Cooperative template-directed assembly of mesoporous metal-organic frameworks

Despite great efforts, the development of a reliable way to assemble mesoporous metal-organic frameworks (mesoMOFs) remains a challenge. In this work, we have designed a cooperative template system, comprising a surfactant (cetyltrimethylammonium bromide) and a chelating agent (citric acid), for the generation of a mesoMOF containing a hierarchical system of mesopores interconnected with microspores. The surfactant molecules form micelles and the chelating agent bridges the MOF and the micelles, making self-assembly and crystal growth proceed under the direction of the cooperative template. However, when the surfactant or the chelating agent was applied individually, no mesoMOF was obtained.     

Separation of CO2 from CH4 using mixed-ligand metal-organic frameworks

The adsorption of CO2 and CH4 in a mixed-ligand metal-organic framework (MOF) Zn 2(NDC) 2(DPNI) [NDC = 2,6-naphthalenedicarboxylate, DPNI = N, N'-di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydiimide] was investigated using volumetric adsorption measurements and grand canonical Monte Carlo (GCMC) simulations. The MOF was synthesized by two routes: first at 80 degrees C for two days with conventional heating, and second at 120 degrees C for 1 h using microwave heating. The two as-synthesized samples exhibit very similar powder X-ray diffraction patterns, but the evacuated samples show differences in nitrogen uptake. From the single-component CO2 and CH4 isotherms, mixture adsorption was predicted using the ideal adsorbed solution theory (IAST). The microwave sample shows a selectivity of approximately 30 for CO2 over CH4, which is among the highest selectivities reported for this separation. The applicability of IAST to this system was demonstrated by performing GCMC simulations for both single-component and mixture adsorption.     

Ligand-to-metal ratio controlled assembly of nanoporous metal-organic frameworks

Two bilayered metal-organic frameworks with nanoporous channels were synthesized at different ligand-to-metal ratios, which demonstrated an interesting crystal-to-crystal transformation property and a special fluorescent response to the different guest molecules included.     

Molecular tectonics of metal-organic frameworks (MOFs): a rational design strategy for unusual mixed-connected network topologies

To systematically explore the higher-dimensional network structures with mixed connectivity, a series of two-dimensional (2D) and three-dimensional (3D) metal-organic frameworks (MOFs) with unusual (3,6)-connected net topologies are presented. These crystalline materials include [{[Mn(btza)2(H2O)2].2 H2O}n] (1), [{[Zn(btza)2(H2O)2].2 H2O}n] (2), [{[Cu(btza)2].H2O}n] (3), and [{[Cd(btza)2].3 H2O}n] (4), which have been successfully assembled through a predesigned three-connected organic component bis(1,2,4-triazol-1-yl)acetate (btza) with a variety of octahedral metal cores based on the modular synthetic methodology. The topological paradigms shown in this work cover the 2D CdCl2, 3D (4(2).6)2(4(4).6(2).8(7).10(2)), and pyrite (pyr) types. That is, when properly treated with the familiar first-row divalent metal ions, btza may perfectly furnish the coordination spheres for effective connectivity to result in diverse (3,6)-connected nets. Beyond this, a detailed analysis of network topology for all known 3D (3,6)-connected frameworks in both inorganic and inorganic-organic hybrid materials is described. Specific network connectivity of these MOFs indicates that the metal centers represent the most significant and alterable factor in structural assembly, although they show reliable and similar geometries. In this context, the combination of the distinct d10 AgI ion with btza in different solvents affords two isomorphous MOFs [{[Ag(btza)].glycol}n] (5) and [{[Ag(btza)]CH3OH}n] (6) with a binodal 4-connected 3D SrAl2 (sra) topology. The network structures of MOFs 1-3 and 5 turn out to be more complicated and interesting if one considers the hydrogen bonding between the host coordination frameworks and the intercalated solvent molecules. Furthermore, the role of the included solvents in the generation and stabilization of MOFs 1-6 is also investigated.     

