Catalysis and CO2Capture by Palladium-Incorporated Covalent Organic Frameworks

Microwave-Assisted One-Step Synthesis of Palladium-Encapsulated Covalent Organic Frameworks for Heterogeneous Catalysis

Metal-encapsulated covalent organic frameworks (metal/COFs) represent an emerging paradigm in heterogeneous catalysis. However, the time-intensive (usually 4 or more days) and tedious multi-step synthesis of metal/COFs remains a significant stumbling block for their broad application. To address this challenge, we introduce a facile microwave-assisted in situ metal encapsulation strategy to cooperatively combine COF formation and in situ palladium(II) encapsulation in one step. With this unprecedented approach, we synthesize a diverse range of palladium(II)-encapsulated COFs (termed Mw-Pd/COF) in the air within just an hour. Notably, this strategy is scalable for large-scale production (~0.5 g). Leveraging the high crystallinity, porosity, and structural stability, one representative Mw-Pd/COF exhibits remarkable activity, functional group tolerance, and recyclability for the Suzuki-Miyaura coupling reaction at room temperature, surpassing most previously reported Pd(II)/COF catalysts with respect to catalytic performance, preparation time, and synthetic ease. This microwave-assisted in situ metal encapsulation strategy opens a facile and rapid avenue to construct metal/COF hybrids, which hold enormous potential in a multitude of applications including heterogeneous catalysis, sensing, and energy storage.

Emerging trends in the development and applications of triazine-based covalent organic polymers: a comprehensive review

Owing to unique structural features, triazine-based covalent organic polymers (COPs) have attracted significant attention and emerged as novel catalysts or support materials for an array of applications. Typically formed by reacting triazine-based monomers or the in situ creation of triazine rings from nitrile monomers, these COPs possess 2D/3D meso/microporous structures held together via strong covalent linkages. The quest for efficient, stable and recyclable catalytic systems globally necessitates the need for a well-structured and comprehensive review summarizing the synthetic methodologies and applications of triazine-based COPs. This review explores the various synthetic routes and applications of these COPs in photocatalysis, heterogeneous catalysis, electrocatalysis, adsorption and sensing. By exploring the latest advancements and future directions, this review offers valuable insights into the synthesis and applications of triazine-based COPs.

Pd NP-loaded covalent organic framework for pH-switched Pickering emulsion catalytic dechlorination

Quinoline carboxylic acid-linked and Pd nanoparticle (NP)-loaded COF nanospheres were constructed via a three-component one-pot Doebner reaction and post-synthetic metalation. The obtained Pd@DhaTAPB-COOH solid stabilizer can greatly promote the pH-switched recyclable Pickering interfacial dechlorination reaction, which sheds light on the bright future of smart Pickering emulsion catalysis.

Exploration of the Performance and Mechanism of Uranium Adsorption by a Covalent Organic Framework Possessing the Thiazole Structure

This study prepared an active 2-D covalent organic skeleton (HDU-27) with a network structure, high crystallinity, considerable specific surface area, excellent pore structure, and excellent stability. Kinetic studies manifested that HDU-27 could effectively capture uranium as monolayer chemisorption within a very short kinetic equilibrium time (10 min). In particular, the temperature significantly and positively impacted the uranium adsorption performance of HDU-27. At 298, 313, and 328 K, the adsorption capacity reached 269.2, 488.8, and 576.2 mg g-1, respectively, suggesting the potential to treat high-temperature industrial wastewater containing uranium. HDU-27 had high stability and recoverability with an adsorption efficiency of 98.5% after five adsorption-desorption cycles. According to X-ray photoelectron spectroscopy, the mechanism of interaction between U(VI) and HDU-27 was mainly the chelation of UO22+ by the N atom in the thiazole structure and the strong coordination of the O atom in the keto structure with UO22+. More excitingly, HDU-27 could chemically reduce soluble U(VI) to insoluble U(IV) and release binding sites for the adsorption of additional U(VI). In conclusion, HDU-27 has outstanding potential for uranium adsorption from industrial wastewater containing uranium.

