Accepted Manuscript

A comparative study on Keggin and Wells-Dawson sandwich type polyoxome-tallates in the oxidation of alcohols with 30% hydrogen peroxide

Mostafa Riahi Farsani, Fariba Jalilian, Bahram Yadollahi, Hadi Amiri Rudbari

PII: S0277-5387(14)00210-1DOI: http://dx.doi.org/10.1016/j.poly.2014.03.060Reference: POLY 10643

To appear in: Polyhedron

Received Date: 27 January 2014Accepted Date: 23 March 2014

Please cite this article as: M.R. Farsani, F. Jalilian, B. Yadollahi, H.A. Rudbari, A comparative study on Kegginand Wells-Dawson sandwich type polyoxometallates in the oxidation of alcohols with 30% hydrogen peroxide,Polyhedron (2014), doi: http://dx.doi.org/10.1016/j.poly.2014.03.060

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1

A comparative study on Keggin and Wells-Dawson sandwich type polyoxometallates in the

oxidation of alcohols with 30% hydrogen peroxide

Mostafa Riahi Farsani, Fariba Jalilian, Bahram Yadollahi*, Hadi Amiri Rudbari

Faculty of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran

Abstract

The tetra-n-butylammonium (TBA) salts of Keggin and Wells-Dawson sandwich type

polyoxotungstates, [M4(PW9O34)2]m-

and [M4(P2W15O56)2]n-

(M = Mn2+

, Fe3+

, Co2+

, Ni2+

and

Zn2+

), proved to be effective catalysts for the oxidation of benzylic alcohols to the corresponding

aldehydes with 30% hydrogen peroxide in acetonitrile. The Wells-Dawson type

polyoxometallates showed lower activity than the Keggin ones, and both the Zn substituted

polyoxometallates depicted higher conversions. Oxidation of different benzylic alcohols with

electron withdrawing and electron donating substituents gave high to excellent yields in the

presence of Zn substituted POMs.

Keywords: Sandwich type polyoxometallatates, catalysis, oxidation, alcohols, hydrogen

peroxide

* Corresponding Author. Tel: +98-311-7932742; fax: +98-311-6689732; e-mail: yadollahi@chem.ui.ac.ir,

yadollahi.b@gmail.com

2

1. Introduction

The selective oxidation of alcohols into the corresponding carbonyl compounds is an important

research field. These most vital functional group transformations are used for the production of

drugs, vitamins and fragrances [1-4]. Various inorganic oxidants such as Na2Cr2O7, NaClO,

MnO2, and KMnO4 have been used for alcohols oxidation [5-11]. However, these oxidants are

not only relatively expensive but they are also commonly hazardous or toxic, for the reason that

these oxidants generate high amounts of heavy-metal waste. Moreover, the reactions are often

performed in environmentally undesirable solvents, typically chlorinated hydrocarbons.

From economic and environmental viewpoints, many studies have been conducted with the aim

of developing clean systems with atom economy. Catalytic oxidation processes for the oxidation

of alcohols into their corresponding carbonyl compounds on an industrial scale which employs

safer, cheaper and more environmentally friendly oxidants remain a main challenge. Hydrogen

peroxide has received much attention from the viewpoint of green chemistry, it is very attractive

and a “green oxidant” for various oxidation reactions [12-16]. Additionally, it provides a high

content of active oxygen species with water as the sole byproduct, safety in storage and

operation, it is much cheaper and safer than inorganic oxidants, and it is also readily available

[17,18].

Over the last two decades, polyoxometallates (POMs), anionic metal-oxygen clusters based on

tungsten or molybdenum and in particular their transition metal substituted derivatives, have

received attention as promising oxidation catalysts for the selective oxidation of organic

substrates by a variety of oxygen sources [19-21]. For example, Mizuno et al. reported that [γ-

SiW10O36]8–

exhibits high catalytic activity for the epoxidation of various olefins and oxidation of

organosilanes by hydrogen peroxide [13,16]. Recently, [WZnZn2(H2O)2(ZnW9O34)2]12-

[22],

3

[SiW11ZnH2O40]6-

[23], [π-C5H5N(CH2)15CH3]3PMo12O40 [24], Na2WO4 [25,26], [γ-

SiW10O36(PhPO)2]4-

[27], [PW11O39]7-

[28] and K8[BW11O39H] [29] have been used as effective

catalysts for the oxidation of alcohols with hydrogen peroxide. Also, Misono et al. reported that

the diiron-substituted [γ-SiW10{Fe(OH2)}2O38]6-

has the highest efficiency for hydrogen peroxide

utilization and conversion [30]. Very recently, the mono transition metal substituted Keggin

phosphomolybdates PMo11M (M = Co, Mn, Ni) have been reported to be very efficient catalysts

for the oxidation of alcohols with hydrogen peroxide [31].

