Cocatalyst-Free Reduction of 4,4′-Dinitrodiphenyl Ether to 4,4′-Diaminodiphenyl Ether Over Twin-Crystal ZnxCd1?xS under Visible Light

Article abstract of DOI:10.1002/cctc.202101016

Semiconductor-based photocatalytic conversion of solar energy is a promising method for the synthesis of high value-added chemicals. In this paper, a cocatalyst-free nano-twin crystal Zn_xCd_1?xS (T?Zn_xCd_1?xS) semiconductor was employed to achieve almost complete conversion of DNDPE and the yield of ODA product achieves >99 % in 40 min reaction time without additional hydrogen source. As far as we know, this is the first time to apply the photocatalytic technology for reducing DNDPE to ODA, and the photocatalytic efficiency has greatly exceeded the result of traditional catalytic method. Theoretical calculation and isotope labeling in situ HPLC-MS analysis demonstrates that the reduction mechanism of DNDPE is two nitro groups of DNDPE are separately instead of simultaneously reduced, following the process of DNDPE→NO₂?C₆H₄?O?C₆H₄?NO→NO₂?C₆H₄?O?C₆H₄?NHOH→NO₂?C₆H₄?O?C₆H₄?NH₂→NO?C₆H₄?O?C₆H₄?NH₂→NH₂?C₆H₄?O?C₆H₄?NHOH→ODA. Hydrogen protons of water, instead of ethanol, provide the hydrogen source for the photocatalytic reduction of DNDPE to ODA.

Full text of DOI:10.1002/cctc.202101016

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Cocatalyst-Free Reduction of 4,4’-Dinitrodiphenyl Ether to
4
,4’-Diaminodiphenyl Ether Over Twin-Crystal Zn Cd S
x
1À x
under Visible Light
[a]
[b]
[a]
[a]
[a]
Yujia Hu, Guiyang Yu,* Chuanwang Xing, Shanshan Liu, Chuangyu Wei,
Heyuan Liu, Jianzhuang Jiang,*
[c]
[a, d]
[b]
and Xiyou Li*
Semiconductor-based photocatalytic conversion of solar energy
is a promising method for the synthesis of high value-added
chemicals. In this paper, a cocatalyst-free nano-twin crystal
calculation and isotope labeling in situ HPLC-MS analysis
demonstrates that the reduction mechanism of DNDPE is two
nitro groups of DNDPE are separately instead of simultaneously
Zn Cd
S
(TÀ Zn Cd S) semiconductor was employed to
reduced,
following
the
process
of
DNDPE!
x
1À x
x
1À x
achieve almost complete conversion of DNDPE and the yield of
ODA product achieves >99% in 40 min reaction time without
additional hydrogen source. As far as we know, this is the first
time to apply the photocatalytic technology for reducing
DNDPE to ODA, and the photocatalytic efficiency has greatly
exceeded the result of traditional catalytic method. Theoretical
NO À C H À OÀ C H À NO!NO À C H À OÀ C H À NHOH!
2
6
4
6
4
2
6
4
6
4
NO À C H À OÀ C H À NH !NOÀ C H À OÀ C H À NH !
2
6
4
6
4
2
6
4
6
4
2
NH À C H À OÀ C H À NHOH!ODA. Hydrogen protons of water,
2
6
4
6
4
instead of ethanol, provide the hydrogen source for the
photocatalytic reduction of DNDPE to ODA.
Introduction
precious metal-based catalyst in expensive organic solvent
(such as DMF/DMSO/DMAC) at strongly harsh reaction con-
4
,4’-aminodiphenyl ether (ODA) is an important intermediate in
ditions (high temperature/ pressure), with a concomitant
[4]
fine chemicals synthesis. It is the main monomer for the
synthesis of polyimide, polymaleimide and other heat-resistant
plastics. In addition, ODA is also used as a curing agent in the
synthesis of epoxy resins and has application in the replace-
ment of carcinogenic benzidine in the production of azo dyes
and reactive dyes. Therefore, it has attracted many interests in
the synthesis of ODA including condensation of nitrophenol
and nitrochlorobenzene, p-dinitrobenzene method, catalytic
hydrogenation of 4,4’-dinitrodiphenyl ether (DNDPE). Above
classical synthetic strategies are currently carried out upon
formation of copious amounts of wastes. The conversion
efficiency and yield of ODA product is unsatisfied. An econom-
ical, environmentally friendly, and efficient catalytic process is
highly required.
Photocatalytic technology is one clean and mild method in
energy conversion and the synthesis of value-added
[1]
[5]
chemicals. Many reports have been reviewed the photoredox
[2]
[3]
[6]
[7]
[8]
reactions, such as H evolution, CO reduction, N fixation,
2 2 2
[9]
amines/ alcohols oxidation and so on. However, as far as we
know, the study of photocatalytic reduction of DNDPE to
synthesize ODA has not been reported. The essence of reducing
DNDPE to ODA is two nitro groups hydrogenated to amino
groups but two phenyl groups remain. Therefore, photocatalyst,
solvent and light source are three important elements to
achieve high selectivity and conversion in photocatalytic
reduction of DNDPE. Based on photocatalytic hydrogenation of
nitrobenzenes to anilines, several semiconductor systems, such
as TiO , g-C N , CdS, CeO and MOF, should be favorable for
[a] Y. Hu, C. Xing, S. Liu, C. Wei, Prof. J. Jiang
College of Science
China University of Petroleum (East China)
Qingdao 266580 (P. R. China)
E-mail: jianzhuang@ustb.edu.cn
b] Dr. G. Yu, Prof. X. Li
[
School of Materials Science and Engineering
Institute of New Energy
China University of Petroleum (East China)
Qingdao, 266580 (P. R. China)
E-mail: xiyouli@upc.edu.cn
2
3
4
2
[10]
reduction of DNDPE. However, some shortcomings still exist
with these photocatalysts, for instance the limited utilization of
[11]
yuguiyang333@upc.edu.cn
visible light for TiO2, overwhelming dependence on noble
[c] Dr. H. Liu
[12]
metal modification for g-C N , CeO and TiO ,
and low
3
4
2
2
College of New Energy
[13]
China University of Petroleum (East China)
Qingdao, 266580 (P. R. China)
d] Prof. J. Jiang
photocatalytic stability of CdS and MOF. More importantly,
the reaction systems usually contain costly organic solvent
[
(
acetonitrile, dichloromethane), even adding additional hydro-
Beijing Key Laboratory for Science and Application of Functional Molecular
and Crystalline Materials
Department of Chemistry
University of Science and Technology Beijing
Beijing 100083 (P. R. China)
[14,15]
gen source (NaBH4).
