WO3 monolayer loaded on ZrO2: Property-activity relationship in n-butane isomerization evidenced by hydrogen adsorption and IR studies

Article abstract of DOI:10.1016/j.apcata.2012.04.039

The property-activity relationship of WO₃ supported on ZrO ₂ (WZ) was evaluated in n-butane isomerization for a series of catalysts with WO₃ loading ranging from 5 to 20 wt% on ZrO ₂. The catalysts were prepared by incipient-wetness impregnation of Zr(OH)₄ with an aqueous solution of (NH₄) ₆[H₂W₁₂O₄₀·nH₂O], followed by drying and calcination at 1093 K. The introduction of WO₃ continuously increased the tetragonal phase of ZrO₂, WO₃ surface density and coverage. The specific surface area and total pore volume passed through a maximum of WO₃ loading at 13 wt%; this loading corresponds to 5.9 WO₃/nm² and is near the theoretical monolayer-dispersed limit of WO₃ on ZrO₂. The IR results indicate that the presence of WO₃ eroded the absorbance bands at 3738 and 3650 cm^-1 corresponding to bibridged and tribridged hydroxyl groups up to near the monolayer-dispersed limit of WO₃. A new broad and weak band appeared, centered at 2930 cm^-1, indicating the presence of bulk crystalline WO₃ for WO₃ coverage exceeding the theoretical monolayer-dispersion limit. In addition to the band at 2930 cm^-1, two WO stretching bands were observed at about 1021 and 1014 cm^-1 for all WZ catalysts, confirming the existence of WO connected to coordinative unsaturated (cus) Zr⁴⁺ through O and to the other W through O, respectively. Pyridine adsorbed IR and NH₃-TPD revealed that the presence of WO₃ modified the nature and concentration of acidic sites. The highest acidity was observed with 13 wt% loading WO₃. The decrease in the intensity of peaks due to increasing WO₃ loading was much higher on Lewis acid sites than on Bronsted acid sites. Hydrogen adsorption isotherms and the IR results for hydrogen adsorption on preadsorbed pyridine were used to evaluate the formation of active protonic acid sites from molecular hydrogen. The catalyst with 13 wt% WO ₃ loading showed the maximum hydrogen uptake capacity and formation of protonic acid sites. These results show a direct correlation with the activity of WZ in n-butane isomerization at 573 K in which 13 wt% WO₃ loading on ZrO₂ yielded the highest amount of isobutane. It is suggested that the presence of strong Lewis acid sites on monolayer-dispersed WO₃ facilitates the formation of protonic acid sites from hydrogen in the gas phase which act as active sites in n-butane isomerization. The presence of permanent Bronsted acid sites could not be directly associated with activity. In fact, no isomerization activity was observed in the absence of hydrogen.

Full text of DOI:10.1016/j.apcata.2012.04.039

Applied Catalysis A: General 433–434 (2012) 49–57
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
WO monolayer loaded on ZrO : Property–activity relationship in n-butane
3
2
isomerization evidenced by hydrogen adsorption and IR studies
Ainul Hakimah Karim , Sugeng Triwahyono , Aishah Abdul Jalil , Hideshi Hattoric
a
a,∗
b
a
Ibnu Sina Institute for Fundamental Science Studies, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
Institute of Hydrogen Economy, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
Catalysis Research Center, Hokkaido University, Sapporo 011-0021, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The property–activity relationship of WO3 supported on ZrO2 (WZ) was evaluated in n-butane iso-
merization for a series of catalysts with WO3 loading ranging from 5 to 20 wt% on ZrO2. The
catalysts were prepared by incipient-wetness impregnation of Zr(OH)4 with an aqueous solution of
Received 28 February 2012
Received in revised form 11 April 2012
Accepted 29 April 2012
(
NH4)6[H2W12O40·nH2O], followed by drying and calcination at 1093 K. The introduction of WO3 con-
Available online 9 May 2012
tinuously increased the tetragonal phase of ZrO2, WO3 surface density and coverage. The specific surface
area and total pore volume passed through a maximum of WO3 loading at 13 wt%; this loading corre-
sponds to 5.9 WO3/nm and is near the theoretical monolayer-dispersed limit of WO3 on ZrO2. The IR
results indicate that the presence of WO3 eroded the absorbance bands at 3738 and 3650 cm corre-
sponding to bibridged and tribridged hydroxyl groups up to near the monolayer-dispersed limit of WO3.
A new broad and weak band appeared, centered at 2930 cm , indicating the presence of bulk crystalline
WO3 for WO3 coverage exceeding the theoretical monolayer-dispersion limit. In addition to the band at
Keywords:
WO3 ZrO2
Protonic acid sites
Lewis acid sites
Hydrogen uptake
n-Butane isomerization
2
−
1
−1
−
1
−
1
2
930 cm , two W O stretching bands were observed at about 1021 and 1014 cm for all WZ catalysts,
4
+
confirming the existence of W O connected to coordinative unsaturated (cus) Zr through O and to
the other W through O, respectively. Pyridine adsorbed IR and NH3-TPD revealed that the presence of
WO3 modified the nature and concentration of acidic sites. The highest acidity was observed with 13 wt%
loading WO3. The decrease in the intensity of peaks due to increasing WO3 loading was much higher on
Lewis acid sites than on Brønsted acid sites. Hydrogen adsorption isotherms and the IR results for hydro-
gen adsorption on preadsorbed pyridine were used to evaluate the formation of active protonic acid sites
from molecular hydrogen. The catalyst with 13 wt% WO3 loading showed the maximum hydrogen uptake
capacity and formation of protonic acid sites. These results show a direct correlation with the activity of
WZ in n-butane isomerization at 573 K in which 13 wt% WO3 loading on ZrO2 yielded the highest amount
of isobutane. It is suggested that the presence of strong Lewis acid sites on monolayer-dispersed WO3
facilitates the formation of protonic acid sites from hydrogen in the gas phase which act as active sites in
n-butane isomerization. The presence of permanent Brønsted acid sites could not be directly associated
with activity. In fact, no isomerization activity was observed in the absence of hydrogen.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
of coke deposits and/or the lack of active sites for isomerization.
