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Title

A simple and cost-effective synthesis of ionicporous organic polymers with excellent porosity forhigh iodine capture

Author(s)Li, Zhongping; Li, He; Wang, Dongjin;
Suwansoontorn, Athchaya; Du, Gang; Liu, Zhaohan;
Hasan, Md. Mahmudul; Nagao, Yuki

Citation

Polymer, 204: 122796

Issue Date

2020-07-22

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/18071

RightsCopyright (C)2020, Elsevier. Licensed under theCreative Commons Attribution-NonCommercialNoDerivatives 4.0 International license (CC BY-NCND 4.0). [http://creativecommons.org/licenses/bync-nd/4.0/] NOTICE: This is the author's version ofa work accepted for publication by Elsevier.
Zhongping Li, He Li, Dongjin Wang, Athchaya
Suwansoontorn, Gang Du, Zhaohan Liu, Md.
Mahmudul Hasan, and Yuki Nagao, Polymer, 204,
2020, 122796,
https://doi.org/10.1016/j.polymer.2020.122796

Description

Japan Advanced Institute of Science and Technology

A simple and cost-effective synthesis of ionic porous organic polymers
with excellent porosity for high iodine capture

Zhongping Li,a†* He Li,b† Dongjin Wang,a† Athchaya Suwansoontorn,a Gang
Du,a Zhaohan Liu,a Md. Mahmudul Hasan,a and Yuki Nagaoa*a Schoolof Materials Science, Japan Advanced Institute of Science andTechnology, Japan.
b StateKey Laboratory of Catalysis, iChEM, Dalian Institute of Chemical

Physics, Chinese Academy of Sciences, China.
c College of Chemistry, Jilin Normal University, Changchun, 130103, Republicof China.

†Z. Li, H. Li, and D. Wang contributed equally.
⁎

Corresponding

author.

ynagao@jaist.ac.jp.

E-mailaddress:

lizhongping2013@163.com;

Highlights⚫

The i-POPs have been synthesized via a simple and cost-effective process.

⚫

The i-POPs showed the highest BET surface area and pore volume.

⚫

The i-POPs exhibited the best iodine uptake value among all reported ionic POPs.

⚫

The capture process could be efficiently recycled and reused for five cycles.

1Abstract
Porous organic polymers (POPs) are excellent adsorbent candidates owing to their highporosity, permanent pore, and well-designed structure. Ionic porous organic polymers (iPOPs) are a special type of POPs, which can greatly improve the capacity of guest
molecules through electrostatic interactions. However, most reported i-POPs haveshortcomings such as low porosity and high-cost synthesis procedure. With these
considerations in mind, several ionic porous organic polymers (i-POP-BP-BPTMs) havebeen successfully designed and synthesized by using the commercial and simplesynthesized building units via a simple and cost-effective process. i-POP-BP-BPTMs
indicated excellent porosity including the highest BET surface area (1491-1753 m2 g–1) andthe best pore volume (2.19-2.94 cm3 g–1) among most reported ionic POPs. Interestingly,
i-POP-BP-BPTM was found to be highly effective in iodine vapor capture up to 415 wt%,
which is higher than the value of all reported ionic POPs. These results offer a new way todesign and construct functional ionic POPs.
Keywords: Ionic porous organic polymers, facile and low-cost synthesis, high porosity,
effective iodine uptake.

