Microporous cobaltporphyrin covalent polymer mediated Co3O4@PNC nanocomposites for efficient catalytic C-H bond activation

Article abstract of DOI:10.1016/j.apcata.2021.118035

A Cobalt porphyrin conjugated porous polymer (Co/PCP) was obtained by hybrid polymerization of a mixture of Co(II)(5,10,15,20-tetrakis(4-bromophenyl)-21H,23H-porphyrin) (CoTBPP) and 5,10,15,20-tetrakis(4-bromophenyl)-21H,23H-porphyrin (TBPP) (3:7 M ratio) with the coupling reagent of p-phenyldiboric acid. This Co/PCP was carbonized at 400 °C and the obtained catalyst (denoted as Co₃O₄@PNC-400) was composed of Co₃O₄ nanoclusters wrapped in the nitrogen-carbon materials. This catalyst has an extremely high catalytic activity for the aerobic oxidation of C–H bonds. It can efficiently catalyze the oxidation of the C–H bonds in toluene and cyclohexane with high conversion and high selectivity of (alcohol + aldehyde), (alcohol + ketone) compounds, respectively. When heat-treated at 400 °C, the porous framework structure of the porphyrin polymer was partially retained, and the highly active Co₃O₄ nanoclusters were generated in situ. The reaction of C[dbnd]N to C–N and connection with the Co₃O₄ nanoclusters also occurred. Co₃O₄@PNC-400 has obvious synergistic catalytic effect between Co₃O₄ and nitrogen-carbon framework during the catalytic reaction. The defective structure caused by the migration of cobalt also greatly improved the catalytic activity. Co/PCPs with different cobalt contents were also synthesized and heat treated at different temperature. These catalysts were a new type of efficient C–H oxidation catalysts with excellent potential application value.

Full text of DOI:10.1016/j.apcata.2021.118035

Applied Catalysis A, General 614 (2021) 118035
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Microporous cobaltporphyrin covalent polymer mediated Co O @PNC
3
4
nanocomposites for efficient catalytic C-H bond activation
a
a
a
a
a
,
b
,
a,b,
Mingyang Tan , Liang Zhu , Hao Liu , Yajun Fu , Shuang-Feng Yin
*, Weijun Yang
*
a
College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China
b
Engineering Research Center of Advanced Catalysis, Ministry of Education, Hunan University, Changsha, 410082, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A Cobalt porphyrin conjugated porous polymer (Co/PCP) was obtained by hybrid polymerization of a mixture of
Co(II)(5,10,15,20-tetrakis(4-bromophenyl)-21H,23H-porphyrin) (CoTBPP) and 5,10,15,20-tetrakis(4-bromo-
Co3O4@PNC
C-H activation
Pyrolysis process
Oxidation
phenyl)-21H,23H-porphyrin (TBPP) (3:7 M ratio) with the coupling reagent of p-phenyldiboric acid. This Co/
◦
PCP was carbonized at 400 C and the obtained catalyst (denoted as Co
3
O
4
@PNC-400) was composed of Co
3
O
4
nanoclusters wrapped in the nitrogen-carbon materials. This catalyst has an extremely high catalytic activity for
the aerobic oxidation of C–H bonds. It can efficiently catalyze the oxidation of the C–H bonds in toluene and
cyclohexane with high conversion and high selectivity of (alcohol + aldehyde), (alcohol + ketone) compounds,
Metalloporphyrin polymer
◦
respectively. When heat-treated at 400 C, the porous framework structure of the porphyrin polymer was
–
N to C–N
partially retained, and the highly active Co
and connection with the Co nanoclusters also occurred. Co
effect between Co and nitrogen-carbon framework during the catalytic reaction. The defective structure
3
O
4
nanoclusters were generated in situ. The reaction of C
–
3
O
4
3 4
O
@PNC-400 has obvious synergistic catalytic
3 4
O
caused by the migration of cobalt also greatly improved the catalytic activity. Co/PCPs with different cobalt
contents were also synthesized and heat treated at different temperature. These catalysts were a new type of
efficient C–H oxidation catalysts with excellent potential application value.
1
. Introduction
The selective oxidation of C–H bonds is an important reaction in
Due to the different preparation methods, the performance of the ob-
tained cobalt oxide catalyst is also different [20–24]. In this paper, a
highly active Co O @PNC catalyst supported on porous nitrogen-carbon
3 4
catalytic chemistry [1], and it is of great significance to the drug syn-
thesis process, fine chemical industry and petrochemical industry [2].
