Iodine capture in porous organic polymers and metal–organic frameworks materials

Abstract

The enrichment of radioactive iodine in the waste of nuclear industries threatens human health, and thus the efficient capture of iodine has attracted a great deal of attention in recent years. Porous organic polymers (POPs) and metal–organic frameworks (MOFs), new classes of porous materials, act as outstanding candidate adsorbent materials in this field due to their high surface areas, permanent tunable porosities, controllable structures, high thermal/chemical stabilities, versatility in molecular design and potential for post-synthetic modification. Herein, this review focuses on the research progress of these two types of porous materials for highly efficient iodine capture. We analyze and discuss some valid strategies for enhancing their iodine uptake, including increasing their surface area and pore volume, using organic building units with unique configurations and functions, introducing chemical functional groups to provide high-enthalpy binding sites, and further processing of POP and MOF materials. Indeed, there are many special structural and functional features found in porous POP and MOF materials, which make them unique and merit further exploration. Thus, we expect to see their usage grow as this field progresses.

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Wei Xie

Wei Xie is a Lecturer at the Key Laboratory of Preparation and Applications of Environmental Friendly Materials in Jilin Normal University. She received her Bachelor's degree in Chemical Engineering of Changchun University of Science and Technology in 2012. In the same year, she joined in Professor Zhongmin Su's group and studied in the Institute of Functional Material Chemistry of Northeast Normal University. She obtained her PhD degree in 2017. Her current scientific interests are focused on the design, synthesis and functions of metal organic frameworks (MOFs) and MOFs-based composite materials.

Di Cui

Di Cui is a Master's student at the School of Jilin Normal University. She received her Bachelor's degree in Chemical Education in Jilin Normal University in 2016. She is now pursuing a Master's degree in Professor Yanhong Xu's group. Her current scientific interests are focused on the design, synthesis and functions of CMPs and organic porous materials.

Shu-Ran Zhang

Shuran Zhang is a Teacher at the Key Laboratory of Preparation and Applications of Environmental Friendly Materials in Jilin Normal University. She received her Bachelor's degree in Chemical Education of Harbin Normal University in 2011. In the same year, she began her studies at the Institute of Functional Material Chemistry of Northeast Normal University and received her PhD degree under the supervision of Professor Zhongmin Su in 2016. Her current scientific interests are focused on the design, synthesis and functions of MOFs.

Yan-Hong Xu

Yanhong Xu received her Bachelor's degree in Harbin Normal University in 2005 and Master's degree in Northeast Normal University in 2009. In 2012, she obtained her PhD degree from Donglin Jiang's group. She then was a postdoctoral researcher until 2014 at the Institute for Molecular Science for Advanced Studies (Japan) under the supervision of Professor Donglin Jiang. In 2014, She joined Jilin Normal University as a Professor and set up an independent laboratory. She was selected as a Changbai Mountain scholar. Her current scientific interests are focused on the design, synthesis and functions of porous organic frameworks materials.

Dong-Lin Jiang

Donglin Jiang received his PhD degree from the University of Tokyo in 1998. In 2005, he took a concurrent associate professorship at the Graduate University for Advanced Studies, Japan, where he started his independent laboratory and research in covalent organic frameworks. In 2016, he was promoted to a full professor at Japan Advanced Institute of Science and Technology. From February 2018, he was appointed the strategic hiring professor at National University of Singapore. His career work has been recognized by awards including the 34th Chemical Society of Japan Award for Creative Works (2017), the Young Scientists' Prize, and so on.

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1. Introduction

With the rapid increase in energy demand, nuclear power, as a green and reliable energy source, has long drawn increasing attention around the world.^1,2 However, radioiodine in the waste of nuclear industries prevents its promotion and poses an exceptional challenge because it is a highly flowing gas, can easily dissolve in water, and has negative impacts on organisms.^3,4 Among the radioisotopes of iodine, ¹²⁹I has a long half-life (1.57 × 10⁷ years)⁵ and ¹³¹I isotope is short-lived (half-life of 8.02 days),⁶ but they both directly affect human metabolic processes due to bioaccumulation through the food chain.^7,8 As is known, the Chernobyl and Fukushima nuclear power disasters involved in radioisotopes of ¹³¹I and ¹²⁹I, which caused thyroid injury and were carcinogenic, highlighting the need for materials and technology to capture radioactive iodine.^9,10 Therefore, dealing with iodine promptly and effectively has become an urgent problem to be solved.

Accordingly, some adsorbents for iodine have been studied in the last few decades, including silica,¹¹ chalcogenide aerogels,¹² activated carbon, zeolites,^13,14 and microporous polymers.^15,16 Presently, the traditional industrial method of capturing iodine is to impregnate an organic amine (such as triethyl diamine) on a porous material (such as activated carbon).¹⁷ The resulting adsorbent has a high affinity for radioactive iodine and purification effect, and the process to capture iodine is relatively simple; however, it has the following shortcomings: (i) sublimation of organic amine occurs easily, leading to a significant reduction in the iodine absorption efficiency of impregnated carbon materials; (ii) iodine adsorption is an exothermic process, and organic amines reduce the ignition point of impregnated carbon, exacerbating the potential safety hazard; and (iii) impregnated carbon is prone to aging and weathering, has poor regeneration capacity, is difficult to re-use, and easily causes the captured iodine to desorb and cause secondary pollution. Therefore, the exploration and development of adsorbent materials with iodine binding ability, high iodine adsorption, and excellent thermal stability have important academic value and application prospects.

Recently, solid porous materials as iodine sorbents have proven to be superior due to their easy operation, low energy consumption, fast speed and recycling. Porous organic polymers (POPs)^18–20 and metal–organic frameworks (MOFs)^21–23 have been studied as excellent candidates for iodine capture. POPs are constructed from various organic building blocks by strong covalent bonds. In contrast, MOFs are comprised of inorganic building units (metal ions or clusters) and organic linkers connected by coordination bonds of moderate strength. POPs and MOFs are new classes within the versatile family of porous materials, which can be synthesized from a wide range of organic monomers using many different chemistries and can incorporate a variety of chemical functionalities. POP and MOF materials possess versatility in molecular design, high surface areas, tunable pore sizes and volumes, adjustable skeletons, good physicochemical robustness, potential for post-synthetic modification (PSM), etc. Therefore, POPs and MOFs provide outstanding platforms for iodine uptake due to their advantages.

This review mainly focuses on the recent advances in the implementation of iodine capture on two types of solid adsorbents, -POP and MOF materials. Although there have been several reviews about the adsorption of iodine on porous solid sorbents,^24–27 POPs and MOFs were simply summarized as two types of adsorbents. However, numerous POPs and MOFs with high iodine uptake have been reported. Based on this, in this tutorial review, we will summarize in detail the current progress of porous POP and MOF materials for iodine adsorption, discuss the effective strategies for increasing their iodine capture capacity, and analyze issues and challenges of porous materials for their application in iodine capture. To some extent, we hope our this work will help researchers design and develop porous adsorbent materials.

2. Iodine capture in porous organic polymers (POPs) materials

As a class of functional porous materials, from the view of structure, POPs can be divided into five categories, including conjugated microporous polymers (CMPs),^28–30 polymers of intrinsic microporous (PIMs),^31–33 hyper-crosslinked polymers (HCPs),^34–37 covalent organic frameworks (COFs),^38–42 and porous aromatic frameworks (PAFs).^43,44 The unique nature of POPs gives them potential applications in the fields of guest adsorption and separation, heterogeneous catalysis, sensing, photo-electricity, energy storage, etc.^45–48 More recently, plenty of research efforts have been devoted to designing POPs for ultrahigh iodine capture. In contrast to the traditional inorganic porous materials, besides large specific surface areas and high porosities, POP materials possess the following unique advantages: (i) wide diversity of POP networks, where the structures and properties of the organic building blocks used to construct POPs are diverse, and their synthetic conditions and other options can be regulated, which easily facilitate the realization of pre- or post-functionalization of POPs; (ii) through the formation of strong covalent bonds, POPs have good thermal/chemical stability; and (iii) POPs are constructed using light elements (such as C, H, N, O, and B), and thus their skeletons densities are very low. To the best of our knowledge, to date, HCPs and PIMs have not been reported for iodine capture. In this section, we discuss in detail the performances of different types of POPs (CMPs, COFs, and PAFs) for iodine adsorption. The relevant structural characteristics, pore properties, and iodine adsorption capacities of selected POPs are summarized and tabulated in Table 1.

Table 1 Iodine adsorption capacities in selected POP and MOF materials

Materials	BET surface area (m^2 g^−1)	Pore size (Å)	Pore volume (cm^3 g^−1)	Iodine uptake (wt%)	Ref.
Note: test conditions:a Iodine vapor adsorption, 75–80 °C, ambient pressure.b Iodine vapor adsorption, 358.15 K, 1.0 bar.c Iodine vapor adsorption, 60 °C, ambient pressure.d Iodine vapor adsorption, 343.3 K, ambient pressure.e Iodine vapor adsorption, room temperature, ambient pressure.f Iodine solution of cyclohexane adsorption, room temperature, ambient pressure.
TPB-DMTP	1927	33	1.28	620^a	98
TPT-BD-COF	—	—	—	543^a	99
TTA-TTB	1733	22	1.01	500^a	98
TTPPA	512.39	19.1	0.2997	490^a	80
SIOC-COF-7	618	11.8	0.41	481^a	96
CalPOF-1	303	—	—	477^a	70
COF-DL229	1762	14	0.64	470^a	97
ETTA-TPA	1822	14, 27	0.95	470^a	98
TPT-DHBD25-COF	—	—	—	465^a	99
TTPB	222	19.4	0.127	443^a	15
NDB-H	116.93	74.6	0.13	443^a	79
TPT-DHBD50-COF	—	—	—	430^a	99
NDB-S	56.45	75.5	0.11	425^a	79
TPT-DHBD75COF	—	—	—	412^a	99
CalPOF-2	154	—	—	406^a	70
TPT-DHBD-COF	—	—	—	388^a	99
POP-2	41	20, 400	—	382^a	73
COP^01	—	—	—	380^c	90
TFBCz-PDA	1441	15	0.74	370^a	98
POP-1	12	20, 400	—	357^a	73
CalPOF-3	91	—	—	353^a	70
ADB-HS	148.20	62.8	0.14	345^a	79
SCMP-II	119.76	20	—	345^a	86
ADB-S	41.53	62.9	0.07	342^a	79
HCMP-3	82	<10	0.08	336^b	72
TTDAB	1.643	17.6–27.0	0.004	313^a	80
CalP4_Li	445	—	0.588	312^a	91
Tm-MTDAB	2.778	27.9	0.007	304^a	80
HCMP-1	430	<10	0.22	291^b	72
AzoPPN	400	5.8–12.7	0.68	290^a	69
HCOF-1	—	—	—	290^a	100
BDP-CPP-1	635	49	0.78	283^a	89
HCMP-2	153	<10	0.06	281^b	72
Zr6O4(OH)4(peb)6	2650	14.2	1.16	279^e	159
COP^02	—	—	—	277^c	90
PAF-24	—	—	—	276^a	104
PAF-23	—	—	—	271^a	104
TTA-TFB	1163	16	0.55	270^a	98
NAPOP-4	626	12.7	0.15, 1.17	265^a	76
PAF-25	—	—	—	260^a	104
COP2^++	—	—	—	258^c	90
CalP3_Li	308	—	0.558	248^a	91
NAPOP-3	702	<4.3, 5.8	0.18, 1.01	241^a	76
NAPOP-2	458	<4.2, 5.9	0.10, 0.78	239^a	76
Azo-Trip	510.4	6.5	0.47	238^a	71
BDP-CPP-2	235	30	0.18	223^a	89
SCMP-2	855	—	1.50	222^a	16
HCMP-4	—	<10	—	222^b	72
NRPP-2	1028	7.0	0.81	222^a	77
CalP4	759	—	1.08	220^a	91
COP1^++	—	—	—	212^c	90
COP2^•+	—	—	—	211^c	90
FCMP-600@2	780	17.4	0.665	209^a	88
CMPN-3	1368	∼20	2.36	208^d	92
NAPOP-1	657	<3.9, 7.2	0.25, 1.49	206^a	76
NiP-CMP	2630	7.0–12.5	2.288	202^a	67
HKUST-1	1850	5.0–13.5	0.74	175^a	118
[(ZnI2)3(tpt)2]	—	—	—	173^e	205
MFM-300(Sc)	1250	8.1	0.50	154^a	164
HMTI-1	—	18 × 21	—	150^f	153
MFM-300(Fe)	1192	7.8	0.46	129^a	164
ZIF-8	1630	3.4–11.6	0.66	125^a	115
UiO-66-PYDC	1030	—	0.43	125^f	170
MFM-300(In)	1050	7.6	0.41	116^a	164
Zr6O4(OH)4(sdc)6	2900	11.9	1.33	107^e	159
Zn3(DL-lac)2(pybz)2	762.5	10.5	0.40	101^f	123
HMTI-2	—	4 × 18	—	100^f	154

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2.1 CMPs for iodine capture

Since Copper's group first reported CMPs in 2007,⁴⁹ a variety of CMPs with different functionalities have been published. CMPs are generally formed by the interconnection of multiple carbon–carbon bonds or aromatic benzene rings and often have large conjugated structures. Due to the rigidity of their conjugated units at the molecular level and the expansion of their conjugated structures, the CMP network skeletons can effectively support porous channels and form inherent pores.^50–53 CMP materials are commonly prepared via a series of polycondensation reactions, including the Suzuki cross-coupling reaction,^54,55 Yamamoto cross-coupling reaction,^56,57 Sonogashira–Hagihara cross-coupling reaction,^58,59 oxidative coupling reaction,^60–62 Schiff-base reaction,^63–65 Friedel–Crafts reaction,⁵⁰ and phenazine ring fusion reaction.⁶⁶ The inherent porosity, extended π-conjugated skeletons, higher surface areas and larger pore volumes make CMPs a great platform as iodine absorbents. Herein, we analyzed the structures of CMPs and illustrate their effect on the capacities of iodine adsorption of the diverse CMPs.

