Three-Dimensional Covalent Organic Frameworks with Dual Linkages for Bifunctional Cascade Catalysis

Article abstract of DOI:10.1021/jacs.6b09563

Covalent organic frameworks (COFs) are an emerging class of porous crystalline polymers with broad potential applications. So far, the availability of three-dimensional (3D) COFs is limited and more importantly only one type of covalent bond has been successful used for 3D COF materials. Here, we report a new synthetic strategy based on dual linkages that leads to 3D COFs. The obtained 3D COFs show high specific surface areas and large gas uptake capacities, which makes them the top COF material for gas uptake. Furthermore, we demonstrate that the new 3D COFs comprise both acidic and basic sites, and act as excellent bifunctional catalysts for one-pot cascade reactions. The new synthetic strategy provides not only a general and versatile approach to synthesize 3D COFs with sophisticated structures but also expands the potential applications of this promising class of porous materials.

Full text of DOI:10.1021/jacs.6b09563

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Article
3D Covalent Organic Frameworks with Dual
Linkages for Bifunctional Cascade Catalysis
Hui Li, Qinying Pan, Yunchao Ma, Xinyu Guan, Ming Xue,
Qianrong Fang, Yushan Yan, Valentin Valtchev, and Shilun Qiu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b09563 • Publication Date (Web): 18 Oct 2016
Downloaded from http://pubs.acs.org on October 18, 2016
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3D Covalent Organic Frameworks with Dual Linkages for Bifunction-
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Hui Li,†,§ Qinying Pan,†,§ Yunchao Ma,† Xinyu Guan,† Ming Xue,† Qianrong Fang,∗,† Yushan
Yan,‡ Valentin Valtchev†,∥ and Shilun Qiu∗,†
†
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, P. R.
China
‡
Department of Chemical and Biomolecular Engineering, Center for Catalytic Science and Technology, University
of Delaware, Newark, DE 19716, USA
∥
Normandie Univ, ENSICAEN, UNICAEN, CNRS, Laboratoire Catalyse et Spectrochimie, 6 Marechal Juin, 14050
Caen, France
ABSTRACT: Covalent organic frameworks (COFs) are an emerging class of porous crystalline polymers with broad poten-
tial applications. So far, the availability of 3D COFs is limited and more importantly only one type of covalent bond has
been successful used for 3D COF materials. Here, we report a new synthetic strategy based on dual linkages that leads to
3D COFs. The obtained 3D COFs show high specific surface areas and large gas uptake capacities, which makes them the
top COF material for gas uptake. Furthermore, we demonstrate that the new 3D COFs comprise both acidic and basic
sites, and act as excellent bifunctional catalysts for one-pot cascade reactions. The new synthetic strategy provides not
only a general and versatile approach to synthesize 3D COFs with sophisticated structures but also expands the potential
applications of this promising class of porous materials.
areas and large gas storage capacities for H2, CH4 and CO2.
ꢀ
INTRODUCTION
Furthermore, we demonstrate that both DL-COFs, consisting
of both acidic and basic sites, can act as excellent bifunction-
al cascade catalysts. To the best of our knowledge, this study
is the first example of 3D COFs with dual linkages and being
applied to acid/base bifunctional catalysis.
Covalent organic frameworks (COFs),1-4 as a new class of
crystalline polymers, have attracted considerable attention
over the past decade with promising performance in gas ad-
sorption and separation,5,6 catalysis,7-9 optoelectronics,10-13
proton conduction,14,15 chemical sensing,16,17 and drug deliv-
er.18,19 COFs are composed entirely of covalent bonds be-
tween light elements, typically H, B, C, N, and O, which crys-
tallize into periodic architectures that can be precisely prede-
termined by the employed molecular building blocks. Since
the pioneering work of Yaghi in 2005,1 a variety of COFs have
been prepared through several condensation reactions, in-
cluding the formation of B−O (boronate, boroxine, and boro-
silicate),20,21 B−N (borazine),22 C−N (triazine and imide),16,23
C=N (imine, hydrazine, and squaraine),24-26 and N=N (azodi-
oxide)27 bond linkages. However, while the design and syn-
thesis of 2D COFs is already well established,28-32 the con-
struction of 3D COFs is limited, and the successful synthetic
methods for 3D COFs have been confined to only one type of
covalent bond formation.20,33 Thus, the exploration of a new
synthetic strategy is of paramount importance for the devel-
opment of 3D COF materials and diversifying their properties.
