Effect of metal-substitution on the redox behaviors of mono-transition metal-substituted Wells-Dawson tungstoarsenates

Article abstract of DOI:10.1016/j.catcom.2015.02.030

Mono-transition metal (Mo, V, and Nb)-substituted Wells-Dawson tungstoarsenates were investigated to elucidate the effect of metal-substitution on their redox behaviors. In the electrochemical analysis, an additional redox transition was observed for moly

Full text of DOI:10.1016/j.catcom.2015.02.030

Catalysis Communications 65 (2015) 66–71
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Effect of metal-substitution on the redox behaviors of mono-transition
metal-substituted Wells–Dawson tungstoarsenates
a
a
a
a
b
a,
Jung Ho Choi , Tae Hun Kang , Yongju Bang , Jong Kwon Lee , Jae Chun Song , In Kyu Song ⁎
a
School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-Gu, Seoul 151-744, South Korea
Hanwha Chemical Corp., Daejeon 305-804, South Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Mono-transition metal (Mo, V, and Nb)-substituted Wells–Dawson tungstoarsenates were investigated to eluci-
date the effect of metal-substitution on their redox behaviors. In the electrochemical analysis, an additional redox
transition was observed for molybdenum- and vanadium-substituted tungstoarsenates, while a negatively
shifted redox transition was observed for niobium-substituted tungstoarsenate. First electron reduction potential
of the catalysts showed the consistent trend with UV–visible absorption edge energy of the catalysts. Oxidative
dehydrogenation of benzylamine to dibenzylimine was carried out as a model reaction to probe oxidation
catalysis. Yield for dibenzylimine increased with increasing first electron reduction potential and with decreasing
UV–visible absorption edge energy.
Received 5 December 2014
Received in revised form 25 February 2015
Accepted 26 February 2015
Available online 27 February 2015
Keywords:
Redox behavior
Tungstoarsenate
Cyclic voltammetry
© 2015 Elsevier B.V. All rights reserved.
UV–visible spectroscopy
Oxidative dehydrogenation of benzylamine
1
. Introduction
Heteropolyacids (HPAs) are polymeric metal–oxygen clusters that
visible spectroscopy measurement was also conducted as a simple diag-
nostic for electronic structure. Gas-phase oxidative dehydrogenation of
benzylamine was carried out to track the oxidation catalysis of mono-
exhibit various physicochemical properties. HPAs are structurally
well-defined than any other metal oxides and their physicochemical
properties can be easily tuned by changing the constituent elements.
This means that the design of HPAs is possible at molecular level to
meet the need for specific catalytic applications.
transition metal-substituted Wells–Dawson tungstoarsenates.
2. Experimental
2.1. Preparation of catalyst
HPAs are strong oxidizing agents or excellent redox catalysts be-
cause the addenda atoms in heteropolyanion frameworks are in their
highest oxidation states. Thus, several structural classes of HPAs have
been investigated as catalysts in the oxidation reactions and photocata-
lytic redox reactions [1–3]. Previous researches [4,5] on the oxidative
dehydrogenation of alcohols over HPAs have suggested that the
redox transition of HPAs occurs during the catalysis and the rate-
determining step involves the reduction of HPAs by hydrogen and
electron from the substrate. It is obvious that systematic researches on
the redox nature of HPAs should be preferentially conducted before
the catalytic applications. However, much progress has not been made
on the redox nature of Wells–Dawson HPAs due to the structural
diversity of mixed-addenda HPAs [6].
α-K
were prepared as described in the previous literature [7]. α
As 17Nb 62 was prepared by direct incorporation of niobium
into mono-lacunary tungstoarsenate (α -K10As 61). For this,
6 2 18 2 6 2 1 2 7 2 17 1 62
As W O62, α -K As W17Mo O62, and α -K As W V O
2
-
K
7
2
W
1
O
2
2 17
W O
0.4 g of oxalic acid (Sigma-Aldrich) was dissolved in 30 ml of deionized
water. After complete dissolution, 0.110 g of niobium chloride (Sigma-
Aldrich) was added to the solution. 1.70 g of mono-lacunary
tungstoarsenate, which was prepared as described in the previous
work [7], was separately dissolved in 20 ml of deionized water. These
two solutions were then mixed. The mixed solution was treated with
hydrochloric acid (Samchun Chem.) until the pH of the solution
becomes ca. 1.0 and it was refluxed for 4 h. The resulting transparent so-
lution was cooled to room temperature. The solution was added to
300 ml of methanol (Sigma-Aldrich) and the resultant was placed at
4 °C overnight. After a few days, white precipitate was collected and
In this work, mono-transition metal (Mo, V, and Nb)-substituted
Wells–Dawson tungstoarsenates were prepared to elucidate the effect
of transition metal-substitution on their redox behaviors. Redox behav-
iors of the catalysts were examined by electrochemical analysis. UV–
2 7 2 1 62
dried under room temperature to obtain α -K As W17Nb O .
Formation of Wells–Dawson heteropolyanion framework was
confirmed by diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopy using a Nicolet 6700 (Nicolet) spectrometer. Chemical
⁎
Corresponding author.
E-mail address: inksong@snu.ac.kr (I.K. Song).
http://dx.doi.org/10.1016/j.catcom.2015.02.030
566-7367/© 2015 Elsevier B.V. All rights reserved.
1