The building blocks and linking units directly affect the structures, compositions, and porosities of CMPs. From the perspective of the core of the skeleton structure, porphyrin-based CMPs facilitate strong interactions between the host adsorbent and guest. For example, Liu and co-workers first reported a metalloporphyrin-based NiP-CMP for the capture of iodine,⁶⁷ which was constructed by utilizing nickel(II) 5,10,15,20-tetrakis(4-bromophenyl) porphyrin as a building block via Yamamoto cross-coupling polymerization (Fig. 1a). NiP-CMP possessed a high BET (Brunauer Emmett Teller) surface area of 2630 m² g⁻¹ and large pore volume of 2.288 cm³ g⁻¹. It exhibited a wide pore size distribution, with pore widths centered at around 0.70, 0.85, 0.95 and 1.25 nm. NiP-CMP could capture 202 wt% iodine vapor, and also capable of trapping iodine in solution. In addition, the iodine loaded NiP-CMP could be recovered by immersing the material in ethanol to remove the adsorbed iodine. Subsequently, another Por-Py-CMP was prepared, in which the porphyrin acted the as core and pyrene units as linkers (Fig. 1b).⁶⁸ Por-Py-CMP had a BET surface area of 1014 m² g⁻¹ and pore volume of 0.81 cm³ g⁻¹, and showed capacity for iodine capture (130 wt%). Thus, the results indicated that CMPs with porphyrin units can effectively absorb iodine molecules in vapor and solution conditions.

Fig. 1 Synthesis of (a) NiP-CMP and (b) Por-Py-CMP.

Inspired by the structural design of strategic molecule manipulation, Han and co-workers designed and prepared a porous azo-bridged CMP containing a large number of porphyrin groups and phthalocyanine units (AzoPPN) (Fig. 2a).⁶⁹ The unique π-conjugated systems of porphyrin and phthalocyanine allowed the enrichment of π-electrons clouds, which make them suitable hosts for the adsorption of iodine guests. AzoPPN had a BET specific surface area of 400 m² g⁻¹ with a wide pore-size distribution of 0.58–1.27 nm and pore volume of 0.68 cm³ g⁻¹. Compared with NiP-CMP (S_BET = 2630 m² g⁻¹ and pore volume = 2.288 cm³ g⁻¹), AzoPPN had a much lower surface area and pore volume; however, it exhibited a prominent adsorption capacity of 290 wt% towards iodine vapor. The high I₂ capture of AzoPPN may be attributed to its azo-linkage and π-electron conjugated aromatic framework with plenty of porphyrin and phthalocyanine units, which could afford high affinity towards iodine. Additionally, X-ray photoelectron spectroscopy measurements indicated the process of adsorbing iodine by AzoPPN is a hybrid of physisorption and chemisorption processes, that is, it captured both iodine and triiodide ions. The above results suggest that incorporating macrocyclic molecules in CMPs including porphyrin and phthalocyanine provides a new approach for designing effective absorbents for volatile iodine.

Fig. 2 Synthesis of (a) AzoPPN, (b) CalPOFs and (c) Azo-Trip.

Considering the performance of AzoPPN, azo-linked CMPs may show potential applications for the capture of iodine. For example, by changing the building units and the reaction conditions, azo-bridged calix[4]resorcinarene and triptycene-based CMPs, CalPOFs (Fig. 2b)⁷⁰ and Azo-Trip⁷¹ (Fig. 2c) have been reported. The porous properties of CalPOFs could be adjusted by varying the alkyl chain lengths. With an increase in the alkyl chain length from methyl and ethyl to propyl, the BET surface area varied from 303 and 154 to 91 m² g⁻¹, respectively. Among these CalPOFs polymers, CalPOFs with a methyl chain exhibited the highest iodine vapor uptake of up to 477 wt%. In addition, CalPOFs with ethyl and propyl moieties captured iodine with a capacity of 406 wt% and 353 wt%. On the other hand, the Azo-Trip polymer network showed a BET surface area, pore size, and pore volume of 510.4 m² g⁻¹, 0.65–5.0 nm, and 0.47 cm³ g⁻¹, respectively. Also, Azo-Trip displayed an efficient guest uptake of 238 wt% iodine vapor. The high capacity of azo-bridged CMPs for iodine may be due to their numerous effective sorption sites including azo (–N=N–) groups, macrocyclic π-rich cavities and high affinity of I₂ to the host network, which favour the enhancement of their interactions with iodine and possible exchange of electrons to a stable state.

Apart from azo-linkers, nitrogen-containing electron-donating –NH– functional groups have been explored for the assembly of CMPs and iodine uptake. Liao et al. designed a series of hexaphenylbenzene-based CMPs, that is, HCMP-1, 2, 3, and 4 (Fig. 3a), which were synthesized via Buchwald–Hartwig cross-coupling of a hexakis(4-bromophenyl) benzene core and aryl diamine linkers.⁷² Although these three-dimensional (3D) imino-linked HCMPs possessed low BET surface areas (S_HCMP-1 = 430 m² g⁻¹, S_HCMP-2 = 153 m² g⁻¹, S_HCMP-3 = 82 m² g⁻¹, and S_HCMP-4 = na) and pore volumes (V_HCMP-1 = 0.22 cm³ g⁻¹, V_HCMP-2 = 0.06 cm³ g⁻¹, V_HCMP-3 = 0.08 cm³ g⁻¹, and V_HCMP-4 = na), they exhibited high iodine affinity with uptake capacities of up to 291, 281, 336, 222 wt%, respectively. Interestingly, the iodine uptake capacities of these HCMPs increased with the increase in number of –NH– groups, which may be attributed to the stronger charge transfer interactions between the –NH– units and the oxidizing iodine. In addition, the two other –NH-containing CMPs, POP-1 and POP-2 reported via the same Buchwald–Hartwig cross-coupling reaction captured iodine vapor of 357 and 382 wt% (Fig. 3b), respectively.⁷³

Fig. 3 Synthesis of –NH-containing HCMP-1,2,3,4, POP-1 and POP-2.

Thomas and Faul⁷⁴ successfully constructed a series of polyaniline-like porous materials, NCMP1, NCMP2 and NCMP3 (Fig. 4), via the facile FeCl₃-catalyzed oxidation polymerization of multi-connected aniline precursors. The NCMPs with high N contents (7.39–11.84 wt%) showed BET surface areas and iodine vapor uptake of 58 m² g⁻¹ and 215 wt% for NCMP1, 280 m² g⁻¹ and 186 wt% for NCMP2, 485 m² g⁻¹ and 161 wt% for NCMP3, respectively. Compared to NCMP2 and NCMP3, NCMP1 with the highest nitrogen content had the highest iodine capture capacity among them. Additionally, our group constructed a series of –NH-containing pore-tunable porous materials (NT-POPs),⁷⁵ and then NT-POPs were further pyrolyzed under nitrogen conditions at 800 °C to form porous materials (NT-POP@800). NT-POP@800 possessed moderate BET surface areas in the range of 475–736 m² g⁻¹ and displayed relatively rapid guest uptake of 56–192 wt% iodine vapor.

Fig. 4 Synthesis of NCMP1–3.

Zhang and co-workers⁷⁶ reported four aminal-linked CMPs (NAPOP-1, 2, 3 and 4), which were synthesized through the Schiff base condensation of 1,4-bis(4,6-diamino-s-triazin-2-yl) benzene with four types of aldehydes substituted with different N-heterocyclic groups (Fig. 5a). The BET surface areas and pore volumes of NAPOP-1, 2, 3 and 4 were 657 m² g⁻¹ and 1.74 cm³ g⁻¹, 458 m² g⁻¹ and 0.88 cm³ g⁻¹, 702 m² g⁻¹ and 1.19 cm³ g⁻¹, 626 m² g⁻¹ and 1.32 cm³ g⁻¹, respectively. Also, their pore size distributions ranged from 0.39 to 20 nm and their capture iodine uptake was about 206, 239, 241, and 265 wt%, respectively. The NAPOP polymers possessed moderate BET surface areas, but they showed a higher iodine capture capacity than that of CMPs with larger surface areas and pore volumes, such as NiP-CMP (S_BET = 2630 and 202 wt%)⁶⁷ and Por-Py-CMP (S_BET = 1014 and 130 wt%).⁶⁸ Based on the same core building block (1,4-bis-(2,4-diamino-1,3,5-triazine)-benzene), the other two polymers NRPP-1 and NRPP-2⁷⁷ were produced. The BET surface areas and total pore volumes were 1579 m² g⁻¹ and 0.91 cm³ g⁻¹ for NRPP-1 and 1028 m² g⁻¹ and 0.81 cm³ g⁻¹ for NRPP-2, respectively. NRPP-1 and NRPP-2 had an adsorption capacity for iodine of 192 wt% and 222 wt%, respectively.

Fig. 5 Synthesis of (a) NAPOP-1, 2, 3, and 4, and (b) pha-H_COP-1.

Furthermore, pha-H_cOP-1⁷⁸ (Fig. 5b) was synthesized via a mixed-linker approach, which made it possible to accommodate the –CO–NH– functionality to enhance the binding affinity between the adsorbents and iodine molecules. Its BET surface area, total pore volume and pore size were 217 m² g⁻¹, 1.048 cm³ g⁻¹ and 2.45 nm, respectively. It could capture iodine of 131 wt% at 353 K and ambient pressure. Compared with Por-Py-CMP (S_BET = 1014 m² g⁻¹ and V_total = 0.81 cm³ g⁻¹),⁶⁸ pha-H_cOP-1 possessed a lower BET surface area and similar pore volume, but it showed an equal adsorption capacity. This result further demonstrates that the specific surface area is not the only factor determining the iodine absorption capacity. Additionally, four other amorphous POPs (NDB-H, NDB-S, ADB-HS and ADB-S) based on Schiff-base chemistry were obtained under different solvothermal conditions.⁷⁹ It is noteworthy that excellent efficiency for removing iodine vapor was observed for NDB-S (∼425 wt%), NDB-H (∼443 wt%), ADB-HS (∼345 wt%) and ADB-S (∼342 wt%).

