(699h) Control of Metal-Support Interaction of Fe@CNTs By Surface Modification of CNT and Its Application to Direct Olefin Synthesis from Syngas

  Rapid
economic development in China arouses the intense interest to produce
increasing amounts of light olefins (e.g., ethylene, propylene) in both
academia and industry. Traditionally, steam cracking of naphtha with dwindling
and non-renewable crude oil is the main route for olefin synthesis [1,2]. Most
recently, direct synthesis of light olefins from coal-/biomass-based syngas is
regarded as the most promising non-petroleum route due to low-energy
consumption, low-cost, high-efficiency, etc [1,3,4]. Generally,
the catalyst design strategy for direct syngas route is mainly from the aspect
of support modification, promoter doping, metal precursor synthesis and construction
of multifunctional active sites [1,3,4]. Regardless of multifunctional
catalyst, Fe is considered as the most promising active metal for FTO with _-Fe₅C₂
commonly recognized as active phase [1,5,6]. The key to high olefin selectivity
is to modulate the Fe catalyst with appropriate capability for CO dissociation
and hydrogenation.

  Carbon
materials possess high surface area, chemical inertness, mechanical strength,
fruitful surface chemistry and electronic properties via covalent modifications.
The type and quantities of surface chemical groups determined the interactions
between carbon support and metal species [7,8]. Previous work demonstrated that iron-carbon
interactions played an important role in Fischer-Tropsch
process. Comparing with other conventional supports, the iron species on carbon
materials are generally more reducible and can be transformed to iron carbides
more facilely in the FTS reaction. In this work, the interaction between Fe and
CNT is regulated by different surface modifications on CNT. The effect of
the chemical and physical properties of the different CNT supports on the FTO
performance of the Fe/CNTs was discussed.

Table 1 FTO performance of various carbon materials supported Fe
catalysts

  The CNTs were
prepared by surface modification under different conditions. Fe supported
catalysts were prepared by incipient wetness
impregnation of the modified CNTs after drying and calcination. Structural and
electronic properties of Fe/CNTs were characterized by BET, FTIR, Raman, XPS,
XRD, TEM, TPR, TPD, XAFS, etc.

Figure 1 Catalytic performance of Fe
catalysts supported on CNTs with different modifications

 
We
evaluated first the FTO performance on multiple carbon materials supported Fe
catalysts and found that CNT is the most favorable carbon support for olefin
synthesis (Table 1). Moreover, we found that there existed large difference in performance
of Fe catalysts supported on CNT with different modification methods (Figure 1).
In similar conversion level, defect-rich HTCNT supported Fe showed highest
initial conversion and olefin selectivity. Besides, OCNT with large amounts of
O-functional groups supported Fe catalyst exhibited the lowest olefin
selectivity but most stable activity. Based on the performance results, we
tried to carry on various characterizations to elucidate the intrinsic effect
of CNT modification on Fe-CNT interaction. BET, zeta potential and point of
zero charge suggested that defect-rich HTCNT possessed highest surface area and
less negative charge, while OCNT is the opposite. Raman and
FTIR spectra evidenced the large amounts of O-contained groups (-C=O, -COOH)
and rich defect on OCNT and HTCNT, respectively. In addition, there existed
obvious electron transfer between Fe and modified CNTs, which is also demonstrated
by XPS, in situ XRD, TPR, XAFS, etc.

Figure 2 Possible structure-performance
relationship for FTO over different kinds of Fe NPs immobilized on CNT

  The presence/absence of
surface functionalities can directly affect the catalytic behavior of the
active phases.
The defects on CNT benefits to the initial activity and selectivity to light
olefins. High concentration of O groups enhanced the stability of Fe during
FTO. Moreover, the performance strongly depends on the degree of intimacy
between CNT and Fe NPs. The surface modification caused the difference in
Fe-CNT interface property, which significantly affected the electronic
environment of Fe, ultimately altering the FTO performance.
An optimal surface chemistry also facilitates catalyst conversion into its
catalytically active state.

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