Cavities Of Metal-organic Framework Research Articles

How Do Deep Eutectic Solvents Form Porous Liquids? The Example of Methyltriphenylphosphonium Bromide: Glycerol and ZIF-8.

Porous liquids are new materials that provide permanent porosity in the liquid phase through the dispersion of nanoporous solid particles in a bulky solvent. Herein, we aim at understanding how new sustainable solvents such as deep eutectic solvent (DES) can be used to form porous stable suspensions for the capture of gases of interest for sustainable chemistry. The properties of an ionic DES, methyltriphenylphosphonium bromide/glycerol in a 1:3 molar composition, and its behavior at the interface with a metal-organic framework (MOF), ZIF-8, are here investigated by polarizable molecular dynamics simulations. The structural and dynamic properties of the DES are analyzed in the bulk liquid and in the interfacial regions with the MOF, namely, in the accessible cavities exposed at the surface. The porosity of the suspension is maintained, and it is caused not only by the Coulomb cohesive energy between cations and anions, which prevents the small anions from entering the pores, but also by the glycerol molecules not penetrating the small aperture of the ZIF-8 structure. This was further verified by simulating a system composed of glycerol and ZIF-8. Simulations with CO2 show its partition between the DES and the MOF, with higher concentrations registered in the MOF cavities.

Fluorine-functionalized MOF modified GCE for highly sensitive electrochemical detection of persistent pollutant perfluorooctanoic acid

Perfluorooctanoic acid (PFOA) is one of typical per- and polyfluoroalkyl substances (PFASs) in water environments, which attracts global concern due to its high persistence, bioaccumulation, and toxicity. Electrochemical detection provides a feasible, cost-effective, in-situ, and real-time detection approach for PFOA. And the fluorine-functionalized metal-organic frameworks (MOFs) have shown great adsorbing affinity towards PFOA with their fluorophilic interaction, electrostatic interaction, anion-π stacking, large specific area and porous structure. In this work, fluorine-functionalized Ce-UiO-66 (Ce-UiO-66-F) modified glass carbon electrode (GCE) was utilized to effectively and sensitively detect PFOA with redox probe strategy. The adsorption of PFOA occupied cavities of MOFs and blocked the active sites on Ce-UiO-66-F/GCE interface, leading to a decreasing probe current with PFOA concentration increasing. The quantitative detection mechanism was investigated with Langmuir isotherm model and Freundlich-Langmuir isotherm model, transducing the decreasing probe signal into PFOA concentration information. A detection range of 0.40–450 nM was obtained, and the limit of detection (LOD) was estimated to be 0.048 nM, which satisfied the domestic water requirements of Untied State Environmental Protection Agency (USEPA). This work provides a rapid and convenient electrochemical approach to detect PFOA and extends the applicability of MOFs as deployable modifiers for PFAS sensors.

Construction of Hydration Layer for Proton Transport by Implanting the Hydrophilic Center Ag0 in Nickel Metal-Organic Frameworks.

The directional arrangement of H2O molecules can effectively regulate the ordered protons transfer to improve transport efficiency, which can be controlled by the interaction between materials and H2O. Herein, a strategy to build a stable hydration layer in metal-organic framework (MOF) platforms, in which hydrophilic centers that can manipulate H2O molecules are implanted into MOF cavities is presented. The rigid grid-Ni-MOF is selected as the supporting material due to the uniformly distributed cavities and rigid structures. The Ag0 possesses potential combination ability with the hydrophilic substances, so it is introduced into the MOF as hydration layer centers. Relying on the strong interaction between Ag0 and H2O, the H2O molecules can rearrange around Ag0 in the cavity, which is intuitively verified by DFT calculation and molecular dynamics simulation. The establishment of a hydration layer in Ag@Ni-MOF regulates the chemical properties of the material and gives the material excellent proton conduction performance, with a proton conductivity of 4.86×10-2 Scm-1.

