Progress of Covalent Organic Frameworks in Iodine Capture

Yunchao Ma; Yuxin Yao; Yue Fu; Chunbo Liu; Bo Hu; Guangbo Che

doi:10.7536/PC221118

Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (7) : 1097-1105 DOI: 10.7536/PC221118

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Progress of Covalent Organic Frameworks in Iodine Capture

Yunchao Ma ¹^,²
, Yuxin Yao ¹^,²
, Yue Fu ¹^,²
, Chunbo Liu ¹^,³^,^*
, Bo Hu ²^,^*
, Guangbo Che ¹^,³^,⁴^,^*

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Abstract

With the development of the nuclear industry, radioactive iodine was identified as one of the most hazardous nuclear wastes. Radioactive iodine capture also plays an important role in reducing the contamination of nuclear wastewater. Covalent organic frameworks (COFs), a crystalline porous organic material formed by covalent bond connection, are considered an ideal candidate for iodine capture materials for their large specific surface area, regular pore structure and high chemical stability. COFs are considered as ideal iodine trapping materials due to their structural characteristics and the fact that the adsorption sites of COFs are easily occupied by iodine molecules. This paper mainly reviews the progress of COFs with periodic porous structure and tunable functions in the field of iodine capture. Firstly, the recent progress in iodine capture of imine bonded COFs was briefly reviewed. Secondly, iodine capture capacity of compound COFs and ionic COFs are discussed. Finally, the potential of efficient iodine capture COFs to scale and the future development of this field.

Contents

1 Introduction

2 Capture of iodine by different types of COFs

2.1 Imine bonded COFs

2.2 Compound-functionalized COFs

2.3 3D COFs

2.4 Ionic-multivariated COFs

3 Conclusion and outlook

Graphical abstract

Keywords

covalent organic frameworks / radioactive materials / functionalized synthesis / iodine capture

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Yunchao Ma, Yuxin Yao, Yue Fu, Chunbo Liu, Bo Hu, Guangbo Che. Progress of Covalent Organic Frameworks in Iodine Capture. Progress in Chemistry, 2023, 35(7): 1097-1105 DOI:10.7536/PC221118

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1 Introduction

Spent fuel reprocessing is a key link to ensure the sustainable development of nuclear energy as a clean energy. If concentrated nitric acid is used in the spent fuel processing, it will lead to strong acidity and high temperature, and a large amount of radioactive iodine will be released from the reprocessing facility. The main substances in the dissolved waste stream produced in this process are the highly volatile diatomic elemental iodine and a small amount of organic iodine. ¹²⁹I and ¹³¹I are typical radioactive isotopes of nuclear waste (where the half-life of ¹²⁹I is as long as 1.57×10⁷ years)^[1]. Radioactive iodine can rapidly diffuse into the air, resulting in persistent radioactive contamination of the environment, increasing the incidence of human thyroid disease and harming human reproductive and metabolic systems. However, due to the complex conditions of fuel reprocessing waste gas, such as high system temperature, high environmental humidity, high irradiation intensity, extremely low iodine concentration and small partial pressure of the system, and a large amount of acid gas, how to effectively remove radioactive iodine in the reprocessing system is still a challenge. Therefore, it is very important to develop iodine adsorption materials with simplicity, effectiveness and easy recycling. In the past decade, porous materials as iodine adsorbents have attracted the attention of researchers, mainly including porous organic cages, ion-exchanged zeolites, silver-based zeolites and activated carbon. However, inorganic solid adsorbents such as silver-based zeolites have high cost, poor adsorption performance and adverse effects on the environment^[2]^[3]^[4]^[5]. Metal-organic frameworks (MOFs) and porous organic polymers (POPs) have stronger adsorption capacity for iodine than zeolites because of their high surface area, but their stability of capturing iodine vapor at high temperature and in solution is low^[6]^[7]. And in the adsorption process, the complex and irregular pores in the material often lead to pore blockage, so the desired adsorption effect can not be achieved. These drawbacks will limit their practical use in iodine capture. Covalent organic frameworks (COFs) are a new type of crystalline organic porous polymers formed by covalent bonding, which have orderly internal structure and controllable pore structure. The regular pores of COFs can be used to adsorb iodine molecules without causing pore blockage, and because of its good thermal stability, it can capture iodine at high temperature and in solution. In the process of irradiation nuclear fuel processing, COFs can be used to replace concentrated acid to effectively adsorb some long-lived radionuclides (VRL nuclides) from spent fuel. Therefore, COFs have great practical value in dealing with the emergency leakage of radioactive iodine or organic iodine in nuclear accidents or spent fuel reprocessing. Covalent organic framework materials with different pore sizes and polarities can be synthesized by selecting building units with efficient capture ability for iodine. Therefore, COFs materials have great potential in capturing radioactive iodine and become ideal materials for capturing radioactive iodine. Since Yaghi et al. First successfully prepared COFs materials in 2005, it has gradually become a hot research field of current science and technology^[8]. In this paper, the research progress of iodine capture by COFs is reviewed.