For example, cheaper toluene can be oxidized to benzaldehyde and has a
higher value. However, due to the high bond energy, the stable C–H
bond oxidation is a relatively difficult process [3]. The simple method of
directly oxidizing the C–H bond is attractive, but the degree of oxidation
is difficult to control and usually resulting in poor selectivity to the
target product [4]. Using molecular oxygen such as air or oxygen as an
oxidant makes it more difficult to oxidize C–H bonds, and there are
many kinds of oxidation products with poor selectivity.
materials was prepared in situ by using cobalt porphyrin conjugated
porous polymer as a raw material. It exhibited excellent oxidation per-
formance of C–H bonds in alkanes with surprisingly high catalytic ac-
tivity and selectivity.
Our previous research shows that the conjugated porous metal
porphyrin polymer itself has good catalytic oxidation activity [25,26],
and the porphyrin conjugated organic polymer has uniform, rich pores
and rich nitrogen content. However, its stability and thermal stability in
the solution phase are insufficient [27,28]. So, the further improvement
of the catalytic activity of metal porphyrin polymer is a subject worthy
of further research. To the best of our knowledge, use the conjugated
porous metal porphyrin polymers as templates to synthesize supported
transition metal catalyst have not been reported.
Transition metals and their oxides are effective catalysts for C–H
bond functionalization [5]. At present, the catalysts used to activate C–H
bonds are mainly concentrated in relatively inexpensive transition
metals [6]. Due to the low bond energy of the Co-O bond, cobalt oxide
catalysts are generally used as an electrocatalyst to catalyze electrode
materials such as CO oxidation, water oxidation, or fuel cells [7–19].
In this study, we used a mixture of monomers tetraphenyl cobalt
porphyrin and uncomplexed metal porphyrin in different proportions as
raw materials. Through the coupling effect of the coupling agent (p-
*
Correspondingauthorsat: College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China.
E-mail addresses: sf_yin@hnu.edu.cn (S.-F. Yin), wjyang@hnu.edu.cn (W. Yang).
https://doi.org/10.1016/j.apcata.2021.118035
Received 22 December 2020; Received in revised form 6 February 2021; Accepted 8 February 2021
Available online 18 February 2021
0
926-860X/© 2021 Elsevier B.V. All rights reserved.

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
3 4
Scheme 1. Co/PCP synthesis from CoTBPP and TBPP, and Co/PCP heated treatment to prepare Co O @PNC-400 catalyst.
400ꢀ 3200 cm 1, England (532 nm excitation wavelength). EPR spectra
were obtain on a JES-FA200 EPR Spectrometer. As for the parameter,
center field intensity, sweep time, sweep width, mod width and ampli-
tude were 335 Mt, 30 s, 1 × 10 mT, 0.2 × 1 mT, 4.00 × 100Mt,
respectively. Elemental Analyzer(C,H,N) were performed on a Vario
MACRO cube. The Co and Br contents were measured by inductively
coupled plasma mass spectrometer Agilent 7900.
ꢀ
phenyldiboric acid), a hybrid porphyrin conjugated polymer (PCP) with
low content and high dispersion of cobalt was obtained. So the porous
3 4
composite catalyst Co O @PNC was prepared by heat treatment of this
polymer. The catalytic performance of the catalyst was tested by
oxidizing different kinds of C–H bonds including toluene and cyclo-
hexane, and the catalytic performance of the catalyst was compared
with the corresponding monomeric cobalt porphyrin and the original
polymer in detail.
2
2
.2. Synthesis of catalyst
2
. Experimental
.2.1. Synthesis of monometallic cobalt porphyrin
2
.1. Experimental instruments and reagents
5,10,15,20-tetrakis(4-bromophenyl)-21H,23H-porphyrin
(TBPP)
was synthesized firstly. The typical procedure: 18.5 g (0.1 mol) of p-
bromobenzaldehyde dissolved in 120 mL of propionic acid and heated
to reflux, within 30 min dropwise addition of 30 mL of propionic acid
solution dissolved in 6.7 g (0.1 mol) of pyrrole. After refluxing for 0.5 h,
it was cooled to room temperature and filtered with suction to obtain a
solid, which was washed three times with 50 mL of methanol. Recrys-
tallized three times with chloroform: methanol (15 mL:60 mL) and dried
CoCl
2
⋅6H
2
O was purchased from Shanghai Titan Scientific Co. Ltd..
