Donor-acceptor covalent organic frameworks of nickel(ii) porphyrin for selective and efficient CO2reduction into CO

Article abstract of DOI:10.1039/d0dt03205k

Donor-acceptor two-dimensional covalent organic frameworks, PD-COF-23 and PD-COF-23-Ni, are constructed and applied for selective CO2 reduction with CO conversion rates of 20.9 μmol g-1 h-1 and 40.0 μmol g-1 h-1, respectively, in the absence of any additional photosensitizers and noble metal co-catalysts within an operation time of 25 h. The multilayer nanosheet structure, efficient charge separation and transport, and internal reductive quenching cycle of the NiTAPP fragments of PD-COF-23-Ni result in its higher photocatalytic efficiency than that of PD-COF-23. This journal is

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ARTICLE
Donor-acceptor covalent organic frameworks of nickel (II)
porphyrin for selective and efficient CO2 reduction into CO
Nanfeng Xu,a,b Yingxue Diao,c Xihao Qin,a Zhengtao Xu,*c Hanzhong Ke,*a and Xunjin Zhu*b
Received 00th January 20xx,
Accepted 00th January 20xx
Donor-acceptor two-dimensional covalent organic frameworks PD- COF-23 and PD-COF-23-Ni are constructed, and applied
for selective CO2 redution with 20.9 µmol g−1 h−1 and 40.0 µmol g−1 h−1 CO conversion, respectively, in the absence of any
additional photosensitizers and noble metal co-catalytst within an operation time of 25 h. The mutlilayer nanosheet
structure, efficient charge separation and transport, and internal reductive quenching cycle of NiTAPP fragments for PD-
COF-23-Ni lead to its higher photocatalytic efficiency than that of PD-COF-23.
DOI: 10.1039/x0xx00000x
synthesized via the dehydration of 1,4-benzenediboronic acid
or the reaction with hexahydroxytriphenylene. Since then, a
variety of 2D and 3D COFs have been prepared through a series
of reversible synthetic reactions, such as boronate ester
formation,22 nitrile trimerization,23 imidization,24 Schiff base,25-
28 and C=C bond formation reaction.29
1. Introduction
Increasing human activities accelerate the consumption of fossil
fuels and the rise of carbon dioxide emission, which results in
the continuous rise of the global average temperature.1 The
conversion of natural CO2 into stored energy is one of the most
promising approaches to solving these problems.2 Solar power
is abundant and renewable in nature, it is of interest to
photosynthetically convert solar power into storable chemical
energy in the form of carbohydrate fuels and oxygen. Various
nanomaterials and molecular complexes have now been widely
investigated as artificial photocatalysts for CO2 reduction under
visible light.3-8 The challenge is to find catalysts that can reduce
carbon dioxide with high activity, selectivity and durability
under visible light.9
Covalent organic frameworks (COFs), a class of crystalline
porous polymers held together by strong and rigid covalent
linkages between light elements (H, B, C, N, and O) via the
thermodynamically controlled reversible bond formation, have
spurred broad interest in gas absorption and separation,10, 11
drug delivery,12 electronic devices,13,
Recently, COFs have been widely reported for the application
of on CO2 reduction.30-32 For an example, Zou et. al. reported
COFs bearing single Ni sites (Ni-TpBpy) for photocatalytic
reduction of CO2 to CO with a 4057 μmol g–1 of CO in 5 h when
2+
using Ru(bpy)3 as photosensitizer. Recently, porphyrin-
tetrathiafulvalene COFs was reported for efficient
photocatalytic CO2 reduction with 12.33ꢀμmol CO evolution in
60ꢀh without additional photosensitizer, sacrificial agents, and
noble metal co-catalysts.33 Porphyrins have widely been studied
as redox-active photosensitizers in photocatalysis since they
can absorb blue and red light corresponding to the Soret and Q-
bands.34-36 Nevertheless, only a few porphyrin-based covalent-
organic frameworks (PCOFs) have been applied in
photocatalytic CO2 reduction reaction in contrast with the more
widely tested metal–organic framework (MOF) analogues.37, 38
It has been believed that 2D porphyrin-based COFs can be ideal
candidates for photocatalytic studies because of their extended
π-systems, high stability, volatile coordination chemistry and
tunable pore channels.39 Herein, we designed and synthesized
2D COFs, denoted as PD-COF-23 and PD-COF-23-Ni from
conjugated electron donating porphyrin MTAPP and electron
deficient moiety DPP-CHO via acid-catalyzed Schiff-base
reaction under reduced pressure (Scheme 1).40
14
and heterogeneous
catalysis,15-18 owing to their diverse organic building units and
regulable pore sizes. Over the past decade, chemists had found
many ways to prepare porous materials, but it remained
difficult to access organic polymer networks with discrete pores
until the concept of reticular chemistry, which was proposed to
construct these porous materials.19,
Since the pioneering
20
work by Omar Yaghi in 2005,21 the first two two-dimension COF
materials with specific structure and properties were
2. Experimental
2.1 synthesis of PD-COF-23 and PD-COF-23-Ni：
a. Faculty of Materials Science & Chemistry, China University of Geosciences,
Wuhan, Hubei 430074, P. R. China. E-mail: kehanz@163.com Tel: (86) 27
67883731
In a pyrex tube, 5,10,15,20-tetrakis(4-aminophenyl)porphyrin
(TAPP) (6.7 mg, 0.01 mmol) and DPP-CHO (9.1 mg, 0.02 mmol)
was dissolved in a mixture of 3 mL of n-butanol and 1 mL of
mesitylene. Then, 0.1 ml of aqueous acetic (6 M) was added.
b. Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Hong
Kong, P. R. China. E-mail: xjzhu@hkbu.edu.hk Tel: (852) 3411-5159
c. Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue,
Kowloon, Hong Kong, P. R. China. E-mail: zhengtao@cityu.edu.hk
Electronic Supplementary Information (ESI) available: Experimental details and
characterization data. See DOI: 10.1039/x0xx00000x
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The reaction mixture was sonicated for five minutes to get as a carrier gas) equipped with a TDX-01(5 Å molecular sieve
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DOI: 10.1039/D0DT03205K
homogeneous solution. The reaction vessel was degassed by column) and a flame ionization detector (FID).
three freeze-pump-thaw a screw cap and heated at 120 °C for
The apparent quantum efficiency (AQE) for CO evolution
five days. The obtained dark precipitate was collected by was measured using monochromatic visible light (420 nm). And
filtration and washed with THF and acetone for three times. The the AQE was calculated as follow equation:
powder obtained was exchanged by acetone with a Soxhlet
2 × N × 푁퐴 × ℎ × 푐
extraction apparatus and subsequently dried under vacuum for
휂(퐴푄퐸) =
12 h to get PD-COF-23 as dark powder (9.4 mg, 52%).
The synthesis of PD-COF-23-Ni was processed similarly to
PD-COF-23 using NiTAPP instead of TAPP, affording PD-COF-23-
I × A × t × λ
N = yield of CO (mol), NA = 6.02 × 10−23 mol−1, I = the intensity of
irradiation light (5.54 × 10−3 W / cm2), A = irradiation area (3.14
cm2) t = photoreaction time (3600 s), λ = the wavelength of the
monochromatic light (420 × 10−9 m)
Ni as red powder (6.2 mg, 34%).
N
N
N
N
N
N
NH2
O
N
M
N
N
N
N
N
N
N
N
N
N
M
N
O
N
M
N
N
N
3. Results and discussion
NH2
H2
N
3.1 Characterization
O
N
NH2
N
O
M = 2H PD-COF-23
M = Ni(II) PD-COF-23-Ni
Powder X-ray diffraction (PXRD) of the PD-COF-23 and PD-
COF-23-Ni were performed to determine the crystalline
structure. The simulation of crystalline structure for PD-COF-23
were implemented using the software Materials Studio 7.0. Its
unit cell parameters were calculated as a = 47.31 Å, b = 48.24 Å,
c = 3.73 Å; α = γ = 90°, β = 88.46°. It was found that the
experimental data is consistent with the simulated PXRD
pattern using AA stacking mode (Rwp = 4.30% and Rp = 3.33%),
but not AB stacking (Rwp = 42.9% and Rp = 33.4%). The strong
and sharp peak at 2.65° is assigned to the diffraction of 110
plane, indicating a high crystalline of PD-COF-23 (Fig. 1a). The
other diffraction peaks at 2.73°, 5.30° and 7.98° correspond to
200, 220 and 330 planes, respectively. Similarly for PD-COF-23-
Ni (Fig. 1b), the diffraction peaks at 2.64°, 2.75°, 5.28° and 7.92°
are assigned to 110, 200, 220 and 330 planes, respectively. Both
two as-prepared COFs showed high crystallinity from their PXRD
patterns.
