Preparation of mesoporous carbon nitride materials using urea and formaldehyde as precursors and catalytic application as solid bases

Article abstract of DOI:10.1016/j.apcata.2017.03.035

A series of mesoporous carbon nitride materials have been fabricated using inexpensive and eco-friendly urea and formaldehyde as precursors and mesocellular silica foam (MCF) as a template through a nanocasting approach. Several techniques, including XRD, TEM, elemental analysis, FT-IR, XPS, and CO₂-TPD have been applied to characterize the physicochemical properties of the mesoCN materials, and the results show that the materials possess high surface areas (331–355?m²?g^?1), relatively concentrated pore size of ca. 6?nm, and abundant and multiple nitrogen-containing species. As heterogeneous base catalysts, mesoCN materials demonstrate high catalytic activity and selectivity in both Knoevenagel condensation and transesterification reactions.

Full text of DOI:10.1016/j.apcata.2017.03.035

Accepted Manuscript
Title: Preparation of mesoporous carbon nitride materials
using urea and formaldehyde as precursors and catalytic
application as solid bases
Authors: Jie Xu, Yue Wang, Jie-Kun Shang, Dan Ma,
Yong-Xin Li
PII:
S0926-860X(17)30144-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.03.035
APCATA 16194
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
19-10-2016
16-3-2017
26-3-2017
Please cite this article as: Jie Xu, Yue Wang, Jie-Kun Shang, Dan Ma, Yong-Xin
Li, Preparation of mesoporous carbon nitride materials using urea and formaldehyde
as precursors and catalytic application as solid bases, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.03.035
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Manuscript title:
Preparation of mesoporous carbon nitride materials using urea and
formaldehyde as precursors and catalytic application as solid bases
Authors’ names:
Jie Xu *, Yue Wang, Jie-Kun Shang, Dan Ma, Yong-Xin Li *
Authors’ affiliation:
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of
Petrochemical Engineering, Changzhou University, Gehu Road 1, Changzhou,
Jiangsu 213164, PR China
Corresponding authors:
Dr. Jie Xu
Tel.: +86-519-86330135; E-mail: shine6832@163.com
Prof. Yong-Xin Li
Tel.: +86-519-86330135; E-mail: liyxluck@163.com
1

Graphical abstarct
Highlights

Mesoporous carbon nitride prepared using eco-friendly urea and formaldehyde as
precursors


Molar ratio of carbon to nitrogen in mesoporous CN samples is ca. 3.4
Good activity in various Knoevenagel condensation and transesterification
reactions
Abstract:
A series of mesoporous carbon nitride materials have been fabricated using
inexpensive and eco-friendly urea and formaldehyde as precursors and mesocellular
silica foam (MCF) as a template through a nanocasting approach. Several techniques,
including XRD, TEM, elemental analysis, FT-IR, XPS, and CO
2
-TPD have been
applied to characterize the physicochemical properties of the mesoCN materials, and
2
−1
the results show that the materials possess high surface areas (331–355 m g ),
relatively concentrated pore size of ca. 6 nm, and abundant and multiple
nitrogen-containing species. As heterogeneous base catalysts, mesoCN materials
demonstrate high catalytic activity and selectivity in both Knoevenagel condensation
and transesterification reactions.
Keywords: Carbon nitride; Solid base; Knoevenagel condensation; Transesterification
2

1
. Introduction
Chemical transformation promoted by base catalysis holds an important position
in the organic and synthetic chemistry [1]. In particular, C–C bond formation
reactions, represented by Knoevenagel condensation, and transesterification, are
extensively applied in the production of numerous chemicals including drugs,
fragrances, and chemical intermediates [2]. The conventional base catalysts are
mainly organic nitrogen-containing compounds (including pyridine, piperidine, etc) [2,
3
] and alkali metal hydroxides (e.g. KOH and NaOH) [4, 5]. However, the major
disadvantage lies in the difficulty in catalyst/product separation as well as catalyst
recycling due to their homogeneous nature. On the other hand, although
heterogeneous metal oxides (such as Mg-Al-O and MgO) exhibit outstanding catalyst
recyclability, there is still potential metal contamination in reaction system [3, 6].
To circumvent the issue, a series of homogeneous organic bases have been
recently immobilized on mesoporous silicas [7-9] and graphene [10]. Owing to their
high surface areas along with tunable pore sizes, such heterogeneous materials show
excellent catalytic activity in a variety of organic reactions, and thus have been
regarded as promising solid base catalysts. Unfortunately, the immobilization
demands high-cost silicane coupling agent and the procedure is relatively complicated
[11]. In this context, in viewpoints of both fundamental research and industrial
application, it is of very interest to develop a new and efficient solid base catalyst.
The past decade has witnessed the robust development of graphitic carbon nitride
(
g-CN) in multiple research fields. Up to now, g-CN and its based materials
demonstrate promising application in photocatalysis [12-14], fuel cells [15, 16],
adsorption [17, 18], and heterogeneous catalysis [1, 19-21]. Of particular note is that
metal-free g-CN contains abundant and various nitrogen-containing species, which
naturally make it to be a typical solid base [3, 22, 23]. More importantly, compared
with the bulk g-CN samples prepared by direct pyrolysis, mesoporous g-CN materials
possess higher surface areas and much more accessible pores, thus upgrading the
catalytic performances of g-CN [24, 25]. Recently, many research groups including
ours have successfully prepared several mesoporous g-CN materials using various
3

