Visible-light-induced aerobic oxidation of alcohols in a coupled photocatalytic system of dye-sensitized TiO2 and TEMPO

Article abstract of DOI:10.1002/anie.200803630

(Chemical Equation Presented) Like a differential gear: Upon irradiation with visible light, excited dye molecules inject electrons into TiO₂ and form dye radicals, which can oxidize TEMPO to TEMPO⁺, which in turn oxidizes alcohols in the presence of O₂ selectively to corresponding aldehydes. One reason for the selectivity is that the irradiation is downshifted to visible light.

Full text of DOI:10.1002/anie.200803630

Communications
DOI: 10.1002/anie.200803630
Photocatalysis
Visible-Light-Induced Aerobic Oxidation of Alcohols in a Coupled
Photocatalytic System of Dye-Sensitized TiO2 and TEMPO**
Miao Zhang, Chuncheng Chen, Wanhong Ma,* and Jincai Zhao*
Selective catalytic oxidation of alcohols is a fundamental
transformation for the synthesis of fine chemicals.[1] Catalysts
based on transition metals, such as palladium, platinium, and
copper, have been used for efficient aerobic oxidation of
alcohols.[2] Selective transformation of alcohols by dioxygen
can also be achieved in the presence of both transition-metal
ions and nitroxyl radicals (such as 2,2,6,6-tetramethylpiper-
idinyloxyl, TEMPO).[3] However, the use of large amounts of
transition-metal complexes could potentially lead to environ-
mental pollution. Therefore, considerable efforts have been
made to find a clean catalytic system without transition metals
for the aerobic transformation of alcohols. Recently, a novel
Scheme 1. The mechanisms of A) TiO2 photocatalysis under UV irradi-
ation and B) the dye/TiO2-sensitized reaction under visible-light irradi-
ation. CB: conduction band, VB: valence band.
transition-metal-free TEMPO-based catalysis system was
developed, in which Br2 and NaNO2 were used as cocatalysts,
and the effective oxidation of alcohols by molecular oxygen
was accomplished at 808C.[4a] To facilitate the removal of the
TEMPO catalyst after use, a heterogeneous variant in which
TEMPO is supported on SBA-15 was prepared; this, together
with NaNO2 as cocatalyst, was also used successfully for the
selective oxidation.[4b]
The TiO2 photocatalyst under UV irradiation is a clean
catalytic system widely applied for degradation of organic
pollutants by dioxygen. However, for organic synthesis, the
selectivity of the TiO2-photocatalyzed oxidation is unsatis-
factory.[5] In photooxidation processes, the photo-generated
holes in the valence band of TiO2 have high oxidative
potential (2.7 V) and propel the oxidation of the organic
compounds (Scheme 1A). But they may oxidize reactant
alcohols and product aldehydes (or ketones) indiscriminat-
ingly, resulting in a poor selectivity for the product alde-
hydes.[5,6] The selectivity problem actually impedes the
application of TiO2 photocatalysis for the synthesis of fine
chemicals. To achieve high product selectivity for alcohol
oxidation using TiO2 photocatalyst, one must avoid the
formation of strongly oxidative holes in the valence band of
TiO2. Novel systems are required that can conduct alcohol
oxidation but not generate holes under irradiation.
We and others have found that the dyes undergo effective
photosensitized degradation in water in the presence of TiO2
under visible-light irradiation.[7] In the dye-sensitized system,
the dye, rather than the TiO2 catalyst, absorbs visible light and
is excited (Scheme 1B). More importantly, the visible-light
irradiation will not create holes in the valence band of TiO2.
This is distinctly different from the situation of TiO2 photo-
catalysis under UV irradiation (Scheme 1A). In the dye/TiO2-
photosensitized reaction, the conduction band of TiO2 still
plays an important role. The appropriate potential relation
enables the excited dye molecules to inject electrons into the
conduction band of TiO2 particles, yielding dye radicals
(Scheme 1B).[7] These dye radicals are transient, active
species and decompose to small molecules and even further
to CO2 by complicated dioxygen-involved reactions.