O
MTAPP
N
N
O
N
O
N
O
N
N
N
N
O
N
M
N
N
N
O
N
N
N
N
N
M
N
N
N
O
O
N
N
DPP-CHO
Scheme 1. Synthetic route for PD-COF-23 and PD-COF-23-Ni.
2.2 Photo-electrochemical measurements
The photo-electrochemical analysis was carried out in a
conventional three electrode cell system, using Pt plates as
counter electrodes and a Ag/AgCl/saturated KCl (aq) electrode
as reference electrode. The working electrode was prepared on
fluorine-doped tin oxide (ITO, 2 cm × 2 cm) glass, cleaned by
sonication in acetone and ethanol for 30 min, and then dried.
The ITO slide was dip-coated with 0.55 mL of slurry, which was
obtained from the mixture of photocatalyst (1 mg), ethanol (0.5
mL) and Nafion solution (0.05 mL, 5% w/w in water and 1-
propanol) under sonication for 30 min. The photocurrent
measurements were conducted with a CHI660C workstation.
The light source was an LED monochromatic point lamp (3 W,
420 nm). The light spot effective area on the working electrode
was set as 2cm2 and 0.5 M Na2SO4 aqueous solution was used
as the electrolyte. The open circuit voltages were set as the
initial bias voltages in the transient photocurrent responses
tests.
(a)
(b)
Experimental
Experimental
AA stacking
AA stacking
AB stacking
AB stacking
5
10
15
20
25
30
35
40
5
10
15
20
25
30
35
40
2 Theta
2 Theta
Fig.1 Experimental and simulated XRD spectra of PD-COF-23 (a) and PD-COF-23
-Ni (b)
2.3 Photocatalytic experiments
The structural integrity of as-prepared COFs was confirmed
The experiments were performed in a sealed quartz bottle by FT-IR and 13C CP-MAS solid-state NMR spectrum. Both PD-
containing 2.5 mg of catalyst dispersed in 3 mL of acetonitrile COF-23 and PD-COF-23-Ni show one band around 1612 cm−1
and 2 mL of water with 0.5 mL of TEOA as a sacrificial regent. corresponding to the C=N stretching mode in their Fourier
Then, the mixture was purged with CO2 for 10 min and then transform infrared spectroscopies (FT-IR) (Fig S1). In the13C CP-
sealed at 1 atm CO2. A 300 W Xe lamp (Zhong Jiao Jin Yuan, MAS solid-state NMR spectrum (Fig S2a and b), the absence of
Model CEL-HXF300) was used as the light source. After any aldehyde signal, expected to appear at 180-200 ppm,
irradiation, the released gas (400 μL) was collected by syringe indicates total consumption of the DPP and the signal at around
from the headspace of the reactor and analyzed by gas 164 ppm for two COFs is attributed to the formation of C=N bond
chromatography (Shimadzu, GC-2014, Japan, with ultrapure Ar from the Schiff-base reaction
. And their XPS spectra also clearly
2 | J. Name., 2012, 00, 1-3
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0.40
(a)
(b)
PD-COF-23
PD-COF-23-Ni
indicate the peaks of N1s, O1s, and C1s for PD-COF-23 and PD-
COF-23-Ni (Figure S2). The three species of nitrogen in high
resolution N 1s spectrum at 400.02 eV, 401 eV and 401.7 eV are
ascribed to pyrrolic N, pyrrolic NH and imine C=N in PD-COF-23,
respectively (Figure S2b). In the N 1s spectrum of PD-COF-23-Ni
(Figure S2c), the new peak at 401.9 eV is attributed to the N-Ni.
Specifically, the banding energy of 856.7 eV and 874.7 eV are
assigned to the Ni 2p3/2 and 2p1/2 (Figure S2d) in PD-COF-23-
Ni.33, 41
On
- 3.61
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-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
-6.5
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
DOI: 10.1039/D0DT03205K
Off
- 4.15
CO/CO2
- 5.28
O2/H2O
- 5.85
PD-COF-23
0
100
200
Time (s)
300
400
PD-COF-23-Ni
(c) 3.0
(d)0.3
PD-COF-23
PD-COF-23-Ni
2.5
As shown in Fig S3 (a−d), the Brunauer-Emmett-Teller (BET)
surface areas were measured as 181 m2 g−1 for PD-COF-23 and
196 m2g−1 for PD-COF-23-Ni, corresponding to two type of pore
sizes at around 1.8 nm and from 4.1 nm to 4.2 nm, which arise
from the channel of A-A stacking in the frameworks. The
relatively lower BET surface areas indicate a low N2 uptake for
the two as-prepared COFs, which is attributed to the tight
staking and side chains protruding into the pores.
