Flower-like Bi2O2CO3-mediated selective oxidative coupling processes of amines under visible light irradiation

Article abstract of DOI:10.1016/j.jcat.2019.05.001

The photocatalytic selective transformation of amines is a green and cost-effective technology to obtain value-added products in chemical industry. In this work, a series of bismuth-based photocatalysts including Bi₂MoO₆, Bi₂WO₆, Bi₅O₇Cl, Bi₅O₇Br, Bi₅O₇I, BiPO₄, BiVO₄, Bi₂O₃ and various morphology Bi₂O₂CO₃ (flower-like, sponge-like, plate-like and spherical)were synthesized and employed in the aerobic oxidative coupling of benzylamine. It is found that flower-like Bi₂O₂CO₃ exhibited the highest photocatalytic activity, in which a 100% conversion of benzylamine with 99.0% selectivity of N-benzylidenebenzylamine was obtained at room temperature. Moreover, the photocatalytic oxidative coupling processes of various aromatic and aliphatic amines were further investigated, and excellent yields and selectivities of corresponding products are attained. Then, based on characterization results (XRD, SEM, BET and XPS, etc.)of catalyst, high photocatalytic activity of flower-like Bi₂O₂CO₃ is attributed to thin nanopetals, low band gap, the morphology and large specific surface area. Finally, a possible reaction mechanism is proposed for the photocatalytic oxidative coupling of benzyl amine.

Full text of DOI:10.1016/j.jcat.2019.05.001

Journal of Catalysis 374 (2019) 257–265
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Flower-like Bi O CO -mediated selective oxidative coupling processes of
2
2
3
amines under visible light irradiation
⇑
Peng Bai, Xinli Tong , Jun Wan, Yiqi Gao, Song Xue
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology,
Tianjin 300384, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The photocatalytic selective transformation of amines is a green and cost-effective technology to obtain
value-added products in chemical industry. In this work, a series of bismuth-based photocatalysts
Received 18 February 2019
Revised 22 April 2019
Accepted 2 May 2019
including Bi
Bi CO (flower-like, sponge-like, plate-like and spherical) were synthesized and employed in the
aerobic oxidative coupling of benzylamine. It is found that flower-like Bi CO exhibited the highest
photocatalytic activity, in which 100% conversion of benzylamine with 99.0% selectivity of
2 6 2 6 5 7 5 7 5 7 4 4 2 3
MoO , Bi WO , Bi O Cl, Bi O Br, Bi O I, BiPO , BiVO , Bi O and various morphology
2
O
2
3
2
O
2
3
a
Keywords:
Oxidation
Photocatalysis
Amines
Molecular oxygen
Visible light
N-benzylidenebenzylamine was obtained at room temperature. Moreover, the photocatalytic oxidative
coupling processes of various aromatic and aliphatic amines were further investigated, and excellent
yields and selectivities of corresponding products are attained. Then, based on characterization results
2 2 3
(XRD, SEM, BET and XPS, etc.) of catalyst, high photocatalytic activity of flower-like Bi O CO is attributed
to thin nanopetals, low band gap, the morphology and large specific surface area. Finally, a possible
reaction mechanism is proposed for the photocatalytic oxidative coupling of benzyl amine.
Ó 2019 Elsevier Inc. All rights reserved.
1
. Introduction
Developing renewable clean energy has become one of the most
have a wide range of applications in the synthesis of pharmaceutical
intermediates or biologically active organic compounds [19,20].
Notablely, from the standpoint of green chemistry, using molecular
oxygen as oxidant to carry out the oxidation of amines is an attrac-
tive approach in which the activation of oxygen is a key point [21–
24]. In traditional, the aerobic oxidative coupling of amines was
often performed under thermal conditions with precious metallic
promising topics of the 21st century [1,2]. Solar energy is abun-
dant, clean, and renewable on earth. At present, researchers have
paid great attentions on the efficient utilization of solar energy.
In the chemical field, photocatalysis is considered as one of the
most desirable technologies for the conversion of solar energy; of
particular note is that photocatalysis can take advantage of solar
energy to trigger chemical reaction [3,4]. Correspondingly, the
application of the semiconductors as photocatalysts including
metal oxides TiO
CdS [9], ZnS [10] and bismuth oxides BiOX [11–13], Bi
BiVO [15] have showed high photocatalytic activity in UV and/
or visible regions.
ruthenium and gold catalysts. For example, the RuCl
3
[25], Ru-
porphyrin [26], Ru/Al [27], Au/Al [28], Au/CeO [29] and Au/
O
2 3
O
2 3
2
graphite [30] have been found to be active catalysts for this reaction.
