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.

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.

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.

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+).

Covalent Organic Framework with Predesigned Single-Ion Traps for Highly Efficient Palladium Recovery from Wastes

The recovery of palladium from waste streams is of importance for metal recycling and environmental remediation.Herein,we present a“single-ion trap”strategy for efficiently recovering Pd(II)from superacidic solutions and laboratory wastes.This was realized by rational design and synthesis of an antiparallel stacked covalent organic framework(ACOF)with hydrazine-carbonyl sites and pyridine sites for cooperative Pd(II)capture.The single-ion traps provided Lewis base sites with a high Pd(II)binding affinity,enabling the trapping of Pd(II)ions under a wide range of conditions.The developed ACOF-1 adsorbent demonstrated fast kinetics,excellent selectivity,and a high adsorption capacity of 412.9±14.2 mg/g for Pd(II)in a 3M HNO_(3) solution.When applied in a packed column,ACOF-1 dynamically captured Pd(II)from3M HNO_(3) solutions or laboratorywastes containing trace amounts of palladium and many other metals,realizing extraction efficiencies of 232.9 and 320.9 mg/g,respectively.Detailed experimental and theoretical studies revealed that the single-ion traps offered exceptionally strong binding of Pd(II)under both acidic and high ionic strength conditions,enabling selective adsorptive behavior not accessible using traditional adsorbents.Importantly,the general design strategy reported here could be used to create porous adsorbents for the capture of other precious metals.

Tribochemical synthesis of functionalized covalent organic frameworks for anti-wear and friction reduction

Tribochemistry can be defined as a field dealing with the chemical reactions occurring in the friction zone,capable of catalyzing mechanical and physico-chemical changes in the friction contact area,facilitating the formation of tribo-films,which is also an efficient approach to fabricate novel innovative materials.In this paper,we report the successful synthesis of the silicon oil(SO)-functionalized covalent organic frameworks(COFs)prepared via the tribochemical method when subjected to the reciprocating friction;during the friction process,the rich aldehyde-terminated COFs can bond with amino SO via the Schiff base reaction between aldehyde group and amino group to obtain the desired functionalized COFs(SO@COF-LZU1).The tribochemical reaction progress was tracked through in-situ monitoring of the friction coefficient and the operating conditions during the entire friction process.Noticeably,the friction coefficient continued to decrease until it finally stabilized as the reaction progressed,which revealed the formation of a protective tribo-film.Herein,an approximate tribochemical model was presented,wherein the reaction mechanism was investigated and analyzed by employing structural analysis techniques like magic angle spinning nuclear magnetic resonance(MAS NMR),transmission electron microscopy(TEM),and X-ray photoelectron spectroscopy(XPS).Furthermore,the tribochemical-induced SO@COF-LZU1 exhibited remarkable tribological performance with a low friction coefficient of 0.1 and 95.5%reduction in wear volume when used as additives of 500SN base oil.The prime focus of our research was on the preparation and functionalization of COF materials via tribochemical reactions,unraveling a new avenue for the rational design and preparation of functional materials.

Designing energetic covalent organic frameworks for stabilizing high-energy compounds

As an emerging high-energy compound,3-nitro-1,2,4-triazol-5-one(NTO)is used in military explosives and rocket propellants.However,the strong acidic corrosion of NTO,and the high sensitivity and poor thermostability of its salts,severely restrict their practical applications.Therefore,a novel strategy to design and construct energetic covalent organic frameworks(COFs)is proposed in this study.We have successfully prepared a two-dimensional crystalline energetic COF(named ECOF-1)assembled from triaminoguanidine salt,in which NTO anions are trapped in the porous framework via the ionic interaction and hydrogen bonds.The results show that ECOF-1 exhibits superior thermal stability than energetic salt of NTO.It also exhibits insensitivity and excellent heat of detonation of 7,971.71 kJ·kg−1.ECOF-1 greatly inhibits the corrosiveness of NTO.In prospect,energetic COFs are promising as a functional platform to design high-energy and insensitive energetic materials.

Substrate orientation effect in covalent organic frameworks/2D materials heterostructure by high-resolution atomic force microscopy

Heterostructures based on covalent organic frameworks(COFs)and other two-dimensional(2D)materials attract considerable attention due to their extraordinary properties and tremendous application potential.Substrate effects play a crucial role in the integration of ultrathin COF films onto 2D materials through direct polymerization.In this study,highly ordered monolayer COFs were successfully constructed on the surfaces of highly oriented pyrolytic graphite(HOPG),hexagonal boron nitride(hBN),and molybdenum disulfide(MoS_(2)).High-resolution atomic force microscopy(HR-AFM)imaging clearly reveals the substrate orientation effect in COFs/2D materials heterostructure.Honeycomb networks formed via Schiff-base reaction and boronic acid condensation reaction can epitaxially grow in specific orientations relative to the underlying substrate lattices.This work provides direct evidence for substrate effects in the on-surface synthesis of COFs and paves the way for further investigation into the intrinsic electronic properties of monolayer COFs and the development of multifunctional hybrid devices.

