N,S co-doped hierarchically porous carbon materials for efficient metal-free catalysis

Article abstract of DOI:10.1039/c9gc03863a

Metal-free carbon catalysts with excellent catalytic performance have drawn much research attention recently. Herein, polymer-derived N,S co-doped carbon catalysts (PDNSC-X) with a hierarchically porous structure were facilely prepared by a cost-effective and convenient strategy via carbonization of a N- and S atom-containing polymer precursor and were subsequently used as efficient metal-free catalysts. The catalytic activity of the as-fabricated PDNSC-800 was greater than those of other reported heteroatom-doped carbon catalysts in catalytic reduction of various nitroarenes. The high catalytic activity of PDNSC-800 was related to the synergistic effects of a high surface area, a hierarchically porous structure, abundant N- and S-containing active sites, and defect formation. In addition, the close relationship between the N species (especially pyrrolic N) and high selectivity in metal-free catalytic synthesis was investigated in the reduction of nitroarenes and selective oxidation of ethylbenzene. This study may provide a new strategy to fabricate specific heteroatom-doped metal-free carbon catalysts for environmentally friendly efficient organic transformation.

Full text of DOI:10.1039/c9gc03863a

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Page 1 of 24
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N, S co-doped hierarchically porous carbon materials for
efficient metal-free catalysis
Xiwei Hu,a Xun Sun,b Qiang Song,a Yangyang Zhu,a Yu Long a* and Zhengping Dong a*
a
State Key Laboratory of Applied Organic Chemistry, Laboratory of Special Function
Materials and Structure Design of the Ministry of Education, Gansu Provincial Engineering
Laboratory for Chemical Catalysis, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, PR China
b
Shandong Applied Research Center of Gold Nanotechnology (Au-SDARC), School of
Chemistry & Chemical Engineering, Yantai University, Yantai 264005, PR China
E-mail address: sunxun@ytu.edu.cn (Xun Sun), longyu@lzu.edu.cn (Yu Long),
dongzhp@lzu.edu.cn (Zhengping Dong), Tel: 86 931 8912577.
1

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Abstract
Metal-free carbon catalysts with excellent catalytic performance are drawn much research
attention recently. Herein, polymer-derived N,S co-doped carbon catalysts (PDNSC-X) with
hierarchically porous structure were facilely prepared by a cost-effective and convenient
strategy via carbonization of a N- and S atoms-containing polymer precursor, which were
subsequently used as efficient metal-free catalysts. The catalytic activity of as-fabricated
PDNSC-800 was greater than those of other reported heteroatom-doped carbon catalysts in
catalytic reduction of various nitroarenes. The high catalytic activity of PDNSC-800 was
related to the synergistic effects of a high surface area, a hierarchically porous structure,
abundant N- and S-containing active sites, and defect formation. In addition, the close
relationship between the N species (especially pyrrolic N) and high selectivity in metal-free
catalytic synthesis was investigated in the reduction of nitroarenes and selective oxidation of
ethylbenzene. This study may provide a new strategy to fabricate specific heteroatom-doped
metal-free carbon catalysts for environmentally-friendly efficient organic transformation.
2

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1. Introduction
As novel heterogeneous catalysts, heteroatom-doped carbon materials (HDCMs) have
2
attracted considerable scientific interest in metal-free catalysis in recent years.1, As an
effective method, the doping of heteroatoms provides additional heteroatom-containing active
species to carbon materials on the basis of their original excellent structural properties (e.g.,
porous structure, high surface area, and excellent stability), making HDCMs ideal alternatives
to traditional metal-based catalysts.3, 4 As the most common dopant, nitrogen has been widely
used for the preparation of HDCMs. As-prepared nitrogen-doped carbon materials exhibit
outstanding catalytic performance in electrocatalysis and organic synthesis, which is related
to the improved electronic transmission capability caused by the doping of nitrogen atoms.5, 6
Moreover, as another dopant, the sulfur atom, which is different from the nitrogen atom in
terms of the atomic size and electronegativity, is beneficial for developing a unique electronic
structure and forming active defective sites in the framework of materials.7, 8 Furthermore, the
synergistic effects caused by co-doping in carbon materials are reported to be effective for
improving the catalytic activity of as-prepared HDCMs.9
Thus far, dual HDCMs have been mainly prepared by the treatment of
heteroatom-containing natural materials and/or addition of external heteroatom-containing
sources into traditional carbon materials.10, 11 This treatment typically involves tedious steps
and involves the use of toxic and corrosive acids or alkalis, making the preparation
cumbersome and environmentally unfriendly.12 Moreover, the addition of external sources is
frequently accompanied by some environmentally harmful processes, e.g., the introduction of
toxic ammonia as a nitrogen source at high temperature, etching with thiourea, or the
pyrolysis of toxic sulfides during doping with sulfur.13, 14 By contrast, the direct carbonization
of the dual-heteroatom-containing polymer precursor is not only eco-friendlier but also
contributes to the in-situ formation of abundant active sites in the resulting materials.
