Chemistry Reference
In-Depth Information
catalysts. This section will examine strategies and classes of catalysts encapsulated
within microcapsules. Generally, there are two ways encapsulated catalysts may act
in a reaction.
First, the catalyst is meant to leach out of the capsules into a reaction solution. In
this case, the capsules are not meant to break open but are semipermeable to the cata-
lyst, which diffuses into the reaction mixture over time. This method is typically used
for metal catalysts or catalyst precursors where the metals leach out and perform the
desired reaction. This method is useful because metal-catalyzed reactions typically
require lower catalyst loading than organocatalysts (,1 mol%), and highly loaded
capsules can be isolated and reused until exhausted. Such metal catalysts are often
touted for their decreased pyrophoricity relative to such catalysts as palladium on
carbon (Coleman and Royer 1980; Bremeyer et al. 2002). One could simply use
resins, microspheres, or other solid supports as catalyst reservoirs, but capsules are
well suited because of their inherently higher surface areas (Royer et al. 1985;
Wang et al. 2006).
Second, the catalyst is meant to be site isolated. Site isolation implies that the catalyst
is not meant to leave the interior of the capsule where all catalysis occurs. This requires
that the catalyst be kept inside the capsule either by anchoring it to the inner shell wall or
by anchoring it to something that cannot diffuse out through the shell under reaction
conditions, like a polymer chain. Encapsulated enzymes fall into this category,
because they are typically much larger than substrates and products, which can
diffuse in and out of the capsules. The reasons for pursuing such materials are to
facilitate catalyst recycling, to create a unique microenvironment inside the capsule,
or to protect the catalyst from other fouling reagents throughout the reaction.
There are several ways to determine if a catalyst leaches or remains inside the
capsule. It must first be determined by microscopy whether the capsule shell walls
have ruptured. If the shell walls remain intact, strategies exist to determine if the cata-
lysis occurs outside the capsule or within it. Of the many approaches (Hagen et al.
2005), we favor the “three-phase test” illustrated in Figure 8.2 (Rebek and Gavina
1974; Rebek et al. 1975; Rebek 1979; Hagen et al. 2005). One of the substrates in
the reaction is bound to a resin and the reaction is carried out in the presence
of the encapsulated catalyst. If the bound substrate undergoes reaction it is
assumed that the catalyst has leached into solution, because the resin-bound substrate
should not have been able to interact with the encapsulated catalyst (Davies et al.
2001; Steel and Teasdale 2004; Okamoto et al. 2005; Broadwater and McQuade
2006). To prevent false positives, the homogeneous reaction should be carried out
in the same reaction vessel. This also serves as an internal control experiment to
show if the reaction with encapsulated catalyst works at all.
Third and finally, one can use the catalytic capsules in a reaction or Soxhlet extract
them in an appropriate solvent and then analyze the supernatant of the reaction for
the presence of catalyst after the solvent and capsules have been removed. Some
form of spectroscopy (NMR, IR, etc.) or elemental analysis is appropriate for this
(Price et al. 2006). This type of evidence is, of course, circumstantial because the
absence of catalyst does not necessarily prove that it did not return to the capsules
before capsule removal.
