Thermal stability and acidic strength of preyssler-type phosphotungstic acid, H14[P5W30O110Na] and it's catalytic activity for hydrolysis of alkyl acetates

Article abstract of DOI:10.1002/zaac.201100427

Full structural analysis of Preyssler-type phosphotungstic acid, H ₁₄[P₅W₃₀O₁₁₀Na], using ¹⁸³W NMR, ³¹P NMR, and IR spectroscopy, as well as complete elemental analysis revealed that H₁₄[P₅W ₃₀O₁₁₀Na]·44H₂O was obtained in pure form. H₁₄[P₅W₃₀O₁₁₀Na] started to decompose by heating at 373 K and was less stable than Keggin-type phosphotungstic acid, H₃PW₁₂O₄₀·6H ₂O, which was stable even after heating at 673 K. Acidic strength measurements using trimethyl phosphine oxide (Me₃P=O) as a probe molecule indicated that the acidic strength of H₁₄[P ₅W₃₀O₁₁₀Na] was greater than that of H ₃PW₁₂O₄₀ on the basis of the amount of acid in an aqueous solution. H₁₄[P₅W₃₀O ₁₁₀Na] was less active than H₃PW₁₂O ₄₀ for hydrolysis of alkyl acetates on the basis of acid amount. However, H₁₄[P₅W₃₀O₁₁₀Na] was more active than H₃PW₁₂O₄₀ for hydrolysis of alkyl acetates with small alkyl groups on the basis of catalyst weight because acid amount per weight of H₁₄[P₅W₃₀O ₁₁₀Na] was about two times higher than that of H₃PW ₁₂O₄₀. Copyright

Full text of DOI:10.1002/zaac.201100427

ARTICLE
DOI: 10.1002/zaac.201100427
Thermal Stability and Acidic Strength of Preyssler-Type Phosphotungstic
Acid, H14[P5W30O110Na] and It’s Catalytic Activity for Hydrolysis of Alkyl
Acetates
Masahiro Sadakane,*[a] Yutaro Ichi,[a] Yusuke Ide,[a] and Tsuneji Sano[a]
Keywords: Heteropolyacids; Preyssler-type structures; Tungsten; Phosphorus; Thermal stability; Acid catalysts
Abstract. Full structural analysis of Preyssler-type phosphotungstic H14[P5W30O110Na] was greater than that of H3PW12O40 on the basis
acid, H14[P5W30O110Na], using 183W NMR, 31P NMR, and IR spec-
of the amount of acid in an aqueous solution. H14[P5W30O110Na] was
troscopy, as well as complete elemental analysis revealed that less active than H3PW12O40 for hydrolysis of alkyl acetates on the
H14[P5W30O110Na]·44H2O was obtained in pure form. basis of acid amount. However, H14[P5W30O110Na] was more active
H14[P5W30O110Na] started to decompose by heating at 373 K and was than H3PW12O40 for hydrolysis of alkyl acetates with small alkyl
less stable than Keggin-type phosphotungstic acid, H3PW12O40·6H2O, groups on the basis of catalyst weight because acid amount per weight
which was stable even after heating at 673 K.
Acidic strength measurements using trimethyl phosphine oxide
(Me3P=O) as a probe molecule indicated that the acidic strength of
of H14[P5W30O110Na] was about two times higher than that of
H3PW12O40.
Introduction
Heteropolyacids (HPAs) are known as metal-oxygen clusters
showing strong acidity and high oxidation ability.[1, 2] Due to
the attractive properties, HPAs have been utilized as catalysts
even in industrial processes, such as hydration of isobutylene
to tert-butanol, polymerization of tetrahydrofurane, and selec-
tive oxidation of methacrolein to methacrylic acid.
Among the various HPAs,
a phosphotungstic acid,
H3PW12O40, is the most widely used acid catalyst and it has a
so-called “Keggin” structure in which the central PO4 tetrahe-
dron is surrounded by 12 WO6 octahedra with Td symmetry
(Figure 1 (a)). The Keggin-type phosphotungstic acid has been
used as an acid catalyst for a variety of organic reactions in a
homogeneous way as well as a heterogeneous way.[1,2] The
phosphotungstic acid shows higher acidity than that of sulfuric
Figure 1. Structures of (a) Keggin-type and (b) Preyssler-type phos-
photungstates. Dark polyhedra indicate PO4, white ochtahedra indicate
WO6, and the circle indicates sodium cation.
acid and can contribute to the development of more effective sists of a cyclic assembly of five [PW6O22]3– units, each de-
and greener reactions.
