A Ta/W mixed addenda heteropolyacid with excellent acid catalytic activity and proton-conducting property

Article abstract of DOI:10.1016/j.jssc.2016.08.003

A new HPAs H₂₀[P₈W₆₀Ta₁₂(H₂O)₄(OH)₈O₂₃₆]·125H₂O (H-1) which comprises a Ta/W mixed addenda heteropolyanion, 20 protons, and 125 crystalline water molecules has been prepared through ion-exchange method. The structure and properties of H-1 have been explored in detail. AC impedance measurements indicate that H-1 is a good solid state proton conducting material at room temperature with a conductivity value of 7.2×10^?3?S?cm^?1 (25?°C, 30% RH). Cyclic voltammograms of H-1 indicate the electrocatalytic activity towards the reduction of nitrite. Hammett acidity constant H₀ of H-1 in CH₃CN is ?2.91, which is the strongest among the present known HPAs. Relatively, H-1 exhibits excellent catalytic activities toward acetal reaction.

Full text of DOI:10.1016/j.jssc.2016.08.003

Journal of Solid State Chemistry 243 (2016) 1–7
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
A Ta/W mixed addenda heteropolyacid with excellent acid catalytic
activity and proton-conducting property
a
a
a
Shujun Li a, Qingpo Peng a, Xuenian Chen a, , Ruoya Wang , Jianxin Zhai , Weihua Hu ,
n
a,
c
Fengji Ma b, , Jie Zhang , Shuxia Liu
n
n
a School of Chemistry and Chemical Engineering, Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, Henan Normal University,
Xinxiang, Henan 453007, China
b College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang, Henan 453000, China
c Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun City, Jilin 130024,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new HPAs H20[P8W60Ta12(H2O)4(OH)8O236] ꢀ 125H2O (H-1) which comprises a Ta/W mixed addenda
heteropolyanion, 20 protons, and 125 crystalline water molecules has been prepared through ion-ex-
change method. The structure and properties of H-1 have been explored in detail. AC impedance mea-
surements indicate that H-1 is a good solid state proton conducting material at room temperature with a
conductivity value of 7.2 ꢁ 10ꢂ3 S cmꢂ1 (25 °C, 30% RH). Cyclic voltammograms of H-1 indicate the
electrocatalytic activity towards the reduction of nitrite. Hammett acidity constant H0 of H-1 in CH3CN is
ꢂ2.91, which is the strongest among the present known HPAs. Relatively, H-1 exhibits excellent catalytic
activities toward acetal reaction.
Received 9 June 2016
Received in revised form
31 July 2016
Accepted 1 August 2016
Available online 2 August 2016
Keywords:
Acid catalysis
Heteropolyacid
Ion-exchange
Proton conduction
Tantalum
& 2016 Elsevier Inc. All rights reserved.
1. Introduction
reasons. Firstly, most POMs compounds especially those with
complex structures, are only stable under specific narrow pH
Heteropolyacids (HPAs) constitute a small but important sub-
class in Polyoxometalates (POMs) family, which are usually acidic
forms of typical POMs [1,2]. Their heteropolyanions (anion clusters
ranges, and tend to convert into stable Keggin or Dawson species
under strong acid conditions. Secondly, it is difficult to obtain
single crystals of some HPAs suitable for X-ray diffraction mea-
surements. Exploring novel type HPAs other than traditional
Keggin or Dawson type is motivating for the farther development
of HPAs and relevant acid catalysis areas.
Ⅵ
Ⅵ
usually made up of W or Mo and oxygen) have lower basicity
on the surface oxygen atoms, so they are strong Brønsted acids and
have significantly higher catalytic activity than mineral acids, such
as sulfonic acid and hydrochloric acid. In particular in organic
media, the molar catalytic activity of HPA is often 100–1000 times
higher than that of H2SO4, and rarely leads to side reactions [3,4].
So, HPAs are highly suitable to catalyze various types of organic
reactions in homogeneous liquid phase, and several processes
with HPAs as the catalysts have been industrialized [1,5,6].
