A knot-linker planarity control strategy for constructing highly crystalline cationic covalent organic frameworks: Decoding the effect of crystallinity on adsorption performance

Article abstract of DOI:10.1039/d0ta01037e

Ionic covalent organic frameworks (iCOFs), a subclass of COFs, offer a functional platform for diverse applications. However, strong charge repulsion between adjacent layers often leads to low-crystalline iCOFs. Herein, we report a knot-linker planarity control strategy to synthesize a highly crystalline iCOF with C3-symmetric cationic units. More planarity of building blocks gives higher crystallinity of iCOFs, leading to a larger surface area and more exposed binding sites of iCOFs. The highly crystalline iCOF in turn gives larger uptake capacity and faster kinetics than the low-crystalline iCOF and the non-crystalline iCOF, uncovering the significance of crystallinity for the removal of pollutants. The prepared highly crystalline TFPT-TGCl-iCOF exhibits larger saturation sorption capacity (893 mg g-1) than previous adsorbents for 2,4-dichlorophenol. The developed strategy provides a new way to construct highly crystalline iCOFs with C3 symmetric-type cationic sites for various applications. This journal is

Full text of DOI:10.1039/d0ta01037e

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A knot-linker planarity control for constructing high-crystalline
cationic covalent organic framework: decoding crystalline
variation on adsorption performance†
Received 00th January 20xx,
Accepted 00th January 20xx
Hong-Ju Da,a Cheng-Xiong Yang,*,a Hai-Long Qianb and Xiu-Ping Yan*,b
DOI: 10.1039/x0xx00000x
Ionic covalent organic frameworks (iCOFs), a subclass of COFs, offer a functional platform for diverse applications.
However, strong charge repulsion between adjacent layers often leads to low-crystalline iCOFs. Herein, we report a knot-
linker planarity control strategy to synthesize a high-crystalline iCOF with C3-symmetric cationic units. More planarity of
building blocks gives higher crystallinity of iCOFs, leading to larger surface area and more exposed binding sites of iCOFs.
The high-crystalline iCOF in turn gives larger uptake capacity and faster kinetics than the low-crystalline iCOF and the non-
crystalline iCOF, uncovering the significance of crystallinity for the removal of pollutants. The prepared high-crystalline
TFPT-TGCl-iCOF exhibits larger saturation sorption capacity (893 mg g-1) than previous adsorbents for 2,4-dichlorophenol.
The developed strategy opens a new way to construct high-crystalline iCOFs with C3 symmetric-type cationic sites for
various applications.
favorable for effective molecule diffusion. So, constructing
high-crystalline iCOFs is of imperative significance to endow
exceptional functionality. However, it is still challenging to
synthesize high-crystalline iCOFs due to strong charge
repulsion between adjacent cationic sites, especially for the 2D
networks with cationic knots.13,14,20
Introduction
Two-dimension covalent organic frameworks (2D COFs), a class
of porous crystalline polymers,1 have great potential in
extensive applications, such as gas separation,2 catalysis,3
sensing,4 electronic devices5 and adsorption.6 A major goal in
the exploration of COFs is to impart more functions into their
frameworks. One simple method to introduce functionality is
to post-modify the networks,7 but the uncontrollable modified
processes and the unpredictable distribution of functional
groups limit the performance of the final materials. Another
feasible approach to precise tuning of functionality is to
incorporate pre-designed building blocks.8,9 Except for neutral
functional skeletons,10,11 implanting ionic moieties to fabricate
ionic COFs (iCOFs) is also an attractive way to introduce
distinctive functions to networks.12-22
Even so, a few efforts have been made to improve the
crystallinity of iCOFs. Jiang et al.20 combined linear ionic linkers
with neutral knots to design high-crystalline iCOFs where the
reversed AA stacking mode effectively reduced charge
repulsion between cationic sites, resulting in high-crystalline
iCOF. This method is suitable for C2 symmetric ionic
linkers.12,18,19,21 C3-symmetric ionic units such as
triaminoguanidinium chloride (TGCl) is also a good ionic
supplier.13-15 The strong alkalinity (pKa 13.6)24 and Y-
delocalized property25 make TGCl an extremely stable cation
over a wide pH range. In a previous work, we incorporated
intra-layer hydrogen bonds to improve the crystallinity of
guanidine-based iCOF, but only to a quite limited extent.26
Unfortunately, all available guanidine-based iCOFs are low-
crystalline or amorphous.6,13-15 In this sense, it is imperative to
develop a viable strategy to synthesize high-crystalline iCOFs
with C3-type cationic knots.
