The role of structure defects in the deformation of anthracite and their influence on the macromolecular structure

Abstract

The impacts of stress on the physical and optical properties of coals are well recognized, while the influence on the chemical structure is seldom considered. In the light of mechanochemistry research that mechanical force can act on the molecule directly to initiate or accelerate reactions by deforming the chemical bonds, it is meaningful to consider how stress works on the macromolecule of coals. In this work, some insights are given based on anthracites with different tectonic deformation from Qinshui Basin, Shanxi Province, China. The deformation degree was measured by bireflectance (Ro,max−Ro,min), and the macromolecular structure was characterized by Raman spectroscopy. For samples from the same colliery, there is a positive relationship between Raman area ratio AD/AG and bireflectance, suggesting that the deformation of anthracite is related with the generation of structure defects at the atomic scale. Further quantum chemistry calculations demonstrate that accompanying the generation of one Stone-Wales (SW) defect (induced via in-plane rotation of C-C bond by 90°), the molecular geometries of anthracite, such as chemical bonds and angles, change. The deviation of atoms from their equilibrium geometries reflects the local force distribution and transfers 303.48kJ/mol mechanical energy into chemical energy. These changes allow chemical bonds to adjust to the applied stress without breakage, so that anthracite will accommodate plastic deformation. Additionally, the existence of SW defect slightly reduces the energy needed to produce carbon monoxide from carbonyl in anthracite. The current study helps to understand the potential influence of stress on the chemical structure evolution of coals.