Visible-light-initiated one-pot clean synthesis of nitrone from nitrobenzene and benzyl alcohol over CdS photocatalyst

Article abstract of DOI:10.1016/j.jcat.2018.12.011

The controlled visible-light mediated conversion of nitroaromatics to versatile nitrogen-containing intermediates is of great significance but still remains a challenge. Herein, we report for the first time a facile visible-light-initiated one-pot strategy for clean and efficient synthesis of nitrone from nitrobenzene and benzyl alcohol. It integrates the controlled photocatalytic reduction of nitrobenzene to phenylhydroxylamine by photogenerated electrons with the selective photocatalytic oxidation of benzyl alcohol to benzaldehyde by photoinduced holes over a low-cost CdS photocatalyst with a suitable reduction capability, followed by spontaneous condensation of the as-formed hydroxylamine and aldehyde at ambient pressure and room temperature. Furthermore, by modulating cocatalyst and illumination time, for the conversion of the same reactants, the other two useful nitrogen-containing compounds, imine and secondary amine, were also successfully synthesized. The reaction mechanisms for flexible synthesis of the three target products are proposed.

Full text of DOI:10.1016/j.jcat.2018.12.011

Journal of Catalysis 370 (2019) 97–106
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Visible-light-initiated one-pot clean synthesis of nitrone from
nitrobenzene and benzyl alcohol over CdS photocatalyst
Weiwei Yu , Xinwen Guo , Chunshan Song a,b, Zhongkui Zhao a,
a
a
⇑
a
State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering and Chemical Engineering, Pennsylvania State
b
University, University Park, PA 16802, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The controlled visible-light mediated conversion of nitroaromatics to versatile nitrogen-containing inter-
mediates is of great significance but still remains a challenge. Herein, we report for the first time a facile
visible-light-initiated one-pot strategy for clean and efficient synthesis of nitrone from nitrobenzene and
benzyl alcohol. It integrates the controlled photocatalytic reduction of nitrobenzene to phenylhydroxy-
lamine by photogenerated electrons with the selective photocatalytic oxidation of benzyl alcohol to ben-
zaldehyde by photoinduced holes over a low-cost CdS photocatalyst with a suitable reduction capability,
followed by spontaneous condensation of the as-formed hydroxylamine and aldehyde at ambient pres-
sure and room temperature. Furthermore, by modulating cocatalyst and illumination time, for the con-
version of the same reactants, the other two useful nitrogen-containing compounds, imine and
secondary amine, were also successfully synthesized. The reaction mechanisms for flexible synthesis of
the three target products are proposed.
Received 5 October 2018
Revised 6 December 2018
Accepted 12 December 2018
Keywords:
One-pot strategy
Clean synthesis
Nitrone
Imine and secondary amine
Nitrobenzene and benzyl alcohol
Photocatalysis
Ó 2018 Elsevier Inc. All rights reserved.
1
. Introduction
The nitrogen-containing compounds including nitrones, imines
deal of chemical reagents. Moreover, a careful control of the reac-
tion conditions is required to obtain high selectivity. Regarding
Route C, the oxime anions as an intermediate is preliminarily syn-
thesized by the stoichiometric reaction of aldehyde with hydroxy-
lamine hydrochloride followed by reacting with sodium hydroxide,
and then the nitrone is synthesized by N-alkylation [10]. The above
multistep processes (Routes A-C) would lead to a high production
cost and a heavy environmental pollution. In a direct and green
approach, nitrone can be obtained by one-pot reaction from nitro
compounds and aldehydes (Route D). Cisneros et al. [11] and Li
et al. [12] successfully prepared nitrone from nitro compounds
and secondary amines, etc. are versatile organic building blocks for
the synthesis of fine chemicals, pharmaceuticals and agricultural
chemicals [1]. Nitrones can be used as radical spin-traps [2],
antioxidants [3], and enzyme inhibitors [4] due to their spin-
trapping ability. Many nitrogen-containing heterocycles, such as
isoxazolidines and isoxazolines, can be obtained from nitrones
through 1,3-dipolar cycloadditions [5]. These heterocycles can be
transformed into amino-alcohols or b-lactams [6], which are used
to produce many prescribed drugs. Therefore, the development of a
facile and clean method for the synthesis of valuable nitrogen-
containing compounds, especially nitrone, is very important.
Up to now, nitrones are commonly prepared by the following
traditional synthetic routes (Fig. 1): (1) oxidation of imines [7] or
secondary amines [8] (Route A); (2) condensation of hydroxylami-
nes with carbonyl compounds (Route B) [9]; (3) N-alkylation of
oximes (Route C) [10]; (4) a tandem process including a reduction
and aldehydes over Pt/C and Pt/MOF under 5–34 bar of high H
2
pressure, in which the tandem process circumvents the unstable
hydroxylamine storage issues and also simplifies synthetic step.
However, the noble metal Pt and harsh reaction conditions (high
2
H pressure) are required. The Pt/MOF system is only applicable
for conversion of nitromethane [12]. Moreover, the deep reduction
of reactants, intermediates and products depresses the selectivity
to nitrone. Thus the large-scale industrial application is limited.
Therefore, it is highly desirable to develop a facile and practical
method to synthesize nitrone.
Heterogeneous photocatalysis has been considered as one of the
most promising alternatives for selective synthesis of organic
chemicals [13]. From references [14], alcohols can not only serve
as substrate to be oxidized to aldehyde by photogenerated holes,
2
of nitro compounds under higher H pressure with a subsequent
condensation with aldehyde (Route D). Routes A and B consist of
multistep oxidation and reduction processes, consuming a great
⇑
Corresponding author.
