Formation of Strong Basicity on Covalent Triazine Frameworks as Catalysts for the Oxidation of Methylene Compounds

Stable Mg Single-Atom Solid Base Catalysts Anchored on Metal-Organic Framework-Derived Nitrogen-Doped Carbon

Solid base catalysts are widely used in the chemical industry owing to their advantages of environmental friendliness and easy separation. However, their application is limited by basic site aggregation and poor stability. In this study, we report the preparation of magnesium (Mg) single-atom catalysts with high activity and stability by a sublimation-trapping strategy. The Mg net was sublimated as Mg vapor at 620 °C, subsequently transported through argon, and finally trapped on the defects of nitrogen-doped carbon derived from metal-organic framework ZIF-8, producing Mg1/NC. Because of the atomically dispersed Mg sites, the obtained Mg1/NC exhibits high catalytic activity and stability for Knoevenagel condensation of benzaldehyde with malononitrile, which is a typical base-catalyzed reaction. The Mg1/NC catalyst achieves a high efficiency with a turnover frequency of 49.6 h-1, which is much better than that of the traditional counterpart MgO/NC (7.7 h-1). In particular, the activity of Mg1/NC shows no decrease after five catalytic cycles, while that of MgO/NC declines due to the instability of basic sites.

Increasing Donor-Acceptor Interactions and Particle Dispersibility of Covalent Triazine Frameworks for Higher Crystallinity and Enhanced Photocatalytic Activity

Covalent triazine frameworks (CTFs) have recently emerged as an efficient class of photocatalysts due to their structural diversity and excellent stability. Nevertheless, the synthetic reactions of CTFs have usually suffered from poor reversibility, resulting in a low crystallinity of the materials. Here, we report the introduction of methoxy groups on the monomer 2,5-diphenylthiazolo[5,4-d]thiazole to reinforce interlayer π-π interactions of the resulting donor-acceptor type CTFs, which improved crystallinity, further increasing the visible light absorption range and allowing for efficient separation and transport of carriers. The morphology is strongly correlated to the wettability, which has a significant impact on the mass transfer capacity and photocatalytic activity in the photocatalytic reaction. To further improve crystallinity and photocatalytic activity, CTF-NWU-T3 photocatalysts in a bowl shape were prepared using a SiO2 template. The energy band structure, photocatalytic hydrogen evolution, and pollutant degradation efficiency of involved materials were investigated. The donor-acceptor type CTF-NWU-T3 with a bowl-shaped morphology, synthesized using the template method and the introduction of methoxy groups, exhibited an excellent photocatalytic hydrogen production rate of 32064 μmol·h-1·g-1. This study highlights the significance of improving donor-acceptor interactions and increasing the dispersibility of catalyst particles in dispersion to enhance the photocatalytic activity of heterogeneous photocatalysts.

Catalytically Stable Potassium Single-Atom Solid Superbases

Solid superbases can catalyze diverse reactions under mild conditions, while they suffer from aggregation of basic sites and poor stability during recycling. Here we report a new generation of solid superbases derived from K single atoms (SAs) prepared by a tandem redox strategy. The initial redox reaction takes place between base precursor KNO₃ and graphene support, producing K₂ O at 400 °C. Further increasing the temperature to 800 °C, the graphene reduces K₂ O to K anchored by its vacancies, leading to the generation of K SAs (denoted as K₁ /G). The source of basicity in the K₁ /G is K SAs, and neighboring single atoms (NSAs) possess superbasicity, which is different from conventional basicity originated from oxygen and nitrogen atoms. Due to the superbasicity as well as high dispersion and anchoring of basic sites, the K₁ /G shows excellent catalytic activity and stability in transesterification reaction, which is much superior to the reported catalysts.

