Biomass chitosan-derived nitrogen-doped carbon modified with iron oxide for the catalytic ammoxidation of aromatic aldehydes to aromatic nitriles

Article abstract of DOI:10.1016/j.mcat.2020.111293

Nitrogen-doped carbon catalysts have attracted increasing research attention due to several advantages for catalytic application. Herein, cost-effective, renewable biomass chitosan was used to prepare a N-doped carbon modified with iron oxide catalyst (Fe₂O₃@NC) for nitrile synthesis. The iron oxide nanoparticles were uniformly wrapped in the N-doped carbon matrix to prevent their aggregation and leaching. Fe₂O₃@NC-800, which was subjected to carbonization at 800 °C, exhibited excellent activity, selectivity, and stability in the catalytic ammoxidation of aromatic aldehydes to aromatic nitriles. This study may provide a new method for the fabrication of an efficient and cost-effective catalyst system for synthesizing nitriles.

Full text of DOI:10.1016/j.mcat.2020.111293

Molecular Catalysis xxx (xxxx) xxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Biomass chitosan-derived nitrogen-doped carbon modified with iron oxide
for the catalytic ammoxidation of aromatic aldehydes to aromatic nitriles
,
,
Wei David Wanga *, Fushan Wangb, Youcai Changc, Zhengping Donga *
a State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, PR China
b Lanzhou Petrochemical Company, PetroChina, Lanzhou 730060, PR China
c Gansu Yinguang Juyin Chemical Co., Ltd, Baiyin, Gansu 730900, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Nitrogen-doped carbon catalysts have attracted increasing research attention due to several advantages for
catalytic application. Herein, cost-effective, renewable biomass chitosan was used to prepare a N-doped carbon
modified with iron oxide catalyst (Fe2O3@NC) for nitrile synthesis. The iron oxide nanoparticles were uniformly
wrapped in the N-doped carbon matrix to prevent their aggregation and leaching. Fe2O3@NC-800, which was
subjected to carbonization at 800 ◦C, exhibited excellent activity, selectivity, and stability in the catalytic
ammoxidation of aromatic aldehydes to aromatic nitriles. This study may provide a new method for the fabri-
cation of an efficient and cost-effective catalyst system for synthesizing nitriles.
Chitosan
Nitrogen-doped carbon
Iron oxide nanoparticles
Catalytic ammoxidation
Aromatic nitriles
1. Introduction
from aldehydes using hexamethyldisilazane, NaN3, and K3[Fe(CN)6] as
the nitrogen source or using I2, sulfuryl fluoride, and TiCl4 as the cata-
Doping with nitrogen atoms can regulate the electron density of
carbon materials, thereby considerably improving the conductivity,
surface activity, and surface hydrophilicity of N-doped carbon (NC)
materials [1,2]. In addition, it also contributes to the excellent perfor-
mance of N-doped carbon catalysts in various catalytic reactions [3–5].
Recent studies have reported that N-doped nanostructured carbon
derived from renewable biomaterials exhibits high activity [6–8].
Notably, as one of the renewable bioresources, chitosan is not only
cost-effective but also abundant, and it can be produced by the deace-
tylation of chitin, which widely exists in the shell or exoskeleton of
shrimps, crabs, and other arthropods in nature. Renewable chitosan is a
more cost-effective precursor for the preparation of N-doped carbon
materials compared with that of other carbon materials such as nano-
tubes and graphene [9–11].
lysts under mild reaction conditions [16–20]. However, the
above-mentioned methods are toxic or produce considerable amounts of
by-products. And a majority of these methods exhibit some disadvan-
tages, such as poor selectivity or narrow range of synthetic substrates for
producing diverse nitriles. Hence, it is crucial to develop an environ-
mentally friendly and efficient strategy to produce nitriles.