E-mail address: zkzhao@dlut.edu.cn (Z. Zhao).
https://doi.org/10.1016/j.jcat.2018.12.011
021-9517/Ó 2018 Elsevier Inc. All rights reserved.
0

9
8
W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
to phenylhydroxylamine by photoinduced electrons over a low-
cost CdS photocatalyst with a suitable reduction capability and
the photocatalytic oxidation of benzyl alcohol to benzaldehyde
by photogenerated holes, followed by a spontaneous condensation
of the as-formed phenylhydroxylamine and benzaldehyde. Both
photocatalyzed reduction and oxidation half-reactions can be effi-
ciently promoted each other owing to the efficient recombination
inhibition of photogenerated charge carriers. In addition, by tuning
cocatalyst and illumination time, for the conversion of the same
reactants, the other two versatile nitrogen-containing intermedi-
ates, imine and secondary amine, have been successfully synthe-
sized. Finally, the tentative reaction mechanisms for flexible
synthesis of the three target products by the developed visibly-
light-initiated one-pot synthetic strategy are proposed. This work
not only presents a novel and efficient method for a facile synthesis
of nitrone, but also reveals the relationship between the properties
of different CdS-based catalysts and the selectivity to nitrone,
imine and secondary amine.
2
. Experimental section
2.1. Chemicals
Fig. 1. Proposed synthetic routes of nitrones.
Cadmium acetate (Cd(CH
3
COO)
2
Á2H
2
O), nickel chloride hexahy-
CSNH ), ammonium hepta-
O) and benzyl alcohol (C O) were
purchased from Tianjin Guangfu Fine Chemical Research Institute.
Thiourea (CH S) was gained from Tianjin Bodi Chemical Co. Ltd.
Ethylene glycol (C ) and acetonitrile (C N) were gained
from Tian in Fuyu Fine Chemical Co. Ltd. Nitrobenzene (C NO
was purchased from Fuchen (Tianjin) Chemical reagent Co. Ltd.
Phenylamine (C N) was purchased from Tianjin Damao Chemi-
cal Reagent Factory. Trifluorotoluene (C ) was gained from
Aladdin. Benzaldehyde (C CHO) was purchased from Xilong
drate (NiCl
molybdate ((NH
2
Á6H
2
O), thioacetamide (CH
Mo
24Á4H
3
2
but also provide hydrogen for reduction reactions. Nitro com-
pounds can be transformed into amines by photoinduced electrons
through the intermediates including hydroxylamines and nitroso
compounds [15]. If both photooxidation and photoreduction
simultaneously take place in a same reaction system, the two
half-reactions can be remarkably promoted each other owing to
the efficient recombination inhibition of photoinduced charge car-
riers [16]. Moreover, from Route B, we know that nitrone can be
prepared via condensation of hydroxylamine with aldehyde.
Therefore, we envisaged the nitrone might be synthesized from
4
)
6
7
O
2
7 8
H
4 2
N
2
6
H O
2
2 3
H
6
H
5
2
)
7
H
9
7
5 3
H F
6
H
5
Chemical Co. Ltd. All the chemicals are analytical grade and
directly used without further processing. Deionized water was
used in all procedures and photocatalytic experiments were imple-
mented under nitrogen atmosphere.
nitro compound and alcohol through
a facile visible-light-
mediated one-pot strategy over low-cost CdS photocatalyst (Route
E). The as-formed hydroxylamines from nitro compounds directly
react with the as-produced aldehydes from alcohols to produce
nitrones. The proposed approach not only takes over the merits
2.2. Preparation of CdS nanoparticles
2
of Route D, but also avoids the use of high H pressure and noble
metals that definitely required in Route D. In addition, benzyl alco-
hol is used to replace benzaldehyde, which further simplifies the
reaction step and also reduces cost. More importantly, the pro-
posed Route E can also overcome the shortcomings of Route D
regarding the deep reduction of the reactants, intermediate and
The CdS NPs were prepared according to the previously
reported literature with some modifications [18]. Briefly, 0.533 g
cadmium acetate (2 mmol) and 0.451 g thioacetamide (6 mmol)
were dissolved in 50 ml ethylene glycol and 5 ml H O mixed solu-
2
tion by ultrasonic processing. Above mixed solution was trans-
ferred into an 80 ml Teflon-lined autoclave and maintained at
2
products under high H pressure, which hopefully allows us to
obtain high selectivity to nitrone. In order to realize the proposed
Route E, to efficiently control reduction of nitrobenzene to hydrox-
ylamine is the key for a facile, low-cost and sustainable production
of nitrone. Till now, only amines and imines can been obtained by
light-driven one-pot synthesis from nitrobenzene and benzyl alco-
hol because of the difficulty of precisely controlling reduction
degree of nitrobenzene to hydroxylamine [17]. Unfortunately, to
the best of our knowledge, there is no report regarding the photo-
catalytic synthesis of phenylhydroxylamine, not to mention
one-pot synthesis of nitrones from nitro compound and alcohol.
Furthermore, the relationship between selectivity of the produc-
tion (imines or secondary amines) and the nature of catalyst
remains unclear.
1
60 °C for 18 h. Next, the autoclave was cooled to room tempera-
ture and the orange precipitates were washed three times with
deionized water and once with absolute alcohol via centrifugation.
The final product was dried at 65 °C in an oven and grinded by
using the agate mortar.
