New extraction procedure for protonated polyoxometalates prepared in aqueous-organic solution and characterisation of their catalytic ability

Article abstract of DOI:10.1016/j.apcata.2014.07.032

Although POMs prepared in aqueous-organic solutions have been reported to exhibit interesting chemical properties and may be efficient catalysts, they could not be isolated in their protonated form using the normal extraction method. In this study, a new procedure of isolating protonated POMs, [S ₂V_xM_18-xO₆₂]^(4+x)- and [AsV_xM_12-xO₄₀]^(3+x)- (M = Mo, W; x = 0-2), which were prepared in an aqueous-organic mixed solution, was developed by modifying and optimising the extraction conditions. Isolated protonated POMs were characterised using IR and Raman spectroscopy, and their acidity was measured using a Hammett indicator. The acidity of H₄S ₂W₁₈O₆₂, H₅S₂VW ₁₇O₆₂, H₄S₂Mo₁₈O ₆₂, and H₃As₂W₁₈O₆₂ was stronger than that of H₃PW₁₂O₄₀. Their catalytic ability was investigated in multiple types of organic reactions, including pinacol rearrangement, acetal formation with benzaldehyde and ethylene glycol, and the Friedel-Crafts acylation of anisole with ethyl pyruvate. They exhibited greater catalytic ability than commercially available POMs such as H₃PM₁₂O₄₀ and H₄SiM ₁₂O₄₀ (M = Mo, W).

Full text of DOI:10.1016/j.apcata.2014.07.032

Applied Catalysis A: General 485 (2014) 181–187
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
New extraction procedure for protonated polyoxometalates prepared
in aqueous-organic solution and characterisation of their catalytic
ability
Tadaharu Uedaa,∗, Keisuke Yamashitaa, Ayumu Ondab
a Department of Applied Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan
b Hydrothermal Research Institute, Faculty of Science, Kochi University, Kochi 780-8520, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 April 2014
Received in revised form 28 June 2014
Accepted 28 July 2014
Available online 6 August 2014
Although POMs prepared in aqueous-organic solutions have been reported to exhibit interesting chemical
properties and may be efficient catalysts, they could not be isolated in their protonated form using the nor-
(4+x)−
]
mal extraction method. In this study, a new procedure of isolating protonated POMs, [S2VxM18−xO62
(3+x)−
(M = Mo, W; x = 0–2), which were prepared in an aqueous-organic mixed solu-
and [AsVxM12−xO40
]
tion, was developed by modifying and optimising the extraction conditions. Isolated protonated POMs
were characterised using IR and Raman spectroscopy, and their acidity was measured using a Hammett
indicator. The acidity of H4S2W18O62, H5S2VW17O62, H4S2Mo18O62, and H3As2W18O62 was stronger than
that of H3PW12O40. Their catalytic ability was investigated in multiple types of organic reactions, including
pinacol rearrangement, acetal formation with benzaldehyde and ethylene glycol, and the Friedel-Crafts
acylation of anisole with ethyl pyruvate. They exhibited greater catalytic ability than commercially
available POMs such as H3PM12O40 and H4SiM12O40 (M = Mo, W).
Keywords:
Polyoxometalate
Extraction
Acidity
Strong acid
Catalysis
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
commercially available, highly stable, and strongly acidic. In gen-
eral, protonated POMs are prepared in an aqueous solution and are
Recently, extensive attention has been directed toward the cat-
alytic applications of POMs, as they exhibit strong acidity and high
stability and are less corrosive than many existing alternatives [1].
In addition, the use of POMs as catalysts rarely leads to side reac-
tions, unlike the use of sulfonic acid and hydrochloric acid in many
organic syntheses. The stronger acidity of POMs compared to that
of common mineral acids, such as sulphonic acid and hydrochloric
acid, makes them highly suitable as catalysts for various organic
reactions such as the Friedel-Crafts, Mannich, and Prins reactions.
