Calix[4]pyrrole-Crosslinked Porous Polymeric Networks for the Removal of Micropollutants from Water

Article abstract of DOI:10.1002/anie.202016364

Calix[4]pyrrole-based porous organic polymers (P1–P3) for removing organic micropollutants from water were prepared. A bowl-shaped α,α,α,α-tetraalkynyl calix[4]pyrrole and diketopyrrolopyrrole monomer were crosslinked via Sonogashira coupling to produce a 3D network polymer, P1. P1 proved too hydrophobic for use as an adsorbent and was converted to the corresponding neutral polymer P2 (containing carboxylic acid groups) and its anionic derivative P3 (containing carboxylate anion groups). Anionic P3 outperformed P2 in screening studies involving a variety of model organic micropollutants of different charge, hydrophilicity and functionality. P3 proved particularly effective for cationic micropollutants. The theoretical maximum adsorption capacity (q_max,e) of P3 reached 454 mg g^?1 for the dye methylene blue, 344 mg g^?1 for the pesticide paraquat, and 495 mg g^?1 for diquat. These uptake values are significantly higher than those of most synthetic adsorbent materials reported to date.

Full text of DOI:10.1002/anie.202016364

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition Chemie
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Accepted Article
Title: Calix[4]pyrrole-Crosslinked Porous Polymeric Networks for
Micropollutant Removal from Water
Authors: Jonathan L. Sessler, Xiaohua Wang, Linhuang Xie, Kunhua
Lin, Weibin Ma, Tian Zhao, Xiaofan Ji, Niveen M. Khashab,
Mram Alyami, and Hongyu Wang
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RESEARCH ARTICLE
Calix[4]pyrrole-Crosslinked Porous Polymeric Networks for
Micropollutant Removal from Water
Xiaohua Wang‡[a], Linhuang Xie‡[a], Kunhua Lin , Weibin Ma , Tian Zhao , Xiaofan Ji* , Niveen M.
[a]
[a]
[c]
[b]
[d]
[d]
[a]
[a],[c]
Khashab , Mram Alyami , Hongyu Wang* , and Jonathan L. Sessler*
[
[
a]
b]
X. H. Wang, L. H. Xie, K. H. Lin, W. B. M, Prof. Dr. J. L. Sessler, Dr. H. Y.Wang
Department of Chemistry, College of Science, and Center for Supramolecular Chemistry & Catalysis, Shanghai University, 99 Shangda Road, Shanghai
00444, China
E-mail: wanghy@shu.edu.cn
Prof. X. F. Ji
2
Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service
Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.
E-mail: xiaofanji@hust.edu.cn
[
[
c]
d]
T. Zhao
Department of Chemistry, The University of Texas at Austin, 105 E. 24th Street A5300, Austin, Texas 78712, United States.
E-mail: sessler@cm.utexas.edu
N. M. Khashab, M. Alyami
Smart Hybrid Materials Laboratory, Physical Science and Engineering Division, King Abdullah University of Sci-ence and Technology (KAUST), 4700 King
Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
Supporting information for this article is given via a link at the end of the document.
Abstract: Three calix[4]pyrrole-based porous organic polymers (P1-
P3) have been prepared with the goal of removing organic
attracted increasing attention in the scientific and industrial
communities in recent years.[ A variety of treatment options have
either been adopted or are under consideration, including
chemical oxidation, photodecomposition, filtration, coagulation,
flocculation, and adsorption. Currently, the latter is generally
viewed as the favored choice in terms of simplicity, efficiency, and
versatility.[4] However, the most commonly used adsorbent
materials, various activated carbons (ACs), suffer from several
drawbacks, such as slow adsorption rates, poor removal
efficiencies in the case of many hydrophilic micropollutants, and
regeneration protocols that are energy-intensive.[5] Compared to
ACs, organic adsorbents are emerging as promising alternatives
due to their potential advantages, including potentially control
over molecular structure, high thermal and chemical stability,
absence of heavy elements, and ease of post-synthetic
modification. Within the context of this effort, porous organic
polymers (POPs), typically cross-linked systems containing small
molecule supramolecular recognition motifs connected by
covalent bonds, have attracted particular attention, including for
applications beyond water purification.[7] Here we report a new set
of POPs wherein calix[4]pyrrole serves as the key macrocyclic
constituent.