Moreover, the clear elemental and bonding information of the as-fabricated polymer
precursor aids in the investigation of the specific role of dopants in HDCMs for metal-free
catalysis.15
In fact, dual and/or multi HDCMs have been widely applied and extensively examined for
3

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electrocatalysis. For example, Kim et al. have reported S,N-co-doped bamboo carbons as
efficient catalysts for application in ORR and revealed superior durability that is comparable
to that of a commercial Pt/C catalyst.16 Favaro et al. have reported that multi-B,N,S-doped
graphene oxide quantum dots exhibited good catalytic performance for ORR with an
extremely low overpotential.17 Wu et al. have reported that N,F,P ternary-doped macroporous
carbon materials are excellent electrocatalysts with a half-wave potential similar to that of
commercial Pt/C in ORR.18 However, few studies have reported the application of such
materials in heterogeneous organic synthesis; besides, it is difficult to clarify the specific role
19
of each dopant; thus, its specific role remains unclear.3,
Accordingly, it is crucial to
investigate the specific role of each dopant in dual and/or multi HDCMs in detail and
beneficial for the research on the synthesis of metal-free catalysts.
As an efficient and convenient method, the catalytic reduction of nitroarenes to obtain key
aromatic amine intermediates for fine chemical products, dyes, and pharmaceuticals has been
widely applied in the chemical industry.20-22 Conventionally, various metals such as Fe,23
Co,24 Ni,25 Pd,26 Au,27 Pt,28 and Ag29 have been frequently used for the homogeneous and
heterogeneous catalytic reduction of nitroarenes. Nevertheless, tedious post-processing steps
and recycling issues related to homogeneous systems and leaching of metals in heterogeneous
systems render the catalytic reduction process unsustainable and environmentally
unfriendly.30, 31 Similarly, as an important C–H activation reaction, the selective oxidation of
ethylbenzene for synthesizing acetophenone, which is frequently applied for the manufacture
of pharmaceuticals and perfumes, exhibits similar limitations in terms of both manufacturing
33
costs and environmental protection.32,
Accordingly, HDCMs with enhanced catalytic
activity and excellent structural stability as novel, green metal-free catalysts are urgently
required in the above-mentioned catalytic reactions.
In this study, a N- and S-atom-containing polymer was synthesized as a precursor for
preparing polymer-derived N,S co-doped carbon catalysts (PDNSC-X, where X refers to the
carbonization temperature) by direct carbonization without the addition of external sources.
As-prepared PDNSC-X catalysts exhibit a hierarchically porous structure, which is expected
to improve their catalytic activity by providing an efficient mass transport pathway. As
excepted, PDNSC-800 exhibited excellent catalytic activity and even remarkable selectivity
4

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for the reduction of various substituted nitrobenzenes and the selective oxidation of
ethylbenzene. In addition, the high reactivity of the PDNSC-800 catalyst was investigated in
detail; besides, for the first time, the relationship between the doping of pyrrolic N atoms and
their high selectivity in metal-free organic synthesis was confirmed. Thus, this study is
expected to provide a convenient, eco-friendly method to prepare dual-heteroatom-doped
carbon catalysts for metal-free catalysis and clarify the role of different doped atoms in
HDCMs.
2. Experimental section
2.1 Materials
Melamine, trithiocyanuric acid, and glyoxal (40 wt%) were purchased from Xilong
Chemical Co., Ltd. (China). Nitroarenes, hydrazine hydrate (80 wt%), ethylbenzene, and
tert-butyl hydroperoxide (TBHP) were purchased from Aladdin Industrial Corporation. All of
the reagents were used as received.
2.2 Preparation of N,S-co-doped carbon catalysts
First, trithiocyanuric acid (14.06 g) and glyoxal (15.14 mL) were added in
anhydrous alcohol (30 mL) with magnetic stirring for 0.5 h to ensure even mixing. Second,
melamine (10 g) was added into the above-mentioned solution to start the polymerization and
maintain the solution for 10 h at room temperature. Third, the polymerized solution was dried
overnight at 60 °C, and the as-obtained yellow solid was ground to obtain a polymer
precursor (PP). The PP powder was subjected to carbonization at 600, 700, 800, and 900 °C
under N2 for 2 h at a heating rate of 5 °C min1, affording PDNSC-X materials.
2.3 Synthesis of N-doped and S-doped catalysts
First, melamine (10 g) and glyoxal (15.14 mL) were added into anhydrous alcohol (30 mL)
with magnetic stirring, and the reaction was maintained for 10 h at room temperature. Second,
the polymerized solution was dried overnight at 60 °C and subjected to carbonization at
800 °C, affording the polymer-derived N-doped carbon catalyst (PDNC-800). In addition, the
S-doped carbon catalyst (SC-800) was prepared by the pyrolysis of a mixture of commercial
activated carbon and sulfuric acid (w/w, 5:1) at 800 °C and subsequent treatment with
deionized water to neutral.
