G Model
CCLET 5213 No. of Pages 5
2
F. Chen et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
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manganese oxalate calcined at a certain temperature (350 C,
s
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4
50 C, 550 C) were denoted M-3, M-4, M-5. Detailed methods for
catalyst preparation, characterization and evaluation were sum-
marized in Supporting information. The structure and morphology
of the catalyst were characterized by XRD, N
and the results showed that M-3 was mesoporous Mn
2
sorption, SEM, TEM,
microrod
2 3
O
with moderate surface area (Fig. 1, Table S1 and Figs. S1–S3 in
Supporting information). As a novel attemptation for oxidation of
benzylamine to imine, 100% conversion of benzylamine and 96.2%
selectivity of imine were successfully achieved with M-3 catalyst in
2
h, which was far more superior than other existing protocols.
Furthermore, this catalytic process was carried out under milder
conditions, no base additives, and air as the only oxidant.
The X-ray diffraction patterns of the samples were showed in
Fig. 1. M-3 exhibited no distinct diffraction peak in Fig. 1, which
suggested the amorphous nature of the material. Sample M-4
showed clearly appeared diffraction peaks, which were consistent
Fig. 2. Mn 2p and O 1s XPS spectra of various catalysts.
with the main cubic crystal Mn
a small part of metastable Mn
Sample M-5 only showed enhanced diffraction peaks of Mn
2
O
3
((JCPDS Card No. 041-1442) and
O (JCPDS Card No. 039-1218).
Table 1
5 8
Parameters of the fitted components on surface from Mn 2p and O 1s XPS spectra.
2 3
O .
Entry Sample Molar fraction
The results implied that the crystal particle size of manganese
oxides increased with calcination temperature elevated (Table S1).
The surface elemental compositions were analyzed by using
XPS. Fig. 2 displayed the Mn 2p and O 1s XPS spectra of various
catalysts. It was well known that the deconvolution of Mn 2p3/2
and Mn 2p1/2 XPS spectra was useful to distinguish the oxidation
Mn
O
O
Mn2+
Mn3+
Mn4+
Mn3+/Mn4+
ads
O
latt
O
sur
1
2
3
4
M-3
M-4
M-5
M-4
0.194 0.420 0.386 1.091
0.260 0.486 0.254 1.913
0.251 0.476 0.273 1.744
0.249 0.497 0.253 1.964
0.250 0.693 0.057
0.227 0.630 0.143
0.203 0.752 0.045
0.227 0.655 0.117
n+
a
states of Mn . As shown in Fig. 2a, the Mn 2p2/3 XPS spectra on
the surface of catalysts showed a broad shoulder peak, which
a
After third reuse.
implied the coexistence of Mn2 , Mn and Mn species. The
+
3+
4+
fitted Mn 2p3/2 peaks at the binding energy (BE) of 643.0 eV,
4
+
3+
2+
6
41.7 eV, and 640.6 eV could be assigned to Mn , Mn , and Mn
,
in surface oxygen compared with the other samples. The role of
water on manganese oxide surface was also believed to be
significant for amine oxidation to imine [19].
The catalytic activities of different manganese oxides were
evaluated using the oxidative self-coupling of benzylamine as a
model reaction. The evaluating results were shown in Table 2. The
reaction hardly happened in the absence of catalyst (Table 2, entry
respectively (Fig. 2a) [29,30]. A spin-orbit energy separation
(
11.7 Æ 0.2 eV) was displayed between Mn 2p1/2 and Mn 2p3/2
states (Fig. 2a). Fig. 2b showed the asymmetrical peak, which
could be seen as the superposition of three peaks of O 1s at 533.0
eV, 531.3 eV, 529.8 eV. The small peak at BE of 533.0 eV was the
characteristic of the molecular water (named as Osuf), the peak at
BE of 531.3 eV corresponded to adsorbed oxygen species (O
À
,
3 2 2 4 2
1). No products were detected using Mn(NO ) , Mn(C O )‧3H O as
2
2
À
À
O
2
, O , named as Oads), and the peak at BE of 529.8 eV was
the catalysts (Table 2, entries 2 and 3). Surprisingly, not only M-4
catalyst achieved 100% conversion of benzylamine, but also the
selectivity of imine was as high as 96.2% (Table 2, entry 5).
Concerning the solvent (Table S2 in Supporting information,
entries 1–5), toluene solvent could provide higher conversion and
selectivity. The by-product was mainly benzonitrile in toluene as
the solvent, in addition to a small amount of benzaldehyde and
benzamide. At 100% conversion of benzylamine, the selectivity to
imine was only 77.2% by M-3 catalyst (Table 2, entry 4). When the
conversion of benzylamine was 80.3%, the highest selectivity
(98.3%) of imine was achieved using M-5 catalyst (Table 2, entry 6),
implying that the imine showed higher selectivity at low
conversion of benzylamine after 2 h of the reaction. These results
suggested manganese oxide calcined at low temperature showed
high catalytic activity for oxidation of amine, and manganese oxide
calcined at high temperature showed high selectivity to imine.
Although this reaction process produced water, the addition of
trace water increased the selectivity of the imine (Table 2, entry 7,
Fig. 3b). This suggested that trace water was beneficial to the
formation of imines during the reaction, which was also consistent
2
À
indexed to lattice oxygen atoms in Mn-O-Mn (O , named as Olatt
)
n+
[
29–31]. Table 1 showed surface Mn and different surface
oxygen species molar fraction from Mn 2p and O 1s XPS spectra.
Surface-active Mn species had an important influence on the
catalytic activity of the oxidative self-coupling of amines to
imines [19]. In our latest research, we had found that Mn easily
led to excessive oxidation of quinoline [28]. As could be
interestingly seen from Table 1, M-4 possessed highest content
of Mn (0.486) and highest Mn /Mn ratio, implying that M-4
possessed the moderate catalytic activity and higher selectivity of
imines. Table 1 showed that M-4 owned highest content of water
3
+
4
+
3
+
3+
4+
with the literature [19]. Under O
enhanced conversion (98.8%) and the decreased selectivity (94.2%)
after 1 h (Table 2, entry 8, Fig. 3a). N atmosphere only gave 6.3%
2
atmosphere, M-4 achieved the
2
conversion (Table 2, entry 9), which ascribed the oxidation role of
active oxygen species in the manganese oxide [32].
The selectivity of imine was an important issue in the oxidation
of benzylamine [19]. Figs. 3a–c showed catalytic activity of M-4, M-
3, and M-5 dependent on the reaction time. M-4 gave 100%
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Fig. 1. XRD patterns of wide angle (10 –80 ) of the samples.
2 3
O catalyst under eco-friendly conditions,