This doesn’t meet environmentally
friendly requirement and could bring difficulties for collection
and separation of the product. Therefore, our approach for the
photocatalytic reduction of DNDPE to ODA is to use (i) visible
light as energy source, (ii) water-ethanol as reaction system for
Supporting information for this article is available on the WWW under
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[
18]
providing hydrogen source and sacrificing hole at the same
time, (iii) a highly reducing and stable photocatalyst, and (iv)
normal pressure and temperature.
structure for Zn Cd S. With the increase of the Cd content,
0.7 0.3
the X-ray diffraction (XRD) peaks of Zn Cd S solid solution
becomes broader, which is due to the larger particle size from
x
1À x
Zn Cd S solid solution is an extensively studied semi-
conductor material due to its adjustable bandgap and band
edge position with the change of Zn/Cd ratio. It shows excellent
60 nm of TÀ Zn Cd S to 80 nm of TÀ Zn Cd S (Figure S1). In
x
1À x
0.1
0.9
0.9
0.1
addition, the XRD peaks continuously shifts to the lower angle
+
2
(Figure 1e), which is due to the larger radius of Cd (0.97 Å)
[19]
photocatalytic H evolution performance with visible light
incorporated into crystal lattice. At the same time, the crystal
phase of Zn Cd S gradually changes from the cubic phase to
2
[14]
excitation.
Among various Zn Cd S structures, nano-twin
x 1À x
x
1À x
crystal Zn Cd S (TÀ Zn Cd S) solid solution, composed of two
the hexagonal phase. The composition of as-prepared
x
1À x
x
1À x
(
zinc-blende (ZB) and wurtzite (WZ)) crystal phases, has inner
TÀ Zn Cd S was determined by inductively coupled plasma
x
1À x
homo-junction, which promoted charge separation, and then
atomic emission spectrometry (ICP-AES) experiments. The result
in Table S1 shows that the practical atomic ratios of Cd to Zn
are very close to the stoichiometric atomic ratio for all
TÀ Zn Cd S materials, which indicates the composition accu-
[
16]
excellent catalytic activity. Moreover, TÀ Zn Cd S are more
x
1À x
stable and more efficient proton reduction ability in comparison
with CdS. Previous work has reported that the apparent
x
1À x
quantum yield (AQY) for photocatalytic H evolution can reach
racy. With the increase of atomic ratio of Cd/Zn, the specific
2
[17]
to 62% at 425 nm without the presence of any co-catalysts.
Therefore, we use Zn Cd S with nano-twin crystal structure as
surface area of TÀ Zn Cd S notably increases and the max-
x
1À x
2
À 1
imum value reached to 14.20 m g for Zn Cd S (Table S1 and
x
1À x
0.5
0.5
catalyst for the photocatalytic reduction of DNDPE to ODA in
water-ethanol solution under normal pressure, room temper-
ature and visible light irradiation. Without any noble metal
cocatalysts or additional hydrogen source, DNDPE shows the
conversion of more than 95% in 10 min and complete
conversion within 40 min, which greatly exceeds traditional
catalysis method. The yield of corresponding ODA product over
TÀ Zn Cd S achieved >99% under acidic condition. The
Figure S4). In addition, X-ray photoelectron spectra (XPS) was
measured to investigate the surface chemical states of
TÀ Zn Cd S photocatalysts. As shown in Figure S5a, the bind-
x
1À x
ing energies of Cd 3d upon TÀ Zn Cd S are found at 404.87
0
.7
0.3
and 411.60 eV, which is negatively shifted in comparison with
CdS. Similarly, the XPS peaks of Zn 2p1/2 and 2p3/2 observed for
TÀ Zn Cd S sample shifted to lower binding energies at
0
.7
0.3
1021.85 and 1044.87 eV (Figure S5b), compared with those of
pure ZnS. Due to the hybridization of Cd and Zn composing the
0.7
0.3
catalyst performed excellent stability under the reaction
conditions. This is the first report of the photocatalytic
reduction of DNDPE to ODA under visible light excitation in
water-ethanol system. According to HPLC-MS analysis and
theoretical calculation, the reduction of the two nitro groups of
DNDPE are reduced separately, following the order of:
DNDPE!NO À RÀ NO!NO À RÀ NHOH!NO À RÀ NH !
[
20]
conduction band (CB) of TÀ Zn Cd S, above negative energy
0
.7
0.3
shifts of Cd 3d and Zn 2p for TÀ Zn Cd S sample indicate that
0
.7
0.3
the hybridization facilitates the increase of electron density,
which could enhance the photocatalytic reduction activity.
The photocatalytic reduction of DNDPE was carried out in
an ethanol-water suspension of TÀ Zn Cd S under visible light
2
2
2
2
x
1À x
NOÀ RÀ NH !NH À RÀ NHOH!ODA. The isotope label experi-
irradiation. The photoreactor was sealed and purged with N for
2
2
2
ment result demonstrates that hydrogen protons of water,
instead of ethanol, provide the hydrogen source for the
photocatalytic reduction of DNDPE to ODA.
