Ruthenium/magnesium-lanthanum mixed oxide: An efficient reusable catalyst for oxidation of alcohols by using molecular oxygen

Article abstract of DOI:10.1016/j.molcata.2012.03.013

An efficient method for the oxidation of benzylic and secondary aromatic alcohols into their corresponding aldehydes or ketones has been achieved by using ruthenium supported magnesium-lanthanum mixed oxide as a heterogeneous catalyst in toluene, with molecular oxygen as the sole oxidant. This catalyst can also be operated in solvent free conditions at 393 K and reused for five cycles with consistent yield and selectivity.

Full text of DOI:10.1016/j.molcata.2012.03.013

Journal of Molecular Catalysis A: Chemical 359 (2012) 1–7
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ruthenium/magnesium–lanthanum mixed oxide: An efficient reusable catalyst
for oxidation of alcohols by using molecular oxygen
a,∗
a
a
a
a
M. Lakshmi Kantam , R. Sudarshan Reddy , Ujjwal Pal , M. Sudhakar , A. Venugopal ,
a
b
c
d
K. Jeeva Ratnam , F. Figueras , Venkat Reddy Chintareddy , Yuta Nishina
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 607, India
Institut de recherches sur la catalyse et l’environnement, 2 avenue A. Einstein 69626, Villeurbanne Cedex, France
311 Gilman Hall, Department of Chemistry, Iowa State University, Ames, IA 50011, USA
a
b
c
1
d
Research Core for Interdisciplinary Sciences, Okayama University, Tsushima, Kita-ku, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient method for the oxidation of benzylic and secondary aromatic alcohols into their correspond-
ing aldehydes or ketones has been achieved by using ruthenium supported magnesium–lanthanum
mixed oxide as a heterogeneous catalyst in toluene, with molecular oxygen as the sole oxidant. This
catalyst can also be operated in solvent free conditions at 393 K and reused for five cycles with consistent
yield and selectivity.
Received 22 December 2011
Received in revised form 12 March 2012
Accepted 13 March 2012
Available online 23 March 2012
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Oxidation
Alcohols
Ru/Mg–LaO mixed oxide
Heterogeneous
Solvent free
1
. Introduction
Sheldon [22–24] and Uozumi [25] in aerobic oxidation of alco-
hols using reusable catalysts in the recent years resulted in great
progress in this area. However, the development of a novel catalytic
system that exhibits a wide range of substrate tolerance under mild
conditions still remains a major challenge. Ruthenium is an attrac-
tive and efficient metal in catalyzing organic reactions. Ruthenium
physico-chemically deposited on various supports such as hydrox-
yapatite (HAP) [29] showed a low activity but treatment with
basic additives such as proline or prolinol significantly increased
its catalytic activity. Recently we reported that ruthenium sup-
ported on nanocrystalline magnesium oxide modified by basic ionic
liquids (Ru–CHNAP–MgO) was an active and recyclable catalyst
whereas ruthenium supported onto pristine nanocrystalline mag-
nesium oxide (Ru–NAP–MgO), though catalytically active was not
recyclable as the metal leached into the solution [30]. It can be
pointed out that ruthenium was stabilized by modifying the sup-
port with base [30]. Matsumoto et al. reported the oxidation of
alcohols with polymer incarcerated ruthenium catalyst and TEMPO,
in dichloroethane as solvent using oxygen [31]. However, leaching
of ruthenium was observed and could be suppressed by treatment
One of the most commonly practiced routes for the synthe-
sis of carbonyl compounds is the oxidation of their corresponding
alcohols [1,2]. Several transition metal-based oxidizing reagents
have been developed, usually used in stoichiometric amounts, and
lead to a large amount of waste. Molecular oxygen is an attractive
and environmentally friendly oxidant. In this context, the oxi-
dation of alcohols using molecular oxygen catalyzed by reusable
heterogeneous catalysis is ideal from an environmental and atom
economical point of view [3–5]. Solutions based on homogeneous
catalysis are known [6–8], but heterogeneous processes are con-
sidered as more economical since they offer advantages such as
ease of recovery and recycling. Most of the heterogeneous cata-
lysts for oxidation reported so far are based on TEMPO supported
by polymers or silicas [9–14], Ru [15–18] and Pd [19–25] employ-
ing molecular oxygen or air as a stoichiometric oxidant. Previously,
we reported an effective method for the oxidation of alcohols using
Ni–Al hydrotalcite catalyst with molecular oxygen [26]. Recently,
Abad et al. reported an efficient method for the selective aerobic
oxidation of alcohols by using Au/CeO2 and Pd/CeO2 [27,28]. Pio-
neering efforts from Baiker [17], Nishimura [19], Kaneda [20,21],
with a strong base (NaOH, 0.5 N) in the preparation step. Ru/Al O3
2
has also been reported to give very good results [32]; however, the
preparation of the catalyst included an additional step of hydrolysis
of the adsorbed RuCl3 precursor at pH = 13.2 by 1 M NaOH for 24 h,
which is believed to increase the basicity of the support and stabi-
lize Ru. It has been demonstrated that ruthenates are active species
∗ Corresponding author. Tel.: +91 40 2719 3510; fax: +91 40 2716 0921.
