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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 3: 475 - 495

DOI: https://doi.org/10.7536/PC220810

Review

The Stability Enhancement of Covalent Organic Frameworks and Their Applications in Radionuclide Separation

  • Zhang Huidi 1, 2 ,
  • Li Zijie , 1, * ,
  • Shi Weiqun , 1, *
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  • 1. Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences,Beijing 100049,China
  • 2. State Key Laboratory of Featured Metal Materials and Life-cycle Safety for Composite Structures, School of Resources, Environment and Materials, Guangxi University,Nanning 530004, China
* Corresponding author e-mail: (Zijie Li)
(Weiqun Shi)

Received date: 2022-08-15

  Revised date: 2022-10-03

  Online published: 2023-02-16

Supported by

National Science Fund for Distinguished Young Scholars(21925603)

National Science Foundation of China(11975016)

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Abstract

Covalent organic frameworks (COFs) are a class of crystalline organic porous polymers with long-range ordered structures prepared by reversible reactions. Due to high radiation resistance, structural designability and functionalization, COFs are expected to play a role in the efficient adsorption of radionuclides and the exploration of interaction mechanism. However, the reversibility of typical linkage bonds causes the limited chemical stability of COFs. This paper reviews the improvement strategies towards chemical stability of COFs (including the decrease of reversibility of linkage bonds, the post synthetic transformation from reversible bonds to irreversible ones, and the construction of hydrophobic environment around linkage bonds), crystalline control (including the influence of synthesis conditions, in layer coplanar and interlayer interaction for two-dimensional COFs and the crystallization of amorphous polymers), functionalization methods and the applications of COFs in the separation and enrichment of radionuclides. The interaction between radionuclides and COFs could be optimized by enhancing the strength of COFs skeleton, introducing special functional groups or changing the size of monomers. The application prospect and research focus of COFs in radionuclide separation are prospected.

Key words： covalent organic framework; chemical stability; crystallinity control; functionalization; radionuclide

Cite this article

Zhang Huidi , Li Zijie , Shi Weiqun . The Stability Enhancement of Covalent Organic Frameworks and Their Applications in Radionuclide Separation[J]. Progress in Chemistry, 2023 , 35(3) : 475 -495 . DOI: 10.7536/PC220810

Contents

1 Introduction
2 Typical reversible reactions of COFs
2.1 B—O bond formation
2.2 C=N bond formation
2.3 C—N bond formation
2.4 C—O bond formation
2.5 C=C bond formation
2.6 Others
3 Improvement of COFs linkage stability
3.1 COFs linkage cyclization reaction
3.2 Oxidation or reduction of imine linkage
3.3 COF to COF transformation via monomer exchange
3.4 Others
4 Regulation of crystallinity
4.1 Effect of synthesis conditions on crystallinity
4.2 Intralayer coplanarity of 2D COFs
4.3 Interlayer stacking force of 2D COFs
4.4 Crystallization of amorphous polymer
5 Functionalized syntheses of COFs
6 Applications of COFs in separation and enrichment of radionuclides
6.1 UO 2 2 +
6.2 I2vapor
6.3 TcO 4 - / ReO 4 -
7 Conclusion and outlook

1 Introduction

Covalent organic frameworks (COFs) are porous organic crystal materials connected by Covalent bonds, which are mainly composed of C, H, O, N, B and other light elements. Different from the traditional polymer synthesis, the formation of COFs is mainly based on the "thermodynamically controlled dynamic reversible reaction", which repairs the imperfect defects on the skeleton in the process of continuous bond formation, bond breakage and bond formation, and finally obtains high crystallinity products. COFs can be divided into two-dimensional and three-dimensional topologies. Two-dimensional COFs, planar building monomers with different symmetries and molecular sizes are connected and expanded in the plane through covalent bonds to obtain ring structures with different shapes and sizes.The conjugated structure is formed by non-covalent bond interactions such as π-π stacking between layers, and one-dimensional channels are also formed. The shape and size of channels are closely related to the stacking mode between layers (positive pair or staggered stacking). For three-dimensional COFs, building monomers with tetrahedral configuration or orthogonal geometry are introduced to form a regular three-dimensional network framework through infinite extension of covalent bonds, mostly with multiple interspersed structures.
COFs have the characteristics of high specific surface area, porosity, low density, highly ordered periodic structure and easy functionalization[1,2][3][4,5][6][7]. Therefore, it shows great application potential in the fields of gas storage, heterogeneous catalysis, separation and purification, energy and biomedicine. Among them, two-dimensional COFs, π-electron conjugated system in the plane and ordered π-π columnar stacking between layers, endow COFs with excellent photoelectric properties and radiation resistance, which have attracted wide attention and research in the field of radionuclide separation.
The reversibility of connecting bonds also reduces the chemical stability of COFs, and the strong acid and irradiation environment of spent fuel reprocessing puts forward higher requirements for the stability of their structures. Based on this, this paper briefly reviews the development of covalent bonding of COFs, which is also a process of decreasing reversibility and increasing stability of bonding. The further improvement of chemical stability, crystal modification and functionalization of COFs, as well as the application of COFs in the removal and detection of radionuclides up to now, are discussed.

