Co-SBA-15-Immobilized NDHPI as a New Composite Catalyst for Toluene Aerobic Oxidation

Article abstract of DOI:10.1007/s10562-016-1967-3

Abstract: A new composite catalyst was constructed by immobilizing active site N,N-dihydroxypyromellitimide (NDHPI) on the co-catalyst Co-doped mesoporous sieve SBA-15 through the chemical bond with the 3-(glycidoxypropyl) trimethoxysilicane used as the s

Full text of DOI:10.1007/s10562-016-1967-3

Catal Lett
DOI 10.1007/s10562-016-1967-3
Co-SBA-15-Immobilized NDHPI as a New Composite Catalyst
for Toluene Aerobic Oxidation
1
1
1
1
2
Xingxing Li · Lulu Guo · Pengcheng He · Xia Yuan · Feipeng Jiao
Received: 17 September 2016 / Accepted: 29 December 2016
©
Springer Science+Business Media New York 2017
Abstract A new composite catalyst was constructed by
immobilizing active site N,N-dihydroxypyromellitimide
catalyst were also investigated. After reuse for three times,
the composite catalyst kept the activity without loss of Co
from SBA-15.
(
NDHPI) on the co-catalyst Co-doped mesoporous sieve
SBA-15 through the chemical bond with the 3-(glycidoxy-
propyl) trimethoxysilicane used as the silylation agent. The
catalyst was characterized by various means. The new cata-
lyst showed significantly higher activity in toluene aerobic
oxidation at 90°C with acetonitrile as a solvent or under
solvent-free condition compared with the NDHPI and co-
catalyst independent system. The effects of the reaction
conditions such as temperature, oxygen pressure, catalyst
amount on the toluene oxidation over the immobilized
Graphical Abstract A new composite catalyst was con-
structed by immobilizing active site N,N-dihydroxypy-
romellitimide (NDHPI) on the co-catalyst Co-doped
mesoporous sieve SBA-15 through the chemical bond with
the 3-(glycidoxypropyl) trimethoxysilicane used as the
silylation agent. The catalyst was characterized by various
means. The catalytic performance of the composite was
evaluated in toluene aerobic oxidation and the reuseability
of the catalyst was also investigated.
O
O
OMe
Si
HON
NO
OHO
O
O
Co
O
OH
CH3
O OH
CHO
CH OH
COOH
2
O
NDHPI-epoxy/Co-SBA-15
+ O
2
+
+
Keywords N,N-dihydroxypyromellitimide · GPTMS ·
Co-SBA-15 · Toluene oxidation
*
*
Xia Yuan
yxiamail@163.com
Feipeng Jiao
jiaofp@163.com
1 Introduction
1
2
College of Chemical Engineering, Xiangtan University,
Xiangtan 411105, Hunan, China
Benzaldehyde and benzyl alcohol are important organic
value-added intermediates that are widely used in dyes,
pharmaceuticals, food stuff and other industries [1–4].
School of Chemistry and Chemical Engineering, Central
South University, Changsha 410083, Hunan, China
Vol.:(0
1
123456789)
3

X. Li et al.
Benzaldehyde is mainly produced from toluene chlorina-
tion followed by hydrolysis in the domestic industry, which
is a serious polluting process and should be modified in
terms of production cost. Compared with the traditional
preparation method, the direct oxidization of toluene to
benzaldehyde and benzyl alcohol with molecular oxygen
used as an oxidant is an environmental-friendly synthesis
route. However, the majority of the oxidation process pro-
ceeds under harsh conditions such as high temperature,
high pressure, large energy consumption, and limitation
due to the recycle of transition metal catalysts [5, 6]. In
industry, liquid-phase toluene oxidation in the SINA pro-
also needs to be reused, immobilization of the metal con-
tents such as M/ZSM-5 [19] and Cosalen/SiO [20] with
2
NHPI were used in the hydrocarbon and alkylarene oxi-
dation. We believe that binding the active site NHPI and
co-catalyst metal component together can actually realize
the reuse of the catalytic system. NHPI incorporated onto
Cu-BTC metal–organic frameworks (BTC=benzene-1,3,5-
tricarboxylic acid) for toluene aerobic oxidation [21] was
a meaningful attempt, but was limited by low activity. We
previously immobilized N,N-dihydroxypyromellitimide
(NDHPI) on pre-grafted glycidoxypropyl chains of SBA-
15, and thereby improved the catalytic performance in the
toluene aerobic oxidation with Cosalen as a co-catalyst
under mild reaction conditions. It is noticeable that the
toluene conversion rate decreased sharply in the absence
of Cosalen [3], but Cosalen is limited by low recyclability
and a strong tendency to form dimers, resulting in deactiva-
tion [22]. In this work, NDHPI was immobilized on the Co-
doped mesoporous sieve SBA-15 through chemical bond-
ing. Then the new composite catalyst was used in toluene
aerobic oxidation and its reusability was investigated.
