Strongly Reducing (Diarylamino)benzene-Based Covalent Organic Framework for Metal-Free Visible Light Photocatalytic H2O2Generation

Article abstract of DOI:10.1021/jacs.0c09684

Photocatalytic reduction of molecular oxygen is a promising route toward sustainable production of hydrogen peroxide (H2O2). This challenging process requires photoactive semiconductors enabling solar energy driven generation and separation of electrons and holes with high charge transfer kinetics. Covalent organic frameworks (COFs) are an emerging class of photoactive semiconductors, tunable at a molecular level for high charge carrier generation and transfer. Herein, we report two newly designed two-dimensional COFs based on a (diarylamino)benzene linker that form a Kagome (kgm) lattice and show strong visible light absorption. Their high crystallinity and large surface areas (up to 1165 m2·g-1) allow efficient charge transfer and diffusion. The diarylamine (donor) unit promotes strong reduction properties, enabling these COFs to efficiently reduce oxygen to form H2O2. Overall, the use of a metal-free, recyclable photocatalytic system allows efficient photocatalytic solar transformations.

Full text of DOI:10.1021/jacs.0c09684

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Article
Strongly Reducing (Diarylamino)benzene-Based Covalent Organic
Framework for Metal-Free Visible Light Photocatalytic H2O2
Generation
Chidharth Krishnaraj, Himanshu Sekhar Jena, Laurens Bourda, Andreas Laemont, Pradip Pachfule,
́
̂
Jerome Roeser, C. Vinod Chandran, Sander Borgmans, Sven M. J. Rogge, Karen Leus,
Christian V. Stevens, Johan A. Martens, Veronique Van Speybroeck, Eric Breynaert, Arne Thomas,*
and Pascal Van Der Voort*
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ABSTRACT: Photocatalytic reduction of molecular oxygen is a promising
route toward sustainable production of hydrogen peroxide (H2O2). This
challenging process requires photoactive semiconductors enabling solar
energy driven generation and separation of electrons and holes with high
charge transfer kinetics. Covalent organic frameworks (COFs) are an
emerging class of photoactive semiconductors, tunable at a molecular level
for high charge carrier generation and transfer. Herein, we report two newly
designed two-dimensional COFs based on a (diarylamino)benzene linker that
form a Kagome (kgm) lattice and show strong visible light absorption. Their
high crystallinity and large surface areas (up to 1165 m2·g−1) allow efficient
charge transfer and diffusion. The diarylamine (donor) unit promotes strong
reduction properties, enabling these COFs to efficiently reduce oxygen to
form H2O2. Overall, the use of a metal-free, recyclable photocatalytic system
allows efficient photocatalytic solar transformations.
hydroxylation of arylboronic acids to form phenols,12 oxidation
of anisoles to sulfoxides,13 reductive dehalogenation of
phenacyl bromide,14 oxidative coupling of amines to imines,15
etc. To the best of our knowledge, COFs enabling photo-
catalytic reduction of molecular oxygen to produce H2O2 have
not yet been reported.
Hydrogen peroxide (H2O2) is a widely used chemical with
applications as an antiseptic, a disinfectant, and a bleaching
agent, and as an oxidizer in synthetic pathways. It is produced
on a large scale through multistep hydrogenation of
anthraquinone moiety using H2, followed by oxidation using
O2.16 Direct production of H2O2 with Au−Pd bimetallic
catalysts using H2 and O2 gas mixtures was also investigated.17
Alternative approaches involving electrochemical, photocata-
lytic, and photoelectrochemical processes were also studied for
this purpose.18,19 Recently, Yamashita and co-workers have
developed a two-phase system for the photocatalytic H2O2
INTRODUCTION
■
Covalent organic frameworks (COFs) are an emerging class of
crystalline porous organic materials, built up from covalently
linked organic molecules.1 Tuning the reticular chemistry
leading to these structures enables synthesis of crystalline,
photoactive, porous, and highly stable materials, providing an
optimal platform for photocatalysis.2 The main factors required
for an efficient photocatalyst are (i) suitable band gaps, (ii)
proper band position alignments, (iii) strong light absorption,
(iv) efficient charge carrier generation, (v) high charge transfer
kinetics, (vi) low recombination, and (vii) efficient diffusion
and active sites.3−5 By modifying the starting linker, COFs can
be functionalized at a molecular level to optimize these
properties. Functionalities such as triazine, porphyrin, and
benzothiadiazole have been incorporated into COFs to
improve charge transfer kinetics and charge carrier separa-
tion.2,6 By incorporating alternating donor−acceptor moieties
into an extended framework, several COFs have been rendered
suitable for photocatalysis.7,8 There remains, however,
extensive potential to design COFs optimized for photo-
catalysis by tweaking their structures at a molecular level.
