Functionality of Covalent Organic Framework (COF) in Gas Storage Application: First Principal Study

Industrial growth in recent years led to air pollution and an increase in concentration of hazardous gases such as O3 and NO. Developing new materials is important to detect and reduce air pollutants. While catalytic decomposition and zeolites are traditional ways used to reduce the amount of these gases. We need to develop and explore new promising materials. Covalent organic framework (COF) has become an attractive platform for researcher due to its extended robust covalent bonds, porosity, and crystallinity. In this study, first principal calculations were performed for gases adsorption using COFs containing nitrogen and π-bonds. Different building blocks (BBs) and linkers (LINKs/LINK1 & LINK2) were investigated by means of density functional theory (DFT) calculations with B3LYP and 3-21G basis sets to calculate the binding energies of gases @COF systems. Electrostatic potential maps (ESPM), Mulliken charges and non-covalent interaction (NCI) are used to understand the type of interactions between gas and COFs fragments. O3 was found to bind strongly with COF system in comparison with NO which could make COF a useful selective material for mixed gases environment for sensing and removal application.

An imine-linked covalent organic framework as the host material for sulfur loading in lithium–sulfur batteries

Lithium–sulfur(Li–S) batteries have high theoretical specific capacity, providing new opportunities for the next generation of secondary battery. Covalent organic framework(COF) as a new porous crystalline material has been used as the host material in Li–S battery to improve the cell's cycling stability. In this paper, an imine-linked TAPB-PDA-COF was applied as the host material for sulfur loading(60%) in Li–S battery. The TAPB-PDA-COF has a beehive-like morphology with high thermal stability(up to 500 ℃).In the electrochemical experiment, the performance of the composite cathode with acetylene black(AB) and super-P(S-P) as the conductive additives was studied individually. The initial discharge capacity under 0.2 A/g current density was 991 mAh/g and 1357 mAh/g for TAPB-PDA-COF/S@A-B and TAPB-PDACOF/S@S-P, respectively. The better result of S-P based cathode than A-B could be due to the better conductivity of the S-P, as proved by the EIS results. When further increased the current density to 2 A/g,the S-P based composite cathode can still deliver a comparable initial discharge capacity of 630 and 274 mAh/g capacity remained after 940 cycles. This results will inspire researchers develop more suitable conductive additives together with the host materials for high performance Li–S battery.

Covalent organic frameworks as heterogeneous catalysts

Covalent organic frameworks （COFs）, established as an emerging class of crystalline porous polymers with high surface area, structural diversity, and esignability, attract much interest and exhibit potential applications in catalysis. In this review, we summarize the use of COFs as a versatile platform to develop heterogeneous catalysts for a variety of chemical reactions. Catalytic COFs are categorized in accordance with the types of active sites, involving single functional active sites, bifunctional active sites, and metal nanoparticles （NPs） embedded in pores. Special emphasis is placed on the deliberate or incidental synthesis strategies, the stability, the heterogeneity, and the shape/size selectivity for COF catalysis. Moreover, a description of the application of COFs as photocatalysts and electrocatalysts is presented. Finally, the prospects of COFs in catalysis and remaining issues in this field are indicated.

Reducing dielectric confinement effect in ionic covalent organic nanosheets to promote the visible-light-driven hydrogen evolution

Ultra-thin two-dimensional(2D)organic semiconductors are promising candidates for photocatalysts because of the short charge diffusion pathway and favorable exposure of active sites plus the versatile architecture.Nonetheless,the inherent dielectric confinement of 2D materials will induce a strong exciton effect hampering the charge separation.Herein,we demonstrated an effective way to reduce the dielectric confinement effect of 2D ionic covalent organic nanosheets(iCONs)by tailoring the functional group via molecular engineering.Three ultra-thin CONs with different functional groups and the same ionic moieties were synthesized through Schiff base condensation between ionic amino monomer triaminoguanidinium chloride(TG)and aldehyde linkers.The integration of the hydroxyl group was found to significantly increase the dielectric constant by enhancing the polarizability of ionic moieties,and thus reduced the dielectric confinement and the corresponding exciton binding energy(E_(b)).The champion hydroxyl-functional iCON exhibited promoted exciton dissociation and in turn a high photocatalytic hydrogen production rate under visible-light irradiation.This work provided insights into the rationalization of the dielectric confinement effect of low-dimensional photocatalysts.

