Molecular simulation studies of separation of CH4/H2 mixture in metal-organic frameworks with interpenetration and mixed-ligand

In our previous work, we have investigated the adsorption selectivity of CH4/H2 in three pairs of isoreticular metal-organic frameworks (IRMOFs) with and without interpenetration to study the effect of interpenetration on gas mixture separation through Monte Carlo simulation. In addition, the self-diffusivities and the diffusion mechanism of single H2 and CH4 in these MOFs were examined by molecular dynamics simulations. In this work, we extend our previous work to mixed-ligand MOFs to investigate the effects of interpenetration as well as mixed-ligand on both equilibrium-based and kinetic-based gas mixture separation. We found that methane adsorption selectivity is much enhanced in the selected mixed-ligand interpenetrated MOFs compared with their non-interpenetrated counterparts, similar to what we found before for IRMOFs with single-ligand. At room temperature and atmospheric pressure, molecular-level segregation was observed in the mixed-ligand MOFs, and the extent of the effects of interpenetration is comparable for single-ligand and mixed-ligand MOFs. In addition, we found that the diffusion selectivity in the interpenetrated MOFs is similar to the one in their non-interpenetrated counterparts, while the permeation selectivity in the former is much higher than that in the latter, which corroborates our expectation that interpenetration is a good strategy to improve the overall performance of a material as a membrane in separation applications based only on the single component diffusion results. Furthermore, the CH4 permeability of the selected MOF membrane was also evaluated.