Tuning effect of amorphous Fe2O3on Mn3O4for efficient atom-economic synthesis of imines at low temperature: Improving [O] transfer cycle of Mn3+/Mn2+in Mn3O4

Article abstract of DOI:10.1039/d0cy01021a

A facile and scaled-up synthesis route to efficient and environment-friendly metal oxide catalysts with desirable properties is of great practical importance, owing to their excellent performance as heterogeneous catalysts in organic synthesis. Herein, a novel amorphous Fe2O3 modified Mn3O4 catalyst (Fe5Mn5-100) has been prepared by adopting a simple co-precipitation method following low temperature baking. Fe5Mn5-100 showed exceptionally high catalytic activity for the atom-economic production of imine from benzyl alcohol with aniline, giving a 98% imine yield at 60 °C in only 3 h, which is higher than all of the reported non-noble and noble metal catalysts. Importantly, Fe5Mn5-100 could still exhibit extraordinary catalytic performance on a large-scale reaction without any solvent, including the frequently used toxic mesitylene, xylene or toluene. Interestingly, the amorphous Fe2O3-100 provided no catalytic activity, and pure Mn3O4-100 showed very inferior catalytic activity towards this reaction. Further detailed characterizations and experimental results revealed that amorphous Fe2O3 plays an important role in expediting the [O] transfer cycle of Mn3+/Mn2+ in Mn3O4, and enhances the oxidation ability and acidity of Fe5Mn5-100. This discovery would provide a new avenue for the atom-economic and environment-friendly industrial synthesis of imine. This journal is

Full text of DOI:10.1039/d0cy01021a

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Tuning effect of amorphous Fe O on Mn O for
2
3
3 4
efficient atom-economic synthesis of imines at
low temperature: improving [O] transfer cycle of
Cite this: DOI: 10.1039/d0cy01021a
3+
2+
Mn /Mn in Mn O †
3
4
a
a
ab
a
a
a
Jiaheng Qin, Yu Long, * Galian Gou, Wei Wu, Yutong Luo, Xiao Cao,
a
a
a
Shicheng Luo, Kaizhi Wang and Jiantai Ma
*
A facile and scaled-up synthesis route to efficient and environment-friendly metal oxide catalysts with desirable
properties is of great practical importance, owing to their excellent performance as heterogeneous catalysts in
organic synthesis. Herein, a novel amorphous Fe
O
2 3
modified Mn O
3 4
5 5
catalyst (Fe Mn -100) has been
prepared by adopting a simple co-precipitation method following low temperature baking. Fe
5
Mn -100
5
showed exceptionally high catalytic activity for the atom-economic production of imine from benzyl alcohol
with aniline, giving a 98% imine yield at 60 °C in only 3 h, which is higher than all of the reported non-noble
5 5
and noble metal catalysts. Importantly, Fe Mn -100 could still exhibit extraordinary catalytic performance on a
large-scale reaction without any solvent, including the frequently used toxic mesitylene, xylene or toluene.
Interestingly, the amorphous Fe O -100 provided no catalytic activity, and pure Mn O -100 showed very
Received 18th May 2020,
Accepted 1st July 2020
2
3
3
4
inferior catalytic activity towards this reaction. Further detailed characterizations and experimental results
3
+
2+
revealed that amorphous Fe
Mn , and enhances the oxidation ability and acidity of Fe
avenue for the atom-economic and environment-friendly industrial synthesis of imine.
2
O
3
plays an important role in expediting the [O] transfer cycle of Mn /Mn in
DOI: 10.1039/d0cy01021a
O
3 4
5
Mn -100. This discovery would provide a new
5
rsc.li/catalysis
7
,8
is water.
Many homogeneous and heterogeneous noble
Introduction
9
10–13
metal catalysts including Ru, Pd,
Pt (ref. 14) and Au (ref.
Imine is an important organic chemical intermediate, which
has been widely applied in the fields of fine chemicals,
15 and 16) have been developed, but the complicated
manufacturing process and high cost of these catalysts limit
their development in industrial applications.
1–3
pharmaceuticals and agriculture.
The traditional methods
of producing imines include unstable aldehyde condensation
Non-noble metal oxide catalysts have been widely applied
in industrial production, owing to their good catalytic activity
4
,5
with amines in the presence of acid catalysts,
and the
1
7,18
production craft is unfriendly to the environment. Therefore,
constructing new methodologies for imine synthesis is of
prime importance. During the past decade, many new
and stability.
been employed for imine synthesis. Tamura et al. found that
CeO showed much higher activity than other metal oxides
Recently, various metal oxide catalysts have
1
9
2
6
20
strategies such as the oxidative coupling of amines and the
towards the oxidative coupling reaction. Liu et al. found that
γ-Fe O displayed a 91% imine yield at 80 °C for 8 h. Wang
oxidative coupling of alcohols and amines have been
developed. Among these methods, the direct oxidative
coupling of alcohols and amines has attracted great attention
as the reactant alcohols are available and the only byproduct
2
3
2
1
2 4
et al. prepared a hierarchically nanostructured MnCo O
nanorod catalyst to catalyze this reaction, obtaining a 94%
yield of imine after 12 h at 80 °C. MnO /HAP was also
x
developed for this reaction, giving a 92% yield of imine after
2
2
2
4 h at 80 °C. However, all of these metal oxide catalysts
a
showed inferior catalytic performance or needed a high
temperature and long reaction time, which were far from
practical applications. In particular, achieving atom-economic
synthesis of imines (alcohols : amines = 1 : 1) is also a difficult
problem for these catalytic systems.
State Key Laboratory of Applied Organic Chemistry (SKLAOC), Gansu Provincial
Engineering Laboratory for Chemical Catalysis, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou 730000, PR China.
E-mail: longyu@lzu.edu.cn, majiantai@lzu.edu.cn; Tel: +86 931 8912577
b
Lanzhou Petrochemical Research Center, Petrochemical Research Institute,
Petrochina, Lanzhou 730060, PR China
As previous researchers revealed, the redox property of the
metal oxides is a crucial factor for the first oxidation step
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy01021a
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(
rate-determining step for synthesis of imines) in this Results and discussion
19
oxidative coupling reaction. So, developing a novel and
efficient catalytic system to enhance the redox property of the
catalyst for the synthesis of imine is really practical and
meaningful. Our group has done several studies on metal
Catalyst composition and structure
3 4
A series of amorphous Fe modified Mn O catalysts with
O
2 3
different Mn content were prepared by adopting the facile co-
precipitation method following low temperature baking
2
3–26
oxide catalysts for the oxidative coupling reaction.
(
shown in Scheme S1†). A certain amount of MnIJCH
3 2
COO)
Thereinto, we found that the redox and amphoteric
properties of CeO2 could be improved by introducing the
second component, thus showing improved catalytic activity
for this reaction. CeO as a typical metal oxide accorded with
the Mars and van Krevelen mechanism,
vacancies play an important role in the catalytic process.