Geng and co-workers also designed a series of nitrogen-rich CMPs (TTPPA, TTPB, TTDAB, and Tm-MTDAB) (Fig. 6),^15,80 which were synthesized via Friedel–Crafts polymerization reaction of 2,4,6-trichloro-1,3,5-triazine (TCT) and derivatives of phenylamine (N,N,N′,N′-tetraphenylbenzidine (TPB), N,N,N′,N′-tetraphenyl-1,4-phenylenediamine (TPPA), 1,3,5-tris(diphenylamino)benzene (TDAB) and 1,3,5-tris[(3-methylphenyl)-phenylamino]benzene (MTDAB)), respectively, catalyzed by methanesulfonic acid. This reaction has the advantages of low cost, low volatility, no metal catalyst residue and strong positioning ability. The four CMPs were hard bulk solids with good chemical and thermal stability. The BET surface areas of TTPB, TTPPA, TTDAB, and Tm-MTDAB were 222, 512, 1.64 and 2.78 m² g⁻¹, respectively. The total pore volumes of TTPB and TTPPA were 0.127 and 0.2997 cm³ g⁻¹, respectively. Also, TTDAB, and Tm-MTDAB were almost non-porous. Interestingly, the resulting polymers TTPB, TTPPA, TTDAB and Tm-MTDAB possessed a nitrogen-rich, twisted structure, microporosity and good stability, and displayed very high affinity for guest uptake of 443, 490, 313 and 304 wt% iodine vapor. This is because the triphenylamine derivatives are nonplanar molecules, and the electrons easily transfer to form free radical cations. Besides, the introduction of triazine rings increases the polarity of the porous network, promotes the electron transfer process, and forms a complex with the formation of polyiodide anions by electrostatic attraction, which can significantly improve the adsorption performances for iodine. Similarly, Chen and co-workers developed three hierarchical CMPs (NOP-53, NOP-54 and NOP-55)⁸¹ based on a tetraphenyladamantane building block, and their pore features were controlled by adjusting the lengths and rigidities of the linkers. At 348 K and ambient pressure, NOP-53, NOP-54 and NOP-55 could capture iodine of 177, 202, and 139 wt%.

Fig. 6 Synthesis of TTPB, TTPPA, TTDAB, and Tm-MTDAB.

Over all, N-containing CMPs^82–85 have shown outstanding performances in iodine capture. Their effective uptake of I₂ can be ascribed to the combination of high nitrogen content, π-electron conjugated structure, and porosity. The electron-rich N atom is preferred for the sorption of I₂ due to the strong interaction between the electron-deficient I₂ and N-enriched host absorbents. The π-electron conjugated structure allows for the enrichment of π-electrons clouds, which make them suitable hosts for the adsorption of iodine guests. Also, the existence of a larger number of pores is beneficial for I₂ sorption. These studies may provide a useful guidance to manipulate CMPs by varying the constitution of their core building blocks.

Besides the abovementioned functional nitrogen heteroatoms, the introduction of the electron-enriched sulfur and fluorine is also an effective strategy for improving the interaction between host adsorbents and guest adsorbates. For example, four thiophene-based CMPs (SCMP-I, SCMP-II, SCMP-1, and SCMP-2) (Fig. 7)^16,86 were prepared via the polymerization of 3,3′,5,5′-tetrabromo-2,2′-bithiophene, 2,3,5-tribromothiophene, and ethynylbenzene monomers through the palladium-catalyzed Sonogashira–Hagihara cross-coupling reaction. SEM (scanning electron microscopy) images clearly showed that the four samples had different morphologies. SCMP-I was composed of agglomerated spheres with different sizes. SCMP-II showed honeycomb-liked porous spheres with penetrated pores on its surface, and SCMP-1 and SCMP-2 were comprised of agglomerated microgel particles. The BET surface areas were 2.72 m² g⁻¹ for SCMP-I, 119.76 m² g⁻¹ for SCMP-II, 413 m² g⁻¹ for SCMP-1, and 855 m² g⁻¹ for SCMP-2. More interestingly, SCMP-II, which had the lowest surface area among the obtained materials, showed the highest adsorption capacity with an uptake of up to 345 wt%. SCMP-1, SCMP-2, and SCMP-I exhibited iodine uptake of 188, 222, and 102 wt%, respectively. This high amount of iodine adsorption capacity of SCMP-II was attributed to its unique macroscopic honeycomb-liked porous morphology and π–π conjugated structure.

Fig. 7 Synthesis of SCMP-I, SCMP-II, SCMP-1, SCMP-2.

Inspired by the excellent properties of CMPs containing thiophene and nitrogen units, Geng and Niu reported another CMP (TTPT)⁸⁷ (Fig. 8a) with thiophene derivatives and nitrogen groups, which was fabricated via the AlCl₃-catalyzed Friedel–Crafts polymerization reaction of 2,4,6-trichloro-1,3,5-triazine and 2,3,4,5-tetraphenylthiophene in dichloromethane. TTPT had a moderate surface area of 315 m² g⁻¹ and total pore volume of 0.232 cm³ g⁻¹. Its dominant pore width was 1.428 nm. The iodine loading of TTPT was 177 wt% at 350 K and ambient pressure. This value is larger than that of the theoretical maximum adsorption capacity (114 wt%) of TTPT itself, indicating the surface adsorption behavior of the polymer.

Fig. 8 Synthesis of TTPT and FCMP-600@1–4.

Besides, our group also reported four fluorine-enriched CMPs (FCMP-600@1–4, Fig. 8b).⁸⁸ Under the same conditions, FCMP-600@1, 2, 3 and 4 exhibited maximum values for iodine uptake ranging from 90 to 141 wt%. Meanwhile, Song and co-workers designed and synthesized two novel BODIPY-based CMPs (BDP-CPP-1 and BDP-CPP-2)⁸⁹ (Fig. 9) via Sonogashira cross-coupling of 1,3,5-triethynylbenzene and dibromo-substituted BODIPY derivatives. The BET surface areas and the total pore volumes were 635 m² g⁻¹ and 0.78 cm³ g⁻¹ for BDP-CPP-1 and 235 m² g⁻¹ and 0.18 cm³ g⁻¹ for BDP-CPP-2, respectively. SEM images revealed that BDP-CPP-1 was composed of spherical particles, while BDP-CPP-2 showed a block morphology. The significant porous porosity and morphology differences between BDP-CPP-1 and BDP-CPP-2 may be closely related to the ethyl substituents at the 2,6-positions of the BODIPY moieties. BDP-CPP-1 and BDP-CPP-2 could adsorb about 283 and 223 wt% iodine. It is clear that BDP-CPP-1 and BDP-CPP-2 polymers have strong affinity with iodine molecules due to the coexistence of a triple bond, aromatic ring and π-conjugated BODIPY units, which contain heteroatoms (N, B, and F), resulting in a high adsorption capacity for iodine.

Fig. 9 Synthesis of BODIPY-based CMPs.

The above results suggest that heteroatoms (such as nitrogen, thiophene, and fluorine) have a positive influence on the iodine capture of CMP materials. The electron-rich CMPs have the potential to increase the capture capacity for iodine molecules because the lone pair electrons of heteroatoms can enhance the interaction between the adsorbents and adsorbates. Despite their low surface areas and pore volumes, the heteroatoms-rich CMPs are highly desirable for the capture of iodine; however, the examples of high uptake of CMPs are quite limited to date.

On the other hand, the integration of free-ions into the CMP skeleton is another strategy to improve its iodine storage capacity. Two non-porous viologen-based cationic CMPs, COP₁⁺⁺ and COP₂⁺⁺, were developed, in which the viologen units crosslink hexatopic cyclotriphosphazene core moieties (Fig. 10).⁹⁰ COP₁⁺⁺ and COP₂⁺⁺ were obtained by using Menshutkin and Zincke reactions. Then, the radical cationic polymers, COP₁˙⁺ and COP₂˙⁺, and the neutral polymers, COP⁰₁ and COP⁰₂, were generated via treatment with sodium dithionite (Na₂S₂O₄) or excess cobaltocene under a nitrogen atmosphere. N₂ gas adsorption studies exhibited that the two di-cationic polymers, the two radical cationic polymers, and the two neutral polymers were non-porous. At 333 K and ambient pressure, COP₁⁺⁺, COP₁˙⁺, COP⁰₁, COP₂⁺⁺, COP₂˙⁺, and COP⁰₂ could adsorb about 212, 195, 380, 258, 2.1, and 2.77 wt% iodine, respectively. Interestingly, the sorption rates of iodine vapor for all six non-porous polymers (in the range of minutes) were faster than that of most porous and non-porous materials (in the range of hours) reported to date. In addition, density functional theory (DFT) calculations suggested that polymer reduction to its diradical form leads to the reduction of I₂ to I⁻, which eventually is tightly bound to the polymer through electrostatic interactions. Raman spectroscopy and X-ray photoelectron spectroscopy also indicated that iodine exists mainly as polyiodides I₃⁻ and I₅⁻.

Fig. 10 Synthesis of COP₁⁺⁺ and COP₂⁺⁺. Reproduced from ref. 90 with permission from the Royal Society of Chemistry.

Subsequently, the same research group utilized another strategy to explore the iodine capture capacity of CMPs. They synthesized a series of hyper-cross-linked π-bond-rich CMPs (CalP2, CalP3, and CalP4) based on a polycalix[4]arene building block (Fig. 11).⁹¹ The CalP2, CalP3, and CalP4 polymers were further lithiated to obtain the corresponding lithiated polymers CalP2-Li, CalP3-Li, and CalP4-Li, respectively. These polymers exhibited excellent thermal stability over 500 °C. The CalPn and CalPn-Li (n = 2–4) polymers displayed type II isothermal curves with H4 type hysteresis loops. The special BET surface areas and total pore volumes were 596 m² g⁻¹ and 0.73 cm³ g⁻¹ for CalP2, 630 m² g⁻¹ and 0.639 cm³ g⁻¹ for CalP3, 759 m² g⁻¹ and 1.08 cm³ g⁻¹ for CalP4, 274 m² g⁻¹ and 0.31 cm³ g⁻¹ for CalP2-Li, 308 m² g⁻¹ and 0.558 cm³ g⁻¹ for CalP3-Li, 445 m² g⁻¹ and 0.588 cm³ g⁻¹ for CalP4-Li, respectively. Compared to the pre-lithiated polymers, these lithium ions tended to agglomerate, which caused their overall surface areas and pore volumes to decrease. However, their pore-size distributions remained unchanged with the average pore diameter in the range of 6.2–7 nm. All the CMPs samples were capable of adsorbing iodine from the solution and vapor phase. At 348 K and ambient pressure, the capture iodine uptake of CalP2, CalP3, CalP4, CalP2-Li, CalP3-Li, and CalP4-Li was 88, 196, 220, 108, 248, and 312 wt%, respectively. Among all the polymers tested, CalP4-Li showed the fastest I₂ uptake, which required only 30 min for complete adsorption. This result implies that the lithiated polymers were more efficient absorbers.

Fig. 11 Synthesis of CalPn and CalPn-Li (n = 2–4).

Additionally, many studies have shown that the functions of porous polymers are not only affected by their skeleton structures, and the different morphologies of polymers also have an obvious impact on their properties. After constant efforts, to date, the functions of polymers have been well adjusted by controlling their structure. More recently, the morphology of porous polymers was finely tuned by adjusting the reaction conditions, such as solvent polarity, types, monomers ratios, and reaction temperature. Some porous polymers with different morphologies have been reported in succession, for example tubular, spherical, and membranous morphology. These polymers exhibit different functional properties, even though they have the same chemical compositions and similar structure skeletons.

Li and Deng⁹² synthesized three nanotube-like CMPs (CMPN-1, CMPN-2, and CMPN-3) (Fig. 12) via Sonogashira–Hagihara cross-coupling polymerization using different molecular structures. The pore volumes and the BET surface areas were 0.14 cm³ g⁻¹ and 230 for CMPN-1, 0.39 cm³ g⁻¹ and 339 m² g⁻¹ for CMPN-2, and 2.36 cm³ g⁻¹ and 1368 m² g⁻¹ for CMPN-3, respectively. SEM and TEM images clearly displayed that these samples possessed a nanotube-like morphology. The tips of CMPN-1 were open, which exhibited a smooth and neat tubular surface with ca. 150 nm diameter and ca. 20 μm length. CMPN-2 and CMPN-3 displayed a hollow and hierarchical porous surface morphology with honeycomb-like pores in their tube walls. CMPN-1, CMPN-2 and CMPN-3 could capture gaseous iodine of 95, 110 and 208 wt% at 343 K and ambient pressure, which are proportional to their BET surface area. In addition, the π–π conjugated structure also increased the affinity of CMPN-3 for iodine molecules. Due to its special tubular morphology, high mechanical strength, not easily deformed by external forces, combined with better film forming, CMPN-3 is more suitable for industrial applications.

