Photocatalytic oxidation of benzyl alcohols by three-dimensional reduced graphene oxide/nano zinc oxide catalyst under visible light

Article abstract of DOI:10.3184/174751915X14476823418480

A three-dimensional reduced graphene oxide/nano zinc oxide (3D-RGO/ZnO) composite photocatalyst has been prepared through an in situ reduction of GO and growth of nano ZnO particles process. The 3D-RGO/ZnO catalyst displayed enhanced photocatalytic activity towards commercial ZnO nanoparticles and can be applied to the oxidation of a broad series of benzyl alcohols including primary and secondary alcohols to corresponding carbonyl compounds under the irradiation of visible light. A plausible photocatalytic mechanism referring to photo electron generation and transfer has been proposed.

Full text of DOI:10.3184/174751915X14476823418480

JOURNAL OF CHEMICAL RESEARCH 2015
VOL. 39 DECEMBER, 727–730
RESEARCH PAPER 727
Photocatalytic oxidation of benzyl alcohols by three-dimensional reduced
graphene oxide/nano zinc oxide catalyst under visible light
Yuxiang Wanga, Xianlong Zhanga, Lihua Wangb and Youjian Chena*
aCollege of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325027, P.R. China
bShimadzu (China) Co., Ltd. Guangzhou, Guangdong 510140, P.R. China
A three-dimensional reduced graphene oxide/nano zinc oxide (3D-RGO/ZnO) composite photocatalyst has been prepared through an in
situ reduction of GO and growth of nano ZnO particles process. The 3D-RGO/ZnO catalyst displayed enhanced photocatalytic activity
towards commercial ZnO nanoparticles and can be applied to the oxidation of a broad series of benzyl alcohols including primary and
secondary alcohols to corresponding carbonyl compounds under the irradiation of visible light. A plausible photocatalytic mechanism
referring to photo electron generation and transfer has been proposed.
Keywords: reduced graphene, zinc oxide, photocatalysis, oxidation, primary and secondary alcohols
One of the most fundamental approaches to synthesise carbonyl
compounds is the oxidation of alcohols via commonly used
inorganic oxidants such as (NH4)2Ce(NO3)6,1 CrO3,2 Corey–
Suggs reagent3 and MnO2,4 and organic oxidants such as
9-azabicyclo[3.3.1]nonane N-oxyl (ABNO),5 2-azaadamantane
To combine the outstanding performances of 3D-RGO and
nano ZnO, we first synthesised a 3D-RGO/ZnO composite
photocatalyst through an in situ reduction of GO and growth
of nano ZnO particles and then applied this photocatalyst to the
photocatalytic oxidation of benzyl alcohols.
N-oxyl
(AZADO),6
2,2,6,6-tetramethylpiperidinyloxy
(TEMPO)7 and 2-iodoxybenzoic acid (IBX),8,9. However, most
existing oxidants have problems including high toxicities,
difficult recovery and dangerous operation which limited
their broad applications. As a clean, abundant, and sustainable
energy source, efficient utilisation of solar energy is of great
importance.10 If we can effectively use solar energy to achieve
the hydroxyl–carbonyl transformation, traditional inorganic
and organic oxidants may not be indispensable. The crucial
factor is to prepare an appropriate photocatalyst. To date, many
photocatalysts have been developed, such as Mott–Schottky
heterojunctions,11 Au–Pd alloy,12-14 and Pd nanostructures,15,16
for photocatalytic organic reactions, as well as semiconductors
for photocatalytic degradation of pollutants.17,18 Compared to
noble metal catalysts, inexpensive semiconductors with a wide-
band gap have competitive potential. Under the irradiation of
light, efficient photogeneration and transfer of electron can take