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2.4 Catalytic reduction of nitrobenzene
Typically, a solution containing 0.5 mmol of nitrobenzene, 500 μL of hydrazine hydrate, 3
mL of ethanol, and 30 mg of the PDNSC-800 catalyst was stirred in a 10-mL two-necked
flask at 90 °C. The samples were collected by using a syringe and filtered to remove the
catalyst, followed by gas chromatography–mass spectrometry (GC–MS) analysis.
2.5 Selective oxidation of ethylbenzene
Briefly, ethylbenzene (1 mmol), TBHP (500 μL), PDNSC-800 catalyst (20 mg), and H2O
(3 mL) were stirred in a 10-mL two-necked flask at 80 °C. The samples were filtered and
extracted, followed by the GC–MS analysis of the organic phase.
2.6 Catalyst characterization
The structure of PDNSC-X catalysts was observed by transmission electron microscopy
(TEM, Tecnai G2 F30) and scanning electron microscopy (SEM, HITACHI S-4800).
Electron energy-loss spectroscopy (EELS) was performed on a FEI Talos F200S TEM system
using a 200-keV monochromated electron beam. X-ray diffraction (XRD, Rigaku
D/max-2400) patterns were recorded in the 2θ range of 10°–70° with Cu-kα radiation. Raman
spectra were recorded on a JobinYvon LabRAM HR800 spectrometer. X-ray photoelectron
spectroscopy (XPS, PHI-5702) curves were recorded to examine the elemental content of the
PDNSC-X catalysts. The Brunauer–Emmett–Teller (BET, Micromeritics, ASAP 2010)
specific surface areas were calculated on the basis of the obtained nitrogen adsorption–
desorption isotherms at 77 K. The non-local density functional theory method was employed
to analyze the pore size distributions with a slit pore model. FTIR spectra were recorded on a
Bruker VERTEX 70 spectrometer. Conversion and selectivity were analyzed by GC–MS
(Agilent 5977E).
3. Results and Discussion
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Scheme 1. The preparation procedures of PDNSC-X catalysts.
PDNSC-X materials were synthesized by a convenient two-steps method of polymerization
and carbonization (Scheme 1). The introduction of N- and S-containing monomers was
favorable for the improvement in the doping content of HDCMs. Moreover, the use of a
polymer precursor contributed to the in-situ formation of abundant active sites and prevented
the loss of heteroatom-containing components during high-temperature carbonization.
3.1 Reactivity test of PDNSC-X catalysts for the reduction of nitrobenzene
The catalytic activity of as-prepared PDNSC-X was examined in the reduction of
nitrobenzene. In addition, the catalyst carbonization temperature, solvent, catalyst amount,
reaction temperature, and hydrogen donors and their concentrations were investigated. The
PDNSC-800 catalyst exhibited excellent reactivity with a TOF value of 1.67 × 102
mol·g1h1, which is higher than those of the catalysts subjected to carbonization at higher or
lower temperatures (i.e., PDNSC-900, PDNSC-700, and PDNSC-600) (Table 1, entries 1–4).
Among all of the solvent compositions, anhydrous alcohol exhibited the best catalytic
performance (Table 1, entries 4–8). Moreover, the reaction system with a lower catalyst
amount and/or lower reaction temperature exhibited decreased catalytic activity (Table 1,
entries 9 and 10, respectively). The reaction using all other hydrogen donors afforded low
catalytic activity, except for NaBH4, which exhibited obvious reactivity with a conversion of
84.9%, although it was still less than that of N2H4·H2O (Table 1, entries 11–13). The
dramatic decrease in the catalytic activity was related to the reaction with a lower N2H4·H2O
concentration; besides, the reaction did not proceed without N2H4·H2O (Table 1, entries 14–
16). In addition, other related catalysts were prepared and investigated for comparison
purposes. The precursor PP exhibited poor catalytic activity, indicative of the importance of
carbonization for the preparation of efficient heteroatom-doped catalysts (Table 1, entry 17).
The catalysts doped only with N (i.e., PDNC-800) or S (i.e., SC-800) exhibited significant
activity; however, the activities observed for these catalysts were less than that observed for
PDNSC-800, indicative of the necessity of N,S co-doping (Table 1, entries 18 and 19). Pure
carbon material showed low catalytic activity, and the reaction did not occur in the absence of
any catalyst (Table 1, entries 20 and 21).
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Table 1. The optimization of reaction conditions for reduction of nitrobenzene.a
PDNSC-X
NO2
NH2
N H ·
H2O
2
4
Conv./Sel.