However, only limited HPAs with unambiguous structures de-
termined by X-ray crystallography analysis or neutron diffraction
analysis, including Keggin-type H3[PW12O40] ꢀ nH2O [7–9], and
H3[PMo12O40] ꢀ nH2O [10], Dawson-type H7[In(H2O)P2W17O61] ꢀ
23H2O [11] and sandwich complex H8[Ti2{P2W15O54(OH2)2}2] ꢀ
31H2O [12], have been reported owing to the following two
On the other hand, although POMs based on VV, MoVI, WVI, and
now even NbV provide frequent exciting advances, little is known
about the POM chemistry of TaV [13]. The first polyoxotantalate
(K8[Ta6O19]) was discovered more than sixty years ago, but the
synthesis of Ta-containing POMs is still challenging, mainly because
8ꢂ
the soluble Ta precursors (e.g. [Ta6O19
]
or TaCl5) tend to form in-
tractable gel-like materials or Ta2O5 precipitate in aqueous solution
[14,15]. Recently our group and Nyman's group have demonstrated
that it is feasible to access mixed metal Ta/W POMs in acidic solution
[16,17]. These Ta/W mixed addenda POMs will provide excellent
opportunity for the further development of Ta-POMs chemistry, be-
cause they exhibit unique property compared with the related Ta-
POMs and W-POMs in term of electronic and electrochemical prop-
erties, solubility, stability, reactivity and photocatalytic performance.
However, the research is still at the early stage, and only few ex-
amples of Ta/W mixed addenda compounds are known so far [18].
Much systematic and extended work still needs to do in this field.
n
Corresponding author.
E-mail addresses: xnchen@htu.edu.cn (X. Chen), fengji.ma@yahoo.com (F. Ma),
jie.zhang@htu.edu.cn (J. Zhang).
http://dx.doi.org/10.1016/j.jssc.2016.08.003
0022-4596/& 2016 Elsevier Inc. All rights reserved.

2
S. Li et al. / Journal of Solid State Chemistry 243 (2016) 1–7
It has been demonstrated that Ta/W mixed addenda monomers
Table 1
Crystal data and structural refinement for compound H-1.
have strong tendency to form aggregates by the formation of
steady Ta–O–Ta bridges in acidic solution [17]. The very good sta-
bility of the polymerized Ta/W POMs in strong acid solution makes
them promising for a new type of HPAs. In this report a full-acid
(free-acid) form of tetrameric Ta/W mixed addenda POM
Compound
H-1
Formula
H20[P8W60Ta12(H2O)4(OH)8O236] ꢀ 125H2O
Formula weight (gmolꢂ1
)
18,729.56
103 (2)
0.71073
Tetragonal
P-421c
24.185(2)
24.185(2)
27.067(3)
90
T (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
H
20[P8W60Ta12(H2O)4(OH)8O236] ꢀ 125H2O (H-1) has been isolated
as yellow crystals through ion-exchange method. The structure
and properties of H-1 have been discussed in detail. The acidity
has been measured by UV–Vis spectrophotometry using a Ham-
mett indicator. H-1 exhibits excellent homogeneous acid catalytic
activity and proton-conducting ability in solid state.
β (°)
90
γ (°)
90
15,832(3)
2
V(Å3)
2. Experimental section
Z
Dcalc (mg mꢂ3
)
3.929
2.1. Materials and measurement
μ(mmꢂ1
F(000)
)
25.978
16,152.0
0.30 ꢁ 0.26 ꢁ 0.20
0.907
Crystal dimensions mmꢂ1
Goodness-of-fit on F2
Final R indices[I 4 2s(I)]
R indices (all data)
The precursor K8Na8H4[P8W60Ta12(H2O)4(OH)8O236] ꢀ 42H2O (1)
was synthesized according to the procedure described in the lit-
erature [17]. All other reagents were readily available from com-
mercial sources and used without further purification. The FTIR
spectra in KBr pellets were recorded in the range 400–4000 cmꢂ1
with a VECTOR 22 Bruker spectrophotometer at room tempera-
ture. Elemental analyses for P, W, and Ta were determined with a
PLASMASPEC (I) ICP atomic emission spectrometer. Powder X-ray
diffraction (PXRD) measurements were performed on a Bruker D8
a
a
R1 ¼ 0.0855 , wR2 ¼0.1581
a
a
R1 ¼0.1585, wR2 ¼ 0.1942
a
2
R1¼∑||Fo|ꢂ|Fc||/∑|Fo|; wR2¼{∑[w(Fo ꢂFc2)2]/∑[w(Fo2)2]}1/2
.