Inspired by this, it is quite attractive to investigate whether
other metal oxides agree with the Mars and van Krevelen
mechanism and also show the similar phenomenon. This
could be followed by a further investigation of the synergistic
effect between the two components, so as to construct a more
efficient and suitable catalytic system towards this reaction.
As is well known, hausmannite trimanganese tetraoxide
and FeIJNO ·9H O were dissolved in the water and stirred
)
3 3
2
for 1 h at room temperature, then ammonia solution was
added to the mixed solution until the pH value reached 9.
After stirring for another 5 h, the formed precipitate was
filtered and washed with de-ionized water. Finally, the
samples were dried at 100 °C instead of roasting at high
2
1
9,27
in which oxygen
2 3
temperature to endow Fe O with its unique amorphous
property. Conversely, Mn O still showed good crystallinity
3
4
though drying at 100 °C, which is the key factor for the
preparation of this unique material. Moreover, the Fe and
Mn content of the prepared catalysts were obtained by ICP
analysis, which are displayed in Table S1.† The contents of Fe
and Mn are very close to the theoretical values. These results
demonstrate that our preparation method can effectively
control the composition of Fe and Mn in the samples,
avoiding the waste of the metal source.
3 4
(Mn O ) is a stable spinel structure oxide with the formula
2
+
3+
28
[Mn (Mn
2 4
)O ], which has attracted great attention in
2
9,30
31–33
34
electrochemistry,
energy storage,
owing to its peculiar physicochemical
properties and low-cost. More importantly, Mn is also a
photocatalysis,
3
5–38
and catalysis
XRD patterns of the prepared samples are shown in Fig. 1.
Pure Fe O -100 showed no diffraction peaks, demonstrating
2 3
3 4
O
typical metal oxide accorded with the Mars and van Krevelen
mechanism. However, in this paper, the experimental result
showed that Mn
towards this reaction because of the inherent inert oxidizing
ability of Mn . In addition, we found that Mn showed a
real inferior ability to activate O and convert it into a
chemisorbed oxygen species for the reaction consumption,
which may be the critical factor for the low reactivity. Even
so, the existence of an active Mn /Mn redox pair and
oxygen vacancy still endows Mn O with a good catalytic
its amorphous structure, which is due to the high
crystallization activation energy and high crystallization
3 4
O displayed inferior catalytic activity
3
9,40
temperature of amorphous Fe
2
O
3
.
3 4
Pure Mn O -100
showed well-defined diffraction peaks, corresponding to the
tetragonal hausmannite structure of Mn O (JCPDS No 24-
O
3 4
3 4
O
3
4
2
4
1
2 3 3 4
0734). For the amorphous Fe O modified Mn O samples,
Fe Mn -100 and Fe Mn -100 displayed no peaks and very
8
2
5
5
3
+
2+
weak diffraction peaks (Fig. S1†), respectively, which is due
to the low content of Mn and the shielding effect of
Fe Mn -100 showed obvious peaks,
3 4
O
3
4
4
2
amorphous Fe O .
2
3
2
8
potential for this reaction. The key is how to tactfully
introduce the right component to tailor the oxidizing ability
and O conversion cycle of Mn O .
which are broader than those of pure Mn O , indicating a
3
4
smaller nanocrystal size and poorer crystallinity. From the
XRD results when increasing the Fe content in the Fe Mn -
2
3 4
X
Y
On this basis, a novel amorphous Fe
O
2 3
modified Mn
3 4
O
100 samples, we found that the diffraction peaks of Mn O
3 4
catalyst (Fe Mn -100) was prepared by
5
5
a
facile and
8 2 5 5 8 2
got weaker and wider (Fe Mn -100, Fe Mn -100 and Fe Mn -
environmental-friendly co-precipitation method following low
temperature baking, and employed for catalyzing the
oxidative coupling of benzyl alcohol and aniline for imine
formation. The prepared samples were further characterized
in detail to investigate the synergistic effect between
2 3 3 4
amorphous Fe O and Mn O . The reaction mechanism over
this catalyst was also studied in depth. In addition, solvent-
free magnification, recycling and universality of the catalyst
was tested. To the best of our knowledge, the amorphous
Fe O modified Mn O catalyst has been synthesized and
2
3
3 4
applied for the oxidative coupling reaction for the first time.
Experimental
The detailed description of materials, catalysts preparation,
characterization measurements and catalyst test are showed
in ESI.†
Fig. 1 XRD patterns of the as-prepared samples.
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00) compared with those of the Mn O -100 sample,
Paper
1
3
4
indicating a smaller nanocrystal size and poorer crystallinity
caused by the crystalline contraction. These results suggest
that the incorporated Fe ions are partly incorporated into the
Mn O lattice, which results in the change of the physical
3
4
and chemical properties of the catalyst, thus influencing the
catalytic activity. More importantly, the XRD peaks of Fe Mn -
2
8
1
00 shift to higher 2θ values than pure Mn O (see the inset
3 4
in Fig. 1). This phenomenon suggests that chemical
interactions between amorphous Fe O and Mn O in the
2
3
3 4
4
3
catalysts certainly exist.
To further determine the morphology and structure of
Fe Mn -100, SEM and TEM measurements were applied. The
5
5
Fig. 3 FT-IR patterns of the as-prepared samples.
result of SEM is shown in Fig. S2,† which indicate that
Fe Mn -100 has a blocky granular shape. From the results
shown in Fig. 2 and S3,† it can be clearly seen that Fe Mn -
5
5
5
5
−
1
1
00 is composed of two parts. The lattice plane with the
regions, the band at about 437 cm is the characteristic
5
0
interplanar spacing of 0.48 nm is attributed to the (101)
plane of the tetragonal hausmannite structure of Mn O . For
Fe–O bond vibration of Fe
2
O
3
.
Peaks at around 615, 492
are the typical Mn–O bond vibration of
Thus, the FT-IR results can also demonstrate that
the catalysts consist of Mn and Fe , which is in
agreement with the XRD and TEM results.
adsorption–desorption characterization was adopted to
−
1
and 411 cm
3
4
5
1
the other part, no crystal lattice could be found in the TEM
image, indicating that the Fe species shows an
3 4
Mn O .