Photophysics of Iridium(III) (2‐phenylpyridine)(2,2’,6,2”‐terpyridine) Chloride Encapsulated within the Zinc(II) Polyhedral MOF, USF‐2

AbstractPhotoactive metal‐organic frameworks (MOFs) provide an important class of functional porous materials for a wide range of applications including light harvesting, photocatalysis and photodynamic therapy. Two strategies have been employed in the development of photoactive MOFs, one in which the photoactive element is incorporated as an element of the framework itself and the other in which the photoactive element serves as a guest within the MOF cavities. Transition metal polyimines have now been non‐covalently incorporated within the cavities of a large number of MOFs with the RuII‐polyimine being the most widely examined guest complex. Previous studies have demonstrated that the nature of the cavity modulates the Ru‐polyimine photophysics. Here, an IrIII(terpyridine)(phenylpyridine)Cl complex has been encapsulated within the Zn‐polyhedral MOF, USF‐2. Unlike the Ru‐polyimines, the excited state photophysics associated with the encapsulated Ir polyimine shows very little change in either the steady state emission and emission lifetime. The slight decrease in emission lifetime is attributed to energy transfer between encapsulated Ir complexes. These results indicate that transition metal polyimines that exhibit excited state structural changes demonstrate the largest perturbations upon confinement.

Guest-Induced Multilevel Charge Transport Strategy for Developing Metal-Organic Frameworks to Boost Photocatalytic CO2 Reduction.

Encapsulating photogenerated charge-hopping nodes and space transport bridges within metal-organic frameworks (MOFs) is a promising method of boosting the photocatalytic performance. Herein, this work embeds electron transfer media (9,10-bis(4-pyridyl)anthracene (BPAN)) in MOF cavities to build multi-level electron transfer paths. The MOF cavities are accurately regulated to investigate the significance of the multi-level electron transfer paths in the process of CO2 photoreduction by evaluating the difference in the number of guest media. The prepared MOFs, {[Co(BPAN)(1,4-dicarboxybenzene)(H2 O)2 ]·BPAN·2H2 O} and {[Co(BPAN)2 (4,4'-biphenyldicarboxylic acid)2 (H2 O)2 ]·2BPAN·2H2 O} (denoted as BPAN-Co-1 and BPAN-Co-2), exhibit efficient visible-light-driven CO2 conversion properties. The CO photoreduction efficacy of BPAN-Co-2 (5598µmol g-1 h-1 ) is superior to that of most reported MOF-based catalysts. In addition, the enhanced CO2 photoreduction ability is supported by density functional theory (DFT). This work illustrates the feasibility of realizing charge separation characteristics in MOF catalysts at the molecular level, and provides new insight for designing high-performance MOFs for artificial photosynthesis.

Interrogating Encapsulated Protein Structure within Metal–Organic Frameworks at Elevated Temperature

Encapsulating biomacromolecules within metal-organic frameworks (MOFs) can confer thermostability to entrapped guests. It has been hypothesized that the confinement of guest molecules within a rigid MOF scaffold results in heightened stability of the guests, but no direct evidence of this mechanism has been shown. Here, we present a novel analytical method using small-angle X-ray scattering (SAXS) to solve the structure of bovine serum albumin (BSA) while encapsulated within two zeolitic imidazolate frameworks (ZIF-67 and ZIF-8). Our approach comprises subtracting the scaled SAXS spectrum of the ZIF from that of the biocomposite BSA@ZIF to determine the radius of gyration of encapsulated BSA through Guinier, Kratky, and pair distance distribution function analyses. While native BSA exposed to 70 °C became denatured, in situ SAXS analysis showed that encapsulated BSA retained its size and folded state at 70 °C when encapsulated within a ZIF scaffold, suggesting that entrapment within MOF cavities inhibited protein unfolding and thus denaturation. This method of SAXS analysis not only provides insight into biomolecular stabilization in MOFs but may also offer a new approach to study the structure of other conformationally labile molecules in rigid matrices.