Recently, several macrocyclic molecules have been used as
crosslinkers to generate macrocycle-containing POPs, several of
which exhibited considerable promise for the removal of
micropollutants from water.[8] Typically, the constituent
macrocycles endow the material with host-guest encapsulation
capacity, which often translates into an ability to target
micropollutant molecules with different sizes, shapes, and
charges (i.e., positive, negative, or neutral). In a seminal 2016
study, Dichtel et al. reported the preparation of porous β-
cyclodextrin-containing polymers (P-CDPs) and demonstrated
that they were considerably more effective in terms of adsorbing
3]
micropollutants from water.
A bowl-shaped α,α,α,α-tetraalkynyl
calix[4]pyrrole and diketopyrrolopyrrole monomer were crosslinked
via Sonogashira coupling to produce a three-dimensional network
polymer P1. This polymer, which proved too hydrophobic for use as
an adsorbent, was converted to the corresponding neutral polymer P2
(containing carboxylic acid groups) and its anionic derivative (polymer
P3 containing carboxylate anion groups) through post-polymerization
structural modification of the pendent tert-butyl esters. The anionic
polymer P3 outperformed its precursor neutral polymer P2 as
determined from screening studies involving a variety of model
organic micropollutants of different charge, hydrophilicity and
functionality, including dyes, simple aromatics, and two cationic
pesticides. Polymer P3 proved particularly effective for cationic
[
6]
micropollutants. The theoretical maximum adsorption capacity (qmax,e
)
of P3 determined from the corresponding Langmuir isotherms
-
1
-1
reached 454 mg g for the dye methylene blue, 344 mg g for the
-
1
pesticide paraquat, and 495 mg g for diquat, respectively. These
uptake values are significantly higher than those of most synthetic
adsorbent materials reported to date. The present findings thus lend
support to the conclusion that calix[4]pyrrole-based porous organic
polymers may have a role to play in water purification.
Introduction
Organic micropollutants, including industrial dyes, pesticides,
hormones, and pharmaceuticals, are prime contributors to water
pollution.[1] Many of these organic micropollutants, in particular
aromatic compounds, are not easily degraded yet can cause
considerable harm to both the environment and human health due
to their potential mutagenic and carcinogenic effects.[2] Certain
organic micropollutants are recognized endocrine disruptors while
others can lead to aesthetic concerns. Not surprisingly, therefore,
wastewater treatments targeting organic micropollutants have
a variety of organic micropollutants from water than other
adsorbents, including ACs.[9] Subsequently, a number of other
macrocycles,
including
calixarenes,
pillararenes,
and
1
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RESEARCH ARTICLE
calix[4]resorcinarenes, were incorporated into POPs frameworks
and shown effective at removing a range of micropollutants,
approaches were employed.[13-15] However, the requirement for
an organic receiving phase raises concerns that a potential
secondary pollutant (the nonaqueous solvent) might be
introduced into the environment when applied in non-specialty
applications (e.g., radioactive waste remediation). We thus
sought to prepare calix[4]pyrrole-based POPs that would achieve
effective adsorption of micropollutants under solid-liquid
conditions, thereby obviating the need for a secondary organic
phase.
including inter alia radioactive iodine, per- and polyfluorinated
8,10]
alkyl
substances,
and
halomethanes.[
However,
calix[4]pyrroles, a well-studied class of non-aromatic tetrapyrrolic
macrocycles,[11] have yet to be used to create macrocycle-based
POPs.[12] In previous work, we showed that certain calix[4]pyrrole
derivatives could be used to extract surrogate micropollutants
from water. In such cases, two phase liquid-liquid extraction
Scheme 1. (a) Synthesis of porous organic polymers P1-P3 based on α,α,α,α-tetraalkynyl calix[4]pyrrole and diketopyrrolopyrrole (DPP) monomers. Also shown is
a cartoon representation of the polymer structure. (b) Photographs of the polymer products. (c) Chemical structures of organic micropollutants studied in this work.
