Preparation of CoNC catalysts: Via heating a mixture of cobaltporphyrin and casein for ethylbenzene oxidation

Article abstract of DOI:10.1039/c6ra17114a

Biomass-derived cobalt-coordinated N-doped carbon (CoNC) for C-H bond oxidation is synthesized by a facile procedure based on pyrolysis of cobaltporphyrin with natural, amino acid-rich biomass casein as a supplementary nitrogen source. The catalysts are characterized by techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The obtained CoNC catalyst has a high metal content and dispersion and shows superior catalytic performance in C-H bond oxidation with molecular oxygen as oxidant under solvent-free conditions. This is attributed to the promotion of the Co-N_x moiety originating from cobaltporphyrin. Moreover, the catalyst shows remarkable stability and can be recycled several times without losing its activity.

Full text of DOI:10.1039/c6ra17114a

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Preparation of CoNC catalysts via heating a mixture
of cobaltporphyrin and casein for ethylbenzene
oxidation
Cite this: RSC Adv., 2016, 6, 75707
a
a
a
b
a
*
Congqiang Yang, Yuan Chen, Sufang Zhao, Runliang Zhu and Zhigang Liu
Biomass-derived cobalt-coordinated N-doped carbon (CoNC) for C–H bond oxidation is synthesized by
a facile procedure based on pyrolysis of cobaltporphyrin with natural, amino acid-rich biomass casein as
a supplementary nitrogen source. The catalysts are characterized by techniques such as transmission
electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron
spectroscopy (XPS). The obtained CoNC catalyst has a high metal content and dispersion and shows
superior catalytic performance in C–H bond oxidation with molecular oxygen as oxidant under solvent-
free conditions. This is attributed to the promotion of the Co–Nx moiety originating from cobaltporphyrin.
Moreover, the catalyst shows remarkable stability and can be recycled several times without losing its activity.
Received 4th July 2016
Accepted 29th July 2016
DOI: 10.1039/c6ra17114a
www.rsc.org/advances
the research focus has shied from N-containing hazardous
inorganic or organic chemicals to renewable and bountiful
1. Introduction
biomass resources as precursors in the preparation of functional
carbon materials with high nitrogen content.19,20 Several N-rich
natural resources, including egg-white protein,21 ginkgo
leaves,22 chitosan,23 silk broin,24 soybean25 have been reported to
prepare N-doped carbon materials and used as effective catalysts.
Casein, main composition of the milk, is rich in amino acids
such as glutamic, proline, leucine, lysine, valine and tyrosine,
which makes it an attractive N source. It as well possesses
advantages of cheaply and readily available, environmental
friendly. In addition, catalytic materials have been concentrated on
composite structure with metallic actives as their performance can
be signicantly improved by the use of a co-catalyst.26,27 As widely
studied biomimetic catalysts, cobaltporphyrins have been found to
be excellent catalysts for aziridination, C–H amination, and
cyclopropanation.28–30 Its unique characteristics similar to models
of cytochrome P450 might be exploited in preparation of efficient
metal-doped carbon nitrides materials for a wider range of
applications.
Oxidation of carbon–hydrogen (C–H) bonds to oxygenated
derivatives is of great importance in the chemical, pharma-
ceutical and petrochemical industries as they are widely used
for the production of intermediates.1–5 Industrially performed
catalytic oxidation processes, such as oxidation of ethylbenzene,
oen suffer from drawbacks such as high cost, negative envi-
ronmental impact and a large amount of corrosive waste.6,7
With a view to a green catalytic oxidation process, molecular
oxygen as an eco-friendly, effective and economical oxidant is
preferred.2,8–10 However, catalytic oxidation of carbon–hydrogen
bonds in an oxygen-based system remains a challenging topic in
contemporary chemical research as the C–H bond is inert.4,11
In recent years, nitrogen-doped carbon materials have
attracted a lot of interest owing to its unique properties such as
high thermal conductivity, excellent electric conductivity and
high binding ability.12,13 Carbon nitrides are promising candi-
dates to complement carbon in diverse materials applica-
tions.14,23 It has been reported that nitrogen-doped carbon
makes catalytic oxidation of ethylbenzene in an oxygen-based
system possible.15–17 However, its catalytic performance needs
to be further improved. Ning et al.18 have conrmed that the
rational selection of N source has a large effect on the activity of
the catalysts, and the nature of nitrogen functionalities is
greatly affected by the structure of nitrogen precursors. Recently,
In this study, a simple approach for the synthesis of CoNC
catalyst with cobaltporphyrin (serves as metal and N source) and
casein (serves as supplementary nitrogen source) is described.
