Partial photooxidation of ethylene with water as oxidant over copper oxide supported on silica

Article abstract of DOI:10.1006/jcat.2001.3287

Selective photooxidation of ethylene, as a model reaction, was studied using water as an oxidant via irradiation with UV light over copper oxide supported on silica after evacuation at 973 K. Ethylene oxide was formed by irradiation of UV-filtered light at a wavelength longer than 280 nm. For 5 hr of irradiation, a high selectivity to ethylene oxide (82.5%) with an ethylene conversion of 1.6% was achieved at 278 K. However, irradiation of UV-unfiltered light led to the disappearance of ethylene oxide, while CO₂, acetaldehyde, ethanol, ethane, and C₃ and C₄ hydrocarbons were formed. The loading of copper oxide on silica led to an increase in the yield of acetaldehyde and the production of ethanol. The presence of Cu⁺ cations in the pore structure of the catalyst was observed, indicating that a mild oxidant could be generated from water on Cu⁺ sites by UV light irradiation.

Full text of DOI:10.1006/jcat.2001.3287

Journal of Catalysis 202, 427–429 (2001)
doi:10.1006/jcat.2001.3287, available online at http://www.idealibrary.com on
RESEARCH NOTE
Partial Photooxidation of Ethylene with Water as Oxidant
over Copper Oxide Supported on Silica
Yuichi Ichihashi�,1
and Yasuyuki Matsumura†
�
Osaka National Research Institute, AIST, Ikeda, Osaka 563-8577, Japan; and †Research Institute
of Innovative Technology for the Earth, Soraku-gun, Kyoto 619-0292, Japan
Received January 2, 2001; accepted May 15, 2001; published online August 9, 2001
that OH radical may be involved in the photooxidation of
Ethylene can be photooxidized selectively with water as an ox- hydrocarbons over titanium oxide with molecular oxygen
idant via irradiation with UV light over copper oxide supported in the presence of water (6). Hence, water is expected to
on silica after evacuation at 973 K. Ethylene oxide was formed by
irradiation of UV-filtered light at a wavelength longer than 280 nm,
and a high selectivity to ethylene oxide (82.5%) with an ethylene
conversion of 1.6% was obtained at 278 K for 5 h of irradiation.
However, irradiation of UV-unfiltered light results in the disappear-
without molecular oxygen over copper oxide supported on
ance of ethylene oxide, while carbon dioxide, acetaldehyde, ethanol,
be an effective oxidant of hydrocarbons with photocata-
lysts and it may provide a new oxidation process. In the
present study, we will show the possibility of selective pho-
tooxidation of ethylene, as a model reaction, using water
silica.
ethane, and C and C hydrocarbons are formed. X-ray absorption
3
4
near-edge structure (XANES) spectroscopy showed the presence of
+
Cu cations in the pore structure of the catalyst, suggesting that a
EXPERIMENTALS
+
mild oxidant can be generated from water on Cu sites by UV light
irradiation. �c 2001 Academic Press
Copper oxide supported on silica (Fuji Silicia, Cariact
2
� 1
Key Words: photooxidation of ethylene; partial oxidation; G-10; BET surface area, 176 m g ; mean pore diameter,
XANES; photocatalyst; copper oxide supported on silica; decom- 13.7 nm) was prepared by impregnation with an aqueous
position of water.
3 3
solution of Cu(NO ) (Wako Pure Chemical Industries).
The solvent was removed by evacuation at ca. 330 K. The
resulting wet solid was freeze-dried, then calcined in air
for 5 h at 673 K. The sample will be designated as n wt%
CuOx /SiO2, where n represents the content of copper (n =
INTRODUCTION
The selective oxidation of hydrocarbons to their respec-
tive oxygenates is one of the goals of chemical industries.
However, the direct oxidation with molecular oxygen of-
ten results in low selectivity of the desired compounds. The
process of using a photocatalyst offers a unique route to
oxidation (1–4), but it is accompanied by over oxidation
due to the high oxidation potential of the photocatalysts;
such as TiO2. In a photocatalytic system, the decomposi-
tion of water can occur, and it is known that hydrogen
formation is promoted in the presence of organic com-
pounds which consume oxygen from water. Taylor et al.
reported the formation of methanol and carbon monoxide
from methane in a photocatalytic decomposition of water
and suggested the formation ofthe OH radical, which iscon-
sidered to be a mild oxidant (5). Gonzalez et al. also showed
0
.05, 0.1, 1, and 3).
The powder catalyst (150 mg) was spread on the flat bot-
2
tom of a quartz reaction cell (12.5 cm ) connected to a con-
ventional vacuum line equipped with a Baratoron vacuum
gauge. The photoreaction was carried out under a static
condition. Prior to the reaction, the sample was heated
in oxygen at 673 K for 1 h, then evacuated at 973 K for
� 4
1
1
h to 10 Pa. After introduction of ethylene (0.5 kPa,
8 �mol) and water (0.5 kPa, 18 �mol) in gas phase the
catalyst was usually irradiated for 5 h with a high-pressure
Hg lamp (100 W) through a water bath of which the bot-
tom was a transparent quartz glass plate. In some exper-
iments, the light shorter than 280 nm was cut off with a
color filter (Hoya UV-28). The reactants were purified by
vacuum distillation. The products were collected in a liquid
nitrogen trap after the irradiation and analyzed using a gas
chromatograph (Shimadzu GC-8A) equipped with a ther-
mal conductivity detector with a helium carrier gas. Active
1
To whom correspondence should be addressed. E-mail: ichihashi-y
@
aist.go.jp.
427
0021-9517/01 $35.00
Copyright �c 2001 by Academic Press
All rights of reproduction in any form reserved.

