Nitrogen, Phosphorus, and Sulfur Co-Doped Hollow Carbon Shell as Superior Metal-Free Catalyst for Selective Oxidation of Aromatic Alkanes

Article abstract of DOI:10.1002/anie.201600455

Metal-free heteroatom-doped carbocatalysts with a high surface area are desirable for catalytic reactions. In this study, we found an efficient strategy to prepare nitrogen, phosphorus, and sulfur co-doped hollow carbon shells (denote as NPS-HCS) with a surface area of 1020 m² g^-1. Using a poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) shell as carbon source and N, P, S-doping source, and the ZIF-67 core as structural template as well as extra N-doping source, NPS-HCS were obtained with a high surface area and superhydrophilicity. All these features render the prepared NPS-HCS a superior metal-free carbocatalyst for the selective oxidation of aromatic alkanes in aqueous solution. This study provides a reliable and facile route to prepare doped carbocatalysts with enhanced catalytic properties.

Full text of DOI:10.1002/anie.201600455

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International Edition: DOI: 10.1002/anie.201600455
German Edition: DOI: 10.1002/ange.201600455
Heterogeneous Catalysis
Hot Paper
Nitrogen, Phosphorus, and Sulfur Co-Doped Hollow Carbon Shell as
Superior Metal-Free Catalyst for Selective Oxidation of Aromatic
Alkanes
Shuliang Yang, Li Peng, Peipei Huang, Xiaoshi Wang, Yongbin Sun, Changyan Cao,* and
Weiguo Song*
Abstract: Metal-free heteroatom-doped carbocatalysts with
a high surface area are desirable for catalytic reactions. In this
study, we found an efficient strategy to prepare nitrogen,
phosphorus, and sulfur co-doped hollow carbon shells (denote
as NPS-HCS) with a surface area of 1020 m2 gÀ1. Using
a poly(cyclotriphosphazene-co-4,4’-sulfonyldiphenol) (PZS)
shell as carbon source and N, P, S-doping source, and the ZIF-
67 core as structural template as well as extra N-doping source,
NPS-HCS were obtained with a high surface area and
superhydrophilicity. All these features render the prepared
NPS-HCS a superior metal-free carbocatalyst for the selective
oxidation of aromatic alkanes in aqueous solution. This study
provides a reliable and facile route to prepare doped carbo-
catalysts with enhanced catalytic properties.
tronic structure of the adjacent carbon atoms and promoted
À
the activity in selective oxidation of benzylic C H bonds
using tert-butyl hydroperoxide as oxidant.[11] However, the
efficiency of this nitrogen-doped graphene was still unsatis-
factory.
Apparently, the activity of carbocatalysts can be enhanced
through structure design, chemical modification or heteroa-
toms doping (e.g., B, N, F, P or S). Fabrication of carboca-
talysts with hierarchical porous structure would increase the
surface area and pore volume, which are beneficial for mass
adsorption and transportation. Doping heteroatoms in car-
bocatalysts could significantly improve the catalytic abilities
in various organic reactions and electrochemical catalysis.[12]
Such heteroatom doping could change the electron features
of carbon for catalysis. Thus, finding a rational method to
produce carbon nanostructures with controllable heteroatom
doping may lead to better carbocatalysts.
