One-pot synthesis of imine from benzyl alcohol and nitrobenzene on visible-light responsive CdS-TiO2 photocatalysts

Article abstract of DOI:10.1039/c4ra06231k

The one-pot synthesis of imine (N-benzylideneaniline) from benzyl alcohol and nitrobenzene was studied on the CdS-TiO₂ photocatalyst under visible-light irradiation (λ > 420 nm) at 298 K. The photocatalytic activity strongly depends on the load

Full text of DOI:10.1039/c4ra06231k

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Photocatalyzed one-pot synthesis of imine from benzyl alcohol and
nitrobenbzene on the CdS-TiO under visible-light irradiation
2
NO
2
NH2
CHO
NH
2
e-
Visible-light
+
CdS
in the dark
h+
CH OH
2
N
TiO2
CHO
Graphical abstract

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DOI: 10.1039/C4RA06231K
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ARTICLE TYPE
One-pot synthesis of imine from benzyl alcohol and nitrobenzene on
visible-light responsive CdS-TiO photocatalysts
2
a
a
a
b
c
Shinya Higashimoto,* Yuta Nakai, Masashi Azuma , Masanari Takahashi and Yoshihisa Sakata
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The oneꢀpot synthesis of imine (Nꢀbenzylideneaniline) from benzyl alcohol and nitrobenzene was studied on the CdSꢀTiO photocatalyst
2
under visibleꢀlight irradiation (λ > 420 nm) at 298 K. The photocatalytic activity strongly depends on the loadings of CdS on the TiO2,
and the preferable amount of CdS was 15 wt%. Photoꢀelectrochemical investigations were also incorporated to demonstrate the reaction
mechanism: the CdS is photosensitized by visibleꢀlight irradiation, and an effective holeꢀelectron separation is generated in the heteroꢀ
1
0
junction between CdS and TiO . It was also confirmed that the photocatalytic oxidation of benzyl alcohol proceeds over the CdS surface,
2
while the hydrogenative reduction of nitrobenzene into aminobenzene proceeds over the TiO surface. Moreover, the production of imine
2
proceeded efficiently by the successive condensation of benzaldehyde and aminobenzene on the catalyst surface under dark conditions.
Introduction
amines. Recently, the photocatalytic reduction of nitrobenzene
1
9, 20
21
5
6
6
7
7
8
8
9
5
0
5
0
5
0
5
0
using TiO2
or Au/TiO2 photocatalysts in the presence of
1
2
2
3
3
4
5
0
5
0
5
0
The design and development of an effective photocatalyst to
address such energy and environmental needs as H production
by water splitting, the degradation of volatile organic compounds
such sacrificial reagents as 2ꢀpropanol and oxalic acid has been
reported. It was confirmed that nitrobenzene is reduced into
aminobenzene with 6 electrons at the reductive sites, while 2ꢀ
propanol and oxalic acid are oxidized into acetone and CO2,
respectively, at the oxidative sites.
Oneꢀpot synthesis has been an efficient strategy in order to
improve successive chemical reactions in just one reactor. This
system can avoid the relatively long separation and purification
processes of the intermediates. Several oneꢀpot photocatalytic
syntheses of amines, imines, benzimidazoles, indazoles and
1
ꢀ6
2
(VOC), and organic syntheses is strongly desired. In particular,
the development of visibleꢀlight sensitive photocatalyst has been
the focus of much attention since solar irradiation is unlimited
and ca. 50 % which reaches the earth’s surface is visible light.
These reactions are induced by photoꢀexcitation of the catalyst to
produce charge carriers (holes and electrons) which participate in
the oxidation and reduction of the substrates, respectively.
The chemoꢀselective oxidation of organic compounds such as
alcohols and amines into the corresponding carbonyl compounds
and imines, respectively, is a fundamental but significant
azobenzenes in alcoholic suspensions of TiO or CdS under UVꢀ
2
22ꢀ24
light irradiation have also been previously reported
.
Along these lines, we have developed an efficient photocatalytic
system for the oneꢀpot synthesis of imines from alcohols and
nitroꢀcompounds under visible light irradiation. This catalytic
system consists of three steps: 1) the oxidation of alcohol into
aldehyde; 2) reduction of nitroꢀcompounds into amine; and 3)
successive condensation of aldehyde and amine to form imine. It
is, thus, desirable that both photoꢀinduced oxidative and reductive
products participate in further reactions to form more useful
products.
