Copper on boehmite: A simple, selective, efficient and reusable heterogeneous catalyst for oxidation of alcohols with periodic acid in water at room temperature

Article abstract of DOI:10.1016/j.apcata.2010.11.012

Oxidation of various aliphatic, aromatic, alicyclic, benzylic and allylic alcohols to corresponding carbonyl compounds is studied in water at room temperature over copper on boehmite [Cu/AlO(OH)] catalyst which is prepared from CuCl₂·2H₂O, pluronic P123 and Al(O-sec-Bu) ₃. The prepared catalyst was characterized by HR-TEM, SEM-EDX and IR spectroscopy. The reaction conditions for catalytic oxidation of alcohols are optimized with different mole ratio, solvents, and oxidants using 1-phenylethanol system as a model. The scope of the reaction is extended to various types of alcohols. Chemoselectivity, heterogeneity and reusability tests were performed. The use of water as a solvent at room temperature makes the reaction interesting from both an economic and environmental point of view.

Full text of DOI:10.1016/j.apcata.2010.11.012

Applied Catalysis A: General 392 (2011) 218–224
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Copper on boehmite: A simple, selective, efficient and reusable
heterogeneous catalyst for oxidation of alcohols with periodic
acid in water at room temperature
S. Ganesh Babu, P. Aruna Priyadarsini, R. Karvembu∗
Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2010
Received in revised form 25 October 2010
Accepted 7 November 2010
Available online 13 November 2010
Oxidation of various aliphatic, aromatic, alicyclic, benzylic and allylic alcohols to corresponding carbonyl
compounds is studied in water at room temperature over copper on boehmite [Cu/AlO(OH)] catalyst
which is prepared from CuCl2·2H2O, pluronic P123 and Al(O-sec-Bu)3. The prepared catalyst was char-
acterized by HR-TEM, SEM-EDX and IR spectroscopy. The reaction conditions for catalytic oxidation of
alcohols are optimized with different mole ratio, solvents, and oxidants using 1-phenylethanol system
as a model. The scope of the reaction is extended to various types of alcohols. Chemoselectivity, hetero-
geneity and reusability tests were performed. The use of water as a solvent at room temperature makes
the reaction interesting from both an economic and environmental point of view.
Keywords:
Oxidation
Alcohols
Cu/AlO(OH)
Periodic acid
Water
© 2010 Elsevier B.V. All rights reserved.
Room temperature
1. Introduction
and need to use toxic solvents. To overcome this, various cat-
alytic methods for the oxidation of alcohols have been developed
Selective oxidation of alcohols to carbonyl compounds is one
of the most pivotal functional group transformations in organic
synthesis [1–8], since the products are valuable both as interme-
diates as well as being high value components for the perfumery
industry [1,9,10]. Aldehydes of low molecular weight are con-
densed in an aldol reaction to produce derivatives used in the
plasticizer industry. Traditionally, oxidation of alcohols are per-
formed with stoichiometric amounts of inorganic oxidants, notably
chromium(VI) reagents [11]. These oxidants are not only relatively
expensive, but they also generate copious amounts of heavy-
metal waste and have toxic issues. Moreover, the reactions are
often performed in environmentally undesirable solvents, typi-
cally chlorinated hydrocarbons. There are some milder oxidation
reactions using Swern and Dess–Martin reagents [12,13]. Swern
oxidation releases toxic volatile by-products such as dimethyl sul-
fide and carbon monoxide. Dess–Martin periodinane was prepared
from 2-iodoxybenzoic acid which usually decomposes explosively.
Although individually having some synthetic advantages, most
methods suffer from one or more experimental drawbacks such
as severe reaction conditions, complicated reaction procedures
recently [14]. Hence, the exploration of a mild but efficient oxi-
dation method with easy handling and nontoxic green solvent
is a challenging area of research in organic synthesis. Many
examples of homogeneous systems make use of palladium [15],
copper [7] or ruthenium compounds [16], typically in toluene as
solvent. The use of an organic solvent, such as toluene, neces-
sitates a tedious distillation and cumbersome recovery of the
catalyst. Furthermore, the method is not suitable for products
that have boiling points close to that of the organic solvent
used.