4
28
ICHIHASHI AND MATSUMURA
carbon (0.5 m) and Porapak Q (4 m) columns were em-
ployed in the analysis.
Profiles of X-ray absorption near-edge structure
(
XANES) for the samples were taken at room tempera-
ture in fluorescence mode for K-edges of Cu at beam-line
BL01B1 of SPring-8. The sample was sealed with polyethy-
lene films in nitrogen atmosphere.
RESULTS AND DISCUSSION
Ethylene was oxidized mainly to carbon dioxide (CO2)
along with a small amount of acetaldehyde (CH3CHO) by
UV light irradiation without the color filter over silica at
2
93 K in the presence of water (Table 1). This shows that
water can function as an oxidant during UV light irradia-
tion. Ethane (C2H6), C3 and C4 hydrocarbons, and hydro-
gen were also detected, but the quantification of hydrogen
was impossible. No methane formation was detected.
The loading of copper oxide on silica resulted in an
increase in the yield of acetaldehyde and the produc-
tion of ethanol (C2H5OH). The conversion obtained with
FIG. 1. XANES spectra of 0.1 wt% Cu/SiO2 catalyst evacuated at
73 K (a) before reaction and (b) after photoreaction in the presence of
0
.05 wt% CuOx /SiO2 was similar to that of SiO2, but the
9
selectivity of acetaldehyde increased from 0.5 to 5.8% . The
highest conversion was obtained with 0.1 wt% CuOx /SiO2
and the selectivity of acetaldehyde (19.6% ) was relatively
high. In the case of 1 wt% CuOx /SiO2, the conversion de-
creased, but the selectivity to acetaldehyde (14.2% ) was
still high. In the absence of water, C4 hydrocarbons with
the yield of 1.5% was mainly produced over 0.1 wt%
CuOx /SiO2 by irradiation, and the yield of carbon diox-
ide by-produced was only 0.2% . This strongly suggests that
water is the major oxidant in the photocatalysis.
ethylene and water for 1 h, (c) Cu2O, (d) CuO.
are formed on 0.1 wt% CuOx /SiO2 during the evacuation
at 973 K. No significant change in the spectrum of 0.1 wt%
CuOx /SiO2 after irradiation of UV light in the presence of
C2H4 and H2O for 1 h (see Fig. 1b) showed that Cu ions
are not oxidized during the photooxidation.
Since the excitation band of the silica photoluminescence
is around 250 nm and no excitation takes place with light
longer than 280 nm (1, 10), the activity of silica was slight
during the irradiation in the absence of the UV light shorter
than 280 nm. However, the band gap energy of Cu2O is
+
The Cu K-edge X-ray absorption near-edge structure
(
XANES) of 0.1 wt% CuOx /SiO2 after evacuation at 973 K
was rather similar to that of Cu2O and dissimilar to that of
CuO (Fig. 1). Since the concentration of copper is very low
in the catalyst, almost all the copper particles are probably
dispersed on the pore surface of the silica support (7). The
presence of the pre-edge peak at 8984 eV is characteristic of
+
ca. 2.0 eV (ca. 620 nm), the catalyst containing Cu ions
may work with light longer than 280 nm (11, 12). In prac-
tice the irradiation of light longer than 280 nm activated
the CuOx /SiO2 catalysts as summarized in Table 2. Sur-
prisingly, ethylene oxide (C H O) was produced during
+
+
2
4
the Cu ion (8, 9). Hence, it can be assumed that Cu ions
irradiation with light longer than 280 nm while the total
conversion of ethylene decreased. No formation of C3 hy-
drocarbons was observed. Since the production of hydro-
carbons is drastically suppressed with the filtered light, it is
supposed that the surface sites of the silica support mainly
produce ethane and other hydrocarbons with the UV light
shorter than 280 nm. The formation of C3 hydrocarbons
under UV light irradiation may be correlated to the for-
mation of ethane because the cleavage of the C–C bonding
in ethylene should be accompanied by the hydrogenation
process. The conversions of ethylene decreased with an in-
crease in the content of copper above 0.1 wt% , suggesting
that high dispersion of copper is advantageous in the pho-
toreaction with both the filtered and unfiltered light; the
TABLE 1
Product Distribution in the Photocatalytic Reaction of C H and
H O during Irradiation Using UV-Unfiltered Light for 5 h over
2
4
2
CuOx/SiO2
Selectivity (% )
Cu contents Conversion
(
wt% )
of C2H4 (% ) C2H6 C3 C4 CO2 CH3CHO C2H5OH
0
7.7
7.2
11.4
7.4
39.8 26.7 2.3 30.7
28.1 32.3 6.1 24.7
22.2 26.2 5.6 21.9
18.0 29.0 7.9 24.4
11.0 36.9 8.2 24.4
0.5
5.8
19.6
14.2
12.6
0
0
0
1
3
.05
.1
3.0
4.5
6.5
6.9
6.3