7
, 8
procedure with great potential in commercial applications . In
particular, imines are important intermediates for the synthesis of
pharmaceutical and agricultural chemicals , and they are utilized
9
10
for the synthesis of secondary amines by hydrogenation , in
1
1
cycloaddition reactions and biologically active substances such
1
2
as antibiotics .
The selective photocatalytic formation of imines by the aerobic
1
3ꢀ18
coupling of amines
has been extensively studied not only on
1
3, 14
15
TiO2
and Au/TiO2 , but also on other semiconductive
Recently, the Pdꢀdeposited CdSꢀTiO photocatalyst was found
1
6, 17
18
2
photocatalysts such as Nb O
visibleꢀlight irradiation. These photocatalytic systems mainly
have proceeded to form products at the oxidative sites, while H O
and C N under UV and/or
2
5
3
4
to exhibit the selective dehydrogenation of aromatic alcohols into
corresponding carbonyl compounds together with H under
2
2
25
visibleꢀlight irradiation (λ > 420 nm) at 298 K . It was clearly
and/or H O are simultaneously formed as the O2 reductive
2
2
demonstrated that the charge carriers (holes and electrons)
products. However, the oxidation of amines by O2 sometimes
causes overꢀoxidation of the products and/or sideꢀproducts which
could lead to catalyst deactivation.
Contrary to oxidation reactions, the hydrogenation of nitroꢀ
compounds is also an important procedure in the formation of
induced on the CdSꢀTiO led to effective photocatalytic reactions
2
under visibleꢀlight irradiation. This photocatalytic system has the
advantage of preventing photoꢀdissociation of the CdS
photocatalyst in the absence of O2.
In this study, we have investigated the oneꢀpot synthesis of
imine from benzyl alcohol and nitrobenzene in an acetonitrile
4
5
5
0
a
College of Engineering, Osaka Institute of Technology, 5-16-1 Omiya,
Asahi-ku, Osaka 535-8585, Japan Fax: 81 6-6957 2135 Tel: 81 6 6954
suspension with the CdSꢀTiO photocatalyst and without the use
2
4
283; E-mail: higashimoto@chem.oit.ac.jp
Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya,
b
of a precious metal coꢀcatalyst under visible light irradiation (λ >
420 nm) at 298 K. Here, we have studied the reaction mechanism
through a combination of photoꢀelectrochemical measurements.
Joto-ku, Osaka 536-0025, Japan
c
Graduate School of Science and Engineering, Yamaguchi University,
2
-16-1 Tokiwadai Ube 755-8611, Japan
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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Experimental
connected to a Potentiostat/Galvanostat (HABF5001, HOKUTO
DENKO). The TiO film was prepared as follows: TiO powder
2
2
6
6
7
0
5
0
(2.85 g of Degussa Pꢀ25 and 0.15 g of STꢀ01) was wellꢀdispersed
in mL of aq. HNO3 (pH 3) involving 0.3 of
polyethyleneglycol (Hw = 20000) and 0.3 mL of Toriton Xꢀ405,
Preparation of the photocatalysts
2
ꢀ1
5
g
Commercially available bulk TiO (320 m g , STꢀ01, Ishihara
2
Co., Ltd.) and CdS (Sigma Aldrich Japan), benzyl alcohol,
nitrobenzene, aminobenzene and Nꢀbenzylideneaniline, Cd(NO3)2
and the paste was then spread onto an ITOꢀglass electrode (10
5
ꢀ2
ꢁ
ꢁcm ) by spin coating, followed by heatꢀtreatment at 673 K for
and Na S (Wako Pure Chemical Industries, Ltd.) were used as
2
1h in air. This process was repeated twice.
received.
A Cd(OH) film was electrochemically deposited onto the ITO
2
The CdSꢀsupported TiO2 (CdSꢀTiO2) was prepared by
impregnation of TiO2 with aq. Cd(NO3)2 at the desired
ꢀ2
at ꢀ1.0 V vs. Ag/AgCl with an electroꢀcharge of 0.2 Cꢁcm ,
followed by the reaction with 0.1 M Na S for 1 h at 298 K to
2
1
0
concentrations. The precursors were dipped in aq. Na S involving
2
2
ꢀ
2+
form CdS. Details on the characterization of the CdS films have
the same amounts of S ions with Cd ions at 298 K for 48 h,
followed by washing with water/acetonitrile several times and
dried under vacuum for 24 h. The obtained sample was referred
27
been previously reported .
