Layered Covalent Organic Frameworks Research Articles

Statistical Representation of Stacking Disorder in Layered Covalent Organic Frameworks

Statistical Representation of Stacking Disorder in Layered Covalent Organic Frameworks

Aggregated Structures of Two-Dimensional Covalent Organic Frameworks.

Covalent organic frameworks (COFs) have found wide applications due to their crystalline structures. However, it is still challenging to quantify crystalline phases in a COF sample. This is because COFs, especially 2D ones, are usually obtained as mixtures of polycrystalline powders. Therefore, the understanding of the aggregated structures of 2D COFs is of significant importance for their efficient utilization. Here we report the study of the aggregated structures of 2D COFs using 13C solid-state nuclear magnetic resonance (13C SSNMR). We find that 13C SSNMR can distinguish different aggregated structures in a 2D COF because COF layer stacking creates confined spaces that enable intimate interactions between atoms/groups from adjacent layers. Subsequently, the chemical environments of these atoms/groups are changed compared with those of the nonstacking structures. Such a change in the chemical environment is significant enough to be captured by 13C SSNMR. After analyzing four 2D COFs, we find it particularly useful for 13C SSNMR to quantitatively distinguish the AA stacking structure from other aggregated structures. Additionally, 13C SSNMR data suggest the existence of offset stacking structures in 2D COFs. These offset stacking structures are not long-range-ordered and are eluded from X-ray-based detections, and thus they have not been reported before. In addition to the dried state, the aggregated structures of solvated 2D COFs are also studied by 13C SSNMR, showing that 2D COFs have different aggregated structures in dried versus solvated states. These results represent the first quantitative study on the aggregated structures of 2D COFs, deepen our understanding of the structures of 2D COFs, and further their applications.

Covalent Organic Framework-Mediated Thin-Film Composite Polyamide Membranes toward Precise Ion Sieving.

Covalent organic frameworks (COFs) have evinced a potential solution that promises for fast and efficient molecular separation due to the presence of orderly arranged pores and regulable pore apertures. Herein, the synthesized COF (TPB-DMTP-COF) with the pore aperture matching the pore size of the nanofiltration (NF) membrane was utilized to modulate the physicochemical characters of the polyamide (PA) membranes. It is demonstrated that COFs with superior polymer affinity and hydrophilicity not only circumvent the nonselective interfacial cavities but also improve the hydrophilicity of the resultant thin-film nanocomposite (TFN) membranes. Furthermore, the predeposited COF layer is able to slow down the diffusion rate toward the reaction boundary through hydrogen bonding, which is consistent with the results of molecular dynamic (MD) and dissipative particle dynamic (DPD) simulations. In this context, COF-modulated TFN membranes show a roughened and thickened surface with bubble-shaped structures in contrast to the nodular structure of original polyamide membranes. Combined with the introduced in-plane pores of COFs, the resultant TFN membranes display a significantly elevated water permeance of 35.7 L m2 h-1 bar-1, almost 4-fold that of unmodified polyamide membranes. Furthermore, the selectivity coefficient of Cl-/SO42- for COF-modulated TFN membranes achieves a high value of 84 mainly related to the enhanced charge density, far exceeding the traditional NF membranes. This work is considered to provide a guideline of exploring hydrophilic COFs as an interlayer for constructing highly permeable membranes with precise ion-sieving ability.

Band gap opening from displacive instabilities in layered covalent-organic frameworks

Symmetry breaking in covalent organic frameworks (COFs) lowers the energy and increases the valence to conduction band separation of the material.

Impact of Imine Bond Orientations on the Geometric and Electronic Structures of Imine-based Covalent Organic Frameworks.

Many efforts are currently devoted to improving the stability and crystallinity of imine-based two-dimensional (2D) covalent organic frameworks (COFs) given their wide range of potential applications. The variation in the relative orientations of the imine bonds has been found to be a critical factor that impacts the stacking of the 2D COF layers, leads to the formation of isomer structures, and influences the crystallinity of the final product. Most investigations to date have focused only on the structural properties, while the role of the imine orientations on the electronic properties has not been studied systematically. Here, we explore this effect by examining how the electronic band structures, electronic couplings, and effective masses evolve when considering four isomeric structures of an imine-linked tetraphenyl-pyrene naphthalene-diimide COF. Our results provide an understanding of the impact of the imine orientations and how they need to be controlled to realize COF inter-layer stackings that can lead to efficient cross-plane electron transport. They can be used to guide the design and synthesis of imine-based COFs for applications where charge transport needs to be optimized.

