Understanding the molecular structure of HSW coal at atomic level: A comprehensive characterization from combined experimental and computational study

Abstract

In this work, the coal samples from Hongshiwan (HSW) mining area, Ningxia, northwest of China, are characterized by using several modern materials characterization techniques, such as proximate and ultimate analyses, solid state 13C nuclear magnetic resonance (13C NMR), X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy (FT-IR). Then the key information about elements, valence, and chemical bonding for coal molecular structural construction is obtained. The results reveal that the main structure of HSW coal has 75.96% aromatic skeleton in mass. The ratio of aromatic bridge carbon to aromatic peripheral carbon of HSW coal is 0.315, indicating more naphthalene than benzene and anthracene in coal structures. Oxygen predominantly presents in the forms of ether (C–O), carbonyl (CO) and carboxyl (–COO). Nitrogen presents in the forms of both pyridine and pyrrole. Methyl (–CH3) group is predominant in cyclic and aliphatic hydrocarbons. Based on obtained structural information and the approaches of average molecular structure, the single molecular formula of HSW coal is defined as C221H148O28N2, with a molecular weight of 3142.32. Also, the 2D and 3D molecular model of HSW coal are built with computer-aided modeling. The model is optimized and further verified by FT-IR and 13C NMR spectra simulation with quantum chemical calculations. Besides, a more complicated structure of complex model for HSW coal containing 10 single-molecules is also obtained. Therefore, molecular structure of HSW coal has been comprehensively depicted and understood at atomic level from both experimental and quantum chemical approaches in the current work.