Hydrogen storage in metal-organic frameworks by bridged hydrogen spillover

The possible utilization of hydrogen as the energy source for fuel-cell vehicles is limited by the lack of a viable hydrogen storage system. Metal-organic frameworks (MOFs) belong to a new class of microporous materials that have recently been shown to be potential candidates for hydrogen storage; however, no significant hydrogen storage capacity has been achieved in MOFs at ambient temperature. Here we report substantially increased hydrogen storage capacities of modified MOFs by using a simple technique that causes and facilitates hydrogen spillover. Thus, the storage of 4 wt % is achieved at room temperature and 100 atm for the modified IRMOF-8. The adsorption is reversible, and the rates are fast. That has made MOFs truly promising for hydrogen storage application.     

Hydrogen storage in microporous metal-organic frameworks with exposed metal sites

Owing to their high uptake capacity at low temperature and excellent reversibility kinetics, metal-organic frameworks have attracted considerable attention as potential solid-state hydrogen storage materials. In the last few years, researchers have also identified several strategies for increasing the affinity of these materials towards hydrogen, among which the binding of H(2) to unsaturated metal centers is one of the most promising. Herein, we review the synthetic approaches employed thus far for producing frameworks with exposed metal sites, and summarize the hydrogen uptake capacities and binding energies in these materials. In addition, results from experiments that were used to probe independently the metal-hydrogen interaction in selected materials will be discussed.     

Hydrogen sorption in functionalized metal-organic frameworks

Five porous metal-organic frameworks based on linking zinc oxide clusters with benzene-1,4-dicarboxylate, naphthalene-2,6-dicarboxylate, 4,5,9,10-tetrahydropyrene-2,7-dicarboxylate, 2,3,5,6-tetramethylbenzene-1,4-dicarboxylate, or benzene-1,3,5-tris(4-benzoate) were synthesized in gram-scale quantities to measure their hydrogen uptake properties. Hydrogen adsorption isotherms measured at 77 K show a distinct dependence of uptake on the nature of the link. At 1 atm, the materials sorb between 4.2 and 9.3 molecules of H2 per formula unit. The results imply a trend in hydrogen uptake with the number of rings in the organic moiety.     

The ionothermal synthesis of cobalt aluminophosphate zeolite frameworks

Ionothermal synthesis, the use of an ionic liquid as both solvent and structure-directing agent, has been used to synthesize three different cobalt aluminophosphate zeolites. Two of the materials are isostructural with solids prepared previously and have the AEI and SOD framework types. SIZ-7 is a novel zeolite structures that is closely related to the family of 8-ring zeolites consisting of the MER, GIS, and PHI frameworks. Single-crystal X-ray diffraction of SIZ-7 indicates that the distribution of cobalt among the four possible tetrahedral sites is unequal.     

Interrupted zeolite LTA and ATN-type boron imidazolate frameworks

Zeolite A (LTA) is of much interest in zeolite family because of its large-scale industrial applications. Making Zeolite A (a typical 4-connected tetrahedral framework material) with a lower connectivity (3-connected) might lead to new open architecture with expanded ring size and enhanced functionality. The first interrupted Zeolite A with 3-connected network has been experimentally realized here by a boron imidazolate (im) framework material (BIF-20) with 3-coordinate BH(mim)(3)(-) building units. Additionally, a new strategy toward the construction of functional microporous metal-organic frameworks with interrupted zeolite-type topologies is presented by both 3-connected boron imidazolate frameworks (BIF-20 and BIF-21). BIF-20 has an unusual tetrahedral framework with both debonded α and β cages, and exhibits high H(2) uptake capacity.     

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.     

Hydrogen storage materials

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The role of symmetry in building up zeolite frameworks from layered zeolite precursors having ferrierite and CAS layers

Layered zeolite precursors represent an emerging class of materials that expand the zeolite field in a new direction and have already produced new intriguing discoveries. One is a new fundamental insight that a zeolite framework can assemble by two pathways, directly in 3-D and via a layered precursor. Also new types of materials such as pillared and delaminated zeolites have been synthesized. Starting with the discovery of MCM-22 precursor in 1990s, other framework structures have now been obtained by topotactic condensation, including classical zeolites such as sodalite and ferrierite. The recent structural diversity observed with FER and CAS layers, which were found to produce two frameworks each, i.e. FER/CDO and CAS/NSI, respectively, are herein rationalized as resulting from the absence of in-plane mirror symmetry. A systematic treatment is envisioned, such as consideration of translational and pseudo-translational interactions in condensation of elementary shapes into close-packed assemblies, providing a blueprint for a general approach to analyze new precursors in the future. Other already documented instances of structural diversity include the presence of both types of packing in one preparation (CAS and NSI) and incomplete pairing of silanols connecting FER layers in ESR-12.     

