Template-free synthesis of graphene-like carbons as efficient carbocatalysts for selective oxidation of alkanes

Article abstract of DOI:10.1039/c9gc03781k

Selective oxidation of aromatic alkanes by C-H activation is one of the key reactions in organic synthesis and the chemical industry. Activation of C-H bonds to obtain high-value added products under mild conditions by using sustainable carbocatalysts is of particular interest. Herein, we report a sustainable, green and template-free strategy towards the fabrication of N-doped and N/S codoped carbon nanosheets by metal-free carbonization of bioprecursors guanine and guanine sulfate. The formation of thin and N/S codoped carbon nanosheets was induced by multiple interactions of the nucleobases. Benefiting from the unique textural structure of the as-synthesized carbons, including ultrathin thickness, optimal porosity, and rich structural defects, and the synergistic coupling effect of multiple dopants, the carbon nanosheets show a high catalytic performance with 85% ethylbenzene conversion and 98% selectivity to acetophenone at 80 °C after 4 h reaction, which outperforms other equivalent benchmarks (e.g. 8.5 times higher conversion and 3.2 times higher selectivity than those of oxidized carbon nanotubes). Density functional theory simulations indicate that the oxidation of ethylbenzene is catalyzed by the synergistic effect of p-N/S and g-N/S catalysts via an OH radical mechanism. This N/S codoped strategy provides guidance for the design of carbon-based catalysts for the selective oxidation of other alkanes.

Full text of DOI:10.1039/c9gc03781k

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Page 1 of 24
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DOI: 10.1039/C9GC03781K
Template-free Synthesis of Graphene-like
Carbons as Efficient Carbocatalysts for
Selective Oxidation of Alkanes
1
1
1
1
2
Xiang Hu , Yuchuan Liu , Haitao Huang , Baobing Huang , Guoliang Chai * and Zailai
Xie1*
1
. College of Chemistry, Fuzhou University, 2 Xueyuan Road, Fuzhou 350116, China
Corresponding author. Email: zlxie@fzu.edu.cn, Tel: +(86)-591-22866102
. State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
2
Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou, 350002, China
Corresponding author. Email: g.chai@fjirsm.ac.cn
Abstract
Selective oxidation of aromatic alkanes by C-H activation is one of the key
reactions in organic synthesis and chemical industry. It is of particular interest to activate
C-H bond to get high-value added products under mild conditions by using sustainable
carbocatalysts. Herein, we report a sustainable, green and template-free strategy towards
the fabrication of N-doped and N/S codoped carbon nanosheets by metal-free
carbonization of bioprecursor guanine and guanine sulfate. The occurrence of thin and
N/S codoped carbon nanosheets was induced by multiple interactions of nucleobase.
Benefiting from the unique textural structure of the as-synthesized carbons, including
ultrathin thickness, optimal porosity, rich structural defects and synergistic coupling
effect of multiple dopants, the carbon nanosheet delivers a high catalytic performance
o
with 85% ethylbenzene conversion and 98% selectivity of acetophenone at 80 C after 4
h reaction, which outperforms other equivalent benchmarks (e.g. 8.5 times in conversion
1

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and 3.2 times in selectivity higher than oxidized carbon nanotubes). Density functional
theory simulations indicate that the oxidation of ethylbenzene is catalyzed by synergic
effect of p-N/S and g-N/S catalysts via OH radical mechanism. This N/S codoped
strategy provides guidance for the design of carbon-based catalysts for selective
oxidation of other alkanes.
Keywords: selective oxidation; N/S codoped carbon nanosheets; ethylbenzene; OH
radical
2

