Merging visible light photocatalysis of dye-sensitized TiO2 with TEMPO: The selective aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c7cy01510k

The selective oxidation of alcohols to aldehydes or ketones is very difficult to accomplish when the oxidant is molecular oxygen (O₂) and the reaction is carried out under ambient conditions. To meet these challenges, visible light photocatalysis was exploited to enable the use of O₂ for the selective oxidation of alcohols at room temperature. In this scheme, eosin Y, an organic dye, was connected with TiO₂ to capture visible light, leading to consequential photocatalytic processes. More importantly, these photocatalytic processes can be neatly merged with the redox catalytic cycles of TEMPO, which in turn induce the oxidation of alcohols under visible light irradiation. This work suggests that the merger of visible light photocatalysis and TEMPO catalysis has great potential for execution of challenging oxidative transformations.

Full text of DOI:10.1039/c7cy01510k

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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
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DOI: 10.1039/C7CY01510K
Journal Name
ARTICLE
Merging the visible light photocatalysis of dye‐sensitized TiO
2
†
with TEMPO: The selective aerobic oxidation of alcohols
b
b
a,b
Received 00th January 20xx,
Accepted 00th January 20xx
Yichi Zhang , Zhan Wang , and Xianjun Lang *
The selective oxidation of alcohols to aldehydes or ketones is very difficult to accomplish when the oxidant is molecular
oxygen (O ) and the reaction is carried out under ambient conditions. To meet these challenges, visible light photocatalysis
was exploited to enable the use of O for the selective oxidation of alcohols at room temperature. In this scheme, eosin Y,
an organic dye, was connected with TiO to capture visible light, leading to consequential photocatalytic processes. More
DOI: 10.1039/x0xx00000x
2
2
www.rsc.org/
2
importantly, these photocatalytic processes can be neatly merged with the redox catalytic cycles of TEMPO, which in turn
induce the oxidation of alcohols under visible light irradiation. This work suggests that the merger of visible light
photocatalysis and TEMPO catalysis has great potential in the execution of challenging oxidative transformations.
These improvements can be achieved by relying on light
energy to further this reaction, aided by a photocatalyst. Even
though semiconductor photocatalysis has been widely
Introduction
The selective oxidation of alcohols to their corresponding
carbonyls is of great importance in terms of both fundamental
research and industrial applications and has therefore been the
employed in the degradation of organic pollutants for a long
time, interest in the selective oxidation alcohols with O by
2
photocatalysis has recently grown rapidly with the
development of new mechanistic insights and an increased
1
‐7
subject of continued research efforts throughout the world.
However, this particular transformation usually requires
inorganic oxidants, such as chromium or manganese oxides, or
organic oxidants in stoichiometric amounts. Consequently,
large amounts of carcinogenic or toxic wastes are produced
concomitantly, which goes against the principles of green
1
2‐15
ability to control product selectivity.
homemade rutile TiO was successfully implemented for the
selective photocatalytic oxidation of alcohols in water under UV
For example,
2
1
6‐17
irradiation.
component of sunlight, cannot be sufficiently used in this
scenario due to the large bandgap of TiO (3.0 eV for rutile). As
However, visible light, which is the major
8
chemistry. Replacing these previously used oxidants with a
2
clean oxidant such as O
has become an urgent task. The direct oxidation of alcohols
is prevented by the inherent properties of O
catalyst is usually required for its activation.
2
to yield water as the only side product
such, a more convenient method to initiate visible light
photocatalysis should be developed to establish an energy
sustainable process. Meanwhile, the activation of O can be
2
carried out at room temperature. The interaction between
heteroatom‐containing substrates such as amines or sulfides
9‐11
2
, and a metal
For this reaction to be carried out, high temperature and
high oxygen pressure are necessary even in the presence of a
and TiO
2
can result in the formation of a surface complex, which
1
catalyst, which makes controlling the selectivity for the
in turn leads to photocatalytic selective oxidation reactions
carbonyl compounds very difficult to achieve. Maintaining the
high temperature and high gas pressure consumes a large
amount of energy. A sustainability issue might arise that makes
1
8‐19
under irradiation by visible light with λ>400 nm.
The visible‐
light‐induced selective oxidation of alcohols can occur via this
20‐21
2
same mode of activation of rutile or anatase TiO .
2
these processes less attractive for the application of O as the
Nonetheless, a small range of visible light spectra can be
used, and the scope of alcohols is quite narrow. The
terminal oxidant in the oxidative transformation of alcohols.
Both environmental and energy aspects should be considered
to improve these chemical processes, favoring reaction
conditions that include room temperature and ambient O
pressure.
