Microporous Metal-organic Frameworks Materials Research Articles

MOF based membranes in gas separation area

Microporous metal organic framework materials are a series of noticeable membrane materials for the separation of gas molecules and liquid molecules. MOFs can be functionalized through tuning pore size through the variation of ligand length, metal nodes and more, or through adjusting adsorption site such as functional sites, linker functional sites, and open metal sites. Membranes can provide higher structure stability for MOFs, however, addressing synthetic, processing, and design challenges is crucial to produce MOF films with excellent mechanical integrity, chemical stability, and high performance under relevant industrial conditions，such as interfacial compatibility issues towards Mixed-Matrix-Membranes (MMMs) and Pure-MOF film issues regarding chemical and thermal robustness, morphology, and orientation. The alkyne/alkene separation, alkane/alkene separation and more gas mixture has been proven can be achieved through the use of MOF-based membranes, moreover, compared to traditional methods, MOF-based membranes possess better performance on separating forementioned gas mixtures with higher selectivity, purity and less harm to the environment.

Fabrication of Hierarchically Porous CuBTC@PA-PEI Composite for High-Efficiency Elimination of Cyanogen Chloride.

Cyanogen chloride (CNCl) is highly toxic and volatile, and it is difficult to effectively remove via porous substances such as activated carbon due to the weak interaction between CNCl and the adsorbent surface. Developing a highly effective elimination material against CNCl is of great importance in military chemical protection. In this work, a new metal-organic framework (MOF) CuBTC@PA-PEI (polyacrylate-polyethyleneimine) composite was prepared and exhibited excellent CNCl elimination performance in the breakthrough tests. PEI was used for the functionalization of PA with amino groups, which is beneficial to anchor with metal ions of MOF. Afterward, the growth of MOF occurred on the surface and in the pores of the matrix by molecular self-assembly via our newly proposed stepwise impregnation layer-by-layer growth method. Breakthrough tests were performed to evaluate the elimination performance of the composites against CNCl. Compared with the pristine CuBTC powder, the CuBTC@PA-PEI composite exhibited better adsorption capacity and a longer breakthrough time. By compounding with the PA matrix, a hierarchically porous structure of CuBTC@PA-PEI composite was constructed, which provides a solution to the mass transfer problem of pure microporous MOF materials. It also solves the problems of MOF molding and lays a foundation for the practical application of MOF.

Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Air‐Steam Etched Construction of Hierarchically Porous Metal‐Organic Frameworks

Main observation and conclusionThe introduction of mesoporosity into the microporous metal‐organic frameworks (MOFs) is expected to expand their applications. Herein, we report a green and facile method to obtain hierarchically porous MOF structures by using an air‐steam etching process. By virtue of the protonation reaction between the imidazole moiety and water vapor, the protonated imidazole related linkers leave the framework, resulting in the formation of mesopores in the zeolitic imidazolate frameworks (ZIFs), as exemplified by ZIF‐8. Given the mild etching process, the materials’ structural integrity and crystallinity are well maintained. Accordingly, the hierarchical porous ZIF‐8 exhibited enhanced performance in the dye removal as well as CO2 cycloaddition reaction with epichlorohydrin in comparison with microporous ZIF‐8, owing to the accelerated mass transfer arising from mesoporous structures. Remarkably, the proposed steam etching approach is generally applicable, which can be readily extended to other ZIFs, such as ZIF‐14, ZIF‐69, and ZIF‐71, thus representing a powerful strategy to construct hierarchically porous MOF materials.

Topological Strain-Induced Regioselective Linker Elimination in a Chiral Zr(IV)-Based Metal-Organic Framework

Summary Zr-carboxylate metal-organic frameworks (MOFs) are structurally robust materials, in part due to their strong coordination bonds. The regioselective Zr–O bond cleavage and formation between 3D architectures are thus challenging and are heretofore unexplored. In this work, by introducing highly flexible 18-crown-6-ether functionalities into a homochiral Zr-MOF, we report an unprecedented topology transition in which a 4,10-connected framework undergoes a rapid solid-state transition into a thermodynamically more stable 4,8-connected analog by a regioselective-linker-elimination under ambient conditions. The transition process was unambiguously unraveled by single-crystal and powder X-ray diffraction studies, and we proposed a possible transition mechanism based on various control experiments and theoretical calculations. The excellent chemical stability and substantially expanded porosity and pore apertures endowed the transformed chiral MOF with an exceptional capacity for the enantioadsorptive and solid-phase extractive separation of the racemic drug molecule of lansoprazole with 98% ee and 93% ee, respectively.