The crystallinity of COFs is very crucial in their diverse
applications,
such
as
environmental
remediation,
biotechnology and energy consumption.23 High-crystalline
materials with well-defined porosity and channels are usually
a. Research Center for Analytical Sciences, Tianjin Key Laboratory of Molecular
Recognition and Biosensing, College of Chemistry, Nankai University, Tianjin
300071, China
Herein, we report a knot-linker planarity control (KLPC)
strategy to construct a high-crystalline guanidine-based iCOF
(Fig. 1a). As a proof of concept, TGCl is prepared as a cationic
knot for iCOFs (Fig. S1 and S2†). Three knot-linkers, including
2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT), 1,3,5-tris(4-
formyl-phenyl) benzene (TFPB) and tris(4-formylphenyl) amine
b. State Key Laboratory of Food Science and Technology, International Joint
Laboratory on Food Safety, Institute of Analytical Food Safety, School of Food
Science and Technology, Jiangnan University, Wuxi 214122, China.E-mail:
xpyan@jiangnan.edu.cn
c. †Electronic Supplementary Information (ESI) available: experimental details,
structural simulation and adsorption performance. See DOI: 10.1039/x0xx00000x
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Fig. 1 (a) Scheme for the synthesis of TFPT-TGCl-iCOF, TFPB-TGCl-iCOF and TFPA-TGCl-iCOF via the KLPC strategy. The optimized structures of planarity modulators obtained by DFT
calculation: (b) TFPT, (c) TFPB, (d) TFPA (C: gray, N: blue, O: red and H: white).
(TFPA),27,28 are chosen as the planarity modulators. High-
crystalline TFPT-TGCl-iCOF is successfully synthesized from
Synthesis of TFPT-TGCl-iCOF
TGCl and TFPT were dispersed in a binary solvent consisting of
more planar TFPT in combination with TGCl. The reduction in
diverse volume ratios of water, 3 mol L-1 HAc or 6 mol L-1 HAc
the density of cationic sites (Table S1
planarity of neutral units greatly enhance
†
) and the increase in the
stacking
and DMAc or NMP. Typically, TGCl (7.03 mg, 0.05 mmol) and
TFPT (19.7 mg, 0.05 mmol) were dispersed in 1.0 mL of binary
solvent in a 20 cm long
-
interaction between adjacent layers. We highlight that well-
defined porosity of high-crystalline iCOF makes abundant open
spaces of cationic sites, hydrogen bonding sites and benzene
rings more accessible to guest molecules. Such a delicate
structure exhibits ultrafast adsorption kinetics and super-high
uptake capacity in capturing anionic organic pollutants.
 8 mm i.d glass tube. Then, the
mixture was ultrasonicated for 5 min. The glass tube was
frozen in a liquid nitrogen bath at 77 K, evacuated and sealed
after three times of freeze-pump-thaw cycles. After warming
to room temperature, the glass tube was placed in an oven at
120 oC for 3 days. The orange precipitate was filtrated out and
Soxhlet extracted by DMF and MeOH for 48 h, respectively.
The precipitate was then dried under vacuum overnight to
yield the orange fluffy powder with the yield of 91%. Anal. Cald.
for (C25H18N9Cl)n (%): C 62.57; H 3.75; N 26.28; Cl 7.40. Found
(%): C 60.68; H 3.14; N 25.21; Cl 5.97. FT-IR: 3431, 2921, 2851,
2304, 1660, 1629, 1598, 1574, 1515, 1412, 1364, 1299, 1100,
1060, 812, 756, 662 cm-1. PXRD: 6.0o, 10.4 o, 15.2 o, 25.7 o.
Experimental section
Synthesis of triaminoguanidinium chloride (TGCl)
TGCl was synthesized according to the previously reported
method.13 Firstly, guanidine hydrochloride (0.023 mol, 1.93 g) was
dispersed in 10 mL of 1,4-dioxane in a 25 mL three necked bottle.
The hydrazine hydrate (0.068 mol, 3.41 g) was then slowly added
via a dropping funnel and the mixture was refluxed for 2 h. After
cooling to room temperature, the white precipitate was filtered and
washed by 1,4-dioxane and EtOH. The product was finally dried in
Structural simulation and PXRD analysis
Structural simulation of three iCOFs was carried out by Material
Studio (ver. 2017). The possible 2D stacking modes were used to
correlate the experimental PXRD patterns with the simulated
profiles. As for TFPT-TGCl-iCOF, the initial lattice was firstly created
with the parameter of a = b = 18.3609 Å and c = 5.0000 Å via
calculating the geometric structure of the original fragments and
o
an oven at 100 C for 12 h. Isolated yield: 96%. FT-IR: 3320, 3210,
1685, 1615, 1334, 1129, 956, 742, 638, 614 cm-1. 13C NMR (D2O, 600
MHz)  (ppm): 160.81 ppm.
