Oxovanadium(IV), copper(II) or cobalt(II) acetylacetone complexes immobilized on amino-functionalized CMK-3 for the aerobic epoxidation of styrene

Article abstract of DOI:10.1002/aoc.3353

Oxovanadium(IV), copper(II) and cobalt(II) acetylacetone complexes have been grafted onto amino-modified CMK-3-O (VO-NH₂-CMK-3, Cu-NH₂-CMK-3 and Co-NH₂-CMK-3,respectively) and the materials thus prepared were used as heterogeneous catalysts for the aerobic oxidation of styrene. X-ray diffraction, nitrogen adsorption-desorption and transmission electron microscopy measurements confirmed the structural integrity of the mesoporous hosts, and spectroscopic characterization techniques (Fourier transform infrared, X-ray photoelectron, Raman) and thermogravimetry confirmed the ligands and the successful anchoring of the acetylacetone complexes to the modified mesoporous support. VO-NH₂-CMK-3 displayed a relatively good catalytic performance with 94.6% of styrene conversion using air as oxidant, while Cu-NH₂-CMK-3 gave 99.6% of styrene conversion using tert-butyl hydroperoxide as oxidant.

Full text of DOI:10.1002/aoc.3353

Full paper
Received: 11 March 2015
Revised: 25 May 2015
Accepted: 21 June 2015
Published online in Wiley Online Library: 6 August 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3353
Oxovanadium(IV), copper(II) or cobalt(II)
acetylacetone complexes immobilized on
amino-functionalized CMK-3 for the aerobic
epoxidation of styrene
a
a
a
a
a
Xiufang Wang , Shujie Wu , Zhifang Li , Xiaoyuan Yang , Jing Hu ,
b
a
a
Qisheng Huo , Jingqi Guan * and Qiubin Kan *
Oxovanadium(IV), copper(II) and cobalt(II) acetylacetone complexes have been grafted onto amino-modified CMK-3-O (VO-NH
2
-
CMK-3, Cu-NH -CMK-3 and Co-NH -CMK-3,respectively) and the materials thus prepared were used as heterogeneous catalysts
2
2
for the aerobic oxidation of styrene. X-ray diffraction, nitrogen adsorption–desorption and transmission electron microscopy mea-
surements confirmed the structural integrity of the mesoporous hosts, and spectroscopic characterization techniques (Fourier
transform infrared, X-ray photoelectron, Raman) and thermogravimetry confirmed the ligands and the successful anchoring of
the acetylacetone complexes to the modified mesoporous support. VO-NH -CMK-3 displayed a relatively good catalytic perfor-
2
2
mance with 94.6% of styrene conversion using air as oxidant, while Cu-NH -CMK-3 gave 99.6% of styrene conversion using
tert-butyl hydroperoxide as oxidant. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: epoxidation; oxovanadium(IV) acetylacetone complexes; styrene; CMK-3
[
31]
producing new catalysts for hydrogenation reactions and cata-
Introduction
[
32]
lytic ammonia decomposition. CMK-n materials have important
characteristics for use as supports for transition metal complexes
in many oxidation reactions, producing new catalysts which can
be easily separated from the reaction mixture and reused in subse-
Olefin epoxidation is a highly important oxidation reaction, because
the resulting epoxides are widely used in the production of epoxy
resins, paints and surfactants, and they are highly valued as syn-
[
1–3]
[33]
thetic intermediates in industrial processes.
Olefin epoxidation
quent cycles. Lin et al. reported the preparation of well-dispersed
can be carried out using a variety of oxidants, such as molecular
tantalum oxide on ordered mesoporous carbon CMK-3 for catalytic
epoxidation of cyclooctene, which showed good conversion, selec-
tivity and reusability. Gaspar et al. immobilized Mn(III) complexes
[4,5]
[6,7]
[8,9]
oxygen,
hydrogen peroxide,
alkyl hydroperoxides and
[10]
[34]
iodosylbenzene.
The utilization of molecular oxygen as an
oxidant has attracted much interest in the epoxidation of olefins
recently, due to its abundance in nature, ease of handling and lack
of toxic byproducts produced during reaction.
onto CMK-3, which exhibited high catalytic performance in the ox-
idation of alkene. These results highlight the potential of CMK-3 as a
support for metal complexes in oxidation reactions. Geng et al.
