Titania-catalyzed H2O2 thermal oxidation of styrenes to aldehydes

Article abstract of DOI:10.3390/molecules24142520

We investigated the selective oxidation of styrenes to benzaldehydes by using a non-irradiated TiO₂–H₂O₂ catalytic system. The oxidation promotes multi-step reactions from styrenes, including the cleavage of a C=C double bond and the addition of an oxygen atom selectively and stepwise to provide the corresponding benzaldehydes in good yields (up to 72percent). These reaction processes were spectroscopically shown by fluorescent measurements under the presence of competitive scavengers. The absence of the signal from OH radicals indicates the participation of other oxidants such as hydroperoxy radicals (?OOH) and superoxide radicals (?O₂^?) into the selective oxidation from styrene to benzaldehyde.

Full text of DOI:10.3390/molecules24142520

molecules
Communication
Titania-Catalyzed H2O2 Thermal Oxidation
of Styrenes to Aldehydes
Satoru Ito 1,
*
, Yoshihiro Kon 2,
*
, Takuya Nakashima 2, Dachao Hong 2, Hideo Konno 2,
*
Daisuke Ino 1 and Kazuhiko Sato 2,
1
Institute for Energy and Material/Food Resources, Technology Innovation Division, Panasonic Corporation,
3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan
Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science
and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
2
*
Correspondence: ito.satoru002@jp.panasonic.com (S.I.); y-kon@aist.go.jp (Y.K.); k.sato@aist.go.jp (K.S.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Kei Saito
Received: 1 June 2019; Accepted: 9 July 2019; Published: 10 July 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: We investigated the selective oxidation of styrenes to benzaldehydes by using a non-irradiated
TiO2–H2O2 catalytic system. The oxidation promotes multi-step reactions from styrenes, including the
cleavage of a C=C double bond and the addition of an oxygen atom selectively and stepwise to provide the
corresponding benzaldehydes in good yields (up to 72%). These reaction processes were spectroscopically
shown by fluorescent measurements under the presence of competitive scavengers. The absence of the
signal from OH radicals indicates the participation of other oxidants such as hydroperoxy radicals ( OOH)
•
and superoxide radicals (•O2−) into the selective oxidation from styrene to benzaldehyde.
Keywords: titanium dioxide; hydrogen peroxide; styrenes; benzaldehydes; non-irradiated oxidation;
heterogeneous catalyst
1. Introduction
The oxidation of olefins is widely known as a key reaction necessary for the production
of various fine chemicals. In manufacturing, oxidation is used in versatile applications such as
epoxidation, dihydroxylation and carboxylation [1]. Among them, the transformation from styrenes
to benzaldehydes has attracted interest because of applicability of this process to the production of
perfumes, pharmaceuticals and agrochemicals. This transformation is usually carried out through
ozonolysis, which uses a toxic ozone as an oxidant and produces an explosive ozonide intermediate [
The oxidative cleavage of olefins by the OsO4–NaIO4 protocol generates hazardous heavy metal
waste [ ]. These classical methods have a severe impact on the environment. Therefore, a green
2].
3
alternative method for the oxidation of styrenes to benzaldehydes has been required [4,5].
Oxidation using hydrogen peroxide (H2O2) as an oxidant has a low environmental load due to
high atom economy and only water as a byproduct. Heterogeneous catalysts, which are superior to
homogeneous catalysts in both separability and reusability, have been developed for the activation of
H2O2. Various heterogeneous catalysts for the H2O2 oxidation of styrenes to benzaldehydes have been
reported [
as a common strategy, for example, polyoxometalates (POMs) [10
materials [13 24] and metal-containing carbon materials [25 26]. However, these catalysts entail a complicated
preparation method and high cost. Alternatively, single-component metal oxides have enough potential to
proceed with the oxidation, such as V2O5 [25], MoO3 [27], Fe2O3, [28 29] and Fe3O4 [30]. These catalysts
6–30]. In recent years, the active metals or metal oxides were embedded in various solid supports
–12], metal-containing mesoporous
–
,
,
were relatively simple to prepare but remained environmentally compatible except for iron oxides.
Titanium dioxide (TiO2), one of the most common metal oxides, is used as a cosmetic pigment
and as a color additive for food owing to its low cost, nontoxicity, and environmental friendliness [31].
Molecules 2019, 24, 2520; doi:10.3390/molecules24142520
www.mdpi.com/journal/molecules

