Metal–Organic Frameworks for the Exploitation of Distance between Active Sites in Efficient Photocatalysis

Discoveries of the accurate spatial arrangement of active sites in biological systems and cooperation between them for high catalytic efficiency are two major events in biology. However, precise tuning of these aspects is largely missing in the design of artificial catalysts. Here, a series of metal–organic frameworks (MOFs) were used, not only to overcome the limit of distance between active sites in bio‐systems, but also to unveil the critical role of this distance for efficient catalysis. A linear correlation was established between photocatalytic activity and the reciprocal of inter active‐site distance; a smaller distance led to higher activity. Vacancies created at selected crystallographic positions of MOFs promoted their photocatalytic efficiency. MOF‐525‐J33 with 15.6 Å inter active‐site distance and 33 % vacancies exhibited unprecedented high turnover frequency of 29.5 h^?1 in visible‐light‐driven acceptorless dehydrogenation of tetrahydroquinoline at room temperature.     

Zirconium–Porphyrin‐Based Metal–Organic Framework Hollow Nanotubes for Immobilization of Noble‐Metal Single Atoms

Single atoms immobilized on metal–organic frameworks (MOFs) with unique nanostructures have drawn tremendous attention in the application of catalysis but remain a great challenge. Various single noble‐metal atoms have now been successfully anchored on the well‐defined anchoring sites of the zirconium porphyrin MOF hollow nanotubes, which are probed by aberration‐corrected scanning transmission electron microscopy and synchrotron‐radiation‐based X‐ray absorption fine‐structure spectroscopy. Owing to the hollow structure and excellent photoelectrochemical performance, the HNTM‐Ir/Pt exhibits outstanding catalytic activity in the visible‐light photocatalytic H₂ evolution via water splitting. The single atom immobilized on MOFs with hollow structures are expected to pave the way to expand the potential applications of MOFs.     

Studies on Photocatalytic CO2 Reduction over NH2‐Uio‐66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal–Organic Frameworks

Metal–organic framework (MOF) NH₂‐Uio‐66(Zr) exhibits photocatalytic activity for CO₂ reduction in the presence of triethanolamine as sacrificial agent under visible‐light irradiation. Photoinduced electron transfer from the excited 2‐aminoterephthalate (ATA) to Zr oxo clusters in NH₂‐Uio‐66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr^III and its involvement in photocatalytic CO₂ reduction was confirmed by ESR analysis. Moreover, NH₂‐Uio‐66(Zr) with mixed ATA and 2,5‐diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO₂ reduction due to its enhanced light adsorption and increased adsorption of CO₂. This study provides a better understanding of photocatalytic CO₂ reduction over MOF‐based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO₂ reduction.     

An Efficient,Visible‐Light‐Driven,Hydrogen Evolution Catalyst NiS/ZnxCd1−xS Nanocrystal Derived from a Metal–Organic Framework

Photocatalytic water splitting for hydrogen production using sustainable sunlight is a promising alternative to industrial hydrogen production. However, the scarcity of highly active, recyclable, inexpensive photocatalysts impedes the development of photocatalytic hydrogen evolution reaction (HER) schemes. Herein, a metal–organic framework (MOF)‐template strategy was developed to prepare non‐noble metal co‐catalyst/solid solution heterojunction NiS/Zn_xCd_1?xS with superior photocatalytic HER activity. By adjusting the doping metal concentration in MOFs, the chemical compositions and band gaps of the heterojunctions can be fine‐tuned, and the light absorption capacity and photocatalytic activity were further optimized. NiS/Zn_0.5Cd_0.5S exhibits an optimal HER rate of 16.78 mmol g^?1 h^?1 and high stability and recyclability under visible‐light irradiation (λ>420 nm). Detailed characterizations and in‐depth DFT calculations reveal the relationship between the heterojunction and photocatalytic activity and confirm the importance of NiS in accelerating the water dissociation kinetics, which is a crucial factor for photocatalytic HER.     

