Improving the Visible Light Photocatalytic Aerobic Oxidation of Sulfides into Sulfoxides on Dye-Sensitized TiO2

Article abstract of DOI:10.1002/cctc.201801304

Visible light-driven interfacial redox reactions during dye-sensitized TiO₂ are essential to meet the energy and environmental challenges in the form of dye-sensitized solar cells and sensitized degradation of organic pollutants. Inspired by th

Full text of DOI:10.1002/cctc.201801304

Accepted Article
Title: Improving the visible light photocatalytic aerobic oxidation of
sulfides into sulfoxides on dye-sensitized TiO2
Authors: Huimin Hao, Zhan Wang, Ji-Long Shi, Xia Li, and Xianjun
Lang
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Improving the visible light photocatalytic aerobic
oxidation of sulfides into sulfoxides on dye-sensitized
TiO2
Huimin Hao a, Zhan Wang b, Ji-Long Shi a, Xia Li a, and Prof. Dr. Xianjun Lang a*
a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
b College of Chemistry, Central China Normal University, Wuhan 430079, China
*Corresponding author at: College of Chemistry and Molecular Sciences, Wuhan University,
Wuhan 430072, China. E-mail: xianjunlang@whu.edu.cn (X. J. Lang)
Keywords: photocatalytic oxidation • visible light • titanium dioxide • TEMPO • sulfoxide
Abstract: Visible light-driven interfacial redox reactions during dye-sensitized TiO2 are essential
to meet the energy and environmental challenges in the form of dye-sensitized solar cell and
sensitized degradation of organic pollutants. Inspired by these prior successes, it is conceivable
that the underlying principles of these processes can guide us to construct selective chemical
transformations in which the control of selectivity is in higher demand. We attempt to improve the
selectivity for sulfoxides during the visible light-driven selective aerobic oxidation of sulfides in
the merger of dye-sensitized TiO2 photocatalysis with TEMPO [(2,2,6,6-tetramethylpiperidin-1-
yl)oxyl] catalysis. Thus, different factors like the TEMPO and its derivatives, initial O2 pressures,
initial sulfide concentrations, solvents, light-emitting diodes (LED) and TiO2 were examined to
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identify the critical factor for selectivity improvement. It was found that an easily synthesized
anatase TiO2, sensitized by 0.33 mol% of alizarin red S, was significant in improving the selectivity
for sulfoxides at high conversions of sulfides with the aid of 2 mol% of TEMPO as a redox
mediator under blue LED irradiation. This work suggests that the engineering of the TiO2 materials
can be a viable solution in improving the selectivity of visible light-driven chemical
transformations by dye-sensitized TiO2 photocatalysis.
Introduction
Oxidative organic reactions are at the core of many important chemical processes for the
synthesis of fine chemicals, pharmaceuticals and industrial commodities [1]. In many practical
oxidation processes, toxic oxidants were needed to carry out the reactions; and resultantly large
quantities of undesired side products were produced, leading to undesired environmental and
ecological impacts [2]. Moreover, many industrial oxidation reactions are energy intensive due to
high operating pressure and temperature. Therefore, the operational conditions for these useful
reactions are under close scrutiny because sustainable development is becoming increasingly
important [3]. Designing innovative paradigm for selective oxidation of organic compounds
becomes a subject of considerable research effort [4], which in turn could enlighten our
understanding on detailed redox processes in addressing either environmental or energy related
problems. It would be very rewarding to uncover an oxidative reaction pathway in a controlled
manner yet without burdening its high redox efficiency.
To this end, one should consider the following criteria in designing a oxidative organic
reaction: (1) clean oxidants like O2 should be preferred with innocuous water as the by-product;
(2) renewable energy should be favoured as a driver for the reaction; (3) inexpensive reaction
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vessel should be applied as the reaction environment; (4) the selectivity for the desired product
should be improved. These criteria could resolve many oxidation related issues in a concerted
fashion. Usually occurring at room temperature with atmosphere O2 as the terminal oxidant, TiO2
photocatalysis can meet the majority of these above criteria with the exception of selectivity factor.
Under UV light irradiation, a plethora of reactive oxygen species (ROS) can be generated at both
the conduction band and valence band of TiO2 during the interfacial reduction of O2 or the
oxidation of H2O and organic compounds [5].
In effect, these ROS are quite useful in mediating the degradation of toxic organic
compound with O2 on the surface of TiO2 under UV light irradiation, one of the most well-
established applications of TiO2 photocatalysis. Fluctuations in the composition of ROS often lead
to activity and selectivity variations in TiO2 photocatalyst [6]. One can curtail the number and
velocity of ROS that got involved in the photocatalytic system to get desirable results. In
circumventing these issues, visible light photocatalysis derived from TiO2 turns out to be a viable
alternative. Visible light photocatalysis provides safety, robustness and environmental friendliness
[7]
necessary for enabling chemical transformations in biocompatible conditions , emerging as an
attractive method in performing selective oxidation reaction. In this context, we recently
demonstrated the potential of visible light photocatalysis of dye-sensitized TiO2 for selective
aerobic oxidation of alcohols [8] and amines [9] in which the ROS generated at the valance band of
TiO2 were completely switched off, leading to high selectivities for the desired products of the
corresponding carbonyl compounds and imines at very high conversions of benzyl alcohols and
amines.