O
2 3
3
O
4
2 3
O
amorphous phase. The corresponding selected area electron
diffraction (SAED) pattern can also confirm this conclusion,
owing to no diffraction being found apart from the crystal
diffraction of Mn O , which draws the same conclusion as
N
2
study the pore structure and specific surface area of the
catalysts, and the results are displayed in Fig. S4.† Mn O -100
3
4
3 4
5
2,53
the XRD results.
shows II-type isotherms with a H3-type hysteresis loop,
and other catalysts show IV-type isotherms with a H2-type
To investigate the surface group species of the catalysts,
FT-IR analysis was employed and the spectra results are
displayed in Fig. 3. All of the prepared catalysts show a
5
4,55
hysteresis loop.
distribution curves, which are displayed in Fig. S4(b),† it can
be concluded that Mn -100 is composed of a macroporous
Based on the results of the aperture
−
1
distinct wide band at about 3410 cm corresponding to the
3 4
O
stretching vibration of the O–H bonds of the hydroxyl
and mesoporous structure, and the other catalysts show a
typical mesoporous structure. In addition, the values of the
specific surface areas and pore volumes for the as-prepared
catalysts are all exhibited in Table S2.† It should be clear that
4
4
−1
groups. A band at around 1625 cm could be attributed to
the bending vibrational mode of the interlayer water
4
5
−1
molecules. Furthermore, the peak at about 950 cm can be
4
6,47
ascribed to the Mn–OH bending vibrations.
The surface
the amorphous Fe
surface areas than Mn
of amorphous Fe O are conducive to adsorbing more oxygen
2 3
O -100 possesses much higher specific
hydroxyl groups of the catalyst can serve as Brønsted acidic
sites. It can interact with the oxygen atom of the benzyl
alcohol, and weaken the chemical bond of O–H in the benzyl
3 4
O -100, and the greater surface areas
2
3
molecules and converting them into the chemisorbed oxygen
species during the reaction process.
4
8,49
alcohol molecule,
which is conducive to facilitating the
oxidation of benzyl alcohol. Moreover, for the low-frequency
Catalytic performance
The oxidative coupling of benzyl alcohol (BA) and aniline for
N-benzylideneaniline (NBA) production was employed when
evaluating the catalytic performance of the catalysts. As we
all know, the reaction conditions are vital for organic
synthesis reactions, so we first optimized the reaction
conditions (shown in Table 1). Fe
NBA at 60 °C for 3 h. When the reaction temperature rose to
0 °C, the conversion of BA was almost unchanged (Table 1,
5 5
Mn -100 gave 98% yield of
8
entries 1, 2). It is also worth noting that when the catalytic
reaction was conducted at 30 °C, Fe Mn -100 still maintained
5
5
high product yields, giving a 95% NBA yield for 16 h (Table 1,
entry 3). Therefore, 60 °C is the optimal temperature towards
this reaction. As for the reaction time, Fe Mn -100 displayed
5
5
Fig. 2 TEM images of Fe
5
Mn
5
-100.
79% conversion of BA for 2 h (Table 1, entry 4) and 99%
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5 5
Table 1 Optimization of the reaction conditions for the oxidative coupling synthesis of imine from benzyl alcohol and aniline over Fe Mn
-100a
b
Entry
BA : aniline (molar ratio)
Solvent
T (°C)
Time (h)
Conv. of BA [%]
Sel. of NBA [%]
1
2
3
4
5
6
7
8
9
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1.5
1 : 2
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
MeCN
60
80
30
60
60
60
60
60
60
60
60
60
60
60
3
3
16
2
6
3
3
3
3
3
3
2
3
98
99
95
79
99
99
99
29
Trace
Trace
Trace
81
99
97
99
99
99
98
99
99
99
48
Trace
Trace
Trace
99
99
99
EtOH
EtOAc
10
11
12
13
14
2
H O
c
Toluene
Toluene
Toluene
c
d
12
a
b
c
Conditions: benzyl alcohol (1 mmol), aniline (1 mmol), 50 mg catalyst, solvent (3 mL), air, 800 rpm. Determined by GC-MS.
2
O as the
d
oxidant. Conditions: benzyl alcohol (10.0 mmol), aniline (10.0 mmol), 500 mg catalyst, no solvent, air, 800 rpm, 60 °C, 12 h.
conversion of BA for 6 h (Table 1, entry 5), demonstrating
that 3 h is the optimal time for this reaction. According to
solvents. We found that toluene showed the best reactivity
(Table 1, entries 8–11), which might be caused by the non-
polar and aprotic properties of toluene. Moreover, when O
2
was used as the oxygen source, there was no difference in
activity compared to air (Table 1, entries 12, 13). From the
results mentioned above, the optimum conditions towards
this reaction are 60 °C, 3 h, toluene as solvent, BA : aniline =
1 : 1, and under air atmosphere.
1
9
the literature, poor imine yield was obtained when the
same equivalent of aniline (BA : aniline = 1 : 1) was applied,
and only adding excess aniline enabled the reaction to
proceed normally. Therefore, from the perspective of atomic
economy, achieving atom-economic synthesis of imines is
necessary for this reaction. Different molar ratios of BA :
aniline were further investigated, and the experiment results
showed that Fe Mn -100 could still reach the same high
Meanwhile, the influence of the composition of the
prepared catalysts on the catalytic activity was also studied.
No imine product was detected when no catalyst was added,
indicating that the oxidative coupling reaction cannot occur
without a catalyst. The pure Mn O -100 showed inferior
5
5
imine yield even with BA : aniline = 1 : 1 (Table 1, entries 1, 6
and 7). This demonstrated the exceptional catalytic
performance of Fe Mn -100 for the atom-economic
5
5
3 4
production of imine. As different solvents have different
effects on the reaction, we further tested various types of
catalytic activity for this reaction, and only 10% yield of
imine was obtained under the optimal reaction conditions
Table 2 N-Benzylideneaniline (NBA) synthesis from benzyl alcohol (BA) and aniline over various as-prepared catalystsa
b
Entry
Catalyst
None
T (°C)
Time (h)
Conv. of BA [%]
Sel. of BA [%]
1
2
3
4
5
6
7
8
9
60
60
60
60
60
60
60
60
60
60
60
60
60
3
3
3
3
3
3
3
3
3
3
3
3
3
Trace
Trace
12
17
32
65
78
98
81
48
23
14
35
Trace
Trace
95
99
99
99
99
99
99
99
99
99
99
2 3
Fe O -100
3 4
Mn O -100
Fe
9
Fe
8
Fe
7
Fe
6
Fe
5
Fe
4
Fe
3
Fe
2
Fe
1
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
1
-100
2
-100
3
-100
4
-100
5
-100
6
-100
7
-100
8
-100
9
-100
1
1
1
1
0
1
2
3
c
Fe : Mn = 5 : 5
a
c
b
Conditions: benzyl alcohol (1 mmol), aniline (1 mmol), 50 mg catalyst, solvent (3 mL), air, 800 rpm, 60 °C, 3 h. Determined by GC-MS.
Mechanical mixed sample, the molar ratio of Fe : Mn is 5 : 5.
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Table 3 Catalytic performance of imine synthesis from oxidative coupling of benzyl alcohol and aniline over different reported catalystsa
b
c
−1 −1
Catalysts
Fe Mn -100
T (°C)
Time (h)
BA : aniline (molar ratio)
Conv. [%]
Sel. [%]
V [mmol h
g
]
Ref.
