Solvent-free aerobic selective oxidation of hydrocarbons catalyzed by porous graphitic carbon encapsulated cobalt composites

Article abstract of DOI:10.1039/c8nj03492c

The development of noble-metal-free heterogeneous catalysts that can catalyze the solvent-free oxidation of C-H bonds with molecular oxygen as an oxidant is a profound challenge. Herein we report a facile strategy for the synthesis of a porous graphitic carbon encapsulated cobalt composite (Co@C₈₀₀) via the pyrolysis of a phenolic resin containing Co²⁺. Catalyst Co@C₈₀₀ is highly active in the selective aerobic oxidation of hydrocarbons under mild and solvent-free conditions (120 °C, 0.8 MPa O₂, 5 h), showing the best conversion of 60.7% with 95.7% selectivity, and can be stably reused at least 6 times. The full catalyst characterization and experimental tests demonstrate that the basic porous graphitic carbon and highly dispersed CoO_x particles can be responsible for the good catalytic performance.

Full text of DOI:10.1039/c8nj03492c

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January 2016 Pages 1–846
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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE
Solvent-free Aerobic Selective Oxidation of Hydrocarbons
Catalyzed by Porous Graphitic Carbon Encapsulate Cobalt
Composites
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Yuchen Jiang, Chenjun Zhang, Yue Li, Pingping Jiang, Jiusheng Jiang and Yan Leng*
www.rsc.org/
The development of noble-metal-free heterogeneous catalysts that can catalyze the solvent-free oxidation of C-H bonds
with molecular oxygen as an oxidant is a profound challenge. Herein we report a facile strategy for the synthesis of porous
2+
graphitic carbon encapsulate cobalt composite (Co@C800) via the pyrolysis of a phenolic resin containing Co . The catalyst
o
Co@C800 is highly active for the selective aerobic oxidation of hydrocarbons under mild and solvent-free conditions (120 C,
0
.8 MPa O
The full catalyst characterizations and experimental tests demonstrate that the basic porous graphitic carbon and highly
dispersed CoO particles can be responsible for the good catalytic performance.
2
, 5 h), showing the best conversion of 60.7% with 95.7% selectivity, and can be stably reused at least 6 times.
x
1
3-15
low cost, easy recovery and separation.
Especially,
Introduction
nitrogen-doped carbon materials with rich pore structure and
excellent electrical conductivity have been recognized as
superior candidates for catalyst supports and catalytic
Selective oxidation of organic compounds, such as alcohols,
saturated or unsaturated hydrocarbons, to the high value-
added aldehydes and ketones is of great importance in green
1
6-18
materials.
developed as catalysts for heterogeneous catalysis.
Various carbon supported metal NPs have been
1
9,20
1
-3
organic synthesis. In recent years, many noble-metal-based
palladium, platinum, ruthenium) homogeneous and
However, the weak interaction between metal particles and
carbon support for these supported catalysts often lead to
aggregation or leaching of active metal species, which
(
heterogeneous catalysts have been developed that show
superior catalytic activity and selectivity for oxidation of
organic compounds, but the ease of separation and reuse or
2
1
inevitably limits the practical applications. Recently, the
-6 encapsulation structure of metal particles in protective
carbon/nitrogen-doped carbon frameworks (M@C) has proved
effective in changing the catalytic performance and enhancing
4
the high price greatly restrict the further usage in industry.
Therefore, heterogeneous catalysts based on more abundant
transition metals such as Fe, Co and Ni have attracted much
attention and been considered as a promising substitute for
noble metals in selective oxidation reactions and other
2
2-25
the stability of noble metal catalysts.
Therefore, it is of
great importance to extend this facial encapsulation approach
to more cheap transition metals.
7
-10
catalytic reactions.
However, in order to improve the
Phenolic resins derived from the polymerization of phenols
and aldehydes, are commonly employed as excellent
precursors for the synthesis of “rigid” carbon materials, and
meanwhile display a strong ability to chelate various metal
activity of these transition metal-based catalysts, various
additives like bases and solvents were usually introduced into
reaction systems, which cause many difficulties in products’
1
1,12
separation and catalysts’ recovery.
Consequently, in terms
2
6
ions. On the other hand, as an earth-abundant metal, Co has
been widely used as catalyst in many chemical processes,
of sustainable and environmentally reaction process, additive-
and solvent-free catalytic systems with molecular oxygen as an
oxidant and effective low coat transition metal catalysts will be
desirable for heterogeneous catalysis.
