UiO-66 is a highly important prototypical zirconium metal−organic framework (MOF) compound because of its excellent stabilities not typically found in common porous MOFs. In its perfect crystal structure, each Zr metal center is fully coordinated by 12 organic linkers to form a highly connected framework. Using high-resolution neutron power diffraction technique, we found the first direct structural evidence showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs. The concentration of the missinglinker defects is surprisingly high, ∼10% in our sample, effectively reducing the framework connection from 12 to ∼11. We show that by varying the concentration of the acetic acid modulator and the synthesis time, the linker vacancies can be tuned systematically, leading to dramatically enhanced porosity. We obtained samples with pore volumes ranging from 0.44 to 1.0 cm 3 / g and Brunauer−Emmett−Teller surface areas ranging from 1000 to 1600 m 2 /g, the largest values of which are ∼150% and ∼60% higher than the theoretical values of defect-free UiO-66 crystal, respectively. The linker vacancies also have profound effects on the gas adsorption behaviors of UiO-66, in particular CO 2 . Finally, comparing the gas adsorption of hydroxylated and dehydroxylated UiO-66, we found that the former performs systematically better than the latter (particularly for CO 2 ) suggesting the beneficial effect of the −OH groups. This finding is of great importance because hydroxylated UiO-66 is the practically more relevant, non-air-sensitive form of this MOF. The preferred gas adsorption on the metal center was confirmed by neutron diffraction measurements, and the gas binding strength enhancement by the −OH group was further supported by our firstprinciples calculations. ■ INTRODUCTIONMetal−organic frameworks (MOFs), consisting of inorganic metal centers connected by organic linkers, are a relatively new family of porous materials that possess rich chemistry and offer great promise for gas adsorption related applications.

We have examined the methane uptake properties of six of the most promising metal organic framework (MOF) materials: PCN-14, UTSA-20, HKUST-1, Ni-MOF-74 (Ni-CPO-27), NU-111, and NU-125. We discovered that HKUST-1, a material that is commercially available in gram scale, exhibits a room-temperature volumetric methane uptake that exceeds any value reported to date. The total uptake is about 230 cc(STP)/cc at 35 bar and 270 cc(STP)/cc at 65 bar, which meets the new volumetric target recently set by the Department of Energy (DOE) if the packing efficiency loss is ignored. We emphasize that MOFs with high surface areas and pore volumes perform better overall. NU-111, for example, reaches ~75% of both the gravimetric and the volumetric targets. We find that values for gravimetric uptake, pore volume, and inverse density of the MOFs we studied scale essentially linearly with surface area. From this linear dependence, we estimate that a MOF with surface area 7500 m(2)/g and pore volume 3.2 cc/g could reach the current DOE gravimetric target of 0.5 g/g while simultaneously exhibiting around ~200 cc/cc volumetric uptake. We note that while values for volumetric uptake are based on ideal single crystal densities, in reality the packing densities of MOFs are much lower. Finally, we show that compacting HKUST-1 into wafer shapes partially collapses the framework, decreasing both volumetric and gravimetric uptake significantly. Hence, one of the important challenges going forward is to find ways to pack MOFs efficiently without serious damage or to synthesize MOFs that can withstand substantial mechanical pressure.

Experimentally refined crystal structures for metal− organic frameworks (MOFs) often include solvent molecules and partially occupied or disordered atoms. This creates a major impediment to applying high-throughput computational screening to MOFs. To address this problem, we have constructed a database of MOF structures that are derived from experimental data but are immediately suitable for molecular simulations. The computationready, experimental (CoRE) MOF database contains over 4700 porous structures with publically available atomic coordinates. Important physical and chemical properties including the surface area and pore dimensions are reported for these structures. To demonstrate the utility of the database, we performed grand canonical Monte Carlo simulations of methane adsorption on all structures in the CoRE MOF database. We investigated the structural properties of the CoRE MOFs that govern methane storage capacity and found that these relationships agree well with those derived recently from a large database of hypothetical MOFs.