However, these processes need high temperature and high pressure,
resulting in large number of energy consumption despite the use of
precious metal. On the contrary, along with the rapid development
of photocatalytic technology, the aerobic oxidative coupling of ami-
nes to imines with photocatalysts can be achieved at a low temper-
2
[5], ZnO [6], SnO
2
[7], ZrO
2
[8], metal sulfides
2 3
O
[14],
4
Although photocatalysis has been studied extensively, the
research and application of photocatalysis now mainly focuses on
photolysis of water to produce hydrogen and degradation of organic
pollutants. The application of photocatalysis on organic synthesis is
limitedly reported, and some efforts have been made toward expan-
sion the scope of organic reactions [16–18]. In recent years, the oxi-
dation of amines to imines is often chosen as a model reaction to
investigate the activity of photocatalyst, where the generated imines
ature and low pressure. In the previous investigations, TiO
2
[31] and
Nb [32] have exhibited relatively high photocatalytic activities
2 5
O
under light irradiation. However, the disadvantage of these systems
is requiring a long reaction time (greater than 24 h) to obtain a high
yield of imines. In addition, Jin et al. also reported the use of homo-
geneous cobalt thioporphyrazine as photocatalyst to accelerate the
oxidation of amines to form imines [33]; however, the recycling
and reuse of cobalt catalyst still keeps a problem.
In the photocatalytic researches, bismuth oxides including
⇑
Corresponding author.
E-mail address: tongxinli@tju.edu.cn (X. Tong).
WO
3 2 6 4 4 4 2 3 2 5
@Bi WO /NiWO [34], BiVO /FeVO [35] and Bi O -V O [36]
were mainly used for photo-degradation of organic pollutants,
https://doi.org/10.1016/j.jcat.2019.05.001
021-9517/Ó 2019 Elsevier Inc. All rights reserved.
0

2
58
P. Bai et al. / Journal of Catalysis 374 (2019) 257–265
and there have been few reports on the application of organic syn-
thesis. Recently, Samanta et al. employed a BiVO /g-C compos-
(1 mol/L) (a transparent solution named as solution A) and
1.0 g CTAB and 8.45 g Na CO (80 mmol) were added to
4
3
N
4
2
3
ite catalyst to accelerate the oxidation of benzylamine, benzyl
alcohol and aniline, and achieved a desired product yield after
90 mL distilled water (a transparent solution named as solu-
tion B). When solution A was added dropwise to solution B
with constant stirring, there was the immediate formation
of a milk-like suspension. After being stirred for 10 min,
the mixture was subject to filtration and the resulting white
precipitate was collected and then washed several times
with distilled water and absolute alcohol. After drying at
1
6 h reaction time [37]. Han et al. reported that the bismuth oxy-
halide could promote the oxidative coupling of amines to imines
with molecular oxygen [38]. Therein, the catalytic efficiency and
the introduction of halogen were not still quite satisfactory, so
exploring a new and green bismuth oxide catalyst is attractive
and promising in the photocatalytic synthesis.
2 2 3
60 °C for 4 h, the flower-like Bi O CO samples were
Bi
showed a good photocatalytic activity in the degradation of organic
pollutants. For example, the Co /Bi CO catalyst was used to
accelerate the degradation of naphthalene [39], and CuS-
Bi CO was employed on the photocatalytic chlorpyrifos degra-
dation [40]. In addition, the single Bi CO was also found to be
efficient for the purification of dye-containing wastewater [41].
Also, different morphologies BiO CO catalysts such as flower-
2
O
2
CO
3
, as a newly developed green photocatalyst, has
obtained.
b) The preparation method of sponge-like Bi
2
O
2
CO
3
is similar
Á5H O and
3
O
4
2
O
2
3
with the reference 42. Firstly, 0.5 mmol Bi(NO
3
)
3
2
0.75 mmol tri-sodium citrate were added to 25 mL deion-
ized water with stirring at room temperature, and then the
solution was subjected to an ultrasonic process (10 min)
and vigorous stirring (3 h). Then the PH value of the solution
2
O
2
3
2
O
2
3
2
3
was adjusted to about 9 using 25% (mass weight) NH
3
ÁH O
2
like, sponge-like, plate-like and spherical were also successfully
prepared with the simple methods [41–43].
solution. After another 2 h of stirring, the final transparent
solution was transferred into a 30 mL Teflon-sealed auto-
clave with about 85% of the total volume and maintained
at 180 °C for 24 h. After being cooled to room temperature,
milk-white precipitates were collected and washed with
deionized water and ethanol several times and finally dried
in a vacuum at 60 °C for 5 h.
