The Preparation Of ZnO By Homogeneous Precipitation Method And Its Applications In Fischer-Tropsch Synthesis

Fischer-Tropsch Synthesis(FT synthesis) is a technology to convert syngas, which derived from coal, natural gas and biomass resources, into environmentally friendly liquid fuel and high additional value chemical materials. It plays an important role in solving the problem of liquid fuel supply. Cobalt-based catalyst is a promising catalyst for FT synthesis with high activity, high selectivity to heavy hydrocarbons and low activity of water–gas shift reaction.ZnO was obtained from Zinc carbonate hydroxide after calcination, which was prepared by homogeneous precipitation method. The mentioned ZnO was used as the supporter of Cobalt-based catalyst. The main work of this dissertation is discussing the formation process of zinc carbonate hydroxide, and the influence of reaction temperature and content of ethanol during the process on the activity of FT synthesis reaction. Promoter Al was added to enhance the activity and stability of Co/ZnO catalyst. Analysis methods of X-ray diffraction(XRD), temperature-programmed reduction(TPR), low temperature N2 physical adsorption and scanning electron microscope(SEM) had been used to characterize the catalyst properties and study the relationship between catalyst structure and its reaction performance. The results are as follows:(1)Zinc carbonate hydroxide was synthesized by the homogeneous precipitation method. The influence of reaction duration on the utilization of Zn atom from solution into precipitate was studied, and it increased with higher temperature, longer time and more urea. ZnO crystal was detected in the precipitates formed during the first 20 minutes, but only Zn4CO3(OH)6·H2O(Zn4) and Zn5(CO3)2(OH)6(Zn5) were found in the precipitates with extended reaction time longer than 40 minutes. The partial transformation of Zn4 into Zn5 was found according to the XRD results. After calcination, ZnO crystal was formed and maintained the same morphology as zinc carbonate hydroxides with much pores in the plate-like unit. The size of ZnO crystal was raised with the increased calcination temperature.(2)The impact of different resultant temperature and ethanol content in the homogeneous precipitation process on the performance of FT synthesis was studied. The crystal of ZnO became smaller with the increased temperature, but the structure of hexagon-flower formed by laminar ZnO became bigger. Different structure made different activities, the best temperature was 90°C. More ethanol in solution could change the hexagon-flower structure into ball, the size of ZnO crystal was also changed. Adding 20% ethanol enhanced the CO conversion by 22%.(3)The deactivation phenomenon of Co/ZnO catalysts was resulted from the sintering of Cobalt. Adding Al as promoter weakened the sintering phenomenon and enhanced catalytic stability in the reaction. The best content of Al is 0.1%, not only improved the stability, but also enhance the activity of FT synthesis reaction.