N-Doped carbon nanofibers derived from bacterial cellulose as an excellent metal-free catalyst for selective oxidation of arylalkanes

Article abstract of DOI:10.1039/c9cc00185a

N-Doped carbon nanofibers derived from one-step pyrolysis of low-cost bacterial cellulose with the assistance of urea were reported. Owing to their interconnected nanofibrous structure and high specific surface area as well as high N doping, they exhibited excellent catalytic performance for selective oxidation of arylalkanes even with O₂ as an oxidant in aqueous solution.

Full text of DOI:10.1039/c9cc00185a

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N-doped carbon nanofibers derived from bacterial cellulose as an
excellent metal-free catalyst for selective oxidation of arylalkanes
Runkun Huang,ab Changyan Cao,*ab Jian Liu,ab Dongping Sunc and Weiguo Song*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
N-doped carbon nanofibers derived from one-step pyrolysis of low- developing a low-cost, environmental-benign and simple
cost bacterial cellulose with the assistance of urea was reported. fabrication method to produce carbocatalysts with excellent
Owing to the interconnected nanofibrous structure and high performance is highly desirable for selective oxidation of
specific surface area as well as high N doping, it exhibited excellent arylalkanes.
catalytic performance for selective oxidation of arylalkanes even
with O2 as oxidant in aqueous solution.
Bacterial cellulose (BC), a polysaccharide like plant cellulose
with a,β-1,4-glycosidic linkage, could be produced by several
species of bacteria in industrial–scale and possesses intrinsic 3D
interconnected nanofibrous network structure as well as N
heteroatoms, which came from culture media and secretions.21,
The selective oxidation of arylalkanes, one type of C-H
activation, is a very important reaction in organic synthesis and
industrial process.1-5 For example, selective oxidation of
ethylbenzene to acetophenone has attracted enormous
attention due to the application of acetophenone in
manufacturing perfumes, pharmaceuticals, resins, alcohols,
esters, and aldehydes.5 Generally, catalysts based on noble
metals5-8 or transition metals9-14 are needed for this reaction.
Carbocatalyst was demonstrated as a “green” option for many
22
All these features make BC a versatile raw biomass material
for fabricating a series of 3D heteroatom-doped carbon
materials, which were widely used in many fields such as oxygen
reduction reaction electrocatalysts,23 supercapacitors,24
lithium-ion batteries25 and absorbents26. For heterogeneous
catalysis, nanofibrous structure is beneficial for internal
diffusion of reaction species and thus increase the catalytic
efficiency. However, there are rare reports about these carbon
materials being used as carbocatalysts for liquid-phase
heterogeneous catalysis.27, 28
Herein, we developed an efficient and low-cost method to
produce N-doped carbon nanofibers (denoted as N-CNF)
through one-step pyrolysis of BC with the assistance of urea.
The as-obtained N-CNF maintained the original natural 3D
interconnected nanofibrous network and possessed high N
content of 5.74% along with high specific surface area of 506
m2/g. When used as a metal-free carbocatalyst for selective
oxidation of arylalkanes with tert-butyl hydroperoxide (TBHP)
as oxidant in aqueous solution, it showed excellent activity,
selectivity and medium recyclability. Especially, it still worked
well when using O2 as oxidant with TBHP as the initiator.
Scanning electronic microscopy (SEM) and Transmission
electronic microscopy (TEM) images show that the obtained N-
CNF maintain the nanofibrous network of BC, while the mean
diameter (~ 38 nm) shrinks to nearly half of the original freeze-
dried BC (Figures 1a-1b and Figure S1). EDS mapping result
reveals the homogeneous distribution of N species in carbon
nanofibers (Figure 1d). High-resolution TEM (HRTEM) image
shows that N-CNF is mostly composed of bending graphene
traditional reactions due to the advantages in metal-free and
16
efficiency.15,
Selective oxidation of ethylbenzene has also
undergone rapid development in recent years. Ma et al.
disclosed that graphitic-type N dopants in layered carbon could
change the electronic structure of the adjacent carbon atoms,
and thus further stimulated C-H activation of ethylbenzene.17 In
our previous works, we reported efficient methods to prepare
nitrogen, phosphorus, and sulfur co-doped carbon materials for
further improving the activity of carbocatalyst for selective
oxidation of arylalkanes in aqueous solution.18-20 Despite the
high efficiency, the preparation methods for above
carbocatalysts have disadvantages in complicate processes and
high cost. From the sustainable chemistry point of view,
a. Beijing National Laboratory for Molecular Sciences, Laboratory of Molecular
Nanostructures and Nanotechnology, CAS Research/Education Center for
Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing, 100190, P. R. China.
