The Regulatory Mechanism Of Heterogeneous Catalyst-microbiome Hybrids For Carboxylic Acids Production From CO₂ Conversion

The continuous consumption of fossil fuels on a global scale has led to the greenhouse effect caused by massive CO₂emissions and energy shortages.Utilizing CO₂as a resource to produce fuels and chemicals can form a carbon cycle economy,achieving the dual goals of CO₂reduction and resource recycling.Therefore,the conversion and utilization of CO₂is imperative.Microbial fermentation technology that selectively produces carboxylic acids through CO₂conversion is a novel CO₂utilization technology that can convert CO₂into high-value carboxylic acid products at low cost and with high selectivity.However,this technology often suffers from problems such as complex reaction pathways,slow rates,and low efficiency.To address these issues,this study proposes a catalytic system that combines chemical and biological catalysis,coupling solid catalysts with microbial systems to efficiently adsorb and activate CO₂,thereby accelerating the production of medium-chain carboxylic acids（C6-C12）through microbial systems.Based on the above,this study aims to explore the microbial fermentation technology that selectively produces carboxylic acids through CO₂conversion.A series of experimental conditions were investigated to determine the optimal operating conditions for producing medium-chain carboxylic acids through CO₂conversion.A catalyst-coupled microbial reaction system was then constructed under these conditions,and the regulatory mechanism of the coupled system was further elucidated through catalyst characterization and microbial metagenomics analysis.The results obtained from this study are as follows:（1）We investigated the optimal operating conditions for producing medium-chain carboxylic acids through CO₂conversion.In this study,a series of experiments were conducted with H₂/CO₂gas ratios of 4:1,2:1,1:1,1:2,and 1:4.The results showed that the optimal performance for producing medium-chain carboxylic acids was obtained with an H₂/CO₂ratio of 2:1,where the production of caproate was 1303.4 mg/L,which was 2.5-3.6 times higher than that of reactors with other gas ratios.Based on the abundance of key enzymes in the acid production process,the highest abundance of hydrogenase related to H₂utilization and formate dehydrogenase related to CO₂conversion was observed in the 2:1 reactor,indicating the highest gas utilization efficiency under the 2:1 condition.Subsequently,a series of experiments were conducted at p H levels of 4,5,6,7,8,9,and 10 at temperatures of 25°C and 35°C.The results showed that the medium-chain carboxylic acid was best prepared at p H 6 at 35 ^oC,while high-throughput sequencing showed the highest abundance of chain elongation microorganisms at this condition,which corroborated with the acid production results.Therefore,the suitable conditions for producing medium-chain carboxylic acids are an H₂/CO₂ratio of 2:1 and a p H of 6at 35°C.（2）Under suitable acid production conditions,a successful coupling system was constructed between catalyst and microorganisms.The coupling system produced 2701.1 mg/L of caproate and 13895.5 mg/L of acetic acid,which were 93.2%and 50.7%higher than the blank group,respectively.The total fixed carbon was 1927.1 m L,which was 1.9 times more than the blank.Therefore,the Pt/Fe₂O₃catalyst had an excellent modulation effect on the microbial preparation of medium-chain carboxylic acid system in terms of product generation and gas consumption.（3）The regulation mechanism of the catalyst and the metabolic mechanism of microorganisms have been clarified.Firstly,the phase composition,structural characteristics,and surface chemical states of the catalyst were characterized and analyzed by XRD,N₂adsorption-desorption analysis,XPS,and other characterization tests.The surface adsorption species of the catalyst were analyzed through In-situ DRIFTS after CO₂was introduced,and the results showed the generation of pyruvic acid species on the catalyst surface.Combined with microbiome analysis,it was found that pyruvic acid was converted into acetyl-Co A in one step after entering the microorganism.Compared with the traditional eight-step acetyl-Co A synthesis pathway,the pyruvic acid conversion pathway established in this study significantly improved the microbial metabolic rate.Acetyl-Co A was further extended to medium-chain carboxylic acids such as caproic acid through the reverseβ-oxidation pathway.The key species,Anaerosalibacter sp.Marseille-P3206,was the most important functional microorganism in the pyruvic acid conversion and chain elongation pathway.