with copper electrodes (the purity of Cu is more than 99.8%) over
the frequency range from 105 to 1 Hz. The powdered crystalline
sample of H-1 was compressed to 1.0–1.2 mm in thickness and
12.0 mm in diameter under a pressure of 12–14 MPa at room
temperature. The conductivities were determined from the Ny-
quist plots. According to the Nyquist plot for H-1 at each tem-
perature and humidity, the proton conductivity was calculated as
s¼(1/R) (h/S), where R is the resistance, h is the thickness, and S is
the area of the disk. The activation energy was calculated from the
Arrhenius plot according to the formula sT¼s0 exp(ꢂEa/kBT).
Real (Z′) and imaginary (Z″) parts of the impedance spectra are
shown in Fig. 6 and Figs. S9–S10.
Advance Instrument with Cu K
α
radiation in the angular range
2
θ
¼3–50° at 293 K. The thermal behavior of H-1 was examined by
synchronousthermal analyses (TG/DSC, Netzsch 449 C Jupiter/QMS
403D). The samples were heated to 700 °C with a heating rate of
5 °C /min, under a flowing N2 atmosphere. UV–Vis absorption
spectra were obtained by using a UV-1700 UV–Vis spectro-
photometer. The 31P NMR spectra were measured on Avance-400
Bruker NMR spectrometers at an operating frequency of
16.66 MHz, with a 2.5 kHz sweep width, and 5-s pulse delay.
Electrochemical measurements were performed with CHI1604E
electrochemical workstation (Chenhua Instruments Co., Shanghai,
China). Three-electrode system was employed in this study, a glass
carbon electrode used as the working electrode, a Ag/AgCl elec-
trode used as the reference electrode and a Pt coil used as the
counter electrode. All the experiments were conducted at ambient
temperature (20–25 °C).
2.4. Acid strength measurement of H-1
Acidity measurements of H-1 were performed in acetonitrile ac-
cording to the procedure described in the literatures [21,22]. Di-
cinnamylideneacetone (pKa value is ꢂ3.0, where Ka is the dissocia-
tion constant of the protonated indicator pKa¼ ꢂlogKa) was used as
indicator. The concentration of Dicinnamylideneacetone and proton
(based on the number of protons per HPAs) were set to
7ꢁ 10ꢂ4 mol/L and 9.72 ꢁ 10ꢂ4 mol/L respectively. UV–vis absorp-
tion spectra were obtained by using a UV-1700 UV–Vis spectro-
photometer. The Hammett acidity function (H0) is defined by
H0¼pKaꢂlog[BHþ]/[B]. [BHþ] and [B] are the concentrations of the
protonated and neutral forms of the indicator in the equilibrium
BH+ ⇌ B + H+. The ratio of the extinction coefficient of the two
forms was estimated to be 1.3 by H3PW12O40 (Fig. S11 and S12).
2.2. Crystal structure determination
Single crystal XRD analysis of H-1 was conducted on a Bruker
Smart Apex CCD diffractrometer with Mo K
α
monochromated ra-
diation (
λ
¼0.71073 Å) at room temperature. The linear absorption
coefficients, scattering factors for the atoms, and anomalous disper-
sion corrections were taken from the International Tables for X-Ray
Crystallography [19]. Empirical absorption corrections were applied.
The structures were solved by using the direct method and refined
through the full matrix least-squares method on F2 using SHELXS-97
[20]. Anisotropic thermal parameters were used to refine all non-
hydrogen atoms. Those hydrogen atoms attached to lattice water
molecules were not located. The crystal data and structure refine-
ment results of H-1 are summarized in Table 1. Further details on the
crystal structure investigation scan be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif (depository number CCDC-1457311).
2.5. Preparation of H20[P8W60Ta12(H2O)4(OH)8O236] ꢀ 125H2O (H-1)
400 mL 1 M HCl solution was poured into a column with an
inner diameter of 15 mm charged with 100 g cationic exchange
resin (Amberlite IR120B NA). A dripping rate of one drop/2 s was
used. After that, the column was washed with deionized water to
neutral.