[
11–13]
[35]
Various catalysts have been developed for the aerobic epoxida-
tion of olefins, among which are catalysts composed of redox-
reported the preparation of iron oxides anchored on mesoporous
carbon CMK-3 for oxidation of benzyl alcohol using molecular oxy-
gen as oxidant giving high conversion (72%) and benzaldehyde se-
lectivity (nearly 100%). Therefore, the immobilization of Schiff base
complexes onto a functionalized CMK-n material seems to be
promising.
[
14]
[15]
[16,17]
[18]
active transition metals such as V,
Mn,
Fe,
Co
and
[19,20]
Cu.
However, the vast majority of these catalysts have
drawbacks such as difficult separation of the product, as well as
deactivation, regeneration and recovery of the catalyst. A possible
strategy to overcome these difficulties is to heterogenize homoge-
neous catalysts by immobilizing them on insoluble solid supports
[21]
[22]
[23]
[24]
such as polymer, clay, zeolite or silica. However, some
of these composite structures lack the desired stability and some
of the active metal species tend to leach from the support during
*
Correspondence to: Jingqi Guan or Qiubin Kan, College of Chemistry, Jilin
University, Changchun 130023, PR China. E-mail: guanjq@jlu.edu.cn;
qkan@mail.jlu.edu.cn
[25]
reaction.
Nanostructured regular carbon materials (CMK-n) have been pre-
[
26]
a College of Chemistry, Jilin University, Changchun 130023, PR China
pared as replicas of several porous materials and tested with suc-
[
27]
[28]
cess as adsorbents, electrode materials, electrocatalysts, acid
catalysts
b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun 130012, PR China
[29,30]
and supports for metal nanoparticles and salts
Appl. Organometal. Chem. 2015, 29, 698–706
Copyright © 2015 John Wiley & Sons, Ltd.

Metal complexes immobilized on CMK-3 for styrene epoxidation
In the work presented here, the ordered nanostructured carbon
CMK-3, a SBA-15 replica, was prepared and used as a support for
stirred until complete dissolution of the polymeric surfactant.
Subsequently, 4.2 ml of Si(OC was added dropwise to the
2 5 4
H )
2+
2+
2+
the immobilization of transition metal (Cu , Co and VO ) Schiff
base complexes. The catalytic performances of heterogeneous cop-
per(II), cobalt(II) and oxovanadium(IV) catalysts in the epoxidation
of styrene were compared to those of their homogeneous ana-
logues, and the oxidation performances using air and tert-butyl hy-
droperoxide (TBHP) as oxidizing agents were also compared. It was
found that the oxovanadium-containing catalyst displayed a rela-
tively good catalytic performance with a styrene conversion of
solution under vigorous stirring at 313 K. Finally, the mixture
was stirred for another 24 h at 313 K and then hydrothermally
treated under static conditions for 24 h at 373 K. The obtained
white precipitate was filtered without washing, dried overnight
at 333 K and calcined in air by increasing temperature from am-
bient to 823 K for 8 h.
The prepared SBA-15 material was used as a hard template for
[
27]
the synthesis of a CMK-3 replica.
Briefly, 1 g of SBA-15 was
9
4.6% using air as oxidant and the copper-containing catalyst gave
impregnated with a solution containing 1.25 g of sucrose and
0.14 g of H SO in 5 g of H O. The mixture was placed in a drying
a much higher styrene conversion of 99.6% using TBHP as oxidant.
2
4
2
oven for 6 h at 373 K, and subsequently the oven temperature
was increased to 433 K and maintained there for 6 h. The silica
sample, containing partially polymerized and carbonized sucrose,
was treated again at 373 and 433 K after the addition of 0.8 g
of sucrose, 0.09 g of H SO and 5 g of H O. Carbonization was
Experimental
Materials
2
4
2
ꢁ
1
The following chemicals were commercially available and were
used as received: 3-aminopropyltriethoxysilane (APTES; Aldrich),
Pluronic P123 (EO20PO70EO20; Aldrich), salicylaldehyde (99%),
completed by increasing the temperature (1 K min ) to 1173 K
in flowing nitrogen. The carbon–silica composite obtained after
pyrolysis was washed with
5 wt% hydrofluoric acid at
tetraethyl orthosilicate (Si(OC
H )
2 5 4
; 99%), styrene (98%), HNO
3
,
room temperature, to remove the silica template. The final
material was washed three times with deionized water and dried
at 393 K.
