Stress response of noncovalent bonds in molecular networks of tectonically deformed coals

Abstract

Investigations on macromolecular evolution of tectonically deformed coals are of great theoretical and practical significances for the coal safe production and coalbed methane exploitation. Alteration of covalent bonds in rigid coal carbon skeletons was studied extensively, while insufficient attention was paid to the variation of noncovalent bonds in macromolecular networks. In present study, some insights about stress response of noncovalent bonds are given by investigating a primary coal and six typical tectonically deformed coals collected around a fault structure. Self-associated n-mers (n > 3), OH-ether, cyclic OH, COOH dimers, OH-SH and OH-N are all disrupted by tectonic stress, which is partially resulted from dissociation of functional groups. Conversely, amount of OH-π is in an increasing trend, indicating that there is a transformation between OH-π and other hydrogen bonds. In general, the total content of hydrogen bonds generally decreases from primary coal to granulitic coal, and then slightly increases from scaly coal to wrinkle coal, which is ascribed to the increase of OH-π transformed from other hydrogen bonds. Furthermore, −ΔHtotal and −ΔHav both decrease with the increasing deformation intensity even in scaly and wrinkle coals with slightly increasing total content of hydrogen bonds. While, the amount of ππ bonds increases with enhancement of coal deformation intensity (especially in brittle-ductile and ductile deformed coals), indicating that free molecules liberated by disruption of hydrogen bonds and ππ bonds are rearranged into a more stable, stacked and ordered configuration accompanied by the formation of new noncovalent bonds.