Organically modified titania having a metal catalyst: A new type of liquid-phase hydrogen-transfer photocatalyst working under visible light irradiation and H2-free conditions

Article abstract of DOI:10.1039/c6cp01847e

Organically modified titania having a metal catalyst (OMTC), 2,3-dihydroxynaphthalene-modified titania having palladium metal, successfully worked as a hydrogen-transfer (CC hydrogenation) photocatalyst in the presence of triethanolamine as the hydrogen source under visible light irradiation and hydrogen-free conditions.

Full text of DOI:10.1039/c6cp01847e

Showcasing research from the Surface Design Chemistry
Laboratory/Applied Chemistry, Kindai University, Higashiosaka,
Osaka, Japan.
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Title: Organically modified titania having a metal catalyst: a new
type of liquid-phase hydrogen-transfer photocatalyst working
under visible light irradiation and H -free conditions
2
Organically modified titania having a metal catalyst (OMTC),
2
,3-dihydroxynaphthalene (DHN)-modified titania having
palladium metal, successfully worked as a hydrogen-transfer
C hydrogenation) photocatalyst in the presence of alcohol
(C
as the hydrogen source under visible light irradiation and
hydrogen-free conditions.
See Hiroshi Kominami et al.,
Phys. Chem. Chem. Phys.,
2016, 18, 16076.
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Organically modified titania having a metal
catalyst: a new type of liquid-phase hydrogen-
transfer photocatalyst working under visible light
Cite this: Phys.Chem.Chem.Phys.,
016, 18, 16076
2
Received 18th March 2016,
Accepted 11th May 2016
irradiation and H -free conditions†
2
Hiroshi Kominami,* Shin-ya Kitagawa, Yuki Okubo, Makoto Fukui, Keiji Hashimoto
and Kazuya Imamura
DOI: 10.1039/c6cp01847e
www.rsc.org/pccp
Organically modified titania having a metal catalyst (OMTC),
,3-dihydroxynaphthalene-modified titania having palladium metal,
2
successfully worked as a hydrogen-transfer (CQC hydrogenation)
photocatalyst in the presence of triethanolamine as the hydrogen
source under visible light irradiation and hydrogen-free conditions.
Scheme 1 Photocatalytic hydrogen-transfer reaction between alkenes
and alcohols over Pd/TiO
2
.
2
When titania (titanium(IV) oxide, TiO ) is irradiated by UV light, found photocatalytic hydrogenation of a vinyl group (styrene) to
6
electrons in the valence band (VB) are excited to the conduction an ethyl group (ethylbenzene) in an alcoholic suspension of
band (CB), leaving a positive hole in the VB. The photogenerated Pd/TiO (Scheme 1). Alcohols used as solvents were oxidized to
2
electrons are captured by a species adsorbed on the surface of give carbonyl compounds, and these reactions proceeded with
TiO , resulting in the formation of a reduced species, whereas both material balance and redox balance close to unity.
2
positive holes directly or indirectly cause oxidation of another
species. Since photocatalytic reactions proceed at room temperature indicating that Pd is an indispensable component driving the
under atmospheric pressure and since light is indispensable, hydrogenation after receiving electrons from the TiO photo-
2
Of course, no reaction occurred over bare (Pd-free) TiO ,
2
the reactions can be operated safely and can be easily controlled by catalyst. This reaction is regarded as a hydrogen-transfer reac-
the light intensity. Therefore, conversion of various substances over tion between alkenes and alcohols, giving alkanes and carbonyl
1
a TiO2 photocatalyst has been extensively studied. However, compounds, and the combination of TiO and a potential metal
2
applications of photocatalytic reduction have been less frequently catalyst will therefore provide various possibilities of photocatalytic
reported than photocatalytic oxidation because most of the organic reaction. However, TiO
substrates are easily oxidized or decomposed by positive holes. irradiation of visible light, which is largely contained in solar light.
