Aqueous Sol-Gel Synthesis and Shaping of Covalent Organic Frameworks.

The intrinsic fragility and insoluble nature of covalent organic frameworks (COFs) have strongly impeded their processability for practical applications. Herein, an aqueous-based sol-gel synthetic strategy is reported for the synthesis and shaping of COFs with task-specific applications that satisfy the principles of green chemistry for gram-scale production of crystalline materials. Our successful approach involves three pivotal aspects: the "prodrug mimic" design of water-soluble monomers, the utilization of hydrolyzable bonds, and the manipulation of reaction kinetics. The generality of the method is demonstrated by the successful preparation of representative high-surface area two-dimensional (2D) COFs with several commonly used amines. By virtue of this strategy, a COF colloidal dispersion is achieved and can be formulated into processable fluids, structured films, and COF monoliths. Remarkably, the obtained lightweight (∼0.020 g cm-3) and robust aerogels displayed outstanding adsorption capacity (exceeding 57 times its own weight) toward a variety of organic solvents and exhibited superior thermal insulating properties compared to the widely used sponge and cotton. This work demonstrates a versatile strategy for the synthesis and shaping of processable COF materials in water that will contribute to the development of COF monoliths for advanced applications.

Porous Ionic Network/CNT Composite Separator as a Polysulfide Snaring Shield for High Performance Lithium-Sulfur Battery.

Lithium-sulfur (Li-S) battery features a high theoretical energy density, but the shuttle of soluble polysulfides between the two electrodes often results in a rapid capacity decay. Herein, a straightforward electrostatic adsorption strategy based on a cross-linked polyimidazolium separator as a snaring shield of polysulfides is reported, which suppresses the undesirable migration of polysulfides to the anode. The porous ionic network (PIN)-modified carbon nanotubes (CNTs) are successfully prepared and coated onto a commercial porous polypropylene membrane in a vacuum-filtration step. The favorable affinity of the imidazolium ring toward polysulfide via the polar interaction and the electrostatic effect of ions mitigates the undesirable shuttle of polysulfides in the electrolyte, improving the Li─S battery in terms of rate performance and cycling life. Compared to the reference PIN-free CNT-coated separator, the PIN/CNT-coated one has an increased initial capacity of 1.3 folds (up to 1394.8 mAh g-1 for PIN/CNT/PP-3) at 0.1 C.

Building Porous Ni(Salen)-Based Catalysts from Waste Styrofoam via Autocatalytic Coupling Chemistry for Heterogeneous Oxidation with Molecular Oxygen.

The development of robust and industrially viable catalysts from plastic waste is of great significance, and the facile construction of high performance heterogeneous catalyst systems for phenol-quinone conversions remains a grand challenge. Herein, a feasible strategy is demonstrated to reclaim Styrofoam into hierarchically porous nickel-salen-loaded hypercrosslinked polystyrene (PS@Ni-salen) catalysts with high activities through an unusual autocatalytic coupling route. The salen is immobilized onto PS chain by Friedel-Crafts alkylation of benzyl chloride derivatives, and the generated hydrogen chloride coordinately promotes the simultaneous crosslinking and bridge formation between aromatic rings via a Scholl coupling route, leading to hierarchically porous networks. After the metallization with Ni, the resultant networks exhibit high catalytic activity for the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-1,4-benzoquinone under mild conditions (303 K, 1 bar of O2 ). This catalyst also demonstrates attractive recycling performance without an obvious loss of catalytic efficiency over five consecutive cycles. This methodology might provide a potential sustainable alternative to construct environmentally benign and cost-effective catalysts for specific organic transformation.

Reprocessible Triketoenamine-Based Vitrimers with Closed-Loop Recyclability.

Development of thermosets that can be repeatedly recycled via both chemical route (closed-loop) and thermo-mechanical process is attractive and remains an imperative task. In this work, we reported a triketoenamine based dynamic covalent network derived from 2,4,6-triformylphloroglucinol and secondary amines. The resulting triketoenamine based network does not have intramolecular hydrogen bonds, thus reducing its π-electron delocalization, lowering the stability of the tautomer structure, and enabling its dynamic feature. By virtue of the highly reversible bond exchange, this novel dynamic covalent bond enables the easy construction of highly crosslinked and chemically reprocessable networks from commercially available monomers. The as-made polymer monoliths exhibit high mechanical properties (tensile strength of 79.4 MPa and Young's modulus of 571.4 MPa) and can undergo a monomer-network-monomer (yields up to 90 %) recycling mediated by an aqueous solution, with the new-generation polymer capable of restoring the material strength to its original state. In addition, owing to its dynamic nature, a catalyst-free and low-temperature reprogrammable covalent adaptable network (vitrimer) was achieved. The design concept reported herein can be applied to the development of other novel vitrimers with high repressibility and recyclability, and sheds light on future design of sustainable polymers with minimal environmental impact.

