Charged Covalent Triazine Frameworks for CO2 Capture and Conversion

Regulating the Layer Stacking Configuration of CTF-TiO2 Heterostructure for Improving the Photocatalytic CO2 Reduction

Herein, covalent triazine frameworks in eclipsed AA and staggered AB stacking modes are respectively used for the in-situ growth of TiO2, and two heterostructures are obtained. Due to the highly organized stacking of the molecular layer in CTF-AA that strengthens the interlayer interaction, the light absorption and carrier migration of CTF-AA/TiO2 are both enhanced in comparison to those of its component or CTF-AB/TiO2. Correspondently, the photocatalytic CO2 reduction reaction (CO2RR) of CTF-AA/TiO2 proffers 9.19 μmol·g-1·h-1 CH4 and 2.32 μmol·g-1·h-1 CO production, about 9.2 and 4.3 times greater than that of pristine TiO2, respectively. Even though the innate photoresponse of the triazine unit endows CTF-AB/TiO2 with augmented light capturing, its photocatalytic CO2 conversion is relatively insignificant. According to the analyses of the planar-averaged electron density difference and Bader charge, the unproductive CO2 efficiency might be due to the insufficient interfacial electron transfer from TiO2 to CTF-AB. Given that the ΔG (-3.22 eV) of CHO intermediate generation is lower than that of CO desorption (-1.23 eV), the reaction tends to further generate CH4 other than yielding CO. This study could shed fresh light over the reasonable design of effective photocatalytic heterostructures.

Viologen-Cucurbit[7]uril Based Polyrotaxanated Covalent Organic Networks: A Metal Free Electrocatalyst for Oxygen Evolution Reaction

Viologen-based covalent organic networks represent a burgeoning class of materials distinguished by their captivating properties. Here, supramolecular chemistry is harnessed to fabricate polyrotaxanated ionic covalent organic polymers (iCOP) through a Schiff-base condensation reaction under solvothermal conditions. The reaction between 1,1'-bis(4-aminophenyl)-[4,4'-bipyridine]-1,1'-diium dichloride (DPV-NH2) and 1,3,5-triformylphloroglucinol (TPG) in various solvents yields an iCOP-1 and iCOP-2. Likewise, employing cucurbit[7]uril (CB[7]) in the reaction yielded polyrotaxanated iCOPs, denoted as iCOP-CB[7]-1 and iCOP-CB[7]-2. All four iCOPs exhibit exceptional stability under the acidic and basic conditions. iCOP-CB[7]-2 displays outstanding electrocatalytic Oxygen Evolution Reaction (OER) performance, demanding an overpotential of 296 and 332 mV at 10 and 20 mA cm-2, respectively. Moreover, the CB[7] integrated iCOP-2 exhibits a long-term stable nature for 30 h in 1 m KOH environment. Further, intrinsic activity studies like TOF show a 4.2-fold increase in generation of oxygen (O2) molecules than the bare iCOP-2. Also, it is found that iCOP-CB[7]-2 exhibits a high specific (19.48 mA cm-2) and mass activity (76.74 mA mg-1) at 1.59 V versus RHE. Operando-EIS study evident that iCOP-CB[7]-2 commences OER at a relatively low applied potential of 1.5 V versus RHE. These findings pave the way for a novel approach to synthesizing various mechanically interlocked molecules through straightforward solvothermal conditions.

Recent advances and applications of ionic covalent organic frameworks in food analysis

Ionic covalent organic frameworks with both crystallinity and charged sites have attracted significant attention from the scientific community. The versatile textural structures, precisely defined channels, and abundant charged sites of ionic COFs offer immense potential in various areas such as separation, sample pretreatment, ion conduction mechanisms, sensing applications, catalytic reactions, and energy storage systems. This review presents a comprehensive overview of facile preparation methods for ionic covalent organic frameworks (iCOFs), along with their applications in food sample pretreatment techniques such as solid-phase extraction (SPE), magnetic solid-phase extraction (MSPE), and dispersive solid-phase extraction (DSPE). Furthermore, it highlights the extensive utilization of iCOFs in detecting various food contaminants including pesticides, contaminants from food packaging, veterinary drugs, perfluoroalkyl substances, and poly-fluoroalkyl substances. Specifically, this review critically discusses the limitations, challenges, and future prospects associated with employing iCOF materials to ensure food safety.

