Heterogenization of functionalized Cu(II) and VO(IV) Schiff base complexes by direct immobilization onto amino-modified SBA-15: Styrene oxidation catalysts with enhanced reactivity

Article abstract of DOI:10.1016/j.apcata.2010.04.018

Two functionalized copper(II) and oxovanadium(IV) Schiff base complexes of type [M(N₂O₂), M = Cu, VO] bearing chloromethyl groups were respectively synthesized, directly anchored onto amino-modified SBA-15 materials and examined as catalysts for styrene oxidation. The purity of each ligand was confirmed by ¹H NMR, FT-IR and elemental analysis. XRD, N₂ adsorption/desorption and TEM results indicated that the mesoporous structure of SBA-15 remained intact throughout the grafting procedure. FT-IR, UV-vis spectroscopy plus TG-DTA data demonstrated the incorporation of copper(II) and oxovanadium(IV) complexes on amino-modified SBA-15. ICP-AES, SEM-EDX combined with XPS data further showed the different anchorage status of copper(II) and oxovanadium(IV) species on amino-modified SBA-15. The copper(II) Schiff base complex was anchored through the coordination of copper atom with the nitrogen atom of the amino group modified on the SBA-15 external surface. The oxovanadium(IV) Schiff base complex, however, was covalently anchored on SBA-15 via the condensation reaction of the chloromethyl group of the Schiff base with the amino group from the modified SBA-15 matrix. The catalytic properties of supported copper(II) and oxovanadium(IV) complexes in the oxidation of styrene with air or H₂O₂ as oxidant were investigated and compared with the properties of their homogeneous analogues. It was found that both heterogeneous copper(II) and oxovanadium(IV) catalysts were more active than their homogeneous analogues and that the product selectivity varied in cases of different oxidants. The supported oxovanadium(IV) complex showed high yield of styrene oxide (56.0%) and good recoverability when using air as oxidant.

Full text of DOI:10.1016/j.apcata.2010.04.018

Applied Catalysis A: General 381 (2010) 274–281
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Applied Catalysis A: General
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Heterogenization of functionalized Cu(II) and VO(IV) Schiff base complexes by
direct immobilization onto amino-modified SBA-15: Styrene oxidation catalysts
with enhanced reactivity
a
b
b
a
a
a
Ying Yang , Ying Zhang , Shijie Hao , Jingqi Guan , Hong Ding , Fanpeng Shang ,
a
a,∗
Pengpeng Qiu , Qiubin Kan
College of Chemistry, Jilin University, Jeifang Road 2519, Changchun 130023, Jilin, PR China
a
b
Department of Materials Science and Engineering, China University of Petroleum, Changping District, Beijing 102249, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two functionalized copper(II) and oxovanadium(IV) Schiff base complexes of type [M(N O ), M = Cu,
2
2
Received 9 November 2009
Received in revised form 27 March 2010
Accepted 9 April 2010
VO] bearing chloromethyl groups were respectively synthesized, directly anchored onto amino-modified
SBA-15 materials and examined as catalysts for styrene oxidation. The purity of each ligand was confirmed
1
by H NMR, FT-IR and elemental analysis. XRD, N2 adsorption/desorption and TEM results indicated that
Available online 24 April 2010
the mesoporous structure of SBA-15 remained intact throughout the grafting procedure. FT-IR, UV–vis
spectroscopy plus TG–DTA data demonstrated the incorporation of copper(II) and oxovanadium(IV)
complexes on amino-modified SBA-15. ICP-AES, SEM-EDX combined with XPS data further showed the
different anchorage status of copper(II) and oxovanadium(IV) species on amino-modified SBA-15. The
copper(II) Schiff base complex was anchored through the coordination of copper atom with the nitrogen
atom of the amino group modified on the SBA-15 external surface. The oxovanadium(IV) Schiff base com-
plex, however, was covalently anchored on SBA-15 via the condensation reaction of the chloromethyl
group of the Schiff base with the amino group from the modified SBA-15 matrix. The catalytic properties
of supported copper(II) and oxovanadium(IV) complexes in the oxidation of styrene with air or H2O2 as
oxidant were investigated and compared with the properties of their homogeneous analogues. It was
found that both heterogeneous copper(II) and oxovanadium(IV) catalysts were more active than their
homogeneous analogues and that the product selectivity varied in cases of different oxidants. The sup-
ported oxovanadium(IV) complex showed high yield of styrene oxide (56.0%) and good recoverability
when using air as oxidant.
Keywords:
Mesoporous materials
Metal Schiff base complexes
Styrene oxidation
Copper
Oxovanadium
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
compounds encapsulated in zeolite-Y ([Cu(sal-ambmz)Cl]-Y and
VO (sal-ambmz)Cl]-Y) exhibited high catalytic activity in the oxi-
[
2
The epoxidation of olefins is of great interest due to the impor-
dation of styrene and hydroxylation of phenol. Recently, many
chemical researchers pursue new oxidation processes using molec-
ular oxygen or naturally cheap air as the oxidant. Several successful
examples of the catalytic epoxidation of styrene by molecular oxy-
gen over cobalt(II)-containing molecular sieve catalysts (CoOx/SiO2,
CoOx/NaX, Co/TS-1 and Co–ZSM-5) have been reported [13–15].
