Epoxidation of styrene by t-butyl hydroperoxide over gold supported on Yb2O3 and other rare earth oxides

Article abstract of DOI:10.1246/cl.2004.400

Gold nanoparticles deposited [by homogeneous deposition-precipitation (HDP) using urea] on Yb₂O₃ and other rare-earth oxides (viz. Sm₂O₃, Eu₂O₃, and Tb ₂O₃) are novel

Full text of DOI:10.1246/cl.2004.400

400
Chemistry Letters Vol.33, No.4 (2004)
Epoxidation of Styrene by t-Butyl Hydroperoxide over Gold Supported on Yb2O3
and Other Rare Earth Oxides
N. S. Patil,y;yy B. S. Uphade,y P. Jana,y S. K. Bhargava,yy and V. R. Choudharyꢀ;y
yChemical Engineering Division, National Chemical Laboratory, Pune-411-008, India
yyDepartment of Applied Chemistry, RMIT, Melbourne-3000, Australia
(Received December 8, 2003; CL-031194)
Gold nanoparticles deposited [by homogeneous deposition-
anhydrous t-butyl hydroperoxide (TBHP) (26% TBHP in ben-
zene) in a stirred batch reactor (capacity: 10 cm3) under reflux
(at 82 ꢁC) and vigorous stirring for a period of 3 h, while
removing continuously the reaction water using the Dean–Stark
trap.5
Results in Figure 1 indicate that, among the rare-earth oxide
supported gold catalysts (prepared by HDP), Au/Yb2O3 shows
the best performance (64% conversion and 55% styrene oxide
selectivity). The Au/Tb2O3, Au/Eu2O3, and Au/Sm2O3 cata-
lysts also showed a comparable performance. The order of the
catalysts for their performance in the epoxidation is as follows:
Au/Yb2O3 > Au/Tb2O3 ꢂ Au/Eu2O3 ꢃ Au/Sm2O3 > Au/
CeO2 > Au/La2O3 ꢄ Au/Er2O3 > Au/Nd2O3 ꢄ Au/Gd2O3.
The Au/Gd2O3 showed very high TBHP decomposition activity
and hence it is not useful for the epoxidation. The gold loading in
all the catalysts was more or less the same (6:5 ꢅ 0:3 wt %).
precipitation (HDP) using urea] on Yb2O3 and other rare-earth
oxides (viz. Sm2O3, Eu2O3, and Tb2O3) are novel highly ac-
tive/selective and reusable catalysts for the selective epoxida-
tion of styrene to styrene oxide by anhydrous or aqueous t-butyl
hydroperoxide and no problems are foreseen for a large scale op-
eration of this green process.
Epoxidation of styrene is a practically important reaction for
producing styrene oxide, an industrially important organic inter-
mediate. It is conventionally carried out using organic peracids
as an oxidizing agent,1 which is very expensive, hazardous to
handle and non-selective for the styrene oxide formation, and al-
so leads to a number of undesirable products, creating a lot of
waste. To avoid the waste production, recently a few solid cata-
lysts, such as TS-1,2 Ti/SiO2,3 TS-2, and TBS-24 and ꢀ-alumi-
na,5 using H2O2,2–5 H2O2-urea adduct6 or TBHP7 as the oxidiz-
ing agent for the epoxidation of styrene have been reported.
However, the catalysts show low activity2–4,6,7 and/or poor se-
lectivity for styrene oxide2–4 or deactivation by the reaction wa-
ter.5 It is, therefore, of both scientific and practical interest to de-
velop a better solid catalyst having both high activity and high
selectivity in the epoxidation.
60
Phenyl acetaldehyde
40
20
8
Recently, supported nano-gold, particularly Au/TiO2 and
Au deposited on Ti-containing micro- and mesoporous silica,9,10
has attracted a lot of attention for the direct vapour phase oxida-
tion to propylene to propylene oxide by mixed H2 and O2. We
report here that gold nanoparticles supported on Yb2O3 and oth-
er rare earth oxides (viz. Sm2O3, Eu2O3, and Tb2O3), prepared
by homogeneous deposition precipitation (HDP), shows both
high activity and high selectivity in the liquid phase epoxidation
of styrene to styrene oxide by anhydrous or aqueous TBHP, re-
quiring much shorter reaction time for achieving styrene conver-
sion of practical interest (>50%).
Other products
0
60
40
20
Styrene oxide
Benzaldehyde
The rare earth oxide supported nano-gold catalysts were pre-
pared by homogeneous deposition precipitation (HDP) by con-
tacting a rare earth oxide under stirring with aqueous urea solu-
tion containing HAuCl4, increasing the temperature of the
mixture up to 95 ꢁC, maintaining the temperature at 95 ꢁC for
6 h, aging at 30 ꢁC for 12 h, filtering, thoroughly washing the sol-
id, drying and then calcining the solid in air at 400 ꢁC for 2 h.
Au/Yb2O3 catalyst was also prepared by deposition precipita-