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the space group was set as P1. The suitable monomer was inserted
into the empty cell with the calculated parameters. Before adding
the chloride atoms, the geometry of structure was optimized by MS
Forcite molecular module (Universal force fields, Ewald
summations). Symmetry of optimized structure was found to be P6.
Then hydrogen atoms and chloride atoms were added based on the
above structure and the simulated PXRD patterns of AA stacking
modes were calculated based on the optimized lattice. Finally,
Pawley refinement was applied to define the lattice parameters.
Moreover, the staggered arrangement for TFPT-TGCl-iCOF was also
constructed by building layers and the staggered distance was set
as a = 0.3333 and b = 0.6667. The simulated PXRD patterns of AB
stacking modes were outputted based on the optimized staggered
modes. The simulated processes for TFPB-TGCl-iCOF and TFPA-TGCl-
iCOF were similar to that of TFPT-TGCl-iCOF.
View Article Online
DOI: 10.1039/D0TA01037E
Adsorption kinetics experiments
1 mg of TFPT-TGCl-iCOF, TFPB-TGCl-iCOF or TFPA-TGCl-iCOF was
dispersed in
1 mL of 2,4-DCP solution at the initial
concentrations of 50-600 mg L-1 in 2 mL centrifuge tubes. All
the centrifuge tubes were kept at 25 oC on a shaker at the
rotate rate of 150 r min-1. After contacting for a certain time,
the mixtures were immediately filtrated by 0.22 μm PES filters.
The concentrations of 2,4-DCP were measured by HPLC
(mobile phase: methanol/water = 9:1). The used adsorbents
were collected for further characterization.
Adsorption isotherms experiments
Fig. 2 Comparison of experimental PXRD patterns (red curve), Pawley refinement (blue
curve), their difference (orange), simulated profiles using AA-stacking, and AB-stacking
modes: (a) TFPT-TGCl-iCOF; (b) TFPB-TGCl-iCOF; (c) TFPA-TGCl-iCOF. Unit cells and side
view with d spacing values of the AA-stacking modes: (d) TFPT-TGCl-iCOF; (e) TFPB-TGCl-
iCOF; (f) TFPA-TGCl-iCOF (C: gray; N: blue; Cl: green; H: white).
1 mg of TFPT-TGCl-iCOF, TFPB-TGCl-iCOF or TFPA-TGCl-iCOF was
dispersed in 1 mL of 2,4-DCP solution at various initial
concentrations of 50-1600 mg L-1 in centrifugal tubes. These
centrifugal tubes were kept in a water bath under different
temperatures (25 oC, 35 oC, 45 oC and 55 oC). The
concentrations of 2,4-DCP were determined by HPLC (mobile
phase: methanol/water = 9:1) after filtrating with 0.22 μm PES
filters.
Solvent optimization
We then applied the planarity modulators TFPT, TFPB, TFPA to
react with TGCi for the synthesis of three corresponding iCOFs
(Fig. 1a and S3-S5†). Solvent is able to control the solubility of
building blocks, which plays a crucial role for the crystallization
of COFs.29 For guanidine-based iCOFs, water should be chosen
as one solvent due to the good water solubility of TGCl.
Suitable polar solvent (DMAc, NMP, THF or dioxane) was
selected as another solvent to adjust the solubility of TFPT,
TFPB and TFPA. TFPT is poorly soluble in THF or dioxane and
instead crystallized from strong polar DMAc or NMP. The high-
crystalline TFPT-TGCl-iCOF was finally obtained in NMP and 3
mol L-1 HAc solution at the ratio of 4/1. TFPB shows good
solubility in THF or dioxane and the optimal crystallinity of
TFPB-TGCl-iCOF was obtained in THF and 3 mol L-1 HAc solution
at the ratio of 4/1. Although TFPA is soluble in both THF and
dioxane, only amorphous TFPA-TGCl-iCOF was obtained in all
solvent systems.
Results and discussion
Planarity evaluation of knot-linkers
Choice of proper monomers with different planarity is the first
step to modulate the crystallinity of iCOFs in our KLPC strategy.
The planarity of knot-linkers may have a significant spatial
effect on the crystallinity of iCOFs by controlling - stacking
interaction between interlayers.27 To address this issue, we
performed the structural optimization via density functional
theory (DFT) calculation to demonstrate the difference in the
planarity of the selected knot-linkers TFPT, TFPB and TFPA (Fig.
