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 CO 2 capture, and the influence of water coadsorption on these processes is particularly relevant, as CO 2 capture generally occurs in humid gas streams. To investigate CO 2 /H 2 O coadsorption, binary adsorption isotherms of CO 2 and H 2 O were measured on MOF-808-TFA (TFA = trifluoroacetic acid). When water was preadsorbed on MOF-808-TFA, and a subsequent CO 2 adsorption isotherm was measured, the CO 2 adsorption was slightly reduced, as expected. However, when CO 2 was adsorbed first and then an H 2 O adsorption isotherm was measured, no significant H 2 O adsorption capacity was observed. The near complete loss of water adsorption capacity was observed even when only a trace amount of CO 2 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.