Highly efficient epoxidation of styrene and α-pinene with air over Co2+-exchanged ZSM-5 and Beta zeolites

Article abstract of DOI:10.1016/j.catcom.2012.01.029

Co²⁺-exchanged zeolites (prepared simply by ion-exchange method) have been used in the epoxidation of styrene and α-pinene with air, in which Co-ZSM-5 and Co-Beta without any organic Schiff-base ligand or additive behaved as highly efficient catalysts. Co-ZSM-5 and Co-Beta achieved 88.4-92.4 mol% conversion with the overall selectivity of 94.6-96.6% for the epoxidation of styrene, and 92.8-90.6 mol% conversion with the selectivity of 86.1-88.3% for the epoxidation of α-pinene, respectively. Recycling studies and control experiments showed the stability and recyclability of Co-ZSM-5 and Co-Beta in the epoxidation reaction.

Full text of DOI:10.1016/j.catcom.2012.01.029

Catalysis Communications 21 (2012) 68–71
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Highly efficient epoxidation of styrene and α-pinene with air over Co2+-exchanged
ZSM-5 and Beta zeolites
⁎
B. Tang, X.-H. Lu, D. Zhou, J. Lei, Z.-H. Niu, J. Fan, Q.-H. Xia
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, China
School of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, China
a r t i c l e i n f o
a b s t r a c t
Co2+-exchanged zeolites (prepared simply by ion-exchange method) have been used in the epoxidation of
styrene and α-pinene with air, in which Co-ZSM-5 and Co-Beta without any organic Schiff-base ligand or
additive behaved as highly efficient catalysts. Co-ZSM-5 and Co-Beta achieved 88.4–92.4 mol% conversion
with the overall selectivity of 94.6–96.6% for the epoxidation of styrene, and 92.8–90.6 mol% conversion
with the selectivity of 86.1–88.3% for the epoxidation of α-pinene, respectively. Recycling studies and control
experiments showed the stability and recyclability of Co-ZSM-5 and Co-Beta in the epoxidation reaction.
© 2012 Elsevier B.V. All rights reserved.
Article history:
Received 3 December 2011
Received in revised form 15 January 2012
Accepted 27 January 2012
Available online 4 February 2012
Keywords:
Co-ZSM-5
Co-Beta
α-Pinene
Styrene
Epoxidation
Air
1. Introduction
work, our new progress firstly shows that, the selective epoxidation
of styrene and α-pinene with dry air can be efficiently conducted
The catalytic epoxidation of alkenes to produce epoxides is an
important industrial reaction, since epoxides are key building blocks
in organic synthesis [1,2]. Over the past decade, with respect to
environmental and economic consideration, the epoxidation of
alkenes by H2O2 [3–5], molecular oxygen or air [6–9] has attracted
much attention, and great effort has focused on the discovery of
efficient catalyst [10–14]. Due to triplet ground state of O2 disfavoring
reactions with singlet organic compounds, the epoxidation of alkenes
with O2 was generally carried out in the presence of co-reductants
like H2, alcohols and aldehydes [15–18]. In 2004, Tang et al. [19,20]
firstly reported that Co2+-exchanged faujasite-type zeolites could
efficiently catalyze the epoxidation of styrene with O2 in the absence
of co-reductant. Later, Co-containing X-type zeolites modified by
alkali and alkaline metals [21,22], Co-TUD-1 [23], cobalt-substituted
SBA-15 [24] and Ag-based catalyst Ag-γ-ZrP [25] were also prepared
and applied in the epoxidation of alkenes with O2. However, these
catalytic systems contain four defects, i.e. high cobalt content (ca.
8.8 wt.%), complicated preparation procedure of catalyst, pure oxygen
as oxidant or low selectivity to epoxide.
over simple Co2+-exchanged zeolites with a low Co content (ca.
1.2 wt.%), in which no organic Schiff-base ligand or additive is
required.
