The synthesis of medium chain carboxylic acids (MCCAs) from syngas provides a promising route for converting coal/organic waste/natural gas into high value-added chemicals. However, neither purely chemical synthesis methods nor purely biological approaches seem poised to realize the potential of syngas-to-MCCAs. Hereby, we developed a hybrid approach and created an alternative pathway for C1 chain elongation to C6, where we combined the efficient carbon fixation of heterogeneous catalysts with the high specificity, low cost, and self-adaptation of anaerobic microbiomes. In the hybrid system, inorganic carbon fixation into organic carbon (adsorbed pyruvic acid) was accomplished by the heterogeneous Pt/Fe2O3 catalysts, and the adsorbed pyruvic acid was subsequently metabolized by the anaerobic microbiome. This hybrid system significantly improved CO conversion and n-caproic acid production (6600 mg COD/L), which was 50–190% higher than that observed in previous studies. The interactions were revealed by macro-omics and in situ diffuse reflectance infrared Fourier transform spectroscopy, demonstrating a manipulated inorganic carbon-to-chemical fixation route.

Bioconversion of CO2 into liquid fuels or chemicals, preferred medium chain carboxylic acids (caproic and caprylic acid), is an attractive CO2 utilization technology. The present study aims to investigate the effects of different ratios of H2/CO2 on regulating the distribution of C2‐C8 carboxylic acid products, while the headspace pressure of 1.5 bar was set to amplify the effect of different ratios. The H2/CO2 ratio of 4:1 was more suitable for preparing acetic acid, where the highest acetic acid yield was 17.5 g/L. And the H2/CO2 ratio of 2:1 showed excellent chain elongation ability with the highest n‐caprylic yield of 2.4 g/L. Additionally, the actual H2/CO2 ratios of 4:1 reactors were higher than that in 2:1 may be course chain elongation often accompanied by H2 production. The 16S rRNA genes analysis shows that the genus Terrisporobacter and Coriobacteriales may be related to acetic acid production enriched in H2/CO2 ratio 4:1 reactors, and the genus Clostridium and Paenibacillaceae may associate with the chain elongation pathway were enriched in H2/CO2 ratio 2:1 reactors.

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