Catalytic oxidation of benzaldehyde to benzoic acid with keggin polyoxometalate [(CH2)5NH2]3PW 12O40

Article abstract of DOI:10.14233/ajchem.2013.12905

The organic-inorganic hybrid catalyst [(CH₂)5NH ₂]₃PW₁₂O₄₀ was prepared by heteropoly acid and piperidine. The heteropoly acid was synthesized by sodium molybdate and sodium silicate and the piperidin

Full text of DOI:10.14233/ajchem.2013.12905

Asian Journal of Chemistry; Vol. 25, No. 2 (2013), 793-796
http://dx.doi.org/10.14233/ajchem.2013.12905
Catalytic Oxidation of Benzaldehyde to Benzoic Acid with
Keggin Polyoxometalate [(CH2)5NH2]3PW12O40
1
2
YUANSHENG DING , FEI-LU1,* and XINBAO- HAN
1Department of Chemical and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin 132022, P.R.China
2Key Lab of Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, P.R. China
*Corresponding author: Fax: +86 432 63967717l; Tel: +86 150 44667696; E-mail: lufei1893@163.com
(Received: 21 October 2011;
Accepted: 20 August 2012)
AJC-11980
The organic-inorganic hybrid catalyst [(CH2)5NH2]3PW12O40 was prepared by heteropoly acid and piperidine. The heteropoly acid was
synthesized by sodium molybdate and sodium silicate and the piperidine was organic ligand. The proposed composition and structure of
the catalyst were evidenced by FT-IR, XRD and TG-DTA. The results indicated that the heteropoly anions reserved their Keggin structure
in the compound. Its catalytic performance was evaluated in the oxidation of benzaldehyde to benzoic acid. Various reaction parameters
were changed to attain the optimal conditions. The optimal reaction conditions were found to be: n(catalyst):n(benzaldehyde) = 3.103 ×
10-3:1; n(H2O2):n(benzaldehyde) = 4.0:1; reaction temperature 80 ºC; reaction time 4 h. The yield of benzoic acid achieved above 93 %,
furthermore and the selectivity was 100 %.
Key Words: [(CH2)5NH2]3PW12O40, Keggin polyoxometalate, Catalysis, Benzoic acid.
INTRODUCTION
EXPERIMENTAL
Heteropoly acids (HPAs) and related polyoxometalate
compounds (POMs) are a class of poly-oxygen metal complexes.
The complexes contain the central atom (such as P, Si, etc.)
and ligands (such as Mo, W, etc.). They combined multi-space
oxygen by bridging oxygen atoms. Heteropoly acids and
polyoxometalates have a clear size, shape and discrete fragments
similar to the structure of metal oxides1. They play great roles
in catalysis, photochemistry, medicine, materials science and
magnetic fields2-6. As green catalysts, they contain both acid
sites and redox active sites that make them to be potential redox
molecular sieves and bifunctional catalysts. Furthermore, they
display high catalytic activity, selectivity, rapid response, mild
reaction conditions, oxidation and optical, magnetic, stability,
micro-light corrosion of equipment. Based on these properties,
they can be used for homogeneous and heterogeneous reactions
and even can be used for phase transfer catalyst7-12. Therefore,
synthesis of new efficient polyoxometalates catalysts has
become a hot spot.
Na2WO4·2H2O, hexahydropyridine and benzaldehyde
obtained from Shanghai Chemical Reagent Co. Ltd., H2O2
(30 %) and H3PO4 (85 %) were purchased from Beijing
Chemical Reagent Co. Ltd. The products were characterized
by X-ray powder diffraction (XRD), microscopic meting point
meter, Differential ThermalAnalysis and Thermo Gravimetric
Analysis (TG-DTA).
Materials synthesis: The 20 g Na2WO4·2H2O were
dissolved by 40 mL deionized water and kept in boiling water
bath for 70 ºC. The 2 mL H3PO4 were added in the solutions in
the conditions of pH 2, 70 ºC, 120 r/min. After 10 min, the
HClO4 were added into the solutions. Firstly, the yellow deposit
emerged. After filtered, the liquid was cooled to ambient
temperature. The total reaction times were 2 h. After moved
to the separatory funnel, the liquid was extracted with ethyl
ether and 6 mol L-1 HCl. The below solutions were one more
extracted with ethyl ether and HCl. The extracted oil phase
with added a small amount of distilled water (10-20 drops)
and a few drops of concentrated HNO3 were evaporated under
the 60 ºC water bath until the films appeared. Then the trans-
parent phosphatotungstic acid was attained after cooled.
The purity of crystal was obtained with nearly 100 % after
recrystallization.
Keggin type coordination [(CH2)5NH2]4PW12O40] polyoxo-
metalates catalyst was synthesized and structurally charac-
terized by means of single crystal X-ray diffraction, IR and
TG. The composite catalyst was applied to the oxidation of
benzoic acid with benzaldehyde. The influence factors of the
reaction were also investigated.