2
2
23
incorporation of a metal complex or organic dye with a metal
oxide enables the selective oxidation of alcohols by O . As such,
2
2
longer wavelength visible light can be absorbed in these two
scenarios. However, the rates of these reactions should be
significantly improved for practical applications. The molar
‐
1
extinction coefficient of eosin Y is 112,000 cm /M at 524.8 nm
in basic ethanol. At an excitation wavelength of 490 nm, the
quantum yield of eosin Y is 0.67. These properties suggest that
this molecule can capture visible light efficiently and shows
a.
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072,
China. E‐mail: xianjunlang@whu.edu.cn
College of Chemistry, Central China Normal University, Wuhan 430079, China
b.
†
TEMPO, (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl
2
4‐25
great potential in visible light photocatalysis.
Like most
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
homogeneous photocatalysts such as metal complexes and
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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
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organic dyes, eosin
Y
shows great promise for the
Relatively good results were obtained using eosin Y‐
View Article Online
DOI: 10.1039/C7CY01510K
transformation of S‐ or N‐containing compounds using O
selective oxidation of alcohols can only be achieved with excess alcohol (
amounts (3 equiv.) of organic peroxide and a very lengthy photocatalysis of dye‐sensitized TiO
2 2 2
. The sensitized TiO (eosin Y‐TiO ) in the selective oxidation of benzyl
e
, Figure 1), suggesting that the visible light
can prompt the formation
. Organic dyes are susceptible to
Herein, we report a very efficient photocatalytic system for degradation on the surface of TiO in the presence of O under
the selective oxidation of alcohols by O under green light visible light irradiation. To increase the robustness of the
2
2
6
reaction time (1‐3 days).
of the benzaldehyde with O
2
2
2
2
irradiation, building upon the previous work mentioned above. anchored organic dye eosin Y, a catalytic amount (2 mol%) of
Eosin Y was selected as the sensitizer to absorb green light and TEMPO was added to smooth the electron transfer dye radical
initiated the subsequent redox processes. The surface of cation and benzyl alcohol substrate (
anatase TiO acted as the photocatalytic platform that can agreement with a previous report stating that cooperative
prompt the activation of O
f, Figure 1). This is in good
2
2
with the conduction band electron. photoredox and TEMPO catalysis can prompt the selective
The oxidation of alcohols was not directly carried out by the oxidation of alcohols with O
2
or other terminal oxidants.2
3, 27‐29
oxidatively quenched eosin Y sensitizer. Instead, TEMPO, an However, a very long reaction time (several hours) was required
organocatalyst for the oxidation of alcohols, was chosen to for the oxidation of alcohols with O in these two previous
mediate the reaction, in which multiple reactive oxygen species reports. The use of quinone as an oxidant in porphyrin and
2
2
9
exist in the oxidative reaction conditions, to ensure the TEMPO catalysis agrees with our finding. The quinone motif in
endurance of eosin Y.
eosin Y imparts its exceptional photoredox activity. Importantly,
the conversion between quinone and quinol is a common
feature of naturally occurring photosynthetic reaction
Results and discussion
30
centers.
The addition of 0.5 equiv. of 1,4‐diazabicyclo[2.2.2]octane
The selective oxidation of benzyl alcohol to benzaldehyde is one
of the most common reactions used to verify the efficiency of
heterogeneous catalytic oxidation systems. Therefore, we
sought to develop a visible light‐induced alternative for
heterogeneous photocatalysis of this reaction. Figure 1 shows
the results of the control experiment involving the aerobic
oxidation of benzyl alcohol under various control conditions.
The reaction does not proceed without irradiation by visible
(
DABCO) as the quencher for single oxygen resulted in only a
slight decrease in the yield of benzaldehyde occurred ( , Figure
), excluding the possibility of the energy transfer pathway
participating in the product formation. The reaction could also
proceed when the oxidant is 0.1 MPa of air ( , Figure 1). But the
reaction rate was slower than that of 0.1 MPa of O . The surface
complexation between benzyl alcohol and anatase TiO did not
guarantee any apparent photocatalytic activity ( , Figure 1)
g
1
h
2
2
i
2 2 2
light, without eosin Y‐TiO , without TiO or without TiO and
because LEDs emitting green light at approximately 530 nm
were used as the light source, under which the redshift of the
surface complex did not reach out to this region of visible light
TEMPO (a‐d, Figure 1).
2
1
absorbtion. Without O
2
, the reaction almost stopped (
j, Figure
1), revealing the pivotal role of O
2
as the electron acceptor in
this reaction. To establish which reactive oxygen species is
essential in regulating the product formation, 0.5 equiv. of para‐
∙
‐
benzoquinone (BQ), a scavenger specialized in capturing O
2
,
was added to the reaction system. The reaction was almost
∙‐
quenched (
2
k, Figure 1), confirming that O is essential in the
entire catalytic cycle.
