Conjugated microporous polymers as a visible light driven platform for photo-redox conversion of biomass derived chemicals

Article abstract of DOI:10.1039/d1gc00762a

Photocatalytic conversion of biomass derived chemicals to valuable products is a highly sustainable process. Herein we report the photocatalytic hydrogenation of maleic acid to succinic acid and oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran using the same conjugated porous polymers (CMPs). The CMPs were constructed from 2,4,6-(tri-2-thienyl)-1,3,5-triazine as the knots and different benzene derivatives as linkers, and their morphologies, redox potentials, charge separation efficiency, and the consequent photocatalytic performance have been controlled. As a result, the CMP with benzene as the linker features the highest photocatalytic activities with production rates of 4.66 mmol g^?1h^?1for succinic acid and 0.53 mmol g^?1h^?1for 2,5-diformylfuran, respectively. Most importantly, high photocatalytic activity has also been achieved under natural sunlight irradiation, implying its feasibility as an efficient photocatalytic platform for solar-to-chemical energy conversion.

Full text of DOI:10.1039/d1gc00762a

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Conjugated microporous polymers as a visible
light driven platform for photo-redox
Cite this: DOI: 10.1039/d1gc00762a
conversion of biomass derived chemicals†
Received 28th February 2021,
Accepted 19th April 2021
Bo Chen, Lang Chen, Zijun Yan, Jinyang Kang, Shanyong Chen, Yongdong Jin,
DOI: 10.1039/d1gc00762a
Lijian Ma,
Hongjian Yan * and Chuanqin Xia*
rsc.li/greenchem
Photocatalytic conversion of biomass derived chemicals to valu- energy.10,11 During a photocatalytic process, a photocatalyst
able products is a highly sustainable process. Herein we report the absorbs light and generates photoexcited electrons and holes,
photocatalytic hydrogenation of maleic acid to succinic acid and which could drive reductive and oxidative reactions, respect-
oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran using ively. Therefore, using carefully designed photocatalysts, many
the same conjugated porous polymers (CMPs). The CMPs were current catalytic redox processes involving harsh reaction con-
constructed from 2,4,6-(tri-2-thienyl)-1,3,5-triazine as the knots ditions and high costs could be replaced by photocatalysis,
and different benzene derivatives as linkers, and their mor- with sunlight as the energy source.12
phologies, redox potentials, charge separation efficiency, and the
The last decade has witnessed tremendous efforts in the
consequent photocatalytic performance have been controlled. As pursuit of novel heterogeneous photocatalysts with excellent
a result, the CMP with benzene as the linker features the highest activities. Among them, organic semiconductors, such as
photocatalytic activities with production rates of 4.66 mmol g−1 g-C3N4, CTFs, CMPs, and COFs, have gained considerable
h−1 for succinic acid and 0.53 mmol g−1 h−1 for 2,5-diformylfuran, attention due to their advantages of lower price and less tox-
respectively. Most importantly, high photocatalytic activity has icity compared to their inorganic counterparts.13–18 In particu-
also been achieved under natural sunlight irradiation, implying its lar, conjugated microporous polymers (CMPs) are attracting
feasibility as an efficient photocatalytic platform for solar-to- rapidly growing attention due to their extended π-conjugation,
chemical energy conversion.
high designability, large specific surface area, excellent
thermal stability and tunable band structure, making them
ideal for the development of efficient porous materials for
photocatalysis, energy storage, energy conversion and photo-
thermal therapy.19–22 A lot of research studies have demon-
strated their outstanding activities on photocatalytic hydrogen
evolution and CO2 reduction.23,24 More recently, CMPs were
also used in the photocatalytic transformation of biomass
derived chemicals. For instance, CTP-Th could be used for
efficient photocatalytic hydrogenation of maleic acid and fur-
fural under visible light.25 Using CTF-Th@SBA-15, HMF could
be selectively oxidized to 2,5-diformylfuran (DFF).26 However,
apart from the relatively low photocatalytic rates, their sole use
in reduction or oxidation limits the practical applications of
these CMPs in the upgrading of biomass derived chemicals.
