Selective Aerobic Oxidation of Primary Alcohols to Aldehydes over Nb2O5 Photocatalyst with Visible Light

Article abstract of DOI:10.1002/cphc.201402343

Primary alcohols are selectively converted into aldehydes by using a Nb₂O₅ photocatalyst under visible-light irradiation. A strong interaction between the alcohol and Nb₂O₅ generates a donor level within the for

Full text of DOI:10.1002/cphc.201402343

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DOI: 10.1002/cphc.201402343
Selective Aerobic Oxidation of Primary Alcohols to
Aldehydes over Nb2O5 Photocatalyst with Visible Light
Shinya Furukawa,[a] Tetsuya Shishido,*[b, c] Kentaro Teramura,[a, c, d] and Tsunehiro Tanaka*[a, c]
Primary alcohols are selectively converted into aldehydes by
using a Nb2O5 photocatalyst under visible-light irradiation. A
strong interaction between the alcohol and Nb2O5 generates
a donor level within the forbidden band of Nb2O5, which pro-
vides a visible-light-harvesting ability. Over oxidation of alde-
hydes into carboxylic acids does not proceed under visible-
light irradiation.
and/or other active oxygen species, such as an ozonide anion
3À
C
radical (O ) or a hydroxyl radical (HO ), triggers the oxidation.
This pathway involves undesired deep oxidation of the pro-
duced carbonyl compounds into CO2, owing to the strong oxi-
dation abilities of the oxidizing species.[7] The other pathway is
an electron-derived path way, in which a superoxide anion rad-
ical (O2À) formed through the reduction of a molecular oxygen
with an excited electron triggers the oxidation.[8] This process
provides greater selectivity towards the carbonyl compounds
compared to the hole-derived pathway, partly because the oxi-
dizing ability of O2À is milder than the hole-derived active
oxygen species.[9] In the oxidation of primary alcohols, howev-
er, over oxidation of the produced aldehydes into carboxylic
acid often occurs,[8] because aldehydes are more easily oxi-
dized than alcohols. Thus, the selective photooxidation of pri-
mary alcohols into aldehydes is challenging and requires
a novel strategy that overcomes such an over oxidation. Re-
cently, Tanaka et al. reported that the photocatalytic oxidation
of primary alcohols to aldehydes selectively proceeded over
Au/CeO2.[10] They also suggested that the active oxygen species
were being removed in the reaction system and implied an in-
volvement of Au in the removal.
Oxidation of alcohols to corresponding carbonyl compounds is
one of the most important chemical conversions in industrial
processes and organic synthesis. In the principle of green
chemistry, development of a system that is catalytic and em-
ploys an environmentally friend oxidant is desired.[1] Catalytic
aerobic oxidation has been one of the most effective systems
for alcohol oxidation. Several heterogeneous catalytic systems
that use noble-metal elements (Au,[2] Ag,[3] Pd,[4] and Ru[5]) have
been reported as effective catalysts for the aerobic oxidation
of alcohols. However, replacement of the noble metals to non-
precious and abundant materials is necessary in terms of eco-
nomic efficiency and ubiquitous element strategy. In this con-
text, heterogeneous photocatalytic systems have a number of
advantages such as reusability, durability, economical efficiency,
and utilization of solar energy. Selective photooxidation of al-
cohols to the corresponding carbonyl compounds by using
molecular oxygen has been a hot topic.[6]
We recently demonstrated that the photooxidation of alco-
hols proceeds over Nb2O5 through a unique photoactivation
mechanism that differs from the classical bandgap excitation
mechanism. Excitation from a donor level consisted of an O2p
orbital, localized at the alcoholic oxygen, binding to a Nb site
in the conduction band of Nb2O5 (Scheme 1).[11]
In general, aerobic photooxidation of alcohols can proceed
by two different pathways. One is a hole-derived pathway, in
which a positive hole that is trapped at a lattice oxygen (OÀ)
A similar mechanism has been proposed in the photooxida-
tion of an amine to an imine over Nb2O5.[12] This photoexcita-
tion style has the unique properties as follows: 1) its excitation
energy is smaller than the bandgap energy of Nb2O5 and
[a] Dr. S. Furukawa,+ Dr. K. Teramura, Prof. Dr. T. Tanaka
Department of Molecular Engineering
Graduate School of Engineering, Kyoto University
Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: tanakat@moleng.kyoto-u.ac.jp
[b] Prof. Dr. T. Shishido
Department of Applied Chemistry, Tokyo Metropolitan University
Minami-Osawa, Hachioji, Tokyo 192-0397 (Japan)
E-mail: shishido-tetsuya@tmu.ac.jp
[c] Prof. Dr. T. Shishido, Dr. K. Teramura, Prof. Dr. T. Tanaka
Elements Strategy Initiative for Catalysts & Batteries (ESICB)
Kyoto University
1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245 (Japan)
[d] Dr. K. Teramura
Precursory Research for Embryonic Science and Technology (PRESTO)
(Japan) Science and Technology Agency (JST)
Honcho Kawaguchi, Saitama 332-0012 (Japan)
[+] Current address:
Tokyo Institute of Technology
2-12-1-E1-10 Ookayama, Meguro-ku, Tokyo 152-8550 (Japan)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201402343.
