Different transition metal (Fe2+, Co2+, Ni 2+, Cu2+ or VO2+) Schiff complexes immobilized onto three-dimensional mesoporous silica KIT-6 for the epoxidation of styrene

Article abstract of DOI:10.1039/c3ra45599h

Highly ordered three-dimensional Ia3d mesoporous silica KIT-6 was functionalized with 3-aminopropyltriethoxysilane (3-APTES), following by post-grafting of various transition metal (Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ or VO<

Full text of DOI:10.1039/c3ra45599h

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Different transition metal (Fe2+, Co2+, Ni2+, Cu2+ or
VO2+) Schiff complexes immobilized onto three-
dimensional mesoporous silica KIT-6 for the
epoxidation of styrene†
Cite this: RSC Adv., 2014, 4, 2310
Jian Sun,a Qiubin Kan,a Zhifang Li,a Guangli Yu,b Heng Liu,a Xiaoyuan Yang,a
b
a
*
Qisheng Huo and Jingqi Guan
Highly ordered three-dimensional Ia3d mesoporous silica KIT-6 was functionalized with 3-
aminopropyltriethoxysilane (3-APTES), following by post-grafting of various transition metal (Fe2+, Co2+
,
Ni2+, Cu2+ or VO2+) Schiff complexes onto amino-functionalized KIT-6. The surface functionalized
materials were analyzed by a series of characterization techniques such as XRD, X-ray photoelectron
spectroscopy (XPS), N2 adsorption–desorption, IR spectroscopy, inductively coupled plasma (ICP),
scanning electron microscopy (SEM), and thermal gravimetric analyses (TGA), etc. The characterization
results demonstrated structural intact of the mesoporous hosts throughout the grafting procedures and
successfully anchoring of transition metal Schiff complexes on the modified KIT-6. The obtained
organic–inorganic hybrid materials were subsequently employed as catalysts for the epoxidation of
styrene under optimized reaction conditions. It was indicated that Cu-NH2-KIT-6 showed very high
substrate conversion (98.6%) and excellent epoxide selectivity (97.8%) when using tert-butyl
hydroperoxide (TBHP) as the oxidant at 80 ꢀC after 6 h. It was also observed that the catalyst could be
recycled three times without obvious loss in catalytic activity and selectivity.
Received 5th October 2013
Accepted 21st November 2013
DOI: 10.1039/c3ra45599h
www.rsc.org/advances
heterogenization of homogeneous catalysis has become a
universal means to solve the above questions.
Introduction
Periodically mesoporous silica materials, such as SBA-15,10,11
Olen epoxidation has attained extensive consideration
because the epoxide product is versatile and indispensable
intermediate in the manufacture of both ne chemicals and
pharmaceuticals.1–3 During the past decade, considerable effort
has been paid to develop inexpensive, recyclable, high active
and selective metal-based catalysts to realize large-scale
production.4–6 Despite homogenous transition metal complexes
exhibit high catalytic activity and selectivity in the epoxidation
reactions, many inherent drawbacks hamper their industrial
application: (a) difficulty in the catalyst regeneration and
product separation, (b) deactivation of the catalysts by forma-
tion of dimeric peroxo- and m-oxo species and (c) increasing the
negative inuence on environment and reducing industrial
interest.7–9 With the rapid development of catalyst separation
and preparation technology, encapsulation various transition
metal Schiff complexes on insoluble solid supports to realize
MCM-41,12 MCM-48 13 and hexagonal mesoporous silica
(HMS),14 have been widely used as catalyst support. Therein, the
cubic phase MCM-48 with a highly branched and inter-
penetrating bicontinuous of network channels exhibits partic-
ular promise for advanced functionality. However, MCM-48 is
fabricated under restrictive synthesis conditions because it is an
intermediate during the transformation from a hexagonal
phase to a lamellar phase. Additionally, the weak hydrothermal
stability of MCM-48 in boiling water might induce structural
collapse due to the rapid hydrolysis of the silica in the amor-
phous atomic scale of the network pores, which severely
restricts its practical application.15,16 Recently, Kleitz et al. have
synthesized a novel type of mesoporous silica with three-
dimensional cubic Ia3d symmetry called KIT-6.17 The material
exhibits excellent textural parameters such as high surface area,
large pore volume, high chemical stability for recyclability,
tunable pore diameter simply by altering the aging temperature.
