Design and synthesis of bi-functional Co-containing zeolite ETS-10 catalyst with high activity in the oxidative coupling of alkenes with aldehydes for preparing α,β-epoxy ketones

Article abstract of DOI:10.1039/c7ra06828j

Developing highly efficient heterogeneous catalysts for organic synthesis is of great importance in modern synthetic chemistry. In this work, Co (or Ni)-containing mesoporous zeolite ETS-10 (Co-METS-10 and Ni-METS-10) with both metal and strong basic site

Full text of DOI:10.1039/c7ra06828j

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Design and synthesis of bi-functional Co-
containing zeolite ETS-10 catalyst with high activity
in the oxidative coupling of alkenes with aldehydes
for preparing a,b-epoxy ketones†
Cite this: RSC Adv., 2017, 7, 41204
Jun Hu,a Feifei Xia,a Fengli Yang,a Jushi Weng,a Pengfei Yao,a Chunzhi Zheng,a
b
Chaojie Zhu,b Tiandi Tangb and Wenqian Fu
*
Developing highly efficient heterogeneous catalysts for organic synthesis is of great importance in modern
synthetic chemistry. In this work, Co (or Ni)-containing mesoporous zeolite ETS-10 (Co-METS-10 and Ni-
METS-10) with both metal and strong basic sites were synthesized and applied for the direct oxidative
coupling of alkenes with benzaldehydes to synthesize a,b-epoxy ketones. Co (or Ni)-METS-10 catalysts
show high activity and product selectivity, as compared to metal-free mesoporous zeolite ETS-10
(METS-10). This feature is attributed to the fact that the highly dispersed Co (or Ni) species could
facilitate the tert-butyl hydroperoxide transformation into more alkyloxy and alkylperoxy radicals, which
triggers the alkenes undergoing radical addition with aldehyde and alkylperoxy to form b-peroxy
ketones. Meanwhile, the basic sites on Co (or Ni)-METS-10 catalysts benefit the formation of a,b-epoxy
ketone from b-peroxy ketone.
Received 19th June 2017
Accepted 15th August 2017
DOI: 10.1039/c7ra06828j
rsc.li/rsc-advances
importance for synthesis of a,b-epoxy ketones in green and
sustainable route.
1. Introduction
It has been reported that the metal site can facilitate tert-
butyl hydroperoxide transformation to alkyloxy radicals, which
can abstract hydrogen from aldehyde to give acyl radical.6,9
Subsequently, alkenes undergoing radical addition with acyl
radical and hydroperoxides deliver b-peroxy ketones that can be
transformed to corresponding epoxides catalyzed by organic
base.10–12 Thus, if the heterogeneous catalyst could contain both
metal sites and basic sites, it would be realizable synthesis of
a,b-epoxy ketones through one-pot process. As well known,
crystalline titanosilicate zeolite ETS-10 is porous materials with
12-membered ring network, and possesses strong basicity,13,14
which can be as a solid basic catalyst to catalyze many reactions,
such as knoevenagel reaction,15 aldol condensation16 and aryl
amines oxidation reaction.17 Therefore, it would be achievable
that introducing metal species into zeolite ETS-10 to prepare bi-
functional catalyst that has radical generation ability and base
catalysis property for synthesis of a,b-epoxy ketone compounds
with high efficiency through one-step procedure.
