Preparation and characterization of preyssler-type phosphotungstic acid, H15-n[P5W30O110Mn +], with different encapsulated cations (M = Na, Ca, Bi, Eu, Y, or Ce), and their thermal stability and acid catalyst properties

Article abstract of DOI:10.1002/zaac.201400038

Preyssler-type phosphotungstic acid, H_15-n[P₅W ₃₀O₁₁₀Mⁿ⁺], with different encapsulated cations (M = Ca²⁺, Bi³⁺, Eu³⁺, Y³⁺, Ce³⁺) were prepared and characterized using ³¹P NMR and IR spectroscopy, acid-base titration, and elemental analyses. The pK_a value of an internal proton of Ca-encapsulated derivative, [P₅W₃₀O₁₁₀CaH_in] ^12-, was estimated to be 5.9. Their thermal stabilities and acid catalyst properties were compared with a parent Na-encapsulated Preyssler-type phoshotungstic acid, H₁₄[P₅W₃₀O ₁₁₀Na]. Thermal stability of the Preyssler-type molecule was increased by exchanging the encapsulated Na⁺ with Bi³⁺, Eu³⁺, or Ce³⁺, with keeping their high acidic properties. Copyright

Full text of DOI:10.1002/zaac.201400038

ARTICLE
DOI: 10.1002/zaac.201400038
Preparation and Characterization of Preyssler-type Phosphotungstic Acid,
H15–n[P5W30O110Mn+], with Different Encapsulated Cations (M = Na, Ca, Bi,
Eu, Y, or Ce), and their Thermal Stability and Acid Catalyst Properties
Keita Takahashi,[a] Tsuneji Sano,[a] and Masahiro Sadakane*[a,b]
Keywords: Polyanion; Acidity; Tungsten; Heteropolyacid; Acid catalyst
Abstract. Preyssler-type phosphotungstic acid, H15–n[P5W30O110Mn+], estimated to be 5.9. Their thermal stabilities and acid catalyst proper-
with different encapsulated cations (M = Ca2+, Bi3+, Eu3+, Y3+, Ce3+
)
ties were compared with a parent Na-encapsulated Preyssler-type
were prepared and characterized using 31P NMR and IR spectroscopy, phoshotungstic acid, H14[P5W30O110Na]. Thermal stability of the
acid-base titration, and elemental analyses. The pKa value of an in- Preyssler-type molecule was increased by exchanging the encapsulated
ternal proton of Ca-encapsulated derivative, [P5W30O110CaHin]12–, was
Na+ with Bi3+, Eu3+, or Ce3+, with keeping their high acidic properties.
Introduction
Heteropolyacids (HPAs) are acidic metal-oxygen clusters of
early transition metals such as W, Mo, and V with other ele-
ments. They show strong acidity,[1,2] and have been utilized
as catalysts even in industrial processes, such as hydration of
isobutylene to tert-butanol and polymerization of tetrahydrofu-
rane.
Among the various HPAs, a Keggin-type phosphotungstic
acid, H3PW12O40, is the most widely used acid catalyst, which
has a central PO4 tetrahedron and surrounding 12 WO6 octahe-
dra with Td symmetry (Figure 1a).
Preyssler-type phosphotungstate, [P5W30O110Na]14–, which
has approximate D5v symmetry and consists of a cyclic as-
sembly of five [PW6O22]3– units, is another phosphotung-
state.[3,4] A Na+ is encapsulated within a cage of the Preyssler-
type phosphotungstate on the fivefold axis with the internal
Na+ shifted off from the center (Figure 1b and c). This anion
was isolated as a potassium-sodium mixed salt, and the counter
cations are able to be ion-exchanged by protons to give the
acidic form.[4]
The research groups of Heravi, Bamoharram, Gharib, Aliza-
deh, Hekmatshorar, and Feizi have reported acid catalytic
activities of the Preyssler-type phosphotungstic acid,
H14[P5W30O110Na], for many organic reactions such as esteri-
fication, tetrahydropyranyl (THP) protection of hydroxyl
groups, syntheses of dibenzo xanthene derivatives and 2,4,6-
Figure 1. Polyhedral representations of (a) Keggin-type and (b)
Preyssler-type phosphotungstates. Dark polyhedra, while octahedra,
and a white circle indicate PO4, WO6, and encapsulated sodium cation,
respectively. (c) Ball-and stick representation of one-fifth of the
Preyssler-type phosphotungstate ([PW6O22] unit) with a encapsulated
sodium cation. A grey ball with a cross line, grey balls, and white
balls indicate P, W, and O atom, respectively.
triarylpyridines, and alkylation of phenol.[5–12] We also re-
ported that the Preyssler-type phosphotungstic acid is effective
for acylation of methyl pyruvate.[13] We furthermore reported
that the Preyssler-type phosphotungstic acid were more active
than the Keggin-type phosphotungstic acid for hydrolysis of
alkyl acetates with small alkyl groups on the basis of catalyst
weight.[14] However, the Preyssler-type phosphotungstic acid
was thermally less stable than the Keggin-type phosphotung-
stic acid. The Keggin-type phosphotungstic acid is stable after
heating at 673 K, whereas the Preyssler-type phosphotungstic
acid started to decompose by heating at 373 K.[14] It is desired
to improve thermal stability of the Preyssler-type phos-
photungstic acid.
