Chemistry Reference
In-Depth Information
atmosphere gave p-tolualdehyde in 97% yield without additives. After
treatment with aqueous K
2
CO
3
, the catalyst was reused at least five times
without a decrease in activity.
Ruthenium supported on ceria (CeO
2
) is an effective catalyst for the aer-
obic oxidation of alcohols.
86,87
Primary and secondary alcohols were oxi-
dized to the corresponding aldehydes (carboxylic acids) and ketones,
respectively, above 140 1C. Surprisingly, allylic alcohols, such as geraniol and
some cyclic alcohols, for example, menthol, were unreactive. The results
suggest that low-valent ruthenium species are possibly involved and that
coordination of the double bond to ruthenium inhibits alcohol oxidation. A
RuO
2
/CeO
2
catalyst with large surface area derived from meso- and macro-
pores had well-dispersed ruthenium species
88
and showed high activities,
much higher than that of Ru/CeO
2
prepared by a conventional coprecipita-
tion method. A trimetallic Ru/CeO
2
/CoO(OH) compound acted as a highly
ecient catalyst for the aerobic oxidation of various alcohols to the corres-
ponding carbonyls, including less-reactive primary alcohols.
89,90
Character-
ization of the catalyst using the K-edge X-ray absorption technique revealed
that a monomeric Ru(
IV
) species was immobilized on the surface of the CeO
2
/
CoO(OH) support. The addition of manganese species as a promoter re-
sulted in a more active alcohol oxidation catalyst than the comparable co-
balt-doped material. The application of Ru/Mn/Ce oxides on redox-active
supports, such as TiO
2
or CeO
2
, by deposition-precipitation caused a further
improvement in catalytic activity. Similarly, a Ru/MnO
x
/CeO
2
catalyst was
prepared by wetness impregnation of MnO
x
/CeO
2
precatalyst with an aque-
ous solution of RuCl
3
nH
2
O.
91
The oxidation of alcohols to aldehydes and
ketones proceeded smoothly with this catalyst at 300 K under an O
2
atmosphere.
Ruthenium-exchanged hydrotalcites (Ru-HT) were found to be ecient
heterogeneous catalysts for the aerobic oxidation of reactive allylic and
benzylic alcohols.
92,93
Hydrotalcites are layered anionic clays consisting of a
cationic brucite layer with anions (hydroxide or carbonate) situated in the
interlayer region. Various cations, such as Ru, Fe, Ni, Mn, V and Cr, can be
introduced into the brucite layer by ion exchange. Among them, Ru-HT with
the formula Mg
6
Al
2
Ru
0.5
(OH)
16
CO
3
showed the highest activity; it was
prepared by treating an aqueous solution of RuCl
3
nH
2
O, MgCl
2
6H
2
O
and AlCl
3
H
2
O with a solution of NaOH and Na
2
CO
3
followed by heating
(60 1C, 18 h), filtration, washing with water and drying (110 1C, 12 h). The
catalytic activity of Ru-HT was significantly enhanced by further introduction
of cobalt(
II
) into the brucite layer.
94
For example, cinnamyl alcohol under-
went complete conversion in 40 min at 60 1C in the presence of Ru/Co-HT,
compared with 31% conversion under the same reaction conditions with
Ru-HT. The trimetallic Ru/Mn
2
-HT was prepared, consisting of Ru
IV
and
Mn
IV
cations on the HT surface (Figure 3.10).
95
In the absence of Ru, no
catalytic activity of Mn
2
-HT was observed. Benzyl alcohol was smoothly
oxidized to benzaldehyde in the presence of RuMn
2
-HT (3 mol%) catalyst
and the catalyst was reused successfully for a further three runs.
d
n
4
r
4
n
g
|
0
.
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