Selective oxidation of benzylic alcohols with molecular oxygen catalyzed by copper-manganese oxide nanoparticles

Article abstract of DOI:10.14233/ajchem.2013.14112

The catalytic activity of copper-manganese (CuMn) mixed oxide nanoparticles (Cu/Mn = 1:1) prepared by co-precipitation method has been studied for the selective oxidation of benzyl alcohol and its derivatives to the corresponding aldehydes using molecular

Full text of DOI:10.14233/ajchem.2013.14112

Asian Journal of Chemistry; Vol. 25, No. 9 (2013), 4815-4819
http://dx.doi.org/10.14233/ajchem.2013.14112
Selective Oxidation of Benzylic Alcohols with Molecular Oxygen
Catalyzed by Copper-Manganese Oxide Nanoparticles
*
ROUSHOWN ALI, M.E. ASSAL, ABDULRAHMAN AL-WARTHAN and M. RAFIQ H. SIDDIQUI
Department of Chemistry, College of Science, King Saud University, PO Box 2455, Riyadh-11451, Kingdom of Saudi Arabia
*
Corresponding author: Tel: +966 14676082; E-mail: rafiqs@ksu.edu.sa
Received: 27 May 2012; Accepted: 8 March 2013)
(
AJC-13079
The catalytic activity of copper-manganese (CuMn) mixed oxide nanoparticles (Cu/Mn = 1:1) prepared by co-precipitation method has
been studied for the selective oxidation of benzyl alcohol and its derivatives to the corresponding aldehydes using molecular oxygen as an
oxidizing agent. A 100 % conversion of the alcohols was achieved with high selectivity (>99 %) to the corresponding aldehydes within a
short reaction period at 102 ºC. The best catalytic activity was obtained for the CuMn mixed oxide pre-calcined at 300 ºC. The catalytic
performance is also obtained to be dependent on the electronic and steric effects of the substituents present on phenyl ring. The catalyst
has been characterized with a combination of various characterization techniques.
Key Words: Copper-manganese oxide, Catalyst, Oxidation, Benzylic alcohols.
17
18
19
20
21
22
23
24
25
such as Cu , Ni , Fe , V , Co , Ag , Cr , Mo , Zn and
INTRODUCTION
26
Re for oxidation of benzyl alcohol with molecular oxygen.
Among the various abundant and low cost transition metals,
Cu and Mn-containing compound as catalysts is continuing
to inspire research for developing newer and green method
for the selective oxidation of benzylic alcohols with molecular
oxygen. There are some reports on Mn-containing catalysts
for oxidation of benzylic alcohols using molecular oxygen,
but those involved supports or ligands such as octahedral
The selective catalytic oxidation of alcohols to the corres-
ponding aldehydes has been a subject of intense research as
these transformations have significant impact for laboratory
1
-4
synthesis and industrial manufacturing . Because of their
extensive applications as an important precursor to pharma-
ceuticals and fine organics, the catalytic oxidation of benzylic
alcohols to the corresponding aldehydes has received renewed
interest over the last few decades. With an increasing environ-
mental concerns, the selective oxidation of alcohols to the
corresponding aldehydes with environmentally benign
inexpensive oxidants, such as molecular oxygen has attracted
great attention from both economic and environmentally point
2
7-29
30-34
molecular sieves , various kinds of alumina as support
,
35
supported on cerium(IV) , combination with TEMPO (2,2,6,6-
33,36
37
tetramethyl-piperidyl-1-oxyl) . Mn(II) complexes immobi-
lized in the pore channels of mesoporous hexagonal molecular
3
8
sieves and hexadentate Q
3
Mn(III) complexes have been
as
5
-8
of view . Although many studies have been reported using
transition metal containing catalysts, but disadvantages of the
catalysts are, use of complex ligands or expensive novel metals
as supports/influence or difficult in preparation. Therefore,
facile preparation method using inexpensive transition metals
with a more selective and environmentally friendly system
for the catalytic oxidation of benzylic alcohols to the corres-
ponding aldehydes is still in demand.
reported for oxidation of alcohols using TBHP and H
2
O
2
oxidant, respectively. Thus, there is renewed interest for
searching inexpensive transition-metal catalyst with facile
preparation method offering more activity and selectivity using
molecular oxygen.
39
Recently we have reported CuMn mixed oxide with Cu/
4
Mn = 1:4 for the oxidation of benzyl alcohol to benzaldehyde
with molecular oxygen. Herein, we report an evaluation of
the CuMn mixed oxide (Cu/Mn molar ratio of 1:1) as catalyst
for the selective oxidation of benzyl alcohol and its derivatives
with molecular oxygen. The effect of the catalytic activity on
the substituents in the benzyl alcohols has been compared.
The catalyst has been characterized with a combination of
various characterization techniques, such as thermogravimetric
Many efforts have been made to improve the catalytic
activity and selectivity for this process, using primarily
precious metal or metal-based compounds as catalysts, such
9
,10
11,12
13,14
15,16
Au, Pd , Ru
and Pt. . These catalysts suffer from
high cost and limited availability. Numerous studies have also
been devoted to non-noble metals or metal-based catalysts,