* Prof. Dr. M. Sadakane
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
[b] PRESTO Japan Science and Technology Agency (JST)
4-1-8 Honcho
Kawaguchi, Saitama, 332-0012, Japan
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n+
Preyssler-type Phosphotungstic Acid, H15–n[P5W30O
M ]
110
It has been known that the encapsulated Na+ is exchangeable and K-Bi dissolved in D2O and chemical shifts are summa-
with other cations such as Ca2+, Y3+, lanthanides, Bi3+, and rized in Table 3. In the case of K-Na, K-Y, K-Eu, K-Ce, and
actinides, and stability was expected to be increased by the K-Bi, only one singlet was observed and their chemical shifts
exchange.[4,15–17] Herein, we report synthesis and characteriza- were close to the reported values.[15] The IR spectrum of
tion of Preyssler-type phosphotungstic acid with different en- K-Na had three bands at 1167, 1084, and 1020 cm–1, which
capsulated cations of Ca2+, Y3+, Ce3+, Eu3+, and Bi3+, and their were in the P–O stretching region and was consistent with a
thermal stabilities, acidic strength, and catalytic activities for local Cs symmetry for PO4 caused by the internal Na+, and
hydrolysis activities of ethyl acetate and cellobiose.
four bands at 985, 941, 914, 789, and 744 cm–1, which were
in the W–O vibration region (Figure 3). The band at 1084 cm–1
for K-Na shifted to shorter wavenumber by exchanging the
encapsulated Na+, whereas wavenumber shifts of other bands
were small.
Results and Discussion
Preparation and Characterization of Preyssler-type
Phosphotungstic Acid with Different Encapsulated Cations
Potassium salts of Preyssler-type phosphotungstates with
different encapsulated cations, K(15–n)[P5W30O110Mn+] [Ca
(n = 2), Y (n = 3), Eu (n = 3), Ce (n = 3), and Bi (n = 3),
designated as K-Ca, K-Y, K-Eu, K-Ce, and K-Bi, respec-
tively], were prepared by exchanging encapsulated Na+ of
K14[P5W30O110Na] (designated as K-Na) according to re-
ported literatures[15,16] with slight modifications. Figure 2 (top)
shows 31P NMR spectra of K-Na, K-Ca, K-Y, K-Eu, K-Ce,
Figure 3. (top) IR spectra of (a) K-Na, (b) K-Ca, (c) K-Y, (d) K-Eu,
(e) K-Ce, and (f) K-Bi. Dashed lines indicate bands at 1167, 1084,
1022, 985, 941, and 914 cm–1 for K-Na. (bottom) IR spectra of (a) H-
Na, (b) H-Ca, (c) H-Y, (d) H-Eu, (e) H-Ce, and (f) H-Bi. Dashed
lines indicate bands at 1167 and 1084 cm–1 for H-Na. The white and
black arrows indicate new peaks at 1045 and 1126 cm–1.
In the case of the 31P NMR spectrum of K-Ca (Figure 2
top, b), two singlet signals were observed at –9.0 and
–11.1 ppm, which were close to the reported 31P NMR spectro-
scopic values of Ca-encapsulated Preyssler-type phosphotung-
state in an aqueous solution with different pH values.[16]
PopeЈs group has reported protonation of inner oxygen in the
Preyssler-type phosphotungstates with encapsulated cations of
Ca2+, Y3+, Ce3+, Eu3+, or Bi3+ under acidic conditions
[Equation (1)].[16] The exchange rate of this internal proton
Figure 2. 31P NMR spectra of (top) (a) K-Na, (b) K-Ca, (c) K-Y, (d)
K-Eu, (e) K-Ce, and (f) K-Bi, and (bottom) (a) H-Na, (b) H-Ca, (c)
H-Y, (d) H-Eu, (e) H-Ce, and (f) H-Bi. Samples (ca. 50 mg) were
dissolved in D2O (1 mL).
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K. Takahashi, T. Sano, M. Sadakane
ARTICLE
(Hin) was slower than NMR time scale, and two 31P NMR group has reported that the inner proton of Eu-encapsulated
singlet signals were observed where one singlet was assignable Preyssler-type phosphotungstic acid, [P5W30O110Eu3+H]11–
to the phosphorus of protonated
compound, was highly acidic.[16]
,
[P5W30O110CaHin]12–, and the other was assigned to the phos-
phorus of non-protonated compound, [P5W30O110Ca]13–
.
n+ (15–n)–
[P5W30O
M
]
+ H+ i [P5W30O110Mn+Hin](14–n)–
(1)
110
IR spectrum of K-Ca was similar to one of K-Na, and there
are additional bands near the P–O stretching band at
1160 cm–1, which were also observed for potassium salt of
Ca-encapsulated Preyssler-type phosphotungstate prepared by
PopeЈs group.[4] Details about these bands are described below.