From Metal–Organic Frameworks to Single‐Atom Fe Implanted N‐doped Porous Carbons: Efficient Oxygen Reduction in Both Alkaline and Acidic Media

It remains highly desired but a great challenge to achieve atomically dispersed metals in high loadings for efficient catalysis. Now porphyrinic metal–organic frameworks (MOFs) have been synthesized based on a novel mixed‐ligand strategy to afford high‐content (1.76 wt %) single‐atom (SA) iron‐implanted N‐doped porous carbon (Fe_SA‐N‐C) via pyrolysis. Thanks to the single‐atom Fe sites, hierarchical pores, oriented mesochannels and high conductivity, the optimized Fe_SA‐N‐C exhibits excellent oxygen reduction activity and stability, surpassing almost all non‐noble‐metal catalysts and state‐of‐the‐art Pt/C, in both alkaline and more challenging acidic media. More far‐reaching, this MOF‐based mixed‐ligand strategy opens a novel avenue to the precise fabrication of efficient single‐atom catalysts.     

Highly Dispersed Polyoxometalate‐Doped Porous Co3O4 Water Oxidation Photocatalysts Derived from POM@MOF Crystalline Materials

Rational design of earth‐abundant photocatalysts is an important issue for solar energy conversion and storage. Polyoxometalate (POM)@Co₃O₄ composites doped with highly dispersive molecular metal–oxo clusters, synthesized by loading a single Keggin‐type POM cluster into each confined space of a metal–organic framework (MOF), exhibit significantly improved photocatalytic activity in water oxidation compared to the pure MOF‐derived nanostructure. The systematic synthesis of these composite nanocrystals allows the conditions to be tuned, and their respective water oxidation catalytic performance can be efficiently adjusted by varying the thermal treatment temperature and the feeding amount of the POM. This work not only provides a modular and tunable synthetic strategy for preparing molecular cluster@TM oxide (TM=transition metal) nanostructures, but also showcases a universal strategy that is applicable to design and construct multifunctional nanoporous metal oxide composite materials.     

Metal–Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production

Metal–organic frameworks (MOFs) are crystalline porous materials formed from bi‐ or multipodal organic linkers and transition‐metal nodes. Some MOFs have high structural stability, combined with large flexibility in design and post‐synthetic modification. MOFs can be photoresponsive through light absorption by the organic linker or the metal oxide nodes. Photoexcitation of the light absorbing units in MOFs often generates a ligand‐to‐metal charge‐separation state that can result in photocatalytic activity. In this Review we discuss the advantages and uniqueness that MOFs offer in photocatalysis. We present the best practices to determine photocatalytic activity in MOFs and for the deposition of co‐catalysts. In particular we give examples showing the photocatalytic activity of MOFs in H₂ evolution, CO₂ reduction, photooxygenation, and photoreduction.     

A Methylthio‐Functionalized‐MOF Photocatalyst with High Performance for Visible‐Light‐Driven H2 Evolution

Recently, the emergence of photoactive metal–organic frameworks (MOFs) has given great prospects for their applications as photocatalytic materials in visible‐light‐driven hydrogen evolution. Herein, a highly photoactive visible‐light‐driven material for H₂ evolution was prepared by introducing methylthio terephthalate into a MOF lattice via solvent‐assisted ligand‐exchange method. Accordingly, a first methylthio‐functionalized porous MOF decorated with Pt co‐catalyst for efficient photocatalytic H₂ evolution was achieved, which exhibited a high quantum yield (8.90 %) at 420 nm by use sacrificial triethanolamine. This hybrid material exhibited perfect H₂ production rate as high as 3814.0 μmol g^?1 h^?1, which even is one order of magnitude higher than that of the state‐of‐the‐art Pt/MOF photocatalyst derived from aminoterephthalate.     

An Efficient,Visible‐Light‐Driven,Hydrogen Evolution Catalyst NiS/ZnxCd1−xS Nanocrystal Derived from a Metal–Organic Framework

Photocatalytic water splitting for hydrogen production using sustainable sunlight is a promising alternative to industrial hydrogen production. However, the scarcity of highly active, recyclable, inexpensive photocatalysts impedes the development of photocatalytic hydrogen evolution reaction (HER) schemes. Herein, a metal–organic framework (MOF)‐template strategy was developed to prepare non‐noble metal co‐catalyst/solid solution heterojunction NiS/Zn_xCd_1−xS with superior photocatalytic HER activity. By adjusting the doping metal concentration in MOFs, the chemical compositions and band gaps of the heterojunctions can be fine‐tuned, and the light absorption capacity and photocatalytic activity were further optimized. NiS/Zn_0.5Cd_0.5S exhibits an optimal HER rate of 16.78 mmol g⁻¹ h⁻¹ and high stability and recyclability under visible‐light irradiation (λ>420 nm). Detailed characterizations and in‐depth DFT calculations reveal the relationship between the heterojunction and photocatalytic activity and confirm the importance of NiS in accelerating the water dissociation kinetics, which is a crucial factor for photocatalytic HER.     

