Accelerated Semihydrogenation of Alkynes over a Copper/Palladium/Titanium (IV) Oxide Photocatalyst Free from Poison and H2 Gas

Article abstract of DOI:10.1002/cctc.201902175

Selective hydrogenation of alkynes to alkenes (semihydrogenation) without the use of a poison and H₂ is challenging because alkenes are easily hydrogenated to alkanes. In this study, a titanium (IV) oxide photocatalyst having Pd core-Cu shell nanoparticles (Pd@Cu/TiO₂) was prepared by using the two-step photodeposition method, and Pd@Cu/TiO₂ samples having various Cu contents were characterized by electron transmission microscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy. Thus-prepared Pd@Cu/TiO₂ samples were used for photocatalytic hydrogenation of 4-octyne in alcohol and the catalytic properties were compared with those of Pd/TiO₂ and Cu/TiO₂. 4-Octyne was fully hydrogenated to octane over Pd/TiO₂ at a high rate and 4-octyne was semihydrogenated to cis-4-octene over Cu/TiO₂ at a low rate. Rapid semihydrogenation of 4-octyne was achieved over Pd(0.2 mol%)@Cu(1.0 mol%)/TiO₂, indicating that the Pd core greatly activated the Cu shell that acted as reaction sites. A slight increase in the reaction temperature greatly increased the rate with a suppressed rate of H₂ evolution as the side reaction. Changes in the reaction rates of the main and side reactions are discussed on the basis of results of kinetic studies. Reusability and expandability of Pd@Cu/TiO₂ in semihydrogenation are also discussed.

Full text of DOI:10.1002/cctc.201902175

Accepted Article
Title: Accelerated Semihydrogenation of Alkynes over a Copper/
Palladium/Titanium(IV) Oxide Photocatalyst Free from a Poison
and H2 Gas
Authors: Shota Imai, Kousuke Nakanishi, Atsuhiro Tanaka, and Hiroshi
Kominami
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ChemCatChem
10.1002/cctc.201902175
FULL PAPER
Accelerated Semihydrogenation of Alkynes over a
Copper/Palladium/Titanium(IV) Oxide Photocatalyst Free from a
Poison and H Gas
2
[a]
[a]
[b][c]
and Hiroshi Kominami*[b]
Shota Imai, Kousuke Nakanishi, Atsuhiro Tanaka
Abstract: Selective hydrogenation of alkynes to alkenes
catalytic systems for semihydrogenation of alkynes have been
developed.[3-10]
(
semihydrogenation) without the use of a poison and H
because alkenes are easily hydrogenated to alkanes. In this study,
titanium(IV) oxide photocatalyst having Pd core-Cu shell
nanoparticles (Pd@Cu/TiO ) was prepared by using the two-step
photodeposition method, and Pd@Cu/TiO samples having various
2
is challenging
2
A TiO photocatalyst generates positive holes in the valence
a
band and electrons in the conduction band by irradiation of UV
light and these cause oxidation and reduction, respectively.
Photocatalytic reactions have various advantages: 1) solar light
can be used for the light source driving the reaction, 2) reactions
proceed at room temperature under an atmospheric pressure, 3)
reactions can be suspended by quenching light irradiated to the
reaction system, 4) a photocatalyst can be recovered easily from
the reaction mixture after the reaction, and 5) the recovered
photocatalyst can be re-used after some treatment if necessary.
2
2
Cu contents were characterized by electron transmission microscopy,
X-ray photoelectron spectroscopy and UV-vis spectroscopy. Thus-
prepared Pd@Cu/TiO
hydrogenation of 4-octyne in alcohol and the catalytic property was
compared with those of Pd/TiO and Cu/TiO 4-Octyne was fully
hydrogenated to octane over Pd/TiO at a high rate and 4-octyne was
semihydrogenated to cis-4-octene over Cu/TiO at a low rate. Rapid
semihydrogenation of 4-octyne was achieved over 0.2 mol%Pd@1.0
mol%Cu/TiO , indicating that the Pd core greatly activated the Cu
shell that acted as reaction sites. A slight increase in the reaction
temperature greatly increased the rate with a suppressed rate of H
2
samples were used for photocatalytic
2
2
.
2
2
Our group has reported that the reduction properties of a TiO
2
photocatalyst can be changed by introduction of metal
nanoparticles working as the electron pool and the reduction sites
for a specific selectivity, e.g., Pd for hydrogenation of alkenes to
alkanes,[11] silver for deoxygenation of epoxides to alkenes and
rhodium for ring hydrogenation of benzoic acid.[13] Besides these
2
[12]
2
evolution as the side reaction. Changes in the reaction rates of the
main and side reactions are discussed on the basis of results of kinetic
studies, various conversions of organic compounds over a TiO
2
14-
[
studies.
Reusability and expandability of Pd@Cu/TiO
2
in
photocatalyst having metal nanoparticles have been reported.
1
6]
semihydrogenation are also discussed.
We also succeeded in semihydrogenation of alkynes over a
copper (Cu)-loaded TiO (Cu/TiO ) photocatalyst without the use
of H
.[17] This photocatalyst showed sharp diastereoselectivity
2
2
2
and chemoselectivity, i.e., internal alkynes were hydrogenated to
cis-alkenes without being further hydrogenated to alkanes.
Introduction
2
However, since the reaction rate of Cu-TiO was small and it took
Semihydrogenation of alkynes to alkenes is one of
important reactions in fine chemistry and polymer chemistry.[
However, since alkenes are easily hydrogenated to alkanes,
selective formation of alkenes is generally difficult. A Lindlar
catalyst[ has been used for semihydrogenation of alkynes to
alkenes. However, this catalytic system requires 1) lead (Pb) to
suppress the activity of a palladium (Pd) catalyst, 2) quinoline for
a long time to obtain alkenes with a high yield, a new co-catalyst
that makes rapid semihydrogenation possible is highly desired.
Catalytic performance has often been improved by the use
of a combination of two different elements and, in some cases, a
1]
2]
new catalytic performance was created.[18,19]
bimetallic catalysts have recently been investigated in detail.
