Intrinsic adsorption behaviour related to the structural and mechanical properties of flexible metal-organic frameworks Co(bdp)

The Co(bdp) family has been widely investigated for the adsorption properties of gas molecules such as hydrogen, nitrogen, and methane. In this paper, the structural and mechanical properties of the Co(bdp) family, including Co(bdp), Co(bdp)-p-F-2, Co(bdp)-Me-2, and Fe(bdp), were studied based on first-principles theory. Using the methane adsorption behaviours of the Co(bdp) family as references, the adsorption properties of the Co(bdp) family were analysed by studying the structural properties. The adsorption properties can be estimated by analysing the accessible porous channel and ligand arrangement of the frameworks. This study can guide the design of flexible organic frameworks for specific adsorption or separation requirements. The effects of mechanical pressure and adsorbate molecules on the structural transition were analysed to reveal the mechanism of the structural transition of Co(bdp) when adsorbing gas molecules. The applied mechanical pressure from 0 to 50 bars will not result in a structural transition. The structural transition from the collapsed phase to the expanded phase induced by adsorbate molecules was simulated by applying expansion strains to the collapsed Co(bdp) phase. An intrinsic barrier of 13.15 eV per unit cell is present in the structural transition of Co(bdp). The energy necessary to overcome the energy barrier can be provided by the adsorbate molecular insertion. The simulation method in the present study is potentially applicable to the structural transition analysis of other flexible metal-organic frameworks.