Cobalt Metal-Organic Framework Based on Two Dinuclear Secondary Building Units for Electrocatalytic Oxygen Evolution.

The synthesis of a new microporous metal-organic framework (MOF) based on two secondary building units, with dinuclear cobalt centers, has been developed. The employment of a well-defined cobalt cluster results in an unusual topology of the Co2-MOF, where one of the cobalt centers has three open coordination positions, which has no precedent in MOF materials based on cobalt. Adsorption isotherms have revealed that Co2-MOF is in the range of best CO2 adsorbents among the carbon materials, with very high CO2/CH4 selectivity. On the other hand, dispersion of Co2-MOF in an alcoholic solution of Nafion gives rise to a composite (Co2-MOF@Nafion) with great resistance to hydrolysis in aqueous media and good adherence to graphite electrodes. In fact, it exhibits high electrocatalytic activity and robustness for the oxygen evolution reaction (OER), with a turnover frequency number value superior to those reported for similar electrocatalysts. Overall, this work has provided the basis for the rational design of new cobalt OER catalysts and related materials employing well-defined metal clusters as directing agents of the MOF structure.

Tailor-Made Microporous Metal-Organic Frameworks for the Full Separation of Propane from Propylene Through Selective Size Exclusion.

Adsorptive separation of olefin/paraffin mixtures by porous solids can greatly reduce the energy consumption associated with the currently employed cryogenic distillation technique. Here, the complete separation of propane and propylene by a designer microporous metal-organic framework material is reported. The compound, Y6 (OH)8 (abtc)3 (H2 O)6 (DMA)2 (Y-abtc, abtc = 3,3',5,5'-azobenzene-tetracarboxylates; DMA = dimethylammonium), is rationally designed through topology-guided replacement of inorganic building units. Y-abtc is both thermally and hydrothermally robust, and possesses optimal pore window size for propane/propylene separation. It adsorbs propylene with fast kinetics under ambient temperature and pressure, but fully excludes propane, as a result of selective size exclusion. Multicomponent column breakthrough experiments confirm that polymer-grade propylene (99.5%) can be obtained by this process, demonstrating its true potential as an alternative sorbent for efficient separation of propane/propylene mixtures.

A Moisture-Stable 3D Microporous CoII -Metal-Organic Framework with Potential for Highly Selective CO2 Separation under Ambient Conditions.

Selective adsorption and separation of CO2 from flue gas and landfill gas mixtures have drawn great attention in industry. Porous MOF materials are promising alternatives to achieve such separations; however, the stability in the presence of moisture must be taken into consideration. Herein, we have constructed a microporous metal-organic framework (MOF) {[Co(OBA)(L)0.5 ]⋅S}n (IITKGP-8), by employing a V-shaped organic linker with an azo-functionalized N,N' spacer forming a 3D network with mab topology and 1D rhombus-shaped channels along the crystallographic 'b' axis with a void volume of 34.2 %. The activated MOF reveals a moderate CO2 uptake capacity of 55.4 and 26.5 cm3 g-1 at 273 and 295 K/1 bar, respectively, whereas it takes up a significantly lower amount of CH4 and N2 under similar conditions and thus exhibits its potential for highly selective sorption of CO2 with excellent IAST selectivity of CO2 /N2 (106 at 273 K and 43.7 at 295 K) and CO2 /CH4 (17.7 at 273 K and 17.1 at 295 K) under 1 bar. More importantly, this MOF exhibits excellent moisture stability as assessed through PXRD experiments coupled with surface area analysis.

Preparation of Luminescent Metal-Organic Framework Films by Soft-Imprinting for 2,4-Dinitrotoluene Sensing.

