Synthesis of nitrogen-containing ordered mesoporous carbon materials with tunable nitrogen distributions and their application for metal-free catalytic synthesis of dimethyl carbonates

Article abstract of DOI:10.1016/j.mcat.2020.110848

Dicyandiamide (DCDA) was utilized as a facile nitrogen source for the fabrication of nitrogen-containing ordered mesoporous carbon (NOMC) samples via a one-pot soft-templating approach under aqueous phase. X-ray diffraction, N₂ adsorption–desorption, Transmission electron microscopy, Scanning electron microscopy, Raman and X-ray photoelectron spectroscopy have been applied to analyze the physicochemical properties of the synthesized NOMC materials. The characterization results showed that the textural parameters (545–589 m² g^?1), graphitic crystallinity and distribution of various nitrogen species of the synthesized NOMC materials were largely dependent on the adding mass of DCDA. Besides DCDA, NOMC materials have been also successfully fabricated by employing urea and melamine as nitrogen sources. As metal-free heterogeneous catalysts, the NOMC materials showed good catalytic activity and selectivity in the transesterification of ethylene carbonate to dimethyl carbonate, affording a maximum yield of dimethyl carbonate up to 76 % at 3 h under 120 °C.

Full text of DOI:10.1016/j.mcat.2020.110848

Molecular Catalysis 485 (2020) 110848
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Synthesis of nitrogen-containing ordered mesoporous carbon materials with
tunable nitrogen distributions and their application for metal-free catalytic
synthesis of dimethyl carbonates
Jie Xu*, Lin-Zhi Wen, Yu-Lin Gan, Bing Xue*
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Gehu Middle Road 21, Changzhou,
Jiangsu, 213164, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Dicyandiamide (DCDA) was utilized as a facile nitrogen source for the fabrication of nitrogen-containing ordered
mesoporous carbon (NOMC) samples via a one-pot soft-templating approach under aqueous phase. X-ray dif-
fraction, N adsorption–desorption, Transmission electron microscopy, Scanning electron microscopy, Raman
2
Mesoporous carbon
Metal-free catalysis
Base catalyst
Transesterification
Dimethyl carbonate
and X-ray photoelectron spectroscopy have been applied to analyze the physicochemical properties of the
synthesized NOMC materials. The characterization results showed that the textural parameters
2
−1
(
545–589 m g ), graphitic crystallinity and distribution of various nitrogen species of the synthesized NOMC
materials were largely dependent on the adding mass of DCDA. Besides DCDA, NOMC materials have been also
successfully fabricated by employing urea and melamine as nitrogen sources. As metal-free heterogeneous
catalysts, the NOMC materials showed good catalytic activity and selectivity in the transesterification of ethylene
carbonate to dimethyl carbonate, affording a maximum yield of dimethyl carbonate up to 76 % at 3 h under
120 °C.
1. Introduction
[8,12–14], as such NOMC materials featuring basicity demonstrate
enhanced performance in CO capture [13,15–17], catalysis
2
The past decades have witnessed the remarkable advances in the
[2,7,18,19], supercapacitors [1,11,20], etc.
synthesis and application of ordered mesoporous carbon (OMC) mate-
rials, because of their multiple physicochemical properties including
large surface areas, adjustable mesopores, mechanical stability and
excellent conductivity [1–3]. To date, OMC materials have been re-
garded as promising candidates to traditional carbon materials (such as
graphite) in various research fields including thermal catalysis, super-
capacitors, gas adsorption, sensors, and energy storage and conversion
The hard-templating procedure is a conventional approach to syn-
thesize NOMC materials [2,8,21]. The method usually employs ni-
trogen-containing molecules (or polymers) as carbon and nitrogen
sources [1], and presynthesized ordered mesoporous siliceous tem-
plates (SBA-15, FDU-12, KIT-6, etc.) as the sacrificial scaffolds [16].
Although the hard-templating method is well developed [7], it is ac-
tually time-consuming, and expensive to carry out at large scale [1,2].
In particular, the detemplating procedure thereof involves hazardous
and very corrosive hydrogen fluoride solution [22–24]. In sharp con-
trast, the soft-templating strategy based on co-assembly of high-mole-
cular-weight co-polymer (structure-directing agent) and nitrogen-con-
taining carbon precursors [2,7], provides a sustainable and facile
technique for the synthesis of NOMC and OMC-related materials.
