Synthesis of two-dimensional mesoporous carbon nitride under different carbonization temperatures and investigation of its catalytic properties in Knoevenagel condensations

Article abstract of DOI:10.1039/c5ra00225g

A two-dimensional mesoporous carbon nitride (MCN-1) material with tunable surface area, pore volume and nitrogen content has been synthesized by using carbon tetrachloride and ethylenediamine as precursors and SBA-15 as a template. The effect of the carbonization temperature on the textural properties, N content and the types of nitrogen species of the MCN-1 samples was also investigated by several characterization techniques. The results reveal that the higher temperature favors the formation of porous structure during the thermal condensation process. In addition, the nitrogen content and the surface N/C ratio in the samples monotonically decrease with the increase of carbonization temperature. The catalytic activity of MCN-1-T is not only related to its surface nitrogen content but also dependent on the structural diversities of the surface nitrogen species. The MCN-1 sample synthesized at 400 °C shows the higher catalytic activity and stability for Knoevenagel condensations. This journal is

Full text of DOI:10.1039/c5ra00225g

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Synthesis of two-dimensional mesoporous carbon
nitride under different carbonization temperatures
and investigation of its catalytic properties in
Knoevenagel condensations
Cite this: RSC Adv., 2015, 5, 22838
ab
a
a
a
a
Lina Zhang, Hao Wang,* Zhangfeng Qin, Jianguo Wang and Weibin Fan*
A two-dimensional mesoporous carbon nitride (MCN-1) material with tunable surface area, pore volume
and nitrogen content has been synthesized by using carbon tetrachloride and ethylenediamine as
precursors and SBA-15 as a template. The effect of the carbonization temperature on the textural
properties, N content and the types of nitrogen species of the MCN-1 samples was also investigated by
several characterization techniques. The results reveal that the higher temperature favors the formation
of porous structure during the thermal condensation process. In addition, the nitrogen content and the
surface N/C ratio in the samples monotonically decrease with the increase of carbonization temperature.
Received 6th January 2015
Accepted 18th February 2015
The catalytic activity of MCN-1-T is not only related to its surface nitrogen content but also dependent
DOI: 10.1039/c5ra00225g
ꢀ
on the structural diversities of the surface nitrogen species. The MCN-1 sample synthesized at 400
shows the higher catalytic activity and stability for Knoevenagel condensations.
C
www.rsc.org/advances
15–17
surface area and nanosized pores.
Vinu et al. prepared a
1
. Introduction
series of mesoporous carbon nitrides (MCN-1) with tunable
Carbon nitride (CN) is a well-known and promising material pore diameters by using SBA-15 as template and ethylenedi-
that has drawn considerable attention due to its unique amine (EDA) and carbon tetrachloride (CTC) as precursors.
combination of various physicochemical properties, which They found that MCN-1 can be used as a metal-free catalyst for
endows it with versatile applications in many elds such as the Friedel–Cras reaction of benzene and showed high cata-
1
8,19
heterogeneous catalysis, photocatalysis, fuel cells and lytic activity.
hydrogen-storage.
Zhao et al. synthesized hierarchical MCN
Incorporation of nitrogen into the carbon spheres using spherical mesostructured cellular silica foams as
matrix can offer different types of nitrogen species, e.g. amine, a hard template and EDA and CTC as precursors. It was found
imine and pyridinic nitrogen. This gives rise to the material that these MCN spheres showed excellent CO capture perfor-
1–11
2
16
with intrinsically different basic functions, and thus, the capa- mance and recycling stability. It should be noted that these
bility of acting as efficient solid base catalyst for various cata- MCN materials mostly have low nitrogen content, i.e. the N/C
lytic reactions, such as transesterication of b-keto esters, ratio is less than 0.20, being far lower than the theoretical
1,12,13
Friedel–Cras reactions and Knoevenagel condensations.
value of C N . Therefore, it is necessary to increase the nitrogen
3 4
Bitter and co-workers reported that the catalytic activity of basic content in the MCN. The nitrogen atoms in the wall structure of
nitrogen-containing carbon nanotubes for the Knoevenagel MCN exist in the form of amine, imine and/or pyridinic
condensation of benzaldehyde and ethyl cyanoacetate was nitrogen species, which determine the basic character, and
14
related to their amount of pyridinic nitrogen species. This thus, the basic catalytic performance of the materials. Many
shows that the catalytic performance of CN materials depends researchers tried to improve the nitrogen content of the MCN
on the type and content of nitrogen species in the carbon materials by using a single-molecule precursor with high
matrix.
