Visible light-induced selective photocatalytic aerobic oxidation of amines into imines on Cu/graphene

Article abstract of DOI:10.1039/c5cy00395d

Graphene can stabilize metallic copper nanoparticles and enable them to exhibit excellent photocatalytic activity for aerobic oxidation of various primary and secondary amines into the corresponding imines. The copper nanoparticles stabilized on graphene absorb the energy of visible light via localized surface plasmon resonance, and produce energetic hot electrons that activate the reactants adsorbed on the surface of copper nanoparticles. The formation of imines involves selective oxygenation of amines to aldehydes and subsequent condensation with amines to form imines.

Full text of DOI:10.1039/c5cy00395d

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Visible-Light-Induced Selectively Photocatalytic
Aerobic Oxidation of Amines into Imines on
Cu/graphene
Cite this: DOI: 10.1039/x0xx00000x
a,b
a
a
a
ZhaoꢀYang Zhai , XiaoꢀNing Guo *, GuoꢀQiang Jin , XiangꢀYun Guo *
Received 00th January 2012,
Accepted 00th January 2012
Graphene can stabilize metallic copper nanoparticles and enable them to exhibit excellent
photocatalytic activity for aerobic oxidation of various primary and secondary amines into
corresponding imines. The copper nanoparticles stabilized on graphene absorb the energy of
visible light via the localized surface plasmon resonance, and produce energetic hot electrons
that activate the reactants adsorbed on the surface of copper nanoparticles. The formation of
imines involves a selective oxygenation of amines to aldehydes and a subsequent condensation
with amines to form imines.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
performance of rhodium phosphine complexes for the aerobic
[
10]
oxidation.
Lin et al. found that some doped metalꢀorganic
Imine derivatives are important building blocks for the
synthesis of fine chemicals, pharmaceuticals and pesticides, and
they are traditionally synthesized through condensation of an
amine with a suitable carbonyl compound, aldehydes or
[11]
frameworks showed photocatalytic activity for the oxidation.
Blechert et al. developed a metalꢀfree, heterogeneous catalysis
protocol for the oxidation using carbon nitride and visible
[12]
light. Tanaka et al. found various amines including primary,
[
1]
ketones. However, the wide application of this method is still
limited by the low selectivity for the condensation of primary
aliphatic aldehydes and amines, low reaction efficiency and
secondary, and cyclic amines could be photocatalytically
transformed into corresponding imines in high yields by using
[13]
Nb O as photocatalyst and O as oxidant under visible light.
[
2]
2
5
2
harsh reaction conditions. The direct oxidation of amines,
which directly uses benign and readily available molecular O2
or air as oxidant, has attracted much interest because it is
hopeful to solve the above problems.
Furukawa et al. found that intermetallic Pd Pb supported on
Localized surface plasmon resonance (LSPR) is the resonant
photonꢀinduced coherent oscillation of charge at the metal–
dielectric interface, established when the photon frequency
matches the natural frequency of metal surface electrons
[
3,4]
For example,
3
[14ꢀ
oscillating against the restoring force of their positive nuclei.
Al O can act as an efficient heterogeneous catalyst for the 16]
2
3
The conduction electrons of the nanoparticles of gold (Au),
silver (Ag) and copper (Cu) can harvest visible light energy
through the LSPR effect to produce “hot” electrons with high
[
5]
oxidation of various amines.
catalysts, the reactivity of dioxygen is difficult to control, the
selectivity of target product is usually poor, and the substrate
scope is limited.
Recently, selectively photocatalytical oxidation of amines
has received increased attention because it allows the oxidation
reactions to occur at lower temperature and use O as oxidant
under renewable solar light irradiation.
al. reported the photocatalytic oxidation of aromatic amines to
corresponding imines using TiO2 as the photocatalyst.
However, most of these reactions were only carried out under
UV light. Therefore, the development of visibleꢀlightꢀdriven
catalysts for aerobic oxidation of amines remains a challenge.
