Selective Oxidation of Aliphatic Alcohols using Molecular Oxygen at Ambient Temperature: Mixed-Valence Vanadium Oxide Photocatalysts

Article abstract of DOI:10.1021/acscatal.6b00753

Here we report a class of photocatalysts: mixed-valence vanadium oxide particles grafted onto a variety of oxide supports. In these catalysts V₆O₁₃ species with mixed oxidation states (V⁴⁺ or V⁵⁺) are believed to be catalytically active sites. These catalysts successfully enable alcohol oxidation to selectively produce aldehydes and ketones using O₂ as the oxidant. The catalytic process is driven by visible light irradiation at room temperature and, most importantly, progresses with negligible overoxidation. The catalysts can even selectively oxidize aliphatic alcohols, which are much more challenging to control in comparison to aromatic analogues. They can also be applied to the activation and oxidation of the otherwise stable C-H bonds of saturated aromatic hydrocarbons, such as toluene and xylene, under irradiation. Both experimental results and density functional theory (DFT) simulations suggest the formation of V₆O₁₃-alkoxide species as the initial step in the catalytic cycle. The V₆O₁₃-alkoxide then acts as the light harvester, being excited by light of wavelength shorter than 550 nm. Facile room-temperature C-H bond cleavage in the excited state V₆O₁₃-alkoxide in the presence of O₂ leads to the carbonyl-containing products. These findings demonstrate an example of light-driven selective oxidation of diverse alcohols via in situ formation of photoresponsive V₆O₁₃-alkoxide species. This catalytic process is especially valuable for the synthesis of temperature-sensitive products and represents an alternative pathway to many conventional thermal oxidation reactions.

Full text of DOI:10.1021/acscatal.6b00753

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Article
Selective Oxidation of Aliphatic Alcohols using Molecular Oxygen at
Ambient Temperature: Mixed-Valence Vanadium Oxide Photocatalysts
Sifani Zavahir, Qi Xiao, Sarina Sarina, Jian Zhao, Steven E. Bottle, Robert Mark Wellard,
Jianfeng Jia, Liqiang Jing, Yiming Huang, James P. Blinco, Hai-shun Wu, and Huaiyong Zhu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00753 • Publication Date (Web): 29 Apr 2016
Downloaded from http://pubs.acs.org on May 5, 2016
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ACS Catalysis
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Selective Oxidation of Aliphatic Alcohols using Molecular Oxygen
at Ambient Temperature: MixedꢀValence Vanadium Oxide
Photocatalysts
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Sifani Zavahir,† Qi Xiao,† Sarina Sarina,† Jian Zhao,† Steven Bottle,† Mark Wellard,† Jianfeng Jia,‡
Liqiang Jing,§ Yiming Huang,† James P. Blinco,† Haishun Wu,‡ and HuaiꢀYong Zhu*,†
†School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of
Technology, Brisbane, QLD 4001, Australia
‡School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China
§Key Laboratory of Functional Inorganic Material Chemistry, School of Chemistry and Materials Science, Heilongjiang
University, Harbin 150080, China
ABSTRACT: Here we report a class of photocatalysts: mixedꢀvalence vanadium oxide particles grafted onto a variety
of oxide supports. In these catalysts V6O13 species with mixed oxidation states (V4+ or V5+) are believed to be
catalytically active sites. These catalysts successfully enable alcohol oxidation to selectively produce aldehydes and
ketones using O2 as the oxidant. The catalytic process is driven by visible light irradiation at room temperature and most
importantly, progresses with negligible overꢀoxidation. The catalysts can even selectively oxidize aliphatic alcohols,
which are much more challenging to control compared to aromatic analogues. They can also be applied to the activation
and oxidation of the otherwise stable CꢀH bonds of saturated aromatic hydrocarbons, such as toluene and xylene, under
irradiation. Both experimental results and density functional theory (DFT) simulations suggest the formation of V6O13ꢀ
alkoxide species as the initial step in the catalytic cycle. The V6O13ꢀalkoxide then acts as the light harvester being excited
by light of wavelength shorter than 550 nm. Facile room temperature CꢀH bond cleavage in the excited state V6O13ꢀ
alkoxide in the presence of O2 leads to the carbonylꢀcontaining products. These findings demonstrate an example of
lightꢀdriven selective oxidation of diverse alcohols via in-situ formation of photoꢀresponsive V6O13ꢀalkoxide species.
This catalytic process is especially valuable for the synthesis of temperatureꢀsensitive products and represents an
alternative pathway than many conventional thermal oxidation reactions.
