Catalytic Hybrid Electrocatalytic/Biocatalytic Cascades for Carbon Dioxide Reduction and Valorization

Carbon dioxide, as a greenhouse gas, has a critical impact on global climate change. Hence, reducing its emissions and/or transforming it into value-added chemicals is of paramount importance to society. Such a goal can be achieved through the development of electrochemical catalytic processes, which showed initial successes in synthesizing CO and cosynthesis of H-2 (syngas) from CO2. However, to further advance the technology toward more complex hydrocarbons (possibly fuels) and more complex organic molecules to be used in industrial chemical synthesis of rubber/resins and plastics or even fine chemical synthesis of pharmaceuticals, catalytic structures have to be designed beyond simple materials. One approach consists of executing the required steps linking CO and small alcohols and the desired products through chemical reactions catalyzed by enzymes or microorganisms, hence combining them with the electrocatalysts used for the initial steps of the CO2 reduction. Here, we discuss the electrocatalysts, enzymes, and bacteria to be used for those cascade reactions. Inorganic catalysts can be directly utilized for initial steps of CO2 reduction resulting in formate, carbon monoxide, and methanol, which then can be further reduced by employing enzymatic catalysts such as carbonic anhydrase and formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase. Formate, carbon monoxide, and methanol can also be converted to acetyl-CoA and pyruvate, which are critical intermediates in the production of various chemicals of critical importance. Furthermore, if a chemolithoautotrophic bacteria is employed, the former can be used as a feedstock for biomass. A special emphasis is here given to the compatibility of the different classes of electrocatalysts used in the cascade: heterogeneous electrocatalysts (metals and metal oxides/carbides/nitrites, atomically dispersed transition metal-nitrogen-carbon, etc.), enzymatic catalysts (individually purified enzymes or multienzymatic complexes), bacterial cells (pure or mixed cultures of different classes, planktonic, or biofilm-forming), and their integration in singular bed reactors or multireactors units required for the formation of the products of interest. The electrocatalysts and related cascade reaction pathways are summarized in a descriptive connectivity map identifying the most important reaction pathways.

Automated summary

The translation discusses the importance of reducing carbon dioxide emissions and transforming them into value-added chemicals through the development of electrochemical catalytic processes. Different classes of electrocatalysts, enzymatic catalysts, and bacterial cells are integrated for the cascade reactions, forming complex hydrocarbons and organic molecules for industrial applications. The interconnected pathways of electrocatalysts and cascade reactions are summarized in a descriptive map highlighting critical reaction routes.