Oxidation of primary and secondary alcohols to the corresponding carbonyl compounds with molecular oxygen using 1,1-diphenyl-2-picrylhydrazyl and WO 3/Al2O3 as catalysts

Article abstract of DOI:10.1016/j.catcom.2014.01.025

The oxidation of primary and secondary alcohols to their corresponding carbonyl compounds proceeds with high efficiency under molecular oxygen in the presence of 1,1-diphenyl-2-picrylhydrazyl (DPPH) and tungsten oxide/alumina (WO₃/Al₂O₃). The method is environmentally benign, because the reaction requires only molecular oxygen as the terminal oxidant and gives water as a side product. Various aromatic, alicyclic, and aliphatic alcohols can be converted to their corresponding carbonyl compounds in excellent yields. It is noteworthy that the oxidative transformation of the alcohols proceeds chemoselectively in the presence of other functional groups. In addition, a plausible catalytic pathway is proposed.

Full text of DOI:10.1016/j.catcom.2014.01.025

Catalysis Communications 48 (2014) 78–84
Contents lists available at ScienceDirect
Catalysis Communications
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Short Communication
Oxidation of primary and secondary alcohols to the corresponding
carbonyl compounds with molecular oxygen using
1,1-diphenyl-2-picrylhydrazyl and WO3/Al2O3 as catalysts
⁎
Yaoqin Zhu, Jian Xu, Ming Lu
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of primary and secondary alcohols to their corresponding carbonyl compounds proceeds with high
efficiency under molecular oxygen in the presence of 1,1-diphenyl-2-picrylhydrazyl (DPPH) and tungsten oxide/
alumina (WO3/Al2O3). The method is environmentally benign, because the reaction requires only molecular oxygen
as the terminal oxidant and gives water as a side product. Various aromatic, alicyclic, and aliphatic alcohols can be
converted to their corresponding carbonyl compounds in excellent yields. It is noteworthy that the oxidative trans-
formation of the alcohols proceeds chemoselectively in the presence of other functional groups. In addition, a plau-
sible catalytic pathway is proposed.
Received 24 October 2013
Received in revised form 24 January 2014
Accepted 27 January 2014
Available online 1 February 2014
Keywords:
DPPH/WO3/Al2O3
Alcohol
© 2014 Elsevier B.V. All rights reserved.
Oxidation
1. Introduction
save the overall implied energy, is strongly encouraged. In this re-
spect, the recovery and reuse of the catalyst is a further important
Catalytic oxidation of alcohols to aldehydes, ketones, and car-
boxylic acids is of fundamental importance. In particular, the oxida-
tion of primary and secondary alcohols to the corresponding
carbonyl compounds is significantly useful in organic synthesis,
due to the wide ranging utility of these products as important pre-
cursors and intermediates for many drugs, vitamins, and fragrances.
As the most widely used class of oxidation reactions in organic
chemistry, many reagents and catalysts exist for these transforma-
tions, including chromium reagents [1], manganese(IV) oxide[2],
activated DMSO[3], hypervalent iodine reagents[4,5], ruthenium re-
agents[6], osmium(VIII) oxide[7], metal [8–22]or TEMPO-catalyzed
oxidation[23–25], etc. As a big field of catalysts, transition metal-
based catalysts have been well developed. Both homogeneous and
heterogeneous transition metal-catalyzed systems are reported.
However, at the same time, they are among the most polluting and
hazardous processes, often occurring with a high E-factor (mass of
waste per mass unit of product). Even some recently reported reac-
tions using transition metal based catalyst such as Cu, Ru, Au, and Pd
also afford considerable amounts of toxic metal salts. And all of
these methods still have poor atom efficiencies. From an environ-
mental point of view, it is of particular importance to develop
methods, which use cleaner oxidants and minimize the amount
and toxicity of the released waste. Moreover, the use of catalysis,
that allows processes to occur under mild conditions in order to
goal.
