Highly efficient and selective aerobic oxidation of alcohols in aqueous media by TEMPO-containing catalytic systems

Article abstract of DOI:10.1016/j.catcom.2013.09.022

The use of molecular oxygen as the oxidant and water as reaction medium is a charming research orientation in the oxidation of alcohols. In this communication, the efficient aerobic oxidation of alcohols to the corresponding carbonyl compounds in aqueous media has been achieved with TEMPO-containing catalytic systems. In the absence of base/acid, a 99% conversion and 94% selectivity for benzaldehyde were obtained for the oxidation of benzyl alcohol by TEMPO-Ce(IV)-NaNO₂ in water. Moreover, the effects of quaternary ammonium salts and anionic surfactants were investigated. The aerobic selective oxidation of different non-heterocyclic alcohols in aqueous media was further investigated, and the moderate to high yields (45.5-98.0%) of products were obtained.

Full text of DOI:10.1016/j.catcom.2013.09.022

Highly Efficient and Selective Aerobic Oxidation of Alcohols in Aqueous
Media by TEMPO-Containing Catalytic Systems
Yongtao Yan, Xinli Tong, Kaixuan Wang, Xueqin Bai
PII:
DOI:
Reference:
S1566-7367(13)00364-6
doi: 10.1016/j.catcom.2013.09.022
CATCOM 3653
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Catalysis Communications
Received date:
Revised date:
Accepted date:
20 July 2013
27 August 2013
6 September 2013
Please cite this article as: Yongtao Yan, Xinli Tong, Kaixuan Wang, Xueqin
Bai, Highly Efficient and Selective Aerobic Oxidation of Alcohols in Aqueous Me-
dia by TEMPO-Containing Catalytic Systems, Catalysis Communications (2013), doi:
10.1016/j.catcom.2013.09.022
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ACCEPTED MANUSCRIPT
Highly Efficient and Selective Aerobic Oxidation of Alcohols in
Aqueous Media by TEMPO-Containing Catalytic Systems
Yongtao Yan, Xinli Tong*, Kaixuan Wang, Xueqin Bai
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion,
School of Chemistry and Chemical Engineering, Tianjin University of Technology,
Tianjin 300384, P. R. China; Tel.: +86-22-60214259; Fax: +86-22-60214259;
E-mail address: tongxinli@tju.edu.cn or tongxli@sohu.com.
Abstract
The use of molecular oxygen as the oxidant and water as reaction medium is a
charming research orientation in the oxidation of alcohols. In this communication, the
efficient aerobic oxidation of alcohols to the corresponding carbonyl compounds in
aqueous media has been achieved with TEMPO-containing catalytic systems. In
absence of base/acid, a 99% conversion and 94% selectivity for benzaldehyde was
obtained for the oxidation of benzyl alcohol by TEMPO-Ce(IV)-NaNO2 in water.
Moreover, the effects of quaternary ammonium salts and anionic surfactants were
investigated. The aerobic selective oxidation of different non-heterocyclic alcohols in
aqueous media was further investigated, and the moderate to high yields (45.5-98.0%)
of products were obtained.
Keywords: Oxidation; Alcohols; TEMPO; Molecular Oxygen; Water
1. Introduction

