Noncryogenic Air Separation Using Aluminum Formate Al(HCOO)3 (ALF)

Separating oxygen from air to create oxygen-enriched gas streams is a process that is significant in both industrial and medical fields. However, the prominent technologies for creating oxygen-enriched gas streams are both energy and infrastructure intensive as they use cryogenic temperatures or materials that adsorb N₂ from air. The latter method is less efficient than the methods that adsorb O₂ directly. Herein, we show, via a combination of gas adsorption isotherms, gas breakthrough experiments, neutron and synchrotron X-ray powder diffraction, Raman spectroscopy, and computational studies, that the metal-organic framework, Al(HCOO)₃ (ALF), which is easily prepared at low cost from commodity chemicals, exhibits substantial O₂ adsorption and excellent time-dependent O₂/N₂ selectivity in a range of 50-125 near dry ice/solvent (≈190 K) temperatures. The effective O₂ adsorption with ALF at ≈190 K and ≈0.21 bar (the partial pressure of O₂ in air) is ≈1.7 mmol/g, and at ice/salt temperatures (≈250 K), it is ≈0.3 mmol/g. Though the kinetics for full adsorption of O₂ near 190 K are slower than at temperatures nearer 250 K, the kinetics for initial O₂ adsorption are fast, suggesting that O₂ separation using ALF with rapid temperature swings at ambient pressures is a potentially viable choice for low-cost air separation applications. We also present synthetic strategies for improving the kinetics of this family of compounds, namely, via Al/Fe solid solutions. To the best of our knowledge, ALF has the highest O₂/N₂ sorption selectivity among MOF adsorbents without open metal sites as verified by co-adsorption experiments..