In photocatalysis, COFs have been mainly limited to
hydrogen evolution reaction through water splitting,9 CO2
reduction,10 degradation of organic pollutants,11 oxidative
Received: September 9, 2020
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A

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production. The photocatalyst is dispersed in one phase
(benzyl alcohol) where oxidation reaction takes place, whereas
in the other phase (water) reduction of O2 to form H2O2
occurs.20,21 This promoted separation and avoided catalytic
decomposition of H2O2. Photocatalytic pathways provide a
sustainable route for H2O2 synthesis as they involve (i) a one-
step reaction, (ii) utilization of green reagents, and (iii) light as
the source of energy. Especially, the use of only visible light for
the production of H2O2 is considered to be particularly
sustainable.22 Photocatalytic H2O2 synthesis is performed by
irradiating water saturated with oxygen in the presence of a
semiconductor, using alcohols as the electron and proton
donors. Excitation of the semiconductor produces electron
(e−) and hole (h+) pairs. The holes oxidize the alcohol and
produce aldehyde and protons (eq 1). Simultaneously, the
electrons promote a two-electron reduction of oxygen to give
H2O2 (eq 2).22 A side reaction, detrimental to the desired two-
electron reduction of oxygen, involves one-electron reduction
to produce a superoxide anion. Hence an efficient system to
selectively perform the two-electron reduction is required.
(TAPD) linker expands the inventory of C4 linkers which can
be used to form Kagome lattices. Considering the high charge
transfer transitions in COFs, for the first time, TAPD-(Me)2
and TAPD-(OMe)2 COFs have been applied for photo-
catalytic production of H2O2 through oxygen reduction. This
photocatalytic system is completely metal-free and green
solvents are used for the reaction, thus paving way for a
sustainable pathway.
RESULTS AND DISCUSSION
■
TAPD-(Me)2 and TAPD-(OMe)2 COFs are synthesized in the
presence of 6 M acetic acid (0.1 mL) by reacting N,N,N′,N′-
tetrakis(4-aminophenyl)-1,4-phenylenediamine (TAPD, 23.6
mg. 0.5 mmol) with 2,5-dimethylbenzene-1,4-dicarboxalde-
hyde ((Me)2, 16.2 mg, 1 mmol) or 2,5-dimethoxybenzene-1,4-
dicarboxaldehyde ((OMe)2, 19.4 mg, 1 mmol), in the presence
of 1:1 o-dichlorobenzene/butanol (0.6 mL) or 1:1 dioxane/
mesitylene (0.6 mL) respectively (Figure 2, section S2 in the
Supporting Information). For the synthesis of the COFs, the
TAPD linker was deliberately utilized because of the possibility
of internal charge transfer transitions leading to strong
absorption bands in the visible light region. In similar
molecular systems (diarylamino(anthracene)), Konishi et al.
showed that distorted donor planes induce interchromophoric
conjugation rather than conjugation within the local structure
which promotes charge transfer between the diarylamine and
anthracene moieties.30 Contrary to the general expectation,
R−CH2OH + 2h+ → R−CHO + 2H+
(1)
O2 + 2H+ + 2e− → H2O2 (0.68 V vs NHE)
(2)
Wurster-type systems are classic examples of phenylenedi-
amine units displaying redox activity (Figure 1a).23,25 Phenyl-
́
Bredas et al. have shown that the strength of electronic
interaction is higher in benzene than in anthracene for such
diarylamino systems due to the dependence on steric
interactions.31 This led us to use diarylamino(benzene) as
the COF linker to promote strong electronic interactions.