tion (DP), using aqueous NaOH or Na2CO3 as precipitating
agents, described earlier.11 The catalysts were characterized
for their Au loading (by ICP–OES) and also for their Au particle
size (by TEM). The epoxidation reaction over the catalysts was
carried at atmospheric pressure by contacting 0.1 g supported Au
catalyst with 1.2 mL (10 mmol) styrene and 5.7 mL (15 mmol)
0
60
40
20
0
La Ce Nd Sm Eu Tb Er Yb
Lanthanide in Au/Lanthanide oxide
Figure 1.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.4 (2004)
401
Table 1. Results of the styrene epoxidation by anhydrous or aqueousTBHP over Au/Yb2O3 prepared by the different methods (DP and
HDP)
Styrene
oxide
Au
loading
Au particle
size
Styrene
conversion
Selectivity/%
Method of
preparation
Catalyst
Styrene Phenyl
oxide acetaldehyde
Other
products
yield
/%
/wt %
/nme
/%
Benzaldehyde
a
Yb2O3
—
—
4.7
3.6
6.6
6.6
6.6
6.6
6.6
—
10.9
42.0
52.2
63.5
64.4
65.3
74.0
73.7
4.2
2.8
30.0
24.5
22.9
23.4
8.6
8.7
7.1
6.6
7.1
7.9
1.4
3.3
3.6
84.3
17.8
32.1
15.2
11.1
30.5
21.0
20.6
0.08
18.9
19.2
34.8
37.1
38.9
53.9
53.8
a
a
a
Au/Yb2O3
Au/Yb2O3
Au/Yb2O3
DP (using NaOH)
DP (using Na2CO3)
HDP (using urea)
HDP (using urea)
HDP (using urea)
HDP (using urea)
HDP (using urea)
11:9 ꢅ 1:3
6:3 ꢅ 0:3
10:8 ꢅ 0:5
10:8 ꢅ 0:5
10:8 ꢅ 0:5
10:8 ꢅ 0:5
10:8 ꢅ 0:5
45.1
36.8
54.8
57.6
59.5
72.8
73.0
a,b
Au/Yb2O3
c
Au/Yb2O3
Au/Yb2O3
d,b
Au/Yb2O3
d
2.9
2.8
aUsing anhydrous TBHP (26% in benzene). bReuse of the catalyst, cUsing anhydrous TBHP (26% in benzene) and 0.5 mL water in the
reaction mixture without continuous removal of the water. dUsing aqueous TBHP (70% in water) without continuous removal of the
water. Determined by TEM.
e
Results in Table 1 reveal that the Au/Yb2O3 prepared by the
DP method is less active and selective in the epoxidation as com-
pared to the one prepared by the HDP method, most probably be-
cause of the lower Au loading in the former case. In case of Au/
TiO2 catalyst prepared by DP and HDP methods, the gold load-
ing was 0.48 and 6.0 wt %, respectively, and the styrene oxide
yield in the epoxidation was 19.5 and 31.6%, respectively. The
Au/Yb2O3 (HDP) showed a better performance than the Au/
TiO2 (HDP) in the epoxidation. The styrene oxide selectivity
and yield in the epoxidation by anhydrous TBHP over the Au/
Yb2O3 (HDP) increased with the reaction period (the selectivity
was 38.2, 54.8, and 67.5% and the yield was 13.0, 34.8, and
49.3%, respectively, for the reaction period of 1, 3, and 5 h). It
is interesting to note that the Au/Yb2O3 (HDP) catalyst shows
even better performance in the presence of water; when aqueous
TBHP was used instead of the anhydrous TBHP, both the con-
version and styrene oxide selectivity were higher and conse-
quently the styrene oxide yield (53.9%) was much higher. The
Au/Yb2O3 (prepared by the HDP method) also showed excellent
reusability without a significant loss in its activity or selectivity
in the epoxidation (Table 1). The styrene oxide yield (percentage
conversion of styrene to styrene oxide) obtained in this study is
much higher than those obtained earlier in the styrene epoxida-
tion over TS-12,6 using aqueous H2O2 (1.2% in 6 h)2 and H2O2-
urea (15.4% in 12 h),6 ꢀ-Al2O3 using anhydrous H2O2 (25.6% in
5 h)5 and over the surface modified Ti/HMS using TBHP (15.8%
in 3 h).7 The beneficial effect of water (Table 1) on the epoxida-
tion is consistent with that observed earlier in case of the oxida-
tion of CO over Au/TiO2.12 Further studies are, however, neces-
sary to understand the beneficial effect of water and also the
epoxidation reaction mechanism.
by HDP method), particularly the Au/Yb2O3, is highly active,
selective and reusable and hence highly promising catalyst for
the selective epoxidation of styrene by TBHP.
N.S.P. is grateful to RMIT, Melbourne, Australia for the
award of fellowship. Dr. B.S. Uphade and P. Jana are grateful
to CSIR, New Delhi, India, for the award of Senior Research
Associateship and Sernior Research Fellowship, respectively.
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In summary, the rare-earth oxide (viz. Yb2O3, Tb2O3,
Eu2O3, and Er2O3) supported nanosize gold catalyst (prepared
Published on the web (Advance View) March 6, 2004; DOI 10.1246/cl.2004.400