Cationic pollutants, anionic pollutants, and neutral pollutants are shown in blue, red, and purple, respectively.
Calix[4]pyrroles are readily accessible by simple acid-catalyzed
condensation of pyrrole with various ketones. This allows
calix[4]pyrrole to be functionalized readily at either the meso- or
β-pyrrolic positions with a variety of functional groups, including
those that facilitate polymerization or endow analyte selectivity.[16]
When pyrrole is reacted with 4′-iodoacetophenone, the
calix[4]pyrrole reaction product is obtained as a mixture of four
configurational isomers, including the tetra-α-isomer, which
effort to create calix[4]pyrrole-based POPs, the α,α,α,α-
[
17]
tetraalkynyl calix[4]pyrrole 5 was chosen as the crosslinker and
reacted with the diketopyrrolopyrrole (DPP) monomer 2[ under
conditions of Sonogashira coupling to give polymer P1 (Scheme
1a). Post-synthetic modification then gave P2 and P3. The
presence of the alkyne linkers was expected to endow the
18]
putative POP materials with high surface areas and enhanced
adsorption capacities.[
10a,19]
We further hypothesized that
[
17]
provides a bowl-shaped cavity in its cone conformation. In an
immobilizing the bowl-shaped calix[4]pyrroles within the polymer
2
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RESEARCH ARTICLE
backbones would provide materials with locally defined spatial
features, as well as recognition sites that might facilitate
interactions with organic micropollutants. This design expectation
was realized in the case of P2 and P3 with the latter proving
particular effective as an adsorbent for cationic micropollutants. In
fact, to our knowledge the maximum adsorption capacity of P3 for
the cationic test micropollutants, methylene blue and paraquat, is
the highest among reported POP adsorbent materials, while that
for diquat exceeds by a factor of 2.5 the capacity any other
absorbent system of which we are aware.
polymer P1 was prepared by subjecting monomers 2 and 5 to
Pd(PPh
toluene and Et
3
)
4
/CuI catalyzed Sonogashira coupling in a mixture of
N (1 : 1) at 90 °C for 72 h. During the
3
polymerization, a red solid gradually precipitated out. At the
completion of the reaction, this precipitate was collected by
filtration and washed successively with 1 M aqueous HCl,
methanol, tetrahydrofuran, and acetone. The resulting solid was
further purified by Soxhlet extraction with dichloromethane for 24
h to remove unreacted monomers and oligomers. Treating
polymer P1 with trifluoroacetic acid served to remove the tert-butyl
ester groups and give polymer P2 with free carboxylic acid groups.
Subsequent treatment with aqueous sodium hydroxide at room
temperature gave the anionic polymer P3 as a brown powder.
Polymers P1-P3 proved completely insoluble in all solvents tested,
Results and Discussion
3
including CHCl , THF, 1,4-dioxane, acetone, acetonitrile, MeOH,
Synthesis and Structural Characterization. The DPP monomer
DMF, DMSO, and water. This insolubility was taken as initial
evidence that covalently cross-linked porous materials were being
formed.
2
and α,α,α,α-tetra(4-[ethynylphenyl])calix[4]pyrrole (monomer 5)
were synthesized according to previously reported procedures
[
17,18]
(
Scheme S1 and Figures S1-S7).
