Enhancing ethanolysis of Dongming lignite into O-rich derived oils: The role of H₂O₂ preoxidation based on structural regulation

Lignite has the dense aromatic-aliphatic cross-linked structure and high O content, which limits its conversion to O-rich derived oils in the solvothermal environment. In view of this, a stepwise conversion strategy of H₂O₂ preoxidation-ethanolysis was developed in this work to promote the conversion of Dongming lignite (DL) into O-rich derived oils by simultaneously enhancing oil yield and modifying O-containing class distribution. With this approach, ethanolysis-derived oil yield markedly increased from 11.7 to 21.4 wt%. The structure change of DL during H₂O₂ preoxidation was systematically evaluated using multiple complementary characterization techniques. Results show that H₂O₂ can effectively break inter-aromatic bridged linkages and also act on aromatic rings, thereby lowering the aromaticity of carbon skeleton and increasing proportion and/or abundance of aliphatic moieties. In addition, O existing forms in DL were reconstructed by preoxidation, enriching >C=O/>C-OH groups and O-containing covalent bridged bonds with low-dissociation energies. These changes are beneficial for enhancing ethanolysis efficiency and increasing derived oil yields. Ethanol directly participates in the conversion of >C-O-, >C=O, and -COO- moieties in DL through cleavage, alkylation, transalkylation, and interesterification, resulting in the release of additional O-containing small molecules, including arenols, ketones, carboxylic acids, and esters. Density functional theory calculations further confirmed that >C-O- cleavage in ethanolysis system exhibits the lowest energy barrier and the most stable transition state, revealing the kinetic advantage of ethanolysis in promoting >C-O- cleavage. The correlation between structural feature changes of raw lignite/oxidized lignite and efficiency optimization of ethanolysis was established in this paper, providing a theoretical basis for the efficient stepwise conversion of lignite to O-rich derived oils.