Single-Site AuI Catalyst for Silane Oxidation with Water

Article abstract of DOI:10.1002/adma.201704720

Single-site Au anchored on mpg-C₃N₄ (519 ppm Au loading) is developed as a highly active, selective, and stable catalyst for the oxidation of silanes with water with a turnover frequency as high as 50 200 h^?1, far exceeding most known catalysts based on total gold content. Other hydrosilanes bearing unsaturated functional groups also lead to corresponding silanols under mild reaction conditions without formation of any side products in good or excellent yields. The spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurements both confirm the atomic dispersion of Au atoms stabilized by mpg-C₃N₄. The coordination of the catalytically active Au^I by three nitrogen or carbon atoms in the tri-s-triazine repeating units not only prevents the Au atoms from aggregation, but also renders the surface Au^I highly active, which is completely different than homogeneous Au^I species.

Full text of DOI:10.1002/adma.201704720

CommuniCation
Single-Site Materials
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I
Single-Site Au Catalyst for Silane Oxidation with Water
Zheng Chen, Qi Zhang, Wenxing Chen, Juncai Dong, Hurong Yao, Xiangbo Zhang,
Xuanjue Tong, Dingsheng Wang, Qing Peng, Chen Chen,* Wei He, and Yadong Li
still remains as a major challenge to mini-
mize the Au aggregation and thereby to
achieve high-performance catalysis.
Single-site Au anchored on mpg-C N (519 ppm Au loading) is developed as
3
4
a highly active, selective, and stable catalyst for the oxidation of silanes with
−1
water with a turnover frequency as high as 50 200 h , far exceeding most
known catalysts based on total gold content. Other hydrosilanes bearing
unsaturated functional groups also lead to corresponding silanols under
mild reaction conditions without formation of any side products in good or
excellent yields. The spherical aberration correction electron microscopy and
extended X-ray absorption fine structure measurements both confirm the
atomic dispersion of Au atoms stabilized by mpg-C N . The coordination of
Silanols are important building blocks
[
9,10]
in polymer chemistry,
which are usu-
ally prepared from silanes. Tradition-
ally, strong oxidants with stoichiometric
amounts were often used in silanols pro-
duction, which, however, usually gave
undesired side products like siloxanes and
other side products that are unfriendly to
3
4
[
11–13]
I
the environment.
Selective oxidation
the catalytically active Au by three nitrogen or carbon atoms in the
of silanes with water has attracted wide
interests owing to its advantages in cost,
safety (nonexplosive, nonflammable, and
nontoxic), and waste production (hydrogen
tri-s-triazine repeating units not only prevents the Au atoms from aggregation,
I
but also renders the surface Au highly active, which is completely different
I
than homogeneous Au species.
[
14–18]
gas as the only by-product).
geneous Au catalysts are preferred owing
Hetero-
Organic transformation using heterogeneous Au catalysts has
evolved as a fascinating area in recent years.[1,2] Au nanoparticles
loaded on suitable supports are the most extensively used form
in gas or liquid catalytic reaction (Figure 1a). To promote mass
to the ease in catalyst separation and recycling, among which
Au nanoparticles and nanoporous Au (AuNPore) have shown
the highest activity and selectivity.
[
19–24]
Herein, we report the preparation of single-site Au anchored
transfer, adsorption, and activation of reactants in catalysis,
on polymeric mesoporous graphitic carbon nitrides (mpg-
porous supports are always preferred.[
3–6]
Meanwhile, nano-
C N ).
[25–30]
We further show that this material is a highly
3
4
porous gold has attracted wide attention as catalyst in organic
reactions (Figure 1b). A remaining problem with these reported
heterogeneous Au catalysts is their relatively low atomic effi-
active, selective, and stable catalyst for the silane oxidation with
−1
water. The turnover frequency (TOF) is as high as 50200 h , far
[
19–27]
exceeding most Au catalysts based on total gold content.
ciency. A possible solution is to develop single-site Au catalyst
(
Based on the indication on process costs and study on process-
ability, combined with the high remarkable recyclability, batch
consistency, and substrate compatibility at large scale, our cata-
lyst has the potential to be considered in industrial application.