Scheme 1. Photoexcitation of alcohol oxidation over Nb2O5. A strategy for
the selective oxidation of primary alcohols to aldehydes is also illustrated.
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reaches the visible region (<3.2 eV); 2) the reaction proceeds
with visible light, even though the bandgap energy of Nb2O5 is
larger than 3.2 eV; 3) the localized excitation only enables the
formation of a positive hole at the alcoholic oxygen and not at
the lattice oxygen that causes the over oxidation. In this con-
text, it is expected that visible-light irradiation of Nb2O5 in the
presence of an alcohol excludes the bandgap excitation of
Nb2O5 itself and causes the localized excitation selectively,
leading to the selective photooxidation of the alcohol to the
aldehyde (Scheme 1).
Table 1. Photooxidation of p-methoxybenzylalcohol over Nb2O5 with vari-
ous conditions.[a]
Entry
Catalyst
l
Time
[h]
Conversion
[%]
Selectivity
[%][b]
[nm]
1
2
3[c]
4
5
6
7
8
+
+
+
À
+
À
+
À
+
>390
>390
>390
>390
>350
>350
>300
>300
dark
6
16
6
48
99
14
0
96
2
99
95
0
>99
>99
>99
–
94 (6)
>99
74 (24)
70 (11)
–
6
Figure 1 shows the time course of the product yield in the
aerobic photooxidation of p-methoxybenzylalcohol over Nb2O5
under the irradiation of UV/Vis (>300 nm) and visible (>
390 nm) light.
10
10
4
4
6
9
[a] Reaction conditions: Nb2O5 catalyst (100 mg), p-methoxybenzylalcohol
(1 mmol), PhCF3 solvent (10 mL), oxygen pressure (1 atm.) [b] Selectivity
towards p-methoxybenzoic acid is shown in parentheses. [c] Under 1 atm
N2.
reaction nor thermal reaction is involved. When the wave-
length of the incident light was expanded to >350 nm, the se-
lectivity towards the aldehyde was slightly decreased, owing
to the formation of the carboxylic acid (Entries 1 and 5). In this
case, very low conversion was obtained in the absence of
Nb2O5. This indicated that the contribution from the photore-
action of the alcohol and/or the aldehyde themselves can be
ignored and, hence, the carboxylic acid is formed through the
photocatalytic reaction by using the bandgap excitation of
Nb2O5. Selectivity towards the aldehyde significantly decreased
because of the formation of the carboxylic acid under the irra-
diation of UV/Vis light of >300 nm. Comparable conversion
and selectivity were obtained in the absence of Nb2O5 in this
wavelength region, indicating a large contribution of the pho-
toreaction of the alcohol and/or the aldehyde. These results
are consistent with the fact that the absorption edges of p-me-
thoxybenzylalcohol and p-methoxybenzaldehyde are at ap-
proximately 320[13] and 350 nm,[14] respectively. Figure 2 shows
the conversion-selectivity curves of the alcohol photooxidation
in different wavelength regions of the incident light.
Figure 1. Time course of the product yields in the aerobic photooxidation of
p-methoxybenzylalcohol over Nb2O5 under irradiation of a) UV/Vis
(>300 nm) and b) visible (>390 nm) light.
In the presence of UV light, p-methoxybenzaldehyde was
yielded as the main product at the initial stage, whereas over
oxidation of the aldehyde to the corresponding carboxylic acid
occurred considerably at the end of the reaction. In contrast,
under the irradiation of visible light (in the absence of UV
light), the aldehyde was obtained exclusively, even at 99%
conversion of the alcohol. It took a longer reaction time to
reach 99% conversion under visible-light irradiation than
under UV/Vis light, which was probably because of the de-
crease in the number of photons by using an L-42 cutoff filter.
