Selective Adsorption of the Super Greenhouse Gas Tetrafluoromethane within a Metal–Organic Framework with Dynamic Corrugated Ultramicropores

Perfluorocompound (PFC) gases play vital roles in microelectronics processing. Requirements for ulta-high purities traditionally necessitate use of virgin sources and thereby hinder the capture, purification, and reuse of these costly gases. Most importantly, gaseous PFCs are incredibly potent greenhouse gases with atmospheric lifetimes on the order of 103-104 years, and thus any environmental emissions have an outsized and prolonged impact on our climate. The development of sorbents that can capture PFC gases from industrial waste streams has lagged substantially behind the progress made over the last decade in capturing CO2 from both point emission sources and directly from air. Herein, we show that the metal–organic framework Zn(fba) (fba2– = 4,4’-(hexafluoroisopropylidene)bis-benzoate) displays an equilibrium selectivity for CF4 adsorption over N2 that surpasses those of all water-stable sorbents that have been reported for this separation. This selectivity is enabled by adsorption within narrow corrugated channels lined with ligand-based aryl rings, a site within this material that has not previously been realized as being accessible to guests. Analyses of adsorption kinetics and X-ray diffraction data are used to characterize sorption and diffusion of small adsorbates within these channels and strongly implicate rotation of the linker aryl rings as a gate that modulates transport of CF4 through a crystallite. Multi-component breakthrough measurements demonstrate that Zn(fba) is able to resolve CF4 and N2 under flowing mixed-gas conditions. Taken together, this work illuminates a more complete picture of the dynamic structure of Zn(fba), and also points toward general design principles that can enable large CF4 selectivities in sorbents with more favorable kinetic profiles.