Cobalt single atoms anchored on nitrogen-doped porous carbon as an efficient catalyst for oxidation of silanes

Article abstract of DOI:10.1039/d0gc03498c

The oxidation reactions of organic compounds are important transformations for the fine and bulk chemical industry. However, they usually involve the use of noble metal catalysts and suffer from toxic or environmental issues. Here, an efficient, environmentally friendly, and atomically dispersed Co catalyst (Co-N-C) was preparedviaa simple, porous MgO template and etching method using 1,10-phenanthroline as C and N sources, and CoCl₂·6H₂O as the metal source. The obtained Co-N-C catalyst exhibits excellent catalytic performance for the oxidation of silanes with 97% isolated yield of organosilanol under mild conditions (room temperature, H₂O as an oxidant, 1.8 h), and good stability with 95% isolated yield after nine consecutive reactions. The turnover frequency (TOF) is as high as 381 h^?1, exceeding those of most non-noble metal catalysts and some noble metal catalysts. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), extended X-ray absorption fine structure (EXAFS), and wavelet transform (WT) spectroscopy corroborate the existence of atomically dispersed Co. The coordination numbers of Co affected by the pyrolysis temperature in Co-N-C-700, Co-N-C-800, and Co-N-C-900 are 4.1, 3.6, and 2.2, respectively. Owing to a higher Co-N₃content, Co-N-C-800 shows more outstanding catalytic performance than Co-N-C-700 and Co-N-C-800. Moreover, density functional theory (DFT) calculations reveal that the Co-N₃structure exhibits more activity compared with Co-N₄and Co-N₂, which is because the Co atom in Co-N₃was bound with both H atom and Si atom, and it induced the longest Si-H bond.

Full text of DOI:10.1039/d0gc03498c

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Cobalt single atoms anchored on nitrogen-doped
porous carbon as an efficient catalyst for oxidation
of silanes†
Cite this: Green Chem., 2021, 23,
026
1
Fan Yang,‡ Zhihui Liu,‡ Xiaodong Liu, Andong Feng, Bing Zhang, Wang Yang and
Yongfeng Li
*
The oxidation reactions of organic compounds are important transformations for the fine and bulk
chemical industry. However, they usually involve the use of noble metal catalysts and suffer from toxic or
environmental issues. Here, an efficient, environmentally friendly, and atomically dispersed Co catalyst
(
Co–N–C) was prepared via a simple, porous MgO template and etching method using 1,10-phenanthro-
line as C and N sources, and CoCl ·6H O as the metal source. The obtained Co–N–C catalyst exhibits
excellent catalytic performance for the oxidation of silanes with 97% isolated yield of organosilanol under
mild conditions (room temperature, H O as an oxidant, 1.8 h), and good stability with 95% isolated yield
2
2
2
after nine consecutive reactions. The turnover frequency (TOF) is as high as 381 h−1, exceeding those of
most non-noble metal catalysts and some noble metal catalysts. Aberration-corrected high-angle annular
dark-field scanning transmission electron microscopy (HAADF-STEM), extended X-ray absorption fine
structure (EXAFS), and wavelet transform (WT) spectroscopy corroborate the existence of atomically dis-
persed Co. The coordination numbers of Co affected by the pyrolysis temperature in Co–N–C-700, Co–
N–C-800, and Co–N–C-900 are 4.1, 3.6, and 2.2, respectively. Owing to a higher Co–N
N–C-800 shows more outstanding catalytic performance than Co–N–C-700 and Co–N–C-800.
Moreover, density functional theory (DFT) calculations reveal that the Co–N structure exhibits more
activity compared with Co–N and Co–N , which is because the Co atom in Co–N was bound with both
H atom and Si atom, and it induced the longest Si–H bond.
3
content, Co–
Received 17th October 2020,
Accepted 21st December 2020
3
DOI: 10.1039/d0gc03498c
4
2
3
rsc.li/greenchem
green and atom-economical feature, most of the studies were
focused on noble metals, which serve as the catalytic centre of
The green oxidation reaction of organic compounds, such as silane oxidations, such as Au, Pd, Pt, Ag, Ir, and Ru.
1
. Introduction
6
7
8
9
10
11
silanes, is among the most important transformations in Although most noble metal catalysts show excellent activity,
organic synthesis to build up key intermediates for the syn- the limited supply and high cost of precious metals restrict
thesis of the agrochemical, pharmaceutical, and other fine their sustainable development. Non-noble metal catalysts
1
12
13
chemicals. Although organosilanes can produce the corres- including Mn(ClO
4
)
2
·6H
2
O,
Cu
3
(BTC)
2
,
nanoporous
1
4
15
1a
ponding products under the action of strong oxidants, such as copper, NaY zeolite, and Ni powder have received great
2
3
4
5
dioxiranes, osmium tetroxide, ozone, and oxaziridines,
consideration for silane oxidations due to their relatively abun-
chemo-selective oxidation of silanes is a great challenge dant reserves and low cost. However, non-noble metal catalysts
because of the condensation or the decomposition of silanol. often require more stringent reaction conditions, and they
Recently, great efforts have been devoted to developing hetero- show poor catalytic performance. Therefore, it is desirable to
geneous catalysis for the oxidation of silanes to silanols, which search for more economical, efficient, and eco-friendly alterna-
represents an eco-friendly approach based on the use of tives to catalyse the oxidation of organosilane.
oxygen, water, or hydrogen peroxide as the oxidant. Despite its
Single-atom catalysts (SACs) are emerging as a new frontier
in heterogeneous catalysis due to their excellent catalytic per-
1
6
formance. As one of the most remarkable noble-metal-free
SACs, M–N–C catalysts (M = Ni, Fe, Co) have been widely inves-
tigated for a wide range of applications, including organic
State Key Laboratory of Heavy Oil Processing, China University of Petroleum
(
Beijing), Beijing 102249, China. E-mail: yfli@cup.edu.cn
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc03498c
These authors contributed equally.
