Mesoporous carbon with high content of graphitic nitrogen for selective oxidation of ethylbenzene

Article abstract of DOI:10.1039/c9ra05386g

Graphitic-nitrogen doped mesoporous carbon (accounting 85% in all nitrogen species) was easily synthesized by using acetonitrile as a precursor and SBA-15 as a hard template through a chemical vapour deposition method and exhibited a better catalytic performance than other nitrogen-doped carbon materials for selective oxidation of ethylbenzene.

Full text of DOI:10.1039/c9ra05386g

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Mesoporous carbon with high content of graphitic
nitrogen for selective oxidation of ethylbenzene†
Cite this: RSC Adv., 2019, 9, 28253
a
a
Xiaoli Panb and Fuwei Li
*
Jia Wang,
Received 14th July 2019
Accepted 22nd August 2019
Graphitic-nitrogen doped mesoporous carbon (accounting 85% in all nitrogen species) was easily
synthesized by using acetonitrile as a precursor and SBA-15 as a hard template through a chemical
vapour deposition method and exhibited a better catalytic performance than other nitrogen-doped
carbon materials for selective oxidation of ethylbenzene.
DOI: 10.1039/c9ra05386g
rsc.li/rsc-advances
Selective oxidation of hydrocarbons to corresponding high exhibited a good catalytic performance in the oxidation of EB.27
value-added oxygen-containing products (alcohols, aldehydes, In our previous work,28 nitrogen-containing ordered meso-
ketones and acids, etc.) is one of the most important reactions porous carbon (NOMC) which was synthesized by using m-
in organic synthesis and chemical industry.1,2 Generally, aminophenol as a carbon and nitrogen co-precursor via a co-
heterogeneous or homogeneous catalysts, such as transition assembly process with F127 in aqueous phase could effec-
metals or organometallic centers, are used to efficiently func- tively convert EB to AcPO. It was worth noting that during the
tionalize C–H bond.3–7 Besides, metal-doped carbon nitrides as synthesis process no metal was induced. Therefore, it could
catalysts and N-alkoxysulfonyloxaziridines have been used for exclude the effects of the metal impurities in nanocarbon
the activation of C–H bonds.8–10 Despite the high efficiency that materials on its performance for the EB oxidation. However, it is
could be obtained in these processes, the presence of metals or necessary to further improve its catalytic performance by regu-
complicated catalytic systems is required for superior lating its surface chemical property because there exist varieties
performance.
of nitrogen species in NOMC.
Nanocarbon materials with or without dopants as a metal-
Herein, we selected SBA-15 as a hard template and acetoni-
free heterogeneous catalysts have been used in many organic trile as precursor and directly synthesized graphitic-N doped
reactions, such as dehydrogenation of short-chain alkanes,11,12 mesoporous carbon (GNMC) in order to introduce high level
hydrogen transfer reductions,13–15 selective oxidation of alco- graphitic nitrogen and avoid any metals. These GNMC with
hols,16–19 amines,20 and arenes,21,22 Friedel–Cras alkylation high graphitic-N content exhibited a better catalytic perfor-
reactions,23 and condensation reactions.24,25 Obviously, in these mance than other nitrogen-doped carbon materials for the
reactions, especially for oxidation process, the introduction of selective oxidation of EB.
heteroatoms (N, B, S etc.) into carbon matrix could enhance its
SEM results shown in Fig. S1† indicate that all the MC and
catalytic performance, due to the improvement of electron GNMC have similar wormlike rod structure with coarse surface,
features. Especially, Ma and co-workers found that layered which derives from the hard template (SBA-15). Obvious
carbon doped with graphitic-type nitrogen could regulate the diffraction peak of stacked graphitic sheet is observed in XRD
electronic structure of the adjacent carbon atoms and facili- patterns (Fig. S2†), and the (002) peak intensity of GNMC is
tated the conversion of ethylbenzene (EB) to acetophenone higher than that of MC, suggesting higher graphitic extent.