cess is carried out at 165°C and 1.0 MPa with Co(OAc)
2
used as the catalyst. This route has a toluene conversion
rate of 14–15% and a benzoic acid selectivity of 92–93%,
only a small amount of byproducts (e.g. benzaldehyde and
benzyl alcohol) [3, 7]. With the growing market demand
for benzaldehyde and benzyl alcohol, it is urgent to develop
a new toluene catalytic oxidation system for selective for-
mation of these two substances [5, 8–10]. As reported, a
liquid-phase air oxidation of toluene to benzaldehyde and
benzyl alcohol in the solvent-free or promoter-free condi-
tion and using cobalt tetraphenylporphyrin as the catalyst
has a conversion rate of 8.9% and total selectivity of 60%
2 Experimental
3
at 160°C, 0.8 MPa, and 0.04 m /h airflow [8]. The man-
ganese tetraphenylporphyrin supported on chitosan used
as a catalyst for toluene catalytic oxidation yielded a 5.9%
conversion and 96% selectivity at 195°C and 0.6 MPa [11].
As is well-known, benzaldehyde and benzyl alcohol are
more susceptible to secondary oxidation to benzoic acid,
so reducing the reaction temperature may be an effective
way to improve the selectivity of intermediates from the
dynamic perspective.
2.1 Preparation of Catalysts
2.1.1 Preparation of Co‑SBA‑15
In
a typical procedure, P123 [triblock copolymer
(PEO PPO PEO )] (3.98 g) and Co(OAc) ·4H O
2
0
70
20
2
2
(0.25 g) were dissolved in a HCl solution (120 mL, pH 1)
under stirring at 40°C to form a homogeneous mixture.
Then the mixture was added with tetraethoxysilane (TEOS)
(9 mL) under vigorous stirring for 24 h, then transferred to
a Teflon reaction kettle, and crystallized at 100°C for 24 h.
The mixture was stirred and evaporated to a viscous mate-
rial, which was then dried overnight at 100°C. Finally, the
polymeric structure-directing agent was removed by calci-
nation at 550°C for 6 h. The resulting carrier was coded as
Co-SBA-15.
N-hydroxyphthalimide (NHPI) and its analogues are
known as radical-generating agents, the O–H bonds break
down easily to form phthalimide N-oxyl (PINO) radicals.
Under mild conditions, PINO abstracts a hydrogen atom
from the substrate to activate the carbon hydrogen bond,
thus promoting the oxidation of hydrocarbon [12–15].