We chose mpg-C N as the support, by virtue of its reported fea-
Figure 1c) with porous structure.[ However, in this regard, it
7,8]
Dr. Z. Chen, Dr. W. Chen, X. Zhang, X. Tong, Prof. D. Wang,
Prof. Q. Peng, Prof. C. Chen, Prof. W. He, Prof. Y. Li
Department of Chemistry
Tsinghua University
Beijing 100084, China
3
4
tures of mesoporosity and high surface area, which are conducive
to mass transfer, adsorption, and activation of reactants in cata-
[3–6]
lysis.
On the other hand, the characteristic N/C-coordinating
E-mail: cchen@mail.tsinghua.edu.cn
vacancies of mpg-C N , formed by the unique tri-s-triazine
3
4
Q. Zhang, Prof. W. He
School of Pharmaceutical Sciences
Tsinghua University
structure provide the binding or interaction sites for single
[28–33]
metal atoms.
Mpg-C N is prepared by thermally-induced
3 4
self-condensation of cyanamide using colloidal silica as the tem-
plate according to the literature (Figure S1, Supporting Infor-
Beijing 100084, China
Dr. J. Dong
[
33]
mation). Au species were deposited on mpg-C N via a modi-
Beijing Synchrotron Radiation Facility
Institute of High Energy Physics
Chinese Academy of Sciences
Beijing 100049, China
3
4
fied impregnation method using the as-prepared mpg-C N and
3
4
HAuCl aqueous solutions, followed by reduction in 5% H /Ar
4
2
[28,30]
atmosphere at 100 °C.
six different initial/nominal Au weight loadings (0.1, 0.3, 0.5,
To compare the effect of Au amount,
H. Yao
CAS Key Laboratory of Molecular Nanostructure and Nanotechnology
Institute of Chemistry
1
, 3, and 5 wt%) were tested, and the actual Au loadings were
Chinese Academy of Sciences
Beijing 100190, China
determined by inductively coupled plasma-atomic emission
spectrometry (ICP-AES) (Table S1, Supporting Information).
According to the results of transmission electron microscopy
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201704720.
(
TEM) and X-ray diffraction (XRD), the morphology and crystal
DOI: 10.1002/adma.201704720
structure of mpg-C N was not affected by the impregnation of
3
4
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quantitatively within 30 min at room temperature in aqueous
acetone (Au in Figure 2d). It is noteworthy that no siloxanes
1
were formed during the transformation. Though Au/mpg-C N
3
4
with other loading can also give 99% yield (Tables S2 and S3
and Figure S5, Supporting Information) with turnover num-
bers of 25100 (Table S4, Supporting Information), the turnover
−
1
frequency of Au-519 could reach 50200 h based on total gold
atoms, far exceeding Au with higher loading and most Au cata-
lysts based on total gold content in the literature (Figure 2e).
Figure 1. Diagram of three types of heterogeneous Au catalysts: a) Au
nanoparticles loaded on a support, b) nanoporous Au, and c) single-site
Au.
[24–29]
When Au/mpg-C N catalysts with lower loadings were used
3
4
in diphenylmethylsilane oxidation, similar TOF values could
be obtained (Table S5, Supporting Information). Blank experi-
ments using Au-free mpg-C N did not afford silanol under the
Au species (Figures S2 and S3, Supporting Information).
The mesoporous structure of the graphitic carbon nitride can
be clearly seen, and the pore diameters are about 10–15 nm
3
4
above conditions (Table S2, entry 7 and 8, Supporting Informa-
tion). The use of HAuCl /mpg-C N (0.1 wt% initial loading)
(Figures S1 and S2, Supporting Information), which are con-
4
3
4
sistent with the pore size distribution images (Figure S4, Sup-
porting Information). Interestingly, the XRD patterns indicated
the formation of Au nanoparticles with 0.3, 0.5, 1, 3, and 5 wt%
loading, and the size of nanoparticles are about 3–5 nm from
TEM images (Figure S2, Supporting Information). However,
the characteristic XRD peaks of Au NPs were not observed
when the initial loading is lower than 0.1 wt%.
without reduction in H /Ar for the oxidation of 1a resulted in
2
only 1.2% yields of 2a, also with the formation of undesirable
disiloxane (Table S2, entries 9, Supporting Information). On
the other hand, when Au-519 reduced at 200 °C is used as cata-
lyst (Au NPs in Figure 2d), only 21% silanol can be obtained
within the same time (30 min) (Table S2, entry 10 and Table S3,
Supporting Information). From the TEM and XRD of Au-519
reduced at 200 °C (Figure S6, Supporting Information), we
could observe the formation of Au nanoparticles. These data
collectively suggested that the Au-519 was more dispersed and
showed higher TOF, implying that a single-site Au is operative
in the catalysis.
