Study On Regulating The Activity And Selectivity Of Fischer Tropsch Synthesis Catalysts

Olefin is an important chemical raw material,and currently the main source is petroleum cracking.Fischer Tropsch synthesis（FTS）provides a non petroleum route for olefin synthesis.Traditional Co-based catalysts have the characteristics of high activity and low reverse water gas reaction.The design and regulation of the active phase of Co-based catalysts in the reaction has always been a key and difficult point in the synthesis and preparation of highly active and selective Co-based catalysts.In this article,alkali metal promoters are used to regulate the active phase of Co in Co₃O₄ catalysts,regulate the product distribution of Co₃O₄ catalysts,significantly improve the selectivity of light olefins and total olefins,and provide a theoretical basis for the development of highly selective Co-based catalysts.Firstly,a cubic structure Co₃O₄ catalyst was prepared by hydrothermal synthesis.The effect of various alkali metal promoters（Na,K,Rb,Cs）on the performance of the catalyst was investigated by adding different types of alkali metal promoters and changing their loading amount.The results showed that the 1.0 wt%Na/Co₃O₄ catalyst had the highest selectivity for light olefins and total olefins.XRD and XPS characterization results showed that the addition of Na promoter changed the type and content of active species of Co on the surface of the catalyst,promoting the conversion of the active center of the reaction from metal Co⁰ to Co₂C.The results of CO-TPD characterization showed that the amount and intensity of CO adsorption on catalysts with high Co₂C content were also enhanced.Therefore,the addition of Na promoter to the Co₃O₄catalyst significantly improved the selectivity of light olefins and total olefins.Secondly,in order to further reduce the methane selectivity and improve the olefin selectivity of the supported 1.0 wt%Na/Co₃O₄ catalyst,Mn promoter was added to the 1.0 wt%Na/Co₃O₄ catalyst and its FTS reaction performance was investigated.When the content of supported Mn promoter was 0.2 wt%,the methane selectivity significantly decreased from 31.8%to 22.7%,while the total olefin selectivity increased from 54.7%to 64.7%.Moreover,the selectivity of this catalyst for light olefins is 31.1%,the selectivity for C₅₊is 40.0%,and the olefin to alkyl ratio of C₂-C₄ is 5.0.The catalysts were characterized by XRD,XPS,H₂-TPR,and CO-TPD.The results showed that Mn promoters as electronic promoters contributed to the adsorption and dissociation of CO,and the interaction between Co Mn changed the reduction performance of the catalysts.This was the main reason for the reduction of methane selectivity and the increase of C₅₊selectivity after loading Mn promoters.The role of Na promoter is to promote the formation of Co₂C active phase,thereby increasing olefin selectivity.The synergistic effect of Mn and Na promoters promotes a significant increase in the selectivity of Co₃O₄ catalysts for total olefins.This article successfully achieved the control of the active phase of Co-based catalysts and significantly changed the FTS product distribution of the catalysts,providing a new idea for the efficient preparation of olefins using Co-based catalysts in the future.