Docking Site Modulation of Isostructural Covalent Organic Frameworks for CO2 Fixation

Article abstract of DOI:10.1002/chem.202000552

Three isostructural covalent organic frameworks (COFs) with either methoxyl, hydroxyl, or both groups on the channel wall, are synthesized and served as metal-free heterogeneous catalysts for chemical fixation of CO₂. Among them, the COF decora

Full text of DOI:10.1002/chem.202000552

A Journal of
Accepted Article
Title: Docking Sites Modulation of Isostructural Covalent Organic
Frameworks for CO2 Fixation
Authors: Fan Yang, Yusen Li, Ting Zhang, Ziqiang Zhao, Guolong
Xing, and Long Chen
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202000552
Link to VoR: http://dx.doi.org/10.1002/chem.202000552
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Docking Sites Modulation of Isostructural Covalent Organic
Frameworks for CO2 Fixation
Fan Yang[a], Yusen Li[a], Ting Zhang[a], Ziqiang Zhao[a], Guolong Xing[a] and Long Chen*[a]
[a]
F. Yang, Y. Li, T. Zhang, Z. Zhao, G. Xing, Prof. L. Chen
Department of Chemistry, Institute of Molecular Plus, and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072,
China.
E-mail: long.chen@tju.edu.cn
Abstract: Three isostructural covalent organic frameworks (COFs)
with either methoxyl, or hydroxyl or bearing both groups on the
channel wall, are synthesized and served as metal-free
heterogeneous catalysts for chemical fixation of CO2. Among them,
the COF decorating with both hydroxyl and methoxyl groups named
OMe-OH-TPBP-COF exhibits the highest catalytic activity and
efficiency for CO2 cycloaddition under mild conditions.
beneficial to activate the first step of the cycloaddition of CO2;22-25
(ii) the synergistic effect of the hydroxyl group with nucleophile
could accelerate the reaction rate of the formation of cyclic
carbonates.22-27 Thus, we hypothesize that decorating hydroxyl
docking sites to the channel walls of COFs could be conducible
to develop efficient, metal-free, heterogeneous COF-based
catalysts for chemical CO2 fixation under mild conditions.
Herein, three new isostructural COFs based on the rigid
3,3’,5,5’-tetrakisphenylbiphenyl (TPBP) aromatic core with
different docking sites on the channel walls (namely OMe-TPBP-
COF, OMe-OH-TPBP-COF, OH-TPBP-COF) were synthesized
and used as heterogeneous catalysts for cycloaddition of CO2
under mild conditions. Owing to the good stability, large porosity,
high crystallinity and proper arrangement of functional groups,
OMe-OH-TPBP-COF exhibited the best performance toward CO2
fixation among these three COFs, even outperforms most other
porous organic polymers.
Over the past decades, the deterioration of the notorious
greenhouse effect has led to many global environmental issues
such as climate warming, sea level rise, etc.1 Carbon dioxide
(CO2) not only constitutes up to 82% of the amount of greenhouse
gases but also served as the prime culprit of the global warming
effect.2 The chemical CO2 fixation with reactive epoxides into
economically important cyclic carbonates represents one of the
most efficient method in CO2 capture and conversion technology.3
Meanwhile, such transformation features 100% atom economy
and wide applications. For example, the product of cyclic
carbonates serve as the raw materials for polycarbonates which
were widely used as green electrolytes in batteries.4 A variety of
homogeneous catalysts involving transition metal complexes,5-7
and alkali metal salts8 have been well investigated and exhibit
appreciable performance on reactions of CO2 and epoxides with
high conversion yields. Although homogeneous catalysts are
efficient, the difficulties in separation and recycling have limited
their real application. In contrast, heterogeneous catalysts have
been widely applied in industry because of easy separation, good
recyclability, less catalyst contamination in the products and low
cost. Metal organic frameworks (MOFs) have also been explored
as efficient heterogeneous catalysts for CO2 fixation,9,10 the harsh
conditions needed for such cycloaddition of CO2 and the
instabilities of MOFs in such conditions also ruled out its practical
Four rigid building blocks based on the TPBP aromatic core
including
2,2’-dimethoxyl-3,3’,5,5’-tetrakis(4-
formylphenyl)biphenyl (OMe-TFBP), 2,2’-dihydroxyl-3,3’,5,5’-
tetrakis(4-formylphenyl)biphenyl (OH-TFBP), 2,2’-dimethoxyl-
3,3’,5,5’-tetrakis(4-aminophenyl)biphenyl (OMe-TABP) and 2,2’-
dihydroxyl-3,3’,5,5’-tetrakis(4-aminophenyl)biphenyl (OH-TABP)
were designed to construct three different TPBP-COFs bearing
varied docking sites under solvothermal conditions (Scheme 1).
