The Mechanism of Ozone Oxidation of Coal and the Revelation of Coal Macromolecular Structure by Oxidation Products

Ozone was injected into a coal-water suspension, and an HRTEM test was carried out on the separated oxidation products. The results show that from the perspective of visualization the macromolecular network structure of coal contains a large number of graphite-like structures. However, the chemical reaction mechanism between the coal surface and O₃ is not clear, and the microscopic formation mechanism of oxygen-containing functional groups in carbon quantum dots has not been explained. As a result, the reaction process between O₃ and methylene on the coal surface was studied by the DFT method. We found that OH• generated by O₃ in water can oxidize two adjacent carbon atoms in methylene into double bonds (C=C), and finally, aldehydes and carboxylic acids were generated. By calculation of thermodynamic parameters ΔG and ΔH, it is found that all reactions are spontaneous exothermic processes. The above chemical reaction is based on the physical adsorption of OH• with Ar-(CH₂)₆-Ar and O₃ with Ar-CH₂-CH=CH-(CH₂)₃-Ar. The calculated adsorption energies of the two systems are -9.41 and -12.55 kcal/mol, respectively. Then, the charge transfer and atomic orbital interaction before and after adsorption are analyzed from the perspectives of Mulliken charge, density of states, deformation density, and total charge density. The results show that the electrostatic attraction is the main driving force of adsorption. The ether bond (C-O-C) in coal is finally oxidized to an ester group (RCOOR'), the hydroxyl group (CH₂-CH-OH) on the aliphatic chain is oxidized to a carbonyl group (CH₂-C=O), and the benzene with two OH• forms phenol hydroxyl and one molecule of water. Finally, the coal and the corresponding coal-based carbon quantum dots were investigated by infrared spectroscopy; the difference in functional groups before and after oxidation was clarified, and the result was in good agreement with the simulation.