Catalytic epoxidation reaction over N-containing sp2 carbon catalysts

Article abstract of DOI:10.1021/cs500062s

Nitrogen-doped graphene treated with ammonia under different temperatures was used as the catalysts for the epoxidation of trans-stilbene and styrene at 373 K. NG-800 (N-doped graphene treated at 800°C for 8 h) performed the best and gave the highest recy

Full text of DOI:10.1021/cs500062s

Letter
pubs.acs.org/acscatalysis
Catalytic Epoxidation Reaction over N‑Containing sp2 Carbon
Catalysts
Wenjing Li,† Yongjun Gao,† Wulin Chen,‡ Pei Tang,† Weizhen Li,† Zujin Shi,† DangSheng Su,§
Jianguo Wang,*,‡ and Ding Ma*,†
†Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing
100871, China
‡College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310032, China
§Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Science, 72 Wenhua Road,
Shenyang 110016, China
S
* Supporting Information
ABSTRACT: Nitrogen-doped graphene treated with ammonia under
different temperatures was used as the catalysts for the epoxidation of
trans-stilbene and styrene at 373 K. NG-800 (N-doped graphene treated
at 800 °C for 8 h) performed the best and gave the highest recyclable
catalytic activity for the epoxidation of trans-stilbene, with 95.8%
conversion and 94.4% selectivity to trans-stilbene epoxide. The catalytic
center has been identified with the reaction mechanism elucidated by
DFT calculation.
KEYWORDS: nitrogen-doped graphene, carbon catalyst, trans-stilbene, epoxidation, DFT calculation
In addition to the reports on metal-based catalysts for alkene
epoxidation, few studies have been published so far on the
investigation of nonmetal catalysts. Recently, extensive studies
have been carried out on graphene-based hybrid materials to
investigate the potential applications of such materials in the
fields of adsorption and catalysis. Metal-modified graphene
hybrid materials have been widely used in oxygen reduction
reactions,14,15 batteries,16 supercapacitors,17 catalysis,18 and
photocatalysis.19 Moreover, it has been reported that the
electronic structure of graphene can be significantly modified
by the introduction of certain heteroatoms, such as N20 and
B.21 Therefore, their catalytic properties were altered as a
consequence, which was also previously reported in carbon
nanotube system for oxidative dehydrogenation of alkanes and
carbon nitride system for cyclohexane oxidation.22−24 Partic-
ularly, graphene and heteroatom-modified graphene were
reported to be able to catalyze various catalytic reactions,
such as hydrogenation, polymerization, alcohol oxidation,
oxidative coupling, and even C−H bond activation.25−33
However, until now, there has been no report on the catalytic
epoxidation of a double bond over a graphene catalyst. In this
current study, we prepared N-containing few-layered graphene
lkene epoxidation is an important industrial process
Abecause the products of the reactions, epoxides, are
versatile intermediates in fine chemical synthesis. They serve as
building blocks for the fabrication of plasticizers, perfumes, and
pharmaceutical products.1 Traditionally, the oxianes were
obtained by the direct oxidation of alkenes with stoichiometric
peracids,2 which produced large amounts of waste and
byproducts. Even worse, the peracids were unsafe and corrosive
to the equipments used in the process. With growing attention
paid to the environmental issue, much more effort was made to
develop a variety of new processes for the epoxidation reactions
by using different environmentally benign oxidants, such as
TBHP,3,4 H2O2,1,5−7 oxygen,8 and air.9 It has been reported
that when TBHP was used as the oxidizing agent for alkene
epoxidation, supported gold catalysts10 or metal-embedded
mesoporous materials, such as MCM-413 and SBA-15,11 were
proven efficient in those epoxidation processes. For such
catalytic systems, the presence of metals was believed to be
pivotal for the construction of the active sites, which facilitate
the adsorption and subsequent reaction for both the alkenes
and oxidants.12 Particularly, titanium-substituted silicalite (TS-
1) was demonstrated to be very effective for styrene
epoxidation, and the highly dispersed Ti species incorporated
in the zeolitic matrix was identified as the key contributor of
this catalytic activity.13
Received: January 16, 2014
Revised: March 17, 2014
Published: March 19, 2014
© 2014 American Chemical Society
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Letter
materials and a series of N-doped graphene/layered carbon
materials by the high-temperature ammonia treatment method.
