Co2+-Exchanged faujasite zeolites as efficient heterogeneous catalysts for epoxidation of styrene with molecular oxygen

Article abstract of DOI:10.1039/b314864e

Co²⁺-Exchanged faujasite zeolites can efficiently catalyze the epoxidation of styrene with molecular oxygen, and the Co²⁺ ions located in supercages are suggested to account for the activation of O ₂ for the epoxidation of

Full text of DOI:10.1039/b314864e

Co2+-Exchanged faujasite zeolites as efficient heterogeneous catalysts for
epoxidation of styrene with molecular oxygen
Qinghu Tang, Ye Wang,* Jun Liang, Ping Wang, Qinghong Zhang and Huilin Wan
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University,
Xiamen 361005, P. R. China. E-mail: yewang@jingxian.xmu.edu.cn; Fax: (+86)-592-2183047;
Tel: (+86)-592-2187470
Received (in Cambridge, UK) 18th November 2003, Accepted 5th January 2004
First published as an Advance Article on the web 23rd January 2004
Co2+-Exchanged faujasite zeolites can efficiently catalyze the
epoxidation of styrene with molecular oxygen, and the Co2+ ions
located in supercages are suggested to account for the activation
of O2 for the epoxidation of styrene.
epoxidation of styrene with O2. Styrene conversion increased
proportionally with an increase in the cobalt content in the sample,
while styrene oxide selectivity and the turnover number remained
almost the same simultaneously. This result further suggests that
the cobalt in the zeolite functions as active sites for the epoxidation
of styrene with O2.
Molecular oxygen is the most desirable oxidant for the epoxidation
of alkenes with respect to environmental and economic considera-
tion. However, little success has been achieved for the selective
epoxidation of alkenes with O2 except for ethylene. Cobalt ions and
complexes are well-known catalysts for the selective oxidation of
alkanes and alkylbenzenes with O2.1 Cobalt complexes have also
been used for the epoxidation of alkenes with tert-butyl hydro-
peroxide and iodosylbenzene,2 but the studies on exploitation of
cobalt catalysts for the epoxidation of alkenes with O2 as oxidant
are scarce. The autooxidation of alkenes in O2 with the first-row
transition metal compound typically resulted in a high degree of
oxidized polymers as well as cleaved products.3 Drago and co-
workers reported the catalytic oxidation of terminal olefins
including styrene by O2 to the corresponding 2-ketones and
2-alcohols using a cobalt(II) complex,4 but no styrene oxide was
observed. Cobalt salen complexes were reported to show catalytic
activity for epoxidation of styrene with O2, but a sacrificial co-
reductant, isobutyraldehyde, was necessary.5 Recently, CoCl2 was
investigated for the oxidation of monoteroenes with O2, and it was
found that the allylic oxidation proceeded dominantly.6 Several
heterogeneous cobalt catalysts have been applied for the selective
oxidation of alkanes, especially cyclohexane,7–11 but very few
contribute to the development of heterogeneous catalysts for
epoxidation of alkenes with O2.12,13 Recently, we found that Co2+-
exchanged molecular sieves could catalyze the epoxidation of
alkenes with O2 in the absence of a co-reductant. Here, we report,
for the first time, the catalytic performances of the Co2+-exchanged
faujasite zeolites for the epoxidation of styrene with O2.
The catalytic properties of various Co-containing samples for the
oxidation of styrene with O2 are compared in Table 1. The weight