Significantly enhanced hydrogen storage in metal-organic frameworks via spillover

The utilization of hydrogen in fuel-cell powered vehicles is limited by the lack of a safe and effective system for hydrogen storage. At the present time, there is no viable storage technology capable of meeting the DOE targets. Porous metal-organic frameworks (MOFs) are novel and potential candidates for hydrogen storage. Until now it is still not possible to achieve any significant hydrogen storage capacity in MOFs at ambient temperature. Here, we report, for the first time, significant amounts of hydrogen storage in MOF-5 and IRMOF-8 at ambient temperature by using a very simple technique via hydrogen dissociation and spillover. Thus, hydrogen uptakes for MOF-5 and IRMOF-8 can be enhanced by a factor of 3.3 and 3.1, respectively (to nearly 2 wt % at 10 MPa and 298 K). Furthermore, the isotherms are totally reversible. These findings suggest that our technique is suitable for hydrogen storage in a variety of MOF materials because of their similar structures as MOF-5 and IRMOF-8.     

Metal-organic frameworks—New materials for hydrogen storage

Published data on the physical sorption of hydrogen by new materials with a large specific surface area, crystalline microporous metal-organic frameworks (MOFs), are systematized and analyzed. The hydrogen-accumulating properties of MOFs are compared with those of traditional materials (charcoals and zeolites) and nanocarbon systems. The role of secondary hydrogen spillover in the development of new approaches to increase the adsorption capacity of hydrogen storage materials is separately considered.     

Design of covalent organic frameworks for methane storage

We designed 14 new covalent organic frameworks (COFs), which are expected to adsorb large amounts of methane (CH(4)) at 298 K and up to 300 bar. We have calculated their delivery uptake using grand canonical Monte Carlo (GCMC) simulations. We also report their thermodynamic stability based on 7.5 ns molecular dynamics simulations. Two new frameworks, COF-103-Eth-trans and COF-102-Ant, are found to exceed the DOE target of 180 v(STP)/v at 35 bar for methane storage. Their performance is comparable to the best previously reported materials: PCN-14 and Ni-MOF-74. Our results indicate that using thin vinyl bridging groups aid performance by minimizing the interaction methane-COF at low pressure. This is a new feature that can be used to enhance loading in addition to the common practice of adding extra fused benzene rings. Most importantly, this report shows that pure nonbonding interactions, van der Waals (vdW) and electrostatic forces in light elements (C, O, B, H, and Si), can rival the enhancement in uptake obtained for microporous materials derived from early transition metals.     

Covalent organic frameworks as exceptional hydrogen storage materials

We report the H2 uptake properties of six covalent organic frameworks (COFs) from first-principles-based grand canonical Monte-Carlo simulations. The predicted H2 adsorption isotherm is in excellent agreement with the only available experimental result (3.3 vs 3.4 wt % at 50 bar and 77 K for COF-5), also reported here, validating the predictions. We predict that COF-105 and COF-108 lead to a reversible excess H2 uptake of 10.0 wt % at 77 K, making them the best known storage materials for molecular hydrogen at 77 K. We predict that the total H2 uptake for COF-108 is 18.9 wt % at 77 K. COF-102 shows the best volumetric performance, storing 40.4 g/L of H2 at 77 K. These results indicate that the COF systems are most promising candidates for practical hydrogen storage.     

Structure-directing roles and interactions of fluoride and organocations with siliceous zeolite frameworks

Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown to depend subtly on the architectures of the organic species, which significantly influence the crystalline structures that result. One- and two-dimensional (2D) (1)H, (19)F, and (29)Si nuclear magnetic resonance (NMR) spectroscopy measurements establish distinct intermolecular interactions among F(-) anions, imidazolium structure-directing agents (SDA(+)), and crystalline silicate frameworks for as-synthesized siliceous zeolites ITW and MTT. Different types and positions of hydrophobic alkyl ligands on the imidazolium SDA(+) species under otherwise identical zeolite synthesis compositions and conditions lead to significantly different interactions between the F(-) and SDA(+) ions and the respective silicate frameworks. For as-synthesized zeolite ITW, F(-) anions are established to reside in the double-four-ring (D4R) cages and interact strongly and selectively with D4R silicate framework sites, as manifested by their strong (19)F-(29)Si dipolar couplings. By comparison, for as-synthesized zeolite MTT, F(-) anions reside within the 10-ring channels and interact relatively weakly with the silicate framework as ion pairs with the SDA(+) ions. Such differences manifest the importance of interactions between the imidazolium and F(-) ions, which account for their structure-directing influences on the topologies of the resulting silicate frameworks. Furthermore, 2D (29)Si{(29)Si} double-quantum NMR measurements establish (29)Si-O-(29)Si site connectivities within the as-synthesized zeolites ITW and MTT that, in conjunction with synchrotron X-ray diffraction analyses, establish insights on complicated order and disorder within their framework structures.     

Hamiltonian graph representation of zeolite frameworks and Si,Al ordering in the framework

In the first part, a topological characterization of various zeolite frameworks is performed on the basis of the Hamiltonian graph. It affords a simple representation of the framework connectivity, and presents some possibility of the classification as well as the prediction of unknown frameworks. In the second part, a simple and direct method based on the connectivity matrix is presented, by which all the possible Al distributions in a given framework are derived exhaustively under the restriction of Loewenstein's rule. An example of Al preference site determination is applied for zeolite ZSM-5 and compared with the result by the SCF-MO method.