Highly Efficient Oxidative Cyanation of Aldehydes to Nitriles over Se,S,N-tri-Doped Hierarchically Porous Carbon Nanosheets

Article abstract of DOI:10.1002/anie.202107996

Oxidative cyanation of aldehydes provides a promising strategy for the cyanide-free synthesis of organic nitriles. Design of robust and cost-effective catalysts is the key for this route. Herein, we designed a series of Se,S,N-tri-doped carbon nanosheets with a hierarchical porous structure (denoted as Se,S,N-CNs-x, x represents the pyrolysis temperature). It was found that the obtained Se,S,N-CNs-1000 was very selective and efficient for oxidative cyanation of various aldehydes including those containing other oxidizable groups into the corresponding nitriles using ammonia as the nitrogen resource below 100 °C. Detailed investigations revealed that the excellent performance of Se,S,N-CNs-1000 originated mainly from the graphitic-N species with lower electron density and synergistic effect between the Se, S, N, and C in the catalyst. Besides, the hierarchically porous structure could also promote the reaction. Notably, the unique feature of this metal-free catalyst is that it tolerated other oxidizable groups, and showed no activity on further reaction of the products, thereby resulting in high selectivity. As far as we know, this is the first work for the synthesis of nitriles via oxidative cyanation of aldehydes over heterogeneous metal-free catalysts.

Full text of DOI:10.1002/anie.202107996

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Accepted Article
Title: Highly Efficient Oxidative-Cyanation of Aldehydes to Nitriles over
Se,S,N-tri-Doped Hierarchically Porous Carbon Nanosheets
Authors: Manli Hua, Jinliang Song, Xin Huang, Huizhen Liu, Honglei
Fan, Weitao Wang, Zhenhong He, Zhaotie Liu, and Buxing
Han
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10.1002/anie.202107996
Angewandte Chemie International Edition
RESEARCH ARTICLE
Highly Efficient Oxidative-Cyanation of Aldehydes to Nitriles over Se,S,N-
tri-Doped Hierarchically Porous Carbon Nanosheets
Manli Hua,[a,b] Jinliang Song,*[a] Xin Huang,[a,b] Huizhen Liu,[a,b] Honglei Fan,[a] Weitao Wang,[c]
Zhenhong He,[c] Zhaotie Liu,[c] and Buxing Han*[a,b]
Dedication ((optional))
[a]
M. Hua, Dr. J. Song, X. Huang, Dr. H. Fan, Prof. H. Liu, Prof. B. Han
Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center
for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: songjl@iccas.ac.cn, hanbx@iccas.ac.cn.
[b]
[c]
M. Hua, X. Huang, Prof. H. Liu, Prof. B. Han
School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China.
Dr. W. Wang, Dr. Z. He, Prof. Z. Liu
Shaanxi Key Laboratory of Chemical Additives for Industry, College of Chemistry & Chemical Engineering, Shaanxi University of Science & Technology,
Xi’an, Shaanxi, 710021, China.
Supporting information for this article is given via a link at the end of the document. ((Please delete this text if not appropriate))
renewable biomass resources[8] and ammonia is an inexpensive
Abstract: Oxidative-cyanation of aldehydes provides a promising
strategy for the cyanide-free synthesis of organic nitriles. Design of
robust and cost-effective catalysts is the key for this route. Herein,
we designed a series of Se,S,N-tri-doped carbon nanosheets with a
and abundantly-available nitrogen resource. Moreover, direct
oxidative-cyanation of aldehydes avoids the use of
cyanides/special nitrogen resources and the pre-synthesis of the
N-containing precursors. These features afford direct oxidative-
cyanation of aldehydes being a highly promising approach to
produce nitriles.
hierarchical porous structure (denoted as Se,S,N-CNs-x,
x
represents the pyrolysis temperature). It was found that the obtained
Se,S,N-CNs-1000 was very selective and efficient for oxidative-
cyanation of various aldehydes including those containing other
oxidizable groups into the corresponding nitriles using ammonia as
the nitrogen resource below 100 oC. Detailed investigations revealed
that the excellent performance of Se,S,N-CNs-1000 originated
mainly from the graphitic-N species with lower electron density and
synergistic effect between the Se, S, N, and C in the catalyst.