1b-d). The optimized TFPT shows coplanar structure with
surrounded phenyl rings, whereas the fused phenyl rings in
TFPB rotated out of the central plane at the angle of 36.7o. The
non-planar triangle-center equipped with twisted phenyl rings
(41.2o) offers the lowest planarity to TFPA. The gradual
decrease in the planarity of TFPT, TFPB and TFPA makes them
ideal planarity modulators for constructing high-crystalline
iCOFs.
Resolution of crystalline structures
The crystalline structures of three iCOFs were characterized by
powder X-ray diffraction (PXRD). TFPT-TGCl-iCOF bearing planar
TFPT gave the highest peak intensity at ca. 6o (ca. 9200 cps) for
(100) facet, and showed other weak peaks at 10.4o, 15.2o and
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Journal of Materials Chemistry A
25.7o corresponding to (110), (210) and (001) facets,
respectively (Fig. 2a).12,19 TFPB-TGCl-iCOF containing lower
planar TFPB showed a considerable decrease in the peak
intensity (ca. 1500 cps) at ca. 6o for (100) facet. The strongly
broad peak at 22.8o was signed to the poor π-π stacking along
the vertical-stacked layers (Fig. 2b).13 In contrast, TFPA-TGCl-
iCOF involving non-planar TFPA had no peak for the (100) facet
(Fig. 2c). The peaks at 25.7o (TFPT-TGCl-iCOF), 22.8o (TFPB-TGCl-
iCOF) and 21.2o (TFPA-TGCl-iCOF) gradually became stronger as
the planarity of knots decreased. The distinct variation in the
peak intensity at ca. 6o and ca. 23o reveals that the crystallinity
of iCOFs was effectively improved by incorporating more
planar knot-linkers into the networks. Notably, TFPT-TGCl-iCOF
shows the highest crystallinity among the reported guanidine-
based iCOFs.13-15,26
View Article Online
DOI: 10.1039/D0TA01037E
Structural simulation and Pawley refinement were
performed with Material Studio (v.2017) to resolve the
crystalline structures of TFPT-TGCl-iCOF, TFPB-TGCl-iCOF and
TFPA-TGCl-iCOF. Considering the structures of original units,
two possible two-dimensional (2D) slipped AA-stacking and
staggered AB-stacking modes were constructed for each iCOF
(Fig. S6-S11†). The parameters of the slipped AA stacking
hexagonal unit cell (P6) were deduced to be a = b = 17.49 Å
and c = 3.496 Å for TFPT-TGCl-iCOF, a = b = 17.55 Å and c =
3.504 Å for TFPB-TGCl-iCOF, and a = b = 15.00 Å and c = 3.756 Å
Fig. 3 Structural optimization of the simulated compounds: (a) TFPT-G; (b)TFPB-G; (c)
TFPA-G. Stacking energies of cationic hexagonal units for the studied iCOFs: (d) TFPT-
TGCl-iCOF; (e) TFPB-TGCl-iCOF; (f) TFPA-TGCl-iCOF (C: gray; N: blue; H: white). SEM
images: (g) TFPT-TGCl-iCOF; (h) TFPB-TGCl-iCOF; (i) TFPA-TGCl-iCOF. TEM images: (j) TFPT-
TGCl-iCOF; (k) TFPB-TGCl-iCOF; (l) TFPA-TGCl-iCOF.
for TFPA-TGCl-iCOF (Fig. 2d-f and Tables S2-S4†). After Pawley
refinement, both experimental PXRD patterns of TFPT-TGCl-
iCOF and TFPB-TGCl-iCOF are well matched with the slipped
AA-stacking modes because of their minor differences and
relatively low values of Rwp and Rp (Rwp 3.37% and Rp 1.28%
for TFPT-TGCl-iCOF, Rwp 4.93% and Rp 2.23% for TFPB-TGCl-
and 3a-c), demonstrating that the more planar knot-linkers
were able to bring more coplanar layers to iCOFs. To
quantitatively assess the - stacking effect for the three iCOFs,
single-layer and bilayer hexagonal units capped with phenyl
groups were constructed for each iCOF and three cationic
charges were added in a hexagonal unit. To simplify the
structural optimization process, we did not add chloride anions
into hexagonal units because of the unpredictable location of
iCOF) (Fig. 2a-c and Tables S2-S4†). In contrast, the PXRD
profiles outputted by AB-stacking modes show great
differences from the experimental patterns (Fig. 2d-f).
