A Copper(II) Schiff base complex immobilized onto SBA-15 silica for selective oxidation of benzyl alcohol

Article abstract of DOI:10.1007/s11243-013-9795-4

A copper(II) Schiff base complex has been immobilized onto SBA-15 silica through a stepwise procedure and tested as an oxidation catalyst. BET surface area, total pore volume and average pore width of the SBA-15 all decrease after stepwise modification of SBA-15, while the structure of the support remains intact. The molar ratio of Cu²⁺/Schiff base is ca. 1/2 in the synthesized material. Catalytic tests showed that the supported copper complex catalyzes the oxidation of benzyl alcohol with 30 % conversion and 89 % selectivity to benzaldehyde when water is used as the solvent.

Full text of DOI:10.1007/s11243-013-9795-4

Transition Met Chem (2014) 39:233–238
DOI 10.1007/s11243-013-9795-4
A Copper(II) Schiff base complex immobilized onto SBA-15 silica
for selective oxidation of benzyl alcohol
Jingqi Guan Jing Liu
•
Received: 16 November 2013 / Accepted: 13 December 2013 / Published online: 21 December 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A copper(II) Schiff base complex has been
immobilized onto SBA-15 silica through a stepwise pro-
cedure and tested as an oxidation catalyst. BET surface
area, total pore volume and average pore width of the SBA-
waste [2]. With the rapid development of catalyst separa-
tion and preparation technology, the encapsulation of var-
ious transition metal Schiff complexes on insoluble solid
supports to allow for the heterogenization of homogeneous
catalysis has become a widely employed method to address
these problems [3].
1
5 all decrease after stepwise modification of SBA-15,
while the structure of the support remains intact. The molar
2
?
ratio of Cu /Schiff base is ca. 1/2 in the synthesized
material. Catalytic tests showed that the supported copper
complex catalyzes the oxidation of benzyl alcohol with
In recent years, there have been a number of reports
about the use of Cu-based catalysts for the oxidation of
benzyl alcohol to benzaldehyde [4–14]. Ji et al. [5] reported
that Ru–Mn–Fe–Cu–O was an efficient heterogeneous cat-
alyst with high selectivity for benzyl alcohol oxidation
3
0 % conversion and 89 % selectivity to benzaldehyde
when water is used as the solvent.
using O as oxidant. Mahdavi and Mardani prepared a series
2
2
?
of Mn(II) bipy complexes with different loadings of Mn
Introduction
supported on hexagonal mesoporous silica (HMS) for the
oxidation of benzyl alcohol [6]. Spasiano et al. [7] synthe-
Selective oxidation of benzyl alcohol has been extensively
studied, because benzaldehyde is a very important organic
industrial intermediate in the production of perfumes,
pharmaceuticals, dyestuffs and agrochemicals [1]. During
the past decade, considerable effort has been put into the
development of inexpensive, recyclable, highly active and
selective metal-based catalysts to realize large-scale pro-
duction of benzaldehyde. Although homogenous transition
metal complexes exhibit high catalytic activity and selec-
tivity in oxidation reactions, many inherent drawbacks
hamper their industrial application, such as difficulties in
the catalyst regeneration and product separation, deacti-
vation of the catalysts by formation of dimeric peroxo- and
l-oxo species and the need to minimize heavy industrial
sized TiO /Cu(II) to convert benzyl alcohol to benzalde-
2
hyde by photocatalytic oxidation. Meanwhile, to meet the
increasing demand for benzaldehyde and satisfy environ-
mental requirements, considerable effort has been put into
the production of benzaldehyde using water as a solvent.
In the current work, a copper(II) Schiff complex was
grafted onto SBA-15 for the selective oxidation of benzyl
alcohol to benzaldehyde. The effects of various parameters,
including the type of solvent, reaction temperature, reac-
tion time, the amount of H O and the recyclability of the
2
2
catalyst, have been investigated.
Experimental
Catalyst preparation
J. Guan (&) Á J. Liu
Key Laboratory of Surface and Interface Chemistry of Jilin
Province, College of Chemistry, Jilin University,
Changchun 130023, People’s Republic of China
e-mail: guanjq@jlu.edu.cn
SBA-15 was synthesized according to the literature [15]. In a
typical synthesis, Pluronic P123 [(EO) (PO) (EO) ,
20
70
20
Aldrich] (2 g) was dissolved in a mixture of 15 mL of H O
2
123