2 COFs connection key type

The strength of the connecting bond is a key factor affecting the stability of the COFs framework. At present, more than a dozen dynamic and reversible reactions have been reported to synthesize COFs with different chemical bonds, such as boric anhydride, borate ester and spiroborate linkages based on B — O, imine, hydrazone, azine and triazine linkages based on C = N.C — N based β-ketoenamine, polyimide and amide linkages, C — O based polyarylether (1,4-dioxin) and ester linkages, in addition to sp2 hybrid carbon-carbon double bonds and cyclic linkages such as phenazine and benzimidazole (Scheme 1)[8⇓~10].
图式1 应用于COFs合成的可逆反应

Scheme 1 Reversible reactions used in COFs synthesis

2.1 B — O bond

The initial synthesized COFs all have B — O as the connecting bond. In 2005, Yaghi et al. First reported boronic anhydride-linked COF-1 synthesized by boric acid self-polymerization and boronic ester-linked COF-5 synthesized by boric acid and polyphenol compound (2,3,6,7,10,11-hexahydroxytriphenyl (HHTP)) co-condensation[11]. COF-1 has a two-dimensional hexagonal planar topology with a dislocated AB stacking between the layers. After the solvent guest molecules are removed, the stacking mode will change to a completely overlapping AA stacking with a pore size of 1.5 nm. COF-5 is also a two-dimensional hexagonal planar topology with AA stacking between layers. In 2007, the research group reported four more examples of three-dimensional COFs, the key to the design of which was the selection of tetrahedral configuration boronic acid monomers with Td symmetry: tetrakis (4-borophenyl) methane (TBPM) and tetrakis (4-borophenyl) silane (TBPS), which were prepared by self-polycondensation or polycondensation with HHTP, respectively[4]. The high reversibility of the boron-oxygen bond makes this kind of COFs usually have high crystallinity. However, boron is an electron-deficient structure, and COFs connected by boron-oxygen bonds are unstable to water and air, and the structure is prone to collapse, which limits their practical application. Therefore, it is necessary to find a connection method with better chemical stability.

2.2 C = N bond

C = N bonding reactions mainly include imine bond, hydrazone bond, azine bond and triazine ring.
Imine linkage. COFs based on imine linkage show better chemical stability, which are the most reported and widely used COFs. In 2009, Yaghi et al. First introduced the Schiff base reaction of aldehyde group and primary amine to form imine bond into the preparation of COFs[12]. They reacted terephthalaldehyde with tetrakis (4-aminophenyl) methane to obtain three-dimensional COF-300 with a five-fold interpenetrating structure. It is stable in water and organic solvents such as methanol, acetone, n-hexane and DMF at a high temperature of 490 ℃. The COFs connected by the imine bond fundamentally solve the defect that the boron-oxygen COFs are unstable to water. In 2011, Wang et al. Reported the synthesis of imine-linked two-dimensional planar COF, COF-LZU-1, from benzenetricarboxaldehyde and p-phenylenediamine[13]. At the same time, palladium acetate was loaded in the interlayer space and applied to the catalysis of Suzuki-Miyao coupling reaction, which is the first time that COFs materials have been used as catalyst supports for heterogeneous catalytic systems. In 2015, Jiang et al. Introduced two methoxy groups to the ortho position of the dialdehyde monomer of the building unit, and successfully synthesized the first TPB-DMTP-COF, which can stably exist in boiling water, strong acid and strong base[14]. The lone pair electrons on the oxygen atom in the electron-donating methoxy group are delocalized through the benzene ring to weaken the polarity of the imine bond and the corresponding interlayer electrostatic repulsion, thereby achieving the purposes of stabilizing the COF and improving the crystallinity.
Hydrazone linkage or azine linkage. The aldehyde and hydrazide or hydrazine react to form a hydrazone bond or azine bond, similar to the Schiff base reaction to give a C = N bond, and the hydrazone and azine bond-linked COFs show better hydrolytic stability than the amine bond-linked COFs. In 2011, Yaghi et al. First reported hydrazone-bonded COFs materials[15]. They synthesized COF-42 and COF-43 by polycondensation of linear terephthaloyl hydrazide with ethoxy side chain with trimesic aldehyde and 1,3,5-tris (p-formylphenyl) benzene, respectively. On this basis, COF-IHEP-1 and COF-IHEP-2 were synthesized by condensation of phosphate functionalized hydrazide monomer (2,5-bis [2- (diethoxyphosphoryl) ethoxy] -terephthalhydrazide) with aromatic aldehydes in our group[16]. The crystal form of COF-IHEP-1 was retained after soaking in 3 M HNO3, 1 M NaOH aqueous solution or organic solvents such as DMF, tetrahydrofuran (THF), n-hexane and acetone for 24 H. At the irradiation dose of 200 kGy, the stable structure can also be maintained, showing extremely high irradiation stability. Up to now, few hydrazone-bonded COFs have been reported, mainly because of the poor solubility of hydrazide monomer, which is not conducive to the condensation reaction.
For the first time, Jiang et al. Condensed 1,3,6,8-tetrakis (4-formylphenyl) pyrene and hydrazine hydrate to obtain azine-linked COF[17]. The COF has a rhombic structure with a pore size of 2 nm, and the crystallinity can be maintained in 1 M NaOH and HCl. Later, Liu et al. Reacted 2,4,6-triformylphloroglucinol (Tp) with hydrazine hydrate to obtain JLU-2, which also has good chemical stability[18].
Triazine ring ligation. Aromatic nitriles can cyclotrimerize in ZnCl2 at 400 ℃ to form triazine-linked COFs, which usually have good chemical and thermal stability, high nitrogen content, but low crystallinity, which limits their development. Baek et al. Proposed a new synthetic method to form triazine-linked pCTF-1 by trimerization condensation of benzenedicarboxamide catalyzed by P2O5[19]. It has high crystallinity, excellent porosity, specific surface area (2034.1 m2/g) and nitrogen content, which make pCTF-1 have strong adsorption capacity of CO2 and H2.

