Efficient chemoselective hydrogenation of nitrobenzene to aniline, azoxybenzene and azobenzene over CQDs/ZnIn2S4 nanocomposites under visible light

Article abstract of DOI:10.1016/j.jcat.2020.05.041

Efficient production of amino-, azo- and azoxy- aromatics is important in chemical industries. In this manuscript, we reported that controllable chemoselective hydrogenation of nitrobenzene to produce aniline, azoxybenzene and azobenzene was realized over CQDs/ZnIn₂S₄ nanocomposite under visible light via simply regulating the reaction medium including the alkalinity and the hydrogen source. An optimized production of aniline was obtained over 3.0 wt% CQDs/ZnIn₂S₄ nanocomposite in a reaction medium of TEOA: MeOH (1:1), while a strong alkaline reaction medium promote N-N coupling to produce azoxybenzene and azobenzene. In addition, azoxybenzne is the intermediate to form azobenzene during the hydrogenation of nitrobenzene, in which a strong hydrogen source like TEOA is indispensable for the transformation. This study not only demonstrated a noble metal-free photocatalytic system for visible light induced efficient and controllable chemoselective hydrogenation of nitrobenzene to produce aniline, azobenzene and azoxybenzene, but also highlights the great potential of photocatalysis in organic syntheses.

Full text of DOI:10.1016/j.jcat.2020.05.041

Journal Pre-proofs
Efficient Chemoselective Hydrogenation of Nitrobenzene to Aniline, Azoxy-
benzene and Azobenzene over CQDs/ZnIn S Nanocomposites under Visible
2
4
Light
Bingqing Wang, Zirong Deng, Zhaohui Li
PII:
S0021-9517(20)30228-1
https://doi.org/10.1016/j.jcat.2020.05.041
YJCAT 13771
DOI:
Reference:
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Journal of Catalysis
Received Date:
Revised Date:
Accepted Date:
30 March 2020
6 May 2020
31 May 2020
Please cite this article as: B. Wang, Z. Deng, Z. Li, Efficient Chemoselective Hydrogenation of Nitrobenzene to
Aniline, Azoxybenzene and Azobenzene over CQDs/ZnIn S Nanocomposites under Visible Light, Journal of
2
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Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.2020.05.041
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Efficient Chemoselective Hydrogenation of Nitrobenzene to
Aniline, Azoxybenzene and Azobenzene over CQDs/ZnIn2S4
Nanocomposites under Visible Light
Bingqing Wang, Zirong Deng, Zhaohui Li
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy
and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, P. R.
China

Authors to whom all correspondences should be addressed.
E-mail: zhaohuili1969@yahoo.com Tel (Fax): 86-591-22865855
1

Abstract
Efficient production of amino-, azo- and azoxy- aromatics is important in chemical
industries. In this manuscript, we reported that controllable chemoselective
hydrogenation of nitrobenzene to produce aniline, azoxybenzene and azobenzene was
realized over CQDs/ZnIn S nanocomposite under visible light via simply regulating
2
4
the reaction medium including the alkalinity and the hydrogen source. An optimized
production of aniline was obtained over 3.0 wt% CQDs/ZnIn S nanocomposite in a
2
4
reaction medium of TEOA: MeOH (1:1), while a strong alkaline reaction medium
promote N-N coupling to produce azoxybenzene and azobenzene. In addition,
azoxybenzne is the intermediate to form azobenzene during the hydrogenation of
nitrobenzene, in which a strong hydrogen source like TEOA is indispensable for the
transformation. This study not only demonstrated a noble metal-free photocatalytic
system for visible light induced efficient and controllable chemoselective
hydrogenation of nitrobenzene to produce aniline, azobenzene and azoxybenzene, but
also highlights the great potential of photocatalysis in organic syntheses.
Keywords: Chemoselective hydrogenation of nitrobenzene; Aniline;
Azoxybenzene; Azobenzene; CQDs/ZnIn2S4 Nanocomposites
2

