ZnIn2S4: A photocatalyst for the selective aerobic oxidation of amines to imines under visible light

Article abstract of DOI:10.1002/cctc.201402360

Flowerlike nanospheres of ZnIn₂S₄ consisting of interweaving nanoflakes were prepared by a rapid microwave-assisted solvothermal method. The photocatalytic performance of ZnIn₂S₄ in the aerobic oxidation of amines to imines is for the first time revealed. As-prepared ZnIn₂S₄ showed high efficiency for the transformations of various amines into their corresponding imines under visible-light irradiation by using air as the oxidant. The photocatalytic performance of ZnIn₂S₄ in the selective oxidation of amines to their corresponding imines is much higher than that of graphitic C₃N₄ and CdS previously reported. A plausible mechanism involving the oxygenation of amines to form aldehydes, followed by the nucleophilic addition of the amines to afford the final imines is proposed. The use of a cheap, environmentally friendly inorganic semiconductor photocatalyst and solar energy makes this transformation green and sustainable.

Full text of DOI:10.1002/cctc.201402360

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DOI: 10.1002/cctc.201402360
ZnIn2S4: A Photocatalyst for the Selective Aerobic
Oxidation of Amines to Imines under Visible Light
Lin Ye and Zhaohui Li*[a]
Flowerlike nanospheres of ZnIn2S4 consisting of interweaving
nanoflakes were prepared by a rapid microwave-assisted solvo-
thermal method. The photocatalytic performance of ZnIn2S4 in
the aerobic oxidation of amines to imines is for the first time
revealed. As-prepared ZnIn2S4 showed high efficiency for the
transformations of various amines into their corresponding
imines under visible-light irradiation by using air as the oxi-
dant. The photocatalytic performance of ZnIn2S4 in the selec-
tive oxidation of amines to their corresponding imines is much
higher than that of graphitic C3N4 and CdS previously reported.
A plausible mechanism involving the oxygenation of amines to
form aldehydes, followed by the nucleophilic addition of the
amines to afford the final imines is proposed. The use of
a cheap, environmentally friendly inorganic semiconductor
photocatalyst and solar energy makes this transformation
green and sustainable.
the aerobic oxidation of amines to form imines under visible-
light irradiation.
Ternary ZnIn2S4 is a semiconductor chalcogenide that has
a band gap corresponding to visible-light absorption with con-
siderable chemical stability.[6] ZnIn2S4 has been extensively
studied for photocatalytic H2 evolution as well as for the deg-
radation of organic dyes.[7] Endowed with a suitable band posi-
tion, ZnIn2S4 can also be a promising photocatalyst for organic
transformations. The top of the valance band of ZnIn2S4 is lo-
cated at 1.37 eV versus the normal hydrogen electrode (NHE),
and the bottom of the conduction band of ZnIn2S4 is located
at À0.73 eV versus NHE.[8] Therefore, the photogenerated elec-
trons have a large enough reduction potential to activate mo-
lecular oxygen, whereas the moderate oxidation potential of
the holes prevents uncontrolled oxidation of the synthesized
organic compounds. Indeed, the photocatalytic oxidation of
benzyl alcohol to benzaldehyde over ZnIn2S4 was recently re-
ported.[9] Herein, we reported the transformations of amines
into imines over ZnIn2S4 under visible-light irradiation. It was
found that ZnIn2S4 can be a highly efficient visible-light-in-
duced photocatalyst for the selective aerobic oxidation of
amines to form imines. The use of a cheap, environmentally
friendly inorganic semiconductor photocatalyst in combination
with solar energy makes this transformation green and sustain-
able.
The selective oxidation of amines to the corresponding imines
is one of the most important functional-group transformations
in organic chemistry, because imines and their derivatives are
important building blocks for the synthesis of fine chemicals
and pharmaceuticals.[1] Traditionally, imines are prepared by
dehydrating condensation of an amine with a carbonyl com-
pound.[2] Costly dehydrating reagents are usually required to
realize this transformation, and sometimes the active nature of
the ketones or aldehydes can makes this reaction difficult to
control,[3a–c] although recently a couple of studies have report-
ed that molecular sieves can be used as a catalyst to transform
amines into imines.[3d,e] A more attractive strategy for the for-
mation of imines is the direct oxidation of amines by using
oxygen or more preferably the oxygen content of air. Recently,
the use of sunlight as a driving force for the selective photo-
catalytic oxidation of organic compounds has received increas-
ing attention.[4] The selective oxidation of amines to imines by
photocatalysis has also been realized in some semiconductor
systems, such as TiO2, Nb2O5, CdS, and graphitic carbon nitride
(g-C3N4).[5] However, some selective oxidation reactions can
only be performed under UV irradiation, whereas others re-
quire a higher pressure of O2 or a higher reaction temperature.