The nitroxyl radical TEMPO can act as an electron donor
and be oxidized to TEMPO+, which is an effective and
selective oxidant for the transformation of primary alcohols
into aldehydes by a two-electron-transfer mechanism.[3] The
produced TEMPOH can regenerate TEMPO by reaction
with the oxidative dye radials or O2.[3a,c] The combination of
dye-sensitized TiO2 and TEMPO thus offers an interesting
hypothesis: if dye-sensitized TiO2 can drive the electron-
transfer cycle of TEMPO (TEMPO!TEMPO+!
TEMPOH!TEMPO), we should be able to develop a
clean catalytic system for the selective oxidation of alcohols.
[*] Dr. M. Zhang, C. Chen, W. Ma, Prof. J. Zhao
Key Laboratory of Photochemistry
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry, Chinese Academy of Sciences
Beijing, 100190 (P.R. China)
Fax: (+86)10-82616495
E-mail: jczhao@iccas.ac.cn
wham@iccas.ac.cn
[**] This work was supported by 973 project (No. 2007CB613306) and
the NSFC (Nos. 20537010, 20772129, and 20777076) and CAS.
TEMPO=2,2,6,6-tetramethylpiperidinyloxyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803630.
Scheme 2. The proposed visible-light-induced reaction cycles in the
dye/TiO2/TEMPO system.
9730
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 9730 –9733

Angewandte
Chemie
Such a photocatalytic system can exhibit good selectivity
(owing to TEMPO), operate under visible-light irradiation at
ambient temperature, and use molecular oxygen (because of
the dye-sensitized TiO2). The prerequisite is that the dye
radicals arising from the dye-sensitized TiO2 under visible
light are effectively and rapidly quenched by TEMPO
through an appropriate intermolecular interaction. Dioxygen
can trap the electrons in the conduction band of TiO2 to form
ꢀ
O2 C, H2O2, or H2O. Scheme 2 illustrates the principles of the
coupling redox cycles for the photocatalytic and selective
oxidation of alcohols under visible-light irradiation.
To verify the hypothesis, we used the commercial anthra-
quinone dye Alizarin Red (AR) as the sensitizer and TEMPO
as the cocatalyst in a a suspension of TiO2 in benzotrifluoride
(BTF). BTF was chosen as solvent because of its inertness to
Figure 1. Control experiments for the transformation of benzyl alcohol
(0.1 mmol in 1.5 mL BTF) under different conditions; the reaction
time was 6 h. Experiments 1: AR/TiO2/TEMPO +O2, in the dark;
2: AR+TEMPO+O2, visible light; 3: AR/TiO2 +O2, visible light;
4: AR/SiO2/TEMPO+O2, visible light; 5: TiO2/TEMPO+O2, visible
light; 6: AR+O2, visible light; 7: TEMPO+O2, visible light; 8: AR/
TiO2/TEMPO+Ar, visible light; 9: AR/TiO2/TEMPO+O2, visible light.
In each experiment the 8 mg of the AR/TiO2 photocatalyst was used
(which contained 6.5ꢀ10ꢀ4 mmol AR); 2ꢀ10ꢀ3 mmol TEMPO and
0.1 MPa O2 were used (if they are listed).
ment 9). The turnover number (TON) is over 100, indicating
that this is a catalytic reaction. However, these TON values
are low compared to those reported for other oxidation
systems. The photonic efficiency is also relatively low for this
reaction (0.2% at 525 nm, for the transformation of benzyl
alcohol), suggesting that the system can be optimized in
further studies.
oxidation and high solubility for dioxygen. AR was chosen
because of its high extinction coefficient (emax ꢁ 28000) at
wavelengths of 400–500 nm in the visible region and its redox
potential which is suitable for effectively driving the corre-
sponding events of electron transfer from the excited state of
AR to the conduction band of TiO2 and from TEMPO to the
dye radical. The redox potential EA0 R
(ꢀ1.57 V vs.