3.0 g precursor (1) was dissolved in 5.0 mL deionized water, the
resulting clear solution was passed through the above mentioned
cation-exchange resin column at a dripping rate of one drop/2 s,
then washed with deionized water to neutral. The collected solu-
tion was evaporated on a rotary evaporator at 80 °C, resulting in a
2.3. Proton conductivity measurement of H-1
Ac impedance spectroscopy measurement was performed on a
chi660d (Shanghai chenhua) electrochemical impedance analyzer

S. Li et al. / Journal of Solid State Chemistry 243 (2016) 1–7
3
yellow solid of H-1 with 94.7% yield (based on precursor 1). Pure
crystals product of H-1 suitable for X-ray crystallography was
obtained by recrystallization in water (2.66 g, 91.1% yield based on
precursor 1). Anal. Calcd (%) for H-1: P 1.26, Ta 11.02, W 55.98;
found P 1.34, Ta 11.14, W 56.10. FTIR (KBr disk): 1091(s), 959(s),
903(sh), 773(vs), 562(w), 520(vw). 31P NMR (ppm)ꢂ11.21, ꢂ14.13
in D2O, and ꢂ11.6, ꢂ13.0 in CD3CN.
3. Results and discussion
3.1. Synthesis and structure of H-1
The
tetrameric
Ta/W
mixed
addenda
POM
K8Na8H4[P8W60Ta12(H2O)4(OH)8O236] ꢀ 42H2O (1), which consists
of four lacunary Wells-Dawson {P2W15O56} segments linked by a
central {Ta12} cluster and possesses remarkable photocatalytic H2
evolution activity had been reported in our previous work [17].
Herein H-1 was prepared by cationic ion exchange method using 1
as the precursor. Generally, HPAs can be prepared in aqueous so-
lution and extracted with acidic diethyl ether, which is called the
“etherate method”. But this method is fussy with low yield for H-1
(shown in SI-1), by contrast, cationic ion exchange method de-
scribed above is practical and simple.
Fig. 2. The diffuse reflectance spectral of H-1 (red line) and 1 (black line), and the
digital photograph of H-1 (top) and 1 (bottom). (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this
article.)
3.2. Solid properties
H-1 possess the same space group (P-421c) and similar cell
parameters with that of 1. As shown in Figs. 1 and S1 the het-
eropolyanions packing in the cell form a body-centered tetragonal
lattice in H-1. As conformed by X-ray crystallography analysis, acid
base titration and TG analysis, there were 125 lattice water mo-
lecules and 20 Hþ ions in H-1. Abundant hydrogen bonds exist
among the crystalline water molecules, the coordinated water
molecules, and partial oxygen atoms of the polyanion. These hy-
drogen bonds extend around forming a three dimensional hy-
drogen bonding network (Figs. S2–S4), which is the potential
channel for proton conduction. As found in other HPAs, the posi-
tion of these protons cannot be determined directly by X-ray data,
but may be delocalized among the hydrogen bonding network and
polyanion, or combine with lattice water molecules forming hy-
drated ion (H3Oþ, H5O2þ, etc.) [1,23].
H-1 possesses similar cell parameters and appearance shape but
different color with that of 1. As shown in Fig. 2, H-1 shows similar UV–
vis diffuse reflectance spectra as that of 1. The intense absorption
bands in the range from 200 to 475 nm are mainly attributed to oxy-
gen to metal (O-M, M¼W6þ and Ta5þ) charge transfer (OMCT)
transitions. The absorption edges of the yellow crystals of H-1 shifted
to a lower energy region compared with that of the colorless crystals of
1. H-1 shows similar PXRD patterns with 1 (Fig. S5). The calculated and
experimental diffraction patterns of H-1 match well, indicating that the
bulk products obtained by cationic ion exchange method are homo-
geneous. The TG/DSC curve of H-1 (Fig. S6) showed a total weight loss
of 11.8% between 25 and 400 °C, which was attributed to the release of
125 lattice water molecules and four coordinated water molecules in
the polyanion of H-1. Two exothermic peaks in the DSC curve above
580 °C were attributed to the decomposition of the polyanion skele-
tons in H-1. The FTIR spectra of H-1 (Fig. S7) showed similar features to
that of the precursor 1, but the obvious shift for the bands arising from
the M–Ob and M–Ot bonds (M¼W and Ta, Ob and Ot represent
terminal oxygen atoms and bridging oxygen atoms) might be due to
the strong interaction between protons and the polyanions.
