Co2+-exchanged MOR and 5A zeolites as efficient solid catalysts for the epoxidation of styrene with air

Article abstract of DOI:10.1016/j.catcom.2013.11.006

Selective oxidation of alkenes with air to corresponding epoxides was performed over simple ion-exchanged Co-MOR and Co-5A. Among all transition metal ions-exchanged M-zeolites, both Co-MOR and Co-5A exhibited the highest activity at 363 K. Notably, for the epoxidation of styrene, α-pinene, α-methyl styrene and cyclooctene, Co-MOR obtained higher conversions than Co-5A, in agreement with the difference of pore sizes of both zeolitic materials. Recycling and control tests showed high durability of Co-MOR as a heterogeneous catalyst in our catalytic system.

Full text of DOI:10.1016/j.catcom.2013.11.006

Co2 +-exchanged MOR and 5A zeolites as efficient solid catalysts for the
epoxidation of styrene with air
D. Zhou, B. Tang, X.-H. Lu, X.-L. Wei, K. Li, Q.-H. Xia
PII:
DOI:
Reference:
S1566-7367(13)00426-3
doi: 10.1016/j.catcom.2013.11.006
CATCOM 3709
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
29 July 2013
24 October 2013
5 November 2013
Please cite this article as: D. Zhou, B. Tang, X.-H. Lu, X.-L. Wei, K. Li, Q.-H. Xia,
Co2 +-exchanged MOR and 5A zeolites as efficient solid catalysts for the epoxidation of
styrene with air, Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.11.006
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ACCEPTED MANUSCRIPT
Co2+-exchanged MOR and 5A zeolites as efficient solid catalysts for
the epoxidation of styrene with air
D. Zhoua,b, B. Tanga,b, X.-H. Lua,b, X.-L. Weia,b, K. Lia,b, Q.-H. Xiaa,b,*
a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials
b Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, Hubei University, Wuhan 430062, P.R. China
Abstract
Selective oxidation of alkenes with air to corresponding epoxides was performed
over simply ion-exchanged Co-MOR and Co-5A. Among all transition metal
ions-exchanged M-zeolites, both Co-MOR and Co-5A exhibited the highest activity at
363 K. Appreciably, for the epoxidation of styrene, α-pinene, α-methyl styrene and
cyclooctene, Co-MOR obtained higher conversions than Co-5A, in agreement with
the difference of pore sizes of both zeolitic materials. Recycling and control tests
showed high durability of Co-MOR as heterogeneous catalyst in our catalytic system.
Keywords: Cobalt ion-exchange; Mordenite; 5A, Epoxidation of styrene; Air
______________________
*Corresponding author.
Fax: +86-27-88663043; E-mail: xia1965@yahoo.com (Q.-H. Xia)
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1. Introduction
The catalytic epoxidation of alkenes to produce epoxides is an important
industrial reaction in the manufacturing of fine chemicals and pharmaceuticals, 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 [3,4] or molecular oxygen [5,6] has attracted much attention; however, the
epoxidation of alkenes with O2 was generally carried out in the presence of
co-reductants like H2, alcohols and aldehydes [1,7,8].
Recently, molecular sieve materials as heterogeneous catalysts have been a
subject of growing interests in the epoxidation of alkenes. Tang et al. firstly reported
that Co2+-exchanged faujasite-type zeolites could catalyze the epoxidation of styrene
with O2 in the absence of co-reductant [9]. Later on, the selective oxidation of alkenes
with oxygen/air have achieved encouraging results over other Co-doped porous
catalysts, like Co-X [10], Co-Y [11], Co-SBA-15 [12], which, however, could merely
obtain a low epoxide selectivity of <70%. Very recently, our group has reported the
efficiencies of simply ion-exchanged Co-ZSM-5, Co-Beta, Co-SAPO-34, Co-SAPO-5
with a low Co content (ca. 1.2 wt.%) for dissociating triplet ground state of O2 to
epoxidize singlet olefins in the absence of co-reductant with a high selectivity of >
90% [13,14].
It is noteworthy that the above-stated ZSM-5, Beta, SAPO-34 and SAPO-5 were
hydrothermally synthesized with organic quaternary ammonium ions or amines as
structure-directing agents, which would definitely increase the environmental
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pollution and the synthesis cost. However, their Co2+-exchanged counterparts could
effectively catalyze aerobic epoxidation of alkenes like styrene and α-pinene. This
arouses our interest to study the activity of Co2+-exchanged mordenite (MOR) and 5A
zeolites, which are cheap and can be simply synthesized without any organic
templates. Herein, our new progress firstly shows high activities of Co2+-exchanged
MOR and 5A for the selective epoxidation of styrene with dry air, comparable to other
Co-zeolites.