ꢀ
RESULTS AND DISCUSSION
Our strategy for preparing 3D COFs with dual linkages in-
volves the formation of imine group and boroxine ring. As
shown in Scheme 1a, the condensation of 1-adamantanamine
(AA) with 4-formylphenylboronic acid (FPBA) results in the
triangular molecule with two kinds of covalent bonds.
Through the same strategy, AA is replaced by an tetrahedral
linker, 1,3,5,7-tetraaminoadamantane (TAA), which reacts
with FPBA or 2-fluoro-4-formylphenylboronic acid (FFPBA)
and thus produces two extended 3D framework structures,
DL-COF-1 and DL-COF-2, respectively (Scheme 1b). In this
case, tetrahedral TAA can be defined as a 4-connected node,
and triangular boroxine ring can act as a 3-connected node.
The association between the 3- and 4-connected building
units results in 3D networks with the cubic carbon nitride
(ctn) topology (Scheme 1c).34
Herein we report a new general way to construct 3D COFs
with the formation of two types of covalent bonds. On the
basis of this strategy, two 3D COFs with dual linkages
(boroxine and imine), denoted DL-COF-1 and DL-COF-2,
were successfully obtained. Both materials show high surface
The synthesis of DL-COFs was carried out by solvothermal
reaction of TAA (9.8 mg, 0.05 mmol) and FPBA (30.0 mg, 0.2
mmol) or FFPBA (33.6 mg, 0.2 mmol) in a 1:1 (v:v) mixture of
dioxane and mesitylene, followed by heating at 120 °C for 3
days. The yield of white crystalline solid was 82% for DL-
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COF-1 and 85% for DL-COF-2. These products were insoluble
structures of the new COFs are built of networks with dual
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in common organic solvents such as tetrahydrofuran (THF),
acetone, hexanes, N,N-dimethylformamide (DMF), dimethyl-
sulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP).
linkages. The atomic precision construction of DL-COFs was
further verified by the 13C cross-polarization magic-angle-
spinning (CP/MAS) NMR spectroscopy (Figures S5 and S6)
that confirmed the presence of carbonyl carbon from the
imide ring at 168 ppm for DL-COF-1 and 166 ppm for DL-
COF-2. The structure of DL-COFs, built up of dual covalent
linkages, showed high thermal stability. According to the
thermogravimetric analysis (TGA) the DL-COFs are stable up
to 400°C (Figures S7 and S8).
Scheme 1. Strategy for preparing 3D COFs with dual
linkages (DL-COFs): a) Model reaction of 1-
adamantanamine (AA) with 4-formylphenylboronic
acid (FPBA) to form the triangular molecule with
dual linkages; b) Condensation of tetrahedral
1,3,5,7-tetraaminoadamantane (TAA) and FPBA or 2-
fluoro-4-formylphenylboronic acid (FFPBA) to give
3D COFs with dual linkages, DL-COF-1 or DL-COF-2;
c) On the basis of triangular and tetrahedral build-
ing units, both DL-COFs show 3D networks with ctn
topology.
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Figure 1. PXRD patterns of DL-COF-1 (a) and DL-COF-2 (b)
with the observed profile in black, refined in red, the differ-
ence (observed minus refined) in dark yellow, and calculated
based on the ctn topology in blue.
Scanning electron microscopy (SEM) study reveals that
that both DL-COFs exhibit rectangular morphology with the
particle size of about 0.2 ꢀm (Figures S1 and S2). The crystals
form random in size and morphology aggregates. Fourier
transform infrared (FT-IR) spectra of DL-COFs (Figures S3
and S4) showed the disappearance of the hydroxyl stretching
band (3215 cm-1 for FPBA and 3217 cm-1 for FFPBA), the car-
bonyl stretching band (1676 cm-1 for FPBA and 1684 cm-1 for
FFPBA), and the N-H stretching band (around 3300 cm-1) of
TAA, indicating a completed conversion of these groups.