2

Introduction
In recent years, many urgent attentions have been paid to environmental risks,
especially nuclear accidents and waste, which stems from widespread use of nuclear
energy. Due to the health effects of radiation, the main safety issue associated with nuclear
energy is proper nuclear waste management.1 127I and 129I are important radioisotope innuclear waste, especially long-lived half-life (1.57×107 years), which has generated great
concern. Standard methods for iodine capture focus on inorganic solid adsorbents such as
silver-based zeolites.2 However, these absorbents usually show low capacities, which
derive from their limited accessible surface areas. Considering these detrimental effects,
designing effective materials for radioactive iodine capture is urgent and essential.
Porous organic polymers (POPs) are a new class of porous polymers that possesshigh porosity and permanent pore topology/structure. POPs have grown into a thrivingfamily, including conjugated microporous polymers (CMPs),3-4 polymers of intrinsic
microporous (PIMs),5 covalent organic frameworks (COFs),6-8 porous aromatic frameworks(PAFs),9 and hypercrosslinked polymers (HCPs).10-12 Due to their strong organic bonds
linked nature, POPs usually have good thermal/chemical stability. Therefore, POPs haveshown potential applications in gas storage, molecular sensing, and catalysis, andpollutants removal.13-37
We are interested in porous hyper-crosslinked polymers because they not only
possess permanent porosity and remarked stability, but also have the advantages tointroduce various functional units by simple and cost-effective synthesis, which can
improve guest molecules capture ability.10 Recently, POPs built-in ionic groups exhibitedstronger electrostatic forces towards specific molecules than neutral polymers.33, 38
However, elaborate synthesis methods and complex organic compounds are required to
introduce ionic units into the porous structure, which limited porosity and structure
diversity.38 There is a strong demand to develop ionic porous materials with high porosity
by a sample and lower-cost way. In this context, we synthesized neutral porous polymer(POP-BP) that is constructed through commercial raw materials, 4,4'-bis(chloromethyl)31,1'-biphenyl (BP), as a crosslinking agent and building unit under solvothermal conditions
by cheap FeCl3 as the catalyst via a facile Friedel–Crafts alkylation reaction (Scheme 1a).
Then, ionic building unit, 1,1'-([1,1'-biphenyl]-4,4'-diyl)bis(N,N,N-trimethylmethanaminium)
chloride (BPTM) easily prepared from BP via one step in a large-scale synthesis, wasintroduced into the skeleton of POP-BP to afford ionic porous organic polymers (i-POPs).
With the increment of ionic unit, the i-POP-BP-BPTM-1, i-POP-BP-BPTM-2, and i-POPBP-BPTM-3 were successfully synthesized under the same condition (Scheme 1b).

Scheme 1 (a) Schematic representation of the synthesis of (a) POP-BP and (b) i-POP-BPBPTMs.

Results and discussion
POP-BP was prepared according to a previous report with a high yield of 99%.11 The
construction of ionic polymers was achieved by using BPTM as the monomer, 4,4'bis(chloromethyl)-1,1'-biphenyl as the crosslinking agent, in the presence of FeCl3 as acatalyst. The component of the polymer was tuned by the variation of the number ofmonomers, and the resultant polymers were denoted as i-POP-BP-BPTMs (For detail, see
experimental section). C, N and H elemental analysis was investigated for the POPs. With
the increment of BPTM, the amount of N element was an obvious improvement of networks
4

(Tab. S1). Among the POPs, i-POP-BP-BPTM-3 showed the highest N content of 2.87%.
The successful crosslinking of i-POP-BP-BPTMs was confirmed by FT IR measurements.
The obvious peaks at 2916-3021 cm-1 could be attributed to the C-H stretching vibration,
which originated from the structure of –CH2– and –CH3 in the frameworks (Fig. 1). The CN bond vibration was observed at 1219 cm-1 of i-POP-BP-BPTMs, which is the same asthat of BPTM. In the 13C NMR spectrum of i-POP-BP-BPTM-3, the signals at 128 and 139
ppm were assigned to aromatic carbon (Fig. S1). There were obvious peaks at 37.1 and53.0 ppm for -CH2- and -CH3- of i-POP-BP-BPTM-3.

Fig. 1. FT IR spectra of BP (green curve), BPTM (sky-blue), POP-BP (black curve), iPOP-BPTM-1 (red curve), i-POP-BP-BPTM-2 (blue curve), and i-POP-BPTM-3 (purple
curve).
Scanning electron microscopy (SEM) images were conducted to investigate themorphology of the materials, and all the POPs exhibited morphology as aggregatedmicrometer-scale particles (Fig. S2). The elemental mapping by energy-dispersive X-ray
spectroscopy (EDS) analysis result of i-POP-BP-BPTMs also proved the existence of N
and Cl elements (Fig. S3). Interestingly, EDS mapping result of i-POP-BP-BPTMs indicateda relatively uniform distribution of N and Cl in networks. The thermal stability of the5polymers was investigated by thermogravimetric analysis (TGA) under a nitrogen
atmosphere (Fig. S4). Among these polymers, POP-BP has good thermal ability, which
could be stable up to 350 oC. For i-POP-BP-BPTMs, as the increasement of BPTM in thepolymer,thedecomposition

temperature

decreased.

PowderX-ray

diffraction

measurements (PXRD) of POPs suggested their amorphous structures (Fig. S5).