30 minutes to remove air before light irradiation. High-perform-
ance liquid chromatography (HPLC) was used to determine the
amount of DNDPE, ODA and other products. External standard
method was carried out to quantify the amount of DNDPE, ODA
and 4-(4-Nitrophenoxy) aniline. The standard curves of DNDPE,
ODA and 4-(4-Nitrophenoxy) aniline in Figure S6 show favorable
linear relationship between response area and standard con-
Results and Discussion
9
9
7
A series of TÀ Zn Cd S photocatalysts with various Zn/Cd
centration, with the slope of 1.08×10 , 1.12×10 , and 6.30×10 ,
respectively. A series of blank experiments (Table S2) reveal that
the photocatalyst and photoirradiation are indispensable for
the reduction of DNDPE. Then we investigated and compared
the catalytic performance under different solvent composition
in ethanol-water mixture using TÀ Zn Cd S as photocatalyst.
x
1À x
molar ratio were prepared via a NaOH-assisted hydrothermal
[5f]
method according to our previous work. Detailed procedures
were described in Experimental Section and various character-
izations were employed to figure out the morphology and twin
crystal structure of Zn Cd S. As shown in the representative
x
1À x
0.7
0.3
scanning electron microscope (SEM) and transmission electron
microscopy (TEM) images (Figure 1 and S1-S3), the average
diameter of particles upon pure CdS, Zn Cd S, Zn Cd S,
As shown in Table 1, the conversion of DNDPE to ODA is hardly
detected in both pure water or ethanol. When photocatalytic
reaction is carried out in pure water, the separation efficiency of
photogenerated charges is poor and most photogenerated
electrons recombined with holes. PL emission spectra in
Figure S7 shows that the emission intensity in pure water is
much stronger than that in ethanol-water system. Therefore,
the amount of photogenerated electrons reacted with DNDPE is
significantly reduced and the conversion is low. At the same
0
.1
0.9
0.3
0.7
Zn Cd S, and Zn Cd S samples is 65�5.0 nm. However,
0.5
0.5
0.7
0.3
Zn Cd S and pure ZnS show obviously large particle size with
0.9
0.1
about 80 and 115 nm. High-resolution TEM image in Figure 1c
exhibits obvious boundary of twin crystal structure, which is
zinc-blende (ZB) and wurtzite (WZ) segments, respectively. The
lattice spacing of 0.320 nm belongs to the (100) diffraction
plane of Zn Cd S and the corresponding fast Fourier trans-
time, pure ethanol possesses much smaller ionization equili-
0.7
0.3
À 15.9
form (FFT) image further suggests the presence of twin crystal
brium constant (1×10
) than water, which indicates that the
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Figure 1. (a) SEM, (b) TEM and (c) HRTEM images of TÀ Zn0.7Cd0.3S sample. (d) Average diameter distribution of TÀ Zn0.7Cd0.3S nanoparticles. (e) XRD patterns of
CdS, ZnS, and TÀ Zn Cd1À xS with different Zn/Cd ratio.
x
concentration of hydrogen ions produced by pure ethanol
ionization is much smaller than that of water. The yield of ODA
is measured to be 41.46% in the ethanol-water (1:9) solvent
and reaches the maximum value of 64.54% at ethanol-water
ratio of 3:7. Further increase in the ethanol fraction dramatically
decreased the yield of ODA. This yield change of ODA indicates
that a suitable amount of ethanol is necessary probably
because of the solubility of reactants in this mixed solvent.
Previous work has reported that the addition of ethanol can
as hole sacrifice reagent for suppressing recombination with
[22]
photoelectrons. However, excess ethanol actually reduces the
yield of ODA, which may be due to the formation of other by-
products. Figure S8 shows that the by-product yield of 4-(4-
Nitrophenoxy) aniline reaches to 87.56% under ethanol-water
(9:1) solvent. Therefore, the optimal ethanol-water composition
(3:7) is selected for the following studies.
Photocatalytic performance of TÀ Zn Cd S photocatalysts
x
1À x
with different Zn/Cd ratio were further examined in the
ethanol-water (3:7) system under visible light irradiation. With
[21]
increase the solubility of nitro organics. Alcohols can also act
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vis diffuse reflection spectra in Figure S10a, the absorption
Table 1. Photocatalytic reduction of DNDPE to ODA under different
solvent composition in ethanol-water mixture using TÀ Zn0.7Cd0.3
S
as
edges of TÀ Zn Cd S samples are red-shifted with increasing
x
1À x
a
photocatalyst.
Cd amount and the corresponding color gradually changed
from white to reddish orange between ZnS and CdS (Fig-
ure S10c). To precisely determine the bandgap of TÀ Zn Cd
S
x
1À x
b
samples, the plots of Kubella-Munk function versus energy of
the incident light were shown in Figure S10b, in which the
bandgap of TÀ Zn Cd S samples were determined from
Entry
Solvent : volume ratio
DNDPE
conversion [%]
ODA
yield [%]
c
d
x
1À x
1
2
3
4
5
6
7
Pure water
1:9
<1
–
49.97
>99
>99
92.62
91.76
<1
41.46
64.54
42.86
8.89
1.42
–
3.64 eV (x=1) to 2.30 eV (x=0). In addition, the absolute
position of the conduction band (CB) for TÀ Zn Cd S samples
3:7
5:5
7:3
9:1
x
1À x
can be calculated by the equation: E =X-E -E /2, where X is the
CB
c
g
absolute electronegativity of the semiconductor, Ec is the
Pure ethanol
energy of free electrons on the hydrogen scale (~4.5 eV), and E
g
[24]
[
(
a] Reaction conditions: 10 mg TÀ Zn0.7Cd0.3S, 1.5 μmol DNDPE, visible light
λ>420 nm), total 2-hour reaction time; reaction mixtures were deter-
mined by HPLC. [b] Total volume amount of solvent is 100 mL. [c]
Conversion is calculated by the amount ratio of reacted DNDPE within 2 h
reaction time to initial DNDPE before the photocatalytic reaction. [d] ODA
yield is calculated by the amount ratio of produced ODA after reaction for
is the band gap of the semiconductor. Based on the above
equation and the calculated bandgaps, the CB position energy
is 2.41 eV for TÀ Zn Cd S, 2.18 eV for TÀ Zn Cd S, 2.03 eV for
0
.9
0.1
0.7
0.3
TÀ Zn Cd S, 1.96 eV for TÀ Zn Cd S, and 1.88 eV for
0
.5
0.5
0.3
0.7
TÀ Zn Cd S, respectively. The valance band (VB) can be
0
.1
0.9
2
h to the amount of initial DNDPE.