2. Experimental
2.1. Preparation of catalysts
Zeolites (Na-ZSM-5, Na-Beta, Na-Y, K-3A, and Na-4A) were commer-
cial products. Co2+ cations or other transition metal ions were introduced
into zeolites by conventional ion-exchange method, i.e. 5.0 g of zeolite
was treated at 363 K overnight with an aqueous solution (250 ml)
containing 1.3 mmol of Co(Ac)2·4H2O or other salts like Zn(Ac)2·2H2O,
Mn(Ac)2·4H2O, Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, Cr(NO3)3·9H2O, and
Fe(NO3)3·9H2O. The solid powder was recovered by filtration, washed
several times with hot distilled water until no acidic residue was detected,
and followed by drying at 373 K for 8 h.
Recently, our group [26,27] has reported that Co-ZSM-5
coordinated with bi-/tridentate Schiff-base ligands catalyzed highly
selective epoxidation of alkenes with dry air. Based on the above
2.2. Characterization of catalysts
The X-ray diffraction (XRD) patterns of solid samples were recorded
with a Rigaku D/MAX−IIIC diffractometer with CuKα (λ=1.54184 Å)
radiation operating at 30 kV and 25 mA. The scanning range was from
2θ=5 to 65° with a scanning speed of 2°/min. The BET specific surface
area was calculated using the BET equation in the range of relative
⁎
Corresponding author. Tel./fax: +86 27 5086 5370.
E-mail address: xiaqh518@yahoo.com.cn (Q.-H. Xia).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.01.029

B. Tang et al. / Catalysis Communications 21 (2012) 68–71
69
Table 1
pressures (p/p0) between 0.05 and 0.25. Cobalt contents of thus-
Physicochemical characterization of zeolite catalysts.
prepared Co-zeolites were quantitatively analyzed by X-ray fluores-
cence (XRF).
a
Catalyst
Co contenta
(wt.%)
SiO2/Al2O3
ratios
Surface areab
(m2/g)
Pore size
(Å)
2.3. Catalytic reactions
Na-ZSM-5
Co-ZSM-5
Na-Beta
Co-Beta
Na-Y
0
1.19
0
1.15
0
25.0
27.5
25.0
27.8
4.7
406.5
375.6
556.5
496.3
821.3
720.4
119.4
97.3
5.4
All the solvents underwent purification treatment by redistillation
prior to use. The epoxidation reaction was carried out in a 50-ml two
necked glass reactor equipped with a cryogenic-liquid condenser
under atmospheric pressure. In a typical run, 3.0 mmol of alkene
and 0.3 mmol of TBHP were mixed with 10 g of DMF solvent, followed
by adding 0.1 g of the catalyst and with bubbling dry air to the bottom
of reactor by an air pump. After reaction at the desired temperature
for 5 h, the solid catalyst was filtered off, and the liquid product was
quantified by a gas chromatograph (GC) equipped with an SE-30
column (30 m×0.25 mm×0.25 μm) and an FID detector, in which
chlorobenzene was used as an internal standard. The conversion
and epoxide selectivity were calculated accordingly.
6.7
Co-Y
K-3A
1.31
0
6.7
2.2
7.4
Co-3A
Na-4A
Co-4A
1.10
0
1.14
2.8
2.0
3.0
~3.0
~4.0
155.7
120.6
a
Results obtained from XRF.
Values obtained from N2-adsorption results.
b
of phenylacetaldehyde. For the epoxidation of α-pinene, Co-ZSM-5
achieved 92.8 mol% conversion and 86.1% selectivity to epoxide,
slightly different from 90.6 mol% and 88.3% on Co-Beta.
3. Results and discussion
In contrast, the catalytic activity of Co-3A, Co-4A and Co-Y for the
epoxidation of styrene and α-pinene with air was much low, as listed
in Table 2. Although the surface areas and pore diameters of five zeolite
catalysts used can be arranged in the order of Co-Y>Co-Beta>Co-ZSM-
5>Co-4A>Co-3A (Table 1), there is no obvious correlation between
the catalytic activity and both of the surface area and pore size.
Meanwhile, those parent zeolites have also been used to catalyze
the epoxidation of styrene and α-pinene with air, but displayed
relatively poor activities with conversions lower than 10 mol%
(Table S1, Supplementary data), which further emphasized the
significance of the introduction of Co2+ into the framework for aerobic
epoxidation reaction.
