Two-phase oxidation of toluene derivatives by dioxygen using the 3-cyano-1-decylquinolinium ion as a photocatalyst

Article abstract of DOI:10.1039/c6ra05993g

The two-phase photocatalytic oxidation of toluene and p-xylene by dioxygen occurred efficiently using 3-cyano-1-decylquinolinium hexafluorophosphate (DeQuCN⁺PF₆^-) as an organic photocatalyst. When toluene and p-xylene were used as the solvent with 2% H₂O, the oxygenated products were produced in the organic phase and hydrogen peroxide was produced in the aqueous phase at high turnover numbers.

Full text of DOI:10.1039/c6ra05993g

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Two-phase oxidation of toluene derivatives by
dioxygen using the 3-cyano-1-decylquinolinium
ion as a photocatalyst†
Cite this: RSC Adv., 2016, 6, 41011
Received 7th March 2016
Accepted 11th April 2016
Kei Ohkubo,*abc Kensaku Hirose and Shunichi Fukuzumi*
a
cd
DOI: 10.1039/c6ra05993g
www.rsc.org/advances
The two-phase photocatalytic oxidation of toluene and p-xylene by
We report here the two-phase photocatalytic oxidation of
dioxygen occurred efficiently using 3-cyano-1-decylquinolinium toluene by dioxygen with 3-cyano-1-decylquinolinium hexa-
+
6ꢀ
+
ꢀ
hexafluorophosphate (DeQuCN PF ) as an organic photocatalyst. uorophosphate (DeQuCN PF ) as an organic photocatalyst.
6
When toluene and p-xylene were used as the solvent with 2% H
oxygenated products were produced in the organic phase and oxygenated products were produced in the toluene and H
hydrogen peroxide was produced in the aqueous phase at high was produced in the aqueous phase with high turnover
2
2
O, the The reaction was carried out in toluene with H O (2%); the
2 2
O
1
1
turnover numbers.
numbers (Fig. 1). Toluene could be replaced by p-xylene when
higher concentrations of H were obtained in the aqueous
phase. Such a two-phase system has been reported previously
2 2
O
The photocatalytic oxidation of toluene and its derivatives by
dioxygen with various organic photocatalysts under light irra-
diation has attracted increasing attention as an atom-economic
and environmentally benign alternative to classical oxidation
12–14
for the catalytic reduction of O to produce H O .
However,
the two-phase photocatalytic oxidation of substrates has not
been reported for the production of H in an aqueous phase.
was synthesized by the alkylation of quinoline
2
2 2
2 2
O
+
ꢀ
6
DeQuCN PF
1
–4
methods using inorganic oxidants.
Hydrogen peroxide
with decyl triate. The product was obtained by salt exchange
with potassium hexauorophosphate and characterized by H
NMR and high-resolution mass spectrometry (Fig. S1†). The
detailed synthetic procedures are given in the ESI.†
The oxygenation of toluene and p-xylene by molecular oxygen
O ) was achieved by using DeQuCN as a photocatalyst. Photo-
(
H O ), which is the reduced product of dioxygen, oen
1
2
2
degrades organic photocatalysts under light irradiation,
precluding a high turnover. The stability of riboavin tetraa-
cetate as an organic photocatalyst has been shown to be
enhanced by combination with a non-haem iron catalyst, which
5,6
+
(
2
7
dissociates H
hydrogen because an aqueous solution of H
2
O
2
. H
2
O
2
is an ideal alternative energy carrier to
can be used
instead of gaseous hydrogen as an fuel in a one-compartment
irradiation of the absorption band (lmax ¼ 330 nm; Fig. S2†) of
2
O
2
DeQuCN (50 mM) in an oxygen-saturated toluene/aqueous
+
(
36 mL/0.72 mL) bilayer solution with a high-pressure
8
–10
fuel cell to generate electricity.
If H O can be separated
2 2
from the reactant solution as soon as it is formed, the degra-
dation of organic photocatalysts is avoided. However, such
a separation of H O during the photocatalytic oxygenation of
2 2
substrates has yet to be achieved.
a
Department of Material and Life Science, Graduate School of Engineering, Osaka
University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Suita,
Osaka 565-0871, Japan. E-mail: ookubo@chem.eng.osaka-u.ac.jp
b
Department of Applied Chemistry, Graduate School of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan
c
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-
7
50, Korea. E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
+
6ꢀ
Fig. 1 Chemical structure of DeQuCN PF
photocatalytic oxidation of toluene by O
and the two-phase
d
+
Faculty of Science and Technology, Meijo University, ALCA and SENTAN, Japan
Science and Technology Agency (JST), Nagoya, Aichi 468-8502, Japan
with DeQuCN . The
2
oxygenated products accumulate in the organic phase, whereas H
is accumulated in the aqueous phase.
2 2
O
†
Electronic supplementary information (ESI) available: Experimental details
spectral data. See DOI: 10.1039/c6ra05993g
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mercury lamp (1000 W) through a UV cutoff lter (l < 310 nm)
resulted in the formation of benzaldehyde and benzyl alcohol
(
(
Fig. 1). The products were identied by gas chromatography
Fig. S3†). Aer 96 h of irradiation, the amounts of benzalde-
hyde and benzyl alcohol produced in the toluene layer were 18
mM (0.66 mmol) and 6.0 mM (0.22 mmol), respectively (Fig. 2).
Hydrogen peroxide accumulated in the aqueous layer and was
4
+
quantied by spectroscopic titration with a [TiO(tpypH
4
)]
complex (Ti-TPyP reagent; oxo[5,10,15,20-tetra(4-pyridyl)
15
porphyrinato titanium(IV)] in acidic solution; see ESI†). The
nal concentration of H O was 550 mM (0.40 mmol). The

