Molybdenum Nitride Nanocatalyst Derived from Melamine and Polyoxometalate-based Hybrid for Oxidative Coupling of Amines to Imines with Air

Article abstract of DOI:10.1002/cctc.201800980

Molybdenum nitride (Mo₂N) is one of the most promising alternative catalysts for a wide range of Pt-catalyzed catalytic applications but is seldom investigated in oxidative coupling of amines to imines. A Mo₂N-based nanocatalyst is successfully achieved by the hybridization of PMo₁₂O₄₀^3? (PMo)-paired ionic pyridine-4-carboxylic acid (PC) and melamine (Mel), followed by calcination in N₂. Melamine acts as an outstanding carburization and nitridation reagent that can convert into carbon nitride, and meanwhile the released N element react with Mo species in PMo to form Mo₂N. The obtained catalyst PC-PMo-Mel-800 exhibits high activity and remarkable stability for the oxidative coupling of amines to imines under solvent-free and atmospheric conditions. This is the first case of Mo₂N-catalyzed oxidative coupling reaction catalyst. The strategy reported herein for fabricating Mo₂N may provide a significant and interesting approach to design more transition metal nitrides for wide catalytic applications.

Full text of DOI:10.1002/cctc.201800980

Accepted Article
Title: Molybdenum Nitride Nanocatalyst Derived from Melamine and
Polyoxometalate-based Hybrid for Oxidative Coupling of Amines
to Imines with Air
Authors: Yue Li, Kang Xiao, Jingjing Li, Pingping Jiang, Yucheng
Jiang, Shengyu Du, and Yan Leng
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ChemCatChem
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FULL PAPER
Molybdenum Nitride Nanocatalyst Derived from Melamine and
Polyoxometalate-based Hybrid for Oxidative Coupling of Amines
to Imines with Air
Yue Li,[a]# Kang Xiao,[b]# Jingjing Li, Pingping Jiang, Yuchen Jiang, Shengyu Du, Yan Leng*
[a]
[a]
[a]
[a]
[a]
Abstract: Molybdenum nitride (Mo
2
N) is one of the most promising
structure to that of noble metals.[15-19] TMNs may hold great
alternative catalysts for a wide range of Pt-catalyzed catalytic
applications but is seldom investigated in oxidative coupling of amines
promise as high-performance solid catalysts for a wide range of
catalytic reactions. However, in most of current studies, TMNs
were usually investigated as electrocatalysts for hydrogen-
to imines. A Mo
hybridization of PMo12
acid (PC) and melamine (Mel), followed by calcination in N
2
N-based nanocatalyst is successfully achieved by the
3-
[20-
O
40 (PMo)-paired ionic pyridine-4-carboxylic
evolution reaction (HER) and oxygen evolution reaction (OER),
2
2]
2
. Melamine
they have received much less attention for organic synthesis
acts as an outstanding carburization and nitridation reagent that can
convert into carbon nitride, and meanwhile the released N element
transformations. Considering that the embracing construction of
various nanostructure, controlling morphology, and doping a
second transition metal can improve the intrinsic property of
2
react with Mo species in PMo to form Mo N. The obtained catalyst
[
23-
PC-PMo-Mel-800 exhibits high activity and remarkable stability for the
oxidative coupling of amines to imines under solvent-free and
TMNs and realize high efficiency toward desired applications,
2
5]
it is thus of great significance to fabricate desired MoN
more specific catalytic reactions.
x
for
atmospheric conditions. This is the first case of Mo
oxidative coupling reaction catalyst. The strategy reported herein for
fabricating Mo N may provide a significant and interesting approach
2
N-catalyzed
By taking advantages of the unique wide composition variety
and chemical reactivity, organic-polyoxometalate (POM) hybrids
constructed by combination of POM and organic groups provide
a platform for the synthesis of a wide variety of POM-based
functional materials for many catalytic applications.[26-30]
Importantly, the in situ carbonization of the POM-based hybrids
2
to design more transition metal nitrides for wide catalytic applications.
Introduction
[31-34]
enables easy formation of transition-metal carbides (TMCs).
