Facile Synthesis of BiVO4 for Visible-Light-Induced C?C Bond Cleavage of Alkenes to Generate Carbonyls

Article abstract of DOI:10.1002/cssc.201900439

BiVO₄ crystals synthesized by an ultrasonic-assisted method (Sono-BiVO₄) showed improved efficiency as a heterogeneous photocatalyst under visible-light irradiation. Sono-BiVO₄ was successfully used for the C?C bond cleavage of alkenes to generate carbonyl compounds. Styrene derivatives were converted into carbonyl compounds in the presence of Sono-BiVO₄ under highly sustainable conditions requiring only natural sources, that is, molecular oxygen, visible light, and water at room temperature. Additionally, Sono-BiVO₄ could be easily separated from the reaction mixture and reused.

Full text of DOI:10.1002/cssc.201900439

Accepted Article
Title: Facile Synthesis of BiVO4 for Visible-Light-Induced C-C Bond
Cleavage of Alkenes to Generate Carbonyls
Authors: Eun Jin Cho, Joon Yong Park, Hye Rin Choe, Ki Min Nam,
Sung Su Han, and Ho Seong Hwang
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ChemSusChem
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COMMUNICATION
Facile Synthesis of BiVO for Visible-Light-Induced C-C Bond
4
Cleavage of Alkenes to Generate Carbonyls
[
a]
[b]
[a]
[b]
,[b]
Sung Su Han, Joon Yong Park, Ho Seong Hwang, Hye Rin Choe, Ki Min Nam,* and Eun Jin
Cho*,
[a]
Abstract: BiVO
ultrasonic-assisted method showed improved efficiency as
heterogeneous photocatalyst under visible-light irradiation. Sono-
BiVO was successfully used for the C-C bond cleavage of alkenes
to generate carbonyl compounds. Styrene derivatives were
converted to carbonyl compounds in the presence of Sono-BiVO
4
crystals (Sono-BiVO
4
)
synthesized by an
a
4
4
under highly sustainable conditions, requiring only natural sources,
i.e., molecular oxygen, visible light, and water, at room temperature.
4
In addition, Sono-BiVO could be easily separated from the reaction
mixture and reused.
With the growing awareness for sustainable development, there
has been much interest in finding greener sources and methods
in the chemistry community.[1] An abundant and easily available
energy source, visible light, has great potential in driving
environmentally benign chemical transformations. In the last
Figure 1. Redox potentials of valence (VB) and conduction (CB) bands, and
bandgap energies of metal oxides.
decade, visible-light photocatalysis has seen
a flourishing
[
2]
renaissance, with various photocatalysts. However, recent
significant developments have focused on homogeneous
catalysis, which requires the use of organic dyes or metal (such
as Ru, Ir, and Cu) complexes. On the other hand,
heterogeneous catalysis remains largely underdeveloped
despite the notable advantages of the catalysts, such as stability
and reusability by easy separation from the reaction mixture,
which is an important consideration for industrial manufacturing
processes.[3]
3 3 2 3
A mixture of 0.2 M Bi(NO ) •5H O and 0.2 M VCl precursor
solution was prepared, and ultrasonication was performed under
ambient conditions (Figure 2a). Performing the ultrasound under
ambient conditions for 1 h resulted in the efficient preparation of
BiVO
at 500 °C for 3 h in air to yield crystallized BiVO
Sono-BiVO crystals). Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM, inset) images confirmed
the high uniformity of the BiVO cubes with an average grain
4
powders. The as-prepared BiVO
4
powders were annealed
4
(denoted as
4
Metal oxides such as TiO
inexpensive, and easily filtered from the mixture.
2
, ZnO, and Nb
2
O
5
are stable,
However,
4
[
3a,4]
size of 426 ± 104 nm (Figures 2b). X-ray diffraction (XRD)
analysis was carried out to obtain reliable structural information
about this material (Figure 2c).
because of their wide bandgap (large gap between the valance
and conduction bands), these oxides function as efficient
photocatalysts only under UV irradiation, and they show a
narrow substrate scope under visible-light irradiation (Figure
1
(
).[5] Given its narrower band gap energy, bismuth vanadate
BiVO ) has great potential for use as a heterogeneous visible-
light photocatalyst that can be easily activated in the visible-light
4
wavelength range (~500 nm).[6,7] Surprisingly, BiVO
been used as a photocatalyst in organic transformations[8] as it
shows poor photoreactivity under visible-light irradiation. Herein,
has rarely
4
4
we prepared BiVO crystals in order to improve the efficiency of
this oxide toward organic transformations under visible-light
irradiation.