Herein, we report novel CMPs prepared by coupling 2,4,6-
(tri-2-thienyl)-1,3,5-triazine (TTT) and different benzene deriva-
tive linkers for photocatalytic reductive and oxidative conver-
sions of biomass-derived chemicals. The linkers in the CMPs
play an important role in affecting their morphologies, and
optical and electrical properties, as well as photocatalytic per-
formance. The photocatalytic hydrogenation of maleic acid to
succinic acid and oxidation of HMF to DFF were realized by
In the past century, the fossil-based industry has drastically
promoted economic growth and greatly changed the face of
the Earth. However, global overexploitation of fossil fuels has
also caused ever increasing concern about serious environ-
mental pollution and climate warming problems.1–3 As a sus-
tainable alternative, biomass, by appropriate upgrading pro-
cesses, could be converted into a wide range of high-valued
chemicals. Biomass refinery has mainly focused on catalytic
reductive and oxidative upgrading of furan-based platform
molecules, such as furfural and 5-hydroxymethylfurfural
(HMF).4–6 In most catalytic research studies, the extensive use
of noble metal catalysts, and high temperature and pressure
substantially increase energy consumption and costs.7–9 In
contrast, photocatalysis is known for its mild reaction con-
ditions under ambient temperature and atmospheric pressure,
and direct conversion of solar energy into chemical
College of Chemistry, Sichuan University, Chengdu 610064, P.R. China.
E-mail: hjyan@scu.edu.cn, xiachqin@163.com
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00762a
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the same CMP with remarkable high production rates of could also be found at 1030 and 720 cm−1. The attenuation of
4.66 mmol g−1 h−1 for succinic acid and 0.53 mmol g−1 h−1 for the C–Br band around 1070 cm−1 of CMPs clearly illustrates
DFF. Most importantly, high photocatalytic activity has also the high degree of polymerization.24,27 In addition, more
been achieved under natural sunlight irradiation, implying its detailed connection of these CMPs was obtained via solid-state
feasibility as an efficient photocatalytic platform for solar-to- 13C CP/MAS NMR. The low-field signals at δ = 167 ppm in the
chemical energy conversion.
spectra of all CMPs are attributed to the carbon atoms in the
To build our CMPs, we chose TTT as the knots because its triazine rings. All the remaining peaks could also be ascribed
inherent donor (three thiophene rings) and acceptor (triazine to specific carbons in the three CMPs. For example, the high-
ring) extend the visible light absorption of all the corres- field peak at 55 ppm of pTTT-DMOB is derived from the
ponding CMPs. TTT was linked with three isomorphic linkers, carbon atom of the methoxy group, while the peak at 140 ppm
benzothiadiazole (BTD), benzene (Ben) and dimethoxybenzene is assigned to the carbon in the thiadiazole ring (Fig. 1b and
(DMOB), to form three CMPs (labeled as pTTT-BTD, pTTT-Ben Fig. S3†).28,29
and pTTT-DMOB, respectively) with adjusted optical and elec-
The surface morphologies of the three CMPs were observed
trical properties. As illustrated in Fig. 1a, all the CMPs were by SEM and TEM (Fig. S6†). Both pTTT-BTD and pTTT-DMOB
synthesized by a C–H direct arylation reaction. Three CMPs exhibited nonuniform bulk particles. However, pTTT-Ben pre-
were obtained as colored powders, which did not dissolve in sented a nanofiber morphology, which intertwined to further
water and general organic solvents.
