Oxygen atom transfer in the photocatalytic oxidation of alcohols by tio2: Oxygen isotope studies

Full text of DOI:10.1002/anie.200900322

Angewandte
Chemie
DOI: 10.1002/anie.200900322
Surface Chemistry
Oxygen Atom Transfer in the Photocatalytic Oxidation of Alcohols by
TiO : Oxygen Isotope Studies**
2
Miao Zhang, Qi Wang, Chuncheng Chen, Ling Zang, Wanhong Ma,* and Jincai Zhao*
The selective oxidation of alcohols into carbonyl compounds
using dioxygen in lieu of toxic or corrosive stoichiometric
process involves a selective cleavage of the CꢀO bond of the
alcohol with concomitant formation of a new C=O bond in
the product aldehyde in which the O atom comes from
dioxygen. This finding adds fundamental insight to the
ꢀ
IV
oxidants such as ClO , Cr , and Cl , is one of the most
2
[
1,2]
challenging functional group transformations.
The oxida-
tion of alcohols using dioxygen as the oxidant has been
successfully realized by using noble-metal and transition-
oxidation process of alcohols on the TiO surface, which is
2
of importance for both the TiO2 photocatalysis and the
selective oxidation of alcohols.
[
2]
metal complexes for catalysis. TiO photocatalysis has also
2
1
8
attracted much attention as a potential and promising strategy
for this aim, because of its high oxidation ability, environ-
O-enriched benzyl alcohol and cyclohexanol were used
for the TiO photocatalytic oxidation (Table 1). The original
2
1
8
18
mentally friendly properties, and the benefit of using O as an
abundance of O in the O-enriched benzyl alcohol was 65%
(Table 1, entries 1–2) and 90% (Table 1, entries 4–7), respec-
2
[
3]
oxidant and light as the driving force. A few successful cases
involving TiO2 photocatalysis in acetonitrile, water, or
[
4]
solvent-free systems have recently been reported.
1
8
[a]
Table 1: Abundance of O in the substrates and products.
Molecular oxygen plays a vital role in the aerobic
oxidation of alcohols in all these systems. Therefore, it is
significant and necessary to reveal how the dioxygen partic-
ipates in the reaction process. In the noble-metal catalysis
system, the role of dioxygen has been proven to oxidize the
1
6
Entry
1
Conversion
%]
Selectivity
[%]
Substrate
( O: O)
Product (% O)
18
16
[
0
n+
reduced noble-metal center (for example, M or M hydride
species) without an O-atom transfer from dioxygen to the
40
43
10
100
99
100
C H CHO (>99)
CHO (>99)
C H CHO (50)
6 5
6
5
C H CH OH
[
b]
6
5
2
[5]
2
3
C H
6
5
products. In the aerobic oxidation of alcohols in the
cytochrome P450 system, a gem-diol intermediate is formed,
in which one hydroxyl group comes from the alcohol substrate
(65:35)
[
c]
4
5
6
7
8
17
42
65
43
52
99
99
99
99
99
C H CHO (>99)
6 5
1
6
18
(
ca. 100% O abundance) and the other from O (98%
O
C H CHO (>99)
2
6
5
C H CH OH
6
5
2
labeled). Such a gem-diol intermediate leads to approxi-
C H
6
5
CHO (>99)
(90:10)
[
[
b]
d]
C H CHO (>99)
mately 50% of the carbonyl product having incoorporated
6
5
1
8
[6]
C H
6
5
CHO (17)
O atoms. Unlike these thermal catalytic systems, the
essential role of dioxygen in the oxidation of alcohols by
9
10
11
20
47
99
99
99
C H O (>99)
6 10
C H O (>99)
6 10
C H O (>99)
6 10
C H OH
TiO photocatalysis has not been completely clarified yet.
6
11
2
(77:23)
Herein we disclose an unexpected phenomenon: when the
photocatalytically oxidative transformation of isotope-
11
[
(
a] The reactions were carried out in BTF solutions (1.5 mL) of alcohols
labeled alcohols was performed over pure anatase TiO in
ꢀ1
2
0.1 mmol) with TiO (0.1 mmol, 5.3 gL ) under 0.1 MPa O , and 100 W
2
2
1
8
18
organic solvents, such as benzotrifluoride (BTF), the oxygen
atom in the substrate alcohol is completely replaced by one of
the oxygen atoms of dioxygen, that is, the photocatalytic
Hg lamp irradiation. [b] TiO was treated by stirring in H O (98% O)
2
2
under UV irradiation for 12 h. [c] The reaction was carried out on a 1 cmꢀ
2 cm TiO electrode under UV irradiation for 12 h under Ar (>99.99%).