The pore structure of KIT-6 is similar to that of MCM-48 but its
pore diameter is much larger than the later.18 The unique
structure of KIT-6 is superior to other solid support because its
three-dimensional interpenetrating bicontinuous networks of
chiral channel offer more active sites and resistance to
aCollege of Chemistry, Jilin University, Changchun, 130023, P.R. China. E-mail:
guanjq@jlu.edu.cn; Tel: +86 431 88499140
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun 130012, P.R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra45599h
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agglomeration, thus allowing more reactant molecules and Halenda (BJH) algorithm. Thermogravimetric analyses of the
precursor solution diffusion through the pore channels.19
samples were performed on a Netzch Sta 449c thermal analyzer
Various transition metal (Fe2+, Co2+, Ni2+, Cu2+, Mn2+ or with a heating rate of 10 ꢀC minꢁ1 under N2 atmosphere. X-ray
VO2+) Schiff complexes modied ordered mesoporous silica photoelectron spectroscopy (XPS) was measured on Scienta
have been reported as active heterogeneous catalyst for styrene ESCA200 spectrometer using Al-Ka radiation.
epoxidation with TBHP as oxidant, which lead to a high
conversion and epoxide selectivity because the catalysts
Preparation of the support
possess both Lewis acid and weak oxidant.20 Koner et al.
The synthesis of pure silica KIT-6 was using P123 as a structure-
reported that copper(II) Schiff modied 2D-hexagonal phase
directing agent and 1-butanol as a co-solvent under mild acidic
MCM-41 material gave 97% conversion of styrene and 86%
conditions. The molar gel composition of the reaction mixture is
epoxide selectivity aer 24 h with TBHP as oxidant.21 Tang
TEOS : P123 : HCl : H2O : BuOH ¼ 1 : 0.017 : 1.83 : 195 : 1.31.23
In a typical synthesis, P123 (6 g) was dissolved in the mixture of
distilled water (216 ml) and 37% HCl (9.8 ml). Stirring at 35 ꢀC for
4 h to make the template completely dissolution and then 1-
butanol (6 g) was added at once with continuous stirring for
another 1 h. Following TEOS (12.8 g) was dropped into the
mixture and constant stirring at 35 ꢀC for 24 h. Subsequently the
mixture was carried out in a closed polypropylene bottle and
aged at 100 ꢀC for 24 h under static hydrothermal condition. The
obtained product was ltered without washing and dried at 60 ꢀC
for 12 h. Finally the resultant white powder was calcined at 550 ꢀC
for 6 h in a owing air for surfactant removal.
et al. reported metal (Co2+, Cu2+ or Mn2+) Schiff modied
mesoporous silica nanoparticles (MSN) with ca. 99% styrene
conversion and over 63% epoxide selectivity aer 24 h with
TBHP as oxidant.22 However, as far as we are aware, hitherto,
no report on the covalent anchoring of transition metal (Fe2+
,
Co2+, Ni2+, Cu2+ or VO2+) Schiff complexes on three dimen-
sional mesoporous silica KIT-6 and the application in the
epoxidation of styrene.
In the present work, we attempt to synthesize a novel
recyclable heterogeneous catalyst Cu-NH2-KIT-6 by post-gra-
ing method for the epoxidation of styrene. The inuence of
catalyst amount, molar ratios of oxidant to styrene, reaction
temperature and time on catalytic performance has also been
investigated. Compared with other congeneric transition metal Preparation of amino-functionalized KIT-6
(Fe2+, Co2+, Ni2+ or VO2+) Schiff complexes graed on KIT-6, the
Amino-functionalized KIT-6 was synthesis according to the
catalytic investigation clearly reveals that Cu-NH2-KIT-6 is a
real efficient heterogeneous catalyst for the epoxidation of
styrene.
following procedure described in our previous work.24,25 3-
APTES (0.54 ml) was added to a suspension of KIT-6 (1 g) in dry
toluene (30 ml) under N2 atmosphere and reuxed at 110 ꢀC for
12 h. Aerwards, the sample was ltered, washed with dry
toluene and anhydrous ethanol for several times, and then
dried under vacuum overnight. The resulting material was
denoted as NH2-KIT-6.
