Visible light-induced diastereoselective semihydrogenation of alkynes to cis-alkenes over an organically modified titanium(IV) oxide photocatalyst having a metal co-catalyst

Article abstract of DOI:10.1016/j.jcat.2019.04.022

Hydrogen (H₂)-free and poison (lead and quinoline)-free semihydrogenation of alkynes to cis-alkenes under gentle conditions is one of the challenges to be solved. In this study, a titanium(IV) oxide photocatalyst having two functions (visible light responsiveness and semihydrogenation activity) was prepared by modification with 2,3-dihydroxynaphthalene (DHN) and a copper (Cu) co-catalyst, respectively. The photocatalyst (DHN/TiO₂-Cu) showed high performance for diastereoselective semihydrogenation of alkynes to cis-alkenes in water-acetonitrile solution under visible light irradiation without the use of H₂ and poisons. Alkynes having reducible functional groups were converted to the corresponding alkenes with the functional groups being preserved. The addition of water to acetonitrile changed the amount of alkynes adsorbed on the photocatalyst, which was a decisive factor determining the rate of hydrogenation. A relatively large apparent activation energy, 27 kJ mol^?1, was obtained by a kinetic study, indicating that the rate-determining step of this reaction was not an electron production process but a thermal catalytic semihydrogenation process over the Cu co-catalyst. Semihydrogenation and hydrogen evolution occurred competitively on Cu metals and the former became predominant at slightly elevated temperatures, which is discussed on the basis of the kinetic parameters of two reactions.

Full text of DOI:10.1016/j.jcat.2019.04.022

Journal of Catalysis 374 (2019) 36–42
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Journal of Catalysis
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Visible light-induced diastereoselective semihydrogenation of alkynes to
cis-alkenes over an organically modified titanium(IV) oxide
photocatalyst having a metal co-catalyst
⇑
Makoto Fukui, Yuya Omori, Shin-ya Kitagawa, Atsuhiro Tanaka, Keiji Hashimoto, Hiroshi Kominami
Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, Higashiosaka, Osaka 577-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrogen (H2)-free and poison (lead and quinoline)-free semihydrogenation of alkynes to cis-alkenes
under gentle conditions is one of the challenges to be solved. In this study, a titanium(IV) oxide photo-
catalyst having two functions (visible light responsiveness and semihydrogenation activity) was prepared
by modification with 2,3-dihydroxynaphthalene (DHN) and a copper (Cu) co-catalyst, respectively. The
photocatalyst (DHN/TiO2-Cu) showed high performance for diastereoselective semihydrogenation of
alkynes to cis-alkenes in water-acetonitrile solution under visible light irradiation without the use of
H2 and poisons. Alkynes having reducible functional groups were converted to the corresponding alkenes
with the functional groups being preserved. The addition of water to acetonitrile changed the amount of
alkynes adsorbed on the photocatalyst, which was a decisive factor determining the rate of hydrogena-
tion. A relatively large apparent activation energy, 27 kJ molꢀ1, was obtained by a kinetic study, indicat-
ing that the rate-determining step of this reaction was not an electron production process but a thermal
catalytic semihydrogenation process over the Cu co-catalyst. Semihydrogenation and hydrogen evolution
occurred competitively on Cu metals and the former became predominant at slightly elevated tempera-
tures, which is discussed on the basis of the kinetic parameters of two reactions.
Received 15 February 2019
Revised 9 April 2019
Accepted 15 April 2019
Keywords:
Photocatalyst
TiO2
Visible light
Co-catalyst
Semihydrogenation
Alkyne
Syn-addition
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
[9] and gold [10–12] have been reported. However, these catalytic
systems require harsh reaction conditions such as high pressure of
Hydrogenation of alkynes is one of the important reactions in
the chemical industry. Semihydrogenation of alkynes to alkenes
has been applied to syntheses of cis-alkenes, which are important
building blocks for fine chemicals such as bioactive molecules, fla-
vors and natural products [1,2]. The Lindlar catalyst consisting of
palladium-loaded calcium carbonate (Pd/CaCO3) treated by the
addition of lead acetate (Pb(OCOCH3)2) and quinoline has been
widely used for the semihydrogenation of alkynes [3,4]. The Lind-
lar catalyst shows high performance for semihydrogenation of
internal alkynes to alkenes, whereas terminal alkynes are unfortu-
nately completely hydrogenated to alkanes because of the high
activity of a Pd catalyst. In addition, this catalyst has serious weak
points including the requirements of undesirable additives such as
a toxic Pb salt and a large amount of quinoline to suppress over-
hydrogenation of alkenes. To avoid the use of undesirable addi-
tives, a number of alternative Pb-free catalytic semihydrogenations
using metal catalysts such as copper (Cu) [5,6], Pd [7,8], ruthenium
dihydrogen (H2) and high temperature in a closed reactor. There-
fore, milder catalytic reaction systems for semihydrogenation of
alkynes are favored.
