Heteropolyaromatic Covalent Organic Frameworks via One-Pot Multicomponent Reactions.

Multicomponent reactions (MCRs) offer a platform to create different chemical structures and linkages for highly stable covalent organic frameworks (COFs). As an illustrative example, the multicomponent Povarov reaction generates 2,4-phenylquinoline from aldehydes and amines in the presence of electron-rich alkenes. In this study, we introduce a new domino reaction to generate unprecedented 2,3-phenylquinoline COFs in the presence of epoxystyrene. This work thus presents, for the first time, structural isomeric COFs produced by multicomponent domino and Povarov reactions. Furthermore, 2,3-phenylquinolines can undergo a Scholl reaction to form extended aromatic linkages. With this approach, we synthesize two thermally and chemically stable MCR-COFs and two heteropolyaromatic COFs using both domino and in situ domino and Scholl reactions. The structure and properties of these COFs are compared with the corresponding 2,4-phenylquinoline-linked COF and imine-COF, and their activity toward benzene and cyclohexane sorption and separation is investigated. The position of the pendant phenyl groups within the COF pore plays a crucial role in facilitating the industrially important sorption and separation of benzene over cyclohexane. This study opens a new avenue to construct heteropolyaromatic COFs via MCR reactions.

Unveiling pH-Dependent Adsorption Strength of *CO2 - Intermediate over High-Density Sn Single Atom Catalyst for Acidic CO2-to-HCOOH Electroreduction.

The acidic electrochemical CO2 reduction reaction (CO2RR) for direct formic acid (HCOOH) production holds promise in meeting the carbon-neutral target, yet its performance is hindered by the competing hydrogen evolution reaction (HER). Understanding the adsorption strength of the key intermediates in acidic electrolyte is indispensable to favor CO2RR over HER. In this work, high-density Sn single atom catalysts (SACs) were prepared and used as catalyst, to reveal the pH-dependent adsorption strength and coverage of *CO2 - intermediatethat enables enhanced acidic CO2RR towards direct HCOOH production. At pH=3, Sn SACs could deliver a high Faradaic efficiency (90.8 %) of HCOOH formation and a corresponding partial current density up to -178.5 mA cm-2. The detailed in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic studies reveal that a favorable alkaline microenvironment for CO2RR to HCOOH is formed near the surface of Sn SACs, even in the acidic electrolyte. More importantly, the pH-dependent adsorption strength of *CO2 - intermediate is unravelled over the Sn SACs, which in turn affects the competition between HER and CO2RR in acidic electrolyte.

Integrating Bifunctionality and Chemical Stability in Covalent Organic Frameworks via One-Pot Multicomponent Reactions for Solar-Driven H2O2 Production.

Multicomponent reactions (MCRs) can be used to introduce different functionalities into highly stable covalent organic frameworks (COFs). In this work, the irreversible three-component Doebner reaction is utilized to synthesize four chemically stable quinoline-4-carboxylic acid DMCR-COFs (DMCR-1-3 and DMCR-1NH) equipped with an acid-base bifunctionality. These DMCR-COFs show superior photocatalytic H2O2 evolution (one of the most important industrial oxidants) compared to the imine COF analogue (Imine-1). This is achieved with sacrificial oxidants but also in pure water and under an oxygen or air atmosphere. Furthermore, the DMCR-COFs show high photostability, durability, and recyclability. MCR-COFs thus provide a viable materials' platform for solar to chemical energy conversion.

Construction of Covalent Organic Framework Nanofiber Membranes for Efficient Adsorption of Antibiotics.

Techniques beyond crystal engineering are critical for manufacturing covalent organic frameworks (COFs) and to explore them for advanced applications. However, COFs are normally obtained as insoluble, unmeltable, and thus nonprocessible microcrystalline powders. Therefore, it is a significant challenge to implement COFs into larger architectures and structural control on different length scales. Herein, a facile strategy is presented to prepare flexible COF nanofiber membranes by in-situ growth of COFs on polyacrylonitrile (PAN) nanofiber substrates via a reversible polycondensation-termination approach. The obtained PAN@COF nanofiber membranes with vertically aligned COF nanoplates combine a large functional surface with efficient mass transport, thus making it a promising adsorbent, for example, for water purification. The antibiotic pollutant ofloxacin (OFX) is removed from water with a superior absorption capacity of ≈236 mg g-1 and removal efficiency as high as 98%. The here presented in-situ growth of COFs on nanofiber membranes can be extended to various Schiff base-derived COF materials with different compositions, providing a highly efficient way to construct flexible COF-based membranes for several applications.

Programmable Photocatalytic Activity of Multicomponent Covalent Organic Frameworks Used as Metallaphotocatalysts.

