Chemistry Reference
In-Depth Information
Figure 8.2 Three-phase test.
8.3.2. Encapsulated Catalysts
We chose to organize the discussion of catalyst encapsulation by separating examples
into catalyst type as opposed to encapsulation method. As previously, we do not
intend to present a comprehensive treatment of the literature and discuss only
examples we consider particularly illustrative.
Metal Catalysts.
An early example of encapsulated metal catalysts are the
Pd-containing “ghosts” developed by Garfield Royer's group in the late 1970s
to early 1980s. The process used colloidal templating (described in Section 8.2.4)
to form capsules using porous alumina beads (Meyers and Royer 1977; Royer
1982) and later porous silica beads (Coleman and Royer 1980) to assemble
a polyethyleneimine (PEI) shell. Specifically, PEI-600, which has the form
(—CH
2
CH
2
NH
X
—)
n
, was dissolved in methanol and allowed to self-assemble
onto porous silica beads (400-mm diameter). The coated beads were exposed to glu-
taraldehyde to cross-link the polymer, then more PEI, and finally sodium borohydride
to reduce the imines formed by the cross-linking. The silica template was dissolved
away by treatment with a concentrated sodium hydroxide solution. To add catalytic
Pd(0) to the microcapsules (ghosts), the shells were soaked in a 1 M hydrochloric
acid solution of PdCl
2
, followed by neutralization and reduction of the palladium
to Pd(0) with sodium borohydride. The catalysts were shown to be active in palla-
dium-catalyzed transfer hydrogenolysis and were recycled 10 times without loss of
activity (Coleman and Royer 1980). As of this writing, they are commercially
available as the Royer
w
catalyst, with analogous platinum and rhodium versions.