(%)
Entry
Catalyst/mg
Solvent
Temp. (°C)
TOF (mol·g-1h-1)h
1
2
PDNSC-600/30
PDNSC-700/30
PDNSC-900/30
PDNSC-800/30
PDNSC-800/30
PDNSC-800/30
PDNSC-800/30
PDNSC-800/30
PDNSC-800/20
PDNSC-800/30
PDNSC-800/30
PDNSC-800/30
PDNSC-800/30
PDNSC-800/30
PDNSC-800/30
PDNSC-800/30
PP/30
Ethanol
Ethanol
Ethanol
Ethanol
E:W=2:1
E:W=1:1
E:W=1:2
Water
90
90
90
90
90
90
90
90
90
80
90
90
90
90
90
90
90
90
90
90
90
15.6/>99
54.3/>99
97.9/>99
>99/>99
57.1/>99
38.0/>99
51.3/>99
79.9/>99
45.2/>99
57.7/>99
84.9/>99
2.14/>99
-
2.60×10-3
9.05×10-3
1.63×10-2
1.67×10-2
9.51×10-3
6.33×10-3
8.55×10-3
1.33×10-2
7.53×10-3
9.62×10-3
1.42×10-2
3.57×10-4
-
4.12×10-3
2.45×10-3
-
4.50×10-4
3.23×10-3
1.42×10-2
1.87×10-3
6.07×10-4
3
4
5
6
7
8
9
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
10
11b
12c
13d
14e
15f
16g
17
18
19
20
21
24.7/>99
14.7/>99
0.3/>99
2.7/>99
19.4/>99
85.3/>99
11.2/>99
3.64/>99
PDNC-800/30
SC-800/30
Graphite powder/30
None
a
Reaction conditions: 0.5 mmol nitrobenzene, 500 μL N2H4·H2O, 30 mg catalyst, 3 mL
solvent, 1 h. unless otherwise mentioned.
b, c, d Using NaBH4, formic acid and H2 as hydrogen donors, respectively.
e 300 μL N2H4·H2O.
f 100 μL N2H4·H2O.
g Without N2H4·H2O.
mole of converted substrate
h Turnover frequency (TOF) =
were determined by GC-MS.
, the conversion and selectivity
g catalyst × h (reaction time)
3.2 Structural information of as-prepared PDNSC-X catalysts
Based on the above preliminary investigation with respect to the catalytic activity,
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a series of physicochemical characterization techniques were performed in detail to
understand the relationship between the physicochemical properties and catalytic activity.
Fig. 1a shows the SEM image of the PDNSC-800 catalyst: It exhibited a highly curled and
multi-tiered morphology. Typically, the high specific surface area and hierarchically porous
structure are formed in materials with such morphology, and these morphologies are typically
closely related to the high activity of heterogeneous catalysts.34, 35 The layered nanosheet and
porous structure were clearly observed in the TEM image of the PDNSC-800 catalyst (Fig.
1b). EDX elemental mapping images of the PDNSC-800 catalyst revealed the uniform
distribution of N and S atoms on the nanosheets (Fig. 1c).
Fig. 1. (a) SEM image of PDNSC-800 catalyst. (b) TEM image of PDNSC-800 catalyst. (c)
EDX mapping images of PDNSC-800 catalyst for C, N, O and S elements.
The BET method was applied to investigate the high reactivity of the PDNSC-800 catalyst
in terms of the surface area and hierarchically porous structure. The physisorption isotherms
of the PDNC-800 and PDNSC-800 catalysts revealed type I and type IV isotherms, indicative
of the presence of micropores and mesopores in the corresponding catalysts, respectively
(Fig. 2a).36, 37 Furthermore, the strong adsorption at a low relative pressure (P/P0<0.01) in the
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38
isotherm of PDNSC-800 catalyst revealed the presence of micropores. Pore size distribution
curves confirmed the above-mentioned results (Fig. 2b). In the curve of the PDNSC-800
catalyst, micropores, mesopores, and macropores were observed, indicative of the
hierarchically porous structure of the PDNSC-800 catalyst, while the condition for
PDNC-800 was quite different from that for PDNSC-800, indicating that PDNC-800 is a
microporous material. Clearly, the introduction of the S-containing monomer can not only
render more active components but also help to form a hierarchically porous structure.
Moreover, the surface area of the PDNSC-800 catalyst (351.3 m2 g1) was greater than that of
the PDNC-800 catalyst (273.4 m2 g1). The high surface area renders additional accessible
active sites, which will help to improve the catalytic activity. In addition, for comparison,
Table S1 summarizes the texture parameters of other PDNSC-X catalysts.
Fig. 2. Nitrogen adsorption-desorption isotherm (a) and pore size distribution curve (b) of
PDNC-800 and PDNSC-800 catalysts.