The hydrolytic instability of the simple and lacunary Keggin and Wells-Dawson type POMs in

the presence of aqueous hydrogen peroxide led to the use of various hydrolytically stable

transition metal-substituted sandwich type POMs as catalysts for hydrogen peroxide activation.

For example, various iron-containing POMs with different structures have been investigated and

reported to be hydrolytically stable and have good activity for oxidation reaction with only

moderate non-productive decomposition of hydrogen peroxide [30-33].

Although a number of methods have been developed, the search for new facile, cost-effective

and environmentally benign procedures that avoid the use of toxic solvents and expensive

oxidants still attracts substantial interest. Here, we want to report a comparison between the

catalytic activity of Keggin and Wells-Dawson sandwich type POMs, [M4(PW9O34)2]m-

and

[M4(P2W15O56)2]n-

(M = Mn2+

, Fe3+

, Co2+

, Ni2+

, and Zn2+

), (Fig. 1) as tetra-n-butylammonium

(TBA) salts for the oxidation of benzylic alcohols.

2. Experimental

2.1. Reagents and methods

4

All chemicals were analytical grade, commercially available and used without further

purification unless otherwise stated.

Infrared spectra (KBr pellets) were recorded on a JASCO, FT/IR-6300 instrument. The elemental

analysis was performed on Leco, CHNS-932 and Perkin-Elmer 7300 DV elemental analyzers.

The oxidation products were quantitatively analyzed by gas chromatography (GC) on a

Shimadzu GC-16A instrument using a 2 m column packed with silicon DC-200 and an FID

detector.

Fig. 1. Polyhedral representation of the sandwich type Keggin [M4(H2O)2(PW9O34)2]n-

(A) and

Wells-Dawson [M4(P2W15O56)2]m-

(B) POMs.

2.2. Preparation of the catalysts

Na8HPW9O34·19H2O, Na12P2W15O56·18H2O, [M4(PW9O34)2]m-

, K6P2W18O62·14H2O and

[M4(P2W15O56)2]n-

(M = Mn2+

, Fe3+

, Co2+

, Ni2+

, and Zn2+

) salts were synthesized according to

published procedures and their syntheses were confirmed by elemental analysis and infrared

spectroscopy [34-41].

Na8HPWO34.24H2O was prepared from Na2WO4.2H2O, 85% H3PO4 and glacial acetic acid

according to the literature [36]. Kl0[Mn4(H2O)2(PW9O34)2], K6[Fe4(H2O)2(PW9O34)2],

K10[Co4(H2O)2(PW9O34)2] and K10[Zn4(H2O)2(PW9O34)2] were synthesized from

A B

5

Na8HPWO34.24H2O and the appropriate salts of Mn2+

, Fe3+

, Co2+

and Zn2+

.

K6Na4[Ni4(H2O)2(PW9O34)2] was also prepared as reported in the literature [40] from

Na2WO4·2H2O, Na2HPO4, Ni(II) acetate and potassium acetate. Na16[Zn4(H2O)2(P2W15O56)2],

Na16[Co4(H2O)2(P2W15O56)2], Na16[Ni4(H2O)2(P2W15O56)2], Na16[Mn4(H2O)2(P2W15O56)2] and

Na12[Fe4(H2O)2(P2W15O56)2] were synthesized from K6P2W18O62·14H2O and the appropriate

salts of Mn2+

, Fe3+

, Co2+

, Ni2+

and Zn2+

by literature methods [36].

The TBA salts of these compounds were prepared by a metathesis reaction [40]. To 0.1 mmol of

the KxNay[M4(PW9O34)2] and KxNay[M4(P2W15O56)2] (M = Mn2+

, Fe3+

, Co2+

, Ni2+

and Zn2+

) salts

in 30 mL H2O, 10 mmol of tetra-n-butylammonium bromide was added with stirring. CH2Cl2

(250 mL) was added to this cloudy solution and the mixture was shaken in a separatory funnel.

After standing for 30 min, the mixture separated into a cloudy colorless upper layer and a clear

yellow lower layer. The bottom organic layer was collected and concentrated to a thick oil with a

rotary evaporator. A yellow solid was then precipitated from the concentrated organic layer by

addition of diethyl ether (100 mL). The resulting solid was collected on a medium frit and dried

under vacuum for 24 h (yield 80%). IR, CHNS and ICP analyses established that the POM

structures did not change when the counter-ion was altered.