[
[
a]
b]
Sung Su Han, Ho Seong Hwang, and Prof. Dr. Eun Jin Cho
Department of Chemistry, Chung-Ang University, 84 Heukseok-ro,
Dongjak-gu, Seoul 06974, Republic of Korea
E-mail: ejcho@cau.ac.kr
Joon Yong Park, Hye Rin Choe, and Prof. Dr. Ki Min Nam
Department of Chemistry and Chemistry Institute for Functional
Materials, Pusan National University, Busan 46241, Republic of
Korea
E-mail: namkimin.chem@gmail.com
Figure 2. (a) Schematic of the preparation of Sono-BiVO
and TEM (inset) images of the Sono-BiVO crystals. (c) XRD pattern of the
Sono-BiVO crystals.
4
crystals. (b) SEM
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
4
4
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Having verified the improved photoreactivity of Sono-BiVO
4
based on the photocatalytic degradation of MO, we next
investigated the reactivity of this catalyst in organic
transformations. The use of molecular oxygen and water as
natural reagents, in addition to this new heterogeneous visible-
light photocatalyst, is expected to attract much attention in
sustainable chemistry.[
1]
Figure 3. UV-visible absorption spectrum and Tauc plot (inset) of Sono-BiVO
4
crystals.
The diffraction peaks of the Sono-BiVO
to the monoclinic scheelite structure (JCPDS No. 75-2480),
indicative of well-crystallized BiVO . The bandgap was estimated
from the onset of UV-visible absorbance (Figure 3). From the
absorbance data, the Sono-BiVO crystals showed direct
transition with band gap of 2.4 eV. To compare the
photocatalytic performance of the Sono-BiVO crystals, various
BiVO materials were prepared by well-known synthetic methods
such as the solid-state reaction and hydrothermal method[9] (the
materials are denoted as SS-BiVO , and HT-BiVO , respectively).
Figure S2 shows the SEM images and XRD patterns of SS-
BiVO , and HT-BiVO . The XRD peaks of these samples were
identical to those of the monoclinic scheelite structure of BiVO
The photocatalytic activities of the BiVO samples were then
4
crystals were indexed
4
4
4
4
4
4
4
4
4
.
4
evaluated in the degradation of methyl orange (MO) dye in water
under UV-visible irradiation (1 sun, 100 mW/cm2).
Figure 4a shows the absorption spectra of the MO solution as a
function of illumination time in the presence of the Sono-BiVO
4
crystals. The absorption peak at 465 nm decreased gradually
and finally disappeared after 180 min, indicating complete
degradation of the dye. Blank test (MO without any catalyst)
under UV-visible light showed the negligible self-photolysis of
Figure 4. (a) Time-dependent UV-Vis absorption spectra of MO solution in the
presence of the Sono-BiVO
MO when using Sono-BiVO
black triangles) crystals, and without BiVO
4
crystals. (b) Photocatalytic degradation rates of
(blue squares), HT-BiVO (red circles), SS-BiVO
(pink stars).
4
4
4
(
4
4 4
MO. For comparison, SS-BiVO and HT-BiVO were also used
for photocatalytic MO degradation under the same conditions.
Figure 4b shows the degradation of MO in the presence of
Carbonyl functionalities such as aldehydes and ketones are
some of the most important and versatile functional groups in
organic chemistry.[11,12] Accordingly, a wide range of synthetic
methods for carbonyls have been developed; in particular,
oxidative cleavage of alkenes to generate carbonyls is a widely
employed strategy both in academia and in the industry. A
widely used traditional approach is ozonolysis.[13] However, this
two-step process requires the toxic ozone as the oxidant, as well
as special equipment and an additional reductant. The other
representative method is Lemieux-Johnson oxidation, where
Sono-BiVO
for 180 min, the degradation percentages were 99%, 23%, and
1% for Sono-BiVO , HT-BiVO , and SS-BiVO , respectively.
The Sono-BiVO crystals exhibited higher photocatalytic activity
4
for MO degradation as compared to SS-BiVO and HT-BiVO .
