CO(2) Dynamics in a Metal-Organic Framework with Open Metal Sites

Metal-organic frameworks (MOFs) with open metal sites are promising candidates for CO(2) capture from dry flue gas. We applied in situ(13)C NMR spectroscopy to investigate CO(2) adsorbed in Mg(2)(dobdc) (H(4)dobdc = 2,5-dihydroxyterephthalic acid; Mg-MOF-74, CPO-27-Mg), a key MOF in which exposed Mg(2+) cation sites give rise to exceptional CO(2) capture properties. Analysis of the resulting spectra reveals details of the binding and CO(2) rotational motion within the material. The dynamics of the motional processes are evaluated via analysis of the NMR line shapes and relaxation times observed between 12 and 400 K. These results form stringent and quantifiable metrics for computer simulations that seek to screen and improve the design of new MOFs for CO(2) capture.     

Effect of Larger Pore Size on the Sorption Properties of Isoreticular Metal–Organic Frameworks with High Number of Open Metal Sites

Four isostructural CPO-54-M metal-organic frameworks based on the larger organic linker 1,5-dihydroxynaphthalene-2,6-dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO-27 (MOF-74) materials. Desolvated CPO-54-Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N₂, H₂, CO₂ and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO-27. The plateau of high heat of adsorption decreases earlier in CPO-54-Mn as a function of loading per mole than in CPO-27-Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.     

Exceptional behavior over the whole adsorption-storage-delivery cycle for NO in porous metal organic frameworks

Two porous metal organic frameworks (MOFs), [M2(C8H2O6)(H2O)2] x 8 H2O (M = Co, Ni), perform exceptionally well for the adsorption, storage, and water-triggered delivery of the biologically important gas nitric oxide. Adsorption and powder X-ray diffraction studies indicate that each coordinatively unsaturated metal atom in the structure coordinates to one NO molecule. All of the stored gas is available for delivery even after the material has been stored for several months. The combination of extremely high adsorption capacity (approximately 7 mmol of NO/g of MOF) and good storage stability is ideal for the preparation of NO storage solids. However, most important is that the entire reservoir of stored gas is recoverable on contact with a simple trigger (moisture). The activity of the NO storage materials is proved in myography experiments showing that the NO-releasing MOFs cause relaxation of porcine arterial tissue.     

Hydrogen adsorption in a highly stable porous rare-earth metal-organic framework: sorption properties and neutron diffraction studies

A highly stable porous lanthanide metal-organic framework, Y(BTC)(H2O).4.3H2O (BTC = 1,3,5-benzenetricarboxylate), with pore size of 5.8 A has been constructed and investigated for hydrogen storage. Gas sorption measurements show that this porous MOF exhibits highly selective sorption behaviors of hydrogen over nitrogen gas molecules and can take up hydrogen of about 2.1 wt % at 77 K and 10 bar. Difference Fourier analysis of neutron powder diffraction data revealed four distinct D2 sites that are progressively filled within the nanoporous framework. Interestingly, the strongest adsorption sites identified are associated with the aromatic organic linkers rather than the open metal sites, as occurred in previously reported MOFs. Our results provide for the first time direct structural evidence demonstrating that optimal pore size (around 6 A, twice the kinetic diameter of hydrogen) strengthens the interactions between H2 molecules and pore walls and increases the heat of adsorption, which thus allows for enhancing hydrogen adsorption from the interaction between hydrogen molecules with the pore walls rather than with the normally stronger adsorption sites (the open metal sites) within the framework. At high concentration H2 loadings (5.5 H2 molecules (3.7 wt %) per Y(BTC) formula), H2 molecules form highly symmetric novel nanoclusters with relatively short H2-H2 distances compared to solid H2. These observations are important and hold the key to optimizing this new class of rare metal-organic framework (RMOF) materials for practical hydrogen storage applications.     

Understanding hydrogen adsorption in metal-organic frameworks with open metal sites: a computational study

Recent experimental investigations show that the open metal sites may have a favorable impact on the hydrogen adsorption capacity of metal-organic frameworks (MOFs); however, no definite evidence has been obtained to date and little is known on the interactions between hydrogen and the pore walls of this kind of MOFs. In this work, a combined grand canonical Monte Carlo simulation and density functional theory calculation is performed on the adsorption of hydrogen in MOF-505, a recently synthesized MOF with open metal sites, to provide insight into molecular-level details of the underlying mechanisms. This work shows that metal-oxygen clusters are preferential adsorption sites for hydrogen, and the strongest adsorption of hydrogen is found in the directions of coordinatively unsaturated open metal sites, providing evidence that the open metal sites have a favorable impact on the hydrogen sorption capacity of MOFs. The storage capacity of hydrogen of MOF-505 at room temperature and moderate pressures is predicted to be low, in agreement with the outcome for hydrogen physisorption in other porous materials.     