Protonated POMs such as Keggin-type H3PW12O40 (Fig. 1(a)) are
directly used as catalysts in homogeneous systems. Furthermore,
methods to adsorb protonated POMs onto silica gel and other
porous materials have been developed to avoid contamination
and to allow repeated re-use [1(g)]. The number of reports on the
catalytic use of POMs has been increasing. In particular, numerous
reports of the catalytic use of HnXM12O40 (X = P, Si; M = Mo, W;
n = 3, 4) have appeared in the literature because these POMS are
extracted with acidic diethyl ether, which is called the “etherate
method” [2]. However, the preparation and isolation of numerous
novel POMs as tetra-alkylammonium salts in an aqueous-organic
2−
mixed solution has been reported; for example, [SM12O40
]
4−
and [S2M18O62
]
(M = Mo, W) [3] have been characterised using
X-ray analysis, spectroscopy, and electrochemical methods [4,5].
6−
The Wells-Dawson-type POM S2Mo18O62
(Fig. 1(b)) exhibits
photocatalytic activity; it catalyses the photocatalytic reaction of
benzyl alcohol with the help of electrochemical oxidation [5(p)].
The acid strength of POMs is generally well known to follow the
order:
H3PW12O40 > H3PMo12O40 > H4SiW12O40 > H4SiMo12O40,
which is also largely consistent with the order of their cat-
alytic ability. In addition, Murakami et al. reported the acid
strength of other Keggin-type POMs: H3PW12O40 > H4SiW12O40
>
H4GeW12O40 > H5BW12O40 > H5FeW12O40 > H6CoW12O40 These
.
results imply that a smaller anion charge of Keggin-type POMs
results in greater acidity. POMs with smaller charge density
are expected to exhibit stronger acidity on the basis of the
general reported trend of POM acidity; thus, the isolation of
2−
4−
(M = Mo, W) in their protonated
[SM12O40
]
and [S2M18O62
]
∗
Corresponding author at: 2-5-1 Akebono-cho, Kochi 780-8520, Japan.
Tel.: +81 88 844 8299; fax: +81 88 844 8359.
forms is important. Moreover, they exhibit high redox potentials,
indicating that they could serve as strong oxidants. Recently,
E-mail address: chuji@kochi-u.ac.jp (T. Ueda).
http://dx.doi.org/10.1016/j.apcata.2014.07.032
0926-860X/© 2014 Elsevier B.V. All rights reserved.

182
T. Ueda et al. / Applied Catalysis A: General 485 (2014) 181–187
Fig. 1. Structure of (a) Keggin- and (b) Wells-Dawson-type POMs.
we reported that water-miscible organic solvents lead to the
4. Isolation of protonated POMs
(3+x)−
stable formation of [AsVxM12−xO40
]
(M = Mo, W; x = 0–2),
(3+x)−
which are less stable than [PVxM12−xO40
]
in aqueous solu-
Reaction mixtures for various POMs were prepared according
to the method reported in the literature [3,6,7,9,10]. Each reaction
mixture was poured into a separatory funnel, followed by the addi-
tion of diethyl ether and n-hexane. The ratio of diethyl ether and
tion [6,7]. The literature contains fewer reports of catalysis by
[AsVxM12−xO40
(3+x)−, most likely because of the toxicity of arsenic.
However, for academic reasons, a comparison between the cat-
]
alytic ability of [AsVxM12−xO40
]
(3+x)− and that of the corresponding
n-hexane to the reaction mixture was as follows: reaction mix-
(3+x)−
(4+x)−
[PVxM12−xO40
]
(x = 0–2) would be important and interesting.
ture:diethyl ether:n-hexane = 1:2:1 for [S2VxW18−xO62
]
,
,
4−
5−
Because an organic solvent such as acetone or acetonitrile
mixes with diethyl ether to form a single phase, POMs syn-
thesised in aqueous-organic solutions cannot be isolated in the
protonated form through the normal etherate method. In this
study, we developed a new procedure for the isolation of pro-
1:2:1.25 for [S2Mo18O62
]
,
1:2:2 for [S2VMo17O62]
1:1:0.5 for [AsW12O40
]
3−, 1:2:0.5 for [AsVW11O40
]
4−, 1:1:0.5
5−
3−
for [AsV2W10O40
]
,
1:1:1 for [AsMo12O40
]
,
1:1:0.2 for
4−
5−
[AsVMo11O40
]
,
and 1:1:0.15 for [AsV2Mo10O40
]
. In the
case of H3AsM12O40 (M = Mo, W), a heavy oil phase containing
the protonated POMs appeared after the separatory funnel was
shaken and was then separated before continuing to the next
procedure, as below. In the case of other POMs, no oil phase
appeared after being shaken only with the addition of diethyl
ether and n-hexane, and acid and water were therefore added as
follows: hydrochloric acid (conc.) was added at a volume ratio
(4+x)−
(3+x)−
(M = Mo,
tonated [S2VxM18−xxO62
]
and [AsVxM12−xO40
]
W; x = 0–2) by modifying the common etherate method. The
resulting protonated POMs were characterised by elemental anal-
ysis and IR spectroscopy; their acidity was measured by UV–Vis
spectrophotometry using a Hammett indicator; and their homo-
geneously catalytic ability was compared with that of various
POMs.