Unveiling the Role of Cationic Pyridine Sites in Covalent Triazine Framework for Boosting Zinc-Iodine Batteries Performance

Rechargeable Zinc-iodine batteries (ZIBs) are gaining attention as energy storage devices due to their high energy density, low-cost, and inherent safety. However, the poor cycling performance of these batteries always arises from the severe leakage and shuttle effect of polyiodides (I3 - and I5 -). Herein, a novel cationic pyridine-rich covalent triazine framework (CCTF-TPMB) is developed to capture and confine iodine (I2) species via strong electrostatic interaction, making it an attractive host for I2 in ZIBs. The as-fabricated ZIBs with I2 loaded CCTF-TPMB (I2@CCTF-TPMB) cathode achieve a large specific capacity of 243 mAh g-1 at 0.2 A g-1 and an exceptionally stable cyclic performance, retaining 93.9% of its capacity over 30 000 cycles at 5 A g-1. The excellent electrochemical performance of the ZIBs can be attributed to the pyridine-rich cationic sites of CCTF-TPMB, which effectively suppress the leakage and shuttle of polyiodides, while also accelerating the conversion reaction of I2 species. Combined in situ Raman and UV-vis analysis, along with theoretical calculations, clearly reveal the critical role played by pyridine-rich cationic sites in boosting the ZIBs performances. This work opens up a promising pathway for designing advanced I2 cathode materials toward next-generation ZIBs and beyond.

The development of catalysts and auxiliaries for the synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) have recently seen significant advancements. Large quantities of structurally & functionally oriented COFs with a wide range of applications, such as gas adsorption, catalysis, separation, and drug delivery, have been explored. Recent achievements in this field are primarily focused on advancing synthetic methodologies, with catalysts playing a crucial role in achieving highly crystalline COF materials, particularly those featuring novel linkages and chemistry. A series of reviews have already been published over the last decade, covering the fundamentals, synthesis, and applications of COFs. However, despite the pivotal role that catalysts and auxiliaries play in forming COF materials and adjusting their properties (e.g., crystallinity, porosity, stability, and morphology), limited attention has been devoted to these essential components. In this Critical Review, we mainly focus on the state-of-the-art progress of catalysts and auxiliaries applied to the synthesis of COFs. The catalysts include four categories: acid catalysts, base catalysts, transition-metal catalysts, and other catalysts. The auxiliaries, such as modulators, oxygen, and surfactants, are discussed as well. This is then followed by the description of several specific applications derived from the utilization of catalysts and auxiliaries. Lastly, a perspective on the major challenges and opportunities associated with catalysts and auxiliaries is provided.

Role of Nitrogenous Functional Group Identity in Accelerating 1,2,3-Trichloropropane Degradation by Pyrogenic Carbonaceous Matter (PCM) and Sulfide Using PCM-like Polymers

Groundwater contamination by 1,2,3-trichloropropane (TCP) poses a unique challenge due to its human toxicity and recalcitrance to degradation. Previous work suggests that nitrogenous functional groups of pyrogenic carbonaceous matter (PCM), such as biochar, are important in accelerating contaminant dechlorination by sulfide. However, the reaction mechanism is unclear due, in part, to PCM's structural complexity. Herein, PCM-like polymers (PLPs) with controlled placement of nitrogenous functional groups [i.e., quaternary ammonium (QA), pyridine, and pyridinium cations (py+)] were employed as model systems to investigate PCM-enhanced TCP degradation by sulfide. Our results suggest that both PLP-QA and PLP-py+ were highly effective in facilitating TCP dechlorination by sulfide with half-lives of 16.91 ± 1.17 and 0.98 ± 0.15 days, respectively, and the reactivity increased with surface nitrogenous group density. A two-step process was proposed for TCP dechlorination, which is initiated by reductive ß-elimination, followed by nucleophilic substitution by surface-bound sulfur nucleophiles. The TCP degradation kinetics were not significantly affected by cocontaminants (i.e., 1,1,1-trichloroethane or trichloroethylene), but were slowed by natural organic matter. Our results show that PLPs containing certain nitrogen functional groups can facilitate the rapid and complete degradation of TCP by sulfide, suggesting that similarly functionalized PCM might form the basis for a novel process for the remediation of TCP-contaminated groundwater.

