共价有机框架材料在多相催化领域的研究进展（英文）

共价有机框架(COFs)材料是近年来在拓扑学基础上发展起来的一类新型有机多孔聚合物,是有机单体通过可逆共价键连接而形成的晶型多孔材料,具有拓扑结构"可设计"、比表面积大、结构规整、孔道均一、孔径可调节以及易于修饰和功能化等优点.与金属有机框架材料(MOFs)相比,由于COFs是以共价键连接形成空间网络结构,具有较好的热稳定性和化学稳定性,又被称为"有机分子筛".COFs的构筑单体为有机小分子,有机小分子来源广泛而且种类繁多,使得构筑单体多样化,便于通过构筑单体来调控目标材料的结构和功能.自2005年首次报道以来,COFs以其独特的结构和优越的性能,吸引了广大科研工作者的极大兴趣,对其结构设计、可控合成、结构解析以及功能探索成为了研究热点,在气体吸附与分离、光电材料等领域展现出了广阔的应用前景.特别是在催化领域,由于COFs材料的多孔性、敞开的孔道结构、良好的稳定性以及易于修饰的特点,采用COFs作为催化剂以及催化剂载体受到了人们普遍的关注.作为催化剂,COFs可分为本征型催化剂和负载型催化剂.本征型催化剂的设计方法是基于"自下而上"策略将催化活性中心嵌入材料骨架之中;负载型催化剂的设计方法是以COFs为载体,通过后修饰方式负载金属颗粒或离子来构建多相催化剂.本征型COFs催化剂是在分子水平上引入催化活性中心,具有活性位点均匀分散、数量可控的特点,而且COFs规整均一的孔道结构有利于底物的传质,也为择形催化提供了可能;负载型催化剂通过后修饰方式引入催化活性中心,由于COFs以共价键连接,催化剂稳定性较高.COFs载体具有较大的比表面积,使得催化活性位点分散性好,也有利于底物与催化活性位点的结合.本文综述了COFs作为多相催化剂在催化领域的发展状况,按照COFs引入催化活性位点的类别,如单催化位点、双催化位点以及负载的金属纳米粒子进行了细致的阐述,重点讨论了COFs催化剂的设计理念、制备方式、功能化策略、材料的稳定性、催化活性以及选择性等内容.此外,对COFs作为光催化剂以及电催化剂方面的研究也进行了详细的介绍.最后,我们讨论了COFs在未来催化领域所面临的问题及挑战,并展望了COFs在超分子催化以及酶催化等方面的应用前景.     

Sophisticated Design of Covalent Organic Frameworks with Controllable Bimetallic Docking for a Cascade Reaction

Precise control of the number and position of the catalytic metal ions in heterogeneous catalysts remains a big challenge. Here we synthesized a series of two‐dimensional (2D) covalent organic frameworks (COFs) containing two different types of nitrogen ligands, namely imine and bipyridine, with controllable contents. For the first time, the selective coordination of the two nitrogen ligands of the 2D COFs to two different metal complexes, chloro(1,5‐cyclooctadiene)rhodium(I) (Rh(COD)Cl) and palladium(II) acetate (Pd(OAc)₂), has been realized using a programmed synthetic procedure. The bimetallically docked COFs showed excellent catalytic activity in a one‐pot addition–oxidation cascade reaction. The high surface area, controllable metal‐loading content, and predesigned active sites make them ideal candidates for their use as heterogeneous catalysts in a wide range of chemical reactions.     

Unveiling the Local Structure of Palladium Loaded into Imine‐Linked Layered Covalent Organic Frameworks for Cross‐Coupling Catalysis

Layered covalent organic frameworks (2D‐COFs), composed of reversible imine linkages and accessible pores, offer versatility for chemical modifications towards the development of catalytic materials. Nitrogen‐enriched COFs are good candidates for binding Pd species. Understanding the local structure of reacting Pd sites bonded to the COF pores is key to rationalize interactions between active sites and porous surfaces. By combining advanced synchrotron characterization methods with periodic computational DFT modeling, the precise atomic structure of catalytic Pd sites attached to local defects is resolved within an archetypical imine‐linked 2D‐COF. This material was synthesized using an in situ method as a gel, under which imine hydrolysis and metalation reactions are coupled. Local defects formed in situ within imine‐linked 2D‐COF materials are highly reactive towards Pd metalation, resulting in active materials for Suzuki–Miyaura cross‐coupling reactions.     