2
34
Transition Met Chem (2014) 39:233–238
CHO
OH
OH
NH2
CH Cl
2 2
+
(EtO) Si
3
Reflux
N
Si(OEt)3
APTES
OH
Toluene
Reflux
OH (SBA-15)
OH
O
O
O
Si
Si
O
O
N
O
O
Cu(NO3)2
Reflux
Cu
O
Si
N
O
N
O
HO
O
S-SBA-15
Cu-S-SBA-15
Fig. 1 Synthetic route for Cu–S-SBA-15
and 60 mL of 2 M HCl, and then tetraethoxysilane (TEOS)
room temperature for 12 h. The green solid thus formed
was filtered off, washed with methanol using a Soxhlet
apparatus and dried under vacuum to give 0.11 g Cu–S-
SBA-15 (yield of 87 %) (Fig. 1).
(4.68 mL, 0.02 mol) was added under stirring at 40 °C. The
molar composition of the mixture was thus TEOS: 0.017
P123: 6 HCl: 192 H O. Afterward, the resultant mixture was
2
stirred at 40 °C for 24 h, followed by aging under static
conditions at 100 °C for 24 h. Finally, the solid product was
recovered by filtration and washed with distilled water. After
air-drying at ambient temperature overnight, it was calcined
at 550 °C for 8 h to remove the template. The resulting
sample was denoted as SBA-15 (1.0 g, yield of 83 %).
The Schiff base organosilane reagent was prepared as
follows; (3-aminopropyl)triethoxysilane (APTES) (1.33 g,
Catalyst characterization
Powder X-ray diffraction patterns (XRD) were obtained on
a Rigaku D/max-2200 diffractometer (0.2°/min) using Cu
Ka radiation (40 kV, 40 mA). N adsorption–desorption
2
isotherms were measured with a Micromeritics ASAP 2010
instrument at liquid N temperature. Before measurements,
2
6
6
mmol) was refluxed with salicylaldehyde (0.73 g,
the sample was outgassed at 130 °C for 6 h. The surface
area was calculated using the Brunauer–Emmett–Teller
(BET) method. SEM photographs were taken on an FE-
SEM XL-30 field emission scanning electron microscope.
IR spectra were recorded in KBr disks using a NICOLET
impact 410 spectrometer. Metal content was estimated by
inductively coupled plasma atomic emission spectroscopy
(ICP-AES) analysis using a Perkin–Elmer emission spec-
trometer. Elemental analyses were obtained with a Vari-
oEL CHN elemental analyzer.
mmol) in ethanol (10 mL) under N atmosphere at 80 °C
2
for 3 h. The solvent was removed by rotary evaporation
under reduced pressure, giving 1.9 g of the Schiff base
organosilane product (yield of 92 %).
S-SBA-15 was synthesized by a post-synthetic grafting
method. SBA-15 was first dried under vacuum at 100 °C
for 5 h. Then, a portion of dried SBA-15 (0.5 g) was
refluxed in dried toluene (10 mL) with the Schiff base
organosilane (0.14 g, 0.4 mmol) for 10 h. The solid was
filtered off and washed with copious amounts of CH Cl ,
2
2
hexane and ethanol to remove the remaining unsupported
Schiff base organosilane reagent. The resulting yellowish
solid, denoted as S-SBA-15, was dried overnight to give
Catalytic tests
A mixture of the catalyst (0.05 g), benzyl alcohol (3 mL),
30 % H O (4 mL) and solvent (water, acetone or aceto-
0
.45 g of product (yield of 87 %).
2
2
Cu–S-SBA-15 was prepared by adding a solution of
nitrile, 10 mL) was vigorously stirred at 80 °C under
nitrogen. After the required reaction time, the catalyst was
separated by filtration. The products were analyzed with a
Cu(NO ) Á3H O (0.04 g, 0.17 mmol) in methanol (10 mL)
3
2
2
to solid S-SBA-15 (0.122 g) and stirring the suspension at
1
23

