Beta-Ketoenamine Linked Covalent Organic Frameworks: Synthesis, Structural Characteristics, And Advanced Applications

Molecular Architecture And Formation Mechanism Of Beta-Ketoenamine Linked Covalent Organic Frameworks

Beta-ketoenamine linked covalent organic frameworks are constructed through a two-step condensation process involving amine-functionalized building blocks and hydroxyl-bearing aromatic aldehydes, most commonly 1,3,5-triformylphloroglucinol (Tp) or 2,5-dihydroxyterephthalaldehyde (Dha)1. The synthesis proceeds via reversible Schiff base (C=N) formation followed by irreversible enol-to-keto tautomerization, generating the thermodynamically stable β-ketoenamine linkage (–C=C–NH–C=O–)1,4. This tautomeric transformation is catalyzed by weak Bronsted acids such as acetic acid (3-6 M) under solvothermal conditions (80-150°C, 6-72 hours)1,4. The resulting frameworks exhibit intramolecular O–H···N=C hydrogen bonding between the hydroxyl group and adjacent imine nitrogen, significantly enhancing structural rigidity and chemical stability10.

Representative β-ketoenamine COFs include the widely studied TpPa series (TpPa-1, TpPa-(CH₃)₂, TpPa-NO₂), where Tp reacts with para-phenylenediamine derivatives, and pyrene-based systems such as Tp-Pyrene COFs incorporating extended π-conjugation for enhanced electronic properties1,6. The crystalline nature of these materials is confirmed by powder X-ray diffraction (PXRD), typically showing characteristic reflections corresponding to (100) planes at 2θ ≈ 3-5° and (001) stacking peaks at 2θ ≈ 25-27°, indicative of AA or AB stacking modes with interlayer distances of 3.3-3.6 Å1,10.

Structural tunability is achieved by varying the geometry and functionality of organic linkers. Triangular Tp nodes combined with linear diamines yield hexagonal 2D networks, while tetrahedral amine cores (e.g., tetra(p-aminophenyl)porphyrin) produce 3D frameworks with interpore distances of 1.8-2.0 nm10. Functionalization strategies include incorporation of electron-donating (–CH₃, –OCH₃) or electron-withdrawing (–NO₂, –SO₃H) substituents on aromatic linkers to modulate electronic properties and host-guest interactions6.

Synthesis Methodologies And Process Optimization For Beta-Ketoenamine COFs

Conventional Solvothermal Synthesis

The standard protocol involves dissolving stoichiometric amounts of amine and aldehyde monomers (typically 1:1.5 molar ratio) in a binary solvent system comprising mesitylene (1,3,5-trimethylbenzene) and 1,4-dioxane (volume ratio 1:1 to 3:1)1,4. Acetic acid (3-6 M, 5-10 vol%) serves as the catalyst to facilitate both Schiff base formation and enol-keto tautomerization1. The reaction mixture undergoes freeze-pump-thaw degassing (≥3 cycles) to remove dissolved oxygen, then is sealed in a Pyrex tube and heated at 120°C for 72 hours1,4. The crude product is collected by centrifugation, washed sequentially with mesitylene, tetrahydrofuran (THF), and acetone to remove unreacted monomers and oligomers, and dried under vacuum at 80-120°C for 12-24 hours, yielding crystalline powders with typical surface areas of 400-1500 m²/g1,10.

Microwave-Assisted Rapid Synthesis

To address the lengthy reaction times and high energy consumption of conventional methods, microwave-assisted synthesis has been developed as a green alternative4. In this approach, Tp and p-phenylenediamine are dispersed in a mesitylene/dioxane/acetic acid mixture, subjected to freeze-pump-thaw degassing, and irradiated at 100°C for 1 hour under microwave heating (300-600 W)4. This method reduces synthesis time by 98% while maintaining comparable crystallinity (PXRD peak intensities >80% of solvothermal products) and surface areas (600-900 m²/g)4. The rapid heating promotes uniform nucleation and accelerates tautomerization kinetics, enabling scalable production with reduced solvent usage.

Solvent-Free Mechanochemical Synthesis

Recent advances have introduced solvent-free synthesis using Lewis acid catalysts (BF₃·OEt₂, GaCl₃, Ga(OTf)₃) under mechanochemical conditions5,11. Solid amine and aldehyde monomers are ground with 5-10 mol% catalyst in a ball mill (400-600 rpm, 30-120 minutes), generating β-ketoenamine COFs without organic solvents5. This method eliminates high-pressure risks associated with sealed-tube reactions and enables gram-scale synthesis with yields >85%5. The resulting materials exhibit high crystallinity (PXRD peak widths <0.3° FWHM) and surface areas exceeding 1200 m²/g5,11. Lewis acids activate carbonyl groups via coordination, lowering the activation energy for enol-keto tautomerization and enabling room-temperature synthesis in some cases11.

