β-Cyclodextrin Covalent Organic Framework for Selective Molecular Adsorption

Article abstract of DOI:10.1002/chem.201802564

Covalent organic frameworks (COF) are complex functional systems constructed with atomic precision by linking well-defined building blocks through robust covalent bonds. β-cyclodextrin (β-CD) is a most employed supramolecule which bears a hydrophobic cavity guiding molecular specific recognitions. Building COF with asymmetric β-CD linkers is challenging and has never been reported. Here, β-CD COF is grown with heptakis(6-amino-6-deoxy)-β-CD and terephthalaldehyde in green solvents of water and ethanol at room temperature. The COF is characterized by powder X-ray diffraction, which matches well with the simulated crystal structure. Weaving β-CD into a framework through reticular chemistry allows the integration of a large amount of β-CD units (50 mol %), much higher than β-CD polymers. The β-CD COF has larger surface area, more uniform pore size, and higher thermal stability than the non-crystalline β-CD polymer produced by the same reagents. Finally, the β-CD COF holds abundant specific interaction sites enabling selective molecular adsorption.

Full text of DOI:10.1002/chem.201802564

A Journal of
Accepted Article
Title: β-Cyclodextrin Covalent Organic Framework for Selective
Molecular Adsorption
Authors: Ren-Qi Wang, Xue-Bing Wei, and Yu-Qi Feng
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Article type: Communication
Title
β-Cyclodextrin Covalent Organic Framework for Selective Molecular Adsorption
Author(s), and Corresponding Author(s)* *
Ren-Qi Wang, Xue-Bing, Wei, Yu-Qi Feng
Dr. R-Q. Wang 1,2
1
. Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education),
Department of Chemistry, Wuhan University, Wuhan 430072, P.R. China.
. College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000,
2
P.R. China.
X-B. Wei
Gansu Institute for Drug Control, Lanzhou 730070, P.R. China.
E-mail: yqfeng@whu.edu.cn
Prof. Dr. Y-Q. Feng
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education),
Department of Chemistry, Wuhan University, Wuhan 430072, P.R. China.
Keywords: adsorption, cyclodextrins, covalent organic frameworks, host-guest inclusion,
molecular recognition
((Abstract text. Covalent organic framework (COF) is a complex functional system
constructed with atomic precision by linking well–defined building blocks through robust
covalent bonds. β-cyclodextrin (β-CD) is a most employed supramolecule which bears a
hydrophobic cavity guiding molecular specific recognitions. Building COF with asymmetric
β-CD linkers is challenging and has never been reported. Here, β-CD COF is grown with
heptakis(6-amino-6-deoxy)-β-CD and terephthalaldehyde in green solvents of water and
ethanol at room temperature. The COF is characterized by powder X-ray diffraction, which
matches well with the simulated crystal structure. Weaving β-CD into a framework through
reticular chemistry allows the integration of a great amount of β-CD units (50 mol%), much
higher than β-CD polymers. The β-CD COF has larger surface area, more uniform pore size,
and higher thermal stability than the non–crystalline β-CD polymer produced by the same
1
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Chemistry - A European Journal
10.1002/chem.201802564
reagents. Finally, the β-CD COF holds abundant specific interaction sites enabling selective
molecular adsorption.))
((Main Text Paragraphs. Covalent organic frameworks (COFs) are a burgeon class of
advanced materials attracting great research interests, due to their fascinating properties, such
as high porosity, well–defined pore structure, large surface area, excellent thermal stability,
[
1]
and facile post-synthetic modifications. The applications of COFs include gas storage,
transport of organic substrates and products in catalysis, photoelectric applications, advanced
[
2]
water treatments, chemical analysis, and biosensing. Recently, COFs have been employed
in molecular separation. Such COFs are typically constructed by small molecular building
blocks with rigid structures and hydrophobic properties. Large hydrophobic surface and well–
defined pore aperture of the produced COFs allowed adsorption separation of non-polar
[
3]
gases. In addition, considering their outstanding thermal stability and insolubility in most
[
4]
solvents, the COFs can be grown on membrane or in capillary of chromatography. Based on
such COFs, the separation mechanisms include molecular sieving, hydrophobic interactions,
[
4a,4c,4e,5]
hydrogen bonding, π-π interactions, and ion exchange.
Cyclodextrins (CDs) have been employed extensively in molecular recognition and
[
6]
separation mainly on the basis of host–guest interaction mechanism. β-CD is the most used
CD because of its easy preparation and favorable cavity size. β-CDs are often immobilized on
[
7]
the surface of supporting materials or in polymeric matrices for adsorption separations.