The multicomponent approach allows to incorporate several functionalities into a single covalent organic framework (COF) and consequently allows the construction of bifunctional materials for cooperative catalysis. The well-defined structure of such multicomponent COFs is furthermore ideally suited for structure-activity relationship studies. We report a series of multicomponent COFs that contain acridine- and 2,2'-bipyridine linkers connected through 1,3,5-benzenetrialdehyde derivatives. The acridine motif is responsible for broad light absorption, while the bipyridine unit enables complexation of nickel catalysts. These features enable the usage of the framework materials as catalysts for light-mediated carbon-heteroatom cross-couplings. Variation of the node units shows that the catalytic activity correlates to the keto-enamine tautomer isomerism. This allows switching between high charge-carrier mobility and persistent, localized charge-separated species depending on the nodes, a tool to tailor the materials for specific reactions. Moreover, nickel-loaded COFs are recyclable and catalyze cross-couplings even using red light irradiation.

Linker Exchange as Facile Method for Post-Synthetic Modification of ß-Ketoenamine-linked Covalent Organic Frameworks.

Post-synthetic linker exchange performed on covalent organic frameworks (COFs) has become an important method to introduce functional building blocks into their backbone and thus to tune their chemical and physical properties. However, the linker exchange method has so far only been described for COFs with relatively weak linkages like imines. Herein, it is shown that this method can be also used for a post-synthetic linker exchange reaction on a ß-ketoenamine linked COF. The time needed to achieve considerable linker exchange is much prolonged compared to other COFs with less stable linkages, however, this enables to achieve very good control on the ratio of the respective building blocks within the framework.

Emerging Materials for Interfacial Solar-Driven Water Purification.

Solar-driven water purification is considered as an effective and sustainable technology for water treatment using green solar energy. One major goal for practical applications is to improve the solar evaporation performance by the design of novel photothermal materials, with optimized heat localization and water transport pathways to achieve reduced energy consumption for water vaporization. Recently, some emerging materials like polymers, metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and also single molecules were employed to construct novel solar evaporation systems. In this minireview, we present an overview of the recent efforts on materials development for water purification systems. The state-of-the-art applications of these emerging materials for solar-driven water treatment, including desalination, wastewater purification, sterilization and energy production, are also summarized.

Solar Light Driven H2 O2 Production and Selective Oxidations Using a Covalent Organic Framework Photocatalyst Prepared by a Multicomponent Reaction.

A chemically stable 2D microporous COF (PMCR-1) was synthesized via the multicomponent Povarov reaction. PMCR-1 exhibits a remarkable and long-term stable photocatalytic H2 O2 production rate (60 h) from pure and sea water under visible light. The H2 O2 production is markedly enhanced when benzyl alcohol (BA) is added as reductant, which is also due to a strong π-π interaction of BA with dangling phenyl moieties in the COF pores introduced by the multicomponent Povarov reaction. Motivated by the concomitant BA oxidation to benzaldehyde during H2 O2 formation, the photocatalytic oxidation of various organic substrates such as benzyl amine and methyl sulfide derivatives was investigated. It is shown that the well-defined micropores of PMCR-1 enable size-selective photocatalytic oxidation.

A Triphenylphosphine-Based Microporous Polymer for a Wittig Reaction Cycle in the Solid State.

The Wittig reaction is a key step in industrial processes to synthesise large quantities of vitamin A and various other important chemicals that are used in daily life. This article presents a pathway to achieve the Wittig reaction in a solid network. A highly porous triphenylphosphine-based polymer was applied as a solid Wittig reagent that undergoes, in a multi-step cycle, in total six post-synthetic modifications. This allowed for regeneration of the solid Wittig reagent and reuse for the same reaction cycle. Of particular industrial relevance is that the newly developed material also enables a simple way of separating the product by filtration. Therefore, additional costly and difficult separation and purification steps are no longer needed.

CuNi Nanoalloys with Tunable Composition and Oxygen Defects for the Enhancement of the Oxygen Evolution Reaction.

Colloidal synthesis is an excellent tool for the study of cooperative effects in nanoalloys. In this work, bimetallic CuNi nanoparticles of defined size and composition are fully characterized and tested for the oxygen evolution reaction. Copper addition to nickel leads to modifications in the structural and electronic properties, showing a higher concentration of surface oxygen defects and formation of active Ni3+ sites under reaction conditions. The ratio OV /OL between oxygen vacancies and lattice oxygen shows a clear correlation with the overpotential, being an excellent descriptor of the electrocatalytic activity. This is attributed to modifications in the crystalline structure, leading to lattice strain and grain size effects. Bimetallic Cu50 Ni50 NP showed the lowest overpotential (318 mV vs RHE), low Tafel slope (63.9 mV dec-1 ), and excellent stability. This work unravels the relative concentration between oxygen defects and lattice oxygen (OV /OL ) as an excellent descriptor of the catalytic activity of bimetallic precatalysts.