2
For example, Li et al reported N-carbon coated P-W C composite
as an electrocatalyst for HER by annealing the mixture of
Imines and their derivatives are versatile chemical intermediates
for the synthesis of pharmaceuticals, polymer materials,
biologicals and so on.[1-3] The oxidative coupling of amines was
demonstrated to be a promising one pot synthetic procedure for
imines, and considerable efforts have been devoted to it. Although
precious metals like Au, Pd, Ru, et al are very active catalysts for
this reaction,[4-6] transition metals catalyzed heterogeneous
catalysis, based on Mn, Mo, W, Co, and so on, are preferable
because of their low cost and earth-abundant.[7-12] Moreover,
[35]
polyoxometalate (H
3
PW12
O
40) and dicyandiamide.
We have
C by
PMo12O40-based poly(imidazolium
previously synthesized N-doped carbon encapsulated Mo
confined carbonization of H
2
3
ionic liquids), and used it as efficient heterogeneous catalyst for
[36]
dehydrogenation of alcohols. Although a huge family of POM-
based hybrids has covered a wide range of organic and metal
species, POM-based hybrids are exclusively used as the
precursors for TMCs, and little attention has been paid to the
fabrication of TMNs to date.
2 2
compared to tert-butyl hydroperoxide, H O , oxygen commonly
used oxidants, air is absolutely more economical and
environment-friendly.[13,14] Therefore, the development of efficient
and recyclable transition metal-based catalysts for the oxidative
coupling of amines using air as oxidant is highly desirable.
Transition-metal nitrides (TMNs), particularly, molybdenum
Herein, we report a facile synthetic strategy for the controlled
synthesis of Mo N-based catalyst for oxidative coupling of amines
2
to imines through the direct pyrolysis of a composite of melamine
3-
and PMo12
O
40 (PMo)-paired ionic hybrid. Melamine, a green and
cheap chemical with high N content, was demonstrated to be an
outstanding carburization reagent and nitridation reagent,[37] by
nitrides MoN
x
(MoN, Mo
2
N, Mo N
3 2
), have aroused ever-growing
attention as alternative catalysts owing to their similar electronic
which well-defined Mo
without using any toxic reactant such as NH
encapsulation of the POM by organic ligands with strong
interaction is also favorable for producing small Mo
2
N nanocrystals can be easily generated
3
. Moreover, the
[
a]
b]
Y. Li, J. Li, Prof. P. Jiang, Y. Jiang, Dr. Y Leng
The Key Laboratory of Synthetic and Biological Colloids, Ministry of
Education, School of Chemical and Material Engineering
Jiangnan University
Wuxi 214122, Jiangsu, China
E-mail: yanleng@jiangnan.edu.cn
2
N
nanoparticles and prevents the aggregation of the Mo
the calcination process.
2
N during
[
School of Materials Science & Engineering, Nanjing University of
Posts and Telecommunications, Nanjing 210023, PR China
Results and Discussion
Supporting information for this article is given via a link at the end of
the document.
3.1 Preparation and characterization of catalysts
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Figure 1. Schematic illustration for the preparation of the PC-PMo-Mel-800 through two steps: hybridization and calcination.
The overall preparation process of PC-PMo-Mel-800
composite was illustrated in Figure 1. Initially, PMo and PC were
protonated with 1:3 molar ratio to form ionic hybrid PC-PMo. Next,
PC-PMo was mixed with Mel in deionized water with 1:1 mass
ratio at 80 °C for 15 min. In this stage, multiple functional groups
demonstrated by XRD (Figures S5B and S5C, SI). Notably, the
mixing time of PC-PMo and Mel for preparing PC-PMo-Mel
complex also played an important role for the following Mo
generation. Strong characterized peaks assigned to MoO phase
at 26, 36, 53° were appealed while the mixing time was longer
than 1 h, and the peak intensity for MoO phase increased with
2
N
2
(
-COOH) of PC-PMo might conjugate with (-NH
2
) groups in Mel to
2
form the PC-PMo-Mel complex. The FT-IR spectrum of PC-PMo
presented stretching vibration peaks at 1724 cm for –C=O and
an increase of the mixing time (Figure S5D, SI). This is because
Mel is a basic compound, and it conjugates with PC-PMo through
–COO …H N – interaction during the PC-PMo-Mel formation
3
stage. However, the PMo anion may broke away from the PC
cations and bonded with Mel if the mixing time is too long, and the
2
exposed PMo anions were more likely to form MoO during the
-1
-
1
-1
-
+
3
450 cm for –O–H. However, the peak at 1636 cm was shifted
-1
to a lower wavenumber of 1676 cm and the intensity was
remarkably enhanced (Figure S1, SI). Moreover, a new peak
assigned to N–H was occurred at 3334-1 in the spectrum of PC-
PMo-Mel. These observations indicate that PC-PMo and Mel
calcination process. The above results indicate that both Mel and
-
N+–
have been tethered into PC-PMo-Mel through –COO …H
3
interaction. The PC-PMo-Mel complex was directly carbonized at
o
4
(
00-800 C in N
2
for 2 h to produce the PC-PMo-Mel-t composites
o
t stands for the calcination temperature: 400, 600, 800 C). TG
analysis revealed that the carbonization temperature of organic
species and Keggin PMo anions were approximately 350 and 450
o
o
C, respectively, and they were completely carbonized at 500 C
Figure S2, SI). The N sorption measurements showed that the
(
2
obtained samples were almost none porous material with low BET
surface areas of less than 10 m2g−1 (Figure S3, Table S1, SI).