Aiming at the syntheses of organic compounds under mild reac-
tion conditions, we have focused on the application of photocat-
alytic reactions over a semiconductor such as titanium(IV) oxide
(TiO2) to conversion of organic compounds. When TiO2 powder is
irradiated by UV light, charge separation occurs and thus-formed
electrons in the conduction band (CB) and positive holes in the
valence band (VB) cause reduction and oxidation, respectively.
Since these photocatalytic reactions are driven by sunlight as the
energy source at room temperature and under atmospheric pres-
sure, in addition to mineralization of volatile organic compounds
and water splitting, photocatalytic conversion of organic com-
pounds has attracted much attention as the third application of
TiO2. Since irradiation of light is indispensable for a photocatalytic
reaction, the reaction can be easily controlled by the intensity of
light and finally can be stopped by turning the light source off. Fur-
thermore, by simple filtration or centrifugation, a TiO2 photocata-
lyst is easily separated from the reaction mixture after the
⇑
Corresponding author.
E-mail address: hiro@apch.kindai.ac.jp (H. Kominami).
https://doi.org/10.1016/j.jcat.2019.04.022
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

M. Fukui et al. / Journal of Catalysis 374 (2019) 36–42
37
reaction and reused. The above-mentioned characteristics of pho-
tocatalysis satisfy almost all of the 12 proposed requirements for
green chemistry [13]. We have found that a TiO2 photocatalyst
exhibited high performance for various conversions of organic
compounds under irradiation of UV light. For example, a TiO2 pho-
tocatalyst chemoselectively reduced nitrobenzenes [14] and ben-
of AHS from H+ and semihydrogenation of alkynes to alkenes with
high selectivity.
Based on the above-described background, visible light-induced
semihydrogenation of alkynes to cis-alkenes over DHN/TiO2 having
a Cu co-catalyst (DHN/TiO2-Cu) without the use of toxic Pb at room
temperature and under atmospheric pressure was investigated in
this study. Here, we report (1) semihydrogenation of 1-phenyl-1-
propyne (PP) over a DHN/TiO2-Cu photocatalyst under irradiation
of visible light, (2) expandability of DHN/TiO2-Cu for semihydro-
genation of various alkynes to corresponding alkenes, (3) the effect
of water content of acetonitrile solutions on semihydrogenation
and adsorption of PP and (4) the effect of reaction temperature
on semihydrogenation of PP.
zaldehydes [15] having other functional groups such as
a
chlorine group to corresponding aminobenzenes and benzyl alco-
hols. We also showed that the combination of photocatalysis of
TiO2 and thermocatalysis of Pd nanoparticles drove difficult reduc-
tions, i.e., a TiO2 photocatalyst having a Pd co-catalyst (Pd-TiO2)
successfully hydrogenated benzonitrile [16], styrene [17] and furan
[18] to corresponding hydrogenation products. In other words,
photogenerated electrons reduced protons to active hydrogen spe-
cies (AHS) on the surface of Pd nanoparticles and formed AHS ther-
mocatalytically hydrogenated target organic compounds.
However, a Pd-TiO2 photocatalyst is not suitable for semihydro-
genation of alkynes to alkenes since it exhibits high performance
in photocatalytic hydrogenation of unsaturated bonds such as
vinyl and nitrile groups due to the strong catalytic activity of Pd.
Recently, we developed an additive (Pb and quinoline)-free photo-
catalytic system for selective semihydrogenation of various alky-
nes to corresponding alkenes in an alcoholic suspension of a
copper-loaded TiO2 (Cu-TiO2) photocatalyst (Scheme 1(a)) [19,20].
The next strategy for photocatalytic conversion of organic com-
pounds is efficient utilization of visible light since sunlight consists
of 5% UV light, 50% visible light and 45% infrared light at the sur-
face of the earth. Of course, no hydrogenation of alkynes occurs
over a Cu-TiO2 photocatalyst under irradiation of visible light
because a TiO2 photocatalyst shows no absorption in the region
of visible light. Recently, we found that a 2,3-dihydroxynaphtha
lene-modified TiO2 (DHN/TiO2) photocatalyst exhibited an activity
for chemoselective reduction of benzaldehyde having other func-
tional groups to corresponding benzyl alcohols under irradiation
of visible light [21], following the reduction of nitrobenzene over
DHN/TiO2 reported by Kamegawa et al. [22]. We chose DHN among
various photosensitizers for TiO2 because DHN is obtained easily
and modification of TiO2 with DHN is very easy without any special
care. Therefore, the strategy for the design of a visible light-
induced photocatalytic system for semihydrogenation of alkynes
to alkenes is simple as shown in Scheme 1(b): (1) charge transfer
from DHN to the CB of TiO2 under visible light irradiation and (2)
utilization of photogenerated electrons in the CB for production
2. Experimental
2.1. Preparation of DHN/TiO2-Cu by impregnation and
photodeposition
TiO2 (ST-01, 330 m2 gꢀ1, Ishihara, Osaka) powder was sus-
pended in a methanol solution containing DHN (0.7 wt%) in an
evaporating dish, and methanol was vapored over a water bath
at 313 K until the powder dried, and DHN/TiO2 was obtained. Then
Cu (1.0 wt%) was loaded on DHN/TiO2 using the photodeposition
method. DHN/TiO2 was suspended in 10 cm3 of water-methanol
(1:9) solution containing copper chloride (CuCl2) in a test tube.
