Covalent organic frameworks derived Single-Atom cobalt catalysts for boosting oxygen reduction reaction in rechargeable Zn-Air batteries

The design and development of high-performance and long-life Pt-free catalysts for the oxygen reduction reaction (ORR) is of great important with respect to metal-air batteries and fuel cells. Herein, a new low-cost covalent organic frameworks (COFs)-derived CoNC single-atoms catalyst (SAC) is fabricated and compared with the engineered nanoparticle (NP) counterpart for ORR activity. The ORR performance of the SAC catalyst (CoSA@NC) surpasses the NP counterpart (CoNP-NC) under the same operation condition. CoSA@NC also achieves improved long-term durability and better methanol tolerance compared with the Pt/C. The zinc-air battery assembled by the CoSA@NC cathode delivers a higher power density and energy density than that of commercial Pt/C catalyst. Molecular dynamics (MD) is performed to explain the spontaneous evolution from clusters to single-atom metal configuration and density functional theory (DFT) calculations find that CoSA@NC possesses lower d-band center, resulting in weaker interaction between the surface and the O-containing intermediates. Consequently, the reductive desorption of OH*, the rate-determine step, is further accelerated.

Quest for a Desolvated Structure Unveils Breathing Phenomena in a MOF Leading to Greener Catalysis in a Solventless Setup: Insights from Combined Experimental and Computational Studies

The crystal structure of the metal-organic framework (MOF), {Mn2(1,4-bdc)2(DMF)2}n (1) (1,4-bdcH2, 1,4-benzenedicarboxylic acid; DMF, N,N-dimethylformamide), is known for a long time; however, its desolvated structure, {Mn2(1,4-bdc)2}n (1'), is not yet known. The first-principles-based computational simulation was used to unveil the structure of 1' that shows the expansion in the framework, leading to pore opening after the removal of coordinated DMF molecules. We have used 1' that contains open metal sites (OMSs) in the structure in cyanosilylation and CO2 cycloaddition reactions and recorded complete conversions in a solventless setup. The pore opening in 1' allows the facile diffusion of small aldehyde molecules into the channels, leading to complete conversion. The reactions with larger aldehydes, 2-naphthaldehyde and 9-anthracenecarboxaldehyde, also show 99.9% conversions, which are the highest reported until date in solventless conditions. The in silico simulations illustrate that larger aldehydes interact with Mn(II) OMSs on the surfaces, enabling a closer interaction and facilitating complete conversions. The catalyst shows high recyclability, exhibiting 99.9% conversions in the successive reaction cycles with negligible change in the structure. Our investigations illustrate that the catalyst 1' is economical, efficient, and robust and allows reactions in a solventless greener setup, and therefore the catalysis with 1' can be regarded as "green catalysis".

Stepwise and Controllable Synthesis of Mesoporous Heterotrimetallic Catalysts Based on Predesigned Al4 Ln4 Metallocycles

The motivation for making heterometallic compounds stemmed from their emergent synergistic properties and enhanced capabilities for applications. However, the atomically precisely controlled synthesis of heterometallic compounds remains a daunting challenge of the complications that arise when applying several metals and linkers. Herein, a stepwise and controlled method is reported for the accurate addition of second and third metals to homometallic aluminum macrocycles based on the synergistic coordination and hard-soft acid-base theory. These heterometallic compounds showed a good Lewis acid catalytic effect, and the addition of hetero-metals significantly improved the catalytic effect and rate, among that the conversion rate of compound AlOC-133 reached 99.9% within half an hour. This method combines both the independent controllability of stepwise assembly with the universality of one-step methods. Based on the large family of clusters, the establishment of this method paves the way for the controllable and customized molecular-level synthesis of heterometallic materials and creates materials customized for preferential application.

