Interfacial engineering by creating Cu-based ternary heterostructures on C3N4tubes towards enhanced photocatalytic oxidative coupling of benzylamines

Article abstract of DOI:10.1039/d0ra03164j

Benzylamine coupling is a very important reaction for the synthesis of imine but still faces many challenges. Herein, we present a highly effective strategy towards the coupling reaction by using environmentally friendly catalysts. These catalysts are com

Full text of DOI:10.1039/d0ra03164j

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Interfacial engineering by creating Cu-based
ternary heterostructures on C N tubes towards
3
4
Cite this: RSC Adv., 2020, 10, 28059
enhanced photocatalytic oxidative coupling of
benzylamines†
a
a
a
b
c
d
Yunqi Fu, Mang Zheng, Qi Li, Liping Zhang, * Shuai Wang,* V. V. Kondratiev
a
and Baojiang Jiang
*
Benzylamine coupling is a very important reaction for the synthesis of imine but still faces many challenges.
Herein, we present a highly effective strategy towards the coupling reaction by using environmentally
friendly catalysts. These catalysts are composed of Cu/Cu
tubes and the composites were synthesized by one-step hydrothermal treatment followed by
calcination. Cu O, Cu N, and C all are responsive to visible light and the heterojunction formed can
greatly enhance the charge separation. When used as photocatalysts for oxidative self-coupling of
benzylamine at a low temperature of 323 K in air, Cu/Cu O/Cu N/C was able to give conversion and
2 3 3 4
O/Cu N heterostructures supported by C N
2
3
3 4
N
Received 8th April 2020
Accepted 18th July 2020
2
3
3 4
N
DOI: 10.1039/d0ra03164j
selectivity values of up to 99% and 98%, respectively. The high efficiency of the catalysts is attributable to
2ꢀ
rsc.li/rsc-advances
their ability to generate large quantities of free radicals (such as $OH and $O ) under visible-light irradiation.
have been explored. However, these catalytic reactions involve
complex systems (oxidants and alkali auxiliaries), long reaction
1
. Introduction
8
Imines are important intermediates in the synthesis of many time, and are carried out at high temperatures and pressures.
1,2
9
2
organic compounds. The traditional synthetic method for Juntrapirom et al. used O as an oxidant to convert benzyl-
imines is the condensation of amines with carbonyl amine with a conversion rate and selectivity of up to 94% and
compounds, which generally needs costly dehydrating agents, 85%, respectively, at room temperature and under visible light
3,4
and Lewis acids as the catalysts. However, this method is not irradiation for 4 hours. This may be due to the fact that ben-
only cumbersome to operate, but also causes some side reac- zylamine reacts with pure oxygen to form a series of by-
ꢁ
11
tions, and even causes serious pollution to the environment. products. At a low reaction temperature (35 C), Chai et al.
Alternatively, the direct synthesis of imines through an oxida- did not greatly improve the rate of benzylamine conversion
tive process such as self-coupling of benzylamine has attracted carried out in air for 5 hours, although the selectivity was high
2
much attention, owing to reduced energy consumption, and (>99%). In the absence of O and at a low reaction temperature
5,6
ꢁ
10
simplied procedures. Furthermore, the utilization of solar (35 C), Zhang et al. achieved a conversion rate of 80% aer
light as the energy source for directly oxidizing benzylamine is conducting the reaction for a long time (200 hours). Therefore,
a green and promising strategy.
it is still a challenge to increase the conversion of benzylamine
For the oxidative coupling of benzylamine, CuCl was rstly under mild reaction conditions. It is well known that the
used as the catalyst in the presence of air, and good activity was pyrolysis of benzylamine releases toxic gases. Currently the
7
achieved. Subsequently, many other catalysts for this reaction most commonly used catalysts are expensive noble metals such
12
13
14
as Ru, Pd, and Au. Thus, researchers have turned their
attention to cheaper catalysts containing non-noble metals such
a
15
16
17
18
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of as g-C
N
,
Cu
excellent photocatalytic performance.
