Effect of Pd loading and precursor on the catalytic performance of Pd/WO3-ZrO2 catalysts for selective oxidation of ethylene

Article abstract of DOI:10.1016/j.cattod.2009.02.041

The structure and properties of Pd/WO₃-ZrO₂ (W/Zr = 0.2) catalysts with different Pd loadings and precursors were investigated. The results indicate that Pd/WO₃-ZrO₂ prepared from a PdCl₂ precursor was optimum for high activity and selectivity. Moreover, ethylene conversion increased with the Pd loading. The structure and nature of the catalysts were characterized using X-ray diffraction, BET N₂ adsorption, H₂ temperature-programmed reduction and H₂ pulse adsorption techniques. The results reveal that the higher catalytic performance of Pd/WO₃-ZrO₂ prepared from PdCl₂ could be related to the formation of polytungstate species and the existence of well-dispersed Pd particles.

Full text of DOI:10.1016/j.cattod.2009.02.041

Catalysis Today 149 (2010) 163–166
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Effect of Pd loading and precursor on the catalytic performance of Pd/WO –ZrO2
3
catalysts for selective oxidation of ethylene
a,b
a,b
a,
a,
Lixia Wang , Shuliang Xu , Wenling Chu *, Weishen Yang *
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
b
Graduate School of the Chinese Academy of Sciences, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The structure and properties of Pd/WO –ZrO (W/Zr = 0.2) catalysts with different Pd loadings and
3
2
Available online 5 April 2009
precursors were investigated. The results indicate that Pd/WO
was optimum for high activity and selectivity. Moreover, ethylene conversion increased with the Pd
loading. The structure and nature of the catalysts were characterized using X-ray diffraction, BET N
adsorption, H temperature-programmed reduction and H pulse adsorption techniques. The results
reveal that the higher catalytic performance of Pd/WO –ZrO prepared from PdCl could be related to the
3 2 2
–ZrO prepared from a PdCl precursor
Keywords:
2
3
Pd/WO –ZrO
2
2
2
Pd loading
3
2
2
Pd precursors
Polytungstates
Pd species
formation of polytungstate species and the existence of well-dispersed Pd particles.
ß 2009 Elsevier B.V. All rights reserved.
1
. Introduction
2 2
X-ray diffraction (XRD), BET N adsorption and H temperature-
programmed reduction (TPR) techniques and the results were
correlated with the catalytic performance.
In recent years, there has been a pronounced tendency to use
one-step ethylene oxidation as an alternative method to produce
acetic acid. In the early stages of development, a number of Pd-
containing solid acid catalysts were investigated [1–3]. Unfortu-
nately, the catalytic performance of Pd-containing solid acid
2
. Experimental
2.1. Catalyst preparation
catalysts, such as Pd–Cr
2 3 2 3 2 5 2 5 2 3
O /Al O [1], Pd–V O –Sb O /Al O [2]
and Pd–H PO /SiO [3], is rather poor. Thus, many investigations
have been carried out to develop a promising catalyst.
In 1997, Showa Denko K.K. commercialized the process of
ethylene oxidation to acetic acid using palladium-based hetero-
polyacid catalysts [4], for which a significantly higher acetic acid
3
4
2
WO
using a previously reported procedure [5]. Pd was impregnated on
the WO –ZrO support using a wet impregnation method and the
Pd loading was varied from 0.5 to 2.0 wt.%. To investigate the
influence of the Pd precursor, PdCl , Pd(NH Cl and Pd(CH COO)
3
2
–ZrO catalysts with a W/Zr ratio of 0.2 were prepared
3
2
2
3
)
2
2
3
2
À1 À1
yield (ꢀ100 g L
h
) was reported for Pd–H SiW12O40/SiO as
4 2
were used. After drying at 373 K, the resulting solids were calcined
at 673 K in static air for 5 h.
the catalyst. In all these examples, a common feature of the
catalysts for selective ethylene oxidation is that bi-functional
catalysts comprising palladium and an acidic support appear to
favor the selective formation of acetic acid [4]. Accordingly, it is
2.2. Catalyst characterization
expected that solid acid supports other than H
also show excellent catalytic performance in combination with Pd.