3.3. Solution properties
The resulting yellow crystals of H-1 are highly soluble in water,
and soluble in organic solvent, such as acetone, methanol, ethanol,
and acetonitrile, but insoluble in dichloromethane and chloroform.
To testify the solution stability of H-1, 31P NMR spectra were taken
in D2O and CD3CN respectively. The 31P NMR spectra of H-1 in D2O
showed two singlets at ꢂ11.2 and ꢂ14.1 ppm with 1:1 ratio (Fig. 3a),
which is similar to that of the precursor 1 (ꢂ11.4, ꢂ14.4 ppm) [17].
The lower-field singlet at ꢂ11.2 ppm was assigned to the P atom
closest to the Ta side. Moreover, adjusting the pH of the solution from
0.1 to 6.0 by adding NaOH solution had no influence to the 31P NMR
spectra of H-1. H-1 displayed similar 31P NMR spectrum in CD3CN
(Fig. 3b), the higher-field signal of H-1 shifted to a much lower field
(ꢂ13.0 ppm), whereas the lower-field singlet only little shifted to a
higher field (ꢂ11.6 ppm). Ta/W mixed addenda POM possess strong
tendency to oligomerize under acidic conditions by the formation of
stable Ta–O–Ta bonding motifs, therefore, H-1 is extremely stable in
both acid aqueous solution and organic solution. The UV/vis spectra
of H-1 in aqueous solution appear at 205.6 nm and 280.1 nm (Fig.
Fig. 1. Ball-and-stick representation of H-1 in a unit cell, the aqua spheres re-
present lattice water molecular.

4
S. Li et al. / Journal of Solid State Chemistry 243 (2016) 1–7
ꢂ
Fig. 5. Electrocatalysis reduction of NO2 in the presence of 1.0 mM H-1 in a pH
4.7 medium (1.0 M NaACþHAC), at a scan rate of 100 mV sꢂ1, containing NO2
ꢂ
concentrations of 0.0 mM (a), 2.0 mM (b), 6.0 mM (c), and 8.0 mM (d). The working
electrode was glassy carbon and the reference electrode was Ag/AgCl.
reservoirs for multielectron reductions. The electron-enriched H-
1 is expected to have good catalytic activity. As shown in Fig. 5,
with the addition of nitrite, all the reduction peak currents in-
crease and the corresponding oxidation peak currents decrease
dramatically, which indicates that the reduced species of the H-1
show good electrocatalytic activities toward the reduction of
nitrite.
Fig. 3. 31P NMR spectra of H-1 in D2O (a) and CD3CN (b).
S8), corresponding to the charge transfer of Ot –W/Ta and Ob/c–W/Ta
respectively [24].
3.4. Electrochemistry
3.5. Proton-conducting performance
The electrochemical behaviors of H-1 were studied in 1.0 M
NaAC þ HAC (pH 4.7) aqueous solution at room temperature. The
obtained cyclic voltammograms (CVs) of H-1 at different scan rates
are shown in Fig. 4a. H-1 showed two reversible redox processes
I-I′, II-II’ at E1/2¼ ꢂ914.5 mV, ꢂ1072.5 mV arisen from the W
centers [25]. Taking the anodic (I) and cathodic peak currents (I′)
as representative, at scan rates lower than 200 mV sꢂ1 the peak
currents increased linearly with the increase of the square root of
scan rate, which indicates the redox process is limited by the
diffusion of H-1 to the electrode surface (Fig. 4b).
AC impedance measurements indicated that H-1 is good solid
state proton conducting materials at room temperature and even
at low humidity. At room temperature (25 °C) and 30% relative
humidity (RH), the conductivity value of H-1 is 7.2 ꢁ 10ꢂ3 S cmꢂ1
,
which is superior to most POMs compounds reported recently
[27–29], and comparable with the Dawson-type vanadium sub-
stituted HPAs H9P2W15V3O62 ꢀ 28H2O [30]. With the increase of RH,
the conductivity of H-1 at 25 °C is gradually enhanced, and reaches
to 5.0 ꢁ 10ꢂ2 S cmꢂ1 at 98% RH (Fig. S9).
