New active and recyclable catalytic systems based on [Zn4(H 2O)2(PW9O34)2] 10- for alcohol oxidation with hydrogen peroxide in water

Article abstract of DOI:10.1139/V10-152

A "sandwich" type tungstophosphate, [Zn₄(H ₂O)₂(PW₉O₃₄)₂]10-, was used as the efficient and reusable catalyst for alcohol oxidation. The catalyst I of K₁₀Zn₄(H₂O)₂(PW ₉O₃₄)₂·20H₂O and the catalyst II of [(C₁₈H₃₇)₂(CH₃) ₂N]₈K₂[Zn₄(H₂O) ₂(PW₉O₃₄)₂] were used to catalyze the oxidation of alcohols in water. High yields of the corresponding carbonyl compounds were obtained. The catalysts were recycled several times without loss in activity.

Full text of DOI:10.1139/V10-152

1
3
New active and recyclable catalytic systems
10–
based on [Zn (H O) (PW O ) ] for alcohol
4
2
2
9
34 2
oxidation with hydrogen peroxide in water
Yong Ding, Wei Zhao, Bao-chun Ma, and Wen-yuan Qiu
Abstract: A ‘‘sandwich’’ type tungstophosphate, [Zn4(H2O)2(PW9O34)2]10–, was used as the efficient and reusable catalyst
for alcohol oxidation. The catalyst I of K10Zn4(H2O)2(PW9O34)2Á20H2O and the catalyst II of
[
(C18H37)2(CH3)2N]8K2[Zn4(H2O)2(PW9O34)2] were used to catalyze the oxidation of alcohols in water. High yields of the
corresponding carbonyl compounds were obtained. The catalysts were recycled several times without loss in activity.
Key words: polyoxometalate, recyclable, catalyst, oxidation, alcohol.
R e´ sum e´ : On a utilis e´ un tungstophosphate de type « sandwich », [Zn4(H2O)2(PW9O34)2]10– comme catalyseur efficace et
r e´ utilisable pour l’oxydation d’alcools. On a utilis e´ les catalyseurs I, de formule K10Zn4(H2O)2(PW9O34)2Á20H2O, et II, de
formule [(C18H37)2(CH3)2N]8K2 [Zn4(H2O)2(PW9O34)2], pour catalyser l’oxydation d’alcools dans l’eau. On a obtenu des
rendements e´ lev e´ s des d e´ riv e´ s carbonyl e´ s correspondants. Les catalyseurs ont e´ t e´ recycl e´ s plusieurs fois sans perte d’ac-
tivit e´ .
Mots-cl e´ s : polyoxom e´ talate, recyclable, catalyseur, oxydation, alcool.
[
Traduit par la R e´ daction]
Introduction
received much attention recently. Although a number of
methods have been developed,1
3–19
the search for effective,
Carbonyl compounds are important precursors of various
chemicals.1 The selective oxidation of alcohols to the cor-
responding carbonyl compounds is an active field of re-
search. Hydrogen peroxide used as the oxidant in liquid-
phase oxidation has attracted much attention from research-
ers from the viewpoint of green chemistry. It overcomes the
new, environmentally benign, and reusable procedures for
alcohol oxidation still remains an important challenge.
To the best of our knowledge, the ‘‘sandwich’’ type tung-
–3
10–
stophosphate, [Zn (H O) (PW O ) ] , has not been re-
4
2
2
9
34 2
ported on the catalytic activity for the oxidation of alcohols
in water. We report here two new catalytic systems for alco-
hol oxidation using it as a recyclable and active catalyst
with hydrogen peroxide in water. (i) The potassium of the
tungstophosphate (K Zn (H O) (PW O ) Á20H O) was
4
disadvantage of the toxic traditional inorganic oxidants, and
it is much cheaper and safer than other organic peroxides or
5
peracids. Polyoxometalates as effective catalysts for oxida-
10
4
2
2
9
34 2
2
tion reactions have attracted wide attention in the past. Most
examples of alcohol oxidation with hydrogen peroxide
employed polyoxometalates as catalysts and have been
used to oxidize several alcohols in water with hydrogen per-
oxide. The catalyst was reused for the next cycle from the
organic product by separation of the aqueous phase. There
was no discernable loss in activity and selectivity even after
10 cycles. (ii) High yields of ketones were obtained in the
oxidation of secondary alcohols in water using
6
reported, e.g., [(n-C H ) NCH ] {PO [WO(O ) ] }, [p-
8
17 3
3 3
4
2 2 4
7
C H N(CH ) CH ] {PO [MoO(O ) ] },
tetrakis(diper-
5
5
2 15
3 3
4
2 2 4
8
oxotungsto)phosphates, lacunary polyoxotungstate of [g-
4
– 9
SiW O (PhPO) ] , sodium tungstate + quaternaryammo-
1
0
36
2
1
0
7– 11
14– 12
[(C H ) (CH ) N] K [Zn (H O) (PW O ) ]. The catalyst
niumhydrogen sulfate, [PW O ] , [NaP W O110]
,
18 37 2 3 2 8 2 4 2 2 9 34 2
1
1
39
5
30
was separated from the reaction mixture by adding diethyl
ether to the system after the reaction. Comparing previous
reports, there are some advantages in using our system: (i)
water (a green solvent) was used as the solvent in the oxida-
tion, (ii) the active catalysts could be easily reused, and (iii)
some solid alcohols like DL-menthol and benzhydrol were
and so on. However, in these catalytic systems, the use of
chlorocarbon poisonous solvents or the difficult recovery
and reuse of those catalysts has restricted their wide applica-
tions in industrial and laboratorial synthesis.
From the viewpoint of green chemistry, water as an excel-
lent solvent used in the catalytic oxidation of alcohols has
Received 6 April 2010. Accepted 24 September 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on
2
2 December 2010.
1
Y. Ding. Lanzhou University, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China, Lanzhou
7
30000, China.
W. Zhao, B.-c. Ma, and W.-y. Qiu. State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou 730000, China.
1Corresponding author (e-mail: dingyong1@lzu.edu.cn).
Can. J. Chem. 89: 13–18 (2011)
doi:10.1139/V10-152
Published by NRC Research Press