We report a first-principles study, which demonstrates that a single Ti atom coated on a singlewalled nanotube (SWNT) binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8-wt% hydrogen. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials. PACS numbers: 61.46.+w,84.60.Ve,81.07.De Developing safe, cost-effective, and practical means of storing hydrogen is crucial for the advancement of hydrogen and fuel-cell technologies [1]. The current stateof-the-art is at an impasse in providing any material that meets a storage capacity of 6-wt% or more required for practical applications [1,2,3,4,5,6,7,8]. Here we report a first-principles computation of the interaction between hydrogen molecules and transition metal atoms adsorbed on carbon nanotubes. Our results are quite remarkable and unanticipated. We found that a single Ti-atom adsorbed on a SWNT can strongly bind up to four hydrogen molecules. Such an unusual and complex bonding is generated by the concerted interaction among H, Ti, and SWNT. Remarkably, this adsorption occurs with no energy barrier. At large Ti coverage we show that a (8,0) SWNT can store hydrogen molecules up to 8-wt%, exceeding the minimum requirement of 6-wt% for practical applications. Finally, we present high temperature quantum molecular dynamics simulations showing that these systems are stable and indeed exhibit associative desorption of H 2 upon heating, another requirement for reversible storage.Recent experiments [9,10] and calculations [11,12,13,14] suggest that it is possible to coat carbon nanotubes uniformly with Ti atoms without metal segregation problems [15]. Here we show that such Ti-coated carbon nanotubes exhibit remarkable hydrogen absorption properties. Below we will present our results in detail for a (8,0) nanotube and briefly for four armchair (n,n) (n=4,5,6, and 7) and five zigzag (n,0) (n=7,8,9,10,11, and 12) nanotubes.A single Ti atom on an (8,0) SWNT has a magnetic ground state with S=1 and a binding energy of 2.2 eV; this will serve as our reference system, denoted t80Ti. In order to determine different reaction paths and products between H 2 and t80Ti, we have carried out a series of single-energy calculations as H 2 molecules approaches t80Ti and when there are large enough forces acting on H 2 molecules, we let the atoms evolve according to the quantum mechanical forces obtained from density functional theory (DFT) calculations [16]. We used the conjugated-gradient (CG) minimization and optimized both the atomic positions and the c-axis of the tube.The energy calculations were performed within the plane-wave implementation [16] −1 k-point spacing resulting in 5 k-points along the tube axis. The cutoff energy of 350 eV is found to be enough for total ...

From all-electron fixed-spin-moment calculations we show that ferromagnetic and checkerboard antiferromagnetic ordering in LaFeAsO are not stable and the stripe antiferromagnetic configuration with M(Fe)=0.48 microB is the only stable ground state. The main exchange interactions between Fe ions are large, antiferromagnetic, and frustrated. The magnetic stripe phase breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaFeAsO recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions suggests that magnetism and superconductivity in doped LaFeAsO may be strongly coupled, much like in the high-T(c) cuprates.

We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni3V2O8. The temperature and magnetic-field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.

We found that metal-organic framework (MOF) compounds M(2)(dhtp) (open metal M = Mg, Mn, Co, Ni, Zn; dhtp = 2,5-dihydroxyterephthalate) possess exceptionally large densities of open metal sites. By adsorbing one CH(4) molecule per open metal, these sites alone can generate very large methane storage capacities, 160-174 cm(3)(STP)/cm(3), approaching the DOE target of 180 cm(3)(STP)/cm(3) for material-based methane storage at room temperature. Our adsorption isotherm measurements at 298 K and 35 bar for the five M(2)(dhtp) compounds yield excess methane adsorption capacities ranging from 149 to 190 cm(3)(STP)/cm(3) (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, Ni(2)(dhtp) exhibits the highest methane storage capacity, approximately 200 cm(3)(STP)/cm(3) in terms of absolute adsorption, potentially surpassing the DOE target by approximately 10%. Our neutron diffraction experiments clearly reveal that the primary CH(4) adsorption occurs directly on the open metal sites. Initial first-principles calculations show that the binding energies of CH(4) 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 CH(4) and the coordinatively unsaturated metal ions.

We report hydrogen and methane adsorption isotherms in two prototypical metal−organic framework compounds (i.e., MOF5 and ZIF8) over a large temperature (30−300 K) and pressure (up to 65 bar) range using a fully computer-controlled Sieverts apparatus. We find that, in a volumetric method, a proper choice of real gas equation of state is critical for obtaining reliable isotherm data. The widely used van der Waals equation of state (EOS) is not adequate to describe H2 and CH4, while the modified Benedict−Webb−Rubin (MBWR) EOS works well, even at very low temperatures and high pressures. With the known sample mass and bulk density, the skeleton density and the specific pore volume of MOF5 and ZIF8 were also measured. In addition to excess and absolute adsorption isotherms, we also introduce an “effective adsorption” which compares the amounts of gas adsorbed in a container with and without the adsorbent. At low temperatures, the maximal excess adsorption capacities of H2 and CH4 in MOF5 are found to be 10.3 wt % and 51.7 wt %, respectively, while they are only 4.4 wt % and 22.4 wt % in ZIF8. From the temperature-dependent isotherm data, the isosteric heat of adsorption (Q st) was also estimated. The excess Q st's for the initial H2 and CH4 adsorption in MOF5 are ∼4.8 kJ/mol and ∼12.2 kJ/mol, respectively. We obtained similar Q st's for ZIF8. We hope that the detailed isotherm curves reported here over a large temperature and pressure range will be a critical test for future grand canonical Monte Carlo simulations and force-field models.