The imine (CN) bond formation was confirmed through
Fourier transform infrared (FT-IR) spectroscopy. Stretching
vibrations at 1616 and 1612 cm−1 correspond to imine linkages
in TAPD-(Me)2 and TAPD-(OMe)2 COFs, respectively
Figure 1. (a) N,N,N′,N′-Tetramethyl-p-phenylenediamine (TMPD)
as a two-stage Wurster-type redox system. (b) Generation of
hydrogen peroxide in erythrocytes in the presence of TMPD.23
1
(Figures S4 and S5). H direct excitation MAS NMR (35
kHz MAS; 800 MHz) confirmed the presence of phenyl, N
CHPh, and methyl and methoxy protons for TAPD-(Me)2
and TAPD-(OMe)2 COFs, respectively. 13C{1H} CPMAS
(Figure 3g−j) revealed nine different carbon signals for TAPD-
(Me)2 COF and TAPD-(OMe)2 COF. All carbon resonances
can be assigned to the local chemical structure forming the
enediamine units are found in several dyes and are relevant in
several biological processes. In erythrocytes, they can react
with molecular O2 to form superoxide radicals and Wurster
salts, which can further react to generate hydrogen peroxide
(Figure 1b).25 For example, N,N,N′,N′-tetramethyl-p-phenyl-
enediamine (TMPD) undergoes autoxidation to generate
“active oxygen” species which could be responsible for toxic
changes in the erythrocytes. In a similar molecular system, Ito
et al. reported one-electron oxidation of 9,10-bis(di(p-anisyl)-
amino)anthracene inducing formation of a mixed-valence
species, i.e., a delocalized cationic radical species, [D−π−
D]•+.24 Recently, diarylamine scaffolds (derivatives of phenyl-
enediamine) have been used for metal-free molecular photo-
catalytic fluoroalkylation.26 The diarylamine (donor) moiety
acts as a strong reducing agent and promotes reduction. These
studies encouraged us to design COFs based on diarylamine/
diaminobenzene units for exploiting their strong reduction
properties in photocatalysis.
1
Kagome sheets. Note that both H MAS NMR and 13C{1H}
CPMAS NMR reveal the presence of methoxy and methyl
1
substitutions in the structure. Combining H double quantum
(DQ)−single quantum (SQ) and 13C{1H} CP heterogeneous
correlation (HETCOR) spectra allows confirmation of the
local chemical environments occurring in the COF structures
(Figure 3c−f). While 13C{1H} CP HETCOR reveals the
substitution of the CA and CB of the phenyl moiety with
1
methyl and methoxy groups, respectively, H DQ−SQ MAS
NMR reveals a strong correlation between the respective
methyl/methoxy protons and the HCα of the phenyl group
(Figures S6−S9). Elemental analyses were performed to reveal
the elemental composition of the COFs. The obtained C/N
and C/H ratios were 7.23/13.49 for TAPD-(Me)2 COF and
7.42/13.90 for TAPD-(OMe)2 COF, respectively. These
values matched well with the theoretically calculated values
(Table S2).
Herein, we report the synthesis of two highly crystalline
COFs: TAPD-(Me)2 and TAPD-(OMe)2. Both COFs are
highly stable (chemically and thermally) and form a Kagome
lattice as a result of the C4 + C2 linkage. There are only a
limited number of COFs with Kagome lattices,27−29 and the
N,N,N′,N′-tetrakis(4-aminophenyl)-1,4-phenylenediamine
Powder X-ray diffraction (PXRD) analyses were performed
to determine the structural features of the synthesized COFs.
Both TAPD-(Me)2 and TAPD-(OMe)2 COFs exhibited
B
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Figure 2. Synthesis of TAPD-(Me)2 and TAPD-(OMe)2 COFs forming a dual pore Kagome lattice.
intense reflections in the low-angle region at 2.3 and 2.4° 2θ (d
spacing ∼ 3.8 and 3.7 nm), respectively. An additional set of
lower symmetry reflections that could not be assigned to the
starting monomers are also obtained in the low-angle region,
indicating the formation of crystalline frameworks (Figure 4,
Figures S21 and S24). Indexing of the PXRD patterns in
primitive hexagonal lattices and the presence of broad
diffraction peaks at ∼20.4 and 18.6° 2θ suggest the formation
of a 2D layered material stacking in the c direction through
π−π interactions. According to the symmetry of the linkers, the
structural models for TAPD-(Me)2 and TAPD-(OMe)2 COFs
were constructed by generating the expected 2D layers with
kgm topology and modeling stacking sequences with eclipsed
(AA) and staggered (AB) arrangement (Figure 4c−f, section
S4 in the Supporting Information, Figures S21−S26). The
models were geometrically optimized, and their corresponding
theoretical PXRD patterns were compared to the experimen-
tally measured patterns. Pawley fitting over the full profile was
carried out to refine the final unit cell parameters (for TAPD-
(Me)2, a = b = 43.26 Å, c = 4.35 Å, Rwp = 3.28%; for TAPD-
(OMe)2, a = b = 41.50 Å, c = 4.76 Å, Rwp = 3.17%), resulting in
low residual values and acceptable profile differences (Figure
4a,b).
