Selective Binding of O₂ over N₂ in a Redox–Active Metal–Organic Framework with Open Iron(II) Coordination Sites

Abstract

The air-free reaction between FeCl 2 and H 4 dobdc (dobdc 4À = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe 2 (dobdc) 3 4DMF, a metalÀorganic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a BrunauerÀEmmettÀTeller (BET) surface area of 1360 m 2 /g and features a hexagonal array of onedimensional channels lined with coordinatively unsaturated Fe II centers. Gas adsorption isotherms at 298 K indicate that Fe 2 (dobdc) binds O 2 preferentially over N 2 , with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O 2 molecule per two iron centers. Remarkably, at 211 K, O 2 uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O 2 molecule per iron center. M€ ossbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O 2 at low temperature and complete charge transfer to form iron(III) and O 2 2À at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O 2 bound to iron in a symmetric sideon mode with d OÀO = 1.25(1) Å at low temperature and in a slipped side-on mode with d OÀO = 1.6(1) Å when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe 2 (dobdc) to be a promising material for the separation of O 2 from air at temperatures well above those currently employed in industrial settings. ' INTRODUCTIONWith over 100 million tons produced annually, O 2 is one of the most widely used commodity chemicals in the world. 1 Its potential utility in processes associated with the reduction of carbon dioxide emissions from fossil fuel-burning power plants, however, means that the demand for pure O 2 could grow enormously. For implementation of precombustion CO 2 capture, pure O 2 is needed for the gasification of coal, which produces the feedstock for the waterÀgas shift reaction used to produce CO 2 and H 2 . 2 In addition, oxyfuel combustion is receiving considerable attention for its potential utility as an alternative to postcombustion CO 2 capture. Here, pure O 2 is diluted to 0.21 bar with CO 2 and fed into a power plant for fuel combustion. Since N 2 is absent from the resulting flue gas, the requirement for postcombustion separation of CO 2 from N 2 is eliminated. 3 The separation of O 2 from air is currently carried out on a large scale using an energy-intensive cryogenic distillation process. 4 Zeolites are also used for O 2 /N 2 separation, 5 both industrially and in portable medical devices; however, this process is inherently inefficient as the materials used adsorb N 2 over O 2 with poor selectivity. By employing materials that selectively adsorb

A Dual−Ion Battery Cathode via Oxidative Insertion of Anions in a Metal–Organic Framework

“…38 The nearest Fe···Fe distance of 3.04 Å in Fe 2 (dobdc), 28 which should match that in the isostructural Fe 2 (dobpdc), is the shortest of any metal−organic framework with infinite iron chains. Those that are the next closest are Fe III (gallate)·2H 2 O and Fe(bdc)(OH) 0.8 (F) 0.2 , possessing vertex-sharing octahedra.…”

“…21 The framework, shown in Figure 1a, has hexagonal channels with five-coordinate Fe 2+ ions lining the vertices in infinite 1D chains of edge-sharing square pyramids. 28 When solvated, the iron centers are pseudooctahedral. Similar phases are known to be redox-active and semiconducting.…”

“…28,64 In a dry, argon-filled glovebox, 24.3 mg (0.0795 mmol) of Fe 2 (dobdc) was suspended in MeCN (∼6 mL) and stirred virgorously in a 20 mL glass scintillation vial. In another vial, 28.7 mg (0.0794 mmol) of C 12 H 8 S 2…”

Understanding Small‐Molecule Interactions in Metal–Organic Frameworks: Coupling Experiment with Theory

“…[ 29 ] These open metal sites are shown to induce framework selectivity in the adsorption of small molecules and provide a mechanism for charge transfer on the framework surface. [30][31][32][33] While open metal sites provide strong interactions allowing gas adsorption at higher temperatures and lower pressures than typically used for energy-consuming cryodistillation processes, the adsorbate-adsorbent interactions are often weak relative to the formation of chemical bonds, providing facile release of the molecules during the regeneration step of a separation process. Recent work by McDonald and co-workers also revealed that certain diamines, grafted to open metal sites, can offer strong, selective binding of CO 2 at low pressures (ca.…”

“…One of the most well-studied MOFs to date is M 2 (dobdc), alternatively known as M-MOF-74 or CPO-27-M, where M = Mg, Mn, Fe, Co, Ni, Cu, or Zn ( Figure 2 ). [ 30,[37][38][39][40][41][42][43][44] The signifi cance of 2 H 6 ; the binding mechanisms (right) were unveiled by neutron powder diffraction. [ 30,31 ] The metal, carbon, oxygen, nitrogen, and deuterium atoms are drawn as orange, grey, red, royal blue, and light blue spheres, respectively.…”