The effect of preparation procedure on the performance of Pd-SiW 12/SiO2 catalysts for the direct oxidation of ethylene to acetic acid

Article abstract of DOI:10.1016/S1872-2067(10)60123-4

Pd-SiW₁₂/SiO₂ catalysts were investigated to elucidate the effect of preparation procedure on the direct oxidation of ethylene to acetic acid. Pd-SiW₁₂/SiO₂ catalysts with a fixed amount of Pd and SiW₁₂ were prepared in various ways. The order in which Pd and SiW₁₂ were loaded on the SiO₂ support and the treatment conditions for the supported Pd have a significant influence on the dispersion of Pd on the Pd-SiW₁₂/SiO₂ catalysts, but no influence on the concentration of B acid sites. The Pd-SiW ₁₂/SiO₂ catalyst prepared by simultaneously loading Pd and SiW₁₂ on the SiO₂ support showed the best catalytic activity. The dispersion of Pd was the main factor in the catalytic activity.

Full text of DOI:10.1016/S1872-2067(10)60123-4

CHINESE JOURNAL OF CATALYSIS
Volume 31, Issue 11, 2010
Online English edition of the Chinese language journal
RESEARCH PAPER
Cite this article as: Chin J Catal, 2010, 31: 1342–1346.
The Effect of Preparation Procedure on the Performance of
Pd-SiW12/SiO2 Catalysts for the Direct Oxidation of Ethylene
to Acetic Acid
XU Shuliang1,2, CHU Wenling2,*, YANG Weishen2,#
1National Engineering Laboratory for Methanol to Olefins, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
Liaoning, China
2State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
Abstract: Pd-SiW12/SiO2 catalysts were investigated to elucidate the effect of preparation procedure on the direct oxidation of ethylene to
acetic acid. Pd-SiW12/SiO2 catalysts with a fixed amount of Pd and SiW12 were prepared in various ways. The order in which Pd and SiW12
were loaded on the SiO2 support and the treatment conditions for the supported Pd have a significant influence on the dispersion of Pd on the
Pd-SiW12/SiO2 catalysts, but no influence on the concentration of B acid sites. The Pd-SiW12/SiO2 catalyst prepared by simultaneously
loading Pd and SiW12 on the SiO2 support showed the best catalytic activity. The dispersion of Pd was the main factor in the catalytic ac-
tivity.
Key words: preparation procedure; palladium; silicotungstic acid; silica; ethylene oxidation; acetic acid
Acetic acid is a very important organic acid, which is used in
a broad range of applications. It is mainly produced from
methanol carbonylation and acetaldehyde oxidation, but both
these processes give rise to many problems of corrosion and
waste disposal [1–4]. The use of Pd-H4SiW12O40(SiW12)/SiO2
for the direct catalytic oxidation of ethylene to acetic acid,
developed by Showa Denko [1], has considerable interest.
SiW12 heteropoly acid has several advantages as a Brönsted
acidic catalyst, which makes it economically and environ-
mentally attractive [5,6]. The Pd-SiW12/SiO2 catalyst is a
promising material for reactions such as the isomerization of
n-pentane and n-heptane [7–9]. Both the palladium catalyst and
an acidic support are common features of the catalysts used for
the selective oxidation of ethylene to acetic acid [10–13]. With
the Pd-SiW12/SiO2 catalyst, Pd and SiW12 act as the redox and
acidic active sites, respectively.
sized. It has been found that the procedure for the preparation
of the Pd-SiW12/SiO2 catalyst, such as the order of application
of Pd and SiW12 on the SiO2 support, led to big differences in
catalytic performance [7,10,16–18]. Therefore, new experi-
mental data on the preparation procedures are needed to elu-
cidate these differences. Furthermore, in the preparation proc-
ess, the role of treatment conditions for the supported Pd is
significant for the dispersion of the active phase. According to
the literature [17,18], before loading the SiW12, reduction of
Pd/SiO2 by H2 led to a lower catalytic activity than reduction by
hydrazine hydrate (N2H4·H2O). Despite extensive research
work on the design and characterization of the Pd-SiW12/SiO2
catalyst, the effect of preparation procedure on the catalytic
performance for the selective oxidation of ethylene to acetic
acid is still unclear. In this report, we describe the influence of
the preparation procedure on the catalytic activity of
Pd-SiW12/SiO2 for the direct oxidation of ethylene to acetic
acid. The catalyst was characterized by X-ray diffraction
(XRD), Fourier transform infrared spectroscopy (FT-IR) of
adsorbed pyridine, and H2-pulse chemical adsorption. The
Various parameters, including the Pd precursor [14] and the
nature of the support [15], are factors that influence the cata-
lytic activity of Pd-SiW12/SiO2. Besides these factors, the im-
portance of the preparation procedure should also be empha-
Received date: 12 May 2010.