Experimental
Materials and methods
P123 (EO20PO70EO20) (Aldrich), 3-aminopropyltriethoxysilane
(3-APTES) (Aldrich), tetraethoxysilane (TEOS 99%), 1-butanol
(99%), hydrochloric acid (HCl 37%), styrene (98%), salicylalde-
Synthesis of heterogeneous catalyst Cu-NH2-KIT-6
hyde (99%), tert-butyl hydroperoxide (TBHP 70%), Cu(CH3COO)2, Covalent attachment of copper(II)–Schiff complex onto amino-
Co(NO3)2$6H2O, VOSO4, Fe(NO3)2$9H2O, Ni(NO3)2$6H2O, and functionalized KIT-6 by a post-graing method. NH2-KIT-6
all organic solvents are A.R. grade. Na/diphenylketone ketyl was (1 g) was reuxed with salicylaldehyde (2.3 mmol) in absolute
ꢀ
used to dry toluene, and then the toluene is distilled under N2 methanol (30 ml) under N2 atmosphere at 60 C for 5 h. The
atmosphere.
resultant yellowish product was ltrated, washed with meth-
Small angle powder XRD patterns were collected with a anol and dried under vacuum. The sample was denoted as
Riguku D/MAX2550 diffractometer with Cu-Ka radiation (l ¼ S-NH2-KIT-6. Finally, the Cu-NH2-KIT-6 was prepared by
˚
1.5418 A) at 50 kV and 200 mA. The diffractions were recorded addition of Cu(CH3COO)2 (1.15 mmol, 0.21 g) to the above
in the 2q range of 0.6–5ꢀ and in steps of 2ꢀ minꢁ1. The yellowish solid in anhydrous methanol (30 ml) stirring at
morphology of the samples was obtained with JEOS JSM 6700F room temperature for 24 h. Then the mixture was ltered,
led-emission scanning electron microscope. The metal washed repeatedly with water, ethanol and extracted with
content of the catalysts was measured by inductively coupled dichloromethane using a Soxhlet for 24 h. The product was
plasma atomic emission analyses (Perkin-Elmer Optima 3300 dried under vacuum overnight (Scheme 1A). The content of Cu
DV). The infrared spectra of the materials were conducted on a loaded onto KIT-6 was determined by ICP-AES which is
Nicolet 6700 instrument in the range of 400–4000 cmꢁ1 using 0.22 mmol gꢁ1
.
KBr pellet technique. The N2 adsorption–desorption isotherms
For comparison, Fe-NH2-KIT-6, Co-NH2-KIT-6, VO-NH2-KIT-6
were recorded at ꢁ196 ꢀC on an adsorption analyzer (Micro- and Ni-NH2-KIT-6 were also synthesized through a similar
meritics ASAP 2010). Prior to this measurement, each sample synthesized process with Cu-NH2-KIT-6. The metal content of
was degassed at 150 C for 12 h. The specic surface area was the synthesized congeneric transition metal (Fe , Co2+, VO2+,
2+
ꢀ
determined using the Brunauer–Emmett–Teller (BET) equation Ni2+) Schiff complex onto KIT-6 is 0.12 mmol gꢁ1, 0.14 mmol
and pore size distribution was calculated using Barrett–Joyner– gꢁ1, 0.21 mmol gꢁ1, and 0.14 mmol gꢁ1, respectively.
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oil bath. At the end of the reaction, the solid catalyst was
ltered, washed with acetonitrile and dichloromethane, dried at
vacuum and reused without further purication. The products
of epoxidation reaction were quantied and monitored via gas
chromatography (Shimadzu, GC-8A) equipped with a HP-5
capillary column and a FID detector.
Results and discussion
Spectroscopic characterization
Fig. 1 shows the IR spectra. In all case, the typical Si–O–Si bands
are observed nearly at 1080, 802, 464 cmꢁ1 due to the formation
of KIT-6 silica framework. The bands around 3430 and 1630
cmꢁ1 are mainly aroused by the bend vibration of O–H and
physical adsorption of water molecular.28 Aer functionaliza-
tion of 3-APTES for KIT-6, the intensity of the O–H stretching
bands markedly weakens which was caused by the interaction
between the surface Si–OH and 3-APTES. The presence of C–H
and N–H bending vibration nearly at 2950 and 687 cmꢁ1
,
respectively can demonstrate that 3-APTES has been success-
fully anchored onto KIT-6.29 In addition, a serious of aromatic
Scheme 1 (A) Preparation of heterogeneous catalyst Cu-NH2-KIT-6,
and (B) Preparation of homogeneous catalyst Cu–Salen.
