Selective oxidation of alcohols with molecular oxygen over Ru/CaO-ZrO2 catalyst

Article abstract of DOI:10.1016/j.molcata.2010.03.018

Selective oxidation of alcohols to carbonyl compounds with molecular oxygen was carried out over ruthenium supported on a CaO-ZrO₂ solid solution prepared by the co-precipitation method. In the oxidation of benzyl alcohol, the Ru/CaO-ZrO₂ catalyst gave benzaldehyde in a yield higher than 98% at 90 °C, and the turnover frequency reached 224 h^-1. The Ru/CaO-ZrO₂ catalyst also exhibited high catalytic activities and selectivities to carbonyl compounds in the oxidation of aromatic ring-substituted benzylic, allylic, and aliphatic alcohols. Moreover, this catalyst exhibited high activities in the oxidation of alcohols at a low temperature (40 °C). The catalytic activity and oxidation state of ruthenium depended on the Ca/Zr molar ratio of the support, and the highest catalytic activity was obtained with Ca/Zr = 0.125. DRIFT and XPS analyses revealed that Ruⁿ⁺-OH (n = 3, 4) on the surface of CaO-ZrO₂ were likely the active species in the oxidation of alcohols.

Full text of DOI:10.1016/j.molcata.2010.03.018

Journal of Molecular Catalysis A: Chemical 323 (2010) 7–15
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Editors choice paper
Selective oxidation of alcohols with molecular oxygen over Ru/CaO–ZrO2 catalyst
Takashi Yasu-eda, Susumu Kitamura, Na-oki Ikenaga, Takanori Miyake, Toshimitsu Suzuki∗
Department of Chemical Engineering and High Technology Research Center, Kansai University, 3-3-35 Yamate Suita, Osaka 564-8680, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective oxidation of alcohols to carbonyl compounds with molecular oxygen was carried out over
ruthenium supported on a CaO–ZrO2 solid solution prepared by the co-precipitation method. In the
oxidation of benzyl alcohol, the Ru/CaO–ZrO2 catalyst gave benzaldehyde in a yield higher than 98% at
90 ◦C, and the turnover frequency reached 224 h−1. The Ru/CaO–ZrO2 catalyst also exhibited high catalytic
activities and selectivities to carbonyl compounds in the oxidation of aromatic ring-substituted benzylic,
allylic, and aliphatic alcohols. Moreover, this catalyst exhibited high activities in the oxidation of alcohols
at a low temperature (40 ◦C). The catalytic activity and oxidation state of ruthenium depended on the
Ca/Zr molar ratio of the support, and the highest catalytic activity was obtained with Ca/Zr = 0.125. DRIFT
and XPS analyses revealed that Run+–OH (n = 3, 4) on the surface of CaO–ZrO2 were likely the active
species in the oxidation of alcohols.
Received 16 January 2010
Received in revised form 10 March 2010
Accepted 12 March 2010
Available online 21 March 2010
Keywords:
CaO–ZrO2 solid solution
Ruthenium
Heterogeneous catalyst
Basic support
© 2010 Elsevier B.V. All rights reserved.
Oxidative dehydrogenation
1. Introduction
In general, Ru-hydroxide species (or hydrated RuOx species) are
active in the oxidation of alcohols, although RuO2 exhibits no cat-
The selective catalytic oxidation of alcohols to the correspond-
ing carbonyl compounds is one of the simplest and most important
reactions in organic synthesis [1]. The oxidation has long been per-
formed using a stoichiometric amount of various oxidants. From a
green chemistry perspective, selective oxidation of alcohols using
molecular oxygen has attracted much attention (Eq. (1)) [2–4]:
alytic activity [15–23]. Yamaguchi et al. reported that coordination
number of nearest-neighbor Ru atoms closely related to the activ-
ity of Ru-loaded TiO2 catalyst [24,25]. It has been reported that a
reaction of RuCl3 with basic sites on the support could generate
Ru-hydroxide species, which in turn act efficient active sites for
the oxidation of alcohols [20,26,27]. In addition, it is known that
the basicity of the support plays an important role in the reaction
[28–31]. As such, supports with a tunable basicity such as hydrotal-
cite or hydrotalcite-like mixed metal oxides seem to be preferable
for the oxidation of alcohols [20,28–32].
(1)
Zirconium dioxide has been used in many catalytic reactions as
a support material [33]. It is well known that the ZrO2 can be sta-
A wide range of homogeneous metal catalysts has been found to
be useful for the oxidation of alcohols with molecular oxygen [5–8].
The use of heterogeneous catalysts, however, offers obvious advan-
tages over homogeneous systems with respect to ease of handling
and catalyst recycling. A wide range of supported platinum and
palladium catalysts has long been reported to exhibit high catalytic
performance in the oxidation of alcohols [2,9–11]. Pertinent review
articles summarized recent development in this field [12,13].