It is highly important to develop efficient photocatalysts for
ZnIn2S4 was prepared through a microwave-assisted solvo-
thermal method from ZnCl2, InCl3·4H2O, and thioacetamide
(TAA) in a mixture of ethylene glycol (EG) and N,N-dimethyl-
formamide (DMF). The XRD diffraction patterns of the product
was indexed to hexagonal ZnIn2S4 (JCPDS 65-2023), in agree-
ment with our previous report (Figure S1, Supporting Informa-
tion).[7a,b] The SEM image shows that the product is composed
of flowerlike microspheres with a dimension of approximately
1 mm, and these microspheres are made up of numerous inter-
weaving nanoflakes (Figure S2). The UV/Vis diffuse reflectance
spectrum of the product shows that it has an absorption edge
at approximately 490 nm, with an estimated band gap of
2.5 eV (Figure S3).
To study the photocatalytic performance of ZnIn2S4 for the
selective aerobic oxidation of amines to form imines under visi-
ble-light irradiation, benzylamine was first selected as the sub-
strate, and the results are shown in Table 1. A medium conver-
sion (38.0–63.2%) of benzylamine and a high selectivity (93.5–
96.8%) toward N-benzylidenebenzylamine was achieved over
the as-prepared ZnIn2S4 in various organic reaction media after
irradiation for only 20 min (Table 1, entries 1–5). A trace
amount of benzaldehyde was detected as the side product in
this reaction. The solvent did show an influence on the reac-
tion rate. CH2Cl2 was found to be the best solvent among
[a] L. Ye, Prof. Z. Li
Research Institute of Photocatalysis
State Key Laboratory of Photocatalysis on Energy and Environment
Fuzhou University
Fuzhou, 350002 (P.R. China)
E-mail: zhaohuili1969@yahoo.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402360.
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tuted benzylamines increased in the order ortho<meta<para,
which is indicative of the presence of a steric effect (Table 2,
entries 2–4). Heteroatom (such as oxygen) containing amines
and even a primary aliphatic amine such as 1-butylamine was
transformed over irradiated ZnIn2S4, although with a slower
conversion and a lower selectivity than benzylic amines
(Table 2, entries 8 and 11). The ability to transform heteroatom-
containing imines is important, as the formation of hetero-
atom-containing imines is usually inaccessible under catalysis
with the use of transition-metal complexes owing to the deac-
tivation of the catalysts caused by the strong coordination of
heteroatom-containing amines to the metal center. Moreover,
secondary amines, such as 1,2,3,4-tetrahydroisoquinoline,
could also be converted into the corresponding imines with
moderate conversions (Table 2, entries 9 and 10). However,
amines without a hydrogen atom at the a carbon atom, such
as aniline, could not be oxidized to the corresponding imines
under similar conditions; this implies that the hydrogen atom
at the a carbon atom is involved in the selective aerobic oxida-
tion of the amines to form imines (Table 2, entry 12).
Table 1. Aerobic photocatalytic oxidation of benzylamine to N-benzyli-
denebenzylamine
over
ZnIn2S4
under
various
conditions.[a]
Entry
Solvent[b]
Conversion [%]
Selectivity [%]
1a
1b
1
2
3
4
CH2Cl2
CH3CN
DMF
C6H5CF3
EtOAc
CH2Cl2
CH2Cl2
CH2Cl2
63.2
61.1
50.3
60.5
38.0
–
96.8
96.1
96.5
93.5
96.0
–
<1.0
<1.0
<1.0
<1.0
<1.0
–
5
6[b]
7[c]
8[d]
–
99.0
–
96.5
–
1.0
[a] Reaction conditions: Benzylamine (0.1 mmol), ZnIn2S4 (8 mg), solvent
(2.0 mL), air, irradiation for 20 min. [b] No light. [c] In the absence of
ZnIn2S4. [d] Irradiation for 45 min.
those examined, as it showed the best conversion of
benzylamine (63.2%) and the highest selectivity for
N-benzylidenebenzylamine (96.8%; Table 1, entry 1).
