Colyliform Crystalline 2D Covalent Organic Frameworks (COFs) with Quasi-3D Topologies for Rapid I2 Adsorption

Article abstract of DOI:10.1002/anie.202010829

Constructing three-dimensional (3D) structural characteristics on two-dimensional (2D) covalent organic frameworks (COFs) is a good approach to effectively improve the permeability and mass transfer rate of the materials and realize the rapid adsorption f

Full text of DOI:10.1002/anie.202010829

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: Colyliform Crystalline 2D Covalent Organic Frameworks with
Quasi-3D Topologies for Rapid I2 Adsorption
Authors: Xinghua Guo, Yang Li, Meicheng Zhang, Kecheng Cao, Yin
Tian, Yue Qi, Shoujian Li, Kun Li, Xiaoqi Yu, and Lijian Ma
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10.1002/anie.202010829
Angewandte Chemie International Edition
RESEARCH ARTICLE
Colyliform Crystalline 2D Covalent Organic Frameworks with
Quasi-3D Topologies for Rapid I2 Adsorption
Xinghua Guo,+[a], Yang Li,+[b], Meicheng Zhang,[a] Kecheng Cao,[c] Yin Tian,[d] Yue Qi,[a] Shoujian Li,[a]
Kun Li,[a] Xiaoqi Yu,[a] and Lijian Ma*[a]
[a]
[b]
[c]
[d]
[+]
Dr. X. Guo, Dr. M. Zhang, Dr. Q. Yue, Prof. S. Li, Prof. K. Li, Prof. X. Yu, Prof. L. Ma
College of Chemistry, Sichuan University, Key Laboratory of Radiation Physics & Technology, Ministry of Education
No. 29 Wangjiang Road, Chengdu, 610064, P. R. China
E-mail: ma.lj@hotmail.com. (Prof. L. Ma)
Dr. Y. Li
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China
Hefei, Anhui 230026, People’s Republic of China
E-mail: liyangcd@ustc.edu.cn
Dr. K. Cao
Electron Microscopy of Materials Science, Central Facility for Electron Microscopy, Ulm University,
Albert-Einstein-Allee 11, Ulm 89081, Germany
E-mail: kecheng.cao@uni-ulm.de
Prof. Y. Tian
School of Pharmacy, Chengdu University of Traditional Chinese Medicine
Chengdu 611137, P.R. China
E-mail: ytian227@outlook.com
(X. Guo, Y. Li) These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract:
Constructing
three-dimensional
(3D)
structural
low-energy γ ray emitted by 125I can be used for simple, high
accuracy and low dose-rate bone mineral density determination.
Various types of iodine-labeled compounds also play important
roles in the diagnosis and treatment of diseases.[3] Therefore,
whether from the point of view of the sustainable development of
nuclear energy and the environmental protection, or from the
perspective of enhancing the added value of the nuclear power, it
is of great practical significance to separate and enrich radioactive
iodine produced in the nuclear reactor quickly and efficiently.
Covalent organic frameworks (COFs)[4] are a class of crystalline
porous material with periodic structure formed by the
polymerization of organic building units. Due to its large specific
surface area, excellent physical and chemical stability,
designability of structure and active adsorption sites, COFs have
great potential applications in gas storage,[5] separation,[6]
electronic devices[4c] and catalysis.[7] In recent years, COFs have
also attracted wide attention in the field of gaseous iodine
adsorption owing to the advantages of simple operation, high
adsorption capacity and easy recycling.[1b] Compared with three-
dimensional (3D) COFs, two-dimensional (2D) COFs have lower
cost, richer monomer species and more designable and
adjustable structures. Therefore, current studies on iodine
enrichment are mainly focused on 2D COFs, with emphases on
improving the adsorption capacity of iodine through structural
design, and exploring the influence of structure on the iodine
adsorption performance.[8] However, while focusing on the
increase of iodine adsorption capacity of these COFs,
researchers ignored or avoided the problem of the very slow
iodine adsorption rate of most of the materials studied, especially
the 2D structural materials. Under the typical iodine adsorption
conditions (75°C, 1bar), it usually takes dozens of hours for 2D
COFs to reach the basic equilibrium of adsorption.[9] Some
materials with high adsorption capacity, such as TPB-DMTP COF,
even need up to 100 hours.[8a] The slow adsorption rate makes it
difficult for these materials to meet the needs of rapid treatment
characteristics on two-dimensional (2D) covalent organic frameworks
(COFs) is a smart approach to effectively improve the pore
permeability and mass transfer rate of the materials and realize the
rapid adsorption for guest molecules, while avoiding the high cost and
monomer scarcity in preparing 3D COFs. Herein, we report for the first
time a series of colyliform crystalline 2D COFs with quasi-three-
dimensional (Q-3D) topologies, consisting of unique “stereoscopic”
triangular pores, large interlayer spacings and flexible constitutional
units which makes the pores elastic and self-adaptable for the object
transmission. The as-prepared QTD-COFs have fairly faster
adsorption rate (2.51 g/h) for iodine than traditional 2D COFs, with an
unprecedented maximum adsorption capacity of 6.29 g/g. The
excellent adsorption performance, as well as the prominent irradiation
stability enable the QTD-COFs to be reliably applied to the rapid
removal of radioactive iodine under special circumstances of
emergency response to nuclear accidents.