Thiol-grafted covalent organic framework-based electrochemical platforms for sensitive detection of Hg(II) ions.

Triazine-based covalent organic frameworks functionalized by thiol and thioether (COFS-CH3/COFS-SH) were designed and served as a platform that could bind with mercury ions specifically based on Hard-Soft-Acid-Base theory. As such, when employing COFs as a modifier in a carbon paste electrode (CPE), the COFS-CH3-modified CPE revealed an extraordinary performance (detection limit of 0.01 ppb; linear range of 0.1 to 1.0 ppb) and repeatability for electrochemical detection of trace mercury, even in real samples collected from tap or lake water. This innovative approach leverages the inherent properties of covalent organic frameworks (COFs) to enable highly sensitive and selective detection of target analytes.

Interfacial Ti-S Bond Modulated S-Scheme MOF/Covalent Triazine Framework Nanosheet Heterojunctions for Photocatalytic C-H Functionalization.

Constructing photocatalyst systems to functionalize the inert C-H bonds has attracted extensive research interest. However, purposeful modulation of interfacial charge transfer in heterostructures remains a challenge, as it usually suffers from sluggish kinetics. Reported herein is an easy strategy to construct the heteroatom-induced interface for developing the titanium-organic frameworks (MOF-902) @ thiophene-based covalent triazine frameworks (CTF-Th) nanosheets S-scheme heterojunctions with controllable oxygen vacancies (OVs). Specifically, Ti atoms were first anchored onto the heteroatom site of CTF-Th nanosheets, and then grown into MOF-902 via an interfacial Ti-S linkage, generating OVs. Using in situ X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT) calculations, the enhanced interfacial charge separation and transfer induced by moderate OVs in the pre-designed S-scheme nanosheets was validated. The heterostructures exhibited an improved efficiency in photocatalytic C3-acylation of indoles under mild conditions with a yield 8.2 times larger than pristine CTF-Th or MOF-902 and enabled an extended scope of substrates (15 examples). This performance is superior to state-of-the-art photocatalyst and can be retained, without significant loss, after 12 consecutive cycles.

Modular and Selective Access to Functionalized Alkynes and Allenes via the Intermediacy of Propargylic Acetates.

We report an efficient one-pot, two-step procedure for the modular synthesis of α-difunctionalized alkynes and trisubstituted allenes by sequential cross-coupling of benzal gem-diacetates with organozinc or -copper reagents in the absence of external transition metals. The intermediacy of propargylic acetates enables the divergent and selective synthesis of these valuable products. This method features its readily accessible substrates, relatively mild conditions, wide scope, and scalability in practical synthesis.

Effect of Tea Polyphenols on the Melt Grafting of Glycidyl Methacrylate onto Polypropylene.

It is considered to be one of the most effective strategies to prepare functionalized polypropylene (PP) materials via the melt grafting of polar monomers onto PP chains. However, the grafting efficiency of functional monomers is generally low. To achieve a high grafting efficiency, we explored the effect of tea polyphenols (C), which are good free radical scavengers, on the melt grafting of glycidyl methacrylate (GMA) onto PP chains initiated by dicumyl peroxide (DCP). Specifically, 0.5~3 wt% of tea polyphenols (C) were introduced to the PP/DCP/GMA melt blending system. The morphology, melt flow rate (MFR), thermal and mechanical properties of tea polyphenols (C) incorporated PP/DCP/GMA blends were investigated systematically. The results showed that the proper amount of tea polyphenols (C) (0.5~2 wt%) promoted the grafting of GMA. Unexpectedly, the PP backbone suffered from more severe degradation with the addition of tea polyphenols (C). The phenomena were ascribed to the reaction between phenolic hydroxyl groups of tea polyphenols (C) and epoxy groups of grafted GMA, which was revealed by the FTIR results. In addition, according to DSC and the tensile test, the co-grafting of GMA and tea polyphenols (C) improved the crystallization ability, yield strength and Young's modulus of the PP matrix.

Structure and Properties of PVDF/PA6 Blends Compatibilized by Ionic Liquid-Grafted PA6.

