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

Abstract

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.

Effect of Cyclical Microwave Modification on the Apparent Permeability of Anthracite: A Case Study of Methane Extraction in Sihe Mine, China

“…The peaks near 2925 and 2855 cm –1 are identified as the stretching vibrations of −CH 2 and −CH 3 , respectively. 67 , 68 The peaks near 2515 and 460 cm –1 are attributed to the vibrations of S–H. 69 The peak at 1600 cm –1 corresponds to the vibrations of C=O and C=C.…”

Effect of the Coal Molecular Structure on the Micropore Volume and the Coalbed Methane Content

“…CBM is mainly stored in coal’s micropores in the adsorbed state, while a small quantity of free-state CBM exists in pores and fractures. − The spatial arrangement of coal’s complex macromolecular polymers and their combinations is widely investigated using X-ray diffraction, − high-resolution transmission electron microscopy (HRTEM), − Raman spectroscopy, numerical simulation, and so forth. HRTEM plays an important role in macromolecular structure studies, providing fringe images and allowing one to directly obtain macromolecular structural parameters .…”

“…Niekerk and Mathews revealed the macromolecular structure of a low-volatility bituminous coal and effectively extracted the aromatic stripes from HRTEM images. Due to the appearance of more effective image-processing methods, scholars gained a deeper insight into the tortuosity degree of coal macromolecules. , Noteworthy is that the evolution of coal molecules was strongly affected by coalification . During the coal deposition process, the stress and temperature variations led to a gradual decline in the contents of hydrogen and oxygen in coal and fat structures, a gradual increase in the shares of C atoms and aromatic structures, a gradual growth of aromatic fringes, accompanied by aromatization, condensation, splicing, and stacking of coal structures. − Accordingly, the length of coal macromolecular fringes increased with coalification, promoting the development of aromatic layers, reducing the fringe separation values, and strengthening the molecular directionality .…”

Response of Molecular Structures and Methane Adsorption Behaviors in Coals Subjected to Cyclical Microwave Exposure

“…49 Many characteristic parameters (f a−F , I 1 , I 2 , DOC 1 , DOC 2 , CH 2 /CH 3 , "A", and "C") in the coal molecular structures can be obtained by the deconvolution of FTIR spectra. 50 Lin et al semiquantitatively analyzed the changes in the functional groups and structural parameters of coal dust before and after explosion using the deconvolution of FTIR spectra. 51 Chen et al quantitatively studied the chemical functional groups in the macerals of coals of various ranks combined with reflectance micro-FTIR and KBr-FTIR spectroscopy analyses.…”

“…Fourier transform infrared (FTIR) spectroscopy analysis, a nondestructive analysis method, can identify the type and relative content of surface groups such as −CH 3 , −CH 2 , CC, CO, −O–, and −OH groups through the vibrations of specific chemical bonds, especially some polar chemical bonds . Many characteristic parameters ( f a–F , I 1 , I 2 , DOC 1 , DOC 2 , CH 2 / CH 3 , “ A ”, and “ C ”) in the coal molecular structures can be obtained by the deconvolution of FTIR spectra . Lin et al semiquantitatively analyzed the changes in the functional groups and structural parameters of coal dust before and after explosion using the deconvolution of FTIR spectra .…”