Hypervalent iodine reagents have attracted increasing interest
as oxidants in organic synthesis due to their mild, selective and
environmentally benign oxidizing properties [17]. Periodic acid
is used as oxidant in several mild and selective oxidation reac-
tions. Chromium trioxide [18], pyridinium chlorochromate [19],
fluorochromate [20], bis(trimethylsilyl)chromate [21], chromi-
umtris(acetylacetonate) [22], Fe(III)/2-picolinic acid [23] and KBr
[24] have been used as catalysts for the oxidation of alcohols
with periodic acid. But those reactions are carried out in chlori-
nated solvents or other organic solvents and sometimes at elevated
temperatures. Hence, we herein report an effective procedure
for the selective oxidation of alcohols to the corresponding car-
bonyl compounds catalyzed by Cu/AlO(OH) using H5IO6 as oxidant
in water at room temperature. Preparation of catalyst is simple
∗
Corresponding author. Tel.: +91 431 2503636; fax: +91 431 2500133.
E-mail address: kar@nitt.edu (R. Karvembu).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.11.012

S.G. Babu et al. / Applied Catalysis A: General 392 (2011) 218–224
219
2.4. Oxidation of alcohols
Catalyst
(15.25 mg,
1 mol%)
and
H5IO6
(0.2279 g,
1 mmol/2 equiv.) were stirred with 10 mL of H2O taken in the
round bottomed flask. The substrate (0.5 mmol/1 equiv.) was
added slowly to the stirring solution. Stirring was continued for
required time at room temperature. After the requisite time, the
catalyst was separated by centrifugation. The centrifugate was
decanted slowly and the product was extracted with diethyl ether.
The ether extract was evaporated. Then the product was dissolved
in double distilled acetone and then analysed by GC. Authentic
samples of both reactant and products were used to verify the
retention times and to confirm the product formation. The ether
extract was concentrated and chromatographed on a silica gel
column with n-hexane:ethanol (4:1) as the eluting solvent to give
carbonyl product. The used catalyst was washed with diethyl ether
and then it was reused for the subsequent oxidation of alcohol for
testing the catalyst reusability.
Scheme 1. Preparation of catalyst, Cu/AlO(OH).
and no toxic by-products are released in the present catalytic
system.
2. Experimental
2.1. Materials
All the reagents used were of chemically pure and analar grade.
Commercial grade solvents were distilled according to normal
procedures and dried over molecular sieves before use. All other
chemicals were purchased from Aldrich and were used without
further purification.
2.5. Product analysis
Gas chromatograph is equipped with 5% diphenyl and 95%
dimethyl siloxane, Restek capillary column (60 m length, 0.32 mm
dia) and a flame ionization detector (FID). The initial column tem-
perature was increased from 60 to 150 ◦C at the rate of 10 ◦C/min
and then to 220 ◦C at the rate of 40 ◦C/min. Nitrogen gas was used
as a carrier gas. The temperatures of the injection port and FID
were kept constant at 150 and 250 ◦C, respectively during product
analysis.
2.2. Catalyst preparation
Catalyst was prepared by following literature procedure
[25]. CuCl2·2H2O (400 mg, 2.3 mmol), pluronic P123 (4.0 g)
[EO20PO70EO20 (EO = ethylene oxide, PO = propylene oxide)], and
absolute ethanol (10 mL) were added in 100 mL round bottomed
flask equipped with a condenser. To get a solution, this mixture
was stirred for 30 min at room temperature. Al(O-sec-Bu)3 (9.1 g,
37 mmol) was added carefully. After being stirred at 160 ◦C for 3 h,
3 mL of water was added. The reaction mixture was stirred further
for 30 min at 160 ◦C, cooled down and kept at room temperature
for 3 h. The resulting bluish solid was filtered, washed with ace-
tone, and dried at 120 ◦C for 2 h to give a bluish green powder (3 g,
3.2 wt.% of Cu) (Scheme 1). The copper content was estimated by
SEM-EDX analysis.