Compared with bulk CN material, mesostructured carbon size,
nitride (MCN) material possesses more active sites on the led to a lower nitrogen content due to the low thermal stability
nitrogen content and a template with small nanoparticles
15,20
and found that the higher carbonization temperature
20
surface and higher mass diffusion rate because of its large of nitrogen species. However, few reports studied the effects of
carbonization temperature on the textural properties, structural
a
diversities and amounts of nitrogen species, and basic catalytic
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China. E-mail: properties of MCN-1 materials.
fanwb@sxicc.ac.cn; wanghao@sxicc.ac.cn; Fax: +86 351 4041153; Tel: +86 351
Knoevenagel condensation is one of the well-known reac-
tions for forming carbon–carbon bond between aldehydes/
4
b
199009
University of Chinese Academy of Sciences, Beijing 100049, PR China
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1
ꢀ
ketones and active methylene-containing compounds. This at 100 C for 24 h in an oven. Finally, the solid product was
ꢀ
reaction is a practical way to attain ne chemical intermediates ltered, washed and dried at 80 C in the oven. The template
and therapeutic and pharmacological products. It is gener- was removed by calcination at 550 C for 10 h.
ally catalyzed by homogenous bases such as ammonia, ammo-
22–24
ꢀ
The ordered mesoporous carbon nitride materials were
prepared according to the method reported by Vinu et al.
25,26
19
nium salts, pyridine, and primary and secondary amines.
Although these catalysts show high catalytic activity, the thorny Typically, 1 g of the calcined SBA-15 was rst added into the
issues associated with the separation and recycling of the previously well-mixed solution containing 6.0 g of CTC and 2.7 g
ꢀ
catalysts cannot meet the increasing demand on green and of EDA. Then, the mixture was reuxed and stirred at 90 C for
2
5
ꢀ
sustainable chemical industry. Therefore, development of 6 h, followed by drying at 80 C overnight. The obtained dark-
heterogeneous catalysts has been paid much attention. These brown solid was ground into ne powder and transferred into
materials may have some advantages in achieving high selec- a crucible, and heated from room temperature to target
tivity to desired product, facilitating the separation and recovery temperature and kept for 5 h under nitrogen atmosphere.
of the catalyst from the reaction mixture and lowering the Aerwards, the as-synthesized dark powder was treated with
27
ꢀ
operation cost. Thus, a number of solid base catalysts such as dilute sodium hydroxide solution at 120 C for 24 h, followed by
silica supported amines, nitrided zeolites and metal–organic ltration and washing with water and ethanol several times.
28
29
30
ꢀ
frameworks have been prepared. However, these base catalysts Finally, the dark sample was dried in oven at 100 C for 12 h.
may still suffer from certain deciencies, e.g. partial blockage of The resultant mesoporous CN material was designated as MCN-
the pores by the basic guests and the possible leaching of the 1-T (T representing the carbonization temperatures ranging
basic components during the reaction. Hence, it is highly from 300 C to 800 C). Attempt to prepare MCN-1-200 by the
desirable to develop an effective heterogeneous catalyst for same method failed maybe due to the low carbonization
Knoevenagel condensations. Recently, MCN materials have temperature.
31
ꢀ
ꢀ
been extensively used as solid base catalysts for Knoevenagel
condensations, since they can somehow overcome these draw-
2
.3 Characterization
backs and show good catalytic performance resulting from their
32
high surface area and basicity of nitrogen-containing groups.