Introduction
With an aim to develop green and sustainable processes for organic syntheses, the
use of visible light to initiate organic transformations have attracted extensive recent
research interest.1-7 Photoredox reactions can be carried out under mild conditions and
therefore can be more selective, which avoid the generation of byproducts due to the
8
presence of undesirable thermal side reactions. During the past several years,
photoredox catalysis has made great progress and is becoming more and more
important in synthetic organic chemistry. A variety of organic transformations that used
to be performed under harsh conditions have been realized under mild irradiations over
both homogeneous and heterogeneous photocatalysts.9-14
Catalytic reduction of aromatic nitro compounds is a very useful reaction in
chemical industry since the main reduction products, like aromatic amines, azo- and
azoxy- aromatics are important intermediates in the syntheses of pharmaceuticals,
agrochemicals, dyes and pigments.15-21 Light initiated reduction of nitro aromatics has
been demonstrated over a variety of photocatalytic systems including semiconductors,
2
2-31
plasmonic metal nanoparticles as well as metal-free dyes and g-C N .
For example,
3
4
nitrobenzene and different nitro aromatics were hydrogenated to aniline and
corresponding functionalized amino aromatics over EY and CdS under visible light.28,
1
7
The conversion of nitrobenzene to produce azoxybenzene was realized over a Rh
loaded CdS photocatalyst (SiO /CdS/Rh) under visible light, but aniline and
2
azobenzene was also obtained as the by-products and the activity is relatively low.24
Under visible light irradiations, selective reduction of nitrobenzene to azobenzene can
be realized over plasmonic Au/TiO at room temperature or cheaper Cu/RGO at an
2
elevated temperature.25, 27 A recent study demonstrated a rare example of photocatalytic
reduction of nitro aromatics to chemoselective syntheses of azo- and azoxy-aromatic
compounds under either purple (410 nm) or blue light (450 nm) excitation over
2
9
graphitic C N . The high activity towards the production of azo- and azoxy-aromatics
3
4
instead of amino aromatics is attributed to weakly bound photo-generated surface
3

adsorbed H atoms, which are favorable for the N-N coupling. Even so, the development
of an highly efficient yet inexpensive photocatalytic system to realize the controllable
and chemoselective syntheses of aromatics amines, azoxy- and azo- aromatics via
reduction of nitroaromatics under mild conditions is still very challenging.
Based on the generally accepted pathways for catalytic reduction of nitrobenzene
proposed by Haber (shown in Scheme 1),32 nitrosobenzene (NBS) and N-
phenylhydroxylamine (NPH), the initially formed hydrogenated products of
nitrobenzene, can be directly reduced to aniline (Pathway I) or via a condensation
pathway to produce azoxybenzene (Pathway II), which can be further hydrogenated in
a series of consecutive steps to azobenzene, hydrazobenzene, and finally to aniline
(Pathway III). Since the hydrogenation of nitrobenzene to produce aniline,
azoxybenzene and azobenzene involves proton-coupled multi-electron reduction
processes, its performance can be influenced not only by the catalytic system, but also
by the reaction medium. However, to the best of our knowledge, to realize an efficient
and controllable syntheses of aniline, azoxybenzene and azobenzene via simply
regulating the reaction medium in photocatalytic reduction of nitrobenzene has not been
reported previously.
In this manuscript, we reported that controllable and efficient chemoselective
hydrogenation of nitrobenzene to aniline, azobenzene and azoxybenzene can be
realized over CQDs/ZnIn S nanocomposites under visible light via simply regulating
2
4
the reaction medium. ZnIn S , with a suitable band gap of 2.4 eV, has been widely used
2
4
in a variety of photocatalytic reactions.33-38 Particularly, the conduction band of ZnIn2S4
locates at -1.03 V vs NHE, which is more negative than the reduction potential of
nitrobenzene to form aniline (ca. -0.48 V vs NHE), azoxybenzene (ca. -0.78 V vs NHE)
and azobenzene (ca. -0.80 V vs NHE).39, 40 Therefore, it is possible that the
photocatalytic reduction of nitrobenzene to produce aniline, azoxybenzene and
azobenzene can be realized over ZnIn S . The self-assembly of ZnIn S in the presence
2
4
2 4
of preformed carbon quantum dots (CQDs) led to the formation of CQDs/ZnIn2S4
4