Different photocatalysts have been proposed to implement the
oxidation reaction. Sadow et al. studied the photocatalytic
For many heterogeneous
[17]
energy at the surface of metallic nanoparticles. These “hot”
[
2]
electrons can facilitate chemical reactions and improve the
[7,18,19]
yield of chemical transformations under mild conditions.
Studies on lightꢀdriven reactions catalyzed by Au or Ag
nanoparticles have formed the basis of a new and fastꢀ
2
[
6ꢀ8]
For instance, Zhao et
[14]
expanding field in green photocatalysis. Recently, Zhu et al.
found that AuꢀPd alloy nanoparticles can strongly absorb
[
9]
visible light and efficiently catalyze the aerobic oxidation
[20]
reactions of primary amines.
Although Cu nanoparticles
exhibit strong LSPR absorption in visible light range and are
[21,22]
catalytically active for many reactions,
few studies on the
photocatalytic performance of Cu nanoparticles have been
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reported due to their poor chemical stability under catalytic 450 and 800 nm). Similarly light with wavelengths in the
reaction conditions. Cu nanoparticles are easily oxidized to ranges of 520ꢀ800 and 600ꢀ800 nm were applied to the reaction
Cu O or CuO in the presence of oxygen.
system using filters with cutꢀoff wavelengths of 520 nm and
2
Recently, we found that Cu nanoparticles can be effectively 600 nm, respectively. After reaction, 2 mL aliquots were
stabilized by using graphene sheets as the support, and the collected, centrifuged, and then filtered through a Millipore
Cu/graphene can controllably catalyze the coupling reactions of filter (pore size 0.22 ꢁm) to remove the catalyst particulates.
nitroaromatics to corresponding azoxy or azo compounds under The filtrates were analyzed by BRUKER SCION SQ 456 GCꢀ
[
19]
visible light irradiation.
Because Cu based catalysts can MS to measure the concentration change of reactants and
thermally catalyze many organic reactions and they are more products. The quantitative analysis of specific analytes was
easily and abundantly available, herein we employ graphene detected by SIM mode in GCꢀMS.
supported Cu nanoparticles as the photocatalyst for aerobic
oxidation of amines under visible light irradiation, and report its Results and Discussion
excellent photocatalytic performance.
Figures 1A and
B show the Transmission Electron
Microscopy (TEM) images of the 5wt% Cu/graphene catalyst.
Spherical Cu nanoparticles with a mean diameter of 10 nm are
Experimental
Preparation of 5wt% Cu/graphene catalyst. According to uniformly dispersed on graphene sheets. The interplanar crystal
[
19]
previous work,
Cu/graphene catalyst with a Cu loading of spacing of Cu nanoparticles is 0.19 nm, corresponding to the
5
wt% has suitable LSPR absorption intensity and exhibits the Cu (111) crystal faces. From the Xꢀray diffraction patterns
best catalytic activity. Therefore, the 5wt% Cu/graphene (XRD) (Figure 1C), the peak at 2θ=43.3° is indexed to Cu
catalyst was employed in this work for the photocatalytic (111), suggesting that the graphene sheets can effectively
aerobic oxidation reaction of benzylamine. The catalyst was stabilize metallic Cu nanoparticles. The graphene used in this
prepared by liquid reduction method. Firstly, 16 mg work is obtained from reduction of graphene oxide. In this
Cu(OAc) •H O and 95 mg graphene were dispersed into 30 mL process, the longꢀrange ordered stacking of graphene oxide was
2
2
oleylamine under sonication for 0.5h. After that, the suspension dissociated of by random interlayer expansion and exfoliation,
o
was slowly heated to 230°C and kept at this temperature for 6 leading to the broad diffraction peak at 23.5 . The existence of
hours. Oleylamine acted as not only solvent but also reducing metallic Cu is also supported by the Xꢀray photoelectron
2+
0
agent. It can transfer electrons to Cu and reduce them to Cu . spectroscopy (XPS) (Figure 2A). The binding energies of Cu
The mixture was separated and put into vacuum drying 2p1/2 at around 951.9 eV and Cu 2p at 932.4 eV, respectively,
3/2
0
chamber at 60°C for 24 h. Then, a 5wt% Cu/graphene catalyst can be attributed to the Cu state, confirming that Cu exists on
was obtained. The crystalline phases of the catalyst were the graphene sheets as the metallic phase. In the UVꢀvis spectra
characterized by Xꢀray diffractometer (XRD, Rigaku Dꢀ (Figure 1D), the absorption peak at approximately 558 nm is
Max/RB). The microstructures of the catalyst were investigated due to the LSPR absorption of Cu nanoparticles. Meanwhile,
by highꢀresolution transmission electron microscope (HRTEM, the absorption intensity of Cu/graphene increases with the Cu
JEMꢀ2010). Xꢀray photoelectron spectroscopy (XPS) was loading increasing.
