Strategies for hydrogen storage in metal--organic frameworks

Increased attention is being focused on metal-organic frameworks as candidates for hydrogen storage materials. This is a result of their many favorable attributes, such as high porosity, reproducible and facile syntheses, amenability to scale-up, and chemical modification for targeting desired properties. A discussion of several strategies aimed at improving hydrogen uptake in these materials is presented. These strategies include the optimization of pore size and adsorption energy by linker modification, impregnation, catenation, and the inclusion of open metal sites and lighter metals.     

The route to a feasible hydrogen-storage material: MOFs versus ammonia borane

The replacement of fossil fuels is one of the greatest challenges that chemistry and material sciences will have to face in the near future. While hydrogen seems to be the most likely candidate for this, a material able to store the hydrogen itself is sorely needed. Intense research in the past decade has narrowed down the field of possible concepts to two materials: ammonia borane with chemically bound hydrogen atoms and metal-organic frameworks with physisorbed hydrogen molecules. Herein we want to give an overview of the strengths and weaknesses of each concept, discuss the challenges that need to be overcome, and try to compare the future capabilities of these two materials.     

Modifying cage structures in metal-organic polyhedral frameworks for H2 storage

Three isostructural metal-organic polyhedral cage based frameworks (denoted NOTT-113, NOTT-114 and NOTT-115) with (3,24)-connected topology have been synthesised by combining hexacarboxylate isophthalate linkers with {Cu(2)(RCOO)(4)} paddlewheels. All three frameworks have the same cuboctahedral cage structure constructed from 24 isophthalates from the ligands and 12 {Cu(2)(RCOO)(4)} paddlewheel moieties. The frameworks differ only in the functionality of the central core of the hexacarboxylate ligands with trimethylphenyl, phenylamine and triphenylamine moieties in NOTT-113, NOTT-114 and NOTT-115, respectively. Exchange of pore solvent with acetone followed by heating affords the corresponding desolvated framework materials, which show high BET surface areas of 2970, 3424 and 3394 m(2) g(-1) for NOTT-113, NOTT-114 and NOTT-115, respectively. Desolvated NOTT-113 and NOTT-114 show high total H(2) adsorption capacities of 6.7 and 6.8 wt%, respectively, at 77 K and 60 bar. Desolvated NOTT-115 has a significantly higher total H(2) uptake of 7.5 wt% under the same conditions. Analysis of the heats of adsorption (Q(st)) for H(2) reveals that with a triphenylamine moiety in the cage wall, desolvated NOTT-115 shows the highest value of Q(st) for these three materials, indicating that functionalisation of the cage walls with more aromatic rings can enhance the H(2)/framework interactions. In contrast, measurement of Q(st) reveals that the amine-substituted trisalkynylbenzene core used in NOTT-114 gives a notably lower H(2)/framework binding energy.     

Molecular tectonics of metal-organic frameworks (MOFs): a rational design strategy for unusual mixed-connected network topologies

To systematically explore the higher-dimensional network structures with mixed connectivity, a series of two-dimensional (2D) and three-dimensional (3D) metal-organic frameworks (MOFs) with unusual (3,6)-connected net topologies are presented. These crystalline materials include [{[Mn(btza)2(H2O)2].2 H2O}n] (1), [{[Zn(btza)2(H2O)2].2 H2O}n] (2), [{[Cu(btza)2].H2O}n] (3), and [{[Cd(btza)2].3 H2O}n] (4), which have been successfully assembled through a predesigned three-connected organic component bis(1,2,4-triazol-1-yl)acetate (btza) with a variety of octahedral metal cores based on the modular synthetic methodology. The topological paradigms shown in this work cover the 2D CdCl2, 3D (4(2).6)2(4(4).6(2).8(7).10(2)), and pyrite (pyr) types. That is, when properly treated with the familiar first-row divalent metal ions, btza may perfectly furnish the coordination spheres for effective connectivity to result in diverse (3,6)-connected nets. Beyond this, a detailed analysis of network topology for all known 3D (3,6)-connected frameworks in both inorganic and inorganic-organic hybrid materials is described. Specific network connectivity of these MOFs indicates that the metal centers represent the most significant and alterable factor in structural assembly, although they show reliable and similar geometries. In this context, the combination of the distinct d10 AgI ion with btza in different solvents affords two isomorphous MOFs [{[Ag(btza)].glycol}n] (5) and [{[Ag(btza)]CH3OH}n] (6) with a binodal 4-connected 3D SrAl2 (sra) topology. The network structures of MOFs 1-3 and 5 turn out to be more complicated and interesting if one considers the hydrogen bonding between the host coordination frameworks and the intercalated solvent molecules. Furthermore, the role of the included solvents in the generation and stabilization of MOFs 1-6 is also investigated.     