Comparison of the behavior of metal-organic frameworks and zeolites for hydrocarbon separations

The objective of this work was to study the adsorption and separation of the most important families of hydrocarbon compounds on metal-organic frameworks (MOFs), in comparison with zeolites. For this purpose, we have selected four probe molecules, each of them representing one of these families, i.e., o- and p-xylene as aromatics, 1-octene as an alkene, and n-octane as an alkane. The separation of these four molecules was studied by binary breakthrough experiments. To represent the large diversity of MOF structures, the experiments were carried out with (i) two MOFs with coordinatively unsaturated metal sites (CUS), i.e., Cu-btc (HKUST-1) and CPO-27-Ni, (ii) a MOF with an anionic framework and extraframework cations, i.e. RHO-ZMOF, and (iii) two rather apolar zeolitic imidazolate framework (ZIF) materials with different pore sizes, i.e. ZIF-8 and ZIF-76. Zeolite NaY and zeolite β were used as polar and apolar reference adsorbents, respectively. The results can be briefly summarized as follows: ZIFs (not carrying any polar functional groups) behave like apolar adsorbents and exhibit very interesting and unexpected molecular sieving properties. CUS-MOFs behave like polar adsorbents but show the specificity of preferring alkenes over aromatics. This feature is rationalized thanks to DFT+D calculations. MOFs with extraframework cations behave like polar (cationic) zeolites.     

Controlled Synthesis of Large Single Crystals of Metal-Organic Framework CPO-27-Ni Prepared by a Modulation Approach: In situ Single-Crystal X-ray Diffraction Studies

The size of single crystals of the metal-organic framework CPO-27-Ni was incrementally increased through a series of modulated syntheses. A novel linker modulated synthesis using 2,5-dihydroxyterephthalic acid and the isomeric ligand 4,6-dihydroxyisophthalic acid yielded large single crystals of CPO-27-Ni (∼70 μm). All materials were shown to have high crystallinity and phase purity through powder X-ray diffraction, electron microscopy methods, thermogravimetry, and compositional analysis. For the first time single-crystal structure analyses were carried out on CPO-27-Ni. High BET surface areas and nitric oxide (NO) release efficiencies were recorded for all materials. Large single crystals of CPO-27-Ni showed a prolonged NO release and proved suitable for in situ single-crystal diffraction experiments to follow the NO adsorption. An efficient activation protocol was developed, leading to a dehydrated structure after just 4 h, which subsequently was NO-loaded, leading to a first NO loaded single-crystal structural model of CPO-27-Ni.     

From microscopic insights of H2 adsorption to uptake estimations in MOFs

The adsorption of hydrogen in MOFs takes place mainly close to the inorganic secondary building unit (IBU). The adsorption capacities on MIL-88, UiO-66, MIL-47 and MFU-1 were investigated. Quantum chemical calculations at the ab initio HF/MP2 theoretical level were employed to estimate the maximum uptake of H(2) molecules per metallic centre. Extrapolating the results on small clusters to the unit cell of each particular MOF, the H(2) uptakes (gravimetric and volumetric) were estimated. The loading of hydrogen per metal atom (H(2) molecules/M-atom) and the density of metal atoms (M-atoms ?(-3)) were defined as useful parameters to assess hydrogen storage properties and to estimate the optimum density that the material should have to be a good H(2) adsorbent. It was found that values above 3 H(2) molecules/M-atom and around 0.004 M-atoms ?(-3) for MOFs with densities around 0.7-1.0 g cm(-3) are required to reach the 2015 storage targets.     

Molecular simulation of adsorption and diffusion of hydrogen in metal-organic frameworks

Metal-organic frameworks (MOFs) are thought to be a set of promising hydrogen storage materials; however, little is known about the interactions between hydrogen molecules and pore walls as well as the diffusivities of hydrogen in MOFs. In this work, we performed a systematic molecular simulation study on the adsorption and diffusion of hydrogen in MOFs to provide insight into molecular-level details of the underlying mechanisms. This work shows that metal-oxygen clusters are preferential adsorption sites for hydrogen in MOFs, and the effect of the organic linkers becomes evident with increasing pressure. The hydrogen storage capacity of MOFs is similar to carbon nanotubes, which is higher than zeolites. Diffusion of hydrogen in MOFs is an activated process that is similar to diffusion in zeolites. The information derived in this work is useful to guide the future rational design and synthesis of tailored MOF materials with improved hydrogen adsorption capability.     

