Covalent Organic Frameworks for Carbon Dioxide Capture from Air: A 2025 Guide

Capturing carbon dioxide directly from the air is one of the harder climate challenges. The concentration of CO₂ in the atmosphere is about 420 parts per million, which is dilute enough that most capture technologies struggle to pull it out efficiently. Covalent organic frameworks (COFs) are a class of porous crystalline materials built from light elements like carbon, hydrogen, nitrogen, oxygen, and boron. Unlike metal-organic frameworks (MOFs), which rely on metal clusters, COFs are constructed entirely from organic building blocks linked by strong covalent bonds. This gives them high thermal stability, low density, and tunable pore structures that can be designed to selectively adsorb CO₂ even at low concentrations.

How COFs differ from MOFs for gas capture

Both COFs and MOFs are porous crystalline materials, but their chemistry is distinct. MOFs consist of metal ions or clusters connected by organic linkers, forming coordination networks. The metal centers give MOFs strong adsorption sites for gases like CO₂ and hydrogen, but they can also be sensitive to moisture or degrade under certain conditions Wikipedia.

COFs replace the metal nodes with organic linkages such as boroxine, imine, or triazine bonds. This makes them lighter and often more stable in humid or acidic environments. For direct air capture, where the air contains water vapor and other contaminants, the chemical robustness of COFs is a practical advantage. The pore sizes and surface chemistry can also be adjusted by choosing different organic monomers, which allows researchers to create frameworks that bind CO₂ selectively without competing strongly with water.

Why COFs are suited to dilute CO₂

Direct air capture requires materials that can pull CO₂ from a stream where it makes up only 0.04% of the gas. Most conventional sorbents, like aqueous amines, work well at higher concentrations found in flue gas but become less efficient at such low partial pressures.

COFs can be engineered with high surface areas, often exceeding 3000 m² per gram. More importantly, their pore walls can be functionalized with amine groups or other basic sites that react with CO₂ to form carbamates or bicarbonates. This chemisorption mechanism works even at low concentrations, because the binding energy is high enough to trap CO₂ molecules from a dilute gas stream. The organic nature of COFs also allows precise control over the density and arrangement of these functional groups, something that is harder to achieve in MOFs or zeolites.

Key properties that affect performance

Several physical and chemical properties determine how well a COF performs for air capture.

Surface area and porosity

High surface area provides more sites for adsorption. COFs with hierarchical porosity, meaning they have both micropores and mesopores, can improve gas diffusion. Micropores trap CO₂ molecules, while mesopores allow air to flow through the material without clogging.

Chemical stability

Air contains water vapor, oxygen, and trace acids. COFs with imine or β-ketoenamine linkages are more resistant to hydrolysis than boroxine-based frameworks. Stability under cycling conditions, where the material is repeatedly exposed to air and then regenerated by heating or vacuum, is critical for practical use.

Regeneration energy

Once CO₂ is captured, it must be released to collect the gas or reuse the sorbent. The energy required for regeneration depends on the binding strength. Strong chemisorption sites require more heat to release CO₂, while weaker physisorption sites release it more easily but may not capture enough at low concentrations. COFs can balance this by combining both types of sites in the same framework.

Functionalization strategies for CO₂ capture

Researchers have developed several ways to modify COFs to improve their CO₂ uptake.

Amine grafting

Amine groups, such as polyethyleneimine or ethylenediamine, can be attached to the pore walls. These groups react with CO₂ to form carbamates under dry conditions or bicarbonates in the presence of water. The amine loading must be optimized: too much can block pores and reduce diffusion, while too little leaves capacity untapped.

Nitrogen-rich frameworks

Some COFs are built from monomers that already contain nitrogen, such as triazine or porphyrin units. These nitrogen sites act as Lewis bases that attract acidic CO₂ molecules. Triazine-based COFs, for example, have shown good CO₂ uptake at low pressures because the nitrogen atoms provide both binding sites and a polar pore environment.

Metal doping

Although COFs are metal-free by design, small amounts of metal ions can be incorporated into the framework without disrupting the covalent structure. For instance, lithium or magnesium ions can be coordinated to oxygen or nitrogen sites in the pores. These metal cations create strong electric fields that polarize CO₂ molecules and increase adsorption.