Nonetheless, such protocols often produce materials with less β-CD units, diluting their
effects. Recent developments of β-CD polymers by polycondensation between pristine β-CD
and reactive monomers under alkali conditions afforded adsorbents containing elevated
amounts of β-CD units, enabling selective and efficient adsorptions of organic molecules
[
8]
from aqueous solutions. However, the reported β-CD polymers are amorphous and
disordered. The integration of supramolecular CD units into a long–range–ordered three–
2
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10.1002/chem.201802564
dimensional crystalline structure in covalent organic frameworks (COFs) would allow such a
material to possess regular and highly tunable porous architecture. In addition to the
successful development of γ-CD metal organic framework (MOF), 3-dimensional γ-CD COF
was recently prepared through microwave–assisted transesterifation between pristine γ-CD
and borate, which had high porosity, ordered structure, superior Li–conductivity, and CO2
[
9]
adsorption capability. By far, the combination of β-CD units into a well–defined framework
still remains an undeveloped area of research. Due to its inherent asymmetry and flexible
topology, prediction and fabrication of β-CD COF are extremely challenging and have never
been reported.
Here, we have for the first time synthesized β-CD COF using heptakis(6-amino-6-
deoxy)-β-CD (Am7CD) and terephthalaldehyde (TPA) through acid catalyzed aldimine
condensation in green solvents (Figure 1). Am7CD and TPA were polymerized in water and
ethanol at room temperature with the addition of acid catalyst (acetic acid) to provide a light
yellow precipitate, which proved to be a COF with the expected chemical bonds (Support
Information, Figure S1–S3). The concentration and molar ratio of Am7CD, TPA, and acid
catalyst in reaction solutions were optimized. The protocols of Am7CD 20 mM and TPA 50
mM reacting for 48 h under the catalysis of 180 mM acetic acid gave moderate yield (47%)
and high intensities of the powder X-ray diffraction (PXRD) signals (Support Information,
Table S1 and Figure S1). The yield was able to be raised up to 54% without acid catalyst,
whereas the precipitate exhibited poor crystallinity. Ammonia (300 mM) catalyzed
polycondensation between Am7CD and TPA resulted rapid precipitation of a non–crystalline
polymer (β-CD NCP) in 70% yield, which served as a useful control to demonstrate the
importance of ordered structure and well–defined morphology for selective molecular
adsorptions.
Fourier-transform infrared (FT-IR) spectra of both β-CD COF and β-CD NCP exhibit
-1
characteristic bands of hydrocarbon at 1047.18 and 1155.18 cm , and typical C=N bonds
3
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10.1002/chem.201802564
-1
conjugated with phenyl group at 1569.80, 1645.01, 1697.08 cm . TPA was completely
reacted during the polymerization as their characteristic vibration bands of TPA at 2866, 2800,
-1
and 1693 cm disappeared from the spectra of products (Support Information, Figure S2).
13
Solid–state C CP/MAS NMR spectroscopy of both materials showed signals of δ = 101.66,
8
1
1.53, 71.34, and 40.35 ppm of carbons in β-CD, while signals of δ = 160.61, 135.31, and
25.13 ppm were assignable to carbons of the conjugated imine substituents and phenyl
1
3
groups (Support Information, Figure S3). Both FT-IR and C CP/MAS NMR spectra
evidence the relative quantity of cyclodextrin towards aromatic linkers in β-CD NCP is
substantially lower than that in β-CD COF. The reason is that acid–mediated reversible
polycondensation allows Am7CD to assemble in the longitudinal direction. Thus, the steric
hindrance of the formed β-CD assemblies results in less amino moieties on CD coupling with
[
10]
TPA in the latitudinal direction.
