Structural survey of metal-covalent organic frameworks and covalent metal-organic frameworks

This review offers an overview of the latest developments in metal-covalent organic framework(MCOF)and covalent metal-organic framework(CMOF)materials,whose construction entails a combination of reversible coordination and covalent bonding adapted from metal-organic frameworks(MOFs)and covalent organic frameworks(COFs),respectively.With an emphasis on the MCOF and CMOF structures,this review surveys their building blocks and topologies.Specifically,the frameworks are classified based on the dimensions of their components(building blocks),namely,discrete building blocks and one-dimensional infinite building blocks.For the first category,the materials are further divided into collections of two-and three-dimensional networks based on their topologies.For the second category,the recently emerging MCOFs with woven structures are covered.Finally,the state-of-the-art in MCOF and CMOF chemistry has been laid out for promising avenues in future developments.

Large-scale computational screening of metal–organic frameworks for D_(2)/H_(2) separation

Deuterium(D_(2)) is one of the important fuel sources that power nuclear fusion reactors. The existing D_(2)/H_(2) separation technologies that obtain high-purity D_(2) are cost-intensive. Recent research has shown that metal-organic frameworks(MOFs) are of good potential for D_(2)/H_(2) separation application. In this work, a high-throughput computational screening of 12020 computation-ready experimental MOFs is carried out to determine the best MOFs for hydrogen isotope separation application. Meanwhile, the detailed structure-performance correlation is systematically investigated with the aid of machine learning. The results indicate that the ideal D_(2)/H_(2) adsorption selectivity calculated based on Henry coefficient is strongly correlated with the 1/ΔAD feature descriptor;that is, inverse of the adsorbility difference of the two adsorbates. Meanwhile, the machine learning(ML) results show that the prediction accuracy of all the four ML methods is significantly improved after the addition of this feature descriptor. In addition, the ML results based on extreme gradient boosting model also revealed that the 1/ΔAD descriptor has the highest relative importance compared to other commonly-used descriptors. To further explore the effect of hydrogen isotope separation in binary mixture, 1548 MOFs with ideal adsorption selectivity greater than 1.5 are simulated at equimolar conditions. The structure-performance relationship shows that high adsorption selectivity MOFs generally have smaller pore size(0.3-0.5 nm) and lower surface area. Among the top 200 performers, the materials mainly have the sql, pcu, cds, hxl, and ins topologies.Finally, three MOFs with high D_(2)/H_(2) selectivity and good D_(2) uptake are identified as the best candidates,of all which had one-dimensional channel pore. The findings obtained in this work may be helpful for the identification of potentially promising candidates for hydrogen isotope separation.

Deep dive into anionic metal-organic frameworks based quasi-solid-state electrolytes

The development and application of high-capacity energy storage has been crucial to the global transition from fossil fuels to green energy.In this context,metal-organic frameworks(MOFs),with their unique 3D porous structure and tunable chemical functionality,have shown enormous potential as energy storage materials for accommodating or transporting electrochemically active ions.In this perspective,we specifically focus on the current status and prospects of anionic MOF-based quasi-solid-state-electrolytes(anionic MOF-QSSEs)for lithium metal batteries(LMBs).An overview of the definition,design,and properties of anionic MOF-QSSEs is provided,including recent advances in the understanding of their ion transport mechanism.To illustrate the advantages of using anionic MOF-QSSEs as electrolytes for LMBs,a thorough comparison between anionic MOF-QSSEs and other well-studied electrolyte systems is made.With these in-depth understandings,viable techniques for tuning the chemical and topological properties of anionic MOF-QSSEs to increase Li+conductivity are discussed.Beyond modulation of the MOFs matrix,we envisage that solvent and solid-electrolyte interphase design as well as emerging fabrication techniques will aid in the design and practical application of anionic MOF-QSSEs.