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TiO2 also demonstrates photocatalytic activity for the degradation of pollutants under water [32] and
air [33]. The photocatalytic properties of TiO2 originate from the formation of a photogenerated electron
and hole, which react with the adsorbed oxygen molecule and water, respectively. The resulting
reactive species such as hydroxyl radicals (•OH), superoxide radical anions (•O2−) and hydroperoxy
radicals ( OOH) can oxidize organic molecules. Lachheb et al. reported the photocatalytic oxidation of
•
styrene to produce styrene oxide and benzaldehyde catalyzed by the TiO2–H2O2 system [34]. On the
other hand, the combination of TiO2 and H2O2 can generate reactive species for the degradation
of organic compounds in the absence of photo-irradiation [35,36]. To the best of our knowledge,
however, no study has focused on the use of a non-irradiated TiO2–H2O2 combination for the molecular
transformation [34]. Herein, we have developed a method to oxidize olefins over a non-irradiated
TiO2–H2O2 combination under cost-effective, nontoxic, and environmentally friendly conditions.
2. Results and Discussion
2.1. Oxidation of 4-Chlorostyrene with H2O2 over a TiO2 Catalyst
To achieve the oxidation under environmentally friendly conditions, we started screening TiO2
catalysts for 4-chlorostyrene 1a as a model substrate. As is well known, TiO2 mainly exists in three
types of crystalline phases: rutile, anatase and brookite. To determine which crystalline phase has the
best catalytic activity for oxidation, a series of reactions were performed using TiO2 catalysts (Table S1).
The screening showed that all types of TiO2 showed moderate selectivity to obtain corresponding
benzaldehyde (2a). These results assumed that TiO2 has efficient catalytic activity for the oxidation
regardless of the type of crystalline phase. In addition, we conducted comparative studies with other
oxide catalysts, and as a result, TiO2 showed the highest yield for the oxidation of 1a (Table S2).
Next, we optimized the reaction conditions for the oxidation, as summarized in Table 1. To determine
the optimal amount of H2O2, a series of experiments were performed using 1, 3, 5, and 7 equivalents of
H2O2. The yield of 2a was improved by increasing of the amount of H2O2 from 1 to 5 mmol (entries 1–3).
Further increasing the amount to 7 mmol reduced the yield (entry 4). The reaction without TiO2 showed
low conversion and yield (entry 5). To determine the optimal temperature, H2O2 concentration, solvent,
and catalyst loading, a series of experiments were performed under various conditions summarized in
Tables S3 and S4, which showed almost the same yield. These results led us to understand that the oxidation
was a powerful reaction that various conditions hardly influenced.
Both the conversion and the yield were increased from 1 to 16 h (Figure S1). When the reaction
time was extended from 16 to 24 h, the conversion was increased, but the yield decreased because
overoxidation of 2a occurred (entry 6). 4-Chlorobenzoic acid and 4-chlorostyrene oxide were obtained
as byproducts at 16 h in 4% and 1% yields, respectively (entry 3). The oxidation of 1a could be
performed in a gram scale to obtain isolated 2a in 45% yield (entry 7).
Table 1. Optimization of the reaction conditions.
Conversion (%) a
Yield (%) a
Entry
TiO2 (mg)
H2O2 (mmol)
1
2
3
4
5
100
100
100
100
0
100
100
1
3
5
7
5
5
5
31
60
65
56
1
47
86
92
90
8
98
85
6 b
7 c
63
45 d
a
All conversions and yields were determined on the basis of 1a by GC-FID using biphenyl as an internal standard.
b
c
For 24 h. Reaction conditions: 1a (7.3 mmol, 1 g), TiO2 (730 mg), MeCN (7.3 mL). d Isolated yield.

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2.2. Scope and Limitations
In order to evaluate the scope and limitations of the oxidation, the reaction was employed on
various olefins under optimal conditions. The results, which are shown in Table 2, reveal several features.
It is worth noting that the electronic nature of the substituents at the ortho- or para-position of styrene
influenced the oxidation results. The electron-donating groups inhibited oxidation (entries 3–4), while the
electron-withdrawing groups facilitated oxidation to increase the selectivity of 2 (entries 5–6). The influence
of the substituted position at the aromatic ring differed little between the ortho- and para- positions in the
yield, indicating that the active species is too small to be affected by steric hindrance (entry 7). The yields
decreased when styrenes had the methyl and phenyl groups at the
considered that the reaction between the active species on TiO2 and the
as the -substituent effectively inhibited oxidation. The oxidation of 1-octene 1l hardly proceeded and the
β- and
α-positions (entries 8–11). It is
β
-position of is a key step,
1
β
corresponding aldehyde (2i) was not detected (entry 12). This result assumed that the oxidation of the
olefins that were conjugated with the aromatic rings proceeded to produce the related aldehydes. A series
of results indicated that the oxidation was initiated by the generation of benzyl radicals on the catalytic
system. The generation of benzyl radicals in the oxidation reaction depended on the electronic properties
of the precursor olefins reacted with
cycloalkenes led to the transformation to the corresponding OOH adducts at the allylic positions instead
of causing C=C double bond cleavage (Entries 13–14). These results implied that
of the reactive species in the oxidation process.
•
OH or OOH as nucleophilic radicals. Surprisingly, the oxidation of
•
•
OOH/
O2− is the one
•
Table 2. The oxidation of various olefins with H2O2 over a TiO2 catalyst.
TiO2 P25 (100 mg)
15% H2O2 aq. (5 equiv.)
R2
R2
R3
R1
O
R1
MeCN (1 mL)
50 °C, 16 h
1
2
1.0 mmol
Entry
Substrate
Product
Conversion (%) a
Yield (%) a
O
1
92
65
2a
1a
Cl
Cl
O
2
3
82
92
54
57
1b
2b
O
1c
2c
Me
MeO
Br
Me
MeO
Br
O
4
5
6
63
91
79
23
65
58
1d
2d
O
1e
2e
O
1f
F3C
2f
F3C
Cl
Cl
7
92
72
O
O
O
2g
1g
Me
8
9
35
22
15
7
2b
2b
1h
Ph
1i