Therefore,
[20]
The forms of a bimetallic catalyst include an alloy type, a core-
shell type and other types,[1] and the activity and selectivity of a
the same purpose and 3) hydrogen (H
2
) gas as a hydrogen
-
+
(
electron (e ) and proton (H )) source. Recently, excellent
bimetallic catalyst differ depending on the composition, state, and
distribution and mixing level of the two elements in the
particles.[21,22]
Excellent photocatalytic performance can be
expected when a bimetallic catalyst is loaded on a photocatalyst
as a co-catalyst. Based on our previous results, we chose Cu[17]
and Pd[ as co-catalyst elements for rapid semihydrogenation of
alkynes because Cu showed excellent selectivity for
semihydrogenation of alkynes and Pd exhibited strong activity for
hydrogenation of unsaturated C–C bonds. As the form of the
bimetallic co-catalyst, formation of a Pd core-Cu shell structure
[
[
[
a]
S. Imai, K. Nakanishi
Department of Molecular and Material Engineering
Graduate School of Science and Engineering, Kindai University
Kowakae, Higashiosaka, Osaka 577-8502 (Japan)
A. Tanaka, H. Kominami
11]
b]
c]
Department of Applied Chemistry,
Faculty of Science and Engineering, Kindai University
Kowakae, Higashiosaka, Osaka 577-8502 (Japan)
Precursory Research for Embryonic Science and Technology
(
Pd@Cu) is very important because the exposed Pd atoms in
(PRESTO), Japan Science and Technology Agency (JST), Honcho,
bimetallic particles would probably induce full hydrogenation of
alkynes, resulting in a decrease in the selectivity of cis-alkenes.
Kawaguchi 332-0012 (Japan)
E-mail: hiro@apch.kindai.ac.jp
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In this study, we prepared a TiO
2
photocatalyst having a Pd
Characterization of photocatalysts.
2
core-Cu shell bimetallic co-catalyst (Pd@Cu/TiO ) by using the
two-step photodeposition method[
23,24]
and we examined
An image of transmission electron microscopy (TEM) and the
particle size distribution of Pd(0.2)/TiO are shown in Figures 2(a)
and (b), respectively. Pd particles loaded on TiO were observed
photocatalytic hydrogenation of alkynes in alcohol suspensions of
2
Pd@Cu/TiO
2
. Here we report 1) characterization of Pd@Cu/TiO
2
,
2
2)
performance and expandability of the Pd@Cu/TiO
2
in the TEM image and the average diameter (Dave) of the Pd
photocatalyst in alkyne hydrogenation and 3) the kinetics.
particles was determined to be 2.5 nm. Nanoparticles were
observed in the TEM image of Pd(0.2)@Cu(1.0)/TiO
2
(Figure
2(c)) and Dave was determined to be 4.7 nm (Figure 2(d)).
Results and Discussion
Dave: 2.5 nm
(a)
60 (b)
2
Effects of modification of Pd/TiO with Cu on photocatalytic
Counts: 117
hydrogenation.
σ: 0.6 nm
4
0
Figure 1(a) shows results of hydrogenation of 4-octyne over
Pd(0.2)@Cu(X)/TiO under irradiation of UV light for 1.5 h. 4-
Octyne was almost quantitatively converted to octane in the case
of Cu-free Pd(0.2)/TiO , indicating that the Pd co-catalyst
possesses a strong activity to hydrogenate alkyne to alkane.
Formation of octane was greatly suppressed and cis-4-octene
and trans-4-octene were observed when Pd(0.2)/TiO was
modified with 0.2 mol%Cu. The amounts of octane and trans-4-
octene decreased when the content of Cu was increased to 0.5
and 0.7 mol%. Finally, only cis-4-octene was obtained over
2
20
0
2
5 nm
0
1 2 3 4 5 6 7
Particle size / nm
2
Dave: 4.7 nm
(c)
6
4
2
(d)
0
Counts: 111
σ: 0.9 nm
0
0
0
Pd(0.2)@Cu(1.0)/TiO
property of Pd(0.2)@Cu(X)/TiO
Cu(1.0)/TiO (Figure 1(b)) with an increase in the Cu content.
However, the yield of cis-4-octene in the case of Cu(1.0)/TiO was
less than 50% after 1.5 h, while almost quantitative conversion of
-octyne to cis-4-octene was achieved over
Pd(0.2)@Cu(1.0)/TiO after 1.5 h. In other words, introduction of
2
.
These results indicate that the catalytic
2
became close to that of
2
2
5
nm
1 2 3 4 5 6 7
Particle size / nm
4
2
a Pd core to the Cu co-catalyst resulted in great acceleration of
the semihydrogenation of 4-octyne with maintenance of high
selectivity for cis-4-octene.
Figure 2. TEM images and size distributions of (a, b) Pd(0.2)/TiO
Pd(0.2)@Cu(1.0)/TiO
2
and (c, d)
2
.
2+
2+
Since reduction potentials of Pd /Pd and Cu /Cu are 0.915 V (vs
SHE) and 0.340 V (vs SHE), respectively, no redox reaction
between Pd and Cu2+ occurs before photoirradiation. During the
photodeposition process, photogenerated electrons were trapped
by Pd nanoparticles and then used for reduction of Cu2+ to Cu.
The increase in Dave from 2.5 nm to 4.7 nm supports the idea that
6
5
4
3
2
1
0
0
0
0
0
0
0
(
a)
(b)
Cu was formed on Pd particles of Pd(0.2)/TiO
photodeposition.
