Visible-light-driven selective oxidation of alcohols using a dye-sensitized TiO2-polyoxometalate catalyst

Article abstract of DOI:10.1016/j.jcat.2017.03.017

This study demonstrated a dye-sensitized TiO₂-polyoxometalate system (denoted TiO₂-(PW₁₂-TH)₈, where PW₁₂?=?PW₁₂O₄₀^3? and TH?=?thionine) for selective oxidation of alcohols under visible light. The results showed that various substituted alcohols were transformed into their corresponding aldehydes with high selectivity. Due to more efficient electrons transfer, large surface area, and enhanced visible light absorption, the photocatalytic activity of TiO₂-(PW₁₂-TH)₈ was superior to any other dye-sensitized system reported to date. The response in the photocurrent-time curves over several on/off cycles of intermittent irradiation showed good reproducibility. The photocurrent response of TiO₂-(PW₁₂-TH)₈ was much higher than that of TiO₂/TH (physical mixture of TiO₂ and TH), SiO₂-(PW₁₂-TH)₈ or P25-(PW₁₂-TH)₈ (P25?=?Degussa P25), attributed to the more efficient transfer and longer lifetime of photoexcited electrons. This photocatalytic process confirmed that efficient electron transfer during photocatalytic oxidation plays a vital role in determining the reaction conversion and obtaining good selectivity. Electron paramagnetic resonance (EPR) spectra and radical scavenging experiments proved that superoxide radicals and electrons were the main reactive species in the proposed system, the absence of hydroxyl radicals and holes is demonstrated to be the key of high reaction selectivity.

Full text of DOI:10.1016/j.jcat.2017.03.017

Journal of Catalysis 351 (2017) 59–66
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Visible-light-driven selective oxidation of alcohols using a
dye-sensitized TiO -polyoxometalate catalyst
2
Xue Yang a,b, Hui Zhao a,b, Jifei Feng , Yanning Chen a,b, Shuiying Gao a, , Rong Cao a,b,
a
⇑
⇑
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
College of Chemistry, Fuzhou University, Fuzhou 350002, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This study demonstrated a dye-sensitized TiO -polyoxometalate system (denoted TiO -(PW12-TH)8,
where PW12 = PW12O40 and TH = thionine) for selective oxidation of alcohols under visible light. The
results showed that various substituted alcohols were transformed into their corresponding aldehydes
with high selectivity. Due to more efficient electrons transfer, large surface area, and enhanced visible
2
2
Received 10 January 2017
Revised 22 March 2017
Accepted 23 March 2017
3À
light absorption, the photocatalytic activity of TiO
2 8
-(PW12-TH) was superior to any other dye-
sensitized system reported to date. The response in the photocurrent-time curves over several on/off
Keywords:
Photo-oxidation
Alcohol
cycles of intermittent irradiation showed good reproducibility. The photocurrent response of TiO
PW12-TH) was much higher than that of TiO /TH (physical mixture of TiO and TH), SiO -(PW12-TH)
or P25-(PW12-TH)
2
-
(
8
2
2
2
8
(P25 = Degussa P25), attributed to the more efficient transfer and longer lifetime of
Visible light
8
TiO
Dye
2
photoexcited electrons. This photocatalytic process confirmed that efficient electron transfer during pho-
tocatalytic oxidation plays a vital role in determining the reaction conversion and obtaining good selec-
tivity. Electron paramagnetic resonance (EPR) spectra and radical scavenging experiments proved that
superoxide radicals and electrons were the main reactive species in the proposed system, the absence
of hydroxyl radicals and holes is demonstrated to be the key of high reaction selectivity.
Ó 2017 Elsevier Inc. All rights reserved.