It was observed that the epoxidation of trans-stilbene could be
triggered over N-containing graphene catalysts. The catalyst is
very stable and easily recycled. By XPS tests and other control
experiments, we identified that the lattice nitrogen species that
stayed at the graphitic site, instead of the iron dopant, is
responsible for the epoxidation reaction.
On the basis of the temperatures at which NG samples were
treated, they were denoted as NG-x (x stands for the
temperature of treatment at 200, 400, 600, and 800 °C).
The XRD profiles of the graphite, NG and NG-x are shown
in Figure 1a. In addition to graphite, all the other samples
very low nitrogen concentration (0.48%) was used as the
catalyst, a conversion of 3.8% could be reached in 1 h, and the
selectivity toward epoxide was 80.9% (Table 1, entry 1). After
a
Table 1. Epoxidation of Alkenes on Different Catalysts
d
selectivity (%)
N
Fe
conv
d
b
c
entry
catalyst
NG
(%)
(ppm)
(%)
epoxide benzaldehyde
e
1
0.48
0.58
1.29
0.66
0.42
0.42
0.42
0.42
323
377
289
347
373
373
373
1%
3.8
6.0
80.9
87.0
89.7
86.4
85.8
94.5
93.5
91.8
19.1
13.0
10.3
13.6
14.2
5.5
e
2
NG-200
NG-400
NG-600
NG-800
NG-800
NG-800
e
3
11.1
25.1
25.5
95.8
94.8
23.1
e
4
e
5
f
6
g
7
6.5
h
8
1%Fe/NG-
800
8.2
i
9
NG
0.48
0.42
323
373
54.5
87.6
87.5
72.8
12.5
27.2
i
10
NG-800
a
Reaction conditions: substrate (1 mmol), catalyst (10 mg), TBHP (3
mmol), urea (3 mmol), CH2Cl2 (2 mL), water (1 mL), T = 373 K.
b
c
d
Determined by elemental analysis. Determined by ICP. GC yield.
e
f
trans-Stilbene as substrate, t = 1 h. trans-Stilbene as substrate, t = 24
g
h. trans-Stilbene as substrate, 10 mg KCN was added, t = 24 h.
h
Fe(NO3)3 as precursor, calcined at 150 °C for 4 h under nitrogen
i
flow. Styrene as the substrate, t = 24 h.
treating NG with ammonia at 200 °C (NG-200), more nitrogen
atoms were incorporated into the structure of graphene. We
then observed an increase in catalytic activity (with conversion
of 6.0%), and a slight increase in selectivity toward epoxide
(87.0%). This demonstrated that ammonia treatment of the
graphene is beneficial to its epoxidation reactivity.
The catalytic activity increases with treatment temperature
(Table 1, entry 3). Significantly, for the NG sample treated with
ammonia at 600 °C, a drastic improvement in its catalytic
performance was observed (Table 1, entry 4), that is, the
selectivity toward epoxide reached 86.4% while the conversion
reached 25.1%. When the treating temperature ramped up to
800 °C, the catalyst thus prepared still exhibited a high catalytic
performance comparable to that of NG-600 catalyst (Table 1,
entry 5). When prolonging the reaction time to 24 h, the
conversion of trans-stilbene over NG-800 catalyst can be as
high as 95.8%, and the selectivity toward epoxide is around 95%
(Table 1, entry 6). This demonstrated that by ammonia
treatment at relatively high temperature, we could dramatically
improve the catalytic performance of the N-doped graphene
catalysts. With the increase in the ammonia treatment
temperature (600 and 800 °C), a drop in the nitrogen
concentration was observed (Table 1). For NG-600 and NG-
800 catalysts, only 0.66% and 0.42 wt %, respectively, of
nitrogen was observed by elemental analysis. This is almost
one-half and one-third that of NG-400. One needs to note that
at the same time, instead, the catalytic performance increased
dramatically (Table 1, entries 3−5).