A novel technique for the creation of metal-organic framework (MOF) films based on soft-imprinting and their use as gas sensors was developed. The microporous MOF material [Zn2(bpdc)2(bpee)] (bpdc = 4,4′-biphenyldicarboxylate; bpee = 1,2-bipyridylethene) was synthesized solvothermally and activated by removing the occluded solvent molecules from its inner channels. MOF particles were characterized by powder X-ray diffraction and fluorescence spectroscopy, showing high crystallinity and intense photoluminescence. Scanning electron microscope images revealed that MOF crystals were mainly in the form of microneedles with a high surface-to-volume ratio, which together with the high porosity of the material enhances its interaction with gas molecules. MOF crystals were soft-imprinted into cellulose acetate (CA) films on quartz at different pressures. Atomic force microscope images of soft-imprinted films showed that MOF crystals were partially embedded into the CA. With this procedure, mechanically stable films were created, with crystals protruding from the CA surface and therefore available for incoming gas molecules. The sensing properties of the films were assessed by exposing them to saturated atmospheres of 2,4-dinitrotoluene, which resulted in a substantial quenching of the fluorescence after few seconds. The soft-imprinted MOF films on CA/quartz exhibit good sensing capabilities for the detection of nitroaromatics, which was attributed to the MOF sensitivity and to the novel and more efficient film processing method based on soft-imprinting.

Bilayer sandwich-like membranes of metal organic frameworks-electrospun polymeric nanofibers via SiO2 nanoparticles seeding

Supported forms of the microporous metal-organic frameworks (MOFs) are of great interest targeting applications in gas separation, small molecules sensing, and in heterogeneous catalysis. In the present work we report the synthesis of a novel composite architecture displaying a sandwich-like structure of MOFs materials supported on a flexible mat of electrospun polymeric nanofibers (NFbs). The newly developed system was designed by using a low-cost and scalable multistep synthesis protocol involving a combination of electrospinning and layer by layer (LBL) synthetic growth of MOFs. In order to enhance the MOFs loading, as the active element in the composite, a double-deck construct was attempted, in contrast to alternative routes where deposition of thicker MOFs layer atop the solid support were previously reported. This highly versatile approach enables construction of multi-layer structures of the same MOFs or potentially, of different MOFs targeting specific application with particular interest in multi-component gas separation and multi-functional detection systems.

A Confined Fabrication of Perovskite Quantum Dots in Oriented MOF Thin Film.

Organic-inorganic hybrid lead organohalide perovskites are inexpensive materials for high-efficiency photovoltaic solar cells, optical properties, and superior electrical conductivity. However, the fabrication of their quantum dots (QDs) with uniform ultrasmall particles is still a challenge. Here we use oriented microporous metal-organic framework (MOF) thin film prepared by liquid phase epitaxy approach as a template for CH3NH3PbI2X (X = Cl, Br, and I) perovskite QDs fabrication. By introducing the PbI2 and CH3NH3X (MAX) precursors into MOF HKUST-1 (Cu3(BTC)2, BTC = 1,3,5-benzene tricarboxylate) thin film in a stepwise approach, the resulting perovskite MAPbI2X (X = Cl, Br, and I) QDs with uniform diameters of 1.5-2 nm match the pore size of HKUST-1. Furthermore, the photoluminescent properties and stability in the moist air of the perovskite QDs loaded HKUST-1 thin film were studied. This confined fabrication strategy demonstrates that the perovskite QDs loaded MOF thin film will be insensitive to air exposure and offers a novel means of confining the uniform size of the similar perovskite QDs according to the oriented porous MOF materials.

Zr(IV) and Ce(IV)-based metal-organic frameworks incorporating 4-carboxycinnamic acid as ligand: Synthesis and properties

Two new microporous metal-organic framework materials constructed from Zr(IV) (1) or Ce(IV) (2) ions and 4-carboxycinnamate (CCA) ligand are presented. 1 was prepared by heating a mixture of ZrCl4, H2CCA ligand and acetic acid with a molar ratio of 1:1:10 in N,N′-dimethylformamide (DMF) at 150 °C for 24 h. 2 was synthesized by solvothermal reaction of a mixture of ammonium cerium(IV) nitrate and H2CCA ligand in a DMF/H2O mixture at 100 °C for 20 min. The as-synthesized and the thermally activated forms (1′ and 2′) of the compounds were characterized by X-ray powder diffraction (XRPD), Fourier transform infrared (FT-IR), thermogravimetric (TG) and elemental analysis. 1 and 2 show high thermal stability up to 390 and 300 °C in air atmosphere, respectively. According to XRPD measurements, both compounds show moderate stability in water, but they lose their crystallinity in 1 M HCl. N2 sorption experiments reveal that 1′ and 2′ possess BET surface areas of 870 and 1210 m2 g−1, respectively. The CO2 uptake values of 1′ and 2′ at 0 °C and 1 bar are 2.0 and 1.6 mmol g−1, respectively.

