Investigation of hydrothermal depolymerization of lignite under different conditions by 13C NMR spectroscopy

Abstract

Efficient depolymerization of lignite under mild conditions is the key to realizing its low carbon conversion and high value utilization. This study focuses on three Chinese lignites: Xilinguole lignite (XL), Xiaolongtan lignite (XLT), and Zhaotong lignite (ZT), investigating the inherent structures effect on their depolymerization processes when subjected to alkali depolymerization (AD) and oxidation depolymerization (OD) under hydrothermal condition. The parent lignites and their depolymerized products were characterized by solid 13C NMR, FTIR, TGA, and elemental analysis. It was found that both AD and OD effectively broke down the macromolecular structure of lignite into humic acid (HA) and small molecular products including water-soluble components, with the highest depolymerization activity observed for ZT followed by XLT and XL, regardless of the method used. Notably, by AD at 80 oC, 91.4 % of ZT was depolymerized, producing 69.4 % of HA as the major product, while higher temperatures were required to efficiently depolymerize XLT and XL by either AD or OD. More than 80% of XLT and XL can be converted by AD at 200 oC, yielding 54.8 % and 52.9 % of HA, respectively. OD at 140 oC resulted in a 57.7 % conversion of XL and an HA yield of 50.2 %. Characterization of the products demonstrated that alkali addition significantly promotes lignite depolymerization by breaking oxygen-containing linkages such as ether and ester bonds, with ether bond hydrolysis occurring predominantly at high temperatures. High-temperature AD of lignite is dominated by the cleavage of oxygen-containing linkages, whereas OD at lower temperatures mainly depolymerizes lignite through oxy-cracking of C−C and C−O weak linkages, reducing carboxyl decomposition to achieve higher HA yields. These findings offer a new perspective on the low-carbon utilization of lignite.