金属有机框架(MOFs)基光催化剂的设计及其在太阳能燃料生产和污染物降解领域的研究进展

金属有机骨架(MOFs)具有较高的比表面积,丰富的金属/有机物种,较大的孔体积以及结构和成分可调节的特性,因此在太阳能燃料生产和污染物的光降解领域具有广泛的应用.根据其结构特点,研究者们主要从有机配体和孔道结构两方面对MOFs进行调控:(1)对有机配体进行修饰,如将杂原子、羟基、卤素原子、金属离子、生物大分子等引入MO...     

N‐Doped Carbon Aerogel Derived from a Metal–Organic Framework Foam as an Efficient Electrocatalyst for Oxygen Reduction

Metal–organic frameworks (MOFs) are promising alternative precursors for the fabrication of heteroatom‐doped carbon materials for energy storage and conversion. However, the direct pyrolysis of bulk MOFs usually gives microporous carbonaceous materials, which significantly hinder the mass transportation and the accessibility of active sites. Herein, N‐doped carbon aerogels with hierarchical micro‐, meso‐, and macropores were fabricated through one‐step pyrolysis of zeolitic imidazolate framework‐8/carboxymethylcellulose composite gel. Owing to the hierarchical porosity, high specific surface area, favorable conductivity, excellent thermal and chemical stability, the as‐prepared N‐doped carbon aerogel exhibits excellent oxygen reduction reaction (ORR) activity, long‐term durability, and good methanol tolerance in alkaline medium. This work thus provides a new way to fabricate new types of MOF‐derived carbon aerogels for various applications.     

Enhanced Photocatalytic Activity of MIL‐125 by Post‐Synthetic Modification with CrIII and Ag Nanoparticles

NH₂‐MIL‐125, [Ti₈O₈(OH)₄(bdc‐NH₂)₆] (bdc^2?=1,4‐benzene dicarboxylate) is a highly porous metal–organic framework (MOF) that has a band gap lying within the ultraviolet region at about 2.6 eV. The band gap may be reduced by a suitable post‐synthetic modification of the nanochannels using conventional organic chemistry methods. Here, it is shown that the photocatalytic activity of NH₂‐MIL‐125 in the degradation of methylene blue under visible light is remarkably augmented by post‐synthetic modification with acetylacetone followed by Cr^III complexation. The latter metal ion extends the absorption from the ultraviolet to the visible light region (band gap 2.21 eV). The photogenerated holes migrate from the MOF’s valence band to the Cr^III valence band, promoting the separation of holes and electrons and increasing the recombination time. Moreover, it is shown that the MOF’s photocatalytic activity is also much improved by doping with Ag nanoparticles, formed in situ by the reduction of Ag⁺ with the acetylacetonate pendant groups (the resulting MOF band gap is 2.09 eV). Presumably, the Ag nanoparticles are able to accept the MOF’s photogenerated electrons, thus avoiding electron–hole recombination. Both, the Cr‐ and Ag‐bearing materials are stable under photocatalytic conditions. These findings open new avenues for improving the photocatalytic activity of MOFs.     

Spray‐Coating of Catalytically Active MOF–Polythiourea through Postsynthetic Polymerization

A UiO‐66‐NCS MOF was formed by postsynthetic modification of UiO‐66‐NH₂. The UiO‐66‐NCS MOFs displays a circa 20‐fold increase in activity against the chemical warfare agent simulant dimethyl‐4‐nitrophenyl phosphate (DMNP) compared to UiO‐66‐NH₂, making it the most active MOF materials using a validated high‐throughput screening. The ?NCS functional groups provide reactive handles for postsynthetic polymerization of the MOFs into functional materials. These MOFs can be tethered to amine‐terminated polypropylene polymers (Jeffamines) through a facile room‐temperature synthesis with no byproducts. The MOFs are then crosslinked into a MOF–polythiourea (MOF–PTU) composite material, maintaining the catalytic properties of the MOF and the flexibility of the polymer. This MOF–PTU hybrid material was spray‐coated onto Nyco textile fibers, displaying excellent adhesion to the fiber surface. The spray‐coated fibers were screened for the degradation of DMNP and showed durable catalytic reactivity.     

