Molecular structure characteristic of coals of different rank

Context: Understanding the structural characteristics of coal at the molecular level is fundamental for its effective utilization. To explore the molecular structure characteristic, the long-flame coal from Daliuta (DLT), coking coal from Yaoqiao (YQ), and anthracite from Taixi (TX) were investigated using various techniques such as elemental analysis, Fourier transform infrared spectroscopy, solid-state ¹³C nuclear magnetic resonance spectroscopy, and X-ray photoelectron spectroscopy. Based on the structural parameters, the coal molecular model was constructed and optimized. The molecular formula of DLT was C₁₉₃H₁₇₈N₂O₄₇, that of YQ was C₂₀₁H₁₇₉N₃O₃₀S, and that of TX was C₁₉₈H₁₁₈N₂O₁₀. With an increase in the degree of metamorphism, the substitution of the benzene ring gradually shifted towards lower levels of substitution. The content of long chain in the aliphatic chain decreased while the content of branched chains kept increasing. The percentage of aromatic ether increased gradually, while the phenolic hydroxyl group initially decreased but then increased. The carboxyl group C = O decreased and eventually disappeared in anthracite coal. The proportion of pyrrole nitrogen gradually increased while that of pyridine nitrogen and protonated pyridine gradually decreased.

Methods: The 2D planar structure of coal was constructed using ChemDraw, ACD/CNMR Predictor, and gNMR programs. The geometry optimization was performed using the COMPASS II force field within the Forcite module in Materials Studio 2020. The annealing process employed NVT ensemble at a simulation temperature of 298 K. The Amorphous Cell module in Materials Studio was used to construct large-scale 3D molecular models, with the set parameters in this paper.