Synthesis, characterization and catalytic performance of a Fe polyoxometalate/silica composite in the oxidation of alcohols with hydrogen peroxide

Article abstract of DOI:10.1016/j.molcata.2014.05.001

Fe(III) substituted Keggin type polyoxometalate in amorphous silica matrix, (n-C₄H₉)₄N)₄[PW ₁₁FeO₃₉]/SiO₂ (PWFe/SiO₂), has been synthesized by sol-gel method. The synthesized PWFe/SiO₂ composite was characterized by various analytical and spectroscopic techniques. Results indicated that the primary Keggin structure remain after incorporation in silica matrix. As a heterogeneous catalyst, PWFe/SiO₂ composite showed an elevated catalytic activity and selectivity with high to excellent yields for oxidation of alcohols into aldehydes by H₂O₂. The stability and reusability of the catalyst was very high, so these composites are suitable for the heterogeneous catalysis by hydrogen peroxide.

Full text of DOI:10.1016/j.molcata.2014.05.001

Accepted Manuscript
Title: Synthesis, characterization and catalytic performance of
a Fe polyoxometalate/silica composite in the oxidation of
alcohols with hydrogen peroxide
Author: Mostafa Riahi Farsani Bahram Yadollahi
PII:
S1381-1169(14)00175-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.05.001
MOLCAA 9099
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Journal of Molecular Catalysis A: Chemical
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Revised date:
Accepted date:
1-3-2014
1-5-2014
3-5-2014
Please cite this article as: M.R. Farsani, B. Yadollahi, Synthesis, characterization and
catalytic performance of a Fe polyoxometalate/silica composite in the oxidation of
alcohols with hydrogen peroxide, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.05.001
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Synthesis, characterization and catalytic performance of a Fe
polyoxometalate/silica composite in the oxidation of alcohols with hydrogen
peroxide
Mostafa Riahi Farsani, Bahram Yadollahi*
Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
Abstract
Fe(III) substituted Keggin type polyoxometalate in amorphous silica matrix, (n-
C4H9)4N)4[PW11FeO39]/SiO2 (PWFe/SiO2), has been synthesized by sol-gel method. The
synthesized PWFe/SiO2 composite was characterized by various analytical and spectroscopic
techniques. Results indicated that the primary Keggin structure remain after incorporation in
silica matrix. As a heterogeneous catalyst, PWFe/SiO2 composite showed an elevated catalytic
activity and selectivity with high to excellent yields for oxidation of alcohols into aldehydes by
H2O2. The stability and reusability of the catalyst was very high, so these composites are suitable
for the heterogeneous catalysis by hydrogen peroxide.
Keywords: Polyoxometalates, silica composite, catalysis, oxidation, alcohols
*Corresponding Author: Tel: +98-311-7932742; fax: +98-311-6689732; e-mail: yadollahi@chem.ui.ac.ir,
yadollahi.b@gmail.com
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1. Introduction
Catalytic oxidation of alcohols into the corresponding aldehydes or ketones is one of the most
important functional group transformations in organic synthesis [1]. Traditionally, the
stoichiometric amounts of metal oxidants - notably permanganate [2] ruthenium or chromium
based reagents [3] were used for oxidation reactions. These processes are not green because
usually generate an abundant amount of environmentally unfriendly heavy-metal waste [4].
Moreover, several transition metals such as palladium [5], ruthenium [6], and molybdenum [7]
have been developed for these transformations with other oxidants such as hydrogen peroxide or
O2 [8]. However, catalytic oxidation processes for converting alcohols to carbonyl compounds on
an industrial scale remains an important challenge because these processes need expensive
metals, toxic organic solvents, long reaction times, and in a majority of cases, they require an
elevated temperature for effective catalytic activity. Additionally, some oxidation reactions have
to be performed at high oxygen pressure, which cause significant inconvenience in handling.
Thus, development of readily accessible, air/moisture stable, inexpensive, and environmentally
acceptable oxidation systems, that can promote selective oxidation under mild reaction
conditions, are still desirable [9]. Over the past decades, oxidation reactions with molecular
oxygen and hydrogen peroxide as “green” methods have gained significant impetus [10].
Aqueous hydrogen peroxide is a well-known oxidant because it is a cheap, mild and
environmentally acceptable reagent with a high content of active oxygen and produces water as a
by-product [11].