After many attempts, we discovered a highly selective and efficient
method for the oxidative transformation of alcohols to aldehydes and
ketones by employing the 1,1-diphenyl-2-picrylhydrazyl (DPPH)
as a catalyst, a tungsten oxide/alumina (WO3/Al2O3) as a cocatalyst
[26], and the oxygen as the terminal oxidant under mild conditions.
DPPH is characterized as a stable free radical by virtue of the delo-
calization of the spare electron over the molecule as a whole, so
that the molecules do not dimerise, as would be the case with
most other free radicals. The delocalization also gives rise to the
deep violet color, which can be characterized by an absorption
band in ethanol solution centered at about 520 nm (slightly depen-
dent on the solvent). When a solution of DPPH is mixed with a sub-
stance that can donate a hydrogen atom, then this gives rise to the
reduced form with the loss of this violet color (although there
would be a residual pale yellow color of the picryl group). Due to
these features mentioned above, DPPH has attracted much atten-
tion of the researchers. It has been employed as a standard sub-
strate [27] for ESR spectroscopy and a radical scavenger [28].
Recently, it also has been developed to estimate the antioxidant ac-
tivity of various substances [29–31]. However, it has never been
used as an organic catalyst for the oxidation of alcohols. Therefore,
our report here provides a new catalytic system for the oxidation
of alcohols.
Full details regarding the scope and reaction mechanism of
the aerobic oxidation of primary and secondary alcohols to alde-
hydes and ketones catalyzed by DPPH and WO3/Al2O3 are
described.
⁎
Corresponding author. Tel.: +86 2584315030.
E-mail address: luming302@126.com (M. Lu).
1566-7367/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2014.01.025

Y. Zhu et al. / Catalysis Communications 48 (2014) 78–84
79
Table 2
2. Experimental section
Catalytic activity of cocatalysts.
2.1. Materials and reagents
Entry
Transition-metal
catalyst
Conversion
of benzyl
Selectivity for
aldehyde (%)
alcohol (%)
All the reagents were commercially available and were used without
further purification.
Instrument used: 500 M NMR Spectrometer (Bruker Avance III
500 MHz), Gas Chromatography Instrument (Bruker GC-450), and
Ultraviolet spectrometer.
1
2
3
4
5
6
7
8
None
27
65
69
19
12
39
68
57
23
28
6
95
95
86
64
9
91
83
74
2
WO3/Al2O3
WO3/ZrO2
WO3/TiO2
WO3
TiO2
TiO(acac)2
Nb2O5
MoO2(acac)2
VO(acac)2
Fe(acac)3
RuCl2(PPh3)3
2.2. General procedure for the catalytic oxidation of primary and secondary
alcohols to aldehydes and ketones
9
10
11
12
2
45
20
A 50 mL glass flask was charged with a mixture of DPPH (98.6 mmg,
0.25 mmol), WO3/Al2O3 (0.844 g, W: 3 mmol), alcohol (50 mmol), and
30 mL toluene. Then a balloon was used to provide the oxygen for these
reactions. Then the mixture was stirred at 80 °C for 4 h. The reaction
mixture was subjected to GC analysis to supervise the reaction process
till the oxidation ended.
4
Reaction conditions: benzyl alcohol (50 mmol), DPPH (0.25 mmol), cocatalyst (3 mmol)
in toluene (30 mL), 80 °C, 4 h. Determined by GC analysis using a normalization method.
[32] (NHPI) have been used as highly efficient catalysts for the oxidation
of alcohols, alkanes, and alkyl aromatics respectively, it occurred to us
that maybe DPPH, which is similar to TEMPO and NHPI, can be effective
for the oxidation of alcohols too. Then we discovered the DPPH/WO3/
Al2O3 system and it worked. By comparison, TEMPO and NHPI were
also examined. The results of oxidations using organocatalysts men-
tioned above are summarized in Table 1. As you can see, TEMPO and
NHPI also showed great conversion and selectivity, but they were still
not as good as the DPPH system (entry 4 and 5). It's noteworthy that
WO3/Al2O3 alone is not effective (entry 2), which indicates that the ox-
idation is mainly based on the catalysis reactivity of DPPH.