rived from the Keggin anion, [PW12O40]3–, by removal of two
On the other hand, a heteropoly acid with a structure other sets of three corner-shared WO6 octahedra. A sodium ion is
than a Keggin structure is rare, and further exploration of new located within the polyanion on the fivefold axis (Figure 1 (b)).
and significant catalyses with HPAs is still desired. In 1985, This anion can be isolated as potassium-sodium mixed salt,
the structure of Preyssler-type HPA[3] was revealed by Pope’s and the potassium and sodium cations can be ion-exchanged
group.[4]
The
Preyssler-type
phosphotungstate, by protons to give the acidic form.[4]
[P5W30O110Na]14–, has approximate D5h symmetry and con-
Since the first report by Pope’s group, several research
groups of Heravi, Bamoharram, Gharib, Alizadeh, Hekmat-
shorar, and Feizi have reported acid catalytic activities of the
Preyssler-type phosphotungstic acid for many organic reac-
tions such as esterification, tetrahydropyranyl (THP) protection
of hydroxygroups, syntheses of dibenzo xanthene derivatives
and 2,4,6-triarylpyridines, and alkylation of phenol.[5–12] We
also reported that the Preyssler-type phosphotungstic acid is
* Prof. Dr. M. Sadakane
Fax: Int.-81-82-424-5494
E-Mail: sadakane09@hiroshima-u.ac.jp
[a] Department of Applied Chemistry
Graduate School of Engineering
Hiroshima University
1-4-1, Kagamiyama, Higashi-Hiroshima, 739-8527, Japan
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201100427 or from the author. effective for acylation of methyl pyruvate.[13]
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Z. Anorg. Allg. Chem. 2011, 637, (14-15), 2120–2124

Preyssler-Type Phosphotungstic Acid, H14[P5W30O110Na]
To obtain further information on the Preyssler-type phos-
photungstic acid, in this study, we investigated the thermal sta-
bility, acidic strength, and hydrolysis activity of alkyl acetates
of the Preyssler-type phosphotungstic acid.
Results and Discussion
Preparation and Structural Characterization of
H14[P5W30O110Na]
The potassium and sodium cations of K12.5Na1.5[P5-
W30O110Na] were exchanged using Dowex-50W proton ex-
change resin to produce the Preyssler-type phosphotungstic
acid. The 183W NMR (Figure 2 (a)), 31P NMR (Figure 3 (a)),
and IR spectra (Figure 4 (a)) are very similar to those of the
parent potassium-sodium mixed salts, indicating that the
Preyssler-type structure can be retained after the proton ex-
change process. No other phosphotungstate species was de-
tected by 31P NMR, 183W NMR, and IR spectroscopy. No po-
tassium was found and one equivalent of sodium to a
Preyssler-type molecule, which occupies the central cavity of
a Preyssler-type molecule, was detected by elemental analysis.
No sulfur or carbon was detected, which indicated that no con-
tamination by proton exchange resin occurred. No chlorine,
which was used for the preparation of the potassium salt, was
detected. These results strongly indicate that Preyssler-type
phosphotungstic acid can be prepared in high purity.
Figure 3. 31P NMR spectra of Preyssler-type phosphtungstic acids (a)
before and (b) after heating at 373 K, (c) 423 K, (d) 473 K, (e) 523
K, (f) 573 K, and (g) 623 K for 1 hour. Extended spectra of (a) and
(b) are presented in a box.
Figure 4. IR spectra of Preyssler-type phosphotungstic acid (a) before
and (b) after heating at 473 K, (c) 523 K, (d) 573 K, (e) 623 K, and
(f) 673 K for 1 hour.
heating temperature, the peak intensity of the Preyssler-type
phosphotungstic acid gradually decreased and the intensities of
unassigned peaks increased (Figure 3), suggesting decomposi-
tion of the Preyssler-type phosphotungstate framework. The IR
spectra also indicate decomposition of Preyssler-type phospho-
tungstate after heating (Figure 4). These results indicate that
the Preyssler-type phosphotungstic acid started to decompose
at 373 K.
Figure 2. 183W NMR spectra of Preyssler-type phosphotungstates: (a)
acid form (H14[P5W30O110Na] (ca. 1 g) dissolved in D2O (3 mL)) and
(b) lithium salt (K12.5Na1.5[P5W30O110Na] (ca. 1 g) dissolved in D2O
(3 mL) with the help of Li-resin).
On the other hand, Keggin-type phosphotungstic acid was
stable up to 723 K. No other obvious 31P NMR signal was
observed even after heating at 723 K (Figure S1), and the IR
spectra after heating at 723 K were unchanged (Figure S2).