3. Results and discussion
3.1. Catalyst preparation and characterization
CuCl2·2H2O, pluronic P123 and absolute ethanol were stirred in
100 mL round bottomed flask equipped with a condenser for 30 min
at room temperature. To the solution, Al(O-sec-Bu)3 was added
carefully. After being stirred at 160 ◦C for 3 h, water was added. The
reaction mixture was stirred further for 30 min at 160 ◦C, cooled
down and kept at room temperature for 3 h. The resulting bluish
solid was filtered, washed with acetone, and dried at 120 ◦C for 2 h
to give a bluish green powder (Scheme 1).
2.3. Catalyst characterization
HR-TEM analysis was done in Hitachi instrument. SEM-EDX data
were recorded in Hitachi SU6600 scanning electron microscope.
The electronic spectra were recorded in PG Instrument T90+ spec-
trophotometer. The IR spectra were recorded in Perkin Elmer FT-IR
spectrometer using KBr plates. Gas chromatographic analysis is
done in Shimadzu-2010 gas chromatograph (GC).
The SEM images of the catalyst suggest that the particle size
of boehmite matrix is in the range of 20–30 ␮m above which Cu
nanoparticles are adsorbed. This was further confirmed by HR-TEM
analysis. The size of Cu nanoparticles was found to be 20–30 nm
from HR-TEM analysis (Fig. 1). From the SEM-EDX result, Cu weight
percentage is found to be 3.2%. This value matches with the already
Fig. 1. HR-TEM Images: (a) low resolution (20 nm bar scale), (b) high resolution (5 nm bar scale).

220
S.G. Babu et al. / Applied Catalysis A: General 392 (2011) 218–224
Table 1
100
Effect of oxidant, catalyst amount and temperature on oxidation of 1-phenylethanol.
Entry
Substrate Catalyst
Oxidant
(equiv.)
Time (h)
Temperature Yielda
(equiv.)
(mol%)
(◦C)
(%)
80
60
40
20
0
1
2
3
4
5
6
1
1
1
1
1
1
2
4
4
4
1
0
0.5
2.0
2.0
4.0
2.0
2.0
15
27
27
60
27
27
27
33
83
11
71
96
12
15
15
5
5
5
a
GC yield.
reportedliteraturevalue[25]. IntheIRspectrum, peakat 3400 cm−1
corresponds to OH stretching frequency. The peak at 1075 cm−1 has
been assigned as due to Al O stretching frequency. The other peaks
at 1644 cm−1, 1111 cm−1, 624 cm−1 and 516 cm−1 correspond to
Al–O stretching and bending frequencies [26].
H2O2
t-BuOOH
NMO
H5IO6
3.2. Optimization of reaction conditions
Fig. 3. Effect of oxidant on oxidation of 1-phenylethanol.
In order to get effective results, the reaction variables such
as solvent, oxidant, amount of oxidant and amount of catalyst
were optimized. For this purpose, 1-phenylethanol was chosen as
a model substrate. The completion of the reaction is monitored
through GC.
The reaction was studied at different mole ratios of 1-
phenylethanol, catalyst, and periodic acid (Table 1). In order to get
good yield, the mole ratio of the oxidant is increased and the mole
ratio of catalyst is decreased. This is because, as the mole ratio of the
oxidant is increased the oxygen supply to the substrate is increased
and hence, the reaction proceeds well with good yield. Similarly,
as the mole ratio of the catalyst is decreased, the substrate may
interact with the oxo species effectively as the distance between
substrate and oxo species might be lower. This favors the oxidation
process. A blank run i.e., the reaction carried out in the absence of
the catalyst gives very little conversion (only 12%).