The selective oxidation of sulfides into sulfoxides with O2 is one of very important
[10]
chemical transformations
because it produces biologically active molecules that have wide
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applications in bio-related platforms. We discovered that the same system of dye-sensitized TiO2
photocatalysis can also be applicable for the oxidation of sulfides into sulfoxides with O2 as the
terminal oxidant [11]. However, the selectivity for the desired sulfoxides dropped along with the
increase of the conversions of sulfides. The decrease of selectivity towards the end of an organic
reaction seems trivial. But eventually this leads to very unintended negative consequences, because
the separation of the products from substrates and side products requires tedious workup
procedures [12]
.
Recently, it was reported that the electronic properties of TiO2 can determine the selectivity
for the photocatalytic aerobic oxidation of toluene [13]. Meanwhile, home prepared TiO2 can
enhance the selective photocatalytic aerobic oxidation of benzyl alcohols [14]. These previous
successes can offer valuable insights for improving the selectivity for sulfoxides by TiO2
[15]
photocatalysis. In this regime of cooperative photocatalysis
of dye-sensitized TiO2 with
TEMPO, the surface of TiO2 was covered with a selected dye; and electron transfer was mediated
by TEMPO. At first, we did not expect the properties of TiO2 can impact the result on product
selectivity of sulfoxides for visible light-driven aerobic oxidation of sulfides. To our surprise, we
found that the synthesized new TiO2 sensitized by alizarin red S (ARS) can improve the selectivity
for sulfoxides, suggesting the intriguing interfacial reaction network of ROS and the formation of
products.
Results and discussion
In dye-sensitized TiO2 photocatalysis for the visible light-driven selective aerobic
oxidation of organic compounds, ARS was proven as one of the most resilient and efficient
sensitizers based on previous report [11, 16]. In this work, we would not take ARS into consideration
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to improve the selectivity for sulfoxides in this regime of cooperative photocatalysis. However,
the other factors would be thoroughly investigated to achieve the desirable target. In our previous
report [11], we selected a commercial anatase TiO2 to construct ARS-sensitized TiO2 photocatalysis
for the selective oxidation of sulfides with pressured pure O2 of 0.1 MPa as the oxidant. The
conversions of sulfides were quite good in several hours. However, towards the end of the reaction,
the formed sulfoxides could be further oxidized into sulfone, resulting in lower selectivities. Thus,
in that report, we could only achieve nearly 90% of selectivities at around 80% of conversions.
This is an unsettled issue, prompting us to begin the current investigation.
To further prove the robustness of ARS in ARS-sensitized TiO2, we lowered the amount
of ARS to 0.33 mol% and concentration of substrate to 0.3 mol/L in performing the selective
oxidation of sulfides with atmosphere O2 under blue LED irradiation (entry 1a, Table 1). As
expected, when the conversion of thioanisole was 95%, the selectivity for methyl phenyl sulfoxide
dropped to 82%. Beforehand, we did not think TiO2 can improve the selectivity for sulfoxide
because it was extensively covered with dye ARS. This view was shattered when we used the
benchmark photocatalyst Aeroxide P25 TiO2 to prepare the ARS-TiO2 photocatalyst to carry out
the same visible light-driven aerobic oxidation of thioanisole. Despite the fact that 4 times of
reaction time was needed, about 9% increase of selectivity for methyl phenyl sulfoxide was
achieved as almost the same level of conversion (entry 1b, Table 1), suggesting the engineering of
TiO2 materials can be breakthroughs to improved selectivity.
But one cannot develop a photocatalytic system at the expanse of efficiency. Thus, we
synthesized new anatase TiO2 at room temperature to conduct the same aerobic oxidation of
thioanisoles. About 10% increase of selectivity for sulfoxide was achieved at 96% conversion of
thioanisole with slight longer reaction time of 1.3 h (entry 1c, Table 1). Based on PXRD result
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(Figure S1), we can see that this new TiO2 is of anatase phase with low crystallinity, which might
contribute to the higher selectivity.
Table 1: The influence of TiO2 on selectivity for sulfoxides in the oxidation of sulfides with air in
the merger of visible light photocatalysis of ARS-TiO2 with TEMPO catalysis [a]
Entry
1a
Substrate
Product
TiO2
T [h]
1.0
Conv.[%][b]
95
Sel.[%][b]
82
ST-01
1b
1c
2a
P25
4.0
1.3
93
96
91
92
New TiO2
ST-01
P25
0.7
2.6
0.7
1.0
3.5
1.7
95
96
94
90
78
87
86
90
95
76
88
89
2b
2c
3a
3b
3c
New TiO2
ST-01
O
S
Br
O
S
P25
Br
New TiO2
[a] Reaction conditions: sulfide (0.3 mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol ARS), TEMPO (0.006
mmol), 1 atm air, CH3OH (1 mL), blue LED irradiation (3 W × 4). [b] Determined by GC-FID using chlorobenzene
as the internal standard, conversion of sulfide, selectivity of sulfoxide.