Tetrazole-functionalized two-dimensional covalent organic frameworks coordinated with metal ions for electrocatalytic oxygen evolution reaction

The search for functionalized covalent organic framework (COF) materials is significant on account of their great promise for frontline applications in various fields. Herein, a novel and convenient tactic is developed to design and fabricate the tetrazole-functionalized COF materials with abundant nitrogen atoms, which can provide active sites, facilitating the incorporation of COFs with metal ions. In particular, a β-ketoenamine-linked COF named COF-TpDb is selected as precursor for postsynthetic modification to introduce the tetrazole moieties to coordinate with metal ions cobalt (Co²⁺) and palladium (Pd²⁺), giving two functional metal-coordinated COFs complexes COF-TpDb-TZ-Co and COF-TpDb-TZ-Pd. The resultant COF-TpDb-TZ-Co displays a higher oxygen evolution reaction activity with a lower overpotential of 390 mV at a current density of 10 mA cm^?2, which is much enhanced compared with COF-TpDb-TZ. The tactic for the fabrication of tetrazole-functionalized COFs with abundant nitrogen atoms implements rational design for the construction of functional COFs and expands the promising application of metal-coordinated COF materials in electrocatalysis.     

Three-Dimensional Covalent Organic Frameworks: From Synthesis to Applications

Three-dimensional covalent organic frameworks (3D COFs) with spatially periodic networks demonstrate significant advantages over their 2D counterparts, including enhanced specific surface areas, interconnected channels, and more sufficiently exposed active sites. Nevertheless, research on these materials has met an impasse due to serious problems in crystallization and stability, which must be solved for practical applications. In this Minireview, we first summarize some strategies for preparing functional 3D COFs, including crystallization techniques and functionalization methods. Hereafter, applications of these functional materials are presented, covering adsorption, separation, catalysis, fluorescence, sensing, and batteries. Finally, the future challenges and perspectives for the development of 3D COFs are discussed.     

Reconstruction of Covalent Organic Frameworks by Dynamic Equilibrium

Covalent organic frameworks (COFs) are periodic two‐ or three‐dimensional polymeric networks with high surface areas, low density, and designed structures. Because COFs are normally prepared based on reversible formation of covalent bonds with relatively weak stability, their structures can be easily broken or damaged due to changes in the surrounding environment. Thus, developing strategies to realize the reconstruction of COFs in order to extend their usage lifetime is crucial for practical applications. In addition, exploring the kinetics of COF growth under varied reaction conditions is important for better understanding the nucleation and growth processes of COFs. In this work, the reformation mechanism of an imine‐based COF using an ex situ characterization method was investigated, disclosing an interesting COF reconstruction progress from disorder to order. The present study shows the regeneration ability of COFs, and the developed method could be generalized for broader use in the field.     

Record Ultralarge-Pores,Low Density Three-Dimensional Covalent Organic Framework for Controlled Drug Delivery**

The unique structural characteristics of three-dimensional (3D) covalent organic frameworks (COFs) like high surface areas, interconnected pore system and readily accessible active sites render them promising platforms for a wide set of functional applications. Albeit promising, the reticular construction of 3D COFs with large pores is a very demanding task owing to the formation of interpenetrated frameworks. Herein we report the designed synthesis of a 3D non-interpenetrated stp net COF, namely TUS-64, with the largest pore size of all 3D COFs (47 Å) and record-low density (0.106 g cm⁻³) by reticulating a 6-connected triptycene-based linker with a 4-connected porphyrin-based linker. Characterized with a highly interconnected mesoporous scaffold and good stability, TUS-64 shows efficient drug loading and controlled release for five different drugs in simulated body fluid environment, demonstrating the competency of TUS-64 as drug nanocarriers.     

Radical Covalent Organic Frameworks: A General Strategy to Immobilize Open‐Accessible Polyradicals for High‐Performance Capacitive Energy Storage

Ordered π‐columns and open nanochannels found in covalent organic frameworks (COFs) could render them able to store electric energy. However, the synthetic difficulty in achieving redox‐active skeletons has thus far restricted their potential for energy storage. A general strategy is presented for converting a conventional COF into an outstanding platform for energy storage through post‐synthetic functionalization with organic radicals. The radical frameworks with openly accessible polyradicals immobilized on the pore walls undergo rapid and reversible redox reactions, leading to capacitive energy storage with high capacitance, high‐rate kinetics, and robust cycle stability. The results suggest that channel‐wall functional engineering with redox‐active species will be a facile and versatile strategy to explore COFs for energy storage.     

