A green and efficient photocatalytic route for the highly-selective oxidation of saturated alpha-carbon C-H bonds in aromatic alkanes over flower-like Bi2WO6

Article abstract of DOI:10.1039/c5cc07586f

A green and efficient route for the highly-selective oxidation of saturated alpha-carbon C-H bonds in aromatic alkanes under visible-light irradiation and solvent-free conditions is developed using flower-like Bi₂WO₆ microspheres as catalysts.

Full text of DOI:10.1039/c5cc07586f

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A Green and Efficient Photocatalytic Route for High-Selective
Oxidation of Saturated Alpha-Carbon C–H Bond in Aromatic
Alkanes over Flower-like Bi2WO6
Received 00th January 20xx,
Accepted 00th January 20xx
Ying Liu a, Lang Chena, Qing Yuan, Jie He, Chak-Tong Au and Shuang-Feng Yin*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
green and efficient route for high-selective oxidation of involvement, above room temperature (60°C) and use of noble
saturated alpha-carbon C-H bond in aromatic alkanes under metal. Hence, it is necessary to design high-efficiency
visible-light irradiation and solvent-free condition is developed photocatalysts for the oxidation of saturated C-H bonds. As a simple
using flower-like Bi2WO6 microspheres as catalyst.
Aurivillius oxide, bismuth tungstate (Bi2WO6) shows good
performance in the degradation of organic compounds under
visible-light irradiation, owing to its suitable band location and
unique layered structure.21-25 The Bi2WO6 materials are low-cost
and easily prepared. Recent reports demonstrated that Bi2WO6
exhibits appreciable results for the selective oxidation of alcohols
under visible-light irradiation.26-30
Among the oxidation reactions of hydrocarbon, the efficient and
selective activation of saturated C-H bonds in alkanes or aromatics
for the generation of value-added products such as aldehydes and
ketones is a significant challenge both in organic synthesis and
chemical industry.1 Because of the low selectivity and/or
conversion, the conventional processes are unsatisfactory. In
addition, the systems are usually environment-unfriendly. Since the
saturated C-H bonds are thermodynamically strong and dynamically
inert, the reaction conditions are usually harsh, requiring high
temperature, high pressure, and extra initiators or toxic reagents.2-5
From the viewpoint of green chemistry, it is desirable to develop an
environment-benign route that is highly efficient for the conversion
of alkanes or aromatics to value-added products.
Herein, we report for the first time the use of flower-like Bi2WO6
as photocatalyst for highly selective and efficient oxidation of
aromatic alkanes using O2 as oxidant under visible-light irradiation
and solvent-free condition. We reveal the pivotal roles of
photogenerated holes (h+) and superoxide radicals, and propose a
possible mechanism for the reaction.
The flower-like Bi2WO6 microspheres were prepared by a simple
hydrothermal method, and the details are provided in the
Electronic Supplementary Information (ESI).† According to the SEM
images (Fig. 1a and b), the flower-like microspheres are 6-7 μm in
diameter and are made up by nano-sheets. Based on the results of
XRD and XPS analysis (Figure S1 and S2), the microspheres are
identified as orthorhombic Bi2WO6. 31, 32
It was demonstrated that heterogeneous photocatalysis has
huge potential in the oxidation of hydrocarbons.6-10 So far, only a
limited amount of catalysts, e.g. TiO2 or TiO2-based,11-15 CdS or CdS-
based materials,16, 17 and Nb2O5,18 were researched for the selective
photo-oxidation of alkanes or aromatic alkanes using O2 as oxidant.
However, the application of these catalysts is hampered by ill
factors such as low selectivity, poor efficiency, and the need of
using ultraviolet light or organic solvents. As far as the use of visible
light is concerned, the works on photocatalytic selective oxidation
of alkanes or aromatic alkanes are rare, especially for those without
the use of noxious solvents.19, 20 Furthermore, for the few known
cases there are shortcomings such as low selectivity, extra-initiator
In UV-Vis DRS study, the flower-like Bi2WO6 microspheres
exhibit intense absorption in both ultraviolet and visible-light
regions (Fig. 1d). The band gap energy (Eg) is estimated to be 2.96
eV based on the absorption spectrum and the (Ahv)=K(hv–Eg)n
formula.33 The calculated position of conduction and valence band
is at -0.41 and 2.55 eV, respectively (Fig. S3), implying a relatively
·–
strong oxidation ability of the material. Since H2O/·OH and O2/O2
display a standard redox potential (vs. NHE) of 2.3 and -0.28 eV
that are within the redox capability of the flower-like Bi2WO6, the
band structure provides the feasibility and theoretical basis for the
formation of hydroxyl radicals (·OH) and superoxide radicals (O2·–).