Manipulation of Cationic Group Density in Covalent Organic Framework Membranes for Efficient Anion Transport

Anion exchange membranes with high anion conductivity are highly desired for electrochemical applications. Increasing ion exchange capacity is a straightforward approach to enhancing anion conductivity but faces a challenge in dimensional stability. Herein, we report the design and preparation of three kinds of isoreticular covalent organic framework (COF) membranes bearing tunable quaternary ammonium group densities as anion conductors. Therein, the cationic groups are integrated into the backbones by flexible ether-bonded alkyl side chains. The highly quaternary ammonium-group-functionalized building units endow COF membranes with abundant cationic groups homogeneously distributed in the ordered channels. The flexible side chains alleviate electrostatic repulsion and steric hindrance caused by large cationic groups, ensuring a tight interlayer stacking and multiple interactions. As a result, our COF membranes achieve a high ion exchange capacity and exceptional dimensional stability simultaneously. Furthermore, the effect of the ionic group density on the ion conductivity in rigid COF channels is systematically explored. Experiments and simulations reveal that the ionic group concentration and side chain mobility jointly determine the ion transport behavior, resulting in the abnormal phenomenon that the anion conductivity is not positively correlated to the ionic group density. The optimal COF membrane achieves the ever-reported highest hydroxide ion conductivity over 300 mS cm-1 at 80 °C and 100% RH. This study offers insightful guidelines on the rational design and preparation of high-performance anion conductors.

In silico screening of nanoporous materials for urea removal in hemodialysis applications

The design of miniaturized hemodialysis devices, such as wearable artificial kidneys, requires regeneration of the dialysate stream to remove uremic toxins from water. Adsorption has the potential to capture such molecules, but conventional adsorbents have low urea/water selectivity. In this work, we performed a comprehensive computational study of 560 porous crystalline adsorbents comprising mainly covalent organic frameworks (COFs), as well as some siliceous zeolites, metal organic frameworks (MOFs) and graphitic materials. An initial screening using Widom insertion method assessed the excess chemical potential at infinite dilution for water and urea at 310 K, providing information on the strength and selectivity of urea adsorption. From such analysis it was observed that urea adsorption and urea/water selectivity increased strongly with fluorine content in COFs, while other compositional or structural parameters did not correlate with material performance. Two COFs, namely COF-F6 and Tf-DHzDPr were explored further through Molecular Dynamics simulations. The results agree with those of the Widom method and allow to identify the urea binding sites, the contribution of electrostatic and van der Waals interactions, and the position of preferential urea-urea and urea-framework interactions. This study paves the way for a well-informed experimental campaign and accelerates the development of novel sorbents for urea removal, ultimately advancing on the path to achieve wearable artificial kidneys.

Design, Preparation, and High Intrinsic Proton Conductivity of Two Highly Stable Hydrazone-Linked Covalent Organic Frameworks

Assembling crystalline materials with high stability and high proton conductivity as a potential alternative to the Nafion membrane is a challenging topic in the field of energy materials. Herein, we concentrated on the creation and preparation of hydrazone-linked COFs with super-high stability to explore their proton conduction. Fortunately, two hydrazone-linked COFs, TpBth and TaBth, were solvothermally prepared by using benzene-1,3,5-tricarbohydrazide (Bth), 2,4,6-trihydroxy-benzene-1,3,5-tricarbaldehyde (Tp), and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (Ta) as monomers. Their structures were simulated by Material Studio 8.0 software and confirmed by the PXRD pattern, demonstrating a two-dimensional framework with AA packing. The presence of a large number of carbonyl groups as well as -NH-NH₂- groups on the backbone is responsible for their super-high water stability as well as high water absorption capacity. AC impedance tests demonstrated a positive correlation between the water-assisted proton conductivity (σ) of the two COFs and the temperature and humidity. Under 100 °C/98% RH, the highest σ values of TpBth and TaBth can reach 2.11 × 10^-4 and 0.62 × 10^-5 S·cm^-1, which are among the high σ values of the reported COFs. Their proton-conductive mechanisms were highlighted by structural analyses as well as N₂ and H₂O vapor adsorption data and activation energy values. Our systematic research affords ideas for the synthesis of proton-conducting COFs with high σ values.

