Porous Covalent Organic Frameworks Research Articles

Dynamic two-dimensional covalent organic frameworks.

Porous covalent organic frameworks (COFs) enable the realization of functional materials with molecular precision. Past research has typically focused on generating rigid frameworks where structural and optoelectronic properties are static. Here we report dynamic two-dimensional (2D) COFs that can open and close their pores upon uptake or removal of guests while retaining their crystalline long-range order. Constructing dynamic, yet crystalline and robust frameworks requires a well-controlled degree of flexibility. We have achieved this through a 'wine rack' design where rigid π-stacked columns of perylene diimides are interconnected by non-stacked, flexible bridges. The resulting COFs show stepwise phase transformations between their respective contracted-pore and open-pore conformations with up to 40% increase in unit-cell volume. This variable geometry provides a handle for introducing stimuli-responsive optoelectronic properties. We illustrate this by demonstrating switchable optical absorption and emission characteristics, which approximate 'null-aggregates' with monomer-like behaviour in the contracted COFs. This work provides a design strategy for dynamic 2D COFs that are potentially useful for realizing stimuli-responsive materials.

Covalent triazine framework-based porous mixed matrix membranes for highly efficient dye/salt separation

Mixed matrix membranes (MMMs) incorporating porous covalent organic frameworks (COFs) as fillers have garnered significant attention in membrane separation for water purification. Covalent triazine frameworks (CTFs) possess high thermochemical stability, an inherent triazine conjugate structure, and tunable pore size. Despite these attributes, CTFs have been underutilized in MMMs. In this study, CTF-based porous MMMs were fabricated by blending 4,4′-biphenyldicarbonitrile (BPCN) monomer with polyacrylonitrile (PAN) and employing titanium dioxide nanoparticles (TiO2 NPs) as a pore-forming agent. The nature of PAN also makes the mixed matrix membranes have better hydrophilicity and mechanical properties. The optimized CTF-MMM exhibited an impressive water permeance of ∼ 204 L m−2 h−1 bar−1, accompanied by a remarkable dye rejection of ∼ 99 % and a low salt rejection. Moreover, the CTF-MMM demonstrated sustained water permeance and separation performance even after 48 h. This work introduces a synthesis strategy for fabricating high-performance CTF-based porous mixed matrix membranes, showcasing significant potential for applications in dye/salt separation.

Unveiling the Potential of Highly Porous Covalent Organic Frameworks for Water-Jet Rewritable Papers.

The massive use of paper has resulted in significant negative impacts on the environment. Fortunately, recent progress has been made in the field of rewritable paper, which has great potential in solving the increasing demand for paper while minimizing its environmental footprint. In this work, we report a green and economic strategy to develop ink-free rewritable paper by introducing hydrochromic covalent organic frameworks (COFs) in paper and using water as the sole trigger. When exposed to water or acidic solvents, two kinds of imino COFs change their colors reversibly from red to black. Additionally, a new visible absorption band appears, indicating that it can be transformed into another structure reversibly. This reversibility may be due to the isomerization from the diiminol to an iminol/cisketoenamine and its inability to doubly tautomerize to a diketoenamine. Specifically, we prepared the rewritable paper by loading these two COFs onto filter paper by using the decompression filtration method. When exposed to water, the paper undergoes a color change from red to black, which shows promising potential for applications in water-jet printing. Additionally, there is no significant performance degradation after 20 uses and 10 days between, further highlighting their potential as rewritable papers. To further improve its uniformity, we take the interface polymerization strategy to yield highly crystalline and more compact membranes, which are then transferred to paper to prepare writable papers. Our research has opened up a way for the application of COFs as a water-based printing material.

