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Selective, High-Temperature O2 Adsorption in Chemically Reduced, Redox-Active Iron-Pyrazolate Metal–Organic Frameworks

preprint
revised on 23.06.2020, 14:20 and posted on 25.06.2020, 07:04 by Adam Jaffe, Michael Ziebel, David M. Halat, Naomi Biggins, Ryan Murphy, Khetpakorn Chakarawet, Jeffrey A. Reimer, Jeffrey R. Long
Developing O2-selective adsorbents that can produce high-purity oxygen from air remains a significant challenge. Here, we show that chemically reduced metal–organic framework materials of the type AxFe2(bdp)3 (A = Na+, K+; bdp2 = 1,4-benzenedipyrazolate; 0 < x ≤ 2), which feature coordinatively saturated iron centers, are capable of strong and selective adsorption of O2 over N2 at ambient (25 °C) or even elevated (200 °C) temperature. A combination of gas adsorption analysis, single-crystal X-ray diffraction, magnetic susceptibility measurements, and a range of spectroscopic methods, including 23Na solid-state NMR, Mössbauer, and X-ray photoelectron spectroscopies, are employed as probes of O2 uptake. Significantly, the results support a selective adsorption mechanism involving outer-sphere electron transfer from the framework to form superoxide species, which are subsequently stabilized by intercalated alkali metal cations that reside in the one-dimensional triangular pores of the structure. We further demonstrate similar O2 uptake behavior to that of AxFe2(bdp)3 in an expanded-pore framework analogue and thereby gain additional insight into the O2 adsorption mechanism. The chemical reduction of a robust metal–organic framework to render it capable of binding O2 through such an outer-sphere electron transfer mechanism represents a promising and underexplored strategy for the design of next-generation O2 adsorbents.

Funding

Interrogating Selective Metal-Adsorbate Interactions in Metal-Organic Frameworks

Basic Energy Sciences

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History

Email Address of Submitting Author

adamjaffe@berkeley.edu

Institution

University of California, Berkeley

Country

United States

ORCID For Submitting Author

0000-0002-9886-0249

Declaration of Conflict of Interest

These authors declare no competing financial interest

Version Notes

Jaffe_Long_ChemRxivmanuscript4 Version 4 changes: minor polishing of crystal structure figures

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