Rapid formation of nitrogen-doped carbon foams by self-foaming as metal-free catalysts for selective oxidation of aromatic alkanes

Article abstract of DOI:10.1016/j.apcata.2019.117400

Porous carbon materials have attracted considerable interest as metal-free catalysts. In this study, we report a nitrogen-doped and nanofiber-based porous carbon foam produced via an efficient and facile self-foaming approach and its subsequent pyrolysis; in this approach, carbon dioxide-rich ethanolamine serves as the foaming agent, N source and polymerization catalyst. Meanwhile resorcinol and formaldehyde are used as carbon sources. Carbon dioxide-rich ethanolamine plays a crucial role in the release of gas as well as initiating polymerization on the interfaces of bubbles, which directs the formation of polymer foam. The N-doped carbon foam can be a highly active metal-free heterogeneous catalyst to promote selective oxidation of the benzyl group to the corresponding phenone. In addition, the carbon foams are easily cast with different morphologies. Notably, the prepared carbon foam is fabricated as a monolithic reactor for the oxidation reaction, which also exhibits good catalytic performances in the scale-up experiment.

Full text of DOI:10.1016/j.apcata.2019.117400

Journal Pre-proof
Rapid formation of nitrogen-doped carbon foams by self-foaming as
metal-free catalysts for selective oxidation of aromatic alkanes
Guo-Xin Qin (Validation) (Formal analysis)
(Investigation)<ce:contributor-role>Writing-original draft) (Data
curation), Yan Hao (Validation) (Formal
analysis)<ce:contributor-role>Writing-original draft) (Data curation)
(Funding acquisition), Shuai Wang (Conceptualization) (Formal
analysis) (Investigation)<ce:contributor-role>Writing-original draft)
(Visualization) (Data curation) (Funding acquisition), Yu-Bin Dong
(Conceptualization)<ce:contributor-role>Writing-reviewing and
editing) (Supervision) (Funding acquisition)
PII:
S0926-860X(19)30555-1
DOI:
https://doi.org/10.1016/j.apcata.2019.117400
APCATA 117400
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
17 August 2019
23 December 2019
24 December 2019
Please cite this article as: Qin G-Xin, Hao Y, Wang S, Dong Y-Bin, Rapid formation of
nitrogen-doped carbon foams by self-foaming as metal-free catalysts for selective oxidation of
aromatic alkanes, Applied Catalysis A, General (2019),
doi: https://doi.org/10.1016/j.apcata.2019.117400

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Rapid formation of nitrogen-doped carbon foams by self-foaming as
metal-free catalysts for selective oxidation of aromatic alkanes
Guo-Xin Qin a, 1, Yan Hao b,1, Shuai Wang a, *, Yu-Bin Dong a, *
a
College of Chemistry, Chemical Engineering and Materials Science, Collaborative
Innovation Center of Functionalized Probes for Chemical Imaging in Universities of
Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education,
Shandong Normal University, Jinan 250014, P. R. China
b Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine
Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of
Education, School of Chemistry and Chemical Engineering, Henan Normal University,
Xinxiang, Henan 453007, P. R. China
1 These authors contributed equally to this work
*Corresponding author: wang_shuai27@163.com, yubindong@sdnu.edu.cn
Graphical abstracts
1

Highlights
 A new nitrogen-doped and nanofiber-based porous carbon foam was
synthesized by self-foaming.
 Carbon dioxide-rich ethanolamine takes roles as both the blowing
agents, N source and structural directing agents.
 The carbon foam is a highly active metal-free heterogeneous catalyst to
promote selective oxidation of the benzyl group to its corresponding
phenone.
2