Introduction
As a major gaseous fission product, radioactive iodine is one of
the most important radioactive pollutants in nuclear waste
disposal and nuclear accident. Due to its characteristics of easy
volatilization, strong fluidity and fast diffusion, it is very difficult to
deal with in the actual environment. In addition, the extremely long
radioactive half-life (129I-1.57 × 107 years), high radiation (131I) and
biocompatibility of radioactive iodine also make it a potential
threat to the safety of the ecological environment and human
health. If not properly handled, it will seriously restrict the
development and application of nuclear energy.[1] On the other
hand, radioactive iodine has important application value in the
medical field. For example, 131I can be used for the examination
of thyroid function and the treatment of malignant tumor.[2] The
1
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RESEARCH ARTICLE
Scheme 1. The design strategy of “quasi-three-dimensional” structure. The inherent stereoscopic characteristics of pentavalent phosphorus bonding in the
phosphazene ring (left) and the diagrams of two theoretically possible configurations, Type I and Type II (right). The partial enlarged diagrams of Type I and Type
II are the views of their pores from different directions (the narrow lateral pore for Type I and the unique oblique triangular pore for Type II).
and disposal of iodine in the case of actual emergency such as
radioactive iodine leakage in nuclear accident, which greatly limits
their practical applications. Therefore, how to reasonably design
the structure of 2D COFs to effectively improve their adsorption
rate for iodine while ensuring their adsorption capacity is one of
the key problems to be solved urgently.
dimensions in 2D COFs through ingenious structural design, so
as to effectively improve the pore permeability and mass transfer
rate of the 2D COFs, and thus increase the adsorption rate to
iodine and other guests. The presence of COF building blocks
centered on cyclotriphosphazene structure provides the
possibility for our hypothesis. In this study, hexa(4-formyl-
phenoxy)cyclotriphosphazene (CTP-6-CHO) with symmetric
structure similar to C6 was used to synthesize COFs. Due to the
stereoscopic bonding properties of P atom in the monomer, the
aromatic ring plane dominated by the cyclotriphosphazene
structure is perpendicular to the plane formed by the two non-
cyclic P-O single bonds (O-P-O plane). This unique structure
causes the 2D planar structure of the formed COFs to twist
regularly, so as to construct new vertical multiple channels
beyond the one-way extension channels of traditional 2D COFs.
This quasi-three-dimensional (Q-3D) structure with 3D pore
characteristics can greatly improve the porous permeability and
mass transfer rate of the materials, and shows excellent
adsorption efficiency and commendable adsorption rate in the
field of enrichment and separation of gaseous iodine.
As is known to all, the dimension and topological structure of
materials have important effects on their potential applications.[10]
3D COFs have three-dimensional open spatial structures and
multidirectional and interconnected pore properties, which
enables the materials to have better permeability. The three-
dimensional pore characteristics are conducive to the diffusion of
the target adsorbate in the structure, accelerating the mass
transfer rate and shortening the equilibrium time.[10e, 11] However,
3D COFs usually use monomers with tetrahedral or similar
stereoscopic configurations as building units, resulting in a very
limited selection of monomer structures and types. Therefore, the
number of 3D COFs reported so far is less than one-tenth of the
total COFs. Moreover, the limited available 3D monomers are
complex and difficult to synthesize, and quite expensive to obtain
commercially. The above disadvantages, as well as the difficulty
in structural analysis of 3D COFs, especially those with
interpenetration structure, have severely restricted their
development and practical application. The conventional 2D
COFs are composed of discrete planar sheets stacked between
non-covalent layers and have been widely studied and reported
owing to the abundance of building monomers and the simplicity
and lower cost of material synthesis. However, due to the high
planarization degree and crystallinity of 2D COFs, their pores are
usually located in the same plane, and the orientations of their
pore channels are extremely uniform and consistent. Compared
with stereo channels of 3D COFs, this unidirectional and uniform
channel characteristics make the 2D COFs have lower intracavity
diffusion and guest adsorption efficiency.
Results and Discussion
In order to achieve the goal of building 2D COFs with 3D
structural characteristics, we elaborately selected CTP-6-CHO
with the structure of six cross side arms outside the central plane
as the node module of the COFs (Scheme 1 and Figure 1a; see
the Supporting Information (SI) for details, Section 2). CTP-6-
CHO
is
formed
by
the
condensation
of
hexachlorocyclotriphosphazene and p-hydroxybenzaldehyde and
has a unique stereoscopic structure in which the O-P-O plane (β
plane) is perpendicular to the aromatic ring plane of
cyclotriphosphazene N3P3 (α plane) (Scheme 1). Because of the
unique structure of CTP-6-CHO, the as-prepared COFs are not
traditional 2D planar structure. Theoretically, to realize the
In view of this situation, we wonder if it is possible to construct
structures and channel characteristics similar to those of three
2
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Figure 1. Structure diagram (a) and crystal structure (b) of QTD-COF-1. Experimental (black), Pawley refined (red) and predicted (blue) PXRD patterns, and the
differences between the experimental and refined PXRD patterns in dark cyan (inset: views of space-filling models along the c-axis with the layer distances). FT-IR
spectra (c), high resolution XPS spectrum of N1s (d) and pore-size distribution (e) for QTD-COF-1.
construction of COFs with long-range ordered structure, there are
mainly two connection configurations between CTP-6-CHO and
the C2-symmetric linkers, namely Type I and Type II (Scheme 1).
Type I is constructed by CTP-6-CHO in a C3-like symmetric way.