Our research group has studied the effects of hydrogen on the
catalytic activities of SZ, MZ and WZ catalysts for acid-catalyzed
reactions [8–10]. Our results suggest that protonic acid sites are
generated from molecular hydrogen and act as catalytically active
sites, not only for the skeletal isomerization of alkanes, but also
for acid-catalyzed reactions such as cumene cracking and toluene
disproportionation. On the basis of pyridine adsorbed IR, hydro-
gen adsorbed ESR and quantitative hydrogen adsorption studies,
we have proposed the concept of “molecular hydrogen-originated
protonic acid sites” that involves the generation of protonic acid
sites on the surface of solid acid materials from molecular hydrogen
[9–13]. Hydrogen molecules are dissociatively adsorbed on specific
Zirconia-based solid acid catalysts such as SO42− ZrO2 (SZ),
MoO3 ZrO2 (MZ) and WO3 ZrO2 (WZ) catalysts have been
explored widely due to their potential for replacing halide-type
solid acids and zeolite for linear alkane isomerization [1–7]. Skele-
tal isomerization over solid acid catalysts is normally carried out
in the presence of hydrogen. Without hydrogen, catalytic activ-
ity decreases rapidly with the reaction time due to the formation
∗ Corresponding author. Tel.: +60 7 5536076; fax: +60 7 5536080.
E-mail addresses: sugeng@utm.my, sugengtw@gmail.com (S. Triwahyono).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.04.039

5
0
A.H. Karim et al. / Applied Catalysis A: General 433–434 (2012) 49–57
active sites to form hydrogen atoms, followed by spillover onto
the support and surface diffusion. Each spiltover hydrogen atom
reaches a Lewis acid site and donates an electron to form H . H
for WZ catalysts. They found that the activity of a WZ catalyst in
n-pentane isomerization was lower than that of zeolite or SZ cata-
lysts, although the WZ catalyst possessed higher acidity than zeolite
or the SZ catalysts [19].
+
+
is then stabilized on an O atom near the Lewis acid site. The Lewis
acid site trapped electron reacts with a second spiltover hydrogen
Although numerous published studies have been devoted to
the establishment of the properties and activity of WZ catalysts,
reports showing direct evidence for the property–activity relation-
ship of WZ catalysts have been lacking. In particular, the effects of
tungsten loading have only been evaluated on the basis of physi-
cal properties, acidity and catalytic activities. In the present study,
we demonstrate the results of IR studies of hydrogen adsorption on
pyridine together with isothermal hydrogen uptake results and the
catalytic isomerization of n-butane in order to clarify the relation-
ship between WO3 loading with the formation of active protonic
acid sites, hydrogen uptake and the activity of WZ catalysts. The
results indicate that 13WZ, which possessed the strongest Lewis
acid sites, had greatest ability and capacity to form and to take up
active protonic acid sites from molecular hydrogen. In the presence
of hydrogen in the gas phase, the activity of the catalyst in n-butane
isomerization passed through a maximum at 13 wt%, showing a
direct relationship between hydrogen uptake and the activity of
the catalyst, whereas the presence of permanent Brønsted acid sites
could not be directly associated with activity. In fact, no activity was
observed for n-butane isomerization in the absence of hydrogen.
−
to form H bound to a Lewis acid site.
Recently, several research groups have extensively focused on
the study of WZ catalysts, as they are more thermally and chemi-
cally stable catalysts compared to SZ and MZ catalysts. WZ catalysts
do not suffer from tungsten loss during thermal treatment, undergo
significantly less deactivation during the reaction and can be oper-
ated at relatively high temperatures with higher selectivity and
fewer cracking products. The amount of tungsten loading has
become an interesting topic of discussion as the properties and
activities of WZ are strongly determined by W loading. There seems
to be a general consensus in the literature that the maximum
activity of WZ is achieved with near monolayer surface tung-
sten oxide coverage of ZrO2 catalysts, although the number of W
atoms required to form a monolayer is dependent on the config-
uration of surface polytungstate on the ZrO2 surface. Scheithauer
et al. have reported that Brønsted acid site strength increases with
increasing WO3 loading up to saturation and remains constant at
higher loading in which the strength of these permanent Brønsted
acid sites determines the activity of WZ in n-pentane isomeriza-
tion [14]. Similarly, Naito et al. have concluded that the Brønsted
∼
2
acid sites on monolayer-dispersed WO3 (=6.4 W atoms/nm ) are
active for the isomerization of n-butane [15]. They have stated
that the maximum activity at 6.4 W atoms/nm2 is in good agree-
ment with the maximum Brønsted acidity of the catalyst. Further
WO3 loading diminishes the activity of both Lewis and Brønsted
acid sites. Lebarbier et al. studied the properties and catalytic
performance of WZ in 2-propanol dehydration and n-hexane
isomerization [16]. They concluded the presence of threshold
2. Experimental
2.1. Preparation of catalyst
Zirconium hydroxide (Zr(OH) ) was prepared from an aque-
4
ous solution of ZrOCl ·8H O (Wako Pure Chemical) by hydrolysis
2
2
with 2.8 wt% NH OH aqueous solution [20]. The final pH value of
4
2
the supernatant was 9.0. The precipitate was filtered and washed
with deionized water. The gel obtained was dried at 383 K to form
of W surface density for 2-propanol dehydration (1.3 W/nm )
2
and n-hexane isomerization (3.4 W/nm ). The difference in the
Zr(OH) . Tungstated zirconia (WZ) with various weight percent-
minimum W loading required for the development of activity
for both reactions was attributed to the necessity of stronger
Brønsted acid sites for the n-hexane isomerization. In addi-
tion, a direct relationship of the permanent Brønsted acid site
and catalytic activity for 2-propanol dehydration was observed.