The catalytic performance of the as-prepared Au/mpg-C N4
3
(Figure 2a–c) was evaluated using diphenylmethylsilane (1a) as
a model substrate (Figure 2d). To ensure the batch consistency,
we did three parallel experiments of each reaction. The yield
data are the average values of three independent experiments.
It is shown that the Au loading has a little effect on the reac-
tion. The 519 ppm loading Au-mpg-C N (denoted as Au-519) is
To probe the existence of single-site Au in Au-519, high-angle
annular dark-field scanning transmission electron microscopy
(HAADF-STEM) was used to confirm the atomic dispersion
3
4
the most active. The corresponding silanol was obtained almost
Figure 2. Structures of three types of Au catalysts: a) HAuCl , b) Au nanoparticle, c) single-site Au, d) Conversion-versus-time results of different Au
4
catalysts in diphenylmethylsilane oxidation with water. e) The comparison of TOF values with different Au catalysts. (The TOF was calculated by the
actual Au loadings with complete conversion.)
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Figure 3. Structural characterizations of Au /mpg-C N : a) Representative aberration-corrected STEM images of bright field. b) Representative aberra-
1
3
4
tion-corrected STEM images of dark field. c,d) Magnified HAADF-STEM images of Au /mpg-C N , showing only Au single atoms are present.
1
3
4
and configuration of Au on mpg-C N . Figure 3a,b presents the
indicates that the valance state of Au /mpg-C N is around +1.
1 3 4
3
4
representative STEM images of the obtained Au-519 catalysts.
Figure 3c,d shows the higher-magnification images, and the Au
species existed almost exclusively as isolated single atoms; nei-
ther sub-nanometer clusters nor nanoparticles were detected
We can get the same result from the first derivative curves of
Au L3-edge normalized XANES spectra illustrated in Figure S8
of the Supporting Information. As known, the peak position in
the first derivative curves points out the absorption threshold of
Au. The threshold value of Au /mpg-C N is 11919.4 eV, almost
(
for more images, see Figure S7, Supporting Information).
The single-site Au structure was further confirmed by X-ray
1
3
4
the same as that of AuCl (11919.4 eV).
absorption fine structure (XAFS) spectroscopy. Figure 4a shows
the X-ray absorption near-edge structure (XANES) spectra of
Au-519 samples reduced at different temperatures. It can be
seen that the overall XANES profile for Au-519 reduced at 200 °C
is nearly identical to that of the Au foil, implying the forma-
tion of Au nanoparticles (denoted as Au NPs/mpg-C N );
The extended X-ray absorption fine structure (EXAFS) anal-
ysis indicated that the AuAu bonding is absent in Au /mpg-
1
C N , confirming the single-site Au structure. Figure 4b shows
3
4
the Fourier transforms of the Au L3-edge EXAFS oscillations
of the as-prepared Au /mpg-C N , compared with the Au NPs/
1
3
4
mpg-C N and Au foil. It can be seen that the Fourier trans-
3
4
3
4
however, the XANES features for Au-519 reduced at 100 °C is
drastically different from that of the Au foil, implying the pos-
sible existence of single Au atoms (denoted as Au /mpg-C N ).
