Covalent organic frameworks: Efficient, metal-free, heterogeneous organocatalysts for chemical fixation of CO2 under mild conditions

Article abstract of DOI:10.1039/c7ta08629f

The cycloaddition of CO₂ to epoxides to form cyclic carbonates is very promising and does not generate any side products. Metal-free, heterogeneous organocatalysts offer an environmentally friendly alternative to traditional metal-based catalysts. Herein two triazine-based covalent organic frameworks (COF-JLU6 and COF-JLU7) were successfully synthesized under solvothermal conditions. The structural and chemical properties of COFs were fully characterized by using powder X-ray diffraction analysis, structural simulation, Fourier transform infrared spectroscopy, ¹³C solid-state NMR spectroscopy, electron microscopy, thermogravimetric analysis and nitrogen adsorption. The two COF materials combine mesopores, high crystallinity and good stability, as well as a large number of hydroxy groups in the pore walls. They possess a high Brunauer-Emmett-Teller (BET) specific surface area up to 1390 m² g^-1 and a large pore volume of 1.78 cm³ g^-1. The COF-JLU7 displays a high CO₂ uptake of 151 mg g^-1 at 273 K and 1 bar. Importantly, COF-JLU7 was found to be a highly effective catalyst to convert CO₂ into cyclic carbonate through the cycloaddition reaction with epoxides under mild conditions. The effect of reaction parameters, such as reaction temperature, reaction time and CO₂ pressure, on the catalytic performance was also investigated in detail. Moreover, the new framework-based catalyst can be recovered and reused five times without a significant loss of catalytic efficiency.

Full text of DOI:10.1039/c7ta08629f

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Covalent organic frameworks: efficient, metal-free,
heterogeneous organocatalysts for chemical fixation of CO2 under
mild conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Yongfeng Zhi,a Pengpeng Shao,c Xiao Feng,c Hong Xia,b Yumin Zhang,*a Zhan Shi,d Ying Mu,a and
Xiaoming Liu*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The cycloaddition of CO2 to epoxides to form cyclic carbonates is very promising and does not generate any side products.
Metal-free, heterogeneous organocatalysts offer an environmentally friendly alternative to traditional metal-based
catalysts. Herein two triazine-based covalent organic frameworks (COF-JLU6 and COF-JLU7) were successfully synthesized
under solvothermal conditions. The structural and chemical properties of COFs were fully characterized by using powder X-
ray diffraction analysis, structural simulation, Fourier transform infrared spectroscopy, 13C solid-state NMR spectroscopy,
electron microscopy, thermogravimetric analysis and nitrogen adsorption. The two COF materials combine mesopores,
high crystallinity and good stability, as well as a large of hydroxy groups in the pore walls. They possess a high Brunauer–
Emmett–Teller (BET) specific surface area up to 1390 m2 g-1 and large pore volume of 1.78 cm3 g-1. The COF-JLU7 displays
high CO2 uptake of 151 mg g-1 at 273 K and 1 bar. Importantly, COF-JLU7 was found to be a highly effective catalyst to
convert CO2 into cyclocarbonates through the cycloaddition reaction with epoxides under mild conditions. The effect of
reaction parameters, such as reaction temperature, reaction time and CO2 pressure on the catalytic performance was still
investigated in detail. Moreover, new framework-based catalyst can be recovered and reused five times without a
significant loss of catalytic efficiency.
carbonates is highly desirable, owing to the transformation process
does not present any side products and the products have wide
application including as excellent aprotic solvents, as electrolytes in
lithium-ion batteries and as intermediates for synthesis of polymers,
Introduction
Over the past decade, carbon dioxide (CO2) capture and its chemical
transformation into high value-added chemicals have attracted
considerable attention due to a consideration of the environment
and sustainable development.1 CO2 is deemed as the primary
greenhouse gas causing global warming and subsequent climate
changes. On the other hand, however, it is still a non-toxic,
inexpensive, abundant and renewable carbon source. Therefore,
the catalytic conversion of waste CO2 into usefully industrial
products is an alternative and attractive strategy for CO2
utilization.2 From the standpoints of green chemistry and atomic
economy, the cycloaddition of CO2 with an epoxide to form cyclic
fine chemicals and pharmaceuticals.3 Thus far,
a variety of
homogeneous catalytic system involving alkali metal salts,4
sulfonium salt5 and organometallic complexes6 were developed and
researched. Although some homogeneous catalysts are effective,
their complicated separation, purification and recycling problems
severely limit their application in industry. In order to overcome the
shortcomings of homogeneous catalysts, several kinds of
heterogeneous catalysts based on porous materials such as
zeolites,7 silica-supported salts,8 microporous organic polymers
(MOPs)9 and metal-organic frameworks (MOFs)10 have been
explored for the synthesis of cyclic carbonates. However, the most
of the catalytic systems still contain toxic metal elements and
demand relatively high pressure and temperature.4-11 Therefore,
the development of metal-free heterogeneous catalysts for the
efficiently converting CO2 into cyclic carbonates under the mild
conditions remains to be a huge challenge.