KEYWORDS: selective oxidation, photocatalysis, aliphatic alcohols, V6O13, visible light
■ INTRODUCTION
reasonable yields, compared to only 24 h for benzyl
alcohol under similar conditions. In another example,
Jensen and coꢀworkers found that Pd(OAc)2 works
together with Nꢀheterocyclic carbene ligands as an
active catalytic mixture for aromatic and allylic alcohol
oxidation at 60 °C.12 The reported aliphatic alcohol
oxidation reactions are almost exclusively driven by
thermal processes to achieve reasonable efficiencies
(except for the complicated Pd(OAc)2 and Nꢀ
heterocyclic carbene system) even though the oxidation
is an exothermic process.11,12 Higher operating
temperatures can accelerate the reaction, but often this
occurs along with a decrease in the selectivity for the
desired products. This is because, at high temperatures,
the partially oxidized products readily undergo further
transformation to the corresponding acids,13 directly
yield the esterified products14 or even undergo
decomposition to CO2. Therefore, the development of
new catalytic systems using novel catalyst materials and
The selective oxidation of alcohols to aldehydes and
ketones is an essential process in organic chemistry for
both academic research and industrial chemical
production. Within the literature there are several
methodologies which have been used for this
transformation, including oxides of chromium,1
manganese,2 activated dimethyl sulfoxide (DMSO)
reagents3 or hypervalent iodine4 as oxidants, either
alone or in the presence of free radical containing
species such as 2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀylꢀoxy
(TEMPO).5ꢀ7 While there are a large number of
aliphatic alcohols available, compared with aromatic
alcohols such as benzyl alcohol,8ꢀ10 their selective
oxidation has seldom been reported, as controlling the
overꢀoxidation is particularly challenging. Recently,
nitroxideꢀdecorated, porous coordination polymers have
been studied as catalysts for the oxidation of a range of
aliphatic alcohols.11 It was found that the oxidation of 1ꢀ
hexanol, 2ꢀhexanol and 2ꢀmethylꢀ4ꢀpentanol required
longer reaction times (96 h) at 80 °C to achieve
driven by
a green energy source for effective
conversion of aliphatic alcohols to aldehydes and
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ketones with molecular oxygen as the benign oxidant at
Page 2 of 11
methods involving ultraꢀsonication in ethanol, whereby
the mixedꢀvalence vanadium oxide clusters ultimately
account for a small fraction of the catalyst, but are well
dispersed on the support. Full details of the preparation
of the catalysts are provided in the Experimental
Section. These grafted solids could be used without
subsequent refinement and directly for testing their
photocatalytic activity to selectively oxidize aliphatic
alcohols.
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ambient temperature and pressure is highly desirable,
but remains a significant challenge. The pioneering
work for the catalytic conversion of various alcohols to
carbonyls using VIII metals/TiO2 photocatalyst through
light irradiation was reported by Pichat et al.,15 this
work opens a new avenue to the photocatalytic
transformation of alcohols. Recently we found that the
direct oxidative esterification of aliphatic alcohols using
catalysts of gold−palladium alloy nanoparticles on a
phosphateꢀmodified hydrotalcite support and molecular
oxygen as a benign oxidant can be significantly
enhanced by visibleꢀlight irradiation at ambient
temperatures and under mild and baseꢀfree conditions.16
This finding encouraged us to explore green
photocatalytic processes for the selective oxidation of
aliphatic alcohols, that use atmospheric oxygen gas as
oxidant and visible light energy to drive the reaction at
ambient temperatures.
The catalytic performance of the asꢀprepared
catalysts for selective oxidation of 3ꢀhexanol to 3ꢀ
hexanone was investigated as a model reaction using
molecular O2 as the oxidant under visible light
irradiation of a lightꢀemitting diode (LED) lamp (λ=460
nm) at 30 °C. No base was added to the reaction
systems. The results are summarized in Table 1. Under
the identical experimental condition VO2 and V2O5
exhibited negligible catalytic activity. Therefore, the
catalytically active component in the grafted mixedꢀ
valence vanadium oxide should be the V6O13 clusters.
The V6O13 clusters on various supports exhibited
different catalytic activities. The best activity was
achieved with V6O13 on γꢀAl2O3 and ZrO2 supports
(denoted as V6O13ꢀγꢀAl2O3 and V6O13ꢀZrO2), with 100%
alcohol conversion and excellent 3ꢀhexanone selectivity
(96%). No reaction was observed in the dark at 30 °C in
the presence of the photocatalysts, which confirms the
essential role of visible irradiation in the oxidation. The
supporting material also plays a key role in the
performance of catalysts, substantially affecting these
reactions even if they are not the catalytic active centers.
Compared with traditional thermal processes,11,12 the
photocatalytic reaction is more efficient and can
proceed without any additives using “greener” ambient
temperatures (for comparison, see Table S1, SI).
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In the present study we devised a new class of
photocatalysts: mixedꢀvalence vanadium oxide (VxOy =
VO2 + V2O5 + V6O13) particles grafted onto a variety of
oxide supports. It is known that inorganic species can
be anchored on the surface of various substrates through
stable chemical bonds.17,18
Compared with traditional wellꢀknown VO2 and
V2O5, mixedꢀvalence V6O13, in which both V4+ and V5+
oxidation states coexist, is relatively lessꢀstudied even
though this form of vanadium oxide possesses excellent
electrochemical performance and high electronic
conductivity.19ꢀ21 To our knowledge, V6O13 materials
have not been previously applied in the area of
photocatalysis for organic transformations. A key
feature of the new photocatalysts in the present study is
that the V6O13 species are the catalytically active
component for the selective oxidation and act in a
different mechanism from that for the widely studied
V2O5 based photocatalysts. V2O5 photocatalysts have a
vanadium (V5+) center and react via a semiconductor
mechanism.22 Although we cannot fully exclude the
presence of VO2 and V2O5, our experimental results
confirmed that these species were ineffective for the
selective oxidation of the aliphatic alcohols under
visible irradiation. In the new photocatalysts vanadium
V6O13 particles grafted on the surface mainly is the
active component, and the photocatalytic reactions
proceed via a new mechanism. The reactant alcohol
molecules tend to chemically adsorb onto the surface of
V6O13 clusters and the resultant surface complexes
exhibit a highly unusual visible light absorption in the
400ꢀ550 nm region. This absorption is the critical aspect
required to drive the selective oxidation under
irradiation.
Table 1. Photocatalytic Performance of Different Catalysts
for the Oxidation of 3ꢀHexanol to 3ꢀHexanone.[a]
OH
O
hv
Photocatalyst
Conv.
[%]
100
Select.