1
4
Can. J. Chem. Vol. 89, 2011
also successfully oxidized with excellent yields in our cata-
lytic system.
Trace DSQ), and some isolated products were characterized
by NMR.
Experimental
Results and discussion
Table 1 summarizes the catalytic oxidation of various al-
Reagents
cohols with 30% H O by catalyst I and II in procedures A
All chemicals were analytical grade, commercially avail-
able, and used without further purification unless otherwise
stated.
2
2
and B, respectively. High yields of ketones were obtained in
the oxidation of cyclohexanol and cyclopentanol in both
procedures. For DL-menthol, a low yield of 8% was detected
using procedure A after 10 h. However, a high yield of men-
thone (97%) was obtained using procedure B after 6 h. This
difference should be attributed to whether the catalyst could
be in good contact with the substrates during the oxidation.
In procedure A, the poor water solubility of DL-menthol re-
sulted in the substrate not being in good contact with the
catalyst dissolving in water. But in procedure B, the cation
of dioctadecyl dimethyl ammonium promoted the contact
between the aqueous phase and the substrate. So, there were
obvious differences between procedures A and B during the
oxidation of these hydrophobic alcohols. This phenomenon
was also presented in the oxidation of linear alcohols. For
the oxidation of sec-butyl alcohol, 2-pentanol, and 2-octanol,
the reaction time in procedure B was shorter than that in
procedure A, but the activity of catalyst II in procedure B
was much higher than that of catalyst I in procedure A.
This was especially true for 2-octanol; the yield of 2-octanone
in procedure A was only 39% in 10 h, but a high yield of
Preparation of catalysts
The potassium of the sandwich type tungstophosphate,
1
0–
[
Zn (H O) (PW O ) ]
(catalyst I), was synthesized by
the procedure reported in ref. 20. The catalyst II of
(C H ) (CH ) N] K [Zn (H O) (PW O ) ] was obtained
4
2
2
9
34 2
[
1
8
37 2
3 2
8
2
4
2
2
9
34 2
from K Zn (H O) (PW O ) Á20H O by the following pro-
10
4
2
2
9
34 2
2
cedure: 0.9 g of K Zn (H O) (PW O ) Á20H O was dis-
1
0
4
2
2
9
34 2
2
solved in 55 mL of water. To this solution, 0.9 g of
dioctadecyl dimethyl ammonium chloride dissolved in
10 mL of tert-butyl alcohol was added slowly. The mixture
was stirred vigorously for 4 h at 40 8C. A white solid was
filtered off and then washed with an excess amount of
water. The yield was 97%. Anal. calcd. for
[
(C H ) (CH ) N] K [Zn (H O) (PW O ) ]: C 39.51, H
18 37 2 3 2 8 2 4 2 2 9 34 2
6
6
.97, N 1.21, P 0.67, W 35.84, Zn 2.82; found: C 40.68, H
.98, N 1.13, P 0.60, W 35.23, Zn 2.74.
Characterization techniques
9
9% was obtained in procedure B after 6 h. Even so, proce-
Infrared spectra were recorded on a Nicolet FTIR-360
FTIR spectrometer. The catalysts were measured using 2%–
dure A based on catalyst I is actually an active catalytic sys-
tem for most of the substrates listed in Table 1. Benzhydrol
and 1-phenylethanol were oxidized to their corresponding
ketones with high yields using procedure A, producing sim-
ilar results with procedure B. For a more active substrate (1-
phenylethanol), a two molar excess of H O using catalyst I
4