These data and elemental analyses indicated that K-Y, K-Ca,
K-Eu, K-Ce, and K-Bi were prepared in pure forms.
The potassium cations of K-Na, K-Ca, K-Y, K-Eu, K-Ce,
and K-Bi were exchanged using a Dowex-50W proton ex-
change resin to produce Preyssler-type phosphotungstic acids
with different encapsulated cations, and they were designated
as H-Na, H-Ca, H-Y, H-Eu, H-Ce, and H-Bi, respectively.
Elemental analyses of the proton exchanged samples indicated
that almost all potassium cations were removed without col-
lapse of cation-encapsulated Preyssler-type phosphotungstates
(Table 1).
Figure 4. Acid-base titration of (black circles) H-Na, (white squares)
H-Ca, (white circles) H-Eu, and (black squares) H-Bi. NaOH solution
(1.0 m) was added into the solution of H-Na, H-Ca, H-Eu, or H-Bi,
(H+ concentration: ca. 0.1 m), and pH values were plotted against
amount of added NaOH.
Table 1. Catalytic activities for hydrolysis of ethyl acetate.
Acid-base titration of H-Ca was different from H-Na,
H-Eu, and H-Bi, and there was a weak proton with a pKa
value of ca. 6.
a)
Catalyst
Conv. /% Rate
Per weight /
mmol·g–1·min–1 mmol·(acid-mol)–1·min–1
Per acid amount /
The 31P NMR spectrum of Ca-encapsulated Preyssler-type
phosphotungstates with different pH values (Figure 5 top) was
measured. In a solution with a pH value of 0.8, one singlet at
ca. –9.0 ppm was observed, and one singlet at ca. –11.1 ppm
was observed in a solution with a pH value of 10.1. In a solu-
tion with pH value of 5.9, two singlets were observed with an
integration ratio of ca. 50:50. Integration ratios of these two
peaks were plotted against the pH values (Figure 5 bottom).
The integration ratio change was well simulated by an equation
assuming that one proton was included with a pKa value of
5.9. This result confirmed that two singlets at –9.0 and
–11.1 ppm were assigned to be phosphorus of Ca-encapsulated
H14P5W30O110Na 46.2
275.7
257.0
255.2
258.5
247.2
254.9
175.2
164.0
171.3
176.2
178.5
170.7
182.1
174.5
H
H
H
H
H
13P5W30O110Ca 48.2
12P5W30O110 49.6
Y
12P5W30O110Eu 50.2
12P5W30O110Ce 48.0
12P5W30O110Bi 51.2
H3PW12O40
Blank
49.1
1.4
a) Amount of protons: 0.042 mmol, 5 wt% ethyl acetate in D2O (total
volume: 3.0 mL, ethyl acetate: 0.15 g), Reaction temperature: 353 K,
Reaction time: 2 h.
Figure 2 (bottom) shows 31P NMR spectra of H-Na, H-Ca, Preyssler-type phosphotungstate with one internal proton,
H-Y, H-Eu, H-Ce, and H-Bi. After the exchange of potassium [P5W30O110CaHin]12–
and without the internal proton
cations with protons, the 31P NMR signals did not shift in the [P5W30O110Ca]13–
case of H-Na. In the case of H-Ca, one singlet at –9.0 ppm
PopeЈs group has investigated the 31P NMR spectroscopic
which corresponded to the Ca-encapsulated Preyssler-type behavior of Eu3+-encapsulated Preyssler-type phosphotung-
phosphotungstates with internal proton, [P5W30O110CaHin]12–
state in aqueous solutions with different pH, however, the pKa
,
.
,
was observed. However, two peaks appeared in the case of value of the internal proton in Eu-encapsulated Preyssler-type
H-Y, H-Eu, H-Ce, and H-Bi, where the chemical shift of the phoshotungstate was not detected because the pKa value was
one peak was similar to the one of K-Y, K-Eu, K-Ce, and close to pKa values of protons on the outside of the Preyssler
K-Bi in D2O,[15,16] and the chemical shift of a new peak was phosphotungstate.[16] Our presented result is the first pKa mea-
similar to the reported chemical shift of K-Y, K-Eu, K-Ce, surement of the inner proton of [P5W30O110Ca2+H]12–
.
and K-Bi in aqueous solution with a pH value of 1.[16] These
new peaks were assigned to the protonated Preyssler-type com- lets, whereas 31P NMR of K-Y, K-Eu, K-Ce, and K-Bi in D2O
pounds, [P5W30O110M3+Hin]11–, (M = Y, Eu, Ce, and Bi).