 The carbon foam exhibits good catalytic performances in the scale-up
experiment.
Abstract
Porous carbon materials have attracted considerable interest as metal-free catalysts. In
this study, we report a nitrogen-doped and nanofiber-based porous carbon foam
produced via an efficient and facile self-foaming approach and its subsequent pyrolysis;
in this approach, carbon dioxide-rich ethanolamine serves as the foaming agent, N source
and polymerization catalyst. Meanwhile resorcinol and formaldehyde are used as carbon
sources. Carbon dioxide-rich ethanolamine plays a crucial role in the release of gas as
well as initiating polymerization on the interfaces of bubbles, which directs the
formation of polymer foam. The N-doped carbon foam can be a highly active metal-free
heterogeneous catalyst to promote selective oxidation of the benzyl group to the
corresponding phenone. In addition, the carbon foams are easily cast with different
morphologies. Notably, the prepared carbon foam is fabricated as a monolithic reactor
for the oxidation reaction, which also exhibits good catalytic performances in the scale-
up experiment.
Keywords: Carbon foam, Self-foaming, Metal-free catalysts, Selective oxidation of
aromatic alkanes, Carbon dioxide-rich ethanolamine, Monolith
1. Introduction
Selective oxidation of hydrocarbons and alcohols to the corresponding high value-added
alcohols, aldehydes and ketones is of crucial importance in the industrial manufacturing
of chemicals and intermediates [1,2,3]. Metal-based catalysts, especially noble metals or
metal oxides, are generally used in these reactions. Despite their high efficiency, metal-
based catalysts are expensive, are sometimes characterized by low selectivity and poor
durability, are sensitive to poisoning and have negative environmental impact [4,5].
3

Therefore, metal-free catalysts are indispensable for large-scale practical application and
environmental remediation. Recently, carbon materials were discovered as metal-free
alternatives for many types of heterogeneous catalytic reactions, such as oxygen
reduction and hydrogen evolution reactions [6], electrochemical reduction of CO2 [7],
oxidative dehydrogenation [8] and selective oxidation of hydrocarbons and alcohols [9-
12]. For superior catalytic performance, a carbon-based catalyst is usually required with
a hierarchical porous structure to improve mass transport [13-14], and doped
heteroatoms to change the adjacent carbon electronic structure and/or produce additional
active sites [15-17]. Thus, the preparation of highly efficient carbon-based metal-free
catalysts with the desired morphology and structure is a great challenge.
Carbon foams are a typical class of carbon materials that feature extended porous
network, low density, high porosity, good thermal and electrical transport ability, high
thermal and chemical stability [18,19]. Hence, they have been widely used as
heterogeneous catalysts or catalyst supports to promote various organic transformations
[20,21]. So far, there are several well-established approaches for preparing carbon foams,
most notably through blowing of carbon precursors or templating carbonization of
precursors [22]. Compared to template carbonization method, gas bubbles used in the
blowing approach are able to freely arrange without the aid of a template. This, can avoid
the tedious template-removing process [23-28].
As is well known, pyrolysis under pressure and foaming with blowing agents are two
methods to blow carbon foams [29]. The former has been mainly applied to pitches,
which usually need high temperature (800-1000 C) and high pressure (2-4 MPa) under
inert atmospheres [30]. Foaming with blowing agents such as supercritical carbon
dioxide also experiences a high pressure (15–30 MPa) and rapid depressurization
process [31]. Therefore, the development of a new foaming approach for the fabrication
of carbon foams under milder conditions is imperative.
In this work, we developed a facile and efficient self-foaming approach to prepare N-
doped carbon foam, in which resorcinol (R) and formaldehyde (F) were used as carbon
sources, and CO2-absorbed ethanolamine (EAC) as the blowing agent and nitrogen
4