determined with the equation of E =E +E , which is
VB
CB
g
À 0.91 eV for TÀ Zn Cd S, À 0.70 eV for TÀ Zn Cd S, À 0.58 eV
0
.9
0.1
0.7
0.3
for TÀ Zn Cd S, À 0.54 eV for TÀ Zn Cd S, and À 0.49 eV for
0
.5
0.5
0.3
0.7
pure ZnS (Table 2, entry 1), the DNDPE conversion efficiency is
less than 5% and only trace amount of ODA is detected.
TÀ Zn Cd S also shows low DNDPE conversion of 35.94% with
TÀ Zn Cd S, respectively. The detailed information of the band
0
.1
0.9
structure is displayed in Figure 2c. It is obvious that the
conduction band position of all TÀ Zn Cd S is more negative
0
.9
0.1
x
1À x
5
.81% ODA yield (Table 2, entry 2). Interestingly, when the
than the redox potential of DNDPE, which meets the prereq-
uisite for photocatalytic reaction. However, the bandgap of
pure ZnS and TÀ Zn Cd S is calculated to be 3.64 eV and
content of Cd exceeds 30% (value of x in TÀ Zn Cd S is smaller
x
1À x
than 0.7), photocatalytic performance of DNDPE reduction is
significantly improved. The DNDPE conversion efficiency were
increased to >99% within 60 min for all TÀ Zn Cd S (x=0.7,
0
.9
0.1
3.32 eV, respectively, which is too large to absorb visible-light
energy. This should be the main reason for their low photo-
catalytic activity. When the content of Cd exceeds 30% (value
of x in TÀ Zn Cd S is smaller than 0.7), the bandgap energy for
x
1À x
0
.5, 0.3, 0.1, 0) (Figure 2a) with ODA yields are larger than 50%
(
Table 2, entry 3–7). Among them, TÀ Zn Cd S exhibits the
0
.7
0.3
x
1À x
optimal ODA selectivity with 64.45% (Table 2, entry 3) ODA
yield.
all TÀ Zn Cd S samples satisfies the requirement of visible light
x
1À x
absorption (Figure 2c), resulting in significant improvement of
ODA yield. However, with the increase of Cd content, the
bandgap structure shrinks and the energy position of CB
gradually decreases. This will lead to weakening the reduction
ability of photogenerated electrons and is unfavorable for
photocatalytic activity. Therefore, the maximum ODA yield
should be seriously balanced between the efficient visible-light
absorbance and the stronger electron reduction capacity. As
the result, the more negative CB of TÀ Zn Cd S possesses
As we all know, thermodynamic match between redox
potential of reactant and excited state (mostly determined by
the position of CB or VB) of photocatalyst is preliminary
[23]
foundation of photo-induced reaction. Figure S9 shows the
redox potential of DNDPE under the reaction condition
measured by cyclic voltammetry is À 0.216 eV. As shown UVÀ
0
.7
0.3
stronger reduction ability of photogenerated electrons and thus
beneficial for the conversion efficiency of DNDPE.
Table 2. Photocatalytic reduction of DNDPE to ODA in the ethanol-water
a
(
3:7) system using various TÀ Zn
x
Cd1À xS photocatalysts.
To evaluate the advantage of TÀ Zn Cd S photocatalysts
x
1À x
Entry
Photocatalyst
DNDPE
conversion [%]
ODA
yield [%]
b
c
for carrier separation, photo/electrochemical properties were
measured. As shown in Figure S11 and Table S1, Both
TÀ Zn Cd S and TÀ Zn Cd S exhibits higher photocurrent
1
2
3
4
5
6
7
ZnS
4.96
trace
5.81
64.45
57.55
54.82
63.08
60.28
0
.7
0.3
0.5
0.5
Zn0.9Cd0.1
Zn0.7Cd0.3
Zn0.5Cd0.5
Zn0.3Cd0.7
Zn0.1Cd0.9
CdS
S
S
S
S
S
35.94
>99
>99
>99
>99
>99
density, smaller charge transfer resistance, and weaker PL
emission intensity. These suggest that TÀ Zn Cd S and
0
.7
0.3
TÀ Zn Cd S possess higher charge separation efficiency and
0
.5
0.5
lower charge recombination rate. Therefore, more photogen-
erated charges could participate in the reaction and display
excellent photocatalytic activity for reduction of DNDPE.
In addition, the reaction equation of DNDPE reduction to
ODA is expressed as follows [Eq. (1)]:
[a] Reaction conditions: 10 mg catalyst, 1.5 μmol DNDPE, 100 mL ethanol-
water (3:7) solvent, visible light (λ>420 nm), total 2-hour reaction time;
reaction mixtures were determined by HPLC. [b] Conversion is calculated
by the amount ratio of reacted DNDPE within 2 h reaction time to initial
DNDPE before the photocatalytic reaction. [c] ODA yield is calculated by
the amount ratio of produced ODA after reaction for 2 h to the amount of
initial DNDPE.