4
816 Ali et al.
Asian J. Chem.
analysis, powder X-ray diffraction, scanning electron micro-
scopy, transmission electron microscopy, Brunauer Emmet
Teller (BET) and Fourier transform infrared (FT-IR) spectro-
scopies.
500 ºC and found that catalyst pre-calcined at 300 ºC led higher
performances in liquid phase selective oxidation of benzyl
alcohol with molecular oxygen. Subsequently, oxidation
reactions of benzyl alcohol derivatives were carried out over
the CuMn mixed oxide pre-calcined only at 300 ºC. In a preli-
minary test, 200 mg of the CuMn mixed oxide was used as
catalyst for 2 mmol of benzyl alcohol. A 100 % conversion of
benzyl alcohol was obtained after 1 h with a specific activity
EXPERIMENTAL
Catalyst preparation: The CuMn mixed oxide (Cu/Mn
=
1:1) nanoparticles were prepared by a facile co-precipitation
method employing Cu(NO .3H O, Mn(NO .4H O and
Na CO as described in previous paper . All reagents used
-
1
-1
3
)
2
2
3
)
2
2
of 10 mmol g h at 102 ºC. Under the same conditions,
benzyl alcohol was converted into benzaldehyde with 100 %
conversion after 35 min. in presence of 300 mg catalyst with a
39
2
3
were analytical grade and purchased from MERCK and BDH
Chemical Ltd. and used without further purification. The
powder oxide was calcined at 300 ºC in air for 12 h and used
for oxidation reaction. The catalyst was prepared with the
nominal composition of CuMnOx which is in good agreement
-1
-1
specific activity of 11.43 mmol g h . Thus, 300 mg catalyst
was used as an optimized amount for oxidation of benzyl
alcohol and its derivatives.
To evaluate the scope of the CuMn mixed oxide as catalyst,
oxidation reactions with various electron withdrawing and
electron donating substituents on benzyl alcohol were carried
out under the same conditions used for benzyl alcohol. Table-1
shows conversion of benzyl alcohol and its derivatives as a
function of time. It can be seen that all alcohols converted
completely (100 %) at different times, but the selectivity to
the corresponding aldehydes remained unchanged (> 99 %)
for all the reactions.
with the chemical analysis result Cu1.00Mn1.005
O .
x
Thermogravimetric analysis was carried out using Perkin-
Elmer Thermogravimetric analyzer 7. The temperature was
-1
raised from 25 ºC to 800 ºC at a heating rate of 10 ºC min .
X-ray powder diffraction pattern was recorded on a diffracto-
meter (Rigaku, Ulttima IV) at 40 kV and 40 mA using CuK
α
radiation (λ = 1.54056 Å) for samples pre-calcined at 300 ºC.
Diffraction data were collected from 5-70º with continuous
-1
mode, scan speed 2.0 deg min , sampling width 0.02 deg.
Scanning electron microscopy together with elemental analysis
The conversion of substituted benzyl alcohol with electron
donating -CH group attached to the phenyl ring proceed little
3
(
Energy Dispersive X-Ray Analysis: EDX) analysis was
faster than that of benzyl alcohol. For a complete conversion,
time required for substrate having electron donating -OCH3
group on the phenyl ring is equal to that of benzyl alcohol
(Table-1). On the other hand, attachment of electron withdrawing
conducted on Jeol-SEM model JSM 6360A (Japan). Trans-
mission electron microscopy was carried out using Jeol TEM
model JEM 1101. Surface area of the CuMn mixed oxide was
measured using low temperature nitrogen adsorption by
Brunauer-Emmett-Teller (BET) method on a NOVA 4200e
surface area and pore size analyzer. The infrared spectrum was
recorded as KBr pellets using a Parkin-Elmer 1000 FT-IR spectro-
photometer. The IR spectrum recorded was in the region of 4000-
groups (-NO , -Cl) to the phenyl ring decrease the efficiency
2
of the oxidation reaction and required longer time for a complete
conversion compared with that of the benzyl alcohol. The
results indicated that electron density on the phenyl ring played
an important role in the reactivity of the oxidation reaction.
-1
4
00 cm .
Substituent (-NO ) attached to the meta-position of the phenyl
2
Oxidation reaction and product analysis: The synthe-
ring affects more than that of para-position. Steric hindrance
is another important factor that affect the reactivity as bulky
electron donating -C(CH ) and electron withdrawing -CF
groups attached to the phenyl ring decreased the efficiency of
the reaction. Therefore, it can be concluded that oxidation
reaction catalyzed by CuMn mixed oxide is influenced by both
electronic and steric effects.
Characterization of CuMn mixed oxide catalyst: The
structure and morphology of the CuMn mixed oxide nano-
particles catalyst were studied with a combination of various
characterization techniques. Thermogravimetric analysis is
showing the thermal stability of the CuMn mixed oxide (Fig. 1).
TGA curve of the oxide showed no significant weight
loss up to 450 ºC, suggesting thermal stability of the compound
up to 450 ºC. The loss of wt % in the temperature 25-300 ºC is
observed to be 4 % which is due to removal of surface-
absorbed water or bound water in the structure of the oxide.
The presence of water in the catalyst was also detected by IR
spectroscopy. The noticeable step of thermal decomposition
is observed between 450º and 615 ºC with a loss of wt. %
sized CuMn mixed oxide nanoparticles were tested for the
catalytic activity of benzyl alcohol and some of its derivatives
in liquid phase oxidation using molecular oxygen. The
reactions were carried out in a three-necked flask connected
with oxygen gas cylinder and condenser under atmospheric
pressure. In a typical catalytic experiment, 300 mg catalyst
and 2 mmol substrates were charged in 10 mL toluene as
solvent. The mixture was then heated on an oil bath at 102 ºC.
3
3
3
-1
Oxygen was bubbled at a flow rate of 20 mL min into the
mixture with continuous stirring using a magnetic bar. The
reaction temperature was maintained at 101-103 ºC. A small
amount of reaction mixture was collected after a certain time.
The collected reaction mixtures were separated by centrifuga-
tion and the liquids were analyzed by gas chromatography (GC)
on GC-7890AAgilent Technologies Inc. equipped with a flame
ionization detector (FID) and a 19019S-001 HP-PONA column.
The conversions of the alcohols and the selectivity to the corres-
ponding aldehydes were calculated by the peak areas.
RESULTS AND DISCUSSION
6.18. This loss of wt. may due to decomposition of CuO and
Catalytic performance: Initially we examined the effect
of calcination temperature on the catalytic performance of the
CuMn mixed oxide nanoparticles pre-calcined at 300 ºC and
MnCO and/or formation of Mn phase above 450 ºC. The
total loss of weight observed to be 16.32 % when the catalyst
was heated up to 800 ºC. It can be concluded that the catalyst
3
2
O
3