The two side chains anchored at the O-P-O bonds are parallel,
forming a heteropore structure with frontal hexagonal main pores
and lateral narrow pores. Type II is constructed in a symmetric
configuration similar to C6, and the six stereoscopic side chains
from three nodes are cross-linked upper and lower to form a
colyliform structure with oblique triangular pores. In fact, since the
formation of Type I needs to overcome the huge spatial tension
of the O-P-O bonds, it is difficult to get such kind of configuration.
The formation of Type II involves a torsion process of the β plane.
Under the influence of the stretching effect and the spatial limiting
effect during the formation of the continuous extensional
framework structure, the β plane rotates laterally relative to the
plane α. However, due to the inherent 3D structural features of
pentavalent phosphorus bonding and the existence of spatial
steric hindrance, it is impossible for the two planes to coincide
completely, resulting in a unique “Q-3D” structure between two
and three dimensions. Compared with the traditional 2D structure,
the novel Q-3D structure has abundant stereoscopic lateral
channels which can greatly increase the pore permeability of the
material and improve its adsorption and mass transfer rate for
guest molecule.
Meanwhile, a small molecule model compound CTP-M with
similar structure was prepared for comparative analysis (SI,
Section 2). It is shown by Fourier transform infrared spectroscopy
(FT-IR) that the characteristic diffraction peaks of the amino group
and the aldehyde group of QTD-COF-1 largely disappeared, and
the characteristic peak of the -C=N- stretching band newly
appeared at 1624 cm−1 is fundamentally the same as that of the
model compound CTP-M (Figures 1c and S1a). X-ray
photoelectron spectroscopy (XPS) analysis revealed that there
are only imine nitrogen (-C=N-, 398.8 eV) and phosphazene
nitrogen (-P=N-, 398.1 eV) peaks in the N1s spectrum, with a ratio
of 2:1 and no amino peak (~400 eV) (Figures 1d and S3b). And
there is an imine carbon peak at ~152 ppm in the solid-state 13
C
NMR spectrum, indicating that the condensation reaction of
aldehydes and amines has been successfully achieved (Figure
S2a). In addition, the result of elemental analysis (EA) shows that
the experimental values of element contents are close to the
theoretical values, suggesting that the monomers have virtually
reacted according to the feed ratio (Table S2).
The crystallinity of QTD-COF-1 was confirmed by powder X-ray
diffraction (PXRD) analysis. As shown in Figure 1b, the obvious
diffraction peaks at 4.72°, 9.52°, 12.12° and 21.08° demonstrate
that QTD-COF-1 has high degree of crystallinity. And this is also
the first synthesis of phosphazene COF with high crystallinity.
Materials Studio (MS) software was used to simulate the crystal
structure of COFs and the results show that the experimental
values are in good agreement with the simulated values of the
eclipsed (AA) stacking structure of the Type II configuration
(Figure S6a), and the corresponding diffraction peaks are
In this study, QTD-COF-1, a quasi-three-dimensional COF,
was prepared by self-assembly polymerization under
solvothermal conditions with CTP-6-CHO as the node and p-
phenylenediamine as the linker (Figure 1a, SI, Section 2).
3
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attributed to (100), (200), (210) and (001) facets, respectively.
Pawley refinement produced unit cell parameters of a = b = 21.38
Å, c = 4.22 Å, α = β = 90°, and γ = 120°, with good agreement
factors of RP = 2.68% and RWP = 3.45% (Table S1a). Therefore, it
can be determined that QTD-COF-1 belongs to Type II
configuration and forms a colyliform structure with oblique
triangular channels, rather than Type I configuration with a
hexagon framework structure (Figure S4c). Different from the
conventional 2D COFs, the atoms of the same layer of QTD-COF-
1 are not in the same plane, and there are abundant oblique
triangular pores observed from both the front and the interlayer
directions of the COF layers. The interlayer stacking distance also
increases significantly from ~3-3.5 Å of the traditional 2D COFs to
4.21 Å (Figure 1b), realizing the transition from 2D structure to the
Q-3D structure.
dimensional COF with a colyliform structure consisting of oblique
triangular pores.
The unique structure of QTD-COF-1 formed by Type II
configuration has abundant frontal-lateral pores which are oblique
triangular and face both the front and the side. Compared with
traditional 2D COFs, the big hexagon framework in Type II
configuration is divided evenly into six small triangular regions,
which effectively increases the density of the active binding sites
on the framework. Moreover, the Q-3D structure with frontal-
lateral pores facing forward and sideways allows the guest
molecules to enter the structural framework of the material
through both the frontal and the lateral pathways, which greatly
increases the permeability and mass transfer rate of the pores
and makes the interaction between host-guest molecules more
orderly and efficiently. Interestingly, we found that due to the
introduction of flexible building units in the COF structure, the
guest solvents such as acetone and tetrahydrofuran can cause a
reversible structural transformation of QTD-COF-1 (Figure S5).
This phenomenon indicates that the COF pores constructed by
the flexible units underwent a self-adaptive reversible structural
change with the entry of guest molecules, which further confirmed
the flexible characteristic of the COF structure.[12] Compared with
the rigid structure, the flexible structure makes the COF more
adaptable to the object, and thus increases its affinity to guest
molecules.
