Dr. Frank Kensy (b.fab GmbH) about Transformate

Combination process development for the production of the biopolymer PHB and crotonic acid

CO₂-WIN Connect: The project goal of TRANSFORMATE is to build a two-stage process for producing the bioplastics polyhydroxybutyric acid (PHB) and crotonic acid. What are these two substances used for and how have they been produced so far? And what distinguishes the process in TRANSFORMATE from previous production methods?

Dr. Kensy: PHB is nowadays largely produced from sugar (glucose, sucrose) either from sugar cane/sugar beet or from cellulose. This is an extremely complex process and sugar production is based on natural photosynthesis, which has a low energy efficiency and, thus, requires large agricultural areas. Crotonic acid is nowadays produced via a multi-step petrochemical process with low efficiency. In TRANSFORMATE, we want to transfer both products to a sustainable raw material base. Our raw material is CO₂ and renewable energy. These are converted to formic acid in an electrolyzer, which we then convert to PHB or crotonic acid in a fermentation process with bacteria. PHB is a biodegradable bioplastic that can be well used for products where material recycling cannot be 100% guaranteed and which can be released uncontrolled into the environment. Crotonic acid, on the other hand, is used as a binder in paints and coatings and can serve as a precursor for methyl methacrylate.

CO₂-WIN Connect: The reaction of carbon dioxide with hydrogen also offers the possibility of producing methanol. Methanol has a higher energy density than formic acid. Why did you choose formic acid as a carbon and energy source rather than methanol?

Dr. Kensy: We chose formic acid because the overall process with the intermediate product formic acid is very simple. It is important to know that formic acid, along with hydrogen and carbon monoxide, is one of the three most efficient products that can be converted in an electrolyzer. Faraday efficiencies of 80-95% are achieved for all three products. At the same time, formic acid is a liquid that can be stored well. Methanol, on the other hand, cannot be produced efficiently electrochemically, but mainly thermo-catalytically. Methanol production requires a complex plant with a pressure reactor and distillation, where again a lot of energy is lost. We consider our combination process of electrolyzer and bioreactor to be the leaner and more efficient solution.

CO₂-WIN Connect: What about the selectivity of formic acid formation? Are there by-products that may have to be filtered out of the solution later? Or does this play a minor role, due to the targeted use of the bacteria afterwards?

Dr. Kensy: Hydrogen and carbon monoxide can also be produced during CO₂ reduction in the electrolyzer. Depending on the operation conditions in the electrolyzer, more or less of these by-products are produced. It is precisely this optimization of the reaction that is the subject of our project. If small amounts of by-products remain at the end, the phases (gas/liquid) can be separated relatively easily, so that we only transfer the liquid phase of the formic acid to the bioreactor.

CO₂-WIN Connect: In the field of biotechnology, you want to introduce a new metabolic pathway into the bacterium Cupriavidus necator so that it uses only formic acid as a source of carbon and energy. Can you please explain how a new metabolic pathway is introduced into a bacterium? And how can you tell if a specific metabolic pathway is promising?

Dr. Kensy: Introducing metabolic pathways into microorganisms is not a simple undertaking because it is possible to completely paralyze the organism. Therefore, before starting the work, it is important to study the metabolic network of microorganisms and then specifically plan the interventions in the network. Our metabolism, the reductive glycine pathway, is a linear metabolic pathway with only limited interaction with central metabolism. We introduce the metabolic pathway into bacteria in modules. In doing so, we work with strains that are auxothrophic for the target products of the modules. By introducing the genes, via plasmid or genome-integrated, we enable the bacteria to overcome the auxothropy while knowing that our modules have been successfully introduced. We repeat the game until all modules of the metabolic pathway are integrated. At the end, we can verify the utilization of the substrates via C13 labeling and their incorporation into the cellular intermediates/end products. The selection of a promising metabolic pathway is done via metabolic models and their material and energetic balancing. This has already been demonstrated for the reductive glycine pathway in the run-up to the project.

CO₂-WIN Connect: What is the advantage of using bacteria to convert formic acid as compared to the reaction over, for example, metallic catalysts?

Dr. Kensy: The advantage of biotechnology in the synthesis of molecules is that thousands of reactions take place in parallel within a bacterial cell and one can carry out multi-step syntheses in one cell and, thus, one reactor. Moreover, the cell synthesizes its own biocatalysts from a simple nutrient solution. With the autocatalytic multiplication of the cells (exponential growth) all biocatalysts are multiplied at the same time, so that in the end you get a highly productive process. Chemistry, on the other hand, would have to optimize each synthesis stage individually, produce the catalyst specially for it, and then build a large plant with many individual reactors. As a result, the developments take a very long time and lose energy and materials via each individual process stage.

CO₂-WIN Connect: What challenges do you see in coupling the formic acid electrolyzer with the fermentation system?

Dr. Kensy: The challenges for process integration are manifold. On the one hand, the formic acid concentrations at the outlet of the electrolyzer must meet the requirements of the bioprocess. Here, we still have to reach higher concentrations and, if necessary, a concentration step has to be interposed here. Furthermore, the flow rate of the formic acid into the bioreactor must be controlled according to the process. Interfaces for coupling the electrolyzer with the bioreactor must be created here. In addition, it is necessary to integrate additional nutrient salts for the bacteria into the formic acid flow. The nutrient salts should be as highly concentrated as possible so as not to additionally dilute the formic acid flow. 

CO₂-WIN Connect: How is it possible to transfer the process to an industrial scale? For example, what would an ideal location have to look like and what factor, for example proximity to renewable energy sources or CO2 sources, do you think would play the biggest role in the choice of location?

Dr. Kensy: The elegant thing about our process is that no special bioreactors are required for it. We work with conventional and industrially proven microorganisms. Basically, all that needs to be done in an established fermentation plant is to swap the sugar feed with a formic acid feed. This makes scale-up and implementation very easy and reduces risk. Establishing the process at a chemical plant would make sense, as they already have CO₂ point sources that could be used in our process. Of course, it is also necessary to have access to renewable energy. However, these do not have to be generated directly on site at the chemical plant but could be supplied via power lines. Therefore, the expansion of power lines in Germany is very crucial. It might also be attractive to link up with a waste incineration plant, as these plants generate CO₂, heat and electricity at the same time and in a continuous mode.

 CO₂-WIN Connect: Dr. Kensy, thank you very much for this interview.

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