2
2
catalytic turnover numbers (TONs) were 490 for toluene and 510
+
16
for p-xylene based on the initial concentration of DeQuCN .
+
Most of the DeQuCN remained aer photo-irradiation for 96 h.
However, the reaction rate decreased at prolonged photo-
irradiation times as a result of the photo-oxidation of the cata-
Fig. 3 Time course of the two-phase photocatalytic oxidation of p-
xylene by O with DeQuCN PF (50 mM) in p-xylene with H O (10%)
under photo-irradiation with a xenon lamp (500 W) (l > 310 nm).
+
2 6 2
ꢀ
2 2 2 2 2
lyst with H O . Because the H O in H O is separated from the
organic catalyst in acetonitrile (MeCN), photodegradation of the
catalyst was avoided in this catalytic system.
1
0
ꢀ1 ꢀ1
10
ꢀ1 ꢀ1
were 1.6 ꢁ 10
M
s
for toluene and 1.9 ꢁ 10
M
s
for
When toluene was replaced by p-xylene, the formation of p-xylene, which are close to the diffusion rate constant in MeCN
1
0
ꢀ1 ꢀ1
p-tolualdehyde and p-methylbenzyl alcohol was also observed (2.0 ꢁ 10
M
s
). The free energy change of the photoin-
17
(
Fig. 3) (see Fig. S4† for gas chromatography data). The duced electron transfer (DGet in eV) from toluene to the singlet
+
2 2
quantum yields of H O
were determined from the initial rate of excited states of DeQuCN is given by eqn (2)
the photocatalytic reaction as 5.6% in toluene and 12% in
p-xylene (Fig. S5†). Table 1 summarizes the TONs and quantum
*
DGet ¼ e(Eox ꢀ Ered
)
(2)
yields of the photocatalytic oxidation of toluene and p-xylene by
*
+
where e is the elementary charge, and Eox and Ered are the one-
electron oxidation potential of toluene (Eox ¼ 2.20 V vs. SCE)
and the one-electron reduction potential of the singlet excited
O with DeQuCN .
2
5,18
+
Irradiation of the absorption band of DeQuCN (lmax
¼
3
30 nm) resulted in a strong uorescence at 432 nm in MeCN.
+
*
19
state of DeQuCN (Ered ¼ 2.72 V vs. SCE), respectively. The DGet
However, the uorescence was signicantly quenched in
toluene and p-xylene. The quenching rate constants in MeCN
values were ꢀ0.52 eV for toluene and ꢀ0.79 eV for p-xylene (Eox
4
¼
1.93 V vs. SCE), indicating that the photoinduced electron
were determined from the slopes of the Stern–Volmer plots
2
0
+
transfer reactions were energetically favourable.
The occurrence of photoinduced electron transfer from the
substrates to DeQuCN was conrmed by laser ash photolysis
(
(
Fig. 4) and the lifetimes of the singlet excited state of DeQuCN
s ¼ 34 ns, Fig. S6†). The k values obtained from eqn (1)
q
+
ꢀ1
experiments (see ESI†). Laser excitation (l ¼ 355 nm from
k
q
¼ KSV
s
(1)
+
ꢀ4
a Nd:YAG laser) of DeQuCN (3.0 ꢁ 10 M) in a deaerated
toluene solution resulted in a transient absorption spectrum at
1
ms with new absorption bands at 540 and 950 nm (Fig. 5).
Similar transient absorption spectra were also observed in
p-xylene (Fig. S7†). The transient absorption band at l
¼ 540
max
19
nm was assigned to DeQuCNc. The broad absorption band in
the near-IR region (lmax ¼ 950 nm) was assigned to the toluene
p-dimer radical cation because similar broad transient
absorption bands in the long-wavelength region have been re-
ported for the radical cations of toluene and other aromatic
19,21,22
compounds.
The decay of the absorbance resulting from