As far as we know, the catalytic performances of copper(II)
and oxovandiun(IV) Schiff base complexes have not been eval-
uated in aerobic selective oxidations. Copper hydroxyphosphate
tance of epoxides in the manufacture of both bulk and fine
chemicals [1,2]. Recently, much attention has been focused on
developing novel catalytic processes based on efficient catalysts
with easy recoverability and inexpensive eco-friendly oxidants, for
example H O , O and air.
2
2
2
Various transition metal compounds (Cu, Fe, Co, V and Mn) have
been reported as active catalysts for the epoxidation of olefins
3–7]. A few reports showed that some copper(II) and oxovana-
[
dium(IV) based catalytic systems were efficient for the oxidation
(Cu (OH)PO ) is one copper based catalyst used for aerobic epoxi-
2 4
of styrene when using H O2 as oxidant [8–12]. For example, Mau-
rya et al. [12] reported that copper(II) and dioxovanadium(V)
dation of styrene, with a low conversion of styrene (30.2%) and poor
selectivity to styrene oxide (16.9%) [16].
2
Schiff base transition metal complexes have been extensively
studied because of their potential use as catalysts in a wide range
of oxidation reactions [17–20]. Many strategies have been adopted
to heterogenize the homogeneous catalysts to increase catalyst
∗ Corresponding author. Tel.: +86 431 88499140; fax: +86 431 88499140.
E-mail address: qkan@mail.jlu.edu.cn (Q. Kan).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.04.018

Y. Yang et al. / Applied Catalysis A: General 381 (2010) 274–281
275
stability and allow for catalyst recycling and product separation.
Conventionally, the Schiff base complexes are immobilized on poly-
meric organic materials such as resins, or polystyrene [21], then
supported on inert porous solid such as alumina [22] and silica or
encapsulated in the pores of zeolite-Y [23]. There are certain dis-
advantages with polymeric supports due to their vulnerability to
some chemicals and solvents. Encapsulation of metal complexes
in porous materials typified by zeolites leads to size restriction
and severe leaching of catalysts. Mesoporous silica materials have
been widely used as useful and versatile solid supports to construct
various hybrid materials in catalysis, enzyme immobilization and
drug delivery due to their large tunable pore dimensions, high sur-
face areas and great diversity in surface functionalization [24,25].
Therefore, covalent anchoring of the Schiff base complexes onto
a functionalized siliceous mesoporous material with large pore
diameters seems to be promising. The mesoporous SBA-15 materi-
als are more versatile than other mesoporous supports because of
their larger pore diameter permitting easy diffusion of bulky reac-
tants and products during the liquid-phase oxidation of styrene.
Herein, we report the immobilization of functionalized Cu(II)
and VO(IV) Schiff base complexes bearing chloromethyl groups
onto amino-modified SBA-15 materials and their catalytic prop-
erties in styrene oxidation. The catalytic performances of both
heterogeneous copper(II) and oxovanadium(IV) catalysts were
compared to those of their homogeneous analogues and the oxi-
in petroleum ether. The extract furnished 5-chloromethyl-2-
hydroxybenzaldehyde as white-colored needles (9.0 g, 32.0%
yield). Found: C, 57.57; H, 4.12%. Calc. for C H7O Cl: C, 56.30; H,
8
2
−
1
4.10%. FT-IR (KBr pellets, cm ): 3240 (OH), 2925, 2873 (Aliph-H),
1659 (C=O), 1260 (CH Cl), 772 (C–Cl) (Supplementary Fig. S1).
2
1
UV–vis, ꢀ (nm): 224, 257, 332. H NMR (CDCl , TMS, ı ppm): 11.07
3
(1H, s, OH), 9.90 (1H, s, CHO), 7.59 (1H, s, Ar–H), 7.26–7.19 (1H, d,
Ar–H), 7.02–6.08 (1H, d, Ar–H), 4.59 (1H, s, CH Cl) (Supplementary
2
Fig. S2).
2.2.3. Synthesis of
ꢀ
N,N -bis(5-chloromethyl-salicylidene)ethylenediamine
(CM–SalenH ) and its metal complexes
2
3.41 g (20.0 mmol) of 5-chloromethyl-2-hydroxybenzaldehyde
was dissolved in 10 ml of CH Cl . A solution of ethylene diamine
2
2
(0.67 ml, 10 mmol) in 10 ml of CH Cl2 was then added. The result-
2
ing solution was stirred at room temperature for 24 h. After the
solvent was removed under vacuum, the product was obtained
as bright yellow crystals. Found: C, 58.50; H, 5.10; N, 7.86%. Cal.
for C18
H N O Cl : C, 59.20; H, 4.90; N, 7.70%. FT-IR (KBr pel-
18 2 2 2
−
1
lets, cm ): 3423 (OH), 2929, 2887 (Aliph-H), 1647 (C=N), 1262
1
(CH Cl), 769 (C–Cl). UV–vis, ꢀ (nm): 252, 340, 381. H NMR (DMSO-
2
d6, TMS, ı ppm): 11.12 (2H, s, OH), 7.83–6.98 (6H, m, Ar–H), 8.40,
8.44 (2H, s, CH=N), 4.44 (4H, s, CH Cl), 3.48–3.38, 3.19–3.10 (4H, t,
2
N–CH –CH –N) (Supplementary Fig. S3).
2
2
dation performances using air and H O2 as oxidizing agent were
To prepare the metal complexes of CM–SalenH , we dissolved
2
2
also compared.