Functional mixed metal-organic frameworks with metalloligands

Immobilization of functional sites within metal-organic frameworks (MOFs) is very important for their ability to recognize small molecules and thus for their functional properties. The metalloligand approach has enabled us to rationally immobilize a variety of different functional sites such as open metal sites, catalytic active metal sites, photoactive metal sites, chiral pore environments, and pores of tunable sizes and curvatures into mixed metal-organic frameworks (M'MOFs). In this Minireview, we highlight some important functional M'MOFs with metalloligands for gas storage and separation, enantioselective separation, heterogeneous asymmetric catalysis, sensing, and as photoactive and nanoscale drug delivery and biomedical imaging materials.     

Tuning Hydrogen Sorption Properties of Metal–Organic Frameworks by Postsynthetic Covalent Modification

Postsynthetic modification is presented as a means to tune the hydrogen adsorption properties of a series of metal–organic frameworks (MOFs). IRMOF‐3 (isoreticular metal–organic framework), UMCM‐1‐NH₂ (University of Michigan crystalline material), and DMOF‐1‐NH₂ (DABCO metal–organic framework) have been covalently modified with a series of anhydrides or isocyanates and the hydrogen sorption properties have been studied. Both the storage capacities and isosteric heats of adsorption clearly show that covalent postsynthetic modification can significantly enhance the sorption affinity of MOFs with hydrogen and in some cases increase both gravimetric and volumetric uptake of the gas as much as 40 %. The significance of the present study is illustrated by: 1) the nature of the substituents introduced by postsynthetic modification result in different effects on the binding of hydrogen; 2) the covalent postsynthetic modification approach allows for systematic modulation of hydrogen sorption properties; and 3) the ease of postsynthetic modification of MOFs allows a direct evaluation of the interplay between MOF structure, hydrogen uptake, and heat of adsorption. The findings presented herein show that postsynthetic modification is a powerful method to manipulate and better understand the gas sorption properties of MOFs.     

Elucidating gating effects for hydrogen sorption in MFU-4-type triazolate-based metal-organic frameworks featuring different pore sizes

A highly porous member of isoreticular MFU‐4‐type frameworks, [Zn₅Cl₄(BTDD)₃] (MFU‐4l(arge)) (H₂‐BTDD=bis(1H‐1,2,3‐triazolo[4,5‐b],[4′,5′‐i])dibenzo[1,4]dioxin), has been synthesized using ZnCl₂ and H₂‐BTDD in N,N‐dimethylformamide as a solvent. MFU‐4l represents the first example of MFU‐4‐type frameworks featuring large pore apertures of 9.1 Å. Here, MFU‐4l serves as a reference compound to evaluate the origin of unique and specific gas‐sorption properties of MFU‐4, reported previously. The latter framework features narrow‐sized pores of 2.5 Å that allow passage of sufficiently small molecules only (such as hydrogen or water), whereas molecules with larger kinetic diameters (e.g., argon or nitrogen) are excluded from uptake. The crystal structure of MFU‐4l has been solved ab initio by direct methods from 3D electron‐diffraction data acquired from a single nanosized crystal through automated electron diffraction tomography (ADT) in combination with electron‐beam precession. Independently, it has been solved using powder X‐ray diffraction. Thermogravimetric analysis (TGA) and variable‐temperature X‐ray powder diffraction (XRPD) experiments carried out on MFU‐4l indicate that it is stable up to 500 °C (N₂ atmosphere) and up to 350 °C in air. The framework adsorbs 4 wt % hydrogen at 20 bar and 77 K, which is twice the amount compared to MFU‐4. The isosteric heat of adsorption starts for low surface coverage at 5 kJ mol^?1 and decreases to 3.5 kJ mol^?1 at higher H₂ uptake. In contrast, MFU‐4 possesses a nearly constant isosteric heat of adsorption of ca. 7 kJ mol^?1 over a wide range of surface coverage. Moreover, MFU‐4 exhibits a H₂ desorption maximum at 71 K, which is the highest temperature ever measured for hydrogen physisorbed on metal–organic frameworks (MOFs).     