It has been reported that Pd supported on WO –ZrO catalysts
exhibit high catalytic activity for selective ethylene oxidation at
23 K [5,6]. In the present study, we investigated the effect of the
4 2
SiW12O40/SiO will
The specific surface area and pore size distribution of the
catalysts were measured by nitrogen adsorption/desorption at
3
2
7
7 K using a Counter Omnisorp-100CX system. Samples were first
À4
degassed at 623 K under high vacuum (1.33 Â 10 Pa) for 3 h
before isotherms were recorded.
4
Pd loading and precursor on the catalytic performance for ethylene
oxidation. Structural analysis of the catalysts was performed using
Powder XRD patterns were recorded on a Rigaku D/Max-2500
diffractometer using Cu K
a
radiation (
l
= 0.1542 nm) in the 2
u
À1
range 10–908 at a scan rate of 0.028 s at 40 kV and 200 mA.
TPR was carried out on freshly prepared catalysts to identify
reducible species on the catalyst surface. The analysis was
conducted on a homemade apparatus. In a typical experiment,
0.05 g of sample was purged with flowing He at 373 K for 1 h and
*
Corresponding authors. Tel.: +86 411 84379073; fax: +86 411 84694447.
E-mail addresses: cwl@dicp.ac.cn (W. Chu), Yangws@dicp.ac.cn (W. Yang).
URL: http://yanggroup.dicp.ac.cn
0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2009.02.041

1
64
L. Wang et al. / Catalysis Today 149 (2010) 163–166
then cooled to room temperature prior to TPR experiments. The
temperature was increased from room temperature to 1273 K at
À1
1
0 K min in a mixture of 5 mol% H
2
/Ar. The rate of hydrogen
consumption was detected by thermal conductivity detector (TCD)
and recorded as the TPR profile of the catalyst.
Pd dispersion was measured using a H
2
pulse adsorption
method on an Autosorb-1/C gas chemisorption system at 313 K. In
a typical experiment, the sample was first pretreated in a He flow
at 673 K for 2 h and subsequently reduced in H
1
2
2
gas at 573 K for
h. Then hydrogen was purged from the system by evacuation for
h. The adsorption isotherm was established by measuring the
adsorbed as a function of pressure at 313 K.
amount of H
2
2.3. Catalytic performance
Oxidation of ethylene was performed in a fixed-bed flow
3
reactor (stainless steel, 10 mm i.d.). A sample of 2 cm of catalyst
3.6 g, 40–60 mesh) was placed in the reactor and pretreated at
73 K for 1 h under a stream of H –He (1:1 mixture) at a flow rate
of 60 cm min . After cooling under a He flow to the reaction
temperature of 423 K, a mixture of the reactant gas (C /O /H O/
He 50:7:30:13 by vol.) was fed into the reactor at a total flow rate of
(
5
2
3
À1
2
H
4
2
2
3
À1 À1
1
00 cm min (SV = 3000 h ) and a total pressure of 0.6 MPa. A
trap filled with water cooled with ice-water (ꢀ273 K) was used to
collect the liquid products, which were analyzed by GC on an
Agilent 6890 instrument equipped with an FFAP column and a
flame ionization detector. Gaseous compounds were analyzed
using an on-line TCD-GC instrument equipped with a Porapak Q
column.
Fig. 1. H -TPR spectra of Pd/WO –ZrO with different Pd loadings: (a) 0%; (b) 0.5%;
2
3
2
(c) 1%; (d) 1.5%; (e) 2%.
the variation in reduction temperature for Pd species with respect
to Pd loading. It can be seen from Fig. 1 that the catalysts exhibited
two types of peak related to Pd species. One is a negative peak at
approximately 360 K that is characteristic of decomposition of the
3
. Results and discussion
3.1. Effect of Pd loading
b
-PdH phase [8], indicating the presence of metallic Pd particles
[9]. The other is a peak for hydrogen consumption at ca. 410 K that
The surface area of Pd/WO
loading varying from 0.5 to 2.0 wt.% is listed in Table 1. The surface
3
–ZrO
2
(W/Zr = 0.2) samples with Pd
can be attributed to the reduction of PdO species on the support
[10]. Notably, no remarkable difference could be observed for the
reduction peaks for metallic Pd and PdO species. This means that,
for Pd loading of <2.0 wt.%, the Pd state is independent of the Pd
2
À1
area of WO –ZrO was found to be 46 m g . On the introduction
3 2
of palladium, a decrease in surface area was observed due to pore
blockage. However, when the Pd loading was increased from 0.5 to
loading. For the WO
peaks were observed [11]. H
attributable to polytungstate species or small WO
the other two peaks at significantly higher temperatures (973 and
1173 K) are assigned to the reduction of crystalline WO oxides
3
–ZrO
2
sample without any Pd, three reduction
consumption at 773 K is probably
clusters, and
2
3
.0 wt.%, the BET surface area decreased slightly from 40 to
2
2
À1
8 m g , implying that the increase in Pd loading has almost no
x
effect on the BET surface area. The Pd dispersion and particle size
were calculated from H uptake data (listed in Table 1). The Pd
dispersion on 0.5 wt.% Pd–WO /ZrO was 22.8%, which was the
2
3
[12]. It is worth noting that the reduction peak due to
polytungstate species (773 K) shifted to lower temperature
(ꢀ613 K) for Pd-containing catalysts due to the interaction
3
2
highest among the catalysts. There were general tendencies for all
the catalysts, with a decrease in Pd dispersion and a consequent
increase in particle size when the Pd loading was increased from
3 2
between palladium and the WO –ZrO support. In particular, it
0
.5 to 2.0 wt.%.