À
S
elective oxidation of C H bonds is one of the most
important reactions in organic synthesis.[1] Generally, both
homogeneous and heterogeneous catalysts for this reaction
need noble or transition metals.[2] Despite the high efficiency,
the presence of metals is associated with high costs and
contamination issues.[3] Therefore, development of metal-free
catalysts is quite desirable.[4] Recently, carbon materials with
or without dopant have shown promising abilities in many
organic reactions as one kind of metal-free heterogeneous
catalysts, including Friedel–Crafts alkylation reactions,[5]
reductive hydrogen atom transfer reactions,[6] and oxidation
of alcohols,[7] amines,[8] and arenes.[9] Two recent outstanding
studies demonstrated that it is also feasible to catalyze
In this study, we found an efficient route to produce
nitrogen, phosphorus, and sulfur co-doped hollow carbon
shells (denote as NPS-HCS) with
a surface area of
1020 m2 gÀ1. The key of this method is to design and prepare
the ZIF-67@poly(cyclotriphosphazene-co-4,4’-sulfonyldiphe-
nol) core–shell composite. The PZS shell acts as carbon
source and N, P, S-doping source, and ZIF-67 core acts as
structural template as well as extra N-doping source. Because
nitrogen, phosphorus, and sulfur atoms are atomically dis-
persed in the PZS,[13] atomic-level doping with these three
heteroatoms can be realized in the carbon shells. While the
decomposition of the ZIF-67 core generates the hollow cavity
and the released gases produce a mesoporous structure of the
carbon shell, resulting in a hierarchical porous structure and
high surface area of the NPS-HCS. Moreover, nitrogen atoms
in the ZIF-67 are also doped into the carbon shells during
thermal calcination. These extra N doping optimizes the
distribution of the nitrogen species and improves the hydro-
philicity of the NPS-HCS. All these features render the
prepared NPS-HCS a superior metal-free carbocatalyst for
selective oxidation of aromatic alkanes in aqueous solution.
The NPS-HCS catalyst was prepared through a three-step
method (Scheme 1): 1) a highly cross-linked PZS was coated
on the surface of ZIF-67 to form a core–shell composite
(denote as ZIF-67@PZS); 2) the prepared ZIF-67@PZS
composite was then calcined in Ar atmosphere to form the
Co2P@NPS-HCS composite; 3) Co2P nanoparticles were
removed by acid etching to form the final nitrogen, phospho-
rus, and sulfur co-doped hollow carbon shells (NPS-HCS).
Transmission electron microscopy (TEM) images and X-ray
À
oxidation of C H bonds with carbocatalysts. Wang et al.
reported boron and fluorine co-doped mesoporous carbon
nitride (B, F-doped C3N4) as a metal-free catalyst for selective
oxidation of cyclohexane to cyclohexanone with H2O2 as
oxidant.[10] Ma and co-workers found that incorporating
graphitic-type nitrogen in graphene could change the elec-
[*] S. Yang, P. Huang, X. Wang, Y. Sun, 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
100190, Beijing (China)
E-mail: cycao@iccas.ac.cn
wsong@iccas.ac.cn
Dr. L. Peng
Institute of Chemical Sciences and Engineering
École Polytechnique FØdØrale de Lausanne (EPFL)
EPFL-ISIC-Valais, Sion, 1950 (Switzerland)
Supporting information for this article can be found under http://dx.
doi.org/10.1002/anie.201600455.
4016
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obtained with shell thickness of about 20 nm. The high-
resolution transmission electron micrograph, X-ray diffracto-
gram, and Raman spectrum showed that the graphitization
content of the NPS-HCS was relatively low (Figure 1e,
Figure 1 f, and Figure S3, respectively).
Energy-dispersive spectroscopy (EDS) mapping revealed
that N, P, S were uniformly doped into the hollow carbon
shells (Figure 2). The uniform doping was due to the PZS
shell, which contains carbon, nitrogen, phosphorus, and sulfur
atoms, thereby atomic-level doping with these three heter-
oatoms was possible in the carbon shell during thermal
calcination. Besides the high dispersion state of the doping
atoms, there are three doped atoms, while most of the
reported doped-carbon materials were limited to two doped
heteroatoms.[15]
Scheme 1. Preparation of NPS-HCS.
