Unlocking the Effect of H₂O on CO₂ Separation Performance of Promising MOFs Using Atomically Detailed Simulations

Metal organic frameworks (MOFs) have been considered as potential adsorbents for adsorption-based CO₂/CH₄ and CO₂/N₂ separations because of their high CO₂ selectivities and high working capacities. H₂O in flue gas and natural gas streams affects the gas uptake capacities of MOFs. However, the presence of H₂O is commonly neglected in high-throughput computational screening studies while assessing the CO₂ separation performances of MOFs. In this study, the impact of the presence of H₂O on the CO₂ separation performances of 13 MOFs that were previously identified as the best adsorbent candidates among several thousands of MOFs was examined. Molecular simulations were used to compute selectivity, working capacity, regenerability, and adsorbent performance score (APS) of MOFs considering separation of binary CO₂/CH₄, CO₂/N₂, and ternary CO₂/CH₄/H₂O and CO₂/N₂/H₂O mixtures. The results showed that introduction of H₂O as the third component into binary CO₂/CH₄ and CO₂/N₂ mixtures significantly affected the adsorbent evaluation metrics of MOFs that have strong affinity toward H₂O because of the presence of specific functional groups and/or extra framework anions in the framework. Remarkable increases in CO₂/N₂ selectivities of MOFs were observed in the presence of H₂O. On the other hand, simulations performed using MOFs that are preloaded with H₂O to mimic the exposure of MOFs to humidity prior to gas adsorption revealed drastic decreases in CO₂ working capacities and APSs of MOFs both for CO₂/CH₄ and CO₂/N₂ separations. These results will be useful for the design and development of efficient MOF adsorbents for CO₂ capture under humid conditions.