Reduced copper salt of Wells-Dawson type heteropolyacid as a bifunctional catalyst

Article abstract of DOI:10.1016/j.cattod.2011.02.068

Copper salt of Wells-Dawson type heteropolyacid when reduced in ethanol vapors at temperatures 210-290 °C becomes active bifunctional catalyst for ethanol conversion. In oxygen free atmosphere (helium), both water and ethylene are formed predominantly. Other side products include small amount of diethyl ether and traces of acetaldehyde. When air is used as a carrier gas instead of oxygen free helium, acetaldehyde, typical product of ethanol dehydrogenation, forms on redox centers. Simultaneously, the formation of ethylene and diethyl ether, the products of ethanol dehydration occurring on Br?nsted acid centers, is observed.

Full text of DOI:10.1016/j.cattod.2011.02.068

Catalysis Today 169 (2011) 150–155
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Catalysis Today
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Reduced copper salt of Wells–Dawson type heteropolyacid as a bifunctional
catalyst
Urszula Fileka,∗, Elz˙ bieta Bielan´ skaa, Robert P. Sochaa, Adam Bielan´ skia,b
a Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Krakow, Poland
b Faculty of Chemistry, Jagiellonian University, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2010
Received in revised form 22 February 2011
Accepted 23 February 2011
Copper salt of Wells–Dawson type heteropolyacid when reduced in ethanol vapors at temperatures
210–290 ◦C becomes active bifunctional catalyst for ethanol conversion. In oxygen free atmosphere
(helium), both water and ethylene are formed predominantly. Other side products include small amount
of diethyl ether and traces of acetaldehyde. When air is used as a carrier gas instead of oxygen free helium,
acetaldehyde, typical product of ethanol dehydrogenation, forms on redox centers. Simultaneously, the
formation of ethylene and diethyl ether, the products of ethanol dehydration occurring on Brønsted acid
centers, is observed.
Paper dedicated to the memory of Professor
Jerzy Haber.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Heteropolyacid
Copper salt
Ethanol conversion
Bifunctional catalyst
1. Introduction
conversion of ethanol on reduced silver salt of Dawson type het-
eropolyacid, Ag6P2W18O62. They showed that the catalyst obtained
Ono and Baba observed [1] that the silver, copper and nickel
salts of Keggin type heteropolyacids, when reduced with hydrogen,
became strong acidic catalysts active in methyl-tert-butyl ether
(MTBE) synthesis conducted in the vapor phase. The activation of
such salts includes silver cations reduction to metal clusters with
the simultaneous regeneration of free heteropolyacid (HPA) [2,3]
according to the equation:
by reduction of the salt with ethanol or methanol vapors contained
silver clusters dispersed over microcrystallites of H6P2W18O62. In
the absence of oxygen, this catalyst was active in gas-phase ethanol
dehydration to ethylene and/or diethyl ether. However, when oxy-
gen was supplied to the feed, not only the dehydration products
but also acetaldehyde – the product of ethanol dehydrogenation
– was obtained. The former reaction occurs typically on Brønsted
acid centers while the latter one on metal redox centers.
The aim of the present study was to investigate the behav-
ior of reduced copper salt of Dawson type heteropolyacid, which
was expected to exhibit bifunctional catalytic activity. It should
be observed that similar bifunctional catalytic systems containing
reduced transition metal atoms or clusters finely dispersed in the
crystal lattice of zeolites were also studied by a number of investiga-
tors [16]. However, our interest in studying the reduced transition
metal as catalysts stems from the fact that free heteropolyacids
exhibit much higher acidic strength than hydrogen forms of zeo-
lites.
Ag3PW12O40 + 1(1/2)H2 → 3Ag + H3PW12O40
(1)
Activated silver salts of Keggin type heteropolyacids was inves-
tigated for both conversion of methanol to hydrocarbons [4–6]
and 1-butene isomerization [7]. Recently, the research was focused
on the bifunctional catalysts based on noble metal (Pt, Pd) salts
of heteropolyacids of the Keggin type and/or acid cesium salts of
H3PW12O40. Their catalytic performance was tested among oth-
ers in skeletal isomerization of n-heptane [8,9] and n-butane [10],
as well as hydroizomerization of pentane [11], and hydration
of ethylene [12]. Copper salt of dodecamolybdophosphoric acid,
Cu3(PMo12O40)2, was found to be active and selective catalyst in
liquid phase oxidation of m-cresol [13] and oxidative dehydrogena-
tion of isobutyric acid [14]. Kasza and Bielan´ ski [15] investigated the
2. Experimental
Copper salt of diphosphooctadecatungstic acid, Cu3P2W18O62
was obtained by dissolving 2.9991 g of H6P2W18O62·26.0H2O in
ca. 10 cm3 of distilled water and slow addition of stoichiomet-
ric amount (0.3871 g) of powdered Cu(CH3COO)2·H2O (p.p.a.,
∗
Corresponding author.
E-mail address: ncfilek@cyf-kr.edu.pl (U. Filek).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.02.068