Recently, Wang et al. reported the one-pot synthesis of NOMC ma-
terials via a soft-templating method under aqueous phase [4,7]. Com-
pared with other evaporation-induced self-assembly (EISA) processes
[1,14], such a synthetic route adopts no volatile organic solvent and
thus is much more convenient. Nevertheless, the adjustment of nitrogen
[
4–6]. Unfortunately, most OMC materials have a limited number of
catalytically active sites, and meanwhile the surface thereof is highly
hydrophobic, therefore restraining their extensive application, espe-
cially in the hydrophilic liquid phase.
Alternatively, the incorporation of heteroatoms (e.g. boron, phos-
phorous, and nitrogen) into the carbon skeleton of OMC materials could
significantly improve the semiconducting, field-emission, and surface
polarity as well as electron-donor properties of the original OMC ma-
terials [1,7–11]. Among the various heteroatoms induced into the
carbon matrix of OMC materials, nitrogen-containing ordered meso-
porous carbon (NOMC) materials have attracted particular interest
⁎
Corresponding authors.
E-mail addresses: shine6832@163.com (J. Xu), xuebing_cn@163.com (B. Xue).
https://doi.org/10.1016/j.mcat.2020.110848
Received 23 December 2019; Received in revised form 12 February 2020; Accepted 16 February 2020
Available online 24 February 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

J. Xu, et al.
Molecular Catalysis 485 (2020) 110848
content, as well as the distribution of various nitrogen species, mainly
relied on the carbonization temperature. Moreover, although the
NOMC materials own multiple nitrogen-containing species and are re-
garded as typical solid bases, the basic strength of the reported NOMC
materials is relatively weak [14,18,25]. Up to now, the application
examples of NOMC materials as heterogeneous basic catalysts only in-
clude transesterification of β-esters [18] and Knoevenagel condensation
2.3. Catalytic evaluation
Transesterification of EC with CH
steel autoclave (25 mL). 200 mmol of CH OH, 25 mmol of EC, and 0.1 g
3
of the pre-dried catalyst were added into the autoclave and mixed well.
After being filled with nitrogen gas up to 0.6 MPa, the reactor was
heated at 120 °C for 3 h under stirring.
After the completion of the reaction, the autoclave was cooled down
in ice water and then exhausted slowly. The liquid-phase mixture was
separated by centrifugation and analyzed by a gas chromatography-
mass spectrometer (GC–MS, GCMS-QP2010, Shimadzu) to qualitatively
confirm the products. The quantitative analysis of the reaction mixture
was determined by a gas chromatograph (GC, SP-6890, Shandong
Lunan Ruihong Chemical Instrument Co.) equipped with a SE-54 ca-
pillary column and FID. Owing to the higher stoichiometric of EC than
3
OH was performed in a stainless
[
25]; however, the wide catalytic application is rarely reported.
In this work, we developed a new one-pot approach to synthesize
NOMC materials by adopting resol and dicyandiamide (DCDA) as
carbon and nitrogen precursors based on the previous liquid-phase soft-
templating strategy. In this synthetic route, DCDA molecules and resol
interact with the F127 template, thereby inducing nitrogen species into
the resin framework assembled on the F127 micelle during the con-
densation procedure. Upon adjusting the amount of DCDA, the final
nitrogen contents along with the distribution of nitrogen species can be
easily tuned. Furthermore, based on the synthetic strategy, other ni-
trogen molecules such as urea and melamine could also be employed
for the production of NOMC materials. As heterogeneous base catalysts,
the synthesized NOMC materials demonstrated good catalytic activities
in transesterification reactions between ethylene carbonate and
3
CH OH adopted in the reaction, the catalytic conversion was calculated
based on EC. The detailed EC conversion and selectivity to the target
molecule (DMC) were calculated based on an area-normalization
method of GC, and the corresponding calculation equations are as fol-
lows:
ADMC × f
+ AHEMC × f
DMC
HEMC
CH
3
OH.
Conv. =
Sel.
AEC × f + ADMC × f
+ AHEMC × f
EC
DMC
HEMC
ADMC × f
DMC
=
,
2. Experimental section
ADMC × f
+ AHEMC × f
DMC
HEMC
where A, and f are the peak areas of GC, and response factor for each
component. HEMC stood for the 2-hydroxyethyl methyl carbonate as a
byproduct.
