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

S Wei and SB Yang and X Zhang and SW Tang and D Shen and WH Xue and W Dong and YK Xia and SL Bai, DIAMOND AND RELATED MATERIALS, 139, 110404 (2023).

DOI: 10.1016/j.diamond.2023.110404

A combination of molecular dynamic (MD) simulation and first-principles calculations (DFT) is utilized to explore the separation performance of N-2/CH4 in different graphene slit-pore sizes. By performing MD simulations, the absorption selectivity and diffusion coefficients of N-2/CH4 are determined, facilitating the analysis of the separation mechanism. The highest selectivity of N-2/CH4 is achieved at a slit-pore size of 6.2 & Aring;, attributing to the sieve effect. As increasing pore-size (6.3-7.0 & Aring;), the slit-pore exhibits preferential N-2 permeation, determined by strong adsorptive interactions. However, as the pore-size expands further (7.0-10.0 & Aring;), 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 & Aring;) 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 N-2 to CH4. Our work primarily focuses on investigating the separation mechanisms of N-2/CH4 at different graphene slit-pores, providing theoretical insights and understanding for the experimental separation of coalbed methane using carbon-based materials.

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