794 Ding et al.
Asian J. Chem.
7000
6000
5000
4000
3000
2000
1000
0
The precipitate was prepared by mixing the H3PW12O40·nH2O
and hexahydropyridine with mole ratio 1:3 and reaction at
70 ºC for 2 h. After filteration, the precipitate was cleaned by
ethanol, ethyl ether and H2O in order and dissolved in the
mixture of the acetonitrile. After filteration, the filtrates were
transferred to the beaker and kept at room temperature for
one week. The [(CH2)5NH2]4PW12O40 crystal would crystallize
at the bottom of the beaker.
Catalytic activity: The crystal [(CH2)5NH2]3PW12O40
were used as the catalyst in the process of the oxidation
benzoic acid with benzaldehyde. The effects of catalyst,
reactants, temperature and reaction time were investigated.
RESULTS AND DISCUSSION
10
20
30
40
2θ (º)
2θ(degree)
The FTIR spectrum of charge transfer polyoxometalates
[(CH2)5NH2]3PW12O40 (Fig. 1) exhibited four bands located at
1078, 978, 892 and 799 cm-1 corresponding to P-Oa, W-Od,
W-Ob-W and W-Oc-W vibrations, respectively. These absor-
ption peaks closed to those reported13,14 for heteropolyanions
[PW12O40]3-. The result showed the syntheses transfer POMs
exhibit Keggin structure. However, relative to the starting
heteropoly acids, the absorption peak of P-Oa and W-Od
showed an obvious blue shift, while that of W-Ob-W and
W-Oc-W showed a blue shift. This may be due to the charge
transfer from protoned piperdine to the heteropolyanions. The
absorption bands at 1581 and 1439 cm-1 are assigned to the
stretching vibrations for piperdine ring. FTIR 2863 cm-1 peak
and 2939 cm-1 peak were assigned to the CH2 symmetric stretch
and asymmetric stretch (CH2-as) of the γ-methylene and
β-methylene groups. The bands corresponding to the axial
amine N-H stretching vibrations appear in the 3174 cm-1
region. The FTIR spectrum showed that the stronger charge
transfer between polyanions and its counterions that result in
forming the charge transfer polyoxometalates.
Fig. 2. X-Ray powder diffraction patterns of charge transfer POMs
[(CH2)5NH2]3PW12O40
The TG curve indicated hat their thermal decomposition
processes of [(CH2)5NH2]3PW12O40 can be divided into four
mass loss steps. According to TG curve, the degradation in
the temperature range of 50-800 ºC consisted of four mass
loss steps.
The first mass loss step was in the temperature range of
50-468.11 ºC with similar mass losses of ca. 0.295 %. In the
process, six crystallization waters that connected with acidic
proton via H-bond were lost. The second decomposition step
in the temperature range of 468.11-481.19 ºC. The polyanions
began to decompose in this process. The third decomposition
step started at 481.19 ºC and continued up to 542.39. The third
step can be assigned to the oxidative decomposition of
protoned piperdine with a strong exothermic peak marking
the collapse of Keggin structure. The mass losses in the third
step were 6.957 %. The four decomposition step in the tempe-
rature range of 542.39-800 ºC. In the four step, the polyanions
were total decomposed intoWO3.According to DTA, the decom-
position temperature of [(CH2)5NH2]3PW12O40 was at 481.19 ºC
that lower than decomposition temperature (465 ºC) of the
starting heteropoly acid15. It can be seen the charge transfer
POMs have more hermodynamically stable structures compare
to starting heteropoly acid.
Effects of reaction conditions: The effect of catalyst
dosage on the yields of products was shown in Fig. 3. The
molar ratio of [(CH2)5NH2]3PW12O40/benzaldehyde was varied
from 1.9 × 10-3-3.7 × 10-3. As can be seen, catalyst dosage
significantly affected the reaction. The yield of the benic acid
was promoted with the increase of molar ratio. The most
beneficial molar ratio of [(CH2)5NH2]3PW12O40/benzaldehyde
was chosen to be 3.1 × 10-3.
The effect of H2O2 on reaction was also studied (Fig. 4).
The conversion of benzoic acid was promoted with the molar
ratio of H2O2/benzaldehyde when the molar ratio was less than
4.0:1. While above the molar ratio 4.0:1, the yield of benzoic acid
was decreased with the molar ratio of H2O2/benzaldehyde.
The effect of reaction temperature on the reaction was
investigated and the experiments were conducted at 60, 70,
80 and 90 ºC, respectively. Fig. 5 showed the effect of the
temperature on the yields of product. It can be observed that
temperature have a significant effect on the reactions. When
4000 3500 3000 2500 2000
1500 1000
500
-1
–1
WaWvaevnenuummbbeer r(cm(cm) )
Fig. 1. FTIR spectrum of charge transfer POMs [(CH2)5NH2]3PW12O40
XRD result of the prepared catalyst before reduction was
presented in Fig. 2. Four peaks at around 8.56, 4.58, 9.78 and
8.76º were observed for catalyst. These four peaks were similar
to the Keggin-type H3PW12O40 phase3.

