Stacked Covalent Organic Framework: Structural Engineering, Synthesis Strategies, And Advanced Applications In Gas Storage And Catalysis

Molecular Composition And Structural Characteristics Of Stacked Covalent Organic Framework

Stacked covalent organic frameworks are constructed from organic building blocks—typically aromatic aldehydes, amines, boronic acids, or hydrazides—that undergo reversible condensation reactions to form extended two-dimensional sheets 2,4. These sheets subsequently assemble into three-dimensional macroscopic solids through π-π stacking interactions and hydrogen bonding. The resulting materials exhibit a unique dual-scale architecture: in-plane covalent networks provide mechanical strength and chemical stability, while out-of-plane stacking modes dictate pore size, crystallinity, and electronic properties 12,14.

In-Plane Covalent Linkages And Topology

The in-plane structure of stacked COFs is determined by the symmetry and connectivity of molecular building units. Common linkage chemistries include:

• Imine (C=N) bonds: Formed via Schiff base condensation between aldehydes and amines, imine-linked COFs (e.g., COF-432, TpPa-1) exhibit high crystallinity and tunable pore diameters ranging from 1.0 to 8.0 nm 1,3. However, imine linkages are susceptible to hydrolysis under humid conditions, necessitating structural modifications such as intramolecular O—H—N═C hydrogen bonding to enhance stability 8,13.
• Boronate ester (B-O) bonds: Early COF materials (COF-1, COF-5) utilized boronic acid trimerization to achieve hexagonal frameworks with pore sizes of approximately 9 Å 15. Despite their pioneering role, boronate ester COFs suffer from limited hydrolytic stability and are prone to decomposition upon exposure to moisture 6,13.
• Acylhydrazone linkages: Recent advances have introduced acylhydrazone bonds with 2-alkoxybenzohydrazidyl moieties, enabling rapid crystallization (within hours) and exceptional chemical robustness 12. These frameworks exhibit X-ray diffraction peaks at 2θ ≈ 3° with full width at half maximum (FWHM) of 0.2–0.4°, indicating high long-range order.
• Triazine (C-N) and β-ketoenamine linkages: Triazine-based COFs (CTF-1, CTF-2) and β-ketoenamine frameworks demonstrate superior hydrolytic stability due to irreversible bond formation, though at the cost of reduced crystallinity 6,13.

The topology of stacked COFs is governed by the symmetry of building units. For instance, C₆-symmetric hexaaldehyde nodes combined with C₃-symmetric triamine linkers yield kgd (kagome) topology with triangular pores, while C₄-symmetric tetraamine nodes produce sql (square lattice) topology with square channels 1,14. The choice of topology directly influences gas adsorption isotherms, diffusion kinetics, and mechanical anisotropy.

Out-Of-Plane Stacking Modes And Interlayer Interactions

The stacking arrangement of two-dimensional COF layers profoundly impacts material properties. Three primary stacking modes have been identified 3,14:

1. AA-eclipsed stacking: Layers are perfectly aligned with overlapping pore channels, maximizing π-π interactions and yielding the highest theoretical surface areas. However, this configuration is thermodynamically less stable and prone to structural collapse under mechanical stress 14.
2. AB-staggered stacking: Adjacent layers are offset by half a unit cell, creating a zigzag pore pathway. This mode balances crystallinity and stability, and is commonly observed in imine-linked COFs synthesized under solvothermal conditions 3,7.
3. ABC-staggered stacking: A three-layer repeating unit with incomplete offset results in ultramicroporous structures (pore diameter < 1 nm) and exceptional gas sieving capabilities. For example, a kgd-topology COF with ABC-staggered stacking exhibited helium/methane selectivity exceeding 100 at 298 K, outperforming conventional molecular sieves 14.

The interlayer spacing in stacked COFs typically ranges from 3.3 to 3.8 Å, comparable to graphite (3.35 Å). This close proximity enables efficient charge transport in semiconducting COFs, with reported hole mobilities up to 8.1 cm² V⁻¹ s⁻¹ in nickel-phthalocyanine-based frameworks 4,5. The stacking mode can be controlled through synthetic parameters such as solvent polarity, temperature, and the presence of structure-directing agents (e.g., acetic acid for promoting ABC-staggered arrangements) 14.

