| A12O3 is often used as the support of metal and metal catalysts, which are used in many important chemical processes such as hydrogenation, reforming, methanation, dehydrogenation, desulfurization and dechlorination, etc. Most of these processes, such as the biomass conversion, hydrogenation of butynediol and 3-hydroxypropionaldehyde, are aqueous systems in which water is the solvent or the product. Al2O3 can be converted to the corresponding hydrates by reacting with water. As a result, the mechanical strength and surface area of the catalyst are decreased, the active ingredient of the catalyst is aggregated, which finally cause the irreversible deactivation of the catalyst. The hydration of Al2O3 limits its application in the aqueous system. Therefore, the preparation of catalysts with excellent catalytic performance in aqueous systems via improving the hydrothermal stability of Al2O3 becomes an important research topic in industry and academia.In previous works, we had studied that the effect of different promoters on hydrothermal stability of Ni/γ-Al2O3 catalyst. The results show that SiO2 can inhibit the hydration of Al2O3, which shows potential applications in the hydrogenation of 1,4-butynediol in the aqueous phase. In this thesis, we have prepared a series of Ni/SiO2-Al2O3 catalysts with different SiO2 contents by the impregnation method. The effects of the SiO2 contents and the calcination temperatures on the hydrogenation activity and stability of Ni/SiO2-Al2O3 in the hydrogenation of 1,4-butynediol in the aqueous phase were systematically studied. The main findings are as follows:1. The effects of the introduction of SiO2 by impregnation on the supports acidity were researched. It is found that SiO2 covers the surface of Al2O3 and the formation of one-to-one bonding between silica and aluminum (Al-O-Si) upon deposition of silica alkoxide on alumina. Supports show Bronsted acid sites because the electronegativity of Si is stronger than that of Al, while Lewis acid sites are ascribed to the Al3+ which isn’t covered by SiO2. With the increase of SiO2 contents, Bronsted acid sites of supports increase and Lewis acid sites decrease. When the SiO2 content is 15.0 wt%, both of the Bronsted acid sites and Lewis acid sites present on the 15.0SiO2-Al2O3 support. It indicates that Al2O3 isn’t covered completely by SiO2, which leads to the simultaneous exposure of Al3+ and Si-OH.2. The influence of different contents of SiO2 on the hydrogenation activity and hydrothermal stability of Ni/Al2O3 catalyst was studied. Results show the introduction of SiO2 can improve the hydrothermal stability of Al2O3, due to the prohibition of the attack of H2O to Al3+, which is caused by the coverage of Al2O3 by SiO2. However, as the increase of SiO2 contents, the interaction between Ni and the support is reduced, which causes the decay of the hydrogenation activity. When the SiO2 content is 6.4 wt%, the catalyst shows the best hydrothermal stability and hydrogenation performance.3. The effects of the calcination temperatures on the surface properties of SiO2-Al2O3 and catalytic performance of Ni/SiO2-Al2O3 were studied. The results show that the calcination temperatures have few significant effects on the state of active component Ni and the hydrogenation activity of the catalyst within the range of 400℃-800℃. However, when the calcination temperature is higher, it causes SiO2 gathering and increases exposed Al3+, which leads to the decrease of hydrothermal stability of the catalyst. When the calcination temperature is 400℃, Ni/SiO2-Al2O3 shows the best activity and hydrothermal stability. |
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