Figure 1. Characterization of catalysts. (a) XRD patterns of different
catalysts; (b) XPS spectrum of N 1s in different catalysts; (c) TEM
image of NG; (d) TEM image of NG-800.
exhibit two well-resolved reflections at 11.6° and 26.4° (2θ).
The former signal suggests lamellar structure with a d-spacing
of 0.7 nm. Compared with that of the pristine graphite, the
intensity of the reflection at 26.4° decreases as a result of the
poorer crystallinity caused by the swelling of graphene sheets.
The morphology of NG and NG-x was determined by
transmission electron microscopy (TEM, SEM) method
(Figure 1c,d and Supporting Information, Figure S1a−c,e,f).
TEM of NG showed randomly aggregated, crumpled sheets
with a relatively large size distribution (from around 100 nm to
few micrometers). After the introduction of nitrogen, the
morphology of graphene sheets did not exhibit noticeable
differences under TEM observations, as shown in Figure 1d.
The epoxidation of trans-stilbene was conducted at 373 K in
a glass pressure vessel, with CH2Cl2 as the solvent and TBHP
(65% in water) as the oxidant. The trans-stilbene epoxide was
the main epoxidation product, with benzaldehyde as the
byproduct possibly resulting from the oxidative cleavage of the
double bond in trans-stilbene. When N-doped graphene with
To understand the phenomenon, the states of N in the
materials were investigated by N 1s XPS. Depending on their
location in the graphene sheets, nitrogen can be classified into
pyrrolic N, pyridinic N, amine groups N, and quaternary
N.34−36 For pristine NG, most of the nitrogen stays as pyridinic
N or amine groups (399.3 eV, Figure 1b). Although the
concentration of nitrogen increased for NG-200 and NG-400,
their N 1s XPS profiles maintained a shape similar to that of
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Letter
NG, indicating that most of the nitrogen is pyridinic N or
amine groups. For NG-600 and NG-800, albeit the morphology
of the samples did not change (Figure 1c,d and Supporting
Information, Figure S1a−c), the distribution of nitrogen over
graphene changed extensively. From Figure 1b, it is clear that
the share of quaternary N increased dramatically over
temperature, and for NG-800, it becomes the main peak of
the spectrum. However, as from elemental analysis, an extensive
loss of nitrogen was observed simultaneously.
Reasonably, at relatively high treatment temperature, the
distribution of nitrogen in graphene was governed by the
balance between nitrogen removal/doping due to the high
temperature treatment and/or the reconstruction of nitrogen
among different sites. The NG-600 and NG-800 that have the
relatively low concentration of nitrogen dopant but the highest
population of quaternary N possess the best epoxidation
catalytic performance. Indeed, when we tried to correlate the
catalytic activity with the concentration of quaternary N in the
catalyst, we observed a linear relationship between the graphitic
nitrogen concentration and the activity (Figure 4). These
results demonstrated very clearly that the quaternary N is
responsible for the high epoxidation activity observed on
ammonia-treated NG materials. In addition, NG and NG-800
were also investigated for the epoxidation reaction of styrene.
The results are listed in Table 1 (entries 9 and 10), suggesting
that NG-800 was very efficient in catalyzing the epoxidation of
styrene, as well.
selectivity of benzaldehyde dropped with the progress of the
reaction while its yield remained at a relatively constant level
for 24 h (Figure 2c). This result suggests that in the induction
period of the reaction, the oxidative C−H bond cleavage
coexisted with the epoxidation process. However, with the
reaction going on, the epoxidation was the dominant reaction
and the formation of benzaldehyde was suppressed.
Figure 2d illustrates the conversion of trans-stilbene and the
selectivity to products with temperature. Clearly, the
epoxidation reaction is favored at higher reaction temperature.
The catalyst is very stable and maintains a high catalytic
performance via a stability check by reusing the NG-800
catalyst seven times. As illustrated in Figure 3, the catalytic
The reaction behavior of the epoxidation of trans-stilbene
over NG-800 under different conditions was investigated
(Figure 2; Supporting Information, Table S1). The yield of
trans-stilbene epoxide was 21.9% within the first hour and
gradually increased to 84.3% with the reaction running for 12 h.
