Phosphorus-doped macroporous carbon spheres for high efficiency selective oxidation of cyclooctene by air

Article abstract of DOI:10.1039/c4ra01972e

The phosphorus-doped macroporous carbon spheres (PMCS) with a high specific surface area (574.68 m² g^-1) were synthesized by a template method. A series of catalytic experiments showed that the PMCS possessed excellent catalytic ability for the selective oxidation of cyclooctene (conversion based on cyclooctene was 50.69% and the selectivity to epoxycyclooctane was 88.47%). This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01972e

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Phosphorus-doped macroporous carbon spheres
for high efficiency selective oxidation of
cyclooctene by air†
Cite this: RSC Adv., 2014, 4, 22419
*
*
Chuanxi Li, Manman Yang, Ruihua Liu, Fangfang Zhao, Hui Huang, Yang Liu
and Zhenhui Kang
Received 6th March 2014
Accepted 24th April 2014
The phosphorus-doped macroporous carbon spheres (PMCS) with a high specific surface area (574.68 m2
À1) were synthesized by a template method. A series of catalytic experiments showed that the PMCS
g
DOI: 10.1039/c4ra01972e
possessed excellent catalytic ability for the selective oxidation of cyclooctene (conversion based on
cyclooctene was 50.69% and the selectivity to epoxycyclooctane was 88.47%).
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Increasing industrial production demands have focused term stability, periodic ordered structure and well-developed
increased attention on the research for the oxidation of alkenes preparation methods, which are extensively applied in super-
with high activity, environmental benignity and low cost. In the capacitors,11 electrocatalysts,12,13 selective methylation14 and gas
oxidation process, the selection of catalysts plays a vital role in adsorption.15 It should be noted that these carbon materials
the types of products, conversion efficiency and selectivity.1–3 with unique porous structure and relatively high specic
Nanocatalysts, as a new generation catalyst, possess excellent surface area make the reaction reagents distribute evenly and
catalytic abilities and great potential in practical applications. efficiently and reduce the resistance of diffusion during reac-
The design and preparation of nanocatalysts are signicant in tions, which could be a kind of effective nanocatalyst in the
industry, energy and environmental problems in order to gain selective oxidation of cyclooctene.16 Nowadays, doping different
relatively high conversion and selectivity. Among the oxidation kinds of heteroatoms into the carbon framework has been
of alkenes, selective hydrocarbon oxidation is momentous in aggressively studied for developing novel carbon materials
industrial production, especially for the selective oxidation of because the introduction of heteroatoms changes parts of the
cyclooctene. Recently, numerous types of nanocatalysts have carbon framework, improving the electrical, physical and
been used to improve the catalytic ability for the selective chemical properties of primitive carbon.17 Phosphorus is a
oxidation of cyclooctene, including Au/C,4 Au/graphite,5 meso- conceivable element for the doping of carbon materials as it
porous mixed oxides (Ga, Ga–Nb and Ga–Mo)6 and CuO/meso- contains ve valence electrons that form strong valence bonds
porous SiO2 nanocomposites.2 In previous studies, our group with the ambient carbon atoms.18,19 Due to the high electron-
has designed several nanocatalysts, such as Au/Cu nanoparticle- donating ability of phosphorus, it is necessary to explore the
modied Si nanowires, Au/SiOx composite thin lms, cobalt- distinctive property of phosphorus-doped carbon materials.20
based 3D porous framework and carbon quantum dots/SiO2 Phosphorus-doped macroporous carbon spheres, as a kind of
porous nanocomposites.7–10 These nanocatalysts enriched and non-metal nanocatalyst, have become a promising candidate
elevated the catalytic activity for the selective oxidation of for the catalysis of hydrocarbon selective oxidation on account
cyclooctene. Despite tremendous efforts being made to design of their unique porous structure and phosphorus doping.
effective nanocatalysts, seeking high activity and low cost
Herein, we report that phosphorus-doped macroporous
nanocatalysts remains a great challenge.
carbon spheres (PMCS) with high specic surface area (574.68
Carbon materials, especially porous ones, have gained m2 gÀ1) have been synthesized by a typical template method
widespread acceptance due to their low price, low toxicity, long- using phytic acid as the precursors of carbon and phosphorus.
