The Study On The Preparation And Performance Of Co-based Catalyst For Highly Selective Synthetic Gasoline Under Fischer-Tropsch Synthesis

In China, the last few years has witnessed an increasing dependence on importing petroleum, which is posing severe challenge to the security of our energy supply. Therefore, the exploration of the substitute energy through utilizing the abundant coal resources in our country and developing the technology of indirect coal liquefaction via Fischer-Tropsch synthesis(FTS) reaction has strategic significance for ensuring our energy security and protecting the environment.Catalysts play a vital role in FTS and ample research has focused on the Co-based catalyst. In this dissertation, the collaborative method including vacuum impregnation under ultrasonic treatment, application of promoter and the mixing with HZSM-5 molecular sieve was initiated to successful prepare the Co-based-zeolite composite catalyst (Co/Zr/SiO2+HZSM-5).In addition, the catalytic activity, the selectivity for C5-C11 gasoline fraction and the stability of the catalyst prepared in this way was proved to be high in a fixed bed reactor.1. The effect of impregnation method on the textural property and performance of Co/SiO2 catalysts was studied firstly.Compared to the Co/SiO2 catalyst impregnated under atmospheric pressure, the more inner pores and channels of the support were utilized when the catalyst was prepared under vacuum conditions. Vacuum impregnation enabled a large number of smaller cobalt particles to distribute in the pores. Meanwhile, a certain interaction was generated between cobalt oxides and the support, which prevented the cobalt oxide crystals from agglomerating into larger grain-clusters during the drying and calcination process of the catalyst. Through evaluating the two catalysts under different reaction conditions, the optimal condition for the vacuum impregnated Co/SiO2 catalyst was concluded as follows:T=493 K, P=2.0 MPa, H2/CO (mole ratio)=2.0 and GHSV=1000 h-1.Under this condition, the initial CO conversion was as high as 90.9% and the selectivity for C5+ hydrocarbon was 58.9%. In contrast, the initial CO conversion of the Co/SiO2 impregnated under atmospheric pressure was as low as 83.2% and the selectivity for C5+ hydrocarbon was only 56.1%. The above results indicated that the initial CO conversion of the Co/SiO2 catalyst had been greatly enhanced through vacuum impregnation.2. The effect of cobalt content (5-25%) on the performance of vacuum impregnated Co/SiO2 was also explored in this study. It was shown that the size of cobalt particles enlarged gradually with the increase of cobalt content, which was helpful to enhance the selectivity for C5+ hydrocarbon. On this basis, the optimal cobalt content was fixed on. Furthermore, the effect of the content of zirconium promoter (2-10%) on the performance of Co/SiO2 catalyst was examined. The addition of Zr promoter conduced to weakening the interaction between cobalt oxides and SiO2 support, which in turn improved the reduction degree of the cobalt particles. After 5% Zr promoter was added to the CO/SiO2 catalyst, the initial CO conversion and the selectivity for the C5+ hydrocarbon enhanced from 76.0% and 72.9% to 92.8% and 77.6%, respectively.3. The Co/Zr/SiO2 catalyst was prepared through vacuum impregnation with the ultrasonic treatment. The effects of ultrasonic power and treatment duration on the catalyst performance and the pH value of Co, Zr nitrate solution were studied. It was indicated that the pH value of the solution dropped significantly with the increase of ultrasonic power and duration, which conduced to improving dispersion and reduction degree of cobalt oxides. Specifically, the initial CO conversion and the selectivity for C5+ hydrocarbon enhanced from 92.8% and 77.6% to 95.5% and 81.0%, respectively. After 20-hour reaction, the declining rate of the CO conversion of Co/Zr/SiO2 catalyst treated by high-power ultrasound was minimized. This also showed that the catalytic activity, stability and selectivity for C5+ hydrocarbon were improved to a certain extent when ultrasound was applied in the process of vacuum impregnation.4. The Co/Zr/SiO2 catalyst was blended with the HZSM-5 zeolite. The effects of the two ingredients mixture ratio and the molecular sieve Si/Al ratio on the C5+ hydrocarbon yield and C5—C11 gasoline weight fraction were examined. The optimal composite catalyst was composed of Co/Zr/SiO2 with HZSM-5(Si/Al=50) according to the volume ratio 1.0:1.0.Under the reaction condition of T=523 K, P=2.0 MPa, H2/CO=2.0(mole ratio) and GHSV=500 h-1, the catalyst was evaluated over 500 hours in the fixed-bed reactor. The results showed that CO conversion remained as high as 90% and the yield of C5+ liquid hydrocarbon was about 130 g/Nm3(CO+H2). The weight fraction of C5+ liquid hydrocarbon in the total hydrocarbons was about 73%. Due to the cracking of partial C12+ long chain hydrocarbons at the acidity center of the molecular sieve, the weight fraction of C5-C11 gasoline was about 57%, which was higher than the maximum value 45% predicted by the ASF model. Furthermore, in the C5+ liquid hydrocarbon, the weight fraction of the gasoline was close to 80%. This result indicated the composite catalyst had good performance and high stability.5. Considering the reaction mechanism of Co-based catalyst and excluding the negligible oxygenated hydrocarbons, the simplified method for calculating the weight fraction of C5+ liquid hydrocarbon in the total hydrocarbons was established on the basis of the weight relation between products water and hydrocarbon in the FTS. This method was proved to be convenient and accurate for calculating the weight fraction of C5+ liquid hydrocarbon.In summary, through the results concluded in this dissertation, a new Co/Zr/SiO2+HZSM-5 composite catalyst with high CO conversion was developed for highly synthesizing C5-C11 gasoline under FTS, which explored a new approach to the catalyst development for conversion of syngas to gasoline.