The 31P NMR spectrum of K-Ca in D2O showed two sing-
showed one singlet. 31P NMR of H-Ca in D2O showed one
Figure 4 shows acid-base titration curves of H-Na, H-Ca, singlet signal, whereas the 31P NMR spectrum of H-Y, H-Eu,
H-Eu, and H-Bi. In the case of H-Na, H-Eu, and H-Ce, and H-Bi in D2O showed two singlet signals. From
H-Bi, pH value suddenly increased when all H+ were con- these results and acid-base titration, we propose that the pKa
sumed, indicating that all protons were highly acidic. PopeЈs values of internal proton of Y, Eu, Ce, and Bi-encapsulated
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Z. Anorg. Allg. Chem. 2014, 1314–1321

n+
Preyssler-type Phosphotungstic Acid, H15–n[P5W30O
M ]
110
spectrum of cesium salt of the Ca-encapsulated Preyssler-type
phosphotungstate isolated from aqueous solution with pH
value of 2.0 (Figure 8 b) was a mixture of IR spectra of H-
Ca and the cesium salt of the Ca-encapsulated Preyssler-type
phosphotungstates isolated from aqueous solution with pH
value of 9.8. These results confirmed that the presence of in-
ternal proton affected IR bands assignable to P–O vibrations,
and strong band at 1176 cm–1 was assignable to a P–O vi-
bration of the non-protonated Ca-encapsulated Preyssler-type
phosphotungstate, [P5W30O110Ca]13–, and a strong band at
1155 cm–1 and a weak band at 1134 cm–1 were assignable to
P–O vibrations of the one-proton protonated Ca-encapsulated
Preyssler-type phosphotungstate, [P5W30O110CaHin]12–. There
were three band at 1176, 1155, and 1134 cm–1 in the IR spec-
trum of K-Ca. Therefore, K-Ca was a mixture of Ca-encapsu-
lated Preyssler-type phosphotungstates with one internal pro-
ton, [P5W30O110CaHin]12–, and without the internal proton,
[P5W30O110Ca]13–
.
Figure 5. (top) 31P NMR spectra of H-Ca in aqueous solutions with
different pH values. (a) 0.8, (b) 5.9, and (c) 10.8. (bottom) 31P NMR
integration ratio vs. pH value. Solid lines indicate theoretical curves
with pKa value of 5.9. NaOH solution (1.0 m) was added into the solu-
tion of H-Na (H+ concentration: ca. 0.1 m) and 31P NMR spectra of
solution with different pH values was measured.
Figure 6. IR spectra of Ca-encapsulated Preyssler-type phosphotung-
state isolated from different pH values of (b) 2.0 and (c) 9.8, and H-
Ca (a). H-Ca was dissolved in water (concentration of H+ was ca.
0.1 m), and pH was adjusted by adding NaOH (1.0 m), and cesium
chloride was added to isolate cesium salts of Ca-encapsulated
Preyssler-type phosphotungstate.
Preyssler-type phoshotungstates were close to those of acidic
protons.
The IR spectrum of H-Na was similar to one of K-Na, and
there was an additional weak band at 1045 cm–1 (Figure 3 bot-
tom, white arrow). IR spectra of H-Y, H-Eu, H-Ce, and H-Bi
were similar to one of K-Y, K-Eu, K-Ce, and K-Bi, but a band
at ca. 1167 cm–1 shifted to shorter wavenumber, and there are
two additional weak bands at ca. 1126 (Figure 3 bottom, black
arrow) and ca. 1040 cm–1. When the IR spectra of H-Y, H-Eu,
These results indicated that H-Na, H-Ca, H-Y, H-Eu,
H-Ce, and H-Bi were isolated in pure forms.
Thermal Stability
The obtained cation-encapsulated Preyssler-type phos-
H-Ce, and H-Bi were compared with one of H-Na, two bands photungstic acids, H-Na, H-Ca, H-Y, H-Eu, H-Ce, and H-Bi,
of H-Na at 1167 and 1084 cm–1 shifted to shorter wavenumber and their potassium salts, K-Na, K-Ca, K-Y, K-Eu, K-Ce, and
by exchanging the encapsulated cations.
K-Bi, were heated in a muffle oven and analyzed by solution
Ca-encapsulated Preyssler-type phosphotungstates were iso- 31P NMR spectroscopic measurements to estimate thermal sta-
lated as a cesium salts from solutions of H-Ca with different bility. In the 31P NMR spectrum of H-Na heated at 373 K for
pH values, and the IR spectra of these solids are presented in 1 h, several small peaks together with the peak of H-Na were
Figure 6. The IR spectrum of H-Ca (Figure 6 a and Figure 3 b) observed, suggesting H-Na started to decompose at 373 K.[14]
was similar to those of H-Y, H-Eu, H-Ce, and H-Bi. The IR We evaluated thermal stability of cation-encapsulated
spectrum of cesium salt of the Ca-encapsulated Preyssler-type Preyssler-type phosphotungstic acids and their potassium salts
phosphotungstates isolated from aqueous solution with pH by comparing residual ratio of the Preyssler-type phosphotung-
value of 9.8 (Figure 6 c) was similar to one of K-Na. The IR states estimated by 31P NMR spectroscopic measurements after
Z. Anorg. Allg. Chem. 2014, 1314–1321
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K. Takahashi, T. Sano, M. Sadakane
ARTICLE
calcination at different temperatures (Figure 7). H-Na, H-Ca, 31P NMR time-scale that only one 31P NMR spectroscopic sig-
and H-Y started to decompose in the range of 373–423 K, nal is detected. The peak moves to a more positive chemical
whereas H-Eu, H-Ce, and H-Bi were stable up to 423 K. Their shift if the acidic strength and amount of proton increase.
potassium salts were more stable than their acid. K-Na and Therefore, the acidic strength can be estimated by the peak
K-Ca started to decompose at 523 K, but K-Y, K-Eu, K-Ce, shift:
and K-Bi were stable up to 623 K.