Another reason is that reduction potentials of many organic Various types of photocatalysts that work under irradiation
compounds are more negative than the potential of the CB of of visible light have been reported. Most of the photocatalysts
TiO . Therefore, target compounds in photocatalytic reduction were developed for decolorization of dyes or degradation of
2
does not work as a photocatalyst under
7
2
2
are basically limited to those having a carbonyl group and a contaminants dissolved in water, and only a few of them were
nitro group, and other photocatalytic reductions of organic active in water splitting (H O - H + 1/2O ) or hydrogen (H )
compounds have scarcely been reported. On the other hand, evolution by proton reduction (2H + 2e - H ). For example,
3
2
2
2
2
+
ꢀ
2
8
we recently reported that introduction of potential metal catalysts Ikeda et al. reported H
2
evolution over TiO
changes the reduction properties and greatly widens the 1,1 -binaphthalene-2,2 -diol and platinum (Pt) co-catalyst in the
applicability of a TiO photocatalyst. For example, a cyano group presence of triethanolamine as an electron donor. On the other
benzonitrile) was converted to an amino group (benzylamine)
over palladium-loaded TiO (Pd/TiO ). Over silver-loaded TiO
2
modified with a
0
0
into TiO
2
2
4
(
hand, application of visible-light-responding photocatalysts for
substance conversion, especially reductive conversion, has been
2
2
2
,
5
9
epoxides were chemoselectively deoxygenated to alkenes. We also less frequently studied probably because target reactions are
limited and strategies for efficient conversion are still unknown.
We have focused on organically modified TiO2 (OMT) as a
photocatalyst responding to visible light. Since the use of a
combination of TiO and a metal catalyst is regarded as a new
2
strategy to expand the possibility of photocatalytic reactions,
OMT can be applied for substance conversion by combining it
Department of Applied Chemistry, Faculty of Science and Engineering,
Kindai University, Kowakae, Higashiosaka, Osaka 577-8502, Japan.
E-mail: hiro@apch.kindai.ac.jp
Electronic supplementary information (ESI) available: Experimental proce-
dures. See DOI: 10.1039/c6cp01847e
†
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with a metal catalyst. We prepared organically modified TiO
2
having catalysts (OMTCs) and found that OMTC works as a new
type of photocatalyst for liquid-phase hydrogen transfer under
visible light irradiation and H -free conditions. We show here the
2
high potential of 2,3-dihydroxynaphthalene (DHN)-modified TiO2
having a palladium catalyst, one of the OMTCs, and a new
application of a visible light-responding photocatalyst for substance
conversion other than water splitting and dye decolorization.
Ishihara ST-01 particles were used as TiO
,3-dihydroxynaphthalene-modified TiO (TiO
TiO –DHN (Pd/TiO –DHN) and the procedure of photocatalytic
2
. Preparation of
2
2
2
–DHN) and Pd-loaded Fig. 2 Time courses of (a) amounts of styrene remaining (squares), ethyl-
benzene formed (circles) and H formed (diamonds), and MB (right axis,
crosses) in photocatalytic hydrogen-transfer reaction over Pd/TiO –DHN
formed in the
2
2
2
2
reactions is provided in the ESI.†
under visible light irradiation at 298 K and (b) amount of H
2
Fig. 1(a) shows absorption spectra of TiO , TiO –DHN and
2
2
absence of styrene under the same conditions.
Pd/TiO
l o 390 nm due to the band-gap excitation. A large increase in
photoabsorption in the range of visible light was observed in were reduced by photogenerated electrons. These results indicate
the spectrum of TiO –DHN. A transmission spectrum of DHN that photocatalytic reduction (hydrogenation) of styrene occurred
dissolved in methanol is shown in Fig. 1(b), indicating that selectively over Pd/TiO –DHN under irradiation of visible light.
2 2
–DHN. Bare TiO exhibited strong photoabsorption at
2
2
DHN itself did not absorb visible light. Ikeda et al. reported that
Material balance (MB) calculated by using eqn (1) is also
0
0
strong interaction between TiO2 and 1,1 -binaphthalene-2,2 - shown in Fig. 2(a).
8
diol resulted in absorption in the visible light region. Based on
nðstyreneÞ þ nðethylbenzeneÞ
n0ðstyreneÞ
the results of this study and the previous study, it can be
concluded that strong absorption observed in TiO –DHN was
caused by interaction between DHN and TiO
of Pd/TiO –DHN, a slight increase in photoabsorption in the and ethylbenzene after photoirradiation, respectively, and
Material balance ¼
;
(1)
2
2
. In the spectrum where n(styrene) and n(ethylbenzene) are the amounts of styrene
2
range of visible light was caused by light scattering by Pd fine
n
0
(styrene) is the initial amount of styrene. The high yield of
particles fixed on TiO –DHN.
