New insights into the coalification of lignite from lignin: A comparison of their chemical structures

Abstract

• The chemical structures of lignin and lignite were investigated via chemical & physical methods. • The distribution of polycyclic aromatic clusters in lignite and lignin are identical. • The yield distributions of benzene carboxylic acids from lignin and lignite are almost identical. • Aliphatic structures in lignin are connected with aromatic clusters only by strong C-C bonds. • There are more C-O bonds and less C-C bonds in lignin than in lignite. The structure of coal is considered to be a highly cross-linked network of macromolecules, which is formed by the polymerization of many structurally similar but not totally identical aromatic structural units. It is generally believed that coal is converted from biomass, which is composed of cellulose, hemicellulose, and lignin. Considering lignin as the only aromatic structure in biomass and lignite as a low-rank coal, we believe that there is an inextricable link between the structures of lignin and lignite. In this work, both Huolinhe lignite and enzymatic lignin can be oxidized to benzene carboxylic acids (BCAs), and the yield distribution of BCAs is very similar. By 13 C NMR characterization, we have found that both lignin and lignite contain polycyclic aromatic clusters and their mole fractions of aromatic bridgehead carbon ( X b ) are very close, which indicates that polycyclic aromatic clusters in lignite may be preserved from lignin. Combining the analysis of the supercritical ethanolysis products of Huolinhe lignite and enzymatic lignin, we found the structural units of enzymatic lignin contained more methoxy, phenolic hydroxyl groups, and C-O ether bonds, but fewer methylene chains than those of Huolinhe lignite.