Thermogravimetric analysis (TGA) reveals the high thermal
stability of PD-COF-23 and PD-COF-23-Ni with less than 5%
weight loss in N2 atmosphere up to 320 °C and 380°C,
respectively (Figure S4). Obviously, the Ni(II) coordination in PD-COF-
2.0
1.5
1.0
0.5
0.0
0.2
0.1
0.0
0
2
4
6
8
0
5
10
15
Time (h)
20
25
Time (h)
Fig. 3 (a) Photocurrent of PD-COF-23 and PD-COF-23-Ni, (b) Band structure
diagram for PD-COF-23 and PD-COF-23-Ni, (c) Amount of CO produced for PD-COF-
23 and PD-COF-23-Ni (2.5 mg of catalyst), (d) Cycle experiments for PD-COF-23-Ni.
3.2 Photocatalytic reaction
23-Ni effectively strengthens its skeleton with a higher thermal
stability than PD-COF-23.
The visible light driven photocatalytic CO2 reduction was
In addition, scanning electron investigated using PD-COF-23 or PD-COF-23-Ni as photocatalyst
40, 42
microscopy (SEM) analysis shows that PD-COF-23 crystallizes in a and triethanolamine (TEOA) as sacrificial electron donor. As
block-like structure with sizes of 5–20 μm (Fig. 2), while PD-COF-23- shown in Fig. 3c, the photocatalytic CO evolution rate was 20.9
Ni exists as multilayer nanosheets. The self-assembly synthetic µmol g−1 h−1 for PD-COF-23 and 40.0 µmol g−1 h−1 for PD-COF-
control, which was beneficial for increasing the layer planarity, 23-Ni in 25 h. The Ni (II) in PD-COF-23-Ni acts as catalytic center
rendered the frameworks at large domain size. Transmission to fix the absorbed CO2 molecules, leading to enhanced
electron microscopy (TEM) images of the two COFs were recorded performance of CO2 reduction. Cycle photocatalytic
with further magnification at nano-meter scale to explore the experiments for PD-COF-23-Ni was carried out using the catalyst
internal morphology of COFs (Figure S6). Due to the strong π-π recycled from the former reaction by centrifugation and drying
stacking interactions between adjacent layers, the aggregated sheet- process under vacuum. As shown in Fig. 3d, the photocatalytic
like structure with uniform porosity was observed for both COFs.
CO evolution rate approached up to 38.8 µmol g−1 h−1 in the
second cycle and 35.3 µmol g−1 h−1 in the third cycle within 3
hours. The result indicates a considerable stability of PD-COF-
23-Ni under visible light irradiation. Moreover, the selectivity of
CO2 reduction into CO is as high as 99% with only trace amount
of CH4 detected for the systems of both COFs. In addition, no
other by-products (H2, CH3OH, and HCOOH) were detected from
the reactions. The control reaction was also carried out under
only Ar atmosphere, and no CO was generated due to the
absence of CO2. This means that CO generated in the
photocatalytic systems comes from the CO2. At the same time,
no CO evolution was detected when using water or
trimethylamine as sacrificial electron donor. In addition, the
apparent quantum efficiency (AQE) for CO evolution was
measured to be 0.036 % for PD-COF-23 and 0.079 % for PD-COF-
23-Ni. It’s known that the efficient charge separation and
transfer play an important role in aqueous photocatalytic CO2
reduction. The conduction band (CB) edge of the photocatalysts
should be more negative than the CO2 reduction potential
(CO/CO2, theoretically −0.53 V vs NHE, pH = 7 and −4.37 eV vs
Ev, vacuum level), and the valence band (VB) edge should be
more positive than the redox potential of H2O (O2/H2O,
theoretically 0.82 V vs NHE, pH = 7 and -5.67 eV vs Ev).
