Metal-Free Photocatalytic Hydrogenation Using Covalent Triazine Polymers

Article abstract of DOI:10.1002/anie.202006618

Photocatalytic hydrogenation of biomass-derived organic molecules transforms solar energy into high-energy-density chemical bonds. Reported herein is the preparation of a thiophene-containing covalent triazine polymer as a photocatalyst, with unique donor-acceptor units, for the metal-free photocatalytic hydrogenation of unsaturated organic molecules. Under visible-light illumination, the polymeric photocatalyst enables the transformation of maleic acid into succinic acid with a production rate of about 2 mmol g^?1 h^?1, and furfural into furfuryl alcohol with a production rate of about 0.5 mmol g^?1 h^?1. Great catalyst stability and recyclability are also measured. Given the structural diversity of polymeric photocatalysts and their readily tunable optical and electronic properties, metal-free photocatalytic hydrogenation represents a highly promising approach for solar energy conversion.

Full text of DOI:10.1002/anie.202006618

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Accepted Article
Title: Metal-Free Photocatalytic Hydrogenation Using Covalent Triazine
Polymers
Authors: Yongpan Hu, Wei Huang, Hongshuai Wang, Qing He, Yuan
Zhou, Ping Yang, Youyong Li, and Yanguang Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006618
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Metal-Free Photocatalytic Hydrogenation Using Covalent Triazine
Polymers
Yongpan Hu† , Wei Huang† , Hongshuai Wang, Qing He, Yuan Zhou, Ping Yang, Youyong Li and
Yanguang Li*
Abstract: Photocatalytic hydrogenation of biomass-derived organic
molecules transforms solar energy to high-energy-density chemical
bonds, and may enable the production of industrial building-block
chemicals with great economical values. Despite its great potential,
photocatalytic hydrogenation remains poorly explored up to now.
Existing researches are mostly based on broad bandgap inorganic
semiconductors and require the assistance of noble metal based
cocatalysts. Herein, we report the preparation of thiophene-containing
covalent triazine polymer as the photocatalyst with unique donor-
acceptor units, and for the first time demonstrate the metal-free
photocatalytic hydrogenation of unsaturated organic molecules
exemplified by maleic acid and furfural. Under the visible light
illumination, our polymeric photocatalyst enables the transformation
of maleic acid to succinic acid with an incredible production rate of ~2
mmol g-1h-1, and furfural to furfuryl alcohol with a production rate of
~0.5 mmol g-1h-1. Great catalyst stability and recyclability are also
measured. Given the structural diversity of polymeric photocatalysts
and their readily tunable optical and electronic properties, metal-free
photocatalytic hydrogenation represents a highly promising approach
for solar energy conversion.
generally performed under room temperature and ambient
pressure, using protic solvents like H2O or alcohols as the
hydrogen source instead of molecular hydrogen. Moreover, it
allows for the on-site decentralized production of chemicals that
can take place close to the location of consumption. Despite its
great potential, studies about photocatalytic hydrogenation are
very scarce. Among a handful of demonstrations, Guo and
coworkers showed that Au-SiC could be used for the selective
hydrogenation of α,β-unsaturated aldehydes to their
corresponding unsaturated alcohols with high turnover frequency
(487 h-1) and selectivity (100%) under visible light irradiation.[15]
Priti Sharma and coworkers reported that Pd-g-C3N4 was efficient
for the selective reduction of a wide range of unsaturated
derivatives and nitro compounds with great yields (>99%).[16]
However, the use of broad bandgap inorganic semiconductors
(>2.5 eV) limited the available solar spectrum that they could
absorb. Most of them also required the assistance of noble metal
(Au or Pd) cocatalysts, which considerably compromised their
potential applications.
Compared to those well-studied inorganic semiconductor
materials, photoactive conjugated polymers are famed for their
structural diversity as well as tunable optical and electronic
properties.[17-24] In particular, covalent triazine polymers (CTPs) —
which earn the name with their characteristic triazine nodes in the
formation of two-dimensional or three-dimensional frameworks —
attract rapidly growing attention, and have been investigated for
photocatalytic hydrogen generation and CO2 reduction.[25-30]
Herein, we for the first time report photocatalytic hydrogenation,
exemplified by the hydrogenation of maleic acid to succinic acid
(hydrogenation of C=C bonds) and furfural to furfuryl alcohol
(hydrogenation of C=O bonds) on a metal-free thiophene-
containing CTP. Both succinic acid and furfuryl alcohol are top-
valued chemicals and have an annual market size of $100-1000
million.[31-33] Remarkably, we demonstrate an incredible
production rate of ~2 mmol g-1h-1 for succinic acid, and ~0.5 mmol
g-1h-1 for furfuryl alcohol as well as great stability under visible light
illumination (λ > 420 nm).
