Hierarchical Porous Covalent Organic Frameworks: The Influence of
Additional Macropores on Photocatalytic Hydrogen Evolution and
Hydrogen Peroxide Production
Islam E. Khalil, Prasenjit Das, Huseyin Kucukkececi, Veit Dippold, Jabor Rabeah, Warisha Tahir,
Jérome Roeser, Johannes Schmidt, and Arne Thomas*
Cite This: Chem. Mater. 2024, 36, 8330−8337
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sı Supporting Information
ABSTRACT: Covalent organic frameworks (COFs), an emerging class of
crystalline porous materials, have garnered significant interest due to their
low density, tunable chemical and physical properties, porous structure, and
high surface area. COFs typically exhibit microporosity, i.e., pores below 2
nm influenced by their specific linkers and binding patterns. In addition,
some COFs are reported to exhibit small mesopores, especially when
extended linkers are used. In applications such as catalysis, where rapid mass
transport is a crucial factor, hierarchical pore structures of catalysts are
beneficial. This involves the presence of small micropores to generate a
large surface area and additional macropores to facilitate the transport of
reactants to catalytic sites and to promote product diffusion. This study
describes the application of such a hierarchical porous COF (macro-
TpBpy) for photocatalysis. The macro-TpBpy architecture seamlessly
integrates intrinsic microporosity with additional macropores, thereby yielding a substantial increase in the surface area. The
hierarchical porous material demonstrates a promising performance in photocatalytic hydrogen evolution reaction, exhibiting a rate
of 4.88 mmol g−1h−1. Notably, this is a fourfold improvement compared to the COF analogue featuring micropores only.
Furthermore, the introduction of macropores proved to be beneficial for the photocatalytic production of hydrogen peroxide
(H2O2). Specifically, macro-TpBpy exhibited a production rate of 2716 μmol g−1h−1, in water without sacrificial hole scavengers,
whereas pristine TpBpy had a rate of 2134 μmol g−1h−1. This work thus contributes to the further development of COFs in
photocatalysis and shows that, in addition to suitable band structure and surface functionality, the pore size and substrate transport
influenced by it must also be considered as important factors.
■INTRODUCTION
In recent years, there has been significant research focus on the
exploration of extended crystalline microporous materials. This
interest stems not only from their intriguing structural
characteristics but also from their broad range of potential
applications, such as catalysis, optics, or drug delivery.
1−10
However, the exclusive presence of micropores in crystalline
porous materials can pose challenges related to diffusion. This
limitation becomes particularly problematic in catalytic
processes that are reliant on efficient mass transfer, resulting
in the limited utilization of active sites. The development of
hierarchical porous materials characterized by micro-, meso-,
and macropores presents a highly promising strategy for
mitigating this issue. Within these hierarchical porous
frameworks, micropores are primarily dedicated to increasing
the overall surface and consequently the number of active sites.
On the other hand, meso- or macropores serve as efficient
transportation channels, yielding a significant reduction in
diffusion restrictions and thus an increase in mass transfer.
11−14
Covalent organic frameworks (COFs) represent an emerg-
ing class of crystalline porous materials, created by covalent
bonds between organic building blocks.
15−17
They offer
significant potential for a wide range of applications in various
areas. During the past decade, substantial efforts have been
made to synthesize novel COF structures, including, for
example, chiral, ionic, and guest-responsive (dynamic)
architectures.
18−20
Concurrently, there have been reports on
the controlled morphology of single-phase COFs, notably in
the form of films and fibers.
21−24
In various applications,
hierarchical porous COFs have demonstrated significantly
better performance compared to their nonhierarchical counter-
Received: May 3, 2024
Revised: July 26, 2024
Accepted: July 26, 2024
Published: August 20, 2024
Articlepubs.acs.org/cm
© 2024 The Authors. Published by
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https://doi.org/10.1021/acs.chemmater.4c01298
Chem. Mater. 2024, 36, 8330−8337
This article is licensed under CC-BY 4.0
parts. These applications span a wide range of areas, including
electrocatalytic applications, battery technologies, and separa-
tion processes.
25−27
However, one noteworthy domain in
which their potential is so far unexplored is in photocatalysis.
It can be reasoned, that also in photocatalytic applica-
tions,
28−32
a hierarchical porous structure should be beneficial
enabling short diffusion pathways, facilitating the rapid
transport of reactants and products to and from the catalytic
sites, thereby ensuring the efficient utilization of photo-
generated charge carrier recombination events.
33,34
This
might especially hold true if gaseous products are formed
from liquids, as in the photocatalytic hydrogen or oxygen
evolution reaction (HER, OER) where gas bubbles have to
escape from the surface, which might be severely difficult in
small microporous channels. Furthermore, a hierarchical pore
arrangement can facilitate the penetration of incident light
deep into the material, expanding the exposure of a substantial
portion of the catalyst to light and thereby enhancing the
overall efficiency of photocatalytic reactions. It is often
reasoned that a similar effect is exploited in nature, for
example, seen in the hierarchical porous structure of the silica
cell walls of diatoms, which might enhance light capture
needed for photosynthesis.
35
Moreover, the high surface area
inherent to hierarchical materials endows them with a greater
number of accessible active sites that improves performance
compared to nonhierarchical porous materials.
13,33,34,36,37
Building upon our previously published research paper
outlining an innovative method for synthesizing crystalline
and hierarchical porous COF structures for application in
electrocatalytic OER,
14
we endeavored to extend the utility of
such micro/macroporous COFs to photocatalysis.
