Synthesis of covalent organic frameworks via in situ salen skeleton formation for catalytic applications

Article abstract of DOI:10.1039/c8ta11058a

Covalent organic frameworks (COFs) with highly ordered crystalline structures and uniform microenvironments have potential applications in the field of catalysis; however, their application is restricted by their harsh synthesis conditions and multi-step synthetic procedures. Herein, we report the facile synthesis of a new COF-salen via in situ salen skeleton formation by heating aldehyde and amine precursors in an air atmosphere. This COF-salen possesses an AA stacking eclipsed layered crystalline structure, micro/macro hierarchical pores, and high stability in acid or base medium and can be efficiently transformed to COF-salen-M (M = Co, Mn, Cu, Zn) with a well-retained ordered crystalline structure. The successful application of COF-salen-Co(iii) and COF-salen-Mn, respectively, in epoxide hydration, which requires the cooperation of two salen-Co(iii), and olefin epoxidation, in which the isolated salen-Mn functions as the active site, can be attributed to their unique layered crystalline structures; these structures can efficiently isolate the active sites by restricting their mobility and generate cooperation between nearby active sites in adjacent layers. COF-salen-Co exhibited much higher activity and stability than the corresponding amorphous polymers in cycloaddition reactions of epoxides with CO₂, demonstrating the advantage of the crystalline structure in catalysis.

Full text of DOI:10.1039/c8ta11058a

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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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Synthesis of covalent organic frameworks via in-situ salen
skeleton formation for catalytic applications
He Li,a Xiao Feng,b Pengpeng Shao,b Jian Chen,a,c Chunzhi Li,a,c Sanjeevi Jayakumar,a,d and Qihua
Received 00th January 20xx,
Accepted 00th January 20xx
Yang*a
DOI: 10.1039/x0xx00000x
Covalent organic frameworks (COFs) with highly ordered crystalline structure and uniform microenvironment have potential
applications in the field of catalysis, however, their application was restricted by the harsh synthesis conditions and multi-
step synthetic procedure. Herein, we reported the facile synthesis of a new COF-salen via in-situ salen skeleton formation
by heating the aldehyde and amine precursors in air atmosphere. This COF-salen had AA stacking eclipsed layered crystalline
structure, micro/macro hierarchical pore, high stability in acid or base medium and could be efficiently transferred to COF-
salen-M (M = Co, Mn, Cu, Zn) with well retained ordered crystalline structure. The successful application of COF-salen-Co(III)
and COF-salen-Mn respectively in the epoxide hydration which needs the cooperation of two salen-Co(III) and olefin
epoxidation which involves the isolated salen-Mn as active site should be attributed to their unique layered crystalline
structure that could efficiently isolate the active sites by restricting their mobility and generate the cooperation among the
nearby active sites in the adjacent layers. The COF-salen-Co exhibited much higher activity and stability than corresponding
amorphous polymers in cycloaddition reactions of epoxides with CO2, demonstrating the advantage of crystalline structure
in catalysis.
www.rsc.org/
solvents and so on, which impedes their practical applications.
Recently, several groups have reported the synthesis of COFs
under not very demanding conditions.19-22 For example,
Introduction
Covalent organic frameworks (COFs) are designable porous,
crystalline materials that are constructed by organic monomers
based on reversible chemical reactions, such as Schiff base
reaction, boronic acid self-condensation or polycondensation
with diols, C=C bond formation reaction, etc.1-5 Nowadays,
numerous COFs have been synthesized with different
topologies such as two-dimensional (2D), three-dimensional
(3D) and woven architectures.6-8 Due to their high surface area,
periodic skeletons and ordered pores, and well-defined
chemical structures, COFs have shown potential applications in
the fields of gas storage and separation,9-10 sensors,11-12 photo
Dichtel’s group reported the two-step synthesis of imine linked
COFs using Lewis acid to catalyze the Schiff-base condensation
in the first step, and supercritical CO2 drying in the second
step.19 Zhao and co-workers reported the successful synthesis
of COF-LZU1, TpPa-1, and N3-COF at room temperature using
both batch and continuous flow.20 COF-42, COF-43, and COF-
LZU1 were synthesized under ambient conditions by Wang and
co-workers.21 However, the facile synthesis of COFs is still
limited to several types.
The “homogeneous” in “heterogeneous” catalysis has been a
long term pursuit in the field of catalysis, however, still faces the
big challenge.23-25 One of the challenges lies in the difficulty in
the synthesis of solid catalysts with well-defined active sites
that have identical local structure and microenvironment. The
COFs are very attractive in bridging homogeneous and
heterogeneous catalysts not only because of their solid nature
in water/organic solvent which allows the easy separation and
recovery, but also the possibility of integrating active sites in a
uniform and identical microenvironment. Furthermore, recent
progresses in the successful resolution of the crystal structure
of COFs by a single crystal diffraction method made the detailed
mechanism study during the catalytic process possible.26-27
The salen ligand which has [O,N,N,O] tetradentate bis-schiff
base structure could coordinate with various kinds of metal ions
to form stable salen-metal complexes which have been widely
used in catalysis such as oxidation, CO2 cycloaddition/co-
and
electronic
devices,13-14
proton
conductions,15
heterogeneous catalysis16-18 and so on.
Generally, the COFs were synthesized under strict conditions
such as vacuum/inert atmosphere, the narrow scopes of
a.State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China.
*E-mail: yangqh@dicp.ac.cn.
b.Key Laboratory of Cluster Science, Ministry of Education of China, School of
Chemistry and Chemical Engineering, Beijing Institute of Technology, 5 South
Zhongguancun Street, Beijing 100081, China.
c. University of Chinese Academy of Sciences, Yuquan Road 19A, Beijing 100049,
China.
d.International College, University of Chinese Academy of Sciences, Yuquan Road
19A, Beijing 100049, China.
