High-Pressure Chemistry of a Zeolitic Imidazolate Framework Compound in the Presence of Different Fluids

Pressure-dependent structural and chemical changes of the zeolitic imid­azolate framework compound ZIF-8 have been investigated using different pressure transmitting media (PTM) up to 4 GPa. The unit cell of ZIF-8 expands and contracts under hydrostatic pressure depending on the solvent molecules used as PTM. When pressurized in water up to 2.2(1) GPa, the unit cell of ZIF-8 reveals a gradual contraction. In contrast, when alcohols are used as PTM, the ZIF-8 unit cell volume initially expands by 1.2% up to 0.3(1) GPa in methanol, and by 1.7% up to 0.6(1) GPa in ethanol. Further pressure increase then leads to a discontinuous second volume expansion by 1.9% at 1.4(1) GPa in methanol and by 0.3% at 2.3(1) GPa in ethanol. The continuous uptake of molecules under pressure, modeled by the residual electron density derived from Rietveld refinements of X-ray powder diffraction, reveals a saturation pressure near 2 GPa. In non-penetrating PTM (silicone oil), ZIF-8 becomes amorphous at 0.9(1) GPa. The structural changes observed in the ZIF-8-PTM system under pressure point to distinct molecular interactions within the pores

Synthesis and Selective CO₂ Capture Properties of a Series of Hexatopic Linker-Based Metal–Organic Frameworks

Four crystalline, porous metal–organic frameworks (MOFs), based on a new hexatopic linker, 1′,2′,3′,4′,5′,6′-hexakis­(4-carboxyphenyl)­benzene (H₆CPB), were synthesized and fully characterized. Interestingly, two members of this series exhibited new topologies, namely, <b>htp</b> and <b>hhp</b>, which were previously unseen in MOF chemistry. Gas adsorption measurements revealed that all members exhibited high CO₂ selectivity over N₂ and CH₄. Accordingly, breakthrough measurements were performed on a representative example, in which the effective separation of CO₂ from binary mixtures containing either N₂ or CH₄ was demonstrated without any loss in performance over three consecutive cycles

Liquid-Like Hydrogen Stored in Nanoporous Materials at 50 K Observed by in Situ Neutron Diffraction Experiments

In addition to surface adsorption, hydrogen molecules stored as liquid-like gas at low temperature in zeolites (Na–X, Ca–X, Mg–X) and metal–organic frameworks (MOF-5, MOF-205) were observed using an in situ neutron diffraction experiment. In situ neutron diffraction data indicate that hydrogen molecules form a <i>loosely bound state</i> at 50 K, which is above the critical temperature of hydrogen; the position and broad shapes of the diffraction patterns are very similar to those of liquid hydrogen (D₂). However, this new state of hydrogen (D₂) cannot be in liquid phases because the critical temperature (<i>T</i>_c = 38.34 K) is much less than 50 K. As a contrastive study, the same measurements were carried out with other types of zeolite (ZSM-5) and MOF (HKUST-1), but no broad diffraction patterns were observed. According to Grand Canonical Monte Carlo (GCMC) simulations on the three model systems of Na–X, HKUST-1, and MOF-205 (six steps of hydrogen loading at 50 K), the origin of the broad peaks is attributed to the short-range ordering (SRO) of the hydrogen molecules which are not tightly bound to the adsorbents. The necessary conditions for the existence of the SRO can be stated as follows: There should be enough interaction potential wells (adsorption sites) that are connected with each other through shallow potential bridges. These potential bridges result from an appropriate superposition of the crystal fields in hydrogen-adsorbed systems

In Situ Neutron Powder Diffraction and X‑ray Photoelectron Spectroscopy Analyses on the Hydrogenation of MOF‑5 by Pt-Doped Multiwalled Carbon Nanotubes