Comparative Study on Proton Conductivity and Mechanism Analysis of Two Imidazole Modified Imine-Based Covalent Organic Frameworks

This work elucidates the potential impact of intramolecular H-bonds within the pore walls of covalent organic frameworks (COFs) on proton conductivity. Employing DaTta and TaTta as representative hosts, it was observed that their innate proton conductivities (σ) are both unsatisfactory and σ(DaTta)<σ(TaTta). Intriguingly, the performance of both imidazole-loaded products, Im@DaTta and Im@TaTta is greatly improved, and the σ of Im@DaTta (0.91×10-2  S cm-1 ) even surpasses that of Im@TaTta (3.73×10-3  S cm-1 ) under 100 °C and 98 % relative humidity. The structural analysis, gas adsorption tests, and activation energy calculations forecast the influence of imidazole on the H-bonded system within the framework, leading to observed changes in proton conductivity. It is hypothesized that intramolecular H-bonds within the COF framework impede efficient proton transmission. Nevertheless, the inclusion of an imidazole group disrupts these intramolecular bonds, leading to the formation of an abundance of intermolecular H-bonds within the pore channels, thus contributing to a dramatic increase in proton conductivity. The related calculation of Density Functional Theory (DFT) provides further evidence for this inference.

Amino-Functionalized Ionic-Liquid-Grafted Covalent Organic Frameworks for High-Efficiency CO2 Capture and Conversion

Rationally integrating desired functional components into a composite material can endow the tailored function to achieve the corresponding purpose. This is the first case where a series of [AeImBr]_X%-TAPT-COFs (X = 0, 17, 33, 50, 67, 83, 100) were fabricated by chemically integrating the amino-functionalized imidazole ionic liquid (NH₂-IL) onto channel walls of mesoporous covalent organic framework materials ([HO]_X%-TAPT-COFs). By virtue of the polar groups (amino groups) and abundant imidazole cations of NH₂-IL and its microporous nature, the obtained [AeImBr]_X%-TAPT-COFs exhibit higher CO₂ capture activity than [HO]_X%-TAPT-COFs. Correspondingly, the CO₂ equilibrium capture capacity increases from 62.6 to 117.4 mg/g, which is crucial to the storage of enough CO₂ around the catalytic active sites. Additionally, the synergistic effect of -NH₂ and Br^- in NH₂-IL can also improve the cycloaddition reaction rate. The characteristics of [AeImBr]_X%-TAPT-COFs contribute to the efficient generation of cyclic carbonate through heterogeneously catalyzing CO₂-epoxide cycloaddition without any solvents and cocatalysts. Specifically, [AeImBr]_83%-TAPT-COF has a CO₂ equilibrium capture capacity of 117.4 mg/g and cyclochloroallyl carbonate yield of 99.1%. As a result of the use of the chemical grafting method, [AeImBr]_X%-TAPT-COFs possess excellent stability and cycle life. The equilibrium capture capacity and cyclochloroallyl carbonate yield reach 112.7 mg CO₂/g adsorbent and 95.0% at the eighth cycle.

Nanopores of a Covalent Organic Framework: A Customizable Vessel for Organocatalysis

Covalent organic frameworks (COFs) as crystalline polymers possess ordered nanochannels. When their channels are adorned with catalytically active functional groups, their highly insoluble and fluffy powder texture makes them apt heterogeneous catalysts that can be dispersed in a range of solvents and heated to high temperatures (80-180 °C). This would mean very high catalyst density, facile active-site access, and easy separation leading to high isolated yields. Different approaches have been devised to anchor or disperse the catalytic sites into the nanospaces offered by the COF pores. Such engineered COFs have been investigated as catalysts for many organic transformation reactions. These range from Suzuki-Miyaura coupling, Heck coupling, Knoevenagel condensation, Michael addition, alkene epoxidation, CO2 utilization, and more complex biomimetic catalysis. Such catalysts employ COF as a "passive" support that merely docks catalytically active inorganic clusters, or in other cases, the COF itself participates as an "active" support by altering the electronics of the inorganic catalytic sites through the redox activity of its framework. Even more, catalytic organic pockets or metal complexes have been directly tethered to COF walls to make them behave like single-site organocatalysts. Here, we have listed most COF-based organic transformations by categorizing them as metal-free non-noble-metal@COF and noble-metal@COF. The initial part of this review highlights the advantages of COFs as a component of a heterogeneous catalyst, while the latter part discusses all of the current literature on this topic.

Triazine 2D Nanosheets as a New Class of Nanomaterials: Crystallinity, Properties and Applications

Based on the recent (2015–2021) literature data, the authors analyze the mutual dependence of crystallinity/amorphism and specific surface area and porosity in covalent triazine frameworks (CTFs), taking into account thermodynamic and kinetic control in the synthesis of these 2D nanosheets. CTFs have now become a promising new class of high-performance porous organic materials. They can be recycled and reused easily, and thus have great potential as sustainable materials. For 2D CTFs, numerous examples are given to support the known rule that the structure and properties of any material with a given composition depend on the conditions of its synthesis. The review may be useful for elder students, postgraduate students, engineers and research fellows dealing with chemical synthesis and modern nanotechnologies based on 2D covalent triazine frameworks.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Electrochemical Recognition of Aromatic Species with Ferrocenylated 1,3,5-Triazine- or 1,3,5-Triphenylbenzene-Containing Highly Organized Molecules