†Electronic Supplementary Information (ESI) available: FT-IR spectra, 13C CP/MAS
NMR spectrum, TGA data, XPS results, N2 sorption isotherms
distributions, SEM pictures, PXRD patterns, additional Tables and experimental
sections. See DOI: 10.1039/x0xx00000x
& pore size
polymerization
with
epoxides,
asymmetric
alkene
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epoxidation/Diels-Alder reaction and so on.28-38 Different types isolation effect simultaneously. The salen-based COF have been
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DOI: 10.1039/C8TA11058A
of salen-M complexes generally involved different reaction reported previously, but the cooperation and isolation effect
pathways in catalysis.39-40 For example, salen-M (M = Ti, Al, Co, have not been investigated in details.49-50
Cr) always needs the cooperation of two molecular catalysts to
catalyze the reactions including epoxide hydration/asymmetric heating
Herein, we reported the facile synthesis of the COF-salen via
1,3,5-tris(3’-tert-butyl-4’-hydroxy-5’-
ring-opening reaction, cyanation of aldehyde, etc.33,41-42 For the formylphenyl)benzene and ethylenediamine under air
olefin epoxidation reaction catalyzed by salen-Mn, the atmosphere. The COF-salen with AA stacking eclipsed layered
prevention of the dimer formation is required because the crystalline structure had micro/macro hierarchical pore. The
dimers are inactive for the reaction.43-45 Previously, salen-Co catalytic performance of COF-salen-M samples (M = Co, Mn, Cu,
complexes were encapsulation in silica-based nanoreactor to Zn) successfully generated by coordination of metal ions with
facilitate the cooperation in the epoxide hydration reaction.38 COF-salen were investigated in epoxide hydration, olefin
To prevent the dimer formation, salen-Mn was immobilized on epoxidation and cycloaddition of CO2 and epoxides with the
the solid support or integrated in the molecular cages.46-48 For emphasis on the elucidation of the cooperation and isolation
example, Cui and coworkers reported the enhanced activity and effect. The advantageous of the crystalline COFs in catalysis was
enantioselectivity over the homogeneous catalyst in chiral demonstrated with control experiments using corresponding
coordination cages derived from salen-Mn.48 However, the amorphous polymers as catalysts.
above strategies are hard to realize both the cooperation and
A
THF, EtOH,
CH3COOH
M(OAc)2
COF-salen
COF-salen-M
B
C
THF : EtOH : 3 M CH3COOH = 4 : 1 : 0
THF : EtOH : 3 M CH3COOH = 4 : 1 : 1
DMF : EtOH : 3 M CH3COOH = 4 : 1 : 1
DMF : EtOH : 3 M CH3COOH = 4 : 1 : 0
Dioxane : EtOH : 3 M CH3COOH = 4 : 1 : 0
THF : EtOH : 3 M CH3COOH = 4 : 0 : 1
Dioxane : EtOH : 3 M CH3COOH = 4 : 1 : 1
10
20
30
40
10
20
30
40
2 Theta (degree)
2 Theta (degree)
Figure 1. (A) Schematic illustration for the synthesis of COF-salen and COF-salen-M, (B) and (C) PXRD patterns of COF-salen
prepared under different conditions.
diffraction peaks in the PXRD patterns (Figure 1C). This suggests
Results and discussion
the wide reaction conditions for the synthesis of COF-salen.
Salen-based covalent organic framework, COF-salen was
synthesized via facile heating of ethylenediamine and 1,3,5-
tris(3’-tert-butyl-4’-hydroxy-5’-formylphenyl)benzene
Among all the conditions screened, the combination of acetic
acid and mixed solvent of THF and EtOH was the best for the
synthesis of COF-salen considering both the high crystallinity
(TBHFPB) in organic solvents in the presence of 3 M aqueous
acetic acid under air atmosphere (Figure 1A). Firstly, the
synthesis conditions were optimized by screening the solvents
while keeping the acetic acid concentration constant (detailed
see Table S1). The yellow solids obtained using ethanol-dioxane,
ethanol-THF and ethanol-DMF as solvent exhibited clearly the
diffraction peaks at low 2-theta degree in their PXRD patterns,
suggesting that all materials had crystalline structure (Figure
1B). Even without acetic acid, the crystalline materials could
also be obtained as evidenced by the appearance of the
and the high yield (86%).
The crystalline structure of COF-salen was resolved using
powder X-ray diffraction (PXRD) combined with the
computational structural simulations and Pawley refinement as
conducted by Materials Studios version 8.0. Eclipsed AA
stacking and staggered stacking (AB and ABC) models were built
(Figure 2A and S1). It is found that the simulated PXRD patterns
of the eclipsed AA stacking is in good agreement with the
experimental obtained PXRD patterns with a P321 space group,
as confirmed by the weak intensity of the difference plot in
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Figure 2A. COF-salen exhibited a set of strong PXRD peaks at = 90°, and γ = 120°, with Rwp = 4.37% and Rp = 3.06V%ie.wTAhrtiicslereOsnulinlet
3.5°, 6.0°, 7.0° and 22.2°, which were assigned to the (100), indicates that the COF-salen has well stacDkeOdI: s10tr.1u0c3t9u/rCe8.TA11058A
(110), (200) and (001) facets, respectively. The Pawley
refinement cell parameters are a = b = 29.86 Å, c = 4.18 Å, α = β
100
Experimental
Pawley Refined
Simulated
A
B
Difference
110
1.9 nm
200
001
10
20
2 Theta (degree)
30
40
Top view
Side view
100
80
60
40
20
0
1 M NaOH
1 M HCl
C
D
MeOH
THF
DMF
Water
200
400
600
800
10
20
30
40
Temperature (oC)
2 Theta (degree)
Figure 2. (A) Experimental, Pawley-refined, simulated PXRD patterns (AA stacking) and a difference plot of COF-salen, (B) top and
side views of COF-salen showing the ideal eclipsed AA structure, (C) TGA curve of COF-salen under air atmosphere, (D) PXRD
patterns of COF-salen after the treatment in different types of solvents.