As a new kind of crystalline porous materials, covalent organic
frameworks (COFs) are constructed from organic building blocks
using reticular chemistry and display periodic architectures and
inherent porosity.12 The most fascinating feature of COFs is easy to
realize the adjustment of the size, shape and type of pore, tailor of
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Fig. 1 (a) Schematic representation of the syntheses of COF-JLU6 and COF-JLU7 via the Shiff-base condensation under solvothermal conditions. (b) Top and (c)
side views of COF-JLU6. (d) Top and (e) side views of COF-JLU7. (red, Oxygen; blue, Nitrogen; gray, Carbon; Hydrogen is omitted for clarity)
structure and functionality through delicate design of molecular
building units.12-16 Owing to large specific surface area, low density
and adjustable compositions, as well as high thermal stability,
COFs have emerged as predesignable porous frameworks for
various applications,13-16 including the adsorption and separation
of small gas molecules like H2, CH4 and greenhouse gas CO2.17
Recently, COFs have also been explored as efficient catalysts in
heterogeneous catalysis reactions.18 Up to now, however, the field
of COFs as heterogeneous catalysts for CO2 chemical conversion
remains largely untrapped.19
easy to enrich the reactants and co-catalyst, and serve also as the
nanoreactors to improve the catalytic efficiency for chemical
fixation of captured CO2. Finally, the robust stability and
crystallinity make COFs as functional materials with an excellent
recyclability. Herein, we report two new imine-linked COFs with
hydroxy groups and abundant heteroatoms on the pore walls,
which are efficient heterogeneous organocatalysts with high
activity for the chemical fixation of CO2 to cyclic carbonates under
mild conditions.
From a standpoint of principle, we envisioned that the rationally
designed COF materials could serve as metal-free, heterogeneous
organocatalysts for the chemical conversion of CO2 to cyclic
carbonates in mild conditions. First, the COF with a large number
of accessible nitrogen active sites in the pore wall can increase the
affinity to CO2, and improve its CO2 storage capacity. Second, a
bottom-up strategy can be utilized to attach the high density of
hydroxy groups into COF framework. Thus, introduced hydroxy
group as a Brønsted acid site can active epoxides through
hydrogen-bond interaction.20 Third, the regular nano-channels are
Results and discussion
We
selected
triazine-based
aniline
(1,3,5-tris-(4-
aminophenyl)triazine and 1,3,5-tris-(4-aminophenoxy)benzene) as
knot and 2,5-dihydroxyterephthalaldehyde as linker for the
construction of the new COFs, because the triazine-based porous
organic polymers have excellent storage ability for CO2 gas.21 As
shown in Fig. 1a, COF-JLU6 was synthesized under solvothermal
conditions by condensation of 1,3,5-tris-(4-aminophenyl)triazine
(28.3 mg, 0.08 mmol) and 2,5-dihydroxyterephthalaldehyde (19.9
mg, 0.12 mmol) in the presence of 6M aqueous acetic acid (0.1 mL)
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using mesitylene/1,4-dioxane (1.0 mL, 4:1 by vol.) as the mixed
mode produced a pattern (Fig. 3a, orange line) that deviates from
the experimentlly observed profile. Similarly, the observed PXRD
patterns of COF-JLU7 exhibited four peaks at 2.67, 4.72, 7.22 and
9.50o, respectively (Fig 3d, black line). The structural simulation of
PXRD pattern showed that the COF-JLU7 possesses also preferably
the AA stacking mode (Fig 3d, blue line). The Pawley refinement
showed that XRD profiles (Fig. 3a and 3d, red line) matched the
experimentally observed patterns, as evidenced by their negligible
difference (Fig. 3a and 3d, green lines; Rwp = 4.80% and Rp = 3.90%
for COF-JLU6; Rwp = 4.01% and Rp = 3.13% for COF-JLU7). In
addition, high-resolution transmission electron microscopy (HR-
TEM) image showed long-ordered channels in COF-JLU6 (Fig. 4b).
