Reaction site evolution during low-temperature oxidation of low-rank coal

• The reaction site evolution was examined by combing multiple characterization methods with quantum chemistry calculation. • Electrostatic potential analysis of macromolecular models of coal with different oxidation degrees was performed. • The main active site is the hydroxyl, methyl and methylene groups, especially the hydroxyl in the carboxyl group. • As the temperature increases, the main active site changes from hydroxyl and methylene to carboxyl. In-depth and comprehensive analysis of reaction sites evolution has important enlightening significance for understanding the chemical reaction sequence during low-temperature oxidation. For this purpose, FT-IR, XPS and 13 C NMR were combined to analyze the changes in reaction sites. As the oxidation temperature increased, the CH 3 /CH 2 ratio continuously increases, indicating that methylene has a higher reactivity. Before the oxidation temperature reached 150 °C, the relative content of C-H decreased slightly while that of C-O increased. Then, the C-O gradually transformed into C=O. When the oxidation temperature exceeds 150 °C, the ratio of aliphatic carbon has been significantly reduced and that of aromatic carbon is greatly increased. The ratio of carbonyl carbon also has a small increase. Based on these results, representative macromolecular models are established and electrostatic potential analysis is performed. The results show that the main active site is the hydroxyl, methyl and methylene groups, especially the hydroxyl in the carboxyl group. In summary, it can be considered that the main active sites for coal oxidation at low temperature are hydroxyl and methylene groups. At higher temperatures, the hydroxyl group will be further oxidized and the main active site will gradually evolve into carboxyl groups.