Dense Metal Organic Framework Research Articles

Gd Metal-Organic Framework Thin Film for On-Chip Local Magnetic Refrigeration.

Dense metal-organic frameworks with high spin paramagnetic nodes are competitive materials for cryogenic magnetic refrigeration, particularly in applications for which local cooling is advantageous. We focus on obtaining thin films of gadolinium formate, which has a large volumetric magnetocaloric effect. Continuous and homogeneous deposits of gadolinium formate are successfully formed on silicon by means of aerosol jet printing, with control over the film thickness from 0.35 μm up to 2.5 μm. The excellent cooling power of the deposits is evidenced via direct measurements of the cooling of a 200 μm silicon wafer down to sub-K temperatures by a single demagnetization from 1 T and 2 K, thereby demonstrating the potential of this approach for on-chip local magnetic refrigeration.

Magnetic order in holmium and erbium formate: Parent frameworks for a potential random spin chain paramagnet

This work uses a combination of neutron diffraction and bulk property measurements to establish the low temperature magnetic states in the dense metal-organic frameworks Ho(HCO2)3 and Er(HCO2)3; whose structures feature chains of face-sharing LnO9 polyhedra packed into a triangular lattice. Below 0.7 K Ho(HCO2)3 is found to adopt a state in which the magnetic moment on its ferromagnetic chains vary from neighbouring chains but the sum around each triangle is constant. Er(HCO2)3 is found to be the first lanthanide formate to adopt an ordered magnetic state with antiferromagnetic coupling within its chains near 50 mK. The potential to combine the ferromagnetic and antiferromagnetic coupling within chains associated with Ho and Er cations, respectively, in the same compound has also been explored via the series Ho1-xErx(HCO2)3. Ho0.5Er0.5(HCO2)3 remains paramagnetic to 0.4 K, suggesting it may be a starting point to search for a random spin chain paramagnet.

In situ electrochemical potential-induced synthesis of metal organic framework membrane on polymer support for H2/CO2 separation

Metal-Organic Framework (MOF) membranes act as selective layers have offered unprecedented opportunities for energy-efficient and cost-effective gas separation. Searching for the green and sustainable synthesis method of dense MOF membrane has received huge attention in both academia and industry. In this work, we demonstrate an in situ electrochemical potential-induced synthesis strategy to aqueously fabricate Metal Azolate Framework-4 (MAF-4) membranes on polypropylene (PP) support. The constant potential can induce the heterogeneous nucleation and growth of MAF-4, resulting an ultrathin membrane with the thickness of only 390 nm. This high-quality membrane exhibits a high H2/CO2 separation performance with the H2 permeance as high as 1565.75 GPU and selectivity of 11.6. The deployment of this environment friendly one-step fabrication method under mild reaction conditions, such as low-cost polymer substrate, water instead of organic solvent, room temperature and ambient pressure shows great promise for the scale-up of MOF membranes.

High-pressure study of a 3d-4f heterometallic CuEu-organic skeleton.

We prepared a 3d-4f heterobimetallic CuEu-organic framework NBU-8 with a density of 1921 kg m-3 belonging to the family of dense packing materials (dense metal-organic frameworks or MOFs). This MOF material was prepared from 4-(pyrimidin-5-yl)benzoic acid (HPBA) with a bifunctional ligand site as a tripodal ligand and Cu2+ and Eu3+ as the metal centres; the molecular formula is Cu3Eu2(PBA)6(NO3)6·H2O. This material is a very promising dimethylformamide (DMF) molecular chemical sensor. Systematic high-pressure studies of NBU-8 were carried out by powder X-ray diffraction, high-pressure X-ray diffraction and molecular dynamics simulation. The high-pressure experiment shows that the (006) diffraction peak of the crystal structure moves toward a low angle with increasing pressure, accompanied by the phenomenon that the d-spacing increases, and as the pressure increases, the (10-2) diffraction peak moves to a higher angle, the amplitude of the d-spacing is significantly reduced and finally merges with the (006) diffraction peak into one peak. The amplitude of the d-spacing is significantly reduced, indicating that NBU-8 compresses and deforms along the a-axis direction when subjected to uniform pressure. This is caused by tilting of the ligands to become more vertical along the c direction, leading to its expansion. This allows greater contraction along the a direction. We also carried out a Rietveld structure refinement and a Birch-Murnaghan solid-state equation fitting for the high-pressure experimental results. We calculated the bulk modulus of the material to be 45.68 GPa, which is consistent with the calculated results. The framework is among the most rigid MOFs reported to date, exceeding that of Cu-BTC. Molecular dynamics simulations estimated that the mechanical energy absorbed by the system when pressurized to 5.128 GPa was 249.261 kcal mol-1. The present work will provide fresh ideas for the study of mechanical energy in other materials.