J.H. Choi et al. / Catalysis Communications 65 (2015) 66–71
67
compositions of the catalysts were analyzed by inductively coupled
plasma-atomic emission spectroscopy (ICP-AES) analysis using
an ICPS-1000IV (Shimadzu) instrumentation. In this work, mono-
transition metal-substituted tungstoarsenates were denoted as denoted
As W W
1
2
17
as As
.2. Characterization
Electrochemical analysis was conducted to examine the electro-
2
W
17
M
1
(M = W, Mo, V, and Nb).
As W Mo
2 17 1
2
M=O
t
As-O
As W V
1
2
17
M-O-M
chemical redox behaviors of mono-transition metal-substituted
tungstoarsenates. Cyclic voltammograms were acquired using a con-
ventional three-electrode system (Autolab 302N, Eco Chemie). Glassy
carbon with a diameter of 3.0 mm, platinum rod, and saturated calomel
electrode (KCl saturated) were used as a working electrode, a counter
electrode, and a reference electrode, respectively. Each tungstoarsenate
was dissolved in 0.5 M sodium sulfate electrolyte to prepare the sample
solution (1 mM). The sample solution was pretreated with He flow prior
to electrochemical measurements. Cyclic voltammograms were obtain-
ed at a scan rate of 25, 50, 100, 150, and 200 mV/s.
As W Nb
1
2
17
1
200
1100
1000
900
800
700
-
1
Wavenumber (cm )
UV–visible spectroscopy measurement was performed as
a
Fig. 1. DRIFT spectra of mono-transition metal-substituted tungstoarsenates.
simple diagnostic for electronic structure of mono-transition metal-
substituted tungstoarsenates. UV–visible spectra were acquired using
a Lambda-35 (Perkin-Elmer) spectrometer. The sample solution
(
1 mM) was prepared by dissolving each tungstoarsenate in deionized
water to minimize the peripheral effect of coordinated water.
Kubelka–Munk function (F(R )) was employed to convert reflectance
spectrum into equivalent absorption spectrum using BaSO as a white
6
via selective removal of WO unit of cap site. Thus, transition metal
could be selectively incorporated in the cap site of Wells–Dawson
tungstoarsenate framework, providing the uniform structure to exam-
ine the effect of transition metal-substitution (in the cap site) on the
redox behavior.
∞
4
standard [8]. Absorption edge energy was directly obtained from Tauc
plot.
3.2. Cyclic voltammetry
2
.3. Catalytic test
Electrochemical analysis was conducted to examine the electro-
Gas-phase oxidative dehydrogenation of benzylamine was carried
chemical redox behaviors of mono-transition metal-substituted
tungstoarsenates. Fig. 2 shows the cyclic voltammograms of mono-
transition metal-substituted tungstoarsenates obtained at a scan rate
of 50 mV/s. All the catalysts exhibited reversible and stepwise redox
transitions. Overall shapes of cyclic voltammograms were nearly un-
changed even after several cycles of measurements, indicating that the
catalysts were hydrolytically stable during the electrochemical mea-
surements. Each peak current monotonically increased against the
square root of scan rate, indicating that redox transitions shown in
Fig. 2 were the diffusion-controlled reversible process that could be
expressed by Randles–Sevcik equation [9].
out over mono-transition metal-substituted tungstoarsenates. 0.4 g of
each catalyst was charged into the tubular quartz reactor and it was
pretreated with a mixed stream of nitrogen (30 ml/min) and oxygen
(
10 ml/min) at 300 °C for 1 h. Benzylamine (7.2 mmol/h) was sufficient-
ly vaporized by passing through the preheating zone and it was contin-
uously fed into the reactor together with a mixed stream of nitrogen
and oxygen. Catalytic reaction was carried out at 300 °C for 5 h. Reaction
products were periodically sampled and were analyzed using a gas
chromatograph (YL6100 GC, Younglin) equipped with a flame ioniza-
tion detector. DB-5 (Agilent, 60 m × 0.32 mm) capillary column was
used for products separation. Conversion of benzylamine and yield for
each product were calculated on the basis of carbon balance.
2 17 1
As W W exhibited four stepwise tungsten-centered redox
couples. However, significantly different redox transitions were
observed for mono-transition metal-substituted tungstoarsenates. For
molybdenum- and vanadium-substituted tungstoarsenates, they exhib-
ited an additional molybdenum- or vanadium-centered redox couple at
higher potential region [10,11]. This indicates that molybdenum and
vanadium centers in the tungstoarsenate frameworks are electrochem-
ically more accessible and easier to be reduced than tungsten center.
However, niobium-substituted tungstoarsenate did not exhibit any
additional redox couple but showed a negatively-shifted redox couple
3
. Results and discussion
3
.1. Formation of heteropolyanion framework
Fig. 1 shows the DRIFT spectra of mono-transition metal-substituted
tungstoarsenates. Several characteristic bands attributed to asymmetric
vibrations of M (metal) = O
(terminal oxygen), As–O, and M–O–M
t
−
1
bonds were observed in the range of 1000–700 cm , in good
agreement with the results of previous literature [7]. For all catalysts,
As–O bands appeared as a weak shoulder due to the intense and
broad M–O–M bands. All the mono-transition metal-substituted
tungstoarsenates exhibited slightly shifted bands compared to non-
2 17 1
compared to non-substituted one (As W W ). In this work, first elec-
tron reduction potential was taken as a representative parameter for re-
ducibility [12]. First electron reduction potential increased in the order
of As
(+0.243 V) b As
2
W
17Nb
1
(−0.162 V) b As
2
2 17 1 2 1
W W (+0.049 V) b As W17Mo
(+0.378 V), indicating that vanadium center
17 1
W V
substituted one (As
sten, and substituted-transition metal in the catalysts determined by
ICP-AES analyses were in good agreement with the designed values
2
W
17
W
1
). Chemical compositions of arsenic, tung-
was more effective to enhance the reducibility of tungstoarsenate
framework than the other metal centers.
(
not shown here), supporting the successful formation of mono-
3.3. UV–visible spectroscopy
transition metal-substituted tungstoarsenates. In this work, mono-
transition metal-substituted tungstoarsenates were prepared by direct
incorporation of transition metal into mono-lacunary tungstoarsenate
Previous research [13] has suggested that energy state and composi-
tion of the lowest unoccupied orbital (LUMO) play important roles in
determining the redox behaviors of heteropolyanions. It is obvious
1
0−
2 2 17 6 2 18 62
(α -K10As W O61 ), which was derived from α-K As W O

