Synthesis of nitrogen-containing ordered mesoporous carbon as a metal-free catalyst for selective oxidation of ethylbenzene

Article abstract of DOI:10.1039/c4cc03372h

Nitrogen-containing ordered mesoporous carbon (NOMC) was synthesized by using m-aminophenol (MAP) as a carbon and nitrogen co-precursor via a co-assembly process with F127 in aqueous phase and exhibited a good catalytic performance for selective oxidation

Full text of DOI:10.1039/c4cc03372h

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Synthesis of nitrogen-containing ordered
mesoporous carbon as a metal-free catalyst
for selective oxidation of ethylbenzene†
Cite this: Chem. Commun., 2014,
50, 9182
Received 6th May 2014,
Accepted 26th June 2014
Jia Wang, Hongyang Liu, Xianmo Gu, Haihua Wang and Dang Sheng Su*
DOI: 10.1039/c4cc03372h
www.rsc.org/chemcomm
Nitrogen-containing ordered mesoporous carbon (NOMC) was Therefore, the successful synthesis of NOMC through the direct
synthesized by using m-aminophenol (MAP) as a carbon and nitro- soft-templating method relies much on the co-assembly of
gen co-precursor via a co-assembly process with F127 in aqueous proper nitrogen-containing carbon precursors with the block
phase and exhibited a good catalytic performance for selective co-polymer. For example, on the basis of the EISA method,
oxidation of ethylbenzene.
Wan et al.15 synthesized NOMC via a step-by-step method by
using phenol, formaldehyde and m-aminophenol as an extra
Ordered mesoporous carbon (OMC) materials have recently nitrogen source and F127 as a structure-directing agent; Zhao
attracted interest because of their high specific surface areas, et al.5 successfully obtained NOMC via self-condensation of
regular mesoporous structure and tunable pore size.1–3 To improve melamine confined in the mesochannels of an OMC matrix
their properties, surface modification is necessary and has been synthesized using the EISA method; Dai et al.14 demonstrated
extensively investigated.4 A nitrogen atom is usually induced into that the pyrolysis of the soft-templating polymeric composites in
the carbon matrix, resulting in the improvement of surface polarity an ammonia atmosphere was a direct, facile way towards NOMC
and electron-donor properties of the carbon matrix, and introduc- synthesis. Although NOMC could be synthesized according to
tion of active basic/catalytic sites on the carbon surface.5 Most these methods, the synthesis process is still time-consuming and
nitrogen-containing ordered mesoporous carbon (NOMC) are is not the real direct soft-templating strategy for the synthesis of
synthesized through a hard-templating method using appropriate NOMC. To the best of our knowledge, there are still few reports
nitrogen sources,6 such as acrylonitriles, pyrroles, anilines, of the direct synthesis of NOMC via the one-pot soft-templating
melamine-formaldehyde resins and ethylenediamine-carbon tetra- method.
chloride. Although the incorporation of nitrogen into the OMC
Herein, we selected a water-soluble m-aminophenol (MAP)
through a hard-templating method is well developed and higher as a carbon and nitrogen co-precursor and directly introduced
nitrogen content with a C/N molar ratio of 4.3–2.3 can be achieved,7 N into the OMC during the co-assembly process between MAP
there is still a demand for new methods to synthesize NOMC and F127. This strategy took advantage of polymerization
because the hard-templating approach is low in efficiency, high in between MAP and formaldehyde.16 We found that the N con-
cost, has a fussy process and is harmful to health.
tent of NOMC could be regulated by adding resorcinol (R) to
In recent years, an organic–organic self-assembly method has change the molar ratio of R/MAP and changing the carboniza-
been successfully used to synthesize OMC through either a tion temperature without the loss of ordered mesoporous
‘‘hydrothermal’’8–10 method or an evaporation-induced self-assembly structure. The produced NOMC had a better catalytic activity
(EISA)11–13 process. These methods provide new opportunities for the than CMK-3 for selective oxidation of ethylbenzene.
design and synthesis of OMC and heteroatom-containing OMC. Many
Fig. 1 illustrates the synthesis process towards NOMC. In the
studies have revealed that the interaction between carbon precursors first stage, MAP molecules interact with the hydrophilic PEO
and block copolymers plays a vital role in the formation of blocks of F127 through the hydrogen-bonding interactions, which
ordered mesoporous structures by the soft-templating method.14 also exist among H2O, NH3 and MAP. 1,3,5-Trimethylbenzene
(TMB) is a non-polar molecule and is preferentially located inside
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
the hydrophobic regions (PPO block) of F127, which easily causes
the formation of stripe-like micelles.17 During the stirring process
Chinese Academy of Sciences, 72 Wenhua road, Shenyang 110016, P. R. China.