2.3. Preparation of MoS
2
/CdS
The MoS was decorated on the CdS NPs by hydrothermal
2
method [19]. In a typical process, 100 mg obtained CdS NPs were
dispersed in 50 ml deionized water by ultrasonic processing, then,
39 mg of (NH
4
)
6
Mo
7
O
24Á4H
2
O and 12 mg thiourea were dissolved
Herein, for the first time, we report a facile visible-light-
initiated one-pot strategy for efficient synthesis of nitrone with
in the above mixture by stirring 10 min. The mixture was trans-
ferred into an 80 ml Teflon-lined autoclave and maintained at
160 °C for 18 h. Next, the autoclave was cooled to room tempera-
ture and the dark green precipitates were washed with deionized
water and alcohol via centrifugation. The final product was dried
9
1% selectivity at 77% conversion after only 25 min of illumination
by visible light (89% selectivity at 91% conversion, 30 min) by using
nitrobenzene and benzyl alcohol as starting reactants. This method
integrates the controlled photocatalytic reduction of nitrobenzene
2
at 65 °C in an oven. The theoretical loading ratio of MoS on CdS

W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
99
Fig. 2. SEM images of CdS (a), MoS
2
2
/CdS (b), and Ni/CdS (c). The photographs of CdS (inset in a), MoS /CdS (inset in b), and Ni/CdS (inset in c).
aqueous (60
l mol, 1 g/100 ml) was added above mixed solution.
The mixed solution was sealed in a flask (100 ml) and thoroughly
replaced the air by N
was irradiated with
2
under stirring. Subsequently, the mixture
300 W Xe lamp (PLSSXE300/300UV,
a
Perfectlight) with a 420 nm cut-off filter for 5 min. The Ni/CdS
was obtained via centrifugation and dried at 65 °C in an oven.
The theoretical loading ratio of metallic nickel on CdS is 3.5 wt%,
and were referred to Ni/CdS-3.5 (abbreviated in Ni/CdS).
Analogously, the resultant Ni-CdS composites with various
amounts of Ni were denoted as Ni/CdS-0.5, Ni/CdS-2, Ni/CdS-5
and Ni/CdS-6.5, respectively.
2.5. Characterization of materials
Scanning electron microscopy (SEM) images were obtained on a
JEOL JSM-5600LV SEM instrument. X-ray diffraction (XRD) was
conducted on a D/Max 2400 diffractometer with monochromatic
2
Fig. 3. XRD patterns of CdS, MoS /CdS and Ni/CdS.
Cu K
ments were performed on an ESCALAB 250 XPS system with a
monochromatic Al K X-ray source. All the binding energies were
a source. X-ray photoelectron spectroscopy (XPS) measure-
is 5 wt%, and were referred to MoS
CdS). Analogously, the resultant MoS
2
/CdS-5 (abbreviated in MoS
2
/
2
-CdS composites with vari-
a
ous amounts of MoS
2
were denoted as MoS
2 2
/CdS-1, MoS /CdS-3,
referenced to the C 1s peak at 284.8 eV. The Brunauer-Emmett-
Teller (BET) method was utilized to calculate the specific surface
area, and the pore size distributions was calculated from an
adsorption branch of the isotherm via the Barrett–Joyner–Halenda
MoS /CdS-8 and MoS
2
2
/CdS-10, respectively.
2.4. Preparation of Ni/CdS catalyst
(
BJH) model (3H-2000PS, BeiShiDe Instrument). The UV–vis diffuse
According to Ref. [14a], 100 mg obtained CdS were dispersed in
0 ml CH CN by ultrasonic processing. Then 1430 l NiCl
Á6H
reflectance spectra (DRS) were recorded on a JASCO V-550 UV–vis
spectrometer. The Mott-Schottky plots measurements were per-
2
3
l
2
2
O

1
00
W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
formed on an electrochemical workstation (CHI660e) in 0.2 M Na
2
-
SO aqueous solution in a standard three electrode system with the
4
as-prepared samples coated indium tin oxide (ITO) glass as the
working electrode, Ag/AgCl as reference electrode, and the Pt wire
as the counter electrode. The alternating current frequency is
1
200 Hz.
2.6. Photocatalytic performance test
A flask (10 ml, Synthware) was used as the reaction container,
and after 0.2 mmol nitrobenzene, 3 ml benzyl alcohol and 10 mg
catalyst had been added, the flask was sealed and thoroughly
replaced the air by N under stirring. Subsequently, the mixture
2
was illuminated by the Xe lamp for a certain time with continuous
stirring. The reaction temperature was kept at 20 °C by a circulat-
2
ing bath during the experiment process. The generated H was
quantified by a gas chromatography (GC 9790 II, FuLi Instruments)
equipped with a thermal conductive detector (TCD) and nitrogen
as the carrier gas. The organic products were quantified by the
GC and liquid chromatogram analysis (LC). The GC with a SE-54
capillary column (30 m  0.32 mm  0.50
lm), flame ionization
detector (FID), argon as the carrier gas and trifluorotoluene as
the internal standard. The LC was performed HP 1100 liquid chro-
matogram with a Nova – Pak C18 column and DAD detector, the
2
mobile phase H O:methanol 3:7 and a flow rate of 1.0 ml/min.
The qualitative analysis of the products was performed by gas
chromatography-mass spectrometry (GC–MS, HP6890/5973MSD
and 7000B, Agilent).
3
. Results and discussion
3.1. Preparation and characterization of CdS based photocatalysts
CdS nanoparticles (NPs) were synthesized by a previously
reported solvothermal method with some modifications [18]. The
MoS or Ni co-catalyst was deposited on the surface of CdS NPs
to prepare MoS /CdS and Ni/CdS by hydrothermal and photodepo-
sition methods, respectively [19,14a]. Scanning electron micro-
scope (SEM) images of the CdS, MoS /CdS and Ni/CdS are shown
2
2
2
in Fig. 2. The CdS features irregular nanoparticles with the size
from 30 nm to 140 nm, and there is no significant effect on the
morphology and size of CdS NPs when decorated MoS
Moreover, the photographs (inset in Fig. 2) of CdS, MoS
Ni/CdS feature the orange color for CdS NPs, while the MoS
and Ni/CdS show palm green1 and yellow green nanoparticles,
respectively. This indicates that the MoS and Ni have been success-
fully decorated on CdS NPs. The X-ray diffraction (XRD) patterns
2
and Ni.
