Near-Perfect CO₂/CH₄ Selectivity Achieved through Reversible Guest Templating in the Flexible Metal–Organic Framework Co(bdp)

Metal–organic frameworks are among the most promising materials for industrial gas separations, including the removal of carbon dioxide from natural gas, although substantial improvements in adsorption selectivity are still sought. Herein, we use equilibrium adsorption experiments to demonstrate that the flexible metal–organic framework Co­(bdp) (bdp^2– = 1,4-benzene­dipyrazolate) exhibits a large CO₂ adsorption capacity and approaches complete exclusion of CH₄ under 50:50 mixtures of the two gases, leading to outstanding CO₂/CH₄ selectivity under these conditions. <i>In situ</i> powder X-ray diffraction data indicate that this selectivity arises from reversible guest templating, in which the framework expands to form a CO₂ clathrate and then collapses to the nontemplated phase upon desorption. Under an atmosphere dominated by CH₄, Co­(bdp) adsorbs minor amounts of CH₄ along with CO₂, highlighting the importance of studying all relevant pressure and composition ranges via multicomponent measurements when examining mixed-gas selectivity in structurally flexible materials. Altogether, these results show that Co­(bdp) may be a promising CO₂/CH₄ separation material and provide insights for the further study of flexible adsorbents for gas separations

Near-Perfect CO₂/CH₄ Selectivity Achieved through Reversible Guest Templating in the Flexible Metal–Organic Framework Co(bdp)

Metal–organic frameworks are among the most promising materials for industrial gas separations, including the removal of carbon dioxide from natural gas, although substantial improvements in adsorption selectivity are still sought. Herein, we use equilibrium adsorption experiments to demonstrate that the flexible metal–organic framework Co­(bdp) (bdp^2– = 1,4-benzene­dipyrazolate) exhibits a large CO₂ adsorption capacity and approaches complete exclusion of CH₄ under 50:50 mixtures of the two gases, leading to outstanding CO₂/CH₄ selectivity under these conditions. <i>In situ</i> powder X-ray diffraction data indicate that this selectivity arises from reversible guest templating, in which the framework expands to form a CO₂ clathrate and then collapses to the nontemplated phase upon desorption. Under an atmosphere dominated by CH₄, Co­(bdp) adsorbs minor amounts of CH₄ along with CO₂, highlighting the importance of studying all relevant pressure and composition ranges via multicomponent measurements when examining mixed-gas selectivity in structurally flexible materials. Altogether, these results show that Co­(bdp) may be a promising CO₂/CH₄ separation material and provide insights for the further study of flexible adsorbents for gas separations

Reversible Capture and Release of Cl₂ and Br₂ with a Redox-Active Metal–Organic Framework

Extreme toxicity, corrosiveness, and volatility pose serious challenges for the safe storage and transportation of elemental chlorine and bromine, which play critical roles in the chemical industry. Solid materials capable of forming stable nonvolatile compounds upon reaction with elemental halogens may partially mitigate these challenges by allowing safe halogen release on demand. Here we demonstrate that elemental halogens quantitatively oxidize coordinatively unsaturated Co­(II) ions in a robust azolate metal–organic framework (MOF) to produce stable and safe-to-handle Co­(III) materials featuring terminal Co­(III)–halogen bonds. Thermal treatment of the oxidized MOF causes homolytic cleavage of the Co­(III)–halogen bonds, reduction to Co­(II), and concomitant release of elemental halogens. The reversible chemical storage and thermal release of elemental halogens occur with no significant losses of structural integrity, as the parent cobaltous MOF retains its crystallinity and porosity even after three oxidation/reduction cycles. These results highlight a material operating via redox mechanism that may find utility in the storage and capture of other noxious and corrosive gases

Nature of Decahydro-<i>closo</i>-decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb₂B₁₀H₁₀