Much attention has been paid to the application of supported
ruthenium catalysts in alcohol oxidation. Various ruthenium-
based catalysts such as Ru-exchanged hydroxyapatite (RuHAP)
[14], Ru/Al2O3 [15–17], RuO2/FAUzeolite [18], Ru-Co/Al2O3 [19],
RuMn2/hydrotalcite [20], RuCoHAP [21,22], RuHAP-␥-Fe2O3 [23],
and Ru/TiO2 [24,25] have been reported to be active and selective
for benzylic, allylic, and aliphatic alcohols.
bilized by admixing with lower valence cations such as Y3+, La3+
,
Mg2+, and Ca2+. Replacement of the Zr4+ cation site with these
cations leads to the formation of solid solutions and increases the
number of oxygen vacancies [34]. Stabilized ZrO2 has been reported
to contain surface hydroxy groups that may behave as basic sites
[35]. In particular, incorporation of Ca2+ into ZrO2 leads to the for-
mation of a homogeneous solid solution (CaO–ZrO2) with a strong
basic site. The basicity of the CaO–ZrO2 solid solution can be con-
trolled by changing its Ca/Zr molar ratio [36]. Various reactions
using CaO–ZrO2 solid solution-based catalyst such as autothermal
reforming of CH4 [37], synthesis of dimethyl carbonate and diethyl
carbonate [38,39], CO2 reforming of CH4 [40], and steam reforming
of ethanol [41] have been reported. In organic syntheses, however,
no CaO–ZrO2 solid solution-based catalysts have been reported.
In this report, we focus on the application of CaO–ZrO2 solid
solution as a support for the selective oxidation of alcohols to car-
bonyl compounds with molecular oxygen. We have found that
∗
Corresponding author. Tel.: +81 6 6368 1121x6807; fax: +80 6 6388 8869.
E-mail address: tmsuzuki5@yahoo.co.jp (T. Suzuki).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.03.018

8
T. Yasu-eda et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 7–15
ruthenium supported on CaO–ZrO2 affords a high catalytic activity
with a high selectivity to carbonyl compounds. To the best of our
knowledge, this work is the first report of liquid-phase oxidation
using CaO–ZrO2 solid solution-based catalysts.
Raman spectra were obtained with a Jasco model NSR-3000
laser Raman spectrometer using 523-nm diode laser excitation
with a CCD detector.
The temperature-programmed desorption of CO2 (CO2-TPD)
was performed with an automatic temperature-programmed des-
orption apparatus equipped with a quadrupole mass spectrometer
as a detector (TPD-1-AT, Bel Japan Inc.). The sample was first treated
at 700 ◦C for 30 min in flowing He. After cooling down to 100 ◦C
in He, CO2 was adsorbed onto the sample and was evacuated to
remove excess CO2. After cooling down to 50 ◦C in He, desorption
of CO2 was monitored at a heating rate of 5 ◦C/min to 850 ◦C in
flowing He.
Diffuse reflectance infrared spectra (DRIFT) were recorded on a
Jeol JIR 7000 in a diffuse reflectance mode. A KBr standard powder
was used as the reference. Sixty-four scans were accumulated for
each spectrum at a resolution of 4 cm−1, and observed spectra were
converted into Kubelka-Munk function units.
2. Experimental
2.1. Catalyst preparation
CaO–ZrO2 solid solution was prepared by co-precipitation
method. ZrO(NO3)2·2H2O (4.5 g; Wako Pure Chemical Industries
Ltd.) and a desired amount of Ca(NO3)2·4H2O (Wako Pure Chem-
ical Industries Ltd.) were dissolved into 100 mL of water. The pH
of the resulting solution was slowly adjusted to 13 by an addition
of an aqueous solution of 1 mol/L NaOH under vigorous stirring.
After 1 h, the resultant mixture was separated by centrifugation
and the residue was washed with a large amount of water till neu-
tral (pH < 8), and it was dried at 110 ◦C overnight. The solid mass
was crushed and calcined at 600 ◦C for 5 h in air. Although the order
of the addition of NaOH solution to the solution of Ca and Zr salts
was reversed to the description in the literature [42], XRD patterns
of the calcined oxides were exactly the same as those reported in
the literature [42].
X-ray photoelectron spectroscopy (XPS) data were obtained on
a Jeol JPS-9000MX using Mg K␣ radiation. Powder samples were
pressed and attached on carbon tape and mounted on a sample
holder (Pt). The peaks were calibrated using the binding energy of
Pt 4f7/2 (70.9 eV).
A Jeol JEM2010 was used to obtain high-resolution transmission
electron microscope (HR-TEM) images at 200 kV.