Morphology Engineering for Covalent Organic Frameworks (COFs) by Surfactant Mediation and Acid Adjustment

Two-dimensional covalent organic frameworks(COFs)with specific morphologies including nanofibers and nanoplates are highly desired in both nanoscience research and practical applications.Thus far,however,morphology engineering for COFs remains challenging because the mechanism underlying the morphology formation and evolution of COFs is not well understood.Herein,we propose a strategy of surfactant mediation coupled with acid adjustment to engineer the morphology of aβ-ketoenamine-linked COF,TpPa,during solvothermal synthesis.The surfactants function as stabilizers that can encapsulate monomers and prepolymers to create micelles,enabling the formation of fiber-like and plate-like morphologies of TpPa rather than irregularly shaped aggregates.It is also found that acetic acid is important in regulating such morphologies,as the amino groups inside the prepolymers can be precisely protonated by acid adjustment,leading to an inhibited ripening process for the creation of specific morphologies.Benefitting from the synergistic enhancement of surfactant mediation and acid adjustment,TpPa nanofibers with a diameter down to~20 nm along with a length of up to a few microns and TpPa nanoplates with a thickness of~18 nm are created.Our work sheds light on the mechanism underlying the morphology formation and evolution of TpPa,providing some guidance for exquisite control over the growth of COFs,which is of great significance for their practical applications.

Combining metal-organic frameworks(MOFs)and covalentorganic frameworks(COFs):Emerging opportunities for new materials and applications

In the past decades,metal-organic frameworks(MOFs)and covalent organic frameworks(COFs)basically enjoy the coordination chemistry and covalent chemistry,respectively,and such uniqueness has become the major obstacle hampering their further scope diversity and application multi-functionalization.Inspired from the principle of organic retrosynthesis,combining coordination bond and covalent bond together offers additional opportunities for constructing novel MOFs,COFs and MOF@COF hybrids as well as confer on them superior performances in versatile application fields.In this review,we firstly classify and summarize the recently reported synthesis strategies based on the integration of metal-ligand coordination and dynamic covalent bonds.Then,the application performances of as-constructed MOFs,COFs as well as MOF@COF hybrids are discussed and highlighted in the fields of adsorption,separation,catalysis,biosensing,energy storage and so on.Last,our personal insights of the remaining challenges and further prospects are also provided,in order to trigger much more inspirations and endeavors for this hot research field.

Covalent Organic Frameworks(COFs) for Sequestration of ^(99)TcO_(4)^(–)

Covalent organic frameworks(COFs),as a class of crystalline porous materials with periodic lattices and porous structures,have received extensive attention in the fields of gas storage and separation,energy storage,catalysis and optoelectronics and so on.However,COFs are still in their infancy in the field of nuclear waste treatment,especially for sequestration of long-live problematic radionuclides,such as 99Tc.Battle of decontamination of pertechnetate(TcO4–),a main existence of 99Tc under aerobic environments,is far from finished.In this review,recent progresses of COFs and some relative materials in the sequestration of pertechnetate,and perspective on surmounting the unmet issues are elucidated.

2D/2D covalent organic framework/CdS Z-scheme heterojunction for enhanced photocatalytic H_(2) evolution:Insights into interfacial charge transfer mechanism

Covalent organic frameworks(COFs)with high crystallinity and flexible designability have been consid-ered as promising candidates for photocatalytic hydrogen evolution.However,the existence of unpropi-tious exciton effects in COFs leads to poor charge separation,and thus results in low photocatalytic effi-ciency.Herein,to improve the photoelectron migration efficiency,we designed a 2D/2D organic/inorganic direct Z-scheme COF-based heterojunction(TpTAP/CdS),by the in-situ growing of CdS nanosheets on the COF copolymerized via 2,4,6-tris(4-aminophenyl)-1,3,5-triazine(TAP)and 1,3,5-triformylphloroglucinol(Tp).The femtosecond transient absorption(fs-TA)decay kinetics of TpTAP-COF and TpTAP/CdS further reveal the processes of shallow electron trapping and the recombination of the free photogenerated electron-hole pairs.In particular,the transient absorption traces for TpTAP-COF and TpTAP/CdS normal-ized to the photoinduced absorption peak can effectively verify the Z-scheme charge transfer between TpTAP-COF and CdS,which could enhance the charge mobility and separation,thus reducing the pho-tocorrosion of CdS.Additionally,ultraviolet photoelectron spectroscopy(UPS),in-situ X-ray photoelec-tron spectroscopy(XPS),transient photovoltage measurements,and electron spin resonance(ESR)spec-troscopy further confirm the establishment of the internal electric field(IEF).This work demonstrates the important role of COFs in the construction of 2D/2D organic/inorganic direct Z-scheme heterojunctions and offers a new avenue to explain the criticality of dynamics of the photogenerated carriers for the construction of Z-scheme heterojunctions.