Elasticity of Dense Metal–Organic Framework Compounds

Elasticity of Dense Metal–Organic Framework Compounds

Geometric Frustration on the Trillium Lattice in a Magnetic Metal-Organic Framework.

In the dense metal-organic framework Na[Mn(HCOO)_{3}], Mn^{2+} ions (S=5/2) occupy the nodes of a "trillium" net. We show that the system is strongly magnetically frustrated: the Néel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Néel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at 1/3-saturation magnetization. A simple model of nearest-neighbor Heisenberg antiferromagnetic and dipolar interactions accounts quantitatively for all observations, including an unusual 2-k magnetic ground state. We show that the relative strength of dipolar interactions is crucial to selecting this particular ground state. Geometric frustration within the classical spin liquid regime gives rise to a large magnetocaloric response at low applied fields that is degraded in powder samples as a consequence of the anisotropy of dipolar interactions.

Geometric frustration on the trillium lattice in a magnetic metal–organic framework

In the dense metal-organic framework Na[Mn(HCOO)$_3$], Mn$^{2+}$ ions ($S=\frac{5}{2}$) occupy the nodes of a `trillium' hyperkagome net. We show that this material exhibits a variety of behaviour characteristic of geometric frustration: the N\'eel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the N\'eel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at $\frac{1}{3}$-saturation magnetisation. We demonstrate that a simple nearest-neighbour Heisenberg antiferromagnet model accounts quantitatively for each observation, and hence Na[Mn(HCOO)$_3$] is the first experimental realisation of this model on the trillium net. We develop a mapping between this trillium model and that on the two-dimensional Shastry-Sutherland lattice, and demonstrate how both link geometric frustration within the classical spin liquid regime to a strong magnetocaloric response at low fields.

Polymer-supported ultra-thin ZIF-67 membrane through in situ interface self-repair

Metal-organic framework (MOF) membranes have significant potential for use in efficient gas separation. However, fabricating polymer substrate-supported ultra-thin MOF membranes with no defects remains a challenge. Herein, we proposed an in situ interface self-repair strategy for fabricating ultra-thin and defect-free ZIF-67 membranes for the first time. This was realized by preparing a polyvinyl alcohol (PVA)-Co2+ interfacial layer containing a Co2+ ion layer and a PVA-Co2+ complex layer. The PVA-Co2+ interfacial layer released Co2+ ions and acted as a metal source to form ZIF-67 membranes by reducing the coordination rate of Co2+ ions with 2-methylimidazole. The embedded Co2+ ions functioned as anchor sites for the nucleation of ZIF-67 membrane assisted by the interface-enhanced deprotonation of 2-methylimidazole. Especially, the slow release of Co2+ ions from the PVA-Co2+ interfacial layer facilitated self-repair of defects to form a continuous and dense MOF layer. ZIF-67 membranes, 80-280 nm thick, were successfully synthesized and demonstrated unprecedented CO2 permeance and good CO2/N2 selectivity. This work provides a promising strategy for the fabrication of ultra-thin and defect-free MOF membranes on polymeric support layers for use in practical applications.

Кристаллическая структура плотных металл-органических координационных полимеров на основе Sc(III) и двух типов лигандов

Взаимодействием солей скандия(III) с 2,2'-бипиридил-4,4'-дикарбоновой и терефталевой кислотами в сольвотермальных условиях получены два новых металл-органических координационных полимера: [Sc2(HCOO)2(OH)2(bpydc)]·2H2O (1) и [Sc(HCOO)(bdc)] (2), где bpydc2– — 2,2'-бипиридил-4,4'-дикарбоксилат, bdc2– — терефталат, HCOO– — формиат. Кристаллические структуры соединений 1 и 2 установлены методом монокристального рентгеноструктурного анализа. Оба соединения содержат в своем составе формиат-анионы в качестве дополнительного линкера и имеют плотную структуру. Химический состав и фазовая чистота соединений подтверждены методами рентгеновской дифракции на порошках, ТГ и элементным анализами, ИК спектроскопией.