(AcPO) using TBHP as oxidant.26 Apparently, the introduction of Besides, the increase of 2q ((002) peak) from 24.7ꢀ for MC to
nitrogen enhanced the activity of the layered carbon. However, 25.9ꢀ for GNMC, indicating that the incorporation of nitrogen
there may be still metal residual because graphene oxide was makes a decrease of interplanar crystal spacing of (002). Typical
made by modied Hummers' method.13 Recently, N, P, S co- N2 adsorption–desorption isotherms and corresponding pore
doped hollow carbon shells (NPS-HCS) was synthesized and size distributions of the GNMC are shown in Fig. 1A and B. A
clear hysteresis loop in the P/Po > 0.4 is observed for GNMC,
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of
suggesting an existence of mesostructure. According to the
Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. E-mail: Brunauer–Emmett–Teller (BET) method, the specic BET
surface area of GNMC is calculated to be 371 m2 gꢁ1. The total
fuweili@licp.cas.cn
bState Key Laboratory of Catalysis, iChEM (Collaborative Innovation Center of
pore volume is 0.47 cm3 gꢁ1. The pore size calculated from the
Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese
adsorption branch (Fig. 1B) using the Barrett–Joyner–Halenda
Academy of Sciences, Dalian 116023, China
(BJH) model is ꢂ4.1 nm. However, as for MC (mesoporous
† Electronic supplementary information (ESI) available: Materials preparation,
carbon without doping nitrogen, using ethanol as precursor,
characterization methods, and additional gures, tables and references. See
DOI: 10.1039/c9ra05386g
see synthesis process in ESI†), weak hysteresis loop is observed
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15 and 3.5 at% for GNMC (Table 1), suggesting that N content
could be hardly affected by removing SBA-15. N 1s spectra in
Fig. 2A shows that the main nitrogen species in GNMC and
GNMC/SBA-15 could be attributed to graphitic nitrogen, which
accounts for 85% in all nitrogen species for GNMC (Table S2†).
The similar method using acetonitrile as precursor has also
been reported to synthesize nitrogen-doped graphene,30
nitrogen-doped carbon nanotubes31 and nitrogen-doped
ordered mesoporous carbon.29 ATR-IR spectroscopy was also
conducted to detect the surface chemical-bonding state
(Fig. S5†). However, due to the black appearance (synthesized at
high temperature 950 ꢀC), no obvious peaks are observed for
these two samples. Besides, 13C NMR spectra and UV-vis DRS
for MC and GNMC were also conducted to understand the
chemical property (Fig. S6 and S7†). Unfortunately, all the
samples exhibit similar 13C NMR spectra, and no obvious peaks
are observed for their UV-vis DRS, which may be due to their
good conductivity.
In order to understand the carbon content and oxidation
stability, the three samples (GNMC, GNMC/SBA-15 and MC) was
investigated by TG technique under 10% O2/Ar atmosphere. As
shown in Fig. 2B, about 37 wt% carbon on SBA-15 is obtained
aer CVD process. Besides, the removal of SBA-15 resulted in
the decrease of oxidation stability. GNMC could be oxidized
more easily than MC, suggesting that the introduction of
nitrogen signicantly promoted the oxidation of carbon.32
The high specic surface area, open mesoporous system and
onefold graphitic nitrogen species for GNMC supported by
above results encourage us to test the synthesized GNMC in
selective liquid-phase oxidation of EB, in which the introduc-
tion of N into carbon materials plays a very important role. In
our experiments, the main products of EB oxidation using tert-
butyl hydroperoxide (THBP) as oxidant are shown in Table S3.†
They are acetophenone (AcPO), 1-phenyl-ethylalcohol (PEA),
benzaldehyde (BA) and benzoic acid (BzA). The effects of reac-
tion temperature, the molar ratio of EB/TBHP, reaction time on
Fig. 1 N2 adsorption–desorption isotherms of MC and GNMC (A) and
corresponding pore size distributions (B). TEM images of MC (C) and
GNMC (D).
and its pore size distributions is wider than that of GNMC. Its
specic surface area is only about 137 m2 gꢁ1 and the total pore
volume is 0.22 cm3 gꢁ1. The obvious mesoporous structure of
MC and GNMC is also evidenced by transmission electron
microscopy (TEM) as shown in Fig. 1C and D, respectively.
However, the ordered mesoporous structure is not maintained
aer removing SBA-15, which may be due to the high steam
concentration of ethanol or acetonitrile on the surface of SAB-15
once the furnace was heated at 90 ꢀC, resulting in the diffusion
difficulty of ethanol or acetonitrile steam into pore channel of
SBA-15 (see synthesis process in ESI†). Unlike this synthesis
process, another manner to bring in liquid precursor by a ow
of nitrogen saturated with acetonitrile vapour at room temper-
ature could obtain ordered mesostructure.29 The evident
graphitic order with bent layer planes in both these materials is
observed in high-resolution transmission electron microscopy
(HRTEM) shown in Fig. S3A and B,† indicating the formation of
pore walls with graphitic character.