of catalyst used was regulated in each run to keep the Co amount the
same (0.299 mmol). It should be noted that Co2+ ions in the liquid
phase added as either Co(NO3)2 or Co(Ac)2 were inactive for the
epoxidation of styrene. The CoOx/SiO2 and CoOx/NaX samples
prepared by the conventional impregnation method could also
catalyze the epoxidation of styrene with O2, but styrene conversion
was remarkably lower than for the Co2+-exchanged X zeolite. Thus
the cobalt ions, which are highly dispersed in the zeolite, played a
key role in the catalysis of epoxidation. The comparison among
different types of Co2+-exchanged zeolites reveals that the Co2+-
exchanged Y zeolite shows similar catalytic performances to Co2+-
exchanged X zeolite, and both exhibit much higher styrene
conversion than other Co2+-exchanged zeolites. Since the window
All the parent zeolites used in this work were prepared via
hydrothermal synthesis and their structures were confirmed by
XRD. Cobalt ions were introduced into the zeolites by ion-
exchange in aqueous solution of Co(NO3)2 for 24 h at temperatures
from 298 to 373 K depending on the zeolites. After filtration, and
thorough washing with deionized water, the samples were dried at
313 K in vacuo for 24 h, and the dried powders were used as
catalysts, unless otherwise stated. The cobalt content in each
sample was determined by atomic absorption spectroscopic
analysis. The epoxidation of styrene was carried out using a batch-
type reactor operated under atmospheric pressure. In a typical
reaction, a measured amount of catalyst was added to a glass flask
pre-charged with the reactant (styrene) and the solvent, typically
N,N-dimethylformamide (DMF). The reaction was started by
bubbling O2 into the liquid. After reaction for 4 h, the catalyst was
filtered off, and the liquid organic products were quantified by a gas
chromatograph with a capillary column, using toluene as an internal
standard.
The catalytic performances of the Co2+-exchanged X zeolite
samples with different cobalt contents are shown in Fig. 1. In these
experiments, the catalyst weight was kept at 0.2 g. Styrene oxide
and benzaldehyde were the two main products along with minor
quantities of styrene glycol, benzoic acid and mandelic acid. NaX
zeolite without cobalt was almost inactive for the oxidation of
styrene, strongly suggesting that cobalt was responsible for the
Fig. 1 Catalytic performances of the Co2+-exchanged X zeolite samples
with different cobalt content: (5) styrene conversion, (-) epoxide
selectivity, (2) turnover number. Conditions: T = 373 K; styrene, 10 mmol;
DMF, 20 ml; flow rate of O2, 3.0 ml min21
.
Table
1
Catalytic performances of Co-containing samples for the
epoxidation of styrene with molecular oxygen
Styrene
conv. (%)
Epoxide
select. (%)
Turnover
no.
Catalyst
NaX
Co(Ac)2
Co(NO3)2
0.32
0.40
0.90
44.2
45.0
45.3
18.2
15.9
25.9
24.9
30.1
42.3
0
0
–
0.12
0.27
13.2
13.4
13.5
4.4
4.0
7.2
6.8
9.0
Co2+-X (8.8 wt%)
Co2+-X (8.8 wt%)a
Co2+-Y (6.2 wt%)
Co2+-L (4.3 wt%)
Co2+-mordenite (2.9 wt%)
Co2+-beta (1.4 wt%)
CoOx/SiO2 (8.8 wt%)
CoOx/NaX (8.8 wt%)
60.0
65.0
62.3
53.7
51.7
47.6
60.1
58.9
Conditions: the amount of Co2+ in each catalyst except for NaX, 0.299
mmol; T = 373 K; styrene, 10 mmol; DMF, 20 ml; flow rate of O2, 3.0 ml
.
min21 a Repeated run after three reaction cycles.
440
C h e m . C o m m u n . , 2 0 0 4 , 4 4 0 – 4 4 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