Me3P=O + H+ i Me3P–OH+
(2)
The chemical shift of trimethyl phosphine oxide was plotted
against proton-trimethyl phosphine mole ratio (Figure 8).
When the amount of acid was increased, the chemical shift
moved in a more positive direction. The shift in the presence
of H-Na was larger than those of Keggin-type phosphotungtic
acid and sulfuric acid. The shifts in the presence of H-Ca,
H-Y, H-Eu, and H-Bi were almost the same except H-Ce,
indicating that the acidic strengths of H-Ca, H-Y, H-Eu, and
H-Bi were similar to one of H-Na.
Figure 8. Change in 31P NMR chemical shift of Me3P=O (1 mM) in
D2O in the presence of the Preyssler-type phosphotungtic acid (open
circles) H-Na, (open squares) H-Ca, (open stars) H-Y, (open triangles)
H-Eu, (crosses) H-Ce, and (open diamonds) H-Bi, (close squares)
Keggin-type phosphotungstic acid and (close circles) sulfuric acid.
Figure 7. Comparison of the thermal stability of (a) Preyssler-type
phsphotungstic acids (H-Na, H-Ca, H-Y, H-Eu, H-Ce, and H-Bi) after
heating at (white bars) 373 K, (grey bars) 423 K, and (black bars)
473 K, and (b) their potassium salts, (K-Na, K-Ca, K-Y, K-Eu, K-
Ce, and K-Bi) after heating at (dotted bars) 523 K, (bars with vertical
stripes) 573 K, and (bars with horizontal stripes) 623 K. After heating
the samples, the 31P NMR spectrum of the sample dissolved in D2O
was measured. Residual ratio was calculated by an equation [residual
ratio = (integration of 31P NMR signal assignable to the Preyssler-type
Phosphotungstate) / (sum of integrations of all 31P NMR peaks)].
Hydrolysis of Ethyl Acetate and Cellobiose
Hydrolysis of ethyl acetate and cellobiose were carried out
in the presence of same amount of protons, and their activities
are summarized in Table 1 and Table 2.[22] For hydrolysis of
ethyl acetate, catalytic activities of H-Ca, H-Y, H-Eu, H-Ce,
Table 2. Catalytic activities for hydrolysis of cellobiose.
a)
Catalyst
Cellobiose conv. /% Glucose yield /% Sel. /%
Acidic Strength Measurement
H14P5W30O110Na 39.5
H13P5W30O110Ca 38.1
39.0
35.5
36.6
39.3
36.1
39.4
38.1
22.1
98.7
93.2
94.1
95.9
96.5
94.2
97.6
97.0
Acidic strength of the ion-encapsulated Preyssler-type phos-
photungstic acid was compared with those of Keggin-type
phosphotungstic acid and sulfuric acid using trimethyl phos-
phine oxide (Me3P=O) as a probe molecule. Trialkyl phos-
phine oxide is known as an acidic strength indicator both in a
solid state and in solution.[18–21] Trialkyl phosphine oxide is
protonated to form trialkyl phosphine hydroxide cation [Equa-
H12P5W30O110
Y
38.9
H12P5W30O110Eu 40.9
H12P5W30O110Ce 37.4
H12P5W30O110Bi 41.9
H3PW12O40
H2SO4
39.1
22.8
a) Amount of proton: 0.10 mmol, Cellobiose: 0.20 g, H2O: 12 mL, Re-
tion (2)]. This reversible reaction is so much faster than the action temperature: 373 K, Reaction time: 24 h.
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n+
Preyssler-type Phosphotungstic Acid, H15–n[P5W30O
M ]
110
solution were added 1.2–3.0 equivalents of Mn+ as the chloride or ni-
trate salt dissolved in H2O (30 mL), 0.1 m HCl, or 0.1 m HNO3. The
mixture was placed in autoclave and heated to 433–453 K for 2 d.
After the solution was cooled down to room temperature, the solution
was filtrated. The potassium salts were isolated by the addition of solid
KCl (10 g) to the filtrate. The produced precipitates were dissolved
with hot water (20 mL) heated by a metal bath at 353 K, and the heater
was switched off to produce the desired products (7.0–9.0 g). The po-
tassium salts of Preyssler-type phosphotungstate with different encap-
sulated metals, Na (n = 1), Ca (n = 2), Y (n = 3), Eu (n = 3), Ce (n =
and H-Bi were similar to one of H-Na, and were much higher
than that of sulfuric acid. Catalytic activities of H-Na, H-Ca,
H-Y, H-Eu, H-Ce, and H-Bi, were similar to that of the Keg-
gin-type phosphotungtic acid[23] on the basis of acid amount,
but became higher on the basis of catalyst amount because
acid concentration of the Preyssler-type phosphotungstic acid
(14–12 per [P5W30O110M]) is higher than one of the Keggin-
type phosphotungstic acid (3 per [PW12O40]).