A Thiazole-linked Covalent Organic Framework for Lithium-Sulphur Batteries.

Lithium-sulphur (Li-S) batteries are a promising alternative power source, as they can provide a higher energy density than current lithium-ion batteries. Porous materials are often used as cathode materials as they can act as a host for sulphur in such batteries. Recently, covalent organic frameworks (COFs) have also been used, however they typically suffer from stability issues, resulting in limited and thus insufficient durability under practical conditions and applications. Herein, we report the synthesis of a crystalline and porous imine-linked triazine-based dimethoxybenzo-dithiophene functionalized COF (TTT-DMTD) incorporating high-density redox sites. The imine linkages were further post-synthetically transformed to yield a robust thiazole-linked COF (THZ-DMTD) by utilizing a sulphur-assisted chemical conversion method, while maintaining the crystallinity. As a synergistic effect of its high crystallinity, porosity and the presence of redox-active moieties, the thiazole-linked THZ-DMTD exhibited a high capacity and long-term stability (642 mAh g-1 at 1.0 C; 78.9 % capacity retention after 200 cycles) when applied as a cathode material in a Li-S battery.

A Covalent Organic Framework/Graphene Dual-Region Hydrogel for Enhanced Solar-Driven Water Generation.

Solar-driven water generation is a sustainable water treatment technology, helping to relieve global water scarcity issues. However, this technology faces great challenges due to the high energy consumption of water evaporation yielding low evaporation rates. Here, a covalent organic framework (COF)/graphene dual-region hydrogel, containing hydrophilic and hydrophobic regions in one material, is developed through a facile in situ growth strategy. The hydrophilic COF is covering parts of the hydrophobic graphene regions. Through accurate control of both wetting regions, the hybrid hydrogel shows effective light-harvesting, tunable wettability, optimized water content, and lowered energy demand for water vaporization. Acting as solar absorber, the dual-region hydrogel exhibits a steam generation rate as high as 3.69 kg m-2 h-1 under 1 sun irradiation (1 kW m-2), which competes well with other state-of-the-art materials. Furthermore, this hydrogel evaporator can be used to produce drinkable water from seawater and sewage, demonstrating the potential for water treatment.

Oxygen-evolving catalytic atoms on metal carbides.

Single-atom catalysts have shown promising performance in various catalytic reactions. Catalytic metal sites supported on oxides or carbonaceous materials are usually strongly coordinated by oxygen or heteroatoms, which naturally affects their electronic environment and consequently their catalytic activity. Here, we reveal the stabilization of single-atom catalysts on tungsten carbides without the aid of heteroatom coordination for efficient catalysis of the oxygen evolution reaction (OER). Benefiting from the unique structure of tungsten carbides, the atomic FeNi catalytic sites are weakly bonded with the surface W and C atoms. The reported catalyst shows a low overpotential of 237 mV at 10 mA cm-2, which can even be lowered to 211 mV when the FeNi content is increased, a high turnover frequency value of 4.96 s-1 (η = 300 mV) and good stability (1,000 h). Density functional theory calculations show that either metallic Fe/Ni atoms or (hydro)oxide FeNi species are responsible for the high OER activity. We suggest that the application of inexpensive and durable WCx supports opens up a promising pathway to develop further single-atom catalysts for electrochemical catalytic reactions.

Covalent organic frameworks (COFs) for electrochemical applications.

Covalent organic frameworks are a class of extended crystalline organic materials that possess unique architectures with high surface areas and tuneable pore sizes. Since the first discovery of the topological frameworks in 2005, COFs have been applied as promising materials in diverse areas such as separation and purification, sensing or catalysis. Considering the need for renewable and clean energy production, many research efforts have recently focused on the application of porous materials for electrochemical energy storage and conversion. In this respect, considerable efforts have been devoted to the design and synthesis of COF-based materials for electrochemical applications, including electrodes and membranes for fuel cells, supercapacitors and batteries. This review article highlights the design principles and strategies for the synthesis of COFs with a special focus on their potential for electrochemical applications. Recently suggested hybrid COF materials or COFs with hierarchical porosity will be discussed, which can alleviate the most challenging drawback of COFs for these applications. Finally, the major challenges and future trends of COF materials in electrochemical applications are outlined.

Superstructures of Organic-Polyoxometalate Co-crystals as Precursors for Hydrogen Evolution Electrocatalysts.