Meanwhile, the field-emission scanning electron microscopy (FE-
SEM) images revealed that COF-JLUs aggregates into banded
crystals (Fig. 4a and Fig. S2).
o
solvent at 120 C for 72 h, which produced a red microcrystalline
solid in 90.5% isolation yield. And COF-JLU7 was constructed
under similar conditions with exception of using a mixed solvent of
o-dicholorobenzene/n-butanol (1:1 by vol.). COF-JLU6 and COF-
JLU7 possess one-dimensional ordered mesoporous channels, and
abundant heteroatoms and hydroxy groups on the pore walls can
enrich CO2 and activate the epoxides simultaneously (Fig. 1b and
1d). They were characterized by Fourier transform infrared (FT-IR)
spectroscopy, solid state 13C cross-polarization/magic-angle-
spinning (CP/MAS) NMR spectroscopy and elemental analysis.
Compared with monomer (1683 cm-1), the characteristic C=O
stretching peak is nearly complete disappearance in the spectra of
two COF-JLUs. The strong stretching peaks at 1584 cm-1 for COF-
JLU6 and 1582 cm-1 for COF-JLU7 were obtained, indicating the
formation of imine linkages (Fig. 2a). More detailed information
was assessed by 13C CP/MAS NMR analysis (Fig. 2b). The peak at
159.8 ppm for COF-JLU6 is assigned to the carbon atoms of imine
groups, and the low-field peak at about 168.8 ppm can be
attributed to the triazine ring. Similarly, the two characteristic
peaks for COF-JLU7 displayed at 161.9 and 173.5 ppm, respectively.
Elemental analysis of COF-JLU6 and COF-JLU7 revealed that the C,
H and N contents are in good agreement with their theoretical
values in infinite 2D structures (Fig. S1).
Fig. 3 XRD profiles of COF-JLU6 (a) and COF-JLU7 (d): experimentally
observed (black), Pawley refinement (red) and their difference (green),
simulated by using AA-stacking (blue) and AB-stacking (orange) models.
Crystal structures of the AA-stacking model for COF-JLU6 (b) and COF-JLU7
(e). Crystal structures of the AB-stacking model for COF-JLU6 (c) and COF-
JLU7 (f).
Fig. 2 (a) FT-IR spectra of 1,3,5-tris-(4-aminophenyl)triazine (black line),
1,3,5-tris-(4-aminophenoxy)benzene
(red
line),
2,5-
dihydroxyterephthalaldehyde (green line), COF-JLU6 (blue line) and COF-
JLU7 (sky blue line). (b) Solid state 13C cross-polarization magic-angel
spinning NMR spectrum of COF-JLU6 (blue line) and COF-JLU7 (sky blue
line).
The crystalline structures of COF-JLU6 and COF-JLU7 were
determined using powder X-ray diffraction (PXRD) analysis in
connection with structural simulations. The observed PXRD
patterns of COF-JLU6 showed a strong peak at 2.82o, and other
relatively weak peaks at 4.82, 5.62 and 7.49o which correspond to
the (100), (200), (210) and (220) reflections, respectively. The
broad peak at 25.7o is due to π-π stacking between the COF-JLU6
layers and corresponds the the (001) plane (Fig. 3a, black line). The
simulation of the XRD profile of the AA stacking mode can
reproduce the experimentally observed data in the peak position
and intensity (Fig. 3a, blue line), while the staggered AB stacking
Fig. 4 (a) FE-SEM and (b) HR-TEM images of COF-JLU6.