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Introduction
Selective oxidation of aromatic alkanes via C-H bond activation is one key
important reactions in organic synthesis and chemical industry, since it enables the
production of high value-added platform molecules such as cyclohexanone,
cyclohexanol, and organic acids.1-5 General approaches for efficient C-H bond activation
are usually mediated by employing of transition metals or organometallic compounds as
heterogeneous or homogeneous catalysts.6, 7 As of now, the transition metal or
8
9
10
11
12, 13
organometallic centers, such as Ru 、Pt 、Au 、Pd 、Co
etc. have demonstrated
grand potential in C-H activation. In spite of highly efficiency of metal-based catalysts,
the high cost and limited resource have prompted worldwide efforts to explore non-noble
1
4
metal or metal-free catalysts and more environment-friendly process. In view of these,
it is desirable to develop recyclable heterogeneous metal-free catalysts with high activity
and low cost.
The past decade have witnessed the rapid development of the carbon-mediated
reactions which was also called carbocatalysis and has gained enormous attention due to
the economical and sustainable nature of the catalyst.15-18 So far, a couple of reactions
such as oxidative dehydrogenation, oxidative coupling, catalytic oxidation, reduction,
and transesterification reactions,1, 19-26 can be effectively catalyzed by carbon materials.
As the pioneering work of cabocatalysis, Su et al. discovered that nanocarbons are an
efficient metal-free catalyst to activate short chain alkanes, in which carbonyl group was
2
7
confirmed to be the active origin. Ma et al. reported that graphitic-type sites in layered
carbon catalysts is pivotal for the C-H bond activation reaction, while nitrogen dopant
2
8
did not participant in the activation of reactant. After that, considerable efforts have
focused on improving the oxidation performance based on carbocatalysts by identifying
and exposing active sites, as well as surface engineering.18, 19, 29 Apart from oxygen
functional groups, structural defects and doped heteroatoms are also potential active
centers or have significant effect on catalytic activity.18, 27, 30-32
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Indeed, heteroatoms-doped nanocarbons have been widely investigated as
carbocatalysts to improve catalytic performance of C-H bond activation.29, 33, 34 Nitrogen,
phosphorus and sulfur were generally used as electron donors to modulate the
3
5
π-conjugated system at the periphery of carbon structures. The altered spin distribution
or charge of the sp2 carbon by heteroatom doping facilitates molecules adsorption by
electrostatic interaction or hydrogen bonding like S-O∙∙∙H, and subsequently the C-H
3
6
bond breaking. Meanwhile, the intrinsic defects (e.g., vacancies, Stone-Wales defects
or edges) can also break the electron-hole symmetry, and inducing the radical production
from oxidant decomposition.37 All above these enable wide spectrum change of the
carbons not only in the composition and structure, but also in diversification of catalytic
functions and applications. However, despite considerable achievements have been made
in recent years, the mechanism interpretation about how multiple heteroatoms affect
catalytic properties of carbons is still unclear and the selective oxidation of C-H bonds
with high efficiency under mild conditions driven by carboncalysts remains to be
explored.
Another key point to boost catalytic activity is to maximum expose active sites. The
two dimensional (2D) carbons possess unique advantages which can satisfy this
requirement, such as their potentially large surface area (theoretically up to ca. 2600
2
m /g), capable of providing a 2D area carrying abundant active sites for redox reactions,
3
8
and easily accessible inner-outer interfaces of the 2D architecture. Such 2D carbons in
some cases can serve as a substitute or beneficial complementation of the traditional
metals or metal oxides, especially noble metals.39-42 Although 2D carbon materials are
widely investigated, a sustainable, green and template-free strategy to synthesis of
well-defined 2D carbon nanosheet has rarely been realized. In this aspect, natural
biomolecules could be used as 2D carbon source, as they often appear in the form of
supramolecular self-assemblies due to their abundant resources and multiple reaction
sites.43 These natural molecules would self-assemble into various supramolecular
4

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assemblies primarily promoted by non-covalent interactions, such as hydrogen bonding,
4
4
π-π stacking, van der Waals forces. Indeed, we previously reported that their unique
molecular morphologies would be transferred into carbon architecture during the
carbonization process, and hence providing a facile method to fabricate nanosheets
nanostructures.45, 46
Herein, we report an occurrence of thin and N/S codoped carbon nanosheets
induced by multiple interactions of nucleobase. The as-synthesized carbon nanosheets
possess a series of merits, such as relatively high surface area, ultrathin thickness,
optimal porosity, rich structural defects and synergistic coupling effect of multiple
dopants, which render it suitable for efficient carbocatalysis. To investigate the effect of
N/S doping on catalytic activity, control experiments by different pyrolysis temperatures
were conducted to screen the optimized catalyst. The selective oxidation of ethylbenzene
(EB) with tert-butyl hydroperoxide (TBHP) as oxidant was selected to evaluate the
catalytic properties of nanocarbon catalysts for the activated oxidation of C-H bonds.
Among all the catalysts, N/S codoped sample that was obtained by thermal pyrolysis at
o
1
9
000 C (GS1000) delivers a high catalytic performance with 85% EB conversion and
8% selectivity of acetophenone (AcPO), which is 8.5 times in conversion and 3.2 times
in selectivity higher than OCNT under identical condition, better than most of previously
reported carbon-based benchmark catalysts. The density functional theory (DFT)
simulations indicate that the oxidation of ethylbenzene is catalysed by synergic effect of
p-N/S and g-N/S catalysts via OH radical mechanism. Moreover, a wide scope of
substituted aromatic hydrocarbons could also be smoothly oxidized under mild condition,
and the selectivity of corresponding products reached as high as 97%. Given the
diversity in the structure of the nucleobase moiety, they represent ideal building blocks
for the catalyst-free and metal-free formation of 2D carbon carbocatalysts with enhanced
catalytic properties for carbocatalysis.
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Experimental
Catalysts synthesis
For the preparation of the N/S codoped 2D carbons, 30 g guanine and 5.4 mL
concentrated sulfuric acid were dispersed into 300 mL deionized water under stirring for
2
4 h. The prepared precursors were dried and transferred into a tube furnace and
pyrolyzed at given temperature for 1 h at a heating rate of 5 °C/min in N flow. Then, the
2
black solids were taken out and calcined for another hour at the same temperature. The
powder obtained finally were named as GSX, where X represent the stipulated pyrolysis
temperature. Similarly, the N-doped 2D carbons were prepared by direct pyrolysis of
guanine at a stipulated temperature for 1 h in N flow at twice, which names as GX (X
2
represent the pyrolysis temperature)
Physical characterization
Nitrogen physisorption measurements were carried out at 77 K by Micromeritics
ASAP 2020 plus and ASAP 2060. Powder X-ray diffraction (XRD) patterns were
collected by RIGAKU Ultima IV, and diffraction patterns were attained by using Cu Kα
-1
(
λ = 1.5406 Å) radiation at a scanning of 10° min from 5° to 80°. The Raman spectra
were measured by Renishaw inVia with a 532 nm laser excitation. XPS measurement
was conducted using ESCALAB 250 with a source gun of Al Kα with spot size of 500
μm. Scanning electron micrograph (SEM) was acquired by Hitachi S-4800 operated at
1
5kV. Transmission electron microscopy (TEM) images, high resolution TEM (HRTEM)
images, and high angle annular dark field scanning TEM (HAADF-STEM) images were
recorded on a FEI Talos F200S G2 microscope operated at 200 kV.
Catalytic experiments
All reactions were carried out in a 5 mL round-bottom flask. Typically, 10 mg
catalysts, 1 mL 70% TBHP, 2 mL H O and 1 mmol ethylbenzene were added to flask
2
o
and the reaction was proceeded in an 80 C oil bath for 4 h. Anisole was added to the
solution as internal standard after the reaction. Gas chromatography and GC-MS were
6