2.1. Synthesis
NOMC materials were synthesized by a one-pot method, using re-
sorcinol and hexamethylenetetramine (HMTA) as the precursors,
pluronic F127 as the structure-directing agent, and DCDA as the ni-
trogen source. In brief, 2 g of F127 was fully dispersed in 50 ml of H O,
2
followed by the addition of DCDA, and 0.56 ml of 1,3,5-trimethyl
benzene (TMB). Next, 1.10 g of resorcinol, 0.7 g of HMTA, and 1.3 ml of
For the recycling experiment, the reaction mixture was filtrated, and
the filtrated catalyst was rinsed by methanol for three times. After
drying in a vacuum (100 °C for 2 h), the recycled catalyst was used for
the next catalytic run without the addition of the fresh catalyst.
NH
3
(28 wt%, a.q.) were added into the above liquid successively under
3. Results & discussion
stirring. The liquid was then heated at 80 °C and stirred for another
2
4 h. After that, the mixture was centrifuged and dried, and the sub-
3.1. Material characterization
−
1
sequent solid was heated from room temperature to 600 °C (1 °C min
)
and kept at this temperature for 2 h, in a tubular furnace under nitrogen
The specific surface areas and porous properties of the materials
−1
(
10 ml min ). The resultant black solid was grounded and labeled as
were analyzed by N
nitrogen-containing precursor, presents type-IV isothermal curves fea-
turing a hysteretic loop in the relative pressure (p/p ) ranging from 0.45
2
adsorption. The OMC synthesized without using
NOMC-xD, where x represents the mass (g) of DCDA.
Likewise, urea or melamine was also used to substitute DCDA, and
the corresponding products were labeled as NOMC-xU, and NOMC-xM,
respectively. The material synthesized without adding DCDA/urea/
melamine was labeled as OMC.
0
to 0.65 (Fig. 1A), indicating the sample has mesopores. Like OMC,
NOMC materials show apparent mesostructure along with narrow pore
distributions (Fig. 1B). However, the adsorption quantity of NOMC
0
materials rises continuously in high relative pressure (p/p > 0.95).
This means that, in addition to mesopores, NOMC materials still possess
a certain amount of macropores, which might be associated with the
packing pores of NOMC materials.
2.2. Characterization
X-ray diffraction (XRD) patterns of the materials were recorded with
The surface areas and pore sizes and volumes are summarized in
2
−1
a D/max 2500 PC X-ray diffractometer (Rigaku) with a graphite
monochromator (40 kV, 40 mA) using Ni-filtered Cu-Kα radiation. The
surface areas and porous information were measured using an ASAP
Table 1. The BET surface area of the pure OMC is 565 m g
and the
3
−1
pore volume thereof is 0.39 cm g . As for NOMC materials, their
surface areas are very similar to that of OMC, whereas the pore sizes
become much smaller for NOMC. It is found that the textural para-
meters of NOMC materials are related to the feeding mass of DCDA.
With the increase of the mass of DCDA, the surface areas, as well as pore
volumes, decrease monotonously (Table 1). More importantly, it is
detected that the product mass of NOMC is very sensitive to the mass of
DCDA. In fact, when the mass of DCDA is above 2.4 g, the product yield
of NOMC is quite low. One probable reason is that, during the sub-
sequent carbonization procedure, the addition of more DCDA would
bring out more volatile intermediates, which therefore promote the
decomposition of nitrogen-containing frameworks of the as-synthesized
NOMC materials (discussed below).
2
020 analyzer (Micromeritics). Before the analysis, the samples were
degassed at 150 °C for at least 4 h. The specific surface areas were
calculated based on the Brunauer–Emmet–Teller (BET) method, and
pore size distributions were determined by the Barret–Joyner–Halenda
method. Scanning electron microscopy (SEM) images were obtained on
a Philips XL 30 microscope. Transmission electron microscopy (TEM)
images were recorded on a JEOL 2010 electron microscope (200 kV). X-
ray photoelectron spectra (XPS) were recorded on an ESCALAB 250XI
spectrometer (Thermo) using an Al anode as an X-ray source (12.5 kV).
Raman spectra were collected on a Lab Ram HR evolution spectrometer
(
Jobin Yvon) using a 532 nm line as the excitation source. Elementary
analysis (EA) was performed with Elementar Vario EL III instrument to
calculate the carbon, nitrogen and hydrogen contents of the samples.
