Page 208 - Adsorbents fundamentals and applications

PREPARATION OF THREE TYPES OF SORBENTS 193

cations need to be spread, at a high dispersion, on solid substrates that have
high surface areas. Accordingly, there are three types of π-complexation sor-
bents:

a. Monolayer or near-monolayer salts supported on porous substrates
b. Ion-exchanged zeolites
c. Ion-exchanged resins

Among these three types, type (a) and type (c) are used for bulk separations,
whereas type (b) is used for purification. Specific applications will be given in
Sections 8.3 and 8.4.

8.1.1. Supported Monolayer Salts
It has been known for a long time that metal oxides and salts can be dispersed on
solid substrates in a monolayer form, that is, as opposed to forming a stoichio-
metric compound with the support, or being dissolved in the support to form a
solid solution. Russell and Stokes (1946) showed the first evidence for the mono-
layer dispersion of MoO 3 on γ -Al 2 O 3 . An extensive discussion on the subject
has been given by Xie and Tang (1990). Monolayer dispersion of many ionic
metal oxides and salts, particularly halides, has been accomplished by Xie and
Tang (1990). Their substrates have included γ -Al 2 O 3 , silica gel, TiO 2 , activated
carbon, and a variety of zeolites.
There are two general approaches for dispersing monolayer or near-monolayer
salts on porous supports: thermal monolayer dispersion and incipient wetness
impregnation. Both will be discussed, although the latter has more practical
use. The technique of spontaneous thermal dispersion has been described in
detail in the literature (Xie and Tang, 1990; Xie et al., 1992). It was successfully
applied to synthesize sorbents capable of π-complexation with olefins (Yang and
Kikkinides, 1995; Cheng and Yang, 1995; Yang and Foldes, 1996; Deng and
Lin, 1997; Rege et al., 1998; Padin and Yang, 2000). The work of Deng and Lin
(1997) involved spreading salts by using microwave heating.
Thermal monolayer dispersion involves mixing a metal salt or oxide with a
substrate at a predetermined ratio. This ratio is determined by the amount of
salt that is required for monolayer coverage on the surface area of the substrate,
assuming two-dimensional hexagonal close-packing. The BET surface area of
the substrate is first measured. After the finely divided powders of the salt and
substrate have been thoroughly mixed, it is heated at a temperature between the
Tammann temperature and the melting point of the salt. The Tammann temper-
ature is the point where the crystal lattice begins to become appreciably mobile,
and it is approximately 1/2 T m ,where T m is the melting point in absolute temper-
ature. If the temperature is too low, the dispersion would take an unacceptably
long time. A high dispersion temperature could cause the metal salt to oxidize or
react with the substrate, and potentially deactivate the sorbent. A typical example
of sample preparation is that for AgNO /SiO 2 (Padin and Yang, 2000). The SiO 2
3