Article pubs.acs.org/EF
Enrichment and Identification of Arylhopanes from Shengli Lignite Xing-Shun Cong,†,‡ Zhi-Min Zong,*,† Zhe-Hao Wei,§ Yan Li,§ Xing Fan,† Yin Zhou,† Min Li,∥ Yun-Peng Zhao,† and Xian-Yong Wei† †
Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, People’s Republic of China ‡ Department of Chemistry, Zaozhuang University, Zaozhuang 277160, Shandong, People’s Republic of China § The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States ∥ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, Shandong, People’s Republic of China ABSTRACT: Four arylhopanes, i.e., 22-phenyl-30-norhopane, 22-(o-tolyl)-30-norhopane, 30-(5-methylthien-2-yl)hopane, and 30-(thien-2-ylmethyl)hopane, were enriched from Shengli lignite, and their structures were tentatively identified by gas chromatography/mass spectrometry. Phenylhopanes were unreported before and thienylhopanes were rarely detected in coals. The nearly equal relative contents of 22-phenyl-30-norhopane and 22-(o-tolyl)-30-norhopane suggest that a hopanoid precursor with a longer side chain compared to bacteriohopanetetrol should exist in nature. A novel type of cyclization reaction (cyclized at C30) of hopanoid polyols was proposed.
1. INTRODUCTION Hopanoids, including geohopanoids and biohopanoids, are the most abundant family of complex organic substances in nature and play an important role in organisms, geochemistry, and petroleum exploration.1−3 As shown in Figure 1, bacteriohopanetetrol (1) can be cyclized at C20 or C16. It is well known that benzo[20,21]hopanes4−7 (C32−35, 2) and benzo[16,17,21]hopanes6,8,9 (C31−35, 3) are formed from compound 1 via the cyclization reactions at C20 and C16, respectively. Some compounds with fluorene and acenaphthene moieties (e.g., compounds 4 and 5) in coals should be partially produced from D-ring monoaromatic 8(14)-secobenzo[20,21]hopanes6,10,11 (C 32−35 , 6) and 8(14)-secobenzo[16,17,21]hopanes 11,12 (C31−35, 7), which are produced from the corresponding benzohopanes (i.e., compounds 2 and 3) via aromatization and C8−C14 bond cleavage, respectively. Coals are rich in the compounds with two aromatic moieties linked by −(CH2)n−, such as 1-benzylnaphthalene (8), but their origin is still unclear. Identifying 22-phenyl-30-norhopane (9) and 22-(o-tolyl)-30-norhopane (10) and understanding their cyclization process may provide some clues to their origin at the molecular level. Organosulfur compounds (OSCs) in coals are the main source of air pollutants during coal combustion.13,14 Hence, understanding of the types of OSCs in coals is of great importance for sulfur removal and subsequent clean use of coals.13 A number of researchers investigated the types of OSCs by various analytical methods,15−20 and many OSCs, such as cyclohexanethiol,16 benzenethiol,17 alkyldibenzothiophenes,17,18,21 and alkylbenzonaphthothiophenes,15,21 were detected from coals. However, to our knowledge, few reports were issued on the isolation and identification of sulfur-containing hopanoids, including 30(5-methylthien-2-yl)hopane22 (11) and 30-(thien-2-ylmethyl)hopane23 (12), from coals. © 2014 American Chemical Society
In the present investigation, we detected four hopanoids with an aromatic ring on the side chain in the extract from Shengli lignite (SL) and proposed a novel type of cyclization reaction of hopanoid polyols.
2. EXPERIMENTAL SECTION SL was collected from Shengli coalfield, Xilinhaote, Inner Mongolia, China, and pulverized to pass through a 200-mesh sieve. The proximate and ultimate analyses of SL were described elsewhere.24,25 About 1.5 kg of SL was exhaustively extracted with isometric carbon disulfide/acetone mixed solvent at room temperature to obtain the extract (ca. 31.8 g) in the yield of 2.9% on a dry and ash-free basis. About 4.2 g of the extract was transferred into a silica gel-packed column (3.6 cm internal diameter and 46 cm height) and subsequently fractionated with a Bü chi preparative medium-pressure liquid chromatography system (eluted with petroleum ether). Every 50 mL of the effluent solution (ES) was collected into a vial. All of the ESs were analyzed with a Hewlett-Packard 6890/5973 gas chromatograph/ mass spectrometer, which is equipped with a HP-5MS capillary column, an electron ionization source operated at 70 eV, and a quadrupole mass analyzer scanned from 33 to 550 amu. The injection and ion source temperatures were set at 250 and 230 °C, respectively. The column was heated from 60 to 300 °C at 30 °C/min and held at 300 °C for 28 min. The ESs with similar composition were incorporated as one fraction. Two adjacent ESs (100 mL) were incorporated and then concentrated as eluted fraction 1 (EF1), and another ES (50 mL) was concentrated as eluted fraction 2 (EF2).
3. RESULTS AND DISCUSSION 3.1. Identification of Arylhopanes. As Figure 2 shows, two phenylhopanes (PHs, peaks 9 and 10) and two thienylhopanes (THs, peaks 11 and 12) were detected in EF1 Received: May 15, 2014 Revised: October 3, 2014 Published: October 6, 2014 6745
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Figure 1. Some structures mentioned in the text.
