Rapid Synthesis of Covalent Organic Frameworks with a Controlled Morphology: An Emulsion Polymerization Approach via the Phase Transfer Catalysis Mechanism

Sponge-like inorganic-organic S-scheme heterojunction for efficient photocatalytic hydrogen evolution

Covalent organic frameworks (COFs)-based S-scheme heterojunction photocatalysts have gained considerable attention for photocatalytic hydrogen evolution. However, challenges such as limited interfacial contact and low stability persist, primarily due to uneven inorganic semiconductor coverage on the COFs surface. Therefore, constructing inorganic-organic S-scheme heterojunction photocatalysts via the in-situ growth of COFs on inorganic semiconductor surfaces shows great promise. Herein, we successfully developed a sponge-like TiO2@BTTA S-scheme heterojunction with a tight contact interface by in-situ growing COF (referred to as BTTA) on the surface of sponge-like TiO2 (referred to as ST). Density Functional Theory (DFT) calculations confirmed that the ST@BTTA hybrids exhibit the optimal adsorption and desorption capabilities for H2O and H2 molecules, respectively. Notably, the ST@BTTA-120 S-scheme heterojunction photocatalyst demonstrates an outstanding hydrogen production rate under simulated sunlight irradiation, surpassing pristine ST and BTTA by factors of 10.3 and 2.6, respectively. The enhanced photocatalytic performance is attributed to improved solar energy utilization efficiency, a larger specific surface area, and an increased interfacial contact area between ST and BTTA. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analyses further verify the S-scheme carrier transfer mechanism in the ST@BTTA hybrids. This research provides a valuable method for designing efficient S-scheme heterojunction photocatalysts with closely integrated interfaces for photocatalytic hydrogen production via water splitting.

Bayesian Optimized Crystallization of a Hydroxamate-Functionalized Covalent Organic Framework for Enhanced Uranyl Uptake

To address the synthetic challenge of covalent organic frameworks (COFs), especially those with interfering functional groups, a Bayesian optimization (BO) centered approach is developed and implemented. Specifically, the crystallinity index for a well-known TAPB-PDA COF is improved by ≈80% via a one-round proof-of-concept BO. For a more complicated task toward the preparation of hydroxamate-functionalized TpPa COF, where improvement of both crystallinity and selectivity (against a crystalline byproduct) is needed, an efficient protocol comprising 6 BO iterations (with 5 experiments each) from an initial 64-experiment dataset is successfully developed. The functional COF, namely SUM-99 (SUM = Sichuan University Materials), with enhanced crystallinity, is subsequently demonstrated to be an effective, reversible, and selective sorbent for aquatic uranyl uptake. The importance of improved crystallinity, reflecting the power of BO, is showcased by a 23.7% increase in uranyl adsorption capacity. Therefore, the BO protocol and toolkit is presented for the efficient evolution of COF synthetic conditions, toward higher crystallinity and enhanced performances for downstream applications.

Surfactant-Assisted Construction of Covalent Organic Frameworks

Covalent organic frameworks (COFs), characterized by their unique ordered pore structures, chemical diversity, and high degree of designability, have demonstrated immense application potential across multiple fields. However, traditional synthesis methods often encounter challenges such as low crystallinity and uneven morphology. The introduction of surfactants has opened up new pathways for the synthesis of COFs. Leveraging their intermolecular interactions and self-assembly properties, surfactants can effectively regulate the nucleation, growth processes, and ultimate structure and properties of COFs. This paper systematically reviews the latest research achievements and future trends in surfactant-assisted COF synthesis, emphasizing the crucial role of surfactants as key additives in the preparation of COFs. Surfactants not only facilitate uniform nucleation and growth of COFs, enhancing the crystallinity and structural order of the products but also enable precise and diverse regulation of the dimensionality, morphology, and structure of COFs. Furthermore, by influencing the dispersion and processability of COFs, surfactants enhance their practicality and workability. Finally, the paper presents some prospects for the challenges and future opportunities in this emerging research area.

Overcoming Boundaries: Towards the Ambient Aqueous Synthesis of Covalent Organic Frameworks

The synthesis of covalent organic frameworks (COFs) has traditionally been carried out under strict solvothermal and anaerobic conditions. The utilization of organic solvents in such reactions not only carries significant costs but also imposes a great burden on the environment. The fabrication of COFs using alternative synthetic pathways has, therefore, seen rapid development in recent years and much attention has been placed on green and sustainable methods in particular. The synthesis of COFs in purely aqueous media, however, remains challenging due to the delicate nature of the chemical reactions and the crystallization process in water. This mini-review discusses different synthetic strategies for the construction of crystalline COFs in aqueous media and offers a perspective on the future development of facile COF synthesis in ambient conditions.

