From wood to huminite: Molecular structural transformations

Abstract The formation of coal has always been a topic of interest for coal geologists and plays an important role in the enrichment of coal and coal-associated resources, such as critical elements and coalbed methane. In this study, wood, peat, lignite and xylite samples were collected to study the molecular transformation from wood to huminite. Dipolar decoupling magic-angle spinning 13 C nuclear magnetic resonance, cross-polarization magic-angle spinning 13 C nuclear magnetic resonance, atomic force microscopy-infrared spectroscopy and X-ray photoelectron spectroscopy experiments were conducted to characterize the molecular structure of different samples. In addition, the molecular model of matrix lignite was reconstructed and shown to align well with the experimental data. The results show that in the process of wood transforming into huminite, the first stage involves the degradation of cellulose and hemicellulose, and some of the ether bonds in lignin also break. In the second stage, the methyl groups from the methoxy structure of lignin are lost, and oxygen-containing linkages such as β-O-4 are degraded to form carboxyl, carbonyl structures and other chemical structures. In the third stage, phenolic hydroxyl groups combine with carboxyl groups to form ester linkages in huminite, while the remaining methyl groups from methoxy continue to be released. Due to the heterogeneity of microbial degradation, textinite preserved structures such as cellulose remnants, and these cellulose remnants were oxidized to form carbonyl groups. However, heavily gelified huminite, such as corpohuminite and densinite, is mainly formed by lignin, with no traces of cellulose found. The study elaborates on the molecular transformation from wood into huminite, which may help in further understanding the formation of coal.