The experiment has been conducted at room temperature in dif-
ferent solvents (dichloromethane, acetonitrile, toluene or water)
taking 1 equiv. of 1-phenylethanol, 1 mol% of the catalyst and
2 equiv. of periodic acid in a round bottom flask (Fig. 2). Since the
solubility of the oxidant periodic acid is good in water the reaction
proceeds well in water solvent when compared to the others. Also,
the catalyst is more dispersed in water medium than any other
solvents. It enhances the catalytic activity.
The experiment has been conducted at room temperature
with different oxidants (2 equiv.) viz. hydrogen peroxide, t-butyl
hydroperoxide, N-methylmorpholine-N-oxide (NMO) or periodic
acid, taking 1 equiv. of 1-phenylethanol and 1 mol% of the catalyst
in a round bottom flask with 10 mL of water (Fig. 3). The yield is
tremendously increased after introducing 2 mmol of periodic acid
as oxidant in shorter reaction time. From the optimization studies,
it is evident that water is the best solvent and periodic acid is the
best oxidant for the present catalytic system.
3.3. Extension of scope
To establish the generality of the method, both primary and sec-
ondary allylic, benzylic and aliphatic alcohols were oxidized with
periodic acid to give corresponding carbonyl compounds in good
to excellent yields and results are shown in Table 2.
Oxidation of alcohols is a dehydrogenation process. The removal
of H from the CH or CH2 group is difficult. When an electron with
drawing group is attached with CH or CH2 group, CH or CH2 pro-
ton(s) become(s) more acidic, highly reactive and hence easily
removed. Active aryl secondary alcohols such as 1-phenylethanol,
diphenylcarbinol and 1-indanol can be smoothly oxidized to ace-
tophenone, benzophenone and 1-indanone with excellent yields,
respectively (Table 2, entry 1, 2 and 8). Considering the mesomeric
effect at the 1-phenylethanol system, electron donating sub-
stituents such as methoxy, phenyl, bromo and chloro groups
decrease the yield considerably (Table 2, entry 3–6). Presence of
chloro group at the second position in 1-(2-chlorophenyl)-ethanol
decreases the yield of the corresponding carbonyl compound due
to steric hindrance (Table 2, entry 7).
Oxidation of ␣-methyl-2-naphthalene methanol (Table 2, entry
3) yielded the aldehyde without the oxidation of naphthalene
ring unlike the CrO3/H5IO6 oxidation [27]. In the conversion of 1-
indanol to 1-indanone (Table 2, entry 8), present catalytic system
(with the yield of 95% after 3 h) is better compared to KBr catalyzed
oxidation [24] as well as Ru-catalyzed oxidation [28] (83% after
5 h) with the same oxidant. It is worthwhile to mention that oxida-
tion of benzyl alcohol proceeds well with this system to give 93%
yield of benzaldehyde without any appreciable over oxidation to
the corresponding carboxylic acid (Table 2, entry 9).
100
80
60
40
20
0
Oxidation of allyllic alcohols to ␣,␤-unsaturated carbonyl com-
pounds is an interesting process in the view of chemoselectivity.
It was found that cinnamaldehyde was obtained from cinnamyl
alcohol in 41% yield after 7 h of stirring (Table 2, entry 10)
Dichloro methane
Acetonitrile
Toluene
Water
Fig. 2. Effect of solvent on oxidation of 1-phenylethanol.

S.G. Babu et al. / Applied Catalysis A: General 392 (2011) 218–224
221
Table 2
Oxidation of alcoholsa.
