Selective, High-Temperature O_2 Adsorption in Chemically Reduced, Redox-Active Iron-Pyrazolate Metal-Organic Frameworks

摘要

Developing O_2-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 A_xFe_2(bdp)_3 (A = Na~+, K~+; bdp~(2-) = 1,4-benzenedipyrazolate; 0 < x < 2), which feature coordinatively saturated iron centers, are capable of strong and selective adsorption of O_2 over N_2 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 ~(23)Na solid-state NMR, Mossbauer, and X-ray photoelectron spectroscopies, are employed as probes of O_2 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 O_2 uptake behavior similar to that of A_xFe_2(bdp)_3 in an expanded-pore framework analogue and thereby gain additional insight into the O_2 adsorption mechanism. The chemical reduction of a robust metal-organic framework to render it capable of binding O_2 through such an outer-sphere electron transfer mechanism represents a promising and underexplored strategy for the design of next-generation O_2 adsorbents.