Study on Separation Performance of Metal-organic Frameworks with Interpenetration and Mixed-ligand

Abstract

In our previous work, we have investigated the adsorption selectivity and diffusion selectivity of CH4/H2 in mixed-ligand interpenetrated metal-organic frameworks (MOFs) to investigate the effects of interpenetration as well as mixed-ligand on both equilibrium-based and kinetic-based gas mixture separation through Monte Carlo and molecular dy- namics simulations. The potential of this kind of materials in equilibrium-based and kinetic-based separation applications was evaluated. In this work, we extend our previous work to CO2-related mixtures, like CO2/CH4, CO2/N2, and CO2/H2, as there is an urgent need to identify porous materials that can efficiently separate CO2 from mixtures of gases, such as flue gas and natural gas purification. To show a clearer correlation between gas mixture separation ability and material properties, CH4/H2 and CH4/N2 were also selected as the model mixtures to separate. A combined Monte Carlo and molecular dynamics simula- tion was carried out in eight MOFs with mixed-ligand and with or without interpenetration. Grand-canonical Monte Carlo (GCMC) simulations were employed to calculate the adsorption of mixtures in MOFs studied. Equilibrium molecular dy- namics (MD) simulations were carried out in the canonical (NVT) ensemble to investigate the effects of interpenetration on the diffusion behaviors of mixtures. The adsorption selectivities of mixtures in the selected MOFs were studied in detail. In addition, the diffusion, the permeation selectivities of CH4/H2, CH4/N2, CO2/N2, and CO2/CH4 as well as the CH4 and CO2 permeability through the selected MOF membranes were calculated. We found for all mixtures studied, the permeation selec- tivities in the interpenetrated MOFs are much higher than those in their non-interpenetrated counterparts due to the larger adsorption selectivities in the former, indicating that interpenetration is a good strategy to improve the overall performance of a material as a membrane in separation applications. The knowledge obtained is expected to serve as the guidance for the development of appropriate interpenetrated MOF adsorbents and membranes. Keywords adsorption separation; gas diffusion; membrane-based separation; metal-organic frameworks; molecular simula- tion