COF-anchored design of nanoporous graphene membranes for ultrafast and selective organic separation

Nanoporous graphene has attracted extensive attention as a type of disruptive membrane materials for organic separation. However, the fabrication of an ideal graphene membrane with high permeability and selectivity remains challenging due to the inevitable non-selective defects. Here, a strategy based on synergetic effect of graphene membranes and covalent organic framework (COF) nanounits is developed. Atomically thin nanoporous graphene provides high solvent flux and stability. To prevent the leakage from inevitable non-selective pores in graphene without sacrificing flux, porous COF nanounits are directly anchored at the defective sites. The fabricated membranes exhibit record-high solvent flux of 49.81 L m−2 h−1 (1–2 orders of magnitude higher than that of state-of-the-art membranes), and relatively low reverse solute flux of 1.69 g m−2 h−1 in organic solvent forward osmosis (OSFO) process. The strategy we proposed gives dramatic impetus to the development of OSFO and other membrane separation processes.

Porous covalent organic framework nanofibrous membrane for excellent enrichment and ultra-high sensitivity detection of trace organochlorine pesticides in water

Developing adsorbents with high performance and long service life for effective extracting the trace organochlorine pesticides (OCPs) from real water is attracting numerous attentions. Herein, a self-standing covalent organic framework (COF-TpPa) membrane with fiber morphology was successfully synthesized by using electrospun nanofiber membranes as template and employed as solid-phase microextraction (SPME) coating for ultra-high sensitivity extraction and analysis of trace OCPs in water. The as-synthesized COF-TpPa membrane exhibited a high specific surface area (800.83 m2 g−1), stable nanofibrous structure, and excellent chemical and thermal stability. Based on the COF-TpPa membrane, a new SPME analytical method in conjunction with gas chromatography-mass spectrometry (GC–MS) was established. This proposed method possessed favorable linearity in concentration of 0.05–2000 ng L−1, high sensitivity with enrichment factors ranging from 2175 to 5846, low limits of detection (0.001–0.150 ng L−1), satisfactory precision (RSD < 10 %), and excellent repeatability (>150 cycles), which was better than most of the reported works. Additionally, the density functional theory (DFT) calculations and XPS results demonstrated that the outstanding enrichment performance of the COF-TpPa membrane was owing to synergistic effect of π–π stacking effects, high specific surface area and hydrogen bonding. This work will expect to extend the applications of COF membrane to captures trace organic pollutants in complex environmental water, as well as offer a multiscale interpretation for the design of effective adsorbents.

Nanocomposite membranes comprising covalent organic framework and polymer of intrinsic microporosity for efficient CO2 separation

The fabrication of new mixed-matrix membranes (MMMs) with superior gas permeability and selectivity are highly sought for industrial development of membrane separation technology for CO2 capture. This study reports improvement in CO2 permeability and selectivity by employing porous covalent organic framework (COF-TpPa-1). Herein, we report a novel PIM-1 doped with COF-TpPa-1 in various concentrations ranging from 0 to 10 wt% to prepare COF-TpPa-1/PIM-1 MMMs. Good compatibility between COF-TpPa-1 particles and PIM-1 matrix rendered an ideal interfacial morphology. The MMMs composed of 7 wt% COF-TpPa-1 exhibit a CO2 permeability of 6336 Barrer with an enhancement of 104 % compared to pristine sample and CO2/N2 ideal selectivity 20.1 among five tested compositions. The CO2 permeability of COF-TpPa-1 7 wt% doped membrane was 88 % more than pristine PIM-1 after 30 days aging, while CO2/N2 selectivity was well maintained, coupled with excellent anti-plasticization and mixed-gas separation properties. The overall gas separation performance of MMMs exceeds the Robeson Upper bound (2008) for CO2/N2 and O2/N2. The measured experimental permeability exhibited excellent agreement with the Singh thermal conductivity model at low loading (<5 wt%). This work provides a potential approach to fabricate PIM-1-based high performance MMMs with COF fillers for efficient CO2 separation.

Increasing the Accessibility of Internal Catalytic Sites in Covalent Organic Frameworks by Introducing a Bicontinuous Mesostructure.