A
B
COF-salen
COF-salen-Co
COF-salen-Co-recycle
*
*
*
*
*
*
*
*
*
*
*
*
*
*
3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
250
200
150
100
50
0
-50
Chemical shift (ppm)
C
D
700
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
600
500
400
300
200
100
0
0.0
0.2
0.4
0.6
0.8
1.0
1
10
100
Relative Pressure (P/P0)
Pore Size (nm)
Figure 3. (A) FT-IR spectra of COF-salen, COF-salen-Co and COF-salen-Co after the fifth recycle, (B) 13C CP-MAS NMR spectrum of
COF-salen (* refers to spinning sidebands), (C) N2 sorption isotherms (measured at 77 K) and (D) NLDFT pore size distribution
curves of COF-salen (black) and COF-salen-Co (red).
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DOI: 10.1039/C8TA11058A
The formation of COF-salen was further characterized by
The permanent porosity of COF-salen was characterized by
Fourier-transform infra-red (FT-IR) and solid-state 13C CP MAS the nitrogen sorption analysis at 77 K (Figure 3C, Table 1). The
NMR spectroscopy (Figure 3). The FT-IR spectrum of COF-salen N2 sorption isotherm of COF-salen was of type I with a sharp
showed a strong vibration band at 1632 cm-1 attributed to the uptake at relative pressure P/P0
 0.05, showing that the
stretching vibration of C=N, indicating the formation of imine material had microporous structure as clarified by IUPAC. COF-
bond (Figure 3A). The disappearance of the characteristic C=O salen had the Brunauer-Emmett-Teller (BET) specific surface
stretching vibration at 1651 cm-1 indicated the complete area of 1646 m2/g and pore volume of 1.07 cm3/g. The pore size
consumption of the aldehydes (Figure S2). In 13C CP MAS NMR distribution calculated based on nonlocal density functional
spectrum of COF-salen (Figure 3B), the peak at 167.9 ppm theory (NLDFT) method was centered at about 1.86 nm, which
assigned to C=N confirmed the formation of imine bond. The agrees well with the theoretical data (1.9 nm) calculated by the
peak at 28.4 and 34.2 ppm was from the methyl and quaternary AA stacking model of COF-salen (Figure 3D). This indicated COF-
carbon of tert-butyl group of COF-salen, respectively. The peak salen has very ordered reticular structure. Besides, some
at 61.4 ppm was from the carbons of methylene group, and the meso/macropores could be observed from the pore size
resonance at 159.9 ppm was from the carbon adjacent to the distribution curve. High-resolution scanning electron
oxygen atom. The resonances from 118.6 ppm to 141.8 ppm microscopy (HR-SEM) revealed that COF-salen had
were assigned to aromatic carbons. Furthermore, comparing microspherical morphology with diameter
with the 13C CP MAS NMR spectrum of the corresponding zooming in, we could see that the microspheres were
amorphous polymer POF-salen (Figure S3), the peaks widths of constructed by aggregation of nanorods with thickness of 30
 2 um. Further

COF-salen were much narrower, indicating the more uniform to 60 nm to form large amounts of macropores (Figure 4). This
chemical environment of COF-salen due to its ordered indicated the meso/macropores are constructed from the
crystalline structure. The combined characterization results packing of the nanorods, and the hierarchical porosity is an
confirmed the successful formation of COF-salen via the advantage for the mass diffusion especially during the catalytic
reaction of TBHFPB and ethylenediamine.
process.
Table 1. Textural properties and metal contents of COF-salen and COF-salen-M.
BET surface area
Vtotal
(cm3g-1)
1.07
Pore size
Theoretical metal
content (wt%)a
-
Metal content
COF sample
(m2 g-1)
1646
(nm)
1.86
analysed by ICP (wt%)
-
COF-salen
COF-salen-Co
1065
836
0.66
0.54
0.59
0.77
0.61
0.59
1.59
1.36, 1.58
1.59
12.2
9.0
11.9
9.5
COF-salen-Co(III)
COF-salen-Mn
961
10.6
12.9
13.3
11.9
10.2
10.9
10.6
11.4
COF-salen-Cu
1258
1032
973
1.59
COF-salen-Zn
1.59
COF-salen-Co after 5th cycle
1.59
a Theoretical metal content was calculated based on quantitative metalation of COF-salen.
A
C
B
E
COF-salen-Co
COF-salen-Mn
D
COF-salen-Cu
COF-salen-Zn
10
20
30
40
2 Theta (degree)
Figure 4. HR-SEM pictures of (A-B) COF-salen and (C-D) COF-salen-Co (For A and C, scale bar is 1 μm; for B and D, scale bar is 100
nm) and (E) PXRD patterns of COF-salen-M.
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DOI: 10.1039/C8TA11058A
COF-salen-Co was also transferred into COF-salen-Co(III) via
COF-salen could be stable up to 300 C under air as revealed
by thermal gravimetric analysis (TGA), which indicated the good the oxidation of COF-salen-Co with air in the presence of p-
thermal stability of COF-salen (Figure 2C). The chemical stability toluenesulphonic acid. The appearance of the vibration peak at
of COF-salen was assessed by dispersing COF-salen in water, 1010 and 1125 cm-1 assigned to –OTs in the FT-IR spectrum of
DMF, methanol, 1 M HCl and 1 M NaOH solutions. The COF- COF-salen-Co(III) confirmed the incorporation of –OTs anions
salen was then collected by filtration and washed with (Figure S9). COF-salen-Co(III) had crystalline structure as
methanol, water and THF, followed by drying under vacuum at verified by the existence of diffraction peaks in its XRD patterns
100 oC for 12 h. To our delight, the recovered COF-salen (Figure S10). The BET surface area, pore volume and pore
exhibited strong diffraction peaks in the PXRD patterns (Figure diameter were slightly lower than those of COF-salen-Co due to
2D), suggesting that COF-salen had high chemical and structure the occupation of –OTs anions in the nanochannel (Figure S11,
stability possibly due to the strong hydrogen bond and π-π Table 1). The successful oxidation of COF-salen-Co to COF-salen-
stacking arrangement of the aromatic rings in COF-salen.