precursors [1, 3, 23, 26, 27]. In several base-mediated reactions, it is unambiguous
reported that such g-CN materials exhibit good catalytic activity and recycling ability.
Despite this success, it should be pointed out that the raw materials adopted for the
synthesis of g-CN materials, even reported in other application, are mainly cyanamide,
and carbon tetrachloride and ethylenediamine [28, 29]. These compounds are either
expensive or highly toxic.
In this work, urea and formaldehyde have been used as cheap and widely
available precursors to synthesize mesoporous g-CN (mesoCN) through a
hard-templating method. In Knoevenagel condensation reaction between
benzaldehyde/acetone and malononitrile, and transesterification reactions of β-keto
esters, the mesoCN samples reveal good catalytic activity and selectivity. Finally, the
catalytic performance and catalyst preparation of mesoCN have been compared with
other reported mesoporous CN based materials.
2
2
2
. Experimental
.1. Catalyst preparation
.1.1. Preparation of MCF material
Mesocellular silica foam (MCF) was prepared using tetraethyl orthosilicate
(TEOS) as a precursor and triblock copolymer P123 (EO20PO70EO20) as a soft
template [30]. Briefly, 4 g of P123 was completely dissolved into 150 mL of HCl
−1
solution (1.6 mol L ), followed by addition of 4.0 g of 1,3,5-trimethylbenzene and 46
mg of NH F. The mixture was vigorously stirred for 1 h at 40 °C. Subsequently, 8.8 g
4
of TEOS was added, and the mixture was further stirred for another 20 h at 40 °C. The
obtained suspension was transferred into an autoclave lined by Teflon, and statically
heated at 130 °C for 24 h. Afterwards, the precipitate was filtered off, dried overnight,
and finally calcined at 550 °C for 5 h.
2
.1.2. Preparation of mesoCN materials
7 mmol of urea, 77 mmol of formaldehyde (35wt%, a.q.), and 5 mL of deionized
7
water were mixed well. 1.0 g of previously dried MCF powder was added into the
solution and stirred for 1 h, and the mixture was then dried at 100 °C for another 9 h
in an oil bath. The obtained white composite was calcinated from room temperature to
4

−1
6
00 °C with a ramping rate of 2 °C min and hold at the temperature for 3 h under N
2
atmosphere. After the carbonization, the black solid was immersed into 100 mL of
−1
NaOH (0.5 mol L ) and stirred at 80 °C for 24 h. Afterwards, the dispersion was
centrifuged and the black precipitate was rinsed by water (until the pH value of the
dispersion reached ca. 7.0 in order to remove any possible residual NaOH) and
ethanol for several times. The obtained material was dried at 60 °C under vacuum for
2
h and the resultant solid was labeled as mesoCN-r, where r indicated the molar ratio
of urea to formaldehyde. The detailed amounts of urea, formaldehyde, and MCF for
various mesoCN-r materials were provided in Table S1.
2
.1.3. Preparation of CN/MCF material
16 mmol of urea, 77 mmol of formaldehyde (35wt%, a.q.) solution, and 5 mL of
1
deionized water were mixed well. 1 g of previously dried MCF powder was added
into the solution and stirred for 1 h, and the mixture was then dried at 100 °C for
another 9 h in an oil bath. The obtained white composite was calcinated from room
−1
temperature to 600 °C with a ramping rate of 2 °C min and hold at the temperature
for 3 h under N atmosphere. The obtained black sample was labeled as CN/MCF-1.5.
.2. Sample characterization
X-ray diffraction (XRD) patterns of the mesoCN materials were analyzed by a
Rigaku D/max 2500 PC X-ray diffractometer using Ni-filtered Cu-Kα radiation (λ =
.5418 Å). Nitrogen adsorption–desorption isotherms were measured using a
2
2
1
Micromeritics ASAP 2020 analyzer. Prior to the analysis, the materials were degassed
at 150 °C for 4 h. The specific surface areas were calculated according to the
Brunauer–Emmet–Teller (BET) method and the pore size distribution was determined
by Barret–Joyner–Halenda (BJH) model. Fourier transform infrared (FT-IR) spectra
were recorded by a Bruker Tensor 27 spectrometer. The mass ratio of each sample to
KBr was constant at 1:200. X-ray photoelectron spectra (XPS) were analyzed by a
Thermo ESCALAB 250XI spectrometer using Mg K
source. Elementary analysis (EA) was tested by an Elementar Vario EL III analyzer.
CO temperature-programmed desorption (CO -TPD) was performed on a
α
radiation as the excitation
2
2
Quantachrome ChemBET-3000 analyzer. Prior to analysis, the sample was heated at
5