Polyaniline-Based Cationic Porous Organic Polymers for Fast and Efficient Anion-Exchange-Driven Capture of Cr2O7 2

Efficient treatment of wastewater contaminated with carcinogenic Cr(VI) has been a long-term challenge for both academic and industrial research efforts. Removal of Cr(VI) species by ion exchange is a relatively simple and efficient method, and its combination with highly tailorable nanomaterials is promising for the treatment of such wastewater. Here, we report a type of cationic porous organic polymer (POP), namely, PTPA-PIP, which can be prepared simply by converting the corresponding aromatic polyamine PTPA to its protonated form, thereby significantly increasing its hydrophilicity and ability to disperse homogeneously in water, crucial for application in water treatment. In addition to detailed characterization of the physicochemical properties of PTPA-PIP (including using Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and solid-state NMR techniques), adsorption experiments demonstrate that PTPA-PIP removes low-concentration dichromate anions with very high performance, including excellent exchange capacity (maximum capacity of 230 mg Cr2O7 2-/g PTPA-PIP), ultrafast removal (initial adsorption rate of 83 mg g-1 min-1), excellent selectivity (∼10% loss of adsorption capacity in the presence of 40-fold concentration of competing anions), as well as superior reusability (reusable for at least 5 cycles without compromised performance). These results demonstrate that PTPA-PIP is an outstanding candidate for application in industrial settings for the effective removal of harmful Cr(VI) pollutants in wastewater.

Fabrication of imidazoline-linked cationic covalent triazine framework for enrichment of environmental estrogens

Imidazoline-linked cationic covalent triazine framework (IM-iCTF) was facilely prepared through the Debus-Radziszewski reaction, involving 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline, formaldehyde and methylglyoxal. The IM-iCTF was applied as a sorbent for cartridge solid-phase extraction (SPE). It provided good adsorption performance for estrogen and estrogen mimics including bisphenol F, bisphenol A, 7β-estradiol, bisphenol B and estrone. The adsorption isotherm, adsorption kinetic model, thermodynamic calculations and adsorption mechanism were investigated to reveal the adsorption behavior. The IM-iCTF was employed for the extraction of the estrogens and estrogen mimics from water, fish and shrimp (fish and shrimp samples were extracted with acetonitrile before the SPE). The analytes were then determined by high-performance liquid chromatography with diode array detection. The limits of detection were 0.008-0.05 ng mL-1 for water, 0.015-0.11 μg g-1 for fish, and 0.012-0.10 μg g-1 for shrimp samples. This research not only offers a new approach to construct cationic covalent triazine framework, but also provides a reliable strategy for the adsorption/enrichment trace level of organic pollutants.

Strategies for Enhancing the Photocatalytic and Electrocatalytic Efficiency of Covalent Triazine Frameworks for CO2 Reduction

Converting carbon dioxide (CO2) into fuel and high-value-added chemicals is considered a green and effective way to solve global energy and environmental problems. Covalent triazine frameworks (CTFs) are extensively utilized as an emerging catalyst for photo/electrocatalytic CO2 reduction reaction (CO2RR) recently recognized for their distinctive qualities, including excellent thermal and chemical stability, π-conjugated structure, rich nitrogen content, and a strong affinity for CO2, etc. Nevertheless, single-component CTFs have the problems of accelerated recombination of photoexcited electron-hole pairs and restricted conductivity, which limit their application for photo/electrocatalytic CO2RR. Therefore, emphasis will then summarize the strategies for enhancing the photocatalytic and electrocatalytic efficiency of CTFs for CO2RR in this paper, including atom doping, constructing a heterojunction structure, etc. This review first illustrates the synthesis strategies of CTFs and the advantages of CTFs in the field of photo/electrocatalytic CO2RR. Subsequently, the mechanism of CTF-based materials in photo/electrocatalytic CO2RR is described. Lastly, the challenges and future prospects of CTFs in photo/electrocatalytic CO2RR are addressed, which offers a fresh perspective for the future development of CTFs in photo/electrocatalytic CO2RR.