IR (KBr pellet, 1300-400 cm-1

) for K10P2W18Zn4O68: 1104 (m), 1032 (w), 980 (m), 940 (m, sh),

864 (s), 820 (m); Anal. Calcd (Found): P, 1.21 (1.24); W, 64.74 (64.70); Zn, 5.12 (5.16).

IR (KBr pellet, 1300-400 cm-1

) for [(n-C4H9)4N]10[Zn4(PW9O34)2]: 1104 (m), 1032 (w), 980 (m),

940 (m, sh), 864 (s), 820 (m); Anal. Calcd (Found): C, 26.90 (26.85); H, 5.08 (5.04); N, 1.96

(1.92); P, 0.87 (0.82); W, 46.31 (46.36); Zn, 3.66 (3.62).

6

IR (KBr pellet, 1300-400 cm-1

) for Na16P4W30Zn4O112: 1093 (s), 1018 (w), 955 (s), 916 (s), 894

(m, sh), 829 (s), 801 (s, sh), 758 (s), 681 (m), 625 (m), 528 (w); Anal. Calcd (Found): P, 1.54

(1.52); W, 68.42 (68.44); Zn, 3.24 (3.28).

IR (KBr pellet, 1300-400 cm-1

) for [(n-C4H9)4N]16[P4W30Zn4O112]: 1095 (s), 1014 (w), 957 (s),

918 (s), 890 (m, sh), 824 (s), 804 (s, sh), 756 (s), 681 (m), 625 (m), 528 (w); Anal. Calcd

(Found): C, 26.57 (26.50); H, 5.02 (5.05); N, 1.94 (1.89); P, 1.07 (1.01); W, 47.66 (47.72); Zn,

2.26 (2.25).

2.3. Typical procedure for catalytic oxidation of benzyl alcohol

The catalytic reactions were performed in a 10 mL round bottom flask equipped with a magnetic

stirring bar and a reflux condenser. The oxidation of benzyl alcohol was carried out as follows:

catalyst (0.01 mmol), CH3CN (3 mL), substrate (1 mmol) and H2O2 (30% aq, 1 mL) were

charged in the reaction flask. The reaction was carried out at 80 °C and the progress of the

reaction was detected by TLC accompanied by GC by an internal standard method. Assignments

of the products were made by comparison with authentic samples.

3. Results and discussion

The catalytic oxidation of benzyl alcohol, as a model compound, by hydrogen peroxide in the

presence of tetra-n-butylammonium salts of [M4(PW9O34)2]m-

and [M4(P2W15O56)2]n-

(M = Mn2+

,

Fe3+

, Co2+

, Ni2+

, and Zn2+

) as catalysts was carried out (Scheme 1). All of the synthesized POM

salts used for catalytic oxidation of benzyl alcohol and gave the desired product in between 5 and

100% yield; the results are summarized in Table 1. Under the reaction conditions, benzaldehyde

was produced as the only oxidation product. (TBA)10[Zn4(PW9O34)2] showed 100% conversion

after 45 min (Table 1, entry 5), and for (TBA)16[Zn4(P2W15O56)2] the same conversion was

7

obtained after 1 h catalytic reaction (Table 1, entry 10). For both of these catalysts, the

conversion curve indicated better activity for (TBA)10[Zn4(PW9O34)2] (Fig. 2).

Scheme 1. Selective oxidation of various benzylic alcohols with [Zn4(PW9O34)2]10-

and

[Zn4(P2W15O56)2]16-

tetra-n-butylammonium salts

Fig. 2. The conversion-time curves for the oxidation of benzyl alcohol with

TBA10[Zn4(PW9O34)2] and TBA16[Zn4(PW15O56)2]. Reaction conditions: benzyl alcohol (1

mmol), catalyst (0.01 mmol), acetonitrile (3 mL) and 30 % H2O2 (9.8 mmol) under reflux.

CH3CN, Reflux

H2O2

CH3CN, Reflux

H2O2

8

In the absence of the polyoxotungstate, the conversion of benzyl alcohol was very low (Table 1,

entry 11). On comparing the performance of different POM catalysts after different reaction

times, the highest conversion was obtained in the presence of [Zn4(PW9O34)2]10-

. The results with

this POM catalyst were quite remarkable, as with 100% conversion of benzyl alcohol, complete

selectivity for the benzaldehyde was obtained.