nanocomposites. The photocatalytic reduction of nitrobenzene over CQDs/ZnIn2S4
nanocomposites not only was influenced by the incorporated CQDs, but also depended
on the reaction medium including the alkalinity and the strength of the hydrogen source.
This study not only demonstrated a noble metal-free photocatalytic system for
controllable and efficient chemoselective hydrogenation of nitrobenzene to produce
aniline, azobenzene and azoxybenzene under visible light, but also highlights the great
potential of photocatalysis in organic syntheses.
Experimental
Preparations
All the reagents were commercial available and used without further purifications.
Carbon Quantum Dots (CQDs) were prepared by thermalizing citric acid according to
literature.41 CQDs/ZnIn S nanocomposite was prepared following a previously
2
4
reported solvothermal method by self-assembly of ZnIn S microspheres in the
2
4
4
2
presence of preformed CQDs. CQDs/ZnIn S nanocomposites with different amount
2
4
of CQDs were denoted as x wt% CQDs/ZnIn S (x = 1.0, 3.0 and 5.0). Bare ZnIn S
2 4
2
4
was synthesized similarly without the introduction of preformed CQDs.
Characterizations
The X-ray diffraction (XRD) patterns of the as-obtained products were carried out
on a D8 Advance X-ray diffractometer (Bruker, Germany) using Cu K (λ= 1.5406 Å)
α
radiation at a voltage of 40 kV and 40 mA. XRD patterns were scanned over the angular
range of 10-60° (2θ) with a step size of 0.02°. The FT-IR analyses were carried out on
a Nicolet IS50 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS)
measurements were performed on a PHI Quantum 2000 XPS system (PHI, USA) with
a monochromatic Al K source and a charge neutralizer. All the binding energies were
a
referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. The scanning
electron microscopy (SEM) was obtained on JSM-6700F. The transmission electron
microscopy (TEM) and high resolution TEM (HRTEM) images were measured by
5

JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. UV-visible
diffraction spectra (UV-vis DRS) of the powders were obtained for the dry pressed disk
samples using a UV-visible spectrophotometer (Cary 7000 Scan Spectrophotometers,
Varian). BaSO was used as a reflectance standard.
4
Photocatalytic reactions
The photocatalytic reactions were carried out in a sealed reaction tube irradiated
with a 5 W LED lamp (Light-emitting diodes, PCX50A, multichannel optical catalytic
reaction system). The catalyst (10 mg) and nitrobenzene (0.2 mmol) were suspended in
a solvent containing different hydrogen sources (2 mL). Before the reaction, the
suspension was degassed and saturated with N to remove any dissolved O . After the
2
2
reaction, the resultant suspension was filtered through a porous membrane (20 μm in
diameter) and the filtrate was analyzed by GC-MS and GC-FID (Shimadzu GC-2014)
equipped with an HP-5 capillary column.
The hydrogenation of nitrobenzene to produce azobenzene was investigated over
3
.0 wt% CQDs/ZnIn S nanocomposite with the presence of different amount of bases,
2 4
while the hydrogenation of nitrobenzene to produce azoxybenzene was carried out in a
reaction medium in absence of TEOA. After irradiated with the LED lamp, the resultant
suspension was filtered through a porous membrane (20 μm in diameter). The filtrate
was extracted with CH Cl and the subnatant was analyzed by GC.
2
2
Results and discussion
ZnIn S and CQDs/ZnIn S nanocomposites were prepared following previously
2
4
2 4
4
0
reported method. CQDs with dimension of ca. 5 nm were synthesized via thermolysis
of citric acid, which was used for the assembly of CQDs/ZnIn S nanocomposites
2
4
(
Supporting Fig. S1). The as-obtained CQDs/ZnIn S nanocomposites (take 3.0 wt%
2 4
CQDs/ZnIn S for example), show similar XRD patterns as that of bare ZnIn S
2 4
2
4
4
3
(
Supporting Fig. S2). The SEM image of the CQDs/ZnIn S nanocomposite shows
2 4
microspheres of ca. 2.5 µm, which are consists of numerous interwoven ultrathin
6

