Selective oxidation of alcohols with H2O2 catalyzed by long chain multi-SO3H functionalized heteropolyanion-based ionic liquids under solvent-free conditions

Article abstract of DOI:10.1016/j.catcom.2015.05.011

Two novel long chain multi-SO₃H functionalized heteropolyanion-based ionic liquids were prepared and characterized. They as homogeneous catalysts showed high catalytic activity in selective oxidation of alcohols with 35% aqueous hydrogen peroxide under solvent-free conditions without adding any phase transfer catalyst. Two ionic liquids can be recovered readily and reused five times without any significant loss in their catalytic activities.

Full text of DOI:10.1016/j.catcom.2015.05.011

Catalysis Communications 69 (2015) 5–10
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Selective oxidation of alcohols with H2O2 catalyzed by long chain
multi-SO3H functionalized heteropolyanion-based ionic liquids under
solvent-free conditions
a
b
Xinzhong Li a,b, , Rong Cao , Qi Lin
⁎
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
Department of Chemistry and Chemical Engineering, University of Minjiang, Fujian Provincial Key Laboratory of Green Functional Materials, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel long chain multi-SO3H functionalized heteropolyanion-based ionic liquids were prepared and
characterized. They as homogeneous catalysts showed high catalytic activity in selective oxidation of alcohols
with 35% aqueous hydrogen peroxide under solvent-free conditions without adding any phase transfer catalyst.
Two ionic liquids can be recovered readily and reused five times without any significant loss in their catalytic
activities.
Received 24 February 2015
Received in revised form 17 May 2015
Accepted 18 May 2015
Available online 19 May 2015
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Selective oxidation
Heteropolyacid
Alcohols
Ionic liquids
H2O2
1. Introduction
However, these reports suffer from using organic solvents, low efficien-
cy of H2O2 utilization, mass transfer limitation, lack of generality, etc. In
Oxidation of alcohols to their corresponding carbonyl compounds is
one of the most important functional group transformations in organic
chemistry and is widely used in academic and industrial processes [1].
Traditionally, this transformation was carried out using stoichiometric
inorganic oxidants (i.e. CrO3, KMnO4, NaClO, MnO2, PCC, etc.) in the
liquid phase [2]. However, these oxidants are toxic, hazardous to handle,
expensive, and lack of selectivity and large amounts of wastes are
generated [3]. From the atom economy and environmentally friendly
point of view, the development of clean and green approaches utilizing
H2O2 or molecular oxygen as oxidant in combination with recyclable
catalysts has attracted increasing attention [4,5], and a lot of novel and
efficient homogeneous and heterogeneous catalysts have been report-
ed, such as organometallic complexes [6–10], polyoxometalates [11],
supported heteropolyacids [12], heteropolyacid hybrids [13–16], noble
metal nanoparticles [17,18], supported Pd [19], and mesoporous oxo-
vanadium Schiff base complex [20].
addition, the used HPAILs are solid at room temperature with high melt-
ing point over 100 °C, so defining them as ionic liquids is wrong.
Herein, we report a facile and efficient approach for selective
oxidation of alcohols catalyzed by two long chain multi-SO3H function-
alized heteropolyanion-based ionic liquids (Scheme 1) with 35% aque-
ous H2O2 under solvent-free conditions without adding any phase
transfer catalyst. The corresponding aldehydes and ketones were
obtained with yields of 63%–100%. For benzyl alcohols, the correspond-
ing benzoic acids were obtained with yields of 64%–94%. After the com-
pletion, two ionic liquids can be reused five times after simple removal
of water and without significant loss of their catalytic activities.
2. Experimental
2.1. Materials and methods
Recently, novel heteropolyacid-based ionic liquids (HPAILs) as green
heterogeneous catalysts showed excellent catalytic performances in
selective oxidation of benzyl alcohol with aqueous H2O2 [21,22].
Melting points were determined by an X6 digital microscope melting
point apparatus. FT-IR spectra were recorded on a Perkin-Elmer FT-IR
240-c spectrophotometer using liquid film. 1H NMR spectra were re-
corded on a Bruker Avance-DRX-400 spectrometer using TMS as inter-
nal standard. The elemental analyses were performed on an Elementar
Vario EL III analyzer. TG analysis was carried out with a STA-409 instru-
ment in dry air flow at a heating rate of 10 °C/min. All chemicals were
commercially available and used as received.
⁎
Corresponding author at: State Key Laboratory of Structural Chemistry, Fujian
Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou
350002, China.
E-mail address: bails_wl@126.com (X. Li).
http://dx.doi.org/10.1016/j.catcom.2015.05.011
1566-7367/© 2015 Elsevier B.V. All rights reserved.

