Metallophthalocyanine intercalated layered double hydroxides as an efficient catalyst for the selective epoxidation of olefin with oxygen

Article abstract of DOI:10.1016/j.apcata.2017.05.029

Varied metallophthalocyanine intercalated layered double hydroxides (LDHs) as bifunctional hybrid catalysts for selective epoxidation have been prepared and characterized. Systematic characterizations suggested the successful intercalation of the metallophthalocyanines into the interlayer of ZnAl LDHs. The synthesized hybrid exhibited excellent catalytic activity in the selective epoxidation of various olefins through O₂/isobutaldehyde system. The basicity of the hybrid benefits to the selectivity of epoxide, and the bifunctional roles of the catalyst in the reaction have been discussed and verified by a series of controlled experiments. On the basis of obtained results, a probable mechanism of the epoxidation by the hybrid has been proposed and detailedly investigated. Under the catalysis of metallophthalocyanines intercalated LDHs in the presence of O₂/isobutaldehyde, the production of epoxide undergoes two reaction paths. And two types of intermediates, namely acylperoxy radical and peroxyacid, are formed in the reaction, and the former is predominant.

Full text of DOI:10.1016/j.apcata.2017.05.029

Applied Catalysis A, General 542 (2017) 191–200
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Feature Article
Metallophthalocyanine intercalated layered double hydroxides as an
efficient catalyst for the selective epoxidation of olefin with oxygen
a
a
a
a
a
a,⁎
Weiyou Zhou , Jiacheng Zhou , Yong Chen , Aijun Cui , Fu’an Sun , Mingyang He ,
b,⁎
a
Zhixiang Xu , Qun Chen
a
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, Changzhou 213164, China
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Varied metallophthalocyanine intercalated layered double hydroxides (LDHs) as bifunctional hybrid catalysts for
selective epoxidation have been prepared and characterized. Systematic characterizations suggested the suc-
cessful intercalation of the metallophthalocyanines into the interlayer of ZnAl LDHs. The synthesized hybrid
Metallophthalocyanine
Layered double hydroxides
Hybrid
Epoxidation
Bifunctional catalyst
2
exhibited excellent catalytic activity in the selective epoxidation of various olefins through O /isobutaldehyde
system. The basicity of the hybrid benefits to the selectivity of epoxide, and the bifunctional roles of the catalyst
in the reaction have been discussed and verified by a series of controlled experiments. On the basis of obtained
results, a probable mechanism of the epoxidation by the hybrid has been proposed and detailedly investigated.
2
Under the catalysis of metallophthalocyanines intercalated LDHs in the presence of O /isobutaldehyde, the
production of epoxide undergoes two reaction paths. And two types of intermediates, namely acylperoxy radical
and peroxyacid, are formed in the reaction, and the former is predominant.
1. Introduction
phthalocyanine counterparts are cheap and readily accessible on an
industrial scale: their worldwide production is about 80 000 t/year,
The selective and efficient epoxidation of olefinic compounds is one
making them very attractive as potential industrial catalysts [22,23]. To
avoid the drawback of homogeneous metallophthalocyanines, such as
self oxidation and formation of inactive di-oxo [24–26], the stabiliza-
tion of the metallophthalocyanines molecular in a support is imperative
for further developments in the field. Some supported metallophtha-
locyanines have been designed for the epoxidation of alkenes. Copper
(II) phthalocyanine encapsulated in zeolite Y has been prepared by
Seelan, et al. and introduced into the styrene oxidation by TBHP [27],
however, the selectivity was only 39.1% with benzaldehyde as the main
by-product. Manna, et al. reported a hybrid by encapsulating a copper-
phthalocyanine complex in silica-nanoparticle-assembled micro-
capsules (CuPcS@MC) [28]. The epoxide was the only product when
the conversion of styrene was 50% with TBHP as the oxidant. Except
of the most important industrial processes, because the epoxide is useful
intermediate in the production of varied pharmaceuticals, flavor &
fragrance and fine chemicals [1,2]. Traditionally, expensive peracids
are always applied in the direct epoxidation of alkenes to produce the
corresponding epoxides, but with a formation of large amounts of ef-
fluents [3,4]. Although varied approaches have been developed for the
transformation [5–8], from the view of economic and sustainable
2 2
chemistry, heterogeneous catalytic process employing oxygen or H O
as the ultimate oxygen is quite attractive. Many kinds of catalysts based
on supported complex [9–11], metal oxide [12,13], transition metal
substituted heteropolyanions (TMS-HPAs) [14,15], and supported cat-
alysts [16,17] have been introduced into the oxidation. However, some
drawbacks exist in these catalytic systems, for example, complex
synthesis procedure for catalysts, harsh reaction conditions, and low
catalytic activity and the selectivity of the products. Development of
efficient, stable and recyclable catalyst with high selectivity of the
corresponding epoxide is still desirable.