for comparison experiment was purchased from
The commercial Co
3 4
O
Shanghai Macklin Reagent Co. Ltd..It was 99.5% metals basis with
average diameter of 30 nm. Pyrrole, p-phenyldiboric acid and 1,4
-
dioxane were purchased from Shanghai Aladdin Biochemical Tech-
nology Co., Ltd. and other chemicals were purchased from commercial
suppliers. All chemicals were analytical reagents and without further
purification before use.
to
obtain
5,101,520-tetrakis(4-bromophenyl)-21H,23H-porphyrin
1
(TBPP) purple crystals. The product was characterized by HMR,
Powder X-ray diffraction data was collected on a Bruker D8 Advance
X-ray diffractometer (Bruker, Germany). The radiation source was Cu-
UV–vis and FT-IR, and proved to be TBPP.
•
2
Then 0.93 g (1 mmol) and 2.85 g CoCl
6H
2
O (12 mmol) of the ob-
◦
◦
K
α
ray, and the scan angle was selected from 10 to 80 . FT-IR spectra
tained TBPP placed in a round bottom flask, add 100 mL of DMF, and
heat to reflux for 12 h. After cooling to room temperature, the precipi-
tate was precipitated by adding water and suction filtered. After
washing with water and methanol several times, it was vacuum dried at
were obtained on a WQF-410 FT-IR spectrometer (Ruili, China) ranging
ꢀ 1
from 400ꢀ 4000 cm . The UV–vis diffuse reflectance spectroscopies of
Co /PCP and Co/PCP (Cobalt porphyrin conjugated porous polymer)
were obtained over a UV–vis spectrophotometer (Cary 100) using BaSO
3 4
O
◦
4
70 C for 8 h to obtain a red-brown cobalt porphyrin complex Co(II)
as reference. Scanning electron microscope (SEM) images were collected
on a Hitachi S-4800 scanning electron microscope (Hitachi, Japan).
Transmission electron microscope (TEM) was acquired from Tecnai G2
F20 S-TWIN. The specific surface area and pore size distribution were
obtained on tri-star 3020. The thermogravimetric analysis (TGA) was
recorded using a DTG-60 thermogravimetric analyzer. X-ray photo-
electron spectroscopy (XPS) was performed on Thermo ESCALAB Xi
spectrometer. A Raman spectrometer was obtained at in Via-Reflex from
(5,10,15,20-tetrakis(4-bromophenyl)-21H,23H-porphyrin) (CoTBPP).
2.2.2. Synthesis of cobalt porphyrin conjugated polymer (Co/PCP)
Cobalt porphyrin conjugated polymer was synthesized by using the
Suzuki-Coupling reaction. The typical procedure is as follows: CoTBPP
and TBPP with different proportions were mixed as reactants to syn-
thesize hybrid porphyrin conjugated polymer Co/PCP with different
cobalt contents. For example, the typical steps for synthesis a polymer
2

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
3 4 3 4
Fig. 1. a) XRD of Co O @PNC-400; b, d)FT-IR spectrum of porphyrin conjugated polymer (Co/PCP) and Co O @PNC-400; c) ultraviolet spectrum of
different samples.
with a raw material ratio of CoTBPP: TBPP = 3:7 (molar ratio) are as
follows: p-phenylenediboronic acid (280.4 mg, 2 mmol), CoTBPP
marked as Co/PCP-300, Co/PCP-500 and Co/PCP-600, respectively.
(
289.5 mg, 0.3 mmol) and TBPP (634.5 mg, 0.7 mmol) were added to a
2.2.4. Catalytic oxidation reaction and experimental procedures
three-necked flask, and 80 mL of 1,4-dioxane was added. After three
cycles of freeze-evacuation-nitrogen protection thaw, 20 mL of deion-
ized aqueous solution in which 1.105 g of potassium carbonate
Toluene and cyclohexane containing different C–H bonds are used as
substrates, and oxygen used as oxidant. The oxidation reaction is carried
out in a miniature autoclave. The steps are as follows: 3 mmol of toluene
or cyclohexane, 1.0% mol of substrate as catalyst amount (based on the
content of cobalt), 5 mL of acetonitrile, was placed in a 25 mL miniature
autoclave. 1.0 MPa oxygen was introduced into the reactor, and the
oxygen pressure was maintained at 1.0 MPa by connecting to an oxygen
(
(
8 mmol) (0.1 mmol) was dissolved and 0.116 g of red-brown tetrakis
triphenyl)phosphine palladium were added. Freeze-evacuate-thaw
cycle three times under the protection of argon, then reflux for 24 h
under argon atmosphere, cool to room temperature and add a large
amount of deionized water to precipitate the product. It was filtered
under reduced pressure and washed with deionized water, methanol and
acetone, respectively. Finally the solid was extracted with Soxhlet with
THF, methanol and acetone for 24 h, respectively. The solid was dried
◦
cylinder during the reaction. The reaction was stirred for 6 h at 110 C.