Besides DCDA, we have also prepared other NOMC materials by
applying urea and melamine as nitrogen-containing sources. The N
2
2

J. Xu, et al.
Molecular Catalysis 485 (2020) 110848
2
Fig. 1. N adsorption–desorption isotherms (A) and the pore size distributions (B) of OMC and NOMC-xD materials.
Table 1
Physical and chemical properties of OMC and NOMC-xD materials.
Sample
S
BET (m2 g−1
)
D
pore (nm)a
V
pore (cm3 g−1
)
a
0
(nm)c
t (nm)d
I
D
/I
G
Product mass (g)
Chemical compositione
OMC
565
589
570
545
524
4.3
3.3
3.4
4.0
3.4
0.39
0.45
0.43
0.40
0.38
10.15
10.29
10.31
10.75
10.35
5.83
6.97
6.87
6.77
6.91
0.89
0.78
0.87
0.88
0.86
0.77
0.70
0.61
0.48
–
C60.7NH20.4
C39.5NH18.1
C32.8NH17.4
C21.5NH13.6
C34.5NH18.3
NOMC-0.8D
NOMC-1.6D
NOMC-2.4D
NOMC-1.6D
b
a
Determined by the adsorption branches.
b
c
Spent catalyst after five consecutive runs.
2
2
) is calculated by d1
=
4 h + hk + k
based on (110) reflection.
The lattice parameter (a
0
2
3
a
2
0
hkl
d
e
The thickness of the mesoporous carbon wall (t) is calculated by t = a
Determined by CHN elemental analysis.
0
− Dpore
.
Scheme 1. A possible synthetic route of NOMC via a one-pot co-assembly method using DCDA as a nitrogen source.
adsorption–desorption isotherms (Fig. S1) indicate that the materials
have mesostructures along with narrow pore size distributions like
NOMC-xD materials. Accordingly, the surface areas of NOMC-0.8U and
analyzed by EA. As listed in Table 1, the molar ratio of carbon to ni-
trogen in OMC is 60.7:1, indicating that the material contains a large
number of carbon species. By comparison, NOMC materials possess
higher contents of nitrogen than OMC. Moreover, with the increase of
the amount of DCDA, the molar ratio of carbon to nitrogen of NOMC is
decreased, suggesting that the nitrogen contents of NOMC materials can
be easily controlled by adjustment of the amount of DCDA.
2
−1
NOMC-0.8 M are 553 and 538 m g
(Table S1), respectively.
Likewise, the NOMC-M material adopting more melamine results in a
slight decline in both surface area and pore volume, which is in ac-
cordance with the above NOMC-xD materials.
The bulk chemical compositions of OMC and NOMC materials were
According to the soft-templating synthetic approaches of NOMC
3

J. Xu, et al.
Molecular Catalysis 485 (2020) 110848
further transforms into ordered mesostructure (Step 3). During the
pyrolysis under nitrogen gas, F127 decomposes by itself (generally
occurring at ca. 400 °C [7]), and the mesostructured NOMC material is
finally obtained under higher carbonization temperature (Step 4). It
should be mentioned that DCDA would also partially decompose below
400 °C and release a large amount of gas, thus inducing the decom-
position or even partial collapse of ordered carbon framework of
NOMC. Therefore, adding the mass of DCDA results in lower pro-
ductivity of NOMC (Table 1).
The small-angle XRD (SAXRD) was applied to further study the
mesostructures of NOMC materials. The SAXRD pattern of OMC shows
two pronounced diffraction peaks at 2θ = 0.8 and 1.44° (Fig. 2). The
ratio of the corresponding d-spacing values of the peaks is close to
3
1 . It can be inferred that the two peaks are indexed as (100) and
(
110) reflections of two-dimensional hexagonal symmetry (p6mm), re-
spectively [5,7]. The similar patterns are also observed in the SAXRD
patterns of NOMC materials, which prove the existence of ordered
mesostructure in NOMC. Moreover, the results of SAXRD patterns in-
dicate that the incorporation of a small amount of DCDA into the ori-
ginal synthetic procedure has not altered the regular mesostructures of
OMC. However, as the mass of DCDA is increased, the position of dif-
fraction peak (110) is shifted towards lower 2θ values, corresponding to
Fig. 2. Small-angle XRD patterns of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6
(c), and 2.4 (d)) materials.
