3
48
Z. Yuan et al. / Journal of Catalysis 370 (2019) 347–356
[
23]. The structure and the properties of the nitrogen-doped car-
scientific ESCA MultiLab-2000 spectrometer with a monochroma-
tized Al K source (1486.6 eV) at constant analyzer pass energy
bon materials supported catalysts are affected by the prepared
methods including the precursors, the pyrolysis temperature and
so on. For example, Co-N was prepared by the pyrolysis of cobalt
x
phtalocyanine and the subsequent acid treatment [24], while
cobalt nanoparticles were formed by the use of other precursors
with the same prepared methods [25].
o-Phenylenediamine (OPDA) is a common polymer monomer
for the preparation of nitrogen-doped carbon materials. In addi-
tion, nitrogen atoms in the as-prepared polymer can anchor transi-
tion metal cations. Nitrogen-doped carbon supported transition
metal (such as iron, cobalt and nickel) catalysts have recently been
used for organic transformations [26–28]. For heterogeneous cata-
lysts, supports with large surface areas and abundant porous struc-
tures facilitate the mass transfer of reaction molecules. Taking the
above thoughts into consideration, nitrogen-doped carbon mate-
rial supported cobalt catalysts with large surface areas were pre-
pared for the synthesis of primary amines via reductive
amination. OPDA was firstly polymerized to generate nitrogen-
a
of 25 eV. The cobalt content was determined by inductively cou-
pled atomic emission spectrometer (ICP-AES) on an IRIS Intrepid
II XSP instrument (Thermo Electron Corporation). Raman spectra
were measured on a confocal laser micro-Raman spectrometer
(Thermo Fischer DXR) equipped with a diode laser of excitation
of 532 nm (laser serial number: AJC1200566). Spectra were
obtained at a laser output power of 1 mW (532 nm), and a 0.2 s
acquisition time with 900 lines/mm grating (Grating serial num-
ber: AJG1200531). Nitrogen physisorption measurements were
conducted at 77 K on a quantachrome Autosorb-1-C-MS instru-
ment. Surface area was determined by the standard BET method
based on the relative pressure between 0.05 and 0.20. The pore size
distribution was calculated using the non-local density functional
theory method.
2.4. General procedure of the reductive amination
containing polymers by the use of H
2
O
2
as the oxidant with silica
The reductive amination of carbonyl compounds was per-
formed in a 50 mL stainless steel autoclave reactor. In a typical
run, benzaldehyde (1 mmol), Co@NC-800 (20 mg), ethanol (8 mL)
and NH ꢀH O (26.5 wt%, 2 mL) were charged into the reactor, and
as the hard template, which was in-situ coordinated with cobalt
cations. After pyrolysis, the silica template together with the
loosely bonded cobalt nanoparticles was washed off.
3
2
then the autoclave reactor was closed. The reactor was flushed
with H
2
for several times to remove air, and then charged with
2
at room temperature. The reaction was then carried
2
. Experimental section
1
MPa H
out at 130 °C for 12 h with a stirring rate of 1000 RPM. After reac-
tion, the reaction mixture was cooled down to room temperature
and then depressurized. Then, the products in the reaction mixture
were detected by gas chromatography by the use of ethylbenzene
as the internal standard. The products were also identified by GC/
MS (Shimadzu GCMS-QP2010) equipped with Agilent capillary col-
umn DB-5MS. The selectivity was calculated with the follow
equations:
2
.1. Materials
The chemicals used in this study were purchased from Aladdin
Chemicals Co. Ltd. (Beijing, China). The solvents used in this study
were supplied from Sinopharm Chemical Reagent Co., Ltd. (Shang-
hai, China). All of the chemicals and solvents were used directly
without any purification.
n
Produced benzylamine
2
.2. Preparation of the catalyst
Selecti
v
v
ityBenzylamine ¼ n
Con
verted benzaldehyde
Co(NO
3
)
2 2
ꢀ6H O (2.9 g, 10 mmol) and OPDA (1.08 g, 0.01 mol)
¼ 2 ꢂ n
Produced Nꢁbenzylidenebenzylamine
Con erted benzaldehyde
were firstly dissolved into distilled water (100 mL). Then, HNO
3
Selecti
ity
Nꢁbenzylidenebenzylamine
was added to make the solvent acidic, and 5 mL of 40 wt% colloidal
silica was added into the mixture. The mixture was then stirred at
a rate of 1000 RMP for 30 min to get a homogenous mixture. Then
n
v
Produced dibenzylamine
Con erted benzaldehyde
Selecti
v
ityDibenzylamine ¼ 2 ꢂ n
H
2
O
2
(20 mL, 30 wt%) was added dropwise into the mixture for
0 min to promote the polymerization of OPDA at room tempera-
ture. After the complete addition of H , the mixture was further
n
v
3
2 2
O
stirred at room temperature for 12 h. The water in the mixture was
removed under a reduced pressure, and the resulted composite
was subjected to the pyrolysis at different temperatures for 2 h
under a nitrogen atmosphere from room temperature to the set
temperature at the heating rate of 3 °C/min. The as-made sample
was then treated with HF solution to simultaneously wash off silica
and loosely bonded cobalt nanoparticles, followed by centrifuga-
tion and washed with distilled water three times. Finally, the
obtained powder was dried at 60 °C in a vacuum oven. The as-
prepared nitrogen-doped carbon materials supported Co nanopar-
ticles were abbreviated as Co@NC-T, in which T represents the
pyrolysis temperature.
2.5. Recycling experiments
After reaction, the Co@NC-800 catalyst was collected with an
external magnet, and the spent catalyst was exhaustively washed
with water and ethanol, respectively. Then it was dried at 50 °C
in a vacuum oven. The spent catalyst was used for the next run
under the same conditions. Other cycles were repeated with the
same procedure.
3
. Results and discussion
3.1. Catalyst preparation and characterization
2.3. Catalyst characterization
Fig. 1 shows the general procedure of the preparation of
Co@NC-T catalysts. Silica was used as the hard template to prepare
the Co@NC-T catalysts with large surface area and abundant por-
ous structure. Firstly, a homogeneous solution of OPDA was pre-
Transmission electron microscope (TEM) was performed on a
FEI Tecnai G20 with accelerating voltage of 200 kV. X-ray powder
diffraction (XRD) measurements were conducted on a Bruker
3 2
pared, and then Co(NO ) was added. Then, the mixture was
advanced D8 powder diffractometer (Cu K
range of 10–80° at a scanning rate of 0.016 °/s. X-ray photoelectron
spectroscopy (XPS) experiments were carried out on a Thermo VG
a
), operating with 2h
stirred at room temperature for 4 h to generate Co–OPDA-
complex. After that, silica was added to the solution of Co–OPDA-
complex, and stirred for another 2 h to get a homogeneous suspen-