Microporous carbon nitride as an effective solid base catalyst for Knoevenagel condensation reactions

Article abstract of DOI:10.1016/j.molcata.2013.02.019

Microporous graphitic carbon nitride (CN-Mic) material has been prepared using carbon tetrachloride and ethylene diamine as precursors, and a siliceous microporous MCM-22 as a hard template. A series of Knoevenagel condensation reactions have been performed over the CN-Mic sample, and the influences of reaction conditions on the corresponding catalytic behavior have been investigated. The results showed that the CN-Mic material exhibited high catalytic activity and versatility for the condensation reactions. XPS and FT-IR analysis reveal a high abundance of the N-containing groups located on the surface of CN-Mic, which accounts for the superior catalytic activity of the carbon nitride material as a promising new type of heterogeneous Lewis base catalyst for the condensation reactions. It is also revealed that the activity of CN-Mic is much higher than those obtained over mesoporous g-CN materials. Meanwhile, the catalytic results of CN-Mic compared favorably with those of other g-CN materials for Knoevenagel reactions reported previously. In addition, a mechanism for the CN-Mic catalyzed Knoevenagel condensation was tentatively proposed.

Full text of DOI:10.1016/j.molcata.2013.02.019

Journal of Molecular Catalysis A: Chemical 372 (2013) 105–113
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Microporous carbon nitride as an effective solid base catalyst for Knoevenagel
condensation reactions
Jie Xu∗, Kang Shen, Bing Xue, Yong-Xin Li∗
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, College of Chemistry and Chemical Engineering, Changzhou University, Gehu Road 1, Changzhou, Jiangsu
213164, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Microporous graphitic carbon nitride (CN-Mic) material has been prepared using carbon tetrachloride
and ethylene diamine as precursors, and a siliceous microporous MCM-22 as a hard template. A series of
Knoevenagel condensation reactions have been performed over the CN-Mic sample, and the influences of
reaction conditions on the corresponding catalytic behavior have been investigated. The results showed
that the CN-Mic material exhibited high catalytic activity and versatility for the condensation reactions.
XPS and FT-IR analysis reveal a high abundance of the N-containing groups located on the surface of
CN-Mic, which accounts for the superior catalytic activity of the carbon nitride material as a promising
new type of heterogeneous Lewis base catalyst for the condensation reactions. It is also revealed that the
activity of CN-Mic is much higher than those obtained over mesoporous g-CN materials. Meanwhile, the
catalytic results of CN-Mic compared favorably with those of other g-CN materials for Knoevenagel reac-
tions reported previously. In addition, a mechanism for the CN-Mic catalyzed Knoevenagel condensation
was tentatively proposed.
Received 10 November 2012
Received in revised form 31 January 2013
Accepted 17 February 2013
Available online 4 March 2013
Keywords:
Carbon nitride
Knoevenagel condensation
Base catalyst
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
of these materials. Owing to the increasing attention toward the
development of green and sustainable heterogeneous catalysis, it
Knoevenagel condensation (the reaction between an aldehyde
and an active methylene-containing reagent) is one of the most
primary processes to synthesize valuable ␣,␤-unsaturated car-
bonyl compounds that are widely utilized in the pharmaceutical
industry [1,2]. Conventionally, Knoevenagel condensation reac-
tions proceed homogenously in the presence of various base
catalysts [3], which can activate the methylene-containing reagent
by abstracting an acidic proton from it. These catalysts are mainly
nitrogen-containing molecules such as aliphatic amines, urea and
piperidine or their corresponding ammonium salts and amino acids
[4]. Notwithstanding their high catalytic activity, the thorny issues
associated with the separation and recycling of the catalyst can-
not meet the increasing demand on green and sustainable catalysis
nowadays. Several heterogeneous inorganic salts, such as phos-
phates and halides, have also been reported as catalysts in the
Knoevenagel condensation. However, the use of such materials
leads to metal contamination of the products [5]. Recently, ionic
liquid or organic amine immobilized mesoporous silica materials
have been extensively used as base catalysts [6]. Nevertheless, the
drawback of limited number of active sites along with easy leach-
ing of anchored catalysts restrains a further catalytic application
is imperative to search a more efficient and sustainable catalyst for
Knoevenagel condensation process.