4 4 4
, HT-BiVO , and SS-BiVO samples. After irradiation
1
4
4
4
4
4
The improved photocatalytic activity is mainly attributed to the
charge separation (hole diffusion length) and surface area. The
is about 100 nm.[
10]
hole diffusion length of BiVO
BiVO
nm (Figure 2), which is effective size of charge separation as
compared to SS-BiVO (~500 nm) and HT-BiVO (~1000 nm)
crystals. Furthermore, the Brunauer-Emmett-Teller (BET,
4
The Sono-
4
crystals prepared in this study had a radius of about 200
OsO
4
-catalyzed direct oxidation of alkenes to diols is followed by
[14]
C-C bond cleavage using NaIO
4
.
This two-step process also
4
4
has an inherent disadvantage because of the toxicity of the
reagents used. Although much effort has been made to develop
environmentally benign methods for the cleavage of alkenes to
carbonyls,[ there is no established method using molecular
oxygen as the oxidant in water at room temperature.
nitrogen, 77K) surface area of the Sono-BiVO
4
sample
2
(
6.79m /g) was nearly 5 times as large as that of SS-BiVO
4
(1.44
is a
15]
2
2
4 4
m /g) and HT-BiVO (0.81 m /g). Thus, the Sono-BiVO
more efficient structure for degradation of organic pollutants in
water.
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COMMUNICATION
corresponding aldehyde products. Substrates with electron-
Table 1. Oxidative cleavage of alkene 1a to acetophenone 2a
donating substituents such as m-, p-methyl (1e, 1f), p-tert-butyl
(
1g), and p-methoxy (1h) groups as well as electron-withdrawing
substituents such as o-, p-bromo (1i, 1j), p-fluoro (1k), and p-
chloro (1l) furnished the corresponding aldehydes (2). In general,
the carboxylic acid compound was generated as a side product
through further oxidation of the aldehyde product under the
conditions. In particular, more amount of carboxylic acid was
generated from the styrene derivatives with electron-donating
groups such as 1f and 1g.
Table 2. Substrate scope for the aerobic C-C bond cleavage
[
a] Reaction conditions: 1a (0.1 mmol), Sono-BiVO4 (3 mg) [b] The yields
were determined by GC-MS using n-dodecane as the internal standard. [c]
Reaction time: 36 h
We envisioned that selective aerobic oxidation[16] of alkenes to
carbonyls would proceed in the presence of Sono-BiVO in
4
water (solvent) under visible-light irradiation at room temperature.
We started our investigation with -methylstyrene 1a as the
model substrate (Table 1). To our delight, oxidative cleavage of
1
a occurred in the presence of Sono-BiVO
temperature under visible-light irradiation from 18 W blue LEDs
450 nm) to give acetophenone 2a in 78% yield (entry 1). None
4
in water at room
(
of the organic solvents we employed showed better reactivity
than water (entries 2-6). Control experiments revealed that the
reaction requires a photocatalyst, visible light, and molecular
oxygen (entries 7-9). The reaction also proceeded well under
irradiation by 18 W green LEDs (entry 10), although a longer
time was required in this case. This heterogeneous reaction was
significantly affected by the amount of water (entries 1, 11, and
1
2).
We further confirmed that this newly prepared Sono-BiVO
shows better reactivity as heterogeneous visible-light
photocatalyst as compared to SS-BiVO and HT-BiVO
Consistent with the results shown in Figure 4, the reaction yield
obtained using Sono-BiVO was higher than that in the case of
HT-BiVO and SS-BiVO (entry 1 vs. entries 13 and 14).
4
a
4
4
.
4
4
4
Next, we investigated the substrate scope of the C-C bond
cleavage to generate the carbonyl products from styrene
derivatives. 1,1-Disubstituted compounds 1a, 1b, and 1c were
converted to the ketone products in good yields. Reactions of
various styrene derivatives proceeded under the optimal
conditions, regardless of the electron density, to give the
[
a] Reaction conditions: 1 (0.5 mmol), Sono-BiVO
4
(15 mg). [b] Isolated yields,
1
GC yields using n-dodecane as the internal standard, or H NMR yields using
bromoform as the internal standard.
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COMMUNICATION
The synthetic utility of the transformation was examined in a
gram-scale synthesis, where product 2a was prepared on a 10
mmol scale. The yield was similar to that obtained in the 0.5
mmol scale reaction, despite the longer reaction time.
The reaction was selective as only terminal alkenes participated
in the C-C bond cleavage. Internal alkenes were not suitable
substrates for this transformation.[17] Interestingly, the reactions
of highly conjugated stilbenes (trans and cis) provided the
corresponding epoxide products despite very low conversion
quencher) also prevented the reaction to give less amount of 2a
in 17% yield, indicating the involvement of superoxide as the
reactive oxygen species. However, it is not possible to rule out a
mechanism where singlet oxygen involve because the use of
DABCO (singlet oxygen quencher) also decreased the reaction
yield to 32%. The proposed mechanism is shown in Scheme S1
of the supporting information.