Whereas, the presence of hydrogen in gas phase is required
in the activation of catalyst and isomerization of n-hexane at
4
ages of WO3 were prepared by incipient-wetness impregnation
of Zr(OH)4 with aqueous solution of (NH ) [H W12O ·nH O], fol-
4
6
2
40
2
lowed by drying at 383 K overnight and calcination at 1093 K for
h in air. 5, 10, 13, 15 and 20 wt% WO3 supported on ZrO2 were
3
prepared and the catalysts are denoted as [wt%]WZ, where [wt%]
indicated the wt% of WO3 on WZ catalyst.
The surface density of WO , ꢀ
is determined by Eq. (1) [10].
3
WO3
ꢀ
ꢁ
WO3 atoms
nm2 sample
wt% WO3/100[g WO3/g sample] × 1/MWWO [mol WO3/g WO3] × NA[WO3 atoms/mol WO3]
3
ꢀWO3
=
(1)
2
SA
[nm sample/g sample]
sample
5
23 K. Soultanidis et al. has explored the effect of tungsten surface
where WWO3 , MWWO3 , NA and SA are the weight of WO3 per unit
density in the properties and catalytic activity of n-pentane iso-
merization [17]. They concluded that the surface Brønsted acidity
of WZrOH sample treated at 973 K was constant at ∼0.035 sites/W-
atom below monolayer dispersed W and decreased gradually above
monolayer dispersed W. However the activity of WZrOH samples
demonstrated a volcano-shape dependence on tungsten surface
weight of catalyst, molecular weight of WO , Avogadro number and
3
the surface area of catalyst, respectively.
The theoretical WO3 surface coverage, ꢁWO3 is determined by
Eq. (2).
ꢂ
ꢃ
ꢀ
ꢁ
nm2 WO3
WO3 atom
WO3 atom
nm2 support
2
ꢁWO3 = ꢀsurf
× AWO3
(2)
density with maximum activity at 5.2 W/nm due to the large pop-
ulation of Zr WOx clusters. Barton et al. have concluded that a large
polytungstate species is required to delocalize the negative charge
counterbalancing the formation protonic species from hydrogen in
the gas phase in which the protonic species increase isomerization
and prevent deactivation [18]. The maximum o-xylene isomer-
where AWO3 is the molecular area of WO3 which was determined
from the bulk density of WO3.
ization turnover rates were obtained at WOx surface densities of
2.2. Characterization of catalyst
2
1
0 W atoms/nm , which exceeds the theoretical monolayer capac-
ity of ZrO . Even though most of these reports agree with the acidic
The crystalline structure of catalysts was determined with XRD
2
property-catalytic activity relationship of solid acid catalysts, Var-
tuli et al. have suggested that the comparisons of catalytic activity
between materials based entirely on acid strength may not be valid
recorded on a Bruker AXS D8 Automatic Powder Diffractometer
using Cu K␣ radiation with ꢂ = 1.5418 A˚ at 40 kV and 40 mA, over
◦
the range of 2ꢁ = 20–40 . The fraction of tetragonal and monoclinic

A.H. Karim et al. / Applied Catalysis A: General 433–434 (2012) 49–57
51
phase of ZrO2 in the catalyst was determined based on the formula
proposed by Toraya et al. [21].
1
.31Xm
Vt = 1 −
Xm =
(3)
(4)
1
+ 0.31Xm
¯
Im(1 1 1) + Im(1 1 1)
Im(1 1 1) + Im(1 1 1) + It(1 1 1)
¯
where Xm is the intensity ratio of monoclinic of ZrO . It(1 1 1),
2
¯
Im(1 1 1) and Im(1 1 1) are the integrated intensity of the (1 1 1)
reflection of the tetragonal phase at 2ꢁ = 30.2 , (1 1 1) reflection of
the monoclinic phase at 2ꢁ = 31.8 and (1 1 1) reflection of the mon-
oclinic phase of zirconia at 2ꢁ = 28.2 , respectively. While, 1.31 is
◦
◦
¯
◦
Toraya’s theoretical deviation from the linearity value.
The specific surface area and BJH pore size distribution of the
catalyst were determined with a Quantachrome Autosorb-1 at 77 K.
Prior to the analysis, the catalyst was outgassed at 573 K for 3 h.
Temperature-programmed desorption of ammonia (NH -TPD)
3
was carried out with ThermoQuest TPD1100 by a procedure similar
to that described in previous report [2]. In brief, the catalyst was
activated with hydrogen flow at 673 K for 3 h followed by purg-
ing with He flow at 673 K for 30 min, to ensure the removal of
adsorbed water and organic contaminants. Then, the activated cat-
alyst was exposed to 10 Torr of dehydrated ammonia at 373 K for
Fig. 1. Powder X-ray diffraction pattern for WZ catalysts containing different
amount of WO3: (a) 5WZ, (b) 10WZ, (c) 13WZ, (d) 15WZ, (e) 20WZ catalysts.
3
0 min followed by purging with He flow at 373 K for 30 min. The
TPD was run at a heating rate of 10 K/min from room tempera-
ture to 1200 K under the He flow, and the desorbed ammonia was
detected by mass spectrometry. In the measurement of IR spectra,
a self-supported wafer placed in an in situ stainless steel IR cell
with CaF2 windows was activated with a hydrogen flow at 623 K
for 3 h, followed by outgassing at 623 K for 2 h [22,23]. The sample
was exposed to 0.13 kPa of pyridine at 423 K, followed by out-
The total conversion of n-butane (Xn-butane) was calculated
according to Eq. (5).
[
C]isobutane
Xn-butane = [
(5)
C]isobutane + [C]residual n-butane
where [C] is a mol number for particular compound which is calcu-
lating based on the Scott hydrocarbon calibration standard gas (Air
Liquide America Specialty Gases LLC).
gassing at 573 K. In the H -exposure process, pyridine was adsorbed
2
on activated sample at 423 K and outgassed at 573 K followed by
exposure of dried hydrogen (13.35 kPa) at room temperature. The
catalyst then was heated stepwise from room temperature in 50 K
increments to 523 K for 30 min each. Since the catalysts have been
activated with hydrogen at 623 K, the catalysts will not be reduced
further on contact with hydrogen at a temperature 523 K and below.