ChemInform Abstract: Synthesis of Metal‐Organic Framework Particles and Thin Films via Nanoscopic Metal Oxide Precursors

AbstractReview: 78 refs.

Perspective of microporous metal–organic frameworks for CO2capture and separation

It is emergent to reduce carbon dioxide emissions from fossil fuel combustion and thereby limit climate destabilization. In order to achieve the industrial scenario of CCS, there is a need for the discovery of better solid CO2 adsorbents that realize great improvement of selective capacity and stability to moisture as well as significant reductions in energy requirements and costs. In this review, we provide an overview of the current status of the emerging microporous metal–organic frameworks (MOFs) for the storage and separation of carbon dioxide. We summarize the main factors for CO2 capture performance of MOF materials under different working conditions, in comparison with those for zeolite materials. At the same time, we analyze the relationship among porous structures, pore/window sizes, capacity, selectivity and enthalpy of porous MOFs for CCS, which will give us clues for the design and synthesis of MOF materials as CO2 adsorbents.

Microporous Metal–Organic Frameworks for Gas Separation

Microporous metal-organic frameworks (MOFs) are comparatively new porous materials. Because the pores within such MOFs can be readily tuned through the interplay of both metal-containing clusters and organic linkers to induce their size-selective sieving effects, while the pore surfaces can be straightforwardly functionalized to enforce their different interactions with gas molecules, MOF materials are very promising for gas separation. Furthermore, the high porosities of such materials can enable microporous MOFs with optimized gas separation selectivity and capacity to be targeted. This Focus Review highlights recent significant advances in microporous MOFs for gas separation.

Facile Deposition of Multicolored Electrochromic Metal–Organic Framework Thin Films

The construction of well-defined organic architectures on solid surfaces is a key step toward the development of more efficient and robust electronic devices. Indeed, a number of self-assembly strategies have been applied to building supramolecular structures for use in devices such as organic lightemitting diodes (OLEDs), organic transistors, and solar cells. Recently, self-assembled coordination polymers, namely crystalline, microporous metal-organic frameworks (MOFs), have received considerable attention for their potential in a number of applications, including electronic devices. The solvothermal methods used to synthesize MOF materials generally afford bulk single crystals or microcrystalline powders and, as a result, a number of methods have been developed to process MOFs as thin films, which are generally more desirable for use in devices. However, MOFs still face a number of challenges in this area, particularly with regard to their insulating nature. The incorporation of redox-active organic linkers represents one strategy toward improving the charge transport properties of MOF materials as well as imparting a response to electrochemical stimuli. In the latter case, the use of colorswitching, redox-active organic linkers might be considered for the design of new, color-tunable electrochromic materials. Inorganic materials such as metal oxides and Prussian blue analogs have been extensively studied for electrochromic applications. More recently, conducting organic polymers such as polythiophenes have received considerable attention owing to their greater color tunability and efficiency, and faster switching times. Despite significant advances, the need still exists for more easily synthesized and processable electrochromic materials with the same or greater degree of tunability. In this sense, the well-defined structures, selfassembly synthesis, and guest-accessible microporosity of MOFs might prove useful for their application in this field. Herein, we describe the solvothermal deposition and electrochemical properties of porous, micron-thick Zn-pyrazolate MOF films containing redox-active naphthalene diimide (NDI) linkers. NDIs have been extensively studied for their electron accepting properties which make them attractive as n-type semiconductors. Furthermore, core substitution of NDIs with electron donor groups results in the appearance of charge-transfer transitions in the visible spectrum, affording color-tunable and sometimes fluorescent dyes with applications in optoelectronics and sensing. A handful of MOFs containing NDI-based linkers have been reported and studied as gas sorption, photochromic, and fluorescent sensor materials. However, studies of their electrochemical properties remain scarce. Here, we show that processing them as homogeneously dispersed thin films enables their use as multicolored electrochromic devices. Our group has been concerned with developing modular syntheses for water-stable pyrazolate-based MOFs, and recently we described a series of core-substituted NDIbased materials. These MOFs (Zn(NDI-X), Figure 1) were obtained as microcrystalline powders by heating 1.1:1

Temperature‐/Pressure‐Dependent Selective Separation of CO2 or Benzene in a Chiral Metal–Organic Framework Material

Presented here is a chiral microporous metal-organic framework material with a three-fold interpenetrating diamond-type structural topology and interesting properties for selective separation. The material has a high storage capacity for CO(2) gas (4.23 mmol g(-1) at 273 K and 1 bar) and shows fantastic temperature-dependent selectivity for CO(2) over N(2). Moreover, this multifunctional material, which has a rich π system, can selectively adsorb benzene over cyclohexane at low pressure (0.05 bar) at 298 K.