PHOTOOXIDATION OF C2H4 WITH H2O OVER CuOx /SiO2
429
TABLE 2
Product Distribution in the Photocatalytic Reaction of C H and H O during Irradiation Using
2
4
2
UV-Filtered Light (� ꢀ 280 nm) over CuOx/SiO2
Reaction
Cu contents temperature
Selectivity/%
Conversion
(
wt% )
(K)
of C2H4 (% )
C2H6
C4
CO2
CH3CHO
C2H4O
C2H5OH
a
0
.05
293
293
293
293
278
278
0.6
1.1
0.8
0.4
1.6
2.5
2.3
1.0
1.9
3.9
0.5
0.4
1.2
0.4
3.7
6.3
0.2
0.4
25.5
39.6
30.8
13.3
2.1
21.4
16.4
32.9
39.5
14.7
14.7
49.6
42.6
28.6
32.9
82.5
81.7
0
0
2.1
4.1
0
a
0
1
3
0
0
.1
a
a
.05a
b
.05
1.8
1.0
a
b
Period of photo irradiation, 5 h.
Period of photo irradiation, 10 h.
+
interaction between copper and silica may stabilize the Cu
species.
reaction temperature, and power of the light source is also
important.
When the reaction was carried out at 278 K over 0.05 wt%
CuOx /SiO2 with light longer than 280 nm, the conversion
of ethylene during a 5-h reaction was significantly higher
than that at 293 K (see Table 2). It is noteworthy that the
selectivity of ethylene oxide increased to more than 80%
while that of carbon dioxide was only 2% . The product dis-
tribution of the reaction for 10 h was similar to that for the
reaction of 5 h, while the conversion increased to 2.5% . It
was also confirmed that the quantity of oxygen atoms in
the products produced in the reaction for 2 days exceeded
the number of copper ions in the 0.05 wt% CuOx /SiO2
catalyst. Thus, it is proved that the formation of ethylene
oxide is a catalytic process and water is the oxidant. The
color of the catalyst, which was yellowish after evacuation
at 973 K, did not obviously change during the reaction. The
reaction was repeatable with the spent catalyst, suggesting
that carbon deposition during the reaction, if present, is
small.
ACKNOWLEDGMENTS
The synchrotron radiation experiments were performed at the SPring-8
with the approval of the Japan Synchrotron Radiation Research Institute
JASRI) (Proposal No. 1999A0030-NX-np).
(
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2
1
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(
Although a detailed mechanism will be the subject of fu-
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for the selective oxidation of hydrocarbons in conjunction
with copper oxide catalysts. Since the catalytic system in
this study was designed as a model reaction to prove it, the
reaction rate obtained was too low as an industrial standard.
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by the further development of the catalyst, which probably
7
8
4
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+
contains a larger number of stable Cu species, while justi-
fication of the reaction conditions such as reaction pressure,