Metal-organic Framework Microcrystals Research Articles

Hybrid organic monolithic column containing MIL-68(Al) for the separation of small molecules by capillary HPLC

A hybrid organic monolithic column made of poly(lauryl methacrylate-co-1,6-hexanediol dimethacrylate) and the metal-organic framework MIL-68(Al) was prepared for the first time. The column was used in capillary liquid chromatography, both in isocratic and gradient elution modes. Separation performance towards small molecules of different chemical nature (polycyclic aromatic hydrocarbons, alkylbenzenes, phenols, etc.) was studied. Monte Carlo simulations were made to both select the proper precursors to obtain empty metal-organic framework micropores in the monolithic polymer and also, to analyze the potential free access of the studied analytes into the micropores (necessary to improve mass transfer and column efficiency). The hereby synthesized metal-organic framework microcrystals allowed obtaining homogeneous hybrid monolithic columns. Adding of MIL-68(Al) (1030 m2 g-1 BET specific surface area) increased the surface area from 3.9 m2 g-1 for the parent monolith to 18.2 m2 g-1 for the hybrid column containing 8 mg mL-1 of the microcrystals. Chromatographic performance of this new column was evaluated by studying retention factors, resolution, and plate counts at room temperature. Different compounds, not completely resolved in the parent monolith, were partially or completely separated after metal-organic framework addition. Using the monolithic column with only 2 mg mL-1 of MIL-68(Al), five alkylbenzenes were completely separated with very symmetrical peak shapes, resolution factors up to 3.60 and plate counts of 4300 plates m-1 for n-hexylbenzene. This value is higher than those obtained by other authors who used organic monolithic columns with embedded metal-organic frameworks to perform separations at room temperature. Additionally, nine polycyclic aromatic hydrocarbons were partially or completely resolved in gradient elution mode. The hybrid monolithic columns exhibited very good intra-day (%RSD=1.9), inter-day (%RSD=2.6), and column-to-column (%RSD=4.3) reproducibility values. Easy and fast column preparation, and versatility to efficiently separate several compounds of different chemical nature in isocratic and gradient mode, makes this new hybrid column a very good option for the analysis of small molecules in capillary (or nano) HPLC.

Switchable Anisotropic/Isotropic Photon Transport in a Double-Dipole Metal-Organic Framework via Radical-Controlled Energy Transfer.

Directional control of photon transport at micro/nanoscale holds great potential in developing multifunctional optoelectronic devices. Here, the switchable anisotropic/isotropic photon transport is reported in a double-dipole metal-organic framework (MOF) based on radical-controlled energy transfer. Double-dipole MOF microcrystals with transition dipole moments perpendicular to each other have been achieved by the pillared-layer coordination strategy. The energy transfer between the double dipolar chromophores can be modulated by the photogenerated radicals, which permits the in situ switchable output on both polarization (isotropy/anisotropy state) and wavelength information (blue/red-color emission). On this basis, the original MOF microcrystal with isotropic polarization state displays the isotropic photon transport and similar reabsorption losses at various directions, while the radical-affected MOF microcrystal with anisotropic polarization state shows the anisotropic photon transport with distinct reabsorption losses at different directions, finally leading to the in situ switchable anisotropic/isotropic photon transport. These results offer a novel strategy for the development of MOF-based photonic devices with tunable anisotropic performance.

Rapid Exciton Transport and Structural Defects in Individual Porphyrinic Metal Organic Framework Microcrystals.

To date, spectroscopic characterization of porphyrin-based metal organic frameworks (MOFs) has relied almost exclusively on ensemble techniques, which provide only structurally averaged insight into the functional properties of these promising photochemical platforms. This work employs time-resolved pump-probe microscopy to probe ultrafast dynamics in PCN-222 MOF single crystals. The simultaneous high spatial and temporal resolution of the technique enables the correlation of spectroscopic observables to both inter- and intracrystal structural heterogeneity. The pump-probe measurements show that significant differences in the excited state lifetime exist between individual PCN-222 crystals of an ensemble. On a single PCN-222 crystal, differences in excited state lifetime and photoluminescence quantum yield are found to correlate to microscale structural defects introduced at crystallization. Pump probe microscopy also enables the direct measurement of excited state transport. Imaging of exciton transport on individual MOF crystals reveals rapid, but subdiffusive exciton transport which slows on the 10s of ps time scale. Time-averaged exciton diffusion coefficients over the first 200 ps span a range of 0.27 to 1.0 cm2/s, indicating that excited states are rapidly transported through the porphyrin network of PCN-222 before being trapped. Together, these single-particle-resolved measurements provide important new insight into the role played by structural defects on the photochemical functionality of porphyrin-based MOFs.

