We also measured reactive performance of precipitated Fe catalysts for CO2 hydrogenation and the results are shown in Table 3.

Comparing the results in Table 3 to Table 2 , it can be found that the reactive performances of studied catalysts depend on reactant composition. For example, catalyst Z6K4C8/FS10-I is more active than Z6K2C2/FS15-I for CO hydrogena­tion, while the former is weaker than the latter to convert reactant in the case of CO2 hydrogenation.

We had found that low K content is helpful to increase hydrocarbon yield and Fe catalyst with high Zn/K ratio shows high C2 + hydrocarbon selectivity for CO2 hydrogenation at lower reaction temperature [42]. The results of catalyst Z6K2C6/ FS15-I in Table 3 still support the above conclusions after SiO2 was introduced into precipitated Fe catalyst. Although the content of promoter K in it is the lowest among studied catalysts, more CO2 is hydrogenated into hydrocarbons rather than terminated as CO. It supports that the CO2 is able to be activated by suitable promoter(s) for hydrocarbon synthesis at lower temperature [42], too. The effect of H2/CO2 ratio on reactive performance of catalyst Z6K4C8/FS15-II is evident according to the results in Table 3. After the H2/CO2 ratio is increased to 5, more hydrocarbon is synthesized from CO2, and most liquid products are in C6-C10 range.

1.4 Conclusions

SiO2 is used commonly as structure promoter for precipitated Fe catalyst in order to enhance its mechanical strength besides to increase its specific surface area. The influences of introducing method and the content of SiO2 on the reactive perfor­mance of Fe catalyst were studied under CO + H2 and CO2 + H2, respectively.

Although the amount of effective potassium is decreased by the introduced SiO2, the correlation between promoter composition and catalyst reactivity found for SiO2-free Fe catalysts is still in effect for SiO2-added Fe catalysts. It is beneficial to improve precipitated Fe catalysts for FT synthesis with CO2-containing syngas.

2 Perspectives

In order to develop precipitated Fe catalyst active to convert CO2-containing syngas, we decompose the work into three steps. The first step is to develop Fe catalyst active to convert CO + H2. The second one is to exploit Fe catalyst propitious for CO2 hydro­genation or to activate CO2 into CO. The third step is to set up guidance on how to couple the above two kinds of catalysts according to contents of CO2 and CO in reactants in order to convert effectively CO2-containing syngas into liquid fuels.

By now, the first step is nearly completed, and several Fe catalysts are found with high CO conversion during FT synthesis reaction. Much work is being done for the second aim. We have observed the relation between the pattern of CO2 — TPD and CO2 hydrogenation activity for Fe catalysts promoted with Zn, K, and Cu. It helps us to find Fe catalysts active to hydrogenate CO2 at low temperature. However, new characteristic tool or method is needed to accelerate making up ideal Fe catalyst after SiO2 is introduced into catalyst as support or binder. Based on the catalysts selected in the first two steps, it will be promise to complete the third step and realize acquiring liquid fuels efficiently from CO2-containing syngas.

Acknowledgments This work is partially supported by the Science and Technology Department of Zhejiang Province (2009C21002), Zhejiang Provincial Natural Science Foundation of China (Y4100410) and National Ministry of Science and Technology of China (2009AA05Z435).