source [32]. In addition, EAC herein also acted as a polymerization catalyst that can lead
to a very fast gelation of R with F on the bubble interface after the release of CO2. The
N-doped carbon foam thus obtained features a nanofiber-based porous nanostructure,
and it can be readily moulded into different shaped monoliths. Notably, it can act as a
metal-free catalyst to highly promote the selective oxidation of ethylbenzene under
atmospheric pressure in water with the aid of the oxidant tert-butyl hydroperoxide
(TBHP). In addition, the catalytic recyclability of the N-doped carbon foam catalyst was
also investigated.
2. Experimental
2.1 Chemical
Resorcinol (denoted R, 99.5%) was purchased from Tianjin Kermel Chemical Reagent
Co., Ltd. Formaldehyde (F, 37 wt%), ethanolamine (EA, 99%), and sodium
carboxymethyl cellulose (CMC 800-1200 mPaS) were all supplied by Sinopharm
Chemical Reagent Co., Ltd. All chemicals were used as received. The carbon dioxide
was bubbled into an aqueous solution of ethanolamine (~0.2 g·mL-1) for 20 min. The pH
of the solution was changed from 12.3 to 8.3 after the introduction of carbon dioxide.
The resulting solution, termed as EAC, was sealed for use.
2.2 Synthesis of the foam-like carbon monolith
After addition of the CMC (0.06 g) to an aqueous solution (6 mL) of R (1.1 g) and F (1.6
mL), EAC (2 mL) was quickly injected with stirring. The white polymer thus generated
was allowed to stand at 90 C (18 h) to obtain the polymer foam NPF-1. NPF-1 was
pyrolyzed at 800 °C (2 h) in argon to afford NCF-1 as the foam-like carbon monolith.
NPF-2 and NPF-3 were prepared in a manner similar to that of NPF-1. In NPF-2, no
CMC was used and its corresponding carbon product is named NCF-2; and in NPF-3,
EA was uses instead of EAC, with the carbonization product named NCF-3.
2.3 Typical catalytic procedure
Ethylbenzene oxidation. A mixture of ethylbenzene (2 mmol, 245 L), carbon foam
catalyst (20 mg, powder), and TBHP (6 mmol, 70%) in water (6 mL) was heated in a
sealed glass tube at 80 C with stirring. After the reaction, 500 µL of the solution was
5

taken out of the reaction system, and ethyl acetate was added as the extraction agent.
The mixture was shaken to transfer the substrate and product into the organic phase. The
reactant and product compositions of catalytic procedures were identified by GC-MS.
The products were quantified using the external calibration method.
Pyridine oxidation. A mixture of pyridine compounds (0.5 mmol), carbon foam catalyst
(20 mg, powder), and TBHP (1.5 mmol, 70%) in water (6 mL) was heated in a sealed
glass tube at 80 C with stirring. After reaction, 500 µL of the solution was taken out of
the reaction system and ethyl acetate was added as the extraction agent. The mixture was
shaken to transfer the substrate and product into the organic phase. The reactant and
product compositions were identified by GC-MS.
Scale-up experiment. A mixture of 4-benzylpyridine (14 mmol), tube carbon catalyst
(0.5590 g, foam tube without grinding), and TBHP (42 mmol, 70%) in water (42 mL)
was heated in a sealed glass tube at 80 C with stirring. After reaction, 500 µL of the
solution was taken out of the reaction system, and ethyl acetate was added as the
extraction agent. The mixture was shaken to transfer the substrate and product into the
organic phase. The reactant and product compositions were identified by GC-MS.
2.4 Characterizations
Scanning electron microscopy (SEM) investigations were carried out with a HITACHI
SU8000 instrument. The mechanical properties of samples were tested under uniaxial
compression using a 2 kN intelligent strength tester (model DL III, Dalian Research &
Design Institute of Chemical Industry). The mechanical strength is the maximum stress
at which macroscopic failure occurs at a strain rate of 1 mm/min. Samples were cylinders
with a length/diameter ratio of 1. FT-IR spectra were obtained using a Bruker ALPHA
FT-IR Spectrometer. Thermogravimetric analyses were carried out on a TA Instrument
Q5000 simultaneous TGA under flowing nitrogen at a heating rate of 10 °C/min. Hg
intrusion isotherms and PSDs of the pores larger than 50 nm were measured using a
Micromeritics AutoPore IV 9500 analyser. The skeletal density and porosity of NCF-1
were measured by Hg porosimetry. Raman spectras were collected on a Bruker Optics
Senterra Raman spectrometer, using a 532 nm laser. N2 sorption isotherms were
6