1,3,5-Triformylphloroglucinol Derived β-Ketoenamine-Linked Functional Covalent Organic Frameworks with Enhanced Crystallinity and Stability-Recent Advances

Crystallinity, stability, and complexity are significant factors to consider in the design and development of covalent organic frameworks (COFs). Among various building blocks used, 1,3,5-triformylphloroglucinol (Tp) is notable for enhancing both crystallinity and structural stability in COFs. Tp facilitates the formation of β-ketoenamine-linked COFs through keto-enol tautomerism when reacted with aromatic amines. This review article examines the stability, crystallinity, and flexibility of synthetic methodologies involving Tp-based COFs, while highlighting their recent applications. We emphasize the critical roles of non-covalent interactions and keto-enol tautomerism in achieving high levels of crystallinity and stability. Additionally, the diverse and straightforward synthesis methods available for Tp-based COFs contribute to the prevalence of 1,3,5-triformylphloroglucinol in COF development. We conclude by addressing the challenges and future prospects in this area, underscoring the significant potential of Tp-based COFs for environmental and energy-related applications due to their exceptional structural tunability and functionality.

Triazine-containing Covalent Organic Polymer-derived Grid-Like Multilocular Spheres for Aqueous Supercapacitors

Triazine-containing covalent organic polymers (TCOPs) with unique structures and physicochemical properties are of great potential in energy storage and conversion applications, yet how to finely tune the morphology, and the accessible active sites, and to enhance capacitive activity remains a challenge. Here, the grid-like multilocular spheres derived from TCOP with abundant redox active sites and unique structures are fabricated via a molecular twist-induced regulation strategy, of which the number and size of cavities can be finely modulated by changing the conformers of the twisted unit and the Ostwald ripening time. The unique structure of the as-fabricated TCOP results in unprecedented high specific capacitance (8412 F g-1 at 1 A g-1) and enables the as-assembled supercapacitor with an ultra-high energy density of 675 Wh kg-1 in redox-active electrolyte (KI-mixed H2SO4), much better than all reported aqueous supercapacitors thus far. It is found that the high electro-activity is due to the synergistic effect of the enhanced accessibility of active sites and the enhanced interaction of the abundant active sites with the redox-active electrolytes. This approach may pave a new way to precise synthesis of COPs with tuned structure and properties for application-inspired cutting-edge electrochemical energy storage and beyond.

Donor-acceptor conjugated porous polymers from truxene and triazine: Effect of connecting units on photocatalytic activity for selective oxidation of amines and sulfides

Donor-acceptor (D-A) conjugated polymers have been widely reported as promising photocatalysts for organic conversion. However, achieving excellent photocatalytic performance still relies on the rational design of molecular structures and the careful selection of appropriate building blocks. In this study, we designed two D-A type conjugated porous polymers (CPPs) using 2,7,12-tribromo-5,5,10,10,15,15-hexamethyl-10,15-dihydro-5H-diindeno[1,2-a:1',2'-c]fluorene (Tx) as the donor unit and two 1,3,5-triazine-based derivatives, namely 2,4,6-tri(thiophen-2-yl)-1,3,5-triazine (TTT) and 2,4,6-triphenyl-1,3,5-triazine (TPT), as the acceptor units. The resulting CPPs are named ThTx-CPP and PhTx-CPP, respectively. The research findings emphasize the profound impact of minute structural changes in the triazine peripheral groups on the photocatalytic activity of the polymers. Compared to PhTx-CPP, ThTx-CPP exhibits superior light-harvesting capabilities, narrower bandgaps, and improved efficiency in charge separation. Specifically, ThTx-CPP demonstrates outstanding activity and selectivity in both amine coupling and sulfide oxidation reactions, surpassing PhTx-CPP by a significant margin. Furthermore, the catalyst retains its consistent activity even after five cycles of reuse, showcasing its high stability and excellent reusability.