E-mail addresses: mlakshmi@iict.res.in, lkmannepalli@gmail.com (M.L. Kantam).
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.03.013

2
M.L. Kantam et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 1–7
◦
for this reaction [33]. In oxidizing atmosphere Ru is easily oxi-
dized, probably to ruthenate species. These species being anionic
are unstable on cation exchangers (solid acids) but are stabilized
by solid bases. In such a case, the nature of the support should
be important for determining its activity and recyclability. Indeed
Ru/MCM-41 has been reported to be a selective catalyst for the
oxidation of alcohols, but it has to be modified by anchored quater-
nary ammonium species to make it recyclable [34]. Mg–La mixed
oxides have received much attention as basic catalysts and support
materials. Recently, we efficiently carried out transesterification of
diethyl carbonate by alcohols [35]. Michael addition reactions [36],
aldol type reaction of aldehydes and imines with ethyl diazoacetate
by using this highly basic Mg–La mixed oxide catalyst under recy-
clable conditions [37]. We have also used the Mg–La mixed oxide as
a support for palladium catalyzed cross-coupling reactions [38,39].
This highly basic magnesium lanthanum mixed oxide was found
to promote the activity of palladium. In this work, we report the
oxidation of benzylic and secondary aromatic alcohols under het-
erogeneous conditions to aldehydes or ketones by O2 or air in the
liquid phase with a high efficiency using Ru/Mg–La mixed oxide
catalyst at 353 K in toluene as solvent, or at 393 K without solvent.
The catalyst was characterized by DSC, powder XRD, BET surface
area, TEM, TPR, SEM-EDX, TPD of CO2 and XPS analyses.
2 /min, with the beam voltage and a beam current of 30 kV and
15 mA, respectively. The basicity of the support was investigated
by calorimetry [40], using CO2 adsorption at 303 K and samples
activated in vacuum at 723 K. The instrument is a Tian–Calvet
calorimeter. CO is an acidic probe, which has been commonly used
2
to measure basicity [41,42]. The heat of adsorption measures the
strength of the interaction with basic sites and therefore the basic
strength as a function of coverage. The TPD of CO2 of Ru/Mg–LaO
fresh and used samples are measured using an Auto Chem 2910
(Micromeritics, USA). In a typical method about 0.1 g of sample is
◦
−1
.
