CD-MOF: A Versatile Separation Medium

Porous metal-organic frameworks (MOFs) have been studied in the context of a wide variety of applications, particularly in relation to molecular storage and separation sciences. Recently, we reported a green, renewable framework material composed of γ-cyclodextrin (γ-CD) and alkali metal salts--namely, CD-MOF. This porous material has been shown to facilitate the separation of mixtures of alkylaromatic compounds, including the BTEX mixture (benzene, toluene, ethylbenzene, and the regioisomers of xylene), into their pure components, in both the liquid and gas phases, in an energy-efficient manner which could have implications for the petrochemical industry. Here, we report the ability of CD-MOF to separate a wide variety of mixtures, including ethylbenzene from styrene, haloaromatics, terpinenes, pinenes and other chiral compounds. CD-MOF retains saturated compounds to a greater extent than their unsaturated analogues. Also, the location of a double bond within a molecule influences its retention within the extended framework, as revealed in the case of the structural isomers of pinene and terpinine, where the isomers with exocyclic double bonds are more highly retained than those with endocyclic double bonds. The ability of CD-MOF to separate various mono- and disubstituted haloaromatic compounds appears to be controlled by both the size of the halogen substituents and the strength of the noncovalent bonding interactions between the analyte and the framework, an observation which has been confirmed by molecular simulations. Since CD-MOF is a homochiral framework, it is also able to resolve the enantiomers of chiral analytes, including those of limonene and 1-phenylethanol. These findings could lead to cheaper and easier-to-prepare stationary phases for HPLC separations when compared with other chiral stationary phases, such as CD-bonded silica particles.

A Simple and Non-Destructive Method for Assessing the Incorporation of Bipyridine Dicarboxylates as Linkers within Metal-Organic Frameworks

As a novel avenue for applications, metal-organic frameworks (MOFs) are increasingly used for heterogenizing catalytic molecular species as linkers into their crystalline framework. These multifunctional compounds can be accessed with mixed linkers synthesis or postsynthetic-exchange strategies. Major limitations still reside in their challenging characterization; in particular, to provide evidence of the genuine incorporation of the functionalized linkers into the framework and their quantification. Herein, we demonstrate that a combination of computational chemistry, spectroscopy and X-ray diffraction allows access to a non-destructive analysis of mixed-linker UiO-67-type materials featuring biphenyl- and bipyridine-dicarboxylates. Our UV/Vis-based methodology has been further applied to characterize a series of Rh-functionalized UiO-67-type catalysts. The proposed approach allows a recurrent key issue in the characterization of similar supported organometallic systems to be solved.

Tuning the properties of the metal–organic framework UiO-67-bpy via post-synthetic N-quaternization of pyridine sites

N-Quaternization of pyridine sites in UiO-67-bpy affords a cationic MOF UiO-67-bpy-Me with faster and improved anionic dye and CO₂adsorption, and extended visible-light absorption properties, which make UiO-67-bpy-Me more efficient in photodegrading organic dyes.

Mixed-linker approach in designing porous zirconium-based metal–organic frameworks with high hydrogen storage capacity

New zirconium based metal–organic framework (UBMOF-31) synthesised using mixed-linker strategy showing permanent porosity, excellent hydrogen uptake, and high selectivity for adsorption of CO₂ over N₂.

MOF-253-Pd(OAc)2: a recyclable MOF for transition-metal catalysis in water

We report palladium(ii)-functionalized MOF-253 (MOF-253-Pd(OAc)₂) as a recyclable catalyst to form all-carbon quaternary centers via conjugate additions of arylboronic acids to β,β-disubstituted enones in aqueous media.

Zr-based metal–organic frameworks: design, synthesis, structure, and applications

This review summarizes the advances in the study of Zr-based metal–organic frameworks in terms of their design, synthesis, structure, and potential applications.

The structural diversity and properties of nine new viologen based zwitterionic metal–organic frameworks

A viologen based ligand has been explored as a functional building block to synthesize a series of nine new zwitterionic metal–organic frameworks with this contribution reporting on their structural, thermal and photochromic properties.

A responsive MOF nanocomposite for decoding volatile organic compounds

A molecular decoding platform, which recognizes aromatic VOCs with distinctly different 2D readouts, has been developed by a responsive luminescent MOF nanocomposite. The VOC recognition is based on an unprecedented dual-readout orthogonal identification scheme.

Four Pb(ii) metal–organic frameworks with increasing dimensions: structural diversities by varying the ligands

Four complexes with 1D, 2D and 3D frameworks were synthesized by three rigid, linear ligands. The ligands' effects and luminescence properties were studied.