Synthesis of homogeneous Cu–Salen complex
The Cu–Salen complex was synthesized by the established
procedure.26,27 An equimolar amount of 3-APTES (2.3 mmol) was
mixed with salicylaldehyde (2.3 mmol) in anhydrous ethanol.
The resultant mixture was reuxed for 3 h under protection of
nitrogen. Removal of the solvent under reduced pressure yields
a yellow semi-viscous liquid. The structure of Salen ligand could
be conrmed by IR spectroscopy (ESI Fig. S1†). Then Cu(CH3-
COO)2 (1 mmol) was dissolved in ethanol, followed by addition
of appropriate Salen ligand in 1 : 2 molar ratio of Cu : Salen.
The mixture was reuxed for 12 h, evaporation of ethanol and
cool to room temperature, washed with cold ethanol and dried
vacuum to obtain a green solid Cu–Salen (Scheme 1B). The
structure of Cu–Salen was determined by IR spectroscopy (ESI
Fig. S1†). The n(C]N) band in Salen ligand appears at 1633
cmꢁ1 and shis to lower wavenumber for Cu–Salen complex
due to coordination of azomethine nitrogen with copper.
Fig. 1 FT-IR spectra of (a) KIT-6, (b) NH2-KIT-6, (c) S-NH2-KIT-6, (d)
Cu-NH2-KIT-6, (e) Fe-NH2-KIT-6, (f) Co-NH2-KIT-6, (g) Ni-NH2-KIT-
6 and (h) VO-NH2-KIT-6.
Catalytic reaction
Styrene epoxidation reactions were performed in a 10 ml round
bottom ask equipped with a magnetic stirrer and reux
condenser. Styrene (2 mmol), catalyst (10 mg) and appropriate
amount of TBHP were added into 2 ml of acetonitrile. Then the
Fig. 2 Small-angle XRD patterns of (a) KIT-6 (b) NH2-KIT-6 and (c) Cu-
mixture was stirred continuously at a desired temperature in an NH2-KIT-6.
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ring in the range of 1350–1600 cmꢁ1 and C]N stretching anchor onto the surface of KIT-6 and the cubic Ia3d symmetry of
vibration at around 1645 cmꢁ1 are presented in S-NH2-KIT-6, the support is synchronously preserved.
indicating the graing of salicylaldehyde precursor onto NH2-
KIT-6. In addition, the peaks of IR spectra in Fig. 1d–h, the C]N
Nitrogen adsorption–desorption measurements
stretching vibration slightly shis to the region of 1643–1629
The nitrogen adsorption–desorption isotherms and pore size
distributions of KIT-6 and Cu-NH2-KIT-6 are showed in Fig. 3.
Both of the samples display type IV isotherms with
cmꢁ1 due to the coordination of the nitrogen with metal. Thus,
the FT-IR results suggest the immobilization of metal Schiff
complex onto KIT-6.
a
pronounced capillary condensation step and H1 hysteresis loop
at high relative pressure, indicating the presence of uniform
mesoporous structure with narrow pore size distribution and
large channel.32 However, aer functionalization, a sharp
reduction in the specic surface area from 887 m2 gꢁ1 to 512 m2
gꢁ1 and a slight decrease in pore diameter from 6.14 nm to
5.75 nm have been found, implying that the organometallic
groups have been successfully anchored onto KIT-6.
XRD studies
The low angle powder XRD patterns of KIT-6, NH2-KIT-6 and Cu-
NH2-KIT-6 are depicted in Fig. 2. The pure KIT-6 sample shows a
sharp intense peak around 2q of 0.88 corresponding to the (211)
plane, several hump ordered reections for the (220) and (420)
planes, indicating that the material is a highly ordered meso-
porous structure with the cubic Ia3d symmetry.30,31 Aer
organic-functionalization and encapsulation of metal complex,
there is a slight decline in the intensity of the diffraction peaks,
thereby conforming that the Cu–Schiff should successfully
SEM studies
The morphologies of KIT-6, NH2-KIT-6 and Cu-NH2-KIT-6 are
presented in Fig. 4. All of the samples are agglomeration of
small random particles without explicit shape on the border.33
Even though KIT-6 experienced a multistep post treatment,
Fig.