reduced at 400 C for 5 h in hydrogen at a flow rate of 30 mL min
After reductive pre-treatment the sample is saturated with 9.9%
◦
−1
CO2 (balance helium) at 60 C, at a flow rate of 50 mL min and
subsequently flushed with helium gas at 60 C for 1 h. The TPD
measurements are carried out from 60 C to 700 C at a ramping
rate of 10 C/min. The amount of desorbed CO2 is calculated using
◦
◦
◦
◦
GRAMS/32 software. GC analysis was done by GC-2010, gas chro-
matograph Shimadzu, with ZB5 capillary column, internal diameter
0.53 mm, film thickness 1.50 ␮m, length 30 m, initial temperature
◦
◦
◦
70 C (hold 2 min), ramp 7 C/min → 279 C (hold 7 min), injection
◦
◦
temperature 250 C, and detector temperature 280 C (FID). For the
transmission electron microscope (TEM) analysis, samples were
dispersed in methanol solution and dropped on a 200-mesh Cu
grid, and images were taken using JEOL JEM 2100F high-resolution
transmission electron microscope at an acceleration voltage of
200 kV. For scanning electron microscopy (SEM) and the energy
dispersive X-ray spectroscopy (EDX) analysis, samples were fixed
on carbon tape and images were taken using JEOL JSM-6700FE.
2
. Experimental
2.1. Materials and methods
All solvents used in the catalytic test were of analytical grade
2.2. Preparation of catalyst
and were used as received from Merck India Pvt. Ltd. High
purity oxygen was used in the oxidation of alcohols. Purification
of reaction products was carried out by flash chromatography
using 100–200 mesh silica gel, and a mixture of ethyl acetate
and petroleum ether as the eluting agent. All the products were
characterized by 1H-NMR spectroscopy. The 1H spectra of samples
were recorded on a Gemini 200 MHz, and/or a Bruker-Avance-
The magnesium-lanthanum mixed oxide was prepared by the
co-precipitation method described earlier. It was calcined at 923 K
in air [35,36]. The powder XRD pattern of the Ru/Mg–LaO calcined
mixed oxide showed the diffraction lines of a lanthanum oxide car-
bonate and lanthanum hydroxides with parameters corresponding
to those reported in the literature. This biphasic solid probably con-
sists of a layer of lanthanum oxide deposited on magnesia. This
3
00 MHz Spectrometer using TMS as an internal standard in CDCl3.
X-ray photoelectron spectra were recorded on a KRATOS AXIS
65 with a dual anode (Mg and Al) apparatus using the Mg K␣
2
−1
mixed oxide has a surface area of 37.6 m g . The calcined sample
of Mg–LaO mixed oxide (1.78 g) was stirred with RuCl3 (0.415 g,
1
−
9
−2
anode. The pressure in the spectrometer was about 10 Torr.
For energy calibration, we used the carbon 1s photoelectron line.
The carbon 1s binding energy was taken to be 284.6 eV. Spectra
were deconvoluted using the Sun Solaris based Vision 2 curve
resolver. The location and the full width at half maximum (FWHM)
for a species were first determined using the spectrum of a pure
sample. The location and FWHM of the products, which were
not obtained as pure species, were adjusted until the best fit was
obtained. Symmetric Gaussian shapes were used in all cases. Bind-
ing energies for identical samples were, in general, reproducible
within ± 0.1 eV. The TPR profile was recorded on Micromeritics
2.67 × 10 M) in 75 mL of freshly prepared deionized water at
298 K for 24 h under nitrogen atmosphere. The catalyst was fil-
tered, washed with deionized water, acetone and dried overnight
at 383 K in an oven. The prepared Ru/Mg–LaO catalyst was grey
in colour and from the elemental analysis the Ru content was
found to be 1.0 mmol/g (Ru content: 10.10 wt%). The same proce-
dure was used to prepare Ru supported on ␥-alumina [Harshaw,
2
−1
2
−1
224 m g ], silica [Aldrich, 500 m g ], and titania [Aldrich P25
2
−1
Anatase, 50 m g ] magnesia [NAP–MgO with a specific surface
2
−1
area (BET) ≥ 600 m g
was purchased from “Nano Scale Corp.,
◦
−1
Manhattan, KS, USA.” It was then calcined at 450 C for four hours
2
(
Auto Chem 2910) using 0.05 g of catalyst sample. In a typical
in air. The BET surface area obtained was 183 m g catalysts.
