C. Liang et al.
MolecularCatalysis453(2018)121–131
2.4. Oxidation of p-cresols
In a typical procedure, NaOH (5.4 g, 135 mmol) was first dissolved
with 20 g EGME in a four-necked round-bottom flask, then 50 mmol
substrate and 0.04 g catalyst (cobalt/substrate molar ratio, 0.4%) were
added to the above solution. The flask was equipped with a condenser,
a stirrer and an air-vent needle for continuous flow of 100 mL min−1
O2. The stirring speed was 600 rpm to minimize the effect of mass
transfer (Fig. S1). The reaction temperature is 80 °C and the reaction
time is 7 h. After the reaction, the CoOx@CN was separated by an ex-
ternal magnet for reuse and the decanted reaction solution was diluted
with water.
A high performance liquid chromatography (HPLC)
(Shimadzu LC-20AT) equipped with a UV detector connected to a C18
reversal pillar (size: 250 × 4.6 mm) was used for product analysis. A
mixture of methanol and water (volume ratio, 40:60) was used as
mobile phase at a flow rate of 1.0 mL min−1. The column temperature
was 40 °C and the UV detection wavelength was 230 nm. HPLC and
high performance liquid chromatography-mass spectrum (HPLC–MS)
were used to test the products and intermediates by comparison with
standard chemicals. The conversions (conv.) of p-cresols, yields (Yi) of
p-hydroxybenzaldehydes, selectivity (Si) of p-hydroxybenzaldehydes
and turnover numbers (TONs) of catalysts are defined as following
equations:
Fig. 2. XRD patterns of Co@CN, CoOx@M, CoOx@G and CoOx@CN.
(+)-glucosamine hydrochloride (GAH), 40 g melamine and 1.3 g cobalt
nitrate hexahydrate (CoNO3·6H2O) were dissolved in deionized water
and then stirred under 80 °C overnight to evaporate the water. The
dried solid was grinded into powder and directly calcined at 800 °C in
N2 atmosphere to give 0.92 g CoOx@CN. For comparisons, CoOx@G and
CoOx@M were made from mixtures of CoNO3·6H2O & GAH and
CoNO3·6H2O & melamine at 800 °C. In a similar way, CN and C were
moles of p-cresols reacted
moles of starting p-cresols
Conv.(%) =
× 100
moles of p-hydroxybenzaldehydes
moles of starting p-cresols
Y(%) =
× 100
× 100
i
moles of p-hydroxybenzaldehydes
moles of p-cresols reacted
made from mixtures of melamine
& GAH and glucose without
Si(%) =
CoNO3·6H2O at 800 °C. Furthermore, the Co@CN, which was made up
of metallic cobalt was acquired by acid treatment with 2 mol/L hy-
drochloric acid for 4 days at 50 °C. The cobalt oxide was removed out
which was confirmed by XRD (Fig. 2).
moles of product produced with catalyst
− moles of product produced without catalyst
TON =
moles of catalyst
2.3. Characterization of catalysts
3. Results and discussions
The Co content of CoOx@CN was 29.81% measured by inductively
coupled atomic emission spectrometer (ICP-AES, Thermo, iCAP6300).
The N content was measured by elemental analysis on Varo MACRO.
The N content of CoOx@CN was 1.66%. The O and C content of
CoOx@CN were 5.77% and 62.81%, respectively, measured by scan-
ning transmission electron microscopy-energy dispersive spectrometer
(STEM-EDS) elemental mapping. Scanning electron microscope (SEM,
Model 8100), transmission electron microscopy (TEM) (Model JEM-
1230), high resolution TEM (HRTEM, Tecnai G2 F30 S-Twin) was op-
erated to observe the catalyst morphology. Powder X-ray diffraction
(XRD) patterns were conducted on a D/tex-Ultima TV wide angle X-ray
diffractometer equipped with Cu Kα radiation (1.54 Å) at room tem-
perature. The Fourier-transform infrared spectroscopy (FT-IR) spectra
of CoOx@CN were acquired with KBr method in the range of
400–4000 cm−1 on a Nicolet FT-IR/Nexus 470. The X-ray photoelec-
tron spectra (XPS) were acquired with an ESCALAB MARK source. C 1s
line at 284.6 eV was used to correct all XPS spectra. The specific surface
area measured by using micromeritics ASAP 2020 HD88 based on the
Brunauer–Emmett-Teller (BET) method. Magnetization was measured
by vibrating sample magnetometer (VSM, J3426, Cryogenic Limited) at
300 K, while the applied magnetic field is from −30 kOe to 30 kOe.
After the oxidation of p-cresols, the amount of cobalt leached from
CoOx@CN was measured by atomic adsorption spectrophotometer
(AAS, AA800, PerkinElmer).
3.1. Catalyst characterization
The morphologies of CoOx@CN were examined by SEM and TEM
techniques (Fig. 1). It showed the flake-like shape (Fig. 1a) morphology
resulting from melamine forming sheets by thermal decomposition
[55,56]. For comparison, several different component materials
(CoOx@M and CoOx@G) were prepared and characterized in detail.
The TEM images of the CoOx@G and CoOx@M demonstrated agglom-
eration phenomena (Fig. S2), but uniformly dispersed nanoparticles in
CoOx@CN (Fig. 1b). As shown in the elemental mapping images (Fig.
S3), the elements (Co, O, C, and N) are distributed evenly across the
whole skeletal structure, which verifying the uniform dispersion of four
elements. Besides, BET analysis (Table S1) reaveals that all these three
hybrids appear to be porous. But the specific surface area of CoOx@CN
is larger than those of the other two materials. Such morphology of
CoOx@CN originates both from melamine soft template [57] and ni-
trogen-doped carbon matrix [54] which enhance the nanoparticles
dispersibility. The particle size distribution histogram presented in
Fig. 1b (inset) demonstrates that the cobalt-based NPs average diameter
is roughly 13 nm. The HRTEM images confirm the component of Co0
and cobalt oxide. As shown in Fig. 1c, lattice fringe spaces of 0.289 and
0.468 nm are accordance with the (220) and (111) plane of cubic Co3O4
spinel-phase, respectively, but can hardly find CoO. The Co0 is also
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