Research on Molecular Structure Characteristics of Vitrinite and Inertinite from Bituminous Coal with FTIR, Micro-Raman, and XRD Spectroscopy

To carry out an in-depth study of the macromolecular differences of vitrinite and inertinite with metamorphism degree, ultimate analysis, Fourier transform infrared (FTIR) spectroscopy, micro-Raman spectroscopy, and X-ray diffraction (XRD) were performed on coal macerals by density gradient centrifugation (DGC). Results show that carbon gradually increases rapidly, hydrogen and oxygen decrease significantly, sulfur gradually decreases, and the change of nitrogen is not obvious with coal rank. Functional groups (ether, carbonyl, hydroxyl, and methylene) gradually fall off with R-o(,max), and the molecular chains are reconnected to larger macromolecular chains, which eventually increases the maturity and aromaticity of vitrinite and inertinite. The shedding of aliphatic side chains and oxygen functional groups makes the arrangement of coal macromolecules more orderly, the lattice defects of C atom less, and the structure closer to graphitization, which finally show as the decrease of d(002) and the increase of L-a and L-c. The aromatic structure parameters (f(a-F), I, DOC) of vitrinite are lower than those of inertinite, which indicates that the maturity and aromaticity degree of vitrinite are lower than those of inertinite; the aliphatic side chains of vitrinite are larger than those of inertinite, which makes the hydrocarbon generation potential of vitrinite higher than that of inertinite. Vitrinite has more functional groups and complex structure than inertinite, which makes its C atom lattice defects larger and graphitization degree lower than inertinite. Finally, the heterogeneity of macromolecular structure of vitrinite is larger than that of inertinite. This study can provide the basis for macromolecular structure evolution and molecular modeling of coal macerals.