Herein, as a continuous effort to developing novel zeolite
catalysts for the organics transformations,18–21 we report a facile
methodology to synthesize Co-containing mesoporous zeolite
ETS-10 (Co-METS-10) catalyst that shows very high activity in
the formation of a,b-epoxy ketones by oxidative coupling of
alkenes with aldehydes through radical mechanism. This
feature should be attributed to that the highly dispersed Co
species on zeolite catalyst could facilitate the formation of more
a,b-Epoxy ketones are highly important and valuable
compounds that have received much attention because of their
wide application as versatile intermediates for transformations
to many natural products and pharmaceuticals.1,2 Generally,
these compounds are synthesized through a two-step proce-
dure: synthesis of a,b-unsaturated ketones through Claisen–
Schmidt condensation reaction and subsequent epoxidation of
a,b-unsaturated ketones with peroxide catalyzed by environ-
mentally unfriendly organic base in a homogenous reaction
system.3–5 Recently, although Li et al. synthesized a,b-epoxy
ketones through iron-catalyzed carbonylation-peroxidation of
alkenes to furnish b-peroxy ketones that were subsequently
transformed into the corresponding epoxides in the presence of
organic base catalyst, this approach is still employed in the two-
step procedure.6 Furthermore, these reaction routes present the
following disadvantages, such as the difficulty in separation of
products and organic base from reaction systems and in reusing
the catalysts,7,8 which severely limits their potential applications
in industry. From a practical point of view, developing highly
active heterogeneous catalysts with functionality is great
aSchool of Chemical and Environmental Engineering, Jiangsu University of Technology,
Changzhou, Jiangsu 213001, PR China
bSchool of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu
213164, PR China. E-mail: fuwenqian@cczu.edu.com; Tel: +86-519-86330253
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra06828j
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alkyloxy radicals, which prompt the alkenes underwent radical
addition with aldehyde and alkylperoxy to form b-peroxy ketone
intermediates, and the basic sites benet the transformation
from b-peroxy ketone to a,b-epoxy ketone.
2.3. Activity test
The oxidative coupling reactions of alkenes with aldehydes in
the liquid-phase were conducted in glass reactor. In a typical
run, 40 mg catalyst, styrenes (0.5 mmol), benzaldehydes (2
equiv., 1 mmol), 2 mL acetonitrile and tert-butyl hydroperoxide
(TBHP, 1.0 mmol, 70% aqueous solution) were injected into the
glass reactor in sequence. The reaction mixture was then heated
to desired temperature for continuous reaction for desired time.
Aer the reaction was nished, the catalyst was separated by
centrifugation to obtain the liquid phase for further analysis by
an Agilent 7890B gas chromatograph equipped with a ame
ionization detector.
2. Experimental
2.1. Material synthesis
Co-containing mesoporous zeolite ETS-10 (Co-METS-10)
was synthesized hydrothermally from a titanosilicate gel
with a molar composition 3.7Na2O/1.3K2O/1.0TiO2/6.7SiO2/
0.6TPOAB/0.18CoO/163H2O. TPOAB is N,N-dimethyl-N-octa-
decyl-N-(3-triethoxysilylpropyl)ammonium bromide as meso-
scale template. In a typical run, 17.8 mL water glass was added
in a 100 mL beaker. Then 10.0 mL NaOH aqueous solution (5.38
M) and 9.4 mL KOH aqueous solution (4.75 M) are mixed with
the above solution in beaker under vigorous stirring until
cooling to room temperature. Aer that, 13.1 g TiCl3 solution
(17 wt% in HCl) was added into the beaker slowly followed by
stirring for 240 min. Finally, 3.5 mL TPOAB and 3 mL cobalt
nitrate aqueous solution (1.1 M) was added and further stirring
for 2 h. The obtained gel was transferred into Teon-coated
stainless-steel autoclave for crystallization at 230 ꢀC for 72 h.
The resulting product was ltered, washed, and dried at 100 ꢀC
3. Results and discussion
3.1. Characterization
The XRD patterns of the ETS-10, METS-10 and metal-
containing METS-10 samples in Fig. 1a give well-resolved
diffraction peaks in the range 5–45ꢀ with the ETS-10 zeolite
structure.14 Notably, Co and Ni crystalline phase are not
detected in XRD pattern for Co-METS-10 and Ni-METS-10
samples, indicating that Ni and Co species are highly
dispersed in the zeolites. Nitrogen adsorption–desorption
isotherms of METS-10 and metal-containing METS-10 samples
ꢀ
overnight and calcined in air at 450 C for 5 h. Ni-containing
mesoporous zeolite ETS-10 (Ni-METS-10) was synthesized by
the same procedure except using nickel nitrate replacement of
cobalt nitrate. For comparison, metal-free mesoporous ETS-10
(METS-10) and microporous ETS-10 zeolites were also synthe-
sized by the same procedure in the absence of metal salts and
template, respectively.