Figure 1 The control experiments for the visible‐light‐induced
selective aerobic oxidation of benzyl alcohol by visible light
photocatalysis. Standard conditions: 0.2 mmol of benzyl alcohol, 0.004
mmol of TEMPO, 54.3 mg of eosin Y‐TiO
Y), 5 mL of CH CN, 3 W green light‐emitting diodes (LEDs) irradiation, 0.1
MPa of O , and 1 h. , Dark reaction; , TEMPO only; , without TiO , eosin
Y only; , eosin Y‐TiO only; , standard conditions; , 0.5 equiv. of DABCO
was added to the standard conditions; , 0.1 MPa of air; , TiO , without O
, 0.5 equiv. of BQ was added to the standard conditions.
(including 1.33×10‐3 mmol of eosin
2
3
2
a
b
c
2
; d
e
2
f
g
Scheme 1 The proposed mechanism for the selective oxidation
of benzyl alcohol with O by visible light photocatalysis of eosin
Y‐TiO and TEMPO
h
i
2
;
j
2
;
2
k
2
2
| J. Name., 2012, 00, 1‐3
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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
Journal Name
Based on the experimental results presented above, we resulting in a very good linear fit (Figure 2
ARTICLE
b
). These results
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tentatively proposed a reaction mechanism for the selective match the key features of pseudo‐first‐orDdOeIr: 1k0i.n10e3t9ic/Cs,7CwYh0i1c5h10iKs
oxidation of alcohols with O
2
by eosin Y‐TiO
2
and TEMPO under in line with most of the reported heterogeneous photocatalytic
equals 2.52×I0‐2 min
‐1
visible light irradiation in Scheme 1. Under irradiation by green systems. The reaction constant k
LEDs, the eosin Y anchored on anatase TiO is excited, leading according to Figure 2
to the injection of an electron into the conduction band of TiO Kinetic isotope effects (KIE), i.e., how the rate of a reaction
H
2
b.
2
,
which leaves a positive charge on eosin Y to generate dye radical changes with an isotopic substitution at the α position of benzyl
cations. The dye radical cation animates the switch from TEMPO alcohol with deuterium, can deepen the understanding of the
+
to TEMPO , which undergoes nucleophilic attack by alcohols to molecular mechanism. The KIE can be determined from the
form an intermediate. Then, the product will be afforded from intramolecular competition of isotopically labelled substrates or
this intermediate. In most thermally induced TEMPO catalytic from a comparison of the absolute rates of two parallel
3
1
processes, excess amounts of base are required. However, reactions. The latter provides a clearer picture of the rate‐
neither organic nor inorganic base is needed in the formation of determining step. Moreover, benzyl‐α,α‐d alcohol can be
2
the aldehyde product, showcasing the straightforwardness of procured from a commercial supplier (see the ESI for details).
our reaction scheme. TEMPOH will be formed along with the Therefore, the parallel reaction kinetics for the oxidation of
∙‐
product, which will be switched back to TEMPO by the O
generated at the conduction band of TiO . At this stage, the reaction constant k
catalytic cycles will be complete, and the system is ready for This KIE is a primary KIEs, unequivocally demonstrating that the
2
benzyl‐α,α‐d
2
alcohol were determined (Figure 2
c
). The
‐
3
‐1
2
equals 6.55×I0 min , and KIE=k /k =3.85.
D H D
another turnover of benzyl alcohol.
α
cleavage of the C ‐H bond of benzyl alcohol is the rate‐
determining step. This agrees with the reaction mechanism of
+
Scheme 1 in which the abstraction of C
α
‐H by TEMPO , a direct
two‐electron transfer process, is involved in the formation of
benzaldehyde.
Table 1 The influence of the amount of CH
aerobic oxidation of benzyl alcohol by the visible light
photocatalysis of eosin Y‐TiO
and TEMPO [a]
3
CN on the selective
2
Entry CH
3
CN [mL]
1
C
0
[M]
Conv. [%][b]
53
Sel. [%][b]
98
1
0.200
0.100
0.067
0.050
0.020
2
3
4
5
2
3
4
5
61
66
72
78
98
98
98
98
Figure 2 The reaction kinetics for the selective oxidation of
benzyl alcohol with O
Y‐TiO and TEMPO.
2
by the visible light photocatalysis of eosin
2
a
, The yield of benzaldehyde against the reaction
time for the oxidation of benzyl alcohol; , linear fit of ln(C /C) against time
for the oxidation of benzyl alcohol; , the yield of benzaldehyde‐α‐d against
reaction time for the oxidation of benzyl‐α,α‐d alcohol; , linear fit of ln(C /C)
against time for the oxidation of benzyl‐α,α‐d alcohol. C denotes for the
initial concentration of alcohol; C denotes for the concentration of alcohol.