Wide-angle X-ray powder diffraction (XRD) patterns were
The aim of this study is to reveal the effect of carbonization collected on a Rigaku MiniFlex II desktop X-ray diffractometer
temperatures of MCN-1 materials on their textural properties, with CuKa radiation (l ¼ 0.154 nm, 30 kV, 15 mA). Small-angle
structural diversities and amounts of nitrogen species, and XRD patterns (SXRD) were recorded on a Bruker D8 ADVANCE
catalytic activity for Knoevenagel condensations. The catalytic X-ray diffractometer with Ni-ltered CuKa radiation (l ¼ 0.154
activity of MCN-1 materials for the Knoevenagel condensation nm, 40 kV, 40 mA). Thermogravimetric analysis (TGA) of MCN-
of benzaldehyde or acetone and malononitrile was also dis- 1-T was performed on a Setaram Setsys Evolution 16/18 analyzer
ꢀ
ꢁ1
cussed in details. This study may shed a new light on the (France) at a heating rate of 10 C min in an argon ow to test
understanding of the effects of textural properties, and struc- their thermal stability. Nitrogen sorption isotherms were
ꢀ
tural diversities and amounts of nitrogen species of MCN-1 measured at ꢁ196 C on a TriStar II 3020 volumetric adsorption
materials on its catalytic properties in Knoevenagel condensa- analyzer manufactured by Micromeritics Instrument Corpora-
tion reactions.
tion. Before the measurement, the samples were outgassed at
ꢀ
150 C for 8 h in order to remove and/or desorb impurities and
moisture on their surfaces. The specic surface areas of the
samples were calculated using the Brunauer–Emmett–Teller
(BET) method within the relative pressure range of 0.05–0.30.
The pore diameter, pore volume and pore size distribution were
determined from the adsorption branch of the nitrogen
isotherms using the Barrett–Joyner–Halenda (BJH) method. The
morphology of the solid was identied on a JEOL JSM-7001F
2
. Experimental
2.1 Materials
Pluronic P123 was purchased from Sigma-Aldrich. Decane was
obtained from Alfa Aesar China (Tianjin) Co., Ltd. Absolute
ethanol, tetraethyl silicate (TEOS), hydrochloric acid (HCl),
ethylenediamine, carbon tetrachloride, benzaldehyde, malo-
nonitrile, acetone and sodium hydroxide (NaOH) were all
obtained from Sinopharm Chemical Reagent Co., Ltd. All
reagents were used as received without further purication.

eld emission scanning electron microscope (SEM). High
resolution transmission electron microscopy (HRTEM) images
were measured on a JEOL JEM-2100F transmission electron
microscope operating at 200 kV. Fourier transform infrared (FT-
IR) spectra of the samples were collected in transmission mode
at room temperature using the KBr self-supported pellet tech-
nique on a Bruker Tensor 27 spectrometer in the region of 400–
2.2 Synthesis of mesoporous silica SBA-15 and ordered
mesoporous carbon nitride materials
ꢁ1
SBA-15 was synthesized following the previous procedures 4000 cm . The mass ratio of the sample to KBr was 1 to 300.
33
reported by Zhao et al. In a typical batch, specic amounts of Raman spectra were recorded on a laser Raman spectrometer
Pluronic P123 were mixed with water and 2 M HCl solution at (Renishaw inVia Reex, U.K.) at room temperature using the
ꢀ
3
8 C under the stirring condition. Then, TEOS was dropwise 532 nm line of an argon ion laser as the excitation source. X-ray
added to the above solution. The mixture was further stirred for photoelectron spectra (XPS) were measured on an AXIS ULTRA
ꢀ
2
4 h at 38 C. Aer that, the mixture was hydrothermally treated DLD spectrometer equipped with a monochromatic AlKa
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radiation source. This analysis was used to identify the chem- resolved peaks that can be indexed as (100), (110), and (200)
33
ical states and estimate the surface contents of nitrogen- facets of p6mm symmetry. All the MCN-1-T samples show a
ꢀ
containing species in the catalysts. The carbonaceous C 1s diffraction peak at 2q of 1.10–1.20 , which is similar to that of
line (284.8 eV) was used as the reference to calibrate the binding SBA-15 mesoporous silica template, and indexed as (100) plane.
energies. Elemental analyses were conducted on an Elementar This indicates that the MCN-1 materials possibly possess a two-
Vario EL CUBE elemental analyzer.
dimensional mesoporous structure replicated from the SBA-15
template. It is clear that increase of the carbonization temper-
ature increases the intensity of this peak, showing that the
structural order of the MCN-1 material increases with the
carbonization temperature. The wide-angle XRD patterns of
MCN-1-T samples (Fig. 1B) exhibit a single broad diffraction
2.4 Catalytic tests
The catalytic properties of the prepared MCN-1-T materials were
tested for the Knoevenagel condensation of benzaldehyde or
acetone and malononitrile. The reaction was conducted in a
Teon-lined stainless steel autoclave without adding any addi-
tional solvent. The reaction conditions were as follows: 1 mmol
of benzaldehyde or acetone, 3 mmol of malononitrile, 20 mg of
ꢀ
peak near 25.3 , which is attributed to the (002) plane, and
characteristic of interplanar stacking peaks of aromatic systems
19,34
in graphitic materials.
