Zinc substituted Keggin-type polyoxometalate on Dowex: a green heterogeneous catalyst for oxidation of alcohols in water

Article abstract of DOI:10.1007/s13738-020-01963-6

In this work, homogeneous and heterogeneous oxidation of alcohols by H₂O₂ in the presence of [(n-C₄H₉)₄?N]₅[PW₁₁ZnO₃₉].3H₂O and [PW₁₁ZnO₃₉]^5? supported on Dowex 22 as catalysts have been investigated. Using water as a green solvent, different alcohols were converted into the corresponding aldehydes and ketones in high to excellent yields. Dowex 22 supported polyoxometalate, PW₁₁Zn@Dowex, was also catalyzed highly robust and selective oxidation of unsaturated alcohols. Leaching and recycling experiments on supported catalyst revealed the excellent stability and reusability of this catalytic system.

Full text of DOI:10.1007/s13738-020-01963-6

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-01963-6
ORIGINAL PAPER
Zinc substituted Keggin‑type polyoxometalate on Dowex: a green
heterogeneous catalyst for oxidation of alcohols in water
Mehdi Aghayi1 · Bahram Yadollahi2 · Mostafa Riahi Farsani3
Received: 25 September 2019 / Accepted: 29 May 2020
© Iranian Chemical Society 2020
Abstract
In this work, homogeneous and heterogeneous oxidation of alcohols by H2O2 in the presence of [(n-
C4H9)4 N]5[PW11ZnO39].3H2O and [PW11ZnO39]5− supported on Dowex 22 as catalysts have been investigated. Using
water as a green solvent, diferent alcohols were converted into the corresponding aldehydes and ketones in high to excellent
yields. Dowex 22 supported polyoxometalate, PW11Zn@Dowex, was also catalyzed highly robust and selective oxidation
of unsaturated alcohols. Leaching and recycling experiments on supported catalyst revealed the excellent stability and reus-
ability of this catalytic system.
Keywords Catalysis · Oxidation · Supported polyoxometalate · Alcohol · Hydrogen peroxide · Water · Dowex 22
Introduction
these oxidants should be replaced with clean oxidants
such as aqueous H2O2 and particularly molecular oxygen.
H2O2-based catalytic oxidation is of great advantage to
the environment and industry because (i) it generates H2O
as a by-product, (ii) it has a high content of active oxygen
species, and (iii) H2O2 is rather inexpensive [6]. From the
viewpoint of sustainable and green chemistry, the develop-
ment of cleaner heterogeneous catalysis oxidation systems
is desirable.
Because of an increasing level of ecological consciousness,
research eforts towards developing viable and environmen-
tally benign synthetic methodologies for the oxidation of
alcohols into their corresponding aldehydes and ketones is
of signifcant importance in the laboratory, production of
favors, fragrances and industrial synthetic chemistry [1].
In the industrial, aldehydes are also synthesized from the
corresponding alcohols in the presence of Co, Ce and Mn
salts in acidic medium. These methods cause severe envi-
ronmental problems owing to the generation of dangerous
wastes and of undesirable byproducts [2]. Stoichiometrically
used toxic oxidants, such as CrO3, KMnO4, MnO2, SeO2,
and Br2 produce large amounts of wastes [3–5]. Obviously,
Regard to environmentally benign synthetic catalytic pro-
cedures, the use of polyoxometalates (POMs) as a signifcant
area in oxidation catalysts is very important [7–10]. Transi-
tion metal substituted POMs have attracted much attention
as oxidation catalysts due to their unique physical–chemical
properties such as thermal and hydrolytic stability, acidities
and redox potentials, etc. [11]. However, the application of
POM has been restricted by the low surface, the difculty of
recovery and low repeatability. To easily recover and recy-
cle homogeneous catalysts, various techniques involving the
immobilization of a catalyst precursor onto an insoluble sup-
port material such as ZrO2 or TiO2 or a porous support such
as silica gel, activated carbon, zeolite Y, ordered mesoporous
silica (e.g. SBA-15 or MCM-41), or cellular foam silica were
developed, so that the catalyst could be quantitatively sepa-
rated by fltration and recycled [12–17]. The immobilization
of catalysts on the ion-exchange resins is the other method
for the synthesis of heterogeneous catalysts. Ion-exchange
resins are readily available, inexpensive, thermally and
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-020-01963-6) contains
supplementary material, which is available to authorized users.