Together with the non-toxicity and availability, it attracts
attention from researchers. As reported, a newly-developed
oxidizing system NHPI/O /Co(II) is able to efficiently cata-
2
lyze the oxidation of various organic compounds under
mild conditions and at moderate oxygen pressure and tem-
perature [16]. Metal salts as co-catalysts play an impor-
tant role in the NHPI-catalyzed oxidation with molecular
oxygen. Owing to large dosage repuired and dissolubil-
ity in polar solvents, more attention has been paid to the
reuse of NHPI immobilization in recent years. NHPI has
been immobilized on silica gels [17], in ionic liquid [18],
aminiomethyl polystyrene and chloromethyl polystyre-
nevia amide or ester bonds [6]. Since metal co-catalyst
2.1.2 Preparation of NDHPI‑Epoxy/Co‑SBA‑15
First, NDHPI was synthesized according to a reported
method [23]. Then NDHPI (1.00 g) and 3-(glycidoxypro-
pyl) trimethoxysilicane (GPTMS) (2.83 g) were added into
ethyl acetate (120 mL). The mixture was refluxed under
a nitrogen atmosphere and at 80°C for 24 h. After reac-
tion, the resulting mixture was centrifuged and the pre-
cipitate was washed with dichloromethane (150 mL) to
form NDHPI-epoxy. Secondly, Co-SBA-15 (4.00 g) was
1
3

Co-SBA-15-Immobilized NDHPI as a New Composite Catalyst for Toluene Aerobic Oxidation
suspended in ethyl acetate (120 mL). The mixture was
stirred fully, added with the NDHPI-epoxy (1.84 g), and
refluxed for 24 h. After cooling to room temperature, the
solids were collected by filtering and washed with an ether
and chloroform mixed solution (volume ratio is 1:1) for
several times. Finally, the resulting product was vacuum-
dried at 50°C for 24 h to form NDHPI-epoxy/Co-SBA-15
with 150 mL of chloroform and ether, and vacuum-dried
at 60°C for 24 h to form the recycled catalyst. The toluene
conversion and product selectivity were calculated based
on an Agilent GC-6820 instrument (HP-Innowax capil-
lary column, 30 m×0.32 mm×0.5 μm, flame ionization
detector, N as carrier gas). Cyclohexanone was used as the
2
internal standard. The injection and detector temperatures
were both 250°C. The column was maintained initially at
(
Scheme 1).
8
0°C for 2.5 min, then heated to 220°C at a rate of 20°C/
2
.2 Characterization of Catalysts
min, and kept at this temperature for 9 min.
Fourier transformed infrared spectra (FTIR) of the cata-
lysts were recorded on a Thermo Nicolet 380 spectrom-
eter in KBr disks. Their porous structures were deter-
mined by analyzing the nitrogen adsorptions at 77 K in a
NOVA-2200e Quanta chrome apparatus. Scanning electron
microscopy (SEM) images were recorded on a Jeol JSM-
3 Results and Discussion
3.1 Characterization of Catalysts
3.1.1 FTIR
6
3
610LV instrument equipped with a tungsten filament at
0 kV. Transmission electron microscopy (TEM) images
were taken on a JEM-2100 microscope. Cross-polarization
magic angle spinning (CP-MAS) nuclear magnetic reso-
nance (NMR) spectra were recorded on an Agilent DD2500
instrument at a frequency of 99.29 MHz. The elemental
composition was characterized with X-ray photoelectron
spectroscopy (XPS) in Kratos AXIS UltraDLD. A Thermo
Jarrell Asch IRIS Advantage 1000 inductively coupled
plasma (ICP)-atomic spectroscopy was used to analyze
metal Co contents. CHN elemental analysis was carried out
on Elementar Vario EL ananlyzer.
The FTIR spectra of NDHPI, unloaded Co-SBA-15, and
NDHPI-epoxy/Co-SBA-15 are shown in Fig. 1. The peaks
−
1
at 1083, 792 and 461 cm correspond to the asymmetric
stretching vibration, symmetric stretching vibration and
bending vibration of Si–O–Si, respectively, in the frame-
work of Co-SBA-15. The C–H stretching peak at 2860 and
−
1
2935 cm both in spectrum a and b illustrated that not
all of the organic template has been removed. Spectrum c
−
1
shows the signals at 3508 and 3401 cm that are attributed
to the asymmetric and symmetric stretching vibrations of
the N–OH on NDHPI, respectively. In comparison of spec-
−1
2
.3 Catalytic Procedures
tra c and d, the weak peaks at 3043 and 2929 cm cor-
respond to the asymmetric and symmetric stretching vibra-
Toluene (5.0 g), NDHPI-epoxy/Co-SBA-15 (1.43 g) and
acetonitrile (4.0 g) were added into a 50-mL Teflon-lined
and magnetically-stirred autoclave equipped with a block
heater and a thermometer. The reaction proceeded at 90°C
under an oxygen atmosphere (1.60 MPa) for 7 h. After
that, the catalyst was separated by filtration, then washed
tions of –CH , respectively, which confirms the presence
2
of glycidoxypropyl groups on NDHPI-epoxy. Nearly all
absorption peaks of NDHPI-epoxy appear on spectrum e,
which illustrates the successful immobilization of NDHPI-
epoxy on Co-SBA-15.
