Chemical Structure Evolution of Thermally Altered Coal during the Preparation of Coal-Based Graphene and Division of Thermally Altered Zone: Based on FTIR and Raman

A suite of coal samples near a diabase dike was collected to investigate the structural and functional group evolution of a series of carbon materials prepared from thermally altered coals, explore the influence of thermal metamorphism distance on the structure of coal and its carbon material products, and divide the thermally altered zones. Using Fourier transform infrared and Raman studies, it was found that after demineralization, the aromatic parameters <i>f</i> _ar ^H and <i>I</i> of the coal structure slightly increase, while the aliphatic parameters CH₂/CH₃ and oxidation parameter <i>'C'</i> slightly decrease, and the degree of order of the coal structure increases. Graphitization can greatly improve aromatic parameters, eliminate aliphatic structures, and enhance orderliness. However, after oxidation and reduction, the aromatic parameters and ordering degree of graphene decrease. Except for the sample closely attached to the dike, the coal-based graphene yield of the other samples first decreases and then stabilizes with the increase of distance from the dike, which is consistent with the trend of changes in the reflectance of raw coal. The thermally altered distance affects the structural changes of coal and carbon material products. The coal attached to the dike has been damaged and polluted, and the aromaticity and orderliness of the prepared carbon material products are relatively poor. The aromaticity and orderliness of coal-based products prepared from other thermally altered coals are relatively high and increase with the closer the thermally altered distance. Based on the characterized parameters of coal samples and products with distance from the dike, the sampling area is divided into four zones, including abnormally altered zone, normal altered zone, transition zone, and original coal zone. Among them, the yield and quality of coal-based graphene prepared from coal in the normal altered zone are the highest, an ideal raw material collection area for making coal-based graphene.