2 3
the drawback of the low yield, a 3 equivalent of Na CO was required,
making the process complexed. Rezaeifard, et al. have prepared the Cu
(II) phthalocyanine-tetrasulfonic acid tetrasodium complex (CuPcS)
immoblizated on the silica coated magnetic nanoparticles (Fe
3 4
O @
SiO , SMNP) via the amine functionality and investigated the catalytic
2
Transition-metal complexes of corrole, salen, porphyrins, and
phthalocyanines have been extensively used to resemble the function-
ality of cytochrome P450 [18–21]. Compared with the other catalysts,
performance in the epoxidation of olefins [29]. A high 93% yield of
cyclohexane oxide was obtained with the tetra-n-butylammonium per-
oxomonosulfate (n-Bu NHSO , TBAOX) as the oxidant.
4 5
⁎
Corresponding authors.
E-mail addresses: hemy_cczu@126.com (M. He), Xuzx_jsu@126.com (Z. Xu).
http://dx.doi.org/10.1016/j.apcata.2017.05.029
Received 5 April 2017; Received in revised form 15 May 2017; Accepted 26 May 2017
Available online 30 May 2017
0926-860X/ © 2017 Elsevier B.V. All rights reserved.

W. Zhou et al.
Applied Catalysis A, General 542 (2017) 191–200
Copperperchlorophthalocyanine has been successfully immobilized
2.3. Characterization of catalysts
in the channels of HSi-MCM-41 molecular sieves through a post-
synthesis method (Cu-AM(PS)) [30], which gave full conversion of
styrene with the highest selectivity of 74.4%. The authors found that
the basicity of the prepared catalyst originated from 3-aminopropyl
group benefited to the enhancement of the initial rate of epoxidation by
O /isobutyraldehyde; furthermore, the base could also accelerate the
2
cleavage of OeO bond to produce the high oxo intermediates, which
could react directly with the alkene to give epoxide.
Layered double hydroxides (LDHs), whose anions in the interlayers
can be modified, is a representative basic material and has been widely
applied as basic catalysts or supports in varied reactions [31–33]. In
this context, we envisioned that intercalation of metallophthalocyanine
anions into LDHs might provide an efficient biomimetic catalyst for the
alkene epoxidation. The obtained hybrid would not only present the
high activity of metallophthalocyanine, but also lead to a supported
metallophthalocyanine, making the catalyst stable and recyclable.
Moreover, according to the reported results [30], the surface basicity of
the hybrid might improve the catalytic activity and the selectivity of
epoxide.
In the study, four tetrasulfonic metallophthalocyanine (MPcTs,
M = Co, Cu, Mn and Fe) intercalated LDHs have been prepared and
investigated in the aerobic epoxidation of alkene in the presence of
isobutaldehyde. The effects of reaction conditions on the oxidation have
been systematically studied. Under the optimized conditions, the rela-
tion of structure-activity and the probable mechanism have also been
discussed.
The XRD patterns were obtained on a Rigaku D/max 2500 PC X-ray
generator with Cu-Kα (1.5402 Å) radiation. FTIR spectra of the pre-
pared catalyst were collected on a Nicolet PROTÉGÉ 460 FTIR spec-
trometer using KBr disks. SEM images were recorded on a JEOL JSM-
6360LA scanning electron microscope. A Perkin-Elmer Instruments
Lambda 35 was used to analyze the diffuse reflectance ultraviolet
visible spectroscopy (DR UV–vis). Thermogravimetric measurements of
these compounds were performed in a Seiko Instrument TG/DTA model
32, in the temperature range 323–1173 K at a heating rate of 10 K/min
2
under an N atmosphere. The metal compositions of samples were
measured via inductively coupled plasma optical emission spectrometry
(ICP-OES) method on a Varian Vista-AX device, and the samples were
prepared by wet-digestion method.
2
2.4. Catalytic epoxidation of styrene over O /IBA system
In a typical procedure, styrene (2 mmol), catalyst (6 mg), acetoni-
trile (10 mL), IBA (isobutyraldehyde, 5 mmol), and chlorobenzene
(inert internal standard, 0.5 mmol) were first introduced into a 25 mL
round-bottom flask equipped with condenser and magnetic stirrer.
Dioxygen was bubbled through the solution at a volume rate of
−1
15 mL min . Then the mixture was stirred at 60 °C and sampled at
different intervals during the reaction. Qualitative and quantitative
analysis of all the products were performed on GC–MS (Shimadzu
GCMS-2010) and GC-FID (Shimadzu GC-2010AF), repectively. The TOF
(
turnover frequency) was calculated by following formula: TOF = mol
of converted styrene/mol of MPcTs in the catalyst/reaction time.
2. Experimental
2
.5. 18O-labeled H 18O experiments
2
All the chemicals used in the experiments were of analytical grade
and were purchased from Energy-chemical.
2
To a mixture of styrene (104 mg, 1 mmol), CoPcTs-Zn Al-LDHs
18
18
(
3 mg), IBA (180 mg, 2.5 mmol), and H
enriched, Aladdin Chemical Co.] in a dried solvent mixture (5 mL) of
CH CN was bubbled dioxygen (15 mL/min). The reaction mixture was
2
O [90 mg (5 mmol), 97%
O
2.1. Synthesis of MPcTs(NH
4 4
)
3
stirred for 1.5 h at 60 °C and then directly analyzed by GC–MS through
calculation of the mass peaks.