To further elucidate the atomic structure of the synthesized
COFs, molecular dynamics (MD) simulations were performed
at 300 K and 1 bar to include peak broadening effects. These
MD results are obtained without fitting or refining any model
parameter, using ab initio derived force fields for the TAPD-
(Me)2 and TAPD-(OMe)2 COFs with QuickFF protocol32,33
(section S5 in the Supporting Information). Furthermore, the
dynamic simulations intrinsically take into account the
movement of adjacent layers. Hence, no separate AA and AB
models need to be considered. However, given that the
functional groups in adjacent layers can be in either a parallel
or an inverted conformation, both possibilities were explicitly
modeled (Figure S28). As illustrated in Figure 5 and Figure
S30, the MD simulations provide similar unit cell parameters
for both COFs in line with the expectations for the almost
identical building units used in both materials and indicate that
the COFs display only limited deviations from a perfect AA
stacking, showcasing the strong interlayer interactions.
Furthermore, the MD simulations indicate that the most likely
ordering of the functional groups is inverted for both COFs
and allow the indexing of the experimentally observed PXRD
peaks, as shown in Figure 5.
The permanent porosity of TAPD-(Me)2 COF and that of
TAPD-(OMe)2 COF were assessed by argon sorption
measurements at 87 K and N2 sorption measurements at 77
K (Figure 6, Figures S32 and S33). Both COFs display a steep
uptake in the low-pressure region (P/P0 < 0.01), correspond-
ing to permanent microporosity. In addition, a step is observed
in the isotherm around P/P0 = 0.22. This feature represents the
filling of the second type of bigger pores. The overall isotherm
is a combination of type I and type IV isotherms, indicating
both microporous and mesoporous characteristics as expected
in the Kagome type structures. With the use of Ar sorption,
Brunauer−Emmett−Teller (BET) surface area values of 1105
and 1165 m2·g−1 were observed for TAPD-(Me)2 and TAPD-
(OMe)2 COFs, respectively. With the use of N2 sorption, the
BET surface area values were calculated to be 1083 and 1153
m2·g−1 for TAPD-(Me)2 and TAPD-(OMe)2 COFs, respec-
tively. Pore size analysis, using an argon sorption-based
quenched solid density functional theory (QSDFT) carbon
kernel for cylindrical-type pores, shows two different pore sizes
as expected from the Kagome type COF structures. The
experimental and theoretical average pore sizes for TAPD-
C
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1
Figure 3. Solid-state NMR spectra of TAPD-(Me)2 (left column) and TAPD-(OMe)2 (right column). Panels a and b show H MAS (35 kHz)
NMR spectra and decompositions which yielded the chemical shifts and fractions. Panels c and d show 1H DQ-SQ 2D correlations indicating the
proximity of one of the phenyl protons to the methyl and methoxy protons. Panels e and f show 1H−13C CP HETCOR 2D correlations indicating
the proximity of one type of carbon atoms at the phenyl rings to the attached methyl and methoxy protons. The 13C{1H} CPMAS spectrum (g)
corresponds to the structure (h), and the spectrum (j) corresponds to the structure (i). All the experiments were carried out on a Bruker Neo 800
MHz spectrometer operating at 18.8 T magnetic field.
(Me)2 and TAPD-(OMe)2 COFs were 1.7/2.7 nm (theoretical
1.5/2.5 nm) and 1.3/2.5 nm (theoretical 1.4/2.4 nm),
respectively (Figure 6, inset), thus confirming the formation
of 2D layers with kgm topology stacking in an eclipsed AA
stacking mode. These characterizations prove the successful
formation of the expected structures of TAPD-(Me)2 and
TAPD-(OMe)2 COFs.
Transmission electron microscopy (TEM) images of both
COFs revealed crystalline features with domain sizes in the
range of >50 nm (Figure 7a,c, Figures S14 and S16). Scanning
electron microscopy (SEM) analyses show rough spherical
morphologies of both COFs (Figure 7b,d, Figures S15 and
S17). To analyze the structural integrity, the chemical and
thermal stabilities of the COFs were tested. Both COFs (20
mg each) were immersed in 1 M HCl (acidic) and 1 M NaOH
(basic) solutions for 72 h and were washed with water and
acetone before further analysis. PXRD patterns collected after
these treatments show that the COFs retain their crystallinities
and remain stable (Figures S18 and S19). The yields of the
COFs after treatment with acid and base were >90%.
Additionally, both COFs remain insoluble in common organic
solvents such as acetone, methanol, ethanol, hexane,
tetrahydrofuran, diethyl ether, and dichloromethane. Further-
more, after the COFs were soaked in dimethylformamide for 8
days, the crystallinity was retained (Figures S18 and S19). In
addition to the chemical stability, no significant loss in mass of
COFs was observed when they were heated to 450 °C as
shown through thermogravimetric analyses (Figure S20).