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Surface‐Mediated Construction of an Ultrathin Free‐Standing Covalent Organic Framework Membrane for Efficient Proton Conduction

AbstractAs a new class of crystalline porous organic materials, covalent organic frameworks (COFs) have attracted considerable attention for proton conduction owing to their regular channels and tailored functionality. However, most COFs are insoluble and unprocessable, which makes membrane preparation for practical use a challenge. In this study, we used surface‐initiated condensation polymerization of a trialdehyde and a phenylenediamine for the synthesis of sulfonic COF (SCOF) coatings. The COF layer thickness could be finely tuned from 10 to 100 nm by controlling the polymerization time. Moreover, free‐standing COF membranes were obtained by sacrificing the bridging layer without any decomposition of the COF structure. Benefiting from the abundant sulfonic acid groups in the COF channels, the proton conductivity of the SCOF membrane reached 0.54 S cm−1 at 80 °C in pure water. To our knowledge, this is one of the highest values for a pristine COF membrane in the absence of additional additives.

Bio-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extraction

Summary As the most sophisticated separation system, biological membranes have served as natural prototypes for the design of artificial membranes because they provide high permeability and solute selectivity, owing to the presence of specialized ion channels. However, developing stable and selective artificial ion channels remains a formidable challenge. Herein, we demonstrate the construction of lithium nanochannels in two-dimensional covalent organic frameworks (COFs) to create biomimetic membranes. Implanted lithiophilic oligoethers conferred specificity and facilitated Li+ diffusion along the pore pathway of the COF. The ion channel characteristics were indicated by reversal potential measurements, showing that the relative permeability decreased in the order Li+ > K+ > Na+ > Ca2+ > Mg2+. The Li+ transfer was enhanced, while other ions were obstructed, allowing for high selectivity and permeability. A Li+/Mg2+ separation factor of 64 was achieved, confirming high lithium affinity. This study may serve as a design principle to develop selective artificial membranes for effective ion separation.

Secondary growth of bi-layered covalent organic framework nanofilms with offset channels for desalination

Processing two-dimensional (2D) covalent organic frameworks (COFs) into nanofilms has gained widespread attention in water treatment. However, the designed synthesis of COF-based membranes for ion separations still remains a huge challenge though the strategies to produce COF molecular separation membranes have been greatly developed. Herein, we report the construction of bi-layered COF nanofilms for efficient desalination through a simple yet effective secondary growth. The suggested strategy is capable of easily regulating the growth of another COF layer on the pre-synthesized first layer, thus producing crystalline and defect-free bi-layered COF nanofilms with distinct laminated structures. Most importantly, the offset channels are involved at the interface of two nanofilms obtained from the first and secondary growth, which leads to a constricted effective aperture that significantly enhances the ion separation performance. Specifically, the bi-layered COF nanofilms composited with macroporous supports exhibit high rejection rates (~95.8%) to Na2SO4, observably surpassing the desalination performance of single-layered COF nanofilms (rejection<10%). This work not only develops an ingenious way to fabricate COF-on-COF nanofilms with narrowed channels by tailoring the laminated structure of 2D COFs at the film interface, but also starts an avenue for the adoption of COF platforms for membrane separations with an improved precision.

Alkynyl-Based sp 2 Carbon-Conjugated Covalent Organic Frameworks with Enhanced Uranium Extraction from Seawater by Photoinduced Multiple Effects

Biofouling is a major obstacle to the efficient extraction of uranium from seawater due to the numerous marine microorganisms in the ocean. Herein, we report a novel amidoxime (AO) crystalline cova...