The test tube was sealed with a rubber septum under argon (Ar)
and then photoirradiated for 1 h at k > 300 nm using a 400 W
high-pressure mercury arc (Eiko-sha, Osaka) with magnetic stir-
ring in a water bath continuously kept at 298 K. The resulting pow-
der was washed repeatedly with distilled water and dried for 1 h in
vaccuo.
2.2. Characterization
Diffuse reflectance spectra of the samples were obtained with a
UV–vis spectrometer (UV-2400, Shimadzu, Kyoto) equipped with a
diffuse reflectance measurement unit (ISR-2000, Shimadzu) in
which barium sulfate (BaSO4) was used as a reference. Fourier
transform infrared (FT-IR) spectra of TiO2, DHN/TiO2 and DHN/
TiO2-Cu were measured on an FT-IR spectrometer (FT-IR8300, Shi-
madzu) in which a potassium bromide (KBr) pellet method was
used. X-ray photoelectron spectroscopy (XPS) spectra of TiO2,
DHN/TiO2 and DHN/TiO2-Cu were measured using a Shimadzu
AXIS-NOVA ESCA in the Joint Research Center of Kindai University.
Powder X-ray diffraction (XRD) was measured using CuK
a radia-
tion by a Rigaku MultiFlex equipped with a carbon monochroma-
tor. Morphology of sample was observed under a high-resolution
transmission electron microscope (JEM-2100F, JEOL, Tokyo) oper-
ated at 200 kV in the Joint Research Center of Kindai University.
2.3. Photocatalytic hydrogenation of alkynes in an aqueous
acetonitrile suspension of DHN/TiO2-Cu under visible light irradiation
In a typical run, prepared DHN/TiO2-Cu (50 mg) was suspended
in water-acetonitrile (1:9) solution (5 cm3) containing alkyne (ca.
50 lmol) and triethanolamine (TEOA, 150 lmol) as a sacrificial
reagent in a test tube, and the test tube was sealed with a rubber
septum under argon and then photoirradiated with visible light
of the two blue LEDs (420–530 nm, 29.0 and 56.8 mW cmꢀ2, Haya-
shi Watch Works, Tokyo) from two directions with magnetic stir-
ring at 329 K. The emission spectra of light from two blue LEDs
are shown in Fig. S1. The amount of H2 was measured with an
FID-type gas chromatograph (GC-8A, Shimadzu, Kyoto) equipped
with an MS-5A column. After the suspension had been filtered to
Scheme 1. Semihydrogenation of alkyne to alkene over photocatalysts charge-
separated by different systems: (a) band-gap excitation of TiO2 and (b) charge
transfer from a sensitizer to the conduction band of TiO2.

38
M. Fukui et al. / Journal of Catalysis 374 (2019) 36–42
remove the catalyst, the amounts of PP unreacted and hydrogena-
tion products (cis-alkene, trans-alkene and alkane) formed in the
liquid phase were determined with an FID-type gas chromatograph
(GC-2025, Shimadzu) equipped with a DB-1 column. Toluene was
used as an internal standard substance. Toluene (5 mm3), water
(1 cm3) and diethyl ether (3 cm3) were added to the reaction solu-
tion (2 cm3). After the mixture had been stirred for 10 min, the
diethyl ether phase of the reaction solution was analyzed. The
amounts of alkyne, alkene and alkane were determined from the
ratios of the peak areas of these products to the peak area of
toluene.
