Nitrogen, Sulfur Co-doped Carbon Materials Derived from the Leaf, Stem and Root of Amaranth as Metal-free Catalysts for Selective Oxidation of Aromatic Hydrocarbons

Article abstract of DOI:10.1002/cctc.201801379

Development of porous heteroatom-doped carbon materials from plant biomass is of great significance for various applications in environment, energy conversion and catalysis process. However, the different natures of carbon materials derived from the leaf,

Full text of DOI:10.1002/cctc.201801379

Accepted Article
Title: Nitrogen, sulfur co-doped carbon materials derived from the leaf,
stem and root of amaranth as metal-free catalysts for selective
oxidation of aromatic hydrocarbons
Authors: Yongbin Sun, Kun Liu, Chao Hou, Jian Liu, Runkun Huang,
Changyan Cao, and Weiguo Song
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ChemCatChem
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Nitrogen, sulfur co-doped carbon materials derived from the leaf,
stem and root of amaranth as metal-free catalysts for selective
oxidation of aromatic hydrocarbons
[
a]
[a]
[a]
[b]
[b]
[b]
Yongbin Sun,* Kun Liu,* Chao Hou, Jian Liu, Runkun Huang, Changyan Cao, and Weiguo
Song[b]
Abstract: Development of porous heteroatom-doped carbon
carbon materials, obtained by simple thermolysis of N-containing
MOFs, exhibited excellent catalytic performances for aerobic
materials from plant biomass is of great significance for various
applications in environment, energy conversion and catalysis process. oxidation of toluene and cyclohexane. Generally, the excellent
However, the different natures of carbon materials derived from the
leaf, stem and root of the same plant biomass are rarely reported.
Herein, we synthesized porous carbon materials from the leaf, stem
and root of amaranth respectively by pyrolysis without any activation
treatment. Detailed characterizations indicated that these three
carbon materials all had nitrogen, sulfur doped atoms and similar
degree of graphitization, but showed distinct differences in the pore
structures and surface compositions. We suggested these differences
were contributed by the different contents of ashes, vascular bundles
and proteins in different parts. When used as metal-free
carbocatalysts for the oxidation of aromatic hydrocarbons, stem-
derived carbon (SDC-A) exhibited the highest catalytic performance
due to its high surface area, profitable pore volume and rich doped
heteroatoms. Moreover, the carbocatalysts derived from the leaf,
stem and root of pumpkin vine and coriander showed similar activity
differences in oxidation of aromatic alkanes, implying the present
results have a certain extent generality, and provide a new guidance
for preparing carbon materials from plant biomass.
catalytic behaviors of these carbon materials are associated with
the high surface area and pore volume, different hybridizations of
carbon atoms, and the tunable electronic properties by doping
with heteroatoms (such as B, N, F, S and P).[5] Fabrication of
carbocatalysts with hierarchical porous structure, high
graphitization degree and rich doping heteroatoms can
significantly improve the abilities to catalyse various organic
reactions.
Among the various synthesis approaches of carbon materials,
silica or surfactant based hard or soft sacrificial template method,
pyrolysis of polymer blends or heteroatom-enriched MOFs have
attracted considerable attention.[6] Although they have the ability
to form large surface area, favorable pore size distribution and
certain heteroatoms doped carbon materials, these synthetic
methods are mostly complex or the precursors are high-cost.[
Thus, development of relatively simple and cost-effective strategy
to synthesize carbon materials that have excellent structural and
functional properties is highly sought and challenging.[8] Recently,
plant biomass as renewable, abundant and low-cost resources
have been considered to be excellent candidates for the
production of heteroatom-doped porous carbon materials due to
the large amount of organic and inorganic compounds present in
their backbone.[9] For example, Ogale et al.[10] synthesized
microporous conducting carbon by single-step pyrolysis of dead
neem leaves without any activations, and they suggested that the
7]
Introduction
Selective oxidation of hydrocarbons in the liquid phase to prepare
the corresponding oxygenated derivatives is an extremely
[1]
important and challenging chemical reaction. Recently, carbon
materials as sustainable metal-free catalysts for hydrocarbon
oxidation have acquired great attention, for they exhibit
porous structure was provided by the natural constituents in the
leaves. Wang et al.[11] found that N-doped fullerene-like carbon
shell could be facilely produced by deliberately pyrolyzing fallen
gingko leaves, which behaved as single precursor for carbon as
well as nitrogen. Generally, plants biomass are mainly composed
of leaves, stems and roots, these three parts have distinct
differences in morphological structure and distribution of nutrient
elements, which endow them with different functions. However,
the study on the differences of carbon materials derived from the
leaf, stem and root of the same plant is rarely reported.