A
to as CdS (x wt%)ꢀTiO , x: wt% as CdS.
2
dark
15
Photocatalytic reactions
An acetonitrile solution (5.0 mL) of benzyl alcohol (150 ꢀmol)
and nitrobenzene (50 ꢀmol) suspended with the photocatalyst (10
75
visible light
mg) was introduced in a pyrex cell under purging with N . It was
2
X
Y
then irradiated by blue LED lamps adjusted to 25000 Lux (λ
460 nm, ca. 10 mW/cm ). After the reactions, the catalysts were
immediately separated from the suspension by filtration through a
=
max
2
20
A
B
0
.20ꢀꢀm membrane filter (Dismicꢀ25, Advantec). The solution
was then analyzed by gas chromatography: GCꢀFID (Shimadzu 80 Figure 1 Schematic illustration of the separated cells; the X
GCꢀ14A, capillary column: INERTCAP 1701).
electrode is illuminated by visibleꢀlight, while Y is placed under
dark conditions.
25
Measurements of apparent quantum yields (AQY)
The AQY was measured in a quartz cell involving an acetonitrile
solution (2 mL) of benzyl alcohol (150 ꢀmol) and nitrobenzene
Results and discussions
(
1
50 ꢀmol) suspended with CdS(15 wt%)ꢀTiO (10 mg) using a
2
85 The photocatalytic reaction of benzyl alcohol and nitrobenzene
00 W Xenon lamp (Lax Cute II, Asahi Spectra Co., Ltd.)
was performed on CdSꢀTiO under visible light irradiation. It was
2
3
0
through appropriate band path filters with a FWHM of 10±2 nm.
The AQY at each centered wavelength of light was calculated
using the following equation (1), where the products represent the
total amounts of benzaldehyde and Nꢀbenzylideneaniline:
confirmed that the reaction does not take place under visible light
without the photocatalyst nor with the photocatalyst but without
irradiation.
9
0
Figure 2 shows the time profile for the synthesis of imine from
benzyl alcohol and nitrobenzene on the CdS(15 wt%)ꢀTiO2
photocatalyst under visible light irradiation (λ > 420 nm). A
decrease in the amount of benzyl alcohol and nitrobenzene was
observed with an increase in the irradiation time, while an
increase in the amount of benzaldehyde and aminobenzene was
observed for up to 6 h. A decrease in the amount of benzaldehyde
and aminobenzene was then observed, while the successive
condensation of benzaldehyde and aminobenzene led to the
(
amounts of product formed)
amounts of photons irradiated)
AQY [%] =
×100
(
35
(eq. 1)
95
It was assumed that benzyl alcohol is converted into
benzaldehyde through two electronꢀtransfers by one photon due
2
6
to the current doubling effect . The flux of incident photons was
1
8
19
ꢀ1
adjusted within a range of 7.61×10 ~ 1.10×10 photonsꢁh ,
measured using a powerꢀmeter (Ophir, ORION/PD).
40
150
1
00
Characterizations
(a)
(c)
UVꢀVis spectroscopic measurements of the samples were carried
1
00
50
0
out in diffused reflectance mode using
a
UVꢀVis
spectrophotometer (UVꢀ3100PC, Shimadzu). Powder Xꢀray
diffraction (XRD) patterns were obtained with a RIGAKU
RINT2000 diffractometer using Cu Kα radiation (λ=1.5417 Å).
The amounts of Cd leached in the solution were analyzed by a
multiꢀtype ICP emission spectrometer (ICPEꢀ9000, Shimadzu).
The elemental composition of the photocatalysts was 110
characterized by scanning electron microscope (SEM) with
energy dispersive Xꢀray (EDX) spectroscopy (JSMꢀ6610, JEOL).
105
45
(e)
(b)
(d)
50
0
5
10
1
5
20
22 5
Time / h
Photo-electrochemical measurements
The electrolysis cells separated by Nafion (Aldrich) were filled
with two types of electrolytes. The acetonitrile solutions involve
Figure 2 Time profile for the photocatalytic reaction of
15 nitrobenzene and benzyl alcohol on the CdS (15 wt%)ꢀTiO2
under visibleꢀlight irradiation. (a) benzyl alcohol, (b)
nitrobenzene, (c) benzaldehyde, (d) aminobenzene, (e) Nꢀ
benzylideneaniline.