By integrating functionalized units or groups such as
triazine,2 diacetylene,9 sulfone,34 etc., in the COF backbone,
D
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Figure 4. PXRD pattern and Pawley refinement of (a) TAPD-(Me)2 and (b) TAPD-(OMe)2 COFs. Top views of (c) TAPD-(Me)2 and (d)
TAPD-(OMe)2 COFs showing the ideal eclipsed (AA) structures. Side views of (e) TAPD-(Me)2 and (f) TAPD-(OMe)2 COFs. H atoms are
omitted for clarity.
Figure 5. Comparison between experimentally observed and MD calculated PXRD patterns for TAPD-(Me)2 and TAPD-(OMe)2, averaged over
the course of a 400 ps MD simulation at 300 K and 1 bar (Cu-kα, λ = 1.5406 Å) using ab initio derived system-specific force fields. The resulting
unit cell parameters and peak positions have been indicated.
several photocatalytic conversions have been achieved
needs to generate electron−hole pairs and avoid recombina-
successfully. For efficient photocatalytic activity, the COF
tion. The (diarylamino)benzene unit in the TAPD-(Me)2 and
E
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photocurrent was measured. As the bias voltage was swept in
the negative direction from +0.4 to −0.4 V at a scan rate of 4
mV/s, the photocurrent decreased in magnitude and then
changed sign. The potential at which the sign change of the
photocurrent occurred allows calculation of a good estimate of
the conduction band minimum (Figures S35−S37). The CB
band positions were determined in three different alkaline
buffers, displaying a Nernstian dependence of the CB position
with pH. From these values, the CB positions at pH 0 were
extrapolated (Figure S38). In combination with the optical
band gaps as derived from the Tauc plots, the band structures
were derived for TAPD-(Me)2 (CB = −0.97 eV, VB = 1.64
eV) and TAPD-(OMe)2 (CB = −0.80 eV, VB = 1.83 eV)
COFs as shown in Figure 8c. The band positions are very
much under the requirements for oxygen reduction to
hydrogen peroxide (0.68 V vs NHE); hence the COFs were
utilized further for H2O2 generation.
Figure 6. Ar sorption isotherms and pore size distributions (inset) of
Photocatalytic H2O2 generation experiments were per-
formed in sealed glass vials with water:ethanol (9:1), where
ethanol acts as the electron and proton donor. Under visible
light (420−700 nm) irradiation, long-term (16 h) reactions
were carried out at room temperature (condenser) with 20 mg
of COF used as the photocatalyst. H2O2 concentration was
determined through iodometric titration.44 After 16 h of
reaction, the TAPD-(Me)2 and TAPD-(OMe)2 COFs
produced 25.3 and 22.6 μmol of H2O2, respectively, when
irradiated under 250 W (5.46 W m−2) light. This is quite
comparable to pure carbon nitride, which produced 30 μmol of
H2O2 but used a 2 kW (26.9 W m−2) light source and with 9
times higher ethanol content.22 Also, in the case of TAPD-
(Me)2 COF when a higher amount of ethanol (water:ethanol
1:9) was used, 57.2 μmol of H2O2 was produced, which is
almost twice more than pure carbon nitride.22 We acknowl-
edge that it is difficult to compare the activities of different
materials due to different reaction conditions (irradiation
wavelength, light intensity, sacrificial agent, reaction time, etc.).
Besides, most of the catalysts used in the literature (Table 1)
for H2O2 production involve the assistance of a metal (Au,
TiO2, etc.), whereas the COFs provide a completely metal-free
platform. The H2O2 production vs time profile was recorded
for the photocatalytic reaction over 5 h (Figure 8d). The
overall H2O2 production rates of 97 10 and 91 10 μmol
h−1 gcat−1 were obtained for TAPD-(Me)2 and TAPD-(OMe)2
COFs, respectively.
TAPD-(Me)2 COF and TAPD-(OMe)2 COF.
Figure 7. TEM and SEM images of (a, b) TAPD-(Me)2 and (c, d)
TAPD-(OMe)2 COFs showing crystalline domains and the
morphology of COF crystallites.