Co(III) indicated that COF-salen was robustness enough to
Inspired by the good chemical stability of COF-salen, we survive the acidic oxidation process.
prepared the metal coordinated COF-salen-M (M = Co2+, Mn2+,
Cu2+, Zn2+) by refluxing the corresponding metal acetate and
Table 2. The comparison of the catalytic performances of
COF-salen in EtOH for 3 days (Figure 1). The metal loading of COF-salen-M and their homogeneous counterpart in EPH
COF-salen-M approached the theoretical value based on ICP- hydration, styrene epoxidation and cycloaddition of PO and
AES (inductively coupled plasma atomic emission spectrometry) CO2.
measurement (Table 1). This shows that almost all salen units
Conv.
(%)
Sel.
(%)
>9
9
TOFa
(h-1)
Reaction
Catalyst
could be coordinated with metal ions attributed to the open
and ordered porous structure of COFs. The formation of metal-
salen was proved by the FT-IR spectroscopy (Figure 1 and S4). In
the FT-IR spectra of COF-salen-M, a red shift of C=N vibration
was observed, which indicated the successful coordination of
metal ions with salen unit of COF-salen. For example, the C=N
vibration of COF-salen-Co shifted from 1632 cm-1 to 1613 cm-1,
accompanied with a new peak at 499 cm-1 assigned to Co-N or
Co-O vibration.51 TGA showed that all the COF-salen-M have
good thermostability (Figure S5).
X-ray photoelectron spectroscopy (XPS) was also employed
for the characterization of COF-salen-M using COF-salen-Co as
a model (Figure S6). The XPS result showed the binding energy
for Co (2p1/2) of COF-salen-Co and salen-Co respectively at
780.6 eV and 779.5 eV, showing that the Co species are in +2
state.50 In comparison with the homogeneous counterpart, the
blue shift in Co (2p1/2) binding energy of COF-salen-Co
indicated the slight change in the local coordination structure of
salen-Co possibly due to the increased rigidity of salen unit
integrated in the framework of COFs.
The PXRD patterns of COF-salen-M are almost identical to
those of Co-salen though the intensity of the diffraction peak
decreased (Figure 4E). The N2 sorption isotherms of COF-salen-
M were of type I similar to those of COF-salen (Figure S7). The
BET surface areas of COF-salen-M were lower than those of
COF-salen, but COF-salen-M still had high BET surface area
varying in the range of 961 to 1258 m2/g. The pore diameter of
COF-salen-M samples was the same and distributed at 1.59 nm,
which was big enough for the free diffusion of most of organic
compounds. The decrease in pore volumes and pore diameter
after coordination of COF-salen with metal ions suggested the
coordination of metal ions in the nanochannel of COF-salen.
SEM and HR-SEM images showed that COF-salen-Co had similar
morphology to COF-salen (Figure 3C-3D, Figure S8). The above
results show that the crystalline structure of COF-salen was well
retained after coordination with metal ions.
COF-salen-Co(III)
salen-Co(III)
98
98
52
87
EPH
hydrationb
>9
9
Styrene
COF-salen-Mn
salen-Mn
74
58
32
24
>9
9
120
33
epoxidationc
COF-salen-Co
COF-salen-Zn
COF-salen-Cu
COF-salen-Mn
POF-salen-Co
salen-Co
91
90
20
46
70
25e
9
972
780
67
>9
9
>9
9
CO2
cycloadditio
n
>9
9
252
420
41
reactiond
>9
9
>9
9
>9
9
TBAB
-
a
b
TOF was calculated with conversion less than 30%.
Reaction
o
conditions: EPH (1 mmol), catalyst (20 μmol), water (2 mmol), 40 C, 3
h, 3-chloropropane-1,2-diol was the only product. c Reaction conditions:
styrene (2 mmol), catalyst (10 μmol), t-butyl hydroperoxide (6 mmol),
acetonitrile (5 mL), 40 °C, 24 h. The products included benzaldehyde,
styrene oxide, benzoic acid, etc. d Reaction conditions: PO (8 mmol),
catalyst (8 μmol), TBAB (80 μmol), 100 C, 2 MPa CO2, 3 h. e Reaction
o
time: 6 h.
The catalytic performance of COF-salen-M was tested in a
series of reactions that have been previously catalyzed by
homogeneous salen-M complexes (Table 2). Firstly, the catalytic
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performance of COF-salen-Co(III) was tested in the epoxide showed that salen-M integrated in the network of COFs could
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DOI: 10.1039/C8TA11058A
hydration for the production of glycol (an important not only work cooperatively during catalysis but also be
intermediate in industry) to investigate the possibility of the effectively isolated to inhibit the dimer formation, which is
cooperation of salen-Co(III) in the network of COFs because the mainly attributed to the unique layered structure of COFs.
efficient epoxide hydration needs the cooperation of two salen- Owing to the rigidity of crystalline COFs, the cooperation effect
Co(III) molecules.37,51-52 COF-salen-Co(III) could smoothly is less efficient than the isolation effect during the catalytic
catalyze the epichlorohydrin (EPH) hydration to afford 98% process.
conversion with >99% selectivity to 3-chloropropane-1,2-diol.