U. Filek et al. / Catalysis Today 169 (2011) 150–155
151
100
98
96
94
92
90
88
Sigma–Aldrich) under intensive mixing. The pale blue solution was
evaporated on water bath and dried in the oven at 50 ◦C for 12 h.
Subsequently, the precipitate was kept over saturated solution of
Mg(NO3)2 at room temperature.
Thermogravimetric analysis (TG) of the salt was carried out
within temperature range of 25–650 ◦C in air flow using Netzsch
STA 409PC Instrument Derivatograph. Heating rate 5◦/min was
applied.
Pristine unreducedand reduced copper saltsamples were exam-
ined using Jeol JSM 7500 field emission scanning microscope
equipped with energy dispersive X-ray (EDX) spectrometer.
The X-ray photoelectron spectroscopy (XPS) and X-ray Auger
excitation spectroscopy (XAES) were applied for the samples anal-
ysis with a hemispherical analyzer SES R4000 Gamma Data Scienta.
The FT-IR spectra of the samples were obtained with Bruker
Equinox 55 Spectrometer using KBr tablets.
5.8%
9.097%
200.95 o
C
9.39%
323 o
C
0
100
200
300
400
500
600
700
temperature [ºC]
Fig. 1. Thermogravimetric analysis of pristine heteropolyacid copper salt.
vapors over saturated solution of Mg(NO3)2 at room temperature.
It can be observed that the weight loss of the sample occurs in two
stages upon heating. The first one occurring between room temper-
ature and 201 ◦C is attributed to the departure of loosely bonded
water. It was estimated that in this stage sample looses 16.1 H2O
molecules per one heteropolyanion. The second stage observed
between approximately 202 and 400 ◦C characterized by almost
linear weight loss may be due to the departure of water coordina-
tively bonded to copper ions. Its amount was estimated as 9.9 H2O
molecules per one heteropolyanion. Hence, the total weight loss of
pristine sample corresponded to 26.0 mole of water per one mole
of heteropolyanions.
The catalyst reduction and catalytic tests were carried out in a
constant flow (35 cm3/min) microreactor connected on line with
a gas chromatograph. Helium or air, both saturated with ethanol
vapors at room temperature, was used as the carrier gas. The
amount of the copper salt used for the experiments was 0.063 g.
The construction of the apparatus enabled to control in situ the
sample reduction by analyzing the gas composition on outlet of the
microreactor and subsequent determination of the activity of the
reduced catalyst at reduction temperature or at other temperature
freely chosen.
3. Results and discussion
Infrared spectroscopic data of the unsupported HPA, pristine
copper salt and the sample reduced with ethanol at 250 ◦C are given
in Table 1. Both spectra exhibited four characteristic peaks of the
P2W18O626− ion [17]. The observed shifts of bands in the spectrum
Fig. 1 shows the TG curve of the sample of copper salt of
H6P2W18O62 heteropolyacid previously equilibrated with water
b
a
0,3
0,3
unreacted ethyl alcohol
unreacted ethyl alcohol
water
water
ethylene
diethyl ether
0,2
0,2
0,1
0,0
ethylene
diethyl ether
0,1
0,0
0
30
60
90
120
150
0
30
60
90
120
150
time [min]
time [min]
c
d
0,3
0,2
0,1
0,0
0,3
0,2
0,1
0,0
unreacted ethyl alcohol
water
ethylene
unreacted ethyl
alcohol
water
ethylene
diethyl ether
0
30
60
90
120
150
0
30
60
90
120
150
time [min]
time [min]
e
0,3
0,2
0,1
0,0
unreacted ethyl alcohol
water
ethylene
diethyl ether
0
30
60
90
120
150
time [min]
Fig. 2. Evolution of catalytic activity of Cu3P2W18O62 sample in the course of reduction period at 210 (a), 230 (b), 250 (c), 270 (d) and 290 ◦C (e).