Polyoxometalate
1
. Introduction
visible light photocatalysis and fast electron-hole recombination
typically results in poor selectivity of the products [13,14]. To
The selective oxidation of alcohols has received considerable
attention in both industrial manufacturing and organic syntheses
1,2]. The photo-assisted transformation of alcohols, which can
be conducted under visible light irradiation, has especially
attracted growing interest due to its promising potential for solar
energy utilization [3–5]. In addition, compared with conventional
industrial oxidation processes, over-oxidation can be avoided in
photocatalysis [6–8]. Various semiconductor metal oxides have
address this concern, many approaches have been proposed that
include compositing TiO with other materials to increase the light
response region [15], doping TiO with metal ions or non-metal
ions [16–18], incorporating carbon nanotubes into TiO [19], sup-
porting photocatalysts on graphene with large surface area, com-
bining TiO with other narrow band gap semiconductors to form
heterojunctions [20–22] or sensitizing with dyes [23]. However,
these doping processes may reduce the redox potential of photo-
electrons or photoholes and lead to lower photocatalytic activity.
2
[
2
2
2
been used in selective photo oxidation reactions, but TiO
most reliable material for green chemistry and energy sustainable
solutions, mainly due to two apparent advantages: (1) TiO is
abundant, stable and easy to prepare and (2) TiO possesses appro-
priate redox potentials for its conduction band (CB) and valence
band (VB) to ensure the reduction of O and oxidation of H O or
organic compounds simultaneously [9–12]. However, the large
band gap of TiO (Eg > 3.0 eV) greatly impedes its application in
2
is the
2
Li et al. showed that Pd-loaded anatase TiO nanotubes have excel-
2
lent visible light photoactivity in the photocatalytic oxidation of
propylene [24]. However, the use of noble metal is relatively
expensive and could potentially generate abundant amounts of
heavy metal waste. These processes have been challenged recently
by a promising strategy. Many of the processes include dyes or
2
2
2
2
2
other colored species adsorbed on the surface of TiO to improve
its photocatalytic activity under visible light by enhanced light
absorption [25–27]. Zhao’s group investigated the aerobic selectiv-
ity in the oxidation of alcohols to aldehydes upon visible light
irradiation using a coupled system consisting of dye alizarin
⇑
Corresponding authors at: State Key Laboratory of Structural Chemistry, Fujian
Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou 350002, China. Fax: +86 591 83796710 (R. Cao).
red-sensitized TiO
2
and TEMPO [28]. Hydroxyl radicals (ÁOH) are
E-mail addresses: gaosy@fjirsm.ac.cn (S. Gao), rcao@fjirsm.ac.cn (R. Cao).
http://dx.doi.org/10.1016/j.jcat.2017.03.017
0
021-9517/Ó 2017 Elsevier Inc. All rights reserved.

6
0
X. Yang et al. / Journal of Catalysis 351 (2017) 59–66
generally regarded as ‘‘non-selective” radical species in otherwise
‘selective” oxidation reactions in heterogeneous photo-catalytic
2
the TiO was formed and separated by centrifugation. Finally, the
resulting products were further dried under vacuum at 60 °C for
‘
processes, were not produced in this system. This pioneering work
represents a notable achievement in green organic synthesis under
solar light energy. However, the efficiency for alcohol oxidation
still requires further improvement. A similar composite was
obtained by Robinson et al. [12,29,30], via combining alizarin
use.
Preparation of the TiO
procedure described in our previous reported publications
[40,41], the prepared porous anatase TiO microspheres were
immersed in poly (ethylene imine) (PEI) aqueous solution
(10 mg/mL) for 30 min, and then the PEI-coated TiO were
immersed in PW12 (5 mM) solution and an TH solution
2 8
-(PW12-TH) composite. Following the
2
red-sensitised ZnO or TiO
2
with AgNO
3
and TEMPO system in an
2
aqueous solution, and the system was appealing due to the high
yields in the oxidation of alcohols to their corresponding aldehydes
and ketones under visible light irradiation. However, not only did
this catalyst contain precious metal, but the selectivity also needs
improvement. To develop an effective photocatalytic system that
simultaneously enhances the activity and selectivity for alcohol
oxidation under visible light irradiation, recent results have argued
for the importance of smooth electron transfer in the conduction
band during photocatalytic oxidation to improve the low selectiv-
ity and sluggish reaction rate [31–33]. In this regard, the presence
of superior electron acceptors is significant and necessary to
achieve higher efficiencies. Polyoxometalates (POMs) are an
intriguing class of photocatalysts that are often compared with
semiconductor materials [34,35] and can be activated by light in
most similar processes. Since Jackson first reported the polyox-
a
(0.5 mM) for 30 min, respectively. After each immersion, the sam-
ples were washed with ethanol for several times. By repeating the
above steps 8 times, the catalyst film TiO
pared. P25-(PW12-TH) was prepared in the same manner, but
the (PW12-TH) composite films were adsorbed on the surface of
commercially available Degussa P25.