Ultraviolet photoelectron spectroscopy (Figure S8) was used to
determine the absolute positions of the conduction and valence
Fig. 2 SEM of PD-COF-23 (left) and PD-COF-23-Ni (right)
Owing to the formation of overall conjugated frameworks
by Schiff-base reaction, the absorption edge at around 743 and
727 nm in visible region were endowed for PD-COF-23 and PD-
COF-23-Ni respectively (Figure S7). Subsequently, their optical
band gaps were determined by the Kubelka-Munk equation
with values of 1.67 eV and 1.70 eV, respectively. Moreover, the
photocurrent response of PD-COF-23-Ni was obviously higher
than that of PD-COF-23, indicative of efficient photogenerated
hole-electron pair separation and more available electrons on
the surface of former (Fig. 3a).43 This result proves that PD-COF-
23-Ni is a better photocatalyst than PD-COF-23.
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band edges of PD-COF-23 (corresponding to their LUMO and hole pairs upon visible light irradiation were separated and
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DOI: 10.1039/D0DT03205K
HOMO energy level, respectively). The VB of PD-COF-23 was transported from electron-donating TAPP or NiTAPP to electron
calculated to be −5.28 eV, therefore the CB was thus calculated deficient DPP for CO2 reduction. On the other hand, NiTAPP can
to be − 3.61 eV based on the result of UV-DRS. Similarly, the VB also reduce CO2 reduction to CO at the Ni(II) centers through an
and CB of PD-COF-23-Ni were calculated to be −5.85 eV and − internal reductive quenching cycle, in which NiTAPP acts as both
4.14 eV. The energy-band alignments, as presented in Fig. 3b, photosensitizer and catalytic center.
indicate that both PD-COF-23 and PD-COF-23-Ni fulfil the
This work was supported by Hong Kong Baptist University
energy band requirements for artificial photosynthesis of CO2 (FRG2-17-18-068, RC-ICRS/15-16/02E, RC-ICRS/1617/02C-CHE,
reduction.
RC-ICRS-18-19-01A, RC-IRMS/16/17/02CHEM).
3.3 Mechanism
Conflicts of interest
There are no conflicts to declare.
Notes and references
Fig. 4 (a) Frontier orbitals from DFT calculation; (b) The proposed photocatalytic
mechanism of PD-COF-23-Ni.
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Density functional theory (DFT) calculations were applied
to understand the photo-excitation process and catalytic
reaction mechanisms. The highest occupied molecular orbital
(HOMO) mainly distributes along the electron donating TAPP
fragment, while lowest unoccupied molecular orbital (LUMO)
locates on DPP segments. The sufficient electron density
separation of HOMO and LUMO energy levels enable the
electron transfer from TAPP to DPP fragments upon visible light
irradiation (Fig. 4a). One possible reaction route was proposed
for the photocatalytic reaction based on PD-COF-23-Ni. As
shown in Fig. 4b, the TAPP-DPP repeating unit undergoes charge
separation and electron transfer from TAPP to DPP,
subsequently, CO2 is then reduced into CO by the two electrons
acuminated nearby DPP. Meanwhile, the TEOA regenerates
TAPP-DPP. In another potential reaction route, the Ni(II)-
porphyrin unit acts as both an photosensitizer and a catalytic
center.44, 45 Due to the insufficient conjugation between TAPP
and DPP, the porphyrin fragments might work as an internal
reductive quenching cycle, first absorb visible light to form the
excited NiTAPP*, then associate with TEOA to form MTAPP•−,
lastly, reduce CO2 into CO at Ni(II) centers. In this case, TEOA
plays an important role as the sacrificial electron donor to
prevent the charge recombination on the porphyrin fragments
and achieve a complete photocatalytic cycle. As a result, the
Ni(II) incorporated PD-COF-23-Ni shows a higher activity than
PD-COF-23 in the photocatalytic CO2 reduction.
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Conclusions
In summary, donor-acceptor 2D COFs of PD-COF-23 and
PDCOF-23-Ni were successfully developed for visible light
photocatalytic CO2 reduction to CO with high selectivity.
Typically, PD-COF-23-Ni showed higher CO production
efficiency of 40.0 µmol g−1h−1 than PD-COF-23 of 20.9 µmol
g−1h−1 within an operation time of 25 h. Due to the donor-
acceptor structure configuration of the two COFs, the electron-
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Y. Zhi, P. Shao, X. Feng, H. Xia, Y. Zhang, Z. Shi, Y. Mu and X.
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DOI: 10.1039/D0DT03205K
Table of Contents
Two 2-D donor-acceptor covalent organic frameworks built from porphyrin and
diketopyrrolopyrrole are capable of efficiently and selectively reducing CO2 into CO,
specifically, without any additional photosensitizers or co-catalysts.