Among them, g-C N is an attractive organic semiconductor
O, Fe
O
,
and TiO
,
which also show
3
4
2
2
3
2
the People's Republic of China, School of Chemistry and Materials Science,
Heilongjiang University, Harbin 150080, China. E-mail: jiangbaojiang88@sina.com;
jbj@hlju.edu.cn
3
4
photocatalyst owing to its outstanding visible light activity,
thermal and chemical stability. Interestingly, it can also be
applied for the oxidative coupling of benzylamine under visible
b
Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242,
USA. E-mail: Lzhang30@kent.edu
c
Department of Food and Environment Engineering, Heilongjiang East University,
19–21
Harbin, China. E-mail: 79652283@qq.com
d
light.
Therefore, in this work, we present a Cu/Cu
2 3
O/Cu N
Institute of Chemistry, Saint Petersburg State University, Russia
ternary composite formed on C N
tubes for catalyzing the
3 4
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra03164j
oxidative coupling reaction of benzylamine under visible-light
1
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light using air as the oxidant. The conversion and selectivity of Elmer-Lambda 950 spectrophotometer using BaSO
4
as the
the reaction were as large as 99% and 98%, respectively. In referencing material. The photoluminescence (PL) spectra of
particular, Cu N, which has shown prominent activity in the the samples were measured with a spectrouoro-photometer
3
electrocatalytic hydrogen evolution and oxygen evolution reac- (Hitachi FLS1000) at an excitation wavelength of 375 nm. The
22,23
tions,
was explored for the rst time as a co-photocatalyst for Scanning Kelvin Probe (SKP) measurements (SKP5050 system,
the coupling reaction.
Scotland) were implemented under normal conditions (i.e.,
room temperature and ambient pressure). Electrochemical
impedance spectroscopy (EIS) measurements were performed
under simulated sunlight irradiation at open circuit voltage
over a frequency range from 105 to 0.05 Hz with an AC voltage of
2. Experimental
2.1 Materials
Copper acetate hydrate (Cu(Ac) $H O; analytically pure reagent, 5 mV. Mott–Schottky analysis was performed by using an elec-
2
2
9
9
9.98%), sodium hydroxide (NaOH; AR, 96%), melamine (AR, trochemical Princeton instrument at different frequencies
9%) and dimethyl sulfoxide (DMSO; chromatographically versus the saturated Ag/AgCl electrode (pH ¼ 7). Benzylamine
pure, $99.8%) were purchased from Aladdin Reagent. Benzyl- and imine products were detected using GC-MS (Thermo
amine (chromatographically pure, $99.0%) was bought from Trace1300).
Sigma Aldrich. Deionized water was used during the experi-
ments. The Cu N and Cu O used for testing and characteriza-
tion in the experiment are purchased commercial products.
3
2
2.5 Photocatalytic oxidative coupling of benzylamine
To begin with, 40 mg of a prepared product was placed in
a Schlenk ask. Subsequently, 3 mL of chromatographically
pure dimethyl sulfoxide (DMSO) was added as the solvent, fol-
lowed by 3 mmol of chromatographically pure benzylamine.
Next, the ask was placed in a sonicator for 5 min until
a homogeneous suspension was obtained. The suspension was
then heated to 323 K using a heating mantle and irradiated with
a 300 W Xe lamp for 12 h. Aer the reaction, 1 mL of the
suspension was taken and passed through a 0.22 mm organic
2
C N
.2 Fabrication of Cu
2
O on a supramolecular precursor of
3
4
First, 5 mL of 0.8 M NaOH solution was slowly added into 30 mL
of 0.1 M Cu(Ac)
ꢁ
2
solution under stirring for 20 min at 25 C.
Next, various amounts of melamine [nCu(Ac)2 : nmelamine ¼ 1 : 1,
1
: 2, 1 : 3 and 1 : 4 (molar ratio)] were each added into the
above solution, and the resultant suspensions were stirred in
a water bath at 60 C for 30 min. Subsequently, the mixture
solutions were transferred into 50 mL polytetrauoroethylene
autoclaves and then kept at 180 C for 10 h. Finally, black-brown
ꢁ

lter to remove the catalyst. The resulting ltrate was extracted
with ethyl acetate and then analyzed by GC-MS.
ꢁ
precipitates were obtained by centrifugation, washed three
ꢁ
times with hot water, and then dried at 60 C for 6 h.
