Oxygen is a critical gas in numerous industries and is produced globally on a gigatonne scale, primarily through energy-intensive cryogenic distillation of air. The realization of large-scale adsorption-based air separations could enable a significant reduction in associated worldwide energy consumption and would constitute an important component of broader efforts to combat climate change. Certain small-scale air separations are carried out using N₂-selective adsorbents, although the low capacities, poor selectivities, and high regeneration energies associated with these materials limit the extent of their usage. In contrast, the realization of O₂-selective adsorbents may facilitate more widespread adoption of adsorptive air separations, which could enable the decentralization of O₂ production and utilization and advance new uses for O₂. Here, we present a detailed evaluation of the potential of metal-organic frameworks (MOFs) to serve as O₂-selective adsorbents for air separations. Drawing insights from biological and molecular systems that selectively bind O₂, we survey the field of O₂-selective MOFs, highlighting progress and identifying promising areas for future exploration. As a guide for further research, the importance of moving beyond the traditional evaluation of O₂ adsorption enthalpy, ΔH, is emphasized, and the free energy of O₂ adsorption, ΔG, is discussed as the key metric for understanding and predicting MOF performance under practical conditions. Based on a proof-of-concept assessment of O₂ binding carried out for eight different MOFs using experimentally derived capacities and thermodynamic parameters, we identify two existing materials and one proposed framework with nearly optimal ΔG values for operation under user-defined conditions. While enhancements are still needed in other material properties, the insights from the assessments herein serve as a guide for future materials design and evaluation. Computational approaches based on density functional theory with periodic boundary conditions are also discussed as complementary to experimental efforts, and new predictions enable identification of additional promising MOF systems for investigation.

High or enriched-purity O₂ is used in numerous industries and is predominantly produced from the cryogenic distillation of air, an extremely capital- and energy-intensive process. There is significant interest in the development of new approaches for O₂-selective air separations, including the use of metal-organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O₂ over N₂ via electron transfer. However, most of these materials exhibit appreciable and/or reversible O₂ uptake only at low temperatures, and their open metal sites are also potential strong binding sites for the water present in air. Here, we study the framework Cu^I-MFU-4l (Cu_xZn_5-xCl_4-x(btdd)₃; H₂btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), which binds O₂ reversibly at ambient temperature. We develop an optimized synthesis for the material to access a high density of trigonal pyramidal Cu^I sites, and we show that this material reversibly captures O₂ from air at 25 °C, even in the presence of water. When exposed to air up to 100% relative humidity, Cu^I-MFU-4l retains a constant O₂ capacity over the course of repeated cycling under dynamic breakthrough conditions. While this material simultaneously adsorbs N₂, differences in O₂ and N₂ desorption kinetics allow for the isolation of high-purity O₂ (>99%) under relatively mild regeneration conditions. Spectroscopic, magnetic, and computational analyses reveal that O₂ binds to the copper(I) sites to form copper(II)-superoxide moieties that exhibit temperature-dependent side-on and end-on binding modes. Overall, these results suggest that Cu^I-MFU-4l is a promising material for the separation of O₂ from ambient air, even without dehumidification.

In nature, nonheme iron enzymes use dioxygen to generate high-spin iron(IV)=O species for a variety of oxygenation reactions. Although synthetic chemists have long sought to mimic this reactivity, the enzyme-like activation of O₂ to form high-spin iron(IV) = O species remains an unrealized goal. Here, we report a metal-organic framework featuring iron(II) sites with a local structure similar to that in α-ketoglutarate-dependent dioxygenases. The framework reacts with O₂ at low temperatures to form high-spin iron(IV) = O species that are characterized using in situ diffuse reflectance infrared Fourier transform, in situ and variable-field Mössbauer, Fe Kβ x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O₂, the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.