3.1. Characterization of catalysts
Fig. 1 showed the XRD patterns of Co2+-exchanged zeolites, in
which there was no obvious change in the crystalline structure of
zeolites that had undergone ion-exchange treatment as compared to
those of the parent zeolites (Fig. S1, Supplementary data).
BET surface areas and pore sizes of various zeolites determined by
BET analysis were given in Table 1, in which both parameters exhibited
the same decrease order of Co-Y>Co-Beta>Co-ZSM-5>Co-4A>Co-
3A. Co contents and Si/Al ratios of Co-zeolites were analyzed by XRF
and shown in Table 1.
3.2. Epoxidation of styrene and α-pinene with dry air over various
3.3. Epoxidation of styrene with dry air over transition metal M-Beta
Co2+-exchanged zeolites
To compare the difference of catalytic activity of various metal
ions, some transition metal ions were introduced into the framework
of Beta zeolite. As shown in Table S2 (Supplementary data), Co-Beta
achieved the highest styrene conversion (92.4 mol%) and epoxide
selectivity (91.7%) under the same reaction conditions. Other M-
Beta catalysts achieved very low conversionsb17.1 mol% and epoxide
selectivityb70.1%, much less than those over Co-Beta. According to
our previous studies [9,26,27], Co2+ ions (exchanged into the zeolitic
framework) coordinated with DMF play key roles in activating
oxygen molecules in air to epoxidize styrene and α-pinene. A deep
investigation for the reaction mechanism is underway to disclose
the nature of high efficiency under the present experimental
conditions.
The catalytic activity of different Co2+-exchanged zeolites for the
epoxidation of styrene and α-pinene (Scheme 1) with air is summa-
rized in Table 2. Under identical experimental conditions, those
zeolite catalysts with similar Co contents showed
a sensible
difference in the catalytic conversion of styrene and α-pinene. Co-
Beta obtained 92.4 mol% conversion of styrene, accompanied with
96.6% of overall epoxidation selectivity, inclusive of desired epoxide
(91.7%) and its isomer phenylacetaldehyde (4.9%); whereas Co-
ZSM-5 converted 88.4 mol% of styrene to 90.1% of epoxide and 4.5%
3.4. Effect of various solvents on the epoxidation of styrene over Co-Beta
Co-Beta
The effect of various solvents on the epoxidation of styrene
was explored, and the results are shown in Table S3 (Supplementary
data). Obviously, the solvents affected the epoxidation reactions. The
conversion of styrene decreased in the sequence of 92.4 mol% (DMF)>
90.7 mol% (1,4-dioxane)>27.2 mol% (acetylacetone)>15.8 mol%
(toluene), consistent with the order of the epoxide selectivity: 91.7%
(DMF)>65.9% (1,4-dioxane)>34.9% (acetylacetone)>9.8% (toluene).
However, the epoxidation of styrene with air in pyridine could not
occur completely.
After the reaction was terminated, the amount of oxo-DMF was
detected by GC and calculated accordingly. Generally, ca. 0.15–
0.30 mmol of oxo-DMF was yielded in the optimal experiments,
merely corresponding to 1/20–1/10 of quantity of alkene oxidized,
i.e. the molar ratios of oxo-DMF/epoxide were in the range of 0.05–
0.11, so that DMF cannot be recognized as a real reductant, different
Co-ZSM-5
Co-Y
Co-3A
Co-4A
5
15
25
35
2 (o)
45
55
65
Fig. 1. XRD patterns of Co2+-exchanged zeolites.

70
B. Tang et al. / Catalysis Communications 21 (2012) 68–71
Scheme 1. Catalytic epoxidation of styrene and α-pinene.
from the case reported in the literature [28], which might be ascribed
to the notable variation in our experimental conditions.