Tremendous efforts have been invested on the pursuit of
artificial photosynthesis for the direct conversion of solar energy
to chemical energy.[1-5] Most of them have focused on
photocatalytic water splitting that produces molecular hydrogen
with the solar-to-hydrogen conversion efficiency up to ~1%.[6-8]
Unfortunately, hydrogen as the energy carrier intrinsically suffers
from low volumetric density under the ambient condition. This
poses a severe challenge to its storage and transportation.[9-10]
More recently, solar-driven organic transformation emerges as a
possible alternative and attracts increasing attention.[11-13] It may
give rise to higher-density products that can be more readily
stored and transported and with have greater economical values,
thereby opening a new avenue toward the effective utilization of
solar energy.
In industry, catalytic hydrogenation holds an essential key to
the synthesis of a variety of fine chemicals and bulk commodities.
The reaction usually proceeds under an elevated temperature
(>400oC) and pressure (up to 150 atm) using molecular hydrogen
as the reductant, and therefore is energy-intensive and potentially
hazardous.[14] It is highly desirable to develop alternative
hydrogenation strategies that operate under milder conditions.
Photocatalytic hydrogenation offers an ideal solution. It is
A thiophene-containing covalent triazine polymer (denoted as
CTP-Th) was prepared by the superacid-catalyzed trimerization
reaction of its corresponding aromatic nitrile precursor to yield
yellow powders at the gram scale (Figure 1a,b). The synthetic
details are described in the Supporting Information. Thiophene
and triazine moieties are designed and incorporated here as the
electron donor (D) and acceptor (A) units respectively in the
conjugated skeleton of CTP-Th (Figure 1a). They are expected to
promote the π-electron delocalization over the molecular
backbones, and enhance the light absorption and charge
separation.[34-35] The final product was first characterized by a
range of spectroscopic techniques to probe its molecular structure.
X-ray diffraction (XRD) analysis of CTP-Th indicates its
amorphous nature like many other CTPs (Figure S1). Solid-state
[*]
Y. P. Hu, Dr. W. Huang, H. S. Wang, Q. He, Y. Zhou, P. Yang, Prof.
Dr. Y. Y. Li and Prof. Dr. Y. G. Li
†
The two authors contribute equally.
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu
Key Laboratory for Carbon-Based Functional Materials and Devices,
Soochow University, Suzhou 215123, China
E-mail: yanguang@suda.edu.cn
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13C cross-polarization magic-angle-spinning nuclear magnetic
resonance (CP-MAS NMR) spectrum confirms the presence of
sp2-hybridized carbons in triazine with its peak at 169.7 ppm,
thereby providing definitive evidence for the successful
polymerization of monomers.[36-37] The signals at 143 ppm and
125 ppm are contributed from the thiophene units in the
conjugated skeleton (Figure 1c).[38] Fourier transform infrared (FT-
IR) spectrum of CTP-Th displays two signature bands of triazine
units at 1506 cm-1 and 1364 cm-1, assignable to the aromatic C-N
stretching and breathing vibrations respectively (Figure 1d).[39]
The absence of the vibrational band from the terminal cyano
group at 2221 cm-1 corroborates that the polymerization is
complete.[40] Elemental analysis result also agrees with the
expectation (Table S1). In addition, the microstructure of CTP-Th
was imaged under transmission electron microscopy (TEM) and
is shown to consist of aggregated nanoparticles of 20–50 nm in
size and with abundant porosity (Figure 1e). Its N2 adsorption and
desorption isotherm reveals the presence of mesopores and
macropores with Brunauer-Emmett-Teller (BET) surface areas
measured to be 72 m2 g-1 (Figure S2). Importantly, CTP-Th is
chemically stable, and exhibits thermal stability up to 400oC under
N2 from thermogravimetric analysis (TGA) (Figure S3).