ethylbenzene and the value of MB close to unity indicate that
2
Fig. 2(a) shows time courses of the amount of styrene remaining only hydrogenation of styrene to ethylbenzene occurred, i.e.,
and the amounts of ethylbenzene and H formed in photocatalytic polymerization of styrene, cracking of ethylbenzene and hydro-
2
hydrogen-transfer reaction between styrene and triethanolamine genation of the aromatic ring did not occur. Fig. 2(b) shows the
over Pd/TiO
2
–DHN under irradiation of blue light. The emission result of reaction in the absence of styrene. In this case, only
evolution. It
spectrum of light from a blue LED is shown in Fig. 1(a). Just reduction of protons occurred, resulting in H
2
after photoirradiation, styrene monotonously decreased, while should be noted that the reaction rate was much smaller than
ethylbenzene was formed as the hydrogenated product of that of hydrogenation shown in Fig. 2(a). The large difference in
styrene, indicating that Pd/TiO –DHN works as a photocatalyst the reaction rates of ethylbenzene production and H production
2
2
for styrene hydrogenation under irradiation of visible light. indicates that the activation energy for styrene hydrogenation is
After 60 min of photoirradiation, styrene was almost completely much smaller than that for H evolution (hydrogen coupling)
2
consumed and ethylbenzene was obtained in a high yield over Pd metal.
(98%). It should be noted that H
2
was formed only after
Table 1 shows effects of catalyst components (Pd and DHN)
consumption of styrene. Formation of H
2
means that protons and blue light on ethylbenzene production in photocatalytic
hydrogenation of styrene after 90 min at 298 K. Hydrogenation
of styrene to ethylbenzene almost quantitatively occurred
(
high selectivity at high conversion) over Pd/TiO –DHN under
2
irradiation of blue light (entry 1), whereas no ethylbenzene was
produced over Pd/TiO2 and TiO –DHN under irradiation of
2
2
blue light (entries 2 and 3) and over Pd/TiO –DHN in the dark
(entry 4). These control experiments indicate that (1) introduction
2
of both a Pd catalyst and DHN into TiO is indispensable for
hydrogenation of styrene, (2) light from a blue LED does not
induce band-gap excitation of TiO , (3) neither photochemical
nor thermal reaction occurs under visible light irradiation, and
4) this reaction is induced by photoabsorption by TiO –DHN,
2
(
2
i.e., hydrogenation of styrene is a photocatalytic reaction occurring
over Pd/TiO –DHN. Hydrogenation occurred under H (1 atm) over
Pd/TiO –DHN and Pd/TiO at 298 K in the dark (entries 5 and 6). In
2 2 2
Fig. 1 (a) Diffuse reflectance spectra of TiO , TiO –DHN and Pd/TiO –
2
2
DHN (left axis) and emission spectrum of blue LED used as the light source
for photocatalytic reactions (right axis) and (b) transmission spectrum of
DHN dissolved in methanol.
2
2
this case, styrene was hydrogenated by hydrogen species formed by
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Table 1 Results of hydrogenation of styrene to ethylbenzene under
Table 2 Photocatalytic hydrogenation of various alkenes to alkanes in an
various conditions
acetonitrile suspension of Pd/TiO
blue LED at 298 K for 30 min
2
–DHN under irradiation of light from a
a
b
c
d
Entry
Catalyst
Blue light
Gas phase
Yield /%
Entry
1
Substrate
Product
Yield/%
98
1
2
3
4
5
6
Pd/TiO
TiO –DHN
2
2
–DHN
Yes
Yes
Yes
No
No
No
Ar
Ar
Ar
Ar
498
ND
ND
ND
498
98
Pd/TiO
Pd/TiO
Pd/TiO
Pd/TiO
Pd/TiO
Pd/TiO
2
2
2
2
2
2
–DHN
–DHN
H
H
2
2
3
94
87
2
e
7
8
–DHN
–DHN
Yes
Yes
Ar
Ar
498
498
f
a
3
Each catalyst (50 mg) was suspended in acetonitrile (5 cm ) containing
b
styrene (50 mmol) in the presence of triethanolamine (80 mmol). Irra-
ꢀ2
c
diated by two sets of blue LEDs (420–530 nm, 137 ꢁ 4 mW cm ). Space
4
5
92
98
3
d
e
f
in a test tube (30 cm ). Determined by GC. Second use. Third use.
dissociative adsorption of H
under an H atmosphere is helpful to understand photocatalytic
hydrogenation of styrene over Pd/TiO –DHN under irradiation of
blue light, and the reaction mechanism will be discussed later.