5
5
60
60
60
60
60
60
80
80
60
90
60
60
60
3
24
9
24
12
6
12
24
12
20
16
8
1 : 1
1 : 1
1 : 1
1 : 2
1 : 1.2
1 : 2
1 : 2
1 : 1
1 : 2
1 : 1.2
1.5 : 1
1 : 1.5
1 : 1
98
89
97
99
99
79
98
99
97
92
91
99
99
99
90
99
99
94
98
96
94
97
99
99
99
99
6.47
0.67
2.13
0.81
0.39
0.43
1.12
0.31
1.56
0.29
0.38
0.78
2.51
This work
CeO
Fe Ce
2
19
25
56
57
58
21
22
59
60
11
15
16
8
2
-100
-5
CeO
CeO
CeO
2
2
2
d
rode
/MC
f
MnCo
MnO /HAP
γ-MnO
Pd/AlO(OH)
2 4
O
-500
g
x
2
h
i
Pd–CeNi0.3O
Y
j
Au/Zn0.01Al
Au/HAP
2
O
3
k
3
a
b
c
Conditions: benzyl alcohol (1 mmol), aniline (1 mmol), 50 mg catalyst, toluene (3 mL), air, 800 rpm. Determined by GC-MS. V: moles of
d
desired product (NBA) formed per catalyst amount per hour. Conditions: benzyl alcohol (0.5 mmol), aniline (0.6 mmol), p-xylene (2 mL), CeO
2
e
f
1
00 mg, 1 atm of O
2
.
300 mg catalyst. Conditions: benzyl alcohol (0.5 mmol), aniline (1 mmol), 35 mg catalyst, toluene (2 mL), air.
Conditions: benzyl alcohol (1 mmol), aniline (1 mmol), 125 mg catalyst, toluene (2 mL), air. Conditions: benzyl alcohol (1.0 mmol) and
g
h
i
2
amine (1.2 mmol), heptane (5 mL), catalyst (4.0 mol% of Pd), 1 atm of O . Conditions: benzyl alcohol (0.75 mmol) and amine (0.5 mmol),
toluene (5 mL), 75 mg catalyst, 1 atm of O
atm. Conditions: benzyl alcohol (1 mmol) and amine (1 mmol), toluene (10 mL), 130 mg catalyst, O
j
2
.
Conditions: benzyl alcohol (0.5 mmol), aniline (0.75 mmol), 80 mg catalyst, toluene (5 mL), air, 1
k
−1
2
bubbling (20 mL min ).
Table 4 Experiments on investigating the reaction mechanisma
b
Entry
Catalyst
Fe -100
T (°C)
Time (h)
Conv. of BA [%]
Sel. of NBA [%]
c
1
O
2 3
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
3
3
3
3
3
Trace
Trace
Trace
Trace
8
18
Trace
8
19
Trace
3
11
Trace
13
Trace
Trace
Trace
Trace
95
99
Trace
97
99
Trace
96
99
Trace
99
c
2
Mn -100
-100
-100
-100
-100
-100
-100
-100
-100
-100
-100
-100
-100
-100
-100
3 4
O
c
3
Fe
Fe
5
Mn
5
d
4
O
2 3
d
5
Mn O
3 4
d
6
Fe
Fe
5
Mn
5
3
d
7
O
2 3
12
12
12
3
3
3
3
3
3
96
d
8
3 4
Mn O
d
9
Fe
Fe
5
Mn
5
e
1
1
1
1
1
1
1
0
1
2
3
4
5
6
O
2 3
e
e
f
Mn O
3 4
Fe
Fe
5
Mn
5
O
2 3
f
Mn
Fe Mn
Mn
3
O
4
f
5
5
32
78
99
98
O
3 4
a
c
b
Conditions: benzyl alcohol (1 mmol), aniline (1 mmol), 50 mg catalyst, solvent (3 mL), air, 800 rpm, 60 °C, 3 h. Determined by GC-MS.
Catalysts collected after O -TPD experiments. Under N conditions. 20 mg catalyst. 100 mg catalyst.
2 2
d
e
f
(
Table 2, entries 1, 3). When the reaction time was extended
to 96 h, the imine yield was only 77% (Table 4, entry 16).
Even more interesting, the pure Fe -100 showed no
4–12). The relationship between the imine yield and Mn
molar content showed a volcano shape (Fig. S5†). For
samples with high Fe content, the few active sites (Mn O )
3 4
O
2 3
reaction activity under the same conditions (Table 2, entry 2).
In contrast, the amorphous Fe O modified Mn O samples
could not support the catalytic system in efficiently
proceeding, thus affording low imine yields. As for samples
with high Mn content, there was not enough amorphous
Fe O to form a synergistic effect with Mn O , thus also
2
3
3 4
exhibited greatly enhanced catalytic activity. These results
indicate that Mn O is the active site towards this reaction,
3
4
2
3
3 4
and amorphous Fe O only plays the role of a co-catalyst to
getting low imine yields. Only Fe and Mn with an appropriate
molar ratio can form a highly efficient catalyst. Among these
catalysts, Fe Mn -100 showed the best catalytic performance,
2
3
3 4
modify the physicochemical property of Mn O . Furthermore,
a series of amorphous Fe O modified Mn O catalysts with
2
3
3
4
5
5
different Mn content were also investigated (Table 2, entries
displaying 98% imine yield under the optimum conditions.
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However, the mechanical mixed sample (Fe : Mn = 5 : 5)
showed a dramatic decrease of imine yields (Table 2, entry
XPS surveys, which are displayed and described in the
following section. The broad scan spectrum exhibits the
signals for Fe, Mn and O (see Fig. S6†). Fig. 5 shows the XPS
spectra results of Fe 2p. It exhibits characteristic Fe 2p3/2 and
Fe 2p1/2 peaks at binding energies of 710.7 and 724.3 eV
accompanied with two weak satellite peaks at the positions
of 718.4 and 732.3 eV, evidencing that the Fe element in all
1
3), demonstrating that the formation of an extraordinary
synergistic effect between amorphous Fe O and Mn O
2
3
3 4
during the catalytic synthesis process should be responsible
for the super catalytic activity.
For further illustrating the super high activity of Fe Mn -
5
5
3
+
61–63
1
00, the catalytic performance of various reported non-noble
samples shows the Fe oxidation state.
Combined with
and noble metal catalysts is displayed in Table 3. It is clear
that Fe Mn -100 exhibits the best catalytic activity over all of
the above results, it further indicated that iron oxide exists in
the samples in the form of Fe O . In addition, the Fe 2p
5
5
2 3
the reported catalysts. For non-noble metals catalysts, all
catalysts displayed inferior activity when the same equivalent
of aniline (BA : aniline = 1 : 1) is applied at low temperature.
Only Fe Ce -100 and MnO /HAP could achieve atom-
peaks of Fe Mn -100 are shifted relative to Fe O -100, which
5 5 2 3
is due to the electron redistribution and chemical interaction
6
4
between Fe and Mn.