3. Results and discussion
3
.1 Crystal structure and morphology
2
2 3 3 4
.3 Fabrication of Cu/Cu O/Cu N on hexagonal C N tubes
As is evident from the SEM image (Fig. 1a), the assembly of
melamine in the presence of NaOH resulted in a supramolec-
ular precursor of C N with a hexagonal rod-like structure, and
3 4
the rods are evenly decorated with a large number of CuO
particles generated from the reaction between copper acetate
and the base. Interestingly, aer calcination these solid rods
became smaller and hollow, producing a tubular structure
The black-brown precipitates were each transferred to a porce-
lain boat and then were calcined at 400 C for 2 h in an argon
atmosphere that prevents the oxidation of Cu O (the ow rate
2
was set at 5 mL min ). As a result, dark-green samples were
obtained. These samples were named as Cu/Cu O/Cu N/C N -1,
Cu/Cu O/Cu N/C N -2, Cu/Cu O/Cu N/C N -3, and Cu/Cu O/
Cu N/C N -4, corresponding to the molar ratio of Cu(Ac) to
melamine of 1 : 1, 1 : 2, 1 : 3 and 1 : 4, respectively.
ꢁ
ꢀ
1
2
3
3 4
2
3
3
4
2
3
3
4
2
3
3
4
2
(Fig. 1b and c). Moreover, the particles on the surface of each
tube aggregated. In the high-resolution TEM (HRTEM) image of
the calcined product, two lattice fringes can be clearly seen
2.4 Characterization
2
(Fig. 1d), which correspond to the (111) crystal plane of Cu O
Morphologies of the samples were characterized through (d111 ¼ 0.24 nm) and the (111) crystal plane of Cu N (d ¼ 0.22
3
111
scanning electron microscopy (SEM) with Hitachi S-4800 oper- nm), respectively. In other words, copper(II) oxide was reduced
ating at 15 KV and transmission electron microscopy (TEM) to highly crystalline copper(I)-containing species during the
with JEMF200 an operating at an acceleration voltage of 200 kV. thermal treatment. The presence of C, N, Cu, and O was veried
The X-ray diffraction (XRD) was conducted at room temperature by elemental mapping and these elements are uniformly
on a Bruker D8 ADVANCE diffractometer with nickel-ltered Cu distributed across the tubular structure (Fig. 1e–h).
˚
Ka radiation (l ¼ 1.5406 A). X-ray photoelectron spectroscopy
Phases of the product were further investigated by XRD
(
XPS) was performed on VG ESCALAB MK II with an achromatic experiments. It turned out that samples synthesized from
Mg Ka radiation (1253.6 eV) to analyze the electronic states of different Cu (Ac) /melamine molar ratios have similar patterns
2
ꢁ
ꢁ
ꢁ
the surface. N adsorption–desorption isotherms were recorded (Fig. 2a). The peaks at 43.2 , 50.4 and 74.1 correspond to the
2
on Tristar II 3020 and surface areas were determined by using (111), (200) and (220) crystal planes of Cu (PDF# 04-0836); those
ꢁ
ꢁ
ꢁ
the Brunauer–Emmett–Teller (BET) method. UV-visible diffuse at 36.4 , 42.2 and 61.3 can be indexed to the (111), (200) and
reectance spectra (UV-vis DRS) were obtained on a Perkin- (220) crystal planes of Cu O (PDF# 05-0667); lastly, the peaks at
2
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Fig. 1 High-magnification SEM images of (a) the precursor and (b) Cu/
Cu
Cu
Cu
2
3
3
O/Cu
3
N/C
3
N
4
-3; (c) TEM and (d) HRTEM images of Cu/Cu
2
O/
O/
N/C
N/C
3
N
N
4
-3; (e–h) elemental mappings of C, N, Cu, O for Cu/Cu
2
3
4
-3 in the region shown in (b).
Fig. 2 (a) XRD patterns. (a) Cu/Cu
Cu N/C -2; (c) Cu/Cu O/Cu N/C
4. (b) XPS survey spectrum of Cu/Cu
XPS (c) C 1s, (d) N 1s, (e) Cu 2p, and (f) O 1s spectra of Cu/Cu
-3.