All spectra were recorded on a Perkin-Elmer Spectrum GX FT-IR
Spectrometer at room temperature.
3
. Results and discussion
Fig. 1 shows the XRD patterns for WZ catalysts. The peak at
◦
2
ꢁ = 30.2 was assigned to the tetragonal phase of ZrO and the
2
◦
◦
peaks at 2ꢁ = 28.2 and 31.8 to the monoclinic phase of ZrO . The
2
peak ascribed to the tetragonal phase of ZrO2 was predominant
and small peaks of the monoclinic phase of ZrO2 were observed
for all WZ catalysts. Weak and broad peaks ascribed to the crys-
2.3. Isothermal hydrogen adsorption
◦
tal structure of bulk WO3 at about 2ꢁ = 23–25 were observed for
catalysts with 15 and 20 wt% WO3 loading. This result may indi-
cate the existence of free tungstate anions on the surface of the
catalyst which formed bulk crystalline WO3 during calcination at
Isothermal hydrogen uptake was measured using the automatic
gas adsorption apparatus Belsorp 28SA [24]. A catalyst sample was
placed in an adsorption vessel and activated in hydrogen flow at
1
093 K. However, the WO -free ZrO2 consisted almost completely
3
6
73 K for 3 h followed by evacuation at 673 K for 3 h, and then cooled
of the monoclinic phase of ZrO2 and the phase of ZrO2 did not
change much if crystalline ZrO2 was used as the support in the
preparation of the catalyst. Increasing the WO3 loading led to a
continuous decrease in peaks ascribed to monoclinic phase of ZrO2
and an increase in the peak ascribed to the tetragonal phase of ZrO2.
The change in the phase of ZrO2 was almost not observed for high
to an adsorption temperature of 373 K and held at that temperature
for 3 h. 6.67 kPa of hydrogen was then introduced into the adsorp-
tion system, and the pressure change was monitored with time to
calculate the hydrogen uptake.
2.4. Catalytic reaction of n-butane
loading of WO . Stabilization of the tetragonal phase of ZrO2 and
3
delaying or inhibiting the sintering of ZrO crystallites are caused by
2
Isomerization of n-butane was caried out in a closed recircula-
the anchoring tungstate anion in Zr(OH) hydroxyl groups. The sta-
4
tion batch reactor at 573 K [2]. 0.3 g of catalyst was activated with
circulating of 40 kPa of hydrogen at 673 K for 3 h followed by evacu-
ation at 673 K for 3 h. 46.67 kPa of mixture containing hydrogen and
n-butane with the ratio 6:1 was allowed to react at 573 K. In order
to evaluate the role of hydrogen, the nitrogen gas was also used as a
carrier gas. The products were analyzed with an on-line 6090N Agi-
lent Gas Chromatograph equipped with VZ-7 packed column and
FID detector. No by-product was observed in this reaction.
bilization effect of tungsten oxo-species in the tetragonal phase of
ZrO2 is consistent with previous reports [25–29]. Scheithauer et al.
reported that the monoclinic phase of ZrO2 dominated at 7.5 wt%
WO3 and was lower when the catalyst was calcined at 1098 K. An
increase in WO3 loading up to 19 wt% WO3 increased the fraction
of the tetragonal phase of ZrO . This result indicates that minimum
2
WO3 loading is required to inhibit the formation of the mono-
clinic phase of ZrO2 [25]. On the contrary, Vu et al. reported that

5
2
A.H. Karim et al. / Applied Catalysis A: General 433–434 (2012) 49–57
Fig. 2. Pore size distribution for WZ catalysts containing different amount of WO.
Fig. 3. IR spectra in the hydroxyl stretching region for activated and pyridine
adsorbed WZ catalysts containing different amount of WO3: (a) 5WZ, (b) 10WZ,
(c) 13WZ, (d) 15WZ, (e) 20WZ catalysts.
tungsten loading did not affect the XRD pattern of a ZrO2 support
due to the stabilization of ZrO2 already achieved by the incorpo-
ration silicon in the preparation [30]. In addition, Vaudagna et al.
reported that bulk crystalline WO3 was formed by calcination of
assigned to bibridged and tribridged hydroxyl groups, indicating
the presence of both the tetragonal and monoclinic phases of zir-
conia on the catalyst [20,31]. Both bands eroded continuously with
increased WO3 loading, and the intensity of the hydroxyl stretch-
ing bands totally disappeared at 13 wt% WO3 loading, indicating
that both bibridged and tribridged hydroxyl groups on the sur-
face were covered by tungstate anions. Further loading of WO3
the catalysts at 1103 K regardless of the degree of WO loading [29],
3
while Scheithauer et al. concluded that crystalline WO was formed
3
only for 19 wt% WO loading on ZrO with calcination temperatures
3
2
of 1023 K and above [25].
The crystal phase, specific surface area, total pore volume, WO3
surface density and WO coverage are summarized in Table 1. With
3
increasing WO3 loading, the ratio of the monoclinic to tetragonal
phases of ZrO2 decreased and, in contrast, the surface density and
−1
led to new broad bands at 3500–3700 and 2800–3000 cm , corre-
sponding to the preferential formation of Zr
O W and W O W
coverage of WO continuously increased. The 13WZ catalyst, which
3
terminations of crystallite planes, respectively. In addition, two
absorbance bands were observed in the W O stretching regions
provided the maximum specific surface area, corresponded to ca.