* Bahram Yadollahi
yadollahi@chem.ui.ac.ir
1
Institute for Advanced Studies in Basic Sciences (IASBS),
Gavazang, 45195-1159 Zanjan, Iran
2
Department of Chemistry, University of Isfahan,
81746-73441 Isfahan, Iran
3
Department of Chemistry, Islamic Azad University,
Shahrekord Branch, Shahrekord, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
mechanically stable and chemically resistant [18]. Oxida-
tion of diferent organic substrates was performed by ion-
exchange-supported homogeneous catalysts particularly by
immobilized metalloporphyrins [19].
Elemental analysis calcd (%) for K5[PW11Zn(H2O)O39]: P,
0.91; Zn, 1.92;W, 59.37. Found (%): P, 1.01; Zn, 2.20; W,
59.24.
In the present study, a strong cation exchanger, Dowex 22,
was selected as a very good support for the immobilization
of zinc substituted POM catalyst (Scheme 1). The prepared
catalyst was evaluated in green oxidation of various alcohols
using H2O2 in water as a green solvent. The catalytic activ-
ity of [(n-C4H9)4 N]5[PW11ZnO39]0.3H2O (PW11Zn) and
[PW11ZnO39]5− on Dowex 22 (PW11Zn@Dowex) in homo-
geneous and heterogeneous systems has also been compared.
Synthesis of the [PW11ZnO39]5− supported on Dowex
22
Ion exchange resin Dowex 22 was packed in a column and
washed with acetone for 20 min. Then the resin washed three
times by water and water/acetone. The resin dried overnight
in vacuum at 65 °C in the presence of P2O5. Five gram of
K5[PW11Zn(H2O)O39]0.25H2O was dissolved in 20 mL of
water, and 4 g of the Dowex 22 was added to this solution.
Ion exchange was performed by shaking at room tempera-
ture. After 10 h, the solid was fltered; washed with water,
water/acetone (50:50), and acetone; and eventually air-dried.
Experimental
Material and methods
Typical procedure for oxidation of alcohols
in the presence of PW11Zn@Dowex catalyst
In this study, tetra-n-butylammonium bromide (TBABr),
hydrogen peroxide, diferent alcohols, metal salts, Dowex
22, and the other chemicals and solvents were of analyti-
cal grade and used as received without further purifcation.
Fourier transform infrared (FTIR) spectra (KBr pellets) were
recorded on a FTIR Bruker Vector 22 instrument. Elemental
analysis for Zn, W and P and the exact POMs loading in
supported catalysts were analyzed on a Perkin-Elmer 7300
DV elemental analyzer. 31PNMR spectra were obtained in
D2O on Bruker 400 ultra-shield (296 K) and referenced to
85% H3PO4 (0.0 ppm) external standard. Thermogravimet-
ric analysis (TGA) was performed on a thermogravimetric
analyzer TG50 in the air from 25 to 600 °C at a heating rate
of 5°C min−1. The obtained products were analyzed by gas
chromatography (GC) on a Varian CP-3800 instrument using
capillary column CP-Wax 52 CB and/or CP-Sil 8 CB and
fame ionization detector (FID).
Catalytic experiments were carried out in a 25 mL round-
bottomed fask equipped with a refux condenser. At frst,
an appropriate amount of catalyst, 3 mL water and 1 mmol
of alcohol were added. After adding 1 mL of 30% H2O2
the mixture was refuxed for appropriate time with stirring.
The progress of the reaction was monitored by GC equipped
with a fame ionization detector. At the end of the reaction,
the catalyst and reaction solution were simply separated by
decantation.
