Transport-Relevant Pore Limiting Diameter For Molecular Separations In Metal-Organic Framework Membranes

Metal-organic frameworks (MOFs) are an emerging class of materials for membrane gas separations. The pore limiting diameter (PLD) of a MOF, i.e., the aperture size of the material, is often used as a key structural property to evaluate its size exclusion capability for a gas mixture. The current computation of a PLD is based on the van der Waals radii outlined by the Cambridge Crystallographic Data Centre (CCDC), and this set of van der Waals radii may fail to capture the strong repulsion exerting on a guest molecule when passing through the bottleneck of a channel. In this work, we propose a new set of van der Waals radii for the framework atoms, whose values are smaller than the existing ones to describe the repulsive adsorbent-adsorbate interaction. The PLD of a MOF computed using this more transport-relevant radius set is referred to as PLDt-r, and that computed based on the widely used set outlined by CCDC is referred to as PLDCCDC. We evaluate the relevance between PLDt-r/PLDCCDC and the transport properties of MOF compounds including their diffusivity as well as diffusive and permeative selectivities for various binary mixtures. From our investigation on approximately 400 MOF structures, the results show that PLDt-r can more effectively identify highly selective MOFs for membrane gas separations. In this work, we also demonstrate the applicability of PLDt-r when the framework flexibility is considered.