MOF-in-COF molecular sieving membrane for selective hydrogen separation

Covalent organic frameworks (COFs) are promising materials for advanced molecular-separation membranes, but their wide nanometer-sized pores prevent selective gas separation through molecular sieving. Herein, we propose a MOF-in-COF concept for the confined growth of metal-organic framework (MOFs) inside a supported COF layer to prepare MOF-in-COF membranes. These membranes feature a unique MOF-in-COF micro/nanopore network, presumably due to the formation of MOFs as a pearl string-like chain of unit cells in the 1D channel of 2D COFs. The MOF-in-COF membranes exhibit an excellent hydrogen permeance (>3000 GPU) together with a significant enhancement of separation selectivity of hydrogen over other gases. The superior separation performance for H2/CO2 and H2/CH4 surpasses the Robeson upper bounds, benefiting from the synergy combining precise size sieving and fast molecular transport through the MOF-in-COF channels. The synthesis of different combinations of MOFs and COFs in robust MOF-in-COF membranes demonstrates the versatility of our design strategy.

Oligomer-Induced Ultrathin Covalent Organic Frameworks Composite Membranes Generated by Catalysis-Interfacial Polymerization

The practical application of covalent organic frameworks (COFs) membrane still faces major challenges emanated from harsh fabrication condition, non-uniform performance and high production cost. To address these issues, catalysis-interfacial polymerization (catal.IP) was proposed to fabricate oligomer-induced COFs composite membranes , in which two reactive monomers having extremely low concentration (0.6 mM) were in advance pre-reacted to generate oligomers with planar structure and then subsequently grew under the catalysis of acetic acid to form an ultrathin and continuous COFs layer. The as-prepared TpPa (catal.IP), TpBd (catal.IP) and TpDp (catal.IP) membranes exhibited superior permeability of 52~85 L·m-2·h-1·bar-1 and congo red rejection of 93~99%, precisely in consistent with the pore sizes of COFs. The catal.IP technique would not only inspire the fabrication of COFs composite membranes with a greater variety but also make their scale-up and application feasible and economical.

Fine tuning of supported covalent organic framework with molecular active sites loaded as efficient electrocatalyst for water oxidation

Covalent organic frameworks (COFs) with internal fixation of molecular active sites provide unprecedent high atom utilization and selectivity in various electrocatalysis. So far, it is still big challenge to exploit their fine tuning to enhance the catalytic performance and have insight into their advantages. In this paper, we report an ideal platform construed by COFs and multiwall carbon nanotubes (MWCNTs) to illustrate key factors in COF-based electrocatalysts towards oxygen evolution. Fine tuning of COF layers reveals the optimized catalytic performance relies on the proper loadings of molecular catalyst on MWCNTs to balance the mass and electron transfer. Moreover, a novel strategy of diluting the active sites to enhance their intrinsic activity is proposed to further enhance the electrocatalytic performance. The generality of the fine regulation is fully demonstrated with different COFs. This work elucidates the fine tuning of COF-based electrocatalysts and sheds light on the broad applications of molecular catalysts.

In situ growth of COF-rLZU1 on the surface of silica sphere as stationary phase for high performance liquid chromatography

The composite materials consist of Covalent Organic Frameworks (COFs) and silica have been regarded as a kind of promising stationary phases due to combination of the large specific surface area and good mechanical strength of porous silica microspheres and the porous structure and the excellent stability of COFs. Herein, a novel COFs-silica composite (SiO2@rLZU1, reduced Lan Zhou University-1) was prepared via an in-situ growth strategy with a 32 nm-thick COFs layer on the surface of silica and a 2.2 nm-thick COFs layer on the inner surface of the mesopores of spherical silica (5 μm, 120 Å). With secondary amine and phenyl groups, the novel stationary phase provided hydrophilic, hydrophobic and π-π interactions when used in HPLC, showing different selectivity from typical reversed-phase stationary phases. Probe molecules with aromatic moieties varying in polarity, including acidic (phenol, pyrocatechol, and pyrogallol), basic (aniline, 4-chloroaniline, and 4-nitroaniline) and neutral (benzene homologues) compounds, were all baseline separated on the SiO2@rLZU1 column, indicating its excellent separation performance. Besides, the SiO2@rLZU1 column also exhibited great repeatability with intraday RSDs of the retention time of three anilines less than 0.31% (n = 6) and peak area less than 1.63% (n = 6). On the SiO2@rLZU1 column, satisfied results were achieved in the separation of real samples such as fullerenes and coking wastewater, suggesting the great potential of the as-synthesized stationary phase in HPLC applications.

