CO_xFixation to Elementary Building Blocks: Anaerobic Syngas Fermentation vs. Chemical Catalysis

Abstract

Heterogeneous catalysis and anaerobic syngas fermentation represent two different approaches for the conversion of synthesis gas into chemicals and fuels. This review provides a unique comparison of different reaction paths for the fixation of CO 2 , CO and H 2 into elementary building blocks such as methanol, acetic acid and ethanol. Operating conditions, reactor engineering, influence of gas impurities, yields, conversion efficiencies as well as downstream product recovery are compared. It was found that mass-specific productivity ranges in the same order of magnitude for both technologies, while space-time yield of heterogeneous catalysis is up to three orders of magnitude higher.

Production of biofuels from C1‐gases with Clostridium and related bacteria—Recent advances

“…However, there are a number of shortcomings that should be overcome. For example, low cell densities and low gas–liquid mass transfer rates are the main limitations of this technology, thus generating low productivities (Perret et al, 2022 ). Likewise, inhibition of the solventogenic biocatalyst could be affected by many factors, for example non‐optimal pH values, metabolites accumulation and presence of oxygen.…”

Influence of Increased Cell Densities on Product Ratio and Productivity in Syngas Fermentation

“…Due to low product concentrations, product separation and purification is associated with high costs. Therefore, an increase of the cell density, which means an increase of the density of the biocatalyst in the reactor, represents an essential key variable in order to increase the space-time yield as part of the process development . By using a biomass retention system, the outflow of the biocatalyst in the product stream during continuous operation can be avoided and, thus, an increase in cell density can be achieved.…”

“…14,15 A major bottleneck for the commercialization of syngas fermentation is the low space-time yield, which is up to 3 orders of magnitude lower than of heterogeneous catalysis. 3 Due to low product concentrations, product separation and purification is associated with high costs. Therefore, an increase of the cell density, which means an increase of the density of the biocatalyst in the reactor, represents an essential key variable in order to increase the space-time yield as part of the process development.…”

“…The possibility of using synthesis gas fermentation to convert hydrogen and carbonaceous gases such as CO and CO 2 into chemicals, fuels, and other valuable products could be of great importance to a future circular economy. , The advantages of syngas fermentation over heterogeneous catalysis are low process pressures and process temperatures, self-regeneration of the biocatalyst, and complete gas conversion without the need for a recirculation loop . H 2 , CO, and CO 2 can be obtained, e.g., by (co-)­electrolysis, , gasification of biomass, from industrial waste gases, or air capture and could be converted to C 2 –C 8 compounds by acetogenic bacteria. − Anaerobic acetogenic bacteria convert CO 2 to acetyl-CoA via the Wood–Ljungdahl pathway and they produce acetic acid and ethanol in a first process step .…”

“…H 2 , CO, and CO 2 can be obtained, e.g., by (co-)­electrolysis, , gasification of biomass, from industrial waste gases, or air capture and could be converted to C 2 –C 8 compounds by acetogenic bacteria. − Anaerobic acetogenic bacteria convert CO 2 to acetyl-CoA via the Wood–Ljungdahl pathway and they produce acetic acid and ethanol in a first process step . Both H 2 and CO serve as reducing agents. , A major bottleneck for the commercialization of syngas fermentation is the low space-time yield, which is up to 3 orders of magnitude lower than of heterogeneous catalysis . Due to low product concentrations, product separation and purification is associated with high costs.…”