†
1
7
molecular transformation reactions, electrochemical oxygen
1
8
19
20
‡
2
reduction reaction (ORR), CO reduction, hydrogenation,
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2
1
22
CO oxidation, proton exchange membrane fuel cells, and sonic agitation. The mixture was boiled for 24 hours in a reflux
2
3
other chemical processes due to their unique advantages apparatus. After filtration and drying, the material obtained
including maximum metal utilization, good recyclability, and was ground into a fine powder. Finally, the powder was cal-
unique electronic structure. Moreover, Co–N–C catalysts with cined at 550 °C for one hour to remove water.
excellent performance have attracted wide attention as a cata-
2
.3. Synthesis of cobalt/nitrogen-doped porous carbon
hybrids
oxidation, Wacker-type oxidation, oxidative cleavage of 1,2- The typical procedure for the preparation of the catalysts is
lyst in organic oxidation reactions such as peroxymonosulfate
2
4
25
oxidation, oxidative esterification of furfural, ethylbenzene
2
6
27
2
8
29
2 2
diols, oxidative esterification of primary alcohols, sulfide described as follows: a mixture of CoCl ·6H O (475.8 mg,
3
0
31
oxidation, and oxidative dehydration of N-heterocycles. We 2 mmol) and 1,10-phenanthroline (720.8 mg, 4 mmol) in
speculate that the Co–N–C catalyst has such good oxidation ethanol (200 mL) was stirred for 30 minutes at room tempera-
performance due to the unique electronic structure among ture. Then, porous MgO layers (2 g) were added, and the whole
cobalt, nitrogen, and carbon. Therefore, it is highly desirable reaction mixture was stirred at 60 °C for 2 hours. The reaction
to develop a SAC Co–N–C catalyst with high efficiency for the mixture was cooled to room temperature, and ethanol was
oxidation of organosilanes to organosilanols under mild removed by rotary evaporation. The remaining solid sample
conditions.
was ground into a powder, and then pyrolyzed at 800 °C for
Herein, we report a facile approach to synthesize SAC Co– 2 hours under an argon atmosphere. After cooling to room
N–C catalysts via the one-step pyrolysis of cobalt–phenanthro- temperature, the obtained black powder (labelled as Co–N–C–
−1
line complexes on porous MgO sheets and acid leaching. MgO) was washed with 5 mol L hydrochloric acid with reflux
Owing to its unique structure, Co–N–C catalysts exhibit excel- for 30 minutes and 2 mol L− H SO at 80 °C for 6 h, followed
1
2
4
lent catalytic activity for the oxidation of organosilanes. by thorough washing with deionized water. Synthesis pro-
Notably, the catalyst is very easily recycled, which can be con- cesses at different calcination temperatures (700, 800, and
veniently reused ten times. HAADF-STEM and EXAFS spec- 900 °C) were performed, and the products were annotated as
troscopy were used to reveal the existence of atomically dis- Co–N–C-700, Co–N–C-800, and Co–N–C-900, respectively. For
persed Co and its coordination number with the N atom in comparison, nitrogen-doped porous carbon (labeled as NPC)
synthesized catalysts. The controlled trials and DFT are carried was prepared was synthesized by a similar process except for
2 2
out to identify the active sites and understand the reaction the absence of CoCl ·6H O, and cobalt/porous carbon hybrids
mechanism of the catalysts for the oxidation of organosilanes. (labeled as Co–PC) using phenanthrene as the carbon source.
Moreover, Co–N–C catalysts also show excellent catalytic
2
.4. Synthesis of Co-nanoparticle decorated nitrogen-doped
activity in the oxidative esterification of alcohols.
carbon
Co nanoparticles decorated nitrogen-doped carbon (Co NC)
were synthesized according to the preparation method of Ni
2
. Materials and methods
3
3
NC by our research group. Firstly, 3 g triphenylmethane and
g MgO template were mixed in ethanol solution under mag-
2
.1. Materials
5
All chemicals and solvents were purchased from commercial
suppliers without further purification: magnesium oxide
netic stirring at room temperature. The obtained composite
was pyrolyzed at 800 °C for 2 h under an argon atmosphere.