Thermal Stability
The obtained Preyssler-type phosphotungstic acids were The thermal stability is in good agreement with the reported
heated in a muffle oven and then analyzed by solution 31P value (741 K).[1,2] Thermal stability of Preyssler-type phos-
NMR and IR. In the 31P NMR spectrum of Preyssler-type photungstic acid is lower than that of Keggin-type phos-
phosphotungstic acid heated at 373 K for 1 hour, several small photungstic acid. Therefore, we focus to use Preyssler-type
peaks together with the peak of Preyssler-type phosphotungstic phosphotungstic acid in an aqueous solution as a homogeneous
acid were observed (Figure 3 in a box). With increase in the catalyst.
Z. Anorg. Allg. Chem. 2011, 2120–2124
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M. Sadakane, Y. Ichi, Y. Ide, T. Sano
ARTICLE
Acidic Strength Measurement
type phosphotungstic acid exhibits higher catalytic activity
than that of sulfuric acid or p-toluenesulfonic acid on the basis
of acid amount.[18]
Acidic strength of Preyssler-type phosphotungstic acid was
compared with those of Keggin-type phosphotungstic acid and
sulfuric acid using trimethyl phosphine oxide (Me3P=O) as a
probe molecule. Trialkyl phosphine oxide is known as an
acidic strength indicator both in a solid state and in solu-
tion.[14–17] Trialkyl phosphine oxide is protonated to form trial-
kyl phosphine hydroxide cation (Equation (1)). This reversible
reaction is so much faster than the 31P NMR time-scale that
only one 31P NMR signal is detected. The peak moves to a
more positive chemical shift if the acidic strength and amount
of proton increase. Therefore, the acidic strength can be esti-
mated by the peak shift:
Catalytic activities of Preyssler-type phosphotungstic acid
and Keggin-type phosphotungstic acid for the hydrolysis of
various alkyl acetates are summarized in Table 1. In all cases,
only acetic acid and corresponding alkyl alcohols were formed
as reaction products. The activity was estimated from the
amount of alkyl alcohols produced after reaction for 2 h and
was expressed in two ways: on the basis of acid amount and on
the basis of catalyst weight. Catalytic activity obtained using
H3PW12O40·6H2O (Entry 2) was similar to the reported value
(Entry 1), which indicated that the accuracy of our method of
1
analysis using H NMR is similar to that of the method re-
ported in the literature.[18]
Me3P=O + H+ [larr ] Me3P-OH+
(1)
Conversions of ethyl acetate with Preyssler-type phos-
photungstic acid were higher than those with Keggin-type
phosphotungstic acid (Entries 2–5). Namely, the reaction rate
per catalyst weight of Preyssler-type phosphotungstic acid was
higher than that of Keggin-type phosphotungstic acid. How-
ever, the reaction rate per acid amount of Preyssler-type phos-
photungstic acid was lower than that of Keggin-type phos-
photungstic acid, because the acid amounts per catalyst weight
of Preyssler-type and Keggin-type phosphotungstic acids are
ca. 1.7 and 1.0 mmol·g–1, respectively.
Alkyl acetates with different alkyl groups such as methyl, n-
propyl, isopropyl, n-butyl, iso-butyl, tert-butyl, and cyclohexyl
were also tested for hydrolysis reaction (Entries 6–19). Con-
versions of alkyl acetates decreased by increasing the size of
the alkyl group in the presence of both catalysts. However, the
degree of conversion drop for Preyssler-type phosphotungstic
acid was larger than that for Keggin-type phosphotungstic
acid. Conversions of methyl acetate, ethyl acetate, and n-pro-
pyl acetate with Preyssler-type phosphotungstic acid were
larger than those with Keggin-type phosphotungstic acid.
There was no difference in conversions of isopropyl acetate
and cyclohexyl acetate between the two catalysts. However,
conversions of n-butyl acetate and iso-butyl acetate with
Preyssler-type phosphotungstic acid were lower than those
with Keggin-type phosphotungstic acid. Although the reason
for the difference in catalytic activity is not clear at the present
time, it seems that the difference is due to the difference in
size and softness of Preyssler-type and Keggin-type phospho-
tungstates.[1, 2] Further investigation is now underway in our
group.