2D/2D S-scheme heterojunction with a covalent organic framework and g-C_(3)N_(4) nanosheets for highly efficient photocatalytic H2 evolution

The fabrication of S-scheme heterojunctions with fast charge transfer and good interface contacts,such as intermolecularπ–πinteractions,is a promising approach to improve photocatalytic performance.A unique two-dimensional/two-dimensional(2D/2D)S-scheme heterojunction containing TpPa-1-COF/g-C_(3)N_(4) nanosheets(denoted as TPCNNS)was developed.The established maximum interfacial interaction between TpPa-1-COF NS and g-C_(3)N_(4) NS may result in aπ–πconjugated heterointerface.Furthermore,the difference in the work functions of TpPa-1-COF and g-C_(3)N_(4) results in a large Fermi level gap,leading to upward/downward band edge bending.The spontaneous interfacial charge transfer from g-C_(3)N_(4) to TpPa-1-COF at theπ–πconjugated interface area results in the presence of a built-in electric field,according to the charge density difference analysis based on density functional theory calculations.Such an enhanced built-in electric field can efficiently drive directional charge migration via the S-scheme mechanism,which enhances charge separation and utilization.Thus,an approximately 2.8 and 5.6 times increase in the photocatalytic hydrogen evolution rate was recorded in TPCNNS-2(1153μmol g^(-1) h^(-1))compared to pristine TpPa-1-COF and g-C_(3)N_(4) NS,respectively,under visible light irradiation.Overall,this work opens new avenues in the fabrication of 2D/2Dπ–πconjugated S-scheme heterojunction photocatalysts with highly efficient hydrogen evolution performance.

Fabrication of edge-curled petals-like covalent organic frameworks and their properties for extracting indole alkaloids from complex biological samples

In this study,a functionalized covalent-organic framework(COF)was first synthesized using porphyrin as the fabrication unit and showed an edge-curled,petal-like and well-ordered structure.The synthesized COF was then introduced to prepare porous organic polymer monolithic materials(POPMs).Two composite POPM/COF monolithic materials with rod shapes,referred to as sorbent A and sorbent B,were prepared in stainless steel tubes using different monomers.Sorbents A and B exhibited relatively uniform porous structures and enhanced specific surface areas of 153.14 m;/g and 80.01 m;/g,respectively.The prepared composite monoliths were used as in-tube solid-phase extraction(SPE)sorbents combined with HPLC for the on-line extraction and quantitative analytical systems.Indole alkaloids(from Catharanthus roseus G.Don and Uncaria rhynchophylla(Miq.)Miq.Ex Havil.)contained in mouse plasma were extracted and quantitatively analyzed using the online system.The two composite multifunctional monoliths showed excellent clean-up ability for complex biological matrices,as well as superior selectivity for target indole alkaloids.Method validation showed that the RSD values of the repeatability(n=6)were≤3.46%,and the accuracy expressed by the spiked recoveries was in the ranges of 99.38%-100.91%and 96.39%-103.50%for vinca alkaloids and Uncaria alkaloids,respectively.Furthermore,sorbents A and B exhibited strong reusability,with RSD values≤5.32%,which were based on the peak area of the corresponding alkaloids with more than 100 injections.These results indicate that the composite POPM/COF rod-shaped monoliths are promising media as SPE sorbents for extracting trace compounds in complex biological samples.