Fig. 12 Structure of nanotube-like CMPs (CMPN-1, CMPN-2, CMPN-3, and NCMP-5,6,7,8).

Subsequently, they prepared four NH₂-functional porous materials (NCMP-5, NCMP-6, NCMP-7, and NCMP-8)⁹³ (Fig. 12) via the Sonogashira–Hagihara cross-coupling of 2-amino-3,5-dibromopyridine and 1,3,5-triethynylbenzene with different solvent systems. SEM images showed that NCMP-5 was composed of amorphous aggregated nanoparticles. NCMP-6 and NCMP-8 displayed a similar tubular-shape morphology, while NCMP-7 exhibited smooth small spheres. The iodine vapor adsorption capacity followed the order NCMP-6 > NCMP-7 > NCMP-5 > NCMP-8, in which NCMP-6 had the highest iodine adsorption amount of 297 wt% at 350 K and 1.0 bar. The results indicated that all the porous materials were greatly influenced by the component of mixture solvent used in the reaction process, and the introduction of nitrogen atoms enhanced the interaction between guest molecules and CMPs adsorbents, but different reaction conditions could regulate the pore properties and morphology of these porous materials.

The foregoing discussion clearly indicates that, as a new class of porous materials, the structure skeleton, surface area, pore volume, pore size, and morphology of CMPs can be finely controlled through the selection of different building blocks (such as different geometry, size, linkage and component) and different polar solvents (such as single component, binary mixture component, ternary mixture component). Although CMPs are amorphous in nature, their structure and functionalization can be precisely tuned using the abovementioned methods. Owing to their unique π-conjugation structure and permanent porosity, CMPs can serve as outstanding absorbents for iodine in vapor and solution. In addition, iodine-loaded CMPs have been widely studied for their luminescence, magnetism, electrical conductivity, etc. For example, a multifunctional carbazolic porous framework (Cz-TPM) with a tetrahedral core synthesized via FeCl₃ oxidative coupling polymerization displayed tandem visual detection of iodide and mercury.⁹⁴ An iodine-functionalized tetrathiafulvalene (TTF)-containing CMP displayed a strong electron-donor network. Due to the excessive tendency of TTF and its derivatives to form iodide/polyiodide salts, a charged analogue (TTFCMPC+-In) was subsequently formed by the addition of iodine to the porous networks, which is further step towards realizing organic (opto)electronic devices.⁹⁵

2.2 COFs for iodine capture

Compared to amorphous CMPs, the preparation of crystalline covalent organic framework (COF) materials is more challenging. COFs are an emerging class of porous ordered materials, which are composed of multidentate organic building blocks and linked by strong covalent bonds. Since the first COF was synthesized by Yaghi and co-workers in 2005,³⁸ it has attracted great interest. The unique combination of crystallinity and organic functionality make COFs highly efficient for guest selective storage and separation, sensing, transportation of guest molecules, etc. Especially recently, COFs have been proven to be exceptional iodine capture materials.

Zhao's group reported a hetero-pore COF (SIOC-COF-7)⁹⁶ (Fig. 13), which consisted two different types of micropores with unprecedented shapes. SIOC-COF-7 had a BET surface area of 618 m² g⁻¹ and total pore volume of 0.41 cm³ g⁻¹. In addition, SIOC-COF-7 exhibited two main pore size distributions at 0.5 and 1.18 nm, which were almost the same as the theoretical values of 0.5 and 1.19 nm estimated by PM3 calculations, respectively. It existed as hollow microspheres and exhibited a high iodine uptake (up to 481 wt%) by encapsulating iodine in the inner cavities and porous shells of its microspheres. This result indicated that the abundant aromatic rings, high nitrogen content and well-ordered network of heteroporous COFs should be favorable for iodine enrichment, and the inner cavities of the hollow microspheres and the highly ordered channels in the shells also make significant contributions to the extraordinarily high iodine capture capacity.

Fig. 13 Synthetic route for SIOC-COF-7.

Jiang's group also reported a stable 3D COF (COF-DL229)⁹⁷ (Fig. 14), which adopted an extended diamond topology, consisted of eight-fold interwoven skeletons and possessed ordered 1D nanochannels with π-conjugated pore walls. The 3D COF-DL229 was synthesized via the condensation reaction of 1,3,5,7-tetrakis(4-aminophenyl)-adamantane and 1,4-phthalaldehyde under solvothermal conditions. The COF-DL229 was thermally stable up to 500 °C under a nitrogen atmosphere. SEM images showed a uniform morphology of aggregates of rod-shaped crystallites with a minimum dimension of approximately 500 nm. COF-DL229 possessed a BET surface area, total pore volume, and pore size of 1762 m² g⁻¹, 0.64 cm³ g⁻¹ and 1.4 nm, respectively, and it could adsorb 470 wt% iodine at 348 K.

Fig. 14 (a) Synthesis of COF-DL229. (b) Adamantane-knotted cage in the diamond net. (c) 8-fold interpenetration of the diamond net with π-conjugated pore walls. Reprinted with permission from ref. 97 Copyright 2018 Wiley-VCH.

Subsequently, they synthesized five stable 2D COFs (TPB-DMTP, TTA-TTB, TTA-TFB, TFBCz-PDA, and ETTA-TPA)⁹⁸ (Fig. 15) containing 1D open channels with different shapes, and found their pore volume determined their uptake capacity of iodine. TPB-DMTP and TTA-TTB with 1D hexagonal channels and higher pore volumes (V_TPB-DMTP = 1.28 cm³ g⁻¹ and V_TTA-TTB = 1.01 cm³ g⁻¹) showed higher iodine capture capacities of up to 620 and 500 wt%, respectively. TTA-TFB also consists of hexagonal channels; however, it showed the lowest iodine uptake capacity of 270 wt% due to its lowest pore volume (V_TTA-TFB = 0.55 cm³ g⁻¹). The channel shapes and pore volumes of TFBCz-PDA and ETTA-TPA were tetragonal and 0.74 cm³ g⁻¹ and trigonal and 0.95 cm³ g⁻¹, and they could uptake iodine of 370 and 470 wt%, respectively. In the five 2D COFs, all the 1D channels were fully accessible to iodine, generalizing a new paradigm that the pore volume determines the uptake capacity.

Fig. 15 2D hexagonal COFs, 2D tetragonal COFs and 2D trigonal COFs. Reprinted with permission from ref. 98 Copyright 2018 Wiley-VCH.

The ordered structure and high crystallinity of COF materials will greatly increase the properties of materials in iodine adsorption. Therefore, effectively controlling the crystallinity of COFs and regulating their related application performance are practical research subjects, especially for COFs based on flexible building blocks, which usually have large lattice sizes and various monomer sources. Based on the above considerations, Ma and Xia et al. designed and prepared a series of 2D COFs from flexible building blocks with different contents of intralayer hydrogen bonds (Fig. 16).⁹⁹ The effect of H-bonding on their crystallinity and adsorption properties showed that the partial structure of the COFs was “locked” by the H-bonding interaction, which consequently improved their degree of microscopic order and crystallinity. The obtained COFs exhibited excellent and reversible adsorption properties for volatile iodine with uptake of up to 543 wt%.

Fig. 16 Synthesis of 2D COFs. Reprinted with permission from ref. 99 Copyright 2018 American Chemical Society.

It's well-known that the capture and separation of iodine from water at room temperature is very important in practical applications, but remains challenging. Ke and co-workers developed a microporous hydrogen-bonded COF (H_COF-1)¹⁰⁰ through photo-irradiated single-crystal-to-single-crystal (SCSC) transformation from its molecular precursor 1_crystal (Fig. 17). Firstly, 1_crystal was soaked in neat ethanedithiol in the dark overnight, and photo-irradiated under a mercury lamp or ambient light, and then unreacted ethanedithiol was removed, and it was finally activated using supercritical CO₂. In this reaction system, ethanedithiol served as cross-linkers to form C–S bonds between two alkynyl groups. The obtained polymer H_COF-1 could adsorb I₂ rapidly in an aqueous environment with a high uptake capacity of 290 wt%, which may be attributed to the hydrophobic nature of its pores. Thus, H_COF-1 was prone to forming N–H⋯I hydrogen bonding and N⋯I and S⋯I halogen bonding interactions with I₂ molecules. Notably, the loaded iodine molecules could be released from H_COF-1 at different rates. The release speed was very fast in dimethyl sulfoxide, within 30 min; however, it was slower in MeOH because the solvent molecules can compete with the hydrogen-bonding interactions between iodine and H_COF-1. In addition, Thomas and co-workers developed Fe₃O₄-based COF composites via a facile method (Fig. 18, Fe₃O₄/COF).¹⁰¹ Fe₃O₄/COF can be used as a magnetically recoverable adsorbent, which could capture iodine with an uptake of 797 mg g⁻¹ in iodine aqueous solution (0.2 g L⁻¹, 5 mL) with the liquid phase becoming colorless upon sonication for 15 s, indicating the efficient adsorption of iodine. To further illustrate the significance and advantage and the current limitations of COFs for iodine capture, the performances of 187 experimentally reported COFs for gaseous I₂ were systematically evaluated by theoretical calculation under real industrial conditions.¹⁰² Among them, 3D-Py-COF was identified with the highest I₂ uptake of 1670 wt%, outperforming all the adsorbents reported to date.

Fig. 17 Synthesis of hydrogen-bonded COF (H_COF-1). Reprinted with permission from ref. 100 Copyright 2017 American Chemical Society.

Fig. 18 Synthesis of Fe₃O₄/COF composites. Reprinted with permission from ref. 101 Copyright 2017 American Chemical Society.

Overall, COFs exhibit excellent performances in capturing iodine. COF materials have many advantages, such as regular pore structure and uniform size, easy to adjust pore size in the range of microporous and mesoporous, and easy to control internal pore environment. Moreover, COFs have high thermal/chemical stability, even in strong acidic or alkaline conditions. COFs are comprised of pure organic building units linked by strong covalent bonds. Thus, their skeleton is chemically robust against iodine to avoid structural collapse. Some researchers suggest that the porous skeletons of COFs are soft by triggering structural fitting to iodine, while keeping the covalent connectivity and degree of interpenetration, which is comparable with that of inorganic porous materials. From the perspective of ordered structure, the ordered arrangement of the skeleton of COFs enables full accessibility of their porous space to iodine, which leads ultrahigh iodine adsorption capacity. The ordered structure of COFs plays a key role in iodine adsorption. Thus, studies on the iodine adsorption of COF materials demonstrate they are new benchmark materials for coping with fission vapor waste issues.

2.3 PAFs for iodine capture

The creation of other members of POPs has broadened the range of porous materials to a large extent. Inspired by the successful construction of artificial porous materials, a number of attempts have been made in exploring new POPs. From the viewpoint of structural characters and performance, porous aromatic framework (PAF) networks generally feature diamond-like structures, ultrahigh surface areas and high physicochemical stability, which are also an important class of adsorbents for iodine capture.

Qiu and co-workers have reported two PAFs with high iodine capture capacities, PAF-1 and JUC-Z2 (Fig. 19).¹⁰³ PAF-1 showed a diamond topology and robust all-carbon scaffold with a pore size in the range of 0.5–1.0 nm and high physicochemical stability (up to 420 °C) with an ultrahigh surface area (S_BET = 5640 m² g⁻¹). JUC-Z2 with a hcb topology possessed a well-defined uniform micropore distribution (∼1.2 nm), large surface area (S_BET = 2081 m² g⁻¹) and high physicochemical stability (>440 °C). The amount of iodine adsorbed by PAF-1 and JUC-Z2 was 186 and 144 wt%, respectively.

Fig. 19 Iodine vapor adsorption of PAF-1 and JUC-Z2. Reproduced from ref. 103 with permission from the Royal Society of Chemistry.

Subsequently, Zhu and co-workers¹⁰⁴ synthesized a series of charged PAFs based on a tetrahedral building unit, lithium tetrakis(4-iodophenyl)borate, and different alkyne monomers as linkers via Sonogashira–Hagihara coupling reaction (PAF-23, PAF-24, and PAF-25, Fig. 20). PAF-23, PAF-24 and PAF-25 showed permanent porosities and excellent thermal/chemical stabilities. In addition, PAF-23, PAF-24 and PAF-25 exhibited high affinities and capacities for iodine molecules due to their effective sorption sites, namely ion bond, phenyl ring, and triple bond together. These materials showed strong polarization–polarization interaction with iodine and displayed very high affinity for iodine in vapor and solution with an adsorption capacity in the range of 260–276 wt%.