After cooling to room temperature, porous carbon was
obtained after removal of the MgO template with hydrochloric
acid. Then, 160 mg porous carbon was dispersed in ethanol by
ultrasonication, followed by the addition of 80 mg of
(Sinopharm Chemical Reagent Co., Ltd); triphenylmethane,
iron(II) oxalate dihydrate (Aladdin Industrial Corporation); palla-
dium acetate (Wako Pure Chemical Industries, Ltd); 1-butyl-
3
-methylimidazolium-tetrafluoroborate and cuprous iodide
(
Tokyo Chemical Co., Ltd); nitrobenzene, 4-nitroanisole, 4-nitro-
CoCl
ground with 400 mg of melamine, followed by pyrolysis at
00 °C for 2 h under an argon atmosphere. The Co NC was syn-
thesized by cooling to room temperature.
2 2
·6H O. After ethanol was removed, the dried product was
toluene, and cesium fluoride (Sinopharm Chemical Reagent
Co., Ltd); 8-nitroquinoline and benzoxazole (Sun Chemical
Technology (Shanghai) Co., Ltd); 1,3-dinitrobenzene, p-nitro-
benzoic acid, o-nitrobenzoic acid, and methyl 4-nitrobenzoate
8
2
.5. Procedure for the oxidation of organosilanes
In a typical catalysis reaction, 2 mg Co–N–C catalyst, 2 ml
acetone, H O (108 μL), and dimethylphenylsilane (DMPS)
92 μL, 0.6 mmol) were added in a glass microreactor at room
temperature under magnetic stirring. The reaction was moni-
tored by thin layer chromatography (TLC). After completion of
the reaction, the catalyst was filtered off and washed
(Xiya Chemical Industry Co., Ltd); 4-nitrobenzonitrile, p-nitroa-
cetophenone, iodobenzene, p-nitrophenol, o-nitrophenol, silver
oxide, and phenylacetylene (J&K Scientific Co., Ltd); acetone,
ethanol, hydrogen peroxide, tetrahydrofuran, sulfuric acid, and
magnesium sulphate (Beijing Chemical Works).
2
(
2
.2. Preparation of the MgO template
Porous layer MgO was prepared according to the previously thoroughly with ethyl acetate and acetone. Afterward, the dried
3
2
reported literature. First, the purchased MgO powder (30 g) catalyst was reused for the next cycle. The solvent from the fil-
was mixed with deionized water (300 mL) followed by ultra- trate was evaporated, and the product was purified in a silica
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column using petroleum ether (60–90 °C) as the eluent. radiation. The Brunauer–Emmett–Teller (BET) surface area
Besides, the recyclability of the Co–N–C-800 catalyst was measurement was characterized by nitrogen adsorption at
further evaluated in the oxidation of DMPS with twice the 77 K using an SSA-4200 Surface Area and Pore Size analyzer
amount of catalyst (4 mg).
(Beijing Builder Electronic Technology Co., Ltd) instrument.
The leaching of Co in consecutive cycles was determined by
2
.6. Procedure for the oxidation of alcohols to esters
inductively coupled plasma optical emission spectrometry
1
(
ICP-OES).
H
NMR spectra were recorded on
a
The Co–N–C-800 catalyst (10 mg), K CO (11.1 mg, 0.1 mmol),
2
3
JNM-LA300FT-NMR for examining the final product of the oxi-
dation of silanes and alcohols. The X-ray absorption fine struc-
ture spectra (Co K-edge) were collected using a 1 W1B station
in Beijing Synchrotron Radiation Facility (BSRF). The storage
rings of BSRF were operated at 2.5 GeV with a maximum
current of 250 mA. Using a Si (111) double-crystal monochro-
mator, data collection was carried out in the transmission
mode using an ionization chamber. All spectra were collected
under ambient conditions. The acquired EXAFS data were pro-
cessed according to the standard procedures using the
ATHENA module implemented in the IFEFFIT software
methanol (3 mL), and benzylic alcohol (0.4 mmol) were mixed
in a micro reaction vial. The reaction was stirred under 1 bar
of oxygen at 60 °C for 12 h, and the reaction process was moni-
tored by TLC. After completion of the reaction, the mixture
was cooled to room temperature diluted with ethyl acetate.
Then, the catalyst was filtered off through a short silica
column with ethyl acetate to remove the catalyst, and the
product was purified with a silica column using ethyl acetate/
petroleum ether (60–90 °C) as the eluent.
2
.7. DFT computational details
3
packages. The k -weighted EXAFS spectra were obtained by
Density functional theory (DFT) calculation was performed by
subtracting the post-edge background from the overall absorp-
tion and then normalizing with respect to the edge-jump step.
3
the DMol code of Materials Studio. The generalized gradient
approximation
of
the
revised-Perdew–Burke–Ernzerhof
3
Subsequently, k -weighted χ(k) data of Co K-edge were Fourier
3
(GGA-RPBE) function, and DFT-D dispersion correction by
transformed to real (R) space using a hanging window (dk =
3
4
Grimme were used to treat all energy changes. The core treat-
ment was the effective core potential (ECP), and the basis set
was DNP v4.4. The k-point was set as 1 × 1 × 1, and the size of
−1
1
.0 Å ) to separate the EXAFS contributions from different
coordination shells. To obtain the quantitative structural para-
meters around the central atoms, least-squares curve para-
meter fitting was performed using the ARTEMIS module of the
IFEFFIT software packages.