As depicted in Scheme 1a,
Figure 1． (a) FT-IR spectra of monomers and P1-P3. (b) Solid-state 13C NMR spectra of polymer P1-P3. (c) N
adsorption curves of P1-P3. (d) TGA curves of P1-
2
-
1
P3 recorded under a nitrogen atmosphere at a heating rate of 10 °C min .
calix[4]pyrrole N-H stretch at 3400 cm-1 is seen, as is one
corresponding to the tert-butoxycarbonyl C=O stretch at 1733
cm-1 in the FT-IR spectrum of P1. Such observations are
consistent with the incorporation of the DPP and calix[4]pyrrole
moieties into the POP material. After TFA treatment, the strong
tert-butoxycarbonyl C=O stretching signal is almost absent from
the FT-IR spectrum of P2. This is consistent with the expectation
that this treatment promotes the elimination of isobutylene giving
rise to the corresponding free carboxylic acid polymer.[20]
Support for the proposed chemical structures of polymers P1-P3
came from FT-IR and solid-state 13C NMR spectroscopic studies
(
Figure 1). The FT-IR spectrum of α,α,α,α-tetra(4-
[
ethynylphenyl])calix[4]pyrrole (monomer 5) shows stretching
-1
vibrations characteristic of a terminal alkyne at about 2108 cm (-
-1
C≡C-) and 3285 cm (terminal alkyne -C-H). After polymerization
to give P1), neither terminal alkyne stretching vibration is visible
in the corresponding IR spectrum. Meanwhile a new signal near
(
-1
The 13C NMR spectrum of P1 exhibits broadened signals with
chemical shift values in the range of 150-120 ppm and 120-110
2
210 cm , ascribed to an asymmetric -C≡C- stretching mode,
appears. Additionally, signal readily assigned to the
a
3
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RESEARCH ARTICLE
ppm corresponding to the carbons of the aromatic phenyl and
pyrrole rings in the polymer frameworks, respectively. A broad
peak near 180 ppm attributed to the carbonyl carbons of the DPP
moiety is also seen, as is a broad peak near 95 ppm ascribed to
provide support for the conclusion that both P2 and P3 are quite
stable under nitrogen.[20]
Polymer P1 was found to be superhydrophobic and to float on
water. The hydrophobicity of P1 is consistent with the hydrophobic
character of its constituents, including the tert-butyl ester groups.
“Water regain” analyses reflected increased hydrophilicity when
P1 was subject to post-synthetic modification (Table S2). In
particular，polymer P3 was found to take up 27.5% of its own
[10a]
the acetylene resonances (-C≡C-) of the linkers.
Signals at
about 50 ppm corresponding to the methylene carbon resonances
of the calix[4]pyrrole are found in the 13C NMR spectra of P1, as
they are in P2 and P3. These observations provide support for the
contention that the DPP and calix[4]pyrrole moieties are present
in all three polymers. The peak at about 30 ppm in the 13C NMR
spectrum of P1, corresponding to the methyl carbons of the tert-
butyl groups, is not found in the spectra of P2 and P3, an
observation taken as evidence that the tert-butyl ester groups
were successfully cleaved during the conversion of P1 to P2.
2
weight when dispersed in H O. The corresponding values are
15.7% and 5.9% for P2 and P1, respectively.
The powder X-ray diffraction (PXRD) patterns of polymers P1-P3
are characterized by the broad peaks seen for most POPs (Figure
S9). This was taken as support for their proposed amorphous
nature. The bulk morphology of the polymers was further
characterized by scanning electron microscopy (SEM). All three
polymers of this study act as dense amorphous monoliths with P2
and P3 showing features consistent with higher relative surface
porosity as inferred from the SEM images (Figure S10).
Micropollutant Uptake and Separations Tests. Due to the poor
hydrophilicity of P1, only P2 and P3 were evaluated as potential
adsorbent materials for micropollutants. For these tests, a range
of surrogate organic pollutants of different charges, hydrophilicity
and functionality were used. As shown in Scheme 1c, our test
series included both cationic (methylene blue) and anionic
(methyl orange and Congo red) dyes, paraquat and diquat
(dicationic, non-selective herbicides), the typical triazine herbicide
atrazine, the plasticizer bisphenol A, and several neutral aromatic
species (1-naphthyl amine and 2-naphthol). For each analysis,
5.0 mg of the polymer sample (P2 or P3) was suspended into an
aqueous solution of the micropollutant (0.10 mM, 5.0 mL). After
constructing appropriate calibration curves, the removal efficiency
and the pollutant uptake could be calculated for each polymer-
micropollutant combination by monitoring the change in the UV-
Vis absorbance intensity of the aqueous phase (Figures S11-S19,
Table S3).