Catalytic performance of this material in the heterogeneous
oxidation of ethylbenzene is demonstrated. The CoNC catalysts
are characterized by techniques such as TEM, XRD, Raman and
XPS.
aState Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry
and Chemical Engineering, Hunan University, Changsha 410082, Hunan, China.
E-mail: liuzhigang@hnu.edu.cn; Tel: +86-731-8882-3327
bGuangdong Provincial Key Laboratory of Mineral Physics and Material Research &
Development, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,
Guangzhou 510640, China
2. Experimental
2.1 Characterizations
The morphologies of samples are examined by TEM and high-
resolution transmission electron microscopy (HRTEM) on an
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FEI Tecnai G2 Spirit electron microscope at an accelerating For comparison, cobaltporphyrin and casein are pyrolyzed
voltage of 120 kV. XPS is conducted using a VG ESCA 2000 with using similar method and the obtained products are designed
an MgKa as the source and using the C 1s peak at 284.6 eV as an as CoNC-PR and NC, respectively.
internal standard. Raman spectra are collected on a Bruker RFS
100/S spectrometer with 514 nm laser excitation. XRD patterns
2.4 Preparation of Co/NC-0.5
are acquired on a D8-Advanced diffractometer system from
The catalyst with Co nanoparticles supported on NC material is
synthesized by following procedures. CoCl2$6H2O (0.18 g) is
dissolved in 20 ml CH2Cl2. Then as-prepared NC material (0.2 g)
is added into the solution, and the mixture is heated to reux at
40 ꢀC for 24 h. The CH2Cl2 is completely removed using a rotary
evaporator. The obtained solid sample is transferred into
a quartz boat and annealed in 100 cm3 minꢁ1 of owing N2 at
500 ꢀC for 1 h. The product is designed as Co/NC-0.5 (the molar
content of metal ions is equal to that of CoNC-0.5).
ꢀ
Bruker with CuKa radiation (l ¼ 1.5418 A). Co content in catalyst
is analyzed by Atomic Absorption Spectrometry (AAS) on
VARIAN (AA240).
2.2 Synthesis of cobaltporphyrin
The compound is synthesized by referring to the following
procedures. 5.56 g benzaldehyde is added into 200 ml propanoic
acid and heated to reux, then 4.03 g freshly distilled pyrrole is
added into the system dropwise. The obtained product is cooled
overnight, collected by ltration and washed with cold water.
The precipitate is puried by column chromatography. Then,
2.5 Catalytic tests
The oxidation of ethylbenzene over CoNC-x using O2 as the
oxidant is carried out in a 50 ml teon-lined stainless steel
batch reactor. Typically, 10.0 ml of ethylbenzene and 30 mg of
catalyst are placed in the reactor. Aer being sealed, the reactor
is heated to the reaction temperature (120 ꢀC), with continuous
magnetic stirring. Upon heating to the desired temperature the
O2 pressure is increased to 0.8 MPa and O2 is fed continuously
to maintain constant pressure. The reaction is carried out at
the product tetraphenylporphyrin is obtained. 1.0
g as-
synthesized sample is added to 100 ml N,N-dimethylforma-
mide (DMF). When mixture is heated to reux, 2.0 g metal
chloride is gradually added into the system. The solution is
allowed to cool overnight. The obtained product is ltered and
washed repeatedly with cold water, and the product is achieved.