3
5
the vacuum region was 20 Å. The detailed calculation para-
3
6
meters were reported in our previous studies. The typical
Co–N –C (x = 2–4) substrates were modified by a 5 × 5 super
x
y
3
7
cell of graphene, and all atoms were relaxed. The adsorption
energy of DMPS on different surfaces was defined by eqn (1).
3
. Results and discussion
Eads ¼ Ecalþmol ꢀ Emol
ð1Þ
3.1. Catalyst characterization
where Ecat+mol is the energy of adsorbed molecule on catalyst
surface, Ecat is the energy of the bare catalyst surface, and Emol
is the energy of the molecule in the gas phase. To locate the
transition state of a certain reaction, the complete linear syn-
chronous transit (LST)/quadratic synchronous transit (QST)
method was applied, the RMS convergence was below 4.0 ×
The Co–N–C catalysts were facilely prepared via thermal pyrol-
ysis of carbon, nitrogen, and Co containing complex precur-
sors (Co(Phen) Cl ) using porous MgO as a template. The
2 2
pyrolysis temperatures are 700, 800, and 900 °C, corresponding
to the products labelled as Co–N–C-700, Co–N–C-800, and Co–
N–C-900, respectively. And Co–N–C catalysts were finally
obtained after the removal of the porous MgO templates and
Co particles by acid leaching (Scheme 1). The morphologies of
Co–N–C-800 were first characterized by SEM and TEM images.
Fig. 1a shows a laminar structure appearance, which is attribu-
−
3
−1
1
0
Ha Å . Then the intrinsic reaction path (IRP) was con-
3
8
firmed by the nudged elastic band (NEB) method.
2
.8. Characterization
The morphology of the catalyst was characterized using a ted to the thin MgO sheets. In Fig. 1b and c, no cobalt-contain-
Hitachi SU8010 scanning electron microscope (SEM) operated
at 3.0 kV and a transmission electron microscopy (TEM, Tecnai
G2, F20). The aberration-corrected scanning TEM image was
taken using a Titan Cubed Themis G2 300 equipped with a
spherical aberration corrector. The crystalline phase of the cat-
alysts was identified by X-ray diffraction (XRD) on a Rigaku D/
Max 2500 X-ray diffractometer with Cu Kα radiation (18 kW).
Raman spectra were recorded with a Renishaw inVia confocal
Raman microscope system using green (532 nm) laser exci-
tation. The elemental composition of samples was obtained by
X-ray photoelectron spectroscopy (XPS) with an ESCA Lab220i-
XL electron spectrometer from VG Scientific using 300 W Al Kα carbon hybrids.
Scheme 1 The synthesis process of cobalt/nitrogen-doped porous
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Fig. 1 (a) SEM image, (b) TEM image, (c) high-resolution TEM image, (d)
aberration-corrected HAADF-STEM image, single atoms are highlighted
by red circles of Co–N–C-800. (e) TEM image and (f) enlarged image
with EDX maps, C (red), N (orange), Co (green) of Co–N–C-800.
Fig. 2 Nitrogen sorption isotherms (a) and pore size distribution (b) of
Co–N–C-700, Co–N–C-800, and Co–N–C-900. XRD patterns (c) and
Raman spectra (d) of Co–N–C-700, Co–N–C-800, Co–N–C-900, NPC,
and Co–PC.
ing nanoparticles but amorphous carbon layers in a highly
porous structure with nanopores of 3–15 nm were clearly
observed in Co–N–C-800. To provide information at an atomic
scale, subångström-resolution HAADF-STEM technique was characterization as shown in Fig. 2c and Fig. S5.† The pro-
used to inspect the Co–N–C-800 (Fig. 1d), which shows that a nounced diffraction peaks at 26.5°, 42.3°, 42.9°, 62.2°, 74.7°,
large amount of uniformly dispersed Co single atoms is clearly and 78.6° are ascribed to the reflections of C (002), C (100),
observed. The energy-dispersive X-ray spectroscopy (EDX) MgO (200), MgO (220), MgO (311), and MgO (222), respectively.
elemental mapping shows that the Co species were detected The diffraction peaks at 44.2°, 51.5°, and 75.8° in Co–N–C–
along with C and N elements all homogeneously distributed MgO can be assigned to the (111), (200), and (220) planes of
throughout the whole area of the porous carbon (Fig. 1e and the Co nanocrystal (PDF 15-0806), respectively. Besides, the
f). Moreover, compared with the commonly used carbon sup- diffraction peak at 36.9° can be ascribed to the (311) planes of
3
9
ports, the use of the porous MgO presents outstanding the Co O nanocrystal (JCPDS 43-1003). These results demon-
3
4
4
0
3 4
advantages of preventing the aggregation of cobalt. For com- strated that the Co and Co O nanocrystals were formed by the
parison, NPC, Co–PC, and Co NC samples were also prepared pyrolysis procedure. However, no diffraction peaks of Co,
by a similar procedure. These control samples (Co–N–C-700, Co O , and MgO can be observed in Co–N–C catalysts, NPC,
3
4
Co–N–C-900, NPC, Co–PC, and Co NC) show similar mor- and Co–PC, implying that the Co, Co
phologies as Co–N–C-800 (Fig. S1–S3†). Fig. S3† shows that Co MgO component are completely removed during acid leaching.