DPPH is well known as a stable radical and has been used as an ESR
standard, polymerization inhibitor, and monitor of chemical reactions
involving radicals. However, it has never been used as an organocatalyst
in the oxidation of alcohols. So we examined the catalytic activity of
DPPH alone. It was found that DPPH alone was also effective in the oxi-
dation, but the yield was low (entry 1). The combination of DPPH and
WO3/Al2O3 gave an excellent result (entry 3). These findings indicate
that WO3/Al2O3 can improve the catalysis reactivity of DPPH, but
DPPH is still the main active one to achieve theses oxidations. And no
decomposition or deterioration of DPPH was observed under the reac-
tion conditions, which is in consistence with its stability.
Then we examined the catalytic activity of the cocatalysts. Represen-
tative results are shown in Table 2. DPPH alone exhibited low catalytic
activity for the oxidation (entry 1), which further supported that
WO3/Al2O3 had the ability to improve its catalytic activity. WO3/Al2O3,
WO3/ZrO2, TiO(acac)2, and Nb2O5 were proved to be excellent cocata-
lysts for the oxidation of benzyl alcohol (Table 2, entries 2,3,7 and 8)
which indicated that W, Al, Zr, Ti, and Nb were active metal centers.
While WO3/TiO2, WO3, TiO2, MoO2(acac)2, and VO(acac)2 showed low
catalytic activity (entries 4,5,6,9 and 10), it turned out that Al could
2.3. Procedure for the catalytic oxidation of benzyl alcohol in acetone
A 50 mL three-necked glass flask was charged with a mixture of
DPPH (98.6 mmg, 0.25 mmol), WO3/Al2O3 (0.844 g, W: 3 mmol), alco-
hol (50 mmol), and 30 mL acetone. One neck was connected with a
water condenser to reflux. Besides, there must be a deflated balloon
on the top of the condenser to collect the evaporated acetone. And an-
other neck was connected with a dropping funnel to add acetone.
Then an oxygen steel cylinder was used to slowly provide the oxygen
for the reaction through the third neck with an air duct. The ventilation
speed must be adjusted according to the size of the balloon. Moreover,
to maintain the airtightness of the whole reaction, the liquid level in
the dropping funnel should be kept in a certain height. Then the mixture
was stirred at 80 °C for 4 h. The reaction mixture was subjected to GC
analysis to supervise the reaction process till the oxidation ended.
2.4. Recycling of the catalyst
The first run was carried out under the same reaction conditions
described in the general procedure. After the reaction, the spent
WO3/Al2O3 could be easily separated from the reaction mixture by
filtration, and the isolated WO3/Al2O3 was reused. Vacuum distillation
gave the solvent and the product, and the residue of DPPH was reused.
These recycling procedures were repeated three times in the same
manner as in the first run.
3. Results and discussion
Catalytic oxidation of primary and secondary alcohols using
transition-metal catalysts usually gives aldehydes, ketones, carboxylic
acids, and esters at the same time. So we attempted to discover a
method that is highly selective to obtain the aldehydes. As 2,2,6,6-
tetramethylpiperidine-N-oxyl (TEMPO) and N-hydroxyphthalimide
Table 3
Effect of solvent on the DPPH-WO3/Al2O3 catalyzed oxidative transformation of benzyl
alcohol.
Entry
Solvent
Conversiona/
Selectivitya/selectivityb
for aldehyde(%)
Table 1
conversionb (%)
Activity of catalysts for the oxidative transformation of benzyl alcohol.