Compatibilization of immiscible blends is critically important for developing high-performance polymer materials. In this work, an ionic liquid, 1-vinyl-3-butyl imidazole chloride, grafted polyamide 6 (PA6-g-IL(Cl)) with a quasi-block structure was used as a compatibilizer for an immiscible poly(vinylidene fluoride) (PVDF)/PA6 blend. The effects of two PA6-g-IL(Cl)s (E-2%-50K and E-8%-50K) on the morphology, crystallization behavior, mechanical properties, and surface resistance of the PVDF/PA6 blend were investigated systematically. It was found that the two types of PA6-g-IL(Cl)s had a favorable compatibilization effect on the PVDF/PA6 blend. Specifically, the morphology of the PVDF/PA6 = 60/40 blend transformed from a typical sea-island into a bicontinuous structure after incorporating E-8%-50K with a high degree of grafting (DG). In addition, the tensile strength of the PVDF/PA6/E-8%-50K blend reached 66 MPa, which is higher than that of PVDF, PA6 and the PVDF/PA6 blend. Moreover, the PVDF/PA6/E-8%-50K blend exhibited surface conductivity due to the conductive path offered by the bicontinuous structure and conductive ions offered by grafted IL(Cl). Differential scanning calorimetry (DSC) and wide-angle X-ray diffractometry (WAXD) results revealed that PA6-g-IL(Cl) exhibits different effects on the crystallization behavior of PVDF and PA6. The compatibilization mechanism was concluded to be based on the fact that the nongrafted PA6 blocks entangled with the PA6 chains, while the ionic liquid-grafted PA6 blocks interacted with the PVDF chains. This work offers a new strategy for the compatibilization of immiscible polymer blends.

A Knitting Copolymerization Strategy to Build Porous Polytriazolium Salts for Removal of Anionic Dyes and MnO4.

Although considerable efforts have been devoted to novel ionic porous networks (IPNs), the development of them in a scalable manner to tackle the issues in pollutant treatment by adsorption remains an imminent challenge. Herein, inspired by natural spider webs, a knitting copolymerization strategy is proposed to construct analogue triazolium salt-based porous networks (IPN-CSUs). It is not only convenient to incorporate the cationic motifs into the network, but easy to control over the contents of ionic pairs. The as-prepared IPN-CSUs displays a high surface area of 924 m2 g-1 , a large pore volume of 1.27 cm3 g-1 and abundant ionic sites, thereby exhibiting fast adsorption rate and high adsorption capacity towards organic and inorganic pollutants. The kinetics and thermodynamics study reveal that the adsorption followed a pseudo-second-order kinetic model and Langmuir isotherm model correspondingly. Specifically, the maximum adsorption capacity of the IPN-CSUs is as high as 1.82 mg mg- 1 for permanganate ions and up to 0.54 mg mg-1 for methyl orange, which stands out among the previously reported porous adsorbents so far. It is expected that the strategy reported herein can be extended to the development of other potential efficient adsorbents in water purifications.

Modulating Carrier Transfer over Carbazolic Conjugated Microporous Polymers via Donor Structural Design for Functionalization of Thiophenols.

Developing photocatalysts to steer conversion of solar energy toward high-value-added fine chemicals represents a potentially viable approach to address the energy crisis and environmental issues. However, enablement of this conversion is usually impeded by the sluggish kinetic process for proton-coupled electron transfer and rapid recombination of photogenerated excitons. Herein, we report a simple and general structural expansion strategy to facilitate charge transfer in conjugated microporous polymers (CMPs) via engineering the donor surrounding the trifluoromethylphenyl core. The resulting CMPs combine high surface area, strong light-harvesting capabilities, and tunable optical properties endowed by extended π-conjugation; the optimized compound CbzCMP-5 generated from 9,9',9″-(2-(trifluoromethyl)benzene-1,3,5-triyl)tris(9H-carbazole) remarkably enhanced the photogenerated carrier transfer efficiency, enabling the functionalization of thiophenols toward thiocarbamates and 3-sulfenylindoles with high photocatalytic efficiency. Most importantly, the in-depth insights into the carrier-transfer processes open up new prospects on further optimization and rational design of photoactive polymers for efficient charge-transfer-mediated reactions.

Polarization-induced charge separation in conjugated microporous polymers for efficient visible light-driven C-3 selenocyanation of indoles.