Entry
Substrate
Product
Time (h)
Conversionb (%)
Selectivityb (%)
Yieldb (%)
O
O
OH
1
2
3
5
3
3
99
97
95
97
97
OH
100
99
97 (91)c
94 (87)c
O
OH
OH
O
CH3O
CH3O
4
5
5
98
99
94
99
92 (87)c
98 (94)c
OH
O
Br
Br
10
OH
O
Cl
Cl
Cl
Cl
6
8
90
100
90 (82)c
O
OH
OH
7
8
13
3
78
99
100
96
78
O
95 (93)c
O
H
OH
9
7
99
94
93
O
CH
CH
H
C
H
OH
C
H
10
11
7
56
34
73
41
34
H
OH
O
24
100
OH
O
12
24
78
95
74
O
O
OH
OH
13
14
10
24
53
35
81
43
35
100
OH
O
15
24
16.1
100
16
a
Reaction conditions: Substrate (1 equiv.), Cu/AlO(OH) (1 mol%), H5IO6 (2 equiv.), water (10 mL), 27 ◦C.
Determined by GC analysis.
Isolated yield is given in parenthesis.
b
c

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S.G. Babu et al. / Applied Catalysis A: General 392 (2011) 218–224
Scheme 2. Chemoselectivity of catalyst, Cu/AlO(OH).
whereas NaIO4/2,2,6,6-tetramethylpiperidine-1-oxy/NaBr system
gives only 18% of aldehyde from the same substrate [29]. In the
same way oxidation of cis-hexen-1-ol gives the corresponding ␣,␤-
unsaturated carbonyl compound, the yield is 34% (100% selectivity)
after 24 h of stirring (Table 2, entry 11). Solubility of few solid
substrates particularly diphenylcarbinol, ␣-methyl-2-naphthalene
methanol and 1-indanol is less in water but reaction proceeds well
without causing any trouble which is difficult in other systems
[30,31].
1-Cyclohexylethanol is converted into its corresponding ketone
in 74% yield after stirring for 24 h (Table 2, entry 12). The only short-
coming of this method is less sensitivity towards aliphatic alcohols.
Cyclohexanol and cyclopentanol are converted to cyclohexanone
(43% yield) and cyclopentanone (35% yield) after 10 h and 24 h
respectively (Table 2, entry 13 and 14). In the case of aliphatic and
alicyclic alcohols, slower oxidation required longer reaction time.
The oxidation of dl-isoborneol cannot be completed within 24 h,
due to the steric hindrance from the bridge head (Table 2, entry
15).
60
40
20
0
0
1
2
3
4
Time (h)
Fig. 4. Heterogeneity test for oxidation of ␣-methyl-2-naphthalene methanol.
[Reaction conditions: ␣-methyl-2-naphthalene methanol (86.11 mg, 0.5 mmol/
1 equiv.), Cu/AlO(OH) (15.25 mg, 1 mol%), H5IO6 (0.2279 g, 1 mmol/2 equiv.), water
(10 mL), 27 ◦C].
3.4. Chemoselectivity
To study the chemoselectivity, a mixture of primary and sec-
ondary alcohols was subjected to oxidation in presence of 1 mol% of
catalyst. When benzyl alcohol and 1-phenylethanol were allowed
to react, the former oxidized to benzaldehyde in 83% yield and the
latter gave acetophenone in 13% yield (Scheme 2). The selective
oxidation of benzyl alcohol in presence of 1-phenylethanol may
be due to the fact that ␣-CH unit is more acidic than that of 1-
phenylethanol [32]. This study clearly reveals that this method can
be applied for the chemoselective oxidation of primary alcohols in
the presence of secondary hydroxy groups.
100
80
60
40
20
0
3.5. Heterogeneous nature of the catalyst
In order to prove the heterogeneous nature of the catalyst and
the absence of Cu leaching, a heterogeneity test was performed,
in which the catalyst was separated from the reaction mixture at
approximately 61% conversion of the starting material through cen-
trifugation. The reaction progress in the filtrate is monitored (Fig. 4).
No further oxidation occurs even at extended times, indicating that
no active species (copper) leach from the support during reaction.
1
2
3
4
Cycle
3.6. Reusability
Fig. 5. Reusability of Cu/AlO(OH) for oxidation of ␣-methyl-2-naphthalene
methanol.
