Methane diffusion path in HKUST-1 metal-organic framework revealed by atomistic simulations

A copper-based metal-organic framework (MOF) HKUST-1 exhibits high methane adsorption capacity, primarily due to strong Coulomb interactions near open metal sites (OMSs) and van der Waals interactions within ligand-enclosed cavities. While the adsorption sites have been widely studied, the diffusion pathways through which methane reaches these sites remain unclear, particularly regarding the influence of OMSs. In this study, molecular dynamics (MD) simulations were conducted to investigate methane diffusion in HKUST-1 with and without OMSs. First-principles calculations and methane uptake experiments were also performed to support and validate the simulation results. We found that methane was trapped in ligand-enclosed sites due to steric hindrance, while regions near OMSs serve as diffusion hubs. Moreover, in the absence of OMSs, methane backflow was observed at the surface of HKUST-1 due to the presence of stable adsorption sites on surface planes. When OMSs are present, these stable sites shift toward the OMS regions via Coulomb interactions, reducing surface backflow and enhancing methane uptake. The experimentally measured increase in methane adsorption for HKUST-1 with OMSs supports the simulation results. This study can guide the future design of MOFs with enhanced gas adsorption capacity.