Reaction conditions: 0.2 mmol of alcohol, 54.3 mg of eosin Y‐TiO
[
a] Reaction conditions: 0.2 mmol of benzyl alcohol, 0.004 mmol of TEMPO, 54.3
mg of eosin Y‐TiO
(1.33×10‐3 mmol of eosin Y), 3 W green LED irradiation, 0.1 MPa
of O , and 1 h. [b] Determined by GC‐FID using chlorobenzene as the internal
b
0
2
c
1
2
2
d
0
standard, conversion of benzyl alcohol, and selectivity of benzaldehyde.
2
0
The influence of the amount of CH CN on the selective
3
(1.33×10‐3 aerobic oxidation of benzyl alcohol by eosin Y‐TiO and TEMPO
2
2
mmol of eosin Y), 5 mL of CH
3
CN, 3 W green LEDs irradiation, and 0.1 MPa of under green light irradiation can be used to empirically verify
that the reaction kinetics follow first‐order or pseudo‐first‐
2
O .
order kinetics. If the reaction follows first‐order kinetics, the
Macrokinetics is a great tool to gain a mechanistic
understanding of a heterogeneous photocatalytic system at the
molecular level. We performed several kinetics investigations of
reaction should be faster with smaller amounts of CH
the higher initial concentration of benzyl alcohol with less
CH CN. In contrast, pseudo‐first‐order kinetics should be more
plausible. The results are shown in Table 1. With increasing
amounts of CH CN, the conversion of benzyl alcohol increased
3
CN due to
3
2 2
the selective oxidation of alcohols with O by eosin Y‐TiO and
TEMPO under green light irradiation, and the results are
depicted in Figure 2. The reaction kinetics for the selective
oxidation of benzyl alcohol under the standard conditions were
3
accordingly (from entries 1 to 5, Table 1), which clearly indicates
that the reaction does not favor higher concentrations. This
goes against the characteristics mentioned previously.
determined first (Figure 2
a). We further analyzed the obtained
kinetics data by plotting ln(C
0
/C) against the reaction time,
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Therefore, we can confidently conclude that the reaction that is characteristic of the dye molecule. In this scenario, only
View Article Online
DOI: 10.1039/C7CY01510K
a limited range of visible light can be efficiently harvested from
follows pseudo‐first‐order kinetics.
Importantly, in all the reactions discussed above, the conventional light sources such as tungsten and halogen lamps,
selectivity for benzaldehyde was excellent (98%), even though which supply continuous light spectra. In contrast, an LED can
a radical mechanism was involved in the product formation in offer a specific light spectrum around a certain wavelength, a
the presence of multiple reactive oxygen species. Even if benzyl trait that matches well with the characteristics of organic dyes.
peroxide was formed, it could be terminated by the conduction Accordingly, LEDs are preferred for this reaction. Table 2
band electron of TiO
2
. Benzyl alcohol might also be involved to illustrates the influence of different LEDs on the visible light‐
avoid the overoxidation of benzaldehyde to enable high induced selective oxidation of alcohols by the photocatalysis of
3
2
selectivity. Hereafter, the conversion of benzyl alcohol is eosin Y‐TiO
compared under different conditions unless otherwise stated. Even though the conversion of the alcohol is markedly slow,
Since TEMPO is indispensable to the successful the oxidation of the alcohol can be carried out using red LED
transformation of alcohols with O in the visible light irradiation with the eosin Y sensitizer (entry 1, Table 3). When
photocatalysis of eosin Y‐TiO , further studies were carried out the LED color was shifted from red to green (entries 1‐3, Table
2
.
2
2
to assess the influence of TEMPO on the visible light‐induced 3), the conversion increased, which agrees well with the
selective oxidation of alcohols. The results are shown in Table 2. absorption by the eosin Y molecules. The green LED spectrum is
Without TEMPO, the selective aerobic oxidation of the alcohol near the absorption peak of eosin Y. However, the best result
proceeded via eosin Y‐TiO
2
under 3 W green LEDs irradiation was obtained using blue LEDs (entry 4, Table 3), which might
in the have been caused by the higher electricity‐to‐photon
(
entry 1, Table 2). However, the endurance of eosin Y‐TiO
2
photocatalytic reaction system is also a major consideration conversion power of blue LEDs. When we surveyed the
because decoloration was observed. When 1 mol% TEMPO was literature, reports of visible light photocatalysis with blue LEDs
added, the reaction rate was evidently increased without a were very common. Nevertheless, surface complexes of benzyl
2
change in selectivity (entry 2, Table 2). Surprisingly, with further alcohols and TiO can be formed with absorption in this region
increases in the amount of TEMPO (from 2 mol% to 10 mol%), which might give birth to significant background reaction.
the conversion of benzyl alcohol steadily decreased. This Therefore, we selected green LEDs to carry out the subsequent
phenomenon might be attributed to the quenching effect of photocatalytic reactions. Note that violet and white LEDs were
TEMPO in the visible light photocatalytic reaction at a higher also suitable candidates for the selective oxidation of alcohols
concentration. Therefore, the suitable amount of TEMPO but yielded lower conversion percentages (entries 5 and 6,
should yield a balance between its negative and positive effects. Table 3).
