Designing optimal core–shell MOFs for direct air capture

Abstract

Metal-organic frameworks (MOFs), along with other novel adsorbents, are frequently proposed as candidate materials to selectively adsorb CO₂ for carbon capture processes. However, adsorbents designed to strongly bind CO₂ nearly always bind H₂O strongly (sometimes even more so). Given that water is present in significant quantities in the inlet streams of most carbon capture processes, a method that avoids H₂O competition for the CO₂ binding sites would be technologically valuable. In this paper, we consider a novel core-shell MOF design strategy, where a high-CO₂-capacity MOF "core" is protected from competitive H₂O-binding <i>via</i> a MOF "shell" that has very slow water diffusion. We consider a high-frequency adsorption/desorption cycle that regenerates the adsorbents before water can pass through the shell and enter the core. To identify optimal core-shell MOF pairs, we use a combination of experimental measurements, computational modeling, and multiphysics modeling. Our library of MOFs is created from two starting MOFs-UiO-66 and UiO-67-augmented with 30 possible functional group variations, yielding 1740 possible core-shell MOF pairs. After defining a performance score to rank these pairs, we identified 10 core-shell MOF candidates that significantly outperform any of the MOFs functioning alone.