Lacunary derivative [HPW9O34]8- As reusable and active catalyst for alcohol oxidation in water

Article abstract of DOI:10.1080/00397911.2010.527420

An effective and reusable catalytic system for the oxidation of alcohols was developed based on Δ-Na₈HPW₉O₃₄. The oxidation was conducted in water using hydrogen peroxide as oxidant. Good yields of ketones were obtained in

Full text of DOI:10.1080/00397911.2010.527420

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8-
Lacunary Derivative [HPW O ] as
9
34
Reusable and Active Catalyst for Alcohol
Oxidation in Water
a
a
a
a
Wei Zhao , Yong Ding , Baochun Ma & Wenyuan Qiu
a
State Key Laboratory of Applied Organic Chemistry, College of
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou,
China
Accepted author version posted online: 04 Aug 2011.Version of
record first published: 17 Oct 2011.
To cite this article: Wei Zhao, Yong Ding, Baochun Ma & Wenyuan Qiu (2012): Lacunary
8-
Derivative [HPW O ] as Reusable and Active Catalyst for Alcohol Oxidation in Water, Synthetic
9
34
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
2:4, 554-562
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Synthetic Communications , 42: 554–562, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.527420
8ꢀ
LACUNARY DERIVATIVE [HPW O ] AS REUSABLE
9
34
AND ACTIVE CATALYST FOR ALCOHOL OXIDATION
IN WATER
Wei Zhao, Yong Ding, Baochun Ma, and Wenyuan Qiu
State Key Laboratory of Applied Organic Chemistry, College of Chemistry
and Chemical Engineering, Lanzhou University, Lanzhou, China
GRAPHICAL ABSTRACT
Abstract An effective and reusable catalytic system for the oxidation of alcohols was
developed based on 4-Na
8 9
HPW O34. The oxidation was conducted in water using hydro-
gen peroxide as oxidant. Good yields of ketones were obtained in oxidation of secondary
alcohols, and the catalyst was recycled 10 times without obvious loss in activity.
Keywords Alcohol; catalysis; heteropolyoxotungstate; oxidation; recyclable
INTRODUCTION
The selective oxidation of alcohols to the corresponding carbonyl compounds
is an active field of research. From the viewpoint of green chemistry, hydrogen
peroxide used as oxidant in liquid-phase oxidation has attracted much attention.
[
1,2]
It is much cheaper and safer than other peroxides or peracids.
Polyoxometalates
recently have been shown to be active for the selective oxidation. Many catalytic
systems for alcohol oxidation with hydrogenperoxide have been disclosed, such as
[
3]
[4]
[
(n-C H ) NCH ] fPO [WO(O ) ] g, [p-C H N(CH ) CH ] fPO [MoO(O ) ] g,
8
17 3
3 3
4
2 2 4
5
5
2 15
3 3
4
2 2 4
[
5]
tetrakis(diperoxotungsto)phosphates, lacunary polyoxotungstate of [c-SiW O
10
36
4ꢀ,[6]
[7]
(
PhPO) ]
sodium tungstate þ quaternaryammoniumhydrogen sulfate, [PW11
14ꢀ [9]
5 30 110 8 10 36 2
2
7
ꢀ [8]
[10]
O ] , [NaP W O
]
,
and K [c-SiW O ] ꢁ 13H O. Some of these systems
3
9
use poisonous organic solvents, and in other cases, the recovery of the catalysts was
difficult or not mentioned.
[11,12]
Shelden et al.
found that a water-soluble palladium-phenanthroline
catalyst may be used in water for liquid alcohol oxidation. The catalyst could be
recycled from the product by separation of the aqueous phase. However, this system
Received December 11, 2009.
Address correspondence to Yong Ding, State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China. E-mail:
dingyong1@lzu.edu.cn
5
54