Post-Synthetic Modification And Composite Fabrication

Beta-ketoenamine COFs can be post-synthetically modified to introduce additional functionalities. For example, thiol groups are grafted onto TpPa-1 frameworks via nucleophilic substitution with cysteamine, yielding –SH functionalized COFs with enhanced heavy metal adsorption capacity (>200 mg/g for Cd²⁺, Co²⁺, Ni²⁺)3. Selenium or tellurium functionalization is achieved by incorporating diselenide or ditelluride-bridged diamine linkers, producing redox-responsive frameworks for drug delivery and platinum recovery from wastewater (adsorption capacity 150-300 mg Pt/g)19.

Composite materials are prepared by in-situ growth of β-ketoenamine COFs on carbon nanomaterials (multi-walled carbon nanotubes, graphene, conductive carbon black) via π-π stacking interactions2. The carbon content is optimized at 40-60 wt% to balance conductivity and active site accessibility2. These COF/carbon nanocomposites exhibit synergistic effects: the carbon network provides electron transport pathways (electrical conductivity 10⁻²-10⁰ S/cm), while the COF layer contributes high surface area (800-1500 m²/g) and redox-active sites, resulting in enhanced electrochemical performance for energy storage applications2.

Physicochemical Properties And Structural Characterization

Porosity And Surface Area

Beta-ketoenamine COFs are characterized by permanent porosity with Brunauer-Emmett-Teller (BET) surface areas ranging from 400 to 2000 m²/g, depending on linker geometry and framework topology1,7,10. The TpPa-1 system exhibits a BET surface area of 535 m²/g and pore volume of 0.25 cm³/g, with a dominant pore size of 1.2-1.8 nm determined by non-local density functional theory (NLDFT) analysis of N₂ adsorption isotherms at 77 K1. Porphyrin-containing DhaTph COFs achieve higher surface areas (1300-2000 m²/g) due to larger pore apertures (1.8-2.0 nm) arising from the extended tetrahedral geometry of the porphyrin core10. Pore volumes typically range from 0.3 to 0.8 cm³/g, with framework densities of 0.4-0.8 g/cm³10,12.

The hierarchical pore structure comprises micropores (<2 nm) within the 2D layers and mesopores (2-10 nm) formed by interlayer stacking defects or exfoliation7. This bimodal porosity facilitates rapid mass transport in catalytic and adsorption applications. Water vapor adsorption isotherms reveal type IV behavior with hysteresis, indicating capillary condensation in mesopores at relative humidity (RH) >40%7. At low RH (10-30%), β-ketoenamine COFs adsorb 15-25 wt% water, significantly higher than imine-linked analogs (5-10 wt%), attributed to the hydrophilic carbonyl groups in the β-ketoenamine linkage7.

Thermal And Chemical Stability

Thermogravimetric analysis (TGA) under nitrogen atmosphere shows that β-ketoenamine COFs remain stable up to 350-450°C, with 5% weight loss temperatures (T_d5%) of 380-420°C for TpPa series and 400-450°C for pyrene-based frameworks1,3. Decomposition occurs in two stages: initial loss (350-450°C) corresponds to framework degradation, while residual char (20-30 wt% at 800°C) indicates high carbon content1. Differential scanning calorimetry (DSC) reveals no phase transitions below 300°C, confirming thermal stability suitable for high-temperature applications3.

Chemical stability is assessed by immersing COF samples in aqueous solutions of varying pH (3 N HCl, 3 N NaOH, pH 1-14) for 7 days at room temperature1,3,10. PXRD patterns and BET surface areas remain unchanged (>95% retention) after acid treatment, whereas imine-linked COFs show 30-50% crystallinity loss under identical conditions1. The enhanced acid stability arises from the irreversible β-ketoenamine linkage, which resists hydrolysis due to resonance stabilization and intramolecular hydrogen bonding10. Base stability is moderate, with 10-20% surface area reduction in 3 N NaOH, attributed to partial hydrolysis of residual imine bonds or framework swelling10. Stability in organic solvents (THF, DMF, acetone, toluene) is excellent, with no structural changes observed after 30 days of immersion3.