measured on a Kratos XSAM800 spectrometer, using Al Ka
(
hk=1486.6 eV) Xꢀray source as the excitation source. The UVꢀ
visible absorption spectra of the catalyst were examined by a
HITACHI Uꢀ3900 spectrometer with Al O as a reference.
2
3
Photocatalytic aerobic oxidation reactions of amines. The
photocatalytic aerobic oxidation reactions of amines were
conducted in 1 atm O atmosphere. The whole reaction system
2
o
was strictly controlled at 40 C during the reaction process by an
oil bath. The reactant mixture consists of 10 mL absolute
ethanol, 1 mmol amines and 100 mg 5wt% Cu/graphene
catalyst. A 300W Xe lamp (wavelength from 400ꢀ800 nm) was
employed as light source to irradiate the reaction system. The
ꢀ
2
irradiation intensity was 0.5 Wcm . The dependence of the
catalytic performance on the wavelength range of light was
investigated by employing various low pass optical filters to
block light below specific cutꢀoff wavelengths while keeping
the light intensity the same with the reaction system without
optical filter. For instance, a filter with the cutꢀoff wavelength
of 450 nm can block light with wavelengths shorter than 450
nm (the system is irradiated by light with wavelengths between
2
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DOI: 10.1039/C5CY0
A
0
R
39
T
5
IC
D
LE
2
In the control experiments using only graphene as the catalyst and the reaction temperture was strictly controlled at 0.5 W/cm
o
in O atmosphere or using the Cu/graphene as the catalyst in Ar, and 40 C, respectively. Then, a series of optical lowꢀpass
2
the oxidation reaction of benzylamine did not occur. However, filters were employed to block light below specific cutꢀoff
the 5wt% Cu/graphene exhibits high photocatalytic activity and wavelengths. The irradiation of the light with wavelength range
selectivity. The conversion of benzylamine is 99% and the from 400 to 800 nm gives a benzylamine conversion of 99%.
selectivity of Nꢀbenzylidenebenzylamine is 93% (Table 1, entry The conversion decreases to 85%, 67% and 31% when the
1
). Control experiments reveal that the Cu/graphene catalyst in wavelength range of the irradiation is 450ꢀ800, 520ꢀ800, and
dark (without irradiation) can only achieve a benzylamine 600ꢀ800 nm, respectively. By deducting the contribution of
conversion of 18%, indicating that the enhanced catalytic thermal reaction, the contribution of 400ꢀ450 nm light accounts
activity results from the light irradiation. The reaction can also for about 17% ((99ꢀ85)/(99ꢀ18)×100%) in the total lightꢀ
occur in air atomsphere with
a
lower conversion of induced conversion. Similarly, the light in the wavelength range
benzylamine (82%) comparing with that in O . Under the same of 450ꢀ520, 520ꢀ600 and 600ꢀ800 nm, respectively accounts for
2
temperature, increasing of irradiation intensity resulted in an 22%, 45% and 16% of the lightꢀinduced conversion. These
almost linear growth of the benzylamine conversion (Figure 3). values agree well to the UVꢀvisible absorption spectrum of the
To ensure that the thermal effect could be discounted, the Cu/graphene catalyst (Figure 4B). Because the Cu/graphene
reaction temperature was carefully controlled and maintained at catalyst has a strong SPR absorption at about 558 nm, the light
4
0°C. The contribution of light induced conversion was in the wavelength range of 520ꢀ600 nm contributions the
calculated by subtracting the conversion in the dark (thermal highest lightꢀinduced conversion. This further confirms that the
effect) from the total one under irradiation. The relative light absorbed by Cu nanoparticles is the major driving force of
contributions of irradiation and thermal effect to the total this reaction. The conduction electrons of Cu nanoparticles gain
conversion are shown in Figure 3. The contribution of light the energy of incident light through the LSPR effect and yield
induced conversion increases with the light intensity. For energetic electrons at the surface of nanoparticles. The
instance, the contribution of light induced conversion was 82% resonance of conduction electrons with the electromagnetic
ꢀ
2
(
(99ꢀ18)/99×100%) at a light intensity of 0.5 Wcm , while it is field of incident light results in a significant enhancement of the
ꢀ
2
only 40% at a light intensity of 0.15 Wcm . These results local electromagnetic field near the surface of Cu nanoparticles.
