Thermal stability of activated alumina support
Activated alumina has been widely used in the catalytic field as a supported catalyst carrier because of its unique pore structure and surface properties. A considerable part of the catalytic reactions are carried out in hydrothermal environment, such as amination, hydration, dehydration, oxychlorination, carbonylation, catalytic combustion and selective catalytic oxidation of methane. In the process of low-temperature catalytic reaction, the active alumina carrier can rehydrate under hydrothermal conditions. The product boehmite is formed below 100 ℃, the hydration product at 100 ~ 110 ℃ is the mixture of boehmite and boehmite, and the product above 110 ℃ is boehmite; At high temperatures, γ- High temperature sintering and phase transformation of Al2O3 activated alumina carrier will cause significant reduction of carrier specific surface area, destruction of pore structure or deactivation of catalyst. In addition, the presence of water vapor will continue to hydrate with alumina, promote the continuous formation of al-o-al bridge bonds between alumina particles, aggravate surface sintering, cause sharp reduction of specific surface area and inactivate the catalyst. Therefore, it is of great significance to prepare activated alumina support with high hydrothermal stability for catalytic reaction in hydrothermal environment.
Scholars have carried out extensive research on the thermal stability of activated alumina. The mechanism of sintering and phase transformation of alumina is that there are many tetrahedral and octahedral vacancies in its bulk phase. At the same time, the coordination of aluminum particles on the surface is unsaturated. When high temperature and water vapor exist, these vacancies become very active, and the hydroxyl groups between alumina particles react, resulting in the reduction of specific surface area and final conversion to α Phase. Therefore, improving the preparation process, adding stabilizers and generating new substances can effectively block the sintering and phase transformation of alumina.