Chiral assembly of dodecahedral cavities into porous metal-organic frameworks

Corner-sharing assembly of dodecahedral cavities with both paddle-wheel [M(2)(CO(2))(4)] units and trimeric [M(3)(μ(3)-OH)(CO(2))(6)] units leads to two isomorphous chiral microporous metal-organic frameworks, where the heterometallic framework has high surface area and excellent CO(2)/N(2) adsorption selectivity.     

Stepwise synthesis of metal-organic frameworks: replacement of structural organic linkers

We demonstrate how a single-crystal to single-crystal transformation resulting from bridging-linker replacement is possible in extended 2D and 3D metal-organic frameworks (MOFs) by introducing pillared paddlewheel MOF structures into a solution containing dipyridyl linkers. No lateral movement of the layers was observed during this transformation, creating a templating effect from the "parent" structure to the "daughter" structure. A previously unattainable structure was obtained by a two-step synthetic method utilizing the bridging-linker replacement transformation method. Additionally, a bridging-linker insertion was observed when excess linker was used with the 2D MOF structure, inducing an overall 2D to 3D transformation.     

Molecular screening of metal-organic frameworks for CO2 storage

We report a molecular simulation study for CO2 storage in metal-organic frameworks (MOFs). As compared to the aluminum-free and cation-exchanged ZSM-5 zeolites and carbon nanotube bundle, IRMOF1 exhibits remarkably higher capacity. Incorporation of Na(+) cations into zeolite increases the capacity only at low pressures. By variation of the metal oxide, organic linker, functional group, and framework topology, a series of isoreticular MOFs (IRMOF1, Mg-IRMOF1, Be-IRMOF1, IRMOF1-(NH2)4, IRMOF10, IRMOF13, and IRMOF14) are systematically examined, as well as UMCM-1, a fluorous MOF (F-MOF1), and a covalent-organic framework (COF102). The affinity with CO2 is enhanced by addition of a functional group, and the constricted pore is formed by interpenetration of the framework; both lead to a larger isosteric heat and Henry's constant and subsequently a stronger adsorption at low pressures. The organic linker plays a critical role in tuning the free volume and accessible surface area and largely determines CO2 adsorption at high pressures. As a combination of high capacity and low framework density, IRMOF10, IRMOF14, and UMCM-1 are identified from this study to be the best for CO2 storage, even surpass the experimentally reported highest capacity in MOF-177. COF102 is a promising candidate with high capacity at considerably low pressures. Both gravimetric and volumetric capacities at 30 bar correlate well with the framework density, free volume, porosity, and accessible surface area. These structure-function correlations are useful for a priori prediction of CO2 capacity and for the rational screening of MOFs toward high-efficacy CO2 storage.     

Molecular simulation of hydrogen diffusion in interpenetrated metal-organic frameworks

In this work a combined molecular dynamics simulation and dynamically corrected transition-state theory (dcTST) study was performed to investigate the effect of interpenetration (catenation) on hydrogen diffusion in metal-organic frameworks (MOFs) as well as their relationships. The results on 10 isoreticular MOFs (IRMOFs) with and without interpenetration show that catenation can reduce hydrogen diffusivity by a factor of 2 to 3 at room temperature, and for the interpenetrated IRMOFs with multi-pores of different sizes, free volume can serve as a measure for hydrogen diffusivity: the bigger the free volume, the larger the hydrogen diffusivity. In addition, the present work shows that dcTST can directly reveal the influence of the MOF structure on hydrogen diffusivity, which is a powerful tool for providing a better understanding of the relationship between gas diffusivity and MOF structure.     

Molecular building blocks approach to the assembly of zeolite-like metal-organic frameworks (ZMOFs) with extra-large cavities

Two novel porous anionic zeolite-like metal-organic frameworks, rho-ZMOF and sod-ZMOF, have been synthesized by metal-ligand-directed assembly of rigid and directional tetrahedral building units, InN4 synthesized in situ, and doubly deprotonated bis(bidentate) imidazoledicarboxylic acid ligands (HImDC) in the presence of different structure directing agents (SDAs).     