Thermogravimetric analysis (TGA) was carried out on the dry
COF samples under nitrogen atmosphere, which revealed that
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they are stable up to 460 oC for COF-JLU6 and 406 oC for COF-JLU7,
respectively (Fig. S3). In addition, we still investigated the chemical
stability of new COF materials. The COF-JLU6 and COF-JLU7
samples were immersed in different organic solvents including
strong affinity between CO2 and the heteroatoms on the pore wall
of COFs. In contrast, COF-JLU7 has larger adsorption capacity than
that of COF-JLU6, which attributed to higher heteroatom density
in the pore wall of COF-JLU7. To get more information of the
interaction between COFs and adsorbate CO2, the isosteric heats
of adsorption (Qst) of COFs were estimated from adsorption data
collected under 273 and 298 K (Fig. S6a). At low adsorption values,
the Qst) was 30.8 KJ mol-1 for COF-JLU7 and 28.1 KJ mol-1 for COF-
JLU6 (Fig. S6b), respectively. The values are similar to that of
previously reported triazine-based porous organic polymers under
the same conditions.21 The enrichment of CO2 in the pores of the
COFs is expected to be beneficial for the enhancement of the CO2
chemical conversion.9b
toluene,
tetrahydrofuran
(THF),
methanol,
N,N-
dimethylformamide (DMF), acetonitrile (CH3CN), aqueous HCl (1
M) and NaOH (1 M) solution at room temperature for 24 h. Then
the COF samples were collected, washed with THF, and dried
under vacuum at 100 oC for 8 h. From FT-IR spectra, we can find
that the same network connection is remained in the COF samples
(Fig. S4). And apart from acidic and alkaline aqueous solutions, all
samples exhibit strong diffraction peaks in XRD patterns (Fig. S5),
indicating that two COF materials have good stability to various
organic solvents and neutral water.
Fig. 6 (a) Kinetic profile for cycloaddition of CO2 to epichlorohydrin.
Reaction conditions: COF-JLU7 (0.051 mmole), epichlorohydrin (10.21
mmole), TBAB (0.51 mmole), 40 oC, 0.1 MPa. (b) Effect of the reaction
temperature on the yield. Reaction conditions: COF-JLU7 (0.051 mmole),
epichlorohydrin (10.21 mmole), TBAB (0.51 mmole), 8h, 0.1 MPa. (c) Effect
of the CO2 pressure on the yield. Reaction conditions: COF-JLU7 (0.051
mmole), epichlorohydrin (10.21 mmole), TBAB (0.51 mmole), 24h, 40 oC. (d)
Effect of the reaction time on the TON. Reaction conditions: COF-JLU7 (2.0
mg), TBAB (32.0 mg), 130 oC, 1.0 MPa.
Fig. 5 (a) N2 adsorption (open squares for COF-JLU6; open circles for COF-
JLU7) and desorption (filled squares for COF-JLU6; filled circles for COF-
JLU7) isotherms measured at 77 K. (b) CO2 adsorption isotherms (open
squares for COF-JLU6; open circles for COF-JLU7) at 273 K.
The permanent porosity of COF-JLU6 and COF-JLU7 were
investigated by using the nitrogen sorption isotherms at 77 K in
the activated samples. As shown in Fig. 5a, the sorption curves of
two COFs exhibited a typical type IV isotherm characterized by a
rapid uptake under low relative pressure at P/P0 < 0.01 followed
by a second step in 0.05 < P/P0 < 0.28, which is characteristic of a
mesoporous material. The Brunauer-Emmett-Teller (BET) surface
areas of COF-JLU6 and COF-JLU7 were calculated to be 1450 and
1392 m2 g-1, respectively. The pore volumes were evaluated to be
0.96 cm3 g-1 for COF-JLU6 and 1.78 cm3 g-1 for COF-JLU7 at P/P0 =
0.99. The calculated pore size distribution by using nonlocal
density functional theory (NLDFT) was average pore widths of 3.1
nm for COF-JLU6 and 3.3 nm for COF-JLU7, which are reasonably
close to their simulate values of 3.3 nm for COF-JLU6 and 3.4 nm
for COF-JLU7. Furthermore, CO2 adsorption isotherms of two COFs
were collected at 273 K (Fig. 5b). Clearly, the new COFs have a
good storage capacity with 129 mg g-1 for COF-JLU6 and 151 mg g-1
for COF-JLU7 at 273 K under 1 bar CO2, respectively. The excellent
CO2 adsorption catacity of COF-JLUs could be ascribed to the
Combining high surface area, excellent stability, large
adsorption capacity of CO2 and high density of the hydroxy groups
within the nanoscopic channels, both COF-JLUs should be highly
promising heterogeneous organocatalyst for cycloaddition of CO2
and epoxides to form cyclic carbonates under the mild conditions.