Synthesis of high quality two dimensional covalent organic frameworks through a self-sacrificing guest strategy

The quality of covalent organic frameworks (COFs) crucially influences their mechanistic research and subsequent practical implementations. However, it has been widely observed that the structure damage induced by the activation procedure could compromise the quality of COFs. Here we develop a self-sacrificing guest method for synthesizing high-quality two-dimensional COFs (SG-COFs) by incorporating salt guests into the pores of the COF structure. These introduced salts play an indispensable role in supporting COF pores and mitigating quality loss during the activation process. Interestingly, due to the unique characteristic of salts decomposing into gases upon heating, this method can ultimately enable the synthesis of highly pure, high-quality COFs without the presence of residual guest molecules. The resulting sixteen COFs display superior crystallinity and porosity compared to those synthesized using conventional routes. Moreover, these high-quality SG-COFs have demonstrated to be highly efficient adsorbents for removal of per- and polyfluoroalkyl substances.

Regulation of Exciton Effects in Functionalized Conjugated Polymers by B-N Lewis Pairs for Visible-Light Photocatalysis

Strong excitonic effects are common in organic conjugated polymer semiconductors, severely hindering the generation of free charge carriers for conducting photocatalysis. Therefore, exploring new channels to modulate exciton dissociation in polymers is far-reaching in facilitating photocatalysis. A series of B-N Lewis pair functionalized conjugated polymers have been developed to minimize exciton effects by modulating charge transfer pathways. Theoretical studies have shown that introducing B-N Lewis pairs can dramatically increase the distance of charge transfer (D index) and the amount of electron transfer and reduce the Coulomb attraction energy (EC), which contributes to breaking the equilibrium of the coexistence of excitons and charge carriers. Further experimental results show that the singlet excitons are efficiently dissociated into more free-charge carriers under photoexcitation to participate in surface reactions. The optimized polymer PyPBM shows an exponential increase in photocatalytic hydrogen and hydrogen peroxide production performance by visible light illumination.

Rapid yet Controlled Synthesis of 2D Covalent Organic Framework Nanocapsules as High-Performance Photocatalytic Carriers

Synthesis and assembly of two-dimensional (2D) polymeric materials present a tricky trade-off between the high reaction rate and precise morphology control. Here we report a nanoconfined synthesis of imine-based 2D covalent organic frameworks (COFs) at the interface of oil-in-water (O/W) emulsion droplets stabilized by cationic surfactants. Highly uniform nanocapsules (NCs) could be prepared without adding extra catalysts at room temperature in just 4.5 h at a yield of 86 %. The NCs have tunable average diameters of 114-565 nm and shell thicknesses of 12-63 nm, depending on the monomer and surfactant types/concentrations. Their BET-specific surface areas are up to 139.0 m2/g, mainly contributed by narrowly-distributed mesopores at ~5.0 nm and micropores at 1.4 nm at a volume ratio (V1.4/V5.0) of 1.68. The surfactant plays the role of a catalyst during the reaction and interestingly, it also regulates the formation of mesopores and their sizes. Both theoretical and experimental studies confirm that the reaction has been accelerated by two orders of magnitude at the microdroplet interface, compared to that without emulsification. The resulting NCs could be well dispersed in water for at least six weeks with little size-distribution change, and they have been demonstrated to be highly efficient nanocatalysts in application of water-based hydrogen evolution, reaching a stable hydrogen production rate at 10.2 mmol ⋅ g-1 ⋅ h-1 for 6 hours. Such microdroplet interface-confined synthesis may facilitate the future development of 2D polymeric materials for more advanced applications.

Vertically Expanded Crystalline Porous Covalent Organic Frameworks

Covalent organic frameworks (COFs) can be developed for molecular confinement and separation. However, their proximate π stacks limit the interlayer distance to be only 3-6 Å, which is too small for guests to enter. As a result, COFs block access to the x-y space and limit guest entry/exit strictly to only the pores along the z direction. Therefore, the extended faces of each layer are hidden between layers, precluding any interactions with guest molecules. Here, we report a strategy for opening interlayer spaces of COFs to attain newly accessible nanospaces between layers. This becomes possible using coordination bonds to replace the conventional π-π stacks between layers. We demonstrate this concept by synthesizing two-dimensional covalent cobalt(II) porphyrin layers through topology-guided polymerization, which were piled up by bidentate axial pillars through coordination bonds with cobalt(II) porphyrin along the z direction, assembling vertically expanded COFs via a one-pot reaction. The resultant frameworks separate the layers with axial pillars and create discrete apertures between layers defined by the molecular length of the pillars. Consequently, the originally inaccessible interlayers are open for guest access, while the polygonal π planes are exposed to trigger various supramolecular interactions. Vapor sorption, breakthrough experiments, and computational studies mutually revealed that the vertically expanded frameworks with optimal interlayer slits induce additional interactions to discriminate benzene and cyclohexane and separate their mixtures efficiently under ambient conditions.

