Alumina carrier refers to a white powdered or shaped alumina solid. It is the most widely used catalyst carrier, accounting for about 70% of industrially supported catalysts. Alumina has many forms, not only different forms have different properties, even the same form has different properties, such as density, pore structure, specific surface area, etc., due to different sources.
These properties have important significance for alumina used as a catalyst support. Alumina is mostly prepared from its hydroxide (also called hydrated alumina or alumina hydrate).
Commercial alumina supports are available in powder form, but most of them are shaped. Strip, spherical, and ingot alumina are mostly used as the support of the catalyst in the fixed bed reactor, and the microspherical alumina support is mostly used in the fluidized bed reactor. It can also be made into special-shaped carriers required for specific catalytic processes, such as ring, trilobal, honeycomb, fibrous (see alumina fiber) and so on. In addition to solids, there are also alumina sols for sale. The important parameters when choosing alumina are crystal form, purity, specific surface area, pore structure and bulk density. The specific surface area of α-alumina is very low (less than 1m2/g), and the specific surface area of transition alumina generally reaches 10～102m2/g.
The molded commercial alumina supports are mostly porous materials, and the pore size and distribution have an important influence on the diffusion properties of the reactants in the catalyst particles during the catalysis process. The pore diameter of the fine pores can be less than 20┱, and the pore diameter of the coarse pores can reach the micron level, and the pore structure varies with varieties. All transition-state aluminas contain water to a greater or lesser extent, and there are some hydroxyl groups and exposed aluminum atoms on the surface, thus exhibiting the characteristics of B acid and L acid (see acid-base catalysts). The nature has an important influence. Surface acidity is related to preparation conditions, especially impurity ions and heat treatment temperature. Negative ions such as halogen will enhance the surface acidity of alumina, and the enhancement of acidity will promote the catalytic function of alumina itself for hydrocarbon cracking and isomerization reactions.
① Support catalytic active components. Alumina with a small specific surface area (or a material with a small amount of coarse pores) is usually used to support catalyst active components with high specific activity. The use of this type of carrier can eliminate the diffusion effect in the pores, and can reduce the deep oxidation side reaction in the process for selective oxidation. For example, the silver catalyst used in the oxidation of ethylene to ethylene oxide uses α-alumina as the carrier. Alumina with a large specific surface area has a developed pore structure, which enables the supported catalyst active components to be highly dispersed into particles, and with the help of the barrier effect of the carrier, prevents the active component particles from sintering and growing during use. Alumina support used. For example, dispersing precious metals palladium, platinum, rhodium, etc. on an alumina carrier to make a hydrogenation catalyst can improve the utilization rate of precious metals.
②The acidity of the alumina surface and the supported active components constitute a dual-functional catalyst. For example, the platinum-alumina catalyst used in platinum reforming, in which alumina serves as a carrier and one of the active components of the catalyst.
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