nanoparticles are uniformly dispersed on the Co NC surface. These results illustrate that N was important for the stabiliz-
The N adsorption–desorption isotherms of the Co–N– ation of a single Co atom. To further estimate the graphitiza-
3 4
O nanocrystals, and
2
C-700, Co–N–C-800, Co–N–C-900, NPC, and Co–PC catalysts tion degree of these carbon materials, Raman spectra of the
show a typical type IV isotherm with a hysteresis loop, indicat- as-prepared porous carbons are recorded (Fig. 2d). Two con-
ing the existence of abundant porous structure (Fig. 2a, b and spicuous peaks at about 1353 cm− can be assigned to the D
1
−
1
Fig. S4†). In good agreement with the TEM result, the Co–N– band for disordered carbon and defects, and 1589 cm is
C-800 samples possess a wide pore size of 2–85 nm. Moreover, regarded as the G band for E vibrational mode present in the
2
g
2
N
2
adsorption–desorption measurements reveal that Co–N– sp -bonded graphitic carbon. The intensity ratio of the D band
C-700, Co–N–C-800, and Co–N–C-900 catalysts exhibit meso- to G band (I /I ) is a significant parameter reflecting the
porous structural features with high surface areas of 716, 625, defect and disorder degree of graphene. The increased ID/IG
D
G
2
−1
and 468 m
g , respectively. The reduction of the specific intensity ratios of Co–N–C-800, Co–N–C-900, NPC, and Co–PC
surface area may be due to the fact that the pores of the cata- catalysts are observed, and among them Co–N–C-800 and Co–
lyst are damaged to a certain extent at high temperature. And N–C-700 exhibit the highest value of I /I , indicating that
D
G
the 3D reconstruction videos of the Co–N–C-800 catalyst are more defective sites or disorders are introduced by Co and N
shown in the ESI (Movies S1 and S2†). Such a porous mor- atoms into the carbon network.
phology, providing a high BET specific surface could maximize
XPS measurements are further carried out to probe the
the exposure of active sites and enhance the mass transfer and chemical compositions and contents of the materials. The XPS
4
1
adsorption of reactant, which is desired for catalysis. Further spectra of Co–N–C-700, Co–N–C-800, and Co–N–C-900 clearly
insights into the structural properties are obtained from XRD reveal the presence of C, O, N, and Co elements (Fig. 3a), and
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XANES results to cobalt phthalocyanine (CoPC) with a small
difference in the edge peaks located at 7710–7716 eV, which
are regarded as the fingerprint of Co–N
4
square-planar struc-
4
5
tures, resulting from a 1s to 4p electron transition (Fig. 3e).
3
The Fourier-transformed k -weighted EXAFS spectra (Fig. 3f)
displays one main peak at 1.5 Å, corresponding to the Co–N/O
first coordination shell, and no Co–Co coordination peak at
4
6
2
.2 Å can be detected. These messages mean that no Co par-
ticles exist in as-synthesized catalysts, and the CoN structure
x
is integrated into the carbon frameworks. Owing to the force-
ful resolution in K and R spaces, WT was used to study the
atomic configuration of Co–N–C samples (Fig. 3g). The WT
plots in Co–N–C shows the maximum peak at 4 Å− , corres-
ponding to the Co–N coordination by contrasting with Co foil
1
−1
and CoPc. In addition, no intensity maximum at 8 Å corres-
ponding to Co–Co coordination can be observed. These results
further illustrate the isolated feature of Co species in Co–N–
C-700, Co–N–C-800, and Co–N–C-900 catalysts. The quantitat-
ive EXAFS fitting results reveal that the coordination numbers
of Co in Co–N–C-700, Co–N–C-800, and Co–N–C-900 are 4.1,
Fig. 3 XPS spectra (a) of Co–N–C-800, Co–N–C-900, NPC, and Co–
PC, and the C 1s spectra (b), high resolution N 1s XPS spectrum (c), and
3.6, and 2.2 (Fig. S7 and Table S3†), respectively, and reduction
Co 2p XPS spectrum (d) of Co–N–C-800. XANES spectra at the Co
3
K-edge (e), Fourier transform (FT) k -weighted χ(k)-function of the of the number with temperature increase due to the destruc-
19a
EXAFS spectra (f) and the wavelet transform EXAFS plots (g) of Co–N– tion of Co–N at high temperatures,
which neatly supports
C-700, Co–N–C-800, Co–N–C-900, CoPC, and Co foil.