3
00 °C for 1 h under helium gas and then cooled down to the room temperature.
.3. Catalytic evaluation
2
Knoevenagel condensation reactions were performed in a two-neck round
bottomed flask (25 mL). 5 mmol of benzaldehyde, 5 mmol of malononitrile, 0.4 mL
of n-dodecane, and 5 mL of acetonitrile as a solvent were mixed well. 75 mg of
catalyst was added into the mixture and the reactions were carried out at 90 °C. After
each hour, a small quantity of the reaction mixture was collected, centrifuged and
analyzed by GC equipped by a FID detector. Another Knoevenagel condensation
reaction was carried out using 10 mmol of acetone and 10 mmol of malononitrile as
reactants.
Transesterification reactions of β-keto ester were conducted in a two-neck round
bottomed flask. 15 mmol of ethylacetoacetate (EAA), 10 mmol of alcohol, 0.2 mL of
n-dodecane, and 3 mL of toluene as a solvent were mixed well. 50 mg of catalyst was
added into the mixture and the test was performed at 100 °C. Like Knoevenagel
condensation, the catalytic conversions of transesterification were analyzed by GC.
3
. Results and discussions
3
.1. Catalyst characterization
Fig. 1 shows the XRD patterns of various mesoCN materials. Each sample
demonstrates a pronounced diffraction peak at ca. 2θ = 25.2 ° with a d-spacing value
of 0.353 nm, and a shoulder located at ca. 42 °. The two peaks correspond to the
reflection of (002) and (100) planes of disordered graphitic carbon [3, 31],
respectively. The assignments of the two diffraction peaks are identical with those
reported in mesoporous g-CN prepared using tetrachloride and ethylenediamine as
raw materials [11, 24, 32]. From the XRD patterns, it can be found that adjusting the
molar ratio of urea to formaldehyde from 0.67 to 1.5, the overall graphitic structure of
the materials shows no apparent variation.
The porous properties of the materials were investigated by N
adsorption–desorption characterization. All the samples present apparent type-IV
isothermal curves (Fig. 2A), along with H2 hysteresis loops in the range of p/p
.6–0.95, indicating that the materials possess typical mesoporous structures.
2
0
=
0
6

However, the adsorption curves still rise in the higher relative pressure (p/p > 0.95).
0
This suggests that besides mesopores, there also exist some macroporous structures in
the mesoCN materials, which may be attributed to the intraparticle void. The
corresponding pore size distribution determined from desorption branches of
isotherms shows that the pore size of each sample is relatively narrow, centered at ca.
6
nm. The surface areas and porous parameter are listed in Table 1. The surface area
2
−1
3
−1
and pore volume of mesoCN-0.67 are 331 m g and 0.38 cm g , respectively. It
can be found that the surface area and pore volume of mesoCN materials depend on
the molar ratio of urea to formaldehyde. That is, as the content of urea is increased,
the surface area and pore volume of the final mesoporous materials are improved
gradually whereas the pore size shows a monotonous decline from mesoCN-0.67 to
mesoCN-1.5.
The reason for this relation between textural parameter and molar ratio of
precursors is probably the effect of pore filling in the nanocasting approach. In this
work, the precursors used for the fabrication are urea and formaldehyde. When the
ratio of urea to formaldehyde (a.q.) is low, the precursor mixture actually contains a
lot of water originating from the formaldehyde solution. Although the liquid-phase
precursors could completely fill into the mesopores of MCF template, there remain
partial solid-phase precursors in the mesopores after drying procedure. This
uncompleted filling style, due to the presence of a certain amount of solvent,
inevitably brings out potential collapse of carbon-nitride walls during the subsequent
etching step [33]. The collapse or shrinkage of walls might damage the partial
architecture of mesopores and then result in a decline of surface area or pore volume.
However, as the molar ratio of urea to formaldehyde is elevated, the content of
original water becomes less, thereby facilitating sufficient filling in the mesopores of
MCF, and finally alleviates the decrease of surface area and pore volume. The similar
phenomenon, i.e. the impact of ratio of precursors on the textural parameter of
negative replica during nanocasting synthetic way, has also been reported by Vinu et
al [34]. Wherein, the textual properties including surface areas, and pore volumes of
mesoporous carbon nitride materials have a close relation to the molar ratios of
7