2
7-29
including aerobic oxidation reactions.
Thus, it is rational to
use phenolic resins as chelating agent for Co and carbon
source to fabricate porous carbon encapsulated Co catalysts
for selective oxidation reactions. Herein, we report a facile
approach in which Co particles were encapsulated into
Carbon-based materials offer many advantages as the
support due to their good physical and chemical properties,
2
+
graphitic carbon framework through pyrolysis of Co -
containing phenolic resins processor derived from
polymerization and chelation of resorcinol, formaldehyde,
cobalt acetate, and glycine. A mesoporous structure with a
2
high surface area (up to 175 m /g) and uniform pore size (4.8
nm) is observed for the obtained samples, which is highly
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ARTICLE
Journal Name
o
advantageous in heterogeneous catalysis. The catalyst rate of 15 C/min. The signals were monitored by a thermal
Co@C800 shows an outstanding activity, selectivity, and conductivity detector (TCD).
stability toward the selective oxidation of hydrocarbons under
o
solvent- free and molecular oxygen conditions (120 C, 0.8
Catalyst preparation
2
MPa O , 5 h, 10 ml ethylbenzene and 25 mg Co@C800).
N-doped porous carbon monolith was prepared by
copolymerization of resorcinol and formaldehyde followed by
3
0
Experimental
a pyrolysis process according to the previous literature. In a
typical synthesis, 3 g of resorcinol was dissolved in 20 mL
water, then 4.42 g of formaldehyde (37 wt%) and 1.5 g cobalt
acetate was added during stirring, noted as solution A.
Solution B contains 1.0 g of L-lysine dissolved in 30 mL water.
Solution A was quickly decanted into solution B with vigorous
stirring to obtain a homogeneous solution. Over a 1-min
Materials and Characterization
All the chemicals used in the experiment were of analytical
grade and used without further purification. The morphology
and the elemental distribution were studied by a field emission
scanning electron microscope (FESEM; Hitachi S-4800,
accelerated voltage: 5 kV) accompanied by Energy dispersive
X-ray spectrometry (EDS; accelerated voltage: 20 kV). Scanning
electron microscopy (SEM) images were performed on a
SUPERSCAN SSX-550 electron microscope. Transmission
electron microscopy (TEM) images were recorded with a JEOL
JEM-2100 electron microscope operated at 200 kV. X-ray
photoelectron spectroscopy (XPS) was conducted on a PHI
period, a dusty pink bulk polymer was prepared and dried at
o
0 C for 24 h. The resultant monolithic precursor was then
o
pyrolysis at the desired temperature (450-800 C) for 2 h with
5
o
-1
a heating rate of 5 C min . The samples were denoted as
Co@C450, Co@C600, Co@C700 and Co@C800 corresponding to
o
activation temperatures of 450, 600, 700 and 800 C. The
control samples without metal ions were denoted as C800, C600
,
5000 Versa Probe Xray photoelectron spectrometer equipped
C
3 4
450. Co O
was obtained by direct pyrolysis of cobalt acetate
o
at 400 C in air, and Co/Cno-N was prepared by pyrolysis the
with Al Karadiation (1486.6 eV). The C 1s peak at 285.2 eV was
used as the reference for the binding energies. The nitrogen
sorption isotherms and pore size distribution curves were
measured at the temperature of liquid nitrogen (77 K) using a
o
mixture of glucose and cobalt acetate at 800 C in Ar.
BELSORP-MINI analyzer. XRD patterns were collected on the Catalytic test
Bruker D8 Advance powder diffractometer using Ni-filtered
The selective oxidation of hydrocarbons was carried out in a 50
Cu/Kα radiation source at 40 kV and 20 mA, from 5 to 90°with
a scan rate of 4°/min. GC (SP-6890A) was equipped with a FID
detector and a capillary column (SE-54 60 m×0.32 mm×0.25
μm). Temperature programmed desorption (TPD) was carried
mL stainless steel autoclave reactor with magnetic stirrer.