We employed polystyrene spheres (PSs) as hard templates
to create interconnected macropores within the COF. The
resultant macroporous COF exhibits high crystallinity and high
surface area and a hierarchical pore structure. Consequently,
this material facilitates rapid mass transport and enhances the
accessibility of active sites, thereby improving the performance
of HER and the production of H2O2.
■RESULTS AND DISCUSSION
Macro-TpBpy was synthesized via a Schiff base reaction
involving the organic linkers 2,2′-bipyridine-5,5′-diamine
(Bpy) and 1,3,5-triformylphloroglucinol (Tp),
38
catalyzed by
p-toluenesulfonic acid (PTSA), to promote COF formation.
39
This synthesis took place in the presence of monodisperse
colloidal PSs, which serve as a hard template (Scheme 1).
40
In
contrast to the first method established by us,
14
further
developments were made in the synthesis of the hierarchical
porous COF involving modifications in the monomer
dispersion procedure, the drying process, and PS removal
(see Supporting Information for details). These modifications
result in an enhanced crystallinity and surface area of the
hierarchical porous COF compared with our previous
approach. As the shaking time during the synthesis of COF
composite can significantly impact the material’s overall
porosity, most probably because prolonged shaking ensures
better dispersion of polystyrene within the COF matrix, this
helps in avoiding large agglomerates, promotes uniform
distribution of PS spheres, and also allows for more thorough
mixing of reactants. This can lead to a uniform nucleation
process, where the COF crystallizes around well-dispersed
polystyrene particles. Furthermore, slow evaporation ensures a
uniform distribution of polystyrene within the COF matrix,
promoting porosity, enhancing interfacial interaction, and
minimizing structural defects. In addition, by enhancing the
washing and extraction process, the complete removal of the
polystyrene can be ensured that would otherwise block the
pores of the TpBpy COF, resulting in enhancing the surface
area.
41
The final material, termed macro-TpBpy, exhibits
crystallinity and macropores reflecting the size of the spherical
PS template of ∼270 nm (Figure 1a,b, S1 and S2). In contrast,
pristine TpBpy, synthesized without the PS template, showed
no characteristic morphologies (Figure S3).
To assess the crystallinity, PXRD analyses were conducted
on pristine TpBpy (prepared without a PS template), PS@
TpBpy (before PS removal), and macro-TpBpy. Macro-TpBpy
exhibits a sharp reflection at 3.6°(2θ), resembling pristine
TpBpy, confirming the retained crystalline structure after PS
template removal. Broad reflections at 26°(2θ) indicate π−π
stacking of the COF layers. PS@TpBpy shows an additional
broad reflection at 19°(2θ), which can be attributed to the
amorphous PS template. This reflection is absent in macro-
TpBpy (Figure 1c). Nitrogen sorption isotherms were
conducted to assess the specific surface area and microporous
characteristics of both pristine TpBpy and macro-TpBpy,
showing that the modified synthesis procedure largely
increased the accessible pore volume and surface area of the
hierarchical COF compared to our former report (Figure
S4).
14
Calculation of the pore volumes confirms the existence
of macroporosity within the hierarchical macro-TpBpy, as
evidenced by the notably higher total pore volume of macro-
TpBpy (0.799 cm3/g) compared to pristine TpBpy (0.468
cm3/g). Moreover, the Brunauer−Emmett−Teller (BET)
surface areas calculated for macro-TpBpy (1384 m2/g) surpass
those of pristine TpBpy (859 m2/g) which indicates that the
macropores yield a better accessibility of the microporous
channels. Thermogravimetric analysis (TGA) of both pristine
and macro-TpBpy COFs under a nitrogen atmosphere shows
no weight loss until 370 °C, supporting their good thermal
stability (Figure S5). Furthermore, the macro-TpBpy COF
exhibited chemical stability when subjected to different
conditions, i.e., in water, acid, and bases (Figure S6). Fourier
transform infrared spectroscopy (FTIR) analysis of both the
as-synthesized COFs aligns well with previously reported data.
The strong peaks at approximately 1565 cm−1(−C�C) and
1258 cm−1(−C−N) clearly indicate the formation of β-
ketoenamine-linked framework structures in both pristine and
macro-TpBpy which indicated that the β-ketoenamine back-
bone is unchanged after introducing the macropores in the
TpBpy COF (Figure S7).
42
These observations were further
confirmed by solid-state 13C NMR spectroscopy, which
Scheme 1. Illustration of the Formation of Macro-TpBpy
Employing Hard PSs as Template and Its Use for
Photocatalytic HER and Hydrogen Peroxide Production
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distinctly showed the characteristic peak of the carbonyl
carbon (C�O) from β-ketoenamine in the range of
approximately 180−184 ppm for both pristine and macro-
TpBpy COFs (Figure S8).
To determine the optical properties of pristine and macro-
TpBpy COFs, solid-state UV−visible diffuse reflection spec-
troscopy and valence band (VB) X-ray photoelectron spec-
troscopy (XPS) were utilized. Pristine and macro-TpBpy
COFs both exhibit broad light absorption, while for the macro-
TpBpy COF, a more pronounced tailing of absorption to
longer wavelengths is observed. The absorption onsets are
located at 598 and 634 nm, respectively, for pristine and
macro-TpBpy COFs (Figure 2a). Using the Tauc plot, the
optical band gaps were found to be 2.21 and 2.13 eV for
pristine TpBpy and macro-TpBpy COFs, respectively. From
VB XPS measurements, the energy values for pristine TpBpy
and macro-TpBpy were measured to be 2.25 and 2.04 eV,
respectively (Figures S9−S10). Using these results, we
determined the energy potentials for the VB and conduction
band (CB) to be −6.60 and −4.39 eV for pristine TpBpy and
−6.39 and −4.26 eV for macro-TpBpy, relative to the vacuum
level (Figure 2b). As expected, it is observed that the
introduction of macroporosity did not significantly alter the
optical properties of the COFs. Indeed, based on the band
positions, it is anticipated that the reduction of 2H+to H2is
feasible on both, pristine TpBpy and macro-TpBpy COFs.