Tetra-sulfonic cobalt phthalocyanine tetra-ammonium was prepared
and purified according to the reported method [34]. Triammonium 4-
sulfophthalate (3.0 g, 0.01 mol), urea (7.5 g, 0.125 mol), ammonium
molybdate (0.03 g, 0.025 mol) and anhydrous cobalt(II) chloride (0.32,
3. Results and discussion
2
.5 mmol) were ground together in a 100 mL flask until homogeneous.
3
.1. Characterization of MPcTs-Zn Al-LDH
2
After stirring for 2 h at 240 °C, blue black solid was obtained. The crude
product was washed alternately with ethanol and acetone for 5 times.
The obtained solid was then dissolved in water, and insoluble im-
purities were separated by filtration. The solution was distilled under
vacuum to remove the water. After drying overnight at 80 °C, the blue
In order to probe the formation and location of macrocyclic MPc
molecules in the hydrotalcites, XRD was used to identify the structure of
LDHs. Compared with CO
Zn Al- LDH depicted in Fig. 1 show common features with reflections
typical of a intercalated hydrotalcite-like phase [35,36]. Sharp and
2−
3
2
-Zn Al-LDH, XRD patterns of MPcTs-
2
black, pure product CoPcTs(NH
8%. MPcTs(NH (M = Cu, Mn and Fe) were prepared and purified
by the similar procedure.
4 4
) was obtained in a yield of about
6
4 4
)
2
2.2. Synthesis of MPcTs-Zn Al-LDH
The hybrid catalysts were obtained through the coprecipitation
method. A typical procedure for the CoPcTs intercalated Zn Al LDH
·6H
O (1.05 g, 0.0028 mol) in 50 mL
2
(
(
CoPcTs-Zn
1.67 g, 0.0056 mol) and Al(NO
2
Al-LDH) would be as follows: a solution of Zn(NO
·9H
3
)
2
2
O
3
)
3
2
of deionized water was slowly added to 10 mL of distillated water
−3
4 4
disolved with sodium CoPcTs(NH ) (1.36 g, 1.54 × 10 mol) with
−
1
mechanic stirring under 25 °C. 0.1 mol L NaOH was then added into
the solution continuously and a pH ≈ 7.5 was maintained. Then the
suspension was stirred for another 24 h at 80 °C under N
After filtration, the product was washed with distilled water and finally
dried at room temperature under vacuum. A black solid CoPcTs-Zn Al-
LDH was obtained. MPcTs-Zn Al-LDHs (M = Cu, Mn and Fe) were
obtained by the similar procedure.
2
atmosphere.
2
2
2
Fig. 1. XRD patterns of MPcTs-Zn Al-LDHs (M = Co, Cu, Mn and Fe).
192

W. Zhou et al.
Applied Catalysis A, General 542 (2017) 191–200
Table 1
Compositions and spacing distance of the prepared LDH samples.
−
4
Samples
Content of MPcTs (×10 mol/g)
Content of Zn (wt%)
Content of Al (wt%)
Mol ratio of Zn/Al
Spacing distance (Å)
CO 2
−
-Zn
Al-LDH
Al-LDH
–
42.3
15.5
10.8
15.6
16.0
19.2
2.9
2.6
3.1
3.3
2.2
2.2
2.6
2.1
2.0
7.58[41]
23.36
23.86
21.3
3
2
CoPcTs-Zn
CuPcTs-Zn
FePcTs-Zn
MnPcTs-Zn
2
6.8
6.1
7.6
8.0
2
Al-LDH
2
Al-LDH
2
Al-LDH
21.02
symmetrical for (003), (006), and (009), and broad and asymmetrical
for (0012), (0015), (0018), (015) and (110), respectively are observed,
of CoPcTs, CO 2
–-Zn
Al-LDH and intercalated samples. The character-
3 2
istic vibration modes associated with the cobalt phthalocyanine are
2
–
−1
quite different from that of CO
crystallinity of the prepared samples was low, similar to that of pre-
vious reported CoTSPP-Zn Al-LDH [39], no impurities were observed
40]. The size of anions between the brucite layers and their orientation
3
-Zn
2
Al-LDH [37,38]. Although the
present in the intercalated hybrid between 600–1500 cm
[36,46].
The sharp strong band at 1360 cm , correpoding to the carbonate
anion, is observed for the Zn Al hydrotalcites. The disappearance of the
band in the hybrid sample indicates that no CO
formed during the precipitation procedure. The other three hybrids
exhibit similar characteristic vibration modes as to CoPcTs-Zn Al-LDH.
−1
2
2
2–
[
3 2
-Zn Al-LDH impurity
determines the basal spacing distance of LDHs. The hybrid LDH’s in-
terlayer distances (Table 1), calculated from the reflection (003), are
2
2–
much higher than that of CO
thickness of the hydroxide layer (i.e. 4.80 Å) from the basal spacing
42], the gallery height of these samples can be estimated. The values
3
-Zn
2
Al-LDH [41]. By subtracting a
These results show that metallophathalocyanines have been inter-
calated intact into the LDH.