8 2
Preparation of the SiO -(PW12-TH) composite. The SiO used
here was prepared according to the reported method [42]. Typi-
cally, cetylpyridinium bromide (CPB: 0.0026 mol) and urea
(0.01 mol) in water (30 mL) were added to
0.012 mol tetraethyl orthosilicate (TEOS), cyclohexane (30 mL)
and pentanol (1.5 mL). Silica was formed by MW irradiation
(400 W maximum power) at 120 °C for four hour. After dried in
air for a day, the prepared material was further calcined in air at
2 8
-(PW12-TH) was pre-
8
8
2
a mixture of
4À
ometalates catalyst [S
W
2 18
O
62
]
for the solar light-induced pho-
550 °C for 6 h. SiO
2
-(PW12-TH)
8
was prepared in the same manner,
tocatalytic oxidation of benzyl alcohol [36], the application of
polyoxometalates has been widely studied in photocatalytic pro-
cesses such as the oxidation of alcohols [37,38]. In particular,
but the (PW12-TH)
surface.
8
composite films were adsorbed on SiO
2
3
Preparation of (TH) PW12. The catalyst was prepared by a direct
semiconductor-polyoxometalates
improved photocatalytic performance. Yoon [39] and co-workers
recently reported enhanced photocatalytic reduction rates for
composites
have
shown
precipitation method from the literature [43]. In detail, 60 mL TH
(0.1 mM) ethanol solution was added dropwise to the mixture of
PW12 solution (20 mL, 0.1 mM) and polyethylene glycol (2 mL).
The mixture was stirred vigorously for 5 h. Then, the precipitates
were filtered, washed with distilled water, and dried in air.
3
À
methyl orange in a composite TiO
2
-PW12
40 played the role of electron acceptor and transmitter.
Nevertheless, to the best of our knowledge, dye-sensitized
TiO -polyoxometalates systems for the oxidation of alcohols under
visible light have not been well studied thus far.
In our previous studies [40], TiO -(PW12-TH)
O40 system, where
3
À
PW12
O
2
2.2. Sample characterization
2
8
films were suc-
cessfully prepared using a layer by layer method and were applied
as photocatalysts in aqueous RhB degradation. Here, in continua-
Powder X-ray diffraction (PXRD) data were collected on a
Rigaku MiniFlex 600 diffractometer working with Cu Ka radiation.
tion of our research endeavors, we employed TiO
2
-(PW12-TH)
8
cat-
The morphology of the samples was characterized by scanning
electron microscopy (SEM, JSM6700) and transmission electron
microscopy (TEM, JEM2010). The optical properties of the samples
were analyzed using a UV–vis spectrophotometer (Shimadzu UV-
alyst for the efficient and selective aerobic oxidation of alcohols
under visible light. The results showed that the combined action
3À
of PW12
2
O40 and TiO can result in better photocatalytic perfor-
mance. TH allowed for the photocatalytic application of both
4
2600) and BaSO was used as the background. The Brunauer-
3
À
PW12
O
40 and TiO
2
under visible light irradiation and avoided the
Emmett-Teller (BET) specific surface area of the samples was
analyzed by nitrogen adsorption in a Micromeritics ASAP 2020
apparatus. The photoluminescence spectra of the as-fabricated
catalysts films were collected by an Edinburgh FLS920 fluorescence
spectrometer with an excitation wavelength at 360 nm. The
photocurrent analysis was carried out in a traditional three-
electrode quartz cell. Pt foils plate as the counter electrode, and
Ag/AgCl as the reference electrode. The fluoride-tin oxide (FTO)
glass had been thoroughly cleaned by ethanol and acetone and
acted as working electrode. The catalyst (10 mg) was ultrasoni-
cated in 0.5 mL of DMF to obtain slurry. Then, 20 mL of the slurry
was spread onto the prepared FTO glass, and the side of the glass
was previously protected using scotch tape to ensure the exposed
production of photogenerated holes in the system. In all cases, var-
ious substituted alcohols were effectively transformed to their cor-
responding aldehydes with almost absolute selectivity.