Reticular synthesis of two-dimensional ionic covalent organic networks as metal-free bifunctional electrocatalysts for oxygen reduction and evolution reactions

Bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are the heart of metal-air batteries, fuel cells, and other energy storage systems. Here, we report a series of a novel class of redox-active viologen-based ionic covalent organic networks (vCONs) which are directly used as metal-free bifunctional electrocatalysts towards ORR and OER applications. These vCONs (named vGC, vGAC, vMEL and vBPDP) were synthesized by the well-known Zincke reaction. The installation of redox-active viologen moieties among the extended covalent organic architectures played a crucial role for exceptional acid/base stability, as well as bifunctional ORR and OER activities, confirmed by the cyclic voltammetry (CV) curves. Among all of them, vBPDP showed high ORR efficiency with a half-wave potential of 0.72 V against a reversible hydrogen electrode (RHE) in 1 M KOH electrolyte. In contrast, vMEL demonstrated high OER activity with an overpotential of 320 mV at a current density of 10 mAcm-2 and a Tafel slope of 109.4 mV dec-1 in 1 M KOH electrolyte solution. This work is exceptional and unique in terms of directly used pristine ionic covalent organic networks that are used as bifunctional (ORR and OER) electrocatalysts without adding any metals or conductive materials.

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.

Synthesis of cyano and ionic dual-functional hypercrosslinked porous polymer for effective adsorption and detection of endocrine disrupting chemicals in milk matrix

Endocrine disrupting chemicals (EDCs) can interfere with the normal function of endocrine system, posing serious risk to human health. The monitoring of EDCs in foods is of great importance to ensure food security. Herein, a cyano and ionic dual-functionalized hypercrosslinked porous polymer (CN-iHCP) was designed and prepared for the first time through hyper-crosslink of 1-(4-cyanophenyl)imidazole and 1,4-bis(chloromethyl)benzene. The adsorption mechanism mainly involves electrostatic interaction, hydrogen bonding and π-π stacking interaction. A sensitive analytical method for simultaneous detection of the four phenolic EDCs was established by coupled CN-iHCP based solid-phase extraction with high performance liquid chromatography. Under optimal conditions, the target EDCs exhibited good linearity with coefficient r > 0.993 and high enrichment factors of 164-243. The detection limits (S/N = 3) of EDCs were 0.20-0.50 ng mL-1 for milk sample. The extraction recoveries for the spiked milk samples were in the range of 85.5%- 116.0%. This work not only highlights the CN-iHCP as a promising adsorbent to efficiently enrich EDCs and other pollutants, but also provides a new strategy for the functionalization of HCP for wide applications.

Covalent organic frameworks: linkage types, synthetic methods and bio-related applications

Covalent organic frameworks (COFs) are composed of small organic molecules linked via covalent bonds, which have tunable mesoporous structure, good biocompatibility and functional diversities. These excellent properties make COFs a promising candidate for constructing biomedical nanoplatforms and provide ample opportunities for nanomedicine development. A systematic review of the linkage types and synthesis methods of COFs is of indispensable value for their biomedical applications. In this review, we first summarize the types of various linkages of COFs and their corresponding properties. Then, we highlight the reaction temperature, solvent and reaction time required by different synthesis methods and show the most suitable synthesis method by comparing the merits and demerits of various methods. To appreciate the cutting-edge research on COFs in bioscience technology, we also summarize the bio-related applications of COFs, including drug delivery, tumor therapy, bioimaging, biosensing and antimicrobial applications. We hope to provide insight into the interdisciplinary research on COFs and promote the development of COF nanomaterials for biomedical applications and their future clinical translations.