Pyrazolate-based cobalt(II)-containing metal-organic frameworks in heterogeneous catalytic oxidation reactions: elucidating the role of entatic states for biomimetic oxidation processes

Crystal structures of two metal-organic frameworks (MFU-1 and MFU-2) are presented, both of which contain redox-active Co(II) centres coordinated by linear 1,4-bis[(3,5-dimethyl)pyrazol-4-yl] ligands. In contrast to many MOFs reported previously, these compounds show excellent stability against hydrolytic decomposition. Catalytic turnover is achieved in oxidation reactions by employing tert-butyl hydroperoxide and the solid catalysts are easily recovered from the reaction mixture. Whereas heterogeneous catalysis is unambiguously demonstrated for MFU-1, MFU-2 shows catalytic activity due to slow metal leaching, emphasising the need for a deeper understanding of structure-reactivity relationships in the future design of redox-active metal-organic frameworks. Mechanistic details for oxidation reactions employing tert-butyl hydroperoxide are studied by UV/Vis and IR spectroscopy and XRPD measurements. The catalytic process accompanying changes of redox states and structural changes were investigated by means of cobalt K-edge X-ray absorption spectroscopy. To probe the putative binding modes of molecular oxygen, the isosteric heats of adsorption of O(2) were determined and compared with models from DFT calculations. The stabilities of the frameworks in an oxygen atmosphere as a reactive gas were examined by temperature-programmed oxidation (TPO). Solution impregnation of MFU-1 with a co-catalyst (N-hydroxyphthalimide) led to NHPI@MFU-1, which oxidised a range of organic substrates under ambient conditions by employing molecular oxygen from air. The catalytic reaction involved a biomimetic reaction cascade based on free radicals. The concept of an entatic state of the cobalt centres is proposed and its relevance for sustained catalytic activity is briefly discussed.     

Characterization of H2 binding sites in prototypical metal-organic frameworks by inelastic neutron scattering

The hindered rotor transitions of H(2) adsorbed in the chemically related and prototypical porous metal-organic frameworks IRMOF-1, IRMOF-8, IRMOF-11, and MOF-177 were studied by inelastic neutron scattering to gain information on the specifics of H(2) binding in this class of adsorbents. Remarkably sharp and complex spectra of these materials signify a diversity of well-defined binding sites. Similarities in the spectral features as a function of H(2) loading and correlations with recent crystallographic studies were used to assign transitions ranging in rotational barrier from <0.04 to 0.6 kcal/mol as corresponding to localized adsorption sites on the organic and inorganic components of these frameworks. We find that binding of H(2) at the inorganic cluster sites is affected by the nature of the organic link and is strongest in IRMOF-11 in accord with our adsorption isotherm data. The sites on the organic link have lower binding energies, but a much greater capacity for increases in H(2) loading, which demonstrates their importance for hydrogen uptake by these materials.     

Control of porosity geometry in amino acid derived nanoporous materials

Substitution of the pillaring ligand in the homochiral open-framework [Ni(2)(L-asp)(2)(bipy)] by extended bipy-type ligands leads to a family of layer-structured, homochiral metal-organic frameworks. The 1D channel topology can be modified by the nature of the organic linker, with shape, cross-section and the chemical functionality tuneable. In addition, the volume of these channels can be increased by up to 36 % compared to the parent [Ni(2)(L-asp)(2)(bipy)]. The linker 1,4-dipyridylbenzene (3rbp) gives access to a new layered homochiral framework [Ni(2)(L-asp)(2)(3rbp)] with channels of a different shape. In specific cases, non-porous analogues with the linker also present as a guest can be activated to give porous materials after sublimation. Their CO(2) uptake shows an increase of up to 30 % with respect to the parent [Ni(2)(L-asp)(2)(bipy)] framework.     

Chemical and thermal stability of isotypic metal-organic frameworks: effect of metal ions

Chemical and thermal stabilities of isotypic metal-organic frameworks (MOFs) like Al-BDC (Al-benzenedicarboxylate called MIL-53-Al), Cr-BDC (MIL-53-Cr) and V-BDC (MIL-47-V), after purification to remove uncoordinated organic linkers, have been compared to understand the effect of the central metal ions on the stabilities of the porous MOF-type materials. Chemical stability to acids, bases, and water decreases in the order of Cr-BDC>Al-BDC>V-BDC, suggesting stability increases with increasing inertness of the central metal ions. However, thermal stability decreases in the order of Al-BDC>Cr-BDC> V-BDC, and this tendency may be explained by the strength of the metal-oxygen bond in common oxides like Al(2)O(3), Cr(2)O(3), and V(2)O(5). In order to evaluate precisely the stability of a MOF, it is necessary to remove uncoordinated organic linkers that are located in the pores of the MOF, because a filled MOF may be more stable than the same MOF after purification.     