Oxygen-assisted Cathodic Deposition of Copper-Carboxylate Metal–Organic Framework Films

Reductive deposition is an attractive strategy to prepare films of metal–organic frameworks (MOFs), but to cathodically deposit the intergrown dense films of MOFs containing readily reduced metal ions (e.g., Cu2+) without the plating of the corresponding metals still remains a significant challenge. Here, we report the cathodic deposition of metallic copper-free films of two copper-carboxylate MOFs (HKUST-1 and MOF-14). Our strategy relies on the utilization of O2 as probase to electrochemically trigger the crystallization of MOFs. Systematical cyclic voltammetry studies indicate that the favorable synergistic catalytic effects of the acidic copper ions and benzene-1,3,5-tricarboxylic acid ligands on the O2 reduction can positively shift the deposition potential of HKUST-1 significantly to +0.05 V (vs Ag/AgCl), avoiding effectively the plating of metallic copper and facilitating greatly the control over the crystallization kinetics of MOFs. By the further optimization of experimental conditions relevant to reaction temperature, supporting electrolyte, O2 concentration, and working potential, the intergrown dense MOF films with tunable thicknesses were deposited successfully on several commonly used electrodes including the highly oriented pyrolytic graphite, carbon fiber, carbon cloth, and fluorine-doped tin oxide.

Synthesis of nano-sheets seeds for secondary growth of highly hydrogen permselective ZIF-95 membranes

Recently, more and more attentions have been focused on the preparation of metal-organic framework (MOF) membranes for gas or liquid separations. However, it is rather difficult to prepare a dense MOF membrane by a direct solvothermal synthesis route because of poor heterogeneous nucleation of MOF on the native ceramic support surface. Chemical modifications or seeds coating are usually carried out to promote nucleation and growth of MOF membranes. In this work, we report the synthesis of ZIF-95 nano-sheets as seeds for secondary growth of highly hydrogen permselective ZIF-95 membranes. Under the action of the vacuum force, the ZIF-95 seeds in the colloidal suspensions are attracted and transported onto the whole area of the support surface continuously and uniformly, thus forming a continuous and uniform ZIF-95 seeding layer, which is helpful to promote the nucleation and growth of ZIF-95 membranes. After solvothermal synthesis at 120 °C for 48 h, a well-intergrown ZIF-95 membrane was formed on the seeding support surface. Attributing to a preferentially affinity to CO2 and a narrow pore size of ~0.37 nm, the developed ZIF-95 membrane shows high hydrogen permselectivity for the separation H2 from other large molecules. At 200 °C and 1 bar, the mixture separation factors of H2/CO2, H2/N2, and H2/CH4 are 41.6, 36.8, and 40.3, respectively, with H2 permeances over 1.7 × 10−7 mol m−2 s−1·Pa−1.

Impact of the Copper-Induced Local Framework Deformation on the Mechanism of Structural Phase Transition in [(CH3)2NH2][Zn(HCOO)3] Hybrid Metal–Formate Perovskite

Dimethylammonium zinc formate [(CH3)2NH2][Zn(HCOO)3] (DMAZn) treated as a model dense metal–organic framework is a compound that has been intensively studied in the last decade. It undergoes a first-order order–disorder structural phase transition at T0 ∼ 167K, considered as the improper ferroelectric one. The complex mechanism of this phase transition is attributed to the ordering of the hydrogen bonds between (CH3)2NH2+ (DMA+) and the formate groups as well as distortion of the zinc–formate framework. Here, the analysis of the impact of zinc substitution by Cu doping on the order–disorder phase transition is presented. The Jahn–Teller effect of Cu2+ ions results in a local framework distortion, which consequently perturbs the structural dynamics. Based on the dielectric, thermal, structural, electron paramagnetic resonance, and IR spectroscopy measurements, it is shown that doping with Cu2+ ions leads to a decrease of the phase transition entropy and temperature as well as a change in the nature of the ...