2.2. Characterization
X-ray diffraction (XRD) patterns of the samples were collected in
a RIGAKU UltimalV diffractometer with Cu Ka radiation (l ¼
˚
1.548 A). Nitrogen adsorption–desorption isotherms were
measured at liquid nitrogen (ꢁ196 ꢀC) temperature using
a Micromeritics ASAP 2020M automated apparatus. The sample
was degassed for 8 h at 350 ꢀC prior to the measurement.
Surface area was calculated according to the BET equation.
Mesopore volume was calculated by t-plot method. The content
of the Ni (or Co) in zeolite was analyzed by inductively coupled
plasma optical emission spectroscopy (ICP-OES) with a Perkin-
Elmer 2100DV emission spectrometer.
The morphology of the sample was observed with a eld
emission scanning electron microscope (SEM) on a SUPRA55
apparatus operated at an accelerating voltage of 5 kV. Trans-
mission electron microscopy (TEM) experiment was performed
on a JEM-2100F microscope with a limited line resolution
˚
capacity of 1.4 A, under a voltage of 200 kV. Before character-
ization, the sample was cut into thin slices and dropped onto
a Cu-grid coated with carbon membrane. Ultraviolet-Visible
(UV-Vis) spectrum was obtained on a Perkin-Elmer Lambda
25 spectrometer. X-ray photoelectron spectroscopic (XPS)
measurement of the catalyst was performed on an ESCALAB MK
II system.
Fig.
1 (a) XRD patterns and (b) nitrogen adsorption–desorption
isotherms and pore size distributions of the zeolite samples.
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Table 1 Texture parameters of the zeolite samples
line) smaller than 1 nm are highly dispersed in microspores and
mesopores (Fig. 2b). The mesopores (bright zones) are also
observed in the TEM image of Ni-METS-10, and small Ni
nanowires (black wire, about 1 nm diameter) with length of 5–
10 nm are irregular dispersed in the micro–mesopores (Fig. 3b
and S1†).
a
b
c
d
SBET
Sext
Vmicro
Vmeso
Metal
Samples
(m2 gꢁ1
)
(m2 gꢁ1
)
(cm3 gꢁ1
)
(cm3 gꢁ1
)
loadinge (wt%)
ETS-10
METS-10
Ni-METS-10 320
Co-METS-10 324
335
330
16
65
75
85
0.12
0.11
0.11
0.11
0.03
0.13
0.12
0.11
—
—
1.8
1.7
The UV-Vis spectra of the METS-10, Co-METS-10 and Ni-
METS-10 samples are shown in Fig. 4. The absorption band at
280 nm can be assigned to the charge transfer transition
involving Ti atoms in octahedral coordination in zeolite
framework, which are in line with the reported literature.22 The
electronic state of Co and Ni species are also investigated by XPS
technique (Fig. 5). The Co 2p spectrum of Co-METS-10 sample
has a main binding energy at 781.5 eV with a shakeup feature at
a higher binding energy, which is consistent with the charac-
teristic of Co2+ species.23 The Ni 2p spectrum of Ni-METS-10
shows a binding energy at 856.6 eV associated with Ni2+
species, along with a shake-up peak at approximately 6.0 eV
higher than that of the Ni2+ species.24 These results indicate the
presence of Co2+ and Ni2+ in the Co-METS-10 and Ni-METS-10
sample, respectively.
a
BET surface area. b External surface area (mesoporous surface area is
c
d
e
included). Microporous volume. Mesoporous volume. The metal
content was analyzed by ICP-OES.
give distinct hysteresis loop at relative pressure of 0.5–0.9,
suggesting the presence of mesoporous structure in the
samples (Fig. 1b).21 The mesoporous sizes was centered at 6.1,
8.1 and 6.7 nm for METS-10, Co-METS-10 and Ni-METS-10
samples, respectively. Sample textual parameters presented
in Table 1 show that Co-METS-10 and Ni-METS-10 have high
mesoporous surface area (85 and 75 m2 gꢁ1) and mesoporous
volume (0.11 and 0.12 cm3 gꢁ1), which benet the reactant
diffusion, as compared to ETS-10 zeolite.