Considering a series merit of using Bi
lyst, in this article, the Bi CO was first employed as photocata-
lyst to promote the selective oxidative coupling of amines with
molecular oxygen. Especially, Bi CO catalysts of different mor-
2 2 3
O CO as the photocata-
2
O
2
3
2
O
2
3
phologies including the flower-like, sponge-like, plate-like and
spherical ones were studied in detail on the oxidation of benzy-
2 2 3
c) The synthesis of plate-like Bi O CO is near to that of
lamine. As a result, it is found that the flower-like Bi
2
O
2
CO
3
showed
sponge-like one,which was obtained by replacing tri-
sodium citrate with 1 mmol ammonium carbonate, with
the other conditions unchanged.
the highest photocatalytic activity with a 100% conversion of ben-
zylamine and 99.0% selectivity to N-benzylidenebenzylamine.
Moreover, the oxidative coupling processes of different aromatic
and aliphatic amines have also been successfully performed at
room temperature. Furthermore, the recycling experiment exhib-
d) The preparation method of spherical Bi
with the reference 43. In a typical procedure, 4.5 mmol of
Bi(NO O was dissolved in 40 mL of dilute HNO solu-
Á5H
2 2 3
O CO is similar
3
)
3
2
3
ited that the Bi
2
O
2
CO
3
catalyst still keeps a high active after being
tion (1 mol/L) and stirred for 10 min. Then, 3 mmol of citric
acid was added to the solution to ensure complete dissolu-
tion. Next, a certain amount of NaOH aqueous solution was
added dropwise into the solution, setting the PH value of
the solution to 4–5. Later, the solution was transferred into
a 100 mL Teflon lined stainless steel autoclave, and milky-
white precipitates were obtained by a hydrothermal reac-
tion at 180 °C for 24 h. Finally, the precipitate was cen-
trifuged and washed with deionized water and ethanol
several times before being dried at 75 °C for 8 h.
reused for 5 times.
2
. Experimental
2.1. Materials and instruments
Bismuth
Na WO
Á2H
molybdate (K
sodium carbonate (Na
nitrate
O), ammonium metavanadate (NH
MoO
(Bi(NO
3
)
3
Á5H
2
O),
sodium
tungstate
), potassium
(
2
4
2
4
VO
3
2
4
), Sodium dihydrogen phosphate (NaH
2 4
PO ),
2
CO ) and potassium chloride (KCl) are pur-
3
2
.2.2. The preparation of Bi
In general, the synthesis of Bi
previous work [44]. For the preparation of Bi
Bi(NO O and 4 mmol of KI was dissolved in 20 mL distilled
Á5H
5 7
O X
chased from Aladdin Chemistry Co., Ltd. Cetyltrimethylammonium
bromide (CTAB), sodium hydroxide (NaOH), sodium citrate, citric
acid, potassium bromide (KBr), potassium iodide (KI) and ammo-
nium carbonate are commercially available from Tianjin Guangfu
Fine Chemical Research Institute. All the solvents and chemicals
are of analytical grade and used received without further
purification.
5
O
7
X is similar with the method in
I, firstly, 4 mmol of
5 7
O
3
)
3
2
water to form a transparent solution under stirring. Then, the
mixed solution was adjusted to PH = 13 with an appropriate con-
centration of NaOH solution. After being stirred for 10 min, the
aforementioned solution was then transferred into a Teflon lined
stainless steel autoclave, sealed and maintained at 160 °C for
The quantitative analyses of the products are performed on a GC
apparatus with FID detector. The capillary column is HP-5,
1
6 h. Subsequently, the as-prepared product was centrifuged,
washed with distilled water and ethanol several times and finally
dried at 60 °C in air overnight. The syntheses of Bi Cl and Bi
Br were similar with that of Bi I except for replacing KI with KCl
or 4 mmol of KBr, respectively.
Moreover, the preparation of Bi
MoO catalysts are provided in the supporting information.
3
0 m  0.25 mm  1.0
lm. The qualitative analysis for the product
is carried out on the Agilent 6890/5973 Gas Chromatography-Mass
Spectrometer (GC–MS) instrument. The light source is a CEL-
HXF300 Xe lamp purchased from Beijing China Education Au-
light Co., Ltd with different cutoff filters.