Switching Amine Oxidation from Imines to Nitriles by Carbon-Hydrogen Bond Activation via Strong Base Modified Strategy

Precisely controlling the product selectivity from the complex reaction is always an attractive topic in the catalysis field. In this paper, the Ru/strong base junction formed by the redox strategy was demonstrated as an efficient catalyst to switch the selectivity in aerobic oxidation of benzylamines. The zirconia-supported ruthenium (Ru-ZrO₂) catalyst could catalyze benzylamine oxidation coupling to imines; however, the potassium oxide strong base modified zirconia-supported ruthenium (Ru-K-ZrO₂) catalyst could catalyze benzylamine oxidation dehydrogenation to nitriles. Insight into the mechanism showed that the base modified catalyst had excellent dehydrogenation ability which was assisted by the C-H bond activation and changed the reaction pathway. The strong base modified strategy may provide a new approach for controlling the performance of heterogeneous catalysts and product selectivity.

Functionalized Triazines and Tetrazines: Synthesis and Applications

The molecules possessing triazine and tetrazine moieties belong to a special class of heterocyclic compounds. Both triazines and tetrazines are building blocks and have provided a new dimension to the design of biologically important organic molecules. Several of their derivatives with fine-tuned electronic properties have been identified as multifunctional, adaptable, switchable, remarkably antifungal, anticancer, antiviral, antitumor, cardiotonic, anti-HIV, analgesic, anti-protozoal, etc. The objective of this review is to comprehensively describe the recent developments in synthesis, coordination properties, and various applications of triazine and tetrazine molecules. The rich literature demonstrates various synthetic routes for a variety of triazines and tetrazines through microwave-assisted, solid-phase, metal-based, [4+2] cycloaddition, and multicomponent one-pot reactions. Synthetic approaches contain linear, angular, and fused triazine and tetrazine heterocycles through a combinatorial method. Notably, the triazines and tetrazines undergo a variety of organic transformations, including electrophilic addition, coupling, nucleophilic displacement, and intramolecular cyclization. The mechanistic aspects of these heterocycles are discussed in a detailed way. The bioorthogonal application of these polyazines with various strained alkenes and alkynes provides a new prospect for investigations in chemical biology. This review systematically encapsulates the recent developments and challenges in the synthesis and possible potential applications of various triazine and tetrazine systems.

Graphitic Aza-Fused π-Conjugated Networks: Construction, Engineering, and Task-Specific Applications

2D π-conjugated networks linked by aza-fused units represent a pivotal category of graphitic materials with stacked nanosheet architectures. Extensive efforts have been directed at their fabrication and application since the discovery of covalent triazine frameworks (CTFs). Besides the triazine cores, tricycloquinazoline and hexaazatriphenylene linkages are further introduced to tailor the structures and properties. Diverse related materials have been developed rapidly, and a thorough outlook is necessitated to unveil the structure-property-application relationships across multiple subcategories, which is pivotal to guide the design and fabrication toward enhanced task-specific performance. Herein, the structure types and development of related materials including CTFs, covalent quinazoline networks, and hexaazatriphenylene networks, are introduced. Advanced synthetic strategies coupled with characterization techniques provide powerful tools to engineer the properties and tune the associated behaviors in corresponding applications. Case studies in the areas of gas adsorption, membrane-based separation, thermo-/electro-/photocatalysis, and energy storage are then addressed, focusing on the correlation between structure/property engineering and optimization of the corresponding performance, particularly the preferred features and strategies in each specific field. In the last section, the underlying challenges and opportunities in construction and application of this emerging and promising material category are discussed.

Generation of Strong Basicity in Metal-Organic Frameworks: How Do Coordination Solvents Matter?

