Turning CO2 into useful chemicals

Turning carbon dioxide into important starting materials for fine chemicals - it actually works: a research team from the Fraunhofer IGB has succeeded for the first time in the Max Planck cooperation project _eBioCO2nin fixing_CO2 in an enzyme cascade based on the transfer of electrons and converting it into a solid starting material for the chemical industry. The process for electrobiocatalytic_{CO2 fixation} has already been published and is considered a "hot paper".

Burning fossil fuels produces climate-damaging carbon dioxide, which plays a major role in global warming as a greenhouse gas. Nevertheless, crude oil is still one of the most important raw materials - not only as an energy source, but also as a raw material for the chemical industry and therefore for numerous everyday items such as medicines, packaging, textiles, cleaning agents and more. Intensive research is therefore being carried out into various alternatives to fossil sources.

Renewable raw materials are a promising option for the future, but not the only alternative raw material basis for covering the availability of green synthetic products in the coming years. A sustainable addition to this in terms of a circular carbon economy is the possibility of fixing_CO2 in a targeted manner and under mild reaction conditions.

Capture from the air for lower CO2 emissions

A research team at the Straubing branch of the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB, together with colleagues from the Max Planck Institute for Terrestrial Microbiology in Marburg and the Technical University of Munich, has now succeeded for the first time in converting CO2 electrobiocatalytically into valuable substances for the chemical industry. By combining various approaches from bioelectrochemistry, enzyme biology and synthetic biology, special bioelectrodes have been developed to drive enzymes with electricity from renewable energy, which produce solid organic molecules from the greenhouse gas in a coupled reaction similar to photosynthesis.

The aim is to capture_CO2 directly from the air: "The process could then not only help industry to dispense with fossil raw materials, but also actively promote climate change by reducing CO2," explains Dr. Michael Richter, Head of the Bioinspired Chemistry Innovation Field at Fraunhofer IGB. "First of all, however, we wanted to show that our idea of driving such a complex biocatalytic multi-enzyme reaction with electricity in this way works at all."

Hydrogel transports electrons for_CO2-fixing enzymes

With success: the researchers were inspired by the metabolism of microorganisms and developed an electricity-based process for_{CO2 fixation}. The main players are_CO2-fixing enzymes, which were developed by colleagues Dr. David Adam and Prof. Tobias Erb, Director at the MPI in Marburg. One challenge was to continuously supply the_CO2-fixing enzymes with the electrons required for the reduction of_CO2, which can be supplied by renewable electricity. This was achieved by embedding the enzymes in a redox-active hydrogel, which enables them to be electrochemically driven in such a way that they bind carbon dioxide to a substrate and thus convert it into a valuable intermediate. "The process is a very efficient reaction pathway, a reductive carboxylation that is very economical and clean because you don't need any other substances in the system - just carbon dioxide, substrate and electrons, preferably from renewable sources," explains Dr. Leonardo Castañeda-Losada, who conducted research in the field of electrobiocatalysis in his doctoral thesis and is now working on the project at Fraunhofer IGB together with Dr. Melanie Iwanow and Dr. Steffen Roth.

The hydrogels in which the enzymes carry out their work, which were specially developed at the TU Munich at the chair of Prof. Nicolas Plumeré, are modified in such a way that they conduct electrons well and at the same time offer the biomolecules optimal working conditions. "This means that we can not only use monolayers of enzymes, but also extend this three-dimensionally many times over, as the electrons are directed to any location in the gel. These are good prerequisites for scaling up the process for the chemical industry in the future," explains Prof. Volker Sieber, who has been pursuing strategies for_{CO2 storage} at the Straubing branch of the Fraunhofer IGB for a long time.

Cofactors are permanently regenerated at the same time

However, the researchers' completely new approach is not only based on the fact that an enzymatic reaction sequence can be successfully driven by electricity, but also includes another extremely innovative module: In order for the reactions to proceed as desired and to achieve the highest possible product yield at the end, a continuous supply of "doping" for the enzyme is required in this case: the appropriate and functional cofactors. These small, organic molecules are used up in the course of each individual reaction and need to be regenerated in order to be ready for use again. Providing them in large quantities is very expensive and therefore uneconomical for industry. That is why the _eBioCO2n expertshave found a way to renew them again using electricity within the same reaction system in the hydrogels - theoretically for an infinite period of time. "Actually, you would only have to add cofactor to the system once and it would then be automatically regenerated again and again. But in practice, this only works nearly as well because the cofactor does not remain stable indefinitely - but it does remain stable for a very long time," says Richter.

For the bioelectrocatalytic recycling process of the cofactors, the researchers even have a whole toolbox of different enzymes at their disposal, which they have tracked down from various organisms. This means that the spectrum of these biomolecules can be modularly expanded for further work depending on the application and can be used as a platform system. "You can select practically any enzymes from bioinformatic databases, produce them biotechnologically and incorporate them into the hydrogels," says Richter. "This would make the production of various bio-based fine chemicals conceivable, which could be diversified via further enzyme cascades practically as required if expanded accordingly." This is where the Marburg MPI in particular contributes its expertise. If this can be successfully scaled up, the platform technology could become a promising business model for the chemical industry.

Platform system to be expandable and scalable as required

With the help of bio-inspired_{CO2 fixation} from the laboratory, the Fraunhofer IGB was able to carboxylate a coenzyme A derivative, a biomolecule that is important for many metabolic processes in living organisms. "This is the most challenging molecule to date to which_CO2 could be fixed by biocatalytic means," says Richter. "Modifying such a large and structurally challenging substance using this technology is far from a matter of course." The researchers now face the final challenge: proving that their idea works reliably and scalably and can be expanded modularly. However, the IGB is optimistic, especially against the background of a well-functioning interdisciplinary team, as the scientist emphasizes. In follow-up projects, industrial partners are to be involved as quickly as possible.