Visible-Light-Driven Photocatalytic Hydrogenation of Olefins Using Water as the H Source

Article abstract of DOI:10.1002/cctc.201900262

In this work, a highly efficient PCN-KCl (KCl-modified polymeric carbon nitride) nanosheet photocatalyst was synthesized with the assistance of KCl. The as-prepared PCN-KCl catalyst shows a more than 30-fold enhancement in the photocatalytic activity for H₂ evolution from water compared to the pristine PCN. More importantly, when PCN-KCl was composited with a second catalyst (Pd nanoparticles), the simultaneous production and utilization of active H species for alkenes hydrogenation was achieved by visible light irradiation under ambient conditions.

Full text of DOI:10.1002/cctc.201900262

Accepted Article
Title: Visible-Light-Driven Photocatalytic Hydrogenation of Olefins
Using Water as the H Source
Authors: Xin Fan, Yanling Yao, Yangsen Xu, Lei Yu, and Chuntian Qiu
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10.1002/cctc.201900262
ChemCatChem
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Visible-Light-Driven Photocatalytic Hydrogenation of Olefins
Using Water as the H Source
Xin Fan,[a], [b] Yanling Yao,[c] Yangsen Xu,[b] Lei Yu,[a] Chuntian Qiu*[b]
Abstract: In this work, a highly efficient PCN-KCl (KCl-modified
polymeric carbon nitride) nanosheet photocatalyst was synthesized
with the assistance of KCl. The as-prepared PCN-KCl catalyst
shows a more than 30-fold enhancement in the photocatalytic
activity for H2 evolution from water compared to the pristine PCN.
More importantly, when PCN-KCl was composited with a second
catalyst (Pd nanoparticles), the simultaneous production and
utilization of active H species for alkenes hydrogenation was
achieved by visible light irradiation under ambient conditions.
utilization of photochemically generated H species for the
synthesis of value-added fine chemicals is attractive for both
fundamental research and industrial production. To achieve this
concept, the most challenging obstacle is the design and
synthesis of efficient bifunctional photocatalysts for both the
water splitting and hydrogenation reactions.
Polymeric carbon nitride (PCN) is a low-cost, nontoxic and
earth-abundant metal-free semiconductor with appropriate band
gap.[7] It is considered to be an efficient photocatalyst for visible-
light-driven water splitting. On the other hand, owing to its
layered structure and abundant nitrogen-containing groups
(such as -NH2 and -NH-), PCN is a good support for anchoring
desired metals (such as Pd nanoparticles) as catalytically active
Photocatalytic water splitting technology (PWST) is one of
the most promising ways to convert solar energy into the
production of green hydrogen energy and contributes to solving
current environmental problems and the energy crisis.[1] Instead
of H2 energy storage, the promotion of utilizing H2 or/and active
hydrogen species (H species, Had) photo-generated from water
for organic synthesis of value-added fine chemicals is becoming
a more valuable and achievable strategy but has barely been
investigated.[2] Hydrogenation of alkenes is one of the most
fundamental transformations in the organic synthesis and
manufacture of industrial fine chemicals. However, the current
hydrogenation process requires high temperature, considerable
pressures and elaborates experimental setups due to the use of
hazardous pressurized H2 gas as the H source.[3] As an
attractive alternative, transfer hydrogenation is becoming
increasingly noticeable, especially in reactions using water as
non-H2 source.[4]
Because of it is green, abundant, safe and low-cost, water is
considered as one of the most ideal H sources. However, the
activation of water to generate hydrogen is a great challenge,
due to the high dissociation energy of the O-H bond. In recent
years, photocatalytic water splitting over semiconductors has
become one of the most promising technologies for hydrogen
production.[5] Under UV and/or visible light illumination,
photoexcited electrons can migrate, and some of them are
transferred to protons and/or water to form H radicals, which
subsequently recombine to produce H2 gas.[6] In addition, the
centres
to
construct
multi-functional
photocatalysts.[8]
Nevertheless, its low efficiency of light transfer and catalytic
performance hinder its further applications.
