Exploring the gas separation of N2/CH4 mixture on nitrogen-modified graphene

Doping represents a widely adopted approach to modulate the electronic characteristics of graphene surfaces. In particular, nitrogen-doped graphene has emerged as a promising, cost-effective adsorbent with high performance potential. In this study, Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were employed to investigate the adsorption and diffusion behaviors of N₂/CH₄ mixtures within slit-shaped nanopores formed by graphic-N dopant graphene (NDG), with systematically varied nitrogen doping concentrations and pore widths. The results reveal that increasing nitrogen content enhances both the N₂/CH₄ selectivity and the adsorption capacity. A maximum selectivity of 5.86 was achieved at 5.6 % NDG with 6.2 Å, where the NDG exhibited pronounced molecular sieving characteristics. At 298 K and 500 atm, the N₂ uptake exceeded 0.006 mmol/m², while CH₄ adsorption was negligible. At narrower pore widths, N₂ adsorption was favored due to the combined effects of stronger thermodynamic interactions and smaller dynamic diameter, whereas in wider pores (beyond two atomic layers), thermodynamic factors dominated. Notably, at 1 atm, CH₄ adsorption reached 0.0094 mmol/m²-surpassing N₂ (0.0091 mmol/m²)-in NDG with 5.6 % doping and a 7.0 Å pore width. Potential of mean force (PMF) calculations further revealed distinct energy barriers for molecular transport, underscoring the critical influence of pore chemistry and geometry on diffusion selectivity. While the self-diffusion coefficient varied with molecular loading, the effect of nitrogen doping on gas diffusion was minimal. These findings underscore the potential of nitrogen-functionalized graphene for efficient N₂/CH₄ separation and provide theoretical guidance for the rational design of advanced carbon-based adsorbents.