2
The porosity of these polymers was characterized by N gas
adsorption/desorption measurements. The nitrogen adsorption
isotherms of the polymers can be categorized as type II,
consistent with the existence of micropores and mesopores in the
polymers. The polymers have an inferred pore size distribution
mainly in the mesopore region, and non-local density functional
theory calculations indicate the average pore diameters are 63 Å
for P1, 34 Å for P2, and 24 Å for P3, respectively (Figure S8, Table
S1). The Brunauer-Emmett-Teller (BET) surface areas are
2
-1
2
-1
2
-1
calculated to be 110 m g for P1, 89 m g for P2, and 32 m g
for P3, respectively. Both the surface area and pore width values
decreased after subjecting P1 and then P2 to post-synthetic
modifications, which may be due to partial filling of the pores or to
the collapse of some of them during the ester hydrolysis and
deprotonation reactions.[10a]
All three polymers were subject to thermogravimteric analysis
(
TGA) (Figure 1d). The gradual weight loss of polymers at
approximate 100 °C is attributed to the loss of adsorbed water.
For P1 evidence of apparent decomposition is seen starting at
about 170 °C and ending at about 230 °C. This is correlated with
a weight loss of about 10%, which corresponds to the theoretical
weight reduction (~12%) expected for the elimination of
isobutylene from the side chains of the DPP moiety. Such a
weight loss is not seen for P2 and P3. These analyses thus
Figure 2. UV-Vis absorption spectra of aqueous solutions of methylene blue (a and e), paraquat (b and f) and diquat (c and g) (0.1 mM in all cases) in the presence
-
1
of polymers P2 and P3 (1.0 mg mL ), respectively, recorded as a function of increasing adsorption times. Insets show the color change of the methylene blue
solutions before and after the corresponding adsorption study. Time-dependence of the adsorption efficiencies for the adsorptive removal of various test
-
1
micropollutants (0.1 mM in each) by POPs P2 (d) and P3 (h) (1.0 mg mL ).
4
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RESEARCH ARTICLE
A first qualitative screening study revealed that polymer P2 was
moderately effective at removing neutral micropollutants, such as
bisphenol A, atrazine, and 2-naphthol, from an aqueous source
phase under the above test conditions. However, it proved
relatively ineffective as an adsorbent for cationic micropollutants,
such as methylene blue, paraquat, and diquat (cf. Figures 2d and
S20 and discussion below). A similar screening study carried out
with the anionic polymer P3 revealed adsorption efficiencies that
could be correlated closely with the micropollutant charge
source phase. In contrast, only 68% of paraquat, and 49.6% of
diquat could be adsorbed by the activated carbon under the same
conditions, whereas P3 effected nearly complete removal.
Adsorption Kinetics and Isotherms. The time required to
achieve maximum removal revealed trends that mirrored the
adsorption efficiencies. In general, P3 took less time to reach
equilibrium in adsorbing cationic pollutants than was required in
the case of the test neutral and anionic micropollutants. For
example, about 90% of the methylene blue is removed by P3
within 10 min under our test conditions with apparent equilibrium
being reached in about 15 min. In contrast, roughly 2 and 4 h were
required to reach equilibrium in the case of the neutral species
bisphenol A and the anionic dye methyl orange, respectively.
To obtain more quantitative information, the adsorption kinetics
were fit using Ho and McKay's pseudo-second-order model
(
Figures 2h and S21). In the case of the cationic micropollutants,
almost complete removal from the aqueous source phase was
seen when P3 was tested under the standard assay conditions
given above, i.e., methylene blue (99.8%), paraquat (98.4%), and
diquat (94.3%). In contrast, P3 proved much less effective as an
adsorbent material for the neutral and anionic test species
considered in the present study, with efficiencies of about
[
21]
(Figures S24-S25 and Table S3a).
