Nature and Tunability of Enhanced Hydrogen Binding in Metal−Organic Frameworks with Exposed Transition Metal Sites

Metal−organic framework (MOF) compounds with exposed transition-metal (TM) sites were recently found to exhibit significantly larger experimental heats of adsorption of H2 than classical MOFs, thus attracting greater attention. Here we show that the hydrogen binding in Mn4Cl−MOF is not of the expected Kubas type because there is (a) no significant charge transfer from TM to H2, (b) no evidence of any H2-σ* Mn-d orbital hybridization, (c) no significant H−H bond elongation, and (d) no significant shift in H−H stretching mode frequency. We make predictions for the magnetic superexchange interactions in Mn4Cl−MOF and determined low- and high-spin states of the Mn ion as local minima with very different hydrogen binding energies. We show that, by replacing Cl with F or Br, one can tune the H2 binding energy. We further reveal that the major contribution to the overall binding comes from the classical Coulomb interaction which is not screened due to the open-metal site and explains the relatively high binding energies and short H2−TM distances observed in MOFs with exposed metal sites compared to traditional ones. Finally, we show that the orientation of H2 has a surprisingly large effect on the binding potential, reducing the classical binding energy by almost 30%

Methane Sorption in Nanoporous Metal−Organic Frameworks and First-Order Phase Transition of Confined Methane

Nanoporous metal−organic frameworks (MOFs) are promising materials for methane sorption and storage. Using neutron powder diffraction, we have directly determined the methane sorption sites in two prototypical MOF materials: zeolitic imidazolate framework-8 (ZIF-8, Zn6(N2C4H5)12) and metal−organic framework-5 (MOF-5, Zn4O13(C8H4)3). The primary methane adsorption sites are associated with the organic linkers in ZIF-8 and the metal oxide clusters in MOF-5. Methane molecules on these primary sites possess well-defined orientations, implying relatively strong binding with the framework. With higher methane loading, extra methane molecules populate the secondary sites and are confined in the framework. The confined methanes are orientationally disordered and stabilized by the intermolecular interactions. Below 100 K a maximum of ∼18 and ∼24 methane molecules per formula unit can be stored in ZIF-8 and MOF-5, corresponding to ∼20 and ∼50 wt % storage capacities, respectively. An unusual reversible methane-induced structural phase transition in MOF-host lattice is observed at ∼60 K in both ZIF-8 and MOF-5 due to strong intermolecular interaction between confined methane molecules in the pores of the host lattice

Exceptional Mechanical Stability of Highly Porous Zirconium Metal–Organic Framework UiO-66 and Its Important Implications

Metal–organic frameworks (MOFs) with high porosity usually exhibit weak mechanical stabilities, in particular, rather low stabilities against shear stress. This limitation remains one of the bottlenecks for certain applications of porous MOFs, such as gas storage or separation that requires dense packing of the MOF powders under mechanical compression without collapsing the pores. We found that UiO-66, a prototypical Zr-MOF with high porosity, exhibits unusually high shear stability. Its minimal shear modulus (<i>G</i>_min = 13.7 GPa) is an order of magnitude higher than those of other benchmark highly porous MOFs (e.g., MOF-5, ZIF-8, HKUST-1), approaching that of zeolites. Our analysis clearly shows that the exceptional mechanical stability of UiO-66 is due to its high framework connections (i.e., the high degree of coordination of Zr–O metal centers to the organic linkers). Our work thus provides important guidelines for developing new porous MOFs targeting at high mechanical stabilities

Exceptional Mechanical Stability of Highly Porous Zirconium Metal–Organic Framework UiO-66 and Its Important Implications

Metal–organic frameworks (MOFs) with high porosity usually exhibit weak mechanical stabilities, in particular, rather low stabilities against shear stress. This limitation remains one of the bottlenecks for certain applications of porous MOFs, such as gas storage or separation that requires dense packing of the MOF powders under mechanical compression without collapsing the pores. We found that UiO-66, a prototypical Zr-MOF with high porosity, exhibits unusually high shear stability. Its minimal shear modulus (<i>G</i>_min = 13.7 GPa) is an order of magnitude higher than those of other benchmark highly porous MOFs (e.g., MOF-5, ZIF-8, HKUST-1), approaching that of zeolites. Our analysis clearly shows that the exceptional mechanical stability of UiO-66 is due to its high framework connections (i.e., the high degree of coordination of Zr–O metal centers to the organic linkers). Our work thus provides important guidelines for developing new porous MOFs targeting at high mechanical stabilities

Exceptional Mechanical Stability of Highly Porous Zirconium Metal–Organic Framework UiO-66 and Its Important Implications

Metal–organic frameworks (MOFs) with high porosity usually exhibit weak mechanical stabilities, in particular, rather low stabilities against shear stress. This limitation remains one of the bottlenecks for certain applications of porous MOFs, such as gas storage or separation that requires dense packing of the MOF powders under mechanical compression without collapsing the pores. We found that UiO-66, a prototypical Zr-MOF with high porosity, exhibits unusually high shear stability. Its minimal shear modulus (<i>G</i>_min = 13.7 GPa) is an order of magnitude higher than those of other benchmark highly porous MOFs (e.g., MOF-5, ZIF-8, HKUST-1), approaching that of zeolites. Our analysis clearly shows that the exceptional mechanical stability of UiO-66 is due to its high framework connections (i.e., the high degree of coordination of Zr–O metal centers to the organic linkers). Our work thus provides important guidelines for developing new porous MOFs targeting at high mechanical stabilities

