Nakeun Ko

Ultrahigh Porosity in Metal-Organic Frameworks

Network Approaches to Highly Porous Materials Metal-organic frameworks (MOFs), in which inorganic centers are bridged by organic linkers, can achieve very high porosity for gas absorption. However, as the materials develop larger void spaces, there is also more room for growing interpenetrating networks—filling the open spaces not with gas molecules but with more MOFs. Furukawa et al. (p. 424, published online 1 July) describe the synthesis of a MOF in which zinc centers are bridged with long, highly conjugated organic linkers, but in which the overall symmetry of the networks created prevents formation of interpenetrating networks. Extremely high surface areas and storage capacities for hydrogen, carbon dioxide, and methane were observed. The large surface areas of these materials would correspond to that of dispersed nanocubes just 3 to 6 nanometers wide. Crystalline solids with extended non-interpenetrating three-dimensional crystal structures were synthesized that support well-defined pores with internal diameters of up to 48 angstroms. The Zn4O(CO2)6 unit was joined with either one or two kinds of organic link, 4,4′,4″-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoate (BTE), 4,4′,44″-[benzene-1,3,5-triyl-tris(benzene-4,1-diyl)]tribenzoate (BBC), 4,4′,44″-benzene-1,3,5-triyl-tribenzoate (BTB)/2,6-naphthalenedicarboxylate (NDC), and BTE/biphenyl-4,4′-dicarboxylate (BPDC), to give four metal-organic frameworks (MOFs), MOF-180, -200, -205, and -210, respectively. Members of this series of MOFs show exceptional porosities and gas (hydrogen, methane, and carbon dioxide) uptake capacities. For example, MOF-210 has Brunauer-Emmett-Teller and Langmuir surface areas of 6240 and 10,400 square meters per gram, respectively, and a total carbon dioxide storage capacity of 2870 milligrams per gram. The volume-specific internal surface area of MOF-210 (2060 square meters per cubic centimeter) is equivalent to the outer surface of nanoparticles (3-nanometer cubes) and near the ultimate adsorption limit for solid materials.

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 qom 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 (pyr) and rutile (rtl) nets were expected instead of qom. 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 (qom, pyr, and rtl), 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%.

Tailoring the water adsorption properties of MIL-101 metal–organic frameworks by partial functionalization

MIL-101 and MIL-101–NH2 were partially modified to incorporate various functional groups that are capable of forming hydrogen bonds with water. Specifically, MIL-101–NH2 was partially functionalized with –NHCONHCH2CH3 (–UR2), –NHCOCHCHCOOH (–Mal), or –NH(CH2)3SO3H (–3SO3H) and MIL-101 was partially functionalized with –COOH in order to investigate the effect of these groups on the water sorption properties when compared to the pristine versions. The MIL-101 derivatives were synthesized by either post-synthetic modification of MIL-101–NH2 or through direct synthesis using a mixed linker strategy. The ratios of the incorporated functional groups were determined by 1H-NMR analyses and the porosity changes were revealed by N2 gas adsorption measurements at 77 K. Water sorption isotherms at 298 K conclude that the incorporation of –3SO3H enhances the water vapour uptake capacity at a low relative pressure (P/P0 = 0.30), whereas –UR2 and –Mal retard water adsorption in MIL-101–NH2. The partial incorporation of –COOH in MIL-101 exhibits a steeper water uptake at lower pressure (P/P0 = 0.40) than MIL-101–NH2. Interestingly, a greater –COOH content within the MIL-101 framework reduces the water uptake capacity. These results indicate that even partial functionalization of MIL-101 induces noticeably large changes in the water adsorption properties.

Enhanced water stability and CO2 gas sorption properties of a methyl functionalized titanium metal–organic framework

A methyl-modified metal–organic framework (m-TiBDC) shows significantly enhanced hydrostability than unmodified TiBDC, and thus can maintain almost intact CO2 gas adsorption capacity even after its immersion in water for 2 h while TiBDC does not.