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After obtaining these good results, we next verified whether it can be applied in the
oxidation of other types of thioanisoles, achieving similar degree of success. The ARS-TiO2
photocatalyst based on P25 TiO2 (Figure S2) was used to this extension as well for comparative
analysis. The results were summarized in Table 1. One can conclude that for ARS-TiO2 (P25)
always requires about 4 times of reaction times to achieve same level of conversion as that of ARS-
TiO2 (ST-01) (entries 1a, 2a and 3a vs.1b, 2b and 3b, Table 1). And the photocatalytic activity of
ARS-TiO2 (new) only needs slight longer reaction time, with magnitude about 10% of increase of
selectivity for sulfoxides (entries 1a, 2a and 3a vs. 1c, 2c and 3c, Table 1). Quite recently, anatase
TiO2 octahedrons with exposed high-index {102} was used to construct ARS-TiO2 for the
photocatalytic aerobic oxidation of sulfides in which exposed high-index facets and hollow
structure was attributed to improved activity, demonstrating the potential of engineering of TiO2
materials in improving the systems of dye-sensitized TiO2 photocatalysis.
Since TiO2 materials play an important role in determining both the selectivities and
conversions for the visible light-driven aerobic oxidation of sulfide. Thus, we performed
experiments to synthesize several ARS-TiO2 photocatalysts with same amount of ARS and varied
amounts of new TiO2. The obtained photocatalysts were used to carry out the visible light-driven
aerobic oxidation of thioanisole. We thought that the amounts of TiO2 might have further impact
on the selectivity for sulfoxide. Our considerations for the rational design of these experiments are
that ARS can be chemically adsorbed onto TiO2 but differed in the amount of TiO2, resulting in
different coverage of the surface of TiO2. These difference in coverage of TiO2 by ARS might give
rise to varied selectivity due to differed interfacial redox reaction pathways. However, we found
that the amount of TiO2 as 80 mg, 60 mg, 40 mg and 20 mg does not influence selectivities which
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were around 95% for methyl phenyl sulfoxide but conversions decreased with reducing the
amounts of TiO2 to be as 81%, 75%, 68% and 63% respectively.
Solvents are used pervasively in studies of visible light-driven selective aerobic oxidation
of sulfides to enhance selectivities for sulfoxide and kinetics for conversions of sulfide. Thus, we
took a survey of literature reports on this particular subject and submitted their preferred solvent
to the merger of dye-sensitized TiO2 with TEMPO to evaluate whether the selectivities could be
further improved to let solvent take some control over the reaction outcome. For porous organic
[17]
photocatalysts like C3N4
or carbazolic conjugated polymer [18], CH3CN was selected as a
solvent; for Pt/BiVO4 photocatalyst [19], a mixture of CH3CN/H2O (2:1) was chosen as the reaction
medium; for a flavin photocatalyst [20], ethanol/water (95:5, v/v) was adopted as the medium; for
Rose Bengal or immobilized Rose Bengal photocatalyst [21], C2H5OH (0.05 mol/L of HCl) was
used as the medium; for several other photocatalysts including CdS/C3N4 [22], TiO2 [10], boron-
dipyrromethene (BODIPY) dyes [23], thioxanthone [24] and metal organic framework [25], CH3OH
was utilized as the reaction medium. Except in the case of CH3CN, the other reaction mediums
were containing protic solvent in one form or another, necessitated by protons in aiding the
formation of sulfoxides.
According to the results in Table 2, the reaction performed well in CH3OH (entry 1, Table
2). However, CH3CN was not suitable to be a solvent in our present system in terms of both
conversion and selectivity (entry 2, Table 2). With a mixture of CH3CN/H2O (2/1, v/v), the
conversion of thioanisole deteriorated significantly (entry 3, Table 2). When adding HCl to ethanol,
color change was observed for ARS-TiO2 photocatalyst due to the change of absorption of ARS
in different pH values. Even though the conversion of thioanisole dropped, the selectivity of
sulfoxide is excellent (entry 5, Table 2), affirming the indispensable role of protons in the
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formation of sulfoxide. Since H2O is the most protic amongst common solvents. Adding some
amount of it into ethanol can result in conversion and selectivity (entry 5, Table 2) comparable to
that in CH3OH (entry 1, Table 2).
Table 2: The influence of solvent on the selective oxidation of thioanisole with air in the merger
of visible light photocatalysis of ARS-TiO2 with TEMPO catalysis
Entry
Solvent
CH3OH
Conv.[%]
Sel.[%]
96
1
2
3
4
5
68
16
3
CH3CN
88
CH3CN/H2O (2/1, v/v)
C2H5OH (0.05 mol/L HCl)
C2H5OH/H2O (95:5, v/v)
99
41
63
99
96
[a] Reaction conditions: thioanisole (0.3 mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol ARS), TEMPO (0.006
mmol), 1 atm of air, CH3OH (1 mL), blue LED irradiation (3 W × 4), 40 min. [b] Determined by GC-FID using
chlorobenzene as the internal standard, conversion of thioanisole, selectivity of methyl phenyl sulfoxide.