Preparation and Dielectric Characterization of P(VDF–TrFE) Copolymer-Based Composites Containing Metal–Formate Frameworks

We report the synthesis and dielectric characterization of novel polyvinylidene fluoride-trifluoroethylene P(VDF-TrFE) composite films containing [(CH3)2NH2][Mg(HCOO)3] (DMAMg) and [NH4][Zn(HCOO)3] (AmZn) dense metal-organic frameworks (MOFs). The optical camera and Raman microscopies are used to map the distribution of the MOF fillers in the prepared films. The dielectric spectroscopy experiments of the DMAMg/P(VDF-TrFE) composite performed in a broad temperature range demonstrate rich dielectric behavior originating from the dipolar dynamics of the (CH3)2NH2+ molecular cations and glassy behavior of the copolymer matrix. An anomalous behavior of the complex dielectric permittivity is also observed because of the structural phase transition of DMAMg fillers. The dielectric properties of the AmZn/P(VDF-TrFE) composite film are mainly determined by the dipolar glass relaxation of the P(VDF-TrFE) polymer. The frequency-dependent dielectric spectra of both composites allow us to characterize the observed dipolar relaxation processes. The (CH3)2NH2+ cation dynamics follows the Arrhenius law, whereas the glassy behavior of P(VDF-TrFE) is described by the Vogel-Fulcher equation. For both composites, we observe a significant increase of the dielectric permittivity compared with the P(VDF-TrFE) film without MOF fillers.

CD-MOFs Crystal Transformation from Dense to Highly Porous Form for Efficient Drug Loading

Transformation of a dense metal–organic framework (MOF) to a highly porous form can radically improve its applications in drug loading. In this study, an environmentally friendly synthesis of potas...

Spectroscopic Study of Structural Phase Transition and Dynamic Effects in a [(CH3)2NH2][Cd(N3)3] Hybrid Perovskite Framework

We present a combined study of a [(CH3)2NH2][Cd(N3)3] hybrid perovskite dense metal–organic framework by infrared (IR), Raman, continuous-wave electron paramagnetic resonance (CW EPR), phase-transi...

Zeolite Imidazolate Framework Membranes on Polymeric Substrates Modified with Poly(vinyl alcohol) and Alginate Composite Hydrogels.

Poly(vinyl alcohol)-sodium alginate composite hydrogels were first introduced to synthesize robust and well-intergrown zeolite imidazolate framework (ZIF)/polymer hollow fiber membranes. Through enough adsorption interaction with metal ions by chelation, sufficient nucleation sites for in situ metal-organic framework (MOF) preparation are provided. Using this method, we can not only easily prepare defect-free MOF membranes ignoring the complex modification process and seed deposition but also structurally fix crystalline MOF layers and greatly improve the stiffness and durability of MOF composite membranes. The strategy also gives the appropriate level of generality for synthesis of versatile dense MOF membranes on a variety of polymeric supports. The fabricated ZIF-8/polyethersulfone membrane presented remarkable gas separation performance with H2 permeance of as high as 9.66 × 10-7 mol m-2 s-1 Pa-1 and a high H2/CO2 separation factor of up to 29.0.

Thermodynamic and Kinetic Effects in the Crystallization of Metal–Organic Frameworks

The evolution of metal-organic frameworks (MOFs) has been one of the most exciting aspects of materials chemistry over the last 20 years. In this Account, we discuss the development during this period in our understanding of the factors that control the crystallization of MOFs from solution. Both classical porous MOFs and dense MOF phases are considered. This is an opportune time at which to examine this complex area because the experimental tools now available to interrogate crystallization processes have matured significantly in the last 5 years, particularly with the use of in situ synchrotron X-ray diffraction. There have also been impressive developments in the use of density functional theory (DFT) to treat not only the energies of very complex structures but also their entropies. This is particularly important in MOF frameworks because of their much greater flexibility compared with inorganic structures such as zeolites. The first section of the Account describes how early empirical observations on the crystallization of dense MOFs pointed to a strong degree of thermodynamic control, with both enthalpic and entropic factors playing important roles. For example, reactions at higher temperatures tend to lead to denser structures with higher degrees of framework connectivity and lower levels of solvation, and polymorphs tend to form according to their thermodynamic stabilities. In the case of metal tartrates, these trends have been validated by calorimetric studies. It has been clear for more than a decade, however, that certain phases crystallize under kinetic control, especially when a change in conformation of the ligand or coordination around a metal center might be necessary to form the thermodynamically preferred product. We describe how this can lead to time-dependent crystallization processes that evolve according to the Ostwald rule of stages and can be observed by in situ methods. We then consider the crystallization of porous MOFs, which presents additional challenges because of solvation effects. In spite of these problems, much has been learned about the energetics of the underlying frameworks, where the relationship between porosity and stability initially seemed to mirror the behavior of zeolites, with more porous structures being less stable. Recently, however, this simple relationship has had to be reconsidered with the emergence of some very flexible structures wherein the open structures are more stable than their denser analogues at finite temperatures because of their large vibrational entropies. In the final section we describe how the concepts developed in the MOF work have been extended into the closely related area of hybrid organic-inorganic perovskites. We describe recent studies on polymorphism in hybrid perovskites, which is amenable to total free energy calculations using a combination of DFT and lattice dynamics methods.

