Breakthrough Analysis. Breakthrough measurements were conducted at 25 °C using pelletized Cu2.7-MFU-4l and compressed air inlet streams (2 mL min -1 ) with varying levels of humidity to assess the selectivity of Cu2.7-MFU-4l for O2 and N2 under more realistic conditions (see Section 1.13 of the Supporting Information for details). When the material was exposed to dry air, sharp breakthrough of N2 occurred before O2 (after 10 and 25 min, respectively), highlighting the selective nature of O2 binding under these conditions (Figure ). We note that following breakthrough of N2 from the column, the normalized outlet flow rate (F/F0) for N2 temporarily exceeded the inlet flow rate, indicative of roll-up 48 from displacement of bound N2 by O2 due to competitive adsorption. After this initial breakthrough run, the material was regenerated with heating at 150 °C under a He purge (until O2, N2, and H2O were no longer detected in the outlet stream), completing the first breakthrough "cycle". The material was then cooled to ambient temperature and two more adsorption-desorption cycles were performed under the same conditions. The breakthrough times and capacities for O2 and N2 did not change over the course of these three cycles (Figures 6a and S49 and Table ).