Significant synergistic effect between supported ruthenium and copper oxides for propylene epoxidation by oxygen

Article abstract of DOI:10.1002/cplu.201100050

This work was supported by the National Basic Research Program of China (No. 2010CB732303), the NSF of China (Nos. 21173172, 20873110, and 21033006), the Key Scientific Projects of Fujian Province (2009HZ002-1), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1036).

Full text of DOI:10.1002/cplu.201100050

DOI: 10.1002/cplu.201100050
Significant Synergistic Effect between Supported Ruthenium and Copper
Oxides for Propylene Epoxidation by Oxygen
[
a]
Wenjing Long, Qingge Zhai, Jieli He, Qinghong Zhang,* Weiping Deng, and Ye Wang*
Propylene oxide (PO), which is one of the most important bulk
chemicals, is currently produced mainly by the chlorohydrin
and the organic hydroperoxide processes. Both processes pro-
duce large amounts of by-products and are not atomically eco-
nomical. Dow Chemical and BASF have recently developed a
new industrial process for the direct epoxidation of propylene
by hydrogen peroxide, which is a green and atomically eco-
for epoxidation reactions because they mainly catalyze the
[3]
complete oxidation of C H . A few studies have demonstrat-
3
6
ed that RuO alone cannot catalyze the epoxidation of C H by
2
2
4
O , although the oxygen species over oxygen-rich RuO surfa-
2
2
[14–15]
ces may be electrophilic.
A theoretical study predicted a
negative effect of the addition of Ru to Cu on the epoxidation
[16]
of C H . Thus, it would be of significance to investigate ex-
3
6
nomical process that leads to the formation of H O as the sole
perimentally whether noble metal oxides such as RuO present
2
x
[
1]
by-product. The epoxidation of propylene by oxygen is the
most ideal route for the production of PO because oxygen is
much cheaper than hydrogen peroxide. However, this route
still remains one of the biggest challenges and is viewed as a
in the Cu-based catalysts in the absence of an alkali metal
halide can exert positive effects on the epoxidation of C H by
3
6
O . Herein, we report our recent finding that there exist signifi-
2
cant synergistic effects between RuO and CuO loaded on SiO
2
x
x
[
2]
“
Holy Grail” in catalysis. There has been little success in the
for the epoxidation of C H by O .
3 6 2
development of efficient catalysts for the epoxidation of C H6
We first examined the effect of various noble metal oxides
on the catalytic behavior of CuO /SiO for the epoxidation of
3
by O although the Ag-catalyzed epoxidation of ethylene by
2
x
2
[
3]
O has been commercialized for several decades. The main
C H . Table 1 shows that acrolein is the main product over the
3 6
2
difficulty is believed to arise from the higher reactivity of the
allylic CÀH bonds, which exist in C H but are not present in
3
6
C H . The allylic oxidation is more competitive over many het-
Table 1. Effect of noble metal oxides on catalytic performances of
2
4
[
a]
CuO
x
/SiO
2
catalysts for C
3
H
6
epoxidation by O
2
.
erogeneous catalysts in O , leading to the formation of acrolein
2
followed by consecutive oxidation to carbon oxides (COx).
Recently, Cu-based catalysts have attracted much attention
for the epoxidation of C H by O . Supported CuO is a well-
[b]
[c]
Catalyst
Conv. [%]
Selectivity [%]
Acrolein
PO yield [%]
PO
COx
3
6
2
x
CuO
x
/SiO
2
1.4
5.2
2.5
1.7
4.8
1.4
0.94
2.7
1.4
36
11
2.5
12
3.3
6.0
2.1
78
7.7
69
92
33
92
88
92
5.1
55
16
2.7
51
1.3
5.4
2.0
0.020
1.9
0.27
0.043
0.58
0.046
0.056
0.057
known C H allylic oxidation catalyst that mainly produces
RuO –CuO /SiO
3
6
x
x
x
x
x
x
2
2
2
[
4]
acrolein. Interestingly, the increase in the dispersion of active
RhO
PdO
–CuO
–CuO
/SiO
/SiO
species or the modification of CuO by alkali metal ions pro-
x
IrO
PtO
Ag–CuO /SiO
x
–CuO
x
/SiO
2
[
5–10]
0
I
vides PO with considerable selectivities.