RuCl3·nH2O (Ru 38.45 wt%, Mitsuwa Chemicals Co. Ltd.) was
used as a ruthenium source. Ruthenium was introduced to the sup-
port (1.0 g) with 30 mL of an aqueous RuCl3 solution (5.1 × 10−3 M)
for 4 h at room temperature under stirring. After 4 h, the solid
powder was again separated by centrifugation and washed with
a large amount of water until no trace of Cl− could be detected
by AgNO3 test. The powder remained was dried at room tempera-
ture under vacuum overnight and typically used without further
pretreatment. The amount of ruthenium in the remaining solu-
tion was analyzed using inductively coupled plasma (ICP, Shimadzu
ICPS-7510) to calculate the loading level of ruthenium on the cata-
lyst. Ruthenium-supported hydrotalcite (Mg6Al2(OH)16CO3·4H2O,
Wako Pure Chemical Industries Ltd.) catalyst was prepared by the
same procedure.
3. Results and discussion
3.1. Characterization of CaO–ZrO2 solid solution by XRD, Raman,
and CO2-TPD
Fig. 1 shows the results of XRD analyses of CaO–ZrO2 mixed
oxides with different Ca/Zr molar ratios (in charged Ca and Zr).
Monoclinic (m-ZrO2) and tetragonal (t-ZrO2) structures exist on
pure ZrO2 (Fig. 1a). However, with the addition of Ca to ZrO2
2.2. Oxidation of alcohols with molecular oxygen
The oxidation of alcohol was carried out in a 100 mL three-neck
flask fitted with a reflux condenser. A catalyst (60–120 mg) was
placed in the reactor and the reactor was purged with oxygen. Then
1–2 mmol of substrate, 5 mL of solvent (o-xylene) and 1–2 mmol
of anisol (an internal standard: only in the cases to obtain time
conversion profiles otherwise after the reaction) were introduced
into the reactor through a septum. The reactor was immersed into
a preheated oil bath, and stirring was started. The reaction was
carried out by feeding pure oxygen directly into the liquid phase
(10 mL/min).
After the reaction, the catalyst was separated by centrifuga-
tion. The products were analyzed by an FID-GC (Shimadzu model
GC-18A) equipped with a 0.53 mm × 30 m capillary column of
DB-WAXETR and were identified by a GC–MS (Shimadzu model
QP2010) equipped with a 0.22 mm × 25 m capillary column of BP-
21.
2.3. Catalyst characterization
Surface area of the supports and the catalysts were measured by
the BET method using N2 at −196 ◦C with an automatic Micromerit-
ics Gemini 2375.
X-ray diffraction (XRD) measurements were performed with
a
Rigaku RINT-TTR III with monochromatized Cu K␣ radia-
Fig. 1. XRD patterns of CaO–ZrO2 solid solutions having various Ca/Zr molar ratios
tion.
(numerals indicate molar ratio).

T. Yasu-eda et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 7–15
9
Fig. 2. High-resolution XRD patterns of CaO–ZrO2 solid solutions having various
Ca/Zr molar ratios (2Â = 29–31◦).
Fig. 3. Raman spectra of CaO–ZrO2 solid solutions having various Ca/Zr molar ratios.
With increasing Ca/Zr molar ratios, the amounts of adsorbed CO2
and the desorption temperatures clearly increased. The result indi-
cates that the strong basic property of CaO affected the solid base
characteristics of ZrO2 [36].
(Ca/Zr = 0.1–0.25 mol/mol), no monoclinic structure existed on the
CaO–ZrO2 mixed oxides, and only the tetragonal structure was
observed (Fig. 1b–e). In these samples, CaO, Ca(OH)2, and CaCO3
could not be detected by XRD, though, with increasing Ca/Zr molar
ratios, the diffraction peaks of t-ZrO2 broadened. These results are
in agreement with those observed in a CaO–ZrO2 solid solution
prepared by the glycothermal method [37]. It was reported that
the formation of a homogeneous solid solution prevented a phase
transformation from t-ZrO2 to m-ZrO2 and the growth of the crys-
tallite size.
High-resolution XRD analyses of CaO–ZrO2 between 2Â = 29–31◦
were carried out, and the results are shown in Fig. 2. The peak
at 2Â = 30.2◦, corresponding to the (1 1 1) reflection, shifted to a
slightly lower angle with increases in the Ca/Zr molar ratio. Similar
behavior has been observed in Y3+-stabilized ZrO2 [43], and also
CaO–ZrO2 solid solution [42]. The replacement of the Zr4+ cation
with the Ca2+ cation slightly changes the lattice parameters of t-
ZrO2. Accordingly, we assumed that homogeneous CaO–ZrO2 solid
solution could be prepared by the precipitation method employed
here.
Raman analyses of CaO–ZrO2 solid solution with various Ca/Zr
molar ratios are shown in Fig. 3. Raman peaks of m-ZrO2 (at 178,
190, 222, 333, 382, 501, 539, 557 and 617 cm−1) and t-ZrO2 (at
268, 317, 475 and 640 cm−1) were clearly observed on the pure
ZrO2 (Fig. 3a) [44]. In contrast, only very broad peaks corresponding
to t-ZrO2 were observed on the CaO–ZrO2 solid solutions, and the
peaks were further broadened with increases in the Ca/Zr molar
ratio (Fig. 3b–e). The broadening of peaks might be ascribed to the
larger number of oxygen vacancies existing in the CaO–ZrO2 solid
solutions.