The chemical composition of these COFs was confirmed by
Fourier transform infrared spectroscopy (FT-IR) spectra and
solid-state 13C NMR spectra (Figures S5-S7). As revealed by the
FT-IR spectra comparison, the nearly complete disappearance of
C=O stretching bands (1700 cm-1 and 1694 cm-1 for OMe-TFBP
and OH-TFBP, respectively) and N-H stretching bands (3302 cm-
1, 3300 cm-1 for OMe-TABP and 3360 cm-1, 3362 cm-1 for OH-
TABP) demonstrated a high polycondensation efficiency between
the monomers. Additionally, new occurrence of strong vibration
bands at ~1600 cm-1 for the three new TPBP-COFs verified the
formation of imine linkages. The carbon chemical shifts of the
characteristic C=N double bonds in these TPBP-COFs were
clearly observed (157, 155, and 150 ppm for OMe-TPBP-COF,
OMe-OH-TPBP-COF, and OH-TPBP-COF, respectively) in the
13C Solid-State NMR spectra, which further confirmed the
formation of imine linkages. In addition, the carbon signals of
methoxyl groups at ca. 60 ppm were observed for OMe-TPBP-
COF and OMe-OH-TPBP-COF, but was absent for OH-TPBP-
COF. Elemental analysis of OMe-TPBP-COF, OMe-OH-TPBP-
COF and OH-TPBP-COF revealed that the C, H and N contents
are in good agreement with their theoretical values calculated for
the infinite 2D framework structures (Table S1).
application.11,12
Therefore,
developing
a
metal-free
heterogeneous catalyst for efficient conversion of cycloaddition of
CO2 under mild conditions is highly desirable.
Covalent organic frameworks (COFs) have attracted much
interests since the first report by Yaghi and coworkers in 2005.13
Thanks to the immense design space for variation of the
topologies, linkages, molecular structures and functions,14-17
COFs with tailored functions have been explored in various
applications like gas separation and storage,18 energy storage
and conversion,19 and catalysis,20,21 etc. The integration of
spatially isolated and uniformly distributed active sites into COF
skeletons provide an ideal platform for the catalytic CO2 fixation.21-
23 It has been demonstrated before that the catalytic efficiency for
CO2 fixation could be well improved by proper molecular design:
(i) the catalysts bearing with hydrogen bond donor (HBD) are
1
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Scheme 1. Schematic representation of the synthesis of OMe-TPBP-COF, OMe-OH-TPBP-COF and OH-TPBP-COF via Schiff-base polycondensation.
The crystallinity of these TPBP-COFs was verified by Powder
X-ray diffraction (PXRD) measurements associated with
structural simulations. Experimental PXRD patterns (Figure 1a,
red curve) showcased strong diffraction peaks at 5.52°, 8.05° and
11.05° which could be indexed to (110), (200) and (220) facets,
respectively. A relatively broad peak at 21.66° assignable to 001
reflection indicated an interlayer distance of ca. 4.49 Å. The
experimental PXRD pattern was in good agreement with that of
simulated ones based on AA stacking mode for OMe-OH-TPBP-
COF (Fig. 1a, light blue line). The lattice model and Pawley
refinement for OMe-OH-TPBP-COF afford optimized unit cell
(space group of P1) with parameters of a = 26.45 Å, b = 21.60 Å,
c = 4.49 Å, a = g = 90.0° and b = 123.9°, and the final Rwp and Rp
values converged to 2.69% and 1.80%, respectively. Similarly,
the structures of OMe-TPBP-COF and OH-TPBP-COF were also
simulated and the details were summarized in Supplementary
Information (Figures S19, S20 and Tables S4, S5).