金属-有机框架材料在液相催化化学制氢中的研究进展

金属-有机框架(MOFs)材料是由金属簇节点或金属离子与有机配体连接而成的典型的无机-有机杂合物, 作为一类新兴的无机多孔晶态材料, MOFs因具有高度有序的多孔性、 结构可裁剪性、 高比表面积及灵活多变的骨架类型等优点而在工业合成、 能源开发、 环境治理和生物制药等领域展现出广阔的应用前景. 本文从氢能源的开发利用出发, 总结了近年来MOFs基纳米复合材料在催化化学制氢方面的研究进展. 讨论了常见的含氢量高的化学储氢材料, 包括氨硼烷、 甲酸和水合肼等; 催化材料主要有单一MOFs、 MOF基贵金属和非贵金属复合材料及MOF基衍生材料等. 最后, 对MOF基复合材料在液相催化化学储氢中的应用前景进行了展望.     

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.     

Enhanced isosteric heat of H2 adsorption by inclusion of crown ethers in a porous metal-organic framework

Inclusion of 18-crown-6 or 15-crown-5 in a porous MOF increased the isosteric heats of H(2) adsorption significantly, which are comparable to MOFs containing open metal sites.     

Interaction of Hydrogen with MOF-5

Hydrogen storage is among the most demanding challenges in the hydrogen-based energy cycle. One proposed strategy for hydrogen storage is based on physisorption on high surface area solids such as metal-organic frameworks (MOFs). Within this class of materials, MOF-5 has been the first structure studied for hydrogen storage. The IR spectroscopy of adsorbed H2 performed at 15 K and ab initio calculations show that the adsorptive properties of this material are mainly due to dispersive interactions with the internal wall structure and to weak electrostatic forces associated with O13Zn4 clusters. Calculated and measured binding enthalpies are between 2.26 and 3.5 kJ/mol, in agreement with the H2 rotational barriers reported in the literature. A minority of binding sites with higher adsorption enthalpy (7.4 kJ/mol) is also observed. These species are probably associated with OH groups on the external surfaces present as termini of the microcrystals.     

Selective Catalytic Chemistry at Rhodium(II) Nodes in Bimetallic Metal–Organic Frameworks

We report the first study of a gas‐phase reaction catalyzed by highly dispersed sites at the metal nodes of a crystalline metal–organic framework (MOF). Specifically, CuRhBTC (BTC^3?=benzenetricarboxylate) exhibited hydrogenation activity, while other isostructural monometallic and bimetallic MOFs did not. Our multi‐technique characterization identifies the oxidation state of Rh in CuRhBTC as +2, which is a Rh oxidation state that has not previously been observed for crystalline MOF metal nodes. These Rh²⁺ sites are active for the catalytic hydrogenation of propylene to propane at room temperature, and the MOF structure stabilizes the Rh²⁺ oxidation state under reaction conditions. Density functional theory calculations suggest a mechanism in which hydrogen dissociation and propylene adsorption occur at the Rh²⁺ sites. The ability to tailor the geometry and ensemble size of the metal nodes in MOFs allows for unprecedented control of the active sites and could lead to significant advances in rational catalyst design.     

Advances of Metal‐Organic Frameworks in Energy and Environmental Applications

Nowadays, energy shortage and environmental pollution issues are increasingly severe and urgent to be solved. The effective storage and use of environmentally friendly fuels and removal of harmful gases from the environment are great challenges and of great importance both for the environment protection and for human health. Porous metal‐organic frameworks (MOFs) are highly ordered crystalline materials formed by the self‐assembly process of metal ions and organic ligands. Their good features such as ultrahigh porosity, large surface area, structural diversity and functionalities make them promising candidates for applications in energy and environmental fields. MOF thin films and MOF composites have also been investigated to further enhance the properties and introduce new functionalities. This review provides an overview of the synthesis methods of pristine MOFs, MOF thin films and MOF composites, and significant advances of MOFs in energy and environment applications such as energy storage (H₂, CH₄), CO₂ capture and separation, adsorption removal and sensing of harmful gases in the environment.     