Besides, the hierarchically porous structure could also promote the
reaction. Notably, the unique feature of this metal-free catalyst is that
it tolerated other oxidizable groups, and showed no activity on
further reaction of the products, thereby resulting in high selectivity.
As far as we know, this is the first work for the synthesis of nitriles
via oxidative-cyanation of aldehydes over heterogeneous metal-free
catalysts.
Initially, direct synthesis of nitriles from aldehydes and
ammonia required stoichiometric amounts of reactive oxidants or
reagents (e.g., I2, and (Bu4N)2S2O8),[9] which would result in the
generation of large amounts of wastes, thereby limiting the wide
application of these routes. In order to overcome this drawback,
O2 has been applied as a green oxidant in the oxidative-
cyanation of aldehydes with water as the only by-product. To
date, metal (e.g., Cu, Ru, Mn, Co, Ni, Ag, Fe)-based catalysts
remain the commonly employed catalysts for aerobic oxidative-
cyanation of aldehydes.[7d, 9c, 10] However, the reaction selectivity
over metal-based catalysts is generally difficult to be controlled
owing to their high activity to catalyze the oxidation of various
groups and promote further conversion of the generated nitriles
into amides. Additionally, metal-based catalysts also suffer from
other drawbacks, including the ease to be deactivated with
ammonia, high cost (especially the noble metals), and metal
contaminants (extremely unwanted in pharmaceuticals). To
address these drawbacks of metal-based catalysts, metal-free
catalytic systems have drawn much interest. Recently, several
metal-free catalytic systems have been developed for the
synthesis of nitriles by the oxidative-cyanation of aldehydes.[11]
Nevertheless, these reported systems were homogeneous
catalysts (i.e., nitroxyl/NOx,[11a] and modified TEMPOs[11b-e]) or
special nitrogen resource (hydroxylamine-O-sulfonic acid[11f]).
Undoubtedly, from the economical and practical perspective,
developing robust, low-cost, and heterogeneous metal-free
catalysts for direct oxidative-cyanation of aldehydes is highly
desirable.
Introduction
Nitriles are very useful compounds and have been widely
applied as versatile intermediates to produce agricultural
chemicals, pharmaceuticals, dyes, fine chemicals, and functional
materials.[1] Generally, nitriles can be synthesized by several
cyanide-involved methods,[2] but the requirement of high toxic
cyanides significantly limited their applications. Thereby, much
effort has been devoted on cyanide-free strategies, and several
cyanide-free approaches for nitrile synthesis have been
developed, including oxidation of amines, amides or azides,[3]
dehydration of amides/oximes,[4] ammoxidation of arenes,[5]
reaction of organic halides with sodium azide,[6] and direct
oxidative-cyanation of alcohols or aldehydes.[7] Among these
cyanide-free methods, direct oxidative-cyanation of aldehydes
using ammonia as the nitrogen resource is a very attractive
route because aldehydes can be abundantly obtained from
In recent years, metal-free carbon materials as typical
nonmetallic catalysts have attracted much interest in the field of
catalysis.[12] The features of earth abundance, diverse origination
and low cost afford carbon material catalysts possessing
practical advantages in comparison with metal catalysts, but
1
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10.1002/anie.202107996
Angewandte Chemie International Edition
RESEARCH ARTICLE
pure carbon materials show low catalytic activity. Generally, the
catalytic activity of carbon materials could be enhanced by
12b, 13]
doping heteroatom (e.g., B, N, F, P, or S)[12a,
or
constructing advanced nanostructures (e.g., carbon quantum dot,
carbon nanosheets, and porous carbon).[14] The former can
adjust the properties of carbon materials or introduce additional
catalytic sites,[15] while the latter are beneficial for mass transport
and more accessible catalytic-sites.[16]
In this work, we synthesized Se,S,N-tri-doped carbon
nanosheets (denoted as Se,S,N-CNs-x, and x represents the
pyrolysis temperature) with a hierarchical porous structure via
the pyrolysis of the mixture of SeO2, chitosan, NH4Cl and L-
methionine. It was discovered that Se,S,N-CNs-1000 showed
excellent performance for the aerobic oxidative-cyanation of
various aldehydes to synthesize nitriles using NH3·H 2O as the N
resource. Besides the metal-free property, Se,S,N-CNs-1000
showed several advantages, including no leaching of active
species (Se, S, and N), heterogeneous nature, no futher
conversion of the formed nitriles, and good tolerance of other
oxidizable groups, confirming the great potential of Se,S,N-CNs-
1000. Notably, the finding of the synergistic effect of doped
multi-elements and the advaned structure (the hierarchical
porous nanosheet structure) provided a promising strategy to
improve the catalytic performance of carbon-based catalysts for
organic reactions. To the best of our knowledge, this is the first
work to realize the synthesis of nitriles via oxidative-cyanation of
aldehydes over heterogeneous metal-free catalysts.
