Methane Adsorption Behavior on Graphene Surfaces Regulated by Imidazole-Based Ionic Liquids: A DFT Investigation

Efficient exploitation of coalbed methane (CBM) is of paramount importance for energy security and coal mine safety. The adsorption-desorption behavior of CBM represents a critical micro-scale process significantly influencing extraction efficiency. To investigate the regulatory mechanism of ionic liquids on gas adsorption behavior, this study employed quantum chemical calculations, utilizing graphene as a model for coal surface, and selected three imidazolium-based ionic liquids with distinct alkyl side chains ([Bmim][BF4], [Emim][BF4], [HOmim][BF4]). The microscopic effects of methane adsorption on graphene surfaces were systematically investigated. The nature of interactions within the ionic liquid-graphene-methane ternary system was elucidated through electrostatic potential analysis, independent gradient model based on Hirshfeld partition (IGMH) analysis, and atom-in-molecule (AIM) topological analysis. The results indicate that physical adsorption between ionic liquids and graphene primarily occurs via π‒π stacking and van der Waals interactions. The molecular structure of ionic liquids exerts a decisive influence on the regulation of methane adsorption behavior. The butyl chain ionic liquid ([Bmim][BF4]) significantly enhances methane adsorption energy, whereas the ethyl chain ionic liquid ([Emim][BF4]) reduces adsorption energy. The hydroxyethyl ionic liquid ([HOmim][BF4]) slightly promotes methane adsorption due to electrostatic interactions introduced by hydroxyl groups. This study elucidated the molecular-level regulation mechanism of ionic liquid structure on methane adsorption, providing a theoretical foundation for the design of functionalized ionic liquids to modulate the adsorption/desorption process of coalbed methane.