A slight increase in the conversion and selectivity to trans-
stilbene epoxide was observed when the reaction ran for a
longer time. At the same time, it was interesting to note that the
Figure 3. (a) Recycling studies of NG-800 on trans-stilbene
epoxidation. Conv. means conversion of trans-stilbene; Select. means
selectivity toward epoxidation product. Reaction conditions: trans-
stilbene (1 mmol), NG-800 (10 mg), urea (3 mmol), CH2Cl2 (2 mL),
water (1 mL), T = 373 K, t = 24 h. (b) TEM picture of NG-800 after
first use.
activity of NG-800 on the epoxidation of trans-stilbene remains
almost unchanged during the first five runs (95%), and it
dropped only slightly after seven recycles, but the selectivity
always stayed at a constant level (95%). The catalyst kept the
same morphology with previous NG-800 after reaction. These
results strongly indicate that nitrogen-doped graphene is a very
active and stable catalyst for the epoxidation reaction.
We then come to the question why this catalyst is so active in
the epoxidation reaction. On the basis of elemental analysis
(EA) and XPS and ICP results, we noticed that, in addition to
carbon and nitrogen, the pristine NG and those after ammonia
treatment all contain an impurity of iron (determined by ICP).
As shown in Table 1, NG and NG-x contain around 350 ppm
Fe, indicating the content of iron in all the tested catalysts is
relatively constant and independent of ammonia treatment.
However, we observed that NG contains 0.48% nitrogen, and
treating this material with ammonia at 200 °C and then at 400
°C will bring the concentration of nitrogen up to 1.29% for the
NG-400 catalyst. Most of these newly incorporated nitrogen
Figure 2. Catalytic activity and selectivity of NG-800 on the
epoxidation of trans-stilbene under different conditions. (a) Effect of
the amount of catalyst, TBHP (3 mmol), T = 373 K, t = 8 h; (b) effect
of the amount of TBHP, NG-800 (10 mg), T = 373 K, t = 12 h; (c)
effect of reaction time, NG-800 (10 mg), TBHP (3 mmol), T = 373 K;
(d) effect of reaction temperature, NG-800 (10 mg), TBHP (3 mmol),
t = 24 h. Reaction conditions: trans-stilbene (1 mmol), urea (3 mmol),
CH2Cl2 (2 mL), water (1 mL). Symbols: blue triangle, conversion of
trans-stilbene; black square, selectivity to trans-stilbene epoxide; red
dot, selectivity to benzaldehyde.
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atoms stayed at the graphitic sites (Figure 1b; Supporting
Information, Figure S2b). More importantly, the yield of
epoxide is in line with the concentration of graphitic nitrogen in
these samples (Figures 1b, 4). This result suggests that
addition of KCN. However, the reaction activity remains almost
the same after CN− is added (Table 1, entry 7). This implies
that the catalytic activity of these materials is not affected by the
iron element, although it exists in all catalysts at different
contents.
Our previous report on the study of selective oxidation of
ethylbenzene with nitrogen-doped sp2-hybridized carbon by
using the soft X-ray adsorption technique suggests that it is the
graphitic nitrogen dopant that modulated the electronic
structure of sp2 carbon material. The density of states
intensities near the Fermi level for the adjacent ortho-carbon
are much stronger than those of undoped graphene carbon,
which confers the nitrogen-neighboring carbon a metallike d
band electronic structure and makes the adjacent carbon more
capable to host the formation of reaction oxygen species; the
peroxide; and, consequently; to activate saturated hydrocarbon.
We believe that this is also the case in the current study because
the presence of graphitic nitrogen dopant is responsible for the
observed epoxidation activity; that is, the catalytic performance
is in line with the concentration of graphitic nitrogen in those
nitrogen-doped sp2 carbon materials (Figure 4).
Figure 4. The relationship between the catalytic activity (yield of
stilbene epoxide) and the concentration of quaternary nitrogen in the
N-doped graphene catalysts.
graphitic nitrogen is critical for the epoxidation reaction,
which is in good agreement with previous reports that nitrogen
incorporation can stimulate the chemical reactivity of adjacent
carbon species, making it an active catalyst for oxidation
reactions.31 However, because iron and nitrogen coexist on all
those catalytically active samples, it is unclear whether it was
iron or nitrogen alone or the combination of these two
components that generated the epoxidation activity.