Fig. 6 shows the high-resolution XPS scan over the Mn 2p
spectral region. The peaks at around 653.0 and 641.3 eV can
be ascribed to Mn 2p1/2 and Mn 2p3/2, respectively. The spin-
energy separation between the two peaks is 11.7 eV, which is
consistent with the earlier report on the mixed valent state of
8
2
x
economic synthesis of imines. However, a long reaction time,
high temperature and large amount of catalyst were needed.
Moreover, Fe Mn -100 still showed better catalytic
5
5
performance than all of the noble metal catalysts. Only Au/
HAP showed almost the same activity as Fe Mn -100, but 130
2
+
3+
65,66
Mn and Mn of Mn O .
The Mn 2p spectra were fitted
5
5
3 4
mg catalyst and O
under the same amount of substrate, only 50 mg Fe
could catalyze this reaction efficiently under air atmosphere,
demonstrating that Fe Mn -100 showed excellent ability to
involve O in the catalytic process. Unfortunately, the surface
area of the active site (Mn O ) in Fe Mn -100 could not be
2
bubbling were indispensable. However,
and refined with four peaks. The two peaks at binding
energies of 641.2 eV and 652.7 eV are the characteristic peaks
5
Mn -100
5
2
+
of the Mn reduction states, and the peaks at around 642.3
3
+
5
5
eV and 654.0 eV can be ascribed to the Mn oxidation
2
1,67,68
2
states.
These results demonstrate that the Mn element
2
+
3+
exhibits the mixed states of Mn and Mn in all of the as-
prepared samples. Furthermore, the percent composition of
Mn in all catalysts was calculated through the formula
[Mn /(Mn + Mn )] on the basis of the fitting peaks area
(Table S2†). Fe
3
4
5
5
measured, so the chemical reaction data were defined by the
2
+
formation rate per catalyst amount (abbreviated as V [mmol
−
1
−1
2+
2+
3+
h
g ]). The more distinct results are shown in Fig. 4.
Mn -100 shows the largest V value in comparison with
2
+
Fe
5
5
5 5
Mn -100 shows the highest Mn content over
2
+
other catalysts even under the mildest of conditions. To our
knowledge, Fe Mn -100 displays the best catalytic
all catalysts. The greater existence of Mn
is closely
correlated with the formation of more oxygen vacancies,
which is very important for the oxidation ability and catalytic
5
5
performance until now towards the imine synthesis from the
oxidative coupling of benzyl alcohol and aniline.
6
9,70
3 4
activity of the Mn O -based catalysts.
For the amorphous
Fe O modified Mn O catalysts, greater oxygen vacancies
2
3
3 4
mean better capacity for capturing oxygen species, thus
Chemical properties characterizations
3+
2+
promoting the Mn /Mn redox cyclic process in Mn
3
O
4
. In
2
+
The elemental valence state composition and change plays
an important role during the catalytic process. Therefore, the
elemental valence states of the samples were determined by
addition, the high content of Mn in Fe Mn -100 may
5
5
3
+
2+
decrease the redox potential between the Mn and Mn
Fig. 5 Fe 2p XPS spectra of the as-prepared (a) Fe
Fe Mn -100, (c) Fe Mn -100, and (d) Fe Mn -100.
2 3
O -100, (b)
Fig. 4 Catalytic performance of different reported catalysts.
8
2
5
5
2
8
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amorphous Fe O -100 exhibits better O mobility than
sur
2
3
Mn O -100, and Fe Mn -100 displays more chemisorbed
3
4
5
5
oxygen species and better Osur mobility than Mn
3 4
O -100 and
Fe O -100.
2
3
As shown in Fig. 8, of particular note is that the Fe 2p
peaks of Fe Mn -100 shifts to a lower position than Fe
00, which is caused by the electron redistribution between
5
5
2 3
O -
1
3
+
3+ 43,75
Mn and Fe .
p XPS spectra of Fe
position than Mn O -100. Additionally, the O 1s peaks of
This mission was further sided by the Mn
2
5
Mn -100, which shifted to a higher
5
3
4
Fe Mn -100 fall in between Mn O -100 and Fe O -100, which
5
5
3
4
2 3
further confirms the presence of a chemical interaction
between Mn O and amorphous Fe O in Fe Mn -100.
3
4
2
3
5
5
3 4
Fig. 6 Mn 2p XPS spectra of the as-prepared (a) Mn O -100, (b)
To find out the structure–function relationship and
synergistic effect between Mn and amorphous Fe in
Fe Mn -100, TPR/TPO/TPD experiments were then applied to
Fe
8
Mn
2
-100, (c) Fe
5
Mn
5
-100, (d) Fe
2
Mn
8
-100.
O
3 4
2 3
O
5
5
the as-prepared samples. On the basis of previous reports,
5
,19,58
cations, which endows Fe
storage capacity.
5
Mn
5
-100 with the best oxygen
the oxidative coupling is a tandem reaction.
Oxidizing
5
6
benzyl alcohol into benzaldehyde is the first step, and
benzaldehyde was also examined by GC-MS during the
reaction process. Then, the produced benzaldehyde is
nucleophile-attacked by amine to get imine. The rate-
controlling step of this tandem reaction is the initial alcohol
As shown in Fig. 7, the O 1s XPS spectra were fitted with
two peaks ascribed to the surface chemisorbed oxygen
species at high binding energy (labeled as Osur) and the
lattice oxygen species at low binding energy (labeled as
36,71,72
19
O
lat).
According to the previous study, the chemisorbed
oxidation. Therefore, the oxidation ability of the catalyst is
oxygen species exhibit higher mobility than the lattice oxygen
species, which deliver better catalytic activity in the oxidation
of great importance for this reaction. As shown in Fig. 9(a),
the oxidation ability of the samples was studied by H
experiments. Fe -100 shows peaks at about 270, 341 and
560 °C, demonstrating the characteristic reduction progress
2
-TPR
3
6,43
reactions.
Osur mobility of the catalyst.
sur in all catalysts was calculated through the formula [Osur
sur + Olat)] on the basis of the fitting peaks area (Table S2†).
The mobility of Osur displays the order as follows: Fe Mn -100
Moreover, higher Osur content means better
2 3
O
7
3,74
The percent composition of
0
25
O
/
as follows: Fe
2 3 3 4 3 4
O → Fe O → FeO → Fe . As for Mn O -100,
(O
two reduction peaks at around 230 and 435 °C can be
76
observed, which is consistent with Y. Yongnian's work. They
demonstrated that Mn shows a normal spinel structure
5
5
>
2 3 3 4
Fe O -100 > Mn O -100. These results indicate that
3 4
O
2
+
3+
with Mn
ions in tetrahedral sites and Mn
ions in
tetragonally distorted octahedral sites, which contain three
3
+
types of Mn –O bonds, including the octahedral long and
3
+
3+
short Mn –O, as well as additional tetrahedral Mn –O
bonds associated with nonstoichiometric oxygen bound to
Fig. 7 O 1s XPS spectra of the as-prepared (a) Fe
00, (c) Fe Mn -100, (d) Fe Mn -100, (e) Mn -100.