Different types of polyoxometalates (POMs) have been used for oxidative transformations with
various oxygen donors [12,13]. Much work has been done on studying POMs as heterogeneous
catalysts and applying them for industrial purposes. However, most of the studies on catalytic
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behavior of POMs have been made with homogeneous solutions [14]. The major difficulty to the
practical applications of these catalytic systems is the high-water solubility of POMs, which
prevent ready reuse and recovery of the catalyst (and therefore, its cost effectiveness) [15]. In
recent years, progresses have been made in preparation of the insolubilized POMs for
preparation of the heterogeneous catalysts via supporting or inclusion of Keggin-type POMs on a
silica matrix [16,17]. For the first time, inclusion of heteropolyacids in silica matrix was reported
by Izumi et al. They showed these structures have several advantages as catalysts for Friedel-
Crafts and hydrolysis of ethyl acetate in liquid-phase reactions. As they claimed, these catalysts
are insoluble, easily separable and thermally more stable than resin catalysts, and show large
surface areas owing to their micro and mesoporous structures and strong acidity [18-20].
Generally, the insoluble POM catalysts are prepared by impregnation or by encapsulating POM
into the silica matrix by means of sol-gel technique [21,22].
In this work, heterogeneous catalysts with mono substituted Keggin type POMs, [n-
C4H9)4N]x[PW11MO39].nH2O (PWM) (M = Cr, Mn, Fe, Co, Ni, and Cu) in silica matrix
(PWM/SiO2), were prepared by sol-gel technique via hydrolysis of tetra-ethyl orthosilicate
(TEOS). The catalytic activity of these heterogeneous catalysts was studied in the oxidation of
alcohols by H2O2 as oxidizing agent.
2. Experimental
2.1. General and methods
All materials such as metal salts (M(NO3)n.xH2O), 1-butanol, tetra-n-butylammonium bromide
(TBABr), hydrogen peroxide 30%, TEOS, various alcohols and solvents were analytical grade,
commercially available and used without further purification unless otherwise stated.
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[PW11MO39]q- (M = Cr, Mn, Fe, Co, Ni, and Cu) salts were prepared according to the literature
[23-25].
The exact POM loading in supported catalysts and the elemental analyses were performed by
inductively coupled plasma atomic emission spectrometer (ICP-AES) on a Perkin-Elmer 7300
DV elemental analyzer. Energy-dispersive X-ray (EDX) analyses of the catalysts, to determine
the presence of iron, silicon, phosphorous and tungsten, were performed on a Philips 515
scanning electron microscope equipped with an EDX Phoenix microanalyzer. Diffuse reflectance
ultraviolet-visible (DRS-UV-vis) spectra were recorded on JASCO V-670 UV-vis
spectrophotometer (190-2700 nm). Fourier-transform infrared (FTIR) spectra were obtained as
potassium bromide pellets in the range of 400-4000 cm−1 with Nicollet Impact 400D instruments.
Scanning electron micrographs (SEM) of the catalyst were taken on SEM Philips XL 30. Powder
X-ray diffraction (XRD) data were obtained on a D8 Advanced Bruker using Cu Kα radiation
(2θ = 5-70◦). Thermogravimetric analysis (TGA) was performed on a Thermogravimetric
Analyzer TG50 in air from 25 to 800 °C at a heating rate of 5 °Cmin−1. The oxidation products
were quantitatively analyzed by gas chromatography (GC) experiments on a Shimadzu GC-16A
instrument with a flame-ionization detector using silicon DC-200 column.
2.2. Synthesis of PWM (M = Cr, Mn, Fe, Co, Ni, and Cu)
Aqueous solution of the [PW11MO39]q- anions were prepared by mixing of Na2HPO4 (9.1 mol),
Na2WO4.2H2O (100 mmol) and the nitrate salts of M (Cr, Mn, Fe, Co, Ni, and Cu) (12 mmol) in
200 mL of water and adjusting the pH to 4.8. An aqueous solution of TBA bromide (45 mmol in
20 mL) was added drop wise, with stirring, at 80-85 °C. The solid was filtered off and
recrystallized from acetonitrile. In the case of iron-substituted POM, a few drops of nitric acid
were added to turn the solution from green to yellow [24,25].