The thermal stabilities of these TPBP-COFs were evaluated by
thermogravimetric analysis (TGA). OMe-TPBP-COF, OMe-OH-
TPBP-COF and OH-TPBP-COF showcased a 5% weight loss at
as high as 497, 433 and 346 °C, respectively (Figure S8). OMe-
TPBP-COF displayed the highest thermal stability, followed by
OMe-OH-TPBP-COF and OH-TPBP-COF which might be
attributed to the steric hindrance induced by the methoxyl
substituents. Generally, the two phenyl rings in a biphenyl moiety
readily rotate along the C−C bond, while the introduction of
substituent at the 2,2’ ortho-positions can significantly limit the
free rotation, thus enhancing the stability of the TPBP-COFs.28
The steric hindrance of hydroxyl substituents is smaller than that
of methoxyl groups, which leading to relatively poor stability of
OH-TPBP-COF compared to OMe-TPBP-COF.29 Moreover, the
chemical stabilities of TPBP- COFs were tested by immersing
these COF samples in boiling water, aqueous NaOH (3 M) and
HCl (3 M) solutions for 3 days. The PXRD patterns (Figure S9)
and FT-IR spectra (Figure S10) remained unchanged after
treatments in the above-mentioned harsh conditions, which
indicates that both the crystallinity and chemical structure were
well maintained. The permanent porosities of these TPBP-COFs
were assessed by N2 sorption measurements at 77 K (Figure 2a).
The adsorption isotherms of these COFs showcased a rapid N2
uptake at low relative pressure range (P/P0 < 0.05) which reveals
the characteristics of microporous materials. The desorption
branch of the isotherms of OMe-OH-TPBP-COF and OH-TPBP-
COF did not well overlap with the adsorption branches at low
relative pressures, which might be originated from the swelling
effect of the frameworks.30 Brunauer–Emmett–Teller (BET)
surface areas were calculated to be 914, 501, 200 m2 g-1 for OMe-
TPBP-COF,
OMe-OH-TPBP-COF
and
OH-TPBP-COF,
respectively. The pore size distribution of the three TPBP-COFs
was calculated by the nonlocal density functional theory (NLDFT).
The average pore width is about 1.3 nm for OMe-TPBP-COF, 1.4
nm for OMe-OH-TPBP-COF and 1.6 nm for OH-TPBP-COF,
which are reasonably close to their simulated values (Figure 2b).
The smallest pore size distribution of OMe-TPBP-COF among the
other two isostructural COFs also manifest that the largest steric
hindrance of the methoxyl docking sites on the pore channel walls.
Figure 1. (a) PXRD profiles of OMe-OH-TPBP-COF: experimentally observed (red), Pawley refinement (blue) and their difference (green), simulated by using AA-
stacking (light blue) models and AB-stacking (pink) models. (b) top and side views of OMe-OH-TPBP-COF.
2
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be realized at relatively low reaction temperature range (25-
100 °C). Considering the energy penalty, practical protocols, and
economic cost, a moderate temperature of 40 °C was fixed as the
typical condition for further investigation.
To systematically investigate and compare the catalytic
performance of these TPBP-COFs with different docking sites,
the cycloaddition of CO2 and epichlorohydrin was performed at
40 °C in the presence of tetra-n-butylammonium bromide (TBAB)
as the co-catalyst and under a balloon of CO2 (0.1 MPa). As
summarized in Table 1, TPBP-COFs containing hydroxyl groups
exhibit prominent catalytic activity toward the CO2 cycloaddition
reaction with a yield of 91% for OMe-OH-TPBP-COF (Entry 2),
89% for OH-TPBP-COF (Entry 3); In contrast, OMe-TPBP-COF
barely possessing methoxyl docking sites but without hydroxyl
groups only exhibits low catalytic activity, and led to a low reaction
yield of 48% (Entry 4). This transformation efficiency is nearly as
low as the reaction only catalysed by TBAB alone without adding
TPBP-COFs (36% in Entry 1). These results also demonstrate
that the OMe-TPBP-COF bearing no hydroxyl groups merely
exert negligible catalytic activity on this organic transformation.