The XRD patterns in Figure 2A provided detailed phase
evolution information for the obtained samples. For PC-PMo-Mel-
8
00, a set of sharp and strong diffraction peaks were observed at
o
37.4, 43.5, 63.3, and 75.9 , corresponding to the typical
N phase.[38,17] At 400 C and 600 C,
o
o
hexagonal structure of Mo
2
2
no Mo N phase was detected in PC-PMo-Mel-400 and PC-PMo-
Mel-600. The FT-IR spectra for PC-PMo-Mel-400 and PC-PMo-
Mel-600 revealed that the Keggin structure of PMo preserved well
in PC-PMo-Mel-400, but it was decomposed into MoO in PC-
x
PMo-Mel-600 (Figure S4, SI). With a relative low content of Mel
in the precursor PC-PMo-Mel0.5, the calcinated composite PC-
PMo-Mel0.5-800 showed weaker diffraction peaks for Mo
indicating the lower crystallinity (Figure S4A, SI). Moreover, when
using Mel-free PC-PMo or PC-free PMo-Mel as the precursors for
2
N,
Figure 2. (A) XRD patterns and (B) Raman spectra of (a) PC-PMo-Mel-
800, (b) PC-PMo-Mel-600, and (c) PC-PMo-Mel-400; The XPS spectrum
of (C) Mo3d and (D) N1s for (a) PC-PMo-Mel-800 and (d) PC-PMo-800.
2
calcination, PMo anions were mostly converted into MoO as
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Figure 3. (A) SEM and (B) TEM images of PC-PMo-800; (C) SEM, (D) TEM, (E) EDS elemental mapping, and (F) HTEM image of PC-PMo-Mel-800.
PC in the precursor PC-PMo-Mel are crucial for the Mo
generation.
Raman spectroscopy in Figure 2B gave a more favorable
characterization. Obvious, these samples displayed a D band at
2
N
2
MoO phase (Figure 3B). SEM image of PC-PMo-Mel-800
revealed the amorphous agglomeration morphology (Figure 3C).
In the TEM image of PC-PMo-Mel-800 (Figure 3D), fluffy
microstructure with quite small particles size of 3-5 nm was
observed. The HTEM image of PC-PMo-Mel-800 indicates that
−
1
−1
about 1330 cm and a very weak G band at 1600 cm for
graphitic carbon. PC-PMo-Mel-800 presented the characterized
the Mo
(Figure 3F). The lattice parameters with an interplanar distance of
2
0.24 nm can be ascribed to the (002) crystal plane of Mo N
2
N NPs were embedded in the nitrogen carbon matrix
−
1
2
peaks for the crystallized Mo N phase at 812 and 993 cm . There
are no bands belong to Mo-N mode were observed in the spectra
of PC-PMo-Mel-600 and PC-PMo-Mel-400. Additionally, the
(Figure 3F). The TEM elemental mapping images validated the
homogeneous distribution of Mo, N, C, O, and P elements in the
sample (Figure 3E), and Mo elements were covered by abundant
C and N elements (Figure S8, SI), which further confirms that
−
1
spectrum of PC-PMo-800 exhibited diffraction peak at 726 cm ,
which overlapped satisfactorily with that of a monoclinic MoO
Figure S6, SI). The phenomena of Raman spectra are in good
accordance with that in XRD patterns.