H
2
SO , sucrose, VOSO , V , Cu(CH
4
4
2
O
5
3
COO) , Co(NO O, NaOH,
2
) ꢀ6H
3 2 2
acetylacetone and all organic solvents were of AR grade. Toluene
was dried using Na/diphenylketone ketyl and distilled under nitro-
gen atmosphere. All air- or moisture-sensitive compounds were
transferred using a standard vacuum line and the Schlenk
technique.
3
The final dried material was oxidized using HNO : 1 g of CMK-3
was added to 10 ml of HNO and the mixture was boiled to dry-
3
ness. It was then washed with deionized water to neutral pH, de-
noted CMK-3-OH. Subsequently, this material was treated with
NaOH: 100 mg of CMK-3-OH was dispersed in 50 ml of 0.04 M
NaOH and refluxed for 4 h. The final material was washed with
deionized water to pH 8 and dried at 120°C under vacuum, being
denoted CMK-3-O.
Synthetic procedures
The synthesis of the heterogeneous catalysts is shown in Scheme 1
and is described in the following.
Synthesis of amino-functionalized CMK-3-O
Synthesis of CMK-3 carbon replica and surface modification
The obtained CMK-3-O (1.0 g, 30 ml) was suspended in anhydrous
toluene under nitrogen atmosphere. After adding 2.3 mmol of
APTES, the mixture was refluxed at 110°C under nitrogen protection
for 24 h, washed with a large amount of toluene and dried in a
vacuum oven at 40°C for 24 h, affording a product denoted
SBA-15 was synthesized under acidic conditions using Pluronic
ꢁ
1
P123 triblock copolymer (EO20PO70EO20, Mav = 5800 g mol
and Si(OC as a silica source according to a procedure de-
scribed elsewhere.
)
2 5 4
H )
[36]
In a polypropylene bottle, 2 g of Pluronic
P123 was dissolved in a solution containing 15 g of distilled wa-
ter and 60 g of 2 M HCl at 313 K. The mixture was vigorously
NH -CMK-3.
2
Synthesis of catalysts
2
An amount of 0.5 g of NH -CMK-3 was added to 30 ml of ethanol
2+
and then 0.57 mmol of M and 6 mmol of acetylacetone were
added. The mixture was refluxed at 70°C for 3 h. After cooling to
room temperature, the mixture was filtered and washed with etha-
nol, followed by drying. The resulting solid powder was denoted
2
M-NH -CMK-3 (M = VO, Cu, or Co).
The homogeneous compound VO(acac)
ported previously.
2
was synthesized as re-
In a typical synthesis, V was added to
00 ml of acetylacetone under nitrogen atmosphere and the mix-
[37]
2 5
O
1
ture was stirred at 80°C for 3 h. The resulting solid was filtered,
washed with acetone and dried at 100°C.
Catalytic tests
The epoxidation of styrene was used as a test reaction for the
catalysts and was carried out as follows. The optimum conditions
of the epoxidation reaction were as follows: 10 mmol of styrene
along with 10 ml of CH
3
CN, 25 mmol of isobutyraldehyde, air at
ꢁ
1
8
0 ml min , 50 mg of catalyst, temperature of 80°C and time of
[38–40]
2 2
Scheme 1. Synthetic schematic outlines of NH -CMK-3 and M-NH -CMK-3.
8 h.
Based on our previous work, aqueous 1.0 M sodium
Appl. Organometal. Chem. 2015, 29, 698–706
Copyright © 2015 John Wiley & Sons, Ltd.
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X. Wang et al.
[41,42]
hydroxide was added to maintain a pH of 7.5–8.0.
reaction was finished, the catalyst was filtered, washed with copi-
ous CH CN, dried at 100°C overnight and reused directly without
After the
3
further purification. The liquid organic products were quantified
using a gas chromatograph (Shimadzu, GC-8A) fitted with a flame
ionization detector and an HP-5 capillary column.
