Morphological structure of alumina carrier

Morphological structure of alumina carrier
Commercial alumina carriers are in powder form, but most of them are already formed. Strip, spherical, and ingot-shaped alumina are mostly used as carriers for catalysts in fixed bed reactors, and microspherical alumina carriers are mostly used in fluidized bed reactors. It can also be made into special-shaped carriers required for specific catalytic processes, such as rings, trilobes, honeycombs, fibers (see alumina fibers), etc. In addition to solids, alumina sols are also available for sale. Important parameters for selecting alumina include 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 ​​transitional alumina is generally 10~102m2/g. Most of the formed commercial alumina carriers are porous materials, and the pore size and distribution have an important influence on the diffusion properties of reactants inside the catalyst particles during the catalytic process. The pore size of fine pores can be less than 20┱, and the pore size of coarse pores can reach micrometers. The pore structure varies with the variety. All transition aluminas contain water to a greater or lesser extent, and there are some hydroxyl groups and exposed aluminum atoms on the surface, thus showing the characteristics of B acid and L acid (see acid-base catalysts), which has an important influence on the properties of alumina carriers. Surface acidity is related to the preparation conditions, especially to impurity ions and heat treatment temperature. For example, negative ions such as halogens will enhance the surface acidity of alumina, and the enhanced acidity will promote the catalytic function of alumina itself in hydrocarbon cracking and isomerization reactions.