Thus, 2 mol% TEMPO was adopted in subsequent investigations.
Table 3 The influence of different LEDs on the selective aerobic
Table 2 The influence of the amount of TEMPO on the selective oxidation of benzyl alcohol by the visible light photocatalysis of eosin
[
a]
aerobic oxidation of benzyl alcohol by the visible light photocatalysis Y‐TiO and TEMPO
2
[
a]
of eosin Y‐TiO
2
ꢀ
Entry
1
LED
Red
λ
max (nm)
Conv. [%][b]
23
Sel. [%][b]
98
Entry
1
TEMPO [μmol]
0
Conv. [%][b]
65
Sel. [%][b]
98
630
2
3
4
5
6
Yellow
Green
Blue
590
530
460
400
63
78
84
67
72
98
98
98
98
98
2
3
4
5
6
7
2
4
82
78
74
73
70
68
98
98
98
98
98
98
6
Violet
White
8
Continuous
10
20
[
a] Reaction conditions: 0.2 mmol of benzyl alcohol, 0.004 mmol of TEMPO,
4.3 mg of eosin Y‐TiO CN, 3 W
(1.33×10‐3 mmol of eosin Y), 5 mL of CH
green LED irradiation, 0.1 MPa of O , and 1 h. [b] Determined by GC‐FID using
5
2
3
2
[
a] Reaction conditions: 0.2 mmol of benzyl alcohol, 54.3 mg of eosin Y‐TiO
1.33×10‐3 mmol of eosin Y), 5 mL of CH
CN, 3 W green LED irradiation, 0.1
, and 1 h. [b] Determined by GC‐FID using chlorobenzene as the
2
chlorobenzene as the internal standard, conversion of benzyl alcohol, and
selectivity of benzaldehyde.
(
3
MPa of O
2
internal standard, conversion of benzyl alcohol, and selectivity of
benzaldehyde.
The solvent can be an important factor in determining the
results of an oxidation reaction. Several inert organic solvents
commonly employed in aerobic oxidation reactions were
screened to determine the best solvent. The results are outlined
in Table 4. Under green light irradiation, the best conversion of
Selection of the light source is important for photocatalytic
reactions. In dye‐sensitized photocatalysis, the visible light
absorption occurs at the anchored dyes, whose absorption
usually has a limited range around a definitive absorption peak
3
benzyl alcohols was obtained in CH CN (entry 1, Table 4). In a
4
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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
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previous report, benzotrifluoride (BTF) was selected as the Table 5 The influence of the amount of TiO
ARTICLE
2
on the selective aerobic
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solvent for the visible‐light‐induced selective oxidation of oxidation of benzyl alcohol by the visible light photocatalysis of eosin
DOI: 10.1039/C7CY01510K
2
3
and TEMPO [a]
In Y‐TiO
2
alcohols because of its superior ability to dissolve O
2
.
disagreement with this report, CH CN as a solvent conveyed a
much better result. Other common solvents such as ethyl
acetate (EtOAc), dichloromethane (DCM) and toluene (PhMe)
3
Entry
1
TiO
2
[mg]
Conv. [%][b]
96
Sel. [%][b]
98
106.7
53.4
26.7
17.8
13.4
6.7
did not confer better results than CH
3
CN (entries 3‐5, Table 4).
2
3
4
5
6
7
78
61
42
28
10
6
98
98
98
98
98
98
In other previous reports, pyridine or DCM was applied as the
2
7, 29
solvent with excess amounts of base
because the
deprotonation of the alcohol mediated by TEMPO normally
necessitates a basic environment. When a basic solvent, N,N’‐
dimethylformamide (DMF), was selected, no improvement in
the conversion of benzyl alcohol was achieved (entry 6, Table 4),
reaffirming the greenness of our reaction scheme that does not
require an adventitious base for aldehyde formation.
4.5
[
a] Reaction conditions: 0.2 mmol of benzyl alcohol, 5 mL of CH
3
CN, 0.004
mmol of TEMPO, 1.33×10 mmol eosin Y, 3 W green LED irradiation, 0.1 MPa
of O , and 1 h. [b] Determined by GC‐FID using chlorobenzene as the internal
standard, conversion of benzyl alcohol, and selectivity of benzaldehyde.