The cycloaddition of CO2 with epichlorohydrin was first selected as
a model reaction to optimize the reaction conditions. As showed
in Table S1, both polymers showed efficiently catalytic activity for
model reaction with a yield of 56% for COF-JLU6 and 67% for COF-
JLU7 at 25 oC and 0.1 MPa CO2 pressure in the presence of tetra-n-
butylammonium bromide (TBAB) as a cocatalyst (entries 1 and 2).
When the reaction temperature is increased to 40oC, a higher
yield was obtained (86% for COF-JLU6 and 92% for COF-JLU7,
entries 3 and 4). The results indicate that COF-JLU7 exhibits higher
catalytic activity than the COF-JLU6 under the same conditions,
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which is attributed to the greater CO2 adsorption capacity of COF-
JLU7. Next, we still selected tetra-n-butylammonium chloride
(TBAC) and tetra-n-butylammonium iodide (TBAI) as co-catalyst
and inverstigated the influence of cocatalyst anion on catalytic
performances. The order of the activity of catalytic system
consisting COF-JLU7 and co-catalyst with diffrent halogen anions
was found to be TBAI (93%, entry 6) > TBAB (92%, entry 4) > TBAC
(89%, entry 5). These experimental results might be ascribed to
the synergistic effects of the nucleophilicity and leaving ability of
halide anions which play a key role in the catalytic cycle.7-10 In
addition, in the absence of the cocatalyst, no product was
detected for the model reaction (entry 7). However, the
cycloaddition reaction catalyzed by the TBAB alone gave a 40%
yield (entry 8). These results of the controlled tests clearly show
that the COF-JLU7 and co-catalyst can synergistically catalyze the
cycloaddition reactions of CO2 with epichlorohydrin with high
temperature-dependent in the range of the research. And the
yield of chloropropene carbonate was drastically enhanced from
12% to 99% with reaction temperature from 25 oC to 100 oC.
Furthermore, the influence of the CO2 pressure on the
cycloaddition reaction was also investigated at 40 oC with a
reaction time for 24 h and the results are shown in Fig. 6c. We can
found that the yield of chloropropene carbonate improves with
increasing pressure from 0.1 MPa to 1.0 MPa. This can be
attributed to an increase of CO2 concentration in the
epichlorohydrin with the pressure facilitates the cycloaddition
reaction. In addition, the reactions with loading of excessive
epichlorohydrin (10.23 mmol) and the unchanged quantity of COF-
JLU7 (2.0 mg) were conducted in an autoclave reactor with CO2
purged to 1 MPa at 130 oC. This reaction gives an initial TOF up to
5076 per mole of catalyst per hour (Fig. 6d). To the best of our
knowledge, this value is greater than some previously reported
data for MOP and MOF-based catalytic systems for the
cycloaddition reaction of CO2 to epoxides under the similar
condition.9,10
efficiency under
a relatively low reaction temperature and
atmospheric pressure.
Table 1. Cycloaddition of CO2 with different substituted epoxides to
form cyclic carbonates over COF-JLU7a
Entry
1
Substrate
Product
Yield (%)b
92
96c
99
99c
2
3
79
99c
61
4
5
6
58c
88c
71c
Fig. 7 (a) Recycling catalytic test of COF-JLU7 for the CO2 cycloaddition with
epichlorohydrin: solvent free, CO2 0.1 MPa, 40 oC, 24 h. (b) PXRD curves of
COF-JLU7 before and after five runs. (c) FT-IR spectra of COF-JLU7 before
and after five runs. (d) SEM images of COF-JLU7 before and after five runs.
A typical kinetic profile of the cycloaddition reaction of CO2 with
epichlorohydrin in the presence of COF-JLU7 and TBAB at 40 oC
and 0.1 MPa CO2 pressure is shown in Fig. 6a. In the model
reaction, the yield of product increased with increasing reaction
time. After 24 h, more than half of the substrates were converted
and a yield of 63% of chloropropene carbonate was detected.