For recycling experiments, after the request reaction time
the round-bottom fask was cooled to room temperature and
the catalyst was separated by decantation. The catalyst was
then thoroughly washed with acetonitrile and acetone fol-
lowed by drying in vacuum at room temperature.
Synthesis of the K5[PW11Zn(H2O)O39]
and TBA5[PW11ZnO39]
Results and discussion
Synthesis and characterization of PW11Zn
and PW11Zn@Dowex
Preparation of K5[PW11Zn(H2O)O39] was adapted from
the literature method [20]. FTIR of the K5[PW11Zn(H2O)
O39]: 1092, 1053, 957, 888, 828, 758, 709, 509 cm−1. The
TBA salt of this POM was prepared by metathesis. The
FTIR spectrum of TBA5[PW11ZnO39] is also in agreement
with published literatures [20–22]. 31PNMR: -11.7 ppm.
In the FTIR spectra, the characteristic fngerprints for POM
clusters in the range 1100–500 cm–1 were used for structural
confrmation. Comparison between FTIR spectra of Dowex
22, PW11Zn, and PW11Zn@Dowex indicated that the struc-
ture of PW11Zn is reserved after supporting on Dowex 22
as cation exchange resin [16]. The W–O stretching modes
are observed at 958, 890, and 803 cm–1 and P–O frequency
at 1100 cm–1 (Fig. 1).
The 31PNMR spectrum of POM structure was also
detected. The 31P resonance for PW11Zn was at -11.7 ppm
which is in consistent with the literature value [21]. Difuse
refectance UV–vis spectra for Dowex, POM and PW11Zn@
Scheme 1. Immobilization of zinc substituted Keggin type POM on
Dowex 22
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Journal of the Iranian Chemical Society
On the other hand, by supporting of POMs on Dowex 22,
the thermal stability of the catalyst is improved.
The determined amounts of PW11Zn on Dowex 22 sup-
port are presented in Table 1. ICP analyses on this heteroge-
neous catalyst confrmed the theoretical values of supported
W, Zn and P and showed very good attachment of PW11Zn
on the Dowex 22 resin. The results confrmed 0.197 mmol
PW11Zn loading per 1 g PW11Zn@Dowex. Furthermore, in
this samples the molar ratio of P:Zn:W was estimated to be
1:1:11. These results vouch that the Keggin structure unit is
well preserved in the catalyst.
The X-ray powder diffraction patterns of Dowex 22,
PW11Zn and PW11Zn@Dowex were also obtained (Fig. S3).
The XRD pattern for crystallite structure of PW11Zn was
observed at 2θ = 3–60° and showed characteristic peaks
at 2θ = 8°, 19°, 29°, 41° and 50°. However, the XRD pat-
tern of PW11Zn@Dowex was not shown any sharp peaks
and only two obviously broadened peaks at 2θ=5–10° and
2θ = 15–35° were observed. This broadened XRD pattern
results from the uniform dispersal of crystallite PW11Zn on
the Dowex 22. It was showed that when heteropoly acids are
dispersed in silica composites it has no opportunity to build
clusters exhibiting XRD patterns [22].
3000
2500
2000
1500
1000
500
Wavenumber(cm-1)
Fig. 1 FTIR spectra of PW11Zn, Dowex 22, and PW11Zn@Dowex
(down to top)
Dowex (Fig. S1) were shown that very broad absorption
band for PW11Zn@Dowex is quite similar to that for the
reference compound PW11Zn. This broad absorption band
in PW11Zn@Dowex attributed to the charge transfer bond
in Keggin type POMs [17].
The results for thermal stability of Dowex 22, and
PW11Zn@Dowex were obtained by TG-DTG, and are
shown in Figs. 2 and S2. For Dowex 22 (Fig. S2) a continu-
ous weight loss from 160 to 450 °C could be seen which is
attributed to the structure decomposing of Dowex 22. In
this temperature range (160–450 °C) for POM supported
on Dowex 22 a relatively small weight loss was observed
(Fig. 2), that is in great contrast to the big weight loss in
Dowex 22. Thus, the thermal stability of POM@Dowex
catalyst is very good and it is more stable than Dowex 22.
