Three-dimensional porphyrin-based covalent organic frameworks with tetrahedral building blocks for single-site catalysis

Article abstract of DOI:10.1039/c9nj04017j

Two three-dimensional (3D) porphyrin covalent organic frameworks, PCOF-1 and PCOF-2, were synthesized via imine condensation of a planar porphyrin tetramine (TAPP or TABPP) and a rigid tetrahedral aldehyde based on the steric hindrance of 3,3′,5,5′-tetrakis(4-formylphenyl)bimesityl (TFBM). The structures of PCOF-1 and PCOF-2 were studied by FT-IR and ¹³C CP-MAS solid state NMR spectroscopy. Powder X-ray diffraction patterns revealed obvious crystallinity with two intense peaks at 3.28° and 3.75° for PCOF-1, and 2.63° and 2.98° for PCOF-2. Structural simulation confirmed their 3D rutile type (pts) topological structures with two different pore sizes. X-ray single crystal diffraction revealed a distorted tetrahedral structure for the building block TFBM with two dihedral angles of 119° and 107.8°, and a planar square structure for the model compound (MC) with an outspread angle of 176.5°. PCOF-1 and PCOF-2 exhibited Brunauer-Emmett-Teller (BET) surface areas of 316 and 234 m² g^-1, respectively. The morphologies of PCOF-1 and PCOF-2 were investigated by scanning electron microscopy and transmission electron microscopy methods. PCOF-1 and PCOF-2 showed a high thermal stability up to 420 °C without decomposition through thermogravimetric analysis (TGA), and high chemical stability with no obvious mass loss after three days of immersion in various solutions. Due to the large surface area and the appropriate pore size, PCOF-Fe exhibited excellent biocatalytic catalytic performance, while PCOF-Co exhibited good electrocatalytic activity towards oxygen evolution reactions. These results indicate that 3D porphyrin-based COFs constructed from the tetrahedral building block with steric hindrance are promising candidates for single-site catalysis.

Full text of DOI:10.1039/c9nj04017j

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Three-dimensional porphyrin-based covalent
organic frameworks with tetrahedral building
blocks for single-site catalysis†
Cite this: New J. Chem., 2019,
43, 16907
Yong Liu,‡a Xiaodong Yan, ‡a Tao Li,a Wen-Da Zhang,a Qiu-Ting Fu,a
ab
Hui-Shu Lu,a Xuan Wanga and Zhi-Guo Gu
*
Two three-dimensional (3D) porphyrin covalent organic frameworks, PCOF-1 and PCOF-2, were synthesized
via imine condensation of a planar porphyrin tetramine (TAPP or TABPP) and a rigid tetrahedral aldehyde
based on the steric hindrance of 3,30,5,50-tetrakis(4-formylphenyl)bimesityl (TFBM). The structures of PCOF-1
and PCOF-2 were studied by FT-IR and 13C CP-MAS solid state NMR spectroscopy. Powder X-ray diffraction
patterns revealed obvious crystallinity with two intense peaks at 3.281 and 3.751 for PCOF-1, and 2.631 and
2.981 for PCOF-2. Structural simulation confirmed their 3D rutile type (pts) topological structures with two
different pore sizes. X-ray single crystal diffraction revealed a distorted tetrahedral structure for the building
block TFBM with two dihedral angles of 1191 and 107.81, and a planar square structure for the model
compound (MC) with an outspread angle of 176.51. PCOF-1 and PCOF-2 exhibited Brunauer–Emmett–Teller
(BET) surface areas of 316 and 234 m2 ꢀ1, respectively. The morphologies of PCOF-1 and PCOF-2 were
g
investigated by scanning electron microscopy and transmission electron microscopy methods. PCOF-1 and
PCOF-2 showed a high thermal stability up to 420 1C without decomposition through thermogravimetric
analysis (TGA), and high chemical stability with no obvious mass loss after three days of immersion in various
solutions. Due to the large surface area and the appropriate pore size, PCOF-Fe exhibited excellent
biocatalytic catalytic performance, while PCOF-Co exhibited good electrocatalytic activity towards oxygen
evolution reactions. These results indicate that 3D porphyrin-based COFs constructed from the tetrahedral
building block with steric hindrance are promising candidates for single-site catalysis.
