Parametric simulations of hierarchical core–shell MOF materials for direct air capture

Abstract

Metal–organic frameworks (MOFs) can separate and adsorb specific gases from a gas mixture, making them an ideal candidate for direct air carbon capture. However, MOFs that adsorb CO2 well often competitively adsorb the more abundant H2O molecules in the atmosphere, degrading their adsorptive performance and operational longevity. MOF-contained-in-MOF (MOF⊂MOF) core–shell materials may resolve this issue if a shell can be synthesized to reject water while the core receives CO2. This paper builds on our team’s prior computational screening and experimental synthesis of individual and core–shell MOFs by performing multiphysics simulations on a spherical MOF⊂MOF core–shell pellet exposed to atmospheric air. This core–shell pellet is endowed with our previously calculated and experimentally measured constituent MOF properties to demonstrate a high throughput methodology for scale-up analysis on composite adsorbent structures. We simulate and screen five UiO-67(Zr) MOF variants for use as either the core or shell materials that lead to 25 MOF⊂MOF core–shell pellet combinations, five of which are pellets whose core and shell are the same MOF. The resulting rankings suggest that adding a shell MOF to a pellet does not necessarily improve the adsorptive performance of that pellet when using these UiO-67(Zr) variants. The highest-ranked core–shell pellet combination was a pellet whose core and shell were the same MOF material. However, when considering only the core domain of the pellet as a frame of reference, the ranking results suggest that using hydrophobic MOFs as the pellet core and hydrophilic MOFs as the pellet shell show improvement compared to without the shell MOF. Using this second frame of reference, we identify the MOF⊂MOF core–shell combination UiO-67(Zr)⊂2(NH2)-UiO-67(Zr) as the highest-performing pellet of those that show improvement over their homogeneous pellet counterparts. A set of parametric simulations are performed comparing the core–shell pellet UiO-67(Zr)⊂UiO-67(Zr) against that of UiO-67(Zr)⊂2(NH2)-UiO-67(Zr) to investigate the impact of air CO2 concentration, relative humidity, and shell thickness on each respective core MOF’s performance. From these studies, the largest improvement in the core MOF’s performance was observed when using elevated CO2 concentrations of 1000 ppm, reduced relative humidity levels of 10%, and an increased shell thickness of 0.03 cm on a pellet core 0.12 cm in radius. Together, these results suggest that additional UiO-67(Zr) MOF variants will need to be screened to determine the use cases where the use of a shell is of water-rejecting benefit to the pellet in the DAC setting. Further adsorption and porosimetry experimentation is necessary to replicate the proposed pellet under DAC conditions and establish a means of comparative analysis between the simulated and measured pellet properties.