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2.3. Preparation of PWM/SiO2 (M = Cr, Mn, Fe, Co, Ni, and Cu) composites
The transition metal substituted Keggin type POM/silica composites were synthesized as the
literature method [26]. A 1 g amount of PWM (M = Cr, Mn, Fe, Co, Ni, and Cu) was dissolved
in a small amount of CH3CN and 5 mL of H2O, and then the acidity of the solution was adjusted
to pH 2.0 with HClO4 (1.5 M). This solution was then added drop wise to the mixture of TEOS
(5 mL) and 1-butanol (4.5 mL). The mixture was stirred 1 h at room temperature, then 3 h at 45
°C, and finally at 80 °C until the hydrogel was formed after one hour. Acidity of the mixture was
maintained at pH 2.0 by adding HClO4. The hydrogel was dehydrated slowly at 45 °C for 12 h
and then dried in a vacuum at 80 °C for 6 h. The dried gel was washed with hot water three times
and then dried at 80 °C for 6 h, which resulted in the silica particulate gel. Before catalytic
investigation, the particulate gel was calcined in vacuum at 150 °C for 3h to fasten the silica
matrix. The loading amount of POM in the composite was obtained about 0.03 mmol POM in 1
g PWM/SiO2.
2.4. Typical procedure for oxidation of alcohols by H2O2
In a 25 mL round-bottomed flask equipped with a reflux condenser, a mixture of 1 g PWM/SiO2
(M = Cr, Mn, Fe, Co, Ni, and Cu) catalyst containing 0.03 mmol POM, 3 mL acetonitrile and 1
mmol of alcohol was added. Then 1 mL of H2O2 30% was added and the mixture was refluxed
for 2 h with stirring. The progress of the reaction was monitored by GC equipped with a flame
ionization detector and a silica pack column. At the end of reaction, the catalyst was filtered out
and 10 mL NaHCO3 10% was added. The organic phase extracted with chloroform and dried.
Flash chromatography on a short column of silica gel with ethylacetate:n-hexane as eluent gave
pure products.
2.5. Recycling experiments of the catalyst
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A typical recycling experiment was carried out as follows: After the request reaction time, the
round-bottom flask was cooled to room temperature. The composite was separated from the
reaction mixture by filtration. The catalyst was then thoroughly washed with CH3CN and acetone
followed by drying in vacuum at room temperature, and used in the other oxidation reaction.
3. Results and discussion
3.1. Characterization of the catalyst
The TBA salts of transition metal substituted POMs, [n-C4H9)4N]x[PW11MO39].nH2O (PWM) (M
= Cr, Mn, Fe, Co, Ni, and Cu), and POM/silica composites were synthesized and characterized as
described elsewhere [23-27]. The FTIR spectra, which provide characteristic finger prints for
POM clusters in the range 1100-500 cm-1, were used for structural confirmation. Figure 1 shows
the infrared spectra of PWFe, PWFe/SiO2 and SiO2.
Fig. 1. FTIR spectra of PWFe (red), PWFe/SiO2 composite (blue) and SiO2 (pink)
The vibration peaks at 1060-1090, 960-970, 870-890, and 808-820 cm−1 are characteristic of a
Keggin type structure. These peaks suggest that Keggin structures of these POMs are preserved
(Table 1). However, in comparison with the parent PWFe the characteristic peak at 1088 cm−1
has been shifted by about 30 cm−1 and broadened. This is owing to the overlapping of P–Oa and
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Si–O–Si vibrations. Therefore, it is believed that a strong chemical interaction, not a simple
physical adsorption, exists between POM and silica matrix [28].
Form the elemental analysis results, which are listed in Table 1, the POMs/SiO2 samples contain
10.85 to 11.1 wt.% of POM. Also, the molar ratio of P:M:W was estimated to be 1:1:11 in all
samples. This suggests that the Keggin structure is well preserved in the silica composites.
Therefore, elemental analysis and FTIR spectra of POMs and PWM/SiO2 composites indicated
that the structure of PWM reserved after incorporation of POMs in the silica matrix.
Table 1. elemental analysis and FTIR data for the POMs/SiO2 composites
Samlpe
POM
Molar ratio
IR data (cm-1)
Loading (wt %) of P:W:M
(P or Si)-O
1080
W=O
957
957
964
963
949
960
954
W-Ob-W W-Oc-W
PW/SiO2
11
11
10.9
11.1
10.85
11
1:10.8:0
1:10.75:0.98
1:10.8:0.97
1:10.75:0.96
1:10.8:0.97
1:10.8:0.98
1:10.8:0.97
1:11:1
885
885
887
892
881
890
886
808
810
812
815
808
809
810
PWCr/SiO2
PWMn/SiO2
PWFe/SiO2
PWCo/SiO2
PWNi/SiO2
PWCu/SiO2
PWM
1080
1081
1086
1080
1084
1080
10.95
-
1050-1060 949-964 881-892 808-820
1093
SiO2
-
-
-
-
-
The DRS UV-vis spectrum of PWFe/SiO2 composite is shown in Figure 2. Characteristic
absorption peaks at 259, 291 and 448 nm for PWFe/SiO2 composite, attributed to the oxygen-to-
metal (O–W) and (O–Fe) charge-transfer bands of Keggin anions, indicates that the primary
Keggin structure is retained after incorporation of PWFe into silica matrix [11].
