Bimetallic oxide nanoparticles confined in ZIF-67-derived carbon for highly selective oxidation of saturated C–H bond in alkyl arenes

Article abstract of DOI:10.1002/aoc.6047

Zeolite imidazolate frameworks (ZIFs) have recently emerged as an ideal type of carbon precursors with abundant tailorability. In this work, a series of ZIF-derived porous carbon catalysts have been prepared with encapsulation of bimetallic oxide nanoparticles via simple thermal treatment. The composition and structure of these catalysts were confirmed in detail by different characterization methods. The bimetallic oxide (Mn/Co, Fe/Co, and Cu/Co) nanoparticles were encapsulated in the nitrogen-doped graphitized carbon matrix. Moreover, the hierarchically porous structure and carbon defects were successfully constructed in the carbon catalysts. Additionally, in the selective oxidation of saturated C–H bonds in alkyl arenes, the carbon catalysts demonstrate outstanding performance for the oxidation of C–H bonds to corresponding carboxyl groups. This was due to their unique structure can greatly promote mass transfer and molecular oxygen activation, resulting in high conversion and high selectivity. Remarkably, this work here could also provide a novel strategy to the controllable synthesis of metal–organic frameworks (MOFs)-derived carbon catalysts for enhanced performance in heterogeneous catalysis.

Full text of DOI:10.1002/aoc.6047

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Received: 24 June 2020
DOI: 10.1002/aoc.6047
Revised: 15 September 2020
Accepted: 22 September 2020
F U L L P A P E R
Bimetallic oxide nanoparticles confined in ZIF-67-derived
carbon for highly selective oxidation of saturated C–H bond
in alkyl arenes
Cheng Huang1
| Xiaoyan Su1 | Xiangyu Gu2 | Rui Liu2 | Hongjun Zhu2
1Department of Chemical and
Pharmaceutical Engineering, Southeast
University ChengXian College, Nanjing,
China
2Department of Applied Chemistry,
College of Chemistry and Molecular
Engineering, Nanjing Tech University,
Nanjing, China
Zeolite imidazolate frameworks (ZIFs) have recently emerged as an ideal type
of carbon precursors with abundant tailorability. In this work, a series of ZIF-
derived porous carbon catalysts have been prepared with encapsulation of
bimetallic oxide nanoparticles via simple thermal treatment. The composition
and structure of these catalysts were confirmed in detail by different character-
ization methods. The bimetallic oxide (Mn/Co, Fe/Co, and Cu/Co)
nanoparticles were encapsulated in the nitrogen-doped graphitized carbon
matrix. Moreover, the hierarchically porous structure and carbon defects were
successfully constructed in the carbon catalysts. Additionally, in the selective
oxidation of saturated C–H bonds in alkyl arenes, the carbon catalysts demon-
strate outstanding performance for the oxidation of C–H bonds to
corresponding carboxyl groups. This was due to their unique structure can
greatly promote mass transfer and molecular oxygen activation, resulting in
high conversion and high selectivity. Remarkably, this work here could also
provide a novel strategy to the controllable synthesis of metal–organic frame-
works (MOFs)-derived carbon catalysts for enhanced performance in heteroge-
neous catalysis.
Correspondence
Cheng Huang, Department of Chemical
and Pharmaceutical Engineering,
Southeast University ChengXian College,
Nanjing 210088, China.
Email: huangcheng@cxxy.seu.edu.cn
Funding information
Strategic Pioneer Program on Space
Science, Chinese Academy of Sciences,
Grant/Award Numbers: XDA15013101,
XDA15013100; Youth Foundation of
Southeast University ChengXian College,
Grant/Award Number: z0017; National
Key Research and Development Program
of China, Grant/Award Number:
2017YFB0307202; Natural Science
Foundation of Jiangsu Province-
Outstanding Youth Foundation, Grant/
Award Number: BK20170104
K E Y W O R D S
bimetallic oxide, C–H bonds oxidation, hierarchical pores, ZIF-67-derived carbon
1 | INTRODUCTION
catalysts usually faced with leaching and aggregation
problems, leading to the loss of catalytic efficiency. Thus,
High activity and selectivity in catalytic aerobic oxidation
of C–H bonds are the current research focuses in both
academic and industrial fields.[1–4] Heterogeneous cata-
lysts are expected to achieve this goal due to their pre-
cisely regulated structure and composition. Notably,
various supported transition-metal nanoparticles (NPs)/
clusters and oxides have attracted considerable attention
in C–H bonds oxidation. It was found that the highly dis-
persed metal sites in solid catalysts would greatly benefit
the catalytic performance.[5–7] However, the supported
developing an appropriate support for dispersing and
anchoring metal is imperative in catalytic oxidation of
C–H bonds.