Molecular tectonics: design,synthesis and structural analysis of thiacalixarene-based tectons

The synthesis of five new coordinating tectons based on tetrathiacalix[4]arene backbone was achieved and their structure analysed and confirmed by X-ray diffraction on single crystal. All tectons were based on tetrafunctionalisation of either tetrathiacalix[4]arene or tetramercaptotetrathiacalix[4]arene derivatives by four pyridine units. The junction between the pyridine units and the calix backbone was ensured by ester or thioester groups. On the pyridine ring, either position 3 or 4, defining the location of the coordination sites, were used to connect the monodentate site to the calix framework. To cite this article: H. Akdas et al., C. R. Chimie 6 (2003).     

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.     

Solvent- and metal-directed lanthanide-organic frameworks based on pamoic acid: observation of slow magnetization relaxation,magnetocaloric effect and luminescent sensing

Two types of lanthanide coordination polymers, namely, [Ln(PA)(NO_3)(DMA)_3]_n(Ln=Gd(1), Dy(2), Eu(3), Tb(4))(type I), and {[Ln_2(PA)_3(DMF)_4]·2DMF}(Ln=Eu(5), Tb(6))(type II)(PA=Pamoic acid, DMA=dimethylacetamide,DMF=N,N-dimethylformamide), have been synthesized by the reaction of Ln(NO_3)_3·6H_2O with pamoic acid through layer diffusion method. These complexes were characterized by single crystal X-ray diffraction, infrared spectroscopy(IR),thermogravimetric analysis(TGA), fluorescence and magnetic measurements. Solvents and lanthanide atoms in the reaction play an important role in controlling different structures. Type I demonstrated 1-D linear chain structure connected by Ln atoms and PA ligands. Type II exhibited non-interpenetrating 3-D 6-connected 4_36~(12) nets based on binuclear [Ln_2(CO_2)_6(DMF)_4] cores.Magnetic properties of complexes 1–4 were investigated in details. Complex 1 shows significant magnetocaloric effect with–ΔS_m=20.37 J kg~(–1) K~(–1) at 3.0 K and 7 T. Complex 2 exhibits slow relaxation of the magnetization. Complexes 3–6 exhibit both ligand- and metal-centered fluorescent properties. Complex 6 demonstrates fluorescent sensing of DMF and Cu~(2+) ion.     

Molecular dynamics simulations of gas diffusion in metal-organic frameworks: argon in CuBTC

The class of coordination polymers known as metal-organic frameworks (MOFs) has three-dimensional porous structures that are considered as a promising alternative to zeolites and other nanoporous materials for catalysis, gas adsorption, and gas separation applications. In this paper, we present the first study of gas diffusion inside an MOF and compare the observed diffusion to known behaviors in zeolites. Using grand canonical Monte Carlo and equilibrium molecular dynamics, we calculate the adsorption isotherm and self-, corrected, and transport diffusivities for argon in the CuBTC metal-organic framework. Our results indicate that diffusion of Ar in CuBTC is very similar to Ar diffusion in silica zeolites in magnitude, concentration, and temperature dependence. This conclusion appears to apply to a broad range of MOF structures.     

Direction of unusual mixed-ligand metal-organic frameworks: a new type of 3-D polythreading involving 1-D and 2-D structural motifs and a 2-fold interpenetrating porous network

The reaction of Cu(II) or Cd(II) acetate with mixed ligands terephthalic (tp) and 3,5-bis(4-pyridyl)-4-amino-1,2,4-triazole (bpt) under the same conditions affords two unusual metal-organic frameworks, in which represents a new type of polythreaded supramolecular architecture consisting of distinct 1-D and 2-D coordination polymers within one crystal, however, has an interpenetrating porous network with two similar laterally interlocking 2-D (4,4) layers.     