6
X. Li et al. / Catalysis Communications 69 (2015) 5–10
O
O
S
O
(1)
N
SO3
N
H4SiW12O40
H3PW12O40
S4SiIL
S3PIL
- 4
=
SiW12O40
- 3
=
PW12O40
=
N
SO3H
= SO3H
Scheme 1. Synthetic route and structures of S4SiIL and S3PIL.
2.2. Preparations of S4SiIL and S3PIL
2.2.1. Compound 1 [23]
Off-white crystal. m.p: 249–252 °C.1H NMR (400 MHz, CDCl3): δ =
0.848 (t, J = 7.2Hz, 3H, CH3), 1.262–1.337 (m, 18H, 9CH2), 1.725 (m,
2H, CH2), 2.141–2.200 (m, 2H, CH2). 2.918 (t, J = 7.2Hz, 2H, CH2),
3.085 (s, 6H, CH3), 3.281 (m, 2H, CH2), 3.428–3.470 (m, 2H, CH2).
Elemental analysis Calcd for C17 H37O3S: C, 80.55; H, 4.69; N, 9.39; S,
12.65. Found: C, 80.60; H, 4.72; N, 9.43; S, 12.57.
A mixture of N, N-dimethyldodecylamine (50 mmol) and 1, 3-
propanesultone (52 mmol) in ethanol (20 mL) was stirred at room tem-
perature for 24 h. On completion, this reaction system was placed in an
ice-water bath and cooled to 0–5 °C, the resulting precipitate was fil-
tered, washed with diethyl ether, and dried at 65 °C under a vacuum
for 8 h to afford precursor zwitterionic salt (1) in a yield of 95%. After
the obtained zwitterionic salt was dissolved in ethanol, H4SiW12O40
(12.5 mmol) or H3PW12O40 (16.7 mmol) aqueous solution was added
dropwise, the resulting mixture was stirred at 40 °C in an oil batch for
24 h. After removal of ethanol and water under vacuum, S4SiIL and
S3PIL were obtained as off-white viscous liquids at room temperature
with yields of 97% and 98%, respectively.
2.2.2. S4SiIL
1H NMR (400 MHz, CDCl3): the values of δ are identical to those of
compound 1. FT-IR, cm−1: 3415, 2924, 1170, 1042, 1013, 974, 919,
883, 796. Elemental analysis Calcd for C68H152N3O52S4SiW12: C, 19.36;
H, 3.63; N, 1.33; S, 3.04. Found: C, 19.32; H, 3.60; N, 1.28; S, 2.97.
2.2.3. S3PIL
1H NMR (400 MHz, CDCl3): the values of δ are identical to those of
compound 1. FT-IR, cm−1: 3400, 2924, 1467, 1079, 1048, 978, 896,
818. Elemental analysis Calcd for C51H144N3O49S3PW12: C,15.64; H,
3.71; N, 1.07; S, 2.46. Found: C, 15.60; H, 3.68; N, 1.04; S, 2.40.
100
S4SiIL
S3PIL
90
Table 1
Hammett acidity function of S4SiIL and S3PIL.
Ionic liquids
Con. (mol/L)
Amax
[B]/%
[BH+]/%
H0
H01
80
70
60
S4SiIL
S3PIL
S4SiIL
S3PIL
S4SiIL
S3PIL
S4SiIL
S3PIL
0.016
0.016
0.032
0.032
0.048
0.048
0.064
0.064
0.978
1.045
0.953
0.976
0.928
0.968
0.916
0.926
57.00
61.43
56.30
57.38
54.56
56.91
53.85
54.44
42.50
38.56
43.97
42.62
45.44
43.09
46.15
45.56
1.121
1.192
1.095
1.119
1.069
1.111
1.057
1.067
1.557
1.557
1.211
1.211
1.112
1.112
1.095
1.095
0
100
200
300
400
500
T (°C)
H0 = PKa + log[B]/[BH+]; indicator: 4-nitroaniline; PKa = 0.99; H10: Hammett function of
H2SO4.
Fig. 1. TG patterns of S4SiIL and S3PIL.