Results and discussion
Structural integrity studies
The ordered mesostructure and framework microstructure of
these materials were investigated using small- and wide-angle
XRD, respectively (Fig. 1 and Fig. S1). It can be seen in Fig. 1 that
all of the carbon materials exhibit one well-resolved reflection at
ca 1.0°, which can be assigned to the (100) reflection according
[
43,44]
to ordered two-dimensional hexagonal microstructure.
The
clear (100) reflection of CMK-3-O reveals that the ordered micro-
structure is well preserved after the modification treatment with
HNO
peak decreases after the introduction of metal–salen complexes
Figs 1(c)–(e)). The reduction in intensity may be mainly due to
3
. The relative intensity of the prominent (100) diffraction
(
contrast matching between the carbon framework and organic
moieties that are located inside the channels of CMK-3-O. Similar
[
45]
conclusions have also been drawn previously.
The (100) dif-
fraction peak (Fig. S2) of the catalysts after catalytic use is also
observed. These results confirm that the structures of the mate-
rials remain intact, which further demonstrates that the catalysts
are chemically and thermally stable. The microstructure of the
carbon framework was further confirmed from the wide-angle
XRD patterns shown in Fig. S1. The peak observed at ca 23° for
all catalysts is attributed to the (002) graphitic plane reflection
of carbon.
Typical transmission electron microscopy (TEM) images of CMK-3
and VO-NH -CMK-3 are shown in Fig. 2. CMK-3 has a hexagonally
2
ordered structure with rod-like walls; in particular, highly one-
dimensional ordered channels are arrayed along the rods. Note that
the image of VO-NH -CMK-3 is similar to that of CMK-3, indicating
2
Fig. 1. Low angle X-ray diffraction patterns of (a) CMK-3-O, (b) CMK-3-NH
c)VO-NH -CMK-3-O, (d) Co-NH -CMK-3-O, and (e) Cu-NH -CMK-3-O.
2
,
Fig. 2. TEM images of (a) CMK-3-O, (b) VO-NH
(c) CMK-3-O, and (d) VO-NH -CMK-3.
2
-CMK-3; and SEM images of
(
2
2
2
2
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Appl. Organometal. Chem. 2015, 29, 698–706

Metal complexes immobilized on CMK-3 for styrene epoxidation
the maintenance of an ordered structure after the grafting of VO
Spectroscopic characterization
(
acac)
2
(Figs 2(a) and (b)). Scanning electron microscopy (SEM) im-
-CMK-3 reveal that no evident change
Figure 4 shows the Fourier transform infrared (FT-IR) spectra of var-
ious catalysts. The bands at 1585, 1460 and 1115 cm correspond
ages of CMK-3 and VO-NH
in morphology is observed, and both materials present a short
rod-like morphology (Figs 2(c) and (d)).
2
ꢁ
1
to the C¼C stretching vibrations of the aromatic carbon network,
[
49]
benzene rings and the skeletal C–C tangential motions, respec-
The nitrogen adsorption–desorption isotherms of CMK-3-O,
tively. These bands can also be observed in the spectra of other car-
2 2
CMK-3-NH and VO-NH -CMK-3 are shown in Fig. 3. These are typ-
ical type-IV isotherms according to the classification of IUPAC with
three distinct well-defined stages at a relative pressure around
[
50]
bon materials. In addition, the medium intense bands near 1190
ꢁ
1
and 1735 cm are attributed to stretching vibrations of the C¼O of
[
50]
[46]
carboxylic acid, ketone, quinone, anhydride and ester groups.
0
.4–0.8. The first stage at P/P < 0.4 is due to a monolayer ad-
0
ꢁ1
Moreover, the band near 2949 cm is attributed to asymmetric –
sorption of nitrogen molecules on the walls of the mesopores.
The second stage, at 0.4 < P/P < 0.8, is characterized by a steep in-
crease in adsorption due to capillary condensation inside the
mesopores. The third stage is a tail portion at P/P > 0.8, associated
ꢁ
1
2
CH stretching vibrations, while the low-intensity band at 1397 cm
0
is assigned to O–H deformation vibrations of phenol groups. The
symmetric and asymmetric C–H stretching vibrations of the propyl
0
with multilayer adsorption on the external surface of the materials.