In order to clarify the surface basicity of the CaO–ZrO2 solid solu-
tions, the CO2–TPDs were performed and the results are shown
in Fig. 4. The pure ZrO2 had a small desorption peak at temper-
atures between 50 and 350 ◦C (Fig. 4a), which could be ascribed
to the small number of weak basic sites on the ZrO2 surface [36].
Fig. 4. CO2-TPD profiles of CaO–ZrO2 solid solutions having various Ca/Zr molar
ratio. CO2 adsorption: CO2 100 Torr, 100 ◦C, 30 min; desorption: He 50 cm3/min,
5
◦C/min.

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T. Yasu-eda et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 7–15
Table 1
It was found that the catalyst pretreatment with air or hydrogen
Surface areas and Ru loading level of catalysts.
at 300 ◦C greatly decreased the catalytic activity of Ru/CaO–ZrO2
catalyst (entry 7 and 8). Opre et al. have reported that the pre-
treatment temperature of the catalyst is critical in the oxidation of
benzyl alcohol with RuHAP and RuCoHAP catalysts due to the dehy-
dration of active ruthenium species such as Ru(OH)2+ [21]. The loss
of catalytic activity in the Ru/CaO–ZrO2 catalyst with pretreatment
at an elevated temperature can be ascribed to the dehydration of
active ruthenium species.
Catalyst
Ca/Zra (mol/mol)
BET (m2/g)
Support
Ru (wt%)
Catalyst
Ru/ZrO2
0
42
125
127
138
132
–
132
130
138
140
<0.3
0.8
1.5
1.5
1.5
Ru/CaO–ZrO2
Ru/CaO–ZrO2
Ru/CaO–ZrO2
Ru/CaO–ZrO2
0.100
0.125
0.167
0.250
a
Ca/Zr charged molar ratio.
3.4. Characterization of catalysts by XRD, HR-TEM, DRIFT, and
XPS
3.2. Immobilization of Ru on CaO–ZrO2 solid solution
In the XRD analyses, no spectral differences between
ruthenium-immobilized CaO–ZrO2 catalysts and bare CaO–ZrO2
solid solution were observed in any of the samples. The HR-TEM
image of Ru/CaO–ZrO2 catalyst was the same as that of bare
CaO–ZrO2 solid solution, and no particles of ruthenium species
were identified. These results seem to indicate that ruthenium
species are highly dispersed on the CaO–ZrO2 solid solution.
In order to clarify the ruthenium species, DRIFT analyses were
performed. A support having a Ca/Zr molar ratio of 0.125 was
employed in this measurement. The left panel of Fig. 5 shows
the DRIFT spectra of CaO–ZrO2 solid solution (a), Ru/CaO–ZrO2
as-prepared (b), Ru/CaO–ZrO2 after treatment with D2O at room
temperature for 18 h (c), and Ru/CaO–ZrO2 after treatment in air (d)
and in H2 at 300 ◦C (e). The difference spectra between CaO–ZrO2
solid solution and respective samples are illustrated in the right
panel of Fig. 5(f–i). The bare CaO–ZrO2 solid solution showed a band
at 3690 cm−1, which can be assigned to the tri-bridging OH group
of ZrO2 [35,45]. Bands between 1300 and 1600 cm−1 are carbonate
species originating from CO2 adsorption from air [27,44]. Compared
to the bare CaO–ZrO2 solid solution (Fig. 5a), the intensities of a very
broad band in the range 2500–3700 cm−1 and a band at 1630 cm−1
increased in the Ru-loaded catalysts (Fig. 5b and f). This behavior is
similar to that reported for Ru/TiO2 catalyst [46], and suggests the
presence of adsorbed water on the catalyst.
Immobilization of ruthenium was performed by simply treat-
ing CaO–ZrO2 solid solution with aqueous RuCl3 solution at room
temperature for 4 h. The amounts of immobilized ruthenium deter-
mined by ICP are shown in Table 1 together with the surface areas.
As seen in Table 1, only a small amount of the ruthenium species
was immobilized on the pure ZrO2. However, with the addition
of Ca2+ to the ZrO2, ruthenium was immobilized onto the sup-
port. This result might be ascribed to the stronger basic property
of the support with the addition of Ca2+, leading to a reaction
between ruthenium chloride and surface –OH group. As a result,
the amount of ruthenium increased to 1.5 wt% (0.15 mmol/g) on
the support at Ca/Zr = 0.125. Further increases in the Ca/Zr ratio
increased numbers of basic site as discussed above, however, no
significant increases in the Ru were observed. This may partly be
ascribed to the increases in the basic site above Ca/Zr ratio 0.125
were small as compared to that without Ca to ratio 0.125. Therefore,
increases in the amount of Ru immobilized to the catalyst surface
seem to be within detection limit of analyses.