Transition Met Chem (2014) 39:233–238
235
a
a
b
c
b
c
2
4
6
0
.0
0.2
0.4
0.6
0.8
1.0
2θ (degree)
Relative Pressure (P/P0)
Fig. 2 XRD patterns of a SBA-15, b S-SBA-15 and c Cu–S-SBA-15
Fig. 4 N adsorption/desorption isotherms of a SBA-15, b S-SBA-15
and c Cu–S-SBA-15
2
a
b
c
2
4
6
8
10
12
14
16
Pore Diameter, D (nm)
Fig. 3 SEM image of Cu–S-SBA-15
Fig. 5 Pore size distributions of a SBA-15, b S-SBA-15 and c Cu–S-
SBA-15
gas chromatograph (Shimadzu, GC-8A) equipped with a
XE60 capillary column and FID detector.
N adsorption/desorption studies
2
Results and discussion
N adsorption–desorption isotherms and pore size distri-
2
XRD and SEM characterization
butions of SBA-15, S-SBA-15 and Cu–S-SBA-15 are
depicted in Figs. 4 and 5, respectively. These isotherms are
classified as type IV, with a sharp capillary condensation
step at high relative pressure and H1 hysteresis loop, which
reveals the presence of large channel-like pores in a narrow
range of size [15]. The BET surface area, pore volume and
pore diameter of SBA-15, S-SBA-15 and Cu–S-SBA-15
are presented in Table 1. The BET surface area and aver-
age pore width significantly decrease after functionali-
zation of SBA-15 with the Schiff base and further
considerably decrease upon reaction with copper. The total
pore volume also decreases after each modification of
SBA-15.
The powder XRD patterns of the synthesized SBA-15,
S-SBA-15 and Cu–S-SBA-15 are shown in Fig. 2. For each
of the samples, three well-resolved peaks can be clearly
observed, indicating that the prepared materials contain
well-ordered hexagonal arrays of one dimensional channel
structure [15]. However, a decrease in intensity of the (1 1
0
) and (2 0 0) peaks for Cu–S-SBA-15 is observed, sug-
gesting the formation of a less ordered mesostructure.
The SEM image of the synthesized Cu–S-SBA-15
(
Fig. 3) shows a worm-like structure, indicating that the
structure of the SBA-15 support has remained intact [15].
123

2
36
Transition Met Chem (2014) 39:233–238
Table 1 Structural and textural parameters of the synthesized
materials
Table 2 Elemental and ICP-AES analyses’ results (%) for S-SBA-15
and Cu–S-SBA-15
2
-1
3
(cm g
-1
)
Sample
S
BET (m g
)
D
p
(nm)
V
p
Sample
C
H
N
Cu
SBA-15
892
336
304
8.9
4.7
3.7
1.30
0.51
0.49
S-SBA-15
6.56 (10.3) 0.66 (12.5) 0.74 (1)
–
S-SBA-15
Cu–S-SBA-15
Cu–S-SBA-15 6.04 (10.2) 0.61 (12.4)
0.69(1) 0.76 (0.53)
The values in the brackets represent the molar ratio of C, H, N and Cu
S
p p
BET surface area, D average pore width, V total pore volume
Table 3 Catalytic data of the benzyl alcohol oxidation over different
catalysts
c
b
a
Catalyst
Conversion (%)
Selectivity (%)
Benzaldehyde
1626
1640
Benzoic acid
2
930
No catalyst
SBA-15
Trace
Trace
Trace
30
–
–
–
–
1632
8
02
a
b
S-SBA-15
Cu–S-SBA-15
–
–
89
11
3
430
464
2 2
Reaction conditions: catalyst (0.05 g), benzyl alcohol (3 mL), H O
c
(
2
4 mL), H O (10 mL), reaction time (6 h) and reaction temperature
(
80 °C)
1
600
1500
1400
1300
1080
analyses (Table 2). The results show that the molar ratio of
N/Cu is close to 2/1, consistent with chelation of two Schiff
base ligands to each copper(II) center, as shown in Fig. 1.
4
000 3500 3000 2500 2000 1500 1000
500
-
1
Wavenumbers (cm )
Fig. 6 FT-IR spectra of a SBA-15, b S-SBA-15 and c Cu–S-SBA-15
Catalytic properties
FTIR studies
The catalytic performance of Cu–S-SBA-15 was evaluated
for the selective oxidation of benzyl alcohol with H O as
2
2
FTIR spectra of SBA-15, S-SBA-15 and Cu–S-SBA-15 are
presented in Fig. 6. In each case, the typical Si–O–Si bands
oxidant. For comparison, the catalytic performances of
SBA-15, S-SBA-15 and a blank test were also investigated,
and the reaction data are listed in Table 3. It is obvious that
the oxidation reaction cannot be carried out without cata-
lysts. In addition, SBA-15 and S-SBA-15 also cannot cat-
alyze this reaction. It is demonstrated that Cu is the
catalytic component for the partial oxidation of benzyl
alcohol.
-
1
are observed at ca. 1,080, 802 and 464 cm due to the
SBA-15 silica framework [16, 17]. The bands around 3,430
-
1
and 1,632 cm are assigned to the bending vibration of
O–H and physically adsorbed water molecules. After
functionalization of SBA-15, the intensity of the O–H
stretching bands is markedly weaker, which is attributed to
the interaction between the surface Si–OH and the Schiff
base organosilane reagent. The presence of C–H vibrations
In order to identify the optimal reaction conditions, the
effects of varying the reaction medium, oxidant amount,
reaction time and temperature were investigated. The
influence of solvent on the catalytic performance was
investigated for water, acetone and acetonitrile, with the
results shown in Table 4. It is interesting to note that Cu–S-
SBA-15 catalyzes the oxidation reaction well when water
is used as the solvent, followed by acetone, while only
6.8 % conversion was achieved when the solvent was
acetonitrile.
-
1
at ca. 2,930 cm plus a series of aromatic ring vibrations
-
1
in the range of 1,350–1,600 cm
and C=N stretching
vibration at around 1,640 cm are observed for S-SBA-
5, indicating grafting of the Schiff base organosilane onto
-
1
1
SBA-15 [16, 17]. In addition, the peak in the IR spectrum
in Fig. 6c due to the C=N stretching vibration is shifted to
-
,626 cm , which is attributed to the coordination of the
1
1
nitrogen with copper(II).
The temperature has a great influence on the catalytic
performance. As shown in Table 4, low temperatures result
in very low benzyl alcohol conversion. For example, only
11 % of benzyl alcohol conversion was achieved at 60 °C.
Increasing the reaction temperature increases the catalytic
Elemental and ICP-AES analyses
Quantification of the functional group loading on SBA-15
was performed using elemental (CHN) and ICP-AES
1
23

