Adsorption Preference Sites and Mechanism of Coal with Different Degrees of Metamorphism

The adsorption preference sites of coal with different metamorphic degrees could provide theoretical guidance for the targeted screening of a modifier that strongly adsorbed to adsorption preference sites or destroyed the adsorption preference sites for the coal-seam-injection-modified water. Radial distribution function analyses showed that the pyrrole nitrogen functional group had the strongest adsorption capacity for methane. Density functional theory electrostatic potential analysis revealed that the extreme difference between the extreme value difference of positive and negative electrostatic potentials on the coal surface was negatively correlated with the degree of metamorphism. The Hirshfeld surface visualization study revealed that the non-uniform distribution of the electron cloud and the synergistic effect of hydrogen bonding in the pyrrole nitrogen sites were the structural basis of the adsorption advantage. The independent gradient model based on Hirshfeld partition analyses indicated that the interaction between pyrrole nitrogen and methane decayed with decreasing coal rank and the electron density gradient characteristics of van der Waals forces and hydrogen-bonding synergy in anthracite were significantly stronger than in lignite. This study provided some theoretical basis for targeted screening and design of modifiers in the coal liquid injection measure.