Highly efficient and reversible iodine capture using a metalloporphyrin- based conjugated microporous polymer

Article abstract of DOI:10.1039/c4cc01783h

A new metalloporphyrin-based conjugated microporous polymer, NiP-CMP, was constructed via a homo-coupling polymerization reaction. NiP-CMP possesses a high BET surface area of over 2600 m² g^-1, a large pore volume of 2.288 cm³ g^-1, good stability, and displays excellent guest uptake of 202 wt% in iodine vapour. We also highlight that the polymer exhibits outstanding performance for the reversible adsorption of iodine in solution. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01783h

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Highly efficient and reversible iodine capture
using a metalloporphyrin-based conjugated
microporous polymer†
Cite this: Chem. Commun., 2014,
50, 8495
Received 10th March 2014,
Accepted 9th May 2014
Sigen A,a Yuwei Zhang,a Zhongping Li,a Hong Xia,b Ming Xue,c Xiaoming Liu*a and
Ying Mua
DOI: 10.1039/c4cc01783h
www.rsc.org/chemcomm
A new metalloporphyrin-based conjugated microporous polymer, shadow on their practical application for the storage of iodine,
NiP-CMP, was constructed via a homo-coupling polymerization since the off-gas of fuel reprocessing plants contains high levels
reaction. NiP-CMP possesses a high BET surface area of over of water vapor. In this context, therefore, the development of a
g g
2600 m2 À1, a large pore volume of 2.288 cm3 À1, good stability, new class of outstanding porous sorbents with a large surface
and displays excellent guest uptake of 202 wt% in iodine vapour. area, large pore volume, and robust chemical and thermal
We also highlight that the polymer exhibits outstanding performance stability for capturing iodine is also an important challenge.
for the reversible adsorption of iodine in solution.
Over the past few years, covalently linked porous organic
polymers (POPs), including crystalline covalent organic frame-
The demand for clean and safe nuclear energy is growing worldwide works,5 triazine-based organic frameworks,6 amorphous hyper-
in order to meet the necessary energy requirements for the crosslinked polymers,7 conjugated microporous polymers,8
future without the emission of greenhouse gases. A main safety polymers of intrinsic microporosity,9 element organic frame-
concern associated with nuclear energy is appropriate nuclear works10 and porous aromatic frameworks11 have been constructed
waste management, due to the effects of radiation on health.1 from well-functionalized organic building blocks, representing a
129I is an important radioisotope in nuclear waste, and is new and fascinating class of nanoporous materials with excellent
particularly long-lived (half-life of 1.57 Â 107 years), and has stability, and they have demonstrated great potential application in
hence generated strong interest and concern. However, a leading energy related fields. Metalloporphyrin complexes are excellent
approach to the efficient capture and reliable storage of radio- catalysts for many molecular transformations and important
active gaseous byproducts such as iodine in nuclear effluents components of photoelectric materials. Generally, porous
remains challenging.2 Currently, inorganic solid adsorbents, materials based on metalloporphyrin building blocks are easily
such as silver-based zeolites, are used to capture radioactive functionalized, and have excellent thermal stability, which
iodine.3 However, these silver-loaded adsorbents have low is attributed to the hybrid properties of metalloporphyrin
adsorption capacities, owing to their limited available surface segments. Indeed, recent research results demonstrated that
areas. In particular, the main disadvantages of silver-based the metalloporphyrin-based POPs are excellent heterogeneous
zeolites are related to their high cost and the adverse environ- catalysts,12 gas adsorbents,13 and high-rate charge carrier
mental impact of silver. Crystalline metal–organic frameworks transporters.14 To date, however, the iodine adsorption and
(MOFs) have recently been exploited as a new type of sorbent for storage properties of covalently linked porous organic polymer
capturing iodine, and they exhibited high uptake capacity.4 But frameworks have not been explored. With these considerations
the poor water and moisture stability of most MOFs casts a in mind, in this contribution, we report herein the construction
of a metalloporphyrin-based conjugated microporous polymer
(NiP-CMP) utilizing nickel(II) 5,10,15,20-tetrakis(4-bromophenyl)-
porphyrin as a building block by a bottom-up strategy. NiP-CMP
a State Key Laboratory for Supramolecular Structure and Materials,
College of Chemistry, Jilin University, 2699 Qianjin Avenue, Changchun, 130012,
possesses a unique microporous character with a large surface
area, large pore volume, ideal chemical and thermal stability
and a large adsorption capacity for iodine vapour. As expected,
the resultant polymer framework also exhibits an exceptional
performance for the reversible adsorption of iodine.