measured with a Micromeritics ASAP 2020 adsorption analyser at 77 K. Prior to each
adsorption experiment, the sample was degassed for 6 h at 200 °C, ensuring that the
residual pressure fell below 510-3 mbar; it was then cooled to the target temperature,
followed by the introduction of pure N2 gas into the system. The Brunauer-Emmett-
Teller (BET) method was used to calculate the specific surface area. Total pore volumes
were calculated from the amount of nitrogen adsorbed at a relative pressure, P/P0, of
0.99. Micropore volumes were calculated using the t-plot method. X-ray photoelectron
spectroscopy (XPS) was carried out with a VG Scientific ESCALAB Mk ii instrument.
The reactant and product compositions of catalytic procedures were identified by GC-
MS (Agilent HP6890/5973N). The expansion ratio in this article is expressed as follows,
expansion ratio = (V2 – V1)/ V1, where, V1 is the initial volume of the precursor solution
and V2 is the volume of the foam.
3. Results and discussion
Fig. 1. Schematic representation of the formation of foam-like carbon monoliths.
7

The synthesis procedure of foam-like carbon monolith is schematically illustrated in Fig.
1. Taking the preparation of NCF-1 as an example, after the addition of EAC to the
solution of resorcinol (R), formaldehyde (F) and CMC (Fig. 2a), the transparent solution
turned into a white phenolic resin oligomer-based emulsion accompanied by the release
of large amount of heat. In this emulsification process, the generated heat helped to bring
about the rapid release of CO2 from EAC and in turn, the CO2-desorbed EA induced the
fast resin oligomer cross-link on the interface of the CO2 bubbles. Due to a synergistic
effect, the stable oligomer foam could be obtained within 1 min (Fig. 2). After curing
and drying at 90 C, the foam-like polymeric monolith of NPF-1 was generated with a
volume of 36.052.57 cm3 (Fig. 3a), which achieved a 2.400.24 volume expansion
compared with that of the precursor solution (Table S1). After the pyrolysis of NPF-1 at
800 °C for 2 h in argon, the resulted NCF-1 retained the foam morphology with a
macrocellular structure (Fig. 3d), with a carbonization yield of ~41% and a total mass
yield of ~32%. Its foam volume, however, decreased from 36.052.57to 11.200.63 cm3
after carbonization. More photographs of NPF-1 and NCF-1 are also shown in Fig. S1.
The densities of NPF-1 and NCF-1 are calculated as ca. 0.046 and 0.059 g cm-3,
respectively. The bulk density of NCF-1 is 0.30 g cm-3, with a skeletal density of 0.56 g
cm-3 and a porosity of 82%. In addition, the preliminary mechanical strength test
indicated that the foam-like monoliths of NPF-1 and NCF-1 can readily bear 0.53 MPa
(76.9 psi) and 0.45 MPa (65.12 psi), respectively. Their high mechanical strength shown
herein should be attributed to their mutually supported foam skeleton.
8

Fig. 2. Photographs of the preparation steps of NPF-1 with time: t= 0 s (a), t= ~5 s (b),
t= ~30 s (c) and t= 60 s (d).
In general, the formation of polymeric foams depends on the complicated interplay
between the initial foaming ability and the subsequent solidification of the precursor
solutions [33]. Herein, the open cell foam preparation approach we developed is very
simple. As shown above, the generated small CO2 bubbles that were triggered by the
exothermic phenol-formaldehyde condensation, could fuse with neighbouring bubbles
to form porous foam materials with open cells.
9