Metal-organic frameworks with mixed-ligands strategy as heterogeneous nucleation center to assist crystallization for efficient and stable perovskite solar cells

Deep-level defects and random oriented configuration in perovskite crystallization process would cause the nonradiative recombination and further affect the performance of perovskite solar cells(PSCs).Herein,two metal-organic frameworks(MOFs)with tunable Lewis-base passivation sites have been constructed(Cd-Httb and Cd-Httb-BDC,Httb=5-(4-(1H-1,2,4-triazole-1-yl)benzyl)-1h-tetrazole,BDC=1,4-dicarboxybenzene)to eliminate deep-level defects and simultaneously as nanostructured heterogeneous nucleation seed to assist the growth of large-grained perovskite films.Compared with the control and Cd-Httb,Cd-Httb-BDC designed with mix-ligands strategy exhibited the enhanced inducted effect on the crystallization and nucleation of high-quality perovskite films during annealing process.Consequently,the resultant Cd-Httb-BDC-modified device achieved higher power conversion efficiency(PCE)(22.18%)than the control(20.89%)and Cd-Httb(21.56%).Meanwhile,the unencapsulated Cd-Httb-BDC-modified device still maintained 90%of initial PCE after 1500 h in ambient conditions and exhibited enhanced thermal stability(85℃ in N_(2) atmosphere).This work presented a successful example of mixligands strategy on construction of high-quality MOF-assisted perovskite films for high-efficient and stable PSCs.

Integrated electrocatalysts derived from metal organic frameworks for gas-involved reactions

Integrated electrocatalysts(IECs)containing well-defined functional materials directly grown on the current collector have sparked increasing interest in the fields of electrocatalysis owing to efficient activity,high stability and the fact that they are easily assembled into devices.Recently,metal organic frameworks(MOFs)provide a promising platform for constructing advanced IECs because of their properties of low cost,large surface area and efficient structural tunability.In this review,the design principles of state-of-the-art IECs based on MOFs are presented,including by hydrothermal/solvothermal,template-directed,electrospinning,electrodeposition and other methods.The high performance of MOF-derived IECs has also been demonstrated in electrocatalytic gasinvolved reactions.This is promising for green energy storage and conversion.The structure-activity relationship and performance improvement mechanism of IECs are uncovered by discussing some in situ technologies for IECs.Finally,we provide an outlook on the challenges and prospects in this booming field.

A Review of Metal–Organic Framework-Based Compounds for Environmental Applications

Metal–organic framework-based compounds have recently gained great attention because of their unique porous structure,ordered porosity,and high specific surface area.Benefiting from these superior properties,metal–organic framework-based compounds have been proven to be one of the most potential candidates for environmental governance and remediation.In this review,the different types of metal–organic framework-based compounds are first summarized.Further,the various environmental applications of metal–organic framework-based compounds including organic pollutant removal,toxic and hazardous gas capture,heavy metal ion detection,gas separation,water harvesting,air purification,and carbon dioxide reduction reactions are discussed in detail.In the end,the opportunities and challenges for the future development of metal–organic framework-based compounds for environmental applications are highlighted.

Recent Progress in Synthesis, Mechanism and Applications of Zinc-Based Metal-Organic Frameworks for Fluorescent Sensing

As more and more pollutants threaten human health, it is necessary and essential to develop sensitive, accurate and rapid methods and sensory materials to detect harmful substance. Metal-organic frameworks (MOFs) are inorganic-organic hybrids assembled from inorganic metal ions or clusters and suitable organic ligands. Zinc-based MOFs (Zn-MOFs) have emerged as one of the most promising sensory material of MOFs for practical applications, and attracted significant attention due to structural diversity and incomparable stability properties. However, there are few reviews on systemic summary of synthesis design, mechanism and application of Zn-MOFs. In this review, we summarize the synthesis design methods, structure types and luminescence mechanism of Zn-MOFs sensor recognition in the past ten years and their applications in metal cations, anions, organic compounds and other analytes. Finally, we present a short conclusion, and look forward to the future development direction of Zn-MOFs.

Nanoengineering Metal–Organic Frameworks and Derivatives for Electrosynthesis of Ammonia

Electrocatalytic synthesis under mild conditions has become increasingly important as one of the practical alternatives for industrial applications,especially for the green ammonia(NH_(3))industry.A properly engineered electrocatalyst plays a vital role in the realization of superior catalytic performance.Among various types of promising nanomaterials,metal–organic frameworks(MOFs)are competitive candidates for developing efficient electrocatalytic NH_(3) synthesis from simple nitrogen-containing molecules or ions,such as N_(2) and NO_(3)^(−).In this review,recent advances in the development of electrocatalysts derived from MOFs for the electrosynthesis of NH_(3) are collected,categorized,and discussed,including their application in the N_(2) reduction reaction(NRR)and the NO_(3)^(−)reduction reaction(NO3RR).Firstly,the fundamental principles are illustrated,such as plausible mechanisms of NH_(3) generation from N_(2) and NO_(3)^(−),the apparatus of corresponding electrocatalysis,parameters for evaluation of reaction efficiency,and detection methods of yielding NH_(3).Then,the electrocatalysts for NRR processes are discussed in detail,including pristine MOFs,MOF-hybrids,MOF-derived N-doped porous carbons,single atomic catalysts from pyrolysis of MOFs,and other MOF-related materials.Subsequently,MOF-related NO3RR processes are also listed and discussed.Finally,the existing challenges and prospects for the rational design and fabrication of electrocatalysts from MOFs for electrochemical NH_(3) synthesis are presented,such as the evolution of investigation methods with artificial intelligence,innovation in synthetic methods of MOF-related catalysts,advancement of characterization techniques,and extended electrocatalytic reactions.