[2]
comparable activity and selectivity to conventional metals. For
example, Song et al.[3] prepared nitrogen, phosphorus, and sulfur
co-doped hollow carbon shells using a poly (cyclotriphosphazene-
co-4,4’-sulfonyldiphenol) shell and the ZIF-67 core, and it showed
excellent catalytic activity for selective oxidation of aromatic
alkanes in aqueous solution. Li et al.[4] reported that N-doped
Amaranth is a kind of annual herbaceous plant, widely
cultivated in China with the characteristics of low-cost,
reproducible and easy to obtain. It contains a lot of protein and
inorganic salts, making itself an ideal precursor for the synthesis
of heteroatom-doped carbon materials with hierarchical structures.
In this work, we synthesized porous carbon materials from the leaf,
stem and root of amaranth respectively by one-step pyrolysis
without any chemical or physical activation. Detailed
characterizations results indicate these three kinds of carbon
materials all have porous structure, nitrogen and sulfur doped
atoms, and similar graphitization degrees, but show obvious
[
[
a]
b]
Dr. Y. Sun, Dr. K. Liu, Dr. C. Hou,
School of chemistry and pharmaceutical engineering, Taishan
Medical University, Taian 271016 (China)
E-mail: sunybin6@iccas.ac.cn, liukun2436@126.com
Dr. J. Liu, R. Huang, Dr. C. Cao, Prof. W. Song
Beijing National Laboratory for Molecular Sciences, Laboratory of
Molecular Nanostructures and Nanotechnology, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190 (China)
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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ChemCatChem
10.1002/cctc.201801379
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difference on the surface area, pore volume, nitrogen and sulfur Results and Discussion
content, and surface composition. When used as metal-free
carbocatalysts for the oxidation of aromatic hydrocarbons with
tert-butyl hydroperoxide as oxidizing agent, these carbon
materials displayed distinct differences. Among them, stem-
derived carbon exhibits the highest catalytic performance, due to
its high surface area, profitable pore volume and rich doped
heteroatoms. Further, it is highly stable and can be reused for at
least six times without obvious loss of catalytic efficiency.
Moreover, the carbocatalysts derived from the leaf, stem and root
of pumpkin vine and coriander show similar activity differences in
oxidation of aromatic hydrocarbons, implying the present results
have a certain degree of universality.
As stated in the introduction, the morphological structure,
distribution of elements and functions of the leaf, stem and root
from the same plant are different. The elemental analysis of the
leaf, stem and root raw biomass from amaranth show the C, H, O
and N element content in these three parts are distinctly different
(Table S1), confirming the above description is reasonable.
Taking into account the differences in the structure and
composition of the leaf, stem and root, amaranth is first split into
three parts, as shown in Figure 1. After drying, crushing,
carbonization and washing, three kinds of carbon materials are
obtained, which are denoted as LDC-A, SDC-A and RDC-A, as
described in experimental section. The yields of the three carbon
materials are 16.6%, 11.8%, and 20.4%, respectively (Table S2).
Figure 1. Flow diagram for the fabrication of carbon materials derived from the leaf, stem and root of amaranth.
SEM analysis was carried out to observe the morphology of
these raw biomass and carbon materials (Figure S1). All the raw
biomass show irregular shape of block and grain, and no collapse
is found after carbonization, indicating the biomass skeleton is
stable during the pyrolysis process. The SEM images with larger
magnification indicate three kinds of carbon materials have
different microstructures (Figure S2). LDC-A shows many holes,
SDC-A shows sponge-like morphology, and RDC-A shows fewer
holes compared with SDC-A. This difference can be attributed to
different precursors. To further understand their microstructure,
TEM and HRTEM analysis were conducted. As shown in Figure
2a-c, these carbon materials are normally composed of randomly
agglomerated nanosheets, and the porous structure is distributed
in the entire samples. Furthermore, HRTEM images (Figure 2d-f)
show that the highly crystalized carbon frameworks of these
carbon materials have an interplanar spacing of approximately
0
.34 nm, which is in accordance with the interlayer distance of
graphite.[
12]
Figure 2. TEM images of (a) LDC-A, (b) SDC-A, (c) RDC-A, and HR-TEM
images of (d) LDC-A, (e) SDC-A and (f) RDC-A.