6

Fig. 2. Nitrogen sorption isotherms of (a) POP-BP, (c) i-POP-BP-BPTM-1, (e) i-POP-BPBPTM-2, and (g) i-POP-BP-BPTM-3 measured at 77 K (●: adsorption, ○: desorption). Poresize distribution of (b) i-POP-BP, (d) i-POP-BP-BPTM-1, (f) i-POP-BP-BPTM-2, and (h) iPOP-BP-BPTM-3.
Table I. Porosity of POPsName

BET surface Area
(m2 g–1) a

Pore Volume(cm3 g-1) b

Pore Size(nm) cPOP-BP

2039

3.38

1.40, 2.93, 5.47

i-POP-BP-BPTM-1

1753

2.94

1.36, 2.93, 5.46

i-POP-BP-BPTM-2

1611

2.44

1.12, 2.92, 4.56

i-POP-BP-BPTM-3

1491

2.19

1.09, 2.91, 4.50

a The BET surface area was calculated through the nitrogen adsorption isotherms by using the BET

method. b Total pore volume was estimated at a P/P0 of 0.99. c The pore size was calculated from nonlocal
density functional theory (NLDFT).

The permanent porosities of POPs were investigated by nitrogen adsorption
measurements at 77 K. As shown in Fig. 2, All the prepared POPs showed type I isothermwith sharp uptake observed in the low pressure, which is a representative character ofmicroporosity according to the IUPAC classification. POP-BP showed the highest
Brunauer–Emmett–Teller (BET) surface area of 2040 m2 g–1 with the pore volume of 3.38
cm3 g–1. Introducing ionic building blocks into the network also gave high porosity for ionicpolymers. i-POP-BPTM-1, i-POP-BPTM-2, and i-POP-BP-BPTM-3 displayed high BET
surface area of 1753, 1611 and 1485 m2 g–1, respectively. To our best knowledge, the BET
surface area of i-POP-BPTMs is higher than that of all reported ionic POPs (Table S2).21.
26, 38-48

The i-POP-BPTM-1, i-POP-BPTM-2, and i-POP-BPTM-3 displayed remarkable

pore volume of 2.95, 2.24, and 2.19 cm3 g–1, respectively, which was also higher than themost reported ionic POPs. The excellent porosity of i-POP-BP-BPTMs is possibly due toits high polymerization degree and reactivity of 4,4'-bis(chloromethyl)-1,1'-biphenyl. The
pore size distribution of POPs was evaluated using nonlocal density functional theory(NLDFT). POP-BP exhibited a wide pore size distribution from 1.40, 2.93, to 5.47 nm, andthe main pore size of POP-BP is at 1.40 nm, which proved its microporous nature. Due tothe introduction of a larger building unit of BPTM into the polymer network, the pore was7partially blocked, and the main pore size was centered at 1.09 to 1.36 nm for i-POP-BPBPTMs.

Fig. 3. Carbon dioxide capture of (a) POP-BP, (b) i-POP-BP-BPTM-1, (c) i-POP-BP-BPTM2, and (d) i-POP-BP-BPTM-3 measured at 298 K (red curve) and 273 K (black curve) under
1.0 bar.
The carbon dioxide uptake properties of the POPs were investigated at 298 K and 273
K (Fig. 3). The carbon dioxide isotherms of POP-BP showed a carbon dioxide uptake of
143 mg g-1 at 273 K and 1 bar. At the same condition, i-POP-BP-BPTM-1, i-POP-BPBPTM-2, and i-POP-BP-BPTM-3 displayed carbon dioxide uptake of 140, 136, and 131
mg g-1 at 273 K and 1 bar, respectively. With the increment of ionic units in the skeleton,
the carbon dioxide capture ability of i-POP-BPTMs decreased. This is because the ionic
units blocked the pores and reduced porosity. The isosteric heats of adsorption (Qst) for
carbon dioxide were estimated from the adsorption data collected at 273 K and 298 K. The
Qst of i-POPs was calculated to be 20-30 kJ mol−1 (Fig. S6). This indicated the physicaladsorption nature of CO2 for all the prepared polymers.