Figure 1. Characterizations of the obtained Se,S,N-CNs-1000. (A) TEM image,
(B) SEM image, (C) HR-TEM image, (D) XRD pattern and (F) N2
adsorption−desorption isotherms (Insert E: distribution of micropore).
The fine structure of Se,S,N-CNs-1000 was further
characterized by Raman spectrum and X-ray photoelectron
spectroscopy (XPS). In the Raman spectrum of Se,S,N-CNs-
1000 (Figure 2A), there were obvious D-band and G-band,
which were characteristic bands of graphitic carbon materials.
Meanwhile, from the spectrum, it could be deduced that there
were many structural defects in Se,S,N-CNs-1000, as can be
known from the strong intensity of D-band. Furthermore, XPS
characterization was performed to determine the chemical states
of various elements in the Se,S,N-CNs-1000. The presence of C,
N, Se and S could be further confirmed from XPS survey
spectrum (Figure S5). The Se 3d spectrum was split into two
well-defined peaks at 55.94 and 58.82 eV (Figure 2B), which
corresponded to the Se-S and C-Se units.[17] From the high-
resolution N 1s spectrum (Figure 2C), there were three types of
N species, including pyridine-type N (398.15 eV), pyrrole-type N
(399.4 eV), and graphitic-type N (400.59 eV). Furthermore, S 2p
spectrum revealed the presence of S2+ (164.05 and 161.08 eV,
assigned to S-Se and S-C units) and S6+ (167.75 eV, assigned
to sulfone units) species (Figure 2D).[18] These results indicated
that Se, S and N were chemically boned in the carbon
framework.
Results and Discussion
Preparation and characterization of the catalysts. The
desired material (Se,S,N-CNs-x) was synthesized from the
pyrolysis of the fine mixture of SeO2, chitosan, NH4Cl and L-
methionine, and the detail procedure is described in the
supporting information. As shown in the SEM image (Figure 1A)
and TEM image (Figure 1B and S1), the morphology of Se,S,N-
CNs-1000 was geometrically a nanosheet. There were plenty of
macropores and mesopores in the prepared Se,S,N-CNs-1000
(Figure 1C), and its thickness was about 1.5 nm from the result
of AFM (Figure S2). Energy dispersive X-ray (EDX) mapping
demonstrated clearly that all elements (C, N, S, Se) were
distributed uniformly in the Se,S,N-CNs-1000 (Figure S3). In
XRD pattern (Figure 1D), two peaks were obviously observed at
~25o and ~44o, which corresponded to the (002) and (101)
planes of graphitic carbon, respectively, and there were no extra
diffractions of SeO2 or NH4Cl, suggesting the complete
conversion of SeO2 and NH4Cl in the pyrolysis process.
Moreover, N2 adsorption-desorption isotherm showed the mixed
mode of type I and type IV (Figure 1E), implying that Se,S,N-
CNs-1000 was a micro- and meso-porous material. Meanwhile,
the BET surface area of Se,S,N-CNs-1000 was 575 m2g-1 (Table
S1), and the average sizes of micropores (Figure 1E) and
mesopores (Figure S4) were 0.48 and 3.37 nm using the
Barrett-Joyner-Halenda and the Horvath-Kawazoe methods,
respectively. Additionally, the contents of Se, S and N in the
Se,S,N-CNs-1000 were 2.34, 0.49 and 4.12 wt% (Table S2),
respectively.
Figure 2. (A) Raman spectra of the Se,S,N-CNs-1000, XPS spectra of (B) Se
3d, (C) S 2p, and (D) N 1s in Se,S,N-CNs-1000.
