Tuning Catalytic Selectivity in Cascade Reactions by Light Irradiation

Article abstract of DOI:10.1007/s10562-018-2320-9

Abstract: Selectivity of cascade redox reactions: the reduction of nitrobenzene to azoxybenzene and then to azobenzene and the oxidation of benzyl alcohol to benzaldehyde and then to benzoic acid, is discovered to be tuneable via light irradiation over plasmonic gold photocatalysts. The representative photocatalyst of Au/CeO₂ was characterized by TEM, EDX, UV–Vis and XPS to determine its morphology, elemental composition, photo absorptivity and oxidation state of gold. The catalytic test results demonstrate that the net contribution of light irradiation correlates with the ability of incident light to excite electrons and light absorption of catalysts. These findings may inspire peer researchers in developing new photocatalytic processes or in designing new photocatalysts for clean chemicals synthesis.

Full text of DOI:10.1007/s10562-018-2320-9

Catalysis Letters
https://doi.org/10.1007/s10562-018-2320-9
Tuning Catalytic Selectivity in Cascade Reactions by Light Irradiation
Xingguang Zhang1 · Jianfeng Yao1 · Xuebin Ke2
Received: 26 November 2017 / Accepted: 5 February 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Selectivity of cascade redox reactions: the reduction of nitrobenzene to azoxybenzene and then to azobenzene and the oxi-
dation of benzyl alcohol to benzaldehyde and then to benzoic acid, is discovered to be tuneable via light irradiation over
plasmonic gold photocatalysts. The representative photocatalyst of Au/CeO2 was characterized by TEM, EDX, UV–Vis and
XPS to determine its morphology, elemental composition, photo absorptivity and oxidation state of gold. The catalytic test
results demonstrate that the net contribution of light irradiation correlates with the ability of incident light to excite electrons
and light absorption of catalysts. These findings may inspire peer researchers in developing new photocatalytic processes or
in designing new photocatalysts for clean chemicals synthesis.
Graphical Abstract
Keywords Photocatalysis · Nanoparticles · Aromatic compounds · Selectivity regulation · Plasmonic effect
1 Introduction
Controlling selectivity in clean chemicals synthesis is cru-
* Xingguang Zhang
cial to obtaining target products with high purity [1, 2], and
x.g.zhang@njfu.edu.cn
it is more challenging in cascade reactions [3, 4]. Apart
* Jianfeng Yao
from optimizing reaction kinetics and mass transfer kinet-
jfyao@njfu.edu.cn
ics to achieve better catalytic efficiency [5], developing new
* Xuebin Ke
heterogeneous catalysts requires controlling active sites
x.ke@hull.ac.uk
(e.g. composition, orientation, and distribution) [6, 7], and
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