Electron paramagnetic resonance (EPR) spectroscopy is an
effective method to assess the charge separation efficiency of
photocatalysts.
43,44
The behavior of the CB electrons (CB e−)
was monitored using EPR before and after visible light
exposure. For pristine and macro-TpBpy COFs, a consistent
EPR signal at g= 2.007 was observed, attributed to the
presence of photoexcited electrons in the CB, which increases
largely after light exposure, affirming the generation of
electron−hole pairs in the COF catalysts. Notably, the
macro-TpBpy COF exhibited a signal intensity substantially
higher than that of pristine TpBpy, suggesting a marked
improvement in charge separation efficiency by the incorpo-
ration of macropores in TpBpy (Figure 2c,d), probably due to
enhanced light capture and photon utilization. To further
elucidate the charge separation efficiency, we measured the
photocurrent in the absence and presence of visible light for
both pristine and macro-TpBpy COFs. The intensity of the
photocurrent notably increased upon light exposure, with
macro-TpBpy exhibiting a higher photocurrent intensity
compared to pristine TpBpy, indicating a higher number of
light-induced charge carriers. However, there is a decreasing
trend of photocurrent density, which might indicate some
photocorrosion of the catalyst. On the other hand, in long-
term and recycling photocatalytic experiments, no significant
decrease in activity is observed (Figure S11), thus the decrease
in photocurrent density might as well be explained with some
loss of the catalyst from the surface of the FTO electrode
during this experiment.
In the next step, both COFs were tested as photocatalysts.
First experiments were conducted to test their photocatalytic
performance in hydrogen evolution from water, using visible
light irradiation (λ> 420 nm) and employing 0.1 M
triethanolamine (TEOA) as a sacrificial electron donor
(SED) for capturing the photogenerated holes produced by
Figure 1. FESEM figures of (a) macro-TpBpy, the yellow square shows a section with higher magnification and (b) PS@TpBpy. (c) PXRD pattern
of pristine TpBpy, PS@TpBpy, and macro-TpBpy COFs.
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the photocatalyst and platinum (Pt) as a cocatalyst (Figures
S12−S14). Details on the experimental procedure can be
found in the Supporting Information. The macro-TpBpy COF
demonstrated a significantly improved photocatalytic activity
in the HER, achieving a rate of 4.88 mmol g−1h−1, which is
approximately 4 times higher than that observed for the
pristine counterpart (1.24 mmol g−1h−1) (Figure 3a). This
superior photocatalytic performance can be attributed to
several factors resulting from the additional introduced
macroporosity. First, the hierarchical porous structure should
facilitate the transport of reactants and products to and from
the catalytic sites. By minimizing mass transport time, we can
also assume a more effective utilization of photogenerated
charge carriers (electron−hole pairs) before recombination
events occur. In the present case, also the formation of
hydrogen gas bubbles should be facilitated through the
macropores, which otherwise would compete with the wetting
of the micropore channels and the surface of the COF with
water molecules. Additionally, the hierarchical porous structure
allows incident light to penetrate deeply into the material,
thereby increasing the exposure of a substantial portion of the
catalyst to light and enhancing the overall photocatalytic
efficiency. Finally, the larger surface area of hierarchical porous
materials provides a greater number of accessible active sites.
An extended 24 h HER experiment was carried out to assess
the stability of the macro-TpBpy COF. A steady production of
Figure 2. (a) Solid-state UV−vis spectroscopy of pristine and macro-TpBpy COFs. (b) Experimental band gap and band positions of pristine
TpBpy and macro-TpBpy COFs. EPR CB electron spectra of macro-TpBpy (c) and pristine TpBpy (d) in the dark and during visible light
irradiation (> 420 nm, 300 W Xe lamp).
Figure 3. (a) Photocatalytic HER rates of the macro- and pristine TpBpy COF. (b) Photocatalytic H2O2production rates of the macro- and
pristine TpBpy COF, in water without the use of a SED under O2purging.
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hydrogen over the entire time frame indicates satisfactory
stability of the catalyst (Figure S15a). The recyclability of the
macro-TpBpy COF was further investigated. After 2 h of the
reaction, the catalyst was isolated by centrifugation, dried, and
then reused in the remaining runs for 2 h in each cycle. As
shown in Figure S15b, the macro-TpBpy COF can be reused
for at least four cycles without significant decrease in activity.
Moreover, XRD analysis showed that the macro-TpBpy COF
retained its crystalline structure following the 24 h reaction
period (Figure S15c). Furthermore, the FTIR spectra remain
consistent before and after 24 h of photocatalysis, confirming
the preservation of the chemical structure (Figure S15d).
Finally, the structure of macro-TpBpy was examined using
FESEM and EDX, showing that the macroporous structure was
fully retained and the existence of metallic Pt nanoparticles
(Figure S16). Control experiments were undertaken to
elucidate the role of the different components in the
photocatalytic experiment. In the absence of platinum (Pt)
and the SED, hydrogen evolution was negligible. Substitution
of TEOA with ascorbic acid as the SED led to a pronounced
decrease in hydrogen production to 1.0 mmol g−1h−1
particularly notable at a pH of 2.2 (Figure S17).