of 0.1 for [S2VMo17O62
]
5−, 0.6 for [AsVMo11O40
]
4−, and 1 for
4−, whereas
[AsV2Mo10O40
]
5−, [AsVW11O40
]
4−, and [S2Mo18O62
]
sulphonic acid at volume ratios of 0.2 and 0.3 were added for
5−
[AsV2W10O40
]
and [S2VxW18−xO62]
(4+x)−, respectively. In the
2. Experimental
case of H3AsMo12O40 and H3AsW12O40, the heavy oil was heated
at ca. 80 ◦C to dryness. For H3+xAsVxM12−xO40 (M = Mo, W; x = 1,2)
and H4S2W18O62, the extracted heavy oil was allowed to stand at
r.t.; a crystal formed and was separated by filtration. In the case of
The 1H NMR measurements were performed on a JEOL Model
JNM-LA400 spectrometer. All yields were determined by 1H NMR.
Raman spectra were recorded on a Horiba Jobin Yvon model HR-
800 spectrophotometer. The argon line at 514.5 nm was used for
excitation. The Raman measurements were obtained at 20 ◦C. The IR
spectra were obtained on a Jasco FT/IR-460 plus spectrophotometer
using the KBr pellet method. UV–Vis spectra were recorded on a
Jasco Model V-670 spectrophotometer using a quartz cell with a
path length of 1 mm. All reagents were of analytical grade and were
used as received.
H4S2Mo18O62, H5S2VMo17O62, H5S2VW17O62, and H6S2V2W16O62
,
after the addition of a few drops of conc. H2SO4 to the extracted
heavy oil with stirring or in an ultrasonic bath, the solution was
allowed to stand at r.t. or dried in vacuo at ca. 80 ◦C. When a crystal
appeared, it was separated by filtration, washed with n-hexane,
and dried in vacuo. Elemental analysis was performed after the
compounds were dried. The detailed results of the elemental
analysis are shown in Tables S1 and S2.
3. Acidity measurement
5. Results and discussion
The acidity measurements were performed according to the lit-
erature method, as follows [8]. Isolated protonated POMs were
added to CH3CN, which had been dried over 3A molecular sieves
containing 3.5 × 10−5 M 1,9-diohenyl-1,3,6,8-nonatetraen-5-one
(dicinnamylacetone: pKa = −3.0). The concentration of the proton-
ated POMs was set to 4.86 × 10−3 M on the basis of the number
of protons per POM. The UV–Vis spectra were measured using a
quartz cell (L = 1 cm) in the 190–850 nm region.
5.1. IR and Raman spectroscopy
Table 1 shows the peak IR and Raman bands for proton-
ated POMs isolated in this study and for the previously prepared
tetraalkylammonium POM salts. The IR spectra (Figs. S1-S11) of the
protonated form and the tetraalkylammonium salt of each POM
were compared. The IR spectra and peak bands for the proton-
ated form were identical to those for the tetraalkylammonium salts

T. Ueda et al. / Applied Catalysis A: General 485 (2014) 181–187
183
Table 1
Peak frequencies (cm−1) of the protonated form and tetra-alkylammonium salt of POMs.
