3
16
Z.-G. Liu et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 315–321
Table 1
2
. Experimental
The surface areas and pore volume of Co–N–C/SiO2 catalysts.
2
2.1. Synthesis of cobalt
Sample
BET surface area (m /g)
Pore volume (ml/g)
(
II)5-(4-carboxyphenyl)-10,15,20-triphenyl porphyrin (CoTPP)
SN
30.8
11.3
14.9
24.6
19.5
17.6
0.092
0.013
0.019
0.031
0.023
0.016
Co–N–C-300/SiO2
Co–N–C-400/SiO2
Co–N–C-500/SiO2
Co–N–C-600/SiO2
Co–N–C-700/SiO2
The compound was synthesized according to the literature
7]. Typically, 4.69 g of distilled pyrrole was added drop-
[
wise into a three-neck flask containing a mixture of 250 mL
propanoic acid, 5.56 g benzaldehyde and 2.62 g 4-carboxy benzal-
dehyde, and then heated to reflux for 30 min. And the resultant
product was cooled overnight, then filtered and purified. 5-
2
.5. Measurement of catalytic activity
(
1
4-Carboxyphenyl)-10,15,20-triphenyl porphyrin was obtained.
.0 g of the as-synthesized sample was dissolved in 100 mL N,N-
The selective oxidation of ethylbenzene with molecular oxygen
was conducted in a 50 mL autoclave. 10 mL of ethylbenzene and
0 mg catalyst were loaded in the reactor and then sealed and raised
pressure to 8.0 atm with O . Following that, temperature was ele-
vated to 120 C and kept for 5 h. The products are analyzed by gas
chromatography (ShimadzuGC-2014 equipped with a capillary col-
umn (RTX-5, 30 m, ꢁ0.25 mm) with internal standard method using
bromobenzene and 1,4-dichlorobenzene as reference. The recov-
ered catalyst was obtained by centrifugation, then washed with
dimethylformamide (DMF). After 2.5 g of CoCl ·6H O was loaded,
2
2
mixture was heated to reflux under stirring until porphyrin was
exhausted. Cooling overnight, the obtained mixture was filtered
and washed repeatedly with hot water, and the product, denoted
as CoTPP, was achieved.
3
2
◦
2
.2. Synthesis of NH –SiO
2 2
◦
ethanol and dried at 80 C for 12 h.
The amine-functionalized SiO2 were prepared as described
in literature [13]. In a typical procedure, SiO2 (1.0 g) and 15 mL
3
-aminopropyltrimethoxysilane (APTES) were added into 30 mL
3. Results and discussion
◦
methylbenzene under vigorous stirring at 60 C for 24 h. The
white solid was centrifuged and washed with methylbenzene
and ethanol in order to remove solvent and residual APTES. And
3.1. Catalyst characterization
◦
then dried overnight at 80 C, the obtained product was denoted
3.1.1. BET
as SN.
The BET surface areas of Co–N–C/SiO2 together with SN are listed
in Table 1. The surface area of SN is 30.8 m /g, much larger than
2
those of Co–N–C/SiO . Evidently, metalloporphyrin and/or their
2
2
.3. Synthesis of Co–N–C/SiO2 catalysts
heat treatment residual have leaded to the elimination of sur-
face area by blocking or covering the pores in SN. Moreover, when
Co–N–C/SiO2 catalyst was prepared by heat treatment of
◦
◦
heat temperature is in the range from 300 C to 700 C, the surface
supported metalloporphyrin. Supported metalloporphyrin was
prepared as following. 0.01 g CoTPP and 0.10 g SN were dispersed
in 20 mL of dichloromethane at 40 C for 24 h. After centrifugation
and drying at 80 C overnight, supported metalloporphyrin was
achieved, denoted as CoTPP/SiO , and heated in nitrogen atmo-
sphere in the range from 300 C to 800 C for 1 h, respectively. And
the products are denoted as Co–N–C-X/SiO2 (where X means heat
temperatures). For example, the sample heated at 500 C is denoted
as Co–N–C-500/SiO2.
2
area of Co–N–C/SiO2 initially keeps increasing from 11.3 m /g of
Co–N–C-300/SiO , and after Co–N–C-500/SiO2 reaches 24.6 m /g,
the surface area of Co–N–C-700/SiO is lowered to 17.6 m /g in the
2
2
◦
2
2
◦
end. Apparently, in this process, heat treatment plays an impor-
tant role in determining the surface area and pore volume of
2
◦
◦
Co–N–C/SiO . This may attributed to the formation of new pores
2
◦
in the surface of Co–N–C/SiO2 at low temperature such as 300 C
or 400 C and the pores will be blocked due to the collapse of the
pores after high temperature such as 600 C or 700 C. Moreover,
the carbonized residual, which may plug into the small pores, has
a negative effect on the BET surface area. The reduction of small
pores results in the decrease of surface areas [14].
◦
◦
◦
◦
2.4. Characterization of catalysts
Surface area was measured by nitrogen adsorption/desorption
◦
3.1.2. FT-IR
at −196 C on an Autosorb-6b apparatus from Quanta Chrome
◦
FT-IR spectra of SN, SiO2 and Co–N–C/SiO2 are shown in Fig. 1,
the peaks around 3400 cm
Instruments. The samples were degassed at 160 C for 2 h prior to
−
1
−1
and 1630 cm are attributed to
the adsorption experiments. FT-IR spectra were carried out on a
Vertex 70 (Bruker) Fourier transform infrared spectrometer. UV–vis
diffuse reflectance spectra of solid samples were collected on the
Shimadzu 2450 spectrophotometer. The morphology of samples
was measured by a transmission electron microscopy (TEM, JEM-
silanol group and adsorbed water [15]. C–H bending vibration
in unhydrolyzed-OEt groups are observed between 1350 and
−
1
−
1
1
500 cm [16]. Bands located at 1101, 945, 802 and 476 cm are
associated with the longitudinal-optical (LO) mode and transverse-
optical (TO) mode of Si–O–Si asymmetric bond stretching vibration,
Si–OH stretching vibration, and network Si–O–Si symmetric bond
stretching vibration, respectively [16].
As shown in Fig. 1, the peak at 945 cm is gradually reduced
with the elevation of heat temperature. This is assigned to the elimi-
nation of Si–OH. As a result, the reduction of Si–OH group will cause
the collapse of pores in SN. This is also correlated with the trend of
2
100F) with an electron microscope operating at an 80 kV voltage.
TG–DTA was carried out on a Shimadzu TG/DTA 60 thermal ana-
3
lyzer. The sample was placed in platinum crucible (0.1 cm ) and
−
1
measured under N2 atmosphere with flowing rate of 30 mL/min.
◦
The heat rate was fixed at 10 C/min when heating temperature
◦
was raised from 50 to 800 C. X-ray photoelectron spectroscopy
(
XPS) measurements were performed with a RBD upgraded
PHI-5000C ESCA system (Perkin Elmer) with Mg K␣ radiation
hꢀ = 1253.6 eV) or Al K␣ radiation (hꢀ = 1486.6 eV). The obtained
surface area of Co–N–C/SiO . In addition, except the characteristic
peaks of SiO , there is no more peak to be found. This may be due to
2
(
2
the content of metalloporphyrin residual lower than the sensitivity
of the detector in FT-IR spectrum instrument.
binding energies were calibrated using the C1s peak at 284.6 eV as
reference.