[%]
96
TON[c]
6.52
A.Q.Y.
[%]
1.30
Photocatalyst
V6O13ꢀγꢀAl2O3
V6O13ꢀZrO2
100
40
96
96
94
96
93
63
ꢀ
6.52
2.61
1.47
4.37
0.95
4.28
ꢀ
1.30
0.52
0.29
0.87
0.19
0.85
ꢀ
V6O13ꢀzeolite Y
V6O13ꢀV2O5
23
V6O13ꢀtitanate
V6O13ꢀTiO2
67
15
[b]
VO(acac)2
100
<1
V6O13ꢀ
hydrotalcite
V2O5
<1
<1
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
VO2
[a] Reaction conditions: 0.5 mmol of 3ꢀhexanol, 50 mg of photocatalyst,
2 mL of α,α,αꢀtrifluorotoluene, reaction time 16 h, reaction temperature
■ RESULTS AND DISSCUSSION
[b]
30 °C, 1 atm O2, LED lamp (λ=460 nm) light intensity 0.5 W/cm2.
In the present study, mixedꢀvalence vanadium oxide
species (see below for structural characterization)
grafted onto a number of oxide supports, such as γꢀ
Al2O3, ZrO2, TiO2, zeoliteꢀY, titanate, and hydrotalcite
[Mg6Al2(OH)16CO3·mH2O], were synthesized by simple
[c]
Vanadyl acetylacetonate [VO(acac)2] 8.3 mg.
Turnover number
(TON) was calculated based on vanadium centers available in 50 mg of
V6O13 grafted solid according to EDX data. The apparent quantum yield
(AQY) was calculated based on the molecule of product formed per
photon involved in the reaction.
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the V6O13ꢀZrO2 sample, (d) Raman spectra of the subtracted
data of ZrO2 from V6O13ꢀZrO2, and (e) XPS spectra for V
species of the V6O13ꢀZrO2 sample.
To better understand the excellent photocatalytic
performance of the catalysts prepared in the present
study, we further investigated the physical
characterization of the V6O13 grafted solid catalysts.
Figures 1a and 1b show the transmission electron
microscopy (TEM) images of the typical V6O13ꢀZrO2
sample. We can clearly see the V6O13 clusters are well
dispersed on the ZrO2 particle surface. Energy
dispersive Xꢀray spectroscopy (EDS) mapping further
confirmed the presence and distribution of V6O13
clusters (Figure 1c). The γꢀAl2O3 nanofibers have a
large specific surface area (Figure S1a, SI), on which
V6O13 particles are also well dispersed and ready for
adsorption of the reactant alcohol molecules. The
vanadium content in the catalyst is 2.8 wt% as
confirmed by EDS results (Figure S1b, SI), which
matches well with the experimental design. The
diffusion of reactants and products among the fibers
should be very efficient compared with that in the pores
of microporous or mesoporous solids because of the
large interꢀfiber voids.23 The supporting material γꢀ
Al2O3 did not show any light absorption within the
visible region of the solar spectrum, but the V6O13ꢀγꢀ
Al2O3 exhibited strong absorption (Figure S1c, SI).
XRD pattern of the sample could be indexed for
alumina phase, and no V6O13 phase could be observed
(Figure S1d, SI). This is due to the low content of V6O13
and its presence in small clusters.
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To further identify the presence of V6O13 clusters on
the solid surface, we carried out Raman scattering for
the V6O13ꢀZrO2 sample. However, due to the very small
amount of vanadium species grafted on to the ZrO2
surface, those signals for the characteristic vanadium
peaks are barely detectable (Figure S2, SI). By
subtracting the signals from the ZrO2 support from the
data for the V6O13ꢀZrO2 sample, as shown in Figure 1d,
we observed some peaks which could be ascribed to
vanadium species (143, 900, and 1000 cm–1) which are
in agreement with previously reported results.24 The
highꢀfrequency peak situated around 1000 cm–1, which
can be assigned to the stretching mode of vanadyl
oxygen, is indicative of the presence of V6O13.20 To
understand the oxidation state of the vanadium in the
asꢀprepared sample, we performed Xꢀray photoelectron
spectroscopy (XPS) measurements. The XPS spectra of
V 2p together with O 1s are presented in Figure 1e. The
peaks of both V 2p3/2 and V 2p1/2 were identified at
516.1, 517.5 eV and around 525 eV, corresponding to
V4+ and V5+ respectively,25ꢀ27 which confirms the
presence of mixedꢀvalence vanadium oxides. The V
2p1/2 peak is relatively low due to the low levels of V
species present. The intensity of the V5+ peak at 517.5
eV is a little higher than that of the V4+ peak at 516.1
eV, which is due to the easy oxidation of V4+ to V5+ on
the surface considering that the detection depth of the
XPS is only a few nanometers.20
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Considering that V6O13ꢀγꢀAl2O3 and V6O13ꢀZrO2
photocatalysts exhibited the best performance according
to the reaction conversion under light irradiation, we
used V6O13ꢀγꢀAl2O3 as the model catalyst to further
evaluate the selective oxidations by extending the
substrate scope. As can be seen from Table 2, V6O13ꢀγꢀ
Al2O3 can successfully drive a wide range of examples
of selective oxidations under visible irradiation. The
yields of primary aliphatic alcohols are >60%, a
significant improvement on those reported in literature
where yields of <44% were only able to be attained for