The d spacing calculated from the
(002) plane of MCN-1-T samples is around 0.35 nm, being
ꢀ
catalyst, 40 C. The products were analyzed by a Shimadzu GC-
similar to the value obtained from nonporous carbon nitrides
materials. This indicates that the wall structures of these
samples are composed of carbon and nitrogen that are arranged
in a turbostratic form.
2014C gas chromatograph equipped with a 60 m DB-1 capillary
column and a ame ionization detector. Aer each reaction run,
the used catalyst was washed with water and ethanol, and dried
ꢀ
overnight at 100 C for next running.
To investigate the thermal stability of MCN-1-T samples, TGA
ꢀ
proles were measured (Fig. 2). The weight loss below 200 C is
attributed to the desorption of water molecules in the pore and/
or on the surfaces of the MCN-1 material. The strong weight loss
takes place when the temperature is higher than the carbon-
ization temperature for all the samples, and the lost weight
signicantly decreased with increasing carbonization tempera-
ture. This shows that the thermal stability of MCN-1-T materials
highly depends on their carbonization temperature, and it is
stable below the carbonization temperature. The weight loss
occurred at the temperature higher than carbonization
temperature may be caused by the thermal decomposition of
3
. Results and discussion
3.1 Characterization results
The pore structure orderings of MCN-1-T materials and their
parent silica templates were investigated by powder XRD tech-
nique. Fig. 1A shows the SXRD patterns of SBA-15 and MCN-1-T
materials. The parent SBA-15 template exhibits three well-
16
the samples into ammonia and nitrogen, and the evaporation
35–37
of carbon species.
The nitrogen sorption isotherms and BJH pore-size distribu-
tions of the samples are displayed in Fig. 3. SBA-15 shows a type
IV curve with a steep H1 hysteresis loop at P/P ¼ 0.65–0.80,
0
indicative of a typical mesoporous structure with large pore size
and narrow pore size distribution (PSD). Table 1 shows that the
2
ꢁ1
surface area and total pore volume of SBA-15 reach 1015.5 m g
3
ꢁ1
and 1.07 cm
g
respectively. All the MCN-1 materials, like
SBA-15, also display type IV sorption isotherms with H1 hyster-
esis loops, implying the presence of mesopores. The MCN-1-T
Fig. 1 Small-angle XRD patterns (A) of SBA-15 (a) and MCN-1-T (T ¼
3
00 (b), 400 (c), 500 (d), 600 (e), 700 (f), and 800 (g)) samples, and
wide-angle XRD patterns (B) of MCN-1-T samples.
Fig. 2 TGA profiles of MCN-1-T samples.
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nm may come from the small mesopores formed aer
dissolution of the silica template, whereas the other peak
should be attributed to the partial collapse of MCN-1 meso-
structures and interparticle voids. It is worth noting that the
pore diameters of MCN-1 samples increase with the carbon-
ization temperature.
The textural parameters, including the specic surface area,
pore volume and pore diameter of MCN-1-T samples are given
2
ꢁ1
in Table 1. MCN-1-300 shows a low surface area (19.4 m g
)
3
ꢁ1
and pore volume (0.016 cm g ). This could be attributed to the
pore collapse during the removal of silica template as a result of
low polymerization degree of the formed mesoporous silica–
carbon nitride composites. Nevertheless, an increase in the
carbonization temperature led to an increase in the surface
areas and pore volumes. When the carbonization temperature
ꢀ
was increased to 700 C, the surface area and pore volume
2
ꢁ1
3
ꢁ1
reached the highest values of 790.3 m g and 0.95 cm g
respectively. Thus, it can be drawn a conclusion that high
temperature favors the formation of porous structure during
the thermal condensation process.