The good fits (correlation
2
20%~50% and 15~40% being seen for these classes of
coefficients R > 99%) to this pseudo-second order kinetic model
micropollutants, respectively.
is consistent with the adsorption of the dye following a
Generally, P3 outperformed its precursor polymer P2 in terms of
removal efficiency. For instance, whereas P3 effected almost
complete adsorption of paraquat and methylene blue, P2
removed only 4.4% and 26.5% of these positively charged
species, respectively, under the same test conditions (Figure 2).
However, similar uptake efficiencies (approaching 50%) were
seen for P2 and P3 in the case of bisphenol A, a neutral
micropollutant for which P2 proved relatively effective as
compared to its performance for the positively charged test
species. In the case of the anionic dyes, Congo red and methyl
orange, contacting aqueous solutions of these surrogate
micropollutants with P2 resulted in an immediate color change of
the mixture, a finding that was ascribed to charge transfer
interactions between P2 and these two anionic dyes. Support for
this latter contention came from the observation of a new
adsorption peak at the longer wavelength region of UV-Vis
spectrum that gradually increased as a function of contact time
chemisorption mechanism, likely driven by electrostatic attraction
effects.[
22,23]
The apparent rate constant (kobs) for the adsorption
of the cationic and neutral surrogate pollutants by P3 are in the
range of 0.01-0.06 g mg-1 min , while the kobs value proved much
lower in the case of the anionic test micropollutants.
-1
In order to gain insight into the rate determining step involved in
the adsorption of the three cationic substrates, methylene blue,
paraquat, and diquat, by P3, the uptake data were analyzed in
terms of Weber’s intra-particle diffusion model (Figure 3b).[
Generally, the adsorption of micropollutant from solution into an
adsorbent includes three consecutive steps: External diffusion,
pore diffusion (intraparticle diffusion), and adsorption on the pore
surface.[25] In the case of P3, a double linear pattern is seen for
each cationic pollutant, and the two straight lines do not pass
through the origin. Such a finding is taken as evidence that
intraparticle diffusion is not the sole rate-limiting step, and the
overall adsorption process may be controlled by multiple-diffusion
steps.[ The first fitting line in this diffusion model may be
attributed to the transfer of the adsorbate molecule from the
solution to the adsorbent surface, while the second fitting line
likely reflects intraparticle diffusion within P3 (Figure 3d). For the
cationic pollutants, ki1 is larger than ki2, indicating external
diffusion is rapid while intraparticle diffusion is more gradual.
The effect of different initial pollutant concentrations on the
adsorption process was studied in the case of P3. The higher the
initial concentration of the pollutant, the higher the adsorption.
This finding is rationalized in terms of a greater driving force for
mass transfer. As shown in Figure 3c, the equilibrium uptake by
P3 as a function of residual pollutant concentration after
adsorption could be fit well to the Langmuir model as expected for
a system where the adsorption sites are identical.[27] The
maximum adsorption capacity (qmax,e) of P3 determined from the
24]
26]
(
Figure S22).
The greater ability of P3 to adsorb cationic surrogate
micropollutants as compared to P2 is rationalized in structural
terms. In general, the uptake of a pollutant by an adsorbent
depends on both the affinity of the molecular components within
the material for the targeted species, as well as the physical
features of the bulk material, such as pore size, effective charge,
and pore volume. In our polymers, a number of putative factors
can regulate the attraction between the adsorbent and adsorbate,
including: (1) electrostatic interactions (dipole-dipole and
monopole-dipole interactions); (2) host-guest effects involving the
calix[4]pyrrole
macrocycle;
(3)–donor-acceptor
and
solvophobic interactions involving, e.g., the aromatic cores of the
pollutants and the hydrophobic surfaces of the polymer. The
strong uptake of methylene blue, paraquat, and diquat by P3
leads us to suggest that electrostatic interactions play a dominant
role in the case of these positively charged species. However, that
anionic dyes were adsorbed by both P2 and P3 provides evidence
for the hypothesis that the anion recognition sites afforded by the
calix[4]pyrrole subunits present in the polymer backbones play a
role in mediating the adsorption characteristics of these two
porous materials.
