The well-known frameworks of the type M2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) have numerous potential applications in gas storage and separations, owing to their exceptionally high concentration of coordinatively unsaturated metal surface sites, which can interact strongly with small gas molecules such as H2. Employing a related meta-functionalized linker that is readily obtained from resorcinol, we now report a family of structural isomers of this framework, M2(m-dobdc) (M = Mg, Mn, Fe, Co, Ni; m-dobdc(4-) = 4,6-dioxido-1,3-benzenedicarboxylate), featuring exposed M(2+) cation sites with a higher apparent charge density. The regioisomeric linker alters the symmetry of the ligand field at the metal sites, leading to increases of 0.4-1.5 kJ/mol in the H2 binding enthalpies relative to M2(dobdc). A variety of techniques, including powder X-ray and neutron diffraction, inelastic neutron scattering, infrared spectroscopy, and first-principles electronic structure calculations, are applied in elucidating how these subtle structural and electronic differences give rise to such increases. Importantly, similar enhancements can be anticipated for the gas storage and separation properties of this new family of robust and potentially inexpensive metal-organic frameworks.

Nanoporous adsorbents are a diverse category of solid-state materials that hold considerable promise for vehicular hydrogen storage. Although impressive storage capacities have been demonstrated for several materials, particularly at cryogenic temperatures, materials meeting all of the targets established by the U.S. Department of Energy have yet to be identified. In this Perspective, we provide an overview of the major known and proposed strategies for hydrogen adsorbents, with the aim of guiding ongoing research as well as future new storage concepts. The discussion of each strategy includes current relevant literature, strengths and weaknesses, and outstanding challenges that preclude implementation. We consider in particular metal-organic frameworks (MOFs), including surface area/volume tailoring, open metal sites, and the binding of multiple H 2 molecules to a single metal site. Two related classes of porous framework materials, covalent organic frameworks (COFs) and porous aromatic frameworks (PAFs), are also discussed, as are graphene and graphene oxide and doped porous carbons. We additionally introduce criteria for evaluating the merits of a particular materials design strategy. Computation has become an important tool in the discovery of new storage materials, and a brief introduction to the benefits and limitations of computational predictions of H 2 physisorption is therefore presented. Finally, considerations for the synthesis and characterization of hydrogen storage adsorbents are discussed. IntroductionStorage of hydrogen with sufficient gravimetric and volumetric capacity for vehicular use remains a significant obstacle to the widespread adoption of hydrogen fuel cell electric vehicles (FCEVs). Several FCEV models are now commercially available in limited locations around the world, and in these vehicles hydrogen is stored as a gas at room temperature with a fill Table 1 along with the current performance of 700 bar systems. These values pertain to the entire storage system, which includes the mass and volume of hydrogen in addition to the tank and associated balance-ofplant (BOP) components. Notably, it is physically impossible to meet the 2025 and ultimate volumetric capacity target with pressurized gas, as the density of H 2 gas at 700 bar and room temperature is just 40 g L À1 without accounting for the BOP.The search for solid-state H 2 storage materials that can supplant compressed gas systems has been ongoing for at least two decades. The development of a viable storage material Broader contextThe widespread use of hydrogen as a clean, sustainable energy carrier has the potential to provide several significant benefits, including a reduction in oil dependency and emissions, improved energy security and grid resiliency, and substantial economic opportunities across many sectors. Hydrogen-fueled vehicles are already appearing internationally, and one of the critical enabling technologies for increasing their availability is on-board hydrogen storage. Stakeholders in developing a hydrogen in...

A joint experimental and computational study of noble gas adsorption in the metal−organic framework (MOF) material HKUST-1 has been carried out. Using a standard gas adsorption analyzer fitted with a cryostat, isotherms were measured for Xe, Kr, Ar, and Ne at optimum temperatures for the determination of loading-dependent heats of adsorption using the Clausius−Clapeyron equation. Direct calorimetric measurements for Kr and Xe adsorption provide comparable heats of adsorption. A detailed analysis of the experimental data alongside complementary grand canonical Monte Carlo (GCMC) simulations led to the conclusion that the strongest binding for noble gases occurs in and around the small tetrahedral pockets and not at the accessible Cu(II) sites in the structure. Synchrotron X-ray and neutron powder diffraction experiments with in situ gas loading confirm the assignment of preferred binding sites inferred from the adsorption measurements and simulations.