% (w/w) KBr pellets prepared by manual grinding. 31
P
MAS NMR spectra were recorded at 9.4 T on a Bruker
Avance-400 wide bore spectrometer. GC analyses were per-
formed on Shimadzu GC-9AM with a flame ionization de-
tector equipped with a FFAP capillary. Mass spectra were
recorded on a Finnigan Trace DSQ (Thermo Electron Cor-
poration) at an ionization voltage of 70 eV and equipped
with a DB-5 capillary column. Chemical elemental analysis
of the catalysts was done on an ICP-atomic emission spec-
trometer (IRIS ER/S), and C, H, and N contents were meas-
ured on a German Elementar Vario EL spectrometer.
2
2
in water was sufficient (not listed here). Interestingly, cata-
lyst I was also active for the primary alcohol of benzyl alco-
hol, and the products of benzaldehyde and benzoic acid
were obtained, respectively, in high yields by controlling
the amount of H O in the system. Benzaldehyde is a very
2
2
important chemical that has widespread applications in the
fields of flavors, odorants, and pharmaceutical intermedi-
ates.21 Therefore, we tried to get benzaldehyde from the ox-
idation of benzyl alcohol in our system. When the ratio of
H O –substrate was 1:2, benzaldehyde as the sole product
Catalytic reaction
The oxidation was carried out in two procedures. Proce-
2
2
dure A: catalyst
I
of K Zn (H O) (PW O ) Á20H O
with a 94% yield was obtained using catalyst I in water. In-
creasing the amount of hydrogen peroxide to a molar excess
five times greater than the substrate, the conversion of ben-
zyl alcohol reached up to 100%, but benzoic acid as the
main product was obtained with 93% selectivity. The results
indicated that increasing the amount of hydrogen peroxide
enhanced the activity of oxidation, but the excessive oxidant
would also bring in deep oxidation.
1
0
4
2
2
9
34 2
2
(9 mmol), water (3 mL), substrate (1 mmol), and H O
2 2
(
3
5 mmol, 30% aq) were added to the reaction flask at
63 K. When the reaction was over, the organic products
were separated from the aqueous phase by extraction. Then,
readdition of alcohols (1 mmol) and H O (30% aq) to the
aqueous phase containing the catalyst were carried out for
the next oxidation. Procedure B: catalyst II of
2
2
[
(C H ) (CH ) N] K [Zn (H O) (PW O ) ] (18 mmol),
K Zn (H O) (PW O ) Á20H O is a ‘‘water-soluble’’ cata-
1
8
37 2
3 2
8
2
4
2
2
9
34 2
10
4
2
2
9
34 2
2
substrate (4 mmol), and H O (20 mmol, 30% aq) were
lyst, and it effectively catalyzes the oxidation of alcohols in
water. After reaction, the organic products were separated
from the aqueous phase by extraction. The aqueous phase
containing the catalyst could be kept to do the next oxida-
tion. The catalytic results in the oxidation of cyclohexanol
on catalyst I for different cycles are shown in Fig. 1. It can
be seen that the activity of the reused catalyst did not
2
2
added to the reaction flask. The reaction was carried out at
63 K. When the reaction was over, the catalyst precipitated
3
with cooling and was collected completely from the solution
by adding diethyl ether to the system. Assignments of prod-
ucts were made by comparison with authentic samples. Se-
lected samples were also analyzed by GC–MS (Finnigan
Published by NRC Research Press

