Article pubs.acs.org/EF
Characterization of Biomarkers and Structural Features of Condensed Aromatics in Xianfeng Lignite Fang-Jing Liu, Xian-Yong Wei,* Juan Gui, Yu-Gao Wang, Peng Li, and Zhi-Min Zong Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou, Jiangsu 221116, People’s Republic of China S Supporting Information *
ABSTRACT: Xianfeng lignite (XL) was sequentially extracted under ultrasonication at room temperature with petroleum ether, carbon disulfide (CDS), methanol, acetone, and isometric CDS/acetone mixed solvent to afford extracts 1−5, respectively. The mixed solvent-inextractable portion was sequentially extracted with cyclohexane, benzene, 1-methylnaphthalene, methanol, and ethanol at 320 °C to afford extracts 6−10, respectively. The extracts were analyzed with a gas chromatography/mass spectrometer (GC/MS) to characterize biomarkers in XL. The biomarkers were significantly enriched in extracts 1 and 6. They can be classified into a series of n-alkanes, isoprenoid alkanes, terpenoids, n-alkenes, methyl alkanones, n-alkylbenzenes, nalkyltoluenes, and n-alkyl-p-xylenes. The biomarker distributions provided important information on the main origin of organic matter in XL. Related mechanisms for the formation of biomarkers during coalification were discussed. The residue from sequential thermal extraction was subjected to ruthenium-ion-catalyzed oxidation along with subsequent product analyses with GC/MS and direct analysis in a real-time ionization source coupled to a time-of-flight mass spectrometer (DARTIS/TOFMS) to understand its structural features. The results show that the residue is rich in condensed aromatics (CAs) and methyl is the dominant alkyl side chain on aromatic rings. The aromatic rings in the residue are mainly connected by −(CH2)3− and −CHCH3(CH2)2−. DARTIS/TOFMS analysis suggests that CAs with alkyl-substituted biphenyl and alkyl-substituted phenylbiphenyl skeletons also exist in the residue. This investigation provides an effective approach for understanding biomarkers and the structural features of the macromolecular network in XL.
1. INTRODUCTION Coals are an abundant fossil resource and were mainly derived from the remains of higher plants that accumulated in waterlogged environments and underwent various physical and chemical alterations through the coalification process.1 Organic geochemistry plays a crucial role in the exploration and utilization of coals. It involves separation and identification of biomarker components. For coals, biomarkers (i.e., biological markers) generally refer to organic molecules that may be used as indicators for the existence of living organisms during coal formation. The biomarkers have great resemblance in the structure of their parent organic molecules in living organisms and provide significant information on the main input of organic matter, estimation of the coal-forming environment, and determination of the thermal maturity of coals. Elemental analysis and other bulk chemical analyses are still common analytical techniques for characterizing coals.2−5 However, investigations on biomarkers based on separation of soluble organic matter (SOM) from coals in combination with gas chromatography/mass spectrometer (GC/MS) analysis attracted great interests of coal chemists.6−10 According to previous investigations, biomarkers in coals mainly included terpenoids,9,11,12 n-alkanes,13−15 fatty acid methyl esters,14 alkylcyclohexanes,7,16 alkylbenzenes,7,17−20 and methyl alkanones.21,22 Organic geochemical analysis of these compounds in coals could provide clues to the botanical input and their formation. SOM, which contains biomarkers, accounts for a portion of coals. The solvent-insoluble organic matter primarily consists of © 2013 American Chemical Society
macromolecular species, including condensed aromatics (CAs). Understanding structural features of CAs is important for efficient utilization of coals and providing important insights into coal structures. Ruthenium-ion-catalyzed oxidation (RICO) was widely applied to characterize alkyl side chains (ASCs) and methylene bridges connecting aromatic rings in coals,23−26 along with subsequent analyses of the resulting carboxylic acids. Muhammad and Abbott27 investigated RICO of asphaltenes from coals with different ranks and proposed that the distributions of ASCs could be related to the thermal maturity of coals. Xianfeng lignite (XL) is a typical lignite in China and was extensively investigated,28−32 but few reports have been issued on the biomarkers and structural features of CAs in XL. In the present study, we tried to understand the biomarker distributions and structural features of CAs in XL by sequential extraction and subsequent RICO, in combination with analyses of the soluble portions with a GC/MS and a direct analysis in a real-time ionization source coupled to a time-of-flight mass spectrometer (DARTIS/TOFMS).
2. GEOLOGICAL SETTING The Xianfeng coal mine is located 15 km to the northwest of Xundian County, in the northeast of Yunnan Province, China. It contains a large reserve of lignite and diatomite mineral and Received: October 8, 2013 Revised: November 27, 2013 Published: November 27, 2013 7369
dx.doi.org/10.1021/ef402027g | Energy Fuels 2013, 27, 7369−7378
Energy & Fuels
Article
Table 1. Proximate and Ultimate Analyses (wt %) of XLa proximate analysis
ultimate analysis (daf)
Mad
Ad
Vdaf
C
H
N
Odiff
St,d
H/C
33.56
18.45
60.60
63.07
6.01
1.79
>28.73
0.40
1.1356
a
diff, by difference; daf, dry and ash-free basis; Mad, moisture (air-dried basis); Ad, ash (dry basis, i.e., moisture-free basis); Vdaf, volatile matter (dry and ash-free basis); and St,d, total sulfur (dry basis).
is a strike-slip fault basin formed during tertiary. The basin covers an area of about 13 km2, which has a coal seam with a maximum thickness of 180 m. The major coal-bearing stratum is mainly composed of several coal seams, carbon mudstone, and silty mudstone.33
3. EXPERIMENTAL SECTION 3.1. Materials. XL was collected from the Xianfeng coal mine. It was pulverized to pass through a 200-mesh sieve (particle size of
