To tackle the challenge of climate change, Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB, with partners from science and industry, has pursued a new path in the EU-funded project Celbicon. Greenhouse gas CO2 is being used as a raw material for chemicals.

The researchers were able to produce a value-added terpenoid dye from CO2 adsorbed from air by a combination of electrochemical and biotechnological conversion. Developing the processes for the utilization of CO2 will be a crucial component for the future of a climate-friendly and resource-efficient circular economy.

Adsorption, electrochemistry, biotechnology

By using the natural synthetic capabilities of bacteria − in addition to CO2 adsorption and electrochemical conversion − we can produce more complex molecules and, thus, value-added products that make the new process economical.

To use atmospheric CO2 as raw material, it must be adsorbed from the air. The project partner Climeworks set up a demonstration plant on the premises of the IGB BioCat branch in Straubing. In the CO2 collectors of the plant, CO2 is adsorbed to selective filter material that is in direct contact with air blown though the system by a ventilator.

CO2 can be converted into simple compounds, such as formic acid, methanol or ethanol, via electrochemical reactions in so-called electrolysis cells powered by electricity. The formed products are so-called C1 or C2 compounds, which contain only one or two carbon atoms.

With particular catalysts containing tin and a phosphate-based buffer electrolyte for the electrolysis cell, best results were achieved producing formic acid in higher concentrations. The electrolyte must neither be toxic nor inhibit enzymes for the subsequent biotechnological conversion step to work.

However, the simple C1 and C2 compounds can hardly be produced in an economical way through this method. Due to local climatic conditions of Germany, there is fluctuation in availability of renewable energies. Therefore, only a partial-load operation of maximum 2000-3000 hours per year is possible. Electrochemical production will only become economically advantageous if the primary products can be further converted into products of higher value.

The C1 compounds serves as the sole carbon and energy source for methylotrophic bacteria applied in the third process step, the microbial fermentation. The Fraunhofer researchers selected Methylobacterium extorquens for the CELBICON process. This organism is capable of forming a complex red dye from methanol or formic acid.

The value-added dye is formed via the microbial terpene metabolism. Other bacteria require energy-rich sugar as substrate, instead of formic acid or methanol utilized here.

Fermentation was established as a fed-batch process on a 10-litre scale. 14 percent of the formic acid used in the fermentation process is converted into terpenoid dye. After extracting and purifying the dye, they are working on clarifying its exact chemical structure.

Goal is to further optimize the applied bacteria by means of metabolic engineering and enzyme engineering to increase the product yield and therefore the efficiency of the overall process.

After valid complete process on laboratory scale, Fraunhofer IGB succeeded in constructing and building an automated electrolyzer demonstration unit.

The core of this unit is an electrochemical cell with an electrode area of 100 cm2. The demonstrator can be used to control important parameters, such as temperature and pH value of the electrolytes used in long-term stability tests. For this purpose, the plant is equipped with an automatic data acquisition system. The integrated system consisting of CO2 adsorber and electrolyzer demonstration unit was validated in continuous operation.

The demonstrator is designed also for the integration of electrode stacks. Allowing to increase the production rate of formic acid and use the demonstrator for the further development of the electrolysis cell to an industrial scale.

Since CO2, just like renewable energy, is mainly generated in a decentralized way, the combined process is particularly suitable for the production of chemicals on a smaller scale. In this way, even the decentralized production of smaller quantities can become economically viable with a product of correspondingly high quality and value.