is evident from Fig. 1 that the amount of polytungstate species at
613 K tended to increase as the Pd loading increased from 0.5 to
2.0 wt.%.
3 2
XRD patterns (data not shown) of the Pd/WO –ZrO catalysts
with different Pd loadings exhibited peaks attributed to diffraction
for the WO –ZrO support alone [7], suggesting that Pd was
Table 1 shows the effect of Pd loading on the catalytic
3
2
present in a well-dispersed state on the support, and therefore
small Pd particles could not be detected by XRD.
performance of Pd/WO
of ethylene. Pd-free WO
3
–ZrO
2
catalysts in the selective oxidation
exhibited almost no catalytic
3
–ZrO
2
TPR profiles for catalysts with Pd content varying from 0.5 to
.0 wt.% are shown in Fig. 1. The profiles reveal information about
activity for the formation of acetic acid. Introduction of palladium
dramatically increased the ethylene conversion and acetic acid
2
Table 1
3 2
Physical properties and catalytic performance of Pd/WO –ZrO catalysts with various Pd loadings.
Pd loading
wt.%)
Surface area
Pd dispersion
(%)
Pd particle
size (nm)
Ethylene
Acetic acid yield
Selectivity (%)
Acetic acid
2
À1
À1 À1
(
(m
g
)
conversion (%)
(g h
L
cat)
Acetaldehyde
CO
x
0.0
0.5
1.0
1.5
2.0
46
40
39
39
38
–
–
0.3
2.7
4.2
4.3
4.9
0.2
2
63
75
77
78
9
26
11
9
87
11
14
14
14
22.8
12.0
8.0
4.9
67
117
130
154
9.3
14.0
15.6
7.2
8

L. Wang et al. / Catalysis Today 149 (2010) 163–166
165
yield, further confirming that a bi-functional catalyst comprising
palladium and an acidic support is favorable for the selective
formation of acetic acid [4]. The catalytic activity (or yield of acetic
acid) clearly increased as a function of Pd loading, with 2.0 wt.% Pd/
3 2
WO –ZrO exhibiting the highest ethylene conversion. Although
Pd dispersion also varied with the Pd loading (Table 1), the
difference in catalytic activity cannot be explained by this
difference, suggesting that Pd dispersion is not responsible for
2
the catalytic activity. Combining the H -TPR data and the results
for catalytic activity, it is noteworthy that the ethylene conversion
increased with the Pd content, in agreement with the polytung-
state species trend at 613 K in Fig. 1. Based on this finding, we
speculate that the presence of polytungstate species at 613 K plays
an important role in increasing the activity of Pd-containing WO
ZrO catalysts. On the other hand, the CO selectivity showed no
obvious change when the Pd loading was increased from 0.5 to
.0 wt.%, perhaps because of the similar Pd state. Only one
3
–
2
x
2
significant difference in product distribution was observed: lower
acetic acid and higher acetaldehyde selectivity for 0.5 wt.% Pd/
3 2
WO –ZrO . The reason for this could be the lower Pd loading,
which may not have been sufficient to oxidize the intermediate
acetaldehyde to acetic acid.