2
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10.1002/anie.202107996
Angewandte Chemie International Edition
RESEARCH ARTICLE
Catalytic activity of various catalysts. The catalytic
performance of the metal-free carbon-based catalysts was
initially evaluated by employing oxidative-cyanation of
benzaldehyde to benzonitrile as a model reaction (Table 1). No
desired product was generated in the absence of catalysts
(Table 1, entry 1). The synthesized Se,S,N-CNs-1000 could
efficiently catalyze the reaction (Table 1, entry 2) with a nearly
quantitative yield of benzonitrile, indicating the excellent
selectivity over Se,S,N-CNs-1000. In comparison, several other
carbon-based materials were synthesized by varying the doping
elements, the usage of precursors, and the preparation
temperatures, and the obtained materials were characterized by
different techniques, including TEM (Figure S6), XRD (Figure
S7), Raman spectra (Figure S8), and N2 adsorption-desorption
(Table S1). It was found that the preparation conditions could
significantly affect the morphology and the properties of the
obtained materials, thus influencing their catalytic activity. For
example, there was trace target product observed when the
reaction was performed over the material without Se doping,
such as N-CNs-1000 (Table 1, entry 3) and S,N-CNs-1000
(Table 1, entry 4), indicating the crucial role of Se-doping on the
catalytic activity. Meanwhile, the yield of benzonitrile was only
43% over the Se,N-CNs-1000 (Table 1, entry 5), implying that
the S-doping was also important to promote the generation of
the desired benzonitrile. Moreover, it was observed that the
catalytic activity of the synthesized carbon-based materials
increased with the increasing amount of both Se (Table 1,
entries 2 and 6-8) and S (Table 1, entries 2, 9 and 10)
precursors, further verifying the crucial role of Se and S on the
reaction. Additionally, the pyrolysis temperature also affected the
catalytic performance of the carbon-based materials, and the
catalytic activity increased with the increasing pyrolysis
Optimization of reaction conditions. Employing Se,S,N-
CNs-1000 as the catalyst, the influence of various reaction
parameters, including reaction temperature (Figure 3A), reaction
time (Figure 3B), and the amount of catalyst usage (Figure 3C),
was subsequently investigated. Clearly, too low temperature (40
oC) was unfavorable for the reaction (Figure 3A), and the yield of
benzonitrile gradually increased when the reaction temperature
increased from 40 to 80 oC. At 80 oC, benzaldehyde could be
quantitatively converted to benzonitrile with a reaction time of 10
h (Figure 3B). Moreover, the catalyst usage also affected the
efficiency of the reaction (Figure 3C). It was observed that the
reaction hardly proceeded without using the catalyst, and the
optimal catalyst usage was 50 mg. Thus, we could deduce that
the optimal reaction parameters were 80 oC, 10 h, and 50 mg
catalyst for the reaction. Besides, O2 pressure could affect the
catalytic activity. However, the influence of O2 pressure was not
significant when the pressure increased from 2 to 5 bar (Table
S4), implying that O2 pressure was not the dominant factor to
affect the catalytic activity in the pressure range. Furthermore,
the recyclability of Se,S,N-CNs-1000 was evaluated by three
sets of experiments. No considerable changes in catalytic
performance was observed in five catalytic cycles with a reaction
time of 6h (Figure S9) or 10 h (Figure 3D) when the initial usage
of Se,S,N-CNs-1000 was 50 mg. As characterized by TEM
(Figure S10), N2 adsorption-desorption (Figure S11A), XRD
(Figure S11B), Raman spectra (Figure S11C) and XPS (Figure
S12) techniques, no obvious difference was observed between
the fresh and used Se,S,N-CNs-1000. Meanwhile, elemental
analysis (Table S2) indicated that there were no obvious
changes for various elements in fresh Se,S,N-CNs-1000 and the
recovered one, and no Se and S were detected in the isolated
product, indicating that there was no leaching of Se and S from
Se,S,N-CNs-1000 into the reaction system. Additionally, when
the catalyst usage was decreased to 30 mg, the Se,S,N-CNs-
1000 could be still reused at least for three catalytic cycles
without considerable change in catalytic performance (Table S5).
These results above confirmed the high stability of Se,S,N-CNs-
1000 under the reaction conditions.
o
temperature from 700 to 1000 C (Table 1, entries 2 and 12-14).
The results above indicated that Se,S,N-CNs-1000 was the best
catalyst for the studied reaction.