Fig. 3. Photographs of NPF-1 (a) and NCF-1 (d) and their SEM images: NPF-1 (b, c)
and NCF-1 (e, f).
As is well known, the bubbles are usually unstable due to their high surface energy in
the gas-liquid interfaces. To minimize this energy, bubbles have a tendency to coalesce
and disproportionate, whereby the gas diffuses from smaller to larger bubbles. The
stability of the foam depends on the hindrance of bubble coalescence. The viscosity of
the precursor is a key factor to control the rate of coalescence and the cell size and struts
of the resultant foam. In this work, gelatin, PVA, and CMC were chosen as the thickeners
to improve the viscosity of the precursor. We found that only CMC can significantly
increase the binding ability of the precursor to bubbles and reduce coalescence of
bubbles in the blowing procedure. Therefore, CMC was indispensable and it ensured the
formation of polymer foam with a high expansion ratio. For example, without the aid of
CMC, the bubbles coalesced and disappeared quickly in NPF-2, and the obtained NPF-
2 showed an irregular foam framework (Fig. S2a-c) with a volume expansion of 1.04,
which is significantly lower than that of NPF-1.
The microstructure of NPF-1 was shown Fig. 3b-c; its cell walls exhibit a three-
dimensional network nanostructure constituted by nanofibers with a diameter of
approximately 500 nm. When the sample was synthesized without CMC, NPF-2 was
obtained as non-uniform polymer spheres with diameters ranging from several hundred
nanometers to approximately 3 μm (see Fig. S3a). When using EAinstead of EAC, NPF-
3 was generated as irregular polymer nanoparticles that had an average diameter of
several hundred nanometers (Fig. S3c). It is said that cellulose forms linear and partially
aggregated crystalline configurations [30]. Based on the above observation, it can be
speculated that CMC and EAC may synergistically change the surface property of the
precursor, leading to the formation of a nanofiber network (Fig. 3c). After thermal
treatment at 800 °C, the SEM images (Fig. 3e-f) showed that the cell walls of NCF-1
featured more interconnected macropores stacked by irregular carbon nanofibers,
although its macroscopic shape was retained with a shrinking volume.
As shown in Fig. 3, NPF-1 and NCF-1 reveal well-developed macroporous frameworks.
10

Mercury intrusion measurement is further performed to comprehensively analyse their
macroporous characteristics (Fig. 4). The mercury intrusion data show that NPF-1 has
macropore sizes concentrated at ∼ 60, 90 and 150 μm. After carbonization, the
macropores of NCF-1 are close to ∼50, 70 and 180 μm. These indicate that both NPF-
1 and NCF-1 possess abundant macroporosity and large macropore volumes.
Fig. 4. Pore size distributions of the pores larger than 50 nm of NPF-1 (a) and NCF-1
(b) .
NPF-1, NPF-2 and NPF-3 show similar characteristic bands in their FT-IR spectra,
which are different from that of CMC. As indicated in Fig. 5a, the broad band at
approximately 3289 cm-1 should be assigned to O-H stretching vibrations or water
molecules, while the peaks approximately 1226 cm-1 is associated with the C-N
stretching vibration. Hence, the existence C-N species in the polymer foam is well
supported by the evidence. Moreover, the addition of CMC does not change the surface
characteristics of the N-doped polymer foam.
The TGA curves of NPF-1, NPF-2 and NPF-3 are shown in Fig. 5b, they show similar
thermal behaviour, and respectively exhibit approximately 43.4, 47.3 and 46.3% of the
residual carbon at 800 °C. Such a high carbon yield indicates these polymers are
excellent precursors for the production of carbonaceous materials [34-36].
The N2 sorption isotherm of NCF-1 is of type I, indicating its microporous structure (Fig.
11

5c). The specific surface area of the NCF-1 was determined to be ~518 m2·g–1, and it is
higher than that of tannin-based carbon foam [25-27]. Moreover, the corresponding pore
size distribution curve display the distribution of micropores mainly at ~0.6 nm (Fig.
S4). Together with the microstructure shown in the SEM images and mercury intrusion
measurement, such carbon foam possesses hierarchical micro-/macropore system, which
is beneficial for mass adsorption and transportation. NCF-2 and NCF-3 exhibited types
of isotherms similar to that of NCF-1 (Fig. 5c) and their porous structural parameters
are shown in Table S2. It seems that the addition of CMC did not change the porous
characters, although the CMC has a carbon residue up to 31.5%.
Raman spectroscopy was used to further examine the microstructure of the carbon foam.
As indicated in Fig. 5d, NCF-1 showed two broad peaks at 1335 and 1584 cm-1, which
are related to the D-band and G-band, respectively. The G-band corresponds to the bond
stretching of sp2 carbon, while the D band reflects the disordered carbon structure of
NCF-1, as also shown by the spectra of NCF-2 and NCF-3 [37]. The intensity ratio of
the D-band to the G-band is 0.98, which combined with their large band widths is
indicative of significant structural disorder. Weak and broad 2D peaks are additional
evidence for their structural disorder [38,39].
12