Received 2nd August 2019,
Accepted 1st October 2019
DOI: 10.1039/c9nj04017j
rsc.li/njc
and excellent catalytic performance.15–21 However, porphyrin-
based COFs reported so far possess mainly two-dimensional
Introduction
Covalent organic frameworks (COFs) are novel crystalline organic layered structures, and the effective contact between the cata-
porous materials formed by dynamic covalent bonding.1–7 Com- lytic active centre and the substrate is limited by the layer-to-
pared with traditional pure inorganic molecular sieves and porous layer stacking.22–25 Three-dimensional (3D) structures could
carbon materials, COFs have unique advantages such as precise solve this issue.26–33 3D porphyrin-based COFs with a control-
design, adjustable structures and well-ordered pores.8–14 lable pore structure and adjustable catalytic performance thus
Porphyrin is one of the ideal building blocks for COFs because have great potential application in the field of heterogeneous
of its conjugated planar structure, strong thermal stability, high catalysis.
chemical stability, easy modification, high skeletal strength
In order to construct 3D porphyrin COFs, T4 + C2 and T4 + C4
polymerization strategies were usually adopted to generate
diamond (dia) and rutile type (pts) topologies by using tetra-
hedral T4 building blocks and two-connected porphyrin C2
building blocks or four-connected porphyrin C4 units.34,35 So
far, most of the 3D porphyrin COFs have excellent photocatalytic
properties due to the open pore structure and sufficient porphyrin
active sites. However, the tetrahedral T4 building blocks (tetra-
(4-aminophenyl)methane or tetrakis(4-formylphenyl)methane) of
3D porphyrin-based COFs are non-conjugated and flexible due to
sp3 carbons. The sp3 carbons in 3D COFs inhibit the transport of
electrons between the conjugated units in the frameworks, and
a Key Laboratory of Synthetic and Biological Colloids, Ministry of Education,
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
P. R. China. E-mail: zhiguogu@jiangnan.edu.cn
b International Joint Research Center for Photoresponsive Molecules and Materials,
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
P. R. China
† Electronic supplementary information (ESI) available: Experimental details,
general characterization, additional plot and discussion. CCDC 1912119 and
1912120. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c9nj04017j
‡ These authors contributed equally.
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o-dichlorobenzene, 0.3 mL mesitylene and 0.2 mL 6 M aqueous
acetic acid. This mixture was sonicated for 10 min to obtain a
homogeneous dispersion. After being frozen in liquid N2, the
reaction tube was degassed by freeze–pump–thaw cycles three
times. Then the tube was sealed under vacuum and heated at
120 1C for 7 days. The precipitate was collected by filtration,
washed with N,N-dimethyl formamide (6 ꢁ 5 mL), extracted
using a Soxhlet extractor with tetrahydrofuran for 24 hours, and
finally dried at 80 1C under vacuum for 12 hours to yield PCOF-1
as a dark purple powder. Yield: 60%.
Synthesis of PCOF-2
PCOF-2 was synthesized using TABPP (19.6 mg, 0.02 mmol)
following the procedures used for preparing PCOF-1. The
PCOF-2 was finally dried as a dark purple powder. Yield: 66%.
Scheme 1 Schematic representation of the design and synthesis of (a)
tetrahedral T4 building block (TFBM) and (b) three-dimensional porphyrin
covalent organic frameworks.
Synthesis of PCOF-1-Fe
PCOF-1 (40 mg) and FeCl2ꢂ4H2O (100 mg) were dispersed in
DMF (20 mL) under a nitrogen atmosphere and refluxed for six
hours. After cooling down to room temperature, powder was
obtained by centrifuging. The resulting precipitate was washed
with deionized water, anhydrous methanol, DMF, and anhydrous
THF and finally dried at 80 1C under vacuum for 12 hours to
obtain PCOF-1-Fe. Yield: 75%.
the flexible structure weakens the stability of the whole 3D
skeletons.36–41 Compared with 3D porphyrin COFs with sp3
carbons, their rigid structure can greatly improve the stability of
the entire skeleton, and the porphyrin catalytic centres in the 3D
framework have a good synergistic effect. However, it is challen-
ging to improve the rigidity of 3D porphyrin-based COF skeletons.