-1
Langmuir isotherm was estimated to be 454 mg g for methylene
2
-1
2
blue (R = 0.9841), 344 mg g for paraquat (R = 0.9907), and
495 mg g-1 for diquat (R = 0.9906). These results provide support
for the conclusion that the adsorption capacities of P3 for these
test cationic micropollutants are superior to those of most recently
reported adsorbents. Particularly, the maximum adsorption
capacity of P3 for paraquat and diquat is the highest among the
reported POPs;[10d] that for diquat by P3 exceeds that of any other
adsorbent of which we are aware. Comparisons between the
adsorption capacity of P3 and other adsorbents are provided in
Tables S4-S6.
2
We also compared the uptake of cationic micropollutants by P3
and activated carbon (Acros, NORIT® A SUPRA) (Figure S23). It
was found that both P3 and this AC (1 mg mL−1 in both) eventually
removed almost all the methylene blue from a 0.1 mM aqueous
5
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RESEARCH ARTICLE
Figure 3. Fits to a pseudo-second-order model (a) and intra-particle diffusion model (b). Langmuir adsorption isotherms (c) for the adsorption of cationic pollutants
by P3. Calculated parameters for the indicated kinetic and adsorption isotherm models of cationic pollutant uptake by P3 (d).
methylene blue and congo red were ca. 96% and 56%,
Binary-Component Adsorption Studies. The ability of P3 to
adsorb cationic micropollutants effectively in single substrate
tests led us to explore how this adsorbent material system (1 mg
respectively. These values are decreased by roughly 4%, and
increased by approximately 14% compared to the corresponding
mono-component experiments. However, for binary mixtures
consisting of diquat and methyl orange, the removal efficiencies
decreased to about 64% and 2.3%, respectively. As noted above,
we postulate that cationic and anionic micropollutants largely
occupy different sites within the polymer. Specifically, we suggest
that the cationic micropollutant is bound predominantly to the
carboxylate anions of polymer P3 through electrostatic attraction
effects, whereas the anionic dye is accommodated via host-guest
interactions involving the calix[4]pyrrole subunits or via less
specific – donor-acceptor interactions involving the POP
structure as a whole. As a result, appreciable competitive
adsorption involving the anionic and cationic micropollutant
surrogates is not expected. However, this is an oversimplification
since interactions between the micropollutant and the POPs, as
well as between the two species in question are expected. Such
effects, which are expected to be a complex function of size,
charge density, solvation effects, and possibly ion pairing, could
account for the generally modest (but readily discernible)
difference in adsorption behavior seen on moving from the single-
component to the two-component source mixtures.
-1
mL ) would function when exposed to an aqueous binary dye
mixture consisting of methylene blue and methyl orange (0.1 mM
in each). As can be seen from an inspection of Figure 4, under
these experimental conditions, roughly 70% of the methylene blue
is adsorbed in the first 10 minutes, a quantity that gradually
increased to an apparent equilibrium value of 92% within 1 h.
Simultaneously, only 3 minutes were required to reach the
maximum removal value for methyl orange (44%). This latter
value then gradually decreased with time before an apparent
equilibrium efficiency of 24% is reached after approximately 30
minutes. The color of the mixed methylene blue-methyl orange
aqueous solution (Figure 4a inset) changed during the course of
this adsorption experiment from an initial deep green to orange
reflecting the dominant residual presence of non-adsorbed methyl
orange at equilibrium. Relative to the corresponding mono-
component experiments, the removal efficiency of methylene blue
in the binary dye study was decreased by ca. 8%, while that for
methyl orange increased by 9%. Similarly, for methylene blue and
congo red binary aqueous solutions, the removal efficiencies of
6
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Angewandte Chemie International Edition
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-
1
Figure 4. Simultaneous adsorption of cationic and anionic micropollutants by P3 (1 mg mL ) seen upon exposure to binary aqueous solutions consisting of the
indicated two surrogate micropollutants (0.1 mM in each). UV-Vis absorbance spectra of respective methylene blue-methyl orange (a), methylene blue-congo red
(b), and diquat-methyl orange (c) binary aqueous solutions in contact with P3 recorded at different time points. Inset shows the color of the aqueous solution before
and after contact with P3 and presumed differential adsorption of the two test dyes. Time-dependent adsorption efficiency of methylene blue-methyl orange (d),
methylene blue-congo red (e), and diquat-methyl orange (f) binary aqueous solutions.