1
2
3
4
5
6
Acetonitrile
N,N-Dimethylacetamide
Toluene
Water
Cyclohexane
Acetone
39/23
61/33
65/48
19/16
66/47
82/48
90/83
12/11
95/93
99/99
38/35
68/63
Entry
Catalyst
Conversion of
benzyl alcohol (%)
Selectivity for
aldehyde (%)
1
2
3
4
5
DPPH
WO3/Al2O3
DPPH/WO3/Al2O3
TEMPO/WO3/Al2O3
NHPI/WO3/Al2O3
27
7
65
57
42
96
99
95
94
78
Reaction conditions: benzyl alcohol(50 mmol), DPPH(0.25 mmol), WO3/Al2O3(W:
3 mmol) in solvent (30 mL),80 °C,4 h. Determined by GC analysis using a normalization
method. Conversiona: conversion of benzyl alcohol with molecular oxygen. Conversionb:
conversion of benzyl alcohol with ambient air. Selectivitya: selectivity of aldehyde with
molecular oxygen. Selectivityb: selectivity of aldehyde with ambient air.
Reaction conditions: benzyl alcohol (50 mmol), organocatalyst (0.25 mmol), WO3/Al2O3
(W: 3 mmol) in toluene (30 mL), 80 °C, 4 h. Determined by GC analysis using a normali-
zation method.

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Y. Zhu et al. / Catalysis Communications 48 (2014) 78–84
Table 4 (continued)
Table 4
DPPH/WO3/Al2O3-catalyzed oxidative transformation of primary alcohols.
Entry Substrate
Product main byproduct
Conversion(%)/
selectivity(%)/
time(h)
Entry Substrate
Product main byproduct
Conversion(%)/
selectivity(%)/
time(h)
6
78/99/3
1
65/95/4
2
73/97/3
7
83/96/4
3
74/99/3
8
73/93/4
9
69/90/4
63/91/4
4
71/96/3
10
Reaction conditions: alcohol (50 mmol), DPPH(0.25 mmol), WO3/Al2O3(W: 3 mmol) in
solvent(30 mL), 80 °C. Determined by GC analysis using a normalization method.
improve the catalytic activity of W, but Ti decreased W's activity. Be-
sides, Mo and V were also two kinds of the active catalytic metal centers.
Fe(acac)3 and RuCl2(PPh3)3 showed poor catalytic activity (entry 11
and 12).
5
74/94/3
The solvent effect of the oxidation of benzyl alcohol catalyzed by
DPPH/WO3/Al2O3 is very important. The results are summarized in
Table 3. Among all the examined solvents, N,N-Dimethylacetamide, tol-
uene, cyclohexane, and acetone gave high conversions(entries 2,3,5 and
6). However, when N,N-dimethylacetamide was used, most of the
resulting products were benzyl benzoates. As DPPH was insoluble in
water, the conversion was low. The reactions in polar solvents, such as
acetonitrile and water resulted in low conversions. In total, when tolu-
ene was used, the conversion and selectivity were both high, so it was
chosen to be the solvent here. Besides, oxidation experiments with am-
bient air were also carried out to compare the reactivity with molecular

Y. Zhu et al. / Catalysis Communications 48 (2014) 78–84
81
Table 5
DPPH/WO3/Al2O3-catalyzed oxidative transformation of secondary alcohols.
Entry
1
Substrate
Product
Conversion(%)/time(h)
88/2
2
3
4
94/12
83/10
81/12
5
93/10
95/12
6
7
8
96/6
86/12
9
83/12
84/12
10
Reaction conditions: alcohol (50 mmol), DPPH (0.25 mmol), WO3/Al2O3 (W: 3 mmol) in solvent (30 mL), 80 °C. Determined by GC analysis using a normalization method.
oxygen. It's obvious that both conversion and selectivity were lower
than that with molecular oxygen, which indicated that higher oxygen
ratio can result in better conversion and less byproducts.