Selecting rigid tetrahedral T4 building blocks may help construct
rigid 3D porphyrin-based COFs.
To develop rigid 3D porphyrin-based COFs, we selected rigid
biphenyl to design tetrahedral T4 building blocks through
limiting the rotation of biphenyl by attaching six methyl groups
at the 2, 20, 4, 40, 6 and 60 positions (Scheme 1a).42 And four
benzaldehyde groups are decorated at the 3, 30, 5 and 50
positions of biphenyl to form rigid tetrahedral T4 building blocks
3,30,5,50-tetrakis(4-formylphenyl)bimesityl (TFBM). TFBM has
strong steric hindrance with six methyl groups limiting the
rotation to the plane, and four benzaldehyde groups can form
dynamic CQN bonds with amine to constitute a rigid 3D frame-
work. Herein, we report two 3D porphyrin-based covalent organic
frameworks (PCOFs) constructed from tetrahedral TFBM building
blocks and two different types of porphyrin tetramine units
(5, 10, 15, 20-tetra(4-aminophenyl)porphyrin and 5, 10, 15,
20-tetra(4-aminobiphenyl)porphyrin) by aldehyde–amine conden-
Synthesis of PCOF-2-Fe
PCOF-2-Fe was synthesized using PCOF-2 (40 mg) following the
procedures used for the preparation of PCOF-1-Fe. Yield: 69%.
Synthesis of PCOF-1-Co
PCOF-1-Co was synthesized using PCOF-1 (40 mg) and excess
Co(CH3COO)2ꢂ4H2O (100 mg) following the procedures used for
the preparation of PCOF-1-Fe. Yield: 73%.
Synthesis of PCOF-2-Co
PCOF-2-Co was synthesized using PCOF-2 (40 mg) and excess
Co(CH3COO)2ꢂ4H2O (100 mg) following the procedures used for
the preparation of PCOF-1-Fe. Yield: 79%.
Results and discussion
sation reactions (Scheme 1b). The metal ions were incorporated PCOF-1 and PCOF-2 were synthesized by T4 + C4 aldehyde–
into the porphyrin centres of 3D PCOFs43,44 to explore the amine condensation polymerization reactions (Fig. 1). Starting
performance of biomimetic catalysis for PCOF-Fe and electro- with metallized porphyrins as monomers would lead to metal
catalytic oxygen evolution for PCOF-Co as single-site catalysts. detachment and amorphous PCOFs, and the method of post-
modification was thus used to coordinate the metal with
the porphyrin centres of as-synthesized PCOFs. In order to
enhance the crystallinity of the imine condensation reaction,
Experimental
it is necessary to adjust the solvent selection and the ratio, and
All reagents and solvents were of reagent grade, purchased from
use a suitable concentration of acetic acid solution as a catalyst
commercial sources and used without further purification.
to trigger the polymerization at a suitable temperature (Tables S1,
S2 and Fig. S11, S12, ESI†). PCOF-1 and PCOF-2 were crystallized
through condensation between TFBM (0.02 mmol) and TAPP
Synthesis of PCOF-1
A Pyrex tube was charged with TFBM (13.0 mg, 0.02 mmol) (0.02 mmol) or TABPP (0.02 mmol) under solvothermal conditions
and TAPP (13.5 mg, 0.02 mmol) in a solution of 1.7 mL in a mixture of o-dichlorobenzene (1.7 mL), mesitylene (0.3 mL)
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Fig. 1 Illustration of the synthesis and structure of PCOF-1 and PCOF-2.
