Unexpectedly selective hydrogenation of phenylacetylene to styrene on titania supported platinum photocatalyst under 385 nm monochromatic light irradiation

Article abstract of DOI:10.1016/S1872-2067(19)63453-4

Conversion of alkynes to alkenes by photocatalysis has inspired extensive interest, but it is still challenging to obtain both high conversion and selectivity. Here we first demonstrate the photocatalytic conversion of phenylacetylene (PLE) to styrene (STE) with both high conversion and selectivity by using the titania (TiO₂) supported platinum (Pt) as photocatalyst under 385 nm monochromatic light irradiation. It is demonstrated that the conversion rate of PLE is strongly dependent on the content of Pt cocatalyst loaded on the surface of TiO₂. Based on our optimization, the conversion of PLE and the selectivity towards STE on the 1 wt% Pt/TiO₂ photocatalyst can unexpectedly reach as high as 92.4% and 91.3%, respectively. The highly selective photocatalytic hydrogenation can well be extended to the conversion of other typical alkynes to alkenes, demonstrating the generality of selective hydrogenation of C≡C over the Pt/TiO₂ photocatalyst.

Full text of DOI:10.1016/S1872-2067(19)63453-4

Chinese Journal of Catalysis 41 (2020) 598–603
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Communication (Special Column for the Youth Innovation Promotion Association, Chinese Academy of
Sciences)
Unexpectedly selective hydrogenation of phenylacetylene to styrene
on titania supported platinum photocatalyst under 385 nm
monochromatic light irradiation
a,b
a
b
b
a
a,
Juhong Lian , Yuchao Chai , Yu Qi , Xiangyang Guo , Naijia Guan , Landong Li *,
b,#
Fuxiang Zhang
a
School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin 300350, China
State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean
b
Energy, Dalian 116023, Liaoning, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Conversion of alkynes to alkenes by photocatalysis has inspired extensive interest, but it is still
challenging to obtain both high conversion and selectivity. Here we first demonstrate the photo-
catalytic conversion of phenylacetylene (PLE) to styrene (STE) with both high conversion and selec-
Received 11 November 2019
Accepted 30 November 2019
Published 5 April 2020
tivity by using the titania (TiO
chromatic light irradiation. It is demonstrated that the conversion rate of PLE is strongly dependent
on the content of Pt cocatalyst loaded on the surface of TiO . Based on our optimization, the conver-
sion of PLE and the selectivity towards STE on the 1 wt% Pt/TiO photocatalyst can unexpectedly
2
) supported platinum (Pt) as photocatalyst under 385 nm mono-
2
Keywords:
Phenylacetylene
Titania
Selective Hydrogenation
Photocatalysis
Cocatalyst
2
reach as high as 92.4% and 91.3%, respectively. The highly selective photocatalytic hydrogenation
can well be extended to the conversion of other typical alkynes to alkenes, demonstrating the gen-
erality of selective hydrogenation of C≡C over the Pt/TiO photocatalyst.
2
©
2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Alkenes are the key building blocks in various natural
products, pharmaceuticals and agrochemicals, which have been
widely investigated and synthesized by selective hydrogena-
tion of alkynes [1,2]. Meanwhile, one of the main challenges is
to suppress the over-hydrogenation of alkynes to alkanes. To
achieve high selectivity of the alkyne semi-hydrogenation, the
conventional thermodynamic catalytic reaction has developed
popular Lindlar’s catalyst, palladium (Pd) supported on calcium
the introduction of high pressure of H may cause the danger of
explosion [4–7]. Therefore, an environmentally friendly cata-
lytic system for the selective hydrogenation of alkynes to al-
2
kenes working free from H is highly desirable.
2
Photocatalysis is a promising approach to replace the con-
ventional thermodynamic catalysis by utilizing the active pho-
togenerated electrons and holes to participate organic reac-
tions [8–10]. In consideration that the potential explosion dan-
carbonate, by employing flammable H
2
as the reducing agent to
ger of high pressure H
2
can be avoided through a mild hydro-
add two hydrogen atoms to the C≡C of alkynes [3]. In this case,
genation reaction, where organic molecules are employed as
*
#
Corresponding author. Tel: +86-22-23500341; E-mail: lild@nankai.edu.cn
Corresponding author. Tel: +86-411-82463370; E-mail: fxzhang@dicp.ac.cn
This work was supported by the National Natural Science Foundation of China (21633009), Dalian Science Foundation for Distinguished Young
Scholars (2017RJ02), and the Liaoning Revitalization Talents Program (XLYC1807241).
DOI: S1872-2067(19)63453-4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 4, April 2020