152
U. Filek et al. / Catalysis Today 169 (2011) 150–155
Table 1
100
FTIR vibration frequencies for pristine Wells–Dawson heteropolyacid and copper
salt.
90
80
70
60
50
40
30
20
10
0
Vibration
type
H6P2W18O62
devoid of
Cu3P2W18O62
pristine [cm−1
Cu3P2W18O62 after
reduction (250 ◦C)
]
crystallization
[cm−1
]
water [cm−1
]
unreacted ethyl alcohol
water
P–Oa
1090
961
908
773
1091
957
914
783
1091
964
917
783
W
Od
ethylene
W–Ob–W
W–Oc–W
diethyl ether
acetaldehyde
of pristine copper salt were similar to those observed by Kasza and
Bielan´ ski [15] for silver salts.
As mentioned before, the reduction of the copper salt of Daw-
son type heteropolyacid was carried out in catalytic reactor with
saturated ethanol vapors mixed with oxygen free helium as carrier
gas. A series of reduction experiments were performed within the
temperature range of 210–290 ◦C for 150 min. The progress of this
activation was controlled by analyzing the gas products leaving the
reactor. Fig. 2a–e shows the examples of such experiments. In all
cases besides the main products, water and ethylene, the formation
of small amounts of diethyl ether as well as traces of acetaldehyde
were observed. The presence of ethylene and water as the prod-
ucts of ethanol dehydration that forms on the acid centers indicate
appearance of free heteropolyacid in the reduced salt At the begin-
ning of reduction the amount of unreacted ethanol changed rapidly
achieving constant value after 90–150 min. In the prolonged time
a certain deactivation (increase of amount of unreacted ethanol) of
the obtained catalyst was observed. This partial deactivation was
presumably due to the formation of coke on strong acid centers
of recrystallized heteropolyacid forming platelet-like crystallites
observed in the scanning microscope images.
The conversions of ethanol and selectivities to ethylene, water
and diethyl ether at reduction temperatures of 210–290 ◦C are pre-
sented in Fig. 3. It shows that the effect of reduction of copper salt of
Wells–Dawson type heteropolyacid was the weakest at reduction
temperature of 210 ◦C and increased with the temperature increas-
ing up to 270 ◦C. However, at temperature higher than 270 ◦C, the
decrease of the conversion of ethanol was observed. This might be
explained by the decomposition of heteropolyacid resulting from
slow loss of its constitutional water at this temperature.
0
20
40
60
80
100
time [min]
Fig. 4. Concentration of ethanol, diethyl ether, ethylene and acetaldehyde observed
for the catalyst sample prereduced for 2.5 h at 250 ◦C. Reaction temperature 250 ◦C,
oxygen free helium as a carrier gas (35 cm3/min).
reaction at 250 ◦C. It is seen that at these conditions ethanol is only
partially used and about 75–85 mol% of it remains in the gas.
The effect of reaction temperature (230–290 ◦C) on the ethanol
conversion and the selectivity of the catalysts prereduced at 250 ◦C
is shown in Fig. 5. It is noticed that in parallel to the increase
of ethanol conversion with temperature, the selectivity to ethy-
lene, the product of an endothermic reaction, increases as well.
On the other hand, the selectivity to diethyl ether, the product
of an exothermic reaction, decreases with an increase of reaction
temperature. Both reactions are typical for Brønsted acidity of the
catalysts. Acetaldehyde, the product of ethanol oxidative dehydro-
genation occurring on catalytic redox centers, was formed only in
small quantities and the selectivity of the catalyst to this prod-
uct was always below 1%. Some explanation should be added to
the difference between conversion of ethanol in the presence of
catalyst reduced at 290 ◦C and reaction performed at 290 ◦C with
copper salt prereduced at 250 ◦C. It is well known that the tem-
perature of reaction strongly affect the morphology, texture and
dispersion of reduced metal on the surface. Our results suggest
that in both cases, dispersion of Cu0 on H6P2W18O62 was to some
extent different. When the prereduced sample was used in reac-
tion carried out at 290 ◦C, initially formed Cu0 nanoparticles could
migrate on the surface, agglomerate improving the accessibility of
H+ in heteropolyacid. The higher acidity of the prereduced cata-
lyst may explain the higher selectivity to the products of ethanol
dehydration.
After reduction at 250 ◦C for 150 min the catalyst was tested at
temperature range of 230–290 ◦C in two series of experiments, one
carried out in ethanol vapor with oxygen free helium gas and the
other one conducted in the presence of air. Fig. 4 shows the com-
position (mol%) of reaction mixture registered during the former
Fig. 3. Ethanol conversion and selectivities to diethyl ether, ethylene and acetalde-
Fig. 5. Ethanol conversion on the sample prereduced at 250 ◦C and selectivity to the
hyde reached after 150 min of salt reduction in ethanol vapor at 210–290 ◦C.
particular products at 230–290 ◦C. Oxygen free helium gas used as a carrier gas.