Vol. 25, No. 9 (2013)
Selective Oxidation of Benzylic Alcohols with Molecular Oxygen 4817
visible peaks can be attributed to the formation of a little
TABLE-1
SELECTIVE OXIDATION OF BENZYL ALCOHOLS INTO
CORRESPONDING ALDEHYDES OVER CuMn
MIXED OXIDE NANOPARTICLES CATALYST USING
MOLECULAR OXYGEN AS OXIDANT
amount of CuMn
2
O phase and low intensity of the diffraction
4
pattern. Most of the peaks that appear in the diffraction pattern
are broad suggesting an amorphous nature or small particle
size of the CuMn mixed oxide. This amorphous nature or small
particle size may be attributed to the high catalytic activity of
the catalyst.
Time
Conversion Selectivity
Entry
Substrate
Product
(min)
(%)
(%)
OH
O
H
1
35
100
>99
100
100
OH
O
H
95
95
2
3
4
30
35
45
100
100
100
>99
>99
>99
CH3
CH3
OH
90
90
O
H
85
85
OCH3
OH
OCH3
H
80
80
O
10
10
0
0
2
20
0
0
0
3
30
0
0
0
4
4
0
00
0
5
5
0
00
0
6
6
0
00
0
7
7
0
0
0
0
8
8
0
0
0
0
T
T
e
e
m
m
p
pe
e
r
r
a
a
tuturere(º
(
C
°
)
C)
Fig. 1. TGA profile of CuMn mixed oxide
NO2
OH
NO2
H
1
1
6
60
0
0
0
ꢀ
∗
ꢀ
•
MnCO (80868)- rhombohedral
3
O
O
O
ꢀ-CuO (43180)- monoclinic
1400
1400
∗
CuMn O4 (93434) Cubic
•
2
5
6
70
50
100
100
>99
>99
1200
1200
Cl
Cl
H
1000
1000
OH
•
ꢀꢀ
ꢀ•
•
•
•
•
ꢀ
800
800
∗ ꢀ
∗
ꢀ
ꢀ
•
ꢀ
∗•
∗
•
600
600
2
0
30
40
50
60
70
OH
H
20
30
40
50
60
70
2θ (º)
2θ
(°)
Fig. 2. Powder X-ray diffraction pattern of CuMn mixed oxide. Each
symbol indicates peak of the corresponding phases
7
8
100
60
100
100
>99
>99
F
F
F
F
Fig. 3 shows image of scanning electron microscopy
F
F
H
(SEM) of the CuMn mixed oxide calcined at 300 ºC.A uniform
OH
O
spherical morphology with the average diameter being ca 1050
nm was observed. To ascertain surface area of the CuMn mixed
oxide BET analysis was carried out and obtained to be 101.86
NO2
NO2
2
m /g.
Reaction conditions: 300 mg catalyst, 2 mmol benzyl alcohol in 10 mL
toluene, reaction temperature 102 °C
TEM image of the catalyst (Fig. 4) shows no indication
of the existence of a unique phase and is not well crystalline.
Copper and manganese oxides as well as other species were
distributed over all the area. Particle size distribution calcu-
lated from the TEM image (Fig. 4) using Image J 1.44 is
shown in Fig. 5. Mean particle size was obtained to be 11.70
is thermally stable up to 300 ºC with the presence of two main
phases CuO and MnCO
two components.
3
as X-ray diffraction confirmed these
40
X-ray diffraction pattern (Fig. 2) of the CuMn mixed oxide
pre-calcined at 300 ºC shows that the oxide consists of two
±
0.02 nm.
FT-IR spectra of the CuMn mixed oxide is shown in Fig. 6.
major phases rhombohedral MnCO
monoclinic CuO (ICSD #43180) and a minor cubic CuMn
ICSD #93434) phase. Peaks of the corresponding phases are
indicated by different symbols in Fig. 2. Some peaks of cubic
CuMn phase are not apparent, but an influence in peak
intensity/width of co-existing phase can be observed. The less
3
(ICSD #80868) and
-1
The IR band appearing at 3420 cm can be due to the presence
of OH group and H O as bound water in the structure. The
absorption band at 1633 cm corresponds to bending vibration
modes of combined water molecules. The presence of absor-
O
2 4
(
2
-1
O
2 4
-1
ption bands at 583 and 539 cm may be assigned to vibration