Elemental analysis confirmed that the contents of C, H,
and N in the β-CD COF were 46.02, 6.80, and 7.27, which were close to the theoretical values
of 48.90, 6.51, and 8.00 expected for an infinite framework. β-CD NCP had C, H, and N
contents of 53.18, 6.46, and 5.87. The molar ratio between CD and aromatic linkers in β-CD
NCP was calculated as 1:5 based on its nitrogen content. The linkages were formed between
amino–substituents on the primary rim of Am7CD and TPA. Due to the steric hindrance, the
derivatization of amino–substituents was incomplete, which was also observed in the
[
8a]
synthesis of other β-CD polymers. Field-emission scanning electron microscopy (FE-SEM)
revealed that β-CD COF adopted cubic pole structure, with a width of around 125 nm. Field-
emission transmission electron microscopy (FE-TEM) images of the β-CD COF indicated the
cubic pole of 125 nm width was actually comprised of several cubic poles of about 40 nm
width (Support Information, Fig. S4). In contrast, β-CD NCP exhibited spherical structure
(Figure 1). The FE-TEM image of β-CD NCP further revealed its particular morphology and
rugged surfaces (Support Information, Fig. S4). β-CD COF immersed in 0.5 M acetic acid for
2
4 h at room temperature gave slimmer poles on the scale of several tens of nanometers. On
4
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Chemistry - A European Journal
10.1002/chem.201802564
the contrary, agglomeration of the cubic poles of β-CD COF was observed when immersing
the material in 0.5 M ammonia for 24 h. Specifically, β-CD COFs broke into small bricks and
aligned tightly in the axial direction after being ultrasonicated in polar organic solvent
tetrahydrofuran (THF) for 3 h (Support Information, Figure S5). The PXRD intensities and
patterns of β-CD COFs after the treatments of various solvents were almost identical,
indicating the high stabilities of their crystalline structures. Meanwhile, the crystal of β-CD
COF was also thermal–stable after heating at 130 °C for 24 h. Thermal gravity analysis
revealed that β-CD COF had a weight loss of merely 1.76% up to 235.6 °C , which was much
more stable than β-CD NCP (Support Information, Figure S6 and S7).
PXRD patterns of the β-CD COF exhibited strong diffraction peaks at 4.44°, 11.51°,
1
3.32°, 17.60°, 19.94°, and 24.08°, assignable to 100, 201, 300, 122, 411, and 213 facets,
respectively (Figure 2). Pawley refinement were performed to determine the final lattice
parameters. Simulation using the P41 space group with a = b = 19.69 Å and c = 12.26 Å gave
a PXRD pattern that was in good agreement with the experimentally observed pattern (Figure
2
a). Figure 2b and c show the derived unit–cell and super–cell structures. Figure 2d and e
show the side views of the crystal and a helix composed of β-CD units in the axial direction.
In this formulation, the CD units are located in helices separated by 17.20 Å in the radius
direction and 12.26 Å in the axial direction, which stack to form cubic poles. Despite the β-
CD skeleton is also relatively rigid, the formation of β-CD COF would cause substantial
conformational variances. In order to confirm the crystallinity of β-CD COF, the same
protocols were also applied for the preparation of γ-CD COF. The PXRD pattern of as–
prepared γ-CD COF proved the similar packing of γ-CD units in a P41 space group, but the
dimension of crystal cell were a = b = 19.87 Å and c = 15.12 Å, which were relatively larger
due to the size difference between the two CDs (Support Information, Figure S8). The above
results indicate that the β-CD COF is a crystalline material with well–organized stacking of
the CD units.
5
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To evaluate the porous structure, nitrogen sorption isotherm measurements were carried
out. Reversible sorption profiles were observed with typical type II sorption characteristics
(Support Information, Figure S9). The Langmuir surface area, Brunauer-Emmett-Teller
2
-1
(
BET) surface area, and pore volume of β-CD COF were calculated to be 108.2 m g , 79.2
2
-1
3
-1
m g , and 0.332 cm g , respectively. The surface area of as–prepared β-CD COF is
comparable to the condensed β-CD polymers recently prepared by Alsbaiee (SBET = 118~263
2
-1
2
-1 [8]
m g ) and Villalobos (S
= 160 m g ). However, compared with other reported
Langmuir
COFs, the framework mainly fabricated with β-CD is less porous, probably because of the
[
11]
network interpenetration.
Meanwhile, γ-CD COF prepared by the same method also
2
-1
3
-1
exhibited a low BET surface area and pore volume of 26.8 m g and 0.0845 cm g (Support
Information, Figure S9). Such interpenetration might be induced by the stable hydrophobic
[
12]
assembly of cyclodextrin reactants in aqueous phases.
Similarly, hydrophilic β-CD
2
-1
polymers prepared in aqueous solutions invarably have surface areas less than 5 m g , but
[
8a,13]
they have effected molecular adsorptions in water.
In this work, β-CD NCP prepared in
2
the aqueous solution of water and ethanol had BET surface area and pore volume of 4.92 m
-1
3
-1
g and 0.0148 cm g , which were both less than β-CD COF grown under acid–mediation.
On the other hand, due to the well–ordered and rigid structure, the pore sizes of β-CD COF
were in the narrow range less than 4 nm, whereas β-CD NCP had largely irregular pores. The
pore–size distribution profile confirmed the intrinsic porosity of β-CD COF was mainly
originated from the CD units in cubic cells.
