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

An international research team led by the 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 be used, for example, to capture carbon dioxide (CO₂), as the researchers describe in Nature communication.

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

A research team led 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, whose members are particularly stable and promise great application potential.

Researchers from Berlin, Bremen, Saarbrücken, Turkey and Great Britain were also involved in the study.

The class of covalent organic polyphosphonate frameworks is characterized by phosphorus-oxygen-phosphorus bonds, which consist of simple organic phosphonic acid building blocks and can be connected together – almost like Lego blocks – by heating them to temperatures as low as approx. 200 degrees Celsius.

Dr. Yücesan said: “The special property of these COFs is that, despite the mild synthesis conditions, they show good water and water vapor stability, which means that – unlike the compounds developed so far – they can be used in water and electrolytes. “

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

A challenge for the researchers was that the compounds did not crystallize well and were amorphous. They managed to find evidence for the bonds using nuclear magnetic resonance.

Professor Schmedt auf der Günne said: “If we had not been able to use the common states of adjacent phosphorus atomic nuclei, the bond structure of the substance would have remained in the dark and its 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 small change in pressure can release it again.

“Such substances are necessary for cleaning flue gases and preventing greenhouse gas emissions,” the study authors said.