U. Filek et al. / Catalysis Today 169 (2011) 150–155
153
100
90
80
70
60
50
40
30
20
10
0
unreacted ethyl alcohol
water
ethylene
diethyl ether
acetaldehyde
Fig. 7. The effect of the reaction temperature on the concentration of ethanol, ethy-
lene, water, acetaldehyde and diethyl ether obtained on the catalysts prereduced at
250 ◦C. Air saturated with ethanol vapor was used as carrier gas (35 cm3/min).
rapidly at the beginning of reaction and after 20 min or so reached
a constant value. At the beginning of catalytic test the acid–base
(ethylene and water formation) type was predominant process and
later on the share of redox reaction increased. Acetaldehyde, the
product of redox type reaction, became the main product. On the
other hand, the selectivities to ethylene and diethyl ether, the prod-
ucts of acid–base type reaction, were low, and the catalyst exhibited
very distinct bifunctional properties.
0
20
40
60
80
100
time [min]
Fig. 6. The concentration of ethanol, diethyl ether, ethylene and acetaldehyde
observed for the catalyst sample prereduced for 2.5 h at 250 ◦C. Reaction tempera-
ture of 250 ◦C, air saturated with ethanol vapor as a carrier gas (35 cm3/min).
The yield of acetaldehyde distinctly increased when the atmo-
spheric air was used as a carrier gas instead of oxygen free helium.
The results of the reaction carried out under such conditions (after
catalyst prereduction at 250 ◦C) are shown in Fig. 6. The activ-
ity of catalyst, which was first contacted with helium saturated
with ethanol vapors, and then with air containing ethanol, changed
The effect of reaction temperature on the conversion of ethanol
and selectivity to acetaldehyde, diethyl ether and ethylene is shown
in Fig. 7. This graph illustrates the situation when air was used as
carrier gas. At first, the selectivity to acetaldehyde increased with
the temperature, and then reached the maximum at 250 ◦C. The
further increase of temperature caused reduction of selectivity to
Fig. 8. SEM image (a) and EDX maps of tungsten (b) and copper (c) distribution in pristine sample of Cu3P2W18O62
.