and 6 M acetic acid (0.2 mL) at 120 1C for 7 days. For the synthesis PXRD pattern (Fig. S14, ESI†). The simulations were carried out by
of PCOFs, o-dichlorobenzene and mesitylene that increase the using Materials Studio version-7. Only a few nets (e.g., pth, pti, pts,
solubility of porphyrin, 6 M acetic acid and a suitable temperature etc.) are reasonable for PCOF-1 and PCOF-2 according to Reticular
(120 1C) promote the Schiff base reaction of aldehyde and amine. Chemistry Structure Resource.45 After comparing these possible
PCOF-1 and PCOF-2 were separated by filtration and washed with spatial structures, it is proposed that both PCOF-1 and PCOF-2
various solvents to afford a dark purple solid. The PCOF products adopt the Pmc21 space group (a = 50.60 Å, b = 26.70 Å, c = 26.69 Å
are insoluble in water and common organic solvents such as for PCOF-1; a = 61.65 Å, b = 33.51 Å, c = 32.99 Å for PCOF-2) (Tables
ethanol, tetrahydrofuran (THF) and N,N-dimethylformamide S7 and S8, ESI†). According to the optimized simulation, the
(DMF). The metal ions (Fe or Co) were introduced into the extension angles between the positions 3 and 30, 3 and 5, 5 and 50,
porphyrin centres of PCOFs by the method of post-modification. 30 and 50 in the tetrahedral unit were about 1091, 1141, 1051 and
PCOFs and excess FeCl2ꢂ4H2O or Co(CH3COO)2ꢂ4H2O were refluxed 1121 in PCOF-1 and PCOF-2. The tetrahedral T4 building blocks
in a DMF solution at 150 1C for six hours under a nitrogen (TFBM) with four joint points and the square C4 units (TAPP for
atmosphere, then filtered and washed to obtain PCOF-Fe and PCOF-1 and TABPP for PCOF-2) with four joint points form a pts
PCOF-Co.
topology with two-fold interpenetration. PCOF-1 mainly possesses
a hexagonal pore with a size of 42.3 Å and quadrilateral pores with
a size of 18.4 Å, while they are 53.3 Å and 25.23 Å for PCOF-2. The
Structural and morphological characterization
In order to determine the structure of the obtained COFs, porphyrin units orientate the channel, helping to maximize
powder X-ray diffraction (PXRD) experiments and theoretical the available active sites. To the best of our knowledge, PCOF-1
simulations were carried out. The experimental results are and PCOF-2 are the first reported full-rigid three-dimensional
similar to the theoretical simulation. As shown in Fig. 2a, the porphyrin COFs.
PXRD pattern of PCOF-1 shows diffraction peaks at 3.301, 3.731,
The structures of TFBM and MC have been characterized by
4.801, 6.841, 8.181 and 9.061, corresponding to the (010), (110), single-crystal X-ray diffraction structural analysis (Tables S3–S6,
(210), (120), (212) and (312) facets, respectively. The peaks at ESI†). Fig. 2c shows the structures of TFBM and MC. TFBM has
2.631, 2.991, 3.891, 4.721, 5.461 and 7.531 for PCOF-2 correspond a distorted tetrahedral structure with two dihedral angles of
to the (010), (110), (210), (211), (120) and (312) facets, respectively 107.81 and 1191 due to the steric effect from its own six methyl
(Fig. 2b). After metalation (PCOF-1-Fe, PCOF-2-Fe, PCOF-1-Co and groups, which are similar to the angles of the theoretical
PCOF-2-CO), the crystallinity was well-maintained as shown in the structure. The crystal structure of MC unambiguously proves
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Fig. 2 (a) PXRD pattern and simulated structure of PCOF-1. (b) PXRD pattern and simulated structure of PCOF-2. (c) Single-crystal X-ray structure of
TFBM (left. grey, C; red, O) and the model compound MC (right. grey, C; blue, N). (d) Infrared spectra of TFBM (red), PCOF-1 (green), PCOF-2 (blue). (e) 13
C
CP/MAS NMR spectra of PCOF-1 (red) and PCOF-2 (black). N2 adsorption/desorption isotherms (77 K) and pore size distribution profiles (insert) of
(f) PCOF-1 and (g) PCOF-2.
the reaction of aldehyde–amine condensation, and it has a the porphyrin rings are coordinated with Fe or Co ions in
planar structure with an approximate straight angle of 176.51. PCOF-Fe or PCOF-Co (Fig. S15, ESI†).34 The powder electrical
The distorted tetrahedral structure of TFBM and the planar conductivity of PCOFs was measured to be 10ꢀ6 S cmꢀ1 and
structure of MC further confirm the feasibility of the theoretical improved to 10ꢀ5 S cmꢀ1 after metal incorporation.