To account for the temperature influence in proton conduction,
the conductivity of H-1 at different temperature (30, 45, 60, 75 and
95 °C, Fig. S10) was also studied under 30% RH. As shown in the
The electrocatalytic reduction of nitrate remains a challenge in
the NOx series because a complete process requires several
electrons [26]. High nuclear POMs can serve as powerful electron
Fig. 4. a: Cyclic voltammogram of H-1 in 1.0 M NaACþHAC of pH¼4.7 at different scan rates (from inner to outer: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200 mV sꢂ1). b: The
relationship of the scan rates vs the oxidation peak currents of I and reduction peak currents of I′.

S. Li et al. / Journal of Solid State Chemistry 243 (2016) 1–7
5
Fig. 6. Nyquist plots of proton conduction for H-1 at 30 °C (a), 45 °C (b), and 60 °C (c) under 30% RH. And the Arrhenius plot of proton conductivity of H-1 (d).
black line of Fig. 6, the conductivity of H-1 increases with the in-
creasing of temperature: 7.1 ꢁ 10–3 S cmꢂ1 at 30 °C (Fig. 6a),
2.2 ꢁ 10–2 S cmꢂ1 at 60 °C (Fig. 6b) and 7.2 ꢁ 10ꢂ2 S cmꢂ1 at 95 °C
(Fig. 6c). Proton transition within the hydrogen bonding network
of H-1 may be accelerated at higher temperature, thus increase the
conductivity. The activation energy calculated from the Arrhenius
plot is 0.39 eV (Fig. 6d), suggesting that Grotthuss mechanism is
dominant in the proton transfer [31]. This result is consistent with
the existence of abundant hydrogen bonds and the hydrogen
bonding network in H-1. Solid HPAs usually contain abundant
lattice water molecules, among which hydrogen-bond networks
for proton conducting can form. Moreover, solid HPAs possess
extremely high proton mobility, so can serve as promising solid
state proton conducting materials [32]. There are 125 lattice water
molecular and 20 Hþ in H-1. Moreover, abundant hydrogen bonds
exist among the crystalline water molecules, the coordinated
water molecules, and partial oxygen atoms of the polyanion (Figs.
S2–S4). These hydrogen bonds extend around to make a three
dimensional hydrogen bonding network in H-1, which is the po-
tential channel and may be the main cause for proton conduction.
Furthermore, the structure type and large size of the Ta/W mixed
addenda polyanion of H-1 may also contributive to the excellent
conductivity. This inspires us to find more mixed addenda HPAs
with different structures and sizes and investigate their proton-
conducting performance, by which searching for a general struc-
ture-function relationship.
3.6. Acidity and acid catalysis
Acidity measurements of H-1 were performed according to
the procedure described in the literature and the detailed
results are shown in SI-5 [33]. Generally, the acidity of HPAs of
the same structure decreases as the number of protons in-
crease, for example, the acidity of Keggin-type HPAs:
H3PW12
O
40 4H4SiW12
O
40 4H4GeW12
O
40 4H5BW12O40
.
The
acidity of known Dawson HPAs series are relatively higher
than that of Keggin series, for example, the Hammett acidity
constant H0 of H3PW12O40 and H6P2W18O62 are ꢂ2.14 and
ꢂ2.77 respectively [12,34]. Unfortunately, HPAs and acidity
with novel structures other than classical Keggin or Dawson
types are scarce reported. The Hammett acidity constant H0 of
H-1 in CH3CN, using dicinnamalacetone as Hammettin dicator
is ꢂ2.91, which is the strongest among present known HPAs
(shown in Table 2).
To evaluate the acid catalytic properties of H-1, acetal reaction
between benzaldehyde and five types of alcohols were in-
vestigated; the results are listed in Table 3. Compared with other
reported HPAs [34,35], H-1 showed significantly higher catalytic
activity for the reactions involving methyl alcohol, ethyl alcohol,
isopropanol and ethanediol. The reaction conversion involving
methyl alcohol, n-butyl alcohol and ethanediol were higher than
80%. But ethyl alcohol, 2-butyl alcohol, and tertiary butanol pro-
vided relatively low conversion.
