Selective Adsorption of Oxygen from Humid Air in a Metal–Organic Framework with Trigonal Pyramidal Copper(I) Sites
Kurtis M. Carsch, Adrian Huang, Matthew N. Dods, Surya T. Parker, Rachel C. Rohde, Henry Z. H. Jiang, Yuto Yabuuchi, Sarah L. Karstens, Hyunchul Kwon, Romit Chakraborty, Karen C. Bustillo, Katie R. Meihaus, Hiroyasu Furukawa, Andrew M. Minor, Martin Head‐Gordon, Jeffrey R. Long
Abstract
High Resolution Image Download MS PowerPoint Slide High or enriched-purity O 2 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 O 2 -selective air separations, including the use of metal–organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O 2 over N 2 via electron transfer. However, most of these materials exhibit appreciable and/or reversible O 2 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-4 l (Cu x Zn 5– x Cl 4– x (btdd) 3; H 2 btdd = bis(1 H -1,2,3-triazolo[4,5- b ],[4′,5′- i ])dibenzo[1,4]dioxin), which binds O 2 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 O 2 from air at 25 °C, even in the presence of water. When exposed to air up to 100% relative humidity, Cu I -MFU-4 l retains a constant O 2 capacity over the course of repeated cycling under dynamic breakthrough conditions. While this material simultaneously adsorbs N 2, differences in O 2 and N 2 desorption kinetics allow for the isolation of high-purity O 2 (>99%) under relatively mild regeneration conditions. Spectroscopic, magnetic, and computational analyses reveal that O 2 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-4 l is a promising material for the separation of O 2 from ambient air, even without dehumidification.