The SEM images in Fig. 4 show that all the MPcTs intercalated LDHs
hybrid formed plate-like agglomerated crystals, although the crystal-
lization of FePcTs-Zn Al-LDH and MnPcTs-Zn Al-LDH are not very
good. These observations represent the character of layered materials
[38,39]. This suggestion was further supported by thermogravimetric
analysis (Fig. 5). After intercalation, the thermal decomposition tem-
perature of these metallophathalocyanines increased to some extent,
indicating the successful coassembly of LDH sheets with metallo-
phathalocyanine molecules.
In conclusion, these characterization results discussed above verify
that tetrasulfonic metallophthalocyanine moleculars have been suc-
cessfully intercalated into the interlayer of ZnAl LDHs and a series of
organic-inorganic hybrid materials formed originated from me-
tallophthalocyanines and LDHs.
[
are between 16.2–19.1 Å, comparable with the calculated dimensions
of the metallophthalocyanine molecular plane (ca. 18 × 18 × 5.5 Å)
2
2
[
35], indicating that MPcTs molecular has been intercalated into the
interlayers of ZnAl LDHs [43].
Fig. 2 shows the absorption spectrum of pure metallophthalocya-
nines and the corresponding intercalated samples. Two strong absorp-
tion region which is characteristic Q (300–350 nm) and B bands
(
[
600–700 nm) were observed for all the metallophthalocyanines
2−
44,45], while only a weak peak at 333 nm is observed for CO
3
-
Zn
2
Al-LDH. For the intercalated hybrids, all the samples display cor-
responding Q and B bands, although the Q bands are not obvious for Co
and Fe samples and the shifts of B bands are observed. The results
further confirm that tetrasulfonic metallophthalocyanines have been
intercalated into the interlayer of ZnAl LDHs.
Despite the evident difference in the DR-UV spectra, the FTIR
spectra of these samples are quite similar. Fig. 3 shows the comparision
Fig. 2. DR UV–vis spectra of CO
MPcTs-Zn Al-LDH and MPcTs.
2–-Zn
Al-LDH,
3 2
2
193

W. Zhou et al.
Applied Catalysis A, General 542 (2017) 191–200
2–
Fig. 3. FTIR spectra of CO
3 2
-Zn Al-LDH, MPcTs-
2
Zn Al-LDH and MPcTs.
2
Fig. 4. The SEM images of MPcTs-Zn Al-LDHs.
3
.2. Catalytic performance of MPcTs-Zn
2
Al-LDH in the epoxidation of
Firstly, varied solvents have been investigated in the epoxidation, and
significant impact of the solvent nature on the conversion of styrene
and the styreneoxide selectivity has been observed. Among these sol-
vents, the styrene completely converted under acetonitrile, di-
chloromethane (DCM), nitroethane, and nitropropane, while
styrene
The epoxidation of styrene by CoPcTs-Zn
2
Al-LDH was selected as
the model reaction in the optimization of the reaction conditions.
2
Fig. 5. TG and DTG curves of MPcTs and MPcTs-Zn Al-LDHs.
194

W. Zhou et al.
Applied Catalysis A, General 542 (2017) 191–200
Table 2
Optimization of reaction conditions a
.
Entry
Solvent
Weight of catalyst (mg)
Equiv. of IBA
Temperature (°C)
time (h)
Conversion (%)
TOF b (h−1
)
Selectivity (%)
1
2
3
4
5
6
7
8
9
acetonitrile
DCM
DMF
DMSO
dioxane
20
20
20
20
20
20
20
20
20
20
20
20
20
20
10
6
6
6
6
6
6
6
6
6
6
6
6
6
5
4
5
5
5
80
80
80
80
80
80
80
80
80
80
40
60
60
60
60
60
60
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
1.5
100
100
28
11
67
100
87
89
100
74
85
100
100
92
100
100
95
59
59
17
7
16
24
10
34
28
17
30
16
18
18
21
10
12
10
11
10
16
84
76
90
66
72
82
68
84
82
81
79
89
88
90
89
90
84
39
59
51
52
59
43
50
59
98
90
196
327
466
nitroethane
toluene
trifluorotoluene
nitropropane
benzonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
10
11
12
13
14
15
16
17
4
a
b
Reaction conditions: styrene 2 mmol, solvent 8 mL, O
Turnover frequency with no allowance for the background.
2
15 mL/min.
acetonitrile gave the highest 84% selectivity of the epoxide. In the re-
action, benzaldehyde was found as the main by-product. Concerning
the reaction temperature, the results summarized in Table 2 show that
the conversion increased as the temperature increase, and 60 °C was
suitable for the reaction. Under the lower temperature (Table 2),
prolonging reaction time could not afford better results. Finally, the
amounts of catalyst and IBA were optimized, and 6 mg of catalyst and
Table 3
Comparison of catalytic activity of different catalytic systems in the oxidation of styrene a
.
Entry Catalytic system
Cb (%) Sc (%) TOFd
Ref.
−1
(h
)
1
2
3
4
5
CoPcTs-Zn
2
Al-LDH/IBA/O
2
2
100
81
90
86
67
67
95
327
318
179
169
184
This
work
This
work
This
work
This
work
[49]
CuPcTs-Zn
2
Al-LDH/IBA/O
2.5 equivalent of IBA were enough for the transformation of 2 mmol of
styrene. Under the optimized conditions, the selectivity of styreneoxide
reached 90% with a complete conversion of styrene.