2
(
2
Mo N was encapsulated in the nitrogen carbon framework.
To probe the surface property of obtained samples, XPS
analysis was performed. The wide XPS analysis indicated the
presence of Mo, C, N, O, P elements in PC-PMo-Mel-800 (Figure
S7B, SI). The signals of Mo in the high-resolution Mo 3d spectrum
3.2 Catalytic activity for oxidative coupling of amines to
imines
In a preliminary experiment, a model imine formation from
benzylamine was performed over the prepared samples under
solvent-free and atmospheric conditions. As can be seen in Table
1, a very low conversion of 14.5% with 95.5% selectivity was
obtained without using any catalyst at 100 °C for 12 h (entry 1).
(
Figure 2C) can be deconvoluted into six peaks, which assigned
to three types of Mo species, that was Mo-C (228.9 and 231.8 eV),
Mo-N (229.9 and 233.1 eV), and Mo-O (235.6 and 232.6 eV).[37]
The N 1s spectrum (Figure 2D) exhibited the presence of Mo-N
(
397.8 and 395.5 eV), pyridinic-N (398.0 eV), pyrrolic-N (399.2 eV) The commercial MoO
and quaternary-N (401.1 eV), clearly confirm the N-doping and
formation of Mo N. The N doping was also confirmed in the C 1s
3
and PMo showed 36.8% and 61.7%
conversions, respectively (entries 2 and 3). Interestingly, PC-
PMo-Mel-800 offered an excellent conversion of 100% with 99.2%
selectivity and caused a heterogeneous catalytic system (entry 4).
The reaction conditions over PC-PMo-Mel-800 were investigated
(Table S2, SI), and it was found that a high conversion of 98.0%
could also be achieved when the reaction was performed at 90 °C.
However, in the case of PC-PMo-Mel-600 and PC-PMo-Mel-400,
approximately 47% conversions were obtained (entries 5 and 6),
2
spectrum (Figure S7A, SI), which showed a peak at 285.66 eV,
the characteristic peak for C=N. In contrast to PC-PMo-Mel-800,
the XPS spectrum of PC-PMo-800 revealed the formation of
MoO
2
(Figures 2C and 2D). Therefore, the above characterization
results provide very strong evidences that the introduction of Mel
into the PC-PMo can cause the generation of Mo
resulting samples.
2
N in the
which are only slightly higher than that of MoO
than PC-PMo-Mel-800. Furthermore, the sample PC-PMo-800
amorphous solid particles with size between 30-50 nm (Figure 3A). also gave a very low conversion of 46.4% (entry 7). From the
3
, but far less active
As shown in Figure 3, the SEM image of PC-PMo-800 showed
The TEM image of PC-PMo-800 was consistent with its SEM
image, and no lattice structure was observed, confirming the
above comparison results, one can deduce that different Mo
species in PC-PMo-Mel-800 (Mo N), PC-PMo-Mel-600 (MoO
2
x
)
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Table 1. Oxidative coupling of benzylamine promoted by different catalysts.[a]
Entry
Catalyst
None
Con[b]
%)
Sel[c]
(%)
Sel[d]
(%)
Sel[e]
(%)
(
1
2
3
4
5
6
7
14.5
36.8
61.7
100
95.5
97.9
97.7
99.2
97.7
97.4
98.3
0.8
0.2
0.9
0.1
0.5
0.4
0.6
3.7
1.9
1.4
0.7
1.8
2.2
1.1
MoO
3
PMo
Scheme 1. Proposed reaction pathway of PC-PMo-Mel-800 catalyzed imine
formation from amines.
PC-PMo-Mel-800
PC-PMo-Mel-600
PC-PMo-Mel-400
PC-PMo-800
47.4
47.0
46.4
Table 2. PC-PMo-Mel-800-catalyzed synthesis of imines from various amines
under solvent-free conditions.[
a]
[
1
a] Reaction conditions: benzylamine (4 mmol), catalyst (0.03 g), air balloon,
00 oC, 12 h. [b] Conversions and selectivity were determined by GC. [c]
Con[b]
Sel[c]
T
Entry
Substrate
Product
(
h)
12
12
(%)
(%)
Selectivity for imines. [d] Selectivity for benzaldehyde. [e] Selectivity for benzyl
cyanide.