A new family of hybrid inorganic-organic materials has been synthesized using a combination of flexible bis-pyridyl ligands in conjunction with a perfluorinated or nonfluorinated benzenedicarboxylate ligand. A significant difference between the carboxylate torsion angles of the fluorinated and nonfluorinated ligands leads to the formation of markedly different structures for the two groups of materials. No isostructural phases were found, and the use of perfluorinated ligands tends to increase the dimensionality of the resulting frameworks.Metal-organic frameworks containing fluorinated ligands are receiving increasing attention due to reports of interesting H 2 adsorption in these materials. 1-4 However, few have been synthesized to date and little is known of the structural chemistry of perfluorinated ligands in hybrid framework materials as compared to their nonfluorinated analogues. 5-13 Our current work involves developing a better understanding of the manner in which perfluorinated benzenedicarboxylates incorporate into hybrid structures and examining structural trends of the resulting materials. We have previously reported structures containing perfluorinated carboxylates in combination with nonfluorinated coligands such as imidazole, 14 triazole, 3 and both 2,2 0 -and 4,4 0 -bipyridine. 15,16 Here we extend this work to include the similar but longer and more flexible 1,2-bis(4-pyridyl)ethane (bpe) and 1,3-bis(4-pyridyl)propane (bpp) ligands in conjunction with tetrafluoroterephthalate (tftpa) and tetrafluoroisophthalate (tfipa). In contrast to our previous work, there are few reports of materials containing bpe or bpp and the nonfluorinated analogues of these dicarboxylates (tpa and ipa) for comparison. We have therefore undertaken a synthetic study of these materials as well.There are two important differences in the chemistry of perfluorinated benzenedicarboxylates as compared to their nonfluorinated analogues. First is their significantly enhanced acidity, which may contribute to the inability of compounds containing only transition metals and perfluorinated benzenedicarboxylates to crystallize, as hybrid frameworks are typically not obtained under strongly acidic conditions. This could explain the relative ease with which perfluorinated benzenedicarboxylates incorporate into hybrid materials when other basic ligands such as triazole and bipyridines are present in the reaction. The second difference involves the effect that the fluorine atoms have on the torsion angle by which the carboxylate groups are twisted out of the plane of the benzene ring. In structures containing tpa and ipa, the carboxylate group typically remains roughly in plane with the benzene ring to which it is attached (i.e., torsion angle near 0°). However, in structures containing tftpa and tfipa, the carboxylate groups are typically rotated between 45°and 60°with respect to the benzene ring. 12 This can be attributed both to an electrostatic repulsion between the highly electronegative fluorine atoms on the ring and the lone-pair ...

A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu3(btc)2 (btc(3-) = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu3(btc)2, and its isostructural analogue Cr3(btc)2, show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu(2+) or Cr(2+) cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu3(btc)2 are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu3(btc)2 itself.

A porous coordination polymer containing zinc, 1,2,4-triazolate, and tetrafluoroterephthalate displays a high physisorptive hydrogen adsorption enthalpy of 8 kJ/mol, as a result of fluorine atoms exposed to the pore surface and the structure's small pore size.

Small-molecule binding in metal–organic frameworks (MOFs) can be accurately studied both experimentally and computationally, provided the proper tools are employed. Herein, we compare and contrast properties associated with guest binding by means of density functional theory (DFT) calculations using nine different functionals for the M2(dobdc) (dobdc4– = 2,5-dioxido,1,4-benzenedicarboxylate) series, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn. Additionally, we perform Quantum Monte Carlo (QMC) calculations for one system to determine if this method can be used to assess the performance of DFT. We also make comparisons with previously published experimental results for carbon dioxide and water and present new methane neutron powder diffraction (NPD) data for further comparison. All of the functionals are able to predict the experimental variation in the binding energy from one metal to the next; however, the interpretation of the performance of the functionals depends on which value is taken as the reference. On the one hand, if we compare against experimental values, we would conclude that the optB86b-vdW and optB88-vdW functionals systematically overestimate the binding strength, while the second generation of van der Waals (vdW) nonlocal functionals (vdw-DF2 and rev-vdW-DF2) correct for this providing a good description of binding energies. On the other hand, if the QMC calculation is taken as the reference then all of the nonlocal functionals yield results that fall just outside the error of the higher-level calculation. The empirically corrected vdW functionals are in reasonable agreement with experimental heat of adsorptions but under bind when compared with QMC, while Perdew–Burke–Ernzerhof fails by more than 20 kJ/mol regardless of which reference is employed. All of the functionals, with the exception of vdW-DF2, predict reasonable framework and guest binding geometries when compared with NPD measurements. The newest of the functionals considered, rev-vdW-DF2, should be used in place of vdW-DF2, as it yields improved bond distances with similar quality binding energies.

Metal(II) formates (Co and Ni) show a significantly larger heat of adsorption for xenon than krypton across all loadings due to size selectivity in the primary adsorption site.