Two ferrocenylated organized molecules comprising 1,3,5-triphenylbenzene (Fc-1) or 2,4,6-triphenyl-1,3,5-triazine skeletons (Fc-2) were used for the first time as receptor layers for the electrochemical recognition of polycyclic aromatic hydrocarbons. While our group recently reported the synthesis of Fc-1, herein the facile synthesis of its 2,4,6-triphenyl-1,3,5-triazine-containing structural analog (Fc-2) is presented. Although the synthesis of Fc-2 was found to be challenging, we achieved a very high yield (89 %) under mild conditions using an acid-catalyzed imine-bond formation reaction in 1,4-dioxane:toluene solvent system. Title compounds were comprehensively characterized with various analytical techniques, including spectroscopic (NMR, FT-IR, FT-Raman spectroscopy) methods, high-resolution mass spectrometry (HRMS), microscopic (SEM) and electrochemical (CV) analyses. Fc-1 and Fc-2 were also used for the construction of the first-of-a-kind recognition layers (electrochemical sensors) dedicated to the recognition of polycyclic aromatic hydrocarbons. Fully constructed innovative sensors enabled the efficient recognition of analytes since the limit of detection (LOD) values were not higher than 2.9 μM. Comparative studies between the working parameters of electrochemical sensors comprising Fc-1 or Fc-2 were also included in this work.

Deciphering the Weak CO2···Framework Interactions in Microporous MOFs Functionalized with Strong Adsorption Sites-A Ubiquitous Observation

Carbon capture from industrial effluents such as flue gas or natural gas mixture (cf. landfill gas), the primary sources of CO₂ emission, greatly aids in balancing the environmental carbon cycle. In this context, the most energy-efficient physisorptive CO₂ separation process can benefit immensely from improved porous sorbents. Metal organic frameworks (MOFs), especially the ultramicroporous MOFs, built from readily available small and rigid ligands, are highly promising because of their high selectivity (CO₂/N₂) and easy scalability. Here, we report two new ultramicroporous Co-adeninato isophthalate MOFs. They concomitantly carry basic functional groups (-NH₂) and Lewis acidic sites (coordinatively unsaturated Co centers). They show good CO₂ capacity (3.3 mmol/g at 303 K and 1 bar) along with high CO₂/N₂ (∼600 at 313 K and 1 bar and ∼340 at 303 K and 1 bar) selectivity, working capacity, and smooth diffusion kinetics (D_c = 7.5 × 10^-9 m² s^-1). The MOFs exhibit good CO₂/N₂ kinetic separation under both dry and wet conditions with a smooth breakthrough profile. Despite their well-defined CO₂ adsorption sites, these MOFs exhibit only a moderately strong interaction with CO₂ as evidenced from their HOA values. This counterintuitive observation is ubiquitous among many MOFs adorned with strong CO₂ adsorption sites. To gain insights, we have identified the binding sites for CO₂ using simulation and MD studies. The radial distribution function analysis reveals that despite the amine and bare-metal sites, the pore size and the pore structure determine the positions for the CO₂ molecules. The most favorable sites become the confined spaces lined by aromatic rings. A plausible explanation for the lack of strong adsorption in these MOFs is premised from these collective studies, which could aid in the future design of superior CO₂ sorbents.

Covalent Organic Frameworks: Synthesis, Properties and Applications-An Overview

Covalent Organic Frameworks (COFs) are an exciting new class of microporous polymers with unprecedented properties in organic material chemistry. They are generally built from rigid, geometrically defined organic building blocks resulting in robust, covalently bonded crystalline networks that extend in two or three dimensions. By strategically combining monomers with specific structures and properties, synthesized COF materials can be fine-tuned and controlled at the atomic level, with unparalleled precision on intrapore chemical environment; moreover, the unusually high pore accessibility allows for easy post-synthetic pore wall modification after the COF is synthesized. Overall, COFs combine high, permanent porosity and surface area with high thermal and chemical stability, crystallinity and customizability, making them ideal candidates for a myriad of promising new solutions in a vast number of scientific fields, with widely varying applications such as gas adsorption and storage, pollutant removal, degradation and separation, advanced filtration, heterogeneous catalysis, chemical sensing, biomedical applications, energy storage and production and a vast array of optoelectronic solutions. This review attempts to give a brief insight on COF history, the overall strategies and techniques for rational COF synthesis and post-synthetic functionalization, as well as a glance at the exponentially growing field of COF research, summarizing their main properties and introducing the numerous technological and industrial state of the art applications, with noteworthy examples found in the literature.

