Unexpected Impacts of CO 2 /H 2 O Adsorption Order on the Mixed-Gas Adsorption Capacity of a Metal–Organic Framework

Metal-organic frameworks (MOFs) have been examined extensively for CO₂ capture, and the influence of water coadsorption on these processes is particularly relevant, as CO₂ capture generally occurs in humid gas streams. To investigate CO₂/H₂O coadsorption, binary adsorption isotherms of CO₂ and H₂O were measured on MOF-808-TFA (TFA = trifluoroacetic acid). When water was preadsorbed on MOF-808-TFA, and a subsequent CO₂ adsorption isotherm was measured, the CO₂ adsorption was slightly reduced, as expected. However, 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 was observed even when only a trace amount of CO₂ was preadsorbed. The results show that unexpected, nonstate function adsorption equilibria can result from dynamic MOF behaviors and defect sites, which may lead to counterintuitive adsorption data compared to traditional materials.