The conversion, selectivity, and TON of the oxidation of a
broad range of alcohols with the proposed system are
summarized in Table 1. Of these substrates, aromatic alcohols
such as p-methoxyl benzyl alcohol could be easily oxidized
with high TON (154) and selectivity (99%, Table 1, entry 4),
while aliphatic alcohols (entry 8) exhibited relatively low
activity. It is worth noting that allylic alcohols could be
oxidized to the corresponding a,b-unsaturated aldehydes with
high selectivity (94–98%) without protection of the double
bond (Table 1, entries 7 and 9). More interestingly, alcohols
containing a N heteroatom, which are generally not reactive
in transition-metal-catalyzed reactions,[10] could also be con-
verted with high selectivity (Table 1, entry 10). However, the
present system exhibits very low activity for the transforma-
tion of secondary cyclic alcohols (Table 1, entry 11) compared
with the activity for primary alcohols. This is attributed to the
inherent limitations of TEMPO.[3] The photocatalytic reaction
could also be carried out on a larger scale. For instance, when
the reaction in Table 1, entry 1, was conducted on a large
scale, it still exhibited high activity (TON = 665) and selec-
tivity (Table 1, entry 2). Thus, a visible-light-induced system
to realize the selective oxidation of alcohols has been
assembled successfully. In this system, all components—
TiO2, dye, TEMPO, dioxygen, and visible-light irradiation—
make unique contributions. A broad range of alcohols were
transformed with greater than 93% selectivity with this
system; this selectivity is much higher than that in the TiO2
photocatalytic oxidation under UV irradiation, especially for
aliphatic alcohols.
=ARþ
*
C
standard hydrogen electrode, SHE) is lower than Ecb of
TiO2 (ꢀ0.50 V vs. SHE), and E0ARþ =AR (0.79 V vs. SHE) is
C
higher than E0TEMPO=TEMPOþ (0.64 V vs. SHE) (see Table S2 in
the Supporting Information).[8] Moreover, AR can be strongly
ꢀ
preanchored on the TiO2 surface by coordination of the SO3
group to the surface sites of TiO2. AR/TiO2 hardly dissociates
into the solution during the photoreaction, so that it can be
easily recovered by filtration for reuse (see Figure S1 in the
Supporting Information). In control experiments (Figure 1),
we found no obvious transformation of benzyl alcohol in the
absence of anyone of TiO2, dye, or dioxygen. The reaction
could not take place without visible-light irradiation. Alco-
hols could also be transferred in this system under UV
irradiation. However, in this case, not only the dye but also
TiO2 was excited by UV light and holes were produced, which
led to uncontrollable deep oxidations and low selectivity.[9]
When TiO2 was replaced with SiO2 nanoparticles, the reaction
did not occur; this indicates that the process of electron
injection from the excited state of the dye to the conduction
band of TiO2 is important. Benzyl alcohol could also be
oxidized by dye/TiO2 under visible-light irradiation in the
absence of TEMPO (Figure 1, experiment 3); however, the
reaction was much slower and the AR degraded quickly. This
is in agreement with the sensitization degradation of dyes by
TiO2 in the presence of dioxygen.[7] The AR/TiO2/TEMPO
triad system exhibited remarkable reactivity and selectivity
for transformation alcohols in the presence of O2 under
visible-light irradiation (l > 450 nm) (Figure 1, experi-
Angew. Chem. Int. Ed. 2008, 47, 9730 –9733
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9731

Communications
Table 1: Aerobic oxidation of alcohols with the AR/TiO2/TEMPO catalyst system under visible-light
irradiation.[a]
The results of electron spin
resonance (ESR) experiments
(Figure 2) also support the photo-
catalytic cycles proposed in
Scheme 2. For the sample contain-
ing only AR/TiO2, a strong single
peak at g = 2.0067 appeared upon
laser irradiation at 532 nm which is
attributed to the radical of AR. The
AR radical is generated by electron
injection from the excited state of
AR to the conduction band of
TiO2.[8] When TEMPO was added
to this system, a triplet peak at g =
2.0079 with a splitting constant of
aN = 16.8 G was observed before
the sample was irradiated. This
triplet peak was assigned to the
free radical TEMPO. Upon laser
irradiation at 532 nm, the intensity
of the triplet peak decreased imme-
diately, and the single peak of the
AR radical appeared. These
changes in the ESR signals corre-
spond to the oxidation of TEMPO
to TEMPO+ by the AR radical. The
broader single peak of the AR
radical can be explained by the
interaction existing between the
two types of radicals.[11] The inten-
sity of the triplet peak of TEMPO
[b]
Entry
Substrate
Product
t [h]
Conv. [%]
TONdye
Sel. [%]
1
18
80
43
123
98
2[c]
100
665
98
3
4
5
6
15
15
15
20
87
100
59
135
154
91
97
99
99
98
49
76
7
13
22
16
66
20
71
101
31
98
93
94
8
9[d]
218
10
11
10
16
23
7
36
10
95
99
[a] Reaction conditions: Alcohol (0.1 mmol), AR/TiO2 (8 mg, containing 6.5ꢀ10ꢀ4 mmol AR), and
TEMPO (2ꢀ10ꢀ3 mmol) were stirred in 1.5 mL of BTF under 0.1 MPa O2 under visible-light irradiation
(l>450 nm). [b] TONdye was calculated as nproduct/nAR [c] Scaled-up reaction: 5 mmol substrate, 40 mg
AR/TiO2, 0.01 mmol TEMPO in 30 mL BTF. [d] 0.2 mmol substrate..