In comparison to the parent CMK-3-O, both CMK-3-NH and VO-
2
NH -CMK-3 samples exhibit a decreased uptake of nitrogen, owing
2
to immobilization of Schiff base complex on the inner wall surface
[47]
of the matrix. As evident from Table 1, summarizing the nitrogen
sorption results, the Brunauer–Emmett–Teller (BET) surface area of
2
ꢁ1
3
ꢁ1
CMK-3-O is 960.8 m g with pore volume of 0.36 cm g . All
the calculated values are in agreement with those reported for
[
48]
mesoporous carbon.
BET surface area of 358.0 m g and pore volume of 0.28 cm g ),
while VO-NH -CMK-3 shows even less nitrogen uptake (BET
CMK-3-NH
2
shows less nitrogen uptake
2
ꢁ1
3
ꢁ1
(
2
2
ꢁ1
3
ꢁ1
surface area of 241.8 m g and pore volume of 0.18 cm g ).
The complex-modified sample possesses a lower pore volume
compared to its parent materials due to the incorporation of the
metal salen complex inside the mesoporous CMK-3 matrix.
Fig. 3. N
2
adsorption-desorption isotherms of (a) CMK-3-O, (b) CMK-3-NH
2
,
2
and (c) VO-NH -CMK-3.
Table 1. Textural properties of samples
2
ꢁ1
3
ꢁ1
Material
S
BET (m ·g
)
V
BJH (cm ·g
)
CMK-3-O
960.84
358.02
241.84
0.36
0.28
0.18
CMK-3-NH
2
2
VO-NH -CMK-3
Fig. 4. FT-IR spectra of (a) CMK-3-O, (b) CMK-3-NH , (c) VO-NH -CMK-3, (d)
2
2
2 2 2
Co-NH -CMK-3, (e) Cu-NH -CMK-3, and (f) VO(acac) .
Appl. Organometal. Chem. 2015, 29, 698–706
Copyright © 2015 John Wiley & Sons, Ltd.
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X. Wang et al.
chain through which the amine is connected to the carbon surface
ꢁ
1 [51]
lead to the appearance of a new absorption band at 1465 cm
.
ꢁ
1
The band at ca 1559 cm is due to C¼N and C¼C stretching vibra-
[52]
tions within M(acac)
2
.
The FT-IR spectra of the complexes exhibit
ꢁ
1
weak bands around 500–1100 cm (corresponding to the asym-
metric and symmetric stretching vibrations of M–O), which can be
[
53,54]
taken as evidence for M(acac)
2
complex immobilization.
Com-
pared with the original catalysts, the majority of the functional
groups of the recycled materials are still present, which indicates
that the synthesized catalysts are chemically stable and truly het-
erogeneous during the reaction (Fig. S3).
X-ray photoelectron spectroscopy (XPS) studies
To gain deeper insight into the surface constitution of the catalysts,
the C 1s, N 1s and M 2p (M = Cu, VO, Co) binding energies were in-
vestigated, and the corresponding spectra are shown in Figs 5–7.
The high-resolution C 1s XPS spectra of these carbon materials
2
(
Fig. 7) indicate five types of sp -hybridized C 1s with five peaks
centered at ca 282.5, 284.3, 285.4, 287.4 and 289.1 eV, which corre-
spond to C–Si, C¼C, C–O–H, C–O–C and O–C¼O groups,
[
55,56]
respectively,
idized by HNO
suggesting that CMK-3 has been successfully ox-
3 2
. The N 1s spectrum of VO-NH -CMK-3 (Fig. 6) shows
two peaks with maxima at 400.1 and 401.5 eV, which can be
assigned to the two types of nitrogen atoms (the NH in amino
groups and the C¼N in salen ligands) in the tethered oxovanadium
[57]
catalyst. It is the same case for Co-NH
2 2
-CMK-3 and Cu-NH -CMK-
3
9
. The binding energies of Cu 2p for Cu-NH
54 and 962.0 eV, which can be assigned to Cu(II).
-CMK-3 (Fig. 7) shows peaks at ca 517.1
and 524.1 eV, which can be attributed to the vanadium(IV) in the
2
-CMK-3 are 934, 943,
[
58]
The M 2p
XPS spectrum of VO-NH
2
[39]
tethered catalyst.