Extra‐Framework Aluminum Species of Zeolite that Surrogate the Growth of Metal Organic Framework from Zeolite Matrix

Constructing a robust hybrid material with a porous inorganic and a porous organic framework is highly intriguing owing to its diverse functionality and porosity. However, the line of synthesis is not straightforward, since their nucleation and crystal growth processes are incompatible. Here, a simple method for the fabrication of hybrid zeolite/metal–organic framework of different framework structures is developed wherein the less‐useful extra‐framework aluminum species present in the zeolite surrogate the growth of metal organic framework (MOF) from the zeolite matrix in the presence of organic linkers of the corresponding MOF. An NMR study confirms that all the octahedral Al species are converted to Al‐MOF. TGA analysis shows that 32 % Al of H‐Beta is converted to Al‐MOF. Furthermore, NH₃ TPD analysis shows that most of the weak acid sites disappear but strong acid sites are preserved suggesting the utilization of weakly bound Al species of H‐Beta in the growth of Al‐MOF. The synthesis strategy is successfully demonstrated using H‐Beta, H‐ZSM‐5, and H‐Y zeolites for the growth of MIL‐53 and MIL‐96 MOFs from the zeolite matrix. This synthesis strategy enables application‐based engineering of the framework structures, functionality, and porosity of the materials.     

Fabrication of ZIF‐8@SiO2 Core–Shell Microspheres as the Stationary Phase for High‐Performance Liquid Chromatography

The unique features of high porosity, shape selectivity, and multiple active sites make metal–organic frameworks (MOFs) promising as novel stationary phases for high‐performance liquid chromatography (HPLC). However, the wide particle size distribution and irregular shape of conventional MOFs lead to lower column efficiency of such MOF‐packed columns. Herein, the fabrication of monodisperse MOF@SiO₂ core–shell microspheres as the stationary phase for HPLC to overcome the above‐mentioned problems is reported. Zeolitic imidazolate framework 8 (ZIF‐8) was used as an example of MOFs due to its permanent porosity, uniform pore size, and exceptional chemical stability. Unique carboxyl‐modified silica spheres were used as the support to grow the ZIF‐8 shell. The fabricated monodisperse ZIF‐8@SiO₂ packed columns (5 cm long × 4.6 mm i.d.) show high column efficiency (23 000 plates m^?1 for bisphenol A) for the HPLC separation of endocrine‐disrupting chemicals (bisphenol A, β‐estradiol, and p‐(tert‐octyl)phenol) and pesticides (thiamethoxam, hexaflumuron, chlorantraniliprole, and pymetrozine) within 7 min with good relative standard deviations for 11 replicate separations of the analytes (0.01–0.39, 0.65–1.7, 0.70–1.3, and 0.17–0.91 % for retention time, peak area, peak height, and half peak width, respectively). The ZIF‐8@SiO₂ microspheres combine the advantages of the good column packing properties of the uniform monodisperse silica microspheres and the separation ability of the ZIF‐8 crystals.     

Direct Fabrication of Free‐Standing MOF Superstructures with Desired Shapes by Micro‐Confined Interfacial Synthesis

Recently, metal–organic frameworks (MOFs) with multifunctional pore chemistry have been intensively investigated for positioning the desired morphology at specific locations onto substrates for manufacturing devices. Herein, we develop a micro‐confined interfacial synthesis (MIS) approach for fabrication of a variety of free‐standing MOF superstructures with desired shapes. This approach for engineering MOFs provides three key features: 1) in situ synthesis of various free‐standing MOF superstructures with controlled compositions, shape, and thickness using a mold membrane; 2) adding magnetic functionality into MOF superstructures by loading with Fe₃O₄ nanoparticles; 3) transferring the synthesized MOF superstructural array on to flat or curved surface of various substrates. The MIS route with versatile potential opens the door for a number of new perspectives in various applications.     