As one of the most appealing and promising metal-free mate-
rials, graphitic carbon nitride (g-CN) has recently received a great
deal of attention in many research fields [7,8]. The success of g-CN
is largely attributed to its unique combination of various physico-
chemical properties. Of particular note is that the incorporation of N
atoms into the C nanostructures endows g-CN with plenty of basic
sites in the forms of amine groups, etc. [9], which dictates an intrin-
sically basic function. Hence, the exploitation of g-CN has opened
new and exciting opportunities in the fields of base catalysis. Park
et al. [5] reported mesoporous g-CN as metal-free base catalyst
for Knoevenagel condensation reactions, wherein g-CN materials
exhibited efficiently catalytic activity under microwave irradiation,
affording high yields of 75–95% within short time of 12 min. Wang
et al. [10] reported the synthesis of high-surface-area g-CN using
cyanamide as precursor. After the pretreatment of deprotonation
by tBuOK or KOH solution, the g-CN materials demonstrated higher
catalytic activity for various Knoevenagel condensation as well as
transesterification reactions.
Although the results above have evidenced the catalytic poten-
tial of g-CN materials in Knoevenagel condensation reactions and
other organocatalysis processes, it should be mentioned that the
high catalytic results were achieved under an assisted condition or
pretreatment, i.e. microwave irradiation or deprotonation. In fact,
∗
Corresponding authors. Tel.: +86 519 86330135.
E-mail addresses: shine6832@163.com (J. Xu), liyxluck@126.com (Y.-X. Li).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.02.019

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J. Xu et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 105–113
poor catalytic yields were obtained if using bare g-CN materials
without any promoting condition. From both practical and sustain-
able points of view, it is highly desired to develop a readily usable
and effective g-CN catalyst capable of facilitating Knoevenagel con-
densation under mild conditions.
Herein, we employ microporous g-CN material (CN-Mic) as a
catalyst for a series of Knoevenagel condensation. The catalytic
results have revealed that CN-Mic can be used readily without any
assisted reaction condition or catalyst pretreatment, and exhibited
high efficiency for the condensation between various aldehydes
and methylene group-containing compounds. Furthermore, the
catalytic performance of CN-Mic has been compared in detail with
those obtained over other g-CN materials reported previously. In
addition, a possible mechanism for the CN-Mic catalyzed Knoeve-
nagel condensation has been proposed.
(b)
1000
(a)
5
10
15
20
25
2
30
θ (°)
35
40
45
50
2. Experimental
2.1. Catalyst preparation
Fig. 1. XRD patterns of MCM-22 (a) and CN-Mic (b) samples.
CN-Mic sample was prepared according to the established
templating-assisted method [11]. In a typical procedure, 2.0 g of
MCM-22 powder was added into the previously well-mixed solu-
tion containing 6.0 g of carbon tetrachloride (CTC) and 3.6 g of
ethylenediamine (EDA). The mixture above was then heated at
90 ◦C for 6 h under refluxing to induce polymerization of the
precursors, followed by drying at 50 ◦C overnight. The obtained
dark-brown solid was heated from room temperature to 600 ◦C
at a ramp of 3 ◦C min−1 and kept in 600 ◦C for another 5 h under
argon atmosphere. Finally, the CN-Mic sample was recovered after
washing with NH4HF2 aqueous solution (4 mol L−1) to remove the
template. The yield of resultant sample was around 0.7 g.
for two times, dried overnight, and then investigated for its next
running.
3. Results and discussions
3.1. Catalyst characterization
The powder XRD patterns of the microporous CN-Mic mate-
rial together with the parent zeolite template (MCM-22) are
shown in Fig. 1. In sharp contrast with the template, which
exhibits a highly intense diffraction peaks, the CN-Mic sam-
ple prepared via nanocasting method demonstrates broad and
weak diffraction peaks. The peak located at 2Â = 25.5◦ correspond-
ing to the (0 0 2) diffraction (interlayer of the graphitic sheets)
2.2. Samples characterization
Structural analysis of MCM-22 and CN-Mic samples was carried
out on a Bruker D8 Advance X-ray diffractometer (XRD) equipped
with a graphite monochromator, operating at 40 kV and 40 mA and
˚
with a d spacing of 3.50 A is similar to the d spacing obtained
˚
in the bulk carbon nitride (d = 3.36 A) materials reported previ-
ously [11,9]. This indicates the formation of turbostratic ordering
of carbon and nitrogen atoms in the CN graphene layers of
CN-Mic [12].