Compared to the staggered AB stacking mode, the AA stacking
feature creates direct contact of planarity modulators, which is
significant for the control of interlayer interaction. As for non-
crystalline TFPA-TGCl-iCOF, the specific structural constitution
cannot be inferred only from the simulated structures, so
additional evidences will be supplied to further confirm the
stacking mode later. In addition, it is worth noting that the
inter-layer distances calculated from the experimental PXRD
patterns apparently increased from 3.5 Å for TFPT-TGCl-iCOF to
3.9 Å for TFPB-TGCl-iCOF and 4.2 Å for TFPA-TGCl-iCOF (Fig. 2d-
f). More planar modulators gave lower steric hindrance and
enabled adjacent layers to align closer to each other for
chloride anions within networks (see more details in ESI†). The
calculated stacking energies of TFPT-TGCl-iCOF (146.7 kcal mol-
1), TFPB-TGCl-iCOF (151.5 kcal mol-1), TFPA-TGCl-iCOF (166.2
kcal mol-1) increased as the planarity of knots decreased (Fig.
3d-f), which indicates that a more planar modulator gave
lower total energy to the cationic fragment and establish a
more stable system. The presence of more planar knots has
stronger - stacking interaction to partially counteract the
electrostatic-repulsion interaction between cationic sites,
leading to the obvious reduction of interlayer spacing, which is
spatially favorable to the crystallization of iCOFs. This result
stronger - stacking interaction.
uncovers the effective control over the
through our KLPC strategy for the construction of high-
crystalline iCOFs.
- stacking effect
Effect of the planarity of knot-linkers on structural features
We then investigated how the planarity of knot-linkers
modulated the structural features of iCOFs. For this purpose,
DFT calculation was performed to construct and optimize the
structures of simulated compounds TFPT-G, TFPB-G and TFPA-
G for the corresponding iCOFs (Fig. 3a-c). The planarity order
for the simulated compounds (TFPT-G > TFPB-G > TFPA-G) is
identical to that of knot-linkers (TFPT > TFPB > TFPA) (Fig. 1b-d
Variation in the - stacking effect for TFPT-TGCl-iCOF, TFPB-
TGCl-iCOF and TFPA-TGCl-iCOF was also reflected in their
different morphologies. Field emission scanning electronic
microscopy (SEM) and transmission electron microscopy (TEM)
images show the twining-nanofiber morphology for TFPT-TGCl-
iCOF (Fig. 3g, j), the short rod-like morphology for TFPB-TGCl-
iCOF (Fig. 3h, k) and the heterogeneous hollow-sphere
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morphology for TFPA-TGCl-iCOF (Fig. 3i, l). The above results
indicate that the stacking effect follows an increasing
order of TFPT-TGCl-iCOF > TFPB-TGCl-iCOF > TFPA-TGCl-iCOF
along the c axis. The strong stacking interaction directed
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DOI: 10.1039/D0TA01037E
-
-
the growth of TFPT-TGCl-iCOF to expand preferentially in the
vertical direction to form long-nanofiber morphology with the
width of ca. 30 nm. A wide range of lattice structure was
observed in the TEM images of TFPT-TGCl-iCOF (Fig. S12
assuring that high crystallinity was achieved by implanting
planar knots into networks. The weaker stacking
†),
-
interaction within TFPB-TGCl-iCOF hindered the continuous
growth along c axis and cut down the length of the final
material, giving rise to the appearance of short rods with the
width of ca. 80 nm for TFPB-TGCl-iCOF with only a little lattice
at edge in the TEM image (Fig. S13†). The weakest interlayer
interaction in TFPA-TGCl-iCOF made the successive expansion
along the vertical direction hard to take place, so the material
was eventually assembled into hollow-sphere morphology by
the Ostwald ripening process (Fig. S14
analysis reveals the uniform distribution of C, N and Cl within
†
).27 Meanwhile, the EDX
the networks of the three iCOFs (Fig. S15†).
Structural characterization
The occurrence of Schiff reaction during the construction of
TFPT-TGCl-iCOF, TFPB-TGCl-iCOF and TFPA-TGCl-iCOF was
confirmed by Fourier transform infrared (FT-IR) and 13C CP-
MAS solid-state nuclear magnetic resonance (13C NMR)
spectroscopy. New stretching vibration bands at ca. 1630 cm-1
in the FT-IR spectra of the as-prepared iCOFs along with the
concomitant disappearance of the primary N-H stretching
band for TGCl (3322 and 3207 cm-1) and C=O characteristic
peaks for TFPT (1686 cm-1), TFPB (1687 cm-1) or TFPA (1694
cm-1) reveal the successful formation of new imide bonds from
the condensation of the original aldehyde and amino groups in
Fig. 4 N2 adsorption-desorption isotherms of the as-synthesized (a) TFPT-TGCl-iCOF, (b)
TFPB-TGCl-iCOF and (c) TFPA-TGCl-iCOF. Pore size distribution of the as-prepared (d)
TFPT-TGCl-iCOF, (e) TFPB-TGCl-iCOF and (f) TFPA-TGCl-iCOF.
