Chemistry Reference
In-Depth Information
There is no doubt that in recent years gold-based catalysts have attracted
the attention of researchers as catalysts for both liquid- and gas-phase oxi-
dations.
26,32-42
Also in the case of alcohol oxidation, when used either in the
form of unsupported nanoporous Au or as supported nanoparticles (NPs),
these catalysts show peculiar reactivity properties. In general, high selectivity
to acetaldehyde is shown, with remarkably low selectivity to CO
2
, which is
unexpected because of the generation of highly electrophilic O species on
the Au surface when the NPs are supported over conventional materials,
such as silica or titania. However, depending on the reaction conditions and
catalyst type, the prevailing product can be either acetic acid (in this case,
however, there is typically also greater selectivity to CO
2
, because of the
higher temperatures needed) or ethyl acetate (formed by consecutive di-
merization of acetaldehyde). The behavior is different from that shown in
liquid-phase oxidation with Au-based catalysts, which may provide fairly
high selectivity to acetic acid and ethyl acetate (by esterification of acetic acid
with ethanol) at moderate temperature and pressure (Scheme 8.4).
43,44
For example, nanoporous Au, prepared by the dealloying of Au-Ag alloys,
has a stable structure that is active without any support. It catalyzes the
selective oxidative coupling of methanol to methyl formate with selectivity
above 97% and high turnover frequency at temperatures below 80
d
n
4
r
4
n
g
|
3
1C,
1/2 O
2
O
OH
O
1/2 O
2
-H
2
O
OH
OH
2O
2
O
-H
2
O
.
-H
2
O
O
2CO
2
+ H
2
O
O
O
OH
OH
-H
2
O
H
2
O
1/2 O
2
OH
O
O
1/2 O
2
-H
2
O
1/2 O
2
OH
OH
OH
-H
2
O
O
OH
1/2 O
2
-H
2
O
O
O
Scheme 8.4 Main reactions occurring during the gas-phase (top) and liquid-phase
(bottom) oxidation of ethanol.
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