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

Abstract

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.

Figure 1

(a) Tetrahedral cage of MIL-101; (b) medium and large cage of MIL-101; (c) topology of MIL-101; (d) atom labels of MIL-101; (e) 3D view of simulation cell; and (f) 2D view of simulation cell (large cage: blue; medium cage: green; small cage: yellow; pentagon window: orange; and triangle window: white).

Figure 2

CH₄ and N₂ adsorption isotherms comparison of GCMC simulations and experimental measurements in MIL-101(Cr) at 298 K.

Figure 3

Isosteric heat of adsorption (Q_st) for N₂ and CH₄ in MIL-101.

Figure 4

RDF of CH₄ around the Cr1 atom and C3 atom.

Figure 5

2D density distribution for CH₄ in MIL-101 with different numbers of CH₄ molecules: (a) number = 100, (b) number = 200, (c) number = 400, (d) number = 600, (e) number = 800, and (f) 2D view of simulation cell.

Figure 6

2D density distribution for N₂ in MIL-101 with different numbers of N₂ molecules: (a) number = 100, (b) number = 200, (c) number = 400, (d) number = 600, (e) number = 800, and (f) 2D view of simulation cell.

Figure 7

(a) 3D density distribution for CH₄, (b) 3D density distribution for N₂, (I) volume visualization mode, and (II) atoms visualization mode.

Figure 8

Self-diffusion coefficients of CH₄ and N₂ molecules in MIL-101(Cr).

Figure 9

CH₄ and N₂ adsorption isotherms comparison of GCMC simulations and experimental measurements in MIL-101(Cr) at 298 K.

Figure 10

Separation selectivity values of CH₄/N₂ (0.3:0.7) mixture for adsorption on MIL-101(Cr) at 298 K.

Figure 11

2D density distribution of CH₄ and N₂ for 200 CH₄ molecules in MIL-101 with different numbers of N₂ molecules: (a) number = 20, (b) number = 50, (c) number = 200, (d) number = 300, and (e) number = 600.

Figure 11

2D density distribution of CH₄ and N₂ for 200 CH₄ molecules in MIL-101 with different numbers of N₂ molecules: (a) number = 20, (b) number = 50, (c) number = 200, (d) number = 300, and (e) number = 600.

Figure 12

Snapshot for 200 CH₄ molecules in MIL-101 with different numbers of N₂ molecules: (a) number = 20, (b) number = 50, (c) number = 200, (d) number = 300, and (e) number = 600 (CH₄: red and yellow; N₂: blue).

Figure 13

(a) Self-diffusion coefficients of CH₄ and N₂ when loading 200 CH₄ molecules in MIL-101 at different loadings of N₂ molecules at 298 K. (b) Self-diffusion coefficients of N₂ when compared loading 0 CH₄ molecules with 200 CH₄ molecules at different loadings of N₂ molecules at 298 K.