2
5
.9 WO /nm as the surface concentration of WO , which is near
−1
3
3
at 1025–2010 cm for all WZ catalysts (Fig. 4). The main band
the theoretical monolayer-dispersed limit of WO3 in this exper-
iment. The specific surface area and total pore volume of ZrO2
at a higher wavenumber was relatively sharp and stronger, and
2
obtained by calcination of Zr(OH)4 at 1093 K were 25 m /g and
0.030 ml/g, respectively. These values are lower than those of ZrO2-
supported WO3 catalysts, indicating the inhibition effect of WO3
species in the sintering of ZrO2 supports. The specific surface area
and total pore volume of the catalysts increased with WO3 loading
and reached a maximum value at 13 wt% WO3 loading. The further
addition of WO slightly decreased both surface area and total pore
3
volume. These decreases were not caused by ZrO2 phase collapse,
but most probably occurred after WO3 coverage reached the the-
oretical monolayer limit, and the excess amorphous or crystalline
WO3 narrowed or plugged the pores of the catalyst, leading to a
decrease in the specific surface area and pore volume after reach-
ing a maximum. The dV/dD function for all WZ catalysts presented
sharp peaks centered at 6–8 nm (Fig. 2). The peak at 7.8 nm for the
5
WZ catalyst intensified and shifted to a smaller pore diameter with
increasing WO3 loading, up to 13 wt%, indicating the penetration
of WO3 into large pores to form pores of relatively small diame-
ter. The further addition of WO3 lowered the small pore diameter
intensities due to the plugging of small diameter pores.
Figs. 3 and 4 show the IR spectra in the O H and W O stretch-
ing regions of the activated and pyridine adsorbed WZ catalysts.
The catalysts were activated at 673 K, followed by the adsorption
of pyridine at 423 K and outgassing at 623 K. The activated 5WZ
−
1
catalyst showed broad bands centered at 3738 and 3650 cm
,
Fig. 4. IR spectra in the W O stretching region for activated and pyridine adsorbed
WZ catalysts containing different amount of WO3: (a) 5WZ, (b) 10WZ, (c) 13WZ, (d)
15WZ, (e) 20WZ catalysts.
inferring the existence of several hydroxyl stretching bands with
different acidic centers and strength (Fig. 3). These bands were

A.H. Karim et al. / Applied Catalysis A: General 433–434 (2012) 49–57
53
Table 1
Composition and physicho-chemical peoperties of tungstated zirconia catalysts.
M/Ta (%/%)
Surface area (m /g)
2
Pore volume (ml/g)
Surface density (WO3/nm2)
WO3 coverage (%)
Catalyst
WO3 content (wt%)
5
1
1
1
2
WZ
5
10
13
15
20
38/62
19/81
11/89
5/95
33
46
57
49
44
0.068
0.086
0.098
0.089
0.070
3.9
5.6
5.9
8.0
11.8
61
87
92
124
171
0WZ
3WZ
5WZ
0WZ
5/95
a
Monoclinic/tetragonal phase of ZrO2.
the shoulder band at a lower wavenumber was much weaker and
reveals that the surface acid strength was widely distributed on
broader. Increased WO loading did not change the position of the
the catalysts. Gaussian deconvolution of NH -TPD spectrum for
3
3
−1
−1
band at 1021 cm , whereas the band at 1014 cm was observed
13WZ indicated the existence of NH3 desorption peaks centered
at 440, 470, 560, 665 and 720 K which inferring that there are
several type of acidic centers with different strengths of acidity
(Fig. 5C). No ammonia was desorbed at 900 K and above. The NH3-
TPD curves also indicated that the amount of adsorbed ammonia
−
1
only for the 13WZ catalyst. The peak at 1014 cm shifted slightly
to a lower wavenumber for the 5WZ, 10WZ, 15WZ and 20WZ cat-
alysts. Based on our previous assigment, the bands at 1021 and
−
1
1
014 cm for 13WZ were assigned to the stretching of W O con-
4+
nected to coordinative unsaturated (cus) Zr through O and to the
other W through O, respectively [32]. No dioxo (O O) tung-
increased with increasing WO loading and reached a maximum at
3
W
13 wt% WO3 loading. This result reveals that the total number of
acid sites was determined by the WO3 coverage in which the max-
sten oxide species were present in any of the catalysts, as seen by
the absence of IR bands for symmetric and antisymmetric stretch-
imum acidity of catalysts was obtained when the ZrO catalyst was
2
ing modes of the O
and wavenumbers (30–50 cm difference). The spectra in the O
W
O bond with different relative intensities
covered by monolayer-dispersed WO . In fact, a further increase
in WO3 loading decreased the intensity of the peaks centered at
453 K. Fig. 5B shows the IR spectra of pyridine adsorbed on WZ
3
−1
H
and W O stretching regions were essentially the same as those
reported by Naito et al. [15] and Scheithauer et al. [14,25] for
dehydrated WZ catalysts. Naito et al. reported a band assigned to
the hydroxyl group observed at 3676 cm for both ZrO2 and WZ
catalysts. Scheithauer et al. reported the O H stretching region
−1
−1
in the range of 3777–3741 cm , although the band position of
the hydroxyl groups varied in the catalysts containing different
amounts of WO loading and following calcination at different tem-
3
peratures. In addition to the hydroxyl groups, Scheithauer et al.
concluded, together with the Raman spectra data, that the two
−1
bands at 1024 and 1005 cm are characteristic of two indepen-
dent W O species. Similar to the hydroxyl groups, the W O band
positions varied in the catalysts containing different amounts of
WO3 loading.
Both O H and W O bands for WZ catalysts in this experiments
were affected by pyridine adsorption due to the inductive effects
by pyridine coordinated to cus Zr4 sites in the vicinity of WOx.
Particularly, W O groups connected to cus Zr4+ should be affected
more extensively than the other types of W O groups. Adsorption
of pyridine at 423 K followed by outgassing at 623 K shifted the
+
W
O bands to a lower frequency and changed the intensity of the
−1
bands. Adsorption bands at 1021 and 1014 cm for all WZ catalysts
shifted to 1014 and 995 cm , respectively (Fig. 4). In addition to the
−1
−
1
shift in the band positions, the intensity of the band at 1021 cm
decreased more extensively compared to the band at 1014 cm−1,
−1
indicating that the peak at 1021 cm is the W O band connected
4
+
to cus Zr sites through an O atom. In addition to the changes in
the W O absorbance bands, the adsorption of pyridine on the 5WZ
catalyst eroded the hydroxyl stretching bands centered at 3738 and
−
1
−
1
3
650 cm and the new band centered at 3085 cm , which may be
4+
related to pyridine coordinated to cus Zr sites (Fig. 3). The bands at
about 3085 cm eroded with increasing WO3 loading and dimin-
ished at higher WO3 loading. However, 15WZ and 20WZ formed
new weak and broad bands centered at 2930 and 2860 cm ; these
bands were unaffected by the adsorption of pyridine, indicating the
−1
−
1
existence of bulk crystalline WO on the catalyst surface. This result
3
is in good agreement with the XRD results in which crystalline WO3
was observed only in the 15WZ and 20WZ catalysts.