The gas chromatography was used to detect the yield and conversion
rate (Scheme 1)[1–9].
◦
under vacuum at 70 C for 12 h to obtain 770 mg of reddish brown
3. Results and discussion
polymer product.
3
.1. Characterization analysis of catalysts
2
3 4
.2.3. Preparation of Co O @PNC catalyst
The above-prepared polymer was heated to a set temperature at a
3.1.1. XRD, FT-IR and UV spectrum analysis
The x-ray diffraction pattern of the sample after 400 C heat treat-
ment (Fig. 1a) shows a distinct diffraction peak corresponding to Co
which can correspond well to the card [Co PDF#43-1003].
Comparing with precursor of Co/PCP (Fig. S4), the catalyst keep
◦
◦
heating rate of 5 C/min in argon atmosphere, and maintained at the
temperature for 1 h to obtain a corresponding heat-treated catalyst. The
3 4
O ,
◦
catalyst after heat treatment at 400 C is marked as Co
3
O
4
@PNC-400,
3 4
O
◦
◦
◦
and the polymer after heat treatment at 300 C, 500 C and 600 C is
3

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
Fig. 2. a) Raman spectrum of Co
3
O
4 2 3 4
@PNC-400；c,d) N adsorption-desorption isotherm and pore size distribution of Co O @PNC-400; b) Thermogravimetric
analysis of Co/PCP in Argon; e,f) N
2
adsorption-desorption isotherm and pore size distribution of Co/PCP.
◦
◦
◦
◦
basically original structure. The peaks at 2θ of 19 , 31.3 , 36.9 , 38.5 ,
discharged completely. Although pyrolysis is carried out in an inert at-
mosphere, the limited amount of oxygen adsorbed in the pores can still
◦
◦
◦
◦
4
4.8 , 55.6 , 59.3 , and 65.3 correspond to (111), (220), (311), (222),
(
400), (422), (511), (440) crystal planes in the Co crystal structure,
3
O
4
oxidize Co atoms into Co
bound to the nitrogen-carbon structure.
The UV–vis spectra of Co @PNC-400 and Co/PCP (Fig. 1c) clearly
3 4
O with high catalytic activity, and is closely
respectively [29]. Because the precursor of the catalyst, Co/PCP, is a
highly microporous material, its pores adsorb much air and cannot be
3 4
O
4

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
3 4
Fig. 3. SEM of Co/PCP (a,b) and Co O @PNC-400 (c,d).
◦
show that the Soret and Q bands of both have not changed significantly
30]. Compared with Co/PCP, the FT-IR spectrum of Co @PNC-400
Fig. 1d) shows a decrease in the peak intensity at 1660 cm , which
8.6%. When the temperature rises to 490 C, the mass loss is only 10%,
which indicates that Co/PCP has high stability. Fig. 2c-f are the specific
[
3 4
O
ꢀ 1
(
surface area and pore size distribution of Co
The N adsorption-desorption isotherm shows that the specific surface
area of Co /PNC-400 and Co/PCP are 384.6 and 380.0 m2/g,
3 4
O /PNC-400 and Co/PCP.
–
may be due to the weak
π
-bonded electron cloud in C
–
–
N loses its
2
overlap and breaks to form C
N. And the present of the peak at 554
3 4
O
ꢀ
1
cm indicates that cobalt oxide formed [31]. This indicates that during
respectively; The pore size distribution charts show that the pore size of
the two catalysts is mainly distributed between 1ꢀ 2 nm, which is mainly
a microporous structure. It is obvious that before or after Co/PCP py-
rolysis, the specific surface area is almost the same, and the pore size
distribution is also almost consistent, although there are slight variations
in specific surface area and pore size. So it is inferred that the two are
very similar in structure.