68
J.H. Choi et al. / Catalysis Communications 65 (2015) 66–71
7
6
1
1
2
0
8
6
4
2
As
W
W
As W Mo
2
17
1
2
17
1
5
4
3
2
4
6
8
10
12
14
16
4
6
8
10
12
14
16
Square root of scan rate
Square root of scan rate
+
0.049 V
+0.243 V
2 17 1
As W W
(
a)
(b)
1
.0
0.5
0
-0.5
-1.0
1.0
0.5
0
-0.5
-1.0
Potential (volts vs. SCE)
Potential (volts vs. SCE)
8
7
6
5
4
3
2
14
12
10
8
As W V
As W Nb
2 17
2
17
1
1
6
4
2
4
6
8
10
12
14
16
4
6
8
10
12
14
16
Square root of scan rate
Square root of scan rate
-
0.162 V
+
0.378 V
2 17 1
As W W
2 17 1
As W W
(
c)
(d)
1
.0
0.5
0
-0.5
-1.0
1.0
0.5
0
-0.5
-1.0
Potential (volts vs. SCE)
Potential (volts vs. SCE)
Fig. 2. Cyclic voltammograms of mono-transition metal-substituted tungstoarsenates obtained at a scan rate of 50 mV/s (each inset figure shows the peak current plotted as a function of
square root of scan rate (for the first electron reduction)).
that energy state of the LUMO strongly depends on the nature
of substituted-metal, because d-orbitals on the metal centers largely
contribute to the LUMO of heteropolyanion. On the other hand,
energy state of the highest occupied molecular orbital (HOMO) is
little perturbed by metal-substitution, because the HOMO of
heteropolyanion mostly comprises 2p-orbitals on the framework
oxygens.
UV–visible spectroscopy measurement was performed as a simple
diagnostic for electronic structure of mono-transition metal-
substituted tungstoarsenates. Fig. 3(a) shows the Tauc plots of mono-
transition metal-substituted tungstoarsenates for indirect-allowed
transition [14]. It is interesting to note that absorption edge energy,
which corresponded to the energy gap between the HOMO and the
LUMO, decreased in the order of As
2
W
17Nb
1
(3.01 eV) N As
2 17 1
W W
(2.97 eV) N As 17Mo (2.90 eV) N As
2
W
1
W V
2 17 1
(2.36 eV). This indicates
that vanadium center is the most effective to stabilize the energy level
of the LUMO. Such stabilization effect by vanadium center has also
been reported in the previous density functional theory (DFT) calcula-
tions [15]. It has been successfully demonstrated that the energy of
tungsten center at cap sites is effectively stabilized by vanadium-
substitution and the added electrons are preferentially delocalized
over three vanadium centers in the reduced species. In this work, the
2
2
2
2
3
3
.2
.4
.6
.8
.0
.2
(
b)
2 17 1
As W V
(
a)
2 17 1
As W V
As
As
2
W
17Mo
17
1
2
W
W
1
2 1
As W17Mo
2 17 1
As W W
2 1
As W17Nb
2
.0
2.4
2.8
3.2
3.6
-0.2
0
0.2
0.4
Photon energy (eV)
First electron reduction potential
volts vs. SCE)
(
Fig. 3. (a) Tauc plots of mono-transition metal-substituted tungstoarsenates for indirect-allowed transition and (b) a correlation between first electron reduction potential and absorption
edge energy.