Fig. 5. FTIR spectra (a) and TGA results (b) of NPF-1, NPF-2, NPF-3 and CMC.
Nitrogen sorption isotherms (c) and Raman spectra (b) of NCF-1 NCF-2 and NCF-3.
The isotherms of NCF-1 and NCF-2 were offset by 40 cm3·g–1and 80 cm3·g–1 (STP),
respectively.
Besides the viscosity of the precursor, the foam solidification rate is another crucial
factor in controlling the excessive coalescence of bubbles. As we know, the rapid
polymerization of phenolic resin should be beneficial to maintaining the foam structure.
In contrast, slow phenolic condensation, however, would result in a weak foam skeleton
that could not effectively prevent the bubble expansion, consequently, causing the
collapse of the foam framework. Compared to EAC, EA showed higher alkalinity, and
led to very fast cross-linking and solidity (5 min). As a result, the self-bubbled EAC,
together with the in situ generated EA, resulted in the foam-like carbon monolith of
NPF-1 with a macrocellular structure under mild reaction conditions. The polymer
monolith of NPF-3 which was prepared based on EA rather than EAC showed no foam
13

structure (Fig. S2d-f).
As EAC played an important role in the blowing of carbon foam, the influence of the
amount of EAC on the formation of polymer and carbon foam was investigated. NPF-1
was obtained with a volume expansion of 2.400.24 at an EAC/resorcinol molar ratio
(denoted EAC/R) of 0.62. When the EAC/R was reduced to 0.31, the polymer foam only
exhibited a lower volumetric expansion of 0.520.16. When the EAC/R was increased
to 0.93, the volumetric expansion of the polymer foam reached 3.360.12, with a volume
of 49.41.45 cm3. The volumetric expansion of the obtained polymer foam increased
with the amount of EAC, which is shown in Fig. 6a and Table S3. The increased amount
of EAC could release more CO2 for foaming. More EAC could generate more EA, which
could accelerate the speed of solidification of the precursor solution. However, when
EAC/R > 0.93, the change in volumetric expansion became constant. That may be
because when the EAC was excess, the speed of the precursor solution solidification was
much faster than that of CO2 release, which would affect the foaming efficiency. Hence,
the formation of foam framework required a match of foaming rate and solidification
rate.
14

Fig. 6. Expansion of polymer foam as a function of EAC/R (a) and XPS spectra of N1s
of NCF-1 (b), NCF-2 (c) and NCF-3 (d).
In addition to being the blowing agent, EAC also played the role of nitrogen source. The
FT-IR analysis has shown the existence of nitrogen species in the obtained polymers of
o
NPF-1, NPF-2 and NPF-3. Upon pyrolysis at 800 C, the corresponding N-doped
carbon foams of NCF-1, NCF-2 and NCF-3 were obtained with nitrogen amounts of
2.11, 2.67 and 2.73% respectively, which was determined by X-ray photoelectron
spectroscopy (XPS). XPS also demonstrated the nature of the nitrogen species on the
surface of the carbon foams. For example, there are two different types of nitrogen
species on the NCF-1 surface, i.e., pyridinic-N (398.7±0.3 eV, 15%) and quaternary
nitrogen (401.4±0.5 eV, 85%) (Fig. 5b and Table S4). Similar results were observed in
NCF-2 and NCF-3, and the corresponding ratios of pyridinic-N/quaternary nitrogen
were 29/71 and 28/72%, respectively (Fig. 5c-d, Table S4). Therefore, the quaternary-N
in the carbon foams was the main N-doped species after high temperature treatment (800
15