nanosheets (Supporting Fig. S3). Although the diffraction peaks corresponding to
CQDs are not observed in the XRD patterns, the successful incorporation of CQDs in
the as-obtained CQDs/ZnIn S nanocomposite can be confirmed by its TEM, FT-IR
2
4
4
4
and XPS (Supporting Fig. S4-S6). As compared with bare ZnIn S , the morphology
2
4
of CQDs/ZnIn S nanocomposite does not change obviously, indicating that the
2
4
existence of CQDs do not influence the self-assembly of ZnIn S microspheres
2
4
(
Supporting Fig. S3 and S4). The UV-vis DRS of CQDs/ZnIn S nanocomposite shows
2 4
absorption edge extended to 520 nm, in accordance with the band gap of ZnIn S (ca.
2
4
2
.4 eV). Although the incorporation of CQDs in ZnIn S does not change the band gap
2 4
of ZnIn S , CQDs/ZnIn S nanocomposite shows a slightly enhanced absorption
2
4
2 4
beyond 500 nm, ascribable to the absorption of CQDs (Supporting Fig. S7).
The performance of CQDs/ZnIn S nanocomposites for the reduction of
2
4
nitrobenzene under visible light was investigated. The reaction was initially carried out
in MeOH over 1.0 wt% CQDs/ZnIn S nanocomposite, with triethanolamine (TEOA)
2
4
as the hydrogen source. It was found that 51% of nitrobenzene was converted after
irradiated for 12 h, with the only detectable product to be aniline in a reaction medium
containing TEOA: MeOH (1:1) (Table 1, entry 1). No conversion of nitrobenzene was
observed in dark or in absence of 1.0 wt% CQDs/ZnIn S nanocomposite, indicating
2
4
that reduction of nitrobenzene to aniline is truly induced by photocatalysis over
CQDs/ZnIn S nanocomposite (Table 1, entries 2-3). Bare ZnIn S showed inferior
2
4
2 4
performance, with only 23% of nitrobenzene converted and a yield of 19% to aniline
under otherwise similar condition (Table 1, entry 4), suggesting that CQDs promotes
the photocatalytic reduction of nitrobenzene to aniline over ZnIn S . In addition, the
2
4
amount of CQDs in the CQDs/ZnIn S nanocomposites influenced the performance.
2
4
The conversion of the nitrobenzene improved with the amount of CQDs in the
nanocomposites and then decreased (Table 1, entries 1, 5-6), with an optimized
performance observed over 3.0 wt% CQDs/ZnIn S nanocomposite, in which 79% of
2
4
nitrobenzene was converted, with a yield of 76% to aniline in 12 h (Table 1, entry 5).
7

Solvents and hydrogen sources also influenced the performance of the reduction
of nitrobenzene to produce aniline. The change of the solvent from MeOH to either
EtOH, MeCN or H O led to lowering of the conversion of nitrobenzene, with 48%, 72%
2
and 38% of nitrobenzene converted for the reactions carried out in EtOH, MeCN and
H O respectively (Supporting Table 1, entries 1-3). Although the reduction of
2
nitrobenzene to aniline occurred when either ammonium formate or formic acid was
used as the hydrogen source, the conversion of nitrobenzene over these two systems
was low, with only 38% and 41% of nitrobenzene converted under otherwise similar
conditions (Supporting Table 1, entries 4-5). Alcohols are even worse hydrogen sources
for this reaction, with only 11% of nitrobenzene converted for reaction carried out in
the presence of benzyl alcohol and no aniline was detected when isoproponal was used
(Supporting Table 1, entries 6-7). Therefore, among the investigated systems, an
optimized performance for photocatalytic reduction of nitrobenzene to produce aniline
was obtained in mixed MeOH/TEOA (1:1) over 3.0 wt% CQDs/ZnIn S .
2
4
The time-dependent reduction of nitrobenzene in a mixed TEOA/MeOH (1:1)
over 3.0 wt% CQDs/ZnIn S nanocomposite showed that the amount of aniline
2
4
increased with the irradiation time, accompanied by the gradual consumption of
nitrobenzene (Fig. 1). No other products were detected during the whole investigated
process although NBS and NPI were assumed to be the intermediates during the
hydrogenation of nitrobenzene to produce aniline, based on the well-known pathways
proposed by Haber. It is either because that the hydrogenation of NBS and NPI are
much faster than the reduction of nitrobenzene to produce NBS, or nitrobenzene can be
hydrogenated to produce aniline directly over CQDs/ZnIn S nanocomposite, a
2
4
1
5
phenomenon previously reported over Au/TiO . In such a reaction system, a full
2
conversion of nitrobenzene can be realized after irradiation for 16 h (Table 1, entry 7).
In addition, it was found that no obvious decrease of the activity of 3.0 wt%
CQDs/ZnIn S nanocomposite was observed after three cycling tests (Fig. S8).
2
4
When base like NaOH was introduced into the above reaction system, in addition
8

