Carbon dioxide (CO₂) separation is critically important across a wide range of technologies, from natural gas purification to hydrogen production and carbon management. One widely used approach relies on thin filtering materials called membranes. However, these membranes face a serious challenge: materials that allow CO₂ to pass through quickly are generally poor at distinguishing it from other gases, while materials with high selectivity slow down CO₂ flow. Overcoming this dilemma, known as the permeability-selectivity trade-off, remains one of the field's fundamental research problems.
Carbon dioxide (CO₂) separation is of critical importance across a broad range of technologies spanning from natural gas purification to hydrogen production and carbon management. One commonly used approach relies on membranes, thin filtration materials. However, these membranes face a serious challenge: materials that allow CO₂ to pass through rapidly are often insufficient at distinguishing it from other gases, while materials showing high selectivity slow down CO₂ flow. Overcoming this dilemma, known as the permeability-selectivity trade-off, remains one of the field's fundamental research problems.
New membranes exceed a long-standing performance boundary
Researchers from Tohoku University and collaborating institutions have developed a new class of heteroatom-engineered covalent organic framework (COF)-based mixed matrix membranes (MMM) that eliminates this limitation. These membranes exhibit exceptional CO₂ separation performance that exceeds the 2008 Robeson upper bound, long considered a performance ceiling for gas separation membranes. The work was published in the Journal of the American Chemical Society.
How do the new materials work?
The researchers achieved this breakthrough by developing two new porous materials specially designed to interact strongly with carbon dioxide (CO₂). These materials were added to a polymer membrane, both attracting CO₂ molecules and creating pathways that allow them to move rapidly through the membrane.
The highest-performing membrane combined rapid CO₂ transport with high-accuracy separation from methane and hydrogen, surpassing the performance metrics that many conventional membranes struggle to achieve.
Why is better CO₂ separation needed?
Carbon dioxide separation is an indispensable process in sectors such as natural gas enrichment, hydrogen purification and carbon capture. Since existing technologies such as amine washing and cryogenic separation are energy-intensive and operationally challenging, developing more energy-efficient membrane-based alternatives is of great importance.
MMMs, which combine porous fillers with polymer matrices, offer a promising strategy for enhancing gas separation performance. However, most membranes remain constrained by the inherent trade-off between permeability and selectivity. Overcoming this boundary requires materials that can simultaneously support both selective adsorption and rapid molecular transport.
Investigating pore chemistry with specialized COFs
COFs are crystalline porous polymers with atomically defined pore architectures and tunable chemical functionality. However, systematically understanding how pore surface chemistry affects gas transport has been difficult, as chemical functionality changes often simultaneously transform framework topology and pore geometry as well.
Dr. Saikat Das from the Institute of Multidisciplinary Research for Advanced Materials at Tohoku University explains: "To isolate the role of pore chemistry, we designed two isomeric COFs that differ only in heteroatom composition. This approach allowed us to directly correlate heteroatom engineering at the molecular level with gas separation performance at the membrane level."
The team developed two similar porous materials named TUS-621 and TUS-622 by using chemical components containing oxygen or sulfur. Among these materials that share nearly identical structures, TUS-621, which is oxygen-rich, showed stronger attraction to CO₂, allowing the gas to pass more easily and exhibiting markedly improved CO₂ separation performance.
Performance tests and molecular insights
Comprehensive mixed gas permeability experiments demonstrated that the optimized TUS-621/Pebax-10% membrane not only exceeded the 2008 Robeson upper bound for CO₂/CH₄ separation, but also maintained superior separation performance across a wide range of pressure and temperature during 30 days of continuous operation.
Computational studies revealed that strong electronic interactions between CO₂ molecules and oxygen-containing pore environments played a decisive role in enhancing selective CO₂ adsorption and transport. Yuichi Negishi from the Institute of Multidisciplinary Research for Advanced Materials evaluates as follows: "This work demonstrates that precise heteroatom engineering within structurally controlled COFs can fundamentally reshape membrane transport behavior. We believe this strategy opens a new door toward practical and energy-efficient carbon capture and gas separation technologies."
Source
Publication details / Tsukasa Irie et al, Heteroatom-Engineered Covalent Organic Frameworks Break the CO₂ Separation Trade-Off in Mixed Matrix Membranes, Journal of the American Chemical Society (2026). DOI: 10.1021/jacs.5c23169 / Journal information: Journal of the American Chemical Society / https://phys.org/news/2026-05-pore-chemistry-carbon-capture-cofs.html
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