X. Li et al. / Catalysis Communications 69 (2015) 5–10
7
45
40
35
30
25
20
15
10
5
0
4000
3000
2000
1000
Fig. 2. IR spectrum of S4SiIL.
2.3. General procedure for oxidation of alcohols
and identified by the comparison of their IR and 1H NMR spectra with
those of authentic samples. The efficiency of H2O2 utilization was deter-
mined by iodometric titration methods [24]. It was found that the
amount of residual H2O2 at the end of reaction is 1–3 mmol, showing
the efficiency of H2O2 utilization in this catalytic system exceeded 90%.
A mixture of S4SiIL or S3PIL (0.05 mmol) and alcohol (30 mmol) in a
25 mL flask fitted with a reflux condenser was heated to the reaction
temperature, then the required amount of aqueous H2O2 was added
dropwise under stirring. The progress of the reaction was monitored
by TLC (GF254 silica gel coloration in phosphomolybdic acid/ethanol
for aromatic alcohol, in KMnO4 solution for aliphatic alcohol). After
the completion, the reaction mixture was extracted with diethyl ether
(3 × 20 mL), solvent was evaporated in a vacuum. The residual was an-
alyzed by gas chromatography (HP 6890) equipped with a flame ioniza-
tion detector and an SE-54 column. The aqueous phase was subjected to
rotary evaporation, and then was dried at 85 °C under a vacuum for 8 h
to give regenerated ionic liquids. Pure oxidation products were obtained
by column chromatography (petroleum ether-ethyl acetate 9:1 (V/V))
3. Results and discussion
3.1. Preparation and characterization of S4SiIL and S3PIL
Two ionic liquids were synthesized using N, N-dimethyldodecylamine,
1, 3-propanesultone, H4SiW12O40 and H3PW12O40 as starting materials by
quaternerization and acidification two step atom economic reactions
(Scheme 1). Their structures were confirmed by 1H NMR, IR and Elemen-
tal analysis. Three or four SO3H-functionalized organic cations together
45
40
35
30
25
20
15
10
5
18
4000
3000
2000
1000
Fig. 3. IR spectrum of S3PIL.

8
X. Li et al. / Catalysis Communications 69 (2015) 5–10
with the counter heteropolyanion made S4SiIL and S3PIL exhibited
strong Brønsted acid and good oxidation catalytic activities at the
same time. Moreover, the structure of SO3H-functionalized long chain
alkylammonium also endowed two ionic liquids with excellent surface
activity and solubility in water, which made the problem of hydrophobic
alcohol immiscible in aqueous H2O2 to be solved. The oxidation can be
carried out under homogenous condition without adding any phase
transfer catalyst or co-solvent.
spectra of S4SiIL and S3PIL (Figs. 2–3) showing absorption bands at
1079, 978, 896, 818 and 1013, 974, 919, 883, and 796 cm−1, respective-
ly, these peaks are perfectly in accordance with the characteristic skele-
tal stretching vibrations of Keggin type phosphotungstate and
silicotungstate. These results indicated that the Keggin structure is
well retained in their structures.
3.2. Selective oxidation of alcohols
S4SiIL and S3PIL are immiscible with diethyl ether, n-hexane,
toluene, and ethyl acetate, but are soluble in methanol, ethanol and
water. Their water content is 1.4% and 2.5%, respectively (Karl-Fischer
titration). The TG patterns of S4SiIL and S3PIL (Fig. 1) indicated
two ionic liquids with good thermal stability before 523 K. Their
acidities were measured using a UV–visible spectrophotometer with
4-nitroaniline as a basic indicator [25], the results were listed in
Table 1. S4SiIL and S3PIL exhibited higher acidity than H2SO4. The
acid amount of S4SiIL and S3PIL was determined by acid–base titration,
the obtained results are 0.93 and 0.75 mmol/g, respectively. These
values are very close to the expected values thereby confirming that
four and three SO3H-functionalized long chain alkylammonium cations
were introduced into their structures successfully. The obtained IR
Initially, the oxidation of benzyl alcohol was chosen as a model reac-
tion, the results were listed in Table 2. For S4SiIL, the benzaldehyde was
obtained with yield of 94%. Interestingly, for S3PIL the benzoic acid was
obtained with yield of 83%. Based on this success, the substitution effect
was investigated, and found that all substituted benzyl alcohols can be
oxidized smoothly to the corresponding benzaldehydes or benzoic
acids with good to excellent yields, but higher oxidation rate and yields
were obtained for alcohols with electron-donating group.
With the optimal conditions in hand, the generality of alcohol sub-
strates was examined. As shown in Table 2. Cyclohexanol, cyclopentanol
and 2-octanol were oxidized to the corresponding ketones with yields
of 63%–87%, and the highest TOF is 130 h−1. For 1-pentanol, which is
Table 2
Results of selective oxidation of alcohols catalyzed by S4SiIL and S3PIL.
Entry
1
Substrate
Product
Conv. (%)
100a
Yield (%)
94a
99b
100a
99b
100a
99b
99a
83b
100a
87b
100b
94b
91b
77b
81b
2
3
4
5
97b
96a
95b
64b
6c
7c
94a/90b
79a/65b
96a/95b
87a/78b
8c
9c
90a/85b
83a/90b
72a/63b
63a/72b
10c
11c
98a/95b
85a/97b
94a/80b
64a/90b
12c
95a/89b
83a/76b
Reaction conditions: 0.05 mmol catalyst, 45 mmol H2O2, 30 mmol alcohol, 70 °C, 4 h.
a
Results of S4SiIL.
Results of S3PIL.
0.05 mmol catalyst, 60 mmol H2O2, 30 mmol alcohol, 70 °C, 4 h.
b
c