/CdS and
/CdS
2
2
2
(
(
Fig. 3) confirm a mixture of cubic (PDF#65-2887) and hexagonal
PDF#41-1049) phases of CdS NPs. There is not visible diffraction
Fig. 4. XPS spectra of CdS: (a) full spectrum, (b) Cd 3d, (c) S 2p.
peak towards molybdenum sulfide or Ni species on MoS
Ni/CdS to be resolved, ascribed to the low loading and/or small size.
X-ray photoelectron spectroscopy (XPS) spectra of CdS, MoS /CdS
and Ni/CdS are shown in Figs. 4–6. The Mo 3d spectrum can be fitted
2
/CdS and
further showing no obvious change in the CdS structure after
loading MoS or Ni cocatalysts.
2
2
The photocatalysis is strongly dependent on light absorption
capacity and the charge-carriers separation efficiency, mainly
affected by the energy band structure of photocatalyst. Further-
more, the position of valence band (VB) and conduction band
(CB) of a photocatalyst determines its oxidation and reduction
capability, which is crucial to the product distribution of photocat-
alytic organic reactions. The band gap and the positions of VB and
CB were determined by combining UV–Vis diffuse reflectance
spectroscopy and VB-XPS (Fig. 8). The UV–Vis diffuse reflectance
spectra (Fig. 8a) indicate the increased visible light absorption with
4
+
into two subsets of peaks at 228.2 and 231.7 eV attributed to Mo
,
and at 232.7 and 235.4 eV ascribed to Mo6 . The Mo was found to
+
4+
6+
be the dominant oxidation state, and the existed Mo can be
ascribed to exposure to air [20]). From Ni 2p spectrum in Fig. 6d,
the peak appearing at 852.2 eV is assigned to metallic Ni, and at
856.0 eV ascribed to NiO. The existence of NiO originates from the
oxidation of the deposited Ni while the catalyst is exposed to air
[
1
2
14a]. The surface area of CdS NPs, MoS /CdS and Ni/CdS are
2
À1
2
À1
and 14.5 m2
À1
6.1 m
g
, 16.5 m
g
g
, respectively (Fig. 7),
2 x
a wide absorption tail if MoS or Ni (NiO ) is loaded on CdS NPs.
1
The corresponding Tauc plots coming from the UV–Vis diffuse
reflectance spectra are shown in Fig. 8a (inset), and the band gaps
For interpretation of color in Fig. 2, the reader is referred to the web version of
this article.

W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
101
2
Fig. 5. XPS spectra of MoS /CdS: (a) full spectrum, (b) Cd 3d, (c) S 2p, (d) Mo 3d.
Fig. 6. XPS spectra of Ni/CdS: (a) full spectrum, (b) Cd 3d, (c) S 2p, (d) Ni 2p.
of CdS, MoS
tively. Based on XPS valence band spectra (Fig. 8b), the top of VB
of CdS, MoS /CdS and Ni/CdS are 1.35, 1.06 and 1.10 eV vs. E
the potential of the Fermi level), respectively. The Fermi levels
2
/CdS and Ni/CdS are 2.28, 2.23 and 2.26 eV, respec-
plots (Fig. S1) are À0.44, À0.48 and À0.57 V vs. RHE (reversible
hydrogen electrode), respectively. The top of VB of CdS, MoS /CdS
and Ni/CdS are calculated to be 1.79, 1.51 and 1.67 eV vs. RHE
[21]. Fig. 8c displays the electronic band structure of CdS, MoS
CdS and Ni/CdS. The CB of CdS is À0.49 eV. MoS /CdS and Ni/CdS
2
2
F
(
2
/
of CdS, MoS
2
/CdS and Ni/CdS are obtained from Mott-Schottky
2

1
02
W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
3.2. Photocatalytic properties for nitrone clean synthesis
Based on the results of controlled experiments, both imine and
nitrone can be formed by the spontaneous condensation of ben-
zaldehyde with aniline (Fig. S2) and phenylhydroxylamine
(Figs. S3 and S4) at ambient pressure and room temperature,
respectively. However, nitrosobenzene cannot react with ben-
zaldehyde (Fig. S5). Therefore, to controllably obtain phenylhy-
droxylamine without further reduction to aniline is a key for the
synthesis of nitrone with high selectivity. With respect to the pro-
posed Route E for nitrone synthesis, we design a one-pot two-stage
process: one is photocatalytic stage containing the controlled
reduction of nitrobenzene to phenylhydroxylamine and the selec-
tive oxidation of benzyl alcohol to benzaldehyde; the other is the
spontaneous condensation between the as-formed phenylhydrox-
ylamine and benzaldehyde. Fig. S6 further confirms that the
designed one-pot two-stage process works well, and the as-
formed phenylhydroxylamine at the 1st stage gradually decreases
while the desired nitrone gradually increases when the reaction
mixture stands for 2 h.
By using nitrobenzene and benzyl alcohol as reactants, the reac-
tion properties of CdS, MoS /CdS and Ni/CdS photocatalysts for
2
nitrone synthesis via one-pot two-stage approach were tested.