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used to analyzed the reaction mixture.
Simulation methods
The density functional theory (DFT) simulations were performed by PWSCF code
in Quantum ESPRESSO to investigate the reaction mechanisms.47 The generalized
gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) was adopted for
4
8
exchange correlation functional in DFT simulations. The kinetic energy cutoffs for the
wavefunction and the charge were set to 40 Ry and 400 Ry, respectively. Spin
polarization was taken into account for all the simulations. The lattice parameters parallel
to the graphene nanosheet are about 12.35 Å × 20 Å, and the vacuum region
perpendicular to the graphene nanosheet is larger than 10.00 Å. The k-point sampling in
the Brillouin zone is set to 3 × 1 × 1. Semiempirical Grimme's DFT-D2 method was
adopted to describe Van der Waals correction.49
Results and discussion
Catalyst synthesis and characterization
Figure 1. The SEM images of different catalysts: (a): G800; (b): G900; (c): G1000; (d): GS800; (e):
GS900; (f): GS1000.
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Figure 2. The TEM images of G1000 and GS1000, and the corresponding EDS element mapping of
C, N, O and S.
Scanning electron microscopy (SEM) images show that all of the carbons have
typical sheet-like morphologies with uniformity and thin properties, no matter which
o
temperature (800-1000 C) used and with or without S doping (Figure 1). Transmission
electron microscopy (TEM) images further manifest the formation of graphene-like
structures, which are very transparent and curly (Figure 2). Moreover, the elemental
mapping images reveal that C, N, and O elements are uniformly distributed over the
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skeletons of both G1000 and GS1000, meanwhile S element has been doped evenly into
the GS1000 skeleton. It is supposed that such heteroatom-doped properties combined
with the unique 2D architecture will be very suitable for heterogeneous catalytic
reactions.
Figure 3. (a) N adsorption/desorption isotherms; (b) Pore size distribution curves; (c) Powder X-ray
2
diffraction patterns; (d) Raman spectra.
The nitrogen sorption behavior was measured to probe the porous structure of the
as-synthesized samples (Figure 3). Besides the common microporous regions, all the
samples present remarkable hysteresis loops at P/P > 0.45, indicating the developed
0
mesoporous properties. When the samples synthesized at the same temperature, GSX
possesses more developed porosity than that of GX, suggesting the pore-forming effect
of pre-ionization process by H SO . Moreover, higher temperature results in more
2
4
desirable porous structure. G1000 and GS1000 have larger specific surface area up to
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2
-1
2
-1
2
37 m g and 427 m g , respectively. The total pore volumes of G1000 and GS1000
3
-1
3
-1
are 0.49 cm g and 0.75 cm g , wherein the proportions of mesopore volumes occupy
up to 86% and 84%, respectively, which are very considerable. Thus, G1000 and
GS1000 are expected to exhibit more ideal effect for the mass transfer.
As shown in Figure 3c, XRD patterns of all the samples have two peaks centered at
ca. 26 ° and 43 °, which can be assigned to (002) and (100) crystal plane of graphite
carbon, respectively. The wide peak shapes indicate the existence of abundant defective
5
0
structure. The structural defects of samples were further analyzed by Raman spectra
Figure 3d) and expressed by the ratio of D1 band to G band. The higher I /I indicates
(
D1
G
that more defective structure exists in the carbon materials. The I /I values of GSX are
D1
G
generally lower than those of GX synthesized under otherwise the identical condition,
suggesting the dopant S has extra influence on the crystallinity of the generated carbon.
Therefore, the XRD and Raman results combined with the SEM and TEM
characterization indicate the ultrathin and defective nature of the as-prepared 2D carbon
nanosheets.
Table 1. Textural properties and elemental compositions.
Porosity data
Vtotal a
Elemental composition c
Sample
Vmicro b
SBET
Vmeso
N1+N3(
%)
C
N
O
S
2
-1
3
-1
3
-1
3
-1
[
m g ]
[cm g ]
[cm g ] [cm g ]
G800
G900
51.99
60.71
0.30
0.21
0.14
0.42
0.30
0.31
0.63
0
76.67 21.04 2.29
0
0
0
7
7
6
7
7
6
7.72
9.73
9.91
4.53
8.39
5.83
0.19
0.01
0.03
0.02
0.02
87.21 10.39
93.46 4.75
2.4
G1000
GS800
GS900
GS1000
237.52
180.08
193.43
0.49
1.79
0.36
79.88 15.54 4.12 0.46
0.37
86.57 9.72
89.8 3.93
3.31 0.41
5.74 0.53
427.07
0.75
0.05
a
b
c
From total N
2
uptake at P/P
0
= 0.95. From the DFT method. Determined by XPS, at%
1
0