Introducing continuous mesochannels into covalent organic frameworks (COFs) to increase the accessibility of their inner active sites has remained a major challenge. Here, we report the synthesis of COFs with an ordered bicontinuous mesostructure, via a block copolymer self-assembly-guided nanocasting strategy. Three different mesostructured COFs are synthesized, including two covalent triazine frameworks and one vinylene-linked COF. The new materials are endowed with a hierarchical meso/microporous architecture, in which the mesochannels exhibit an ordered shifted double diamond (SDD) topology. The hierarchically porous structure can enable efficient hole-electron separation and smooth mass transport to the deep internal of the COFs and consequently high accessibility of their active catalytic sites. Benefiting from this hierarchical structure, these COFs exhibit excellent performance in visible-light-driven catalytic NO removal with a high conversion percentage of up to 51.4 %, placing them one of the top reported NO-elimination photocatalysts. This study represents the first case of introducing a bicontinuous structure into COFs, which opens a new avenue for the synthesis of hierarchically porous COFs and for increasing the utilization degree of their internal active sites.

Flexible Linker-Based Triazine-Functionalized 2D Covalent Organic Frameworks for Supercapacitor and Gas Sorption Applications.

Covalent organic frameworks (COFs) having a large surface area, porosity, and substantial amounts of heteroatom content are recognized as the ideal class of materials for energy storage and gas sorption applications. In this work, we have synthesized four different porous COF materials by the polycondensation of a heteroatom-rich flexible triazine-based trialdehyde linker, namely 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TPT-CHO), with four different triamine linkers. Triamine linkers were chosen based on differences in size, symmetry, planarity, and heteroatom content, leading to the synthesis of four different COF materials named IITR-COF-1, IITR-COF-2, IITR-COF-3, and IITR-COF-4. IITR-COF-1, synthesized within 24 h from the most planar and largest amine monomer, exhibited the largest Brunauer-Emmett-Teller (BET) surface area of 2830 m2 g-1, superior crystallinity, and remarkable reproducibility compared to the other COFs. All of the synthesized COFs were explored for energy and gas storage applications. It is shown that the surface area and redox-active triazene rings in the materials have a profound effect on energy and gas storage enhancement. In a three-electrode setup, IITR-COF-1 achieved an electrochemical stability potential window (ESPW) of 2.0 V, demonstrating a high specific capacitance of 182.6 F g-1 with energy and power densities of 101.5 Wh kg-1 and 298.3 W kg-1, respectively, at a current density of 0.3 A g-1 in 0.5 M K2SO4 (aq) with long-term durability. The symmetric supercapacitor of IITR-COF-1//IITR-COF-1 exhibited a notable specific capacitance of 30.5 F g-1 and an energy density of 17.0 Wh kg-1 at a current density of 0.12 A g-1. At the same time, it demonstrated 111.3% retention of its initial specific capacitance after 10k charge-discharge cycles. Moreover, it exhibited exceptional CO2 capture capacity of 25.90 and 10.10 wt % at 273 and 298 K, respectively, with 2.1 wt % of H2 storage capacity at 77 K and 1 bar.

Mercapto-β-cyclodextrin covalent organic frameworks for enantioselective liquid-liquid extraction

β-Cyclodextrin (β-CD) exhibits exceptional chiral recognition ability, and covalent organic frameworks (COFs) have been reported as highly promising chiral carriers. Therefore, incorporating cyclodextrin into COFs may result in chiral covalent organic frameworks (CCOFs) suitable for chiral applications. In this work, a solvothermal method was employed to synthesize a classical porous covalent organic framework (COF-V), while β-CD underwent a two-step modification process, resulting in the synthesis of mercapto-β-cyclodextrin (β-CD-SH). Subsequently, a β-CD-based covalent organic framework (COF-S-CD) was synthesized through a Thiol-Ene “click” reaction. The synthesized COF-S-CD was successfully applied in enantioselective liquid-liquid extraction for five racemic arylcarboxylic acids, including 2-phenylpropionic acid, 2-(4-methylphenyl) propionic acid, 2-(3-chlorophenyl) propionic acid, 2-(4-nitrophenyl) propionic acid and α-cyclohexylmandelic acid. Under optimized conditions, enantioseparation factors for the five racemates reached 1.78, 2.20, 1.84, 1.76 and 1.96, respectively, which indicated a significant enhancement in chiral recognition ability upon the incorporation of cyclodextrin into the COF material. This new application suggested that the synthesis of covalent organic framework materials incorporating β-CD, with the ability to uniformly disperse in the aqueous phase, holds promising potential for highly effective enantioselective liquid-liquid extraction.