2
by the
Electronic states of Pd and Cu loaded on TiO
2
were
analyzed by using an XPS technique. Figure 3(a) shows Cu 2p
spectra of various samples. In the Cu 2p spectra, peaks due to
Cu 2p3/2 and 2p1/2 were observed at around 932.2 eV and 952.1
eV, respectively, and the peaks became intense with an increase
in the Cu content. In Pd 3d spectra (Figure 3(b)), peaks were
observed at around 335.0 eV and 340.3 eV, which are assignable
to Pd0.[25]
decreased with increasing Cu content (X), and peaks were barely
observed for Pd(0.2)@Cu(1.0)/TiO . These XPS spectra indicate
the formation of Pd@Cu core-shell structure on TiO Since
broad peaks were observed on Pd 3d spectra, we carried out
deconvolution of the Pd 3d peak on Pd(0.2)@Cu(X)/TiO (Figure
(b)). In Pd 3d spectra of Cu-free Pd(0.2)/TiO (Figure 4(a)), 3d5/2
Peak intensity of Pd 3d for Pd(0.2)@Cu(X)/TiO
2
0
0.2 0.5 0.7 1.0
1.0
Amount of Cu loaded / mol%
2
2
.
Figure 1. Amounts of 4-octyne, cis-4-octene, trans-4-octene and octane in 2-
propanol suspensions of (a) Pd(0.2)@Cu(X)/TiO (X=0, 0.2, 0.5, 0.7, 1.0) and
b) Cu(1.0)/TiO under Ar (irradiation time: 1.5 h).
2
2
(
2
3
2
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0
and 3d3/2 peaks due to Pd were observed at 334.9 eV and 340.3
(
a)
(b)
2
+
eV, and the peaks due to Pd were observed at 336.8 and 342.5
2
1
2
1
0
MB
RB
[
25, 26]
eV.
Pd in the Pd(0.2)/TiO
Figures 4(b)-(e)), peaks due to Pd were also observed, and the
These results indicate the presence of electron-deficient
MB
RB
2
sample. In the Pd@Cu/TiO samples
2
0
600
400
200
0
600
400
200
0
2
+
(
60
60
40
20
0
peaks became intense with increase in the Cu content, indicating
that electron donation from Pd to Cu occurred when two elements
formed the core-shell structure.
40
Acetone
Acetone
20
(
a) Cu 2p
(b) Pd 3d
3d3/2
H2
2
p 1/2
2p3/2
3d 5/2
H2
0
0
1
2
3
0
1
2
3
Cu(1.0)/TiO2
Irradiation time / h
Irradiation time / h
Pd(0.2)@Cu(1.0)/TiO2
Pd(0.2)@Cu(0.7)/TiO2
Pd(0.2)@Cu(0.5)/TiO2
Pd(0.2)@Cu(0.2)/TiO2
Pd(0.2)/TiO2
Figure 5. Time courses of amounts of 4-octyne (●), cis-4-octene (◆), trans-4-
octene (■), octane (▼), H (○) and acetone (◇) formed and values of MB (▲)
and RB (△) in 2-propanol suspensions of (a) Pd(0.2)@Cu(1.0)/TiO and (b)
Cu(1.0)/TiO under Ar at 293 K.
2
2
2
9
60
950
940
930
345
340
335
330
Binding energy / eV
Binding energy / eV
0
2+
Ratios of the Pd peak and Pd peak of the Pd(0.2)@Cu(X)/TiO
2
2+
0
samples are summarized in Table 1. The ratios of Pd and Pd
peaks for Pd(0.2)/TiO and Pd(0.2)@Cu(0.2)/TiO samples were
almost the same within the experimental error, indicating that the
Figure 3. XPS spectra of Pd(0.2)@Cu(X)/TiO
Cu(1.0)/TiO : (a) Cu 2p and (b) Pd 3d.
2
(X=0, 0.2, 0.5, 0.7, 1.0) and
2
2
2
2
core-shell structure of Pd(0.2)@Cu(0.2)/TiO was incomplete and
parts of Pd may be exposed outside. The ratio of the Pd2+ peak
increased with increase in the Cu content (in other words, with
(
a) Pd(0.2)/TiO2
(b) Pd(0.2)@Cu(0.2)/TiO 2
(c) Pd(0.2)@Cu(0.5)/TiO 2
2
increase in the thickness of the Cu shell) of Pd@Cu/TiO . These
results suggest that the Pd species near the interface of Cu was
in an electron-deficient state, in other words, the Cu shell of
3
45
340
335
330
345
340
335
330
345
340
335
330
Pd@Cu/TiO
activity for semihydrogenation of 4-octyne.
2
was in an electron-rich state, resulting in higher
Binding energy / eV
Binding energy / eV
Binding energy / eV
(
d) Pd(0.2)@Cu(0.7)/TiO 2
(e) Pd(0.2)@Cu(1.0)/TiO 2
Photocatalytic
hydrogenation
of
4-octyne
over
Pd(0.2)@Cu(1.0)/TiO
2
.
Figure 5(a) shows the time courses of formation of 4-octyne and
products in a 2-propanol suspension of Pd(0.2)@Cu(1.0)/TiO
2
3
45
340
335
330
345
340
335
330
Binding energy / eV
Binding energy / eV
under irradiation of UV light. The amount of 4-octyne decreased,
while cis-4-octene was formed selectively. After 1.5 h, most of
the 4-octyne had been consumed and cis-4-octene was obtained
in a high yield (99%). Almost quantitative formation of cis-4-
octene means that only syn-addition to 4-octyne occurred over
Figure 4. Deconvolution of Pd 3d peaks of (a) Pd(0.2)/TiO
2
,
(b)
Pd(0.2)@Cu(0.2)/TiO , (c) Pd(0.2)@Cu(0.5)/TiO , (d) Pd(0.2)@Cu(0.7)/TiO
2
2
2
2
and (e) Pd(0.2)@Cu(1.0)/TiO .
Pd(0.2)@Cu(1.0)/TiO
2
.
Even under light irradiation after
consumption of 4-octyne, there was no formation of trans-4-
octene or octane, indicating that isomerization of cis-4-octene and
sequential hydrogenation of octenes did not occur over
0
2+
Table 1. Ratios of Pd peak and Pd peak of Pd(0.2)@Cu(X)/TiO
2
samples.