2 Iodine capture by different types of COFs

Due to their regular structure, easy surface modification, low density, large specific surface area and high chemical stability, COFs have attracted wide attention in the fields of gas adsorption and storage, catalysis and optoelectronics, and have also attracted the interest of scientists and researchers in related fields. In this paper, we will analyze the iodine capture ability of different kinds of COFs from the perspectives of structure, building unit and the introduction of different functional groups to form functionalized organic ligands.

2.1 Imine-linked COFs.

Zhao's team used density functional theory (DFT) and crystal orbital Hamilton population (COHP) calculations to systematically study the adsorption mechanism of iodine on COFs materials, and then Jiang et al. Synthesized chemically stable PBDMTP COF and TTA-TTB COF, with iodine uptake capacities as high as 6.26 and 4.95 G/G, respectively^[9]^[10]. This higher adsorption capacity is attributed to the one-dimensional channel of two-dimensional COFs, which allows iodine molecules to fully enter. Wen et al. developed ETTA-PyTTA-COF through Schiff base condensation reaction, as shown in Figure 1^[11]. Due to its high BET surface area (1519 m²/g), large π-conjugated structure and a large number of imine groups in the covalent organic framework as effective iodine adsorption sites, the iodine adsorption can reach 4.6 G/G at 348 K, which is among the top of all porous adsorbents reported so far. After the first cycle, the iodine adsorption capacity of the material decreased from 4. 6 G/G to 3. 9 G/G, and then the iodine adsorption capacity of the material remained almost unchanged after several cycles, which indicated that the ETTA-PyTTA-COF material had good recyclability.

In 2021, Chen et al. Reported a high-efficiency iodine adsorbent TAPB-BPDA COF synthesized by the condensation reaction of 1,3,5-tris (4-aminophenyl) benzene (TAPB) and 4,4 ′ -diphenylbenzaldehyde (BPDA), as shown in Fig. 2^[12]. The weak interaction between the benzene ring of the main chain of the material and the iodine molecule and the electron transfer between the nitrogen atom and the iodine can enhance the iodine adsorption performance of the material. The iodine adsorption capacity of the material for aqueous solution at room temperature can reach 988.17 mg/G.

Chen et al. Used Tris (4-aminophenyl) amine (TAPA) and p-phthalaldehyde (PDA) as monomers to synthesize the nitrogen-containing covalent organic framework material TAP-PDA COF by Schiff base reaction design, as shown in Fig. 3^[13]. The TAPA-PDA COF prepared by a simple, rapid, low-temperature reaction has good crystallinity, high thermal stability, and exhibits an ultra-high capture capacity of 5. 09 G/G for iodine vapor. Although the Raman spectrum confirms that the charge transfer between TAPA-PDA COF and iodine molecules will form a by-product, multi-iodine compounds, TAPA-PDA COF can also capture multi-iodine compounds in addition to iodine because of its one-dimensional open pore. The successful synthesis of TAPA-PDA COF provides an effective strategy to solve the difficult problem of commercializing COFs and using them for practical applications.

