Unlocking the effect of H2O on CO2 separation performance of promising MOFs using atomically detailed simulations

Abstract

Metal organic frameworks (MOFs) have been considered as potential adsorbents for adsorption-based CO2/CH4 and CO2/N-2 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/N-2, and ternary CO2/CH4/H2O and CO2/N-2/H2O mixtures. The results showed that introduction of H2O as the third component into binary CO2/CH4 and CO2/N-2 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/N-2 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/N-2 separations. These results will be useful for the design and development of efficient MOF adsorbents for CO2 capture under humid conditions.