Owing to the unique porous structure and phosphorus doping,
a series of catalytic experiments conrmed that the PMCS
Institute of Functional Nano & So Materials (FUNSOM), Jiangsu Key Laboratory for showed excellent catalytic ability for the selective oxidation of
Carbon-based Functional Materials and Devices, Collaborative Innovation Center of
cyclooctene (conversion based on cyclooctene was 50.69% and
the selectivity to epoxycyclooctane was 88.47%) under mild
conditions.
Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123,
China. E-mail: hhuang0618@suda.edu.cn; yangl@suda.edu.cn; Fax: +86 512-
65882846; Tel: +86 512-65880957
In a typical experiment, the PMCS were prepared by a typical
† Electronic supplementary information (ESI) available: Preparation of all
template method. Firstly, the silica beads were synthesized by
catalysts, additional gures, FT-IR spectra and Raman spectra; detailed
conversion and selectivity data. See DOI: 10.1039/c4ra01972e
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indicates that the pores among the PMCS (red dotted circles) are
layered upon layers and orderly arraying, which further
conrms the macroporous structure of the PMCS. Fig. 1(c)
shows the Fourier transform infrared (FT-IR) and Raman
spectra of the PMCS. In the FT-IR spectrum (black line), the
peaks at 3442 and 1632 cmÀ1 correspond with the O–H
stretching and bending vibrations, respectively. The peaks at
1394 and 1058 cmÀ1 show the asymmetric and symmetric
stretching vibrations of C–O–C bond,21,22 while the peaks at 1210
and 687 cmÀ1 can be attributed to P–O and P–C stretching
vibrations, respectively.23,24 In the Raman spectrum (red line),
two characteristic peaks located at 1320 and 1585 cmÀ1 are
observed, which represent the D-band and G-band of carbon,
respectively.25 Fig. 1(d) shows the X-ray diffraction (XRD)
patterns of the PMCS. The typical peak at 26ꢀ indicates that the
PMCS are made up of carbon.26
Scheme 1 Schematic diagram of the preparation process for PMCS.
The successful doping of phosphorus into macroporous
carbon spheres was further veried by the X-ray photoelectron
spectroscopy (XPS) measurements and EDS mapping. The full
XPS survey spectrum in Fig. 2(a) shows a peak at 284.6 eV cor-
responding to C 1s; a peak at 532.4 eV, to O 1s; and a peak at
133.2 eV, to P 2p.19 The high-resolution P 2p XPS spectrum in
Fig. 2(b) reveals that phosphorus was doped into the macro-
porous carbon spheres in two bonding types: P–O bonding at
133.6 eV and P–C bonding at 132.5 eV, respectively.20 The value
of 133.6 eV is generally known as the oxidation state of P.27,28
The peak at 132.5 eV has been discussed by other research
groups,20,29,30 and the accepted understanding is that the peak is
identied as P–C bonding.31,32 The EDS mapping of the PMCS
was also carried out, and the results are shown in Fig. 2(c)–(f).
The red, green, and yellow hues represent the distribution of C,
O and P elements, respectively; evidently, element P is
uniformly distributed in PMCS. These results conrm that
phosphorus has been successfully doped in PMCS.
stirring the colloidal silica crystals. Fig. S1† shows the SEM
images of monodispersed silica particles and silica crystal
beads. Then, appropriate phytic acid was injected into the pore
space of silica templates. Finally, aer the carbonization of
phytic acid under nitrogen protection, the silica templates were
removed by hydrouoric acid; consequently, the PMCS formed
(detailed experiment shown in ESI†). The schematic diagram of
the preparation steps for PMCS is shown in Scheme 1.
Fig. 1(a) shows the typical scanning electron microscopy
(SEM) image of the prepared PMCS, from which the spherical
structure is distinctly visible with a diameter of about 20 mm.