oC) [40-41]. The C 1s spectrums of carbon foams are shown in Fig. S5.
Based on this EAC-based self-foaming and EA-catalysed in situ polymerization
approach, the shape of polymeric foams could be easily handled on demand by changing
the mould. For example, NPF-1 could be readily processed into tubes with different
thicknesses, sticks and plates. Their morphologies, however, could be completely
maintained during the carbonization processes (Fig. 7). This feature would significantly
expand the application scope of the carbon foams. As shown in Fig. S6, a cuplike reactor
could be readily realized by this templating approach, and it could be a simplified fixed-
bed reactor model for the organic transformation being studied.
Fig. 7. Photograph of polymeric foam tube and stick (a) and their corresponding carbon
monoliths (b), polymeric foam tube with thinner wall thickness and its corresponding
carbon monoliths (c), and polymeric foam plate (d) based on NPF-1 and NCF-1,
respectively.
As we know, the N atoms can activate the benzylic C-H bond, and phenolic resin
oligomer can effectively adsorb aromatics from aqueous solution [42-48]. By taking
16

advantage of the N-doped species, together with the high surface area, we assumed that
the obtained NCF-1, NCF-2 and NCF-3 should be good catalysts for activation of the
benzylic C-H in aqueous medium. Their catalytic performances were evaluated using
the selective oxidation of ethylbenzene in the presence of TBHP oxidant under
atmospheric pressure in water. We investigated the effect of stirring speed on the activity
of ethylbenzene oxidation with NCF-1 to eliminate the external diffusion effect. The
results (Fig. S7a) showed that the catalytic activity increased with increasing of stirring
speed up to 500 rpm, but reduced with a higher stirring speed. This may be because in a
higher stirring speed a relatively large vortex was formed, and instability of droplet
limited the increasing trend of activity with a higher stirring speed [49-50]. Thus, 500
rpm was chosen for further experiment. We also investigated the effect of reaction
temperature on the activity of ethylbenzene oxidation with NCF-1. As shown in Fig.
S7b, 80 oC and 90 oC show a small difference in catalytic activity. So, we choose 80 oC
as reaction temperature for energy saving. After optimization, it was found that
ethylbenzene was oxidized to acetophenone in 93% conversion (selectivity, 91%) in
water at 80C within 24 h in the presence of NCF-1 (Table 1, entry 1). Without the aid
of NCF-1, the oxidation conversion of ethylbenzene is only ~15% with 78% selectivity
(Table 1, entry 2). In contrast, NCF-2 and NCF-3 showed selective oxidation
conversions of 69% and 91% under the same reaction conditions (Table 1, entries 3 and
4). The acetophenone formation rates based on the molar amount of acetophenone per
mass of catalyst indicated that the catalytic activity of the carbon foams followed the
order of NCF-1>NCF-3>NCF-2 (Fig. S8). The high catalytic activity of the carbon
foams is owing to the doping of N and hierarchical porous structure. [51-54]. NCF-1 has
a continuous porous framework with more interconnected pores, which might be more
beneficial for the mass transfer of reactant and product, and consequently improves the
efficiency of ethylbenzene oxidation. In addition to the porous structure, NCF-1 shows
the largest number of active sites, as shown in the CO2-temperature programmed
desorption files (Fig. S9). For practical application, the recyclability of NCF-1 was
examined. NCF-1 still showed excellent activity and the conversion was above 80%
17