Figure 2. SEM images (a-c), TEM images (d, e, inset: SAED patterns) and
ACTEM images (f, inset: the corresponding lattice distances and the fast Fourier
transformation) of QTD-COF-1
Scanning electron microscopy (SEM) analysis shows that
QTD-COF-1 is composed of homogeneous spherical particles
formed by stacking of sheets, with a diameter of 2 μm (Figure 2a-
c). The high-resolution transmission electron microscope (HR-
TEM) images also show that it is formed by ordered stacking of
layers, with good lattice patterns and obvious electron diffraction
spots (Figures 2d,e and S8a). In order to further reflect the
structure of QTD-COF-1 accurately, the spherical aberration
corrected transmission electron microscope (ACTEM) was
adopted and the existence of the interlayer lateral pores can be
clearly observed in the ACTEM images (Figures 2f and S9), which
is very similar to the interlayer image simulated by MS (Figures
1b and 2f). And the interlayer distance is found to be 4.28 Å which
is consistent with the theoretical value. The pore-size distribution
curve shows that the average pore diameters are 1.35 nm and
1.71 nm, and the two sizes of pores are actually consistent with
the triangular pores of 1.36 nm and 1.66 nm in the simulated
wheel-shaped structure (Figure 1e). SEM, HR-TEM and pore-size
distributions further prove that QTD-COF-1 is a quasi-three-
Figure 3. Iodine adsorption and desorption experiments. (a) Gravimetric iodine
uptake of QTD-COF-X (X= 1, 2, 3, 4, V) as a function of time at 75°C and
ambient pressure. (b) Comparison of the saturated adsorption capacity and the
K80% of various COF materials. (c) Photographs show the color change of iodine
enrichment progress of QTD-COF-1 in hexane. (d) Schematic diagram of iodine
adsorption by QTD-COF-1 in an open atmosphere. (e) Controlled release of
iodine upon heating the I2-laden QTD-COFs at 125°C .
Since the Q-3D structure can increase the permeability and
mass transfer rate of the channel, theoretically, the adsorption
rate of the sorbent to the target adsorbate will also be improved
effectively. To verify this ratiocination, the adsorption performance
of QTD-COF-1 was investigated in detail using iodine as the
target adsorbate. As shown in Figure 3a, the iodine adsorption
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Figure 4. Structure diagram (a) and crystal structures (b-e) of QTD-COF-X (X= 2, 3, 4, V). Experimental (black), Pawley refined (red) and predicted (blue) PXRD
patterns, and the differences between the experimental and refined PXRD patterns in dark cyan. (f) Interlayer distances of QTD-COF-X (X=1, 2, 3, 4, V) calculated
with Materials Studio.
capacity of QTD-COF-1 increased almost linearly in the initial
stage and reached 80% of the saturated adsorption capacity
within just 3 hours. And an obvious turning point of adsorption
occurred at about 5 hours, indicating that the adsorption
equilibrium was basically reached. The measured saturated
adsorption capacity of QTD-COF-1 is 4.62 g/g, which is at a high
level in the iodine adsorption materials reported. In order to
compare and evaluate the adsorption performance of the
materials more scientifically and reasonably, the average
adsorption rate before the adsorption capacity reaches 80% of the
saturated adsorption capacity was determined as the benchmark
and defined as K80% (K80% = 80% of saturated adsorption capacity
/ adsorption time, and the unit is g h−1) (See SI Section 16 for
detail). Figure 3b shows the relationship between the saturated
adsorption capacity and K80% of the typical 2D COFs for iodine
adsorption reported previously. It can be seen from the diagram
that QTD-COF-1 has a large K80% value of 1.23 g h−1, which is
better than all the 2D COFs reported so far (Figure S13).
To further prove the rapid and efficient adsorption performance
of QTD-COF-1, we compared the adsorption of iodine at room
temperature between QTD-COF-1 and TPT-BD COF, a reported
iodine adsorbent with outstanding adsorption performance which
contains similar flexible ether bond structure and has a high
saturated adsorption capacity of 5.43 g/g and K80% of 0.75 g h−1
(Figure 3b).[8b] It was found that the adsorption rate and
adsorption capacity of QTD-COF-1 are much higher than TPT-BD
COF at room temperature (Figure S14c), which may be attributed
to the blockage of the characteristic one-dimensional channel in
the traditional 2D TPT-BD COF by iodine. However, the QTD
structure of QTD-COF-1 overcomes this shortcoming and the
guest iodine can enter the COF structure through unique
stereoscopic channels, which leads to the significant
improvement in adsorption capacity and rate. Moreover, QTD-
COF-1 also showed a fast iodine adsorption rate in hexane
solution. The iodine can be adsorbed completely within about 4 h
by QTD-COF-1. While, TPT-BD COF did not fully adsorb the
iodine after 48 h and the solution was still pale red (Figures 3c
and S15). Therefore, the construction of Q-3D COFs is an
effective method to improve the iodine adsorption rate of 2D
COFs. In addition, the iodine adsorption experiments in an open
atmosphere also confirmed the faster adsorption rate and higher
affinity of QTD-COF-1 for iodine, along with quick and obvious
color changes (Figure 3d), which is expected to be applied to the
enrichment and disposal of radioactive iodine under various
emergency conditions.