Table 1. Oxidative-cyanation of benzaldehyde to benzonitrile over various
catalysts.[a]
Entry
Catalyst
Conversion (%)[b]
Yield (%)[b]
1
2
Blank
6
>99
5
0
99
Se,S,N-CNs-1000
N-CNs-1000
3
trace
trace
45
4
S,N-CNs-1000
8
5
Se,N-CNs-1000
Se(1),S,N-CNs-1000
Se(2),S,N-CNs-1000
Se(3),S,N-CNs-1000
Se,S(0.1),N-CNs-1000
Se,S(0.2),N-CNs-1000
Se,S,N-CNs-700
Se,S,N-CNs-800
Se,S,N-CNs-900
46
16
57
83
62
86
15
39
94
6[c]
7[c]
8[c]
9[c]
10[c]
12
13
14
15
56
82
59
84
13
38
Figure 3. Influence of reaction temperature (A), time (B), and amount of
catalyst (C) on the conversion of benzaldehyde, and the reusability of Se,S,N-
CNs-1000 (D). Reaction conditions: benzaldehyde, 0.5 mmol; NH3·H 2O, 100
μL; O2, 5 bar; acetonitrile, 3 mL; Se,S,N-CNs-1000, 50 mg for A, B and D;
80 °C for B-D; 10 h for A, C and D. It should be pointed out that the amount of
the reactant in each cycle to examine the recyclability was added based on the
weight ratio of the recovered catalyst and the fresh one (Table S6), and the
carbon balances in these experiments were all >96% because the detected
by-products were trace.
91
[a] Reaction conditions: benzaldehyde, 0.5 mmol; catalyst, 50 mg; NH3·H 2O,
100 μL; O2, 5 bar; acetonitrile, 3 mL; 80 °C; 10 h. [b] Conversions and yields
were determined by GC using n-dodecane as the internal standard. [c] The
values in the parentheses were the amount (g) of the used Se or S precursors,
and the amounts of Se and S precursors were 4g and 0.3 g, respectively, in
the catalyst without the specific mark. The results of the calculated carbon
balance were provided in Table S3.
3
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10.1002/anie.202107996
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RESEARCH ARTICLE
Scope of substrates. Delighted by the excellent
performance of Se,S,N-CNs-1000 for the oxidative-cyanation of
benzaldehyde to benzonitrile, the reactivity of various
benzaldehydes, including some lignin derivatives (e.g., 3,4-
dimethoxybenzaldehyde, and 3,4,5-trimethoxybenzaldehyde),
was examined (Table 2). It was observed that the substrates
with electron donating groups, i.e., alkyl (methyl, ethyl and
isopropyl), alkoxyl, and phenolic hydroxyl, showed good
reactivity at 80 oC, and the yields of the corresponding para-
substituted nitriles were all above 90% with a reaction time of 10
h. The lower reactivity of 4-tert-butylbenzaldehyde may be
caused by the steric hindrance of the tert-butyl group, and 82%
yield was obtained. When the substituents were halogen (weak
electron-withdrawing groups), longer reaction time (24 h) was
needed to achieve satisfactory yields (>90%) of nitriles.
Moreover, when strong electron-withdrawing groups (-COOH, -
CN, -CONH2 and -NO2) were involved, the reaction temperature
[a] Reaction conditions: substrate, 0.5 mmol; Se,S,N-CNs-1000, 50 mg;
NH3·H 2O, 100 μL; O2, 5 bar; acetonitrile, 3 mL; 80 °C; 10 h. Conversions (C)
and yields (Y) were determined by GC using n-dodecane as the internal
standard. [b] Substrate, 0.5 mmol; catalyst, 50 mg; NH3·H 2O, 100 μL; O2, 5
bar; acetonitrile, 3 mL; 80 °C; 24 h. [c] Substrate, 0.5 mmol; catalyst, 50 mg;
NH3·H 2O, 100 μL; O2, 5 bar; acetonitrile, 3 mL; 90 °C; 24 h. The carbon
balances in these experiments were all >95%.
o
Scheme 1. Oxidative-cyanation of 5-hydroxymethylfurfural and furfural.
