Support Effects And Promoter Effects Of Fe-Based Catalysts On The Fischer-Tropsch Synthesis Of Lower Olefins

Lower olefins (C2=-C4=) are key building blocks in chemical industry. In view of the supply limitations of oil and the development of C1 chemistry, direct conversion of syngas to lower olefins (i.e., FTO) as a non-oil route has recently attracted increasing attention. Fe-based catalysts, allowing for the use of coal-or biomass-derived syngas without a H2/CO ratio adjustment, are promising for the FTO process. In this thesis, influences of the microstructure of support and the content of sulfur promoter on catalytic activity, selectivity and stability of Fe/a-Al2O3 catalysts were focused, aiming to provide the principle of the rational design and optimization of Fe-based FTO catalysts.(1) Hierarchical structured (α-Al2O3 support (i.e., α-Al2O3-H) was obtained from the calcination of hierarchical structured boehmite prepared by a two-step solvothermal method, and commercial α-Al2O3 (α-Al2O3-C) was used as a reference support. Effects of the microstructures of both supports on the dispersion and immobilization of iron catalysts were investigated by H2-TPR, HRTEM and Elemental Mapping. α-Al2O3-H is shown to be able to effectively disperse and immobilize iron species.(2) The as-obtained Fe/α-Al2O3-H and Fe/α-Al2O3-C catalysts were tested for the FTO process at ambient and high pressure, respectively. Fe/α-Al2O3-H is found to have higher catalytic activity under different pressures, while the two catalysts show similar product selectivity. Subsequently, the two used catalysts under high pressure were characterized by HRTEM, TGA, Raman and FTIR. The results showed that the used Fe/α-Al2O3-H catalyst exhibits higher content of carbon deposits with higher graphitization degree and mainly in the form of "Fibrous Carbon". However, on the used Fe/α-Al2O3-C catalyst, the type of carbon deposits is mainly "Encapsulating Carbon". Meanwhile, in most cases, on the former used catalyst, the iron particles are located at the tips of "Fibrous Carbon" and have more exposed surface, whereas on the latter used catalyst, the iron particles are covered by "Encapsulating Carbon". This can provide an interpretation for the differences in the FTO performances of Fe/α-Al2O3-H and Fe/α-Al2O3-C catalysts.(3) Effects of the sulfur promoters concentrations on FTO performance and formation of carbon deposits of Fe/α-Al2O3-H catalysts were investigated at ambient pressure. It is found that the addition of low sulfur content (i.e.,0.03 and 0.05 wt%) to Fe/α-Al2O3-H catalyst (i.e., Fe/α-Al2O3-H-3S and Fe/α-Al2O3-H-5S) can significantly enhance the activity and C2=-C4= selectivity, but lead to the higher content of carbon deposits and the change in their types from "Encapsulating Carbon" to "Fibrous Carbon". More interestingly, the further increase in the sulfur content to 0.08 wt%, the corresponding Fe/α-Al2O3-H-8S catalyst shows higher stability and higher C2=-C4=selectivity (i.e.,68 C%). Meanwhile, on the used catalyst, the content of carbon deposits is remarkably reduced, and its type is changed into "Isolated Carbon". It should be pointed out that although Fe/α-Al2O3-H-8S catalyst has lower activity than Fe/α-Al2O3-H-3S and Fe/α-Al2O3-H-5S catalysts, it is more active than Fe/α-Al2O3-H catalyst. These findings might shed new light on the development of highly efficient and carbon-deposits-resistant Fe-based FTO catalysts by adjusting promoter content.