Design and Synthesis of a Water‐Stable Anionic Uranium‐Based Metal–Organic Framework (MOF) with Ultra Large Pores

Ionic metal–organic frameworks (MOFs) are a subclass of porous materials that have the ability to incorporate different charged species in confined nanospace by ion‐exchange. To date, however, very few examples combining mesoporosity and water stability have been realized in ionic MOF chemistry. Herein, we report the rational design and synthesis of a water‐stable anionic mesoporous MOF based on uranium and featuring tbo‐type topology. The resulting tbo MOF exhibits exceptionally large open cavities (3.9 nm) exceeding those of all known anionic MOFs. By supercritical CO₂ activation, a record‐high Brunauer‐Emmett‐Teller (BET) surface area (2100 m² g^?1) for actinide‐based MOFs has been obtained. Most importantly, however, this new uranium‐based MOF is water‐stable and able to absorb positively charged ions selectively over negatively charged ones, enabling the efficient separation of organic dyes and biomolecules.     

Self‐assembly of metal–organic framework thin films containing metalloporphyrin and their photocatalytic activity under visible light

With [5,10,15,20‐tetra(4‐carboxyphenyl)porphyrin]Mn(III) and sterically controlled 2,2¢‐dimethyl‐4,4¢‐pyridine as the main raw materials, metal–organic framework thin films containing metalloporphyrin (MnPor‐MOF) with catalytically active sites were built up on functionalized quartz glass surfaces using a layer‐by‐layer self‐assembly method. Retaining active catalytic sites and having a porous reticular structure, the MnPor‐MOF films possessed remarkable photocatalytic activity for oxidative degradation of methylene blue in the presence of hydrogen peroxide under visible‐light irradiation. Most meaningfully, the MnPor‐MOF films were highly stable and simply and conveniently reusable, and are thus a potentially new organic material for photocatalytic wastewater treatment. Copyright © 2015 John Wiley & Sons, Ltd.     

Crystal Dynamics in Multi‐stimuli‐Responsive Entangled Metal–Organic Frameworks

An understanding of solid‐state crystal dynamics or flexibility in metal–organic frameworks (MOFs) showing multiple structural changes is highly demanding for the design of materials with potential applications in sensing and recognition. However, entangled MOFs showing such flexible behavior pose a great challenge in terms of extracting information on their dynamics because of their poor single‐crystallinity. In this article, detailed experimental studies on a twofold entangled MOF ( f‐MOF‐1) are reported, which unveil its structural response toward external stimuli such as temperature, pressure, and guest molecules. The crystallographic study shows multiple structural changes in f‐MOF‐1 , by which the 3 D net deforms and slides upon guest removal. Two distinct desolvated phases, that is, f‐MOF‐1 a and f‐MOF‐1 b , could be isolated; the former is a metastable one and transformable to the latter phase upon heating. The two phases show different gated CO₂ adsorption profiles. DFT‐based calculations provide an insight into the selective and gated adsorption behavior with CO₂ of f‐MOF‐1 b . The gate‐opening threshold pressure of CO₂ adsorption can be tuned strategically by changing the chemical functionality of the linker from ethanylene (?CH₂?CH₂?) in f‐MOF‐1 to an azo (?N=N?) functionality in an analogous MOF, f‐MOF‐2 . The modulation of functionality has an indirect influence on the gate‐opening pressure owing to the difference in inter‐net interaction. The framework of f‐MOF‐1 is highly responsive toward CO₂ gas molecules, and these results are supported by DFT calculations.     

Cu3(hexaiminotriphenylene)2: An Electrically Conductive 2D Metal–Organic Framework for Chemiresistive Sensing

The utility of metal–organic frameworks (MOFs) as functional materials in electronic devices has been limited to date by a lack of MOFs that display high electrical conductivity. Here, we report the synthesis of a new electrically conductive 2D MOF, Cu₃(HITP)₂ (HITP=2,3,6,7,10,11‐hexaiminotriphenylene), which displays a bulk conductivity of 0.2 S cm^?1 (pellet, two‐point‐probe). Devices synthesized by simple drop casting of Cu₃(HITP)₂ dispersions function as reversible chemiresistive sensors, capable of detecting sub‐ppm levels of ammonia vapor. Comparison with the isostructural 2D MOF Ni₃(HITP)₂ shows that the copper sites are critical for ammonia sensing, indicating that rational design/synthesis can be used to tune the functional properties of conductive MOFs.