In recently years, carbon materials known as their
superior chemical durability, large specific surface area,
and low-cost have been applied as supports in heteroge-
neous catalysis.[8–10] In particular, carbon embedded with
metal NPs and oxides exhibit excellent catalytic activity
towards C–H bond catalytic oxidation.[11,12] Generally,
doping the electron-rich heteroatom N in these catalysts
Appl Organomet Chem. 2020;e6047.
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https://doi.org/10.1002/aoc.6047

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HUANG ET AL.
can tune their electronic properties and redox abilities
through charge transfer. Moreover, the basicity of the car-
bon catalyst could be improved by the incorporated N dop-
ant, thus enhanced the catalytic performance in oxidation
reaction.[13,14] Zhang et al.[15] reported Pd NPs supported
on the N-doped carbon catalyst, which was showed much
higher catalytic activity than other Pd catalyst in aerobic
oxidation of C–H bonds. The nitrogen dopant activated
the electronic effect of Pd NPs and further enhanced the
adsorption and activation of the substrates. Sun et al.[16]
deposited Cu NPs on the N-doped graphene and used in
inert C–H bonds selective oxidation. The enrichment of
electron density in the carbon matrix by the N atoms pro-
moted the charge transfer between metal and graphene,
which greatly increased the conversion of substrates.
Meanwhile, the interaction between pyridinic-N and Cu
NPs improved the stability of catalyst and ensured the cat-
alytic performance in several runs. However, these
supported metal NPs are easy to leach from the carbon sur-
face during the oxidation process, and thus, the durability
of catalyst remains a great challenge. Besides, the synthe-
sis of N-doped carbon catalysts in these methods required
extra nitrogen sources, which increases the amount of
organic reagent in the preparation of catalyst.
Metal–organic frameworks (MOFs) are a class of
porous materials that construct from metal cations and
organic linkers.[17–19] It is widely known as their high sur-
face area, abundant metal sites, ordered pore structure,
and readily tailorable framework, making MOFs desirable
catalyst supports.[20] Recently, MOFs concerned as poten-
tial materials for preparing functional porous carbons in
many fields.[21–24] Remarkably, some MOF-derived carbon
matrices incorporated with metals, for example, gold, plat-
inum, cobalt, and manganese, have been extensive
explored for catalytic reaction, which showed superior cat-
alytic efficiency.[25–27] Zeolitic imidazolate frameworks
(ZIFs), especially ZIF-67, owe to the high density of Co
sites, ultrahigh porous properties, and rich N elements
that enable usage of ZIF-67 for an ideal sacrificial template
for the N-doped carbon support.[28,29] The metal sites
could uniformly disperse and firmly anchor in the carbon
framework by direct carbonization of ZIF-67. Besides, the
N atoms are also doped in the formed porous carbon after
pyrolysis of ZIF-67, and thus, the cobalt-nitrogen synergis-
tic effect facilitates the catalytic activity.[13] Unfortunately,
a drawback restricts the ZIF-67-derived carbon catalyst
use in catalytic reaction: microporous-dominated struc-
ture in the carbon matrix, which hinders mass transfer
and the accessibility of Co sites.[30] Lately, some strategies
have developed rational carbonization of ZIF-67 with hier-
archical pores for enhancing catalytic activity. An effective
method is that the mesoporous SiO2 induces the formation
of mesopores in ZIF-67-derived carbon.[31] However, the
SiO2 as hard template needs to be removed by hydrofluoric
acid (HF) or sodium hydroxide (NaOH), increasing risk
and difficulty in the preparation process. Therefore, it is
essential to obtain hierarchical pores in the ZIF-67-derived
carbon catalyst via a simple and universal strategy.