Fig. 2 shows the N2 adsorption isotherms and the corresponding
pore distribution of the parent MCM-22 sieve and CN-Mic sample.
˚
employing nickel-filtered Cu K␣ radiation (ꢀ = 1.5418 A).
Nitrogen adsorption at −196 ◦C was measured using
a
Micromeritics ASAP 2020 after the samples were degassed
(1.33 × 10−2 Pa) at 150 ◦C for 4 h. The specific surface area (SBET
)
was calculated using the Brunauer–Emmett–Teller (BET) method.
The pore diameter and pore size distribution were determined from
the Horvath–Kawazoe (H–K) method.
The X-ray photoelectron spectra (XPS) of the CN-Mic sample was
measured using a Perkin-Elmer PHI 5000C spectrometer working
in the constant analyzer energy mode with Mg K␣ radiation as the
excitation source. The carbonaceous C 1s line (284.6 eV) was used
as the reference to calibrate the binding energies (BE).
FT-IR spectrum of the sample was collected on a Bruker Tensor
spectrometer via the usual KBr pellet technique. Elementary anal-
ysis (EA) was performed with EA2400 II (PerkinElmer) instrument
to determine the carbon, nitrogen, hydrogen, and sulfur content of
the sample.
2.3. Catalytic test
Catalytic tests for Knoevenagel condensation reaction were car-
ried in a two-neck round bottomed flask (25 mL). 10 mmol of
aldehyde, 10 mmol of methylene group-containing nitrile, 0.5 mL
of n-decane as internal standard, and 5 mL of solvent (CH3CN or
n-butanol as desired) were mixed well. 100 mg of CN-Mic sample
was added into the mixture and the catalytic tests were performed
at 90 ◦C for 4 h. After the reaction, the mixture was centrifuged and
analyzed by GC–MS. The filtered catalyst was washed with ethanol
Fig. 2. N2 adsorption isotherms of MCM-22 (a) and CN-Mic (b) samples. The insert
shows the corresponding pore distribution of the samples.

J. Xu et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 105–113
107
(a)
(b)
C 1s
N 1s
O 1s
F 1s
294 292 290 288 286 284 282 280
Binding energy (eV)
1000 900 800 700 600 500 400 300 200 100
Binding energy (eV)
0
(d)
(c)
1590 1286
3423
4000 3500 3000 2500
1500
1000
500
408 406 404 402 400 398 396 394
Binding energy (eV)
Wavenumber (cm 1)
Fig. 3. XP spectra of the CN-Mic sample: survey spectrum (a), C 1s spectrum (b), and N 1s spectrum (c) and FT-IR spectrum of the CN-Mic sample (d).
According to the IUPAC classification, both isotherms are of type
I, which is characteristic of uniform microporous solids. The steep
uptake in the adsorption branch of CN-Mic below p/p◦ = 0.02 indi-
cates an existence of the microporous structures after the removal
of the template. Unlike MCM-22, the CN-Mic sample demonstrates
an apparent capillary condensation at p/p◦ > 0.2, reflecting that CN-
Mic also possesses considerable mesopores besides micropores, e.g.
interparticle voids. As pointed out by Bao et al. [13], the occurrence
of mesoporosity could be attributed to interparticle voids. Mean-
while, parts of MCM-22 remain unfilled during the polymerization
of precursors, and after HF extracting, large mesopores are created.
The surface areas and pore information of the two samples
are summarized in Table 1. It is noteworthy that, unlike those
pervasively mesoporous size distribution methods, H–K method
is derived independently from the Kelvin equation and the cal-
culation of pore distribution is based on the very low pressure
(p/p◦ < 0.01) [11,13]. As shown in the inset of Fig. 2, the pore
size of CN-Mic is 0.71 nm, larger than that of its parent template.
Moreover, the surface area and the micropore volume of CN-Mic
are 212 m2 g−1 and 0.06 cm3 g−1, appreciably lower than those of
MCM-22 (Table 1).
