Activated Alumina and Silica Gel

Alumina, silica and many other metal oxides are insulators. However, recent experiments indicate that the surfaces of these insulators are mainly ionic (Masel, 1996). The pristine or freshly cleaved surfaces of single crystals of these oxides (cleaved under ultrahigh vacuum) are fairly inert and do not have significant adsorption capacities for even polar molecules such as CO and S02 (Masel, 1996 Henrich and Cox, 1994). However, the surface chemistry and adsorption properties are dominated by defects on real surfaces. For example, oxide vacancies on alumina expose the unsaturated aluminum atoms, which are electron acceptors, or Lewis acid sites. 

The commercial alumina and silica gel sorbents are mesoporous, i.e., with pores mostly larger than 20 A (see Fig. 1). Activated alumina is produced by thermal dehydration or activation of aluminum trihydroxide, A1 (OH)3 (Yang, 1997), and is crystalline. Commercially, silica is prepared by mixing a sodium silicate solution with a mineral acid such as sulfuric or hydrochloric acid. The reaction produces a concentrated dispersion of finely divided particles of hydrated Si02, known as silica hydrosol or silicic acid  

The silica gel is amorphous. Using high-resolution electron microscopy, it is known that its amorphous framework is made up of small globular (primary) particles having sizes of 10 to 20 A (Rouquerol, Rouquerol and Sing, 1999). An alternative route involves reactions of silicon alkoxides with water, and a wide variety of materials can be made this way (Jones 1989 Brinker and Sherer, 1990). The processes based on this route are referred to as sol-gel processing, and they offer many promising possibilities. For silica gel, the reaction is 

The silicic acids thus formed can then polymerize via 

Different mechanisms for the interactions between the silicate precursors and the organic template as well as the liquid crystal templating mechanisms have been discussed (Ying, Mehnert, and Wong, 1999 Tanev and Pinnavaia, 

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Adsorption is the property of certain extremely porous materials to hold vapors in the pores until the desiccant is either heated or exposed to a drier gas. The material is a solid at all times and operates alternately through drying and reactivation cycles with no change in composition. Adsorbing materials in principal use are activated Alumina and silica gel. Molecular sieves are also used. Atmospheric dew points of minus 1000°F are readily obtained using adsorption. 

Fig. 1 compares the pore size distributions of major commercial adsorbents discussed in this section. Activated carbons have a broad pore size distribution like activated alumina and silica gel. Although activated carbon is thought to be hydrophobic, it does adsorb... 

Existing commercial sorbents including activated carbon, zeolites, activated alumina, and silica gels will 2. 

Adsorbent costs vary with time, supplier, and other factors, even for the same exact material. Prices often range from 0.50 per pound for some commodity adsorbents to 100 per pound or more for more exotic materials. Some that are impregnated with expensive metals are much higher priced. Price is also sensitive to quantity. For example, from a laboratory supply house, in 100-g quantities, activated alumina and silica gel cost as much as 70 to 140 per pound. But in tonnage quantities, they may sell for less than 1 to about 4 per pound. 

Practically all the internal surface and a large fraction of the pore volume of finely porous materials such as activated aluminas and silica gels are contained in pores smaller than 300 A diam. (micropores). The average diameter of the micropores is usually of the order of 50 A, so that pore-size distributions cannot be measured directly even using an electron microscope. Of the indirect approaches possible, low-temperature adsorption isotherms appear to provide the most complete data. 

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