X. Zhang et al.
structures (e.g. pores, sizes, and shapes) [1, 8]. Electrons
(e.g. density, interaction, and transfer) also play an indispen-
sable role in manipulating catalytic performances, particu-
larly in photocatalysis which involves light-excited electrons
[9, 10]. In parallel, strategies associated with energy input
are developed to enhance product selectivity, such as the use
of microwave, ultrasound, an additional electric or electro-
magnetic field, and light irradiation [11–13].
support (nanopowder, 99.95%, CeO2) was also purchased
from Sigma-Aldrich and used without further purification.
2.2 Catalyst Preparation
In a glass beaker with a magnetic stirring bar, 2.5 g of sup-
port powders, such as CeO2, were dispersed into the 100 ml
of 3.8×10−3 mol/L HAuCl4 aqueous solution. Subsequently,
20 ml of 0.53 mol/L lysine was added dropwise to the mix-
ture under stirring, and after that the stirring was prolonged
for 30 min. To this suspension, 10 ml of 0.35 mol/L NaBH4
solution was added dropwise, followed by adding 10 ml of
0.3 mol/L hydrochloric acid (to achieve a pH 9.5). The mix-
ture was stirred for 1 h and aged for 24 h. Then the solid was
separated, washed firstly with 600 ml deionized water for
three times in a 250-ml centrifuge bottle to remove the chlo-
rine ions, and the residual solution was tested by 0.1 mol/L
AgNO3 solution and no AgCl precipitate was observed.
The solids were collected, dried at 60 °C for 16 h, and used
directly as the photocatalyst.
Photocatalysis offers a promising opportunity to obtain a
high product selectivity due to its capacity to influence cata-
lytic reactions at ambient temperatures. More significantly, the
unique property of incident light interacting with reactants or
photocatalysts is able to initiate or accelerate reactions with
low energy barriers and to give a certain product [14–17].
Recent studies demonstrate that plasmonic photocatalysts
based on the localized surface plasmon resonance (LSPR)
effect can actually achieve these goals [18–20]. The LSPR
effect features a collective and coherent oscillation of the free
conduction electrons with the incident photons on the surface
of metal nanoparticles (NPs) [21–23]. These strong oscillat-
ing electrons can drive the target reaction locally (around
active sites) and affect the product selectivity fundamentally
[24–26]. However, the functional mechanism of light irradia-
tion to tune reactive procedure remains ambiguous.
2.3 Photocatalytic Tests
The reaction was conducted in a 50 ml round-bottomed
Pyrex glass flask with a sealed spigot and a magnetic stirrer.
The reaction temperature was controlled by an air-condi-
tioner within a sealed wooden box. The light intensity in
the reaction position was set at 0.1 W/cm2 and could be
adjusted by changing the distance between the reactor and
the light source. The wavelength range was tuned by using
various glass filters to cut off the irradiation below a certain
value of wavelength. A tuneable laser source (OPOTECK
VIBRANT, 350 II) and a solar simulator (NEWPORT,
150 W) were used as light source, respectively.
Herein, a tuneable laser source and a solar simulator are
used to explore the processing mechanism involving incident
light, light absorption, electron excitation, electron transfer
and activating the reactants to obtain targeted products. In
clean chemicals synthesis, the catalytic hydrogenation of
nitro compounds is an ideal process to produce amines,
mainly azobenzene (other products of azoxybenzene and
aniline can also be produced via this route) [14], and the
catalytic oxidation of benzyl alcohol plays a significant role
to produce aromatic aldehydes [27]. Conventional catalytic
processes of these two consecutive reactions always neces-
sitate complicated methodologies to minimize the production
of side-products [28–31]. This study highlights that light irra-
diation over supported plasmonic gold catalysts [14, 32] can
effectively tune the catalytic selectivity of the aforementioned
two cascade reactions, with the net contribution depending
upon light wavelength and intensity at a certain temperature.
Catalytic reduction of nitrobenzene was conducted under
the argon (Ar) atmosphere. Typically, 3 mmol of nitroben-
zene, 30 ml of isopropanol as solvent, and 3 ml of 0.1 mol/L
KOH solution in isopropanol were mixed in the reactor, fol-
lowed by adding 100 mg of the catalyst and purging with
argon gas, and then stirred during reaction and illuminated
with the light source.
Catalytic oxidation of benzyl alcohol was conducted
under the oxygen (O2) atmosphere. Typically, 3 mmol of
benzyl alcohol was dissolved into 30 ml of isopropanol, and
3 ml of 0.1 mol/L KOH solution in isopropanol and 100 mg
of the catalyst were added into the mixture. The mixture was
stirred during reaction and illuminated with the light source.
During these reactions, 0.5 ml aliquots were collected at
given irradiation time intervals and filtered through a Milli-
pore filter (pore size 0.45 µm) to remove the catalyst particu-
lates. Then flask was purged with Ar or O2 again for more
than 3 min to remove air and then sealed. The filtrates were
analyzed by an Agilent 6890 gas chromatograph with the
2 Experimental
2.1 Raw Materials
Chemicals of chloroauric acid (99.999%, HAuCl4), l-Lysine
(98.0%), sodium borohydride (99.99%, NaBH4), hydrochloric
acid (37%, HCl), nitrobenzene (99.0%), potassium hydrox-
ide (99.0%, KOH), isopropanol (99.5%), and azobenzene
(98.0%), were purchased from Sigma-Aldrich. The inert gas
of argon was from the BOC (99.99%). The cerium (IV) oxide
1 3