precursors. Consequently, upon adjusting the molar ratio of the precursor, a series of
mesoporous materials with tunable textural parameters could be obtained.
The small-angle XRD (SAXRD) patterns of MCF and mesoCN materials are
provided in Fig. S1. No apparent diffraction peak has been detected in MCF as MCF
itself is foam-like disordered mesoporous silica [35]. The morphology of the materials
was observed by TEM. The TEM image of the MCF template (Fig. 3A) reveals a
disordered arrangement of polygonal pores (20–30 nm), which is characteristic
structure of the MCF material. Also, the pore size detected in the image agrees well
with the result of N adsorption–desorption characterization (Table 1 and Fig. S2). In
2
the case of mesoCN, both the images (Fig. 3B) of mesoCN-1.0 and mesoCN-1.5
exhibit irregular foam-like structure. Accordingly, like MCF, the mesoCN-1.5 material
also shows no obvious diffraction peak in the SAXRD pattern (Fig. S1). However, at
the edges of the samples, there present polygonal-like slices and mesopores between
them. Obviously, the mesoCN materials have negatively replicated the original
mesostructures of MCF. Meanwhile, the corresponding selected area electron
diffraction (SAED) pattern (the inset) shows clear diffraction rings. This confirms that
the present mesoCN materials own at least partly graphitic structures, consistent with
the above XRD characterization result.
In order to probe their chemical functionality, the mesoporous materials were
characterized by FT-IR and the transmittance spectra are depicted in Fig. 4. It is
evident that the transmittance for each spectrum is dominated mainly in two regions,
−
1
i.e. 3500–3200 and 1600–1200 cm . The first region is attributed to the stretching
vibration of N–H, and O–H bonds of adsorbed water. The other two sharp bands at
1
570 and 1295 cm−1 are indexed as N–H deformation modes, and C–N stretching
−
1
vibration, respectively. In addition, a minor band has also been detected at 2202 cm ,
which is associated with the –N=C=O vibration. In this study, mesoCN materials were
prepared using urea and formaldehyde as raw materials. As presumed by Liu et al [31],
the two precursors react via a series of self-condensation reactions in the low
temperature (~100 °C) and transform into urea-formaldehyde resin. Wherein, the
motif unit for the polymer is mainly amide (–CO–NH–) groups. During the
8

subsequent calcination (~600 °C), the as-synthesized resin undergoes cross-linking
stage and final carbonization. Given in this synthetic propose, the above various
chemical functionalities, especially C–N groups, can be explained for the mesoCN
samples. Despite the resemblance in main bands, the intensity of transmittance
−1
(
especially the band bands located in 3500–3200 cm ) seems be related to the molar
ratios of the precursors of mesoCN. Naturally, the mesoCN samples prepared by using
higher content of urea might possess much more nitrogen-containing species, which
are then reflected in the nitrogen-related bands of FT-IR spectra.
The chemical composition of mesoCN materials was measured by EA
characterization. As listed in Table 1, the calculated chemical formula based on the
values of C/N/H element for mesoCN-0.67 is C3.68NH2.00. Further comparing the
chemical compositions of various mesoCN specimens, it can be seen that the molar
ratio of the precursors has significant impact on the content of N element; with the
increase of the amount urea fed, the content of N increases progressively. Obviously,
the correlation originates from the fact that urea itself serves as the sole N source in
the synthesis of mesoCN.
XPS technique was further employed to analyze the surface chemical
environment of the materials. For concise, Fig. S3 provides the survey of mesoCN-1.0
and mesoCN-1.5 materials. Each mesoCN sample was constituted by C, N, and O
elements. Wherein, the O element is derived from the adsorbed water on the surface
of materials, as also detected in the abovementioned FT-IR result. The absence of Si
confirms that the original MCF siliceous template has been thoroughly eliminated by
NaOH solution. Based on the peak area integrated in Fig. S3, the chemical
compositions for mesoCN-1.0 and mesoCN-1.5 materials are C3.41NO0.28 and
C2.85NO0.22, respectively (note: the hydrogen element cannot be detected by XPS).
Apparently, the mesoCN materials own higher concentration of N element on their
surface than in the bulk phase (Table 1).
To probe the chemical bonding information, deconvolution of detailed C 1s and N
1
s spectra was performed. As depicted in Fig. 5A, the C 1s spectra of mesoCN-1.0
and mesoCN-1.5 can be divided into four independent peaks. The primary peak with
9

the lowest binding energy at 284.7 eV is assigned to graphitic carbon atoms, i.e. sp2
C–C bond [26]. The peaks located at 286.1 and 287.4 eV are indexed as sp2 C=N and
sp3 C–N signals [36], respectively. The last broad peak with the highest binding
energy signifies the presence of C–O groups. The multiple carbon bonding states are
consonance with the abovementioned FT-IR spectra. In the case N element in
mesoCN (Fig. 5B), it can be also divided into four species. The majority is attributed
to the N atoms existing in sp3 C–N groups, as reflected by the major peak with a
binding energy of 398.2 eV. The second type of N element owns an energy
contribution at 400.2 eV, corresponding to sp2 C=N groups [36]. The weak peak at
4
02.0 eV is associated with the N atoms bonded with O (i.e. N–O), whereas the last
peak is assigned with quaternary N species or charging effect of XPS [37]. The above
deconvolution results of both C and N 1s spectra agree well with the results reported
by Park [36] and Liu [31].
Since all the FT-IR, EA, and XPS characterization results have verified that the
present mesoCN materials have abundant and various nitrogen-containing species, it
is reasonably deduced that such materials are typical solid bases. Concerning this
point, CO
described in Fig. 6, the corresponding CO
broad desorption peaks mainly in the range of 150–400 °C. In order to understand the
detailed base distribution of the solid bases, the desorption peaks of CO -TPD has
2
-TPD was applied to study the basicity of the mesoCN materials. As
2
-TPD curves of mesoCN materials exhibit
2
been deconvoluted. The desorption curve for each material can be separated into three
independent peaks. The first peak with the lowest desorption temperature at ca. 125