However, for high conversion of 92%, an extended reaction time
of 48 h was necessary. Fig. 6b showed the catalytic performance
of COF-JLU7 as a function of reaction temperature. COF-JLU7
aReaction conditions: COF-JLU7 (0.051 mmole), epoxide (10.21 mmole),
TBABr (0.51 mmol), CO2 (0.1 MPa), 40oC, 48 h. bDetermined by 1H-NMR
spectroscopic analysis. c80oC, 12 h.
After proving the high catalytic activity of COF-JLU7 for
chloropropene carbonate formation, we next explored the
substrate scope of the COF-JLU7 under the reaction conditions
developed above (0.051 mmol COF-JLU7, 0.51 mmol TBAB, 0.1
MPa CO2). The cycloaddition reaction of CO2 with epibromohydrin
gave excellent yields (Table1, entry 2), which are slight greater
than those of epichlorohydrin (Table1, entry 1). A good activity
o
exhibited catalytic active, even at temperature as low as 25 C. In
sharp contrast, however, for the most of zeolite-based and
mesoporous solid acid catalysts, this reaction often occurs at
reaction temperature above 100 oC.22 The reaction is
o
was showed for cycloaddition of butylene oxide and CO2 at 40 C
under 0.1 MPa with a yield of 79% over 48h. When the reaction
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temperature was increased to 80oC, a nearly quantitative cyclo
carbonate was obtained, even in the short reaction time (Table1,
entry 3). Interestingly, when the epoxide with sterically hindered
was utilized as a substrate, an obvious decrease in the yield of the
corresponding cyclic carbonates such as 61% for 3-allyoxyl-1,2-
propylene was observed at 40 oC under 0.1 MPa (Table1, entries 4).
This phenomenon may be due to the limited diffusion of the
substrate with large molecular size into the the one-dimensional
channels of COF.9,10 Compared with the long chain substituted
epoxide, however, the aromatic epoxides styrene oxide and
phenyl glycidyl ether as reactant afforded a high yield of 88% for
1-phenyl-1,2-ethanediol cyclic carbonate and 71% for
(phenoxymethyl)ethylene carbonate at 80 oC under 0.1 MPa,
respectively (Table1, entries 5 and 6). When R- or S-styrene oxide
was used as the substrate, the cycloadditon reaction of CO2
exhibited excellent enantioselectivity (an ee value of 97% for R-
styrene oxide and 93% for S-styrene oxide, Table S2), indicating
the selective ring opening occurred preferentially at the
methylene carbon-oxygen bond of the epoxides.
through a hydrogen bond. Simultaneously, the Br— anion
attacks the less sterically hindered carbon atom of the
epoxide which forms intermediate I. And then the opening of
the epoxide forms the oxyanion which is stabilized by the
hydroxy group on adjacent layer II. The nucleophilic addition
of the oxyanion intermediate to CO2 and subsequent form an
alkylcarbonate anion III. Then the ring-closing of intermediate
III affords the cyclic carbonate and regenerates the COF.
Conclusions
In summary, two triazine-based covalent organic frameworks
were successfully designed and constructed via imine
formation under solvothermal conditions. The two COFs have
a high crystallinity, a good porosity with a high surface area, a
large pore volume and an outstanding stability. COF-JLU7
with more heteroatoms in the pore wall gives a higher
storage of 151 mg g-1 at 273 K and 1 bar. Using TBAB as co-
catalyst, importantly, the COF-JLU7 as a metal-free organocatalyst
displayed an efficient catalytic performance for the cycloaddition
of CO2 with epoxides under mild conditions with a broad
substrate scope. Furthermore, COF-JLU7 can be recycled and
reused for multiruns without obvious decrease in catalytic
efficiency, which represents an important advantage for
industial catalytic processes. We also highlight that this work
opens up a new way for the construction of excellent solid
catalysts at a molecular level. Currently, such researches are
underway in our laboratory.