Area ratio
2
Pd(0.2)@Cu(1.0)/TiO , i.e., the strong catalytic property of Pd in
Amount of Cu loaded
/
mol%
hydrogenation was completely controlled by introduction of a Cu
shell covering Pd particles. To evaluate the hydrogenation of 4-
octyne during the reaction, material balance (MB) was calculated
0
Pd2+ / %
20
Pd / %
0
80
83
71
61
57
by Equation (1):
0.2
0.5
0.7
1.0
17
n(4-Octyne)+n(cis/trans-4-Octene)+n(Octane)
MB =
(1),
n0(4-Octyne)
29
39
where n(4-Octyne), n(cis/trans-4-Octene) and n(Octane) are the
amounts of 4-octyne, cis/trans-4-octene and octane during the
photocatalytic reaction, respectively, and n (4-Octyne) is the
0
43
amount of 4-octyne before the photocatalytic reaction. As also
shown in Figure 5(a), the values of MB were always ca. 1.0 during
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the reaction, which indicating that no stable intermediates formed
and no side reaction occurred. To evaluate the stoichiometric
balance between photocatalytic oxidation and reduction, redox
balance (RB) was calculated by Equation (2):
5
4
3
2
1
0
14
12
10
8
(a)
(b)
n(cis/trans-4-Octene)+2n(Octane)+n(H2)
RB =
(2),
6
n(Acetone)
4
2
where n(Acetone) is the amount of acetone. The values of RB
were always ca. 1.0 within the experimental error except in the
early stage of the reaction, indicating that acetone was the sole
product of oxidation. From these results, hydrogenation of 4-
0
300 350 400 450 500
Wavelength / nm
300 350 400 450 500
Wavelength / nm
2
Figure 7. (a) Absorption spectrum of TiO and (b) Action spectrum in the
octyne and evolution of H
2
in a 2-propanol suspension of
photocatalytic hydrogenation of 4-octyne to cis-4-octene in
suspension of Pd(0.2)@Cu(1.0)/TiO
a
2-propanol
Pd(0.2)@Cu(1.0)/TiO
respectively.
2
are shown in Equations (3) and (4),
2
.
OH
O
+
+
(
3)
We noted that the color of Pd(0.2)@Cu(1.0)/TiO
after the reaction.
recorded absorption spectra of the 2-propanol suspension of
changed
2
To investigate the change of color, we
OH
O
H2
+
(4)
Pd@Cu/TiO
2
.
Pd(0.2)@Cu(1.0)/TiO
2
(20 mg) was suspended in
3
2
cm of 2-propanol containing 4-octyne (ca. 25 µmol) in a quartz
Figure 5(b) shows the results for hydrogenation of 4-octyne
over Cu(1.0)/TiO As also shown in Figure 1(b), the Cu co-
cell. Then the quartz cell was sealed with a silicon rubber septum
and photoirradiated under Ar at 293 K with the high-pressure
2
.
catalyst showed high selectivity for cis-4-octene formation.
However, it took more than 3 h to complete the reaction and the
rate of cis-4-octene formation was ca. 20 µmol h- as shown in
mercury lamp for various periods of time.
Diffuse reflection
spectra of the suspensions are shown in Figure 6. By irradiation
of UV light, a new peak appeared at around 580 nm. The peak
was attributed to extinction due to surface plasmon resonance
1
Figure 5(b).
production over Pd(0.2)@Cu(1.0)/TiO
Figure 5(a)), which was 3-times larger than that over Pd-free
Cu(1.0)/TiO These results show that the Pd core did not work
On the other hand, the rate of cis-4-octene
reached 59 µmol h-1
2
(
SPR) of Cu. The change in the spectra indicates that the Cu
(
shell had been partially oxidized under exposure to air in the
preparation process, and the partially oxidized Cu shell was
reduced to metal at the early stage of the photocatalytic reaction.
Therefore, the decrease in the value of RB at the initial stage (0.8)
can be explained by consumption of photogenerated electrons
due to reduction of the Cu shell. To evaluate the effect of SPR
of Cu, photocatalytic reaction under irradiation of visible light (λ>
2
.
as active sites for hydrogenation of 4-octyne but greatly
contributed to activation of the Cu shell as active sites for the
semihydrogenation of 4-octyne. The results of XPS indicate that
electron donation from the Pd core to the Cu shell accelerates the
generation of active hydrogen species by H+ reduction and
subsequent hydrogenation of 4-octyne over the Cu shell.
430 nm) from a Xe lamp (Ushio, Tokyo) filtered with a Y43 filter
(
Sigma Koki, Tokyo) was carried out under the same condition
used for Figures 1 and 5 except the light condition. No 4-octyne
was hydrogenated, indicating that there is no contribution of SPR
to hydrogenation of alkyens.
An action spectrum often clarifies whether the observed
results are induced by photoabsorption or not. To understand
this reaction system, semihydrogenation of 4-octyne in a 2-
5
3
3
2
1
5
0
5 min
0 min
0 min
0 min
min
4
3
2
1
0
min
propanol suspension of Pd(0.2)@Cu(1.0)/TiO
2
was carried out at
298 K under irradiation of monochromated light from a Xe lamp.
A
multi-wavelength irradiation monochromator (MM-3,
Bunkoukeiki, Tokyo) was used to obtain apparent quantum
efficiency (AQE), and light intensity was determined by using a
spectroradiometer (USR-45D, Ushio, Tokyo).
AQE at each
centered wavelength of light was calculated from the ratio of twice
the amount of cis-4-octene formed and the amount of photons
irradiated using Equation (5):
4
00
500
600
700
Wavelength / nm
2
× amount of cis-4-octene formed
AQE=
× 100
(5).
amount of incident photons
Figure 6. Change in absorption spectra of Pd(0.2)@Cu(1.0)/TiO
suspended in 2-propanol (2 cm ) containing 4-octyne (ca. 25 µmol) in a quartz
2
(20 mg)
3
cell under Ar with irradiation of UV light.