2.3 C — N bond

There are three main types of C-N bond forming reactions: the first is imine-based tautomerization, the second is imidization of phthalic anhydride with primary amines to form phthalimide, and the third is the reaction of squaric acid or dimethyl succinylsuccinate (DMSS) with aromatic amines. In 2012, when Banerjee et al. used Tp and p-phenylenediamine monomers to synthesize COF materials through Schiff base reaction, they observed that the imine bond and phenolic hydroxyl group formed an imine-enol structure, which was transformed into a ketone-enamine C-N bond structure through tautomerism, and COF stable in 9 M HCl, 9 M NaOH and boiling water was obtained[20]. In addition, during tautomerization, the atomic positions remain almost unchanged, involving only the transformation of chemical bonds, thus not changing the crystallinity of COF. After that, the research group also used Tp as an aldehyde monomer to react with terephthalic acid dihydrazine to obtain COFs mixed with keto-enamine and hydrazone bonds[21].
Yan et al. First reported a series of polyimide-linked COFs[22]. PI-COF-1, PI-COF-2 and PI-COF-3 were prepared from aromatic tetracarboxylic anhydrides and triamine monomers under isoquinoline catalysis. Among them, PI-COF-3 has a pore size of 5.3 nm and a specific surface area of up to 2346 m2/g, which surpasses all amorphous porous polyimide polymers at that time. This series of polyimide COFs have good thermal stability and can maintain the skeleton structure at 530 ℃. In 2018, Zhu et al. Synthesized a naphthalene diimine porous aromatic skeleton with stable skeleton and good crystallinity from naphthalene-1,4,5,8-tetracarboxylic dianhydride and Tris (4-aminophenyl) amine in the same solvent[23]. Thermogravimetric analysis and powder XRD showed that PAF-110 had good thermal stability and could maintain crystallinity after exposure to air for more than one year. Acetylene can be adsorbed efficiently and selectively in the environment of ethylene through the electrostatic interaction between acetylene and carbonyl oxygen atoms in the framework.
In addition, in 2013, Jiang et al. Successfully synthesized a planar zigzag undulating porphyrin COF (CuP-SQ COF) connected by squaraine units through the condensation of squaric acid and amino monomers[24]. The CuP-SQ COF has good thermal stability and chemical stability, and the steric effect formed by its unique structure can prevent the slippage between layers. In addition, the fully conjugated structure endows the Cu-SQ COF with a wider light absorption range, which has potential applications in the field of photocatalysis. Chen et al. Successfully prepared two C-N linked COFs: AAm-TPB and AAm-Py through the condensation reaction between DMSS and amino derivatives[25]. They are all AA opposite stacking structures, forming hexagonal and quadrilateral topologies, respectively. Immersion in 1 M H2SO4, 6 M HCl and 6 M NaOH at room temperature for 24 H did not change the crystal form, pore structure and chemical composition, highlighting the stability of the linkage.

2.4 C — O bond

The C — O bond is mainly in the form of aryl ether bond and ester bond. In 2018, Yaghi et al. Reported that poly (aryl ether) COF-316 and COF-318 linked by benzo-1,4-dioxin were prepared by base-catalyzed nucleophilic aromatic substitution reaction between o-diphenols and o-difluorobenzene monomers[26]. This kind of COFs has super acid and alkali resistance, strong oxidant and strong reductant stability. In 2019, Fang et al. Also reported aryl ether linked ultrastable COFs: JUC-505/506, in which JUC-505 was refluxed in lithium aluminum hydride or sodium hydroxide solution, respectively, and the cyano group in the structure was reduced to an amine group or hydrolyzed to a carboxyl group to give JUC-505-NH2 and JUC-505-COOH[27]. These functionalized COFs all maintain the crystallinity, chemical and pore structure of the parent framework, and have a good adsorption effect on antibiotics in sewage at pH values ranging from 1 to 13. The functionalized COFs can be recycled for 5 times with stable performance. Huang et al. Successfully introduced phthalonitrile and its metal derivatives into the structure of poly (arylene ether) COFs and demonstrated high activity, selectivity, and chemical stability in electrocatalytic CO2 reduction experiments[28].
In 2020, Yaghi et al. First prepared ester-linked COFs by transesterification between phenolic monomers and ester monomers[29]. Due to the short research time, the understanding of it needs to be deepened.

2.5 sp2 carbon conjugated COF (C = C bond connection)

In 2016, Zhang et al. First reported sp2 hybrid carbon-carbon double bond (C = C) -linked COF: 2DPPV by using Knoevenagel reversible condensation reaction and base-catalyzed reaction of phenylacetonitrile and aromatic aldehyde monomer[30]. Because C = C is highly consistent with the π-π conjugation degree of the benzene ring constituting the COF skeleton, the electron delocalization ability and stability of the framework are greatly enhanced, which has incomparable advantages over traditional materials. Subsequently, the feasibility of 2,4,6-trimethyl-1,3,5-triazine (TMT) as a building monomer was confirmed by the research group[31]. Jiang et al. Also reported that a series of similar sp2COFs,C=C bonds connected pyrene and benzene/biphenyl into a two-dimensional lattice and formed an ordered overlapping layer structure[32]. The magnetic, radical, and photoactive semiconductor properties of the fully sp2 conjugated COFs are described in detail. However, due to the low reversibility of Knoevenagel condensation, the synthesis of sp2 carbon conjugated COFs is very difficult and challenging.
In 2019, Thomas et al. Synthesized two sp2 carbon COFs by the reaction of TMT with terephthalaldehyde and 1,3,5-tris (p-formylphenyl) benzene, respectively[33]. The methyl hydrogen of TMT has a certain acidity and is easily attacked by alkali to form a stable carbanion, which further undergoes Aldol condensation with aromatic aldehydes. Yaghi et al. Also successfully prepared C = C double bond-bridged COF-701 by Aldol condensation reaction using TMT and 4,4 '-biphenyldialdehyde (BPDA) as monomers[34]. The specific surface area of the material is up to 1715 m2/g, and the crystal form is still maintained under strong acid and strong alkaline conditions. A strong Lewis acid, BF3·OEt2, was immobilized in the pore of COF-701structure as a heterogeneous Lewis acid catalyst, and its high catalytic activity was proved in the Diels Alder reaction.
After quaternization of the pyridine nitrogen, the methyl substituent at the ortho/para position is activated to undergo Knoevenagel condensation with aldehydes to produce various C = C linked molecules. Inspired by this, in 2021, Zhang et al. Used 2,4,6-trimethylpyridine as a precursor to obtain framework positively charged sp2 conjugated COFs materials through quaternization and condensation with different aldehydes[35].