The recyclability of catalyst is the most important advantage in
the heterogeneous reaction for industrial applications. Thus we
studied the recyclability of COF-JLU7 in the model reaction using
epichlorohydrin as substrate. After each cycle, the COF catalyst
was recovered by centrifugation, washed, dried and reused in the
next run of the reaction by charging with the same substrate. It
was found that the catalyst can be efficiently recycled and reused
for five repeating cycles without significant loss of catalytic activity
and selectivity (Fig. 7a). To our delight, the COF-JLU7 after
recycling remained well high crystallinity (Fig. 7b), original
skeleton connectivity and morphology (Fig. 7c and 7d), which
attributed to the good stability of framework material. The highly
catalytic activity, good reusability and simple separation process
made the catalyst expectant for commercial applications.
Experimental section
Instrumentations. 1H spectra were recorded on a Avance III-400
NMR spectrometer, where chemical shifts (δ in ppm) were
determined with a residual proton of the solvent as standard. FT-
IR spectra were recorded from 400 to 4000 cm-1 on an Avatar FT-
IR 360 spectrometer by using KBr pellets. Solid-state 13C CP/MAS
NMR measurement was recorded using a Bruker AVANCE III 400
WB spectrometer at a MAS rate of 5 kHz and a CP contact time of
2 ms. Elemental analyses were carried out on an Elementar model
vario EL cube analyzer. Field emission scanning electron
microscopy was performed on
a SU8020 model HITACHI
microscope. Transmission electron microscopy was performed on
a JEOL model JEM-2100 microscope. Powder X-ray diffraction data
were recorded on a PANalytical BV Empyrean diffractometer
diffractometer by depositing powder on glass substrate, from 2θ =
1.5° to 45° with 0.02° increment at 25 °C. Thermogravimetric
analysis (TGA) was performed on a TGA Q500 thermogravimeter
by measuring the weight loss under nitrogen. Nitrogen sorption
isotherms were measured at 77 K with a JW-BK 132F or ASIQ (iQ-2)
analyzer. Before measurement, the samples were degassed in
vacuum at 120 °C for more than 10 h. The Brunauer-Emmett-
Teller (BET) method was utilized to calculate the specific surface
areas and pore volume. The nonlocal density functional theory
(NLDFT) method was applied for the estimation of pore size
distribution. Enantiomeric excess (ee) was determined by HPLC
(Waters 2048) with a Chiralcel OD-H (250 mm × 4.6 mm) column;
Fig. 8 A proposed reaction mechanism for the cycloadditon reaction of CO2
and epoxide catalyzed by COF-JLU7 and TBAB.
Based on the previous studies on related systems2,19,20 and
the current results, a plausible mechanism for the COF-JLU7
catalyzed cycloaddition reaction of CO2 with epoxides is
proposed as shown in Fig. 8. First, the hydroxy groups on the
pore wall of COF activate the C-O bond of the epoxide ring
6 | J. Name., 2012, 00, 1-3
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a mixture of n-hexane and Isopropanol (90:10) was used as mobile
phase with a flow of 1.0 mL/min.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 51473064, 61435005 and
21274050).
Synthesis of COF-JLU6. A dioxane/1,3,5-trimethylbenzene (0.2
mL/0.8 mL) mixture of 1,3,5-tris-(4-aminophenyl)triazine (28.3 mg,
0.08 mmol) and 2,5-dihydroxy-1,4-benzenedicarboxaldehyde (19.9
mg, 0.12 mmol) in the presence of an acetic acid catalyst (0.1 mL,
6 M) in a 10 mL Pyrex tube was degassed via three freeze–pump–
thaw cycles. The tube was flame-sealed and heated at 120 oC for
three days. The precipitate was collected through centrifugation,
washed with anhydrous THF (3 x 5.0 mL) and anhydrous acetone
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(3 x 5.0 mL). The powder was collected and dried at 120 C under
vacuum overnight to produce COF-JLU6 as a red solid in 90.5%
isolated yield. Anal. Cald for (C12H9N2O)n: C 73.08; H 4.60; N 14.20.
Found: C 71.53; H 4.94; N 14.15.
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(0.5/0.5 mL) mixture of 2,4,6-tris(4-aminophenoxy)triazine (32.2
mg, 0.08 mmol) and 2,5-dihydroxy-1,4-benzenedicarboxaldehyde
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an orange solid in 91% isolated yield. Anal. Cald for (C12H9N2O)n: C
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Covalent organic frameworks with hydroxy groups in the pore wall, which exhibits
efficient, metal-free, heterogeneous catalytic performances for chemical fixation of
CO2 under mild conditions, are reported.