place at the interfaces of semiconductor to organic reactants,
thus displaying excellent photocatalytic effects. For example,
titanium dioxide (TiO2) can effectively catalyse the oxidation
of benzyl alcohols into corresponding aldehydes under visible
light irradiation.19 Zinc oxide (ZnO) which, like TiO2, is a
common wide bandgap semiconductor, has many applications
in the fields of electroluminescence,20 photodetector,21 dye
degradation,22 and photocatalysis.23,24 ZnO is possesses excellent
optical activities and light-energy transformations, implying it
has a great potential for the photocatalytic oxidation of benzyl
alcohols.
Three-dimensional reduced graphene oxide (3D-RGO), a
kind of two-dimensional (2D) carbon material, usually prepared
from graphene oxide (GO) through thermal,25 chemical,26 and
hydrothermal27 processes has shown great potential in a broad
range of applications, such as polymer composites,28,29 chemical
and biological sensors,30 energy-storage,31 electrocatalysis,32
and environmental disposal.33 Due to their high specific surface
area and OH groups, many nanoparticles can be deposited on
the surface of RGO through a in situ growth process, and thus
3D-RGO/nanoparticles composites form which can display
enhanced performances toward single nanoparticles including
electromagnetic absorption,34 and photoelectric conversion.35
Experimental
All reagents were purchased from commercial companies without
purification. Distilled water was obtained from Direct-Q3 UV,
Millipore.
Synthesis and purification of graphene oxide
Graphite oxide was prepared by a modified Hummers method. In brief,
concentrated H2SO4 (50 mL) was added to graphite (1.0 g) under an
ice bath and stirred for 3 h. Then, KMnO4 (4.0 g) was slowly added.
After stirring for 2 h, distilled water (100 mL) was added dropwise.
After additional stirring for 30 min, H2O2 (10 mL) was slowly added to
this solution and bright yellow graphite oxide appeared. The graphite
oxide was filtered and washed four times with distilled water and
freezed-dried at −50 oC for 24 h to obtain graphite oxide powder. The
graphite oxide powder was dispersed in distilled water as 1 mg mL-1,
ultrasonicated for 30 min, and centrifuged at 10,000 rev min-1 for 30
min to obtain GO as the supernatant. This was freeze-dried at −50 oC
to furnish the GO sponge.
Synthesis of 3D-RGO/ZnO photocatalyst
GO sponge (120 mg) was dispersed in distilled water (40 mL) by
sonification for 30 min to form solution A. Anhydrous Zn(OAc)2
(120 mg) was dissolved in ethanol (80 mL) to form solution B.
Ethylenediamine (0.5 mL) was added to the mixture of solution A
and B, and the resultant mixture was sealed in a 200-mL Teflon-lined
o
autoclave and maintained at 180 C for 12 h. Then the autoclave was
naturally cooled to room temperature and the as-prepared 3D-RGO/
ZnO was washed with distilled water and freezed-dried to obtain the
3D-RGO/ZnO photocatalyst.
Photocatalytic oxidation of benzyl alcohols by 3D-RGO/ZnO
photocatalyst
A 25 mL round-bottomed flask was charged with alcohol (1 mmol),
3D-RGO/ZnO (40 mg) and N,N-dimethyl formamide (5 mL). The
resultant mixture was stirred under O2 with two white LED lamps (12
W). After completion of the reaction, the 3D-RGO/ZnO catalyst was
recycled by filtration and the organic phase of the filtrate was extracted
with EtOAc, washed three times with water and dried over Na2SO4.
The pure product was then isolated by silica chromatography using
petroleum ether/EtOAc mixtures as the eluent.
* Correspondent. E-mail: chenyoujian1985@163.com