Table 4 The influence of the solvent on the selective aerobic
‐
3
oxidation of benzyl alcohol by the visible light photocatalysis of eosin
and TEMPO [a]
2
Y‐TiO
2
Entry
1
Solvent
CH CN
Conv. [%][b]
78
Sel. [%][b]
98
The eosin Y‐TiO
the same procedure (see the ESI for details). TiO
greater than the gram scale was used as the starting material
for a series of eosin Y‐TiO samples to avoid the discrepancies
arising from weighing an amount of eosin Y dye that is too small
and obtain a more credible conclusion. Thus, the amount of
2
photocatalysts were prepared according to
3
in an amount
2
BTF[c]
2
3
4
5
6
32
31
42
25
42
97
97
99
99
96
2
EtOAc
PhMe
DCM
DMF
eosin Y was fixed to investigate the impact of TiO
with a decreasing amount of TiO in eosin Y‐TiO , the conversion
of benzyl alcohol under green light irradiation sharply
decreased (entry 1 to 7, Table 5). We believed that TiO is not a
photoredox‐active component in the visible light photocatalysis
of dye‐sensitized TiO . However, as indicated in Scheme 1, after
accepting an electron from the excited dye, the conduction
band of TiO can be considered a room‐temperature‐active
catalyst for the activation of O
2
. Surprisingly,
2
2
2
[
a] Reaction conditions: 0.2 mmol of benzyl alcohol, 0.004 mmol of TEMPO,
4.3 mg of eosin Y‐TiO
(1.33×10‐3 mmol of eosin Y), 5 mL of the solvent, 3
W green LED irradiation, 0.1 MPa of O , and 1 h. [b] Determined by GC‐FID
5
2
2
2
using chlorobenzene as the internal standard, conversion of benzyl alcohol,
and selectivity of benzaldehyde. [c] Using biphenyl as the internal standard.
BTF, benzotrifluoride; EtOAc, ethyl acetate; PhMe, toluene; DCM,
dichloromethane; DMF, N,N’‐dimethylformamide.
2
2
.
Commercially available and economically viable anatase
TiO
2
with a very high specific surface area (BET specific surface
2
area, 275 m /g) was employed in this study, conveniently
circumventing the tedious materials synthesis of TiO
importantly, the quality of TiO was consistent, which is
essential for systematic investigations of its influence on the
selective aerobic oxidation of alcohols by eosin Y‐TiO under
green light irradiation. As shown in Scheme 1, the conduction
band of TiO plays a pivotal role in channeling the electron
transfer from the excited dye to O , and an oxidatively
2
. More
2
2
2
2
quenched process is essential to driving the ensuing catalytic
cycles of TEMPO. The conduction band position should be
important in determining the final results. However, no
conclusive results can be obtained with commercially available
TiO
2
with differing in the conduction band positions. Thus, we
on dye‐
Figure 3 The diffuse reflectance UV‐visible spectra of the
different eosin Y‐TiO samples listed in Table 5 and anatase TiO
a. u., arbitrary units)
only consider the influence of the amount of TiO
sensitized visible light photocatalysis, with the results
presented in Table 5.
2
2
2
(
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Journal Name
Figure 3 shows the diffuse reflectance UV‐visible spectra of in CH
different eosin Y‐TiO samples and TiO
absorption of the solid samples of eosin Y‐TiO
3
CN solution under ultrasonication and then subjected to
2
. The UV‐visible the standard conditions for the selective aerobic oxidation of
View Article Online
DOI: 10.1039/C7CY01510K
2
2
decreased with benzyl alcohol under green LEDs irradiation.
the increasing activity listed in Table 5. This result is
counterintuitive because in most cases, stronger absorption
results in better activity of a visible light photocatalyst, which is
quite reasonable because the amount of eosin Y is finite.
Increasing the amount of TiO can have dilution effect. Even
2
though they can be easily overlooked, non‐photochemically
active factors can be the key to improving the photocatalytic
activity according to the strategy of cooperative Figure 5 A photograph of the organic dye‐TiO visible light
2
3
3
photocatalysis. Redox‐active metal oxides such as TiO
very important in the visible light photocatalytic aerobic
oxidation of alcohols. Redox‐inert metal oxides such as Al
can prompt the photocatalytic aerobic oxidation of alcohols by
2
can be photocatalysts listed in Table 6
Their activities in the photocatalytic aerobic oxidation of
benzyl alcohol under visible‐light irradiation are shown in Table
2 3
O
6
. The absorption peaks of eosin Y, Rose Bengal and alizarin red
2
2
3 2
a [Ru(bpy) ]Cl photocatalyst. In addition, it can be observed
that the relative ratio between the peaks of 485 nm and 523 nm
shifts gradually in Figure 3, an indication that the adsorption of
eosin Y onto TiO
TiO . Although the highest amount of TiO
the benzaldehyde formation (entry 1, Table 5), some products
can adsorb to eosin Y‐TiO . Therefore, the second‐best
S are near each other, which matches well with the peak of the
green LED employed as the light sources. Rational conclusions
can be drawn. For comparison, we first prepared alizarin red S‐
2
might differ with the change of amounts of
sensitized TiO
entry 8, Table 6). However, the conversion of benzyl alcohol
was quite low, indicating that TiO is very important in
conferring the activity. As the amount of TiO was increased,
2
for the selective oxidation of benzyl alcohol
2
2
can be beneficial to
(
2
2
2
condition was selected for further reactions (entry 2, Table 5,
BET specific surface area, 186 m /g). When we replaced this
good results were obtained with the sensitizers eosin Y, Rose
Bengal and alizarin red S (entries 2, 4 and 7, Table 6). No
significant difference occurred between the sodium salt and
lactone or acidic form of the same dye (entries 1, 3 and 6 vs.
entries 2, 4 and 7, Table 6). Both eosin Y and Rose Bengal are
fluorescein dyes. We also used the core motif of both dyes,
fluorescein, to conduct the reaction and obtained a comparable
conversion (entry 5, Table 6).