Raman spectra were recorded to obtain information about structural defects in the
as-prepared PDNSC-X catalysts. Two clear peaks observed at 1325 cm1 (D band) and 1595
cm1 (G band) correspond to the defects and graphitic phase in the materials, respectively
(Fig. 3a).39, 40 The maximum intensity ratio of the D and G bands (ID/IG) in PDNSC-800
reflected the highest density of defects. Moreover, lattice defects in heteroatom-doped carbon
42
catalysts have been frequently reported as catalytically active sites.41,
Therefore,
PDNSC-800 is expected to exhibit high catalytic activity with more defective sites. In
addition, all PDNSC-X catalysts exhibited two characteristic peaks at 26° and 43.3°,
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43
corresponding to the (002) and (101) planes of graphitic carbon, respectively (Fig. 3b).
Fig. 3. Raman spectra (a) and XRD patterns (b) of PDNSC-600, PDNSC-700, PDNSC-800
and PDNSC-900 catalysts.
3.3 Probing the causes of defect formation and possible factors for high catalytic
activity
XPS spectra were analyzed in detail to provide important information about the element
contents, bonding configuration of N and S atoms, and the cause of defect formation in
PDNSC-X catalysts. All of PDNSC-X catalysts contained C, N, O, and S (Fig. S1). Table 2
summarizes the XPS element content. With the increase in the carbonization temperature, the
N doping contents decreased, while S doping contents did not exhibit any clear regularity.
Notably, the S doping contents exhibited a linear relationship to the ID/IG values in different
PDNSC-X catalysts (Fig. S2); besides, the ID/IG value calculated for the PDNC-800 catalyst
(without S doping) was 1.00; this value was considerably less than that (1.13) calculated for
the PDNSC-800 catalyst (Fig. S3). The above results indicated that the formation of defects
in the PDNSC-X catalysts is mainly associated with the doping of S atoms rather than N
atoms. Furthermore, the PDNSC-800 catalyst exhibited the highest density of defects and the
best catalytic performance, indicative of the close relationship between the formation of
defect sites and catalytic activity. In addition, the EELS C K-edge spectrum of the
PDNSC-800 catalyst revealed that the peaks observed at 286.6 and 294.2 eV correspond to
the 1s-π* and 1s-σ* electronic transition of sp2 hybrid carbon, respectively; besides, the sp2
carbon content was calculated to be 82.3% (Fig. S4).
High-resolution N 1s and S 2p XPS spectra were recorded to illustrate the detailed bonding
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configuration. All of the catalysts contained two nitrogen species: pyridinic N (398.2 eV) and
pyrrolic N (400.1 eV) (Fig. 4a).44, 45 Three main peaks were observed at 163.7 eV, 164.8 eV,
and 167.6 eV in the S 2p spectra (Fig. 4b), corresponding to the S 2p3/2 and S 2p1/2 of C–S–C
and SOx species.16, 46 In addition, C–N, C=N, N=C=S, and S–C stretching vibration peaks
were observed at 1250 cm1, 1585 cm1, 2214 cm1, and 738–818 cm1, respectively, in the
FTIR spectra of PDNSC-X catalysts (Fig. S5). The coexistence of N and S active species in
the PDNSC-X catalysts is expected to improve the catalytic reactivity. Indeed, the
PDNSC-800 catalyst exhibited excellent activity with a TOF value of 2.78 × 102 mol·g1h1,
which is higher than those observed for mono-heteroatom-doped (NC-700) and even
dual-heteroatom-doped catalysts (BNC and BN-HCS-800) (Table S2).47-49
Based on the above-mentioned important findings and the analysis of related
characterization data, the high catalytic activity of PDNSC-800 can be attributed to the
synergistic effects of following factors: (1) The additional accessible active sites rendered by
the relatively high surface area. (2) The formation of a hierarchically porous structure by the
introduction of the S-containing monomer, which promotes the mass transfer. (3) The
presence of active defects caused by the doping of N and S atoms (mainly by the doping of S
atoms) in the carbon lattice. (4) The abundant active sites resulting from the co-doping of N
and S atoms.
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Fig. 4. XPS high-resolution N 1s (a) and S 2p (b) spectra of PDNSC-X catalysts, i.e.,
PDNSC-600, PDNSC-700, PDNSC-800 and PDNSC-900, respectively.
Table 2. XPS element contents in different PDNSC-X catalysts.
