STRUCTURAL-CHANGES IN COALS DUE TO PYRIDINE EXTRACTION

Three coals (Bruceton, Illinois No. 6, and Rawhide) were exhaustively Soxhlet extracted with pyridine at 85 °C. The insoluble residues from these extractions were then refluxed with pyridine for 7 days at 113 °C. More coal dissolved under reflux. The refluxed coal residues were swollen in a series of solvents. The refluxed residues swelled less in all solvents than did the Soxhlet extraction residues. Heating the extraction residues for 7 days at 113 °C (no pyridine) had no effect on solvent swelling. These observations seem to be contradictory in that the increased extraction suggests bond (or noncovalent interaction) breaking while the decreased swelling indicates bond (or noncovalent interaction) making. We reject explanations based on covalent bonds and propose an explanation based on disassociation of groups connected by noncovalent interactions, a rearrangement of the coal, and reassociation. We suggest that noncovalent interactions are broken, allowing more material to be slowly extracted by the boiling pyridine. Later, as the pyridine is removed from the swollen network, it is free to rearrange as it shrinks. We believe it finds a lower free energy conformation containing more noncovalent interactions. The new conformation has a greater noncovalent cross-link density and so swells less in nonpolar solvents. © 1990, American Chemical Society. All rights reserved.