8

Fig. 4. (a) Iodine vapor capture profiles of the POP-BP (black curve), i-POP-BP-BPTM-1(sky curve), i-POP-BP-BPTM-2 (blue curve), and i-POP-BP-BPTM-3 (red curve) as afunction of exposure time at 350 K and ambient pressure. Inset: the photographs reveal
the color change of the samples of i-POP-BP-BPTM-3 before and after iodine adsorption.
Owing to their high BET surface area and pore volume, and abundant ionic sourcesof i-POP-BP-BPTMs, we investigated the iodine vapor capture property by exposing the iPOP polymers into iodine vapor at 350 K under ambient pressure, which are typical nuclear
fuel reprocessing conditions. As time progressed, i-POP samples showed the quick
adsorption towards iodine and steep increase within 6 h, and the maximum adsorption
capacity of i-POP polymers was reached relatively quickly (Fig. 4). No significant weightgain of the samples was observed after 12 h, which indicated the system was saturated.
The neutral POP-BP showed iodine uptake of 305 wt% at 24 h. With increment of ionic
units, the maximum adsorption capacity was greatly improved. For example, the saturated
iodine loading is up to 342 and 375 wt% for i-POP-BPTM-1 and i-POP-BPTM-2,
respectively. Interestingly, i-POP-BPTM-3 showed the highest capture ability of 415 wt%,
which is higher than most reported i-POPs (Tab. S3) such as PAF-25 (260 wt%)44 PAF-24(276 wt%),44 COP1++ (212 wt%),45 CalP4-Li (312 wt%),46 PHF-1-Ct (405 wt%),47 CTF-Cl-1(268 wt%).48 This value is comparable to various neutral POPs 49-66 such as NiP-CMP (202wt%),49 TPT-TAPB-COF (251 wt%),51 Tm-MTDAB (304 wt%),52 NRPP-2 (222 wt %),54
NOP-5 (202 wt%),59 NTP (180 wt%),61 BDP-CPP-1 (283 wt%),64 HFCMP (TTPB) (443wt%).669

Fig. 5. I 3d XPS spectra of (a) POP-BP@I2 and (b) i-POP-BPTMs@I2.
We investigated the mechanism of the iodine capture process by XPS results of POPs.
POP-PB@I2 showed only a valence of zero were detected, which confirmed neutral iodine
remained in the network (Fig. 5a). This indicated that a purely physical adsorption processof iodine in POP-BP. Meanwhile, both neutral I2 and anion I3– were observed in i-POPBPTMs@I2 complex (Fig. 5b), which indicate that some iodine molecules were transformed
into I3–. The improvement of iodine capture ability is attributed to ionic units on the walls of
networks. Increasing the ionic units of networks may produce electrostatic interactions
between the positively charged ionic groups and iodine molecules.33, 48

Fig. 6. Cycle performances of iodine capture for (a) POP-BP, (b) i-POP-BPTM-1, (c) i-POPBPTM-2, and (d) i-POP-BPTM-3.
10

We investigated the cycle performance for I2 capture. POPs@I2 samples weredispersed in ethanol and then filtered and washed by ethanol for several times until nocolor is in the solution. The regenerated POPs were tested for the next absorption. To oursurprise, all networks could be efficiently recycled and reused for five cycles without
significant loss of iodine uptake (Fig. 6). FT IR analysis results indicated that POPs
remained structure before and after I2 capture after five cycles (Fig. S7).

Conclusions
Several novel ionic porous organic polymers were successfully designed and constructedby using the commercial and simple-synthesized building units via the Friedel–Crafts
alkylation reaction, which is an easy and cost-effective method. The synthesized i-POPs
showed high BET surface area (1491-1753 m2 g–1) and pore volume (2.19-2.94 cm3 g–1),
which is much higher than that of all reported i-POPs. i-POPs improved the iodine capture
than neutral material through the electrostatic forces. Importantly, i-POPs also showed highiodine capture ability up to 415 wt% and five cycles of performance without losing ability.
We also highlight that this research opens a new way for the construction of ionic porous
organic polymers with low-price and simple-synthesis. Currently, the new design andsynthesis of functional i-POPs for energy, environment, and catalysis-related fields areunderway in our laboratory.

Experimental
Materials and methods

4,4'-Bis(chloromethyl)-1,1'-biphenyl,

trimethylamine,

iodine,hydrochloric

acid,

dimethylacetamide, tetrahydrofuran, acetone, methanol, and ethanol were obtained from
TCI and Wako. Anhydrous iron (III) chloride and 1,2-dichloroethane were bought from
Sigma-Aldrich.

1H

NMR spectra were recorded on a Varian Mercury-500 NMR

spectrometer, where chemical shifts (δ in ppm) were determined with a residual proton of11

the solvent as standard. Fourier transforms Infrared (FT IR) spectra were recorded from
650 to 4000 cm–1 on a Fourier-transform infrared spectrometer (Nicolet 6700; Thermo
Fisher Scientific Inc.). Elemental analyses were carried out on a CHNS element analysis
VarioEL3. Scanning electron microscope (SEM) was performed on HITACHI Miniscope
TM3030. Energy-dispersive X-ray spectroscopy (EDS) mapping was measured by
TM3030Plus miniscope. Powder X-ray diffraction (PXRD) data were recorded on fully
automatic horizontal multipurpose X-ray diffractometer (Rigaku Smartlab) (Cu Kα, λ =0.154 nm) by depositing powder on the glass substrate, from 2θ = 1.5° up to 30° with 0.02°
increment. Nitrogen sorption isotherms were measured at 77 K by BELSORP-Max. The
Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas.
By using the non-local density functional theory (NLDFT) model, the pore volume wasderived from the sorption curve.