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Figure 4. X-ray photoelectron spectroscopy. (a) Survey spectra; (b), (c), (d) N1s XPS spectra; (e) S
2
p XPS spectra; (f) The content percentages of different nitrogen species.
The surface chemistry of the catalyst is an important parameter for the catalytic
reaction, because it has great influence on the adsorption, desorption and activation of
the substrate during the reaction. In order to understand the surface chemical properties
of the catalyst, we used X-ray photoelectron spectroscopy (XPS) to detect the surface
composition of the catalysts. As shown in Figure 4a, wide range scanning spectra
indicate that C, N and O elements exist in all the catalysts, meanwhile S element is also
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doped into GSX which is confirmed by the corresponding high resolution S 2p spectra
Figure 4e). The elemental composition determined by XPS is shown in Table 1. The
high-resolution N1s spectra can be deconvoluted into four typical peaks located at 397.8,
99.3, 400.5, and 401.5 eV, respectively, belonging to N-1 (pyridine nitrogen), N-2
(
3
5
1
(
pyrrole nitrogen), N-3 (graphitic nitrogen), and N-4 (nitrogen oxide) . The ratios of
four N species in different samples are shown in Figure 4f. According to previous studies,
2
8
N-3 species play an important role at the activation of C-H. We found that the total
content of N species decreases with the increase of synthesis temperature, while the ratio
of N-3 keeps relatively stable. The G1000 has a high N-3 ratio up to 34.59% in GX, and
GS1000 has the same value 32.28% of N-3 ratio among GSX. Figure 4e shows the
deconvolution results of high-resolution S 2p spectra of GSX, wherein the peaks at 163.2,
1
2
64.0, 167.1 and 168.6 eV belong to C-S-C (S 2p3/2), C-S-C (S 2p1/2), -C-SOx- (S
p3/2) and -C-SOx- (S 2p1/2) bond, respectively.29, 36, 53 The –C-S-C structure dominates
in GS1000, while -C-SOx- dominates in both GS900 and GS800. -C-SO - groups can
x
serve as potential adsorption sites for the construction of S-O∙∙∙H, leading to enhanced
interaction with EB. Thus the distinct surface statuses are believed to exert different
influences in catalytic reactions.
Catalytic performance
To explore the catalytic potential of the presented carbons, we performed the liquid
phase EB selective oxidation reaction catalyzed by these carbons under the same reaction
condition. As shown in Table 2, TBHP can hardly oxidize EB in the absence of catalyst
(Table 2, Entry 1). When oxidized carbon tubes (OCNT) and commercial graphene were
used as catalysts, the EB conversion was below 15%, together with quite low selectivity
for AcPO (45%), which indicating low performance of these two nanocarbons. In
comparison, when different graphene-like GX and GSX samples were introduced, they
exhibit notable enhanced catalytic performance. Moreover, the performance can be
1
2

