Structural evolution of lignite from heterogeneous particles to homogeneous products during ruthenium ion catalytic oxidation depolymerization

Disclosing the structure evolution of lignite from heterogeneous particles to homogeneous products during depolymerization is of great significance to understand the depolymerization process of lignite. In this work, the structure evolution of lignite during ruthenium ion catalytic oxidation (RICO) depolymerization was studied. Special attention was paid on the changes of the size of lignite particles and molecular weight of the soluble products. The results showed that the depolymerization of lignite into benzene polycarboxylic acids (BPCAs) involved the conversion of lignite from microparticles into nanoparticles, which can be further depolymerized and released the soluble products into the solution. Around 93% of the organic matter in lignite can be converted from heterogeneous particles to homogeneous products. The molecular weight of the soluble products had a wide distribution from dozens of thousand Dalton to several hundred Dalton, indicating that the macromolecules released from lignite particles were gradually decomposed and formed a series of intermediate products. These results explain that why the composition of the depolymerization products is complex. Finally, the microstructural evolution process of lignite during depolymerization was speculated. Lignite particles mainly experienced the microparticles, nanoparticles, macromolecules, and then small molecules such as BPCAs. The formation of macromolecules proceeded fast and the subsequent formation of BPCAs from macromolecules was the rate-determining step. This work contributes deep insights into the structural evolution process of lignite during RICO depolymerization, and provides basic data for the regulation and control of the depolymerization process