with high >90% selectivity after three catalytic cycles (Fig. S10), indicating that NCF-
1 featured good stability under the given reaction conditions. In addition to ethylbenzene
oxidation, NCF-1 was also used to promote the selective oxidation of 2-benzylpyridine,
4-benzylpyridine and 4-(4-nitrobenzyl) pyridine. The corresponding products of 4- or 2-
benzoylpyridines were obtained in moderate to good yields ranging from 64 to 84%
without by products (Table 1, entries 5 and 6). Notably, for the 4-(4-nitrobenzyl) pyridine
oxidation, the reactant was almost completely converted to the corresponding product
(Table 1, entry 7). In contrast, the 2-(4-chlorobenzyl) pyridine oxidation afforded much
lower conversion but with excellent selectivity (Table 1, entry 8). The performance of
NCF-1 on selective oxidation of benzylpyridines is better than that of NCF-3 (Table S5).
The NCF-1 based oxidation described herein might provide an alternative way to obtain
benzoylpyridines, which are known as very useful intermediates in the synthesis of
pharmaceuticals [55]. For practical application, a cuplike reactor (Fig. S6), which can be
used as a simplified fixed-bed catalytic model, was fabricated and used in the scale-up
of the oxidation of 4-benzylpyridine (catalyst 0.5590 g, pyridine 14 mmol, water 42 mL,
and 70% TBHP 42 mmol), and the result showed that an 89% conversion
(selectivity, >99%) was achieved after 30 h. Thus, the catalytic activity of NCF-1 was
well maintained within the carbon foam-based cuplike reactor. After reaction, the
product and catalyst were easily separated, and the cuplike reactor could be directly used
for the next catalytic cycle by simply washing with water/ethanol and vacuum-drying.
The conversions and products were determined by GC-MS (Fig. S11-S13).
Table 1 Catalytic activities of the carbon foams in the oxidation of ethylbenzene a and
pyridine compounds c.
Conversion Selectivity
Entry Catalyst Substrate
Product
(%) d
(%)
O
1
93
91
NCF-1
18

O
2 b
15
69
78
90
--
O
3
NCF-2
O
4
5
6
91
84
64
92
NCF-3
NCF-1
NCF-1
O
O
>99
>99
N
N
N
N
O
N
7
8
>99
24
>99
>99
NCF-1
NCF-1
N
NO2
NO2
O
N
N
Cl
Cl
a Reaction conditions: carbon foam catalyst (20 mg), ethylbenzene (2 mmol), 70% TBHP
b
c
(6 mmol), and H2O (6 mL), 80 °C and 24 h. Catalyst (0 mg). Carbon foam catalyst
(20 mg), pyridine compounds (0.5 mmol), 70% TBHP (1.5 mmol), H2O (6 mL), 80 °C,
24 h. d Conversion was determined by GC-MS.
4. Conclusions
In summary, we demonstrated a first-of-its-kind facile strategy to create N-doped carbon
foam via self-bubbling with carbon dioxide-rich ethanolamine as the blowing agent and
structural directing agents, which could cause the RF precursor to rapidly generate a
19

great volume expansion at room temperature and atmospheric pressure. The obtained N-
doped carbon foam exhibits a three-dimensional network nanostructure constituted by
nanofibers. The hierarchical porous structures provide a highly accessible surface area
full of active sites, while the nitrogen-doping transport could change the electronic
features of the carbon. These advantages make them good metal-free catalysts for
selective oxidation of C-H in ethylbenzene and benzylpyridines. The prepared carbon
tube can be directly used as a monolithic reactor for the oxidation reaction, which
exhibits good catalytic performance in scale-up experiments. This study provides a
reliable and facile route to prepare doped carbon-based metal-free catalysts with superior
catalytic properties.
Credit Author Statement
Guo-Xin Qin: Validation, Formal analysis, Investigation, Writing-Original Draft, Data
curation
Yan Hao: Validation, Formal analysis, Writing-Original Draft, Data curation, Funding
acquisition
Shuai Wang: Conceptualization, Formal analysis, Investigation, Writing-Original Draft,
Visualization, Data curation, Funding acquisition
Yu-Bin Dong: Conceptualization, Writing-Reviewing and Editing, Supervision,
Funding acquisition
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (Nos. 21701103 and 21671122), Natural Science Foundation of Henan province
20

(162300410175), Key Scientific Research Project of Colleges and Universities in Henan
(17A150031), Henan Normal University Doctoral Research Funds (5101039170150).
21

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