the selective oxidation of 1ꢀheptanol and 1ꢀdecanol to
the corresponding aldehydes using a V2O5 catalyst at
100 °C and a base.28 Using the new photocatalysts
described here conversion rates for the oxidation of 1ꢀ
pentanol, 1ꢀhexanol, 1ꢀoctanol, 1ꢀnonanol and 1ꢀ
decanol are excellent, with more than 90% of the
alcohols consumed and up to 70% selectivity achieved
for the desired aldehyde products. Also base addition to
the reactions was not required. Such selectivity and
efficacy is far superior to other oxidations reported in
the literature using other catalysts. For example
Dimitratos and coꢀworkers reported yields of only 13%
at 60 °C using a bimetallic catalyst of AuꢀPd and Auꢀ
Pt29 while Lu et al. reported30 much lower aldehyde
yields (~3%) for 1ꢀhexanol, 1ꢀpentanol and 1ꢀoctanol at
90°C using Pt/Bi2O3. Very recently, we found that Auꢀ
Pd alloy nanoparticles on phosphate anionꢀexchanged
hydrotalcite could achieve 94% conversion rates for 1ꢀ
octanol for direct oxidative esterification under visible
Figure 1. (a) The TEM image of the V6O13ꢀZrO2 sample, (b)
High resolution (HR)ꢀTEM image of the V6O13ꢀZrO2 sample,
the red circles indicate the V6O13 clusters, (c) EDS mapping of
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light irradiation,16 however this reaction required
Page 4 of 11
A simulated sunlight source was used for the
oxidation of 3ꢀhexanol and it was found that at an
irradiance of 0.12 W/cm2 (equivalent to 1.2 sun) V6O13ꢀ
ZrO2 converted 92% of 3ꢀhexanol to 3ꢀhexanone within
16 h reaction. This conversion rate is comparable to that
of 460 nm monochromatic LED lamp (96%). This result
demonstrates that we can drive the selective oxidation
with focused sunlight.
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heating to 55 °C for good conversion. In contrast, the
photocatalysts described in the present study can
achieve very good aldehyde yields at only 30 °C. The
yields of ketones from secondary aliphatic alcohols
using the new catalyst are even better, lying in the range
of 70% to 95%, much higher than for the primary
alcohols. The oxidation of aromatic alcohols under
similar conditions provided excellent aldehyde yields in
shorter reaction times. It is worth noting that allylic
alcohols such as cinnamyl alcohol and citranellol gave
relatively low selectivity for the desired aldehyde target.
This is possibly due to the catalyst’s ability to attack the
alkene double bond leading to unwanted oxidative
cleavage and the formation of smaller fragments such as
acetone.
Furthermore, we found that V6O13 grafted solid
photocatalysts exhibit strong oxidative capacity for the
activation and oxidation of the otherwise stable CꢀH
bonds of the saturated sideꢀchains of aromatic
hydrocarbons such as toluene and xylene under light
irradiation at ambient temperature (Table 3). Insertion
of an oxygen atom into hydrocarbons to yield alcohols,
aldehydes and ketones is highly desirable because
partially oxidized derivatives represent key raw material
feedstocks for manufactured products such as polymers,
perfumes, agrochemicals, synthetic compounds and
novel drugs.10 However, such reactions remain a
synthetic challenge. In light of this several aromatic
hydrocarbons were also subjected to selective oxidation
with the V6O13ꢀZrO2 photocatalyst at 30 °C. It was
observed that ethylbenzene can be selectively oxidized
with a very high selectivity (>95%) to acetophenone
after 48 h. Toluene oxidized predominantly to
benzaldehyde, however benzoic acid, benzyl alcohols
and esters were also observed. The catalysts were also
able to selectively oxidize one of the three methyl
groups of 1,3,5ꢀtrimethylbenzene while preserving the
other two. It is noteworthy that the oxidation reactions
proceed without the need for solvent.
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Table 2. Photocatalytic Selective Oxidation of Various
Aliphatic and Benzylic Alcohols with V6O13ꢀγꢀAl2O3.[a]
Entry
Substrate
Conv.
[%]
Select.
[%]
Yield
[%]
OH
1
2
100
97
58
70
58
68
OH
OH
OH
3
100
87
87
4
5
6
97
78
76
82
82
92
80
64
70
OH
OH
OH
OH
7
8
100
55
96
58
96
32
Table 3. Selective Oxidation of Aromatic Hydrocarbons
with V6O13ꢀZrO2 under Visible Light Irradiation at 30 °C.[a]
OH
9
97
88
69
97
67
85
Rate
Product selectivity [%]
Substrate
10
[µmol
g
HO
OH
Aldehyde Alcohol
/ Ketone
Acid
Ester
ꢀ1 hꢀ1]
11
12
96
89
98
98
94
87
Toluene[b]
Ethylbenzene[b]
pꢀxylene[c]
mꢀxylene[c]
1,3,5ꢀ
22.4
86.8
57.5
9.3
48.2
15.2
ꢀꢀ
10.0
0.9
4.3
ꢀꢀ
26.6
1.9
OH
97.2
65.6
84.3
55.4
OH
OH
13
14
92
93
68
71
63
66
16.7
6.6
13.4
7.7
15
16
100
100
35
98
35
98
OH
OH
39.4
18.0
11.1
15.5
trimethylbenzene
[b]
OH
17
18
98
86
100
100
98
86
O
OH
[a]
Reaction conditions: 1 mmol of substrate without solvent, 25 mg of
V6O13ꢀZrO2 photocatalyst, temperature 30 °C, 1 atm O2, LED lamp
(λ=400ꢀ800 nm) light intensity 0.5 W/cm2; reaction time [b] 48 h and
[c]
OH
19
20
97
70
100
69
97
48
24 h.
