Evolution of molecular structure during coalification and graphitization: Evidence from multiple characterization methods

The molecular structure of coal plays a critical role in coalbed methane enrichment, carbon capture, utilization and storage (CCUS), as well as in the preparation of coal-based carbon materials. However, few studies have examined the continuous, overall evolution of coal molecular structure throughout coalification and graphitization. In this work, ultimate analysis, X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the molecular structure of coals ranging from low-rank to coaly graphite. The results reveal distinct spectral features corresponding to different coal maturities. During the elimination of heteroatoms, deoxidation occurs primarily in the early stage of coalification, as evidenced by the disappearance of C=O structure from low-rank to medium-rank coals. Dehydrogenation proceeds throughout both coalification and graphitization, especially at high maturity stage, marked by the loss of aliphatic side chains and aromatic C–H bonds. The evolutionary trends of L_a, L_c and d₀₀₂ parameters derived from XRD as a function of R_{o, m} indicate that aromatic layers begin forming in low-rank coals, with vertical stacking continuing through the medium-rank stage and accompanied by a decrease in d₀₀₂. Re-stacking of aromatic layers and re-reduction of d₀₀₂ occur during the early stage of graphitization. Polycondensation of naphthalene and 3 × 3, 4 × 4 aromatic rings proceed continuously during coalification and graphitization, increasing the lateral size of crystallites. The trends in G-FWHM from Raman data and alignment degree derived from HRTEM with increasing R_{o, m} suggest that the molecular structure become progressively more directionally aligned from graphitized coal to coaly graphite. The stepwise evolution of multiple parameters obtained from XRD, HRTEM and Raman indicates that coal graphitization is limited to R_{o, m} values between 4.0 % and 5.0 %. Overall, the results enhance the theoretical framework of coalification and graphitization, and have indicative significance for coal-to-oil, CCUS and preparation of coal-based carbon materials.