Page 165 - Adsorbents fundamentals and applications

150 SILICA GEL, MCM, AND ACTIVATED ALUMINA

accept electrons) are usually abundant on aluminas. These are the Al 3+ sites on
the surfaces. There are both tetrahedral and octahedral Al 3+ sites. Fully hydrated
surfaces of aluminas also have Brønsted acid sites (i.e., −OH groups that can
donate protons). The high-temperature forms of alumina, for example, θ-alumina
and δ-alumina, contain only Lewis acid sites. On γ -alumina and η-alumina,
however, both types of acidity may exist, depending on the degree of hydration.
Peri has shown models of surface of alumina as a function of its dehydration
level (Peri, 1976).
The acidity of the surface can be increased by treatment with acid, such as
HCl and HF. Partial conversion of a fully hydrated alumina surface by the acid
−
−
treatment would form a surface that contains both Cl and OH groups (Gates
et al., 1979):
OH − OH − OH − OH − OH − OH −
Al 3+ Al 3+ Al 3+ Al 3+ Al 3+
⇒ Cl − OH − Cl − OH − OH − Cl −
Al 3+ Al 3+ Al 3+ Al 3+ Al 3+

From the above discussion, it can be seen that both pore structure and surface
chemistry of the activated alumina can be manipulated and controlled. As a result,
activated alumina is a very versatile sorbent and can be tailored for specific
applications.

6.7. ACTIVATED ALUMINA AS SPECIAL SORBENTS

Desiccation remains to be a major application for activated alumina. Like sil-
ica gel, activated alumina have higher ultimate capacities for water vapor than
zeolites do. When high removals of water vapor (to very low dew points) are
required, zeolites are used because their water capacities at very low concentra-
tions are higher, as shown in Figure 6.16.
Activated alumina is a versatile sorbent that can be tailored for many special
applications. New applications continue to be developed, mainly by the aluminum
companies. Little is disclosed on the details of their modifications. However,
the modifications follow simple general principles of surface chemistry, such as
acid–base chemistry. Two methods are used for tailoring: (1) variation of the
activation process, and (2) use of dopants. The following are proven applications
of various tailored aluminas:
• Removal of HCl and HF from gases and liquids
• Removal of acidic gases (COS, CO 2 ,H 2 S, CS 2 ) from hydrocarbons
• Removal of oxygenates and Lewis Bases
• Removal of polar organic compounds
5+ 3− − −
• Removal of As ,PO 4 ,Cl ,and F from water