The Effect of Oxidation on Coal’s Molecular Structure and the Structure Model Construction of Oxidized Coal Molecular

Three kinds of 1/3 coking coals with different degrees of oxidation were used for this study in Inner Mongolia, China. Using analytical testing methods such as X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance carbon spectroscopy (13C-NMR) and Fourier transform infrared spectroscopy (FT-IR), combined with computer-aided software such as Chemdraw, Materials Studio (2017), and MestRenova, three coal samples were characterized and analyzed. On this basis, the molecular formulas of three coal samples with different degrees of oxidation were constructed by optimizing the model energies: we used C228H165N3O21S4 for Suhaitu9# coal, C244H171N3O31S2 for Guoyu coal, and C225H177N3O33S2 for Lu9# coal. The results showed that, at the same coal rank, the oxidation degrees of S9, GY, and L9 coal samples were 21.10%, 48.30%, and 53.12%, respectively. As the oxidation degree increased, the proportion of oxygen-containing functional groups and nitrogen oxides in the coal macromolecular structure gradually increased. The bridge circumference ratios were 0.3786, 0.3351, and 0.2228, respectively, showing a gradual decrease. The average methylene chain lengths were 4.9569, 2.6843, and 1.9055, respectively, showing a gradual decrease. This indicates that the condensation degree of the compounds decreases with the increase in the degree of oxidation. These findings reflect the effect of oxidation on the modeling of coal’s macromolecular structure and lay a theoretical foundation for the further study of impact of the degree of oxidation on the physicochemical properties of coal.