1
50
40
30
20
10
0
cis-MS
PP
0.5
0
H2
trans-MS
PB
0 1 2 3 4 5 6
Irradiation time / h
Fig. 1. Time courses of amounts of PP, cis-MS, trans-MS, PB and H2 in photocatalytic
hydrogenation of PP in a water-acetonitrile (1:9) suspension of DHN/TiO2-Cu under
visible light irradiation with magnetic stirring in a water bath continuously kept at
329 K.
2.4. Adsorption experiment of PP on DHN/TiO2-Cu in the dark
A DHN/TiO2-Cu sample (300 mg) was suspended in 5 cm3 of
water-acetonitrile solution containing TEOA (150 lmol) in a test
tube, and the test tube was sealed with a rubber septum under
argon. To reduce partially oxidized Cu on the surface to the metal,
the suspension was photoirradiated with visible light of the same
blue LEDs with magnetic stirring at 298 K for 1 h. After water-
PP, there was no formation of trans-1-phenyl-1-propene (trans-b-
methylstyrene, trans-MS) or propylbenzene (PB) as a result of iso-
merization or sequential hydrogenation of cis-MS. These results
indicate that semihydrogenation of PP over DHN/TiO2-Cu pro-
ceeded with high diastereoselectivity. In order to reveal the selec-
tivity for the target compound, an indicator, i.e., material balance
(MB) calculated by using Equation (1) is also shown in Fig. 1.
acetonitrile solution (0.1 cm3) containing PP (50
lmol) had been
injected into the test tube, the test tube was magnetically stirred
in a water bath continuously kept at 298 K for 20 h in the dark.
After the suspension had been filtered to remove particles, the
amount of PP in the liquid phase was determined with an FID-
type gas chromatograph (GC-2025) equipped with a DB-1 column.
Toluene (5 mm3), water (1 cm3) and diethyl ether (3 cm3) were
added to the reaction solution (2 cm3). After the mixture had been
stirred for 10 min, the diethyl ether phase of the reaction solution
was analyzed. The amount of PP was determined from the ratio of
the peak area of PP to the peak area of toluene.
nðPPÞ þ nðcis ꢀ MSÞ þ nðtrans ꢀ MSÞ þ nðPBÞ
MB ¼
ð1Þ
n0ðPPÞ
where n(PP), n(cis-MS), n(trans-MS) and n(PB) are the amounts of
PP, cis-MS, trans-MS and PB during the photocatalytic reaction,
respectively, and n0(PP) is the amount of PP before the photocat-
alytic reaction. The values of MB close to unity before and after
the complete consumption of PP indicate that only hydrogenation
of an alkynyl group to an alkenyl group occurred. As also shown
in Fig. 1, in addition to semihydrogenation of PP, AHS over Cu were
coupled, resulting in H2 evolution. This means that semihydrogena-
tion and H2 evolution are competitive reactions in utilization of
photogenerated electrons.
3. Results and discussion
3.1. Characterization
Fig. S2 shows the FT-IR spectra of unmodified TiO2, DHN/TiO2
and DHN/TiO2-Cu. A band attributable to DHN fixed on the surface
of TiO2 [23] was observed at 1284 cmꢀ1 for the DHN modified sam-
ples. Fig. S3 shows the Cu 2p XPS spectra of unmodified TiO2, DHN/
TiO2 and DHN/TiO2-Cu. Peaks assignable to Cu 2p3/2 and Cu 2p1/2
[24] were observed at 954.9 and 934.5 eV. These results indicate
that DHN and Cu are loaded on the surface of TiO2 in DHN/TiO2-
Cu. Fig. S4 shows XRD patterns of unmodified TiO2, DHN/TiO2
and DHN/TiO2-Cu. Peaks attributable to the anatase phase were
observed in all samples, while other peaks were not observed, indi-
cating that Cu particles were highly dispersed on the surface of
TiO2 having a large specific surface area of 330 m2 gꢀ1. Fig. S5
shows a TEM photograph of DHN/TiO2-Cu. TEM observation
revealed that fine Cu particles were loaded on TiO2, which was in
agreement with the result of XRD analysis.
The effects of various reaction conditions on the photocatalytic
semihydrogenation of PP to cis-MS were investigated, and the
results are summarized in Table 1. Photoirradiation for 4 h pro-
duced cis-MS almost quantitatively as shown in Fig. 1 (Entry 1).
Entries 2–5 show the results of four blank reactions: (1) dark reac-
tion (thermal catalytic reaction) over DHN/TiO2-Cu (Entry 2), (2)
photocatalytic reaction over DHN/TiO2 (effect of a Cu co-catalyst,
Entry 3), (3) photocatalytic reaction over Cu-TiO2 (effect of DHN
as a photoabsorption site, Entry 4) and (4) in the absence of
DHN/TiO2-Cu with irradiation of blue light (photochemical reac-
tion, Entry 5). The results indicate that TiO2, DHN, Cu co-catalyst
and photoirradiation are indispensable for the semihydrogenation
of PP under visible light irradiation. We also carried out thermal
catalytic hydrogenation of PP over DHN/TiO2-Cu under an H2
atmosphere (1 atm) at various temperatures (Entries 6–9). Even
at 323 K after 6 h, no cis-MS was formed, indicating that no PP
was hydrogenated by AHS formed from H2 over a Cu co-catalyst
under these conditions and that more severe conditions are
required for activation of H2 to thermocatalytically hydrogenate
PP over Cu [5,6].
