Chemistry Reference
In-Depth Information
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Scheme 3.3 Homogeneous oxidation precipitation of RuO
2
on herringbone CNTs.
(HOP) method using H
2
O
2
as both oxidant and precipitant at room tem-
perature (Scheme 3.3). The resultant composite is a very active and selective
catalyst for the aerobic oxidation of alcohols to the corresponding aldehydes
and ketones.
76
In addition, CNT-supported ruthenium catalysts, assembled
at the interfaces of emulsion droplets, show excellent activity, selectivity and
stability for the aerobic oxidation of benzyl alcohol to benzaldehyde with
molecular O
2
or air in the presence of H
2
O.
77
After the reactions, the cata-
lysts can be easily separated and recycled by sedimentation. The activity of
RuO
2
/CNT for the oxidation of benzyl alcohol can be predicted by the spe-
cific capacitance (SC) of RuO
2
, suggesting that highly active RuO
2
nanoca-
talysts are also good supercapacitor materials with SC high up to 1500
Fg
RuO
2
1
.
78
White and co-workers synthesized RuO
2
nanoclusters (1 nm) in the
supercages of faujasite (FAU) zeolite (RuO
2
/FAU) and they showed high ac-
tivity and selectivity for the aerobic oxidation of various alcohols.
79
Although
bulk RuO
2
showed low activity (16% yield), benzyl alcohol was oxidized
quantitatively to benzaldehyde by RuO
2
/FAU (7.8 mol%) catalyst under the
same reaction conditions. No decrease in yield was observed after the fifth
run when using the recovered catalyst. Another interesting feature of the
zeolite-based catalysts derived from their uniform pore size is that they
behave as a shape-selective catalyst. It was shown that a competitive reaction
of benzyl alcohol over 9-hydroxyfluorene resulted in no formation of
9-fluorenone.
Polymer-supported perruthenate (PSP), prepared by anion exchange of
KRuO
4
with a basic anion-exchange resin (Amberlyst A-26), has emerged as a
versatile catalyst for the aerobic oxidation of alcohols.
80
However, the activity
was about four times lower than that of homogeneous TPAP and this catalyst
could not be recycled, which was attributed to oxidative degradation of the
polystyrene support.
A heterogeneous TPAP catalyst that could be recycled successfully and
displayed no leaching was developed by tethering the tetraalkylammonium
perruthenate to the internal surface of mesoporous silica (MCM-41).
81,82
It
was shown to catalyze the selective aerobic oxidation of primary and
secondary allylic and benzylic alcohols, but surprisingly, both cyclohexanol
and cyclohexenol were unreactive, although these substrates can easily be
accommodated in the pores of MCM-41 (Figure 3.9).
.
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