Recent Advances of Covalent Organic Frameworks in Chemical Sensing

With the rapid development of reticular chemistry, an increasing number of covalent organic frameworks(COFs) have been designed and synthesized over the past decades. Owing to the large surface areas, numerous active sites, and high chemical stability, recent effects gradually were made to investigate the interaction with various small molecules. Among the reported application areas, sensorics is an attractive field, where COFs have exhibited tremendous potential and acquired high- performance sensitivity and selectivity due to their structural merits. In this review, we highlighted the recent progress of COFs as sensors for the detection of various analytes, mainly depending on the analysis of change of fluorescence signals. The basic principles of physics for fluorescence-based sensors were briefly discussed for better understanding of the relationship between structures and functions of COFs. Moreover, we reviewed various classes of small molecule analytes that have been successfully detected by COFs, including explosives, gases, humidity, metal ions, pH, and biological molecules. In this work, we detailedly discussed the components of COFs, functional sites, and sensing performance in each sensing application, aiming to disclose their intrinsic connection. This review also concluded with several issues to be solved and provided the outlook for the future development direction for practical applications.     

Efficient gaseous iodine capture enhanced by charge-induced effect of covalent organic frameworks with dense tertiary-amine nodes

Based on the outstanding application advantages of nitrogen-rich materials with regular porous frameworks in the capture of gaseous radioactive iodine, a series of covalent organic frameworks (COFs) with dual channels and abundant tertiary-amine active sites were constructed herein via a unique multi-nitrogen node design. The high density of up-to-six nitrogen adsorption sites in a single structural unit of the products effectively improved the adsorption capacities of the materials for iodine. Moreover, the adsorption affinity of the active sites can be further regulated by charge-induced effect of different electron-donating groups introduced into the COFs. Adsorption experiments combined with DFT theoretical calculations confirmed that the introduction of electron-donating groups can effectively increase the electron density around the active sites and enhance the binding energy between the materials and iodine, and thus improve the iodine adsorption capacity to 5.54 g/g. The construction strategy of multi-nitrogen node and charge-induced effect proposed in this study provides an important guidance for the study of the structure-activity relationship of functional materials and the design and preparation of high-performance iodine adsorption materials.     

Dual Microenvironment Modulation of Pd Nanoparticles in Covalent Organic Frameworks for Semihydrogenation of Alkynes

The chemical microenvironment modulation of metal nanoparticles (NPs) holds promise for tackling the long-lasting challenge of the trade-off effect between activity and selectivity in catalysis. Herein, ultrafine PdCu₂ NPs incorporated into covalent organic frameworks (COFs) with diverse groups on their pore walls have been fabricated for the semihydrogenation of alkynes. The Cu species, as the primary microenvironment of Pd active sites, greatly improves the selectivity. The functional groups as the secondary microenvironment around PdCu₂ NPs effectively regulate the activity, in which PdCu₂ NPs encapsulated in the COF bearing −CH₃ groups exhibit the highest activity with >99 % conversion and 97 % selectivity. Both experimental and calculation results suggest that the functional group affects the electron-donating ability of the COFs, which successively impacts the charge transfer between COFs and Pd sites, giving rise to a modulated Pd electronic state and excellent catalytic performance.     

Divergent Synthesis of Chiral Covalent Organic Frameworks

Featuring the simultaneous generation of a library of compounds from a certain intermediate, divergent synthesis has found increasing applications in the construction of natural products and potential medicines. Inspired by this approach, presented herein is a general strategy to introduce functionality, in a divergent manner, into covalent organic frameworks (COFs). This modular protocol includes two stages of covalent assembly, through which functional COFs can be constructed by a three‐step transformation of a key platform molecule, such as 4,7‐dibromo‐2‐chloro‐1H‐benzo[d]imidazole (DBCBI). Constructed herein are four types of chiral COFs (CCOFs) from DBCBI by nucleophilic substitution, Suzuki coupling, and imine formation. The unique array of eight isoframework CCOFs allowed investigation of their catalytic performance and structure–activity relationship in an asymmetric amination reaction.     

Covalent Organic Framework Materials for Photo/ Electrocatalytic Carbon Dioxide Reduction北大核心CSCD

Covalent organic frameworks （COFs）are a class of emerging materials connected by covalent bonds,which have high thermal/chemical stability （except boric acid COFs）,permanent porosity,large specific surface area and good crystallinity. In addition,the structure of the monomer unit in COFs is adjustable and can coordinate with many transition metal ions to provide catalytic active sites. These advantages make COFs helpful to catalyze various reactions. Among them,COFs have an excellent catalytic effect on the CO2 reduction reaction（CO2 RR）. This is mainly because the adjustable pore structure of COFs allows them to adsorb a large amount of CO2 and the π-π stacking structure in COFs can promote charge transfer, which can greatly improve the efficiency of CO2 reduction. COFs can be used as photo/ electrocatalysts to efficiently reduce CO2 to CO,CH4 ,HCOOH and other products. This review discusses the important achievements of CO2 RR catalyzed by COFs, including photo/electrocatalytic CO2 RR and photoelectric coupling CO2 RR. In addition,the future development of COFs as CO2 RR catalysts is also prospected. © 2022, Science Press (China). All rights reserved.     