Unveiling the Local Structure of Palladium Loaded into Imine-Linked Layered Covalent Organic Frameworks for Cross-Coupling Catalysis.

Layered covalent organic frameworks (2D-COFs), composed of reversible imine linkages and accessible pores, offer versatility for chemical modifications towards the development of catalytic materials. Nitrogen-enriched COFs are good candidates for binding Pd species. Understanding the local structure of reacting Pd sites bonded to the COF pores is key to rationalize interactions between active sites and porous surfaces. By combining advanced synchrotron characterization methods with periodic computational DFT modeling, the precise atomic structure of catalytic Pd sites attached to local defects is resolved within an archetypical imine-linked 2D-COF. This material was synthesized using an in situ method as a gel, under which imine hydrolysis and metalation reactions are coupled. Local defects formed in situ within imine-linked 2D-COF materials are highly reactive towards Pd metalation, resulting in active materials for Suzuki-Miyaura cross-coupling reactions.

Unveiling the Local Structure of Palladium Loaded into Imine‐Linked Layered Covalent Organic Frameworks for Cross‐Coupling Catalysis

AbstractLayered covalent organic frameworks (2D‐COFs), composed of reversible imine linkages and accessible pores, offer versatility for chemical modifications towards the development of catalytic materials. Nitrogen‐enriched COFs are good candidates for binding Pd species. Understanding the local structure of reacting Pd sites bonded to the COF pores is key to rationalize interactions between active sites and porous surfaces. By combining advanced synchrotron characterization methods with periodic computational DFT modeling, the precise atomic structure of catalytic Pd sites attached to local defects is resolved within an archetypical imine‐linked 2D‐COF. This material was synthesized using an in situ method as a gel, under which imine hydrolysis and metalation reactions are coupled. Local defects formed in situ within imine‐linked 2D‐COF materials are highly reactive towards Pd metalation, resulting in active materials for Suzuki–Miyaura cross‐coupling reactions.

Incorporation of photocatalytic Pt(II) complexes into imine-based layered covalent organic frameworks (COFs) through monomer truncation strategy

A new photoactive Pt (II) hydroxyquinoline complex has been covalently linked into the structure of an imine-based layered covalent organic framework (COF) through the monomer truncation strategy. Such strategy allows the incorporation of molecular fragments with only one functional group able to condensate into the imine-framework. The photocatalytic activity of the resulting Pt@COF has been applied to the oxidation of sulfides to sulfoxides, obtaining excellent results for all the studied cases, and showing that this reaction goes through both photoredox and energy transfer processes. The results obtained showed a great enhancement of the catalytic activity (up to 25,000 turnover number, TON), stability and a significant decrease on the reaction times, as a consequence of immobilization and isolation of Pt(II) centers into the organic framework. In addition, Pt@COF has been proved to be an excellent heterogeneous photocatalyst also in exclusive photoredox processes, reaching 7500 TON in hydrodebromination reactions.

MOF@COFs with Strong Multiemission for Differentiation and Ratiometric Fluorescence Detection

Aggregation-caused quenching (ACQ) is often observed in covalent organic frameworks (COFs) for their low emission. Here, we propose that limited COF layers form on UiO-66 to eliminate the ACQ by the formation of UiO@COF composites. UiO-66 is selected because this metal-organic framework (MOF) is easily prepared in nanosize with Zr4+ ion and 2-aminoterephthalic acid (BDC-NH2). High affinity of Zr4+ ion to phosphate species improves sensing selectivity. The surface -NH2 reacts 2,4,6-triformylphloroglucinol (Tp) to integrate COF1 and COF2, which are prepared with Tp and phenylenediamine or tetraamino-tetraphenylethylene, respectively. Hydrogen bond formed between hydroxyl group in Tp and imine nitrogen realizes excited state intramolecular proton transfer, so multi-emission is observed from the enol and keto states of the COFs and UiO-66 at 360, 470 and 613 nm for UiO@COF1 and 370, 470, and 572 nm for UiO@COF2. When added phosphate ion in the composites, the emissions from the COFs keep stable, while that from UiO-66 is enhanced. However, adenosine-5'-triphosphate (ATP) improves the emissions from UiO-66 and COF's enol state, but that from keto state keeps stable. The differentiation and ratiometric fluorescence detection of ATP and phosphate ion are therefore realized with the multi-emission, the affinity of Zr4+ ions, and the structural selectivity of the COFs. Thus, UiO@COF is a novel strategy to integrate multi-emission, affinity, and structural selectivity to improve sensing performance for differentiation and ratiometric detection.