J.H. Choi et al. / Catalysis Communications 65 (2015) 66–71
69
effect of vanadium- or molybdenum-substitution on the energy state of
the LUMO may be understood in the similar manner. The niobium
center, which exhibits greater predisposition than vanadium or molyb-
denum center to lose its electrons (hard to be reduced), does not stabi-
lize the energy level of the LUMO [15]. This trend was also consistently
observed in a series of tri-transition metal (Mo, V, and Nb)-substituted
Keggin tungstosilicates [13], indicating that metal-substitution showed
the analogous effect on the redox behavior of heteropolyanion even in
the different structures.
Fig. 3(b) shows the correlation between first electron reduction
potential and absorption edge energy. Reduction potential increased
with decreasing absorption edge energy. Although the effect of metal-
substitution on the electronic structure is much complicated due to
the presence of oxygen contribution to the LUMO, the correlation clearly
shows that energy state of the LUMO depends on the nature of
substituted metal. It also suggests that absorption edge energy can be
utilized as an alternative parameter to estimate energy level of the
LUMO and reduction potential of heteropolyanions in the series of
mono-transition metal-substituted Wells–Dawson tungstoarsenates.
shows the reaction pathway in the oxidative dehydrogenation of
benzylamine over HPAs [16]. Dibenzylimine, benzonitrile, and benzal-
dehyde can be formed from the dehydrogenated intermediate
(phenylmethanimine). Unstable phenylmethanimine reacts with
another molecule of benzylamine to form aminal and then releases
ammonia to form dibenzylimine. Benzonitrile can be formed by further
oxidative dehydrogenation of phenylmethanamine. Benzaldehyde can
be produced by hydrolysis of phenylmethanimine.
In this work, dibenzylimine was mainly formed over the catalysts.
Trace amounts of benzonitrile and benzaldehyde were also detected
during the reaction. It is noteworthy that none of them was detected
without oxygen, supporting that dibenzylimine, benzonitrile, and benz-
aldehyde were formed via oxidative dehydrogenation reaction. It was
also found that benzylamine readily reacted with benzaldehyde
to form dibenzylimine even at room temperature, indicating that as-
produced benzaldehyde could react with benzylamine to form
dibenzylimine. Fig. 5(a) shows the catalytic performance of As
2 17 1
W W
during a 5 h-reaction. Fig. 5(b) shows the conversion of benzylamine
and product yield after a 5 h-reaction. All the catalysts exhibited a stable
catalytic performance during a 5 h-reaction. Yield for dibenzylimine
3
.4. Gas-phase oxidative dehydrogenation of benzylamine
increased in the order of As
2
W
17Nb
1
b As
2
W
17
W
1
b As
2
W
17Mo
1
b
2 17 1
As W V .
Gas-phase oxidative dehydrogenation of benzylamine was carried
Fig. 6 shows the DRIFT spectra of fresh and used As W W catalysts.
2 17 1
out over mono-transition metal-substituted tungstoarsenates. Fig. 4
Although slight changes in band position and signal intensity were
H O
2
NH
2
½
O2
*
HPAnn-
HPAn(n+2)-
Benzylamine
H O
2
NH
½
O2
HPAnn-
O
- NH3
HPAn(n+2)-
Phenylmethanimine
unstable)
(
Benzaldehyde
-
NH3
N
+
Benzylamine
-
H O
2
+
Benzylamine
N
Benzonitrile
*
HPAn: heteropolyanion
Dibenzylimine
Fig. 4. Reaction pathway in the oxidative dehydrogenation of benzylamine over HPAs [16].

