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

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.