Typically, substrates (10 mL) and catalyst (25 mg) were added
2 2
into the reactor and filled with 0.8 MPa of O , and O was fed
continuously to maintain constant pressure. The reaction was
o
kept at 120 C for 5 h with continuous magnetic stirring at 900
out by using CO
treated in He flow at 150 C for 3h and cooled to room
temperature. After being saturated with CO , the sample was
2
as the probe molecule. Sample was pre-
o
rpm. After reaction, the catalyst was separated and recovered
via centrifugation, and the dual internal standard of 1,4-
dichlorobenzene and bromobenzene as well as the as obtained
2
purged with He for 1h at room temperature to sweep the
o
physical molecule. Then sample was heated by 950 C at the
2
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clarified reaction mixture were added into the absolute alcohol which are denoted as Co@C450, Co@C600, Co@C700, Co@C800
.
solvent. Finally, the products were analyzed quantitatively by TG analysis reveals that the organic species start to
o
gas chromatography (GC). The recovered catalyst was washed decompose at 300 C and were completely carbonized at 700
o
three times with absolute alcohol, and then dried at 50 C, and
o
C (Figure S1). The Co loading on Co@C800 was 4.25 wt %, as
directly reused for the next run. For the retreatment of determined by inductively coupled plasma atomic emission
catalyst, the recovered catalyst at the fifth run was calcinated spectroscopy (ICP-AES).
o
at 600 C in Ar and then reused.
2
The N adsorption isotherms and corresponding pore size
distributions of obtained samples are shown in Figure 1, and
the specific surface areas and pore volumes are summarized in
o
prepared at 450-700 C show
Results and discussion
Table 1. As can be seen, Co@C
t
type III adsorption isotherms, a characteristic of nonporous
o
materials. When the temperature reached 800 C, the
Catalysts preparation and characterization
t
The detailed route to synthesize Co@C is shown in Scheme 1.
adsorption isotherm of Co@C800 was transformed into type IV
First, the Co-containing composite was synthesized through a
solution-based process using resorcinol, formaldehyde, cobalt
acetate, and lazy glycine as the monomers. It should be
emphasized that no organic solvents were used in this process.
The -OH groups of resorcinol and N-H on lazy glycine
with obvious H hysteresis loops in the relative pressure range
2
of 0.5-1.9, indicating an interconnected porous structure. The
specific surface area increased with carbonization temperature
o
below 700 C (376-446 m²/g). However, with the temperature
o
of 700 C as the dividing line, Co@C has a very narrow pore
2
+
800
coordinate to Co , producing uniform Co(II)-PR hybrid. To
optimize the synthetic process, the effect of the lazy glycine
amount on the hybrid yield was investigated (Table SI,
Supporting information). The Co(II)-PR hybrid was then
size distribution of about 4 nm in the mesopore region, and
the surface area and micropore volume decreased to 175 m²/g
3
-1
and 0.024 cm g , respectively (Table 1). Obvious, Co@C from
microporous materials transformed to mesoporous materials,
which can be attributed to continuous rearrangement of
o
thermally treated in Ar at 450-800 C, during which Co(II) is
reduced to Co-based nanoparticles (Co@Co
3
O
4
NPs) and PR
o
carbon frameworks during carbonization at 800 C. In addition,
polymer gradually restructured into N-doped graphitic carbon
1,32
the increase of BET surface area could be achieved (from 175
to 198 m²/g, Figure S2) by decreasing the Co loading amount in
the phenolic resin processor. The specific surface area of Co-
free graphic carbon (519 m²/g, Figure S2) is much larger than
that of Co@C800 due to the release of space occupied by Co
particles.
3
framework.
Thus, the pyrolysis of Co(II)-PR renders a
NPs encapsulated in graphitic carbon,
composite of Co@Co
3 4
O
SEM and TEM images were taken to investigate the
morphology of Co@C (Figure 2). Co@C shows a sphere-like
t
800
morphology that the particles size is around 200-300 nm with
rough surface (Figure 2A and 2B). Co@Co O NPs are clearly
3
4
seen as uniformly embedded inside the granular-like carbon
sphere. The well-dispersed Co particles (10-40 nm) completely
encapsulated inside the carbon framework are also seen in the
TEM image of Co@C800 (Figure 2D). Besides, the EDS elemental
mapping images indicate the homogeneous distribution of Co,
N, O, and C on Co@C800 (Figure 2C). The HTEM image of
Co@C800 reveals a core-shell structure that Co particle was
encapsulated in Co O (Figure 2E). The carbon framework is
3
4
multiple layers with the d-spacing of 0.3 nm, corresponding to
the typical layer spacing of high-quality graphitic carbon
(Figure 2F). Such mesoporous graphite shells can improve the
electric conductivity and lead to efficient mass transport
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capability, and thus enhance catalytic activity of metal/metal
The full survey spectrum of Co@C800 indicates the
oxides toward the selective oxidation reactions. Inside the existence of main elements C, O, Co, and N (Figure S4B and
graphitic carbon, two types of lattice plane distance measured Table S2 ). The C 1s spectrum reveals the formation of C-C
to be 0.2 and 0.25 nm are distinguishable, corresponding to (284.78 eV), C-OH (285.58 eV), and C-N (289.94 eV) in Co@C800
4
2,43
the planes of Co and Co
3
3
O
4
(Figure 2F, Figure S3), (Figure S4 A).