and Th. Compared to the latter, CTP-Th exhibits a slightly red-
shifted absorption edge at ~530 nm and an extended tail up to
~700 nm. Their optical band gaps are estimated to be 2.26 eV
and 2.38 eV for CTP-Th and Th respectively based on the
Kubelka–Munk equation (Figure 2b). The enhanced light
absorption and reduced band gap of CTP-Th is believed to result
from the increased electron delocalization within its conjugated
framework. In order to accurately determine the electronic band
structure of CTP-Th, electrochemical Mott–Schottky analysis was
performed by casting its thin film on fluorine-doped tin oxide (FTO)
conductive glass as the working electrode. As shown in Figure S4
(Supporting Information), the inverse of its square capacitance (C-
2) is found to linearly correlate with the working potential between
0.2 V and 0.5 V (versus saturated calomel electrode or SCE) in
0.2 M Na2SO4. The extrapolation of this linear region to the
potential axis is derived to be -1.36 eV, corresponding to the flat
band potential or lowest unoccupied molecular orbital (LUMO)
level. The highest occupied molecular orbital (HOMO) level of
CTP-Th is accordingly determined to be +0.9 eV. Furthermore,
we
performed
steady-state
photoluminescence
(PL)
spectroscopy analysis with an excitation wavelength of 450 nm in
order to probe the electron transfer dynamics. Th exhibits a
pronounced PL emission peak centered at 521 nm. When
polymerized, the PL intensity of CTP-Th becomes red-shifted to
536 nm and significantly quenched by more than five times
(Figure 2c). Time-resolved PL spectroscopy reveals
a
photoexcited carrier lifetime (τ) of 0.35 and 0.77 ns for Th and
CTP-Th, respectively (Figure 2d). The attenuated PL emission
and prolonged carrier lifetime of CTP-Th in comparison with those
of Th unambiguously support that the radiative recombination of
photoexcitons is effectively retarded upon polymerization, due to
the enhanced photoexciton dissociation as facilitated by the D-A
structure. The enhanced charge separation within CTP-Th was
also confirmed using the density functional theory (DFT)
simulations. As shown in Figure S5 (Supporting Information), the
HOMO and LUMO electron-state-density distribution of Th
spatially overlap and evenly distribute throughout the molecule.
As for CTP-Th (here modelled using a trimer), its HOMO is mainly
located on the electron-donating thiophene struts, while the
LUMO is more delocalized over the conjugated framework owing
to the presence of the electron-withdrawing triazine ring. The
spatial asymmetric distributions of HOMO and LUMO evidence
facile photoexciton dissociation (Figure 2e). Such advantageous
optical property and charge dynamics render CTP-Th an ideal
metal-free candidate for photocatalytic applications as would be
discussed below.
The photocatalytic performance of CTP-Th for the selective
hydrogenation of unsaturated C=C and C=O double bonds
(exemplified using the reduction of maleic acid and furfural
respectively) were carried out by dispersing the photocatalyst
powder in corresponding aqueous solution under the irradiation of
a 300 W Xe lamp equipped with a 420 nm cutoff filter (see
Supporting Information for details). Additionally, ascorbic acid
was added to the reaction solution here as the sacrificial electron
donor in order to unveil the intrinsic activity and full potential of
CTP-Th for photocatalytic hydrogenation. Of note, it should be
highlighted that no cocatalyst was introduced so the entire
photocatalytic system was essentially metal-free. Since the
measured LUMO level of CTP-Th is higher than the theoretical
Figure 1. Preparation and characterization of CTP-Th. (a) Schematic illustration
of the synthesis of CTP-Th from the monomer (Th); (b) photograph of as-
prepared CTP-Th powders; (c) solid-state 13C NMR spectrum of CTP-Th; (d)
FT-IR spectra of CTP-Th and Th; (e) TEM image of CTP-Th.
We next interrogated the optical property and electronic
structure of CTP-Th, and compared them with those of the
thiophene monomer (denoted as Th). Figure 2a depicts the
ultraviolet-visible diffuse reflection (UV-vis DR) spectra of CTP-Th
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10.1002/anie.202006618
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reduction potential of maleic acid (-0.03 V at pH = 7 solution) and
furfural (-0.34 V at pH = 7 solution), photogenerated electrons on
CTP-Th are sufficiently energetic to drive their reductive
transformation (Figure 2f).
(Figure S8). The Th monomer is also not photocatalytic active.
Replacing ascorbic acid with methanol or lactic acid as the
sacrificial electron donor quenches the photocatalytic activity.
Moreover, our control experiments show that the photocatalytic
hydrogenation of maleic acid could not proceed on TiO2 P25 or
C3N4 free of any noble metal cocatalyst.
Photocatalytic hydrogenation may involve H+ or water molecule
as the hydrogen source. Even though a detailed mechanistic
study is beyond the scope of this work, we are interested at the
pH-dependence of the photocatalytic activity in order to shed light
on the possible reaction pathway. To this end, experiments were
conducted in solution with pH adjusted from ~1.6 (the standard
condition) to >12 under otherwise identical conditions. We find
that the photocatalytic activity (measured by the accumulated
amount of succinic acid after 6 h irradiation) monotonically
declines with the increase of solution pH, and become completely
suppressed at pH > 7 (Figure 3d). This trend leads us to conclude
that H+ directly participates in the rate-determining step while H2O
is not an effective proton donor. Interestingly, when H2O is
replaced with dimethyl sulfoxide (DMSO) as the solvent, the
production rate of succinic acid is lowered to 0.92 mmol g-1h-1. It
suggests that H2O promotes hydrogenation presumably by
facilitating the proton dissociation of ascorbic acid (Figure S9).