Under the optimized conditions, the catalytic performances of me-
tallophthalocyanines intercalated LDHs with different complexed me-
tals have been compared. The results summarized in Table 3 (entries
MnPcTs-Zn
FePcTs-Zn
2
Al-LDH/IBA/O
2
69
2
Al-LDH/IBA/O
2
61
Iron “helmet” phthalocyanines/ 100
2 2
H O
1
–4) show that CoPcTs-Zn
2
Al-LDH gave the highest yield of styrene-
6
7
8
9
1
1
Fe-Salen-SBA/IBA/O2
Silylated [Nb]-MCM-41/TBHP
80.9
62
36
59
97.5
69
78.6
76.5
31.5
94.2
50
59.7
94
96
26
[50]
[51]
[52]
[53]
[54]
[55]
[55]
[20]
[24]
[27]
[28]
[56]
[57]
oxide, and the order was: Co > Cu > Mn > Fe, which should be
related to the redox potentials of different metallophthalocyanines
Ti-SiO
2
/H
O
2 2
100
100
17.3
58.2
71.2
49.8
50.6
39.1
–
4.2
1.97
352
63.4
63
175.5
18.7
50.3
–
OMS-2/TBHP
Silylated Cu-SBA-15/H
[
47,48], and different accelerating activity in the production of iso-
0
1
2
O
2
butyryl radical (Section 3.3).
VO-CM-salen/IBA/O
VO-salen-SBA/IBA/O
2
Subsequently, recycle tests were conducted in the epoxidation of
12
2
1
1
1
1
17
18
3
4
5
6
Fe-salen-GO/IBA/O
CuCl16Pc/TBHP
2
styrene by O
ability of the CoPcTs-Zn
version and selectivity was observed even after six runs (Fig. 6A). The
ICP analysis of the filter of the reaction mixture containing no catalyst
excluded the potential leaching of CoPcTs. XRD pattern of the reused
2
/IBA with shorter reaction time (1 h) to assess the dur-
Al-LDH. No significant decrease of the con-
2
CuPcY-1(e)/TBHP
CuPcS@MC/TBHP
FeP-CMP/IBA/O2
Cobalt(III)-Schiff base
complexes
55
78
69
55
23
537
2
CoPcTs-Zn Al-LDH shows that the layered structure was completely
preserved (Fig. 6B), suggesting that the prepared hybrid was stable
under the reaction.
To elucidate the efficiency of the studied samples, some reported
catalytic systems for the epoxidation of styrene have been summarized
a
See the reference for the detailed structure of catalyst.
Conversion of styrene.
Selectivity of styreneoxide.
b
c
d
Turnover frequency with no allowance for the background.
2
in Table 3 for comparision. It is obvious that the CoPcTs-Zn Al-LDH
exhibits more excellent catalytic performance in comparison with these
catalytic systems. Although some catalysts present comparable con-
versions of styrene (Table 3, entries 6, 9, 11), the selectivities of the
epoxide are even lower. Homogeneous iron “helmet” phthalocyanines
reported by Skobelev et al. shows excellent catalytic result for the
Encouraged by the obtained results, the generality of this catalytic
system was further investigated in the epoxidation of different olefins
under the optimized reaction conditions (Table 4). For the styrene and
α-methylstyrene, the corresponding benzaldehyde and acetophenone
were formed as the main by-products, indicating that the selected
conditions supply enough energy to break the C]C bond [46]. From
the results of substituted styrenes, the substrate with electro-donating
group enhanced the reaction (entry 3), while the reaction rate de-
creased in case of electro-withdrawing substituted styrene (entry 4).
However, excellent yields of these substrates were obtained under
2 2
epoxidation of styrene by H O [49], however, the catalyst is difficult to
be recycled. To precisely compare the activity of different catalysts, the
turnover frequency (TOF) of active site has been adopt in the study. To
our delight, high TOF values were obtained for the prepared catalysts,
strongly indicating the high-efficiency of the catalytic system.
195

W. Zhou et al.
Applied Catalysis A, General 542 (2017) 191–200
Fig. 6. Recycled results (A) and the XRD pattern (B)
2
of reused CoPcTs-Zn Al-LDH.
Table 4
different reaction times.
The oxidation of cyclohexene gave high 98% conversion and 89%
selectivity (Table 4, entry 5), superior to many catalytic systems
Catalytic epoxidation of varied olefins under CoPcTs-Zn
2
Al-LDH a
.
