High Methane Storage Capacity in Aluminum Metal–Organic Frameworks

The use of porous materials to store natural gas in vehicles requires large amounts of methane per unit of volume. Here we report the synthesis, crystal structure and methane adsorption properties of two new aluminum metal–organic frameworks, MOF-519 and MOF-520. Both materials exhibit permanent porosity and high methane volumetric storage capacity: MOF-519 has a volumetric capacity of 200 and 279 cm³ cm^–3 at 298 K and 35 and 80 bar, respectively, and MOF-520 has a volumetric capacity of 162 and 231 cm³ cm^–3 under the same conditions. Furthermore, MOF-519 exhibits an exceptional working capacity, being able to deliver a large amount of methane at pressures between 5 and 35 bar, 151 cm³ cm^–3, and between 5 and 80 bar, 230 cm³ cm^–3

High Methane Storage Working Capacity in Metal–Organic Frameworks with Acrylate Links

High methane storage capacity in porous materials is important for the design and manufacture of vehicles powered by natural gas. Here, we report the synthesis, crystal structures and methane adsorption properties of five new zinc metal–organic frameworks (MOFs), MOF-905, MOF-905-Me₂, MOF-905-Naph, MOF-905-NO₂, and MOF-950. All these MOFs consist of the Zn₄O­(−CO₂)₆ secondary building units (SBUs) and benzene-1,3,5-tri-β-acrylate, BTAC. The permanent porosity of all five materials was confirmed, and their methane adsorption measured up to 80 bar to reveal that MOF-905 is among the best performing methane storage materials with a volumetric working capacity (desorption at 5 bar) of 203 cm³ cm^–3 at 80 bar and 298 K, a value rivaling that of HKUST-1 (200 cm³ cm^–3), the benchmark compound for methane storage in MOFs. This study expands the scope of MOF materials with ultrahigh working capacity to include linkers having the common acrylate connectivity

Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology

The adsorption of H₂ in ZIF-76, a zeolitic imidazolate framework (ZIF) with lta topology, was investigated in a combined experimental and theoretical study. Each Zn²⁺ ion in the structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains a large amount of structural disorder, which makes this a challenging material for modeling. We therefore chose to parametrize and simulate H₂ adsorption in two distinct crystal structure configurations of ZIF-76 that differ by only the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. The simulated H₂ adsorption isotherms for both structures are in satisfactory agreement with the newly reported experimental data for the ZIF, especially at low pressures. The experimental initial isosteric heat of adsorption (<i>Q</i>_st) value for H₂ in ZIF-76 was determined to be 7.7 kJ mol^–1, which is comparable to that for other ZIFs and is fairly high for a material that does not contain open-metal sites. Simulations of H₂ adsorption in one of these structures resulted in <i>Q</i>_st values that are in very good agreement with experiment within the loading range considered. Two notable H₂ binding sites were discovered from simulations in both structures of ZIF-76; however, the preferential regions of H₂ occupancy are reversed for the two structures. The inelastic neutron scattering (INS) spectra for H₂ adsorbed in ZIF-76 contain several peaks that arise from transitions of the hindered H₂ rotor, with the lowest energy peak occurring in the range of 6.0–7.2 meV. Two-dimensional quantum rotation calculations for H₂ adsorbed at the considered sites in both structures yielded rotational transitions that are in good agreement with the peaks that appear in the INS spectra. Despite the large degree of disorder in the ZIF-76 crystal structure, the overall environment in the ZIF still gives rise to interconnected INS features as discerned from our calculations. This study demonstrates how important details of the H₂ adsorption mechanism in a ZIF with structural disorder can be obtained from a combination of experimental measurements and theoretical calculations

Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology

The adsorption of H₂ in ZIF-76, a zeolitic imidazolate framework (ZIF) with lta topology, was investigated in a combined experimental and theoretical study. Each Zn²⁺ ion in the structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains a large amount of structural disorder, which makes this a challenging material for modeling. We therefore chose to parametrize and simulate H₂ adsorption in two distinct crystal structure configurations of ZIF-76 that differ by only the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. The simulated H₂ adsorption isotherms for both structures are in satisfactory agreement with the newly reported experimental data for the ZIF, especially at low pressures. The experimental initial isosteric heat of adsorption (<i>Q</i>_st) value for H₂ in ZIF-76 was determined to be 7.7 kJ mol^–1, which is comparable to that for other ZIFs and is fairly high for a material that does not contain open-metal sites. Simulations of H₂ adsorption in one of these structures resulted in <i>Q</i>_st values that are in very good agreement with experiment within the loading range considered. Two notable H₂ binding sites were discovered from simulations in both structures of ZIF-76; however, the preferential regions of H₂ occupancy are reversed for the two structures. The inelastic neutron scattering (INS) spectra for H₂ adsorbed in ZIF-76 contain several peaks that arise from transitions of the hindered H₂ rotor, with the lowest energy peak occurring in the range of 6.0–7.2 meV. Two-dimensional quantum rotation calculations for H₂ adsorbed at the considered sites in both structures yielded rotational transitions that are in good agreement with the peaks that appear in the INS spectra. Despite the large degree of disorder in the ZIF-76 crystal structure, the overall environment in the ZIF still gives rise to interconnected INS features as discerned from our calculations. This study demonstrates how important details of the H₂ adsorption mechanism in a ZIF with structural disorder can be obtained from a combination of experimental measurements and theoretical calculations