In 2021, Sun et al. Synthesized three kinds of CCOFs (COF-TpgDB, COF-TpgBD, and COF-TpgTd) by the reaction of 2,4,6-trihydroxybenzene-1,3,5-trifurfural (Tpg) and linear amino linkage units (DB, BD, and Td) at room temperature and pressure^[14]. This kind of COFs has a strong binding ability to I₂, I⁺, I^3- and I^5-. DFT calculations show that the benzene ring, C = O and — C — (NH) — C — units in CCOFs have strong interactions with iodine^[15,16]. It is found that the three COFs have large surface area, high chemical stability and thermal stability. In addition, COF-TpgDB, COF-TpgBD and COF-TpgTd achieved high adsorption capacities of 260, 181 and 166 wt% for iodine, respectively. The outstanding contribution of this research group is to study the adsorption sites of different iodine substances in CCOFs by using theoretical calculation methods, which clearly illustrates the relationship between different functional groups and different iodine substances, and provides a basis for the design and development of more advanced iodine adsorbent materials^[17,18].

In 2021, Song et al. Successfully synthesized a series of imine-linked COFs (TFB-DB COF, TFB-BD COF, and TFB-Td COF), as shown in Fig. 4^[19]. The results show that the large pore size of the three COFs allows iodine to pass through the one-dimensional open channel of the COFs, and the Lewis acid-base interaction between the polar C-N bond and the iodine molecule also increases the ability to capture iodine. The iodine adsorption capacities of TFB-DB COF, TFB-BD COF and TFB-Td COF were 6.4, 6.23 and 4.97 G/G, respectively, and reached 99.9, 99.8 and 99.8 mg/G in n-hexane solution, respectively. In addition, after five iodine adsorption cycles, the three COFs can still remove more than 99% of iodine in n-hexane solution, which indicates that the three COFs have good recoverability for iodine adsorption in both vapor and solution. The results show that the binding energy of three kinds of COFs with iodine is different due to the different pore size, which ultimately affects the amount of iodine adsorption. Therefore, it is an important challenge to further obtain the appropriate pore size through theoretical calculation for the development of efficient iodine adsorbents.

In 2021, Zhai et al. Synthesized two new types of TTA-TMTA-COF and TTA-FMTA-COF with large porosity and good chemical stability by introducing methoxy functional groups into the framework, as shown in Figure 5^[20]. This kind of material has high specific surface area, large pore volume and high chemical stability. The electron cloud density on the benzene ring skeleton in the molecule is denser under the electron-donating action of the methoxy group, and the adsorption performance of the iodine molecule in the organic solvent is enhanced due to the high-polarity methoxy functional group, so that the obtained COFs material has higher iodine adsorption capacity, and the iodine adsorption capacity of TTA-TMTA-COF and TTA-FMTA-COF is 3.21 and 5.07 G/G, respectively.

In 2021, Zhao et al. Designed and synthesized a new adsorbent COF-PA with double adsorption of iodine containing quinoline and phenylacetylene units by post-modification, as shown in Figure 6^[21]. The adsorption rate of the material for iodine and iodine in solution is very fast, and the adsorption capacity for iodine in solution is up to 1. 3 G/H. When the weight ratio of adsorbent to iodine was 1:1, the adsorption efficiency of COF-PA reached 90%. And COF-PA can still maintain good crystallinity after soaking in organic solvents, strong acids and strong bases for a week. In addition, COF-PA can emit bright yellow fluorescence under 365 nm ultraviolet (UV) excitation, and the fluorescence is quickly extinguished when iodine is added, which verifies the effectiveness of iodine adsorption. Therefore, COFs with strong stability, high adsorption and sensitive detection function will have great application prospects in industry in the future.