Solvent-Free Mechanochemical Preparation of Metal-Organic Framework ZIF-67 Impregnated by Pt Nanoparticles for Water Purification

In this study, the crystalline metal-organic framework (MOF) ZIF-67 was obtained using the solvent-free ball milling method, which is a fast, simple, and economical green method without the need to use solvents. Using the impregnation method, platinum metal ions were loaded in the MOF cavities. Various descriptive methods have been used to explain the prepared Pt@ZIF-67 compound, such as FTIR, BET, TEM, SEM, EDS, XRD, TGA, and ICP. Based on this, the results showed that Pt nanoparticles (0.26 atom%) were located inside the pores of ZIF-67. In addition, no evidence supports their accumulation on the MOF surface. The efficiency of Pt@ZIF-67 was approved in the reduction of toxic and harmful nitrophenol compounds in water. The results showed that the removal of 4-nitrophenol in aqueous medium was successfully achieved with 94.5% conversion in an optimal time of 5 min with the use of NaBH4, and catalyzed by Pt@ZIF-67. Additionally, the Pt@ZIF-67 was recoverable and successfully tested for five qtr runs, with reasonable efficiency.

Transforming an insulating metal–organic framework into electrically conducting metal–organic framework⊃conducting polymer composites

Owing to their diverse potentials to help advance modern electronics and energy technologies, electrically conducting metal–organic frameworks (MOFs) have emerged as one of the most coveted functional materials within the past decade. The key to developing electrically conducting MOFs is to equip them with mobile charge carriers and facilitate long-range charge movement. Circumventing the challenges and unpredictability associated with the construction of intrinsically conducting MOFs, herein, we have converted a structurally robust and porous but intrinsically insulating Zn-dpzNDI MOF based on an electron deficient dipyrazolate-naphthalenediimide (dpzNDI) ligand into electrically conducting MOF⊃conducting-polymer (MOF⊃CP) composites via oxidative polymerization of preloaded redox-active 3,4-ethylenedioxythiophene (EDOT) and pyrrole (Py) monomers to corresponding PEDOT and PPy polymers, which are well-known hole-transporters. After monomer loading and in-situ polymerization, the resulting MOF⊃CP composites remained crystalline but became less porous, suggesting that the amorphous CP chains were mostly confined to the MOF cavities. The presence of CPs was confirmed by infra-red (IR), diffuse-reflectance UV–Vis–NIR and STEM-EDX analyses. Whereas the pristine MOF had immeasurably low conductivity (σ < 10−12 S/m), the MOF⊃PEDOT and MOF⊃PPy composites displayed significantly higher electrical conductivity: 1.8 × 10−5 and 2.5 × 10−3 S/m, respectively. Thus, we have transformed an intrinsically insulating MOF into electrically conducting MOF⊃CP via in-situ oxidative polymerization of electron-rich monomers, a versatile strategy that could be adopted to engineer this much coveted but elusive electronic property practically in any porous MOFs.

Constructing proton selective pathways using MOFs to enhance acid recovery efficiency of anion exchange membranes

Efficient recovery of acids from industrial wastewater containing metal ions is crucial for resource recycling and environmental safety. Dialysis based on anion exchange membranes (AEMs) is a promising method for acid recovery but typically suffers from low selectivity. Herein, we develop highly stable AEMs based on metal-organic frameworks (MOFs) to improve the acid recovery performance. For membranes based on quaternary ammonium polysulfone (QAPSF) and quaternary ammonium poly (2,6-dimethyl-1,4-phenylene oxide) (QPPO), embedded MOFs can provide selective proton transport paths because of a precise size-sieving effect and abundant hydrogen-bonding networks, thus improving both the acid dialysis selectivity and flux. Remarkably, the QPPO membrane incorporated with 20 wt% UiO-66 exhibits a high dialysis coefficient of 16 mm/h and a separation factor of 683. The MOF-hybrid AEMs are sufficiently stable and retain their original structure and morphology after dialysis tests. In addition, molecular dynamics simulations suggest that the competitive Fe2+ ions are immobile and present a high energy barrier to diffuse in UiO-66, whereas water molecules can hop between the cavities of MOFs, thereby facilitating fast proton conduction and thus improving proton selectivity. Therefore, Zr-MOFs can be incorporated as porous sieving fillers into AEMs to develop advanced hybrid membranes for acid recovery.