3 N2 adsorption–desorption isotherms and pore diameter
distributions of (a) KIT-6 and (b) Cu-NH2-KIT-6.
Fig. 5 TG profiles of (a) KIT-6 and (b) Cu-NH2-KIT-6.
Fig. 4 SEM images of (a) KIT-6, (b) NH2-KIT-6 and (c) Cu-NH2-KIT-6. Fig. 6 XPS spectra of (a) Cu(CH3COO)2 and (b) Cu-NH2-KIT-6.
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virtually no tremendous variation has been found compared ratio of ligand/metal is close to the nominal value of 2 : 1. All
with the parental morphology.
these results indicate that Cu–Schiff complex has been
anchored onto KIT-6.
Thermal analyses
XPS measurements
To estimate the amount of covalently anchored complex on KIT-
6 and its thermal stability, thermogravimetric analysis was
employed and the relevant thermograms are presented in Fig. 5.
The weight lose below 250 ꢀC is ascribed to physically absorbed
water molecules.34 A slight mass loss of 2.1% is observed in
calcined KIT-6 between 250 ꢀC and 800 ꢀC, which is attributed to
the loss of condensation of hydroxyl groups. For Cu-NH2-KIT-6,
3.9% mass loss in the region of 250 ꢀC to 450 ꢀC due to
decomposition of aminopropyl groups, suggesting the immo-
bilization of 3-APTES on KIT-6. Thermograms of Cu-NH2-KIT-6
also appears a relatively large weight loss of 8.7% in the range
Fig. 6 displays the Cu 2p XPS spectra of Cu(CH3COO)2 and Cu-
NH2-KIT-6. The binding energies of Cu 2p1/2 and Cu 2p3/2 in
Cu(CH3COO)2 appear at 934.8 and 954.8 eV, respectively. For
the Cu-NH2-KIT-6, the Cu 2p1/2 and Cu 2p3/2 peaks respectively
shi to lower values of 934.3 and 954.0 eV, which can be
explained through the coordination of copper atoms with azo-
methine nitrogen of anchored Salen resulting in the increase of
the electronic density around copper atoms.34 Thus we can
conclude that copper(II) complex was successfully covalently
anchored onto KIT-6 by post-graing method.
ꢀ
from 450 to 700 C, which is mainly related to the decomposi-
tion of Cu–Schiff ligand.35 It can be approximately calculated
that the loading of Schiff base groups is 0.47 mmol gꢁ1 on
Catalytic properties
account of the weight loss, which is nearly in agreement with The catalytic performance of Cu-NH2-KIT-6 was carried out in
the Cu content of 0.22 mmol gꢁ1 estimated by ICP. The molar the epoxidation of styrene with TBHP as oxidant. In order to get
Table 1 Epoxidation of styrene under various reaction conditions
Product
selectivityc (mol%)
Catalyst amount
(mg)
Molar ratio
of styrene/TBHP
Temperature (ꢀC)
Time (h)
Conversationa (%)
TOFb (hꢁ1
)
Soc
Others
10
10
10
—
30
10
10
10
10
10
10
10
80
80
80
80
80
80
80
80
80
25
40
60
1 : 1
1 : 2
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
6
6
6
6
6
3
4
5
8
6
6
6
40.6
85.1
98.6
—
96.2
79.6
92.4
96.6
99.1
6.8
61.5
128.8
149.4
—
48.6
241.2
210
175.6
112.6
10.3
21.6
88.5
92.4
100
97.8
—
89.3
90.4
96.6
96.4
91.3
6.9
7.6
0
2.2
—
10.7
9.6
3.4
3.6
8.7
93.1
80.1
23
14.3
58.4
19.9
77
a
Reaction conditions: styrene (2 ml); acetonitrile (2 ml). b TOF, hꢁ1: (turnover frequency) moles of substrate converted per mole metal ion per hour.