Heterogeneous catalysts with transition-metal@N-doped carbons
exhibit excellent properties for the further improvement of their activity
and stability, and the metal particles wrapped within often exhibit su-
perior synergistic effects [21–25]. Remarkably, iron-based N-doped
carbons have been widely investigated because the transition-metal iron
is abundant, and it exhibits low toxicity and a low price [26–28]. Hence,
iron-based N-doped carbons can be a suitable candidate for replacing
some current precious-metal-based catalysts. Due to these advantages,
in-depth studies have been performed to improve the activity of
iron@N-doped carbons and realize environmentally benign syntheses in
recent years [29–31]. Clearly, iron oxide exhibits exquisite selectivity
for nitriles, which is the precursor for advanced chemicals, bioactive
molecules, and natural materials [32].
Conventional nitrile synthesis methods require highly poisonous
metal cyanides as precursors and an additional carbon atom from the
cyanide nucleophile at high temperature. In addition, nitriles can be
obtained from corresponding substrates containing amines [12], amides
[13], alcohols [14], and aldoximes [15] by the dehydration of oxime
intermediates or the oxidation of imine intermediates. However, these
methods often require high temperature and strong oxidants. An
attractive alternative pathway is the direct oxidative synthesis of nitriles
In this study, Fe2O3@NC catalysts were successfully synthesized
using renewable biomass chitosan, cost-effective urea, and abundant
ferric chloride as starting materials (Scheme 1). Fe2O3 nanoparticles
* Corresponding authors.
E-mail addresses: ww@lzu.edu.cn (W.D. Wang), dongzhp@lzu.edu.cn (Z. Dong).
https://doi.org/10.1016/j.mcat.2020.111293
Received 3 August 2020; Received in revised form 18 October 2020; Accepted 24 October 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Wei David Wang, Molecular Catalysis, https://doi.org/10.1016/j.mcat.2020.111293

W.D. Wang et al.
Molecular Catalysis xxx (xxxx) xxx
ammonia as a green nitrogen source for the ammoxidation reaction
represents the green synthesis advantage. Thus, this study may open a
cost-effective and environmentally friendly strategy for aromatic nitrile
synthesis.
2. Results and discussion
2.1. Catalyst preparation and characterization
The processes of the catalyst preparation and the catalytic ammox-
idation of benzaldehydes to benzonitriles are shown in Scheme 1. The
detailed synthesis procedures are provided in the Supporting Informa-
tion. And the successful preparation of Fe2O3@NC catalysts is further
investigated as following.
Scheme 1. The preparation procedure and catalytic ammoxidation application
of Fe2O3@NC-800 catalyst.
(NPs) were wrapped within chitosan-derived N-doped carbons, which
enhanced the stability of Fe2O3 in catalysis. Notably, Fe2O3@NC-800
(Fe2O3@NC catalyst calcinated at 800 ◦C) exhibited excellent catalytic
activity, selectivity, and stability for nitrile synthesis from the ammox-
idation of benzaldehydes. In this heterogeneous catalysis, using
TGA analysis of the as-made Fe2O3@NC catalyst precursor shows the
evolution of precursor material through heating (Fig. S1). Marginal
◦
weight loss at temperatures of less than 144 C was observed for the
precursor material probably due to water loss. With increase in calci-
nation temperature, the urea in the precursor started to decompose.
Fig. 1. Electron microscopy analysis of Fe2O3@NC catalysts: TEM images of (a) Fe2O3@NC-500, (b) Fe2O3@NC-600, (c) Fe2O3@NC-700, (d) Fe2O3@NC-800, (e)
Fe2O3@NC-900; (f) HRTEM images of Fe2O3@NC-600, Fe2O3@NC-700, Fe2O3@NC-800 and Fe2O3@NC-900; (g) EDX analysis of Fe2O3@NC-800; (h) element
distribution diagram of Fe2O3@NC-800.
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Molecular Catalysis xxx (xxxx) xxx
Fig. 2. (a) Raman spectra and (b) XRD patterns of a series of Fe2O3@NC catalysts.
Hence, a weight loss of 44.1 % is observed between 144 ◦C and 245 ◦C.