XPS results. These results demonstrate the homogeneous dis-
persion of Co throughout the whole material. Such finely dis-
persed Co species coordinate with the N atoms to form Co–N
the peaks of C 1s, N 1s, and O 1s are at 283.8, 398.0, 530.7, moieties, which are highly beneficial for the catalysis of
4
2
7
79.6, and 795.6 eV, respectively. Generally, the C 1s spec- organic transformations.
trum of Co–N–C-800 in Fig. 3b shows four peaks with binding
energies of 284.6, 285.0, 286.1, and 288.2 eV, which can be
3.2. Catalytic performance
assigned to the graphitic C, C–N, CvO, and OvC–O, respect- We used Co–N–C catalysts for the oxidation of organosilanes
ively. This result illustrates that N was successfully doped into to organosilanols, and the results are summarized in Table 1.
the carbon materials. The high-resolution N 1s spectra of Co– The catalysis of 0.6 mmol of DMPS with 6 mmol H O in
2
N–C-800 can be deconvoluted into five different peaks with different solvents, such as acetone, ethyl acetate, tetrahydro-
binding energies of 397.9, 398.4, 399.2, 400.5, and 404.4 eV, furan, and H O, using 2 mg Co–N–C-800 was initially per-
2
corresponding to pyridinic N, Co–N, pyrrolic N, graphitic N, formed at room temperature. The catalytic activity of Co–N–
2
5
and oxidized N (Fig. 3c), respectively. Table S1† outlines the
atomic surface concentrations, and high N content determined
by XPS of Co–N–C-700, Co–N–C-800, and Co–N–C-900 are 8.24,
a
Table 1 The oxidation of organosilanes under different conditions
8
.46, and 6.71 at%, respectively. The Co content in Co–N–
C-700, Co–N–C-800, and Co–N–C-900 are 2.1, 2.5, and 2.1 wt%,
respectively. The amount of Co is high in Co–N–C catalysts
owing to the fact that a high nitrogen content helps anchor
4
3
more cobalt atoms. The contents of Co–N are decreased with
the increase in temperature (Fig. 3c, Fig. S6, and Table S2†),
indicating that some Co–N bonds were broken at high temp-
erature. The Co 2p spectrum of Co–N–C-800 exhibits two peaks
at 780.0 and 795.5 eV, corresponding to the Co 2p3/2 and Co
d
Entry
Catalyst
Solvent
Time (h)
Yield (%)
1
2
3
4
5
Co–N–C-800
Co–N–C-800
Co–N–C-800
Co–N–C-800
NPC
NPC + CoCl
Co–PC
L + CoCl
Co NC
Co–N–C-700
Co–N–C-900
Acetone
Ethyl acetate
THF
1.8
2.1
7.8
25
2
3
2
3
3
1.8
1.8
97
96
94
82
7
20
0
2
H O
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
2
p
1/2, respectively (Fig. 3d). The binding energy at 780.0 eV and
satellite peaks at 783.8 and 801.9 eV are characteristics of Co
species coordinating with N atoms in the graphitic layer. The
X-ray absorption spectroscopy (XAS) measurements, contain-
ing EXAFS and X-ray absorption near-edge structure (XANES), 11
b
6
2
·6H
2
O
+
2
7
8
c
4
4
2
2
·6H O
9
9
22
84
80
10
were performed to further understand the local atomic coordi-
nation and electronic structure of the Co species at the atomic
level. Co–N–C-700, Co–N–C-800, and Co–N–C-900 show similar 1,10-phenanthroline. L (1.8 mg), CoCl
a
Reaction conditions: Silanes (0.6 mmol), cat. Co–N–C-800 (2 mg), and
b
c
H
2
O (108 μL) in 2 mL solvent. NPC (2 mg), CoCl
2 2
·6H O (1.5 mg). L =
d
2
·6H O (1.2 mg). Isolated yield.
2
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a
C-800 decreased in different solvents in the following order: Table 2 Co–N–C-800 catalyzed the oxidation of organosilanes
acetone > ethyl acetate > tetrahydrofuran (THF) > water.
Among the above solvents, acetone proved to be the best reac-
tion medium mainly because it shows good performance to
well dissolve reactants and provides a highly dispersed system
for the Co–N–C-800 catalyst. Ethyl acetate and THF show
inferior performance, which is possibly because the water solu-
bility and catalyst dispersibility are a little bit lower than
acetone. The worst catalytic activity of the oxidation of DMPS
in water perhaps owing to poor dispersibility performance of
the Co–N–C-800 catalyst in water and the immiscibility of the
1
0b,14
DMPS with water (entries 1–4).
species of the catalyst, a series of control experiments, using
For identifying the active
a
Reaction conditions: 0.6 mmol silanes, 108 μL H
2
O in 2 mL acetone,
b
2
mg Co–N–C-800 catalyst, room temperature. Isolated yield. Reaction
NPC, NPC and CoCl ·6H O; Co–PC, 1,10-phenanthroline and
2
2
conditions: 0.6 mmol silanes, 1 mL methanol, 2 mg Co–N–C-800 cata-
lyst, room temperature. Reaction conditions: 0.6 mmol silanes, 1 mL
c
2 2
CoCl ·6H O; Co–N–C-700; and Co–N–C-900 as catalysts were
d
carried out. NPC, NPC and CoCl
2
·6H
2
O; Co–PC, 1,10-phenan- ethanol, 2 mg Co–N–C-800 catalyst, room temperature. Reaction con-
ditions: 0.6 mmol silanes, 1 mL isopropanol, 2 mg Co–N–C-800 cata-
throline and CoCl ·6H O; and Co NC showed extremely low
2
2
lyst, room temperature.