The following reaction mechanism is proposed (Figure
S39). The reaction proceeds with the formation of electron−
hole carriers through visible light irradiation on the COF. The
holes are utilized by ethanol to produce acetaldehyde and two
protons along with it. Molecular oxygen utilizes the electrons
from the COF and the protons from the alcohol trans-
formation to produce H2O2. Electron and hole trapping
experiments were performed using silver nitrate (AgNO3) and
triethylamine (TEA), respectively (Table S3). In the case of
the electron trapping experiment there was a drastic decrease
in the formation of H2O2, whereas in the case of the hole
trapping experiment there was no formation of H2O2. This
emphasizes the role of photocatalytic electron−hole carriers in
the overall reaction. Further, benzoquinone was used as a
radical scavenger in the reaction system to determine the role
of radicals in the reaction (Table S3). However, the reaction
proceeded without much hindrance which means the radicals
formed are quickly utilized for the formation of H2O2. A
similar fast step was also observed in the carbon nitride
TAPD-(OMe)2 COFs is a strong donor and has strong
reducing properties.26 Moreover, high crystallinity of the COFs
can enhance the charge transfer kinetics.34 Diffuse reflectance
ultraviolet−visible (UV−vis) spectroscopy was used to analyze
the optical properties of the COFs, where it was observed that
each COF exhibits a broad and strong absorbance peak in the
visible region (Figure 8a). The optical band gaps calculated
from Tauc plots are 2.61 and 2.63 eV, respectively, for TAPD-
(Me)2 and TAPD-(OMe)2 COFs (Figure 8b). This makes
them ideal materials for visible light activation and photo-
catalytic H2O2 generation.
The positions of the conduction bands (CBs) and valence
bands (VBs) of both COFs were estimated on the basis of
linear sweep voltammetry measurements under chopped
illumination.35,36 A photocatalyst-coated FTO substrate was
used as the working electrode in a three-electrode setup,
together with an Ag/AgCl (3 M NaCl) reference electrode and
a Pt coil counter electrode. Under an applied bias of +0.4 V vs
Ag/AgCl and chopped white light illumination, an anodic
F
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Figure 8. (a) Solid state UV−vis spectra, (b) Tauc plot analyses for band gap calculations, (c) band potential alignments of the COF vs NHE (V)
at pH 0, and (d) H2O2 production per gram of catalyst over 5 h of reaction time of TAPD-(Me)2 and TAPD-(OMe)2 COFs.
Table 1. Photocatalytic Production of H2O2
entry
material
H2O2 production rate
irradiation conditions
solvent system
reference
63.00 μmol h−1 gcat
λ = 420−500 nm
λ > 420 nm
solar simulator
(AM = 1.5)
H2O:EtOH (1:9)
H2O
H2O
22
37
38
−1
1
2
3
g-C3N4
g-C3N4/PDI-BN0.2-rGO0.05
PEI/C3N4
30.80 μmol h−1 gcat
−1
208.10 μmol h−1 gcat
−1
4
Si/TiO2-Au
14.80 μmol h−1
λ = 365 nm
0.01 M HCl, Y3+
cocatalyst
propan-2-ol/H2O (1:9)
propan-2-ol/H2O (1:9)
propan-2-ol/H2O (1:9)
39
361.00 μmol h−1 gcat
λ > 400 nm
λ > 400 nm
λ > 420 nm
40
40
41
−1
5
6
7
g-C3N4/AQ-COOH
g-C3N4/rGO (10 wt %)
boron nitride quantum
dots−carbon nitride (BNQD/UPCN)
C-N-g-C3N4
OPA/Zr92.5Ti7.5-MOF
74.30 μmol h−1 gcat
72.30 μmol h−1 gcat
−1
−1
0.98 μmol h−1 gcat
420 < λ < 700 nm
λ > 420 nm
H2O
benzyl
alcohol:H2O (2.5:1)
H2O
H2O:EtOH (9:1)
H2O:EtOH (9:1)
H2O:EtOH (1:9)
42
20
−1
8
9
9700.00 μmol L−1 h−1
a
10
11
12
13
resorcinol−formaldehyde resins
TAPD-(Me)2 COF
TAPD-(OMe)2 COF
TAPD-(Me)2 COF
∼40.00 μmol h−1
λ > 420 nm
43
97.00 μmol h−1 gcat
λ = 420−700 nm
λ = 420−700 nm
λ = 420−700 nm
this work
this work
this work
−1
91.00 μmol h−1 gcat
−1
234.52 μmol h−1 gcat
−1
a
Extrapolated from graph.
photocatalyst,22 where the one-electron reduction of O2 to
release superoxide (•OOH) is suppressed. To determine the
H2O2 selectivity, the reaction was tested with benzyl alcohol as
the proton source instead of ethanol, as the quantification of
benzaldehyde is more reliable than that of acetaldehyde using
NMR analysis. The reaction was performed with water:benzyl
alcohol (1:9) for 16 h at room temperature by using TAPD-
(Me)2 COF. A 53.7 μmol amount of H2O2 was produced,
which is comparable to the reaction with ethanol under the
1
same conditions. H NMR analysis showed the production of
benzaldehyde, and correspondingly H2O2 was produced with
96% selectivity (Figure S40). In addition, this also confirms the
mechanism of the reaction. As expected, alcohol to aldehyde
conversion occurs which provides protons for reducing oxygen
G
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Article
to H2O2. A control reaction was performed with all substrates
excluding the COF yielding no significant amount of H2O2.