5
O
7
5 7
O -
5 7
O
2 3 4 4 2 6
O , BiVO , BiPO , Bi WO and
Bi
2
6
2
2
.2. Catalyst preparation
2
.3. The characterization of catalyst
2 2 3
.2.1. The preparation of Bi O CO
À1
IR spectra were recorded in the range of 4000–400 cm on a
Perkin-Elmer spectrometer with KBr pellets. X-ray powder diffrac-
tion (XRD) intensities of the different samples were measured on a
a) The preparation method of flower-like Bi
similar with the reference 41. In a typical process, 4.85 g Bi
NO O (10 mmol) was dissolved in 10 mL HNO
Á5H
2
O
2
CO
3
is
(
3
)
3
2
3
Rigaku D/max-IIIA diffractometer (Cu Ka, k = 1.54056 Å) in the

P. Bai et al. / Journal of Catalysis 374 (2019) 257–265
259
range from 5° to 80° (2h). TG experiments were performed in flow-
ing N on a NETZSCH TG 209 instrument with a heating rate of
0 °C/min. The morphology of catalytic materials was determined
by the scanning electron microscope (SEM: JSM-6301F, JEOL)
equipped with a JED-2300 (JEOL) EDXS spectrometer for chemical
element analysis. X-ray photoelectron spectra (XPS) were recorded
on a KRATOS AXIS 165 with a dual X-ray anode (Mg and Al) and all
(JCPDS card No. 41-1488) and other morphology of Bi
ples can be assigned to orthorhombic Bi CO
1752). The sharp and narrow diffraction peaks indicated that the
Bi CO samples are of high crystallinity, and there is no any sight
2
O
2
CO
3
sam-
2
2
O
2
3
(JCPDS card No. 84-
1
2
O
2
3
of impurity phases.
Moreover, it is found that the specific surface area and pore vol-
2
3
2 2 3
ume of flower-like Bi O CO are 20.2 m /g and 0.104 cm /g,
XPS spectra were recorded using the Mg K
areas, pore volumes, and average pore diameters of the prepared
samples were obtained from N adsorption measurement using a
Micro-meritics ASAP2020M system.
a
line. The BET surface
respectively (Shown in Table 1), which are much larger than most
of other bismuth oxides (Table S1). For different morphology of
2
Bi
slightly lower than the flower-like, the pore volume is smaller than
that of the flower shape. In case of the plate-like Bi CO material,
both the specific surface area and pore volume are much smaller
than the flower-like Bi CO material, however, its band gap is
much larger than that of the flower-like one. As for the spherical
Bi CO material, all the surface area, pore volume and band
gap are much larger than the flower-like Bi CO material. Fur-
thermore, thermogravimetric analysis of the flower-like Bi CO
showed that it can keep stable between room temperature to
00° (Fig. S2). On the other hand, there was no significant change
in the structure as well as in phase after 4 photocatalytic runs
Fig. S3), indicating that the flower-like Bi CO showed good sta-
bility during the catalytic process.
In the following, the surface composition of the flower-like
CO sample was further investigated by XPS technique.
2 2 3
O CO materials, although band gap of the sponge-like is
2
O
2
3
2.4. Photocatalytic oxidative coupling process of amines
2
O
2
3
All photocatalytic experiments are performed in a 120 mL auto-
clave equipped with a glass window and magnetic stirring. A typ-
ical procedure for the oxidative coupling of benzylamine is as
2
O
2
3
2
O
2
3
2
O
2
3
follows: 0.1 g of benzylamine, 0.025 g of flower-like Bi
2 2 3
O CO cat-
alyst and 10 mL of CH CN solvent were charged into the reactor,
3
3
and the atmosphere inside was replaced with oxygen for three
times after the reactor was sealed. Then, pure oxygen was charged
to 0.3 MPa at room temperature. Subsequently, the mixed solution
was irradiated through the window of autoclave with a Xe lamp,
and then was kept for 6 h under stirring. After reaction, the excess
gas was purged. The mixture was transferred into a 100 mL volu-
metric flask, in which the reactor was washed with ethanol for
(
2
O
2
3
Bi
2
O
2
3
Fig. 2 presents the detection results of the high-resolution XPS
spectra. As shown in Fig. 2a, the relative peaks of the Bi, C and O
elements have been successfully detected. Therein, the C1s peak
at a binding energy of 284.6 eV can be attributed to contaminant
carbon, and the peak observed at 288.5 eV should be ascribed to
3
–5 times in order to transfer completely. The obtained products
were analyzed with internal standard technique by GC with a
flame ionization detector (all products were determined on GC–
MS with an Agilent 6890N GC/5973 MS detector).
2 2 3
the carbon of carbonate in Bi O CO material (Shown in Fig. 2b).