2
TiO
the same reaction, only 8% conversion of benzyl alcohol was
obtained, proving the effectiveness of the chosen anatase TiO
2 2
with equal amount of benchmark P25 TiO to carried out
2
.
Table 6 The influence of different sensitizers on the visible light‐
[
a]
induced selective aerobic oxidation of benzyl alcohol on TiO
2
λ
max
Entry
Sensitizer
Conv. [%]
Sel. [%]
(nm)
1
2
Eosin Y disodium salt
Eosin Y
525
525
78
78
98
98
Figure 4 The organic dyes used as sensitizers in the visible light
photocatalysis of TiO
Rose Bengal sodium
salt
2
3
540
72
98
The sensitizer should play a more significant role than TiO
in determining the activity of dye‐sensitized TiO in visible light
2
4
Rose Bengal
520
490
530
530
530
64
75
82
83
34
98
98
98
98
98
2
photocatalysis. Several typical organic dyes containing quinone
motifs (Figure 4) were investigated: fluorescein, eosin Y, Rose
Bengal and alizarin red S. Since the adsorption of the organic
5
Fluorescein
6
Alizarin red S
Acidic alizarin red S
Alizarin red S
7
8[c]
2
dye onto the surface of anatase TiO is essential to initiating the
visible light photocatalytic reaction, the adsorption modes of
sodium salts and lactones might differ. Both the sodium salts
and lactones of eosin Y and Rose Bengal were screened for
comparison, and the sodium salt and the acidic form of alizarin
red S were compared.
[
a] Reaction conditions: 0.2 mmol of benzyl alcohol, 5 mL of CH
mmol of TEMPO, 1.33×10‐3 mmol of organic dye, 53.4 mg of TiO
, 3 W green
LEDs irradiation, 0.1 MPa of O , and 1 h. [b] Determined by GC‐FID using
chlorobenzene as the internal standard, conversion of benzyl alcohol, and
selectivity of benzaldehyde. [c] 16.1 mg of TiO
3
CN, 0.004
2
2
2
.
2
In this regard, eight samples of organic dye‐TiO visible light
photocatalysts were prepared for detailed comparison. Figure 5
shows an optical image of the solid samples, whose colors
varied according to the absorption peaks of the dye molecules
Next, we explored the scope of alcohols that could
participate in the visible light photocatalysis by eosin Y‐TiO and
TEMPO. The results of the selective oxidation of benzyl alcohol
and its derivatives by eosin Y‐TiO and TEMPO under green light
2
2
anchored to anatase TiO . These samples were finely dispersed
2
6
| J. Name., 2012, 00, 1‐3
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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
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ARTICLE
irradiation are shown in Table 7. In the presence of a 1/150 protocol (entries 3‐11, Table 7). Generally, excellenVtiesweAlertcictleivOitnilienes
equivalent of eosin Y, most of the benzyl alcohol was were obtained in all cases in short reactionDOtiIm: 1e0s.1.0T3o9/tCh7eCYb0e1s5t10oKf
successfully transformed to benzaldehyde, with a conversion of our knowledge, these good results for the selective oxidation of
9
5% and excellent selectivity achieved in 1.5 h (entry 1, Table 7). alcohols with
O
2
by heterogenous photocatalysis are
With the scale‐up of the amounts of benzyl alcohol and TEMPO unprecedented. Benzyl alcohols with electron‐donating groups
to twice those in the standard conditions, 77% conversion of aided the formation of the corresponding aldehydes and
benzyl alcohol was achieved in 4.0 h, resulting in a turnover required shorter reaction times or higher conversions with
number (TON) of 231 for the eosin Y sensitizer. However, a similar reactions times (entries 3‐5 vs. entry 1, Table 7). In
further attempt to increase the TON failed, suggesting that the contrast, benzyl alcohols with electron‐withdrawing groups
organic dye is depleted during the product formation process. required longer times (entries 6‐11, Table 7) to achieve
Thus, a better redox mediator, other than TEMPO, that matches comparable conversions of the benzyl alcohol (entry 1, Table 7).