In order to further verify the general applicability of the Q-3D
COFs construction strategy, we prepared
a
series of
phosphazene QTD-COF-X (X = 2, 3, 4, V) with high crystallinity
and different cell sizes by replacing the amine linking units, as
shown in Figure 4. FT-IR and EA analysis were used to confirm
the bonding modes and compositions of the materials, and the
analysis results are similar to those of QTD-COF-1 (Figure S1 and
5
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Table S2). That is to say, the aldehyde and amine monomers of
this series of QTD-COF-X (X = 2, 3, 4, V) have virtually reacted
completely to form imine bonds according to the feed ratio. Solid-
state 13C NMR spectrum and XPS analysis further corroborated
this conclusion (Figures S2 and S3).
Furthermore, in addition to the fast adsorption rate, the adsorption
capacity of QTD-COF-X (X = 1, 2, 3, 4, V) are also satisfactory,
which are 4.67, 2.87, 5.16, 4.85 and 6.29 g/g, with the
corresponding iodine percentage contents of 82.3 wt%, 74.2 wt%,
83.8 wt%, 82.9 wt% and 86.3 wt%, respectively (Figure S14b).
The adsorption capacity of the QTD-COFs are at a high level
among all the reported iodine adsorption materials, which was
also proved by TGA analysis (Figures 5a and S11).
The crystallinity and structure of QTD-COF-X (X = 2, 3, 4, V)
were analyzed and studied by PXRD and MS simulation. As
shown in Figure 4b-e, QTD-COF-X (X = 2, 3, 4, V) have a number
of obvious diffraction peaks. Regardless of the different positions
of the peaks, the patterns are almost the same as that of QTD-
COF-1, indicating that the stacking modes of the series of
materials are consistent except for the different cell sizes. The MS
simulation in Figures 4 and S6 also proves this result. The
experimental values of PXRD patterns coincide well with the
simulation values of AA stacking mode of the Type II configuration
which has very small refined parameters. The results confirm that
QTD-COF-X (X = 2, 3, 4, V) take CTP-6-CHO as the node and
form Q-3D COFs with the colyliform framework structure
equipped with abundant oblique triangular channels. It was found
from the pore-size-distribution curves that the pore diameters of
QTD-COF-X (X = 2, 3, 4, V) increase from 1.36 to 1.72 nm with
the increasing length of the linking monomers (Figure S10b). The
pore diameters are fundamentally consistent with the theoretical
oblique triangular pore structure, which further confirms their
topological structure. Therefore, the strategy of constructing Q-3D
COFs by monomers possessing flexible six-arm structure similar
to CTP-6-CHO is feasible and universal. In addition, TGA
analyses show that QTD-COF-X (X = 1, 2, 3, 4, V) have excellent
thermal stability in air or nitrogen atmosphere (Figures 5a and
S11), which may be related to the flame retardant property of
organic-inorganic phosphazene heterocyclic ring. Moreover,
there are almost no changes in the PXRD patterns and FT-IR
spectra of the QTD-COFs before and after γ-ray irradiation, which
indicates that the as-prepared COFs possess excellent anti-
irradiation performance and can withstand 105 Gy γ-ray irradiation,
Figure 5. TGA curves of QTD-COF-V before (in N2 and air atmosphere) and
after iodine adsorption (in N2 atmosphere). (b) Raman spectra of iodine and
QTD-COF-V before and after volatile iodine adsorption. (c) FT-IR spectra of
QTD-COF-V before and after iodine adsorption. (d) Reusability of QTD-COF-1
and QTD-COF-V for iodine capture.
In particular, it is the first time that two flexible monomers, a
flexible aldehyde and a flexible amine, were combined and
copolymerized to prepare the highly crystalline QTD-COF-V with
V-shaped linking units and spiral fan structure (Figure S6e,f).
Compared with the traditional linear monomers, the introduction
of V-shaped monomer not only increases the overall flexibility and
self-adaptive ability of the material’s framework, but also reduces
the π-π interaction between the interlayers to a greater extent,
resulting in the decrease of interlayer stabilization and the obvious
increase of interlayer spacing. Therefore, among the series of
QTD-COFs, the interlayer distance of QTD-COF-V is the largest
and up to 0.45 nm (Figure 4b,f and Table S1b). It was also found
through iodine adsorption experiments that the adsorption rate
and capacity of QTD-COF-V are much higher than that of all the
reported 2D COFs, with the adsorption rate of 2.51 g/h (K80%) and
the adsorption capacity of 6.29 g/g (75°C , 1bar) (Figure 3a,b). The
excellent adsorption performance of QTD-COF-V can also be
proved by iodine adsorption at room temperature, with an
adsorption capacity of 4.56 g/g, which is also better than QTD-
COF-1 (Figure S14c). Therefore, the increase of lateral pore
diameter, interlayer spacing and flexible self-adaptive ability can
significantly increase the adsorption rate and capacity of COFs to
iodine. More importantly, the iodine adsorption capacity of QTD-
COF-V after irradiation was 6.02 g/g which was not different from
that before irradiation, indicating its application potential for
radioactive iodine adsorption in practical environment. Of course,
similar phenomena and results were also observed in the other
QTD-COFs (Figure S14d).
revealing their potentials to capture radioactive iodine, such as 129
I
or 131I, in specific emergency circumstance of nuclear accident
(Figure S12).