Pore Structure And Surface Area Characteristics

Stacked COFs exhibit hierarchical porosity with pore sizes spanning from micropores (< 2 nm) to mesopores (2–50 nm), depending on the length of organic linkers and stacking configuration. Key structural metrics include:

• BET surface area: Ultrahigh values exceeding 3000 m² g⁻¹ have been reported for three-dimensional COFs with large pore apertures (e.g., COF-103, COF-108) 6,9. Two-dimensional stacked COFs typically exhibit surface areas in the range of 1000–2500 m² g⁻¹ 1,8.
• Pore volume: Total pore volumes range from 0.5 to 2.0 cm³ g⁻¹, with micropore volumes contributing 40–80% of the total 9,10.
• Pore size distribution: Narrow distributions (FWHM < 0.5 nm) are characteristic of highly crystalline stacked COFs, enabling size-selective molecular sieving 14. In contrast, amorphous porous organic polymers (POPs) exhibit broad distributions and lower selectivity 13.

The cylindrical pore geometry in stacked COFs facilitates unidirectional mass transport, making them ideal for applications requiring anisotropic diffusion, such as ion conduction in solid-state electrolytes 7.

Precursors And Synthesis Routes For Stacked Covalent Organic Framework

The synthesis of stacked COFs requires careful selection of precursors and reaction conditions to balance the kinetics of covalent bond formation with the thermodynamics of crystallization. The reversibility of linking reactions is critical for error correction during framework assembly, enabling the formation of long-range ordered structures 2,12.

Molecular Building Blocks And Design Principles

Stacked COF synthesis employs rigid, planar aromatic precursors to minimize conformational flexibility and promote π-π stacking. Representative building blocks include:

• Aldehydes: 1,3,5-Triformylphloroglucinol (Tp), 2,5-dihydroxyterephthalaldehyde (Da), and terephthalaldehyde derivatives are widely used for constructing hexagonal and square lattices 1,8,10. The presence of hydroxyl groups adjacent to aldehyde functionalities enables intramolecular hydrogen bonding, which stabilizes imine linkages against hydrolysis 8,13.
• Amines: Benzidine (BZ), tetra(p-aminophenyl)porphyrin (Tph), and o-tolidine (BD(Me)₂) serve as linear or tetrahedral linkers 1,8,10. Amine precursors with electron-donating substituents (e.g., methoxy groups) accelerate imine formation but may reduce framework crystallinity 10.
• Boronic acids: Hexahydroxytriphenylene (HHTP) and phenylboronic acid derivatives form boronate ester linkages under dehydrating conditions 6,15. However, their limited stability has prompted a shift toward more robust chemistries.
• Hydrazides: 2-Alkoxybenzohydrazides react with aldehydes to form acylhydrazone linkages, which exhibit superior hydrolytic stability and rapid crystallization kinetics (< 6 hours at 120°C) 12.

The molar ratio of precursors must be precisely controlled to achieve stoichiometric balance. For example, a [6+3] condensation between a C₆-symmetric hexaaldehyde and a C₃-symmetric triamine requires a 1:2 molar ratio to form a kgd-topology network 14.

Solvothermal Synthesis And Reaction Conditions

Solvothermal synthesis is the most widely adopted method for preparing stacked COFs, involving the heating of precursors in sealed vessels under autogenous pressure. Key parameters include:

• Solvent selection: Polar aprotic solvents such as 1,4-dioxane, mesitylene, and N,N-dimethylacetamide (DMAc) are preferred for dissolving aromatic precursors and facilitating reversible bond formation 1,8,12. Solvent mixtures (e.g., dioxane/mesitylene 1:1 v/v) can modulate solubility and crystallization rates.
• Temperature and duration: Typical reaction temperatures range from 90 to 120°C, with durations of 12–72 hours 1,10,12. Lower temperatures (< 100°C) favor crystallinity but require extended reaction times (up to 7 days), while higher temperatures (> 120°C) accelerate polymerization but may yield amorphous products 2,12.
• Catalysts and additives: Brønsted acids (e.g., acetic acid, p-toluenesulfonic acid) catalyze imine condensation and promote error correction by protonating intermediate iminium ions 1,8. The concentration of acetic acid (typically 0.1–0.5 M) influences the stacking mode: higher concentrations favor ABC-staggered arrangements by stabilizing specific layer offsets 14.
• Pressure and atmosphere: Reactions are conducted under inert atmospheres (N₂ or Ar) to prevent oxidation of amine precursors. Autogenous pressure generated by solvent vapor enhances precursor solubility and suppresses side reactions 12.