C (Peak I) is associated with the physical adsorption, while the peak at ca. 200–250
C (Peak II) is attributed to weak chemical adsorption that can be assigned to weak
basic sites on the surface of the sample. The desorption peak centered at ca. 325 C
Peak III) is ascribed to relatively strong chemical adsorption associated with the
medium-strength basic sites. In the case of CN/MCF-1.5, the predominant desorption
of CO occurs at the ca. 210 C. In sharp comparison, the peak position pertaining to
(
2
the medium-strength base moves towards higher temperatures with respect to
mesoCN samples, signifying that mesoCN materials owns superior basic intensity to
10

the supported CN/MCF-1.5 material. From mesoCN-0.67 to mesoCN-1.5, the Peak II
shows no apparent change whereas the intensity of Peak III undergoes substantial
improvement. According to the titration experiment of CO
calculation based on Peak III, the quantity of medium-strength base for mesoCN-1.0,
and mesoCN-1.5 is ca. 48, 72, and 115 μmol CO
catal−1, respectively. Therefore, it
2
and subsequent quantity
2
g
can be inferred that, the quantity of medium-strength basic sites can be smoothly
tuned by adjusting the molar ratio of urea to formaldehyde. More specifically, as the
amount of nitrogen-containing urea is increased, the basic quantity of the mesoCN
materials is enhanced progressively. Additionally, the variation in basic quantity
coincides well with the content of nitrogen element (Table 1) of mesoCN materials.
3
.2. Catalyst performance
Initially, Knoevenagel condensation between benzaldehyde and malononitrile
was selected as a base-mediated reaction to examine the catalytic performance of
mesoCN. A blank test without any catalyst was first performed under 90 °C and the
result (Table 2) show the catalytic conversion of benzaldehyde was only 4% at 5 h.
Applying mesoCN-0.67 as a catalyst instead, the conversion increases up to 32%.
Concerning various mesoCN materials, as the molar ratio of urea to formaldehyde
increases, the final catalytic activity increases progressively. The highest catalytic
conversion is achieved over mesoCN-1.5, affording
a
yield of
2
-benzylidenemalononitrile as much as 78% at 5 h. In this study, we have also
prepared MCF-supported CN catalyst (CN/MCF-1.5) using urea and formaldehyde as
starting materials, and the corresponding textural information and chemical
composition are summarized in Table 1. Fig. S2 indicates that CN/MCF-1.5 has a
remarkable adsorption of N , together with a large pore size of ca. 20 nm. Also, the
2
main chemical functionality of CN/MCF-1.5 (Fig. S4) is very similar with that of
mesoCN-1.5. Although the CN/MCF-1.5 sample has higher surface area and larger
pore volume than mesoCN-1.5 (Table 1), under the same reaction conditions, the
catalytic conversion of benzaldehyde obtained over CN/MCF-1.5 is 36%, much lower
than that of its counterpart. Since Knoevenagel condensation is a base-mediated
reaction, a catalyst having more content of nitrogen species especially along with
11

higher quantity of active basic sites (Fig. 6) could naturally provide superior activity.
As illustrated above, from mesoCN-0.67 to mesoCN-1.5, the quantity of base
increases monotonously, which agrees well with the variation in catalytic conversion
received over various mesoCN materials. As far as the supported CN/MCF-1.5
material, despite its comparable surface area, either the content of nitrogen element or
basic intensity is limited. In this sense, it can be speculated that the essential active
sites of mesoporous CN materials in Knoevenagel condensation are medium-strength
basis sites.
The mesoCN-1.5 sample was chosen as a reference catalyst for further catalytic
evaluation. The influence of reaction time and catalyst weight on the catalyst
performance has been investigated subsequently (Fig. 7). At the first 1 h, the
conversion of benzaldehyde and selectivity to 2-benzylidenemalononitrile are 48%
and 98%, respectively. Prolonging the reaction, the selectivity presents no clear
change, whereas the conversion elevates gradually but levels off after 5 h. Likewise,
the catalytic performance is observed to be dependent on the catalyst amount. The
selectivity gained without any catalyst is ca. 81%, probably due to the oxidation of
benzaldehyde with ambient oxygen. Upon feeding the catalyst, the conversion
increases drastically. As the catalyst weight is up to 100 mg, the reaction offers a best
yield of 90%. In addition, the reproducibility of mesoCN-1.5 was examined by
consecutive catalytic runs. After each run, the catalyst was rinsed by ethanol for at
least three times and then dried in a vacuum oven for 2 h. As shown in Table 2, the
catalytic conversion acquired on mesoCN-1.5 is ca. 77%, together with the same
selectivity (96%), indicating that the mesoCN catalyst could provide stable catalytic
activity. Indeed, according to the various characterization results including XRD, N
2
adsorption–desorption, FT-IR, EA, and N 1s XPS (Fig. S5), the structure, textural
properties, chemical compositions, and nitrogen species states of regenerated sample
after five catalytic runs reveal no significant change. Therefore, the stable
physicochemical properties of mesoCN materials are responsible for their high
tolerance and recyclability in liquid-phase heterogeneously catalytic reaction.
Besides the Knoevenagel condensation reaction between benzaldehyde and
12