Fig. 20 Synthesis of PAF-23, PAF-24, and PAF-25. Reprinted with permission from ref. 104 Copyright 2015 Wiley-VCH.

As stated above, POPs have been found to be a class of potential and effective iodine sorbents due to their high surface area, inherent porosity (pore size, pore volume, and pore shape), and light weight. The iodine adsorption capacity of POPs can be enhanced via several techniques such as selecting core building blocks with different functional groups (porphyrin, phthalocyanine, thiophene, –NH₂, –NH–, OH, –CO–NH–, fluorine, etc.) for their synthesis, different preparation methods (Yamamoto reaction, Sonogashira coupling polymerization, Suzuki coupling polymerization, azo-linked reaction, Friedel–Crafts reaction, and Schiff-base reaction) and activation routes (sodium dithionite, cobaltocene, and nBu-Li), and changing the functional groups and electronegativity of the polymers. POPs have prominent iodine sorption capacity. However, in laboratory studies, the test conditions and corresponding performance evaluations do not always represent the expected real conditions during a serious nuclear accident (temperature > 80 °C and relative humidity > 40%). Therefore, the main factors such as temperature and relative humidity should be considered in data analysis, which will provide an important basis for the practical application of porous adsorbents in the iodine industry. Moreover, it can be noted that the capture of iodine by POPs sorbents has been depicted as physisorption and chemisorption mechanisms. Physisorption has been found to be reversible, in contrast, chemisorption is irreversible due to the strong chemical bond formation between the adsorbate and adsorbent. In addition, although COFs show excellent performances as iodine adsorbents, there are still several problematic issues, including the synthesis of COFs is cumbersome, monomers are not easily obtain, and COFs cannot be produced on an industrial scale. Thus, these factors will limit the practical application of POP iodine adsorbents.

3. Metal–organic frameworks (MOFs) for iodine capture

As is known, MOFs are effective porous adsorbents and are widely applied in the adsorption/separation of guest molecules such as gases,¹⁰⁵ water,¹⁰⁶ organic vapors,¹⁰⁷ and dyes,¹⁰⁸ and others.¹⁰⁹ The use of porous MOFs for iodine sorption is motivated by the capturing and sequestering of the volatile radioactive iodine produced in nuclear energy enterprises. Porous MOFs are good candidates for iodine capture, and many excellent explorations of iodine adsorption have been carried out, in which iodine was introduced into MOFs by guest exchange in iodine solution or by diffusion in high concentration iodine vapor. Moreover, some researchers indicate most MOF materials show good recycling because the encapsulated iodine can be released, and the frameworks can be retained. In addition, iodine assembly in their restricted spaces has been widely studied for luminescence,¹¹⁰ magnetism,¹¹¹ and electrical conductivity.¹¹²

Herein, we simply divide MOFs for iodine adsorption into two categories of non-iodine MOFs and iodine-templated MOF materials based on the presence or absence of iodine. In some systems, we found that iodine species are introduced as raw materials to obtain iodine-templated MOF materials. We will present the iodine adsorption performances of these two types of MOFs. The relevant structural characteristics and iodine adsorption capacities of the selected MOFs are summarized and tabulated in Table 1.

3.1 Iodine capture in non-iodine MOFs

In the field of capture iodine, a large number of MOFs have been developed and utilized, of which non-iodine MOFs account for the majority. A wide variety of metals and organic ligand units offer the possibility of constructing diverse and versatile MOFs materials. Their large surface areas, adjustable skeletal structures, facile post-processing and functionalization provide an excellent platform for adsorbing iodine. In addition, the morphology, size, and further processing of MOFs can also affect their uptake capacities of iodine.

As early as 2003, Robson and co-workers¹¹³ reported a chiral zinc d-saccharate network [Zn(C₆H₈O₈)]˙ ≈ 2H₂O containing two different characteristics of hydrophobic and hydrophilic parallel channels, which were isolated from each other by seemingly impenetrable walls. The color of the crystals became deep red after exposure to I₂ vapor for a few days. Single crystal X-ray diffraction indicated that the hydrophilic channels were still assigned to the H₂O molecules; however, disordered iodine centres were defined in the hydrophobic channels. The absorption amount was approximately 0.30 I₂ molecules per zinc atom or 2.6 I₂ molecules per unit cell. The results showed that the hydrophobic/hydrophilic characteristics of its pores and channels affected the adsorption of iodine. Similarly, Ghosh et al.¹¹² synthesized a 3D biporous anionic MOF {[(Me₂NH₂)₂]·[Cd₃(5-tbip)₄]·2DMF}_n (5-tbipH₂ = 5-tert-butylisophthalic acid), which possessed hydrophobic and hydrophilic channels due to the 5-tbip linker containing a hydrophobic t-butyl group and hydrophilic carboxylate groups. The activated sample exhibited reversible iodine uptake in vapor and solution, and 58 wt% iodine was included. Furthermore, the iodine-loaded MOF showed ∼76 times higher electrical conductivity compared with the guest-free MOF.

Inspired by the potential iodine adsorption of MOFs, some well-known classic MOFs have been extended as iodine sorbents. For example, zeolitic imidazolate framework ZIF-8,¹¹⁴ which was purposely selected due to its relatively large cavities (11.6 Å), large specific surface area, high chemical and thermal stability and suitable apertures (3.4 Å) close to the I₂ kinetic diameter (∼3.35 Å). Nenoff et al.¹¹⁵ presented detailed structural evidence of I₂ capture within ZIF-8 by employing a combination of experiments and molecular simulations. ZIF-8 had a high sorption capacity for I₂, up to 125 wt%, where only 25 wt% I₂ binds to the surface, while ∼100 wt% I₂ is efficiently contained within the sodalite cages of ZIF-8. The I₂ adsorption is mainly due to the favorable interactions with the imidazolate-based linker. Moreover, cage-trapped I₂ is secured in the pores of ZIF-8 until the framework decomposes at ∼575 K. Thus, the ZIF-8 material can be used to sequestrate iodine. The cages of ZIF-8 strongly bind iodine within the framework, while I₂ bound on the surface mirrors binding energies of traditional iodine sorbents. ZIF-8 binds iodine 4 times more strongly than activated carbon (Fig. 21a).¹¹⁶ In a broader sense, this thermodynamic analysis of iodine-ZIF-8 binding offers clues for designing effective functional MOFs for specific applications, which are centered on guest–host interactions.

Fig. 21 (a) I₂ capture within ZIF-8 and (b) competitive I₂ sorption by Cu-BTC from Humid Gas Streams. Reproduced with permission from ref. 116 and 118 Copyright and 2013 American Chemical Society.

The other classic MOF Cu-BTC, also known as HKUST-1,¹¹⁷ is a hydrophilic open-pored MOF containing coordinatively unsaturated metal centers, which offer the potential for open metal sites to directly interact with the targeted capture molecule I₂. Cu-BTC has been used as I₂ “getters” for competitive sorption of molecular I₂ from a mixed stream of iodine and water vapor (Fig. 21b).¹¹⁸ The combination of simulation with crystallography showed that HKUST-1 substantially adsorbs I₂ in preference to water vapor because I₂ forms an effective hydrophobic barrier to minimize water sorption. I₂ molecules are adsorbed in two principal areas, first, in the smallest cage close to the copper paddle wheel, then second, it adsorbs within the main pore with close interactions with the benzene tricarboxylate organic linker. The maximum I₂ uptake was ∼175 wt%. Furthermore, the wider 6.5 Å window openings, as well as interactions of I₂ with the framework are sufficiently weak to allow the “on-demand” release of I₂ upon moderate heat treatment. Also, Lobanov and co-workers¹¹⁹ reported the iodine adsorption by two microporous MOFs, SBMOF-1 (Ca(sdb), sdb = 4,4′-sulfonyldibenzoate) and SBMOF-2 (Ca(tcpb), tcpb = 1,2,4,5-tetrakis(4-carboxyphenyl)-benzene),^120,121 which preferentially adsorbed I₂ over water in a humid environment due to the specific phenyl ring-halogen interactions in their channels. The competitive I₂ sorption from Humid Gas Streams is relevant to mixed gas streams in nuclear energy industrial processes and accident remediation. Thus, moisture-stable porous MOFs that can selectively adsorb I₂ in the presence of water vapor are of great interest. Besides ZIF-8 and HKUST-1, MIL-101 has been used as a sorbent due to its large surface area and high stability, and it shows efficacy for the capture of radioiodine.¹²²

In addition to the classic MOF materials, many outstanding MOFs with distinct structures have been designed and synthesized for iodine adsorption. In 2010, a highly stable microporous MOF, {[Zn₃(DL-lac)₂(pybz)₂]·2.5DMF}_n (DL-lactic acid (DL-lac), 4-(pyridin-4-yl)benzoic acid (pybz)), was constructed by rigid metal–organic pillars (Zn-lactate) connected through organic groups (pybz) forming double walls (Fig. 22).¹²³ The desolvated crystals could uptake iodine (101 wt%) in a cyclohexane solution of I₂, and the excellent and promising I₂ affinity may be attributed to the structural character of the regular π-electron walls made of pybz. Its uptake capacity (101 wt%) was remarkable and clearly exceeded that of zeolite 13× (0.32–0.38 g g⁻¹, with 10 Å pore)¹²⁴ and activated carbon (∼0.84 g g⁻¹). Conversely, the I₂ could be released, which was governed by the host–guest interaction. Furthermore, donor–acceptor (I₂-π-electron of the surface of the walls) interactions resulted in cooperative anisotropic electrical conductivity. Recently, a 4-fold interpenetrated double-walled MOF {[Ni₄(44pba)₈]·sol}_n, (44pba⁻ = 4-(4-pyridyl)benzoate)¹²⁵ was reported to absorb 110 wt% iodine. Its high uptake for I₂ was attributed to the high porosity of its network, in which the aromatic π-electron-rich walls formed by 44pba ligands allowed the interaction of iodine molecules with the framework. Another porous MOF, [Zn₂(tptc)(apy)_2−x (H₂O)_x]·H₂O (where x ≈ 1, apy = aminopyridine, H₄tptc = terphenyl-3,3′′,5,5′′-tetracarboxylic acid), displayed efficient, reversible adsorption of I₂ in vapor and solution.¹²⁶ The I₂ loading was 216 wt%. Its strong affinity for I₂ was ascribed to its large porosity, conjugated π-electron aromatic system, halogen bonds, electron-donating aminos, and unsaturated Zn(II) sites. These results indicated that π-electron-rich MOFs facilitate the binding of their skeleton and guest iodine, and further support that the judiciously assembled MOFs with predesigned regular and suitable structures can find many applications in the encapsulation of iodine substances.

Fig. 22 (a)–(c) Structure of {[Zn₃(DL-lac)₂(pybz)₂]·2.5DMF}_n. (d) Schematic of I₂ molecules diffusing in the channels. Reproduced with permission from ref. 123 Copyright 2010 American Chemical Society.

In the field of iodine adsorption, an increasing number of MOFs have been studied, showing varying capacity, reversibility and stability. The thermally stable Hofmann-type clathrate Ni^II(pz)[Ni^II(CN)₄] (pz = pyrazine)¹²⁷ could efficiently and reversibly capture I₂ both in solution and the gaseous phase of 83 wt% and 59 wt%, respectively. The seemingly nonporous highly permeable discrete metallocyclic complex ([Cu₂(bitmb)₂Cl₄]·CH₃OH·H₂O, bitmb = 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene)¹²⁸ could absorb iodine from iodine vapor, which is because the host molecules may cooperate with each other in a dynamic and concerted fashion to create windows of opportunity, thus allowing mobile guest iodine molecules to freely traverse the crystals. Moreover, it is worth noting that the locations of I₂ in the pores of Cu-MOF ({[Cu^II(btz)]·0.5H₂O}_n, H₂btz = 1,5-bis(5-tetrazolo)-3-oxapentane)¹²⁹ and porphyrin-based Co-MOF, [Co(DpyDtolP)]₆·12H₂O (DpyDtolP, 5,15-di(4-pyridyl)-10,20-di(4-methylphenyl)porphyrin)¹³⁰ were unambiguously determined by single crystal diffraction. More importantly, the successful location determination of the I₂ molecules may provide new insight for the design and application of MOF materials. Besides, many other MOFs have been used for the capture of iodine molecules, including calixarene-based cages CIAC-103 and CIAC-104,¹³¹ MOFs, IFMC-10,¹³² IFMC-15,¹³³ IFMC-69,¹³⁴ members of the [M₃(HCOO)₆] family,^111,135,136 JLU-Liu14,¹³⁷ CityU-7,¹³⁸ and others.^{110,139–152} They consist of interpenetrating, non-interpenetrating, cationic, anionic, chiral, achiral, and other networks.