Reaction conditions: substrate, 0.5 mmol; Se,S,N-CNs-1000, 50 mg; NH3·H 2O,
100 μL; O2, 5 bar; acetonitrile, 3 mL; 80 °C; 24 h.
should be increased to 90 C to obtain satisfactory yields of the
corresponding products with a reaction time of 24 h. These
results indicated that electron-withdrawing groups were
unbeneficial for the oxidative-cyanation of benzaldehydes. More
importantly, furfural and 5-hydroxymethylfurfural (two important
biomass-derived platform compounds) could also be selectively
converted into 2-furonitrile and 5-hydroxymethy-2-cyanofuran
over Se,S,N-CNs-1000 at 80 oC with a reaction time of 24 h
(Scheme 1), indicating that a wide range of functional groups,
including other oxidizable groups, were tolerated under the
reaction conditions. Additionally, for all reactions, the generation
of amides was not observed, which was a general phenomenon
in most of reported catalytic systems, further verifying the
superiority of Se,S,N-CNs-1000 for the oxidative-cyanation of
aldehydes.
Discussion for the reason on the excellent performance
of Se,S,N-CNs-1000. As discussed above, Se,S,N-CNs-1000
showed superior catalytic activity for the oxidative-cyanation of
aldehydes. It is of significant importance to reveal the
catalytically active sites or the reason on the superior catalytic
activity of Se,S,N-CNs-1000. As characterized by Raman
spectra, the intensity ratio of D/G in N-CNs-1000 (Figure S8K,
ID/IG=0.98), S,N-CNs-1000 (Figure S8G, ID/IG=1.00), Se,N-CNs-
1000 (Figure S8C, ID/IG=1.00) and Se,S,N-CNs-1000 were
similar (Figure 2A, ID/IG=1.01), indicating that the amount of
defects in these materials was similar. However, N-CNs-1000
and S,N-CNs-1000 showed no catalytic activity. Thus, the
catalytic role of the defects could be excluded. Subsequently,
several control experiments were conducted to reveal the
probable catalytic-active site. No reaction occurred over SeO2,
diphenyl selenide, Se-doping carbon (synthesized from SeO2
and cellulose, denoted Se-CNs-1000), or Se,S-co-doped carbon
(synthesized from SeO2, cellulose, and 4,4’-sulfonyldiphenol,
denoted Se,S-CNs-1000) (Table S7), implying that the doped Se
and S were not the catalytically active sites. Considering the
obvious activity of Se,N-CNs-1000 and Se,S,N-CNs-1000 and
the ignorable catalytic role of the doped Se and S, the doped N
species were the probable catalytically-active sites. Moreover, at
a higher reaction temperature (120 oC), N-CNs-1000 showed
obvious catalytic activity for the reaction, while the activity of Se-
CNs-1000, Se,S-CNs-1000, and CNs-1000 (prepared using
cellulose) was very low (Table S8). These results further
indicated that the N species were the active sites in Se,S,N-
Table 2. Oxidative-cyanation of various benzaldehyde derivatives to
]
corresponding nitriles.[a
Y: 99%
C: 99%
Y: 99%
C: 99%
Y: 82%
C: 86%
Y: 97%
C: 99%
Y: 95%
C: 99%
Y: 85%b
C: 89%b
Y: 88%b
C: 92%b
Y: 99%
C: 99%
o
CNs-1000. Additionally, the low activity of N-CNs-1000 at 80 C
Y: 83% b
C: 86%b
Y: 95%
C: 99%
Y: 79%b
C: 84%b
Y: 75%b
C: 79%b
was probably caused by the insufficient ability to activate O2 at
lower reaction temperature. As well-accepted, the graphitic-N
species or the carbon atoms adjacent to the graphitic-N are the
catalytically active sites for oxidations in N-doped carbon-based
catalysts.[13a, 19] Indeed, Se,N-CNs-1000 and Se,S,N-CNs-1000
contained graphitic-N species, as detected by XPS (Figure 4
and Figure 2D). Therefore, we deduced that the graphitic-N
species played the catalytic role in Se,S,N-CNs-1000. However,
the contents of graphitic-N species in N-CNs-1000, Se, N-CNs-
1000 and S,N-CNs-1000 were similar (Figure 4), but they
showed no activity (for N-CNs-1000 and S,N-CNs-1000) or
Y: 95%b
C: 96%b
Y: 92%b
C: 95%b
Y: 91%b
C: 91%b
Y: 97%b
C: 99%b
Y: 86%c
C: 91%c
Y: 87%c
C: 87%c
Y: 91%c
C: 94%c
Y: 82%c
C: 85%c
4
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10.1002/anie.202107996
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RESEARCH ARTICLE
much lower activity (for Se, N-CNs-1000), indicating that the
doped Se and S could influence the catalytic activity of graphitic-
N although they were not the direct catalytically active sites.