*Corresponding author. Tel: +86-411-84379301; Fax: +86-411-84694447; E-mail: cwl@dicp.ac.cn
#Corresponding author. Tel: +86-411-84379073; Fax: +86-411-84694447; E-mail: yangws@dicp.ac.cn
Copyright © 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60123-4

XU Shuliang et al. / Chinese Journal of Catalysis, 2010, 31: 1342–1346
relationship between Pd dispersion, surface acidity, and cata-
For procedure (D), first, SiO2 was added to an aqueous so-
lution of H4SiW12O40 in the appropriate concentration. The
mixture was stirred vigorously for 3 h, then dried at 343 K until
the wet sample became colloidal. Then the sample SiW12/SiO2
was dried in an oven at 383 K overnight. The resulting dry solid
was calcined at 523 K in static air for 3 h. Subsequently, Pd was
supported using the incipient wetness method from a solution
of the PdCl2 dissolved in 0.1 mol/L HCl. The same procedures
were used for drying and calcination as used in procedure (A).
This catalyst was denoted Pd-SiW12/SiO2(D).
lytic properties of the Pd-SiW12/SiO2 catalyst is discussed.
1 Experimental
1.1 Preparation of the catalysts
Pd-SiW12/SiO2 catalysts with fixed amounts of SiW12 and
Pd, mass loadings 30.0 wt% and 1.0 wt%, respectively, were
prepared by various procedures. Commercially available SiO2
(ABET = 384.7 m2/g) was used as the support. The Pd-SiW12/
SiO2 catalysts were prepared according to the following dif-
ferent procedures shown in Fig. 1.
For procedure (E), simultaneously, Pd and SiW12 were
supported onto SiO2 using a mixed solution of PdCl2 dissolved
in 0.1 mol/L HCl and an aqueous solution of H4SiW12O40. The
mixture was stirred vigorously for 3 h, then dried at 343 K until
the wet sample became colloidal. After impregnation, the
sample was dried in an oven at 383 K overnight and then cal-
cined at 523 K in static air for 3 h. This catalyst was denoted
Pd-SiW12/SiO2(E).
For procedure (A), Pd was first impregnated onto SiO2 using
a solution of PdCl2 dissolved in 0.1 mol/L HCl. The mixture
was stirred vigorously for 3 h, then dried at 343 K until the wet
sample became colloidal. Then the Pd/SiO2 sample was dried
in an oven at 383 K overnight. The resulting dry solid was
calcined at 523 K in static air for 3 h. Subsequently, SiW12 was
supported on the Pd/SiO2 by the incipient wetness method from
aqueous H4SiW12O40. After vigorously stirring for 3 h, the
same procedures as those used in preparing the Pd/SiO2 sample
were used for drying and calcination. This catalyst was denoted
as Pd-SiW12/SiO2(A).
1.2 Characterization of the catalysts
Powder XRD patterns were recorded with a Rigaku
D/Max-2500 diffractometer employing Cu KĮ radiation (Ȝ =
0.154 2 nm) in the 2ș range 5°–70° with a scan rate of 0.02º/s at
40 kV and 200 mA.
For procedure (B), Pd was first impregnated onto SiO2 with
the preparation process being the same as procedure (A) but the
sample was reduced in an aqueous solution of 5% hydrazine
hydrate at room temperature for 24 h. The resulting mixed
sample was filtered and washed until no N2H4·H2O was de-
tected. The reduced Pd/SiO2 sample was dried in an oven at
383 K overnight. Finally, SiW12 was supported by the same
incipient wetness method as that in procedure (A). This catalyst
was denoted Pd-SiW12/SiO2(B).