0.367 g of CM–SalenH2 (1 mmol) and 1 mmol of Cu(CH COO)2 or
3
VOSO4 in 20 ml THF under N2 atmosphere and we stirred the mix-
ture at room temperature for 24 h. The solvent was removed by
filtration, and the resulting solid was washed with copious tetrahy-
drofuran and dried in vacuum. Found for Cu–CM–Salen: C, 50.50;
2
. Experimental
2.1. Materials
H, 3.82; N, 6.32; Cu, 13.50%. Calc. for C18H16N O Cl Cu: C, 50.61; H,
2
−
2
1
2
The following chemicals were commercially available and were
3.75; N, 6.56; Cu, 14.88%. FT-IR (KBr pellets, cm ): 3423 (OH), 2929,
2887 (Aliph-H), 1621 (C=N), 767 (C–Cl), 526 (Cu–O), 470 (Cu–N).
UV–vis, ꢀ (nm): 240, 345, 389, 572. Found for VO–CM–Salen: C,
49.92; H, 3.63; N, 6.42; V, 11.50%. Calc. for C18H16N2O3Cl2V: C,
50.20; H, 3.72; N, 6.51; V, 11.84%. FT-IR (KBr pellets, cm ): 3423
(OH), 2929, 2887 (Aliph-H), 1631 (C=N), 769 (C–Cl), 976 (V=O), 534
(V–O), 456 (V–N). UV–vis, ꢀ (nm): 263, 323, 379, 554.
used as received: 3-aminopropyltriethoxysilane (Aldrich), Pluronic
P123 (EO20PO70EO20) (Aldrich), salicylaldehyde (99%), tetraethyl
orthosilicate Si(OC H5)4 (99%), styrene (98%). 30% H O , VOSO ,
2
2
2
4
−
1
Cu(CH COO) and all organic solvents are A.R. grade. Toluene was
3
2
dried by Na/diphenylketone ketyl and distilled under N2 atmo-
sphere. All air or moisture-sensitive compounds were transferred
using a standard vacuum line and the Schlenk technique.
2.2.4. Immobilization of Cu–CM–Salen and VO–CM–Salen on
2.2. Synthetic procedures (Scheme 1)
amino-functionalized mesoporous SBA-15
Metal Schiff base complexes were anchored onto SBA-15 matrix
based on the nucleophilic reaction between the chloromethyl
modified metal complexes and the aminopropyl-functionalized
SBA-15. A known amount of Cu–CM–Salen (0.33 g, 0.8 mmol) or
VO–CM–Salen (0.32 g, 0.8 mmol) was added to a suspension of the
amino-functionalized SBA-15 (1.0 g) in dry toluene and stirred at
reflux temperature for 24 h. The resulting samples were filtered
off, Soxhlet-extracted with CH2Cl2 to remove untethered species
2
.2.1. Synthesis of amino-functionalized SBA-15
The mesoporous support SBA-15 (1.0 g), prepared by a lit-
◦
erature method [26], was activated by heating at 120 C for
2
aminopropyltriethoxysilane (1.6 mmol in 20 ml of dry toluene)
under N2 atmosphere and the mixture was stirred at room
temperature for 24 h. The resulting solid was filtered, washed,
Soxhlet-extracted with CH Cl2 for 24 h and dried under vacuum.
Found for APS–SBA-15: C, 5.37; H, 1.41; N, 1.39%. FT-IR (KBr pel-
lets, cm ): 1510 (NH ), 1000–1130 (Si–O–Si), 960 (Si–OH), 687
h. After cooling, the activated SBA-15 was added to the 3-
−
1
and dried in vacuum. Cu–Salen–SBA, FT-IR (KBr pellets, cm ):
3413 (OH), 3236 (N–H), 2926 (Aliph-H), 1636 (C=N), 1087, 808, 468
(Si–O–Si). UV–vis, ꢀ (nm): 265, 349, 380, 576. VO–Salen–SBA, FT-IR
2
−
1
2
−
1
(
N–H).
(KBr pellets, cm ): 3407 (OH), 3220 (N–H), 2931 (Aliph-H), 1629
C=N), 1086, 804, 464 (Si–O–Si). UV–vis, ꢀ (nm): 243, 325, 389.
(
2.2.2. Synthesis of 5-chloromethyl-2-hydroxybenzaldehyde
5
-Chloromethyl-2-hydroxybenzaldehyde was synthesized
2.3. Characterization
from salicylaldehyde by the classical chloromethylation method.
