Exploring the promotion behavior of manganese in Fischer–Tropsch synthesis by changing the interaction patterns between manganese and iron

Abstract

Mn promoter is widely investigated and employed in Fe-based catalysts. However, it remains a substantial challenge to elucidate how the Mn promoter tunes the product selectivity because the Mn promoter can change crystal structure, particle size, phase composition, etc. simultaneously. Hence, three kinds of newly designed catalysts with different interaction patterns between Mn and Fe were developed in this work to explore the promotion mechanism, including Mn-Fe spinel, core–shell Fe3O4@MnO2 and physical mixing of Fe3O4 and MnO2. Texture structure, reduction behavior and phase transformation of the catalysts were carefully investigated. The Mn content and Mn-Fe interaction pattern determine the strength of the Mn-Fe interaction, which affects the Fe species and FTS performance. Strong Mn-Fe interaction in spinel promotes the generation of ε-Fe2C, reducing C5+ selectivity while enhancing C2-C4 selectivity. C2-C4 O/P ratio of Mn-Fe spinel is dramatically elevated while simultaneously rapidly decreasing over time. Although the Mn-Fe interaction patterns are different, the medium content MnO2-coated Fe3O4 catalyst with strong interaction strength exhibits the similar Fe species and FTS performance feature as Mn-Fe spinel. The decrease in the Mn-Fe interaction strength at high MnO2 coating content contributes to the formation of χ-Fe5C2 and displays the alkaline effect of Mn promotion, i.e., a considerable drop in methane selectivity and an increase in C5+ selectivity. The effect on the FTS performance is relatively moderate for physical mixing of Fe3O4 and MnO2 due to the weak Mn-Fe interaction strength. Combining Fe species and product distribution, χ-Fe5C2 is favorable to produce long-chain hydrocarbons while ε-Fe2C is favorable to produce C2=-C4=. Most importantly, our work is expected to open up a new avenue for guiding the design of efficient catalysts.