The ordered monoclinic phase of the alkali-metal decahydro-<i>closo</i>-decaborate salt Rb₂B₁₀H₁₀ was found to be stable from about 250 K all the way up to an order–disorder phase transition temperature of ≈762 K. The broad temperature range for this phase allowed for a detailed quasielastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) study of the protypical B₁₀H₁₀^2– anion reorientational dynamics. The QENS and NMR combined results are consistent with an anion reorientational mechanism comprised of two types of rotational jumps expected from the anion geometry and lattice structure, namely, more rapid 90° jumps around the anion <i>C</i>₄ symmetry axis (e.g., with correlation frequencies of ≈2.6 × 10¹⁰ s^–1 at 530 K) combined with order of magnitude slower orthogonal 180° reorientational flips (e.g., ≈3.1 × 10⁹ s^–1 at 530 K) resulting in an exchange of the apical H (and apical B) positions. Each latter flip requires a concomitant 45° twist around the <i>C</i>₄ symmetry axis to preserve the ordered Rb₂B₁₀H₁₀ monoclinic structural symmetry. This result is consistent with previous NMR data for ordered monoclinic Na₂B₁₀H₁₀, which also pointed to two types of anion reorientational motions. The QENS-derived reorientational activation energies are 197(2) and 288(3) meV for the <i>C</i>₄ fourfold jumps and apical exchanges, respectively, between 400 and 680 K. Below this temperature range, NMR (and QENS) both indicate a shift to significantly larger reorientational barriers, for example, 485(8) meV for the apical exchanges. Finally, subambient diffraction measurements identify a subtle change in the Rb₂B₁₀H₁₀ structure from monoclinic to triclinic symmetry as the temperature is decreased from around 250 to 210 K

Nature of Decahydro-<i>closo</i>-decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb₂B₁₀H₁₀

The ordered monoclinic phase of the alkali-metal decahydro-<i>closo</i>-decaborate salt Rb₂B₁₀H₁₀ was found to be stable from about 250 K all the way up to an order–disorder phase transition temperature of ≈762 K. The broad temperature range for this phase allowed for a detailed quasielastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) study of the protypical B₁₀H₁₀^2– anion reorientational dynamics. The QENS and NMR combined results are consistent with an anion reorientational mechanism comprised of two types of rotational jumps expected from the anion geometry and lattice structure, namely, more rapid 90° jumps around the anion <i>C</i>₄ symmetry axis (e.g., with correlation frequencies of ≈2.6 × 10¹⁰ s^–1 at 530 K) combined with order of magnitude slower orthogonal 180° reorientational flips (e.g., ≈3.1 × 10⁹ s^–1 at 530 K) resulting in an exchange of the apical H (and apical B) positions. Each latter flip requires a concomitant 45° twist around the <i>C</i>₄ symmetry axis to preserve the ordered Rb₂B₁₀H₁₀ monoclinic structural symmetry. This result is consistent with previous NMR data for ordered monoclinic Na₂B₁₀H₁₀, which also pointed to two types of anion reorientational motions. The QENS-derived reorientational activation energies are 197(2) and 288(3) meV for the <i>C</i>₄ fourfold jumps and apical exchanges, respectively, between 400 and 680 K. Below this temperature range, NMR (and QENS) both indicate a shift to significantly larger reorientational barriers, for example, 485(8) meV for the apical exchanges. Finally, subambient diffraction measurements identify a subtle change in the Rb₂B₁₀H₁₀ structure from monoclinic to triclinic symmetry as the temperature is decreased from around 250 to 210 K

Nature of Decahydro-<i>closo</i>-decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb₂B₁₀H₁₀

The ordered monoclinic phase of the alkali-metal decahydro-<i>closo</i>-decaborate salt Rb₂B₁₀H₁₀ was found to be stable from about 250 K all the way up to an order–disorder phase transition temperature of ≈762 K. The broad temperature range for this phase allowed for a detailed quasielastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) study of the protypical B₁₀H₁₀^2– anion reorientational dynamics. The QENS and NMR combined results are consistent with an anion reorientational mechanism comprised of two types of rotational jumps expected from the anion geometry and lattice structure, namely, more rapid 90° jumps around the anion <i>C</i>₄ symmetry axis (e.g., with correlation frequencies of ≈2.6 × 10¹⁰ s^–1 at 530 K) combined with order of magnitude slower orthogonal 180° reorientational flips (e.g., ≈3.1 × 10⁹ s^–1 at 530 K) resulting in an exchange of the apical H (and apical B) positions. Each latter flip requires a concomitant 45° twist around the <i>C</i>₄ symmetry axis to preserve the ordered Rb₂B₁₀H₁₀ monoclinic structural symmetry. This result is consistent with previous NMR data for ordered monoclinic Na₂B₁₀H₁₀, which also pointed to two types of anion reorientational motions. The QENS-derived reorientational activation energies are 197(2) and 288(3) meV for the <i>C</i>₄ fourfold jumps and apical exchanges, respectively, between 400 and 680 K. Below this temperature range, NMR (and QENS) both indicate a shift to significantly larger reorientational barriers, for example, 485(8) meV for the apical exchanges. Finally, subambient diffraction measurements identify a subtle change in the Rb₂B₁₀H₁₀ structure from monoclinic to triclinic symmetry as the temperature is decreased from around 250 to 210 K