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Figures 7(a) and 7(b) show photoabsorption spectra of TiO
2
and
hydrogenation
Pd(0.2)@Cu(1.0)/TiO
presence of H
. Therefore, used, most of the 4-octyne was converted to octane due to the
of
4-octyne
, Pd(0.2)/TiO
2
over
and Cu(1.0)/TiO2, in the
2
in the dark at 293 K. When Pd(0.2)/TiO was
three
catalysts,
an action spectrum of Pd(0.2)@Cu(1.0)/TiO
Change in AQE in 4-octyne hydrogenation showed a tendency
similar to that in the photoabsorption spectrum of TiO
it can be concluded that hydrogenation of 4-octyne was induced
by photoabsorption of TiO and that photoabsorption (extinction)
2
,
respectively.
2
2
2
strong hydrogenation activity of Pd nanoparticles (Entry 5). The
thermal catalysis of Pd was controlled by introducing a Cu shell,
2
due to Pd and Cu did not contribute to the hydrogenation. As
also shown in Figure 7(b), AQE reached 12.5% at 326 nm,
indicating that photocatalytic semihydrogenation of 4-octyne
occurred with high efficiency of photon utilization in addition to
high selectivity of cis-4-ontene.
and Pd(0.2)@Cu(1.0)/TiO
octene (Entry 4), although the reaction rate at 293 K was much
2
showed high selectivity for cis-4-
lower than that of the photocatalytic reaction.
occurred over Cu(1.0)/TiO under H (Entry 6).
suggests that H
No reaction
This result
2
2
2
is not dissociatively adsorbed on Cu, not forming
active hydrogen species.
Effects of reaction conditions.
Reusability and applicability of the Pd@Cu/TiO
2
Table 2 shows the effects of photoirradiation, kind of catalyst and
atmosphere of the gas phase on the reaction at 293 K.
Conversion (Conv.) and yield were calculated by Equations (6)
photocatalyst.
The reusability and applicability of Pd@Cu/TiO
2
for
and (7), respectively:
semihydrogenation of alkynes were examined, and the results are
summarized in Table 3. As shown in Figure 5(a), Pd@Cu/TiO2
almost quantitatively converted 4-octyne to cis-4-octene (Entry 1)
and showed the same performance even in the second and third
uses (Entries 2 and 3). We also examined hydrogenation of
n0(4-Octyne)-n(4-Octyne)
Conv.=
×100
(6),
n0(4-Octyne)
n(Product)
Yield=
×100
(7).
n0(4-Octyne)
various
alkynes
in
2-propanol
suspensions
of
Pd(0.2)@Cu(1.0)/TiO
2
at 293 K (Entries 4-11). 2-Hexyne was
No reaction occurred when Pd(0.2)@Cu(1.0)/TiO
absent (Entries 2 and 3). These results show that hydrogenation
2
or light was
converted to cis-2-hexene, although a small amount of cis-2-
hexene was eliminated to the gas phase due to the high vapor
pressure (Entry 4). We succeeded in quantitative hydrogenation
of alkynol to the corresponding cis-alkenol, indicating that
oxidation of the hydroxyl group by holes was prevented (Entry 5).
of 4-octyne was neither
a
photochemical reaction nor
a
thermocatalytic reaction and that both Pd(0.2)@Cu(1.0)/TiO
light are indispensable, being in agreement with the action
spectrum (Figure 7). Entries 4-6 show results for thermocatalytic
2
and
We investigated the applicability of Pd@Cu/TiO
2
for
semihydrogenation of terminal alkynes. 1-Hexyne was converted
to 1-hexene with almost the same yield of 2-hexyne (Entry 6). 3-
Phenyl-1-propyne and phenylacetylene was hydrogenated to
allylbenzene and styrene, respectively (Entries 7 and 8). In both
cases, hydrogenation of alkenes also occurred, resulting in the
Table 2. Effect of reaction conditions on hydrogenation of 4-octyne for 2 h
at 293 K.
[
a]
Catalyst
IPA (5 cm
93 K, 2 h
3
), Ar or H
2
1
2
3
4
2
decreased yields.
Pd(0.2)@Cu(1.0)/TiO
Entries
9
and 10 show that
2
can be applied for hydrogenation of 1-
Entry
UV
Catalyst
Gas
phase
Conv.
Yield
of 2
/ %
Yield
of 3
/ %
Yield
of 4
/ %
[
b]
[b]
[b]
[b]
alkynes having nitrile and chloro groups to corresponding 1-
alkenes. These results have two impacts on semihydrogenation
of alkynes: 1) 1-alkenes formed in a short time, although
hydrogenation of 1-alkynes is generally difficult and the reaction
rate is low and 2) nitrile and chloro groups remained in the
products, although these groups are easily reductively eliminated
over a Pd catalyst. No reactivity to these groups indicates that
the Pd core is almost completely covered with the Cu shell.
Semihydrogenation of 1-alkyne having a carboxyl group was
achieved (Entry 11). The results of Entries 1-11 clarified that
chemoselective semihydrogenation of alkynes without reduction
of
1
/
%
1
2
3
4
5
6
On
Pd@Cu
Ar
>99
0
>99
0
0
0
0
1
0
0
0
/TiO
2
On
-
Ar
0
-
-
-
-
Pd@Cu
TiO
Ar
0
0
0
/
2
Pd@Cu
TiO
H
H
H
2
2
2
32
>99
0
34
0
0
/
2
of other functional groups is possible over Pd@Cu/TiO
examined the expandability of Pd@Cu/TiO in alcohols (Entries
2-15) and we found that semihydrogenation of 4-octyne
2
. We also
2
Pd/TiO
2
93
0
1
occurred in various alcohols besides 2-propanol. We confirmed
production of butanal and 2-petanone in the photocatalytic
reactions in 1-butanol and 2-pentanol, respectively (Entries 14
and 15). As shown in Table 3, the values of RB were close to
unity, indicating that semihydrogenation and alcohol oxidation
occurred almost stoichiometrically.
Cu/TiO
2
0
[
a] Reaction conditions: Pd(0.2)@Cu(1.0)/TiO
2
(50 mg), 4-Octyne (ca. 50
. [b] Determined by GC
3
µmol), 2-propanol (5 cm ), 293 K and under Ar or H
2
using an internal standard.
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ChemCatChem
10.1002/cctc.201902175
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that the value of E
a
of alkene hydrogenation is much larger than
that of alkyne hydrogenation.