The surface morphology of PWFe/SiO2 composite was determined by SEM. A typical SEM
picture of the as-prepared PWFe/SiO2 catalyst is shown in Figure 3. This picture shows non
regular morphology and so this composite has an amorphous structure. The particles size was
between 50 and 500 nm, as determined. EDX spectrum revealed the presence of Fe, W and P
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after incorporation of PWFe in the silica matrix. The EDX and elemental analysis results for
PWFe/SiO2 catalyst indicated that the amount of mono substituted POM in the silica matrix is
about 0.03 mmol/g.
Fig. 2. DRS-UV-vis spectrum of PWFe/SiO2 composite
Fig. 3. SEM image of the PWFe/SiO2 composite
The PWFe/SiO2 showed two distinct steps of weight loss in the combined TG-DTG curves (see
Figure 4), when it is heated from room temperature to 800 ºC under airflow. First weight loss in
the temperature range of 50-250 ºC is due to removal of the surface-adsorbed water and for the
loss of H2O molecules per Keggin unit. The later corresponds to the dehydration of a relatively
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stable hydrate. The other weight loss in the temperature range of 280-510 ºC is logical assign to
the decomposition of the organic part of PWFe/SiO2. The weight loss in this temperature range
(ca. 31 wt. %) attributed to the loss of 4 TBA molecules. Decomposition was completed at about
600-650 ºC to form the constituent inorganic oxides. This observation was also confirmed by the
IR spectrum of PWFe/SiO2 after heating at 650 oC (Figure 5).
Fig. 4. TG-DTG curves of the PWFe/SiO2 composite
2000
1800
1600
1400
1200
1000
800
600
400
Wavenumber(cm-1)
Fig. 5. FTIR spectra of fresh PWFe/SiO2 (pink) and PWFe/SiO2 after heating at 650 ºC (blue)
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The XRD patterns of PWFe and PWFe/SiO2 are shown in Figure 6. For PWFe the characteristic
picks in the XRD pattern are at 2θ = 7, 8, 19-25 and 30. In the XRD pattern of PWFe/SiO2 the
pattern of PWFe is repeated at the same 2θ. This observation also say that the structure of POM
preserved after incorporation in the silica matrix [28].
Fig. 6. XRD patterns of PWFe (pink) and PWFe/SiO2 (blue)
3.2. Catalytic activities of PWM and PWM/SiO2 (M = Cr, Mn, Fe, Co, Ni, Cu)
The catalytic activities of PWM (M = Cr, Mn, Fe, Co, Ni, Cu) salts were tested in the oxidation
of benzyl alcohol under reflux condition; results are summarized in Table 2. Among transition
metal substituted POMs, PWFe showed higher activity exhibiting nearly 75% yield of
benzaldehyde (Table 2, entry 5) and PWCu showed the lowest activity through 3% yield (Table
2-
2, entry 8). Also, the catalytic activity of WO4 and [PW11O39]7- in the oxidation of benzyl
alcohol was very low (Table 2, entries 1 and 2) and for (TBA)3[PW12O40].3H2O only 45% yield
was obtained (Table 2, entry 9).
Then, the catalytic activity of PWM/SiO2 (M = Cr, Mn, Fe, Co, Ni, and Cu) composites in the
oxidation of benzyl alcohol were tested under reflux condition; the results are summarized in
Table 3. Among these catalysts, PWFe/SiO2 composite showed the highest activity (80% yields)
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for preparation of benzaldehyde after 2 h catalytic reaction (Table 3, entry 8). The catalytic
performance of (TBA)5H2[PW11O39]/SiO2, Fe(NO3)3/SiO2, Na2WO4/SiO2 for oxidation of benzyl
alcohol was very low (Table 3, entries 1, 8-10) and by the other POM catalysts low to moderate
catalytic yields was obtained. From these results, PWFe/SiO2 was selected as the best in this
catalytic system.