2.6 Other connecting key

Other N-based COFs materials also show significant chemical stability, such as the direct synthesis of heterocyclic (phenazine and benzimidazole) linked COFs with limited reversibility under the action of catalysts, which can maintain chemical stability and crystallinity in strong acid and alkali environments[5][36]. Li et al. Also synthesized CCU, CCTU and CCTS COFs connected by C — N, C — S and C — O bonds[37]. Later, the research group synthesized MPCOF connected by P-N bond and CPF connected by P-O bond, both of which expanded the bond types of COF[38].

3 Improved bond stability of COFs

The use of reversible covalent bonds not only ensures the formation of crystalline structure, but also brings about the problem of reducing chemical stability, which has a great impact on the practical application of COFs. In recent years, researchers have found that reversible covalent bonds (especially imine bonds) can be converted into cyclic structures or other more stable bonds through "post-synthesis modification strategy" or even "multi-component one-pot method", while maintaining the high crystallinity of materials. It not only develops new and more stable bond types, but also may endow COFs with new functions. In addition, most COFs materials are generally highly resistant to common organic solvents (such as dichloromethane, THF, n-hexane, etc.), and their chemical stability is mainly limited by their stability in water or acid-base media. Therefore, hydrophobic treatment of the environment around the connecting bonds of COFs is also a common strategy to improve chemical stability.

3.1 Linkage cyclization reaction

The reversible covalent bond is expected to be converted into a more stable aromatic heterocyclic (such as thiazole, oxazole, or quinoline) linker with stronger π electron delocalization through the cyclization reaction of the linker itself or with the adjacent group (Scheme 2), which can also enhance the π-π interaction between the layers of COFs. Lotsch et al. Treated the imine-linked TTI-COF with molten S8 at 350 ° C, and successfully oxidized and cyclized the imine bond into benzothiazole-linked units to obtain the isostructural TTT-COF[39]. Compared with TTI-COF, the crystal structure of TTT-COF was not significantly changed after treatment in NaOH, hydrazine and sodium borohydride solutions, which highlighted its chemical stability. Moreover, TTT-COF has higher electron contrast and electron beam stability, and can be used to observe the grain boundaries and defects of the framework structure by electron diffraction and transmission electron microscopy (TEM). Cooper et al. Also synthesized a series of thiazole-linked TZ-COFs with high crystallinity, high specific surface area and chemical stability by reacting aldehyde monomer, amine monomer and S8 under COFs synthesis conditions (120 ℃) through imine ortho-C sulfurization and oxidative cyclization using a "multi-component one-pot method"[40]. The reaction process is simple, does not require high temperature, and is suitable for a wider range of monomers, thereby providing a diversified and efficient preparation method for the synthesis of thiazole COFs.
图式2 若干COFs连接键稳定化处理方法