Selective CO2 adsorption by a triazacyclononane-bridged microporous metal-organic framework

Metal-organic frameworks constructed by self-assembly of metal ions and organic linkers have recently been of great interest in the preparation of porous hybrid materials with a wide variety of functions. Despite much research in this area and the large choice of building blocks used to fine-tune pore size and structure, it remains a challenge to synthesise frameworks composed of polyamines to tailor the porosity and adsorption properties for CO(2). Herein, we describe a rigid and microporous three-dimensional metal-organic framework with the formula [Zn(2)(L)(H(2)O)]Cl (L=1,4,7-tris(4-carboxybenzyl)-1,4,7-triazacyclononane) synthesised in a one-pot solvothermal reaction between zinc ions and a flexible cyclic polyaminocarboxylate. We have demonstrated, for the first time, that a porous rigid framework can be obtained by starting from a flexible amine building block. Sorption measurements revealed that the material exhibited a high surface area (135 m(2) g(-1)) and was the best compromise between capacity and selectivity for CO(2) over CO, CH(4), N(2) and O(2); as such it is a promising new selective adsorbent for CO(2) capture.     

Charge Control in Two Isostructural Anionic/Cationic CoII Coordination Frameworks for Enhanced Acetylene Capture

Two isostructural Co^II‐based metal–organic frameworks (MOFs) with the opposite framework charges have been constructed, which can be simply controlled by changing the tetrazolyl or triazolyl terminal in two bifunctional ligands. Notably, the cationic MOF 2 can adsorb much more C₂H₂ than the anionic MOF 1 with an increase of 88 % for C₂H₂ uptake at 298 K in spite of more active nitrogen sites in 1 . Theoretical calculations indicate that both nitrate and triazolyl play vital roles in C₂H₂ binding and the C₂H₂ adsorption isotherm confirms that the enhanced C₂H₂ uptake for 2 (225 and 163 cm³g^?1 at 273 and 298 K) is exceptionally high for MOF materials without open metal sites or uncoordinated polar atom groups on the frameworks.     

Supramolecular assemblies based on organometallic quinonoid linkers: a new class of coordination networks

Self-assembly of coordination frameworks exhibiting original architectures is an active area of research. Generally, such assemblies are constructed from organic spacers and transition metals of different geometrical structures. Herein, we report a novel class of supramolecular coordination assemblies with organometallic linkers based on metalated quinonoid and thioquinonoid complexes that serve as spacers. The organometallic ligands are stable and have the general formula [Cp*M(eta(4)-benzoquinone)] (o- and p-benzoquinone, Cp*=C(5)Me(5), M=Rh, Ir) and [Cp*Ir(eta(4)-thiobenzoquinone)] (o- and p-thiobenzoquinone). These units bind through both oxygen or sulfur atoms to metal ions of different coordination geometry, such as Cu(I), Ag(I), and Pt(II), to generate supramolecular coordination networks, with the metalated quinonoid or thioquinonoid linkers acting as backbones and the metal centers as nodes. This novel family of supramolecular assemblies exhibits short pi-pi and MM interactions. These results illustrate successfully the role of the organometallic linkers to produce an impressive range of novel supramolecular architectures that hold promise for the development of functional materials.     

Structure and charge control in metal-organic frameworks based on the tetrahedral ligand tetrakis(4-tetrazolylphenyl)methane

Use of the tetrahedral ligand tetrakis(4-tetrazolylphenyl)methane enabled isolation of two three-dimensional metal-organic frameworks featuring 4,6- and 4,8-connected nets related to the structures of garnet and fluorite with the formulae Mn(6)(ttpm)(3)5 DMF3 H(2)O (1) and Cu[(Cu(4)Cl)(ttpm)(2)](2)CuCl(2)5 DMF11 H(2)O (2) (H(4)ttpm=tetrakis(4-tetrazolylphenyl)methane). The fluorite-type solid 2 displays an unprecedented post-synthetic transformation in which the negative charge of the framework is reduced by extraction of copper(II) chloride. Desolvation of this compound generates Cu(4)(ttpm)(2)0.7 CuCl(2) (2 d), a microporous material exhibiting a high surface area and significant hydrogen uptake.     