45
The
apparent quantum efficiency (AQE) was measured for both
pristine and macro-TpBpy. An AQE of 0.71 and 0.15% at 420
nm was found for macro- and pristine TpBpy COFs,
respectively.
Considering the measured photophysical properties, it is
expected that the reduction of O2to •O2−and oxidation of
H2O to H2O2are also feasible, as their redox potentials are
falling between the CB and VB levels of both macro- and
pristine TpBpy COFs. To verify this, photocatalytic H2O2
production experiments were conducted by using both pristine
and macro-TpBpy COFs as photocatalysts. The experiments
were performed with 5 mg of COF in 11 mL of pure water
measured after 2 h at 20 °C under λ> 420 nm light irradiation
with 10 min of O2purging. The macro-TpBpy COF exhibited
a production rate of 2716 μmol h−1g−1, which is higher than
the rate of 2134 μmol h−1g−1that is achieved by the pristine
TpBpy COF under the same conditions (Figure 3b), even
though the effect is not as substantial as observed for the
photocatalytic HER. One reason might be that in this case, no
gaseous products are formed and no sacrificial agents have to
be employed, so that mass transport limitations in the pristine
COF play a less important role. To gain further insights into
the photocatalytic mechanism for producing H2O2, different
conditions were tested. Under an Ar atmosphere, H2O2
production was negligible, suggesting that it occurs primarily
through oxygen reduction. Additionally, in the absence of light,
no H2O2was detected even after 24 h, confirming the necessity
of light for the photocatalytic process. Furthermore, a long-
term H2O2experiment was carried out to evaluate the stability
of pristine and macro-TpBpy COFs. A steady production of
H2O2over 8 h indicates a satisfactory stability of the catalyst
(Figure S18). Additionally, the reusability of the macro-TpBpy
COF catalyst for H2O2production was systematically
evaluated. After 2 h photocatalytic reaction, the catalyst was
recovered via centrifugation, subjected to a drying process, and
then reused in subsequent 2 h reaction cycles. As
demonstrated in Figure S19, the macro-TpBpy COF catalyst
retained its activity for at least four cycles without a significant
decrease in performance. Additionally, AQE was measured for
both pristine and macro-TpBpy for H2O2production. AQE
values of 0.3 and 0.17% at 420 nm were found for macro- and
pristine TpBpy COFs, respectively. The solar-to-chemical
conversion (SCC) efficiency in pure water (no sacrificial
agents) for H2O2was calculated to be 0.03% for the macro-
TpBpy COF.
To further understand the mechanism, an experiment with
the radical trap benzoquinone was performed, and it was
observed that H2O2production was quenched suggesting the
involvement of superoxide (•O2−) radicals. Furthermore,
liquid-state EPR measurements were conducted using water,
and 5,5-dimethyl-1-pyrrolidine N-oxide, which served as a spin
trap. Upon light illumination, distinct signals of •O2−and •OH
were detected, confirming the generation of •O2−by the
reduction of O2(Figure S20). However, the EPR intensity of
macro-TpBpy was much higher than for the pristine TpBpy
COF. These results show that H2O2formation follows the
reduction of O2, while macro-TpBpy shows a superior
generation of •O2−and •OH leading to greater H2O2
production compared to the pristine COF (Figure S21). In
addition, PXRD and SEM conducted on macro-TpBpy after 2
h of light exposure in water (Figures S22−S23) gave no
noticeable changes in crystallinity and macroporous structure
after light exposure. This indicates that the macro-TpBpy COF
remains stable during photocatalysis. However, nitrogen
sorption isotherms showed a slight decrease in the surface
area, which might be due to some pore blockage during the
experiments (Figure S24).
46
■CONCLUSIONS
In conclusion, a hierarchical porous COF that exhibits an
additional interconnected and uniform macroporous structure
besides its intrinsic microporosity was prepared by a PS-
mediated templating method. It was shown that the
hierarchical porous structure has a profound effect on the
photocatalytic performance. Macro-TpBpy exhibits an HER
rate of 4.88 mmol h−1g−1which is a fourfold increase
compared to the pristine, that is, only microporous COF.
Similarly, macro-TpBpy exhibited a high H2O2production rate
of 2716 μmol h−1g−1, in water without sacrificial hole
scavengers again outperforming the pristine COF; these
activities in both H2evolution and H2O2production are
comparable to other previous reported works (Tables S1 and
S2). This work shows that, in addition to the many factors
already describedthat influence the photocatalytic performance
of COFs, the porous architecture should be also considered, as
additional macroporosity can enhance light capture and
photon utilization and especially the mass transport within
the porous structure, an essential feature for all catalytic
applications.
■EXPERIMENTAL SECTION
Synthesis of Triformylphloroglucinol (Tp).
47
Phloroglucinol
(6.0 g, 49 mmol), hexamethylenetetramine (15.1 g, 108 mmol), and
trifluoroacetic acid (90 mL) were refluxed at 100 °C under N2for 2.5
h. After that, 150 mL of 3 M HCl was added slowly and the mixture
was heated at 100 °C for 1 h. After cooling to room temperature, the
solution was filtered through Celite and extracted with 350 mL of
dichloromethane, and the solution was evaporated under reduced
pressure to yield 1.6 g of an off-white powder. 1H NMR indicated
near 99% purity; a pure sample was obtained by sublimation (Scheme
S1 in Supporting Information).