Tuning Catalytic Selectivity in Cascade Reactions by Light Irradiation
HP-5 column. Quantities of the products and reactants were
calculated from the peak areas of the compounds using cali-
bration curves. An Agilent HP5973 mass spectrometer was
used to determine and analyze the product compositions.
At a higher temperature (80 °C), the influence of light is
negligible and both light reaction and thermal reaction can
achieve a complete conversion. Generally, the light irradia-
tion can increase the reaction conversion compared with the
reaction with light off at lower reaction temperatures and
have significant influence on product selectivity, thereby
playing a threshold/switch role in tuning the catalytic reac-
tion procedure.
2.4 Characterization
TEM images were taken with a JEOL JEM-2100 transmis-
sion electron microscope employing an accelerating voltage
of 200 kV. The specimens were sample powders deposited
onto a copper microgrid coated with a holey carbon film.
The element composition of some samples was determined
by energy-dispersive X-ray spectroscopy attached on an FEI
Quanta 200 scanning electron microscopy (The average
gold loading was 2.8 wt%). The diffuse reflectance UV–Vis
spectra of the samples were recorded on a Cary 5000 spec-
trometer. The XPS data were recorded on an ESCALAB
250 spectrometer and Al Ka radiation was used as the X-ray
source. The C1s peak at 284.5 eV was used as a reference
for the calibration of the binding energy scale.
To explain this phenomenon, the physiochemical prop-
erties of catalysts are characterized by transmission elec-
tron microscopy (TEM), X-ray photoelectron spectroscopy
(XPS), and UV–Vis spectroscopy. In Fig. 1a, the typical
TEM images of the Au-NPs on the CeO2 support show that
Au NPs are well dispersed and have narrow size distribu-
tions within 2–6 nm. The XPS spectra (Fig. 1b) provides the
spectra of Au 4f and confirms that the binding energies of
Au4f7/2 and Au4f5/2 electrons are 84.0 and 87.7 eV, respec-
tively, corresponding to the oxidation state of gold metal
[33]. The UV–Vis spectra of Au/CeO2 shows the absorption
band centres at about 539 nm in wavelength owing to the
LSPR effect; on the contrary, the CeO2 support itself shows
no absorption in the visible light region, which substantially
demonstrates that the contribution of light irradiation to
affecting the catalytic activity and selectivity via the LSPR
effect of Au NPs only.
3 Results and Discussion
The catalytic performances of the hydrogenation of nitroben-
zene are presented in Table 1. The effect of light irradiation
on tuning the product selectivity is evident. In particular, at
40 °C and with the light off, the selectivity of azoxybenzene
can be up to 91%; whereas with the light on, azoxybenzene
disappears and the yield of azobenzene can be up to >99%.
It would be exciting if the underlying mechanistic rea-
sons for the light irradiation which affects catalytic selec-
tivity are unravelled. At the current stage of this study,
we propose that the distinct differences of reaction path-
way at a certain reaction temperature can be ascribed to
Table 1 Catalytic performances of 3%Au/CeO2 for selective reduction of nitrobenzene under the simulate sunlight source (a solar simulator)
O
NO2
N
N
NH2
N
N
Au/CeO2
Isopropanol
Azoxybenzene
Nitrobenzene
Azobenzene
Aniline
T (°C)
Light
Selectivity (%)
Azoxybenzene
Conversion (%)
Nitrobenzene
Net change (%)
Conversion
Azobenzene
Aniline
Azobenzene
91
40
60
80
On
Off
On
Off
On
Off
–
91
–
92
–
–
>99
9
96
6
80
98
–
>99
86
>99
93
>99
>99
14
–
4
2
20
2
7
90
18
0
Bold values highlight the catalytic results with light on
Reaction conditions time=6 h, solvent=isopropanol (30 ml), catalyst=3%Au/CeO2 (100 mg), reactant=nitrobenzene (3 mmol), KOH
(0.1 mol/L, 3 ml), light wavelength (full), and light intensity (0.1 W/cm2). Control experiments no products were detectable over the CeO2 sup-
port as catalyst
1 3

X. Zhang et al.
Fig. 1 a TEM images of 3%Au/
CeO2 with Au size distribution,
b XPS spectra and c UV–Vis
spectra of 3%Au/CeO2
30
25
20
15
10
5
a
0
2
3
4
5
6
7
8
Particle size / nm
87.7
Au4f5/2
b
c
84.0
Au4f7/2
539 nm
3%Au/CeO2
CeO2
90
87.5
85
82.5
80
400
500
600
700
800
Binding energy / eV
Wavelength / nm
“the reduction barriers of intermediate products” and “the
light-induced electrons interaction” between reactant mol-
ecules and Au NPs, and these proposed associations have
been initially explored in our previous studies [24, 25, 34].
To illustrate, the reduction of nitrobenzene can be induced
by both heat and the light irradiation over supported gold
catalysts [14, 28], undergoing different reaction pathways
and thus giving different product selectivity. In the light-
induced process, the solvent of isopropanol also serves
as a hydrogen donor to form the intermediate species of
Au–H bonds [14], and these species can break the N–O
bonds to reduce nitrobenzene. Meanwhile, the resonating
electrons of Au NPs strongly interact with the electrophilic
nitro groups of nitrobenzene to facilitate the cleavage of
the N–O bonds by Au–H species. For these reasons, even
at a lower temperature of 40 °C, the reduction of nitroben-
zene still occurs efficiently; however, at a higher tempera-
ture of 80 °C, both heat and light irradiation take effect,
and it is difficult to identify the individual contribution of
heat or light, thereby showing identical conversions and
similar selectivity. Hence, the similar experimental results
at 80 °C with light on and off convince the switch role of
light irradiation at lower reaction temperatures of 60 °C
and 40 °C (Table 1).
As the light irradiation essentially affects the catalytic
reaction procedure, then it is sensible to deduce that the cata-
lytic performances should differ with the light source, light
absorption and energy transfer. It is shown in Fig. 2a that the
Fig. 2 a The change in selectiv-
a
b
ity with the laser wavelength
under the same light intensity
of 80 mW/cm; b the change in
the product selectivity with the
laser intensity at the same wave-
length of 539 nm. AZY azoben-
zene, azoxy azoxybenzene
1 3