2
O/Cu
-3; (d) Cu/Cu
O/Cu N/C -3; high-resolution
O/Cu N/
3
N/C
3
N
4
-1; (b) Cu/Cu
2
O/
3
N
3 4
2
3
3
N
4
2
O/Cu N/C
3
3 4
N -
ꢁ
ꢁ
ꢁ
2
3
3 4
N
2
3.2 , 40.8 and 47.5 correspond to the (100), (111) and (200)
24–29
2
3
crystal planes of Cu
broad peak at about 27 is visible, which is attributable to the X-
3
N (PDF# 47-1088).
Further, a weak
3 4
C N
ꢁ
ray reections from the (002) crystal planes of C N (corre-
3
4
3
0,31
sponding to interlayer stacking).
acteristic peaks are sharp and strong due to the good
crystallinity of Cu, Cu O, and Cu N, which is in line with the
TEM result. However, the crystallinity of carbon nitride is poor.
Although the diffraction of X-rays by C is not well resolved
due to the presence of Cu O with higher crystallinity (Fig. 2a
and S1†), in the XRD pattern of pure C synthesized without
Moreover, all these char-
stretching vibration of N–H and O–H owing to adsorbed water
molecules.
The chemical state of the synthesized materials and the
elemental composition on the surfaces were studied by X-ray
photoelectron spectroscopy (XPS). The characteristic peaks of
C, N, Cu, and O are clearly visible in the survey spectrum
2
3
3 4
N
2
3 4
N
(Fig. 2b), suggesting that the surface regions of the materials
ꢁ
ꢁ
copper acetate pure, the characteristic peaks at 13 and 27 can
contain the same elements as the bulk parts. The high-
resolution XPS C 1s spectrum exhibits two peaks at 284.6 eV
and 288.1 eV (Fig. 2c), which can be assigned to carbon in C–C
be clearly seen (Fig. S2†), verifying that C N indeed can be
3
4
synthesized by the thermal polymerization of melamine. As the
Cu(Ac) /melamine molar ratio was increased, the content of
increases, the crystallinity of the composite will also
2
2
32,33
and N–C]N (namely sp -hybridized C), respectively.
The
3 4
C N
XPS N 1s spectrum can be deconvoluted into two core lines,
featuring three peaks (Fig. 2d). The peak at 398.5 eV corre-
sponds to N of the triazine ring (C]N–C) or Cu–N, and the peak
decrease, as a result, Cu XRD peaks became weaker, indicating
a decrease in the Cu content. This result was further veried by
inductively coupled plasma (ICP) measurements, which show
at 400.2 eV is due to surface-adsorbed NH
during the thermal polymerization of melamine) and tertiary
nitrogen in the N–(C) or HN(C) groups. The peak at 404.5 eV
Therefore, the XPS results verify the
successful preparation of C
In the XPS Cu 2p spectrum (Fig. 2e), a pair of peaks at
32.7 eV and 953.7 eV can be ascribed to 2p and 2p1/2
3
(which was produced
that the mass percentage was reduced from 66.46% (Cu/Cu O/
2
Cu N/C N -1) to 33.47% (Cu/Cu O/Cu N/C N -4, Table S1†). In
3
3
4
2
3
3 4
ꢀ
1
3
2
the FT-IR spectrum (Fig. S3†), the peak at 808 cm is attrib-
34,35
belongs to C–N–H.
utable to the vibration of the triazine ring, further supporting
3 4
N .
the presence of CN in Cu/Cu
melamine increases, the vibration intensity of C
strengthens. The peaks appearing in the range of 1200–
2
O/Cu
3
N/C
3
N
4
. As the amount of
3 4
N
also
9
,
3
/2
+
36
respectively, of Cu . Another pair of peaks at 934.2 eV and
9
ꢀ
1
1
600 cm are due to the tensile vibration of C–N and C]N. The
0
2+ 37
54.6 eV are attributed to metallic Cu or Cu . According to
ꢀ
1
broad peak in the range of 3100–3300 cm is formed by the
XRD characterization, there are characteristic peaks of Cu and
2
+
no characteristic peaks of Cu are found. Therefore, these two
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peaks are Cu, while the peak of Cu belongs to Cu
Paper
+
2
O in the Photocatalysts with small band gaps are more favorable owing
catalyst. Hence, XPS further conrms the XRD result that during to their enhanced light absorption ability.