Synthesis of PS Suspension. Monodisperse colloidal PSs with a
diameter of 270 nm were synthesized according to the literature.
48
Briefly, 39 mL of styrene was washed with 12 mL of NaOH aqueous
solution (10 wt %) and subsequently deionized water to remove the
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stabilizer. Then a defined amount of 1.5 g of poly(vinylpyrrolidone)
(Mw ∼29000) was added to a triple-neck, 500 mL round-bottom
flask containing 300 mL of water. The mixture was bubbled with
nitrogen for 15 min and followed by refluxing at 75 °C for 30 min
under magnetic stirring. Then, 0.5 g of K2S2O8with 50 mL of water
was added into the flask to initiate the polymerization of styrene. After
constant stirring (<500 r.p.m.) for 24 h at this temperature, a turbid
dispersion was observed to form from monodisperse colloidal PS
spheres.
Synthesis of Macro-TpBpy. The synthesis of macro-TpBpy was
carried out similar to that in ref 14 with some crucial modifications.
To a clean 10 mL centrifuge tube, 5 mL of colloidal PS with a solid
content of ∼10 wt % PTSA (500 mg, 2.5 mmol) and Bpy powder
(83.8 mg, 0.45 mmol) was added and mixed thoroughly in a vortex
shaker for 10 min. Then, 63 mg of 1,3,5-triformylphloroglucinol (Tp,
0.3 mmol) was added and further shaken for approximately 40 min
until an obvious color change is observed (orange−yellow). The
solution was poured into a Petri dish and covered tightly overnight
until the water evaporated completely (the solvent must evaporate
slowly), followed by transferring the covered Petri dish into an oven at
80 °C for 24 h. Then, the sample (PS@TpBpy) was thoroughly
washed with hot water to remove PTSA, DMAc, and acetone, and
subsequently, Soxhlet extraction was conducted to remove the PS
template with first DMAC overnight and tetrahydrofuran for 3 days to
obtain macro-TpBpy.
Synthesis of the Pristine COF. The synthesis of the pristine
COF was carried out similar to macro-TpBpy, but instead of colloidal
PS, we used deionized water (Scheme S2 in Supporting Information).
■ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.chemmater.4c01298.
Details on photocatalytic experiments; SEM, TGA, 13C
NMR and IR spectroscopy, and N2sorption measure-
ments of the polymers; and long-term photocatalysis and
recycling and stability tests (PDF)
■AUTHOR INFORMATION
Corresponding Author
Arne Thomas −Department of Chemistry, Functional
Materials, Technische Universität Berlin, Berlin 10623,
Germany; orcid.org/0000-0002-2130-4930;
Email: [email protected]
Authors
Islam E. Khalil −Department of Chemistry, Functional
Materials, Technische Universität Berlin, Berlin 10623,
Germany
Prasenjit Das −Department of Chemistry, Functional
Materials, Technische Universität Berlin, Berlin 10623,
Germany
Huseyin Kucukkececi −Department of Chemistry, Functional
Materials, Technische Universität Berlin, Berlin 10623,
Germany
Veit Dippold −Department of Chemistry, Functional
Materials, Technische Universität Berlin, Berlin 10623,
Germany; orcid.org/0009-0006-7871-6158
Jabor Rabeah −State Key Laboratory of Low Carbon
Catalysis and Carbon Dioxide Utilization, Lanzhou Institute
of Chemical Physics (LICP), Chinese Academy of Sciences,
Lanzhou 730000, P. R. China; orcid.org/0000-0003-
2162-0981
Warisha Tahir −Department of Chemistry, Functional
Materials, Technische Universität Berlin, Berlin 10623,
Germany
Jérome Roeser −Department of Chemistry, Functional
Materials, Technische Universität Berlin, Berlin 10623,
Germany
Johannes Schmidt −Department of Chemistry, Functional
Materials, Technische Universität Berlin, Berlin 10623,
Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemmater.4c01298
Author Contributions
I.E.K. conceived the project, performed the experiments, and
wrote the manuscript with the support of P.D. and A.T. I.E.K.
designed, synthesized, and characterized the COFs. V.D.
conducted XPS and photocurrent experiments. HER measure-
ments were performed together by I.E.K. and H.K. H2O2
measurements were performed together by I.E.K. and P.D.
W.T. carried out UV−vis spectroscopy. J.R. carried out EPR
measurements. J.R. and J.S. helped with BET measurements
and gave fruitful suggestions. All the authors revised the paper.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The authors gratefully thank Germany’s Excellence Strategy-
EXC 2008-390540038-UniSysCat for financial support. I.E.K.
and A.T. thank the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) for financial support (TH
1463/18-1). H.K. and W.T. thank the Einstein Center of
Catalysis/Berlin International Graduate School of Natural
Sciences and Engineering. H.K. acknowledges support by the
IMPRS for Elementary Processes in Physical Chemistry. The
authors are furthermore grateful to the BMBF (Fördermaβ-
nahme CO2WIN, Förderkennzeichen 033RC02-4PRODIGY).
The authors thank Christina Eichenauer and Maria Un-
terweger for their assistance.
■REFERENCES
(1) Coté, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger,
A. J.; Yaghi, O. M. Porous, Crystalline, Covalent Organic Frameworks.
Science 2005,310, 1166−1170.
(2) Khalil, I. E.; Xue, C.; Liu, W.; Li, X.; Shen, Y.; Li, S.; Zhang, W.;
Huo, F. The Role of Defects in Metal−Organic Frameworks for
Nitrogen Reduction Reaction: When Defects Switch to Features. Adv.