E-mail: dangsheng@fhi-berlin.mpg.de, dssu@imr.ac.cn
at 80 1C, formaldehyde molecules are gradually produced by the
† Electronic supplementary information (ESI) available: Materials preparation,
hydrolysis of hexamine (HMT). MAP molecules undergo conden-
sation with formaldehyde molecules forming nitrogen-containing
characterization methods, and additional figures and references. See DOI:
10.1039/c4cc03372h
9182 | Chem. Commun., 2014, 50, 9182--9184
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Fig. 1 Schematic representation of the synthesis route towards NOMC.
resole–F127 composites. At the same time, –NH2 groups of MAP
can react with the aldehyde groups via elimination to generate a
Schiff base, which is stable because the –CHQN– bond is con-
jugated with a –CHQCH– bond.15 During the calcination, most of
the F127 molecules are removed below 400 1C (Fig. S1, ESI†) and
the thermosetting nitrogen-containing resoles are left forming
Fig. 3 (A) Small angle XRD pattern and (B) N2 adsorption–desorption
isotherms with the pore size distribution (inset) of the NOMC-800. (C) XPS
spectra of the as-made and NOMC-800 and (D) the variation of the nitrogen
content with the molar ratio of R/MAP and carbonization temperature.
pore walls. The loss of some small molecules (such as water and of d-spacing values for these peaks is about 1 : 1/O3 and can be
+
NH4 ) as well as the decomposition of functional groups due to the indexed as (100) and (110) reflections associated with 2-D
elevation of temperature lead to the formation of NOMC. There- hexagonal p6mm symmetry. The cell parameter, a, is calculated
fore, the basic principle relies on the polymerization of MAP and to be B13 nm, using the formula a = 2d100/O3, where d100
HCHO molecules around F127 to form the nitrogen-containing represents the d-spacing value of the (100) diffraction, consistent
resole–F127 composites and the removal of F127 to obtain ordered with above TEM results. N2 adsorption–desorption isotherms of
mesoporous structures made of the nitrogen-containing pore walls NOMC-800 show typical type-IV curves, with a clear condensa-
after calcination. Here, for clarity, we took nitrogen-containing tion step at P/P0 = 0.4–0.6, suggesting a uniform mesopore
ordered mesoporous carbon with a molar ratio of R/MAP = 0 : 1 (Fig. 3B). It should be noticed here that the isotherms do not
after carbonization at 800 1C (denoted as NOMC-800) for example, give a good H1-type hysteresis loop, suggesting that some pore
unless special instructions are given.
distortion may exist and partially block the channels.11 Accor-
TEM images (Fig. 2) show that NOMC-800 has an ordered ding to the Brunauer–Emmett–Teller (BET) method, the specific
2-D hexagonal mesoporous structure. The cell parameter, a (the BET surface area is calculated to be as high as 857 m2 gÀ1. The
distance between the two adjacent pore centres), is estimated micropore and mesopore volumes are 0.27 and 0.64 cm3 gÀ1
,
from the TEM images to be B12.7 nm. SEM images (Fig. S2, respectively. The pore size calculated from the adsorption
ESI†) of the sample indicate that the sample is composed of branch using the Barrett–Joyner–Halenda (BJH) model is about
irregularly polyhedral particles with the size ranging from B5.1 nm (see Table S1, ESI†).
hundreds of nanometres to a few micrometres.
XPS spectra were employed to detect the surface chemistry
The ordered mesoporous structure could be further con- properties of these samples. Fig. 3C shows the spectra of
firmed by small angle XRD analysis (Fig. 3A). The XRD pattern NOMC-800 and the as-made sample (synthesized without
of NOMC-800 shows two resolved peaks at 2y = 0.6–2.0. The ratio carbonization, namely, nitrogen-containing resole–F127 com-
posites) and clear signals from C, N, O of these two samples can
be observed. The N content of the as-made sample is about
5.1 at%, suggesting that N can be induced into the resole–F127
composites via polymerization of MAP and HCHO around F127.
After carbonization at 800 1C, the N content decreases to
3.3 at% and the O content decreases from 20.6 to 5.7 at%
because of the decomposition of many oxygen and nitrogen
functional groups and the removal of F127. Interestingly, a
noticeable change in the N type is observed after carbonization
at 800 1C (Fig. S3, ESI†). In the case of the as-made sample,
there is one obvious peak at B397.6 eV. After carbonization at
800 1C, the N1s peak shifts to higher binding energy, indicating
an obvious change in N types which originates from rearrange-
Fig. 2 TEM images of NOMC-800 with its ordered hexagonal meso-
porous structure viewed from the (A) [110] and (B) [001] directions. The
ment and transformation of the different N species.18,19 The
insets show the corresponding fast Fourier transform (FFT) patterns.