2
The bias potential was 0.6 V vs. SCE. [d] The reaction was conducted
under Ar, 0.1 mol% Pt/TiO was used as the photocatalyst.
2
[
*] Dr. M. Zhang, Dr. Q. Wang, Prof. C. Chen, Prof. W. Ma, Prof. J. Zhao
Key Laboratory of Photochemistry, Beijing National Laboratory for
Molecular Sciences, Institute of Chemistry
Chinese Academy of Sciences, Beijing, 100190 (China)
Fax: (+86)10-8261-6495
1
6
tively. However, the O abundance in the product, benzyl-
aldehyde, was approximately 100% in all these experiments
when analyzed at different conversions. This result indicates
that an oxygen atom transfer from the dioxygen to the
product during the reaction process. Moreover, little change
E-mail: jczhao@iccas.ac.cn
whma@iccas.ac.cn
Prof. L. Zang
Department of Materials Science and Engineering
University of Utah, 122S. Central Campus
Salt Lake City, UT 84112 (USA)
1
8
in the abundance of O in the unreacted alcohols was
observed, indicating no isotope exchange between the
substrate and product, or between the substrate and dioxygen.
[
**] This work was supported by 973 project (No. 2007CB613306), NSFC
(
Nos. 20537010, 20772129 and 20877076) and CAS.
In entries 2 and 7 of Table 1, the photocatalyst TiO was
treated with O-enriched water (98% O abundance) under
2
1
8
18
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900322.
UV irradiation for 12 hours to replace the surface H O/OH
2
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6081

Communications
1
8
18
groups with O-enriched H O/OH groups. When the O-
Table 2: The oxygen atom transfer in the photocatalytic oxidation of
benzyl alcohol in different solvents.
2
[
a]
enriched TiO2 surface was used as the photocatalyst, the
oxygen atom in the product was still replaced with
completely. This observation implies that the O atom trans-
ferred into the product did not originate from the surface-
bound water or OH groups of TiO , also excluding the
1
6
1
6
O
Entry Solvent Substrate Conversion Selectivity Product (% O)
1
8
16
( O: O)
[%]
[%]
1
2
3
4
BTF
CH CN C H CH OH
51
44
31
40
99
99
99
99
>99
>99
>99
>99
2
3
6
5
2
involvement of the common COH radicals as the active
oxidizing species in the aqueous TiO photocatalysis.
CH Cl (90: 10)
2
2
[3a]
C
6
H
14
2
In a photo-electrochemical experiment under an anaero-
bic Ar atmosphere (Table 1, entry 3), the photo-generated
electrons on the conduction band were transferred to the
counter electrode, and hence the alcohol could also be
oxidized into the corresponding aldehyde in the absence of
[
a] The reactions were carried out under 0.1 MPa O and 100 W Hg lamp
2
ꢀ1
irradiation for 4 h. Benzyl alcohol (0.1 mmol), TiO (0.1 mmol, 5.3 gL ),
solvents (1.5 mL).
2
O . In this process, a two-electron transfer (TET) process
1) The TET mechanism for the oxidation of alcohols
could be easily ruled out. In the TET process (Table 2,
entries 3 and 8), the oxygen atom in dioxygen is not
incorporated into the product and no oxygen atom transfer
will occur. When dioxygen exists in the reaction system, the
O2 molecule reacts easily with the alcohol radical and
therefore limits the TET process. Thus, the contribution of
the TET process can be ignored under the aerobic atmos-
phere.
2
[
8a]
(
Scheme 1) occurs,
and the oxygen atom of the alcohol
Scheme 1. The two-electron transfer mechanism in the oxidation of
alcohols in the absence of O2.
2) The photocatalytic reaction in organic solvents such as
BTF was dramatically different from that in water. The ESR
trapping experiments showed that the main active oxidative
species in aqueous system is the COH radical, which was,
however, not detected in the BTF system (see Figure S4 in the
Supporting Information). The experiments on kinetic isotope
[
9]
should remain in the product. As shown in entry 3 of Table 1,
1
8
the product was found to retain the original O-enriched
content after reacting for 12 hours. The little increase (from
effect (KIE) were carried out for the TiO photocatalytic
2
1
6
3
5% to 50%) in the abundance of O in the product may
oxidation of C H CH OH/C H CD OH under aerobic con-
6
5
2
6
5
2
come from the dioxygen mixed into Ar and dissolved in the
solvent (BTF). In another anaerobic experiment using Pt/
ditions. The KIE values were 1.2 and 4.6 in BTF and the
aqueous system, respectively. The significant difference in the
KIE values indicates different photocatalysis mechanisms in
the two systems, that is, the abstraction of the b-hydrogen
atom of the alcohol by COH radical is the rate-determining
TiO as the photocatalyst (Table 1, entry 8), the alcohol could
2
also be oxidized by a TET process because the electrons on
Pt/TiO could combine with protons to lead to the continuous
2
[8b,c]
[10]
production of H under the catalysis of Pt clusters.
abundance of O in the product was found to be 83%. These
results, in comparison with those under aerobic conditions
The
step in the aqueous system, but not in the BTF system.