Cu and Cu have
x
–CuO
x
/SiO
2
both been claimed to be the active sites for the epoxidation of
x
2
2
[
5–9]
C H by O .
Very recently, with the aid of high-throughput
x
Au–CuO /SiO
3
6
2
[
11]
catalyst screening technique, Senkan et al.
communicated
À1
[
a] Reaction conditions: T=498 K, W=0.20 g, F=60 mLmin , P(C
3
H
6
)=
that a RuO –CuO –NaCl/SiO catalyst exhibited very promising
2.5 kPa, P(O
2
)=98.8 kPa. [b] CuO and noble metal oxide loadings were
2
x
2
5
.0 and 2.0 wt%, respectively; the noble metal additive may exist in met-
performances for the epoxidation of C H by O ; PO selectivi-
3
6
2
allic state. [c] The other products were allyl alcohol, propanal, acetone,
and acetaldehyde.
ties of 40–50% were obtained at C H conversions of 10–15%.
3
6
The roles of alkali metal cations and halide anions in enhanc-
[
11,12]
ing the PO selectivity have mainly been discussed.
The de-
activation of the catalyst, especially the decrease in PO selec-
tivity, was observed with time on stream in the absence of gas-
eous halide modifiers; for example, PO selectivity decreased
5 wt% CuO /SiO catalyst, consistent with the knowledge that
x
2
the supported CuO mainly catalyzes the allylic oxidation of
x
[12]
[4]
from 45% to approximately 30% after 10 hours of reaction.
C H . Most of the noble metal oxide (maybe in a metallic
3
6
The roles of RuO in PO formation in this system are unclear.
state) additives listed in Table 1 increased the conversion of
C H . The presence of PdO , PtO , and Au remarkably promoted
x
Noble metal oxides are generally believed not to be suitable
3
6
x
x
the selectivity of acrolein. On the other hand, the incorporation
of RuO , IrO , and RhO into the CuO /SiO significantly in-
x
x
x
x
2
[
a] W. Long, Q. Zhai, J. He, Prof. Dr. Q. Zhang, Dr. W. Deng, Prof. Dr. Y. Wang
State Key Laboratory of Physical Chemistry of Solid Surfaces
National Engineering Laboratory for Green Chemical Productions
of Alcohols Ethers and Esters, Department of Chemistry
College of Chemistry and Chemical Engineering
Xiamen University, Xiamen, 361005 (P. R. China)
Fax: (+86)592-2183047
creased the selectivity to PO. The RuO –CuO /SiO catalyst, par-
x
x
2
ticular, was outstanding for PO formation; a PO selectivity of
36% was achieved at a C H conversion of 5.2% at 498 K. This
3
6
combination of PO selectivity and C H conversion was even
3
6
better than those over the alkali metal ion-modified CuOx-
based catalysts, where PO selectivity decreased to <30% as
E-mail: zhangqh@xmu.edu.cn
[5–10]
wangye@xmu.edu.cn
C H conversion increased to >5%.
3 6
ChemPlusChem 2012, 77, 27 – 30
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
27

Then, we investigated the catalytic behavior of supported
binary oxides containing RuOx and another transition-metal
oxide. As displayed in Table 2, the 3 wt% RuO /SiO provided a
x
2
Table 2. Effect of transition metal oxides on catalytic performances of
[
a]
RuO
x
/SiO
2
catalysts for C
3
H
6
epoxidation by O
2
.