O
O
O
O
OMe
Si
2
4h reflux
HON
NO
OMe
Si OMe
OMe
O
O
HON
NOH
+ MeO
Ethyl acetate
O
O
O HO
NDHPI-epoxy
OMe
O
O
NDHPI
GPTMS
O
O
OMe
Si
HON
O
NO
O HO
+
O
24h reflux
Co
O
OH
Co
O HO H O OHOH
O OH
Toluene
O
Co-SBA-15
NDHPI-epoxy/Co-SBA-15
Scheme 1 The synthesis routes of NDHPI-epoxy/Co-SBA-15
1
3

X. Li et al.
3
volume decreased from 0.756 to 0.543 cm /g, which fur-
ther confirm the successful immobilization of NDHPI on
Co-SBA-15.
a
2
860
2
935
b
The heterogeneous catalyst NDHPI-epoxy/Co-SBA-15
was also characterized by elemental analyses. The C, H, N
mass percent were 14.14, 2.27 and 2.18%, respectively. The
loading amount of NDHPI was calculated as 0.8 mmol/g.
The final Co mass content was 1.53% quantified by the ICP
analysis.
c
d
e
2929
3
043
792
5
68
3508
3401
461
3
.1.3 29Si(CP‑MAS) NMR and XPS
1083
The silica framework of the materials was characterized via
4
000
3500
3000
2500
2000
11500
1000
500
2
9
-
Si CP-MAS NMR experiments. Mesoporous silica is gen-
erally characterized by the presence of three silicon sites
Wavenumber/cm
2
3
4
Q , Q and Q , representing the species Si(OSi) (OH) ,
2
2
Fig. 1 FTIR spectra of SBA-15 (a), Co-SBA-15 (b), NDHPI (c),
NDHPI-epoxy (d) and NDHPI-epoxy/Co-SBA-15 (e)
2
9
Si(OSi) (OH) and Si(OSi) , respectively [24]. The Si CP-
3
4
MAS NMR spectrum of NDHPI-epoxy/Co-SBA-15 shows
three main peaks centered at −93, −102 and −111 ppm
2
3
4
0
0
0
0
0
0
0
0
0
.40
.35
.30
.25
.20
.15
.10
.05
.00
(
Fig. 3a), corresponding to Q , Q and Q silicon reso-
nances. The alkyl linkage to surfaces can be characterized
1
2
3
by T , T and T resonances, and normally, the Si–C bond
resonances at −46, −56 and −67 ppm stand for the reac-
tions of 1, 2 and 3 methoxy groups, respectively, with the
silanols on surfaces [25]. As the dominant resonance is at
-0.05
1
0
100
−
56 ppm, two methoxy groups of one GTPMS molecule
Pore diameter(nm)
react with the surface of Co-SBA-15, and another methoxy
group is unchanged.
Figure 3b shows the XPS spectra of N1s and Co(2p)
of NDHPI-epoxy/Co-SBA-15. As showed in Fig. 3b-2,
the two larger bands centered at 781.3 and 797.6 eV with
the respective satellites at 787.1 and 803.8 eV are related
NDHPI-epoxy/Co-SBA-15
Co-SBA-15
0
.0
0.2
0.4
0.6
0.8
1.0
2
+
to those of Co in CoO from the Co(2p) spectrum of the
catalyst [26, 27]. The XPS spectra of N1s fit to two peaks
at BE values of 400.7 and 401.3 eV (Fig. 3b-1), which are
assigned to N–O–C and N–OH, respectively. However, the
peak area ratio of N–O–C to N–OH is 4:1, which indicates
that the majority of N–OH groups were condensed with
epoxy groups after immobilization, but a small part was
left for activation reaction because of the steric effect [3].