Two‐Dimensional Tetrathiafulvalene Covalent Organic Frameworks: Towards Latticed Conductive Organic Salts

The construction of a new class of covalent TTF lattice by integrating TTF units into two‐dimensional covalent organic frameworks (2D COFs) is reported. We explored a general strategy based on the C₂+C₂ topological diagram and applied to the synthesis of microporous and mesoporous TTF COFs. Structural resolutions revealed that both COFs consist of layered lattices with periodic TTF columns and tetragonal open nanochannels. The TTF columns offer predesigned pathways for high‐rate hole transport, predominate the HOMO and LUMO levels of the COFs, and are redox active to form organic salts that exhibit enhanced electric conductivity by several orders of magnitude. On the other hand, the linkers between the TTF units play a vital role in determining the carrier mobility and conductivity through the perturbation of 2D sheet conformation and interlayer distance. These results open a way towards designing a new type of TTF materials with stable and predesignable lattice structures for functional exploration.     

Linkage Microenvironment of Azoles-Related Covalent Organic Frameworks Precisely Regulates Photocatalytic Generation of Hydrogen Peroxide

Artificial H₂O₂ photosynthesis by covalent organic frameworks (COFs) photocatalysts is promising for wastewater treatment. The effect of linkage chemistry of COFs as functional basis to photoelectrochemical properties and photocatalysis remains a significant challenge. In this study, three kinds of azoles-linked COFs including thiazole-linked TZ-COF, oxazole-linked OZ-COF and imidazole-linked IZ-COF were successfully synthesized. More accessible channels of charge transfer were constructed in TZ-COF via the donor-π-acceptor structure between thiazole linkage and pyrene linker, leading to efficient suppression of photoexcited charge recombination. Density functional theory calculations support the experimental studies, demonstrating that the thiazole linkage is more favorable for the formation of *O₂ intermediate in H₂O₂ production than that of the oxazole and imidazole linkages. The real active sites in COFs located at the benzene ring fragment between pyrene unit and azole linkage.     

Ionothermal Synthesis of Imide‐Linked Covalent Organic Frameworks

Covalent organic frameworks (COFs) are an extensively studied class of porous materials, which distinguish themselves from other porous polymers in their crystallinity and high degree of modularity, enabling a wide range of applications. COFs are most commonly synthesized solvothermally, which is often a time‐consuming process and restricted to well‐soluble precursor molecules. Synthesis of polyimide‐linked COFs (PI‐COFs) is further complicated by the poor reversibility of the ring‐closing reaction under solvothermal conditions. Herein, we report the ionothermal synthesis of crystalline and porous PI‐COFs in zinc chloride and eutectic salt mixtures. This synthesis does not require soluble precursors and the reaction time is significantly reduced as compared to standard solvothermal synthesis methods. In addition to applying the synthesis to previously reported imide COFs, a new perylene‐based COF was also synthesized, which could not be obtained by the classical solvothermal route. In situ high‐temperature XRPD analysis hints to the formation of precursor–salt adducts as crystalline intermediates, which then react with each other to form the COF.     

Two‐Dimensional Covalent Organic Frameworks for Carbon Dioxide Capture through Channel‐Wall Functionalization

Ordered open channels found in two‐dimensional covalent organic frameworks (2D COFs) could enable them to adsorb carbon dioxide. However, the frameworks’ dense layer architecture results in low porosity that has thus far restricted their potential for carbon dioxide adsorption. Here we report a strategy for converting a conventional 2D COF into an outstanding platform for carbon dioxide capture through channel‐wall functionalization. The dense layer structure enables the dense integration of functional groups on the channel walls, creating a new version of COFs with high capacity, reusability, selectivity, and separation productivity for flue gas. These results suggest that channel‐wall functional engineering could be a facile and powerful strategy to develop 2D COFs for high‐performance gas storage and separation.     

