Construction of Pingdingshan coal molecular model based on FT-IR and 13C-NMR

To construct a molecular model of coal and study the adsorption characteristics from the perspective of microscopic molecules, the microstructure of coal samples from Pingdingshan mining area in China was studied by means of Fourier transform infrared spectroscopy (FT-IR) and carbon-13 nuclear magnetic resonance spectroscopy (13C-NMR). The coal mainly possesses an aromatic molecular structure with fa (aromaticity), faH and XBP (the ratio of aromatic bridge carbon to surrounding carbon) being 0.86, 0.29 and 0.54, respectively. According to the results, the aromatic structure, with a high degree of condensation, is mainly dominated by pyrene and chrysene, while the aliphatic structure is mainly composed of methyl and methylene whose ratio is about 0.52. Furthermore, it contains a small amount of oxygen-containing functional groups such as carbonyl group, phenolic hydroxyl group and ether bond. In light of the above-mentioned molecular structure characteristics, the periodic structure model of Pingdingshan coal with the molecular formula C108H71O4N was constructed based on molecular mechanics and molecular dynamics by using the Materials Studio software. The model has a density of 1.3 g/cm3, a porosity of 4.1% and a specific surface area of 614.5 m2/g, faithfully reflecting the structural information of coal sample. Moreover, methane isothermal adsorption curves were simulated by the model at the temperatures of 318, 333 and 348 K. The experimentally measured and simulated isothermal methane adsorption curves both exhibit an error of smaller than 5% under high pressure. The results verify rationality and accuracy of the model structure. The study provides a model basis for further research on coal on the microscopic level.