ASSOCIATION OF AROMATIC STRUCTURES IN COALS

Three different experimental probes were used to detect the presence of noncovalent interactions between aromatic groups in coals. The pyridine extractability of coals is greater at 115 °C than at 25 °C, even though the solvent is not saturated in either case. This is due to greater dissociation of the extractable molecules at the higher temperature. Since room temperature pyridine breaks the coal-coal hydrogen bonds, another noncovalent interaction must be responsible for the temperature dependence observed. Immersing Pittsburgh No. 8 coal in pyridine at room temperature or warming it in chlorobenzene or toluene causes the coal to rearrange into a more stable configuration, resulting in a decrease in pyridine swelling and extractability. If the macromolecular chains gain some freedom of motion, they rearrange into a lower free energy configuration involving more noncovalent interactions. The formation of adducts between maleic anhydride and coals sharply increases the pyridine extractability of coals containing more than about 88% (daf) carbon. Adduct formation sterically blocks some noncovalent interactions and decreases associations. Non-hydrogen-bond, noncovalent interactions play an important role in coal structure, and this role appears to increase with rank. © 1990, American Chemical Society. All rights reserved.