Catalytic activities of H-Na, H-Ca, H-Y, H-Eu, H-Ce, and
H-Bi, for the hydrolysis of cellobiose were much higher than 3), and Bi (n = 3), were designated as K-Na, K-Ca, K-Y, K-Eu,
K-Ce, and K-Bi, respectively.
one of sulfuric acid, and were similar to one of the Keggin-
type phosphotungstic acid on the basis of the acid amount.
ShimizuЈs group reported that the Keggin-type phosphotungtic
acid was active catalyst for the hydrolysis of cellobiose and
their derivatives.[24] Acid concentration of the Preyssler-type
phosphotungtic acid is higher than that of the Keggin-type
phosphotungtic acid. Therefore, less amount of the Preyssler-
type phosphotungtic acid catalyst is enough to achieve the
same activity.
Metal salts, amount of metal salts, reaction conditions, elemental
analysis results, IR and 31P NMR spectroscopic data are summarized
in Table 3.
n+
Preparation of H15–n[P5W30O
M
]·nH2O. K15–n[P5W30O110Mn+]·
110
nH2O (10 g) was dissolved in H2O (200 mL) and passed through 50 g
of Dowex 50Wϫ8 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.).
Conclusions
The Preyssler-type phosphotungstic acid with different encapsulated
metals, H(15–n)[P5W30O110Mn+], Na (n = 1), Ca (n = 2), Y (n = 3), Eu
(n = 3), Ce (n = 3), and Bi (n = 3), were designated as H-Na, H-Ca,
H-Y, H-Eu, H-Ce, and H-Bi, respectively.
Preyssler-type phosphotungstic acid, H15–n[P5W30O110Mn+],
with different encapsulated cations (M = Ca2+, Bi3+, Eu3+, Y3+,
Ce3+) were prepared and fully characterized using 31P NMR
and IR spectroscopy, acid-base titration, and elemental analy-
ses. The Ca-encapsulated derivative with a internal proton
(Hin), [P5W30O110CaHin]12–, and without internal proton,
[P5W30O110Ca]13–, were characterized using IR and 31P NMR
spectroscopy, and pKa value of the internal proton was esti-
mated to be 5.9 by acid-base titrations. Thermal stability of the
Elemental analysis results, IR and 31P NMR spectroscopic data are
summarized in Table 3.
Hydrolysis of Ethyl Acetate: Hydrolysis of ethyl acetate was carried
out at 353 K with 5 wt% ethyl acetate in D2O (total volume: 3.0 mL,
ethyl acetate: 0.15 g) for 2 h.[22] The amount of protons used was kept
Preyssler-type structure was increased by exchanging Na+ with to be 0.042 mmol.
Bi3+, Eu3+, or Ce3+, without loosing high acid catalyst activi-
Conversion and yield were estimated using 1H NMR spectroscopy.
ties.
Signals corresponding to ethyl acetate, ethyl alcohol, and acetic acid
were observed. No other signal was observed. Therefore, we assumed
that selectivity of this hydrolysis was 100%. Since signals of methyl-
ene (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:
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.[23]
K14[P5W30O110Na]·23H2O and H14[P5W30O110Na]·44H2O were pre-
pared according to a published method[14] and the purity was con-
firmed by IR, 31P NMR, and 183W NMR spectroscopy.
Conversion = (integration of CH2O of ethyl alcohol) / (integration of
CH2O of ethyl acetate + integration of CH2O of ethyl alcohol).
1H NMR of ethyl acetate in D2O (HOD peak at δ = 4.75 ppm):
1.13 ppm (CH3CH2, triplet, 3 H), 1.97 ppm (CH3CO, singlet, 3 H),
4.03 ppm (CH2O, quartet, 2 H).
Characterizations: FT-IR spectra were recorded with a Nicolet 6700
FT-IR spectrometer with 2 cm–1 resolution. 183W NMR spectra were
recorded with a Varian System 500 (500 MHz) spectrometer (W reso-
nance frequency of 20.825 MHz) using an external sat. Na2WO4 refer-
ence. 31P NMR spectra were recorded with a Varian System 500
(500 MHz) spectrometer (P resonance frequency of 202.327 MHz)
1H-NMR of ethyl alcohol in D2O (HOD peak at δ = 4.75 ppm):
1.06 ppm (CH3CH2, triplet, 3 H), 3.52 ppm (CH2O, quartet, 2 H).
using an external H3PO4 reference. 1H NMR spectra were recorded 1H NMR of acetic acid in D2O (HOD peak at δ = 4.75 ppm): 1.97 ppm
with a Varian System 500 (500 MHz) spectrometer (H resonance fre-
quency of 499.827 MHz). Elemental analyses of K-Na, K-Ca, K-Y,
K-Eu, K-Ce, K-Bi, and H-Na, were performed by Microanalytisches
Labor Pascher (Remagen, Germany). Elemental analyse of H-Ca,
H-Y, H-Eu, H-Ce, and H-Bi were performed with a ICP-AES (SPS-
7700, SII).