2
1
8
Therefore, in the BTF system, a broad range of alcohols
including aryl, aliphatic, and enolic alcohols could be oxidized
to their corresponding aldehydes or ketones with high
selectivity (see Table S2 in the Supporting Information). In
comparison, much poorer selectivity in the aqueous system
was observed (using benzyl alcohol as a substrate, 38%
selectivity and 45% conversion at 8 h; and 45% total organic
carbon (TOC) was removed after 12 h of photoreaction).
We therefore propose the possible reaction mechanism as
shown in Scheme 2: an alcohol molecule adsorbs onto the
(
Table 1, entries 4–6), clearly indicate that an O atom transfer
is not involved in the oxidative transformation of the alcohol
when the reaction is run in the absence of dioxygen, although
the alcohol can still be oxidized into the corresponding
aldehyde by a TET process.
A similar oxygen atom transfer phenomenon could also
1
8
be observed in the photocatalytic oxidation of O-enriched
1
8
cyclohexanol (77% O) into cyclohexanone at different
conversions (Table 1, entries 9–11). In other solvents, such as
CH CN, CH Cl , and C H , the oxygen atom transfer process
surface of TiO to form the structure I via a deprotonation
2
+
ꢀ
process. TiO is excited by UV light to produce h /e pairs.
3
2
2
6
14
2
was also observed (Table 2). These results unambiguously
The adsorbed alcohol first reacts with the photo-generated
hole and subsequent deprotonation to form a carbon radical,
demonstrate a selective cleavage of the a-CꢀO bond of the
IV
alcohol, with the incorporation of an oxygen atom from the
whereas the photo-generated electron is captured by Ti to
III
III
dioxygen to form a new C=O bond in the aerobic photo-
form Ti (II). Both of the carbon radical and the Ti are easy
[
3]
oxidation of alcohols by TiO photocatlysis. Such an oxygen
to combine with dioxygen. The oxygen bridge structure III
may form through two possible routes: 1) the electron/Ti (in
2
III
atom transfer mechanism has not been proven either in the
photocatalytic or noble-metal catalytic transformation of
alcohols.
To reveal the oxygen atom transfer mechanism, some
possible reaction processes can be ruled out based on our
investigations:
the conduction band) first reduces O2 to the superoxide,
which then attacks the carbon radical to form intermediate III
or 2) the carbon radical first combines with dioxygen to form
an organic superoxyl radical which additionally reacts with
III
Ti to form structure III. The concerted cleavage of CꢀO
6
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ꢀ
1
structure I. The isotopic shift (peak at 762 cm in Figure 1C)
ꢀ
1
ꢀ1
fits the calculated value (762 cm ), and the peak at 830 cm
in Figure 1B could be assigned to the n(CꢀO) stretch
vibration of the adsorbed alcohol, which has an isotopic
ꢀ
1
ꢀ1
shift of 19 cm (811 cm in Figure 1C), very close to the
ꢀ1
ꢀ1
calculated value of 20 cm . The 890 cm band in Figure 1B
1
6
16
is assigned to the n( Oꢀ O) of a side-on coordinate peroxide
[
12a]
(
es.
IV).
Similar bands have been reported for Ti complex-
Scheme 2. The proposed oxygen transfer process in the TiO photo-
2
[
12b]
ꢀ1
16
16
catalytic oxidation of alcohols in BTF solvent in the presence O2.
In Figure 1C, the 890 cm band of the n( Oꢀ O) still
ꢀ1
remained, but was much weaker. The new band at 866 cm is
1
6
18
assigned to the n( Oꢀ O) in structure IV. The isotopic shift
ꢀ1
ꢀ1
bond of the alcohol and the OꢀO bond of dioxygen is likely
realized through such an oxygen bridged structure III. A
similar structure was also observed when the dioxygen reacts
of 24 cm for this mode was very close to the shift of 25 cm
predicted by Hookeꢀs law for a diatomic OꢀO stretch. The
1
6
18
existence of the Oꢀ O bond confirms the concerted
cleavage process III!IV (Scheme 2) of the CꢀO bond of
the alcohol and the OꢀO bond of the dioxygen.