70
J.H. Choi et al. / Catalysis Communications 65 (2015) 66–71
1
00
100
80
60
40
20
0
100
90
Conversion of benzylamine
Yield for dibenzylimine
Yield for benzonitrile
(
a)
(b)
As W17W
2
1
8
6
4
2
0
0
0
0
0
80
Yield for benzaldehyde
Yield for other products
7
0
60
5
0
40
3
0
20
1
0
0
0
60
120
180
240
300
As
2
W17Nb
1
As
2
W
17
W
1
As
2
W17Mo
1
As W V
2 17 1
Time on stream (min)
2 17 1
Fig. 5. (a) Catalytic performance of As W W during a 5 h-reaction and (b) conversion of benzylamine and product yield after a 5 h-reaction.
observed due to the loss of water of crystallization, characteristic bands
of used catalyst corresponding to W = O , As–O, and W–O–W were
4. Conclusions
t
maintained as observed for fresh catalyst. This result suggests that
Wells–Dawson tungstoarsenate was thermally stable during the reac-
tion at 300 °C. The other catalysts also showed similar results in the
DRIFT measurements.
Fig. 7 shows the first electron reduction potential and UV–visible ab-
sorption edge energy plotted as a function of yield for dibenzylimine
after a 5 h-reaction. Yield for dibenzylimine increased with increasing
first electron reduction potential and with decreasing UV–visible ab-
A series of mono-transition metal-substituted tungstoarsenates
were prepared and their redox behaviors were investigated by electro-
chemical analysis and UV–visible spectroscopy measurement.
For molybdenum- and vanadium-substituted tungstoarsenates, they
exhibited an additional molybdenum- and vanadium-centered
redox couples at higher potential region. However, niobium-
substituted tungstoarsenate showed a negatively-shifted redox couple
2 17 1
compared to non-substituted one (As W W ). Among the catalysts,
sorption edge energy. Among the catalysts, As
2
W
17
V
1
(which was easier
vanadium center was more effective to enhance the reducibility of
tungstoarsenate framework than any other metal centers. The results
of UV–visible spectroscopy measurements revealed that energy
state of the unoccupied orbitals, which depended on the nature of
substituted transition metal, was closely related to redox behaviors of
heteropolyanions. In the gas-phase oxidative dehydrogenation of
benzylamine, yield for dibenzylimine (oxidation product) increased
with increasing first electron reduction potential and with decreasing
UV–visible absorption edge energy. It was successfully demonstrated
that first electron reduction potential and UV–visible absorption edge
energy could be utilized as alternative parameters to estimate the cata-
lytic activity of mono-transition metal-substituted tungstoarsenates in
the oxidative dehydrogenation of benzylamine.
to be reduced than the other catalysts) showed the highest yield for
dibenzylimine. Previous researches [4,5] have suggested that hydrogen
and electron of reactant are transferred to reducible metal center and
the rate-determining step involves the reduction of HPAs by hydrogen
and electron in the oxidative dehydrogenation reactions. Therefore, it
is inferred that the substituted vanadium center provided efficient
active site for oxidative dehydrogenation by contributing to the density
of accessible electronic states (unoccupied orbitals). We have
demonstrated that reducibility of mono-transition metal-substituted
tungstoarsenates plays an important role in determining the catalytic
activity in the oxidative dehydrogenation of benzylamine.
2
2
2
2
3
.2
.4
.6
.8
.0
0
0
.4
.2
0
Fresh As W W
1
As W V
2 17 1
2
17
Used As W W
1
2
17
-
0.2
0.4
As W17Mo1
2
As W17W1
2
As W17Nb1
2
-
2
8
32
36
40
44
1
200
1100
1000
900
800
700
Yield for dibenzylimine after a 5 h-reaction (%)
-
1
Wavenumber (cm )
Fig. 7. First electron reduction potential and UV–visible absorption edge energy plotted as
Fig. 6. DRIFT spectra of fresh and used As
W W
2 17 1
catalysts.
a function of yield for dibenzylimine after a 5 h-reaction.

J.H. Choi et al. / Catalysis Communications 65 (2015) 66–71
71
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Acknowledgments
This work was supported by the Mid-career Researcher Program of
National Research Foundation (NRF) grant funded by the Korea govern-
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