ALCOHOL OXIDATION
555
required expensive noble metal and a water-soluble organic ligand. Subsequently,
the ‘‘self-assembled’’ catalyst of Na -[WZnZn (H O) (ZnW O ) ] has been investi-
1
2
2
2
2
9
34 2
13–15]
[
gated as an active catalyst for alcohol oxidation by Neumann.
This sandwich-
type polyoxometalate was an effective and recyclable catalyst for alcohol oxidation
in water. The catalyst was used over three reaction cycles without discernible loss in
activity or selectivity.
Here, we report an environmentally friendly and efficient procedure for the
secondary alcohol oxidation catalyzed by 4-Na HPW O in water with aqueous
8
9
34
hydrogen peroxide. Good yields of ketones were obtained, and the catalyst was
recycled 10 times without obvious loss in activity or selectivity.
RESULTS AND DISCUSSION
It would be very advantageous for practical applications if the catalyst could be
effective and recyclable for the oxidation of alcohols. Table 1 summarizes the cata-
lytic oxidation of various alcohols by the catalyst with 30% H O . Secondary alcohols
2
2
including cyclic alcohols, linear alcohols, a-phenylethyl alcohol and benzhydrol were
all selectively oxidized to the corresponding ketones in good yields. Several trends
may be noted. Cyclohexanol, cyclopentanol, and a-phenylethyl alcohol were effec-
tively converted to the corresponding ketones in water. For other alcohols, the
sodium of this tungstophosphate dissolved in water without phase-transfer catalyst
was not active enough. Poor yield (only 5%) of the product was detected in the
oxidation of racemate (DL)–menthol after 10 h. When cetylpyridinium chloride
was added to the system, good yield of 83% of menthone was obtained in 7 h. There
is an obvious effect of the hydrophobicity of the substrate. The poor water solubility
of DL–menthol meant that the substrate was not well touched with the catalyst in
water, but the cation of cetylpyridinium allowed the oxidant of H O to react well
2
2
with the catalyst and substrate. This phenomenon was also present in the oxidation
of linear alcohols. For example, the yield of 2-octanone in the oxidation of 2-octanol
was only 35% in 10 h, but a good yield of 96% after 6 h was obtained by adding
cetylpyridinium chloride. The experimental results also show that the shorter linear
alcohol (2-pentanol) was more active than the longer one (2-octanol) in this oxidation
system. Besides, the solid alcohol of benzhydrol was also completely oxidized to the
corresponding ketone with the help of the phase-transfer catalyst. The substituent
secondary alcohols of 4-bromo-a-phenylethyl alcohol and 4-nitro-a-phenylethyl alco-
hol were used to check the activity of the catalyst. Although they were more difficult
to oxidize than a-phenylethyl alcohol, good yields of the corresponding ketones were
obtained based on the catalyst within 8 h (Table 1, entries 11 and 12). The represen-
tative primary alcohol of benzyl alcohol was also used for the oxidation based on the
catalyst. It was found that benzyl alcohol could be successfully oxidized with high
conversion (99%) in our catalytic system. However, the product for the oxidation
of benzyl alcohol was not the only one. Benzaldehyde and benzoic acid were obtained
together. In the previous system reported by Venturello et al., [3] benzonic acid was
nearly the sole product (aldehyde was less than 3%). In our catalytic system, the sel-
ectivity of benzaldehyde was 24% in the oxidation of benzyl alcohol with hydrogen
peroxide, which was different from the previous report. A combination of H O =urea
2
2
mixture as the oxidant was tried to avoid the overoxidation of benzaldehyde. The

556
W. ZHAO ET AL.
a
Table 1. Oxidation of various alcohols catalyzed by 4-Na
8
HPW
9
2 2
O34 with H O
Time
(h)
Yield
(mol%)
Selectivity
(mol%)
Entry
1
Substrate
Product
6
8
99
97
99
99
2
3
10
5
99
99
b
4
7
83
5
10
6
74
97
98
98
b
6
7
10
6
35
96
98
98
b
8
9
5
94
99
b
10
11
12
7
8
8
99
78
85
99
99
99
(
Continued )

ALCOHOL OXIDATION
557
Table 1. Continued
Time
(h)
Yield
(mol%)
Selectivity
(mol%)
Entry
Substrate
Product
13
8
8
24
75
24
76
8
8
31
24
56
44
c
4
1
a
Reaction conditions: 7 mmol catalyst, 2 ml water, 1 mmol alcohol, 5 mmol (30% aq.) H
temperature: 85 C. The selectivity was based on ketone. Yields and selectivities were determined by gas
2
O
2
; reaction
ꢂ
chromatography using an internal standard technique.
b
1
0 mg cetylpyridinium chloride were added.
c
2 2
H O =urea (1:1) as the oxidant.
selectivity of benzaldehyde reached 56%, but the conversion of benzyl alcohol was
decreased to 55%.
The catalyst of 4-Na HPW O ꢁ 19H O is a water-soluble catalyst. In this
8
9
34
2
biphasic system, the catalyst is dissolved in water and oxidized alcohols. After the
reaction, the organic products were separated from the aqueous phase by extraction
and the organic phase was taken for analysis. The aqueous phase containing the
Table 2. Oxidation of cyclohexanol by catalyst for
a
different cycles
Cycles
Yield (mol%)
Selectivity (mol%)
1
2
3
4
5
6
7
8
9
99
99
99
99
97
97
97
98
98
97
99
99
99
99
99
99
99
99
99
99
1
0
a
Reaction conditions: 0.028 mmol catalyst, 10 ml
water, 4 mmol alcohol, 20 mmol (30% aq.) H
ꢂ
2
2
O ; reac-
tion temperature: 85 C; reaction time: 6 h.