3.6. Recycling studies
The recycling experiments of Co-ZSM-5 and Co-Beta catalysts for
the epoxidation of styrene have been conducted. As can be seen
from Fig. 2, the reuses of Co-Beta (seven times) did not appreciably
decrease the conversion of styrene and the selectivity of epoxide;
however, the conversion on Co-ZSM-5 showed a little reduction in
the initial two recycles, but remained basically unchanged in the
following five recycles. ICP analysis of reaction mixture and the used
catalysts did detect the leaching of cobalt from these two Co-
zeolites, leading to a big reduction of the cobalt content from 1.19
to 1.02 wt.% for Co-ZSM-5 and a small drop from 1.15 to 1.14 wt.%
for Co-Beta, respectively. Once free Co2+ ions were totally leached
out of the pores, the activity of both catalysts tended to maintain a
normal fluctuation in an almost constant range.
In order to further verify whether the observed catalysis was truly
heterogeneous or not, control experiments have been carried out. As
shown in Fig. S2 (Supplementary data), after aerobic epoxidation
reactions were initially conducted for 1 h, the obtained conversions
were 23.1 mol% over Co-ZSM-5 and 22.6 mol% over Co-Beta, then
solid catalysts were filtered off and the reactions were continued for
another 4 h. Finally, the conversions were analyzed by GC to be
33.8 mol% on Co-ZSM-5 (with an increment of 10.7 mol%) and
3.5. Effect of different oxidants and initiators on the epoxidation of styrene
and α-pinene
As demonstrated in Table 3, various oxidants have been tested for
the epoxidation of styrene over Co-Beta. When the reaction was carried
out under inert N2 atmosphere, no epoxidation reaction occurred at all.
Oxidants H2O2, NaClO, and NaIO4 exhibited very poor reactivities for the
epoxidation of styrene, in which NaIO4 converted 13.1 mol% of styrene
(82.2% the epoxide selectivity), H2O2 acquired 14.0 mol% conversion
(61.6% the epoxide selectivity), while the effect of NaClO was negligible.
When either only TBHP or air was used as unique oxidant, no high con-
version or selectivity was obtained. However, if air was used as an oxi-
dant together with a small amount of TBHP added as initiator,
92.4 mol% conversion of styrene and 91.7% selectivity to epoxide were
achieved, indicating the enhanced effect of a small amount of TBHP as
initiator on the titled reaction.
Also, other initiators such as H2O2 and NHPI molecules have been
applied in the epoxidation of styrene and α-pinene with air over Co-
ZSM-5 and Co-Beta (Table 4). TBHP showed the best promoting
performance for the epoxidation of alkenes with air. The rapid
decomposition of H2O2 over the Co-catalyst at 363 K could be
responsible for its low efficiency on the reaction, while homolysis of
NHPI to phthalimido-N-oxyl (PINO) radical which plays a key role
in the aerobic epoxidation could not proceed in the absence of
induced molecules under the present mild conditions, thus resulting
in a poor reactivity [29].
26.9 mol% on Co-Beta (with
a small increment of 4.3 mol%),
attributed to the contribution of a small amount of Co2+ ions leaching
from Co-zeolites and the autoxidation of alkenes by air. This study
further revealed the essentiality of heterogeneous catalysis over Co-
ZSM-5 and Co-Beta.
Table 2
Table 3
Epoxidation of styrene and α-pinene with air over Co2+-exchanged zeolitesa.
Effect of different oxidants on the epoxidation of styrene over Co-Betaa.
Selectivity (%)
Selectivity (%)
Catalyst
Alkene
Conv. (mol%) Epoxide
A
B
Others Total
Oxidant
Conv. (mol%) Epoxide Benzaldehyde Phenylacetaldehyde Others
Co-ZSM-5 Styreneb
88.4
92.4
59.3
53.5
33.4
90.1
91.7
80.0
83.5
79.5
86.1
88.3
86.0
86.3
81.8
5.4 4.5
3.4 4.9
14.5 5.5
13.1 3.4
0
0
0
0
0
94.6
96.6
85.5
86.9
79.5
86.1
88.3
86.0
86.3
81.8
Noneb
Airc
0
0
0
0
0
0
0
11.3
–
Co-Beta
Co-3A
Co-4A
Co-Y
35.5
22.0
14.0
Trace
13.1
12.2
72.8
78.1
61.6
–
21.9
18.2
24.8
–
5.3
3.7
2.3
–
TBHPd
d
H2O2
20.5
0
NaClOd
Co-ZSM-5 α-Pinenec 92.8
3.9 8.3 1.7
3.3 7.7 0.7
4.0 8.3 1.7
3.9 7.9 1.9
4.2 8.6 5.4
NaIO4
82.2
84.2
91.7
15.9
9.4
3.4
1.9
6.4
4.9
0
0
0
d
Co-Beta
Co-3A
Co-4A
Co-Y
90.6
62.2
68.6
66.9
TBHPe
Air+TBHP 92.4
a
Alkene, 3 mmol; DMF, 10 g; Catalyst, 0.1 g; Initiator (TBHP), 0.3 mmol; Time, 5 h;
Temperature, 363 K; Flow rate of air, 40 ml/min.