◦
the inert atmosphere pyrolysis at 400 C, the porphyrin polymer skel-
–
N bond to C–N, and the Co
eton mainly undergoes the reaction of C
–
transform to Cobalt tetroxide. Therefore, conbining with previous XRD
analysis results, we believe that the structural characteristics of the
organic framework of Co
Co/PCP, but the oxidation, migration and fusion of cobalt atoms
occurred during the pyrolysis process, forming active Co clusters.
N into C–
3 4
O @PNC-400 are basically the same as
3 4
O
–
–
During this process, the conversion of C
N single bond and
3.1.3. SEM/TEM analysis
connection with Co
3
O
4
nanoclusters also occurred, thus forming a
3 4
The morphology of Co O @PNC-400 was studied by scanning elec-
highly active, well-developed, porous nitrogen-carbon-coated Co
catalyst.
3
O
4
tron microscope (SEM) and transmission electron microscope (TEM). It
can be seen from the SEM pictures of the sample that there are no
obvious changes seen on the surface (Fig. 3) after pyrolyzed. However,
3
.1.2. Raman spectroscopy, thermogravimetry, nitrogen adsorption and
the high-resolution TEM image of Co
nanoclusters are generated after pyrolysis. Fig. 4a shows Co
particles encapsulated in a carbon structure. Fig. 4d shows the 0.289 nm
lattice spacing, which can be assigned to the (220) plane of Co . The
lattice spacing of 0.244 nm is consistent with the Co (311) plane.
Fig. 4c shows the obvious interplanar spacing (0.23 nm), which corre-
sponds to the (222) plane of Co [32]. Fig. 4d shows that the obvious
cobalt oxide nanoparticles are embedded in the N-C structure formed by
the porphyrin polymer framework, and the size of Co particles is
3
O
4
@PNC-400 shows that Co
3 4
O
desorption, and pore size distribution analysis
3 4
O
nano-
3 4
From the Raman spectra (Fig. 2a), the Co O @PNC-400 curve does
not show a clear G band, but it shows obvious characteristic peaks of
some functional groups like FT-IR spectra. This indicates that there was
3 4
O
3 4
O
◦
no graphitization occurred when the sample heat-treated under 400 C.
However, the Co/PCP-500 clearly shows the G band, indicating that the
sample heat treated at 500 degrees has a certain degree of graphitiza-
3 4
O
tion. The Raman spectra of Co/PCP and Co
3
O
4
@PNC-400 overlap well,
3 4
O
which also confirms that the framework structure of the porphyrin
polymer basically remains integrality. The stability of Co/PCP was
tested by thermogravimetric analysis (TGA). When the temperature rises
about 2ꢀ 5 nm. The raw material ratio of CoTBPP: TBPP = 3:7 can
reduce the distribution density of cobalt ions in the polymer and ensure
the production of uniform and fine Co
3 4
O nanoparticles after pyrolysis,
◦
to 100 C, the mass loss is about 7.6% (Fig. 2b), which results from
thus obtaining the highly active catalysts.
◦
absorbed water molecules before TGA. From 160–430 C, the sample
quality remains relatively stable, with a mass loss of approximately
5

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
3 4 3 4
Fig. 4. a-d) HRTEM images of Co O @PNC-400; e)HAADF-DTEM and f-i) corresponding element mapping images in STEM mode of Co O @PNC-400.
3
.1.4. X-ray photoelectron spectroscopy
The element valence state of the catalyst surface was studied by X-
ray photoelectron spectroscopy. Fig. 5a and b are the energy spectrum of
the original catalyst Co @PNC-400 and the catalyst after five cata-
x
(403 eV) [33]. Among them, the content of Co-N is relatively high,
which is consistent with the previously speculated structural charac-
teristic of the catalyst. The three peaks at 531.9, 533.5, and 535.0 eV in
Fig. 5d can be assigned to lattice oxygen (O1), adsorbed oxygen (O2),
and hydroxyl oxygen (O3) [34]. The presence of adsorbed oxygen may
be the result of the catalyst being preserved in the air. The high lattice
oxygen content indicates that cobalt oxide crystals do exist in the cata-
lyst. Fig. 5f shows that the atomic content of C, N, O, Co is approximately
87.54, 8.81, 3.16, 0.49% respectively, which is basically consistent with
the results of the elemental analysis (SI).
