A succinct strategy for construction of nanoporous ionic organic networks from a pyrylium intermediate

Article abstract of DOI:10.1039/c9cc06767a

Hydroxyl group and pyridinium salt-bifunctionalized nanoporous ionic organic networks prepared via a simple two-step strategy under metal-and template-free conditions are presented. The structural features of the resultant polymer (e.g. high surface area,

Full text of DOI:10.1039/c9cc06767a

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A succinct strategy for construction of
nanoporous ionic organic networks from a
pyrylium intermediate†
Cite this: Chem. Commun., 2019,
55, 13450
Received 30th August 2019,
Accepted 17th October 2019
bc
c
d
Siying Che,a Zhenzhen Yang,
*
Ilja Popovs,
Huimin Luo,c Yali Luo,
Wei Guo,b Hao Chen,b Tao Wang,
Kecheng Jie,b Congmin Wang *a and
b
bc
DOI: 10.1039/c9cc06767a
Sheng Dai
*
rsc.li/chemcomm
Hydroxyl group and pyridinium salt-bifunctionalized nanoporous NIONs can be achieved by judicious choice of ionic monomers
ionic organic networks prepared via a simple two-step strategy under and suitable synthetic routes.9,10 However, most of the direct
metal- and template-free conditions are presented. The structural synthetic approaches are conducted in the presence of metal
features of the resultant polymer (e.g. high surface area, abundant catalysts through coupling reactions, leading to coordination of
hydroxyl groups and ionic functionalities) made it a promising candi- metal species with the ionic functionalities on the polymer
date as an efficient scavenger of toxic oxo-anions from water.
skeletons.11,12 Although free radical polymerization is an easy
fabrication procedure of NIONs under metal-free conditions,
Nanoporous materials, e.g. activated carbons,1 zeolites,2 metal ionic monomers with multi-vinyl groups are required and it is
organic frameworks (MOFs),3 covalent organic frameworks difficult to delicately tailor the pore structures for polymers
(COFs),4 etc., have been regarded as one of the potential candidates obtained in this way.13,14 Therefore, the development of novel
to meet the ever-increasing energy requirements in various systems. and simple synthetic methodologies for the fabrication of NIONs
Among them, porous organic polymer (POP)-based species are with easy introduction of functionalities among the pore wall
attractive due to the easy processing and wide variability of mono- under metal-free conditions would propel further advancement
mers, and show typical features of high specific area, diverse pore in this field, despite still being a challenging task.
dimensions, the use of lightweight elements only, strong covalent
Pyrylium-based small-molecule compounds possess high
linkages and addressable chemical functions.5,6 Nanoporous ionic oxidative power and can be easily synthesized from aldehydes
organic networks (NIONs) are one kind of POP containing extra and acetylbenzenes in the presence of acid, and have been
charges, either positive or negative, within the polymer network, as utilized as a photocatalyst for promoting organic reactions under
well as free counterions electrostatically bound to maintain the visible-light irradiation.15,16 In addition, air and moisture stable
overall electrical neutrality.7,8 The existence of charge on the pore Katritzky N-alkylpyridinium salts can be easily prepared from
wall endows the polymer skeleton with selective interactions with primary amines by reaction with pyrylium salts.17 We envisaged
guest molecules due to the intrinsic charge repulsion/affinity that this simple and metal-free process could be adopted for the
effects. Electrostatic interactions therefore can play critical roles fabrication of NIONs with pyridinium salts within the backbone,
in amplifying separation and sorption efficiency. For the prepara- using multi-substituted aromatic aldehydes and acetylphenyl-group
tion of NIONs, compared with the hard/soft templating synthetic containing compounds as the starting materials. This strategy
method, in which the requirement of sacrificial templates and later also allowed introduction of other functionalities considering
removal make scaled-up production difficult, direct synthesis of the abundant variability of amine structures.
In this work, a hydroxyethyl group and pyridinium salt-
bifunctionalized NION (denoted as HE-Py-NION) prepared via a
succinct two-step strategy under metal-free conditions is pre-
a Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University,
Hangzhou 310027, China. E-mail: chewcm@zju.edu.cn
b Department of Chemistry, The University of Tennessee, Knoxville, TN, 37996, USA.
sented (Fig. 1). The whole process included the construction of a
pyrylium-functionalized polymer through a BF3ꢀEt2O-catalyzed
condensation reaction and subsequent transformation of the
pyrylium groups into pyridium groups in the presence of amine.
The introduction of additional functionality was easily realized
through this approach by employing functional amines (e.g.
ethanolamine). The resultant material HE-Py-NION possessed
porous structures with a Brunauer–Emmett–Teller (BET) surface
E-mail: ZYANG17@utk.edu
c Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008,
Oak Ridge, TN 37831, USA. E-mail: dais@ornl.gov
d College of Materials Science and Engineering, Nanjing Tech University, Nanjing,
211816, P. R. China
† Electronic supplementary information (ESI) available: Synthetic procedures of
monomers and polymers, characterization, and adsorption of methylene blue
hydrate and oxo-anions. See DOI: 10.1039/c9cc06767a
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Fig. 1 Synthetic pathways of Py-POP and HE-Py-NION, and the structure
of the monomers in this work; reaction conditions: step I BF3ꢀEt2O,
1,4-dioxane, 120 1C, 72 h; step II ethanolamine, ethanol, 100 1C, 24 h;
inset: photograph of HE-Py-NION.
area of 398 m2 gꢁ1 and rapid adsorption of methylene blue
hydrate owing to the existence of abundant hydroxyl groups
among the backbone. The typical structural features of the
polymer made it a promising candidate as an efficient scavenger
of toxic oxo-anions from water.
