Chemical Structure and Electronic Properties of Thermally Altered Coals: Insights from Multi-Technique Analysis and Modeling

With the progressive depletion of shallow coal resources, the study of thermally altered coals has gained increasing importance, particularly in understanding their chemical structures and properties. In this paper, the molecular models of two thermally altered coals were established based on results obtained from ultimate analysis, ¹³C NMR, XPS, and HRTEM. Based on the experimental results, molecular models for SL-A and SL-B were constructed, with molecular chemical formulas of C₂₀₇H₁₁₈N₃O₅ and C₂₄₂H₁₃₀N₄O₅, respectively. The simulated ¹³C NMR spectra were in good agreement with experimental data, validating the reliability of the two models. Three-dimensional molecular structures in energy-minimum conformations were further obtained by molecular mechanics and dynamics simulations. The aggregate models indicated unit cell dimensions of 3.31 × 3.31 × 3.31 nm³ for SL-A and 3.54 × 3.54 × 3.54 nm³ for SL-B. Using quantum chemistry methods, the electrostatic potential (ESP) and frontier molecular orbitals (FMO) were calculated to evaluate the reactivity of the models. The results demonstrated that the negative potentials in both coal models were primarily associated with oxygen-containing functional groups (e.g., -CO) and aromatic rings, whereas nitrogen and hydrogen atoms exhibited strongly positive potentials. FMO analyses revealed that high metamorphism degrees in thermally altered coals correlate with increased reactivity. Thus, investigating the chemical structure and electronic properties of thermally altered coals provides valuable insights for their further processing and utilization, including gasification, combustion, and coal graphitization.