Scheme 2 Several stabilization strategies towards COFs linkage

Baek et al. First prepared imine-linked I-COF with high crystallinity and porosity through the condensation reaction of 1,3,5-triamino-2,4,6-trisphenol and terephthalaldehyde, and then treated I-COF with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) as an oxidant, resulting in the cyclization-oxidative dehydrogenation reaction between the imine bond and the ortho-phenolic hydroxyl group to obtain benzoxazole rigid-linked BO-COF[41]. The BO-COF retains the framework structure of I-COF, but the specific surface area decreases slightly, which is attributed to the decrease of pore size and crystallinity to a certain extent caused by imine cyclization, but its thermal stability and chemical stability are greatly improved. In 2018, Wang et al. Added basic benzimidazole to the reaction system through the cascade reaction of "multi-component one-pot method", which could not only promote the final dehydrogenation aromatization, but also greatly improve the crystallinity of COFs compared with other bases[42]. The formation of imine bond and the transformation to benzoxazole heterocycle were realized, and a series of ultrastable benzoxazole heterocycle-linked COFs: LZU-190/191/192 were obtained. These COFs showed long-lasting stability under 9 M HCl, 9 M NaOH, or light conditions. In addition, the benzoxazole is constructed into a conjugated framework, so that the visible light absorption of the material is enhanced, and the material shows excellent photocatalytic activity and recycling performance in the reaction of oxidative hydroxylation of phenylboronic acid to phenol driven by visible light, with up to 20 catalytic cycles and a yield of more than 99% each time. Therefore, the series of COFs are prepared to achieve stability and functionality at the same time, that is, the so-called "two birds with one stone" effect.
Li et al. Obtained two imine-linked COFs by Schiff base reaction of 2,4,6-tris (4-formylphenyl) -1,3,5-triazine (TATTA) and 1,3,5-tris (p-formylphenyl) benzene with 2,5-bis (5-methylthiophen-2-yl) benzene-1,4-diamine, respectively[43]. Ultrastable fully π-conjugated fused aromatic thieno [3,2-c] pyridine-linked COFs, B-COF-2 and T-COF-2, were obtained by Pictet-Spengler oxidative cyclization between the imine carbon and the β-carbon adjacent to the thiophene group. Powder XRD, HRTEM and N2 adsorption characterization confirmed that the crystal forms of these COFs were well preserved, and the pore size and specific surface area were slightly reduced. The powder XRD pattern did not change significantly after immersion in 12 M HCl or 12 M NaOH solution at 50 ° C for 24 H. Further studies have clarified the photocatalytic potential of the obtained COFs as semiconductor materials, and found that the photoactivities of triazine-centered and benzene-centered COFs are completely different. Triazine-centered T-COF-2 showed the best NADH regeneration rate (74%) in 10 min, which was more efficient than most reported materials.
In 2018, Liu et al. Used Povarov cycloaddition reaction to react COF-1 linked by imine bond with 4-substituted phenylacetylene in toluene solution of BF3·Et2O and chloranil to obtain MF-1 series COFs linked by quinoline six-membered ring, while retaining crystallinity and porosity, which also provided an opportunity for COF to add additional functional groups[44]. The π conjugation of the modified MF-1 framework is strengthened, the degree of polarization is weakened, and the hydrophobicity is significantly enhanced. Compared with COF-1, it maintained stability under strong acid, strong alkaline and redox conditions, and was the most chemically stable COF material at that time. In 2020, Dong et al. Similarly prepared quinoline-linked COFs through a "multi-component one-pot method"[45]. Specifically, TAPB, DMTPA, and styrene were heated at 120 ° C for 3 days in a mixed solvent of o-DCB and n-BuOH containing BF3·OEt, DDQ, and acetic acid to obtain P-StTaDm-COF (2), which has the same structure as MF-1a.
In addition to transforming the imine bond into a more stable ring structure to enhance the stability of the skeleton, researchers have also tried to transform the hydrazone bond and C = C bond, but due to the difficulty, there are few reports at present. Wang et al. Used DMF as solvent to irreversibly oxidatively cyclize the hydrazone-linked unit in the H-COF structure to the more stable oxadiazole-linked ODA-COF with Cu(CF3SO3)2[46]. The crystallinity and periodicity of COF were retained in the post-synthesis modification process, and the chemical stability was further improved. The extended π electron delocalization, which suppresses the recombination of photogenerated charge carriers and promotes electron transfer, significantly improves the photocatalytic hydrogen production activity compared with the hydrazone bond-linked H-COF. Gu et al. Reacted a hydroxyl-containing aldehyde monomer with a cyano monomer in an O2 and alkaline environment, which first underwent Knoevenagel condensation to form a C = C linked skeleton, followed by cyano migration and oxidative cyclization to form an irreversible benzofuran heterocycle-linked skeleton[47].

3.2 Oxidation or reduction of imine bond

Stabilization of the imine bond can also be achieved by conversion to a C — N single bond, i.e., oxidation to an amide or reduction to a secondary amine (Scheme 2). Polyamide (commonly known as nylon) is a kind of high-performance polymer with high stability and mechanical properties, which is widely used in industry and daily life. COFs with amide as the connecting unit have the properties of both polyamide and COFs, and are considered to have high practical application value. In 2016, Yaghi et al. Used TPB-TP-COF (1) and 4PE-1P-COF (2) linked by imine bond as precursors, sodium chlorite as oxidant, 2-methyl-2-butene to remove hypochlorous acid, a reduction by-product of sodium chlorite, and acetic acid as catalyst, and finally reacted in 1,4-dioxane for 48 H to obtain the corresponding amide-linked COFs[48]. The products were characterized by FT-IR, 13C CP-MAS NMR and powder XRD, which confirmed the complete oxidation of the imine and the preservation of the crystalline framework structure, and the chemical stability was significantly improved. In 2022, Zhao et al. Used a simpler and milder method, using KHSO5(Oxone) as oxidant, acetic acid as buffer and DMF as solvent, to convert the imine bond in the precursor COF into amide bond in only a few hours, while maintaining the crystalline framework structure of the precursor[49]. The method has strong universality, seven tested imine COFs with different structures are successfully converted into corresponding amide COFs, and polyamide COFs are prepared on a gram scale under the same reaction conditions, thereby demonstrating the high efficiency and scalability of the synthesis method.
The reduction of the imine bond to the secondary amine not only improves the stability, but also provides a new possibility for the functional modification of the post-synthesis. In 2018, Deng et al. Used NaBH4 as a reducing agent to react with imine-linked COF-300 or COF-366-M (M = Co, Zn, Cu) in methanol solution at 0 ℃ overnight, and successfully converted them into the corresponding secondary amine linked COF-300-AR and COF-366-M-AR, which retained crystallinity and improved chemical stability in strong acid and alkali environments[50]. Furthermore, the researchers deposited COF-300-AR on the surface of silver foil electrode, making full use of the selectivity provided by the abundant secondary amine functional groups on the wall of COF channel and the high activity of silver electrode to achieve highly selective and efficient electrocatalytic reduction of CO2. In 2021, Lotsch et al. Successfully reduced two-dimensional PI-3-COF to secondary amine-linked rPI-3-COF using milder formic acid based on the Leuckart-Wallach reaction[51]. The material retains the high crystallinity and hexagonal symmetry of the imine precursor, but the relatively flexible C-N bond and the steric repulsion of the benzylic hydrogen in the adjacent layer increase the layer-by-layer stacking spacing. The porosity and pore size distribution were also completely retained, but the specific surface area decreased by 31. 2% after vacuum drying in dichloromethane medium, which may be due to the increase of flexibility and polarity of the connecting unit after reduction, which led to the increase of capillary effect of the framework structure and the interaction with solvent molecules, resulting in the disorder and collapse of the pore structure caused by the drying process. The authors also attempted a "one-pot" synthesis strategy using formic acid as the imine framework to construct the catalyst and reducing agent, and obtained crystalline rPI3-COF, but the structural order was slightly poor, and the specific surface area was only 174 m2/g.
Ma et al. Also used formic acid as a catalyst instead of acetic acid, and successfully realized the reaction process of imine framework structure-imine double bond reduction in a "one-pot" system through Eschweiler-Clarke reaction by using the dual catalytic and reductive properties of formic acid, and finally obtained a series of FAL-COFs with high crystallinity and flexibility connected by C — N single bonds[52]. Beyzavi et al. Successfully synthesized secondary amine-linked COF-366-R with high crystallinity by "one-pot" method using H3PO3 as acidic catalyst and reducing agent[53]. However, unlike the AA stacking mode of the imine precursor, the stacking mode of COF-366-R was changed to AABB due to the protonation of porphyrin during reduction, and the specific surface area of COF-366-R was only 135 m2/g.