2
O
3
-100, (b) Fe
8
Mn
2
-
Fig. 8 The peaks migration of (a) Fe 2p, (b) Mn 2p, (c) O 1s in the XPS
spectra.
1
5
5
2
8
3
O
4
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Fig. 9 (a) H
2
-TPR results of the as-prepared samples, (b) O
2
-TPO
2 3
Fig. 10 (a) O -TPD results of the as-prepared samples, (b) NH -TPD
results of the as-prepared samples.
results of the as-prepared samples.
the tetrahedral Mn. Thus, it can be concluded that the two
reduction peaks are ascribed to the reduction of three
oxygen species, while peaks at higher temperatures are
5
6,80
bound to the desorption of lattice oxygen.
The O
2
3
+
77
different kinds of Mn –O bonds in Mn
3 4
O .
5 5
For Fe Mn -
desorption of Fe Mn -100 starts at low temperatures around
5
5
1
00, when amorphous Fe O was introduced into Mn O , the
2
3
3
4
50 °C, and Fe Mn -100 desorbs more chemisorbed oxygen
5
5
first two peaks became overlapped and less steep, which were
2 3 3 4
species than Fe O -100 and Mn O -100 at the same
7
8
caused by the co-reduction of Mn
O
3 4
2 3
and Fe O .
temperature (as seen by the green dotted line in Fig. 10(a)),
which may be also caused by the synergistic effect between
Furthermore, Fe Mn -100 showed a lower first reduction
5
5
3
+
2+
3+
2+
temperature (T ) than Fe O -100 and Mn O -100. The T value
1
2
3
3
4
1
the Mn /Mn and Fe /Fe redox. Given the above, we know
that Fe Mn -100 possesses both higher amount of
faster chemisorbed oxygen
-100 and Mn -100 at the
low temperature range, implying that Fe Mn -100 shows good
represents the temperature at which the surface oxygen
begins reacting with H , where the lower T value is ascribed
a
5
5
2
1
chemisorbed oxygen and
a
5
6
to the better oxidation ability of the catalyst. According to
the above results, it can be known that the oxidation ability
desorption rate than pure Fe
O
2 3
3 4
O
5
5
of the samples shows the order as follows: Fe Mn -100 >
5
5
catalytic potential towards the low temperature oxidative
process.
Mn O -100 > Fe O -100. The above results confirm the
3
4
2 3
3
+
2+
3+
existence of a synergistic effect between Fe /Fe and Mn /
According to the previous works reported, when the same
equivalent of aniline (BA : aniline = 1 : 1) was applied for the
oxidative coupling reaction, most of the catalysts displayed
2
+
3+
2+
Mn redox in Fe Mn -100. The Mn /Mn redox could be
5
5
3
+
2+
expedited by the Fe /Fe redox via the redox equilibrium of
2
+
3+
3+
2+
3+
2+
1
9,56
Mn + Fe ↔ Mn + Fe . This is because the Fe /Fe
inferior catalytic activity, especially at low temperatures.
For our catalytic system, Fe Mn -100 showed excellent
3
+
redox (0.77 V) shows much lower redox potentials than Mn /
5
5
2
+
43,79
Mn redox (1.54 V).
ability of Fe Mn -100 is attributed to the modification of the
unique amorphous property of Fe O -100, as well as the
Therefore, the enhanced oxidation
catalytic performance for the atom-economic production of
imine at 60 °C, giving 98% yield of imine for only 3 h.
Previous studies have shown that the acid sites of the catalyst
could promote the second aldehyde–amine condensation
5
5
2
3
3
+
2+
3+
2+
synergistic effect between the Fe /Fe redox and Mn /Mn
redox.
7
,25,81
reaction.
So, the NH -TPD experiment was applied to
3
Oxygen plays an indispensable part on the catalytic
measure the surface acidity sites of the catalysts. As shown in
Fig. 10(b), Fe Mn -100 possesses much more acidic sites than
process. Therefore, O -TPO measurements were employed to
2
5
5
evaluate the ability of different catalysts to obtain and
Fe O -100 and Mn O -100 irrespective of the low temperature
2
3
3 4
transform O (shown in Fig. 9(b)). The oxygen consumption
2
zone or high temperature zone. The abundant acidic sites of
Fe Mn -100 endow the catalyst with the strong ability to
of the same amount of different catalysts displays the order
5
5
5 5 2 3 3 4
as follows: Fe Mn -100 > Fe O -100 > Mn O -100. The unique
adsorb aniline at even BA : aniline = 1 : 1. The adsorbed
aniline further attack the produced benzaldehyde to achieve
the efficient atom-economic synthesis of imines.
amorphous property endows the amorphous Fe O -100 much
2
3
better ability to acquire O2 than Mn O -100. More
3
4
5 5
interestingly, Fe Mn -100 is surprisingly better at capturing
oxygen than pure Fe O -100 and Mn O -100, even though the
2
3
3 4
Catalytic mechanism study
molar content of Fe is only 50%. It demonstrates that the
3+
2+
3+
2+
synergistic effect between Fe /Fe and Mn /Mn redox can
greatly promote Fe Mn -100 to capture and activate O , and
Based on the above characterization test results, we have
gained a better understanding of the physical and chemical
properties of the catalysts. To further uncover the oxidative
5
5
2
then convert it into the chemisorbed oxygen species, which
can greatly expedite the catalytic process for the oxidation
5 5
coupling reaction mechanism over the Fe Mn -100 catalyst,
2
5
coupling reaction.
other mechanism experiments were also necessary. In the
first place, Fe O -100, Mn O -100 and Fe Mn -100
An O -TPD measurement was then carried out to compare
2
2
3
3
4
5
5
the amount of chemisorbed oxygen of the as-prepared
samples (displayed in Fig. 10(a)). The peaks at about 130 °C
and 345 °C are ascribed to the desorption of the chemisorbed
2
measurements (following O -TPD) were carried out on the
oxidative coupling reaction, and all samples gave no imine
yield. This is because the samples of Fe O -100, Mn O -100
2
3
3 4
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and Fe Mn -100 would lose their chemisorbed oxygen species
was carried out under air atmosphere, it showed a slight
increase in the imine yield, giving a 10% imine yield for 3 h
(Table 2, entry 3). Even when the reaction time was extended
to 96 h, the imine yield was only 77% (Table 4, entry 16).