[
11]
with iron complexes such as deoxyhemerythrin. The TiO2-
bound peroxide bridge structure IV, as one of the two
products of the concerted bond-cleavage process (III!IV),
may then combine with the protons to form H O , which was
In summary, it is reported for the first time that an oxygen
atom transfer, from the dioxygen to the a-carbon atom of the
alcohol, dominates the reaction process of the photocatalytic
2
2
evidenced by iodometric titration method (see Figure S5 in
the Supporting Information). An analogous side-on peroxide
of Ti site (IV) has been reported in the tilanosilicate
oxidation of alcohols by TiO in organic solvents such as BTF.
2
This new discovery helps us to better understand of the TiO2-
based photocatalysis and the selective oxidation of alcohols,
and to find broad applications in the synthesis of organic
compounds.
ꢀ
[4d]
photocatalysis in the presence of F ions.
Isotope-labeled resonance Raman spectroscopy was used
to detect the peroxide OꢀO bond formation in the photo-
catalytic oxidation of benzyl alcohol. Figure 1B shows the
Experimental Section
Photocatalysis: The photocatalytic reactions were carried out under
irradiation by a 100 W high-pressure Hg lamp (Toshiba SHL-
1
(
00UVQ) with continuous stirring in a 10 mL Pyrex glass bottle
cut-off light below 300 nm, thus TiO2 can be excited but not the
substrates ) under O (0.1 MPa). The radiant flux received by the
2
ꢀ2
reactor was 12.1 mWcm . A typical reaction system contained
0
ꢀ1
.1 mmol substrate and 0.1 mmol TiO (5.3 gL ) in 1.5 mL benzotri-
2
fluoride (BTF) solvent. The products were detected by GC-MS
methods (Thermo-Finingan; Trace 2000/Trace DSQ) with a DB-5MS
column (30 m ꢁ 0.25 mm).
The photocatalyst TiO was prepared by a hydrothermal process
2
(
see the Supporting Information). Pure anatase particles with average
2
ꢀ1
size of 10 nm and surface area of 210 m g were obtained.
In this work, we defined the conversion and selectivity for the
alcohol oxidation as:
Conversion = (c ꢀc )/c 100%
r0
r
r0
Selectivity = c /(c
c : the initial concentration of the reactant;
r0
r
0
ꢀc )100%
p
r
cr: the concentration of the reactant during the reaction;
cp: the concentration of the product during the reaction.
Figure 1. Low-frequency resonance Raman spectra: A) the Raman
Photo-electrochemical studies: The photo-electrochemical
experiments were carried out using a potentiostat at an applied bias
of 0.6 V vs. SCE, under the protection of Ar. A Pyrex glass bottle (cut-
off light < 300 nm) was used as the electrochemical cell. The
spectra of TiO photocatalyst used in this work. B) Raman spectra of
2
1
6
the TiO photocatalyst with O benzyl alcohol after three hours of
photoreaction. C) Raman spectra of the TiO photocatalyst with
2
1
8
O
2
+
ꢀ
benzyl alcohol after three hours of photoreaction.
supporting electrolyte used was 0.05m (tBu) N (PF ) . A TiO film
4 6 2
electrode was prepared by coating the precursor TiO gel (prepared
2
2
by the hydrothermal process) onto a 1 ꢁ 2 cm ITO glass slide and
1
6
Raman spectrum of the TiO catalyst with O-benzyl alcohol
subsequent heating at 4508C for 3 h. In this system, a TiO2 film
electrode was used as the working electrode, a saturated calomel
electrode (SCE) as the reference electrode, and a platinum plate as
the counter electrode. The experiments were carried out under
irradiation using a 100 W Hg lamp; 0.2 mmol of the substrate was
added into 10 mL BTF, and the reaction time was 12 h.
The Raman spectra were recorded in the backscattering geom-
etry by using a Renishaw-2000 Raman spectrometer with the
514.5 nm line of an Ar ion laser as the excitation source. TiO2
2
as the substrate after the photocatalytic reaction for three
hours, and Figure 1C gives the Raman spectrum of the TiO
catalyst with O benzyl alcohol (90% O abundance) after
three hours of photoreaction. Comparing with the Raman
spectrum of TiO (Figure 1A), new peaks at 890 cm ,
2
1
8
18
ꢀ1
2
ꢀ1
ꢀ1
8
30 cm , and 800 cm appeared in Figure 1B, respectively.
The peak at 800 cm could be assigned to the n(O-Ti) in
ꢀ
1
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Communications
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Keywords: alcohols · isotopic labeling · oxidation ·
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