Changes in the BET surface areas were not observed in any of
the samples before and after the immobilization of ruthenium.
3.3. Effect of Ca/Zr molar ratio and catalyst pretreatment on the
oxidation of 1-phenylethanol
After treatment with D2O, the intensity of the band at 1630 cm−1
greatly decreased and the band ꢀO–H in the range 2500–3700 cm−1
slightly weakened (Fig. 5c and g). Magnification of intensities in
the region at 2200–2700 cm−1 revealed that adsorbed water on the
catalyst surface was partly exchanged with D2O and as a result the
absorption shifted to 2500–2700 cm−1. However, a large portion
of the ꢀO–H band in the range 2900–3700 cm−1 still remained after
treatment with D2O. This broad band in the range 2900–3700 cm−1
would be assigned to Ru–OH species [15–17] that could not be
exchanged by the treatment with D2O at an ambient temperature.
In contrast to the treatment with D2O, the band in the range
2900–3700 cm−1 greatly decreased after the treatment in air and
H2 at 300 ◦C (Fig. 5d, e, h and i). These results indicate that the
adsorbed water and Ru–OH were mostly removed by the treat-
ment at an elevated temperature. These findings suggest that the
active sites in the oxidation of alcohols are highly dispersed Ru–OH
species on the CaO–ZrO2 solid solution [15–23].
The effects of the Ca/Zr molar ratio and catalyst pretreatment
on the oxidation of 1-phenylethanol over Ru/CaO–ZrO2 catalyst
were examined, and the results are shown in Table 2. Note that no
by-product formation was observed and that the selectivity to ace-
tophenone was higher than 98% with all the Ca/Zr molar ratios. The
Ca/Zr molar ratio apparently influenced the catalytic performance
of the Ru/CaO–ZrO2, and the highest conversion of 1-phenylethanol
(95%) was obtained with the molar ratio at Ca/Zr = 0.125 (entry
2). The relation between the catalytic activity and the Ca/Zr molar
ratio may be ascribed to the changes in the basicity of the support
[28–31].
Table 2
Effect of Ca/Zr molar ratio and catalyst pretreatment on the oxidation of 1-
phenylethanol with molecular oxygen.a.
In addition, a new absorption was observed at 1840 cm−1 after
treatment with air at 300 ◦C (Fig. 6c). This band seems to be the
Ru4+ O mode of the RuO2 phase [27,46,47]. The result strongly
supports that no RuO2 species exists on the as-prepared catalyst.
Interestingly, the oxidation state of ruthenium species on
CaO–ZrO2 solid solution depended on the molar ratio of Ca/Zr. XPS
profiles of the Ru/CaO–ZrO2 catalysts with different Ca/Zr molar
ratios are shown in Fig. 7. The Ru 3p3/2 peak of the Ru/CaO–ZrO2 at
the molar ratio of Ca/Zr = 0.1, appeared at a binding energy (BE) of
465.2 eV (Fig. 7a), which agrees with that reported for the RuHAP-
␥-Fe2O3 catalyst involving monomeric Ru4+ species [23]. Oxidation
of catalyst precursor Ru3+ to Ru4+ during the preparation of the
Entry
Catalyst
Ca/Zr (mol/mol)
Ru (wt%)
Conv. (%)
1
2
3
4
5
6
7
Ru/CaO–ZrO2
Ru/CaO–ZrO2
Ru/CaO–ZrO2
Ru/CaO–ZrO2
CaO–ZrO2
Ru/CaO–ZrO2
Ru/CaO–ZrO2
0.100
0.125
0.167
0.250
0.125
0.125
0.125
0.8
1.5
1.5
1.5
–
85
95
91
77
b
No reaction
17
24
c
1.5
1.5
d
a
2 mmol 1-phenylethanol, Ru/CaO–ZrO2 (11.9 ␮mol Ru), substrate/Ru = 168 mol/
mol, 90 ◦C, 5 mL o-xylene, 2 h and O2 flow (10 mL/min).
b
200 mg.
c
Pretreated with air at 300 ◦C for 1 h.
d
Pretreated with H2 at 300 ◦C for 1 h.

T. Yasu-eda et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 7–15
11
Fig. 5. Left panel: DRIFT spectra of a) CaO–ZrO2 solid solution, b) as-prepared Ru/CaO–ZrO2, c) Ru/CaO–ZrO2 treated with D2O, d) Ru/CaO–ZrO2 treated in air at 300 ◦C for
1 h and e) Ru/CaO–ZrO2 treated in H2 at 300 ◦C for 1 h. Right panel: The difference spectra between CaO–ZrO2 solid solution and respective spectrum shown in the left panel.
