Study On The Performance Of Composite Oxide Catalysts In Fischer-tropsch Synthesis Of Light Olefins

Fischer-Tropsch to light olefins（FTO）refers to the method of syngas（CO+H₂）made from carbon-rich raw materials such as coal,natural gas and biomass as the feedstock to produce light olefins through catalytic conversion.Light olefins are important fundermental chemicals,and they are mainly produced by petroleum industry.In recent years,the international situation is tense and complex.Considering the actual situation of"rich coal,meager oil and deficient gas"in China,developing FTO is of great strategic significance to alleviate the contradiction between oil supply and demand,ensure national energy security,balance utilization of coal resources and change the coal consumption structure.Fe-based catalysts have gained a renewed interest in FTO due to their low cost,good toxicity resistance,low methane selectivity,weak hydrogenation and C—C coupling capabilities.In this thesis,three series of Fe-based catalysts were prepared.The effects of support element composition,support Al/Zn ratio,promoter effect,and support calcination temperature on FTO performance are investigated in details.（1）Fe/K/Zn-O-M（M=Zr,Cr）catalysts were prepared.Results show that,compare with the reference precipitated iron catalyst,the zinc zirconate（or zinc chromate）support effectively adjust the surface alkaline of Fe-based catalysts.It is found that the moderate strength basic sites on the catalyst surface promote C—C coupling and suppress hydrogenation,while the strong base sites on the catalyst surface weaken C—C coupling and facilitate hydrogenation.The zinc zirconate supported Fe/K catalyst achieve 42.4%on C₂⁼-C₄⁼hydrocarbon selectivity,which is 55%higher than the reference precipitated iron catalyst.（2）Fe/K/Zn-O-Al catalysts were prepared.Results show that,ammonium carbonate precipitant increase the specific surface area and porosity of the Fe-based catalysts,and adjust the surface alkalinity which can improve FTO activity.The support Al/Zn ratio has a positive directional relationship with the number of the weak and moderate strength basic sites,which act to promote the C—C coupling and suppress hydrogenation.Compare to the other two catalysts,the catalyst with an Al/Zn ratio of 8 exhibits the lowest C₅₊hydrocarbon selectivity and the highest target product C₂⁼-C₄⁼hydrocarbon selectivity.（3）Seven catalysts were prepared by calcining Zn Al₂O₄·3Al₂O₃ support at seven temperatures and carrying Fe/K.Results show that,the calcination temperature of support is a key factor affecting the surface alkalinity of Fe-based catalysts.As the support is calcined at 900^oC,the C₂-C₄ olefin/paraffin ratio of the catalyst reaches an extraordinary ratio of 10.2.As the support is calcined at 1200^oC,the C₂⁼-C₄⁼hydrocarbon selectivity of the catalyst achieve 59.2%.As the support is calcined at 1300^oC,the C₂⁼-C₄⁼hydrocarbon selectivity of the catalyst varies in a relatively narrow and high numerical range with the change of reaction temperature.