The utility of MOF-based materials in direct air capture (DAC) application to ppm-level CO₂

Metal-organic frameworks (MOFs) are well-suited materials for CO₂ removal and have robust capture capacity and selectivity. Although the adsorption of CO₂ in MOFs has been studied, the implementation of ppm-level CO₂ uptake in MOFs and the effects of the pore size and charge have not been fully explored. We performed grand canonical Monte Carlo (GCMC) simulations combined with the Density Functional Theory plus U (DFT + U) charge method to investigate MOF screening for ppm-level CO₂ uptake and its application in a direct air capture (DAC) system. Three types of MOFs containing eight members were studied: i.e., ZIF-68, 69, 70; UiO-66, 67, 68; CAU-10; and MIL-125. The pore landscape characterization, electrostatic field-induced enhancement, and preferential binding sites of these MOFs were examined for CO₂ capture. MOFs with pore limited diameters (PLD) 1.5 times the size of CO₂ molecules and with large cavity diameters (LCD) smaller than 10 Å exhibit robust confinement capacity. Polar functional groups and metal ions dominate the electrostatic contributions and subsequently enhance the surface adhesion of CO₂ molecules. For a given framework, favorable CO₂ binding occurs in the following order: small pores/cages > polar functional group/metal ions > larger pores/cages. ZIF-69 which comprises smaller pores (7.5 Å) and robust polar functional groups (-Cl) collectively enhances CO₂ capture; thus, ZIF-69 outperforms other MOFs; the performance of ZIF-69 is followed by that of CAU-10 which has an optimal pore size of 6 Å. These findings are of fundamental and practical importance for the application of MOFs in DAC technologies for CO₂ removal.