Porous organic polymers (POPs) for environmental remediation

Modern global industrialization along with the ever-increasing growth of the population has resulted in continuous enhancement in the discharge and accumulation of various toxic and hazardous chemicals in the environment. These harmful pollutants, including toxic gases, inorganic heavy metal ions, anthropogenic waste, persistent organic pollutants, toxic dyes, pharmaceuticals, volatile organic compounds, etc., are destroying the ecological balance of the environment. Therefore, systematic monitoring and effective remediation of these toxic pollutants either by adsorptive removal or by catalytic degradation are of great significance. From this viewpoint, porous organic polymers (POPs), being two- or three-dimensional polymeric materials, constructed from small organic molecules connected with rigid covalent bonds have come forth as a promising platform toward various leading applications, especially for efficient environmental remediation. Their unique chemical and structural features including high stability, tunable pore functionalization, and large surface area have boosted the transformation of POPs into various macro-physical forms such as thick and thin-film membranes, which led to a new direction in advanced level pollutant removal, separation and catalytic degradation. In this review, our focus is to highlight the recent progress and achievements in the strategic design, synthesis, architectural-engineering and applications of POPs and their composite materials toward environmental remediation. Several strategies to improve the adsorption efficiency and catalytic degradation performance along with the in-depth interaction mechanism of POP-based materials have been systematically summarized. In addition, evolution of POPs from regular powder form application to rapid and more efficient size and chemo-selective, "real-time" applicable membrane-based application has been further highlighted. Finally, we put forward our perspective on the challenges and opportunities of these materials toward real-world implementation and future prospects in next generation remediation technology.

Eliminating Concentration Polarization with Cationic Covalent Organic Polymer to Promote Effective Overpotential of Nitrogen Fixation

Electrocatalytic nitrogen reduction reaction offers a sustainable alternative to the conventional Haber-Bosch process. However, it is currently restricted by low effective overpotential due to the concentration polarization, which arises from accumulated products, ammonium, at the reaction interface. Here, a novel covalent organic polymer with ordered periodic cationic sites is proposed to tackle this challenge. The whole network exhibits strong positive charge and effectively repels the positively charged ammonium, enabling an ultra-low interfacial product concentration, and successfully driving the reaction equilibrium to the forward direction. With the given potential unchanged, the suppressed overpotential can be much liberated, ultimately leading to a continuous high-level reaction rate. As expected, when this tailored microenvironment is coupled with a transition metal-based catalyst, a 24-fold improvement is generated in the Faradaic efficiency (73.74 %) as compared with the bare one. The proposed strategy underscores the importance of optimizing dynamic processes as a means of improving overall performance in electrochemical syntheses.

Rational Fabrication of Ionic Covalent Organic Frameworks for Chemical Analysis Applications

The rapid development of advanced material science boosts novel chemical analytical technologies for effective pretreatment and sensitive sensing applications in the fields of environmental monitoring, food security, biomedicines, and human health. Ionic covalent organic frameworks (iCOFs) emerge as a class of covalent organic frameworks (COFs) with electrically charged frames or pores as well as predesigned molecular and topological structures, large specific surface area, high crystallinity, and good stability. Benefiting from the pore size interception effect, electrostatic interaction, ion exchange, and recognizing group load, iCOFs exhibit the promising ability to extract specific analytes and enrich trace substances from samples for accurate analysis. On the other hand, the stimuli response of iCOFs and their composites to electrochemical, electric, or photo-irradiating sources endows them as potential transducers for biosensing, environmental analysis, surroundings monitoring, etc. In this review, we summarized the typical construction of iCOFs and focused on their rational structure design for analytical extraction/enrichment and sensing applications in recent years. The important role of iCOFs in the chemical analysis was fully highlighted. Finally, the opportunities and challenges of iCOF-based analytical technologies were also discussed, which may be beneficial to provide a solid foundation for further design and application of iCOFs.