Juhong Lian et al. / Chinese Journal of Catalysis 41 (2020) 598–603
599
hydrogenation source, such as isopropanol, triethanolamine,
and methanol [11–14]. Due to the environmentally friendly
1(b) show the typical transmission electron microscopy (TEM)
images of the pristine TiO and Pt/TiO . It clearly shows that Pt
nanoparticles are uniformly deposited on the surface of TiO by
2
2
application of TiO
2
photocatalysis, various photocatalytic reac-
2
tions over TiO photocatalyst have been reported [15–19], es-
2
photodeposition method, and the size of Pt nanoparticles is
about 5 nm. X-ray diffraction patterns (XRD) in Fig. 1(c) indi-
cate a mix phase of anatase (JSPDS #21-1272) and rutile (JSPDS
pecially for the photocatalytic reduction of organic compounds
by using the photogenerated electrons. For example, it was
reported that nitrobenzenes having reducible group such as
#21-1276) for TiO
Pt peaks are observed due to the few amount of cocatalyst on
the semiconductor. It is general to know that the TiO is a
2
. While for the Pt/TiO
2
sample, no obvious
vinyl group are selectively reduced over bare TiO
lyst to the corresponding aminobenzenes [14]. Kominami et al.
20] reported that the copper loading on the surface of TiO
2
photocata-
2
[
2
widely investigated UV-responsive photocatalyst with absorp-
photocatalyst is effective to improve the selective hydrogena-
tion of 4-octyne with 2-propanol as the hydrogenation source.
Xiong and co-works [21] reported a selective light-driven
tion edge of ca. 400 nm [22–25]. The UV-vis absorption spectra
in Fig. 1(d) shows the absorption edge of TiO is consistent
2
with previous work [26]. After loading Pt cocatalyst, the ab-
sorption intensity between 400 to 800 nm increases as com-
2
-methyl-3-butyn-2-ol semi-hydrogenation reaction by using
PdPt alloy as cocatalyst loaded on TiO photocatalyst. Although
various selective hydrogenation reactions have been reported
with TiO photocatalyst, an effective photocatalytic selective
hydrogenation of PLE on TiO has not been reported yet. Here-
in, we report the mild photocatalytic conversion of PLE to STE
with both high conversion and selectivity by using the TiO
2
2
pared to TiO , which can be attributed to the loading of Pt co-
catalyst.
2
The photocatalytic hydrogenation of PLE was performed in
the methanol suspension of 1.0 wt% M/TiO with 385 nm
monochromic light (“M” stands Pt, Pd, Au, Rh and Ag). As
shown in Fig. S1, the Pt/TiO catalyst shows the best hydro-
2
2
2
2
supported Pt as photocatalyst and methanol as hydrogenation
source under 385 nm monochromatic light irradiation. The
highly selective hydrogenation can be well extended to the
conversion of other typical alkynes to alkenes, demonstrating
its generality.
genation performance of PLE and high selectivity towards STE.
Fig. 2(a) shows the time-dependent curves of photocatalytic
hydrogenation of PLE on the Pt/TiO photocatalyst, in which
2
the photocatalytic conversion rate of PLE is almost linearly
increased as a function of irradiation time. And the selectivity
towards STE is maintained for 91.3% at the experimental re-
gion. Conversion rate of PLE reaches 100% after 8 h irradia-
tion, which is consistent with the decreased amounts of PLE in
The cocatalyst of Pt nanoparticles was loaded on the surface
of TiO
2
by the photodeposition method, and the corresponding
sample was denoted as Pt/TiO
2
(1 wt% Pt). Fig. 1(a) and Fig.
(
a)
(b)
(
c)A
Pt/TiO2
(d)
Pt/TiO2
TiO2
TiO2
R
A
A
R
A
A
R
A
A
R
R
20
30
40
50
60
70
80
200
300
400
500
600
700
800
2 (degree)
Wavelength (nm)
Fig. 1. TEM images of TiO
Pt/TiO
2
(a) and Pt/TiO (b); (c) XRD patterns of TiO
2
2
and Pt/TiO2, A: anatase; R: rutile; (d) UV-visible absorption spectra of TiO and
2
2
.

