Anchoring CoII Ions into a Thiol-Laced Metal–Organic Framework for Efficient Visible-Light-Driven Conversion of CO2 into CO

Article abstract of DOI:10.1002/cssc.201900338

Using solar energy to convert CO₂ into valuable fuels or chemicals offers a powerful solution to urgent energy and environmental problems. However, the development of efficient and selective catalysts remains a considerable scientific challenge. To address this, catalytically active Co^II centers can be anchored into the porous matrix of metal–organic frameworks (MOFs) by utilizing a robust Zr-based MOF (Zr-DMBD) functionalized with freestanding thiol groups to enable efficient post-synthetic metal insertion. The thus-prepared Zr-DMBD?Co MOF solids are modified by well-defined Co–thiolate units and have the capability of photocatalytically converting CO₂ into CO with high efficiency and selectivity under visible-light irradiation in a water-containing system. The turnover number and CO selectivity reach as high as 97 941 and 98 %, respectively.

Full text of DOI:10.1002/cssc.201900338

Accepted Article
Title: Anchoring Co(II) Ions into a Thiol-Laced Metal-Organic
Framework for Efficient Visible-Light-Driven CO2-to-CO
Conversion
Authors: Di-Chang Zhong, Dong-Cheng Liu, Ting Ouyang, Ran Xiao,
Wen-Ju Liu, Zhengtao Xu, and Tong-Bu Lu
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To be cited as: ChemSusChem 10.1002/cssc.201900338
Link to VoR: http://dx.doi.org/10.1002/cssc.201900338
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10.1002/cssc.201900338
ChemSusChem
DOI staff
CO2 Reduction
Anchoring Co(II) Ions into a Thiol-Laced Metal-Organic Framework
for Efficient Visible-Light-Driven CO2-to-CO Conversion
Dong-Cheng Liu,† Ting Ouyang,† Ran Xiao,† Wen-Ju Liu, Di-Chang Zhong,* Zhengtao Xu* and Tong-
Bu Lu*
Abstract: Using solar energy to convert CO2 into valuable fuels or
chemicals offers a powerful solution to the urgent energy and
environment problems. However, the development of efficient and
selective catalysts remains a considerable scientific challenge.
Herein, we report a versatile strategy for anchoring catalytically
active Co(II) centers into the porous matrix of metal-organic
frameworks (MOFs), by utilizing a robust Zr-based MOF (Zr-
DMBD) functionalized with self-standing thiol (−SH) groups to
enable efficient post-synthetic metal insertion. The Zr-DMBD-Co
MOF solids thus prepared are modified by well-defined Co-thiolate
units and possess the capability of photocatalytically converting
CO2 to CO with high efficiency and selectivity under visible-light
irradiation in a water-containing system. The turnover number
(TON) and CO selectivity reach as high as 97941 and 98%,
respectively.
proton reduction in the photocatalytic process and thus decease the
activity and selectivity of catalysts.[7] Heterogeneous catalysts used
in photocatalytic CO2 reduction mainly include semiconductor
materials[8] and metal-incorporated zeolites[9]. They show better
durability and can be employed in water-containing systems, but
most of them are active only in the UV region and their efficiency
for photocatalytic CO2 reduction needs to be further enhanced.[10]
Therefore, it is still a challenge to develop highly efficient and
selective catalysts composed of earth-abundant transition metals and
capable of reducing CO2 under visible-light irradiation in a water-
containing system.
Metal-organic frameworks (MOFs), constructed from metals or
metal clusters interconnected by multidentate organic linkers, are a
new class of three-dimensional crystalline porous materials.[11] Their
inherent large surface areas, uniform but tunable cavities, and
tailorable chemistry impart notable properties[12] including gas
adsorption,[13] chemical sensing,[14] heterogeneous catalysis,[15]
proton conduction,[16] and drug delivery.[17] Recently, MOFs have
also been explored for electrochemical[18] and photochemical[19] CO2
reduction. Their excellent CO2 adsorption capacities, high-density
catalytically active sites, and tailorable light-absorption abilities
contribute to accelerate photocatalytic CO2 reduction. [19a, 20] In these
studies, the MOF systems comprise robust metal clusters such as
Zr6O4(OH)4 (e.g. NH2-UiO-66(Zr))[21] and Fe3O/Ti3O (e.g. MIL-
101(Fe), MIL-125 (Ti))[22]; moreover, they incorporate ligands
functionalized with photosensitive groups to enhance light
harvesting and expedite charge separation,[19a] as showcased in
Re(CO)3(dcbpy)Cl-UiO-67,[23] PCN-222,[24] and NH2-MIL-125(Ti)
[25]. MOFs can also be composited with metal nanoparticles (e.g.