With the above in mind, a facile approach was used
to encapsulate bimetallic oxide NPs into mixed-metal
ZIF-67 (Co/Zn)-derived carbon with hierarchical pore.
Cobalt and zinc nitrates were firstly used as metal nodes
to synthesize mixed-metal ZIF-67 (Co/Zn) precursors. To
ensure the metal NPs can well disperse in the carbon
matrix, the metal (Mn, Fe, and Cu) aqueous were then
impregnated into the ZIF-67 (Co/Zn) pores at room tem-
perature by double-solvent method. Subsequently, these
precursors were carbonized under high temperature to
obtain porous carbon catalysts. During the carbonization
of these presynthesized precursor materials, the Zn atoms
were evaporated away from the framework, which forms
mesoporous structure in the carbon catalysts. As a result,
a series of bimetallic carbon catalysts were obtained
(M/Co-ZNC), and the as-prepared samples were detailed
characterized to confirm structure and composition.
These carbon materials exhibit high activity for selective
aerobic oxidation of substituted toluene to corresponding
carboxyl product. This work suggests that MOF-derived
carbon material could be an ideal candidate for heteroge-
neous catalytic oxidation.
2 | EXPERIMENTAL
2.1 | Chemicals
2-Methylimidazole (HMIM), cobalt nitrate hexahydrate
(Co (NO3)2Á6H2O), manganese nitrate hexahydrate
(Mn
(Fe
(NO3)2Á6H2O),
(NO3)3Á9H2O),
iron
cupric
nitrate
nitrate
nonahydrate
trihydrate
(Cu (NO3)2Á3H2O), and zinc nitrate hexahydrate
(Zn (NO3)2Á6H2O) were obtained by Energy Chemical
Co., Ltd. Dichloromethane (DCM), tert-butyl hydroperox-
ide (TBHP), methanol, and ethanol were provided from
Sinopharm Chemical Co., Ltd. and used without further
purification. The various substituted toluenes were pur-
chased from J&K Scientific Co., Ltd.
2.2 | Synthesis of mixed-metal
ZIF-67 (Co/Zn)
The synthesis of mixed-metal ZIF-67 was prepared by
modified previous work.[32] A 3.28 g of HMIM (40 mmol)
was dissolved in 120-mL methanol, and 1.19 g of Zn
(NO3)2Á6H2O (4 mmol) and 1.75-g Co (NO3)2Á6H2O

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HUANG ET AL.
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(6 mmol) were dissolved in 60-mL methanol, respec-
tively. And then, the metal solutions slowly added into
the HMIM solution with continuous stirring. After the
mixture continued to be stirred for 24 h, the purple
precipitates were collected by centrifugation, washed
with methanol for several times, and activated in vacuum
at 120ꢀC.
in the range of 400–4,000 cm−1. The X-ray photoelectron
spectroscopy (XPS) of the catalysts was acquired on an
Imaging Photoelectron Spectrometer (Escalab 250 Xi,
Thermo Scientific) with Al Kα radiation (1,486.7 eV).
Metal contents were determined by inductively coupled
plasma mass spectrometry (ICP-MS) spectroscopy
measured on an Agilent 7700 instrument.
2.3 | Synthesis of metal@ZIF-67 (Co/Zn)
2.6 | Catalyst activity tests
The metal precursors (Mn, Fe, and Cu) encapsulated in
ZIF-67 (Co/Zn) were based on double-solvent method
with some modifications.[33] In a typical procedure,
0.40 g of ZIF-67 (Co/Zn) was dispersed in 50-mL n-
hexane by ultrasonic treatment for 30 min. Then, 0.2-mL
aqueous solution dissolving 0.20 g of Mn (NO3)2Á6H2O
was dropped into the former hydrophobic solution in
30 min with vigorous stirring. After further oscillating
the mixture for 12 h on a shaker, the resulting purple
solid powders were collected by centrifugation and
washed with methanol for three times and then dried in
vacuum for 6 h. For the synthesis of other materials, Mn
(NO3)2Á6H2O was replaced by Cu (NO3)2Á3H2O and Fe
(NO3)3Á9H2O.