Table 3. Photocatalytic hydrogenation of various alkynes to alkenes in
alcohol suspensions of Pd(0.2)@Cu(1.0)/TiO
2
at 293 K.[
a]
1
0.5
0
1
00
(a)
100
80
60
40
20
0
(b)
(c)
3
28 K
Conv.[b]
/ %
Sel.[b]
/ %
318 K
308 K
Entry
Substrate
Product
Time
RB
8
6
4
2
0
0
0
0
0
298 K
/
h
-0.5
2
3
3
3
98 K
08 K
18 K
28 K
-1
1
2
2
2
1
2
1
2
2
2
2
3
1
1
2
3
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
92
1.1
1.1
0.9
0.8
0.8
0.8
0.8
0.8
1.1
0.8
0.8
-
-1
-1.5 E =25 kJ mol
a
2[c]
-2
0
20
40
60
80
0
20
40
60
80
3
3.1
3.2
/(T / K)
3.3
3.4
Irradiation time / min
Irradiation time / min
10
3
3[d]
4
Figure 8. Time courses of the amounts of (a) cis-4-octene formed and (b) 4-
octyne remaining in a 2-propanol suspension of a Pd(0.2)@Cu(1.0)/TiO
photocatalyst at various temperatures. (c) Arrhenius plot (logarithm of k vs.
reciprocal of T).
2
5
HO
HO
>99
88
6
7
89
To evaluate the hydrogenation over Pd@Cu/TiO
slightly elevated temperature in detail, photocatalytic reaction was
carried out at 328 K. Figure 9 shows the time courses of
formation of 4-octyne and products in a 2-propanol suspension of
Pd(0.2)@Cu(1.0)/TiO at 328 K under irradiation of light from a
UV LED, indicating that almost quantitative formation of cis-4-
octene was achieved at 45 min at 328 K. Since the intensities of
UV light in Figure 5 and Figure 9 were different (25.8 mW cm
2
at a
8
66
N
N
9
83
2
1
1
1
1
1
1
0
Cl
Cl
>99
88
O
O
1
OH
OH
-
2
2[e]
94
-
2
and 17.0 mW cm , respectively), the results include factors other
than the reaction temperature. However, the time necessary to
complete the reaction was shortened by half at 328 K despite the
3[f]
98
-
4[g]
98
1.1
1.0
smaller intensity of UV light. We noted that production of H
2
was
that
5[h]
greatly
Pd(0.2)@Cu(1.0)/TiO
suppressed
at
328
a
K,
indicating
very sharp selectivity for
reduction (hydrogenation) of 4-octyne at 328 K, although H can
>99
2
showed
+
[a] Reaction conditions: Pd(0.2)@Cu(1.0)/TiO
2
(50 mg), substrate (ca. 50
µmol), 2-propanol (5 cm ), 293 K and under Ar. [b] Determined by GC using
an internal standard. [c] second use. [d] third use. [e] solvent: methanol (5
3
also be reduced to H
2
.
3
3
3
2
cm ). [f] solvent: ethanol (5 cm ). [g] solvent: 1-butanol (5 cm ). [h] solvent: 2-
3
MB
RB
pentanol (5 cm ).
1
0
0
Kinetic study and thermal effect.
200
6
For kinetic discussion of semihydrogenation of alkynes over
2
Pd@Cu/TiO , photocatalytic reactions were carried out at various
1
50
00
temperatures under irradiation of UV light with a UV LED being
used as the light source for the reaction at controlled elevated
temperatures. Figures 8(a) and 8(b) show time courses of the
amounts of cis-4-octene formed and 4-octyne remaining,
respectively. The reaction rate increased with elevation of the
reaction temperature, and the reactions proceeded almost linearly
40
1
Acetone
20
50
0
H2
with photoirradiation time after the induction period.
Rate
constants were determined from the linear parts of the time
courses based on zero-order kinetics, and an Arrhenius plot is
shown in Figure 8(c). From the slope of the plot, activation
0
0
0.5
Irradiation time / h
1
energy (E
a
)
of the hydrogenation over Pd@Cu/TiO
2
was
-1
estimated to be 25 kJ mol . This value is much smaller than the
value of E
a
of hydrogenation over Cu/TiO
in methanol (54 kJ mol-
2
1)
,[27] indicating that the Pd core greatly contributed to the
Figure 9. Time courses of amounts of 4-octyne (●), cis-4-octene (◆), trans-4-
octene (■), octane (▼), H (○) and acetone (◇) formed and values of MB (▲)
and RB (△) in 2-propanol suspensions of Pd(0.2)@Cu(1.0)/TiO under Ar at 328
2
2
decrease in E
a
of the Cu co-catalyst in hydrogenation. The
difference in the values of E over Pd@Cu/TiO and Cu/TiO
explains the large difference in the reaction rates at 293 K (Figure
). Sharp selectivity for cis-alkenes over Pd@Cu/TiO suggests
a
2
2
K.
5
2
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ChemCatChem
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-
For comparison, photocatalytic reactions in 2-propanol
suspensions of Pd(0.2)@Cu(1.0)/TiO in the absence of 4-octyne
were examined at various temperatures (Figure 10(a)). Under
this condition, H was solely evolved because there was no other
electron acceptor. The results were used for estimation of E
over Pd(0.2)@Cu(1.0)/TiO , and an Arrhenius plot is shown in
Figure 10(b). A linear correlation was observed in the Arrhenius
plot between 298 K and 328 K. From the slope of the plot, E of
2-propanol to protons and acetone, 3) e in the conduction band
of TiO
2
2
transfer to the Pd@Cu co-catalyst, reduce the protons and
form active hydrogen species (AHS) on Pd@Cu co-catalyst, 4)
AHS hydrogenate alkynes to cis-alkenes (4a) and produce H by
2
2
a
self-coupling(4b).
2
a
Conclusions
-
1
H
2
evolution from 2-propanol was estimated to be 22 kJ mol .