Ding et al.
15
Table 1. Oxidation of various alcohols catalyzed by [Zn4(H2O)2(PW9O34)2]10– with H2O2.
change. Even after 10 consecutive cycles of the reaction, a
7% yield of the cyclohexanone with 99% selectivity was
still kept in this catalytic system, indicating that this system
is a robust and reusable oxidation system.
To evaluate the reusable property of catalyst II, the oxida-
tion system in procedure B was investigated. The counter-
cation of dioctadecyl dimethyl ammonium made the catalyst
disperse in the mixture to form an emulsion with heating.
After the reaction finished with the temperature dropping,
the system changed to turbid. Then, some diethyl ether was
added to the system to precipitate the catalyst completely.
The catalyst was recovered by centrifugation and used for
the next cycle. Table 2 shows the oxidation of 2-octanol us-
ing procedure B for different cycles. A yield of 98% was
9
Published by NRC Research Press

1
6
Can. J. Chem. Vol. 89, 2011
Fig. 1. Oxidation of cyclohexanol by catalyst I in procedure A for
different cycles. Reaction conditions were similar to that in Table 1.
Fig. 3. 31P MAS NMR spectra of the two catalysts before and after
an oxidation reaction: (a) fresh catalyst I, (b) used catalyst I, (c)
fresh catalyst II, and (d) used catalyst II.
Table 2. Oxidation of 2-octanol by catalyst II
for different cycles.
Yield
(mol%)
98
98
97
98
Catalyst
Fresh
Cycle 1
Cycle 2
Cycle 3
Selectivity (mol%)
99
99
99
99
Note: Reaction conditions were the same as for
procedure B.
Fig. 2. IR spectra of the two catalysts before and after oxidation
reaction: (a) fresh catalyst I, (b) used catalyst I, (c) fresh catalyst II,
and (d) used catalyst II.
the fresh catalyst K Zn (H O) (PW O ) Á20H O (Fig. 2a)
10
4
2
2
9
34 2
2
exhibited characteristic peaks at 1034, 942, 886, and
756 cm–1 and were similar to the reference reports. The
20
kept after three cycles, indicating that this catalytic system
based on catalyst II is also a recyclable oxidation system.
The fresh catalysts and the used ones were characterized
with FTIR spectra. As shown in Fig. 2, the IR spectrum of
–1
single band at ~1034 cm , near the P–O band position
–1
3–
(1080 cm ) of [PW O ] , suggests that significant resto-
12
40
ration of tetrahedral symmetry to the phosphate group has
Published by NRC Research Press

Ding et al.
17
occurred on metal substitution. There were obvious differen-
ces between the IR spectra of the fresh catalyst I and the
used one. Several peaks were moved to new positions at
at a high conversion rate and selectivity by controlling the
amount of H O . The catalysts were recycled several times
without an obvious loss in activity. These catalytic systems
are environmentally friendly and practical systems for alco-
hol oxidation.
2
2
–
1
1054, 955, 889, 831, and 762 cm , indicating that the struc-
ture of the catalyst has changed. It was not surprising that it
could be ascribed to the recovery method of the catalyst.
1
6,22
Acknowledgements
There have been several papers
that reported that the al-
2
teration of polyoxotungstate catalyst in the presence of H O2
This work was financially supported by the National Nat-
ural Science Foundation of China (Grant No. 20803032).
We gratefully thank Professor Wei Wang for the measure-
ment of solid-state NMR.
followed by the recovery of the initial structure after evapo-
ration of solvents has always taken place, and the change of
the structures cannot be ruled out. Considering the activity
of
the
used
catalyst,
the
fresh
catalyst
of
K Zn (H O) (PW O ) Á20H O seems to act as a precata-
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(
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2
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[
Zn (H O) (PW O ) ]
(shown in Fig. 3c) at –5.0 ppm.
4
2
2
9
34 2
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