Further, the reaction was also tested with all the substrates and
the COF but in the absence of light. No H2O2 was produced,
thus confirming the requirement of light (Table S3). To check
the reusability of the catalyst, the reaction was performed for
five consecutive cycles with TAPD-(Me)2 COF and no
significant loss of activity was observed (Figure S41). The
PXRD patterns of the COFs after 16 h of photocatalytic
reactions show that they retain crystallinity, confirming that the
structure is stable under the reaction conditions (Figures S42
and S43). SEM analyses before and after catalysis of TAPD-
(Me)2 and TAPD-(OMe)2 COFs show no changes in the
overall morphologies (Figures S44 and S45). Additionally, the
reaction was performed for 96 h continuously and 142.3 μmol
of H2O2 was obtained. The production was still linear
compared to the overall kinetics in the short-term reactions
(Figure S46). The effect of different alcohol sources in the
reaction was tested by performing the photocatalytic reactions
using isopropanol and methanol in addition to ethanol and
benzyl alcohol. The production of H2O2 was in the order
isopropanol > ethanol > methanol > benzyl alcohol (Figure
S47). These results imply that the COFs are active for metal-
free visible light photocatalytic production of H2O2 by oxygen
reduction.
AUTHOR INFORMATION
■
Corresponding Authors
Pascal Van Der Voort − COMOC − Center for Ordered
Materials, Organometallics and Catalysis, Department of
Chemistry, Ghent University, 9000 Ghent, Belgium;
orcid.org/0000-0002-4874-0943;
Email: pascal.vandervoort@ugent.be
Arne Thomas − Department of Chemistry/Functional
Materials, Technische Universität Berlin, 10623 Berlin,
Germany; orcid.org/0000-0002-2130-4930;
Email: arne.thomas@tu-berlin.be
Authors
Chidharth Krishnaraj − COMOC − Center for Ordered
Materials, Organometallics and Catalysis, Department of
Chemistry, Ghent University, 9000 Ghent, Belgium;
Department of Chemistry/Functional Materials, Technische
Universität Berlin, 10623 Berlin, Germany; orcid.org/
0000-0001-8625-7322
Himanshu Sekhar Jena − COMOC − Center for Ordered
Materials, Organometallics and Catalysis, Department of
Chemistry, Ghent University, 9000 Ghent, Belgium;
orcid.org/0000-0002-5869-5226
Laurens Bourda − COMOC − Center for Ordered Materials,
Organometallics and Catalysis, Department of Chemistry and
XStruct − Bio-Inorganic Chemistry, Department of
Chemistry, Ghent University, 9000 Ghent, Belgium
Andreas Laemont − COMOC − Center for Ordered
Materials, Organometallics and Catalysis, Department of
Chemistry, Ghent University, 9000 Ghent, Belgium
Pradip Pachfule − Department of Chemistry/Functional
Materials, Technische Universität Berlin, 10623 Berlin,
Germany
CONCLUSION
■
Inspired by Wurster-type systems, we have synthesized a new
series of imine COFs consisting of (diarylamino)benzene
units. These COFs are highly crystalline and arrange into a
Kagome lattice. 13C−1H and 1H−1H NMR experiments
confirmed the local chemical environments of the COFs.
The COFs remained stable under various thermochemical
conditions. Molecular dynamics simulations on the COF
structures further confirmed the AA stacked crystalline nature.
Both COFs displayed high surface areas, strong absorption in
the visible light region, and ideal band gaps for photocatalysis.
The absolute positions of the conduction and valence bands
were determined by linear sweep voltammetry under chopped
illumination. Photocatalytic reactions using oxygen-saturated
water under visible light irradiation showed a high H2O2
production rate. The catalysts are stable and reusable for
several runs of the reaction. This study is the first example of
utilizing COFs for photocatalytic H2O2 production and opens
a new avenue toward solar driven metal-free photocatalysis.