Acid site distribution and strength for WZ catalysts were stud-
ied by NH -TPD and pyridine adsorbed IR spectroscopy (Fig. 5). The
3
NH -TPD results show that ammonia desorbed in one large enve-
lope centered at 453 K for all WZ catalysts (Fig. 5A). The shoulder
band which emerged when the catalysts were heated to 1200 K
3
Fig. 5. (A) NH3-TPD and (B) pyridine adsorption plots for WZ catalysts containing
different amount of WO3: (a) 5WZ, (b) 10WZ, (c) 13WZ, (d) 15WZ, (e) 20WZ catalysts.
(
C) NH3-TPD spectrum and Gaussian deconvolution peaks for 13WZ catalyst.

5
4
A.H. Karim et al. / Applied Catalysis A: General 433–434 (2012) 49–57
catalysts. The IR bands at 1540 and 1450 cm−1 assigned to
pyridinium ions adsorbed on protonic acid sites and pyridine
coordinated to Lewis acid sites appeared for all WZ catalysts,
respectively. As WO loading increased, the bands at 1540 and
3
−1
1
450 cm
continuously intensified and reached a maximum at
1
3 wt% WO3 loading. A further increase in WO3 loading decreased
−
1
the intensity of both peaks at 1540 and 1450 cm . The decrease
in the intensity was much greater on Lewis acid sites than on
Brønsted acid sites for 15 and 20 wt% WO3 loaded catalysts
due to the presence of WO3 coverage exceeding the theoretical
monolayer-dispersion limit on the surface of ZrO , which lowered
2
the accessibility of Lewis acid sites to pyridine. This result resem-
bles the NH -TPD result in which the acidity of the catalyst was
3
determined by the WO coverage. Studies on the acidity of WZ have
3
been reported by Sun et al. [33] and Naito et al. [15] using ammo-
nia TPD and IR spectroscopy. Based on NH -TPD, Sun et al. reported
3
that WZ catalysts containing 3.8–34.1 wt% WO3 possess two kinds
of acidic centers from which ammonia desorbs at 400 and 500 K.
However, Naito et al. reported that the introduction of WO3 on
ZrO2 developed new bands assigned to the interaction of hydroxyl
−
1
groups with ammonia at 3344, 3262 and 3180 cm . Other peaks
corresponding to the ammonia species adsorbed on WO -loaded
Fig. 6. Variations of hydrogen uptake as a function of time for WZ catalysts con-
3
−
1
taining different amount of WO : (a) 5WZ, (b) 10WZ, (c) 13WZ, (d) 15WZ, (e) 20WZ
ZrO2 were observed at 1608, 1433 and 1262 cm . In contrast, dif-
ferent results of pyridine adsorbed IR were found by Naito et al., in
which they reported that an increase in WO3 loading continuously
3
catalysts.
decreased the Lewis acid sites, while the Brønsted acid sites showed
adsorption on WZ, MZ and Pt/MoO3 catalysts evidenced by IR
spectroscopy and quantitative hydrogen uptake measurements
[24,34–36]. The hydrogen adsorption on those catalysts was char-
acterized by the dissociation of molecular hydrogen into hydrogen
atoms, spillover of hydrogen atoms, surface diffusion of spiltover
hydrogen atoms and the formation of H+ or HxMoO3. The pres-
ence of specific active sites and strong Lewis acid sites are required
for hydrogen adsorption in which specific active sites such as Pt,
acidic sites or reduced WO3 or MoO3 facilitate the dissociative
adsorption of molecular hydrogen and Lewis acid sites stabilize a
proton by trapping an electron from a hydrogen atom. Therefore,
stronger Lewis acid sites form a larger number of protonic acid
sites from molecular hydrogen. The hydrogen adsorption kinetic
behavior is different between WZ, MZ and Pt/MoO3 catalysts. The
rate-controlling step of hydrogen adsorption is surface diffusion of
spilt-over hydrogen atoms for all WZ, MZ and Pt/MoO3 catalysts
with apparent activation energies of 25.9, 62.8 and 83.1 kJ/mol,
respectively [34–36]. The difference in the apparent activation
energies may be caused by differences in the acidity of the cata-
lyst, which leads to differences in hydrogen-surfaces interaction
and/or the ease of surface diffusion of hydrogen atoms. Other dif-
ference among WZ, MZ and Pt/MoO3 catalysts is the necessity
for specific active sites to facilitate the dissociative adsorption of
hydrogen. Pt is required for MoO3 catalysts, but hydrogen uptake
is considerable for Pt-free WZ and MZ. It seems that acidic sites on
the catalysts act as active sites to dissociate hydrogen molecules
into hydrogen atoms [34]. Barton et al. [18] have reported on the
relationshipbetweentungstenloadingandhydrogen uptake capac-
ity of tungstated zirconia catalysts. They concluded that hydrogen
uptake increases with tungsten loading due to the stabilization of
hydrogen by polytungstate and crystalline WO3 species on the sur-
a maximum at 6 W atoms/nm2 catalyst. Both acidic sites totally
2
diminished for the catalyst containing 17.3 W atoms/nm . This con-
clusion is rather similar to the CO adsorption results reported by
Scheithauer et al., in which the Lewis acid sites were not observed
in all WZ samples with WO loading beyond the saturation value as
3
4
+
all the Zr on the surface was covered by WO [14,25]. Our research
3
group has also reported on a pyridine adsorbed IR study of WZ and
Pt-loaded WZ catalysts [9]. For both WZ and Pt/WZ, all the protonic
acid sites were strong; pyridine molecules remained at the protonic
acid sites after outgassing at 673 K. In addition to strong Lewis acid
sites, there existed a number of weak Lewis acid sites from which
the pyridine molecules were desorbed in the temperature range
of 423–673 K. The ratio of Lewis acid sites to protonic acid sites
increased with an increase in the treatment temperature for both
WZ and Pt/WZ. The presence of Pt did not considerably change the
permanent Lewis and Brønsted and acid sites; however, it markedly
enhanced the formation rate of protonic acid sites from molecular
hydrogen through a hydrogen spillover mechanism.