OH
Teramura et al. reported the oxidation of CꢀH bonds
in both aliphatic and aromatic hydrocarbons using a
V2O5 catalyst.22 In the present study the grafted V6O13
catalyst exhibited much higher photocatalytic activity
than V2O5 catalyst for this reaction. This fact implies
that the two catalysts work in different mechanisms.
This is supported by the observation that the vanadium
[a]
Reaction conditions: 0.2 mmol of alcohol, V6O13ꢀγꢀAl2O3 50 mg of
photocatalyst, 2 mL of α,α,αꢀtrifluorotoluene, reaction time 24 h for
aliphatic alcohols and 16 h for benzylic alcohols, reaction temperature
30 °C, 1 atm O2, LED lamp (λ=400ꢀ800 nm) light intensity 0.5 W/cm2.
Conversion and yield were calculated based on the product formed and
reactant consumed as measured by gas chromatography (GC).
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precursor VO(acac)2 in the present study also exhibit
whereas zeolite Y is an acidic silicate. V6O13 grafted on
zeolite Y was active for this reaction while hydrotalcite
was not. Usually, a basic support surface is beneficial to
the oxidation process,16,30,31 however in the present
study, this effect was not observed. We noted that the
light absorption by catalysts formed from V6O13 grafted
onto hydrotalcite, zeolite Y and TiO2 is less significant
compared to those grafted onto γꢀAl2O3 fibers, titanate
and ZrO2 (Figure 2). Since the γꢀAl2O3, ZrO2 and
titanate have negligible visible light absorption, the
difference in light absorption is due to the interaction
between the support surface and the grafted V6O13
particles. The weak light absorption is the likely basis
for the poor performance of a photocatalyst. Thus one
can optimize the catalyst performance by manipulating
the properties of the supports.
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significant photocatalytic activity as shown in Table 1.
We found that using an equivalent amount of vanadium
precursor VO(acac)2 as the photocatalyst can achieve a
complete conversion of alcohols, however the yield of
the desired product 3ꢀhexanone was substantially lower
(63%) compared to V6O13 grafted on oxides (>90%).
The results also reveal that VꢀOꢀC bonding can be
excited by visible light, and the lightꢀexcited bonding
state plays a role in the photocatalytic reaction. We
found that the acetylacetonate ligands in VO(acac)2
decompose and couple with 3ꢀhexanol forming overꢀ
oxidized 3ꢀhexyl esters of acetic acid during the
reaction. Thermogravimetric analysis (TGA) of
VO(acac)2 and V6O13ꢀtitanate catalysts reveals the loss
of the actylacetonate ligands in the 140ꢀ200 °C range
for VO(acac)2 while no such weight loss was observed
with the V6O13ꢀtitanate photocatalyst (Figure S3, SI).
Based on this observation, the presence of esters in the
product mixtures is attributed to the acetylacetonate
ligand of VO(acac)2. Moreover, VO(acac)2 forms a
homogeneous solution with the reactants and products,
causing problems for catalyst recycling, whereas the
grafted V6O13 photocatalyst was successfully recycled
without further treatment for up to five cycles with well
retained photocatalytic activity and selectivity (Figure
S4, SI). This is an important aspect in the development
of practical and costꢀeffective catalytic oxidation
processes. These results reveal a potential route towards
a commercial process for clean and efficient production
of aliphatic aldehydes and ketones using relatively earth
abundant vanadium and clean energy sources.
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Vanadium oxide (V2O5) has been extensively
studied and is recognized as possessing a remarkable
visible light absorption (Figure 2b), V2O5 was also used
as a support material to graft V6O13. Significant
enhancement in catalytic activity was expected through
synergism between V6O13 and V2O5. However, this
catalyst combination exhibited only
a
modest
performance towards the reaction. Conversion by
V6O13ꢀV2O5 was only 23%, although a 94% selectivity
to give 3ꢀhexanone was achieved (Table 1).
It was not possible to achieve higher overall
conversions by increasing the reaction temperature. 1ꢀ
Octanol oxidation catalyzed by V6O13ꢀZrO2 under
visible light irradiation at 40 °C and 60 °C gave octanal
yields of 68% and 63% respectively, while no reaction
in the dark was detected at 40 °C or 60 °C. This
indicated that the reaction is relatively unaffected by
reaction temperature, and is primarily driven by light
excitation.
Figure 2. UVꢀVisible diffuse reflectance spectra of V6O13
grafted catalysts (a), and pristine supporting materials (b).
The supporting materials play an important role in
the performance of catalysts. When investigating the
support, it is known that hydrotalcite is slightly basic
Figure 3. The action spectra of V6O13ꢀZrO2 (a) and V6O13ꢀγꢀ
Al2O3 (b) for the selective oxidation of 1ꢀoctanol.
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grafted V6O13 active species, as the response of V6O13ꢀ
Page 6 of 11
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To further study the impact of light irradiation on
the catalytic performance of the photocatalysts we
applied action spectra to show the relationship between
the incident light wavelength and the product yield.
Action spectrum of a reaction provides insight into the
role of lightꢀexcitation on the reaction activity by
explicitly depicting the influence of the irradiation
within narrow energy ranges.16 The action spectra of 1ꢀ
octanol oxidation to the corresponding carbonyl
products at 30 °C with two catalysts (V6O13ꢀZrO2 and
V6O13ꢀγꢀAl2O3) are illustrated in Figure 3, in which the
apparent quantum yield (AQY) is plotted against the
irradiation wavelength. A clear dependence of the yield
on the wavelength of the light absorption can be seen
for both cases. Higher activity is always observed at
stronger light absorption that occurs as the wavelength
decreases. Significantly lower yields are observed at
longer wavelengths (590 nm and 620 nm).