ꢀ
2.3 Preparation of CoNC-x composites
120 C for 5 h. Aer reaction, the samples are analyzed by GC
(ShimadzuGC-2014) equipped with a capillary column (RTX-5,
30 m, 4 0.25 mm) and FID detector with 1 ml minꢁ1 nitrogen
as carrier gas. The programmed temperature was conduced
from 60 C to 120 C and then kept at 120 C for 3 min. The
product is quantied by internal standard method using bro-
mobenzene and 1,4-dichlorobenzene as reference.
The synthesis of CoNC-x samples has been carried out by
loading different amount of cobaltporphyrin (x ¼ 0.1, 0.2, 0.3,
0.5, 0.8 g, where x is the exact weight of cobaltporphyrin used
per 0.5 g casein) onto the casein and then calcinated under
nitrogen atmosphere. In a typical procedure, cobaltporphyrin is
dissolved in 20 ml CH2Cl2 and stirred for 10 min. Then 0.5 g
casein is added into the solution, and the mixture is heated to
reux at 40 ꢀC for 24 h. The CH2Cl2 is completely removed using
a rotary evaporator. The obtained solid sample dried at 60 ꢀC for
ꢀ
ꢀ
ꢀ
3. Results and discussion
3.1 Characterizations
100 cm3 minꢁ1 of owing N2 at 500 C for 1 h. The sample is As illustrated in Fig. 1, samples obtained are denoted as CoNC-x
then allowed to cool in the tube furnace to room temperature. (where x is the exact quality of cobaltporphyrin used per 0.5 g
4 hours is transferred into a quartz boat and annealed in
ꢀ
Fig. 1 Schematic illustration of the synthesis of CoNC-x from cobaltporphyrin and casein under N2 protection.
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casein). For comparison, cobaltporphyrin and casein are pyro-
lyzed using similar method and the obtained products are
designed as CoNC-PR and NC, respectively. The morphology
and structural features of the as-prepared samples are exam-
ined by TEM and high-resolution TEM (HRTEM). The spongy
N-doped carbon layer deriving from casein with hollow carbon
nanocages can be clearly observed in Fig. 2a. As shown in Fig. 2b
and e, the Co nanoparticles (NPs) of CoNC-x are uniformly
embedded in the carbon matrix with the dark spots depicting
metal crystallite formation. The Co NPs are rarely aggregated,
indicating a good dispersion in the carbon structure. The size
distribution histograms (Fig. 2d–f) demonstrate the average size
of cobalt NPs is about 3.6, 3.8 and 6.3 nm for CoNC-0.2, CoNC-
0.5 and CoNC-PR, respectively. Co particle size in CoNC-x is
smaller than that of CoNC-PR. This implies that casein can
effectively prevent Co NPs from agglomerating. This can be
attributed to the introduction of extra nitrogen atoms
increasing the number of anchoring sites, improving the
dispersion of metal ions.31 HRTEM images clearly reveal that
the as-synthesized CoNC-x possess a crystalline structure with
a lattice spacing of 0.21 nm, which is in good agreement with
the (111) lattice spacing of metallic Co, and also uncovers
the lattice fringes of Co3O4 (220) planes with a spacing of
0.29 nm.32,33
Fig. 3 XRD patterns of NC, CoNC-x (x ¼ 0.2, 0.3, 0.5), and CoNC-PR.
The X-ray diffraction (XRD) patterns of samples in Fig. 3
show a broad characteristic peaks located at about 27.4ꢀ which
can be indexed to the (002) planes of graphite.4,33,34 The XRD
pattern of CoNC-PR exhibits XRD peaks at around 44.3ꢀ, 51.6ꢀ
and 75.8ꢀ implying the presence of metallic cobalt in the
Fig. 2 TEM images of (a) NC, (b) CoNC-0.5, (c) HRTEM images of CoNC-0.5, (d) particle size distribution histogram of CoNC-0.5, (e) TEM images
of CoNC-0.2, (f) TEM images of CoNC-PR, inset (e), (f) are corresponding particle size distribution histogram.