So: styrene epoxide. Others: benzaldehyde, benzoic acid.
c
Table 2 Epoxidation of styrene with TBHP catalyzed by a variety of catalysts
Catalyst
Conversation (%)
Epoxide selectivity (%)
TOF (hꢁ1
)
Time (h)
Reference
KIT-6
Cu–Salen
—
—
—
6
6
6
6
6
6
6
24
24
24
24
This study
This study
This study
This study
This study
This study
This study
21
89.7
98.6
12.7
14.8
37
36.8
97
99
84.9
97.8
88.7
88.1
89.3
90.4
86
79
95
6
20.2
149.4
30.2
41.1
88.1
55.8
124
Cu-NH2-KIT-6
Ni-NH2-KIT-6
Fe-NH2-KIT-6
Co-NH2-KIT-6
VO-NH2-KIT-6
Cu-NH2-MCM-41
Cu-NH2-MSN
Cu-NH2-MSa
Cu-Yb
103
64.2
22
37
38
86.3
21
a
Cu-NH2-MS: copper(II) Schiff anchored onto amino-functioned 2D-hexagonal mesoporous silica. b Cu-Y: copper(II) phthalocyanine encapsulated
in zeolite Y.
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the optimal reaction conditions, different parameters such as Cu–Salen onto three dimensional mesoporous silica KIT-6
catalyst amount, substrate/oxidant molar ration, reaction time efficiently avoiding the dimerization of active sites.36
and temperature were investigated. From the results of different
We have investigated the recyclability and stability of the
styrene/TBHP molar rations in Table 1, it can be seen that the modied material for styrene epoxidation with TBHP as
conversion of styrene remarkably improves from 40.6% to oxidant. The solid catalyst was easily recovered by ltration aer
98.6% aer 6 h by increasing the styrene/TBHP molar ratio from each reaction cycle, washed thoroughly with acetonitrile and
1 : 1 to 1 : 3. It is also found that the epoxide selectivity does not
change much by increasing the styrene/TBHP molar ration.
Above results indicate that TBHP plays a crucial role in the
epoxidation of styrene and the styrene/TBHP molar ratio of 1 : 3
is optimal for running the epoxidation reaction.
The inuence of catalyst amount on the catalytic perfor-
mance was performed by varying the catalyst amount from 10 to
30 mg and the results are showed in Table 1. It is interesting to
note that without catalyst exhibited negligible catalytic activity
attributed to no active sites. However, a sharp increase in the
styrene conversion (98.6%) and epoxide selectivity (97.8%) was
attained by using 10 mg Cu-NH2-KIT-6 as catalyst, which
suggest Cu–Schiff complex successfully covalently anchored
onto KIT-6 and the presence of active sites. Nevertheless, when
increasing catalyst amount up to 30 mg, epoxide selectivity
reduces to 89.3% and the styrene conversion also slightly
Fig. 7 Reuse of the Cu-NH2-KIT-6 catalyst.
decreases to 96.2%. Increasing the catalyst amount could
provide more active sites, but inevitably results in further
oxidation of the product styrene oxide. Thus, the catalyst
amount (10 mg) is sufficient for the good conversion of styrene.
The reaction time has some inuence on the catalytic
behavior. As can be seen from Table 1, the epoxide selectivity
gradually increased from 90.4% to 97.8% by prolonging the
reaction from 3 h to 6 h, while further prolonging reaction time
has no signicant inuence on the styrene conversion but
results in the decrease of epoxide selectivity.
The temperature has great inuence on the catalytic
performance. As shown in Table 1, low temperature causes very
low styrene conversion and epoxide selectivity. For example,
only 6.8% of styrene conversion and 6.9% of epoxide selectivity
can be achieved at 25 ꢀC. Increasing reaction temperature could
synchronously raise the catalytic activity.
Compared with a series of copper based heterogeneous
catalyst conducted by previous reports (Table 2), it is obvious
that Cu-NH2-KIT-6 possesses the highest conversation, epoxide
selectivity and turnover frequency. Most importantly, our reac-
tion was carried out only needing 6 h, while the reported cata-
lysts needed 24 h for the epoxidation of styrene. The excellent
catalytic performance of Cu-NH2-KIT-6 can attribute to the three
dimensional mesoporous structures to avoid pore blockage and
facilitate reactants diffusion into the pores. However, it should
be noted that the other congeneric transition metal (Fe2+, Co2+,
VO2+, Ni2+) Schiff complexes graed on KIT-6 have not so good
catalytic performance. For example, only 12.7% conversion of
styrene and 88.7% selectivity to epoxide can be obtained over
Ni-NH2-KIT-6. This result may means that the synergistic effect
between Cu(II) Schiff complex and KIT-6 support are superior to
the others. In addition, the pure KIT-6 showed negligible cata-
lytic performance since no active sites. The homogeneous Cu–
Salen complex exhibited lower catalytic activity than its
Scheme 2 Proposed mechanism for epoxidation of styrene with
heterogeneous catalyst, which is probably due to the anchoring TBHP over copper(II) catalyst.
