(281a) Activation Studies of An Iron-Based Fischer-Tropsch Catalyst In a Slurry Reactor

1. Introduction

As the alternative route
to synthesize hydrocarbons, Fischer-Tropsch synthesis (FTS) has been the subject of
renewed interest for conversion of coal and natural gas to liquid fuels. Before
Fischer-Tropsch synthesis (FTS), the catalyst precursor is subjected to an
activation treatment [1-5]. Co, Ni and Ru
catalyst are almost reduced in flowing H₂ at 200~450 ^oC
to the zero valent metallic state and remain in the metallic state during FTS.
However, the pretreatment for the iron catalyst is not so clear. The iron catalyst
can be successfully activated with H₂, CO or syngas [1,3-8]. Significant
changes in iron catalyst compositions, structure and activity can result from
different pretreatment parameters prior to exposure to FTS conditions.

In the present study we
investigated the effects of activation ambient
on textural properties, the bulk compositions of the iron catalyst. The results clearly
showed that activation ambient had a significant effect on subsequent FTS
activity. And the conclusions from this paper may give us some specific
indications on how to use an iron-based catalyst in a slurry phase FTS process.

2. Experimental

A typical Fe/Cu/K/SiO₂
catalyst was prepared by a combination method of continuous
co-precipitation and spray-drying technique for the slurry phase FTS
application [9]. All activations were conducted in
situ, using CO, H₂, CO followed by H₂, H₂
followed by CO, and syngas (H₂/CO=0.67 and 2.0) at 270 ^oC,
l NL/g_-cat.h and 0.1 MPa. The detailed pretreatment conditions were
summarized in Table 1. The FTS was performed in
a 1 dm³ continuous flow STSR equipped with a wax and catalyst
separation system [10].

Table 1 Pretreatment conditions of the catalyst and
test designations

*The catalyst was
firstly reduced by H₂ for 6.5 h, and then treated with CO for 6.5 h.

** The catalyst was firstly reduced by CO for 6.5 h, and then
treated with H₂ for 6.5 h.

The gas, liquid (oil and
liquid) and solid (heavy wax) products were analyzed quantitatively. Mössbauer effect spectroscopy(MES)
was measured with a CANBERRA series 40MCA constant-acceleration Mössbauer
spectrometer (CANBERRA, USA) at room temperature.

3. Results and discussion

3.1 The catalyst reduction

The CO₂
content in tail gas during reduction process is measured periodically on-line
using GC. The
changes of CO₂ concentration with time on stream under the different procedures are
shown in Fig. 1.

Fig 1 The change of CO₂
content during reduction process

Reduction conditions: 270 ^oC, 0.1 MPa, 1.0 NL/gcat.h

3.2 Catalyst phase characterization

MES can provide
quantitative information about the amount of various iron phases present. Bulk
compositions in the catalyst samples after the activation under different
reduction ambient are determined by MES. The MES spectra of corresponding contents
of the reduced catalysts are summarized in Table 2.
The MES results show that the differences in phase compositions of the reduced
catalysts are obvious. The effect of activation ambient on the catalyst bulk
compositions is more complicated than those of reduction temperature and
pressure. ¦Á-Fe, Fe₃O₄, iron
carbide, and superparamagnetic (spm) phases in the reduced catalyst are
coexist. The amount of carbides in reduced catalyst with syngas is much more
than that of activation with CO. The content of carbide in reduced catalysts
using syngas decreases with the increase of H₂/CO ratios.

Table 2 Iron
phases identified by MES in reduced
catalysts under
different ambient

3.3 Activity and stability

To systematically
investigate the effect of reduction ambient on the FTS performances, a series
of the FTS tests for more than 500 h under the baseline conditions (250 ^oC,
1.5 MPa and 2.0 NL/g_-cat.h) are carried out. After then, FTS
reaction temperature for each test was increased. The FTS activity measured by
carbon monoxide conversion is shown in Figure 2.
As shown in Figure 2, there have been two areas of the higher catalyst activity
(A-H₂ and A-H₂+CO) and the lower catalyst activity A-CO,
A-CO+H₂, A-0.67, and A-2.0). Reduction by syngas with high H₂/CO
ratios of precipitated iron-based catalysts can achieve the similar activity of
the catalyst reduced by CO.

Fig 2 COconversion and
stability of catalysts after reduction at different ambients

Reaction conditions: 250-280 ^oC, 1.5 MPa, H₂/CO=0.67, 2.0
NL/gcat.h

There is a relationship between the
catalyst activity and activation ambient. No iron carbide (A-H₂), or iron carbide
content less than 30% (A-H₂ + CO) in the reduced catalyst, the FTS
activity is very low. However, the iron carbide content in the reduced catalyst
achieved greater than 30%, the FTS activity is relative high. Meanwhile, the
FTS activity also depended on the phase equilibrium of the iron phases when the
iron carbide content in the reduced catalyst increases to certain level.

Some researchers
suggested that Fe₃O₄ was the active phase [13] while numerous studies supported that iron
carbides were the active phases [12,13] for FTS.
Dictor et al. [14] reported that the mixture of ¦Ö-,
¦Å΄-iron carbides and a small amount of ¦Á-iron was the active phase. The results
of this study showed that the active phase for FTS was a mixture of carbides and
corresponding amounts of superparamagnetic phase. Different preparation method
and different pretreatment conditions of the iron-based catalyst led to
different understanding results.

4. Conclusions

There is a relationship between the
catalyst activity and activation ambient. The carbides formation may be
controlled primarily activation ambient. Multi-phase coexistence becomes more
apparent. The changes in the bulk compositions resulted in the variation
in catalyst activity during FTS. The results of this study showed that the
active phase for FTS was a mixture of carbides and corresponding amounts of superparamagnetic
phase. The
highest FTS activity was obtained when the iron catalyst was reduced using CO. However,
the activation with H₂, or with H₂ followed by CO resulted in the
low FTS activity.

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