forms of Au foil and Au NPs/mpg-C N reveal main peaks in
3 4
1.8–3.3 Å arising from AuAu bonding, while the Au /mpg-
1
C N demonstrates a strong Au–ligand peak located at 1.7 Å,
1
3
4
3
4
Besides, the valance state of Au species can be investigated
through the absorption threshold and white line peak inten-
which can be attributed to AuN and/or AuC bonds and no
AuCl bond is observed (as shown in Figure S9, Supporting
Information). The X-ray photoelectron spectroscopy (XPS) also
shows no peaks of Cl 2p in Au /mpg-C N and Au NPs/mpg-
sity of Au L3-egde.[ Au NPs/mpg-C N and Au foil (Au ) have
34]
0
3
4
no obvious white line peak, and the peak intensity of HAuCl4
1
3
4
III
(
Au ) is quite strong. By contrast, the intensity of Au /mpg-
C N samples (Figure S10, Supporting Information). Neverthe-
1
3 4
I
C N is approximately equal to that of AuCl (Au ), which
less, the Au /mpg-C N also show a minor peak at 2.2–2.8 Å
1 3 4
3
4
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Figure 4. a) The normalized Au L3-edge XANES spectra. b) Fourier transform magnitudes of the EXAFS spectra and c) Wavelet transforms for the
k2-weighted EXAFS signals of Au /mpg-C N , Au NPs/mpg-C N and Au foil. In (c), the vertical dashed lines that denote the k-space positions of the
1
3
4
3
4
AuN and AuAu contribution are provided for guidance. d) Au L3-edge EXAFS analysis of Au /mpg-C N . The black line denotes the experimental
1
3
4
signal and the red line represents the best-fit one. The Au-N(C) two-body signals in the first two coordination shells are also shown. e) Schematic
model of Au /mpg-C N (yellow balls represent Au, blue balls represent N, and gray balls represent C).
1
3
4
that just overlaps with the AuAu bonding distribution region.
In order to confirm the atomic dispersion of Au in the pre-
pared Au /mpg-C N sample, wavelet transform (WT) analysis
not only a radial distance resolution, but also resolution in the
[
35]
k-space, and thus the coordination atoms can be discrimi-
nated even when they overlap substantially in R-space. Two
1
3
4
−
1
of the Au L3-edge EXAFS oscillations of the as-prepared Au1/
mpg-C N (Figure 4c) was applied. WT-EXAFS analysis is a
intensity maxima at 4.1 and 7.9 Å , which are associated with
the AuAu contribution, are well-resolved for the Au NPs/
mpg-C N and Au foil, in good agreement with the theoretical
3
4
powerful tool for separating backscattering atoms that provides
3
4
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Table 1. Single-site Au catalyzed oxidation of various organosilanes.
1 3 4
Au /mpg-C N
RS i( R′)3−n-Hn + nH2O  → RS i( R′)3−n-(OH)n + nH2 (n = 1or 2)
Acetone,RT
1
b - h
2b - h
Entrya)
1
Silane
PhMe2SiH
PhMe2SiH
Ph3SiH
2
Silanol
t [h]
0.5
0.3
6
Yieldb) [%]
TOF [h−1
50200
29700
4180
]
1
1b
1b
1c
1d
1e
1f
2b
2b
2c
2d
2e
2f
PhMe2SiOH
PhMe2SiOH
Ph3SiOH
99
99
98
96
97
95
97
98
2c)
3
4
Et3SiH
Et3SiOH
0.5
3
48647
8192
5
Ph2SiH2
Ph2Si(OH)2
Et2Si(OH)2
6
Et2SiH2
3
8023
7
1g
1h
PhC CMe2SiH
(CH2CH)Ph2SiH
2g
2h
PhC CMe2SiOH
1
24576
4180
8
(CH2CH)Ph2SiOH
6
a)
Conditions unless otherwise stated: silane (0.5 mmol), Au1/mpg-C3N4 (0.004 mol% based on Au) dispersed in water, acetone (2 mL) as solvent, 25 °C; b)Yield of isolated
c)
product with three parallel experiments (Table S7, Supporting Information); Silane (0.2 mmol), Au1/mpg-C3N4 (0.004 mol% based on Au) dispersed in water (50 µL),
THF (2 mL) as solvent, 25 °C. Yield determined by gas chromatography (GC) analysis of the reaction mixture using dodecane as an internal standard.
prediction as shown in Figure S11 of the Supporting Information.
breaking of N-3C bridging. The change of the peak intensity
−
1
By contrast, Au /mpg-C N shows one maximum near 4.0 Å
of typical π* resonances in the Au /mpg-C N indicate that Au
1
3
4
1
3
4
[39–42]
for the first and second coordination shells, consistent with
the predicted position of AuN/C contributions, Hence, in
Au /mpg-C N , the AuAu bonding is absent and all Au atoms
introduction may locally affect the C N matrix.