FT-IR spectra of adsorbed pyridine were obtained using an
FT-IR spectrometer. The samples were pressed into a
self-supporting wafer (~10 mg/cm2) and mounted into an in
situ quartz IR cell with CaF2 windows. The nature of the acid
sites was investigated using pyridine as the probe molecule.
Prior to the experiment, the sample was degassed at 523 K at a
pressure of 10–3 Pa. Pyridine was introduced to the evacuated
samples for 10 min at room temperature, followed by evacua-
tion at a fixed temperature (room temperature, 423 K, or 523
K) for 0.5 h. Then, a difference IR spectrum was recorded at
room temperature.
For procedure (C), Pd was first impregnated onto SiO2 with
the preparation process the same as that of procedure (A) but
the sample was reduced in a flow of 5% H2-95% Ar (30
ml/min) at 523 K for 3 h. Finally, SiW12 was supported by the
same incipient wetness method as used in procedure (A). This
catalyst was denoted Pd-SiW12/SiO2(C).
The palladium dispersion was measured by a H2-pulse
chemical adsorption method using an Autosorb-1/C system at
313 K. In a typical experiment, the sample was first pretreated
H4SiW12O40
(A)
(B)
Pd-SiW12/SiO2
PdCl2
5%N2H4•H2O
H4SiW12O40
Pd/SiO2
Pd-SiW12/SiO2
Pd-SiW12/SiO2
Pd/SiO2
Pd/SiO2
5%H2-95%Ar
H4SiW12O40
(C)
(D)
SiO2
H4SiW12O40
PdCl2
SiW12/SiO2
Pd-SiW12/SiO2
PdCl2, H4SiW12O40
(E)
Pd-SiW12/SiO2
Fig. 1. Schematic of the preparation procedures for Pd-SiW12/SiO2 catalysts.

XU Shuliang et al. / Chinese Journal of Catalysis, 2010, 31: 1342–1346
in a flow of He at 523 K for 6 h and subsequently reduced in H2
at 523 K for 1 h. Hydrogen was removed from the system by
evacuating for 2 h. The adsorption isotherm was obtained by
measuring the amount of H2 adsorbed as a function of pressure
at 313 K.
Pd-SiW12/SiO2(E)
Pd-SiW12/SiO2(D)
The specific surface area and pore size distribution of the
catalysts were derived from nitrogen adsorption/desorption
isotherms at 77 K using a Counter Omnisorp-100CX apparatus
using the BET and BJH methods, respectively. The samples
were pretreated at 523 K under high vacuum (1.33 × 10–4 Pa)
for 3 h before the isotherms were recorded.
Pd-SiW12/SiO2(C)
Pd-SiW12/SiO2(B)
1.3 Catalytic reaction
Pd-SiW12/SiO2(A)
10
20
30
40
2T/(o )
50
60
70
The direct oxidation of ethylene to acetic acid was per-
formed in a fixed-bed flow reactor (stainless steel, 10 mm i.d.).
The catalyst (2 ml, 40–60 mesh) was placed in the reactor and
pretreated at 523 K for 1 h under a mixed stream of H2/He (1:1,
vol%) at a flow rate of 60 ml/min. After cooling in a flow of He
to the reaction temperature of 423 K, a mixture of the reactant
gases (C2H4:O2:H2O:He = 50:7:30:13, vol%) was fed into the
reactor at a total flow rate of 100 ml/min (SV = 3 000 h–1) and a
total pressure of 0.6 MPa. A cooling trap filled with water and
ice (~273 K) was used to collect the liquid products, which
were later analyzed by FID-GC (Agilent 6890) equipped with
an FFAP column (30 m × 0.25 mm, film thickness 0.3 ȝm). The
outlet gaseous compounds were analyzed by an online
TCD-GC (Agilent 6890) with 13X (3 m × 2 mm) and Porapak
Q (3 m × 2 mm) columns.
Fig. 2. XRD patterns of Pd-SiW12/SiO2 catalysts prepared by various
procedures.