These results indicate that Mn O -100 showed a real inferior
5
5
(
Table 4, entries 1–3) after O -TPD. Then, fresh samples of
2
2 3 3 4 5 5
Fe O -100, Mn O -100 and Fe Mn -100 were investigated
under N2 atmosphere, displaying no yield, 8% and 18%
imine yields, respectively (Table 4, entries 4–6). When
extending the reaction time to 12 h, the imine yields barely
increased (Table 4, entries 7–9). Furthermore, different
amounts of catalysts were also carried out under N2
atmosphere. When the amount of catalyst was reduced to 20
mg, Fe O -100, Mn O -100 and Fe Mn -100 obtained no yield,
3
4
2
ability to activate O and convert it into the chemisorbed
oxygen species for the reaction consumption, which is
consistent with the O -TPO and O -TPD results.
2
2
According to the Mars and van Krevelen mechanism and
2
7,69,70
the above experimental results,
the proposed reaction
2
3
3
4
5
5
3
1
% and 11% imine yields, respectively (Table 4, entries 10–
2). As for when the amount of catalyst was increased to 100
mechanism over Mn O -100 is shown in Scheme 1. The
chemisorbed oxygen species, Mn /Mn redox and O form
2
3
4
3+
2+
mg, Fe O -100, Mn O -100 and Fe Mn -100 showed no yield,
a unique catalytic cyclic process: benzyl alcohol reacts with
the chemisorbed oxygen species of Mn O to form
2
3
3
4
5
5
1
1
3% and 32% imine yields, respectively (Table 4, entries 13–
5). From all of these results, we can draw the conclusion
3
4
3
+
2+
benzaldehyde, and Mn
will be reduced to Mn . The
that the oxidative coupling reaction can be only triggered in
the presence of Mn O -100, while Fe O -100 showed no
formed benzaldehyde reacts with aniline to produce imine.
2
+
3+
Mn is oxidized to Mn by means of O . However, the
3
4
2
3
2
reactivity. This is due to the better oxidation ability of Mn
3
O
4
-
inherent inert physicochemical and redox properties of
Mn O causing the cyclic process are very inefficient. When
3
+
2+
1
00 than Fe O -100, and Mn O -100 has unique Mn /Mn
2 3 3 4
3 4
redox and oxygen vacancies. Furthermore, the chemisorbed
oxygen on Mn -100 is also indispensable for oxidizing
benzyl alcohol to benzaldehyde. At the initial time of this
5 5
Fe Mn -100 was conducted under air atmosphere, it
displayed much more enhanced catalytic activity than that
O
3 4
under N atmosphere, giving a 98% imine yield for 3 h. This
2
reaction, benzyl alcohol is adsorbed on the surface of Mn
3
O
O
4
-
-
result demonstrates that introducing amorphous Fe
2 3
O into
3
+
2+
1
1
00. The Mn /Mn redox and oxygen vacancies of Mn
00 can interact with the oxygen atom of benzyl alcohol. This
3
4
the Mn catalytic system could greatly expedite the [O]
3 4
O
3
+
cyclic process. Owing to the good oxidation ability (Fe ) and
abundant moveable chemisorbed oxygen species of
will weaken the chemical bond of the O–H benzyl alcohol
8
2,83
molecule,
so that the benzyl alcohol reacts with the
2 3 3 4 5 5
amorphous Fe O -100, as well Mn O , Fe Mn -100 possesses
surface chemisorbed oxygen species belonging to Mn O to
a large amount of oxygen vacancies to capture oxygen
3
4
2
+
3+
form benzaldehyde, along with the production of one H
2
O
species. Thus, the Mn
amorphous Fe -100 instead of
chemisorbed oxygen of amorphous Fe O -100 transports to
will be oxidized to Mn
by
molecule. Furthermore, the adsorbed aniline quickly attacks
the produced benzaldehyde to form imine. Therefore, for
O
2 3
2
O .
Meanwhile, the
2
3
3
+
2+
3 4
Mn O -100 under 1 atm N
2
atmosphere, when using up the
the oxygen vacancies of Mn
3 4
O and Fe is reduced to Fe .
only certain amount of chemisorbed oxygen species on
Mn O -100, the oxidation of benzyl alcohol with chemisorbed
oxygen species will be terminated. This is because there is no
continuous supply of chemisorbed oxygen species to keep the
reaction going on, thus giving only 8% imine yield.
Furthermore, O will instantly oxidize the reduced product of
2
2
+
3+
2+
3+
Fe to Fe rather than oxidizing Mn to Mn , owing to
3
4
3
+
2+
the Fe /Fe redox (0.77 V) showing much lower redox
3+
2+
potentials than the Mn /Mn redox (1.54 V). In addition,
amorphous Fe O -100 shows much greater ability to capture
2
3
Interestingly, Fe
the same reaction conditions (1 atm N
twice as much as Mn O -100, even though the molar content
of Mn is only 50%. According to the atomic conservation law,
one molecule of benzyl alcohol reacts with one molecule
chemisorbed oxygen to form one molecule of benzaldehyde
5
Mn
5
-100 showed an 18% imine yield under
and activate O
2
, as well as convert it into the chemisorbed
2
), which is almost
oxygen species than Mn
Based on all of the results above, the proposed reaction
mechanism over Fe Mn -100 is displayed in Scheme 2. In the
first place, benzyl alcohol reacts with the chemisorbed oxygen
species belonging to Mn O to form benzaldehyde, along with
3 4
O -100 (Fig. 9b).
3
4
5
5
3
4
3
+
and H
than Mn
amorphous Fe O in Fe Mn -100 must also be involved in this
2
O. Therefore, to form about twice the amount of imine
2
the H O molecule. In the meantime, Mn is reduced to
3 4
O -100, the chemisorbed oxygen species of
2
3
5
5
reaction process. This is because the amount of chemisorbed
oxygen species on Mn is not enough to form 18% imine
yield. However, Fe O -100 cannot catalyze the oxidation of
3 4
O
2
3
benzyl alcohol alone, even though it contains a lot of the
chemisorbed oxygen species. The only possibility is that the
unique physicochemical property of Fe Mn -100, as well as
5
5
3
+
2+
3+
2+
the synergistic effect between the Fe /Fe and Mn /Mn
redox, plays an important role in transferring the
chemisorbed oxygen species of amorphous Fe O to Mn O
3 4
2
3
during the catalytic process. Furthermore, when Mn
3 4
O -100
3 4
Scheme 1 Proposed reaction mechanism over Mn O -100.
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5 5
Fig. 11 (a) Recycle test of Fe Mn -100 for the synthesis of
benzalaniline from benzyl alcohol and aniline. Reaction conditions:
benzyl alcohol (1 mmol), aniline (1 mmol), catalyst (50 mg), solvent
5 5
Scheme 2 Proposed reaction mechanism over Fe Mn -100.
(
toluene 3 mL), 800 rpm, 60 °C, air, 3 h. (b) Hot filtration experiment of
Fe Mn -100 by continuing the reaction after filtration of the catalyst.
Reaction conditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst
50 mg), solvent (toluene 3 mL), 800 rpm, 60 °C, air.