The uniqueness of the catalysis which is visible light
driven and does not require elevated temperatures, but
which is O2 and substrate support controlled, suggests a
unique mechanistic process is involved. Whilst a
complete understanding of the mechanism remains
elusive, some insight into the mechanism can be
determined. For instance the photoꢀgenerated charge
transfer and separation properties of the V6O13ꢀZrO2 and
V6O13ꢀTiO2 catalysts can be assessed using atmosphereꢀ
ZrO2 is much weaker (Figure 4b, approximately 1/10 of
that of V6O13ꢀTiO2 catalyst,). However, the strong
response of the TiO2 support did not contribute to the
photocatalytic performance. We conducted the selective
oxidation of 3ꢀhexanol using V6O13ꢀZrO2 and V6O13ꢀ
TiO2 catalyst respectively under UV irradiation (365 nm,
light intensity 0.12 W/cm2). The reaction with V6O13ꢀ
ZrO2 catalyst gave 30% conversion, whereas the
reaction with V6O13ꢀTiO2 catalyst gave only 9%
conversion. These results suggest that the mechanism
by which light drives the selective oxidation is different
from that of the conventional processes for
semiconductor photocatalysts.
When focused on the grafted V6O13 species, we
noted that the SSꢀSPS intensity of V6O13ꢀZrO2 is
strongly affected by the atmosphere in the wavelength
range between 350 nm and 500 nm. There is a
significant SSꢀSPS response in pure O2 atmosphere, but
no SSꢀSPS response in an N2 atmosphere (Figure 4b).
This result suggests that the photoꢀgenerated charge
transfer to the adsorbed O2 molecules play an important
role. The ability of the supports to adsorb and activate
O2 may also affect the selective oxidation. Zeolite Y has
a poor oxygen adsorption capability whereas titanates
have excellent ability to activate O2.33 It is important to
note then that V6O13ꢀzeolite Y gave no activity in air
(Figure S5, SI), while the activity of V6O13ꢀtitanate in
air was only slightly lower (70%) than that in a pure O2
atmosphere. Also the three grafted photocatalysts that
exhibit high photocatalytic activity, V6O13ꢀγꢀAl2O3,
V6O13ꢀZrO2 and V6O13ꢀtitanate, can substantially absorb
visible light (Figure 2). The visible light absorption
arises from the grafted V6O13 species as the γꢀAl2O3,
ZrO2 and titanate supports themselves all have
negligible visible light absorption. Hence, the visible
light absorption of the grafted V6O13 species and the
adsorbed O2 molecules on the catalyst surface result in
the SSꢀSPS response, which indicates photoꢀgenerated
charge separation. The SSꢀSPS response is relatively
weak, which is an important indicator of activating O2
molecules for the photocatalytic oxidation.
Beyond the photoꢀgenerated charge driving
mechanism, there are other plausible mechanistic
pathways by which the photocatalytic oxidation
reaction may proceed. The light absorption could
generate radicals that ultimately drive the oxidation at
room temperature.34 Alternatively, upon light
illumination, excited states of the chemically adsorbed
alcohol molecules formed on the V6O13 particles, may
transfer a proton to adsorbed O2, before further reaction
leads to the products. In this regard, radical scavengers
and excited state quenchers were used in an attempt to
give insight into the reaction pathway (Table S2, SI).
We found that introducing diꢀtertꢀbutylphenol (DTBP, a
powerful radical scavenger) of the same molar number
as that of vanadium atoms in the catalyst to the reaction
system (i.e. using a molar ratio of DTBP:V of 1:1) had
no obvious influence on the reaction. Whereas, adding
an equimolar amount of pyridine, an excited state
quencher, resulted in a decrease in the 3ꢀhexanone yield
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controlled
steadyꢀstate
surface
photovoltage
spectroscopy (SSꢀSPS) (Figure 4). The strong SPS
response indicates high charge transfer and separation,32
and this is always related to high photocatalytic activity
for conventional semiconductor photocatalysts.
Figure 4. The SSꢀSPS responses of V6O13ꢀTiO2 (a) and V6O13ꢀ
ZrO2 (b) catalyst in different atmosphere.
The strongest SPS response from the V6O13ꢀTiO2
catalyst appears in the UV range (Figure 4a). However
this is attributed to the TiO2 support rather than the
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to 51% under otherwise identical conditions. This effect
the V6O13 surface was modelled through the adsorption
of 3ꢀhexanol on a V6O13 cluster using density functional
theory (DFT) calculations. Details for the calculations
are described in the SI.
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may also arise from pyridine behaving as a base.
This further suggests that the reaction proceeds
through a lightꢀexcited state. An electron spin resonance
(ESR) spectrum of the freshly prepared V6O13ꢀγꢀAl2O3
photocatalyst indicates the presence of vanadium
species35 (Figure 5). When the nitroxide TEMPO,
which can quench excited state and scavenge radicals, is
introduced, the signature 3ꢀline ESR signal of TEMPO
decreases and interferes with the vanadium ESR signal.
Furthermore, when this mixture was irradiated with
blue light (460 nm), the TEMPO signals disappeared
completely, indicating that TEMPO interacts with light
excited V6O13ꢀalkoxide and transforms to an ESR silent
state (Figure 5). Similar results were also obtained when
using the V6O13ꢀZrO2 catalyst (Figure S6, SI). However,
the same ESR test on V2O5 catalyst shows that the
TEMPO signals were still observed clearly after
irradiation (Figure S7, SI). Given that the V2O5 catalyst
exhibited negligible photocatalytic activity, the ESR
results further confirmed that the light excited V6O13ꢀ
The calculation results show that 3ꢀhexanol readily
adsorbs on the V6O13 cluster with adsorption energy of
21.2 kcal/mol (Figure 6a). The adsorbed structure was
first present as isomer 1 as shown in the figure. When
the 3ꢀhexanol was adsorbed on the V6O13 cluster surface,
the OꢀH bond was easily cleaved to form the isomer 2.