It is important to confirm that an observed reaction was driven
by photocatalysis. An action spectrum obtained by using
monochromatic light is useful tool for determining whether an
observed reaction occurs by a photoinduced or a thermal catalytic
process. To obtain an action spectrum in this reaction system,
hydrogenation of PP in a water-acetonitrile (1:9) suspension of
DHN/TiO2-Cu was carried out at 298 K under irradiation of
monochromatic visible light from a Xe lamp with a light width of
15 nm using a multi-wavelength irradiation monochromator
3.2. Photocatalytic hydrogenation of PP in a water-acetonitrile (1:9)
suspension of DHN/TiO2-Cu
Fig. 1 shows the time courses of photocatalytic hydrogenation
of PP in a water-acetonitrile (1:9) suspension of DHN/TiO2-Cu with
irradiation of light from a blue LED under deaerated conditions
with magnetic stirring in a water bath continuously kept at
329 K. The amount of PP monotonically decreased with photoirra-
diation, while cis-1-phenyl-1-propene (cis-b-methylstyrene, cis-
MS) was formed in an amount corresponding to consumption of
the amount of PP. When PP had been completely consumed after
4-h photoirradiation, cis-MS was obtained in a high yield (>99%).
During the blue light irradiation and even after consumption of

M. Fukui et al. / Journal of Catalysis 374 (2019) 36–42
39
Table 1
Effects of various reaction conditions on photocatalytic production of cis-MS from PP in a water-acetonitrile (1:9) suspension of DHN/TiO2-Cu.
Entry
Catalyst
Blue light
Gas phase
Temperature/K
Time/h
PP conv. /%
cis-MS yield /%
1
2
3
4
5
6
7
8
9
DHN/TiO2-Cu
DHN/TiO2-Cu
DHN/TiO2
Cu-TiO2
–
DHN/TiO2-Cu
DHN/TiO2-Cu
DHN/TiO2-Cu
DHN/TiO2-Cu
On
(Dark)
On
On
On
(Dark)
(Dark)
(Dark)
(Dark)
Ar
Ar
Ar
Ar
Ar
H2
H2
H2
H2
329
329
329
329
329
298
303
313
323
4
4
4
4
4
6
6
6
6
>99
–
–
–
–
–
–
–
–
>99
–
–
–
–
–
–
–
–
(MM-3PK, Bunkoukeiki, Tokyo). The light intensity determined
with a spectroradiometer (USR-45D, Ushio, Tokyo) and the time
course of the amount of cis-MS formed under irradiation of
monochromatic light are shown in Fig. 2(a) and (b), respectively.
cis-MS was formed linearly along with irradiation of light having
wavelengths of 416, 437, 469, 492 and 521 nm, whereas no forma-
tion of cis-MS was observed under irradiation of light having wave-
lengths of 542 and 572 nm. These results indicate that DHN/TiO2-
Cu was induced by visible light having k < 521 nm. The apparent
quantum efficiency (AQE) at each centered wavelength of light
was calculated from the ratio of the amount of cis-MS and the
amount of incident photons by using Eq. (2).
A durability test of a (photo)catalyst is very important for its
practical applications of (photo)catalyst. To evaluate the durability
of DHN/TiO2-Cu, after photocatalytic hydrogenation of PP to cis-
MS, DHN/TiO2-Cu was recovered from the reaction mixture by sim-
ple filtration and washed with distilled water and used repeatedly
for the same reaction. Conversion and selectivity were maintained
at ca. 100% in the second, third and fourth reactions (Fig. 3(a)), and
light absorption spectra of DHN/TiO2-Cu after the repeated reac-
tions were almost the same as the spectrum before the reaction
(Fig. 3(b)). These results indicate that DHN/TiO2-Cu worked as a
reusable photocatalyst at least 4 times without losing its activity
and diastereoselectivity.
In addition, to evaluate the possibility of practical use of
diastereoselective semihydrogenation of alkynes to cis-alkenes
under visible light irradiation, we carried out photocatalytic hydro-
2 ꢁ the amount of cis ꢀ MS formed
AQE ¼
ꢁ 100
ð2Þ
the amount of incident photons
genation of a large amount of PP (550 lmol) with the volume of
the reaction mixture being unchanged (5 cm3). The results are
shown in Fig. S6. After photoirradiation for 80 h, PP was almost
quantitatively hydrogenated to cis-MS over the DHN/TiO2-Cu pho-
tocatalyst even in the presence of a large amount of PP. Under this
condition, the turnover numbers (TON) of cis-MS formation based
The results are shown in Fig. 2(c). Values of AQE measured at
416, 437, 469, 492 and 521 nm were determined to be 2.3%, 1.4%,
0.52%, 0.32% and 0.11%, respectively. Wavelength-dependency of
AQE was similar to the photoabsorption of DHN/TiO2-Cu (Fig. 2
(d)), indicating that hydrogenation of PP in a water-acetonitrile
(1:9) suspension was induced by visible-light absorption of DHN/
TiO2-Cu.
on the amounts of DHN (0.7 wt%, 2.2
lmol) and Cu (1.0 wt%,
7.9 mol) after 80-h photoirradiation were calculated to be 250
l
and 70, respectively. These values indicate that DHN/TiO2-Cu
works as a catalytic material for semihydrogenation of alkynes to
cis-alkenes.