Ag⊂Re3-MOF),[26] semiconductors (e.g. Cu3(BTC)2@TiO2),[27] and
photosensitizers (e.g. [Ru(bpy)3]Cl2)[28] to boost the photocatalytic
efficiency for CO2 reduction.
The utilization of solar energy to convert CO2 into valuable
chemical feedstocks is one of the most promising ways to solve the
problems of global warming and renewable solar fuel production. [1]
The key is to develop artificial photosynthetic systems from which
solar fuels can be produced from CO2 and H2O.[2] The ideal artificial
photosynthetic systems for CO2 reduction, just as the chloroplasts of
natural green plants, should be highly-efficient, highly-selective and
highly-stable. Over the past several decades, tremendous efforts
have been devoted to addressing one or more of these challenges. A
large number of homogeneous molecular catalysts[3] and
heterogeneous catalysts[4] for photocatalytic CO2 reduction have
been reported. Homogeneous catalysts possess advantages of
definite structures and thus easier mechanism investigation.[5]
However, they often entail expensive precious metal catalysts, and
show low efficiency and stability in aqueous-containing catalytic
systems.[6] Water participation also tends to cause competitive
Several issues, however, still need to be addressed in the
development of MOF-based catalysts for photochemical CO2
reduction. Firstly, a universal approach to catalyst preparation is
needed for conveniently and systematically modifying the
composition and other features, so as to optimize the catalytic
performances. Such a need is more stark in view of the poor stability
and efficiency/selectivity in most MOF-based catalysts, especially in
water-containing conditions (Table S1). Also, the active sites of the
MOF catalysts, like in other heterogeneous catalysts, are often not
well defined. In this connection, anchoring catalytically active metal
centers into the porous MOF matrix is advantageous: one can easily
insert variable amounts/types of metal guests; and the crystalline
MOF structure can also offer well-defined anchoring sites to
generate the active centers, and to help elucidate the catalytic
mechanisms. Herein, we anchored the catalytically active Co(II) into
the MOF matrix of Zr-DMBD[29] through the coordination of self-
standing thiol (−SH) groups, to form a Co(II)-modified MOF Zr-
DMBD-Co (Figure 1). Zr-DMBD was chosen as a support on the
[*]
Mr. D. C. Liu, Dr. T. Ouyang, Ms. W. J. Liu, Dr. Prof. D. C. Zhong, Dr.
Prof. T. B. Lu
Institute for New Energy Materials and Low Carbon Technologies,
School of Materials Science and Engineering, Tianjin University of
Technology, Tianjin 300384, China
E-mail: zhong_dichang@hotmail.com
E-mail: lutongbu@tjut.edu.cn
Dr. R. Xiao, Dr. Prof. Z. Xu
Department of Chemistry, City University of Hong Kong, 83 Tat Chee
Avenue, Kowloon, Hong Kong, China
E-mail: zhengtao@cityu.edu.hk
Mr. D. C. Liu, Dr. T. Ouyang, Ms. W. J. Liu, Dr. Prof. T. B. Lu
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School
of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China
These authors contributed equally to this work.
†
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/anie.201xxxxxx.
1
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10.1002/cssc.201900338
ChemSusChem
Figure 1. Synthetic scheme for the Zr-DMBD-Co network.
basis of the following reasons: (1) the framework of Zr-DMBD has
good stability toward water and common organic solvent, even
30]
under mild acid/base conditions.[21c,
(2) Zr-DMBD possesses
Figure 2. (a) TEM image and (b-f) EDS elemental mapping images of Zr-
suitable pores capable of CO2 adsorption. (3) the free-standing −SH
groups of Zr-DMBD strongly bind guests of metal ions to create
highly active metal-thiolate units on its extensive pore surface to
boost catalytic efficiency.[31] As expected, the resulting Zr-DMBD-
Co exhibits high activity and stability for visible-light-driven CO2-
DMBD-Co-0.002%.