1
2
100
99.1
81.0
and PC-PMo-Mel-400 (Keggin PMo) may account for their
85.9
2
different catalytic performance.The Mo N in PC-PMo-Mel-800
should act as the main active center toward the oxidation coupling
of amines. It was reported in the previous literature that graphene
can catalyze the oxidative coupling reactions.[39, 40] To make clear
3
4
12
12
99.8
88.7
>99
97.8
the possible effects of carbon nitride in PC-PMo-Mel-800 for the
oxidative coupling reaction, we have tried to synthesize carbon
nitride material via the direct pyrolysis of PC-Mel composite.
However, no product was obtained after the calcination step due
to the complete decomposition of PC and Mel. In spite of this,
during the PC-PMo-Mel carbonization process, PC-Mel enabled
highly dispersed Mo N active centers for the catalyst PC-PMo-
2
Mel-800 that is essential for the high activity.
As well known, the Mo-based carbides and nitrides can
promote the cleavage of the C-H bond and abstraction of proton
from substrates.[41,42] Therefore, based on the experimental
results and previous reports,[43-45] a possible mechanism was
5
6
7
12
12
12
13.2
>99
-
-
-
-
8
9
12
-
-
12
12
12
99.1
98.2
99.7
10.8(89.2)
24.9(75.1)
97.5(2.5)
10
11
12
12
99.2
87.0(13.1)
proposed in Scheme 1 for the air-based oxidative coupling of
amines to benzylamine over PC-PMo-Mel-800. After the initial
oxidative dehydrogenation of benzylamine on Mo N under air,
2
[
a] Reaction conditions: catalyst (0.03 g), amine substrate (4 mmol), air
o
balloon, 100 C, 12 h. [b] conversions and [c] selectivity were determined by
GC. For entries 9-12, the molar ratio of benzylamine and different amines is
there are three possible competitive reactions that can take place.
Step (a), hydrolyzed to give aldehyde and then condensed with
another benzylamine molecule to produce the product imine. Step
1
/3, and the selectivity was calculated based on the cross-coupled and self-
coupled (in bracket) products of benzylamine.
(
b), coupled with another benzylamine molecule and then
Not only the catalytic efficiency of PC-PMo-Mel-800 is higher or
comparable to the previously reported noble metal catalysts (such
deaminated to produce imine. Step (c), further oxidized to
generate benzyl cyanide. Since the main byproducts
benzaldehyde and benzyl cyanide for the oxidative coupling of
benzylamine were detected by GC-MS over PC-PMo-Mel-800, it
thus indicates that the reaction can be assumed to mostly proceed
through path a and c.
Subsequently, we tested the reusability and stability of PC-
PMo-Mel-800 in the oxidative coupling of benzylamine (Figure 4).
PC-PMo-Mel-800 can be easily recovered after the reaction by
centrifugation and steadily reused at least 7 times. HTEM
spectrum of the recovered catalyst revealed the well retained
as Au-Pd alloy,[46] Pd(II)–P(Oquin)
,[47] Au/TiO
,[48] Au/C[49]) and
3
2
[50,51]
transition metal catalysts
β-Tetrakis(trifluoromethyl)-mesotetraphenylporphyrin,[
Cs/MnO
,[54] nitrogen-doped carbons[17,40,55]) but also the reaction
(such as cobalt(II)
52]
[53]
2
TiO ,
x
condition is more energy saving and environmental protection, i.e.,
the reaction was performed in a static air at ambient pressure and
needless to use any solvents.
The substrate scope and limitations of PC-PMo-Mel-800
catalyzed oxidative coupling reactions were explored. As shown
in Table 2, various benzylamine derivatives with the presence of
electronic donating or withdrawing functionalities could be
coupled to the corresponding imines with high conversions and
Mo
structure for the excellent sustainability and recyclability. This is
the first case of Mo N-based oxidation coupling reactions catalyst.