Transition Met Chem (2014) 39:233–238
237
Table 4 Catalytic data of the
benzyl alcohol oxidation over
Cu–S-SBA-15
Solvent
2
H O
2
(mL)
T (°C)
Time (h)
Conversion (%)
Selectivity (%)
Benzaldehyde
Benzoic acid
Water
4
4
4
4
4
4
4
4
4
2
3
5
80
80
80
60
70
90
80
80
80
80
80
80
6
6
6
6
6
6
2
4
8
6
6
6
30
10
7
89
*100
*100
88
11
0
Acetone
Acetonitrile
Water
0
11
18
31
11
18
33
16
16
32
12
10
11
0
Water
90
Water
89
Water
*100
90
Water
10
11
7
Water
89
Water
93
Water
91
9
Reaction conditions: catalyst
0.05 g), benzyl alcohol (3 mL)
Water
85
15
(
systems for the substrate to contact the surface of the cat-
alyst and for the product to depart from the surface of the
catalyst.
Table 5 Reuse of Cu–S-SBA-15 for the selective oxidation of benzyl
alcohol
Cycle
Conversion (%)
Selectivity (%)
Benzaldehyde
The recyclability and stability of Cu–S-SBA-15 for
benzyl alcohol oxidation with H O as oxidant and water as
Benzoic acid
2
2
1
2
3
30
29
29
89
90
89
11
10
11
solvent were investigated (Table 5). The solid catalyst was
easily recovered by filtration after each reaction cycle; it
was then washed thoroughly with acetone and water, dried
under vacuum at 60 °C for 12 h and reused for subsequent
cycles. The reused catalyst exhibited a negligible decrease
in the catalytic activity and selectivity over three consec-
utive runs.
2 2
Reaction conditions: catalyst (0.05 g), benzyl alcohol (3 mL), H O
(
2
4 mL), H O (10 mL), reaction time (6 h) and reaction temperature
(
80 °C)
activity. The reaction time also influences on the catalytic
behavior. As can be seen from Table 4, the benzyl alcohol
conversion gradually increases from 11 to 30 %, while the
selectivity for benzaldehyde decreases from *100 to
Conclusions
A heterogeneous catalyst, Cu–S-SBA-15, has been syn-
thesized by covalent anchoring of a copper(II) Schiff
complex onto SBA-15 support. The pore structure of SBA-
8
9 %, when the reaction is increased from 2 to 6 h; while
more prolonged reaction times have relatively little influ-
ence on the benzyl alcohol conversion, but result in the
decrease of selectivity for benzaldehyde.
15 remains intact throughout the synthesis. The reaction
conditions for the oxidation of benzyl alcohol to benzal-
dehyde using this catalyst have been determined. Cu–S-
SBA-15 exhibits better catalytic performance with water as
the solvent, which is attributed to the distinctive pore
structure of SBA-15 and synergistic effect between the
Cu(II) Schiff base complex and the SBA-15 support.
As shown in Table 4, the conversion of benzyl alcohol
remarkably improves from 16 to 30 % after 6 h when the
amount of H O (30 %) is increased from 2 to 4 mL. Fur-
2
2
ther increases in H O have relatively little influence on the
2
2
conversion, but decrease the selectivity for benzaldehyde.
Mahdavi et al. [6] have investigated the effect of sol-
vents on the oxidation of benzyl alcohol with tert-butyl-
Acknowledgments This work was supported by the National Nat-
ural Science Foundation of China (21303069), Jilin province
(201105006) and Specialized Research Fund for the Doctoral Pro-
gram of Higher Education (20100061120083).
2
?
hydroperoxide (TBHP) in the presence of [Mn(bipy)2]
/
HMS (HMS: hexagonal mesoporous silica) and found that
the best conversion (40 %) was achieved in the aprotic
solvent acetonitrile, while the worst conversion (only ca.
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