form a reticular structure (Fig. 1c). TEM images (Fig. 1d) showed
Firstly, we applied elemental analysis, Fourier transform that the diameter of the nanofibers ranged from 20 to 40 nm. It
infrared (FT-IR) spectroscopy, and solid-state 13C CP/MAS is believed that this 1D nanofiber structure enables better
NMR to confirm the chemical structures of the obtained exposure of active sites, which leads to more efficient charge
CMPs. Elemental analysis results revealed that the carbon, transfer compared to 3D bulk particles.30 The different mor-
hydrogen, nitrogen and sulfur contents are close to their calcu- phologies may be caused by the negative effect of side groups in
lated values (Table S1†). As shown in Fig. S2,† the intense benzothiadiazole and dimethoxybenzene on π–π stacking inter-
peaks at 1500 and 1370 cm−1 in the infrared spectra could be action, which made pTTT-Ben grow faster along the Z direction
ascribed to the C–N stretching and breathing mode in the tri- rather than X and Y directions.31–33 According to the N2 sorption
azine ring, respectively. The characteristic bands of thiophene experiment at 77 K, the Brunauer–Emmett–Teller (BET) specific
surface areas of pTTT-BTD, pTTT-Ben and pTTT-DMOB were
found to be 66, 307 and 39 m2 g−1, respectively (Fig. S4†). As
known, a high surface area plays a propitious role for catalysts in
heterogeneous catalysis.34 Thermogravimetric analysis was con-
ducted to study thermal stability (Fig. S10†). Thanks to the fully
conjugated aromatic backbones, these CMPs are chemically
stable and exhibit high thermal stability.35–37 Especially,
pTTT-Ben has an excellent thermal stability of up to 600 °C.
The UV-Vis diffuse reflectance spectra (DRS) and solid-state
fluorescence spectra of the CMPs are displayed in Fig. 2a and
Fig. S11,† respectively. These CMPs exhibited different visible-
light absorption properties. Compared with pTTT-Ben, both
pTTT-DMOB and pTTT-BTD have red-shifted absorption and
emission, but the reasons are very different. For pTTT-DMOB,
it can be explained by the more electron-rich linker of di-
methylbenzene than benzene, while for pTTT-BTD, it is mainly
the influence of the D–A structure consisting of benzothiadia-
zole and adjacent thiophenes. Calculated from the corres-
ponding Tauc plot (Fig. 2b), their optical band gaps were
determined to be 2.01 eV for pTTT-BTD, 2.50 eV for pTTT-Ben
and 2.23 eV for pTTT-DMOB, which are in good agreement
with their colors (insets of Fig. 2c). To further investigate the
relative band structures, the electrochemical Mott–Schottky
analysis was performed (Fig. S12†). The flat band potentials,
or conduction bands (CB), were approximately −0.39, −0.48
and −0.44 V (vs. SCE), with corresponding valence band (VB)
positions at +1.62, +2.02 and +1.79 V for pTTT-BTD, pTTT-Ben,
and pTTT-DMOB, respectively (Fig. 2c).
It is known that organic semiconductors have a high
exciton binding energy, which makes charge separation a key
Fig. 1 Synthetic routes (a) of CMPs and (b) 13C solid state NMR spectra
of TTT-based CMPs, spinning side bands are marked with an asterisk (*).
(c) SEM and (d) TEM images of pTTT-Ben.
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(−0.03 V vs. SCE)25 and HMF (+0.89 V vs. SCE),26 indicating
that photogenerated electrons/holes in TTT-based CMPs can
kinetically drive these redox transformations.
Firstly, we explored the photocatalytic hydrogenation of
maleic acid to succinic acid over the obtained CMPs using
ascorbic acid as the sacrificial agent and a LED (40 W, 460 nm)
lamp as the light source. The yields of succinic acid were deter-
mined by 1H-NMR spectroscopy. As depicted in Fig. 3b, the
succinic acid production rates were 0.16 mmol g−1 h−1 for
pTTT-BTD, 2.44 mmol g−1 h−1 for pTTT-DMOB and 4.66 mmol
g−1 h−1 for pTTT-Ben. Among the three CMPs, pTTT-Ben
exhibited the best photocatalytic performance. A time-depen-
dent experiment demonstrated that the production of succinic
acid correlates approximately linearly with the irradiation time
in the initial stage and finally the yield reached 40% at 20 h
(Fig. S15 and S16†). It is important to note that without light,
photocatalyst or ascorbic acid, no hydrogenation product was
detected, demonstrating the nature of the photocatalytic
process (Fig. S18†).