Furthermore, the active species in the photocatalytic oxidation
reaction process were explored by EPR and radical scavenging
experiments. For the first time, a comprehensive mechanism for
the oxidation of alcohols under visible light by the dye-sensitized
2
TiO -polyoxometalates system was proposed.
2
. Experimental
.1. Sample preparation
Porous anatase TiO
2
2
area of the working electrode was controlled at 0.25 cm . Then, the
coated working electrode was dried at room temperature in air
without any heating process. The electrolyte was a 0.2 M aqueous
2
microspheres were synthesized using the
microwave-assisted method [38]. In a typical process, 0.2 mL tita-
nium tetrachloride was dropwise added into the mixture contain-
ing ethanol (12 mL) and acetic acid (6 mL). After magnetic stirring,
the clear solution was formed and then transferred into a 20 mL
vial for reaction. The synthesis was performed in a 400-W micro-
wave oven heated at 120 °C (Initiator 8 EXP, Biotage Corp). After
being breacted for 15 min and cooled to the room temperature,
2 4
Na SO solution without additives. The visible light irradiation
source was the same light source as that used in the photoactivity
tests in the following section.
The electron paramagnetic resonance (EPR) signal of the radical
species was measured by a Bruker-BioSpin E500 spectrometer
under room temperature. 5-Tert-Butoxycarbonyl-5-Methyl-1-
Pyrroline N-oxide (BMPO) was used as spin trap. The irradiation

X. Yang et al. / Journal of Catalysis 351 (2017) 59–66
61
source was same light source used in our photocatalytic experi-
ments described below.
2.3. Typical experimental procedure for photocatalytic test
In a general procedure for the photocatalytic test, a suspension
of 8 mg catalyst，0.1 mmol aryl alcohol and 10 mL dodecane in
.5 mL of reagent grade benzotrifluoride (BTF) was transferred into
a 20 mL quartz bottle and magnetically stirred at 1000 rpm during
the reaction. A stream of pure O at 1 atm pressure was bubbled
into the reaction system. The suspension was irradiated by a
00-W Xenon Illuminator System with a UV cut-off filter in an
air-conditioned room to maintain the reaction temperature at
0 °C. The photocatalytic reactions of fresh benzyl alcohol-BTF-
1
2
3
3
dodecane mixture under the same reaction conditions were
repeated 5 times to confirm the reliability of the data. After the
reaction, reaction solutions (60 mL) were filtered by a one-time
plastic filter and added into 0.5 mL ethanol for gas chromatograph
analysis (Agilent 7890A equipped with an FID detector). The speci-
fic GC parameters were as follows: HP-5 capillary column
Fig. 1. XRD patterns of pure anatase TiO
2
, P25 and the as-prepared catalyst films of
TiO -(PW12-TH) and P25-(PW12-TH) (A = anatase, R = rutile).
2
8
8
(
30 m  0.32 mm  0.25
lm); detector temperature (300 °C); split
ratio (20:1); injector temperature (300 °C). The oven temperature
was maintained at 50 °C for 5 min, and then raised to 120 °C at a
À1
rate of 10 °C min , finally the temperature increased to 300 °C.
À1
Continuous nitrogen gas (20 mL min ) was used as a carrier gas.
Here, dodecane was used as an internal standard to calculate the
conversion of alcohol and selectivity for aldehyde. The carbon bal-
ance was evaluated to be >99% for all substrates. Thus, there is neg-
2
ligible formation of volatile products, such as CO and CO . The
products quantification were confirmed by comparison with the
retention time of commercial samples and further confirmed by
gas chromatography mass spectrometry (GC–MS) (430 GC Varian,
USA). Controlled photoactivity experiments using a series of radi-
cal scavengers, ammonium oxalate (AO) as the scavenger for pho-
togenerated holes, tertbutyl alcohol (TBA) as the scavenger for
2 2 8
hydroxyl radicals, K S O as the scavenger for electrons, and ben-
zoquinone (BQ) as scavenger for superoxide radical species were
performed in a similar manner to the above photocatalytic exper-
iment with radical scavengers (0.1 mmol) added to the reaction
system.