In response to the growing importance of photocatalysis and organocatalytic in chemical
transformations, the merger of visible light photocatalysis with TEMPO catalysis has produced
very exciting results in implementing chemical transformation [26]. One unique advantage of
TEMPO is its tunability in oxidation potentials and electron transfer capability. We selected a
series of 4-substituted derivatives of TEMPO (Figure 1) to figure out whether these redox
mediators can further improve the photocatalytic oxidation of sulfide by ARS-sensitized TiO2
photocatalysis apart from the two factors like TiO2 materials and protic solvent in improving the
selectivity for sulfoxide.
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Figure 1: The molecular structures of TEMPO and its derivatives
Table 3: The influence of TEMPO derivatives on the selective oxidation of thioanisole with air in
[a]
the visible light photocatalysis of ARS-TiO2
Entry
TEMPO or derivatives
TEMPO
Conv.[%]
Sel.[%]
96
1
2
3
4
5
6
7
68
64
61
22
68
67
60
HO-TEMPO
97
CH3O-TEMPO
Oxo-TEMPO
97
98
NH2-TEMPO
96
CH3CONH-TEMPO
PhCOO-TEMPO
97
97
[a] Reaction conditions: thioanisole (0.3 mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol ARS), TEMPO or
derivatives (0.006 mmol), 1 atm of air, CH3OH (1 mL), blue LED irradiation (3 W × 4), 40 min. [b] Determined by
GC-FID using chlorobenzene as the internal standard, conversion of thioanisole, selectivity of methyl phenyl sulfoxide.
TEMPO is a organocatalyst that can be commonly used in mediating the oxidation of
alcohols and amines with O2 [27]. But it is not so common to applying TEMPO into the aerobic
oxidation of sulfides. Only several reports were found and reported quite recently [11, 28]. TEMPO
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and its 4-substituted derivatives can all be commercially procured to test their influence on the
visible light-driven aerobic oxidation of thioanisole (Table 3). From these results we can conclude
that TEMPO is one of the best redox mediator that gave one of the best conversion of sulfide. And
4-substituted derivatives of TEMPO show only comparable conversions and improvement of
selectivies cannot be observed in the present study (entries 2-7, Table 3). Notably, CH3CONH-
TEMPO was reported as an efficient co-catalyst for oxidation of alcohols in catalytic systems [29]
or oxidative functionalization of alcohols in photocatalytic system [30]. However, the activity
CH3CONH-TEMPO (entry 6, Table 3) was only comparable with that of TEMPO.
Table 4: The influence of the amount of TEMPO on the selective oxidation of thioanisole with air
in the merger of visible light photocatalysis of ARS-TiO2 with TEMPO catalysis [a]
Entry
TEMPO [μmol]
Conv.[%][b]
Sel.[%][b]
1
2
3
4
5
6
7
0
3
33
63
68
67
66
65
65
98
96
96
96
96
96
96
6
9
12
15
18
[a] Reaction conditions: thioanisole (0.3 mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol ARS), 1 atm of air,
CH3OH (1 mL), blue LED irradiation (3 W × 4), 40 min. [b] Determined by GC-FID using chlorobenzene as the
internal standard, conversion of thioanisole, selectivity of methyl phenyl sulfoxide.
We arrived at a conclusion that TEMPO itself was the best co-catalyst in terms of
selectivity for the visible light-driven aerobic oxidation of thioanisole by ARS-sensitized TiO2
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photocatalysis. Next, more thorough studies were carried on to decide the amount of TEMPO on
the selective photocatalytic aerobic oxidation (Table 4). With the synergistic effect of new TiO2
and CH3OH, the selectivity is very good even without the presence of TEMPO (entry 1, Table 4).
But significant increase of conversion was achieved after adding 1mol% of TEMPO (entry 2, Table
4). These results accord well with the results in Table 3 in which TEMPO derivatives do not
influence the selectivity for sulfoxide but have direct effect on conversion of sulfide. Nevertheless,
the benefit of further increase of TEMPO amounts do not appear (from entry 3 to entry 7, Table
4). Thus, a redox mediator is an accurate description of the role of TEMPO in the system of
cooperative photocatalysis.
Table 5: The influence of initial O2 pressure on the selective oxidation of thioanisole in the merger
of visible light photocatalysis of ARS-TiO2 with TEMPO catalysis [a]
Entry
Initial O2 pressure [atm]
Conv.[%]
Sel.[%]
96
1
2
3
4
5
6
0.2
0.5
1.0
1.5
2.0
2.5
68
80
88
88
87
87
94
93
93
93
93
[a] Reaction conditions: thioanisole (0.3 mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol of ARS), TEMPO
(0.006 mmol), CH3OH (1 mL), blue LED irradiation (3 W × 4), 40 min. [b] Determined by GC-FID using
chlorobenzene as the internal standard, conversion of thioanisole, selectivity of methyl phenyl sulfoxide.
In photo-induced interfacial events at TiO2, the time scale of electron transfers from
conduction band electron (ecb ) to O2 is about 10-100 µs [31], much longer than other redox events.
-
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Thus, prompting the efficient electron transfer from ecb- to O2 is crucial in improving the overall
dye-sensitized photocatalytic efficiency in which the valence band holes induced events are
ꞏ-
irrelevant. More importantly, the consequential superoxide radical (O2 ) is active in participating
in the formation of sulfoxide which might have some leverage on the selectivity for the sulfoxide.
