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, π–π interactions between aromatic rings, and electrostatic interactions. No clear evidence for the contribution of charge transfer interactions has been obtained.