E-mail: cycao@iccas.ac.cn; wsong@iccas.ac.cn.
b. University of Chinese Academy of Sciences, Beijing, 100049, P. R. China.
c. Institute of Chemicobiology and Functional Materials
Nanjing University of Science and Technology, Nanjing, 210094, P. R. China.
Electronic Supplementary Information (ESI) available: SEM images, N2 adsorption-
desorption isotherms and XPS analysis of controlled samples; reusability test of
catalyst. See DOI: 10.1039/x0xx00000x
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layers accompanying with amorphous and defective carbon
area (Figure 1c). Raman spectroscopy also confirms the
disordered sp3 carbon (D bands) at around 1350 cm-1 and
graphitic sp2 carbon (G bands) at 1580 cm-1 with ID/IG of 1.08
(Figure 1e). The interlayer distance between two graphitic
layers was measured to be about 0.42 nm, which was larger
than that of graphite (0.34 nm). Figure 1f shows the XRD pattern
of the obtained N-CNF. Two broad diffraction peaks centered at
~21.6° and ~44° can be observed, corresponding to the (002)
and (100) planes of carbon, respectively. Compared with
standard carbon materials, the diffraction peak of (002) planes
of N-CNF was apparently shift to lower 2θ angle. The
corresponding interlayer distance was calculated to be 0.41 nm
based on the Bragg’s law, which agreed well with HRTEM result.
The larger interlayer lattice distance demonstrated that N
atoms were successfully doped into the framework of carbon.
X-ray photoelectron spectroscopy (XPS) was then employed
to investigate the surface status of N-CNF (Figure 2). N atoms
content was measured to be 5.74% according to the full XPS
spectra (Figure 2a). The high-resolution N1s spectrum (Figure
2b) could be fitted well with four different N species. Peaks at
398.5, 400.0, 401.3 and 403.0 eV were corresponding to
pyridinic N (28.4%), pyrrolic N (11.3%), graphitic N (49.0%) and
pyridinic N+-O- (11.3%), respectively. This data further
confirmed that N atoms have been incorporated into the carbon
nanofibers.
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DOI: 10.1039/C9CC00185A
The surface area and porosity structure were determined by Figure 1 (a) SEM, (b) TEM, (c) HRTEM, (d) compositional EDS
N2 adsorption-desorption. As shown in Figure S2, N-CNF mappings of images of N-CNF; (e) Raman spectrum of N-CNF
showed a typical type II isotherm. The Brunauer-Emmett-Teller and (f) XRD pattern of N-CNF.
(BET) surface area and pore volume were calculated to be 506
m2/g and 0.62 cm3/g, respectively. In addition, the external
surface area ratio was calculated to be 64.8%. Therefore, large
external surface area and pore volume were beneficial for mass
transfer, which would promote reaction efficiency.
The key of this method is the special location of BC and urea:
BC is put on the top of and separated from urea in a crucible
with cover (apparatus illustration shown in Scheme S1). Four
different N-doped carbon materials were also prepared for
comparison: N-doped carbon nanofibers obtained by directly
pyrolysis of BC without urea under Ar (denoted as N-CNF
without urea); N-doped carbon nanofibers derived from
pyrolysis of mixed BC and urea (denoted as N-CNF-mix); N-
Figure 2 (a) Full XPS spectrum of N-CNF, (b) high resolution XPS
spectrum of N1s of N-CNF.
doped carbon nanofibers prepared by directly pyrolysis of BC in
better than those comparing catalysts (Table 1, entries 5-8) and
NH3 atmosphere (denoted as N-CNF-NH3); N-doped graphene
was comparative to the best result of reported carbocatalysts
prepared from GO through the same method as N-CNF
in the literatures (Table S1).17-20, 29-31 Especially, when using O2
(denoted as N-rGO).
as oxidant with TBHP as the initiator, 75% conversion and 94%
Ethylbenzene was first chosen as the model substrate to
selectivity to acetophenone could also be obtained with N-CNF
explore the efficiency of the prepared N-CNF with TBHP as
(Table 1, entries 3-4). When replacing O2 with N2, only 18.4%
oxidant (Table 1). The reaction condition was similar to the
conversion and 45.9% selectivity were obtained, further
literatures.18 The catalytic performances were listed in Table 1.
indicating the involvement of O2 in the former reaction and
About 29.1% conversion and 57.9% selectivity to acetophenone
initiator function of TBHP.