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further improved with the elevation of synthesis temperature independent of S doping.
Figure 5a compares the reactivity of as-synthsized carbons with OCNT and commercial
graphene. We can see that the prepared carbon nanosheets show a high catalytic
performance with 85% EB conversion and 98% selectivity of AcPO, which is 8.5 times
in conversion and 3.2 times in selectivity higher than OCNT under identical condition.
Moreover, the catalytic performance of S-doped carbon nanosheet is obviously better
than that of GX (without S doped) synthesized under the same condition. The EB
conversion and AcPO selectivity reach up to 99% over GS1000 catalyst after 6 h
reaction, indicative of effectively C-H activation ability of our predesigned carbon
nanosheets. The participation of S changes the electrical characteristics and surface
functional groups of the catalyst surface, which is responsible for improved chemical
properties. Since GS1000 seems to be the best catalyst among all of the samples, we
used it as a reference and optimized the reaction conditions. As can be seen from Table 2,
high conversion as well as high AcPO selectivity can be obtained in the GS1000 catalyst
by changing of reaction time and temperature, which are much better than other
carbon-based benchmark catalysts.
The catalytic stability of catalysts is a very important indicator in industrial
applications. To this purpose, we explored the recyclability of GS1000 for EB oxidation
under the optimal reaction conditions. As shown in Figure 5b, after five times of reaction,
there is minimal deactivation in the activity and selectivity, indicative of good stability of
the catalyst GS1000 under current mild reaction condition.
On the basis of the preliminary success of the EB oxidation with TBHP as the
oxidant, we also studied the oxidation of substituted aromatic hydrocarbons. Since
GS1000 was found to be the best catalyst for EB oxidation, it was used in subsequent
experiments, and the experimental results are shown in Table 3. GS1000 can effectively
catalyze the oxidation of various benzyl aromatic hydrocarbons into corresponding
ketone products, with the conversion ranging from 72% to 99%, and selectivity almost
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over 95%. Both electron-donor MeO- and electron-acceptor -NO substituted substrates
2
have relatively good conversion and selectivity. Aromatic substrates with large volume
or steric hindrance also have good conversion, such as propyl benzene, isopropyl
benzene, butyl benzene, iso-butyl benzene, diphenylmethane, fluorene and so on. This
indicates that our new metal-free catalyst is applicable to the phenyl substrate.
Table 2. Screening of catalysts and optimization of reaction conditions
O
catalyst, TBHP
H O, 80℃
2
o
Entry
Catalyst
Temp. ( C)
T (h)
Conv. (%)
Sel. (%)
1
2
3
4
5
6
7
8
9
Blank
OCNT
80
80
80
80
80
80
80
80
80
80
80
80
60
100
4h
4h
4h
4h
4h
4h
4h
4h
4h
2h
5h
6h
4h
4h
3
10
14
8
96
31
45
70
83
96
82
95
98
95
99
>99
79
97
Graphene
G800
G900
28
79
24
83
85
68
92
>99
69
99
G1000
GS800
GS900
GS1000
GS1000
GS1000
GS1000
GS1000
GS1000
1
1
1
1
1
0
1
2
3
4
5
mg (98%)
H SO
1
1
5
6
80
80
4h
4h
0
0
-
-
2
4
Guanine
Reaction condition: 10 mg catalyst, 1 mmol ethylbenzene, 2 mL H O, 1 mL TBHP.
2
1
4

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As revealed above, the activities of GSX all got improved compared to that of GX
at different degrees. Based on available data, it can be reasonable to propose that both
S-doping and the developed porosity are responsible for the remarkably enhanced
activity. Among various N dopants, graphitic N have been demonstrated to be the most
efficient dopant for improving the activity of carbon-based materials, which has been
confirmed by the previous report 4, 28. In our case, with the increase of the synthesis
temperature, the ratio of N3 appears relatively constant (Figure 4f), but the activity of the
catalyst increased significantly (Table 2), indicating that graphitization degree and
surface areas could play more pronounced role for EB oxidation. In contrast to G1000,
the GS1000 has similar N configuration, while it shows superior activity (see Table 1
and 2). The enhanced activity could be induced by N/S codoped effect from the
incorporation of S dopant. XPS results indicate the presence of -C-SOx groups, which
could facilitate the adsorption of EB by -S-O∙∙∙H interaction, which is confirmed by
calculation below.
(b)
(a)
Figure 5. (a) activity comparison of respective catalysts, (b) the recycling test of GS1000 for
ethylbenzene oxidation.
1
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1
6