When the temperature reached 356 ◦C, the weight of the precursor
material decreased by 28.0 %, probably due to the pyrolysis of a ma-
jority of the oxygen-containing functional groups in the precursor. At a
temperature greater than 356 ◦C, pyrolysis was almost completed, and
the material weight slowly decreased.
approximately 15 nm (Fig. 1d). N-doped carbon carriers were continu-
ously pyrolyzed and transferred toward carbon nanotubes. Fe2O3@NC-
800 is a type of a floccule porous material between the folded nanosheet
and nanotube. The TEM image of Fe2O3@NC-900 revealed that the N-
doped carbon carrier changes to nanotubes and that Fe2O3 NPs with an
average diameter of 20 nm are wrapped within the nanotube (Fig. 1e).
Therefore, the calcination temperature affects the particle size of Fe2O3
NPs in the Fe2O3@NC catalysts and the morphology of N-doped carbon
materials. High calcination temperatures afford large Fe2O3 NPs, and
because Fe is one of the catalysts used to prepare carbon nanotubes, the
carbon matrix in the material is gradually converted into carbon nano-
tubes [33]. HRTEM images of Fe2O3@NC-600, Fe2O3@NC-700,
Fe2O3@NC-800, and Fe2O3@NC-900 revealed that Fe2O3 NPs are coated
with N-doped carbon (Fig. 1f). The lattice fringe of the C(002) crystal
surface wrapped around Fe2O3 NPs is observed. The EDX analysis of
Fe2O3@NC-800 confirmed the presence of Fe, C, N, and O in
Fe2O3@NC-800 (Fig. 1g). The elemental distribution of Fe2O3@NC-800
confirmed the successful embedding of Fe2O3 NPs into N-doped carbon
TEM analysis revealed that the calcination temperature obviously
affects the morphology of the prepared catalyst (Fig. 1a-e). When cal-
cinated at 500 ◦C, Fe2O3@NC-500 exhibited a pleated nanosheet, which
makes it difficult to observe Fe2O3 NPs (Fig. 1a). This is probably related
to the difficulty in transferring Fe3+ to Fe2O3 NPs at 500 ◦C and failure to
form the related Fe2O3 NPs. TEM image of Fe2O3@NC-600 revealed that
the morphology of the catalyst prepared at 600 ◦C remains unchanged,
but the Fe2O3 NPs started to form (Fig. 1b). Fig. 1c shows TEM image of
Fe2O3@NC-700. The average particle size of Fe2O3 NPs in Fe2O3@NC-
700 is about 12 nm. However, the morphology of the carbon matrix was
changed. With increase in the carbonization temperature to 800 ◦C, the
Fe2O3 NPs still maintained a smaller size with an average diameter of
Fig. 3. XPS analysis of Fe2O3@NC-800 (a),C 1s (b), N 1s (C) and Fe 2p (d).
3

W.D. Wang et al.
Molecular Catalysis xxx (xxxx) xxx
(Fig. 1h).
Table 1
Raman spectra mainly showed that D (1350 cm–1) and G (1587
Preparation of benzonitrile from benzaldehyde and ammonia by catalysts:
comparison of catalytic activity under different conditions.a.
cm–1) peaks for a series of Fe2O3@NC catalysts varied with the tem-
perature (Fig. 2a). The relative intensity ratio of ID/IG represents the
density of defects in the catalysts. The ID/IG value exerted a certain
relationship with the carbonization temperature. The ID/IG firstly
increased and then decreased. Moreover, with increase in carbonization
temperature to 700 ◦C, the ID/IG value was the maximum, and then it
decreased with increased carbonization temperature. The higher the
carbonization temperature, the higher the graphitization degree of the
prepared materials. However, due to the effect of Fe, additional defects
were observed in the vicinity of Fe2O3 NPs. Therefore, this change is
caused by the effect of changes from carbon matrix to carbon nanotubes
with increase in carbonization temperature. XRD patterns of different
Fe2O3@NC materials shows a wide diffraction peak at 26◦, corre-
sponding to the presence of the C(002) crystal plane in all samples
(Fig. 2b). Especially, Fe2O3@NC-900 prepared at a high carbonization
temperature of 900 ◦C exhibited strong peak strength. By comparing the
XRD patterns of different Fe2O3@NC samples, a new peak at 44.7◦ which
attributed to Fe(110) crystal plane was observed in Fe2O3@NC-900
pattern [34]. In addition, it was difficult to observe the characteristic
peak for the Fe(110) crystal plane in Fe2O3@NC-500, Fe2O3@NC-600,
Fe2O3@NC-700, and Fe2O3@NC-800, because Fe NPs were hardly
formed at such carbonization temperatures.