A molar ratio of Ca/Zr = 0.125 was employed in all samples.
ruthenium-supported catalysts has been reported [18,20,23,26].
With an increase in the Ca/Zr molar ratio from 0.1 to 0.25, the Ru
3p3/2 peak shifted to a lower BE of 463.5 eV, assignable to Ru3+
species [14,17]. Two oxidation states of ruthenium species (Ru4+
and Ru3+) seem to exist on the Ru/CaO–ZrO2 at a molar ratio of
0.125 (Fig. 7b); this catalyst exhibited the highest catalytic activity
in the oxidation of 1-phenylethanol.
A higher oxidation state of ruthenium (Ru4+) has been reported
to exhibit a high catalytic activity in the oxidation of alcohols
[18–20,23]. In our study, however, no significant relation between
the catalytic activity and the oxidation state of the ruthenium could
Fig. 6. DRIFT spectra of a) CaO–ZrO2 solid solution, b) as-prepared Ru/CaO–ZrO2,
Fig. 7. XPS patterns of Ru 3p3/2 in Ru/CaO–ZrO2 catalyst with various Ca/Zr molar
ratios.
and c) Ru/CaO–ZrO2 treatment in air at 300 ◦C for 1 h. The molar ratio Ca/Zr = 0.125
was employed in all samples.

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T. Yasu-eda et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 7–15
be observed, although the relative abundance of Ru3+ and Ru4+
shown in Fig. 7 obtained by curve fitting was not quantitative.
A probable explanation for the change in the BE (oxidation state
of ruthenium) with the Ca/Zr molar ratio is the change in the inter-
actions between ruthenium species and CaO–ZrO2 solid solution.
The dependency of the amount of immobilized ruthenium on the
Ca/Zr molar ratio shown in Table 1 would support the change in
such interactions. Slight increases in the amount of the more basic
site on CaO–ZrO2 at the ratio of 0.125 (compare the CO2 desorption
pattern at 350–500 ◦C) would likely be associated with the increase
in the Ru loading level. Based on the XRD patterns of Ru/CaO–ZrO2,
the crystallite sizes of CaO–ZrO2 decreased with an increase in the
CaO–ZrO2 ratio from 0.1 to 0.125, as seen in Fig. 1. Broadening of the
diffraction peaks and the shift of the diffraction angle are more pro-
nounced at Ca/Zr ratios from 0.1 to 0.125, indicating a co-relation
between the CaO–ZrO2 surface and the dispersion of loaded Ru
species.
Fig. 8. Time vs. conversion profiles for the oxidation of benzyl alcohol with
molecular oxygen over Ru/CaO–ZrO2 catalyst and the effect of catalyst removal
on conversion. Reaction conditions: 2 mmol benzyl alcohol, 80 mg Ru/CaO–ZrO2
(Ca/Zr = 0.125 mol/mol, 11.9 ␮mol Ru), 90 ◦C, 5 mL o-xylene, O2 flow (10 mL/min).
3.5. Oxidation of alcohols with molecular oxygen over
Ru/CaO–ZrO2 catalyst
(Ca/Zr = 0.125 mol/mol) at 90 ◦C. The oxidation of benzyl alcohol
was completed within 45 min, and benzaldehyde was obtained
with ca. 100% yield without further oxidation to benzoic acid. The
turnover frequency (TOF) based on the total amount of ruthenium
Fig. 8 shows a conversion versus time profile for the oxidation of
benzyl alcohol with molecular oxygen over Ru/CaO–ZrO2 catalyst
Table 3
Results of oxidation of various alcohols with molecular oxygen over Ru/CaO–ZrO2 catalyst (Ca/Zr = 0.125).a.
Entry
1
Substrate
Product
Substrate/Ru (mol/mol)
168
Time (h)
0.75
Conv. (%)
100
Yield (%)
100
TOF (h−1
224
)
2b
168
168
0.75
0.75
Trace
100
Trace
100
–
3
224
4
5
168
168
0.75
2.0
100
100
100
100
224
84
6
225
168
0.5
2.0
100
95
100
94
449
80
7
8
168
56
3.5
2.5
86
89
83
89
42
20
9c
a
2 mmol substrate, 60–120 mg Ru/CaO–ZrO2 (Ca/Zr = 0.125), 90 ◦C, 5 mL o-xylene and O2 flow (10 mL/min).
Under Ar atmosphere.
1 mmol substrate.
b
c

T. Yasu-eda et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 7–15
13
Table 4
The oxidation of benzyl alcohol,1-phenylethanol and cinnamyl alcohol with molecular oxygen over Ru/CaO–ZrO2 catalyst (Ca/Zr = 0.125) at 40 ◦C.a.