Fig. 6 shows the variation in isothermal hydrogen adsorption as
a function of time at 373 K for activated WZ catalysts. Similar fea-
tures of hydrogen adsorption were observed for WZ catalysts with
different amounts of WO3 loading. For the initial few minutes, fast
adsorption occurred and the hydrogen uptake achieved near equi-
librium within 10 h. At equilibrium, the hydrogen uptake increased
with increasing WO3 loading and reached a maximum at 13 wt%
1
8
2
with a hydrogen uptake capacity of 6.09 × 10 H atoms/m cat. The
high concentration of hydrogen uptake on 13WZ may be related to
the mechanism and structure of H bounded to O atom on WZ. We
have proposed that H from molecular hydrogen bounds to three
+
+
O atoms, two O atoms bridging Zr and W, and one O atom in the
+
2
x
face ZrO . However, WO species, which are present below the
W
O of which W is connected to Zr through O. The H may be
mobile among the three O atoms [32]. Therefore, the maximum
capacity of WZ to adsorb H is at least similar to the number of W
atom connected to Zr through O. Beyond the monolayer-dispersed
theoretical monolayer coverage, cannot accommodate hydrogen
due to the inability of WO species to delocalize the negative charge
required to stabilize the H concentration.
+
x
˛
+
WO , increasing of the number of W did not necessarily increase the
capacity of WZ to uptake H In fact, the hydrogen uptake capacity
The hydrogen uptake capacity of WZ catalysts was confirmed
by the formation of protonic acid sites from molecular hydrogen
as evidenced by IR spectroscopy. Fig. 7 shows the features of the
formation of protonic acid sites from molecular hydrogen for 5WZ,
13WZ and 20WZ. The results for 10WZ and 15WZ are not shown
here. A similar trend was observed for all catalysts in which the
3
+.
decreased with the addition of 15 and 20 wt% WO . This decrease
3
may have been caused by a reduction in Lewis acid sites as the
WO3 on the catalyst surface had exceeded the theoretical limit of
monolayer-dispersed WO . We have reported studies of hydrogen
3

A.H. Karim et al. / Applied Catalysis A: General 433–434 (2012) 49–57
55
Fig. 7. IR spectra of pyridine adsorbed on WZ catalysts (above) and temperature dependencies of formation of protonic acid sites from molecular hydrogen (bottom). Spectral
changes when pyridine-preadsorbed sample was heated in hydrogen at (b) room temperature, (c) 323 K, (d) 373 K, (e) 423 K, (f) 473 and (g) 523 K. (a) Before exposure to
hydrogen.
heating of pyridine preadsorbed catalysts in the presence of hydro-
The isomerization of n-butane was carried out at 573 K in a
closed recirculation batch reactor with a mixture of H2 (40 kPa) or
N2 (40 kPa) and n-butane (6.67 kPa). In the presence of hydrogen
gas, the product consisted of only isobutane and residual n-butane.
It is interesting to note that neither lower nor higher linear alka-
nes were observed, indicating that dimerization, hydrogenolysis
or cracking did not take place. The different result was observed
when the isomerization was done in a pulse or continuous flow
reactor in which the isomerization took place either in the pres-
ence or absence of molecular hydrogen [10,23]. The differences in
the reaction system and activation technique may be resulted in
the different observation. The time dependencies and the effect of
−
1
gen in gas phase increased the intensity of the band at 1540 cm
assigned to the protonic acid sites and simultaneously decreased
−
1
the intensity of the band at 1450 cm assigned to the Lewis acid
−
1
sites. Two absorbance bands at 1460 and 1475 cm assigned to
adsorbed piperidine were not observed in the heating of WZ up to
5
23 K indicating the hydrogenation of pyridine did not take place.
−
1
The changes of absorbance bands at 1450 and 1540 cm reveals
that the protonic acid sites were formed from molecular hydrogen
through a hydrogen spillover mechanism facilitated Lewis acidic
sites as electron acceptors. Based on our previous assignment, it
+
is plausible that the H is loosely bound to three O atoms, i.e. two
O atoms bridging Zr and W, and one O atom in W O of which W
is connected to Zr through O. The H may be mobile among the
WO loading in the yield of the reaction are shown in Fig. 8A and B,
3
+
respectively. In the presence of H , the activity of WZ in n-butane
2
three O atoms [32]. The effect of WO3 loading in the formation of
protonic acid sites is clearly seen if the number of protonic acid
sites formed is plotted against the heating temperature. Although
the results for the 10WZ and 15WZ catalysts are not shown here,
the steepest incline of the plot of [A1540]T − [A1540]BG versus T was
observed for the 13WZ catalyst, indicating the highest ability and
capacity to form and to take up protonic acid sites from molecu-
lar hydrogen. This result agrees with the acidity of the catalysts in
which the 13WZ catalyst possessed the highest acidity of the cat-
alysts, particularly Lewis acid sites. This also confirmed that the
acidity of the catalyst plays an important role in hydrogen uptake
or the formation of protonic acid sites over WZ catalysts.
isomerization was strongly determined by the amount of WO3
loading. Fig. 8A shows that increasing WO3 loading improved the
yield of isobutane and reached a maximum for 13WZ, which may
have been due to the highest formation rate of protonic acid sites or
hydrogen uptake capacity. In addition, the 13WZ catalyst possessed
the highest number of strong Lewis acid sites corresponding to the
tetragonal phase of ZrO2 in which this type of Lewis acid site may
play an important role in the formation of active protonic acid sites
for isomerization [23]. This activity was not observed for the WO3-
free ZrO2 catalyst (data not shown), which may have been due to
the absence of strong Lewis acid sites corresponding to the tetrago-
nal phase of zirconia [10,37]. In general, high activity was observed

5
6
A.H. Karim et al. / Applied Catalysis A: General 433–434 (2012) 49–57
Fig. 8. Time dependences of the yield of isobutane for WZ catalysts containing dif-
ferent amount of WO3: (a) 5WZ, (b) 10WZ, (c) 13WZ, (d) 15WZ, (e) 20WZ catalysts.