Functionalized five-membered cyclic carbonates have wide
This suggests that the successful cooperation of salen-Co(III) applications as nonprotic solvents, precursors for
integrated in the network of COFs, possibly realized by the polycarbonate synthesis, intermediates in pharmaceuticals and
nearby Co-salen in the adjacent layers of the COFs. Under fine chemicals synthesis.57-59 Salen-M complexes with Lewis
similar reaction conditions, the homogeneous counterpart acidity have been considered as promising catalysts in
(molecular structure see Scheme S1) with TOF of 87 h-1 was cycloaddition of CO2 and epoxides for the production of five-
more active than COF-salen-Co(III) with TOF of 52 h-1. This membered cyclic carbonates with the aid of co-catalyst such as
indicates that cooperation of salen-Co(III) of COF-salen-Co(III) is tetrabutylammonium bromide (TBAB).31 The catalytic
not as free as in homogenous system due to the rigidity of the performance of COF-salen-M was investigated in cycloaddition
crystalline structure of the COFs.
of CO2 and propylene epoxide (PO) with TBAB as co-catalyst
(Table 2). Almost no product could be detected using COF-Co
without TBAB and only 9% conversion was obtained using only
TBAB. The combination of COF-salen-Co and TBAB could
smoothly catalyze the cycloaddition reaction to obtain 91%
conversion with 99% selectivity to propylene carbonate (PC)
(Figure 5B), showing that the efficient cycloaddition reaction
needs the cooperation of Lewis acid and TBAB. A control
experiment was performed with the combination of COF-salen
and TBAB, but only 10% yield of PC was obtained. This confirms
that salen-M was the active sites for the cycloaddition reaction.
80
100
B
A
80
60
40
20
0
60
40
20
0
COF-salen-Mn
salen-Mn
0
5
10
15
20
25
0
50
100
150
200
250
Time (min)
Time (h)
Figure 5. (A) Kinetic curves for the epoxidation of styrene
catalyzed by COF-salen-Mn and salen-Mn. Reaction conditions:
styrene (2 mmol), catalyst (10 μmol), t-butyl hydroperoxide (6
mmol), acetonitrile (5 mL), 40 °C. (B) Kinetic curves for the
cycloaddition reactions of PO and CO2 catalyzed by COF-salen-
Co. Reaction conditions: PO (8 mmol), catalyst (8 μmol), TBAB
(80 μmol), 100 oC, 2 MPa CO2.
COF-salen-M
Salen-Mn is an efficient catalyst for epoxidation of olefins,
but suffers the deactivation by forming dimer complexes during
the catalysis.53-56 The integration of salen-Mn in the crystalline
structure of COFs may inhibit the dimer formation. To
investigate if the salen-Mn could be efficiently isolated in COFs,
the catalytic performance of COF-salen-Mn was tested in the
styrene epoxidation reactions (Table 2 and Figure 5A). As the
kinetic curves showed, the styrene conversion increased
sharply over COF-salen-Mn in the initial reaction time and about
50% conversion could be achieved in 1.5 h. After that, the
reaction rate was slowed down, possibly due to the low
concentration of styrene in the reaction system. The
homogeneous salen-Mn afforded much slower reaction rate
than COF-salen-Mn, about 58% conversion of styrene was
achieved in 24 h. The TOF of COF-salen-Mn was ca 3.6 fold that
of homogeneous counterpart, which indicated that the salen-
Mn isolated in well-ordered pore channels of COFs was hard to
form dimers due to the restricted movement during the
catalysis. Moreover, the selectivity to styrene oxide over COF-
salen-Mn is higher than that over salen-Mn, which further
proved the crystalline structure could inhibit the deactivation,
at least more effective than that of salen-Mn. The above results
Scheme 1. Proposed reaction mechanism for the cycloaddition
reaction of CO2 with PO over COF-salen-M in the presenece of
TBAB.
A tentative mechanism of CO2 cycloaddition reaction was
illustrated in Scheme 1. Initially, PO was activated by
coordination with COF-salen-M serving as a Lewis acid center.
Then the Br- of TBAB attacked on the less sterically hindered
side of PO as a nucleophile, affording a ring-opening alkoxide
intermediate. Subsequently, the M-carbonate intermediate
formed by the insertion of CO2 into the M-O bond of the
alkoxide was converted to PC through a ring-closing step and
COF-salen-M was regenerated. Under similar conditions, the
activity increased in the order of COF-salen-Cu < COF-salen-Mn
< COF-salen-Zn < COF-salen-Co. Considering that COF-salen-M
had similar textural parameters and crystalline structure, the
reaction sequence may be related with the Lewis acid strength
6 | J. Name., 2012, 00, 1-3
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Figure 6. (A) The catalytic performances of COF-salen-Co in PO
cycloaddition reactions at different temDpOerI:a1t0u.1r0eV3si9,e/wC(BA8r)TticAkl1ei1nO0en5lt8iniAce
curve for the cycloaddition reactions of EPH and CO2 catalyzed
by COF-salen-Co at 25 oC and 1 bar CO2.
and coordination ability of salen-M with ring-opening alkoxide
intermediate.
The counter anions of the co-catalysts have great influence
on the activity. Thus, apart from TBAB, potassium iodide (KI),
tetrabutylammonium chloride (TBAC), tetrabutylammonium
iodide (TBAI) and tetraoctylammonium bromide (TOAB) were
selected as nucleophilic co-catalysts. The activity decreased in
the order of TBAB > TBAI > TOAB > TBAC > KI (Figure S12).
Among the three tetrabutylammonium halides (TBAB, TBAC and
TBAI), TBAB gave the highest activity, this result is a
combination of nucleophilicity (Cl-> Br-> I-) and leaving ability (I-
> Br- > Cl-). Encouragingly, TOAB with a much larger size could
also give a high PO conversion of 76% with a TOF of 450 h-1. This
is possibly due to the uniform and large pore channels of COF-
salen-Co, which benifits the transmission of co-catalyst with
larger sizes.