Hollow spherical covalent organic framework supported gold nanoparticles for photocatalytic H2O2 production

Covalent organic frameworks (COFs) are porous crystalline materials with promising applications in photocatalysis, but their performance is greatly hampered by the fast recombination of photogenerated carriers. Herein, a hollow spherical COF was synthesized via the condensation polymerization of 1,3,5-tris(4-aminophenyl)benzene and 1,3,5-benzenetricarbaldehyde, and Au nanoparticles were in-situ deposited onto the COF through NaBH4 reduction to ameliorate the photocatalytic performance. The composite exhibits impressive photocatalytic H2O2-production performance with the highest H2O2-evolution rate of 6067 μmol g−1 h−1 under visible light irradiation, which is about 1.9 times that of the pristine COF. The enhanced photocatalytic activity was ascribed to improved light absorption, abundant active Au sites, and efficient transfer and separation of photogenerated carriers in space. This work provides an avenue for designing high-performance H2O2-production photocatalysts by decorating porous COFs with active noble-metal sites.

Creating amphiphilic porosity in two-dimensional covalent organic frameworks via steric-hindrance-mediated precision hydrophilic-hydrophobic microphase separation

Creating different pore environments within a covalent organic framework (COF) will lead to useful multicompartment structure and multiple functions, which however has been scarcely achieved. Herein we report designed synthesis of three two-dimensional COFs with amphiphilic porosity by steric-hindrance-mediated precision hydrophilic-hydrophobic microphase separation. Dictated by the different steric effect of the substituents introduced to a monomer, dual-pore COFs with kgm net, in which all hydroxyls locate in trigonal micropores while hydrophobic sidechains exclusively distribute in hexagonal mesopores, have been constructed to form completely separated hydrophilic and hydrophobic nanochannels. The unique amphiphilic channels in the COFs enable the formation of Janus membranes via interface growth. This work has realized the creation of two types of channels with opposite properties in one COF, demonstrating the feasibility of introducing different properties/functions into different pores of heteropore COFs, which can be a useful strategy to develop multifunctional materials.

Ionic Liquid‐Mediated Dynamic Polymerization for Facile Aqueous‐Phase Synthesis of Enzyme‐Covalent Organic Framework Biocatalysts

AbstractUtilizing covalent organic framework (COF) as a hypotoxic and porous scaffold to encapsulate enzyme (enzyme@COF) has inspired numerous interests at the intersection of chemistry, materials, and biological science. In this study, we report a convenient scheme for one‐step, aqueous‐phase synthesis of highly crystalline enzyme@COF biocatalysts. This facile approach relies on an ionic liquid (2 μL of imidazolium ionic liquid)‐mediated dynamic polymerization mechanism, which can facilitate the in situ assembly of enzyme@COF under mild conditions. This green strategy is adaptive to synthesize different biocatalysts with highly crystalline COF “exoskeleton”, as well evidenced by the low‐dose cryo‐EM and other characterizations. Attributing to the rigorous sieving effect of crystalline COF pore, the hosted lipase shows non‐native selectivity for aliphatic acid hydrolysis. In addition, the highly crystalline linkage affords COF “exoskeleton” with higher photocatalytic activity for in situ production of H2O2, enabling us to construct a self‐cascading photo‐enzyme coupled reactor for pollutants degradation, with a 2.63‐fold degradation rate as the poorly crystalline photo‐enzyme reactor. This work showcases the great potentials of employing green and trace amounts of ionic liquid for one‐step synthesis of crystalline enzyme@COF biocatalysts, and emphasizes the feasibility of diversifying enzyme functions by integrating the reticular chemistry of a COF.