Further analysis of PXRD shows that the (001) diffraction peaks
of QTD-COF-X (X = 1, 2, 3, 4, V) were 21.1°, 21.8°, 20.6°, 20.3°,
19.5°, respectively. In other words, with the increase of the length
of linking monomers and the number of substituent groups, the
diameters of the Q-3D lateral pores and the related interlayer
stacking distances increase gradually. And the interlayer
spacings of QTD-COF-X (X = 1, 2, 3, 4, V) are significantly higher
than that of the traditional COFs (0.34 nm) and reach ~0.42, 0.41,
0.43, 0.44 and 0.45 nm (Figures 1 and 4b-e), respectively, which
are consistent with the interlayer distances calculated by MS
(Figure 4f). TEM images of QTD-COF-V proved that the material
is layered with better lattice patterns and regular electron
diffraction spots, and the interlayer distance is ~4.53 Å which is
fairly close to the experimental value of the PXRD pattern and the
MS simulated value (Figure S8b). Studies on iodine adsorption of
the QTD-COFs show that the iodine adsorption rate (K80%) mainly
keeps increasing with the increase of the interlayer spacings,
suggesting that the iodine adsorption rate of 2D materials can be
controlled through the regulation of the interlayer distance. The
adsorption rate of QTD-COF-3 is lower than that of QTD-COF-1,
which may be due to the strong interlayer stabilizing effect[12a, 13]
of the biphenylene linker which limits the adaptive structural
transformation of the material during the adsorption process.
6
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10.1002/anie.202010829
Angewandte Chemie International Edition
RESEARCH ARTICLE
molecules enter the Q-3D channels of the adsorbent through both
the frontal and the lateral pathways and interact with the electron-
−
rich conjugated systems and groups to form I5 charge-transfer
complex.
In addition, QTD-COFs can be recycled through thermal
desorption and solvent desorption. Under the thermal desorption
condition of 125°C , the desorption equilibrium can be reached
within 2 h and the desorption rate is as high as over 90% (Figure
3e). The materials are also capable of being recovered by ethanol
desorption, and agitation and ultrasound can greatly accelerate
the desorption rate (Figures 3e and S16). Meanwhile, QTD-COFs
have the advantages of good reusability (Figure 5d and S14g) and
easy preparation in large scale (See SI Section 2 and 17 for detail).
Based on the results, it is clear that the QTD-COFs are a kind of
excellent iodine adsorbent with high efficiency, large capacity and
recyclability, and have great application potentials.
Scheme 2. Schematic diagram of the rapid iodine adsorption process and
mechanism by QTD-COFs.
Conclusion
Through the analyses of Raman spectroscopy (RM), FT-IR and
XPS, the mechanism of iodine capture and transmission path
were studied. During the adsorption process, iodine is dispersed
in the air in form of molecular iodine and interacts with the
adsorbent, thus being absorbed and immobilized by the material.
It was found in the literature that the straight-chain diameter of the
iodine molecule is 3.35 Å[14] which is far less than the lateral pore
diameters and interlayer spacings of all the QTD-COFs from 0.41
to 0.45 nm. This implies that iodine molecule can enter the
materials from not only the same frontal channels of the QTD-
COFs as the traditional 2D material, but also the Q-3D lateral
channels between the layers during the adsorption process.
Meanwhile, due to the strong self-adaptive ability of the flexible
COFs to guest molecule, the pore sizes of the materials can be
adjusted according to the size of guest molecule within a certain
range, so that the guest iodine can pass through the frontal-lateral
channels at a faster rate, enter into the structural frameworks of
the COFs, and be loaded at the adsorption active sites. The
species of iodine loaded on the adsorbents was detected through
RM analysis. It was found that after the adsorption of iodine by
the QTD-COFs, the characteristic peaks of I5− appeared at ~110
and 167 cm−1 in the RM spectra, which was quite different from
the 182 cm−1 of molecular iodine (Figures 5b and S17), indicating
the charge transfer interaction between the adsorbents and
iodine.[15] The FT-IR spectra of the QTD-COFs show that the peak
positions of the C=N bond, the C=C/C-H bonds on benzene ring
and the P=N/P-O-Ar bonds on phosphazene ring have obvious
shifts before and after adsorption (Figures 5c and S1).[16]
Moreover, the binding energy of I2-laden QTD-COFs in the XPS
In conclusion, to improve the permeability and mass transfer
rate of 2D COFs, a series of phosphazene COF materials with
high crystallinities was prepared for the first time in this study by
using CTP-6-CHO as the node module and stereo source center.
The as-prepared QTD-COFs have unique oblique triangular
frontal-lateral pores and larger interlayer distances (0.41~0.45
nm), presenting a colyliform Q-3D structure, which successfully
achieves the construction of 3D structural features on 2D
materials, and greatly increases the permeability and mass
transfer rate of 2D COFs. The unique pore structure of the COFs
enables the guest molecules to enter the framework of the
materials through both the frontal and the lateral pathways, thus
making the interaction between the host-guest molecules more
orderly and efficiently. Adsorption experiments show that the
iodine adsorption rate of QTD-COFs are fairly better than
traditional 2D COFs, and the adsorption capacity are also very
prominent. Especially for QTD-COF-V, the adsorption rate
parameter of K80% can reach 2.51 g/h and the adsorption capacity
is up to 6.29 g/g, with excellent material reusability and irradiation
stability. It was found by comparative study that the lateral pore
diameter and interlayer spacing, as well as the iodine adsorption
rate of QTD-COFs increased along with the increasing length and
flexibility of the linking monomers, indicating that the regulation of
the guest adaptive ability and adsorption property of the COFs
were successfully realized through the control of chain length and
flexibility of the monomers. The design and construction strategy
of Q-3D materials proposed in this study can not only greatly
reduce the preparation cost and limitation of COF materials, but
also effectively improve the iodine adsorption rate and capacity of
2D COFs, which provides an applicable solution for the rapid
enrichment and separation of radioactive iodine in emergencies
such as nuclear accident. And this strategy is also of great
scientific significance for the rapid adsorption of other gas
molecules and the design and preparation of other mass storage
materials.