Chemical and thermal stability of isotypic metal-organic frameworks: effect of metal ions

Chemical and thermal stabilities of isotypic metal-organic frameworks (MOFs) like Al-BDC (Al-benzenedicarboxylate called MIL-53-Al), Cr-BDC (MIL-53-Cr) and V-BDC (MIL-47-V), after purification to remove uncoordinated organic linkers, have been compared to understand the effect of the central metal ions on the stabilities of the porous MOF-type materials. Chemical stability to acids, bases, and water decreases in the order of Cr-BDC>Al-BDC>V-BDC, suggesting stability increases with increasing inertness of the central metal ions. However, thermal stability decreases in the order of Al-BDC>Cr-BDC> V-BDC, and this tendency may be explained by the strength of the metal-oxygen bond in common oxides like Al(2)O(3), Cr(2)O(3), and V(2)O(5). In order to evaluate precisely the stability of a MOF, it is necessary to remove uncoordinated organic linkers that are located in the pores of the MOF, because a filled MOF may be more stable than the same MOF after purification.     

Synthesis, structural characterization and selectively catalytic properties of metal-organic frameworks with nano-sized channels: A modular design strategy

Modular design method for designing and synthesizing microporous metal-organic frameworks (MOFs) with selective catalytical activity was described. MOFs with both nano-sized channels and potential catalytic activities could be obtained through self-assembly of a framework unit and a catalyst unit. By selecting hexaaquo metal complexes and the ligand BTC (BTC=1,3,5-benzenetricarboxylate) as framework-building blocks and using the metal complex [M(phen)₂(H₂O)₂]²⁺ (phen=1,10-phenanthroline) as a catalyst unit, a series of supramolecular MOFs 1-7 with three-dimensional nano-sized channels, i.e. [M¹(H₂O)₆]·[M²(phen)₂(H₂O)₂]₂·2(BTC)·xH₂O (M¹, M²Co(II), Ni(II), Cu(II), Zn(II), or Mn(II), phen=1,10-phenanthroline, BTC=1,3,5-benzenetricarboxylate, x=22−24), were synthesized through self-assembly, and their structures were characterized by IR, elemental analysis, and single-crystal X-ray diffraction. These supramolecular microporous MOFs showed significant size and shape selectivity in the catalyzed oxidation of phenols, which is due to catalytic reactions taking place in the channels of the framework. Design strategy, synthesis, and self-assembly mechanism for the construction of these porous MOFs were discussed.     

Catalysis by metal-organic frameworks: fundamentals and opportunities

Crystalline porous materials are extremely important for developing catalytic systems with high scientific and industrial impact. Metal-organic frameworks (MOFs) show unique potential that still has to be fully exploited. This perspective summarizes the properties of MOFs with the aim to understand what are possible approaches to catalysis with these materials. We categorize three classes of MOF catalysts: (1) those with active site on the framework, (2) those with encapsulated active species, and (3) those with active sites attached through post-synthetic modification. We identify the tunable porosity, the ability to fine tune the structure of the active site and its environment, the presence of multiple active sites, and the opportunity to synthesize structures in which key-lock bonding of substrates occurs as the characteristics that distinguish MOFs from other materials. We experience a unique opportunity to imagine and design heterogeneous catalysts, which might catalyze reactions previously thought impossible.     

Toward templated metal-organic frameworks: synthesis, structures, thermal properties, and luminescence of three novel lanthanide-adipate frameworks

Three novel praseodymium-adipate frameworks were synthesized hydrothermally. GWMOF-3 ([Pr(2)(adipic acid)(3)(H(2)O)(4)].adipic acid.4H(2)O) and GWMOF-6 ([Pr(2)(adipic acid)(3)(H(2)O)(2)].4,4'-dipyridyl) formed three-dimensional structures, whereas GWMOF-4 ([Pr(2)(adipic acid)(3)(H(2)O)(2)].H(2)O) produced a more dense, two-dimensional topology. Single-crystal X-ray and powder diffraction, IR spectroscopy, fluorescence spectroscopy, thermogravimetric analysis, and elemental analysis were employed to characterize all samples. GWMOF-6 represents an innovative step forward in metal-organic framework synthesis where a neutral molecular species not used in the construction of the framework is utilized as a structure-directing agent, or template. Furthermore, this template molecule (4,4'-dipyridyl) is shown to sensitize the fluorescence of lanthanide metal centers in a europium analogue of GWMOF-6.