On the other hand, the hydroxyl docking sites might be the
primary effective catalytic sites for the cycloaddition of CO2.27,31
Among these three TPBP-COFs, OMe-OH-TPBP-COF exhibits
the best catalytic performance on the cycloaddition. This could be
properly attributed to better stability, larger BET surface arear32
and higher crystallinity compared that of OH-TPBP-COF (Figures
S16, S17, S18). In addition, only trace amount of product was
detected when OMe-OH-TPBP COF was applied as the catalyst
but without addition of the co-catalyst TBAB (Entry 5). The above
control experiments verified that the high catalytic performance of
TPBP-COFs toward cycloaddition of CO2 can only be realized by
the synergistic effect of TPBP-COF and co-catalyst TBAB under
mild conditions.
Figure 2. (a) Nitrogen adsorption (solid) and desorption (open) isotherms at 77
K for OMe-TPBP-COF (violet), OMe-OH-TPBP-COF (orange) and OH-TPBP-
COF (pink); (b) pore size distribution profiles for OMe-TPBP-COF (violet), OMe-
OH-TPBP-COF (orange) and OH-TPBP-COF (pink).
The morphologies of TPBP-COFs are visualized by scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM). SEM images revealed that all the three TPBP-COFs
exhibited rod-like morphologies (Figure S11). TEM images further
verified the inherent porosity of these TPBP-COFs (Figure S12).
Inspired by the high surface area, excellent stability, and the
abundant hydroxyl groups decorated at the channel walls, these
TPBP-COFs are supposed to be promising heterogeneous
catalyst candidates for the cycloaddition of CO2 with epoxide
under mild conditions. The cycloaddition of CO2 with
epichlorohydrin was selected as the model reaction using OMe-
OH-TPBP-COF as the catalyst. In order to optimize the reaction
conditions, the influence of reaction time was firstly investigated.
As displayed in Figure S13a, the typical kinetic profile indicates
that the reaction yield increased with the elongation of reaction
time. Nearly half of the substrates were converted to the target
carbonate product after 6 h, and the yield of chloropropene
carbonate is 52%. It is clearly that the complete transformation
could be achieved when the reaction time is prolonged to about
30 h. Subsequently, the influence of reaction temperature on the
model reaction was also investigated. The catalytic activity of
OMe-OH-TPBP-COF increased with elevating the temperature
(Figure S13b). To our delight, decent catalytic activities could also
To test the generality of the TPBP-COFs catalyst system on
other epoxide substrates and assess the tolerance of different
functional groups in this CO2 cycloaddition transformations,
various epoxides with different substituents were applied to
synthesize the corresponding cyclic carbonates using OMe-OH-
Table 2. Catalytic cycloaddition of CO2 with different epoxide substrates using
OMe-OH-TPBP-COF. [a]
Table 1. Catalytic cycloaddition of CO2 with epichlorohydrin using TPBP-
COFs.[a]
Entry
1
R
Substrate
Product
Yield (%) [b]
96
Br
Entry
Co-
Catalyst
TBAB
Catalyst
-
T(°C)
Time(h)
Yield (%) [b]
2
3
4
5
Cl
H
91
90
86
67
1
2
40
40
24
24
36
91
TBAB
OMe-OH-TPBP-
COF
3
4
5
TBAB
TBAB
-
OH-TPBP-COF
40
40
40
24
24
24
89
48
CH3
C2H5
OMe-TPBP- COF
OMe-OH-TPBP-
COF
Trace
[a] Reaction conditions: Epichlorohydrin (1 mmol), TBAB (0.05 mmol), TPBP-
COFs (0.01 mmol), CO2 (0.1 MPa), T (40 °C), Time (24 h); [b] Determined by
1H-NMR spectroscopic analysis.
[a] Reaction conditions: Epichlorohydrin (1 mmol), TBAB (0.05 mmol), TPBP-
COFs (0.01 mmol), CO2 (0.1 MPa); [b] Determined by 1H-NMR spectroscopic
analysis.
3
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TPBP-COF catalyst under the optimum conditions (Table 2). The
reaction yield of cycloaddition of CO2 with epibromohydrin
reached as high as 96% (Entry 1), and 91% for chloropropene
carbonate (Entry 2). The reaction yield of the transformation for
the most commercially important epoxypropane was 90% (Entry
3), which is slightly higher than that for 1,2-epoxybutane (86%,
Entry 4). A significant decrease in the rection yield was observed
for the epoxide with longer alkyl chain length. For example, only
67% of 1,2-epoxypentane was converted to the corresponding
carbonate under identical conditions (Entry 5). The declined
reaction yields can be reasonably explained by the steric
resistance of the epoxides with enlarged alkyl chain substituents.