−
spectra also indicates the formation of I5 . The binding energy of
I3d5/2 and I3d3/2 of the QTD-COFs after iodine adsorption are
located at ~617.9 eV and ~629.4 eV, respectively, which have
larger shifts compared with the values of 618.6 eV and 630.1 eV
−
of iodine elements and can be attributed to the formation of I5
(Figure S3e).[17] In addition, the binding energy of C, N, P and O
in the COFs also shifted to some degrees, indicating their
interaction with iodine, which is consistent with the FT-IR results
(Figure S3f-i). Thus, it can be concluded that the charge transfer
between iodine and the adsorbents occurred in the electron-rich
groups and conjugated systems. The possible mechanism of
iodine enrichment by the QTD-COFs was proposed based on the
conclusion. As shown in Scheme 2 and Video S1, the iodine
Acknowledgements
The financial support from National Natural Science Foundation
of China (Grants 21771128 and 21976125), Science Challenge
7
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10.1002/anie.202010829
Angewandte Chemie International Edition
RESEARCH ARTICLE
[13] L. Ascherl, T. Sick, J. T. Margraf, S. H. Lapidus, M. Calik, C. Hettstedt, K.
Karaghiosoff, M. Döblinger, T. Clark, K. W. Chapman, F. Auras, T. Bein,
Nat. Chem. 2016, 8, 310-316.
Project (TZ2016004), Sichuan Science and Technology Program
(2020JDRC0014) and China Postdoctoral Science Foundation
(2020M671906) is gratefully acknowledged. We thank Jiqiu Wen
for XRD analysis of the Analytical & Testing Center, Sichuan
University. And we are grateful for the support from the
Comprehensive training platform of specialized laboratory
College of chemistry, Sichuan University.
[14] a) D. F. Sava, M. A. Rodriguez, K. W. Chapman, P. J. Chupas, J. A.
Greathouse, P. S. Crozier, T. M. Nenoff, J. Am. Chem. Soc. 2011, 133,
12398-12401; b) L. Chen, J.-Y. Qian, D.-D. Zhu, S. Yang, J. Lin, M.-Y. He,
Z.-H. Zhang, Q. Chen, ACS Appl. Nano Mater. 2020, 3, 5390-5398.
[15] a) P. H. Svensson, L. Kloo, Chem. Rev. 2003, 103, 1649-1684; b) C.
Wang, Y. Wang, R. Ge, X. Song, X. Xing, Q. Jiang, H. Lu, C. Hao, X. Guo,
Y. Gao, D. Jiang, Chem. - Eur. J. 2018, 24, 585-589; c) J. T. Hughes, D.
F. Sava, T. M. Nenoff, A. Navrotsky, J. Am. Chem. Soc. 2013, 135,
16256-16259.
Keywords: covalent organic frameworks • quasi-three-
dimension • mass transfer rate • rapid adsorption • iodine
[16] a) L. Sun, T. Liu, H. Li, L. Yang, L. Meng, Q. Lu, J. Long, ACS Appl. Mater.
Interfaces 2015, 7, 4990-4997; b) M. Zhang, Y. Li, C. Bai, X. Guo, J. Han,
S. Hu, H. Jiang, W. Tan, S. Li, L. Ma, ACS Appl. Mater. Interfaces 2018,
10, 28936-28947.
[1]
[2]
a) B. J. Riley, J. D. Vienna, D. M. Strachan, J. S. McCloy, J. L. Jerden, J.
Nucl. Mater. 2016, 470, 307-326; b) W. Xie, D. Cui, S.-R. Zhang, Y.-H.
Xu, D.-L. Jiang, Mater. Horiz. 2019, 6, 1571-1595; c) N. R. Soelberg, T.
G. Garn, M. R. Greenhalgh, J. D. Law, R. Jubin, D. M. Strachan, P. K.
Thallapally, Sci. Technol. Nucl. Install. 2013, 2013, 1-12.
[17] a) J. Wang, Z. Li, Y. Wang, C. Wei, K. Ai, L. Lu, Mater. Horiz. 2019, 6,
1517-1525; b) Y. Lin, X. Jiang, S. T. Kim, S. B. Alahakoon, X. Hou, Z.
Zhang, C. M. Thompson, R. A. Smaldone, C. Ke, J. Am. Chem. Soc.
2017, 139, 7172-7175.
S. American Thyroid Association Taskforce On Radioiodine, J. C. Sisson,
J. Freitas, I. R. McDougall, L. T. Dauer, J. R. Hurley, J. D. Brierley, C. H.
Edinboro, D. Rosenthal, M. J. Thomas, J. A. Wexler, E. Asamoah, A. M.
Avram, M. Milas, C. Greenlee, Thyroid 2011, 21, 335-346.
a) M. J. Adam, D. S. Wilbur, Chem. Soc. Rev. 2005, 34, 153-163; b) W.
Wei, Z. T. Rosenkrans, J. Liu, G. Huang, Q. Y. Luo, W. Cai, Chem. Rev.
2020, 120, 3787-3851.