Table 2. Aerobic photocatalytic oxidation of various amines over ZnIn2S4.[a]
Therefore, CH2Cl2 was chosen as the reaction
medium for the following studies. No detectable
products were obtained in the absence of light or
ZnIn2S4, which indicates that the transformation of
benzylamine into N-benzylidenebenzylamine is truly
induced by ZnIn2S4 photocatalysis (Table 1, entries 6
and 7). Extension of the irradiation time to 45 min
led to almost complete conversion of benzylamine
into N-benzylidenebenzylamine (Table 1, entry 8). Al-
though a similar transformation was also achieved
over visible-light irradiated CdS and g-C3N4, a higher
temperature (808C) and a high pressure of O2
(0.5 MPa O2) were required over g-C3N4 photocataly-
sis.[5c,d] In addition, our current ZnIn2S4 system
showed higher efficiency, as almost complete conver-
sion of benzylamine took less time (45 min vs. 4.5 h
over CdS) under similar reaction conditions, even
with a lower molar ratio of the photocatalyst to sub-
strate (1:5) relative to that that used with CdS (3:5).
With the optimum reaction conditions in hand, we
then examined the selective aerobic oxidation of
other amines under visible-light irradiation, and the
results are shown in Table 2. The selective aerobic ox-
idation of all the benzylamine derivatives to their cor-
responding imines proceeded efficiently under the
reaction conditions with medium to high conversions
(73.8–99.9%) and high selectivity (94.0–97.0%;
Table 2, entries 1–7). Electron-donating substituents
(CH3À and CH3OÀ) and electron-withdrawing sub-
stituents (FÀ and ClÀ) on the phenyl ring did not
have a clear effect on the conversion rate and the se-
lectivity, and this indicates that there is no clear elec-
tronic effect in this reaction (Table 2, entries 2–7). The
conversion of ortho-, meta-, and para-methoxy-substi-
Entry Substrate
Product
Conver- Selec-
sion [%] tivity [%]
1
2
99.0
73.8
94.0
97.0
3
77.8
96.1
4
5
6
7
98.9
99.9
99.0
97.9
96.0
96.2
96.4
95.2
8
58.7
77.3
90.3
83.3
9[b]
10
77.0
82.8
11[c]
12
92.5
–
67.1
–
–
[a] Reaction conditions: Substrate (0.1 mmol), ZnIn2S4 (8 mg), CH2Cl2 (2.0 mL), air, irra-
diation for 45 min. [b] Dibenzylamine (0.05 mmol). [c] Irradiation for 2 h.
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Two different mechanisms for the selective aerobic oxidation
of amines to imines exist. The widely accepted one is that the
imine is generated through oxidative dehydrogenation of the
amine,[5d,10] whereas the other mechanism involves oxygena-
tion of the amine with O2 to form an aldehyde, which is fol-
lowed by nucleophilic addition of the amine to the aldehyde
to afford the final imine.[11] Zhao et al. proposed that these two
pathways can be differentiated by comparison of the products
obtained in the selective oxidation of benzylamine and diben-
zylamine. They proposed that if the selectivity to N-benzyli-
denebenzylamine for dibenzylamine oxidation is higher than
that for benzylamine oxidation, then the reaction primarily pro-
ceeds through a dehydrogenation pathway, because oxidation
of dibenzylamine through this pathway is a one-step reaction,
whereas the oxygenation pathway is a detour in which benzal-
dehyde would accumulate.[11] In our reaction system, the selec-
tive aerobic oxidation of dibenzylamine was also performed,
and the result is shown in Table 2 (entry 9). It was found that
both the conversion of dibenzylamine and the selectivity to N-
benzylidenebenzylamine decreased, and this implies that the
formation of imine over ZnIn2S4 preferably proceeds through
the oxygenation pathway.
after the photocatalytic reaction the sample showed an XRD
pattern that was similar to that of the fresh catalyst (Figure S4),
the photocatalyst showed slightly decreased photocatalytic ac-
tivity upon recycling.
