Chemistry Reference
In-Depth Information
imidazolium salt, [Imim-TEMPO][FeCl
4
], containing cooperative function-
alities (see also Chapter 6, Scheme 6.8) was easily synthesized and applied to
the selective aerobic oxidation of aromatic alcohols. In each run, [Imim-
TEMPO][FeCl
4
] could be readily recovered by extraction with diethyl ether,
then subjected to a subsequent run by charging with fresh substrate and
NaNO
2
. The catalyst could be reused at least five times with retention of high
activity and selectivity.
Bimetallic nanoclusters have also been found to work well in solvent-free
conditions, as reported by Hutchings and co-workers using Au/TiO
2
alloyed
with Pd
26
(see also Chapters 5 and 7).
d
n
4
r
4
n
g
|
6
10.7.5 Supercritical Carbon Dioxide
Supercritical carbon dioxide (scCO
2
) has been demonstrated to offer inter-
esting opportunities as a medium for performing various catalytic reactions.
In addition to its benign nature, the good miscibility with gaseous reagents
and organic compounds allows remarkable accelerations of many reactions
due to increased diffusion rates.
27
A representative example of aerobic oxidation in scCO
2
was reported by
Leitner and co-workers.
28
They found that the giant palladium cluster
[Pd
561
phen
60
(OAc)
180
], dispersed in poly(ethylene glycol) (PEG), eciently
catalyzes the aerobic oxidation of alcohols in scCO
2
(Scheme 10.4).
The use of scCO
2
with Pd as catalyst is discussed in Chapter 4. In
particular, the use of this alternative solvent in continuous-flow micro-
reactors is described in Section 4.2.2, and an example of the oxidation of
cinnamyl alcohol is reported in Section 4.2.5.
.
10.8 Catalyst Recovery
The main advantage of heterogeneous over homogeneous catalysts is the
easy recovery by simple filtration of the catalyst and the opportunity to reuse
it. This is probably one of the main reasons why industry prefers hetero-
geneous catalysts. However, there is another interesting approach, that
offers promising opportunities. This consists in the immobilization of the
homogeneous catalysts in multiphase operation (liquid-liquid biphasic
catalysis).
29
Examples of this technique have already been provided in
previous sections dealing with fluorinated solvents, ionic liquids and scCO
2
.
Some other interesting examples of catalyst recovery working in a homo-
geneous phase are briefly mentioned in this section.
OH
O
O
2
/scCO
2
Pd
561
phen
60
(OAc)
180
PEG-1000
R
1
R
2
R
1
R
2
Scheme 10.4 Oxidation with palladium nanoparticles in scCO
2
.
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