XPS was performed to analyze the surface information of the
CN-Mic sample. As illustrated in Fig. 3a, the main composition of
the sample are carbon and nitrogen, accompanied with a small
amount of oxygen and fluorine which may come from trace of
water and HF adsorbed on the surface of CN-Mic. The absence of
silicon or aluminum confirms that the template has been almost
thoroughly removed by NH4HF2 etching. Furthermore, detailed
resolution spectra of C 1s and N 1s were collected to further inves-
tigate the chemical bonding of the CN-Mic sample. The C 1s peak
(Fig. 4b) can be deconvoluted into four bands at 289.3, 287.5,
285.7, and 284.1 eV, respectively. The band centering at 284.1 eV
can be assigned to pure graphitic sites in a CN matrix [14,15],
while the peak at 285.7 eV can be attributed to the sp2 carbon
atom bonded to N inside the aromatic ring [16]. The highest peaks
located 287.5 and 289.3 eV suggest the formation of sp3-hybridized
and sp2-hybridized carbon in the aromatic ring attached to NH2
groups, respectively [17]. Additionally, two peaks centered at 400.2
and 397.8 eV, are observed in the N 1s spectrum (Fig. 4c), which
corresponds to the N atoms trigonally bonded to sp2 carbon in
an amorphous C N network and nitrogen sp2-bonded to carbon,
respectively [15].
Table 1
Textural properties of MCM-22 and CN-Mic samples.
Sample
Surface area
Pore size
(nm)
Micropore volume
(m2 g−1
)
(cm3 g−1
)
MCM-22
CN-Mic
357
212
0.46
0.71
0.16
0.06

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J. Xu et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 105–113
100
90
80
70
60
50
40
30
20
10
0
100
90
80
sel
sel
70
60
50
40
30
20
10
0
conv
conv
(a)
100
(b)
70
80
90
0
1
2
3
4
5
6
Reation temperature (ºC)
Reaction time (h)
Fig. 4. Effect of reaction temperature (a) and reaction time (b) on Knoevenagel condensations over the CN-Mic catalyst.
Moreover, FT-IR spectrometry was applied to analyze the bulk
is assigned to the stretching mode of N H groups in the aro-
matic ring and/or residual water molecule [12,13]. According to the
information, it can be inferred that that CN-Mic are mainly com-
prised of pyridine and benzene rings interconnected by nitrogen
atoms, in line with those reported in literatures [14,18]. In addition,
chemical information of CN-Mic. Fig. 3d shows three major bands
centered at 3423, 1590, and 1286 cm−1. The bands at 1286 and
1590 cm−1 are assigned to aromatic C N stretching bands and aro-
matic ring modes, respectively, while the broad peak at 3423 cm−1
100
100
(a)
conv
sel
(b)
conv
sel
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
0
60
80
100
1
2
3
4
5
Amount of catalyst (mg)
Running cycles
Fig. 5. Influence of amount of catalyst on the catalytic performance (a) and evolution of catalytic performance of Knoevenagel condensations during five consecutive runs
(b).

J. Xu et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 105–113
109
Table 2
Various Knoevenagel condensations catalyzed by the CN-Mic catalyst.a
Entry
Aldehyde
Nitride
Time (h)
4
Conv. (%)
87.1
Sel. (%)
95.1
CHO
CHO
H2
C
1b
NC
NC
CN
H2
C
2c
4
3
53.6
92.4
96.1
96.3
COOC2H5
CHO
H2
C
3b
NC
NC
CN
CN
Cl
CHO
H2
C
4b
6
82.1
97.4
CH3O
H2
C
5b
6b
7b
6
4
4
84.5
90.3
41.6
98.2
96.3
97.1
CHO
NC
NC
NC
NC
CN
H2
C
CHO
CHO
O
O
CN
H2
C
COOC2H5
H2
C
COOC2H5
8c
4
87.8
97.2
CHO
O
a
Reaction condition: 10 mmol of aldehyde, 10 mmol of methylene group-containing nitrile, 0.5 mL of n-decane as internal standard, 100 mg of CN-Mic. Catalytic tests were
performed at 90 ◦C.
b
5 mL of CH3CN as solvent.