®
1
55
0
.1 M LiClO4 with 30 mM benzyl alcohol (A) and 10 mM
nitrobenzene (B), as shown in Fig. 1. Two electrodes were
2
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C4RA06231K
formation of Nꢀbenzylideneaniline. The carbon and nitrogen
benzylideneaniline was observed (Runs 2, 3). Moreover, CdS (15
massꢀbalance were almost stoichiometric within 5% for each 55 wt%)ꢀTiO was found to improve the activity, accompanied by
2
reaction time. This reaction exhibited a turnover number (TON),
i.e., the total number of photoꢀformed benzaldehyde and Nꢀ
benzylideneaniline per Cd atom for 24 h was determined to be ca.
the formation of Nꢀbenzylideneaniline (Run 4). It was also
observed that the activity of the CdS(15 wt%)ꢀsupported SiO2
5
0
5
0
5
0
5
was much lower than that of the CdS (15 wt%)ꢀTiO (Run 5).
2
1
4, indicating that this reaction system takes place
These results indicate that CdS works as the photoꢀsensitizer, and
photocatalytically. It should be noted that the concentration of Cd 60 the TiO surface plays a significant role in the enhancement of
2
in the solution after the reaction was below the detection limit for
ICP analysis (< 0.01 ppm), suggesting that the anodic photoꢀ
corrosion of CdS was successfully suppressed.
the activity for the production of Nꢀbenzylideneaniline.
Table 2 shows the results of the condensation of benzaldehyde
and aminobenzene under dark conditions. This reaction proceeds
even in the absence of catalysts under dark conditions. The
1
1
2
2
3
3
The dependence of the amount of CdS loaded on TiO upon
2
photocatalytic activity is shown in Fig. 3. Only traceable Nꢀ 65 activity remarkably increases in the presence of the active
benzylideneaniline was formed when both TiO2 and CdS by
themselves were applied as the photocatalyst. However, it was
observed that the photocatalytic activity increases with an
increase in the amount of CdS up to 15 wt% and then decreases.
These results indicate that the synergetic effects between TiO2 70 dark conditions. It should be noted that the photocatalytic activity
and CdS play significant roles in the enhancement of the activity.
surfaces of CdS, TiO and CdS(15 wt%)ꢀTiO . Therefore, the
2
2
production of Nꢀbenzylideneaniline was confirmed to proceed
efficiently by the successive condensation of the photoꢀproduced
benzaldehyde and aminobenzene on the catalyst surface under
for the synthesis of imine on the CdS surface is quite low, as
shown in Table 1 (Runs 2, 3) in spite of the high activity for
condensation on the CdS surface. This is probably due to the
blocking effect of the active sites for condensation reactions by
the reactants and/or intermediates during the reaction.
40
75
30
Table 2. Reactions of benzaldehyde and aminobenzene on
various catalysts under dark conditions.
Amounts / ꢀmol
20
[a]
Run Catalyst
Time / h
Nꢀbenzylideneaniline
10
1
2
3
4
ꢀ
24
24
24
24
25.8
42.5
42.8
43.4
CdS
TiO
CdS(15 wt%)ꢀTiO
2
0
2
[a]
0
20
40
60
80
100
Reaction conditions: catalyst (10 mg); benzaldehyde (150 ꢀmol), aminobenzene
Amounts of loaded CdS / wt%
80 (50 ꢀmol) in acetonitrile (5 mL).
Figure 3 Effect of the amount of CdS (x wt%) on the photoꢀ
formed Nꢀbenzylideneaniline. Reaction conditions: photocatalyst
The characterization of the CdS(15 wt%)ꢀTiO photocatalyst
2
was performed by XRD, SEMꢀEDX and UVꢀVis spectroscopies.
(
10 mg), nitrobenzene (50 ꢀmol), benzyl alcohol (150 ꢀmol),
The XRD patterns of TiO , CdS(15 wt%)ꢀTiO2 and CdS are
2
blue LED (λmax = 460 nm, 25000 lux), reaction time (24 h).