2. Experimental
2.1 Preparation of catalysts
Co2+ and other transition metal ions were introduced into zeolite by a simple
ion-exchange method as described elsewhere [13]. In a typical run, 5.0 g of zeolite
was treated with an aqueous solution (250 ml) of metal salts (2.6 mmol) at 363 K
overnight. The solid powder was recovered by filtration, washed several times with
hot distilled water until no acidic residue was detected, followed by drying at 373 K
for 8 h, in which no calcination was required. Fe-MOR, Cr-MOR, Cu-MOR, Ni-MOR,
Mn-MOR and Zn-MOR were prepared through the same procedure stated above.
2.2 Charaterization of catalysts
X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku
D/MAX-IIIC diffractometer with Cu Kα radiation (λ = 1.54184 Å) operating at 30 kV
and 25 mA. Cobalt content in each sample was analyzed on a Shimadzu XRF-1800
spectrometer. The morphology and size of crystals were imaged on a JEOL
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JSM-6510A scanning electron microscope (SEM). X-ray photoelectron spectroscopy
(XPS) data was acquired on a Thermol ESCALAB-250 system using Al Kα
monochromatic radiation (1486.6 ev). The BET specific surface area was calculated
using the BET equation in the range of relative pressures (p/p0) between 0.05 and
0.25.
2.3 Catalytic reactions
The epoxidation of alkenes with air was performed in a 50-mL two-necked
round-bottom glass flask equipped with a cryogenic-liquid condenser under
atmospheric pressure. For reaction and analysis details, please refer to the supporting
information.
3. Results and discussion
3.1 Characterization of catalysts
The XRD patterns of ion-exchanged MOR and 5A have been shown in Figure 1.
Clearly, the diffraction patterns of Co2+-exchanged MOR and 5A show the
characteristics of highly crystalline materials, with typical reflections of MOR and
LTA structures. However, the intensity of diffractions of Co-5A within 5–45° shows a
slight reduction, as compared to that of the parent 5A, which is probably due to partial
removal of Al from the framework of 5A zeolite. The majority of Co species in the
Co2+-exchanged samples exists in the form of Co2+ cations, as characterized by the
appearance of only one peak with the binding energy of ca. 782 eV in the X-ray
photoelectron spectra, characteristic of Co2+-zeolites [15]. BET surface areas and pore
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sizes of various zeolites determined by BET analysis are given in Table 1. It can be
seen that the ion-exchange treatment has led to a slight decrease in the surface area of
catalysts, which is easily understandable because such an ion-exchange can remove
partial Al species from the framework of zeolites, and some smaller Na+ ions have
been substituted by larger Co2+ ions [13,14,16]. The XRF results have shown that the
SiO2/Al2O3 ratios in Co-MOR and Co-5A are increased to 25.2 and 3.0 from 20.6 and
2.0 of Na-MOR and 5A, probably due to partial removal of Al atoms from the
frameworks of zeolites by ion-exchange treatment. The Co contents of both Co-MOR
and Co-5A are determined to be 1.03 wt% and 0.43 wt%, respectively.
The scanning electron micrographs of catalysts are shown in Figure 2, indicating
that the morphologies and particle sizes of catalysts that have undergone the
ion-exchange treatment are still similar to those of the parent zeolites.
3.2 Epoxidation of styrene with air over various Co2+-exchanged zeolites
The catalytic activity of various Co2+-exchanged zeolites for the epoxidation of
styrene has been compared in Table 2. When Co(Ac)2·4H2O was used as a
homogeneous catalyst, only 12.7 mol% of styrene was converted. When Na-MOR or
5A was used, an extremely low conversion of 3.7 mol% or 9.4 mol% was achieved. In
contrast, Co-MOR and Co-5A catalysts presented considerably catalytic activity,
remarkably higher than those of Co-4A and Co-13X. Co-MOR catalyzed the
oxidation of 94.2 mol% styrene to 90.5% styrene epoxide, 3.7% benzaldehyde, and
5.8% phenylacetaldehyde. Likewise, Co-5A catalytically converted 89.4 mol%
styrene to 90.2% styrene epoxide, 4.1% benzaldehyde, and 5.7% phenylacetaldehyde.
