(297d) Revealing the Critical Impact of Adsorption Order on Mixed-Gas Adsorption in a MOF

MOFs have been extensively examined for CO₂ capture, and the influence of water co-adsorption on these processes is particularly relevant, as CO₂ capture commonly occurs in humid gas streams. However, little information is available on the binary interactions of CO₂ and H₂O and how mixed gas adsorption impacts the surface characteristics of MOFs. Therefore, to investigate CO₂/H₂O co-adsorption, binary CO₂ and H₂O adsorption isotherms were measured on MOF-808-TFA (TFA = trifluoroacetic acid). When water was pre-adsorbed on MOF-808-TFA, and a subsequent CO₂ adsorption isotherm was measured, the CO₂ adsorption was slightly reduced, similar to adsorbents such as zeolite 13X, as expected. Unexpectedly, when CO₂ was adsorbed first and then an H₂O adsorption isotherm was measured, no significant H₂O adsorption capacity was observed. The near-complete loss of water adsorption capacity occured even when only a trace amount of CO₂ was pre-adsorbed. The results show that unanticipated non-state function adsorption equilibria can arise from dynamic MOF behaviors and defect sites, potentially leading to counterintuitive adsorption data. These data indicate that non-ideal behavior can be challenging to predict, and even when the adsorption mechanism of a single component species is understood, it may be difficult to estimate the impact of trace amounts of a secondary adsorbate. Complex non-ideal behavior may not be limited to uncommon adsorbates or adsorbents, and such behavior should be considered when explaining CO₂ capture adsorption mechanisms on porous materials.

These results have significant implications for the industrial application of MOFs in processes such as pressure or temperature swing adsorption, which are particularly relevant for direct air capture or post-combustion capture of CO₂. For example, slightly different gas phase compositions could produce significant changes in adsorption capacity, or changes in cyclic working-capacity . Normally, one might attribute cyclic capacity losses to the degradation of the adsorbent. However, for MOF-808-TFA, the results indicate that these changes could be attributed to adsorbate interactions that alter the surface of the adsorbent.