the thermal treatment of the supramolecular precursor CuO can Mott–Schottky plots of Cu N and Cu O at three frequencies
be reduced to Cu and Cu O, possibly due to the reducing gases (i.e., 1.0 kHz, 1.5 kHz, and 2.0 kHz) were recorded to determine
3
2
2
such as NH
namely lattice oxygen and oxygen in the water adsorbed on the and C
surface, is evidenced by the peaks at 531.3 eV, and 532.9 eV, NHE), respectively, by extrapolation. In turn, the valence band
3
released. The presence of two types of oxygen, the at-band potentials. The at-band potentials of Cu
3 2
N, Cu O
3 4
N
were assessed to be about 0.04, ꢀ0.76 and ꢀ1.12 (vs.
3
8
respectively, in the XPS O 1s spectrum (Fig. 2f). Quantitative edges of Cu
analysis by using elemental analysis revealed that the C/N ratio band gaps. Based on the potential difference, electrons in Cu
for all the composites (Table S2†) is smaller than the theoretical transfer to Cu O. To further conrm this electron transfer
3 2 3 4
N, Cu O and C N were calculated by using the
3
N
2
39
value for C N (i.e., 0.6428). Particularly, Cu/Cu O/Cu N/C N - direction, scanning Kelvin probe (SKP) was utilized to deter-
3
4
2
3
3 4
3
has the smallest C/N ratio (0.5767). Carbon nitride with mine the work functions, which were calculated according to
a larger percentage of N is believed to have more active sites and the following formula: W ¼ 5.1 + Wave/1000, where W and Wave
therefore give better photocatalytic performance. Hence, Cu/ refer to the work function and work function average, respec-
Cu
2
O/Cu
3
N/C
3
N
4
-3 was selected for the remaining experiments. tively. It turned out that the work function decreases in the
order of Cu N > Cu/Cu O/Cu N/C -3 > Cu O > Cu (Fig. 3e). In
3
2
3
3
N
4
2
other words, the Fermi level relative to the vacuum level
increases in the order Cu N < Cu/Cu O/Cu N/C N -3 < Cu O <
3
.2 Optical properties and charge dynamics
Fig. 3a and S4a† show the diffuse reectance spectra (DRS) of
Cu O, Cu N, C and Cu/Cu O/Cu N/C -3 in the 300–
00 nm range. The absorption spectrum of Cu/Cu O/Cu N/
-3 is similar to that of Cu N but is slightly shied toward
larger absorbance. Cu N, Cu O and Cu/Cu O/Cu N/C -3 show
stronger optical absorption than C N at wavelengths above the
3
2
3
3
4
2
Cu; therefore, in the Cu/Cu O/Cu N/C N -3 heterojunction
2
3
3 4
2
3
3
N
4
2
3
3 4
N
electrons transfer from Cu to Cu O and nally to Cu N. The
2
3
9
C
2
3
band diagram of Cu/Cu
2
O/Cu
3
N/C
3
N
4
-3 is shown in Fig. 3f and
3
N
4
3
2 3 3 4
S5.† Under the irradiation of visible light, Cu O, Cu N, and C N
3
2
2
3
3 4
N
are excited and electron–hole pairs are generated. According to
the potential difference, the electrons on C N are transferred to
3
4
3
4
absorption edge of C N , namely 579.9 nm. The optical band
3
4
2 3
Cu, then to Cu O and nally to Cu N. In the meanwhile, holes
gaps of Cu O, Cu N, C N and Cu/Cu O/Cu N/C N -3 were
2
3
3
4
2
3
3
4
are transferred from Cu N to Cu O and nally transported to
3 2
determined to be 1.89 eV, 1.12 eV, 2.19 eV and 0.82 eV,
respectively, from the Tauc plots displayed in Fig. 3b and S4b.†
C N . Subsequently, electrons react with O and holes react with
water to produce highly reactive $O2 and $OH, respectively,
3
4
2
ꢀ
which may induce the coupling reaction of benzylamine. The
specic surface area (SSA) and pore size of Cu/Cu
3
2
O/Cu
were determined to be 45.6 m g and 24.0 nm, respectively,
from nitrogen adsorption–desorption isotherms (Fig. S5†). SSA
of Cu/Cu O/Cu N/C -3 is larger than those of Cu, Cu O, and
3 3 4
N/C N -
2
ꢀ1
2
3
3
N
4
2
Cu N, indicating that the former has the most reaction sites on
3
its surface, which is benecial to the oxidation of benzylamine.