[
b]
[c]
Catalyst
Conv. [%]
Selectivity [%]
Acrolein
PO yield [%]
PO
CO
x
RuO
RuO
RuO
RuO
RuO
RuO
RuO
RuO
RuO
RuO
RuO
x
x
x
x
x
x
x
x
x
x
x
/SiO
–VO
–CrO
2
1.1
0.63
2.2
3.3
0.17
2.3
0.44
13
1.0
0.27
2.0
3.9
0
0
0.13
3.7
1.5
0
26
0
16
20
29
2.4
52
4.6
27
3.1
32
13
20
73
36
50
97
10
94
71
69
59
8.4
71
0.020
0
0
0.0043
0.0063
0.035
0
3.4
0
0.0073
0.16
x
/SiO
2
x
/SiO
/SiO
/SiO
/SiO
/SiO
/SiO
2
–MnO
–FeO
–CoO
–NiO
–CuO
–ZnO/SiO
–MoO /SiO
–Ag/SiO
x
2
x
2
x
2
x
2
x
2
2
x
2
2.7
7.8
2
Figure 1. Effect of the addition of RuO
x
with different contents into 5 wt%
epoxidation. A) C conver-
À1
[
2
a] Reaction conditions: T=498 K, W=0.20 g, F=60 mLmin , P(C
3
H
6
)=
CuO /SiO on catalytic performances for C
x
2
3
H
6
3 6
H
2 2
.5 kPa, P(O )=98.8 kPa. [b] RuO and transition metal oxide loadings
were 3.0 and 5.0 wt%, respectively. [c] The other products were allyl alco-
hol, propanal, acetone and acetaldehyde.
sion and PO yield; B) product selectivity. Reaction conditions: T=498 K,
À1
W=0.20 g, F=60 mLmin , P(C
3
H
6
)=2.5 kPa, P(O
2
)=98.8 kPa.
C H conversion of 1.1% under our reaction conditions, and as
3
6
expected, the selectivity to CO over this catalyst was quite
x
high. Acrolein was the main component in the partial oxida-
tion products. The addition of most of the transition-metal
oxides (maybe in the metallic state) listed in Table 2 into the
RuO /SiO catalyst did not exert positive effects on PO forma-
x
2
tion. The combination of RuO with only CuO and Ag en-
x
x
hanced the PO selectivity, and the RuO –CuO combination
x
x
also markedly increased C H conversion. C H conversion and
3
6
3
6
PO selectivity of 13% and 26% were achieved over the 3 wt%
RuO –5 wt% CuO /SiO catalyst.
x
x
2
Our results have demonstrated that the combination of
RuO and CuO is very unique for the epoxidation of C H by
x
x
3
6
O . To gain further insights into the synergistic effect between
2
RuO and CuO , we have investigated the effect of the compo-
x
x
sition of the supported binary metal oxides. Figure 1 shows
that the increase in the RuO content up to 3 wt% in the
x
RuO –5 wt% CuO /SiO catalysts significantly increases C H6
x
x
2
3
conversion. On increasing the RuO content from 0 to 2 wt%,
x
Figure 2. Effect of the addition of CuO with different contents into 2 wt%
x
the selectivity to acrolein decreased significantly from 78% to
x 2 3 6 3 6
RuO /SiO on catalytic performances for C H epoxidation. A) C H conver-
sion and PO yield; B) product selectivity. Reaction conditions: T=498 K,
<
8%. At the same time, the selectivities to PO rose from 1.4%
À1
W=0.20 g, F=60 mLmin , P(C
3
H
6
)=2.5 kPa, P(O
2
)=98.8 kPa.
to 36%. A further increase in RuO content decreased PO selec-
x
tivity. The selectivity to CO increased monotonically with the
x
RuO content.
tion (2 wt% RuO and 5 wt% CuO) but prepared by different
x
2
Figure 2 displays that C H conversion increases significantly
methods. Table 3 shows that the physical mixture of single
3
6
on increasing the CuO content in the supported binary cata-
RuO /SiO and single CuO /SiO [denoted as (RuO + CuO )/
x
x
2
x
2
x
x
lysts with a fixed RuO content (2 wt%). The increase in the
SiO ] only exhibits a very low PO selectivity as well as a lower
x
2
CuO content from 0 to 5 wt% increased PO selectivity from
C H conversion. The performances of the catalysts prepared
x
3
6
4
.3% to 36%. At the same time, the selectivities to acrolein
by the two-step impregnation depended on the sequence
used for impregnation. The preparation of RuO /SiO in the
and CO decreased. A further increase in the CuO content de-
x
x
x
2
creased the selectivity to PO and mainly increased that to COx.