Ethylene purification in a metal–organic framework over a wide temperature range via pore confinement

The separation of C2H4from C_(2)H_(6)/C_(2)H_(4)mixture is of great importance but difficult and energy intensive. Adsorptive separation provides an alternative approach to ameliorate this situation. Here, we report a microporous metal–organic framework(MOF) BUT-315-a as a C_(2)H_(6)-selective adsorbent for the separation of C2H6/C2H4gas mixture. BUT-315-a combines good IAST selectivity of 2.35 with high C_(2)H_(6)uptake of 97.5 cm^(3)g^(-1), giving superior high separation potential ΔQ(2226 mmol L^(-1)) for equimolar C_(2)H_(6)/C_(2)H_(4) at 298 K. Impressively, such excellent performance can be preserved at higher temperatures of 313 and 323 K to accommodate industrial conditions. Efficient dynamic separation performance of BUT-315-a has been demonstrated by column breakthrough experiments under varied temperatures and gas ratios. Theoretical calculations further reveal multiple synergistic interactions between C_(2)H_(6) and the framework. This work highlights a new benchmark material for C_(2)H_(6)/C_(2)H_(4)separation and provides guidance for designing adsorbent for separation applications.

Metal–Organic Framework-Based Sensors for Environmental Contaminant Sensing

Increasing demand for timely and accurate environmental pollution monitoring and control requires new sensing techniques with outstanding performance, i.e.,high sensitivity, high selectivity, and reliability. Metal–organic frameworks(MOFs), also known as porous coordination polymers, are a fascinating class of highly ordered crystalline coordination polymers formed by the coordination of metal ions/clusters and organic bridging linkers/ligands. Owing to their unique structures and properties,i.e., high surface area, tailorable pore size, high density of active sites, and high catalytic activity, various MOF-based sensing platforms have been reported for environmental contaminant detection including anions, heavy metal ions,organic compounds, and gases. In this review, recent progress in MOF-based environmental sensors is introduced with a focus on optical, electrochemical, and field-effect transistor sensors. The sensors have shown unique and promising performance in water and gas contaminant sensing. Moreover, by incorporation with other functional materials, MOF-based composites can greatly improve the sensor performance. The current limitations and future directions of MOF-based sensors are also discussed.

Metal-organic frameworks for electrochemical reduction of carbon dioxide: The role of metal centers

Direct electrochemical reduction of CO2 into valuable chemicals and fuel is one of the most promising approaches to address the current energy crisis and lower CO2 emission.Recently,numerous metal-organic framework(MOF)and their derived materials have extensively been developed as electrocatalysts for CO2 reduction owing to their unique structure including porosity,large specific surface area,and tunable chemical structures.In this review,the recent progress of MOF-based electrocatalysts for CO2 reduction was summarized and discussed.Detailed discussions mainly focus on the synthesis and mechanism of pristine MOFs and MOF-derived materials for electrocatalytic CO2 reduction.These examples are expected to provide clues to rational design and synthesis of stable and high-performance MOFs-based electrocatalysts for CO2 reduction.

Metal–Organic Framework-Assisted Synthesis of Compact Fe_2O_3 Nanotubes in Co_3O_4 Host with Enhanced Lithium Storage Properties

Transition metal oxides are promising candidates for the high-capacity anode material in lithium-ion batteries.The electrochemical performance of transition metal oxides can be improved by constructing suitable composite architectures. Herein, we demonstrate a metal–organic framework(MOF)-assisted strategy for the synthesis of a hierarchical hybrid nanostructure composed of Fe_2O_3 nanotubes assembled in Co_3O_4 host. Starting from MOF composite precursors(Fe-based MOF encapsulated in a Cobased host matrix), a complex structure of Co_3O_4 host and engulfed Fe_2O_3 nanotubes was prepared by a simple annealing treatment in air. By virtue of their structural and compositional features, these hierarchical composite particles reveal enhanced lithium storage properties when employed as anodes for lithium-ion batteries.