N1s spectra of NOMC-800 could be fitted to four components,
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pyridinic-N (398.3 eV), pyrrolic-N (400.2 eV), graphitic-N (401.1 eV) the carbon catalysis research to exclude the effects of the metal
and nitrogen-oxides (403.3 eV).14,18,19 In order to regulate the impurities in the carbon catalyst.28
nitrogen content, resorcinol (R) was added into the reaction
In conclusion, using MAP as a carbon and nitrogen co-precursor,
system, because R was usually used to synthesize OMC.17,20,21 nitrogen-containing ordered mesoporous carbon has been success-
The N content decreases with the rise of carbonization tempera- fully synthesized via a co-assembly process between MAP and F127.
ture and the molar ratio of R/MAP (Fig. 3D). For example, the N Our results show that this approach is simpler for direct introduc-
content of NOMC-800 changes from 3.3 to 0.9 at% upon tuning tion of nitrogen into the OMC compared with traditional NH3
the weight fraction of R from 0 to 100 wt%. We propose that NH3 post-treatment, step-by-step strategies and other hard-templating
from ammonia water or the hydrolysis of HMT is also partially methods. Furthermore, this work enlarges the application of OMC
involved in the polymerization and/or co-assembly.22 Therefore, in catalysis especially by using OMC as a metal-free catalyst.
there is still some N remaining when only R is used. Nevertheless,
This work was supported by the National Natural Science
the main source of N comes from MAP molecules (Fig. S4, ESI†). Foundation of China (21203214, 21133010, 51221264, and
Alternatively, small XRD patterns (Fig. S5, ESI†), N2 adsorption– 21261160487), MOST (2011CBA00504), the ‘‘Strategic Priority
desorption isotherms (Fig. S6, ESI†), TEM images (Fig. S7 and S9, Research Program’’ of the Chinese Academy of Sciences (Grant
ESI†) and XPS characterization data (Fig. S8 and S10, ESI†) of No. XDA09030103), and the China Postdoctoral Science Foun-
other products given in the ESI† also confirm that we successfully dation (2012M520652). Dr Zhenhua Sun is gratefully acknowl-
synthesized NOMC with a tunable N content.
edged for his stimulating discussions.
NOMC has been used in adsorption, separation, and energy
storage/conversion, but there are few reports of NOMC as a
metal-free catalyst.4,23 It has been demonstrated that N in
N-doped graphene plays a vital role in the C–H activation.24
This progress encouraged us to test the produced NOMC in
selective liquid-phase oxidation of ethylbenzene. The results of
various carbon catalysts are summarized in Table 1. In our
experiment, acetophenone (AcPO) is the main product with
benzaldehyde, benzoic acid and 1-phenyl-ethylalcohol as
the by-products, which is in line with CNTs and/or N-doped
graphene as catalysts.24,25 The blank test indicates that there is a
low reaction activity because of the EB autoxidation.26 CMK-3
exhibits higher catalytic performance than that observed in the
blank test, indicating that the mesoporous carbon material itself
has a certain catalytic ability for the formation of AcPO. However,
O-CMK-3 (CMK-3 treated with HNO3) has a lower activity than
CMK-3, which may be attributed to the side-effects of oxygen-
functional groups introduced via HNO3-treatment.25,27 There is
an obvious increase of catalytic performance when NOMC-800 is
used as a catalyst. We propose that N introduced into the OMC
plays a very important role in the reaction, which is in agreement
with a previous report.24 Moreover, although NOMC prepared
via a similar literature method14 exhibits approximate activity
(see properties of NH3-OMC-800 in Table S2, ESI†), in the view of
the synthesis process, NOMC-800 prepared in this work has an
advantage over the NH3 treatment. It is worth noting that no
metal is induced into the material which is very important for
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Table 1 Catalytic activity of different carbon materials for the liquid phase
oxidation of ethylbenzenea
b
Sample
Conversion (%) Selectivity (%) Rs (mmol mÀ2 hÀ1
)
Blank
13.8
40.8
34.9
63.3
14.1
40.2
30.7
84.1
86.1
—
40.8
39.1
168.7
108.1
CMK-3
O-CMK-3c
NOMC-800
NH3-OMC-800 63.1
a
The conversion and selectivity for (AcPO) were determined by GC.
b
c
Yield rate of AcPO per m2 of the catalyst surface at 12 h. CMK-3 is
28 J. Zhang, X. Liu, R. Blume, A. Zhang, R. Schlogl and D. S. Su, Science,
2008, 322, 73.
treated with concentrated HNO3 for 3 h, at 60 1C and denoted as O-CMK-3.
9184 | Chem. Commun., 2014, 50, 9182--9184
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