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Figure 2. (a) The conversion of DNDPE and (b) ODA yield in the ethanol-water (3:7) system using various TÀ Zn
including CB and VB position of CdS, ZnS, and TÀ Zn Cd1À xS with different Zn/Cd ratio.
x
Cd1À xS photocatalysts. (c) Bandgap structure
x
(
1)
improved. Under the alkaline condition (pH~10.20), the yield of
ODA is only 26.58%. More importantly, the stability of ODA is
poor at this condition (Figure 3b). When the value of pH adjusts
to 2.12, it is surprising that DNDPE is completely converted
within 20 min (Figure 3a) and achieves >99% ODA yield
(Figure 3b). This has greatly exceeded the result of traditional
catalytic method. Considering the presence of the benzene ring
in DNDPE structure, we monitored the formation of gases
during the reaction by gas chromatograph (GC) technology.
The GC result shows that there is no signal of carbon-containing
It can be seen that besides the effect of photogenerated
electrons, the H concentration should also have important
effect on the reduction progress of DNDPE. Therefore, photo-
catalytic activity was further evaluated in ethanol-water (3:7)
system with different pH value. As shown in Table 3, with the
+
+
increase of H concentration (reduced pH value), DNDPE
conversion efficiency and selectivity of ODA are dramatically
gases such as CO . Therefore, it is reasonable to speculate that
2
Table 3. Photocatalytic reduction of DNDPE to ODA in ethanol-water (3:7)
system with different pH value.
the benzene ring in DNDPE structure remains stable without
producing or consuming carbon element. The carbon content
involved in the reaction is equal to the carbon in the product.
Moreover, the apparent quantum efficiency (AQE) under several
monochromic lights was measured with the ratio of the amount
of ODA and the amount of incident photons (see equation S3 in
Experimental Section). The calculated AQE is 0.931%, 0.472%,
and 0.01% at wavelength of 420, 520, and 550 nm, respectively.
It is observed that the wavelength-dependent hydrogen
evolution efficiency was in well agreement with the absorption
spectrum of TÀ Zn Cd S (Figure 3c), indicating that the
a
b
c
d
Entry
pH value
DNDPE conversion [%]
ODA yield [%]
1
2
3
4
5
10.20
7.61
6.47
4.06
2.12
>99
>99
>99
>99
>99
26.58
64.45
85.53
88.75
>99
[
1
a] Reaction conditions: 10 mg TÀ Zn0.7Cd0.3S catalyst, 1.5 μmol DNDPE,
00 mL ethanol-water (3:7) solvent, visible light (λ>420 nm), total 2-hour
reaction time; reaction mixtures were determined by HPLC. [b] The
regulation of pH value was carried out by adding different amount HCl
0
.7
0.3
À 1
À 1
(
0.1 mol L ) or NaOH (0.1 molL ) solution. [c] Conversion is calculated by
reduction of DNDPE in ethanol-water system was induced by
photo-absorption of catalyst.
the amount ratio of reacted DNDPE with 2 h reaction time to initial DNDPE
before the photocatalytic reaction. [d] ODA yield is calculated by the
amount ratio of produced ODA to the amount of initial DNDPE.
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Figure 3. (a) The conversion of DNDPE and (b) ODA yield in ethanol-water (3:7) system with different pH value using TÀ Zn0.7Cd0.3S photocatalysts. (c)
Wavelength dependence of apparent quantum efficiency and (d) Cycle stability of ODA yield for TÀ Zn0.7Cd0.3S sample in ethanol-water (3:7) system with
pH=2.12.
In addition, the effect of solvent to substrate ratio on
activity was investigated. As shown in Table S5, after appropri-
ately increases the amount of DNDPE substrate (5 μmol), the
DNDPE conversion is 99% and 82% ODA yield is obtained in
characteristic peaks are well-maintained, and the peak position
did not obviously shift (Figure S13, 14). The absorption property
of the recycled TÀ Zn Cd S kept the similar absorption edge
0
.7
0.3
with the fresh sample (Figure S15). These characterization
2
h reaction time (entry 1). Similar result was obtained in
results indicate that TÀ Zn Cd S are stable under acidic
x
1À x
reduced solvent volume (80 mL solvent) and 1.5 μmol substrate,
which shows the slightly decreased ODA yield (95.44%) in
comparison with the original condition. Therefore, suitable
solvent to substrate ratio is the critical factor for efficiently
photocatalytic activity. Considering the practical application
condition in future, ethanol can be recovered by distillation
under reduced pressure to ensure a clean and environmentally
friendly reaction process.
conditions during photocatalytic reaction.
In order to understand the reaction mechanism of photo-
catalytic reduction of DNDPE, time courses of amounts of
DNDPE, ODA, intermediate and by-product were monitored.
Figure 4a–b shows that the amount of DNDPE quickly
decreased within 10 min irradiation of visible light with the
conversion efficiency exceed 95%. Different from DNDPE
conversion, ODA yield follows the monotonical linear increase
in the initial stage. The yield of ODA is only 66.7% within the
initial 10 min and continuously increases to 99% after 20 min
light irradiation. We calculated an indicator for labelling
material balance (MB) by using the equation as follows
The stability of TÀ Zn Cd S catalyst under the acidic
x
1À x
conditions should be crucial, therefore, the cyclic stability of
TÀ Zn Cd S was tested. Fresh prepared TÀ Zn Cd S sample
x
1À x
0.7
0.3
was stored in ethanol-water (3:7) system with pH value of 2.12
[25]
for 24 h. Then the above treated TÀ Zn Cd S was evaluated for
[Eq. (2)].
0
.7
0.3
photocatalytic reduction of DNDPE with five recycling experi-
ments under visible light irradiation. The result in Figure 3d
shows that the photocatalytic reduction of DNDPE to ODA of
treated TÀ Zn Cd S maintains stable and the yield of product
n ðODAÞþn ðDNDPEÞ
MB ¼
(2)
n ðDNDPEÞ
0
0.7
0.3
ODA has no obvious change. Furthermore, the morphology,
where n(ODA) and n(DNDPE) are the amounts of ODA and
DNDPE during the photocatalytic reaction, respectively, and
n0(DNDPE) is the amount of DNDPE before the photocatalytic
reaction. As shown in Figure 4b, the value of MB is smaller than
unity in the initial 10 min and then gradually increases to 1
within 20 min. This result indicates the presence of intermediate
during the reduction of DNDPE to ODA. DNDPE contains two
structure and surface chemical state of the used TÀ Zn Cd S
0
.7
0.3
photocatalyst are characterized. The SEM, TEM and EDS images
in Figure S12 show that the morphology and size of the used
TÀ Zn Cd S sample did not change obviously in comparison
0.7
0.3
with the freshly prepared TÀ Zn Cd S sample (Figure 1a–c).