Entry
Catalyst
T
Ammonia
(μL)
Solvent
Yield
(%)
(◦C)
1
2
3
4
5
Fe2O3@NC-500
(30 mg)
60
60
60
60
60
800
800
800
800
800
EtOH
EtOH
EtOH
EtOH
EtOH
0.15
Fe2O3@NC-600
(30 mg)
0.42
Fe2O3@NC-700
(30 mg)
32.00
69.25
59.79
Fe2O3@NC-800
(30 mg)
Fe2O3@NC-900
(30 mg)
6
7
NCNS (50 mg)
Fe2O3@NC-800
(10 mg)
60
60
800
800
EtOH
EtOH
trace
13.26
8
Fe2O3@NC-800
(50 mg)
60
60
60
60
60
50
70
60
60
800
800
800
800
800
800
800
600
700
EtOH
DMSO
DMF
99.45
78.61
49.37
29.93
27.25
96.39
69.65
46.82
63.64
9
Fe2O3@NC-800
(50 mg)
10
11
12
13
14
15
16
Fe2O3@NC-800
(50 mg)
Fe2O3@NC-800
(50 mg)
1, 2-
To investigate the specific surface area, pore diameter and pore
volume of Fe2O3@NC-800, nitrogen adsorption–desorption isotherms
were recorded (Fig. S2a). A typical type IV nitrogen adsorption isotherm
was observed for Fe2O3@NC-800, and hysteresis was observed in the
region of the medium and high pressures, near P/P0 = 0.4, the relative
pressure of closed, indicative of a mainly mesoporous aperture distri-
bution for Fe2O3@NC-800 [35]. The specific surface area of
Dichloroethane
n-Hexane
Fe2O3@NC-800
(50 mg)
Fe2O3@NC-800
(50 mg)
EtOH
EtOH
EtOH
EtOH
Fe2O3@NC-800
(50 mg)
Fe2O3@NC-800
(50 mg)
Fe2O3@NC-800 was 418.8 m2
g
–1. Such a high specific surface area
Fe2O3@NC-800
(50 mg)
plays an active role in catalysis. Fig. S2b shows the aperture distribution
curve of Fe2O3@NC-800. Consistent with the conclusion from the ni-
trogen adsorption–desorption isotherm analysis, the types of pores in
Fe2O3@NC-800 are mainly mesoporous: at 4.2 nm and 20–50 nm.
Mesopores are more favorable for the diffusion of the substrate within an
abundant pore size distribution.
a
Reaction condition: benzaldehyde (1 mmol), solvent (5 mL), ammonia (28
%–30 %) and at O2 atmosphere for 24 h.
ICP–OES were 1.73 %, 1.88 %, 4.22 %, 5.43 %, and 6.03 %.
