2.5D Skeletons Boost CO2 Separation in Organic Crystals

Porous organic crystals with superior properties as CO2 adsorbents were created by researchers at Institute of Science Tokyo. Owing to the novel 2.5-dimensional skeleton, the materials feature ultrahigh-density amines. The covalently-bonded microporous skeleton and high crystallinity realize fast CO2 adsorption and high thermal stability. Their low adsorption heat, only one-fourth of the current amine scrubbing method, and their light-elemental nature can reduce the cost for CO2 separation from flue gases.

To mitigate climate change, CO₂ emissions from large industrial facilities have to be reduced. To separate CO₂ from the flue gases, the major current technology is the amine scrubbing method, in which an aqueous solution of amine molecules is circulated in the capture facility to cyclically perform capture and release of CO₂. However, this method suffers from reportedly high running costs (e.g., a report by Institute for Energy Economics and Financial Analysis, October 2022 ) and several problematic aspects of amine solutions such as high environmental risk and corrosivity to steels (e.g., Renewable and Sustainable Energy Reviews, vol. 168, article no. 112902, 2022 ). The high cost arises from the necessity to also heat up the liquid water, which is a solvent of the amine molecules, and to generate steam that requires a large energy input to generate, in the regeneration process to strip the captured CO₂ from the amine molecules. The high cost is also caused by the excessively large heat of reaction (denoted Q; typically, in 80-100 kJ/mol range).

Therefore, to avoid the high cost, one should (1) cease the use of aqueous amine solution and (2) decrease Q as low as possible. To achieve (1), the use of solid sorbents is reasonable. However, if we use non-porous solids, the capturing speed would be intolerably slow. Thus, we should use porous solids. The use of solid sorbents would also resolve the issues of corrosion and environmental risk. Achieving (2) is more difficult, because a high Q assures the speed of CO₂ capture and high selectivity to CO₂ over other species in air like nitrogen and oxygen. In other words, Q at a level that is too low will cause an intolerably low CO₂ capturing speed and low selectivity to CO₂. Therefore, to decrease the cost, we have to decrease Q without scarifying the CO₂ capturing speed and selectivity to CO₂. That is the technical dilemma.

The organic porous materials recently reported by the researchers of Institute of Science Tokyo (Science Tokyo) have a slightly peculiar structure. Initially, the researchers were trying to polymerize two kinds of monomers, which are a tetrahedral molecule with four primary amines (-NH₂) on the vertexes (TAM) and a triangle molecule with aldehydes (-CHO) on the vertexes (TFPT/TFPB), as shown in the upper left portion of Figure 1. Here, -NH₂ and -CHO serve as "hands" to form a covalent bond; a "shaking-of-hands" between -NH₂ and -CHO is known to form an imine bond (-HN=C-), a type of covalent bond, with a release of one H₂O molecule. The researcher thought that they would obtain solids with granular shapes as a result of a three-dimensional (3D) network formation, as geometrically expected from the "four hands" of the tetrahedron monomer. In short, they planned to generate 3D covalent organic frameworks (3D-COFs).

However, their expectation did not come true. Instead, they obtained two-dimensional (2D) solids composed of stacked layers, as shown in the bottom right portion of Figure 1. The morphology is similar to graphite that is composed of stacked layers of graphene (an atomic layer of hexagonally-bonded carbon atoms). The researchers were puzzled by this morphology, which is a typical morphology of 2D covalent organic frameworks (2D-COFs).

The answer was revealed by the single-crystal X-ray diffraction analysis. The layered solids were composed of corrugated framework layers constructed with imine bonds of 3D connectivity, ending up in a macroscopically two-dimensionally extended polymer for one layer (Figure 1). The stacking of the layers resulted in the formation of the layered solids. Because such a framework structure was unexpected from the geometry of the monomers and did not match previous depictions or definitions of 2D-COFs nor 3D-COFs, the researchers thought it would be appropriate to call such materials 2.5-dimensional COFs (2.5D-COFs).

Because this structure was constructed using only three hands of a tetrahedral monomer, one hand is left unused per the monomer in the material (Figure 1). Consequently, the material uniquely has ultrahigh density amine (-NH₂) moieties, where all -NH₂ are regularly arrayed and pointing normal to the 2D layer. This material is microporous (pore size: 6-7 Å) and -NH₂ is the moiety that has, more or less, an ability to capture CO₂ molecules.

Project leader Professor Yoichi Murakami commented, "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."

The researchers found that the heat of adsorption, Q, was much lower (about 25 kJ/mol) than that in the amine scrubbing method (typically 80-100 kJ/mol) and other porous organic materials (Supplementary Section 3 of the Reference below). Importantly, these 2.5D-COFs do not suffer from the aforementioned dilemma. Despite their significantly low Q, they exhibited sufficiently high selectivity to CO₂ over N₂ (100 or higher) and a high speed of CO₂ adsorption (equilibrium time constant < 10 s). They have high thermal stability in air to around 300 °C. Simultaneous achievements of these advantages render these 2.5D-COFs promising materials that can efficiently capture and separate CO₂ at a lower cost than the current technology, whether the separated CO₂ is reused or buried underground, ultimately to mitigate climate change.

About Institute of Science Tokyo (Science Tokyo)

Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the

merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of

Technology (Tokyo Tech), with the mission of "Advancing science and human wellbeing to

create value for and with society."