Representative results of the oxidation of primary alcohols in tolu-
ene at 80 °C in the presence of the DPPH/WO3/Al2O3 catalyst using O2
as the oxidant are displayed in Table 4. As you can see, primary alcohols
were found to be effective substrates. Under these reaction conditions,
various benzylic alcohols with different substituents on the phenyl
ring can be efficiently oxidized to the corresponding aldehydes in
65–83% conversions (Table 4, entries 1–7). Heterocyclic primary alco-
hols were smoothly oxidized to give the aldehydes in good yields
(Table 4, entry 8). These results indicated that the oxidative reactions
catalyzed by DPPH/WO3/Al2O3 were highly chemo-selective without
oxidizing other groups. In addition, alkyl alcohols were also suitable

82
Y. Zhu et al. / Catalysis Communications 48 (2014) 78–84
100
90
80
70
60
50
40
30
20
10
0
Yield (%)
Selectivity
(%)
1
2
3
Cycle
Fig. 1. Recycling of the DPPH/WO3/Al2O3 catalyst for the oxidation of benzyl alcohol. Conditions: benzyl alcohol (50 mmol), DPPH (0.25 mmol), WO3/Al2O3 (W: 3 mmol) in solvent (30 mL),
80 °C, 4 h. Determined by GC analysis using a normalization method.
substrates giving the aldehydes in 69%, 63% in conversions respectively
(Table 4, entries 9 and 10).
cyclohexanol is not only the cyclohexanone. A little amount of the adipic
acid was detected as well. It may result from the over-oxidation of the
cyclohexanone. But the other homologues of cyclohexanol were not
over-oxidized, which may due to the steric effects of their substituents
that make them harder to be attacked. And acyclic, aliphatic alcohols
were efficiently transferred into ketones in 83–86% conversions
(Table 5, entries 8–10). Comparing the oxidations of primary and sec-
ondary alcohols, it's obvious that the oxidation of secondary alcohols
takes much longer time, which may due to the steric effect of secondary
alcohols. Because the steric effect of secondary alcohols is stronger than
that of the primary alcohols, it makes the secondary alcohols more dif-
ficult to be attacked, which results in the prolonged time.
Stability and recyclability are key issues in catalysis. The DPPH/WO3/
Al2O3 catalyst can be reused without loss of catalytic activity or selectiv-
ity. After the reaction, the WO3/Al2O3 could be easily separated from the
reaction mixture by filtration. Then distillation under reduced pressure
gave the solvent, the product, and the DPPH residue. Afterwards, the re-
covered WO3/Al2O3 and DPPH were reused directly. The results obtain-
ed in a stability study of DPPH/WO3/Al2O3-catalyzed oxidation of benzyl
alcohol are shown in Fig. 1. As we can see, the yield of benzyl aldehyde
was maintained at a similar value for at least three rounds of recycling,
which indicated the excellent stability and recyclability of the catalyst.
We investigated the effect of the DPPH/WO3/Al2O3 concentrations
on catalytic performance. As shown in Fig. 2(a), a linear increase
was observed with respect to the DPPH concentration in the range of
0.05–0.25 mmol. On the other hand, with respect to the effect of the
concentration of WO3/Al2O3, no significant increase in rate was ob-
served above a W concentration of 3 mmol (Fig. 2(b)). The kinetics
study showed a first-order relationship for the amount of DPPH catalyst
Next, we also examined the oxidations of secondary alcohols in tol-
uene at 80 °C in the presence of the DPPH/WO3/Al2O3 catalyst. The re-
sults are shown in Table 5 and high yields were obtained for a wide
range of alcohols. Various secondary benzylic alcohols with alkyl groups
were effectively oxidized to ketones in 88–94% conversions (Table 5, en-
tries 1 and 2). The oxidation can also be extended to a variety of
nonbenzylic secondary alcohols, giving the corresponding ketones in
81–96% (Table 5, entries 3–10), which proved that the DPPH/WO3/
Al2O3 system had a wide utility range for the oxidative transformations.