Synthesis of covalent organic framework materials and their application in the field of sensing

Covalent organic frameworks(COFs)are an emerging type of porous crystalline polymers formed by combining strong covalent bonds with organic building blocks.Due to their large surface area,high intrinsic pore space,good crystallization properties,high stability,and designability of the resultant units,COFs are widely studied and used in the fields of gas adsorption,drug transport,energy storage,photoelectric catalysis,electrochemistry,and sensors.In recent years,the rapid development of the Internet of Things and people’s yearning for a better life have put forward higher and more requirements for sensors,which are the core components of the Internet of Things.Therefore,this paper reviews the recent progress of COFs in synthesis methods and sensing applications,especially in the last five years.This paper first introduces structure,properties,and synthesis methods of COFs and discusses advantages and disadvantages of different synthesis methods.Then,the research progress of COFs in different sensing fields,such as metal ion sensors,gas sensors,biomedical sensors,humidity sensors,and pH sensors,is introduced systematically.Conclusions and prospects are also presented in order to provide a reference for researchers concerned with COFs and sensors.

A General Synthesis Method for Covalent Organic Framework and Inorganic 2D Materials Hybrids

Two-dimensional(2D)inorganic/organic hybrids provide a versatile platform for diverse applications,including electronic,catalysis,and energy storage devices.The recent surge in 2D covalent organic frameworks(COFs)has introduced an organic counterpart for the development of advanced 2D organic/inorganic hybrids with improved electronic coupling,charge separation,and carrier mobility.However,existing synthesis methods have primarily focused on few-layered film structures,which limits scalability for practical applications.Herein,we present a general synthesis approach for a range of COF/inorganic 2D material hybrids,utilizing 2D inorganic materials as both catalysts and inorganic building blocks.By leveraging the intrinsic Lewis acid sites on the inorganic 2D materials such as hexagonal boron nitride(hBN)and transition metal dichalcogenides,COFs with diverse functional groups and topologies can grow on the surface of inorganic 2D materials.The controlled 2D morphology and excellent solution dispersibility of the resulting hybrids allow for easy processing into films through vacuum filtration.As proof of concept,hBN/COF films were employed as filters for Rhodamine 6G removal under flow-through conditions,achieving a removal rate exceeding 93%.The present work provides a simple and versatile synthesis method for the scalable fabrication of COF/inorganic 2D hybrids,offering exciting opportunities for practical applications such as water treatment and energy storage.

A new strategy for the fabrication of covalent organic framework‐metal‐organic framework hybrids via in‐situ functionalization of ligands for improved hydrogen evolution reaction activity

The development of novel porous materials have attracted significant attention owing to its possible application in several fields.In this study,we designed a novel covalent organic framework‐metal‐organic framework(COF‐MOF)material through an in‐situ ligand self‐assembly method.The in‐situ modified ligands not only act as nucleation sites to form Ti‐MOF,but also as a channel to rapidly transfer photogenerated electrons without introducing additional chemical bonds.The photocatalytic hydrogen production rate achieved over B‐CTF‐Ti‐MOF(1:1)was 1975μmol·g^(–1)·h^(–1) with an apparent quantum efficiency of 4.76%,which is 11.8 times higher than that of the pure CTF‐1.In addition,compared with the sample prepared by separating the ligands(CTF‐1/Ti‐MOF),B‐CTF‐Ti‐MOF shows excellent activity and stability.Finally,a reasonable photocatalytic mechanism was proposed using the results of electrochemical tests and spectral analyses.This study provides a universal method for the construction of highly efficient and stable COF/MOF materials with excellent properties.

Amide-linked covalent organic frameworks as efficient heterogeneous photocatalysts in water

Semiconductor photocatalysts play an indispensable role in the photocatalytic process.Two-dimensional covalent organic frameworks(2D-COFs),as a kind of innovative photocatalyst,have garnered tremendous attention.Herein,we report an amide-linked 2D-COF(COF-JLU19)with outstanding photocatalytic performance in water,designed through a multi-synergistic approach.The synergistic effects of the high porosity,photoactive framework,high wettability,and stability of COF-JLU19 led to an unprecedented enhancement in the photocatalytic activity and recyclability in water upon illumination by visible light.More importantly,amide-linked 2D-COF based electrospinning membranes were prepared,which also exhibited superior photocatalytic activity for the degradation of Rhodamine B in water with sunlight.This study highlights the potential of the multi-synergistic approach as a universal rule for developing COF-based photocatalysts to address environmental and energy challenges.