Table 1 Catalytic activity of different carbon materials for the liquid
phase oxidation of EBa and their textural propertyb and chemical
compositionc
In order to understand these samples' composition, element
analysis (EA) was conducted (Table S1†). EA reveals that the N
content of MC is very low (0.111 wt%, close to the detection limit
of the instrument), while 7.3 wt% of N content is obtained for
GNMC. The composition of C/N/H/O is 100/0.1/3.7/0.1 and 100/
8.6/3.4/6.9 for MC and GNMC, respectively, indicating that the
framework is carbon due to the high synthesis temperature.
XPS spectra were further employed to detect the surface
chemistry properties of these samples, especially the nature and
coordination of the nitrogen atoms, due to the bulk elemental
composition shown by EA results. Fig. S4† shows the spectra of
MC, GNMC and GNMC/SBA-15. The obvious N signal is
observed in the XPS spectra of GNMC and GNMC/SBA-15 indi-
cating that nitrogen could be introduced into carbon using
acetonitrile as precursor. Moreover, GNMC and GNMC/SBA-15
contain the similar nitrogen content, 3.1 at% for GNMC/SBA-
XPS
Cat.
SBET C 1s N 1s O 1s Con.d Sel.e Yield rateg
1
2
3
4
5
6
7
8
Blank
MC
CMK-3
GNMC
MCN
—
—
—
—
—
13.8
20.2
28.5
79.5
31.9
41.3
56.9
14.1 —
137 90.9
1153 95.4 0.2
371 87.5 3.5
558 82.6 12.6 4.8
406 95.7 1.9
87.8 94.8 1.8
9.1
4.3
8.9
18.5 22.6
35.3 122.1
97.6 932.6
40.2 154.1
45.4 229.7
94.2 669.9
12.9 34.9
25.9 36.6
NOLC
NCNT
2.4
2.6
10.8 21.9
10.8 30.5
GNMC/SBA-15
—
—
86.1 3.1
86.1 3.1
9f GNMC/SBA-15
a
Reaction condition: 2 mL acetonitrile, 1.5 mmol EB, 10 mg catalyst,
b
EB/TBHP ¼ 1/3, 80 ꢀC, 12 h. Calculated according to BET method
(unit: m2 gꢁ1) from N2 adsorption–desorption data. Measured from
c
d
e
XPS spectra (unit: at%). EB conversion (unit: %). AcPO selectivity
(unit: %). f 27 mg catalyst. g Yield rate of AcPO (unit: mmol gꢁ1 hꢁ1).
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Fig. 3 (A) The effect of reaction temperature on the catalytic oxidation
of EB with TBHP as oxidant in the presence of GNMC. Reaction
conditions: 1.5 mmol EB, 2 mL CH3CN, 10 mg GNMC, 12 h, molar ratio
of EB/TBHP ¼ 1/3. (B) The effect of molar ratio of EB/TBHP on the
catalytic oxidation of EB with TBHP as oxidant in the presence of
GNMC. Reaction conditions: 1.5 mmol EB, 2 mL CH3CN, 10 mg GNMC,
12 h, 80 ꢀC. (C) The effect of reaction time on the catalytic oxidation of
EB with TBHP as oxidant in the presence of GNMC. Reaction condi-
tions: 1.5 mmol EB, 2 mL CH3CN, 10 mg GNMC, molar ratio of EB/
TBHP ¼ 1/3, 80 ꢀC.
increase of the TBHP consumption rate as shown in Fig. S8.†
The values of carbon balance of the reaction on GNMC catalyst
under different reaction temperature are also shown Table S3.†
We could observe that all of the carbon balance value is close to
98 ꢃ 5%, suggesting a good mass balance and few unknown
products.
Fig. 2 (A) N 1s spectra of MC, GNMC and GNMC/SBA-15. (B) TPO
curves of GNMC/SBA-15, MC and GNMC.
the catalytic performances of GNMC were systematically inves-
tigated in the selective oxidation of EB. The blank test indicates
that there is a low reaction activity due to the EB autoxidation33
(Table 1, entry 1).
Fig. 3B shows the effect of molar ratio of EB/TBHP on the
catalytic performance of GNMC for EB oxidation. With the
decrease of EB/TBHP ratio from 1/1 to 1/3, EB conversion
increases from 51.1% to 79.5% accompanied with the increase
of AcPO selectivity from 86.8% to 97.6%. Nevertheless, further
decrease of EB/TBHP ratio to 1/5, the AcPO selectivity slightly
decreased with the slight increase of EB conversion. Therefore,
the appropriate molar ratio of EB/TBHP is 1/3. With the increase
of TBHP concentration, the amount of active oxygen groups also
increases, resulting in the increase of EB conversion. However,
high concentrated active oxygen groups might produce the side-
products, bringing about the decrease of AcPO selectivity. The
effect of reaction time on the catalytic activity of GNMC for EB
oxidation is shown in Fig. 3C. EB conversion and AcPO selec-
tivity initially increases sharply in the period 6–12 h. And with
further increase of reaction time, AcPO selectivity had no
obvious increase and the conversion for EB gradually increase.