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dichloromethane, dried in the vacuum at 60 ꢀC for 12 h and
reused for the subsequent cycles. The reused catalyst exhibited
a negligible decrease in the catalytic activity and epoxide
selectivity in three consecutive runs (shown in Fig. 7). To test if
copper is leaching out of the catalyst in liquid phase reaction;
the reaction mixture was hot ltered out at the end of the
reaction and subjected to inductively coupled plasma analysis
techniques. The result suggested almost no detectable copper
metal ions in the ltrate solution.
6 D. Saha, T. Maity, T. Dey and S. Koner, Polyhedron, 2012, 35,
55–61.
7 M. Salavati-Niasari, E. Esmaeili, H. Seyghalkar and
M. Bazarganipour, Inorg. Chim. Acta, 2011, 375, 11–19.
8 S. M. Islam, A. S. Roy, P. Mondal, S. Paul and N. Salam, Inorg.
Chem. Commun., 2012, 24, 170–176.
9 B. Bahramian, V. Mirkhani, M. Moghadam and
S. Tangestaninejad, Appl. Catal., A, 2006, 301, 169–175.
10 Y. Yang, Y. Zhang, S. Hao and Q. Kan, Chem. Eng. J., 2011,
171, 1356–1366.
Based on our experimental results and other related litera-
ture.39,40 A reasonable reaction mechanism for epoxidation of 11 P. Wang, Z. Dong, Y. Lei, Y. Du, H. Li, H. Yang, Y. Nie and
styrene with TBHP over Cu-NH2-KIT-6 catalyst (LCuII) has been
postulated in Scheme 2. In the rst step, LCuII was coordinated 12 S. Jana, S. Bhunia, B. Dutta and S. Koner, Appl. Catal., A,
with TBHP to form an active copper(III)-peroxo species I. Then, 2011, 392, 225–232.
the activated oxygen of I consecutively interacted with C]C of 13 A. Sakthivel, W. Sun, G. Raudaschl-Sieber, A. S. T. Chiang,
J. Ma, J. Porous Mater., 2012, 20, 277–284.
¨
styrene molecule to form cyclic intermediate III. Finally, III
subsequently released to the epoxide, simultaneously accom-
M. Hanzlik and F. E. Kuhn, Catal. Commun., 2006, 7, 302–
307.
panied with the regeneration of copper(II) catalyst. The forma- 14 A. R. Silva, K. Wilson, J. H. Clark and C. Freire, Microporous
tion of benzaldehyde and benzoic acid may be caused by deep
oxidation of TBHP.
Mesoporous Mater., 2006, 91, 128–138.
15 Y. Xia and R. Mokaya, J. Phys. Chem. B, 2003, 107, 6954–6960.
16 S. A. El-Say, T. Hanaoka and F. Mizukami, Adv. Mater.,
2005, 17, 47–53.
17 F. Kleitz, S. H. Choi and R. Ryoo, Chem. Commun., 2003,
2136–2137.
Conclusions
Novel heterogeneous catalysts have been synthesized by cova-
lently anchored various transition metal (Fe2+, Co2+, Ni2+, Cu2+ 18 A. Vinu, N. Gokulakrishnan, V. V. Balasubramanian, S. Alam,
or VO2+) Schiff complexes onto amine functionalized KIT-6. The
M. P. Kapoor, K. Ariga and T. Mori, Chemistry, 2008, 14,
11529–11538.
characterization results demonstrate that the pore structure of
KIT-6 remain intact aer multiple synthetic procedures. In 19 H. Oveisi, C. Anand, A. Mano, S. S. Al-Deyab, P. Kalita,
addition, a detail research was carried out to optimize the
reaction conditions of styrene epoxidation over Cu-NH2-KIT-6.
A. Beitollahi and A. Vinu, J. Mater. Chem., 2010, 20, 10120–
10129.
Compared with copper(II) complex graed onto other supports 20 Y. Yang, S. Hao, Y. Zhang and Q. Kan, Solid State Sci., 2011,
(e.g. zeolite Y, MCM-41, MSN and MS) and different metal (e.g. 13, 1938–1942.