Moreover, the peaks located at 158.7 and 164.0 eV are assigned
to Bi 4f7/2 and Bi 4f5/2 of the Bi component, respectively (Shown
3
. Results and discussion
3+
2 2 3
in Fig. 2c). In other words, the bismuth of Bi O CO exists as the
3
.1. Characterization
3+
Bi form. Also, the IR spectrum of Bi
characteristic peaks at 1400 cm and 800 cm , respectively, due
2
O
2
CO
3
exhibites two obvious
À1
À1
The phase structure of different bismuth oxides were investi-
to the stretching vibration of Bi-O bond and CAO bond (Fig. S4). For
the UV–Vis spectra of different catalysts, the Bi O CO material has
2 2 3
two significant absorption peaks which are at 210 nm and 300 nm,
respectively (shown in Fig. S5).
gated by means of XRD and the patterns are shown in Fig. 1. It
can be seen that the diffraction peaks of the Bi CO samples
are sharper, indicating that Bi CO has better crystal form than
those of other bismuth oxides. In addition, for the XRD pattern of
Bi CO with different morphologies, the diffraction peaks of
the flower-like Bi CO matched those of tetragonal Bi CO
3
2
O
2
3
2
O
2
3
The SEM images of the as-prepared Bi
played in Fig. 3. From the image of magnification, it can be seen
that the Bi CO materials with different preparation methods
have the flower-like (a, b), sponge-like (d), plate-like (e) and spher-
ical (f) structure. In the flower-like Bi CO material, with the
2 2 3
O CO samples are dis-
2
O
2
3
2
O
2
3
2
O
2
2
O
2
3
2
O
2
3
intercrossing of nanosheets, there is the formation of open pores
that allow reactant access to reach the inner surface of the catalyst.
What is more, the open pores could promote multiple scattering of
the UV–vis light, resulting in greater light-harvesting capacity. It is
suggested that the excellent photocatalytic activity of the flower-
like Bi O CO material is possibly due to the flower-like morphol-
2 2 3
ogy, thin nanopetals, low band gap, and large specific surface area.
During the cycling experiment, the morphology of the flower-like
2 2 3
Bi O CO catalyst was basically unchanged (Fig. 3(c)), which
proved that the prepared catalyst could keep stable and recyclable
in the reaction. Furthermore, the results of EDX analysis of the
Table 1
2 2 3
Results of BET surface areas and band gaps of Bi O CO sample.
Bi
2
O
2
CO
3
samples
Surface area
Pore volume
(cm /g)
Band gap (eV)
2
3
(
m /g)
Flower-like
Sponge-like
Plate-like
20.2
29.6
3.8
0.104
0.093
0.013
0.150
2.90 [41]
2.87 [42]
3.34 [42]
3.25 [43]
Spherical
37.1
Fig. 1. XRD patterns of different bismuth-based catalysts.

2
60
P. Bai et al. / Journal of Catalysis 374 (2019) 257–265
2 2 3
Fig. 2. XPS spectra of Bi O CO samples: (a) survey scan, (b) C1s core and (c) Bi4f core.
flower-like Bi
catalyst surface, which confirms that the material is of high purity
Fig. 3 (g)).
2
O
2
CO
3
indicated that only the Bi, O and C exist on the
respectively. This proves that the morphology of Bi
nificant factor to affect the catalytic reactivity. Therefore, the
flower-like Bi CO exhibited the best photocatalytic activity in
oxidative coupling of amine to imine.
2 2 3
O CO is a sig-
(
2
O
2
3
3
3
.2. Photocatalytic oxidation of amines
3
.2.2. Effects of different reaction conditions
The effects of different solvents were investigated in the oxida-
.2.1. The oxidation of benzylamine with different catalysts
The photocatalytic oxidation of benzylamine (1) was carried out
2 2 3
tive coupling process of 1 with flower-like Bi O CO as the catalyst,
at room temperature with molecular oxygen as the oxidant. The
reaction equation is given in Scheme 1. The generated reaction
products include N-benzylidenebenzylamine (2), benzaldehyde
and the corresponding results are presented in Table 3. It can be
seen that a 94.3% conversion of 1 was obtained after 6 h in DMF
solvent, while the selectivity of 2 was 92.0% (entry 2). With DMSO
as the solvent, the conversion of 1 and the selectivity of 2 was
respectively 78.2% and 94.0% under the similar conditions (entry
3). In THF, the conversion of 1 was 96.9% whereas the selectivity
of 2 was 62.9% (entry 4), limiting its usage due to the noticeable
amount of by-products. Moreover, using benzotrifluoride and
dichloromethane (DCM) as solvents, both the conversion and
selectivity were lower than those in acetonitrile, where the conver-
sion of 1 was 57.1% and 72.4%, respectively (entries 5 and 6). It was
therefore concluded that acetonitrile is the most suitable solvent
for the catalytic oxidation reaction. Moreover, when the wave-
length was controlled at more than 400 nm with the filter, the con-
version of 1 was decreased to 60% in acetonitrile for 6 h (entry 7);
however, a 100% conversion of 1 in a 99% selectivity of 2 was
obtained when the time was further prolonged to 21 h at room
temperature (entry 8). It shows that the oxidative coupling process
should be driven by the visible light.