Solvothermal Metal Metathesis on a Metal-Organic Framework with Constricted Pores and the Study of Gas Separation

Metal-organic frameworks (MOFs) with constricted pores can increase the adsorbate density of gas and facilitate effective CO2 separation from flue gas or natural gas due to their enhanced overlapping of potential fields of the pores. Herein, an MOF with constricted pores, which was formed by narrow channels and blocks of functional groups, was fabricated from the assembly of a methyl-functionalized ligand and Zn(II) centers (termed NPC-7-Zn). Structural analysis of the as-synthesized NPC-7-Zn reveals a series of zigzag pores with pore diameters of ∼0.7 nm, which could be favorable for CO2 traps. For reinforcing the framework stability, a solvothermal metal metathesis on the pristine MOF NPC-7-Zn was performed, and a new Cu(II) MOF (termed NPC-7-Cu) with an identical framework was produced. The influence of the reaction temperatures on the metal metathesis process was investigated. The results show that the constricted pores in NPC-7-Zn can induce kinetic issues that largely slow the metal metathesis process at room temperature. However, this kinetic issue can be solved by applying higher reaction temperatures. The modified MOF NPC-7-Cu exhibits significant improvements in framework stability and thus leads to a permanent porosity for this framework. The constricted pore structure enables enhanced potential fields for these pores, rendering this MOF with high adsorbate densities for CO2 and high adsorption selectivity for a CO2/N2 gas mixture. The adsorption kinetic studies reveal that CH4 has a faster diffusion rate constant than CO2, showing a surface diffusion controlled mechanism for CO2 and CH4 adsorption.

Stabilization of volatile Ti(BH4)3 by nano-confinement in a metal-organic framework

Volatile Ti(BH₄)₃ molecules stabilized on the surface of a MOF.

Liquid complex hydrides are a new class of hydrogen storage materials with several advantages over solid hydrides, e.g. they are flexible in shape, they are a flowing fluid and their convective properties facilitate heat transport. The physical and chemical properties of a gaseous hydride change when the molecules are adsorbed on a material with a large specific surface area, due to the interaction of the adsorbate with the surface of the host material and the reduced number of collisions between the hydride molecules. In this paper we report the synthesis and stabilization of gaseous Ti(BH₄)₃. The compound was successfully stabilized through adsorption in nanocavities. Ti(BH₄)₃, upon synthesis in its pure form, spontaneously and rapidly decomposes into diborane and titanium hydride at room temperature in an inert gas, e.g. argon. Ti(BH₄)₃ adsorbed in the cavities of a metal organic framework is stable for several months at ambient temperature and remains stable up to 350 K under vacuum. The adsorbed Ti(BH₄)₃ reaches approximately twice the density of the gas phase. The specific surface area (BET, N₂ adsorption) of the MOF decreased from 1200 m² g^–1 to 770 m² g^–1 upon Ti(BH₄)₃ adsorption.

Concentration Dependent Dimensionality of Resonance Energy Transfer in a Postsynthetically Doped Morphologically Homologous Analogue of UiO-67 MOF with a Ruthenium(II) Polypyridyl Complex

A method is described here by which to dope ruthenium(II) bis(2,2'-bipyridine) (2,2'-bipyridyl-5,5'-dicarboxylic acid), RuDCBPY, into a UiO-67 metal-organic framework (MOF) derivative in which 2,2'-bipyridyl-5,5'-dicarboxylic acid, UiO-67-DCBPY, is used in place of 4,4'-biphenyldicarboxylic acid. Emission lifetime measurements of the RuDCBPY triplet metal-to-ligand charge transfer, (3)MLCT, excited state as a function of RuDCBPY doping concentration in UiO-67-DCBPY are discussed in light of previous results for RuDCBPY-UiO-67 doped powders in which quenching of the (3)MLCT was said to be due to dipole-dipole homogeneous resonance energy transfer, RET. The bulk distribution of RuDCBPY centers within MOF crystallites are also estimated with the use of confocal fluorescence microscopy. In the present case, it is assumed that the rate of RET between RuDCBPY centers has an r(-6) separation distance dependence characteristic of Förster RET. The results suggest (1) the dimensionality in which RET occurs is dependent on the RuDCBPY concentration ranging from one-dimensional at very low concentrations up to three-dimensional at high concentration, (2) the occupancy of RuDCBPY within UiO-67-DCBPY is not uniform throughout the crystallites such that RuDCBPY densely populates the outer layers of the MOF at low concentrations, and (3) the average separation distance between RuDCBPY centers is ∼21 Å.