3.3 "COF-to-COF" conversion based on monomer exchange

The reversible nature of the connecting bonds of COFs makes it possible to "isomorphically" replace the monomers after synthesis. In 2017, Zhao et al. First proposed the "COF-to-COF" conversion strategy of organic monomer exchange, and reacted the imine-linked TP-COF-BZ with 10 times equivalent of p-phenylenediamine to replace the benzidine monomer in the COF skeleton in situ, and successfully obtained the crystalline TP-COF-DAB (Figure 1), whose structure is consistent with that of the material obtained by direct polycondensation, thus confirming the feasibility of this strategy[54]. Since then, "monomer exchange" has been used to prepare COFs that cannot be synthesized by direct polymerization, which provides a new idea for improving the stability of COFs and realizing structural modification, modification and functionalization.
图1 基于单体交换的“COF-to-COF”转化[54]

Fig. 1 “COF to COF” transformation via monomer exchange[54]. Copyright 2017, American Chemical Science

Zhao et al. Reacted the COF linked by imine bond with p-dinitrosobenzene to replace the terephthalaldehyde unit in the COF structure in situ, and completely converted the imine bond into an azo bond (N = N) to obtain an azo-linked COF with a corresponding framework structure and higher stability[55]. The formation of the product COF structure is guided by the template effect of the precursor COF, which overcomes the problem that the azo construction reaction with low reversibility is not conducive to the self-repair and crystallization of the defect structure. The researchers also found that the azo COF has a narrower optical band gap and a photocatalytic degradation performance of organic dyes different from that of the imine COF. In 2020, Yan et al. First synthesized the imine-linked TzBA through the condensation polymerization of 4,4 '-biphenyl dialdehyde (BA) and 2,4,6-tris (4-aminophenyl) -1,3,5-triazine (Tz) catalyzed by Sc(OTf)3[56]. The replacement of acetic acid catalyst by Sc(OTf)3 is to avoid the destruction of the subsequent acyl chloride monomer by water in the acetic acid solution. Terephthaloyl chloride with stronger nucleophilicity is added into the system to react for 2 days at 4 deg C to realize the slow replacement of the aldehyde unit in the skeleton, and the JNU-1 with high crystallinity and irreversible amide bond connection is successfully obtained. The crystal form and structure of JNU-1 were basically unchanged after soaking in 10 M HCl or 1 M NaOH for 1 day. The structure of TzBA under the same condition collapses. The JNU-1 can rapidly and highly selectively adsorb Au (Ⅲ) through the amide site, and the maximum adsorption capacity reaches 1124 mg/G, which improves the stability and functionality at the same time.
Thiol or hydroxyl functionalized aromatic amine monomers, such as 2,5-diamino-1,4-benzenedithiol dihydrochloride and 2,5-diamino-1,4-dihydroxybenzene dihydrochloride, cannot be directly used in the preparation of imine COFs. Yaghi et al. Reacted these functionalized aromatic amines with ILCOF-1 to replace the p-phenylenediamine in the structure, and then formed benzothiazole or benzoxazole connecting units by oxidative cyclization of the imine bond with the adjacent sulfhydryl or hydroxyl groups to obtain crystalline COF-921 and LZU-192 with the overall framework structure unchanged[57]. The three COFs were immersed in 10 M NaOH, 12.1 M HCl, 18 M H2SO4, 14.8 M H3PO4, and DMSO containing 9 M H2SO4 for 1 day, respectively, and the results of powder XRD and FTIR analysis confirmed the stability of azole-linked COF-921 and LZU-192, while after the treatment of ILCOF-1 in H2SO4 solution, new phases were formed according to the results of powder XRD, and the signal of imine bond was weakened and the signal of aldehyde peak was enhanced in the FTIR spectrum, indicating that the framework had collapsed.