previously reported in the literature [1,4,5,7], a possible synthetic route
of NOMC using DCDA, urea, and melamine as nitrogen sources has been
proposed in this study. As illustrated in Scheme 1, the present synthetic
method for NOMC is indeed based on the co-assembly of nitrogen-
containing sources and structure-directing agent (tri-block PEO-PPO-
PEO F127 polymer). First, F127 molecules transform into stripe-like
micelle in the aqueous solution and TMB prefers to locate inside and
swell the hydrophobic domain (PPO) of F127 micelle (Step 1) [5]. Si-
multaneously, DCDA molecules are adsorbed on the hydrophilic section
0
larger lattice parameters (a ) of NOMC (Table 1). This means that the
lattice parameters of the p6mm symmetry of NOMC materials undergo
expansion. Accordingly, the thickness of the carbon wall of mesos-
tructures becomes thinner from NOMC-0.8D to NOMC-2.4D. Further-
more, the SAXRD patterns of NOMC-0.8U and NOMC-0.8 M (Fig. S2)
manifest that the materials fabricated using urea and melamine as ni-
trogen source also possess ordered mesoporous structures.
(
PEO) of F127 via hydrogen-bonding interaction. On the other hand,
The micro-morphology of NOMC-0.8D and NOMC-1.6D samples are
observed by SEM and TEM techniques. The SEM images of NOMC
HMTA hydrolyzes under 80 °C, thus generating formaldehyde. The li-
quid phase with basic condition originating from ammonia induces the
condensation of resorcinol and formaldehyde, thereby gradually
forming resorcinol-formaldehyde resin. Afterwards, the resin interacts
with the hydrophilic section of F127 micelle by a hydrogen bond (Step
(
Fig. 3) reveal rod-like domains with a mean width of ca. 100 nm. The
aggregated particles of NOMC are like that of siliceous SBA-15 [26].
The existence of mesopores in NOMC materials detected by N ad-
2
sorption–desorption analysis should originate from the interparticle
voids. TEM images of NOMC materials exhibit well regular shutter-like
strips (Fig. 4), further confirming the ordered mesoporous structures of
2). Namely, both resin and DCDA are located at the exterior of the F127
strip-type micelle. The co-assembly of DCDA-resol/F127 micelles
Fig. 3. SEM images of NOMC-0.8D (A&B) and NOMC-1.6D (C&D) materials..
4

J. Xu, et al.
Molecular Catalysis 485 (2020) 110848
Fig. 4. TEM images of NOMC-0.8D (A&B) and NOMC-1.6D (C&D) materials.
Fig. 7. XPS survey of OMC and NOMC-xD materials.
NOMC materials. Directly judged from the images, the pore sizes of
Fig. 5. Wide-angle XRD patterns of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6
NOMC are ca. 4 nm, close to the results gained from N
lysis in Table 1.
2
sorption ana-
(c), and 2.4 (d)) materials.
Fig. 5 depicts the wide-angle XRD (WAXRD) patterns of OMC and
NOMC. The two diffraction peaks appearing at 2θ = ca. 22.5 and 43.1°,
which are ascribed to (002) and (100) reflections of discorded graphite
[
13,15,27] in NOMC. For NOMC-0.8D, the intensity of the diffraction
peak is comparable to that of OMC. Whereas, further adding the mass of
DCDA, the eventual intensity of peak is weaken.
Raman spectroscopy was used to further investigate the graphitic
character of the samples. The resultant spectra of both OMC and NOMC
−
1
(
Fig. 6) reveal two primary signals at 1356 and 1586 cm , which are
assigned to the disordered (D) and graphitic (G) modes of graphite-
analogous material [15,22,27]. The intensity ratio of the two peaks (I
) is commonly applied to appraise the graphitic crystallinity. The
calculated I /I values of NOMC materials (Table 1) are 0.78–0.89, il-
D
/
I
G
D
G
lustrating that the synthesized NOMC materials possess comparatively
highly sp2-hybridized carbon species, The results of Raman spectra
agree with the WAXRD pattern described above.
The chemical compositions of OMC and NOMC were probed by XPS
technique. The spectrum of OMC (Fig. 7) shows carbon and oxygen
elemental signals detected at binding energies of 285 and 533 eV. The
occurrence of the oxygen element of OMC is possibly owing to the
Fig. 6. Raman spectra of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6 (c), and 2.4
d)) materials.