Magnetic metal organic framework for pre-concentration of ampicillin from cow milk samples

The aim of this study is a present of a simple solvothermal synthesis approach to preparation of Cu-based magnetic metal organic framework(MMOF)and subsequently its application as sorbent for ultrasound assisted magnetic solid phase extraction(UAMSPE)of ampicillin(AMP)from cow milk samples prior to high performance liquid chromatography-Ultraviolet(HPLC-UV)determination.Characteristics of prepared MMOF were fully investigated by different techniques which showed the exclusive properties of proposed sorbent in terms of proper functionality,desirable magnetic property and also high specific surface area.Different influential factors on extraction recovery including sorbent dosage,ultrasonic time,washing solvent volume and eluent solvent volume were assessed using central composite design(CCD)based response surface methodology(RSM)as an operative and powerful optimization tool.This is the first report for determination of AMP using MMOF.The proposed method addressed some drawbacks of other methods and sorbents for determination of AMP.The presented method decreases the extraction time(4 min)and also enhances adsorption capacity(250 mg/g).Moreover,the magnetic property of presented sorbent(15 emu/g)accelerates the extraction process which does not need filtration,centrifuge and precipitation procedures.Under the optimized conditions,the proposed method is applicable for linear range of 1.0-5000.0 μg/L with detection limit of 0.29 μg/L,satisfactory recoveries(≥95.0%)and acceptable repeatability(RSD less than 4.0%).The present study indicates highly promising perspectives of MMOF for highly effective analysis of AMP in complicated matrices.

Sulfonic groups functionalized Zr-metal organic framework for highly catalytic transfer hydrogenation of furfural to furfuryl alcohol

The highly selective catalytic transfer hydrogenation(CTH)of furfural(FF)to furfuryl alcohol(FOL)is a significant route of biomass valorization.Herein,a series microporous Zr-metal organic framework(ZrMOF)functionalized by sulfonic groups are prepared.Based on the comprehensive structural characterizations by means of X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),N2 physisorption,Thermogravimetric(TG)and Fourier transformed infrared spectroscopy(FTIR),we find that sulfonic acid(–SO_(3)H)functional groups are tethered on the UIO-66 without affecting the structure of the framework.Systematic characterizations(NH_(3)-TPD,CO_(2)-TPD,and in-situ FTIR)demonstrate that modifying of sulfonic groups on UIO-66 results in the formation of stronger Lewis acidic-basic and Brnsted acidis sites.The cooperative role of the versatile Lewis acidic-basic and Brnsted acidic sites in 60%mol fraction of sulfonic acid-containing UIO-66(UIO-S_(0.6))retain high surface area and exhibit excellent catalytic performance of 94.7%FOL yield and 16.9 h^(-1).turnover number(TOF)under mild conditions.Kinetic experiments reveal that the activation energy of the CTH of furfural(FF)over UIO-S_(0.6) catalyst is as low as 50.8 k J mol^(-1).Besides,the hydrogen transfer mechanism is investigated through isotope labeling experiments,exhibiting that theβ-H in isopropanol is transferred to the a-C of FF by forming six-membered intermediates on the Lewis acidic-basic and Brnsted acidic sites of the UIO-S_(0.6),which is the rate-determining step in the formation of FOL.

Facile Synthesis of the Magnetic Metal Organic Framework Fe_3O_4@UiO-66-NH_2 for Separation of Strontium

A magnetic metal organic framework（MMOF） was synthesized and used to separate Sr^2＋ in aqueous solution. The shape and structure of prepared Fe3O4@UiO-66-NH2 were characterized, and the absorbed concentration of strontium was determined through inductively coupled plasma mass spectrometry. The results indicated that Fe3O4 and UiO-66-NH2 combined through chemical bonding. The experimental adsorption results for separation of Sr^2＋ in aqueous solution indicated that the adsorption of Sr^2＋ to Fe3O4@UiO-66-NH2 increased drastically from pH 11 to pH 13. The adsorption isotherm model indicated that the adsorption of Sr^2＋ conformed to the Freundlich isotherm model（R2 = 0.9919）. The MMOF thus inherited the superior qualities of magnetic composites and metal organic frameworks, and can easily be separated under an external magnetic field. This MMOF thus has potential applications as a magnetic adsorbent for low level radionuclide （90）Sr.

