In addition to a high capacity and selectivity for O2 over the other components of air, an ideal O2-selective adsorbent would exhibit robustness to humidity and a low affinity for water, thus potentially enabling air separations without the need for pre-treatment to remove moisture. The Cu I -MFU-4l framework was previously reported to be air stable, and in our hands, a sample of Cu2.7-MFU-4l was found to be robust to ambient air for three months, based on powder x-ray diffraction analysis (Figure ). Furthermore, DRIFTS data collected for a sample of Cu2.7-MFU-4l exposed to the atmosphere at 300 K revealed stable coordination of both N2 and O2 over the course of at least 10 h without any additional changes in color (Figure ). A water isotherm collected for Cu2.7-MFU-4l at 298 K at relative humidity levels ranging from 2% to 80% revealed that the material has a low affinity for water (Figure ), in contrast to the parent framework MFU-4l. Consistent with this result, DFT calculations predict an electronic energy of -27 kJ/mol for water adsorption at the copper(I) sites in the framework, indicating a weaker metal-adsorbate interaction compared to the binding of O2 or N2 to the exposed copper(I) site. Additionally, while the O2, N2, and Ar adsorption isotherms of Cu2.7-MFU-4l were completed within several hours, completion of the water adsorption isotherm required approximately 50 h, indicative of sluggish water uptake kinetics. A small amount of hysteresis upon water desorption may be due to the formation of water clusters within the pores. Powder x-ray diffraction analysis of Cu2.7-MFU-4l following water adsorption/desorption isotherms collected at 298 and 308 K revealed that the material remains crystalline (Figure ). These observations contrast the behavior of Cu2.2-MFU-4l, which exhibits substantial loss of crystallinity following water adsorption/desorption isotherms at 298 K. Adsorption-Desorption Cycling Performance under Dry Air. To gain initial insight into the cycling stability of Cu2.7-MFU-4l under more realistic O2 capture conditions, we performed thermogravimetric analysis adsorption/desorption cycling experiments by exposing a sample of the material to flowing dry air (10 min at 30 °C) followed by desorption under simulated vacuum (Ar purge, 10 min at 30 °C). Remarkably, the material retained >99.9% of its total capacity over the course of 40 cycles, although we note that the weight change measured under these conditions reflects both adsorbed O2 and N2 (Figure ). Adsorption/desorption cycling data were also collected under simulated temperature swing conditions, by exposing the material to dry air (5 min at 30 °C) followed by desorption under an O2 purge at higher temperature (30 s at 100 °C). Under these conditions, incomplete desorption was observed, and a capacity of ~95% (O2 and N2) was retained over 40 cycles (Figure ). The slightly lower capacity relative to that measured under simulated pressure swing conditions is attributed to the highly oxidizing conditions used for desorption.