728 JOURNAL OF CHEMICAL RESEARCH 2015
Characterisation of catalyst and oxidation products
is much more convenient than using UV light. Obviously, no
reaction took place without a 3D-RGO/ZnO catalyst even with
enhanced visible light (Table 1, entry 4). Commercial nano ZnO
particles can promote the oxidation reaction to furnish desirable
aldehyde in good yield (Table 1, entry 5), but not so effectively
as 3D-RGO/ZnO, indicating an enhanced catalytic effect when
using 3D-RGO. 3D-RGO should play a role in acceleration of
the reaction but cannot catalyse the reaction (Table 1, entry
6). The solvents can influence the oxidation efficiencies of 1a.
Apart from dimethylformamide (DMF), other solvents such
as tetrahydrofuran (THF), acetone, ethyl acetate (EtOAc),
and dimethyl sulfoxide (DMSO) have been also examined
The detailed morphologies of the 3D-RGO/ZnO were observed with
a field emission scanning electron microscope (FE-SEM, S4800,
Hitachi) and field emission high resolution transmission electron
microscope (FE-HRTEM, Tecnai G2 F30 S, FEI). The crystal
structure of the as-synthesised 3D-RGO/ZnO was identified by X-ray
diffractometer (XRD, X’ Pert Pro, Philips), using Cu K ( = 1.54 Å)
radiation. X-ray photoelectron spectra (XPS) was carried out in a
Thermo Scientific ESCALAB 250Xi Xray photoelectron spectrometer
equipped with a monochromatic Al K X-ray source (1486.6 eV). NMR
spectra were obtained with a Bruker Avance 500 spectrometer or
Bruker Avance 300 spectrometer.
Results and discussion
Table 1 Optimising conditions for the oxidation of alcoholsa
GO nanosheets were self-assembled to form a 3D structure and
reduced to RGO under hydrothermal conditions. Meanwhile, ZnO
nanoparticles were supported on 3D-RGO sheets through an in
situ growth process and dispersed evenly in the interconnected
3D porous network (Fig. 1a). TEM was used to investigate the
crystal size as well as morphology of ZnO nanoparticles. Figure
1b shows that ZnO nanoparticles with a size less than 500 nm
are symmetrically attached to the surface of RGO without
disassociated ZnO. In Fig. 1c, the broad diffraction peak of graphite
(002) implied sufficient removal of oxygen functional groups from
the surface of GO.36 The strong and sharp peaks at 2 = 31.61, 34.50,
36.33, 47.57, 56.59° were consistent with the standard XRD data
for the (100), (002), (101), (102) and (110) planes of ZnO. Figure 1d
shows the XPS spectrum of 3D-RGO/ZnO photocatalyst. Except
for the apparent C1s and O1s peaks, the Zn2p peak is very weak in
the XPS spectrum, which may be attributed to the low content of
the ZnO in the catalyst.
To examine the photocatalytic activity of 3D-RGO/ZnO catalyst,
(4-methoxyphenyl)methanol (1a) and 1-(4-methoxyphenyl)
ethanol (3a) were used as model compounds and the results are
summarised in Table 1. Trace of oxidation product (2a) can be
obtained without using light (Table 1, entry 1). But the oxidation of
alcohol (1a) proceeded smoothly to furnish corresponding aldehyde
(2a) in excellent yields under irradiation of UV light as well as
visible (Vis) light (Table 1, entries 2 and 3). We next explored the
activity of 3D-RGO/ZnO catalyst under visible light because this
R
R
H
O
Li ht
g
O
O2, r.t.
O
O
1a
3a
2a
4a
R = H
R = Methyl
R = H
R = Methyl
Entry Catalyst
Light
None
UV
Solvent
DMF
DMF
DMF
DMF
DMF
DMF
THF
Acetone
EtOAc
DMSO
DMF
Yield /%b
1
2
3
4
5
6
7
8
3D-RGO/ZnO
Trace
91
96
0c
85
0
82
76
78
87
93d
3D-RGO/ZnO
3D-RGO/ZnO
3D-RGO/ZnO
Nano ZnO
Vis
Vis
Vis
Vis
Vis
Vis
Vis
Vis
Vis
3D-RGO
3D-RGO/ZnO
3D-RGO/ZnO
3D-RGO/ZnO
3D-RGO/ZnO
3D-RGO/ZnO
9
10
11
a Reaction conditions: (4-methoxyphenyl) methanol 1a (1 mmol), 3D-RGO/ZnO catalyst
(40 mg),solvent (5 mL), O2 atmosphere, room temperature, 6 h.
b Isolated yield.
c Without 3D-RGO/ZnO.
d
1-(4-Methoxyphenyl) ethanol 3a (1 mmol) was used instead of (4-methoxyphenyl)
methanol.
Fig. 1 (a) SEM image, (b) TEM image, (c) XRD pattern, and (d) XPS spectrum of 3D-RGO/ZnO catalyst.

