F. Yue, et al.
Molecular Catalysis 495 (2020) 111149
were shown in Fig. S3. It can be seen from the figure that all Co-ZIF-9
materials are square block crystals having a side length of 15–0 μm. As
the crystallization temperature increased, the regular morphology of
Co-ZIF-9 was gradually strengthened. At the temperature of lower than
which is a mesoporous material with relatively narrow pore size dis-
tribution, also confirmed by SEM and TEM with uniformly spherical
aggregate.
As shown in Fig. 7, the thermogravimetric analysis of Co-ZIF-9 and
Co-ZIF-x materials was tested under air atmosphere. It can be seen from
the figure that the weight loss between 100 and 300 °C was attributed
to the loss of water. The collapse process of the Co-ZIF structure oc-
curred in the range of 300–450 °C. In this process, the Co species and
1
40 °C, Co-ZIF-9 was not crystallized completely, but when the tem-
perature was 160 °C, the crystallization was very complete. When the
synthesis temperature was lifted to 180 °C, the ligand benzimidazole
was easily decomposed to affect the crystallinity of the material, as
disclosed by XRD (Fig. 1). The effect of crystallization time on the
morphology of Co-ZIF-9 is depicted in Fig. S4. With the extension of
time, the block morphology of Co-ZIF-9 material gradually formed.
When the crystallization time was ≥36 h, the Co-ZIF-9 material could
be successfully synthesized. It is apparent that the optimal synthesis
conditions for Co-ZIF-9 in static solvothermal synthesis were 160 °C and
the organic ligands were oxidized to CoOx, CO and NxOy; when the
2
temperature was higher than 450 °C, the Co species was totally con-
verted to Co . After being thermally treated, the final percentage of
3 4
O
Co
3
O from Co-ZIF-9 precursor, Co-ZIF-250, Co-ZIF-350, Co-ZIF-450,
4
Co-ZIF-550 was 24.7, 29.1, 74.9, 84.3 or 94.5 %.
The reducibility of Co-ZIF-x materials and the interaction between
4
8 h.
The effect of reduction temperature on the surface morphology
SEM) of the catalyst was shown in Fig. 3. When the reduction tem-
Co and the support were further characterized by H -TPR analysis. The
2
reducible ability of the catalytic material and the interaction strength of
each component correspond to different reduction temperature posi-
tion. As can be seen in Fig. 8, the TPR profile of the Co-ZIF-9 precursor
shows three different signal peaks, attributable to the structural col-
lapse of the Co-ZIF material and the transformation of the Co species to
CoO and Co°. The Co-ZIF material was firstly pyrolyzed and carbonized
to form CoO and hydrocarbons, and then CoO was further reduced by
(
perature was lower than 250 °C, the morphology of the sample did not
change much. At the temperature of ≥350 °C, the ZIF structure
changed and the surface of the material began to become defective and
rough. In order to further study the morphology and microstructure of
the materials, TEM images were detected in Fig. 4. As the reduction
temperature was ≥350 °C, the Co-ZIF crystal structure collapsed to
form a mixture of nanoparticles and carbon nanotubes, as shown by the
XRD result in Fig. 2. When the pyrolysis temperature further increased
to 450–550 °C, the amount of Co nanoparticles increased, also con-
sistent with the diffraction peaks of some Co nanoparticles formed by
Co-ZIF-x at high pyrolysis temperature (Fig. 2). According to statistical
results, the average particle sizes of Co-ZIF-350, Co-ZIF-450 and Co-ZIF-
H
2
in a high temperature region to Co°. For Co-ZIF-250, the TPR curve
reduction tem-
was similar to that of Co-ZIF precursor, but the H
2
perature was relatively lower than that of Co-ZIF precursor. For the
samples Co-ZIF-350, Co-ZIF-450 and Co-ZIF-550, the majority of the
ligand moiety had been largely dissociated during the pyrolysis process,
so that those merely showed the signal peak of CoO reduced to Co°. In
addition, for the samples pyrolyzed at 350, 450, 550 °C, the reduction
temperature of the ZIF-derived material gradually rose with increasing
the pyrolysis temperature. As a result, if the dispersion of species was
lowered, the reduction temperature was increased.
5
50 were 9.4, 10.8 and 13.7 nm, respectively. As observed from the
elemental mapping of Co-ZIF-350, there was a good spatial overlap of
C, N and Co elements, showing homogeneous distribution of Co NPs on
the sample rather than simple physical mixing.
Fig. 5 showed the XPS spectra of the catalysts prepared at different
reduction temperatures. When the pyrolysis temperature was 250 °C,
the crystal structure of Co-ZIF-9 was roughly preserved, and the cobalt
3.2. Catalytic performance
2
+
3.2.1. Effect of catalysts on the hydrogenation of cyclohexanone
elements were basically composed of Co
ions. As the pyrolysis tem-
Table 1 showed the results of the cyclohexanone hydrogenation
with different catalysts. In the catalyst screening study, Co-ZIF pre-
cursor had no catalytic activity for cyclohexanone hydrogenation. In
addition, cobalt powder, CN material (obtained by filtering, washing
perature increased to ≥350 °C, the N in the ZIF material was greatly
2
+
reduced by hydrogen to NH . On the other hand, one part of Co was
3
reduced to Co° on the surface of the material, so that the total metal
ratio of Co° was gradually increased, as confirmed in the corresponding
XRD patterns (Fig. 2).
and drying Co-ZIF-350 treated with hydrochloric acid) and Co
3
O (Co-
4
ZIF-350 treated with 400 °C air for 3 h) were used as comparative
catalysts. It was found that under the same reaction conditions, the
hydrogenation of cyclohexanone could hardly be catalyzed by Co
The N sorption analysis and the pore size distribution of the re-
2
presentative sample Co-ZIF-350 is shown in Fig. 6. The adsorption and
desorption behavior of the sample was a typical type IV isotherm. It has
a hysteresis loop when the relative pressure range is P/P° = 0.4–0.9,
which showed that there were mesopores in the sample and the average
pore size was 3.8 nm. This sample has a specific surface area of 136.3
powder and Co
3
O , and the conversion of cyclohexanone was only
4
5
5.8% over CN material. At the same time, the catalytic properties of
nitrogen-doped carbon-supported metals such as Ni, Cu, or Fe and re-
lated materials have also been studied. The conversion on Ni-ZIF-350,
Fe-ZIF-350 or Cu-ZIF-350 was only 53.5 %, 1.4 % or < 1 % at 50 °C. In
contrast, Co-ZIF-350 catalyst exhibited 100 % conversion of cyclohex-
anone and > 99 % selectivity of cyclohexanol at 50 °C. As the reaction
temperature was increased to 70 °C, Ni-ZIF-350 and Cu-ZIF-350 could
2
-1
m
g
as calculated by the Brunauer-Emmett-Teller (BET) method,
which is typical for Co@carbon based composites. Co-ZIF-350 material
2
−1
has a specific surface area of 136.3 m
g
(calculated by Brunauer-
type,
Emmett-Teller (BET) method). Hysteresis loop belongs to H
1
Fig. 4. TEM images of Co-ZIF-x with different pyrolysis temperatures: a: 350 °C, b: 450 °C, c: 550 °C.
4