remained at a certain level, suggesting that TEMPO is in the
catalytic cycle.
The catalytic cycle involves two important steps of
electron transfer: 1) the electron injection from the excited
dye molecule to the conduction band of TiO2 to generate dye
radical; and 2) the dye-radical-promoted oxidation of
TEMPO to TEMPO+, which can selectively oxidize the
alcohols to aldehydes. Dioxygen reacts with ecbꢀ to form O2Cꢀ
(which reacts further to give H2O2 or H2O) and enables the
excited state of the dye molecules to inject electrons into the
conduction band of TiO2 incessantly.[7,8] The byproduct H2O2
ꢀ
produced by the trapping of ecb by dioxygen was also
detected by idiometric titration method (see Table S1 in the
Supporting Information).
In summary, the selective oxidation of alcohols was
achieved under visible-light irradiation in a system containing
dye-sensitized TiO2 and TEMPO. This photocatalytic system
employs neither transition-metal ions nor deleterious oxi-
dants such as Br2 and NO2ꢀ. The reaction is driven by visible-
light irradiation at ambient temperature and uses molecular
oxygen as the oxidation agent. It exhibits very good selectivity
because the process does not generate holes in valence band
of TiO2. Our work indicates that the photocatalytic synthesis
of many fine chemicals can potentially be achieved with high
product selectivity when the catalysis systems are properly
assembled.
Figure 2. A) The ESR spectrum of the cation radical of ARC. AR/TiO2
(8 mg, containing 6.5ꢀ10ꢀ4 mmol AR) in benzotrifluoride (BTF)
(0.5 mL) was stirred and then put into a quartz tube. B) Changes in
the ESR spectra of TEMPO and ARC. TEMPO (1ꢀ10ꢀ6 m), AR/TiO2
(8 mg, containing 6.5ꢀ10ꢀ4 mmol AR), and benzyl alcohol (0.1 mmol)
were added to BTF (0.5 mL). The mixture was stirred and then put
into a quartz tube. In situ irradiation from a laser (532 nm ) was used
in this experiment.
9732
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 9730 –9733

Angewandte
Chemie
Semmelhack, C. R. Schmid, D. A. Cortꢁs, C. S. Chou, J. Am.
Experimental Section
Chem. Soc. 1984, 106, 3374; d) A. Cecchetto, F. Fontana, F.
Minisci, F. Recupero, Tetrahedron Lett. 2001, 42, 6651.
[4] a) R. Liu, X. Liang, C. Dong, X. Hu, J. Am. Chem. Soc. 2004,
126, 4112; b) B. karimi, A. Biglari, H. C. James, V. Budarin,
Angew. Chem. 2007, 119, 7348; Angew. Chem. Int. Ed. 2007, 46,
7210.
[5] a) J. H. James, J. M. Miller, Tetrahedron Lett. 1977, 7, 599; b) P.
Pichat, M. Mozzanega, H. Courbon, J. Chem. Soc. Faraday
Trans. 1987, 83, 697; c) G. Palmisano, V. Augugliaro, M. Pagliaro,
L. Palmisano, Chem. Commun. 2007, 3452; d) S. Yurdakal, G.
Palmisano, V. Loddo, V. Augugliaro, L. Palmisano, J. Am. Chem.