The chemical shift of trimethyl phosphine oxide was plotted
against proton-trimethyl phosphine mole ratio (Figure 5 and
Figure S3). When the amount of acid was increased, the chem-
ical shift moved in a more positive direction. The shift in the
presence of Keggin-type phosphotungstic acid was larger than
that in the presence of sulfuric acid, indicating that the acidic
strength of Keggin-type phosphotungstic acid is greater than
that of sulfuric acid. It was also found that the shift in the
presence of Preyssler-type phosphotungtic acid was larger than
that in the presence of Keggin-type phosphotungstic acid, indi-
cating that the acidic strength of Preyssler-type phosphotung-
stic acid per one proton in aqueous solution is greater than that
of Keggin-type phosphotungstic acid per one proton.
Figure 5. Change in 31P NMR chemical shift of Me3P=O (1 mM) in
D2O in the presence of Preyssler-type phosphotungstic acid (open
circles), Keggin-type phosphotungstic acid (open squares) and sulfuric
acid (open triangles).
Conclusions
Preyssler-type phosphotungstic acid starts to decompose at
373 K and is much less stable than Keggin-type phos-
photungtic acid, which is stable up to 773 K. Therefore,
Preyssler-type phosphotungstic acid is not suitable as a hetero-
Hydrolysis of Alkyl Acetate
The fact that Preyssler-type phosphotungstic acid had geneous catalyst but should be used as a homogeneous cata-
greater acidic strength than that of Keggin-type phoshotungstic lyst.
acid in aqueous solution motivated us to use Preyssler-type
Acidic strength of Preyssler-type phosphotungstic acid esti-
phosphotungstic acid as an acid catalyst for hydrolysis of alkyl mated by using Me3P=O is greater than that of Keggin-type
acetate in water. Okuhara’s group already found that Keggin- phsphotungstic acid on the basis of acid amount. Acid catalyst
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Preyssler-Type Phosphotungstic Acid, H14[P5W30O110Na]
Table 1. Catalytic activities of phosphotungstic acids for hydrolysis of alkyl acetates.
Alkyl acetate
Cat.a)
Conc.
/%
Rate
per weight /mmol·g–1·min–1
per acid amount /mmol (mmol acid)–1·min–1
Ethyl acetate
H3PW12O40
H3PW12O40
H14[NaP5W30O110
H3PW12O40
H14[NaP5W30O110
Solvent: D2O
70.4b,c)
68.7b)
b,c)
1
2
3
4
5
70.4
36.6
52.3
71.2
80.6
68.7b)
57.8b)
134.7
95.3
]
]
98.2b)
134.7
152.5
Methyl acetate
H3PW12O40
H14[NaP5W30O110
Solvent: D2O
175.0
194.4
6
7
77.8
86.4
175.0
121.5
]
]
]
]
]
n-propyl acetate
H3PW12O40
H14[NaP5W30O110
Solvent: D2O, [D6]acetone (2:3)
8
9
58.7
60.1
95.8
98.1
95.8
61.3
isopropyl acetate
10 H3PW12O40
11 H14[NaP5W30O110
Solvent: D2O, [D6]acetone (2:3)
40.7
40.6
66.4
66.2
66.4
41.4
n-butyl acetate
12 H3PW12O40
13 H14[NaP5W30O110
Solvent: D2O, [D6]acetone (2:3)
55.4
51.2
79.5
73.5
79.5
45.9
iso-butyl acetate
14 H3PW12O40
15 H14[NaP5W30O110
Solvent: D2O, [D6]acetone (2:3)
33.3
31.1
47.8
44.6
47.8
27.9
cyclohexyl acetate
16 H3PW12O40
Solvent: D2O, [D6]acetone (2:3)
35.6
35.4
41.9
41.6
41.9
26.0
17 H14[NaP5W30O110
]
tert-butyl acetate
18 H3PW12O40
19 H14[NaP5W30O110
Solvent: D2O, [D6]acetone (2:3)
Trace
Trace
]
a) Amount of catalyst: 0.075 g, 5 wt-% ethyl acetate in D2O (total volume: 3.0 mL, ethyl acetate: 0.15 g), Reaction temperature: 353 K, Reaction
time: 2 hours. b) Reaction temperature: 337K. c) Data in reference [18].
ian System 500 (500 MHz) spectrometer (W resonance frequency of
20.825 MHz) using an external saturated NaWO4 reference. 31P NMR
spectra were recorded with a Varian System 500 (500 MHz) spectrom-
eter (P resonance frequency of 202.327 MHz) using an external H3PO4
activity of Preyssler-type phosphotungstic acid for hydrolysis
of alkyl acetate is lower on the basis of acid amount but higher
on the basis of catalyst weight than that of Keggin-type phos-
photungstic acid.