The inset in the top right corner shows the enlarged SEM image
of the PMCS, and the single pore diameter of PMCS is about 150
nm. The pores among the PMCS are uniform in size, nearly
spherical and interconnected closely. The energy dispersive X-
ray spectroscopy (EDS) spectrum in Fig. 1(a) shows that the
PMCS only consist of C, O and P elements. The transmission
electron microscopy (TEM) image of the PMCS in Fig. 1(b)
Fig. 1 (a) SEM image and EDS spectrum of PMCS, and the inset is the Fig. 2 (a) Full XPS spectrum of PMCS. (b) High-resolution P 2p XPS
enlarged SEM image of the pores in PMCS. (b) TEM image of PMCS, and spectrum of PMCS. (c–f) EDS element mapping images of C, O and P
the red dotted circles show the pore structures. (c) FT-IR and Raman elements. (g) Nitrogen adsorption–desorption isotherm of PMCS. (h)
spectra of PMCS. (d) XRD pattern of PMCS.
Pore size distribution of PMCS.
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To study the macroporous characteristics of PMCS, the ESI†). The different contents of phosphorus were 0%, 2%, 4%,
nitrogen adsorption–desorption measurements were carried and 6%, which were abbreviated to NPMCS, PMCS-2, PMCS-4
out. The obtained curve in Fig. 2(g) shows the nitrogen and PMCS, respectively. The maximum phosphorus-doped (6%)
adsorption–desorption isotherm of PMCS, which exhibits a carbon material without a macroporous structure was abbrevi-
typical type-IV isotherm with an H1 hysteresis loop. This ated to NPC. The elemental analyses of all the samples were
adsorption behaviour is generally attributed to the capillary calculated by the EDS spectra (Fig. S2 and Table S2†). The FT-IR
condensation of N2 in the pores.33 The specic surface area of and Raman spectra of NPMCS, PMCS-4, PMCS-2 and NPC are
PMCS is 574.68 m2 gÀ1 obtained from the nitrogen BET (Bru- shown in Fig. S3 and S4,† respectively. Both these spectra
nauer–Emmett–Teller) adsorption measurements, indicating indicate that the basic chemical compositions (mainly carbon)
that the PMCS have a relatively large specic surface area. This of the four contrasts are in accordance with the PMCS. In the
porous characteristic of PMCS may further improve the activity following catalytic experiments, the selective oxidation of
of catalytic reaction. Fig. 2(h) shows the pore size distributions cyclooctene was also carried out with NPMCS, PMCS-2, PMCS-4
of PMCS, revealing a typical curve of macroporous structure and PMCS as catalysts to verify if the content of phosphorus had
with a characteristic of multiple pore diameters.
an inuence on the catalytic activity. The detailed conversion
The catalytic ability of PMCS over the selective oxidation of efficiency of cyclooctene and selectivity to two oxidation prod-
cyclooctene was investigated with tert-butyl hydroperoxide ucts are shown in Table S3† and Fig. 4(a). As shown in Fig. 4(a),
(TBHP) as a radical initiator and oxygen in air as the oxidant at the conversions of cyclooctene using NPMCS, PMCS-2, PMCS-4
353 K. The conversion of cyclooctene and selectivity to oxidation and PMCS as catalysts reached 25.01%, 31.73%, 37.56% and
products were measured by gas chromatography measurements 50.69%, respectively, aer 48 h. The highest selectivity to
(GC-MS). The detailed conversions of cyclooctene and selectivity epoxycyclooctane was 88.47% obtained by using PMCS as a
to oxidation products by using PMCS as catalyst are shown in catalyst, while the lowest selectivity to epoxycyclooctane was
Table S1.† Fig. 3 reveals the relationship between the conver- 77.43% by using NPMCS as a catalyst. The macroporous carbon
sion efficiency of cyclooctene and selectivity to oxidation prod- spheres containing lesser amounts of phosphorus (PMCS-2 and
ucts with reaction time by using PMCS as catalyst. As shown in PMCS-4) showed an intermediate conversion efficiency and
Fig. 3, the conversion of cyclooctene reached 50.69%, while the selectivity aer 48 h. The conversions of cyclooctene and
selectivities to epoxycyclooctane and 2-cyclooctenone were selectivity to epoxycyclooctane in all of the reactions increased
88.47% and 8.37%, respectively, aer 48 h. The selectivity to
epoxycyclooctane increased from 65.57% aer 6 h to 88.47%
aer 48 h while the selectivity to 2-cyclooctenone decreased
from 32.60% aer 6 h to 8.37% aer 48 h. The results indicate
that the conversion efficiency of cyclooctene and the selectivity
to epoxycyclooctane increased, while the selectivity to 2-cyclo-
octenone decreased simultaneously. From the experimental
results, we can conclude that by controlling the reaction time,
we can obtain different kinds of oxidation products.