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To further investigate the degree of graphitization and
doping status of CoNC-x, CoNC-PR and NC, Raman spectros-
copy is used. As shown in Fig. 4, all these materials have an
obvious D band at 1338 cmꢁ1 which is associated with defects
including bonding disorders and vacancies in the graphite
lattice, and a G band at 1578 cmꢁ1 which is corresponding to
graphitic carbon.38–41 The intensity ratio of D band versus G
band (ID/IG) is a signicant parameter to reect the defect and
disorder level in the graphitic carbon layers.42,43 The ID/IG value
of NC (1.09) is higher than those of CoNC-0.2 (0.97), CoNC-0.3
(0.85), CoNC-0.5 (0.81) and CoNC-PR (0.79). The ratio of ID/IG
increases accompanied by a increase in the relative content of
casein indicating that casein can improve defect level of the
catalyst. The higher ratio of CoNC-x than that of CoNC-PR
demonstrates excellent structure of CoNC-x derived from
biomass casein.
The X-ray photoelectron spectroscopy (XPS) analysis is used
to investigate surface compositions and surface chemical states
of the catalysts. As shown in Fig. 5a, a set of peaks corre-
sponding to C 1s (284.6 eV), N 1s (401.0 eV), O 1s (531.6 eV) and
Co 2p (780.0 eV) is observed. The spectral deconvolution of the
C 1s high-resolution XPS spectrum (Fig. 5b) reveals the chem-
ical environment of carbon. The C 1s spectrum shows three
peaks at 283.6, 284.8, 285.4 eV, which are assigned to C]C, C]
N, and C–N, respectively.38,44 The high-resolution XPS spectrum
(Fig. 5c) of the N 1s peaks at 397.5, 398.8, and 400.2 eV, is
Fig. 4 Raman spectra of NC, CoNC-x (x ¼ 0.2, 0.3, 0.5), and CoNC-
PR.
CoNC-PR hybrid composite.35,36 Peaks of metallic cobalt have
not been detected in XRD patterns of CoNC-x probably due to
the smaller size and higher dispersion of Co NPs in nitrogen-
doped carbon.37 These characters are attributed to casein
which restrains the growth of Co NPs and improves metal
dispersion through increasing anchoring sites.
Fig. 5 (a) XPS survey of samples, high-resolution XPS spectra of (b) C 1s, (c) N 1s and (d) Co 2p regions of CoNC-x.
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corresponding to the pyridinic C]N p* state, amino-type and suggests that casein can act as a novel and efficient N source.
graphitic-type N, respectively..44,45 It has been reported that CoNC-x possesses higher N content than that of CoNC-PR
amino-type N inherits functional groups such as catechol and demonstrates the advantageous role of casein in improving
N–H groups could effectively coordinate with Co in formation of nitrogen content of catalysts. Meanwhile, the relative content of
Co–Nx species.35 In cobalt region (Fig. 5d), the rst peak at Co ions increases, total nitrogen content slightly decreases with
around 778.2 eV is consistent with the Co 2p3/2 binding energy an increase in the amount of cobaltporphyrin. The results above
of Co in a zero-valent state.36 Generally speaking, thermal suggest that the N and Co NPs dosage can be facilely tuned by
treatment of metalloporphyrin can generate groups containing changing the mass ratio of casein and cobaltporphyrin.