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3.3 Epoxidation of styrene with air over various M-MOR zeolites
As shown in Figure 3, among various M-MOR catalysts, only Co-MOR could
achieve very high styrene conversion of 94.2 mol% and epoxide selectivity of 90.5%.
Other M-MOR catalysts exhibited much lower activities, in which all the conversions
were < 24.4 mol% (see Table S1). The catalytic activity of different M-MOR catalysts
decreased in the following order of Co-MOR (94.2 mol%) > Zn-MOR (24.4 mol%) >
Cr-MOR (13.8 mol%) > Cu-MOR (13.5 mol%) > Mn-MOR (12.5 mol%) > Ni-MOR
(9.7 mol%) > Fe-MOR (7.8 mol%). The epoxide selectivity presented a different
decreasing trend: Co-MOR (90.5%) > Cu-MOR (80.7%) > Fe-MOR (77.9%) >
Zn-MOR (73.4%) > Cr-MOR (73.0%) > Ni-MOR (70.6%) > Mn-MOR (70.1%).
Clearly, cobalt is an effective component for catalyzing the epoxidation of
styrene with oxygen, and hence widely used in the field of epoxidation of alkenes
with O2/air [6,9,10,12-14,17-26]. A comparison of our results with the earlier data
reported for the epoxidation of styrene is summarized in Table 3, where further shows
the excellent catalytic activity of our aerobic epoxidation system using
Co2+-exchanged zeolites as the catalysts.
3.4 Effect of solvents and oxidants on the epoxidation of styrene over Co-MOR
The effect of different solvents on the activity of Co-MOR was compared in
Table S2. The epoxidation of styrene with air in pyridine could not occur at all. When
MIBK, toluene or acetylacetone was used as the solvent, the styrene conversion was
low (< 29.0 mol%), and the main product was benzaldehyde (61.1%, 65.6% or
74.0%). Although a high styrene conversion of 95.8 mol% could be achieved in the
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polar solvent 1,4-Dioxane, the selectivity to epoxide was 48.5%. In contrast, DMF
was especially efficient in obtaining both high conversion (94.2 mol%) and good
epoxide selectivity (90.5%), probably ascribed to the strong coordination among DMF
(solvent), Co2+ and O2 that can promote the activation of O2 molecules to form an
active intermediate, which further reacts with styrene to produce epoxide
[6,13,14,23].
It is observed from Table 4 that no any conversion of styrene took place in the
presence of NaClO or only inert N2. When NaIO4 was used, the styrene conversion
was very low (13.2 mol%). When only air was bubbled to act as unique oxidant, 10.3
mol% conversion of styrene was achieved. When using H2O2 as the oxidant, the
conversion of styrene was only 21.4 mol%, mainly ascribed to the rapid
decomposition of H2O2 into O2 at 363 K. However, when air was bubbled into the
reactor with 0.3 mmol of TBHP or CHP (ca. 2 drops) as the initiator, the highest
conversion of styrene and selectivity of epoxide were obtained. For the purpose of
comparison, even 3 mmol of TBHP (10 times as much as that used as the initiator)
was added as the oxidant, but only 20.4 mol% conversion was obtained. Also, we
attempted to use H2O2 as the initiator; however, only 37.6 mol% of styrene was
oxidized. In contrast, the epoxidation could not occur at all when adding TEMPO as
the initiator. Obviously, the addition of TBHP or CHP in small amounts is the most
beneficial to the catalytic epoxidation of styrene with air.
3.5 Catalytic epoxidation of various alkenes over Co-MOR and Co-5A
The catalytic epoxidation of other alkenes (like α-pinene, α-methyl styrene, and
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cyclooctene) over Co-MOR and Co-5A is described in Table 5. It is observed that the
reactivity of styrene over both Co-MOR and Co-5A was the highest, with a
conversion of 89.4-94.2 mol%, and an epoxide selectivity of 90.2-90.5%. Co-MOR
could convert almost 100% of α-methyl styrene (with 75.3% selectivity); however,
Co-5A converted only 55.8 mol% substrate (96.8% selectivity). The reactivity of
α-pinene over Co-MOR and Co-5A was lower than that of styrene, for which
70.8-86.4 mol% α-pinene was oxidized to obtain 81.5-83.6% selectivity. The
reactivity of cyclooctene over Co-MOR and Co-5A was the lowest, in which only
28.1-44.2 mol% cyclooctene was converted, but the epoxide selectivity could reach
93.5-96.9%. This can be ascribed to the negative effect of microporous pore sizes of
Co-MOR and Co-5A on the diffusion of cyclooctene, which seriously limits the
approaching of cyclooctene molecules to catalytic active sites forming the
intermediate states.