3 4
To obtain quantitative structural parameters around Au in
the prepared Au /mpg-C N sample, a least-squares EXAFS
1
3
4
1
3
4
exist as mononuclear centers.
curve fit was carried out (see Supporting Information for
details). The best-fit results of Au /mpg-C N sample, as well
The binding state was investigated by XPS. As shown in
Figure S12 of the Supporting Information, the oxidation state
1
3
4
as Au NPs/mpg-C N and Au foil, are given in Figure 4d and
3
4
I
of Au is mainly +1 (Au ), which is in good agreement with
Figures S15 and S16 (Supporting Information), together with
corresponding structure parameters listed in Table S6 (Sup-
porting Information). We can clearly see that the fitting curves
agree perfectly/ideally with the experiment results. It clearly
shows that three nearest ligand atoms (N or C) on average are
around the center atom Au, with the distance of 2.04 Å. The
coordination number of 4.1 and the average atom distance of
2.92 Å are obtained for the second shell. To further confirm
coordination structure, we performed the XANES calcula-
tions (Figure S17, Supporting Information). The above results
indicate that isolated Au atoms could be anchored with three
nitrogen atoms of tri-s-triazine repeating units in mpg-C N , as
the XAFS results. This can be attributed to the formation of
AuN(C) coordination bonds. As seen in Figure S13 of the
Supporting Information, the peaks of N 1s spectra which cor-
2
respond to the sp -bonded nitrogen in CNC (≈398.6 eV), the
nitrogen in tertiary N-(C)3 groups (≈399.7 eV), and the pres-
[
36,37]
ence of amino groups (CNH, ≈401.2 eV)
will shifted
to higher-energy side to a certain extent, indicating that the
electron density of the N atoms becomes lower in the presence
of Au atoms.[ Also we can see that the C 1s spectra which is
assigned to sp -bonded carbon in CC (≈285.2 eV) and NCN
≈288.4 eV) shifted to higher-energy side, indicating the inter-
38]
2
(
3
4
action of C and Au atoms. Synchrotron-based near edge X-ray
absorption fine structure (NEXAFS) was also used to further
probe the metal-support interaction by exploring the electronic
structure of Au/mpg-C N sample, as exhibited in Figure S14
illustrated in Figure 4e.
The Au /mpg-C N was demonstrated to be a highly active
1
3
4
and selective catalyst in the oxidation of various organosilanes
(Table 1). In all reactions listed, silanols were obtained as the
only products and no siloxanes were detected. Au /mpg-C N
3
4
of the Supporting Information. In the carbon K-edge NEXAFS
spectra (Figure S14a, Supporting Information), the Au/mpg-
C N samples show characteristic resonances including struc-
1
3
4
was exceptionally active for the oxidation of dimethylphenyl-
silane (1b), and the calculated TOF could also be as high as
3
4
−
1
tural defects at ≈284 eV and π*CNC at 288.5 eV. The change
50200 h . If the TOF was calculated based on the total gold
loading, Au /mpg-C N is the most active catalyst compared
of the peak intensity of the Au /mpg-C N at 288.5 eV represents
1
3
4
1
3
4
[
19,20,22]
a corresponding reduction in the number of π* bonds, which
with all the reported Au NPs catalysts.
To compare its
indicate that Au introduction may locally affect the C N matrix.
catalytic efficiency with previously reported results under the
same reaction conditions, TOF of the Au /mpg-C N was meas-
3
4
In nitrogen K-edge region (Figure S14b, Supporting Informa-
1
3
4
tion), Au /mpg-C N shows two typical π* resonances at 399.7
ured on the commonly studied silane 1b. The oxidation of 1b
was catalyzed with 0.01 mol% of the Au /mpg-C N to reach
1
3
4
and 402.6 eV corresponding to aromatic CNC coordination
in one tri-s-triazine heterocyclic ring and N-3C bridging among
three tri-s-triazine moieties (N2, see the inset of Figure S13b,
Supporting Information). As compared with pure mpg-C N , a
1
3
4
−
1
[19]
TOF of 29700 h which is higher than the reported results.
Oxidation of less reactive triphenylsilane (1c) afforded triphe-
−1
nylsilanol (2c) in excellent yields, and the TOF (4180 h ) was
3
4
[
19]
different intensity of N2 resonance in the hybrid reveals a partial
more than tenfold higher than that with AuCNT catalyst. Not
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only aromatic silanes were successfully transformed into the
corresponding silanols, but also alkyl silanes (such as triethylsi-
lane 1d) gave the product triethylsilanol in high yields. Besides
the trisubstituted silanes, the single-site Au could also catalyze
the conversion of disubstituted silanes to disilanols (1e and 1f).
