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

ABSTRACT: 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 A_xFe₂(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 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 O₂ uptake behavior similar to that of A_xFe₂(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.

SUBMITTER: Jaffe A

PROVIDER: S-EPMC7484140 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Covalent organic frameworks (COFs) have high potential in gas separation technologies because of their porous structures, large surface areas, and good stabilities. The number of synthesized COFs already reached several hundreds, but only a handful of materials were tested as adsorbents and/or membranes. We used a high-throughput computational screening approach to uncover adsorption-based and membrane-based CO2/H2 separation potentials of 288 COFs, representing the highest number of experimentally synthesized COFs studied to date for precombustion CO2 capture. Grand canonical Monte Carlo (GCMC) simulations were performed to assess CO2/H2 mixture separation performances of COFs for five different cyclic adsorption processes: pressure swing adsorption, vacuum swing adsorption, temperature swing adsorption (TSA), pressure-temperature swing adsorption (PTSA), and vacuum-temperature swing adsorption (VTSA). The results showed that many COFs outperform traditional zeolites in terms of CO2 selectivities and working capacities and PTSA is the best process leading to the highest adsorbent performance scores. Combining GCMC and molecular dynamics (MD) simulations, CO2 and H2 permeabilities and selectivities of COF membranes were calculated. The majority of COF membranes surpass Robeson's upper bound because of their higher H2 permeabilities compared to polymers, indicating that the usage of COFs has enormous potential to replace current materials in membrane-based H2/CO2 separation processes. Performance analysis based on the structural properties showed that COFs with narrow pores [the largest cavity diameter (LCD) < 15 Å] and low porosities (ϕ < 0.75) are the top adsorbents for selective separation of CO2 from H2, whereas materials with large pores (LCD > 20 Å) and high porosities (ϕ > 0.85) are generally the best COF membranes for selective separation of H2 from CO2. These results will help to speed up the engineering of new COFs with desired structural properties to achieve high-performance CO2/H2 separations.

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

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

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OmicsDI is part of the ELIXIR infrastructure

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