Trifluoromethyl-Functionalized 2D Covalent Organic Framework for High-Resolution Separation of Isomers

Development of novel functional materials for effective isomer separation is of great significance in environmental science, chemical industry, and life science due to the different functions of isomers. However, the similar physicochemical properties of isomers make their separation greatly challenging. Here, we report the fabrication of trifluoromethyl-functionalized 2D covalent organic framework (COF) TpTFMB with 2,2'-bis(trifluoromethyl)benzidine (TFMB) and 1,3,5-triformylphloroglucinol (Tp) for the separation of isomers. TpTFMB was in situ-grown on the inner surface of a capillary for the high-resolution separation of isomers. The introduction of hydroxyl and trifluoromethyl functional groups with uniform distribution in 2D COFs is a powerful tactic to endow TpTFMB with various functions such as hydrogen bonding, dipole interaction, and steric effect. The prepared TpTFMB capillary column enabled the baseline separation of positional isomers such as ethylbenzene and xylene, chlorotoluene, carbon chain isomers such as butylbenzene and ethyl butanoate, and cis-trans isomers 1,3-dichloropropene. The hydrogen-bonding, dipole, and π-π interactions as well as the structure of COF significantly contribute to the isomer separation. This work provides a new strategy for designing functional 2D COFs for the efficient separation of isomers.

Accurate binding of porous aluminum molecular ring catalysts with the substrate

Metal molecular rings are a class of compounds with aesthetically pleasing symmetry and fundamentally useful properties. The reported work generally focuses on the ring center cavity, and there is little known about those on the ring waist. Herein, we report the discovery of porous aluminum molecular rings and their performance and contribution to the cyanosilylation reaction. We develop a facile ligand induced aggregation and solvent regulation strategy towards AlOC-58NC and AlOC-59NT with high purity, high yield (75% and 70%, respectively) and gram-level scale-up. These molecular rings exhibit a "two-tier" pore feature involving the general central cavity and newly observed equatorial semi-open cavities. AlOC-59NT with two types of one-dimensional channels showed good catalytic activity. The interaction of the aluminum molecular ring catalyst with the substrate has been crystallographically characterized and theoretically confirmed, showing a ring adaptability process that involves the capture and binding of the substrate. This work provides new ideas for the assembly of porous metal molecular rings and to understand the overall reaction pathway involving aldehydes and is expected to inspire the design of low-cost catalysts through structural modifications.

Palladium nanoparticles-confined pore-engineered urethane-linked thiol-functionalized covalent organic frameworks: a high-performance catalyst for the Suzuki Miyaura cross-coupling reaction

Covalent organic frameworks (COFs) are potential templates for the synthesis of nanomaterials owing to the versatility of their structure. Most of the reported COFs comprise imine linkages. Herein, we report for the first time the synthesis of a urethane-linked COF (UCOF) using monoformylphloroglucinol and 1,4-phenylene diisocyanate as monomers. Furthermore, the UCOF was functionalized with cysteamine to introduce free dangling thiol groups into the cavity. The latter played a critical role in fixing the active metal efficiently and facilitating the confined growth of small metal nanoparticles (∼4-6 nm) with a high surface area leading to a pore-engineered heterogeneous Pd catalyst (PdNPs@UCOF-SH). The COF and Pd catalyst were characterized using various analytical techniques such as CP-MAS NMR, FTIR, PXRD, BET, FEG-SEM, HRTEM, XPS, TGA, and ICP-AES. The as-prepared UCOF-SH-supported Pd nanoparticles showed excellent catalytic activity in the Suzuki Miyaura cross-coupling reaction under mild conditions with low catalyst loading and eco-friendly solvents. The scope was extended to various aryl boronic acids and aryl halides (I, Br, and Cl). The halo-substituted and non-halo biaryl derivatives were obtained in good to excellent yields, within a shorter reaction time, avoiding the homocoupling of aryl boronic acid. The pore-engineered COF-derived catalyst is selective and easily recycled up to 10 runs without significant loss of catalytic activity. This reveals the robust nature of the PdNPs@UCOF-SH catalyst and the sustainability of the process which opens a new frontier for several catalytic applications.