Fig. 2. UV–visible diffuse reflectance spectra (DRS) of TiO -(PW12-TH)8 and TiO2.
2
might have photocatalytic activity for the target reactions in visible
light range.
3
. Results and discussion
The oxidant O
reaction product is H
meet engineering requirements and minimize a crucial drawback
of TiO as a photocatalytic catalyst, it is highly desirable to apply
as the oxidant. For sake of comparison, the photocatalytic oxida-
tion of benzyl alcohol using the TiO -(PW12-TH) , P25-(PW12-TH)
SiO -(PW12-TH) , (TH) PW12 and TiO /TH photocatalysts were
explored under the same reaction conditions. Morphological infor-
mation of the mentioned photocatalytic nanomaterials was shown
in the supporting information (Figs. S1 and S2). It can been clearly
2
is abundant and economical. Moreover, its only
Characterizations were performed to understand the causes of
2
O, and it produces no secondary pollution. To
the enhanced photocatalytic performance of the ternary hybrid
TiO -(PW12-TH) catalyst film. Anatase TiO usually shows supe-
rior photoactivity to that of rutile crystal phase TiO [5]. Therefore,
the crystal phases of the catalyst films were characterized by XRD.
As shown in Fig. 1, major peaks for TiO -(PW12-TH) and pure ana-
tase TiO match well, and peaks assignable to the rutile phase at
h = 54.1 and 68.6° were detected in the P25-(PW12-TH) pattern.
It should be noted that peaks in the pattern of P25-(PW12-TH) film
were well defined, while those in the pattern of the porous anatase
TiO film were broad, indicating that its crystallinity was low. UV–
2
8
2
2
2
O
2
2
8
8
,
2
8
2
8
3
2
2
2
8
8
2 8
seen from Fig. 3 that among various catalysts, TiO -(PW12-TH)
2
showed highest catalytic activity without lowering the selectivity.
It was found that the yield ratio of benzaldehyde was approxi-
mately 3–4 times higher for the film prepared using our porous
vis diffuse reflectance spectrum (DRS) showed optical absorption
properties for the catalyst films. As the results show in Fig. 2,
where the reflectance date were converted into absorbance using
the Kubelka–Munk function (K-M) [44], TH molecules have a
strong absorption in the visible region, which successfully extends
anatase TiO
on commercial Degussa P25. The improved photocatalytic perfor-
mance can be attributed to the porous structure of TiO [43], which
provided a larger specific surface area and more active sites for the
photocatalytic reaction [45,46]. Pore diameters of TiO -(PW12-TH)
are in the range of 1.5–9.1 nm (Fig. S7). In addition, the BET surface
2
microspheres than that of the catalyst film supported
the photo response of the TiO
UV region to the visible region. There was no absorption within
00–450 nm, which accounts for the purple color of the TH-
sensitized films. Moreover, the TiO -(PW12-TH) nanocomposite
exhibited enhanced light absorption intensity in the range of
00–800 nm compared with the pristine TiO , suggesting that it
2
-POMs co-catalyst system from the
2
4
2
8
2
8
2
area of TiO
the Degussa P25-(PW12-TH)
2 8
-(PW12-TH) (200 m /g) (Fig. S3) was larger than that of
2
film (59 m /g) (Fig. S4). Due to the
5
2
8

6
2
X. Yang et al. / Journal of Catalysis 351 (2017) 59–66
the PL spectra in Fig. S5. Under an excitation wavelength of
60 nm, the PL intensity follows the order SiO -(PW12
TH) > P25-(PW12-TH) > TiO -(PW12-TH) which indicates the
longer lifetime of photogenerated electrons in TiO -(PW12-TH)
compared with that in SiO -(PW12-TH) and P25-(PW12-TH) . This
3
2
-
8
8
2
8
,
2
8
2
8
8
can be understood to be a result of the dye having the excellent
capability to adsorb visible light for excitation, and inject electrons
into the conduction band of TiO
of strongly oxidizing holes in the valence band of TiO
2
, this process avoid the formation
[28]. The
2
lifetime of charge carriers can be efficiently prolonged as there is
no hole to recombination with electron. Most importantly, POMs
as a photocatalyst share many aspects of similarity with TiO
2
in
their operating mechanisms [47–49]. The incorporation of
3À
2
PW12O40 into the photoactive TiO /TH catalyst imparts more
efficient photoactivity in the visible light photo oxidation of
alcohols. Based on the above results, we choose the optimal
TiO -(PW12-TH) catalyst for the following tests and analysis.