So, here 12 h reaction period is chosen because of too long
period of 24 h.
Fig. 3A shows the effect of reaction temperature on the
catalytic activity of GNMC for the oxidation of EB. The conver-
sion of EB obviously increases from 26.4% to 79.5% with the
increase of temperature from 30 ꢀC to 80 ꢀC, and the selectivity
for AcPO increases from 34.2% to 97.6%. However, when the
temperature increases to 90 ꢀC, EB conversion slightly increases
to 80.9% and AcPO selectivity further increases to 99.1%. This
phenomenon is probably ascribed to the increasing effective
conversion of THBP to active oxygen groups when raising
temperature. As shown in Table S3,† BzA is the main side
ꢀ
product at 30 C, which signicantly decreases from 52.8% to
2.9% with the increase of temperature from 30 ꢀC to 90 ꢀC, and
the selectivity for the other two side products also decreases
with the increase of temperature. Therefore, these results show
that the reaction temperature is a key factor for the conversion
of EB to AcPO, and the appropriate temperature is 80 ꢀC.
Besides, the conversion of TBHP also increase with the
enhancement of reaction temperature (Fig. 3A). However, the
consumption rate of EB does not linearly increase with the
The results of various carbon catalysts for selective liquid-
phase oxidation of EB are summarized in Table 1. MC
exhibits higher catalytic performance than that observed in the
blank test, indicating that the carbon material itself could
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convert EB to AcPO. However, AcPO selectivity on MC is only draw into adequate nitrogen species (for example, graphitic
35.3%. CMK-3 is an ordered mesoporous carbon which nitrogen is a key for the oxidation of EB).
possesses high specic surface area and shows a little better
performance than that of MC (entry 3). This result suggests that
the catalytic activity is not directly related to the specic surface
Conflicts of interest
area of the catalysts, that is to say specic surface are is not a key
factor on catalytic activity of carbon material. There is an
There are no conicts to declare.
obvious increase of catalytic performance using GNMC as
a catalyst (entry 4), on which the selectivity for AcPO reaches to Acknowledgements
97.6% at EB conversion of 79.5%. MCN which is synthesized
The authors are grateful for the supports from National Key
according the reported method,34 is a mesoporous carbon
R&D Program of China (2018YFB1501600), the National Natural
nitrides and contains 12.6 at% nitrogen content (entry 5).
Science Foundation of China (21802149, 21773271, 21522309),
However, its catalytic activity (31.9% of EB conversion and
the China Postdoctoral Science Foundation (2017M623280), the
40.2% of AcPO selectivity, entry 5) is far lower than that of
Cooperation Foundation of Dalian National Laboratory for
GNMC, although the nitrogen content of GNMC (3.5 at%, entry
Clean Energy (DNL180303), the Natural Science Foundation of
4) is much lower than that of MCN. Besides, other nitrogen-
Jiangsu Province (BK20160395), and the Chinese Academy of
doped nanocarbon materials were also synthesized and tested
Sciences.
for EB oxidation. Nitrogen-doped onion-like carbon (NOLC)
synthesized according to literature35 only exhibits 41.3% of EB
conversion and 45.4% of AcPO selectivity (entry 6), although it
shows superior catalytic performance for the expoxidation of
Notes and references
styrene.35 Nitrogen-doped carbon nanotubes (NCNT), which
shows a good catalytic activity in some oxidation reactions,36–38
was synthesized at 900 ꢀC using imidazole as a precursor
performs 56.9% of EB conversion and 94.2% of AcPO selectivity
(entry 7). As shown in Fig. 2A, the main species of nitrogen in
GNMC is graphitic nitrogen (400.8 eV).39,40 However the main
nitrogen for MCN is not graphitic nitrogen but nitrogen trigo-
nally bonded to all sp2 carbon atoms34,41,42 as shown in Fig. S9.†
The deconvolution analysis results of N 1s for GNMC, NOLC
and NCNT are shown in Table S1,† which indicates that NOLC
and NCNT contains various nitrogen species with very low
graphitic nitrogen content. Consequently, these results suggest
that the introduction of nitrogen, especially for graphitic
nitrogen, plays a very important role in promoting the conver-
sion of EB to AcPO, which is in agreement with a previous
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