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.

One-Pot Fabrication of Pd Nanoparticles@Covalent-Organic-Framework-Derived Hollow Polyamine Spheres as a Synergistic Catalyst for Tandem Catalysis

Facile fabrication of nanocatalysts consisting of metal nanoparticles (NPs) anchored on a functional support is highly desirable, yet remains challenging. Covalent organic frameworks (COFs) provide an emerging materials platform for structural control and functional design. Here, a facile one-pot in situ reduction approach is demonstrated for the encapsulation of small Pd NPs into the shell of COF-derived hollow polyamine spheres (Pd@H-PPA). In the one-pot synthetic process, the nucleation and growth of Pd NPs in the cavities of the porous shell take place simultaneously with the reduction of imine linkages to secondary amine groups. Pd@H-PPA shows a significantly enhanced catalytic activity and recyclability in the tandem dehydrogenation of ammonia borane and selective hydrogenation of nitroarenes through an adsorption-activation-reaction mechanism. The strong interactions of the secondary amine linkage with borane and nitroarene molecules afford a positive synergy to promote the catalytic reaction. Moreover, the hierarchical structure of Pd@H-PPA allows the accessibility of active Pd NPs to reactants.

Postsynthetic functionalization of covalent organic frameworks

Covalent organic frameworks (COFs) have been at the forefront of porous-material research in recent years. With predictable structural compositions and controllable functionalities, the structures and properties of COFs could be controlled to achieve targeted materials. On the other hand, the predesigned structure of COFs allows fruitful postsynthetic modifications to introduce new properties and functions. In this review, the postsynthetic functionalizations of COFs are discussed and their impacts towards structural qualities and performances are comparatively elaborated on. The functionalization involves the formation of specific interactions (covalent or coordination/ionic bonds) and chemical reactions (oxidation/reduction reaction) with pendant groups, skeleton and reactive linkages of COFs. The chemical stability and performance of COFs including catalytic activity, storage, sorption and opto-electronic properties might be enhanced by specific postsynthetic functionalization. The generality of these strategies in terms of chemical reactions and the range of suitable COFs places them as a pivotal role for the development of COF-based smart materials.

An unsymmetrical covalent organic polymer for catalytic amide synthesis

Herein, we present the first report on the Covalent Organic Polymer (COP) directed non-classical synthesis of an amide bond. An economical route has been chosen for the synthesis of APC-COP using p-aminophenol and cyanuric chloride. APC-COP acts as a smart, valuable and sustainable catalyst for efficient access to the amide bond under mild conditions at room temperature in 30 min. APC-COP exhibits selectivity towards carboxylic acids over esters. The key features of this protocol involve the variety of parameters, viz. wider substrate scope, no use of additive and recyclability, which makes this approach highly desirable in gramscale synthesis. Moreover, we have shown the practical utility of the present method in the catalytic synthesis of paracetamol.

A Cobalt-Modified Covalent Triazine-Based Framework as an Efficient Cocatalyst for Visible-Light-Driven Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction into carbonaceous feedstock chemicals is a promising renewable energy technology to convert solar energy and greenhouse gases into chemical fuels. Here, a covalent triazine-based framework (CTF) is demonstrated as an efficient cocatalyst to reduce CO₂ under visible-light irradiation. The nitrogen-rich triazine moieties in CTF contribute to CO₂ adsorption, while the periodical pore structure of CTF favors the accommodation of CO₂ and electron mediator. Immobilization of cobalt species onto CTF promotes the photocatalytic activity with a 44-fold enhancement over pristine CTF and the optimal CO production rate of the obtained Co/CTFs was up to 50 μmol g^-1 h^-1 . The results of solid-state UV-vis diffuse reflectance spectra (UV-vis DRS), CO₂ adsorption and electrochemical impedance spectroscopy (EIS) illustrated that the increased activity was ascribed to the enhanced CO₂ capture capacity, improved absorption of visible-light and facilitated the transfer of charge from CTF to CO₂ molecules. The CTF not only serves as a substrate for active Co species, but also bridges the photosensitizer with cobalt catalytic sites for the efficient transfer of photoexcited electrons. This work highlights the capability and ease of fabricating covalent organic framework-based photocatalytic systems that are potentially useful for energy-conversion applications.