Therefore, we studied the impact of initial O2 pressure on the visible light-driven aerobic oxidation
of thioanisole (Table 5). The selective photocatalytic aerobic oxidation proceeds smoothly with
atmosphere O2 as the oxidant (entry 1, Table 5). With the elevation of initial O2 pressures, the
conversion of thioanisole increases without influencing the selectivity for sulfoxide (entries 2-3,
Table 5). This momentum lost with the further increase of initial O2 pressures to the limit of our
common glass apparatus of 2.5 atm (entries 4-6, Table 5), suggesting the elevation of O2 pressure
-
in prompting the ecb transfer might not be a wise option in the dye-sensitized TiO2 photocatalysis.
According to conventional wisdom, a photochemical reaction should be carried out under
a diluted solution to ensure a high efficiency [32]. Moreover, ARS in ARS-TiO2 preferred a diluted
condition to avoid the aggregation-caused weakening or quenching of fluorescence of ARS at high
concentrations. Whilst the fluorescence property of a dye is proportional to its photocatalytic
activity. Most of the reported photocatalytic systems based on TiO2 were applied in dealing with
organic pollutants in very diluted conditions. This might not be a critical issue in aqueous condition.
This turns out to be an issue of major concern because most photocatalytic chemical
transformations are performed in organic solvent. In this manner, large amount of organic solvent
is used which is disadvantageous in terms of environmental impact. We carried out the reaction at
a very high concentration of thioanisole (0.30 mol/L) in CH3OH for the photocatalytic oxidation
(68% conversion, 96% selectivity, 40 min). Based on the reasons listed above, we would envision
that diluting the photocatalytic systems with increase the amount of CH3OH would improve the
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visible light-driven oxidation of thioanisole. Against the conventional wisdoms, it was discovered
that conversions of thioanisole decrease to be 53%, 44%, 38%, 33% respectively with the dilutions
of thioanisole as 0.15 mol/L, 0.10 mol/L, 0.075 mol/L and 0.06 mol/L with slight variations of
selectivities for methyl phenyl sulfide from 96% to 99%. It should mention that this phenomenon
is substrate-dependent. In our previous report, in the merger of eosin Y-sensitized TiO2 with
TEMPO for visible light-driven selective oxidation of alcohols, the dilution of alcohol can have
positive effect with quite similar experimental setup [8a], differing from the present work. We
cannot fully understand the characteristics underpinning this phenomenon at this stage. But it was
an unexpectedly good result in terms of environmental friendliness of the present system.
Table 6: The influence of different LED on the selective oxidation of thioanisole with air in the
merger of visible light photocatalysis of ARS-TiO2 with TEMPO catalysis [a]
Entry
LED
Blue
λmax [nm]
460
Conv.[%][b]
Sel.[%][b]
96
1
2
3
4
5
6
68
55
22
15
45
41
Green
Yellow
Red
510
97
590
>99
>99
97
630
Violet
White
400
Continuous
97
[a] Reaction conditions: thioanisole (0.3 mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol ARS), TEMPO (0.006
mmol), 1 atm of air, CH3OH (1 mL), LED irradiation (3 W × 4), 40 min. [b] Determined by GC-FID using
chlorobenzene as the internal standard, conversion of thioanisole, selectivity of methyl phenyl sulfoxide.
After the consideration of the above internal factors of photocatalysis, we take one
important external factor into account for the sake of further improving selectivity for sulfoxide.
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[33]
One technological breakthrough is the invention and dissemination of inexpensive LED
,
emitting high-energy light at certain wavelengths that can be used for dye-sensitized TiO2
photocatalysis. For visible light LED, there are emitting three-primary colors, i.e. red, green and
blue. LED emitting other colors are synthesized from these three-primary colors. Blue LED is
quite powerful in driving the selective aerobic oxidation of thioanisole (entry 1, Table 6). With the
increase of maximum emitting wavelengths, the conversions decreased (from entry 2 to entry 4,
Table 6). We also considered a more powerful 400 nm LED to drive this oxidation reaction with
lower conversion of 45% (entry 5, Table 6). The might ascribe to this range is near to the absorption
edge of TiO2 that leads to the partial oxidation of solvent CH3OH and slows down the oxidation
of substrate thioanisole. Meanwhile, white LED, whose spectrum contains peaks the same as the
blue ones, can also be suitable as a driver for this oxidation reaction (entry 6, Table 6).
Figure 2: The correlation between photocatalyst and light sources, (a) the UV-visible light
absorbance of TiO2, ARS and ARS-TiO2; (b) the relative spectrum distribution of blue LED
For a photochemical reaction, the light absorption and consequential photo-induced redox
reactions are interrelated. Thus, we want to establish relationship between the visible light
absorption of the photocatalyst and the applied light sources (Figure 2). TiO2 is a photocatalyst
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that does not absorb visible light (TiO2, Figure 2a); and the visible light absorbance of ARS-TiO2
was caused by the sensitizer ARS (ARS, Figure 2a). Moreover, there is red-shift of absorbance
(ARS-TiO2) due to the interaction catechol motifs of ARS and TiO2. In this fashion, large amount
of light of visible range could be captured by be ARS-TiO2. This property underpins the results of
Table 6 in which different LED can be used to perform the reaction. There is very large overlap
between the absorbance of ARS-TiO2 and the relative spectrum distribution of blue LED. One can
envision that green LED might be a better light sources because there are more overlap between
green LED [8b] and the absorbance of ARS-TiO2. This should be the case if the output energy of
green and blue LED were near to each other. However, in this case, the output energy of blue
radiation energy is about twice to that of green LED. The technological side of blue LED caused
higher conversion of thioanisole.