3.4 Other factors to improve bond stability

Du et al. Dispersed borate ester-linked COF-5 and COF-10 in acetone solution with pyridine, and replaced the benzene ring linked to borate ester with pyridine to obtain pyridine-modified p-COF-5 and p-COF-10[58]. The chemical structure, crystallinity and porosity did not change significantly after exposure to humid air for 7 days. Du et al. Introduced 3-aminopropyltriethoxysilane (APTES) into boroxane-linked COF-1 to obtain APTES-COF-1[59]. The COF is stable in air for more than 4 months, while the unmodified COF-1 decomposes within a few hours. According to the analysis, this stability improvement is due to the formation of Lewis acid-base pair complex between electron-rich pyridine/amino group and electron-deficient boron, which inhibits the nucleophilic attack of water molecules on boron, thus improving the stability of B — O bond.
Jiang et al. synthesized an imine-linked TPB-DMeTP-COF with a pore size of 3.36 nm by polycondensation of 1,3,5-tris (4-aminophenyl) benzene (TPB) and 2,5-dimethylp-phthalaldehyde (Figure 2)[60]. The two methyl substituents on the benzene ring of the aldehyde monomer trigger the hyperconjugation and induction effect, which weakens the polarity of C = N, weakens the interlayer repulsion, and is more conducive to the stability of the structure, which is similar to the mechanism of the methoxy group on the benzene ring in the TPB-DMTP-COF structure. Phosphoric acid is loaded into a TPB-DMeTP-COF pore canal through a vacuum perfusion method to obtain a H3PO4@TPB-DMeTP-COF, and imine nitrogen atoms on the wall of the pore canal can generate hydrogen bond anchoring with a phosphoric acid hydrogen bond network, so that the proton conduction capacity is 2 to 8 orders of magnitude higher than that of the existing similar anhydrous proton conduction system.
图2 超共轭和诱导效应增强TPB-DMeTP-COF层间相互作用[60]

Fig.2 The enhanced interlayer interaction of TPB-DMeTP-COF by hyperconjugation and induced effects[60]. Copyright 2020, Springer Nature

The introduction of hydrophobic groups on the COFs skeleton can not only increase the steric hindrance, but also provide a hydrophobic environment to protect the connecting bonds and further improve the stability of COFs. In 2011, Lavigne et al. Introduced a series of hydrophobic alkyl chains (methyl, ethyl, and propyl) on the COF-5 skeleton. After soaking in water for 20 min, the ethyl-modified COF-14 14 Å retained 57% of the powder XRD intensity and 76% of the specific surface area, while the unmodified COF-18 18 Å retained only 4% of the powder XRD intensity and 5.5% of a specific surface area[61]. The authors believe that although these borate ester-bonded materials are still sensitive to water, they also prove the feasibility of hydrophobic protection to improve the water stability of COFs.
In addition, researchers have also introduced alkyl chains into imine COFs to improve their chemical stability. Cui et al. Prepared imine-linked CCOF-3/4 containing a Zn (salen) structure by condensation of unsubstituted or tert-butyl-substituted trisalicylaldehyde with chiral 1,2-diaminocyclohexane in the presence of zinc acetate, both of which were AA opposite-stacked structures (Fig. 3)[62]. The tert-butyl-substituted CCOF-4 remained stable in 1 M HCl and 9 M NaOH, highlighting the protection of the hydrophobic tert-butyl group on the imine bond, while the unsubstituted CCOF-3 was almost completely soluble. Through the subsequent ion exchange method, a variety of other transition metal elements can be loaded, and efficient and recyclable heterogeneous catalysis can be achieved in a variety of organic reactions.
图3 烷基修饰的CCOF-3/4合成和结构[62]

Fig. 3 Synthesis and structure of alkyl modified CCOF-3/4[62]. Copyright 2017, American Chemical Science

Subsequently, a series of 2/3/4-component two-dimensional COFs were obtained by condensing triamine monomer or ortho-alkyl (ethyl or isopropyl) substituted triamine with dialdehyde or trialdehyde in different ratios[63]. The types and contents of alkyl substituents hanging on the channel wall of COFs are different, which can adjust the steric hindrance between layers and realize the regulation of the stacking mode (AA/AB/ABC) and chemical stability of COFs. COFs with a high proportion of alkyl substituents tend to adopt AB or ABC stacking structure, while those with a low proportion of AA stacking structure. Isopropyl-modified COFs can still maintain crystallinity and porosity after treatment in boiling 20 M NaOH solution, highlighting the protection of large steric hindrance alkyl groups on the imine bond. Among the COFs containing metallated bipyridine moieties, isopropyl COFs exhibit higher heterogeneous catalytic aromatic C — H borylation than unsubstituted or ethyl-substituted COFs, which is attributed to the higher chemical stability and developed pore structure of isopropyl COFs.
Cui and Huang et al. Successfully prepared three kinds of two-dimensional COFs with AA or ABC stacking structure with controllable hydrophobicity (water contact angle of 111.5 ° ~ 145.8 °) for ultrafast oil-water separation by condensation reaction of fluorine or isopropyl-substituted triamine and perfluorodialdehyde[64]. These COFs all showed good resistance to boiling water, alkali and stability in conventional organic solvents, but the order of acid resistance was isopropyl and fluorine co-substituted COF > single isopropyl substituted COF > single fluorine substituted COF, which was consistent with the hydrophobicity. The authors attributed this to the inhibition of hydrolysis of the ortho-imine bond by the isopropyl group and the change in the stacking pattern of the COF by the isopropyl group, resulting in smaller pore sizes.
Perfluoroalkyl chains with strong hydrophobicity were also introduced into the COFs structure. In 2018, Ma et al. Used a post-synthetic modification strategy to chemically graft perfluoroalkyl chains onto the surface of COF-V channels using click chemistry between vinyl and sulfhydryl groups to obtain COF-VF[65]. The modified COF-VF has similar crystalline structure, slightly lower specific surface area (COF-V:1152 m2/g;COF-VF:938 m2/g) and superhydrophobicity (water contact angle of 167 °) as the precursor. The superhydrophobicity of COF-VF also endows it with ultrahigh tolerance to water, acid, and alkali. COF-VF can maintain crystallinity and porosity after being treated with 12 M HCl, 14 M NaOH solution and boiling water for one week; After exposure to hydrogen chloride or ammonia atmosphere with 100% relative humidity for 48 H, the structure of COF-V remained crystalline, while it collapsed in 2 M HCl or humid hydrogen chloride atmosphere for 12 H. Horike and Zhang et al. Synthesized a series of perfluoroalkylated hydrazone linked COF-F6/8/10 with different alkyl chain lengths[66]. These COFs are equally hydrophobic (water contact angle: ∼ 142 °) and extremely stable in 85 wt%H3PO4, 38 wt% HCl, and 65 wt%HNO3. Among them, COF-F6 has been developed as a phosphate-based anhydrous proton conducting material with both high stability and high proton conductivity.