to aniline, azoxybenzene and azobenzene were also obtained. Not only the conversion
of nitrobenzene, but also the distribution of the products varied with the amount of
NaOH introduced. When 2 mmol of NaOH was added, 52% of nitrobenzene was
converted after irradiated for 12 h, with the yield to aniline, azobenzene and
azoxybenzene to be 9%, 5% and 36% respectively, indicating that the alkaline condition
promotes the condensation pathway to produce azoxybenzene and azobenzene (Table
1
, entry 8). The increase of the amount of NaOH to 4 mmol led to enhanced conversion
of nitrobenzene (80%), with the products obtained to be aniline (37%), azobenzene
31%) and azoxybenzene (11%) (Table 1, entry 9), suggesting that enhanced alkalinity
(
promoted the conversion of nitrobenzene as well as the hydrogenation of azoxybenzene
to produce azobenzene over CQDs/ZnIn S . The further increase of the amount of
2
4
NaOH to 6 mmol, by simultaneous addition of water to dissolve the base, led to a full
conversion of nitrobenzene, with a significantly improved yield to azobenzene (73%),
although a small amount of azoxybenzene (9%) and aniline (18%) was still present after
irradiated for 12 h (Table 1, entry 10). Prolonging the irradiation time to 13 h, all the
azoxybenzene was converted, with a yield of 84% to azobenzene. However, aniline was
still present as a minor product (16%) in the system. Time-dependent reactions in a
reaction medium of TEOA/MeOH/H O (2:1:1) in the presence of 6 mmol of NaOH
2
revealed that the amount of nitrobenzene decreased with the irradiation time, with a full
conversion of nitrobenzene achieved after irradiated for 7 h (Fig. 2). The amount of
azoxybenzene increased in the first 7 h, and then decreased with the concomitant
increase of azobenzene, with a maximum yield to azoxybenzene in the system to be 82%
after irradiated for 7 h, suggesting that azoxybenzene is the intermediate during the
transformation of nitrobenzene to azobenzene. After an optimum yield of 84% to
azobenzene was obtained after irradiated for 13 h, the further irradiation led to a
decrease of the yield to azobenzene, together with an slightly improved yield to aniline,
indicating that the further hydrogenation of azobenzene led to the production of aniline,
although no intermediates like hydrazobenzne was observed in this process.
9

Controlled experiment using azoxybenzene as the substrate revealed that
azoxybenzene was totally converted to azobenzene in a similar reaction medium over
3
.0 wt% CQDs/ZnIn S after irradiated for 13 h (Table 1, entry 11), although such a
2 4
transformation did not occur over CQDs/ZnIn S nanocomposites without light
2
4
irradiations (Table 1, entry 12), indicating that the hydrogenation of azoxybenzene to
produce azobenzene is really induced by photocatalysis over CQDs/ZnIn2S4
nanocomposite. In addition, the transformation of azoxybenzene to azobenzene did not
occur over bare ZnIn S , suggesting that CQDs in the nanocomposites is indispensable
2
4
for the hydrogenation of azoxybenzene (Table 1, entry 13). The promoting of the
hydrogenation of azoxybenzene to produce azobenzene by introduction of CQDs is not
unexpected since our previous studies have also revealed that electron-rich CQDs can
promote the hydrogenation of imines to amines due to the presence of Frustrated Lewis
acid-base Pairs (FLPs)-like separated positive and negative charge centers in their
structure.42
In addition, the hydrogenation of nitrobenzene carried out in a reaction medium of
MeOH/H O (1:1) in the presence of 6 mmol of base (NaOH), i.e., a TEOA-free reaction
2
medium, led to a full conversion of nitrobenzene to yield azoxybenzene after irradiated
for 12 h (Table 1, entry 14). An increase of the reaction time to 16 h did not lead to the
transformation of azoxybenzene to azobenzene, indicating that the hydrogenation of
azoxybenzene to produce azobenzene can only be realized using a strong hydrogen
source like TEOA. This is not difficult to understand since TEOA, with a pKa value of
ca. 7.77, is a much stronger hydrogen source as compared with MeOH, with a pKa
value of 15.49. 45, 46
Based on what Haber has proposed, the hydrogenation of nitrobenzene follows
different pathways as shown in Scheme 1, leading to the formation of different products.
In the direct pathway (Pathway I in Scheme 1), nitrobenzene is first reduced to NBS
and then to corresponding NPH, which can be further hydrogenated to produce aniline.
Meanwhile, the alkaline-promoted condensation between NBS and NPH also occurs to
1
0