Fig. 2 Optical properties and band structure analysis. (a) UV/Vis DR
spectra of three CMPs. (b) Tauc plot and their optical band gaps. (c)
Calculated CB and VB positions. (d) Time-resolved photoluminescence
spectroscopy.
factor influencing the photocatalytic efficiency of organic
semiconductors.38,39 The carrier lifetime (τ) in time-resolved
photoluminescence spectroscopy can evaluate the separation
of the photogenerated electrons and holes. As shown in
Fig. 2d and Table 1, pTTT-Ben has the longest carrier lifetime
(τ = 0.36 ns), implying that it possesses the highest charge sep-
aration efficiency among the three CMPs. The highest photo-
current response of pTTT-Ben also shows its high production
of photogenerated carriers (Fig. S13†).40,41
Subsequently, the CMPs were applied to the photocatalytic
transformation of organic chemicals. In order to study the
application potential of these CMPs in biomass refinery,
maleic acid and HMF were chosen as model molecules,
because they are both important biomass derivatives. More
importantly, their reduction/oxidation products, succinic acid
and DFF, are also value-added chemicals. Succinic acid is
widely used in food, pharmaceutical and plastic industries,
while DFF can be used as a furan-based chemical for polymer
materials, drug intermediates, antifungal agents and fluo-
rescent agents.42,43 To our delight, the measured CB/VB levels
of TTT-based CMPs match redox potentials of maleic acid
Table 1 Porous and optical properties of three CMPs
CMPs
SBETa [m2 g−1
]
Egb [eV]
CBc [V]
VBc [V]
τd [ns]
pTTT-Ben
pTTT-DMOB
pTTT-BTD
307
39
66
2.50
2.23
2.01
−0.48
−0.44
−0.39
+2.02
+1.79
−1.62
0.36
0.30
0.30
a Surface area calculated from the N2 adsorption isotherm using the
BET equation. b Optical band gaps calculated using the Kubelka–Munk
function. c Conduction band valence band positions calculated from
band gaps and flat band potentials vs. SCE. d Fluorescence lifetime cal-
culated from the equation τ = t (I0/e) − t (I0).
Fig. 3 Photocatalytic activities of the CMPs. (a) and (b) show the photo-
catalytic production rates of succinic acid and DFF with different cata-
lysts, respectively.
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Encouraged by the excellent photocatalytic hydrogenation activity after 5 cycles, with its structure unaltered (Fig. S25 and
activity of these CMPs, we further tested their photocatalytic oxi- S26†). The slight decay of activity is more likely due to photo-
dation of HMF. Atmospheric air was used as the oxidant because catalyst mass loss during each solid–liquid separation process.
it is cheaper, safer and more convenient than pure oxygen.44
In summary, three CMPs with different isomorphic linkers
1H-NMR spectroscopy revealed that DFF was the sole product, were successively synthesized by C–H direct arylation polymer-
with formation rates of 0.06, 0.04 and 0.53 mmol g−1 h−1 for ization. The linkers in the CMPs have huge multiple impacts
pTTT-BTD, pTTT-DMOB and pTTT-Ben, respectively (Fig. 3c). on their morphologies, redox potentials and charge separation
The time-dependent experiment showed that the production efficiency. The photocatalytic hydrogenation of maleic acid to
yield of DFF was linearly correlated with irradiation time and the succinic acid and oxidation of HMF to DFF were realized by
yield reached 25% at 60 h (Fig. S20 and S21†). Control experi- using these CMPs as photocatalysts. The benzene linked
ments also confirmed that no production of DFF occurred in the pTTT-Ben has the highest surface area, nanofiber morphology,
dark, under a nitrogen atmosphere or in the absence of a photo- high thermal stability, relatively wide band gap, and strong
catalyst (Fig. S23†). A pH-dependent experiment demonstrated charge separation effect, and therefore show the best photo-
that DFF production was totally inhibited when the reaction catalytic properties in both reductive and oxidative reactions.
system was strongly basic (pH = 12, Fig. S22†).