558
W. ZHAO ET AL.
catalyst could be recycled to do the next oxidation. Because the catalyst was still dis-
solving in water after extraction, new catalyst and new solvent were not needed for the
next cycle. To the aqueous phase, addition of alcohols and H O was sufficient for the
2
2
next oxidation. The catalytic activity in the oxidation of cyclohexanol for different
cycles is shown in Table 2. It can be seen that the catalytic activity and selectivity
of the reused catalytic system were nearly the same as that of the first oxidation. Even
after 10 consecutive cycles of the reaction, 97% yield of the cyclohexanone with 99%
selectivity was still obtained in this catalytic system, indicating that this system was a
robust and reusable oxidation system.
The fresh catalyst and recovered catalysts were characterized with Fourier
transform–infrared (FT-IR) spectra. As shown in Fig. 1, the IR spectrum of 4-Na
8
-
HPW O ꢁ 19H O [Fig.1(a)] exhibited characteristic peaks at 1171–994 and
9
34
2
ꢀ
1
[16]
8
96–742 cm that were attributed to n (P-O) and n (W-O), respectively.
There
were obvious differences in the IR spectrum between the fresh catalyst and the cata-
lyst of cycle 1, indicating that the structure of the catalyst after the first reaction was
changed. However, when the catalyst was recycled again, we found that the IR
spectrum of cycle 2 catalyst was similar to that of cycle 1 catalyst, indicating that
Figure 1. IR spectra of (a) the fresh catalyst of 4-Na
8 9
HPW O34; (b) the catalyst with hydrogen peroxide;
(
c) the catalyst of cycle 1; and (d) the catalyst of cycle 2.

ALCOHOL OXIDATION
559
Scheme 1. The possible mechanism for oxidation of alcohols by catalyst 4-Na
8
HPW
9
2
O34 ꢁ 19H O.
the structure of the catalyst would not changed after the first cycle. The mixture of the
catalyst with hydrogen peroxide was also characterized with the IR spectrum. There is
ꢀ
1
a broad peak in the spectrum at 932–806 cm , including three peaks: 932, 888, and
ꢀ
1
ꢀ1
8
(
40 cm . The IR bands at 1029, 932, and 888 cm can be assigned to n (P–O), n
[17,18]
ꢀ1
The band at 840 cm
W ¼ O), and n (W–Ob–W) (corner-sharing), respectively.
was attributed to n (O–O), which is responsible for the oxidation. Compared with
the activity of the used catalyst showed in Table 2, we proposed a possible
mechanism for alcohol oxidation by the catalyst of 4-Na HPW O ꢁ 19H O in water
8
9
34
2
with hydrogen peroxide (shown in Scheme 1). When the catalyst was treated with
hydrogen peroxide, the structure of the catalyst changed into a mixture of polytungsto-
phosphate species. With the effect of H O , they could be transferred to each other and
2
2
finally became the true active peroxotungstophosphate species. When the oxidation
catalyzed by the active complexes was finished, the catalyst changed back to the mixture
of polytungstophosphate species.
In summary, we demonstrated a water-soluble recyclable catalytic system in
the oxidation of alcohols with hydrogen peroxide. Secondary alcohols were selec-
tively oxidized to the corresponding ketones in good yields. The catalyst was reused
for 10 cycles without loss in activity. This system is an environmentally friendly and
reusable oxidation system for alcohol oxidation.
EXPERIMENTAL
[16]
Preparation of the Catalyst 4-Na HPW O ꢁ 19H O
8
9
34
2
First 30 g of Na WO ꢁ 2H O were dissolved in 37 ml of distilled water with
2
4
2
stirring. Then 0.75 ml of 85% H PO , followed by 5.5 ml of glacial acetic acid, were
4
3
added to the stirring solution. After a few seconds, the solution became cloudy and
after about 1 min a heavy white precipitate had formed. The solid was collected
and first dried under aspiration for 24 h, and then the aspirated solid was left in the
ꢂ
air for 24 h. After that, the solid was dried at 140 C for about 1.5 h. The yield of
4
ꢀ1
-Na HPW O ꢁ 19H O was about 80%. IR spectrum (KBr, cm ): 1171, 1057,
8
9
34
2
9
94, 947, 896, 818, 742, 653, 624, 588, 507, 470, 417.