were observed without catalyst (Table 1, entry 1). To our
Furthermore, various arylalkanes can also be oxidized to the
delight, N-CNF showed excellent catalytic performance with
corresponding ketone compounds with high yields (Table 2),
99.6% conversion and 99.7% selectivity to acetophenone (Table
indicating the efficiency of N-CNF carbocatalyst. The catalyst
1, entry 2). This catalytic property of N-CNF was significantly
can be easily separated from the solution by centrifugation for
the next runs. It was found that a slightly loss of activity and
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selectivity during the recycle runs (Figure S3).32 The decrease of graphitic-type N atoms), which were two important features for
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DOI: 10.1039/C9CC00185A
N content from 5.74 at% to 3.87 at% (Figure S4) may be carbocatalysts in oxidation of arylalkanes. N content in N-CNF-
responsible for the loss of catalytic activity.
NH3 was measured to be only 2.99% (Figure S5), which was
lower than that in N-CNF (5.74%). Thus, urea was a cheaper and
safer N source compared to the common NH3 gas. In addition,
after pyrolysis of mixed BC and urea, the obtained N-CNF-mix
possessed lower BET specific surface area (332 m2/g) due to the
collapse of 3D interconnected network caused by the melting of
urea at around 133 °C (Figures S6-S9). This suggests the
placement location of urea and BC in our approach is critical to
prepare N-CNF with controllable N doping and nanofibrous
structure. Besides the excellent activity and selectivity, the
abundant urea and BC together make the N-CNF an efficient
and low-cost carbocatalyst for oxidation of arylalkanes.
Table 1 Catalytic performances for the selective oxidation of
ethylbenzene. [a]
To elucidate the reaction mechanism, the reaction was
stopped at reaction times of 3 h and 4.5 h, and the products
were analyzed with gas chromatography-mass spectrometer
(GC-MS) and electron paramagnetic resonance spectroscopy
(EPR). As the results shown in Figures S10-S12, 1-phenylethanol
and
1-phenylethyl-tert-butyl-peroxide
(the
possible
intermediates) were detected by GC-MS and tert-butyl
hydroperoxide radical (t-BuOO·) was observed by EPR. In order
to further confirming the intermediates, two reactions were
carried out using 1-phenylethanol and 1-phenylethyl-tert-butyl-
peroxide as starting materials under the same conditions
(Figure S12). 99% of yields were also obtained in both
situations. Obviously, according to the results of toluene
oxidation (Table S2), the intermediate benzaldehyde (in
contrast to ketones formed in all other cases) easily undergoes
further oxidation to form the corresponding carboxylic acid.
Based on the results above and the literatures, the possible
reaction mechanism was proposed in the Figure S13.
[a] Reaction conditions: catalyst (5.0 mg), TBHP (500 μL, 70 wt
% in water), ethylbenzene (0.5 mmol), H2O (3.0 mL), 80 °C, 12
h.
[b] Reaction conditions: catalyst (10 mg), TBHP (500 μL, 70 wt%
in water) as an initiator, ethylbenzene (10.0 mmol), H2O (10
mL), O2 (2.4 MPa), 100 °C, 48 h.
[c] Reaction conditions: replace O2 (2.4MPa) with N2 (2.4MPa).
[d] Reaction conditions: catalyst (3.0 mg).
[e] Reaction conditions: TBHP (300 μL).
In summary, nitrogen-doped carbon nanofibers were
successfully prepared through a one-step pyrolysis of bacterial
cellulose with the assistance of urea. Owing to the
interconnected nanofibrous structure and high specific surface
area as well as high N doping, the as-prepared nitrogen-doped
carbon nanofibers exhibited excellent activity, selectivity and
medium recyclability for selective oxidation of arylalkanes in
aqueous solution. The low-cost raw materials and one-step
pyrolysis process together with the excellent catalytic
performance make it a promising metal-free carbocatalyst.
Table 2. Oxidation of arylalkanes in the aqueous phase with N-
CNF as catalyst. [a]
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank the financial supports from the National Natural
Science Foundation of China (NSFC 21333009, 21573244,
21573245) and the Youth Innovation Promotion Association of CAS
(2017049).
[a] Reaction conditions: catalyst (5.0 mg), TBHP (500 μL, 70 wt
% in water), substrate (0.5 mmol), H2O (3.0 mL), 80 °C, 12 h.
The excellent catalytic activity of N-CNF can be attributed to
the high surface area and N doping content (especially
Notes and references
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