Efficient oxidative depolymerization of lignite through selective cleavage of cross-linked structures for producing oxygen-containing chemicals

The non-fuel utilization of lignite is of great significance, and mild oxidative depolymerization is a promising approach for producing oxygen-containing chemicals. In this work, Xiaolongtan lignite (XLT) was used to investigate selective oxidation under mild conditions, with the aim of avoiding excessive oxidation and reducing alkali consumption. The results demonstrate that O2 oxidation can efficiently depolymerize XLT into humic acid (HA) in diluted KOH aqueous solution. Under the optimized conditions (XLT/KOH mass ratio = 3, 3 MPa O2, 80 °C, 3 h), the HA yield can reach as high as 65.8%, with only a minimal amount of CO2 (3.1%) generated. In addition, the structure-oxidative depolymerization relationship was further examined through FTIR characterization and the oxidation of model compounds. The cleavage of cross-linked bonds, including methylene and ether bridge bonds as well as hydroaromatic rings, is essential for the depolymerization of lignite by O2 oxidation. The high stability of aliphatic substituents resulted in their enrichment in the oxidized residue (OR). Under mild conditions, the aromatic ring and aromatic carboxylic acids are stable and favors the production of HA. By combining with the oxidation of model compounds, a better understanding of the oxidative depolymerization of XLT to oxygen-containing chemicals is achieved.