Figure 2. Total ion chromatograms of EF1 and EF2.
Figure 3. Mass spectra of four arylhopanes. 6746
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Figure 4. Possible formation pathways of compounds 9, 10, and 12.
Compound 11 is tentatively identified as 30-(5-methylthien2-yl)hopane via the characteristic ions and retention time. 3.2. Possible Formation Pathways of Arylhopanes. As shown in Figure 4, compound 9 should be formed from compound 1 by cyclization and subsequent dehydration. Identifying the precursor of 22-tolyl-30-norhopane (TNH) is difficult because a C36-hopanoid precursor with polyhydroxyls like compound 1 has not been detected until now. There are at least two possible pathways for TNH formation: via intermolecular dehydration (IMDH) between a cyclized intermediate of compound 1 and methane or the reaction of an unknown C36-hopanoid polyol. Because IMDH is more difficult than intramolecular dehydration, the relative content (RC) of TNH should be obviously lower than that of compound 9 if TNH was produced by IMDH. In fact, the RCs of compound 9 (35.8%) and TNH (33.9%) are nearly equal, indicating that two PHs underwent similar or the same formation pathways and a C36-hopanoid precursor with polyhydroxyls like compound 1 should exist in nature. The detection of homohopanes27,28 (> C35) also indicates that some polyhydroxy-hopanoid precursors with a longer side chain compared to compound 1 must exist in nature. According to the possible precursor and formation pathways, compound 10 should be 17β(H),21β(H)-22-(o-tolyl)30-norhopane.
(ca. 0.3 mg) and EF2 (ca. 0.1 mg), respectively. As exhibited in Figure 3, the peaks at m/z 191 and 369 of compound 9 indicate that compound 9 belongs to hopanoids and has a intact 22,29,30-trinorhopane moiety. The base peak at m/z 105 is typical 1-phenylethan-1-yl ion or benzylidyneoxo ion. The absence of fragmental ion (FI) at m/z 77 indicates that the most abundant FI (base peak at m/z 105) is 1-phenylethan-1-yl ion. The peak at m/z 191 has a lower intensity than that at m/z 253, suggesting that compound 9 could be 17β(H),21β(H)-22phenyl-30-norhopane. The increments by 14 units comparing peaks at m/z 105, 253, and 474 of compound 9 to those at m/z 119, 267, and 488 of compound 10 indicate that compounds 9 and 10 belong to the same family of hopanoids and that compound 10 has a methyl on the benzene ring. Compound 10 could be 17β(H),21β(H)-22-tolyl-30-norhopane because the intensity of FI at m/z 267 is higher than that at m/z 191. Valisolalao et al.22 first detected 30-(thien-2-ylmethyl)17β(H),21β(H)-hopane in immature sediments and fully identified it by synthesis. In comparison to their work, we identified compound 12 as 30-(thien-2-ylmethyl)-17β(H),21β(H)-hopane. In 1989, Sinninghe Damsté et al.23 first detected 30-(5-methylthien-2-yl)hopane in oil shale bitumens, and then Olivella et al.26 detected it in a sulfur-rich coal. However, a detailed mass spectrum of the compound was not provided. 6747
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Sulfur-containing cyclized hopanoids should be formed by inorganic sulfur (IOS) incorporation into hopanoid precursors. Gug et al.29 and Schouten et al.30 proposed a formation pathway of OSCs by sulfur incorporation into polyenes and subsquent aromatization. Compound 11 is possibly formed from a hopene precursor via the above pathway. However, compound 1 is more likely the precursor of compound 12 than pentakishomohop-32(33),34-diene, which possibly did not occur in a large amount in the geosphere. The fact that compound 11 has a obviously lower RC (5.9%) than that of compound 12 (52.9%) supports our analysis. The detection of THs proved the transformation of IOS in SL into organic sulfur (OS) at the molecular level.
4. CONCLUSION Two novel PHs and two THs were enriched from SL and tentatively identified by gas chromatography/mass spectrometry. PHs should be produced from hopanoid precursors by a novel type of cyclization reaction, i.e., cyclization at C30. Identifaction of compound 10 with 36 carbon atoms indicates that an unknown C36-hopanoid polyol should be present in nature. Some OSCs in SL should result from the transformation of IOS in SL into OS. The results show that the combination of solvent extraction and preparative mediumpressure liquid chromatography is effective for isolating trace organic compounds from coals. Further work aiming at the full identification the structures of PHs by high-resolution mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance is under investigation.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was subsidized by Natural Basic Research Program of China (Grant 2011CB201302), the Fund from the Natural Science Foundation of China for Innovative Research Group (Grant 51221462), the Key Project of Coal Joint Fund from Natural Science Foundation of China and Shenhua Group Co., Ltd. (Grant 51134021), National Natural Science Foundation of China (Grants 21206187 and 21206188), Strategic Chinese− Japanese Joint Research Program (Grant 2013DFG60060), the Fundamental Research Funds for the Doctoral Program of Higher Education (Grant 20120095110006) and from Zaozhuang University, and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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