The SEM images display that the Co-METS-10 and Ni-METS-
10 catalysts have similar morphologies with particle size of 2–3
mm (Fig. 2a and 3a). It seems that these particles are aggregated
by nanosheets. TEM image of Co-METS-10 shows that the
mesoporous (marked with white lines) are present in zeolite
crystals, and very small Co clusters (black dots, marked with red
3.2. Catalytic performance
In order to evaluate the catalyst activity, the direct coupling of
alkenes with aldehydes to synthesize a,b-epoxy ketones was
performed by choosing styrene and benzaldehyde as substrates
at different temperatures. The results are shown in Fig. 6. At all
Fig. 2 (a) SEM and (b) TEM images of the Co-METS-10 zeolite.
41206 | RSC Adv., 2017, 7, 41204–41209
Fig. 3 (a) SEM and (b) TEM images of the Ni-METS-10 zeolite.
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Fig. 4 UV-vis spectra of the zeolite samples.
Fig. 6 Styrene conversions over Co-METS-10, Ni-METS-10, METS-10
and ETS-10 catalysts at different reaction temperature in the reaction
time of 6 h.
while the target product selectivity over Co-METS-10 and Ni-
METS-10 is 97 and 87%. In addition, METS-10 shows the
lowest styrene conversion and product selectivity at different
reaction time among the three catalysts. Compared with
METS-10, Co-METS-10 and Ni-METS-10 have equivalent meso-
porous volume and external surface area (Table 1), but a higher
activity and product selectivity are obtained on Co-METS-10 and
Ni-METS-10 catalysts. These results indicate that Co(Ni)-METS-
10 as bi-functional catalysts not only facilitate the enhanced
reaction activity, but also benet the formation of target
product at relatively reaction temperature.
The superior catalytic performance of the Co(Ni)-METS-10
can be explained as follows. Generally, the oxidative coupling
of alkenes with aldehydes over basic catalyst underwent radical
reaction mechanism.6,12,14 The radical trapping experiment
result also demonstrated that acyl radical was generated (the
details please see ESI, Fig. S2†), suggesting that this oxidative
coupling reaction over Co-METS-10 may perform through
a radical mechanism. In this case, the generation of alkyloxy
radical is crucial. Previous studies have reported that metal
such as Fe2+, Co2+ and Cu2+ catalysts can promote the formation
Fig. 5 XPS for Co 2p and Ni 2p of the zeolite samples.
reaction temperature, ETS-10 catalyst shows the lowest styrene
conversion. This could be attributed to the mesoporous pres-
ence in METS-10, which could benet the mass transfer. For the
metal-containing METS-10 catalysts, Co-METS-10 (30%) and Ni-
METS-10 (23%) has poor styrene conversion at reaction
ꢀ
temperature of 60 C, and are lower than metal-free METS-10
(37%). Nevertheless, when the reaction was carried out at
70 ꢀC, the styrene conversion over Co-METS-10 and Ni-METS-10
was obviously improved (56% and 64%), while that over METS-
10 is only 46%. This result indicates that Co-METS-10 and Ni-
METS-10 hꢀave high reaction activity at relatively high tempera-
of alkyloxy radical from t-BuOOH by at relatively temperature
2+
(80–120 C).6,25,26 In our case, the metal Co or Ni2+ species in
ꢀ
METS-10 could facilitate the formation of the alkyloxy and
alkylperoxy radicals at reaction temperature of 70–80 ꢀC, which
facilitate the formation of acyl radical that attack the styrenes to
generate the b-peroxy ketones. In addition, our previous study
reported that basic sites (marked as ^TiO62ꢁ) on the METS-10
under heating not also help the t-BuOOH split into alkyloxy and
alkylperoxy radicals, but also favor the transformation from
b-peroxy ketones to a,b-epoxy ketones.14 Thus, the formation of
b-peroxy ketone intermediate was facilitated by the basic sites to
transform a,b-epoxy ketones over Co(Ni)-METS-10 catalyst. As
a conclusion, the presence of metal Co (or Ni) and basic sites on
the Co(Ni)-METS-10 catalyst play a synergistic effect enhance-
ment its catalytic activity at relatively high reaction temperature
(70–80 ꢀC). The low activity of Co-METS-10 catalyst at 60 ꢀC
could be attributed to that the density of basic sites on METS-10
ꢀ
ture of 70 C. When the reaction was performed at 80 C, the
styrene conversions on Co-METS-10 and Ni-METS-10 are further
improved, reaching 84% and 95%, respectively.
