Page 161 - Adsorbents fundamentals and applications

146 SILICA GEL, MCM, AND ACTIVATED ALUMINA

O
CH 2 CH 2 CH 2 NH 2 CH 2 CH 2 CH 2 NHC
O −
+ CO 2
+
CH 2 CH 2 CH 2 NH 2
CH 2 CH 2 CH 2 NH 3
+
CO 2
+
+
2 CH 2 CH 2 CH 2 NH 3 HCO 3 - H 2 O

Figure 6.11. Adsorption of CO 2 on amine grafted silica with and without water vapor, showing
that the amount adsorbed is doubled in the presence of water (Huang et al., 2003).

aminosilane groups. By this route, Katz and Davis could synthesize amorphous
silicas with binding sites comprising one, two, or three propylamine groups that
are covalently bonded to the silicon atom of the silica.
This covalent bonding is an advantage over imprinting with bulk polymers
because it results in heterogeneity due to noncovalent bonding. The pore-size
distribution of the imprinted silica was measured by adsorption of argon at 77 K.
The pore volume indicated by the crosses in Figure 6.13 were those of the poly-
merized TEOS, with the imprint molecules remaining inside the structure. This
was the pore volume of the micropores that was filled by the solvent used in the
sol-gel process. Upon removal of the imprint, additional porosity was created,
indicated by circles in the figure. This pore volume was confined to a narrow
range of micropore sizes, between 0.8 and 1.0 nm. The differences in isotherms
of Ar at 77 K are also shown in Figure 6.13 (inset).
This route can potentially lead to design and synthesis of silicas, not only
with desired pore sizes, but, more importantly, imprinted with features modeled
on those of enzymes and antibodies. Hence it is possible to mimic some of the
essential features of binding and catalysis found in biological systems.

6.6. ACTIVATED ALUMINA

Commercial production of activated alumina is performed exclusively by thermal
dehydration or activation of aluminum trihydrate, Al(OH) 3 , or gibbsite (MacZura
et al., 1977). The oldest form, which is still widely used, is made from Bayer
α-trihydrate, which is a by-product of the Bayer process for aqueous caustic
extraction of alumina from bauxite. The trihydrate, in the form of gibbsite, is
◦
heated or activated in air to ∼400 C to form crystalline γ /η-alumina with a
2
minor amount of boehmite and has a surface area of ∼250 m /g. Alternatively,
◦
the trihydrate is heated very rapidly at 400–800 C to form an amorphous alumina
2
with a higher surface area, 300–350 m /g. The major impurity in these products,
besides water (typically 6%), is Na 2 O at nearly 1%. The micropore volume is
shown in Figure 6.1, but there is a considerable number of pores with sizes