As reported in our previous work about the fabrication of
phosphabenzen-functionalized POPs,18 pyrylium salt-containing
polymers were obtained via a BF3ꢀEt2O-promoted condensation
reaction in 1,4-dioxane. In this work, tetra(4-acetylphenyl)methane (A)
and 9,90-spirobi[9H-fluorene]-2,20,7,70-tetracarboxaldehyde (B)
were selected as building blocks for the synthesis of pyrylium
tetrafluoro borate containing POP (denoted as Py-POP) in the
presence of BF3ꢀEt2O (Fig. 1, step I) (for the detailed synthetic
process, see the ESI†). The formation of Py-POP was verified
by Fourier transformation infrared spectroscopy (FTIR), X-ray
photoelectron spectroscopy (XPS), and cross-polarization magic-angle
spinning (CP/MAS) 13C NMR. In the FTIR spectrum of Py-POP
(Fig. 2A), the characteristic bands in the range of 1400–1600 cmꢁ1
belonged to the benzene rings in the skeleton and the small
peak centred at 1660 cmꢁ1 arose from the CQO+ bond stretch-
ing mode in the pyrylium ring. The successful formation of the
pyrylium frameworks in Py-POP was further confirmed by XPS
analysis (Fig. 2B), with C, O, B and F elements observed in the
survey spectra. As shown in Fig. 2C, signals for B1s with
binding energy (BE) of 193.8 eV and F1s with BE of 686.9 eV
corresponded to the B–F bond in the ꢁBF4 anion. In the C1s
spectrum, three peaks with BEs at 284.8, 285.8 and 289.0 eV can
be deconvoluted, which were ascribed to carbons of aromatic
rings/quaternary carbons, the CQO bond in terminal
Fig. 2 Structural characterization of Py-POP and HE-Py-NION: (A) FTIR
spectra. (B) XPS survey spectra. (C) C1s, O1s, F1s and B1s spectra of Py-POP.
(D) C1s, O1s, F1s, B1s and N1s spectra of HE-Py-NION. (E) CP/MAS 13C NMR.
(F) TGA result conducted under air from 25 to 700 1C with a ramping rate of
10 1C minꢁ1. (G) N2 sorption/desorption isotherm analysis of HE-Py-NION at
77 K. (H) Pore size distribution in HE-Py-NION.
unreacted acetyl or aldehyde groups and CQO+ in the pyrylium The peaks with chemical shift in the range of 127.0–150.7 ppm
ring, respectively. The O1s spectrum also shows the presence of belonged to the aromatic carbons in the skeleton. The peak at
both a CQO+ bond with BE of 533.7 eV and the residual 64.9 ppm was attributed to the quaternary carbon. Thermo-
unreacted CQO bond with BE of 532.0 eV. As evidenced gravimetric analysis (TGA) results indicated that Py-POP was
by CP/MAS 13C NMR (Fig. 2E), the formation of a pyrylium stable in air up to 370 1C (Fig. 2F). The N2 adsorption/
ring was confirmed by the presence of the small peak around desorption analysis result of Py-POP at 77 K (Fig. S1A, ESI†)
170.2 ppm corresponding to the carbon in the CQO+ bond. showed that the BET surface area was 606 m2 gꢁ1 with a total
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pore volume of 0.35 cm3 gꢁ1. The pore size distribution is in
the range of 0.5–2.0 nm together with a small amount of
mesopores around 3.2 nm (Fig. S1B, ESI†), obtained from the
adsorption branches using the non-local density functional
theory (NLDFT) model.
Subsequently, Py-POP was subjected to reaction with aliphatic
amine in ethanol to afford the N-hydroxyehtylpyridium tetra-
fluoro borate-functionalized NION (denoted as HE-Py-NION) in
the presence of ethanolamine (Fig. 1, step II). The transforma-
tion of the pyrylium functionality to a pyridium ring is evidenced
by the FTIR spectrum of HE-Py-NION (Fig. 2A), in which besides
the characteristic peaks for the aromatic benzene ring in
the range of 1400–1600 cmꢁ1, a new small peak appeared at
Fig. 3 (A) Hydrogen bonding between MB and HE-Py-NION. (B) Residual
capacity of MB in solution as a function of time.