Regulation of Coordinating Anions around Single Co(II) Sites in a Covalent Organic Framework for Boosting CO2 Photoreduction

While photocatalytic CO2 reduction has been intensively investigated, reports on the influence of anions coordinated to catalytic metal sites on CO2 photoreduction remain limited. Herein, different coordinated anions (F-, Cl-, OAc-, and NO3 -) around single Co sites installed on bipyridine-based three-component covalent organic frameworks (COFs) were synthesized, affording TBD-COF-Co-X (X = F, Cl, OAc, and NO3), for photocatalytic CO2 reduction. Notably, the presence of these coordinated anions on the Co sites significantly influences the photocatalytic performance, where TBD-COF-Co-F exhibits superior activity to its counterparts. Combined experimental and theoretical results indicate that the enhanced activity in TBD-COF-Co-F is attributed to its efficient charge transfer, high CO2 adsorption capacity, and low energy barrier for CO2 activation. This study provides a new strategy for boosting COF photocatalysis through coordinated anion regulation around catalytic metal sites.

CO2/N2 Triggered Aqueous Recyclable Surfactants for Biphasic Catalytic Reactions in the Pickering Emulsions

The utilization of Pickering emulsions in interfacial catalysis offers a promising environmental platform for biphasic reactions. However, complicated surface coating or chemical grafting methods are always required to prepare the surface-active catalysts for the Pickering emulsions, since most of them are commercially unavailable. Here, we report CO2-switchable Pickering emulsions for biphasic reactions, in which Pd@Al2O3 nanoparticles are in situ modified by a CO2/N2 responsive surfactant. Compared with the chemical grafted methods, the in situ formed Pickering interfacial catalysts avoid complex chemical modification. Furthermore, efficient demulsification and separation of the oil phase and the products without surfactant contaminations can be achieved by CO2 trigger. The Pickering interfacial catalysis system can also be reformed after the aqueous phase containing the catalyst nanoparticles, and the surfactant is recycled and reused. The strategy is universal for nitrobenzene reductions and alcohol oxidations, providing a convenient and green method for the preparation of Pickering catalysts with commercially available nanoparticles, efficient emulsion separation, and recovery of the catalyst nanoparticles and emulsifiers in various two-phase organic reactions.

Ultrafast Interfacial Self-Assembly toward Bioderived Polyester COF Membranes with Microstructure Optimization

The precise manipulation of the microstructure (pore size, free volume distribution, and connectivity of the free-volume elements), thickness, and mechanical characteristics of membranes holds paramount significance in facilitating the effective utilization of self-standing membranes. In this contribution, the synthesis of two innovative ester-linked covalent-organic framework (COF) membranes is first reported, which are generated through the selection of plant-derived ellagic acid and quercetin phenolic monomers in conjunction with terephthaloyl chloride as a building block. The optimization of the microstructure of these two COF membranes is systematically achieved through the application of three different interfacial electric field systems: electric neutrality, positive electricity, and negative electricity. It is observed that the positively charged system facilitates a record increase in the rate of membrane formation, resulting in a denser membrane with a uniform pore size and enhanced flexibility. In addition, a correlation is identified wherein an increase in the alkyl chain length of the surfactants leads to a more uniform pore size and a decrease in the molecular weight cutoff of the COF membrane. The resulting COF membrane exhibits an unprecedented combination of high water permeance, superior sieving capability, robust mechanical strength, chemical robustness for promising membrane-based separation science and technology.

Fluorescence visualization of CO2-responsive phase transfer materials targeting at the heterogeneous interfacial reactions in advanced oxidation of naphthenic acid in wastewater