Entry
1
Substrate
tb(h)
1.5
Product
Cc(%)
100
Sd(%)
90
[
30,58–60]. In case of other cycloolefines, excellent conversions and
selectivities could also be obtained under the optimized conditions
entries 6 and 7). Terminal alkenes, especially for the linear olefines
(
like 1-pentene and 1-octene, are always thought to be difficult to be
epoxidized. To our delight, the present catalyst also gave the high re-
sults for them, although 5 equivalent of IBA was needed for the 1-oc-
tene (entries 8 and 9). When cinnamic alcohol was introduced into the
reaction, high yield of corresponding expoxide was obtained, indicating
that the presence of alcohol hydroxyl has little effect on the epoxidation
in the structure (entry 10). However, in the case of cyclohexenol, the
main product was cyclohexenone, in contrast to the Ni(II) complexes
[61]. These results suggest that the presence of phenyl at the α-site
improves the epoxidation reactivity of the olefinic double bond. Fur-
ther, an electron-withdrawing compound 2,3,5-trimethylcyclohexa-2,5-
diene-1,4-dione (entry 12), which is even less active, was tested under
the catalytic system. Only a 58% conversion of the substrate was ob-
served, although 5 equivalent of IBA was added with a prolonged re-
action time. These results indicate that the low electron concentration
of the olefinic double bond retards the epoxidation, and the epoxidation
is an electrophilic procedure.
2
2
100
81
3
4
1.5
2
100
98
82
85
5
6
2
2
98
98
89
99
α-Pinene oxide is an important intermediate in fine chemicals,
especially for flavours and fragrances [62,63]. Therefore, the epoxida-
tion of α-pinene has also been investigated in the study. To our delight,
almost a quantitative yield of epoxide was obtained, markedly superior
to some reported catalytic systems [49,64–66]. From the above results,
7
2
2
99
98
99
99
8
9b
2
it can be obviously concluded that the CoPcTs-Zn Al-LDH hybrid cat-
alysts can catalyze the aerobic oxidation of various olefins to corre-
sponding epoxides under such mild reaction conditions.
4
2
86
99
82
1
0
100
To discuss the relation between the catalytic performance and
1
1
1
2
4
99
58
90
98
2
structure of the hybrid CoPcTs-Zn Al-LDH, a series of controlled reac-
tions have been performed with a shorter reaction time. The results of
the blank reaction showed that the aldehyde can activate molecular
oxygen to lead to the epoxidation of olefins even without catalysts.
However, the conversion and selectivity without catalyst were even low
2e
(
Table 5, entry 1), indicating that the prepared hybrid performed as
catalyst in the reaction.
2–
CoPc and CO
3 2
-Zn Al-LDH have also been investigated in the
1
3
1.5
100
99
epoxidation of styrene as comparision, and the results indicate that
2
–
CO
3 2
-Zn Al-LDH restrained the reaction, which may be related to its
basicity. It can be easily concluded that the active site for the epox-
idation is CoPcTs molecules in the interlayer of the hybrid. On the other
hand, intercalated sample exhibited higher cataltyic efficiency than
CoPc, as well as the higher styreneoxide selectivity. We speculated that
the improvement of the epoxide selectivity might be due to the basicity
of the intercalated LDHs, which is a represent basic material. To verify
a
2
Reaction conditions: substrate 2 mmol, CoPcTs-Zn Al-LDH 6 mg, acetonitrile 8 mL,
6
0 °C, IBA 5 mmol, O
2
15 mL/min, 1.5 h.
b
Reaction time.
Conversion of styrene.
Selectivity of styreneoxide.
c
the dedution, CoPc combining with Na
2 3
CO has been introduced into
d
e
the reaction under the optimized conditions. As we espected, an
5
equivalent of IBA was used.
196

W. Zhou et al.
Applied Catalysis A, General 542 (2017) 191–200
Table 5
The performance of different catalytic system in the aerobic oxidation of styrene a.
Cb (%)
Sc (%)
TOF (h−1)
d
Entry
Catalyst
1
2
3
4
5
6
7
8
–
CO
CoPc
CO
CoPcTs-Zn
CoPc + Na
73
0
65
0
–
0
2
–
Al-LDHe
Al-LDH
3
- Zn
2
f
94
25
93
95
94
0
83
68
88
91
94
–
286
–
365
289
369
–
2–- Zn
3
2
2
Al-LDH
g
2
CO
CoPc-Zn
CoPcTs-Zn
2
Al-LDH + Na
COg
2
Al-LDHh
2
a
Reaction conditions: styrene 2 mmol, catalyst 6 mg, acetonitrile 8 mL, 60 °C, IBA
15 mL/min, 75 min.
Conversion of styrene.
Selectivity of styreneoxide.
Turnover frequency with no allowance for the background.
Without IBA.
5
mmol, O
2
b
c
d
e
f
The amount of CoPc 3 mg.
g
h
2 3
The amount of Na CO 1 mg.
2
equivalent of BHT was added.
improved selectivity of styreneoxide was observed. Furtherly, Na
2 3
CO
was also introduced into the reaction catalyzed by CoPcTs-Zn Al-LDH,
2
and the selectivity went up to as high as 94%. These results indicate
that the base is beneficial to the selectivity of epoxide, and the surface
basiciy of the hybrid can accelerate the production of epoxide. The
possible reason is that the basic site can restrain the transfer from the
radical (c) to the species (g) (Section 3.3).
Summary, the hybrid originated from CoPcTs and LDHs can catalyze
the epoxidtion of olefins smoothly. The active site should be the CoPcTs
molecular, rather than the ZnAl LDHs, while the high selectivity of the
epoxide should be due to the basicity of the hybrid. The prepared hy-
brid material may play as a bi-functional catalyst in the epoxidation of
Scheme 1. The reported reaction paths for the formation of epoxide.
served as the catalyst the initiation reaction of IBA, namely the for-
mation of isobutyryl radical.