Table S1 summarizes the photocatalytic results for diverse illumi-
nation times, and the results for the combined photocatalytic pro-
cess and subsequent condensation is presented in Table 1. Fig. S7
shows the time-dependent change in the amounts of products
and no visible change in products distribution if the standing time
reaches up to 8 h. In order to make sure the complete condensa-
tion, we stand the photocatalytic reaction mixture more than
1
2 h. To our delight, the results verify the flexible synthesis of
nitrone, imine, and secondary amine from nitrobenzene and benzyl
alcohol over the low-cost CdS-based photocatalysts by tuning
cocatalyst and illumination time. The blank experiments
(
Table S1 and 1, entry 5,6,11,12,18,19) reveal only negligible trans-
formation of nitrobenzene and benzyl alcohol. Interestingly, in
combination of the photocatalytic process over bare CdS NPs and
the subsequently spontaneous condensation process, nitrone is
successfully synthesized. However, only trace nitrone is obtained
in the absence of subsequent condensation process (entry 1,2 in
Table S1). The extension of illumination time heightens the conver-
sion of nitrobenzene and benzyl alcohol, but it depresses the selec-
tivity to nitrone (Table 1, entry 1–4). Thanks to the suitable
reduction capability of CdS photocatalyst and the efficient inhibi-
tion of recombination of photogenerated charge carriers by the
integrating effect of the simultaneous photoreduction of nitroben-
zene and photooxidation of benzyl alcohol half-reactions, the 91%
selectivity to nitrone at 77% conversion (89% selectivity at 91% con-
version) is obtained after only 25–30 min of illumination by visible
light (Table 1, entry 1,2). In a word, we develop a facile visible-
light-initiated one-pot strategy for efficient synthesis of nitrone
at ambient pressure and room temperature. To the best of our
knowledge, this is the first report on efficient synthesis of nitrone
from nitrobenzene and benzyl alcohol. This discovery presents a
practical method for the industrial production of nitrone, a versa-
tile intermediate. In order to obtain functionalized nitrones, the
substituted nitrobenzenes were used as reaction substrates to
replace the nitrobenzene, and the results are listed in Table S2. It
can be found that the developed strategy regarding nitrone synthe-
sis can be extended to the substituted nitrobenzenes with diverse
methyl, chloro, and methoxyl groups. However, if the hydroxyl and
carboxyl groups substituted nitrobenzenes as substrates, only the
corresponding imines can be obtained, owing to the amines being
formed but not hydroxylamine. It was established that the submit-
ted benzyl alcohols could be easily oxidized to benzaldehydes.
Therefore, it can be safely say that a series of submitted nitrones
Fig. 7. N
2 2
sorption isotherms and BET surface area of CdS (a), MoS /CdS (b) and Ni/
CdS (c). Inset: pore size distribution.
indicate a more negative CB by 0.2 eV and 0.1 eV than that of CdS,
respectively. This implies that CdS has a relatively low reduction
capability [19]. On the other hand, CdS shows higher VB than
2
MoS /CdS and Ni/CdS by 0.25 and 0.12 eV, respectively, which
endows CdS with higher oxidation capability than the other two
photocatalysts. The relatively low reduction capability makes it
possible for CdS to controllably reduce nitrobenzene to phenylhy-
droxylamine by photogenerated electrons. As a result, the pro-
posed visible-light-initiated one-pot method (Route E in Fig. 1)
can be realized.

W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
103
Fig. 8. UV–Vis diffuse reflectance spectra (a) XPS valence band spectra (b), and electronic band structures (c) of the CdS, MoS
2
/CdS and Ni/CdS catalysts. Inset in a: Tauc plots.
can be obtained by our developed strategy by using the selected
substituted nitrobenzenes with diverse substituted benzyl alcohols
as starting materials. It is of great significance to elucidate the reac-
tion mechanism behind the developed visible-light-initiated one-
pot strategy for efficient synthesis of nitrone. On the basis of the
aforementioned controlled experiments (Fig. S3-6) and the reac-
tions results (entry 1–6 in Tables 1 and S1), a tentative mechanism
is proposed as depicted in Fig. 9. Upon illumination, the photoin-
duced electrons in VB migrate to CB, which leaves the equal pho-
toinduced holes in VB. Then, benzyl alcohol is oxidized to form
entry 10). The 6 h for photocatalytic process is long enough for
the subsequent condensation of the as-formed aniline with ben-
zaldehyde, confirmed by the similar results with and without sub-
sequent standing period after photocatalytic reaction (entry 10 in
Tables 1 and S1). The developed process for imine synthesis in this
work eliminates the required non-polar solvent like toluene and
cyclohexane in the previous reports [17a,b], since benzyl alcohol
serves as both reactant and solvent. The influence of amount of
MoS
Fig. S8) was also studied, and the highest selectivity was obtained
when the optimized theoretical weight ratio of MoS was 5 wt%.
The proposed mechanism for imine evolution over MoS /CdS is
2
decorated on CdS on the selectivity of imine (shown in
+
benzaldehyde and to release H by the holes. At the same time,
2
nitrobenzene is reduced to nitrosobenzene by the photogenerated
electrons, and then further hydrogenated to phenylhydroxylamine
2
illustrated in Fig. 10, similar to the previously reported process
over different photocatalysts [17a,b]. Upon illumination, the
photo-excited electrons in VB move to CB and leave the equal
amount of photo-excited holes. Then, benzyl alcohol is oxidized
+
by the electrons and H . Finally, nitrone can be obtained by the
spontaneous condensation reaction (no light, no catalyst) of the
as-formed phenylhydroxylamine with benzaldehyde at ambient
pressure and room temperature (Fig. S3,4,6) [9]. The controlled
reduction of nitrobenzene to phenylhydroxylamine but not to ani-
line by combining suitable reduction capability of CdS and illumi-
nation time is crucial to the selective synthesis of nitrone.
