Diffusion mechanism and adsorbed-phase classification-molecular simulation insights from Lennard-Jones fluid on MOFs

Abstract

Physisorption of gases has been widely applied in thermal energy utilization and purification processes. Diffusion in porous media has been well studied. However, molecular-scale adsorbate diffusion mechanism remains unexplored. In this study, molecular dynamics have been employed to elucidate the diffusion behaviors of liquid and gaseous methane adsorbed in Cu-BTC (Copper(2+) 1,3,5-benzenetricarboxylate). Based on the energy distribution and trajectories of adsorbed molecules, a hypothesis is proposed that the adsorbed phase can be classified into four types: bound molecules (oscillate around a specific region of the adsorbent), generally adsorbed molecules (within the range of surface interaction and possess negative total energy), non-adsorbed molecules (within the range of surface interaction, but having positive total energy), and free molecules (beyond the range of surface interaction). To support this hypothesis, further simulation of methane adsorption in MOF-5 (Zn₄O(BDC)₃) has been conducted and compared with existing experimental data, indicating the hypothesis has broader applicability.

Keywords: Computational chemistry; Computational materials science; Materials science.

Graphical abstract

Figure 1

Physical properties of adsorbed methane within Cu-BTC under the bulk phase of low-temperature liquid, low-temperature gas, and room temperature gas (A–C) Number of adsorbed molecules, (D–F) heat capacity, (G–I) kinetic energy, (J–L) kinetic energy per atom.

Figure 2

The schematic of sub-boxes separation

Figure 3

Filling ratio of methane adsorbed in Cu-BTC in the start period (A) Filling ratio under low-temperature liquid, (B) filling ratio under low-temperature gas, and (C) filling ratio under room temperature gas.

Figure 4

Radial distribution function of CH₄-CH₄ and Cu-CH₄ (A) CH₄-CH₄ under low-temperature liquid, (B) CH₄-CH₄ under low-temperature gas, (C) CH₄-CH₄ under room temperature gas, and (D) Cu-CH₄ under the three bulk phases.

Figure 5

Positive ratio of total energy among adsorbed methane and displacement distribution (A) Positive ratio under low-temperature liquid, (B) positive ratio under low-temperature gas, (C) positive ratio under room temperature gas, and (D) the displacement distribution of adsorbed methane with 200 ps intervals under low-temperature gas-phase adsorption.

Figure 6

Adsorbed-phase classification of methane in Cu-BTC

Figure 7

Adsorption result of methane adsorbed in MOF-5 Adsorption isotherm of methane adsorbed in MOF-5 at (A) 200 K and (B) 300 K. The positive ratio of the total energy of methane molecules at (C) 200 K and (D) 300 K.