5
5
2
+
Mn . Then, the formed benzaldehyde quickly reacts with
aniline to synthesize imine. Owing to the good oxidation
(
3
+
ability (Fe ) and abundant moveable chemisorbed oxygen
species of amorphous Fe O , Mn O in Fe Mn -100 also
2
3
3
4
5
5
possesses a large amount of oxygen vacancies to capture
also detected by SEM, XRD, N2 adsorption–desorption
measurement and XPS, which are displayed in Fig. S7–S10.†
The morphology, structure, pore structure and element
2+
3+
oxygen species. Mn
will also be oxidized to Mn
by
amorphous Fe O -100 instead of O . Meanwhile, the
2
3
2
2 3
chemisorbed oxygen of amorphous Fe O -100 transported to
valence state composition of the reused Fe
obvious change compared with the fresh Fe
Only the surface area and pore volume show a small decrease
shown in Table S3†), which is due to the adsorption of the
5 5
Mn -100 shows no
3
+
2+
the oxygen vacancies of Mn
3 4
O and Fe is reduced to Fe .
5 5
Mn -100 catalyst.
Furthermore, O will instantly oxidize the reduced product of
2
2
+
3+
2+
3+
Fe to Fe rather than oxidize Mn to Mn . This is due to
the Fe /Fe redox (0.77 V) showing much lower redox
(
3+
2+
residual organic substance on the surface and pores of the
reused Fe5Mn5-100. These results show that Fe Mn -100 has
3+
2+
potentials than Mn /Mn
amorphous Fe -100 shows a much greater ability to capture
and activate O , as well as convert it into the chemisorbed
redox (1.54 V). Furthermore,
5
5
2 3
O
good stability during the reaction. Furthermore, the hot
filtration experiments were also carried out (shown in
Fig. 11(b)), and it is obvious that no subsequent benzyl
alcohol was converted after the catalyst was filtered from the
liquid phase. Moreover, the filtrate was also examined by ICP
analysis, and only negligible amounts of Fe (0.4 ppm) and
Mn (0.2 ppm) were detected, further demonstrating the
2
oxygen species, than Mn O -100. We can draw the conclusion
3
4
that the amorphous Fe
expediting the [O] transfer cycle of Mn /Mn in Mn O , as
3 4
2
O
3
plays an important part in
3+
2+
well as enhancing the oxidation ability and acidity of the
catalyst.
5 5
stability and heterogeneous nature of Fe Mn -100 during
the reaction process. These results indicate that the
Solvent-free magnification, recycling and universality of the
catalyst
5 5
Fe Mn -100
catalyst
is
a
highly
efficient
and
environmentally-friendly
catalyst,
showing
favourable
In consideration of both green technology and industrial
industrial application prospect.
applications,
developing
highly
effective
and
To gain more insight into the general applicability of the
Fe5Mn5-100 catalytic system, a series of substrates were
investigated on a 10-fold large-scale reaction at 60 °C without
using toxic mesitylene, xylene or toluene as the solvent. It
was observed that high imine yields were obtained regardless
of electron-donating or electron-withdrawing aniline
derivatives (Table 5, entries 1–6). In addition, the substitution
position of the aniline derivatives showed a slight influence
on the catalytic performance (Table 5, entries 1–3). As for the
benzyl alcohol derivatives, when they reacted with aniline
under the same reaction conditions, the benzyl alcohol
derivatives could also bear whatever electron-withdrawing
and electron-donating groups to achieve high imine yields
(Table 5, entries 7–13). The ortho-methyl derivative exhibited
environmentally-friendly catalysts with low cost is necessary.
For our catalyst, the low-cost Fe Mn -100 could be prepared
by a facile and low energy consumption approach, as well as
achieve a feasible scaling-up preparation. In addition, it is
essential to achieve a solvent-free reaction because the
reported works always use toxic mesitylene, xylene or toluene
5
5
as the solvent. Therefore, Fe
5 5
Mn -100 was carried out on a
1
0-fold large-scale reaction at 60 °C without solvent,
displaying a 96% imine yield for 12 h (Table 1, entry 14). In
addition, the recyclability of the catalyst is also an important
factor for environmental protection and industrial catalytic
application. From the results shown in Fig. 11(a), we know
5 5
that the catalyst Fe Mn -100 dried at 100 °C could still
maintain 80% imine yield after 4 catalytic cycles even if the
catalyst was not calcinated at high temperature,
demonstrating the good recyclability of Fe Mn -100 towards
5 5
this low-temperature organic synthesis reaction. The
morphology and structure of the catalyst after reaction was
a lower imine yield than the meta- and para-methyl
derivatives, which is due to the steric hindrance effect of the
ortho-methyl group (Table 5, entries 7–9). The above results
demonstrate that Fe Mn -100 has good substrate universality
5
5
towards this reaction.
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Table 5 Scope of benzyl alcohols and anilines for the imine synthesisa
b
Entry
1
Alcohol
Amine
Conv. [%]
94
Sel. [%]
98
2
97
99
3
4
98
96
99
99
5
6
98
99
99
99
7
8
85
93
96
99
9
97
98
98
99
99
99
99
99
1
1
1
0
1
2
13
98
99
a
b
Conditions: benzyl alcohol (10 mmol), aniline (10 mmol), 500 mg catalyst, no solvent, air, 800 rpm, 60 °C, 12 h. Determined by GC-MS.
Conclusions
simple catalyst could afford excellent imine yield on the
magnification experiment without solvent rather than using
In summary, we demonstrated the novel amorphous Fe O
2
3
toxic mesitylene, xylene or toluene. Furthermore, Fe
5 5
Mn -
modified Mn O as an efficient catalyst for the atom-
3
4
100 has the advantages of easy manufacturing, low energy
economic synthesis of imines from benzyl alcohol and
aniline at low temperature, which exhibits better catalytic
performance than all of the reported non-noble and noble
metal catalysts. Based on the more in-depth studies, we
learned that the amorphous Fe O plays an important part in
consumption, practical scaling-up as well as good
universality and recyclability, providing an environmental-
friendly and economical protocol for industrial production
of imines.
2
3
3
+
2+
expediting the [O] transfer cycle of Mn /Mn in Mn O , and
3
4
5 5
enhances the oxidation ability and acidity of Fe Mn -100,
which should be responsible for the excellent catalytic
performance of the catalyst. In addition, this inexpensive and
Conflicts of interest
There are no conflicts to declare.
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Catalysis Science & Technology
Acknowledgements
24 Y. Long, Z. Gao, J. Qin, P. Wang, W. Wu, L. Zhang, Z. Dong
and J. Ma, J. Colloid Interface Sci., 2019, 538, 709–719.
This research was supported by the National Natural Science
Foundation of China (China, 21872066 and 21802058) and
the Fundamental Research Funds for the Central Universities
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2
5 J. H. Qin, Y. Long, W. Wu, W. Zhang, Z. K. Gao and J. T. Ma,
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