Encapsulation of Mono- or Bimetal Nanoparticles Inside Metal-Organic Frameworks via In situ Incorporation of Metal Precursors

A facile, in situ metal precursor incorporation strategy is established for good control over the location and composition of metal nanoparticles within metal-organic frameworks (MOFs). This one-step metal precursor incorporation route is successfully applied to the fabrication of ultrafine Pd, Ni, and PdNi alloys to be selectively encapsulated inside the pores of MOFs, achieving superior catalytic activity and stability in the hydrogenation of nitrobenzene.

Stable porphyrin Zr and Hf metal-organic frameworks featuring 2.5 nm cages: high surface areas, SCSC transformations and catalyses

Two isostructural porphyrin Zr and Hf metal-organic frameworks (FJI-H6 and FJI-H7) are rationally synthesized, and are constructed from 2.5 nm cubic cages. Notably, they both possess high water and chemical stability and can undergo single-crystal to single-crystal transformations to embed Cu2+ ions into the open porphyrin rings. FJI-H6 has a high BET surface area of 5033 m2 g-1. Additionally, they exhibit promising catalytic abilities to convert CO2 and epoxides into cyclic carbonates at ambient conditions.

Methane storage in metal-organic frameworks

Natural gas (NG), whose main component is methane, is an attractive fuel for vehicular applications. Realization of safe, cheap and convenient means and materials for high-capacity methane storage can significantly facilitate the implementation of natural gas fuelled vehicles. The physisorption based process involving porous materials offers an efficient storage methodology and the emerging porous metal-organic frameworks have been explored as potential candidates because of their extraordinarily high porosities, tunable pore/cage sizes and easily immobilized functional sites. In this view, we provide an overview of the current status of metal-organic frameworks for methane storage.

A robust, catalytic metal-organic framework with open 2,2'-bipyridine sites

We report two synthetic approaches to prepare a highly crystalline Zr(IV)-based metal-organic framework (MOF) containing open 2,2'-bipyridine (bpy) chelating sites. The resulting UiO-67-bpydc readily forms complexes with PdCl2 to produce a MOF that exhibits efficient and recyclable catalytic activity for the Suzuki-Miyaura cross-coupling reaction.

Lanthanides post-functionalized nanocrystalline metal-organic frameworks for tunable white-light emission and orthogonal multi-readout thermometry

We demonstrate tunable white-light emission and multi-readout thermometry in two respective nanocrystalline luminescent metal-organic frameworks (MOFs), which are prepared via postsynthetic functionalization with lanthanide cations of a robust UiO type MOF bearing the 2,2'-bipyridyl moiety (UiO-67-bpydc, 1). The white-light emitting framework Eu(3+)@1 can be conveniently applied as a thin film onto a commercial UV-LED chip for practical white lighting applications. The multi-readout orthogonal thermometry is illustrated in relation to the emission intensity ratio as well as the decay time and luminescence color of Eu(3+)/Tb(3+)@1 nanocrystals. This work highlights the opportunity for designing white-light emitters and nanothermometers based on lanthanide functionalized MOFs.

Single-crystal-to-single-crystal metalation of a metal-organic framework: a route toward structurally well-defined catalysts

Metal-organic frameworks featuring ligands with open chelating groups are versatile platforms for the preparation of a diverse set of heterogeneous catalysts through postsynthetic metalation. The crystalline nature of these materials allows them to be characterized via X-ray diffraction, which provides valuable insight into the structure of the metal sites that facilitate catalysis. A highly porous and thermally robust zirconium-based metal-organic framework, Zr6O4(OH)4(bpydc)6 (bpydc(2-) = 2,2'-bipyridne-5,5'-dicarboxylate), bears open bipyridine sites that readily react with a variety of solution- and gas-phase metal sources to form the corresponding metalated frameworks. Remarkably, Zr6O4(OH)4(bpydc)6 undergoes a single-crystal-to-single-crystal transformation upon metalation that involves a change in space group from Fm3̅m to Pa3̅. This structural transformation leads to an ordering of the metalated linkers within the framework, allowing structural characterization of the resulting metal complexes. Furthermore, Zr6O4(OH)4(bpydc)6 yields an active heterogeneous catalyst for arene C-H borylation when metalated with [Ir(COD)2]BF4 (COD = 1,5-cyclooctadiene). These results highlight the unique potential of metal-organic frameworks as a class of heterogeneous catalysts that allow unparalleled structural characterization and control over their active sites.