method the catalyst was loaded in an isothermal zone of a quartz
reactor (i.d = 6 mm, length = 300 mm) heated by an electric furnace
2.3. General procedure for the oxidation of alcohols
◦
◦
at a rate of 10 C/min to 300 C/min in flowing helium gas at a
3
rate of 30 cm /min, which facilitates desorption of the physically
An oven dried 50 mL two-necked round bottomed flask
equipped with a reflux condenser connected with a balloon filled
with oxygen was charged with Ru/Mg–LaO catalyst (0.102 g), alco-
hol (1.0 mmol) and toluene (5.0 mL). The resulting mixture was
stirred at 353 K. After completion of the reaction (as monitored by
TLC and GC), the reaction mixture was centrifuged to separate the
solid catalyst. The catalyst was washed with toluene (2 × 20 mL)
for complete recovery of the organic materials. In case of recy-
cling studies, the catalyst was washed with deionized water and
dried prior to the reusability experiments. The combined filtrate
adsorbed water. After degassing, the sample was cooled to room
3
temperature and the helium gas was switched to 36 cm /min
reducing gas of 5% H2 in argon and the temperature was increased
◦
◦
to 725 C at a ramping rate of 10 C/min. Hydrogen consumption
was measured by analyzing effluent gas by means of thermal con-
ductivity detector. The consumption of hydrogen was calibrated
measuring the TPR of Ag O (20 mg), with the same protocol. The
2
XRD patterns of Mg–LaO and Ru/Mg–LaO samples were obtained
on a Rigaku miniflex X-ray diffractometer using Ni filtered Cu
◦
K␣ radiation (ꢀ = 0.15406 nm) from 2ꢁ = 2–80 , at a scan rate of
was dried over anhydrous Na SO4 and the solvent was removed
2

M.L. Kantam et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 1–7
3
Fig. 2. XRD patterns of the (a) Ru/Mg–LaO fresh, (b) Mg–LaO and (c) Ru/Mg–LaO
used samples.
Fig. 1. The basicity of various support materials measured by CO2 adsorption at
03 K.
3
samples. The CO2 uptakes of the fresh and used catalysts are found
to be 240 and 214 ␮mol/g respectively.
under reduced pressure. The crude product was purified by column
chromatography (silica gel: 100–200 mesh using ethyl acetate and
petroleum ether) to give the corresponding aldehyde or ketone. For
the solvent free oxidation of benzyl alcohol, a dry 100 mL three-
neck round bottomed flask, equipped with a reflux condenser was
charged with Ru/Mg–LaO catalyst (0.490 g) and freshly distilled
benzyl alcohol (1.08 g, 10 mmol). The resulting mixture was stirred
at 393 K under one atmosphere of oxygen. Conversion and selec-
tivity were determined by GC using undecane as external standard.
The XRD diffractogram (Fig. 2a) of Ru/Mg–LaO fresh is similar
to that of the support Mg–LaO (Fig. 2b) and used (Fig. 2c) samples,
and it can then be concluded that the Ru oxide is dispersed as small
particles.
This high dispersion of Ru is confirmed by the examina-
tion of scanning electron microscopy–energy dispersive X-ray
spectroscopy (SEM–EDX) (Fig. 3). The ratio of Ru:Mg:La did
not change dramatically before and after the reaction (before,
2
1.75:55.48:22.77, used, 21.49:60.33:18.18, respectively).
Fig. 4 shows the TEM micrographs of the fresh and used
0%Ru/Mg–LaO catalysts. It can be seen that the particles are almost
The combined organic layers were dried using anhydrous Na SO ,
2
4
filtered. The distillation of the residue gave pure benzaldehyde (75%
yield).
1
spherical shape with a mean diameter of 15 and 30 nm for fresh
and used catalysts respectively. Although some aggregation was
observed in the used catalyst, it did not affect the product yield.
The oxidation state of Ru was investigated by two methods: TPR
and XPS. The TPR profiles of the fresh and used Ru/Mg–LaO cata-
lysts, reported in Fig. 5 shows one large peak at about 550 K, with a
shoulder at 750 K. The high temperature peak at about 750 K, linked
to a very stable Ru species, is assigned to the oxychloride, and the
low temperature peak to Ru cations well dispersed at the surface
[41,43].