Chemical vapor deposition of guest-host dual metal-organic framework heterosystems for high-performance mixed matrix membranes

Designing metal-organic framework (MOF) architectures and regulating their porous microenvironments are of greatly scientific interests. In this study, we report a kind of guest-host dual MOF@MOF heterosystems with guest MOF cells in other host MOF cavities, and construct them by vapor linker processing. Thanks to the simplified mass transfer process from solvent-free chemical vapor deposition, the pre-impregnated metal precursors in confined MOF pores can be in situ bridged by linkers to form MOF cages with well porosity. These heterosystems exhibit tailored dual pore geometries, enhanced affinities to gas molecules, and combined advantages of different MOFs. We further demonstrate that the MOF@MOF composites have substantially improved compatibility toward polymers and can serve as molecular sieving to adjust membrane transport pathways for achieving outstanding carbon capture performance, with 162% CO2 permeability (14,366 Barrer, 1 Barrer=10−10 cm3 cm cm−2 s−1 cmHg−1) and 152% CO2/N2 selectivity as that of pure polymeric membranes. Our findings provide an alternative perspective for developing heterogeneous and porous materials.

MOFs-based nanoagent enables dual mitochondrial damage in synergistic antitumor therapy via oxidative stress and calcium overload

Targeting subcellular organelle with multilevel damage has shown great promise for antitumor therapy. Here, we report a core-shell type of nanoagent with iron (III) carboxylate metal-organic frameworks (MOFs) as shell while upconversion nanoparticles (UCNPs) as core, which enables near-infrared (NIR) light-triggered synergistically reinforced oxidative stress and calcium overload to mitochondria. The folate decoration on MOFs shells enables efficient cellular uptake of nanoagents. Based on the upconversion ability of UCNPs, NIR light mediates Fe3+-to-Fe2+ reduction and simultaneously activates the photoacid generator (pHP) encapsulated in MOFs cavities, which enables release of free Fe2+ and acidification of intracellular microenvironment, respectively. The overexpressed H2O2 in mitochondria, highly reactive Fe2+ and acidic milieu synergistically reinforce Fenton reactions for producing lethal hydroxyl radicals (•OH) while plasma photoacidification inducing calcium influx, leading to mitochondria calcium overload. The dual-mitochondria-damage-based therapeutic potency of the nanoagent has been unequivocally confirmed in cell- and patient-derived tumor xenograft models in vivo.

Protonated Emeraldine Polyaniline Threaded MIL-101 as a Conductive High Surface Area Nanoporous Electrode

The low electrical conductivity of metal organic frameworks (MOFs) is currently the major hurdle for their electrochemical applications. Herein, we render an MOF with a 9-order magnitude higher electrical conductivity by threading a conductive polymer in the MOF cavities at molecular scale. Such electrically conductive MOF–protonated emeraldine polyaniline (PANI) threaded in MIL-101(Cr), PANI∼MIL-101, demonstrates superb electrical conductivity of 0.01 S cm–1 with the ultrahigh surface area retained (2065 m2 g–1) because of the full penetration of PANI and its uniform distribution within MIL-101 cavities. Effective electron transport is established in the PANI∼MIL-101 structure via π–π stacking among polyaniline chains and n → π* and π–π interactions between polyaniline and MIL-101. We demonstrate PANI∼MIL-101 is an effective electrode in Fe-ion sensors and all-Fe redox flow batteries. One coating of this conductive MOF can serve as alternate for a microelectrode array. This work now opens up a new class of conductive MOFs with high surface area and nanoporosity, which are promising for electrochemical applications.

Fabricating polyoxometalates-stabilized single-atom site catalysts in confined space with enhanced activity for alkynes diboration

Effecting the synergistic function of single metal atom sites and their supports is of great importance to achieve high-performance catalysts. Herein, we successfully fabricate polyoxometalates (POMs)-stabilized atomically dispersed platinum sites by employing three-dimensional metal-organic frameworks (MOFs) as the finite spatial skeleton to govern the accessible quantity, spatial dispersion, and mobility of metal precursors around each POM unit. The isolated single platinum atoms (Pt1) are steadily anchored in the square-planar sites on the surface of monodispersed Keggin-type phosphomolybdic acid (PMo) in the cavities of various MOFs, including MIL-101, HKUST-1, and ZIF-67. In contrast, either the absence of POMs or MOFs yielded only platinum nanoparticles. Pt1-PMo@MIL-101 are seven times more active than the corresponding nanoparticles in the diboration of phenylacetylene, which can be attributed to the synergistic effect of the preconcentration of organic reaction substrates by porous MOFs skeleton and the decreased desorption energy of products on isolated Pt atom sites.