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U. Filek et al. / Catalysis Today 169 (2011) 150–155
Fig. 9. SEM image (a) and EDX map of copper (b) and line scan of copper and tungsten distributions (c) of Cu3P2W18O62 reduced at 250 ◦C.
acetaldehyde in favor to ethylene. It can be seen that the temper-
ature rise results in the increase of ethanol conversion (increasing
the yield of products of acid–base type reaction) with almost con-
stant yield of acetaldehyde (around 23%).
copper for the reduced sample. This feature agrees well with EDX
analysis of the SEM images.
The analysis of Cu 2p core excitation (BE value and lack of shake-
up satellites) allows exclusion of Cu2+ presence at the surface layer
of both samples [18,19]. The values of Cu L3M45M45 excitation
maximum and the Auger parameter (˛ꢀ) obtained from XPS and
XAES spectra may indicate the presence of Cu+ ions in both sam-
ples (Fig. 10). However, the shape of the Cu L3M45M45 XAE spectra
can be rather assigned to the system containing metallic copper
[20]. Taking into account the specific electrophilicity (electronega-
tivity) of the Wells–Dawson unit, it can be assumed that copper in
the ethanol reduced salt, is present in metallic state but the Auger
parameter is shifted to lower value mainly due to a substrate effect.
According to scanning electron microscopy images the second
component of reduced catalyst, free heteropolyacid, forms plate-
like microcrystals, which are the support of copper containing
aggregates. The infrared spectrum of the reduced salt corresponds
to the spectra of Wells–Dawson type anions in the salt and in the
free crystalline heteropolyacid (Table 1). It can be assumed that
such free acid liberated from the copper salt plays the role of acid
component in the bifunctional catalyst. Similarly, it is believed
that the aggregates of reduced copper are the catalytic redox
centers.
Fig. 8b and c shows the distribution of copper and tungsten on
the surface of the unreduced copper salt, and Fig. 8a presents its
microphotograph. It is seen that both distributions are distinctly
homogeneous. However, after reduction, the local enrichments in
copper can be seen in Fig. 9b, where patches of needle-like crys-
tallites appear at the salt surface. The graph in Fig. 9c shows the
results of the EDX line scan. The concentration of copper, propor-
tional to number of X-ray counts, shows distinct increase along
the line when crossing the patch. It is clear that copper uniformly
distributed over the surface of pristine copper salt in the case
of unreduced sample is migrating during reduction. Moreover, it
forms separate clusters or patches of the size of about 1 ␮m com-
posed of nanometric needle-like crystallites.
The surface state of Cu3P2W18O62 prereduced in ethanol at
250 ◦C and after reaction with ethanol was analyzed by XPS and
XAES techniques. The lack of changes in the values of electron bind-
ing energies for W 4f7/2, P 2p3/2 and Cu 2p3/2 (Table 2) suggests
that the surfaces of both studied samples are qualitatively the same
although the atomic ratios indicate surface local enrichment with
Table 2
XPS and XAES data for Cu3P2W18O62 prereduced in ethanol at 250 ◦C (Et) and after reaction with ethanol at 250 ◦C (EtEt).
Sample
Binding energy (eV)
Kinetic energy (eV)
Cu L3M45M45
Auger parameter (eV)
Atomic surface ratio
ꢀ
W 4f7/2
P 2p3/2
Cu 2p3/2
˛
Cu/P
Cu/W
P/W
Et
EtEt
35.8
35.9
133.9
133.9
932.8
932.9
915.0
915.1
1847.8
1848.0
1.36
0.65
0.15
0.12
0.11
0.17

U. Filek et al. / Catalysis Today 169 (2011) 150–155
155
ence of both metallic copper and copper Cu+ ions. The formation
of metallic copper allows accepting that reduction is accompanied
by the formation of free heteropolyacid in the process analogous
to that shown by Eq. (1). In the oxygen free helium, the main prod-
ucts of catalytic ethanol conversion are ethylene, diethyl ether and
water. Furthermore, the typical products of reactions are formed
on Brønsted acid centers. In the present case the latter are supplied
by free heteropolyacid. On the other hand, in the presence of air,
acetaldehyde is predominantly obtained in catalytic reaction. Such
reaction occurs on redox centers and is ascribed to the presence of
catalytic redox centers formed by reduced copper.
L3M45M45
B
A
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4. Conclusions
X-ray microanalysis shows that both tungsten and copper are
uniformly distributed on the surface of pristine copper salt of
Wells–Dawson heteropolyacid H6P2W18O62. However, after the
reduction in ethanol vapors at 250 ◦C the form of patches composed
of needle-like crystallites appear at the surface leading to local
enrichments in copper. The XPS measurements indicate the pres-