2 8
Benzyl alcohol was chosen as a test substrate, the time-online
reaction profile with respect to irradiation time was shown in
Fig. S6. By prolonging the reaction time the conversion was slightly
improved. However, the selectivity for aldehydes decreased (car-
boxylic acids were detected as by-products). When oxygen was
removed during the reaction (3.0 h for photooxidation of benzyl
alcohol), the experiments exposing to illumination under the same
conditions for another 2 h did not lead to observation of any car-
boxylic acids, demonstrating that there was no contribution of
photon-assisted autoxidation.
Considering the balance between reaction rate and aldehyde
selectivity, all reaction time for the various activated and non-
activated alcohols was fixed at 2.5 h to compare and determine
the generality of the catalyst. The photocatalytic reactions of fresh
benzyl alcohol-BTF-dodecane mixture under the same reaction
conditions were repeated 5 times to confirm the reliability of the
data. As shown in Fig. S10, conversion of benzyl alcohol has no
noticeable alteration for 5 repetitions of the operation.
Fig. 3. The oxidation of benzyl alcohol over different photocatalysts.
poor photoactivity of SiO
showed a lower catalytic effect when adsorbed on a SiO
2
, the SiO
2
-(PW12-TH)
8
composite film
substrate.
2
In addition, POMs have wide band-gap energy of 3.5 eV [34] and
2
low specific surface areas (<10 m /g), alone with less active sites
on their surfaces. Accordingly, the reaction with (TH)
ceed negligible activity compared with that of other photocata-
lysts. On the other hand, similar to TiO , polyoxometalate as a
3
PW12 pro-
2
homogeneous photocatalyst can harvest light energy and convert
it into chemical energy to oxidize organic compounds photocat-
alytically. When using the TiO
imum yield of benzaldehyde (61.3%), which was nearly 3 times
that in the reaction using TiO /TH, was achieved in polyoxometa-
late and TiO co-catalyst system. This further proves that the
PW12 composite is a promising material for improving the photo-
catalytic activity of TiO -based photocatalysts.
2 8
-(PW12-TH) catalyst film, the max-
2
2
More encouraging was the performance of our TiO
catalyst over a wide range of alcohols. As shown in Table 1, with
only 1 atm of O , various substrates alcohols were converted to
2 8
-(PW12-TH)
2
In Fig. 4, the response of the photocurrent-time curves over sev-
eral on/off cycles of intermittent irradiation was observed to have
good reproducibility. A sharp attenuation in photocurrent was
observed when the light was off. The higher photocurrent response
2
the corresponding benzaldehydes with almost absolute reaction
selectivity. As the result showed, by visible light irradiation for
2
.5 h under the stated reaction conditions, a desirable benzyl alco-
2 8 2 8
of TiO -(PW12-TH) compared with TiO /TH and P25-(PW12-TH)
hol conversion of 61.3% with an unprecedented selectivity in ben-
indicated the more efficient transfer and longer lifetime of pho-
toexcited electrons. This result matches well with the observed
catalytic activity order. This result is also strongly supported by
zotrifluoride towards benzaldehyde was obtained. The TOF
À1
observed for TiO
2
-(PW12-TH)
8
was 0.245 h (TOF: mol of sub-
strate converted per mol of catalyst per h, with respect to TiO
2
).
This value is much higher than that of previously reported dye-
TiO
2
systems for the oxidation of benzyl alcohol under otherwise
À1
similar reaction conditions (TOF = 0.044 h ) [28]. Aromatic alco-
hols such as p-methoxyl benzyl alcohol could be easily oxidized
with high yield and selectivity (Table 1, entry 5). Moreover, ben-
zylic alcohols that were para-substituted with electron-donating
3 3
groups -OCH and -CH (Table 1, entries 5 and 7) increased the
activity, while para-substitution with electron-withdrawing
groups -Cl, and -F (Table 1, entries 3 and 6) decreased the activity.