Investigation on the catalytic activity of PW11Zn
and PW11Zn@Dowex
To obtain the optimal reaction conditions, the catalytic
behavior of Dowex 22, PW11Zn, and PW11Zn supported on
Dowex 22 were examined through the selective oxidation of
benzyl alcohol by diferent amounts of 30% H2O2 and cata-
lyst (Table 2). Results showed that the catalytic oxidation
of benzyl alcohol over the support alone (Dowex 22) under
the reaction conditions is negligible (Table 2, entry 13 and
Fig. 3). The 0.078 mmol of PW11Zn and 0.4 g of PW11Zn@
Dowex—contains 0.078 mmol PW11Zn—was obtained as
the optimum catalyst amounts. These optimal amounts of
catalyst along with 9.8 mmol hydrogen peroxide (as the opti-
mum amount of oxidant) have been chosen for the oxidation
of various alcohols.
100
80
By the optimized reaction conditions, catalytic per-
formance of PW11Zn and PW11Zn supported on Dowex
22 (PW11Zn@Dowex) in the green oxidation of diferent
60
Table 1 Elemental analyses data for PW11Zn and PW11Zn@Dowex
Sample
Molar ratio of P:W:M
PW11Zn
loading (wt
%)
40
100
200
300
Temperature( C)
400
500
600
PW11Zn (calculated)
PW11Zn (found)
PW11Zn@Dowex
1:11:1
1:10.98:1
0.98:10.98:1
–
–
57
o
Fig. 2 TG/DTG analysis of PW11Zn@Dowex
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Journal of the Iranian Chemical Society
Table 2 Oxidation of benzyl alcohol in the presence of PW11Zn@
Dowex at diferent amount of H2O2 and catalysta
Both of the catalysts selectively catalyzed the green oxi-
dation of various substituted benzyl alcohols to the cor-
responding carbonyl compounds (Table 3 entries 1–8). In
addition, the reactivity of substituted benzyl alcohols with
electron-donating substituents is higher than that of benzyl
alcohol. The better activity in the oxidation of 3-Br, 3-Cl
and 2-nitro substituted benzyl alcohols was obtained for
PW11Zn@Dowex catalyst (Table 3 entries 1, 2, 4), and
PW11Zn showed high conversions for CH3 and t-butyl
substituted benzyl alcohols (Table 3 entries 3, 6). In other
substituted benzyl alcohols both of catalysts showed simi-
lar activity in the oxidation reaction.
Entry
H2O2 (mmol)
Catalyst (mmol)
Yields (%)b
1
2
3
4
5
6
7
8
9.8
9.8
9.8
9.8
9.8
1
3
5
7
12
14
16
9.8
0.005
0.01
0.02
25
41
60
78
92
20
35
48
65
75
78
80
5
0.04
0.078
0.078
0.078
0.078
0.078
0.078
0.078
0.078
0.4 g
9
These two catalysts were also efective for selective
oxidation of cyclooctanol, cycloheptanol, cyclohex-
anol, 4-t-butyl-cyclohexanol, 4-phenyl cyclohexanol,
and 2-methyl cyclohexanol to the corresponding ketones
(Table 3 entries 9–14). However, PW11Zn@Dowex showed
better activity for oxidation of 4-t-butyl-cyclohexzanol,
4-phenyl cyclohexanol and 2-methyl cyclohexanol with
hydrogen peroxide in water.
10
11
12
13c
aReaction conditions: substrate (1 mmol), water (3 mL), H2O2 and
catalyst in 120 min at refux
bYields refer to GC yields
c0.4 g Dowex 22 was used as catalyst
When cycloheptanol and cyclooctanol were oxidized,
both of the conversion and the ketone selectivity were higher
than 99%. In contrast, cyclohexanol only gave a conversion
of 88% with PW11Zn. In cycloalcohols the tensions of rings
increase with their sizes or carbon atoms. This factor causes
the ring structure to be more unstable but more reactive in
the oxidation reaction. As a consequence, the conversions of
cycolalcohols decreased in the order of cyclooctanol>cyclo-
heptanol > cyclohexanol. Also, PW11Zn@Dowex showed
similar activity for oxidation of diferent cycloalcohols with
hydrogen peroxide in water.
