Reaction Mechanisms of Single Metal Site Catalysts Supported on Covalent Organic Frameworks

Biofouling is a major obstacle to the efficient extraction of uranium from seawater due to the numerous marine microorganisms in the ocean. Herein, we report a novel amidoxime (AO) crystalline cova...

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

ConspectusSolar-driven CO2 reduction into value-added chemicals, such as CO, HCOOH, CH4, and C2+ products, has been regarded as a potential way to alleviate environmental pollution and the energy crisis. In the past decades, numerous pioneered homogeneous catalytic systems composed of soluble photosensitizers (PSs) and catalytic active sites (CASs) have been explored for CO2 photoreduction. Nevertheless, inefficient electron migration based on random collision between CASs and PSs in homogeneous catalytic systems usually causes mediocre performance. Moreover, the relatively poor separation/recycling capability of the homogeneous systems has inevitably reduced their reusability and practicality. The rational combination of PSs and CASs have been proven to play critical roles in the development of highly efficient heterogeneous catalysts to improve their performance, such as anchoring them onto the solid matrixes or connecting them through bridging ligands. However, developing effective assembly strategies to achieve the ordered orientation and uniform heterogenization of PSs and CASs remains a great challenge, mainly due to the lack of crystallinity heterogeneous transformation and structural tailoring ability of traditional solid catalysts. Moreover, due to the lack of assembly and synthesis strategies, many efficient homogeneous photocatalytic systems are still unable to achieve high crystallinity heterogeneous transformation.Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) have recently attracted broad interest toward CO2 photocatalysis because of their diverse precursors, well-defined and tailorable structures, abundant exposed CASs and high surface areas, etc. Especially, the highly ordered orientation and uniform combination of PSs and CASs in MOFs and COFs are beneficial for improved light harvesting and charge separation, greatly helping to address the aforementioned challenges. Moreover, the well-defined crystalline structures of MOFs and COFs facilitate the establishment of the structure-activity relationship. Therefore, it is increasingly important to summarize the integration of PSs and catalysts to provide deep insight into MOF/COF-based photocatalysts.In this Account, we summarize the ordered integration of PSs and CASs in MOFs and COFs for CO2 photoconversion and describe the structure-activity relationships to guide the design of effective catalysts. Given the unique structural features of MOFs and COFs, we have emphasized the integration of PSs and CASs to optimize their photocatalytic performance, including the confinement of catalytic active nanoparticles (NPs) into photosensitizing frameworks, co-coordination of PSs and CASs, and ligand-to-metal charge-transfer and anchoring CASs on the secondary building units of the photosensitizing frameworks. The catalytic activity, selectivity, sacrificial agent, and stability of these systems were then discussed. More importantly, MOFs and COFs provide powerful platforms to understand the key steps for boosting CO2 photoreduction and exploring the catalytic mechanism, involving light harvesting, electron-hole separation/migration, and surface redox reactions. Finally, the perspective and challenge of CO2 photoreduction in MOF/COF platforms are further proposed and discussed. It is expected that this Account would provide deep insight into the integration of PSs and catalysts in COFs and MOFs with well-defined structures and afford significant inspiration toward enhanced performance in heterogeneous catalysis.

To achieve high-efficiency catalysts for CO2 reduction reaction, various catalytic metal centres and linker molecules have been assembled into covalent organic frameworks. The amine-linkages enhance the binding ability of CO2 molecules, and the ionic frameworks enable to improve the electronic conductivity and the charge transfer along the frameworks. However, directly synthesis of covalent organic frameworks with amine-linkages and ionic frameworks is hardly achieved due to the electrostatic repulsion and predicament for the strength of the linkage. Herein, we demonstrate covalent organic frameworks for CO2 reduction reaction by modulating the linkers and linkages of the template covalent organic framework to build the correlation between the catalytic performance and the structures of covalent organic frameworks. Through the double modifications, the CO2 binding ability and the electronic states are well tuned, resulting in controllable activity and selectivity for CO2 reduction reaction. Notably, the dual-functional covalent organic framework achieves high selectivity with a maximum CO Faradaic efficiency of 97.32% and the turnover frequencies value of 9922.68 h−1, which are higher than those of the base covalent organic framework and the single-modified covalent organic frameworks. Moreover, the theoretical calculations further reveal that the higher activity is attributed to the easier formation of immediate *CO from COOH*. This study provides insights into developing covalent organic frameworks for CO2 reduction reaction.