diffraction (XRD) patterns of samples obtained after each
step are shown in Figure 1. The as-prepared ZIF-67 core
template was polyhedral with relative uniform size (Figure 1a
and Figure S1).[14] With the assistance of triethylamine, the
PZS shells were coated on the surface of ZIF-67 polyhedrons
through in situ polymerization of the phosphonitrilic chloride
trimer and 4,4’-sulfonyldiphenol. The thickness of the PZS
shell was approximately 20 nm (Figure 1b). After calcination
at 9008C for 2 h in Ar atmosphere, hollow carbon shells
containing nanoparticles were produced (Figure 1c). The
nanoparticles in the inner carbon shells were ascribed to Co2P,
which were confirmed by XRD (Figure 1 f) and HRTEM (see
Figure S2 in the Supporting Information). The nanoparticles
were likely produced from the reaction between cobalt atoms
in ZIF-67 and phosphorus atoms in PZS during thermal
treatment. After etching by acid, hollow carbon shells were
Figure 2. Compositional EDS mapping of the NPS-HCS using scanning
transmission electron microscopy.
The states of the N, P, and S atoms in the NPS-HCS were
then analyzed by X-ray photoelectron spectroscopy (XPS).
The full spectrum of NPS-HCS (Figure 3a) confirmed the
existence of the N, P, and S atoms. As shown in Figure 3b, the
Figure 1. TEM images of a) ZIF-67 polyhedrons, b) ZIF-67@PZS,
c) Co2P@NPS-HCS, d,e) NPS-HCS, and f) XRD patterns of samples
obtained at each step.
Figure 3. a) Full spectrum and high-resolution XPS spectra of b) N1s,
c) P2p, and d) S2p of NPS-HCS.
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high-resolution N1s spectrum of NPS-HCS can be fit well
with four kinds of N species. Peaks at about 398.8, 400.0,
401.0, and 404.9 eV were corresponding to pyridinic nitrogen,
pyrrolic nitrogen, graphitic nitrogen, and pyridinic N+-OÀ,
respectively.[16] Besides above four peaks, another peak at
N+-OÀ, while there were four nitrogen species (pyridinic
nitrogen, pyrrolic nitrogen, graphitic nitrogen, and pyridinic
N+-OÀ) in NPS-HCS. Finally, the existence of a ZIF-67 core
during the calcination improved the wettability of the NPS-
HCS in water. Contact angle (CA) measurement (Figure S8)
showed that the sample produced after calcinations of pure
ZIF-67 under the same conditions displayed superhydrophi-
licity with a CA close to 08, while the sample produced after
calcinations of pure PZS displayed hydrophobicity with a CA
of about 1358. And the NPS-HCS also showed superhydro-
philicity with a CA of about 08. A digital photo showed that
NPS-HCS was well-dispersed in water (Figure S9). This is also
beneficial for catalytic processes in water.
398.6 eV can also be seen, which may be ascribed to the
[17]
=
À
pyridinic nitrogen or P N/P N bonds. A similar peak was
also found in the pure PZS (Figure S4). The P2p spectrum
showed two peaks at 132.7 and 133.9 eV, corresponding to P-
C and P-O, respectively (Figure 3c).[18] The S2p spectrum
displayed three peaks at 164.0, 165.2, and 168.3 eV, which
were ascribed to 2p3/2, 2p1/2 splitting of the S2p spin orbital
(-C-S-C-) and oxidized S, respectively (Figure 3d).[19] These
data adequately confirmed that N, P and S atoms have been
successfully incorporated into the hollow carbon shells.
The porous structure of NPS-HCS was determined by the
N2 adsorption–desorption method. As shown in Figure 4a,
a typical type-IV isotherms with a distinct hysteresis loop was
observed, indicating a mesoporous structure of hollow carbon
shells.[20] The pore size distribution calculated by the Barrett–
Joyner–Halenda (BJH) method showed a peak centered at
around 4.3 nm (Figure 4b). The NPS-HCS had a Brunauer–
Emmett–Teller (BET) surface area of 1020 m2 gÀ1 with a total
pore volume of 1.31 cm3 gÀ1. The high surface area and porous
structure are very beneficial for catalysis.
To study the performance of this NPS-HCS carbocatalyst
À
for the selective oxidation of C H bonds, ethylbenzene was
first chosen as a substrate to explore the efficiency (Table 1).