Hexaazatrinaphthalene-Based Covalent Triazine Framework-Supported Rhodium(III) Complex: A Recyclable Heterogeneous Catalyst for the Reductive Amination of Ketones to Primary Amines

The development of efficient and recyclable heterogeneous catalysts is an important topic. Herein, a rhodium(III) complex Cp*Rh@HATN-CTF was synthesized by the coordinative immobilization of [Cp*RhCl₂]₂ on a hexaazatrinaphthalene-based covalent triazine framework. In the presence of Cp*Rh@HATN-CTF (1 mo l% Rh), a series of primary amines could be obtained via the reductive amination of ketones in high yields. Moreover, catalytic activity of Cp*Rh@HATN-CTF is well maintained during six runs. The present catalytic system was also applied for the large scale preparation of a biologically active compound. It would facilitate the development of CTF-supported transition metal catalysts for sustainable chemistry.

One-step synthesis of nitrogen-rich organic polymers for efficient catalysis of CO2 cycloaddition

Nitrogen-rich organic polymer poly(chloride triazole) (PCTs) was synthesized by a one-step method as metal-halogen-free heterogeneous catalyst for the solvent-free CO2 cycloaddition. PCTs had abundant nitrogen sites and hydrogen bond donors, exhibited great activity for the cycloaddition of CO2 and epichlorohydrin, and achieved 99.6% yield of chloropropene carbonate under the conditions of 110 ℃, 6 h, and 0.5 MPa CO2. The activation of epoxides and CO2 by hydrogen bond donor and nitrogen sites was further explained by density functional theory (DFT) calculations. In summary, this study showed that nitrogen-rich organic polymer is a versatile platform for CO2 cycloaddition, and this paper provides a reference for the design of CO2 cycloaddition catalysts.

Cationic Covalent Triazine Network: A Metal-Free Catalyst for Effective Acetylene Hydrochlorination

Vinyl chloride, the monomer of polyvinyl chloride, is produced primarily via acetylene hydrochlorination catalyzed by environmentally toxic carbon-supported HgCl2. Recently, nitrogen-doped carbon materials have been explored as metal-free catalysts to substitute toxic HgCl2. Herein, we describe the development of a cationic covalent triazine network (cCTN, cCTN-700) that selectively catalyzes acetylene hydrochlorination. cCTN-700 exhibited excellent catalytic activity with initial acetylene conversion, reaching ~99% and a vinyl chloride selectivity of >98% at 200 °C during a 45 h test. X-ray photoelectron spectroscopy, temperature programmed desorption, and charge calculation results revealed that the active sites for the catalytic reaction were the carbon atoms bonded to the pyridinic N and positively charged nitrogen atoms (viologenic N+) of the viologen moieties in cCTN-700, similar to the active sites in Au-based catalysts but different from the those in previously reported nitrogen-doped carbon materials. This research focuses on using cationic covalent triazine polymers for selective acetylene hydrochlorination.

Porous Polymer Materials for CO2 Capture and Electrocatalytic Reduction

Efficient capture of CO₂ and its conversion into other high value-added compounds by electrochemical methods is an effective way to reduce excess CO₂ in the atmosphere. Porous polymeric materials hold great promise for selective adsorption and electrocatalytic reduction of CO₂ due to their high specific surface area, tunable porosity, structural diversity, and chemical stability. Here, we review recent research advances in this field, including design of porous organic polymers (POPs), porous coordination polymers (PCPs), covalent organic frameworks (COFs), and functional nitrogen-containing polymers for capture and electrocatalytic reduction of CO₂. In addition, key issues and prospects for the optimal design of porous polymers for future development are elucidated. This review is expected to shed new light on the development of advanced porous polymer electrocatalysts for efficient CO₂ reduction.

Advances in the Synthesis of Covalent Triazine Frameworks

Covalent triazine frameworks (CTFs) are a class of organic polymer materials constructed by aromatic 1,3,5-triazine rings with planar π-conjugation properties. CTFs are highly stable and porous with N atoms in the frameworks, possessing semiconductive properties; thus they are widely used in gas adsorption and separation as well as catalysis. The properties of CTFs strongly depend on the type of monomers and the synthesis process. Synthesis methods including ionothermal polymerization, amino-aldehyde synthesis, trifluoromethanesulfonic acid catalyzed synthesis, and aldehyde-amidine condensation have been intensively studied in recent years. In this review, we discuss the recent advances and future developments of CTFs synthesis.

