toluene. Similar behaviour has been observed for [n-Pr4N]RuO4
in a homogeneous system.8
affect the activity of RuO2 in the oxidation of alcohols.9 The
mechanism of the enhancing effect of cobalt in alcohol
oxidation is not yet clear. The Ru–Co synergism has been found
in the homogeneous co-oxidation of alcohols and aldehydes4
and in the oxidation of alcohols on Ru–Co–Al hydrotalcite.10
From X-ray diffraction (XRD) and TGA data, the hydrous Ru–
Co (1 : 1.5) oxide could be approximated as a binary oxide
RuO2·1.5CoO(OH)·3–5H2O comprising the amorphous RuO2
hydrate and the crystalline cobaltic acid CoO(OH). It might also
include a mixed Ru–Co oxide phase. Detailed characterisation
of this catalyst is in progress.
Being highly active for alcohol-to-aldehyde oxidation (oxi-
dative dehydrogenation), Ru–Co oxide also accelerated the
consecutive oxidation of the aldehyde to acid (oxygenation), as
can be seen in Fig. 1. Hence a strict control of the reaction
course is required. Addition of a radical scavenger (e.g., 2,6-di-
tert-butyl-p-cresol) almost completely inhibited the over-
oxidation (Fig. 1; Table 1, entries 7 and 8).9 This is indicative of
a free radical mechanism for the aldehyde-to-acid oxidation.
The mechanism of alcohol oxidation on the hydrous Ru–Co
oxide may be viewed as an oxidative dehydrogenation involv-
ing the formation of a RuIV alkoxide intermediate from RuIV
hydroxo species, e.g. RuIV 2 OH + RCH2OH ? RuIV
2
OCH2R + H2O, followed by b elimination to give the aldehyde
and a ruthenium hydride species. The latter is then oxidised by
oxygen. Cobalt might play a significant role in the activation of
oxygen in catalyst reoxidation.
Financial support from Synetix, Quest International and
EPSRC (grant GR/R53760) is gratefully acknowledged. Thanks
are due to Johnson Matthey for their kind donation of ruthenium
compounds.
Notes and references
Fig. 1 Yield of cinnamaldehyde vs. time for the aerobic oxidation of
cinnamyl alcohol (2.5 mmol) catalysed by RuO2 or by Ru–Co (1 : 1.5) oxide
with and without radical scavenger (0.11 mmol) in toluene (10 ml) at 110 °C
(alcohol : Ru = 10 : 1).
†
Experimental. All alcohols were used as received without further
purification. Solvents were dried over 4 Å molecular sieves. Ru dioxide
hydrate was prepared by precipitation from 0.2 M aqueous solution of
RuCl3 with 1 M NaOH at pH 10. The RuIV–CoIII binary oxides were
prepared similarly by co-precipitation of 0.2 M RuCl3 solutions containing
appropriate amounts of CoCl2. The suspensions were aged with stirring for
2 h, filtered off, washed with water until Cl2 was removed (test with
AgNO3) and finally dried at 60 °C/0.5 Torr for 2 h. During the preparation,
RuIII was oxidised to RuIV and CoII to CoIII by air. The oxidation of alcohols
was carried out in a 50 ml round-bottom three-neck glass flask equipped
with a reflux condenser, a magnetic stirrer and a gas inlet allowing bubbling
a flow of oxygen or air (25 ml min21) into the reaction mixture. Because it
is inherently unsafe to mix oxygen with hot organics, appropriate
precautions should be taken when carrying out this work, particularly if
scaling it up. Typically, a mixture of an alcohol (2.5 mmol), Ru catalyst
(alcohol : Ru = 10 : 1 mol/mol) and decane (GC internal standard) in
toluene (10 ml) was charged in the reactor and saturated with oxygen at
room temperature for 5 min while being intensely stirred. Then the reactor
was placed into the oil bath preheated to a certain temperature to start the
reaction. Samples of the reaction mixture were taken out at appropriate time
intervals to monitor the reaction by GC.
The Ru–Co oxide was also found to be active for the aerobic
oxidation of non-activated saturated and unsaturated primary
alcohols such as 1-dodecanol and 9-decenol to the correspond-
ing aldehydes (entries 10 and 11). In the latter case, no double
bond migration was observed. It should be noted, however, that
the cobalt-free catalysts, Ru–hydroxyapatite and Ru/Al2O3,
give higher selectivities to aldehydes in the oxidation of
saturated primary alcohols.11,12
It should be pointed out that only hydrous oxides, RuO2 or
Ru–Co, were catalytically active (Fig. 2). These were obtained
by a mild thermal pre-treatment of the precursor hydroxides at
60 °C/0.5 Torr/2 h. From thermogravimetric analysis (TGA),
the active oxides contained 3–5 water molecules per Ru atom.
Thoroughly dehydrated oxides were inactive in the aerobic
oxidation of alcohols. The hydrous RuO2 has been reported to
be different from the anhydrous form.15 As shown by powder
XRD, the hydrous RuO2 was amorphous; it had a Brunauer–
Emmett–Teller (BET) surface area of ca. 200 m2 g21. After
dehydration at 130 °C/10 h, the Ru dioxide transformed to the
inactive crystalline RuO2 with the rutile structure, which had a
low surface area (ca. 10 m2 g21), in agreement with the
literature.15 The hydrous RuO2 has been found to chemisorb a
significant amount of oxygen, whereas the anhydrous form has
little chemisorbed oxygen.15 These differences might greatly
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10 T. Matsushita, K. Ebitani and K. Kaneda, Chem. Commun., 1999,
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11 (a) K. Yamaguchi, K. Mori, T. Mizugaki, K. Ebitani and K. Kaneda, J.
Am. Chem. Soc., 2000, 122, 7144; (b) H. Ji, K. Ebitani, T. Mizugaki and
K. Kaneda, Catal. Commun., 2002, 3, 511; (c) H. Ji, K. Ebitani, T.
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14 A. Beloch, B. F. G. Johnson, S. V. Ley, A. J. Priece, D. S. Shephard and
A. W. Thomas, Chem. Commun., 1999, 1907.
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H. Rand, J. Chem. Soc. A, 1968, 653.
Fig. 2 Effect of pre-treatment temperature (0.5 Torr, 2 h) on catalytic
activity of hydrous RuO2 and Ru–Co (1 : 1.5) oxide in the oxidation of
cinnamyl alcohol by O2 in toluene (110 °C, alcohol : Ru = 10 : 1, 2 h for
RuO2 and 0.5 h for Ru–Co oxide).
CHEM. COMMUN., 2003, 1414–1415
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