Determining the crystal structures of covalent organic frameworks (COFs) with atomic precision is pivotal for uncovering their properties and optimizing functionalities. However, the synthesis of high-quality single crystals of COFs suitable for X-ray diffraction analysis, especially chiral COFs (CCOFs), remains a formidable challenge. In this work, we report two three-dimensional (3D) CCOFs synthesized via imine condensation of tetrahedral tetraamine and tetraaldehydes derived from optically active 1,1'-biphenol phosphoryl chloride or thiophosphoryl chloride. Single crystals of varying sizes are obtained through either a low-temperature modulation strategy, yielding large crystals up to 100 μm, or a solvothermal method. The large single crystals are structurally characterized by single-crystal X-ray diffraction, achieving a resolution of 0.90 Å. These two CCOFs are isostructural and each features a 4-fold interpenetrated diamondoid open framework with all phosphoric acid groups periodically aligned within tubular helical channels, displaying enhanced Brønsted acidity compared to non-immobilized acids. The frameworks exhibit permanent porosity, chemical resistance in boiling water, 14 M NaOH, and 0.1 M HCl, and thermal stability up to 400 °C. Notably, these CCOFs serve as efficient and recyclable heterogeneous Brønsted acid catalysts in the asymmetric addition to aromatic aldehydes, enantioselective transfer hydrogenation of ketimines, and three-component direct asymmetric Mannich reactions involving aldimines and cyclic ketones, achieving good to high enantioselectivities (up to 99.5% ee) that surpass those obtained in analogous systems with homogeneous catalysts. This work represents the first successful demonstration of single-crystal structures of homochiral COFs, paving the way for in-depth investigations into structure-property relationships in enantioselective processes and facilitating the design of novel functional chiral organic materials.

Covalent organic frameworks (COFs), characterized by their unique ordered pore structures, chemical diversity, and high degree of designability, have demonstrated immense application potential across multiple fields. However, traditional synthesis methods often encounter challenges such as low crystallinity and uneven morphology. The introduction of surfactants has opened up new pathways for the synthesis of COFs. Leveraging their intermolecular interactions and self‐assembly properties, surfactants can effectively regulate the nucleation, growth processes, and ultimate structure and properties of COFs. This paper systematically reviews the latest research achievements and future trends in surfactant‐assisted COF synthesis, emphasizing the crucial role of surfactants as key additives in the preparation of COFs. Surfactants not only facilitate uniform nucleation and growth of COFs, enhancing the crystallinity and structural order of the products but also enable precise and diverse regulation of the dimensionality, morphology, and structure of COFs. Furthermore, by influencing the dispersion and processability of COFs, surfactants enhance their practicality and workability. Finally, the paper presents some prospects for the challenges and future opportunities in this emerging research area.

Covalent organic frameworks (COFs)have found wide applicationsdue to their crystalline structures. However, it is still challengingto quantify crystalline phases in a COF sample. This is because COFs,especially 2D ones, are usually obtained as mixtures of polycrystallinepowders. Therefore, the understanding of the aggregated structuresof 2D COFs is of significant importance for their efficient utilization.Here we report the study of the aggregated structures of 2D COFs using 13C solid-state nuclear magnetic resonance (13CSSNMR). We find that 13C SSNMR can distinguish differentaggregated structures in a 2D COF because COF layer stacking createsconfined spaces that enable intimate interactions between atoms/groupsfrom adjacent layers. Subsequently, the chemical environments of theseatoms/groups are changed compared with those of the nonstacking structures.Such a change in the chemical environment is significant enough tobe captured by 13C SSNMR. After analyzing four 2D COFs,we find it particularly useful for 13C SSNMR to quantitativelydistinguish the AA stacking structure from other aggregated structures.Additionally, 13C SSNMR data suggest the existence of offsetstacking structures in 2D COFs. These offset stacking structures arenot long-range-ordered and are eluded from X-ray-based detections,and thus they have not been reported before. In addition to the driedstate, the aggregated structures of solvated 2D COFs are also studiedby 13C SSNMR, showing that 2D COFs have different aggregatedstructures in dried versus solvated states. These results representthe first quantitative study on the aggregated structures of 2D COFs,deepen our understanding of the structures of 2D COFs, and furthertheir applications.