We further compared the catalytic performances of the sup-
ported RuO and CuO binary oxides with the same composi-
first step and then impregnating the calcined RuO /SiO with
x 2
Cu(NO ) aqueous solution followed by calcination at 823 K in
3
2
the second step (denoted as CuO /RuO /SiO ) provided a cata-
x
x
x
x
2
2
8
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2012, 77, 27 – 30

selectivity of 5.7% and 32% were sustained after a reaction
time of 25 h, demonstrating that the present supported binary
metal oxide catalyst was quite stable.
Table 3. Effect of preparation methods on catalytic performances for
C H epoxidation by O .
3 6 2
[
a]
[
b]
[c]
Catalyst
Conv. [%]
Selectivity [%]
PO yield [%]
Furthermore, we found a unique dependence of catalytic
performances on the partial pressure of O [P(O )] over our
PO
Acrolein
CO
x
2
2
5
2
wt% CuO
wt% RuO
+CuO
x
/SiO
/SiO
)/SiO
2
1.4
0.76
1.9
3.2
5.5
5.2
1.4
4.3
4.7
2.8
23
78
27
54
72
13
7.7
5.1
64
39
24
60
55
0.020
0.033
0.089
0.090
1.3
supported binary oxide catalyst. At a fixed partial pressure of
x
2
C H [P(C H )=20 kPa], the increase in P(O ) increased not only
3
6
3
6
2
(RuO
x
x
2
C H conversion but also PO selectivity. At the same time, the
CuO
RuO
RuO
x
/RuO
/CuO
x
x
/SiO
/SiO
2
2
3
6
selectivity to acrolein decreased (Figure 4). The selectivity to
x
x
–CuO
x
/SiO
2
36
1.9
À1
[
a] Reaction conditions: T=498 K, W=0.20 g, F=60 mLmin , P(C
3
H
6
)=
2.5 kPa, P(O
2 2
)=98.8 kPa. [b] RuO and CuO loadings were 2.0 and
5.0 wt%, respectively. [c] The other products were allyl alcohol, propanal,
acetone, and acetaldehyde.
lyst with a low PO selectivity. On the other hand, the prepara-
tion of CuO /SiO in the first step and impregnating the cal-
x
2
cined CuO /SiO with RuCl aqueous solution followed by calci-
x
2
3
nation in the second step (denoted as RuO /CuO /SiO ) result-
x
x
2
ed in a catalyst with a considerable PO selectivity (23%). How-
ever, the PO selectivity over the RuO /CuO /SiO was still lower
x
x
2
than that over the catalyst prepared by the co-impregnation
method (denoted as RuO –CuO /SiO ), which was employed as
x
x
2
a standard method in the present work.
All these results clearly suggest that significant synergistic
effects exist between RuO and CuO for PO formation, and the
x
x
direct contact between RuO and CuO may be a key factor.
x
x
We performed further studies for the 2 wt% RuO –5 wt%
x
Figure 4. Effect of partial pressure of O
wt% RuO –5 wt% CuO /SiO catalyst. Reaction conditions: T=498 K,
W=0.20 g, F=60 mLmin , P(C )=20 kPa.
2
on catalytic performances over the
CuO /SiO catalyst. Considering that chlorine may play a role in
x
2
2
x
x
À1
2
[
12]
enhancing the PO selectivity, we have measured the chlorine
content on the surface of the 2 wt% RuO –5 wt% CuO /SiO
2
3
H
6
x
x
catalyst by the X-ray photoelectron spectroscopic (XPS) tech-
nique. Our results revealed that no chlorine existed on our cat-
alyst. Moreover, the chlorine-enhanced PO formation was un-
stable in the absence of gaseous organic chlorides such as
CO increased moderately with increasing P(O ). It is generally
x
2
accepted that acrolein is formed via the allylic oxidation of
[4]
C H by the lattice oxygen over supported CuO catalysts,
3
6
x
[
12]
EtCl2. Thus, we measured the stability of the 2 wt% RuOx–
wt% CuO /SiO catalyst. Figure 3 shows that both C H con-
whereas the epoxidation requires the electrophilic oxygen spe-
[3]
5
cies derived from the adsorption of O2. Figure 4 implies that
both types of reactions occurred over our catalyst. The obser-
x
2
3
6
version and PO selectivity did not undergo significant changes
with time on stream over this catalyst. C H conversion and PO
vation that higher P(O ) favors the selectivity to PO suggests
3
6
2
the crucial role of adsorbed O species derived from gaseous
2
O in the epoxidation of C H to PO.