In comparison, the macroporous carbon spheres with
different contents of phosphorus and the maximum phos-
phorus-doped (6%) carbon materials with different structures
were also synthesized (detailed synthesis processes shown in
Fig. 4 The relationship between the conversion of cyclooctene/
selectivity to epoxycyclooctane and reaction time with (a) the mac-
roporous carbon spheres containing different contents of phosphorus
Fig. 3 The relationship between the conversion of cyclooctene/ (0%, 2%, 4% and 6%) as the catalysts, (b) the porous and nonporous
selectivity to two oxidation products and reaction time with PMCS as structural carbon materials with the maximum content of phosphorus
the catalyst.
(6%) as the catalysts.
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with the reaction time. The results indicated that with an surface area, which improved the conversion efficiency of
increase in the phosphorus content, both the conversion of cyclooctene tremendously and manifested a connement effect
cyclooctene and the selectivity to epoxycyclooctane increased that may greatly decrease the resistance of the reaction reac-
gradually. The role of phosphorus should be attributed to the tants transfer among the porous structure so as to accelerate the
breaking of C–C bond by phosphorus doping and the formation catalytic process.16
of abundant defect-induced active sites among the macro-
porous structure,19,20 which accelerate the oxidation of surface area of PMCS, which gave numerous active sites
cyclooctene. including oxygen-containing groups on the surface as well as
The doping of phosphorus resulted in the high specic
To further conrm the inuence of the macroporous struc- inside the pores. To further evaluate the effects of oxygen-con-
ture on the catalytic activity, the selective oxidation of cyclo- taining groups on the catalytic reaction, we utilized hydrazine
octene was also performed by using NPC and PMCS as the hydrate as the reductant to eliminate the oxygen-containing
catalysts. The detailed conversion efficiency of cyclooctene and groups of PMCS in order to verify their catalytic effects. As
selectivity to two products are shown in Table S4† and Fig. 4(b). shown in Table S5,† the conversion of cyclooctene decreased to
As shown in Fig. 4(b), the conversion efficiency of cyclooctene 18.70% and selectivity to epoxycyclooctane was 80.14%. The
using NPC as the catalyst only reached 20.14% aer 48 h, which results show that the oxygen-containing groups indeed have an
was much lower than that of PMCS (50.69%). The selectivity to effect on the catalytic activity of PMCS, which is consistent with
epoxycyclooctane was 81.31% aer 48 h, which was lower than previous reports.34–36
the selectivity by using PMCS (88.47%) as well. The porous
The blank reactions (without catalyst PMCS) did not result in
structure among the carbon spheres is analogous to a kind of obtaining any epoxide or other products (detailed conversion of
microreactor, which can form connement effect and accelerate cyclooctene and selectivity to oxidation products shown in
the diffusion of the reaction reagents and further enhance the Table S6†). Moreover, the ltrate solution test (additional 48 h
conversion efficiency. The control experiments above revealed reaction aer removing the PMCS from the reaction system) was
that the higher amount of phosphorus in PMCS and the unique also performed, which showed that almost no conversion effi-
porous structure both led to the optimal conversion of cyclo- ciency could be obtained aer 48 h. The results indicated that
octene and higher selectivity to epoxycyclooctane.
leaching did not play an important role in the reaction system.10
In order to further compare the effects of catalysts above in The control experiments above showed that PMCS acted as a
the selective oxidation of cyclooctene, the detailed information valid catalyst in the selective oxidation of cyclooctene.