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Table 3. The conversion of substituted aromatic hydrocarbons to corresponding ketone with GS1000
as catalyst.
Entry
Substrates
Products
Conversion(%) Selectivity(%)
O
1
85
83
87
99
98
99
O
O
2
3
4
O
O
97
O N
2
O N
2
O
97
O
OH
3
2 (ketone)
5
99
6
8 (alcohol)
O
6
7
8
9
72
81
82
81
99
99
99
99
O
O
O
1
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O
1
0
OH
81
99
Reaction condition: 1 mmol substrate, 10 mg catalyst, 1 mL TBHP (70%), 2 mL H O, 80 ℃, 4h
2
Reaction mechanism
Figure 6. The elementary steps of OH radical mechanism for the ethylbenzene oxidation
on pyridinic N and S codoped carbon nanosheet catalyst (p-N/S).
1
8

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Figure 7. The elementary steps of OH radical mechanism for the ethylbenzene oxidation
on graphitic N and S codoped carbon nanosheet catalyst (g-N/S).
To understand the origin of catalytic activity, the free radical scavenger
p-benzoquinone was added to the catalytic system. Negligible conversion of EB was
observed after the addition of p-benzoquinone, indicating the aliphatic C-H oxidation
process in EB over GS1000 follows a free radical mechanism, which is in agreement
5
2
with previous reports. DFT simulations were performed for S/N co-doped graphene
nanosheet catalysts. The S doped at graphene edges is oxidized and the N can be doped
as pyridinic N (p-N) or graphitic N (g-N), which are named as p-N/S and g-N/S catalysts,
respectively. There are also two possible mechanisms for the oxidation of ethylbenzene,
namely, OH radical and OOH radical mechanisms, respectively. The OH radical
mechanism for p-N/S and g-N/S catalysts are shown in Figure 6 and 7, and the OOH
radical mechanism for p-N/S and g-N/S catalysts are shown in Figure S1 and S2 in
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Supporting Information. As shown in Figure 6, we can find that ethylbenzene molecule
is adsorbed on the oxidized S group by Van der Waals interactions (step (1)), which is a
favorable exothermic process. The OH radical is formed via BuOOH and BuO., and
subsequently adsorbed on edge site close to p-N (step (2) and (6)). However, this step is
slightly endothermic for p-N/S catalysts. The step for 1-phenylethyl alcohol to
1
-ethylbenzene oxygen radical (step (5)) is also slightly endothermic. The rest steps are
exothermic and favorable for oxidation of ethylbenzene on p-N/S catalysts. As shown in
Figure 7, it also indicates that the oxidized S group is favorable for adsorption of
ethylbenzene. It is notable that all the elementary steps for g-N/S catalyst are exothermic
till the formation of 1-hydroperoxyethylbenzene intermediate (step (7)). However, this
step is favorable for p-N/S catalyst as shown in Figure 6. So, the best reaction
mechanism should be the synergic effect of p-N/S and g-N/S catalysts, that is, g-N/S
catalyst is favorable for the steps (1)-(6) while the g-N/S catalyst is responsible for the
rest steps. The results for OOH radical mechanism for p-N/S and g-N/S catalysts are
shown in Figure S1 and S2 in Supporting Information. For both catalysts, we can find
that the second step for formation and adsorption of OOH radical is highly endothermic.
So, the OOH radical mechanism is impossible, and the oxidation of ethylbenzene is
catalysed by synergic effect of p-N/S and g-N/S catalysts via OH radical mechanism.
Conclusion
Through direct carbonization of the pre-selected bioprecursor or its sulfate, a kind
of thin N and N/S codoped carbon nanosheets can be obtained. Due to the stability of the
bioprecursor nucleobases, carbon bond formation is restricted to result in a high local
order carbon which as such are highly sp2-conjugated, heteroatom-doped carbons. The
successful doping of S leads to the formation of “noble” carbon with more defects and
optimized electronic configuration. With TBHP as oxidant, these noble carbon performs
high activity to selective oxidation of C-H. Benefiting from the textural structure of the
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noble carbons, such as ultrathin thickness, optimal porosity, rich structural defects and
synergistic coupling effect of multiple dopants, GS1000 delivers superior catalytic
performance for selective oxidation of ethylbenzene with good stability, and the
applicability for the oxidation of the substituted phenyl substrate. It was proved that
GS1000 catalyzed ethylbenzene oxidation is a OH radical reaction. The improved
activity is likely due to the fact that the resulting electron-deficient sites from N/S
codoping enhance the adsorption ability towards TBHP and smooth electron transfer on
the surface of carbons. DFT calculation indicates that the oxidation of ethylbenzene is
catalysed by synergic effect of p-N/S and g-N/S catalysts. The present work may open a
new avenue for rational design and massive synthesis of heteroatom-doped 2D carbons
as efficient metal-free catalysts for applications in selective oxidation.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of
China (21571035), the Min Jiang scholar professorship Project, and the Strategic Priority
Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000).
References
1
2
3
4
4
5
3
6
7
.
.
.
J. Zhang, X. Liu, R. Blume, A. Zhang, R. Schlogl and D. S. Su, Science, 2008, 322, 73-77.
M. Christmann, Angew. Chem. Int. Ed., 2008, 47, 2740-2742.
Q. H. Xie, H. M. Zhang, J. C. Kang, J. Cheng, Q. H. Zhang and Y. Wang, ACS Catal., 2018, 8,
902-4916.
.
.
B. Liu and Z. H. Zhang, ACS Catal., 2016, 6, 326-338.
Z. Guo, B. Liu, Q. H. Zhang, W. P. Deng, Y. Wang and Y. H. Yang, Chem. Soc. Rev., 2014, 43,
480-3524.
.
.
R. G. Bergman, Nature, 2007, 446, 391.
A. G. Macedo, S. Fernandes, A. A. Valente, R. A. S. Ferreira, L. D. Carlos and J. Rocha,
Molecules, 2010, 15, 747-765.
Y. Hirai, T. Kojima, Y. Mizutani, Y. Shiota, K. Yoshizawa and S. Fukuzumi, Angew. Chem. Int.
8
.
2
1