Tuning Catalytic Selectivity in Cascade Reactions by Light Irradiation
Table 2 Catalytic performances of 3%Au/CeO2 for the selective oxidation of benzyl alcohol under simulate light source (a solar simulator)
CHO
CH OH
COOH
Au/CeO
2
2
Isopropanol
Benzyl alcohol
Benzaldehyde
Benzoic acid
T (°C)
Light
Selectivity (%)
Benzaldehyde
Conversion (%)
Benzyl alcohol
Net change (%)
Conversion
Benzoic acid
Benzaldehyde
–
40
60
80
On
Off
On
Off
On
Off
>99
>99
91
>99
–
93
–
90
31
28
6
–
59
9
–
>99
7
97
69
>99
93
9
93
Bold values highlight the catalytic results with light on
Reaction conditions time=6 h, solvent=isopropanol (30 ml), catalyst=3%Au/CeO2 (100 mg), reactant=benzyl alcohol (3 mmol), KOH
(0.1 mol/L, 3 ml), light wavelength (full), and light intensity (0.1 W cm2). Control experiments no products were detected over CeO2 as catalysts
selectivity towards azoxybenzene, azobenzene and aniline
varies as the function of light wavelength under the same
light intensity. But the ability of incident light to excite ener-
getic electrons decreases with longer wavelength and thus
the reaction activity declined. The excited electron distribu-
tions are different even with the same (macroscopic) light
intensity. In general, with the shorter wavelength, there have
more electrons in a high energy level. For instance, with the
same light absorption, at 505 and 582 nm, the conversions
are roughly the same, but the reaction proceeds to different
stages, namely, the selectivity is different. In the case of 460
and 623 nm, the latter only proceeds at the initial stage, hav-
ing a>99% of selectivity towards azoxybenzene.
to control the production of benzaldehyde or benzoic acid
at 80 °C. This observation is inspiring because it not only
proves the “switch role” of light irradiation for these two
selected representative cascade reactions but also demon-
strates that this phenomenon applies to both reductive and
oxidative cascade reactions.
4 Conclusion
This study highlights the contribution of light irradiation to
tuning the selectivity of redox cascade reactions by means
of supported plasmonic gold photocatalysts. The reduction
of nitrobenzene and the oxidation of benzyl alcohol exhibit
different selectivity with light on and off at a certain tem-
perature. The net contribution correlates to light wavelength
and intensity. To further identify or even quantify the con-
tribution of light irradiation and to clarify their underlying
mechanisms, more investigation will be performed on such
as photocatalysts with different gold loadings or sizes and
other redox cascade reactions. Overall, this study presents
a promising approach to refining the product selectivity in
clean chemicals synthesis, and could inspire the design of
new catalytic materials and processes.
Besides light wavelength, the light intensity also has
influence on the catalytic selectivity as shown in Fig. 2b.
Increasing light intensities at the fixed wavelength (at
539 nm) boost more energetic electrons, thus resulting in
different product selectivity; the conversions also increase
and undergo latter stages, that is, much higher yields of
azobenzene and aniline are obtained.
Further to test the generality of this finding that light irra-
diation can tune the product selectivity, the selective oxi-
dation of benzyl alcohol to benzaldehyde and then to ben-
zoic acid was also investigated, and the catalytic results are
shown in Table 2. At 40 °C, the oxidation of benzyl alcohol
mainly produces benzaldehyde, but the conversion is much
higher under visible light irradiation. On the other hand,
the heat effect is significant at 80 °C, and benzyl alcohol
is oxidized into benzaldehyde with light off, but it further
transforms to benzoic acid if the light irradiation applies;
these results show that the light irradiation acts as a “switch”
Acknowledgements We thank the National Natural Science Founda-
tion of China (NNSFC 21706134), the Young Natural Science Foun-
dation of Jiangsu province (BK20170918) and Natural Science Key
Project of the Jiangsu Higher Education Institutions (15KJA220001)
for financial support.
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X. Zhang et al.
17. Yang X-J, Chen B, Zheng L-Q, Wu L-Z, Tung C-H (2014) Green
Chem 16:1082–1086
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Affiliations
Xingguang Zhang1 · Jianfeng Yao1 · Xuebin Ke2
1
2
College of Chemical Engineering, Jiangsu Key Lab
School of Engineering and Computer Science, University
for the Chemistry & Utilization of Agricultural and Forest
Biomass, Nanjing Forestry University, Nanjing 210037,
Jiangsu, People’s Republic of China
of Hull, Hull HU6 7RX, UK
1 3