Structural characteristics of Mile lignite and its molecular model construction

The understanding of the structural characteristics of lignite is of very importance to the lignite utilization. The structure parameters of Mile lignite in Yunnan were ascertained via Fourier transform infrared spectroscopy, ¹³C solid-state nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and ultimate analysis. The results indicate that the aromaticity and number of aromatic ring substituents of Mile lignite are 38.79% and 3, respectively. The aromatic carbon structure typically contains benzene and naphthalene, while the aliphatic carbon structure mainly includes methyl and methylene. Oxygen dominantly exists in ether oxygen, carboxyl and carbonyl; nitrogen occurs in the form of pyrrole nitrogen and pyridine nitrogen; and sulfur is mainly present in thiophenol and mercaptan. According to the analysis results, a molecular structure model of Mile lignite is constructed, with a molecular formula of C₁₄₇H₁₄₈O₃₆N₂S. The semi-empirical PM3 basis set and the density functional theory M06-2X were adopted to optimize the molecular configuration. The optimized model has a significant three-dimensional configuration, in which the aromatic layers are arranged irregularly in space and the aromatic rings are connected by methyl, methylene, methoxy and alicyclic rings. The simulated FT-IR spectrum and simulated ¹³C NMR spectrum are in great agreement with the experimental spectrum, which proves the accuracy and rationality of the molecular model of Mile lignite.