́
̂
Jerome Roeser − Department of Chemistry/Functional
Materials, Technische Universität Berlin, 10623 Berlin,
Germany
C. Vinod Chandran − NMRCoRe, 3001 Leuven, Belgium;
Center for Surface Chemistry and Catalysis −
Characterisation and Application Team (COK-kat),
Department of Microbial and Molecular Systems (M2S), KU
Leuven, 3001 Leuven, Belgium
Sander Borgmans − Center for Molecular Modeling (CMM),
Ghent University, B-9052 Zwijnaarde, Belgium
Sven M. J. Rogge − Center for Molecular Modeling (CMM),
Ghent University, B-9052 Zwijnaarde, Belgium;
orcid.org/0000-0003-4493-5708
Karen Leus − COMOC − Center for Ordered Materials,
Organometallics and Catalysis, Department of Chemistry,
Ghent University, 9000 Ghent, Belgium
ASSOCIATED CONTENT
■
sı
* Supporting Information
Christian V. Stevens − Synthesis, Bioresources and Bioorganic
Chemistry Research Group (SynBioC), Department of Green
Chemistry and Technology, Ghent University, 9000 Ghent,
Belgium; orcid.org/0000-0003-4393-5327
Johan A. Martens − NMRCoRe, 3001 Leuven, Belgium;
Center for Surface Chemistry and Catalysis −
Characterisation and Application Team (COK-kat),
Department of Microbial and Molecular Systems (M2S), KU
Leuven, 3001 Leuven, Belgium
Veronique Van Speybroeck − Center for Molecular Modeling
(CMM), Ghent University, B-9052 Zwijnaarde, Belgium;
orcid.org/0000-0003-2206-178X
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c09684.
Experimental details on organic synthesis, NMR, FT-IR,
elemental analyses, TEM, SEM, structural modeling, ab
initio PXRD modeling, photo(electro)chemical charac-
terization, catalytic evaluations, and stability tests (PDF)
Crystal structure data of TAPD-(Me)2 COF with Pawley
refinement (CIF)
Crystal structure data of TAPD-(OMe)2 COF with
Pawley refinement (CIF)
Eric Breynaert − NMRCoRe, 3001 Leuven, Belgium; Center
for Surface Chemistry and Catalysis − Characterisation and
Force fields (ZIP)
H
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(7) Lei, K.; Wang, D.; Ye, L.; Kou, M.; Deng, Y.; Ma, Z.; Wang, L.;
Kong, Y. A Metal-Free Donor−Acceptor Covalent Organic Frame-
work Photocatalyst For Visible-Light-Driven Reduction of CO2 With
H2O. ChemSusChem 2020, 13 (7), 1725−1729.
Application Team (COK-kat), Department of Microbial and
Molecular Systems (M2S), KU Leuven, 3001 Leuven,
Belgium
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c09684
(8) Pachfule, P.; Acharjya, A.; Roeser, J.; Sivasankaran, R.; Ye, M.;
Bruckner, A.; Schmidt, J.; Thomas, A. Donor−Acceptor Covalent
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Organic Frameworks For Visible Light Induced Free Radical
Polymerization. Chemical Science 2019, 10 (36), 8316−8322.
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Author Contributions
All authors have given approval to the final version of the
manuscript.
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M.; Schomacker, R.; Thomas, A.; Schmidt, J. Diacetylene Function-
alized Covalent Organic Framework (COF) For Photocatalytic
Hydrogen Generation. J. Am. Chem. Soc. 2018, 140 (4), 1423−1427.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Katrien Haustraete for the TEM measure-
ments. C.K., S.B., P.V.D.V., V.V.S., and C.V.S. thank the
Research Board of Ghent University (GOA010-17, BOF
GOA2017000303) for funding. C.K. acknowledges the support
from the FWO travel abroad grant V427919N. H.S.J. thanks
FWO [PEGASUS]2 Marie Sklodowska-Curie grant agreement
No. 665501 for Incoming postdoctoral fellowship. S.M.J.R.
wishes to thank the Fund for Scientific Research Flanders
(FWO) for a postdoctoral fellowship (Grant No. 12T3519N).
This work was supported by the Hercules Foundation
(AKUL/13/21) and by the Flemish Government, department
EWI, via the Hermes Fund (AH.2016.134). J.A.M. and
P.V.D.V. acknowledge VLAIO for Moonshot funding
(ARCLATH, n° HBC.2019.0110). J.A.M. acknowledges the
Flemish Government for long-term structural funding
(Methusalem Beneficiary Johan A. Martens). J.A.M. and
V.V.S. have received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020
research and innovation program under Grant Agreement Nos.
834134 and 647755 (DYNPOR), respectively. A.T., P.P., and
J.R. acknowledge the funding by the Deutsche Forschungsge-
meinschaft (DFG, German Research Foundation) under
Germany’s Excellence StrategyEXC 2008-390540038
UniSysCat and the project TH 1463/15-1. The computational
resources and services used in this work were provided by the
VSC (Flemish Supercomputer Centre), funded by the
Research Foundation Flanders (FWO) and the Flemish
Government, department EWI.
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