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Table 2. Cont.
Entry
Substrate
Product
Conversion (%) a
Yield (%) a
O
10
5
3
Ph
1j
2b
Ph
O
Ph
11
12
13
76
7
43
0
2k
1k
1l
2l
n-C7H15
n-C7H15
O
OH
O
30 b
84
1m
3m
OH
O
23 b
14
65
1n
3n
by GC-FID using biphenyl as an internal standard.
a
Conversions and yields were determined on the basis of
1
b
Yields were determined by NMR spectra using biphenyl as an internal standard.
2.3. Detection of Active Species for the Oxidation and Catalyst Recyclability
In order to investigate the mechanism underlying the oxidation catalyzed by the non-irradiated
H2O2–TiO2 combination, a series of control experiments were performed, as summarized in Table 3.
When this reaction was carried out under an argon atmosphere, the yield was almost the same result
as under air (entry 2). This clearly indicated that H2O2, rather than O2, worked as the major oxidant
for this reaction. To confirm the contribution of TiO2 as a photocatalyst, oxidation was performed
in complete darkness, resulting in almost the same conversion and yield as entry 1 (entry 3). It was
shown that the oxidation did not contribute to the photocatalytic reaction. This was supported by
the result of a photocatalytic reaction performed without H2O2 (entry 4). Although the oxidation
proceeded in the presence of light irradiation, the yield was almost the same as entry 1 (entry 5). In the
presence of butylhydroxytoluene (BHT) as a well-known radical scavenger, oxidation decreased the
conversion from 82% to 11% and the yield from 54% to 1% (entry 6). These results suggested that the
cleavage of the C=C double bond is initiated by radical species produced by the reaction with H2O2
on the TiO2 surface. The use of an OH scavenger tert-butyl alcohol (t-BuOH) as a solvent instead
•
of acetonitrile (MeCN) showed no effect on oxidation (entry 7). On the other hand, the addition of
–
1,4-benzoquinone (BQ) as a scavenger for
•OOH/
•
O2 led to a decrease in both the conversion and the
yield (entry 8) [37]. These results demonstrated that the major reaction pathway for oxidation was
through the •OOH/•O2– process.
Table 3. Control experiments for the oxidation.
Entry
Additive
Atmosphere
Irradiation
Solvent
Conversion (%) a
Yield (%) a
1
2
3
-
-
-
-
-
Air
Argon
Air
Air
Air
Room light
Room light
Darkness
MeCN
MeCN
MeCN
MeCN
MeCN
82
82
83
1
31
51
52
0
hv (365 nm) c
hv (365 nm) c
4 b
5
91
51