Meanwhile, the characteristic stretching bands of the imine
group (1644 cm-1 for DL-COF-1 and 1641 cm-1 for DL-COF-2)
and the B-O and B3O3 ring (715, 1297 and 1336 cm-1 for DL-
COF-1 and 717, 1303 and 1340 cm-1 for DL-COF-2) emerged in
the IR spectra.1,30 These data unambiguously show that the
The crystalline nature of both DL-COFs was confirmed
by powder X-ray diffraction (PXRD) analysis (Figure 1).
After a geometrical energy minimization by using the
Materials Studio Software package35 based on the ctn to-
pology the unit cell parameters were determined. Both
materials exhibit cubic unit cells with a = b = c = 41.0115 Å
and α = β = γ = 90° for DL-COF-1; and a = b = c = 41.0142 Å
and α = β = γ = 90° for DL-COF-2. Simulated PXRD pat-
terns show good match with the experimental ones (Fig-
ure 1, black and blue curves). Furthermore, the full profile
pattern matching (Pawley) refinements are performed
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which are a consequence of the agglomeration of na-
from the experimental PXRD patterns. Strong PXRD
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peaks at 5.30, 6.15, 8.10, 8.71, 10.20 and 13.40° for DL-COF-1
can be assigned to the (211), (220), (321), (400), (420) and
(541) Bragg peaks of I-43d (No. 220); and 5.30, 6.15, 6.85,
8.71, 10.20 and 13.40° for DL-COF-2 correspond to the
(211), (220), (310), (400), (420) and (541) Bragg peaks of I-
43d (No. 220), respectively. The refinement results re-
vealed unit cell parameters nearly equivalent to the pre-
dictions with excellent agreement factors (a = b = c =
40.8123 Å, α = β = γ = 90°, wRp = 3.21%, and Rp = 1.73% for
DL-COF-1; a = b = c = 40.8324 Å, α = β = γ = 90°, wRp =
3.03%, and Rp = 1.66% for DL-COF-2). Besides the ctn
network, we also considered alternative topologies based
on the boracite (bor) net, which were set up from the
space groups of P23 (No. 195). However, these calculated
patterns did not match the experimental PXRD patterns
(Figures S9-14 and Tables S1-4). On the basis of the above
results, both DL-COFs were proposed to have the ex-
pected architectures with the ctn topology, which show
microporous cavities with a diameter of about 14.2 Å (Fig-
ure 2).
nosized COF crystals (Figures S1 and S2). The lack of hys-
teresis indicates that the adsorption and desorption
mechanisms are similar and that the adsorption is re-
versible. The application of the Brunauer-Emmett-Teller
(BET) model in the low pressure region (0.005 < P/P0 <
0.07) showed a specific surface areas of 2259 m2 g-1 for DL-
COF-1 and 2071 m2 g-1 for DL-COF-2 (Figures S15 and S16).
The pore-size distributions of DL-COFs were calculated
by nonlocal density functional theory (NLDFT). Both DL-
COFs showed a narrow pore width (13.6 Å for DL-COF-1
and 12.8 Å for DL-COF-2, Figures S17 and S18), which was
in good agreement with the pore size predicted from their
crystal structures (14.2 Å for both DL-COFs).
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Figure 3. N2 adsorption-desorption isotherms for DL-COF-1
(a) and DL-COF-2 (b) at 77 K.
Because of the high BET surface area of DL-COFs, hy-
drogen (H2), carbon dioxide (CO2) and methane (CH4)
adsorption was also studied to explore their potential ap-
plication in gas storage (Figure 4). The gas adsorption
isotherms were collected at 77 K for H2, and 273 K for CO2
and CH4. At 1.0 bar, DL-COF-1 shows uptake as high as
234 cm3 g−1 (2.09 wt%) of H2, 136 cm3 g−1 (26.7 wt%) of CO2
and 36 cm3 g−1 (2.57 wt%) of CH4. Similarly, at 1.0 bar DL-
COF-2 can store up to 193 cm3 g−1 (1.73 wt%) of H2, 111 cm3
g−1 (21.8 wt%) of CO2 and 30 cm3 g−1 (2.10 wt%) of CH4.