structures of PCOF-1 and PCOF-2. During polymerization, the
The porous structures of PCOF-1 and PCOF-2 were investigated
four amino groups on the square TAPP and the four aldehyde by the physisorption of nitrogen at 77 K. The N2 adsorption/
units on the tetrahedral TFBM control the growing direction of desorption isotherms of PCOF-1 and PCOF-2 demonstrate a
the segments and result in 3D frameworks.
combination of type I and type IV sorption isotherms according
The Fourier transform infrared (FT-IR) spectra of PCOF-1 to the IUPAC classification, indicative of the presence of both
and PCOF-2 showed significantly weakened signals from the micropores and mesopores. The sharp nitrogen adsorption at low
CQO vibration (1701 cmꢀ1) and the appearance of the CQN relative pressures (P/P0 o 0.001) indicates the presence of abun-
bond (1622 cmꢀ1) (Fig. 2d). As shown in the solid-state dant micropores. The slight adsorption hysteresis may be due to
13C cross-polarization magic-angle-spinning (CP/MAS) NMR the spatial network interspersed structure or the restricted access
spectra of PCOF-1 and PCOF-2 (Fig. 2e), a resonance signal at of adsorbate molecules through a narrow pore opening.48 The
155 ppm from the carbon in the CQN group was observed. Brunauer–Emmett–Teller (BET) surface areas (Fig. 2f and g) are
These results also indicate the formation of PCOF-1 and PCOF- 316 and 234 m2 gꢀ1 for PCOF-1 and PCOF-2, respectively. The
2. The UV/vis absorption spectrum of PCOF-1 or PCOF-2 nearly pore size distribution calculated by the non-local density func-
corresponds to TAPP or TABPP, except for stronger Q-bands at tional theory (NLDFT) of PCOF-1 shows the coexistence of two
l = 590 and 650 nm (Fig. S13, ESI†). In addition, the ratio of the different pores, 2.3 and 4.3 nm, which was consistent with the
relative intensities of the Q-bands versus the Soret band simulated structures (42.3 and 18.4 Å). The pore size of PCOF-2 is
increased from 0.49 and 0.35 for TAPP and TABPP to 0.71 concentrated at 1.9 nm, similar to the theoretical simulation
and 0.6 for PCOF-1 and PCOF-2, respectively. This enhanced (25.23 Å). After metalation (PCOF-1-Fe, PCOF-2-Fe, PCOF-1-Co
absorption can be attributed to the fact that the 3D pore and PCOF-2-Co), the porous structures and BET surface areas
structure avoids the accumulation of porphyrins and promotes were well-maintained according to the nitrogen adsorption iso-
the interaction of porphyrins.46,47 Moreover, compared with therms (Fig. S16, ESI†).
PCOFs, the stretching vibrational frequency of the N–H bond
Fig. 3 shows the scanning electron microscopy (SEM) and
(3320 cmꢀ1) of the porphyrin ring disappeared, indicating that transmission electron microscopy (TEM) images of PCOF-1 and
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Fig. 3 (a) SEM and (b) TEM images of PCOF-1. (c) SEM and (d) TEM images
of PCOF-2.
Fig. 4 High resolution XPS spectra for Fe 2p of the (a) PCOF-1-Fe and
(b) PCOF-2-Fe hybrid. The high resolution XPS spectra for Co 2p of the
(c) PCOF-1-Co and (d) PCOF-2-Co hybrid.
PCOF-2. Both PCOF-1 and PCOF-2 are agglomerates of tiny
spheres (Fig. 3a and c). The TEM images show that there are
nanopore channels throughout the whole networks of PCOF-1
and PCOF-2 (Fig. 3b and d). Thermogravimetric analysis (TGA)
showed that PCOF-1 and PCOF-2 have good thermal stability
with small mass losses below 420 1C, and only about 30% mass
loss in a nitrogen atmosphere up to 800 1C (Fig. S19, ESI†).