The performance of a photocatalyst greatly depends on its
charge dynamics, which can be investigated by photo-
luminescence (PL) spectroscopy and electrochemical imped-
40
ance spectroscopy (EIS). The measurement of PL intensity for
different samples at an excitation wavelength of 375 nm is
shown in Fig. 4a. As can be seen, the PL intensity of Cu/Cu
2
O/
Cu N/C N -3 is much lower than that of other samples, illus-
3
3 4
trating that the recombination of the electron–hole pairs in Cu/
Fig. 3 (a) Diffuse reflectance spectra (DRS) and (b) Tauc plots of Cu
Cu N and Cu/Cu O/Cu N/C -3. (c) Mott–Schottky plots of Cu
and (d) Cu
2
O,
2
O
3
2
3
N
3 4
3
N at various frequencies (pH ¼ 7). (e) Scanning Kelvin probe Fig. 4 (a) Photoluminescence spectra of Cu, Cu
2
O, Cu N and Cu/
3
O, Cu N and Cu/
3
2
mappings (SKP) of Cu O, Cu
band diagram.
3 2 3 3
N and Cu/Cu O/Cu N/C N
4
-3. (f) Energy Cu
Cu
2
O/Cu
O/Cu
3
N/C
N/C
3
N
N
4
-3. (b) Nyquist plots of Cu, Cu
-3.
2
2
3
3
4
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Cu O/Cu
RSC Advances
2
3
N/C
3
N
4
-3 is suppressed as compared with other and benzaldehyde. All the above data are derived from Gas
samples. The conclusion is further conrmed by EIS (Fig. 4b). Chromatography-Mass Spectrometer (GC-MS). This paper uses
The Nyquist plot of Cu/Cu O/Cu N/C N -3 displays the smallest external standard method to t the line graph of benzylamine
2
3
3 4
semicircle among all samples studied, which indicates that it concentration and peak area as shown in the Fig. S6.† The
has the smallest interfacial charge transfer resistance.
benzylamine peak area acquired by the GC-MS test was calcu-
lated by a one-time function formula (Fig. S6†) to obtain the
benzylamine concentration, and the benzylamine conversion
rate was calculated. With reasonable test conditions, the
retention time of chromatographic grade benzylamine is
3.3 Photocatalytic tests
Photocatalysis is typically conducted at mild and environmen-
tally friendly conditions, and it involves the generation of
electrons and holes that trigger a series of redox reactions. The
performance of the synthesized materials toward photocatalytic
coupling reaction of benzylamine in the presence of air and at
1.69 min, as shown in the Fig. S7.†
4
1
The photocatalytic activity of Cu/Cu O/Cu N/C N -3 was
2
3
3 4
further studied by using many benzylamine derivatives with
electron withdrawing or donating groups as the substrates. The
2
323 K is shown in Table 1. It can be seen that both Cu O and
electron-withdrawing groups include –F, –Cl, –Br, –OCH
3
and
and
Cu N are able to induce the oxidative coupling reaction of
3
–
–
CF
(CH
3
, while the electron-donating groups include –CH
) C. The conversion rates involving the electron-donating
3 3
3
benzylamine and the conversion was 12% and 80%, respec-
tively. However, Cu/Cu
higher activities. Although Cu
reaction, Cu O and Cu can catalyze the oxidation of benzyl-
amine and thereby promote the reaction. Additionally, it was
observed that for Cu/Cu O/Cu N/C -3 the selectivity and
2
O/Cu
3
3 4
N/C N photocatalysts exhibited
groups are 98% for 4-methylbenzylamine and 78% for 4-tert-
butylbenzylamine, while those involving the electron-
withdrawing groups are 89% (–F), 85% (–Cl), 82% (–Br), 72%
3
N plays a major role in the
2
(
–CF ) and 88% (–OCH ). Generally, electron-donating groups
3 3
2
3
3 4
N
have stronger oxidizing ability than electron-withdrawing
conversion of benzylamine oxidation were 98% and 99%,
respectively. In order to further study the effect of light on the
catalytic reaction, a benzylamine oxidative coupling reaction