size of faujasite zeolites is comparable to that of mordenite and
zeolite beta, the particular location of cobalt ions in the faujasite
zeolites, or their particular porous structure with supercages may
account for their high activity. Several other transition metal ions,
such as in Mn2+-, Ni2+-, Cu2+-, Zn2+- and Cr3+-exchanged X zeolite
samples, were also investigated for the epoxidation of styrene with
O2, but styrene conversion and styrene oxide selectivity were both
remarkably lower than those for the Co2+-exchanged samples.
Therefore, the Co2+-exchanged faujasite zeolites are unique for the
epoxidation of styrene with O2.
It has been confirmed that no leaching of Co2+ from the Co2+-
exchanged X zeolite occurs during the reaction. The cobalt content
in the sample after several reaction cycles did not change, and
cobalt could not be detected in the filtrate after the reaction. No
significant conversion of styrene was observed when the liquid
filtrate was used instead of the solid catalyst for further reaction.
Furthermore, the repeated use of the Co2+-exchanged X zeolite did
not show any decrease in the catalytic activity (Table 1). These
observations strongly suggest that the reaction proceeds heteroge-
neously over the catalyst.
Fig. 2 Effect of O2 pressure on catalytic performances of the Co2+-X (8.8
wt%) sample for the epoxidation of styrene: (5) styrene conversion, (2)
epoxide selectivity. Conditions: T = 373 K; styrene, 10 mmol; DMF, 20 ml;
total flow rate, 6.0 ml min21 (O2 was diluted with N2).
Table 3 Comparison of different oxidants for the epoxidation of styrene
using the Co2+-X (8.8 wt%) sample
Styrene
conv. (%)
Epoxide
select. (%)
Turnover
no.
Oxidant
A change in the calcination temperature was found to exert an
influence on the catalytic performances for the Co2+-exchanged X
zeolite samples (Table 2). The sample dried at 313 K in vacuo
without calcination exhibited the highest styrene conversion. An
increase of the calcination temperature gradually decreased styrene
conversion and thus the turnover number. It is known that the Co2+
ions exchanged in faujasite zeolites may migrate from the
supercages to the sodalite cages and further to the hexagonal prisms
with an increase in the calcination temperature.14 Therefore, the
results in Table 2 suggest that the Co2+ ions located in the
supercages of faujasite zeolites are responsible for the epoxidation
of styrene with O2.
Oxygen has been found to be crucial for the epoxidation reaction
catalyzed by the Co2+-exchanged faujasite zeolites. No reaction
occurred without bubbling oxygen to the liquid, and the conversion
of styrene increased proportionally with the pressure of O2 (Fig. 2).
Hydrogen peroxide, tert-butyl hydroperoxide (TBHP) and NaClO
have also been applied for the epoxidation of styrene using the
Co2+-exchanged X zeolite sample. Although epoxide could be
obtained, it is of interest that styrene conversion or styrene oxide
selectivity with these oxidants were remarkably lower than those
with O2 (Table 3). The rapid decomposition of H2O2 over the
catalyst may be responsible for the lower activity and selectivity
with H2O2. On the other hand, TS-1, a well-known epoxidation
catalyst, showed higher activity and selectivity for the epoxidation
of styrene with H2O2 but almost no activity with O2.15 Therefore,
oxygen species derived from the activation of O2 by the Co2+ sites
in the supercages probably account for the epoxidation of styrene.
Further investigations have revealed that the solvent is also
important for the epoxidation reactions. It is found that the solvents
with higher polarity such as DMF and N,N-dimethylacetamide
show higher conversion of styrene while those with lower polarity
a
O2
44.2
20.6
73.9
2.6
60.0
55.5
12.2
27.1
13.2
6.2
24.8
0.87
b
H2O2
TBHPc
NaClOd
a Reaction conditions are the same with those in Table 1. b Adding 9.7 mmol
H2O2 instead of bubbling O2. c Adding 10 mmol TBHP instead of bubbling
O2. d Adding 10 mmol NaClO instead of bubbling O2.
such as chlorobenzene give much lower conversion. The nature of
the role of solvent and the oxygen species are under investiga-
tion.
In conclusion, we have found that the Co2+ ions exchanged in
faujasite zeolites show unique heterogeneous catalytic properties in
the epoxidation of styrene with O2 in the absence of sacrificial
reductant.
This work was supported by the NSF of China (Nos. 20021002
and 20373055) and the Chinese Ministry of Science and Technol-
ogy (No. G1999022408).
Notes and references
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Table
2 Influence of the calcination temperature on the catalytic
performance of the Co2+-X (8.8 wt%) sample for the epoxidation of styrene
with molecular oxygena
Styrene
conv. (%)
Epoxide
select. (%)
Turnover
no.
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Temp./K
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393
573
673
823
44.2
37.2
35.4
31.4
25.8
60.0
55.5
62.5
61.1
68.4
13.2
10.8
10.4
9.2
7.2
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a Conditions are the same with those in Table 1.
C h e m . C o m m u n . , 2 0 0 4 , 4 4 0 – 4 4 1
441