100
80
60
40
20
0
It is worth noting that the PW11Zn@Dowex was also
a highly robust and selective catalyst for the oxidation of
unsaturated alcohols. With both of the catalysts second-
ary alcohols (phenyl methyl methanol, cyclohexane methyl
methanol, phenyl ethyl methanol, biphenyl methanol and
biphenyl methyl ethanol were selectively oxidized to ketones
with high yields (Table 3 entries 14–19). In these two cata-
lytic systems, less catalytic activity for oxidation of linear
alcohols (Table 3 entry 20) was also obtained. From these
results, PW11Zn and PW11Zn@Dowex are efective catalysts
for selective oxidation of diferent alcohols and PW11Zn@
Dowex showed better activity compared to PW11Zn. More-
over, PW11Zn@Dowex showed a better catalytic activity
than some of the Keggin and Wells–Dawson sandwich-type
POMs, Fe substituted POM/silica and Zinc substituted POM
on activated carbon [12, 23, 24].
0
20
40
60
80
100
120
Time(min)
Fig. 3 The conversion-time curves for the oxidation of benzyl alco-
hol with PW11Zn (square), PW11Zn@Dowex (circle) and Dowex 22
(triangle). Reaction conditions: 0.078 mmol PW11Zn (in PW11Zn
or PW11Zn/Dowex) or 0.4 g Dowex 22, 1 mmol benzyl alcohol,
9.8 mmol H2O2 at refux in water
alcohols by hydrogen peroxide in water was investigated.
From results in Table 3, for both catalysts excellent catalytic
activity in the oxidation of diferent alcohols could be seen.
Among them, PW11Zn@Dowex catalyst gave remarkably
better catalytic performances than PW11Zn. Therefore, the
support in our catalytic system mainly exerts efects on the
catalytic activity of POM and it is reasonable to consider
that the variation in catalytic behaviors of PW11Zn has been
occurred.
The catalytic performance of PW11Zn@Dowex as a het-
erogeneous catalyst has also been appraised on the basis
of the system recycling. To examine the activity of POM
supported on Dowex 22, a set of experiments was carried
out along 10 oxidation runs. After each run, the catalyst was
recovered and treated according to washing cycles to remove
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Journal of the Iranian Chemical Society
Table 3 Oxidation of diferent
alcohols catalyzed by PW11Zn
and PW11Zn@Dowexa
Entry Substrate
1
Product
Time (h) Yields (%)b
2
80 (69)
2
3
4
2
2
2
94 (54)
75 (97)
98.5 (82)
5
6
7
2
2
2
100 (100)
79 (100)
100 (100)
8
1
100 (100)
9
2
2
97 (88)
10
99.5 (99)
11
12
2
2
100 (99.5)
100 (99.5)
13
1
100 (95)
14
15
2
2
94 (77)
100 (98)
16
2
98 (100)
17
18
2
2
100 (100)
99.5 (100)
19
20
2
2
98 (100)
22 (19)
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Journal of the Iranian Chemical Society
Acknowledgements Support for this research by the University of
Isfahan and Institute for Advanced Studies in Basic Sciences (IASBS)
is acknowledged.