The molten salts method, based on the ionothermal process
with mixed salts (e.g., KCl/LiCl, KBr/LiBr, and LiCl/KCl/NaCl), is
approved to obtain high condensation PCN products with
enhanced photocatalytic activity.[9] To achieve the concept of
utilizing active H species for hydrogenation reactions, the
hydrogen generation from photocatalytic water splitting is an
essential prerequisite. Herein, we have synthesized a PCN-KCl
nanosheet photocatalyst via an ionothermal method by using
only KCl, and this photocatalyst is expected to show good
activity towards hydrogen evolution from water. Importantly, the
KCl is reusable, and more than 95 wt% of the salt can be
recovered, suggesting a sustainable method for the production
of modified PCN (Figure S1). The crystal structures of the as-
synthesized PCN catalysts were first characterized by XRD
(Figure 1a). Two distinct diffraction peaks at approximately 13.0o
and 27.4o were observed in the pattern of bulk PCN, which can
be attributed to the (100) in-planar repeating units of tri-s-triazine
and the (002) interlayer reflection of PCN, respectively.[10] In
contrast, in the pattern of PCN-KCl, the (100) peak disappeared,
and the (002) peak intensity of dramatically decreased. This
indicated that damage and/or substantial changes in the in-
planar structure were caused by the presence of KCl.
Accordingly, the Pd/PCN-KCl catalyst showed a similar XRD
pattern as that of PCN-KCl, and no peaks belonging to Pd were
observed, suggesting a high dispersion of Pd nanoparticles with
small crystal sizes. This was further confirmed by transmission
electron microscope characterization (Figure 1).
[a]
[b]
X. Fan, Prof. L. Yu,
School of Chemistry and Chemical Engineering,
Yangzhou University,
Yangzhou, 225002, P. R. China.
X. Fan, Dr. Y. Xu, Dr. C. Qiu,
SZU-NUS Collaborative Center and International Collaborative
Laboratory of 2D Materials for Optoelectronic Science & Technology
of Ministry of Education, College of Optoelectronic Engineering,
Shenzhen University,
Shenzhen 518060, P. R. China.
E-mail: qiuct@szu.edu.cn
[c]
Dr. Y. Yao,
School of Chemistry and Chemical Engineering,
Huizhou University,
Huizhou 516007, P. R. China.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900262
ChemCatChem
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into the PCN structure via the ionothermal process. Due to the
existence of N-H2 and/or N-H groups in the PCN structure, K+
may be anchored to these amino species forming N-K bonds,
and thus producing the PCN-KCl nanosheets.[11]
Figure 1. (a) XRD patterns of bulk PCN, PCN-KCl, and Pd/PCN-KCl; (b-e)
TEM images of Pd/PCN-KCl; (f) size distribution of the Pd nanoparticles.
Further insight into the morphologies and microstructures of
the as-synthesized PCN and Pd/PCN-KCl were obtained by
SEM and TEM (Figure 1 and Figures S2, S3 and S4). A PCN-
KCl nanosheet with a length on the micron-scale and a
thickness of approximately 100 nm can be clearly observed from
the TEM images (Figures S3 and S4). The sheet-like
morphology was composed of numerous nano layers stacked
along the plane (Figure 1c, 1d) and thus contributes to
increasing the surface area, which is highly related to the
catalytic activities. To construct a bifunctional catalyst for both
hydrogen evolution and utilization, Pd, acting as a secondary
catalyst, was composited onto the surface of the PCN-KCl
nanosheet. Pd nanoparticles at approximately 2 nm were evenly
Figure 2. (a) UV-Vis absorption spectra and the Kubelka-Munk function plot of
bulk PCN and PCN-KCl (inset in a); (b) Time dependence of the photocatalytic
H2 evolution over bulk PCN and the PCN-KCl catalysts under light irradiation
(λ≥280 nm).
In addition to the crystal structure, physical properties such
as the light absorptivity and electronic band structure could be
changed as well by the use of KCl. Thus, UV-Vis diffuse
reflectance spectra were measured to investigate the band gap
of the PCN-KCl nanosheets. From Figure 2a, PCN-KCl shows a
red shift in its UV-Vis absorption spectrum compared to that of
bulk PCN. The band gap of as-prepared PCN-KCl in this work
was determined to be 2.68 eV (Figure 2a, inset), which is lower
than that of bulk PCN (2.72 eV). Therefore, PCN-KCl, with
higher efficiency of visible light absorption, was expected to
show a higher photocatalytic activity than that of bulk PCN.
After obtaining Pd/PCN-KCl nanosheet catalyst, we then
evaluated the H2 production activity by performing a light-
induced hydrogen evolution assay, which is an indispensable
former step prior to the hydrogenation reaction. Notably, the
dispersed over the nanosheet, and
a lattice fringe of
approximately 0.234 nm was clearly observed. The chemical
state of Pd was mainly Pd0, which was confirmed by X-ray
photoelectron spectroscopy (XPS) (Figure S5b). The
compositions of the PCN-KCl nanosheets contained C, N, O, K
and their chemical states in the Pd/PCN-KCl photocatalyst were
also measured and are shown in Figure S5. In the survey
spectra (Figure S5a), C, N, O and Pd were all detected in the
catalyst. Interestingly, K 2p peaks were clearly observed while
no peak belonging to Cl was observed, indicating K+ insertion
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PCN-KCl sample exhibited a much higher photocatalytic activity
towards hydrogen evolution than that of bulk PCN. The average
hydrogen evolution rate of the PCN-KCl sample was 3053
μmol·g-1·h-1, which was more than 30 times greater than that of
bulk PCN (92.5 μmol·g-1·h-1). This expected higher H2
generation rate may greatly enhance the activity of the
subsequent hydrogenation reaction. In fact, under the optimized
reaction conditions, the styrene substrate can be smoothly
converted into its corresponding ethyl benzene over Pd/PCN-
KCl instead of Pd/PCN photocatalyst (Figure 3 Condition 6).