In summary, the highly efficient selective aerobic oxidation
of amines to imines by using air as the oxidant was achieved
over ZnIn2S4 under visible-light irradiation. The transformation
of amines into imines over ZnIn2S4 involves oxygenation of the
amines with O2 to form aldehydes, which is followed by nucle-
ophilic addition of the amines to the aldehydes to afford the
final imines. This study shows the high potential of using
ZnIn2S4 as a visible-light-induced photocatalyst for organic syn-
thesis and its application to other organic synthetic reactions
is still ongoing in our laboratory.
Experimental Section
Preparation
All chemicals were analytical grade and were used as received
without further purification. ZnIn2S4 was synthesized as follows:
ZnCl2 (0.136 g, 1 mmol), InCl3·4H2O (0.586 g, 2 mmol), and TAA
(0.3 g, 4 mmol) were dissolved in DMF (30 mL) and EG (30 mL).
Then, the solution was sealed in a Teflon-lined double-walled di-
gestion vessel. After treating the solution at a controllable temper-
ature of 2008C for 90 min by using a microwave system (Ethos TC,
Milestone, Italy), the vessel was cooled to room temperature. The
product was collected by centrifugation, washed with deionized
water and absolute ethanol, and dried at 608C.
Therefore, the proposed mechanism for the selective aerobic
oxidation of amines to imines over ZnIn2S4 is shown in
Scheme 1. Upon irradiation of ZnIn2S4 with visible light, elec-
trons (eÀ) and holes (h+) are generated. Amines can react with
the photogenerated holes, which is followed by deprotonation
to generate a neutral carbon-centered radical (step 1 in
Scheme 1). In the meantime, molecular oxygen can capture
the photogenerated electrons to generate a superoxide radical
anion (step 2 in Scheme 1). The interaction between the pho-
Characterization
Powder XRD data were collected by using a Bruker D8 Advance X-
ray diffractometer (CuKa irradiations). The morphology of the sam-
ples was characterized by field-emission SEM (JSM-6700F). UV/Vis
diffuse reflectance spectra (UV-DRS) of the powders were obtained
for dry-pressed disk samples by using a Cary 500 Scan Spectropho-
tometer (Varian, USA). BaSO4 was used as a reflectance standard in
the UV/Vis diffuse reflectance experiment.
À
togenerated carbon-centered radical and O2 will lead to the
formation of aldehydes (step 3 in Scheme 1). Such a mechanism
is similar to that proposed for the formation of aldehyde over
TiO2 photocatalysis under UV irradiation.[11] Nucleophilic attack
of the unreacted amines on the aldehydes will thus yield the
corresponding imines though dehydration (step 4 in
Scheme 1). The simultaneous consumption of the photogener-
ated electrons and holes ensures the stability of ZnIn2S4 during
the aerobic oxidation of the amines. However, complete inhibi-
tion of the photocorrosion of ZnIn2S4 is not possible. Although
Photocatalytic activity test
The photocatalytic selective oxidation of amines to imines was per-
formed as follows: In a typical process, a mixture of the catalyst
(8 mg) and the organic compound (0.1 mmol) was dissolved in sol-
vent (2.0 mL) in air at room temperature. The mixture was trans-
ferred into a 10 mL Pyrex glass bottle and stirred to make a homo-
geneous solution. The suspension was irradiated with a 300 W Xe
arc lamp (CEL-HXF300/CEL-HXUV300, Beijing Aulight Co. Ltd.) with
a UV cutoff filter (l>420 nm). The mixture was filtered through
a porous membrane (20 mm in diameter) and then injected into
the GC equipped with a flame ionization detector (Shimadzu GC-
2014) with a RTX-5 capillary for analysis. The chemical structures of
the products were confirmed by comparison with standard chemi-
cals.
Acknowledgements
Scheme 1. Proposed mechanism for the selective aerobic oxidation of
amines to imines over ZnIn2S4 under visible-light irradiation. CB=Conduc-
tion band, VB=valence band.
The work was supported by the National Natural Science Foun-
dation of China (NSFC) (21273035), 973 Programs
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support.
Keywords: amines · imines · photooxidation · visible light ·
zinc
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Received: May 20, 2014
Published online on July 23, 2014
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