0
.7
0.3
XRD and XPS characterizations show that the corresponding
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Figure 4. (a and b) Time courses of material analysis of DNDPE and ODA in photocatalytic reduction of DNDPE in ethanol-water (3:7) system under visible
light irradiation. (c) HPLC analysis of intermediate products in the preliminary reduction process of DNDPE.
nitro groups (À NO ) and the reduction of each À NO to the
absolute reduction potential of Eabs(NO À RÀ NOjNOÀ RÀ NO) and
2
2
2
amino group (À NH ) could produce two intermediates, i.e., a
Eabs(NO À RÀ NOjNO À RÀ NHOH) as following formula. Detailed
2
2
2
[26]
nitroso compound (À NO) and a hydroxyl amine (À NHOH).
calculation is discussed in Supporting information [Eq (3)].
Therefore, various intermediates may be formed during the
photocatalytic reaction, which has been listed in Table S3.
To analyze the structures of intermediate products, HPLC
technology was applied to identify the preliminary reduction
intermediate of DNDPE by comparing with the standard
substance and the results are shown in Figure 4c. The retention
time at 2.77 min and 11.85 min is the signal of ODA and DNDPE,
respectively. In the reaction time of initial 5 min, it can be seen
that the peak intensity of DNDPE dramatically decreases and
ODA peak appears. At the same time, three other peaks at
retention time of 3.63 min (peak I), 6.95 min (peak II) and
D GðOjRÞ
r
E
absðOjRÞ¼ À
(3)
n F
e
where O refers to an oxidation substance, R refers to a reduced
substance, n is the number of electrons exchanged between
e
the oxidized and reduced species, Eabs is the absolute reduction
potential, Δ G refers to the free energy changes of reaction (gas
r
[
28]
phase Gibbs free energy and solvation free energy). As shown
in Table 4, the absolute reduction potential of Eabs(NO À RÀ NOj
2
NO À RÀ NHOH) is 4.67 V, which is larger than that of E
-
2
abs
1
3.05 min (Peak III) are observed, which corresponding to 4-(4-
(NO À RÀ NOjNOÀ RÀ NO). It is well known that the larger
2
nitrophenoxy)
benzene
hydroxylamine
(labelled
as
algebraic value of reduction potential means that the oxidation
substance has stronger ability for obtaining electrons and
NO À RÀ NHOH, where R represents the structure of
), 4-
2
[
23]
(
4-nitrophenoxy) aniline (NO À RÀ NH ), and 4-(4-nitrophenoxy)
produces corresponding reduction products.
Therefore,
2
2
nitrosobenzene (NO À RÀ NO), respectively (Figure S16). The
above result demonstrates NO À RÀ NO is more likely to obtain
2
2
presence of three intermediates indicates that during DNDPE
reduction process, two nitro groups are reduced separately one
by one instead of simultaneously reduced. Within 15 min, peaks
electrons and forms NO À RÀ NHOH rather than NOÀ RÀ NO (part I
2
of Scheme 1). The intermediate NO À RÀ NHOH is also detected
2
by HPLC mass spectrometry (HPLC-MS) as shown in Figure S16,
of both DNDPE and NO À NO disappeared completely. Mean-
17. Similarly, the comparison between Eabs(NO À RÀ NHOHj
2
2
while, peaks of NO À RÀ NHOH and NO À RÀ NH were barely
NO À RÀ NH ) and E (NO À RÀ NHOHjNOÀ RÀ NHOH) is 4.82 V vs.
2
2
2
2
2
abs
2
observed. After irradiation of 20 min, only ODA product is
detected and the ODA yield is close to 100% without any other
intermediates or by-product, indicates the intermediates have
gradually converted into ODA.
4.20 V, which indicates the formation of NO À RÀ NH is much
2
2
easier than NOÀ RÀ NHOH (path II of Scheme 1).
Considering the reduction of DNDPE involves hydrogena-
tion, it is necessary to study the source of hydrogen. Ethanol
and water are both possible hydrogen source in the present
reaction system. During photocatalytic reduction of DNDPE,
In addition, DFT calculations were performed to further
reveal the reduction possibility of two nitro group in DNDPE.
The detailed calculated process is shown in Table S4 of
two nitro groups (À NO ) of DNDPE could be hydrogenated with
2
[29]
supporting information.
Herein, we first calculated the
hydrogen proton to form amino groups (À NH ) and two phenyl
2
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a
Table 4. The absolute reduction potential of different redox reactions with DFT calculation.
À
b
Entry
Reaction ([O]+ne ![R])
Δ
r
G(OjR) (Hartree)
E
abs(OjR) (V)
À
þ
þ
1
2
3
4
NO
NO
NO
NO
2
2
2
2
À R À NO þ 2e þ2H ¼ NOÀ R À NOþH
2
O
À 0.32
À 0.34
À 0.35
À 0.31
4.36
4.67
4.82
4.20
À
À R À NO þ 2e þ2H ¼ NO
2
À R À NHOH
À
À
þ
þ
À R À NHOH þ 2e þ2H ¼ NOÀ R À NH
2
þH
2
O
2
O
À R À NHOH þ 2e þ2H ¼ NOÀ R À NHOHþH
[
a] The calculation is performed with the Gaussian programs. Gibbs free energy is calculated with zero-point energy and thermal contributions to the
[27]
energy with vibrational frequencies by B3LYP/6-31G*. [b] ΔrG(OjR) represents the free energy changes during the reaction. Hartree is the unit of energy
À 1 [28]
value. The conversion relationship between Hartree and the International standard unit is 1 Hartree=2625.5 kJmol
.