+
to benzaldehyde and release H by the holes. Simultaneously,
nitrobenzene is reduced to nitrosobenzene and phenylhydroxy-
lamine, and ultimately hydrogenated to amine by the electrons
+
and H . Finally, imine can be obtained by condensation amine
and benzaldehyde. The as-formed nitrone can be photoreduced
to imine under visible light irradiation.
3.3. Imine and secondary amine from nitrobenzene and benzyl alcohol
2
Distinct from CdS and MoS /CdS, using Ni/CdS as photocatalyst,
It can be clearly observed that the main product is imine but not
nitrone over MoS /CdS photocatalyst even at a low conversion of
2
nitrobenzene (Table 1, entry 7,8), ascribed to the deep reduction
of nitrobenzene to aniline owing to more negative CB potential
the photocatalytic reaction mixture mainly consists of condensa-
tion products including nitrone, imine, and secondary amine. No
obvious change in the reaction results for the process including
the subsequent condensation or not can be observed Ni/CdS photo-
catalyst while illumination time is not less than 1 h (Table S1 and
1, entry 13–17), suggesting the complete condensation in the pho-
tocatalytic reaction process. With the increase in the illumination
time, nitrone gradually decreases and then disappears after 3 h of
of MoS
of MoS
2
/CdS than that of CdS that comes from the introduction
. The selectivity to imine increases gradually with the
2
extension of illumination time, and up to 95% at 100% of nitroben-
zene conversion after 6 h of illumination by visible light (Table 1,

1
04
W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
Table 1
Photocatalytic performance of various catalysts under different visible-light irradiation times.a
Entry
Catalyst
Time (min/h)
Conv.b (%)
Benzaldehyde 6 (
l
mol)
H
2
(lmol)
Sel.c (%)
Nitrone
1
Imine
2
Amine
3
Nitrosobenzene
4
Aniline
5
1
2
3
4
CdS
CdS
CdS
CdS
–
25 min
30 min
35 min
1 h
25 min
25 min
30 min
1 h
3 h
6 h
6 h
6 h
30 min
1 h
3 h
4 h
5 h
77
91
100
100
–
138
184
205
320
–
–
–
–
0.08
–
–
–
–
91
89
79
52
–
5
9
18
45
–
–
–
4
2
3
2
–
–
5
2
2
2
–
–
2
1
–
–
–
–
–
–
–
–
1
–
–
5
2
3
3
–
–
3
3
3
3
3
–
–
–
–
–
–
–
–
–
–
–
–
1
39
57
56
–
d
5
6
e
CdS
–
–
–
–
7
8
9
MoS
MoS
MoS
MoS
–
2
2
2
2
/CdS
/CdS
/CdS
/CdS
31
63
100
100
–
100
172
312
462
–
20
37
6
0
–
70
59
89
95
–
0.05
0.08
–
10
11
12
13
14
15
16
17
18
19
d
e
MoS
2
/CdS
–
–
–
–
–
Ni/CdS
Ni/CdS
Ni/CdS
Ni/CdS
Ni/CdS
–
66
95
100
100
100
–
145
321
452
649
1151
–
0.1
0.4
2.3
18.3
28.6
–
36
20
–
–
–
59
75
58
34
25
–
f
d
e
4 h
4 h
–
–
Ni/CdS
–
–
–
–
–
a
2
Reaction condition unless otherwise noted: 0.2 mmol nitrobenzene, 3 ml benzyl alcohol, 10 mg catalyst, N 1 atm, k > 420 nm, 20 °C. After photocatalytic reaction, the
mixture was detected after a period (more than 12 h) of spontaneous condensation process without catalyst in dark.
b
Determined by GC with trifluorotoluene as an internal standard.
Determined by GC–MS.
No catalyst.
Without light irradiation.
c
d
e
f
The by-product is 2-hydroxy-N-benzylideneaniline (9%) and N-benzyl-4-hydroxyaniline (10%). Balance corresponds to small amount of unidentified products.
Fig. 10. Plausible reaction mechanism for the visible-light-initiated one-pot
synthesis of imine.
Fig. 9. Proposed mechanism for visible-light-initiated one-pot synthesis of nitrone.
illumination time, while imine increases and reaches the maxi-
mum at 1 h of illumination time and then decreases along with
an increase in secondary amine. The 57% selectivity to secondary
amine (the left is mainly imine) at 100% nitrobenzene conversion
can be obtained after 4 h of illumination time (Table 1, entry 15).
We also studied the influence of amount of Ni decorated on CdS
on the selectivity of secondary amine (shown in Fig. S9), and the
highest selectivity was obtained when the optimized theoretical
weight ratio of Ni was 3.5 wt%. This approach for visible-light pho-
tocatalytic secondary amine production over Ni/CdS shows its
advantages in contrast to the previously reported methods regard-
ing the elimination of the required noble metals and UV light [17c,
d]. The further hydrogenation of imine to secondary amine over Ni/
CdS cannot be ascribed to the higher reduction capability owing to
2
its lower reduction capability than that of MoS /CdS. Controlled
experiments were performed to understand the origin of the pro-
moted formation of secondary amine over Ni/CdS photocatalyst.
