Network structure of coals and association behavior of coal-derived materials

The characteristics of two kinds of network structure models, i.e., covalent and non-covalent network models, were given. The applicability of these network models was examined mainly from the amount of solvent-soluble components that originally existed in coal. More than 50% of extraction yields obtained with carbon disulfide-N-methyl-2-pyrrolidinone (CS2-NMP) mixed solvent with an additive for several bituminous coals suggest that the non-covalent network model is better than the covalent one for those coals. But network structures for other coals, which gave a low extraction yield such as lignites, are not clear. Association behaviors of coal-derived materials are also reviewed from their solubility, molecular weight, viscosity, and vapor pressure. Small-angle neutron scattering (SANS) measurements of coal extracts in the solution show the formation of large associates. Interactions responsible for the association of coal-derived materials such as coal extracts and liquefaction products are considered to be hydrogen bonds, pi-pi interactions between aromatic rings, and electrostatic interactions. No clear evidence for the contribution of charge transfer interactions has been obtained. (C) 2000 Elsevier Science B.V. All rights reserved.