Scheme 1. The proposed photocatalytic reduction pathways of DNDPE to ODA over TÀ Zn
x
Cd1À xS photocatalysts.
groups keep unchanged. Several literatures also report that the
benzyl groups are usually hydrogenated under the condition of
high temperature and pure hydrogen gases, which needs to
solvent is D-ethanol-H O, the signal of HPLC-MS exhibits m/z=
2
201 and is similar to that of ethanol-H O solvent (Figure 5c).
2
This result implies that ODA is composed with the common
hydrogen atom (H) and D-ethanol don’t provide protons for
hydrogenation of DNDPE. This comparison result demonstrates
that the hydrogen source of ODA mainly comes from water
instead of ethanol. In addition, it should be mentioned that the
added HCl (for regulating pH) is also suspected of providing
hydrogen atoms. The comparison experiment with the presence
overcome
hydrogenation.
the
higher
energy
barrier
than
nitro
[
30]
If the hydrogen source is replaced with
deuterated hydrogen, such as deuteroxide D O or deuterated
2
ethanol (D-ethanol), the amino groups of formed ODA product
should be involved deuterium (D) element and the molecular
weight of ODA will be heavier than that of original hydrogen
atom. Based on the above principle, isotope labeling in
combination with in situ HPLC-MS analysis was measured to
monitor the molecular weight change of formed ODA product
under different composition of ethanol and water and
determine the source of hydrogen. Photocatalytic reduction of
DNDPE to ODA is carried out in following three ethanol-water
solvents, which is the common ethanol-H O, ethanol-D O, and
of deuterium chloride (DCl) was carried out in ethanol-H O
2
solvent. The result is similar to ethanol-H O system without DCl
2
(Figure 5d) and shows the signal of m/z=201 assigned to ODA
with common hydrogen, which excludes the possibility of the
hydrogen source from HCl.
Besides the sacrificial reagent oxidized by photogenerated
holes were also studied. Using 2,4-dinitrophenylhydrazine
(DNPH) as the detection reagent for aldehyde compound, we
monitored the acetaldehyde product during whole reaction
time from 10 min to 120 nm. Figure S19 shows the gradual
increase of acetaldehyde. This phenomenon is similar to the
2
2
D-ethanol-H O, respectively. For comparison, the standard ODA
2
was dissolved in ethanol-H O solvent and shows the m/z of 201
2
(Figure S18). After photoirradiation for 120 min, Figure 5a shows
that the strongest signal of m/z=201 is observed in ethanol-
H O system, which indicates the formation of ODA product. At
2
sacrifice reagent role in photocatalytic H evolution reaction,
2
the same time, the strong signals at m/z=202, 203 are detected
such as TiO semiconductor in methanol sacrificial reagent, g-
2
in ethanol-D O solvent (Figure 5b), which should be ascribed to
C3N4 photocatalyst in TEOA sacrificial reagent, et al.
2
the presence of heavier deuterium atom (D) in ODA. When the
TÀ Zn Cd S is photoexcited to generate electrons in the
0
.7
0.3
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2 2 2
Figure 5. Isotope in situ HPLC-MS signal of reaction mixture after reaction for 120 min in (a) common ethanol-H O, (b) ethanol-D O, (c) D-ethanol-H O, and (d)
ethanol-H O-DCl solvent, respectively.
2
conduction band to reduce DNDPE, while photoexcited holes Conclusion
oxidize ethanol to acetaldehyde.
According to above analysis, the possible photocatalytic
reduction pathways of DNDPE to ODA are proposed and
depicted in Scheme 1. Under the visible light irradiation,
TÀ Zn Cd S photocatalyst is photoexcited to generate elec-
In summary, we firstly found that nano-twin crystal Zn Cd
S
1À x
x
(TÀ Zn Cd S) semiconductor, activated by visible light irradi-
x
1À x
ation, promoted highly efficient photocatalytic reduction of
DNDPE to ODA without any noble metal cocatalysts or addi-
tional hydrogen source. This photocatalytic strategy breaks
through the limitations of thermo-catalytic reaction process
which overly relies on high temperature, high pressure, and
expensive organic solvents. TÀ Zn Cd S shows excellent
x
1À x
tron-hole pairs. Due to the existence of twin crystal structure
upon TÀ Zn Cd S, photogenerated charges achieve effective
x
1À x
separation, in which electrons of CB transport from zinc-blende
to wurtzite phase. Then photogenerated electrons and holes
respectively transfer to the surface of the photocatalyst. The
sacrifice reagent of ethanol could capture holes and be oxidized
to acetaldehyde. At the same time, the reduction ability of
photogenerated electrons upon TÀ Zn Cd S is stronger than
0
.7
0.3
DNDPE conversion of >99% and the yield of ODA product
achieves >99% within 40 min reaction time in ethanol-water
(3:7) solvent of pH=2.12. As far as we know, this is the first
time to report such high efficiency conversion DNDPE and ODA
yield and it has exceeded the reported results of classical
chemical/thermal catalytic reduction (representative result: 90%
ODA yield upon Pd/C catalysts). The more negative CB position
of TÀ Zn Cd S could provide stronger reduction ability of
x
1À x
the reduction potential of DNDPE (E(
=À 0.216 V). There-
DNDPE/ODA)
fore, the nitro group (À NO ) of DNDPE could be reduced by
2
photogenerated electrons and hydrogenated with protons of
water. According to the experiment result and theoretical
calculation, the reduction mechanism of DNDPE is two nitro
groups of DNDPE are separately instead of simultaneously
reduced. One nitro group of DNDPE is firstly transformed to
x
1À x
photogenerated electrons, and acid reaction condition are
more favorable to hydrogenation of DNDPE. These two effects
facilitate rapid DNDPE to ODA reduction while suppressing side
reactions. The reaction mechanism reveals that two nitro
groups in DNDPE are separately reduced one by one, which
demonstrates the order DNDPE!NO À RÀ NO!NO À RÀ NHOH!