In 2022, Zhang et al. Successfully prepared a 2D COFs material with high stability, high specific surface area and specific iodine adsorption, named JUC-609, as shown in Figure 7^[22]. JUC-609 has high crystallinity and large specific surface area. The adsorption capacity of JUC-609 for iodine was 5. 9 G/G at 353 K and high pressure. In addition, JUC-609 can be recycled many times, and the crystal structure and morphology are not changed. The higher adsorption capacity can be attributed to the combined effect of large porosity, effective adsorption sites, and extended π-conjugated structure.

Tetrathiafulvalene (TTF) derivatives, as superior electron donors, are able to form radical cations with the acceptor I₂ through strong chemical interactions^[23]. In 2021, Wang et al. Successfully synthesized TTF-TD-COF and TTF-TAPT-COF by introducing the tetrathiafulvalene functional group into the building unit, as shown in Fig. 8^[24]. The calculated BET surface area of TTF-TD-COF and TTF-TAPT-COF is 235 and the pore volume of 461 m²/g,TTF-TD-COF is about 0.23 cm³/g,TTF-TAPT-COF and the pore volume is about 0.28 cm³/g. The higher BET surface area and large pore volume indicate the porosity of the resulting COFs. In addition, the obtained TTF-TAPT-COF and TTF-TD-COF have the maximum iodine adsorption capacity of 5.02 and 4.38 G/G at 348 K ambient pressure, respectively, which exceed most other materials reported so far, and still maintain a high adsorption capacity after six cycles. In addition, the experimental results prove that iodine molecules will diffuse into the pore channels of COFs to produce electron transfer species (i.e., I^3- and I^5-),The synergistic effect of physical adsorption and chemical adsorption will accelerate the transport rate and adsorption rate of iodine molecules, which is due to the fact that the TTF group can enhance the lipophilicity of the framework, thus enhancing the active sites in contact with iodine molecules and improving the adsorption capacity of iodine molecules. Because of its high adsorption capacity and strong recovery, it provides a feasible method for the rational design and construction of novel and effective iodine adsorbents. This study also provides fundamental guidance for the design and exploration of advanced crystalline porous materials for rapid and efficient adsorption of radioactive iodine and other harmful molecules.

In 2021, in order to achieve high iodine adsorption, Chang et al. Introduced a TTF-based derivative, tetrathiafulvalene-tetrabenzaldehyde (TFP-TTF, Fig. 9 B) as a planar four-connected building unit, TFP-TTF and electron acceptor (I₂) are easy to form radical cations to improve iodine adsorption capacity^[25]. Our research group selected 2 ', 5' -dimethyl- [1 '' -terphenyl] -4,4 '' diamine (DTDA, Fig. 9a) as the linear building unit and 2,4-tris (4-aminophenyl) amine (TAPA, Fig. 9c) as the three-linked building unit, and successfully synthesized tetrathiafulvalenyl COFs (JUC-560 and JUC-561), as shown in Fig. 9de. The strong chemical interaction between iodine and the framework of the material was revealed by DFT calculations, X-ray photoelectron spectroscopy and Raman spectroscopy. At the same time, the results show that the TTF-based COFs have high specific surface area, JUC-560 and JUC-561 are 1815 and 2359 m²/g, respectively, and the adsorption kinetics of JUC-560 and JUC-561 at 75 ℃ are 0. 49 G/ (G · H) and 0. 70 G/ (G · H), respectively, which are significantly better than those of most reported iodine adsorbents.

In 2021, in order to enhance the host-guest interaction between the COFs skeleton and iodine, He et al. Selected 2,2 ′ -bipyridine-5,5 ′ -dialdehyde as a functional ligand to construct a two-dimensional two-pore SCU-COF-2, as shown in Fig. 10^[26]. The experimental results show that the absorption capacity of SCU-COF-2 is as high as 5. 1 G/G at 348 K for 24 H, and the maximum absorption capacity is 6. 0 G/G at 96 H. The excellent iodine capture ability of SCU-COF-2 was demonstrated by breakthrough experiments at high humidity and temperature. At 348 K, the absorption capacity of SCU-COF-2 was up to 5. 1 G/G at 24 H, and the maximum absorption capacity was 6. 0 G/G at 96 H. The excellent iodine capture ability of SCU-COF-2 was demonstrated by breakthrough experiments at high humidity and temperature. At 348 K, the absorption capacity of SCU-COF-2 was up to 5. 1 G/G at 24 H, and the maximum absorption capacity was 6. 0 G/G at 96 H. The excellent iodine capture ability of SCU-COF-2 was demonstrated by breakthrough experiments at high humidity and temperature.