85 shown in Fig. 4. CdS(15 wt%)ꢀTiO
2
is observed to involve CdS
with a cubic structure and TiO with an anatase structure. By the
2
4
4
5
0
5
0
In order to understand the role of the components of the
photocatalyst, various examinations were carried out under
visible light irradiation and the results are listed in Table 1. It was
Scherer equation, the crystal size of TiO
to be ca. 7.5 and 3.6 nm, respectively.
and CdS were estimated
2
4
000
observed that the TiO exhibited the formation of small amounts
90
○
●: TiO (anatase)
2
2
○
: CdS (cubic)
of benzaldehyde and aminobenzene, but only trace amounts of
●
○
imine (Run 1), suggesting that the TiO by itself does not exhibit
3000
2
○
significant photocatalytic reaction. On the other hand, CdS by
itself showed photocatalytic activity for the formation of
benzaldehyde and aminobenzene, while little formation of Nꢀ
●
●
●
(
c)
b)
(a)
95
2
1
000
000
0
(
Table 1. Reaction of benzyl alcohol and nitrobenzne on various
photocatalysts under visible light irradiation.
Amounts / ꢀmol
Time
[a]
100
Run Photocatalyst
Phꢀ
CHO
5.7
Phꢀ
NH
PhꢀC=Nꢀ
Ph
/
h
20
30
40
50
60
2
degree / 2
θ
1
2
3
TiO
2
24
12
24
0.8
10.0
16.7
trace
CdS
CdS
51.3
64.3
trace
0.7
Figure 4 XRD patterns of (a) TiO , (b) CdS(15 wt%)ꢀTiO and
2
2
1
05 (c) CdS.
CdS(15 wt%)ꢀ
TiO
CdS(15 wt%)ꢀ
4
5
12
12
121.2
16.8
19.2
1.8
22.1
2
Figure 5 [I] and [II] show the SEM photograph and
distributions of Ti, O, Cd, and S elements obtained by EDX
trace
[
b]
analysis, respectively, on CdS(15 wt%)ꢀTiO . The red and white
2
SiO
2
1
10 colors represent the high concentration of the corresponding
elements in Fig. 5 [II]. It was observed that the elements of Cd
and S are randomly dispersed at locations of low Ti and O
[
a]
Reaction conditions: catalyst (10 mg); benzyl alcohol (150 ꢀmol), nitrobenzene
50 ꢀmol) in acetonitrile (5 mL). SiO (390 m ꢁg , S5130ꢀ500G, Aldrich)
2
[b]
2
ꢀ1
(
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concentrations, and the ratio of S/Cd was ca 1.0. These results
indicate that stoichiometric but partially aggregated CdS species
CdS(15 wt%)ꢀTiO2 is also shown in Fig. 6. The AQY were
determined as follows: 17.3% at 480 nm, 8.7% at 540 nm, and ca.
with an island structure is formed on the TiO surface. Therefore, 65 1.5% at 600 nm. The AQY plots form the action spectrum for the
2
such reactants as benzyl alcohol and nitrobenzene can interact not
photocatalytic reaction on CdS(15 wt%)ꢀTiO . These results
2
5
0
5
0
5
0
5
0
5
only with the TiO but also CdS surface on the CdS(15 wt%)ꢀ
show that the action spectrum is in good agreement with the
2
TiO photocatalyst.
photoꢀabsorption of CdS(15 wt%)ꢀTiO . It could, thus, be
2
2
Figure 6 shows the UVꢀVis spectra of CdS(15 wt%)ꢀTiO (a)
concluded that the CdS on the TiO2 works as the photoꢀ
2
together with the pure TiO (b) and CdS (c). It was observed that 70 responsible center for the visibleꢀlight induced photocatalytic
2
the TiO exhibits a direct bandꢀgap excitation under UVꢀlight
dehydrogenation of benzyl alcohol.
2
1
1
2
2
3
3
4
4
irradiation. However, the CdS (15 wt%)ꢀTiO2 photocatalyst
exhibits visibleꢀlight absorption above 400 nm almost identical to
absorption of the pure CdS. It was observed that the absorption
The photocatalytic activity for the reaction of (a) normal benzyl
alcohol and (b) α, αꢀd2 benzyl alcohol with nitrobenzene is
shown in Fig. 7 [I]. It was observed that the photocatalytic
edges of all CdSꢀTiO photocatalysts are identical. These results 75 activity of normal benzyl alcohol with nitrobenzene was much
2
indicate that polycrystalline CdS is formed on the TiO . The
higher than that of α, αꢀd2 benzyl alcohol. The reaction rate
constants of kH and kD for (a) and (b), respectively, in Fig. 7 [I]
have been evaluated from the firstꢀorder rate law in equation (2).