Table 1: Optimization for the selective oxidation of ethylbenzene.[a]
[a] Reaction conditions: catalyst (0.010 g), TBHP (1.0 mL, 70 wt% in
water), ethylbenzene (1.0 mmol), H2O (2.0 mL), 808C.
For comparison, control samples (including pure PZS, ZIF-
67, and PZS-HCS after calcination at 9008C for 2 h in Ar
atmosphere) were also tested. The NPS-HCS catalyst showed
high activity and excellent selectivity during 6 h (entry 5),
much better than those of the test samples (entries 2–4) and
nitrogen-doped graphene reported in the literature.[11] When
the reaction time was prolonged to 12 h, conversion of 99%
with a selectivity 99.5% for acetophenone was achieved
(entry 6). The NPS-HCS catalyst could be easily separated
from the solution by centrifugation and was reused five times
without obvious loss of activity and selectivity (Figure S10).
The morphology, structure, and chemical states of the NPS-
HCS after five runs were nearly the same relative to that of
the fresh catalyst (Figures S11–S13). A little decrease of
activity can be ascribed to the unavoidable sample loss during
the recycle process and partial oxidation of carbon active sites
under the oxidative conditions (Table S2).
Various benzyl aromatic hydrocarbons could also be
oxidized with high yields (Table 2), indicating the superior
efficiency of the NPS-HCS catalyst. The electron-donating
group-substituent (MeOÀ) showed a higher reactivity relative
to those of the electron-withdrawing groups (NO2À; Table 2,
entries 2–3). A high reaction activity could also be obtained
for bulky and sterically hindered molecules, such as 2-
ethylnaphthalene, cumene, fluorine, and diphenylmethane
Figure 4. a) N2 adsorption–desorption isotherms and b) mesopore
size distribution curve of NPS-HCS.
The ZIF-67 core template plays a very important role in
the formation of the final NPS-HCS. First, ZIF-67 was the
structural template for PZS coating. ZIF-67 also helped to
maintain the structural integrity of the hollow carbon shells
during the calcination process. Some broken hollow carbon
shells can be clearly seen in the TEM images of pure PZS
hollow shells after calcination (denoted as PZS-HCS; Fig-
ure S5). Second, ZIF-67 helped to generate a mesoporous
structure and to increase the surface area of NPS-HCS by the
released gases after the decomposition of ZIF-67. For
comparison, PZS-HCS showed no mesopores and a signifi-
cantly lower surface area of 667 m2 gÀ1 (Figure S6). Third,
nitrogen atoms in the ZIF-67 core were also doped into the
hollow carbon shells during thermal calcination, increasing
the total amount of N doping and optimizing the distribution
of nitrogen species in the final NPS-HCS. According to the
XPS result, the N atomic ratio increased from 1.98% in PZS-
HCS to 3.15% in NPS-HCS (Table S1). Furthermore, the N1s
spectrum of PZS-HCS (Figure S7) can be divided into three
peaks of pyridinic nitrogen, graphitic nitrogen, and pyridinic
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Table 2: Oxidation of arylalkanes in the aqueous phase with NPS-HCS as
NPS-HCS a superior metal-free catalyst for selective oxida-
tion of aromatic alkanes in aqueous solution.
catalyst.[a]
In summary, we produced nitrogen, phosphorus, and
sulfur co-doped hollow carbon shells with a mesoporous
structure and a surface area of 1020 m2 gÀ1. When used as
a metal-free carbocatalyst for selective oxidation of aromatic
alkanes in aqueous solution, the prepared NPS-HCS showed
excellent activity, selectivity, and good recyclability. This
study provides a reliable and facile route to prepare doped
carbocatalysts with enhanced catalytic properties.