Ionic Liquid-Mediated Dynamic Polymerization for Facile Aqueous-Phase Synthesis of Enzyme-Covalent Organic Framework Biocatalysts.

Utilizing covalent organic framework (COF) as a hypotoxic and porous scaffold to encapsulate enzyme (enzyme@COF) has inspired numerous interests at the intersection of chemistry, materials, and biological science. In this study, we report a convenient scheme for one-step, aqueous-phase synthesis of highly crystalline enzyme@COF biocatalysts. This facile approach relies on an ionic liquid (2 μL of imidazolium ionic liquid)-mediated dynamic polymerization mechanism, which can facilitate the in situ assembly of enzyme@COF under mild conditions. This green strategy is adaptive to synthesize different biocatalysts with highly crystalline COF "exoskeleton", as well evidenced by the low-dose cryo-EM and other characterizations. Attributing to the rigorous sieving effect of crystalline COF pore, the hosted lipase shows non-native selectivity for aliphatic acid hydrolysis. In addition, the highly crystalline linkage affords COF "exoskeleton" with higher photocatalytic activity for in situ production of H2 O2 , enabling us to construct a self-cascading photo-enzyme coupled reactor for pollutants degradation, with a 2.63-fold degradation rate as the poorly crystalline photo-enzyme reactor. This work showcases the great potentials of employing green and trace amounts of ionic liquid for one-step synthesis of crystalline enzyme@COF biocatalysts, and emphasizes the feasibility of diversifying enzyme functions by integrating the reticular chemistry of a COF.

Hierarchically Porous Covalent Organic Frameworks: Synthesis Methods and Applications.

Covalent organic frameworks (COFs) with high porosity have garnered considerable interest for various applications owing to their robust and customizable structure. However, conventional COFs are hindered by their narrow pore size, which poses limitations for applications such as heterogeneous catalysis and guest delivery that typically involve large molecules. The development of hierarchically porous COF (HP-COF), featuring a multi-scale aperture distribution, offers a promising solution by significantly enhancing the diffusion capacity and mass transfer for larger molecules. This review focuses on the recent advances in the synthesis strategies of HP-COF materials, including topological structure design, in-situ templating, monolithic COF synthesis, defect engineering, and crystalline self-transformation. The specific operational principles and affecting factors in the synthesis process are summarized and discussed, along with the applications of HP-COFs in heterogeneous catalysis, toxic component treatment, optoelectronics, and the biomedical field. Overall, this review builds a bridge to understand HP-COFs and provides guidance for further development of them on synthesis strategies and applications.

Porous covalent organic frameworks in photocatalytic ROS-mediated processes

Porous covalent organic frameworks towards photocatalytic ROS-mediated hydrogen peroxide production and organic substances degradation.

Tuning the electrostatic energy landscape within the pores of covalent organic frameworks: post-synthetic modification reactions and structural imperfections

Polar substituents decorating the pore walls of covalent organic frameworks can be used to control the electrostatic potential within the pores and to tune the alignment of the electronic states in guest molecules and the COF scaffold.

CuI-anchored porous covalent organic frameworks for highly efficient conversion of propargylic amines with CO2 from flue gas

A unique CuI-anchored covalent organic framework heterogeneous catalyst was designed and synthesized, and the catalyst exhibited highly efficient conversion of propargylic amines with flue gas CO2 into 2-oxazolidones.

Tannin-assisted interfacial polymerization towards COF membranes for efficient dye separation.

