Page 206 - Adsorbents fundamentals and applications

8

π-COMPLEXATION

SORBENTS

AND APPLICATIONS

The π-complexation bond is typically a weak bond that can be formed between
the sorbent and sorbate. The sorbents that are used for separation and purifica-
tion based on π-complexation are called π-complexation sorbents. Development
of π-complexation sorbents began only recently. A number of such sorbents
have already been used commercially, and tremendous potential exists for future
applications in separation and purification, both for the chemical/petrochemical
industry and environmental applications. For this reason, π-complexation sor-
bents are discussed in a separate chapter.
All major industrial adsorption processes are based on van der Waals and elec-
trostatic interactions between the sorbate and sorbent. Chemical bonds have yet
to be exploited in a significant way. Chemical complexation has been studied and
used on a large scale in a number of other separation and purification processes
by using mass separating agents (King, 1980). As suggested by King (1987),
chemical complexation bonds are generally stronger than van der Waals interac-
tions (thus giving rise to higher selectivities), yet many of them are weak enough
to be reversible (i.e., to be broken by simple engineering means). This picture is
well illustrated by the bond-energy-bond-type diagram of Keller (Humphrey and
Keller, 1997). Indeed, a number of important separations have been proposed by
King and co-workers, who used solvents with functional groups to form reversible
chemical complexation bonds between the solute and solvent molecules (King,
1987). The π-complexation is a special class of chemical complexation. For π-
complexation sorbents, it pertains to the main group (or d-block) transition metals
(and there are 27 such elements). When interacting with a gas or solute molecule,
these metals and their ions can form the usual σ bonds with their s-orbitals and,
in addition, their d-orbitals can back-donate electron density to the antibonding
π-orbitals of the molecule to be bonded. The π-complexation has been seri-
ously considered for olefin/paraffin separation and purification by use of liquid

Adsorbents: Fundamentals and Applications, Edited By Ralph T. Yang
ISBN 0-471-29741-0 Copyright  2003 John Wiley & Sons, Inc.

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