The above-mentioned MOFs are all composed of common transitions metals (such as Zn, Cd, Cu, Co, and Ni). Besides, some MOFs constructed from the other relatively rare metals (for instance Pb, In, and Zr) have shown excellent performances in the field of iodine adsorption. Three 3D porous lead(II) coordination polymers, HMTI-1¹⁵³ ([Pb(4-bpdh)(NO₃)(H₂O)]_n, 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene), HMTI-2¹⁵⁴ ([Pb(4-bpdh)(NO₃)₂]_n), and TMU-15¹⁵⁵ ([Pb(4-bpdb)(μ-NO₃)(μ-SCN)]_n·1.5CH₃OH, 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) could reversibly absorb approximately 3I₂ (150 wt%), 2I₂ (100 wt%), and 3I₂ for one lead(II) atom, respectively. Their exceptional affinity for I₂ may be attributed to the structural character of their regular π-electron walls made of organic ligands. Moreover, HMTI-2 could complete a reversible crystal to crystal transformation to HMTI-1 by hydration and dehydration. The presence of H₂O molecules in HMTI-1 plays an effective role in increasing its iodine adsorption and speeding up the adsorb rate, while the delivery of iodine from HMTI-2 was faster than HMTI-1 because the coordinated H₂O can increase the affinity for I₂.

Zr(IV) MOFs, as in the UiO-66 series,¹⁵⁶ have been receiving widespread research interest due to their enhanced chemical and mechanical stabilities.^157,158 Forgan et al.¹⁵⁹ investigated the iodine storage capacities from iodine vapors of the stable Zr-stilbene MOFs [Zr₆O₄(OH)₄(sdc)₆]_n, [Zr₆O₄(OH)₄(edb)₆]_n,¹⁶⁰ [Zr₆O₄(OH)₄(bdb)₆]_n, and [Zr₆O₄(OH)₄(peb)₆]_n (sdc = 4,4′-stilbenedicarboxylate, edb = 4,4′-ethynylenedibenzoate, bdb = 4,4′-(buta-1,3-diyne-1,4-diyl)-dibenzoate, and peb = 4,4′-[1,4-phenylenebis(ethyne-2,1-diyl)]-dibenzoate) (Fig. 23). Unsaturated alkene, alkyne and butadiyne units can serve as reactive sites for PSM by halogenation. Although both bromination and bromohydrination can occur in a facile manner, [Zr₆O₄(OH)₄(sdc)₆]_n is unable to chemisorb I₂ across its integral alkene units, and its maximum I₂ physisorption storage capacity was 107 wt%. In contrast, [Zr₆O₄(OH)₄(edb)₆]_n and [Zr₆O₄(OH)₄(bdb)₆]_n could both chemi- and physisorb I₂, demonstrating a maximum of 57 wt% and 88.5 wt% trapping of I₂. The higher adsorption of [Zr₆O₄(OH)₄(bdb)₆]_n is due to the presence of twice as many alkyne units. Among them, the interpenetrated [Zr₆O₄(OH)₄(peb)₆]_n showed the highest storage value, with a maximum uptake of 279 wt%, which is comparable to the benchmark capacity of 276 wt% set by the charged PAF-24.¹⁰⁴ The superior uptake of [Zr₆O₄(OH)₄(peb)₆]_n is the result of the high tendency of I₂ to physisorb within pores, possibly due to the high density of Zr₆ clusters as a result of interpenetration. By comparison, the I₂ chemisorption capacity of [Zr₆O₄(OH)₄(peb)₆]_n was lower than that of [Zr₆O₄(OH)₄(bdb)₆]_n, even though both materials contained a similar alkyne content. Chemisorption by [Zr₆O₄(OH)₄(bdb)₆]_n may also lead to partial pore collapse, thus hindering later I₂ uptake. Generally, the chemical stability of Zr(IV) MOFs, combined with their high porosity in concert with reactive chemisorptive sites on the linkers, generates potential candidates for I₂ storage/capture applications.

Fig. 23 (a) Scheme of the ligands used and (b) representation of the crystal structure of [Zr₆O₄(OH)₄(sdc)₆]_n. (c) Summary of the physisorption and chemisorption I₂ storage capacities of the four [Zr₆O₄(OH)₄L₆]_n MOFs. Reproduced with permission from ref. 159 Copyright 2016 Wiley-VCH.

More recently, MFM-300(M) (M = Al, Sc, Fe, and In)^161–164 with a high storage density have been developed for I₂ capture. The maximum uptake of I₂ by MFM-300(M) was 94, 154, 129, and 116 wt% for Al, Sc, Fe, and In, respectively. Among them, MFM-300(Sc) exhibited fully reversible I₂ uptake of 154 wt%, and its structure remained completely unperturbed upon the inclusion/removal of I₂. Both experimental and theoretical investigations rationalized that at a low loading, the adsorbed I₂ molecules form specific interactions with the metal-bound hydroxyl groups within the pores, supplemented by intermolecular interactions between adsorbed I₂ molecules. In contrast, at a high loading, the intermolecular guest–guest interaction becomes the dominant feature and leads to the unusual self-aggregation of adsorbed I₂ molecules into ordered triple-helical chains within the confined channels, generating an exceptional density of 3.08 g cm⁻³ in MFM-300(Sc), 63% of that for solid I₂ (4.93 g cm⁻³ at 298 K). The widespread implementation of sustainable nuclear energy requires the development of efficient iodine stores that have simultaneously high capacity, stability and more importantly, storage density (and hence minimized system volume).

The choice and design of organic ligand units are crucial for the synthesis of MOFs, which directly affect the structures of the obtained MOFs, and then influence their ability to adsorb iodine. Using organic linkers with functional groups to construct MOFs or functionalizing post-treatments of MOFs can both change the porous environments, and thus improve the interaction between the host and the guests. The use of amine-tagged ligands can enhance the adsorbent-adsorbate interactions through host–guest hydrogen-bonding to enhance iodine adsorption. Morsali et al.¹⁶⁵ constructed two isoreticular two-fold interpenetrated MOFs, [Zn₂(BDC)₂(4-bpdh)]·3DMF (TMU-16) and [Zn₂(NH₂-BDC)₂(4-bpdh)]·3DMF (TMU-16-NH₂, NH₂-BDC = amino-1,4-benzenedicarboxylate) (Fig. 24a). Both unmodified and amino functionalized MOFs could serve as hosts for encapsulating I₂. Their adsorption amounts of I₂ were similar (∼45 wt%), while, the amino functionalized TMU-16-NH₂ could absorb iodine 1.4 times faster than the non-functionalized analogue. The differences in adsorption rate between the non- and amine-functionalized frameworks probably resulted from the different host–guest interactions. Thus, assembling MOFs with –NH₂ groups tagged on the linkers as a hydrogen bond donor can find more applications for the encapsulation of iodine. Subsequently, using a similar strategy, two isoreticular 3D pillared-bilayer interpenetrated MOFs, {[Cd(bdc)(4-bpmh)]}_n·2n(H₂O) and {[Cd(2-NH₂bdc)(4-bpmh)]}_n·2n(H₂O) (4-bpmh = N,N-bis-pyridin-4-yl-methylene-hydrazine),¹⁶⁶ were developed (Fig. 24b), in which the iodine capture capacity of the amino-functionalized MOF was two times that of the non-functionalized MOF. A similar phenomenon was observed in the amino-functionalized Al-based MOF.¹⁶⁷ These works demonstrate that amino-functionalized MOFs exhibit enhanced iodine adsorption, faster I₂ adsorption rate and controlled delivery of I₂ over the non-functionalized homologs, which indicate that the electro-donor –NH₂ group is favorable for the adsorption and storage of iodine.

Fig. 24 (a) and (b) Schematic views of the comparative synthesis of amino-functionalized MOF and non-functionalized MOF, respectively. Reproduced from ref. 165 with permission from the Royal Society of Chemistry, and ref. 166 with permission from 2015 Wiley-VCH.

The introduction of thiol (–SH) groups influence iodine capture. The thiol-modified MIL-53(Al)¹⁶⁸ was capable of iodine adsorption from the vapor phase and from solution with an equilibrium uptake of ∼32.5 wt%, which is higher than that of other reported modified forms of MIL-53. The formation of covalent S–I bonds allows the sequestration of iodine by the porous solid. Analogously, ZrDMBD (Zr₆O₄(OH)₄·(C₈H₆O₄S₂)₆·(DMF)₇·(H₂O)₁₈) appended with free-standing thiol (–SH) groups could uptake iodine from solution to form Zr₆O₄(OH)₄·(C₈H₆O₄S₂)₆·(DMF)₃·(I)₁₅·(H₂O)₂₅.¹⁶⁹ ZrDMBD could react readily with I₂ to form sulfenyl iodide (S–I) units (Fig. 25a), which anchored onto the rigid MOF skeleton and thus prevented them from approaching each another to undergo the dismutation reaction, exhibiting distinct stability even at elevated temperatures (e.g. 90 °C). Besides, Gu and co-workers¹⁷⁰ developed UiO-66-PYDC constructed by utilizing the pyridine-containing linker PYDC (Fig. 25b). The abundant and inherent pyridine moieties in the developed adsorbent worked as active adsorption sites to capture I₂. Due to the strong affinity of pyridine-containing ligands to I₂ and high porosity, the adsorption capacities of UiO-66-PYDC for I₂ could reach as high as 125 wt%. Additionally, UiO-66-PYDC exhibited excellent renewable adsorption properties, prefiguring their great promise as green adsorbents for I₂ removal in nuclear waste management. These results further prove that the introduction of functional groups (–NH₂, –SH, and pyridine) can enhance iodine adsorption.

Fig. 25 (a) Schematic of I₂ oxidation of the –SH groups in the ZrDMBD net (left) into the ZrDMBD-I₂ net (right). The host net is simplified as a tetrahedral cage, with each Zr₆O₄(OH)₄ cluster shown as a sphere. (b) Schematic illustration of the I₂ removal mechanism with the developed adsorbent of UiO-66-PYDC using 2,5-pyridine-dicarboxylic acid (PYDC) as the inherent bridging struts and active adsorption sites. Reproduced from ref. 169 and 170 with permission from the Royal Society of Chemistry.

Besides, due to their adjustable structures, some MOFs can be easily subjected to post-synthetic modification, which plays an important role in adjusting the interaction between host and guests. Zeng's grpoup¹⁷¹ described an MOF containing uncoordinated hydroxyethyl groups, {Zn₃[(L)₂(μ₂-OH)₂]·6H₂O}_n (1) (H₂L = 2-(1-hydroxyethyl)-1H-benzo[d]imidazole-5-carboxylic acid), which could undergo unusual tandem PSM reactions (Fig. 26). Heating crystals of 1 at 120 °C gave rise to dehydrated 1′, and then heating crystals of 1′ at 250 °C resulted in a new MOF 1′′ with vinyl groups in its channels, which was associated with the elimination of H₂O from the hydroxyethyl groups to give vinyl groups. After PSMs, 1′ and 1′′ could absorb comparable iodine of 28 wt% and 26 wt%, respectively, from solution, while the iodine sorption rate and release rate of 1′ were quicker than that of 1′′.

Fig. 26 Scheme of the transformations of 1: post-synthetic elimination of H₂O from the hydroxyethyl groups in the channels to form vinyl groups, and photos showing the iodine sorption of 1′ from a saturated solution of iodine in cyclohexane. Reproduced with permission from ref. 171 Copyright 2013 Wiley-VCH.