malononitrile, a condensation between acetone and malononitrile has also been
carried out using mesoCN-1.5 as a heterogeneous catalyst. Fig. 8 illustrates the
resultant catalytic performance as a function of time. The catalyst demonstrates
apparent activity even at 50 °C, and the temperature has an apparent positive effect
over the activity of mesoCN. Regarding the product selectivity, the target product is
isopropylidenemalononitrile whereas the byproduct is solely isobutenyl methyl ketone,
which is derived from self-aldol condensation of acetone catalyzed by basic mesoCN
material. With the increase of the temperature, the catalytic conversion increases
monotonously at the same reaction time. Meanwhile, it can be observed that the
selectivity decreases under higher temperatures, evidencing that mesoCN could also
promote aldol condensation besides Knoevenagel condensation. Under 90 °C, the
catalytic conversion of acetone reaches ca. 80% and the selectivity is 81% at 3 h of
reaction time.
Following the Knoevenagel condensation reactions, we have also carried out a
series of transesterification reactions between a β-keto ester (EAA) and alcohols. The
transesterification reaction of β-keto ester is a practical transformation to synthesize
complex β-keto esters starting from simple β-keto esters [38]. The conventional
catalysts are mainly homogeneous organic bases. Herein, the mesoCN-1.5 was
subjected to transesterification reactions to further testify its basic activity. As
displayed in Fig. 9, for a simple n-butanol, the material exhibits a high conversion of
6
2% under 100 °C at 6 h (The selectivity to the target complex ester is 100%).
Moreover, in addition to n-butanol, various alcohols, including aliphatic and aromatic
alcohols, can be smoothly converted by the catalyst under the identical conditions.
For comparison, the catalytic performances of various mesoporous CN materials
for transesterification have been summarized in Table 3. To the best of our knowledge,
it covers almost all the previously reported relevant catalyst. It is worth noting that
their activity is determined all based on the conversions of alcohols. Under similar
reaction conditions (including temperature and catalyst amount), the catalytic activity
13

received on mesoCN-1.5 is slightly lower than those obtained over other mesoporous
materials, although it owns relatively higher nitrogen content to its counterparts. We
think that the difference in catalytic activity of CN materials is not merely determined
by their nitrogen concentration. For a mesoporous solid base in catalytic application,
the contribution of textual properties (including surface area and pore volume) also
can substantially improve the final catalytic activity, because a solid base with higher
surface area could expose much more accessible active sites in a heterogeneously
catalytic reaction. In fact, in the case of textural nature, the surface areas of the
2
−1
present mesoCN samples are ca. 331–355 m g , while the values for other
2
−1
mesoporous carbon nitride materials are almost higher than 500 m g , along with
3
−1
larger pore volume (ca. > 0.65 cm g ). Therein, the higher catalytic conversion
obtained over other mesoporous carbon nitride samples should be attributed to their
large surface areas and pore volumes than the present mesoCN materials.
Notwithstanding, it should be pointed out that, the most advantage of the present
mesoCN comparable to other materials lies in its preparation. As mentioned above,
the precursors adopted for the mesoporous CN material are generally cyanamide, and
CTC and EDA. Although these compounds are classical raw materials, it should be
stressed that they are highly toxic, and meanwhile the etching procedure to eliminate
silica templates involves hazardous and volatile HF solution. By contrast, the present
mesoCN materials are fabricated using inexpensive and relatively safe urea and
formaldehyde as precursors. In this viewpoint, the preparation route of mesoCN
materials is much more eco-friendly than other reported mesoporous CN samples.
Finally, regarding the catalytic versatility in both Knoevenagel condensation and
transesterification reaction, it can be assumed that such mesoporous materials could
serve as a new solid base for wide base-mediated organic reactions.
4
. Conclusion
To conclude, we have prepared a series of mesoCN materials using urea and
14

formaldehyde as cheap and widely available starting materials via a nanocasting route.
The mesoCN materials possess high surface areas and relatively narrow pore size
distribution. The average molar ratio of C/N in mesoCN is ca. 3.4, depending on the
molar ratio of urea to formaldehyde. In Knoevenagel condensation reactions between
benzaldehyde/acetone and malononitrile, such mesoCN samples exhibited good
catalytic activities with high selectivities. The catalytic active sites of mesoCN
materials are possibly medium-strength basic sites. Furthermore, transesterification
reactions between EAA with various alcohols could be smoothly promoted using
mesoCN materials, affording a maximum conversion of 68%. It can be anticipated
that the metal-free mesoporous g-CN materials could enrich the finding in the
heterogeneous base catalysis, and provide a new strategy for the synthesis of
mesoporous g-CN catalysts towards wide base-mediated organocatalysis.
Acknowledgments
This work was supported by National Natural Science Foundation of China
(21203014), Jiangsu Key Laboratory of Advanced Catalytic Materials and
Technology (BM2012110), Advanced Catalysis and Green Manufacturing
Collaborative Innovation Center (ACGM2016-06-28), and the Project Funded by the
Priority Academic Program Development of Jiangsu Higher Education Institutions.
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17