the photoredox properties of eosin Y is needed to create a more Very strong electron‐withdrawing groups such as ‐CN and ‐NO
2
robust system. Alternatively, the spent eosin Y can be required longer reaction time and resulted in lower conversions
replenished with fresh dye solution to easily resume the (entries 10 and 11, Table 7). This phenomenon agrees with
+
photocatalytic reactions. This is a very viable alternative Scheme 1, in which the nucleophilic attack of TEMPO by the
because organic dyes are rather inexpensive.
substrate of alcohol is a key step. Benzyl alcohol with very
strong electron‐donating group of para‐OCH required shorter
reaction times (entry 3, Table 7); benzyl alcohol with a meta‐
OCH group required a longer time due to the electron‐
withdrawing effect of this substituent (entry 12, Table 7); benzyl
alcohol with an ortho‐OCH group required a comparable
3
Table 7 The selective oxidation of benzyl alcohols to aldehydes with
O by the visible light photocatalysis of eosin Y‐TiO and TEMPO
2 2
[
a]
3
3
amount of time due to the balance between the electron‐
donating and steric effects (entry 13, Table 7).
Conv.
[
Sel.
[%]
Entry
Substrate
Product
T (h)
%][
b]
[b]
Table 8 The selective oxidation of other representative alcohols to
1
2[c]
3
1.5
95
98
98
99
98
aldehydes or ketones with O
eosin Y‐TiO
and TEMPO [a]
2
by the visible light photocatalysis of
2
4.0
1.0
1.5
77
99
98
Sel.
Entry
Substrate
Product
T (h)
Conv. [%][b]
4
[b]
[%]
1
2
3
2.0
29
62
39
50
88
88
5
6
1.5
2.0
96
75
98
98
1.0
1.5
7
8
9
2.0
2.0
2.0
2.5
3.0
2.0
87
91
97
73
69
81
98
98
98
97
98
98
4
5
1.5
0.5
72
57
75
34
10
11
12
6
2.5
64
96
7
8
2.5
2.5
56
27
100
98
13
1.5
92
98
9
1.0
2.0
14
18
73
89
[
a] Reaction conditions: 0.2 mmol of alcohol, 0.004 mmol of TEMPO, 54.3 mg of eosin Y‐TiO
1.33×10‐3 mmol of eosin Y), 5 mL of CH
CN, 3 W green LEDs irradiation, and 0.1 MPa of O . [b]
2
10
(
3
2
Determined by GC‐FID using chlorobenzene as the internal standard, conversion of alcohol, and
selectivity of the corresponding aldehyde. [c] 0.4 mmol of benzyl alcohol and 0.008 mmol of
TEMPO.
[
a] Reaction conditions: 0.2 mmol of alcohol, 0.004 mmol of TEMPO, 54.3 mg of eosin Y‐
TiO CN, 3 W green LEDs irradiation, and 0.1
(1.33×10‐3 mmol of eosin Y), 5 mL of CH
MPa of O . [b] Determined by GC‐FID using chlorobenzene as the internal standard,
2
3
2
Benzyl alcohol derivatives with para‐substituted groups
conversion of alcohol, and selectivity of the corresponding aldehyde or ketone.
were subjected the standard visible light photocatalytic
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Lastly, the visible light photocatalytic aerobic oxidation of
other representative alcohols by eosin Y‐TiO and TEMPO was
carried out to further demonstrate the generality of our
protocol, and these results are illustrated in Table 8. When
redox‐active substituents such as ‐OH were located at the para‐
position of the benzyl alcohol, both the conversion and
Conflicts of interest
There are no conflicts of interest to declare.
View Article Online
DOI: 10.1039/C7CY01510K
2
Acknowledgements
selectivity were significantly lower than those of unsubstituted Financial support from the National Natural Science Foundation
benzyl alcohol (entry 1, Table 8). If a heteroatom was present in of China (grant number 21503086), the Fundamental Research
the aromatic ring, the substrate could be transformed with a Funds for the Central Universities (grant number
lower conversion and slight lower selectivities (entries 2 and 3, CCNU16A05003) and the start‐up fund of Wuhan University is
Table 8). When activated allylic alcohols were subjected to the gratefully acknowledged.
standard conditions, the conversions were moderate, the
selectivity for phenyl allylic alcohol was good (entry 4, Table 8),
and the selectivity for aliphatic allylic alcohol was low (entry 5,
Notes and references
Table 8). For the TEMPO‐mediated selective oxidation of
alcohols, the selective oxidation of secondary alcohols is usually
challenging due to steric hindrance in the nucleophilic attack of
1
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3
4
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(
entries 9 and 10, Table 8).
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This work resonates with the intense and increasingly
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8
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Catalysis Science & Technology
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Table of contents entry
Efficient visible-light-induced selective aerobic oxidation of alcohols
was accomplished by merging the photocatalysis of dye-sensitized
TiO with TEMPO catalysis.
2