The catalytic performance of COF-salen-Co was compared
with that of corresponding amorphous polymer POF-salen-Co
and the homogenous counterpart. POF-salen-Co with BET
surface area of 860 m2/g, pore volume of 0.84 cm3/g and pore
diameter of 0.85 nm has identical chemical composition to COF-
salen-Co (Figure S13). Under similar conditions, POF-salen-Co
afforded 70% conversion of PO. The TOF of COF-salen-Co was
ca. 2.3 fold that of POF-salen-Co. The much higher activity of
COF-salen-Co was mainly attributed to its ordered crystalline
structure and large pore diameter. Homogeneous salen-Co with
TOF of 41 h-1 could only afford PO conversion of 25% after 6 h.
The much higher activity of COF-salen-Co and POF-salen-Co
than the homogeneous counterpart reflected the superiority of
heterogenization of salen-Co, which is possibly related with
increased rigidity of salen-Co complex in the network of COFs.
The PO conversion and TOF sharply increased with increasing
reaction temperature over COF-salen-Co (Figure 6A). The TOF
of COF-salen-Co as high as 1875 h-1 could be obtained at 140 oC.
This result indicated the good stability of COF-salen-Co at high
temperature. Encouragingly, COF-salen-Co could smoothly
catalyze the cycloaddition of epichlorohydrin (EPH) and CO2
even under ambient conditions (25 oC and 1 bar of CO2) to
afford 96% conversion of EPH with a TOF of 26 h-1 (Figure 6B).
The high catalytic activity of COF-salen-Co under ambient
conditions is not only attributed to the uniform distribution of
salen-Co but also the enrichment effect of the porous structure
for the low concentration CO2. The activity of COF-salen-Co in
cycloaddition reactions is comparable or higher than most
reported solid catalysts, such as metal-organic frameworks
(MOFs) and porous organic polymers (POPs) (Table S2).
Scheme 2. Cycloaddition of different types of epoxides with CO2
catalyzed by COF-salen-Co.
We further investigated the substrate scope of COF-salen-Co
and the results are summarized in Scheme 2. All the substrates
employed could be converted to the corresponding carbonates.
For the less steric hindrance substrates, such as EPH, COF-salen-
Co afforded a high TOF of 1056 h-1 and 97% of EPH conversion.
The activity of COF-salen-Co decreased as the alkyl length of
epoxides increased with an order of butyl glycidyl ether < 1,2-
epoxyhexane < propylene oxide. Styrene oxide with a large
molecular size could be smoothly converted with a conversion
of 78% and TOF of 246 h-1. Surprisingly, a challenging substrate,
2-[(1-Naphthyloxy)methyl]oxirane (size of 0.8 * 0.9 nm2) could
also be efficiently converted to corresponding cyclic carbonate
with high conversion of 94% and TOF of 296 h-1. The above
results indicate that COF-salen-Co with uniform pore structure
could accommodate large molecules. COF-salen-Co even could
catalyse the most challenging internal epoxide, cyclohexene
oxide (CHO), to afford 77% conversion and a TOF of 59 h-1. These
results confirmed that the COF-salen-Co has a wide substrate
scope.
Conv. (%)
Sel. (%)
A
B
100
80
60
40
20
0
1
2
3
4
5
10
20
30
40
2 Theta (degree)
Run
Figure 7. (A) Recycling stabitity of COF-salen-Co for the
cycloaddition reaction, (B) PXRD pattern of COF-salen-Co after
5 cycles.
120
100
80
60
40
20
0
2000
1500
1000
500
0
100
80
60
40
20
0
Conv. (%)
Sel. (%)
B
A
The stability of COF-salen-Co was investigated using
cycloaddition reaction as a model. To confirm the heterogeneity
of COF-salen-Co during the catalytic process, a hot filtration
experiment was carried out by filtrating COF-salen-Co out of the
reaction system after 3 h using 1,2-epoxyhexane as substrate
when the conversion achieved up to 66%. No obviously increase
in the conversion (67% vs 66%) was observed by stirring the
filtrate under 2 MPa CO2 for another 6 h, proving that the
reaction is catalyzed by the solid catalyst. In contrast to COF-
80
100
120
140
0
20
40
60
80 100 120 140
Temperature (oC)
Time (h)
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salen-Co, an obvious increase in the conversion from 38% to 47% FT-IR spectra in the range of 400-4000 cm-1 were collected with
View Article Online
was observed for amorphous POF-salen-Co in the same hot a Nicolet Nexus 470 IR spectrometer usiDnOgI:K1B0.r10p3e9l/lCe8tTs.A1S1o05li8dA-
filtration experiment. This strongly proved the excellent state NMR spectra were performed on a Bruker 500 MHz
stability of COFs comparing to amorphous polymers, possibly spectrometer equipped with a magic-angle spin probe using a
due to the ordered π-π stacking arrangement of the aromatic 4-mm ZrO2 rotor. 13C signals were referenced to Glycine
rings in the network of COFs. Furthermore, COF-salen-Co could (C2H5NO2). The experimental parameters are as follows: 8 kHz
be stably recycled for 5 times in CO2 cycloaddition reaction with spin rate, 5 s pulse delay, and 2500 scans. Scanning electron
PO and no obvious decrement in PO conversion and selectivity microscopy (SEM) was undertaken using a FEI Quanta 200F
was observed (Figure 7A). The cobalt content of COF-salen-Co scanning electron microscope operating at an acceleration
after the fifth cycle analyzed by ICP was 11.4 wt%, which is voltage of 2-30 kV. High-resolution scanning electron
comparable to the fresh sample (11.9 wt%). The BET surface microscopy (HRSEM) was undertaken by using a HITACHI S5500
area (1065 m2/g vs 973 m2/g) of the COF-salen-Co after five apparatus operating at an acceleration voltage of 30 kV. Before
cycles only decreased slightly and pore size distribution was measurement, the samples were fully dispersed in ethanol and
almost the same compared to the fresh one (Figure S14). The then deposited on a holey carbon film on a Cu grid. Powder X-
crystalline structure was well maintained even after five cycles ray diffraction (PXRD) patterns of the samples were measured
(Figure 7B). Meanwhile, the morphology and FT-IR spectrum of on a Rigaku RINT D/Max-2500 powder diffraction system
the reused sample were almost identical to the fresh one operated at 40 kV and 200 mA using Cu Kα radiation (The scan
(Figure S15 and 1A). The above characterization results of the rate of 5° min−1 was applied to record the patterns in the range
reused COF-salen-Co confirmed its stability during the catalytic of 2 - 40°). The thermo gravimetric analysis (TGA) was
process.
performed from 25 to 900
a heating rate of 5 C/min using a NETZSCH STA 449F3 analyzer.
