Exploring separation mechanism of graphene slit-pore for N2/CH4 in coalbed methane via DFT and MD simulations approaches

A combination of molecular dynamic (MD) simulation and first-principles calculations (DFT) is utilized to explore the separation performance of N2/CH4 in different graphene slit-pore sizes. By performing MD simulations, the absorption selectivity and diffusion coefficients of N2/CH4 are determined, facilitating the analysis of the separation mechanism. The highest selectivity of N2/CH4 is achieved at a slit-pore size of 6.2 Å, attributing to the sieve effect. As increasing pore-size (6.3–7.0 Å), the slit-pore exhibits preferential N2 permeation, determined by strong adsorptive interactions. However, as the pore-size expands further (7.0–10.0 Å), a higher diffusion coefficient indicates that the system is converted to preferential permeation of CH4. Moreover, the separation mechanism of larger slit-pore (>10.0 Å) size is not a mere selective mechanism but a synergistic interplay between adsorption and diffusion. Notably, we demonstrate that the dominant separation mechanism can be transitioned sequentially from size sieving to thermodynamic adsorption and dynamic diffusion by adjusting bilayer graphene slit-pores. This various mechanism determines the crossover preferential permeation for N2 to CH4. Our work primarily focuses on investigating the separation mechanisms of N2/CH4 at different graphene slit-pores, providing theoretical insights and understanding for the experimental separation of coalbed methane using carbon-based materials.