relative pressure,P/Po
Fig. 2 N adsorption–desorption isotherms and BJH pore size distri-
2
bution of Co-SBA-15 and NDHPI-epoxy/Co-SBA-15
3
.1.2 N Adsorption and Elemental Analysis
2
As showed in Fig. 2, the N adsorption–desorption iso-
2
therms of Co-SBA-15 and NDHPI-epoxy/Co-SBA-15
both pertain to type IV of the International Union Pure
and Applied Chemistry (IUPAC) Classification, as they
have obvious H1 hysteresis loops and belong to the typi-
cal adsorption curve of mesoporous structures. The Bar-
rett–Joyner–Halenda (BJH) pore size curves show that both
Co-SBA-15 and NDHPI-epoxy/Co-SBA-15 display narrow
pore size distributions with a center peak, indicating the
immobilization of NDHPI did not destroy the pore struc-
ture of Co-SBA-15. After the immobilization, the specific
3.1.4 SEM and TEM
In the SEM images, the Co-SBA-15 (Fig. 4a) and NDHPI-
epoxy/Co-SBA-15 (Fig. 4b) are both wheat-like. In conclu-
sion, the immobilization of NDHPI-epoxy did not destroy
the structure of Co-SBA-15. In the TEM images, the Co-
SBA-15 (Fig. 4c) has a highly-ordered two-direction hex-
agonal structure, which is typical of the SBA-15. After
NDHPI loaded on Co-SBA-15, the structure of the carrier
was clearly maintained.
2
surface area decreased from 453 to 296 m /g and the pore
1
3

Co-SBA-15-Immobilized NDHPI as a New Composite Catalyst for Toluene Aerobic Oxidation
100
A
B2
CoO
satellites
781.3
Q3
Co 2p
8
0
0
0
0
7
97.6
787.1
790
8
03.8
6
4
2
820
810
800
780
770
Q4
B1
Q2
N 1s
400.7
T2
401.3
0
-
20
410
405
400
395
390
5
0
0
-50
-100
ppm
-150
-200
-250
B.E.(eV)
Fig. 3 29Si CP-MAS NMR (a) and XPS spectra of N1s (b-1) and Co 2p (b-2) of NDHPI-epoxy/Co-SBA-15
Fig. 4 SEM and TEM images of Co-SBA-15(a, c) and NDHPI-epoxy/Co-SBA-15(b, d)
3
.2 Activity Evaluation
in the solvent-free condition. Some contrast tests were
also carried out for comparison. All the results are listed
in Table 1. Co-SBA-15 as a carrier was not reactive in the
oxidation reaction. NDHPI catalyzed the toluene oxidation
and the use of Cosalen improved the toluene conversion
The catalytic performance of the NDHPI-epoxy/Co-
SBA-15 was evaluated in the toluene aerobic moderate
oxidation at 90°C with acetonitrile as the solvent or even
1
3

X. Li et al.
Table 1 The performances of different catalysts in toluene oxidation
CH
3
CHO
CH
2
OH
COOH
+
O2
+
+
A
B
C
Entry
Catalyst
N %
Conv. %
Selectivity %
N–OH mmol/g
TONg
A
B
C
1a
2
Co-SBA-15
NDHPI
NDHPI+Cosalen
NDHPI/SBA-15+Cosalen
NDHPI-epoxy/Co-SBA-15
NDHPI-epoxy/Co-SBA-15
B-PS-NHPI
–
–
–
–
–
–
–
b
4.71
4.71
1.98
2.18
2.62
1.13
5.1
7.6
14.8
22.1
29.9
8.5
72.2
59.6
37.3
24.4
8.4
14.7
11.7
8.7
5.7
2.3
52
13.1
28.7
54.0
69.9
89.3
17
3.4
3.4
0.7
0.8
0.9
0.81
0.8
1.2
11.6
15.2
18.3
6.2
3
4
5
6
c
d
e
f
7
31
Reaction conditions: 5 g (55 mmol) toluene, 2 mol% NDHPI, 0.2 mol% Cosalen, 4 g acetonitrile, 1.60 MPa O , 90°C, 7 h, mol%:n/n(toluene)
2
a
Contrast test used only Co-SBA-15 as catalyst
In the absence of Cosalen
b
c
1
.43 g NDHPI/SBA-15, quoted from previous work [3]
d
e
f
1
.43 g NDHPI-epoxy/Co-SBA-15 as catalyst
1
.43 g NDHPI-epoxy/Co-SBA-15, in the absence of solvent
Quoted from reference [6]
g
TON, turn over number, moles of substrate converted per mole of N–OH
just as reported elsewhere [3]. NDHPI-epoxy/Co-SBA-15
was more active in the toluene oxidation, as it improved the
toluene conversion to 22.1% and the total selectivity of ben-
zaldehyde and benzyl alcohol to 30.1%. Surprisingly, the
toluene conversion was improved to 29.9% in the absence
of solvent even though the total selectivity decreased to
some extent. The turnover number (TON) of the NDHPI-
epoxy/Co-SBA-15 was significantly higher compared with
the co-catalyst system and two references [3, 6], indicating
the high efficiency of the composite catalyst, which resulted
from the combined action of N–OH active site and the Co
doped in the framework of SBA-15.