Figure Captions
Fig. 1 Wide-angle XRD patterns of mesoCN-0.67 (a), mesoCN-1.0 (b), mesoCN-1.25
(c), mesoCN-1.5 (d), and recycled mesoCN-1.5 (e) samples.
Fig. 2 N adsorption–desorption isotherms (A) and pore size distributions (B) of
2
mesoCN materials (up triangle: mesoCN-0.67; circle: mesoCN-1.0; left triangle:
mesoCN-1.25；square: mesoCN-1.5; down triangle: recycled mesoCN-1.5 subjected
to five catalytic runs).
Fig. 3 TEM images of MCF (A), mesoCN-1.0 (B), and mesoCN-1.5 (C) materials.
The inset is the corresponding SAED pattern of mesoCN-1.5.
Fig. 4 FT-IR spectra of mesoCN-0.67 (a), mesoCN-1.0 (b), mesoCN-1.25 (c),
mesoCN-1.5 (d), and recycled mesoCN-1.5 (e) materials.
Fig. 5 C 1s (A) and N 1s (B) spectra of mesoCN-1.0 and mesoCN-1.5 materials.
Fig. 6 CO -TPD profiles of CN/MCF-1.5 (a), mesoCN-0.67 (b), mesoCN-1.0 (c), and
2
mesoCN-1.5 (d) materials.
Fig. 7 Effect of reaction time (A) and weight (B) on the Knoevenagel condensation
reaction over mesoCN-1.5. Reaction conditions: 5 mmol of benzaldehyde, 5 mmol of
malononitrile, 0.4 mL of n-dodecane, 5 mL of CH CN as solvent, and Wcatal. = 75 mg.
3
The left was collected at 90 °C and the right was tested at 5 h.
Fig. 8 Catalytic performance of mesoCN-1.5 in Knoevenagel condensation reactions
between acetone and malononitrile. Reaction conditions: 10 mmol of acetone, 10
mmol of malononitrile, 0.3 mL of n-dodecane, 3 mL of toluene as solvent, and Wcatal.
=
50 mg.
Fig. 9 Catalytic performances of mesoCN-1.5 in transesterification reactions between
EAA with various alcohols. There was no byproduct in each case; namely the
selectivity to the target complex β-keto was 100%. Reaction conditions: 15 mmol of
EAA, 10 mmol of alcohol, 0.2 mL of n-dodecane, 5 mL of CH
0 mg, and T = 100 °C.
3
CN as solvent, Wcatal. =
5
18

1
000
e
d
c
b
a
10
20
30
40
50
60
70
80
2
(°)
Fig. 1 Wide-angle XRD patterns of mesoCN-0.67 (a), mesoCN-1.0 (b), mesoCN-1.25
c), mesoCN-1.5 (d), and recycled mesoCN-1.5 (e) samples.
(
B
A
0
.5
1
00
0.0
0.2
0.4
0.6
0.8
1.0
0
5
10 15 20 25 30 35
p/p0
Pore size (nm)
Fig. 2 N adsorption–desorption isotherms (A) and pore size distributions (B) of
2
mesoCN materials (up triangle: mesoCN-0.67; circle: mesoCN-1.0; left triangle:
mesoCN-1.25；square: mesoCN-1.5; down triangle: recycled mesoCN-1.5 subjected
to five catalytic runs).
19

A
B
C
Fig. 3 TEM images of MCF (A), mesoCN-1.0 (B), and mesoCN-1.5 (C) materials.
The inset is the corresponding SAED pattern of mesoCN-1.5.
2
202
3
500-3200
e
1
570
1
295
d
c
b
a
1
0
4000
3500
3000
2500
2000
1500
1000
500

1
Wave number (cm )
Fig. 4 FT-IR spectra of mesoCN-0.67 (a), mesoCN-1.0 (b), mesoCN-1.25 (c),
mesoCN-1.5 (d), and recycled mesoCN-1.5 (e) materials.
20

A
B
mesoCN-1.0
284.7
mesoCN-1.0
3
98.2
4
00.2
2
86.1
2
87.4
402.0
403
2
90.6
404.8
293
291
289
287
285
283
281
407
405
401
399
397
395
mesoCN-1.5
mesoCN-1.5
294
292
290
288
286
284
282
280 408
406
404
402
400
398
396
394
Binding energy (eV)
Binding energy (eV)
Fig. 5 C 1s (A) and N 1s (B) spectra of mesoCN-1.0 and mesoCN-1.5 materials.
Peak II
Peak III
a
b
Peak I
2
c
d
100
150
200
250
300
350
400
450
Temperature (C)
Fig. 6 CO
2
-TPD profiles of CN/MCF-1.5 (a), mesoCN-0.67 (b), mesoCN-1.0 (c), and
mesoCN-1.5 (d) materials.
21