2
3
6
We have made preliminary characterizations for the 2 wt%
RuO –5 wt% CuO /SiO catalyst. The in situ X-ray diffraction
x
x
2
(
XRD) measurements uncovered that almost no changes oc-
curred during the reaction and the catalyst contained crystal-
line RuO2 and CuO phases (Figure 5). No formation of new
compound was observed. From transmission electron micros-
copy (TEM) images (Figure 6), we observed both isolated RuO2
nanoparticles (Figure 6B) and the nanocomposites containing
both RuO and CuO nanoparticles (Figure 6C). The mean size
2
of these particles was 7.3 nm. We speculate that the nanocom-
posites containing RuO and CuO in direct contact may ac-
2
count for the epoxidation of C H to PO by O .
3
6
2
In conclusion, we have found a strong synergistic effect be-
tween CuO and RuO loaded on SiO for the epoxidation of
Figure 3. Changes of C
stream over the 2 wt% RuO
T=498 K, W=0.20 g, F=60 mLmin , P(C
3 6
H conversion and PO selectivity with time on
-.5 wt% CuO
x
/SiO
2
catalyst. Reaction conditions:
)=98.8 kPa.
x
x
2
x
À1
3
H
6
)=2.5 kPa, P(O
2
C H by O . While both supported single CuO and single RuO
3
6
2
x
x
ChemPlusChem 2012, 77, 27 – 30
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chempluschem.org
29

pared by the impregnation method using Cu(NO ) or RuCl as the
3
2
3
precursor. The co-impregnation method was employed for the
preparation of the RuO –CuO /SiO or SiO -supported other binary
x
x
2
2
metal oxides. Briefly, the powdery SiO was added into the mixed
2
aqueous solution containing the precursors, and the suspension
was stirred for 4 h, followed by resting for 20 h. After being evapo-
rated to dryness at 353 K, the mixture was further dried at 393 K in
air, followed by calcination in air at 823 K for 6 h. XRD patterns
were collected on a Panalytical X’pert Pro diffractometer using
CuKa radiation (40 kV, 30 mA). TEM measurements were performed
on a JEM-2100 electron microscope operated at an acceleration
voltage of 200 kV.
Catalytic reactions were performed by using a fixed-bed quartz re-
actor operated at atmospheric pressure. Typically, the catalyst was
pretreated in the quartz reactor with a gas flow containing He and
O at 623 K, followed by purging with He before reaction. The reac-
2
Figure 5. In situ XRD patterns for the 2 wt% RuO
Reaction conditions: T=498 K, F=120 mLmin , P(C
P(O )=98.8 kPa.
2
x
–5 wt% CuO
x
/SiO
2
catalyst.
À1
tion was started by introducing a reactant gas flow containing
C H and O to the reactor at a determined reaction temperature.
3
H
6
)=2.5 kPa,
3
6
2
The products were analyzed by on-line gas chromatography.
Acknowledgements
This work was supported by the National Basic Research Pro-
gram of China (No. 2010CB732303), the NSF of China (Nos.
21173172, 20873110, and 21033006), the Key Scientific Projects of
Fujian Province (2009HZ002–1), and the Program for Changjiang
Scholars and Innovative Research Team in University (No.
IRT1036).
Keywords: copper · epoxidation · heterogeneous catalysis ·
propylene · ruthenium
[
[
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[
[
1
Experimental Section
2
À1
SiO with a specific surface area of 678 m g was synthesized by a
Received: November 16, 2011
Published online on January 4, 2012
2
[8]
sol–gel method. SiO -supported single CuO or RuOx was pre-
2
x
3
0
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ChemPlusChem 2012, 77, 27 – 30