XRD measurements were conducted to investigate the
structural order of the present carbon materials. As shown in
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Figure 3a, two peaks between 20° and 25°, and at approximately
process, the differences in the specific surface area and pore
volume should be attributed to their precursors. We consider the
natural ash in these biomass play an important role. The ash is
mainly composed of inorganic salts, including potassium salts and
calcium salts, which can create pores in the carbon matrix during
carbonization as reported in many literatures.[16] To explore the
influence of ash, the ash was collected by calcination in air, then
44° are found in these three carbon materials, which can be
indexed to the (002) and (101) planes of graphite respectively,
indicating the disorderly oriented tiny graphitic type fragments in
these structures.[13] To further illustrate the degree of
graphitization and amorphous carbons, Raman analysis was
conducted. Generally, the ratio of I
degree of the carbon materials. As can be seen in Figure 3b, the
/I ratio of LDC-A, SDC-A, RDC-A is determined to be 0.98, 0.97, and desorption analysis (Figure S3) shows that the BET surface
.96 respectively, implying the existence of amorphous carbon
D G
/I represents the graphite
2
mixed with fresh biomass and pyrolysis in nitrogen. N adsorption
I
D G
0
areas and pore volumes of the carbon materials increased greatly
after adding more ash (Figure S4 and Table S3), which
demonstrates that ash in biomass make a great influence on the
pore structure of their derived carbon materials. As shown in
Table S4, the ash content in stem is highest among these raw
biomass, and there are more well defined pores and larger
surface area in SDC-A compared to LDC-A and RDC-A. In
addition, we speculate the vascular bundles in plant biomass also
make significant impact on the pore structure of their derived
carbon materials. The vascular bundles are consisted of many
tubular structures, which may leave pore structures in their
derived carbon material. As vascular bundles in stem are more
abundant than that in leaf, SDC-A has higher pore volume and
specific surface area than LDC-A. To clarify this point further, the
main veins and lateral veins which marked as V are separated
from the leaf, and the rest part which mainly composed of
mesophyll are marked as M (Figure S5). After carbonization, two
kinds of carbon materials are obtained, which are denoted as
VDC-A and MDC-A respectively. The analysis shows that VDC-A
has much higher specific surface area and pore volume than that
of MDC-A (Figure S6 and Table S3), for V has more vascular
bundles than M, which has a structure similar to stem.
structures with multiple defects.[14] These results demonstrate that
all these three carbon materials have porous structure and similar
degree of graphitization.
Figure 3. (a) XRD and (b) Raman spectra of LDC-A, SDC-A and RDC-A.
To gain a greater understanding of the porosity of these carbon
materials, N adsorption and desorption measurements were
2
conducted, and the isotherms and pore-size distribution of the
three samples are shown in Figure 4. All these samples display
type-IV isotherms with a distinctive hysteresis loop (Figure 4a),
implying the presence of both micropores and mesopores.[15] The
Brunauer–Emmett–Teller (BET) surface areas of SDC-A (1023
2
-1
2 -1
m g ) is much higher than that of LDC-A (392 m g ) and RDC-A
(
2
-1
160 m g ), which is quite intriguing. The pore size distribution
calculated by the Barrett–Joyner–Halenda (BJH) method are
nearly all the same for all the samples, but the pore volumes of
3
-1
SDC-A (1.18 cm g ) calculated by DFT method are much larger
3
-1
3 -1
than that of LDC-A (0.42 cm g ) and RDC-A (0.12 cm g ) (Table
S3). Given that none activator was added in the preparation
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the percentage of each species is different (Table S6 and Table
S7), which can be attributed to the different composition of the
precursors. These results imply that different parts of plant
biomass make important influence on the pore structure and
surface composition of their derived carbon materials, even
though they come from the same plant.
2
Figure 4. (a) N adsorption–desorption isotherms and (b) pore-size
distributions of LDC-A, SDC-A and RDC-A.