indicate the indispensable role of light irradiation.
The lightꢀexcited electrons can activate adsorbed reactant
The wavelength dependence of the catalytic activity was molecules on the surface of Cu nanoparticles. The enhanced
illustrated in Figure 4A. Without any filters, the light intensity local field may also promote the reaction.
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oxidation of aliphatic primary amines with dioxygen usually
has a poor selectivity.
To study the mechanism of the photocatalytic oxidation, the
reaction profile for the aerobic photocatalytic oxidation of
benzylamine was investigated (Figure 5). In the beginning, little
benzylamine converted into Nꢀbenzylidenebenzylamine with
high selectivity. As the reaction time was prolonged, the
conversion of benzylamine increased, but the selectivity of the
imine slightly decreased step by step and small amounts of
benzaldehyde could appear in the products. This is mainly
because the increased consumption of benzylamine in the
solution could not ensure the full condensation of nascent
aldehyde and amine. Based on these results, a similar reaction
[
9]
mechanism to the previous reports was proposed and shown
in Equation (1). This represents an involvement of a selective
oxygenation step to form aldehydes, which then undergo a
subsequent condensation step to produce imines. The high
selectivity of imines (Table 1) is mainly because highly
selective formation of aldehydes from amines and easy
nucleophilic attack of such nascent aldehydes by the unreacted
amines to yield corresponding imines.
To demonstrate the general applicability of the catalyst, the
photocatalytic oxidation of a variety of amine substrates was
investigated. The results were summarized in Table 1. The
oxidation of benzylamine derivatives proceeded efficiently
under visibleꢀlight irradiation with high conversion of amines
and high selectivity of imines (Table 1, entries 1–4). The
substituents on benzene ring have only a small influence on the
conversion and selectivity. This photocatalyst also has good
applicability for heteroatomꢀcontaining substrates (Table 1,
entries 6 and 7), whose imine products are usually difficult to
be obtained by transitionꢀmetal complex catalysts due to the
rapid deactivation of catalysts caused by the strong
(1)
(2)
A series of aromatic secondary amines converting into
dehydrogenated imines were also investigated (Table 2).
Comparing with the oxidation of primary amines, both
conversion and selectivity become low. Significant amounts of
benzaldehyde and its derivative from oxidative cleavage of the
CꢀN bond, were also formed as byproducts in the oxidation of
secondary benzylic amines. These clearly indicate that the
reaction proceeded via an oxygenation pathway. That is, a
detour for aldehyde and benzylamine occurred [Equation (2)].
Because the selfꢀcoupling of inꢀsitu generated benzylamines
could also happen during the reaction, some accumulated
aldehydes can be detected as the final product in addition to
[
23]
coordination of these amines with metal centers.
When
employing aliphatic primary amines as reactants, no imines
were produced and only fragmentation products were detected
(
Table 1, entries
8 and 9). Because the unselective
autooxidation caused by the activation of multiple reactive sites
[
23]
can easily occur during the photocatalytic reaction,
the
4
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the condensation between benzaldehyde and aniline led to the
low selectivity of Nꢀbenzylideneaniline.