A Covalent Organic Framework that Exceeds the DOE 2015 Volumetric Target for H₂ Uptake at 298 K

Physisorption in porous materials is a promising approach for meeting H₂ storage requirements for the transportation industry, because it is both fully reversible and fast at mild conditions. However, most current candidates lead to H₂ binding energies that are too weak (leading to volumetric capacity at 298 K of <10 g/L compared to the DOE 2015 Target of 40 g/L). Using accurate quantum mechanical (QM) methods, we studied the H₂ binding energy to 48 compounds based on various metalated analogues of five common linkers for covalent organic frameworks (COFs). Considering the first transition row metals (Sc though Cu) plus Pd and Pt, we find that the new COF-301-PdCl₂ reaches 60 g total H₂/L at 100 bar, which is 1.5 times the DOE 2015 target of 40 g/L and close to the ultimate (2050) target of 70 g/L. The best current materials, MOF-200 and MOF-177, are predicted to store 7.6 g/L (0.54 wt % excess) and 9.6 g/L (0.87 wt % excess), respectively, at 298 K and 100 bar compared with 60 g/L (4.2 wt % excess) for COF-301-PdCl₂

High Methanol Uptake Capacity in Two New Series of Metal–Organic Frameworks: Promising Materials for Adsorption-Driven Heat Pump Applications

Two new series of metal–organic frameworks (MOFs), termed M-VNU-74-I and -II (where M = Mg, Ni, Co; VNU = Vietnam National University) were designed to expand the methanol uptake capacities with polar amide functionalities. The resulting MOFs, isoreticular to MOF-74, exhibited high porosity (up to 3000 m² g^–1) as well as the highest reported methanol uptake [>1 g g^–1 or >400 cm³ cm^–3]. As a representative example, Mg-VNU-74-II was shown to maintain a remarkably high stability and methanol mass transfer capacity for at least 42 ad/desorption cycles (3 days). Indeed, these findings highlight the potential of such materials for practical use in adsorption heat pump applications

A Titanium–Organic Framework as an Exemplar of Combining the Chemistry of Metal– and Covalent–Organic Frameworks

A crystalline material with a two-dimensional structure, termed metal–organic framework-901 (MOF-901), was prepared using a strategy that combines the chemistry of MOFs and covalent–organic frameworks (COFs). This strategy involves <i>in situ</i> generation of an amine-functionalized titanium oxo cluster, Ti₆O₆(OCH₃)₆(AB)₆ (AB = 4-aminobenzoate), which was linked with benzene-1,4-dialdehyde using imine condensation reactions, typical of COFs. The crystal structure of MOF-901 is composed of hexagonal porous layers that are likely stacked in staggered conformation (<b>hxl</b> topology). This MOF represents the first example of combining metal cluster chemistry with dynamic organic covalent bond formation to give a new crystalline, extended framework of titanium metal, which is rarely used in MOFs. The incorporation of Ti­(IV) units made MOF-901 useful in the photocatalyzed polymerization of methyl methacrylate (MMA). The resulting polyMMA product was obtained with a high-number-average molar mass (26 850 g mol^–1) and low polydispersity index (1.6), which in many respects are better than those achieved by the commercially available photocatalyst (P-25 TiO₂). Additionally, the catalyst can be isolated, reused, and recycled with no loss in performance

Hydrogen Storage in New Metal–Organic Frameworks

Five new metal–organic frameworks (MOFs, termed MOF-324, 325, 326 and IRMOF-61 and 62) of either short linkers (pyrazolecarboxylate and pyrazaboledicarboxylate) or long and thin alkyne functionalities (ethynyldibenzoate and butadiynedibenzoate) were prepared to examine their impact on hydrogen storage in MOFs. These compounds were characterized by single-crystal X-ray diffraction, and their low-pressure and high-pressure hydrogen uptake properties were investigated. In particular, volumetric excess H₂ uptake by MOF-324 and IRMOF-62 outperforms MOF-177 up to 30 bar. Inelastic neutron-scattering studies for MOF-324 also revealed strong interactions between the organic links and hydrogen, in contrast to MOF-5 where the interactions between the Zn₄O unit and hydrogen are the strongest. These data also show that smaller pores and polarized linkers in MOFs are indeed advantageous for hydrogen storage

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

Seven Post-synthetic Covalent Reactions in Tandem Leading to Enzyme-like Complexity within Metal–Organic Framework Crystals

The design of enzyme-like complexity within metal–organic frameworks (MOFs) requires multiple reactions to be performed on a MOF crystal without losing access to its interior. Here, we show that seven post-synthetic reactions can be successfully achieved within the pores of a multivariate MOF, MTV-IRMOF-74-III, to covalently incorporate tripeptides that resemble the active sites of enzymes in their spatial arrangement and compositional heterogeneity. These reactions build up H₂N-Pro-Gly-Ala-CONHL and H₂N-Cys-His-Asp-CONHL (where L = organic struts) amino acid sequences by covalently attaching them to the organic struts in the MOFs, without losing porosity or crystallinity. An enabling feature of this chemistry is that the primary amine functionality (−CH₂NHBoc) of the original MOF is more reactive than the commonly examined aromatic amines (−NH₂), and this allowed for the multi-step reactions to be carried out in tandem within the MOF. Preliminary findings indicate that the complexity thus achieved can affect reactions that were previously accomplished only in the presence of enzymes