Sustainably produced covalent organic frameworks can be used for efficient CO₂ capture

An international research team headed by Heinrich Heine University Düsseldorf (HHU) and the University of Siegen has synthesized a new compound, which forms a so-called covalent organic framework. The compound, which is based on condensed phosphonic acids, is stable and can, for example, be used to capture carbon dioxide (CO₂), as the researchers describe in Nature Communications.

Covalent organic frameworks (COFs) are a class of porous crystalline materials which form scaffold-like structures. The term “covalent” denotes that chemical bonds between the individual building blocks of the framework are formed via shared electron pairs.

A research team headed by Dr. Gündoğ Yücesan, Heisenberg Junior Research Group Leader at the Section for Nanoporous and Nanoscale Materials at HHU and Professor Dr. Jörn Schmedt auf der Günne, leader of the Inorganic Materials Chemistry group at the University of Siegen, now presents a simple approach to this family of frameworks, the members of which are particularly stable and promise great application potential.

Researchers from Berlin, Bremen, Saarbrucken, Turkey and the United Kingdom were also involved in the study.

The class of polyphosphonate covalent organic frameworks is characterized by phosphorus-oxygen-phosphorus bonds, which comprise simple organic phosphonic acid building blocks and—almost like Lego bricks—can be joined together by heating them to temperatures of just approx. 200 degrees Celsius.

Dr. Yücesan said, “The special property of these COFs is that, despite the mild synthesis conditions, they exhibit good water and water vapor stability, meaning that—by contrast with compounds developed to date—they can be used in water and electrolytes.”

A further milestone was the development of a sustainable synthesis route. Yücesan said, “For the first time, a solid-state synthesis process has been developed for COFs, which can be realized completely without solvents. This method enables low-cost, scalable production from kilograms to tons, making it more cost-effective compared with other microporous materials.

One challenge for the researchers was that the compounds did not crystallize well and are amorphous. They succeeded in finding evidence for the bonds by means of nuclear magnetic resonance.

Professor Schmedt auf der Günne said, “If we had not been able to use the common states of neighboring phosphorus atom nuclei, the bonding structure of the substance would have remained in the dark and the properties would not have been understood.”

Polyphosphonates of this type have great application potential. The framework structures can capture the harmful greenhouse gas CO₂. A slight change in pressure can release it again.

“Such substances are needed for waste gas cleaning and to prevent greenhouse gas emissions,” the authors of the study noted.