In a typical synthesis, 17.5 g (160 mmol) of salicylaldehyde was
treated with 24 ml of 38% aqueous formaldehyde and 1.2 g of
ZnCl2 in 100 ml of con. HCl. The mixture was stirred at room
temperature under N2 atmosphere for 24 h. The resulting white
solid was filtered and repeatedly extracted with diethyl ether. The
combined organic phases were washed with saturated aqueous
Powder XRD was collected with a Rigaku X-ray diffractome-
ter with nickel filtered CuK␣ radiation (ꢀ = 1.5418 Å). The samples
◦
◦
◦
were scanned in the range 2ꢁ = 0.4–5.0 and in steps of 2 /min. N
2
adsorption/desorption isotherms were recorded at 196 C with a
Micromeritics ASAP 2020. Before measurements, the samples were
outgassed at 120 C for 12 h. The specific surface area was calculated
◦
NaHCO3 and water and then dried over MgSO . The viscous oil
was obtained by distillation and then subjected to crystallization
using the Brunauer–Emmett–Teller (BET) method and the pore size
distributions were measured using Barrett–Joyner–Halenda (BJH)
4

2
76
Y. Yang et al. / Applied Catalysis A: General 381 (2010) 274–281
analyses from the desorption branch of the isotherms. The infrared
spectra (IR) of samples were recorded in KBr disks using a NICO-
LET impact 410 spectrometer. UV–vis spectra were recorded on
a PerkinElmer UV–vis spectrophotometer Lambda 20 using bar-
ium sulfate as the standard in the range 200–800 nm. TG–DTA was
carried out on Shimadzu DTG-60 instrument. X-ray photoelectron
spectra (XPS) were recorded on a VG ESCA LAB MK-II X-ray electron
spectrometer using AlK␣ radiation (1486.6 eV, 10.1 kV). The spectra
were referenced with respect to the C 1 s line at 284.7 eV. The mea-
1
surement error of the spectra was ± 0.2 eV. H NMR experiments
were carried out in sealed NMR tubes on a Varian Mercury-300
NMR spectrometer. Microanalyses for C, H, N were performed at
the PerkinElmer 2400. Metal content was estimated by inductively
coupled plasma atomic emission spectroscopy (ICP-AES) analysis
conducted on a PerkinElmer emission spectrometer. The chemical
microanalyses by EDX were performed on a JSM-6301F scanning
microscope FEI with an EDX detector. Samples were coated with
gold using a sputter coater. Transmission electron microscopy
(
TEM) results were recorded on a JEOL JEM 2010 operated at 200 kV.
Fig. 1. XRD patterns of (a) SBA-15, (b) Cu–Salen–SBA and (c) VO–Salen–SBA.
2.4. Catalytic reactions
As shown in Table 1, the pore diameter decreased from 6.35 nm
for pure silica SBA-15 to 6.19 nm for Cu–Salen–SBA and to 5.18 nm
for VO–Salen–SBA. This result is in good agreement with the con-
clusion drawn from the TEM studies. Compared to the pure silica
SBA-15, a pronounced decrease of the BET surface area and the pore
volume and an increase of the wall thickness occurred due to the
introduction of copper(II) or oxovanadium(IV) complexes. Similar
trend has also been observed previously [29].
All catalysts were dried overnight before they were subjected
to epoxidation reactions. Typically, a 100 ml two-necked flask sup-
plied with a magnetic stirrer and a backflow condenser was kept
in a constant temperature oil bath. Ten mmol styrene, 50 mg of
catalyst (5 mg for the homogeneous catalyst) were suspended in
1
0 ml of CH CN. The reaction was started by adding 3.06 ml of 30%
3
H O (3 equiv.) or 2.28 ml of isobutyraldehyde (2.5 equiv.), com-
2
2
bined with bubbling air (80 ml/min) as oxidant at the reaction
temperature. After the reaction was finished, the catalyst was fil-
3.2. Spectroscopic characterization
◦
tered, washed with CH CN, dried at 100 C overnight and reused
3
directly without further purification. The liquid organic products
were quantified by using a gas chromatography (Shimadzu, GC-
Fig. 4 shows the FT-IR spectra of the pure ligand and unsup-
ported complexes of copper(II) and oxovanadium(IV). The ligand
8
A) equipped with a flame detector and an HP-5 capillary column.
−1
CM–SalenH2 exhibited a band at 1647 cm
due to azomethine
The products were identified by GC–MS, and calibration of GC peak
areas of products was carried out with quantitative authentic sam-
ples.
ꢂ(C=N) stretch (Fig. 4a). This band registered a low frequency shift
−1
of ca. 16–26 cm in the spectra of unsupported copper (Fig. 4b)
and oxovanadium (Fig. 4c) complexes, indicating the coordination
of azomethine nitrogen to the metal ions [30,31]. The coordina-
tion of the phenolic oxygen could not be ascertained unequivocally
3
. Results and discussion
−
1
due to the appearance of a strong band in the 3400 cm
shown). In the lower frequency region, the bands appeared at
(not
3.1. Structural integrity studies
−
1
5
26 and 470 cm due to ꢂ(Cu–O) and ꢂ(Cu–N) vibrations in the
Cu–CM–Salen material [32]. Similarly, VO–CM–Salen presented
The powder XRD patterns for SBA-15, Cu–Salen–SBA and
−
1
bands at 976, 534 and 456 cm assigned to ꢂ(V=O), ꢂ(V–O) and
ꢂ(V–N), respectively [8]. However, these bands became vague upon
homogeneous complex anchorage due to the appearance of strong
VO–Salen–SBA are depicted in Fig. 1. The SBA-15 sample showed
three peaks, indexed as the (1 0 0), (1 1 0) and (2 0 0) diffraction
peaks, associated with the typical two-dimensional hexagonal
symmetry of the SBA-15 material (Fig. 1a) [27]. For the hybrid
materials, the relative intensities of the prominent diffraction peak
−
1
and broad bands at 1086, 807 and 402 cm assigned to SBA-15
−
1
framework [33]. Moreover, in the region of 1600–1300 cm , the
spectra of hybrid materials showed some weak peaks, which could
be due to the existence of organic ligands in the hybrid materi-
als (Fig. 4d and e). The UV–vis spectra of the samples are shown
(
1 0 0) decreased after the introduction of bulky organometallic
groups. The intensity reduction may be mainly due to contrast
matching between the silicate framework and organic moieties
which are located inside the channels of SBA-15 [28]. TEM images
provide a direct visualization of well-ordered hexagonal arrays of
Table 1
2
−1
1
D mesoporous channels particularly along the direction of the
Characteristics of support and catalysts, specific surface area, SBET (m
volume, VBJH (cm
g
); pore
−1
); pore diameter, DBJH (nm); cell parameters, Ao (nm); inter-
3
g
pore axis or in the direction perpendicular to the pore axis (Fig. 2).