Fig. 3a shows the wide-spectrum XPS of Fe2O3@NC-800 catalyst. The
result was consistent with the EDX results: C, N, O, and Fe were present
in Fe2O3@NC-800. The characteristic signal of Fe 2p was relatively
weak, possibly caused by Fe2O3 NPs being surrounded by N-doped
carbon. Fig. 3b shows the high-resolution C 1s spectrum of Fe2O3@NC-
2.2. Catalytic ammoxidation of benzaldehyde to benzonitrile
The catalytic properties of the prepared Fe2O3@NC materials were
examined by ammoxidation of benzaldehyde to benzonitrile. Table 1
summarizes the optimization of reaction conditions. Fe2O3@NC cata-
lysts prepared at different calcination temperatures exhibited different
catalytic properties under the same reaction conditions, and Fe2O3@NC-
800 exhibited the highest catalytic activity (entries 1–5). When N-doped
carbon nanosheets (NCNS) without iron was used as the catalyst, there
was no conversion of benzaldehyde (entry 6), indicating that the Fe2O3
should be the active sites for the catalytic ammoxidation of benzalde-
hyde to benzonitrile. With the reduction in the amount of the
Fe2O3@NC-800 catalyst to 10 mg, the conversion rate of benzaldehyde
was low at 24 h (entry 7). By using 50 mg of the Fe2O3@NC-800 catalyst,
almost all of the benzaldehyde was converted into benzonitrile at 24 h
(entry 8). Low-polarity solvents gave poor reaction results, and the yield
of benzonitrile accordingly decreased with the gradual decrease in the
solvent polarity (entries 8–12). Therefore, ethanol is the optimal solvent
herein. Reaction temperature certainly affected the reaction yield. The
yield of benzonitrile was the highest at 60 ◦C, and the yield decreased
with the decrease in the temperature to 50 ◦C (entry 13). However, with
increase in reaction temperature to 70 ◦C, the benzonitrile yield reduces
to about 70 %. The reason is that, in a high reaction temperature of
–
–
800. The C in Fe O @NC-800 mainly existed as C C (284.6 eV) and
–
2
3
–
–
–
CN (285.3 eV). The characteristic signal for O CO in the sample
–
was observed at 288.7 eV with weak strength, confirming that the
considerable pyrolysis of oxygen-containing functional groups occurs in
the catalyst during carbonization. Fig. 3c shows the high-resolution N 1s
spectrum of Fe2O3@NC-800. The two peaks at 398.1 and 400.7 eV
corresponded to pyridine N and graphite N, respectively [36]. Through
XPS analysis, the N content of Fe2O3@NC-800 was 12. 28 %, and the
corresponding contents of pyridine N and graphite N were 7.98 % and
4.3 %, respectively. Fig. 3d shows the high-resolution Fe 2p spectrum of
Fe2O3@NC-800. The Fe 2p3/2 signal with a combined energy of 710.7 eV
and the Fe 2p1/2 signal with a combined energy of 723.0 eV were
observed in the spectrum, indicating that Fe element is existing as Fe2O3
in Fe2O3@NC-800 sample [37,38]. It is worth mentioning that the sat-
ellite peak around 719.0 eV also suggests that the iron oxide is the main
Fe species in Fe2O3@NC-800 sample [39]. In addition, Fe-Nx was not
observed in Fe@NC-800 (whose characteristic signal was observed at
713.4 eV), providing supplementary evidence for the active component
in the catalytic reaction. Table S1 summarizes the contents of N and Fe
in Fe2O3@NC-800. As determined by elemental analysis, the contents of
N in Fe2O3@NC-500, Fe2O3@NC-600, Fe2O3@NC-700, Fe2O3@NC-800,
and Fe2O3@NC-900 were 27.20 %, 25.15 %, 15.12 %, 14.06 %, and 7.14
%, respectively, and the corresponding Fe contents estimated by
◦
above 70 C, the volatilization of NH3 from the reaction mixture will
lead the reduction of the NH3H2O concentration, which is not conducive
to the ammoxidation reaction. The ammonia content of ammonia water
led to the significant reduction in the yield of benzonitrile due to
4

W.D. Wang et al.
Molecular Catalysis xxx (xxxx) xxx
Table 2
Fe2O3@NC-800 catalyzed ammoxidation of aromatic aldehydes to benzonitriles.a.