Compounds
Cation
IR
X
Raman
Oa
M
Od
M
Ob
M
Oc
M Od
4−
S2W18O62
H+
TPA
1178; 1074
1180; 1076
990; 968
986; 966
(sh)
906
795
804
1003
1007
5−
S2VW17O62
H+
1-TBA
4-TBA
1195; 1174; 1070
1175; 1075
1200; 1174; 1072
982; 960
978
978; 958
(sh)
901
901
793
803
802
994
990
989
6−
5−
S2V2W16O62
H+
TBA(H)
1198; 1174; 1071
1202; 1160; 1070
981; 959
977; 955
(sh)
901
788
798
1002
995
S2Mo18O62
H+
TPA
1169; 1068
1170; 1070
969
966
862
880
773
792
989
4−
a
–
S2VMo17O62
H+
TBA
1168; 1069
1183; 1166; 1069
966
956
866
876
773
792
989
975
3−
AsW12O40
H+
TBA
912
914
986
984
865
875
794
796
1001
1001
AsVW11O40
AsV2W10O40
H+
TBA
909
906
983
971
862
870
778
792
1009
991
4−
H+
905
905
983
969
865
870
780
790
1018
992
5−
TBA(H+)
3−
AsMo12O40
H+
TBA
894
896
963
962
846
856
769
792
992
984
4−
AsVMo11O40
H+
TBA
894
891
963
951
841
852
768
789
989
989
5−
AsV2Mo10O40
H+
893
890
963
948
845
851
769
785
1001
980
TBA(H+)
Oa: Oxygen bonded with sulfur atom; Ob: Corner-shared Oxygen; Oc: Edge-shared Oxygen; Od: Terminal Oxygen.
a
No peak was obtained because of combustion by laser.
within a small shift. C. Rocchiccioli et al. reported the features of
the IR spectra of various salts of Keggin-type POMs and indicated
that stronger electrostatic anion–anion interactions and smaller
counter-cation size lead to an increase in the stretching frequen-
cies, particularly to higher M-Od (terminal) stretching frequencies
[11]. The M-Od stretching frequencies in the spectra of all of the
protonated POMs in this study were higher than those of the cor-
responding tetraalkylammonium salts. In addition, in the case of
spectra of all of the protonated POMs, no peaks appeared at ca.
1480 and 1380 cm−1, which are associated with the bending vibra-
tions of the C–H bond of the tetraalkylammonium cation. These
results indicate that the protonated forms of all of the investigated
POMs could be isolated with our newly developed procedure with-
out inducing a structural change. With respect to the spectrum of
H5[S2VW17O61], three peaks appeared at 1200–1140 cm−1 because
two isomers, resulting from substitution of vanadium at different
positions, are always included [10] and because isolation of the pure
isomers is not currently possible.
Keggin-type POMs. However, compared with the acidity of
series of protonated POMs with the same structure that
were isolated in this study, the acidity decreased in the order
H4S2W18O62 > H5S2VW17O62 > H6S2V2W16O62 H4S2Mo18O62
H5S2VMo17O62 H3AsW12O40 > H4AsVW11O40 > H5AsV2W10O40
H3AsMo12O40 > H4AsVMo11O40 > H5AsV2Mo10O40. Similar to the
Keggin-type series HnXM12O40, the acidity of the protonated POMs
decreases with increasing total charge or with increasing number
of vanadium atoms in the POM.
a
;
>
;
;
5.3. Catalytic ability of protonated polyoxometalates
Numerous reports on the acidic and oxidative catalytic ability
of POMs have appeared in the literature [1]. They exhibit excel-
lent catalytic properties, as they are stronger but less corrosive
Table 2
Acidity of protonated POMs obtained from UV–Vis spectra with Hammett indicator.
Compounds
−H0
5.2. Acidity of POMs
H4S2W18O62
H5S2VW17O62
H3AsW12O40
H4S2Mo18O62
H3PW12O40
H3AsMo12O40
H4AsVMo11O40
H4AsVW11O40
H4SiW12O40
H5AsV2W10O40
H5AsV2Mo10O40
H6S2V2W16O62
H4GeW12O40
H5S2VMo17O62
H5BW12O40
2.70
2.26
2.19
2.17
2.14
2.07
2.03
2.00
1.98
1.94
1.85
1.82
1.69
1.65
1.55
1.48
0.88
The acidity of the POMs was measured using the Ham-
mett indicator, and the obtained data are presented in Table 2.