To better understand the mechanism of the outstanding
catalyic activity of the nitrogen-doped sp2 carbon catalyst and
the oxidation of the carbon−carbon double bonds, we report
here the density functional theory (DFT) calculations using
opt88 functionals. As shown in Figure 5, the ortho-carbon in
To check whether iron is critical for the epoxidation, we
prepared the 1% Fe/NG-800 catalyst, where the concentration
of iron is around 30 times of that of NG-800 (although the
chemical state of iron may be different from those of fresh NG).
As shown in Table 1 (entry 8), the conversion of stilbene and
selectivity to epoxide are almost the same as that of parent NG-
800. This result implies that the presence of iron alone is
irrelevant to the epoxidation activity. This conclusion is also
confirmed by the fact that although NG, NG-600, and NG-800
have similar iron contents, they exhibited a huge difference in
reaction activity. To verify this conclusion, we prepared three
pristine NG catalysts, each containing the same amount of
nitrogen and different amounts of iron impurity (Supporting
Information, Table S2 and Figure S3). The reaction results
show that the catalytic performance is not in line with the
content of iron impurity and confirm that iron alone is not the
center for the catalytic epoxidation reaction.
Is it possible that iron and graphitic nitrogen function
synergistically to generate the catalytic active center? It has
been reported that Pd nanoparticles supported on nitrogen-
doped nanoporous carbon could be bound to nitrogen atoms
incorporated in carbon to enhance the catalytic activity of
hydrocarbon and alcohol oxidation.37 Furthermore, over
nitrogen-doped CNT-based catalysts, the iron impurity
cooperates with the nitrogen to form structures containing an
iron−nitrogen bond as well as Fe−N proximity. These iron/
nitrogen species were believed to be responsible for the good
activity in the oxygen reduction reaction.14 To determine
whether this also happens in the current study, NG-800 was
used to investigate the epoxidation reaction with 10 mg KCN
added into the reaction system. Because CN− can coordinate
strongly with iron and consequently poison the iron-centered
catalytic sites, if iron alone or the Fe−N moiety is the active
center for epoxidation reaction, the catalytic performance
should be observed with a remarkable decrease with the
Figure 5. The reaction mechanism for the epoxidation of trans-stilbene
on nitrogen-doped graphene catalyst.
nitrogen-doped graphene is the most stable adsorption site of
peroxide-like species,31,38,39 OOH. The α-oxygen (that does
not connect with hydrogen) in the pristine reactive oxygen
species stays straight above one of the ortho-carbons, and the β-
oxygen atom is pointing toward another ortho-carbon around
the graphitic nitrogen (Figure 5I). Because graphene has a big π
system, reactants with π orbitals such as stilbene or styrene are
able to adsorb on its surface via a strong π−π interaction
(Figure 5a).40,41 With the benzene rings stabilized by the
graphene domain, the locked internal C−C double bond lies on
top of reactive oxygen species with a favorite adsorption energy
of −0.43 eV.
In the transition state (Figure 5b), the carbon−carbon
double bond is attacked by the α-oxygen of the reactive oxygen
species. Because of the existence of three equivalent ortho-
carbons (Supporting Information, Figure S4), the reaction
between the α-oxygen with and the double bond, the oxygen−
oxygen bonds were elongated while the terminal hydroxyls
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* Supporting Information
Experimental procedures, the effects of different reaction
conditions, as well as the iron impurity on the epoxidation
catalytic activity. Figures S1−S4 and Tables S1−S2. This
material is available free of charge via the Internet at http://
pubs.acs.org.”
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AUTHOR INFORMATION
■
Corresponding Authors
*Phone: +86-571-88871037. Fax: +86-571-88871037. E-mail:
jgw@zjut.edu.cn.
*Phone: +86-10-62758603. Fax: +86-10-62758603. E-mail:
dma@pku.edu.cn.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work received financial support from the 973 Project
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91334013).
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