3 4
O
lytic cycles (toluene oxidation). Two main peaks and two satellite peaks
can be observed in the spectra. The peaks of 780.3 eV and 795.9 eV are
assigned to Co 2p3/2 and Co 2p1/2, respectively. The main peak can be
3
+
2+
divided into three sub-peaks, corresponding to Co , Co , and Co-N.
3
+
The peaks at 779.9 eV and 795.4 eV can be assigned to Co , and the
2
+
peaks at 780.7 eV and 796.2 eV can be attributed to Co . The content of
3
+
Co changed from 56.49% before the reaction to 56.4% after the re-
3
+
action. It can be seen that the content of Co remained basically un-
changed before and after the catalytic reaction. The high-resolution
spectrum of N1 s in Fig. 5c shows that it can be fitted to five peaks,
3.1.5. Catalytic Oxidation of C–H bonds
Toluene and cyclohexane were selected as substrates for oxidation.
First of all, catalysts with different cobalt content were obtained by
◦
including pyridine N (397.9 eV), Co-N
x
(398.8 eV), pyrrolic nitrogen
400 C heat treatment for the polymers prepared from different raw
(
399.9 eV), graphitic nitrogen (400.7 eV) and oxidized Nitrogen
material ratios, and the catalysts were used in the catalytic oxidation
6

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
3 4
Fig. 5. XPS high resolution spectrum of Co 2p before (a) and after (b) reaction, N 1s (c), O 1s (d) and C 1s (e), of Co O @PNC-400 and survey spectrum (f).
7

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
Table 1
cyclohexane, the conversion rate of toluene is lower, and the selectivity
of oxidation products is different. This is mainly because that the C–H
bonds of methyl have stronger bond energy than methylene, and are
more difficult to oxidize.
Catalytic performance for selective oxidation of toluene.
Selectivity (%)
Catalyst
Conv.%
alcohols
aldehydes
others
As shown in Table 1, toluene cannot be oxidized under a catalyst-free
oxygen atmosphere. The heat treatment product of the porphyrin con-
jugated polymer without metal cobalt, PCP-400 (0:1), exhibits a very
low conversion rate for toluene oxidation. A mixture of commercial
no catalyst
0.8
7.5
92.5
83.8
73.0
85.9
86.2
64.6
86.1
86.2
80.5
61.7
83.7
0
a
Co/PCP-400 (1:0)
Co/PCP-400 (1:1)
17.1
19.3
22.0
1.4
9.5
6.7
17.8
6.9
3.4
6.1
8.9
13.8
9.3
16.0
11.7
b
15.2
7.2
3 4
Co O @PNC-400
c
PCP-400 (0:1)
Co/PCP
10.4
29.3
5.0
Co
oxidation than Co
can infer that the high activity of Co
synergistic effect of the in-situ generated Co
3
O
4
and PCP-400 has a much worse catalytic effect on toluene
@PNC-400. From these experimental results, we
@PNC-400 is caused by the
and the heat-treated
9.0
3 4
O
d
(
Co
3
O
4
+PCP)-400
10.3
9.7
3 4
O
Co/PCP-300
Co/PCP-500
CoTBPP
0
3 4
O
15.1
8.6
10.2
22.3
4.6
porous porphyrin polymer framework. In Table 1, it can be found that
the catalyst has better selectivity for toluene C–H activation to benzal-
dehyde comparing with other cobalt catalysts.
Co(acac)
2
14.3
Reaction conditions: 3 mmol of substrate, 1.0% mol of substrate as catalyst
3 4
Co O @PNC-400 has better performance in the selective oxidation
amount (based on the content of cobalt), 5 mL of acetonitrile, 1.0 MPa O
2
,
◦
1
10 C, 6 h.
of C–H bonds under oxygen conditions, and the comparation results with
other literatures were listed in Table 3 and Table S1. It can be seen that
the selectivity of benzaldehyde is better than that of most catalysts when
the conversion is similar. It also has good reuse performance (Fig. S3).
After five cycles, the performance of the catalyst almost did not
decrease. Its high conversion rate and selectivity are mainly attributed
a
◦
Prepared from 400 C heat treatment of porphyrin polymer(a raw material
ratio of CoTBPP: TBPP = 1:0 (molar ratio)).
b
◦
Prepared from 400 C heat treatment of porphyrin polymer (a raw material
ratio of CoTBPP: TBPP = 1:1 (molar ratio)).
c
◦
Prepared from 400 C heat treatment of porphyrin polymer without cobalt in
the structure (a raw material ratio of CoTBPP: TBPP = 0:1 (molar ratio)),
catalyst weight equals to Co @PNC-400.