(
5

J. Xu, et al.
Molecular Catalysis 485 (2020) 110848
Fig. 8. N 1s spectra of NOMC-xD materials.
Scheme 2. Chemical structure of NOMC with various nitrogen functionalities.
Table 2
Distributions of various nitrogen species in NOMC materials.
Fig. 9. Correlation between the catalytic conversion of EC and the proportion of
graphitic nitrogen species of various NOMC materials.
Sample
Nitrogen component (mol%)
pyridinic
pyrrolic
graphitic
pyridinic oxide
residual water molecules adsorbed on the surface of OMC. In addition
to the two elements, a weak peak at 400 eV is observed in each spec-
trum of NOMC. Calculated based on the peak areas integrated, the
surface atomic ratios of carbon to nitrogen of NOMC materials are in
the range of 20.7:1–37.8:1, depending on the adding mass of DCDA in
the synthetic procedure of NOMC. The XPS survey spectra of NOMC-
NOMC-0.8D
NOMC-1.6D
NOMC-2.4D
NOMC-0.8 M
NOMC-0.8U
29.1
27.1
22.2
28.9
27.0
39.6
41.0
54.4
41.5
42.1
14.3
20.7
14.1
16.4
16.3
17.0
11.2
9.3
13.2
14.6
0.8 M and NOMC-0.8U are depicted in Fig. S3. The carbon, nitrogen,
Table 3
Catalytic performances of OMC and NOMC materials in transesterification reactions of EC with CH
3
OHa.
Catalyst
Conv. (%)
Sel. (%)
Yield (%)
–
41
53
76
86
71
83
82
40
61
69
89
69
80
82
16
32
52
76
49
66
67
OMC
NOMC-0.8D
NOMC-1.6D
NOMC-2.4D
NOMC-0.8 M
NOMC-0.8U
a
3
200 mmol of CH OH, 25 mmol of EC, T =120 °C, Wcatal. = 0.1 g, and t =3 h.
6

J. Xu, et al.
Molecular Catalysis 485 (2020) 110848
3.2. Catalyst evaluation
Dialkyl carbonates represented by dimethyl carbonate (DMC) and
diethyl carbonate (DEC) have been extensively applied for numerous
applications in the contemporary chemical industry [28]. Due to its
high octane number and excellent biodegradability [29–31], DMC is a
sustainable additive to gasoline. Furthermore, DMC is considered as a
potential candidate to replace phosgene and dimethyl sulfate in car-
bonylation, and methylation processes [32–34], respectively. Among
various synthetic processes, including phosgenation and oxidative car-
bonylation of methanol (CH
nates (such as ethylene carbonate and propylene carbonate) with
CH OH has been proposed as the most convenient and eco-benign ap-
3
OH), transesterification of cyclic carbo-
3
proach for the manufacture of DMC [32,35,36].
The transesterification is an acid/base-catalyzed reaction. Up to
now, the most efficient catalysts suggested for the transesterification of
EC to DMC or DEC are mainly acidic and basic ionic liquids [37,38].
Despite their high catalytic activities, such homogeneous catalysts
suffer from difficulty in product separation and catalyst recycling. In
sharp contrast, heterogeneous catalysts are more convenient to be re-
cycled, and more importantly, nanostructured and mesoporous het-
erogeneous catalysts have been well developed in recent years [39–45].
However, it should be pointed out, most of the heterogeneous catalysts
including metal oxides [46] and their composites [33,47–49], and
dawsonites [36,50], proposed for the transesterification reactions,
contain metal cations. The potential metal contamination is un-
doubtedly unfavorable for the further application of DMC as the elec-
trolyte [51]. Regarding these viewpoints, it is highly desired to develop
a metal-free solid catalyst for the clean production of DMC via trans-
esterification.
Fig. 10. Effects of reaction temperature (A) and time (B) on the transester-
ification on catalytic performances Reaction conditions: 200 mmol of CH
5 mmol of EC, 0.1 g of NOMC-1.6D, T =120 °C (A), and t =3 h (B).