[29, 31, 33]
These marked changes indicate that the disappearance of
the S−H signal in the Zr-DMBD-Co-x samples is directly correlated
to the formation of the S−Co bonds. This observation was further
confirmed by XPS studies. The S2p spectrum of Zr-DMBD shows
two main peaks at 163.5 and 164.6 eV indicative of the unbound
thiols (−SH),[34] and one small peak at 168.1 eV pointing to trace
amount of oxidized sulfur centers (Figure S8).[35] No cobalt species
was detected by XPS in Zr-DMBD-Co-0.002% (Figure S9a), as the
very low Co(II) loading was likely beyond the detection limit of
XPS. For Zr-DMBD-Co-3.11% and Zr-DMBD-Co-8.96% (Figure
S9b and S9c), the high resolution Co 2p XPS spectra show the main
peaks of Co 2p3/2 and Co 2p1/2 at 781.0 and 796.9 eV, respectively,
to-CO conversion in
a
water-containing catalytic system
(CH3CN/H2O, v/v = 4:1). The TON and TOF values reach as high as
97941 and 2.72 s-1, respectively, and the selectivity to CO reaches
98%.
Zr-DMBD was synthesized according to the literature
procedure.[29] To investigate the influence of the loaded amounts of
Co(II) on the catalytic activity, a series of Zr-DMBD-Co with Co(II)
contents of 0.002, 0.02, 0.3, 1, 3.11, 8.69%, designated as Zr-
DMBD-Co-x (x is the weight percentages of Co(II) ions, Figure S1),
were prepared by three methods (see the synthetic details in the
Supporting Information). Zr-DMBD and Zr-DMBD-Co-x revealed
by the distinct powder X-ray diffraction (PXRD) studies showed
that the patterns of Zr-DMBD-Co-x are the same as those of Zr-
DMBD, demonstrating that the framework of Zr-DMBD keeps
stable after anchoring Co(II) (Figure S2). Scanning electron
microscopy (SEM) results show that after uptake of Co(II) ions, Zr-
DMBD-Co-x retains homogenous sizes as that of Zr-DMBD
(Figures S3). The elemental mapping of Zr-DMBD-Co-0.002%
further reveals the homogeneous distribution of C, S, Co, and Zr
elements in the bulky crystals (Figure 2). The permanent porosity of
Zr-DMBD-Co-0.002% was confirmed by the N2 and CO2
adsorption/desorption. At 77 K, the N2 adsorption isotherm of the
activated Zr-DMBD-Co-0.002% exhibits a typical type-I curve, with
a BET surface area of 428 m2/g (Figure S4). This value is slightly
smaller than that of the Zr-DMBD (ca.500 m2/g),[29] indicating that a
little amount of Co(II) loading occupies a tiny pore cavity of the
framework. Zr-DMBD-Co-0.002% exhibits good CO2 adsorption
capacity at 273 K, with the adsorption amount of 40 cm3/g at 1 atm
(Figure S5), which is similar to that of Zr-DMBD (49 cm3/g).[29] The
capacity of CO2 adsorption of Zr-DMBD-Co may improve its
catalytic performance for CO2 reduction.
indicating the presence of Co(II) in the metalated Zr-DMBD.[36]
A
new peak is also noted at 162.5 eV in the S2p spectra of Zr-DMBD-
Co-3.11% and Zr-DMBD-Co-8.96%, which can be attributed to the
thiol groups coordinating with Co(II).[35b, 37] The S2p peaks of the
−SH groups monotonically weakened and the Co2p peaks
correspondingly intensified with the increasing weight percentages
of Co(II) in the Zr-DMBD-Co-x samples (Figure S9), indicating the
steady conversion of S−H bonds into S−Co units. These results thus
corroborate that the Co(II) ions are immobilized on the pore surface
of Zr-DMBD by the strongly-binding thiol groups.