金属有机骨架复合材料在样品预处理中的研究进展

金属有机骨架（MOFs）材料是近几年涌现出的一类新型多功能多孔材料,以金属离子或金属簇为配位中心,与含氧或氮的有机配体通过配位作用形成多孔骨架结构。相比于其他传统无机多孔材料,MOFs具有比表面积高、孔隙率大、热稳定性好和结构与功能多样化的特点,因而被广泛用于气体存储、催化、吸附和分离等领域。MOFs复合材料在样品预处理方面的应用引起了研究者们的极大兴趣和广泛关注。由于MOFs材料和不同功能材料如高分子聚合物、碳基材料以及磁性材料组装复合,使MOFs复合材料的性能优于原来的MOFs材料。综述了近年MOFs复合材料在样品预处理的研究应用,尤其是在固相微萃取、固相萃取以及磁性固相萃取等方面的应用。     

On the mechanism of hydrogen storage in a metal-organic framework material

Monte Carlo simulations were performed modeling hydrogen sorption in a recently synthesized metal-organic framework material (MOF) that exhibits large molecular hydrogen uptake capacity. The MOF is remarkable because at 78 K and 1.0 atm it sorbs hydrogen at a density near that of liquid hydrogen (at 20 K and 1.0 atm) when considering H2 density in the pores. Unlike most other MOFs that have been investigated for hydrogen storage, it has a highly ionic framework and many relatively small channels. The simulations demonstrate that it is both of these physical characteristics that lead to relatively strong hydrogen interactions in the MOF and ultimately large hydrogen uptake. Microscopically, hydrogen interacts with the MOF via three principle attractive potential energy contributions: Van der Waals, charge-quadrupole, and induction. Previous simulations of hydrogen storage in MOFs and other materials have not focused on the role of polarization effects, but they are demonstrated here to be the dominant contribution to hydrogen physisorption. Indeed, polarization interactions in the MOF lead to two distinct populations of dipolar hydrogen that are identified from the simulations that should be experimentally discernible using, for example, Raman spectroscopy. Since polarization interactions are significantly enhanced by the presence of a charged framework with narrow pores, MOFs are excellent hydrogen storage candidates.     

Metal–Organic Frameworks for Oxygen Storage

We present a systematic study of metal–organic frameworks (MOFs) for the storage of oxygen. The study starts with grand canonical Monte Carlo simulations on a suite of 10 000 MOFs for the adsorption of oxygen. From these data, the MOFs were down selected to the prime candidates of HKUST‐1 (Cu‐BTC) and NU‐125, both with coordinatively unsaturated Cu sites. Oxygen isotherms up to 30 bar were measured at multiple temperatures to determine the isosteric heat of adsorption for oxygen on each MOF by fitting to a Toth isotherm model. High pressure (up to 140 bar) oxygen isotherms were measured for HKUST‐1 and NU‐125 to determine the working capacity of each MOF. Compared to the zeolite NaX and Norit activated carbon, NU‐125 has an increased excess capacity for oxygen of 237 % and 98 %, respectively. These materials could ultimately prove useful for oxygen storage in medical, military, and aerospace applications.     

Tailor-made metal-organic frameworks from functionalized molecular building blocks and length-adjustable organic linkers by stepwise synthesis

In this work, we have demonstrated a family of diamondoid metal-organic frameworks (MOFs) based on functionalized molecular building blocks and length-adjustable organic linkers by using a stepwise synthesis strategy. We have successfully achieved not only "design" and "control" to synthesize MOFs, but also the functionalization of both secondary building units (SBUs) and organic linkers in the same MOF for the first time. Furthermore, the results of N(2) and H(2) adsorption show that their surface areas and hydrogen uptake capacities are determined by the most optimal combination of functional groups from SBUs and organic linkers, interpenetration, and free volume in this system. A member of this series, DMOF-6 exhibits the highest surface area and H(2) adsorption capacity among this family of MOFs.     

Design for hydrogen storage materials via observation of adsorption sites by computer tomography

An effective method denoted as "computer tomography for materials" (mCT) was employed to study the adsorption sites inside metal-organic frameworks (MOFs) at any positions and any view angles. For MOF-5, the first adsorption site alpha(-COO)3 was clearly observed from the mCT images; it locates at the position where three -COO groups joined like a cup. There are four alpha(-COO)3 sites around the Zn4O cluster. Two of them located at the diagonal of the Zn4O cluster are in the same plane "A", whereas the other two equivalent adsorption sites are in another plane "B", which is about 5.4 A away from the plane A. It was found that the electronegativity of oxygen atoms is very important to the adsorption of hydrogen molecules. The hydrogen amount adsorbed in MOFs might be enhanced by introducing some strong electronegative atoms to the organic linkers or frameworks. On the basis of this point of view, five new MOF materials were designed. The adsorbed amounts both in number of hydrogen molecules per unit cell and weight uptake for all of the designed MOFs were calculated. The adsorption amounts of designed MOFs were improved, and the amount for MOF-d5 at 1 bar is as high as 3.7 wt %. It is nearly 5-6 times of that of MOF-5 as a whole. It can be observed that extra adsorption sites were formed in the pores and the effective occupation rate of pore space was obviously improved viewing from the mCT images. These results may give helpful suggestions for the synthetic experimentalists.