In 2022, Liu et al. Developed a mild and efficient microwave irradiation method instead of the traditional solvothermal method to prepare copper phthalocyanine-based covalent organic framework Cu_xPc-COFs in only 15 min, as shown in Fig. 11^[27]. Nitrogen-rich 1,2,4,5-tetramethylbenzene (TCNB) was chosen as the sole organic ligand to construct 2D COFs of copper phthalocyanine. The obtained Cu_xPc-COFs exhibited good iodine adsorption capacity of 2.99 G/G for volatile iodine and 492.27 mg/G for iodine in cyclohexane solution, which is better than most reported porous adsorbents. Spectral analysis and DFT calculations show that this good adsorption performance is attributed to the charge transfer between the nitrogen-rich phthalocyanine structure and the iodine molecule. In addition, the strong electrostatic interaction between the Cu (II) in the chelating center and the polyiodide anion (I^x^-) also plays an important role in trapping the radioactive iodine. According to the experimental data and theoretical analysis, there are three interaction mechanisms between the Cu_xPc-COFs and iodine molecule: the charge transfer between the nitrogen atom on the imine bond and the electron-rich π-conjugated system and iodine molecule^[28⇓~30]; Electrostatic Interaction between Cu-N₄ and Polyiodide Anion; Trace redox reaction (cuprous iodide). In addition, Cu_xPc-COFs is considered to be a promising host material for the uptake of radioiodine from nuclear waste due to its rigid structure and inhomogeneously charged two-dimensional conjugate network structure. The success of this experiment also provides a simple and intelligent method to achieve effective iodine adsorption by metal-based COFs.

2.2 Composite functionalized COFs

In 2020, Li et al. successfully prepared COFs @ cotton composite by post-modification method, as shown in Fig. 12^[31]. Firstly, the surface of cotton fiber was oxidized by NaIO₄ to obtain aldehyde groups, and then the COFs were attached to the skeleton of cotton fiber by Schiff base reaction. The composite material has high specific surface area and good pore structure, and also has high iodine absorption capacity in solution, and can show fast adsorption rate, and the iodine adsorption capacity can reach 533.9 mg/G, which is much higher than that of the untreated cotton fiber.

In 2012, Zhu et al. Successfully compounded COF with SOF (supramolecular organic framework) to obtain composite material BTM with bicrystal, as shown in Fig. 13^[32]^[33⇓~35]. The iodine adsorption capacity of TTPT-DHBD COF is only 70% of that of BTM, and its saturated adsorption capacity can only reach 3. 88 G/G, indicating that the adsorption capacity of the composite has been effectively improved. The design strategy of bicrystal composites proposed in this study can not only effectively solve the problem of poor stability of traditional SOFs, but also greatly increase the types of COFs and the number of functional sites, which creates conditions for improving and enhancing the application performance of materials, and has great application value.

2.3 3D COFs

Three-dimensional covalent organic frameworks (3D COFs) have received considerable attention due to their high specific surface area and open pore structure. In 2018, Wang et al. Explored and discovered a stereotetra-linked adamantane structure, which can maintain a strong porous framework due to its rigid structure and chemical stability^[36]. The research group used this characteristic to design and synthesize three-dimensional COF-DL229 containing adamantane building units, as shown in Figure 14. The material has an eight-fold interpenetrating diamond topology and ordered one-dimensional nanopores. In addition, the edge part has phenyl groups connected by two imine bonds, giving the skeleton structural flexibility. These characteristics determine that this is an ideal porous material for iodine adsorption, and the iodine adsorption capacity can reach 4. 7 G/G at 75 ℃ and ambient pressure.