5 mL of n-butanol as solvent.
c
calculated by elemental analysis (EA), the bulk composition of
CN-Mic is C3.97N1H1.61 and the amount of N (22.1 wt%) thereof is
remarkably higher than those of reported N-doped carbon mate-
rials [19] as well as that microporous carbon nitride prepared by
ZSM-5 template [13].
The effect of the amount of catalyst on the catalytic behavior
has also been evaluated (Fig. 5a). Blank experiment shows that
almost no benzaldehyde or malononitrile converts into the target
molecule in the absence of the catalyst. Upon increasing the amount
of catalyst, a remarkable improvement of the conversion is clearly
observed as expected. Meanwhile, it is also found that the selectiv-
ity is sensitive to the amount of catalyst. Taking into account of the
auto-oxidation of benzaldehyde, higher concentration of base cat-
alyst could suppress the formation of the byproduct (benzoic acid)
[20], thereby offering a spontaneous increment at the conversion
together with selectivity.
A series of repetitive experiments were performed to test the
reusability of the catalyst and reproducibility of catalytic perfor-
mance. In each cycle, CN-Mic was recovered by filtration and then
rinsed with ethanol. After drying, the catalyst was reused for the
next run. The catalytic behavior of the condensation reaction during
the first consecutive five runs is displayed in Fig. 5b. The selec-
tivities obtained in these cycles are all above 95%. In the case of
the conversion, a distinct decrease of ca 10% is observed in the
first recycling, which is mainly due to the leaching of N-containing
3.2. Catalyst activity
Initially, benzaldehyde and malononitrile are chosen as mode
substrates to examine the catalytic activity of CN-Mic for Kno-
evenagel condensation reactions. The influence of the reaction
temperature on the catalytic activity was first investigated
(Fig. 4a). With increase of temperature, the conversion of reagent
increases drastically from 70 to 90 and smoothly over 90 ◦C.
At the elevated temperature of 100 ◦C, the conversion is up to
93.6%. On the other hand, the selectivities to the products (2-
benzylidenemalononitrile) at different reaction temperatures are
all around 95%. The byproduct is mainly benzoic acid that ori-
ginates from the auto-oxidation of benzaldehyde under ambient
atmosphere. Fig. 4b plots the conversion and selectivity as a func-
tion of reaction time. At 0.5 h, the reaction processes with a
moderate conversion of 56%. Upon prolonging the reaction over
4 h, the conversion reaches its maximum. Regarding the effect
of reaction conditions on their corresponding catalytic perfor-
mance, the temperature of 90 ◦C and reaction time of 4 h were
chosen as optimal reaction conditions for the further catalytic
investigation.
fragments ( NH2 or
N ) located at the brink of the graphitic lay-
ers [9,18]. In spite of the initial decline, the subsequent running
tests present a stable catalytic activity, implying the CN-Mic can
be served as a heterogeneous and stable catalyst for Knoevenagel
condensation.
To probe the versatility of CN-Mic for Knoevenagel condensa-
tion reaction, various aldehyde and methylene group-containing

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J. Xu et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 105–113
nitride with
a
molar ratio of 1:1 in the presence of CN-
ability of the substituent carbonyl group is weaker than that of the
CN group. However, a high yield could be retrieved if the reaction
was carried out under a protic solvent such as n-butanol (entry 8).
For a comparative purpose, we have also prepared three meso-
porous g-CN materials adopting SBA-15, mesoporous cellular silica
foam (MCF), and commercial nanosized silica sphere (∼15 nm) as
template. The preparation routes and the corresponding textural
properties of synthesized CN-SBA-15, CN-MCF, and CN-SiO2-15 nm
materials are given in the supporting information. As depicted
in Fig. 6, CN-Mic demonstrates superior catalytic conversion to
its counterparts under the same catalytic condition. The order
of the conversion of the four g-CN catalysts is as follows: CN-
Mic > CN-MCF > CN-SBA-15 > CN-SiO2-15 nm. Considering this fact,
Mic were studied and the results are summarized in Table 2.