Alkali and Alkaline-Earth Metal Amidoboranes: Structure, Crystal Chemistry, and Hydrogen Storage Properties

Alkali- and alkaline-earth metal amidoboranes are a new class of compounds with rarely observed [NH2BH3]− units. LiNH2BH3 and solvent-containing Ca(NH2BH3)2·THF have been recently reported to significantly improve the dehydrogenation properties of ammonia borane. Therefore, metal amidoboranes, with accelerated desorption kinetics and suppressed toxic borazine, are of great interest for their potential applications for hydrogen storage. In this work, we successfully determined the structures of LiNH2BH3 and Ca(NH2BH3)2 using a combined X-ray diffraction and first-principles molecular dynamics simulated annealing method. Through detailed structural analysis and first-principles electronic structure calculations the improved dehydrogenation properties are attributed to the different bonding nature and reactivity of the metal amidoboranes compared to NH3BH3

High-Capacity Methane Storage in Metal−Organic Frameworks M₂(dhtp): The Important Role of Open Metal Sites

We found that metal−organic framework (MOF) compounds M2(dhtp) (open metal M = Mg, Mn, Co, Ni, Zn; dhtp = 2,5-dihydroxyterephthalate) possess exceptionally large densities of open metal sites. By adsorbing one CH4 molecule per open metal, these sites alone can generate very large methane storage capacities, 160−174 cm3(STP)/cm3, approaching the DOE target of 180 cm3(STP)/cm3 for material-based methane storage at room temperature. Our adsorption isotherm measurements at 298 K and 35 bar for the five M2(dhtp) compounds yield excess methane adsorption capacities ranging from 149 to 190 cm3(STP)/cm3 (derived using their crystal densities), indeed roughly equal to the predicted, maximal adsorption capacities of the open metals (within ±10%) in these MOFs. Among the five isostructural MOFs studied, Ni2(dhtp) exhibits the highest methane storage capacity, ∼200 cm3(STP)/cm3 in terms of absolute adsorption, potentially surpassing the DOE target by ∼10%. Our neutron diffraction experiments clearly reveal that the primary CH4 adsorption occurs directly on the open metal sites. Initial first-principles calculations show that the binding energies of CH4 on the open metal sites are significantly higher than those on typical adsorption sites in classical MOFs, consistent with the measured large heats of methane adsorption for these materials. We attribute the enhancement of the binding strength to the unscreened electrostatic interaction between CH4 and the coordinatively unsaturated metal ions

Enhanced H₂ Adsorption in Isostructural Metal−Organic Frameworks with Open Metal Sites: Strong Dependence of the Binding Strength on Metal Ions

Enhanced H2 Adsorption in Isostructural Metal−Organic Frameworks with Open Metal Sites: Strong Dependence of the Binding Strength on Metal Ion

Hydrogen Storage in a Prototypical Zeolitic Imidazolate Framework-8

Using the difference Fourier analysis of neutron powder diffraction data along with first-principles calculations, we reveal detailed structural information such as methyl group orientation, hydrogen adsorption sites, and binding energies within the nanopore structure of ZIF8 (Zn(MeIM)2). Surprisingly, the two strongest adsorption sites that we identified are both directly associated with the organic linkers, instead of the ZnN4 clusters, in strong contrast to classical MOFs, where the metal-oxide clusters are the primary adsorption sites. These observations are important and hold the key to optimizing this new class of ZIF materials for practical hydrogen storage applications. Finally, at high concentration H2-loadings, ZIF8 structure is capable of holding up to 28 H2 molecules (i.e., 4.2 wt %) in the form of highly symmetric novel three-dimensional interlinked H2-nanoclusters with relatively short H2−H2 distances compared to solid H2. Hence, ZIF compounds with robust chemical stability can be also an ideal template host-material to generate molecular nanostructures with novel properties

Exceptional Mechanical Stability of Highly Porous Zirconium Metal–Organic Framework UiO-66 and Its Important Implications

Metal–organic frameworks (MOFs) with high porosity usually exhibit weak mechanical stabilities, in particular, rather low stabilities against shear stress. This limitation remains one of the bottlenecks for certain applications of porous MOFs, such as gas storage or separation that requires dense packing of the MOF powders under mechanical compression without collapsing the pores. We found that UiO-66, a prototypical Zr-MOF with high porosity, exhibits unusually high shear stability. Its minimal shear modulus (<i>G</i>_min = 13.7 GPa) is an order of magnitude higher than those of other benchmark highly porous MOFs (e.g., MOF-5, ZIF-8, HKUST-1), approaching that of zeolites. Our analysis clearly shows that the exceptional mechanical stability of UiO-66 is due to its high framework connections (i.e., the high degree of coordination of Zr–O metal centers to the organic linkers). Our work thus provides important guidelines for developing new porous MOFs targeting at high mechanical stabilities