Energy & Fuels 1995,9, 1003-1010
1003
A Model Structure of Zao Zhuang Bituminous Coal Kazuo Nakamura Fundamental Research Laboratories, Osaka Gas Co., Ltd., 19-9, 6-chome Torishima, Konohana-ku, Osaka 554, Japan
Toshimasa Takanohashi* and Masashi Iino Institute for Chemical Reaction Science, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-77, Japan
Haruo Kumagai, Masaaki Sato, Susumu Yokoyama, and Yuzo Sanada Center for Advanced Research of Energy Technology, Hokkaido University, Kita-ku, Sapporo 060, Japan Received May 16, 1995. Revised Manuscript Received August 8, 1995@
A model structure of Zao Zhuang bituminous coal (Shan Tong province in China, 86.9 w t % C) based on the structural analyses and the computer simulation was constructed. The coal was extracted with pyridine or carbon disulfide-N-methyl-2-pyrrolidinone(CS2-NMP) mixed solvent, and the extracts obtained were further fractionated. The extract fractions were hydrogenated using Adkins catalyst under mild condition a t 430 "Cfor 1h. The structure of the oil (n-hexane solubles) obtained from each extract fraction was analyzed by mass spectrometry after column separation and the molecular models of the fractions including the extraction residue were constructed assuming associated structure of coal molecules. Finally, a model structure of Zao Zhuang coal which consists of a huge associate of coal molecules was constructed threedimensionally using computer-aided molecular design (CAMD) by assuming an anisotropic model structure formed from a periodic boundary cell. Good agreement of the physical density (1.22 g/cm3) of the constructed model with that (1.29g/cm3) observed experimentally suggests that Zao Zhuang coal has a possibility t o have associated structure of coal molecules having a continuous distribution of molecular weight from the lighter fraction t o the solvent-insoluble residue.
Introduction The nature of macromolecular structure of coals has not been fully understood. Shinn' has constructed a model structure of Illinois No. 6 bituminous coal based on analytical data on the coal and its liquefaction products and showed various structural components. Shinn described that considerations of the secondary structure such as hydrogen bonding interaction and the three-dimensional nature of the coal would be necessary. Spiro2ahas constructed three-dimensional coal models proposed by Given: Wiser,5 Solomon,6 and Heredy,' using space-filling models. The three models of them were found to include spatially or sterically inaccessible moieties. Carlsona has determined three-dimensional minimum-energy conformations of coal models suggested by Given,4 Wiser,5 Solomon,6 and Shinn' and * To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, October 1, 1995. (1)Shinn, J. H.Fuel 1984,63, 1187. (2)Spiro, C. L.; Kosky, P. G. Fuel 1981,60, 1121. (3)Spiro, C. L. Fuel 1982,61, 1080. (4)Given, P.H. Fuel 1960, 39, 147. ( 5 )Wiser, W. H. NATO ASI Ser. C 1984,124, 325. (6) Solomon, P.R. New Approaches in Coal Chemistry; ACS Symp. Ser. 169;American Chemical Society: Washington, DC, 1981;p 61. (7)Heredy, L. A.; Wender, I. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1980;28,38. (8)Carlson, G. A. Energy Fuels 1992,6, 771.
found that secondary bondings, in particular, van der Waals interactions and hydrogen bonding, are strong driving forces to form and to maintain the threedimensional structure of coals. Nomura et al. has constructed a bituminous coal structural model based on the data of pyrolysis GCNS and CP/MAS 13C NMR.g Faulon and co-workerslOJ1developed a new simulation technique including CASE (computer-aided structure elucidation) and CAMD and calculated statistically structural, energetic, and physical characteristics for five coal models with different cross-linking densities. Takanohashi and ~ o - w o r k e r s determined ~~J~ the minimum-energy structure of solvent-soluble molecules by the computer-aided molecular design (CAMD) and reported that the most stable conformation was an associated structure of several coal molecules through noncovalent interactions and these molecules seemed to form a three-dimensional cross-link structure which mainly consists of physical cross-links. The solvent~
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(9)Nomura, M.; Matsubayashi, K.; Ida, T.; Murata, S. Fuel Process. Technol. 1992,31, 169. (10)Faulon, J.-L.; Carlson, G. A.; Hatcher, P. G. Energy Fuels 1993, 7 -. , 1062 (11)Faulon, J.-L.; Mathews, J. P.; Carlson, G. A,; Hatcher, P. G. Energy Fuels 1994,8,408. (12)Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1994, 8,395. (13)Takanohashi, T.;Iino, M.; Nakamura, K. Kagaku Kogaku Ronbunshu 1994,20,959.
0887-0624/95/2509-1003$09.00/00 1995 American Chemical Society
1004 Energy & Fuels, Vol. 9, No. 6, 1995
soluble molecules after extraction seem t o have the minimum-energyconformation, since the strained structure of coal has been relaxed and stabilized during extraction and fractionation. The structure of raw coal is considered to be anisotropic with increasing rank of coal. The understanding of the macromolecular structure of coals seems to be a key factor in coal handling and reactivities of coal conversion processes, such as liquefaction and pyrolysis. A concept of “two-phase (component) structure” of coal, Le., a main three-dimensional covalently crosslinked network and a small amount of low-molecular weight components trapped in the network, has been widely accepted.14J5 Significant contributions of noncovalent interactions among coal molecules to coal structure, such as hydrogen bondings,16 n-x interact i o n ~ , ’ and ~ , ~ charge ~ transfer i n t e r a c t i ~ n s , l ~ have -~l been recently reported. It has been found that carbon disulfide-N-methyl-2-pyrrolidinone(CS2-NMP) mixed solvent gave very high extraction yields more than 50% (daf) for some bituminous coals even at room temperature22,23and that the addition of a small amount of tetracyanoethylene or p-phenylenediamine greatly increased the extraction yield, suggesting that a large part of coals is composed of associates of solvent-soluble molecules which are dissociated by the reagents a b ~ v e . Physical ~ ~ , ~ ~cross-links by locally cooperative noncovalent interactions for the formation of threedimensional cross-linked structure have been indicated in synthetic and biological polymers. Cody et al.25,26 have reported that a pyridine-swollen coal shows viscoelastic properties and that it behaves like an entangled network unlike a covalently cross-linked macromolecular network. In the present paper, Zao Zhuang coal (Chinese bituminous coal) was extracted with pyridine in a Soxhlet extractor, or with CSz-NMP mixed solvent at room temperature. For the fractions obtained by solvent fractionation after the extraction, structural analyses were carried out. A molecular model of the raw coal which consists of a huge associate of coal molecules was constructed three-dimensionally using the CAMD method.
Experimental Section Extraction and Fractionation. Zao Zhuang coal (Shan Tong province in China, 86.9 w t % C,