2
relationship between the photocatalytic activity as apparent
quantum yields (AQY) and the wavelength of the irradiated light
was then examined in order to identify the photoꢀresponsible
center. The AQY at various wavelengths of irradiated light on
a
k t = ln
(
eq. 2)
(
a – x)
80 Equation (2) shows that when ln [
the firstꢀorder reaction gives a straight line for slope
amount of added benzyl alcohol and photoꢀformed products
benzaldehyde and Nꢀbenzylideneaniline in total) are defined as
and , respectively. The reaction rate constants for normal benzyl
85 alcohol and α, αꢀd benzyl alcohol were roughly estimated to be
[I]
a
/ (
a
ꢀ
x)] is plotted against
t,
k. Here, the
(
a
x
5
ꢀm
2
ꢀ
5
ꢀ1
ꢀ5 ꢀ1
8
.94 × 10
s
and 3.43 × 10
s
for (c) and (d), respectively (cf.
kD) was, therefore,
Fig. 7 [II]). The ratio of the reaction rates (kH
/
[IIꢀa]
[IIꢀb]
estimated to be ca. 2.6. The first order isotope effect shows that
the CꢀD bond is cleaved more slowly than the CꢀH bond. It
90 should be noted that the value of kCꢀH
when the CꢀH bond is completely cleaved at the transitional state.
Here, from the value of kCꢀH kCꢀD at ≈ 2.6, the kinetic isotope
effect shows that an abstraction of the αꢀhydrogen atom from the
/ kCꢀD is theoretically ≈ 8
/
10 ꢀm
Ti
10 ꢀm
O
8
6
4
2
0
0
0
0
0
[IIꢀd]
[IIꢀc]
[I]
9
5
(
a)
1
00
(b)
10 ꢀm
Cd
10 ꢀm
S
0
0.5
1
1.5
2
Figure 5 SEM image [I], and elemental mappings [II] (a) Ti, (b)
O, (c) Cd and (d) S of the CdS (15 wt%)ꢀTiO2.
Time / h
105
0
.8
.6
[
II]
1
25
20
0
50
0
0
0
0
.8
(c)
(
d)
110
0.4
.6
.4
.2
0
15
10
5
(
a)
0.2
(
b)
(d)
(
c)
×
0.65
600
0
55
115
0
2000
4000
6000
8000
0
Time / s
3
50
400
450
500
550
650
Wavelength / nm
Figure 7 Time profile for the photocatalytic reaction of (a)
Figure 6 UVꢀVis spectra of: (a) CdS(15 wt%)ꢀTiO , (b) TiO , 120 normal benzyl alcohol and (b) α, αꢀd2 benzyl alcohol with
60
2
2
(
c) CdS and (d) AQY plots for the formation of benzaldehyde and
nitrobenzene on CdS(15 wt%)ꢀTiO under visibleꢀlight irradiation
2
Nꢀbenzylideneaniline in total.
[I], and the plots for ln [a/(aꢀx)] against the time [II] for (c)
normal benzyl alcohol and (d) α, αꢀd benzyl alcohol.
2
4
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DOI: 10.1039/C4RA06231K
ꢀ
CH OH group is a rateꢀdetermining step while the CꢀH bond is
proceed by way of the formation of a as a minor species only at
the initial reaction time. Furthermore, Nꢀbenzylideneaniline is
readily formed by the successive reaction of aminobenzene and
benzaldehyde on the catalyst surface under dark conditions.
2
partially left in the transitional state.
In order to understand the reaction mechanism for the
photocatalytic reaction, a photoꢀelectrochemical technique was
5
0
5
0
5
incorporated. We fabricated the photoꢀelectrochemical cell, i.e.
,
70
NO2
+ 6 H+
the electrodes X and Y participate in the oxidation of benzyl
alcohol under visibleꢀlight irradiation, and the reduction of
nitrobenzene under dark conditions, respectively, as shown in Fig.
Visibleꢀlight
6 eꢀ
CB
NH2
CB
1
. It should be noted that the influence of the chemical bias can
be ignored since the pH of the electrolytes A and B are 5.8 and
.0, respectively. Several types of controlled experiments were
1
1
2
2
75
CH OH
2
6
h+
6
3
carried out under no bias and the results are shown in Fig. 8. First,
when the TiO electrode for X, and TiO or CdS for Y were used,
VB
2
2
.