a
b
Alkene, 3 mmol; DMF, 10 g; Catalyst, 0.1 g; Initiator (TBHP), 0.3 mmol; Time, 5 h;
Temperature, 363 K; Flow rate of air, 40 ml/min.
Flow rate of N2, 40 ml/min.
Flow rate of air, 40 ml/min.
Oxidant, 3 mmol.
Oxidant, 0.3 mmol.
c
b
d
A = benzaldehyde, B = phenylacetaldehyde.
A = verbenol, B = verbenone.
c
e

B. Tang et al. / Catalysis Communications 21 (2012) 68–71
71
Table 4
exerted notable impacts on the epoxidation reaction. Recycling studies
showed that Co2+-exchanged ZSM-5 and Beta zeolites could be repeat-
edly used as excellent heterogeneous catalysts for the epoxidation of
alkenes.
Effect of different initiators on the epoxidation of styrene and α-pinene over Co-ZSM-5
and Co-Beta.
Selectivity (%)
Catalyst
Alkene
Initiator Conv. (mol%) Epoxide
A
B
Others
Acknowledgments
Co-ZSM-5 Styrene
Co-Beta
Co-ZSM-5
Co-Beta
TBHPa
88.4
92.4
77.2
75.8
41.2
42.4
92.8
90.6
67.7
64.3
66.8
63.0
90.1
91.7
84.4
84.8
76.1
76.1
86.1
88.3
68.4
79.9
75.3
76.9
5.4
3.4
7.9
10.1
17.2
19.5
3.9
4.5
4.9
4.2
3.2
5.0
4.4
8.3
7.7
0
0
NHPIb
3.5
1.9
1.7
0
1.7
0.7
The authors acknowledge the funding supports provided by the
National Natural Science Foundation of China (Nos. 20901023,
21173073), by the 2007 excellent mid-youth innovative project of
HPED of China (no. T200701), and by the key project of HPSTD of
China (no. 2008CAD030). Dr. D. Zhou thanks the financial support
by Chen Guang Scheme of Wuhan City (no. 201050231087).
c
Co-ZSM-5
Co-Beta
H2O2
Co-ZSM-5 α-Pinene TBHPa
Co-Beta
Co-ZSM-5
Co-Beta
Co-ZSM-5
Co-Beta
3.3
NHPIb
7.0 13.9 10.7
5.9 12.4
4.6 13.9
4.3 12.7
1.8
6.2
6.1
c
H2O2
Appendix A. Supplementary data
a
Alkene, 3 mmol; DMF, 10 g; Catalyst, 0.1 g; Initiator (TBHP), 0.3 mmol; Time, 5 h;
Temperature, 363 K; Flow rate of air, 40 ml/min.
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2012.01.029.
b
Adding 0.3 mmol NHPI instead of TBHP.
c
Adding 0.3 mmol H2O2 instead of TBHP.
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4. Conclusions
Transition metal ions (Co2+, Fe3+, Ni2+, Cu2+, Mn2+, Cr3+, and Zn2+
)
exchanged zeolites have been prepared by a simple route. Co-ZSM-5 and
Co-Beta displayed the highest activities for the epoxidation reaction, in
which Co-ZSM-5 and Co-Beta achieved 88.4–92.4 mol% conversion with
the overall selectivity of 94.6–96.6% for the epoxidation of styrene, and
90.6–92.8 mol% conversion with the selectivity of 86.1–88.3% for the ep-
oxidation of α-pinene, respectively. Solvents, oxidants and initiators