Recent research on hydrogen storage in metal–organic frameworks focuses on how to achieve increased hydrogen binding energies by using doped metal, using unsaturated metal ions, or forming composites comprising Pt-doped carbon materials. In particular, noticeable progress using MOFs and Pt-doped carbons has been achieved to enhance the hydrogen storage capacity near room temperature. A three-component composite material, Pt-MWCNT-MOF5 (PMM5), which is a metal–organic framework (MOF-5) hybridized with Pt nanoparticles on multiwalled carbon nanotubes (MWCNT), stores 0.22 wt % hydrogen at 320 K and 30 bar, which is larger than 0.13 wt % at 300 K and 30 bar. Although the increased quantity is small, it is possible to detect the origin of uptake increase based on various analyses. In situ neutron diffraction experiments of deuterium-sorbed PMM5 with a temperature cycling process (D₂ loading at 50 K → 4 K → 320 K, 2 h → 50 K → 4 K) result in a significant background increase owing to both chemisorbed deuterium atoms and a local deformation of the MOF-5 framework. Hydrogen loading in PMM5 induces significant binding energy shifts in C 1s and Zn 2p_3/2 electrons in the X-ray photoelectron spectra, suggesting the chemical environment change in Zn₄O­(COO)₆ coordination sphere in MOF-5. All accumulated experimental data support the fact that the hydrogen receptor is the oxygen atoms of benzene-1,4-dicarboxylates of MOF-5, which is facilitated by the embedded Pt-MWCNT

Mixed-Metal Zeolitic Imidazolate Frameworks and their Selective Capture of Wet Carbon Dioxide over Methane

A presynthesized, square planar copper imidazole complex, [Cu­(imidazole)₄]­(NO₃)₂, was utilized as a precursor in the synthesis of a new series of zeolitic imidazolate frameworks, termed ZIF-202, -203, and -204. The structures of all three members were solved by single-crystal X-ray diffraction analysis, which revealed ZIF-203 and -204 having successfully integrated square planar units within the backbones of their respective frameworks. As a result of this unit, the structures of both ZIF-203 and -204 were found to adopt unprecedented three-dimensional nets, namely, <b>ntn</b> and <b>thl</b>, respectively. One member of this series, ZIF-204, was demonstrated to be highly porous, exhibit exceptional stability in water, and selectively capture CO₂ over CH₄ under both dry and wet conditions without any loss in performance over three cycles. Remarkably, the regeneration of ZIF-204 was performed under the mild conditions of flowing a pure N₂ gas through the material at ambient temperature

Introduction of Functionality, Selection of Topology, and Enhancement of Gas Adsorption in Multivariate Metal–Organic Framework-177

Metal–organic framework-177 (MOF-177) is one of the most porous materials whose structure is composed of octahedral Zn₄O­(−COO)₆ and triangular 1,3,5-benzenetribenzoate (BTB) units to make a three-dimensional extended network based on the <b>qom</b> topology. This topology violates a long-standing thesis where highly symmetric building units are expected to yield highly symmetric networks. In the case of octahedron and triangle combinations, MOFs based on pyrite (<b>pyr</b>) and rutile (<b>rtl</b>) nets were expected instead of <b>qom</b>. In this study, we have made 24 MOF-177 structures with different functional groups on the triangular BTB linker, having one or more functionalities. We find that the position of the functional groups on the BTB unit allows the selection for a specific net (<b>qom</b>, <b>pyr</b>, and <b>rtl</b>), and that mixing of functionalities (-H, -NH₂, and -C₄H₄) is an important strategy for the incorporation of a specific functionality (-NO₂) into MOF-177 where otherwise incorporation of such functionality would be difficult. Such mixing of functionalities to make multivariate MOF-177 structures leads to enhancement of hydrogen uptake by 25%

Interplay of Metalloligand and Organic Ligand to Tune Micropores within Isostructural Mixed-Metal Organic Frameworks (M′MOFs) for Their Highly Selective Separation of Chiral and Achiral Small Molecules