(CH3CO, singlet, 3 H).
Hydrolysis of Cellobiose: Hydrolysis of cellobiose was carried out at
373 K with cellobiose (0.20 g) in H2O (12 mL) for 24 h.[22] The
amount of protons used was kept to be 0.1 mmol.
Conversion of cellobiose and yield of glucose was estimated using
HPLC with a column (Shodex Sugar SH1011) and RI detector (RID-
10A, Shimadzu).
n+
Preparation of K15–n[P5W30O
M ]·nH2O. K14[P5W30O110Na]·
110
23H2O (10 g, ca. 1.21 mmol) was dissolved in H2O (120 mL) . To this
Z. Anorg. Allg. Chem. 2014, 1314–1321
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1319

K. Takahashi, T. Sano, M. Sadakane
Table 3. Reaction condition for the preparation of K15–n[P5W30O110Mn+] and characterization of H15–n[P5W30O110Mn+].
ARTICLE
Encapseled cation
Na+ [14]
Metal salt (mmol)
Solvent
Temp. /°C
Time /h
K14[P5W30O110Na]·23H2O (K-Na)
Elemental analysis: found: P, 1.83;W, 65.5; Na, 0.28; H, 0.54; Cl, Ͻ0.05;K, 6.31%; calcd: P, 1.84;W, 65.5; Na, 0.27; H, 0.59; Cl, 0;K, 6.48 0%.
IR: ν˜ = Ͼmax = 1167 (s), 1084 (s), 1020 (m), 985 (m), 941 (vs), 914 (vs), 789 (vs), 744 (sh) cm–1
31P NMR (D2O): δ = –9.39 ppm
H14[P5W30O110Na]·44H2O (H-Na)
Elemental analysis, found: P, 1.72;W, 67.02; Na, 0.26; H, 1.1; Cl, 0.008;K, Ͻ0.002%; calcd: P, 1.88;W, 66.77; Na, 0.28; H, 1.24; Cl, 0;K, 0%.
IR: ν˜ = Ͼmax = 1167 (s), 1084 (s), 1045 (m), 1024 (m), 987 (m), 936 (vs), 918 (vs), 787 (vs), 746 (sh) cm–1
31P NMR (D2O): δ = –9.39 ppm
Ca2+
CaCl2 (1.44 mmol)
H2O
160
48
K12.2[P5W30O110CaH0.8]·25H2O (K-Ca)
Elemental analysis, found: P, 1.84;W, 65.4; Ca, 0.46; H, 0.6; Cl, Ͻ0.05;K, 5.94; Na, Ͻ0.01%; calcd: P, 1.84;W, 65.7; Ca, 0.48; H, 0.61; Cl, 0;K,
5.68; Na, 0%.
IR: ν˜ = Ͼmax = 1176 (m), 1157 (s), 1134 (w), 1078 (s), 1022 (m), 985 (m), 943 (vs), 914 (vs), 784 (vs), 739 (sh) cm–1
31P NMR (D2O): δ = –9.02, –11.1 ppm
H13[P5W30O110Ca]·85H2O (H-Ca)
Elemental analysis, found: P, 1.81;W, 61.1; Ca, 0.50;K, 0.04%; calcd: P, 1.72;W, 61.2; Ca, 0.44;K, 0.00%.
IR: ν˜ = Ͼmax = 1154 (s), 1134 (w), 1074 (s), 1040 (w) 1024 (m), 985 (m), 943 (vs), 916 (vs), 787 (vs), 748 (sh) cm–1
31P NMR (D2O): δ = –9.05 ppm
Y3+
Y(NO3)3·6H2O (1.44 mmol)
H2O
160
48
K12[P5W30O110Y]·25H2O (K-Y)
Elemental analysis, found: P, 1.84;W, 65.4; Y, 1.04; H, 0.55; Cl, Ͻ0.05;K, 5.74; Na, Ͻ0.01; N, Ͻ0.03%; calcd: P, 1.84;W, 65.4; Y, 1.05; H, 0.59; Cl,
0;K, 5.78; Na, 0%.
IR: ν˜ = Ͼmax = 1160 (s), 1067 (s), 1020 (m), 984 (m, sh), 939 (vs), 912 (vs), 784 (vs), 742 (sh) cm–1
31P NMR (D2O): δ = –10.1 ppm
H12[P5W30O110Y]·90H2O (H-Y)
Elemental analysis, found: P, 1.80;W, 60.0; Y, 1.11;K, 0.04%; calcd: P, 1.69;W, 60.3; Y, 0.97;K, 0.00%.