nitrogen that serve as coordination centers for metal ions where
metal–Nx is formed. The binding energy of 781.5 eV of the Co
2p3/2 is generally ascribed to Co–Nx.46 Peak characteristic for
3.2 Activity measurement
oxidic Co with the typical binding energy of 795.3 eV of the Co
2p1/2 is found.47 Furthermore, a shake-up satellite peak at 802.9
eV suggests that the oxidic Co is mainly composed of Co3O4.48
Surface species composition is depicted in Table 1. The metal
content of CoNC-PR is up to 1.70 at% indicating that cobalt-
porphyrin is an excellent metal precursor. The higher N content
of NC (8.47 at%) compared to that of CoNC-PR (5.05 at%)
Catalytic testing is performed for oxidation of ethylbenzene over
CoNC-x, CoNC-PR and NC. The corresponding catalytic perfor-
mances are summarized in Table 2. The blank run (entry 1,
Table 2) gives conversion of 7%. However, NC exhibits
remarkably improved catalytic activity with the conversion of
15% (entry 2, Table 2). This observation suggests that NC
material can help to promote the activation of C–H bond.
Table 1 The ratio analysis of the surface species of the catalysts
N (at%)
Samples
C (at%)
O (at%)
Co (at%)
Total
PDa (%)
PR (%)
GH (%)
NC
81.30
86.26
81.27
85.84
83.94
10.22
6.99
9.76
7.91
8.59
—
8.47
5.05
7.63
5.84
5.45
43.69
62.40
58.02
58.00
62.60
49.66
30.25
34.69
34.71
30.15
3.95
7.35
7.29
7.29
7.25
CoNC-PR
CoNC-0.2
CoNC-0.3
CoNC-0.5
1.70
0.90
1.20
1.22
a
PD ¼ pyridinic N, PR ¼ pyrrolic N, GH ¼ graphitic N.
Table 2 Catalytic performances of ethylbenzene oxidation over NC, CoNC-x and CoNC-PRa
Sel.b (%)
AP
Entry
Catalyst
Conv. (%)
PA
BD
1
2
3
4
5
6
7
8
Blank
NC
7
77
76
73
74
75
75
75
76
73
75
78
20
18
21
18
19
19
20
21
22
20
20
3
6
6
8
6
6
5
3
5
5
2
15
17
18
19
20
24
19
9
CoNC-PR
CoNC-0.1
CoNC-0.2
CoNC-0.3
CoNC-0.5
CoNC-0.8
CoNC-0.5
CoNC-0.5
Co/NC-0.5
9c,d
10e
11
20
17
a
Typical conditions: 10 ml of ethylbenzene, 30 mg of catalyst, O2 (0.8 MPa), 120 ꢀC, 5 h. b Corresponding selectivities (%) to acetophenone (AP),
phenethyl alcohol (PA), benzaldehyde (BD). c Catalyst was immersed in l mol Lꢁ1 KSCN solution at 80 ꢀC for 5 hours. d 0.1 g of KSCN powder should
be added into the reaction system. e A thermal treatment was applied to catalyst cycling 6 times at 300 ꢀC for an hour.
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CoNC-0.5 gives conversion of 24% which is higher than that conversion of ethylbenzene increases accompanied by
of blank and NC. This may be attributed to the promotion of a increase in relative content of cobaltporphyrin may be
Co–Nx particles.49,50 The role of Co–Nx moiety can be conrmed attributed to increasing content of Co–Nx (entry 4–7, Table 2).
by using small molecules to form highly stable metal–ligand However, the catalytic performance decreases when content of
coordination intermediate compounds to interrupt the reac- cobaltporphyrin further increases (entry 8, Table 2). This may be
tion.51–54 When Co–Nx species are poisoned, signicant loss of attributed to the aggregation of excess metal ions, and thus lead
catalytic activity is observed (entry 9, Table 2). To further verify to a decline in activity. Since CoNC-0.5 was found to be the best
the role of Co–Nx, a control-experiment is carried out. As catalyst for the oxidation of ethylbenzene, it was also used in
a comparison, Co/NC-0.5 with Co NPs supported on NC mate- further experiments and showed excellent catalytic perfor-
rial gives conversion up to 17% (entry 11, Table 2) which is mance, the results of which are summarized in Table 3.
much lower than that of CoNC-0.5 (entry 7, Table 2). This may
be attributed to that Co ions doped in N–C structure can
promote the generation of free radicals by interaction of the
metal clusters with oxygen.55,56 The higher catalytic performance
of CoNC-0.5 shows that Co–Nx moieties which act as a co-cata-
lyst can effectively improve the catalytic activity of NC catalyst.
