Structural rearrangement of strained coals

Native coals are strained and glassy. When coals are swollen, this strain is relieved as the coal structure rearranges to a lower free energy and more highly noncovalently associated state. Four coals ranging in carbon content from 75% C to 84% C were warmed in the weak swelling solvent chlorobenzene at 132 degrees C for 2 weeks and samples were withdrawn at,intervals. After evaporation of the chlorobenzene, the pyridine extractability of the treated coals had decreased, sometimes by a factor of 2. The pyridine swelling of Pittsburgh No. 8 coal was sharply reduced. The extractability and swelling decreases with time demonstrate that changes in coal structure occurred with the rearranged coal being more associated. This increased association is not due to hydrogen bond formation because pyridine is known to break most if not all, of the hydrogen bonds which occur in coals, The rearranged Pittsburgh No. 8 coal was studied by differential scanning calorimetry. Over the 2 week chlorobenzene reflux period, the heat, capacity decreased by a factor of 2, demonstrating that the coal rearranged to a more highly associated, more rigid structure. X-ray diffraction-studies show enhanced intensity for a regular structural feature occurring at about 20 Angstrom with no other alterations, including the aromatic face to face stacking. The observation that the rearrangement occurs in a day or two in pyridine at room temperature and the absence of a decrease in the radical population argue against increases in covalent bonding as the source of the observed changes. We believe the driving force for the rearrangement is the release of stored elastic strain. Coal swelling provides the macromolecule with the opportunity to undergo conformational rearrangements and to adopt a lower free energy more highly associated structure. The behavior of the high-rank Upper Freeport coal is opposite in direction to the lower rank coals. It apparently rearranges to a less associated structure.