Vol. 25, No. 2 (2013)
Catalytic Oxidation of Benzaldehyde to Benzoic Acid with Keggin Polyoxometalate 795
the reaction temperature was increased from 60-80 ºC, there
was a significant increase in the yield of benzoic acid. When
the reaction temperature rose above 80 ºC, the yield of benzoic
acid began to fall and higher temperatures thus could accelerate
the rate of chemical reactions, but unwanted side reactions
also appeared at the same time. For example, H2O2 could
decompose, thus the reaction temperature was set to 80 ºC in
this experiment.
Time course of oxidation was shown in Fig. 6. The amount
of benzoic acid grew very fast for the first 2 h. Then it was
observed slightly to decrease after 4 h. It was found that 4 h
were sufficient for completion of the oxidation reaction.
n(catalyst):n(benzaldehyde)/10
90
Fig. 3. Catalytic effect of [(CH2)5NH2]3PW12O40 dosage on the conversion
of benzoic acid (reaction conditions: nH2O2:benzaldehyde = 4.0:1,
temperature 80 ºC, time 4 h)
85
80
75
70
2
3
4
5
6
Time (h)
Time (hour)
Fig. 6. Effect of reaction time on the catalytic activity (reaction conditions:
n[(CH2)5NH2]3PW12O40:n-benzaldehyde = 3.1 × 10-3, nH2O2:
benzaldehyde = 4.0:1, temperature 80 ºC)
n(H2O2):n(benzaldehyde)
To estimate the potential [(CH2)5NH2]3PW12O40 contribu-
tion in the reaction, benzaldehyde oxidation was also carried
out at mole ratio (H2O2/benzaldehyde) of 4:1 at 80 ºC without
[(CH2)5NH2]3PW12O40, the product yield was only 27.5 %
that lower than the product conversion of reaction with
[(CH2)5NH2]3PW12O40. While H2O2 was not added into the
system, no product was formed. It can conclude that the
[(CH2)5NH2]3PW12O40 was not an oxidizing agent, however it
played great role in promoting the benzaldehyde oxidation
reaction as catalyst.
Fig. 4. Effect of H2O2 dosage on the conversion of benzoic acid (reaction
conditions: n[(CH2)5NH2]3PW12O40:n-benzaldehyde = 3.1 × 10-3,
temperature 80 ºC, time 4 h)
92
90
88
86
84
82
80
78
76
74
72
Above the about analysis, the optimal reaction conditions
were drawn. For verification three parallel experiments were
carried out, the results were shown in the Table-1.
Conclusion
The proton-transfer reactions between hexahydropyridine
and Keggin-type heteropolyacid produced solid charge transfer
hybrid materials [(CH2)5NH2]3PW12O40. The charge transfer
interactions are strong enough to cause evident changes
in FTIR spectrum from heteropolyacids to hybrid
materials. The optimal reaction conditions were as follows:
n[(CH2)5NH2]3PW12O40: n-benzaldehyde = 3.1 × 10-3, nH2O2:
benzaldehyde = 4.0:1, 80 ºC, 4 h. Under the optimum
conditions, the yield of the product is 93.1 %.
60
65
70
75
80
85
90
Temperature (ºC)
Temperature (¡ãC)
Fig. 5. Effect of temperature on the catalytic activity (reaction conditions:
n[(CH2)5NH2]3PW12O40:n-benzaldehyde = 3.1 × 10-3, nH2O2:
benzaldehyde = 4.0:1, time 4 h)

796 Ding et al.
Asian J. Chem.
TABLE-1
REACTION CONDITIONS (VERIFICATION)
Number
nH2O2:n-benzaldehyde
Reaction temperature (ºC)
80
n[(CH2)5NH2]3PW12O40:n-benzaldehyde × 10-3
Time (h)
Yeild (%)
93.03
1
2
3
4.0:1
4.0:1
4.0:1
3.1
4
4
4
80
80
3.1
3.1
93.15
93.87
7. P. Liu, Y. Liang, X.Z. Lin, C.X. Wang and G.W. Yang, AcCS Nano, 5,
4748 (2011).
ACKNOWLEDGEMENTS
8. A. Aouissi, Asian J. Chem., 22, 4924 (2010).
9. S.T. Zheng, J. Zhang and G.Y. Yang, Angew. Chem. Int. Ed., 47, 3909
(2008).
The authors are grateful for the financial support provided
by the Jilin Education Department Science Foundation of
China (Nos. 2008174 and 2009152).
10. X.L. Wang, C. Qin, E.B. Wang, Z.M. Su, Y.G. Li and L. Xu, Angew.
Chem. Int. Ed., 45, 7411 (2006).
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