(760b) Effects of Mn Promoter in Controlling Geometric and Electric Properties of Fe-Based Catalysts for Syngas-to-Olefins Reaction

The Critical Role of Mn Promoter in
Controlling Geometric and Electric Properties of Fe-based FTS Catalysts  

Zixu Yang¹, Zhengpai Zhang¹, Yitao Liu¹, Jing Xu¹*, Yifan Han¹,²*

¹ State Key Laboratory of Chemical Engineering, East China University
of Science & Technology, Shanghai 200237 (China)

²Research Center of Heterogeneous Catalysis and Engineering Sciences,
School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou
450001(China)

*yifanhan@ecust.edu.cn;
xujing@ecust.edu.cn

Introduction

Direct conversion of syngas to
olefins via Fischer-Tropsch Synthesis (FTS) is a
promising technology to meet the growing demand for chemical commodities using non-petroleum
feedstock (e.g. shale gas, coal and biomass). Fe-based FTS catalysts has been
widely employed for FTS process due to its tunable product distribution, low
cost, and high water-gas-shift activity which makes it suitable for syngas with
low H₂/CO ratios [1]. The major challenge to achieve high C2~C4
olefin selectivity is to decouple chain growth rate from over-hydrogenation and
methane formation. Attempts have been made to address this issue by modifying the
Fe-based FTS catalysts with promoters and supports to tune the CO dissociation
and hydrogenation activities [2,3]. Manganese has been widely used as a
promoter for Fe-based FTS catalysts. The higher C2~C4 olefins selectivity and
suppressed formation of methane provided by Mn-promoted Fe-based FTS catalysts
have been well documented in the literature. Mn was found to enhance the
dispersion of Fe NPs, which results in higher number of active sites [4]. In
addition, the introduction of Mn may lead to variations in textural structure of
Fe catalysts and the electronic state of the surface carbonaceous species,
which would change the particle size, reducibility of iron species and
evolution of active phase [5]. Although Mn as a promoter has been extensively
studied, the effects of Mn on the dynamic structure of Fe catalysts during its calcination-activation-reaction
cycle, and how these effects were important to the FTS performance were not well
understood.

In
this study, monodisperse FeMn NPs with tunable Fe/Mn
ratios were prepared and tested for FTS reaction. Combined with various
in-situ/ex-situ catalyst characterization techniques, the role of Mn in dynamic
structural evolution of iron species was elucidated.

Materials and Methods

FeMn catalysts with various Fe/Mn
ratios were prepared by low-temperature co-precipitation of FeCl₂
and MnCl₂ solutions. The as-prepared catalysts were calcined and activated
in H₂ flow before subjected to FTS reaction. Table 1 summarizes the results
of catalytic performance of FeMn catalysts. The surface
and bulk structure of FeMn catalysts were characterized by TEM, HRTEM, in-situ/ex-situ XRD,
Raman, UV-Vis, XPS, XAFS, H₂-TPR and CO-TPSR.

Results and Discussion

The addition of
Mn effectively promoted the activity and C2~C4 olefins selectivity in FTS
reaction. Among the catalysts, the FeMn (4:1) provided
the best performance (CO conversion: 5.49%, C2~C4 olefins selectivity: 48.7%). TEM
and XRD results revealed that the Mn was incorporated into the framework of Fe₂O₃
to yield a binary oxide (Fe, Mn)₂O₃, whose particle size was
reduced with increasing Mn amount. The H₂-TPR and in-situ XRD
results evidenced that phase transition of FeMn oxide
NPs followed the step of (Fe,Mn)₂O₃¡ú(Fe,Mn)₃O₄¡úFe_1-xMn_xO¡úFe⁰¡úFe_xC
during activation and reaction. Mn enhanced the reduction of (Fe,Mn)₂O₃¡ú(Fe,Mn)₃O₄, but inhibited
the reduction of Fe_1-xMn_xO and formation of iron carbides.
In addition, Mn acted as an electron donor for Fe catalysts, enhancing the CO
dissociation as observed by CO-TPSR experiment.

Table
1. Catalytic performance of FeMn catalysts with various
Fe/Mn ratios.

^a Catalysts activation conditions:
UHP H₂ (99.999 vol. %), 25 ml/min, 0.1 MPa, 350 ¡ãC, 5h

^b Reaction conditions: 25mg
catalysts diluted in 75 mg SiC, syngas (CO/H₂/N₂=45/45/10
v/v%), 15ml/min, 260 ¡ãC, 2.0 MPa

^c Excluding CO₂
selectivity

Significance

The study proposes new insights for the understanding
of the promotional mechanisms of the promoters from a catalyst life cycle
perspective.

Figure 1. Characterizations of FeMn catalysts. (a)
XRD patterns of calcined FeMn catalysts, (b) TEM
image of calcined FeMn (4:1) catalyst, (c) H₂-TPR
profiles of FeMn catalysts.

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