Methane Adsorption Behaviors in Coal: New Insights into Functional Groups and Reaction Pathways

As the mining depth increases, the pressure and temperature within the coal seams rise correspondingly. This change has a non-negligible impact on the adsorption characteristics of gas within the coal seams. There exists a research gap regarding the chemical interaction between methane molecules and the active functional groups of coal under the influence of temperature and pressure. This work examined the alterations of functional groups before and after methane adsorption under different temperature and pressure conditions by Fourier transform infrared spectroscopy. Moreover, based on the experimental results of elemental analysis, carbon-13 nuclear magnetic resonance, and X-ray photoelectron spectroscopy experiments, a coal molecular model was constructed for simulating saturated adsorption and identifying dominant adsorption sites of methane. Finally, based on density functional theory, a transition state search was conducted for the reaction between the functional groups and methane at the dominant adsorption positions. The results show the following: (i) the structural parameters of functional groups during the methane adsorption process by coal under different temperature and pressure conditions were calculated by means of infrared spectral peaks. A potential chemical interaction between the oxygen-containing functional groups within coal and methane transpired at 4 MPa and 140 °C. (ii) The methane saturation adsorption energy under different conditions demonstrates that neither the valence energy nor the nonbonding energy is zero during the process of coal adsorbing CH4, implying that both physical adsorption and chemical adsorption occur during the process of coal adsorbing CH4. (iii) The simulation results of reaction pathways disclose that methane molecules may chemically interact with −OCH3, −COOH, and −COH in coal molecules at specific temperatures. The research is intended to augment the theory of methane adsorption in coal and establish a theoretical basis for the safe mining of deep coal and the prevention and control of gas disasters.