There are also reports that the frameworks of MOF materials can react with guest iodine (e.g. redox reaction) to realize iodine capture. This type of MOF is usually constructed of metals with redox activity (such as Fe and Ni). The pillared bilayer open framework BOF-1 ([Ni₂(C₂₆H₅₂N₁₀)]₃ [BTC]₄·6C₅H₅N·36H₂O) had unprecedented sponge-like dynamic behavior in response to guest molecules.¹⁷² Upon reaction with I₂, two-thirds of the Ni(II) ions in the BOF-1 framework were oxidized to low spin Ni(III) and the I₂ molecules were reduced to I₃⁻ anions, which were included in the channels. Previously, the Hofmann clathrate Fe^II(pz) [Pt^II(CN)₄]¹⁷³ was used as a host for the chemisorptive uptake of I₂ molecules involving associative oxidation of Pt^II to Pt^IV and reduction of the dihalogen to the corresponding halide to give to Fe^II(pz)[Pt^II/IV(CN)₄](I) with the formation of a Pt^IV–I bond. Also, Kitagawa and co-workers¹⁷⁴ prepared a 2D layer porous framework [Fe(isophthalate)(bpy)] (bpy = 4,4′-bipyridyl) containing redox active Fe²⁺ centers. The activated samples could uptake I₂ after exposure to I₂ vapor, and the adsorbed amount corresponded to five iodine atoms per Fe²⁺ dimer. Of importance is that there was no adsorption of I₂ for isostructural frameworks that included Zn²⁺, Mn²⁺ or Ni²⁺ with the same ligand systems. The iodine accommodation by only the Fe²⁺ compound is presumably due to the electron-donating property of the Fe²⁺ frameworks. The charge transferred from the Fe²⁺ framework to iodine, and half of the Fe ions in the framework were oxidized to Fe³⁺. The iodine inside was predominantly of the pentaiodide (I₅⁻) form.¹⁷⁵ Furthermore, almost all the Fe³⁺ was reduced to Fe²⁺ as iodine was released, which proved its redox reversibility. These results demonstrate that some frameworks with redox active metal centers can react with iodine to achieve the trapping of iodine. These designs may be applied to prepare a new class of versatile multifunctional crystalline materials, including host lattices for guest molecular and chemical reactions.

Besides the storage of molecular iodine, porous MOFs can also be applied to uptake other iodine species such as I⁻, I₃⁻ IO₃⁻, and organic iodides. In 2011, Dong et al.¹⁷⁶ developed a porous 3D MOF, [Cd(L)₂(ClO₄)₂]·H₂O (L = 4-amino-3,5-bis(4-pyridyl-3-phenyl)-1,2,4-triazole), for iodine enrichment in vapor- and liquid-phases via both molecular sorption and anion-exchange approaches (Fig. 27). The Cd(II)-triazole MOF could uptake around 46 wt% of I₂ to generate 2I₂⊂CdL₂ in vapor, and is also capable of trapping I₂ in solution (0.8I₂⊂CdL₂). More interestingly, the MOF was capable of enriching other iodine anions. I₃⁻ and IO₃⁻ were included via ion-exchange by immersion in an aqueous solution of I₂/KI and an aqueous solution of NaIO₃. Also the other porous hydrogen-bonding Cu(II) coordination framework [Cu₆(AcNTB)₆]·6ClO₄·nH₂O¹⁷⁷ was used for accumulating iodine species I⁻, I₃⁻ or I₂ in an aqueous environment and another robust and water-stable MOF TMBP·CuI (TCuI, TMBP = 3,3′,5,5′-tetramethyl-4,4′-bipyrazol)¹⁷⁸ was capable of reversibly binding the volatile molecules of ICl and I2.

Fig. 27 Schematic representation of iodine species enrichment based on [Cd(L)₂(ClO₄)₂]·H₂O via sorption and anion-exchange approaches. Reproduced from ref. 176 with permission from the Royal Society of Chemistry.

Organic iodides, as a class of radioactive species, should also be selectively captured and sequestered to ensure safe nuclear energy usage. The different MOFs, MIL-53(Al), MIL-100(Al), MIL-120(Al), CAU-1(Al), UiO-66(Zr), HKUST-1(Cu) and ZIF-8(Zn) have been studied for CH₃I adsorption.¹⁷⁹ Li and co-workers¹⁸⁰ recently created organic iodide molecular traps through post-functionalization of the MIL-101-Cr MOF with tertiary amine-binding sites. The obtained molecular trap MIL-101-Cr-TED (TED = triethylenediamine) exhibited a high CH₃I uptake capacity of 71 wt% at 150 °C, which is more than 340% higher than that of the industrial adsorbent Ag⁰@MOR (15 wt%, MOR = mordenite) under identical conditions. Furthermore, the resulting adsorbent could be recycled multiple times without loss in capacity. In combination with its chemical and thermal stability, high capture efficiency, and relatively low cost, the adsorbent demonstrated promise for industrial radioactive organic iodide capture from nuclear waste. Then they investigated the effect of amine molecules (N,N-dimethylethylenediamine (DMEDA), N,N-dimethyl-1,3-propanediamine (DMPDA), and N,N-dimethyl-1,4-butanediamine (DMBDA) selected for MOF functionalization) with varying lengths on organic iodide capture.¹⁸¹ The results revealed that DMEDA functionalized MIL-101-Cr gave rise to a record-high CH₃I saturation uptake capacity of 80% at 150 °C, which is 5.3 times that of Ag⁰@MOR, a leading adsorbent material for capturing radioactive organic iodides during nuclear fuel reprocessing.

Furthermore, the size,^130,153 crystallinity, morphology^182–184 and further processing of MOF materials^185–188 may affect their uptake of guests. For example, the adsorption and desorption rates of Nano-HMTI-1 are faster than that of HMTI-1 with a larger size. It seems that the pores in nano-HMTI-1 are more accessible than HMTI-1. Usually, amorphous crystals do not exhibit good adsorption capacity. However, ZIF-8 maintains its high adsorption capacity in extruded pellet form. Besides, there are some works of further processing of MOFs to prepare MOF-based composites for iodine uptake.^185–188 For instance, the growth of HMTI-1 on silk fibers resulted in superior adsorption capacity and recovery of I₂ under ambient conditions.¹⁸⁵ Also, Ag@MIL-101 is a potentially attractive material for the highly efficient adsorption of radioactive iodide anions from water.¹⁸⁸

3.2 Iodine capture in iodine-templated MOFs

One effective strategy to obtain MOFs is the introduction of templates (for example polyoxometalate anion and iodine molecule) introduced into the assembly process. Compared with polyoxometalate anions, the iodine molecule possesses particular chemistry such as oxidizability and conductivity. Besides, iodine has a lower melting point and good solubility, and can be removed easily through heating or guest molecule exchange. From another perspective, polyiodide-MOFs have earned much deserved attention not only due to their tremendous structural variety but also their applications in a wide variety of areas such as electrical devices, fuel cells, batteries, solar cells, and optical devices. To date, numerous MOFs containing small polyiodides such as I₃⁻, I₄²⁻, and I₅⁻ have been reported, and even a few large polyiodide species including I₇⁻, I₉⁻, I₁₀⁻, and I₁₂²⁻ are also known.¹⁸⁹

More than ten years ago, Horn et al. described a system combining the supramolecularity of M-phen (phen = 1,10-phenathroline) complexes and polyiodides, [CuI(phen)₂]I₃,¹⁹⁰ [Fe(phen)₃]I₁₂,¹⁹¹ [M(phen)₃]I₇ (M = Mn, Fe),¹⁹² [M(phen)₃]I₈ (M = Mn, Fe),¹⁸⁹ [Fe(phen)₃](I₃)₂¹⁹³ [Fe(phen)₃]I₁₄ and [M(phen)₃]I₁₈(M = Mn and Fe).¹⁹⁴ The [Fe(phen)₃]²⁺-polyiodide crystal system is particularly superior. Successively, three iodine-containing coordination polymers, [CuI(C₅H₃NI₂)·1/2I₂],¹⁹⁵ {[Cu₂(IN)₃]·I₅⁻·2/3I₂·H₂O}_∞ (IN: isonicotinato)¹⁹⁶ and [Cu(IN)₂]·I₂¹⁹⁷ were obtained in the presence of iodine under hydrothermal conditions. [CuI(C₅H₃NI₂)·1/2I₂] is the first novel copper(I) diiodopyridine coordination polymer produced via the simultaneous reduction of copper(II) to copper(I) and substitution of carboxylato groups by iodo nucleophiles in a self-assembly chemical process. {[Cu₂(IN)₃]·I₅⁻·2/3I₂·H₂O}_∞ is a unique 3D nano-size open-channel framework polymer with polyiodide guests synthesized via an oxidation reaction route. Interestingly, [Cu(IN)₂]·I₂ is the removable iodine included open-framework polymer with an eclipsed 2D structure, in which each iodine molecule has weak donor–acceptor electrostatic attractions to oxygen atoms in the neighbouring layers. By evacuating the crystals at 195 °C, the iodine molecules could be removed to form [Cu(IN)₂], which was still stable after the removal of the iodine molecules from the open-channel of its eclipsed layers. Besides, a copper(I) halide-based cationic framework, [Cu₄I₃(DABCO)₂]I₃ (DABCO = N,N′-dimethyl-1,4-diazabicyclo[2.2.2]octane),¹⁹⁸ was constructed, and its channels were occupied by I₂ and I⁻. The guest I₂ molecules could move freely in and out of the host-framework and the guest I⁻ could be exchanged by SCN⁻. Thus, this compound exhibited iodine release/adsorption and ion-exchange properties.

By attempting to use iodine molecules as precursor templates, a Cu(II)-MOF with polyiodides {[Cu₆(pybz)₈(OH)₂]·I₅⁻·I₇⁻}_n (pybz, 4-pyridyl benzoate)¹⁹⁹ was obtained successfully. It is an unexpectedly interdigitated and 2-fold interpenetrated bipillared-bilayer MOF (Fig. 28). Its cationic framework contains two types of zigzag 1D channels with the largest dimensions of ca. 4.8 and 5.1 Å, and segregate I₅⁻ and I₇⁻ polyiodides in its channels, respectively. Its iodine content was about 43.2 wt%, which is competitive with the traditional methods of introducing iodine. Moreover, it could slowly release iodine in dry methanol to give [Cu₆(pybz)₈(OH)₂](I⁻)₂·3.5CH₃OH and partially recover iodine from cyclohexane to form [Cu₆(pybz)₈(OH)₂](I⁻)₂·xI₂. The three MOFs could mutually transform into one another by the release and recovery of I₂. In addition, the polyiodide anion-containing {[Cu₆(pybz)₈(OH)₂]·I₅⁻·I₇⁻}_n had good electrical conductivity and nonlinear optical activity. Therefore, rational synthesis using target guest (iodine) opens a promising approach for the construction of functional guest-encapsulated MOFs with new structural topologies and interesting properties.

Fig. 28 Schematic presentation of the Cu(II)-MOF {[Cu₆(pybz)₈(OH)₂]·I₅⁻·I₇⁻}_n using iodine as the precursor template. Reproduced with permission from ref. 199 Copyright 2012 American Chemical Society.

Additionally, Cu(II)-MOF {[Cu₄(TTTMB)₄(NO₃)₄₋(H₂O)₅]·(NO₃)₂·(I₃)₂·H₂O}_n (TTTMB, 1,3,5-tris(triazol-1-ylmethyl)-2,4,6-trimethylbenzene)²⁰⁰ containing polyiodide I₃⁻ was obtained through SCSC by immersing a Cd^II complex into an aqueous solution of Cu(NO₃)₂. Also, Ma and co-workers²⁰¹ reported a 2D I₂-containing [Cu(IN)₂]·I₂, which strikingly could transform into a known 3D compound [Cu(IN)₂] through lattice iodine release. This SCSC transformation process represents an example involving the breakage/formation of chemical bonds via lattice iodine release. The iodine content was high at about 45.2 wt%. Subsequently, they have presented an I₂-containing In(III)-MOF, [In₂(pydc)₃(H₂O)]·0.5I₂·0.5H₂O (H₂pydc = pyridine-2,5-dicarboxylic acid),²⁰² which could release its lattice iodine molecules.