The value of E was very close to that of E of photocatalytic
evolution of H from aqueous solutions of 2-propanol over
platinum-loaded TiO
for hydrogenation over Pd(0.2)@Cu(1.0)/TiO
evolution over Pd(0.2)@Cu(1.0)/TiO explains the thermal
a
a
We prepared a TiO
bimetallic nanoparticles (Pd@Cu/TiO
photodeposition method and we examined photocatalytic
hydrogenation of alkynes in alcohol suspensions of Pd@Cu/TiO
photocatalyst having Pd core-Cu shell
2
2
) by using the two-step
2
-
1 [28]
2
(20 kJ mol ).
The slightly larger value of
than that for H
E
a
2
2
2
.
2
Introduction of a Pd core to the Cu co-catalyst resulted in great
acceleration of the semihydrogenation of 4-octyne (about 3-times
greater than that in the case of Cu/TiO ) with maintenance of high
2
selectivity for cis-4-octene. XPS measurement showed that the
Pd core gradually became covered with the Cu shell with an
increase in the Cu content and that parts of the Pd core in
acceleration (increase in the reaction rate at an elevated
temperature) of hydrogenation (Figure 9); however, this does not
explain the high selectivity in hydrogenation of alkynes at 293 K
(
Figure 5(a)). The Arrhenius plot also indicates an important
parameter, i.e., the frequency factor, A. The values of A were
4
-1
3
-1
determined to be 1.9×10 µmol min and 6.2×10 µmol min for
hydrogenation and H evolution, respectively. The almost three-
Pd@Cu/TiO
revealed that introduction of a Pd core greatly decreased the
activation energy in semihydrogenation from 54 kJ mol-1 (Cu/TiO
to 25 kJ mol-1 (Pd@Cu/TiO
), which is the reason for the large
increase in the rate of hydrogenation over Pd@Cu/TiO . A slight
increase in the reaction temperature further increased the
reaction rate and suppressed the side reaction (H evolution),
which is explained by the large difference in the frequency factors
2
were in an electron-deficient state. A kinetic study
2
fold larger value of A is probably caused by strong adsorption of
alkynes on Pd@Cu co-catalyst, which explains the high selectivity
in hydrogenation of alkynes.
2
)
2
2
2
(a)
(b)
1
.5
2
6
4
2
0
0
0
0
3
28 K
18 K
08 K
98 K
1
3
3
2
evolution (1.9×104 µmol min-1 and
photocatalyst can be
of hydrogenation and H
6
2
0.5
0
.2×103 µmol min ). The Pd@Cu/TiO
-1
2
applied for semihydrogenation of various alkynes having other
functional groups and works in various alcohols, producing
corresponding carbonyl compounds.
-
0.5 E =22 kJ mol-1
a
-
1
0
10
20
30
40
3
3.1
3.2
3.3
3.4
Irradiation time / min
10
3
/(T / K)
Figure 10. Time courses of the amounts of (a) H
suspension of a Pd(0.2)@Cu(1.0)/TiO photocatalyst at various temperatures.
b) Arrhenius plot (logarithm of k vs. reciprocal of T)
2
formed from a 2-propanol
Experimental Section
2
(
2 2 2
Preparation of Cu/TiO , Pd/TiO and Pd@Cu/TiO .
2
Pd and Cu as co-catalysts were loaded on TiO (P 25 supplied by Nippon
2
-1
Aerosil Co. Ltd, 50 m g ) by using the photodeposition method. TiO
2
was
3
suspended in 10 cm of a 50 vol% aqueous 2-propanol solution in a Pyrex
test tube, and an aqueous palladium(II) chloride or copper(II) chloride was
added as a metal source. Then the test tube was sealed with a rubber
septum under argon (Ar) and photoirradiated for 2 h at λ> 300 nm using a
high-pressure mercury lamp (Koike Precision Instruments, Hyogo) with
magnetic stirring in a water bath continuously kept at 293 K. The resulting
powder was washed repeatedly with distilled water and dried for 2 h in
vacuo. Pd@Cu/TiO
method[15, 16]: Cu was loaded on 0.2 mol%Pd/TiO
similar way as that used for the preparation of Cu/TiO
Cu (X) was changed up to 1.0 mol% and each thus-prepared sample is
designated as Pd(0.2)@Cu(X)/TiO (X= 0, 0.2, 0.5, 0.7, 1.0 mol%).
2
was prepared by using the two-step photodeposition
(Pd(0.2)/TiO ) in a
The content of
2
2
Figure 11. Proposed mechanism for photocatalytic semihydrogenation of
alkynes to cis-alkenes over Pd@Cu/TiO under irradiation of UV light.
2
.
2
2
The proposed reaction mechanism.
Photocatalytic hydrogenation of 4-octyne.
The proposed reaction mechanism for photocatalytic
semihydrogenation is shown in Figure 11. This reaction system
The details of experimental procedure were described in supporting
information.
A multi-wavelength irradiation monochromator (MM-3,
+
consists of following four steps: 1) positive holes (h ) and
Bunkoukeiki, Tokyo) was used to obtain apparent quantum efficiency
-
+
electrons (e ) are formed under irradiation of UV light, 2) h oxidize
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ChemCatChem
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(
(
AQE), and light intensity was determined by using a spectroradiometer
USR-45D, Ushio, Tokyo).
[7]
[8]
G. Li, R. Jin, J. Am. Chem. Soc. 2014, 136, 11347-11354.
A. Fedorov, H.-J. Liu, H.-K. Lo, C. Coperet, J. Am. Chem. Soc. 2016, 138,
6502-16507.
Y. Liu, A. J. McCue, C. Miao, J. Feng, D. Li, J. A. Anderson, J. Catal.
018, 364, 406-414.
1
[
[
[
[
[
[
[
[
[
9]
Characterization.
2
10] Y. Lu, X. Feng, B. S. Takale, Y. Yamamoto, W. Zhang, M. Bao, ACS
Catal. 2017, 7, 8296-8303.
