Pd/WO3-ZrO2 as an efficient catalyst for the selective oxidation of ethylene to acetic acid in the vapor phase

Article abstract of DOI:10.1246/cl.2005.642

Pd supported on WO₃-ZrO₂ calcined at temperatures of 973-1073 K exhibits a high activity and selectivity for the oxidation of ethylene to acetic acid in the vapor phase. The space time yield (STY) per unit volume of catalyst is higher for Pd/WO₃-ZrO₂ than for Pd/H₄SiW₁₂O₄₀-SiO₂, the best catalyst reported to date. Copyright

Full text of DOI:10.1246/cl.2005.642

642
Chemistry Letters Vol.34, No.5 (2005)
Pd/WO3–ZrO2 as an Efficient Catalyst for the Selective Oxidation
of Ethylene to Acetic Acid in the Vapor Phase
Wenling Chu,y Yasunobu Ooka,yy Yuichi Kamiya,yy Toshio Okuhara,ꢀyy and Hideshi Hattoriyyy
yOn leave from State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, P. R. China
yyGraduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810
yyyEmeritus Professor of Hokkaido University
(Received February 2, 2005; CL-050149)
lution (40 mmol dmꢁ3) of PdCl2 H2O (Wako Chemical Co.) The
Pd loading was adjusted to 1.0 wt %. After drying at 373 K, the
resulting solid was calcined at 673 K for 5 h, with the tempera-
ture being raised at a rate of 2 K minꢁ1. 1.5 wt % Pd/40 wt %
H4SiW12O40–SiO2 was prepared by impregnation according to
a method described previously.3 The solid was calcined at
523 K in air for 5 h. Cs2:5H0:5PW12O40 and MoO3–ZrO2 (Mo/
Zr = 0.2, calcined at 1073 K) were prepared according to litera-
ture procedures.4,5 SiO2–Al2O3 (JRC-SAL-2), H-ꢀ (Zeolist,
Si/Al ¼ 12:5), H–mordenite (Zeolist, Si/Al ¼ 10) were used af-
ter calcination at 773 K. Nb2O5 (CBMM, HY-340) calcined at
573 K was also used. Pd was loaded onto these supports by the
method described for Pd/WO3–ZrO2.
.
Pd supported on WO3–ZrO2 calcined at temperatures of
973–1073 K exhibits a high activity and selectivity for the oxida-
tion of ethylene to acetic acid in the vapor phase. The space time
yield (STY) per unit volume of catalyst is higher for Pd/WO3–
ZrO2 than for Pd/H4SiW12O40–SiO2, the best catalyst reported
to date.
Recently, it was reported that a catalyst composed of Pd and
a heteropoly acid exhibited a high yield of acetic acid in the one-
step gas-phase oxidation of ethylene.1 Among the heteropoly
acids, silicotungstic acid (H4SiW12O40) exhibited the highest
space time yield (STY) of acetic acid. Sano et al. suggested that
the acidic nature of H4SiW12O40 contributed to its high activity
in the oxidation of ethylene.1 They also reported that the addition
of Se and Te to the catalyst increased the STY of acetic acid, due
to the suppression of CO2 formation.1 A plant for the production
of acetic acid based on the catalyst Pd/H4SiW12O40–SiO2 was
commercialized in 1998. In the present publication we present
Pd/WO3–ZrO2 as an efficient catalyst for the production of
acetic acid from ethylene. Pd/WO3–ZrO2 is a more efficient
catalyst than a number of other catalysts, including Pd/
H4SiW12O40–SiO2.
WO3–ZrO2 with different W/Zr ratios was prepared by an
incipient wetness method from an aqueous solution (16
mmol dmꢁ3) of (NH4)10W12O41 (Wako Chemical Co.) and
Zr(OH)4 (Dai-ich Kigenso Kagaku Kogyo Ltd., dried overnight
at 373 K).2 The resulting solid was dried at 373 K and calcined at
a pre-determined temperature for 3 h, with the temperature being
gradually increased at a rate of 10 K minꢁ1. Pd/WO3–ZrO2 was
prepared by the impregnation of WO3–ZrO2 with an aqueous so-
The oxidation of ethylene was performed in a fixed-bed flow
reactor (stainless steel SUS 317, 10 mm inside diameter). Cata-
lyst (2 cm3, 60–80 mesh) was placed in the reactor and pretreated
at 573 K for 1 h under a stream of H2–He (1:1 mixture) at a flow
rate of 60 cm3 minꢁ1. After cooling under the He flow to a reac-
tion temperature of 423 K, a mixture of the reactant gas
(C2H4:O2:H2O:He = 50:7:30:13 in vol. %) was fed into the re-
actor at a total flow rate of 100 cm3 minꢁ1 (SV ¼ 3000 hꢁ1) and
total pressure of 0.6 MPa. The effluent gas was introduced to a
trap kept at 200 K in order to collect the liquefied products.