The SEM images of MCN-1-T samples are shown in Fig. 4. All
the samples exhibit a rod-like morphology with uniform parti-
cles, the diameters of which are around 200 nm, being similar
to those of the parent SBA-15 templates. This shows that the
replication process of MCN-1 from SBA-5 template is successful
and the morphology of MCN-1 samples is highly similar to that
of the SBA-15 even aer SBA-15 was removed. The HRTEM
images of MCN-1-400, MCN-1-600 and MCN-1-800 are shown in
Fig. 5. Hexagonally arrayed mesostructure was clearly observed
for all the samples, and the structural order increases with the
carbonization temperature. This is consistent with the results
obtained with powder XRD patterns. The pore diameters
obtained from the HRTEM images are similar to those calcu-
lated from nitrogen sorption isotherm.
Fig. 3 Nitrogen sorption isotherms (a) and BJH pore-size distributions
(
b) of MCN-1-T samples.
samples carbonized at different temperatures show signicant
differences in the mesostructures. The MCN-1-300 sample has a
very low nitrogen adsorption capacity, while MCN-1-400
exhibits a broader capillary condensation step, indicative of a
slight disordered mesoporous structure. In addition, the sorp-
tion hysteresis loop of MCN-1-400 located at P/P of 0.4–0.85,
0
suggesting that the pore size of MCN-1-400 should be smaller
than that of SBA-15. In contrast, the other four MCN-1-T
samples exhibit two capillary condensation steps in the sorp-
tion isotherms. One is at a low relative pressure and the other is
at a high relative pressure region. This indicates that MCN-1
materials prepared at high carbonization temperatures
possess ordered mesoporous structures with bimodal pores, as
conrmed by the BJH pore-size distribution (Fig. 3b). Two
peaks were observed in the PSD curves of the samples carbon-
Fig. 6 shows the FT-IR spectra of MCN-1-T samples. All the
ꢁ1
samples exhibit three intense bands around 1260 cm , 1610
ꢁ1
ꢁ1
cm and 3420 cm , which are corresponding to the aromatic
C–N stretching vibration, aromatic ring modes, and the
stretching mode of the N–H groups in the aromatic ring and/or
13,19
the O–H stretching vibration of residual water molecules.
This suggests that the MCN-1-T samples are mainly composed
ꢀ
ized at temperature higher than 500 C. The rst peak near the
Table 1 Textural properties and chemical compositions of MCN-1-T and SBA-15 samples
c
Pore diameter (nm)
Composition (wt%)
Primary mesopore
diameter
Large mesopore
diameter
Other possible
elements (Si, O, Cl and Na)
a
2
ꢁ1
b
3
ꢁ1
Sample
S
BET (m g
)
V
pore (cm g
)
C
N
H
MCN-1-300
MCN-1-400
MCN-1-500
MCN-1-600
MCN-1-700
MCN-1-800
SBA-15
19.4
306.4
491.4
618.4
790.3
711.4
0.016
0.22
0.42
0.67
0.95
0.93
1.07
—
—
—
56.91
57.29
61.83
65.76
68.54
75.67
—
24.15
23.52
21.69
15.75
12.97
8.69
3.80
3.25
2.94
2.55
2.22
1.45
—
15.14
15.94
13.54
15.94
16.27
14.19
—
6.27
7.30
8.08
9.27
9.41
—
3.36
3.56
3.71
3.92
8.06
1015.5
—
a
b
c
BET surface area. Total pore volume obtained at P/P
0
¼ 0.99. Most probable pore size.
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Fig. 6 FTIR spectra of MCN-1-T samples.
that all the MCN-1-T samples show similar C 1s, N 1s and O 1s
spectra, indicative of similar surface compositions. For
simplicity, only the C 1s and O 1s spectra of MCN-1-300 are
presented in Fig. 8b and c. The XPS survey spectrum of the
MCN-1-300 sample conrms that its surface is mainly
composed of C, N and O atoms (Fig. 8a). The O species may
come from the moisture, ethanol and atmospheric O
2
or CO
2
19
adsorbed on the surface, and partial oxidation of the precur-
16
Fig. 4 SEM images of MCN-1-300 (a), MCN-1-400 (b), MCN-1-500
sors. The C 1s spectrum (Fig. 8b) of MCN-1-300 was divided
into three peaks at 288.6, 285.6 and 284.4 eV. The intense peak
at 284.4 eV results from the pure graphitic species in the
(
c), MCN-1-600 (d), MCN-1-700 (e), and MCN-1-800 (f).
1
2,40,41
amorphous CN matrix,
while the peak located at 285.6 eV
of pyridine and benzene rings interconnected by nitrogen
atoms.