Keggin-type phosphotungstic acid is used as an acid catalyst
for a wide range of reactions. Development of an acid catalyst
other than Keggin-type phosphotungstic acid is highly desired,
and Preyssler-type phosphotungstic acid is one of the promis-
ing candidates. Our findings are important for the development
of an acid catalyst using Preyssler-type phophotungstic acid.
1
reference. H NMR spectra were recorded with a Varian System 500
(500 MHz) spectrometer (H resonance frequency of 499.827 MHz).
Complete elemental analyses were performed by Microanalytisches
Labor Pascher (Remagen, Germany).
Preparation of H14[P5W30O110Na]·44H2O
K12.5Na1.5[P5W30O110Na]·15H2O (20 g) was dissolved in H2O
(300 mL) and passed through 50 g of Dowex 50W ϫ 8 in the proton
form packed in a glass tube (inner diameter: 15 mm), and the obtained
eluent was evaporated in vacuo at 318 K (Caution: Evaporation at a
higher temperature caused slight decomposition. See also Results and
Discussion.).
Experimental Section
Materials
All of the chemicals used were of reagent grade, and they were used as
supplied. Keggin-type phosphotungstic acid, H3PW12O40·6H2O, was
purified by using a published method.[18] K12.5Na1.5[P5W30O110Na]
was prepared according to a published method[19] and the purity was
confirmed by IR, 31P NMR, and 183W NMR spectroscopy.
Elemental analysis for H14[P5W30O110Na]·44H2O, found: P, 1.72; W,
67.02; Na, 0.26; O, 30.1; H, 1.1; Cl, 0.008; K, Ͻ0.002, S, Ͻ0.005%
total 100.20%; calcd: P, 1.88; W, 66.77; Na, 0.28; O, 29.83; H, 1.24;
Cl, 0%. IR: ν˜max = 1164 (m), 1079 (m), 1020 (w), 980 (sh), 939 (vs),
916 (vs), 785 (vs) cm–1.
Characterizations
FT-IR spectra were recorded with a Nicolet 6700 FT-IR spectrometer 183W NMR (D2O): δ = –207.8 (2 W), –209.7 (2 W), –275.9 (W),
with 2 cm–1 resolution. 183W NMR spectra were recorded with a Var- –288.3 (W). 31P NMR (D2O): δ = –9.41.
Z. Anorg. Allg. Chem. 2011, 2120–2124
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M. Sadakane, Y. Ichi, Y. Ide, T. Sano
ARTICLE
Hydrolysis of Alkyl Acetate
Acknowledgments
Hydrolysis of ethyl acetate was carried out at 333 K with 5 wt-% ethyl
acetate in D2O (total volume: 3.0 mL, ethyl acetate: 0.15 g) for 2 h.[20]
The weight of the catalyst used was 0.075 g.
This study was supported by Grants-in-Aid for Scientific Research
(Scientific Research “C”) of the Ministry of Education, Culture,
Sports, Science, Japan for financial support. M. S. would like to thank
Mitsubishi Rayon Co. for financial support.
Conversion and yield were estimated using 1H NMR spectroscopy.
Signals corresponding to ethyl acetate, ethyl alcohol, and acetic acid
were observed. No other signals were observed. Therefore, we as-
sumed that the selectivity of this hydrolysis was 100%. Since signals
of methylene (CH2O) for ethyl acetate (4.03) and ethyl alcohol (3.52)
were well separated, the conversion of ethyl acetate was calculated
using the integration ratio of these two signals as follows:
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Hydrolysis of other alkyl acetates was carried out by a procedure sim-
ilar to that described above. In the cases of n-propyl acetate, isopropyl
acetate, n-butyl acetate, iso-butyl acetate, cyclohexyl acetate, and tert-
butyl acetate, a mixed solvent (D2O : [D6]acetone = 3:2) was used to
obtain a homogeneous solution.
Supporting Information (see footnote on the first page of this article):
(Figure S1) 31P NMR spectra of Keggin-type phosphtungstic acid (a)
before and after heating at (b) 723 K and (c) 773 K (Figure S2) IR
spectra of Keggin-type phosphotungstic acid (a) before and after heat-
ing at (b) 673 K, (c) 723 K and (d) 773 K. (Figure S3) 31P NMR
spectra of Me3P=O (1 mM) in D2O. (bottom) Me3P=O in the absence
of acid. From bottom to top, amount of Preyssler-type phosphotungtic
acid was increased.
[20] K. Sasaki, M. Sadakane, W. Ninomiya, W. Ueda, Chem. Lett.
2010, 39, 426–427.
Received: September 26, 2011
Published Online: October 27, 2011
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Z. Anorg. Allg. Chem. 2011, 2120–2124