Brönsted acid sites, obtained from the integrated absorbance
peaks of pyridine at 1 450 and 1 540 cm–1 [19], is shown in
Table 1. As can be seen in Table 1, the Pd-SiW12/SiO2 catalysts
prepared using the various procedures possessed mainly
Brönsted acid sites although small numbers of Lewis acid sites
were also observed. It can be seen that the preparation proce-
dure had little influence on the concentration of B acid sites on
the Pd-SiW12/SiO2 catalysts, but had significant influence on
the L acid sites. It has been reported [20–22] that the appear-
ance of Lewis acid sites on Pd-SiW12/SiO2 catalysts was due to
the presence of Pd2+ electron-deficient metal sites, which ex-
hibited Lewis acidity as a result of the interaction between the
metal sites and the acid sites. The difference in preparation
procedures can lead to different interactions between the metal
sites and acid sites, which explained the difference in the
2 Results and discussion
The XRD patterns of the Pd-SiW12/SiO2 catalysts prepared
using the various procedures are shown in Fig. 2. No diffrac-
tion peak of Pd appeared for all the catalysts, which suggested
that Pd was well dispersed on the surface of the catalysts. The
samples displayed a wide peak at 2ș = 8o (d = 1.1 nm) that can
be attributed to X-ray scattering by non-ordered hydrated
SiW12 polyanions present on the surface as isolated molecular
moieties or small clusters containing a few Keggin units [7].
The absence of signals from bulk SiW12 suggested that it was
also well dispersed on the surface of the silica support. This
observation is significant because the acidity of supported
SiW12 was related to its dispersion on the support [7,10]. It was
reasonable to consider that with a higher SiW12 dispersion,
there are more protons present on the surface of the support.
In order to investigate the nature of the surface acid sites
(Brönsted and Lewis) of the catalyst, the FT-IR spectra of
adsorbed pyridine on the Pd-SiW12/SiO2 catalysts were re-
corded. As shown in Fig. 3, pyridine molecules bonded to
Lewis acid sites give a peak at 1 450 cm–1, while those that
interact with Brönsted acid sites (pyridinium ions) displayed
absorbance at 1 540 cm–1. The concentration of the Lewis and
Pd-SiW12/SiO2(E)
Pd-SiW12/SiO2(D)
Pd-SiW12/SiO2(C)
Pd-SiW12/SiO2(B)
Pd-SiW12/SiO2(A)
1700
1600
1500
1400
Wavenumber (cmꢀ1)
Fig. 3. FT-IR spectra of pyridine adsorbed on Pd-SiW12/SiO2 catalysts
prepared by various procedures.

XU Shuliang et al. / Chinese Journal of Catalysis, 2010, 31: 1342–1346
Table 1 Concentrations of B and L acid sites on Pd-SiW12/SiO2 catalysts
Table 2 Specific surface area and Pd dispersion of Pd-SiW12/SiO2
catalysts prepared by various procedures
prepared by various procedures
Acid site concentration (mmol/g)
ABET
/
APd
/
Pd dispersionb Pd crystallite
a
b
Catalyst
Catalyst
(m2/g)
206.2
203.5
189.1
210.0
210.1
(m2/g)
1.96
2.40
1.60
2.97
3.44
(%)
sizeb (nm)
B acid
0.23
0.24
0.28
0.22
0.26
L acid
0.067
0.024
0.054
0.075
0.089
Pd-SiW12/SiO2(A)
Pd-SiW12/SiO2(B)
Pd-SiW12/SiO2(C)
Pd-SiW12/SiO2(D)
Pd-SiW12/SiO2(E)
44.04
53.91
35.97
66.64
77.22
2.5
Pd-SiW12/SiO2(A)
Pd-SiW12/SiO2(B)
Pd-SiW12/SiO2(C)
Pd-SiW12/SiO2(D)
Pd-SiW12/SiO2(E)
2.1
3.1
1.7
1.5
aMeasured by nitrogen adsorption.
bMeasured by H2-pulse chemical adsorption.
concentration of L acid sites listed in Table 1.
Table 2 shows the specific surface areas measured by the
BET method, and the dispersion of Pd determined from
H2-chemical adsorption. On comparing the different
Pd-SiW12/SiO2 catalysts, no significant variation of the surface
area was observed. This result indicated that the preparation
procedures had little effect on the bulk physical structure of the
catalysts. However, it is found that the Pd-SiW12/SiO2(E)
catalyst prepared by simultaneously supporting Pd and SiW12
on the SiO2 support showed a better dispersion than those
catalysts prepared by the other procedures. This clearly indi-
cated that the dispersion of the Pd metal was strongly de-
pendent on the preparation procedures, and decreased in the
following order: (E) > (D) > (B) > (A) > (C). It is obvious that
impregnating Pd first on the SiO2 support or reduction of
supported Pd was unfavorable for the dispersion of Pd on the
Pd-SiW12/SiO2 catalysts.