XPS element contents
Entry
Samples
C [at. %]
62.34
N [at. %]
31.68
S [at. %]
0.61
O [at. %]
5.37
1
2
3
PDNSC-600
PDNSC-700
PDNSC-800
71.92
22.42
0.62
5.04
71.98
15.71
1.02
11.29
4
PDNSC-900
79.85
10.19
0.48
9.48
3.4 Substrate scope of the reduction of substituted nitroarenes catalyzed by
PDNSC-800
The reduction of different substituted nitroarenes was performed to examine
the substrate scope of the PDNSC-800 catalyst and key factors for its high selectivity. First,
halogenated nitroarenes were examined, and all substrates were reduced with high selectivity
in a relatively short reaction time (Table 3). The reaction rate for the reduction of halogenated
nitroarenes followed the decreasing order of fluoro-substituted nitrobenzene
>
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chloro-substituted nitrobenzene > bromo-substituted nitrobenzene > iodo-substituted
nitrobenzene; this result was mainly attributed to the difference in the electron-withdrawing
abilities of different functional groups (Table 3, entries 1–7). Notably, almost no
dehalogenation was observed during catalytic reduction. All substrates containing
electron-donating groups (i.e., -CH3, -OH, and -NH2) were converted into corresponding
products with extremely high conversion and selectivity, although with a longer reaction time,
which is probably related to the strong electron-donating effect (Table 3, entries 8–16).
Moreover, the substrate containing both electron-withdrawing and electron-donating groups
was efficiently reduced with high selectivity, while for the substrate containing both
electron-donating groups, an extremely long reaction time was needed to complete catalytic
reduction (Table 3, entries 17–18). In addition, dinitro-substituted benzene was reduced to
the main product 1,3-benzenediamine, with a high selectivity of 82.7% (Table 3, entry 19).
3.5 Probing the relationship between the chemoselectivity and dopant by control
experiments
The key factors for the high selectivity of the PDNSC-800 catalyst were further
investigated by control experiments catalyzed by PDNC-800 (without S doping) and SC-800
(without N doping) catalysts. First, the reduction of several halogenated nitroarenes catalyzed
by PDNC-800 was carried out to illustrate the pivotal role of the doped nitrogen species in
terms of selectivity. Unexpectedly, all of the substrates were converted into corresponding
products with high selectivity similar to that observed in the reaction catalyzed by
PDNSC-800, though with a relatively low conversion (>37.5%), indicative of the significant
role of doped nitrogen species in selectivity (Table S3). The above-mentioned results are
quite different from those obtained in a study previously reported by our group for the same
reaction catalyzed by an N-doped carbon material (NC-700), where clear dehalogenation was
observed for the catalytic reduction of halogenated nitroarenes.47 To clarify this phenomenon,
the XPS curve of the PDNC-800 catalyst was analyzed in detail. Fig. S6 shows the element
content of PDNC-800. Notably, the N content of the PDNC-800 catalyst is 11.11 at%; this
value is less than that of NC-700 (17.25 at%). The PDNC-800 catalyst with a lower N content
exhibited higher selectivity, suggesting that the types of N species, instead of the content of
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N, constitute the key factor for the high selectivity. Therefore, the high-resolution N 1s XPS
spectrum of the PDNC-800 catalyst is further analyzed. The N 1s spectrum was composed of
pyridinic N (398.2 eV) and pyrrolic N (400.1 eV), which was different from the condition that
the N 1s spectrum was composed of pyridinic N and graphitic N in case of the NC-700
catalyst (Fig. S7). From the above comparison, it is reasonable to assume that pyrrolic N
plays a key role in chemoselectivity. Indeed, N-doped carbon materials derived from a
melamine–chitosan composite and N-doped hollow carbon spheres, which contain pyrrolic N,
have been reported as efficient catalysts that exhibit excellent selectivity (>99%) for the
reduction of halogenated nitroarenes.50, 51 Accordingly, from the above results, the presence of
pyrrolic N in catalysts is beneficial to improving chemoselectivity.
Similarly, the SC-800 catalyst was applied to catalyze the reduction of several halogenated
nitroarenes to further illustrate the role of S species in terms of selectivity. All of the
substrates were converted into corresponding products with a reaction rate similar to that of
the PDNSC-800 catalyst, though with decreased selectivity (>91%), indicating that S doping
plays a crucial role in the catalytic activity but not in selectivity (Table S4). However,
considering that a selectivity of greater than 91% was still observed for the SC-800 catalyst,
the high selectivity of PDNSC-800 catalyst should be attributed to the pyrrolic N and S
species in its framework, in which pyrrolic N atoms should be the main active components for
the chemoselectivity. Similarly, the high catalytic activity of PDNSC-800 should be attributed
to the presence of both doped N and S species in its framework, in which doped S species is
the main active component responsible for its catalytic activity.