In 2020, Guo et al. Selected hexane (4-formylphenoxy) cyclotriphosphazene (CTP-6-CHO) with a symmetrical structure similar to that of C₆ and p-aminobenzene to synthesize QTD-COFs with quasi-three-dimensional (Q-3D) topology, as shown in Fig. 15^[37]. The iodine adsorption rate of the prepared QTD-COFs is faster than that of the traditional COFs, and the adsorption capacity is 6. 29 G/G. The experimental results show that the construction of 2D COFs with 3D structural characteristics is an effective method to improve the permeability and mass transfer rate of materials and achieve rapid adsorption of guest molecules, while avoiding the shortcomings of high cost and monomer scarcity in the preparation of 3D COFs.

2.4 Ionized multicomponent COFs

A series of ionic COFs (iCOFs) for iodine capture were designed and synthesized by using a "multi-component" strategy combined with post-modification method, which have unique structural and compositional characteristics^[38,39]. In 2021, Xie et al. Optimized COFs through multiple experiments, and finally obtained ionic COFs (iCOF-AB-50), as shown in Fig. 16^[40]. First, the class of materials exhibits high crystallinity and large specific surface area (>2000 m²/g), which ensures that significant porosity can be maintained after ionic modification. Second, there is a high density of imine and triazine groups throughout the framework, thus providing a large number of binding sites for I₂. The static I₂ absorption capacity of iCOF-AB-50 was measured to be 10.21 G/G at 75 ° C, and the dynamic absorption capacity was measured to be 2.79 G/G at 25 ° C, far exceeding the performance of previously reported adsorbents under similar conditions. In addition, iCOF-AB-50 also has fast adsorption kinetics, good moisture tolerance, and sufficient reusability. Ionic COFs will also occupy a place in the future industrial development as high-iodine adsorbents.

3 Conclusion and prospect

In this paper, based on the structural design of COFs, the efficient adsorption of iodine by different types of COFs and the change of iodine adsorption capacity in different environments were discussed in detail. The performance of COFs materials for the adsorption and removal of iodine in solution has been fully demonstrated. The maximum adsorption capacity of COFs can reach more than 6 times the mass of the adsorbent itself. At the same time, COFs materials have a very fast kinetic process, and the adsorption effect is far superior to that of existing commercial adsorbents. Although many researchers have made great breakthroughs in the direction of efficient iodine adsorption, COFs still face challenges in iodine adsorption.In order to be applied in the actual environment as soon as possible, a series of potential problems still need to be solved, such as the harsh synthesis conditions of COFs, the poor stability of COFs in the strong radiation environment, the complex environmental factors affecting the iodine adsorption capacity, and whether the efficient iodine adsorbents can be recycled many times. In the previous design concept of iodine adsorption materials, iodine molecules are mostly filled into the open space, reaching high adsorption capacity under static conditions. However, this leads to relatively weak host-guest interactions when iodine is at low concentrations, as well as poor specific selectivity when coexisting gaseous molecules are present in large amounts. Therefore, in the future, the following three problems should be focused on in the research and development of COFs materials with efficient iodine adsorption from the laboratory testing stage to the actual industry: first, most of the solvents used in the synthesis of COFs materials are organic solvents harmful to human body, so a green and simple synthesis path should be selected; Second, although COFs have achieved excellent adsorption capacity for iodine in solution, their application in real water environment needs to be explored.In order to improve the adsorption capacity, metal atoms, electron-rich atoms and electron-donating groups can be introduced into COFs to increase the electron density around the active site and strengthen the interaction between the host and the guest, thus improving the adsorption capacity of iodine. Thirdly, laboratories usually use strong acid and alkali solutions to treat COFs after iodine adsorption, but these strong acid and alkali solutions may be dangerous in practical application, so their concentrations should be reduced or milder regeneration reagents should be used instead in future studies.

In addition, due to the rapid development of the application of COFs materials in the environmental field, the related ecological risk assessment needs to be carried out. COFs are expected to play a more important role in iodine treatment technology as people understand more about COFs.

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