X-ray photoelectron spectroscopy (XPS) was recorded on a
Thermofisher ESCALAB 250Xi apparatus by using AlKα (hλ
C under nitrogen atmosphere with

Conclusions
=
1486.6 eV) as the excitation light source. The nitrogen sorption
isotherms were performed on a Micromeritics ASAP 2020
system volumetric adsorption analyzer at 77 K. All samples were
degassed at 100 oC for 6 h prior to the sorption measurements.
The BET surface area was calculated from the adsorption data
at a relative pressure P/P0 in the range of 0.05-0.25. Pore size
distributions were determined from the adsorption branches
using nonlocal density functional theory (NLDFT) method. Metal
content of the samples was determined using a PLASAM-SPEC-
II inductively coupled plasma atomic emission spectrometry
(ICP) by digesting the COFs in HNO3/H2SO4 (1:1, v/v).
In summary, we synthesized a new salen-based COF via a facile
solvothermolysis under air. COF-salen has high surface area
(1646 m2/g), uniform pore size (1.86 nm) and AA stacking
eclipsed layered crystalline structure. After metalation with
metals (Co, Mn, Cu, Zn), COF-salen-M with well-maintained high
BET surface area and crystalline structure could be formed. In
the EPH hydration reaction, COF-salen-Co(III) afforded a TOF of
52 h-1, slightly lower than the homogeneous counterpart,
indicating that salen-Co (III) in the adjacent layers of COFs could
work cooperatively, but not as free as homogeneous
counterpart. COF-salen-Mn exhibited much higher activity than
salen-Mn in styrene epoxidation reaction, possibly due to the
site isolation effect of COFs. Moreover, COF-salen-Co showed
enhanced activity and recycling stability for cycloaddition
reactions of epoxides with CO2 (TOF up to 1875 h-1) in
Synthesis of COF-salen.
1,3,5-Tris(3’-tert-butyl-4’-hydroxy-5’-formylphenyl)benzene
(182 mg, 0.30 mmol) was dissolved in amixture of THF (6.4 mL)
and EtOH (1.6 mL) in a 30 mL stainless autoclave. Then, 1.6 mL
3 M CH3COOH and ethylenediamine (30 μL, 0.45 mmol) were
added and the autoclave was placed in an oven and heated at
comparison
with
amorphous
POF-salen-Co,
which
demonstrated the advantages of COFs in catalysis. The
synthesis of COFs with catalytic active sites in the network
provides a new approach for the bridging “homogeneous” and
“heterogeneous” catalysis.
120 C for 3 days. The resultant product was collected by
fltration, washed extensively with ethanol, THF, CH2Cl2, water
and DMF. Then the yellow powder was dried under vacuum at
60 oC for 12 h, the yield of COF-salen was estimated about 86%.
Synthesis of COF-salen-Co.
In a 100 mL round flask, 200 mg of COF-salen (fully evacuated
at 100 °C for 3 h before used) was added and dispersed in 12 mL
ethanol, then a ethanol/water (6 mL/0.5 mL) solution of
Co(OAc)2∙4H2O (160 mg, 0.64 mmol) was added. The mixture
was refluxed at 80 ºC for 72 h under nitrogen atmosphere. After
filtration, the obtained material was fully washed by ethanol,
and then dried under vacuum at 60 ºC for 12 h. The red powder
was denoted as COF-salen-Co. The cobalt content was 11.9 wt%
determined by ICP, the theoretical value was 12.2 wt%.
Synthesis of COF-salen-Mn.
Experimental section
Chemicals and Reagents.
All chemicals and solvents were used as received without any
further purification. 1,3,5-Tris(3’-tert-butyl-4’-hydroxy-5’-
formylphenyl)benzene,60
ethanediamine cobalt (salen-Co (II)),50 N,N’-bis(3-tert-
butylsalicylidene)-ethanediamine manganese chloride (salen-
Mn)61 and POF-salen-Co51 were synthesized according to the
literature method.
N,N’-bis(3-tert-butylsalicylidene)-
Characterizations.
200 mg of COF-salen (fully evacuated at 100 °C for 3 h before
used) was dispersed in 20 mL DMF, then a 5 mL ethanol solution
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of Mn(OAc)2∙4H2O (229 mg, 0.93 mmol) was added. The mixture Naphthyloxy)methyl]oxirane was analysed byView1AHrticleNOnMlinRe
DOI: 10.1039/C8TA11058A
was stirred at 90 ºC for 18 h under nitrogen atmosphere. Then spectroscopy.
a 5 mL ethanol solution of lithium chloride was added and the
For the room temperature reaction, epichlorohydrin (8
mixture was pumped with oxygen with a flow rate of 20 mmol), COF-salen-Co (8 μmol), TBAB (80 μmol) and the internal
mL/min. The mixture was further stirred at 90 ºC for 6 h. After standard biphenyl were fixed in a Schlenk tube equipped with a
filtration, the obtained material was fully washed by ethanol, magnetic stirrer. Then, the tube was purged with a balloon filled
DMF and water, then dried under vacuum at 60 ºC for 12 h. The with CO2 at 0.1 MPa and stirred at a speed of 550 rpm at 25 °C
brown powder was denoted as COF-salen-Mn. The manganese for 144 h.
content was 10.2 wt% determined by ICP, the theoretical value
was 10.6 wt%.