Besides the comparable catalytic performance for the reduction
of imine to secondary amine can be obtained in the absence and
presence of the added H
the yield of secondary amine over Ni/CdS is much higher than that
over MoS /CdS or CdS under either H atmosphere (Table S3, entry
2
(Table S3, entry 2,3), we can find that
2
2
2,6) or nitrogen atmosphere (Table S3, entry 3,7). This implies that
the reduction of imine to secondary amine over Ni/CdS doesn’t
result from the released H from benzyl alcohol. Even if extra H
2 2
was introduced, imine still cannot be converted into secondary
amine while acetonitrile acted as solvent to replace benzyl alcohol,
suggesting the further reduction of imine to secondary amine by

W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
105
Fig. 11. Plausible reaction mechanism for the visible-light-initiated one-pot synthesis of secondary amine.
the Ni-H hydrids that comes from the reduction of the adsorbed
alcohol on the Ni species by photogenerated electrons. Based on
the above controlled experiments, a tentative mechanism is pro-
posed as presented in Fig. 11. Initially, benzyl alcohol can be
absorbed on the surface of Ni particles. When CdS is illuminated,
the photo-excited electrons in VB will move to CB and leave equal
amount of holes, and the electron is localized from CdS to the
deposited Ni NPs. A proton from benzyl alcohol absorbed on Ni sur-
face is reduced by electron, resulting in a Ni-H hydrid and an alkox-
ide anion. Subsequently, this alkoxide anion is oxidized by the hole,
between the properties of different CdS-based catalysts and the
selectivity of nitrone, imine and secondary amine has been
revealed. This work not only presents a new and efficient method
for the synthesis of nitrone, but also opens a new avenue for a
facile one-pot synthesis of the other chemicals.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (21676046 and U1610104), the Chi-
nese Ministry of Education via the Program for New Century Excel-
lent Talents in Universities (NCET-12-0079), and the Natural
Science Foundation of Liaoning Province (grant no. 2015020200).
and the Ni-assisted homolytic cleavage of a-C-H occurs, affording
another Ni-H hydrids and the corresponding aldehyde [17c,d]. A
part of Ni-H hydrids and electrons are used to reduce nitrobenzene
to aniline and reduce imine to secondary amine and the left Ni-H
2
hydrides are used to release H .
Appendix A. Supplementary material
4
. Conclusions
In summary, we for the first time realize the visible-light- initi-
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2018.12.011.
ated one-pot synthesis of nitrone from nitrobenzene and benzyl
alcohol under ambient pressure and room temperature conditions.
Thanks to the suitable reduction capability of CdS photocatalyst
and efficient inhibition of recombination of photogenerated charge
carriers by the integrating effect of the spontaneous photoreduc-
tion of nitrobenzene and photooxidation of benzyl alcohol half-
reactions, 91% selectivity to nitrone at 77% conversion (89% selec-
tivity at 91% conversion) has been achieved after only 25–30 min
of illumination by visible light. Furthermore, by tuning cocatalyst
and illumination time, for the conversion of the same reactants,
the other two useful nitrogen-containing compounds including
imine and secondary amine have been successfully synthesized.
The tentative reaction mechanisms for flexible synthesis of the
three target products are proposed. In addition, the relationship
References
[1] a) D.R. Sun, L. Ye, Z.H. Li, Appl. Catal., B. 164 (2015) 428–432;
b) T. Song, J.E. Park, Y.K. Chung, J. Org. Chem. 83 (2018) 4197–4203;
c) M. Vellakkaran, K. Singh, D. Banerjee, ACS Catal. 7 (2017) 8152–8258;
d) L. Tang, H.Y. Sun, Y.F. Li, Z.G. Zhao, Z.Y. Wang, Green Chem. 14 (2012) 3423–
3428;
e) Y. Shiraishi, K. Fujiwara, Y. Sugano, S. Ichikawa, T. Hirai, ACS Catal. 3 (2013)
312–320.
[
2] F.A. Villamena, S.J. Xia, J.K. Merle, R. Lauricella, B. Tuccio, C.M. Hadad, J.L.
Zweier, J. Am. Chem. Soc. 129 (2007) 8177–8191.
[3] G. Durand, M. Rosselin, P.A. Klein, K. Zéamari, F. Choteau, B. Tuccio, J. Org.
Chem. 82 (2017) 135–142.
[
[
4] C.W. Lin, B.C. Hong, W.C. Chang, G.H. Lee, Org. Lett. 17 (2015) 2314–2317.
5] K.V. Gothelf, K.A. Jørgensen, Chem. Rev. 98 (1998) 863–909.
[6] E. Rodrigo, S.R. Waldvogel, Green Chem. 20 (2018) 2013–2017.

1
06
W. Yu et al. / Journal of Catalysis 370 (2019) 97–106
[
7] a) C. Gella, E. Ferrer, R. Alibés, F. Busqué, P. de March, M. Figueredo, J. Font, J.
Ed. 53 (2014) 2951–2955;
Org. Chem. 74 (2009) 6365–6367;
b) D. Christensen, K.A. Jorgensen, J. Org. Chem. 54 (1989) 126–131.
8] S.I. Murahashi, H. Mitsui, T. Shiota, T. Tsuda, S. Watanabe, J. Org. Chem. 55
d) Y. Su, Z.K. Han, L. Zhang, W.Z. Wang, M.Y. Duan, X.M. Li, Y.L. Zheng, Y.G.
Wang, X.L. Lei, Appl. Catal., B. 271 (2017) 108–114.
[15] a) S.C. Jensen, S.B. Homan, E.A. Weiss, J. Am. Chem. Soc. 138 (2016) 159–1600;
b) K. Tsutsumi, F. Uchikawa, K. Sakai, K. Tabata, ACS Catal. 6 (2016) 4394–
4398;
[
[
(
1990) 1736–1744.
9] S. Yavuz, H. Ozkan, N. Colak, Y. Yildirir, Molecules 16 (2011) 6677–6683.
[
[
[
10] E. Buehler, J. Org. Chem. 32 (1967) 261–265.
c) Q. Xiao, S. Sarina, E.R. Waclawik, J.F. Jia, J. Chang, J.D. Riches, H.S. Wu, Z.F.