The selective oxidation of typical primary aromatic alkanes was
carried out at room temperature in the absence of solvent over
various catalysts under visible-light irradiation for 5 h. Under N2
atmosphere or in the absence of a photocatalyst and/or light
a.The authors contributed equally to this work
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry
and Chemical Engineering, Hunan University, Changsha 410082, Hunan, People’s
Republic of China
*Corresponding Author: Phone (Fax):86-731-88821171
E-mail address: sf_yin@hnu.edu.cn
†Electronic Supplementary Information (ESI) available: experimental details,
photocatalyst characterization figures, photocatalytic activity experiments. See
DOI: 10.1039/x0xx00000x
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Scheme 1 Substrate scope for photocatalytic oxidation of aromatic
alkanes in this study a
(a)
(b)
5ꢀm
1ꢀm
(c)
(d)
2c
2d
Conv. 0.8 %
Sel. 100 %
2a
Conv. 2.2 %
Sel. 82%
2b
Conv. 2.1 %
Sel. 92 %
Conv. 3.0 %
Sel. 93 %
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
hν (eV)
5 nm
300
400
500
600
700
800
Wavelength (nm)
2g
Conv. 1.8 %
Sel. 100 %
2h
Conv. 1.6 %
Sel. 100 %
2e
Conv. 1.3 %
Sel. 100 %
2f
Fig. 1 (a-b) SEM images, (c) TEM image, and (d) UV-Vis DRS of
flower-like Bi2WO6; the inset is the corresponding (Ahν)2 versus
photon energy plot
Conv. 1.2 %
Sel. 100 %
2i b
Conv. 3.4 %
Sel. 100 %
2j c
Conv. 2.0 %
Sel. 100 %
illumination, there is no detection of reactivity. The results of the
control experiments suggest that the reaction is photocatalytic, and
an oxidant such as O2 is needed.
The photocatalytic activity of selective oxidation of toluene was
Conversion:
2.0%
16
25, 34, 35
examined over CdS and Bi2WO6 of different morphologies
Acetophenone:
556 ꢀmol·gꢁ1·hꢁ1
Selectivity: 88%
(prepared according to the reported literatures) (Table 1). It was
observed that the flower-like Bi2WO6 performs the best, showing
the highest reaction rate (464 μmol·g-1·h-1) and conversion (1.5%).
The only by-product of the reaction is benzyl alcohol. Since all the
prepared samples exhibit good response of visible light and the
band gaps of Bi2WO6 with different morphologies are similar (Fig.
S4), the high performance of flower-like Bi2WO6 is unlikely to be
due to the good response of visible light or the appropriateness of
band gap. Besides, the structure properties and photoluminescence
(PL) results of the as-prepare Bi2WO6 samples (Fig. S5 and S6, and
Table S1) do not suggest that the better photoactivity of flower-like
Bi2WO6 can be ascribed to factors such as surface area, pore volume
or separation ability of photogenerated charge carries. Considering
Conversion: 1.5%
Propiophenone:
488 ꢀmol·gꢁ1·hꢁ1
Selectivity: 100%
a Substrate (8 mmol), catalyst (50 mg), O2 flow rate (3 mL·min-1)
b
P-cresol (8mmol), catalyst (50 mg), benzotrifluoride (2 mL), O2
flow rate (3 mL·min-1)
c
P-methoxytoluene (1 mmol), catalyst (50 mg), acetonitrile (2 mL),
O2 flow rate (3 mL·min-1)
that the as-prepared Bi2WO6 samples are greatly different in
morphology (Fig. S7), it is deduced that the high performance is
ascribable to the unique flower-like structure of Bi2WO6, and the
enhancement of visible light absorption as a result of multi-
reflection of light across the structure.
Table 1 Selective oxidation of toluene over different photocatalysts
under visible-light irradiation a
Benzaldehyde
(μmol·g-1·h-1)
Conv.
(%)
Sel.
(%)
Catalyst
It was found that the selective oxidation of toluene derivatives
over flower-like Bi2WO6 is also effective (Scheme 1). It is noted that
the oxidation of ethylbenzene or n-propylbenzene produces
ketones rather than aldehydes. Thus, only the alpha-carbon of C-H
bond is activated in this photocatalytic system. Compared with the
oxidation of xylenes, the lower transformation of chlorotoluenes
can be associated with the presence of electron withdrawing group
CdS
240
172
160
140
0.7
0.6
0.5
0.4
98
97
95
96
Nanoparticle Bi2WO6
Nanoplate Bi2WO6
Nest-like Bi2WO6
Flower-like Bi2WO6
464
1.5
96
a
Photocatalyst (50 mg), substrate (8 mmol), no solvent, room (–Cl) that inhibits the oxidation activity of the methyl. That is, the
temperature, O2 flow rate (3 mL·min-1), visible-light irradiation
methyl of xylenes is more active than that of chlorotoluenes. For
the same reason, the selectivity to chlorobenzaldehydes is 100%,
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(λ > 400 nm, 5 h).