Solid strong bases with an ordered pore structure (OPS-SSBs) have attracted much attention because of their high catalytic activity and shape selectivity as heterogeneous catalysts in various reactions. Nevertheless, high temperatures are required to fabricate OPS-SSBs by using traditional methods. Herein, we report for the first time that the coordination solvents affect basicity generation in metal-organic frameworks (MOFs) greatly and that strong basicity can be formed at comparatively low temperatures. A typical MOF, MIL-53, was employed, and three different solvents, namely, water, methanol, and N,N-dimethylformamide (DMF), were coordinated, respectively, by means of solvent exchange. Thermogravimetry-mass spectrometer analysis shows that the conversion temperature of base precursor KNO₃ is quite different on MIL-53 coordinated with different solvents. The conversion of KNO₃ to basic sites takes place at 350, 300, and 250 °C on MIL-53 coordinated with water, methanol, and DMF, respectively. It is fascinating to observe the generation temperature of strongly basic sites at 250 °C, which is noticeably lower than that on various supports, such as mesoporous silica SBA-15 (600 °C), zeolite Y (700 °C), and metal oxide ZrO₂ (730 °C). This is due to the redox interaction between coordination solvents and KNO₃, leading to a significant decrease in the temperature for KNO₃ conversion. Consequently, OPS-SSBs were prepared successfully with an ordered pore structure and strong basicity. The obtained OPS-SSBs show good shape selectivity in Knoevenagel condensation of aromatic aldehydes with different active methylene compounds. Moreover, these solid bases are highly active in the synthesis of dimethyl carbonate through transesterification reaction. This work might open up a new avenue for the fabrication of various functional materials at low temperatures through redox interactions.

Covalent triazine frameworks for the dynamic adsorption/separation of benzene/cyclohexane mixtures

High adsorption selectivities for benzene and cyclohexane of three covalent triazine frameworks have been prepared via Friedel–Crafts reactions.

Tricycloquinazoline-containing 3D conjugated microporous polymers and 2D covalent quinazoline networks: microstructure and conductivity

The isomers of 3D TQ-CMP and 2D TQ-CQN was made toward the applications of gas adsorption and hydrogen evolution reaction, which clarified the interrelation between microstructure and conductivity for structural design and functional exploration.

Direct Z-Scheme Heterojunction of SnS2 /Sulfur-Bridged Covalent Triazine Frameworks for Visible-Light-Driven CO2 Photoreduction

Solar-driven reduction of CO₂ into renewable carbon forms is considered as an alternative approach to address global warming and the energy crisis but suffers from low efficiency of the photocatalysts. Herein, a direct Z-Scheme SnS₂ /sulfur-bridged covalent triazine frameworks (S-CTFs) photocatalyst (denoted as SnS₂ /S-CTFs) was developed, which could efficiently adsorb CO₂ owing to the CO₂ -philic feature of S-CTFs and promote separation of photoinduced electron-hole pairs. Under visible-light irradiation, SnS₂ /S-CTFs exhibited excellent performance for CO₂ photoreduction, yielding CO and CH₄ with evolution rates of 123.6 and 43.4 μmol g^-1  h^-1 , respectively, much better than the most catalysts reported to date. This inorganic/organic hybrid with direct Z-Scheme structure for visible-light-driven CO₂ photoreduction provides new insights for designing photocatalysts with high efficiency for solar-to-fuel conversion.

Covalent triazine framework with efficient photocatalytic activity in aqueous and solid media

Covalent triazine frameworks (CTFs) have been recently employed for visible light-driven photocatalysis due to their unique optical and electronic properties. However, the usually highly hydrophobic nature of CTFs, which originates from their overall aromatic backbone, leads to limitations of CTFs for applications in aqueous media. In this study, we aim to extend the range of the application media of CTFs and design hybrid material of a CTF and mesoporous silica (SBA-15) for efficient photocatalysis in aqueous medium. A thiophene-containing CTF was directly synthesized in mesopores of SBA-15. Due to the high surface area and the added hydrophilic properties by silica, the hybrid material demonstrated excellent adsorption of organic molecules in water. This leads not only to high photocatalytic performance of the hybrid material for the degradation of organic dyes in water, but also for efficient photocatalysis in solvent-free and solid state. Furthermore, the reusability, stability and easy recovery of the hybrid material offers promising metal-free heterogeneous photocatalyst for broader applications in different reaction media.