This result is in good agreement with the conclusion drawn from
the XRD patterns and confirms that characteristic pore dimen-
sions and channel structures of the support materials remain intact
after the introduction of metal complexes to the support SBA-
planar spacing, D100 (nm); wall thickness, W (nm); C (M), initial concentration of
−1
metal species (mmol g ).
Materials
C (M)
SBET
VBJH
DBJHa
Ao
D100
Wb
SBA-15
−
892
411
299
276
1.01
0.68
0.59
0.50
6.35
6.23
6.19
5.18
11.87
11.41
12.37
11.67
10.28
9.88
10.71
10.01
5.52
5.18
6.18
6.49
APS–SBA-15
Cu–Salen–SBA
VO–Salen–SBA
−
1
5. The N2 adsorption/desorption isotherms of SBA-15 and the
0.180
0.312
hybrid materials shown in Fig. 3 further confirm that the chan-
nels of the SBA-15 support remain accessible. Obviously, the hybrid
materials maintain the characteristics of type IV isotherms and
show a uniform pore size distribution in the mesoporous region.
a
Calculated from the desorption branch.
ꢀ
ꢁ
b
W = Ao − DBJH Ao = 2D100
√
⁄
3
.

Y. Yang et al. / Applied Catalysis A: General 381 (2010) 274–281
277
Fig. 2. TEM images of (a, b) SBA-15, (c, d) Cu–Salen–SBA and (e, f) VO–Salen–SBA.
*
*
in Fig. 5. Intense n–␲ , ␲–␲ ligand charge transfer bands in the
range of 220–380 nm were present in all cases. The expected d–d
6
3
transitions at 630 nm for Cu–Salen–SBA (tg eg ) as well as those
at 560 nm for its homogeneous analogues indicate that copper(II)
complexes have been incorporated onto the surface of SBA-15. A
weak band usually appears in the ca. 600 nm region in oxovana-
dium(IV) complexes [34].
3.3. TG–DTA studies
Quantification of weight loss at various steps is possible because
of nonoverlapping. TG–DTA analysis results of Cu–Salen–SBA are
depicted in Fig. 6. The TG curve recorded under N2 atmosphere
showed a three-step weight loss at temperatures ranging from 30
◦
◦
to 800 C. The weight loss below 250 C was due to the physically
adsorbed water and solvent inside the pores. The 20% weight loss at
◦
2
50–450 C was mainly related to the combustion of amine. Weight
◦
loss (6.0%) in the temperature range from 450 to 670 C was prob-
ably attributed to the decomposition of Schiff base groups [35].
The loading of Schiff base ligand was 0.22 mmol/g based on 6.0%
Fig. 3. N2 adsorption/desorption isotherms and pore size distribution profiles of (a)
SBA-15, (b) APS–SBA-15, (c) Cu–Salen–SBA and (d) VO–Salen–SBA.

2
78
Y. Yang et al. / Applied Catalysis A: General 381 (2010) 274–281
of weight loss and the copper content was 0.180 mmol/g estimated
by ICP-AES. The molar ratio of ligand to copper was approximately
.2:1 and was close to the nominal value of 1:1, indicating that
1
the homogeneous copper complexes remained intact after being
introduced to the SBA-15 matrix. This result demonstrates that
heterogenization of functionalized Cu(II) Schiff base complexes by
direct immobilization ensures the purity of the metal complex
present on the solid. The DTA curve corresponds with the TG result.
◦
The intense exothermal peak at 387 C along with a shoulder peak
◦
at 429 was observed. The higher temperature peaks of supported
copper(II) complexes, in comparison with those of APS–SBA-15
(Supplementary Fig. S4), give another piece of evidence for the exis-
tence of copper complexes in hybrid materials. Similar results were
obtained for VO–Salen–SBA (Supplementary Fig. S5).
3.4. Anchorage status analysis
The anchorage status of supported copper(II) and oxovana-
dium(IV) complexes may closely relate to their catalytic behavior.
In our study, the initial concentration of copper species in
Cu–Salen–SBA was 0.180 mmol/g and the surface concentration
was 0.100 mmol/g as estimated by XPS; such values suggest that
some copper(II) complexes were located on the external surface
of hexagonal pores. But for VO–Salen–SBA, the total concentra-
tion of vanadium was 0.312 mmol/g and its surface concentration
was neglectable. These results indicate that most oxovanadium(IV)
complexes were located in the mesoporous channels. The interior
location may directly lead to the thicker walls and smaller pore
diameters of oxovanadium(IV) complex anchored SBA-15 in com-
parison with those of copper (II) complex immobilized materials
Fig. 4. FT-IR spectra of (a) CM–SalenH2, (b) Cu–CM–Salen, (c) VO–CM–Salen, (d)
Cu–Salen–SBA and (e) VO–Salen–SBA.
