Easy and bulk-scale syntheses of two-dimensional (2D) covalent organic frameworks (COFs) represent an enduring challenge in material science. Concomitantly, the most critical aspect is to precisely ...control the porosity and crystallinity of these robust structures. Disparate complementary approaches such as solvothermal synthesis have emerged recently and are fueled in part by the usage of different modulators and acids that have enriched the COF library. Yet, the fundamental understanding of the integral processes of 2D COF assembly, including their growth from nucleating sites and the origin of periodicity, is an intriguing chemical question that needs to be answered. To address these cardinal questions, a green and easy-to-perform approach of COF formation has been delineated involving acid-diamine salt precursors. The role of hydrogen bonding d av(Namine–H···Oacid); d av signifies the average Namine–H···Oacid distances, i.e., the average distance from the H atom of the amine to the O atom of the acid present in the acid-diamine salts in improving the COFs’ crystallinity and porosity has further been decoded by thorough crystallographic analyses of the salt molecules. What is particularly noteworthy is that we have established the hydrogen-bonding distances d av(Namine–H···Oacid) in the acid-diamine salts that are pivotal in maintaining the reversibility of the reaction, which mainly facilitates highly crystalline and porous COF formation. Moreover, this reactant-structure to the product-quality relationship has further been utilized for the synthesis of highly crystalline and porous COFs that are unattainable by other synthetic means.
Pyrolysis of a bimetallic metal–organic framework (MIL‐88‐Fe/Ni)‐dicyandiamide composite yield a Fe and Ni containing carbonaceous material, which is an efficient bifunctional electrocatalyst for ...overall water splitting. FeNi3 and NiFe2O4 are found as metallic and metal oxide compounds closely embedded in an N‐doped carbon–carbon nanotube matrix. This hybrid catalyst (Fe‐Ni@NC‐CNTs) significantly promotes the charge transfer efficiency and restrains the corrosion of the metallic catalysts, which is shown in a high OER and HER activity with an overpotential of 274 and 202 mV, respectively at 10 mA cm−2 in alkaline solution. When this bifunctional catalyst was further used for H2 and O2 production in an electrochemical water‐splitting unit, it can operate in ambient conditions with a competitive gas production rate of 1.15 and 0.57 μL s−1 for hydrogen and oxygen, respectively, showing its potential for practical applications.
I've got to split: An efficient bifunctional electrocatalyst for overall water splitting was synthesized by pyrolyzing a bimetallic metal–organic framework (MOF; MIL‐88‐Fe/Ni)–dicyandiamide composite. The resulting N‐doped carbon material with embedded FeNi3 and NiFe2O4 nanoparticles show high activity and stability for both H2 and O2 production from water.
Abstract
The fabrication of macroscopic objects from covalent organic frameworks (COFs) is challenging but of great significance to fully exploit their chemical functionality and porosity. Herein, ...COF/reduced graphene oxide (rGO) aerogels synthesized by a hydrothermal approach are presented. The COFs grow in situ along the surface of the 2D graphene sheets, which are stacked in a 3D fashion, forming an ultralight aerogel with a hierarchical porous structure after freeze-drying, which can be compressed and expanded several times without breaking. The COF/rGO aerogels show excellent absorption capacity (uptake of >200 g organic solvent/g aerogel), which can be used for removal of various organic liquids from water. Moreover, as active material of supercapacitor devices, the aerogel delivers a high capacitance of 269 F g
−1
at 0.5 A g
−1
and cycling stability over 5000 cycles.
Research on covalent organic frameworks (COFs) has recently gathered significant momentum by the virtue of their predictive design, controllable porosity, and long-range ordering. However, the lack ...of solvent-free and easy-to-perform synthesis processes appears to be the bottleneck toward their greener fabrication, thereby limiting their possible potential applications. To alleviate such shortcomings, we demonstrate a simple route toward the rapid synthesis of highly crystalline and ultraporous COFs in seconds using a novel salt-mediated crystallization approach. A high degree of synthetic control in interlayer stacking and layer planarity renders an ordered network with a surface area as high as 3000 m2 g–1. Further, this approach has been extrapolated for the continuous synthesis of COFs by means of a twin screw extruder and in situ processes of COFs into different shapes mimicking the ancient terracotta process. Finally, the regular COF beads are shown to outperform the leading zeolites in water sorption performance, with notably facile regeneration ability and structural integrity.
Two 2D covalent organic frameworks (COFs) linked by vinylene (−CH=CH−) groups (V‐COF‐1 and V‐COF‐2) are synthesized by exploiting the electron deficient nature of the aromatic s‐triazine unit of ...C3‐symmetric 2,4,6‐trimethyl‐s‐triazine (TMT). The acidic terminal methyl hydrogens of TMT can easily be ed by a base, resulting in a stabilized carbanion, which further undergoes aldol condensation with multitopic aryl aldehydes to be reticulated into extended crystalline frameworks (V‐COFs). Both V‐COF‐1 (with terepthalaldehyde (TA)) and V‐COF‐2 (with 1,3,5‐tris(p‐formylphenyl)benzene (TFPB)) are polycrystalline and exhibit permanent porosity and BET surface areas of 1341 m2 g−1 and 627 m2 g−1, respectively. Owing to the close proximity (3.52 Å) of the pre‐organized vinylene linkages within adjacent 2D layers stacked in eclipsed fashion, 2+2 photo‐cycloadditon in V‐COF‐1 formed covalent crosslinks between the COF layers.
Come on over vinylene: The synthesis of two vinylene‐linked (−CH=CH−) covalent organic frameworks (COFs) by base‐catalyzed aldol condensation is reported. Structural properties of the highly crystalline COF were investigated.
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 β‐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.
A new family of porous crystalline COFs bearing acridine moieties was synthesized and applied as photocatalysts in metallaphotocatalytic C−N cross‐coupling. Among these materials the fully β‐ketoenamine‐linked COF showed the highest catalytic activity and was shown to be recyclable and even catalyzed the cross‐coupling efficiently under green light irradiation.
A rapid and scalable synthesis of six new imine‐linked highly porous and crystalline COFs is presented that feature exceptionally high chemical stability in harsh environments including conc. H2SO4 ...(18 m), conc. HCl (12 m), and NaOH (9 m). This is because of the presence of strong interlayer C−H⋅⋅⋅N hydrogen bonding among the individual layers, which provides significant steric hindrance and a hydrophobic environment around the imine (−C=N−) bonds, thus preventing their hydrolysis in such an abrasive environment. These COFs were further converted into porous, crystalline, self‐standing, and crack‐free COF membranes (COFMs) with extremely high chemical stability for their potential applications for sulfuric acid recovery. The as‐synthesized COFMs exhibit unprecedented permeance for acetonitrile (280 Lm−2 h−1 bar−1) and acetone (260 Lm−2 h−1 bar−1).
The rapid and scalable construction of highly porous and crystalline imine‐linked covalent organic frameworks (COFs) with ultra‐high stability (H2SO4 18 m, HCl 12 m, NaOH 9 m) is presented. The chemical stability was examined regarding the presence of significant interlayer C−H⋅⋅⋅N H‐bonding among the adjacent COF layers, which protects and prevents imine (−C=N−) bond hydrolysis through the steric and hydrophobic environment.
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 (−CHCH−) covalent organic framework, COF-701, directly from acetonitrile, a cheap commodity solvent, by combining/telescoping two consecutive reactionscyclotrimerization of nitrile and subsequent aldol condensation with aldehydesin 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.
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