Moreover, substrates containing carbon–carbon double bond
(Table S10, Supporting Information). HAADF-STEM and
EXAFS images showed that the spent catalyst remained the
feature of single-site Au structures (Figure 5b,c).
To shed light on the true key essential catalytic species, we
further characterize the XPS of the catalyst recovered at low and
high conversions of the catalytic reactions. No obvious change
in the binding energy of Au 4f in XPS spectra was observed
(Figure S18, Supporting Information), and the Au species still
(1g) and triple bond (1h) can also be successfully oxidized to the
corresponding silanols. The TOF of the Au /mpg-C N for the
1
3
4
I
I
oxidation of silane (1g) in acetone, which was calculated based
existed as Au . These observations suggested that Au in the
catalyst was not reduced by silane during the reaction. On the
other hand, the XPS of Au NPs/mpg-C N (Figure S19, Sup-
−1
on the surface Au atoms, was as high as 24576 h . This value
[
29]
was almost 20-fold as high as that with nanoporous gold. It
is clearly seen that the activity of the Au /mpg-C N catalyst is
3
4
porting Information) shows that about 25% is Au(I), which
account for the obtained catalytic results (21%). Therefore,
1
3
4
one order of magnitude higher than the reference catalyst if the
TOF is normalized to each Au atom. For Au /mpg-C N , each
I
no matter it is for single-site Au or Au nanoparticle, the Au
1
3
4
and every single-site Au could potentially serve as the catalytic
center, and therefore the highest TOF was obtained.
is likely the catalytic active species for the silane oxidation.
I
Interestingly, when homogeneous Au salt Au(PPh )Cl and
3
III
The Au /mpg-C N catalyst is highly stable in the reaction.
Au salt were used as the catalysts, almost no conversion was
1
3
4
Hot filtration experiments shows that the leached Au was less
than 0.08%, and the reaction was not catalyzed by the leached
soluble Au species. No further conversion of 1a to 2a was
detected in the absence of Au catalyst. The recycled catalyst
retained its activity for at least ten cycles (Figure 5a; Table S8,
Supporting Information). We also used ICP-AES to monitor
the Au leaching in every recycle reaction (Table S9, Supporting
Information), which showed that the residual Au was at nano-
gram level (about 0.01 ppm). Not surprisingly, when the Au
loadings of the fresh and spent catalysts were determined by
ICP-AES, the result demonstrated that the Au /mpg-C N
observed (Table S11, Supporting Information). The inactivity
of Au could be easily understood as it could not be further
III
oxidized, which is required for the SiH activation. However,
I
the conspicuous failure of homogeneous Au salt points to the
I
high probability that the surface-bound Au drastically changed
I
the reaction kinetics. We propose that the surface-bound Au ,
upon ligand dissociation, renders easier binding of the silane
[
19–24]
hence the SiH activation.
We attempted to determine
whether the concerned SiH activation proceeds through a
radical mechanism by employing electron paramagnetic reso-
nance (EPR) experiments. However, no obvious radical signal
was recorded/detected. Although this negative result could not
1
3
4
largely retained its Au content even after ten recycles
Figure 5. a) Recycling the single-site Au catalyst for 10 cycles. b) HAADF-STEM image of Au /mpg-C N single-site catalyst after ten-run reactions.
1
3
4
The high magnification images clearly reveal the presence of isolated Au single atoms (indicated by the white dots). c) EXAFS spectra of single-site
Au before catalysis and after 10 cycles show no AuAu bonds were found. d) The mechanism of single-site Au catalyzed silane oxidation with water.
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conclusively rule out a radical mechanism, we tend to believe
Keywords
I
that the Au undergoes oxidative insertion into the SiH bond
carbon nitrides, coordination environments, gold, mesoporous,
single-site catalysis
III
to form SiAu H intermediate. This intermediate in turn
reacts with water to give silanol and hydrogen gas which was
I
detected by GC. This latter process regenerates Au (Figure 5d).
Received: August 18, 2017
Revised: October 18, 2017
Published online:
This plausible mechanism accounted very well for the struc-
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mpg-C N catalyst. On the other hand, no silanol was formed
3
4
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Conflict of Interest
The authors declare no conflict of interest.
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