Process And Mechanism Of Anaerobic Microbial Conversion Of Syngas To Produce Medium-chain Alcohols

With the rapid development of industrial level and the increasing scarcity of fossil energy,it has been an inevitable trend to develop new renewable energy processes.China is rich in biomass energy resources,but the utilization rate is low,and there is great potential for bioconversion of syngas from biomass pyrolysis gasification into chemicals with high added value,such as medium-chain alcohols.The use of anaerobic mixed bacterial flora to convert syngas to prepare medium-chain alcohols has the advantages of simple operation,low cost and high resistance to shock loading,but the research is still in the initial stage,and the metabolic pathways are not clear,the influencing factors and influence laws are not clear,and the mechanism of action is not clear.This study aims to construct a system for the preparation of medium-chain alcohols by converting syngas with anaerobic mixed bacterial flora,and on this basis,to study the effects of reaction operating conditions of temperature,CO partial pressure and p H on the synthesis of medium-chain alcohols,and then to reveal the mechanism of synthesis and conversion of medium-chain alcohols and the mechanism of different operating conditions from microbiological and thermodynamic perspectives,with the following main conclusions.(1)It was determined that low temperature(25°C),high CO partial pressure(1.0-1.5 bar)and low p H(5.5)were more favorable for medium-chain alcohol synthesis.It was found that:lowering the temperature was more favorable to the carbon chain process,which could obtain higher medium-chain acid accumulation and thus higher medium-chain alcohol concentration;increasing the CO partial pressure could effectively increase the rate of medium-chain acid synthesis and obtain higher medium-chain acid accumulation and thus higher medium-chain alcohol concentration and production rate;lowering the p H was also favorable to medium-chain alcohol synthesis,but promoted in a manner inconsistent with lowering the temperature and increasing the CO partial pressure,without the need to accumulate High concentrations of medium-chain acids were not required.(2)The results of 16s r RNA analysis showed that genus Clostridium sensu stricto 12 and Clostridium sensu stricto 1 were in the highest abundance,and the changes in abundance of both were positively correlated with the concentrations of medium-chain acids and medium-chain alcohols,respectively,and may both play important roles in the synthesis of medium-chain alcohols.The metabolic pathway for the preparation of medium-chain alcohols by syngas conversion within the system was clarified by determining the carbon chain elongation within the system only through the reverseβ-oxidation pathway by metagenomics and metaproteomics techniques.The metabolic abundance of related enzymes within the system was analyzed under different operating conditions,and it was found that increasing the partial pressure of CO effectively promoted the synthesis of acetyl coenzyme A and carbon chain elongation process,accelerated the synthesis of medium-chain acids,and most importantly,increased the abundance of aldehyde dehydrogenase to promote the production of medium-chain alcohols during the conversion of acids to alcohols;lowering the p H had a negative effect on the synthesis of acetyl coenzyme A,carbon chain elongation process.It is possible that the aldehyde/alcohol dehydrogenases in Ruminococcaceae bacterium AM07-15and Spirochaetae bacterium HGW-Spirochaete-4,which specifically catalyze the direct conversion of acyl coenzyme A to aldehydes,are up-regulated,reducing the metabolic steps and increasing the rate of medium-chain alcohol synthesis;lowering the temperature is beneficial for some key enzymes in the carbon chain elongation process,and increases the abundance of aldehyde dehydrogenases to promote the synthesis of medium-chain alcohols.(3)Thermodynamic analysis showed that lowering the temperature,increasing the CO partial pressure,and lowering the p H all made the synthesis of medium-chain alcohols more spontaneous;the results of the actual Gibbs free energy changes during the reaction showed thatΔ_f~′eventually converged to0 under different reaction conditions,reaching a quasi-thermodynamic equilibrium state,indicating that there is a thermodynamic upper limit in the preparation of medium-chain alcohols,and all of them have reached the thermodynamic Therefore,thermodynamics is also a key factor limiting the preparation of medium-chain alcohols.(4)The stability of the long-term operation of the syngas bioconversion system for the preparation of medium-chain alcohols was demonstrated by semi-continuous experimental studies combined with thermodynamic analysis.This study reveals the synthesis mechanism of medium-chain alcohols prepared by anaerobic microbial conversion of syngas,and reveals the metabolic pathways of syngas bioconversion to medium-chain alcohols,the mechanism of action of different factors and the key microorganisms in the process by combining 16s r RNA,macro-genomics and macro-proteomics techniques,and also explores the stability of the long-term operation of the system,which provides some theoretical support for the industrial application of syngas bioconversion to medium-chain alcohols.The long-term stability of the system was also investigated,which provided some theoretical support for the industrial application of syngas bioconversion to medium chain alcohols.