Metal–organic frameworks as O2-selective adsorbents for air separations

Abstract

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.