Green Chemistry
Page 22 of 24
View Article Online
DOI: 10.1039/C9GC03781K
Ed., 2008, 47, 5772-5776.
9
2
1
.
R. P. Doherty, J. Krafft, C. Methivier, S. Casale, H. Remita, C. Louis and C. Thomas, J. Catal.,
012, 287, 102-113.
0. L. Kesavan, R. Tiruvalam, M. H. A. Rahim, M. I. Saiman, D. I. Enache, R. L. Jenkins, N.
Dimitratos, J. A. Lopezsanchez, S. H. Taylor and D. W. Knight, Science, 2011, 331, 195-199.
1
1
2
1
1
1
1
1
1. P. Zhang, Y. Gong, H. Li, Z. Chen and Y. Wang, Nat. Commun., 2013, 4, 1593.
2. X. Lin, Z. Nie, L. Zhang, S. Mei, Y. Chen, B. Zhang, R. Zhu and Z. Liu, Green Chem., 2017, 19,
164-2173.
3. X. Lin, S. Zhao, Y. Chen, L. Fu, R. Zhu and Z. Liu, J. Mol. Cataly. A-Chem., 2016, 420, 11-17.
4. A. K. Suresh, M. M. Sharma and T. Sridhar, Ind. Eng. Chem. Res., 2000, 39, 3958-3997.
5. V. Georgakilas, J. A. Perman, J. Tucek and R. Zboril, Chem. Rev., 2015, 115, 4744-4822.
6. D. S. Su, G. Wen, S. Wu, F. Peng and R. Schlogl, Angew. Chem. Int. Ed., 2017, 56, 936-964.
7. K. Zhou, B. Li, Q. Zhang, J. Huang, G. Tian, J. Jia, M. Q. Zhao, G. Luo, D. S. Su and F. Wei,
ChemSusChem, 2014, 7, 723-728.
1
1
5
2
5
2
8. M. Antonietti, N. Lopez-Salas and A. Primo, Adv. Mater., 2019, 31.
9. G. Wen, Q. Gu, Y. Liu, R. Schlogl, C. Wang, Z. Tian and D. S. Su, Angew. Chem. Int. Ed., 2018,
7, 16898-16902.
0. Y. Ding, X. Huang, X. Yi, Y. Qiao, X. Sun, A. Zheng and D. Su, Angew. Chem. Int. Ed., 2018,
7, 13800-13804.
1. F. Chen, W. Li, B. Sahoo, C. Kreyenschulte, G. Agostini, H. Lund, K. Junge and M. Beller,
Angew. Chem. Int. Ed., 2018, 57, 14488-14492.
2. J. Zhu, T. Diao, W. Wang, X. Xu, X. Sun, S. A. C. Carabineiro and Z. Zhao, Appl. Catal.
B-Environ., 2017, 219, 92-100.
2
2
2
2
2
6
2
2
2
2
3. C. Tang and Q. Zhang, Adv. Mater., 2017, 29, 1604103.
4. J. X. Zhang, L. Qu, G. Shi, J. Liu, J. Chen and L. Dai, Angew. Chem. Int. Ed., 2016, 55,
230-2234.
5. B. Frank, J. Zhang, R. Blume, R. Schlogl and D. S. Su, Angew. Chem. Int. Ed., 2009, 48,
913-6917.
6. D. Begin, G. Ulrich, J. Amadou, D. S. Su, C. Phamhuu and R. Ziessel, J. Mol. Catal. A-Chem.,
009, 302, 119-123.
7. D. S. Su, S. Perathoner and G. Centi, Chem. Rev., 2013, 113, 5782-5816.
8. Y. Gao, G. Hu, J. Zhong, Z. Shi, Y. Zhu, D. S. Su, J. Wang, X. Bao and D. Ma, Angew. Chem.
Int. Ed., 2013, 52, 2109-2113.
2
2