Table 3. The reduction of different substituted nitroarenes catalyzed by PDNSC-800 catalyst
under the optimal conditions. a
Entry
1
Substrate
Product
Time (h) Conv./ Sel. (%)b TOF (mol·g-1h-1)
NO2
F
NH2
F
0.5
1
96.6/>99
97.1/>99
1.51×10-2
1.61×10-2
NH2
Cl
NO2
Cl
2
NO2
NH2
3
4
1
1
99.9/>99
98.8/>99
1.67×10-2
1.65×10-2
Cl
Cl
Cl
Cl
NO2
NH2
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NH2
Br
1.10×D1O0I-:210.1039/C9GC03863A
NO2
Br
5
1.5
99.0/>99
NO2
NH2
6
7
1.5
2
99.6/>99
99.1/98.6
1.11×10-2
7.99×10-3
Br
Br
I
NO2
NO2
I
NH2
NH2
CH3
8
9
8.5
8.5
99.2/>99
99.9/>99
1.94×10-3
1.96×10-3
CH3
NO2
NH2
H3C
H3C
H3C
H3C
10
11
8.5
9
99.6/>99
99.9/>99
1.95×10-3
1.84×10-3
NO2
NH2
NO2
OH
NH2
OH
NO2
NH2
12
13
14
10
11
7
99.0/>99
99.7/>99
99.3/>99
1.64×10-3
1.50×10-3
2.29×10-3
HO
HO
HO
HO
NO2
NH2
NO2
NH2
NH2
NH2
NO2
NH2
15
16
7
8
92.7/>99
98.3/>99
2.08×10-2
1.97×10-3
H2N
H2N
H2N
H2N
NO2
NH2
NH2
H3C
H3C
17
18
2
99.9/>99
99.2/>99
8.33×10-3
1.16×10-3
NO2
Cl
Cl
NH2
NO2
14
CH3
H2N
O2N
CH3
H2N
H2N
NO2
NH2
17.3
2.62×10-4
1.25×10-3
NO2
19
11
99.9
82.7
H2N
a Reaction conditions: 0.5 mmol substrate, 30 mg PDNSC-800 catalyst, 3 mL of ethanol, 500
μL N2H4·H2O, 90 °C. b Conversion and selectivity were determined by GC-MS.
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3.6 Proposed mechanism for the reduction of nitrobenzene catalyzed by
PDNSC-X
A reasonable catalytic mechanism for the reduction of nitrobenzene was proposed on the
basis of the above conclusions and several control experiments. Generally, the reduction of
nitrobenzene occurs by two pathways: the direct pathway with N-phenylhydroxylamine as the
intermediate product and the indirect pathway with azobenzene as the intermediate product.50
As a stable intermediate product, azobenzene was easily detected by GC–MS, whereas it was
not detected during the catalytic process.52, 53 Accordingly, the reduction of nitrobenzene
catalyzed by PDNSC-800 should proceed in the direct pathway. Moreover, the active
hydrogen species produced by the decomposition of N2H4·H2O were identified by some
control experiments. The control experiment in the presence of a radical inhibitor (such as
BHT) exhibited a conversion (97.3%) similar to that observed from the original experiment
(>99%) under the same conditions, suggesting that hydrogen radicals (H·) are not generated
in the reaction (Table 4). Besides, the reaction did not occur by using H2 as the hydrogen
donor, indicating that the real active hydrogen species should not be the H2 produced by
N2H4·H2O. Based on the above results, Hδ+ and Hδ generated from the decomposition of
N2H4 should be the real active hydrogen species.
Table 4. The control experiment of reduction of nitrobenzene in the presence of a radical
inhibitor.a
Entry
1
Catalyst
Radical inhibitor
none
Conv./Sel. (%)
>99/>99
PDNSC-800
PDNSC-800
2b
BHT
96.2/>99
a Reaction conditions: 0.5 mmol substrate, 30 mg PDNSC-800 catalyst, 3 mL of ethanol, 500
μL N2H4·H2O, 90 °C. b 0.25 mmol butylated hydroxytoluene (BHT) was added in the reaction
solution.
Scheme 2 shows the detailed catalytic mechanism. First, N2H4 was adsorbed on the catalyst
and further activated at the N- and S-containing active sites. Second, the reversible adsorption
and desorption of reactants occurred; besides, the active hydrogen species bonded with
electronegative N and S (N–Hδ+ and S–Hδ+) and bonded with electropositive C (C–Hδ) were
generated after the heterolytic cleavage of N2H4. In addition, based on the above detailed
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analysis of N, S dopants and the formation of defects, the main active sites are located at
defects. Third, the absorbed reactants were reduced by the assistance of N–Hδ+, S–Hδ+, and C–
Hδ species at defects by the above-mentioned direct pathway. Finally, the desorption of
products (aniline) from the PDNSC-800 catalyst was completed, indicative of the completion
of the whole catalytic cycle.
Scheme 2. The proposed mechanism in reduction of nitrobenzene catalyzed by PDNSC-X
catalysts.