NO À RÀ NO (where R represents the structure of
) via
2
reduction-dehydration reaction, in which NO À RÀ NO intermedi-
2
ate is detected by HPLC of Figure 4c. NO À RÀ NO is then
2
2
2
hydrogenated to form NO À RÀ NHOH. NO À RÀ NHOH can be
NO À RÀ NH !NOÀ RÀ NH !NH À RÀ NHOH!ODA. At the same
2
2
2
2
2
2
transformed to NO À RÀ NH through hydrogenation-dehydra-
time, the hydrogen atom of DNDPE hydrogenation is derived
from the hydrogen protons of water rather than ethanol,
determined by the isotope labeling HPLC-MS analysis. The
ethanol acts as the sacrifice reagent of photogenerated holes
for suppressing recombination with photoelectrons and enhan-
ces the catalytic activity.
2
2
tion reaction. The second nitro group of DNDPE also follows the
above reduction process, producing NOÀ RÀ NH , NH À RÀ NHOH,
2
2
and ODA product.
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Experimental Section
using 10 mg catalyst. AQE was calculated using the following
formula [Eqs. (6) and (7)]:
Chemicals
number of reacted electrons
number of incident photons
All chemicals were analytical grade and used without any further
purification. The 4,4’-Dinitrodiphenyl ether (>99.0%), 4,4’-Diamino-
diphenyl ether (>98.0%), and 4-(4-nitrophenoxy) aniline (>98.0%)
were supplied by aladdin. Cd(CH COO) ·2H O, Zn(CH COO) ·2H O,
thioacetamide (98%, Sigma Aldrich), HCl, and NaOH were pur-
chased from Tianjin Guangfu.
AQE ¼
ꢁ100 %
(6)
¼ 1
2 ꢁ number of evolved DNDPE molecules
ꢁ100 %
3
2
2
3
2
2
(
7)
number of incident photons
Acknowledgements
Preparation of photocatalysts
This work was financially supported by the National Natural
Science Foundation of China (91233108, 21703287), the Natural
Science Foundation of Shandong Province (ZR2017MB006,
ZR2017BB027, ZR2020QB057), the Major Program of Shandong
Province Natural Science Foundation (ZR2017ZB0315) and the
Fundamental Research Funds for the Central Universities
A series of TÀ Zn Cd S (x=0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) samples
x
1À x
were synthesized via NaOH assisted hydrothermal method accord-
[
5f]
ing to the previous literatures.
As for TÀ Zn Cd S sample,
0.7
0.3
7
mmol of Zn(CH COO) ·2H O and 3 mmol of Cd(CH COO) ·2H O
3
2
2
3
2
2
were added into 20 mL deionized water by constantly stirring to
form a homogeneous solution, and then 12.5 mmol of thioaceta-
mide was drop into the above solution while stirring. The mixture
was stirred for 1 h and then 5 mL of 4 M NaOH aqueous solution
was dispersed into the reaction by constantly vigorous stirring for
(
18CX02052A, 19CX05002A) are acknowledged. The author ac-
knowledges Taishan Scholar Program of Shandong Province
ts201712019), the fellowship of China Postdoctoral Science
(
1
5
1
h. Subsequently, the mixture solution was transferred into a
0 mL Teflon-lined stainless-steel autoclave and was heated to
80°C for 24 h. After the system was then cooled down to room
Foundation (2020M672162), Talent Introduction Program of China
University of Petroleum (East China) (yj20190039), and Independ-
ent Innovation Project of China University of Petroleum (East
China) (20CX06035A) for the financial support.
temperature. The final precipitates were collected by centrifugal
and washed with distilled water and absolute alcohol for 5 times,
and then was dried overnight in vacuum at 60°C. Other
TÀ Zn Cd S samples were prepared with the same above method
x
1À x
except for the amount of adding Zn(CH COO) ·2H O and Cd-
3
2
2
(
CH COO) ·2H O.
Conflict of Interest
3
2
2
The authors declare no conflict of interest.
Photocatalytic reduction of DNDPE
In
a
typical operation, the TÀ Zn Cd
S powder (10 mg) is
1À x
x
Keywords: Cocatalyst-free · Nano Catalysis · Photocatalysis ·
Reduction · Twin crystal
suspended in a water-ethanol solution (70:30) (100 mL) containing
DNDPE (about 1.5 μmol) in a 250 mL quartz reactor, and 1 mL
1
6
mol/L HCl is added. The suspension was fully stirred at a speed of
00 r/min, and degassed for 30 minutes before light irradiation to
remove air. Seal the reactor with a balloon under nitrogen (N ), and
2
then irradiate light at
a wavelength >420 nm. During the
irradiation, the temperature of the reactor was maintained at room
temperature by circulating water in the outer interlayer. A 300W Xe
lamp (Beijing Perfectlight Technology Co., Ltd. PLS-SXE 300D) with
a 420 nm cut-off filter was selected as the light source. The
measured effective scattering area was 16.5 cm , and the obtained
reaction solution was analyzed qualitatively and quantitatively by
HPLC.
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FULL PAPERS
Twin Crystal Catalysis: A highly-
efficient cocatalyst-free nano-twin
Y. Hu, Dr. G. Yu*, C. Xing, S. Liu, C. Wei,
Dr. H. Liu, Prof. J. Jiang*, Prof. X. Li*
crystal Zn Cd S is employed to
x
1À x
1
– 12
achieve highly efficient conversion of
DNDPE with ~99% and the yield of
ODA product with >99% within
Cocatalyst-Free Reduction of 4,4’-
Dinitrodiphenyl Ether to 4,4’-Diami-
nodiphenyl Ether Over Twin-Crystal
4
0 min reaction time under mild and
environmentally friendly reaction
condition.
Zn Cd1À xS under Visible Light
x