Fourier Transform Infrared Spectroscopy Evidence of the Nanoscale Structural Jump in Medium-Rank Tectonic Coal

Coal is a pressure-sensitive organic rock. The effect of tectonism on the structural evolution of medium-rank coal has been confirmed by the change in the crystal state of tectonic coal, but the organic molecular level response has not been reported. In this paper, three sets of medium-rank tectonic coals and symbiotic nontectonic coals were selected. The distributions of their functional groups and their molecular structure evolution were assessed using Fourier Transform Infrared Spectroscopy (FTIR), and their structural parameters were determined from the curve-fitting analysis. The nanoscale structural jump characteristics and mechanisms of medium-rank tectonic coal were revealed. Compared with symbiotic nontectonic coal, tectonism accelerated the exfoliation of side chains (groups) in the macromolecular structure, enlarged the aromatic system, and removed the unstable groups such as associative hydrogen bonds at first, which indicated that the molecular structure of tectonic coal was affected by nanoscale deformation, showing obvious advanced evolution characteristics. For the fat coal, the removal of side chains (groups) during the formation of tectonic coal makes the aromatic ring condensation obvious. For the coking coal, the formation of tectonic coal is dominated by cycloaliphatic dehydrogenation and aromatization, accompanied by the condensation of the aromatic rings. The tectonic coal formed from lean coal shows obvious aromatization characteristics. The molecular depolymerization and chemical tailoring caused by tectonism promotes the removal of hydrophobic side chains (groups) and activates some polar structure sites in coal. It is considered that the nanoscale structural jump of medium-rank tectonic coal is the result of the competition between the aromatic system and aliphatic structures.