The O 1s spectrum exhibits two types of O
3,34
respectively.
Figure 3A illustrates the XRD patterns of Co@C
environments with binding energies at 532.36 and 529.94 eV
4-46
. As for attributed to C=O and O-Co, respectively (Figure 3D).
4
t
The N
Co@C800, the peak at 26° is attributed to the (002) plane of 1s can be deconvoluted into two peaks with the binding
graphitic carbon, the diffraction peaks at 44.2, 51.5, and 75.9° energies of 401.30 and 397.45 eV, which are attributed to
4
7,48
can be assigned to cubic reflections of Co, and the peaks at graphitic N and pyridinic N, respectively (Figure 3E).
3
The
Only strong basic sites in Co@C800 may be related to the generation
5-37
3 4
36.8 and 65.7° were assignable to the Co O phase.
o
of these N species C, and may also have influence on the
very weak peaks at 36.8 and 44.2° were observed for Co@C700
.
o
At lower carbonization temperatures (< 700 C), almost no catalytic performance.The peaks in the range of 770-810 eV
3
peaks were observed, which proves the absence of crystalline can be assigned to Co 2p and Co 2p of Co (Figure 3F). The
/2
1/2
2
metal/metal oxide phases. The presence of two characteristic deconvoluted Co 2p profile accounts for Co (781.04 eV), Co
+
3+
-
1
-1
peaks of D band (1310 cm ) and G band (1596 cm ) in the (779.67 eV)
49-51
, and their shake-up (satellites) peaks.
Due to
0
Raman spectroscopy of Figure 3B confirms the existence of the very limited depth of election of XPS, the Co encapsulated
3
8-40
graphitic carbon structure for Co@Ct.
The relative ratios of in Co O and carbon hardly showed any recognizable signals in
3 4
5
2
D band to G band integrated intensities were increased, the Co 2p spectrum.
respectively, indicating that the crystallization degree of
graphitic carbon was increased with an increase in pyrolysis
Catalytic oxidation of hydrocarbons
temperature. Additionally, the appearance of the 2D band at
-
1
2
615 cm indicates that graphitic structures were well The catalytic performances of the obtained samples for
41
developed in Co@C800
.
selective oxidation of ethylbenzene was first carried out under
o
The CO -TPD technique was employed to examine the solvent-free conditions at 120 C and 0.8 MPa O . The results
2
2
basicity of obtained samples (Figure 3C). For Co@C800, a broad are listed in Table 2, the main product was acetophenone and
o
peak between 700 and 800 C was observed, which was due to by-products were phenethyl alcohol and benzaldehyde. Poor
the strong base site. However, only sharp peaks were conversion and selectivity were obtained in the absence of
observed for Co@C
and Co@C700 at temperatures lower catalyst. Co@C450 exhibited a very low conversion of 7.0% with
00
6
o
than 500 C. These results clearly indicate the existence of 73.6% selectivity. It is similar to that of Co O . Co@C and
3
4
600
strong basic sites for Co@C800
.
Co@C700 presented significantly improved catalytic activity
4
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showing 38-40% conversions and 93% selectivity. Interestingly, system. Although the higher calcination temperature for C-t
the catalyst Co@C800 with strong basicity and good play a positive influence on the catalytic performance, only 10-
mesoporous structure displayed the best activity of 60.7% 30% conversions with 70-80% selectivity was obtained under
conversion and 95.7% selectivity. In contrast, the N-free the present reaction conditions.
sample Co/Cno-N prepared by pyrolysis the mixture of glucose
It is noteworthy that in the previously reported metal-
and cobalt acetate displayed only 42.7% conversion and 83.3% catalyzed oxidation of ethylbenzene systems, a low nsub/nmetal
selectivity, which further confirms the important role of N. It ratio usually required to achieve a high catalytic efficiency
was reported that N-doped carbon materials could catalyze (Table S3). In this work, only 25 mg catalyst was added in the
5
3-55
the oxidation of hydrocarbons.