As well-known, the doping of Se could adjust the active
sites to possess higher spin density, and charge
delocalization,[20] resulting from the larger size and greater
polarizability of Se, thereby promoting the catalytic activity of the
catalytic materials. Indeed, as detected by XPS (Figure 4), the
binding energy of graphitic-N in Se,N-CNs-1000 shifted to higher
energy in comparison of those in N-CNs-1000 and S,N-CNs-
1000. The shift would be more obvious in Se,S,N-CNs-1000
because bonding Se with S could further enhance the ability to
tune the electronic structures of graphitic-N. These results
indicate that the graphitic N in the Se-doped materials was more
positively charged, affording the ortho-carbon of the graphitic-N
being more positive, which was beneficial for the generation of
active oxygen species on this carbon to promote the reaction.[21]
In another aspect, 13C MAS NMR spectra was conduct to
determine the chemical state of C in the catalysts doped with
different elements, including N-CNs-1000, Se,N-CNs-1000, S,N-
CNs-1000, and Se,S,N-CNs-1000. It was observed that the
chemical shift for the C in these materials changed as the
following order: Se,S,N-CNs-1000 > Se,N-CNs-1000 > S,N-
CNs-1000 ≈ N-CNs-1000 (Figure S13), indicating that the C in
the Se,S,N-CNs-1000 was most positive among these materials.
Meanwhile, Se,S,N-CNs-1000 showed a higher shift than Se,N-
CNs-1000 because the formation of Se-S bond could increase
the ability of Se to adjust the electronic structures of the
materials. These results were consistent with the XPS results of
graphitic-N, which revealed the role of Se on enhancing the
catalytic activity. Based on the discussion above, in Se,S,N-
CNs-1000, Se bonded with S played the crucial role of affording
the active sites (graphitic-N) being more positively charged, thus
improving the ability to activate O2.
Moreover, the preparation temperature could significantly
affect the catalytic activity of the Se,S,N-CNs-x materials. On the
basis of XPS results (Figure S14), the content of graphitic-N
species in these materials increased with the increase of the
calcination temperature from 700 to 1000 oC, suggesting more
catalytically active sites were generated at higher temperature.
More importantly, the binding energy of graphitic-N increased
with the preparation temperature, and the graphitic-N in Se,S,N-
CNs-1000 showed the highest binding energy. As discussed
above, graphitic-N with higher binding energy could promote the
formation of active oxygen species for the reaction, which was
beneficial for the reaction.
Generally, higher porosity is beneficial for the accessibility of
catalytically active sites in heterogeneous catalysts, thus
resulting in higher catalytic activity. Herein, the porosity of the
prepared catalysts was also an important factor to affect the
reaction activity. First, Se,N-CNs-1000 and Se,S,N-CNs-1000
showed the hierarchically porous structure (Figure 1E and S4),
affording these two materials having higher BET surface areas
(Table S1), while N-CNs-1000 and S,N-CNs-1000 did not form
the hierarchically porous structure (Figure S1G and K), resulting
in the lower surface areas (Table S1). Therefore, Se,N-CNs-
1000 and Se,S,N-CNs-1000 were deduced to have higher
porosity than N-CNs-1000 and S,N-CNs-1000, making more
active sites be exposed to catalyze the reaction, which may be a
reason for the higher activity of Se,N-CNs-1000 and Se,S,N-
CNs-1000. More obviously, from the results of BET surface
areas (Table S1), Se,S,N-CNs-1000 had much higher surface
area than Se,N-CNs-1000, which was a probable reason for the
higher catalytic activity of Se,S,N-CNs-1000. Second, as
characterized by TEM (Figure S6), Se,S,N-CNs-1000 and
Se,S,N-CNs-900 possessed the hierarchically porous structure,
which was not formed in both Se,S,N-CNs-800 and Se,S,N-
CNs-700. Correspondingly, Se,S,N-CNs-1000 and Se,S,N-CNs-
900 showed much higher catalytic activity than Se,S,N-CNs-800
and Se,S,N-CNs-700, implying the important role of
hierarchically porous structure on the catalytic activity.