which is well-matched with the simulated pore size of the AA-
stacking mode (1.3 nm) (Fig. 1a), confirming the slipped
stacking mode of TFPA-TGCl-iCOF. Additionally, the pore
volumes of TFPT-TGCl-iCOF, TFPB-TGCl-iCOF and TFPA-TGCl-iCOF
were determined to be 0.52, 0.35 and 0.03 cm3 g-1,
respectively. All above results clearly show that the
improvement of crystallinity helps to increase the accessible
surface and pore volume of the iCOFs, making more binding
sites available for target molecules.
networks (Fig. S16-S18†). Furthermore, the signals at ca. 155-
158 ppm in the patterns of solid 13C NMR represent the
characteristic peak of imide (-C=N-) carbon atoms formed via
Schiff reaction for the three iCOFs (Fig. S19-S21 †). The
Adsorption performance
chemical shifts at ca. 152 ppm were contributed to the carbon
atom of -C=N in TGCl. The peaks in the range of 127-148 ppm
were assigned to the carbon atoms of the fused phenyl groups
in three knot-linkers.
To figure out the effect of crystallinity on adsorption
performance, we used the prepared three iCOFs with different
crystallinities to adsorb a polar ionic organic pollutant 2,4-
dichlorophenol (2,4-DCP) and to investigate the adsorption
behaviors of these iCOFs. The high bio-toxicity and the wide
distribution properties make 2,4-DCP be a priority pollutant.30
Efficient adsorbents are highly desired to trap 2,4-DCP from
environmental systems.31-34 As a phenyl contained organic
pollutant (pKa, 7.8), 2,4-DCP is partially ionized under typical
ambient pH and exists in both molecules and anions.35
Therefore, the iCOFs with abundant benzene rings, cationic
sites and hydrogen bonding sites have great potential as
outstanding adsorbents for 2,4-DCP remediation.
The crystalline difference of the prepared iCOFs motivated
us to get more information on their porosity and the nature of
channels from N2 adsorption-desorption measurements at 77K.
TFPT-TGCl-iCOF exhibited the largest Brunauer-Emmett-Teller
(BET) surface area of 454 m2 g-1 among the three iCOFs, which
was two times that of TFPB-TGCl-iCOF (228 m2 g-1), thirty-four
times that of TFPA-TGCl-iCOF (13 m2 g-1) (Fig. 4a-c and S22
†)
and four times that of our previously prepared guanidine-
based iCON (96 m2 g-1).26 The analysis of pore size based on the
nonlocal density functional theory (NLDFT) demonstrates that
the pore sizes were mainly distributed in the range of 1.2-1.5
nm for TFPT-TGCl-iCOF and TFPB-TGCl-iCOF (Fig. 4d, e), which
are consistent with the theoretical pore widths. From the
analysis of pore distribution, we found that the pore size of
TFPA-TGCl-iCOF was mainly distributed around 1.5 nm (Fig. 4f)
To assess the adsorption capacity of the three iCOFs,
adsorption equilibrium experiments were performed for 2,4-
DCP in an initial concentration range of 50-1600 mg L-1 (Fig. 5a
and S23-S25
†
). The adsorption isotherms fitted well with the
).
Langmuir model (R2 > 0.99) for the three iCOFs (Table S5
†
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Journal of Materials Chemistry A
TFPT-TGCl-iCOF gave the largest capacity (893 mg g-1) among
the three iCOFs studied at 298 K. To the best of our knowledge,
the adsorption capacity of TFPT-TGCl-iCOF is also larger than
previous adsorbents for 2,4-DCP, such as KOH activated char
derived carbon (KOH/char 3.0) (728 mg g-1),31 carbon
nanotubes/porous polyimide composites (1/1) (506 mg g-1),32
activated carbon fiber (372 mg g-1)33 and activated carbon AP-
View Article Online
DOI: 10.1039/D0TA01037E
42 (339 mg g-1)34 (Table S6
†). Such an excellent adsorption
capacity of TFPT-TGCl-iCOF for 2,4-DCP mainly originates from
the high-crystalline nature of TFPT-TGCl-iCOF with sufficient
phenyl-containing interfaces, well-aligned cationic and
hydrogen bonding sites exposed to 2,4-DCP.
The adsorption capacity of low-crystalline TFPB-TGCl-iCOF
(578 mg g-1) and non-crystalline TFPA-TGCl-iCOF (467 mg g-1) is
significantly lower than that of TFPT-TGCl-iCOF (893 mg g-1) (Fig.