P.R. China. E-mail: xm_liu@jlu.edu.cn
b State Key Laboratory on Integrated Optoelectronics, College of Electronic Science
and Technology, Jilin University, Changchun, 130012, P.R. China
c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, Changchun, 130012, P.R. China
† Electronic supplementary information (ESI) available: Synthesis of monomer
and polymer, FT-IR, PXRD, TGA, UV-Vis, SEM, TEM and 13C CP/MAS NMR of
NiP-CMP and sorption kinetics. See DOI: 10.1039/c4cc01783h
NiP-CMP was prepared in high yields by the well-established
Yamamoto homocoupling polycondensation reaction (Scheme 1).11
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Fig. 1 Nitrogen adsorption/desorption isotherms (filled circles: adsorp-
tion, open circles: desorption) (a) and pore size distribution (b) of NiP-CMP.
Scheme 1 Synthetic route towards NiP-CMP using nickel(II) 5,10,15,20-
tetrakis(4-bromophenyl)porphyrin as a building block.
the presence of small particles. In addition, a hysteresis can be
NiP-CMP is a red-brown powder with a low density and is observed at low relative pressures for NiP-CMP based on the
insoluble in water and common organic solvents. It is chemically isotherms; this phenomenon is associated with the irreversible
stable, even upon exposure to dilute solutions of acid and base, uptake of gas molecules in the pores. It probably means a
suggesting a highly cross-linked and robust structure. The NiP-CMP swelling in the flexible framework at 77 K due to nitrogen.10a,15
was characterized by infrared spectroscopy (IR), 13C cross Derived from the N2 adsorption data, NiP-CMP has a BET specific
polarization magic angle spinning (CP/MAS) NMR, thermo- surface area of 2630 cm2 gÀ1. To the best of our knowledge, this
gravimetric analysis (TGA), powder X-ray diffraction (PXRD), surface area is the highest in metalloporphyrin-based organic
scanning electron microscopy (SEM) and transmission electron porous materials that has been reported, and even competitive
microscopy (TEM).
with the best reported metalloporphyrin-based MOFs (Table S1,
NiP-CMP is stable under a nitrogen atmosphere and no ESI†). The NiP-CMP has a wide pore size distribution, with the
mass loss is observed upon heating up to 300 1C by TGA pore widths centering around 0.70, 0.85, 0.95 and 1.25 nm,
(Fig. S1, ESI†). It gradually decomposed at elevated temperatures, respectively, as calculated by the Saito–Flory method (Fig. 1b).
but it also retained 85% of its initial mass at 700 1C. SEM images The total volume calculated with nitrogen gas adsorbed at
displayed that the polymer adopts a spherical shape with sizes of P/P0 = 0.99 was 2.288 cm3 gÀ1
50–100 nm (Fig. S2a, ESI†). TEM revealed the homogeneous
.