was performed under a single reaction condition only, and the
42
groups; however, it is not clearly reected in the present
work. Interestingly, the conversion rate of 4-tert-butylbenzyl-
amine coupling was 20% lower than that of 4-methylbenzyl-
amine coupling, indicating that the tert-butyl group is not
conducive to the photocatalytic benzylamine coupling reaction,
results are included in Table S3.† When Cu/Cu
2
O/Cu
3
N/C
3
N
4
-3
ꢁ
was heated to 50 C (323 K) without light, the conversion and
selectivity were very low, reaching only 33% and 94%, respec-
tively. Aer light was introduced, the conversion and selectivity
were increased to 74% and 98%, respectively. The results of
oxidative coupling of benzylamine in different atmospheres are
listed in Table S4.† It was found that under inert gas, the
conversion rate of benzylamine is very low, only 44%, but the
selectivity reaches 98%. In an atmosphere containing oxygen,
the conversion of benzylamine can be close to 100%, but the
selectivity of benzylamine under pure oxygen is lower than that
under air. This may be due to the reaction of benzylamine with
oxygen that produces some by-products such as benzylalcohol
43
which may be ascribed to the steric effect. Furthermore, both
the conversion and selectivity for the coupling reaction of any
benzylamine with a methyl group were larger than 90%
regardless of the position of the group. From the table, it nds
that the conversion and selectivity of benzylamine with different
groups are at the upper levels, indicating that the catalyst has
a promoting effect on the organic reaction (Table 2).
3.4 Mechanism of photocatalytic benzylamine coupling
The underlying mechanism of the coupling reaction of benzyl-
amines was studied by electron spin resonance (ESR). To detect
ꢀ
$
OH and $O2 , 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) that
captures these radicals was added. Based on the ESR spectra
shown in Fig. 5a and b, the amount of these two radicals
a
Table 1 Coupling reaction of benzylamine with different catalysts
decreases in the sequence of Cu/Cu
Cu O, which is consistent with their oxidation performance
discussed above. $O
and therefore plays a major role in the photocatalytic coupling
2 3 3 4 3
O/Cu N/C N -3 > Cu N > Cu
>
2
ꢀ
2
has a stronger oxidizing ability than $OH,
rd
th
th
Sample
Conversion
Selectivity of benzylamine. Samples for GC-MS were taken at 3 , 6 , 9
th
and 12 hour during the oxidative coupling reaction of ben-
zylamine. The mass spectrum of the 3 -hour sample shows
Cu
Cu
Cu
2
O
12%
77%
80%
93%
97%
99%
60%
7%
99%
89%
85%
96%
92%
98%
98%
62%
rd
peaks at m/z ¼ 103, 105, 106.12, 107.15 and 108.13, corre-
sponding to benzonitrile, intermediate I, benzaldehyde, ben-
zylamine and benzylalcohol, respectively (Fig. S8a†). At the
same time, a peak at m/z ¼ 194.15, which belongs to imine, can
3
N
Cu/Cu
Cu/Cu
Cu/Cu
Cu/Cu
—
2
2
2
2
O/Cu
O/Cu
O/Cu
O/Cu
3
3
3
3
N/C
N/C
N/C
N/C
3
3
3
3
N
N
N
N
4
4
4
4
-1
-2
-3
-4
rd
be observed in the mass spectrum of the 3 -hour sample
(
Fig. S8b†). In the chromatogram (Fig. S8c–f†), peaks with
a
Reaction condition: 3 mmol benzylamine, 12 h, 40 mg Cu/Cu
2
O/Cu
3
N/
retention times of 1.69 and 1.71 are the characteristic peaks of
benzylamine, while peaks with retention times of 5.41 and 5.42
belong to imines. As benzylamine was consumed while the
ꢀ2
C
3
N
4
-3, cut 420 nm with 300 mW cm , Xe lamp, 3 mL dimethyl
sulfoxide (DMSO), 323 K and air atmosphere.
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Table 2 Selective oxidation of different aromatic amines with Cu/
a
Cu
2 3 3 4
O/Cu N/C N -3 as the catalyst
Substrate
Produce
Con.