Fig. 9. Hydrogen uptake and W atoms loading dependences of the yield of isobutane
for WZ catalysts containing different amount of WO3. Isomerization was carried out
at 573 K under the presence of hydrogen gas.
(
B) n-Butane isomerization at 573 K as a function of WO3 loading (data was taken
at t = 120 min in (A)).
a higher calcination temperature is required to obtain high activ-
ity of WZ catalysts. In addition, they stated that hydrogen gas has
a role in the removal of coke by inhibiting the dehydrogenation
process and in shortening the induction period, indicating the for-
mation of active hydroxyl groups from hydrogen in the gas phase.
Naito et al. reported the effect of WO3 loading on n-butane iso-
merization in which the maximum activity was obtained at 6.4 W
atoms/nm2 catalyst; further W loading diminished the activity of
the catalyst. They also concluded that the presence of Brønsted acid
sites on the monolayer was responsible for high activity in n-butane
isomerization.
in the initial 30 min of the reaction with no-induction periode was
observed in this WZ which may be due to reaction system and/or
the activation method in this experiment (H -circulating at 673 K
2
for 3 h, followed by outgassing at 673 K for 3 h) [38]. The activity
of the catalysts decreased gradually and totally lost their activity
within 120 min for all WZ catalysts. The rapid deactivation may
have been caused by the poor ability of WZ catalysts to form or to
maintain active protonic acid sites and/or to remove coke deposits
on the catalyst surface due to the absence of strong specific active
sites like Pt metal for dissociative adsorption of molecular hydro-
gen. Indeed, none of the WZ catalysts showed any activity when
hydrogen gas was changed to nitrogen gas (Fig. 8B). Thus, we sug-
gest that the introduction of specific active sites for dissociative
adsorption of molecular hydrogen, such as Pt or Pd, is necessary in
order to improve and to maintain the activity and stability of WZ
catalysts.
The activity of WZ was also observed in n-pentane and xylene
isomerization. Santiesteban et al. concluded that the activity of
WZ in n-pentane isomerization passed through a maximum as W
loading increased [28]. The activity reached a maximum at approx-
imately 16 wt% W loading on ZrO ; this loading represents about
2
2−
twofold the monolayer coverage of the WO4
species. In addition,
they also concluded that the activity of WZ was strongly dependent
Fig. 9 shows the plots of the dependence of activity of WZ upon
on the reducibility of tungsten oxospecies on ZrO , i.e. WZ catalysts
the hydrogen uptake capacity and WO loading. The results showed
2
3
containing higher tungsten oxospecies are relatively active due to
the ease of reduction compared to lower tungsten oxospecies cat-
alysts. Scheithauer et al. reported that an increase in the tungsten
loading continuously increased the number of Brønsted acid sites
such that the activity of the catalyst for n-pentane isomerization
corresponded to the strength and number of permanent Brønsted
acid sites [14]. However, the Lewis acid sites reached a maximum
and started to decline as the loading reached saturation. The role
of hydrogen gas was clarified with o-xylene isomerization in which
hydrogen gas played an important role in promotion of the reaction
rate and a decrease in the catalyst deactivation rate by maintain-
ing the Brønsted acid site concentration and reversing the xylene
dehydrogenation steps [40].
a direct correlation between hydrogen uptake and the activity of
WZ in n-butane isomerization at 573 K. The yield ratio of isobu-
tane to hydrogen uptake capacity did not change much for any
of the WZ catalysts, indicating that the activity of WZ catalysts is
clearly correlated with the number of hydrogen uptake on WZ cat-
alysts. While, the dependence of activity of WZ on the number of W
atom loading was only observed at and below monolayer-dispersed
WO . Beyond the monolayer coverage, the activity of WZ decreased
3
markedly due to the formation of multilayer-dispersed WO3 on the
surface ZrO which hindering the accessibility of all W atoms to the
2
reactant.
The activity of WZ catalysts has been explored widely in the last
two decades. Hino and Arata reported that the activity of WZ is
not high enough to isomerize n-butane compared to SZ catalysts
[
26]. Therefore, they mechanically mixed WZ with Pt-loaded metal
4. Conclusion
oxides to enhance n-butane isomerization. The results showed that
the effect of mixing was specific only to Pt/ZrO . Mixing WZ with
The introduction of WO3 on ZrO2 continuously increased the
tetragonal phase of ZrO2 as well asWO3 surface density and cover-
age. The specific surface area and total pore volume passed through
a maximum near the theoretical monolayer-dispersed limit of
WO3 at 13 wt% WO3 loading; this amount of WO3 corresponds
2
Pt/TiO , Pt/Al O , Pt/Fe O or Pt/SnO2 did not show any activity
2
2
3
2
3
in n-butane isomerization. They concluded that only Pt on ZrO2
formed cationic Pt, which was active in the isomerization of n-
butane. Rossi et al. reported the effect calcination temperature and
the role of hydrogen in the gas phase [39]. They concluded that
2
to 5.9 WO /nm . The IR results indicate that WO interacted with
3
3

A.H. Karim et al. / Applied Catalysis A: General 433–434 (2012) 49–57
57
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[
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This work was supported by The Ministry of Higher Education,
Malaysia through the Fundamental Research Grant Scheme No.
[
[
7
8670. We also thank the Hitachi Scholarship Foundation for the
Gas Chromatograph Instruments Grant.
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