560
W. ZHAO ET AL.
Catalytic Reaction
The catalytic reactions were performed in a 25-ml, two-necked, round-
bottomed flask equipped with a septum, a magnetic stirring bar, and a reflux
condenser. The oxidations were carried out in the following procedure: catalyst
(
7 mmol), water (2 ml), substrate (1 mmol), and H O (5 mmol, 30% aq.) were
2
2
charged in the reaction flask. The reaction was carried out at 358 K. When the
reaction was over, the organic products were separated from the aqueous phase
by extraction, and then the organic layer was analyzed by gas chromatography
(
(
GC) with the internal standard method. After separation, addition of alcohols
1 mmol) and H O (30% aq.) to the aqueous phase containing the catalyst was
2
2
carried out for the next oxidation. Assignments of products were made by com-
parison with authentic samples based on retention time in GC. Selected samples
were also analyzed by NMR.
Data
GC (FFAP capillary column, 30 m ꢃ 0.32 mm ꢃ 0.5 mm, GC-9AM, Shimadzu
2
Corporation) carrier gas: N , 1.2 kg=cm , column temperature: 125 C, injection tem-
ꢂ
2
ꢂ
ꢂ
perature: 200 C, detection temperature: 200 C, and retention time: 2.3 min.
GC (FFAP capillary column, 30 m ꢃ 0.32 mm ꢃ 0.5 mm, GC-9 AM, Shimadzu
2
Corporation) carrier gas: N , 1.2 kg=cm , column temperature: 130 C, injection tem-
ꢂ
2
ꢂ
ꢂ
perature: 200 C, detection temperature: 200 C, and retention time: 2.2 min.
GC (FFAP capillary column, 30 m ꢃ 0.32 mm ꢃ 0.5 mm, GC-9 AM, Shimadzu
2
Corporation) carrier gas: N , 1.2 kg=cm , column temperature: 135 C, injection tem-
ꢂ
2
ꢂ
ꢂ
perature: 200 C, detection temperature: 200 C, and retention time: 3.7 min.
GC (FFAP capillary column, 30 m ꢃ 0.32 mm ꢃ 0.5 mm, GC-9 AM,
2
Shimadzu Corporation) carrier gas: N , 1.2 kg=cm , column temperature: 85 C,
ꢂ
2
ꢂ
ꢂ
injection temperature: 200 C, detection temperature: 200 C, and retention time:
.7 min.
2

ALCOHOL OXIDATION
561
GC (FFAP capillary column, 30 m ꢃ 0.32 mm ꢃ 0.5 mm, GC-9 AM, Shimadzu
2
Corporation) carrier gas: N , 1.2 kg=cm , column temperature: 130 C, injection tem-
ꢂ
2
ꢂ
ꢂ
perature: 200 C, detection temperature: 200 C, and retention time: 2.4 min.
GC (FFAP capillary column, 30 m ꢃ 0.32 mm ꢃ 0.5 mm, GC-9 AM, Shimadzu
2
Corporation) carrier gas: N , 1.2 kg=cm , column temperature: 155 C, injection
ꢂ
2
ꢂ
ꢂ
temperature: 200 C, detection temperature: 200 C, and retention time: 4.0 min.
The pure product was obtained by distillation or silica-gel column chromato-
1
graphy. H NMR (400 MHz; CDCl ) d 7.80 (4H, d d, J ¼ 8.4, 1.2 Hz), 7.59 (2H,
3
1
3
m), 7.48 (4H, m). C NMR (100 MHz; CDCl ) d 197.7, 138.6, 133.38, 131.0, 129.2.
3
1
H NMR (400 MHz; CDCl ) d 8.23 (4H, d, J ¼ 8.8 Hz), 8.06 (2H, d,
3
1
3
J ¼ 8.8 Hz), 2.64 (3H, s). C NMR (100 MHz; CDCl ) d196.17, 150.14, 141.26,
3
1
29.12, 123.61, 26.73.
1
H NMR (400 MHz; CDCl ) d 7.78 (2H, d, J ¼ 8.5 Hz), 7.57 (2H, d,
3
1
3
J ¼ 8.4 Hz), 2.55 (3H, s). C NMR (100 MHz; CDCl ) d 196.80, 135.76, 131.77,
3
1
29.73, 128.16, 77.32, 77.00, 76.68, 26.39.
ACKNOWLEDGMENT
This work was financially supported by the National Natural Science Foun-
dation of China (Grant No. 20803032).
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3 2 17 2 3 2 3
(CH ) ) N(CH ) ]
[
4
O
2 2
O