Metal/covalent organic frameworks for aqueous rechargeable zinc-ion batteries

Lithium-ion batteries(LIBs)have become one of the most successful energy storage systems due to their high operating voltage,high energy density,and long cycle life.However,with the widespread use of LIBs in recent decades,lithium resources are at risk of being exhausted.Therefore,it is necessary to find a substitute for LIBs to meet the needs of future large-scale energy storage systems.Because of their competitiveness,low cost,and high safety,aqueous rechargeable zinc-ion batteries(ARZIBs)are regarded as promising components in the post-lithium-ion-battery era.Given the tunable composition,ordered porous channels,and controllable structure of metal-organic frameworks(MOFs)and covalent organic frameworks(COFs),these frameworks are viewed as potential materials for developing high-performance ARZIBs.In this review,we focus on the recent developments in the applications of MOF-/COF-based materials in ARZIBs,including in electrode materials,anode modifications,separators,and solid electrolytes.We then focus on the critical factors and optimization techniques of MOF-/COF-based materials that affect the performance of ARZIBs.Finally,we conclude with some projections for the expansion of ARZIBs containing MOF-/COF-based materials.

Imide-based covalent organic framework with excellent cyclability as an anode material for lithium-ion battery

Covalent organic frameworks(COFs)exhibiting reversible redox behaviors have been identified as promising candidates for constructing electrode materials in lithium-ion batteries(LIBs).However,their extensive application has been limited due to finite redox sites and poor structural stability.In this study,we design and synthesize a novel polyimide covalent organic framework(PI-COF)using the traditional solvothermal method and successfully apply it as an anode material for LIBs.The large conjugated structure of PI-COF accelerates charge transfer,while its large surface area provides more active sites,making PI-COF an attractive anode material for LIBs.Furthermore,the PI-COF anode material demonstrates high reversible specific capacity and excellent long-term cycling stability due to its COF characteristics.Specifically,the PI-COF electrodes deliver a specific capacity of 800 m Ah/g at a current density of 200 m A/g after 200 cycles,while a specific capacity of 450 m Ah/g at a current density of 1000 m A/g is sustained after 800 cycles.The outstanding lithium storage capacity,particularly the satisfactory long-term cycling stability,establishes PI-COF as a promising material for LIBs.

Universal synthesis of metallophthalocyanine covalent organic frameworks as ultra-sensitive multifaceted electrochemical sensor

The universal synthesis of highly stable covalent organic frameworks(COFs)for ultra-sensitive and multi-component electrochemical detection in different scenarios remains a great challenge.Herein,a series of metallophthalocyanine-based twodimensional(2D)dioxin(DXI)-linked metalophthalocyanine(MPc)-n DXI-COFs(M=Ni,Zn;n=1,2)are afforded in high yield(80%-96%)by a facile trace-quinoline assisted one-pot condensation of tetracarbonitrile precursors.Powder X-ray diffraction and electron microscopy investigations disclose their lamellar texture 2D network with AA stacking mode.Experiments and calculation results elucidate that the 2DXI-linked MPc-2DXI-COFs provide the stronger built-in electronic field and more electrostatic/hydrogen bonding adsorption sites than DXI-linked MPc-DXI-COFs,and the lower electrode reaction Gibbs free energy and stronger adsorption of analytes at Ni Pc than Zn Pc unit,which grants Ni Pc-2DXI-COF excellent sensing properties for various analytes including neurotransmitters,organic pollutants,and heavy metal ions,with high sensitivity and low detection limit of 0.53 to 25.66 nM.Especially in binary and ternary systems and even in real-world conditions,simultaneous multi-component detection could be achieved.