2D and 3D Porphyrinic Covalent Organic Frameworks: The Influence of Dimensionality on Functionality

The construction of 2D and 3D covalent organic frameworks (COFs) from functional moieties for desired properties has gained much attention. However, the influence of COFs dimensionality on their functionalities, which can further assist in COF design, has never been explored. Now, by selecting designed precursors and topology diagrams, 2D and 3D porphyrinic COFs (2D‐PdPor‐COF and 3D‐PdPor‐COF) are synthesized. By model building and Rietveld refinement of powder X‐ray diffraction, 2D‐PdPor‐COF crystallizes as 2D sheets while 3D‐PdPor‐COF adopts a five‐fold interpenetrated pts topology. Interestingly, compared with 2D‐PdPor‐COF, 3D‐PdPor‐COF showed interesting properties, including 1) higher CO₂ adsorption capacity; 2) better photocatalytic performance; and 3) size‐selective photocatalysis. Based on this study, we believe that with the incorporation of functional moieties, the dimensionality of COFs can definitely influence their functionalities.     

Precision Construction of 2D Heteropore Covalent Organic Frameworks by a Multiple‐Linking‐Site Strategy

Integrating different kinds of pores into one covalent organic framework (COF) endows it with hierarchical porosity and thus generates a member of a new class of COFs, namely, heteropore COFs. Whereas the construction of COFs with homoporosity has already been well developed, the fabrication of heteropore COFs still faces great challenges. Although two strategies have recently been developed to successfully construct heteropore COFs from noncyclic building blocks, they suffer from the generation of COF isomers, which decreases the predictability and controllability of construction of this type of reticular materials. In this work, this drawback was overcome by a multiple‐linking‐site strategy that offers precision construction of heteropore COFs containing two kinds of hexagonal pores with different shapes and sizes. This strategy was developed by designing a building block in which double linking sites are introduced at each branch of a C₃‐symmetric skeleton, the most widely used scaffold to construct COFs with homogeneous porosity. This design provides a general way to precisely construct heteropore COFs without formation of isomers. Furthermore, the as‐prepared heteropore COFs have hollow‐spherical morphology, which has rarely been observed for COFs, and an uncommon staggered AB stacking was observed for the layers of the 2D heteropore COFs.     

二维共价有机框架材料中引入含氮配体用于络合金属催化

共价有机骨架材料(COF)也被称为“有机分子筛”,具有孔道结构开放有序、易于进行化学修饰改性、化学/热稳定性好等优点,是一种新型的有机聚合物多孔材料.近年来,以COF材料为催化剂载体负载金属化合物用于制备多相反应催化剂已经成为材料领域研究的热点,表现出高活性和高选择性.但是到目前为止,仍未找到简单有效地控制骨架中金属负载量和分散性的方法,这已成为该领域一个具有挑战性的课题.
　　本文以2,2’-联吡啶-5,5’-二甲醛作为其中一个结构基元,成功把联吡啶配体引入到二维材料中.除此之外,由于COF是以亚胺键联接构筑形成的,因此框架中同时存在联吡啶和亚胺键两种含氮配体.我们通过红外光谱、结构模拟、元素分析、热失重分析、透射电镜(TEM)、X-射线光电子能谱、电感耦合等离子体色谱等手段详细表征了所制备的二维共价有机框架材料对醋酸钯(Pd(OAc)2)分子的络合负载行为.
　　研究发现,联吡啶和亚胺键均可参与配位Pd(OAc)2,与亚胺键配位的Pd(OAc)2分布于框架的层与层之间,而与联吡啶配位的则部分占据了框架的孔道,导致孔径减小.另外,由于框架中的联吡啶配体含量可通过加入2,2’-联吡啶-5,5’-二甲醛含量的变化实现线性调控,因此也可调节与其配位的Pd(OAc)2含量,其负载量可控制在14.3–18.7 wt%,是目前已报道的二维COF中的最高值;另外, COF材料中调控金属负载量尚未见报道. TEM结果显示,负载在框架中的催化剂分子没有发生团聚,框架的孔道仍处于开放状态,因而反应底物可以自由地出入一维孔道并与络合的催化剂充分接触.另外,由于催化剂在框架内部可以达到分子级别的分散,而且其负载量和负载位置都易于控制,因而对有机反应表现出了优异的催化性能.
　　我们尝试了以不同Pd负载量的COF为多相催化剂催化Heck反应.结果发现, Pd(II)@75%BPy COF(Pd负载量为最高值18.7 wt%)的催化活性最高,对不同底物均表现出优异的催化性能,产率达73–96%,反应速率遵循一级动力学曲线.且催化剂经多次循环利用仍能保持高活性,框架的有序结构也未被破坏,因此该材料有望用于各种类型优异的多相反应催化剂.