Table 2. Catalytic oxidation of benzyl alcohol by various POM salts
Entry Catalyst
Yield (%)
2-
1
2
3
4
5
6
7
8
9
10
WO4
5
(TBA)5H2[PW11O39].3H2O
(TBA)4[PW11CrO39].3H2O
(TBA)5[PW11MnO39].3H2O
(TBA)4[PW11FeO39].3H2O
(TBA)5[PW11CoO39].3H2O
(TBA)5[PW11NiO39].3H2O
(TBA)5[PW11CuO39].3H2O
(TBA)3[PW12O40].3H2O
No catalyst
10
55
19
75
20
65
3
45
5
aReaction condition: benzyl alcohol (1 mmol), catalyst
(0.03 mmol), H2O2 30% (1 mL), time (1 h) in CH3CN
(3 mL) at reflux.
bYields refer to GC yields.
Table 3. Oxidation of benzyl alcohol by various transition metal substituted POMs in silica
matrix
Entry Catalyst
Yield (%)
1
2
3
4
5
6
7
8
(TBA)5H2[PW11O39]/SiO2
3
40
11
80
20
17
15
5
(TBA)4[PW11CrO39]/SiO2
(TBA)5[PW11MnO39]/SiO2
(TBA)4[PW11FeO39]/SiO2
(TBA)5[PW11CoO39]/SiO2
(TBA)5[PW11NiO39]/SiO2
(TBA)5[PW11CuO39]/SiO2
Na2WO4/SiO2
9
Fe(NO3)3/SiO2
4
10
SiO2
3
aReaction condition: benzyl alcohol (1 mmol), catalyst
(0.03 mmol), H2O2 30% (1 mL), time (2 h) in CH3CN (3
mL) at reflux.
bYields refer to GC yields.
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In the presence of appropriate amount of silica-included POM, the effect of various amounts of
catalyst (0.005, 0.01, 0.02, 0.03, 0.04, 0.05, and 0.1 mmol) in the oxidation of benzyl alcohol
was studied. After 4 h oxidation reaction, maximal conversion was obtained when 0.03 mmol
from POM catalyst was used (93%). Results also indicated that the conversion of benzyl alcohol
decreased with increasing in the amount of catalyst, may be due to increasing in aggregation of
PW11M in the silica matrix.
The effect of H2O2 amounts as another important factor in this oxidation system was
investigated. The conversions were increased by increasing in the amount of H2O2 from 0.5 to 1,
1.5, and 2 mL (Figure 7). The yield of benzaldehyde in the presence of 1 mL H2O2 and 0.03
mmol catalyst after 4 h oxidation reaction was 93%. By using from higher amounts of H2O2,
selectivity was decreased and a large amount of acid produced. According to this, 1 mL H2O2 is
optimum amount for this oxidation reaction.
Fig. 7. The effect of hydrogen peroxide amounts in the oxidation of benzyl alcohol with PWFe/SiO2
In the same reactions, the effect of different solvents was studied. Results indicated that among
acetonitrile, methanol, chloroform, dichloromethane, ethyl acetate and n-hexane, the best solvent
for oxidation of benzyl alcohol is CH3CN.
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As mentioned above, the optimal operational condition for the catalytic oxidation of benzyl
alcohol into benzaldehyde is: 1 mL H2O2 30% and an appropriate amount of silica-included
POM containing 0.03 mmol (TBA)4[PW11FeO39] in acetonitrile as solvent at reflux.
The time courses of PWFe and PWFe/SiO2 at the optimized reaction condition are shown in Figure
8.
Fig. 8. Time courses for the selective oxidation of benzyl alcohol with PWFe and PWFe/SiO2
For establishing this protocol in the oxidation reaction of various alcohols, oxidation of different
aliphatic and aromatic alcohols with aqueous 30% H2O2 catalyzed by PWFe/SiO2 at reflux
condition was studied which their results are listed in Table 4. Oxidation of aliphatic alcohols
such as 1-heptanol and 1-hexanol (Table 4, entries 25, 26), and cyclohexanol and cyclooctanol as
cyclic alcohols (Table 4, entries 18, 19) gave low conversions to the corresponding carbonyl
compounds. Cynamyl alcohol as an allylic alcohol showed excellent conversion and selectivity
for oxidation of OH group (Table 4, entry 22). Yields for aromatic alcohols were comparatively
higher than the aliphatic alcohols. Benzyl alcohol and substituted benzylic alcohols with electron
withdrawing and donating groups in para-, orto- and meta-positions, gave excellent yields.