4
In conclusion, an efficient heterogeneous catalyst, BiVO was
synthesized by an ultrasonic-assisted method and successfully
applied for the C-C bond cleavage of alkenes. Complementary
to the traditional two-step processes such as ozonolysis and
Lemieux-Johnson oxidation, this method is expected to be a
practically viable alternative. Remarkably, the transformation in
(
Scheme 1).
the presence of our newly synthesized BiVO
sustainable, requiring only natural sources, i.e., molecular
oxygen, visible light, and water. The BiVO crystals can be easily
4
is highly
4
Scheme 1. Reactions of stilbene. 3a/3b (0.1 mmol), Sono-BiVO
4
(3 mg), H
2
O
separated from the reaction mixture and reused with the same
efficiency.
(1 mL). The yields were determined by GC-MS using n-dodecane as the
internal standard.
4
Note that the heterogeneous Sono-BiVO could be easily
Experimental Section
separated from the reaction mixture and reused (Table 3). The
recovered photocatalyst was subjected to the same reaction
more than 5 times. In all cases, the catalyst provided 2a, without
notable loss of reactivity, demonstrating the most significant
An oven-dried resealable tube equipped with a magnetic stir bar was
charged with Sono-BiVO (15 mg) in H O (5 mL) and the tube was
4 2
sealed with a silicone septum screw cap. Molecular oxygen was bubbled
through the reaction mixture for 5 min; then, styrene derivatives (0.5
mmol) were added. (Please add styrenes after oxygen bubbling due to
the volatility of some styrenes.) An oxygen-filled balloon was fixed to the
tube, which was then placed under 18 W 450 nm blue LEDs (420-480
nm) at room temperature, with stirring for 8-24 h. The reaction progress
advantage for
turnover number (TON) of 2100 was obtained in the presence of
.01% Sono-BiVO
a heterogeneous catalyst. Furthermore, the
0
4
.
1
was monitored by gas chromatography or H NMR spectroscopy. After
Table 3. Reusability of Sono-BiVO
4
crystals[a]
completion of the reaction, dichloromethane or ethyl ether was added to
the mixture, and the organic phase was extracted and concentrated on a
rotary evaporator (carefully evaporated using a vacuum controller owing
to the volatility of some carbonyl products). The carbonyl products was
purified by silica gel flash column chromatography, using hexanes and
ethyl ether as the eluent.
Acknowledgements
This research was supported by the Chung-Ang University
Graduate Research Scholarship in 2018 for S. S. Han. We
gratefully acknowledge the National Research Foundation of
Korea [NRF-2012M3A7B4049657, NRF-2014-011165, NRF-
2017R1E1A1A01074224 and NRF-2017R1A2B2004082].
[
4
a] Reaction conditions: 1a (0.5 mmol), Sono-BiVO (15 mg). [b] The yields
were determined by GC-MS using n-dodecane as the internal standard.
Keywords: Heterogeneous catalysis • Bismuth Vanadates •
Oxygen • Photocatalysis • C-C bond cleavage
Next, to ascertain mechanistic insight into the source of oxygen,
1
8
isotopic labeling experiment was conducted with water- O in the
reaction of 1a. The 18O-labelled product was not formed from the
reaction, indirectly confirming that molecular oxygen is the
oxygen source for the carbonyl products. Additionally, several
experiments were conducted to propose a mechanism for the
oxidative cleavage of alkenes (Table S2). The reaction of 1a
was conducted in the presence of various additives such as
TEMPO, 1,4-dinitrobenzene, benzoquinone, and DABCO.
TEMPO (free radical quencher) and 1,4-dinitrobenzene (single
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electron
transfer
inhibitor)
completely
inhibited
the
transformation.
The use of benzoquinone (superoxide
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ChemSusChem
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COMMUNICATION
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[
4
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COMMUNICATION
Sung Su Han, Joon Yong Park, Ho
Seong Hwang, Hye Rin Choe, Ki Min
Nam,* and Eun Jin Cho*
Page No. – Page No.
4
Facile Synthesis of BiVO for Visible-
Light-Induced C-C Bond Cleavage of
Alkenes to Generate Carbonyls
4
As an efficient heterogeneous catalyst, BiVO crystals were synthesized by an
ultrasonic-assisted method and successfully applied for the C-C bond cleavage of
alkenes.
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