Macromolecular response to tectonic deformation in low-rank tectonically deformed coals (TDCs)

Abstract

The nano-porous and optical properties of tectonically deformed coals (TDCs) are well characterized while their macromolecular structures have been minimally reported in recent years. In this study, the structural transformations of low-rank TDC macromolecular structures were identified utilizing 13C nuclear magnetic resonance (13C NMR) and Fourier transform infrared spectroscopy (FTIR). Both the NMR aromaticity (fa’) and the ratio of aromatic bridge carbon to aromatic peripheral carbon (xBP) increase with the increasing deformation intensity, indicating the aromatization process induced by the stress condensation. The carbonyl first increases then decreases with the increasing deformation intensity, leading the maximum value in granulitic coal. The decrease of A (CH2)/A (CH3) (defined by FTIR A2940-2900/A3000-2940) indicates the increasing branching degree and shorting aliphatic chains especially for the ductile- and strong brittle deformation processes. The Al/-OX (defined by FTIR A3000-2800/A1800-1520) firstly increases until the granulitic coal and then decreases, indicating that the loss rate of the oxygen functional groups is higher than that of the aliphatic carbons during the brittle deformation process but lower than the latter during the shear- and ductile deformation stages. The ductile- and strong brittle deformations (especially for the deformation process of mortar- to granulitic coal) have a more significant promote effect for aliphatic loss than the weak brittle deformations (i.e. deformation process of primary-, cataclastic-, and mortar coal). The loss of oxygen functional groups is significant during shear- and brittle deformation stages but rather slight during the ductile deformation stage. The strong shear TDCs (i.e. mortar- and granulitic coal) have a relative abnormal high content of ether, hydroxybenzenze, and CH2CO, indicating that the shear deformation process has a significant promote effect for the diversification in the aliphatic carbons and oxygen functional groups. Then, the relative content of ether, hydroxybenzenze, and unsaturated carboxylic acids overall decreases with the increasing deformation intensity and the reduction rate all manifests as high at the shear deformation stage and low at the ductile deformation stage.