Four porous isostructural mixed-metal–organic frameworks (M′MOFs) have been synthesized and structurally characterized. The pores within these M′MOFs are systematically tuned by the interplay of both the metalloligands and organic ligands which have enabled us not only to direct their highly selective separation of chiral alcohols 1-phenylethanol (PEA), 2-butanol (BUT), and 2-pentanol (2-PEN) with the highest ee up to 82.4% but also to lead highly selective separation of achiral C₂H₂/C₂H₄ separation. The potential application of these M′MOFs for the fixed bed pressure swing adsorption (PSA) separation of C₂H₂/C₂H₄ has been further examined and compared by the transient breakthrough simulations in which the purity requirement of 40 ppm in the outlet gas can be readily fulfilled by the fixed bed M′MOF-<b>4a</b> adsorber at ambient conditions

Introduction of Functionality, Selection of Topology, and Enhancement of Gas Adsorption in Multivariate Metal–Organic Framework-177

Metal–organic framework-177 (MOF-177) is one of the most porous materials whose structure is composed of octahedral Zn₄O­(−COO)₆ and triangular 1,3,5-benzenetribenzoate (BTB) units to make a three-dimensional extended network based on the <b>qom</b> topology. This topology violates a long-standing thesis where highly symmetric building units are expected to yield highly symmetric networks. In the case of octahedron and triangle combinations, MOFs based on pyrite (<b>pyr</b>) and rutile (<b>rtl</b>) nets were expected instead of <b>qom</b>. In this study, we have made 24 MOF-177 structures with different functional groups on the triangular BTB linker, having one or more functionalities. We find that the position of the functional groups on the BTB unit allows the selection for a specific net (<b>qom</b>, <b>pyr</b>, and <b>rtl</b>), and that mixing of functionalities (-H, -NH₂, and -C₄H₄) is an important strategy for the incorporation of a specific functionality (-NO₂) into MOF-177 where otherwise incorporation of such functionality would be difficult. Such mixing of functionalities to make multivariate MOF-177 structures leads to enhancement of hydrogen uptake by 25%

Interplay of Metalloligand and Organic Ligand to Tune Micropores within Isostructural Mixed-Metal Organic Frameworks (M′MOFs) for Their Highly Selective Separation of Chiral and Achiral Small Molecules

Four porous isostructural mixed-metal–organic frameworks (M′MOFs) have been synthesized and structurally characterized. The pores within these M′MOFs are systematically tuned by the interplay of both the metalloligands and organic ligands which have enabled us not only to direct their highly selective separation of chiral alcohols 1-phenylethanol (PEA), 2-butanol (BUT), and 2-pentanol (2-PEN) with the highest ee up to 82.4% but also to lead highly selective separation of achiral C₂H₂/C₂H₄ separation. The potential application of these M′MOFs for the fixed bed pressure swing adsorption (PSA) separation of C₂H₂/C₂H₄ has been further examined and compared by the transient breakthrough simulations in which the purity requirement of 40 ppm in the outlet gas can be readily fulfilled by the fixed bed M′MOF-<b>4a</b> adsorber at ambient conditions

Interplay of Metalloligand and Organic Ligand to Tune Micropores within Isostructural Mixed-Metal Organic Frameworks (M′MOFs) for Their Highly Selective Separation of Chiral and Achiral Small Molecules

Four porous isostructural mixed-metal–organic frameworks (M′MOFs) have been synthesized and structurally characterized. The pores within these M′MOFs are systematically tuned by the interplay of both the metalloligands and organic ligands which have enabled us not only to direct their highly selective separation of chiral alcohols 1-phenylethanol (PEA), 2-butanol (BUT), and 2-pentanol (2-PEN) with the highest ee up to 82.4% but also to lead highly selective separation of achiral C₂H₂/C₂H₄ separation. The potential application of these M′MOFs for the fixed bed pressure swing adsorption (PSA) separation of C₂H₂/C₂H₄ has been further examined and compared by the transient breakthrough simulations in which the purity requirement of 40 ppm in the outlet gas can be readily fulfilled by the fixed bed M′MOF-<b>4a</b> adsorber at ambient conditions