3
. Results and discussion
3.1. Catalyst characterization
The Ru/Mg–LaO catalyst was characterized by powder XRD,
TPR, XPS and SEM–EDX in order to determine the nature of the
ruthenium dispersed on the solid. The support was characterized
by calorimetry in order to determine the basicity. The results are
reported in Fig. 1, in which a nano MgO sample of high surface area
The peak at about 550 K has been assigned due to the reduction
2
−1
−1
2
−1
2
−1
(
(
183 m g ), TiO2 (50 m g ), SiO2 (500 m g ) and ␥-alumina
of RuO . The integration of the area under the peaks in the interval
2
2
224 m g ) surface areas are added as reference. It appears clearly
400–900 K, coupled with the calibration of the hydrogen consump-
tion, permits to determine the number of H atoms required for
the reduction of Ru. This determination gives a ratio H/Ru = 3.86,
corresponding to the reduction of a mixture of 80% Ru4+ and 20%
that the mixed oxide shows a higher basic strength than MgO.
The CO2 uptakes of the fresh and used form of Ru/Mg–LaO are
carried out by TPD of CO2 in order to measure the basicity of the
Fig. 3. Images of (a) fresh and (b) used Ru/Mg–LaO catalyst. SEM images of (a-1) fresh and (b-1) used catalyst. Element mapping of (a-2) and (b-2) Ru, (a-3) and (b-3) La, and
a-4) and (b-4) Mg.
(

4
M.L. Kantam et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 1–7
Fig. 4. TEM images of the (a) fresh Ru/Mg–LaO and (b) used Ru/Mg–LaO catalysts.
Scheme 1. Oxidation of alcohols over Ru/Mg–LaO catalyst.
Ru3+. In the XPS spectrum, the comparison of the surfaces of Ru
d peaks at 285 eV with that of the 2p Cl peak at 199 eV gives a
3
Ru/Cl atomic ratio of 61/38; using the Ru 3p line at 466 eV, a sim-
ilar result is obtained with Ru/Cl = 63/36. It can then be concluded
that drying the solid at oven temperature induces hydrolysis of
Ru Cl bonds with formation of oxides. The strong basicity of the
mixed oxides has induced the same effect as that of by NaOH treat-
ment. It has been however known that chlorine is not completely
eliminated from Ru supported on alumina [40] or silica [41,42]
after calcination or even after calcination followed by reduction.
The retention of Cl was attributed to the formation of an oxychlo-
ride, which should be present here in rather small amounts. This
oxychloride can be responsible for the high temperature shoulder
appeared in the TPR curve. The deconvolution of the XPS lines of
Ru 3d level (for fresh and used Ru/Mg-LaO mixed oxide catalysts)
is reported in Fig. 6. It is observed that both fresh and used cata-
lysts show common features, indicating that there is no change in
the surface structure of the fresh catalyst after the reaction. These
spectra contain three lines attributed to C; the one at 284.6 and
Fig. 5. TPR patterns of the (a) fresh and (b) used Ru/Mg–LaO catalyst.
2
86.8 eV are due to pollution carbon, and a smaller peak at 289 eV
Table 1
Effect of the support on the catalytic properties of ruthenium for the selective oxi-
a
dation of 1-phenylethanol to acetophenone.
Entrya
Catalyst
Solvent
Yield (%)b
n.r.c
1
2
3
4
5
6
7
8
9
MgLaO
RuCl3
Toluene
Toluene
Toluene
Trifluorotoluene
DMF
Toluene
Toluene
Toluene
Toluene
c
n.r.
Ru/MgLaO
Ru/MgLaO
Ru/MgLaO
Ru/SiO2
Ru/Al2O3
Ru/MgO
Ru/TiO2
96, 95d
95
80
45
40
36
22
a
Reaction conditions: alcohol (1.0 mmol), catalyst (0.1 g), toluene (5.0 mL), 353 K,
O2 balloon.
b
Isolated yield after column chromatography.
Determined by NMR spectroscopic integration.
