| The crises of energy shortage and environmental pollution have attracted ever-increasing concern in the world. In recent years, the oil consumption has been increasing rapidly in China. However, the contradiction between the shortage and demand of oil has significantly inhibited the development of China. Fischer-Tropsch synthesis(FTS), which converts coal, natural gas and biomass to clean liquid fuel, is an alternative way to ease the tension between supply and demand for oil.For the supported catalyst, the support is crucial for designing an efficient catalyst in Fischer-Tropsch synthesis. Owing to the large specific surface area, ordered mesopore and adjustable pore size distribution, ordered mesoporous materials have become the most commonly used support to prepare the FTS catalyst. Mesostructured cellular silica foams(MCF) have some similar properties as compared to the conventional ordered mesoporous materials, such as high surface area and large pore volume. On the other hand, the MCF has novel pore structure that aerogel-like, uniformly large spherical cell pores interconnected with uniformly small window pores. The large pore size and 3D open mesostructure of MCF are beneficial to the transport and diffusion of reactants in the channel. This thesis describes the preparation and hydrothermal stability of a series of cobalt catalysts supported on MCF and MCF modified by Al and zeolite. The effects of the different MCF supports on the activity, selectivity and stability of Co/MCF catalysts were investigated in order to prepare the efficient FTS catalyst. The main contents of this thesis are as follows:1. 3D foam-like mesoporous MCF was synthesized using P123 as template and benzene as microemulsion. TEOS was used as a silica source. Cobalt catalysts(15wt.%) with different pore structure were prepared using different materials(MCF, SBA-16, KIT-6, SBA-15) having similar pore size. Four Co catalysts showed different reduction extent(39.1%-63.7%) and dispersion(9.6%-11.1%) due to the different pore structure, which indicated that the pore structure significantly affected the dispersion and reduction degree of cobalt catalysts. Co/MCF, Co/SBA-16 and Co/KIT-6 catalysts with 3D pore structure showed higher CO conversion(46.0%-49.4%) than Co/SBA-15 catalyst with 2D pore structure(22.3%). Additionally, Co/MCF catalyst exhibited higher activity compared to Co/SBA-16 and Co/KIT-6 catalysts. We concluded that the MCF with 3D open pore channel is beneficial to the diffusion of the syngas and the products in the pore of Co/MCF catalyst, resulting in lower methane selectivity, higher C5+ selectivity and C20+ selectivity(31.5%) in FTS.2. Cobalt catalysts supported on MCF with different cobalt particle sizes were prepared using citric acid as the complexion agent. With the increase in the amount of citric acid, the cobalt particle size decreased from 9.4 nm to 3.9 nm, in parallel with the decrease of reduction extent from 63.5% to 49.6% and the increase of the dispersion from 10.2% to 24.6%. The Co/MCF catalyst with Co crystallite size of 6.9 nm showed the highest CO conversion(62.7%) and good stability for FTS due to good dispersion(14.0%) and appropriate reduction degree(54.2%). There is a linear relationship between TOF and cobalt particle size in the range of 3.9 nm-8.0 nm. The TOF value increased with the cobalt particle size firstly. When the cobalt particle size is larger than 8.0 nm, the TOF is constant.3. Al-MCF(AMCF) supports with Al/Si ratio from 0.05 to 0.3 have been synthesized using “pH-adjusting” method. The results showed that the aluminum can be doped into the wall of MCF while the foam-like mesostructure of MCF was maintained. With the increase of aluminum content, the dispersion of obtained cobalt catalyst increased from 7.5% to 12.0%. The FTS reaction showed that the CO conversion of the catalyst supported on Al-doping(Co/AMCFs)(31.6%-37.1%) were higher than that of Co/MCF catalyst(19.5%) at the reaction temperature of 220 °C. When increasing the reaction temperature from 220 °C to 250 °C, the CO conversion of the catalyst with Al-doping increased from 53.4% to 57.6%. We found that the catalytic activity(30.9%) and C5+ selectivity(58.4%) of undoped Co/MCF catalyst were low and the methane selectivity(24.3%) was high due to the catalyst deactivation, which resulted from the cobalt sintering and the formation of cobalt-silica compound in reaction. However, the addition of aluminum into MCF suppressed the deactivation of the Co/AMCF catalyst. It was found that Co/AMCF-3 catalyst showed the highest selectivity to liquid product(60.8%) and hydrocarbon selectivity(17.4%) due to the highest content of tetrahedral-coordinate framework aluminum and the strongest acidity.4. 3D foam-like mesoporous material Z-MCFs with large surface area and double pore size distributions have been synthesized by assembling different amounts of ZSM-5 seed with MCF(Co/Z-MCF). The Co/Z-MCF catalyst showed higher CO conversion(76.5%-79.0%) than that of the Co/MCF catalyst(68.1%) prepared from pure silica MCF in FTS. With the increase of the ZSM-5 seed content, the activity of Co/Z-MCF catalysts gradually increased firstly and then decreased when the ZSM-5 seed content increased further. It was noted that Co/Z-MCF-3 catalysts with the highest ZSM-5 seed content exhibited the lowest deactivated rate, suggesting the highest stability. This was ascribed to the strong interaction between cobalt and Z-MCF support. It was found that the decrease in the space velocity from 8 to 4 NL·h-1·g-1 led to the high alkane selectivity for the Co/Z-MCF-3 catalyst. In addition, the hydrocarbon selectivity(64.2%) toward middle distillate products were also increased with the decrease of space velocity.5. The synthesis method of MCF was investigated using TEOS as silica source. In this study, MCF was synthesized using P123 as template and cyclohexane as microemulsion. The effects of the amount of cyclohexane, aging temperature and Al doping on the structure and hydrothermal stability of MCF were investigated. The pore size and pore volume of MCF increased with the increase of the amount of cyclohexane, as result the specific surface area is > 570 m2·g-1. When the amount of cyclohexane added is 12 g, the specific surface area of the MCF obtained is 800.7 m2·g-1. The pore size of MCF and pore volume increased with the increase of aging temperature. The specific surface area of MCF was increased up to 1000 m2·g-1 and the foam structure was maintained while the aging temperature was 130 °C. The hydrothermal stability of the synthesized MCF was investigated in hot water. It was found that the specific surface area of pure silica MCF decreased by 73.5% after hydrothermal treatment for 12 h, whereas that of Al-MCF decreased by only 38.7% after the same treatment. This indicated that the Al doping improved the hydrothermal stability of MCF. |
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