[3]
[4]
a) C. S. Diercks, O. M. Yaghi, Science 2017, 355, eaal1585; b) P. J.
Waller, F. Gandara, O. M. Yaghi, Acc. Chem. Res. 2015, 48, 3053-3063;
c) M. S. Lohse, T. Bein, Adv. Funct. Mater. 2018, 28, 1705553; d) S. Y.
Ding, W. Wang, Chem. Soc. Rev. 2013, 42, 548-568; e) X. Feng, X. Ding,
D. Jiang, Chem. Soc. Rev. 2012, 41, 6010-6022.
[5]
[6]
a) H. Furukawa, O. M. Yaghi, J. Am. Chem. Soc. 2009, 131, 8875-8883;
b) Z. Wang, S. Zhang, Y. Chen, Z. Zhang, S. Ma, Chem. Soc. Rev. 2020,
49, 708-735.
a) Y. Li, X. Guo, X. Li, M. Zhang, Z. Jia, Y. Deng, Y. Tian, S. Li, L. Ma,
Angew. Chem. Int. Ed. 2020, 59, 4168-4175; b) X. Wang, L. Chen, L.
Wang, Q. Fan, D. Pan, J. Li, F. Chi, Y. Xie, S. Yu, C. Xiao, F. Luo, J. Wang,
X. Wang, C. Chen, W. Wu, W. Shi, S. Wang, X. Wang, Sci. China: Chem.
2019, 62, 933-967; c) V. A. Kuehl, J. Yin, P. H. H. Duong, B. Mastorovich,
B. Newell, K. D. Li-Oakey, B. A. Parkinson, J. O. Hoberg, J. Am. Chem.
Soc. 2018, 140, 18200-18207.
[7]
[8]
a) H. Wang, H. Wang, Z. Wang, L. Tang, G. Zeng, P. Xu, M. Chen, T.
Xiong, C. Zhou, X. Li, D. Huang, Y. Zhu, Z. Wang, J. Tang, Chem. Soc.
Rev. 2020, 49, 4135-4165; b) Q. Yang, M. Luo, K. Liu, H. Cao, H. Yan,
Appl. Catal., B 2020, 276, 119174.
a) P. Wang, Q. Xu, Z. Li, W. Jiang, Q. Jiang, D. Jiang, Adv. Mater. 2018,
30, 1801991; b) X. Guo, Y. Tian, M. Zhang, Y. Li, R. Wen, X. Li, X. Li, Y.
Xue, L. Ma, C. Xia, S. Li, Chem. Mater. 2018, 30, 2299-2308; c) Z. J. Yin,
S. Q. Xu, T. G. Zhan, Q. Y. Qi, Z. Q. Wu, X. Zhao, Chem. Commun. 2017,
53, 7266-7269.
[9]
S. An, X. Zhu, Y. He, L. Yang, H. Wang, S. Jin, J. Hu, H. Liu, Ind. Eng.
Chem. Res. 2019, 58, 10495-10502.
[10] a) R.-R. Liang, F.-Z. Cui, R.-H. A, Q.-Y. Qi, X. Zhao, CCS Chem. 2020, 2,
139-145; b) Q. Gao, X. Li, G.-H. Ning, H.-S. Xu, C. Liu, B. Tian, W. Tang,
K. P. Loh, Chem. Mater. 2018, 30, 1762-1768; c) R. R. Liang, S. Y. Jiang,
R. H. A, X. Zhao, Chem. Soc. Rev. 2020, 49, 3920-3951; d) X. Wu, X.
Han, Y. Liu, Y. Liu, Y. Cui, J. Am. Chem. Soc. 2018, 140, 16124-16133;
e) Y. Meng, Y. Luo, J. L. Shi, H. Ding, X. Lang, W. Chen, A. Zheng, J.
Sun, C. Wang, Angew. Chem. Int. Ed. 2020, 59, 3624-3629.
[11] a) C. Gao, J. Li, S. Yin, G. Lin, T. Ma, Y. Meng, J. Sun, C. Wang, Angew.
Chem. Int. Ed. 2019, 58, 9770-9775; b) X. Chen, K. Geng, R. Liu, K. T.
Tan, Y. Gong, Z. Li, S. Tao, Q. Jiang, D. Jiang, Angew. Chem. Int. Ed.
2020, 59, 5050-5091; c) H. M. El-Kaderi, J. R. Hunt, J. L. Mendoza-
Cortes, A. P. Cote, R. E. Taylor, M. O'Keeffe, O. M. Yaghi, Science 2007,
316, 268-272.
[12] a) C. Zhao, C. S. Diercks, C. Zhu, N. Hanikel, X. Pei, O. M. Yaghi, J. Am.
Chem. Soc. 2018, 140, 16438-16441; b) Y. Chen, Z. L. Shi, L. Wei, B.
Zhou, J. Tan, H. L. Zhou, Y. B. Zhang, J. Am. Chem. Soc. 2019, 141,
3298-3303.
8
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RESEARCH ARTICLE
Entry for the Table of Contents
A novel type of COFs with colyliform quasi-three-dimensional (Q-3D) topologies equipped with “stereoscopic” oblique triangular pores
and larger interlayer spacings are reported. The unique Q-3D structure effectively improve the pore permeability and mass transfer
rate of the COFs and lead to a fairly faster adsorption rate (2.51 g/h) for iodine than traditional 2D COFs, with an unprecedented
maximum adsorption capacity of 6.29 g/g.
9
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