It is well known that the b-hydrogen atom in benzylic alcohols is
more active and it generally showed superior transformation than
that of aliphatic alcohols (Table 1, entry 8). More interestingly,
most systems are applicable only to the oxidation of activated ben-
zylic and allylic alcohols; here, alcohols containing nitrogen or sul-
fur atoms, which are generally not reactive, could also be
converted to the corresponding aldehydes with certain yields
(
Table 1, entries 9 and 10). Notably, secondary benzylic substrates
especially those with increased steric hindrance from aryl groups
Table 1, entries 12–14), showed lower activities (Table 1, entries
1–14) compared with primary benzylic alcohols. In addition, the
existence of a chlorine group (Table 1, entry 13) leads to a lower
(
1
Fig. 4. Photocurrent transient response of different catalysts under visible light
irradiation.

X. Yang et al. / Journal of Catalysis 351 (2017) 59–66
63
Table 1
Selective oxidation of various substrates alcohols over TiO
2
-(PW12-TH)
8
under visible light irradiation for 2.5 h.
Product
Entrya
1
Substrate
Conversion (%)b
61.3
Selectivity (%)
>99
OH
O
2
3
51.8
38.4
>99
>99
OH
O
Cl
Cl
Cl
Cl
Cl
OH
OH
O
4
5
43.1
41.3
>99
>99
O
Cl
OH
O
MeO
F
MeO
F
6
7
32.5
45.0
>99
>99
OH
OH
O
O
8
9
26.6
21.3
>99
>99
OH
O
N
S
N
OH
OH
OH
O
1
1
0
1
30.0
26.7
>99
>99
S
O
O
1
1
2
3
23.2
17.1
>99
>99
OH
O
OH
O
Cl
Cl
1
4
25.1
>99
OH
O
a
2 8
Reaction conditions: 0.1 mmol aryl alcohol, 10 mL dodecane, 1.5 mL BTF, 8 mg TiO -(PW12-TH) , and visible light illumination for 2.5 h under ambient conditions.
Conversation of the alcohols calculated by GC analysis.
b
activity than that of a methyl group (Table 1, entry 14), which may
be attributed to electron-withdrawing effects.
Using photocatalytic selective oxidation of benzyl alcohol to
benzaldehyde as a model reaction, control experiments in the
absence of visible light showed no conversion of benzaldehyde,
similar to the reaction without photocatalyst, indicating that the
reaction was driven by a photocatalytic process. In the control
2
experiments, the reaction performed in nitrogen (N ) atmosphere

6
4
X. Yang et al. / Journal of Catalysis 351 (2017) 59–66
showed the conversion of benzyl alcohol is only 3.2%, which indi-
cated that O was directly involved in alcohol oxidation. It has been
2
well recognized that POMs share a parallel process to the band-gap
excitation in semiconductor photocatalyst [50]. Here, TH directly
absorbs visible light to be excited and then injected electrons into
the lower-lying LUMO orbits of TiO
2
or POMs. Finally, molecular
À
oxygen scavenged e to generate superoxide radical species.
The spin trap BMPO was used to confirm the formation of reac-
tive oxygen species. In Fig. 5a, BMPO showed no EPR signals under
visible light irradiation. Upon irradiation the mixture of TiO
2
-
(
PW12-TH) and BMPO for 3 min, a four-line spectrum with relative
8
intensities of 1:1:1:1 for the BMPO-superoxide radical was
observed (Fig. 5b). BMPO is a novel spin trap that also often used
for capturing hydroxyl radical. There is a need to further investi-
gate whether the EPR signals of Fig. 5b in part produced from
hydroxyl radical. It is well known that SOD (Superoxide Dismu-
tase) can scavenge superoxide radicals efficiently and specifically
by catalyzing its dismutation. In contrast, SOD has no effect on
hydroxyl radical. When 300 U/ml SOD added into the mixture of
Fig. 6. Controlled experiments using different radical scavengers in the oxidation of
benzyl alcohol over TiO -(PW12-TH) after visible light irradiation for 2 h.