The CO2 reduction reactions were conducted in a visible-light-
driven catalytic system, by using Zr-DMBD-Co-x as a catalyst,
[Ru(phen)3](PF6)2 as a photosensitizer, and triethanolamine (TEOA)
as a sacrificial reductant.[7a, 38] Typically, a glass reactor containing a
mixture of 5 mL CO2-saturated CH3CN/H2O (v/v = 4:1), Zr-DMBD-
Co-x, [Ru(phen)3](PF6)2 and TEOA, was irradiated by a 450 nm
LED light with an intensity of 100 mW·cm-2. The generated gases
were analyzed by a gas chromatography (GC). The results show that
the visible-light photoredox cycle produced a significant amount of
CO and a small amount of H2 (Table 1, Table S2, and Figure S10),
along with a trace amount of formate detected in the liquid phase by
ion chromatograph (IC). Under the same conditions, Zr-DMBD-Co-
x with different Co(II) contents exhibit impressive photocatalytic
CO2 reduction activity and CO selectivity. Notice that even the
catalyst content is as low as 0.002%, CO generated by
photochemical CO2 reduction is substantial and can be readily
quantified. This result further confirms that Zr-DMBD-Co-x clearly
possess good catalytic activity for photocatalytic CO2 reduction. For
example, by using 0.1 mg of Zr-DMBD-Co-0.002%, 3.33 μmol of
CO and 0.041 μmol of H2 were produced within 10 h, corresponding
to the TON and TOF values of 97941 and 2.72 s-1, respectively, and
IR, Raman and X-ray photoelectron (XPS) spectra verify the
successful immobilization of Co(II) into the pores of Zr-DMBD, i.e.,
via S−H bond cleavage and S−Co bond formation. The IR and
Raman spectra of the as-prepared Zr-DMBD-Co-x samples
demonstrate that the intensity of the characteristic S−H stretching
vibration at 2563 cm−1 steadily diminishes and even disappears with
the weight percentages of Co(II) increasing from 0 to 8.69% (Figure
S6 and S7).[29, 32] In addition, a new peak at 362 cm-1 corresponding
to S−Co stretching vibration emerges in Zr-DMBD-Co-8.69%,
which is consistent with its highest Co(II) loading (Figure S7g).
2
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10.1002/cssc.201900338
ChemSusChem
Table 1. The results of photochemical CO2-to-CO conversion catalyzed by Zr-
DMBD-Co-x.[a]
Zr-
DMBD
/μM
Co(II)/
%
CO/μm H2/
CO/
%
Entry
Co(II)/μmol
TON
TOF/s-1
ol
μmol
1
2
8.96
3.11
0.15
7.51
7.99
8.59
0.33
96
97
57
0.0016
0.0053
-2
10.08 0.28
190
5.3 × 10
-2
1.7 × 10
-3
3
4
1
8.16
8.22
9.45
6.75
0.16
0.11
98
98
556
0.0154
0.038
0.3
1350
5.0 × 10
-4
5
6
0.02
8.245 3.69
8.247 3.33
0.10
97
98
10853
97941
0.30
2.72
3.4 × 10
-5
0.002
0.041
3.4 × 10
[a] Reaction conditions: LED light (λ
= 450 nm), catalyst (0.1 mg),
[Ru(phen)3](PF6)2 (0.4 mM), TEOA (0.3 M), 1 atm CO2 atmosphere. All the
reactions were performed at room temperature (25 C). TON values of CO for
Zr-DMBD-Co-x were calculated based on per cobalt ion. To confirm the
reliability of the data, each photocatalytic reaction was repeated at least three
times. The TON value presented is the average one of three repeated
experiments; all the data with deviations below 4.5%.