Elemental analysis results show these carbon materials all
contain N and S element (Table S5). In addition, FTIR spectra
-1
(
Figure S7) show two weak peaks at around 1220 cm and 1110
-1
cm , which can be assigned to sulfur groups (-C-S-) and amine
groups (-C-N-).[ These results suggest the obtained samples
are nitrogen and sulfur co-doped carbon materials. The nitrogen
and sulfur atoms most probably come from the proteins in
precursors, for N and S element are mainly found in proteins in
plants. The content of protein was assayed by Kjeldahl
determination. The nitrogen content in leaf, stem and root of
amaranth are 40.4 g/kg, 19.2 g/kg and 2.0 g/kg respectively,
indicating the order of protein content in amaranth is leaf > stem
17]
Figure 5. High-resolution XPS spectra of a) N1s and b) S2p of LDC-A, SDC-A
and RDC-A.
In order to investigate the performance of these three carbon
materials for the oxidation of hydrocarbons, ethylbenzene was
first selected as a substrate to explore their efficiencies. Figure 6
show the time dependent yields of acetophenone over LDC-A,
SDC-A and RDC-A catalysts for oxidation of ethylbenzene with
>
root, which is consistent with the order of nitrogen content in
their derived carbon materials (Table S5). It implies that protein
content in biomass affects the content of heteroatoms in their
derived carbon materials. To investigate the states of the sulfur
and nitrogen atoms in these three carbon materials, XPS analysis
were conducted. The full spectra of these samples (Figure S8)
further confirm the presence of the nitrogen and sulfur atoms. As
Figure 5 shows, high-resolution N1s spectra (Figure 5a) of these
samples can be adapted well with three kinds of N species. Peaks
at about 398.3, 399.8 and 401.1 eV are corresponding to pyridinic
nitrogen, pyrrolic nitrogen, and graphitic nitrogen, respectively.[18]
The S2p spectra (Figure 5b) display three peaks at 163.6, 164.8
and 168.2 eV, which are ascribed to 2p3/2, 2p splitting of the S2p
spin orbital (-C-S-C-) and oxidized S, respectively.[19] All three
carbon materials display similar N1s spectra and S2p spectra, but
o
TBHP as oxidizing agent at 80 C. It can be seen that the yield of
acetophenone was very low (10%) within 12 h without catalyst.
After adding the carbocatalyst, the reaction rate is greatly
increased. Especially, SDC-A displays the highest activity, and
nearly 95% yield can be achieved in 12 h. In comparison, LDC-A
exhibits a lower activity (80% yield), and RDC-A shows the worst
ativity (25% yield).
1/2
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carbonyl compounds with high conversion and selectivity within
2
12 h. The electron-withdrawing groups (—NO ) shows a higher
reactivity than that of the electron- donating group-substituent (—
MeO; Table 1, entries 2–3). For bulky and sterically hindered
molecules,
such
as
fluorine,
diphenyl
methane,
tetrahydronaphthalene and indane (Table 1, entries 4–7), high
reactivity can also be achieved. These results confirm the
excellent catalytic performance of SDC-A.
Table 1. SDC-A catalyzed oxidation of benzyl aromatic hydrocarbons to
carbonyl compounds using TBHP as oxidant.[
a]
Figure 6. Time dependent yields of acetophenone over LDC-A, SDC-A and
RDC-A catalysts for oxidation of ethylbenzene with TBHP as oxidizing agent at
o
8
0 C.
Generally, the catalytic activity of these carbocatalysts can be
attribute to the synergistic effect between the high surface area
and the heteroatom dopants of the carbon materials.[20] The high
specific surface area promotes the mass transportation, while the
nitrogen and sulfur atom dopants change the electronic structure
of carbon atoms and produce active sites. Especially,
incorporating graphitic-type nitrogen in carbocatalysts is pivotal
for the C-H bond activation.[5b] In this study, about 54 percent of
the N species in SDC-A is graphitic nitrogen. In addition, the sulfur
[
(
a] Reaction conditions: substrates 1.0 mmol, catalyst 20.0 mg, TBHP 1.0 mL
70wt% in water), 80 C, 12 h.
o
dopants also play a role in modulating electronic structure of the
carbon atoms.[21] Further, the SDC-A catalyst has high surface
What’s more, we prepared carbocatalysts from leaf, stem and
root of pumpkin vine and coriander in the same method, and they
show similar activity differences in oxidation of aromatic alkanes
under the same reaction conditions (Figure S13 and Figure S14).
Further, the ash content in leaf, stem and root of pumpkin vine
and coriander is in the same order to that of amaranth (Table S4).