Cu/Cu2O Nanoparticles Supported on a Phenol-Pyridyl COF as a Heterogeneous Catalyst for the Synthesis of Unsymmetrical Diynes via Glaser-Hay Coupling

Covalent organic frameworks (COFs) are a new class of porous crystalline polymers with a modular construct that favors functionalization. COF pores can be used to grow nanoparticles (nPs) with dramatic size reduction, stabilize them as dispersions, and provide excellent nP access. Embedding substrate binding sites in COFs can generate host-guest synergy, leading to enhanced catalytic activity. In this report, Cu/Cu₂O nPs (2-3 nm) are grown on a COF, which is built by linking a phenolic trialdehyde and a triamine through Schiff bonds. Their micropores restrict the nP to exceptionally small sizes (∼2-3 nm), and the pore walls decorated with strategically positioned hydrogen-bonding phenolic groups anchor the substrates via hydrogen-bonding, whereas the basic pyridyl sites serve as cationic species to stabilize the [Cu_clusterCl₂]^2- type reactive intermediates. This composite catalyst shows high activity for Glaser-Hay heterocoupling reactions, an essential 1,3-diyne yielding reaction with widespread applicability in organic synthesis and material science. Despite their broad successes in homocoupled products, preparation of unsymmetrical 1,3-diynes is challenging due to poor selectivity. Here, our COF-based Cu catalyst shows elevated selectivity toward heterocoupling product(s) (Cu nP loading 0.0992 mol %; turn over frequency: ∼45-50; turn over number: ∼175-190). The reversible redox activity at the Cu centers has been demonstrated by carrying out X-ray photoelectron spectroscopy on the frozen reactions, whereas the crucial interactions between the substrates and the binding sites in their optimized configurations have been modeled using density functional theory methods. This report emphasizes the utility of COFs in developing a heterogeneous catalyst for a truly challenging organic heterocoupling reaction.

Covalent Organic Frameworks: Promising Materials as Heterogeneous Catalysts for C-C Bond Formations

Covalent organic frameworks (COFs) are defined as highly porous and crystalline polymers, constructed and connected via covalent bonds, extending in two- or three-dimension. Compared with other porous materials such as zeolite and active carbon, the versatile and alternative constituent elements, chemical bonding types and characteristics of ordered skeleton and pore, enable the rising large family of COFs more available to diverse applications including gas separation and storage, optoelectronics, proton conduction, energy storage and in particular, catalysis. As the representative candidate of next-generation catalysis materials, because of their large surface area, accessible and size-tunable open nano-pores, COFs materials are suitable for incorporating external useful active ingredients such as ligands, complexes, even metal nanoparticles deposition and substrate diffusion. These advantages make it capable to catalyze a variety of useful organic reactions such as important C-C bond formations. By appropriate pore-engineering in COFs materials, even enantioselective asymmetric C-C bond formations could be realized with excellent yield and ee value in much shorter reaction time compared with their monomer and oligomer analogues. This review will mainly introduce and discuss the paragon examples of COFs materials for application in C-C bond formation reactions for the organic synthetic purpose.

Highly Stable COF-Supported Co/Co(OH)2 Nanoparticles Heterogeneous Catalyst for Reduction of Nitrile/Nitro Compounds under Mild Conditions

Ordered nanoporosity in covalent organic framework (COF) offers excellent opportunity for property development. Loading nanoparticles (nPs) onto them is one approach to introducing tailor-made properties into a COF. Here, a COF-Co/Co(OH)₂ composite containing about 16 wt% of <6 nm sized Co/Co(OH)₂ nPs is prepared on a N-rich COF support that catalyzes the release of theoretical equivalence of H₂ from readily available, safe, and cheap NaBH₄ . Furthermore, the released H₂ is utilized for the hydrogenation of nitrile and nitro compounds to amines under ambient conditions in a facile one-pot reaction. The COF "by choice" is built from "methoxy" functionalized dialdehydes which is crucial in enabling the complete retention of the COF structure under the conditions of the catalysis, where the regular Schiff bonds would have hydrolyzed. The N-rich binding pockets in the COF ensure strong nP-COF interactions, which provides stability and enables catalyst recycling. Modeling studies reveal the crucial role played by the COF in exposing the active facets and thereby in controlling the activation of the reducing agent. Additionally, via density functional theory, we provide a rational explanation for how these COFs can stabilize nanoparticles which grow beyond the limiting pore size of the COF and yet result in a truly stable heterogeneous catalyst - a ubiquitous observation. The study underscores the versatility of COF as a heterogeneous support for developing cheap and highly active nonnoble metal catalysts.

1,3,5-Triphenylbenzene: a versatile photoluminescent chemo-sensor platform and supramolecular building block

Rich and diverse chemistry of 1,3,5-triphenylbenzene is discussed with emphasis on fluorescence based chemo-sensors, apart from a discussion on its use in building a number of supramolecular assemblies and fluorescent covalent-organic-frameworks.