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Evonik and Siemens launch phase 2 of Rheticus: butanol, hexanol from CO2 and water using renewable electricity and bacteria

Evonik and Siemens launched phase two of their joint research project Rheticus. The goal is to develop an efficient and powerful test plant that will use carbon dioxide and water as well as electricity from renewable sources and bacteria to produce specialty chemicals.

In the Rheticus I project, the two companies worked for two years to develop the technically feasible basis for artificial photosynthesis using a bioreactor and electrolyzers. Evonik and Siemens are now combining these two, previously separate, plants in a test facility at Evonik’s site in Marl (Germany). Rheticus II will run until 2021 and will receive funding of around €3.5 million from Germany’s Federal Ministry of Education and Research (BMBF).


Process design for technical synthesis of butanol and hexanol from CO2, H20 and electricity. From the Supplementary Information for a 2018 paper in Nature Catalysis by the Evonik-Siemens team (Haas et al.) describing the approach.
The plant consists of 5 main units and operates continuously. The first unit comprises two electrolyzers which convert H2O and CO2 to H2 and CO. These product gases together with excess CO2 are fed to the second unit, the gas fermenter, in which C. autoethanogenum converts the product gases of the electrolyzer (syngas) to acetic acid (reaction 1) and ethanol (reaction 2). This unit is equipped with a microfiltration system to retain the acetogenic cells. In addition fresh cells are fed to the fermenter and a small purge stream is taken out of the unit to keep the cell concentration and activity constant.
The gas fermenter also converts butyrate and hexanoate to butanol (reaction 5) and hexanol (reaction 6). Butyrate (reaction 3) and hexanoate (reaction 4) are formed in the third unit, the acetate/ethanol fermenter, by the fermentative activity of chain elongation cells -C. kluyveri. This unit is also equipped with a microfiltration system to retain C. kluyveri.
Fresh cells are fed to the fermenter and a small purge stream is taken out of this reactor to keep the cell concentration and activity constant. The product stream of unit two is extracted in the fourth unit with a high boiling solvent like oleyl alcohol in which butanol and hexanol dissolves. The raffinate of the extractor contains acetate and ethanol and is fed to the second unit, the acetate/ethanol fermenter. The extract is distilled in the fifth unit. The distillate contains the products butanol and hexanol, which can easily be separated. The bottoms liquid contains the solvent being recycled to the extractor unit 4.

The innovative technology used for Rheticus has the potential to contribute to the success of Germany’s energy transition. In the future, this platform could be installed anywhere CO2 is available—for example, at power plants or biogas plants. We use available CO2 as the raw material for the production of high-value chemicals using artificial photosynthesis.

—Thomas Haas, responsible for the Rheticus project at Evonik

Siemens is contributing the world’s first CO2 electrolyzer to the Rheticus project.

We are developing a flexible system that can provide answers to various questions raised by the energy transition. We are making it possible to store renewable energy by converting it into useful substances such as specialty chemicals or fuel. We are also contributing to the stability of the grid because production is so flexible that we can respond to fluctuations in power supply.

—Karl-Josef Kuhn,in charge of Power2X research at Siemens

The test facility, which comprises electrolyzers and a bioreactor, is scheduled to start operating in early 2020. In a first step, carbon dioxide and water are converted into carbon monoxide (CO) and hydrogen in electrolyzers with the aid of electricity.

Special microorganisms then convert the CO in the gases synthesized in this way into chemicals. Siemens and Evonik are each contributing their core competencies—electrolysis and biotechnology—to this artificial photosynthesis process. Artificial photosynthesis means combining chemical and biological steps so that energy can be used to produce viable chemicals from CO2 and water.

The Rheticus technology platform also contributes to the reduction of carbon dioxide levels in the atmosphere, as it uses CO2 as a raw material. Three tons of carbon dioxide would be needed to produce one tonne of butanol, for example.

The synthesis module came on stream at Evonik in spring 2019. At its heart is an 8-meter high stainless steel bioreactor with capacity of 2,000 liters. Microorganisms work continuously in the reactor. Their main nutrients are hydrogen and carbon monoxide. Siemens has developed a fully automated CO2 electrolyzer which was integrated into a container in summer 2019. The world’s first CO2 electrolyzer comprises 10 cells and the total surface area of the electrodes is 3,000 cm2.

The electrolyzer and the bioreactor will be combined in the coming months. In addition, a unit to process the liquid from the bioreactor is being built to obtain pure chemicals.

In the test facility, bacteria will produce butanol and hexanol for research purposes. These substances are used as starting products, for example, for specialty plastics and food supplements. However other specialty chemicals are conceivable, depending on the bacterial strain and conditions.


  • Thomas Haas, Ralf Krause, Rainer Weber, Martin Demler & Guenter Schmid (2018) “Technical photosynthesis involving CO2 electrolysis and fermentation” Nature Catalysis volume 1, pages 32–39 doi: 10.1038/s41929-017-0005-1



This could become an effective way to better use excess energy from REs?


Nothing on efficiency, meaning it must be bad.  Given the number of steps in this process (fermenting CO + H2 to alcohols and acetate with loss of CO2, another fermentation to convert EtOH to heavier products, then distillation) it's going to be far less efficient than straight electrolysis.

We just saw the pre-commercial application of the copper-CNT CO2 electrolysis scheme.  It would be nice to have numbers for both in order to compare them.

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