These results are among the highest values for COFs, such
as NTU-COF-2 (1.55 wt% of H2),30 TDCOF-5 (1.6 wt% of
H2),36 TpPa-COF (21.8 wt% of CO2)37 and FCTF-1-600 (24.4
wt% of CO2),38 which places DL-COFs amongst the top
Figure 2. Structural representations of 3D DL-COF-1 (a) and
DL-COF-2 (b).
The porosity of DL-COFs was demonstrated by measur-
ing nitrogen-gas (N2) adsorption at 77 K. As can be seen
in Figure 3, both DL-COFs exhibit the characteristic for
microporous materials type I isotherm with sharp uptake
below 0.1 P/P0. The inclination of the isotherm in the 0.1-
1.0 P/P0 range reveals the presence of textural mesopores,
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COF material for gas uptake.
are key characteristics those allow a heterogeneous cata-
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lyst to achieve high activity. Hence the 3D COFs are ex-
pected to be a promising bifunctional catalyst. In this
work, the employed reaction involves hydrolysis of the
acetal catalyzed by the acidic sites of boroxine group fol-
lowed by Knoevenagel condensation catalyzed by the
basic sites of imine bonds (Scheme 2).
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Figure 4. H2, CO2 and CH4 adsorption isotherms of DL-COF-
1 (a) and DL-COF-2 (b). Filled and empty symbols represent
the adsorption and desorption branches, respectively.
Scheme 2. Catalytic activity of DL-COFs in acid-base
catalyzed one-pot cascade reactions.
Figure 5. Conversion (%) versus time (h) for bifunctional
catalytic reactions on DL-COF-1 (a) and DL-COF-2 (b) based
on entry 1 and 2 in Scheme 2, respectively. Inset: the struc-
tures of reactant, intermediate and the final product. H, gray;
C, green; N, blue; O, red.
Typically, the catalytic reaction was performed with 1
mmol of each reactant in CDCl3 (3 mL) with DL-COF-1 or
DL-COF-2 (10 mol%) at room temperature for 20 h. The
conversion and the product distribution were monitored
1
by H NMR spectroscopy based on the starting materials.
As shown in Scheme 2, reactant A (benzaldehyde dime-
thyl acetal) was hydrolyzed to give product B (benzalde-
hyde), which further reacted with C (malononitrile, ethyl
cyanoacetate or acetylacetone) to form the final product
Encouraged by high crystallinity and porosity as well as
by the proximity of acidic and basic catalytic sites of DL-
COFs, we have explored the catalytic potential of new
COFs for the acid-base catalyzed one-pot cascade reac-
tions. Up to now, only a 2D catechol-porphyrin COF with
a single type linkage was used for a bifunctional catalytic
reaction.39 To the best of our knowledge, such exploration
has not been extended to 3D COF materials. Compared
with 2D COFs, 3D COF materials possess interrelated
porous structures and higher specific surface areas, which
D
(benzylidene
malononitrile,
ethyl
trans-α-
cyanocinnamate or 3-benzylidene-2,4-pentanedione). As
mentioned, the first reaction step requires acidic sites,
which are supplied by boroxine group, whereas the sub-
sequent condensation step involves the basic sites from
imine bonds in DL-COFs. Figure 5 showed the conversion
of benzaldehyde dimethyl acetal (100%) and the for-
mation of benzaldehyde intermediate that is reacted
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afford a white precipitate which was isolated by filtration
through consecutive Knoevenagel condensation with
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malononitrile to give the final product, benzylidene
malononitrile (98% for DL-COF-1 and 96% for DL-COF-
2 ). Furthermore, by replacing malononitrile reactant with
ethyl cyanoacetate or acetylacetone, and keeping all other
reaction conditions the same, similar results can be ob-
tained (Scheme 2 and Figures S19-22). After the reactions,
the results of PXRD and N2 adsorption confirmed the
structural integrity of DL-COFs, thus revealing the high
stability of new COF materials in the acid-base catalytic
reaction (Figures S23–S26). The COF crystals can be easily
isolated from the reaction mixture by a simple filtration
and reused at least three times with almost no loss of ac-
tivity (Figures S27 and S28).