activities, affording methyldiphenylsilanol in 7%, 20%, 0%,
%, and 22% respectively (entries 5–9). It is demonstrated that
9
the N doped carbon, the Co salt, and Co nanoparticles contrib-
ute little to this reaction. Co–N–C-700 and Co–N–C-900 show a
lower performance than Co–N–C-800, giving methyl-
diphenylsilanol in 84% and 80%, respectively (entries 10 and
Although the turnover frequency (TOF) value of the catalyst cal-
culated from the oxidation of organosilanes to organosilanols
1
(
381 h− based on the total amount of Co species) is lower
than that of precious metal single-site Au catalyst and Pd
nanocatalyst (Pd/XC-72-700-Ar) (entries 2 and 3), Co–N–C-800
is prepared by a facile one-step calcination. Impressively, Co–
N–C-800 shows better catalytic activity than some noble–metal
catalysts (Pd/MgO catalyst and Au nanoparticles) (entries 4 and
1
1). Based on the control experiments and characterization
results, it can be concluded that the Co single atoms bonded
to N atoms in the graphitic sheets play a vital role in the
overall catalytic performances. It is possible that the Co–N
species show better activation performance than Co–N and
Co–N owing to the fact that the higher content of Co–N exists
3
2
5
) and other heterogeneous catalysts (Cu (BTC) , nanoporous
3 2
4
3
copper, NaY zeolite, and Ni powder) (entries 6–9) under com-
parative reaction conditions. Compared with homogeneous
in Co–N–C-800 compared with that in Co–N–C-700 and Co–N–
C-900. In addition, the gram-scale reaction also shows an
excellent catalytic effect within 3 h, which indicates that our
catalyst has the potential for large-scale application
catalysts (TC-6, Mn(ClO ) ·6H O, and lacunary polyoxotung-
4 2
2
state), it also exhibits comparative catalytic activity under com-
parative reaction conditions (entries 10–12). In short, Co–N–
C-800 delivers superior performance in the oxidation of PMDS.
Moreover, the recyclability of the Co–N–C-800 catalyst is
further examined in the oxidation of DMPS. After the reaction,
the catalyst was filtered off and washed thoroughly with ethyl
acetate and acetone for the next run. The isolated yield
obtained from each run is shown in Fig. 4a. Although the
chemical selectivity remains great all the time, more time is
needed to maintain the good yield of dimethylphenylsilanol.
We found that the yield of dimethylphenylsilanol remained
(Scheme 2).
In view of sustainable development, we further explored the
generality of the oxidation of organosilanes in acetone by
using Co–N–C-800 as a catalyst (Table 2). Satisfyingly, the oxi-
dation of phenyl substituted silanes, such as triphenylsilane
and methyldiphenylsilane, giving triphenylsilanol and methyl-
diphenylsilane in good to high yield (entries 1 and 2).
Unfortunately, we discover that the catalyst exhibits an inferior
reactivity for the hydrolytic oxidation of aliphatic silanes (entry
3
), which was also observed in some reported noble metal-
1
5
based catalysts. Noteworthily, the oxidation of DMPS with
alcohols in the corresponding alcohol solution were also per-
formed, producing the corresponding silyl ethers in high
yields (entries 4–6).
We also compared the efficiency of Co–N–C-800 with that of
the ever-reported catalysts in the literature (Table S4†).
Fig. 4 The yields and reaction times for 10 cycles of the Co–N–C-800
catalyzed oxidation of DMPS (a) and TEM image (b) of Co–N–C-800
after 10 cycles.
Scheme 2 Gram-scale oxidation reaction of DMPS.
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95% in 13 h after the fifth cycle. This is probably because the
Green Chemistry
∼
adhesion of some organic substances on the catalyst surface
prevented the active center from coming in contact with
DMPS. However, the Co–N–C-800 catalyst can be reused ten
times without obvious deactivation. These results illustrate
that the Co–N–C-800 catalyst exhibits good catalytic activity
and recyclability. To analyse the reason for the inactivation of
the catalyst and confirm that the reaction was indeed catalyzed
by solid Co–N–C rather than by homogeneous Co species, the
leaching test was performed. After the catalytic oxidation of
DMPS was carried out for 1 h under standard conditions, the
Fig. 5 The investigated stable structures of DMPS adsorption on Co–
N –C (x = 2–4), as well as the adsorption energy and bond length of
x y
catalyst was removed from the vessel by filtering with the Si–H. The gray, blue, light blue, yellow and white balls represent carbon,
product produced in 56% yield at this time (Fig. S8†). No nitrogen, cobalt, silicon, and hydrogen atoms, respectively.
further reaction took place after removing the catalyst from the
reaction mixture. After the reaction, 1.53 ppm of Co was
detected in the reaction supernatant, which was confirmed by was retained in the basal plane, and the adsorption energy was
ICP-OES. Thus, these results illustrate that leaching Co cannot only ∼1.71 eV. All the bond lengths of Si–H were increased,
catalyse the oxidation of the DMPS reaction, and the decreased compared to the Si–H of DMPS in the gas phase (1.496 Å),
activity of the Co–N–C-800 catalyst is primarily due to metal suggesting the activation of the Si–H bond. In particular, for
leaching during the recycle reactions. In addition, the Co–N– Co–N , the Co atom was bonded with both the H atom and Si
3
C-800 catalyst after cycling test has also been examined by atom, and it induced the longest Si–H bond, indicating the
TEM (Fig. 4b), and the morphology of the recycled catalyst highest catalytic activity, which is coincident with the experi-
does not change.
mental observations mentioned above.