3.6 Recycling studies
The recycling experiments of Co-MOR and Co-5A catalysts for the epoxidation
of styrene were conducted. As observed from Figure 4(a), the reuses of Co-MOR
(five times) did not appreciably decrease the conversion of styrene and the
selectivity of epoxide, in which all the reactions maintained almost constant levels of
the conversion and selectivity of about 94.2 mol% and 90.5%. However, the
conversion on Co-5A showed a little reduction in the initial recycle, but remained
basically unchanged in the following four recycles with a small fluctuation of
85.8-86.3 mol% conversion and of 89.2-89.5% selectivity.
In order to further verify whether the observed catalysis was truly heterogeneous
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or not, control experiments were carried out (see Figure 4 (b)). After reactions were
initially conducted for 1 h, the styrene conversions were 15.6 mol% over Co-MOR
and 10.3 mol% over Co-5A, then solid catalysts were filtered off and the reactions
were continued for another 4 h. Finally, the conversions were analyzed by GC to be
19.7 mol% for Co-MOR (with an increment of 4.1 mol%) and 19.2 mol% for Co-5A
(with an increment of 8.9 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 heterogeneity of catalysis occurred over Co-MOR and
Co-5A.
4. Conclusions
Various transition metal ions exchanged zeolites were prepared by a simple route,
and tested for the aerobic epoxidation of styrene to styrene oxide, in which Co-MOR
and Co-5A displayed highly catalytic activities to achieve 89.4–94.2 mol% conversion
with the selectivity of > 90%. Solvents, the catalyst amount, the reaction time, the
reaction temperature, oxidants and initiators exerted notable impacts on the
epoxidation reaction. Recycling studies and control experiments showed the
recyclability and stability of Co-MOR and Co-5A as heterogeneous catalysts.
Acknowledgment
The authors thank the financial supports by National Natural Science Foundation
of China (Nos. 20901023, 21173073, 21273064).
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Captions:
Table 1 Physicochemical characterization of zeolite catalysts. (a Result obtained from
XRF. b Values obtained from N2-adsorption results.)
Table 2 Epoxidation of styrene with air over various Co2+-exchanged zeolite catalysts
and Co(Ac)2. (Reaction conditions: 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.)
Table 3 Comparison of different Co-containing catalysts for the epoxidation of
styrene with O2 or air in the absence of reductant.
Table 4 Effect of various oxidants on the epoxidation of styrene with air over
Co-MORa. (a Reaction conditions: styrene, 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.
b flowrate of air or N2, 40 ml/min; c 3 mmol; d 0.3 mmol; e 0.3 mmol.)
Table 5 Catalytic epoxidation of various alkenes over Co-MOR and Co-5A. (Reaction
conditions: 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.)
Figure 1 XRD patterns of Co2+-exchanged MOR and 5A zeolites.
Figure 2 SEM images of Co2+-MOR (a), Na-MOR (b), Co-5A (c), and 5A (d)
zeolites.
Figure 3 Epoxidation of styrene with air over various transition metal ion-exchanged
MOR zeolites. (Reaction conditions: styrene, 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.)
Figure 4 (a) Recycling tests of Co-MOR (◆ and ◇) and Co-5A (■ and □) in the
epoxidation of styrene (Reaction conditions: DMF, 10 g; styrene, 3 mmol; TBHP, 0.3
mmol; catalyst, 0.1 g; temperature, 363 K; time, 6 h; flowrate of air, 40 ml/min), (b)
Control experiments of Co-MOR (■&□) and Co-5A (▲&△).