Spectroscopic And Crystallographic Characterization

Fourier-transform infrared (FTIR) spectroscopy confirms β-ketoenamine formation through characteristic absorption bands: C=O stretch at 1580-1620 cm⁻¹, C=C stretch at 1450-1500 cm⁻¹, and N–H bend at 1250-1280 cm⁻¹1,4. The absence of aldehyde C=O peaks (1680-1720 cm⁻¹) and primary amine N–H stretches (3300-3500 cm⁻¹) indicates complete monomer conversion1. Solid-state ¹³C cross-polarization magic-angle spinning (CP-MAS) NMR spectroscopy reveals resonances at δ 180-190 ppm (ketone C=O), δ 150-160 ppm (aromatic C–N), and δ 100-120 ppm (enamine C=C), consistent with the proposed structure1,10.

Powder X-ray diffraction (PXRD) patterns exhibit sharp reflections indexed to hexagonal (P6 or P6/m) or tetragonal (P4/mmm) space groups, with unit cell parameters a = b = 20-30 Å, c = 3.3-3.6 Å for 2D frameworks1,10. Pawley refinement yields goodness-of-fit values (R_wp) of 3-8%, confirming high crystallinity10. Transmission electron microscopy (TEM) images show layered morphologies with lattice fringes corresponding to (100) planes (d-spacing 1.5-2.0 nm), while selected-area electron diffraction (SAED) patterns display hexagonal symmetry1,10.

Electrochemical Properties And Energy Storage Applications

Supercapacitor Electrode Materials

Beta-ketoenamine COFs serve as pseudocapacitive electrode materials due to reversible redox reactions of carbonyl (C=O) and imine (C=N) groups1,2. The first β-ketoenamine COF applied in electrochemical energy storage, reported by Dichtel and coworkers in 2013, achieved a specific capacitance of 48 ± 10 F/g in 1 M H₂SO₄ electrolyte at a scan rate of 5 mV/s1. Subsequent optimization through composite engineering has significantly enhanced performance. COF/multi-walled carbon nanotube (MWCNT) composites with 40-60 wt% MWCNT content exhibit specific capacitances of 120-180 F/g at 1 A/g, with 85-90% capacitance retention after 5000 charge-discharge cycles2. The carbon network reduces charge transfer resistance (R_ct = 2-5 Ω) and facilitates ion diffusion, while the COF layer provides high pseudocapacitive contribution (60-70% of total capacitance)2.

Cyclic voltammetry (CV) profiles in acidic electrolytes (1 M H₂SO₄) display quasi-rectangular shapes with redox humps at 0.2-0.4 V vs. Ag/AgCl, corresponding to proton-coupled electron transfer (PCET) at carbonyl sites2. Galvanostatic charge-discharge (GCD) curves exhibit triangular shapes with slight curvature, indicating combined electric double-layer capacitance (EDLC) and pseudocapacitance2. Rate capability tests show 70-80% capacitance retention at 10 A/g relative to 1 A/g, attributed to the hierarchical pore structure enabling rapid electrolyte access2.

Lithium-Ion And Sodium-Ion Battery Anodes

Functionalized β-ketoenamine COFs with redox-active groups (quinone, triazine, anthraquinone) serve as organic anodes for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs)2,17,18. Triazine-containing COFs synthesized from 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and Tp deliver reversible capacities of 200-300 mAh/g at 0.1 A/g in LIBs, with lithiation occurring at 1.5-2.0 V vs. Li/Li⁺ via reduction of triazine nitrogen and carbonyl oxygen2. The irreversible capacity loss in the first cycle (20-30%) is attributed to solid-electrolyte interphase (SEI) formation and electrolyte decomposition2.

Thioether-linked COFs incorporating benzoquinone moieties exhibit enhanced sodium storage capacity (250-350 mAh/g at 0.05 A/g) due to multi-electron redox reactions (C=O + 2Na⁺ + 2e⁻ → C–ONa)9,17. Pre-sodiation treatment increases initial Coulombic efficiency from 60-70% to 85-90% by pre-forming the SEI layer9. Cycling stability is improved by composite formation with conductive carbon (graphene, carbon black), achieving 80-85% capacity retention after 200 cycles at 0.5 A/g2,17. The main challenges include low electronic conductivity (10⁻⁸-10⁻⁶ S/cm for pristine COFs) and dissolution of small oligomers in organic electrolytes, necessitating further structural optimization and electrolyte engineering17,18.

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