The liquefied components were analyzed by gas chromatography
(Simadzu 8A, FID with Porapak QS column). The gaseous prod-
ucts passing through the trap were directly injected into a gas
chromatograph (Aera, Micro GC M200).
The results of catalytic reactions with different types of cat-
alysts are summarized in Table 1. It is noted that Pd/WO3–ZrO2
gave the highest STY of acetic acid per unit volume of catalyst.
In addition, Pd/WO3–ZrO2 exhibited high selectivity for acetic
Table 1. Catalytic data for gas-phase oxidation of ethylene over various catalystsa
STY of AcOHc
Catalystb
Conversion
of O2/%
Selectivity/%
AcHd EtOHd
/mol hꢁ1 dmꢁ3 (mol hꢁ1 kgꢁ1
)
AcOHd
COx
e
Pd/WO3–ZrO2
Pd–Te/WO3–ZrO2
1.21 (0.69)
0.91 (0.56)
0.69 (0.90)
0.44 (0.21)
0.42 (0.26)
0.03 (0.06)
0.02 (0.05)
0.01 (0.01)
0.03 (0.03)
74
81
55
17
36
29
38
22
24
8
10
15
7
19
43
10
18
52
0
0
10
15
0
0
4
4
1
18
9
32
27
21
68
25
5
6
4
4
f
g
Pd/H4SiW12O40–SiO2
Pd/Cs2:5H0:5PW12O40
20
61
45
28
48
56
23
h
Pd/MoO3–ZrO2
Pd/SiO2–Al2O3
Pd/H–ꢀi
Pd/H–mordenitej
Pd/Nb2O5
aReaction conditions: C2H4:O2:H2O:He = 50:7:30:13, GHSV ¼ 3000 hꢁ1, temperature 423 K, and pressure 0.6 MPa. The data was col-
lected at 3 h. bLoading amount of Pd was 1.0 wt % except for 1.5 wt %Pd/40 wt %H4SiW12O40–SiO2; cSpace time yield of acetic acid;
dAcOH: acetic acid, AcH: acetaldehyde, EtOH: ethanol; eW/Zr ¼ 0:2 and calcined at 1073 K; fTe/Pd atomic ratio was 0.1;
h
i
j
g1.5 wt %Pd/40 wt %H4SiW12O40–SiO2; Mo/Zr ¼ 0:2 and calcined at 1073 K; Si/Al ¼ 12:5; Si/Al ¼ 10.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.5 (2005)
643
acid among the catalysts examined. As reported by Sano et al.,1
although Pd/H4SiW12O40–SiO2 was a highly active catalyst for
the reaction, its selectivity for acetic acid was moderate. Thus,
Table 1 demonstrates that Pd/WO3–ZrO2 has potential for prac-
tical use. As a matter of fact, the addition of Te to Pd/WO3–
ZrO2 greatly enhanced its selectivity for acetic acid, producing
81% acetic acid and 10% acetaldehyde. Time courses for this re-
action over Pd/WO3–ZrO2 and Pd/H4SiW12O40–SiO2 showed
that changes in the activity and selectivity were small for a peri-
od up to at least 9 h for both catalysts.
Pd/Cs2:5H0:5PW12O40 and Pd/MoO3–ZrO2 also exhibited
high activities, comparable to that of Pd/H4SiW12O40–SiO2,
however the selectivity of this catalyst was very low. Both Pd/
H-ꢀ and Pd/H-mordenite were less active and less selective.
Pd/SiO2–Al2O3 and Pd/Nb2O5 were inferior to the other
catalysts.