Figure 3: Control experiments for the selective oxidation of thioanisole in the merger of visible
light photocatalysis of ARS-TiO2 with TEMPO catalysis. a, blank reaction; b, TiO2 only; c, ARS
only; d, TEMPO only; e, without ARS; f, without TiO2; g without TEMPO; h, standard conditions;
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i, without blue LED irradiation; j, without O2; k, adding 0.5 equiv. of DABCO (1,4-
diazabicyclo[2.2.2]octane); l, adding 0.5 equiv. of p-BQ (p-benzoquinone). Reaction conditions:
of thioanisole (0.3 mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol ARS), TEMPO (0.006
mmol), 1 atm of air, CH3OH (1 mL), blue LED irradiation (3 W × 4), 40 min, yield of methyl
phenyl sulfoxide was determined by GC-FID.
In order to gain more insights on the reaction mechanism, we further carried out a series of
control experiments. Since thioanisole cannot absorb light of visible range in CH3OH [10a], no
reaction was observed under blank reaction (Figure 3, a), suggesting the importance of
photocatalytic components. There are three constituted components namely, ARS, TiO2 and
TEMPO in this regime of cooperative photocatalysis. We can find that none of the three constituted
components can independently carry out the photocatalytic reactions (Figure 3, b-d). Next, we
excluded one component out of the three to further verify the essential reaction factors. With the
absence either ARS or TiO2, the visible light photocatalytic reactions almost stopped (Figure 3, e
and f), indicating these two components underpins the photochemical reaction. Without TEMPO,
the selective oxidation of thioanisole can still be carried on (Figure 3, g), implying TEMPO is not
an essential part of the photochemical reaction.
The standard condition result reinforces that the catalysis of TEMPO can significantly
improve the outcome (Figure 3, h) in a cooperative catalytic manner. Without blue LED irradiation,
there is almost no yield of sulfoxide (Figure 3, i), affirming the photocatalytic nature of the reaction.
Several further control experiments were performed to establish the nature of ROS in this reaction
regime. With the reaction atmosphere was filled with N2 rather than air, the reaction ceased to
proceed (Figure 3, j). When we added 1 equiv. of DABCO, a trap of singlet oxygen, into the
reaction system, there is some drop in the yield of sulfoxide (Figure 3, k). We attributed this drop
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of yield phenomena to the competitive oxidation of DABCO and thioanisole, demonstrating
energy transfer is not the deciding factor in formation of this product. However, further experiment
is needed to prove this point. Furthermore, we added 0.5 equiv. of p-BQ to act as a scavenger for
ꞏ-
O2 . It was found that the formation of sulfoxide disappeared after adding p-BQ (Figure 3, l),
ꞏ-
revealing O2 is indispensable ROS in the photocatalytic reaction.
Figure 4: Reaction kinetics plots for the selective oxidation of thioanisole with air in the merger
of visible light photocatalysis of ARS-TiO2 with TEMPO catalysis (a) in 1 mL of CH3OH; (b) in
l mL of CD3OD. Reaction conditions: thioanisole (0.3 mmol), ARS-TiO2 (40.1 mg, containing
1×10-3 mmol ARS), TEMPO (0.006 mmol), 1 atm of air, blue LED irradiation (3 W × 4). Taking
a sample at regular intervals of 10 minutes and concentration of thioanisole was determined by
GC-FID.
After gaining some mechanistic understanding based on control experiments, we made
further efforts in kinetics studies to get more insights on the detailed reaction processes. We first
establish the relation between conversions of thioanisole and selectivity for sulfoxide (Figure S5).
Although the selectivity for sulfoxide have been apparently improved compared with our previous
report, there are some sulfone formed if one wants to achieve 100% conversion of thioanisole,
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leaving room for future improvement. The kinetics plot follows C=C0-kt for the photocatalytic
aerobic oxidation of thioanisole (Figure 4, a). A zero-order reaction relationship was established,
suggesting that the oxidation reaction rate does not dependent on the concentration of thioanisole.
Next, we carried out the reaction using CD3OD as the solvent (Figure 4, b). The reaction constant
in CH3OH is kH=0.0044 molꞏL-1ꞏmin-1; while the reaction constant in CD3OD is kH=0.0042 molꞏL-
1ꞏmin-1. The reaction constants in CH3OH and CD3OD is almost the same excluding some
experimental variations. The purpose of these results can be two-fold. In the above control
experiments, we thought that energy transfer of singlet oxygen does not contribute to the formation
of sulfoxide. Since the lifetime of singlet oxygen is significantly longer in deuterated solvent, it
should be much faster in CD3OD than in in CH3OH. This prediction contradicts result in Figure 4,
implying electron transfer is the sole pathway for formation of sulfoxide. This kinetics studies
demonstrate that even though the protons from CH3OH is helpful in the formation of sulfoxide,
abstraction of protons from CH3OH is not the rate-determining step of overall process. Otherwise,
the reaction should become much slower in CD3OD.