Yield in fifth cycle.
c
Fig. 6. X-ray photoelectron spectra of the (a) Ru 3d5/2 fresh and (b) Ru 3d5/2 used
catalysts.
d

M.L. Kantam et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 1–7
5
Table 2
a
Oxidation of various alcohols using molecular oxygen using Ru/Mg–LaO mixed oxide catalyst.
a
Reaction: alcohol (1.0 mmol), catalyst (0.1 g), toluene (5.0 mL), 353 K, O2 atmosphere, reaction time 4 h.
Isolated yield.
b
can be attributed to carbonates adsorbed at the basic sites of the
support. Due to spin–orbit interactions, the transition correspond-
ing to Ru is composed of two doublets [44], the relative weight
of which are 60% for Ru 3d5/2 at 283.06 eV attributed to Ru3+ and
non-reduced catalyst at 353 K (Scheme 1) and the results are
reported in Table 1.
As can be seen from Table 1, Ru supported on acidic silica or
on weak solid bases such as Al O , MgO and TiO afforded low
2
3
2
4
0% for Ru 3d3/2 at 284.2 eV ascribed to Ru4+ [45–48] (Fig. 6). This
yields reaching 45%, 40%, 36% and 22% yields after 4 h (Table 1,
entries 6–9). It can be pointed out that RuCl3 has not suffered
here any treatment with NaOH, so that the low basicity of alu-
mina is retained, and this puts in evidence the strong effect of
this treatment. The best result was obtained with the catalyst
Ru/Mg–LaO (96%, Table 1, entry 3). The efficiency and stabil-
ity of this newly established catalytic system was examined in
detail with 1-phenylethanol as substrate. Oxidation proceeds to
XPS analysis therefore confirms that Ru could exist as RuO2 at the
surface.
3.2. Catalytic activity of Ru/Mg–LaO for oxidation of alcohols
The oxidation of various alcohols in oxygen atmosphere on
different supported Ru catalysts has been performed with the

6
M.L. Kantam et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 1–7
Table 3
4. Conclusions
a
Reusability of the Ru/Mg–LaO catalyst.
Trial
Time (h)
Yield (%)b
In conclusion, mild and efficient green oxidation of various ben-
zylic and secondary aromatic alcohols to aldehydes or ketones can
be obtained by using Ru supported on a strong basic Mg–La mixed
oxide and molecular oxygen as an oxidant. The catalyst can be used
for five cycles with consistent activity and selectivity.
1
2
4
4
4
4
4
96
96
95
95
95
4
5
a
Reaction conditions: 1-phenyl ethanol (1 mmol), catalyst (0.1 g), toluene (5 mL),
53 K, O2 atmosphere
Isolated yields.
Acknowledgements
3
b
R.S. Reddy, U. Pal and M. Sudhakar thank the Council of Scientific
and Industrial Research, New Delhi, for their research fellowships.
We also thank P. Delichère for the analysis of the XPS spectrum
completion giving excellent isolated yields through five successive
cycles (Table 1, entry 3).
(IRCE Lyon, France). This research has been performed as part of
the Indo-French “Joint Laboratory for Sustainable Chemistry at
Interfaces”. We thank CNRS, MESR, French Ministry for foreign
affairs and CSIR for support of this research. MLK thanks DST-JSPS
for the exploratory visit to Okayama University, Japan as part of
India–Japan Cooperative Science Program (IJCSP).
The particularity of Mg–LaO is tentatively attributed to its strong
basicity which would stabilize Ru as ruthenates at the surface of
the solid. The activity pattern of Ru/Mg–LaO was investigated on a
series of alcohols (Table 2). All aromatic secondary alcohols were
converted to their corresponding ketones in short reaction times,
with nearly quantitative yields (Table 2, entries 1–5 and 9–12). It is
worth mentioning here that benzyl alcohol was cleanly converted
to benzaldehyde and was obtained in 92% isolated yield (Table 2,
entry 6), which is a significant improvement compared to our pre-
vious protocol which involved the use Ni–Al hydrotalcite wherein
only 36% yield was obtained [26]. Substituted benzyl alcohols
also afforded the corresponding aldehyde product in good yields
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2012.03.013.
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(
Table 2, entries 7 and 8). We were also pleased to find that nitro-
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