FTIR and XRD microscopic characterisation of coal samples with different degrees of metamorphism

To study the influence of coalification on the microstructural properties of coal, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and Density functional theory (DFT) simulations were used to analyse the microstructural properties of coal with different degrees of metamorphism. The results show that the degree of aromatic ring substitution of high-ranking coals is lower than that of low-ranking coals, and the lower the degree of aromatic ring substitution, the more stable the aromatic-substituted hydrocarbon structure. The phenolic hydroxyl group CO occupies a dominant position among the oxygen-containing functional groups of the coal samples, and the relative proportion of -COOH is the lowest. The O atoms linked by double bonds are more susceptible to electrophilic and nucleophilic reactions, but the structural stability is lower than that of the O atoms linked by single bonds. The aliphatic hydrocarbons in low-ranking coals are mainly in the form of long chains complemented by side chains, whereas high-ranking coals show the opposite tendency. The fatty chain structure of –CH2 is more stable than that of –CH and -CH3, and the C and H atoms in -CH3 are more susceptible to nucleophilic attack. The relative proportion of cyclic hydrogen bonds in coal samples is high, and the relative proportion of OHOH and OH-π hydrogen bonds increases with the degree of coalification. Stacking degree (Lc), number of aromatic layers (Nave) and aromaticity (fa-XRD) increase with increasing metamorphic degree, and interlayer spacing (d002) decreases with increasing coalification degree. Based on the results of FTIR and XRD analyses, coal slit-hole models with different structures are constructed, providing model construction methods and reference values for researchers in this field.