These studies suggest that there is certain degree of generality in
the present results, which provides a new guidance for preparing
carbon materials from plant biomass.
area, large proportion of mesopores and profitable pore volume,
which is beneficial for the diffusion and adsorption of substrates
in catalysis.[22] All these characteristics together make SDC-A an
excellent metal-free catalyst for the oxidation of aromatic
hydrocarbons. LDC-A exhibits a slightly lower activity, which can
be ascribed to its lower surface area, even though it has a higher
nitrogen content. RDC-A shows the worst activity, which can be
attributed to its low nitrogen content and low specific surface area.
We further investigated the effect of calcine temperature on the
catalysis performance of these three carbon materials. As shown
in Table S8, with the rising of temperature, the product yields Conclusions
decrease, while the catalytic activity became higher (Figure
S9~S11). We suggest the carbon matrix reforms and releases
more volatile compounds with the increase of temperature, which
may create more pores and increases the graphitization degree.
These are all beneficial for the oxidation of hydrocarbons.
Moreover, the SDC-A catalyst can be easily re-collected from
the reaction system by centrifugation. After being recycled for 6
times, SDC-A shows a slight decrease in activity (Figure S12),
which can be ascribed to the inevitable loss of catalyst in the
recovery process and partial oxidation of active sites during the
oxidation process. It indicates that SDC-A is a highly stable
carbocatalyst for the oxidation of aromatic hydrocarbons.
In conclusion, three kinds of carbon materials were prepared from
the leaf, stem and root of amaranth without any additional
activator. These carbon materials all have porous structure,
nitrogen and sulfur doped atoms, and graphitized carbon atom,
but show distinct differences in pore structure and surface
composition. We suggest these differences are contributed by the
different contents of ashes, vascular bundles and proteins in
different parts. When used as metal-free carbocatalysts for the
oxidation of aromatic hydrocarbons, stem-derived carbon showed
the best activity, which can be ascribed to its high surface area,
profitable pore volume and rich doped heteroatoms. Moreover,
the carbocatalysts derived from the leaf, stem and root of pumpkin
vine and coriander show similar activity differences in oxidation of
The oxidation of a variety of benzyl aromatic hydrocarbons with
SDC-A as catalyst was also studied. As shown in Table 1, all
benzyl aromatic hydrocarbons are oxidized to the corresponding
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ChemCatChem
10.1002/cctc.201801379
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aromatic alkanes, indicating there is certain degree of generality
in the present results, and provide a new guidance for preparing
carbon materials from plant biomass.
a Rigaku D/max-2500 diffractometer with Cu Kα radiation (λ = 1.5418 Å)
at 40 kV and 40 mA.
Acknowledgements
Experimental Section
We thank the Natural Science Foundation of Shandong Province
(ZR2018LB009), National Natural Science Foundation of China
Materials
(
NSFC 21501130) and the Science Foundation of Shandong
Amaranth and coriander were bought from the vegetable market, pumpkin
vine was obtained from the farm. Hydrochloric acid and ethyl acetate were
purchased from Sinopharm Chemical Reagent Beijing Co., Ltd.
Ethylbenzene, TBHP (tert-butyl hydroperoxide, 70wt% in water), 4-
ethylnitrobenzene, 4-ethylanisole, anisole, fluorine, diphenylmethane,
Province for Excellent Young Scholars (ZR2017JL015) for
financial support. The instrumental analysis were carried out in
the Institute of Chemistry, the Chinese Academy of Sciences
(
ICCAS) and shiyanjia lab. Haijiao Xie and Weiwei Chen are
acknowledged for their support.
1,2,3,4-tetrahydronaphthalene and indane were purchased from Beijing
InnoChem Science & Technology Co., Ltd. All the reagents were used
without further purification.
Keywords: carbon • heterogeneous • leaf, stem and root •
metal-free catalysts • selective oxidation
Synthesis of carbon materials
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Amaranth was thoroughly washed, split into leaf, stem and root three parts,
and dried at 80 C for 24 h in an oven. These dried samples were crushed
o
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Yongbin Sun,* Kun Liu,* Chao Hou, Jian
Liu, Runkun Huang, Changyan Cao, and
Weiguo Song
Page No. – Page No.
Nitrogen, sulfur co-doped carbon
materials derived from the leaf, stem
and root of amaranth as metal-free
catalysts for selective oxidation of
aromatic hydrocarbons
Nitrogen, sulfur co-doped carbon materials are prepared from the leaf, stem and root
of amaranth respectively by pyrolysis without any activation treatment. Stem-derived
carbon exhibits the highest catalytic performance for selective oxidation of aromatic
hydrocarbons.
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