2
N crystallinity (Figure S7, SI), indicating a durable catalyst
2
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2.2 Preparation of catalysts
Synthesis of hybrid PC-PMo
Pyridine-4-carboxylic acid (12 mmol, 1.5 g) was dissolved in 40 mL distilled
water with stirring, then the aqueous solution of H 40 (4 mmol, 3.6
3
PMo12O
g) was added into it. The mixture was stirred at room temperature for 12 h.
On completion, the solid powder was centrifuged, washed with distilled
water and dried at 70 °C in vacuum to afford the hybrid PC-PMo.
Synthesis of hybrid PC-PMo-Mel
The obtained PC-PMo (2 g) was mixed with melamine (2 g) in distilled
water (50 mL) and then stirred thoroughly at 80 °C for 15 min. After that,
the solid was filtered out, washed with water and dried at 80 °C for 12 h to
give the hybrid PC-PMo-Mel. Based on the similar method, the sample PC-
PMo-Mel0.5 can be obtained when the ratio of PC-PMo/Mel was 1:0.5.
Figure 4. Recycling tests of PC-PMo-Mel-800 for the oxidative coupling
of benzylamine (the reaction condition is similar to that in Table 1).
Carbonization process
selectivity. Heterocyclic amine could also be converted into imine
effectively. However, in the case of phenylamine, very low
conversion of 13.2% was observed. Additionally, the oxidative
coupling of cyclohexane, butylamine, and dodecylamine did not
proceed at all. Therefore, attempts were subsequently made to
explore the feasibility of using PC-PMo-Mel-800 to synthesis of
asymmetrical imines through cross-coupling of benzylamine with
different amines. It was found that the conversions were always
higher than 99%, and aliphatic amines reacted efficiently with
benzylamine to produce the corresponding asymmetrical imines
in high selectivity. However, when aniline or cyclohexane was
tested, relatively low selectivity for asymmetrical imine was
obtained.
Finally, the solid product was pyrolyzed in a tube furnace at 400, 600, or
8
00 oC for 2 h in N with heating rate of 5 oC/min, and the samples PC-
2
PMo-Mel-t (t stands for the carbonization temperature) was achieved. The
sample PC-PMo-800 was obtained by direct calcination of PC-PMo at 800
o
C in N2.
2.3 General procedure for catalytic reaction
Typically, amine (4 mmol) and catalyst (0.03 g) was added into a 25 mL
sealed round-bottomed flask and vigorously stirred at 100 °C under air
balloon for 12 h. After completion, the catalyst was separated by
centrifugation with N, N-Dimethylformamide, washed with ethanol, dried in
a vacuum, and reused for the next time. The filtrate was identified by GC-
MS, and the conversion and yield were analyzed by GC (SP-6890A)
equipped with a FID detector.
Conclusions
Author Contributions
In summary, we have developed a novel catalyst (PC-PMo-Mel-
#
Yue Li and Kang Xiao contributed equally
800) for the air-based oxidative coupling of amines to synthesis
imines. The PC-PMo-Mel-800 was prepared by thermolysis of
3
-
melamine and PMo12
The melamine in the primary structure of precursor enabled the Acknowledgements
generation of highly dispersed Mo N active phase due to that the
O
40 (PMo)-based ionic hybrid at 800 oC.
2
released N elements interact with Mo species to form Mo-N bond
at high temperature. PC-PMo-Mel-800 was demonstrated to be
excellent catalyst for oxidative coupling of amines with air without
solvent, affording good to excellent conversions and selectivity.
Besides, the catalyst can be steadily reused at least seven times.
This work may provide a facile way to fabricate more stable
transition-metal nitrides with desired performance for more
practical catalytic applications.
We thank the Fundamental Research Funds for the Central
Universities (JUSRP51623A) and MOE & SAFEA for the 111
Project (No. B13025).
Keywords: molybdenum nitrides • melamine • polyoxometalates
•
oxidative coupling reactions
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ChemCatChem
10.1002/cctc.201800980
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A Mo N-based nanocatalyst prepared by calcination of melamine and PMo12O40 (PMo)-based ionic hybrid was demonstrated to be a
highly active and stable heterogeneous catalyst for the oxidative coupling of amines to imines under solvent-free and atmospheric
conditions. The strategy in this work may provide a facile way to fabricate more stable transition-metal nitrides with desired
performance for more practical catalytic applications.
This article is protected by copyright. All rights reserved.