3.2. Effect of Pd precursor
The above results indicate that the catalyst with 1.0 wt.% Pd
loading exhibited satisfactory catalytic activity and acetic acid
yield. Thus, to minimize the amount of Pd used for economic
reasons, subsequent investigations focused on the 1.0 wt.% Pd/
2
Fig. 2. H -TPR spectra of Pd/WO
3
–ZrO
2
catalysts prepared with different Pd
WO
3
–ZrO
2
catalyst. The BET surface area and Pd dispersion
chemisorption results for this catalyst prepared
precursors: (a) PdCl
2
; (b) Pd(NH Cl ; (c) Pd(CH COO) .
3
)
2
2 3 2
calculated from H
2
from different Pd precursors are summarized in Table 2. No
appreciable change in BET surface area was observed, indicating
that the Pd precursor has almost no influence on the textural
precursors exhibited a very sharp negative peak at 360 K. This
suggests that PdO agglomeration occurred, leading to the
formation of large particles that were easily reduced to metallic
Pd, for which the characteristic negative TPR peak (360 K) is much
structure of the catalysts. An interesting observation from the H
2
chemisorption results is that a change in palladium precursor
yielded different Pd dispersion. Higher dispersion was obtained
larger than that for the sample prepared from PdCl
2
. In particular,
the sample prepared from Pd(CH COO) exhibited two negative
3
2
using PdCl
2
as the palladium source compared to Pd(NH
3
)
2
Cl
2
and
peaks, which may be due to the formation of considerably larger Pd
particles. This result is in good agreement with the dispersion and
particle size of palladium in Table 2. On the other hand, it is
important to note that the Pd precursor has an effect on the state of
Pd(CH COO)
3
2
precursors, clearly indicating that differences in
dispersion are related to the nature of the Pd precursor. It has been
reported that precursors with organic ligands yield lower
dispersion than the precursors containing ammonia [13]. The
reason is that localized heating due to organic ligand oxidation
during calcination may be sufficient to result in sintering of novel
metal particles. In addition, metal–support interaction is an
important factor that contributes to the overall dispersion on
tungsten, especially polytungstate species, which exhibit
reduction peak at approximately 613 K with intensity decreasing
in the order PdCl > Pd(CH COO) > Pd(NH Cl
Comparison of the catalytic performance of 1.0 wt.% Pd/WO
ZrO catalysts for direct oxidation of ethylene reveals a significant
differencefordifferentPdprecursors(Table 2).Thecatalystprepared
from PdCl was the most active and selective, with catalytic activity
decreasing in the order PdCl > Pd(CH COO) > Pd(NH Cl , which
a
2
3
2
3
)
2
2
.
3
–
2
catalysts. For PdCl
2
and Pd(NH
3
)
2
Cl
2
it is known that Pd exists as
2
À
2+
PdCl
4
and Pd(NH
3
)
4
, respectively. Anion exchange reactions
À
2
2
À
between PdCl
4
and OH groups on the support result in strong
2
3
2
)
3 2
2
interaction between Pd species and the support [14,15], which
could inhibit metal agglomeration on the surface and result in
highly dispersed Pd, as confirmed by particle size data.
is consistent with the order for the reduction peak at 613 K assigned
to polytungstate species (Fig. 2). This result further confirmsthat the
catalytic activity of Pd/WO
is strongly dependent on the amount of polytungstate species. In
addition, the sample prepared from PdCl , with the lowest-intensity
3 2
–ZrO for selective oxidation of ethylene
2
H -TPR measurements (Fig. 2) clearly demonstrate that the Pd
precursor has an important influence on the state of palladium
species. In comparison to the TPR profile for the sample prepared
2
negative peak at 360 K (Fig. 2) clearly indicating the presence of
well-dispersed Pd particles, showed the highest acetic acid
2 3 2 2 3 2
using PdCl , the samples for Pd(NH ) Cl and Pd(CH COO)
Table 2
Physical properties of 1.0 wt.% Pd/WO
3
–ZrO
2
catalysts prepared with different Pd precursors.
a
Pd precursors
Surface
area (m
Pd dispersion
(%)
Pd particle
Ethylene
Acetic acid yield
Selectivity (%)
Acetic acid
2
À1
a
À1 À1
g
)
size (nm)
conversion (%)
(g h
L
cat)
Acetaldehyde
CO
x
PdCl
Pd(NH
2
39
32
37
12.0
6.4
9.3
17.5
4.2
1.8
2.1
117
47
75
65
69
11
19
9
14
16
22
3
)
2
Cl
COO)
2
Pd(CH
3
2
0.6
192.5
59
a
2
Calculated from H chemisorption.