To optimize the conditions for Pd/WO3–ZrO2, the effect of
the Pd starting material, the Pd loading, the calcination temper-
ature of the WO3–ZrO2 support, the W/Zr ratio and the pretreat-
ment of Pd/WO3–ZrO2 (calcination and reduction temperatures)
were investigated. The Pd starting material affected the catalytic
activity of the resulting catalysts. The catalyst prepared using
PdCl2 exhibited a higher STY than those prepared using
(NH4)2PdCl4 and Pd(NO3)2. In particular, the use of the latter
two catalysts generated significant amount of CO2. PdCl2 is
therefore the preferred source of Pd. Although the Pd loading
did not greatly affect the STY, the catalyst loaded with 1 wt %
Pd showed the highest STY.
Figure 2. Effect of atomic ratio of W/Zr for WO3–ZrO2 on STY
of acetic acid and selectivity for oxidation of ethylene over
1 wt %Pd/WO3–ZrO2. WO3–ZrO2 was calcined at 1073 K. (
)
STY and selectivity to ( ) acetic acid, ( ) acetaldehyde, and
( ) CO2.
and selectivity. Pd/WO3–ZrO2 exhibited the highest STY with
W/Zr = 0.2 and the highest selectivity for acetic acid with
W/Zr ¼ 0:05. The surface areas of WO3–ZrO2 were measured
to be 46, 72, 63, 61, and 45 m2 gꢁ1 for 0.05, 0.1, 0.2, 0.3, and
0.4 of W/Zr ratio, respectively. The surface densities of WO3
were 5.1, 5.7, 11.2, 15.4, and 24.8 W-atom nmꢁ2 for 0.05, 0.1,
0.2, 0.3, and 0.4 of W/Zr ratio, respectively, calculated using
the surface areas and the W/Zr loadings. Since, in theory, 7
W-atom nmꢁ2 is just sufficient to cover the ZrO2 surface with
a monolayer WO3,8 it is considered that monolayer WO3 species
and/or three-dimensional WO3 clusters9 would be formed on
Pd/WO3–ZrO2 with the W/Zr range of 0.05–0.3, the catalysts
which showed the high STY and high selectivity for acetic acid.
Since WO3–ZrO2 with a WO3-surface density of 7–15 W-atom
nmꢁ2 shows the highest activity for acid catalyzed reactions
such as the benzoylation of toluene,6 and isomerizations of pen-
tane6 and o-xylene,7 acidity of WO3–ZrO2 would much contrib-
ute to the formation of the active sites. From contact time de-
pendencies and indispensableness of water to the reaction we
have previously demonstrated that ethylene is oxidized to acetal-
dehyde over Pd/WO3–ZrO2 through a Wacker-type reaction,
with acetaldehyde being further oxidized to acetic acid.10 Con-
sidering the similarity to the Wacker oxidation using homogene-
ous PdCl2–CuCl2 catalyst, the acidity of WO3–ZrO2 thus may
play an important role in forming active sites such as Pd2þ
Figure 1. Effect of calcination temperature of WO3–ZrO2 on
STY of acetic acid and selectivity for oxidation of ethylene over
1 wt %Pd/WO3–ZrO2(W/Zr ¼ 0:2). ( ) STY and selectivity to
10
.
This work was supported by Core Research for Evolution
Science and Technology (CREST) of the Japan Science and
Technology Corporation (JST).
(
) acetic acid, ( ) acetaldehyde, and ( ) CO2.
Figure 1 shows the influence of the calcination temperature
of the WO3–ZrO2 support on the activity and selectivity. The op-
timum temperatures were 1073 K for the greatest selectivity for
acetic acid and 973 K for the highest activity. It is remarkable
that the selectivity for acetic acid increases instead of decrease
of the selectivity for acetaldehyde as the calcination temperature
increased. The surface area of the catalyst decreased monotoni-
cally as the calcination temperature increased (124, 109, 82, and
72 m2 gꢁ1 for 773, 873, 973, and 1073 K, respectively). Thus, the
observed change in activity (Figure 1) probably relates to the
change in acidity of WO3–ZrO2, since the activities for acid-cat-
alyzed reactions were higher at higher WO3–ZrO2 calcination
temperatures.6,7
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Figure 2 shows effect of the W/Zr atomic ratio on the STY
Published on the web (Advance View) March 26, 2005; DOI 10.1246/cl.2005.642