(3) and N-benzylbenzamide (4). In the absence of catalyst, only
1
8.6% conversion of benzylamine was obtained (Table 2, entry 1),
indicating that benzylamine (1) is not easily reacted under the
light irradiation. In particular, no reaction occurs when the oxida-
tion is performed in dark (Table 2, entry 2) which demonstrates
that light irradiation is very important for the oxidation of 1.
Firstly, a series of bismuth-based catalysts were employed to pro-
mote the oxidation of 1. Therein, the conversion was respectively
6
(
5.0%, 62.7% or 37.4% with Bi
Table 2, entries 3–5), proving that the catalytic ability of Bi
for the oxidation of 1 was decreased with the decreasing of elec-
tronegativity of the halide atom. Then, the flower-like Bi CO
3
5 7 5 7 5 7
O Cl, Bi O Br or Bi O I as the catalyst
5 7
O X
2
O
2
was used as the catalyst for the oxidation of 1 (Table 2, entry 6).
To our surprise, a 95.3% conversion of 1 and more than 99% selec-
tivity of 2 are obtained after 6 h. Moreover, when the BiVO
4 2
, Bi -
WO , Bi MoO , BI and BiPO were employed, less than 44.0%
6
2
6
2
O
3
4
conversion of 1 was attained though the selectivity of 2 can reach
2
In addition, the effects of O pressure and reaction time were
more than 99% (Table 2, entries 7–11). These results showed that
also investigated and the obtained data are provided in the
Table S2 and Fig. S6 of supporting Information.
the Bi
aerobic oxidation of 1. Furthermore, the catalytic performances
of various morphology Bi CO materials are also studied, and a
0.4%, 65.8% and 53.2% conversion of 1 were acquired in the pres-
ence of sponge-like, plate-like and spherical Bi CO materials,
2 2 3
O CO is the most efficient to accelerate the photocatalytic
2
O
2
3
3.2.3. The recycling experiment of catalyst during the oxidation
7
2 2 3
After the oxidative coupling reaction, Bi O CO catalyst was fil-
2
O
2
3
tered, and washed with deionized water and ethanol and then was

P. Bai et al. / Journal of Catalysis 374 (2019) 257–265
261
Fig. 3. SEM images of Bi
2 2 3
O CO samples: (a–b) flower-like sample, (c) recycled flower-like sample, (d) sponge-like sample, (e) plate-like sample, (f) spherical sample and (g)
EDX spectrum of flower-like sample.
2
Scheme 1. The equation for the selective oxidation of benzylamine by O .

2
62
P. Bai et al. / Journal of Catalysis 374 (2019) 257–265
Table 2
The oxidation of benzylamine using the Bi-containing photocatalyst.a
employed for next cycle. As shown in Fig. 4, both the conversion of
and selectivity of 2 still kept more than 95.0% even after being
reused for five times. It indicates that the Bi CO was recyclable
1
Entry
Catalyst
Conversion (%)b
Selectivity (%)b
2
O
2
3
and stable during the photocatalytic oxidation process.
2
3 + 4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
.