JOURNAL OF CHEMICAL RESEARCH 2015 729
for the photocatalytic oxidation reaction, but lower yields of
target product were obtained (Table 1, entries 7–10). Under the
optimised condition, 1-(4-methoxyphenyl) ethanol 3a can also
be oxidised to ketone, implying an application for the oxidation
of secondary alcohols (Table 1, entry 11).
Under optimised conditions, a series of primary alcohols have
been used as substrates to explore the scope of 3D-RGO/ZnO
photocatalysed oxidation. As shown in Table 2, the aryl alcohols
which bear electron-donating groups can be smoothly oxidised
to the corresponding aldehydes in high yields (Table 2, entries
1–5). When the halogen was introduced into the structure of
substrates, the yields of products 2f and 2g decreased slightly
(Table 2, entries 6 and 7). The oxidation reactions were seriously
suppressed with the existence of strong electron-withdrawing
groups such as –CN, and –NO2. The above results show that
the electron density of the substrate plays the key role in the
photocatalytic oxidation of primary alcohols.
Furthermore, various secondary alcohols have been used
to extend the scope of the 3D-RGO/ZnO photocatalytic
method. As shown in Table 3, the secondary alcohol 3a and
3b bearing m- and p-methoxyl groups afford excellent yields
of the products (Table 3, entries 1 and 2). When using –H
instead, the efficiency was apparently not affected (Table 3,
entry 3). However, electron-withdrawing groups obviously
Table 3 Photocatalytic oxidation of secondary alcohols to ketonesa
R2
R2
H
O
O
Viable Light
O2, r.t.
R1
R1
3
4
Table 2 Photocatalytic oxidation of primary alcohols to aldehydesa
H
O
Viable Light
O
Entry
1
Substrate
Product
Yield/%b
93
R
R
O2, r.t.
H
O
1
2
4a
4b
4c
4d
Entry
1
Substrate
Product
Yield /%b
96
O
H
O
3a
2a
O
H
O
1a
1b
O
2
3
4
89
92
87
H
O
2
3
2b
2c
92
94
3b
3c
O
H
O
H
O
O
O
H
l
O
1c
H
O
C
4
5
6
2d
2e
2f
91
95
88
O
1d
3d
H
O
H
O
5
6
4e
4f
61
95
1e
N
O2
3e
H
O
H
O
I
1f
H
O
3f
7
2g
81
l
OH
C
1g
1h
7
8
4g
4h
90
86
H
O
O
8
9
2h
2i
76
64
N
C
3g
3h
H
O
H
O
N
O2
1i
a
Reaction conditions: primary alcohol 1 (1 mmol), 3D-RGO/ZnO catalyst (40 mg),
DMF (5 mL), O2 atmosphere, room temperature, 6 h.
bIsolated yield.
a Reaction conditions: secondary alcohol 3 (1 mmol), 3D-RGO/ZnO catalyst (40 mg), DMF
(5 mL), O2 atmosphere, room temperature, 6 h.
b Isolated yield.

730 JOURNAL OF CHEMICAL RESEARCH 2015
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Conclusion
We prepared a 3D-RGO/ZnO photocatalyst through an in
situ reduction of GO and the growth of nano ZnO particles.
The 3D-RGO/ZnO displayed an enhanced photocatalytic
effect towards commercial ZnO nanoparticles and can be
applied to the oxidation of a broad series of benzyl alcohols
including primary and secondary alcohols to the corresponding
carbonyl compounds under the irradiation of visible light.
A plausible photocatalytic mechanism referring to photo
electron generation and transfer has been proposed. Further
studies on the application of 3D-RGO/ZnO catalyst, such as
photodegradation, are in progress.
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Received 6 August 2015; accepted 3 November 2015
Paper 1503529 doi: 10.3184/174751915X14476823418480
Published online: 1 December 2015
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