Secondary growth of covalent organic frameworks (COFs) on porous substrates for fast desalination

Covalent organic frameworks (COFs) has emerged as a highly promising platform material for the manufacturing of molecule-separating membranes. However, COF membranes are predominantly used in the separation of dyes from water or organic solvents and there are very few reports on COF-enabled membranes for desalination. In this work, the secondary growth approach based on unidirectional diffusion synthesis is developed to grow continuous thin films of a COF with an intrinsic pore size of 0.8 nm on macroporous substrates to prepare COF membranes for desalination. The amino-modified polymer microfiltration substrate is fixed in a diffusion cell to separate the oil phase containing the aldehyde precursor (1,3,5-triformylphloroglucinol, Tp) and the aqueous phase containing both the amino precursor (hydrazine, Hz) and the catalyst. Because of the immiscible bi-phase solutions and the high molar ratio of Hz to Tp, the unidirectional diffusion of Hz molecules in the pore channel of the substrate is achieved and the polycondensation between the two precursors occurs at the phase interface. The unidirectional diffusion synthesis is sequentially performed for two times and the secondary growth is essential to ensure the formation of a defect-free, continuous thin film of COF (TpHz) on the top surface of the substrate. Cladding COF coatings are also formed along the skeleton in the near-surface region of the substrate. Such an asymmetric structure ensures both a good permselectivity and a strong adhesion between the grown COF layers and the substrates. The continuous COF thin film grown on the substrates serves as the selective layer, and the COF membrane prepared under the optimized condition exhibits an appreciable Na2SO4 rejection of 58.3% and a water permeance of 40.5 L∙m−2∙h−1∙MPa−1 which are ~ ten times higher than other membranes with similar rejections. This study not only demonstrates the potential of COFs in desalination but also showcases an efficient strategy to grow continuous thin films of COFs on porous substrates which are desired in diverse applications.

Covalent Organic Framework-Functionalized Magnetic CuFe2O4/Ag Nanoparticles for the Reduction of 4-Nitrophenol.

In this work, magnetic CuFe2O4/Ag nanoparticles activated by porous covalent organic frameworks (COF) was fabricated to evaluate the heterogenous reduction of 4-nitrophenol (4-NP). The core-shell CuFe2O4/Ag@COF was successfully prepared by polydopamine reduction of silver ions on CuFe2O4 nanoparticles, followed by COF layer condensation. By integrating the intrinsic characteristics of the magnetic CuFe2O4/Ag core and COF layer, the obtained nanocomposite exhibited features of high specific surface area (464.21 m2 g−1), ordered mesoporous structure, strong environment stability, as well as fast magnetic response. Accordingly, the CuFe2O4/Ag@COF catalyst showed good affinity towards 4-NP via π-π stacking interactions and possessed enhanced catalytic activity compared with CuFe2O4/Ag and CuFe2O4@COF. The pseudo-first-order rate constant of CuFe2O4/Ag@COF (0.77 min−1) is 3 and 5 times higher than CuFe2O4/Ag and CuFe2O4@COF, respectively. The characteristics of bi-catalytic CuFe2O4/Ag and the porous COF shell of CuFe2O4/Ag@COF made a contribution to improve the activity of 4-NP reduction. The present work demonstrated a facile strategy to fabricate COF-activated nano-catalysts with enhanced performance in the fields of nitrophenolic wastewater treatment.