Funct. Mater. 2021,31 (17), 2010052.
(3) Krishna, R. Diffusion in porous crystalline materials. Chem. Soc.
Rev. 2012,41 (8), 3099−3118.
(4) Kuhn, P.; Antonietti, M.; Thomas, A. Porous, covalent triazine-
based frameworks prepared by ionothermal synthesis. Angew. Chem.,
Int. Ed. Engl. 2008,47 (18), 3450−3453.
(5) Pan, T.; Khalil, I. E.; Xu, Z.; Li, H.; Zhang, X.; Xiao, G.; Zhang,
W.; Shen, Y.; Huo, F. Spatial compartmentalization of metal
nanoparticles within metal-organic frameworks for tandem reaction.
Nano Res. 2022,15 (2), 1178−1182.
(6) Ren, Z.; Zhou, W.; Weng, J.; Qin, Z.; Liu, L.; Ji, N.; Chen, C.;
Shi, H.; Shi, W.; Zhang, X.; Khalil, I. E.; Zheng, B.; Wu, J.; Zhang, W.;
Huo, F. Phase transition of metal−organic frameworks for the
encapsulation of enzymes. J. Mater. Chem. A 2022,10 (37), 19881−
19892.
(7) Serrano, D. P.; Escola, J. M.; Pizarro, P. Synthesis strategies in
the search for hierarchical zeolites. Chem. Soc. Rev. 2013,42 (9),
4004−4035.
Chemistry of Materials pubs.acs.org/cm Article
https://doi.org/10.1021/acs.chemmater.4c01298
Chem. Mater. 2024, 36, 8330−8337
8335
(8) Shen, Y.; Pan, T.; Wu, P.; Huang, J.; Li, H.; Khalil, I. E.; Li, S.;
Zheng, B.; Wu, J.; Wang, Q.; Zhang, W.; Wei, W. D.; Huo, F.
Regulating Electronic Status of Platinum Nanoparticles by Metal−
Organic Frameworks for Selective Catalysis. CCS Chem. 2021,3(5),
1607−1614.
(9) Xin, X.; Qu, Q.; Khalil, I. E.; Huang, Y.; LiuWei, W. M.; Chen, J.;
Zhang, W.; Huo, F. Hetero-phase zirconia encapsulated with Au
nanoparticles for boosting electrocatalytic nitrogen reduction. Chin.
Chem. Lett. 2024,35, 108654.
(10) Zhang, W.; Ji, W.; Li, L.; Qin, P.; Khalil, I. E.; Gu, Z.; Wang, P.;
Li, H.; Fan, Y.; Ren, Z.; Shen, Y.; Zhang, W.; Fu, Y.; Huo, F.
Exploring the Fundamental Roles of Functionalized Ligands in
Platinum@Metal-Organic Framework Catalysts. ACS Appl. Mater.
Interfaces 2020,12 (47), 52660−52667.
(11) Lee, S. H.; Kim, J.; Chung, D. Y.; Yoo, J. M.; Lee, H. S.; Kim,
M. J.; Mun, B. S.; Kwon, S. G.; Sung, Y. E.; Hyeon, T. Design
Principle of Fe-N-C Electrocatalysts: How to Optimize Multimodal
Porous Structures? J. Am. Chem. Soc. 2019,141 (5), 2035−2045.
(12) Sun, Q.; Dai, Z.; Meng, X.; Xiao, F. S. Porous polymer catalysts
with hierarchical structures. Chem. Soc. Rev. 2015,44 (17), 6018−
6034.
(13) Yang, X. Y.; Chen, L. H.; Li, Y.; Rooke, J. C.; Sanchez, C.; Su,
B. L. Hierarchically porous materials: synthesis strategies and
structure design. Chem. Soc. Rev. 2017,46 (2), 481−558.
(14) Zhao, X.; Pachfule, P.; Li, S.; Langenhahn, T.; Ye, M.;
Schlesiger, C.; Praetz, S.; Schmidt, J.; Thomas, A. Macro/Micro-
porous Covalent Organic Frameworks for Efficient Electrocatalysis. J.
Am. Chem. Soc. 2019,141 (16), 6623−6630.
(15) Geng, K.; He, T.; Liu, R.; Dalapati, S.; Tan, K. T.; Li, Z.; Tao,
S.; Gong, Y.; Jiang, Q.; Jiang, D. Covalent Organic Frameworks:
Design, Synthesis, and Functions. Chem. Rev. 2020,120 (16), 8814−
8933.
(16) Zhu, D.; Li, X.; Li, Y.; Barnes, M.; Tseng, C.-P.; Khalil, S.;
Rahman, M. M.; Ajayan, P. M.; Verduzco, R. Transformation of One-
Dimensional Linear Polymers into Two-Dimensional Covalent
Organic Frameworks Through Sequential Reversible and Irreversible
Chemistries. Chem. Mater. 2021,33 (1), 413−419.
(17) Tan, K. T.; Ghosh, S.; Wang, Z.; Wen, F.; Rodríguez-San-
Miguel, D.; Feng, J.; Huang, N.; Wang, W.; Zamora, F.; Feng, X.;
Thomas, A.; Jiang, D. Covalent organic frameworks. Nat. Rev. Methods
Primers 2023,3(1), 1.
(18) Han, X.; Yuan, C.; Hou, B.; Liu, L.; Li, H.; Liu, Y.; Cui, Y.