Advanced metal–organic frameworks for aqueous sodium-ion rechargeable batteries

Inexpensive and abundant sodium resources make energy storage systems using sodium chemistry promising replacements for typical lithium-ion rechargeable batteries(LIBs).Fortuitously,aqueous sodium-ion rechargeable batteries(ASIBs),which operate in aqueous electrolytes,are cheaper,safer,and more ionically conductive than batteries that operate in conventional organic electrolytes;furthermore,they are suitable for grid-scale energy storage applications.As electrode materials for storing Na~+ ions in ASIBs,a variety of multifunctional metal-organic frameworks(MOFs) have demonstrated great potential in terms of having porous 3 D crystal structures,compatibility with aqueous solutions,long cycle lives(≥1000 cycles),and ease of synthesis.The present review describes MOF-derived technologies for the successful application of MOFs to ASIBs and suggests future challenges in this area of research based on the current understanding.

A dual metal-organic framework strategy for synthesis of FeCo@NC bifunctional oxygen catalysts for clean energy application

Developing high efficient bifunctional oxygen electrocatalysts for clean energy applications like Zin-air battery(ZAB)is highly desired,because it would reduce the cost and speed up the practical application of ZAB.Here we use a dual metal-organic framework(MOF)synthesis strategy to prepare the N-doped carbon supported bimetallic FeCo nanoparticle catalysts(marked as FeCo@NC)by pyrolysis of Zn CoZIF/MIL-101(Fe)composite.The FeCo@NC exhibits remarkable electrocatalytic activity for ORR with half-wave potential of 0.89 V vs.the reversible hydrogen electrode(RHE)and robust durability for both ORR and OER(oxygen reduction reaction and oxygen evolution reaction),which is attributed to the generation of Fe_(0.26)Co_(0.74) crystalline phase and mesopores due to the dual-MOF synthesis strategy.The rechargeable ZAB based on FeCo@NC air electrode shows a maximum energy density of139.6 mW·cm^(-2) and excellent cyclic stability over 130 h,significantly surpassing the Pt and Ir-based ZAB.The present work provides a useful dual-MOF synthesis strategy for preparing high-performance multifunctional catalysts for ORR,OER and hydrogen evolution reaction(HER).

Hierarchical Metal-Organic Frameworks with Macroporosity:Synthesis, Achievements,and Challenges

Introduction of multiple pore size regimes into metalorganic frameworks(MOFs)to form hierarchical porous structures can lead to improved performance of the material in various applications.In many cases,where interactions with bulky molecules are involved,enlarging the pore size of typically microporous MOF adsorbents or MOF catalysts is crucial for enhancing both mass transfer and molecular accessibility.In this review,we examine the range of synthetic strategies which have been reported thus far to prepare hierarchical MOFs or MOF composites with added macroporosity.These fabrication techniques can be either pre-or post-synthetic and include using hard or soft structural template agents,defect formation,routes involving supercritical CO2,and 3D printing.We also discuss potential applications and some of the challenges involved with current techniques,which must be addressed if any of these approaches are to be taken forward for industrial applications.

Ultrathin two-dimensional metal–organic framework nanosheets for efficient electrochemical CO_(2) reduction

Electrochemical CO_(2) reduction into CO or high-value products is regarded as a feasible pathway for energy conversion,which has attracted universal attention in recent years [1-3].However,the reduction of CO_(2) molecule is a thermodynamically uphill process,which involves multiple elemental steps and the competition of hydrogen evolution reaction(HER) in aqueous solution.

Metal–organic framework derived hierarchical porous TiO_2 nanopills as a super stable anode for Na-ion batteries

Hierarchical porous TiOnanopills were synthesized using a titanium metal-organic framework MIL-125（Ti） as precursor. The as-synthesized TiOnanopills owned a large specific surface area of 102 m/g and unique porous structure. Furthermore, the obtained TiOnanopills were applied as anode materials for Na-ion batteries for the first time. The as-synthesized TiOnanopills achieved a high discharge capacity of 196.4 m Ah/g at a current density of 0.1 A/g. A discharge capacity of 115.9 m Ah/g was obtained at a high current density of 0.5 A/g and the capacity retention was remained as high as 90% even after 3000 cycles. The excellent electrochemical performance can be attributed to its unique hierarchical porous feature.