Unexpected Impacts of CO₂/H₂O 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.