Treatment of naphthenic acids (NAs) in wastewater is necessary due to its high toxicity and difficult degradation. In the heterogeneous Fenton-like advanced oxidation of organic pollutant system, the insufficient accessibility of oxidizing agent and NAs greatly hamper the reaction efficiency. CO2-responsive phase transfer materials derived from polyethylene glycol (PEG)-based deep eutectic solvents were specific targeted at the immiscible-binary phase system. The NAs oxidative degradation process was optimized including the kinds of catalyst (Molecular weight of PEG, constitute of DESs, and dosage.), temperature, flow rate of CO2, et al. With the help of fluorescence properties of catalyst, the hydrophilic-hydrophobic interaction was visual-monitored and further studied. The amphipathic property of PEG-200/Sodium persulfate/Polyether amine 230 (PEA230) greatly reduced the aqueous/organic phase transfer barrier between sodium persulfate and NAs (up to 84 %), thus accreting oxidation rate. The surface tension decreased from 35.364 mN/m to 28.595 mN/m. To control the reaction rate, the CO2 respond structure of amido played an important role. In addition, the interfacial transfer intermediates and oxidation pathways were also explored by nuclear magnetic resonance, flourier transform infrared spectroscopy, surface tension, and radical inhibition experiments. The mechanism of advanced oxidation of NAs catalyzed by CO2-responsive phase transfer catalyst was proposed, which would made up for the deficiency of the system theory of heterogeneous chemical oxidation of organic pollutants.

Biomineralization-Inspired Synthesis of Hybrid COF Nanosheets toward Efficient Desalination Membranes

Synthesis of covalent organic framework nanosheets (CONs) with high aspect ratio is crucial to their assembly into advanced membranes. Nonetheless, the π-π stacking between covalent organic framework (COF) layers often leads to thick CONs. Herein, inspired by biomineralization process, a series of aspect ratio CONs >15 000 is synthesized by multifunctional polyelectrolytes which not only provide the nucleation sites for pre-assembly with COF monomer, but also suppress π-π interaction for anisotropic growth through protonation. The membrane assembled from CONs exhibited water permeance of 341 kg m-2 h-1 and salt rejection of 99.5% in desalination, outperforming ever-reported membranes. This method establishes a platform for the synthesis of crystalline nanosheets.

Phenanthroline-functionalized donor-acceptor covalent organic frameworks as photo-responsive nanozymes for visual colorimetric detection of isoniazid

Development of metal-free nanozymes has raised concern for their extensive applications in photocatalysis and sensing fields. As novel metal-free nanomaterials, covalent organic frameworks (COFs) have engendered intense interest in the construction of nanozymes due to their structural controllability and molecular functionality. The formation of the molecular arrangement by embedding orderly donor-acceptors (D-A) linked in the framework topology to modulate material properties for highly efficient enzyme mimicking activity is of importance but challenging. Here, a strong D-A type of COF was designed and synthesized by integrating electron donor units (pyrene) and electron acceptor units (phenanthroline), named Py-PD COF. Using experiments and theoretical calculations, the introduction of a phenanthroline ring endowed the Py-PD COF with a narrowed band gap, and efficient charge transfer and separation. Further, the Py-PD COF exhibited a superior light-responsive oxidase-mimicking characteristic under visible light irradiation, which could catalyze the oxidation of 3,3',5,5-tetramethylbenzidine (TMB) and give the corresponding evolution of color. The nanoenzymatic activity of the Py-PD COF was light-regulated, which offers a fascinating advantage because of its high efficiency and spatial controllability. Based on previously mentioned characteristics, an "on-off" sensing platform for the colorimetric analysis of isoniazid (INH) could be constructed with a good linear relationship (2-100 μM) and a low limit of detection (1.26 μM). This research shows that not only is Py-PD COF an environmentally friendly compound for the colorimetric detection of INH, but it is also capable of providing the interesting D-A type COF-based material for designing an excellent nanozyme.

Constructing ordered and tunable extrinsic porosity in covalent organic frameworks via water-mediated soft-template strategy

As one of the most attractive methods for the synthesis of ordered hierarchically porous crystalline materials, the soft-template method has not appeared in covalent organic frameworks (COFs) due to the incompatibility of surfactant self-assembly and guided crystallization process of COF precursors in the organic phase. Herein, we connect the soft templates to the COF backbone through ionic bonds, avoiding their crystallization incompatibilities, thus introducing an additional ordered arrangement of soft templates into the anionic microporous COFs. The ion exchange method is used to remove the templates while maintaining the high crystallinity of COFs, resulting in the construction of COFs with ordered hierarchically micropores/mesopores, herein named OHMMCOFs (OHMMCOF-1 and OHMMCOF-2). OHMMCOFs exhibit significantly enhanced functional group accessibility and faster mass transfer rate. The extrinsic porosity can be adjusted by changing the template length, concentration, and ratio. Cationic guanidine-based COFs (OHMMCOF-3) are also constructed using the same method, which verifies the scalability of the soft-template strategy. This work provides a path for constructing ordered and tunable extrinsic porosity in COFs with greatly improved mass transfer efficiency and functional group accessibility.