A mixture of commercial Co and PCP-400, the Co content and PCP-400
content equals to Co @PNC-400.
2+
3+
to the redox characteristics of Co /Co cobalt tetroxide embedded in
the porphyrin-based nitrogen-carbon framework (PNC). [47] The elec-
tron spin resonance test (EPR) found that the intensity of the spin
3 4
O
d
3 4
O
3 4
O
resonance signal (g = 2.0032) of Co
Co/PCP (Fig. 6). This shows that part of the trivalent cobalt was con-
verted into high-spin bivalent cobalt, thus Co is generated [48,49].
3 4
O @PNC-400 is about twice that of
reaction of toluene (Table 1). It can be seen from Table 1 that the
@PNC-400,
3 4
O
catalyst obtained by CoTBPP: TBPP = 3:7, namely Co
3 4
O
Bivalent cobalt has higher intensity of spin signals than trivalent Cobalt,
which actually proves to be equivalent to XPS test results. The EPR
signal of the catalyst before and after oxidation of toluene and cyclo-
hexane hardly changed, indicating that the total amount of magnetic
field sensitive substances in the catalyst remained basically constant.
had the best catalytic performance. So the polymer with this ratio was
selected to continue heat treatment at different temperatures
◦
(
300ꢀ 500 C) to investigate the effect of heat treatment temperature on
the performance of the catalyst. As can be seen from Table 1, heat
◦
treatment at 400 C produced the best catalytic effect (Scheme 2).
The catalytic performance of Co
oxidation was further tested and compared with Co/PCP and CoTBPP.
As can be seen from Tables 1 and 2, Co @PNC-400 has excellent
catalytic oxidation activity of C–H bond. Compared with the polymer
before heat treatment (Co/PCP), Co @PNC-400 not only improves
3
O
4
@PNC-400 for C–H bond
3 4
O
3 4
O
Table 2
the conversion rate of each substrate several times (such as 5.5 times for
toluene and 1.5 times for cyclohexane), but also improve the selectivity
of target products. For example, the selectivity of benzaldehyde
increased from 64.6%–85.9%. We know that the metalloporphyrin
conjugated polymer itself is an excellent catalyst [18–21], and in this
study, CO /PCP did have a good catalytic oxidation effect on toluene and
cyclohexane (Tables 1 and 2). After heat treatment, its catalytic per-
formance has been greatly improved. This is mainly due to the in situ
Catalytic performance for selective oxidation of cyclohexane.
Selectivity (%)
Catalyst
Substrate
Conv.%
ketones
alcohols
others
Co
3
O
4
@PNC-400
cyclohexane
cyclohexane
cyclohexane
cyclohexane
16.5
11.2
6.8
50.4
41.7
39.3
33.6
40.1
42.3
45.0
48.2
9.5
Co/PCP
CoTBPP
Co(acac)
16.0
15.7
18.2
2
6.4
Reaction conditions: 3 mmol of substrate, 1.0% mol of substrate as catalyst
generated Co
Co oxygen bonding rate, so that molecular oxygen can be activated
by Co and transferred to the substrate [19]. Compared with
3 4
O which has a lower Co-O bond energy and a higher
amount (based on the content of cobalt), 5 mL of acetonitrile, 1.0 MPa O
2
,
3 4
O
◦
1
10 C, 6 h.
3 4
O
Scheme 2. Catalytic oxidation equation.
8

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
Table 3
3 4
A comparison of toluene oxidation activity of the Co O @PNC-400 catalyst with other reported catalysts.
Catalyst
T/P
t(h)
Solvent
Conv.%
Selectivity (%)/aldehydes
References
a
◦
◦
Co
3
O
4
@PNC-400
110 C /1MPa
6
CH
CH
\
3
CN
CN
22
85.9
92
This work
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
a
Co(acac)
2
100 C /10atm
5
3
11
a
◦
sub-Au/LC-CeO
2
160 C /1.2MPa
10
4
6.5
81.2
38.9
99
a
◦
MnTPPCl
160 C /1.5MPa
CH
3
CN
12
a
◦
8
-CN-600
160 C /1MPa
16
6
Acetone
CH CN
2
a
◦
◦
◦
MnCo-MOF-74
80 C /1atm
3
17.6
44.19
59.5
12.4
6.3
98.3
100
90
a
CeO
2
nanocube
90 C /1atm
12
18
2
2
H O
b
MnWO
4
NFs
80 C /1atm
CH
3
CH
3
CH
3
\
CN
CN
CN
b
◦
CNB0.15
Cu-CNB
150 C /-
99
b
◦
◦
70 C/-
16
4
99
b
Fe
2
O
3
/HZSM-5
90 C /-
17.3
62.7
22
51.4
39
b
◦
FeCl16Pc-Na-X
120 C /-
8
\
◦
MPAV
2
/Nb
2
O
5
*
25 C /1atm
8
\
76
a
2
Oxidant was O .