(Table 1). Furthermore, the amino groups were not completely
depleted by chloromethyl in the Cu–Salen–SBA material. SEM-EDX
results (Supplementary Fig. S6) confirm this fact, since the molar
ratio of N/Cu was 14.4 (N/V = 13.8), which was much higher than
the theoretical value of 4:1. Moreover, the molar ratio of Cu/Cl in
Cu–Salen–SBA was 1:2.1, suggesting the possibility that the amino
groups bound to the copper atoms instead of producing the nucle-
ophilic substitution on the benzyl halide (Scheme 1). This probably
leads to some extent of leaching of the copper(II) species to the
solution during the progress of the reaction. The binding energy of
Cu 2p1/2 (933.8 eV) in unsupported copper complexes shifted to a
lower value of 933.0 eV after immobilization. It can be speculated
that the amino groups coordinated to copper and thus increased the
electronic density around copper atoms (Supplementary Fig. S7).
On the contrary, chlorine was not detected in the VO–Salen–SBA
material by EDX analysis (Supplementary Fig. S8) and the binding
energy of vanadium(IV) (V 2p3/2, 516.4 eV) was almost unchanged
after immobilization. This result indicates that all the oxovanadium
complexes were covalently anchored on SBA-15 matrix.
Fig. 5. UV–vis spectra of (a) CM–SalenH2, (b) Cu–CM–Salen, (c) VO–CM–Salen and
(
d) Cu–Salen–SBA.
3.5. Catalytic properties
The catalytic performances of various catalysts are shown in
Table 2. When the oxidant air was used in combination with CH CN
3
as the solvent (Entries 1–10), all catalysts were active for the aerobic
oxidation of styrene. The fresh VO–Salen–SBA catalyst showed high
−
1
activity with 78.6% conversion of styrene after 8 h (TOF 63.0 h ),
while the fresh Cu–Salen–SBA catalyst showed low activity with
−
1
2
6.9% conversion after 8 h (TOF 37.4 h ). Moreover, the catalytic
performances of the recycled catalysts also differed significantly.
The catalytic activity of Cu–Salen–SBA decreased rapidly after the
first run. To test for leaching, we filtered the catalyst at the reac-
◦
tion temperature (80 C) after 2 h (5.0% styrene conversion). At this
time, half the volume was filtered and the resulting clear solution
was allowed to react. The percentage of leaching was estimated by
comparing the time–conversion plot of the twin reactions with and
Fig. 6. TG and DTA curve of Cu–Salen–SBA.

Y. Yang et al. / Applied Catalysis A: General 381 (2010) 274–281
279
Scheme 1. Schematic outlines of syntheses of (a) homogeneous metal complexes of copper(II) and oxovanadium(IV) and (b) anchored metal complexes.
without solid (not shown). It was found that after the hot filtration,
the mother liquor reacted further at roughly the same rate as that
observed when the Cu–Salen–SBA catalyst was not filtered, which
indicates that the leaching of active species should be the main
reason for the rapid deactivation of supported copper(II) catalysts.
According to the literature [36,37] and to our experimental results,
metal complexes heterogenized by a coordination bond are not sta-
ble during the process of preparation and can be easily leached into
solutions. But VO–Salen–SBA could be successfully recycled for four
times without significant decrease of activity or selectivity under
test conditions. The absence of vanadium in the filtrate solution of
subsequent runs and the inactivity of filtrate indicate that the sup-
ported vanadium catalyst is stable and that any leaching of active
species into solution is insignificant. The ICP analysis also showed
that the vanadium content for the used catalysts just decreased
slightly (0.295 mmol/g of vanadium after 4 cycles) compared with
the fresh one. These results suggest that the large majority of the
catalysis is carried out by a truly heterogeneous vanadium catalyst.
For comparison, the catalytic properties of the homogeneous
complexes were also examined under identical reaction conditions.
When VOSO4 or VO–CM–Salen was used as catalyst, 42.9 or 69.0%
conversion of styrene (Table 2, Entries 1 and 2) could be obtained
after 8 h; such values were much lower than the results obtained
when we used supported oxovanadium catalyst VO–Salen–SBA.
Similarly, the fresh Cu–Salen–SBA showed higher activity than the
homogeneous Cu(CH COO)2 and Cu–CM–Salen catalysts (Table 2,
3
Entries 7–9). The same trend was also observed when using H O as
2
2
oxidant for both copper(II) and oxovanadium(IV) catalysts (Table 2,
Entries 11–13). These results suggest that the heterogenization of
homogeneous metal complexes probably isolatedtheactive species
and prevented their dimerization. The enhanced activity of het-
erogeneous catalyst may also be due to the synergistic beneficial
catalytic interaction between the metal complexes and the SBA-15
support. The synergistic effect was also observed in other catalytic
systems. For example, González-Arellano et al. [38] reported that
the heterogenized Rh(I) and Ir(I) metal complexes with chiral tri-
aza donor ligands showed much higher activity than that observed
under homogeneous conditions for the hydrogenation reactions.
The influence of the oxidants on the catalytic properties of cop-
per(II) and oxovanadium(IV) catalysts was also investigated. With

2
80
Y. Yang et al. / Applied Catalysis A: General 381 (2010) 274–281
Table 2
Catalytic activity and selectivity of various catalysts in the oxidation of styrene.