The reaction of benzaldehyde and ethanediol was chosen to
compare the catalytic activity of H-1 with other HPAs. As shown in

6
S. Li et al. / Journal of Solid State Chemistry 243 (2016) 1–7
Table 2
Table 4
Acidity of HPAs obtained from UV–vis spectra with
The reactions of benzaldehyde and ethanediol catalyzed by different HPAs.a
Hammettin dicator.
c
HPAs
Conversion (%)
TONb
TOF (hꢂ1
)
HPAs
-H0
1-A
1
83.5
9.2
4810
527
1603
176
H-1
2.91
H6P2W18O62
H4S2W18O62
2.77 [12]
2.70 [31]
H6[P2W18O62
]
32.9
26.9
1900
1552
633
517
H3[PW12O40
]
H8[Ti2{P2W15O54(OH2)2}2] 31H2O 2.72 [12]
a
H3PW12O40
H4SiW12O40
H5BW12O40
2.14 [29]/2.13a
Reaction conditions: benzaldehyde 3.3 mmol, ethanediol 25 mmol, HPAs
0.17 mol%, room temperature, 3 h.
1.98 [34]
1.55 [34]
b
TON¼mol of product/mol of HPAs.
c
TOF¼mol of product/(mol of HPAs ꢀ time (h))
a
Tested value in our experiments.
negative charge may endow the protons relatively independent. This
may be the primary cause for the stronger acidity and acid catalytic
activity of H-1 comparing with other traditional HPAs.
Table 4, at room temperature, H-1 showed a significantly higher
catalytic activity than the traditional HPAs H6[P2W18O62],
H3[PW12O40], and H4[SiW12O40] (32% yield at room temperature,
24 h) [35].
It had been demonstrated that the catalytic activity was corre-
lated with both the strong Brønsted acid strength and the softness of
the anion in homogeneous Heteropolyacid system [35,36]. The soft-
ness of the polyanions in H-1 resulting from the large size and large
4. Conclusion
A new HPAs (H-1) which comprises a Ta/W mixed addenda
heteropolyanion [P8W60Ta12(H2O)4(OH)8O236
]
20ꢂ, 20 protons, and
Table 3
Acetal reactions of benzaldehyde and different alcohols with H-1 as catalyst.a
c
Entry
1
Reagent
Product
Conversion (%)
84
TONb
4828
TOF (hꢂ1
3218
)
Methyl alcohol
2
Ethyl alcohol
37
2126
1418
3
4
Propyl alcohol
Isopropanol
77
59
4425
3391
2950
2261
5
n-Butyl alcohol
81
4655
3103
6
7
Isobutanol
75
29
4310
1667
2874
1111
2-Butyl alcohol
8
9
Tertiary butanol
Ethanediol
25
83
1437
4770
958
3180
a
Reaction conditions: benzaldehyde 3.3 mmol, alcohol 25 mmol, H-1 10 mg, 90 min. The reaction temperatures are 100 °C for entry 5 and 9, and refluxing temperatures
for entry 1, 2, 3, 4, 6, 7, and 8.
b
TON¼mol of product/mol of HPAs.
c
TOF¼mol of product/(mol of HPAs ꢀ time (h))

S. Li et al. / Journal of Solid State Chemistry 243 (2016) 1–7
7
125 crystalline water molecules has been prepared and re-
presented. AC impedance measurements indicate that H-1 is a
good solid state proton conducting materials at room temperature
with conductivity value of 7.2 ꢁ 10ꢂ3 S cmꢂ1 (25 °C, 30% RH).
Hammett acidity constant H0 of H-1 in CH3CN is ꢂ2.91, which is
the strongest among present known HPAs. Relatively, H-1 exhibits
excellent catalytic activities toward acetal reaction. The present
work offers a new approach to the development of novel HPAs,
and indicates that Ta/W mixed addenda HPAs are a potential
subclass of acid catalysts. Our future work will be extended to
exploring more Ta/W or Nb/W mixed addenda HPAs and relevant
acid catalysis studies.
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of Göttingen, Göttingen Germany, 1997.
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This work was supported by the National Natural Science
Foundation of China (Grant nos. 21401047, 21301010, and
21371051), Science and Technology Key Project of Education De-
partment of Henan Province (Grant no. 14A150019).
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found in
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