The reactions between benzaldehyde/4-chorobenzaldehyde/4-
methoxybenzaldehyde and malononitrile afforded high yields of
the corresponding products (entries 1, 3 and 4). Besides aromatic
aldehyde, aliphatic aldehyde such as pentanal, and furfural could
undergo the condensation with malononitrile, giving a final prod-
uct with high efficiency (entries 5 and 6). It is worthy to note
that furfural is a typical biomass-derived compound. Furthermore,
different active methylene compounds were compared for Knoeve-
nagel condensation at the same catalytic conditions. As listed in
Table 2, ethyl cyanoacetate presents less reactivity than malonon-
itrile (entries 2 and 7), possibly because the electron withdrawing
Fig. 6. N2 adsorption–desorption isotherms and the corresponding pore size distributions of mesoporous CN-SBA-15, CN-MCF, and CN-SiO2-15 nm samples.

J. Xu et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 105–113
111
N
N
N
N
N
N
N
N
NH2
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NH
N
N
N
(b)
(a)
Scheme 1. Proposed structures of g-C3N4 and g-CN based on the building blocks of tri-s-triazine (a) and N-substituted graphitic rings or cross-linking of graphitic C N (b),
respectively.
LB
CN-Mic
H2O
H
H
C
+
NC
CN
N
N
NH2
N
N
N
N
N
CN
H
C
C
CN
N
N
N
NH
N
N
H
C
H
H
NC
CN
LB
LB
CN-Mic
CN-Mic
O
O
CN
CN
C
H
C CH
H
Scheme 2. A possible reaction mechanism for Knoevenagel condensation catalyzed over the CN-Mic material.
the catalytic activity for Knoevenagel condensation is necessarily
associated with the porous structures (Fig. 6 and Table 3) of the
g-CN materials.
photocatalyst for water splitting [28], degradation of pollutants
[29], etc. Alternatively, the other species of g-CN materials posses
a substantially higher content of carbon, which makes the solids
pure black [13,30]. Typically, the precursors used for synthesis of
the material are CTC) and EDA [31]. Unlike those of yellow g-C3N4,
the motifs of the second species of g-CN materials are speculated as
Bearing in the mind that the condensation reaction is catalyzed
in the presence of base catalyst, to gain further insight into the
base nature of the four porous g-CN samples above in relation
to their corresponding catalytic activity, the N/C molar ratios are
listed in Fig. 7. Although CN-SBA-15 and CN-MCF with a pore size
of 4–5 nm have higher total N/C ratios (calculated by EA), the con-
versions are moderate, approximately 60%. However, it is found
the catalytic activity correlates well with the corresponding super-
ficial N/C ratios (based on XPS profiles). This points to that the N
species located over the surface, rather than bulk, of g-CN con-
tributes directly to the catalytic activity.
N-substituted graphitic rings and/or cross-linking of graphitic C
planes (Scheme 1b) [31].
N
Table 4 summarizes the catalytic performances of Knoevenagel
condensation reactions over various g-CN materials; the informa-
tion has covered almost all recently reported results involving the
g-CN-catalyzed Knoevenagel reactions. Mpg-C3N4 (entries 1 and
2) and MCN (entry 3) were prepared by thermal condensation
In general, the reported g-CN materials can be categorized
into two groups. The first species of g-CN are prepared by ther-
mal condensation using cyanamide [21,22], dicyandiamide [23]
or melamine [24] as precursor, and the C/N ratios of such com-
pounds are close to the theoretical ratio of g-C3N4 (0.75). As
suggested by Kroke et al. [25], the tectonic units of 2D ꢁ-conjugated
g-C3N4 sheets are tri-s-triazine (Scheme 1a) rings linked by N
bridges. Featured with a band gap of 2.7 eV (consistent with yel-
low color) [26,27], g-C3N4 has been intensively used as a robust
Table 3
Textural properties of mesoporous CN-SBA-15, CN-MCF, and CN-SiO2-15 nm
samples.
Sample
Surface area
Pore size
(nm)
Pore volume
(m2 g−1
)
(cm3 g−1
)
CN-SBA-15
CN-MCF
CN-SiO2-15 nm
753
440
475
3.9
5.2
12.7
1.13
0.72
1.11

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J. Xu et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 105–113
Table 4
Comparison of the catalytic performances of Knoevenagel reactions catalyzed over g-CN materials.
Entry
Catalyst
Amount of aldehyde (mmol)
Temperature (◦C)
Time (h)
Con. (%)
Assisted treatment/condition
Ref.