In the literature [8–10], it was suggested that the direct
oxidation of ethylene on Pd-SiW12/SiO2 catalysts take place
bifunctionally by cooperation between Pd species and the acid
sites of the support. In the present study, the B acid sites of the
catalysts prepared by various procedures were rather similar. It
is well known that Pd itself plays a vital role in the selective
oxidation of ethylene to acetic acid [23,24]. Usually, the cata-
lytic properties of supported Pd catalysts were dependent on
the Pd dispersion. Thus, the catalytic performance of the Pd-
SiW12/SiO2 catalysts prepared by the various procedures was
mainly controlled by the dispersion of palladium for similar
concentrations of B acid sites. This is in agreement with the
result of our experiment, which showed that the catalytic per-
formance of the Pd-SiW12/SiO2 catalysts prepared by the
various procedures was dependent on the dispersion of palla-
dium.
Table 3 shows the catalytic performance for the oxidation of
ethylene to acetic acid on the various Pd-SiW12/SiO2 catalysts
after 2 h reaction time. The catalytic properties strongly de-
pended on the preparation procedure. The conversion of eth-
ylene, space-time yield (STY) of acetic acid, and selectivity for
acetic acid decreased in the order: Pd-SiW12/SiO2(E) >
Pd-SiW12/SiO2(D) > Pd-SiW12/SiO2(C) > Pd-SiW12/SiO2(A) >
Pd-SiW12/SiO2(B). The Pd-SiW12/SiO2(E) catalyst prepared by
simultaneously loading Pd and SiW12 on the SiO2 support gave
the best catalytic activity. The Pd-SiW12/SiO2(D) catalyst,
which was obtained by first impregnating SiW12 on the SiO2
support, also exhibited better catalytic properties than those
prepared by impregnating Pd first. As was concluded about the
Pd dispersion, it can be seen that impregnating Pd first on the
SiO2 support or reduction of supported Pd was unfavorable for
the catalytic performance of the Pd-SiW12/SiO2 catalysts.
3
Conclusions
A series of Pd-SiW12/SiO2 catalysts prepared by various
procedures and their catalytic performance in the selective
oxidation of ethylene to acetic acid was investigated. The
numbers of B acid sites of the catalysts were similar. The order
in which Pd and SiW12 were supported on SiO2 and the treat-
ment conditions for the supported Pd catalysts had a significant
influence on the dispersion of Pd. The catalytic activity was
dependent on the dispersion of palladium. The Pd-SiW12/SiO2
catalyst prepared by simultaneously supporting Pd and SiW12
on SiO2 gave the best catalytic performance for the selective
oxidation of ethylene. This was attributed to the better disper-
sion of palladium. For fixed amounts of SiW12 and Pd, the
Table 3 Catalytic performance of Pd-SiW12/SiO2 catalysts prepared by various procedures
Conversion of
STY of acetic acid
(g/(L·h))
79.0
Selectivity (%)
Catalyst
ethylene (%)
Acetic acid
53.5
Acetaldehyde
Ethanol
3.0
Ethyl acetate
CO2
24.5
24.9
22.4
15.0
18.7
CO
3.3
2.1
2.8
2.7
2.7
Pd-SiW12/SiO2(A)
Pd-SiW12/SiO2(B)
Pd-SiW12/SiO2(C)
Pd-SiW12/SiO2(D)
Pd-SiW12/SiO2(E)
3.8
3.2
4.2
5.4
5.8
10.5
12.1
9.3
5.2
5.0
5.9
7.9
6.5
64.4
52.4
3.4
90.6
56.2
3.3
134.0
63.9
7.0
3.6
145.2
64.0
5.6
2.5
Reaction conditions: C2H4:O2:H2O:He = 50:7:30:13, GHSV = 3 000 hꢀ1, 150 oC, p = 0.6 MPa, 2 h.

XU Shuliang et al. / Chinese Journal of Catalysis, 2010, 31: 1342–1346
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