To verify this, Co-free reaction system with 10 mL ethylbenzene. The nsub/nmetal ratio
carbon samples were prepared and applied to catalyze the of 38857 is much higher than those of the previous reports.
oxidation of ethylbenzene with O as oxidant in solvent-free The TOF was calculated to be 4514 and 510 based on the
2
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surface Co species and total Co species in Co@C800
respectively. This result is almost the highest TOF reported
thus far, representing clear advantage for practical
,
Conflicts of interest
The authors declare no conflicts of interest.
a
applications. Moreover, the catalytic activity of Co@C800 is also
comparable or better than those of previous reported
heterogeneous catalysts (Table S3).
Acknowledgements
We thank the Fundamental Research Funds for the Central
Universities (JUSRP51623A) and MOE & SAFEA for the 111
Project (No. B13025).
3 4
The above control experiments indicate that Co@Co O
NPs act as the main active centres toward the selective
oxidation of ethylbenzene. The basic sites in mesoporous N-
doped carbon framework also have an important role in
5
6,57
enhancing the catalytic performance.
Actually, the opinion
Notes and references
that strong basic support can facilitate the conversion and
selectivity for the oxidation of ethylbenzene has been reported
1
2
3
4
5
6
Y. Leng, J. J. Li, C. J. Zhang, P. P. Jiang, Y. Li, Y. C. Jiang and S.
Y. Du, J. Mater. Chem. A 2017, , 17580-17588.
P. Zhang, H. Lu, Y. Zhou, L. Zhang, Z. Wu, S. Yang, H. L. Shi, Q.
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5
8
5
by many research groups. It is believed that the strong basic
sites could assist metal to activate the substrate, and made it
easier for the intermediate to transform into oxidative
6
5
9
product. Therefore, both the highly dispersed Co@Co
and basic sites in mesoporous carbon framework may be
account for the good catalytic performance of Co@C800
Accordingly, a possible reaction mechanism for the oxidation
of ethylbenzene with O over Co@C800 was proposed in
3 4
O NPs
2
013, 4, 1593.
.
Y. Z. Chen, Z. U. Wang, H. Wang, J. Lu, S. H. Yu and H. L. Jiang,
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[29,58]
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Co@C800 also exhibited appropriate conversions and
selectivity in selective oxidation of other commonly
hydrocarbons, such as cumene, n-propylbenzene, tetralin,
indane, and diphenylmethane (Table 3). The catalytic
reusability of Co@C800 for oxidation of ethylbenzene is shown
in Figure S5. As can be seen, the catalyst could be separated
easily by filtration, and the recycled catalyst in the fourth run
showed conversion of 27% with selectivity of 82%, indicating a
slow decrease in activity and selectivity for reused one. The FT-
7
8
9
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1
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1
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o
recovered catalyst through calcination at 600 C in Ar. To our
5903.,
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1
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2
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2
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hydrocarbons, displaying 60.7% conversion and 95.7%
o
selectivity (solvent-free, 120 C, 0.8 MPa O
2
, 5 h). In addition,
2
the catalyst Co@C800 possesses convenient recovery, good
Environ. Sci. 2014,
7, 1919-1923.
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,
graphitic carbon framework were proved to be advantageous
for the oxidation of hydrocarbons, which led to a superior
catalytic activity. Such fabrication strategy of porous graphitic
carbon encapsulated Co particles would have more hopeful
prospects for supported metal catalysts in catalytic
applications.
4
2
2
3
6
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New Journal of Chemistry
Page 8 of 8
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DOI: 10.1039/C8NJ03492C
Graphical Abstract
Solvent-free Aerobic Selective Oxidation of Hydrocarbons
Catalyzed by Porous Graphitic Carbon Encapsulate Cobalt
Composites
Yuchen Jiang, Chenjun Zhang, Yue Li, Pingping Jiang, Jiusheng Jiang and Yan Leng*
The Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material
Engineering, Jiangnan University, Wuxi 214122, Jiangsu Province, China. Fax: +86-510-85917763; Tel:
+86-510-85917090; E-mail: yanleng@jiangnan.edu.cn.
Cobalt particles encapsulated in graphitic carbon (Co@C800) was demonstrated to be an effective
catalyst for the selective oxidation of hydrocarbons.