600
Juhong Lian et al. / Chinese Journal of Catalysis 41 (2020) 598–603
1
00
100
80
60
40
20
0
500
400
300
200
100
0
10
8
(
a)
PLE
STE
EBE
(b)
8
6
4
2
0
0
0
0
0
H2
6
4
2
0
0
1
2
3
4
5
6
7
8
0
2
4
6
8
Photoirradiation time (h)
Photoirradiation time (h)
Fig. 2. Time courses of the conversion of PLE hydrogenation and the selectivity towards STE (a), and the amounts of PLE, STE, EBE and the amounts
of H (b) produced in the methanol suspension of Pt/TiO . Reaction conditions: PLE 0.5 mmol, Pt/TiO 50 mg, methanol 25 mL, LED irradiation (λ =
85 nm) 5 mW·cm .
2
2
2
‒2
3
Fig. 2(b). PLE is almost completely consumed after 8 h. The
selectivity of STE in the products always remains at 91.3% be-
fore 6 h. But it decreases after 6 h photoirradiation, indicating
the intermediate product of STE is hydrogenated further to EBE
after a prolonged reaction time. As shown in Fig. 2(b), the
amount of STE increases slowly after 6 h irradiation, while the
amount of EBE increases obviously. It is clear to see that the
intermediate product of STE is hydrogenated further to EBE
creases to 1.5 wt%, the conversion rate decreases because of
the light-shading effect of the excessive cocatalyst. To illustrate
the process of hydrogen evolution, the chemical states of de-
posited Pt are further examined by X-ray photoelectron spec-
troscopy (XPS). According to the binding energies shown in Fig.
3(b), the deposited Pt by photodeposition method exists as
both metallic Pt and PtO
evolution than metallic Pt [26–28]. The Pt/TiO
prepared by photodeposition method in Fig. 3(b) shows more
PtO . According to the peaks area of metallic Pt and PtO , the
content of PtO is calculated to be 59% approximately, which is
disadvantageous for H evolution. The byproduct is only EBE.
For comparison, the comparative experiment is performed
over Pt/TiO using H as the hydrogenation source (1 atm) at
x
. It is reported PtO
x
is less reactive for
H
2
2
sample
after a prolonged irradiation time. The TiO
has no hydrogenation activity towards PLE (as shown in entry
, Table S1). It is inferred that the cocatalyst is essential for the
photocatalytic hydrogenation of the PLE substrate. It should be
pointed out that the byproduct of H is barely observed in Fig.
(b).
The effect of the Pt loading content was also investigated in
2
sample without Pt
x
x
4
x
2
2
2
2
2
room temperature free of light irradiation. The reaction time (6
h) is similar to that of photocatalytic hydrogenation. Interest-
ingly, the result in Fig. 4 shows that EBE is detected as the only
product. The conversion rate of PLE and the selectivity towards
Fig. 3(a). The conversion rate of PLE increases with adding the
Pt amount up to 1.0 wt%, which is due to the increase of active
sites for hydrogenation reaction. With the amount of Pt in-
100
100
(a)
(b)
Pt 4f
95
90
85
80
75
70
80
60
40
20
0
4
+
2+
Pt
Pt
Pt
0.4
0.6
0.8
1.0
1.2
1.4
1.6
78
76
74
72
70
Pt (wt%)
Binding Energy (eV)
Fig. 3. (a) Effect of amounts of Pt loading on TiO
2
on the photocatalytic hydrogenation of PLE (0.5 mmol) in methanol suspension for 6 h under 385
nm monochromatic light; (b) XPS spectra of the Pt 4f of Pt/TiO
2
.