The role of pore topology in adsorption behavior

The internal geometry of a COF is not just about surface area. The shape and connectivity of the pores directly influence how CO₂ molecules move through the material and how strongly they bind. COFs can be designed with different topologies, such as hexagonal, square, or kagome lattices, depending on the symmetry of the building blocks.

For direct air capture, pore size matters as much as pore chemistry. If pores are too small, CO₂ molecules diffuse slowly and the material takes too long to saturate. If pores are too large, the binding energy per molecule drops because the adsorbate is farther from the pore walls. An ideal pore diameter for CO₂ capture at ambient conditions is roughly 0.6 to 1.0 nanometers, which is large enough for diffusion but small enough to create overlapping potential fields from opposite pore walls.

An example is the COF known as COF-1, which forms a hexagonal structure with pore diameters around 1.5 nanometers. While this material has good surface area, its pores are too wide for strong CO₂ binding at low pressures. Researchers have addressed this by interpenetrating two identical frameworks within the same space, effectively halving the pore size and increasing the number of adsorption sites per volume.

Humidity and the role of water in CO₂ capture

Water is present in ambient air at concentrations far higher than CO₂. Relative humidity of 50% at 25°C corresponds to roughly 15,000 ppm of water vapor, compared to 420 ppm of CO₂. Any sorbent for direct air capture must handle this imbalance.

In amine-functionalized COFs, water can play a dual role. Under dry conditions, CO₂ reacts with two amine groups to form a carbamate, which consumes two binding sites per CO₂ molecule. In the presence of water, the reaction shifts toward bicarbonate formation, which requires only one amine per CO₂ molecule. This means that moderate humidity can actually increase the CO₂ capacity of amine-grafted COFs, because each amine site becomes more efficient.

However, too much water can cause problems. Condensed water inside the pores can block access to adsorption sites or hydrolyze certain linkages. Boroxine-based COFs, for example, degrade rapidly in moist air. Imine-linked COFs are more resistant but can still undergo hydrolysis over many cycles. The most robust COFs for humid conditions are those with β-ketoenamine linkages, which form a tautomeric structure that resists water attack.

Researchers have tested these materials under controlled humidity chambers to measure capacity loss over time. Some COFs retain over 90% of their initial CO₂ uptake after 100 cycles at 70% relative humidity, while others lose half their capacity within 20 cycles. The choice of linkage chemistry is therefore one of the first design decisions for any practical air capture application.

Current limitations and research directions

COFs for direct air capture are still largely at the laboratory stage. Several challenges remain before they can be deployed at scale.

Synthesis cost and scalability

Most COFs are synthesized under solvothermal conditions using expensive organic precursors and large amounts of solvent. Scaling up production while maintaining crystallinity and porosity is an active area of research. Some groups are exploring mechanochemical synthesis, where the building blocks are ground together without solvent, as a lower-cost alternative.

Water competition

Water vapor in air competes with CO₂ for adsorption sites. In some amine-functionalized COFs, water can actually help by facilitating bicarbonate formation, but in others it blocks pores or degrades the framework. Designing COFs that work well under realistic humidity levels, which vary by location and season, is a priority.

Long-term cycling

Repeated adsorption and desorption cycles can cause gradual loss of capacity due to amine oxidation, pore collapse, or accumulation of impurities. Accelerated aging tests under realistic conditions are needed to evaluate durability.

Comparison with other direct air capture materials

COFs are not the only porous materials being studied for air capture. A quick comparison helps clarify where they fit.

COFs offer a middle ground: better stability than most MOFs in wet conditions and more tunability than zeolites. They are not yet competitive with amine solutions on cost, but their lower regeneration energy could be a long-term advantage.

Key Takeaways

• Covalent organic frameworks are porous crystalline materials made entirely from light elements, offering high thermal and chemical stability for direct air capture.
• Their pore surfaces can be functionalized with amines or nitrogen groups to selectively adsorb CO₂ at low atmospheric concentrations.
• COFs avoid the moisture sensitivity and metal toxicity issues that affect many MOFs, making them attractive for real-world air capture.
• Key challenges include scaling up synthesis, managing water competition, and maintaining capacity over many cycles.
• Research is focused on reducing cost through solvent-free synthesis and improving durability under realistic operating conditions.