Page 146 - Adsorbents fundamentals and applications

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SILICA GEL, MCM,

AND ACTIVATED ALUMINA

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, acti-
vated alumina is more versatile than silica gel and has been applied more often
as a sorbent.

6.1. SILICA GELS: PREPARATION AND GENERAL PROPERTIES
The commercial silica gel sorbents are mesoporous, that is, with pores mostly
larger than 20 ˚ A. Silica gel can be formed by two routes: (1) polymerization

Adsorbents: Fundamentals and Applications, Edited By Ralph T. Yang
ISBN 0-471-29741-0 Copyright  2003 John Wiley & Sons, Inc.

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