Besides, the presence of iodide groups in the frameworks also can affect iodine adsorption. Kawano et al.²⁰³ obtained two networks using a labile Cu₄I₄ cubane cluster [Cu₄I₄(PPh₃)₄] (1) and T_d-symmetry ligand tetra-4-(4-pyridyl)phenylmethane (2). By controlling the cooling rate, a yellow needle crystal isomer 3a {[(CuI)₂(2)]·solvent}_n was produced exclusively under rapid cooling, while slow cooling produced orange prism crystals {[(Cu₂I₂)(2)]·solvent}_n (isomer 3b) (Fig. 29). The isomer 3a is a non-interpenetrated porous network and has 1D channels with a pore window of 5.8 × 5.5 Å. The salient feature of this network is that the bridging iodide in the framework faces into the 1D channel. Isomer 3b is a quadruply interpenetrated network and possesses 1D channels with a pore window of 4.0 × 3.9 Å. Upon exposure to iodine vapor, the desolvated isomer 3a′ and 3b′ adsorbed iodine to give iodine-loaded samples. The crystal-structure analysis revealed that the pores of the isomer 3a encapsulated I₂ by chemisorption through the formation of I₃⁻ groups from bridging iodide units, giving a reasonable geometry for the I₃⁻ ion. However, isomer 3b showed physisorption and the I₂ molecules were arranged linearly along the 1D channels and highly disordered. Their work indicates that frameworks with iodine groups can encapsulate I₂ by chemisorption.

Fig. 29 Selective preparation of network isomers 3a and 3b. Structure of iodine-encapsulating network isomer 3a′ and 3b′. Reproduced with permission from ref. 203 Copyright 2013 Wiley-VCH.

The well-known crystalline sponge porous framework [(ZnI₂)₃(tpt)₂]²⁰⁴ was assembled from the reaction of ZnI₂ nodes with 2,4,6-tris(4-pyridyl)-1,3,5-triazine (tpt) ligand, in which iodide functional groups lining the one-dimensional channels could act as anchor sites for I₂ adsorption. Woo, Murugesu and co-workers²⁰⁵ presented the stepwise crystallographic visualization of the incorporation of I₂ and identified the preferred binding motifs throughout the uptake process. The guest I₂ molecules initially bind with terminal iodide atoms of the framework to form [I₄]²⁻ units, and similar I₄²⁻ ions were observed in MOF [Tb(Cu₄I₄)(ina)₃(DMF)] (ina = isonicotinate) containing Cu₄I₄ nodes.²⁰⁶ However, as the adsorption progresses, the I₂ molecules form less energetically favorable I₃⁻ groups with the same framework iodide, thereby allowing more guest molecules to be chemisorbed. At near saturation, even more binding motifs are observed in the same pores, including both physisorbed and chemisorbed guest molecules. The participation of both physisorption and chemisorption in the uptake of gaseous guests is another fascinating feature of MOFs since chemisorption is generally associated with frameworks exhibiting open metal sites that can bind guest molecules.²⁰⁷ The MOF [(ZnI₂)₃(tpt)₂] could load an excess of 173 wt% of I₂ at room temperature, which is comparable with the high I₂ uptake capacity reported for Cu-BTC (175 wt%).¹¹⁸ This high uptake of I₂ can be attributed to a number of factors, including favorable guest–host interactions, framework flexibility, and high porosity (∼60%). More specifically, this MOF offers strong adsorption site for I₂ through the terminal iodide ion of the framework as well as favorable π-halogen interaction with the tpt ligand. Their study presented the successful identification of a unique set of host–guest interactions, which will drive the improvement of high capacity iodine capture materials. In contrast, when the reaction between ZnI₂ and tpt took place in the presence of triphenylene, a different triphenylene-loaded network [(ZnI₂)₃(tpt)₂]·triphenylene²⁰⁸ with two types of channels was formed. Recently, Burrows, Raithby et al.²⁰⁹ evaluated the iodine uptake in a triphenylene-loaded framework using dynamic X-ray crystallography. The included iodine molecules interacted with the iodine atoms of the ZnI₂ nodes, forming coordinated I₃⁻ ions. Also, the porous polymer [(Na₂I₂CB[6])·8H₂O]_n (CB[6] = Cucurbit[6]uril)²¹⁰ has discrete iodide anions located in its channel regions, which are surrounded by {Na₂CB[6]} chains, playing both space-filling and charge-compensating roles. After I₂ vapor exposure, it could uptake ca. 25 wt% I₂. The formation of halogen bonds between discrete iodide ions and iodine molecules is predominant driving forces for iodine adsorption. These works further indicate that iodine groups in the crystal lattice are ideal and preferential molecular docks for iodine accommodation.

Briefly, MOF materials possess large surface areas, high porosities, tunable chemical compositions, potential for post-synthetic modification and other structural and chemical features, which make MOFs promising candidates for iodine capture. With the research progress, more and more MOFs with different iodine adsorption capacities have been reported. Some well-known classic MOFs (ZIF-8 and HKUST-1) have been explored as iodine sorbents. The introduction of functional groups (–NH₂ and –SH, pyridine) can enhance iodine adsorption. Besides the storage for molecular iodine, porous MOFs can also be applied to uptake other iodine species such as I⁻ and organic iodides. In addition, the size, morphology, and further processing of MOFs can also affect their uptake capacities for iodine. Also, rational synthesis using iodine species as templates is a promising approach for constructing functional guest-encapsulated MOFs. However, only a few materials can remain stable in wet conditions, and these materials have limited thermal stability and are not produced on a large scale, which limit their application.

4. Analysis and improvement strategies for iodine capture of POP and MOF materials

POP and MOF materials are becoming promising candidates for iodine capture due to their high surface areas, large pore volumes, precisely adjustable pore structures, good physicochemical stabilities, facile pre- or post-synthetic modification, etc. More relevant to the present study is the increasing number of POPs and MOFs reported to possess various iodine uptake properties, including irreversible and reversible trapping and even chemisorption. MOFs are organic–inorganic hybrid materials that consist of inorganic metal ions and organic ligands linked by coordination bonds, while POPs are constructed from various pure organic building blocks by strong covalent bonds. Most MOFs are unstable under humid conditions, which limits their application. POPs are usually amorphous solids, unlike crystalline MOFs; however, their excellent physicochemical stability makes POPs more suited for real applications. In addition, compared to MOFs, POPs usually have higher iodine adsorption capacity. However, in the highly ordered crystalline MOF system, it is helpful to elucidate the relationship between the structure and the adsorption behavior of iodine. Furthermore, the synthetic costs of POP and MOF materials are high and their production is in the laboratory stage; thus, their actual application is far away.

It has been demonstrated that both molecular porosity and preferential binding sites in porous materials play crucial roles in determining their efficiency and capacity for iodine adsorption. There are a number of potential routes for increasing the iodine uptake capacity in POPs and MOFs, which include: (i) increasing their surface areas and pore volumes. Generally, the high surface areas and large pore volumes of POPs and MOFs can be beneficial in improving iodine capture by providing more sites with interactions between sorbent and iodine. (ii) Tuning pore size and pore structure. A combination of suitable pore size (6–8 Å) and shape/geometry (squared-shaped pore) of channels and suitable pore decorations (e.g., hydroxyl groups) can provide a unique platform to stabilize the formation of a complex assembly of I₂, resulting in highly efficient packing and hence exceptional storage of I₂. (iii) Increasing the affinity of host materials to iodine. Some networks with lower surface areas and pore volumes but strong affinity have led to increased adsorption uptake. Thus, the introduction of desirable functional groups and open metal sites into the pore structures to modify the environment of the pores and surfaces will allow tuning of the interactions with guests, which is regarded as an effective design to enhance affinity. Open-pored POPs and MOFs containing unsaturated active centers offer potential for active sites to directly interact with the targeted capture iodine. Porous materials with pore functionality of electron-donating groups, for example –OH, –NH₂, –SH, and pyridine, typically achieve high adsorption efficiency for iodine molecules. Accordingly, materials with discrete iodide moieties can be an excellent choice to realize high iodine adsorption efficiency since it is well-known that electron-rich iodide moieties can serve as electron-donating groups to bind iodine molecules via strong halogen bonds to form polyiodide, in which, iodide serves as a halogen bond acceptor and iodine is the halogen bond donor. (iv) Increasing the physicochemical stability. The excellent stability of materials is a prerequisite for ensuring their practical application. Especially, the strong covalent bonds of POP materials afford high thermal and chemical stability, and their constituent light elements add gravimetric advantage. POPs are stable in different polar solvents, and moisture and water conditions, and they can be generally reused for manifold cycles via the use of a simple filter. The use of specific porous materials has often been dependent on sorption material characteristics such as size selectivity into a pore opening, chemical bonding with framework sites, such as metal centers or functional sites, and geometric “pockets” that hold the guest species preferentially.

To date, the research on POPs and MOFs for high efficiency capture of iodine still faces the following problems and challenges: (i) presently, the preparation of porous materials requires complex monomers, expensive transition metal catalysts and harsh reaction conditions. Besides, the synthesis and screening have a certain degree of randomness, and the understanding of the various factors that affect the final structure is still in its infancy. Thus, the controllable synthesis and assembly of POPs and MOFs are still key scientific issues to be solved. (ii) Compared with activated carbon with organic amine impregnated, there is still a large gap between the binding capacity and adsorption capacity for iodine. The interaction between POP and MOF materials and iodine molecules is weak, and iodine adsorption in the low-pressure zone is low, and thus it is difficult to meet practical application requirements. (iii) Most of the current research focuses on the synthetic strategies and functions of materials. The structure–activity relationship between the structure and the adsorption properties of iodine, and the adsorption behavior of iodine have not been elucidated. The mechanism of action on iodine, the interaction between absorbent and adsorbate, and the relationship between properties and macroscopic solid materials also need to be further studied. There has been a small number of theoretical research articles focused on iodine adsorption by MOFs using periodic dispersion density functional theory and Grand Canonical Monte Carlo simulations,^211–213 which help deepen our understanding. For POP materials, most are amorphous or microcrystalline and their structures cannot be accurately characterized by experimental means. For MOFs, although their crystal structures can be unambiguously determined, host–iodine interactions are still difficult to characterize due to the loss of crystallinity and/or the poor crystal quality after the loading of guest molecules, the serious disorder of guest molecules, etc. Presently, it is rather rare to determine the location of guest iodine within pores. Nevertheless, to better understand and improve the adsorption function of POP and MOF materials, it is necessary to investigate the locations of the guest molecules within their pores and the interactions between the guest molecules and pore structures.

5. Summary and concluding remarks

Presently, the storage of iodine and the treatment of radioactive iodine have caused great concerns to the population. POP and MOF materials show outstanding application prospects in the field of adsorption of iodine. Compared with traditional porous materials, POP and MOF materials have some obvious advantages. The types of building blocks are diversified, which can form various structures. Their surface areas, pore sizes and pore volumes are adjustable, and it is easy to modify their inner pores and surfaces. In recent years, a large number of POPs and MOFs have been reported, and these materials have demonstrated potential applications in the capture and storage of iodine. Herein, we focused on two relatively new classes of materials, POPs and MOFs, as platforms for iodine capture.

Although POP and MOF sorbents are highly praised for their good performance in iodine capture, they are difficult to apply industrially for the trapping and treatment of iodine. Furthermore, they are still new laboratory materials and are therefore produced in very small quantities at high prices. Thus, the further development of efficient sorbents that combine a high iodine adsorption capacity over a wide temperature range, high stability and selectivity in the capture of iodine at high temperatures and humidity and in the presence of other molecules, and low cost still remains a challenge. We recommend that new sorbent materials should have controlled morphology, well-defined particle sizes, the absence of any blocked bulk volume, high sensitivity with fairly reduced purge volumes, low energy and overall costs, and high chemical durability. Thus, the synthesis of composite materials combining the advantageous properties of several types of iodine sorbents can be a promising way to fulfil the requirements of real applications. We hope this review can provide some useful references for researchers, and further broaden the research scope of adsorbent materials.

We tried to present an up-to-date overview of this rapidly growing field; however, this subject is very active, and many papers are contributed each year (even during the preparation of this article) from chemists, energy, environmental and materials scientists. Therefore, it is hard to consider all publications in this field in limited pages. We apologize here if some significant contributions were omitted.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The financial support of the Science and Technology Program of Jilin Province (Grants No. 20180520159JH, 20180520152JH), the Research Program on Science and Technology from the Education Department of Jilin Province (JJKH20190995KJ, JJKH20191011KJ), the National Natural Science Foundation of China (Grants No. 21501065), and the Changbai Mountain Scholars Program (Grant No. 2013073) are acknowledged.

Notes and references

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