Page 209 - Adsorbents fundamentals and applications

194 π-COMPLEXATION SORBENTS AND APPLICATIONS

2
had a surface area of 670 m /g, which would require ∼0.55 g of AgNO 3 for
monolayer close-packing. However, the surface area of the impregnated sample
unavoidably decreases from that of the starting support. Hence the monolayer
amount had to be determined empirically, which was 0.32 g/g. Thus, after thor-
ough mixing of 0.32 grams of AgNO 3 pergramofSiO 2 , the sample was heated
◦
in air at 200 Cfor ∼4 days to ensure complete dispersion. The BET surface
2
area of this sample was 384 m /g, indicating some pore plugging as a result of
AgNO 3 impregnation.
The thermal monolayer dispersion technique requires the use of fine powder
and thorough mixing. Thus, pelletizing is needed when sorbents in the pellet form
are to be used. An advantage of this technique is that salts that are insoluble in
water can be dispersed directly, as in the case with the important salt CuCl. This
technique is suitable for laboratory experiments.
The other technique involves incipient wetness impregnation, which is used
at an industrial scale for catalyst preparation. It involves preparing a solution of
the salt to be dispersed. The solution is then mixed with the substrate. It is then
absorbed by the substrate due to incipient wetness. After the substrate has imbibed
the solution containing the salt into its pore structure, the sample is heated to
remove the solvent. Care must be taken when selecting solvents for use in this
technique. First, the salt needs to be sufficiently soluble in the solvent to allow
enough salt to be dissolved in the volume of solution that can be imbibed by the
substrate pores. Second, the solvent selected needs to be able to wet the surface
of substrate. The preparation of the AgNO /SiO 2 is used again to illustrate this
3
technique (Padin and Yang, 2000). Because AgNO 3 is highly soluble in water,
water was used as the solvent. Also, the high affinity of SiO 2 for water also
ensures proper wetting of the surface. The pore volume and surface area of the
3 2
SiO 2 were 0.46 cm /g and 670 m /g, respectively. A 1.2 M solution of AgNO 3
was first prepared. A volume of solution equal to the total pore volume of the
SiO 2 support was mixed with the substrate such that an AgNO 3 /SiO 2 weight ratio
◦
equal or close to 0.32 was achieved. The sample was then heated for 4 h at 105 C
in air to remove the water. The ratio of AgNO 3 /SiO 2 in the resulting sample was
2
calculated at 0.27. The BET surface area of this sorbent was 398 m /g, which
indicated some pore plugging.

Water-Insoluble Salts. Many important salts for π-complexation are water-
+
insoluble. The best examples are cuprous (Cu ) salts, e.g., CuCl. The most
practical technique for preparing monolayer CuCl is by a two-step process: incip-
ient wetness impregnation of CuCl 2 followed by reduction to CuCl. This process
also applies to other cuprous salts, as the cupric salts are generally water-soluble.
Attempts have also been made for direct impregnation of CuCl. This could be
accomplished by two ways: using acid or basic solutions or dissolving CuCl
with the aid of ammonium chloride. These two techniques will be first briefly
described, and the two-step process will be then discussed in more details.
Cuprous salts are generally soluble in acid or base solutions. Thus, one could
impregnate a porous support with CuCl directly. For example, following the