A newly proposed view on coal molecular structure integrating two concepts: Two phase and uniphase models

Concept of coal molecular structure was reexamined on the basis of detailed information concerning aliphatic moieties of coal and the reactions occurred during coalification process. Based on the observed similarity of the distribution of chain length between alkylene bridge bonds and alkyl pendant groups on coal aromatic cluster, the authors have already proposed [Nomura, M., Murata, S., Kidena, K. 2004a. Some view on the solubilities of coals toward solvents. Proceedings of 21st Annual International Pittsburgh Coal Conference, No. 12-4.] the presence of common intermediates in the coalification process which results in uniphase structure under reductive conditions and two phase structure under oxidative conditions, respectively. The present paper proposes, the concept that the reactivity of the coal aromatic moieties for common intermediates and/or the stability of common intermediates govern the reactions leading to a two phase structure or uniphase structure in more rationalized way as coalification is believed to proceed under reductive conditions, especially at its late stage. Brown and subbituminous coals have relatively many oxygen-containing functionalities on aromatic moieties so that their aromatic rings are activated for radical reactions. Common intermediate radicals, aryl alkyl radicals, from cleavage of alkyl pendant groups on aromatic rings and alkylene bridge bonds attack neighboring aromatics to form entanglement-like structure in an easy way. These structures are less soluble to organic solvents due to its entanglement character. Bituminous coals with high solubility toward CS2/NMP solvents tend to have carbon contents from 85 to 87% (daf. basis), their aromatic moieties being so stable that intermediate radicals cited above can attack more selectively the neighboring aromatic moieties: they have less tendency to make such bridge bonds to form cross linked structures, the resulting coal molecules being not entangled like uniphase. In the case of coals having more carbon than 87% or bituminous coals with less carbon than 85%, the resulting coal molecules become less soluble toward organic solvents: In the higher rank coals, the aromatic rings are so large and so stable that their solubility toward solvent are intrinsically low. In addition to this, it is supposed that bridge bonds are cleaved to make the structure with less cross linking. Their stacking tendency should be also considered as other reason for low solubility to solvent. on the other hand, less solubility of the lower ranked bituminous coals can be rationalized partly, in an analogy with the behavior that the brown and subbituminous coals show, despite of their relatively large aromatic clusters. (c) 2007 Elsevier B.V. All rights reserved.