is shown in Table 1. As shown in Table 1, the conversion effi-
To conrm the reactive oxygen species in the selective
ciency of cyclooctene increased with the increase in the oxidation of cyclooctene, the reactive species trapping experi-
contents of phosphorus as well as the specic surface areas of ment was also performed. Here, the p-benzoquinone (BZQ, a
the porous carbon spheres increasing gradually (1–4). The radical scavenger) was used to verify that TBHP acted as a
conversion efficiency increased from 25.01% with the specic radical initiator as well as oxygen in air as the oxidant. In the
surface area of 195.60 m2 gÀ1 to 50.69% with the specic surface experiment, the reaction conditions were the same without BZQ
area of 574.68 m2 gÀ1. The increasing amount of P (0 to 6%) before 24 h, yielding a conversion efficiency of about 24% as
resulted in a larger specic surface area of the catalysts (195.60 before. Aer an appropriate amount of BZQ (0.02 g) was added
to 574.68 m2 gÀ1), which provided many active sites and had into the reaction system, the conversion efficiency still stayed at
positive effects on the selective oxidation of cyclooctene. As 24%. The result indicated that the reactive oxygen species
shown in Table 1, the conversion of cyclooctene was 20.14% produced by TBHP was consumed by BZQ and the subsequent
using NPC as the catalyst and the specic surface area was only reaction using O2 as the oxidant stopped.37
33.40 m2 gÀ1, while the conversion of cyclooctene was 50.69%
The possible reasons for the enhanced activity and selectivity
using PMCS as the catalyst and the specic surface area reached are as follows: the porous structure among the carbon spheres
up to 574.68 m2 gÀ1 (4 and 5). The results showed that the is analogous to a kind of microreactor, which can form a
porous structure among the catalysts led to a large specic connement effect and accelerate the diffusion of the reaction
reagents and further enhance the conversion efficiency and
selectivity.12–14 In addition, the XPS and FT-IR spectra results
show that the P–O and P–C bonds form in PMCS, resulting in
defect-induced active sites among PMCS and the excellent
activity and selectivity.24,38 Furthermore, phosphorus doping
results in high specic surface area of PMCS and gives many
active sites including oxygen-containing groups among the
pores, leading to the enhanced activity and selectivity. The
plausible mechanism is that the tert-butyl hydroperoxide
(TBHP) acts as a radical initiator, while the oxygen in air acts as
the oxidant. During the reaction, the cyclooctene can diffuse
evenly to both the surface as well as inside the porous spheres,
which act as the microreactors. Then, the C]C bonds in
Table 1 The effects of specific surface areas and contents of phos-
phorus on the selective oxidation of cyclooctene
COPa
(wt%)
SSAc
Conv.
(%)
d
No.
Catalysts
MPSb
(m2 gÀ1
)
Ssel C8
1
2
3
4
5
NPMCS
PMCS-2
PMCS-4
PMCS
0
2
4
6
6
Yes
Yes
Yes
Yes
No
195.60
267.45
395.66
574.68
33.40
25.01
31.73
37.56
50.69
20.14
93.62
94.03
95.90
96.84
92.80
NPC
a
b
Content of phosphorus was abbreviated as COP. Macroporous
c
structure was abbreviated as MPS. Specic surface area was
abbreviated as SSA. d Total selectivity to C8 partial oxidation products.
cyclooctene are directly oxidized by a peroxide species
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Conclusions
We demonstrate a template method for the preparation of PMCS
with a pore size of about 150 nm and high specic surface area of
574.68 m2 gÀ1. The PMCS exhibited high conversion efficiency of
cyclooctene and selectivity to epoxycyclooctane with TBHP as a
radical initiator and oxygen in air as the oxidant at 353 K. The
conversion efficiency of cyclooctene came to 50.69% and the
selectivity to epoxycyclooctane reached 88.47% aer 48 h. The
control experiments above revealed that the higher doped
phosphorus and higher specic surface area in PMCS with the
unique porous structure led to the optimal conversion of cyclo-
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Acknowledgements
This work was supported by the National Basic Research Program
of China (973 Program) (2012CB825800, 2013CB932702), the
National Natural Science Foundation of China (51132006), the
Specialized Research Fund for the Doctoral Program of Higher
Education (20123201110018), a Suzhou Planning Project of
Science and Technology (ZXG2012028), and a project funded by
the Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD).
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