Two porous metal–organic frameworks containing zinc–calcium clusters and calcium cluster chains

A two-dimensional metal–organic framework (MOF) containing both Zn(II) and Ca(II) centres and a three-dimensional MOF containing only Ca(II) centres have the largest surface areas (BET 1560 and 914 m2 g−1, respectively) among the reported Ca-based MOFs and also exhibit high gas uptake up to 1 bar for H2 at 77 K and CO2 at 298 K.

Separation of Acetylene from Carbon Dioxide and Ethylene by a Water‐Stable Microporous Metal–Organic Framework with Aligned Imidazolium Groups inside the Channels

Separation of acetylene from carbon dioxide and ethylene is challenging in view of their similar sizes and physical properties. Metal-organic frameworks (MOFs) in general are strong candidates for these separations owing to the presence of functional pore surfaces that can selectively capture a specific target molecule. Here, we report a novel 3D microporous cationic framework named JCM-1. This structure possesses imidazolium functional groups on the pore surfaces and pyrazolate as a metal binding group, which is well known to form strong metal-to-ligand bonds. The selective sorption of acetylene over carbon dioxide and ethylene in JCM-1 was successfully demonstrated by equilibrium gas adsorption analysis as well as dynamic breakthrough measurement. Furthermore, its excellent hydrolytic stability makes the separation processes highly recyclable without a substantial loss in acetylene uptake capacity.

SuFEx in Metal–Organic Frameworks: Versatile Postsynthetic Modification Tool

A new type of click reaction, sulfur(VI) fluoride exchange (SuFEx), has been utilized to prepare five postsynthetically modified UiO-67 series metal-organic frameworks (MOFs). The postsynthetic modification (PSM) via SuFEx can be achieved selectively for the sulfonyl fluoride (R-SO2F) without degrading the MOF structure as confirmed by X-ray crystallographic analysis. The present SuFEx method provides a straightforward tool for introducing new functionality inside MOFs. Introduction of an imidazolium group into the MOF afforded a heterogeneous catalyst for the benzoin condensation reaction.

Poly[bis­(μ4-2,3,5,6-tetra­fluoro­benzene-1,4-di­carboxyl­ato-κ4O1:O1′:O4:O4′)bis­(tetra­hydro­furan-κO)dizinc]

The title compound, [Zn2(C8F4O4)2(C4H8O)2]n, has a three-dimensional metal-organic framework structure. The asymmetric unit consists of two ZnII atoms, two tetrahydrofuran ligands, one 2,3,5,6-tetrafluorobenzene-1,4-dicarboxylate ligand and two half 2,3,5,6-tetrafluorobenzene-1,4-dicarboxylate ligands, which are completed by inversion symmetry. One ZnII atom has a distorted trigonal–bipyramidal coordination geometry, while the other has a distorted octahedral geometry. Two independent tetrahydrofuran ligands are each disordered over two sets of sites with occupancy ratios of 0.48 (4):0.52 (4) and 0.469 (17):0.531 (17).

Poly[bis­(ethanol)(μ4-2,3,5,6-tetra­fluoro­benzene-1,4-di­carboxyl­ato)cadmium]

In the title compound, [Cd(C8F4O4)(C2H5OH)2]n, the CdII cation sits on an inversion centre and is coordinated by six O atoms from four tetrafluorobenzene-1,4-dicarboxylate anions and two ethanol molecules in a distorted octahedral geometry. The anionic ligand is also located on an inversion centre, and connects four CdII cations, generating a two-dimensional polymeric layer parallel to the ab plane. Within the layer, the ethanol molecule links F and O atoms of the nearest anionic ligands via O—H⋯O and O—H⋯F hydrogen bonds. The ethyl group of the ethanol molecule is disordered over two positions with an occupancy ratio of 0.567 (10):0.433 (10).