Figure 2. Optical property and charge dynamics analysis of CTP-Th. (a) UV-Vis
DR spectra of CTP-Th and Th; (b) band gap determination of CTP-Th and Th
according to the Kubelka–Munk equation; (c) steady-state PL emission spectra
of CTP-Th and Th excited at 450 nm; (d) time-resolved PL spectra of CTP-Th
and Th; (e) simulated HOMO and LUMO electron-state-density distributions on
CTP-Th; (f) HOMO and LUMO levels of CTP-Th plotted together with the
theoretical reduction potentials of maleic acid and furfural.
We first explored the photocatalytic hydrogenation of maleic
acid to succinic acid (Figure 3a). The reaction product was
analyzed and quantified using 1H NMR (Figure 3b and Figure S6).
It is found that in the presence of maleic acid, the hydrogen
production rate is suppressed from 6.61 mmol g-1h-1 (normalized
to the mass of CTP-Th) to 0.18 mmol g-1h-1 (Figure S7a,b). By
contrast, 2.05 mmol g-1 of succinic acid is yielded in the first hour
under the visible light illumination. The product linearly
accumulates with the irradiation time, and at the end of 30 h,
reaches a great amount of 56.5 mmol g-1 (or a total of 0.28 mmol
of succinic acid). This translates to a remarkable average
production rate of 1.88 mmol g-1h-1. We demonstrate that this
reaction is completely light-controlled. Under the alternating light-
dark period, the amount of succinic acid correspondingly
increases at the rate of ~2 mmol g-1h-1 when irradiated, and
remains unchanged when not (Figure 3c), ultimately leading to
the accumulation of ∼0.058 mmol of succinic acid after 12 h
reaction. Worth noting is that no hydrogenation product is
measured in the absence of CTP-Th, light or ascorbic acid within
the instrumental detection limit under otherwise identical
conditions, suggesting their indispensable roles in photocatalysis
Figure 3. Photocatalytic hydrogenation on CTP-Th. (a) Schematic showing the
photocatalytic hydrogenation of maleic acid and furfural to corresponding
products on CTP-Th; (b) photocatalytic production rates of succinic acid and
furfuryl alcohol over the reaction time; (c) photocatalytic hydrogenation of maleic
acid under the alternating light-dark period; (d) pH-dependent photocatalytic
activities in the hydrogenation of maleic acid; (e,f) recyclability of CTP-Th for the
photocatalytic hydrogenation of (e) maleic acid and (f) furfural.
Following the successful demonstration about the
photocatalytic transformation of maleic acid, we went on to study
the photocatalytic hydrogenation of furfural to furfuryl alcohol. The
resultant liquid product was quantitatively analyzed by high
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performance liquid chromatography (HPLC) (Figure S10). Its
amount is also observed to follow a linear dependence on the
irradiation time with an average production rate of 0.5 mmol g-1h-
1 in 18 h (Figure 3b). The competing hydrogen production rate is
measured to be only ~0.09 mmol g-1h-1 (Figure S7c). The
photocatalytic activity for the hydrogenation of furfural is still
significant, but admittedly lower than that of maleic acid probably
owing to its smaller thermodynamical driving force as indicated in
Figure 2f.
Science and Technology. W. Huang thanks the support from
China Postdoctoral Science Foundation (2018M640518).
Keywords: metal-free photocatalytic hydrogenation • covalent
triazine polymers • donor-acceptor units • maleic acid • furfural
References
In addition to excellent activities, catalyst recycling is important
in heterogeneous catalysis as this would greatly reduce the
material cost. We here examined the recyclability of our polymeric
photocatalyst for both reactions. This was done by filtrating the
catalyst out from the solution after 6 h light experiment (defined
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and re-dispersion of our polymeric photocatalyst. CTP-Th
collected after the recycling experiments exhibits no apparent
change in its molecular structure as revealed from UV-vis DR, FT-
IR and TEM image (Figure S11).
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Yongpan Hu†, Wei Huang†, Hongshuai Wang, Qing He, Yuan Zhou,
Ping Yang, Youyong Li and Yanguang Li*
Metal-Free Photocatalytic Hydrogenation Using Covalent
Triazine Polymers
Covalent triazine polymeric photocatalyst with unique donor-
acceptor units could enable the metal-free photocatalytic
hydrogenation of maleic acid to succinic acid as well as furfural to
furfuryl alcohol with an incredible production rate, great stability and
recyclability.
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