Molybdenum-based carbides and nitrides have been considered as catalysts for the hydrogen evolution reaction (HER). One of the challenges in using Mo-based HER electrocatalysts is establishing well-defined precursors which can be transformed into Mo-based carbides/nitrides with controllable structure and porosity. We report the synthesis of a series of superstructures consisting of organic-polyoxometalate co-crystals (O-POCs) as a new type of metal-organic precursor to synthesize Mo-based carbides/nitrides in a controlled fashion and to use them for efficient catalytic hydrogen production. This protocol enables to create electrocatalysts composed of abundant nanocrystallites and heterojunctions with tunable micro- and nanostructure and mesoporosity. The best performing electrocatalyst shows high HER activity and stability with a low overpotential of 162 mV at 100 mA cm-2 (in comparison to Pt/C with 263 mV), which makes it one of the best non-noble metal HER catalysts in alkaline media and seawater.

Acridine-Functionalized Covalent Organic Frameworks (COFs) as Photocatalysts for Metallaphotocatalytic C-N Cross-Coupling.

Covalent organic frameworks (COFs) are structurally tuneable, porous and crystalline polymers constructed through the covalent attachment of small organic building blocks as elementary units. Using the myriad of such building blocks, a broad spectrum of functionalities has been applied for COF syntheses for broad applications, including heterogeneous catalysis. Herein, we report the synthesis of a new family of porous and crystalline COFs using a novel acridine linker and benzene-1,3,5-tricarbaldehyde derivatives bearing a variable number of hydroxy groups. With the broad absorption in the visible light region, the COFs were applied as photocatalysts in metallaphotocatalytic C-N cross-coupling. The fully ß-ketoenamine linked COF showed the highest activity, due to the increased charge separation upon irradiation. The COF showed good to excellent yields for several aryl bromides, good recyclability and even catalyzed the organic transformation in presence of green light as energy source.

Covalent Organic Framework (COF) Derived Ni-N-C Catalysts for Electrochemical CO2 Reduction: Unraveling Fundamental Kinetic and Structural Parameters of the Active Sites.

Electrochemical CO2 reduction is a potential approach to convert CO2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel-nitrogen-doped carbon (Ni-N-C) is an efficient catalyst for CO2 reduction to CO, and the single-site Ni-Nx motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)-derived Ni-N-C catalysts, for which the Ni-Nx content could be adjusted by the pyrolysis temperature. The combination of high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure evidenced the presence of Ni single-sites, and quantitative X-ray photoemission addressed the relation between active site density and turnover frequency.

Intraligand Charge Transfer Enables Visible-Light-Mediated Nickel-Catalyzed Cross-Coupling Reactions.

We demonstrate that several visible-light-mediated carbon-heteroatom cross-coupling reactions can be carried out using a photoactive NiII precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two carbazole groups (Ni(Czbpy)Cl2 ). The activation of this precatalyst towards cross-coupling reactions follows a hitherto undisclosed mechanism that is different from previously reported light-responsive nickel complexes that undergo metal-to-ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl2 with visible light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross-coupling reactions. The heterogeneous catalyst shows stable performance in a packed-bed flow reactor during a week of continuous operation.

Protonated Imine-Linked Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution.

Covalent organic frameworks (COFs) have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction (HER)from water. To optimize their photocatalytic activity, typically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However, the effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor-acceptor (D-A) type imine-linked COFs can produce hydrogen with a rate as high as 20.7 mmol g-1 h-1 under visible light irradiation, upon protonation of their imine linkages. A significant red-shift in light absorbance, largely improved charge separation efficiency, and an increase in hydrophilicity triggered by protonation of the Schiff-base moieties in the imine-linked COFs, are responsible for the improved photocatalytic performance.

Synthesis of Vinylene-Linked Covalent Organic Frameworks from Acetonitrile: Combining Cyclotrimerization and Aldol Condensation in One Pot.

Combining two or more consecutive reactions in one pot is a common approach for process development, as such a method involves cheap starting materials and allows in situ generation of a reactive intermediate, to undergo further reaction, without isolation. Herein, we report the synthesis of a vinylene-linked (-CH═CH-) covalent organic framework, COF-701, directly from acetonitrile, a cheap commodity solvent, by combining/telescoping two consecutive reactions-cyclotrimerization of nitrile and subsequent aldol condensation with aldehydes-in one pot. Acetonitrile is trimerized to generate protonated 2,4,6-trimethyl-s-triazine tautomers in situ, which undergo Aldol condensation with 4,4'-biphenyldicarbaldehyde in one pot to form crystalline COF-701. COF-701 is obtained as a polycrystalline powder and possesses permanent microporosity and a BET surface area (SABET) of 736 m2·g-1. This strategy can be further extended to generate other porous vinylene-linked frameworks.