Especially, the catalytic activity of Se,S,N-CNs-900 with
hierarchically porous structure was much higher than that of
Se,S,N-CNs-800 without the hierarchically porous structure
although they showed similar electronic state of graphitic-N,
further confirming the importance of forming hierarchically
porous structure. In addition, the order of the catalytic activity for
different catalysts at different temperatures was consistent with
their BET surface areas (Table S1): Se,S,N-CNs-1000 > Se,S,N-
CNs-900 > Se,S,N-CNs-800 > Se,S,N-CNs-700, indicating that
higher porosity was indeed favorable to the reaction. These
results above proved that the formation of the hierarchically
porous structure could increase the porosity of the catalysts,
thereby enhancing the catalytic activity to some extent.
On the basis of the above discussion, the graphitic-N
species were the catalytically active sites, and their electronic
state, which could be well tuned by the doping of Se and S, was
the crucial factor for the catalytic activity. Meanwhile, the
formation of the hierarchically porous structure could result in
higher BET surface area and more porosity, which caused that
more active sites were exposed to catalyze the reaction.
Therefore, it could be deduced that the synergistic effect of
graphitic-N with suitable electronic state and the hierarchically
porous structure afforded Se,S,N-CNs-1000 possessing the best
catalytic performance.
Figure 4. XPS spectra of N 1s in N-CNs-1000, Se,N-CNs-1000, S,N-CNs-
1000 and Se,S,N-CNs-1000. In this figure, a, b and c represented graphitic-
type N, pyrrole-type N and pyridine-type N, respectively.
5
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10.1002/anie.202107996
Angewandte Chemie International Edition
RESEARCH ARTICLE
Pobable reaction pathway. As well-accepted,[7e,10c] the
oxidative-cyanation of aldehydes to nitriles follows the imination-
oxidation pathway, and imines are the intermediates. However,
the imines were not detected in the catalytic systems with the
active catalysts and O2 because the imines were very unstable,
resulting in that they were rapidly oxidized to form nitriles.[10c] In
contrast, very small amounts of dimeric or trimeric imines
(Scheme S1), generated from the in situ formed imines, were
detected in the catalytic systems with inactive catalysts or in the
absence of catalysts, implying that the imines were the potential
intermediates. In addition, aldehydes were always detected
during the whole reaction process, suggesting that the formation
of imines was the probable rate-determining step for our
catalytic system. Based on the results above, a probable
reaction pathway was proposed for the oxidative-cyanation of
aldehydes to nitriles over Se,S,N-CNs-1000 using O2 and
aqueous ammonia (Scheme 2). Initially, imines were formed as
the intermediates by the condensation of aldehydes and
ammonia. Then, the formed imines were rapidly oxidized to
generate the final products (nitriles). In the catalytic cycle,
Se,S,N-CNs-1000 played the role of activating O2, which was
highly crucial for the reaction because nitriles could not be
synthesized when the ability of the catalysts (N-CNs-1000 and
S,N-CNs-1000) to activate O2 was low. Therefore, the activation
of O2 could also determine the reactivity of the reaction.
The work was supported by National Natural Science
Foundation of China (22072157, 21733011), the National Key
Research
and
Development
Program
of
China
(2018YFB1501602), Beijing Municipal Science & Technology
Commission (Z191100007219009), the Chinese Academy of
Sciences (QYZDY-SSW-SLH013), and Youth Innovation
Promotion Association of CAS (2017043).
Keywords: Oxidative-cyanation of aldehydes • Organic nitriles •
Metal-free catalysts • Graphitic-type N • Cyanide-free
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Scheme 2. Proposed reaction pathway for the synthesis of nitriles by
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RESEARCH ARTICLE
Entry for the Table of Contents
Insert graphic for Table of Contents here. ((Please ensure your graphic is in one of following formats))
The metal-free Se,S,N-tri-doped carbon nanosheets (Se,S,N-CNs-1000) showed high activity and selectivity for oxidative-cyanation
o
of various aldehydes to synthesize organic nitriles using ammonia as the nitrogen resource at below 100 C. Other oxidizable groups
were tolerated, and no further reaction of the product was observed over Se,S,N-CNs-1000. Systemic study revealed that the
graphitic-N species with lower electron density and synergistic effect between the Se, S, N, and C resulted in the excellent
performance of Se,S,N-CNs-1000.
8
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