Carboxyl Functionalized Covalent Organic Framework: Synthesis, Structural Engineering, And Advanced Applications In Catalysis And Separation

Molecular Design Principles And Structural Characteristics Of Carboxyl Functionalized Covalent Organic Framework

Carboxyl functionalized covalent organic frameworks are constructed by incorporating carboxylic acid (-COOH) or carboxylate-bearing organic building blocks into the framework topology through reversible covalent bond formation, typically via imine (C=N), boronate ester (B-O), or β-ketoenamine linkages 1. The introduction of carboxyl groups serves multiple strategic purposes: (i) enhancing framework hydrophilicity and water stability, (ii) providing Brønsted acidic sites for catalysis and proton transport, (iii) enabling post-synthetic metalation or covalent modification, and (iv) improving guest molecule binding through hydrogen bonding and electrostatic interactions 23. Two-dimensional (2D) carboxyl-functionalized COFs typically adopt layered structures with π-π stacked aromatic sheets, creating one-dimensional channels perpendicular to the layers; the interlayer spacing (typically 3.3–3.8 Å) and pore aperture (ranging from 1.2 to 4.5 nm depending on linker length) can be precisely tuned by selecting appropriate building blocks 14. Three-dimensional (3D) carboxyl-functionalized COFs, though less common, offer interconnected pore networks in all spatial dimensions, facilitating isotropic mass transport and higher structural rigidity 7. The carboxyl groups can be installed either as integral components of the organic linkers (e.g., using terephthalic acid derivatives) or introduced via post-synthetic oxidation of aldehyde or alcohol precursors, with the former approach generally yielding higher functionalization density and structural uniformity 38.

The chemical stability of carboxyl functionalized COFs is critically dependent on the linkage chemistry and the local environment of the carboxyl groups. Imine-linked COFs, while offering high crystallinity and reversibility during synthesis, are susceptible to hydrolysis under strongly acidic or basic conditions; however, the presence of intramolecular hydrogen bonding between carboxyl groups and imine nitrogen atoms can significantly enhance hydrolytic stability, as demonstrated in frameworks stable in boiling water and pH 1–14 solutions for extended periods 18. β-Ketoenamine-linked COFs exhibit superior chemical robustness due to the resonance stabilization of the enamine tautomer, making them particularly suitable for applications in aqueous media and under oxidative conditions 14. Boronate ester linkages, traditionally considered hydrolytically labile, can be stabilized through the formation of tetra-coordinated borate species in the presence of Lewis bases or by encapsulation within hydrophobic pore environments 11. The thermal stability of carboxyl functionalized COFs typically ranges from 300 to 500 °C under inert atmosphere, with decomposition temperatures influenced by the strength of the covalent linkages and the degree of framework interpenetration 46. Thermogravimetric analysis (TGA) of representative carboxyl-functionalized imine COFs shows initial weight loss below 150 °C corresponding to solvent or water desorption, followed by framework decomposition onset at 350–400 °C, indicating suitability for most catalytic and separation processes operating below 300 °C 818.

Structural characterization of carboxyl functionalized COFs relies on a combination of powder X-ray diffraction (PXRD), solid-state nuclear magnetic resonance (ssNMR), Fourier-transform infrared spectroscopy (FTIR), and gas sorption analysis. PXRD patterns reveal the long-range crystalline order, with characteristic low-angle reflections (2θ < 10°) corresponding to the periodic pore structure; successful carboxyl functionalization is confirmed by the appearance of C=O stretching bands at 1680–1720 cm⁻¹ in FTIR spectra and ¹³C NMR signals at 170–180 ppm 818. Brunauer-Emmett-Teller (BET) surface areas for carboxyl functionalized COFs typically range from 500 to 2500 m²/g, with pore size distributions determined by non-local density functional theory (NLDFT) analysis of N₂ adsorption isotherms at 77 K showing predominantly mesoporous character (2–10 nm) for frameworks designed with extended linkers 24. The presence of carboxyl groups introduces additional complexity to the sorption isotherms, often manifesting as hysteresis loops indicative of capillary condensation and framework flexibility, particularly in water vapor sorption measurements where hydrogen bonding interactions between water molecules and carboxyl sites lead to stepped isotherms with distinct pore-filling transitions 4.