Their chemical stability was investigated by immersing 15 mg
of PCOF-1 and PCOF-2 in THF, DMF, HCl solution (6 M) or
NaOH solution (6 M). After three days, the polymer was filtered,
washed and dried, and showed a slight mass loss and a similar
FT-IR spectrum (Fig. S20, ESI†) to the original polymers. These
results indicate that PCOF-1 and PCOF-2 have high chemical
stability.
In order to confirm the elemental composition of the polymer
and the oxidation states of the Fe and Co elements, X-ray
photoelectron spectroscopy (XPS) (Fig. S17, ESI†) and energy
dispersive X-ray spectroscopy (EDX) (Fig. S18, ESI†) analyses were
performed. The XPS survey spectra clearly showed the presence of
carbon, nitrogen and oxygen in both COFs, while Fe(III) and Co(II)
were detected in PCOF-Fe and PCOF-Co, respectively. As shown in
Fig. 4a and b, the binding energy of Fe 2p1/2 in PCOF-1-Fe and
PCOF-2-Fe is B724.5 eV, while the binding energy of Fe 2p3/2 in
PCOF-1-Fe and PCOF-2-Fe is B711.0 eV, suggesting the presence
of Fe(III)–Nx species.49 The peaks at 781.0 eV (Co 2p3/2) and
796.0 eV (Co 2p1/2) in PCOF-1-Co and PCOF-2-Co (Fig. 4c and d)
are assigned to the Co(II)–Nx species.50
centres. We loaded the porphyrin centres of PCOF-1 and PCOF-2
with ferric iron and evaluated their biomimetic catalytic
performance by catalytic oxidation of two substrates, that is
2,20-azinodi(3-ethylbenzothiazoline)-6-sulfonate (ABTS) and 3,30,5,50-
tetramethylbenzidine (TMB) (Scheme 2). These are commonly used
to characterize the catalytic properties of heme-like enzyme mimics.
The contents of the active centres in PCOF-1-Fe and PCOF-2-Fe
were found to be 0.663 ꢁ 10ꢀ3 mol gꢀ1 (the ratio of metalliza-
tion was 91%) and 0.554 ꢁ 10ꢀ3 mol gꢀ1 (the ratio of metalliza-
tion was 93%) by ICP-MS, respectively.
The two catalytic oxidation reactions followed the conven-
tional enzymatic dynamic regulation of the Michaelis–Menten
equation and were monitored by absorption spectroscopy.
These reactions are catalyzed by PCOF-1-Fe with constant
hydrogen peroxide and catalyst concentrations, but variable
substrate concentrations. We performed the same experiment
on PCOF-2-Fe for comparison. The Lineweaver–Burk plots
can be obtained based on reaction rates at varied substrate
concentrations, and important kinetic parameters such as kcat
and Km can be obtained. The kcat value is the maximum number
of substrate molecules transformed per catalyst molecule per
unit time under optimal conditions, and directly measures the
catalytic activity. Km is the Michaelis constant usually related to
the affinity of the catalyst molecule for the substrate and a
measure of the substrate concentration required for efficient
catalysis. The catalytic efficiency (kcat/Km) was obtained using a
Michaelis–Menten curve fit.
Biocatalytic properties
Porphyrin is a good single-site catalytic unit due to its
conjugated planar structure. The biomimetic catalysis and electro-
catalysis of porphyrin are studied in this paper. Fe–porphyrin is
an important class of biomimetic catalysts, which can simulate
cytochrome P450 monooxygenase and activate molecular oxygen
under mild conditions, so that hydrocarbons can be catalytically
oxidized with high efficiency and selectivity in air.51 PCOF-1 and
PCOF-2 meet all the prerequisites of a biomimetic system: large
pore size, high chemical stability, and potentially numerous active
Scheme 2 Biomimetic catalysis of the ABTS oxidation and TMB oxidation.