over a medium glass frit and washed with anhydrous THF
(20.0 mL). The product was immersed in anhydrous THF
(20.0 mL) for 8 h, during which the activation solvent was
decanted and freshly replenished four times. The solvent
was removed under vacuum at 80°C to afford DL-COF-2
as a white powder (30.9 mg, 85%).
Further detailed experimental procedures and charac-
terization are described in the Supporting Information.
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ꢀ
ASSOCIATED CONTENT
Supporting Information
Synthetic procedures, SEM, FTIR, solid state 13C NMR, TGA,
BET plot, and pore size distribution. This material is availa-
ble free of charge via the internet at http://pubs.acs.org.
ꢀ
CONCLUSIONS
ꢀ
AUTHOR INFORMATION
In conclusion, we have developed a new strategy to con-
struct 3D COFs by employing two types of covalent bonds.
Two novel 3D COFs with both boroxine and imine dual
linkages were for the first time prepared, and showed
large surface areas as well as high H2, CH4 and CO2 uptake
capacities. In addition, we explored the catalytic activity
of new COF materials, comprising both acidic and basic
sites, in acid-base catalyzed one-pot cascade reactions. 3D
bifunctional COF catalysts successfully prepared in this
work may not only expand the development of COFs with
more sophisticated structures and compositions, but also
promote the applications of COFs as multi-functional
materials.
Corresponding Author
*qrfang@jlu.edu.cn
*sqiu@jlu.edu.cn
Author Contributions
§
These authors contributed equally to this work.
Notes
The authors declare no competing financial interests.
ꢀ
ACKNOWLEDGMENT
This work was supported by National Natural Science Foun-
dation of China (21390394, 21261130584, 21571079, and
21571076), “111” project (B07016), Guangdong Science and
Technology Department Project (2009B090300432) and Min-
istry of Education, Science and Technology Development
Center Project (20120061130012). V.V. and Q.F. acknowledge
the Thousand Talents program (China).
ꢀ
EXPERIMENTAL SECTION
Starting Materials. All starting materials and solvents,
unless otherwise noted, were obtained from J&K scientific
LTD. and used without purification. 1,3,5,7-
tetraaminoadamantane (TAA) was synthesized by the
method of literatures.8
ꢀ
REFERENCES
Synthesis of DL-COF-1. A Pyrex tube measuring o.d. ×
i.d. = 10 × 8 mm2 was charged with FPBA (30.0 mg, 0.2
mmol) and TAA (9.8 mg, 0.05 mmol) in a mixed solution
of dioxane (0.5 mL) and mesitylene (0.5 mL). The tube
was flash frozen at 77 K (LN2 bath), evacuated to an inter-
nal pressure of 0.15 mmHg and flame sealed. Upon sealing
the length of the tube was reduced to ca. 13 cm. The reac-
tion mixture was heated at 120 °C for 3 days to afford a
white precipitate which was isolated by filtration over a
medium glass frit and washed with anhydrous tetrahydro-
furan (THF, 20.0 mL). The product was immersed in an-
hydrous THF (20.0 mL) for 8 h, during which the activa-
tion solvent was decanted and freshly replenished four
times. The solvent was removed under vacuum at 80 °C to
afford DL-COF-1 as a white powder (26.7 mg, 82%).
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Synthesis of DL-COF-2. Similar to DL-COF-1, FPBA was
treated with FFPBA (33.6 mg, 0.2 mmol) in a mixed solu-
tion of dioxane (0.5 mL) and mesitylene (0.5 mL). The
tube was flash frozen at 77 K (LN2 bath), evacuated to an
internal pressure of 0.15 mmHg, and flame sealed. Upon
sealing the length of the tube was reduced to ca. 13 cm.
The reaction mixture was heated at 120 °C for 3 days to
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