To further investigate the catalytic performance, the target
product selectivity of different zeolite catalysts was also
compared. Fig. 7 shows styrene conversion and product selec-
tiviꢀty with different reaction time at reaction temperature of
80 C. At the same reaction time, although Ni-METS-10 shows
much higher styrene conversion than Co-METS-10, the target
product selectivity over Ni-METS-10 is lower than that over Co-
METS-10 catalyst. For example, at reaction time of 8 h, styrene
conversion over Co-METS-10 and Ni-METS-10 is 94 and 100%,
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Alkyloxy and alkylperoxy radicals are generated over Co-METS-
10 catalyst (steps a and b). Subsequently, aldehydic hydrogen
was abstracted from 2 by alkyloxy radical, giving acyl radical 4,
which underwent radical addition with alkene giving the radical
5 and followed by radical coupling with alkylperoxy leading to
the intermediate b-peroxy ketone 6. b-peroxy ketone 6 was
catalyzed by basic sites (^TiO62ꢁ) on Co-METS-10 to form the
nal product 3.
Encouraged by the promising results, the scope of this
oxidative coupling reaction over Co-METS-10 catalyst was
investigated through choosing different aldehydes and
styrenes, and the results are summarized in Table 2. Co-METS-
10 catalyst tolerates various benzaldehydes with electron-
donating (methoxy, methyl) and electron-draw (chlorine,
bromine) substituents, giving high activity and target product
selectivity (3a–3e). Meanwhile, styrenes with halogen
Table 2 The oxidative coupling of styrenes with benzaldehydes at
80 ꢀC for 8 ha
Entry Alkenes
1
Aldehydes
Products
Conv.b (%)
96 (92)
Fig. 7 (a) Styrene conversion and (b) product selectivity over Co-
METS-10, Ni-METS-10 and METS-10 catalysts on different reaction
time at reaction temperature of 80 ꢀC.
2
3
4
5
95 (90)
93 (92)
88 (89)
95 (92)
is higher than on Co-METS-10 (Fig. S3, the details please
see ESI†).
On the basis of the above results and literature reports,6,14,25
a tentative reaction mechanism for Co-METS-10-catalyzed
oxidative coupling reaction of styrene is given in Fig. 8.
6
7
96 (92)
94 (90)
8
96 (92)
91 (90)
9
a
Conditions: styrenes (0.5 mmol), benzaldehydes (1.0 mmol), TBHP
(1.0 mmol), Co-METS-10 (40 mg), MeCN (2.0 mL), 80 ꢀC, 8 h.
b
Obtained by GC analysis, the data out of parenthesis is conversion,
and in parenthesis is selectivity.
Fig. 8 Proposed reaction mechanism.
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´
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suitable for this transformation and give the products in good
yields (3f–3h). In addition, the reagent 4-methyl styrene is also
successfully applied to this transformation, affording the
desired product in gratifying yield (3i).
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4. Conclusions
In summary, the Co (or Ni)-containing mesoporous zeolite ETS-
10 was directly synthesized by templating with a mesoscale
silane surfactant through hydrothermal method. Co-METS-10
and Ni-METS-10 catalysts show high activity in the oxidative
coupling reaction of styrenes with benzaldehydes through one-
pot synthetic procedure. In addition, Co-METS-10 exhibits the
highest product selectivity to form a,b-epoxy ketones. Co (or Ni)-
METS-10 has bi-functional characteristic with transition-metal
and strong basicity sites. The highly dispersed Co and Ni
species facilitate the generation of the alkyloxy and alkylperoxy
radicals from t-BuOOH, leading to the occurrence of radical
addition for the alkenes with aldehyde and alkylperoxy to form
b-peroxy ketones. Meanwhile, the basic sites on Co (or Ni)-
METS-10 catalyst favor the transformation from intermediate
b-peroxy ketone to a,b-epoxy ketones, resulting in the enhanced
catalytic activity.
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18 W. Fu, T. Liu, Z. Fang, M. Ma, X. Zheng, W. Wang, X. Ni,
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There are no conicts to declare.
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