To examine the possible active intermediate, 3-chloroperbenzoic
2
acid (mCPBA) was used instead of IBA/O without catalyst to examine
the reaction 4. Both epoxide and benzaldehyde were obtained by
GC–MS, and the results are shown in Table 6, indicating that peroxyacid
might be one of the intermediate. When one equivalent of BHT was
added into the reaction, the conversion decreased and only styrene-
oxide was obtained (Table 6, entry 2). These results indicate that both
radical and nonradical paths (reaction 2 and 4) existed in the catalytic
system, and the benzaldehyde should be produced by radical path.
To examine if the benzaldehyde was produced from styreneoxide,
styreneoxide was used as the substrate in the reaction under the same
conditions. No benzaldehyde was observed in the reaction after 2 h,
strongly indicating that the styreneoxide and benzaldehyde are formed
along two different parallel reaction pathways, in contrast to the im-
mobilized Vitamin B12 and Ti-containing molecular sieves systems
59,80]. A possible reaction paths (path I) for the production of ben-
zaldehyde has been proposed by Haber, et al. as shown in Scheme 2
71]. To confirm the reaction, the effect of the volume rate of oxygen on
the selectivities of styreneoxide and benzaldehyde has been in-
vestigated. One can observed that the selectivity of benzaldehyde de-
creased with the enhancement of the oxygen rate (Table 6, entries 3–5),
inconsistent with the reported reaction path I, indicating the formation
of benzaldehyde might be through path II [81].
2
olefins by IBA/O system.
3.3. Discussion of mechanism
In the epoxidation of olefin by IBA/O
2
system, the mechanism has
been thoroughly investigated in some reported literatures [67–70], and
some reaction paths have been proposed by researchers under different
catalysts. To test the radical pathway, BHT (2,6-Di-tertbutyl-4-methyl-
phenol) as the radical scavenger was first introduced into the reaction
in the present study. The result in Table 5 (entry 8) shows that almost
no product was observed, indicating that the radical species was formed
as the key intermediate. Actually, initiation of IBA can produce iso-
butyryl radical, which can then combine an oxygen molecular to form
an isobutyric acrylperoxy radical, has been generally accepted (Scheme
[
[
1
, reaction 1). Besides the peroxy acryl radical, (reaction 2) another two
active intermediate, namely high-valent cobalt intermediate (reaction
) and peroxyacid (reaction 4) may also form as the active species ac-
3
cording to the reported results [58,71,72]. In case of metalloporphyrin,
the oxidized porphyrin metal-oxo species has been reported to transfer
oxygen to olefins forming epoxides [58,71]. Another accept fact is that
the high-valent metal-peroxide complex species usually formed for
metallo complexes in the presence of peroxide [73–77]. In the study,
high-valent cobalt phthalocyanine species was firstly tested by in-
To further confirm the radical reaction pathways, cis- and trans-
stilbene were introduced into the reaction as the substrates (Scheme 3).
A higher reaction activity was observed for stilbene for that only 1 h
was needed for the fully conversion. Generally, the reaction (4) gives
the configuration maintained product for its concerted reaction, while
reversions in configuration will happened in reaction (2).
1
8
troducing O labeled water into the reaction. The oxygen can exchange
Examination of the product distribution of the epoxidation cis- and
trans-stilbenes showed that, in the both cases, almost same ratios of
trans/cis- stilbeneoxide were observed (Table 7, entries 1 and 2). The
observation strongly verified that the epoxidation of olefins occured by
acylperoxy radical intermediate. Peroxy acids known to epoxidize cis-
stilbene stereoselectively to give only the cis-oxide as product, and some
content of the configuration maintained product for the cis-stilbene
suggested that peracid might also formed as the epoxidation regent.
between high-valent cobalt and water [78,79], therefore, if the reaction
(
served. However, the analysis results showed that no such product
formed, indicating the reaction (3) did not happen. On the other hand,
3) existed in the system, the 1 O labeled styreneoxide should be ob-
8
4
TBHP and NaIO have also been used as the oxidant for the reaction,
1
8
and also no O labeled styreneoxide were detected in both cases. These
results further verified that high-valent cobalt was not the reactive in-
termediate for the formation of epoxide, consistent with the Nam’s re-
2
These results are significantly different from the IBA/O system without
2
sults [70]. Therefore, the hybrid CoPcTs-Zn Al-LDH in the epoxidation
197

W. Zhou et al.
Applied Catalysis A, General 542 (2017) 191–200
Table 6
The results of compared experiments under different catalytic system for the oxidation of styrene a.
Entry
Catalyst
Oxidant
Additive
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
–
–
mCPBA
–
BHT
58
29
80
84
> 99
27
–
86
100
23
18
9
14
0
b
mCPBA
c
CoPcTs- Zn
CoPcTs- Zn
CoPcTs- Zn
CoPcTs- Zn
CoPcTs- Zn
2
2
2
2
2
Al-LDH
Al-LDH
Al-LDH
Al-LDH
Al-LDH
IBA/O
IBA/O
IBA/O
TBHP
2
2
2
–
–
–
H
H
77
82
90
76
–
d
e
1
1
8
8
2
2
O
O
24
–
NaIO
4
a
Reaction conditions: styrene 2 mmol, catalyst 6 mg, acetonitrile 8 mL, oxidant 5 mmol, 60 °C, 1.5 h.
b
c
2
mmol.