Fe2+, Co2+, Ni2+, or VO2+) Schiff complexes graed onto KIT-6, 21 S. Jana, B. Dutta, R. Bera and S. Koner, Langmuir, 2007, 23,
Cu-NH2-KIT-6 exhibits excellent catalytic performance such as
2492–2496.
high conversion, excellent epoxide selectivity, high TOF and 22 D. Tang, W. Zhang, Y. Zhang, Z. A. Qiao, Y. Liu and Q. Huo,
good recyclability due to its unique pore structure of KIT-6 and J. Colloid Interface Sci., 2011, 356, 262–266.
efficient synergistic effect between Cu(II) Schiff and the KIT-6 23 D.-H. Choi and R. Ryoo, J. Mater. Chem., 2010, 20, 5544–
support.
5550.
24 Y. Yang, J. Guan, P. Qiu and Q. Kan, Appl. Surf. Sci., 2010,
256, 3346–3351.
25 H. Wang, J. Huang, S. Wu, C. Xu, L. Xing, L. Xu and Q. Kan,
Mater. Lett., 2006, 60, 2662–2665.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21303069), Jilin province (201105006), 26 S. J. J. Titinchi and H. S. Abbo, Catal. Today, 2013, 204, 114–
and Jilin University (450060445017).
124.
27 N. Malumbazo and S. F. Mapolie, J. Mol. Catal. A: Chem.,
2009, 312, 70–77.
28 X. Wang, Y. Tseng, J. Chan and S. Cheng, J. Catal., 2005, 233,
266–275.
29 A. Di Giuseppe, C. Di Nicola, R. Pettinari, I. Ferino,
D. Meloni, M. Passacantando and M. Crucianelli, Catal.
Sci. Technol., 2013, 3, 1972–1984.
30 D. Zhang, A. Duan, Z. Zhao and C. Xu, J. Catal., 2010, 274,
273–286.
Notes and references
1 L. Ma, F. Su, W. Guo, S. Zhang, Y. Guo and J. Hu, Microporous
Mesoporous Mater., 2013, 169, 16–24.
2 X. H. Lu, Q. H. Xia, H. J. Zhan, H. X. Yuan, C. P. Ye, K. X. Su
and G. Xu, J. Mol. Catal. A: Chem., 2006, 250, 62–69.
3 X. Lu and Y. Yuan, Appl. Catal., A, 2009, 365, 180–186.
4 S. Tangestaninejad, M. Moghadam, V. Mirkhani,
I. Mohammadpoorbaltork and K. Ghani, Catal. Commun., 31 S. Alam, C. Anand, S. M. Zaidi, T. S. Naidu, S. S. Al-Deyab and
2009, 10, 853–858. A. Vinu, Chem.–Asian J., 2011, 6, 834–841.
5 J. Sebastian, K. Jinka and R. Jasra, J. Catal., 2006, 244, 208– 32 C. Yuan, Z. Huang and J. Chen, Catal. Commun., 2012, 24,
218.
56–60.
2316 | RSC Adv., 2014, 4, 2310–2317
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View Article Online
Paper
RSC Advances
33 Y. Q. Dou, Y. Zhai, F. Zeng, X. X. Liu, B. Tu and D. Zhao, J. 37 M. Nandi, P. Roy, H. Uyama and A. Bhaumik, Dalton Trans.,
Colloid Interface Sci., 2010, 341, 353–358. 2011, 40, 12510–12518.
34 Y. Yang, Y. Zhang, S. Hao, J. Guan, H. Ding, F. Shang, P. Qiu 38 S. Seelan, A. Sinha, D. Srinivas and S. Sivasanker, J. Mol.
and Q. Kan, Appl. Catal., A, 2010, 381, 274–281. Catal. A: Chem., 2000, 157, 163–171.
35 G. R. Bardajee, R. Malakooti, F. Jami, Z. Parsaei and 39 B. Singh, B. S. Rana, L. N. Sivakumar, G. M. Bahuguna and
H. Atashin, Catal. Commun., 2012, 27, 49–53. A. K. Sinha, J. Porous Mater., 2012, 20, 397–405.
36 Y. Luo and J. Lin, Microporous Mesoporous Mater., 2005, 86, 40 H. Cui, Y. Zhang, L. Zhao and Y. Zhu, Catal. Commun., 2011,
23–30.
12, 417–420.
This journal is © The Royal Society of Chemistry 2014
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