A novel zirconium-based metal-organic framework covalently modified by methyl pyridinium bromide for mild and co-catalyst free conversion of CO2 to cyclic carbonates

Building metal-organic frameworks (MOFs) covalently modified by onium halides is a promising approach to develop efficient MOF-based heterogeneous catalysts for the cycloaddition of CO₂ to epoxides (CCE) into cyclic carbonates. Herein, we report a novel zirconium-based MOF covalently modified by methyl pyridinium bromide, Zr₆O₄(OH)₄(MPTDC)_2.2(N-CH₃-MPTDC)_3.8Br_3.8 ((Br-)CH3-Pyridinium-MOF-1), where MPTDC denotes 3-methyl-4-pyridin-4-yl-thieno[2,3-b] thiophene-2,5-dicarboxylate. The structure and composition of this complex were fully characterized with PXRD, NMR, XPS, TEM and so on. CO₂ adsorption experiments show that (Br-)CH3-Pyridinium-MOF-1 has a higher affinity for CO₂ than its electrically neutral precursor, which should be attributed to the fact that charging frameworks containing pyridinium salt have stronger polarization to CO₂. (Br-)CH3-Pyridinium-MOF-1 integrated reactive Lewis acid sites and Br^- nucleophilic anions and exhibited efficient catalytic activity for CCE under ambient pressure in the absence of co-catalysts and solvents. Furthermore, (Br-)CH3-Pyridinium-MOF-1 was recycled after five successive cycles without substantial loss in catalytic activity. The corresponding reaction mechanism also was speculated.

Ordered Macro/Microporous Ionic Organic Framework for Efficient Separation of Toxic Pollutants from Water

In case of pollutant segregation, fast mass diffusion is a fundamental criterion in order to achieve improved performance. The rapid mass transport through porous materials can be achieved by availing large open pores followed by easy and complete accessibility of functional sites. Inducing macroporosity into such materials could serve as ideal solution providing access to large macropores that offer unhindered transport of analyte and full exposure to interactive sites. Moreover, the challenge to configure the ionic-functionality with macroporosity could emerge as an unparalleled avenue toward pollutants separation. Herein, we strategized a synthetic protocol for construction of a positively charged hierarchically-porous ordered interconnected macro-structure of organic framework where the size and number of macropores can easily be tuned. The ordered macropores with strong electrostatic interaction synergistically exhibited ultrafast removal efficiency towards various toxic pollutants.

Porous organic polymers for CO2 capture, separation and conversion

Porous organic polymers (POPs) have long been considered as prime candidates for carbon dioxide (CO2) capture, separation, and conversion. Especially their permanent porosity, structural tunability, stability and relatively low cost are key factors in such considerations. Whereas heteratom-rich microporous networks as well as their amine impregnation/functionalization have been actively exploited to boost the CO2 affinity of POPs, recently, the focus has shifted to engineering the pore environment, resulting in a new generation of highly microporous POPs rich in heteroatoms and featuring abundant catalytic sites for the capture and conversion of CO2 into value-added products. In this review, we aim to provide key insights into structure-property relationships governing the separation, capture and conversion of CO2 using POPs and highlight recent advances in the field.

Immobilization of Ionic Liquid on a Covalent Organic Framework for Effectively Catalyzing Cycloaddition of CO2 to Epoxides

Transforming CO2 into value-added chemicals has been an important subject in recent years. The development of a novel heterogeneous catalyst for highly effective CO2 conversion still remains a great challenge. As an emerging class of porous organic polymers, covalent organic frameworks (COFs) have exhibited superior potential as catalysts for various chemical reactions, due to their unique structure and properties. In this study, a layered two-dimensional (2D) COF, IM4F-Py-COF, was prepared through a three-component condensation reaction. Benzimidazole moiety, as an ionic liquid precursor, was integrated onto the skeleton of the COF using a benzimidazole-containing building unit. Ionization of the benzimidazole framework was then achieved through quaternization with 1-bromobutane to produce an ionic liquid-immobilized COF, i.e., BMIM4F-Py-COF. The resulting ionic COF shows excellent catalytic activity in promoting the chemical fixation of CO2 via reaction with epoxides under solvent-free and co-catalyst-free conditions. High porosity, the one-dimensional (1D) open-channel structure of the COF and the high catalytic activity of ionic liquid may contribute to the excellent catalytic performance. Moreover, the COF catalyst could be reused at least five times without significant loss of its catalytic activity.