One-dimensional metal–organic framework photonic crystals used as platforms for vapor sorption

We present the fabrication of one-dimensional photonic crystals (Bragg stacks) based on a microporous metal–organic framework material and mesoporous titanium dioxide. The Bragg stack heterostructures were obtained using two complementary synthesis approaches utilizing the bottom-up assembly of heterogeneous, i.e. two-component photonic crystal multilayer structures. Zeolitic imidazolate framework ZIF-8 and mesoporous titanium dioxide were chosen as functional components with different refractive indices. While ZIF-8 is intended to impart molecular selectivity, mesoporous TiO2 is used to ensure high refractive index contrast and to guarantee molecular diffusion within the Bragg stack. The combination of micro- and mesoporosity within one scaffold endows the 1D-MOF PC with characteristic adsorption properties upon exposure to various organic vapors. In this context, the sorption behavior of the photonic material was studied as a function of partial pressure of organic vapors. The results show that the multilayered photonic heterostructures are sensitive and selective towards a series of chemically similar solvent vapors. It is thus anticipated that the concept of multilayer heterogeneous photonic structures will provide a versatile platform for future selective, label-free optical sensors.

Highly Selective CO2 Capture by a Flexible Microporous Metal–Organic Framework (MMOF) Material

Caught in a CO2 trap: A highly flexible microporous metal–organic framework material exhibits a remarkable ability to capture and separate carbon dioxide from other small gases, such as N2, H2, CH4, CO, and O2, with separation ratios of 294, 190, 257, and 441 for CO2/N2, CO2/H2, CO2/CH4, and CO2/CO, respectively, at 0.16 atm and 25 °C, and 768 for CO2/O2 at 0.2 atm and 25 °C.

Interaction of Molecular Hydrogen with Microporous Metal Organic Framework Materials at Room Temperature

Infrared (IR) absorption spectroscopy measurements, performed at 300 K and high pressures (27-55 bar) on several prototypes of metal organic framework (MOF) materials, reveal that the MOF ligands are weakly perturbed upon incorporation of guest molecules and that the molecular hydrogen (H(2)) stretch mode is red-shifted (30-40 cm(-1)) from its unperturbed value (4155 cm(-1) for ortho H(2)). For MOFs of the form M(bdc)(ted)(0.5) (bdc = 1,4-benzenedicarboxylate; ted = triethylenediamine), H(2) molecules interact with the organic ligands instead of the saturated metal centers located at the corners of the unit cell. First-principles van der Waals density functional calculations identify the binding sites and further show that the induced dipole associated with the trapped H(2) depends sensitively on these sites. For M(bdc)(ted)(0.5) systems, the strongest dipole moment is of the site that is in the corner of the unit cell and is dominated by the interaction with the benzene ligand and not by the metal center. For MOFs of the M(3)[HCOO](6) type with relatively short ligands (i.e., formate) and 1-D pore structures, there is a weak dependence of H(2) vibrational frequency on the cations, due to a small change in the unit cell dimension. Furthermore, translational states of approximately +/-100 cm(-1) are clearly observed as side bands on the H(2) stretch mode in these 1-D channels interconnected by very small apertures. The H(2) stretch IR integrated areas in all the MOFs considered in this work increase linearly with H(2) pressure, consistent with isotherm measurements performed in similar conditions. However, the IR intensity varies substantially, depending on the number of benzene rings interacting with the H(2) molecules. Finally, there is no correlation between H(2) binding energies (determined by isotherm measurements) and the magnitude of the H(2) stretch shift, indicating that IR shifts are dominated by the environment (organic ligand, metal center, and structure) rather than the strength of the interaction. These results highlight the relevance of IR spectroscopy to determine the type and arrangement of ligands in the structure of MOFs.