Sel.
9
8
9%
9%
98%
98%
8
8
7
8
5%
2%
2%
8%
96%
96%
97%
98%
Fig. 5 Electron spin resonance (ESR) spectra of radicals trapped by
5
,5-dimethyl-1-pyrroline-N-oxide (DMPO) in (a) aqueous dispersion
ꢀ
for DMPO-$OH and (b) methanol dispersion for DMPO-$O
2
. (c)
Proposed processes toward the formation of an imine from benzyl-
amine over Cu/Cu O/Cu N/C -3.
2
3
3 4
N
7
9
8%
6%
94%
imine through three ways. The rst way is the hydrolysis of
intermediate I that releases NH molecules and creates benz-
3
46
92% aldehyde with m/z ¼ 106.12. Same as the last step of the rst
pathway, the reaction between benzaldehyde and benzylamine
47
results in the imine with the release of H O. Also, a simpler
2
9
9
8%
8%
95%
96%
and more dominant way is that intermediate I directly reacts
with benzylamine and imine is produced as one of the two
4
8
products (NH
3
being the other product). The last way starts
ꢀ
with the dehydration of intermediate I with the help of $O
which yields benzonitrile (m/z ¼ 103). Next, benzonitrile reacts
2
,
49
9
2%
95% with benzylamine to form a diamine (intermediate II), which
then undergoes deamination to give an imine. This third path
49
mainly occurs as a side reaction.
a
Reaction condition: 3 mmol different aromatic amines, 12 h, 40 mg
Cu/Cu O/Cu N/C N -3, cut 420 nm with 300 mW cm , Xe lamp, 3 mL
2 3 3 4
ꢀ2
dimethyl sulfoxide (DMSO) and air atmosphere.
4. Conclusions
In summary, ternary Cu/Cu
successfully prepared on C
and heat treatment. Cu and Cu
Cu O from photo-corrosion and improve the photocatalytic
activity. Particularly, Cu/Cu O/Cu N/C -3 is responsive to
2
O/Cu
tubes by hydrothermal synthesis
N in the composites protect
3
N heterostructures have been
3 4
N
imine was produced, the relative intensity of the imine peak to
the benzylamine peak was increased. And it is also accompa-
nied by the conversion of some by-products to imine.
Therefore, two possible oxidation pathways are proposed in
Fig. 5c. The rst pathway is a traditional synthesis method,
namely oxidative condensation of benzylalcohol and benzyl-
3
2
2
3
3 4
N
a wide range of light from visible to near-infrared, and is able to
ꢀ
produce highly reactive $O2 . Therefore, under visible light
irradiation, Cu/Cu O/Cu N/C N -3 can induce the synthesis of
2
3
3 4
amine to obtain imine. In the rst pathway, benzylamine is
imine at 323 K by using benzylamine. The conversion and
selectivity of benzylamine coupling reaction are up to 99% and
ꢀ
oxidized by $O
2
to benzylalcohol with m/z ¼ 108.13 and then to
benzaldehyde with m/z ¼ 106.12, which is subsequently dehy-
drated and condensed with a benzylamine molecule through
nucleophilic substitution, producing an imine with m/z ¼
98%, respectively. This work not only demonstrates the poten-
tial of Cu N and other copper-based semiconductors for organic
3
synthesis, but also presents a promising strategy involving
photocatalysis towards imine production through the coupling
reaction of benzylamine.
1
94.15. Specically, the O atom bonded with a-C is replaced by
44
2
the N atom of –NH in a benzylamine molecule.
In the second pathway, a benzylamine molecule becomes
a positively charged benzylamine radical with the help of a hole
ꢀ
and then the benzylamine radical is oxidized by $O
2
to form an Conflicts of interest
45
imine intermediate I with m/z ¼ 105, which is converted to
There are no conicts to declare.
28064 | RSC Adv., 2020, 10, 28059–28065
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2
3 H. L. Liu, Q. Wu, L. M. Li and X. S. Tai, Dalton Trans., 2019,
Acknowledgements
16, 5131–5134.
This work was supported by the National Natural Science 24 Y. M. Juan, S. J. Chang, H. T. Hsueh, S. H. Wang, T. C. Cheng,
Foundation of China (21771061).
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