Study on molecular structure characteristic and optimization of molecular model construction of coal with different metamorphic grade

To investigate the structural properties of coal molecules with different degrees of metamorphism, in this study, we used Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and solid-state nuclear magnetic resonance carbon spectroscopy (13C NMR) to investigate the structural properties of three coal molecules with different degrees of metamorphism and finally constructed and optimized the coal molecular model. The experimental results showed that with the deepening of the deterioration degree, the proportion of oxygen-containing functional groups in the coal molecules was gradually reduced, the long fatty chains were shortened and reduced, and the aromaticity was enhanced. With increasing coalification, pyridine nitrogen and thiophene sulfur are becoming important components of the nitrogen- and sulfur-containing organic matter in coal. Coal metamorphism promotes the aromatization and condensation degree increase of coal molecular structure, which makes the coal molecular structure evolve into a microcrystalline structure with higher order degree. With this increase of coal rank, the carbon ratio around the bridge (XBP) increased from 0.12 to 0.47. The low-rank coal was dominated by benzene rings with low polymerization degree, and the high rank coal was dominated by aromatic structures, such as tetracene with a high degree of polymerization. The molecular formula of lignite finally constructed was C219H159O21N3, that of coking coal was C235H164O7N4S, and that of anthracite was C286H108O5N4S. The coal molecules constructed in this study provide a theoretical basis for the study of the molecular structure properties of coal with different degrees of metamorphism.