3
OH,
2
Herein, as the present NOMC materials contain rich nitrogen species
and more importantly, no metallic component has been involved in the
overall synthetic procedure, the NOMC materials are applied as cata-
lysts for the metal-free catalytic production of DMC. Initially, the re-
action was performed without any catalyst (Table 3), and the blank
result revealed that the conversion of EC was low (41 %). In the case of
catalytic selectivity, the main product was mainly 2-hydroxyethyl me-
thyl carbonate (HEMC) instead of the desired DMC. The monoester
byproduct was probably because of the incomplete transesterification
of EC [32,52]. After the addition of 0.1 g of OMC, the catalytic activity
is obviously improved. The conversion of EC is 53 %, and the selectivity
to DMC is increased to 61 %, respectively. Unfortunately, the yield of
DMC (32 %) is still not satisfactory. Using NOMC materials instead, the
catalytic conversion is remarkably enhanced. Among the three NOMC-
xD materials, NOMC-1.6D demonstrates the highest catalytic conver-
sion and selectivity.
In addition to NOMC-xD materials, the catalytic performances of
other NOMC materials prepared using urea, and melamine as nitrogen
precursors were tested under identical reaction conditions. The corre-
sponding catalytic results (Table 3) show that the transesterification
reactions could also be smoothly promoted in the presence of these
NOMC materials. The EC conversion and DMC selectivity for each case
are ca. 80 %. The catalytic results evidence that using other typical
nitrogen-containing compounds, the synthesized NOMC samples could
also act as efficient catalysts to promote the transesterification reactions
Fig. 11. Recycling catalytic performance of NOMC-1.6D in transesterification
of EC with CH OH. Reaction condition: 200 mmol of CH OH, 25 mmol of EC,
catal. = 0.1 g (the first run), T =120 °C, and t =3 h.
3
3
W
and oxygen elements are observed in the two spectra, and the atomic
ratios of carbon to nitrogen of NOMC-0.8 M and NOMC-0.8U are 29.6:1
and 30.5:1, respectively.
For most of the nitrogen-containing carbon materials, there exist
various nitrogen species in the forms of pyridine, pyrrole [8,9,15], etc.
In order to elucidate the detailed chemical environment of nitrogen
species, the N 1s spectra with high resolution of NOMC samples are
deconvoluted. As described in Fig. 8, the N 1s spectra can be separated
into four independent peaks. The intensive peak appearing at 398.4 eV
is attributed to pyridinic nitrogen atoms [6,13], and the signal at
400.4 eV is attributed to pyrrolic nitrogen species (Scheme 2). The two
nitrogen species account for the major proportion of the whole nitrogen
species. The broad peak centered at 401.2 eV is assigned to graphitic
nitrogen whereas the signal of 403.0 eV is ascribed to pyridinic oxide on
the surface of NOMC [6,7]. Additionally, the deconvoluted N 1s spectra
of NOMC-0.8U and NOMC-0.8M are presented in Fig. S4 and the cor-
responding molar proportions of various nitrogen components are listed
in Table 2. From the full-range XPS spectra and deconvoluted N 1s
spectra, it can be concluded that NOMC materials own multiple ni-
trogen-containing species, which thus dictate the basic character of the
materials, which thus could be applied for the base-mediated organo-
catalysis.
3
between EC and CH OH.
The above XPS analysis (Fig. 8) has revealed that there are four
kinds of nitrogen species existing in NOMC, and on the other hand, the
3
transesterification of EC with CH OH in this work undoubtedly belongs
to base-mediated catalysis. To find out possible catalytic sites for the
reactions promoted by NOMC, the correlation between the catalytic
activity of various NOMC materials with their corresponding nitrogen
species is tentatively elucidated. As described in Fig. 9, the catalytic
activity has a good relationship with the proportion of graphitic ni-
trogen species of NOMC materials, suggesting that these graphitic ni-
trogen species have a positive effect for NOMC in the transesterification
7

J. Xu, et al.
Molecular Catalysis 485 (2020) 110848
Table 4
a
Catalytic performances of NOMC-1.6D for transesterification reactions with various substrates .
Cyclic carbonate
Alcohol
CH OH
T (°C)
t (h)
Conv. (%)
86
Sel. (%)b
89
3
120
3
C
2
2
H
H
5
OH
140
140
140
3
6
6
63
76
37
41
78
84
C
5
OH
3
CH OH
a
2
00 mmol of alcohol, 25 mmol of cyclic carbonate, and Wcatal. = 0.1 g.
b
Selectivities to the corresponding dialkyl carbonates.
reactions. The attribution of such graphitic nitrogen to catalytically
active sites agrees well with the relevant literature involving base-
mediated reactions (e.g. Knoevenagel condensation [53–55] and cy-
EC and C
selectivity to the target molecular (diethyl carbonate) is only 41 %. The
dissociation of the OeH bond in C OH is more difficult than that in
CH OH [48,52], which is responsible for the lower selectivity in the
cases of C OH than CH OH. Despite this, upon prolonging the re-
2 5
H OH, even the temperature is elevated up to 140 °C, the
2 5
H
cloaddition of CO
2
with epoxides [56]) catalyzed by mesostructure
3
carbon nitride materials.
