Molecular structural differences and molecular structure models of the vitrain and durain in Low-Rank coal

To elucidate the molecular heterogeneity of low-rank coal lithotypes and its control mechanism on methane adsorption, vitrain and durain samples from the Huanglong Jurassic coalfield were investigated. A combination of solid-state 13 C NMR, FTIR, XPS, XRD, and high-pressure methane isothermal adsorption was employed to characterize the physicochemical structure and adsorption performance. Results indicate that vitrain is hydrogen-rich with higher volatile matter, whereas durain is carbon- and oxygen-rich, characterized by higher moisture, ash, and mineral contents. Microstructurally, durain exhibits significantly higher aromaticity ( f a ) and bridgehead carbon ratio ( X BP ) than vitrain. XRD analysis reveals that durain possesses larger aromatic crystallite dimensions ( L a , L c ) and smaller interlayer spacing ( d 002 ), suggesting a more compact, highly aromatized structure with substantial lateral extension. Conversely, vitrain features a higher aliphatic carbon ratio ( f al ), abundant in aliphatic hydrocarbons and longer alkyl side chains. Macromolecular models for both lithotypes were constructed and energetically optimized based on the spectroscopic data. Adsorption experiments demonstrate that the Langmuir volume ( V L ) of vitrain is significantly superior to that of durain. Correlation analysis reveals a distinct structure–property relationship: adsorption capacity correlates positively with aliphatic carbon ratio and chain length, suggesting that the developed aliphatic side chains in vitrain provide primary adsorption sites and free volume. In contrast, a negative correlation with X BP and L a indicates that the dense aromatic stacking in durain compresses the effective molecular accommodation space. This study confirms that maceral composition fundamentally determines reservoir gas potential, emphasizing that the heterogeneity of coal macerals is a critical determinant in coalbed methane exploration.