give azoxybenzene (Pathway II in Scheme 1). The azoxybenzene can further be
hydrogenated in a series of consecutive steps to produce azobenzene, hydrazobenzene
and finally to aniline (Pathway III in Scheme 1). Therefore, in addition to the catalyst,
the modifications on the reaction medium as well as the hydrogen source can also tune
the rate of the different steps during the hydrogenation of nitrobenzene, which can
ensure a highly chemoselective hydrogenation of nitrobenzene to produce aniline,
azoxybenzene and azobenzene.
With regard to the current CQDs/ZnIn S nanocomposite, the chemoselective
2
4
hydrogenation of nitrobenzene to produce aniline, azoxybenzene and azobenzene can
be rationalized as following (shown in Scheme 2, Path 1). ZnIn S is a visible light
2
4
responsive semiconductor. When irradiated with visible light, ZnIn S is excited to
2
4
generate electrons and holes. Coupled with a strong hydrogen source like TEOA, the
photogenerated electrons can hydrogenate nitrobenzene to produce aniline, probably
through NBS and NPH as the intermediates, even though these two compounds were
not detected during the time-dependent reactions. However, the reactions carried out
using either NBS and NPH instead of nitrobenzene occurs quickly to produce aniline,
implying that the absence of NBS and NPH during the hydrogenation of nitrobenzene
may be due to their fast hydrogenation to produce aniline as compared with their
production from nitrobenzene. TEOA also acts as the electron donor to react with the
photogenerated holes in ZnIn S and complete the whole photocatalytic cycle. When
2
4
CQDs are deposited on the surface of ZnIn S , the electrons photogenerated in ZnIn S
2 4
2
4
can transfer to CQDs to form electron-rich CQDs, due to a more negative conduction
band of ZnIn S as compared with that of CQDs. With the assistance of TEOA as a
2
4
hydrogen source, the electron-rich CQDs can act like FLPs to promote the stepwise
hydrogenation of nitrobenzene to NBS, NPH and finally to aniline, resulting in an
improved yield to aniline over CQDs/ZnIn S nanocomposite as compared with that
2
4
over bare ZnIn S .
2
4
The introduction of the base like NaOH to the reaction system promotes the
1
1

condensation between NBS and NPH to produce azoxybenzene over CQDs/ZnIn2S4
4
7
nanocomposite (shown in Scheme 2, Path 2). Although azoxybenzene can be further
hydrogenated to produce azobenzene over CQDs/ZnIn S nanocomposite, such a
2
4
transformation is completely inhibited over CQDs-free bare ZnIn S or a reaction
2
4
medium without TEOA, indicating that only a strong enough catalyst for hydrogenation
CQDs) and a strong hydrogen source (TEOA) can realize the hydrogenation of
(
azoxybenzene to produce azobenzene. As mentioned above, electron-rich CQDs can
act as a catalyst for hydrogenation due to the presence of FLPs-like separated positive
and negative charge centers. Based on Scheme 2, it is therefore possible to realize a
controllable and an efficient chemoselective hydrogenation of nitrobenze to produce
aniline, azoxybenzene and azobenzene over CQDs/ZnIn S nanocomposite.
2
4
Conclusion
In summary, the self-assembly of ZnIn S in the presence of preformed CQDs led
2
4
to the formation of CQDs/ZnIn S nanocomposites, which can be used for visible light
2
4
induced hydrogenation of nitrobenzene. A controllable and efficient chemoselective
hydrogenation of nitrobenzene to produce aniline, azobenzene and azoxybenzene can
be realized over 3.0 wt% CQDs/ZnIn S nanocomposite under visible light via simply
2
4
regulating the reaction medium including the alkalinity and the hydrogen source. This
study not only demonstrated a noble metal-free photocatalytic system for efficient
visible light initiated chemoselective hydrogenation of nitrobenzene to produce aniline,
azobenzene and azoxybenzene, but also highlights the great potential of photocatalysis
in organic syntheses.
Acknowledgements
This work was supported by the NSFC (21872031 and U1705251). Z. Li
acknowledges the Award Program for Minjiang Scholar Professorship for financial
support.
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1
5