Synthetic procedures

Synthesisof

1,1'-([1,1'-biphenyl]-4,4'-diyl)bis(N,N,N-trimethylmethanaminium)

chloride (BPTM): 4,4'-Bis(chloromethyl)-1,1'-biphenyl (BP) (2.5 g, 0.01 mmol) was
dissolved into dimethylacetamide (20 mL) at room temperature. Then, the trimethylamine(5.4 mL, 0.06 mmol) was added into the solution. The system was stirred for 48 hours atroom temperature. The mixture was poured into tetrahydrofuran 150 mL, and stirred at 10
mins. The precipitate was collected and washed by THF, and dried under vacuum at room
temperature overnight to afford white powder in 93% isolated yield. 1H NMR (400 MHz,D2O): δ 7.75-7.77 (d, 4H), 7.56-7.58 (d, 4H), 4.45 (s, 4H), 3.03 (s, 18H) ppm.
Synthesis of POP-BP: The POP-BP was synthesized using reported methods.11 The
12

bis(chloromethyl)-1,1'-biphenyl (70 mg, 0.28 mmol), anhydrous FeCl3 (91 mg, 0.56 mmol),and 1,2-dichloroethane 4 mL were added the flask. The mixture was stirred for 1 min,degassed through three freeze-pump-thaw cycles, and heated at 80 ºC for 48 h under Ar
atmosphere. The precipitate was collected and washed with water and acetone, andsoxhleted with methanol for 24 h. The initial polymer was stirred with HCl (1M 20 mL)
overnight, filtered and washed by water and acetone, and dried under vacuum at room
temperature for 24 hours to afford brown powder in 99% isolated yield.
Synthesis of i-POP-BP-BPTM-1: Bis(chloromethyl)-1,1'-biphenyl (70 mg, 0.28 mmol),
1,1'-([1,1'-biphenyl]-4,4'-diyl)bis(N,N,N-trimethylmethanaminium) chloride (BPTM) (51.5
mg, 0.14 mmol), anhydrous FeCl3 (91 mg, 0.56 mmol) and 1,2-dichloroethane 4 mL wereadded the flask. The mixture was stirred for 1 min, degassed through three freeze-pumpthaw cycles, and heated at 80 ºC for 48 h under Argon atmosphere. The precipitate wascollected and washed with a larger amount of water and acetone, and soxhleted withmethanol for 24 hours. The initial polymer was stirred by HCl 1M 20 mL overnight, filtered
and washed by water and acetone, and dried under vacuum at room temperature for 24hours to afford light-brown powder in 78% isolated yield. The synthesis method of i-POPBP-BPTM-2, and i-POP-BP-BPTM-3 was the same as the i-POP-BP-BPTM-1. The rationof BP and BPTM was 1:1 and 1:3 for i-POP-BP-BPTM-2, and i-POP-BP-BPTM-3,
respectively. (There is a larger amount molar ratio of BPTM than reactivity sites of BP toenhance the ionic units for i-POP-BP-BPTM-3).

Iodine vapor sorption

Three open vials (2 mL) with POP samples were placed in a large vial (20 mL) containing13

iodine (3 g). This system was sealed and kept in an oven at 350 K. After a period of time,
the system was cooled down to room temperature. The small vial containing the POP
samples were weighted and placed back into the system. Then, the system was put backin the oven at 350 K to continue the adsorption until the mass of the small vial containing
the POP samples did not change. The POPs@I2 samples were added to EtOH (10 mL) at
2 h. This operation was repeated until the solution showed no color changes. The POP
samples were filtered and washed with ethyl alcohol, dried under vacuum at room
temperature at 24 h and reused for the next cycle.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
Z. L. appreciates the support by JSPS KAKENHI grant number JP18J13699. Z. L. also
expresses gratitude to Prof. Shun Nishimura, Prof. Noriyoshi Matsumi, and Mr. Xianzhu
Zhong for nitrogen adsorption measurement and other experiments.

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