The XPS surface atomic percentages of the various elements
are summarized in Table 2. For the M-NH -CMK-3-O materials,
2
the complexes are slightly more concentrated on the external
surface, since the M bulk contents (inductively coupled plasma
atomic emission spectroscopy, ICP-AES) are lower than the M
surface contents (XPS). This suggests that the tethered metal
species are mainly heterogeneously located on the outer surface
[59]
of CMK-3.
Thermogravimetric (TG) studies
Thermal analysis was performed for CMK-3-O, VO-NH -CMK-3, Cu-
2
NH -CMK-3 and Co-NH -CMK-3 and the results are shown in Fig.
2
2
S4. The TG curve of CMK-3-O exhibits two obvious weight losses
in the range 50–800°C (Fig. S4a). The first weight loss observed in
the range 100–600°C is assigned to oxygen groups that decompose
at lower temperatures, such as carboxylic acid groups, carboxylic
anhydrides and lactones. The mass loss at temperatures higher
than 600°C can be assigned to the decomposition of phenols and
carbonyl/quinones. The TG profile also confirms that the resulting
CMK-3-O material has excellent thermal stability and a significant
amount of the oxygen-containing functional groups are chemically
grafted on the carbon surface following treatment with HNO
3
,
which is consistent with the results of FT-IR spectroscopy.
The functionalized materials show higher total weight losses, as
expected, which can be divided into three steps. Weight loss below
250°C is due to loss of physically adsorbed water and solvent inside
the pores. The following weight loss at 250–500°C is mainly related
Fig. 5. X-ray photoelectron spectra of C1s for (a) CMK-3-O, (b) VO-NH2-
to the combustion of amine and oxygen groups. The larger loss
2 2
CMK-3, (c) Co-NH -CMK-3, and (d) Cu-NH -CMK-3.
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Appl. Organometal. Chem. 2015, 29, 698–706

Metal complexes immobilized on CMK-3 for styrene epoxidation
2
Fig. 6. X-ray photoelectron spectra of N1s for (a) VO-NH -CMK-3, (b) Co-
NH -CMK-3, and (c) Cu-NH -CMK-3.
2
2
above 500°C can probably be attributed to the decomposition of
[
60]
tetrahydro-salen or salen ligands.
Raman spectroscopy studies
Figure 8 shows the Raman spectra of CMK-3-O and VO-NH -CMK-
2
ꢁ
1
3
1
, in which a strong peak at about 1580 cm and a weak peak at
350 cm can be clearly seen. The peak at 1580 cm (G band) is
attributed to the vibration of sp -bonded carbon atoms in a two-
dimensional hexagonal lattice, namely the stretching modes of
ꢁ
1
ꢁ1
2
ꢁ
1
C¼C bonds typical of graphite, while the peak at 1350 cm
(
D band) is associated with vibrations of carbon atoms with dan-
gling bonds of plane terminations of disordered graphite and re-
lated to the defects and disorders in structures in carbon
materials. The relative intensities of these two bands depend on
the type of graphitic material and reflect the graphitization de-
D G
gree. The value of I /I (the Raman intensity ratio of D and G
bands) for VO-NH -CMK-3 (2.56) is notably greater than that for
2
2
CMK-3-O (1.67), suggesting that the size of the in-plane sp do-
mains decreases and the covalent attachment of salen complexes
to CMK-3-O causes many defects or deoxygenation.
2
Fig. 7. X-ray photoelectron spectra of M2p for (a) VO-NH -CMK-3, and (b)
Cu-NH -CMK-3.
2
[
61]
one hand, the synergistic beneficial catalytic interaction between
Cu(II) Schiff complex and CMK-3-O support is superior to the others,
which can promote the separation of styrene oxide from the active
Catalytic properties
A study of styrene epoxidation with air as the oxidant and
isobutyraldehyde as the co-reductant in acetonitrile over various
catalysts was performed (Table 3). For VO-NH -CMK-3, 94.6% of
the styrene is converted into products with 70.5% selectivity to
the epoxide (entry 1). Cu-NH -CMK-3 gives 73.0% styrene conver-
sion and 63.3% styrene oxide selectivity (entry 5), while Co-NH2-
CMK-3 gives 79.9% conversion of styrene (entry 6) with a styrene
oxide selectivity of 73.8%.