共价有机骨架化合物(COFs)储氢材料研究进展

简要介绍了二维和三维COFs的结构,重点介绍了COFs作为储氢材料的研究现状和提高其储氢性能的改性方法,并对COFs在储氢方面存在的不足和未来的研究方向作出了总结与展望。

共价有机骨架聚合物(COFs)的研究进展

共价有机骨架聚合物(COFs)是一类结晶微孔聚合物,具有优异的孔性质、高的热及化学稳定性和大的比表面积,在气体储存、催化、光电材料等诸多领域中有重要的应用前景,已成为国内外的研究热点。简要介绍了共价有机骨架聚合物的合成方法,最后对该领域未来发展趋势进行了展望。

Superhigh intrinsic proton conductivity in densely carboxylic covalent organic framework

Herein,we developed for the first time two carboxylic acid based intrinsic proton conductors(COOHCOF-1 and COOH-COF-2)via pre-assembly approach.The obtained COOH-COF-1 and COOH-COF-2 not only show outstanding chemical and thermal stabilities,but also exhibit superhigh intrinsic proton conductive behaviors.Especially,the intrinsic proton conductivity of COOH-COF-2 is up to 2.6×10^(−3) S/cm at 353 K and 98%RH,which is the highest value among all the reported acid functionalized COFs.This work lights up the way for the rational design of functional COFs with remarkably intrinsic proton conducting performance and related practical applications.

基于Pd/COF-LZU1非标记型C-反应蛋白免疫传感器的研制

采用溶剂热法,通过有机单体合成了一种亚胺键连接的共价有机框架材料（COF-LZU1）;在常温常压条件下,通过后合成的方法将贵金属钯（Ⅱ）引入到COF材料中,合成了复合材料Pd/COF-LZU1,该材料具有优良的催化性能.利用Pd/COF-LZU1多孔复合材料将C-反应蛋白（CRP）抗体（anti-CRP）固定在玻碳电极表面,构建了一种非标记型CRP免疫传感器.当抗体与抗原发生免疫反应时,形成的免疫复合物会阻碍电化学探针[Fe（CN）6]^4-/3-的电子传递,降低其响应电流,从而实现CRP的快速检测.采用交流阻抗和差示脉冲伏安法（DPV）考察了免疫传感器的电化学特性,同时考察了测试底液的p H值、抗原培育时间和抗体固定浓度等实验条件对传感器性能的影响.在最优的实验条件下,采用DPV法对CRP进行检测的线性范围为5-180 ng/m L,检出限为1.66 ng/m L,线性相关系数为0.992.

以Pt/COF-LZUI为固定基底和Pd NPs/MnO_2为标记物的C反应蛋白免疫传感器的研制

采用溶剂热法合成了共价有机框架材料,并使铂(Ⅳ)通过电化学还原沉积于此材料上,制成作为固定化基质的Pt/COF-LZUI。另据文献分别制备了MnO_2纳米材料,Pd NPs/MnO_2纳米复合材料,及以此为标记物的CRP抗体。用上述材料按规定程序修饰玻碳电极并制成夹心型CRP免疫传感器。利用固体核磁、X射线粉末衍射对COF-LZUI的结构和晶型结构进行了表征,利用透射电镜对COF-LZUl、Pt/COF-LZUl和Pd NPs/MnO_2纳米复合材料的形貌进行了表征。采用循环伏安法和计时电流法(i-t)研究该传感器的电化学特性及该电极对过氧化氢的电化学响应。该传感器的线性范围为1~150μg·L-1,检出限(3σ)为0.33μg·L^(-1)。

Cobalt-containing covalent organic frameworks for visible light-driven hydrogen evolution

Covalent organic frameworks(COFs) have recently emerged as a new class of photocatalysts.However,integrated design is crucial to maximizing the performance of COF-incorporating photocatalytic systems.Herein,we compare two strategies of installing earth-abundant metal-based catalytic centers into the matrice of a 2 D COF named NUS-55.Compared to NUS-55(Co)prepared from the post-synthetic metalation of coordination sites within the COF,the molecular co-catalyst impregnated NUS-55/[Co(bpy)3]Cl2 achieves a seven-fold improvement in visible light-driven H2 evolution rate to 2,480 μmol g^-1h^-1,with an apparent quantum efficiency(AQE) of 1.55% at 450 nm.Our results show that the rational design of molecular anchoring sites in COFs for the introduction of catalytic metal sites can be a viable strategy for the development of highly efficient photocatalysts with enhanced stability and photocatalytic activities.