Design principles based on intramolecular interactions for hydroxyl-functionalized covalent organic frameworks

Bio-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extraction

Covalent Organic Frameworks (COFs) are a newly emerged class of porous materials consisting of organic building blocks linked by strong covalent bonds. The physical and chemical properties of COFs, i.e., modularity, porosity, well-developed specific surface area, crystallinity, and chemical-thermal stability, make them a good application material, especially in the aspects of adsorption and gas separation. The organic compositions of their building blocks also render them with biocompatible properties; therefore, they also have potential in biomedical applications. Depending on the symmetry of the building blocks, COF materials form two-dimensional (2D COF) or three-dimensional (3D COF) crystal structures. 3D COF structures have a higher specific surface area, they are much lighter due to their low density, and they have a larger volume than 2D COF crystals, but, unlike the latter, 3D COF crystals are less frequently obtained and studied. Selecting and obtaining suitable building blocks to form a stable 3D COF crystal structure is challenging and therefore of interest to the chemical community. Triptycene, due to its 3D structure, is a versatile building block for the synthesis of 3D COFs. Polymeric materials containing triptycene fragments show good thermal stability parameters and have a very well-developed surface area. They often tend to be characterized by more than one type of porosity and exhibit impressive gas adsorption properties. The introduction of a triptycene backbone into the structure of 3D COFs is a relatively new procedure, the results of which only began to be published in 2020. Triptycene-based 3D COFs show interesting physicochemical properties, i.e., high physical stability and high specific surface area. In addition, they have variable porosities with different pore diameters, capable of adsorbing both gases and large biological molecules. These promising parameters, guaranteed by the addition of a triptycene backbone to the 3D structure of COFs, may create new opportunities for the application of such materials in many industrial and biomedical areas. This review aims to draw attention to the symmetry of the building blocks used for COF synthesis. In particular, we discussed triptycene as a building block for the synthesis of 3D COFs and we present the latest results in this area.

Advances in Emerging Crystalline Porous Materials.

Over the past decade, covalent organic frameworks (COFs) have garnered significant attention as supporting materials for the immobilization of radical species, showing great promise in applications such as catalysis, energy storage, and dynamic nuclear polarization. While considerable progress has been made in developing monoradical COFs, the creation of COFs with embedded biradicals remains a substantial challenge. In this study, we present a novel pyridine N-oxide ylide COF, featuring an electron-withdrawing dicarboxamide group designed to facilitate the formation of a biradical COF (pyridine N-oxide biradical COF). This biradical generation occurs through a spontaneous intramolecular single-electron transfer process under ambient conditions. By integrating both electron-withdrawing and π-conjugated units into the pyridine ring, we enhance the stability and formation of biradical species. The electron paramagnetic resonance results demonstrate that the COF structure is pivotal in stabilizing and promoting biradical species formation. Further analyses, including Fourier-transform infrared, X-ray photoelectron, and 13C cross-polarization magic angle spinning nuclear magnetic resonance spectroscopies, confirm the coexistence of ylides and biradical species within the COF material. Additionally, the COF exhibits promising catalytic activity, serving as an efficient catalyst in the dehydrogenation of nitrogen heterocycles. This work bridges the gap between ylide COFs and biradical COFs, expanding our understanding of porous materials and their potential applications in advanced chemistry.