Eu(III)-functionalized MIL-124 as fluorescent probe for highly selectively sensing ions and organic small molecules especially for Fe(III) and Fe(II)

A layerlike MOF (MIL-124, orGa2(OH)4(C9O6H4)) has been prepared and chosen as a parent compound to encapsulate Eu(3+) cations by one uncoordinated carbonyl group in its pores. The Eu(3+)-incorporated sample (Eu(3+)@MIL-124) is fully characterized, which shows excellent luminescence and good fluorescence stability in water or other organic solvents. Subsequently, we choose Eu(3+)@MIL-124 as sensitive probe for sensing metal ions, anions, and organic small molecules because of its robust framework. Studying of the luminescence properties reveals that the complex Eu(3+)@MIL-124 was developed as a highly selective and sensitive probe for detection of Fe(3+) (detection limit, 0.28 μM) and Fe(2+) ions through fluorescence quenching of Eu(3+) and MOF over other metal ions. In connection to this, a probable sensing mechanism was also discussed in this paper. In addition, when Eu(3+)@MIL-124 was immersed in the different anions solutions and organic solvents, it also shows highly selective for Cr2O7(2-)(detection limit, 0.15 μM)and acetone. Remarkably, it is the first Eu-doped MOF to exhibit an excellent ability for the detection of Fe(3+) and Fe(2+) in an aqueous environment without any structural disintegration of the framework.

An amino-decorated NbO-type metal–organic framework for high C2H2 storage and selective CO2 capture

A novel amino-decorated NbO-type metal–organic framework (ZJU-8) has been designed and synthesized, exhibiting high acetylene storage and selective CO₂ capture and separation capacities at room temperature.

The low dielectric constant and relaxation dielectric behavior in hydrogen-bonding metal–organic frameworks

A 3D hydrogen-bonding metal–organic framework shows a low dielectric constant and relaxation dielectric behavior at high temperature.

Pre-concentration and energy transfer enable the efficient luminescence sensing of transition metal ions by metal–organic frameworks

The 2,2′-bipyridyl moieties in designer MOFs pre-concentrate metal ion analytes and transfer energy to lanthanide reporters for luminescence sensing.

A new NbO type metal–organic framework for high acetylene and methane storage

A novel NbO-type metal–organic framework (ZJU-9) has been designed and synthesized, exhibiting high acetylene and methane storage capacity at room temperature.

Synthesis of zirconium oxycarbide powders using metal–organic framework (MOF) compounds as precursors

ZrC_xO_y oxycarbides were synthesized for the first time by using metal–organic framework (MOF) compounds as precursors.

Luminescent zinc metal–organic framework (ZIF-90) for sensing metal ions, anions and small molecules

We synthesize ZIF-90, which exhibits an intense blue luminescence excited under visible light. Luminescent studies indicate that ZIF-90 could be an efficient multifunctional fluorescence material for sensing metal cations, anions and organic small molecules.

A Zr metal–organic framework based on tetrakis(4-carboxyphenyl) silane and factors affecting the hydrothermal stability of Zr-MOFs

A new Zr-MOF based on tetrakis(4-carboxyphenyl) silane was synthesized, and factors affecting the hydrothermal stabilities of Zr-MOFs are discussed.

Solvent-controlled formation of four Ni(ii) coordination polymers based on a flexible bis(imidazole) ligand: syntheses, structural diversification, properties

Four new isomeric Ni(ii) coordination polymers with different architectures and properties were successfully synthesized by tuning the solvent media.

Turn-on luminescence based discrimination of protic acids using a flexible layered metal–organic coordination polymer

A flexible layered metal–organic coordination polymer used as probe for turn-on luminescence based discrimination of protic acids and amino acids.

A Zn(ii) coordination polymer and its photocycloaddition product: syntheses, structures, selective luminescence sensing of iron(iii) ions and selective absorption of dyes

Nanospheres of a Zn(ii) complex are used to selectively sense iron(iii) ions while its photocycloaddition product can selectively absorb dyes.

Pillared-bilayer porous coordination polymers of Zn(ii): enhanced hydrophobicity of pore surface by changing the pillar functionality

Two new isostructural porous coordination polymers of Zn(ii) have been synthesized using a mixed ligand system. A huge difference in water adoption is observed by introducing a small hydrophobic group in the organic linker.

Tuning the cavities of zirconium-based MIL-140 frameworks to modulate CO2adsorption

A combined experimental and computational study on the CO₂adsorption performance of zirconium-based MIL-140 frameworks has revealed the crucial influence of pore-confinement effects.

Synthesis of an exceptional water-stable two-fold interpenetrated Zn(ii)-paddlewheel metal–organic framework

An exceptional water-stable two-fold interpenetrated Zn(ii)-paddlewheel metal–organic framework was obtained via the facile conversion of its non-interpenetrated isomer at ambient temperature, showing high CO₂ and CH₄ uptake capacities.

Four super water-stable lanthanide–organic frameworks with active uncoordinated carboxylic and pyridyl groups for selective luminescence sensing of Fe3+

Water-stable Ln–MOFs with uncoordinated carboxyl and pyridyl groups have been proved to exhibit efficient luminescence sensing for Fe³⁺ in aqueous solutions.