Experimental Section
Synthesis of NPS-HCS: ZIF-67 polyhedrons were first prepared
according to a previous report with minor modification.[14] In the
second step, the above-synthesized ZIF-67 polyhedrons were dis-
persed into 80 mL methanol using ultrasound for five minutes. Then
another solution containing 300 mg of hexachlorocyclophosphazene
and 675 mg of 4,4’-sulfonyldiphenol in 20 mL of methanol was added
dropwise. After stirring for five minutes, 1 mL of triethylamine was
added dropwise and the solution was continued to stir for 18 h. The
purple precipitates were collected and washed with methanol three
times and dried in vacuum at room temperature for 12 h. In the third
step, the obtained ZIF-67@PZS was calcined at 9008C for 2 h
(28CminÀ1) in Ar atmosphere. Finally, Co2P@NPS-HCS was pre-
leached in 0.5 molLÀ1 H2SO4 at 808C for 12 h, and washed thoroughly
with deionized water/ethanol and then dried in vacuum at room
temperature for 12 h.
General procedure for the catalytic test: The substrate
(0.5 mmol), catalyst (5.0 mg), TBHP (tert-butyl hydroperoxide,
500 mL, 70 wt% in water), and water (1 mL) were introduced into
a 15 mL glass reaction tube sealed with a Teflon lid. The reaction
mixture was stirred in a preheated 808C oil bath for 12 h. After that,
77 mL anisole was added to the system as internal standard. Next,
10 mL CH2Cl2 was added to extract organic compounds in the
reaction system. The organic phase was analyzed using a gas
chromatograph (Shimadzu GC-2010) equipped with a flame ioniza-
tion detector (FID) and an Rtx-5 capillary column (0.25 mm in
diameter, 30 m in length).
[a] Reaction conditions: catalyst (5.0 mg), TBHP (500 mL, 70 wt% in
water), ethylbenzene (0.5 mmol), H2O (1.0 mL), 808C, 12 h.
(Table 2, entries 4–7). Besides aromatic molecules, aliphatic
molecules can be oxidized. As can be seen in Table 2, entry 8,
57% conversion of n-hexane can be reached after 12 h at
À
808C. For aromatic hydrocarbons, usually the benzyl C H
bond adjacent to the aromatic ring is much more reactive than
others, thus a high selectivity can be obtained. However, all
Acknowledgements
À
C H bonds of long-chain hydrocarbons show a similar
selectivity. Therefore, it is difficult to gain high selectivity
for the oxidation of aliphatic molecules.
We thank for financial support from the National Natural
Science Foundation of China (NSFC grant number 21333009,
21573244, and 21573245) and the Chinese Academy of
Sciences. The XRD, SEM, TEM and XPS measurements
were performed at the Center for Physicochemical Analysis
and Measurements in the Institute of Chemistry, the Chinese
Academy of Sciences (ICCAS). Dr. Zhijuan Zhao (XPS
discussion) and Dr. Ye Tian (CA measurement) are acknowl-
edged for their support.
The superior activity of the NPS-HCS catalyst can be
ascribed to several features. Ma et al. suggested that the
incorporated nitrogen (mostly of graphitic-type sites) in
À
layered carbon dopant catalysts was pivotal for the C H
bond activation.[11] The nitrogen itself was not the active site,
but instead changed the electronic structure of the adjacent
carbon atoms and promoted the catalytic reactivity.[21] In this
study, about 39 atom% of the N species on NPS-HCS was
graphitic nitrogen. Besides the nitrogen dopants, the elec-
tronic structure of the carbon atoms can also be modulated by
phosphorus and sulfur dopants.[17c,22] Thus, the N, P, and
S dopants are an important feature of this NPS-HCS catalyst.
In addition, the NPS-HCS catalyst has a high surface area,
mesoporous shells, and a hollow cavity structure, which is
usually beneficial for mass adsorption and transformations in
catalysis. Moreover, the surface of the NPS-HCS was super-
hydrophilic. As a consequence, the NPS-HCS was highly
dispersible in water. All these features together render the
Keywords: carbon · doping · heterogeneous catalysis ·
metal-free catalysts · selective oxidation
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Received: February 1, 2016
Published online: February 17, 2016
4020 www.angewandte.org
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