Flexible three-dimensional diacetylene functionalized covalent organic frameworks for efficient iodine capture

The construction of functionalized covalent organic frameworks (COFs) is of great significance for broadening their potential applications, but is yet challenging to achieve, especially for three-dimensional (3D) COFs, because the connection of the building organic skeleton must strictly follow the pre-designed topology. Here we present the synthesis of two diamondyne-like 3D COFs (CPOF-2 and CPOF-3) functionalized with acetylene (-CC-) and diacetylene (-CC-CC-), respectively. The obtained COFs show a high crystallinity, permanent porosity, and chemical stability. Furthermore, CPOF-3 exhibited an extremely high volatile iodine uptake (as high as 5.87 g g^-1), much higher than that of most reported COF-based adsorbents for iodine capture. Therefore, this study provides a new design principle to obtain high-performance iodine loading porous materials to solve the environmental pollution problem caused by radioactive iodine in the waste of the nuclear industry.

Heterogeneous Photoredox Catalysis Based on Silica Mesoporous Material and Eosin Y: Impact of Material Support on Selectivity of Radical Cyclization

Heterogenization of the photocatalyst appears to be a valuable solution to reach sustainable processes. Rapid and efficient synthesis of supported photocatalyst is still a remaining challenge and the choice of the support material is crucial. The present study aims at preparing a new generation of hybrid inorganic/organic photocatalysts based on silica mesoporous material and Eosin Y. These results highlight the influence of non-covalent interactions between the material support and the reagent impacting the selectivity of the reaction.

Low-temperature and gram-scale synthesis of chemically stable covalent organic frameworks in an aqueous medium

A facile and green scalable approach for production of chemically stable covalent organic frameworks (COFs) in aqueous medium at room temperature was reported by exploring an ionic liquid ([Bmim][N(CN)2) as the superior catalyst.

Experimental and computational evidence for stabilising parallel, offset π[C(O)N(H)NC]⋯π(phenyl) interactions in acetohydrazide derivatives

Stabilising π[C(O)N(H)NC]⋯π(phenyl) interactions are described.

Synthesis of a covalent organic framework with hydrazine linkages and its application in open-tubular capillary electrochromatography

Recently, covalent organic frameworks (COFs), owning to their excellent and unique properties, are attracting the attention of numerous researchers in some areas, especially the domain of chromatographic separation. However, the application of hydrazine linkages COFs in open-tubular capillary electrochromatography (OT-CEC) lies in the early stage at present. Herein, a well-crystallized hydrazine-linked COF (Tf-DHzOH) was synthesized successfully from 2,5-dihydroxyterephtalohydrazide and 1,3,5-triformyl-benzene. Tf-DHzOH was firstly regarded as a stationary phase material to prepare the Tf-DHzOH coated capillaries with different coating thickness by covalent bonding in this work. The characterization results showed that Tf-DHzOH was successfully synthesized. The separation performance and stability of the Tf-DHzOH coated capillary were evaluated by considering amino acids, sulfonamides, tetracyclines and benzene compounds as analytes. The relative standard deviations (RSDs) of separation time in the intra-day (n = 9), inter-day (n = 6), column-to-column (n = 3) and batch-to-batch (n = 3) were 0.76-4.97%, 1.59-5.94%1.78-8.72% and 1.66%-8.23%, respectively, the RSDs of peak areas were 1.90-5.16%, 1.73-5.24%, 1.26-7.33% and 3.77%-11.24%, respectively. It was confirmed that there was no visible change of separation efficiency after the Tf-DHzOH-coated capillary was used more than 200 runs. The results make clear that 2D hydrazine-linked COF (Tf-DHzOH) has superior potential as the stationary phase in OT-CEC for chromatographic separation.

