Noncryogenic Air Separation Using Aluminum Formate Al(HCOO)<sub>3</sub> (ALF)
Dinesh Mullangi, Hayden A. Evans, Taner Yildirim, Yuxiang Wang, Zeyu Deng, Zhaoqiang Zhang, T. Thuc, Fengxia Wei, John Wang, Angela R. Hight Walker, Craig M. Brown, Dan Zhao, Pieremanuele Canepa, Anthony K. Cheetham
Abstract
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 N 2 from air. The latter method is less efficient than the methods that adsorb O 2 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 O 2 adsorption and excellent time-dependent O 2 /N 2 selectivity in a range of 50–125 near dry ice/solvent (≈190 K) temperatures. The effective O 2 adsorption with ALF at ≈190 K and ≈0.21 bar (the partial pressure of O 2 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 O 2 near 190 K are slower than at temperatures nearer 250 K, the kinetics for initial O 2 adsorption are fast, suggesting that O 2 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 O 2 /N 2 sorption selectivity among MOF adsorbents without open metal sites as verified by co-adsorption experiments..