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The selective oxidation of alcohols to their corresponding carbonyl compounds is
one of the most important transformations in industrial production and fine chemical
processes [1-2]. The traditional methods involve stoichiometric oxidants such as
permanganate, chromic acid and potassium dichromate, etc. [3-5]. These reagents are
toxic and hazardous, and the processes will generate many inorganic or organic waste.
Considering both economical and environmental benefits, the use of molecular
oxygen as terminal oxidant received great attentions in recent years. Correspondingly,
many catalytic systems have been developed for the aerobic alcohol oxidation [4, 6].
Among various catalytic systems, the notable heterogeneous catalysts include
ruthenium catalyst [7], palladium nanoclusters [8], gold nanoparticles [9, 10],
manganese oxides [11] and cobalt oxide [12], etc.. While, in homogeneous catalyst
systems, the combination of 2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPO) and
some mediators is attractive and promising for the aerobic oxidation of alcohols [13].
However, in TEMPO-catalyzed processes, organic solvents or ionic liquids are often
employed as the solvents. With toluene as the solvent, both Punniyamurthy’s group
and Repo’s group [14, 15] found that Cu(II) complex and TEMPO was efficient for
the aerobic oxidation of aromatic and allylic alcohols. With acetic acid as the solvent,
Mn(II)–Co(II) nitrates and TEMPO could promote the selective oxidation of alcohols
to aldehydes and ketones by oxygen [16]. In acetonitrile solvent, TEMPO and
Cu(II)-triethanolamine complexes allowed efficient oxidation of primary alcohols to
the corresponding aldehydes [17]. Furthermore, Stahl et al. [18] found that a
(2,2-bipyridine)CuI/ TEMPO catalyst system with N-methylimidazole catalyzed the

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oxidation of benzylic, allylic and aliphatic primary alcohols to aldehydes in excellent
yields. In dichlormethane solvent, the TEMPO and hydroxylamine could efficiently
catalyze the aerobic oxidation of alcohols [19]. Besides, in trifluorotoluene solvent,
Liang and co-workers [20] found that FeCl3-NaNO2- TEMPO system could promote
the oxidation of benzylic alcohols and cinnamyl alcohol in excellent yields. In ionic
liquids, the CuCl/TEMPO/base catalytic system showed good activity for the
oxidation of benzylic alcohols, and 57-99% conversion were obtained [21]. Also, the
same group found that molecular sieve 3A could replace the base and promote the
oxidation of alcohol in [bmim][PF6] ionic liquid [22]. On the other hand, the
two-phase reaction systems have been reported for the aerobic oxidation of alcohols.
For example, in the presence of TEMPO, the fluorous biphasic system [23, 24] and
the mixture of acetonitrile and water [25, 26] were employed as the solvents for the
oxidation of alcohol.
In order to avoid the safety issues associated with the flammability of mixture of
oxygen and organic solvent in the gas phase [27], employing the green and
sustainable water as the solvent are more promising and favorable [28]. In the former
investigations, several catalytic systems have been found to be useful in water;
however, the processes need to be performed in aqueous alkaline solution [29], in the
presence of AcOH [30] or with hypervalent iodine as the additive [31]. Very recently,
the efficient oxidation of primary benzylic alcohols in distilled water was performed
in the presence of CuBr2, N-isopropyl-3,5-di-tert-butylsalicyl aldimine (HL3) and
TEMPO [32]. But, the preparation of HL3 ligand is not easy.

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In this communication, a new TEMPO-containing catalytic system has been
developed for efficient aerobic oxidations of different alcohols in water, in which the
corresponding aldehydes are obtained as main products in the presence of the
TEMPO-Ce(IV)-NaNO2 system under mild conditions.
2. Experimental
2.1. General reagents and equipments
Benzyl alcohol, TEMPO, benzaldehyde, Ceric ammonium nitrate (CAN) and
NaNO2 are of analytical grade and purchased from Alfa Aesar. The organic solvents
and tetraethyl ammonium bromide (TEAB), tetraethylammonium chloride (TEAC),
tetramethyl ammonium bromide (TMAB), tetramethyl ammonium chloride (TMAC),
tetra butyl ammonium bromide (TBAB) , tetra butyl ammonium chloride (TBAC),
dodecyl trimethyl ammonium bromide (DTAB), sodium dodecyl sulfonate (SDS),
sodium dodecyl benzene sulfonate (SDBS) are of analytical grade and obtained from
commercial. The deionized water is purchased from commercial and used without
further treatment. Oxygen supplied in a high-pressure cylinder was used through a
reducing valve without further treatment.
2.2 General Procedure for the Oxidation of Alcohols
All oxidation experiments are performed in a 120 mL autoclave equipped with
the magnetic stirring and temperature control. A typical procedure for the oxidation of
benzyl alcohol is as follows: 1.08 g (10.0 mmol) of benzyl alcohol, 0.0156 g (0.1
mmol) of TEMPO, 0.274 g (0.5 mmol) of CAN, 0.0690 g (1.0 mmol) of NaNO2,
additive in suitable amount and 10 mL of H2O were charged into the reactor, and the