2+
sites and then Cu sites can continue activating the terminal oxy-
[
62]
gen of TBHP which then can react with the styrene. On the other
hand, probably peroxide has a marked ability to dissolve the redox
component from the solid support to the liquid phase, like
2
2
[
63]
vanadium.
The catalytic properties of the homogeneous complexes were
also examined under identical reaction conditions. It is worth not-
For comparison, the effect of TBHP as the oxidant on the epoxi-
dation of styrene over the various catalysts under standard condi-
ing that when VO(acac)
and selectivity (Tables 3 and 4) are much lower than those over
VO-NH -CMK-3. These results may be for two reasons. On the one
2
is used as catalyst, conversion of styrene
tions was also investigated (Table 4). The Cu-NH
2
-CMK-3 catalyst
2
gives 99.6% conversion of styrene in 7 h with an epoxide selectivity
of 63.8%, which is much higher than that using air as oxidant. VO-
hand, the heterogenization of homogeneous metal complexes
probably isolates the active species and prevents their dimerization.
On the other hand, the supported catalyst probably isolates the ac-
tive species and prevents the formation of m-oxo, m-peroxo di-
NH
conversion after 7 h, but with an excellent epoxide selectivity of
9.1 and 97.5%, respectively. The reasons are as follows. On the
2 2
-CMK-3 and Co-NH -CMK-3 give only 48.2 and 49.6% styrene
[
58]
9
meric or other polymeric species.
Appl. Organometal. Chem. 2015, 29, 698–706
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X. Wang et al.
Table 2. Surface chemical composition of CMK-3-based materials obtained using XPS and ICP-AES
a
ꢁ1
Material
XPS (at.%)
V (μmol g )
b
c
C 1s
N 1s
O 1s
Si 2p
M 2p
XPS
ICP-AES
Before test
After test
CMK-3-O
93.84
79.71
81.34
76.90
1.92
2.42
2.85
2.67
3.21
13.8
1.03
3.84
3.60
4.89
—
—
—
—
VO-NH
Co-NH
2
-CMK-3
0.25
0.15
0.77
187.76
113.74
556.94
165
97
132
83
2
-CMK-3
-CMK-3
12.07
14.76
Cu-NH
2
354
325
a
Determined by the areas of the respective peaks in the high-resolution XPS spectra.
P
b
Metal surface content determined by XPS: μmol M/weight of material = at.% M/ [at.% (i) × Ar(i)].
c
Metal bulk content determined by ICP-AES before and after use.
catalytic performance. Although some catalysts may also achieve
high yield, they need more time or higher reaction temperature
(Table 4, entry 9).
The recoverability and stability of the supported catalysts was
investigated in repeated epoxidation of styrene. From the recycling
results of representative catalysts, VO-NH -CMK-3 and Cu-NH
2
2
-
CMK-3 (Tables 3 and 4), it is clear that both of them have good
recoverability with little significant loss of activity or selectivity.
The results of ICP-AES analysis show that the metal contents for
ꢁ
1
ꢁ1
VO-NH -CMK-3 (132 μmol g ) and Cu-NH -CMK-3 (325 μmol g
)
2
2
after two cycles decrease slightly compared with values for fresh
catalysts, implying that the large majority of the catalysis is carried
out by truly heterogeneous catalysts. From an economic consider-
ation, the amount of catalyst required to achieve the process
goals and the number of times of reuse of the catalyst are the keys
to control industrial profits. Our synthesized catalysts possessing
good recoverability and selectivity have potential application
prospects.
Fig. 8. Raman spectra of (a) CMK-3-O, and (b) VO-NH
2
-CMK-3.
Based on our experimental results and other related
[
64,65]
literature,
a reasonable reaction mechanism for epoxidation
II
From the listed results of previously reported heterogeneous cat-
alysts for epoxidation of styrene (Table 3, entries 8–12; Table 4, en-
tries 5–7), it is notable that our prepared catalysts have superior
of styrene with TBHP or oxygen over M-NH -CMK-3 catalyst (LM )
has been postulated, as shown in Scheme 2. (1) LM is coordinated
2
II
with TBHP to form an active M(III) peroxo species I. (2) The activated
Table 3. Activity of various catalysts used for styrene epoxidation reaction in the presence of oxygen
b
Selectivity (%)
a
Entry
Catalyst
Conversion (%)
So
Others
Ref.