1
00
100
90
80
70
60
50
40
30
20
10
0
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
CN
CN
H
CHO
C C
H2
C
+
NC
CN
Conv.
Sel.
A
B
1
2
3
4
5
6
0
25
50
75
100
Reaction time (h)
Weight (mg)
Fig. 7 Effect of reaction time (A) and weight (B) on the Knoevenagel condensation
reaction over mesoCN-1.5. Reaction conditions: 5 mmol of benzaldehyde, 5 mmol of
malononitrile, 0.4 mL of n-dodecane, 5 mL of CH
3
CN as solvent, and Wcatal. = 75 mg.
The left was collected at 90 °C and the right was tested at 5 h.
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
5
6
7
8
9
0 C
0 C
0 C
0 C
0 C
O
H3C
3
CN
CN
H2
C
+
H3C
CH3 NC
CN
H C
50
1
2
3
4
5
1
2
3
4
5
Time (h)
Time (h)
Fig. 8 Catalytic performance of mesoCN-1.5 in Knoevenagel condensation reactions
between acetone and malononitrile. Reaction conditions: 10 mmol of acetone, 10
mmol of malononitrile, 0.3 mL of n-dodecane, 3 mL of toluene as solvent, and Wcatal.
=
50 mg.
22

80
70
60
50
40
30
20
10
0
n-butanol
cyclohexanol
benzyl alcohol
n-octanol
furfuralcohol
O
O
O
O
+
CH3
ROH
C2H5O
RO
CH3
1
2
3
4
5
6
7
Reaction time (h)
Fig. 9 Catalytic performances of mesoCN-1.5 in transesterification reactions between
EAA with various alcohols. There was no byproduct in each case; namely the
selectivity to the target complex β-keto was 100%. Reaction conditions: 15 mmol of
EAA, 10 mmol of alcohol, 0.2 mL of n-dodecane, 5 mL of CH
3
CN as solvent, Wcatal. =
5
0 mg, and T = 100 °C.
Table 1 Texture parameters and chemical composition of mesoCN materials.
S
BET (m2 Pore size
V
p
b
(cm3
N
C
H
Chemical
Sample
−
1
a
−1
formula c
g )
362
331
338
347
355
367
410
(nm)
20.1
7.0
g )
2.08
0.38
0.40
0.42
0.55
0.60
[wt%] [wt%] [wt%]
MCF
0
0
1.61
2.79
3.34
3.35
3.01
4.25
–
mesoCN-0.67
mesoCN-1.0
mesoCN-1.25
mesoCN-1.5
mesoCN-1.5 d
19.52 61.62
20.15 61.37
20.56 59.03
21.49 55.82
20.57 56.71
C
3.68NH2.00
3.55NH2.32
3.35NH2.28
3.03NH1.96
3.21NH2.89
6.6
C
C
C
C
C
6.3
6.2
6.4
CN/MCF-1.5
22.9
0.77
7.84
25.15 1.147
3.74NH2.05
a
b
c
d
Determined by the desorption branches. Total pore volume. Measured by EA.
Recycled catalyst subjected to five catalytic runs.
23

Table 2 Catalytic results of Knoevenagel condensation reactions over mesoCN and
CN/MCF-1.5 samples a.
Sample
–
Conv. (%)
Sel. (%)
81
Yield (%)
4
3
mesoCN-0.67
mesoCN-1.0
mesoCN-1.25
mesoCN-1.5
CN/MCF-1.5
mesoCN-1.5 b
32
45
62
81
36
77
96
31
44
60
78
34
74
97
96
96
95
96
a
Reaction conditions: 5 mmol of benzaldehyde, 5 mmol of malononitrile, 0.4 mL of
n-dodecane, 5 mL of CH
CN as solvent, Wcatal. = 75 mg, t = 5 h, and T = 90 °C. b
3
Recycled catalyst in the fifth catalytic run.
Table 3 Comparison of catalytic activity over various mesoporous CN catalysts for
transesterification reaction between EAA and n-butanol.
Catalyst
Precursor
T
S
BET
V
p
t
Conv. Ref.
(
°C) (m2
(cm3
(h) (%)
−
1
−1
g )
702
788
440
505
645
355
g )
CN-FDU12 a
CN-H-SBA15 b
CTC+EDA
HMTA f
100
110
110
110
110
1.40
0.68
1.20
0.55
0.67
0.55
7
6
5
6
2
6
74
70
98
69
90
60
[39]
[40]
c
mpg-C
3
N
4
cyanamide
CTC+EDA
CTC+EDA
[1]
MCN-1 d
MCN-3 e
[41]
[22]
mesoCN-1.5
urea + formaldehyde 100
this work
a
mesoporous carbon nitride using mesoporous silica FDU-12 as a template. b
prepared using mesoporous silica SBA-15 as a template. c prepared using colloid
silica nanoparticles as hard template; the reactant alcohol was benzyl alcohol. d
e
prepared using SBA-15 as a template. prepared using mesoporous silica IBN-4 as a
f
template. hexamethylenetetramine.
24