2
can be assigned to the sp C atoms bonded to N inside the
42,43
aromatic structures.
As for the peak with the binding energy
The relative intensity ratio of D-band to G-band (I /I ) in the
2
D
G
at 288.6 eV, it could be ascribed to the presence of sp C atoms
Raman spectrum is a measurement of the ordering of nitrogen-
2
bonded to aliphatic amine (–NH or –NH–) in the aromatic
rings.
The N 1s spectra (Fig. 8d–i) indicate that several types of
nitrogen species with different electronic states exist in the
sample. Two peaks were observed at 397.8 and 399.5 eV in the N
38
containing carbon solid. Fig. 7 shows, as an example, the
Raman spectrum of MCN-1-400 since all the MCN-1 samples
exhibit similar spectra. Two intense bands were observed at
12,44
ꢁ
1
ꢁ1
1
371 cm and 1575 cm , which are ascribed to the D (disor-
2
dered) and G (graphitic) bands of the sp -based carbon. The
presence of the D band is characteristic of some disordered
graphitic carbons in the walls of material.
1
s spectrum of MCN-1-300 sample (Fig. 8d). The one at 399.5 eV
2
is ascribed to the N atoms trigonally bonded to all sp carbons
or bonded with the sp carbon atoms and hydrogen atoms,
22,39
Fig. 7 shows that
2
13,19
the intensity of the G band is higher than that of the D band (ID/
viz. tricoordinated nitrogen species and/or amino groups. The
I ¼ 0.83), evidencing a high graphitization degree in the MCM-
2
G
other one at 397.8 eV arises from the sp N atoms bonded to C
2
1-400 structure. This reveals that many sp -hybridized C species
16,19
atoms in aromatic rings, i.e. pyridine nitrogen.
The surface
are present in the pore walls, being consistent with the XRD
results.
In order to further investigate the chemical and electronic
states of nitrogen and carbon atoms in the MCN-1-T samples,
XPS analysis (Fig. 8) was carried out. It should be mentioned
N/C ratio, the relative concentration of the functional groups in
the C 1s, O 1s and N 1s, and the relative fractions of different
types of nitrogen species were estimated by deconvolving the
XPS spectra, and the results are summarized in Table 2. The
surface nitrogen content gradually decreases with the
Fig. 5 HRTEM images of MCN-1-400 (a), MCN-1-600 (b) and MCN-1-800 (c).
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increasing carbonization temperature. This suggests that the
pyridine nitrogen should be chemically transformed into
nitrogen species with higher binding energies through the
condensation reactions during the carbonization treatment. As
a result, the surface pyridine nitrogen content gradually
decreases with the increase in the carbonization temperature,
but the content of the surface tricoordinated nitrogen and/or
amino groups increases until the carbonization temperature
ꢀ
reached 400 C, despite that a further increase in the carbon-
ization temperature led to a signicant decrease.
It should be noted that the depletion rates of nitrogen and
carbon atoms are different during the high temperature
carbonization process, according to the C, H, and N elemental
analyses (Table 1). The nitrogen content continuously decreases
with the carbonization temperature, and all the samples have
lower nitrogen content than the theoretical carbon nitride. This
can be accounted for by the low thermodynamic stability of
nitrogen species in the carbon matrix and the preferable state of
nitrogen species at higher temperature being nitrogen mole-
cules. Trace amounts of H comes from either the moisture or
ethanol adsorbed on the surface, or the amino groups on the
MCN-1. The hydrogen contents correspondingly decrease
with increasing carbonization temperature because most of
Fig. 7 Raman spectrum of MCN-1-400.
carbonization temperature due to release of more nitrogenous
volatiles at higher carbonization temperature. MCN-1-300
shows the highest surface N/C ratio of 0.34. The peak analyses
of N 1s show that the MCN-1-T samples all contain several types
of nitrogen species, but with different amounts. The fraction of
pyridine nitrogen decreases but the proportion of the tri-
coordinated nitrogen and/or the amino groups increases with
13
Fig. 8 XPS survey spectrum (a) and deconvolved C 1s (b), and O 1s (c) of MCN-1-300 and N 1s spectra of MCN-1-T samples (d–i) (T ¼ 300 (d),
00 (e), 500 (f), 600 (g), 700 (h), and 800 (i)).
