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3.2 Heterogeneous Ruthenium-Based Catalysts
3.2.1 Introduction
As described in Section 3.1, many homogeneous ruthenium-based catalysts
have been developed for the aerobic oxidation of alcohols to the corres-
ponding aldehydes and ketones and they are usually dissolved in reaction
solutions making all catalytic sites accessible to the substrates, which
realized high catalytic activity and selectivity. Despite these advantages,
homogeneous catalysts have a share of only about 20% in industrial
processes,
57
since the recovery and reuse of the expensive catalysts are often
dicult. In this respect, the development of easily recoverable and recyclable
heterogeneous catalysts has attracted considerable research interest and
a number of solid-supported catalysts such as ruthenium, palladium,
platinum and gold have been investigated and reported.
58,59
The main
purpose of this section is to provide an analysis of recent advances in
research into the heterogeneous ruthenium-catalyzed oxidation of alcohols
with molecular O
2
and the different nature of the catalysts to obtain the
desired products in an ecient manner, which are helpful in designing
guidelines for next-generation catalysts and developments in this field.
d
n
4
r
4
n
g
|
0
3.2.2 Heterogeneous Ruthenium-Catalyzed Aerobic Oxidation
of Alcohols
Among the oxides of ruthenium, RuO
4
is well known as a powerful oxidizing
agent of alcohols. It is, however, too strong to be used for the selective
oxidation of allylic alcohols to the corresponding unsaturated carbonyl
compounds. A lower oxidation state than RuO
4
, namely RuO
2
hydrate, acts
as an oxidizing agent and a catalyst for the aerobic oxidation of allylic
alcohols with higher eciency than MnO
2
.
60,61
A zeolite was used to confine
RuO
2
hydrate nanoparticles in its cages, and both saturated and unsaturated
alcohols were aerobically oxidized with high activity and selectivity.
62
Zeolite-
confined (ZC) nanostructured dinuclear ruthenium clusters (Figure 3.6) were
prepared by the following simple three-step procedure: (i) ion exchange of
Ru
31
ions with the extra-framework Na
1
ions in zeolite-Y, (ii) reduction of
.
OH
OH
HO
OH
HO
OH
Ru
Ru
O
O
Si
Al
Si
A
l
O
O
O
O
O
O
O
O
Figure 3.6 Local structure of zeolite-confined (ZC) Ru obtained from Ru K-edge
EXAFS analyses.
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