The energy barrier to break the OꢀH bond is 9.7
kcal/mol. We also calculated the optical absorption
properties of V6O13 and isomer 2. The excited states
with energies in the range from 400 to 800 nm are listed
in Table S2, ESI. There are significantly more visible
light excited states for the V6O13ꢀalkoxide species
compared to pristine V6O13. We plotted the calculated
excited states with the light absorption spectrum of the
V6O13ꢀγꢀAl2O3 catalyst (Figure 6b). The V6O13 particles
and alcohol molecules preferably form V6O13ꢀalkoxide
in the reaction system and this V6O13ꢀalkoxide has a
stronger optical absorption compared to naked V6O13
particles in 400ꢀ800 nm wavelength regions. It is
therefore likely that visible light excites the V6O13ꢀ
alkoxide and the resultant excited state reacts with O2
adsorbed in the vicinity.
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alkoxide species play
a dominate role in the
photocatalytic reactions, which is different from those
catalysts via a semiconductor mechanism.
Figure 6. (a) The DFT calculated stepwise adsorption of 3ꢀ
hexanol on V6O13 cluster. The red spheres represent oxygen
atoms; large grey spheres, vanadium atom; in the adsorbed
molecule, grey spheres are for carbon atom white small spheres
for hydrogen atoms, (b) DFT simulated oscillator strength for
the allowed excited states of 3ꢀhexanol adsorbed on V6O13
(green) in 400ꢀ800 nm wavelength range compared with the
light absorption spectrum of the V6O13ꢀγꢀAl2O3 catalyst (left
axle). The strength for the excited states of the V6O13 particles
is relatively low (red).
The strong interaction of the reactant with the
surface of the V6O13 grafted surface catalysts is
confirmed by infrared emission spectroscopy (IES)
analysis with the V6O13ꢀZrO2 catalyst and 3ꢀhexanol as
the reactant (Figure 7). We can clearly see the presence
of a strong peak around 1000 cmꢀ1 in panel of Figure 7,
Figure 5. ESR spectra of the V6O13ꢀγꢀAl2O3 catalyst under
different reaction conditions.
To simulate the formation of V6O13ꢀalkoxide species
on the support surface, the behavior of the alcohol on
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which is attributed to V=O bonds, and the peak
Page 8 of 11
lightꢀdriven catalytic selective oxidation of alcohols can
be attained (Scheme 1). The first step of the likely
catalytic cycle would involve alcohol adsorbing on the
V6O13 clusters (I). Subsequent insertion into the OꢀH bond
and loss of a proton would give the V6O13ꢀalkoxide on the
cluster surface as supported by DFT simulation results.
The release proton is captured by adsorbed O2 on the
catalyst surface (II). Irradiation with wavelength <520 nm
can efficiently excite the V6O13ꢀalkoxide and cleavage of
the CꢀH bond of αꢀC (the reaction step yielding IV from III)
in the lightꢀexcited state of V6O13ꢀalkoxide would be much
easier than that in unexcited state. Hence, the reaction
proceeds under light irradiation, but not in the dark. The
presence of adsorbed O2 in the vicinity of the photoꢀ
excited V6O13ꢀalkoxide may facilitate the cleavage (III),
producing the desired carbonyl compound and regenerating
the V6O13 surface (IV).
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disappeared above 250 °C (Figure 7a). The strong C=O
carbonyl stretching was observed in Figure 7a, which
can be ascribed to the presence of the oxidation product
3ꢀhexanone, since the 3ꢀhexanol was adsorbed on the
catalyst prior to the IES analysis and the alcohol
adsorbed catalysts were exposed to air during the
adsorption and analysis. The interaction of 3ꢀhexanol
with the surface of V6O13ꢀZrO2 catalyst is much
stronger than that without the V6O13 species according
to comparison of Figure 7a and 7c. Therefore, the
oxidation is likely to take place on the V6O13 clusters of
the V6O13ꢀZrO2 catalyst, and the mechanism of the
reaction is likely to involve the reactant molecule
activation on the grafted V6O13 species. The strong
chemisorption of the reactant molecule on the surface of
the catalyst would significantly promote the catalytic
activity in photocatalysis.36
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Scheme 1. Proposed mechanism of alcohol oxidation by V6O13
grafted surface catalysts.
■ CONCLUSIONS
In summary, a new lightꢀdriven catalytic process has
been developed that exploits a variety of stable,
reusable and readily prepared catalysts of mixedꢀ
valence vanadium oxide clusters dispersed on oxide
supports. A key feature of these new photocatalysts is
that V6O13 species with mixed oxidation states (V4+ or
V5+) are the active component for the visible light
photocatalysis. The catalysts are highly effective for
selective oxidation of aliphatic and aromatic alcohols to
the corresponding aldehydes and ketones under visible
light irradiation using molecular oxygen as a benign
oxidant. The reaction is likely to proceed via an excited
state V6O13ꢀalkoxide intermediate and subsequent
elimination of αꢀH in the bound alkoxide to produce the
desired carbonyl compound. This represents a new
mechanism distinct from conventional processes for
semiconductor photocatalysts. It was also demonstrated
that the catalytic selective oxidation can be driven with
focused sunlight at ambient temperature. Furthermore,
these catalysts are able to oxidize relatively stable CꢀH
Figure 7. IES spectra of 3ꢀhexanol adsorbed on V6O13ꢀZrO2
catalysts (a), V6O13ꢀZrO2 catalysts (b), and 3ꢀhexanol adsorbed
on ZrO2 (c).