Host-Guest Interactions in a Metal-Organic Framework Isoreticular Series for Molecular Photocatalytic CO2 Reduction.

A strategy to improve homogeneous molecular catalyst stability, efficiency, and selectivity is the immobilization on supporting surfaces or within host matrices. Herein, we examine the co‐immobilization of a CO2 reduction catalyst [ReBr(CO)3(4,4′‐dcbpy)] and a photosensitizer [Ru(bpy)2(5,5′‐dcbpy)]Cl2 using the isoreticular series of metal–organic frameworks (MOFs) UiO‐66, ‐67, and ‐68. Specific host pore size choice enables distinct catalyst and photosensitizer spatial location—either at the outer MOF particle surface or inside the MOF cavities—affecting catalyst stability, electronic communication between reaction center and photosensitizer, and consequently the apparent catalytic rates. These results allow for a rational understanding of an optimized supramolecular layout of catalyst, photosensitizer, and host matrix.

Recent Progresses in Metal–Organic Frameworks Based Core–shell Composites

AbstractEncapsulation of active guest compounds inside metal–organic frameworks (MOFs) architectures is one of the most promising routes to reach properties beyond those of the bare MOFs and/or guest species. In contrast with the conventional host/guest composites that rely on the encapsulation of guest species into MOF cavities, core–shell composites display a better accessibility to the pores ensuring an optimal diffusion of the substrate while presenting a unique structure that prevents the aggregation and the runoff of the active guests and ensures a tight interaction between core and shell, leading to synergistic effects. Herein, the recent advances in this field are summarized. The main synthetic strategies are first discussed before highlighting a few potential applications, such as heterogeneous environmental catalysis, gas separation, and sensing, while others (bio‐applications…) are briefly mentioned. This review is concluded by a critical perspective in order to promote new generations of MOFs based composites for energy‐related applications.

High-performance membrane with angstrom-scale manipulation of gas transport channels via polymeric decorated MOF cavities

Gas separation performance of mixed matrix membrane heavily depends on the pore structure of the nanofillers. Metal-organic frameworks (MOFs) are promising platform materials for constructing molecular-selective pores for specific applications. In this work, deliberately-selected polymers with CO2 affinity (PVAm, Pebax and PEI) are employed as pore regulators to manipulate the pore chemistry and size of MOF UiO-66 nanoparticles and consequently control gas transport rate of CO2 and N2 molecules. The branched polymer (polyethyleneimine (PEI)) grafted UiO-66, denoted as UKI, is beneficial to enhancing the membrane selectivity. The UKI doped Pebax/mPSf membranes exhibit CO2/N2 selectivity up to 278, 6.5 times of the bare Pebax/mPSf membranes. Meanwhile, the CO2 permeance is boosted from around 690 to 1120 GPU (1 GPU = 10−6 cm3 (STP)·cm−2·s−1·cmHg−1 = 3.35 × 10−10 mol m−2 s−1·Pa−1). The block copolymer (poly(ether block amide) (Pebax)) grafted UiO-66, denoted as UKX, is conducive to increasing the membrane permeance. The UKX doped Pebax/mPSf membranes exhibited CO2 permeance up to 1683 GPU, 2.45 times of the bare Pebax/mPSf membranes. Meanwhile, CO2/N2 selectivity increased from around 42 to 146. Additionally, excellent pressure-resistant property and outstanding stability are observed under simulated flue gas.