Cyclic alcohols with variable ring sizes and substituents were found to
be effective substrates (Table 5, entries 3–7). However, it should be
noted that among these cyclic alcohols, the oxidation product of
a
100
90
95
51
95
65
94
94
93
37
80
70
60
50
40
30
20
10
0
29
24
Conversion(%)
Selectivity(%)
0.05
0.1
0.15
0.2
0.25
DPPH(mmol)
b
100
90
80
70
60
50
40
30
20
10
0
1.0
96
95
95
65
95
95
95
DPPH
Reaction mixture
0.8
65
65
50
0.6
0.4
0.2
0.0
42
Conversion(%)
Selectivity(%)
27
0
1
2
3
4
5
W(mmol)
Fig. 2. Effect of DPPH/WO3/Al2O3 concentrations on the DPPH/WO3/Al2O3-catalyzed
oxidation of benzyl alcohol: (a) different concentrations of DPPH with WO3/Al2O3
(W: 3 mmol); (b) different concentrations of WO3/Al2O3 (W) with DPPH (0.25 mmol).
Conditions: benzyl alcohol (50 mmol), DPPH (0.25 mmol), WO3/Al2O3 (W: 3 mmol) in
toluene (30 mL), 80 °C, 4 h. Determined by GC analysis using a normalization method.
150 200 250 300 350 400 450 500 550 600 650
Wavelength(nm)
Fig. 3. The UV–vis spectra of DPPH in acetonitrile and reaction mixture in acetonitrile.

Y. Zhu et al. / Catalysis Communications 48 (2014) 78–84
83
OH
C
R1
R2
.
OO
6
O
OH
C
OH
w-OH
R2
Ph
Ph
dehydration
R2
C
O2
R1
N
R1
R1
R2
OH
9
OOH w-OOH
7
NH
10
O2N
NO2
OH
Ph
Ph
.
C
4
N
R1
R2
.
N
NO2
5
O2N
NO2
Ph
Ph
.+
OH
N
NO2
8
N-
R1
R2
O2N
NO2
3
OH
R1
R2
NO2
2
1
Scheme 1. Proposed mechanism for DPPH/WO3/Al2O3-catalyzed oxidation of alcohols.
and a zero-order dependence of the reaction rate on the amount of
WO3/Al2O3 catalyst (W N 3 mmol).
or base required), making it compatible with acid- or base-sensitive
substrates; and (3) the method is amenable to gram scale with no
special precautions to exclude air or moisture. This strategy provides
an efficient and environmentally benign method for the synthesis of
aldehydes and ketones.
When we finished adding all the substrates and started the reaction,
the color of the reaction mixture changed immediately from purple to
brown. In order to explore the reason why the color changed, the UV–
vis experiment was carried out. Because toluene also has ultraviolet ab-
sorption, it was replaced by acetonitrile here to avoid the interference
arising from benzene ring. And the UV–vis absorption at 500 nm of
DPPH in acetonitrile was not observed as shown in Fig. 3. It meant
that the radical of DPPH was transferred. While a precise understanding
of the reaction mechanism awaits further study, a plausible catalytic
pathway referring to literatures [26,33–35] is proposed in Scheme 1. Ini-
tially, fast electron transfer from alcohols to DPPH (8) occurs to give the
DPPH anion (2) and hydroxyl cation radical (3). Then the hydroxyl cat-
ion radical thus formed the hydroxyl radical (4) and DPPH-H (5). The
latter radical reacted with the molecular oxygen to afford a peroxy rad-
ical, which underwent the abstraction of hydrogen from DPPH-H (5) to
give the product (7) and regenerate the DPPH (8). Then the reaction of
product (7) with tungsten oxide/alumina (WO3/Al2O3) gives the inter-
mediate (9) and tungstate hydroperoxide w-OOH. After dehydration,
the final product (10) was obtained.
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4. Conclusion
In summary, we have developed a novel oxidation method of alco-
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