Many protonated POMs, including H3PW12O40 and H3PMo12O40
are well known to exhibit stronger acidity than mineral acids
such as H2SO4 and HCl [8,12]. Furthermore, H4S2W18O62
H5S2VW17O62, and H4S2Mo18O62 are more strongly acidic than
commercially available H3PW12O40. The acidity of Keggin-type
POMs decreases as the number of protons increases, as fol-
,
,
lows:
H3PW12O40 > H4SiW12O40 > H4GeW12O40 > H5BW12O40
>
H5FeW12O40 > H6CoW12O40
.
The total size of the Keggin-
type POMs is nearly identical, even when the central atom,
referred to as the heteroatom, is changed. The difference
in acidity should be dependent on the total charge of the
H5FeW12O40
H6CoW12O40

184
T. Ueda et al. / Applied Catalysis A: General 485 (2014) 181–187
O
O
POM(0.1 mol%)
423K, 30 min
HO
OH
O
O
H
POM
+
OH
+
H
HO
r.t. 24 h
Scheme 1. Transformation reaction of pinacol into 3,3-dimethyl-2-butanone and
2,3-dimethyl-1,3-butadiene [15].
Scheme 2. Acetal formation reaction of benzaldehyde and ethylene glycol [16].
than mineral acids, and they exhibit high stability and can be used
repeatedly. Many POM-catalysed organic synthesis reactions are
high yielding (>90%), so a comparison of the catalytic ability of
POMs isolated in this study against reported reactions with high
yields is not appropriate. Therefore, we surveyed adequate catalytic
reactions that proceed with moderate yield (approximately 50%)
under mild conditions and finally selected four reactions in order
to compare the catalytic activity of the POMs.
reaction, 3,3-dimethyl-2-butanone was formed only in trace pro-
portions. We surmised that the POMs were reduced during the
pinacol rearrangement, as the colour of the reaction mixtures
changed to blue, and that the acidity of the protonated POMs was
lowered by the increasing anion charge that resulted from the
reduction of POMs. The effect of reaction time on the yield was
not investigated because of the low boiling point of 3,3-dimethyl-
2-butanone (106 ◦C).
Elimination of a hydroxy group is a very important reaction
in various electrophilic transformations, as a variety of products,
including alkenes, dienes, ethers, and carbonyl compounds, can be
formed depending on the reaction conditions and the structure of
the starting reagents. Although diols are well known to undergo
electrophilic dehydration reactions under both homogeneous con-
ditions with strong mineral acids and heterogeneous conditions
with Al2O3 and zeolites, the literature contains few reports of the
transformation of diols catalysed by POMs. Baba and Ono first
reported a kinetic study on the dehydration of 1,4-butanediol into
tetrahydrofuran [13]. The heterogeneous pinacol rearrangement
of 1,1,2-triphenyl-1,2-ethanediol to triphenylacetaldehyde and
diphenylacetophenone catalysed by Cs2.5H0.5PW12O40 has also
been investigated [14].
5.5. Acetal formation
Sato et al. systematically investigated various acetal reactions
between four types of carbonyl compounds and six types of alcohols
using H4SiW12O40 as a catalyst [16]. The advantage of this reaction
is that the reaction mixture is ultimately divided into two phases:
the upper phase contains the products, whereas the lower phase
contains the POMs; consequently, the product selectivity should be
improved and the POMs could be easily separated and re-used sev-
eral times. In this study, the acetal reaction between benzaldehyde
and 1,2-ethanediol was chosen to compare the catalytic activity
of the POMs (Scheme 2). The results are listed in Table 4. In most
cases of the use of protonated POMs and in the case of the report
on H4SiW12O40, the yield of 2-phenyl-1,3-dioxolane was approxi-
mately 30%. However, the yield was 50% when H5AsV2W10O40 was
used. In contrast to the aforementioned systems, the number of
vanadium atoms in the POMs slightly affected the product yield,
with the exception that the yield tended to increase according to
the number of vanadium atoms in H3+xAsVxW12−xO40. Moreover,
the effect of reaction time on the product yield was examined for
H5AsV2W10O40. The product yield increased with increasing reac-
tion time up to 45 min and then remained constant for 48 h. This
result indicates that the reaction was completed within an hour,
even at room temperature.