conversion
1
00
80
benzaldehyde and benzyl alcohol 100
benzoic acid
80
60
40
20
0
6
4
2
0
0
0
0
3
.3 Effects of Reaction Conditions on the Performance
of Catalyst
70
75
80
85
90
95
100
Reaction temperature, c
B
The effect of temperature on toluene oxidation over
NDHPI-epoxy/Co-SBA-15 is shown in Fig. 5. As
expected, the conversion of toluene increased with reac-
tion temperature, however, the total selectivity of ben-
zaldehyde and benzyl alcohol decreased all along, the
selectivity of benzoic acid went up with increasing tem-
perature. These phenomena could be explained by the
followed reasons: at low temperature, the energy was not
sufficient for the activation of oxygen molecules, toluene
Fig. 5 Effect of reaction temperature on oxidation of toluene.
Condition: 5 g (55 mmol) toluene, 1.43 g NDHPI-epoxy/Co-
SBA-15 (2 mol% NDHPI), 4 g acetonitrile, 1.60 MPa O , 7 h,
2
mol%:n(NDHPI)/n(toluene)
was hardly oxidized. As increasing temperature, oxygen
molecular was apt to form the oxotransition metal reac-
tive intermediate to oxidize toluene. So the conversion
1
3

Co-SBA-15-Immobilized NDHPI as a New Composite Catalyst for Toluene Aerobic Oxidation
toluene was so low that little oxygen could be activated
conversion
benzaldehyde and benzyl alcohol
benzoic acid
70
60
50
40
30
20
10
70
60
50
40
30
20
10
by catalysts, which resulted in the lower conversion.
However excessive oxygen pressure could lead to oxidi-
zation of benzaldehyde and benzyl alcohol into benzoic
acid. To our surprise, when the oxygen pressure increased
to 2.0 MPa, the conversion of toluene decreased obvi-
ously. It may be the higher oxygen pressure accelate the
reaction rate and also accelate the deactivation of catalyst
caused by the products. It’s worth to have an intensive
study to find the reason in our following work. 1.6 MPa
was a proper oxygen pressure in the present work.
Figure 7 shows the effect of the amount of catalyst
on oxidation of toluene. It was clear that the conversion
increased linearly with the increasing amount of catalyst.
However, the total selectivity of benzaldehyde and ben-
zyl alcohol decreased all along, the selectivity of benzoic
acid went up with increasing amount of catalyst, which
was mainly due to the probability of effective collision
increased with catalyst amount increasing. However, with
the conversion increasing, benzaldehyde and benzyl alco-
hol were oxidized into benzoic acid.
0
.8
1.0
1.2
1.4
1.6
1.8
2.0
oxygen pressure,MPa
Fig. 6 Effect of oxygen pressure on oxidation of toluene. Condition:
5
g (55 mmol) toluene, 1.43 g NDHPI-epoxy/Co-SBA-15 (2 mol%
NDHPI), 4 g acetonitrile, 90°C, 7 h, mol%:n(NDHPI)/n(toluene)
increased. Furthermore, at high temperature, benzalde-
hyde and benzyl alcohol were oxidized to benzoic acid
easily, which decreased the total selectivity of benzalde-
hyde and benzyl alcohol and increased the selectivity of
benzoic acid. In addition, taking the conversion of tolu-
ene and the total selectivity of benzaldehyde and benzyl
alcohol into consideration, the temperature of the reac-
tion should be kept at 90 °C.