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atmosphere inside is replaced with oxygen after the reactor is sealed. Then, pure
oxygen is charged to 0.3 MPa at room temperature. In the following, the autoclave is
heated to 80 ºC under stirring, and then kept for 2 h. After reaction, the autoclave was
cooled to room temperature and excess gas was purged. The mixture was transferred
into a flask, in which the reactor was washed with CH2Cl2 for 3-5 times in order to
transfer completely. Next, the products are extracted with 6 mL CH2Cl2 three times.
The obtained products were analyzed with internal standard technique by GC with a
flame ionization detector (All products were determined on GC-MS with Agilent
6890N GC/5973 MS detector).
3. Results and discussion
3.1. Oxidation of benzyl alcohol with TEMPO in water
Scheme 1. The oxidation of benzyl alcohol with TEMPO-containing catalytic systems
Table 1. Oxidation of benzyl alcohol with different catalytic systems a
Entry
CAN
NaNO2
Conv. (%) b
Sel. (%) c
TEMPO
1
2
3
×
√
×
×
×
√
×
×
×
2.7
11.5
8.6
34.8
36.1
13.8
4
5
×
×
√
10.8
14.6
31.9
50.3
√
√
×
6
7
×
√
√
√
12.0
61.1
18.5
89.5
√
×
8
√
√
√
99.0
94.0
a
Reaction condations: 1.08 g benzyl alcohol in water (10 mL), in the presence of 1.0 mol%
TEMPO, 5.0 mol% CAN, 10.0 mol% NaNO2 and ETAB (20 mol%) under 0.3 MPa of O2,

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temperature 80 ºC, time 6h. The symbol “√” represents “being added”; the symbol “×”
represents “not being added” in the oxidation.
b
Conv. means the conversion of benzyl alcohol. The results are obtained by GC with internal
standard method.
c
Sel. Means the selectivity of benzaldehyde. The results are obtained by GC with internal
standard technique.
In order to investigate the performance of different catalytic systems in water, the
oxidation of benzyl alcohol in the presence of TEAB is selected as a model reaction
(Scheme 1).
From the obtained experimental results in Table 1, the conversion of benzyl
alcohol is only 2.7% in the absence of any catalyst at 80 ºC for 6 h (entry 1). The
conversion is elevated to 11.5% when 1.0 mol% TEMPO is added (entry 2). Besides,
in the presence of single CAN or NaNO2, an 8.6% or 10.8% conversion is obtained,
respectively (entries 3 and 4). In the following, the combination of two compounds is
used for the oxidation of benzyl alcohol, in which the conversion of benzyl alcohol is
respectively 14.6% or 12.0% when the combination of TEMPO-CAN or CAN-NaNO2
is employed (entries 5 and 6). While, a 61.1% conversion in 89.5% selectivity for
benzaldehyde is obtained when the TEMPO-NaNO2 is used (entry 7). Furthermore,
the conversion and selectivity for benzaldehyde are respectively increased to 99.0%
and 94.0% when the TEMPO-CAN-NaNO2 catalytic system is employed (entry 8).
Moreover, no acids are detected from the obtained products by GC-MS technique.
These results indicate that there exists the synergistic effect between the component
TEMPO, CAN and NaNO2 in water. The oxidation process is suggested as follows:
the TEMPO oxoammonium salt reacts with beznyl alcohol to give the product
benzaldehyde. In the following, Ce(IV) functions with TEMPOH to produce the