sacrificial agent accompanying with metal sulfide photocatalysts,
such as CdS and ZnS, for water splitting, was unfavourable in
the above reaction system and gave trace yield of ethyl benzene
product (Conditions 11). Obviously, the hydrogenation reactions
were driven by light (Condition 5), and with an increase in the
intensity of blue light intensity (λ=420 nm), the catalytic
performance was gradually enhanced (Figure S7). Although UV-
Vis light catalyses the H2 evolution reaction to achieve a rather
high activity, the effective utilization of visible light is a more
attractive option. Furthermore, electrons with high energy will be
excited by UV light and then participate in the activation of more
stable bonds, such as carbonyls, ether groups or even aromatic
rings. Thus, undesired side reactions, e.g., photodegradation,
could occur. Nevertheless, these results provide insight into new
reactions based on PWST that could be further developed by
adjusting the reaction system, including the photocatalyst and/or
the driving light.
Scheme 1. Model reaction of the PWST-based olefin hydrogenation.
Table 1. Reaction scope of the PWST-based olefins hydrogenation.a
Figure 3. Reaction conditions optimization: catalyst Pd/PCN-KCl 10 mg;
substrate (styrene) 0.1 mmol; total liquid volume 5 ml, containing 2 ml ethyl
acetate (EA) as the solvent and the rest of the volume being H2O (1.5 ml);
sacrificial agent (1.5 ml) of either CH3OH, triethylamine (TEA), triethanolamine
(TEOA), or Na2S (0.3 mol/L); an additive of either HCOOH (0.1 ml), CH3COOH
(0.1 ml), or H2SO4 (0.05 ml); light irradiation with blue light 20 W (λ=420 nm);
25 °C, and a reaction time of 1h. GC yields were calculated from standard
calibration curves.
Reaction
Time (h)
Conversion
(%)
Yield
(%)
Entry
R1
R2
1a
2a
3a
4a
5a
6a
7a
8a
Ph
Ph
H
Me
H
H
H
3
3
3
3
3
4
12
12
>99
>99
>99
>99
>99
97
>99
>99
93
96
90
95
81
90
4-MeC6H4
4-MeOC6H4
4-MeOCOC6H4
Ph
Ph
Stilbene
Chalcone
>99
>99
To effectively utilize the in situ generated H species, we set
up a mixed homogeneous liquid reaction solution that was more
beneficial for the proton transfer from water to the organic phase.
From Figure 3 Condition 1, it could be found that pure H2O
conditions gave trace yield of hydrogenated product.
Surprisingly, when methanol was added as a sacrificial agent, a
greater than 10% yield of ethyl benzene was obtained within 1h
(Condition 3). This result successfully confirmed our strategy of
utilizing the hydrogen photogenerated from water decomposition
for the hydrogenation of alkenes under ambient conditions.
Furthermore, the photocatalytic reactivity could be greatly
enhanced by adding HCOOH, CH3COOH or H2SO4 (Conditions
4, 7 and 8). Owing to a higher concentration of protons
dissociated from the additives, the generation of active H
[a] Reaction condition: catalyst Pd/PCN-KCl 10 mg, substrate 0.1 mmol, 2 ml
EA, 1.5 ml H2O, 1.4 ml CH3OH, 0.1 ml HCOOH, light irradiation blue light 20
W (λ=420nm), 25 ℃, reaction time 3~12 h.
After obtaining the optimized reaction conditions, we have
further conducted the reaction scope and effectiveness of the
PWST-based olefin hydrogenation. As shown in Table 1, typical
olefins were converted into their corresponding alkenes with
high conversion and good selectivity over the Pd/PCN-KCl
photocatalyst. During the photocatalytic water decomposition,
active H species were generated and could be adsorbed onto
the surface of the catalyst. Pd nanoparticles as a bifunctional
catalyst, not only promoted the electron transport to the
dissociated protons to form active H species, but also adsorbed
and activated unsaturated organic substrates such as styrene to
synergistically react with hydrogen. In addition to styrene (1a),
its derivatives with both electron-donating and electron-
species, e.g.,
H
radicals, was greatly increased, thus
accelerating the subsequent hydrogenation reaction (Figure S6).