Table 1 shows the results of several control experiments. Re-
placement of the reaction atmosphere from O2 to N2 signifi-
cantly lowered the catalytic activity (Entries 1 and 3). We previ-
ously reported that the photooxidation of alcohols proceeded
through oxidative dehydrogenation to form the aldehyde,
water, a vacancy in the lattice oxygen, and reduced Nb4+ sites,
followed by recovery of Nb5+ and the lattice oxygen by O2
(see Scheme S1 in the Supporting Information).[11] The lower
catalytic activity in the absence of O2 supports the idea that
the reoxidation process by O2 is necessary to function the cata-
lytic cycle effectively. The reaction did not proceed at all in the
absence of Nb2O5 under visible light (>390 nm, Entry 4) or in
the presence of Nb2O5 in the dark (Entry 9). These results sug-
gest that the reaction is photocatalytic and neither any photo-
In the presence of UV light (>300 and>350 nm), the selec-
tivity gradually decreased with increasing conversion. Irradia-
Figure 2. Conversion-selectivity curves for the photooxidation of p-methoxy-
benzylalcohol to the corresponding aldehyde in different wavelength re-
gions of the incident light.
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tion of the light >300 nm showed a steeper drop in selectivity
than that at >350 nm. In contrast, >99% selectivity was main-
tained, even at 99% conversion, under the irradiation of visible
light (>390 nm). Thus, the photooxidation of primary alcohols
to aldehydes was successfully achieved by using Nb2O5, O2,
and visible light. We subsequently investigated the substrate
scope of this photocatalytic system, as shown in Table 2. Ben-
molecules. We have already confirmed this in a previous study,
as no oxidized product was formed during the reoxidation of
Nb4+ to Nb5+ by using molecular oxygen.[15] Molecular oxygen
is consumed in the reoxidation of Nb4+ and is used to recover
the oxygen vacancies that are generated by oxidative dehydro-
genation (see Scheme S1 in the Supporting Information). Re-
cently, a similar photocatalytic system for alcohol oxidation
with TiO2 and visible light has also been reported.[16] Although
the proposed photoexcitation mechanism differs slightly from
that of our system, they are identical in that a strong interac-
tion between an alcohol and a photocatalyst induces a visible-
light-harvesting ability.
Table 2. Photooxidation of various alcohols over Nb2O5 under visible-light ir-
radiation.[a]
Entry Substrate
Product
Time Conversion Selectivity
[%][b]
In summary, the aerobic photooxidation of primary alcohols
into aldehydes selectively proceeded over Nb2O5 by using visi-
ble light. The obtained results not only indicate an attractive
photocatalytic system, but also demonstrate the validity of our
strategy for selective photocatalytic oxidation.
[h]
16
16
16
[%]
99
96
80
1
2
3
4
>99
97 (3)
>99
>99
Keywords: alcohols · aldehydes · Nb2O5 · photooxidation ·
visible light
16
16
64
59
5
6
>99
>99
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[a] Reaction conditions: Nb2O5 catalyst (100 mg), alcohol (1 mmol), PhCF3 sol-
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zylalcohol and its p-substituted derivatives (methoxy-, methyl-,
and chloro-) were selectively converted into the corresponding
aldehydes (Entries 1–4). Carboxylic acid was not formed at all
in each case, even at high conversion levels. In the oxidation
of p-methylbenzylalcohol, only a small amount of terephtalal-
dehyde was yielded as a byproduct. Other aromatic secondary
alcohols, 1-phenylethanol and diphenylmethanol, were selec-
tively converted to the corresponding ketones (Entries 5 and
6). Although the reaction rate was lower than those of aromat-
ic alcohols, cyclohexanol was also oxidized to cyclohexanone
with excellent selectivity (Entry 7). Thus, this photocatalytic
system can be applied to the oxidation of various alcohols, in-
cluding primary, secondary, aromatic, and aliphatic alcohols. As
expected, the key points of this photocatalytic system involve
the localized excitation from the donor level (alcoholic O2p or-
bital), which enables the oxidation of adsorbed alcohols (alkox-
ide) into the corresponding aldehydes by using visible light,
even though it is not absorbed by Nb2O5 itself. The lack of UV
light excluded the involvement of bandgap excitation of
Nb2O5 and photoreaction of an alcohol itself, both of which
cause over oxidation of an aldehyde, forming the correspond-
ing carboxylic acid. It should also be noted that the excited
electrons and the trapped electrons at Nb5+ sites (namely,
Nb4+ sites) do not trigger any oxidation reaction of organic
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Received: May 15, 2014
Published online on && &&, 2014
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S. Furukawa, T. Shishido,* K. Teramura,
T. Tanaka*
Primary alcohols are selectively con-
verted into aldehydes by using a Nb2O5
photocatalyst under visible-light irradia-
tion. A strong interaction between an
alcohol and Nb2O5 generates a donor
level within the forbidden band of
&& – &&
Selective Aerobic Oxidation of Primary
Alcohols to Aldehydes over Nb2O5
Photocatalyst with Visible Light
Nb2O5, which provides the visible-light-
harvesting ability. Overoxidation of alde-
hydes into carboxylic acids does not
proceed under visible-light irradiation.
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