Tröger's Base Network Polymers of Intrinsic Microporosity (TB-PIMs) with Tunable Pore Size for Heterogeneous Catalysis

Heterogeneous catalysis plays a pivotal role in the preparation of value-added chemicals, and it works more efficiently when combined with porous materials and supports. Because of that, a detailed assessment of porosity and pore size is essential when evaluating the performance of new heterogeneous catalysts. Herein, we report the synthesis and characterization of a series of novel microporous Tröger’s base polymers and copolymers (TB-PIMs) with tunable pore size. The basicity of TB sites is exploited to catalyze the Knoevenagel condensation of benzaldehydes and malononitrile, and the dimension of the pores can be systematically adjusted with an appropriate selection of monomers and comonomers. The tunability of the pore size provides the enhanced accessibility of the catalytic sites for substrates, which leads to a great improvement in conversions, with the best results achieving completion in only 20 min. In addition, it enables the use of large benzaldehydes, which is prevented when using polymers with very small pores, typical of conventional PIMs. The catalytic reaction is more efficient than the corresponding homogeneous counterpart and is ultimately optimized with the addition of a small amount of a solvent, which facilitates the swelling of the pores and leads to a further improvement in the performance and to a better carbon economy. Molecular dynamic modeling of the copolymers’ structures is employed to describe the swellability of flexible chains, helping the understanding of the improved performance and demonstrating the great potential of these novel materials.

3D Ionic Olefin-Linked Conjugated Microporous Polymers for Selective Detection and Removal of TcO4-/ReO4- from Wastewater

Technetium (⁹⁹Tc) is a highly toxic radioactive nuclear wastewater contaminant. Real-time detection of ⁹⁹Tc is very difficult due to its difficult-to-complex nature. Herein, a novel three-dimensional ionic olefin-linked conjugated microporous polymer (TFPM-EP-Br) is constructed using tetrakis(4-aldehyde phenyl)methane (TFPM) as the central monomer. The unique cationic cavity and highly hydrophobic framework enable TFPM-EP-Br to act as a fluorescent sensor for TcO₄^-. The fluorophores of TFPM-EP-Br can be quenched due to electron transfer from TFPM-EP-Br to TcO₄^- and the formation of strongly nonfluorescent complexes. Meanwhile, the regular pore channels are beneficial for the fast mass transfer of TcO₄^-, resulting in an ultrafast response time (less than 2 s) with an ultralow detection limit (33.3 nM). In addition, the ultrahigh specific surface area enables TFPM-EP-Br to combine the ability to synergistically detect and remove radioactive ⁹⁹Tc. From this perspective, the novel conjugated microporous polymer has made a breakthrough in the detection and extraction of radioactive contaminants.

Porous Organic Polymers: Promising Testbed for Heterogeneous Reactive Oxygen Species Mediated Photocatalysis and Nonredox CO2 Fixation

Catalysts play a pivotal role in achieving the global need for food and energy. In this context, porous organic polymers (POPs) with high surface area, robust architecture, tunable pore size, and chemical functionalities have emerged as promising testbeds for heterogeneous catalysis. Amorphous POPs having functionalized interconnected hierarchical porous structures activate a diverse range of substrates through covalent/non-covalent interactions or act as a host matrix to encapsulate catalytically active metal centers. On the other hand, conjugated POPs have been explored for photoinduced chemical transformations. In this personal account, we have delineated the evolution of various POPs and the specific role of pores and pore functionalities in heterogeneous catalysis. Subsequently, we retrospect our journey over the last ten years towards designing and fabricating amorphous POPs for heterogeneous catalysis, specifically photocatalytic reactive oxygen species (ROS)-mediated organic transformations and nonredox chemical fixation of CO₂ . We have also outlined some of the future avenues of POPs and POP-based hybrid materials for diverse catalytic applications.

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.