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Figure 5: Hammett plot for the aerobic oxidation of para-substituted thioanisoles in the merger of
visible light photocatalysis of ARS-TiO2 with TEMPO catalysis. Reaction conditions: sulfide (0.3
mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol ARS), TEMPO (0.006 mmol), 1 atm of air,
CH3OH (1 mL), blue LED irradiation (3 W × 4).
After the establishment of electron transfer rather energy transfer pathway governing the
formation of product, the electronic effect of substituted groups in thioanisole and its derivatives
was considered. Thus, we carried out the kinetics studies for the oxidation of thioanisole and its
para-substituted derivatives. The oxidation of all these sulfides obey the zero-order reaction plot.
After obtaining the reaction constant, a Hammett plot can be constructed to determine the value of
ρ (Figure 5). Based on the ρ value of -0.70, we can conclude mechanisms involves the formation
of positive charge after the electron transfer. This result agrees well with the mechanism we
proposed that TEMPO+ (2,2,6,6-tetramethylpiperidine-1-oxoammonium) accepting electron from
thioanisole, leaving the positive charge at the radical cation of thioanisole.
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We did not delve into the special effect of ARS sensitizer in implementing this
photocatalytic aerobic reaction. The electron ejection from ARS to conduction band of TiO2 of
ARS-sensitized TiO2 belongs to the type II pathway [34] due to the fact that ARS is a catechol dye.
Type II sensitizer shows strong new absorption band in the longer wavelength region which is in
good agreement with UV-visible absorbance of ARS-TiO2 (Figure 2, a). Under visible light
irradiation, type II dye does not experience pathway from the ground HOMO state to the excited
LUMO state of dye*. In contrast, the direct injection of electron from HOMO of type II dye to the
conduction band of TiO2 occurs. Both the broadening of wavelength absorption and direct electron
injection pathway are all due to the bidentate mononuclear chelating linkage of the catechol motif
of ARS and the Brønsted acid sites of TiO2. Recently, we found that catechol-TiO2 complex could
be an efficient photocatalytic system for selective oxidation reaction, implying the merit of this
linkage[35]
.
-O
O
O
H2 + HCHO
S+
O
S
CH3OH
-
O2
LUMO
e
Dye
S
O2
N
O
TiO2
HOMO
Dye
+
N
O
S
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Figure 6: The proposed mechanism for the selective oxidation of sulfide into sulfoxide with air
in the merger of visible light photocatalysis of ARS-TiO2 with TEMPO catalysis
The mechanism can be tentatively rationalized in Figure 6. Under blue LED irradiation,
ARS will directly inject electron to the conduction band of TiO2, without undergoing an excited
state of ARS*. Resultantly, ARS radical cation ARSꞏ+ was generated in this step. ARSꞏ+ can further
drive the conversion of TEMPO to TEMPO+, returning to its ground state of ARS. Then TEMPO+
will be restored to TEMPO by interaction with thioanisole, engendering radical cation of
thioanisole. This was supported by the Hammett plot relationship. In addition, we synthesized a
-
stable salt of TEMPO+BF4 to replace TEMPO as a starting redox mediator to merge with the
visible light photocatalysis of ARS-TiO2. We found that 93% conversion of thioanisole was
achieved after photocatalytic reaction of 2 h. This result is inferior to that of TEMPO, 1.3 h, for
the same conversion, which might be caused the difficult of abstracting electron from sulfide. In
order to keep the stability of TEMPO, bulky side groups existed in TEMPO. Thus, it is not a good
catalyst for steric hindered substrates, verifying this predication in expanding the scope of
ꞏ-
substrates. At the conduction band of TiO2, O2 will be produced by reduction of O2. The newly
ꞏ-
generated O2 will attack radical cation of thioanisole, supported by the p-BQ trapping control
experiment. Protons from solvent assist the formation of sulfoxide, supported by different proton-
containing solvent experiments (Table 2).
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Figure 7: The ESR signals of superoxide captured by DMPO (DMPO-ꞏOOH) (a) and TEMPO
(b)
Spectroscopic studies would give more direct evidence to the reaction pathways. Therefore,
we carried out ESR investigation to understand the essential roles of ROS and TEMPO in
managing the oxidation outcome. Since there is overlap of ESR signals between the spin trapping
ꞏ-
of superoxide radicals (O2 ) with DMPO (5,5-dimethyl-1-pyrroline n-oxide) of DMPO-ꞏOOH and
TEMPO. We can only obtain the ESR signals of DMPO-ꞏOOH on ARS-TiO2 without TEMPO
under visible light irradiation. The signals agrees well with the hyperfine coupling constant of
radicals of DMPO-ꞏOOH (AN=13.7G, AH=11.2G) and TEMPO (AN=17.1G), tracing the time
course of DMPO-ꞏOOH and TEMPO (Figure 7) [36]. The signal of DMPO-ꞏOOH increased with
time from 0 min to 10 min (Figure 7 a), giving further evidence to the proposed mechanism in
ꞏ-
Figure 7 that O2 is the essential ROS in the formation of sulfoxide. After adding TEMPO into the
reaction system, the ESR signals of TEMPO becomes weaker and weaker under visible light
irradiation from 0 min to 4 min (Figure 7 b). This is in good agreement that the occurrence of
TEMPO to TEMPO+, transforming ESR active species into a silent one. It is very evident that all
the in situ ESR results support the mechanism in Figure 6.