5.4. Pinacol rearrangement
Torok et al. conducted systematic studies on the transformation
of various diols catalysed by H3PM12O40 and H4SiM12O40 (M = Mo,
W) [15]. Among these transformations, the conversion of 2,3-
dimethyl-2,3-butanediol (pinacol) into 3,3-dimethyl-2-butanone
and 2,3-dimethyl-1,3-butadiene was chosen to compare the cat-
alytic ability of our isolated POMs (Scheme 1). All reactions
were performed with 0.02 mol of 2,3-dimethyl-2,3-butanediol and
0.1 mol% POM at 423 K for 30 min, and the results are presented
in Table 3. In the cases of H4SiW12O40 and H3PW12O40 (entries
1 and 2), nearly identical yields for 3,3-dimethyl-2-butanone and
2,3-dimethyl-1,3-butadiene were observed, although selectivity
was only reported for H3PW12O40 [15]. However, 3,3-dimethyl-
2-butanone was formed in higher yield when H4+xS2VxW18−xO62
(x = 0–2) was used (entries 5–7). As the number of vanadium
atoms in the POMs increased, the yield of 3,3-dimethyl-2-butanone
increased. In particular, the highest yield and highest selectiv-
ity were obtained with H6S2V2W16O62. When POMs containing
molybdenum, such as H3+xAsVxMo12−xO40, were used for this
5.6. Friedel-Crafts acylation
Various types of Friedel-Crafts reactions, such as alkylation
and acylation, require strong Lewis acids such as AlCl3 or zeo-
lites [17]. Both protonated forms and salts of POMs have also
been used in catalytic Friedel-Crafts reactions [1(l)]. Izumi et al.
investigated acidic alkali salts of Keggin-type POMs as solid
Table 4
Acetal formation catalyzed by various protonated POMs.
Table 3
Entry
POMs
Yield (%)
Pinacol rearrangement catalyzed by various protonated POMs.
1
2
3
4
5
6
7
8
9
H4SiW12O40
H3AsW12O40
27(32a)
30
35
51
Trace
29
30
23
28
28
Entry
POMs
Yield (%)
A
H4AsVW11O40
H5AsV2W10O40
H3AsMo12O40
H4AsVMo11O40
H5AsV2Mo10O40
H4S2W18O62
H5S2VW17O62
H6S2V2W16O62
H4S2Mo18O62
H5S2VMo17O62
B
1
2
3
4
5
6
7
H4SiW12O40
H3PW12O40
59(71a)
63(78a)
67
<50
68
17(29a)
15(22a)
12
<15
13
H3AsW12O40
H4AsVW11O40
H4S2W18O62
H5S2VW17O62
H6S2V2W16O62
77
82
15
16
10
11
12
Trace
24
A: 3,3-Dimethyl-2-butanone; B: 2,3-Dimethyl-1,3-butadiene.
a
a
The yield in parenthesis of entry 1 means reported result [16].
The yield in parenthesis of entry 1 and 2 means reported result [15].
Reaction conditions: benzylaldehyde (2.5 mmol), ethylene glycol (2.5 mmol), POMs
(0.003 eq), room temperature, 24 h.
Reaction conditions: 2,3-dimethyl-2,3-butanediol (0.02 mol), POMs (0.1 mol%),
423 K, 30 min.

T. Ueda et al. / Applied Catalysis A: General 485 (2014) 181–187
185
O
O
O
O
OMe
O
OMe
O
O
POM
reflux, 2 h
O
O
O
O
POM
CH3COOH
343K, 1 h
O
+
OEt
+
O
OEt
OEt
O
O
O
O
(2 mmol)
(20 mmol)
Scheme 3. Friedel-Crafts acylation reaction of anisole and acetic anhydride [18].