With [Co4(PW9O34)2]10-

and [Co4(P2W15O56)2]16- TBA salts, the benzyl alcohol completely

disappeared after a 4 h oxidation reaction. But after one hour the conversion was very low (Table

1, entries 2, 7). The yields for sandwich type POMs with a Wells-Dawson structure were 5-100%

(Table 1, entries 5-10), and for sandwich type POMs with a Keggin structure they were 35-100%

(Table 1, entries 1-5). From these results, the importance of transition metals in the central belt

of the sandwich type POMs could be seen. Also, the catalytic effect of Keggin and Wells-

Dawson type POMs in this catalytic oxidation reaction could be compared. As in previously

reported works [42], among the various transition metals used in the central belt of sandwich

type POMs, the catalytic effect of Zn is better. Transition metal substituted POMs may have a

significant effect on the rate of dismutation of hydrogen peroxide and thus on the yield of

aqueous biphasic oxidation of organic substrates [43]. The decomposition of H2O2 in the

presence of various transition metal substituted POMs was examined and for Zn substituted

POM less reactivity in the dismutation of hydrogen peroxide was obtained. This transition metal

substituted POM also appeared to be stable in aqueous solution and in the presence of hydrogen

peroxide [32]. The results also indicated higher catalytic activity of Keggin type POMs

compared to Wells-Dawson type POMs. As stated in previous works, various transition metal-

substituted Keggin type POMs are more stable than Wells-Dawson ones, especially in the

presence of an oxidant for oxidation reactions [43].

9

Table 1. Catalytic performance of various Keggin and Wells–Dawson sandwich type

polyoxometallates in the selective oxidation of benzyl alcohol to benzaldehydea

Entry Catalyst Time (min) Yields (%)b

1 TBA10[Ni4(PW9O34)2] 60 88

2 TBA10[Co4(PW9O34)2] 60 8

3 TBA6[Fe4(PW9O34)2] 60 43

4 TBA10[Mn4(PW9O34)2] 60 34

5 TBA10[Zn4(PW9O34)2] 45 100

6 TBA16[Ni4(P2W15O56)2] 60 37

7 TBA16[Co4(P2W15O56)2] 60 5

8 TBA12[Fe4(P2W15O56)2] 60 24

9 TBA16[Mn4(P2W15O56)2] 60 11

10 TBA16[Zn4(P2W15O56)2] 60 100

11 No catalyst 60 5 aReaction conditions: benzyl alcohol (1 mmol), catalyst (0.01 mmol),

acetonitrile (3 mL) and H2O2 30% (9.8 mmol) at reflux. bYields refer to GC yields.

For the results which were obtained with 1 to 9.8 mmol of H2O2 and 0.1 to 0.005 mmol of

[Zn4(PW9O34)2]10-

and [Zn4(P2W15O56)2]16-

TBA salts, in most cases larger amounts of catalyst

and/or oxidant gave an improvement in the conversion of benzyl alcohol to benzaldehyde (Table

2). The best conversion for benzyl alcohol was obtained when 0.01 mmol of catalyst and 9.8

mmol of hydrogen peroxide were used.

Table 2. Oxidation of benzyl alcohol with different amounts of the catalysts and oxidanta

Entry Cat. (mmol) H2O2 (mmol) [Zn4(PW9O34)2]10-

Yields (%)b

[Zn4(P2W15O56)2]16-

Yields (%)b

1 0.1 9.8 95 80

2 0.05 9.8 98 82

3 0.02 9.8 97 87

4 0.01 9.8 100 94

5 0.005 9.8 92 84

6 0.01 1 50 35

7 0.01 2 56 39

8 0.01 4 70 55

9 0.01 6 77 60

10 0.01 8 92 80 aReaction conditions: benzyl alcohol (1 mmol), catalyst, acetonitrile (3 mL) and H2O2

30% at reflux for 45 min. bYields refer to GC yields.

10

Table 3. Selective oxidation of various benzylic alcohols in the presence of [Zn4(PW9O34)2]

10-

and [Zn4(P2W15O56)2]16-

TBA salts as catalystsa

Entry Substrate Time (min) [Zn4(PW9O34)]10-

Yields (%)b

[Zn4(P2W15O56)]16-

Yields (%)b

1 CH2OH

45 100 94

2

CH2OH

NO2

75 99 87

3

CH2OH

NO2

75 98 87

4

CH2OH

Cl

60 100 96

5 CH2OH

Cl

60 99 95

6

CH2OH

Br

60 100 96

7

CH2OH

OMe

30 100 98

8 CH2OH

OMe

35 100 96

9

CH2OH

Me

32 100 98

10

CH2OH

Me

35 100 97

aReaction conditions: alcohol (1 mmol), catalyst (0.01 mmol), acetonitrile (3 mL) and H2O2

30% (9.8 mmol) at reflux. bYields refer to GC yields.