Bipyridinium-Based Ionic Covalent Triazine Frameworks for CO2, SO2, and NO Capture

The exploitation of novel porous materials for capturing/adsorption of harmful gases is considered a very promising approach to deal with air pollution. Herein, bipyridinium-based ionic covalent triazine frameworks (ICTFs) were synthesized via ZnCl₂-catalyzed ionothermal polymerization. The as-prepared ICTFs had a satisfactory total pore volume and specific surface of approximately 0.4582 cm³ g^-1 and 1000 m² g^-1, respectively. Moreover, the specific surface area, pore size and distribution, and total pore volumes of ICTFs could be adjusted via ion-exchange of the anion. The obtained ICTFs were explored as the adsorbent for the separation/adsorption of the mixed gases (SO₂, CO₂, NO, and N₂), and they showed the strong adsorption ability for CO₂ (2.75 mmol g^-1), SO₂ (9.22 mmol g^-1), and NO (4.05 mmol g^-1) at 1 bar and 298 K. This unique design provides a new insight to prepare high-efficiency porous materials for CO₂, SO₂, and NO capture.

Effect of Building Block Transformation in Covalent Triazine-Based Frameworks for Enhanced CO2 Uptake and Metal-Free Heterogeneous Catalysis

Covalent triazine frameworks (CTFs) have provided a unique platform in functional material design for a wide range of applications. This work reports a series of new CTFs with two new heteroaromatic building blocks (pyrazole and isoxazole groups) through a building-block transformation approach aiming for carbon capture and storage (CCS) and metal-free catalysis. The CTFs were synthesized from their respective building blocks [(4,4'-(1H-pyrazole-3,5-diyl)dibenzonitrile (pyz) and 4,4'-(isoxazole-3,5-diyl)dibenzonitrile (isox))] under ionothermal conditions using ZnCl₂ . Both of the building blocks were designed by an organic transformation of an acetylacetone containing dinitrile linker to pyrazole and isoxazole groups, respectively. Due to this organic transformation, (i) linker aromatization, (ii) higher surface areas and nitrogen contents, (iii) higher aromaticity, and (iv) higher surface basicity was achieved. Due to these enhanced properties, CTFs were explored for CO₂ uptake and metal-free heterogeneous catalysis. Among all, the isox-CTF, synthesized at 400 °C, showed the highest CO₂ uptake (4.92 mmol g^-1 at 273 K and 2.98 mmol g^-1 at 298 K at 1 bar). Remarkably, these CTFs showed excellent metal-free catalytic activity for the aerobic oxidation of benzylamine at mild reaction conditions. On studying the properties of the CTFs, it was observed that organic transformations and ligand aromatization of the materials are crucial factor to tune the important parameters that influence the CO₂ uptake and the catalytic activity. Overall, this work highlights the substantial effect of designing new CTF materials by building-block organic transformations resulting in better properties for CCS applications and heterogeneous catalysis.

Significant Decrease in Activation Temperature for the Generation of Strong Basicity: A Strategy of Endowing Supports with Reducibility

Mesoporous solid strong bases are quite attractive due to their good catalytic performance for applications as environmentally friendly catalysts in various reactions. However, pretty harsh conditions are usually compulsory for the fabrication of strong basicity by using traditional thermal activation (e.g., 700 °C for the activation of base precursor KNO₃ supported on mesoporous Al₂O₃). This is energy intensive and harmful to the mesoporous structure. In this study, we report a strategy of endowing supports with reducibility (ESWR) by doping low-valence Cr³⁺ into mesoporous Al₂O₃, so that the activation temperature for basicity generation is decreased significantly. Fascinatingly, KNO₃ on mesoporous Al₂O₃ can be motivated to basic sites completely at the temperature of 400 °C via the ESWR strategy, which is much lower than the conventional thermal activation (700 °C). We have demonstrated that the redox reciprocity between KNO₃ and Cr³⁺ is responsible for the low-temperature conversion, and Cr⁶⁺ is formed quantitatively as the oxidation product. The obtained solid bases possessing ordered mesostructure and strong basicity provide promising candidates for base-catalyzed synthesis of dimethyl carbonate via transesterification. The catalytic activity is obviously higher than a typical solid base like MgO as well as a series of reported basic catalysts containing alkali metal and alkaline-earth metal oxides.