Soc. 2008, 130, 1568.
Oxidation of alcohols: A mixture of the alcohol (0.1 mmol), AR/TiO2
(8 mg, containing 6.5 ꢀ 10ꢀ4 mmol AR), and TEMPO (0.002 mmol) in
benzotrifluoride (BTF) (1.5 mL) was transferred into a 10 mL Pyrex
bottle. The container was filled with pure oxygen at a pressure of
0.1 MPa. The mixture was stirred under irradiation from a 500 W
halogen lamp with a light filter to cut off light of wavelength
< 450 nm. After the reaction, the reaction mixture was filtered
through a membrane with pore diameter of 20 mm and then analyzed
with a HITACHI Gas Chromatograph (GC 3900). The structures of
the products were confirmed by comparison with standard samples
and by GC-MS (Thermo-Finingan; Trace 2000/Trace DSQ).
ESR experiments were conducted with a Bruker EPR ELEXSYS
500 spectrometer equipped with an in situ irradiation source (a
Quanta-Ray Nd:YAG laser system with l = 532 nm). The same
quartz tube was used for all the measurements to minimize errors.
[6] a) P. V. Kamat, Chem. Rev. 1993, 93, 267; b) M. R. Hoffmann,
S. T. Martin, W. Choi, D. W. Bahnemann, Chem. Rev. 1995, 95,
69.
[7] a) J. Zhao, T. Wu, K. Wu, K. Oikawa, H. Hidaka, N. Serpone,
Environ. Sci. Technol. 1998, 32, 2394; b) T. Wu, G. Liu, J. Zhao,
H. Hidaka, N. Serpone, J. Phys. Chem. B. 1999, 103, 4862; c) K.
Vinodgopal, D. E. Wynkoop, P. V. Kamat, Environ. Sci. Technol.
1996, 30, 1660; d) W. R. Duncan, W. M. Stier, O. V. Prezhdo, J.
Am. Chem. Soc. 2005, 127, 7941.
[8] a) H. P. Dai, K. K. Shiu, Electrochim. Acta 1998, 43, 2709;
b) S. D. Rychnovsky, R. Vaidyanathan, T. Beauchamp, R. Lin,
P. J. Farmer, J. Org. Chem. 1999, 64, 6745; c) G. Liu, X. Li, J.
Zhao, S. Horikoshi, H. Hidaka, J. Mol. Catal. A 2000, 153, 221.
[9] O. S. Mohamed, A. E.-A. M. Gaber, A. A. Abdel-Wahab, J.
Photochem. Photobiol. A 2002, 148, 205.
Received: July 25, 2008
Published online: November 4, 2008
Keywords: photooxidation · radical reactions · TEMPO · TiO2 ·
.
visible light
[1] M. Hudlicky, Oxidation in Organic Chemistry, American Chem-
ical Society, Washington, DC, 1990.
[2] a) T. Mallat, A. Baiker, Chem. Rev. 2004, 104, 3037, and
references therein; b) J. A. Mueller, M. S. Sigman, J. Am.
Chem. Soc. 2004, 126, 9724; c) S. Velusamy, T. Punniyamurthy,
Org. Lett. 2004, 6, 217.
[10] M. Kꢂnigsmann, N. Donati, D. Stein, H. Schꢂnberg, J. Harmer,
A. Screekanth, H, Grꢃtzmacher, Angew. Chem. 2007, 119, 3637;
Angew. Chem. Int. Ed. 2007, 46, 3567.
[3] a) R. A. Sheldon, I. W. C. E. Arends, Adv. Synth. Catal. 2004,
346, 1051, and references therein; b) F. Recupero, C. Punta,
Chem. Rev. 2007, 107, 3800, and references therein; c) M. F.
[11] X. Li, J. Han, M. L. Pang, Y. Chen, J. X. Zhang, H. Ma, Z. J. He,
J. B. Meng, Tetrahedron Lett. 2007, 48, 6044.
Angew. Chem. Int. Ed. 2008, 47, 9730 –9733
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9733