1
2
3
4
5
6
7
8
Mpg-C3N4
Mpg-C3N4
MCN
mp-PBI
[CTA]-MCM-41
CuBTC
1
1
1
1
4.6
N.A.
10
10
70
70
120
100
30
130
90
90
2
2
0.3
6
1
25
4
33
80
87
100
5.9
100
87
–
[20]
[20]
[24]
[32]
[33]
[34]
This work
This work
Deprotonation
Microwave
–
–
–
–
–
CN-Mic
Bulk CN
4
19
using cyanamide and melamine as precursor, respectively, and the
materials belong to the first species of g-CN, as noted previously.
Although the yellow g-C3N4 materials possess a large quantity
of N atoms, there is no electron localization in ꢁ state in per-
fect carbon nitride sheets [26]. Indeed, when the g-C3N4 materials
are employed as catalysts for base-catalyzed reactions, only sur-
face defects of terminal amino groups and bridging N atoms [20]
(originated from incompletely condensation) could serve as Lewis-
base-type active sites. Under this circumstance, the pristine g-C3N4
materials need assisted treatment or special reaction condition,
such as deprotonation and microwave irradiation to inspire their
catalytic potential.
On the other hand, the black g-CN materials fabricated using
CTC and EDA as precursor could be used readily as catalysts for
Knoevenagel condensation reactions. Compare with bulk CN sam-
ple fabricated without template (entry 8), microporous CN-Mic
catalyst (entry 7) provides a considerably higher conversion of alde-
hyde, which could be owing to the introduction of porous structures
as well as accessible surface area. In addition, other base catalysts,
including mesoporous poly(benzimidazole) (mp-PBI, entry 4), N-
containing MCM-48 ([CTA]-MCM-41, entry 5), and metal organic
frameworks (CuBTC, entry 6) for Knoevenagel condensation as a
base catalyst is also listed in Table 4. Taking into account the final
conversions and catalytic conditions, the catalytic activity of CN-
Mic is obviously superior to that obtained in any other catalyst.
Scheme 2 depicts a possible mechanism for Knoevenagel
condensation catalyzed over the microporous carbon nitride.
The active sites of the catalyst, CN-Mic, can be attributed to
Lewis-base-type nitrogen species located in the CN matrixes,
as discussed in the XPS and FT-IR characterization. First, an
acidic proton is abstracted from the active methylene group by
a Lewis-base site [35], yielding a stable anion. Spontaneously,
the original base site is transformed into a protonated site
(conjugate acid). Subsequently, this anion makes nucleophilic
attack to the carbonyl-carbon atom of the aldehyde, forming an
oxyanion. After abstracting H+ from the protonated site of CN-Mic
and eliminating a molecule of H2O, the oxyanion turns into the cor-
responding condensation product. Meanwhile, the CN-Mic catalyst
is regenerated.
4. Conclusion
In summary, microporous carbon nitride, CN-Mic has been
prepared using MCM-22 sieve as template and investigated as
a heterogeneous base catalyst for Knoevenagel condensation.
The catalytic results exemplified by various kinds of substrates
have shown that the CN-Mic sample demonstrates efficient activ-
ity. The yield obtained was over 80% and the catalyst can
be reused at least five runs without appreciable loss in cat-
alytic activities. The catalytic active sites could be attributed to
Lewis-base-type nitrogen species. We envision that the study
constitutes an insight for further applications of porous car-
bon nitride materials toward metal-free and heterogeneous
base organocatalysis, e.g. aldol-condensation between biomass-
derived aldehyde and ketone, and synthesis of biodiesel via
transesterification.
Acknowledgments
This work was supported by National Natural Science
Foundation of China (21203014), CNPC Innovation Foundation
(2011D-5006-0508), Open Foundation of Shanghai Key Laboratory
of Molecular Catalysts and Innovative Materials attached to Fudan
Univ. (2011MCIMKF01), Open Foundation of Jiangsu Key Labo-
ratory of Fine Petrochemical Engineering attached to Changzhou
Univ. (KF1201), and the Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions. Jie
Xu also thanks Prof. Wei-lin Dai, Fudan Univ. for his kind help in
the characterization of XPS.
100
80
60
40
20
0
0.30
0.25
0.20
0.15
0.10
0.05
EA
XPS
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2013.02.019.
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