ Table 1. Organobromines and organochlorines identified using GC/MS

    Organobromines 2,4-dibromophenol; 2,4,6-tribromophenol

    Organochlorines o-dichlorobenzene; 2,4,6-trichloroaniline;

    3,3’,4,4’,5,5’-hexachlorobiphenyl;

    2-chlorocyclohexanol; 4-chlorobenzophenone; 2-chloroethyl

    palmitate

Table 2. Ten species identified in hydrotreated demineralized coal liquefaction residues

    No. Compound

    1 1,2-Dipropyldisulfide

    2 Ethoxytetrahydrofuran

    3 C4-Benzene

    4 Naphthalene

    5 C5-Benzene

    6 Propoxytetrahydrofuran

    7 C2-2,3-Dihydroindene

    8 Methylnaphthalenes

    9 3-(Pent-4-enyloxy)tetrahydrofuran

    10 Hexa-3,5-dienyloxytetrahydrofuran

Table 3. Ten alkanoic acids and benzene polycarboxylic acids produced from RICO of SFC and

    its residues

    No. Compound

    1 Butanoic acid

    2 2-Methylbutanoic acid

    3 4-Oxopentanoic acid

    4 Succinic acid

    5 3-Methylheptanedioic acid

    6 Tetracosanedioic acid

    7 Propane-1,2,3-tricarboxylic acid

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