P3 exhibited excellent adsorption capacity for cationic
We studied the desorption of methylene blue from P3 in an effort
to assess its potential reusability. Simply rinsing with polar
solvents (DMF or ethanol) or dilute protic acid (1 M H SO ) at
2 4
room temperature failed to release appreciable quantities of the
bound methylene blue dye as inferred from UV-Vis spectral
studies. This finding was taken as further evidence that this
organic cationic dye was adsorbed effectively, presumably as the
result of strong electrostatic interactions. However, effective dye
release and regeneration of the POP system could be achieved
by physically removing the polymer (ca. 5 mg) from the initial
micropollutants. This ability is ascribed to dipolar interactions
between the carboxylate anions of the DPP moieties, as well as
nonpolar interactions involving the constituent aromatic rings and
alkynes of the polymers. Furthermore, polymer P3 could be
treated with strongly acidic aqueous media to release the bound
micropollutant methylene blue. It could then be regenerated by
treating with aqueous NaOH and reused with only modest loss in
function. The results reported here illustrate the structural
tunability of calix[4]pyrrole-based POPs, and provide support for
the contention that systems such as P2 and P3 may have
potential utility in the area of water purification.
aqueous solution, stirring in 1 M H
2 4
SO solution (5 mL) for about
10 h at 80 °C and then treating with aqueous NaOH (0.5 M) to
convert the adsorbent back to its carboxylate anion form. Three
consecutive methylene blue adsorption/desorption cycles were
performed (cf. Figure S26). It was found that the methylene blue
removal efficiency decreased about 10% after the three
recycling/regeneration cycles.
Acknowledgements
H. Wang is grateful for support from the National Natural Science
Foundation of China (21975153) and the Shanghai Pujiang
Program (2019PJD017). X. Ji. acknowledges initial funding from
the Huazhong University of Science and Technology, where he is
being supported by Fundamental Research Funds for the Central
Universities (grant 2020kfyXJJS013). X. Ji. is also grateful for
support from the National Natural Science Foundation of China
Conclusion
In summary, we report here the first calix[4]pyrrole-based porous
organic polymers and tests of their potential for water purification.
The structure of the present porous polymers could be easily
modified by converting the pendent tert-butyl esters to carboxylic
acid and carboxylate groups. The anionic polymer P3 was found
to out-perform its neutral polymer precursor P2 in terms of
removing the majority of the surrogate micropollutants considered
in this study from an aqueous source phase. In particular, polymer
(
No. 22001087), and the Open Fund of Hubei Key Laboratory of
Material Chemistry and Service Failure, Huazhong University of
Science and Technology (2020MCF08). Acknowledgment is also
made to Shanghai University, the Robert A. Welch Foundation
(
grant F-0018 to J.L.S.), a ConTex grant provided by The
University of Texas System (2019-06A to J.L.S.), and the King
7
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Abdullah University of Science and Technology (OSR-2019-
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Entry for the Table of Contents
The calix[4]pyrrole-based porous organic polymers have been prepared for water purification. The structure of the polymer P1 could
be easily modified by converting the pendent tert-butyl esters to carboxylic acid and carboxylate groups. The anionic polymer P3
showed particularly effective adsorption for cationic micropollutants. And the theoretical maximum adsorption capacity (qmax,e) of P3
determined from the corresponding Langmuir isotherms reached 454 mg g for the dye methylene blue, 344 mg g-1 for the pesticide
-1
paraquat, and 495 mg g-1 for diquat, respectively.
9
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