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TEMPO oxoammonium salt and Ce(III). Then, NO2 that is originated from NaNO2
drives the oxidation of Ce(III) to Ce (IV), followed by the reaction of NO with O2
generating NO2 in which the catalytic cycle is realized.
3.2. Effect of the amount of CAN
Figure 1. Effect of the amount of CAN in the oxidation of benzyl alcohol.
Figure 1 shows the effect of different amounts of CAN in the oxidation of benzyl
alcohol. Both the conversion and product yield are gradually increased along with the
amount of CAN being increased from 1.0 mol% to 5.0 mol%. However, the
selectivity of benzaldehyde almost kept unchanged. The maximum conversion and
yield of benzaldehyde is 99.0% and 93.0% in the presence of 5.0 mol% CAN,
respectively. When the amount of CAN was further increased to 7.0 mol%, the
conversion and product yield are hardly changed. Thus, the suitable amount of CAN
is 5.0 mol% for the oxidation of benzyl alcohol.
3.3. Effect of different additives

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As we know, some surfactants can often accelerate the substrate to dissolve in
aqueous media and promote the oxidation reaction based on the formation of the
micro-reaction environment. Thus, the effects of different additives including
quaternary ammonium salts and several anionic surfactants have been studied. Table 2
presents the effect of different additives in the oxidation of benzyl alcohol.
Table 2. Effects of different additives in the oxidation of benzyl alcohol a
Entry Additive Conversion (%)b Selectivity (%) Yield (%) TOF (h-1) c
1
2
3
4
5
6
7
8
9
TMAC
63.1
97.8
61.7
42
TMAB
TEAC
TEAB
TBAC
TBAB
DTAB
SDBS
SDS
99.0
65.9
99.0
98.0
98.0
64.6
66
44
99.0
40.3
93.7
96.6
92.8
38.9
66
27
99.0
72.3
99.0
55.3
92.9
93.1
99.0
87.6
92.0
67.3
98.0
48.4
66
48
66
37
a Reaction conditions: 1.08 g benzyl alcohol, in 10 mL water, 1.0 mol% TEMPO, 5.0 mol% CAN
and 10.0 mol% NaNO2, in the presence of 1.0 mol% additive, under 0.3 MPa of O2, temperature
80 ºC, time 1.5 h.
b The results are obtained by GC with the internal standard method.
c The values of TOF (turnover frequency) are obtained from the formula:
TOF = the mole amount of reactant / (the mole amount of TEMPO · reaction time).
In the case of quaternary ammonium salts, the bromide salts are more useful than
chloride salts in oxidation of benzyl alcohol, in which only a 61.7%, 64.6% or 38.9%
yield of benzaldehyde is respectively obtained when TMAC, TEAC or TBAC is used
as the additive (entries 1, 3, 5). While, 98.0%, 92.8% and 92.0% yields of
benzaldeyde are obtained in the presence of TMAB, TEAB and TBAB (entries 2, 4,

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6). Otherwise, with DTAB as the additive, the yield of benzaldehyde is only 67.3%
(entry 7), which showed that the long alkyl chain is disadvantageous to the alcohol
oxidation in water. On the other hand, the influences of anionic surfactants are also
examined. Therein, SDBS exhibits the outstanding promotion where the yield of
benzaldehyde reaches 98.0%. However, the mediation of SDS is not too obvious, and
only 48.4% yield of benzaldehyde is obtained. Moreover, the value of turnover
frequency (TOF) arrives at 66 in the presence of several additives, which shows the
suitable additive has obvious promotion effect on the oxidation of benzyl alcohol.
3.4. The selective aerobic oxidation of different alcohols
The oxidation of various alcohols is investigated with TEMPO-Ce(IV)-NaNO2
catalytic system in aqueous media, and the results are listed in Table 3. In the
oxidation of 4-methyl benzyl alcohol or 4-ethoxyl benzyl alcohol, 91.3% yield of
4-methyl benzaldehyde or 98.0% yield of 4-ethoxyl benzaldehyde is obtained,
respectively (entries 1 and 2). Similarly, 3-methoxy benzyl alcohol is oxidized with a
98.0% yield of 3-methoxy benzaldehyde (entry 3), which shows that substituted
position has no special effect for the electron donating groups. In case of phenyl
ethanol, a 98.0 yield of acetophenone is obtained (entry 4). Furthermore, 3-chloro
benzyl alcohol is also efficiently oxidized with a 95.9% yield of 3-chloro
benzaldehyde (entry 5). These obtained results are comparable to the best literature
data for the oxidation of different benzylic alcohols in aqueous media [30-32].
Besides, 55.6% yield of cyclohexanone or 45.5% yield of benzophenone is got from
the oxidation of cyclohexanol or diphenylmethanol with this catalytic system (entries

