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

Run No.	Reductant	Duration (h)
A-CO	CO	13
A-H2	H2	13
A-H2+CO*	H2+CO	6.5+6.5
A-CO+H2**	CO+H2	6.5+6.5
A-0.67	H2/CO=0.67	13
A-2.0	H2/CO=2.0	13

*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

Run No.	Phase identification	Area (%)
A-CO	c-Fe5C2	15.3
	¦Å'-Fe2.2C	16.2
	Fe3O4(A)	3.2
	Fe3O4(B)	13.3
	Fe3+ (spm)	34.5
	Fe2+ (spm)	17.4
A-H2	¦Á-Fe	9.6
	Fe3O4(A)	6.6
	Fe3O4(B)	40.9
	Fe3+ (spm)	19.6
	Fe2+ (spm)	23.3
A-CO+H2	¦Á-Fe	2.8
	c-Fe5C2	28.2
	¦Å'-Fe2.2C	12.4
	Fe3O4(A)	9.6
	Fe3O4(B)	6.5
	Fe3+ (spm)	24.2
	Fe2+ (spm)	16.2
A-H2+CO	c-Fe5C2	22.5
	¦Å'-Fe2.2C	3.0
	Fe3O4(A)	3.8
	Fe3O4(B)	32.0
	Fe3+ (spm)	22.0
	Fe2+ (spm)	16.7
A-0.67	c-Fe5C2	22.4
	¦Å'-Fe2.2C	54.9
	Fe3+ (spm)	1.5
	Fe2+ (spm)	21.1
A-2.0	c-Fe5C2	17.6
	¦Å'-Fe2.2C	21.7
	Fe3O4(A)	1.5
	Fe3O4(B)	9.8
	Fe3+ (spm)	20.0
	Fe2+ (spm)	29.4

 

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 CO conversion 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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