Entry
Substrate
Product
Time (h)
Conv. (%)
Yield (%)
TOF (h−1
)
1
2
>99
>98
34
2
6
>99
>99
>98
>98
11
3
12
5.6
a
1 mmol substrate, 100 mg Ru catalyst (14.8 ␮mol Ru), substrate/Ru = 68 mol/mol, 40 ◦C, 5 mL o-xylene and O2 flow (10 mL/min).
reached 224 h−1. This value is slightly lower than that reported on
Ru/HAP-modified with benzoic acid (TOF = 242 h−1 at 60 ◦C) [48]
and higher than those reported for various ruthenium catalysts,
including RuHAP (TOF = 2 h−1, at 80 ◦C) [14], Ru/Al2O3 (40 h−1, at
80 ◦C) [15], RuCoHAP (78 h−1, at 90 ◦C) [21], RuCo/Al2O3 (80 h−1, at
110 ◦C) [19], and RuHAP-␥-Fe2O3 (196 h−1, at 90 ◦C) [23].
3.6. Reaction mechanism
The addition of 2,6-di-tert-butyl-p-cresol as a radical scavenger
did not influence the rate and selectivity of the oxidation of benzyl
alcohol. This result indicates that free-radical intermediates are not
involved in the oxidation.
In order to better our understanding of the reaction mechanism,
competitive oxidation of benzyl alcohol and 1-phenylethanol was
performed (Scheme 1). The Ru/CaO–ZrO2 catalyst gave benzalde-
hyde and acetophenone with 85 and 14% yield, respectively. Similar
relative reactivity of the alcohols has been described in the litera-
ture with Ru/Al2O3 catalyst [15–17], and a larger oxidation rate
of primary alcohols than secondary one seemed to indicate that
sterically hindered Ru–alcoholate is involved as an intermediate
[50,51]. Also, electron donating effect of alkyl group attached to
–OH functional group may contribute smaller rate of H− abstraction
reaction.
The oxidation of several p-substituted benzyl alcohols was per-
formedat60 ◦C, andthe Hammettplot at60 ◦C isshownin Fig. 9. The
slope of the Hammett plot (ꢂ = −0.41) was in agreement with those
of RuHAP (ꢂ = −0.43) [14] and Ru/Al2O3 (ꢂ = −0.46) [16]. This result
clearly indicates that ␤-hydride elimination is the rate-limiting
step, as reported in the above references. In addition, the extremely
high TOF for diphenyl methanol (entry 6 in Table 3), even the
sterically hindered secondary alcohol, supports that ␤-hydrogen
abstraction from Ru–alcohlate species is the rate determining step
of this reaction, where inductive and resonance effects are involved.
Recently Yamaguchi et al. provided clear evidence that ␤-hydride
elimination is the rate-limiting step of Ru/TiO2 catalyzed oxida-
tion of alcohol, by using ␣-monodeuterobenzyl as substrate, where
large kinetic isotope effects of kH/kD of ca. 5 were observed [25].
The effect of oxygen partial pressure on the oxidation of benzyl
alcohol was examined at 90 ◦C (Fig. 10). Under 20%-O2 in Ar, the
reaction rate was smaller than that under a 100%-O2 atmosphere.
When the Ru/CaO–ZrO2 catalyst was separated after the run
for 10 min (conv. = ca. 50%), the oxidation of benzyl alcohol was
completely terminated, as seen in Fig. 8 (ꢁ). This result strongly
indicates that the oxidation of benzyl alcohol proceeded over the
surface of the Ru/CaO–ZrO2 catalyst, and that no leaching out of
ruthenium species into the liquid phase occurred. Analysis of the
supernatant solution by IPC after the reaction did not show even
trace amount of Ru.
The Ru/CaO–ZrO2 catalyst (Ca/Zr = 0.125 mol/mol) exhibited
high performance in the oxidation of various alcohols (Table 3).
Aromatic ring-substituted benzyl alcohols were converted into cor-
responding benzaldehydes in high yields (>98%) without further
oxidation to substituted benzoic acids (entry 1 and 3–5). On the
other hand, the oxidation of benzyl alcohol could not proceed under
an Ar atmosphere (entry 2). The Ru/CaO–ZrO2 catalyst also exhib-
ited high catalytic activities in the oxidation of secondary benzylic
alcohols (entry 6 and 7). It is noteworthy that the outstanding TOF
value (449 h−1) was obtained in the oxidation of diphenylmethanol
(entry 6). The Ru/CaO–ZrO2 catalyst also exhibited high activity
in the oxidation of cinnamyl alcohol, which is commonly used as
an example of allylic alcohol (entry 8). However, the selectivity to
cinnamaldehyde slightly decreased (96%), since small amounts of
by-products (3-phenyl-1-propanol and dihydrocinnamaldehyde)
were formed by the hydrogenation of the C C bond. This result
suggests that Ru–H species were formed as an intermediate. A
secondary aliphatic alcohol, 2-octanol, was also converted into 2-
octanone with 89% yield, although the reaction rate was smaller
than that of benzylic alcohols (entry 9).