Membrane separation has been shown to have significant potential in addressing the global shortage of clean water. Covalent organic frameworks (COFs) have gained significant attention in the field of membrane separation due to their structural stability and controllable pore size. Here, a modification of polyethersulfone ultrafiltration membranes with TA-assisted COFs is prepared by interfacial polymerization and co-deposition. Intriguingly, in comparison to the conventional COF synthesis method, the interfacial polymerization reaction used n-butanol as the oil-phase monomer to prevent substrate corrosion. More importantly, the TA-assisted co-deposition not only introduces a large number of environmentally friendly hydrophilic groups to enhance the hydrophilicity of the membrane surface, but also the phenolic hydroxyl group contained in TA generates a quinone group upon oxidation. This group can undergo a Michael addition reaction with the amine group, followed by interfacial polymerization to regulate the COFs pore size. Consequently, the optimized membrane exhibited a high permeation flux of 122.03 L m-2 h-1 bar-1 without altering the pore size structure of the original membranes and demonstrated separation performance for various dyes (Mw: 300-1300 g mol-1), with a retention rate of over 98%. Despite multiple filtrations of methyl blue dye, the membrane prepared by simple rinsing still exhibited high retention rates (>98%) with exceptional stability and retention performance. The optimized membrane demonstrated good hydrophilicity and dye separation performance, indicated promising potential for dye separation applications.

Large–size porous spherical 3D covalent organic framework for preconcentration of bisphenol F in water samples and orange juice

Large–size spherical sorbents with particle size of 10–50 μm are widely applied in separation fields, however it is still a great challenge to synthesize such large–size spherical covalent organic framework (COF). In this work, a type of large–size porous 3D COF was size–controablly synthesized via a two–step strategy, in which a large–size porous 3D spherical polymer was prepared first through a Pickering emulsion polymerization using nano silica as the stabilizer, and subsequently it was converted into porous spherical 3D COF by a solvothermal method. The as–prepared porous spherical COF (COF–320 as a model) showed size–controllable uniform spherical morphology within 15–45 μm, large specific surface area, fine crystalline structure, and good chemical stability. When used as the sorbent for dispersive solid–phase extraction (d–SPE) of bisphenol F (BPF), the porous spherical COF–320 (15 μm) displayed high adsorption capacity (Qmax = 335.6 mg/g), high enrichment factor (80 folds), and good reusability (at least five cycles). By coupling the d–SPE method to HPLC, a new analytical approach was developed and successfully applied to the determination of trace BPF in two water samples, an orange juice and a standard sample with recoveries of 96.0–102.2 % (RSD = 1.1–1.5 %), 95.7–97.4 % (RSD = 1.4–4.4 %) and 98.7 % (RSD = 2.3 %), respectively. The limit of detection (S/N = 3) and limit of quantification (S/N = 10) were 0.1 and 0.3 ng/mL, respectively. The new synthesis strategy opens a viable way to prepare large–size porous spherical COFs, and the developed analytical method can be potentially applied to sensitively detect the trace BPF in water samples and beverages.

Pore Space Partition Synthetic Strategy in Imine-linked Multivariate Covalent Organic Frameworks.

Partitioning the pores of covalent organic frameworks (COFs) is an attractive strategy for introducing microporosity and achieving new functionality, but it is technically challenging to achieve. Herein, we report a simple strategy for partitioning the micropores/mesopores of multivariate COFs. Our approach relies on the predesign and synthesis of multicomponent COFs through imine condensation reactions with aldehyde groups anchored in the COF pores, followed by inserting additional symmetric building blocks (with C2 or C3 symmetries) as pore partition agents. This approach allowed tetragonal or hexagonal pores to be partitioned into two or three smaller micropores, respectively. The synthesized library of pore-partitioned COFs was then applied for the capture of iodine pollutants (i.e., I2 and CH3I). This rich inventory allowed deep exploration of the relationships between the COF adsorbent composition, pore architecture, and adsorption capacity for I2 and CH3I capture under wide-ranging conditions. Notably, one of our developed pore-partitioned COFs (COF 3-2P) exhibited greatly enhanced dynamic I2 and CH3I adsorption performances compared to its parent COF (COF 3) in breakthrough tests, setting a new benchmark for COF-based adsorbents. Results present an effective design strategy toward functional COFs with tunable pore environments, functions, and properties.