CdS
they exhibited little photocurrent. These results indicate that TiO2
in itself is not sensitized by visibleꢀlight irradiation. When the
CdS electrodes for X and Y were used, respectively, it exhibited
stable photocurrent (i). That is, CdS induces the photoꢀoxidation
of benzyl alcohol at the X, while the photoꢀformed electrons
reduce nitrobenzene at the Y. Moreover, when Y involved the
VB
CHO
+ 6 H+
3
TiO2
C
H
NH2
+
TiO film instead of CdS, the photocurrent (ii) was significantly
in the dark
N
2
80
enhanced five times higher than (i). This is because the
2
8
conduction band of CdS is more cathodic than that of TiO2 , and
the photoꢀinduced electrons on the CdS electrode effectively
transfer into the TiO2.
Figure 9 Schematics for the oneꢀpot synthesis of imine from
benzyl alcohol and nitrobenzene on the CdSꢀTiO photocatalyst.
2
When the uptakes in a mixture of benzyl alcohol and
benzaldehyde (150 ꢀmol for each) on CdS (15 wt%)ꢀTiO (10
2
Conclusions
mg) were investigated, the amount of benzaldehyde adsorbed on
the photocatalyst was negligible compared to that of benzyl
alcohol (ca. 30 ꢀmol). These results indicate that the interaction
8
9
9
5
0
5
The photocatalytic synthesis of imine (Nꢀbenzylideneaniline)
from benzyl alcohol and nitrobenzene was successfully
demonstrated on the CdSꢀTiO photocatalyst by a oneꢀpot method
at 298 K under visibleꢀlight irradiation. The photocatalytic
activity strongly depends on the loadings of CdS on the TiO , and
the amount of CdS was optimized to be 15 wt%. CdS particles
with an island structure were found to form on the TiO . Photoꢀ
electrochemical investigations were also incorporated to
demonstrate the reaction mechanism, i.e., that the photoꢀ
oxidation of benzyl alcohol into benzaldehyde on the CdS and
3
3
4
4
5
5
6
6
0
5
0
5
0
5
0
5
between benzaldehyde and TiO is fairly weak. Therefore, once
2
benzaldehyde is produced by the oxidation of benzyl alcohol, it is
immediately released into the bulk solution. This may be one of
the reasons benzaldehyde is selectively formed and not oxidized
2
2
further to benzoic acid or CO . Moreover, it was confirmed that
2
photoꢀformed aminobenzene is rarely oxidized on the CdS(15
wt%)ꢀTiO2 surface in the absence of O2 under visibleꢀlight
irradiation. From these results, a possible mechanism for imine
synthesis on the CdSꢀTiO2 is shown in Fig. 9. The CdS is
photosensitized under visibleꢀlight irradiation, and an effective
holeꢀelectron separation is generated in the heteroꢀjunction
2
reduction of nitrobenzene into aminobenzene on the TiO surface
2
between CdS and TiO . The photoꢀinduced holes participate in
2
takes place efficiently. The process for abstraction of αꢀhydrogen
of the benzyl alcohol was found to be the rateꢀdetermining step.
Moreover, the successive condensation of the photoꢀproduced
benzaldehyde and aminobenzene into Nꢀbenzylideneaniline was
the oxidation of three benzyl alcohol molecules into three
benzaldehyde molecules. On the other hand, the reaction of 6
electrons left in the TiO2 together with 6 protons with one
nitrobenzene molecule will afford one aniline molecule on the
TiO surface since the reductive potential of nitrobenzene (+0.16 100 found to take place on the catalyst surface under dark conditions.
2
V vs. SHE) is more anodic than that of the conduction band (CB)
This work is the first to achieve the oneꢀpot synthesis of imine
2
8, 29
of TiO (ꢀ0.18 V vs. SHE)
. These reactions consecutively
2
from benzyl alcohol and nitrobenzene on the visibleꢀlight
light on
sensitive CdSꢀTiO photocatalyst without the use of any precious
2
metals as a coꢀcatalyst at 298 K. Both oxidative and reductive
05 products formed in the photocatalytic reaction here participated in
the formation of useful products, leading to a new strategy for
photocatalysisꢀbased oneꢀpot organic synthesis.
1
off
on
2
ꢀA/cm2
Notes and references
off
30 s
1
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configurated electrodes: (i) X: CdS, Y:CdS and (ii) X: CdS, Y:
TiO , respectively
2
008, 9, 157.
2
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Journal Name, [year], [vol], 00–00 | 5

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DOI: 10.1039/C4RA06231K
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