2
8
2 8
TiO -(PW12-TH) and BMPO, all EPR signal totally disappeared
(Fig. 5c). The results demonstrated that EPR signals were produced
from superoxide radicals but not from hydroxyl radicals [51–53].
To get more information of the reaction mechanism, a series of dif-
ferent radical scavengers were applied in controlled experiments.
As shown in Fig. 6, the conversion did not change upon addition
of TBA, which scavenges hydroxyl radicals, into the reaction sys-
tem. Recent study has emphasized that the absence of so-called
‘
‘nonselective” hydroxyl radicals is significant for high selectivity
in alcohols photooxidation to aldehydes. The presence of superox-
ide radicals was also confirmed by adding the scavenger BQ into
the system, after which the conversion of benzyl alcohol was con-
siderably diminished (Fig. 6). This is consistent with the results of
EPR. It was found to be highly different from systems in aqueous
solution where both hydroxyl radicals and superoxide radical are
involved [54]. When the holes scavenger AO is added, the conver-
sion of benzyl alcohol shows no change (Fig. 6). This result is in
agreement with previous works regarding photocatalytic selective
oxidation of alcohols in a coupled photocatalytic system of dye-
2
Fig. 7. The proposed mechanism for the selective oxidation of alcohols by TiO -
(PW12-TH)8 nanocomposite under ambient conditions. (LUMO = lowest unoccupied
orbital, HOMO = highest occupied orbital).
sensitized TiO
of electrons, K
result showed that the conversion of benzyl alcohol was hindered
Fig. 6). The above results suggest that superoxide radical species
2
proceeded in BTF [28]. To confirm the existence
2 2 8
S O was added as a photoelectron trap, and the
(
and the absence of hydroxyl radicals contributes to the high reac-
tion selectivity.
and electrons are the main reactive species in the proposed system,
According to the above analysis and previously reported mech-
anisms, a rationale for the improvement of the visible light pho-
toactivity of TiO -(PW12-TH) catalyst films was proposed and
2 8
was illustrated in Fig. 7. It is acknowledged that POMs share the
same photochemical properties of semiconductor photocatalysts
2
especially TiO in a variety of similar processes [43,55]. Under vis-
ible light irradiation, dye TH directly absorbs visible light and is
excited to form electronically excited states [28]. Then, electrons
injected from the excited states of TH into the lower-lying LUMO
orbits of TiO
2
or POMs to create strong reducing sites (electrons).
and POMs
This process opens a unique route for band-gap TiO
2
to use visible light. Ultimately, the electrons are scavenged by
molecular oxygen to produce superoxide radical species, which
has been verified by electron paramagnetic resonance (EPR) result.
In this mechanism, TH absorbs visible light for excitation, which
averts the formation of strongly oxidizing holes in the system.
Moreover, such electron transfer can avoid the generation of
electron-hole pairs, which is detrimental to the development of
highly selective organic synthesis systems. In addition, as the most
active and nonselective initiators of photocatalytic oxidation in
organic substrates, especially alcohols, non-selective hydroxyl rad-
icals were not detected in the proposed system, which further
improves the selectivity of the photocatalytic aerobic oxidation
Fig. 5. The electron paramagnetic resonance (EPR) spectrum of (a) 5 mg/mL BMPO
in BTF solution under visible light irradiation, (b) 5 mg/mL BMPO and 0.2 mg/mL
TiO
2 8
-(PW12-TH) in BTF solution under visible light for 3 min, (c) 300 U/mL SOD was
added into (b) solution under visible light irradiation.

X. Yang et al. / Journal of Catalysis 351 (2017) 59–66
65
of alcohols. This photocatalytic process facilitated migration of the
photogenerated electrons and high reaction selectivity.
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extended spectral response for the selective photooxidation of
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We acknowledge the financial support from the 973 Program
2014CB845605 and 2013CB933200), National Science Foundation
of China (NSFC) (21521061, 21331006 and 21520102001). Strate-
gic Priority Research Program of the Chinese Academy of Sciences
XDB20000000).
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in
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2
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