4
818 Ali et al.
Asian J. Chem.
41
100
of Cu-O in monoclinic CuO. Morales et at. observed similar
band in copper-manganese mixed oxide. Peak appeared at 861
9
5
-1
2-
cm may be resulting from the distortion vibration of CO .
3
-1
42
90
The characteristic bands at ca 672 and 524 cm for Mn
2
O
3
1633.33
were not observed, suggesting the absence of Mn phase
which is in agreement with the X-ray diffraction pattern.
2
O
3
85
80
75
70
65
60
8
61.90
27.32
7
1
400.08
3419.72
583.28
5
39.17
4
000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm )-1
-
1
Wavenumber (cm )
Fig. 6. Fourier transform infrared (FT-IR) spectroscopy of CuMn mixed
oxide (Cu/Mn = 1:1) pre-calcined at 300 ºC
Conclusion
The CuMn mixed oxide nanoparticles were prepared by
a facile method and found to be highly selective catalyst for
oxidation of benzyl alcohol to benzaldehyde using molecular
oxygen. The CuMn mixed oxide provided high selectivity (>
Fig. 3. Scanning electron microscopy image of CuMn mixed oxide pre-
calcined at 300 ºC
99 %) to aldehyde at a complete conversion of benzyl alcohol.
The catalyst also afforded 100 % conversion with high selec-
tivity (>99 %) to the corresponding aldehydes for the benzyl
alcohol derivatives substituted with various electron donating
[
-CH
3
, -OCH
3
, -C(CH
3
)
3
] and electron withdrawing (-Cl, -NO
2
,
-
CF ) groups on the phenyl ring. The selectivity remained
3
unchanged for all the reactions although the conversions were
found to be affected by the substituent groups. Substrates
having electron withdrawing and/or bulky groups required
longer reaction time compared with those having electron
donating groups on the phenyl rings.
ACKNOWLEDGEMENTS
This work is supported by the Research Center, College
of Science, King Saud University, Riyadh, Kingdom of Saudi
Arabia.
Fig. 4. Transmission electron microscopy image of CuMn mixed oxide
pre-calcined at 300 ºC
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