One-, two- and three-periodic metal-organic rotaxane frameworks (MORFs): linking cationic transition-metal nodes with an anionic rotaxane ligand

A new singly charged pyridinium axle was prepared and combined with disulfonated dibenzo[24]crown-8 ether to form a [2]pseudorotaxane. The reaction of this new, anionic ligand with Zn(II) ions, under various crystallization conditions, resulted in the formation of three metal-organic rotaxane framework (MORF) solids; a one-periodic ML coordination polymer and two, two-periodic ML(2) square grid frameworks. The layers of square grids can be pillared to create full three-periodic MORF structures, which have completely neutral frameworks and are porous. These three-periodic materials represent the first examples of neutral porous MOFs in which one (or more) of the linkers is a mechanically interlocked molecule (MIM).     

Protein side chains facilitate Mg/Al exchange in model protein binding sites.

Among the most frequent protein binding sites served by Mg(II), we identify those which have higher affinity towards Al(III). We also estimate the free energies of metal exchange for all these binding sites taking into account solvent effects explicitly. The obtained results show that thermodynamically favored Mg(II)/Al(III) exchange reactions take place at a number of these metal binding sites, which could possibly be related to some dysfunctions of Mg(II)-dependent biological processes. Additionally, they shed light on the molecular basis of the toxicity of Al(III) in living organisms.     

Homo‐Helical Rod Packing as a Path Toward the Highest Density of Guest‐Binding Metal Sites in Metal–Organic Frameworks

In porous materials, metal sites with coordinate solvents offer opportunities for many applications, especially those promoted by host–guest chemistry, but such sites are especially hard to create for Li‐based materials, because unlike transition metals, lithium does not usually possess a high‐enough coordination number for both framework construction and guest binding. This challenge is addressed by mimicking the functional group ratio and metal‐to‐ligand charge ratio in MOF‐74. A family of rod‐packing lithium–organic frameworks (CPM‐47, CPM‐48, and CPM‐49) were obtained. These materials exhibit an extremely high density of guest‐binding lithium sites. Also unusual is the homo‐helical rod‐packing in the CPM series, as compared to the hetero‐helical rod packing by helices of opposite handedness in MOF‐74. This work demonstrates new chemical and structural possibilities in developing a record‐setting high density of guest‐binding metal sites in inorganic–organic porous materials.     

Lithium‐Functionalized Metal–Organic Frameworks that Show >10 wt % H2 Uptake at Ambient Temperature

We have used grand canonical Monte Carlo simulations with a first‐principles‐based force field to show that metal–organic frameworks (MOFs) with Li functional groups (i.e. C? Li bonds) allow for exceptional H₂ uptake at ambient temperature. For example, at 298 K and 100 bar, IRMOF‐1‐4Li shows a total H₂ uptake of 5.54 wt % and MOF‐200‐27Li exhibits a total H₂ uptake of 10.30 wt %, which are much higher than the corresponding values with pristine MOFs. Li‐functionalized MOF‐200 (MOF‐200‐27Li) shows 11.84 wt % H₂ binding at 243 K and 100 bar. These hydrogen‐storage capacities exceed the 2015 DOE target of 5.5 wt % H₂. Moreover, the incorporation of Li functional groups into MOFs provides more benefits, such as higher delivery amount, for H₂ uptake than previously reported Li‐doped MOFs.     

Effects of surface area, free volume, and heat of adsorption on hydrogen uptake in metal-organic frameworks

Grand canonical Monte Carlo simulations were performed to predict adsorption isotherms for hydrogen in a series of 10 isoreticular metal-organic frameworks (IRMOFs). The results show acceptable agreement with the limited experimental results from the literature. The effects of surface area, free volume, and heat of adsorption on hydrogen uptake were investigated by performing simulations over a wide range of pressures on this set of materials, which all have the same framework topology and surface chemistry but varying pore sizes. The results reveal the existence of three adsorption regimes: at low pressure (loading), hydrogen uptake correlates with the heat of adsorption; at intermediate pressure, uptake correlates with the surface area; and at the highest pressures, uptake correlates with the free volume. The accessible surface area and free volume, calculated from the crystal structures, were also used to estimate the potential of these materials to meet gravimetric and volumetric targets for hydrogen storage in IRMOFs.