Covalent organic frameworks (COFs) have been a potential candidate for applications in photocatalysis due to its periodically porous structures and tunable structure. The COF skeletons consisted of different building blocks may result in different performance. Investigating the effects of different building blocks on energy levels and excitons for COF can provide some insight for designing excellent COF catalysts. Based on the first-principles many-body Green’s function theory, the electronic structures and optical properties of the three donor-acceptor COFs by employing the monomer 2,4,6-trimethyl-1,3,5-triazine (TMT) as the key acceptor subunit and the trigonal aldehyde monomers including the tris(4-formylphenyl) amine (TPA), 1,3,5-tris(4-formylphenyl) benzene (TFPB) and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) as the donor subunit are calculated in this work. Regulation of the donor unit and interlayer interactions on the electronic structures and excitonic properties are analyzed. The results show that the valence band maximum (VBM) and conduction band minimum (CBM) energies of the system are varied by the alteration of donor subunit. From TPA to the TFPB or TFPT, the bandgaps of the system increase, the light absorption blue shift, and the exciton binding energies gradually increase. There is little effect on the band gap and excitation energy by replacing the TFPB with the TFPT. Among the three COFs, the positions of both CBM and VBM of the TFPT-TMT COF only match well with the chemical reaction potential of H<sub>2</sub>/H<sup>+</sup> and O<sub>2</sub>/H<sub>2</sub>O, which is capable of photocatalytic overall water splitting. But the photocatalytic performance for the TFPT-TMT COF might be inhibited by the higher exciton binding energy. The exciton for the TPA-TMT COF is easier to separate according to the exciton distributions and the exciton binding energy. The effect of different building units on the electronic structure, excitation energy, and excitonic properties of COFs in monolayer COFs are in line with that in multilayer and bulk COFs. The variation of the energy levels and excitation energies of all the three COFs as the number of layers are consistent. With the increasing number of layers, the VBM and CBM shift up and down with respect to the vacuum level, respectively. The band gap gradually decreases. The energy tend to decrease slower with the more layer. The exciton energy for multilayer COFs is close to the bulk state. These results are significant to design and modify COFs.

Due to their high structural tunability, remarkable internal surface areas, readily accessible pore space, and host of possible applications, covalent organic frameworks (COFs) remain at the forefront of materials science research. Unfortunately, many COFs suffer from structural distortions or pore collapse during activation, which can lead to a substantial loss of crystallinity and functionality. Thus, herein, we demonstrate a facile method to address this issue by introducing polymer guests. The polymer adheres to the COF internal pore wall, acting as a supporting pillar during activation and effectively preserving the COF porosity and crystallinity. In fact, the surface area of one COF/polymer composite, known as TAPB-TA/PDA, was boosted by a factor of 16 when compared to the parent COF, TAPB-TA. More importantly, the now robust COF structure was able to resist layer shifting and order loss during both solvent immersion and removal. The introduction of functional polymer guests not only solidifies the COF structure and preserves its high porosity but is also shown to enhance the transport and separation of photogenerated charge carriers, thereby facilitating hydrogen evolution during photocatalytic water splitting. Molecular dynamics simulations further support experimental observations that the incorporation of PDA within the COF pores reinforces the walls, preventing its collapse. The proposed mechanism is based on the adsorption of PDA oligomers along the c direction of the unit cell, fastening the COF layers in place via van der Waals interactions. This kind of interaction locks -N═CH-Ph-CH═N- units in a trans-configuration in the COF pores.

Covalent organic frameworks (COFs) have received much attention in the biomedical area. However, little has been reported about stimuli-responsive COF for drug delivery. Herein, we synthesized a hypoxia-responsive azo bond-containing COF with nanoscale size and immobilized both photosensitizers chlorin e6 (Ce6) and hypoxia-activated drug tirapazamine (TPZ) into the COFs. When such a COF entered the hypoxic environment and tumor, the COF structure was ruptured and loaded drugs were released from the COF. Together, upon near-infrared (NIR) light irradiation, Ce6 consumed oxygen to produce cytotoxic reactive oxygen species, leading to elevated hypoxia. Such two-step hypoxia stimuli successively induced the deintegration of COF, drug release and activation of TPZ. This promoted the TPZ to generate massive biotoxic oxyradical. In vitro and in vivo evaluation indicated that this two-step hypoxia-activated COF drug delivery system could kill cancer cells and inhibit the growth of tumors effectively.