Conjugated microporous polymers (CMPs) are cost-effective photocatalysts in organic transformations, while they are usually limited by the insufficient separation of photogenerated charges. Here we report a polarization strategy through molecular geometry optimization to promote the charge separation of CMPs. Three CMP photocatalysts with an alternative donor-acceptor skeleton and tunable symmetry were synthesized by the oxidative coupling of bis-carbazoles with electron-deficient bridges (benzene/pyridine/pyrimidine). Simply regulating the polarization of the starting monomers leads to tailorable porosity, photoelectric properties, and photocatalytic activity of the CMPs. They exhibited high efficiency in C-3 selenocyanation of indoles under visible-light and at room temperature, and pyridine-based CMPs with the largest dipole moment gave a yield of up to 94%, superior to their state-of-the-art photocatalyst counterparts. Photo-physical experiments combined with theoretical calculations further supported that the incorporation of the polarized linker introduced an internal electric field, benefitting efficient charge separation. This offered new insight into developing high-performance photocatalysts.

Visible-light-driven Cr(VI) reduction by ferrocene-integrated conjugated porous polymers via dual catalytic routes.

Conjugated porous polymers with rapid separation of photogenerated charges and multiple catalytic pathways remain a great challenge. Herein, two ferrocene-based polymers (Fc-CPPs) with high charge separation efficiency and unique dual catalytic routes for Cr(vi) reduction were developed. They exhibited an excellent efficiency, with almost 99% of Cr(vi) readily converted to Cr(iii) under 15 min of visible light illumination (λ > 420 nm).

Co(III)-Salen immobilized cellulose nanocrystals for efficient catalytic CO2 fixation into cyclic carbonates under mild conditions.

Searching for green, recyclable and highly efficient catalyst for the synthesis of cyclic carbonates from CO2 is of great importance because it is profitable for reducing the greenhouse effects and meets the principles of green chemistry. Herein, a series of cellulose nanocrystals, either the pristine or modified ones (TEMPO oxidized and Co(III)salen immobilized), were explored as catalysts for cycloaddition of epoxides and carbon dioxide. The impact of surface properties on the performance of the as-made catalysts was investigated. Co(III)-salen grafted cellulose nanocrystals was proven to be the most effective catalyst in this study, which could afford excellent yield up to 99 % after 24 h even under low CO2 pressures of 0.1 MPa. They can be easily recovered and reused for at least 4 times, demonstrating their excellent stability. We found that the surface functional groups such as enriched sulfate or carboxylic groups could also account for the enhanced catalytic activity. This work highlights the applications of green and sustainable nanoparticles in a cycloaddition reaction and offers a sustainable solution in industrial catalysis related to CO2 conversions.

Design of well-defined shell-core covalent organic frameworks/metal sulfide as an efficient Z-scheme heterojunction for photocatalytic water splitting.

Development of a covalent-organic framework (COF)-based Z-scheme heterostructure is a promising strategy for solar energy driven water splitting, but the construction of a COF-based Z-scheme heterostructure with well-defined architecture, large contact area and intimate contact interfaces is scarce. Herein, we fabricated a direct Z-scheme heterostructure COF-metal sulfide hybrid (T-COF@CdS) with shell-core architecture by self-polymerization of 1,3,5-benzenetricarboxaldehyde and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine in situ on CdS. The formed C-S chemical bonding between T-COF and CdS could provide a very tight and stable interface. Owing to the properly staggered band alignment, strong interfacial interaction and large interfacial contact area between T-COF and CdS, a Z-scheme route for charge separation and transfer is realized, resulting in electron accumulation in CdS for H2O reduction. The obtained Z-scheme heterostructure T-COF@CdS-3 exhibits a high apparent quantum efficiency of 37.8% under 365 nm monochromatic light irradiation, and long-term stability arising from shell-core structures in which the T-COF shell protects the catalytic centers of CdS against deactivation, as well as acts as oxidation sites to avoid the photocorrosion of CdS. This work provides a strategy for the construction of a shell-core direct Z-scheme heterostructure photocatalyst for water splitting with high performance.

One-pot construction of nitrogen-rich polymeric ionic porous networks for effective CO2 capture and fixation.

Facile preparation of ionic porous networks (IPNs) with large and permanent porosity is highly desirable for CO2 capture and transformation but remains a challenge. Here we report a one-pot base-mediated construction of nitrogen-rich IPNs through a combination of nucleophilic substitution and quaternisation chemistry from H-imidazole. This strategy, as proven by the model reactions of 1H-imidazole or 1-methyl-1H-imidazole with cyanuric chloride, allows for fine regulation of porosity and physicochemical properties, leading to nitrogen-rich IPNs featuring abundant ionic units and radicals. The as-prepared networks, termed IPN-CSUs, efficiently capture CO2 (80.1 cc g-1 at 273 K/1 bar) with an ideal CO2/N2 selectivity of 139.7. They can also effectively catalyse the cycloaddition reaction between CO2 and epoxides with high yields of up to 99% under mild conditions (0.1 MPa, 298 K), suggesting their possible applications in the fields of both selective molecular separation and conversion. Unlike the previously known strategies generally involving single coupling chemistry, our strategy combining two coupling routes in one pot appears to be unique and potentially applicable to other building blocks.