Remarkable resilience of the formate cage in a multiferroic metal organic framework material: dimethyl ammonium manganese formate (DMAMnF).

Dimethyl ammonium (DMA) metal formate, an important member of the dense metal organic framework (MOF) family, is known to exhibit a low temperature ferroelectric transition, caused by the ordering of the hydrogen bonds. In this study, we probed the effect of pressure on the disordered hydrogen bond and the HCOO- linkers of DMA manganese formate, with the help of XRD, IR and Raman spectroscopic studies up to ∼20 GPa. We observed that though a phase transition was initiated at ∼3.4 GPa, it was complete only by 6 GPa, indicating its first order nature. Beyond 7 GPa, this compound becomes highly disordered and shows an almost amorphous character, indicating a total collapse of the formate network. The reversibility of the initial structure of DMAMnF on the release of pressure from 20 GPa (i.e. from a highly disordered phase) shows the remarkable resilience of the formate cage. At the first crystal to crystal transition at 3.4 GPa, the distortion of the formate cage causes the ordering of the dynamically disordered hydrogen bond, resulting in a rearrangement of the DMA+ cation. Lifting of the mutual exclusivity of the Raman and IR modes (C-H out of plane and O-C-O bending modes) of HCOO- linkers, at this transition, indicates that the high pressure phase may be non-centro-symmetric.

Pulse EPR and ENDOR Study of Manganese Doped [(CH3)2NH2][Zn(HCOO)3] Hybrid Perovskite Framework

We present a pulse electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) study of a manganese-doped [(CH3)2NH2][Zn(HCOO)3] dense metal–organic framework which exhibits a structural phase transition at 163 K. The echo-detected field sweep Mn2+ EPR spectrum of the low-temperature phase is in a perfect agreement with the previous continuous-wave EPR results, while the spectrum of the disordered phase reveals a significant EPR transition-dependent relaxation. The 1H ENDOR pattern indicates several protons in the vicinity of the Mn2+ ion. The experimental ENDOR spectrum is successfully simulated using the proton hyperfine tensors calculated by the density functional theory. A multifrequency electron spin echo envelope modulation (ESEEM) spectroscopy shows a peculiar signal which is unaffected by the external magnetic field. The modulation depth of this signal starts to decrease above 40 K, coinciding with the temperature at which the methyl groups of the (CH3)2NH2+ cations start t...

Tuning optical properties of MOF-based thin films by changing the ligands of MOFs

The preparation and development of novel optical thin films are of great importance to functional optical and opto-electric components requiring a low refractive index. In this study, a typical metal-organic framework (MOF), MIL-101(Cr), is selected as the research model. The corresponding MOF nanoparticles are prepared by a hydrothermal method and the optical thin films are successfully prepared by spin-coating. The optical properties of the corresponding MOF thin films are controlled by changing the type of functional groups on the benzene ring of the ligand (terephthalic acid) on MOFs. The functional groups are hydrogen atoms (H), electron donating groups (–NH2, –OH) and electron withdrawing groups (–NO2, –(NO2)2 or F4), respectively. It is found that the effective refractive index ( n eff) of MOF thin films decreases along with the increasing voids among MOF nanoparticles. In addition, the extinction coefficient ( k ) increases with the addition of electron donating groups, and decreases with the addition of electron withdrawing groups. Among the MOFs used in this study, the n eff of NO2-MIL-101(Cr) containing electron withdrawing groups is as low as ~1.2, and value of k is particularly low, which suggests its potential application in antireflective devices. In addition, the intrinsic refractive index ( n dense) of the dense MOF materials evaluated according to their porosity increases with the number of the functional groups, and the n dense of the two nitro-substituted MOFs is greater than that of the single nitro-substituted one, and the latter is bigger than that of hydroxyl-substituted one, which is close to that of amino-functionalized one. The diversity of ligands in MOFs makes them a promising new generation of optical materials.