Table 1 TONs and quantum yields of photocatalytic oxidation of
+
ꢀ
toluene and p-xylene by O
2
with DeQuCN PF
6
(50 mM)
Substrate
Toluene
Product (TON)
Quantum yield (%)
Benzyl alcohol (120)
Benzaldehyde (370)
p-Methylbenzyl alcohol (170)
p-Tolualdehyde (340)
5.6
12
Fig. 2 Time course of the two-phase photocatalytic oxidation of
+
ꢀ
toluene by O
2
with DeQuCN PF
6
(50 mM) in toluene with H
2
O (2%) p-Xylene
under photo-irradiation with a high-pressure mercury lamp (1000 W)
l > 310 nm).
(
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Scheme 1 Reaction mechanism of photocatalytic oxygenation of
+
toluene derivatives and formation of H
2
O
2
catalysed by DeQuCN .
The photocatalytic reaction was initiated by photoinduced
electron transfer from toluene to the singlet excited state of
+
DeQuCN (Scheme 1). The toluene radical cation, which is in
equilibrium with the p-dimer toluene radical cation formed by
photoinduced electron transfer, produces a benzyl radical via
deprotonation. This is followed by the rapid addition of O to
2
give the peroxyl radical, leading to the nal oxygenated prod-
2
3
ucts, benzaldehyde and benzyl alcohol.
However, the
to form
c disproportionates in the presence of protons to yield
. The overall stoichiometry of the photocatalytic reaction is
given by eqn (3):
DeQuCNc produced by electron transfer reacts with O
2
ꢀ ꢀ
2 2
O c . O
H O
2 2
2
PhCH
3
+ 3O
2
/ 2PhCHO + 2H
2
O
2
(3)
+
6ꢀ
Fig. 4 Upper panel: emission spectra of DeQuCN PF (20 mM) in the
presence of toluene (0–5.0 mM) in MeCN at 298 K. Lower panel:
+
Stern–Volmer plots for the electron transfer quenching of DeQuCN
+
by toluene and p-xylene in MeCN.
DeQuCN thus acts as an efficient organic photocatalyst for
the two-electron reduction of O to produce H O as well as the
2
2 2
oxygenation of organic substrates in a two-phase solution. The
photochemical reaction is initiated by photoinduced electron
transfer to produce a radical ion pair. The reaction intermedi-
ates were detected by time-resolved transient absorption
measurements. This study provides a valuable method for the
selective oxygenation of organic substrates with the simulta-
2 2
neous formation of high concentrations of H O .
Acknowledgements
This work was supported by Grants-in-Aid (no. 26620154 and
6288037 to K. O.) from the Ministry of Education, Culture,
2
Sports, Science and Technology (MEXT), ALCA and SENTAN
projects from JST, Japan (to S. F.).
+
6ꢀ
(100 mM) in
Fig. 5 Transient absorption spectra of DeQuCN PF
deaerated toluene at 298 K taken at 1, 10 and 60 ms after nanosecond
laser excitation at 355 nm.
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bimolecular back electron transfer between DeQuCNc and the
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RSC Adv., 2016, 6, 41011–41014 | 41013

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2
(
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photoexcited DeQuCN occurred to give benzaldehyde.
+
1
1 The dosage of the water employed in the present reaction
system was chosen to maximize the concentration of H
2 2
O
in water keeping the two phase separated.
41014 | RSC Adv., 2016, 6, 41011–41014
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