Synthesis of COF-salen-Cu.
For the hot filtration test, COF-salen-Co or POF-salen-Co was
filtrated out of the system after 3 h of cycloaddition reaction
using 1,2-epoxyhexane as substrate (Conditions: catalyst, 4
90 mg of COF-salen (fully evacuated at 100 °C for 3 h before μmol; TBAB, 40 μmol; 1,2-epoxyhexane, 4 mmol; 100 oC, 2 MPa
used) was dispersed in 6 mL ethanol, then a 6 mL ethanol CO2). The yield of corresponding carbonate was analyzed by GC.
solution of Cu(OAc)2∙H2O (55 mg, 0.27 mmol) was added. The The filtration was put into the reactor and repressurized with 2
mixture was refluxed at 80 ºC for 72 h under nitrogen MPa CO2. The reaction went on for another 6 h then analyzed
atmosphere. After filtration, the obtained material was fully using GC.
washed by ethanol, THF and then dried under vacuum at 60 ºC
For catalyst recycling, COF-salen-Co obtained by
for 12 h. The brown powder was denoted as COF-salen-Cu. The centrifugation was washed three times using EA, dried under
o
cooper content was 10.9 wt% determined by ICP, the vacuum at 100 C for 6 h and then reused directly for the next
theoretical value was 12.9 wt%.
Synthesis of COF-salen-Zn.
catalytic cycle (Conditions: catalyst, 8 μmol; TBAB, 80 μmol; PO,
8 mmol; 100 oC; 2 MPa CO2; 2 h).
150 mg of COF-salen (fully evacuated at 100 °C for 3 h before General procedure for the EPH hydration reactions.
used) was dispersed in 12 mL ethanol, then a ethanol/methanol COF-salen-Co(III) (20 μmol), EPH (1 mmol), and H2O (2 mmol)
(4 mL/2 mL) solution of Zn(OAc)2∙2H2O (105 mg, 0.48 mmol) was were added in a 10 mL schlenk tube, then the reaction mixture
added. The mixture was refluxed at 80 ºC for 48 h under was stirred at 40 °C for 3 h. After cooling to room temperature,
nitrogen atmosphere. After filtration, the obtained material was octanol (internal standard, 48 μL, 40 mg) was added to the
fully washed by ethanol, THF and then dried under vacuum at mixture and the solid catalyst was separated by filtration. The
60 ºC for 12 h. The light yellow powder was denoted as COF- liquid phase was diluted with ethanol then analysed by GC to
salen-Zn. The zinc content was 10.6 wt% determined by ICP, the calculate the conversion of EPH and yield of diol.
theoretical value was 13.3 wt%.
General procedure for the styrene epoxidation reactions.
Synthesis of COF-salen-Co(III).
COF-salen-Mn (10 μmol), styrene (2 mmol), t-butyl
280 mg of COF-salen-Co was dispersed in EtOH (10 mL) hydroperoxide (6 mmol) and 5 mL of acetonitrile were added
containing p-toluenesulfonic acid (150 mg) and the mixture was into a 25 mL flask, then the reaction mixture was stirred at 40
stirred under air atmosphere at room temperature for 12 h. The °C for 24 h. After cooling to room temperature, biphenyl (as the
solid was collected by filtration, washed with EtOH and dried internal standard) was added to the mixture and the solid
under vacuum at room temperature. The obtained brown catalyst was separated by filtration. The liquid phase was
powder was denoted as COF-salen-Co(III).
diluted with EA then analysed by GC to calculate the conversion
General procedure for the coupling reactions of CO2 and of styrene and the yield of styrene oxide.
epoxides.
The coupling of CO2 and epoxides was performed in a high-
pressure stainless steel autoclave (15 mL) equipped with a
Conflicts of interest
magnetic stirrer. Typically, COF-salen-Co (4 mg, 8 μmol), TBAB
(26 mg, 80 μmol) and propylene oxide (470 mg, 8 mmol) were
put into the autoclave. After purging the autoclave with CO2 to
exclude air for three times, the reactor was pressurized with 2
MPa CO2. Then the temperature of the autoclave was increased
and maintained at 100 oC with a stirring speed of 550 rpm for a
certain time. After reaction, the autoclave was quickly cooled by
putting it into an ice-water bath. Then the gas was released
slowly, the reaction mixture was added with ethyl acetate (EA)
for dilution followed by the addition of butyl acetate as internal
standard. After that, the catalyst was separated by
centrifugation. The supernatant of the reaction was analysed by
gas chromatography (GC). Similar procedure were utilized when
using EPH, 1,2-epoxyhexane, butyl glycidyl ether, styrene oxide,
cyclohexane oxide or 2-[(1-Naphthyloxy)methyl]oxirane as
substrate. The yield of carbonate and conversion of 2-[(1-
There are no conflicts to declare.
Acknowledgements
This work is dedicated to the 70th anniversary of Dalian Institute
of Chemical Physics (DICP), CAS. This work was supported by the
National Key R&D Program of China (2017YFB0702800), the
National Natural Science Foundation of China (21733009,
21621063) and the Strategic Priority Research Program of the
Chinese Academy of Sciences (XDB17020200).
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Journal of Materials Chemistry A
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DOI: 10.1039/C8TA11058A
Table of Contents
High activity
Reactant
Product
COF-salen
A facile synthesis of COF-salen via in-situ salen skeleton formation under air atmosphere is described herein. COF-
salen-M prepared via metal ions coordination could efficiently catalyse chemical reactions, e.g. styrene epoxidation,
epoxide hydration and cycloaddition reactions of epoxides with CO2. More interestingly, both the cooperation and
isolation effects are found in COF-salen-M.