Single-Pore versus Dual-Pore Bipyridine-Based Covalent-Organic Frameworks: An Insight into the Heterogeneous Catalytic Activity for Selective CH Functionalization

Exponential growth in the field of covalent-organic frameworks (COFs) is emanating from the direct correlation between designing principles and desired properties. The comparison of catalytic activity between single-pore and dual-pore COFs is of importance to establish structure-function relationship. Herein, the synthesis of imine-linked dual-pore [(BPyDC)]_{x %} -ETTA COFs (x = 0%, 25%, 50%, 75%, 100%) with controllable bipyridine content is fulfilled by three-component condensation of 4,4',4″,4'″-(ethene-1,1,2,2-tetrayl)tetraaniline (ETTA), 4,4'-biphenyldialdehyde, and 2,2'-bipyridyl-5,5'-dialdehyde in different stoichiometric ratio. The strong coordination of bipyridine moieties of [(BPyDC)]_{x %} -ETTA COFs with palladium imparts efficient catalytic active sites for selective functionalization of sp² CH bond to CX (X = Br, Cl) or CO bonds in good yield. To broaden the scope of regioselective CH functionalization, a wide range of electronically and sterically substituted substrates under optimized catalytic condition are investigated. A comparison of the catalytic activity of palladium decorated dual-pore frameworks with single-pore imine-linked Pd(II) @ Py-2,2'-BPyDC framework  is undertaken. The finding of this work provides a sporadic example of chelation-assisted CH functionalization and disclosed an in-depth comparison of the relationship between superior catalytic activity and core properties of rationally designed imine linked frameworks.

An Ultrastable Crystalline Acylhydrazone-Linked Covalent Organic Framework for Efficient Removal of Organic Micropollutants from Water

As an important member of crystalline porous polymers, acylhydrazone-linked covalent organic frameworks (COFs) have gained much attention in recent years. However, the low structural stability imparts a limit on their practical applications. To tackle this problem, we report a simple strategy to increase the chemical stability of acylhydrazone-linked COFs by incorporating azobenzene groups in the conjugated framework. Through reinforcing the π-π stacking interactions between the adjacent layers with increased π-surface, it is surprising to find that the resulting materials exhibit extreme stability in harsh environments, such as in strong acid, strong base, aqueous educing agent and boiling water, even exposed to air for one year. As a proof-of-concept, such frameworks have been used to remove various organic micropollutants such as antibiotics, plastic components, endocrine disruptors, and carcinogens from water with high capacity, fast speed and excellent reusability over a wide pH range at environmentally relevant concentrations. The results provide a new avenue to significantly enhance the stability of COFs for practical applications.

Conductive Metallophthalocyanine Framework Films with High Carrier Mobility as Efficient Chemiresistors

The poor electrical conductivity of two-dimensional (2D) crystalline frameworks greatly limits their utilization in optoelectronics and sensor technology. Herein, we describe a conductive metallophthalocyanine-based NiPc-CoTAA framework with cobalt(II) tetraaza[14]annulene linkages. The high conjugation across the whole network combined with densely stacked metallophthalocyanine units endows this material with high electrical conductivity, which can be greatly enhanced by doping with iodine. The NiPc-CoTAA framework was also fabricated as thin films with different thicknesses from 100 to 1000 nm by the steam-assisted conversion method. These films enabled the detection of low-concentration gases and exhibited remarkable sensitivity and stability. This study indicates the enormous potential of metallophthalocyanine-based conductive frameworks in advanced stand-off chemical sensors and provides a general strategy through tailor-make molecular design to develop sensitive and stable chemical sensors for the detection of low-concentration gases.

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended 2D and 3D polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores.

Multi-Stimuli-Triggered Shape Transformation of Polymeric Filaments Derived from Dynamic Covalent Block Copolymers