Eventually, to extend the applications of the catalyst, we
In order to understand the nature of the highest catalytic
further use the Co–N–C-800 catalyst for the esterification of activity of Co–N
alcohol, and the results are shown in Table S5.† The reaction the DMPS molecule, bare Co–N
was performed at 60 °C using 1 bar of oxygen in the presence adsorbed on the Co–N –C surface as shown in Fig. 6. The elec-
of K CO
. The oxidation of alcohols proceeds relatively comple- trical properties of the Co atom were positive on the Co–N –C
tely, yielding esters with high selectivity in excellent yields surface in the first place, during the process of adsorption,
entries 1–8). Both electron-donating (–CH and –OCH
) and charge transfer occurred between the Co atom and the DMPS
3
–C, we also plotted the electron densities of
–C substrate, and DMPS
3
3
2
3
3
(
3
3
electron-withdrawing groups (–Cl) on the alcohols have little molecule, by which the adsorption strength was augmented.
effect on the reaction yield or selectivity. In addition, the Therefore, we can conclude that the coordinative unsaturation
spatial group effect has little impact on the reaction. However, of the Co atom and the bulge of the Co atom above the Co–N
–
3
alcohols substituted with –NO
2
need a long time to obtain a C surface is beneficial for reducing the steric hindrance
high yield (entry 9). The results show that Co–N–C-800 has between the reactant and the basal plane. Furthermore, the
excellent properties, which can be used in many kinds of mole- charge transfer between dimethylphenyl–silane and the Co–
cular transformation reactions.
3
N –C surface also augment the activation of the Si–H bond,
which can be considered as the essential reason for the
highest catalytic activity of Co–N3.
3
.3. DFT calculations
To understand the incorporation effect of Co, several possible
Combined with the experimental results and the literature
configurations of Co–N –C models were considered and opti- of the metal-catalysed oxidation of DMPS, a possible mecha-
4
7
x
y
mized as shown in Fig. S9.† For the aspect of relative energy, nism is given in Fig. 7a. A representative Co–N –C structural
3
the substrates with plane quadrilateral coordination of Co model was used to evaluate the whole reaction cycle. The reac-
atom (four-fold coordination) were always the more stable con-
figurations. However, for catalytic reactions, catalyst surface
with coordinatively unsaturated sites (CUS) possess high cata-
lytic activity, and full coordination of the Co atom in the gra-
phene plane may not improve the reactivity. As a result, we
further calculated the adsorption energy of DMPS on different
substrates. As shown in Fig. 5, energetically favoured configur-
ations of typical Co–N
Besides, the other possible configurations of Co–N
different Co–N coordination are also considered and shown in
Fig. S10.† Among these, the bulge of the Co atom in Co–N –C
Co–N –C, and Co–N –C significantly increased the adsorption
strength of DMPS, while for Co–N2+2–C of Co–N , the Co atom adsorbed DMPS on Co–N
x
–C
y
(x = 2–4) structures are presented.
x
–C with
y
2
1
,
3
3
2
Fig. 6 The electron density of (a) DMPS, (b) Co–N
–C. Isovalue = 0.5 a.u.
–C substrate, and (c)
3
4
3
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good catalytic activity for the oxidative esterification of alcohols.
This work may provide a cheaper and more efficient catalyst for
the oxidation of organosilane at a large scale.
Author contributions
Fan Yang, Zhihui Liu conceived and designed the experiments.
Fig. 7 (a) Proposed pathway for the hydrolysis of organosilanes cata- Zhihui Liu and Andong Feng performed laboratory experi-
3
lysed by Co–N–C-800. (b) Oxidation of DMPS over Co–N –C. TS, tran-
ments. Bing Zhang and Wang Yang conducted part of the
characterization analysis of the material. Xiaodong Liu pro-
vided theoretical calculation and discussion. Yongfeng Li and
Fan Yang were responsible for planning and supervising the
project.
sition state. The values denote the relative energy referenced to the
initial state.
tion began with the insertion of the Si–H bond into the active
Co centre to generate a silyl–metal hydride intermediate. The
silyl–metal hydride intermediate was subsequently attacked by
a nucleophile derived from water to generate the silanol and
release hydrocarbon gases. Moreover, to reveal the catalytic
Conflicts of interest
There are no conflicts to declare.
3
activity of Co–N –C, the energy (the values denote the relative
energy referenced to the initial state) of each step along the
reaction cycle has been calculated (Fig. 7b). At the beginning
of the reaction, a DMPS molecule dissociatively adsorbed on
Acknowledgements
the graphene surface. The bulk molecule was bonded to the This work was supported by the National Natural Science
Co atom of one Co–N –C site through the Si atom and the H Foundation of China (No. 21776302 and 21776308), and the
3
proton was captured by another Co–N
3
–C site with an adsorp- Science Foundation of China University of Petroleum, Beijing
tion energy of −6.43 eV, which is even more than the adsorp- (No. 2462020YXZZ033).
tion energy of DMPS on a single Co–N –C site. Then, a water
3
3
molecule approached the surface of Co–N –C through van der
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