Chiral covalent organic frameworks: design, synthesis and property.
Chem. Soc. Rev. 2020,49 (17), 6248−6272.
(19) Ishida, Y.; Sakata, H.; Achalkumar, A. S.; Yamada, K.;
Matsuoka, Y.; Iwahashi, N.; Amano, S.; Saigo, K. Guest-Responsive
Covalent Frameworks by the Cross-Linking of Liquid-Crystalline
Salts: Tuning of Lattice Flexibility by the Design of Polymerizable
Units. Chem. Eur. J. 2011,17 (52), 14752−14762.
(20) Yu, F.; Ciou, J.-H.; Chen, S.; Poh, W. C.; Chen, J.; Chen, J.;
Haruethai, K.; Lv, J.; Gao, D.; Lee, P. S. Ionic covalent organic
framework based electrolyte for fast-response ultra-low voltage
electrochemical actuators. Nat. Commun. 2022,13, 390.
(21) Liu, M.; Liu, Y.; Dong, J.; Bai, Y.; Gao, W.; Shang, S.; Wang, X.;
Kuang, J.; Du, C.; Zou, Y.; Chen, J.; Liu, Y. Two-dimensional covalent
organic framework films prepared on various substrates through vapor
induced conversion. Nat. Commun. 2022,13 (1), 1411.
(22) Xiao, C.; Yao, Y.; Guo, X.; Qi, J.; Zhu, Z.; Zhou, Y.; Yang, Y.;
Li, J. Ultralight and Robust Covalent Organic Framework Fiber
Aerogels. Small 2024, 2311881.
(23) Zuo, H.; Lyu, B.; Yao, J.; Long, W.; Shi, Y.; Li, X.; Hu, H.;
Thomas, A.; Yuan, J.; Hou, B.; Zhang, W.; Liao, Y. Bioinspired
Gradient Covalent Organic Framework Membranes for Ultrafast and
Asymmetric Solvent Transport. Adv. Mater. 2024,36, 2305755.
(24) Bag, S.; Sasmal, H. S.; Chaudhary, S. P.; Dey, K.; Blatte, D.;
Guntermann, R.; Zhang, Y.; Polozij, M.; Kuc, A.; Shelke, A.;
Vijayaraghavan, R. K.; Ajithkumar, T. G.; Bhattacharyya, S.; Heine,
T.; Bein, T.; Banerjee, R. Covalent Organic Framework Thin-Film
Photodetectors from Solution-Processable Porous Nanospheres. J.
Am. Chem. Soc. 2023,145 (3), 1649−1659.
(25) Zhao, X.; Pachfule, P.; Thomas, A. Covalent organic
frameworks (COFs) for electrochemical applications. Chem. Soc.
Rev. 2021,50 (12), 6871−6913.
(26) Wang, Z.; Zhang, S.; Chen, Y.; Zhang, Z.; Ma, S. Covalent
organic frameworks for separation applications. Chem. Soc. Rev. 2020,
49 (3), 708−735.
(27) Yang, J.; Lin, C.; Wang, Y.; Xu, Y.; Pham, D. T.; Meng, X.;
Tran, K. V.; Cao, S.; Kardjilov, N.; Hilger, A.; Epping, J. D.; Manke, I.;
Thomas, A.; Lu, Y. Enhancing ionic conductivity and suppressing Li
dendrite formation in lithium batteries using a vinylene-linked
covalent organic framework solid polymer electrolyte. J. Mater.
Chem. A 2024,12 (3), 1694−1702.
(28) Zhao, W.; Yan, P. Y.; Yang, H. F.; Bahri, M.; James, A. M.;
Chen, H. M.; Liu, L. J.; Li, B. Y.; Pang, Z. F.; Clowes, R.; Browning,
N. D.; Ward, J. W.; Wu, Y.; Cooper, A. I. Using sound to synthesize
covalent organic frameworks in water. Nat. Synth. 2022,1, 87−95.
(29) Fu, G.; Yang, D.; Xu, S.; Li, S.; Zhao, Y.; Yang, H.; Wu, D.;
Petkov, P. S.; Lan, Z. A.; Wang, X.; Zhang, T. Construction of
Thiadiazole-Bridged sp2Carbon-Conjugated Covalent Organic
Frameworks with Diminished Excitation Binding Energy Toward
Superior Photocatalysis. J. Am. Chem. Soc. 2024,146, 1318−1325.
(30) Qin, C.; Wu, X.; Tang, L.; Chen, X.; Li, M.; Mou, Y.; Su, B.;
Wang, S.; Feng, C.; Liu, J.; Yuan, X.; Zhao, Y.; Wang, H. Dual donor-
acceptor covalent organic frameworks for hydrogen peroxide
photosynthesis. Nat. Commun. 2023,14, 5238.
(31) Xu, H.; Xia, S.; Li, C.; Li, Y.; Xing, W.; Jiang, Y.; Chen, X.
Programming Tetrathiafulvalene-Based Covalent Organic Frame-
works for Promoted Photoinduced Molecular Oxygen Activation.
Angew. Chem., Int. Ed. Engl. 2024,63, No. e202405476.
(32) Yu, H.; Zhang, F.; Chen, Q.; Zhou, P. K.; Xing, W.; Wang, S.;
Zhang, G.; Jiang, Y.; Chen, X. Vinyl-Group-Anchored Covalent
Organic Framework for Promoting the Photocatalytic Generation of
Hydrogen Peroxide. Angew. Chem., Int. Ed. Engl. 2024,136,
No. e202402297.