5a and Table S5†). This distinct capacity loss is attributed to
the out-of-order dominated structures within TFPB-TGCl-iCOF
or TFPA-TGCl-iCOF with significantly reduced binding sites
accessible for 2,4-DCP. Furthermore, only a little higher
amount of 2,4-DCP adsorbed on TFPB-TGCl-iCOF than TFPA-
TGCl-iCOF indicates that the majority of 2,4-DCP was adsorbed
on the outer surface of TFPB-TGCl-iCOF despite the much larger
surface area of TFPB-TGCl-iCOF (228 m2 g-1) than TFPA-TGCl-
iCOF (13 m2 g-1). Low-crystalline TFPB-TGCl-iCOF with irregular
channels greatly reduced the accessibility of binding sites
within pores. These results highlight the significance of the
high crystallinity of adsorbents to achieve a superior uptake
capacity.
Fig. 5 Comparison of TFPT-TGCl-iCOF (red), TFPB-TGCl-iCOF (blue) and TFPA-TGCl-iCOF
(green) for the adsorption of 2,4-DCP at 298 K: (a) adsorption isotherms in an initial 2,4-
DCP concentration range of 50-1600 mg L-1; (b) adsorption kinetics for 2,4-DCP at an
initial concentration of 50 mg L-1. Adsorption efficiency of 2,4-DCP at an initial
concentration of 200 mg L-1 on TFPT-TGCl-iCOF at 298 K: (c) effect of pH; (d) effect of ion
strength. (e) Reusability of TFPT-TGCl-iCOF for the adsorption of 2,4-DCP at an initial
concentration of 100 mg L-1. (f) Adsorption efficiency of TFPT-TGCl-iCOF for the
adsorption of 2,4-DCP in various water samples (solid / liquid = 1:1; initial 2,4-DCP
concentration, 200 mg L-1).
Thermodynamic studies show that the adsorption of 2,4-
DCP on these three iCOFs was an entropy-driven spontaneous
process (∆G < 0, ΔH > 0, ΔS > 0) (Table S7†). The positive values
of ΔS imply the increased randomness during the adsorption of
2,4-DCP on iCOFs probably because the number of desorbed
water molecules from the surface of iCOF is larger than that of
adsorbed 2,4-DCP molecules.36 Similar positive enthalpy and
positive entropy changes were also reported for the removal
of 2,4-DCP on maize cob derived activated carbon37 and red
mud.38 Moreover, the enthalpy change (∆H) for the adsorption
of 2,4-DCP on TFPT-TGCl-iCOF, TFPB-TGCl-iCOF and TFPA-TGCl-
iCOF was 10.1, 19.4 and 26.4 kJ mol-1, respectively. The lower
energy is required to overcome the intrinsic structural
obstacles during the molecular diffusion process within more
ordered pore channels of higher-crystalline iCOFs.
Time-dependent adsorption was further studied to show the
effect of the crystallinity of iCOFs on adsorption kinetics. The
adsorption equilibrium for 50 mg L-1 2,4-DCP was achieved in
10 s, 1 min and 5 min on TFPT-TGCl-iCOF, TFPB-TGCl-iCOF and
TFPA-TGCl-iCOF, respectively (Fig. 5b). The difference in
equilibrium time among the three iCOFs become more obvious
as the initial concentration of 2,4-DCP increases to 600 mg L-1.
The time required to reach adsorption equilibrium for 600 mg
L-1 2,4-DCP was 1, 5 and 8 h on TFPT-TGCl-iCOF, TFPB-TGCl-iCOF
determined to be 3.13
min-1 (Table S8
).
  
10-3, 4.85 10-4 and 7.42 10-5 g mg-1
†
Notably, high-crystalline TFPT-TGCl-iCOF gave faster
adsorption kinetics than low-crystalline TFPB-TGCl-iCOF and
amorphous TFPA-TGCl-iCOF, especially at higher concentrations.
Such change in adsorption kinetics can result from the
differences in morphology and the degree of crystallinity of
the three iCOFs. Different morphology, including nanofiber (W
30 nm), rod (W 80 nm) and sphere (r 500 nm) (Fig. 3g-i), makes
the diffusion length of 2,4-DCP within channels obviously
increase from TFPT-TGCl-iCOF to TFPA-TGCl-iCOF. Shorter
diffusion distance within one-dimentional channels can lead to
faster adsorption knietics according to Fickian diffusion
model.39 Additionally, the well-ordered pore channels within
high-crystalline TFPT-TGCl-iCOF offer an efficient path for
molecule diffusion which is also conductive for ultrafast
adsorption kinetics.40 Moreover, TFPT-TGCl-iCOF offers a larger
rate constant (2.85
mg L-1 2,4-DCP than previous adsorbents such as empty fruit
bunch derived active carbon (1.9
10-1 g mg-1 min-1)41,
ammonia modified activated carbon (1.7
10-1 g mg-1 min-1)42
and non-living mycelial pellets (9.4
10-2 g mg-1 min-1)43 (Table
S9 ).