20
416 nm
(a)
(b)
437 nm
469 nm
492 nm
521 nm
542 nm
572 nm
300
200
100
0
15
10
5
3.3. Effect of water content in an acetonitrile suspension of DHN/TiO2-
Cu on photocatalytic hydrogenation of PP
In a catalytic reaction, a reaction solvent, not appearing in the
chemical reaction formula, often affects the reaction rate, product
selectivity, adsorption of the substrate and product and so on. Pre-
viously, we reported that various types of solvents such as water,
methanol and acetonitrile have positive and negative effects on
0
400
500
600
0
10 20 30 40 50
Irradiation time / h
Wavelength / nm
3
2
1
0
1
(c)
(d)
2
(b)
(a)
100
50
0
0.5
0
Fresh
2nd
3rd
1
0
4th
400 500 600
Wavelength / nm
400 500 600
Wavelength / nm
400
500
600
Fresh 2nd 3rd 4th
Wavelength / nm
Fig. 2. (a) Light intensity of visible light from a multi-wavelength irradiation
monochromator (MM-3PK, Bunkoukeiki Co., Ltd), (b) time course of the amount of
cis-MS formed in a water-acetonitrile (1:9) suspension of DHN/TiO2-Cu under
irradiation of each monochromatic light, (c) action spectrum in the hydrogenation
of PP (ca. 50
DHN/TiO2-Cu.
Fig. 3. (a) Durability test of DHN/TiO2-Cu in photocatalytic hydrogenation of PP to
cis-MS in a water-acetonitrile (1:9) suspension of DHN/TiO2-Cu for 6-h photoirra-
diation of visible light and (b) light absorption spectra of DHN/TiO2-Cu before and
after hydrogenation of PP.
lmol) to cis-MS over DHN/TiO2-Cu and (d) absorption spectrum of

40
M. Fukui et al. / Journal of Catalysis 374 (2019) 36–42
photocatalytic ring hydrogenation of aromatics over a rhodium
(Rh)-loaded TiO2 photocatalyst [25]. Here, we investigated effects
of the water content of water-acetonitrile solution on the produc-
tion of cis-MS over DHN/TiO2-Cu and adsorption of PP and TEOA on
DHN/TiO2-Cu (Fig. 4). The addition of a small amount of water to
acetonitrile drastically increased the yield of cis-MS as shown in
Fig. 4(a). The maximum yield of cis-MS (15 mmol) was obtained
in water-acetonitrile (1:9) solution, and that yield was about 6-
times larger than that obtained in water-free acetonitrile
(2.4 mmol). Positive effects of the addition of water to acetonitrile
have been observed in various photocatalytic reductions [14,26–
28], photocatalytic hydroxylation of aromatic compounds [29,30]
and hydration of alkenes [31] in the presence of water over
metal-loaded TiO2 have recently been reported. On the other hand,
hydroxylated and hydrated compounds were not observed in the
present system, i.e., the addition of water to acetonitrile did not
affect the selectivity of cis-MS but affected the rate of the reaction.
Similar results were obtained in the case of water-2-propanol solu-
tion (Fig. S7).
In photocatalytic ring hydrogenation of aromatics over Rh-TiO2,
solvents greatly influenced the behavior of adsorption of aromatics
and hole scavengers, which resulted in a large change in the pho-
tocatalytic performance [25]. Hence, we observed the effect of
the water content on adsorption of the substrate and hole scav-
enger in the dark for 24 h in this system. Fig. 4(b) and (c) show
the amounts of PP and TEOA adsorbed on the surface of the
DHN/TiO2-Cu photocatalyst in aqueous acetonitrile solutions hav-
ing various water contents. The addition of a small amount of
water greatly increased the amount of PP adsorbed on DHN/TiO2-
Cu. This result is probably caused by the change in solubility of
PP in the solvents, i.e., PP was concentrated on the surface of the
photocatalyst due to the slightly decreased solubility in acetoni-
trile having a small amount of water. On the other hand, the
amount of TEOA adsorbed on DHN/TiO2-Cu decreased with an
increase in the water content due to the increase in solubility in
solvents containing water. Since a similar tendency was observed
in yields of cis-MS and the amount of PP adsorbed in the dark,
the yields of cis-MS were plotted against the amounts of PP
adsorbed on DHN/TiO2-Cu (Fig. 4(d)). An almost linear correlation
between them was obtained, indicating that adsorption of the sub-
strate controlled the rate of the photocatalytic reaction and that
the amount of adsorption can be easily increased only by the addi-
tion of a small amount of water to acetonitrile.