(33) Ma, D.; Song, Y.; Zhao, H.; Yu, C.; Zhang, Y.; Li, C.; Liu, K.
Ordered Macro−Microporous Covalent Organic Frameworks as
Bifunctional Catalysts for CO2Cycloaddition. ACS Sustainable
Chem. Eng. 2023,11, 6183−6190.
(34) Tan, W.; Zhu, L.; Tian, L.; Zhang, H.; Peng, R.; Chen, K.;
Zhao, S.; Ye, F. Preparation of cationic hierarchical porous covalent
organic frameworks for rapid and effective enrichment of perfluori-
nated substances in dairy products. J. Chromatogr. A 2022,1675,
463188.
(35) Wang, H.; Liu, X.; Niu, P.; Wang, S.; Shi, J.; Li, L. Porous Two-
Dimensional Materials for Photocatalytic and Electrocatalytic
Applications. Matter 2020,2(6), 1377−1413.
(36) Li, X.; Yu, J.; Jaroniec, M. Hierarchical photocatalysts. Chem.
Soc. Rev. 2016,45 (9), 2603−2636.
(37) Liang, Z.; Shen, R.; Ng, Y. H.; Fu, Y.; Ma, T.; Zhang, P.; Li, Y.;
Li, X. Covalent organic frameworks: Fundamentals, mechanisms,
modification, and applications in photocatalysis. Chem Catal. 2022,2
(9), 2157−2228.
(38) Karak, S.; Kumar, S.; Pachfule, P.; Banerjee, R. Porosity
Prediction through Hydrogen Bonding in Covalent Organic Frame-
works. J. Am. Chem. Soc. 2018,140 (15), 5138−5145.
(39) Kandambeth, S.; Biswal, B. P.; Chaudhari, H. D.; Rout, K. C.;
Kunjattu H, S.; Mitra, S.; Karak, S.; Das, A.; Mukherjee, R.; Kharul, U.
K.; Banerjee, R. Selective Molecular Sieving in Self-Standing Porous
Covalent-Organic-Framework Membranes. Adv. Mater. 2017,29 (2),
1603945.
(40) Homaeigohar, S.; Kabir, R.; Elbahri, M. Size-Tailored
Physicochemical Properties of Monodisperse Polystyrene Nano-
particles and the Nanocomposites Made Thereof. Sci. Rep. 2020,10
(1), 5191.
(41) Wang, X.; Mu, Z.; Shao, P.; Feng, X. Hierarchically Porous
Covalent Organic Frameworks: Synthesis Methods and Applications.
Chem. Eur. J. 2024,30 (14), No. e202303601.
Chemistry of Materials pubs.acs.org/cm Article
https://doi.org/10.1021/acs.chemmater.4c01298
Chem. Mater. 2024, 36, 8330−8337
8336
(42) Kandambeth, S.; Mallick, A.; Lukose, B.; Mane, M. V.; Heine,
T.; Banerjee, R. Construction of crystalline 2D covalent organic
frameworks with remarkable chemical (acid/base) stability via a
combined reversible and irreversible route. J. Am. Chem. Soc. 2012,
134 (48), 19524−19527.
(43) Indra, A.; Acharjya, A.; Menezes, P. W.; Merschjann, C.;
Hollmann, D.; Schwarze, M.; Aktas, M.; Friedrich, A.; Lochbrunner,
S.; Thomas, A.; Driess, M. Boosting Visible-Light-Driven Photo-
catalytic Hydrogen Evolution with an Integrated Nickel Phosphide-
Carbon Nitride System. Angew. Chem., Int. Ed. Engl. 2017,56 (6),
1653−1657.
(44) Hollmann, D.; Karnahl, M.; Tschierlei, S.; Kailasam, K.;
Schneider, M.; Radnik, J.; Grabow, K.; Bentrup, U.; Junge, H.; Beller,
M.; Lochbrunner, S.; Thomas, A.; Bruckner, A. Structure−Activity
Relationships in Bulk Polymeric and Sol−Gel-Derived Carbon
Nitrides during Photocatalytic Hydrogen Production. Chem. Mater.
2014,26 (4), 1727−1733.
(45) Yan, J.; Zhang, X.; Zheng, W.; Lee, L. Y. S. Interface
Engineering of a 2D-C3N4/NiFe-LDH Heterostructure for Highly
Efficient Photocatalytic Hydrogen Evolution. ACS Appl. Mater.
Interfaces 2021,13, 24723−24733.
(46) Liu, R.; Chen, Y.; Yu, H.; Polozij, M.; Guo, Y.; Sum, T. C.;
Heine, T.; Jiang, D. Linkage-engineered donor−acceptor covalent
organic frameworks for optimal photosynthesis of hydrogen peroxide
from water and air. Nat. Catal. 2024,7, 195−206.
(47) Chong, J. H. S. M.; Sauer, M.; Patrick, B. O.; MacLachlan, M. J.
Highly Stable Keto-Enamine Salicylideneanilines. Org. Lett. 2003,5,
3823−3826.
(48) Shen, K.; Zhang, L.; Chen, X.; Liu, L.; Zhang, D.; Han, Y.;
Chen, J.; Long, J.; Luque, R.; Li, Y.; Chen, B. Ordered macro-
microporous metal-organic framework single crystals. Science 2018,
359 (6372), 206−210.
Chemistry of Materials pubs.acs.org/cm Article
https://doi.org/10.1021/acs.chemmater.4c01298
Chem. Mater. 2024, 36, 8330−8337
8337