IR: ν˜ = Ͼmax = 1143 (s), 1126 (w), 1067 (s), 1038 (m), 1025 (m), 944 (vs), 920 (vs), 780 (vs), 744 (sh) cm–1
31P NMR (D2O): δ = –8.46, –10.1 ppm
Eu3+
Eu(NO3)3 (1.44 mmol)
H2O
160
48
K12[P5W30O110Eu]·25H2O (K-Eu)
Elemental analysis, found: P, 1.82;W, 64.8; Eu, 1.81; H, 0.53; Cl, Ͻ0.05;K, 5.8; Na, Ͻ0.01; N, Ͻ0.03%; calcd: P, 1.82;W, 64.9; Eu, 1.79; H, 0.59;
Cl, 0;K, 5.74; Na, 0%.
IR: ν˜ = Ͼmax = 1162 (s), 1067 (s), 1020 (m), 981 (m), 939 (vs), 912 (vs), 784 (vs), 739 (sh) cm–1
31P NMR (D2O): δ = 0.60 ppm
H12[P5W30O110Eu]·65H2O (H-Eu)
Elemental analysis, found: P, 1.80;W, 62.8; Eu, 1.77;K, 0.01%; calcd: P, 1.77;W, 62.9; Eu, 1.73;K, 0.00%.
IR: ν˜ = Ͼmax = 1143 (s), 1125 (m), 1068 (s), 1041 (m), 1028 (m), 944 (vs), 920 (vs), 784 (vs), 748 (sh) cm–1
31P NMR (D2O): δ = 0.32, –9.51 ppm
Ce3+
(NH4)2[Ce(NO3)6]2 (3.6 mmol)
0.1 M HNO3
180
48
K12[P5W30O110Ce]·25H2O (K-Ce)
Elemental analysis, found: P, 1.81;W, 64.7; Ce, 1.63; H, 0.6; Cl, Ͻ0.05;K, 5.78; Na, Ͻ0.01; N, Ͻ0.03%; calcd: P, 1.83;W, 65.0; Ce, 1.65; H, 0.59;
Cl, 0;K, 5.74; Na, 0%.
IR: ν˜ = Ͼmax = 1165 (s), 1069 (s), 1020 (m), 985 (m), 940 (vs), 916 (vs), 779 (vs), 744 (sh) cm–1
31P NMR (D2O): δ = –15.9 ppm
H12[P5W30O110Ce]·120H2O (H-Ce)
Elemental analysis, found: P, 1.76;W, 56.8; Ce, 1.42;K, 0.00%; calcd: P, 1.59;W, 56.6; Ce, 1.44;K, 0.00%.
IR: ν˜ = Ͼmax = 1147 (s), 1126 (m), 1070 (s), 1040 (m), 1026 (m), 984 (sh), 945 (vs), 921 (vs), 778 (vs, br) cm–1
31P NMR (D2O): δ = –10.1, –15.9 ppm
Bi3+
BiCl3 (2.4 mmol)
0.1 M HCl
160
48
K12[P5W30O110Bi]·25H2O (K-Bi)
Elemental analysis, found: P, 1.8;W, 64.6; Bi, 2.41; H, 0.54; Cl, Ͻ0.05;K, 5.73; Na, Ͻ0.01%; calcd: P, 1.81;W, 64.5; Bi, 1.65; H, 0.58; Cl, 0;K, 5.70;
Na, 0%.
IR: ν˜ = Ͼmax = 1159 (s), 1039 (s), 1024 (sh), 985 (sh), 937 (vs), 924 (vs), 782 (vs), 752 (sh) cm–1
31P NMR (D2O): δ = –8.2 ppm
H12[P5W30O110Bi]·60H2O (H-Bi)
Elemental analysis, found: P, 1.86;W, 63.4; Bi, 2.68;K, 0.02%; calcd: P, 1.78;W, 63.2; Bi, 2.39;K, 0.00%.
IR: ν˜ = Ͼmax = 1161 (s), 1140 (sh) 1041 (s), 939 (vs, br), 783 (vs), 746 (sh) cm–1
31P NMR (D2O): δ = –7.44, –8.25 ppm
1320
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 1314–1321

n+
Preyssler-type Phosphotungstic Acid, H15–n[P5W30O
M ]
110
[12] N. Feizi, H. Hassani, M. Hakimi, Bull. Kor. Chem. Soc. 2005, 26,
2087–2088.
Acknowledgements
[13] W. Ninomiya, M. Sadakane, Y. Ichi, T. Yasukawa, K. Ooyachi,
T. Sano, W. Ueda, Catal. Today 2011, 164, 107–111.
[14] M. Sadakane, Y. Ichi, Y. Ide, T. Sano, Z. Anorg. Allg. Chem. 2011,
2125–2128.
This research was supported by the Japan Science and Technology
Agency, PRESTO program.
[15] I. Creaser, M. C. Heckel, R. J. Neitz, M. T. Pope, Inorg. Chem.
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Received: January 22, 2014
Published Online: March 14, 2014
Z. Anorg. Allg. Chem. 2014, 1314–1321
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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