Page 207 - Adsorbents fundamentals and applications

192 π-COMPLEXATION SORBENTS AND APPLICATIONS

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solutions containing silver (Ag ) or cuprous (Cu ) ions (Quinn, 1971; Ho et al.,
1988; Keller et al., 1992; Blytas, 1992; Eldridge, 1993; Safarik and Eldridge,
1998). It has also been considered seriously for CO separation by using Cu +
solutions, such as the COSORB process (Haase and Walker, 1974; Kohl and
Reisenfeld, 1979). Although gas/solid operations can be simpler as well as more
efficient than gas-liquid operations, particularly by pressure swing adsorption,
the list of attempts for developing solid π-complexation sorbents was a short
one until the recent work that will be described in this chapter. CuCl, which is
insoluble in water, has been considered for olefin/paraffin separations by using
CuCl powder as the sorbent (Gilliland et al., 1941; Long, 1972). The only suc-
cessful solid sorbent of this nature, before our recent work, was CuCl/γ -Al 2 O 3
for binding with the π bond of CO (Xie and Tang, 1990; Golden et al., 1992a;
Golden et al., 1992b; Kumar et al., 1993). It should also be noted that the com-
mercially available sorbents do not have significant selectivities for olefins (over
corresponding paraffins) and that the use of these sorbents (e.g., 13X zeolite, with
a olefin/paraffin separation factor of ∼1.3, Da Silva and Rodrigues, 1999) would
require additional, substantial operations (Kulvaranon et al., 1990; Jarvelin and
Fair, 1993; Ghosh et al., 1993; Da Silva and Rodrigues, 2001).
Efficient solid π-complexation sorbents have been developed recently (Hirai
et al., 1986b; Yokoe et al., 1987; Golden et al., 1992a; Yang and Kikkinides,
1995; Chen and Yang, 1996; Wu et al., 1999; Rege et al., 1998; Huang et al.,
1999a, 1999b; Padin et al., 1999; Padin and Yang, 2000; Yang et al., 2001) for
a number of applications in separation and purification. The bond between the
sorbent and sorbate needs to be strong. However, excessively strong bonds would
lead to either reaction or irreversible adsorption. Empirically, the adsorption is
“reversible” when the bond is below 15–20 kcal/mol, that is, desorption can be
achieved easily by simple engineering operations such as mild changes in pressure
and temperature. The bonding strength between sorbate and sorbent depends on:
• Emptiness of the outer-shell s-orbital of the cation that is on the sor-
bent surface;
• The amount of π electrons in the target adsorbate molecule and the ease
with which these π electrons can be donated to the s-orbital of the cation;
• The amount of d-orbital electrons of the cation and the ease with which
they can be donated to the adsorbate molecule.
Molecular orbital theory has been used to study π-complexation (Chen and
Yang, 1996; Huang et al., 1999a, 1999b; Yang et al., 2001; Takahashi et al., 2002).
Molecular orbital theory can also serve as an ideal tool for designing sorbents for
π-complexation for any given target adsorbate molecule. For this reason, a section
(Section 8.2) will be devoted to the basics of molecular orbital theory calculations.

8.1. PREPARATION OF THREE TYPES OF SORBENTS
Although cations of many of the d-block metals can be used for π-complexation,
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Ag and Cu have been used most frequently. To prepare good sorbents, these
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