Figure 3. Photocatalytic evolution of CO (blue) and H2 (black) catalyzed by Zr-
DMBD-Co-0.002% (★, 0.1 mg) and blank (●) under irradiation of a LED light
(450 nm, 100 mW·cm-2, irradiation area, 0.8 cm2) in the presence of
[Ru(phen)3](PF6)2 (0.4 mM) and TEOA (0.3 M) in
5 mL CO2-saturated
CH3CN/H2O (v/v = 4:1) solution at 25 ºC.
the selectivity to CO of 98%, as well as the quantum yield of 0.06%
(see more details in the Supporting Information). Compared with the
reported MOF-based catalysts for photochemical CO2 reduction
(Table S1), Zr-DMBD-Co-0.002% exhibits the highest efficiency
and activity, as well as the highest atomic economy. This suggests
that the Zr-DMBD-Co-0.002% solid may likely enable highly
efficient single-site catalysis in photochemical CO2 reduction.
Table 2. Control experiments for the photocatalytic CO2-to-CO conversion.[a]
CO
/μmol
TOF/
Entry
Cat
H2/μmol
CO%
TON
s-1
2.72
0
1
2
3
4
5
6
7
8
9
Zr-DMBD-Co-0.002%
blank
3.33
0
0.041
0.049
0
98%
97941
0
0
0
A series of control experiments were performed to thoroughly
investigate the photochemical CO2-to-CO conversion catalyzed by
Zr-DMBD-Co-0.002%. Firstly, the photocatalytic reaction was
carried out in the absence of Zr-DMBD-Co-0.002% (Figure 3, and
Table 2, entry 2). The result shows that no CO was detected, and
0.049 μmol H2 was recorded, indicating that the production of CO
originates from the catalytic reduction of CO2 by Zr-DMBD-Co-
0.002% rather than by the photosensitizer of [Ru(phen)3](PF6)2.
Secondly, the photocatalytic reaction was performed without
[Ru(phen)3](PF6)2, TEOA, or light irradiation (Table 2, entries 3-5).
The results show that no CO was detected, suggesting that
photosensitizer, sacrificial reductant, and visible-light irradiation are
necessary parts in building the photocatalytic CO2 reduction system.
Thirdly, the photocatalytic reaction was performed under an Ar
atmosphere (Table 1, entry 6). The result show that no CO was
generated, suggesting that the CO formed in entry 1 comes from the
reduction of CO2 rather than the decomposition of Zr-DMBD-Co-
0.002% and/or photosensitizer/sacrificial reductant. This result was
further confirmed by an isotope tracer experiment. As shown in
Figure S11, using 13CO2 instead of CO2, the generated gas is 13CO
rather than CO, strongly evidencing that the generated CO originates
from the reduction of CO2 by Zr-DMBD-Co-0.002%. Fourthly, the
photocatalytic reaction was operated using Zr-DMBD as catalyst
(Table 2, entry 7). There was no CO detected in the reaction system,
indicating the Zr-DMBD support has no photocatalytic activity for
CO2-to-CO conversion. We also performed the photocatalytic
reaction by using the mixture of H2DMBD and Co(II) ions as
catalyst. The result shows that the mixture also exhibits the ability to
catalyze CO2 reduction to CO (Table 2, entry 8), but only 0.22 μmol
of CO was determined, corresponding a relatively lower TON value
(6470) and CO selectivity (78%). Besides, experiment carried out in
the presence of Hg(0) gave the lack of change in catalytic activity
(Table 2, entry 9), indicating that the formation of CO was due to
the molecular catalyst. These observations clearly demonstrate that
Zr-DMBD-Co-0.002%
Zr-DMBD-Co-0.002%
Zr-DMBD-Co-0.002%
Zr-DMBD-Co-0.002%
Zr-DMBD
0
0
0
0
0.16
0
0
0
0
0
0
0
0
0
0.21
0.11
0.062
0.072
0
0
0
0
0
0
0
CoCl2 + H2DMBD
Zr-DMBD-Co-0.002%
0.22
3.15
78%
98%
6470
92647
0.18
2.57
[a] Reaction conditions: LED light (λ = 450 nm), [Ru(phen)3](PF6)2 (0.4 mM),
TEOA (0.3 M), Zr-DMBD-Co-0.002% (0.1 mg), 1 atm CO2 atmosphere. All the
reactions were performed at 25 C. 2: without catalyst; 3: without
[Ru(phen)3](PF6)2; 4: without TEOA; 5: without light; 6: with Zr-DMBD-Co-0.002%
in Ar; 7: with Zr-DMBD (0.1 mg) as catalyst; 8: with CoCl2 (3.4 × 10-5 μmol) and
ligand H2DMBD as catalysts; 9: with Hg(0) (0.2 mL).