The catalyst activity was evaluated under oxygen atmo-
sphere, and the process includes the following steps: A
certain amount of carbon catalysts, 28 mmol of sub-
strates, and 10 mol% of additive TBHP were mixed and
added to a 15-mL sealed stainless reactor with oxygen
flow (0.6 MPa), under magnetic stirring at 100ꢀC for 12 h.
The oxygenated products were analyzed by high-
performance liquid chromatography (HPLC, Dionex,
P680A LPG-4) with nitrobenzene as the internal standard
substance. Each product was referenced to authentic
samples. After each reaction, the catalyst was isolated by
filtration, followed by washing with methanol, dried at
120ꢀC for 24 h and then used to evaluate the recyclability
of catalyst.
2.4 | Synthesis of M/Co-ZNC catalysts
3 | RESULTS AND DISCUSSION
To synthesize the carbon catalysts, the metal@ZIF-67
precursors were thermally treated in a tube furnace at
950ꢀC under Ar atmosphere for 3 h, and the ramp rate
was 5ꢀC/min. The resulting black solids were washed
with dilute sulphuric acid and denoted as Mn/Co-ZNC,
Fe/Co-ZNC, and Cu/Co-ZNC, respectively.
All M/Co-ZNC catalysts were obtained with high yield by
the procedure described in the Section 2. The PXRD anal-
ysis was used to confirm the crystal structure of the as-
prepared samples, and the spectra are shown in Figure 1.
The patterns of ZIF-67 (Co/Zn) are in accordance with
that reported by other groups (Figure 1a),[34,35] which
indicate the mixed metal ZIF-67 has the correct structure
with a high crystallinity. Meanwhile, no any impure
phase was detected in ZIF-67 (Co/Zn), which confirms
that the Co and Zn atoms were coordinated in the frame-
work. In addition, the Mn@ZIF-67 (Co/Zn) patterns are
very similar with ZIF-67 (Co/Zn) in the 2 theta range of
5.0–39.0ꢀ, proving the structure of ZIF-67 (Co/Zn) was
not destroyed after the encapsulated process. On the
other hand, the other four diffraction peaks at
2 theta = 40.6ꢀ, 42.9ꢀ, 45.3ꢀ, and 47.7ꢀ can be belonged to
the (220), (221), (310), and (311) crystalline planes of Mn
species, respectively.[36] These characteristic peaks
strongly indicate that the Mn is successfully encapsulated
in the pores of ZIF-67 (Co/Zn). After the thermal treat-
ment, the XRD pattern of Mn/Co-ZNC shows some new
diffraction peaks (Figure 1b). The board peak at
2 theta = 22.0ꢀ can assign to characteristic peak of par-
tially graphitized carbon.[37] The intense peaks at 19.1ꢀ,
2.5 | Catalyst characterization
Powder X-ray diffraction (PXRD) analyses were recorded
on a Rigaku MiniFlex 600 (Cu Kα, 5–80ꢀ). Specific sur-
face areas were measured on a Micromeritics ASAP 2460
apparatus using N2 adsorption–desorption with the
Brunauer–Emmett–Teller (BET) model. The pore sizes
were calculated by density functional theory (DFT)
method. The pretreatment of all samples was performed
at 100ꢀC for 12 h under vacuum. The morphology and
microstructure of as-synthesized catalysts were operated
by transmission electron microscopy (TEM) on a JEOL
JEM-2100F at 30 kV. The adsorption and desorption of
toluene were verified by Fourier transformation infrared
spectroscopy (FT-IR) on a Thermo Scientific Nicolet iS5
spectrophotometer using the standard KBr disk method

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HUANG ET AL.