An action spectrum is a strong tool for determining whether
an observed reaction occurs via a photoinduced process or a
thermocatalytic process. To obtain an action spectrum in this
reaction system, hydrogenation of styrene in an acetonitrile
2
over Pd metal. The result obtained
three times and, based on eqn (3), the turnover number (TON)
of Pd for styrene hydrogenation was calculated to be 316,
indicating that Pd worked as a catalyst.
2
2
Amount of ethylbenzene formed
TON ¼
:
(3)
Amount of Pd catalyst
Applicability of the photocatalytic hydrogenation of alkenes to
alkanes was investigated using various aromatic alkenes
2
suspension of Pd/TiO –DHN was carried out at 298 K under
irradiation with monochromated visible light from a Xe lamp
(entries 1–4) and aliphatic alkenes (entry 5). Table 2 shows
with a light width of ꢁ5 nm. The apparent quantum efficiency
the results of photocatalytic hydrogenation over Pd/TiO –DHN under
2
(AQE) at each centered wavelength of light was calculated from
irradiation of visible light. Hydrogenation reaction proceeded in all
the ratio of the amount of ethylbenzene and the amount of
photons irradiated using eqn (2), and the results are shown in
Fig. 3 together with a subtraction spectrum obtained from spectra
cases, and high yield was obtained, indicating that Pd/TiO –DHN is
2
applicable for hydrogenation of various alkenes. We also examined
the effect of the type of sacrificial reagent on hydrogenation of
styrene. Very small reaction rates were obtained when methanol,
formic acid and oxalic acid were used, indicating that the oxidation
of Pd/TiO
tendency as that of the subtraction spectrum, indicating that the
observed reaction is caused by photoabsorption of TiO –DHN.
2 2
–DHN and Pd–TiO . The value of AQE showed the same
2
power of TiO
applicable to TiO
Photocatalytic hydrogen evolution over TiO modified with a
–DHN is relatively weak and sacrificial reagents
2
However, photoabsorption at a longer wavelength (4550 nm) did
not contribute to hydrogenation of styrene (discussed later).
2
–DHN are limited.
2
0
0
2
ꢂ Amount of ethylbenzene formed
1,1 -binaphthalene-2,2 -diol and a platinum (Pt) co-catalyst in
the presence of triethanolamine under irradiation of visible
light has been reported, and the reaction mechanism has been
AQE ¼
:
(2)
Number of incident photons
To evaluate the durability of the Pd/TiO
in this reaction system, the reaction was repeated. As shown in
Table 1, the Pd/TiO –DHN photocatalyst was reusable at least
three times without loss of activity (entries 7 and 8). In total,
49 mmol of ethylbenzene was formed by using Pd/TiO –DHN
2
–DHN photocatalyst
8
proposed. Our group reported photocatalytic hydrogen-transfer
2
reaction between alkenes and alcohols over Pd–TiO under
2
5
irradiation of UV light and proposed the reaction mechanism.
Based on the proposed mechanisms of the two different reaction
systems and results of blank experiments shown in Table 1, the
mechanism of the present photocatalytic hydrogen-transfer reac-
tion can be considered as follows (Scheme 2): (1) excitation of the
surface complex by visible light followed by electron injection
1
2
2
into the conduction band of TiO , (2) migration of the injected
electrons in the TiO particles to the Pd catalyst followed by
2
+
reduction of H to give active H species (probably H-Pd), (3)
insertion of H species into a CQC double bond on Pd, resulting
in the formation of ethylbenzene, and (4) reduction of the
oxidized form of the surface complex by triethanolamine as a
sacrificial electron donor, resulting in the formation of aldehyde
+
and H . In the absence of alkenes in the system, H
2
was formed
Fig. 3 Subtraction spectrum obtained from spectra of Pd/TiO
and Pd–TiO (left) and action spectrum in ethylbenzene formation over
Pd/TiO –DHN under visible light irradiation.
2
–DHN
as a coupling product of the active H species, although the
reaction rate is small probably due to the large activation energy
2
2
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Scheme 2 Proposed mechanism for photocatalytic hydrogen-transfer
reaction between alkenes and alcohols over Pd/TiO
light irradiation.
2
–DHN under visible
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2,3-Dihydroxynaphthalene (DHN)-modified titania with a palla-
dium catalyst was active toward various alkenes, and the reaction
rate was larger than that of simple hydrogen evolution. OMTCs
can be applied for another liquid-phase reaction by using an
appropriate catalyst for the reaction.
4
5
6
7
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Acknowledgements
This work was partly supported by Grants-in-Aid for Scientific
Research (No. 26289307 and 26630415) from the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT) of
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for the Strategic Research Foundation at Private Universities
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