An approach to MOFaxanes by threading ultralong polymers through metal–organic framework microcrystals

Mechanically interlocked architecture has inspired the fabrication of numerous molecular systems, such as rotaxanes, catenanes, molecular knots, and their polymeric analogues. However, to date, the studies in this field have only focused on the molecular-scale integrity and topology of its unique penetrating structure. Thus, the topological material design of such architectures has not been fully explored from the nano- to the macroscopic scale. Here, we propose a supramolecular interlocked system, MOFaxane, comprised of long chain molecules penetrating a microcrystal of metal–organic framework (MOF). In this study, we describe the synthesis of polypseudoMOFaxane that is one of the MOFaxane family. This has a polythreaded structure in which multiple polymer chains thread a single MOF microcrystal, forming a topological network in the bulk state. The topological crosslinking architecture is obtained by simply mixing polymers and MOFs, and displays characteristics distinct from those of conventional polyrotaxane materials, including suppression of unthreading reactions.

Graphene oxide assisted assembly of superhydrophilic MOF-based membrane with 2D/3D hybrid nanochannels for enhanced water purification

Assembling metal–organic framework (MOF) microcrystals into macroscopic membrane structure is of vital importance for the development of high-performance MOF-based membranes. However, most MOFs possess poor solution processibility due to their low colloidal and chemical stability. Herein, we report a facile approach that uses graphene oxide (GO) as two-dimensional (2D) nano-surfactant to assist the dispersion and assembly of superhydrophilic amino acid-based Zr-MOF microcrystals, MIP-202(Zr), into a bulge-structured membrane. The incorporation of superhydrophilic MIP-202(Zr) with GO laminates endowed the resultant membrane with desired two-dimensional/three-dimensional (2D/3D) hybrid nanochannels and stronger hydrophilicity. Benefiting from these features, the obtained GO/MIP-202(Zr) membrane exhibited superior water permeance than pure GO membrane and many other GO/MOF composite membranes. The strengthened permeation performance could also be well-maintained with satisfied rejection performances when utilized for dye molecule rejection. The dye molecule rejection process was closely associated with their size, charge and configuration. Moreover, the GO/MIP-202(Zr) membrane showed improved antifouling performance compared with pure GO membrane, demonstrating its superiority in the practical separation process. Our results provide essential reference values for the assembly of MOF microcrystals into macroscopic membrane structures and their application in water treatment.

Steric-Hindrance-Controlled Laser Switch Based on Pure Metal–Organic Framework Microcrystals

Herein, we demonstrated a steric-hindrance-controlled laser switch in pure metal-organic framework (MOF) microcrystals. The well-faceted MOF microwires with aggregation-induced emission (AIE) lumnogens as linkers function as typical Fabry-Pérot microlasers. The steric hindrance around the AIE linkers can be reduced by the loss of guest molecules, which lead to the enhanced rotation of linkers with red-shifted gain behavior. On this basis, the gain region was readily switched through changing the steric hindrance via the desorption/adsorption of guests. As a result, the reversible switching of the dual-wavelength lasing from MOF microwires was achieved. The results provide a promising route to the development of versatile micro-/nanolasers with desired applications.

Dual-wavelength lasing from organic dye encapsulated metal-organic framework microcrystals.

We demonstrate efficient and stable dual-wavelength microlasers fabricated by encapsulating different organic dyes into metal-organic framework (MOF) microcrystals. The intermolecular interactions between the organic dyes are suppressed effectively by the spatial confinement effect of the MOF pores, enabling a simultaneously efficient optical gain, and therefore lasing at different wavelengths. The results represent an important step towards a function-oriented laser material design.

A Zinc(II) Porous Metal-Organic Framework and Its Morphologically Controlled Catalytic Properties in the Knoevenagel Condensation Reaction.

A new metal-organic framework (MOF), {Zn(oba)(3-bpdh)0.5 }∝ ⋅2 H2 O (1 Zn; oba=4,4'-oxybis(benzoic acid), 3-bpdh=N,N'-bis(1-pyridine-3-yl-ethylidene)hydrazine), was successfully assembled in a solvothermal system. The Zn2+ ion center located in a five-coordinate environment combined with oba to produce a layer-like structure, which is again linked with 3-bpdh to yield a 3D channel framework. MOF 1 Zn microcrystals with different morphologies were synthesized through coordination modulation and solvent-dependent methods. The effects of the concentration of the modulator, the reaction temperature, and the volume ratio of mixed solvents have been investigated. Furthermore, the catalytic performance in Knoevenagel condensation was studied for different morphologies of 1 Zn microcrystals; this revealed that the MOF microcrystals exhibited excellent and morphology-dependent catalytic properties.

Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor

Porous Co3O4 microflowers were successfully synthesized by calcinations of novel flower-like metal-organic framework microcrystals. A supercapacitor was conducted based on this porous Co3O4 microflowers as the electrode material, which shows that the porous Co3O4 electrode exhibit a specific capacitance of 240.2 F g−1 at the current density of 0.625 A g−1 and remains more than 96.3% after 2000 cycles. The results demonstrate that porous Co3O4 microflowers can be acted as a promising electrode candidate in supercapacitors.

Imparting functionality to biocatalysts via embedding enzymes into nanoporous materials by a de novo approach: size-selective sheltering of catalase in metal-organic framework microcrystals.

We develop a new concept to impart new functions to biocatalysts by combining enzymes and metal-organic frameworks (MOFs). The proof-of-concept design is demonstrated by embedding catalase molecules into uniformly sized ZIF-90 crystals via a de novo approach. We have carried out electron microscopy, X-ray diffraction, nitrogen sorption, electrophoresis, thermogravimetric analysis, and confocal microscopy to confirm that the ~10 nm catalase molecules are embedded in 2 μm single-crystalline ZIF-90 crystals with ~5 wt % loading. Because catalase is immobilized and sheltered by the ZIF-90 crystals, the composites show activity in hydrogen peroxide degradation even in the presence of protease proteinase K.

Compression-induced deformation of individual metal-organic framework microcrystals.

The deformation and mechanical behavior of individual zeolitic-imidazolate framework (ZIF-8) micro- and sub-microcrystals were observed under compression. Young's modulus and volume changes as a function of applied pressure were determined on individual single crystals, offering insights in the relationship among structure, morphology, and mechanical properties. Dramatic volume decreases and amorphization were detected during compression over a pressure range of 0-4 GPa for individual 1.2 μm ZIF-8 microcrystals, and the deformed microcrystals partially recovered after pressure release. The orientation and size effects on the mechanical behavior of ZIF-8 nano- and microcrystals were also investigated. The presence of solvates within the pores of the ZIF-8 has a dramatic effect on the mechanical properties of the single crystals. Methanol-solvated ZIF-8 microcrystals are much less deformable than the desolvated microcrystals and shatter completely at very low applied force.

Selectively catalytic micro- and nanocrystals of metal–organic framework [Co(4-bpdh)(HIA)

Metal–organic framework micro-crystals [Co(4-bpdh)(HIA)]∝ (1Co), crystallized in the monoclinic system (space group P12/n1, a=10.0009(16)Å, b=15.472(3)Å, c=18.563(3)Å, β=91.81(0)°, and Z=4), were controllably synthesized through the adjustment of the solvent system. The Co2+ ion center located in a six-coordinated environment combined with 5-hydroxyisophthalic acid (HIA) to produce ladder-like structures, which again linked with 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hxadiene (4-bpdh) to yield a double-layer network. Micro-crystals 1Co exhibited selectively oxidation-catalytic properties for the degradation of methyl orange (conversion 90%) owing to ligand to metal charge transfer. Our work determined that the synthesized catalyst is not only highly selective for degradation of organic dyes, but also very efficient.

Metal–organic frameworks post-synthetically modified with ferrocenyl groups: framework effects on redox processes and surface conduction

Metal-organic framework (MOF) materials based on zinc(II) and aluminium(III) dicarboxylate frameworks with covalently attached ferrocene functional redox groups were synthesised by post-synthetic modification and investigated by voltammetry in aqueous and non-aqueous media. In the voltammetry experiments, ferrocene oxidation occurs in all cases, but chemically reversible and stable ferrocene oxidation without decay of the voltammetric response requires a "mild" dichloroethane solvent environment. The voltammetric response in this case is identified as "surface-confined" with fast surface-hopping of electrons and without affecting the bulk of MOF microcrystals. In aqueous media a more complex pH-dependent multi-stage redox process is observed associated with chemically irreversible bulk oxidation and disintegration of the MOF framework. A characteristic 30 mV per pH unit dependence of redox potentials is observed attributed to a "framework effect": the hydroxide-driven MOF framework dissolution.

自己組織化単分子膜を利用した機能性配位高分子薄膜の形成

Porous metal-organic framework (MOFs) and porous coordination polymers have attracted much attention and extensively studied for past decades because of their interesting properties applicable to gas storage, gas sensing, catalyst and so on. Although lots of studies have focused on the creation of new porous materials or new functions, it is also necessary to develop new techniques that utilize MOFs’ interesting properties to some applications. Especially, the method of thin film formation is important to apply MOFs’ occlusion and sensing properties to some devices such as chemical sensors, electrocatalysts and so on. Recently, some research groups have reported on the formation of MOFs thin films on Au substrate utilizing self-assembled monolayers (SAMs). Also they have succeeded in the control of the crystal orientation on SAMs. They have suggested that the chemical bonding interactions between components of MOFs and functional group on the surface of SAMs contributes significantly not only to fixing the MOF thin film to the substrate, but also to controlling the crystal orientation of MOF microcrystals on SAMs.