2 | J. Name., 2012, 00, 1-3
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Visible Light
140
120
100
80
Initial
AO
TBA
O2
eꢁ eꢁ eꢁ
K2S2O8
h+ h+ h+
60
Bi2WO6
40
Fig. 3 Schematic of the plausible mechanism for photocatalytic
selective oxidation of toluene to benzaldehyde over flower-like
Bi2WO6 under visible-light irradiation
20
0
1
2
3
4
5
Reaction time (h)
Based on the above results, it is deduced that the major active
species for the photocatalytic selective oxidation of toluene is h+
rather than ·OH or e–. To a certain degree the deduction is in
agreement with the active species commonly found in systems for
the oxidation of hydrocarbons.16, 27, 28 Accordingly, we propose a
plausible mechanism for the catalytic reaction (Fig. 3). Before the
start of visible light illumination, toluene gets in touch with Bi2WO6.
After then, the photogenerated holes from valence band and
electrons from conduction band are transferred to the Bi2WO6
surface under visible light irradiation. The direct oxidation of
toluene for the formation of toluene cationic radicals as facilitated
by the holes is a vital step. This step involves the activation of
saturated C-H bond and is often regarded as the rate-determining
step for selective oxidation of alkanes. 1 Finally, the O2 dissolved in
toluene combines with the toluene cationic radicals to generate the
target product benzaldehyde. In this reaction system the holes play
a key role in the direct dehydrogenation of toluene to the radical
intermediate, and O2 is the main oxidizing species.
Fig. 2 Kinetic-experiments for selective oxidation of toluene under
visible light in the presence or absence of various radical scavengers
over flower-like Bi2WO6 (AO as hole scavenger, K2S2O8 as electron
scavenger, and TBA as hydroxyl radical scavenger).
whereas in the oxidation of xylenes, there is the generation of the
corresponding alcohols as by-product.
Furthermore, time-dependent scale-up experiments on
toluene oxidation over flower-like Bi2WO6 show that there is
effective increase of benzaldehyde yield but no significant change
of the high selectivity upon the extension of irradiation time (Fig.
S8). With such performance, it is clear that the flower-like Bi2WO6 is
a photocatalyst effective for the oxidation of aromatic alkanes to
the corresponding aldehydes. In all cases of using flower-like
Bi2WO6, there is no detection of carbon dioxide, indicating the
absence of over oxidation during the photocatalytic processes.
We performed experiments to prove that the photocatalytic
oxidation of toluene over Bi2WO6 involves free radicals. By adding
0.01 mmol tetra-methylpiperidine N-oxide (TEMPO) which is a
typical radical scavenger, we quench the transformation of toluene
to benzaldehyde almost completely. Furthermore, the effects of
different active species in the reaction system were explored.
Kinetic-experiments were performed with an appropriate amount
of ammonium oxalate (AO), potassium persulphate (K2S2O8) and
tert-butanol (TBA) as scavenger for holes (h+), photogenerated
electrons (e–) and hydroxyl radicals (·OH), respectively.36-38 As
shown in Fig. 2, the conversion of toluene is almost prohibited in
the presence of AO, and there is no increase of conversion with
reaction time. The results indicate that the holes are vital for the
activation of toluene. Besides, both the addition of TBA and K2S2O8
results in a certain decrease of toluene conversion. The results
suggest that ·OH and e– are also involved in the reaction, but their
influences are apparently moderate. Since O2 accepts
In summary, a green and efficient system has been developed
for the selective oxidation of aromatic alkanes into valuable
products under mild conditions. This is the first report on the use of
easily synthesized flower-like Bi2WO6 as active photocatalyst for the
selective oxidation of aromatic alkanes under visible-light
irradiation without the need of any solvents. The good performance
of flower-like Bi2WO6 toward the oxidation of aromatic alkanes is
attributed to its unique morphology. We investigated the specific
functions of active intermediate species, and proved that the
photogenerated holes are significantly involved in the oxidation
process. A plausible reaction mechanism is proposed. The visible-
light driven method using flower-like Bi2WO6 opens an
environment-friendly way for efficient synthesis of organic
compounds that requires the activation of saturated alpha-carbon
C-H bonds.
This project was financially supported by NSFC (Grant nos.
21401054, 21476065), the China Postdoctoral Science Foundation
(2014M562098) and the Fundamental Research Funds for the
Central Universities. C.-T. Au thanks the Hunan University for an
adjunct professorship.
·–
photogenerated electrons easily to produce superoxide radical (O2
39
), it was considered as oxidant for the transformation of toluene.
16
In this study, the conversion of toluene increase with time in the
presence of electron scavenger K2S2O8. In other words, the
oxidation by O2·– is unlikely to occur or only plays a minute role in
this system.
Notes and references
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