Using dynamic polymers to achieve the morphology transformation of polymeric assemblies under different conditions is challenging. Herein, we reported diversiform shape transformation of multi-responsive polymer filaments, which were self-assembled by a new kind of amphiphilic block copolymer (PVEG-PVEA) possessing dynamic and reversible acylhydrazone bonds through reacting benzaldehyde-containing block copolymers poly(vinylbenzaldehyde)-b-poly(N-(4-vinylbenzyl)-N,N-diethylamine) (PVBA-PVEA) with acylhydrazine-modified oligoethylene glycol. It was found that the resulting amphiphilic and dynamic PVEG-PVEA was capable of hierarchically self-assembling into intriguing core-branched filaments in aqueous solution. Notably, the features of acylhydrazone bonds and PVEA block endow the filaments with multi-responsiveness including acid, base, and temperature, leading to the multiple morphological transformations under such stimuli. Moreover, the core-branched filaments would further transform into polymeric braided bundles driven by hydrogen-bonding interactions of amide bonds. It is noteworthy that both core-branched filaments and braided bundles made from polymers are quite rare. These diversiform polymeric assemblies and their morphological evolution were characterized by TEM, Cryo-TEM, SEM, and DLS. Finally, we used PVBA-PVEA as a platform to facilely prepare functional polymers, such as glycopolymers via the reaction of amino-containing sugars and aldehyde groups. The obtained glycopolymers self-assembled into glycofibers for the biomimicry of glycans via binding with lectins. These findings not only are conducive to understanding of the stimulated shape change process of dynamic polymeric assemblies in water but also provide a new method for the facile fabrication of smart and functional polymeric assemblies for different potential applications, such as biomimicry and targeted drug nanocarriers or delivery vehicles.

Hydrazone-Linked Heptazine Polymeric Carbon Nitrides for Synergistic Visible-Light-Driven Catalysis

Heptazine-based conjugated polymeric carbon nitrides (PCNs) are promising metal-free photocatalysts, yet their synthesis is challenging due to the electron-deficiency and insolubility of heptazine units. Indeed, heptazine-containing polymers have only been prepared through nucleophilic substitution with amines by using toxic cyameluric chloride as the starting material. Herein, we report the novel and environmentally friendly method for preparing heptazine-based mesoporous PCNs with hydrazone links formed through a simple Schiff base condensation of melem-NH₂ and aldehydes. Unlike cyameluric chloride, melem-NH₂ is non-toxic, stable, and can be readily obtained from melem and hydrazine in solution. We demonstrate that the hydrazone linkages and the heptazine units synergistically enhance the photocatalytic activity of PCNs in visible-light-driven aerobic oxidation of benzyl alcohol to benzaldehyde. In particular, the polymer constructed from melem-NH₂ and p-phthalaldehyde shows 17 times more activity than graphitic carbon nitride (g-C₃ N₄ ).

Pore surface engineering of covalent organic frameworks: structural diversity and applications

Connecting molecular building blocks by covalent bonds to form extended crystalline structures has caused a sharp upsurge in the field of porous materials, especially covalent organic frameworks (COFs), thereby translating the accuracy, precision, and versatility of covalent chemistry from discrete molecules to two-dimensional and three-dimensional crystalline structures. COFs are crystalline porous frameworks prepared by a bottom-up approach from predesigned symmetric units with well-defined structural properties such as a high surface area, distinct pores, cavities, channels, thermal and chemical stability, structural flexibility and functional design. Due to the tedious and sometimes impossible introduction of certain functionalities into COFs via de novo synthesis, pore surface engineering through judicious functionalization with a range of substituents under ambient or harsh conditions using the principle of coordination chemistry, chemical conversion, and building block exchange is of profound importance. In this review, we aim to summarize dynamic covalent chemistry and framework linkage in the context of design features, different methods and perspectives of pore surface engineering along with their versatile roles in a plethora of applications such as biomedical, gas storage and separation, catalysis, sensing, energy storage and environmental remediation.

Opportunities of Covalent Organic Frameworks for Advanced Applications

Covalent organic frameworks (COFs) are an emerging class of functional nanostructures with intriguing properties, due to their unprecedented combination of high crystallinity, tunable pore size, large surface area, and unique molecular architecture. The range of properties characterized in COFs has rapidly expanded to include those of interest for numerous applications ranging from energy to environment. Here, a background overview is provided, consisting of a brief introduction of porous materials and the design feature of COFs. Then, recent advancements of COFs as a designer platform for a plethora of applications are emphasized together with discussions about the strategies and principles involved. Finally, challenges remaining for this type material for real applications are outlined.

Post-synthetic modification of covalent organic frameworks

This review is aimed at providing an in-depth understanding of the potential of post-synthetic strategies for the modification of covalent organic frameworks.