2
H
5
3
Since NOMC-1.6D exhibits superior catalytic activity to other
NOMC materials under the same reaction conditions, the influences of
reaction conditions on the eventual catalytic results have been eval-
uated adopting NOMC-1.6D as a representative catalyst. After the first
hour, the catalytic EC conversion is only 50 % while the DMC selectivity
is 53 % (Fig. 10A). Prolonging reaction, both the conversion and se-
lectivity are increased progressively. After 3 h of reaction time, the EC
conversion shows no further incensement. The catalytic reaction is also
found to be sensitive to the reaction temperature (Fig. 10B). In the case
of product distribution, the major product is HEMC rather than the
target DMC as the temperature is below 100 °C. This means that, under
low temperatures, the second step of transesterification between HEMC
action to 6 h, higher selectivity along with enhanced conversion is ob-
tained. Additionally, propylene carbonate could also be moderately
converted into EC.
4. Conclusion
In summary, adopting DCDA as a nitrogen source, a series of NOMC
materials have been synthesized by a one-pot co-assembly approach
under the liquid phase. The NOMC materials possess ordered mesos-
tructures (two-dimensional hexagonal symmetry), with surface areas of
2
−1
5
45–589 m g . Besides DCDA, urea and melamine could be also
successfully applied as nitrogen sources for the fabrication of NOMC.
The introduction of nitrogen into carbon materials is probably based on
the interaction between nitrogen sources and F127 micelle via hy-
drogen bonding. The textural properties, graphitic crystallinity, and
distribution of nitrogen species are related to the adding amount of
DCDA. In the transesterification of EC, the synthesized NOMC materials
as solid bases showed high catalytic activity and selectivity. The cata-
lytic active sites might originate from the graphitic nitrogen atoms, the
proportion of which correlated with the final catalytic activity. The
work offers a convenient method to prepare NOMC materials, also
enlarges the catalytic application of NOMC for the metal-free synthesis
of dialkyl carbonates.
3
and CH OH is difficult to proceed. Elevating the temperature could
effectively promote the transfer of EC and HEMC into the DMC. Not-
withstanding, the catalytic activity and selectivity level off as the
temperature is above 120 °C. The catalytic results using various
amounts of NOMC-1.6D catalyst are given in Fig. S5. Upon increasing
the feeding amount of the catalyst, the EC conversion is gradually en-
hanced. It is apparent that more catalyst together with higher tem-
perature is beneficial to convert more EC and/or HEMC to DMC.
Fig. 11 shows the catalytic performances of the recycled NOMC-1.D
material during five consecutive runs. The mass of the recycled catalyst
for each run was almost 95 mg. For the second run, the catalytic con-
version decreases to 72 %. This might result from the leaching of some
nitrogen-containing fragments unsteadily adsorbed on the surface of
NOMC into the liquid phase. A similar phenomenon has been also found
in our previous work involving the transesterification of β-esters [18]
and Knoevenagel condensation [25] catalyzed by NOMC materials.
Nevertheless, the catalyst still demonstrates stable conversion and high
selectivity within the subsequent catalytic runs.
CRediT authorship contribution statement
Jie Xu: Conceptualization, Writing - review & editing. Lin-Zhi Wen:
Formal analysis, Writing - original draft. Yu-Lin Gan: Validation. Bing
Xue: Supervision.
As mentioned above, catalytic transesterification of cyclic carbo-
3
nates with CH OH is a more sustainable approach for the production of
dialkyl carbonates. Herein, in order to examine the catalytic versatility
of NOMC, other cyclic carbonate and alcohol have been used in the
transesterification reactions. As summarized in Table 4, in the case of
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
8

J. Xu, et al.
Molecular Catalysis 485 (2020) 110848
Acknowledgments
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Jiangsu Higher Education Institutions (19KJA430003), and Advanced
Catalysis and Green Manufacturing Collaborative Innovation Center
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