Scheme 4. Acylation reaction of ethyl pyruvate with acetic anhydride [20].
catalysts for liquid-phase Friedel-Crafts reactions [18]. Accord-
ing to their report, anisole reacted with anhydrous acetic acid,
catalysed by H3PW12O40, to form 4ꢀ-methoxyacetophenone in 50%
yield (Scheme 3), although the main topic of their study was
the catalytic activity of CsxHx−3PW12O40 and the catalytic per-
formance of Cs2.5H0.5PW12O40. Using the same procedure, we
examined the catalytic activity of all POMs isolated in this study
when anisole (100 mmol), acetic anhydride (5 mmol), and a POM
(0.05 mol) were refluxed for 2 h. Table 5 shows the yields of 1-
(4-methoxyphenyl)ethanone using various POMs. In general, the
protonated polyoxotungstates exhibited better catalysis than the
protonated polyoxomolybdates. The use of H5S2VW17O62 led to
a higher yield than that achieved with H3PW12O40 and other
POMs in this study, although 1-(4-methoxyphenyl)ethanone was
Table 6
Alylation of ethyl pyruvate catalyzed by various protonated POMs.
Entry
POMs
Yield (%)
A
B
1
2
3
4
5
H3PW12O40
H3PMo12O40
H4S2W18O62
H4S2Mo18O62
H3AsW12O40
9(7a)
0.3 (2a)
37
21
25
56(61a)
3(6a)
49
12
12
A: Ethyl 2,2-diacetoxypropionate; B: Ethyl ␣-acetoxyacrylate.
a
The yield in parenthesis of entry 1 and 2 means reported result [20].
Reaction conditions: ethyl pyruvate (2 mmol), aceticanhydrate (20 mmol), POM
(7.5 mol% as H+ vs. ethyl pyruvate), acetic acid (20 mmol), 343 K, 1 h.
obtained in higher yields as the number of vanadium atoms in
(3+x)−
series increased. When molybdenum-
20 mmol of anhydrous acetic acid at 343 K for 1 h to form ethyl
␣-acetoxyacrylate in acetic acid containing various protonated
POMs. Table 6 shows the results of these reactions. Although all
protonated POMs were tested, no reaction proceeded or ethyl
2,2-diacetoxypropinate was formed in low yield in most cases;
these results are not listed in Table 6. Better results were obtained
the [AsVxW12−xO40
]
containing POMs were used, the colour of the solution changed
to dark blue shortly after the start of the reaction. This colour
change indicates that the POMs were reduced and that their
acidity was decreased, resulting in
a lower yield of 1-(4-
methoxyphenyl)ethanone. To examine the effect of reaction time
on the yield, the yield was checked every 30–60 min until 360 min
after the reaction catalysed by H5AsV2W10O40 had started. The
yield increased up to ca. 150 min, then remained constant (ca. 74%).
For other POMs, a longer reaction time might also have improved
the product yield.
for three types of protonated POMs: H4S2W18O62, H4S2Mo18O62
,
and H3AsW12O40. Indeed, only H3PW12O40 and H4SiW12O40 gave
good yields of ethyl ␣-acetoxyacrylate, although a variety of acids,
including POM, were also tested in the literature. Mineral acids
such as H2SO4, HCl, and HNO3 are strong acids, but they are not
strong enough to promote the reaction of ethyl pyruvate to ethyl
␣-acetoxyacrylate [20]. Notably, strong acidity is required to facil-
itate this reaction. In particular, H4S2W18O62, which is more acidic
than H3PW12O40, gave higher conversion (86% for H4S2W18O62 and
65% for H3PW12O40), although the yield of ethyl ␣-acetoxyacrylate
with H4S2W18O62 was less than that with H3PW12O40 in a 1 h reac-
tion (49% for H4S2W18O62 and 56% for H3PW12O40). In general, the
protonated polyoxotungstates exhibited better catalytic proper-
ties than the protonated polyoxomolybdates, similar to the results
obtained for the Friedel-Crafts acylation. Furthermore, the yields
of ethyl ␣-acetoxyacrylate and ethyl 2,2-diacetoxypropinate were
investigated with H4S2W18O62 for longer reaction times. When the
conversion ratio and yield were checked at regular time intervals
up to 24 h, the conversion ratio increased to 90% over 7 h, and the
yield of ethyl ␣-acetoxyacrylate ultimately reached 69%. In the case
of H3PW12O40, the conversion ratio (68%) and yield of ethyl ␣-
acetoxyacrylate (61%) obtained in a 1 h reaction would be nearly
the same as in a 24 h reaction on the basis of the data reported in
the literature, even if the reaction proceeded at a higher temper-
ature. The catalytic reaction rate with H4S2W18O62 appears to be
slower than that with H3PW12O40. However, the catalytic activity
of H4S2W18O62 is much higher than that of H3PW12O40, which can
5.7. Acylation of ethylpyruvate
The production of useful chemicals and energies from biomass is
an important goal [19]. The synthesis of polyacyloxyacrylate ester
from lactic acid through lactate ester, pyruvate ester, and acyloxy-
acrylate ester has been developed, although polylactic acid can
result from the polymerisation of lactic acid, which is obtained
from the fermentation of glucose as a polymer from biomass.