Generally, the reaction temperature greatly affects the
catalytic activity. The oxidation reaction of benzylamine was
o
o
respectively conducted at room temperature (25 C) and 60 C
ꢀ
2
under a constant light intensity of 0.5 Wcm . The reaction can
be completed in 7.5 and 5 h, respectively. Both conversions of
benzylamine are 99%. The conversion is only 13% and 27%
without irradiation at 25 C and 60 C, respectively. The
conversion of dibenzylamine can increase to 69% if the
o
o
o
oxidation of dibenzylamine was carried out at 80 C for 12 h
ꢀ
2
under irradiation with intensity of 0.5 Wcm . The above results
suggest that lightꢀinduced conversion can contribute the most of
total yield even at higher temperture. Elevated temperature can
cause a redistribution of the conduction electrons of Cu
[
26,27]
nanoparticles into higher energy levels.
Those electrons at
higher levels can gain additional energy through light
absorption when irradiated. This increase in energy further
enhances the probability that the energetic electrons activate the
reactant molecules adsorbed on the surface of nanoparticles.
Moreover, the relative population of reactant molecules in
excited states increases with temperature rising according to the
[
20]
BoseꢀEinstein distribution.
This means that the reactant
molecules will require less energy to overcome the activation
barrier and this energy could be easily provided by light
irradiation. These results indicate that the electrons of Cu
nanoparticles can effectively couple thermal and photonic
energies to drive the chemical reactions.
To test the recyclability of the Cu/graphene photocatalyst in
the oxidation of benzylamine, the catalyst was reused for 5
times after filtering and drying and the experiments were
conducted for 4 h to ensure that the reactions did not reach
equilibrium. A slow decrease in the activity was found after
five reaction cycles, from 76% at the first cycle to 70% at the
th
5
(Figure 6A). TEM images of the used catalyst show no
obvious change in morphology of the Cu nanoparticles (Figure
B). However, the binding energy of Cu 2p1/2 (952.1 eV) in the
6
XPS of the used Cu/graphene slightly shifts to higher value
comparing with the fresh catalyst (Figure 2B). Moreover, the
activity of used Cu/graphene can be recovered to its initial level
imines. The low conversion is mainly due to that the energy of
energetic electrons is relatively low comparing with the
required energy of oxidative cleavage of the CꢀN bond.
Conducting the oxidation of dibenzylamine under the UV
irradiation but keeping the other reaction conditions unchanged,
a conversion of 95% of dibenzylamine was achieved. The UV
absorption of Cu nanoparticles can induce the interꢀband
electron transition. Thus, these UV excited electrons have high
after it is reduced in H ꢀcontaining atmosphere, indicating that a
2
slight oxidation of Cu nanoparticles occurs during the
photocatalytic process.
Conclusion
The present work demonstrates a novel photocatalytic route of
the aerobic oxidation of amines into imines over a Cu/graphene
catalyst. The Cu/graphene shows an excellent photocatalytic
activity for oxidation of benzylamine, and achieves a 99%
conversion of benzylamine with a 93% selectivity of Nꢀ
benzylidenebenzylamine. Meanwhile, Cu/graphene has an
excellent general applicability for the aerobic oxidation of
various primary and secondary amines into corresponding
imines. The enhanced catalytic activity of the Cu/graphene
under light irradiation results from the localized surface
[24,25]
energy and are capable of cracking of the CꢀN bond.
For
asymmetric dibenzylamine derivative, because C ꢀH bonds on
α
both sides adjacent to the nitrogen atom have equal
opportunities to activate and the condensation between different
aldehydes and amines also have equal reaction rates, four
different imines with molar ratio close to 1:1:1:1 and two
accumulated aldehydes were observed (Table 2, entry 4). For
the Nꢀphenylbenzylamine (Table 2, entry 5), the difficulty of
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Jou5CrnY0al Na5Dme
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ARTICLE
Cu/graphene can selectively photocatalyze the aerobic oxidation of amines into imines at mild conditions by harvesting visible
light via the localized surface plasmon resonance of Cu nanoparticles.
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