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Table 1
a
Co contenta SiO2/Al2O3
Surface areab
(m2/g)
249.3
Pore size
(Å)
6.6
6.7
~5
Catalyst
(wt%)
ratios
20.6
25.2
2.0
Na-MOR
Co-MOR
5A
0
1.03
0
235.6
485.0
~5
~4
~4
10
Co-5A
Na-4A
Co-4A
13X
0.43
0
1.14
0
3.0
2.0
3.0
2.3
396.3
155.7
120.6
862.4
10
Co-13X
1.21
4.0
750.4
Table 2
Sele. (%)
Catalyst
Conv. (mol%)
Epoxide Benzaldehyde Phenylacetaldehyde
Na-MOR
Co-MOR
5A
Co-5A
Co-4A
Co-13X
Co(Ac)2
3.7
94.2
9.4
89.4
53.5
40.3
12.7
70.1
90.5
75.7
90.2
83.5
83.1
57.0
29.9
3.7
24.5
4.1
13.1
14.7
41.6
0
5.8
0
5.7
3.4
2.3
1.4
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Table 3
Reaction condition
Tempt. (K) Oxidant
363 air
Conv.
(mol%)
Sele.
(%)
Catalyst
Ref.
CoOx/SiO2
Co2+-NaX
Co-MCM-41
Co-X
85.7
44.0
45.0
45.0
86.0
60.0
62.0
65.0
6
9
373
373
373
O2
O2
O2
9
10
CaCoX, SrCoX, BaCoX
Co-SBA-15-20-5.2
Co-ZSM-5
373
373
363
363
363
363
373
373
393
373
373
O2
>99
92.4
88.4
92.4
91.9
92.0
42.1
94.1
62.0
47.0
97.0
85.0
63.2
90.1
91.7
90.5
89.5
86.6
65.5
82.0
61.0
83.0
10
12
13
13
14
14
17
18
19
11
20
21
22
23
24
25
26
O2
air
Co-Beta
air
Co-SAPO-34
air
Co-SAPO-5
air
[Co(Bzo2[14]aneN4)]2+-NaY
Co-SBA-1520
O2
O2
O2
S-8CoYZr
NaCsCoY20
O2 (5.5 bar)
O2
Co-Al LDH
Co-SSZ-S-5
Ag-Co-SiO2
Co-ZSM-5(L2)
Co-ZSM-5(L2)
CoAlPO-36
Co/TS-1
373
373
363
363
323
373
363
363
O2
47.5
52.0
90.5
93.9
46.0
94.5
94.2
89.4
78.3
81.0
91.1
89.7
34.0
74.3
90.5
90.2
air
air
air
air (30 bar)
O2
air
air
Co-MOR
this study
this study
Co-5A
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Table 4
Sele. (%)
Conv.
(mol%)
Oxidant
Epoxide Benzaldehyde Phenylacetaldehyde
None
Airb
0
10.3
0
—
66.7
—
—
33.3
—
—
0
b
N2
—
—
0
NaClOc
0
—
—
c
NaIO4
TBHPc
13.2
20.4
79.0
79.7
21.0
20.3
0
c
H2O2
21.4
11.0
15.8
0
69.4
83.9
85.1
—
27.8
11.9
10.5
—
2.8
4.2
4.4
—
TBHPd
CHPd
Airb+TEMPOe
e
Airb+H2O2
37.6
94.3
94.2
78.5
90.9
90.5
17.5
1.5
4.0
7.6
5.8
Airb+CHPe
Airb+TBHPe
3.7
Table 5
Conv.
(mol%)
Sele.
(%)
Catalyst
Co-MOR
Co-5A
Alkene
94.2
89.4
90.5
90.2
Co-MOR
Co-5A
86.4
70.8
100
81.5
83.6
75.3
96.8
96.9
93.5
Co-MOR
Co-5A
55.8
44.2
28.1
Co-MOR
Co-5A
15

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Figure 1
Figure 2
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100
Styrene conversion
Epoxide selectivity
80
60
40
20
0
C
F
N
M
C
Z
C
e
r
n
i
u
o
n
-
-
M
-
-
M
-
-
M
-
M
M
M
M
O
O
O
O
O
O
O
R
R
R
R
R
R
R
Figure 3
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100
90
80
70
60
50
40
30
20
100
90
80
70
60
50
40
30
20
10
Conv.
Sele.
Conv.
Sele.
0
1
2
3
4
5
Number of cycles
(a)
100
80
60
40
20
0
Co-MOR
Co-MOR
Co-5A
Co-5A
Control experiments
0
1
2
3
4
5
6
7
Time (h)
(b)
Figure 4
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Research Highlights
 Co2+-exchanged MOR and 5A are firstly used in the aerobic epoxidation of
alkenes.
 89.4–94.2 mol% conversion of styrene with epoxide selectivity of > 90% is
obtained.
 A route to catalyze epoxidation of alkenes over simply-prepared cheap catalysts.
19