3.4. Applicability of photocatalytic hydrogenation of alkynes in a
water-acetonitrile (1:9) suspension of DHN/TiO2-Cu
In order to evaluate the applicability of a DHN/TiO2-Cu photo-
catalyst in semihydrogenation under visible light irradiation, vari-
ous alkynes were used as substrates and the results are shown in
Table 2. Three internal alkynes including PP were converted to
the corresponding cis-alkenes in high yields (Entries 1–3). In these
reactions, no trans-alkenes and alkanes were produced, indicating
that the DHN/TiO2-Cu photocatalyst has wide applicability for
diastereoselective hydrogenation of alkynes to the corresponding
cis-alkenes. Terminal alkynes were also hydrogenated to the corre-
sponding terminal alkenes in high yields (Entries 4–7). In addition,
this photocatalytic system exhibited high chemoselectivity for an
alkynyl group because other functional groups such as phenyl,
hydroxy, chloro and cyano groups were not influenced. There were
possibilities that a hydroxy group is oxidized to a carbonyl group
by positive holes and that chloro and cyano groups are reductively
eliminated or hydrogenated by excited electrons in the CB of the
Table 2
Photocatalytic hydrogenation of various alkynes in water-acetonitrile (1:9) suspen-
sions of DHN(0.7)/TiO2-Cu(1.0) under visible-light irradiation.
12
10
8
6
4
(b)
(a)
15
10
5
2
0
0
0
10 20 30 40 50
0
10 20 30 40 50
Water content / vol%
Water content / vol%
150
100
50
20
50
40
30
20
10
0
50
40
30
20
10
0
(c)
(d)
(a)
(b)
10
30
50
20
15
10
5
328 K
313 K
303 K
5
2
328 K
315 K
312 K
1
299 K
301 K
0
0
0
0
2
4
6
8 10 12
0
10 20 30 40 50
Water content / vol%
0
1
2
0
1
2
Adsorbed PP / mol
Irradiation time / h
Irradiation time / h
Fig. 4. Effect of water content of water-acetonitrile solution on (a) cis-MS yield in
photocatalytic hydrogenation of PP over DHN/TiO2-Cu under visible light irradia-
tion, (b) amount of PP adsorbed on the surface of DHN/TiO2-Cu in the dark and (c)
amount of TEOA adsorbed on the surface of DHN/TiO2-Cu in the dark. (d)
Correlation between the yield of cis-MS and amount of PP adsorbed. The values
mean water content.
Fig. 5. Effects of reaction temperature on (a) hydrogenation of PP (s) to cis-MS (d)
in a water-acetonitrile (1:9) suspension of a DHN/TiO2-Cu photocatalyst in the
presence of TEOA under visible light irradiation and (b) evolution of H2 from TEOA
in a water-acetonitrile (1:9) suspension of a DHN/TiO2-Cu photocatalyst in the
absence of PP under visible light irradiation.

M. Fukui et al. / Journal of Catalysis 374 (2019) 36–42
41
photocatalyst. For example, we have reported that Pd-TiO2 photo-
catalytically hydrogenated benzonitrile to benzyl amine [16] and
that DHN/TiO2 having Pd as a co-catalyst eliminated a chloro group
in chlorobenzaldehyde to form benzaldehyde [21]. As far as we
know, this is the first report showing the achievement of diastere-
oselective semihydrogenation of alkynes to cis-alkenes under visi-
ble light irradiation without the use of H2 and undesirable
additives.
3.5. Effect of reaction temperature on photocatalytic hydrogenation of
PP over DHN/TiO2-Cu
In recent years, we have reported several selective hydrogena-
tions of organic compounds having various functional groups such
as a vinyl group [17], cyano group [16] and aromatic ring [18,25]
using a TiO2 photocatalyst having a metal co-catalyst such as Pd
or Rh. In this photocatalytic process, photogenerated electrons
reduced protons to form AHS on the surface of the metal co-
catalyst, and AHS hydrogenated organic compounds selectively.