Carbodiimide coupling versus click chemistry for nanoparticle surface functionalization: A comparative study for the encapsulation of sodium cholate by cellulose nanocrystals modified with ß-cyclodextrin.

Grafting beta-cyclodextrin (ß-CD) onto cellulose nanocrystals (CNC) with the formation of well-dispersed nanoparticles (CNC-CD) and understanding their physicochemical properties are appealing but still challenging in controlled-release applications. Two immobilization methods were proposed and examined in this study; (i) copper (I) catalyzed click chemistry (CuACC) and (ii) carbodiimide coupling. Fourier-transform infrared spectroscopy (FTIR), UV-vis, elementary analysis, contact angle measurements, and thermogravimetric analysis (TGA) were conducted to elucidate the surface modifications. Phenolphthalein (PHTH) titration was used to quantify the grafting efficiency of ß-CD on the CNC surface. The carbodiimide coupling in dimethyl sulfoxide was effective to introduce the highest amounts of ß-CD (0.17 mmol/g sample) to the CNC in this study. The encapsulation process of bile surfactant, sodium cholate (NaC) was investigated by isothermal titration calorimeter (ITC), and the thermodynamic parameters were determined. The "molecular docking" brought by ß-CD offers possible new applications of this sustainable nanohybrid system in the environmental, biomedical and pharmaceutical sectors.

A Vinylene-Bridged Conjugated Covalent Triazine Polymer as a Visible-Light-Active Photocatalyst for Degradation of Methylene Blue.

The development of new photocatalytic platforms using novel semiconductor material is an important challenge. Herein, a sp2 carbon-conjugated covalent triazine polymer (sp2 c-CTP-4), featuring a vinylene bridge and extended π-conjugation, is prepared as a highly efficient photocatalyst for degradation of methylene blue. sp2 c-CTP-4 exhibits substantial semiconducting properties such as enhanced charge transfer and prolonged lifetime of carriers compared to its counterparts with CN or CC connections, likely due to its extended π-delocalization with an unencumbered CC bridge. Moreover, benefiting from its high chemical stability, the as-made catalyst can be recycled five times with good retention of photocatalytic activity. This study provides a new pathway for constructing a robust platform for efficient photocatalysis and gives insight into the structure-property relationship of conjugated polymers.

Self-healing stimuli-responsive cellulose nanocrystal hydrogels.

A facile and universal approach to prepare cellulose nanocrystal reinforced functional hydrogels is proposed. An organic solvent-free and eco-friendly method was adopted, where both the modification and polymerization were conducted in an aqueous solution. Cellulose nanocrystal (CNC) and sodium alginate (SA) were first oxidized under mild conditions to introduce aldehyde groups. Subsequently, amine-containing vinyl functionalized monomers were introduced to the surface of CNC or backbone of oxidized SA via a dynamic Schiff-base reaction. The bio-based hydrogels were then prepared via a one-pot in-situ polymerization, where the functional CNC and SA served as novel macro-cross-linkers that contributed to the structural integrity and mechanical stability of the hydrogels. The hydrogels displayed uniform chemical and macroscopic structures and could self-heal within several hours at room temperature. The design of specific monomers will allow the introduction of stimuli-responsive properties to the functional hydrogels and a chemically robust thermally-triggered actuator was demonstrated. Due to its flexible design and practical approach, the hydrogels could find potential uses in agricultural and pharmaceutical products.

Facile preparation of CoO nanoparticles embedded N-doped porous carbon from conjugated microporous polymer for oxygen reduction reaction.

Developing cost-effective approaches for fabricating porous carbon (PC) based catalysts with favourable oxygen reduction reaction (ORR) performance is highly significant for fuel-cell devices. Herein, we reported a precursor controlled, molten salt-templated approach to prepare ultrafine CoO nanoparticles embedded nitrogen-doped PC materials with high surface area (1236 m2 g-1) and large pore volume (0.68 cm3 g-1). This method is simple and feasible, which produce CoO nanoparticles that were uniformly distributed on carbon skeleton with diameters in the range of 5-10 nm. The unexpected collapse of porous structures and agglomeration of metal nanoparticles were suppressed in the synthetic process. The as-made sample not only showed efficient catalytic activity towards ORR in alkaline media with a half wave potential (E1/2) of 0.85 V (vs. RHE), but also exhibited better stability and stronger resistance to methanol than Pt/C.