Interestingly, although the relative content of Co NPs
decreases, CoNC-x gives higher conversion (entry 4–8, Table 2)
4. Reusability of the catalyst
For metal catalysts, stability is a strongly important factor as
metal ions are easily oxidized during oxidation reaction which
will result in the decrease of catalytic activity. However, the as-
prepared catalyst in this paper exhibits outstanding stability
which can be reused for several cycles without loss of activity. As
shown in Fig. 6, there remains 91% activity aer recycling
6 times. A thermal treatment on reused catalyst is applied to
than CoNC-PR. According to the analysis above, this can be
attributed to (i) casein enhances defect and disorder level in the
catalyst, which usually dominate their properties and also the
burn the covered intermediates on the active sites and edges of
catalytic activity; (ii) casein improves dispersion of Co NPs
the catalyst (entry 11, Table 2).39 It gives conversion of 20%,
through increasing the number of anchoring sites in the carbon
a little lower than that of 21% at sixth run. The mild decrease in
structure; (iii) casein can increase the nitrogen content in
catalytic activity may be attributed to the loss of catalyst in the
material and enhance its catalytic activity by tailoring its
process of transfer. The result shows that there almost no
electron-donor properties.
intermediate covers on the surface of catalyst. The structure and
composition of the used catalyst is investigated by XPS. As
shown in Fig. 7, no apparent structural changes are found in XPS
According to the results above, both casein and Co NPs can
enhance the catalytic performance. In CoNC-x catalysts,
spectra of CoNC-0.5 before and aer reaction. The Co content in
fresh and reused CoNC-0.5 catalysts was determined by AAS
(Table 4). About 6.2% metal particles detached from the catalyst
and migrated to the liquid phase. The slight loss of metal
Table 3 Molecular oxygen oxidation of substituted hydrocarbons to
ketone using CoNC-0.5a
particles may be attributed to the powerful interaction between
metal particles and N–C moiety, which stabilize Co species in the
electron-rich N-doped carbon structure mainly through Co–N
bonds and effectively prevent metal particles from detaching
Entry Substrate
Main product
T (ꢀC) Conv. (%) Sel. (%)
1
2
3
160
100
160
14
30
5
67
69
63
4
100
61
84
5
6
100
100
77
67
92
56
7b
120
32
86
a
Fig. 6 Recyclability of CoNC-0.5 in ethylbenzene oxidation. Reac-
tions are carried out at 120 ꢀC for 5 h with 10 ml ethylbenzene and 30
mg CoNC-0.5 under 0.8 MPa oxygen pressure.
Typical conditions: 10 ml of substrate, 30 mg of catalyst, O2 (0.8 MPa),
5 h. b 3 g of substrate, 10 mg of catalyst, O2 (0.8 MPa), 5 h.
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Mineralogy and Metallogeny in Chinese Academy of Sciences
(No. KLMM20150103).
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5. Conclusions
In this work, cobalt-coordinated N-doped carbon is prepared by
pyrolyzing cobaltporphyrin under N2 atmosphere with casein as
supplementary nitrogen source. The CoNC catalyst originated
from cobaltporphyrin has high metal content, dispersion and
small particle size. The catalyst shows promising catalytic
activity for C–H bond oxidation with Co–Nx plays the role of
active site. Moreover, it is highly stable and can be recycled
several times without losing its activity. Thus, this study has
demonstrated that pyrolysis of cobaltporphyrin with casein as
supplementary nitrogen source might be a viable route to
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