After the formation of electrons by photocatalysis, the reaction
proceeded by thermal catalysis of the metal co-catalyst. In fact,
we have reported that semihydrogenation over a Cu-TiO2 photo-
catalyst under UV light irradiation was greatly accelerated by a
slight increase in the reaction temperature [20]. Based on the
above finding, we expected that the reaction rate of photocatalytic
semihydrogenation of alkynes to alkenes over DHN/TiO2-Cu can be
increased at elevated temperatures. In order to examine the effect
of reaction temperature on hydrogenation over a DHN/TiO2-Cu
photocatalyst, the reaction was carried out at various temperatures
(301–328 K), and the results are shown in Fig. 5(a). At any temper-
ature, semihydrogenation of PP to cis-MS proceeded linearly with
maintenance of high selectivity. As expected, the rate of cis-MS
production increased with increase in the reaction temperature
and the reaction rate at 328 K was three-times larger than the rate
at 301 K, indicating that catalytic hydrogenation of PP over Cu met-
als was the rate-determining step of this reaction. For comparison,
we also examined simple H2 evolution from TEOA over DHN/TiO2-
Cu in the absence of PP (Fig. 5(b)). The rate of H2 evolution
increased with an increase in the reaction temperature as did PP
hydrogenation. Assuming these reactions were zero-order reac-
tions, rate constants (k) at these temperatures were determined
from the slopes in Fig. 5, and Arrhenius plots (logarithm of k vs
reciprocal of T) for both reactions are shown in Fig. 6. Two linear
correlations were observed in the Arrhenius plots. From the slope
of the Arrhenius plot (Fig. 6(a)), the apparent activation energy
(Ea) for cis-MS formation was determined to be 27 kJ molꢀ1. The
value of Ea for H2 evolution from TEOA was calculated to be
15 kJ molꢀ1, which is close to values (ca. 10 kJ molꢀ1) previously
reported [32–36], indicating that H2 evolution over Cu nanoparti-
cles was also the rate-determining step in the latter reaction. From
a comparison of Figs. 1 and 5(b) at 328 K, we noted that the yield of
H2 in the presence of PP (Fig. 1) was much smaller than that in the
absence of PP (Fig. 5(b)). This fact suggests that both cis-MS
3
2
1
3
2
1
0
lnk = -3.2/T +12
R =0.99
lnk = -1.9/T+7.7
R2=0.99
(a)
(b)
3
3.2
3.4
3
3.2
3.4
103 / (T / K)
103 / T / K)
Fig. 6. Arrhenius plots (logarithm of k vs reciprocal of T) for (a) hydrogenation of PP
to cis-MS and (b) H2 evolution. The reaction conditions are shown in Fig. 5.
1
0.8
0.6
0.4
0.2
0
300 310 320 330
Temperature / K
Fig. 7. Ratio of cis-MS and H2 in hydrogenation of PP in a water-acetonitrile (1:9)
suspension of DHN/TiO2-Cu under irradiation of blue light for 2 h at various
temperatures.
(1) Photocatalyꢀc reacꢀon
(2) Adsorpꢀon
(3) Hydrogenaꢀon
: Rate-determing step
Acꢀve hydrogen
species (AHS)
2H+
AHS
AHS
H
H
H
H
Cu
e-
Cu
Cu
Visible
light
CB
CB
CB
h+
DHN
DHN
DHN
Ox
TEOA
Fig. 8. Expected photocatalytic mechanism over DHN/TiO2-Cu for semihydrogenation of alkynes under visible light irradiation.

42
M. Fukui et al. / Journal of Catalysis 374 (2019) 36–42
formation (hydrogenation of PP with AHS) and H2 evolution
(coupling of AHS) occurred competitively on the same active sites,
i.e., Cu metals. In other words, the selectivity of cis-MS formation
(PP hydrogenation) can be controlled by the reaction temperature
because the reaction rate was dependent on Ea, frequency factor
(A) and temperature. To examine the effect of the reaction temper-
ature on the selectivity of cis-MS formation (PP hydrogenation),
semihydrogenation of PP over DHN/TiO2-Cu was carried out under
blue light irradiation at various temperatures, and the selectivity of
cis-MS formation is shown in Fig. 7. At around room temperature,
cis-MS formation was inferior to H2 evolution due to the larger Ea.
However, cis-MS formation became predominant at elevated tem-
peratures and the selectivity reached 86% at 328 K. The increase in
selectivity can be explained as follows: elevation of the reaction
temperature decreased the negative effect of the large activation
energy on the reaction rate, while the effect of A on the rate
became larger at high temperatures. In fact, the value of A for
cis-MS formation (2.2 ꢁ 105 mmol hꢀ1) was much larger than that
of A for H2 evolution (2.2 ꢁ 103 mmol hꢀ1).
Research Foundation at Private Universities 2014-2018 from MEXT
and Kindai University. M. F. is grateful to the Japan Society for the
Promotion of Science (JSPS) for a Research Fellowship for young
scientists (grant number: 18J12706). A. T. is grateful for financial
support from the Faculty of Science and Engineering, Kindai
University.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.04.022.
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Acknowledgements
This work was partially supported by Grant-in-Aid for Scientific
Research (No. 17H03462 and 17H04967) from the Japan Society for
the Promotion of Science (JSPS) and Program for the Strategic
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