A representative synthesis protocol for COF-432 involves dissolving 1,3,5-triformylphloroglucinol (Tp) and benzidine (BZ) in a 1:1 mixture of dioxane and mesitylene, adding 3 M acetic acid, sealing the mixture in a Pyrex tube, and heating at 120°C for 72 hours. The resulting yellow precipitate is collected by filtration, washed with anhydrous tetrahydrofuran, and activated under vacuum at 150°C for 12 hours, yielding a crystalline powder with a BET surface area of 2066 m² g⁻¹ 1.

Mechanochemical And Ionothermal Synthesis

Alternative synthetic routes have been developed to address the limitations of solvothermal methods, including long reaction times, low yields, and scalability challenges.

• Mechanochemical synthesis: Ball milling of solid precursors in the presence of catalytic amounts of liquid (liquid-assisted grinding, LAG) enables solvent-free COF synthesis at room temperature 7. This method produces COF powders with anisotropic ordering, where mechanical pressing induces preferred orientation between hk0 and 00l crystallographic planes. Pellets prepared by uniaxial compression (10 MPa) of mechanochemically synthesized COFs exhibit bulk densities of 0.6–0.8 g cm⁻³ and ionic conductivities up to 0.26 mS cm⁻¹ when impregnated with LiClO₄ 7.
• Ionothermal synthesis: Heating precursors in molten ionic liquids (e.g., ZnCl₂ at 400°C) yields triazine-based COFs (CTF-1) with exceptional thermal stability (> 500°C) and high nitrogen content (> 20 wt%) 2. However, the harsh conditions limit the scope of compatible building blocks.
• Microwave-assisted synthesis: Microwave irradiation accelerates imine condensation by providing rapid, uniform heating. Reaction times can be reduced to 30–60 minutes, though careful optimization is required to prevent localized overheating and amorphization 12.

Interfacial Synthesis And Thin Film Fabrication

For applications in optoelectronics and sensing, stacked COFs must be deposited as oriented thin films on conductive substrates. Interfacial synthesis techniques include:

• Langmuir-Blodgett (LB) assembly: Amphiphilic COF precursors are spread at the air-water interface, compressed to form monolayers, and transferred onto substrates via vertical dipping. Repeated transfers yield multilayer films with controlled thickness (10–100 nm) and preferential in-plane orientation 4,5.
• Liquid-liquid interfacial polymerization: Aldehyde and amine precursors are dissolved in immiscible solvents (e.g., dichloromethane and water), and the COF film grows at the interface. This method produces free-standing membranes with thicknesses of 50–500 nm, suitable for gas separation 4.
• Epitaxial growth on graphene: Single-layer graphene serves as a template for COF nucleation and growth, yielding highly crystalline films with improved π-π stacking and charge mobility. The graphene substrate can be transferred to arbitrary surfaces, enabling integration into flexible devices 5.

Thermal Stability, Hydrolytic Resistance, And Mechanical Properties Of Stacked Covalent Organic Framework

The practical utility of stacked COFs hinges on their ability to withstand harsh operating conditions, including elevated temperatures, humid environments, and mechanical stress. Comprehensive characterization of stability and mechanical properties is essential for guiding material selection and process optimization.

Thermal Stability And Decomposition Behavior

Stacked COFs exhibit exceptional thermal stability due to the strength of covalent bonds and the rigidity of aromatic backbones. Thermogravimetric analysis (TGA) reveals that most imine-linked COFs remain stable up to 300–400°C under inert atmospheres, with mass loss onset temperatures (T₅%) ranging from 350 to 450°C 1,3,8. For example:

• COF-432: TGA under N₂ shows no significant mass loss below 380°C, with a residual mass of 60% at 600°C, indicating partial carbonization of the organic framework 1.
• Porphyrin-based COFs: Frameworks incorporating tetra(p-aminophenyl)porphyrin (Tph) exhibit T₅% values of 420–450°C, attributed to the thermal stability of the porphyrin macrocycle 8,13.
• Triazine COFs: CTF-1 and CTF-2 display extraordinary thermal stability (T₅% > 500°C) due to the aromatic triazine core and irreversible C-N linkages 2,6.