1694 cmꢁ1 owing to the presence of the CQN+ bond in the A faster absorption rate and greater dye adsorption was observed
framework. The introduction of the hydroxyethyl group was for HE-Py-NION compared to Py-POP (Fig. 3B), indicating that
confirmed by the characteristic peaks for –CH2– units around –OH groups were successfully introduced to the polymeric net-
2914 cmꢁ1 and a broad peak for –OH groups at 3400 cmꢁ1. The work of HE-Py-NION. The MB adsorption by Py-POP to some
XPS survey spectrum of HE-Py-NION (Fig. 2B) also verified the extent was probably caused by the hydrogen bonding formation
formation of a pyridium ring, with the peak for N element being between the BF4ꢁ anions and MB molecules. Hence, the strategy
observed besides the signals for elements of B, C, O and F. As developed in this work should be an effective approach to further
shown in Fig. 2Dꢁ, in the B1s and F1s spectra, signals for B–F functionalize the pyridium NION networks by employing various
bonds in the BF4 anion almost showed no change compared aliphatic amines for task-specific applications.
with that in Py-POP. In the C1s spectrum, three peaks with BEs
To extend the application scope of the two-step methodology
of 284.6, 285.7 and 287.7 eV were deconvoluted, which belonged developed in this work, other NIONs were also fabricated. By
to the aromatic ring/quaternary carbon, the CQO bond and treating Py-POP with 2,2,2-trifluoroethanamine, trifluoromethyl
the formed CQN+ in the pyridium ring, respectively. The O1s group and pyridinium salt-bifuntionalized NION (denoted
spectrum also showed the presence of the –OH group with BE as TFM-Py-NION) can be easily obtained with a surface area
of 533.4 eV, together with the residual CQO bond with BE of of 386 m2 gꢁ1 (Scheme S1 and Fig. S2 (ESI†)). The structure of
532.0 eV. Notably, the successful formation of pyridium units Py-POP could also be easily changed by choosing the corres-
was observed in the N1s spectrum with BE of 399.8 eV for a ponding functionality-containing aldehydes. Besides fluorinated
CQN+ bond. In the CP/MAS 13C NMR spectrum of HE-Py-NION aldehydes previously reported by our group,18 hydroxyl group-
(Fig. 2E), a new small peak appeared at 40.8 ppm, indicating the functionalized aldehyde was also applicable for the construction
introduction of a –CH2CH2OH group to the skeleton. The N2 of hydroxyl group-containing Py-POP (OH-Py-POP) with a surface
adsorption/desorption analysis result of HE-Py-NION at 77 K area of 212 m2 gꢁ1 (Scheme S2 and Fig. S3, ESI†), which can be
(Fig. 2G) showed that the BET surface area was 398 m2 gꢁ1 with a used then for the preparation of pyridinium salt-containing
total pore volume of 0.23 cm3 gꢁ1. The pore size distribution is in NIONs by treating with amines.
the range of 1.1–1.9 nm together with a small amount of meso-
Increasing water pollution caused by metal derived oxo-anions
pores around 3 nm (Fig. 2H). The surface area of HE-Py-NION (CrO42ꢁ, TcO4ꢁ, SeO32ꢁ, AsO43ꢁ, etc.) has drawn much attention
was decreased compared with the pyrylium-functionalized poly- worldwide,23 and these oxo-anions have been included in the
mer Py-POP, which was probably owing to the introduction priority pollutant list by the EPA (Environmental Protection
of flexible hydroxyethyl groups into the skeleton. The thermal Agency, U.S.).24 Until now, various techniques have been
stability of HE-Py-NION was well retained compared with that of developed for removal of oxo-anions including ion exchange,
Py-POP, being stable under air up to 370 1C, as detected by TGA chemical precipitation, adsorption, electrodialysis, photo-
(Fig. 2F). According to the above results, pyridium-functionalized catalysis and so on, among which an ion exchange method
NION was successfully produced through a novel two-step path- showed the advantages of being low cost, comparatively simple
way. The process was conducted under metal-free conditions, and and safe, and efficient performance.25 In this respect, materials
there was no need to synthesize the ionic precursors in advance. It such as anion exchange resins, layered double hydroxides
was easy to introduce other functional groups into the polymer (LDHs), and cationic metal–organic frameworks (MOFs) have been
backbone. Herein, hydroxyl group-containing NION was obtained developed.26–28 However, anion exchange resins had drawbacks
by simply applying ethanolamine in the second step.
of poor exchange kinetics and lack of stability. Although MOFs
The presence of –OH groups in the skeleton of HE-Py-NION have been employed to capture oxo-anions from water, lack of
can be easily probed using a dye absorption experiment.19 It sufficient physiochemical stability and difficulties in the bulk scale
has been reported that methylene blue hydrate (MB) dye with a synthesis have hindered their wide application.29 In contrast, porous
nitrogen-containing aromatic ring can selectively interact with organic materials, especially NIONs, are constructed from strong
–OH groups through strong hydrogen bonding, rendering it a covalent bonds, leading to high physiochemical stability, which
suitable reagent for the detection of –OH groups (Fig. 3A).20–22 made them promising candidates in oxo-anion capture.9,12,30
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This research was supported financially by the Division of
Chemical Sciences, Geosciences, and Biosciences, Office of
Basic Energy Sciences, US Department of Energy.
Conflicts of interest
There are no conflicts to declare.
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