Entry
Catalyst
Styrene conversiona (%)
TOFb (h−1
)
Selectivityc (mol%)
So
Bza
Others
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
VOSO4
42.9
69.0
78.6
82.3
69.6
64.6
16.8
22.5
26.9
10.2
12.8
53.2
84.1
9.0
27.5
63.4
63.0
−
10.1
58.2
71.2
62.5
55.4
51.6
57.8
59.5
48.0
46.7
7.3
85.2
41.8
28.3
30.5
42.6
42.8
41.4
40.4
26.6
25.3
92.7
83.3
83.3
11.1
57.4
45.2
4.7
−
VO–CM–Salen
VO–Salen–SBA (1st)
VO–Salen–SBA (2nd)
VO–Salen–SBA (3rd)
VO–Salen–SBA (4th)
Cu(CH3COO)2
0.5
7.0
2.0
5.6
0.8
0.1
25.4
28.0
−
−
54.7
7.6
Cu–CM–Salen
27.7
37.4
−
Cu–Salen–SBA (1st)
Cu–Salen–SBA (2nd)
VOSO4
VO–CM–Salen
VO–Salen–SBA
Cu(CH3COO)2
1
1
1
1
1
1
1
8.2
51.2
67.4
4.0
87.2
134.6
9.6
7.1
4.8
−
11.9
88.9
15.3
13.5
Cu–CM–Salen
Cu–Salen–SBA
83.7
96.9
27.3
41.3
a
◦
Reaction conditions (Entries 1–10): styrene 1.14 ml (10 mmol), CH3CN 10 ml, flow of air 80 ml/min, isobutyraldehyde 25 mmol, temperature 80 C and duration 8 h;
◦
Reaction conditions (Entries 11–16): styrene 1.14 ml (10 mmol), CH3CN 10 ml, 30% H2O2 30 mmol, temperature 80 C and duration 8 h.
b
−1
TOF, h : (turnover frequency) moles of substrate converted per mole metal ion per hour.
c
So, styrene oxide, Bza, benzaldehyde and others: including benzoic acid, phenylacetadehyde and 1-phenylethane-1,2-diol.
H O as the oxidant, benzaldehyde was dominant in all cases.
Acknowledgements
2
2
According to the literature [39], the high yield of benzaldehyde
is possibly due to further oxidation of styrene oxide by nucle-
ophilic attack of H O on styrene oxide followed by cleavage of the
Financial assistance from the National Basic Research Program
of China (2004CB217804) and from the National Natural Science
Foundation of China (20673046) is gratefully acknowledged. We
also greatly appreciate the suggestions from editors and referees
concerning the improvement of this paper.
2
2
intermediate hydroperoxistyrene. This result is in good agreement
with H O oxidized styrene reactions catalyzed by polystyrene sup-
2
2
ported Cu(hpbmz)2 and VO(hpbmz)2 catalysts [8]. When air was
used as the oxidant, the selectivity to styrene oxide was relatively
higher than that of benzaldehyde except for VOSO4 catalyst. This
indicates that the product distribution depends on oxidants and is
not related to metal species. We also found that the VO–Salen–SBA
catalyzed reactions produced more styrene oxide (56.0% yield) than
their homogeneous counterparts, which suggests that heteroge-
nization results in elevated styrene oxide selectivity in aerobic
oxidation of styrene. Cu–Salen–SBA, however, displayed lower
yields of styrene oxide (12.9%) with air as oxidant and of benzalde-
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2010.04.018.
References
hyde (43.8%) with H O2 as oxidant compared to its homogeneous
2
[1] G. Grigoropoulou, J.H. Clark, J.A. Elings, Green Chem. 5 (2003) 1–7.
[2] B.S. Lane, K. Burgess, Chem. Rev. 103 (2003) 2457–2474.
analogues and VO–Salen–SBA. In this aspect, Cu–Salen–SBA is an
inferior oxidation catalyst under all the conditions explored. The
better selectivity of supported oxovanadium complexes can be
attributed to the homogeneity of active sites covalently anchored
on the internal surfaces of hexagonal pores, since unselective
reactive sites (external surface sites) would disproportionately con-
tribute to selectivity and would erode the selectivity of the catalyst,
as demonstrated by Becker and Gagné [40].
[
[
3] C. Bowers, P.K. Dutta, J. Catal. 122 (1990) 271–279.
4] P.-P. Knops-Gerrits, D.D. Vos, F. Thibaut-Starzk, P.A. Jacobs, Nature 369 (1994)
5
43–550.
[5] D.D. Agrawal, R.P. Bhatnagar, R. Jain, S. Srivastava, J. Chem. Soc., Perkin Trans.
(1990) 989–992.
6] M.J. Sabater, A. Corma, A. Domenech, V. Fornés, H. Garcia, Chem. Commun. 1997
1997) 1285–1286.
[7] S. Koner, Chem. Commun. 998 (1998) 593–594.
2
[
(
[
[
8] M.R. Maurya, M. Kumar, U. Kumar, J. Mol. Catal. A: Chem. 273 (2007) 133–143.
9] M.R. Maurya, U. Kumar, P. Manikandan, Eur. J. Inorg. Chem. (2007) 2303–2314.
[
10] M.R. Maurya, A.K. Chandrakar, S. Chand, J. Mol. Catal. A: Chem. 274 (2007)
92–201.