The NiP-CMP has excellent porous character and remark-
distribution of nanometer-scale pores in the structure (Fig. S2b, able stability, and these features prompted us to examine its
ESI†). PXRD profiles did not show any strong signals, indicating adsorption properties for iodine. The samples of NiP-CMP
that NiP-CMP is an almost amorphous polymer (Fig. S3, ESI†). powder were placed in a sealed vessel in the presence of solid
The success of the phenyl–phenyl coupling was confirmed by iodine. The iodine sublimed into the porous absorbent over
FT-IR measurements. Compared with the monomer, the peak time at 350 K and ambient pressure, which are typical fuel
at 481 cmÀ1 of the C–Br band vibration modes of the starting reprocessing conditions. As time progressed, the color of the
material almost disappeared after polymerization. In addition, samples changed from red-brown to almost black. Gravimetric
the bands at 3029 cmÀ1, 1606 cmÀ1 and 1492 cmÀ1 are measurements were taken at various time intervals during
associated with the C–H stretch in phenyls, the CQC stretch the iodine uptake, and the results showed that the mass of
in phenyls and CQC stretch in the porphyrin, respectively. The NiP-CMP samples increased significantly with prolonged contact
peak at 1013 cmÀ1 can be ascribed to the Ni–N stretch (Fig. S4, time. The maximum adsorption capacity of NiP-CMP was
ESI†). Furthermore, the structure of NiP-CMP was characterized reached relatively quickly. No further change in the iodine
at the molecular level by solid state 13C CP/MAS NMR (Fig. S5, loading amounts was observed after 24 h, indicating that the
ESI†). In addition, elemental analysis confirmed that the weight system was basically saturated (Fig. 2a). Pleasingly, the iodine
percentages of C, H, and N were close to the theoretical values
of its infinite 2D polymer. The electronic adsorption spectro-
scopy of NiP-CMP also showed that the peak features of the
metalloporphyrin building block were retained and a broad
Soret band appeared at 448 nm, which is red-shifted by 24 nm
from that of the monomer (Fig. S6, ESI†).
In order to characterize the porosity parameters of NiP-CMP,
the nitrogen sorption isotherms were measured at 77 K. As
shown in Fig. 1a, NiP-CMP exhibits a combination of type-I and II
nitrogen sorption isotherm features, according to the IUPAC
classification, and shows a steep nitrogen gas uptake at low
relative pressures, thus reflecting the microporous nature of
Fig. 2 (a) Gravimetric uptake of iodine as a function of time at 350 K.
the network. The nitrogen sorption increases with increasing
pressure, which indicates a large external surface area owing to
Inset: the photographs reveal the color change of the samples of NiP-CMP
before and after iodine adsorption. (b) TGA trace of I2@NiP-CMP.
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loading of 202 wt% is higher than previously reported values
The adsorption isotherm is a significant factor in determining
of iodine uptake for porous adsorbents (Table S2, ESI†).3,4,16 the saturated adsorption capacity when the equilibrium is
Compared with NiP-CMP, no significant spectroscopic shifts attained. As shown in Fig. 3b, the adsorption plot of equilibrium
were observed for the iodine-loaded polymer I2@NiP-CMP in concentration versus adsorption capacity indicated two processes.
Fourier transform infrared spectroscopy (Fig. S7, ESI†). X-ray In one, the adsorption was a function of the initial concentration.
photoelectron spectroscopy (XPS) of I2@NiP-CMP indicated that Next, the adsorption reached the maximum uptake, becoming a
the valence of the encapsulated iodine species is zero, which concentration-independent process. From the kinetic studies,
confirmed that the included iodine species exists as I2 (Fig. S8, NiP-CMP presents a high sorption capacity with an iodine uptake
ESI†). The thermogravimetric analysis (TGA) graph of I2@NiP-CMP of 326 mg gÀ1. Indeed, the same mass (85 mg) of different
displayed a broad mass loss step from 90 to 300 1C (Fig. 2b), and adsorbents was immersed in a 3 mL cyclohexane solution of iodine
the calculated iodine mass loss was 194 wt%, which is close to with the same concentration of 2 mg mLÀ1. Fig. 3c displays that the
the saturated adsorption value. Pleasingly, the recovered polymer uptake of iodine on NiP-CMP is remarkable and clearly exceeds that
was washed with ethanol to remove the adsorbed iodine, of activated carbon and zeolite 13X, and quantitative results for
dried in vacuum, and reused in the next round of adsorption. iodine adsorption are given in Table S4 (ESI†). The high uptake
The NiP-CMP can be efficiently recycled and reused for five capacity of NiP-CMP for iodine may be attributed to its excellent
cycles without significant loss of iodine uptake (Fig. S9, ESI†). porosity; additionally, the conjugated p-electron framework may
In addition, the NiP-CMP is capable of trapping iodine in result in a weak interaction with iodine, thereby increasing the
solution. The adsorption kinetics of iodine at 25 1C were amount of iodine adsorbed.4d,f In addition, different models have
presented in Fig. 3a. It can be seen that two stages of adsorption been considered for the sorption isotherm curve (Fig. S14 and Table
kinetics were obtained: the adsorption capacity for iodine S5, ESI†). The Langmuir equation well fits the sorption curve,
increased quickly during the first 20 h, and after that a slow suggesting a monolayer adsorption of iodine on the polymer surface.