References
1. S. Higashimoto, R. Shirai, Y. Osano, M. Azuma, H. Ohue, Y.
Sakata, H. Kobayashi, J. Catal. 311, 137 (2014)
2. L. Özcan, S. Yurdakal, V. Augugliaro, V. Loddo, S. Palmas, G.
Palmisano, L. Palmisano, Appl. Catal. B 132–133, 535 (2013)
3. R. Sheldon, I. Arends, A. Dijksman, Catal. Today 57, 157 (2000)
4. H. Kargar, Transit. Met. Chem. 39, 811 (2014)
5. H. Wang, W. Fan, Y. He, J. Wang, J.N. Kondo, T. Tatsumi, J.
Catal. 299, 10 (2013)
Fig. 4 Recyclability of PW11Zn@Dowex in the oxidation of benzyl
alcohol with H2O2
6. K. Kamata, K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi,
N. Mizuno, Science 300, 964 (2003)
7. J. Tan, X.B. Liu, W.F. Chen, Y.L. Hu, Chem. Select. 4, 8477
(2019)
absorbed products. Data in Fig. 4 indicate an excellent recy-
cling performance for PW11Zn@Dowex.
8. I. Kozhevnikov, Catalysts for fne chemical synthesis, catalysis by
polyoxometalates (Wiley, Hoboken, 2002)
The FTIR spectrum of recovered PW11Zn@Dowex cat-
alyst still shows the typical bands of the embedded POM
(Fig. S4). So, PW11Zn@Dowex catalyst appears to have the
most stability, high activity and excellent selectivity in the
oxidation reaction runs.
9. R. Tan, C. Liu, N. Feng, J. Xiao, W. Zheng, A. Zheng, D. Yin,
Micropor. Mesopor. Mater. 158, 77 (2012)
10. T.J. Wilke, M.A. Barteau, J. Catal. 371, 357 (2019)
11. V. Panchenko, I. Borbáth, M. Timofeeva, S. Gőbölös, J. Mol.
Catal. A 319, 119 (2010)
12. M.R. Farsani, B. Yadollahi, J. Mol. Catal. A Chem. 392, 8 (2014)
13. B.J. Johnson, A. Stein, Inorg. Chem. 40, 801 (2001)
14. W. Kaleta, K. Nowińska, Chem. Commun. 535 (2001).
15. H.-L. Li, N. Perkas, Q.-L. Li, Y. Gofer, Y. Koltypin, A. Gedanken,
Langmuir 19, 10409 (2003)
Conclusion
16. X. Qu, Y. Guo, C. Hu, J. Mol. Catal. A 262, 128 (2007)
17. C. Shi, R. Wang, G. Zhu, S. Qiu, J. Long, Eur. J. Inorg. Chem.
4801 (2005).
In this work, homogeneous and heterogeneous catalytic oxi-
dation of alcohols by H2O2 in the presence of PW11Zn and
PW11Zn@Dowex has been reported. Very good activity was
obtained for Zn substituted Keggin type polyoxometalate,
PW11Zn, in a green reaction system. The observed experi-
mental data indicated that in the presence of POM supported
on Dowex 22, PW11Zn@Dowex, the catalytic activity of
PW11Zn is preserved and also the catalyst stability and reus-
ability are very improved. So, this work provides important
insight into the heterogeneous and homogeneous catalytic
alcohols oxidation by hydrogen peroxide in the presence
of PW11Zn, which is one of the most convenient, cheap
and green methods for the synthesis of various aldehyde
compounds.
18. P. Barbaro, Chem. Eur. J. 12, 5666 (2006)
19. P. Barbaro, F. Liguori, Chem. Rev. 109, 515 (2008)
20. T. J. R. Weakley, J. Chem. Soc. Dalton Trans. 341 (1973).
21. I. Kozhevnikov, A. Sinnema, R. Jansen, K. Pamin, H. Van Bek-
kum, Catal. Lett. 30, 241 (1994)
22. A. Kukovecz, Z. Balogi, Z. Konya, M. Toba, P. Lentz, S.-I. Niwa,
F. Mizukami, A. Molnar, J. Nagy, I. Kiricsi, Appl. Catal. A 228,
83 (2002)
23. M.R. Farsani, F. Jalilian, B. Yadollahi, H.A. Rudbari, Polyhedron
76, 102 (2014)
24. E. Assady, B. Yadollahi, M.R. Farsani, M. Moghadam, Appl.
Organomet. Chem. 29, 561 (2015)
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