Porous Crystals Excel at CO2 Capture

Researchers from the Institute of Science Tokyo developed porous organic crystals with outstanding CO₂ adsorption characteristics, as reported in a Nature Communications study.

The materials include ultrahigh-density amines thanks to their innovative 2.5-dimensional structure. The covalently linked microporous skeleton and high crystallinity enable rapid CO₂ adsorption and thermal stability. Their low adsorption heat, which is one-fourth that of the existing amine scrubbing approach, and their light-element nature can help to minimize the cost of CO₂ removal from flue gases.

Reducing CO₂ emissions from large-scale industrial operations is essential for addressing climate change. The primary technology currently used for CO₂ separation from flue gases is amine scrubbing, where an aqueous solution of amine molecules cyclically captures and releases CO₂.

However, this method has high operational costs, as noted in an October 2022 report by the Institute for Energy Economics and Financial Analysis. Additionally, amine solutions pose environmental risks and contribute to steel corrosion, as outlined in Renewable and Sustainable Energy Reviews (Vol. 168, Article No. 112902, 2022).

The high costs associated with amine scrubbing stem from the energy-intensive process of heating the water solvent and generating steam for CO₂ desorption. The process also has a high heat of reaction (Q), typically in the range of 80–100 kJ/mol, further contributing to its inefficiency.

To reduce costs, two key strategies should be implemented: (1) eliminating the use of aqueous amine solutions and (2) minimizing the heat of reaction (Q) while maintaining efficient CO₂ capture.

For (1), solid sorbents offer a viable alternative to liquid amine solutions. However, using non-porous materials would result in an unacceptably slow capture rate, making porous materials a necessary choice. Solid sorbents also help mitigate issues related to corrosion and environmental risks.

For (2), achieving a low Q presents a technical challenge. While a high Q facilitates fast CO₂ capture and strong selectivity over nitrogen and oxygen, excessively reducing Q could significantly slow the capture rate and reduce selectivity. The goal is to lower Q while maintaining both capture efficiency and selectivity.

Researchers at the Institute of Science Tokyo (Science Tokyo) have developed a novel class of organic porous materials with an unconventional structure. They attempted to polymerize two monomers: a tetrahedral molecule with four primary amines (-NH₂) at its vertices (TAM) and a triangular molecule with aldehydes (-CHO) at its vertices (TFPT/TFPB).

In this reaction, the –NH₂ and –CHO groups act as reactive sites, forming imine bonds (-HN=C-) through condensation, releasing one H₂O molecule per bond. The researchers initially expected a three-dimensional (3D) covalent organic framework (3D-COF) to form due to the tetrahedral monomer's geometry. They anticipated a granular solid structure resulting from the formation of an extended 3D network.

However, the expected three-dimensional structure did not form. Instead, the resulting solids exhibited a stacked, two-dimensional (2D) structure, similar to graphite, which consists of layered graphene sheets. This unexpected outcome puzzled the researchers, as the structure aligned more closely with known 2D covalent organic frameworks.

Single-crystal X-ray diffraction analysis provided the explanation. The material formed corrugated framework layers where imine bonds established three-dimensional connectivity within each layer, leading to a polymer structure that extended macroscopically in two dimensions. The layers are then stacked together, producing a layered solid.

Since this structure differed from both conventional 2D-COFs and 3D-COFs and did not match prior classifications, the researchers introduced the term 2.5-dimensional COFs to describe these materials.

In this structure, only three of the four reactive sites from the tetrahedral monomer participated in bonding, leaving one site unpaired. As a result, the material features an ultrahigh density of amine (–NH₂) groups, all uniformly aligned perpendicular to the 2D layers. Given that –NH₂ groups serve as CO₂ adsorption sites, and the material has a microporous structure with a pore size of 6–7 Å, this unique arrangement enhances its potential for efficient CO₂ capture.

Although that structure was anticipated when I first looked at the layered morphology, I was excited when the results of the single-crystal X-ray diffraction analysis actually exhibited such an unprecedented network structure. Noticing such an ultrahigh-density array of primary amines in these materials, our group soon decided to investigate the CO₂ adsorption properties. The properties were very good, as we anticipated.

Yoichi Murakami, Project Leader and Professor, Institute of Science Tokyo

Compared to the conventional amine scrubbing method, which typically requires 80–100 kJ/mol, and other porous organic materials, the researchers found that the heat of adsorption (Q) for these 2.5D-COFs was significantly lower, approximately 25 kJ/mol (Supplementary Section 3 of the referenced study). These materials do not face the trade-offs commonly associated with lowering Q.

Despite their low Q, the 2.5D-COFs exhibited a high CO₂ adsorption rate, with an equilibrium time constant of less than 10 seconds, and a CO₂ /N₂  selectivity of 100 or greater. Additionally, they demonstrated thermal stability up to 300 °C in air.

Whether for underground storage or industrial reuse, these materials offer an efficient and cost-effective approach to CO₂ capture and separation, presenting a viable alternative to current technologies for mitigating climate change.

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Journal Reference:

Kitano, T. et. al. (2025) 2.5-dimensional covalent organic frameworks. Nature Communications. doi.org/10.1038/s41467-024-55729-2