Page 23 of 24
Green Chemistry
View Article Online
DOI: 10.1039/C9GC03781K
2
2
3
3
9. S. Yang, L. Peng, P. Huang, X. Wang, Y. Sun, C. Cao and W. Song, Angew. Chem. Int. Ed.,
016, 55, 4016-4020.
0. B. Huang, Y. Liu, X. Huang and Z. Xie, J. Mater. Chem. A, 2018, 6, 22277-22286.
1. M. Titirici, R. J. White, N. Brun, V. L. Budarin, D. S. Su, F. D. Monte, J. H. Clark and M. J.
Maclachlan, Chem. Soc. Rev., 2015, 44, 250-290.
2. R. Arrigo, M. E. Schuster, Z. Xie, Y. Yi, G. Wowsnick, L. Sun, K. Hermann, M. Friedrich, P.
Kast and M. Havecker, ACS Catal., 2015, 5, 2740-2753.
3. Y. Gao, P. Tang, H. Zhou, W. Zhang, H. Yang, N. Yan, G. Hu, D. Mei, J. Wang and D. Ma,
Angew. Chem. Int. Ed., 2016, 55, 3124-3128.
3
3
3
2
3
9
3
1
3
4. W. Liu, C. Wang, F. Herold, B. J. M. Etzold, D. S. Su and W. Qi, Appl. Catal. B-Environ., 2019,
58.
5. Z. Y. Li, Q. M. Gao, X. Liang, H. Zhang, H. Xiao, P. Xu and Z. P. Liu, Carbon, 2019, 150,
3-100.
6. X. Hu, Y. Chen, B. Huang, Y. Liu, H. Huang and Z. Xie, ACS Sustain. Chem. Eng., 2019, 7,
1369-11376.
7. C. Tang, H. Wang, X. Chen, B. Li, T. Hou, B. Zhang, Q. Zhang, M. Titirici and F. Wei, Adv.
Mater., 2016, 28, 6845-6851.
3
3
4
1
4
1
4
4
4
4
4
1
4
8. S. Li, C. Cheng, H. W. Liang, X. L. Feng and A. Thomas, Adv. Mater., 2017, 29.
9. W. Qi, P. Q. Yan and D. S. Su, Acc. Chem. Res., 2018, 51, 640-648.
0. X. L. Guo, W. Qi, W. Liu, P. Q. Yan, F. Li, C. H. Liang and D. S. Su, ACS Catal., 2017, 7,
424-1427.
1. W. Qi, W. Liu, X. L. Guo, R. Schlogl and D. S. Su, Angew. Chem. Int. Ed., 2015, 54,
3682-13685.
2. W. Qi and D. S. Su, ACS Catal., 2014, 4, 3212-3218.
3. B. B. Huang, M. Xia, J. G. Qiu and Z. L. Xie, ChemNanoMat, 2019, 5, 682-689.
4. Y. Oaki, S. Kaneko and H. Imai, J. Mater. Chem., 2012, 22, 22686-22691.
5. B. B. Huang, X. Hu, Y. C. Liu, W. Qi and Z. L. Xie, J. Power Sources, 2019, 413, 408-417.
6. Y. C. Liu, B. B. Huang, X. F. Zhang, X. Huang and Z. L. Xie, J. Power Sources, 2019, 412,
25-133.
7. P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L.
Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U.
Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R.
Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P.
2
3

Green Chemistry
Page 24 of 24
View Article Online
DOI: 10.1039/C9GC03781K
Seitsonen, A. Smogunov, P. Umari and R. M. Wentzcovitch, J. Phys. Condens. Mat., 2009, 21.
4
4
2
5
1
5
8. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865-3868.
9. V. Barone, M. Casarin, D. Forrer, M. Pavone, M. Sambi and A. Vittadini, J. Comput. Chem.,
009, 30, 934-939.
0. L. Yu, C. Falco, J. Weber, R. J. White, J. Y. Howe and M. Titirici, Langmuir, 2012, 28,
2373-12383.
1. L. Lai, J. R. Potts, D. Zhan, L. Wang, C. K. Poh, C. Tang, H. Gong, Z. Shen, J. Lin and R. S.
Ruoff, Energy and Environ. Science, 2012, 5, 7936-7942.
5
5
2. K. Qu, Y. Zheng, S. Dai and S. Qiao, Nano Energy, 2016, 19, 373-381.
3. W. Liu, L. Zhang, X. Liu, X. Liu, X. Yang, S. Miao, W. Wang, A. Wang and T. Zhang, J. Amer.
Chem. Soc., 2017, 139, 10790-10798.
Table of contents
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