Silica gel has become the most widely used as desiccant because of its large water adsorption capacity (about 40% mass fraction) and easy regeneration (regeneration temperature is about 150 degrees, and zeolite regeneration requires 350 degrees). In addition, its surface is easy to pass through and organic Ligand monolayer reaction (or grafting) for modification. Modified silica gel has been used more and more widely in chromatography.
Activated alumina has similar advantages as silica gel and is also widely used as a desiccant. Unlike amorphous silica gel, activated alumina has a crystalline structure. Oxygen vacancies (defects) are easily formed on the surface of activated alumina, so alumina has both Lewis acid and Bronsted acid centers. The surface composition and properties and pore structure of activated alumina can be modified, such as acid (HCI or HF) or alkali (for changing acidity) treatment and controlled heat treatment (adjusting pore structure). Therefore, activated alumina has more uses and functions than silica gel, and is more commonly used as an adsorbent.
Commercial silica gel has a mesoporous structure, and most of its pores are larger than 20A. The most common use of silica gel is as a desiccant, that is, a water vapor adsorbent. This characteristic of silica gel is related to its ability to form relatively weak chemical bonds with water and its macroporous and mesoporous properties. The reason why silica gel is often used as a desiccant is because it can absorb a large amount of water under low humidity conditions and has the highest total water adsorption capacity, and also because silica gel is easy to regenerate.
The main purpose of activated alumina is still drying. Like silica gel, the saturated water vapor adsorption capacity of activated alumina is larger than that of zeolite. If the requirement for the removal of water vapor is relatively high (removal to a very low dew point), zeolite is required as an adsorbent, because zeolite has a higher water adsorption capacity at low concentrations.
Activated alumina is a widely used adsorbent. After modification and adjustment, it can be applied to many special applications. It can be used for acid gas removal, oxidant removal and water treatment.
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