Thursday, 30 April 2009

Silica Gel, MCM, and Activated Alumina

From the e-book, that I see, define of different silica gel, MCM, and activated alumina. What’s is them…? Let’s see…

Silica gel is the most widely used desiccant because of its large capacity for water (40% by weight) and ease in regeneration (150 ◦C, compared with 350 ◦C for regenerating zeolites). In addition, its surface can be readily modified by reacting (or grafting) with a monomolecular layer of organic ligand, and these modified silica gels are being applied in an increasing number of applications in chromatography. All aspects of silica gel and its modification have been reviewed

and discussed extensively (Iler, 1979; Unger, 1979; Vansant et al., 1995). The MCM-type materials belong to a new family of ordered, mesoporous silicate/aluminosilicate prepared by hydrothermal formation of silica gels in the presence of surfactant templates (Beck et al., 1992). They were discovered only recently, by Beck et al. in 1992, and hold promise for a number of interesting applications. Hence they are included in this chapter. Activated alumina is also widely used as a desiccant because of the same advantages for which silica gel is used. Unlike silica gel, which is amorphous, activated alumina is crystalline. Oxygen vacancies (defects) are easily formed on its surfaces, thus alumina has both Lewis and Brønsted acid sites. The surface

chemistry, as well as the pore structure of activated alumina, can be modified, for example, by treatment with acid (HCl or HF) or alkaline (to alter the acidity) and controlled thermal treatment (to tailor the pore structure). As a result, activated alumina is more versatile than silica gel and has been applied more often as a sorbent.

Adsorbent, fundamentals and application, Yang, R.T., John Wiley and Son’s

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