X. Li et al. / Catalysis Communications 69 (2015) 5–10
9
Table 3
Selective oxidation of benzyl alcohol catalyzed by different catalysts.
Table 4
Influence of reaction conditions on the oxidation of benzyl alcohol catalyzed by S4SiIL.
Catalyst
Product
Conv.
(%)
Sel.
(%)
Yield
(%)
Entry Temp.(°C) Time Amount of catalyst H2O2
Conv. Sel. Yieldb
(h)
(mmol)
(mmol) (%)
(%) (%)
–
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
3
6
100
100
97
67
68
94
95
90
96
3
6
34
9
43
89
57
86
90
1
2
3
4
5
6
7
8
RT
50
70
90
70
70
70
70
70
70
70
70
4
4
4
4
3
6
8
4
4
4
4
4
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.02
0.08
0.10
30
30
30
30
30
30
30
45
60
45
45
45
21
56
85
87
79
100 21
99 55
98 83
90 78
99 78
92 83
89 82
94 94
89 89
99 77
92 92
90 90
[bmim]HSO4
[(CH2)3SO3Hmim]HSO4
H4SiW12O40
H3PW12O40
35
13
63
95
60
96
94
[15]
[n-C16H33N(CH3)3]PW12
O
90
92
40
[16]
[bPy]4W10
O
32
[DpyAM]PW[22]
[(C18H37N)2(CH3)2N]10[SiW9O34
100
100
78
100
100
[29]
]
9
10
11
12
Reaction conditions: 0.05 mmol catalyst, 45 mmol H2O2, 30 mmol benzyl alcohol, 70 °C, 4
h.
Yields refer to GC yields.
Reaction conditions: 30 mmol benzyl alcohol.
difficult to be oxidized can also be converted to n-pentanal with
moderate yields (63% and 72%). For 2-phenylethyl alcohol, the desired
aldehyde was obtained in the yield of 90%.
intermediate was responsible for the oxidation of alcohol to the corre-
sponding aldehyde, ketone or carboxylic acid.
In order to explain these good performances, the oxidation of benzyl
The effect of reaction temperature, reaction time, the amount of
H2O2 and catalyst on benzyl alcohol oxidation was summarized in
Table 4. The conversion of benzyl alcohol increased with the reaction
temperature, but decreased in selectivity due to the deep oxidation of
benzaldehyde. With the increase of the reaction time, the conversion
of benzyl alcohol increased from 79% to 92%, while the selectivity to
benzaldehyde passed through a maximum value of 99%, and decreased
slightly for the deep oxidation. The yield of benzaldehyde was positively
dependent on the amount of H2O2, with increasing the molar ratio of
H2O2 with benzyl alcohol from 1.0 to 2.0, the conversion increased dras-
tically from 85% to 100%, however, the selectivity decreased from 98% to
89% due to the deep oxidation of benzaldehyde. Obviously, the
preferred molar ratio for the oxidation of benzyl alcohol was 1:1.5.
Increasing catalyst amount will lead to the increase in conversion and
decrease slightly in selectivity.
The reusability of S4SiIL and S3PIL for oxidations of benzyl alcohol
was investigated, the results were shown in Fig. 4. Two ionic liquids
can be reused five times without significant loss in conversion and se-
lectivity. The IR spectra, TG patterns, acidity and acid content of the
recycled ionic liquids were also determined, and found that no obvious
changes were exhibited by comparison with those of fresh samples.
These results indicated that S4SiIL and S3PIL are stable in this reaction
system, and the active species can be regenerated during catalytic
recycling.
alcohol using [bmim]HSO4, [(CH2)3SO3Hmim]HSO4, H4SiW12O40
,
H3PW12O40 and heteropolyacid ionic hybrids as homogeneous and het-