We found that the Ru/CaO–ZrO2 catalyst could be applied in the
oxidation of alcohols even at 40 ◦C, as shown in Table 4. The ben-
zylic and allylic alcohols were quantitatively converted into the
corresponding carbonyl compounds with yields >98% with moder-
ate reaction rates. In the oxidation of benzyl alcohol, the reaction
was completed in 2 h and TOF up to 34 h−1 was obtained (entry 1).
More recently, it was reported that Ru/MnOx/CeO2 catalyst exhibits
a high catalytic activity in the oxidation of alcohols at 30–60 ◦C
[49]. Considering the TOF value in the oxidation of benzyl alco-
hol, our Ru/CaO–ZrO2 catalyst system seems to be superior to the
Ru/MnOx/CeO2 catalyst (TOF = 22 h−1 at 40 ◦C). From these results,
we believe that the Ru/CaO–ZrO2 catalyst should be a preferred
choice in the oxidation of alcohols to carbonyl compounds.
Scheme 1. Competitive oxidation of benzyl alcohol and 1-phenylethanol. Reaction
conditions: 2 mmol benzyl acohol, 2 mmol 1-phenylethanol, 80 mg Ru/CaO–ZrO2
(Ca/Zr = 0.125 mol/mol), 90 ◦C, 1 h, 5 mL o-xylene, O2 flow (10 mL/min).

14
T. Yasu-eda et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 7–15
Scheme 2. Proposed reaction scheme for the oxidation of alcohols over
Ru/CaO–ZrO2 catalyst. Run+ should be Ru3+ ∼ Ru4+
.
immediately and detection of it seems to be difficult. The same reac-
tion scheme was proposed by Sheldon et al. [12]. Yamaguchi et al.
proposed similar scheme without H2O2 formation [25].
Opre et al. have proposed a different pathway for this reaction.
In their reaction path, a Run+(H)–OH to Run+(OH)2 species has been
proposed [22]. However, based on this route, a direct formation of
molecular oxygen and Ru–H to Ru–OH from a Ru containing inter-
mediate must be considered. Direct formation of molecular oxygen
from the intermediate seems to be difficult, and our path involves
more reasonable reaction courses. In the direct formation of H2O2
from oxygen and hydrogen, no Ru-based catalyst was proposed,
indicating that free H2O2 could not be detected in the oxidation of
alcohols.
Fig. 9. Hammett plot of p-substituted benzyl alcohols. Reaction conditions:
1 mmol p-X-benzyl alcohol (X = OMe, Me, H, Cl, NO2), 60 mg Ru/CaO–ZrO2
(Ca/Zr = 0.125 mol/mol), 60 ◦C, 5 min, 5 mL o-xylene, O2 flow (10 mL/min). ꢃ values
were taken from Ref. [52].
If pure O2 was bubbled into the reaction mixture (10 mL/min), the
reaction rate further increased. The dependence of the reaction rate
on the oxygen partial pressure at a higher temperature suggests
that the rate-limiting step changed from ␤-hydrogen abstraction
to the re-oxidation of Ru–H species or oxygen mass transport [22].
Based on the results of our findings and literature data, we pro-
pose the reaction mechanism shown in Scheme 2 [15–20,22,23,25].
The first step is the formation of Ru–alcoholate species between
Ru–OH species and alcohol together with the formation of H2O
(step 1). The Ru–alcoholate species undergoes ␤-hydride elimi-
nation to give a carbonyl compound and Ru–H species (step 2).
The Ru–H species is then re-oxidized with molecular oxygen to
give Ru–OOH species (step 3). Finally, the Ru–OOH species react
with adsorbed H2O on the support, followed by regeneration of the
Ru–OH active phase via H2O2 formation (step 4). It was reported
that H2O2 is unstable in the presence of a catalyst, and that decom-
position of H2O2 is suppressed with H+ [53]. Thus under the
conditions employed here, even if H2O2 was formed, it decomposed
4. Conclusion
Basic CaO–ZrO2 solid solution was applied as a new support for
Ru-loaded catalyst in the liquid-phase oxidation of alcohols. The
highest activity was obtained with the support with a molar ratio
of Ca/Zr = 0.125, and the Ru/CaO–ZrO2 catalyst exhibited high cat-
alytic performance in the oxidation of alcohols even at 40 ◦C. DRIFT
and XPS analyses revealed that the Run+–OH (n = 3, 4) seemed to be
active species in the oxidation of alcohols.
We also found that the oxidation state of ruthenium species was
affected by the Ca/Zr molar ratio. This relationship can be ascribed
in part to the changes in the basicity of the solid solution, which
change the interactions between loaded ruthenium and the sup-
port.
Acknowledgment
This work was financially supported in part by “High-Tech
Research Center” Project for Private Universities: matching fund
subsidy from MEXT, 2007-2011.
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