no
18.6
<0.5
65.0
62.7
37.4
95.3
31.2
43.5
33.4
7.9
>99
0
<1
0
c
Bi
Bi
Bi
Bi
Bi
2
5
5
5
2
O
O
O
O
O
2
7
7
7
2
CO
Cl
Br
I
3
3
(flower)
(flower)
3.2.4. The investigation on reaction mechanism
Upon visible light irradiation, electron-hole pairs are generated
2 2 3
in Bi O CO . In order to clarify the role of electron and holes in the
oxidation of 1, the silver nitrate or ammonium oxalate solution
was added during the oxidation to suppress electron effect and
hole effect, respectively. The experimental results exhibited that
the conversion of 1 was drastically reduced to 31.1% when
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
98.7
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
1.3
CO
BiVO
4
Bi
Bi
Bi
2
2
2
WO
MoO
O
3
6
6
0
1
2
3
4
1
mmol/L silver nitrate solution was added into reaction system
BiPO
4
17.3
70.4
65.8
53.2
(Table 4, entry 1), which showed that electron plays a key role dur-
Bi
Bi
Bi
2
2
2
O
O
O
2
CO
CO
CO
3
3
3
(sponge)
(plate)
(spherical)
2
2
ing the oxidation. When 1 mmol/L ammonium oxalate solution
was added, the conversion of 1 was also greatly reduced, which
was reduced to 43.0% (entry 2). This provides evidence that holes
are also involved during the oxidation. In the following, tert-
butanol was used to capture hydroxyl radical. The result showed
that the conversion of 1 was reduced to 46.7% (entry 3), indicating
that hydroxyl radical species also promote the oxidative coupling
reaction.
a
Reaction condition: 0.1 g benzylamine, 25 mg catalyst, in 10 mL acetonitrile,
under 0.3 MPa of oxygen, Xe lamp with a light intensity of 160 mW/cm , 6 h, 25 °C.
2
b
The data are obtained by GC with internal standard technique.
Reaction was performed in dark.
c
In general, there are two ways to activate oxygen in which one
Table 3
Effects of different reaction conditions on oxidative coupling of benzylamine.a
2 3
CO photocatalyst as sensitizer to create energy
is that the BiO
transfer to O to form O
excited from the HOMO of the BiO
1
2
species, and another is that electron is
Entry
Solvent
Conversion(%)b
Product distribution
2
b
2
CO
3
and jump to the corre-
(
%)
Å À
sponding LUMO, and further transferred to O
2
to produce O
2
2
3 + 4 + Others
[
45]. In order to further confirm the activation of oxygen, TEMPO
1
2
3
4
5
6
CH
DMF
DMSO
THF
Benzotrifluoride
DCM
CH
CH
3
CN
95.3
94.3
78.2
96.9
57.1
72.4
60.0
100
>99
92
<1
8
Å À
1
and b-carotene were added as scavengers for O
2
and O
2
during
catalytic oxidation, respectively. As shown in entry 4 of Table 4,
94.0
62.9
95.2
>99
>99
>99
6.0
37.1
4.8
<1
<1
<1
when 0.1 mmol of TEMPO was added, the conversion of 1 was
Å À
slightly reduced to 61.0%, indicating that the O
2
species is helpful
for oxidation of 1. Moreover, both the conversion of 1 and the
selectivity of 2 were reduced to 49.4% and 35.7%, respectively
when 0.1 mmol of b-carotene was added into reaction system
c
7
8
3
CN
CN
d
3
a
Reaction condition: 0.1 g benzylamine, 25 mg catalyst, in 10 mL acetonitrile,
(
entry 5). The decrease of product selectivity was considered as
2
under 0.3 MPa of oxygen, Xe lamp with a light intensity of 160 mW/cm , 6 h, 25 °C.
the formation of benzoic acid and compound 4 as by-products dur-
b
The data are obtained by GC with internal standard technique.
The wavelength is controlled at more than 400 nm with the filter and light
1
c
2
ing the oxidation. This provides evidence that O also has a crucial
2
effect on oxidative coupling of 1. Finally, when both 0.1 mmol of
TEMPO and b-carotene were added to reaction mixture, it is found
that the conversion of 1 was reduced to 23.7%, and the selectivity
of 2 was 92.4% (Table 4, entry 6). These data indicated that there
intensity of Xe lamp is 260 mW/cm .
d
The condition is same with that of entry 7 except that the reaction time is 21 h.
are two forms of O
2
activation mode during the reaction, and the
1
activation mode of producing O
2
is dominant. When the formation
2
O is suppressed, the compound 1 is excessively oxidized to
1
of
Table 4
The control experimental result for studying reaction mechanism with BiO
catalyst.
2
CO
3
a
Entry
Additive
Conversion (%)b
Product distribution
b
(
%)
2
3 + 4 + Others
1
2
3
4
5
6
Silver nitrate
Ammonium oxalate
t-butanol
TEMPO
b-carotene
TEMPO + b-carotene
Benzaldehyde
b-phenylethylamine
31.1
43.0
46.7
61.0
49.4
23.7
95.0
32.8
97.5
97.6
>99
2.5
2.4
<1
>99
<1
35.7
92.4
98.9
49.5
64.3
7.6
1.1
50.5
c
7
d
8
a
Reaction condition: 0.1 g of 1, 25 mg catalyst, in 10 mL acetonitrile, under
2
0
.3 MPa of oxygen, Xe lamp with a light intensity of 160 mW/cm , 6 h, 25 °C.
b
The data are obtained by GC with internal standard technique.
