Molecular model construction of Danhou lignite and study on adsorption of CH₄ by oxygen functional groups

In view of the frequent occurrence of gas accidents in coal mines, the mechanism of oxygen-containing functional groups (OCFGs) in Danhou lignite adsorbing gas was studied by experiment and simulation. Elemental analysis, X-ray photoelectron spectroscopy (XPS), solid-state ¹³C nuclear magnetic resonance spectroscopy (¹³C-NMR), and adsorption experiment of CH₄ were applied to establish the macromolecular model of Danhou lignite. Then, molecular mechanics (MM) and molecular dynamics (MD) were utilized to optimize the coal macromolecular model, and the density of coal was determined via adding periodic boundary conditions. The mechanism of gas adsorption by OCFGs was studied by grand canonical Monte Carlo (GCMC) and density functional theory (DFT). The results showed that the aromatic structures mostly exist in the form of pyrenes; the structure of aliphatic carbons are mostly methylene and methine groups; the alkanes are mostly long chains; oxygen atoms are mainly in the form of hydroxyl groups and ether groups; nitrogen atoms are mainly in the form of pyridines; and the density of Danhou lignite is 1.25 g/cm³. The isotherm adsorption curve and Langmuir adsorption curve have a good fit, a single coal molecule reaches saturation after absorbing four CH₄ molecules, and the error between experiment and simulation is small. The results of DFT calculation showed that the adsorption of CH₄ by OCFGs is affected by the adsorption positions and adsorption directions. Due to CH₄ molecules are affected by different electrostatic forces, the adsorption capacities of OCFGs are different, and the order is carbonyl groups > ether bonds > hydroxyl groups > carboxyl groups. The results can be used for reference in the prevention and control of coal and gas outburst.