The recycling of the catalyst is very important for industrial
application. After centrifugation and washing with diethyl ether,
we could reuse Cu/AlO(OH) catalyst with 3.2 wt% Cu for the oxida-
tion of ␣-methyl-2-naphthalene methanol and results are shown
in Fig. 5. The oxidation reaction was carried out 4 times under iden-
tical reaction conditions with the recycling of Cu/AlO(OH) catalyst.
The yield of product was 88% at the 1st run. No significant decrease
was observed in the 2nd run as the yield of product was 87%. There
is a slight decrease in the yield of product during 3rd (83%) and
4th (82%) run. The yield, however was still good and indicating an
excellent reusability of the catalyst.
4. Expected mechanism
Based on the observation, typical mechanism proposed for
Cu/AlO(OH) catalysed oxidation of alcohols with periodic acid is
given in Fig. 6. In the first step the oxidant supplies oxygen to Cu
present in catalyst to form an oxo species. In the second step the
oxo species carry out the conversion of alcohols to the correspond-
ing carbonyl compounds and the catalyst is regenerated. The result
of chemoselectivity experiment is also in line with the proposed

S.G. Babu et al. / Applied Catalysis A: General 392 (2011) 218–224
223
[Cu/AlO(OH)] does not give any absorption peak in UV–vis region
(190–800 nm). A band at 222 nm was observed for periodic acid.
But the electronic spectrum of catalyst after being stirred with
periodic acid shows an absorption maximum at 192 nm. This new
absorption band may be due to the formation of Cu-oxo species.
The FT-IR spectra of pure catalyst [Cu/AlO(OH)], periodic acid
and the catalyst after stirring with periodic acid (for 5 h) were
recorded (Fig. 8). In the spectrum of pure catalyst, there is no sharp
peak around 750 cm−1. But the catalyst exhibits a weak band in this
region which may be due to the presence of trace amount of CuO
and Cu2O. Periodic acid shows a strong band at 854 cm−1 due to
I
O. Whereas in the spectrum of catalyst after stirring with peri-
Fig. 6. Expected mechanism for the Cu/AlO(OH)-H5IO6 catalytic system.
odic acid, there is a sharp peak at 757 cm−1 which may be due to
the formation of Cu-oxo species. Similar spectral changes (UV–vis
and FT-IR) have been observed during the conversion of Ru(II) to
Ru(IV)-oxo species [32]. This suggests the formation of oxo species
and also supports the proposed mechanism.
Catalyst stirred with periodic acid
1.2
Pure catalyst
Periodic acid
1.0
5. Conclusion
In conclusion, Cu/AlO(OH) was found to be a simple, selective, air
stable, efficient and reusable heterogeneous catalyst for the oxida-
tion of alcohols. Hence a new catalytic system which is mild, green
and effective for oxidation of variety of alcohols into carbonyl com-
pounds by using periodic acid as oxidant and Cu/AlO(OH) as catalyst
in water at room temperature, has been developed. This procedure
is environmentally benign, general, efficient, high yielding, safe and
operationally simple.
0.8
0.6
0.4
0.2
Acknowledgement
200
300
400
500
600
700
800
We acknowledge Department of Science and Technology, Gov-
ernment of India for financial support under the Nanomission
project (SR/NM/NS-27).
Wavelength (nm)
Fig. 7. UV–vis spectra of catalyst before and after adding periodic acid.
Appendix A. Supplementary data
mechanism. The second step of the mechanism includes removal
of ␣-hydrogen which is difficult in the case of 1-phenylethanol
because of lower acidity of ␣-CH unit. Hence, benzyl alcohol is
chemoselectively oxidized in presence of 1-phenylethanol.
The electronic spectra were analysed after sonicating 0.2 g of
catalyst per liter of water for 30 min (Fig. 7). The pure catalyst
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2010.11.012.
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