Alkali/alkaline earth metals doped holey penta-hexagonal graphene for ultrahigh-performance CO2 capture and separation over N2/CH4

Abstract

Developing adsorbent materials with superior CO₂ adsorption and separation capabilities is crucial for addressing pressing environmental challenges such as the greenhouse effect and global warming. Employing density functional theory and grand canonical Monte Carlo methods, this study evaluates the potential of two-dimensional holey penta-hexagonal graphene doped with alkali and alkaline-earth metals (AM-HPhGs, where AM = Li, Na, K, Mg, Ca) for CO₂ adsorption and separation. Structural analyses reveal AM-HPhGs possess high binding energies (−2.12 to −5.15 eV) and high cohesive energies (6.37 to 6.77 eV/atom), reflecting their significant structural stability. Electronic structure analysis demonstrates robust covalent interactions, substantial charge transfer, pronounced electropositivity/electronegativity, and clear orbital overlapping between AM atoms and HPhG. These characteristics suggest robust interactions between AM atoms and HPhG, providing a stable and conducive environment for CO₂ uptake. Moreover, incorporating AM atoms significantly enhances both CO₂ adsorption capacities and selectivities in AM-HPhGs. Among AM-HPhG structures, Ca-HPhG demonstrates an outstanding CO₂ adsorption capacity of 11.51 mmol g⁻¹ and CO₂ selectivity over CH₄/N₂ of 2385/1366 at 298 K and 1.0 bar, surpassing many reported metal-doped adsorbent materials. Analysis of gas distribution indicates that AM atom doping not only provides more effective CO₂ adsorption sites on HPhG but also influences the distribution and packing pattern of CO₂. Interaction analysis reveals that AM atom doping increases the affinity of the framework for CO₂, thereby enhancing CO₂-framework interactions. The CO₂-framework isosteric heat, van der Waals, and Coulomb interactions surpass those between framework and CH₄/N₂, successfully accounting for the high CO₂ adsorption performance and selectivity in AM-HPhGs. These findings suggest that AM-HPhGs, particularly Ca-HPhG, hold great promise as excellent CO₂ adsorbent materials.