Imidazolium-Salt-Functionalized Covalent Organic Frameworks for Highly Efficient Catalysis of CO2 Conversion

The conversion of CO₂ into valuable chemicals is an ideal pathway for CO₂ utilization in industry, although the development of highly efficient catalysts remains a challenge. Herein, the design and synthesis of two covalent organic frameworks (COFs) functionalized with imidazolium salts were reported as catalysts for CO₂ conversion. The resultant COFs possessed highly crystalline structures, showed high stability and surface area, and contained dense catalytic active sites on the pore walls. They exhibited outstanding catalytic performances for the reaction of CO₂ with epoxides without any solvent or cocatalyst under mild conditions and afforded a record turnover number of 495 000. In addition, the COFs could serve as effective catalysts in the reductive reaction of CO₂ with amines. The results presented here thus demonstrate the exceptional potential of the functionalized COFs for various challenging CO₂ transformations.

Controlling Monomer Feeding Rate to Achieve Highly Crystalline Covalent Triazine Frameworks

The synthesis of highly crystalline covalent triazine frameworks (CTFs) with ultrastrong covalent bonds (aromatic CN) from the triazine linkage presents a great challenge to synthetic chemists. Herein, the synthesis of highly crystalline CTFs via directly controlling the monomer feeding rate is reported. By tuning the feeding rate of monomers, the crystallization process can be readily governed in a controlled manner in an open system. The sample of CTF-HUST-HC1 with abundant exposed {001} crystal facets has the better crystallinity and thus is selected to study the effect of high crystallinity on photoelectric properties. Owing to the better separation of photogenerated electron-hole pairs and charge transfer, the obtained highly ordered CTF-HUST-HC1 has superior performance in the photocatalytic removal of nitric oxide (NO) than its lesser crystalline counterparts and g-C₃ N₄ .

Recent Advancements in the Synthesis of Covalent Triazine Frameworks for Energy and Environmental Applications

Covalent triazine frameworks (CTFs) are a unique type of porous materials, comprised of triazine units. Owing to the strong linkage of triazine, the most important advantage of CTFs lies in their high chemical and thermal stabilities and high nitrogen content as compared to other porous organic polymers (POPs). Therefore, CTFs are one of the most promising materials for practical applications. Much research has been devoted to developing new methods to synthesize CTFs and explore their potential applications. Nowadays, energy and environmental issues have attracted enormous attention. CTFs are particular promising for energy- and environment-related applications, due to their nitrogen-rich scaffold and robust structure. Here, we selected some typical examples and reviewed recent advancements in the synthesis of CTFs and their applications in gas adsorption, separation, and catalysis in relation to environment and energy issues.

Crystalline Covalent Triazine Frameworks by In Situ Oxidation of Alcohols to Aldehyde Monomers

Covalent triazine frameworks (CTFs) with aromatic triazine linkages have recently received increasing interest for various applications because of their rich nitrogen content and high chemical stability. Owing to the strong aromatic C=N bond and high chemical stability, only a few CTFs are crystalline, and most CTFs are amorphous. Herein we report a new general strategy to give highly crystalline CTFs by in situ formation of aldehyde monomers through the controlled oxidation of alcohols. This general strategy allows a series of crystalline CTFs with different monomers to be prepared, which are shown to have higher thermal stability and enhanced performance in photocatalysis as compared with the less crystalline or amorphous CTFs. This open-system approach is very simple and convenient, which presents a potential pathway to large-scale industrial production of crystalline CTFs.