Zheng, H.Y. Zhu, ACS Catal. 6 (2016) 1744–1753;
d) J.J. Song, Z.F. Huang, L. Pan, K. Li, X.W. Zhang, L. Wang, J.J. Zou, Appl. Catal.,
B. 227 (2018) 386–408;
e) S. Giri, R. Das, C. van der Westhuyzen, A. Maity, Appl. Catal., B. 209 (2017)
669–678.
11] L. Cisneros, P. Serna, A. Corma, Angew. Chem. Int. Ed. 53 (2014) 9306–9310.
12] X.L. Li, B.Y. Zhang, L.L. Tang, T.W. Goh, S.Y. Qi, A. Volkov, Y.C. Pei, Z.Y. Qi, C.K.
Tsung, L. Stanley, W.Y. Huang, Angew. Chem. Int. Ed. 56 (2017) 16371–16375.
13] a) L. Marzo, S.K. Pagire, O. Reiser, B. Koenig, Angew. Chem. Int. Ed. 57 (2018)
[
1
0034–10072;
b) J. Kou, C. Lu, J. Wang, Y. Chen, Z. Xu, R.S. Varma, Chem. Rev. 117 (2017)
445–1514;
c) N. Corrigan, S. Shanmugam, J. Xu, C. Boye, Chem. Soc. Rev. 45 (2016) 6165–
212;
d) Y.Z. Chen, Z.U. Wang, H. Wang, J. Lu, S.H. Yu, H.L. Jiang, J. Am. Chem. Soc.
39 (2017) 2035–2044;
[16] a) N. Salam, B. Banerjee, A.S. Roy, P. Mondal, S. Roy, A. Bhaumik, S.M. Islam,
Appl. Catal., B. 477 (2014) 184–194;
e) S.J. Zhang, W.X. Huang, X.L. Fu, X.Z. Zheng, S.G. Meng, X.J. Ye, S.F. Chen, Appl.
Catal., B. 233 (2018) 1–10.
[17] a) S. Higashimoto, Y. Nakai, M. Azuma, M. Takahashi, Y. Sakata, RSC Adv. 4
(2014) 37662–37668;
1
6
1
e) F. Su, S.C. Mathew, L. Möhlmann, M. Antonietti, X. Wang, S. Blechert,
Angew. Chem. Int. Ed. 50 (2011) 657–660;
f) X. Guo, C. Hao, G. Jin, H.Y. Zhu, X.Y. Guo, Angew. Chem. Int. Ed. 53 (2014)
b) H. Hirakawa, M. Katayama, Y. Shiraishi, H. Sakamoto, K. Wang, B. Ohtani, S.
Ichikawa, S. Tanaka, T. Hirai, ACS Appl. Mater. Interfaces 7 (2015) 3797–3806;
c) Y.J. Song, H. Wang, S.J. Liang, Y. Yu, L.Y. Li, L. Wu, J. Catal. 361 (2018) 105–
115;
1
973–1977;
g) H. Liu, T. Wang, H. Zhang, G. Liu, P. Li, L. Liu, D. Hao, J. Ren, K. Chang, X.
Meng, H. Wang, J. Ye, J. Mater. Chem. A. 4 (2016) 1941–1946;
h) S. Sarina, H. Zhu, E. Jaatinen, Q. Xiao, H. Liu, J. Jia, C. Chen, J. Zhao, J. Am.
Chem. Soc. 135 (2013) 5793–5801;
d) K. Selvam, H. Sakamoto, Y. Shiraishi, T. Hirai, New J. Chem. 39 (2015) 2467–
2473.
[18] W. Yu, D. Zhang, X. Guo, C. Song, Z. Zhao, Catal. Sci. Technol. 8 (2018) 5148–
5154.
i) S.C. Jensen, S.B. Homan, E.A. Weiss, J. Am. Chem. Soc. 138 (2016) 1591–
[19] B. Han, S.Q. Liu, N. Zhang, Y.J. Xu, Z.R. Tang, Appl. Catal., B. 202 (2017) 298–304.
[20] L.L. Zhao, J. Jia, Z.Y. Yang, J.Y. Yu, A.L. Wang, Y.H. Sang, W.J. Zhou, H. Liu, Appl.
Catal., B. 210 (2017) 290–296.
[21] a) N. Zhang, X.Y. Li, H.C. Ye, S.M. Chen, H.X. Ju, D.B. Liu, Y. Lin, W. Ye, C.M.
Wang, Q. Xu, J.F. Zhu, L. Song, J. Jiang, Y.J. Xiong, J. Am. Chem. Soc. 138 (2016)
8928–8935;
1
600;
j) H. Liu, C. Xu, D. Li, H.L. Jiang, Angew. Chem. Int. Ed. 57 (2018) 5379–5383.
14] a) Z.G. Chai, T.T. Zeng, Q. Li, L.Q. Lu, W.J. Xiao, D.S. Xu, J. Am. Chem. Soc. 138
[
(
2016) 10128–10131;
b) H. Wang, X.S. Sun, D.D. Li, X.D. Zhang, S.C. Chen, W. Shao, Y.P. Tian, Y. Xie, J.
Am. Chem. Soc. 139 (2017) 2468–2473;
c) S.J. Liang, L.R. Wen, S. Lin, J.H. Bi, P.Y. Feng, X.Z. Fu, L. Wu, Angew. Chem. Int.
b) S.B. Yang, Y.J. Gong, J.S. Zhang, L. Zhan, L.L. Ma, Z.Y. Fang, R. Vajtai, X.C.
Wang, P.M. Ajayan, Adv. Mater. 25 (2013) 2452–2456.