Sacrificial agents are another important factor that is highly
related to the photocatalytic activity. For example, TEA and
TEOA can be used in the above optimized reaction system
(Conditions 9 and 10). However, Na2S, a commonly used
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(0.1 mmol) and HCOOH (0.1 ml) were dispersed in a mixed solution of
ethyl acetate/H2O/CH3OH=2 ml/1.5 ml/1.4 ml, and then sonicated for 1
min. The reaction mixture was then irradiated with an LED lamp (20 W,
λ=420 nm, Suncat Instruments Co., Ltd., Beijing, China) for 3~12 h under
Argon at 25 °C by using a flow of cooling water during the reaction. After
the reaction, the mixture was centrifuged to remove photocatalyst. The
supernatant was extracted by adding 5 ml of CH2Cl2 and the organic
phase was analysed by GC-MS (Agilent 7890A). The conversions and
yields were calculated from standard calibration curves.
withdrawing groups such as α-methyl styrene (2a), 4-methyl
styrene (3a), 4-methoxy styrene (4a), 4-formyl styrene (5a) and
1,1-diphenylethene (6a), could achieve yields greater than 90%
yields within 3h. In contrast, prolonged reaction times were
needed to convert stilbene (7a) and chalcone (8a) into their
corresponding hydrogenated products, due to their less active
C=C bonds located at non-terminal positions. Interestingly,
chalcone (8a) could be selectively transformed into the 1,3-
diphenyl-1-propanone suggesting the selective reduction of α,β-
unsaturated ketones which was an attractive result and will be
further investigated in a following study.
In summary, we have synthesized a PCN-KCl nanosheet by
means of KCl. By compositing these nanosheets with a second
catalyst, Pd nanoparticles, the as-prepared Pd/PCN-KCl catalyst
shows a greater than 30-fold enhanced catalytic activity for
photocatalytic water splitting compared to that of the Pd/PCN,
and thus successfully achieved the strategy of utilizing
photogenerated H species for olefin hydrogenation by using
visible light irradiation under mild conditions. Hopefully, this work
could provide a prospective route to effectively utilize an in situ
Safety notes: To avoid the unpredictable danger of hydrogen that
generated from water splitting, conducting this hydrogenation reaction
under inert atmosphere is highly recommended. Furthermore, we
recommend connecting an empty balloon onto the reaction flask. It is
helpful to release the pressure that caused by the excess H2 gas from
hydrogen evolution process.
Acknowledgements
This work was supported by National Natural Science
Foundation of China (51502174, 21401190), Science and
Technology Planning Project of Guangdong Province
(2016B050501005), Educational Commission of Guangdong
Province (2016KSTCX126), Shenzhen Peacock Plan (Grant No.
827-000113), Science and Technology Project of Shenzhen
(ZDSYS201707271014468).
H
source, generated from water splitting, for synergistic
reduction reactions.
Experimental Section
Synthesis of catalysts
The authors declare no conflicts of interest.
Typically, the precursor melamine 2 g (Aladdin, China) was first mixed
with KCl (Aladdin, KCl/melamine=10) and ground for 5 min in an agate
mortar. Then the mixture was transferred into a tube furnace and
calcined at 550 °C for 2 h (heating rate 2 °C·min-1). After naturally cooling
to room temperature, the obtained yellow solid was washed with hot
water three times to remove KCl and impurities. After centrifugation and
drying at 80 °C overnight, the as-prepared solids were labelled PCN-KCl.
Bulk PCN was prepared via the same procedure without the use of KCl.
Keywords: PCN • Photocatalysis • PWST • Hydrogen evolution
• Olefins hydrogenation
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Entry for the Table of Contents (Please choose one layout)
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COMMUNICATION
One-step
generation of hydrogen from
water and its subsequent
utilization for reduction reactions
was demonstrated with typical
photocatalytic
[b]
Xin Fan,[a],
Lei Yu,[a] Yanling
Yao,[c] Yangsen Xu,[b] Qitao
Zhang,[b] Chuntian Qiu* [b]
Page No. – Page No.
examples
hydrogenation
of
olefin
the
Visible-Light-Driven
into
Photocatalytic Hydrogenation of
Olefins Using Water as the H
Source
corresponding alkanes over a
Pd/PCN-KCl
photocatalyst
conditions.
bifunctional
under mild
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Author(s), Corresponding Author(s)*
((Insert TOC Graphic here))
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Title
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