3
.4 Catalyst Stability and Characterization
of the Recycled Catalyst
To evaluate the stability of NDHPI-epoxy/Co-SBA-15 in
the toluene aerobic oxidation, we recycled the catalyst and
reused the catalyst two times under the same condition as
the fresh catalyst. As showed in Fig. 8, the activity of the
catalyst was not significantly changed and the total selectiv-
ity of benzaldehyde and benzyl alcohol stabilized after two
recycles. ICP showed that the Co mass percent contents
in the catalyst after zero, one and two recycles were 1.53,
The effect of oxygen pressure on oxidation of tolu-
ene was shown in Fig. 6. It was clear that the conversion
increased with increasing oxygen pressure at first, while
the total selectivity of benzaldehyde and benzyl alcohol
decreased. In general, the higher the oxygen pressure, the
higher the dioxygen solubility in the liquid phase. Hence,
at a lower oxygen pressure, the concentration of O in
2
1
.48 and 1.61%, respectively, indicating no loss of Co from
SBA-15.
80
70
60
50
40
30
20
10
0
80
conversion
benzaldehyde and benzyl alcohol 70
benzoic acid
60
100
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
conversion of toluene
selectivity of benzaldehyde
selectivity of benzyl alcohol
selectivity of benzoic acid
90
80
70
50
40
30
20
10
0
60
50
40
30
20
10
0
.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0
Catalyst concentration,mol%
1
2
3
run
Fig. 7 Effect of catalyst concentration (NDHPI concentration) on
oxidation of toluene. Condition: 5 g (55 mmol) toluene, 4 g acetoni-
Fig. 8 Results of recycle experiments in selective oxidation of tolu-
ene. 5 g toluene, 1.43 g recycled NDHPI-epoxy/Co-SBA-15, 4 g ace-
tonitrile, 90°C, 1.60 MPa, 7 h
trile, NDHPI-epoxy/Co-SBA-15 (x mol% NDHPI), 1.60 MPa O , 7 h,
2
mol%:n(NDHPI)/n(toluene)
1
3

X. Li et al.
4
Conclusions
A composite catalyst was prepared by immobilizing
NDHPI on the Co-doped mesoporous sieve SBA-15
through the chemical bond with GPTMS used as the silyla-
tion agent. The composite catalyst NDHPI-epoxy/Co-
SBA-15 was characterized by various means. The existence
of active site N–OH and the Co state was confirmed, and
the mesoporous structure also remained. The composite
catalyst was more active in the toluene aerobic moderate
oxidation at 90°C with acetonitrile as the solvent or under
the solvent-free condition. The TON was significantly
higher than the separate co-catalyst system. The composite
catalyst kept the activity and no Co was lost from SBA-15
after three reaction runs.
a
b
c
a:NDHPI-epoxy/Co-SBA-15
b:r1-NDHPI-epoxy/Co-SBA-15
c:r2-NDHPI-epoxy/Co-SBA-15
4
000
3500
3000
2500
2000
1500
1000
500
-
1
Wavenumber,cm
Acknowledgements This study was financially supported by the
National Natural Science Foundation of China (No. 21376200) and
Hunan 2011 Collaborative Innovation Center of Chemical Engi-
neering & Technology with Environmental Benignity and Effective
Resource Utilization.
Fig. 9 FT-IR spectra of NDHPI-epoxy/Co-SBA-15 (a), r1-NDHPI-
epoxy/Co-SBA-15 (b) and r2-NDHPI-epoxy/Co-SBA-15 (c) (r the
recycled catalyst)
a
b
c
d
Co-SBA-15
NDHPI-epoxy/Co-SBA-15
r1-NDHPI-epoxy/Co-SBA-15
r2-NDHPI-epoxy/Co-SBA-15
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