2 2
Morphology of the Pd/TiO and Pd@Cu/TiO samples was observed under
a JEM-3010 (JEOL, Tokyo) TEM operated at 200 kV in the Joint Research
Center (JRC) at Kindai University. X-ray photoelectron spectroscopy
11] K. Imamura, Y. Okubo, T. Ito, A. Tanaka, K. Hashimoto H. Kominami,
RSC Adv. 2014, 4, 19883-19886.
(XPS) spectra of Cu/TiO
2 2 2
, Pd/TiO and Pd@Cu/TiO were measured using
12] H. Kominami, S. Yamamoto, K. Imamura, A. Tanaka, K. Hashimoto,
Chem. Commun. 2014, 50, 4558-4560.
an AXIS-NOVA ESCA (Shimadzu) in JRC at Kindai University.
13] K. Nakanishi, R. Yagi, K. Imamura, A. Tanaka, K. Hashimoto, H.
Kominami, Catal. Sci. Technol. 2018, 8, 139-146.
2
Catalytic hydrogenation of 4-octyne under H .
14] B. Ohtani, K. Iwai, S. Nishimoto, S. Sato, J. Phys. Chem. B 1997, 101,
Metal-loaded TiO
containing 4-octyne (ca. 50 µmol) in a test tube, which was sealed with a
rubber septum under H Then the mixture was stirred in a water bath at
93 K in the dark. The amount of 4-octyne was determined in the same
(50 mg) was suspended in 5 cm3 of 2-propanol
2
3349-3359.
15] Y. Sugano, K. Fujiwara, Y. Shiraishi, S. Ichikawa, T. Hirai, Catal. Sci.
Technol. 2013, 3, 1718-1724.
2
.
2
16] H. Hirakawa, Y. Shiraishi, H. Sakamoto, S. Ichikawa, S. Tanaka, T. Hirai,
Chem. Commun. 2015, 51, 2294-2297.
way as that for the photocatalytic reaction.
17] H. Kominami, M. Higa, T. Nojima, T. Ito, K. Nakanishi, K. Hashimoto, K.
Imamura, ChemCatChem 2016, 8, 2019-2022.
[
18] P. Reyes, G. Pecchi, J. L. G. Fierro, Langmuir 2001, 17, 522-527.
19] K. Kusada, H. Kobayashi, R. Ikeda, Y. Kubota, M. Takata, S. Toh, T.
Yamamoto, S. Matsumura, N. Sumi, K. Sato, K. Nagaoka, H. Kitagawa,
J. Am. Chem. Soc. 2014, 136, 1864-1871.
Acknowledgements
[
This work was partly supported by JSPS KAKENHI Grant
Numbers 17H03462, 15J11412 and 17H04967. This work was
also supported by MEXT-Supported Program for the Strategic
Research Foundation at Private Universities 2014-2018, subsidy
from MEXT and Kindai University. K. N. is grateful to the Japan
Society for the Promotion of Science (JSPS) for a Research
Fellowship for young scientists. A.T. is grateful for financial
support from the Faculty of Science and Engineering, Kindai
University.
[20] M. Sankar, N. Dimitratos, P. Miedziak, P. Wells, C. Kielye, G. Hutchings,
Chem. Soc. Rev. 2012, 41, 8099-8139.
[
21] S. Alayoglu, B. Eichhorn, J. Am. Chem. Soc. 2008, 130, 17479-17486.
22] T. Mitsudome, T. Urayama, K. Yamazaki, Y. Maehara, J. Yamasaki, K.
Gohara, Z. Maeno, T. Mizugaki, K. Jitsukawa, K. Kaneda, ACS Catal.
[
2
016, 6, 666-670.
23] A. Tanaka, K. Hashimoto, H. Kominami, ChemCatChem 2011, 3, 1619-
623.
[24] A. Tanaka, K. Hashimoto, H. Kominami, Chem. Eur. J. 2016, 22, 4592-
599.
25] S. H. Y. Lo, Y. Y. Wang, C. C. Wan, J. Colloid Interface Sci. 2007, 310,
90-195.
[
1
4
[
[
Keywords: photocatalyst • TiO
2
• alkyne • semihydrogenation •
1
cis-alkene • core-shell nanoparticle • ensemble effect
26] L. S. Kibis, A. I. Titkov, A. I. Stadnichenko, S. V. Koscheev, A. I. Boronin,
Appl. Surf. Sci. 2009, 255, 9248-9254.
[1]
M. Crespo-Quesada, F. Cardenas-Lizana, A.-L. Dessimoz, L. Kiwi-
Minsker, ACS Catal. 2012, 2, 1773-1786.
[27] H. Kominami, M. Shiba, A. Hashimoto, S. Imai, K. Nakanishi, A. Tanaka,
K. Hashimoto, K. Imamura, Phys. Chem. Chem. Phys. 2018, 20, 19321-
[
2]
3]
H. Lindlar, Helv. Chim. Acta. 1952, 35, 446−450.
19325.
[
K. C. K. Swamy, A. S. Reddy, K. Sandeep, A. Kalyani, Tetrahedron Lett.
[28] F. H. Hussein, R. Rudham, J. Chem. Soc., Faraday Trans. 1 1984, 80,
2018, 59, 419-429.
2817-2825.
[4]
[5]
[6]
L. Lin, X. Zeng, B. Xu, J. Org. Chem. 2019, 84, 11240-11246.
S. Liang, G. B. Hammond, B. Xu, Chem. Commun. 2016, 52, 6013-6016.
T. Mitsudome, Y. Takahashi, S. Ichikawa, T. Mizugaki, K. Jitsukawa, K.
Kaneda, Angew. Chem. Int. Ed. 2013, 52, 1481-1485.
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S. Imai, K. Nakanishi, A. Tanaka, and H.
Kominami*
Page No. – Page No.
A “healthy” and “strong” photocatalyst:
Rapid selective hydrogenation of
alkynes to alkenes was achieved over
Accelerated Semihydrogenation of
Alkynes over a
Copper/Palladium/Titanium(IV) Oxide
Photocatalyst Free from a Poison and
a Cu/Pd/TiO
the use of poisons and H
used in the case of Lindlar’s catalyst.
2
photocatalyst without
2
gas often
2
H Gas
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