Climate-neutral chemistry: researchers produce dye from CO2

08/24/2020 CO2 is not only a climate killer, it can also serve as a raw material for chemicals. In the Celbicon project, researchers have succeeded in producing a dye from the greenhouse gas by means of electrochemical and biotechnological conversion.

In order to reduce the concentration of carbon dioxide in the atmosphere, numerous research groups are investigating how the greenhouse gas CO2 can be used as a raw material for chemicals. "The development of processes for the utilization of CO2 will be a crucial component of a future climate- and resource-saving circular economy," says Dr. Arne Roth, who heads the innovation field of catalysts at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB.

The product in three stages: adsorption, electrochemistry, biotechnology

The electrolyzer synthesizes formic acid from atmospheric CO2.

The development of combined electrochemical-biotechnological processes represents a new way to make CO2 usable as a raw material for fuels and chemicals. The Fraunhofer Institute for Interfacial Engineering and Bioprocess Engineering IGB followed this up with partners from science and industry in the EU-funded Celbicon project and demonstrated an exemplary process chain on a pilot scale. The advantage of this approach: "By using the synthesis power of bacteria from nature - in addition to CO2 adsorption and electrochemical conversion - we can produce more complex molecules and thus value-adding products that make the new process economical," says Dr. Lénárd-István Csepei, who coordinated the project at Fraunhofer IGB.

Adsorption in the CO2 collector
In order to be able to utilize atmospheric CO2, it has to be adsorbed from the air in a first step. To this end, the project partner Climeworks installed a demonstration system on the premises of the IGB Biocat branch in Straubing. The core of the system are CO2 collectors. Air is sucked into this via a fan. Inside the collector there is a selective filter material to which CO2 is bound. The Swiss company's technology is already being used on an industrial scale at various pilot sites. But how does CO2 become a marketable product?

Production of formic acid from CO2
In so-called electrolysis cells, which are operated with electricity, CO2 can be converted into simple compounds such as formic acid, methanol or even ethanol via electrochemical reactions. These are so-called C1 or C2 compounds that contain only one or two carbon atoms. "However, the electrochemical conversion of CO2 only makes ecological sense if renewable energies are used for this," explains Csepei.

To ensure that the electrochemical conversion of CO2 takes place efficiently and that formic acid is formed in the highest possible concentration, the Fraunhofer researchers screened hundreds of different catalysts. "With special tin-containing catalysts and a phosphate-based buffer electrolyte for the electrolysis cell, we were able to achieve the best results and produce formic acid in higher concentrations," explains electrochemistry expert Dr. Luciana Vieira. “Because the electrolyte must neither be toxic nor inhibit enzymes so that the subsequent biotechnological conversion step works,” says the scientist.

Dye to add value with biotechnology

In the fermentation following the electrolysis, formic acid is converted into a valuable terpenoid dye. (Images: Fraunhofer IGB)

The simple C1 and C2 compounds can hardly be produced economically in this way. The reason: The availability of regenerative energies in Germany is subject to strong fluctuations - mainly due to climatic factors. Therefore, only partial load operation of a maximum of 2,000 to 3,000 h / a is possible. "Electrochemical production only becomes economical if it is possible to convert the compounds into higher-quality products," explains Csepei.

The C1 compounds such as methanol or formic acid produced in the second, electrochemical process step, in the third step, serve methylotrophic bacteria as the sole source of carbon and energy. The Fraunhofer researchers selected the bacterium Methylobacterium extorquens for the Celbicon process. This organism is able to form a complex red dye from the simple C1 compounds. "The value-adding color is formed by the microbial terpene metabolism," explains Dr. Jonathan Fabarius, who led the fermentation work at the IGB. Other bacteria require more energy-rich sugar molecules instead of formic acid or methanol.

The fermentation was established as a fed-batch process on a 10-liter scale. "We were able to show that 14% of the formic acid used in the fermentation is converted into the terpenoid dye," explains Fabarius. After the Straubing researchers were able to extract and purify the dye, they are currently in the process of clarifying its exact structure. Fabarius looks ahead: "Our goal is to further optimize the metabolic pathways and enzymes required for product formation by means of metabolic engineering and enzyme engineering in order to increase the product yield and thus the efficiency of the overall process."

Evaluation in a demonstration facility
After the individual process steps were first integrated into a continuous process chain on a laboratory scale, the project was completed by setting up an automated electrolyser demonstration system, the core of which is an electrochemical cell with an electrode area of ​​100 cm2. “With the demonstrator system, we can control important parameters such as temperature and pH value of the electrolytes used in long-term tests. For this purpose, the system is equipped with automatic data recording, ”explains Dr.-Ing. Carsten Pietzka, who is researching the electrosynthesis of basic chemicals at the IGB site in Stuttgart. With the demonstrator, the integrated system of CO2 adsorber and electrolyser could be validated in continuous operation.

In addition, the demonstrator is designed in such a way that so-called stacks, i.e. electrode stacks, can also be integrated. "This enables us to increase the formic acid production rate and use the demonstrator to further develop the electrolysis cell on an industrial scale," says Pietzka.

High-priced chemicals - climate-neutral and decentralized
"With our new technology, CO2 can be electrochemically converted into C1 intermediates and these can then be converted into value-adding compounds using a combined fermentation," summarizes project manager Csepei. With further optimization of the organisms and the fermentation step, it is also possible to produce basic chemicals such as lactic acid, isoprene or the biopolymer polyhydroxybutyric acid - completely climate-neutral.

Since CO2 - just like renewable energy - is mainly generated locally, the combined process is particularly suitable for the production of chemicals on a smaller scale. With a correspondingly high-quality product, decentralized production of smaller quantities can also become economical.