Captions for Figures
Fig. 1 Time-dependent changes in the amounts of nitrobenzene and aniline during
hydrogenation of nitrobenzene in a mixed solvent of TEOA/MeOH (1:1) over irradiated
3
.0 wt% CQDs/ZnIn S nanocomposite.
2 4
Fig. 2 Time-dependent changes in the amounts of nitrobenzene and the products during
hydrogenation of nitrobenzene in a reaction medium of TEOA/MeOH/H O (2:1:1) and
2
6
mmol of NaOH over irradiated 3.0 wt% CQDs/ZnIn S nanocomposite.
2 4
Caption for Table
Table 1 Hydrogenation of nitrobenzene under different conditions.
Captions for Schemes
Scheme 1 Generally accepted pathways for catalytic hydrogenation of nitrobenzene
proposed by Haber.
Scheme 2 Proposed mechanism for chemoselective hydrogenation of nitrobenzene to
produce different products over CQDs/ZnIn S nanocomposite.
2
4
1
6

Fig. 1
1
7

Fig.2
1
8

Table 1
Yield/%
1b
Entry
Catalyst
Solvent
Base
Light Time/h Conv./%
1
a
1c
1
2
3
4
5
6
7
8
9
1.0 wt% CQDs/ZnIn2S4
1.0 wt% CQDs/ZnIn2S4
-
TEOA/MeOH (1:1)
TEOA/MeOH (1:1)
TEOA/MeOH (1:1)
TEOA/MeOH (1:1)
TEOA/MeOH (1:1)
TEOA/MeOH (1:1)
TEOA/MeOH (1:1)
TEOA/MeOH (1:1)
TEOA/MeOH (1:1)
-
ON
OFF
ON
ON
ON
ON
ON
ON
ON
ON
ON
OFF
ON
ON
12
12
12
12
12
12
16
12
12
12
13
13
13
12
51
-
50
-
-
-
-
-
-
-
-
-
-
-
ZnIn2S4
-
23
79
56
>99
52
80
19
76
53
>99
9
-
-
3.0 wt% CQDs/ZnIn2S4
5.0 wt% CQDs/ZnIn2S4
3.0 wt% CQDs/ZnIn2S4
3.0 wt% CQDs/ZnIn2S4
3.0 wt% CQDs/ZnIn2S4
3.0 wt% CQDs/ZnIn2S4
3.0 wt% CQDs/ZnIn2S4
3.0 wt% CQDs/ZnIn2S4
ZnIn2S4
-
-
-
-
-
-
-
-
-
2 mmol NaOH
4 mmol NaOH
6 mmol NaOH
6 mmol NaOH
6 mmol NaOH
6 mmol NaOH
6 mmol NaOH
5
31
36
11
37
1
0
TEOA/MeOH/H O (2:1:1)
2
>
>
99
99
-
18
-
73
9
1
1
1
1a
2a
3a
TEOA/MeOH/H O (2:1:1)
2
>99
-
-
TEOA/MeOH/H O (2:1:1)
2
-
-
-
-
TEOA/MeOH/H O (2:1:1)
2
-
-
-
1
4
3.0 wt% CQDs/ZnIn2S4
MeOH/H O (1:1)
2
>
99
-
>99
a
Reaction conditions: 0.2 mmol nitrobenzene, 10 mg catalyst, 2 mL solvent, N , LED irradiation. azoxybenzene as substrate.
2
1
9

Scheme 1
2
0

Scheme 2
2
1

Graphical abstract
2
2

The authors declared that they have no conflicts of interest to this work.
Highlights


Efficient hydrogenation of nitrobenzene over CQDs/ZnIn S under visible light
2 4
Chemoselective production of aniline, azoxybenzene and azobenzene over
CQDs/ZnIn S under visible light
2
4

Chemoselectivity can be tuned via regulating the reaction medium including
alkalinity and the hydrogen source
2
3