[11] M.R. Maurya, S. Sikarwar, M. Kumar, Catal. Commun. 8 (2007) 2017–2024.
1
4
. Conclusions
[
12] M.R. Maurya, A.K. Chandrakar, S. Chand, J. Mol. Catal. A: Chem. 263 (2007)
27–237.
13] Q.H. Tang, Q.H. Zhang, H.L. Wu, Y. Wang, J. Catal. 230 (2005) 384–397.
2
A series of new copper(II) and oxovandium(IV) complexes with
[
salen ligands have been anchored on SBA-15 matrix and fully
characterized by various techniques. It was found that copper(II)
and oxovandium(IV) complexes were active catalysts for the oxi-
dation of styrene. Heterogenization increased the activity of the
homogeneous catalysts due to site-isolation and cooperative effects
between the SBA-15 support and the metal complexes. The sup-
ported oxovanadium catalyst showed high selectivity to styrene
oxide with air as oxidant and better selectivity to benzaldehyde
[14] M.L. Kantam, B.P.C. Rao, R.S. Reddy, N.S. Sekhar, B. Sreedhar, B.M. Choudary, J.
Mol. Catal. A: Chem. 272 (2007) 1–5.
[
15] Q.H. Tang, Y. Wang, J. Liang, P. Wang, Q.H. Zhang, H.L. Wan, Chem. Commun. 4
2004) 440–441.
[16] X.J. Meng, K.F. Lin, X.Y. Yang, Z.H. Sun, D.Z. Jiang, F.S. Xiao, J. Catal. 218 (2003)
60–464.
(
4
[
[
17] L. Canali, D.C. Sherrington, Chem. Soc. Rev. 28 (1999) 85–93.
18] G.-J. Kim, J.-H. Shin, Catal. Lett. 63 (1999) 83–90.
[19] T. Katsuki, Coord. Chem. Rev. 140 (1995) 189–214.
[20] K.J. O’Connor, S.J. Wey, C.J. Burrows, Tetrahedron Lett. 33 (1992) 1001–1004.
[21] M.R. Maurya, A. Kumar, J. Mol. Catal. A: Chem. 250 (2006) 190–198.
[22] M. Salavati-Niasari, A. Amiri, Appl. Catal. A: Gen. 290 (2005) 46–53.
when using H O2 as oxidant. Moreover, the heterogenized oxo-
2
vanadium catalyst was quite stable and could be recycled many
times. The excellent performance of heterogenized oxovanadium
catalyst can be attributed to the homogeneity of active species as
well as its covalent bonding to the interior mesoporous channels.
[23] S.P. Varkey, C. Ratnasamy, P. Ratnasamy, J. Mol. Catal. A: Chem. 135 (1998)
295–306.
[
[
[
24] R. Liu, Y. Zhang, P.Y. Feng, J. Am. Chem. Soc. 131 (2009) 15128–15129.
25] R. Liu, X. Zhao, T. Wu, P.Y. Feng, J. Am. Chem. Soc. 130 (2008) 14418–14419.
26] P. Sutra, D. Brunel, Chem. Commun. 21 (1996) 2485–2486.

Y. Yang et al. / Applied Catalysis A: General 381 (2010) 274–281
281
[27] N.K.K. Raj, S.S. Deshpande, R.H. Ingle, T. Raja, P. Manikandan, Catal. Lett. 98
[35] C. Jin, W.B. Fan, Y.J. Jia, B.B. Fan, J.H. Ma, R.F. Li, J. Mol. Catal. A: Chem. 249 (2006)
(
2004) 217–223.
23–30.
[
[
28] M.J. Jia, A. Seifert, W.R. Thiel, Chem. Mater. 15 (2003) 2174–2180.
29] T. Joseph, M. Hartmann, S. Ernst, S.B. Halligudi, J. Mol. Catal. A: Chem. 207 (2004)
[36] A.R. Silva, M.M.A. Freitas, C. Freire, B. de Castro, J.L. Figueiredo, Langmuir 18
(2002) 8017–8024.
1
31–137.
[37] C. Baleizão, B. Gigante, M.J. Sabater, H. Garcia, A. Corma, Appl. Catal. A: Gen. 228
(2002) 279–288.
[
[
30] G.G. Mohamed, Spectrochim. Acta Part A 64 (2006) 188–195.
31] E.F. Murphy, L. Schmid, T. Bürgi, M. Maciejewski, A. Baiker, Chem. Mater. 13
[38] G. González-Arellano, A. Corma, M. Iglesias, F. Sánchez, Inorg. Chim. Acta 357
(2004) 3071–3078.
(
2001) 1296–1304.
[
[
32] A. Kilic, E. Tas, B. Deveci, I. Yilmaz, Polyhedron 26 (2007) 4009–4018.
33] M.R. Maurya, A.K. Chandrakar, S. Chand, J. Mol. Catal. A: Chem. 270 (2007)
[39] M.R. Maurya, A.K. Chandrakar, S. Chand, J. Mol. Catal. A: Chem. 263 (2007)
227–237.
2
25–235.
[40] J.J. Becker, M.R. Gagné, Acc. Chem. Res. 37 (2004) 798–804.
[34] N.F. Choudhary, P.B. Hitchcock, G.J. Leigh, Inorg. Chim. Acta 310 (2000) 10–20.