Unusual Moisture-Enhanced CO Capture within Microporous PCN-250 Frameworks.

Developing metal-organic frameworks (MOFs) with moisture-resistant feature or moisture-enhanced adsorption is challenging for the practical CO capture under humid conditions. In this work, under humid conditions, the CO adsorption behaviors of two iron-based MOF materials, PCN-250(Fe) and PCN-250(FeCo), were investigated. An interesting phenomenon is observed that the two materials demonstrate an unusual moisture-enhanced adsorption of CO. For PCN-250 frameworks, HO molecule induces a remarkable increase in the CO uptake for the dynamic CO capture from CO/N (15:85) mixture. For PCN-250(Fe), its CO adsorption capacity increases by 54.2% under the 50% RH humid condition, compared with that under dry conditions (from 1.18 to 1.82 mmol/g). Similarly, the CO adsorption uptake of PCN-250(FeCo) increases from 1.32 to 2.23 mmol/g, exhibiting a 68.9% increase. Even up to 90% RH, for PCN-250(Fe) and PCN-250(FeCo), obvious increases of 43.7 and 70.2% in the CO adsorption capacities are observed in comparison with those under dry conditions, respectively. Molecular simulations indicate that the hydroxo functional groups (μ-O) within the framework play a crucial role in improving CO uptake in the presence of water vapor. Besides, partial substitution of Fe by Co ions in the PCN-250 framework gives rise to a great improvement in CO adsorption capacity and selectivity. The excellent moisture stability (stable even after exposure to 90% RH humid air for 30 days), superior recyclability, as well as moisture-enhanced feature make PCN-250 as an excellent MOF adsorbent for CO capture under humid conditions. This study provides a new paradigm that PCN-250 frameworks can not only be moisture resistant but can also subtly convert the common negative effect of moisture to a positive impact on improving CO capture performance.