Donor-acceptor covalent organic frameworks-confined ultrafine bimetallic Pt-based nanoclusters for enhanced photocatalytic H_(2)generation

Photocatalytic hydrogen generation from hydrogen storage media is an effective and promising approach for the green hydrogen industry as well as for achieving carbon neutrality goals.However,the lower photocatalytic efficiency due to the limited light trapping capacity,low electron transfer rate,and severe aggregation of nanoparticles caused by high surface energy seriously restricts their practical application.Herein,we constructed a series of donor–acceptor(D–A)type covalent organic frameworks to confine ultrafine bimetallic Pt-based nanoclusters for photocatalytic hydrogen generation from ammonia borane(AB)hydrolysis.Under visible light irradiation at 20℃,PtCo_(2)@covalent organic framework(COF)showed the highest photocatalytic activity with a turnover frequency(TOF)of 486 min−1.Experiments and density functional theory(DFT)calculations reveal that the high catalytic activity is mainly attributed to the strong electronic interactions between D–A type COF and ultrafine PtCo_(2)nanoclusters.Specifically,the D–A type COF can significantly enhance the light-trapping ability by fine-tuning the electron-acceptor type in the framework,and accelerate the photogenerated electron transfer from D–A type COF to PtCo_(2)nanocluster,which promotes the adsorption and activation of H_(2)O and AB molecules and accelerates hydrogen release.Furthermore,PtCo_(2)@COF also exhibited ultra-high durability due to the significantly enhanced resistance to nanocluster aggregation caused by the nanopore confinement effect of D–A type COF.We believe that this work will provide a theoretical guide for the rational design of efficient D–A COFbased catalysts for photocatalysis.

Constructing energetic covalent organic frameworks with high stability and low sensitivity

Developing new functional explosives that display high stability,good energy performance,and low sensitivity are one of the key directions of energetic materials research.In this work,two-dimensional(2D)Schiff-based energetic covalent organic frameworks(COFs)are prepared based on triaminoguanidine salts with different anions as building blocks.Benefiting from the robust covalent bond in 2D extended polygons and strongπ-πinteractions in the eclipsed interlayers,the synthesized energetic COFs showed higher thermal stability and lower mechanical sensitivity than their precursor salts.More importantly,incorporating triaminoguanidine salts into COFs effectively increase the corrosion resistance to metal under high humidity conditions,which is due to the imine moieties in COFs functioning asπacceptors and offering strong bonding with metallic ions.This work provides a new pathway for the development of high-performance energetic materials.

Polyimide covalent organic frameworks as efficient solid-state Li^(+) electrolytes

Covalent organic frameworks(COFs) are attractive porous crystalline materials with extremely high stability, easy functionalization, and open channels, which are expected to be unique ion conductors/transporters in lithium ion batteries(LIBs). Despite recent advances, low ion conductivity and low transference number, resulting in low charging/discharging rate, low energy density, and short battery life, are the main issues that limit their direct application as solid electrolytes in LIBs. Here, we designed and synthesized a novel polyimide COF, namely, TAPA-PDI-COF, with abundant C=O groups, which has been successfully employed as high-performance solid electrolytes by doping TAPA-PDI-COF and succinonitrile(SN). Both the well-defined nanochannels of COFs and SN confined in the well-aligned channels restricted the free migration of anions, while C=O on COFs and CN groups of SN enhanced Li^(+) transport, thus achieving a high ion conductivity of 0.102 m S cm^(-1)at 80 °C and a high lithium-ion transference number of 0.855 at room temperature. According to density functional theory(DFT)calculations, Li-ion migration mainly adopted in-plane transport rather than the axial pathway, which may be due to the shorter hopping distances in the planar pathway. The results suggest an effective strategy for the design and development of all-solidstate ionic conductors for achieving high-performance LIBs.