4
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Table 2 Relative surface concentrations of C, O and N of MCN-1-T samples obtained from XPS data
a
Relative surface concentrations (at.%) and relative fraction of nitrogen species (%)
Tricoordinated nitrogen
and/or amino groups
Surface N/C
ratio
Sample
C 1s
N 1s
O 1s
Pyridine nitrogen
MCN-1-300
MCN-1-400
MCN-1-500
MCN-1-600
MCN-1-700
MCN-1-800
67.3
69.1
71.5
73.7
80.2
82.8
22.6
22.6
21.0
16.7
11.9
8.3
10.1
8.4
7.5
9.6
7.9
8.9
8.6 (37.9)
8.3 (36.9)
7.6 (36.2)
6.0 (35.8)
4.1 (34.8)
2.6 (31.7)
14.0 (62.1)
14.3 (63.1)
13.4 (63.8)
10.7 (64.2)
7.8 (65.2)
5.7 (68.3)
0.34
0.33
0.29
0.23
0.15
0.10
a
Values in parentheses are relative fractions of different types of nitrogen species.
hydrogen atoms are bound to the edges and defects of the carbonization temperature. With respect to the Knoevenagel
layers. It can be seen from Table 1 that other elements such as condensation of benzaldehyde and malononitrile, MCN-1-300
Si, Na, O or Cl are also present in the MCN-1-T aer removal of showed a benzaldehyde conversion of 95.9%, MCN-1-400 gave
silica template. The XPS analysis results reveal the presence of a benzaldehyde conversion of 99.3% (entries 2 and 3), but a
only a small amounts of silica (ca. 1 wt%), Na and Cl but a large further increase in the carbonization temperature resulted in a
amount of O in the obtained carbon nitride matrix (Fig. 8a).
decrease of the benzaldehyde conversion. Nonetheless, the
MCN-1-800 sample carbonized at 800 C still exhibited high
ꢀ
catalytic activity, giving a benzaldehyde conversion of 91.5%.
This shows that all the MCN-1-T samples can effectively catalyze
this condensation reaction. When acetone, instead of benzal-
dehyde, was used as the starting material, a similar effect of the
carbonization temperature on the catalytic activity of MCN-1-T
was observed (entries 9–14 in Table 3), although they show a
lower catalytic activity for the condensation of acetone with
malononitrile due to the lower reactivity of ketones than alde-
hydes (entries 10–14). Nevertheless, even for such a tough
reaction, MCN-1-400 still gave an acetone conversion of 92.1%.
Table 3 shows that the product (benzylidene malononitrile and
isopropylidenemalononitrile) selectivities in both the conden-
sations are close to 100% for all the MCN-1-T samples.
3
.2 Catalytic activity of MCN-1-T in Knoevenagel
condensation
The catalytic properties of MCN-1-T were investigated by using
Knoevenagel condensations of benzaldehyde or acetone and
malononitrile as model reactions, and the obtained results are
shown in Table 3. It should be pointed out that the water was
not removed during the reaction. The blank test (without cata-
lyst) gave a quite low benzaldehyde or acetone conversion
45
(entries 1 and 8). In contrast, all the MCN-1-T samples show
high catalytic activity and selectivity. Irrespective of this, the
catalytic activity of MCN-1-T is highly dependent on its
As mentioned above, Knoevenagel condensation reactions
can be catalyzed by the basic catalysts, and the catalytic activity
is related to the amounts and types of active sites and textural
properties of the catalysts. By comparing Table 1 with Table 3, it
can be found that MCN-1-400 shows higher activity than MCN-
Table 3 Catalytic results of MCN-1-T for different Knoevenagel
condensation reactions
a
b
Entry
Catalyst
X (%)
S (%)
I. Benzaldehyde + malononitrile
1-300 although its nitrogen content is slightly lower than that of
1
2
3
4
5
6
7
None
9.8
95.9
99.3
94.9
94.1
92.0
91.5
89.5
100
100
100
100
100
100
MCN-1-300
MCN-1-400
MCN-1-500
MCN-1-600
MCN-1-700
MCN-1-800
MCN-1-300. This is certainly related to its high surface area or
developed porosity. However, the catalytic activity of the MCN-1-
T samples monotonically decreases with increasing carboniza-
ꢀ
ꢀ
tion temperatures from 400 C to 800 C (entries 3–7 and entries
0–14), although their surface areas are all much higher than
1
that of MCN-1-400. This implies that the textural properties of
the catalysts should be not the dominant factor determining the
catalytic activity for the Knoevenagel condensations, as
conrmed by the fact that MCM-1-300 shows signicantly