In accordance with the experimental and
computational results in the present study and
literature,35,37 some insight into the mechanism for the
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bonds present in alkyl aromatics at ambient temperature
detector, which was modified by replacing the IR source
with an emission cell.
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using molecular oxygen under visible light irradiation.
The catalytic process is temperature independent and
driven entirely by light. The prospect of nonꢀnoble
metal catalyzed selective oxidation using an earth
abundant element and molecular oxygen at room
temperature has the potential to deliver greener
industrial processes in the future.
Photocatalytic activity test. The light source for
photocatalytic activity test was light emitting diode (LED).
Single wavelength LEDs (purple: 390ꢀ410 nm, blue: 460ꢀ
462 nm, green: 515ꢀ517 nm, yellow: 587.5ꢀ590 nm, red:
620ꢀ625 nm) and white light LEDs (400ꢀ800 nm) were
employed appropriately in this study. For a typical
photocatalytic reaction, 50 mg of the oxide supported
V6O13 catalyst, 0.2 mmol of an alcohol and 2 mL of α,α,αꢀ
trifluorotoluene (as solvent) were mixed in a closed glass
tube reactor, purged with oxygen for 2 min. The reaction
mixture was magnetically stirred and illuminated with light
9
■ EXPERIMENTAL SECTION
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Synthesis of V6O13 grafted on different oxide supports
.
All precursor chemicals were purchased from Sigmaꢀ
Aldrich except laponite (Kindly supplied by Fernz
Specialty Chemicals, Australia), and used without further
purification. Generally, one gram of the oxide support
(either prepared in the lab (γꢀAl2O3 nanofibres, zeolite Y,
o
of a particular wavelength range at 30±2 C for a desired
time period. For the reactions directly using VO(acac)2 as
photocatalyst, 8 mg of VO(acac)2 was used as the number
of vanadium centers is equivalent to that in the 50 mg
photocatalyst of V6O13 grafted solid. Specimens were taken
from the reaction mixture at designed irradiation time
intervals, and filtered through a Millipore filter (400 nm,
Teflon) to remove the catalyst particles prior to the
analysis. The filtrate was analyzed by Agilent 7820A gas
chromatography (GC) equipped with a HPꢀ5 column.
protonatedꢀtitanate,
hydrotalcite)
or
commercially
available (ZrO2, laponite, V2O5) was mixed with 0.165 g of
VO(acac)2 and 100 mL of 95% ethanol. The suspensions
were sonicated in an ultrasonic bath for 1 h and aged for
about 16 h (overnight). The precipitate (V6O13ꢀgrafted
oxide) was recovered, washed with 95% ethanol for three
times. The product was dried at 60 °C for 12 h under
vacuum
ASSOCIATED CONTENT
Characterization of catalyst. The TEM study on the
catalysts was conducted using a Philips CM200 JEOL
2100 TEM with an accelerating voltage of 200 kV. XRD
patterns of the samples were recorded on a Philips
PANalytical X’Pert PRO diffractometer using Cu Kα
radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA
with a fixed slit. To investigate the light absorption and
emission behavior of the samples as well as their energy
band gap, we measured the diffuse reflectance UVꢀVisible
(DRꢀUVꢀVIS) spectra of the samples on a Varian Cary
5000 spectrometer. The nitrogen sorption isotherms were
measured by volumetric method on an automatic
adsorption instrument (Micromeritics, Tristar 3000) at
liquid nitrogen temperature (77 K). Electron paramagnetic
resonance (EPR) spectra were recorded with a Bruker EPR
ELEXSYS 500 spectrometer operating at a frequency of
9.5 GHz in the Xꢀband mode. Measurements were
performed with an ER 4131 VT variable temperature
accessory at 135 K. The infrared (IR) spectra were
recorded on Nicolet Nexus 870 IR spectrophotometer
equipped with a deuterated triglycine sulfate (DTGS)
detector and a diamond attenuated total reflectance (ATR)
smart accessary. For each measurement 64 scans were
collected over the spectral range of 4000ꢀ525 cmꢀ1 with a
resolution of 4 cmꢀ1. Xꢀray photoelectron spectroscopy
(XPS) data was acquired using a Kratos Axis ULTRA Xꢀ
ray Photoelectron Spectrometer incorporating a 165 mm
hemispherical electron energy analyzer. The incident
radiation was Monochromatic Al Kα Xꢀrays (1486.6 eV) at
225 W (15 kV, 15 ma). Narrow highꢀresolution scans were
run with 0.05 eV steps and 250 ms dwell time. Base
pressure in the analysis chamber was 1.0×10ꢀ9 torr and
during sample analysis 1.0×10ꢀ8 torr. Peak fitting of the
highꢀresolution data was carried out using the CasaXPS
software. The IES measurements were conducted on a
Digilab FTSꢀ60A spectrometer equipped with a TGS
Supporting Information. Figures S1ꢀ8, Tables S1ꢀS3, and
detailed DFT simulation parameters. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: hy.zhu@qut.edu.au
Notes
The authors declare no competing finical interest.
ACKNOWLEDGMENT
The authors gratefully acknowledge financial support from the
Australian Research Council (ARC DP110104990 and
DP150102110). The electron microscopy work was performed
through a user project supported by the Central Analytical
Research Facility (CARF), Queensland University of
Technology.
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