Both β-CD COF and NCP contained functionalized CD units, which effected rapid
adsorption of various polar micropollutants from water at environmental concentration levels
-1
(
μg L ) (Figure 3). β-CD NCP showed indiscriminate adsorption of the adsorbates as their
removal efficiencies were all higher than 80% in 10 min. Differently, β-CD facilitated specific
molecular recognition towards (S)-naproxen, 4-nonyl phenol, bisphenol A, and Rhodamine B.
These molecules contain carboxylic or phenolic functionalities and free phenyl or naphthyl
6
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10.1002/chem.201802564
moieties to complex with β-CD cavity (Support Information, Table S2). The adsorption rates
of selected adsorbates on both β-CD COF fit well to the pseudo–second–order kinetic model
[
14]
(Support Information, Table S3).
β-CD COF contains abundant amino- and hyroxyl-
moieties. Thus, very thin boundary layer could be formed between the aqueous solution and
[
15]
the polar surface of adsorbents, resulting in high sorption rates.
In neutral water, β-CD
COF enabled 78% and 98% removal efficiencies for ibuprofen and naproxen in 10 min. The
-1
-1
adsorption rate for naproxen was extremely high with the kobs value of 25 g mg min , which
[
16]
was much more rapid than other adsorbents. In contrast, the adsorption of ibuprofen was
-1
-1
relatively sluggish as the kobs value was merely 1.4 g mg min . Considering similar
molecular weight, pK , and logK values of the two adsorbates, the large difference between
a
ow
their adsorption behaviors should arise from their different molecular structures. The slowest
-1
-1
adsorption rate on β-CD COF was observed for acridine orange (kobs 0.36 g mg min ),
because of its basic characteristics. As such, electron donor–acceptor interactions played an
important role in the adsorption mechanism. β-CD COF also showed rapid removal
-1
efficiencies for neutral compounds of bisphenol A and 4-nonyl phenol (kobs 1.7–6.8 g mg
-1
min ). However, acid molecules such as tetracycline and Rhodamine B had relatively slower
-1
-1
adsorption rates (kobs 0.9–1.5 g mg min ) probably due to the bulky and rigid molecular
structures. At adsorption equilibrium, β-CD COF exhibited high equilibrium constants (K) for
-1
2
(
S)-naproxen and bisphenol A (5,933–8,400 M ) with high R values (0.865–0.983),
indicating 1:1 hydrophobic inclusions formed between the CD cavity and adsorbates (Support
[
17]
Information, Table S4 & Figure S10).
The two adsorbates contain phenyl or naphthyl
moieties to form host–guest influsion with β-CD cavity. β-CD COF differed from β-CD NCP
as the K values for such adsorbates were generally one order magnitude lower. However, the
maximum adsorption amounts (qmax,e) of the adsorbates on β-CD COF were twice higher than
those on β-CD NCP. Thus, it can be concluded that much more hydrophobic inclusion sites
reside on β-CD COF.
7
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In summary, we have for the first time developed supramolecular covalent organic
framework based on asymmetric β-CD building blocks. The β-CD COF was synthesized
through acetic acid catalyzed polycondensation between Am7CD and TPA, whereas the same
reagents produced non–crystalline β-CD polymer (NCP) under ammonia catalysis. The
fabrication of COF allowed a great amount of CD units to be integrated in the material. β-CD
COF exhibited higher thermal stability, uniform pore size distribution, and large surface area.
In addition, the molecular specific adsorption sites on β-CD COF were abundant. β-CD COF
featured favorable molecular adsorption towards molecules containing free aromatic moieties
and acidic functionalities. The material with such unique properties might find broad
applications of sensing, imaging, and delivery of specific compounds in the biological
organisms.)
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
((Funding from the National Natural Science Foundation of China (Grant 21635006,
2
1475098, 31700312) in support of this project is gratefully acknowledged.))
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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Figure 1. ((β-CD covalent organic framework (COF) and β-CD non-crystalline polymer
NCP) prepared under varied catalysis conditions and the high resolution SEM images of both
(
materials.))
Figure 2. Powder X-ray diffraction patterns for the as-synthesized β-CD COF compared with
simulated patterns derived from its crystal structure (b) unit cell (c) super-cell (d) side view of
the crystal (e) side view of a β-CD helix in the crystal.
1
1
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Figure 3. Rapid adsorption of a broad range of polar micropollutants by (a) β-CD COF and
b) β-CD NCP. ))
(
Keyword
adsorption, cyclodextrins, covalent organic frameworks, host-guest inclusion, molecular
recognition
R-Q. Wang, X-B. Wei, Y-Q. Feng*
Title
β-Cyclodextrin Covalent Organic Framework for Selective Molecular Adsorption
ToC figure
Porous β-CD cubic poles
1
2
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