In addition, high photocatalytic activity under natural sunlight
Notably, using pTTT-Ben, both the photocatalytic rates of irradiation has also been achieved. Our results showed that,
hydrogenation of maleic acid and oxidation of HMF are much according to rational design, CMPs could act as a promising
higher than most reported works including some inorganic sunlight-driven photocatalytic platform for efficient transform-
semiconductors (Table S5†).25,26,45,46 It is known that a photo- ation of biomass derived chemicals.
catalytic reaction is controlled by three key factors, namely, light
absorption, charge separation and surface reaction.13 From
Fig. 2a, we find that pTTT-Ben absorbs less or similar light than
the other two CMPs at 460 nm, while pTTT-Ben is quite
different from them in surface properties and charge separation
gation, writing – original draft, visualization. Lang Chen, Zijun
(Table 1 and Fig. S6†). So, we suspect that surface reaction and
charge separation are indispensable reasons for the best photo-
Ma: validation, resources. Hongjian Yan and Chuanqin Xia:
catalytic performance of pTTT-Ben among the three CMPs.
Since the CMPs were synthesized by the Pd catalyzed reaction,
the residual Pd was detected by inductively coupled plasma
Author contributions
Bo Chen: conceptualization, methodology, resources, investi-
Yan, Jinyang Kang, Shanyong Chen, Yongdong Jin and Lijian
project administration, supervision, funding acquisition, visu-
alization, writing – review & editing.
atomic emission spectrometry (ICP-AES), which showed that Pd
contents in all three CMPs were lower than 0.02% (Table S3†).
Such low values were below the detection limit of energy-disper-
sive X-ray spectroscopy (EDX) (Table S2†). These contents are
Conflicts of interest
The authors claim no conflicts of interest.
much lower than most other CMPs prepared by Pd catalyzed
reactions.23,28 Several studies also show that there is no clear
correlation between residual Pd and photocatalytic activity.47,48
Acknowledgements
Therefore, it is unlikely that the Pd contributes significantly to
the observed high photocatalytic rates.
This work was supported by the National Natural Science
Foundation of China (grant no. 11475120, 11575122,
Considering that sunlight is free, ubiquitous and inexhausti-
ble and one of the goals of photocatalysis is to directly convert
11775153, U1732107, 21790372, 22072100 and 21501123
solar energy to chemical energy,49,50 we conducted the two reac-
2020YFS0070), the International Collaboration Project of
tions with pTTT-Ben under natural sunlight irradiation. The
Science and Technology Program of Sichuan Province, China
reactions were carried out with test tubes half submerged in a
water bath (Fig. S24†). The sunlight intensity was detected at
every half-hour interval and is shown in Table S4.† As expected,
(project no. 2017HH0056, 2018HH0063 and 2021YFH0170),
and the Fundamental Research Funds for the Central
Universities (no. 20826041D4117). We also thank the
pTTT-Ben also exhibited good photocatalytic ability. The
Comprehensive training platform of specialized laboratory and
average production rates of succinic acid and DFF were 0.57 and
the Testing Center, College of Chemistry, Sichuan University
0.28 mmol g−1 h−1, respectively, lower than those under LED
for financial and technical support.
light irradiation. However, even under low solar irradiation and
dense cloud in winter, the results are still impressive and
demonstrate its high portability to other seasons and districts.
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catalyst pTTT-Ben in the photocatalytic conversion of maleic
acid. The photocatalyst maintained over 85% of its original
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