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Table 1 Kinetic parameters for the oxidation of substrates by different
catalysts
Substrate Catalyst
Km [mM] kcat [minꢀ1
]
kcat/Km [Mꢀ1 minꢀ1
]
ABTS
PCOF-1-Fe
PCOF-2-Fe
CHF-152
HRP53
1.56
0.75
0.022
0.64
23.4
3.96
0.45
1.5 ꢁ 104
0.5 ꢁ 104
2.0 ꢁ 104
2.7 ꢁ 103 4.26 ꢁ 106
TMB
PCOF-1-Fe
PCOF-2-Fe
Hemin54
1.14
1.2
3.8
2.44
0.1
3.3 ꢁ 103
2.0 ꢁ 103
1.26 ꢁ 103
8.59 ꢁ 103
0.78
PCN-222(Fe)55 1.63
14.0
diffusion rate of the substrate from the solution to the PCOF-Fe
channel, the actual kcat value of the catalytic centres in PCOF-Fe
is even higher. After consecutive reactions, the resultant powders
could be easily recovered by centrifugation and were thoroughly
washed with CH3CH2OH. It could be reused for three cycles
without losing catalytic activity (Fig. S21, ESI†). The infrared
spectrum (Fig. S22, ESI†) and PXRD (Fig. S23, ESI†) pattern of
PCOF-Fe showed no significant changes after three cycles of
catalytic reaction, indicating that the catalysis remained stable.
Moreover, the contents of Fe in PCOF-1-Fe and PCOF-2-Fe were
found to exhibit only a little reduction by ICP-MS after three cycles
of catalytic reaction.
For oxidation of 3,30,5,50-tetramethylbenzidine (TMB), acidic
aqueous solution (pH = 3) was selected due to the favorable
reaction conditions. The concentrations of the substrate varied
from 0.18 to 2.10 mM along with a fixed amount of PCOF-1-Fe
or PCOF-2-Fe catalysts (10 mg) and a hydrogen peroxide concen-
tration of 40 mM (Fig. 5c, d, g and h). For the TMB oxidation
reaction, the derived kcat values of the PCOF-1-Fe and PCOF-2-Fe
catalyst were 3.8 and 2.44 minꢀ1, respectively (Table 1), which are
dozens of times higher than the kcat of free hemin (0.1 minꢀ1).54
Moreover, the Km values of PCOF-1-Fe (1.14 mM) and PCOF-2-Fe
(1.2 mM) are lower than those of porphyrin-based metal–organic
frameworks PCN-222(Fe) (1.63 mM),55 which are indicative of a
better affinity of the substrate to PCOF-Fe. At the same time, the
metallized porphyrin monomers Fe-TAPP were tested for the
Fig. 5 (a) The fitting curves of the initial ABTS oxidation profiles at marked
concentrations by PPOP-1-Fe. (b) The Lineweaver–Burk plots of ABTS
oxidation catalyzed by PPOP-1-Fe. (c) The fitting curves of the initial TMB
oxidation profiles at marked concentrations by PPOP-1-Fe. (d) The Line-
weaver–Burk plots of TMB oxidation catalyzed by PPOP-1-Fe. (e) The
fitting curves of the initial ABTS oxidation profiles at marked concentra-
tions by PPOP-2-Fe. (f) The Lineweaver–Burk plots of ABTS oxidation
catalyzed by PPOP-2-Fe. (g) The fitting curves of the initial TMB oxidation
profiles at marked concentrations by PPOP-2-Fe. (h) The Lineweaver–
Burk plots of TMB oxidation catalyzed by PPOP-2-Fe.
+
For the oxidation reaction of ABTS with ABTSꢃ which was oxidation of ABTS and TMB, and exhibited lower catalytic
confirmed by scanning the UV-vis absorbance using a spectro- efficiency than PCOF-Fe (Fig. S24, ESI†). The excellent catalytic
photometer, we studied a series of reactions that proceeded performance of PCOF-Fe is attributed to the stable 3D structure
with different concentrations of ABTS (2 to 10 mM) in the with high-density porphyrin active centres which effectively
presence of H2O2 (40 mM) and PCOF-1-Fe or PCOF-2-Fe catalyst promote the reaction.