the anchoring of Co(II) in Zr-DMBD greatly enhances the catalytic
activity of the Co(II) catalytic centres, and Zr-DMBD is an ideal
support for preparation of Zr-DMBD-Co-x heterogeneous catalysts
that may low the reduction potential of CoII/CoI, stabilize the low-
valent Co species, or improve the charge-separation efficiency to
dramatically boost the photocatalytic CO2-to-CO conversion .
The stability of Zr-DMBD-Co-0.002% was examined by repeated
photocatalytic reactions. As shown in Figure S12, the amount of CO
generated from the photocatalytic system in the second/third run
was almost the same as that in the first run, demonstrating that Zr-
DMBD-Co-0.002% possesses good durability. This result is further
evidenced by the powder X-ray diffraction profile of Zr-DMBD-Co-
0.002% after the CO2 photoreduction reactions. As shown in Figure
S13, the PXRD patterns of Zr-DMBD-Co-0.002% after the
photocatalytic tests closely match that of the as-made Zr-DMBD
sample. Besides, the result of ICP-MS experiment shows that no
Co(II) ions was detected in the reaction solution after the
photocatalytic CO2 reduction by Zr-DMBD-Co-0.002%. Based on
the above observations, we conclude that Zr-DMBD-Co-0.002% can
3
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10.1002/cssc.201900338
ChemSusChem
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reduction.
In summary, we have used a versatile post-synthetic metalation
method to prepare a series of MOF solids (Zr-DMBD-Co)
functionalized by Co-thiolate interactions. These MOF materials
show high photocatalytic activity and selectivity for visible-light-
driven CO2-to-CO conversion in a water-containing system in the
presence of [Ru(phen)3](PF6)2 as a photosensitizer, and TEOA as a
sacrificial agent. The high catalytic efficiency originates from the
strong-binding thiol groups (−SH) that effectively anchor Co(II)
guest ions into the pore surface of Zr-DMBD to create the catalytic-
active sites. The experimental results presented here point to a new
direction of using thiol-equipped MOF materials to anchor various
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Acknowledgements
This work was financially supported by National Key R&D
Program of China (2017YFA0700104), NSFC (21861001,
21331007 and 21790052), and 111 Project of China (D17003). ZX
acknowledges a GRF grant (Project 11305915) from the Research
Grants Council of Hong Kong SAR.
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Keywords: Metal-organic framework • Thiol group • Photocatalysis
• CO2-to-CO conversion • CO2 reduction
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5
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10.1002/cssc.201900338
ChemSusChem
Using solar energy to convert CO2 into
CO2 Reduction
valuable fuels or chemicals offers
a
powerful solution to the urgent energy and
environment problems. However, the
development of efficient and selective
catalysts remains a considerable scientific
challenge. Herein, we report a versatile
strategy for anchoring catalytically active
Co(II) centers into the porous matrix of
metal-organic frameworks (MOFs), by
Dong-Cheng Liu,† Ting Ouyang,† Ran
Xiao,† Wen-Ju Liu, Di-Chang Zhong,*
Zhengtao Xu* and Tong-Bu Lu*
Page – Page
Anchoring Co(II) Ions into a Thiol-Laced
Metal-Organic Framework for Efficient
utilizing
a robust Zr-based MOF (Zr-
DMBD) functionalized with self-standing
thiol (−SH) groups to enable efficient post-
synthetic metal insertion. The Zr-DMBD-Co
MOF solids thus prepared are modified by
well-defined Co-thiolate units and possess
Visible-Light-Driven
Conversion
CO2-to-CO
the
converting CO2 to CO with high efficiency
and selectivity under visible-light
capability
of
photocatalytically
irradiation in a water-containing system.
The turnover numbers (TON) and CO
selectivity reach as high as 97941 and 98%,
respectively.
6
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