Adsorption and Diffusion of CH₄, N₂, and Their Mixture in MIL-101(Cr): A Molecular Simulation Study

A comprehensive quantitative grasp of methane (CH₄), nitrogen (N₂), and their mixture's adsorption and diffusion in MIL-101(Cr) is crucial for wide and important applications, e.g., natural gas upgrading and coal-mine methane capturing. Previous studies often overlook the impact of gas molecular configuration and MIL-101 topology structure on adsorption, lacking quantitative assessment of primary and secondary adsorption sites. Additionally, understanding gas mixture adsorption mechanisms remains a research gap. To bridge this gap and to provide new knowledge, we utilized Monte Carlo and molecular dynamics simulations for computing essential MIL-101 properties, encompassing adsorption isotherms, density profiles, self-diffusion coefficients, radial distribution function (RDF), and CH₄/N₂ selectivity. Several novel and distinctive findings are revealed by the atomic-level analysis, including (1) the significance of C=C double bond of the benzene ring within MIL-101 for CH₄ and N₂ adsorption, with Cr and O atoms also exerting notable effects. (2) Density distribution analysis reveals CH₄'s preference for large and medium cages, while N₂ is evenly distributed along pentagonal and triangular window edges and small tetrahedral cages. (3) Calculations of self-diffusion and diffusion activation energies suggest N₂'s higher mobility within MIL-101 compared to CH₄. (4) In the binary mixture, the existence of CH₄ can decrease the diffusion coefficient of N₂. In summary, this investigation provides valuable microscopic insights into the adsorption and diffusion phenomena occurring in MIL-101, thereby contributing to a comprehensive understanding of its potential for applications, e.g., natural gas upgrading and selective capture of coal-mine methane.