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Table 3. Cont.
Entry
Additive
Atmosphere
Irradiation
Solvent
Conversion (%) a
Yield (%) a
BHT
(1 equiv.)
-
BQ
(0.5 equiv.)
6
7
8
Air
Air
Air
Room light
Room light
Room light
MeCN
t-BuOH
MeCN
11
85
2
1
55
1
a
All conversions and yields were determined on the basis of 1a by GC-FID using biphenyl as an internal standard.
b
c
Addition of water instead of H2O2. Irradiated intensity: 10 mW/cm2.
To gain further insight into the oxidation, we performed fluorescence probe experiments
for detecting OH. It was reported that the reaction with terephthalic acid (TA) and OH gave
2-hydroxyterephthalic acid (TAOH), which showed strong fluorescence at 440 nm in MeCN solution [38].
•
•
TA acts as just the scavenger for
•
OH, because of no reaction with other reactive oxygen species such as
−
•
OOH,
•
O2 and H2O2. The fluorescence spectra are shown in Figure 1. On the basis of these spectra,
the fluorescence intensity of TAOH formed by the irradiated H2O2–TiO2 system appeared at around
440 nm, indicating that it worked as a photocatalyst to generate OH. In contrast, the non-irradiated
H2O2–TiO2 system showed very weak fluorescence at that region. The two systems differed by
an order of magnitude. OH was probably not a major reactive species in oxidation because it had
little influence on the yield of the oxidation of 1a despite the 20-fold difference in the amount of
generated OH between the irradiated and the non-irradiated H2O2–TiO2 systems. Therefore, it is
appropriate that the H2O2–TiO2 combination in dark produced OOH as the major active species
•
•
•
•
for oxidation. In accord with the literature, the non-irradiate−d H2O2–TiO2 combination generated
−
•
OOH/
•
O2 to degrade methylene blue (MB), and
•OOH/
•
O2 was detected by an ESR study using
,36]. Three types of reacted H2O2 with the
5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap [35
TiO2 surface were identified from the IR spectra [39]. From these studies, we considered that H2O2
reacted with TiO2 to form peroxo-Ti(IV), followed by the generation of OOH associated with the
•
Ti(IV)/Ti(III) redox process.
Recyclability, an important characteristic of heterogeneous catalysts, was evaluated. After each
recycle run was performed under optimal conditions, the catalyst was recovered by centrifugation and
washed with MeCN, then dried at 110 ◦C overnight. The catalytic results of TiO2 after each recycle run
showed little difference compared with a fresh one (Figure 2a). No change in the XRD patterns between
five-times-reused and fresh catalyst showed that using the catalytic system preserved the catalyst’s
structure (Figure 2b). This result demonstrated that the catalytic system for oxidation had significant
stability and recyclability. When the catalyst was removed from the reaction solution at 50 ◦C after 1 h,
no further reaction occurred (Figure S2). This result suggested that the reaction occurred on the surface
of the TiO2 catalyst.

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Figure 1. Detection of OH by the molecular probe method: the formed TAOH fluorescence spectra
•
in MeCN; non-irradiated reaction condition: TA (1 mmol, 170 mg), TiO2 P25 (100 mg), 35% H2O2
(5 equiv.), MeCN (6.7 mL), 80 ◦C, 1 h; irradiated reaction condition: TA (1 mmol, 170 mg), TiO2 P25
(100 mg), 35% H2O2 (5 equiv.), MeCN (6.7 mL), 80 ◦C, 1 h, hv (365 nm, 10 mW/cm).
Figure 2.
(a) The catalytic performance of recycled catalyst for oxidation; Reaction condition: 1a
(1 mmol), TiO2 P25 (100 mg), MeCN (7.3 mL). (
b
) XRD patterns of fresh TiO2 and five-times- reused TiO2.
3. Conclusions
In conclusion, we have developed a method for the TiO2-catalyzed thermal oxidation of styrenes
to the corresponding benzaldehydes using H2O2 as an oxidant in good yield. Our oxidation method
using the combination of green catalyst TiO2 and green oxidant H2O2 made UV irradiation unnecessary
and allowed us to efficiently cleave the C=C double bond of styrenes along with the generation of
radical species. Notably, unlike a photocatalytic reaction, oxidation efficiently proceeded regardless of
the type of crystalline phase of TiO2. From the fluorescence probe and the competitive scavenging
−
experiments,
•
OOH/
•
O2 are thought to be key active species for oxidation derived from the thermal
H2O2 reaction with TiO2. In addition, our oxidation protocol allowed the reuse of the catalyst and ease
of purification. Therefore, these conditions provided cost effectiveness, non-toxicity, and environmental
compatibility. Our method should be highly feasible for industrial applications. Investigation of the
detail of the reaction mechanism is under way.

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Supplementary Materials: The following are available online, Table S1: Comparison of the type of TiO2 catalysts
for the oxidation; Table S2: Comparison of catalysts for the oxidation; Table S3: Additional optimization of
the reaction conditions; Table S4: Comparison of various solvents for the oxidation; Figure S1: Time profile
of the oxidation; reaction condition: 1a (1 mmol), TiO2 (100 mg), 15% H2O2 (5 equiv.), MeCN (7.3 mL), 50 ◦C;
Figure S2: The hot filtration experiment for the oxidation with TiO2 catalyst; reaction condition: 1a (1 mmol), TiO2
(100 mg), 15% H2O2 (5 equiv.), MeCN (7.3 mL), 50 ◦C.
Author Contributions: S.I., Y.K., D.H., D.I. and K.S. conceived and designed this study; T.N. performed the main
experiments; H.K. performed the fluorescent probe experiments; all authors contributed to the manuscript preparation.
Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-
profit sectors.
Acknowledgments: The authors acknowledge Shota Tsurumi for the NMR measurements.
Conflicts of Interest: The authors declare no conflict of interest.
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