Synthetic Strategies And Precursor Selection For Carboxyl Functionalized Covalent Organic Framework

The synthesis of carboxyl functionalized COFs employs solvothermal or room-temperature condensation reactions between multifunctional aldehydes or ketones and amine or hydrazine building blocks, conducted under conditions that promote reversible bond formation and error correction to achieve high crystallinity 38. A representative synthesis involves the Schiff base condensation of triformylphloroglucinol (Tp) with a diamine bearing carboxyl substituents (e.g., 2,5-diaminoterephthalic acid) in a mixture of mesitylene and 1,4-dioxane (1:1 v/v) with 6 M aqueous acetic acid as catalyst, heated at 120 °C for 72 hours in a sealed Pyrex tube 818. The acetic acid serves dual roles: (i) catalyzing the imine formation through protonation of the carbonyl oxygen, and (ii) modulating the reaction kinetics to favor reversible bond formation over irreversible precipitation, thereby enabling crystalline domain growth 13. Alternative synthetic approaches include mechanochemical synthesis, where solid precursors are ground together with catalytic amounts of liquid additives, offering solvent-free and scalable routes but often yielding materials with lower crystallinity and surface area compared to solvothermal methods 8. Microwave-assisted synthesis has been explored to accelerate COF formation, reducing reaction times from days to hours while maintaining comparable structural quality, though careful control of heating profiles is required to prevent localized overheating and framework degradation 3.

The selection of carboxyl-bearing building blocks is governed by several design criteria: (i) symmetry and rigidity to promote ordered framework assembly, (ii) appropriate pKa values (typically 3–5 for carboxylic acids) to enable desired protonation states under reaction and application conditions, (iii) solubility in the reaction medium to ensure homogeneous nucleation, and (iv) compatibility with the chosen linkage chemistry to avoid side reactions 28. Common carboxyl-functionalized linkers include 2,5-diaminoterephthalic acid, 4,4'-dicarboxybiphenyl derivatives, and pyrene-based tetracarboxylic acids, each offering distinct geometric and electronic properties 718. For 3D COFs, tetrahedral or octahedral building blocks with carboxyl substituents are required; for example, tetrakis(4-carboxyphenyl)methane can serve as a four-connected node, though its synthesis and purification require multi-step organic chemistry and careful handling to prevent esterification during storage 67. Post-synthetic modification strategies provide an alternative route to carboxyl functionalization: aldehyde-functionalized COFs can be oxidized using mild oxidants such as sodium chlorite (NaClO₂) in aqueous acetonitrile at room temperature, converting -CHO groups to -COOH with retention of framework crystallinity, as confirmed by PXRD and FTIR analysis showing disappearance of aldehyde C=O stretch at 1700 cm⁻¹ and appearance of carboxylic acid O-H stretch at 2500–3300 cm⁻¹ 38.

Reaction conditions critically influence the crystallinity, porosity, and functional group accessibility of the final carboxyl functionalized COF. Temperature optimization typically involves screening from 80 to 150 °C, with higher temperatures accelerating reaction kinetics but potentially reducing reversibility and crystalline quality; for imine-linked COFs, 120 °C is often optimal, balancing reaction rate and error correction 18. Solvent selection impacts both solubility of precursors and the thermodynamic stability of the growing framework; binary or ternary solvent mixtures (e.g., mesitylene/dioxane, DMF/DMSO, or o-dichlorobenzene/n-butanol) are commonly employed to fine-tune polarity and boiling point 35. The concentration of acid or base catalyst must be carefully controlled: excessive acidity can protonate amine nucleophiles and inhibit condensation, while insufficient catalysis leads to slow reaction rates and amorphous products 818. Reaction time varies from 24 hours to 7 days depending on the system, with longer times generally favoring higher crystallinity but also increasing the risk of framework degradation or side reactions 13. Purification involves sequential washing with the reaction solvent, followed by activation in lower-boiling solvents (e.g., acetone, methanol) and drying under vacuum at 80–120 °C for 12–24 hours to remove residual guests from the pores; supercritical CO₂ drying can be employed for frameworks prone to pore collapse during conventional drying, preserving mesoporosity and surface area 48.

Physicochemical Properties And Performance Metrics Of Carboxyl Functionalized Covalent Organic Framework

The physicochemical properties of carboxyl functionalized COFs are characterized by a combination of porosity metrics, chemical stability, and functional group reactivity. BET surface areas typically range from 500 to 2500 m²/g, with the lower end corresponding to frameworks with high carboxyl loading or interpenetrated structures, and the higher end achieved in non-interpenetrated frameworks with extended linkers 24. Pore volumes range from 0.3 to 1.5 cm³/g, with the majority of pore volume residing in mesopores (2–10 nm) for frameworks designed with linkers longer than 1.5 nm 47. The presence of carboxyl groups introduces hydrophilicity, manifested in water vapor sorption isotherms showing uptake capacities of 0.2–0.5 g H₂O per g COF at 40% relative humidity (RH) and 25 °C, significantly higher than non-functionalized analogs (typically <0.1 g/g under the same conditions) 4. The isosteric heat of water adsorption (Qst) for carboxyl functionalized COFs ranges from 45 to 55 kJ/mol, indicating moderate binding strength suitable for reversible adsorption-desorption cycles without excessive energy input for regeneration 4. This property is particularly advantageous for atmospheric water harvesting applications, where low regeneration temperatures (40–60 °C) enable solar-driven operation 4.