The compound 1 was reacted with benzaldehyde in the dark for 10 h.
0.1 g of b-phenylethylamine was added to verify the intermediate
c
Fig. 4. The results for the recycling of flower-like Bi
2
O
2
CO
3
catalyst in the oxidation
d
[
Reaction conditions: 0.1 g benzylamine, 25 mg catalyst, in 10 mL acetonitrile,
2
under 0.4 MPa of dioxygen, Xe lamp with a light intensity of 160 mW/cm , for 6 h
1–2 run) or 8 h (3–5 run), 25 °C].
benzaldehyde; therein, generated by-products include 3,
4
and N-
(
benzylidenephenethylamine.

P. Bai et al. / Journal of Catalysis 374 (2019) 257–265
263
Å À
benzoic acid with O
2
species, resulting in the decrease of
in the dark, the conversion of 1 and selectivity of 2 were 95.0%
and 98.9%, respectively (entry 7). Moreover, when same amount
of 1 and b-phenylethylamine were added into the reaction system
(entry 8), both the compound 2 and N-benzylidenephenethylamine
were generated. These results further showed that benzaldehyde
was an intermediate during oxidative coupling of 1, and the role
of visible light is to promote the oxidation of 1 to benzaldehyde
during the reaction.
compound 2 during oxidation. In order to verify the reaction mech-
anism of oxidative coupling of 1, several control experiment were
carried out. When the compound 1 and benzaldehyde were reacted
Based on above investigations and catalytic principle, the
possible reaction mechanism for oxidation of 1 with flower-like
Bi
Bi
2
O
O
2
CO
CO
3
as photocatalyst is proposed. As seen in Fig. 5, the
catalyst firstly produced electrons and holes under visi-
2
2
3
ble light irradiation, and then the photogenerated electron trans-
Å À
1
mitted to O
2
to form O
2
2
and O species. Meanwhile, the a-H
bond of compound 1 was attacked to produce reactive intermedi-
Å À
ate Ⅰ. The generated intermediate Ⅰ reacted with O
2
species to
form intermediate Ⅱ during reaction. Next, the intermediate Ⅱ
removed NH OH to generate benzaldehyde with the help of
2
proton. Finally, another molecule 1 was dehydrated with in situ
generated benzaldehyde to obtain product 2 (route 1). In addition,
there exists a second pathway, in which the compound 1 was
firstly activated to form intermediate III, and then reacted with
1
2
O to produce intermediate IV and hydroxyl radical. In the follow-
ing, the intermediate IV is further oxidized to generate benzalde-
hyde, and dehydrated with another molecular 1 to attain product
2
(route 2).
3.2.5. The oxidative coupling processes of different amines
Fig. 5. Proposed mechanism for the oxidative coupling of 1 in the presence of
Bi
2
O
2
CO
3
catalyst.
Selective oxidative coupling reactions of different amines were
further investigated with the flower-like Bi
2
O
2
CO
3
as the catalyst,
Table 5
The selective oxidation of different aromatic amines with Bi
as catalyst.a
Product
2
O
2
CO
3
Entry
1
Amine
Conversion (%)b
95.3
Selectivity (%)b
>99
2
3
89.0
97.5
>99
>99
4
5
6
7
87.4
97.6
97.8
97.4
>99
>99
>99
>99
8
9
100
>99
94.9
94.2
82.7
>99
>99
>99
1
1
0
1
1
1
2
3
100
69.4
>99
31.0
1
1
4
5
100
87.9
>99
51.4
a
2 2 3 2
Reaction condition: 0.1 g substrate, 25 mg catalyst (flower-like Bi O CO ), in 10 mL acetonitrile solvent, under 0.3 MPa of O , Xe lamp with a light intensity of
2
1
60 mW/cm , for 6 h, at 25 °C.
b
The data are obtained by GC with internal standard technique.

2
64
P. Bai et al. / Journal of Catalysis 374 (2019) 257–265
and the results are presented in Table 5. It was found that all the o-,
m- and p- fluorine substituted benzyl amines can be efficiently
transformed to the corresponding oxidative coupling products, in
which the selectivities of target products kept more than 99%
and the conversions of o-, m- and p- fluorine substituted benzyl
amines were 89.0%, 97.5% and 87.4%, respectively. Moreover, for
the oxidative coupling reaction of p-chloro benzylamine and p-
bromobenzyl amine, the substrate conversions were respectively
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