higher catalytic activity than the samples carbonized at $
600 C, but its surface area is far lower. This suggests that the
catalytic activity of MCN-1-T should be also related to the
amount of nitrogen species. In addition, the XPS analyses show
II. Acetone + malononitrile
8
9
1
1
1
1
1
None
0.17
88.6
92.1
83.5
30.8
25.2
14.0
100
100
100
100
100
100
100
MCN-1-300
MCN-1-400
MCN-1-500
MCN-1-600
MCN-1-700
MCN-1-800
0
1
2
3
4
ꢀ
a
b
Benzaldehyde or acetone conversion. Selectivity to benzylidene that several types of nitrogen species exist on the surface of
malononitrile (I) or isopropylidenemalononitrile (II). Reactions
MCN-1-T materials. Xu et al. reported that tertiary N atoms in
the three-dimensional mesostructured carbon nitride have a
positive effect on the catalytic activity for the reaction of
conditions: 1 mmol benzaldehyde or acetone, 3 mmol malononitrile,
ꢀ
2
0 mg catalyst, 40 C, 2 h.
22844 | RSC Adv., 2015, 5, 22838–22846
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2
benzaldehyde and malononitrile. Indeed, it was found that repeated runs with regeneration that the obtained benzalde-
MCN-1-400 has a higher content of tricoordinated nitrogen and/ hyde conversions are all higher than 90% with benzylidene
or amino groups than MCN-1-300 (Table 2). Thus, the higher malononitrile selectivity of about 100%, showing that MCN-1-T
catalytic activity of MCN-1-400 than MCN-1-300 may be also materials are stable heterogeneous catalysts and can be reused
attributed to its higher content of the tricoordinated nitrogen for Knoevenagel condensation reactions.
and/or amino groups on the surface. In contrast, when the
ꢀ
carbonization temperature is higher than 400 C, both the bulk
and surface nitrogen contents decrease with the carbonization 4. Conclusions
temperature. This is consistent with the decrease of the catalytic
activity. This shows that the catalytic activity of MCN-1-T is not
A series of two-dimensional mesoporous carbon nitride (MCN-
1) materials have been prepared by using carbon tetrachloride
only dependent on the amount of available basic sites on the
surface, but also related to the types of nitrogen species. The
tricoordinated nitrogen and amino groups might show higher
catalytic activity than the other type of nitrogen species.
To further explore the active sites, the benzaldehyde and
acetone conversions were plotted against the surface concen-
trations of tricoordinated nitrogen and/or amino groups, and
total nitrogen species (Fig. 9). It is clear that the catalytic
activities of MCN-1-T samples are in a good agreement with the
contents of the tricoordinated nitrogen and/or amino groups
but slightly different from those of total surface nitrogen
species, further conrming that tricoordinated nitrogen and
amino groups are more active than other type of nitrogen
species.
and ethylenediamine as precursors and SBA-15 as a template.
ꢀ
By altering the carbonization temperature between 300 C and
ꢀ
8
00 C, the MCN-1 samples were obtained with tunable surface
area, pore volume, and structural diversities and amounts of
nitrogen species. Among the prepared MCN-1-T samples, MCN-
1-400 shows the highest catalytic activity for Knoevenagel
condensations, giving benzaldehyde and acetone conversions
of 99.3% and 92.1% respectively in the reactions of benzalde-
hyde or acetone and malononitrile. The catalytic activity of
MCN-1-T is not only related to its surface nitrogen content but
also dependent on the structural diversities of the surface
nitrogen species.
To investigate the catalytic stability of MCN-1-T, MCN-1-400
as an example was recycled to catalyze the reaction between
benzaldehyde and malononitrile. It was found within ve
Acknowledgements
This work is nancially supported by the National Basic
Research Program of China (2011CB201400), the National
Natural Science Foundation of China (21273263, 21273264), the
Natural Science Foundation of Shanxi Province (2012011005-2),
and the Research Project Supported by Shanxi Scholarship
Council of China (2014-102).
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22846 | RSC Adv., 2015, 5, 22838–22846
This journal is © The Royal Society of Chemistry 2015