Tunable nonlinear optical responses based on host-guest MOF hybrid materials

The realization of tunable nonlinear optical (NLO) responses in a single nano-/micro-structure is extremely important. However, in lack of effective ways to integrate multiple performances, it still faces severe limitations during applications. Herein, we demonstrate a wavelength-dependent NLO micro-structure based on host-guest metal-organic framework (MOF) materials through encapsulating linear dye molecules into periodic one-dimensional (1D) channels. The confinement to non-centrosymmetric polar dye molecules enhances the second-/third-order NLO responses of the hybrid crystals, causing obvious two-photon luminescence (TPL), second harmonic generation (SHG) and third harmonic generation (THG) responses in the as-prepared composites. The highly ordered structures of MOFs impart spatial regulation on the linear dye molecules to realize orientation alignment, resulting in the polarized anisotropy emission. NIR-to-NIR (NIR, near-infrared region) two-photon pumped lasing is realized with the natural whispering gallery mode resonance cavities of MOFs under the excitation of a 1200-nm fs laser. Furthermore, tunable NLO properties such as TPL, SHG and THG are achieved through switching the incident excitation wavelength from 800 to 1500 nm. Such hybrid materials with tunable NLO responses may open a new avenue toward designing multifunctional NLO devices in the future.

Electrochemical preparation of Cu/Cu2O-Cu(BDC) metal-organic framework electrodes for photoelectrocatalytic reduction of CO2

Abstract Thin films of the metal-organic framework (MOF) Cu(BDC) were electrochemically grown by anodic deposition of the ligand 1,4-benzenedicarboxylate (1,4-BDC) on metallic copper to form Cu/Cu2O-Cu(BDC) electrode. The construction of the electrode was optimized by investigating parameters such as current density, time of anodization, and temperature that directly affect its stability and photoactivity. The best performing electrode was prepared when a current density of 2.5 mA cm−2 was applied for 6.5 min at 110 °C. A methanol concentration of 234 μmol L-1 was produced from the photoelectron reduction of CO2 under UV–vis irradiation for 3 h, an applied potential of +0.10 V, and 0.1 mol L-1 aqueous sodium sulfate solution saturated with CO2 as supporting electrolyte. The rate of CO2 reduction to methanol was found to be ∼20 times that obtained using Cu/Cu2O electrode alone presumably due to preconcentration of dissolved CO2 in the MOF. The capture of CO2 in the MOF surface and/or cavities was confirmed by ATR and DRIFT spectroscopy.

In Situ One-Step Synthesis of Platinum Nanoparticles Supported on Metal-Organic Frameworks as an Effective and Stable Catalyst for Selective Hydrogenation of 5-Hydroxymethylfurfural.

A facile in situ one-step route for the preparation of platinum nanoparticles supported on metal–organic frameworks (MOFs) without adding stabilizing agents was developed. The obtained 10% Pt@MOF-T3 material possessed a large surface area and high crystallinity. Meanwhile, uniform and well-dispersed platinum nanoparticles were formed inside the cavities of MOFs, which could be attributed to the efficient complexation and stabilization effect derived from the dipyridyl groups. The as-synthesized 10% Pt@MOF-T3 sample showed high activity and selectivity in the hydrogenation of 5-hydroxymethylfurfural (HMF). This excellent catalytic performance could be attributed to the synergistic effects of well-dispersed platinum nanoparticles and electron donation offered by MOFs. Meanwhile, the presence of bipyridine ligands in the MOF framework avoided the irreversible adsorption of the hydrocarbon compounds, leading to the enhanced catalytic efficiency. Besides, it was easily recycled and reused at least five times, showing good recyclability.

Using MOF-808 as a Promising Support to Immobilize Ru for Selective Hydrogenation of Levulinic Acid to γ-Valerolactone

A stable Zr-based metal–organic framework (MOF), MOF-808, was used as a promising support to immobilize metallic Ru for the first time. The skeleton structure of MOF-808 can be well preserved during the immobilization process. Ru nanoparticles existed both inside and outside of the MOF cavities with good dispersion. The developed Ru/MOF-808 composite was found to be an efficient catalyst for the hydrogenation of biomass-derived levulinic acid (LA) to γ-valerolactone (GVL). Under mild reaction conditions, full conversion of LA (100%) with high selectivity to GVL (> 99%) was achieved. The catalyst recycling and hot filtration experiments indicate that Ru/MOF-808 possesses a good catalytic stability for the LA hydrogenation reaction, although some catalyst structure changes were observed. The good immobilization effect of MOF-808 for Ru may be related to the high binding affinity between the Zr6 nodes and Ru particles.