(40 mg) (Fig. 5a, b, e and f). The solvent was 3 mL HEPES buffer
solution. The color of the solution rapidly changed in a short
Electrocatalytic properties
time when PCOF-1-Fe was added. This indicates that the ABTS Co–porphyrin has attracted much attention in the field of
reaction can occur rapidly with PCOF-1-Fe catalysts. For electrocatalysis. PCOF-1 and PCOF-2 with partial conjugation
the ABTS oxidation reaction, as shown in Table 1, the kcat of facilitate the transport of electrons. We loaded the porphyrin
PCOF-1-Fe and PCOF-2-Fe shows a value of 23.4 minꢀ1 and centres of PCOF-1 and PCOF-2 with Co(II) and evaluated their
3.96 minꢀ1, respectively (Table 1). Since kcat represents the electrocatalytic activities towards oxygen evolution reactions
initial rate, PCOF-Fe has a higher initial rate than the recently (OERs) in 1 M KOH using a three-electrode system.56
reported excellent enzyme mimics CHF-1.52 The catalytic effi-
Fig. 6a shows the linear sweep voltammograms (LSVs)
ciency of PCOF-1-Fe (kcat/Km E 1.5 ꢁ 104) was comparable to of PCOF-Co. To achieve a current density of 10 mA cmꢀ2, an
that of CHF-1 (kcat/Km E 2.0 ꢁ 104). Compared with that of overpotential of 473 mV was required for PCOF-1-Co, which is
horseradish peroxidase (HRP),53 the lower catalytic efficiency of lower than that of PCOF-2-Co (487 mV). Notably, this value is
PCOF-Fe may be due to its hydrophobicity. Considering the comparable to that of cobalt porphyrin-based conjugated
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and electrocatalytic activity by adjusting the coordination metal.
The design and synthesis of other 3D porphyrin-based COFs with
interesting features and promising applications are underway
in our lab.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21771089), the Fundamental Research Funds
for the Central Universities (JUSRP21936 and JUSRP51725B), the
Project for Jiangsu Scientific and Technological Innovation Team,
and MOE & SAFEA for the 111 Project (B13025).
Fig. 6 OER activity: (a) LSV curves using PCOF-1-Co and PCOF-2-Co as
catalysts under 1.0 M KOH at 20 mV sꢀ1 with iR compensation. (b) Tafel
plot of PCOF-1-Co and PCOF-2-Co in 1.0 M KOH. (c) Nyquist plots of
PCOF-1-Co and PCOF-2-Co with a potential of 1.65 V (vs. RHE). (d)
Chronopotentiometry data for PCOF-1-Co and PCOF-2-Co as the catalyst
at a current density of 10 mA cmꢀ2 for 50 hours.
Notes and references
ˆ ´
1 A. P. Cote, A. I. Benin, N. W. Ockwig, M. O’Keeffe, A. J.
mesoporous polymers CoP-4ph-CMP-800.51 The higher catalytic
activity of PCOF-1-Co can be attributed to its higher active site
content. Overpotential values of 386 mV (PCOF-1-Co) and
396 mV (PCOF-2-Co) were needed to generate an anodic current
density of 1 mA cmꢀ2 (Fig. S25, ESI†), which could be found to
be in good agreement with other documented Co-based OER
catalysts such as cobalt-modified laminar COF Co-TpBpy.57
Under the same experimental conditions, the metallized
porphyrins showed a higher overpotential (Fig. S26, ESI†),
indicating the advantages of the three-dimensional structure
of PCOFs exposing more active sites. To further understand the
OER dynamics of PCOF-Co, the Tafel plot (Fig. 6b) was
acquired. The values of the Tafel slope were estimated to be
89 mV decꢀ1 (PCOF-1-Co) and 95 mV decꢀ1 (PCOF-2-Co), which
were comparable to and even lower than those of many other
reported catalysts such as CoP-2ph-CMP-800,50 Ni3S2/Ni foam58
and CsCo9/carbon composites.59 Electrochemical impedance
spectroscopy (EIS) analysis was also performed. Fig. 6c depicts
the Nyquist plot of PCOF-Co. The charge transfer resistance
(Rct) of the catalysts is determined by the diameter of the high
frequency semicircle. PCOF-1-Co and PCOF-2-Co have similar
Rct values. In addition, catalyst stability is a key factor for their
practical applications. The long-term chronopotential curve showed
no significant changes in potential over 50 hours at a catalytic
current density of 10 mA cmꢀ2 (Fig. 6d), suggesting good
catalytic stability.
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16914 | New J. Chem., 2019, 43, 16907--16914 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019