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Similar reactivity was observed for the oxidation of primarily aromatic alcohols to the
corresponding aldehydes.
a
Table 4. Oxidation of alcohols with Hydrogen peroxide catalyzed by PWFe/SiO2
Entry
1
Alcohol
Product
Time (h) Yield (%)b
CH2OH
CH3
CH2OH
CHO
4
92
CH3
2
3
3
98
CHO
H3C
H3C
3
100
CH2OH
CH2OH
CH2OH
CHO
H3C
H3C
CHO
4
5
2.5
4
100
65
C
C
CHO
OCH3
CH2OH
OCH3
6
7
4.5
2.5
3.15
3
92
100
98
CHO
H3CO
Br
CH2OH
H3CO
Br
CHO
8
CH2OH
CH2OH
CHO
Br
Br
CHO
9
99
Br
Br
10
4.5
96
CH2OH
CHO
NO2
CHO
NO2
CH2OH
11
12
3
3
60
55
O2N
CH2OH
O2N
CHO
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HO
CH2OH
HO
CHO
13
14
15
16
4
5
85
75
99
96
OH
CH2OH
OH
CHO
Cl
CH2OH
CH2OH
Cl
CHO
CHO
3.15
50
Cl
Cl
Cl
I
Cl
I
17
4
90
CH2OH
CHO
OH
OH
O
O
18
19
3
3
60
60
OH
O
20
21
2.5
3
99
98
CH3
CH3
O
O
OH
OH
O
O
22
23
24
2.15
4.5
6
99
98
30
OH
OH
O
OH
OH
25
26
4
5
15
18
O
aReaction condition: alcohol (1 mmol), catalyst (0.03 mmol), H2O2 30% (1 mL) in CH3CN (3
mL) at reflux.
bYields refer to GC yields.
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The catalytic performance of this POM composite as a heterogeneous catalyst has also been
appraised on the basis of the system recycling. A set of experiments was carried out on benzyl
alcohol by PWFe/SiO2 catalyst in order to examine their activity along 5 oxidation runs. After
each run the catalyst was recovered, and treated according to washing cycles to remove absorbed
products. Results in Figure 9 indicate an excellent recycling performance for PWFe/SiO2. The
XRD pattern and FTIR spectrum of the recovered PWFe/SiO2 catalyst after each run still shows
the typical bands of the embedded POMs (Figures 10, 11). The PWFe/SiO2 appears to be the
most stable catalyst with very good activity and excellent selectivity in the oxidation reaction
runs.
Fig. 9. The reusability of PWFe/SiO2 in the oxidation of benzyl alcohol. Reaction condition:
benzyl alcohol (1 mmol), catalyst (0.03 mmol), H2O2 30% (1 mL), time (4 h) in CH3CN (3 mL)
at reflux.
4. Conclusion
In this work by sol-gel synthesis method, PW11M/SiO2 (M = Cr, Mn, Fe, Co, Ni, and Cu)
composites have been synthesized. These heterogeneous catalysts were used in the oxidation of
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alcohols with hydrogen peroxide 30% as oxidant under reflux. Results showed that PWFe/SiO2
is an excellent catalyst for selective oxidation of various aliphatic, aromatic and allylic alcohols
to the corresponding carbonyl compounds. This catalyst is a cheap and easily prepared
compound with high to excellent catalytic activity and selectivity. High reusability of
PWFe/SiO2 composite is another advantage for this catalytic system. The retention of
PWFe/SiO2 structure after catalytic runs was demonstrated by FTIR and XRD.
Fig. 10. XRD pattern of PWFe/SiO2 recovered after 5 catalytic run
Fig. 11. FT-IR spectrum of the PWFe/SiO2 recovered in run1 to 5
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Acknowledgements
Support for this research by the University of Isfahan is acknowledged.
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The (TBA)4[PW11FeO39] in amorphous silica matrix (PWFe/SiO2) has been synthesized and carefully
characterized. This composite as a heterogeneous catalyst showed an elevated catalytic activity and
selectivity with high to excellent yields for oxidation of alcohols by H2O2. The stability and reusability of
the catalyst was also very high.
PWFe/SiO2
H2O2
RCH2OH
RCOH
CH3CN, Reflux
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Highlights
The use of (TBA)4[PW11FeO39]/SiO2 composite as heterogeneous catalyst
Oxidation of various aromatic and aliphatic alcohols to aldehydes
Heterogeneous oxidation by hydrogen peroxide
High reusability of polyoxometalate/silica composite catalyst
High to excellent yields and selectivity for producing aldehydes
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