1
2
3
5
6
7
8
9
VO-NH
VO-NH
VO-NH
2
2
2
-CMK-3 (1st)
-CMK-3 (2nd)
-CMK-3 (3rd)
94.6
93.2
85.3
73.0
79.9
46.6
4.3
70.5
64.8
63.0
63.3
73.8
66.8
94.5
51.7
18.5
34.2
41.1
0
29.5
35.2
37.0
36.7
36.2
33.2
5.5
This study
This study
This study
This study
This study
This study
This study
[58]
Cu-NH
Co-NH
2
-CMK-3
-CMK-3
2
CMK-3-O
VO(acac)
2
Co(acac)
Cu(acac)
2
85.9
28.6
71
48.3
81.5
55.5
53.9
>99
50.6
28.3
10
11
12
13
14
15
2
[58]
VO{Me
VO{Me
4
(Et)
(Et)
2
[14]tetraneN
[14]tetraneN
4
}-Y
[59]
4
2
4
}-MT
65.9
80.2
81.2
78.6
[60]
VO(acac)
VO(8-Q)
VO-Salen-SBA
2
-SBA-15
[61]
2
-SBA-15
45.9
71.2
[61]
[62]
a
ꢁ1
Reaction conditions: catalyst, 50 mg; styrene, 1.14 ml (10 mmol); CH
temperature, 80°C; time, 8 h.
3
CN, 10 ml; air flow rate, 80 ml min ; isobutyraldehyde, 2.28 ml (25 mmol);
b
So: styrene oxide; others: benzaldehyde, benzoic acid, phenylacetaldehyde, 1-phenylethane-1,2-diolbenzoic acid, etc.
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 698–706

Metal complexes immobilized on CMK-3 for styrene epoxidation
Table 4. Oxidation of styrene using TBHP over various catalysts
b
Selectivity (%)
a
Entry
Catalyst
Conversion (%)
So
Bza
Ref.
1
2
3
4
5
6
7
8
9
Cu-NH
Cu-NH
Cu-NH
2
2
2
-CMK-3 (1st)
-CMK-3 (2nd)
-CMK-3 (3rd)
99.6
98.6
99.7
48.2
49.6
99.9
99.4
96
63.8
65.2
59.7
99.1
97.5
7.6
36.2
34.8
40.3
0.9
2.5
58.9
0.8
75
This study
This study
This study
This study
This study
This study
[63]
VO-NH
Co-NH
2
-CMK-3
-CMK-3
2
VO(acac)
Cu(acac)
2
2
99.2
21
Polymer-anchored Cu(II) catalyst
Cu-AMM
[64]
86.3
61
95.0
26
—
[65]
10
11
12
[Cu
[Cu
2
(bipy)
(bipy)
2
(btec)]∞
(btec)]∞
32
[66]
2
2
55
32
37
[66]
CuO/nanotubes-450
94.5
46.9
2.7
[67]
a
3
Reaction conditions: catalyst, 25 mg; styrene, 5mmol (0.57 ml); CH CN, 5 ml; TBHP, 15mmol (2.31 ml); temperature, 80°C; time, 7 h.
b
So, styrene oxide; Bza, benzaldehyde.
oxovanadium(IV), copper(II) and cobalt(II) catalysts were confirmed
using XRD, FT-IR and Raman spectroscopies, TEM, TG and XPS. The
catalytic properties of supported oxovanadium(IV), copper(II) and
cobalt(II) acetylacetone complexes in the oxidation of styrene with
air or TBHP as oxidant were investigated and compared with the
properties of their homogeneous analogues. The catalytic perfor-
2 2 2
mance of the resulting VO-NH -CMK-3, Cu-NH -CMK-3 Co-NH -
CMK-3 was evaluated for styrene epoxidation in the presence of
air in combination with isobutyraldehyde as a sacrificial co-
reductant and in the presence of TBHP. It was found that VO-NH2-
CMK-3 displayed a relatively good catalytic performance with a
styrene conversion of 94.6% using air as oxidant and Cu-NH2-
CMK-3 gave a much higher styrene conversion of 99.6% using TBHP
as oxidant.
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (21303069) and Jilin province (20150520013JH).
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