Juhong Lian et al. / Chinese Journal of Catalysis 41 (2020) 598–603
601
phatic alkynes were investigated. As seen in Table 1, the
Pt/TiO photocatalyst is effective for selective hydrogenation of
aliphatic and aromatic alkynes to the corresponding cis-alkenes
Table 1, entries 1–5), well demonstrating its generality. The
conversion of alkynes decreases when the functional groups
attach to benzene ring or triple bond (entries 2 and 3) com-
pared with PLE, which is possibly caused by the decreased ad-
sorption of substrate on the surface of photocatalyst due to
steric hindrance. Alkynes having chloro group is also converted
into the corresponding alkenes, and the chloro group is pre-
served (entry 5). The photocatalytic hydrogenation activity of
the aliphatic alkyne having longer carbochain is a little lower
1
00
2
(
8
6
4
2
0
0
0
0
0
(entries 4 and 5).
Fig. 5 schematically illustrates the photocatalytic hydro-
Photocatalysis Comparative experiment
genation of PLE and comparative experiment over the Pt/TiO
2
catalyst. As shown in Fig. 5(a), the main product of photocata-
lytic hydrogenation of PLE is STE. While the product of the
comparative experiment is only EBE in Fig. 5(b). The detailed
reaction mechanism is shown in Fig. 5(c). Both the photocata-
lytic hydrogenation and comparative experiment involve three
steps: PLE adsorption, active hydrogen species (H-Pt) forming
and attack of C≡C, and the product desorption. For the photo-
catalytic hydrogenation of PLE, the photogenerated electrons
and holes are produced under UV light. Then the photogener-
ated electrons would migrate to the surface of Pt cocatalyst.
Fig. 4. Comparison of the selectivity towards STE of photocatalytic
hydrogenation of PLE and comparative experiment.
EBE are both 100%, which is in sharp contrast with the photo-
catalytic hydrogenation result. The comparison clearly shows
that the photocatalytic hydrogenation of PLE to STE has high
selectivity of 91.3%, demonstrating the unique superiority of
the photocatalytic hydrogenation method. What’s more, the
photocatalytic selective hydrogenation of alkynes to alkenes is
working at room temperature free of direct H source. A series
2
CH
3
OH as hydrogenation source and electron donor can be
of blank experiments related to the light irradiation and usage
of photocatalyst were also carried out with results given in
+
oxidized by photogenerated holes of TiO
2
, and thus the H dis-
sociates into the solution. Afterwards, active hydrogen species
Table S1, based on which both Pt/TiO and UV light are
2
(
H-Pt) are formed on the surface of Pt cocatalyst by reducing
demonstrated to be indispensable for the photocatalytic hy-
drogenation of PLE to STE.
+
protons (H ) through photogenerated electrons, and PLE is
hydrogenated by active hydrogen species (H-Pt). Finally, the
formed STE will detach from the surface of catalyst. As report-
ed by previous works [29,30], the increasing electron density of
catalyst will cause the decreased adsorption strength of inter-
mediate STE. Desorption of STE is favored, so the selectivity
towards STE is enhanced. It should be pointed out that the
electron density of Pt under photocatalytic hydrogenation pro-
cess is more than the comparative experiment, because the
To examine the generality of selective hydrogenation of al-
kynes over the Pt/TiO photocatalyst, other aromatic and ali-
2
Table 1
Photocatalytic semi-hydrogenation of alkynes to the corresponding
a
alkenes in the methanol suspension of Pt/TiO
2
at room temperature .
Time Con.b
Alkene c
Select.b
Entry
Alkyne
(h) (%)
(%)
1
6
92.4
91.3
photogenerated electrons transfer from the TiO
2
to the Pt co-
catalyst under UV light. Thus we infer that the high electron
2
3
6
76.8
93.2
89.4
(a)
(b)
2
H
+
H
2
O
O
Pt
Pt
e- TiO
6
26.2
TiO
2
2
(c)
4
5
6
6
44
100
C
C
H
C
H
CH2
C
CH2
H
H
C
CH
H
H

H
Cl
76.6
87.9
Cl
Pt
Pt
Pt
e-
Pt
e-
a
Reaction conditions: Pt/TiO
2
(50 mg), substrate (0.5 mmol), methanol
e- TiO
e- TiO2
TiO2
2
TiO2
or
(25 mL), 385 nm LED monochromatic light, room temperature, Ar
atmosphere.
I
II
III
b
The conversion of alkynes and the selectivity of corresponding alkenes
were determined by GC equipped with a DB-5 column using mesitylene
as an internal standard.
The produced alkenes were determined by gas chromatography-mass
Fig. 5. (a) Schematic illustration of the photocatalytic hydrogenation of
PLE to STE over the Pt/TiO photocatalyst in the methanol suspension;
(b) the comparative experiment over the Pt/TiO with H as hydro-
genation souece; (c) Proposed photocatalytic reaction mechanism for
semi-hydrogenation of PLE over Pt/TiO
2
c
2
2
spectrometry (Shimadzu GCMS-QP2010 SE) with a RXI-5MS column
(30 m, 0.25 mm i.d., stationary phase thickness of 0.25 μm).
2
.