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Table 7: The selective oxidation of sulfides into sulfoxides with air in the merger of visible light
photocatalysis of ARS-TiO2 with TEMPO catalysis [a]
Entry
1[c]
Substrate
Product
T [h]
1.6
Conv.[%][b]
94
Sel.[%][b]
92
2[c]
3[c]
4[d]
5
1.2
2.0
5.0
1.0
98
89
87
97
94
88
94
92
6
7
8
1.3
1.5
1.5
94
88
85
91
89
85
9
4.0
2.0
1.3
1.3
37
49
97
94
85
96
98
92
10
11
12
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13
14
1.7
1.3
82
90
90
88
15
16
3.0
0.5
17
99
93
95
[a] Reaction conditions: sulfide (0.3 mmol), ARS-TiO2 (40.1 mg, containing 1×10-3 mmol ARS), TEMPO (0.006
mmol), 1 atm of air, CH3OH (1 mL), blue LED irradiation (3 W × 4). [b] Determined by GC-FID using chlorobenzene
as the internal standard, conversion of sulfide, selectivity of sulfoxide. [c] Sulfide (0.4 mmol), TEMPO (0.008 mmol).
[d] thioanisole (1.5 mmol), TEMPO (0.015 mmol). Ph, phenyl; n-Bu, n-butyl.
Being scalable could be one step towards practical application, thus we attempted to scale
up the amounts of sulfides to 0.4 mmol in the presence of 2 mol% of TEMPO, we found that slight
prolonging the reaction time can achieve the same level of conversions and selectivities (entries 1-
3, Table 7) in comparison with 0.3 mmol scale (entry 3, entries 4c and 5c, Table 1). With these
results in hand, we further increase the sulfide to 1.5 mmol and decrease the amount of TEMPO
to 1 mol% and less than 5 times of reaction time of 5 h, 87% conversion and 94% selectivity of
sulfoxide can be obtained (entry 4, Table 7). In this manner, turnover number in terms of ARS is
1305, an indicator of the resilience of ARS in this system of cooperative photocatalysis. At 60%
conversion of thioanisole, the second cycles for the oxidation can achieve conversion of 65%,
further demonstrating that the robustness of the ARS-TiO2 photocatalyst. The characterizations by
XRD and DR-UV-visible spectra of the used photocatalyst suggests the slight loss of ARS part
and the stability of TiO2 part during the recycle photocatalytic reactions.
After establishing that our protocol is scalable, we want to demonstrate that our protocol is
a very general one. Therefore, we carried out the selective aerobic oxidation of a series of para-
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10.1002/cctc.201801304
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substituted thioanisoles (entries 5-9, Table 7). It was found that excepted for nitro substituted
thioanisole (entry 9, Table 7), the selectivities for sulfoxides are quite good at high conversions of
substrates (entries 5-8, Table 7). The ortho-substituted Cl group can be detrimental to the
conversion of sulfide (entry 10, Table 7) due to steric hindrance of substrate. However, the
influence of ortho-substituted CH3O group in sulfide is not so apparent (entry 11, Table 7). For
meta-substituted Cl and CH3O group, slight longer time can guarantee very good selectivities and
conversions (entries 12 and 13, Table 7). If the methyl side of thioanisole was replaced with an
ethyl group, the impact on the reaction can be minor (entry 14, Table 7). In contrast, If the methyl
side of thioanisole was replaced with a phenyl group, the conversion of sulfide is very
unsatisfactory (entry 15, Table 7), attributing to the steric hindrance of diphenyl sulfide. Besides,
the selective oxidation of aliphatic sulfide was successful with high conversion and selectivity
(entry 16, Table 7).
Conclusion
In summary, we have successfully improved the selectivities for sulfoxides in the oxidation
of sulfides with air in the merger of visible light photocatalysis of ARS-TiO2 with TEMPO
catalysis. In the current regime of cooperative photocatalysis, both solvent and TiO2 can be pivotal
in terms of selectivities for sulfoxides and conversions for sulfides amongst all the investigated
factors. Our research demonstrates that the potential mechanistic insight guided improvement of a
cooperative photocatalysis system employing the engineering of TiO2 material. Dye-sensitized
semiconductor photocatalysis for visible light-driven selective chemical transformations is an
emerging area of research, whose future development can rely upon similar strategies deployed in
improving dye-sensitized solar cell and sensitized degradation of organic pollutants.
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Acknowledgment
This work was financially supported by the National Natural Science Foundation of China
(grant numbers 21503086 and 21773173), the Fundamental Research Funds for the Central
Universities (grant number 2042018kf0212) and the start-up fund of Wuhan University.
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