b
*
Oxidant was H
2 2
O (30%).
Oxidant was TBHP.
Fig. 6. EPR measurement of various samples.
◦
3 4
migration in the pyrolysis process under 400 C produced active Co O
4
. Consideration on the reaction pathways
In order to confirm whether the mechanism of Co
nanoparticles, and the cobalt migration from the porphyrin ring caused
defect site. This low temperature process made the defect not been
completely repaired during the pyrolysis and partly retained. The
retained defective structure were alkaline site containing nitrogen,
which played an important catalytic role in the hydrogen peroxide in-
termediate decomposition to form benzaldehyde and phenylcarbinol.
3 4
O @PNC-400
catalytic oxidation of toluene C–H bond is a free radical process, we
conducted a free radical suppression test. In the catalytic reaction sys-
tem, 0.2% of free radical inhibitor TEMPO (2, 2, 6, 6-tetramethylpiper-
idinyloxy) was added, and the reaction conditions was the same as
Table 1. No oxidation product was detected. This shows that
◦
Pyrolysis under 400 C can not only trigger the migration and
Co
3
O
4
@PNC-400 catalytic oxidation of toluene C–H bond is a free
3 4
diffusion of cobalt ions and fusion to produce highly active Co O
radical reaction. According to the aforementioned experimental
research results and related reports in the comprehensive literatures
nanoparticles, but also preserve part of the defective structure and
◦
generate part of the nitrogen-carbon structure. 400 C is a proper py-
[
50–55], the process of this catalytic reaction can be described as follows
Fig. 7).
The initiation of this reaction is the trivalent cobalt metal cluster to
extract an H on the methyl group of toluene to produce a benzyl radical
56]. At the same time, the nitrogen-carbon framework based on poly-
rolysis temperature, which is an important reason for the high catalytic
(
3 4
performance of CO O @PNC-400.
5. Conclusions
[
merized cobalt porphyrin can activate oxygen and transfer it to benzyl
radicals to form benzyl peroxy radicals [25,57,58]. It then reduced to
the product benzaldehyde by the defect site containing nitrogen and the
divalent cobalt metal cluster. As the reaction proceeds, benzaldehyde
will be further oxidized to benzoic acid.
For the first time, a metal porphyrin conjugated porous polymer was
used as a template and a cobalt source to form in-situ Co nano-
3 4
O
clusters coated with porphyrin-based nitrogen-carbon porous materials
by low-temperature heat treatment. The experimental results show that
3 4
the composite material Co O @PNC-400, which obtained by heat-
The raw material ratio of CoTBPP: TBPP = 3:7 can reduce the dis-
tribution density of cobalt ions in the polymer and ensure to generate
treatment the polymer polymerized from CoTBPP: TBPP = 3:7 at
◦
400 C, had the best catalytic performance on alkane oxidation. It was
ultra fine Co
3
O
4
nanoclusters after pyrolysis. Cobalt diffusion and
found that when pyrolysis the cobalt porphyrin conjugated porous
9

M. Tan et al.
Applied Catalysis A, General 614 (2021) 118035
Fig. 7. Possible pathways for catalytic toluene oxidation.
◦
polymer at 400 C, fine and highly active Co
3
O
4
nanoclusters can be
Appendix A. Supplementary data
generated in situ, while the porphyrin polymer still retains the main
characteristics of the porphyrin framework structure. The porous com-
posite catalytic material obtained in this way has ultra-high catalytic
activity in the selective catalytic reaction of oxygen to oxidize C–H
bonds in alkane such as toluene and cyclohexane. Its performance is
significantly better than the unpyrolyzed polymer Co/PCP and mono-
mer metalloporphyrin CoTBPP at other ratios and other heat treatment
temperatures. It is also superior to the results reported in many litera-
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118035.
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1