Dual polarization strategy to enhance CH_(4) uptake in covalent organic frameworks for coal-bed methane purification

The purification of low-grade coal-bed methane is extremely important,but challenging,due to the very similar physical properties of CH_(4)and N2.Herein,we proposed a dual polarization strategy by employing triazine and polyfluoride sites to construct polar pores in COF materials,achieving the efficient separa-tion of CH_(4)from N2.As expected,the dual polarized F-CTF-1 and F-CTF-2 exhibit higher CH_(4)adsorption capacity and CH_(4)/N_(2)selectivity than CTF-1 and CTF-2,respectively.Especially,the CH4 uptake capacity and CH_(4)/N_(2)selectivity of F-CTF-2 is 1.76 and 1.42 times than that of CTF-2.This work not only developed promising COF materials for CH4/N_(2)separation,but also provided important guidance for the separation of other adsorbates with similar properties.

Functional decoration on a regenerable bifunctional porous covalent organic framework probe for rapid detection and adsorption of copper ions

Developing fluorescence porous probe for detecting and eliminating Cu^(2+) contamination in water or biosystem is an essential research project that has attracted considerable attention.However,improving the fluorescence detecting efficiency while enhancing the adsorption capacity of the porous probe is of great challenge.Herein,a bifunctional two-dimensional imine-based porous covalent organic framework(TTP-COF)probe was designed and synthesized from 1,3,5-tris(4-aminophenyl)benzene(TAPB)and 2,4,6-Triformylphloroglucinol(TP)ligand.TTP-COF displayed rapid detection of Cu^(2+)(limit of detection(LOD)=10 nmol·L^(−1) while achieving a high adsorption capacity of 214 mg·g^(−1)(pH=6)at room temperature with high reusability(>5 cycles).The key roles and contributions of highπ-conjugate and delocalized electrons in TABP and functional–OH groups in TP were proved.More importantly,the fluorescence quenching mechanism of TTP-COF was studied by density functional theory theoretical calculations,revealing the crucial role of intramolecular hydrogen bonds among C=N and–OH groups and the blocking of the excited state intramolecular proton transfer process in detecting process of Cu^(2+).

Flexible Covalent Organic Frameworks:Design,Synthesis,and Applications

Structural flexibility is an intriguing characteristic observed in materials that endows them with dynamic properties,allowing for intelligent adaptation and deformation in response to varying conditions.Covalent organic frameworks(COFs),a promising class of crystalline porous polymers,have garnered significant attention due to their unique features,including well-ordered,predesignable,and tunable structures,as well as their light-weighted nature and high thermal/chemical stability.Integrating flexibility into COFs offers a captivating pathway for developing advanced materials with dynamic properties and versatile functionalities.The flexible COF field is currently experiencing rapid expansion,and there is a growing need for systematic review articles that offer a comprehensive overview of the developments in this domain.In this mini-review,we delve into the factors that contribute to structural flexibility in COFs from the point of structural design.We summarize and categorize various modes of achieving flexibility across different dimensions,and discuss the potential applications of flexible COFs in vapor/gas adsorption,sensing,and smart membrane separations;further,we highlight their prospects and future research advancement.

Imidazole-Linked Fully Conjugated Covalent Organic Framework for High-Performance Sodium-Ion Battery

Covalent organic frameworks(COFs)have garnered increasing attention as promising electrode materials for sodium-ion batteries(SIBs)due to their ordered backbones,uniform pore sizes,and high surface areas.However,they also face challenges,including low capacity,unsatisfactory rate performance,and limited cycling stability,which pose the primary obstacles to their practical use.Herein,a pyrenoimidazole-based COF,denoted as PyIm-COF,has been synthesized as a novel electrode material for highperformance SIBs using an efficient synthetic strategy.With extended fully conjugated backbones and readily accessible redox-active sites,PyIm-COF demonstrates remarkable high-rate performance,delivering around 250 m Ah g^(−1)and excellent cycling stability,retaining 97.2%of its capacity even after 2500 cycles at 5 A g^(−1),which is higher than that of most previously reported COF-based SIBs.This work provides valuable insights into the development of nitrogen-rich conjugated COF electrode materials for rechargeable SIBs.