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6 and 7). A 58.2% yield of 3-methyl-2-butenal is obtained for the oxidation of
3-methyl-2-butenol (entry 8). These results showed that the naphthenic and aliphatic
alcohols were more difficult than aromatic alcohols with TEMPO-CAN-NaNO2
catalyst systems, in which the moderate yield of aldehydes were obtained. On the
other hand, 5-hydromethylfurfural is oxidized with a 12.4% yield of furan-2,
5-dicarbaldehyde (entry 9), which is much better than the oxidation with vanadyl
phosphate as the catalyst in water [33, 34]. This shows a promising technique for the
selective oxidation of heterocyclic alcohols.
Table 3. The aerobic oxidation of various alcohols in aqueous media a
Entry
Substrate
TEMPO Time Conv. (%) b Sel. (%) c Yield (%)
(mol%)
1
(h)
1.5
1
2
3
92.2
99.0
99.0
99.0
99.0
99.0
91.3
98.0
98.0
1
1
2
1.5
4
3
4
99.0
99.0
98.0
5
6
3
3
4
6
99.0
56.1
96.9
99.0
95.9
55.6
7
3
4
46.0
99.0
45.5
8
9
2
2
3
3
58.8
12.5
99.0
99.0
58.2
12.4
a
Reaction conditions: 1.08 g benzyl alcohol, in 10 mL water, 5.0 mol% CAN and 10.0 mol%
NaNO2, in the presence of 20.0 mol% TEAB, under 0.3 MPa of O2, temperature 80 ºC.
Conv. means the conversion of benzyl alcohol. The results are obtained by GC with internal
b
standard method.

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c
Sel. Means the selectivity of benzaldehyde. The results are obtained by GC with internal
standard technique.
4. Conclusion
In summary, we have developed a new TEMPO-containing catalytic system
which allows the aerobic oxidation of different alcohols to the corresponding
aldehydes or ketones in aqueous media. The yields of corresponding aldehydes add
up to 91.3-98.0% in the oxidation of benzyl alcohol and substituted benzyl alcohols.
For the oxidation of naphthenic and aliphatic alcohols, 55.6-58.2% yields of
aldehydes are obtained. This will generate a promising application strategy for green
synthesis of the carbonyl compounds.
Acknowledgements
This work is financially supported from the Natural Science Foundation of China
(Grant No. 21003093) and the State Key Program of National Natural Science of
China (Grant No.21336008).
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Highly Efficient and Selective Aerobic Oxidation of Alcohols in
Aqueous Media by TEMPO-Containing Catalytic Systems
Yongtao Yan, Xinli Tong*, Kaixuan Wang, Xueqin Bai
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of
Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R.
China; Tel.: +86-22-60214259; Fax: +86-22-60214259;
E-mail address: tongxinli@tju.edu.cn or tongxli@sohu.com.
Graphical Abstract
An efficient and selective oxidation of different alcohols has been successfully performed for
the synthesis of the corresponding aldehydes and ketones which is catalyzed by TEMPO-
containing catalytic systems with molecular oxygen as the oxidant in aqueous media.

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Highly Efficient and Selective Aerobic Oxidation of Alcohols in
Aqueous Media by TEMPO-Containing Catalytic Systems
Yongtao Yan, Xinli Tong*, Kaixuan Wang, Xueqin Bai
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of
Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R.
China; Tel.: +86-22-60214259; Fax: +86-22-60214259;
E-mail address: tongxinli@tju.edu.cn or tongxli@sohu.com.
Research Highlights
 A new catalytic system is developed for aerobic oxidation of alcohols.
 The efficient oxidation of different alcohols is achieved in aqueous media.
 The catalytic system is composed of TEMPO, Ce(IV) and NaNO2.
 45.5-98.0% yields of aldehydes or ketones are obtained in the reaction.