Ninomiya et al. studied the catalytic acylation of pyruvate ester
with carboxylic anhydrides using a few types of POMs in addi-
tion to strong mineral acids such as H2SO4, HCl, and p-TsOH in
the synthesis of polyacyloxyacrylate from lactic acid (Scheme 4)
[20]. Among the various acids tested, H3PW12O40 gave the high-
est product yield (61%). In this study, according to the reported
reaction conditions, 2 mmol of ethyl pyruvate was reacted with
Table 5
Friedel-Crafts Acylation catalyzed by various protonated POMs.
Entry
POMs
Yield (%)
1
2
3
4
5
6
7
8
9
H3PW12O40
H3AsW12O40
50
46
55
63
19
0
be attributed to the high acidity of H4S2W18O62
.
H4AsVW11O40
H5AsV2W10O40
H3AsMo12O40
H4AsVMo11O40
H5AsV2Mo10O40
H4S2W18O62
H5S2VW17O62
H6S2V2W16O62
H4S2Mo18O62
H5S2VMo17O62
6. Conclusion
0
In this study, several POMs prepared in aqueous-organic solu-
tions were isolated in their protonated form via a modified etherate
method. The detailed extraction conditions were tuned for each
individual POM. Isolated protonated POMs were characterised by IR
and Raman spectroscopy, and the resulting spectra were compared
with the corresponding tetraalkylammonium salts. Many of the
POMs were observed to be more strongly acidic than H3PW12O40
39
72
51
14
8
10
11
12
Reaction conditions: anisole (100 mmol), acetic anhydride (5 mmol), POM
(0.05 mmol), reflux, 2 h.

186
T. Ueda et al. / Applied Catalysis A: General 485 (2014) 181–187
and H4SiW12O40 on the basis of results obtained using a Hammett
indicator. Four types of catalytic reactions—pinacol rearrangement,
acetal formation using benzaldehyde and ethylene glycol, and
the Friedel-Crafts acylation of anisole and ethyl pyruvate—that
were previously reported were selected to compare the catalytic
behaviours of the protonated POMs isolated in this study. Based on
the results of a series of catalytic ability tests, the catalytic ability of
the newly isolated POMs should be higher than that of previously
reported POMs for all cases, although different POMs exhibited bet-
ter performance for different reaction systems. We concluded that
the use of the modified etherate method and optimisation of the
extraction conditions allow the isolation of POMs formed in an
aqueous-organic mixed solvent in their protonated forms. In addi-
tion, these POMs could improve the conversion ratios and product
yields in other organic syntheses. The yields of the products tested
in this study are closely related to the acidity of the protonated
POMs. However, this relationship is not simple. The framework
components of the POMs should be essential for their yield and
acidity, which implies that the POMs could directly interact with
materials during catalytic reactions. However, detailed justifica-
tion for this mechanism is currently unavailable. In the future,
reaction conditions should be optimised according to the POMs
that will be compared with the results. This process would allow
researchers to elucidate the effects of acidity and the POM com-
ponents on the respective catalytic reactions. In addition, further
study of the extraction conditions, characterisation, and catalytic
ability of other POMs is in progress, and the results will be reported
in due course.
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Acknowledgement
This work was supported in part by a Grant-in-aid for Scientific
Research (C) (25410095) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan, a Special Research Grant
for Rare Metals and Green Technology, and the Kochi University
President’s Discretionary Grant from Kochi University.
(u) A.M. Bond, J.C. Eklund, N. Fay, P.J.S. Richardt, A.G. Wedd, Inorg. Chim. Acta
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Appendix A. Supplementary data
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Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.apcata.2014.07.032.
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