To further confirm the main active components in the PDNSC-800 catalyst in terms of the
catalytic activity and chemoselectivity, the selective oxidation of ethylbenzene was carried
out as a supporting experiment. PDNSC-800 exhibited remarkable catalytic activity with high
conversion (>99%) and outstanding selectivity (98.7%) (Table 5, entry 1). PDNC-800
exhibited a similarly high selectivity (96.3%), though with a relatively low conversion
(75.3%), during the catalytic process (Table 5, entry 2). Moreover, the SC-800 catalyst
exhibited a similarly high conversion (98.7%), though with a relatively low selectivity
(87.4%), during catalytic oxidation (Table 5, entry 3). Pure carbon material (graphite
powder) showed low catalytic reactivity in the reaction (Table 5, entry 4). The reaction
performed with a longer reaction time (24 h) showed similar catalytic activity and selectivity
with original reaction (Table 5, entry 5). In addition, the absence of TBHP and/or the catalyst
led to the obvious decline in the conversion and selectivity to acetophenone (Table 5, entries
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6 and 7). The above results well confirmed the conclusions that were observed in the
reduction of nitrobenzene, that is, the pyrrolic N atoms and S species are mainly responsible
for the high chemoselectivity and catalytic activity in the reaction, respectively.
Table 5. The selective oxidation of ethylbenzene catalyzed by PDNSC-800 catalyst.a
O
OH
O
Catalyst
TBHP
+
+
Sel. (%)
Acetophenone Benzaldehyde
Conv.
(%)
Entry
Catalysts
α-Phenylethanol
1
2
PDNSC-800
PDNC-800
>99
75.3
98.7
24.9
>99
12.1
0.3
98.7
96.3
87.4
52.3
98.3
33.2
1.0
2.3
2.9
29.0
1.5
4.4
1.4
3
SC-800
9.7
4
Graphite powder
PDNSC-800
PDNSC-800
18.7
0.2
5b
6c
62.4
7
-
23.3
20.3
49.1
30.6
a
Reaction conditions: 1 mmol ethylbenzene, 20 mg catalyst, 3 mL H2O, 500 μL TBHP,
b
80 °C, 10 h. The reaction was performed with a longer reaction time (24 h).c Without
adding of TBHP.
3.7 Recyclability and structural stability of the PDNSC-800 catalyst
The recyclability and stability of the as-prepared PDNSC-800 catalyst were well
demonstrated by cyclic experiments and related characterization of the reused catalyst. The
PDNSC-800 catalyst exhibited outstanding recyclability for the reduction of nitrobenzene and
selective oxidation of ethylbenzene with no clear decline in the conversion and selectivity
(Fig. 5a and b). Moreover, other related characterization of the reused PDNSC-800 catalyst
was performed to further illustrate its remarkable structural stability. The reused PDNSC-800
catalyst exhibited the same layered nanosheet and porous structure in its TEM image (Fig.
S8a). Similarly, the unchanged highly curled and multi-tiered morphology was also observed
in its SEM image (Fig. S8b). The same peaks located at 26° and 43.3° were observed in the
PXRD spectrum of the reused catalyst (Fig. S9a). The Raman spectrum of the reused
PDNSC-800 catalyst exhibited an ID/IG value (1.11) similar to that of the fresh catalyst (1.13)
(Fig. S9b). In addition, the contents of N and S elements in the reused PDNSC-800 catalyst
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were similar to those in the fresh catalyst, indicating that there is no clear loss of active
components after cyclic tests (Table S5). Furthermore, the comparison of the fresh and reused
PDNSC-800 catalysts by the C 1s, S 2p, and N 1s spectra revealed almost unchanged peaks
(Fig. S10a, b and c). All of the above results indicated remarkable recyclability and structural
stability for the PDNSC-800 catalyst.
Fig. 5. Cyclic experiments of PDNSC-800 catalyst in reduction of nitrobenzene (a) and
selective oxidation of ethylbenzene (b).
4. Conclusions
In conclusion, PDNSC-X catalysts were synthesized by a convenient two-step method by
employing a polymer comprising abundant N and S active species as the precursor, which is
beneficial for the in-situ formation of heteroatom-containing active sites. As-fabricated
PDNSC-800 exhibited high catalytic activity and outstanding chemoselectivity for the
reduction of different nitroarenes, especially for halogenated nitroarenes, and the selective
oxidation of ethylbenzene. By the detailed analysis of the related characterization and
experimental results, the high catalytic activity of PDNSC-800 should be attributed to the
synergistic effects of a high surface area, a hierarchically porous structure, abundant N- and
S-containing active sites, and the formation of defective active sites in its framework. In
addition, PDNSC-X catalysts showed outstanding structural stability and recyclability in both
reactions even after 8 times cyclic tests. This study is expected to provide a facile,
cost-effective method to prepare N,S co-doped materials for metal-free catalysis and to clarify
the relationship between the catalytic selectivity and active dopants in N,S co-doped catalysts.
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Conflicts of interest
There are no conflicts to declare.
Supporting Information
Supporting Information is available from the publication website.
Acknowledgements
This work was supported by the Natural Science Foundation of Gansu (No. 18JR3RA274).
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Graphical Abstract