Chemical stability assessments involve immersion tests in aqueous solutions spanning pH 1–14, organic solvents (e.g., DMF, DMSO, THF, toluene, hexane), and oxidative media (e.g., 30% H₂O₂). High-quality carboxyl functionalized imine COFs with β-ketoenamine tautomerization exhibit retention of crystallinity (as evidenced by unchanged PXRD patterns) and porosity (BET surface area loss <10%) after 7 days in boiling water, 12 M HCl, and 14 M NaOH 18. In contrast, boronate ester-linked carboxyl COFs show hydrolytic degradation in neutral water within 24 hours unless stabilized by conversion to tetra-coordinated borate species through addition of Lewis bases such as pyridine or imidazole 11. Oxidative stability is generally excellent for aromatic carboxyl COFs, with no detectable degradation after exposure to air at 150 °C for 100 hours or treatment with 30% H₂O₂ at room temperature for 48 hours 818. Mechanical stability, assessed by compression testing of pelletized COF powders, reveals Young's moduli in the range of 0.5–2.0 GPa for 2D frameworks, with 3D frameworks exhibiting higher values (2–5 GPa) due to their interconnected topology 614. The elastic modulus can be tuned by metal ion incorporation: for example, copper(I)-coordinated woven COFs show a ten-fold increase in elasticity upon demetalation, demonstrating the potential for stimuli-responsive mechanical properties 14.

The acidity of carboxyl groups in COFs, quantified by potentiometric titration or NH₃ temperature-programmed desorption (TPD), typically yields pKa values of 3.5–5.0, slightly higher than the corresponding molecular carboxylic acids due to the electron-donating effect of the aromatic framework and potential intramolecular hydrogen bonding 18. This moderate acidity is advantageous for Brønsted acid-catalyzed reactions such as esterification, acetalization, and aldol condensation, where strong acids may cause substrate decomposition or undesired side reactions 818. The density of accessible carboxyl sites, determined by titration with NaOH or by quantitative ¹H NMR after digestion of the framework, ranges from 1.5 to 4.5 mmol/g depending on the linker composition and framework topology 818. Post-synthetic metalation of carboxyl groups with transition metal ions (e.g., Cu²⁺, Zn²⁺, Fe³⁺) can be achieved by soaking the COF in metal salt solutions, yielding metal-COF composites with enhanced catalytic activity; for instance, Cu²⁺-loaded carboxyl COFs exhibit Lewis acidity and redox activity, enabling oxidation reactions such as aerobic alcohol oxidation with turnover frequencies (TOF) of 50–200 h⁻¹ at 80 °C 13.

Applications Of Carboxyl Functionalized Covalent Organic Framework In Heterogeneous Catalysis

Carboxyl functionalized COFs serve as versatile platforms for heterogeneous catalysis, leveraging their high surface area, tunable pore environment, and reactive carboxyl sites to facilitate a broad range of organic transformations 89. In acid-catalyzed reactions, the Brønsted acidity of carboxyl groups enables esterification of carboxylic acids with alcohols, acetalization of aldehydes with diols, and dehydration of alcohols to alkenes, with reaction rates and selectivities often surpassing those of conventional solid acids such as sulfated zirconia or Amberlyst resins due to the uniform distribution and accessibility of active sites within the COF pores 818. For example, a carboxyl-functionalized imine COF synthesized from 2,5-diaminoterephthalic acid and triformylphloroglucinol catalyzes the esterification of acetic acid with ethanol at 80 °C, achieving 92% conversion in 6 hours with a catalyst loading of 5 wt%, and retains >85% activity after five recycling cycles with simple filtration and washing 8. The reaction mechanism involves protonation of the carbonyl oxygen by the carboxyl group, followed by nucleophilic attack of the alcohol and elimination of water, with the confined pore environment stabilizing transition states and enhancing reaction kinetics 818.

Post-synthetic metalation of carboxyl COFs generates bifunctional catalysts combining Lewis acidity (from metal centers) and Brønsted acidity (from residual carboxyl groups), enabling tandem or cascade reactions 13. A representative example is the synthesis of a Cu²⁺-loaded carboxyl COF by soaking the framework in Cu(NO₃)₂ solution, followed by washing and drying, yielding a material with 8 wt% Cu content (determined by ICP-OES) and retention of crystallinity (PXRD) and porosity (BET surface area 1200 m²/g, compared to 1450 m²/g