increase was observed until equilibrium was reached. Removal
Notably, this iodine sorption of NiP-CMP is reversible.
efficiencies of 56 and 48% were achieved for iodine solutions of The encapsulated iodine could be easily removed from the
2 and 6 mg mLÀ1, respectively, which were comparable with that framework upon immersion of I2@NiP-CMP in organic solvents.
of functionalized MIL-53-NH2.17 The adsorption over NiP-CMP The iodine-loaded samples of NiP-CMP were soaked in fresh
was analyzed using Lagergren pseudo-first-order and pseudo- ethanol, the color of the polymer changed gradually from dark to
second-order kinetic models.18 For the pseudo-second-order red-brown and, correspondingly, the color of the ethanol
kinetic models, good linear fits with correlation coefficient R2 solution deepened from colorless to purple, which clearly
values of greater than 0.999 and 0.926 were achieved for iodine indicates that the guests are dissociated from the NiP-CMP
solutions of 2 and 6 mg mLÀ1 (Table S3, ESI†), respectively. This framework (Fig. S15, ESI†). To investigate the kinetics of iodine
suggested that the present adsorption system can be represented delivery by NiP-CMP, the UV/Vis spectra were recorded at 25 1C.
by the pseudo-second-order kinetic model of the adsorption The absorbance of iodine in ethanol increased linearly with
data. Indeed, the samples of polymer were then immersed in a treatment time (Fig. S16, ESI†). Typically, 15 mg of the
cyclohexane solution of iodine at 25 1C, and the color of the I2@NiP-CMP samples were soaked in 1 mL ethanol. The iodine
polymer gradually changed from red-brown to dark brown. release rate becomes slower after 2 h owing to the fact that the
Moreover, the color of the solution of iodine in cyclohexane concentration of iodine in ethanol increased with time. After
became paler.
8 h, however, about 96% of the initial iodine had been released
from I2@NiP-CMP, indicating the reversibility of the iodine
adsorption process. In addition, NiP-CMP, after reuse five
times, also retains a large surface area and a large pore volume
of over 2100 cm2 gÀ1 and 2.0 cm3 gÀ1 (Fig. S17, ESI†), respectively,
which is attributed to its highly skeletal structure.
In summary, a metalloporphyrin-based porous organic polymer
framework NiP-CMP was successfully designed and synthesized
by a bottom-up strategy. The new polymer has an excellent
porosity with a large BET surface area of over 2600 m2 gÀ1, a
large pore volume and displays the highest reported value of
iodine uptake for porous adsorbents to date. The absorbed
iodine could be recovered, and the NiP-CMP sorbent was
easily recycled for reuse. In addition, we also highlight that
the new polymer exhibits excellent performance in the rever-
sible adsorption of iodine in solution. Currently, the design and
construction of new functional POPs for energy, environment
and catalytic-related applications is underway in our laboratory.
This work was supported by the National Natural Science
Foundation of China (No. 51203058, 51173061 and 21074043).
Fig. 3 (a) Kinetic studies of iodine adsorption by NiP-CMP in cyclohexane
solutions. (b) Adsorption isotherm of iodine onto NiP-CMP. 30 mg
NiP-CMP was stirred in 3 mL of solution for 48 h. Curve: Langmuir. (c)
The photographs display the different inclusion rates and extents for
zeolite 13X (Z-13X), activated carbon (AC) and NiP-CMP (CMP).
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