Given the superior adsorption performance for 2,4-DCP,

10-1 g mg-1 min-1) for the adsorption of 50


and TFPA-TGCl-iCOF, respectively (Fig. S26†). On the basis of

the pseudo-second-order kinetics model, the rate constants (k)
corresponding to the adsorption of 600 mg L-1 2,4-DCP on
TFPT-TGCl-iCOF, TFPB-TGCl-iCOF and TFPA-TGCl-iCOF were
†
TFPT-TGCl-iCOF was adopted as a representative for further
6 | J. Name., 2012, 00, 1-3
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Page 7 of 8
Journal of Materials Chemistry A
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Chemical Science
ARTICLE
adsorption exploration. The structural stability for adsorbents selectivity of TFPT-TGCl-iCOF for the adsorption ofV2ie,w4-ADrtiCclPe O(nFliinge.
DOI: 10.1039/D0TA01037E
is prerequisite for practical applications. The crystal structure 5f).
of TFPT-TGCl-iCOF is stable in a pH range of 2-10 (Fig. S27
and still retained even after the adsorption of 2,4-DCP (Fig. S28
). The adsorption performance of TFPT-TGCl-iCOF under
†),
Conclusions
†
different pH conditions was further investigated because the
species of 2.4-DCP (pKa 7.8) varies from the neutral molecule
at pH < 6 to the zwitterion in the pH range of 6-8, and the
anion at pH > 8.44 The adsorption efficiency of TFPT-TGCl-iCOF
did not change significantly in the pH range of 2-10 (Fig. 5c).
The strong alkalinity of guanidine groups (pKa 13.6) makes TGCl
as a stable cationic hydrogen-bonding donor at pH < 10.24
Multiform affinity including electrostatic, hydrogen-bonding,
In conclusion, we have shown a knot-linker planarity control
strategy for the synthesis of a high-crystalline iCOF with C3-
symmetric cationic guanidine units. Implanting planar units
into the iCOF enables effective increase in the π-π stacking
efficiency between adjacent layers to construct high-crystalline
TFPT-TGCl-iCOF. The improvement of crystallinity brought
larger BET surface and pore volume to iCOFs, exposing more
well-ordered binding sites including cationic, hydrogen
bonding and phenyl sites to targets. The highest uptake
capacity and ultrafast kinetics of the prepared high-crystalline
TFPT-TGCl-iCOF reveal the significant role of crystallinity in
adsorption application. We believe that further investigation
of the construction of crystalline iCOFs with C3-symmetric
ionic units, along with diverse functional designs will open up a
new research field of iCOFs.
hydrophobic and
- interaction offered by the network of
TFPT-TGCl-iCOF enables to trap both neutral and anionic 2,4-
DCP in the pH range of 2-10. The high adsorption capacity over
a wide pH range is crucial in the practical applications, so TFPT-
TGCl-iCOF has great potential as a high-performance adsorbent
for the removal of 2,4-DCP.
The adsorption efficiency of TFPT-TGCl-iCOF for the
adsorption of 2,4-DCP at pH 7.0 slightly increased with higher
ion strength (Fig. 5d), illustrating that electrostatic interaction
played a negligible role in the adsorption because 2,4-DCP
primarily exists as its neutral form at pH 7.0.44 Increase of ion
strength enables the double layer between host and guest to
compress, making 2,4-DCP closer to the surface of the iCOF
with sufficient binding sites and giving rise to slight increase in
the adsorption efficiency. The uptake capacity of TFPT-TGCl-
iCOF, TFPB-TGCl-iCOF and TFPA-TGCl-iCOF slightly decreased for
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by National Natural Science
Foundation of China (No. 21775056 and 21777074), the
National First-class Discipline Program of Food Science and
Technology (No. JUFSTR20180301), and the Fundamental
Research Funds for Central Universities (No. JUSRP51714B).
the adsorption of 2,4-DCP in the presence of HA (Fig. S29†).
This phenomenon mainly resulted from the much larger
average size of HA (ca. 5 nm) than the pore widths of the three
iCOFs (Fig. 2d-f), which makes the binding sites on the outer
surfaces of iCOFs partially occupied by HA.45 Except for 2,4-
DCP, TFPT-TGCl-iCOF also exhibited good adsorption of other
phenolic pollutants, including 2,3-dichlorophenol, 2,6-
dichlorophenol, 3-chlorophenol, 4-chlorophenol and 1-
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