images are displayed in Figures 2 and 3. It can be seen
that the samples still keep their morphologies after the
pyrolysis process, indicating that the organic sources
including C and N in ZIF precursor are sufficient for for-
ming the N-doped carbon matrix. The images in
Figure 2a,b show that the high-dispersed Mn/Co oxide
NPs are embedded in the carbon matrix. This confirms
that the Mn/Co oxide NPs are well confined in the cavi-
ties of the sample without any agglomeration. Interest-
ingly, it was found that a few NPs diffused out of
carbonized support, which might be attributed to
Ostwald ripening effect.[41] The mean diameter of Mn/Co
oxide NPs was found to be approximately 13.9 nm
(Figure 2c). It can be clearly observed that there was a
homogeneous distribution of Mn and Co elements in the
Mn/Co oxide NPs, strongly confirming the formation of
bimetallic oxide composite. From the high-resolution
TEM image (Figure 2d), the fringe distances of Mn/Co
oxide are calculated to be 0.2 nm, which corresponds to
the (111) crystal planes of Mn/Co oxide.[42] This well
coincides with PXRD results. Additionally, TEM analyses
in Figure 3 display that other two bimetallic oxide NPs
(Cu/Co and Fe/Co) were also highly dispersed encapsu-
lated by the carbon support, and the mean sizes were
14.0 and 13.4 nm, respectively. This can prove that
Cu/Co-ZNC and Fe/Co-ZNC samples were successfully
obtained by the pyrolysis process. The confirmed bimetal-
lic oxide NPs in the carbon support may be promoting
the electron transfer between two phases,[43] resulting in
outstanding catalytic oxidation performance.
The structure of carbon matrix was detected by
Raman spectra and depicted in the Figure 4. It was found
that all three samples exhibited two characteristic peaks,
which attributed to the D band at 1,351 cm−1 and G band
at 1,590 cm−1, respectively. The D band represents disor-
dered carbon that comes from the structural defects and
the doped heteroatoms in the matrix. The G band is origi-
nated from the graphitic carbon, and the graphitization
degree is decided by the temperature of pyrolytic treat-
ment.[44] Thus, the intensity ratio of D/G band (ID/IG) in
Raman spectra is a powerful evidence for studying struc-
tural derivation of the carbon samples. In the three as-
prepared carbon catalysts, the ratios of Mn/Co-ZNC,
Cu/Co-ZNC, and Fe/Co-ZNC were 1.04, 0.96, and 1.00,
respectively. This result implies that the relative high
degree graphitized of carbon has formed in these mate-
rials due to the high thermal temperature. Nevertheless,
some defects were generated in the carbon matrix by the
doped nitrogen atoms and the volatilized Zn atoms.
These existed defects would interact with the encapsu-
lated metal oxide to establish a metal–defect interaction
and is considered beneficial to the activation of oxygen
during the oxidation processes.[45]
FIGURE 1 Powder X-ray diffraction (PXRD) spectra of
as-prepared samples (a) ZIF-67 precursors and (b) Mn/Co-ZNC
catalyst
31.3ꢀ, 36.8ꢀ, 38.5ꢀ, 44.7ꢀ, 55.4ꢀ, 59.1ꢀ, and 64.9ꢀ were well
matched with the (111), (220), (311), (222), (400), (422),
(511), and (440) crystal planes of MnCo2O4.5 (JCPDS:
No. 32-0297).[38] This reveals the manganese-cobalt oxide
was formed in the carbon matrix and had good crystallin-
ity. No other diffraction peaks were detected, which
suggested the Zn atoms were completely removed by
vaporizing and washing procedure. Furthermore, the
Cu/Co-ZNC and Fe/Co-ZNC were characterized by XRD,
and their patterns were shown in Figures S1 and S2. The
characteristic peaks of graphitized carbon were also
detected in the Cu/Co-ZNC and Fe/Co-ZNC samples.
And the metal oxides could be indexed to the CuCo2O4
(JCPDS: No.01-1155) and FeCo2O4 (JCPDS: No.80-1540)
in the samples, respectively.[39,40] These above results
indicate that the bimetallic oxide NPs were successfully
encapsulated in ZIF-derived carbon.
TEM was used to investigate the morphologies and
microstructures of the M/Co-ZNC catalysts, and the