1
66
L. Wang et al. / Catalysis Today 149 (2010) 163–166
selectivity. Onthecontrary, the catalystpreparedfromPd(CH
precursor showed significantly higher CO selectivity, which could
be due to the presence of large Pd particles. It is known that, starting
from large Pd particles, combustion of ethylene leads to CO as the
3
COO)
2
owing to the presence of larger Pd particles clearly exhibited
higher CO selectivity, suggesting that well-dispersed Pd species
could be responsible for the higher acetic acid selectivity.
x
x
x
predominant product [16]. The above results suggest that the use of
different precursors may result in different Pd species and surface
structures that are directly related to the active catalyst sites. Since
large Pd particles act as a combustion catalyst, the existence of well-
dispersed Pd species appears to be responsible for the higher acetic
acid selectivity. Furthermore, the amount of polytungstate species
Acknowledgements
The authors gratefully acknowledge financial support from the
National Science Foundation for Distinguished Young Scholars
(
Grant No. 20725313) and the Ministry of Science and Technology
of China (Grant No. 2005CB221404).
ontheZrO
2
surface, whichvariedfordifferentPdprecursors,governs
–ZrO catalysts.
the activity of Pd/WO
3
2
References
4
. Conclusions
In the present study, a series of Pd/WO
[
[
[
[
[
1] S. Takaharu, F. Masayuki, M. Takashige, N. Hiroshi, JP Patent 09-048744 (1997).
2] S. Takaharu, F. Masayuki, M. Takashige, N. Hiroshi, JP Patent 09-151158 (1997).
3] C.W. Capp, H.B. Walton, GB Patent 1,142,897 (1969).
4] K. Sano, H. Uchida, S. Wakabayashi, Catal. Surv. Jpn. 3 (1999) 55.
5] W.L. Chu, Y. Ooka, Y. Kamiya, T. Okuhara, H. Hattori, Chem. Lett. 34 (2005) 642.
3
2
–ZrO (W/Zr = 0.2)
catalysts with different Pd loadings and precursors prepared by the
wet impregnation method was evaluated for selective oxidation of
[6] W.L. Chu, Y. Ooka, Y. Kamiya, T. Okuhara, Catal. Lett. 101 (2005) 225.
[
[
[
7] X.R. Chen, C.L. Chen, N.P. Xu, C.Y. Mou, Catal. Today 93–95 (2004) 129.
8] H. Lieske, J. Volter, J. Phys. Chem. 89 (1985) 1841.
9] M. Boudart, H.S. Hwang, J. Catal. 39 (1975) 44.
ethylene to acetic acid. The catalytic properties of Pd/WO
were strongly influenced by the Pd loading and the type of Pd
precursor. For all samples, Pd/WO –ZrO prepared from a PdCl
precursor showed the best activity and selectivity. Moreover, the
ethylene conversion increased with an increase in Pd loading from
3 2
–ZrO
3
2
2
[10] A. Barrera, J.A. Montoya, M. Viniegra, J. Navarrete, G. Espinosa, A. Vargas, P. del
Angel, G. Perez, Appl. Catal. A 290 (2005) 97.
11] S.R. Vaudagna, S.A. Canavese, R.A. Comelli, N.S. F ı´ goli, Appl. Catal. A 168 (1998) 93.
[
[
12] D.G. Barton, M. Shtein, R.D. Wilson, S.L. Soled, E. Iglesia, J. Phys. Chem. B 103
0
.5 to 2 wt.%. The results may be due to a higher amount of
polytungstate species on the ZrO surface, which was found to play
an important role in increasing the catalytic activity of Pd/WO
ZrO for selective oxidation of ethylene to acetic acid. Furthermore,
the catalyst with a larger negative signal in the H -TPR profile
(1999) 630.
[13] L.J. Hu, K.A. Boateng, J.M. Hill, J. Mol. Catal. A: Chem. 259 (2006) 51.
14] R.S. Monteiro, L.C. Dieguez, M. Schmal, Catal. Today 65 (2001) 77.
15] R. Gopinath, N.S. Babu, J.V. Kumar, N. Lingaiah, P.S.S. Prasad, Catal. Lett. 120 (2008)
12.
[16] W. Unterberger, H. Gabasch, K. Hayek, B. Klotzer, Catal. Lett. 104 (2005) 1.
2
[
[
3
–
2
3
2