11

Using this procedure, the reaction of various benzylic alcohols (Scheme 1) was clean and all of

the alcohols were converted into the corresponding aldehydes in excellent yields without over-

oxidation to carboxylic acids (Table 3). On comparing benzyl alcohol with benzylic alcohols

containing an electron-releasing substituent (Table 3, entries 7-10), it was found that the reaction

times for the electron-rich alcohols are shorter. Benzylic alcohols with electron-withdrawing

groups (Table 3, entries 2-6) required longer reaction times and gave lower yields. However both

electron withdrawing and electron donating substituents, such as p-NO2, m-NO2, o-Cl, p-Cl, p-Br

and p-OMe, on the benzene ring gave the desired products in 98-100% yields with

(TBA)10[Zn4(PW9O34)2] and 87-98% yields with (TBA)16[Zn4(P2W15O56)2]. Thus, both catalysts

showed excellent selectivity in the oxidation of benzylic alcohols, and aldehyde was the only

product.

The recyclability of the catalyst in the oxidation reaction of benzyl alcohol was investigated. For

three cycles, benzyl alcohol was completely converted to benzaldehyde without appreciable loss

in catalytic activity. However, in the forth and fifth cycles the product yields decreased from

100% to 70% (Table 4). The decrease in catalytic activity may be due to the structure

decomposing in the recycled catalyst.

Table 4. Recycling of the catalysts in the oxidation of benzyl alcohola

Run [Zn4(PW9O34)2]10-

Yields (%)b

[Zn4(P2W15O56)2]16-

Yields (%)b

1 100 100

2 100 100

3 100 95

4 95 80

5 90 70 aReaction conditions: benzyl alcohol (1 mmol), catalyst

(0.01 mmol), acetonitrile (3 mL) and H2O2 30% (9.8

mmol) at reflux for 60 min. bYields refer to GC yields.

12

Fig. 3. FT-IR spectra of TBA16[Zn4(P2W15O56)2] recovered in runs 1 to 5

The FT-IR spectra for fresh pre-catalysts and recycled catalysts of TBA10[Zn4(PW9O34)2] and

TBA16[Zn4(P2W15O56)2] for the first 3 runs are very similar and exhibit characteristic peaks

(Figs. 3 and 4). The similarity in FT-IR spectra suggest that the structures of the recycled

catalysts are preserved and they are stable during catalytic recycling. However, in comparison,

the FT-IR spectra for the recycled catalysts after cycles 4 and 5 show some differences.

4. Conclusion

Keggin and Wells-Dawson sandwich type POMs with different transition metal in the central

belts proved to be active catalysts for the oxidation of benzyl alcohol with hydrogen peroxide as

the oxidant and acetonitrile as the solvent at reflux. With this catalytic system, high to excellent

yields of aldehydes were obtained. The Wells-Dawson type POMs were shown to be less active

than the Keggin ones and the Zn substituted POMs depicted higher conversions for both types. In

13

the presence of [Zn4(PW9O34)2]10-

and [Zn4(P2W15O56)2]16-

TBA salts, high to excellent

conversions for different substituted benzylic alcohols, and 100% selectivity for aldehyde were

obtained.

Fig. 4. FT-IR spectra of TBA10[Zn4(PW9O34)2] recovered in runs 1 to 5

Acknowledgement

Support for this research by the University of Isfahan is acknowledged.

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[42] Y. Kikukawa, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 49 (2010) 6096-6100.

[43] J.E. Bäckvall (Ed.), Modern Oxidation Methods, WILEY-VCH Verlag. 2004, pp, 223-248.

17

Graphical Abstract Synopsis:

A comparative study on Keggin and Wells–Dawson sandwich type polyoxometallates in the

oxidation of alcohols with 30% hydrogen peroxide Mostafa Riahi Farsani, Fariba Jalilian, Bahram Yadollahi

*, Hadi Amiri Rudbari

Chemistry Department, University of Isfahan, Isfahan 81746-73441, Iran.

The TBA salts of [M4(PW9O34)2]m-

and [M4(P2W15O56)2]n-

were used as effective catalysts for the

selective oxidation of benzylic alcohols. The Wells-Dawson type polyoxometallates were shown

to be less active than the Keggin ones and the Zn substituted polyoxometallates depicted higher

conversions for both types.

18

Graphical Abstract pictogram:

A comparative study on Keggin and Wells–Dawson sandwich type polyoxometallates in the

oxidation of alcohols with 30% hydrogen peroxide Mostafa Riahi Farsani, Fariba Jalilian, Bahram Yadollahi

*, Hadi Amiri Rudbari

Chemistry Department, University of Isfahan, Isfahan 81746-73441, Iran.

CH3CN, Reflux

H2O2

CH3CN, Reflux

H2O2