erogeneous catalysts were carried out under the same reaction condi-
tions, the results were shown in Table 3. Compared to these control
catalysts, the high catalytic activity of S4SiIL and S3PIL exhibited in ox-
idation could be ascribed to the following facts: (1) High Brønsted acid-
ity and acid content. The oxidation capacity of H2O2 was enhanced
under strong acidic conditions, and with the increase of acidity, the
oxidation capacity of H2O2 is improved [26]; and (2) synergetic catalytic
effect. Oxidation and acid catalytic site exist in the structures of
S4SiIL and S3PIL at the same time, and two catalytic sites were com-
bined with each other by the strong ionic interaction between the or-
ganic cation and heteropolyanion. The oxidation catalytic activity of
silicotungstate anion was improved by four strong acidic organic cat-
ions, and caused the inactive silicotungstate anion showed higher oxi-
dation catalytic activity than phosphotungstate anion [27]. So benzyl
alcohol was oxidized into benzaldehyde with excellent yield. As for
S3PIL, the active phosphotungstate anion was further to be enhanced
by this synergetic catalytic effect, and caused the generated benzalde-
hyde continued to be oxidized into benzoic acid. (3) Excellent surface
activity and solubility in water. For hydrophilic alcohols, the oxidations
carried out under homogeneous condition, for hydrophobic alcohols,
the good results could be attributed to the formation of micro-emulsion.
In order to determine the active species in oxidation, the FT-IR spec-
tra of S4SiIL and S3PIL during the oxidation and reused for cycle 1 and
cycle 2 were examined. A new band at 841–843 cm−1 appeared during
the reaction, which can be assigned to the stretching vibration of
tungsten-peroxo bond (W-(O2)) [28]. Based on this fact and literatures
[15,29], a plausible mechanism for alcohol oxidation is proposed
in Scheme 2. In oxidation, H2O2 was activated by the multi-SO3H func-
tionalized organic cation, the activated H2O2 then reacted with
heteropolyanion to generate active peroxo intermediate, this active
4. Conclusions
Two novel long chain multi-SO3 functionalized heteropolyanion-
based ionic liquids were prepared and characterized, they as recyclable
homogeneous catalysts realized selective oxidation of alcohols with
S4SiIL Product:benzaldehyde
100
S3PIL Product:benzoic acid
95
90
85
O
O
O
W
80
75
70
65
60
Yields (%)
R1
R2
H2O
Peroxo-POM
H +
- H2O
OH
R2
1
2
3
4
5
R1
H2O2
POM
Run number
Scheme 2. A possible mechanism for oxidation of alcohols catalyzed by S4SiIL and S3PIL.
Fig. 4. Recyclability of S4SiIL and S3PIL for oxidation of benzyl alcohol.

10
X. Li et al. / Catalysis Communications 69 (2015) 5–10
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Acknowledgments
The authors are grateful for financial supports from the Natural
Science Foundation of Fujian Province (No. 2013J01053), the Research
Project of Education Department of Fujian Province (No. JA13252), the
Research Project of Fuzhou City (No. 2012-G-138), and the Research
Project of Minjiang University (No. MJK14005).
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