O
O
2
5 mL/min.
10 mL/min.
20 mL/min.
d
e
2
O
2
Table 7
Epoxidation of trans/cis- stilbene under different reaction conditions a.
Entry Substrate
Catalyst
Oxidant Conversion (%) Selectivity (%)b
trans-
cis-
1
2
trans-stilbene CoPcTs-
Zn Al-
IBA/O
IBA/O
2
2
100
100
90
10
2
LDH
cis-stilbene
CoPcTs-
88
12
2
Zn Al-
LDH
Scheme 2. Two possible paths for the formation of benzaldehyde.
3
4
5
trans-stilbene
cis-stilbene
trans-stilbene CoPcTs-
Zn Al-
–
–
mCPBA
mCPBA
mCPBA
100
100
100
78
38
82
22
62
18
catalyst,68 where peracid was the predominant species for the epox-
idation (Scheme 4).
The above results and analysis have shown that peroxyacid maybe
an intermediate in the epoxidation, therefore, mCPBA was also used as
2
LDH
CoPcTs-
6
cis-stilbene
mCPBA
100
49
51
the oxidant in the epoxidation of cis- and trans-stilbene under N
avoid the influence of oxygen. In the case of trans-stilbene, a ratio of
8/22 for the trans/cis- products, while a ratio of 38/62 were observed
for cis-stilbene, indicating that both reaction 2 and 4 were involved in
the epoxidation by mCPBA (Table 7, entries 3 and 4). However, the
ratios of trans/cis- stilbeneoxides were needed to be further explained,
because as the routine consideration, the ratio of the trans/cis- should
2
to
Zn
LDH
2
Al-
7
a
Reaction conditions: substrate 2 mmol, catalyst 6 mg, acetonitrile 8 mL, oxidant
5
mmol, 60 °C, 1.5 h.
b
Rea.
predominant. Further, the reactions have also been performed in the
be similar to the result of that under IBA/O
2
. We thought that the re-
presence of CoPcTs- Zn Al-LDH. The results in Table 7 (entries 5 and 6)
2
sults might be due to the steric effect of mCPBA. The production of cis-
stilbeneoxide from trans-stilbene should be through radical path, and
the species e first formed in the case of trans-stilbene by reacting with
mCPBA. However, compared with species e, the radical f might be more
stable for the steric effect of mCPBA, therefore, the cis-intermediate f
formed during the reaction by the rotation of α-bond (Scheme 4). The
similar situation was observed in the reaction of cis-stilbene. The above
results and analysis indicate that both radical and peroxyacid paths
show that no appreciable change of the ratio of trans/cis- products was
observed for trans-stilbene, while the ratio increased to 49/51 in case of
cis-stilbene. These observations obviously indicate that the CoPcTs-
Zn Al-LDH may also accelerate the radical epoxidation of olefins when
2
using peroxyacid as the oxidant.
In addition, the concentrations of different components in the re-
action system time have been plotted to the reaction time (Fig. 7). It can
be easily observed that an induction period exist for the benzaldehyde,
in consistent with the radical path. The concentration of iso-butyric acid
2
exist in the epoxidation under IBA/O system, and the radical path is
Scheme 3. Epoxidation of cis- and trans- stilbene.
198

W. Zhou et al.
Applied Catalysis A, General 542 (2017) 191–200
Scheme 4. Transformation the trans- intermediate to cis- configuration.
2 2
Fig. 7. Epoxidation of styrene over CoPcTs-Zn Al-LDH under IBA/O .
was evidently higher than the total of benzaldehyde and styreneoxide,
indicating that iso-butyric acid also formed by other ways.
Finally, a probable mechanism is proposed (Scheme 5) on the basis
of the results presented in the previous sections and some literatures
Scheme 5. A probable mechanism of the epoxidation of styrene over CoPcTs-Zn
by O /IBA.
2
Al-LDH
2
[
61,71]. Isobutyryl radical was first formed from isobutyraldehyde
radical path was predominant.
under the catalysis of the CoPcTs-Zn Al-LDH or through autoxidation,
2
and then reacted with molecular oxygen to produce acylperoxy radical.
On one hand, the radical abstract a hydrogen from IBA to lead to per-
oxyacid. The peroxyacid could react with styrene to produce the ep-
oxide. On the other hand, the acylperoxy radical could also react with
styrene and a new radical intermediate formed (c). The species then
transferred into styrene oxide combined with a radical (b) released. The
radical intermediate (c) could also transfer to benzaldehyde through a
species (g).
Acknowledgments
This work was supported by Advanced Catalysis and Green
Manufacturing Collaborative Innovation Center of Changzhou
University, National Science Foundation of China (21403018) and
Prospective Joint Research Project on the Industry, Education and
Research of Jiangsu Province (BY2014037-17).
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