602
Juhong Lian et al. / Chinese Journal of Catalysis 41 (2020) 598–603
genation source in the controllable hydrogenation extent. This
work emphasizes the advantage of photocatalysis in the
semi-hydrogenation of alkynes in a cleaner, safer, and highly
selective manner, which may be useful for the design of highly
selective hydrogenation systems in the alkyne conversion.
TiO2
Pt/TiO2
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (21633009), Dalian Science
Foundation for Distinguished Young Scholars (2017RJ02), and
the Liaoning Revitalization Talents Program (XLYC1807241).
400
450
500
550
600
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Graphical Abstract
Chin. J. Catal., 2020, 41: 598–603 doi: S1872-2067(19)63453-4
Unexpectedly selective hydrogenation of phenylacetylene to
styrene on titania supported platinum photocatalyst under 385
nm monochromatic light irradiation
(
a)
(b)
Juhong Lian, Yuchao Chai, Yu Qi, Xiangyang Guo, Naijia Guan,
Landong Li *, Fuxiang Zhang *
H+
H2
2
Nankai University;
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Pt
Pt
TiO2
Compared with the traditional catalytic hydrogenation of phenyla-
e- TiO2
cetylene using H as hydrogenation source, photocatalytic phenyla-
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cetylene hydrogenation with methanol as hydrogenation source is a
cleaner, safer, and highly selective manner.

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铂负载二氧化钛光催化剂在单波长下苯乙炔高选择性加氢
a,b
a
b
b
a
a,*
b,#
连菊红 , 柴玉超 , 祁 育 , 郭向阳 , 关乃佳 , 李兰冬 , 章福祥
a
南开大学, 材料科学与工程学院, 国家新材料研究院, 天津300350
b
中国科学院大连化学物理研究所, 催化基础国家重点实验室, 洁净能源国家实验室(筹),
能源材料化学协同创新中心, 辽宁大连116023
摘要: 苯乙烯是一种非常重要的化工原料, 它是工业上生产聚苯乙烯、树脂和丁苯橡胶的重要单体, 而苯乙烯单体中经常
含有苯乙炔杂质, 影响苯乙烯的聚合性能, 因此研究苯乙炔选择性加氢生成苯乙烯具有十分重要的工业意义. 传统的热催
化苯乙炔加氢反应会用到易燃易爆的氢气, 引起操作的危险性, 因此, 开发具有环境友好型的加氢反应体系具有很重要的
意义. 光催化加氢反应利用光生电子强的还原能力, 还原质子产生活性的氢物种加氢, 相较于传统的热催化使用氢气作为
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加氢源加氢, 能够在温和的条件下实现高选择性加氢. 基于此, 本文利用Pt/TiO 作为光催化剂, 甲醇作为加氢源实现了在
385 nm单波长光照下苯乙炔的高选择性加氢.
首先, 我们利用光沉积的方法将Pt负载在TiO
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的表面, 通过透射电子显微镜图像和紫外可见吸收光谱表征了负载在
TiO 表面Pt的颗粒分布和光学性质. 结果表明负载的Pt的颗粒大约在5 nm左右, Pt的负载改变了TiO 在可见光区的吸收性
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能. XPS结果显示, 通过光沉积得到的Pt的价态为金属态和氧化态共存. 光催化苯乙炔加氢实验表明, Pt/TiO
常压条件下不仅具有高的苯乙炔光催化转化率, 当光照达8 h后, 苯乙炔完全转化, 而且在6 h之内苯乙烯选择性保持在
1.3%, 具有高的苯乙烯选择性. 通过对负载的Pt的含量进行了优化, 筛选出当Pt的负载量为1 wt%时, 苯乙炔的转化率最
高. 为了对比, 利用传统热催化的方法氢气作为加氢源进行了苯乙炔加氢实验, 结果发现, 使用氢气作为加氢源时, 虽然苯
乙炔的转化率为100%, 但产物是过加氢的产物乙苯. 这主要是因为在光催化反应过程中, TiO 导带上的电子迁移至Pt颗粒
上, 导致Pt的电子密度增加, Pt颗粒表面高的电子密度有利于加氢中间产物苯乙烯的脱附, 因此, 在光催化加氢过程中不会
发生过加氢反应, 具有高的苯乙烯选择性. 同时, 扩展实验表明, Pt/TiO 光催化剂对其他类型的炔烃加氢也具有高的选择
性, 表明Pt/TiO 光催化炔烃加氢具有普适性. 由此可见, 光催化炔烃加氢未来将成为一种环境友好而高效的方法.
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催化剂在室温
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关键词: 苯乙炔; 二氧化钛; 选择性加氢; 光催化; 铂
收稿日期: 2019-11-11. 接受日期: 2019-11-30. 出版日期: 2020-04-05.
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基金来源: 国家自然科学基金(21633009); 大连市杰出青年科学基金项目(2017RJ02); 兴辽人才计划(XLYC1807241).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).