The sizes of epoxides substrate molecules with longer alkyl
chains are not fitted well with the micropores of OMe-OH-TPBP-
COF (Figure S14), which making it more difficult to access the
activity sites on the pore wall.33-36
Recyclability represents one of the most important advantages
of heterogeneous catalysts. To evaluate the recyclability of OMe-
OH-TPBP-COF, the recycling experiments were performed upon
the model reaction of epichlorohydrin under the optimum
conditions. After five repeating catalytic cycles, the yield of 1,3-
Dioxolan-2-one still maintained up to 82%. Both the PXRD pattern
and FT-IR spectra of OMe-OH-TPBP-COF showed no obvious
changes, which manifesting both the chemical composition and
ordered framework structure retained after the catalysis cycles
(Figure S15). In addition, OH-TPBP-COF is partially degraded
during the catalytic reaction revealed lower stability (Figure S16).
These results indicate that OMe-OH-TPBP-COF features not only
high catalytic efficiency but also excellent reusability, which
render OMe-OH-TPBP-COF as a promising heterogeneous
catalyst for CO2 fixation.
To get more insights into the structure-property correlation of
these TPBP-COFs toward CO2 fixation, a reasonable mechanism
of cycloaddition of CO2 with epoxides catalysed by OMe-OH-
TPBP-COF was proposed (Figure 3) based on previous reports.21-
23,37-40 At first step, the neighbouring hydroxyl groups between the
adjacent layers of OMe-OH-TPBP-COF polarize the C-O bond of
epoxide via the formation of hydrogen bond between the
hydrogen atoms of the hydroxyl groups (a) and the oxygen atom
of the epoxide which indicated that ordered structure is conducive
to the catalysis. After that, the less hindered carbon atom in the
epoxide is attacked by the Br− originated from the co-catalyst
(TBAB), meanwhile the epoxide ring was opened and affords the
oxyanion intermediate (b) which could be stabilized by the
hydroxyls. It further undergoes a nucleophilic addition with CO2 to
form an alkyl-carbonate anion intermediate (c). Finally, the ring-
closing via intramolecular nucleophilic substitution affords the
cyclic carbonate, and thus leaving the free catalytic sites for the
next catalysis cycle. According to this plausible catalytic
mechanism, it is obvious that the hydroxyl groups acted as the
essential active sites for the CO2 cycloaddition reaction with
epoxides, and the TPBP-COFs with higher crystallinity and
stability would result in better activity and performance.
In summary, we have designed and synthesized a series of
isostructural TPBP-COFs with different docking sites on the
channel walls as metal-free heterogeneous catalysts for CO2
cycloaddition with various epoxides under mild conditions. The
introduction of hydroxyl docking sites is conducive to the
polarization of C-O bond of epoxides and the incorporation of
methoxyl substituents improves the stability of TPBP-COFs.
Additionally, the high porosity and regular nanochannels are
beneficial to the mass transfer during the catalysis and enable the
easy accessibility of the substrates with CO2 also improve the
concentration and capturing capacity of CO2. OMe-OH-TPBP-
COF not only exhibited high catalytic activities toward CO2 fixation
under mild conditions with a broad scope of epoxide substrates,
but also can be recycled and reused for at least five cycles without
obvious deterioration of performance. Both the prominent activity,
high porosity, excellent stability, and superior recyclability render
OMe-OH-TPBP-COF as a promising heterogeneous catalyst for
chemical fixation of CO2.
Acknowledgements
This work was financially supported by National Key Research
and Development Program of China (2017YFA0207500),
National Natural Science Foundation of China (51973153), and
the Natural Science Foundation of Tianjin City (17JCJQJC44600).
Keywords: docking sites modulation • covalent organic
frameworks • cycloaddition of CO2
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COMMUNICATION
Entry for the Table of Contents
By decorating either methoxyl, or hydroxyl or bearing both groups on the channel walls of TPBP-COFs, highly efficient metal-free
heterogeneous catalysts for chemical fixation of CO2 under mild conditions is realized. Specifically, the COF with both hydroxyl and
methoxyl groups (OMe-OH-TPBP-COF) exhibits the highest catalytic activity and efficiency for CO2 cycloaddition.
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