Entry
Substrate
Product
Conv. (%)
62.92
Sel. (%)
1
2
>99
63.78
>99
3
4
5
78.72
86.49
89.97
>99
>99
>99
6
7
92.41
>99
49.38
80.70
>99
>99
8
a
Reaction condition: substrate (1 mmol), EtOH (5 mL), 800
μ
L ammonia (28 %–30 %), catalyst (50 mg) and under O2 atmosphere for 24 h at 60 ℃.
capacity, the yield of aromatic aldehydes significantly decreased. Under
the same reaction conditions, only 49.38 % of m-nitrobenzaldehyde was
converted into m-nitrobenzonitrile (entry 7). With the increase in the
number of aldehyde groups, the reaction still maintained good conver-
sion and excellent selectivity. Without the increase in the amount of
ammonia, the yield of p-diphenylnitrile reached 80.70 % at 24 h (entry
8). Thus, the Fe2O3@NC-800 exhibits good general applicability for
ammoxidation of aromatic aldehydes to nitriles.
The reusability of Fe2O3@NC-800 was investigated by the catalytic
ammoxidation of benzaldehyde to benzonitrile. All of the catalytic cycle
experiments were completed within 24 h, and the yield of benzonitrile
was maintained to be greater than 95 % (Fig. 4). There is no Fe element
was detected in the reaction solution after the catalytic ammoxidation
reaction using ICP-OES measurement. The TEM image of the recycled
Fe2O3@NC-800 also showed that, the morphology of the spent
Fe2O3@NC-800 catalyst was almost the same as the fresh Fe2O3@NC-
800 catalyst (Fig. S3). These repeated experiments confirmed that as-
prepared Fe2O3@NC-800 catalyst exhibits good chemical stability and
recyclability in the catalytic ammoxidation of benzaldehyde.
Fig. 4. Recyclability of Fe2O3@NC-800 for catalytic ammonia oxidation of
benzaldehyde.
ammonia evaporation (entry 14). This study also examined the effect of
the amount of ammonia on the reaction yield, and the decrease in the
amount of ammonia led to the decrease in the yield of benzonitrile
(entries 15–16). The Scheme in Table 2 shows the possible reaction
mechanism for the formation of benzonitrile from benzaldehyde. Imine
intermediates were formed from aldehyde and ammonia. The elimina-
tion of a H2 molecule from this produced intermediate finally afforded
the corresponding nitriles in the presence of the Fe2O3@NC-800 catalyst
[24].
3. Conclusion
In this study, an N-doped carbon-coated iron oxide nanoparticles
catalyst (Fe2O3@NC-800) was successfully prepared using renewable
biomass chitosan, cost-effective urea, and abundant ferric chloride as
precursors. Iron oxide NPs uniformly and considerably dispersed in the
Fe2O3@NC-800 catalyst and wrapped within N-doped carbon.
Fe2O3@NC-800 exhibited prominentcatalytic activity and selectivity for
the ammoxidation of aromatic aldehydes. This study provides a cost-
effective, simple strategy for the synthesis of non-noble-metal-
modified N-doped carbon catalysts for the ammoxidation of aldehydes.
In addition, Fe2O3@NC-800 was used to prepare nitrile by the
ammoxidation of other aromatic aldehydes under optimal reaction
conditions (Table 2). For various halogen-substituted benzaldehydes,
good conversion and eminent selectivity at 24 h were observed (entries
1–4). The results revealed that the aromatic aldehydes substituted by
different halogens are affected by the halogen species. When the sub-
stituent is changed to an electron-donating methyl group, a high sub-
strate conversion was observed (entry 5). With the increase in the
number of methyl groups, the substrate conversion increased to 92.41 %
while still maintaining excellent selectivity (entry 6). However, when
the substituent is a nitro group, with a higher electron withdrawing
Declaration of Competing Interest
There are no conflicts of interest to declare.
Acknowledgements
We acknowledge the financial support from the National Natural
5

W.D. Wang et al.
Molecular Catalysis xxx (xxxx) xxx
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