4 Crystal form control

Crystal form is an important feature that distinguishes COFs from other porous polymers, and researchers have made a lot of efforts on how to improve the crystallinity of COFs. The chemical composition/reactivity/solubility of the organic monomer, the degree of reversibility of the linkage, the synthesis reaction conditions, and the activation process of the product removal from the guest molecule will all affect the polymerization, self-assembly, crystallization behavior between monomers, and the acquisition of ordered structures[9].

4.1 Effect of Synthesis Conditions on Crystal Form

The formation process of COFs is mainly a reversible reaction, which follows the principle of "Dynamic covalent chemistry" controlled by thermodynamics. In order to obtain highly ordered crystalline COFs, the selection of appropriate catalysts, solvents and reaction conditions (temperature and pressure) is particularly critical for the formation of thermodynamically stable crystalline COFs.
Ketoenamine COFs synthesized by traditional acetic acid catalysis usually have weak crystallinity and small porosity. Taking the preparation of TpBD-COF by the reaction of Tp and benzidine as an example, Guo et al. Used a series of common organic bases (including pyrrolidine, 1-methylpyrrolidine, 2-methylpyrrolidine, piperazine and piperidine) as structural regulators.The effect on the crystallinity and porosity of COF was studied under solvothermal conditions (Fig. 3), and it was found that most organic bases had better performance compared with acetic acid aqueous solution, and pyrrolidine with a volume ratio of 1.96% could obtain the best crystal quality of TpBD-COF (specific surface area 2157 m2/g)[67]. As for the regulation mechanism, the authors believe that pyrrolidine is more basic than the aromatic amine monomer, and can preferentially react with Tp to form a relatively stable tertiary amine product, and then the aromatic amine monomer undergoes reversible amine exchange with Tp to obtain an alcohol imine structure, which is irreversibly converted into a ketoenamine. Compared with the faster Schiff base reaction, the amine exchange involving pyrrolidine has better controllability, which is conducive to improving the crystallinity through reversible molecular structure rearrangement, thus significantly improving the crystallinity and porosity of ketoenamine COF.
图式3 可逆的分子结构重排来提高结晶性[67]

Scheme 3 Reversible molecular structure rearrangement for the improvement of crystallinity[67]

Recently, Cooper et al. Used a high-throughput sonochemical synthesis method to screen efficient imine or ketoenamine COFs materials for the photocatalytic reduction of O2 to produce H2O2 through the polycondensation reaction between 11 aldehyde monomers and 11 amine monomers[68]. The authors emphasize the need to consider the reactivity of the monomer and the stability of the product, and to optimize the synthesis conditions and the activation method of the product. But in general, higher acetic acid concentration (such as 12 M) is beneficial to the synthesis of imine-linked COFs, which can not only increase the solubility of monomers, but also catalyze the condensation reaction. Excessive water in the system is not conducive to the formation of COFs, especially when the product is directly dried and activated, the high surface tension of water may cause the collapse of the pore structure, leading to the amorphization of the product. For these fragile COFs, low surface tension solvent (e.g., n-hexane) or supercritical CO2 activation is critical.
In fact, the organic monomer is only partially dissolved in the solvent thermal polycondensation process, so the selection of the appropriate solvent type and ratio, so that the monomer can maintain the ideal stoichiometric ratio, is undoubtedly conducive to the full two-dimensional or three-dimensional polymerization extension. Based on this, Chen et al. Proposed a "two-in-one" strategy to improve the stability, crystallinity, solvent adaptability and reproducibility of COFs[69]. They incorporated two aldehyde groups and two amine groups into the same pyrene-centered molecule. In the self-polycondensation process, the 1: 1stoichiometric ratio of the functional groups of the bifunctional monomer is strictly maintained, and the imine-linked Py-COF with high crystallinity and porosity can be obtained by the reaction under the condition of a mixed solvent or a single solvent (such as CH2Cl2, CHCl3, THF, methanol, ethanol, acetonitrile and N, N-dimethylacetamide), so that the solvent screening process in the reaction process is greatly simplified.
It has been reported that the type and ratio of solvents can also have an important impact on the crystal structure of porous materials, such as MOFs and molecular cages. Recently, Zhao et al. Successfully realized the controllable synthesis of COF structural isomers by changing the solvent conditions of polycondensation reaction (Fig. 4)[70]. Tetraaldehyde monomer was polymerized with 2-methyl-p-phenylenediamine in mesitylene/1,4-dioxane mixed solvent using acetic acid aqueous solution as catalyst and regulator to obtain SP-COF-ED with single tetragonal pore structure, while n-butanol/o-dichlorobenzene was used as solvent to obtain DP-COF-ED with double pores with periodic distribution of triangular and hexagonal pores. The infrared spectra and solid-state NMR spectra of the two COFs are almost the same. Although they are both AA stacked structures, they show different powder XRD diffraction peaks, indicating that they have the same chemical composition but different framework structures. In mesitylene/1,4-dioxane solvent, the bisporous isomer can also be completely converted to the monoporous isomer by co-heating with an excess of 2-methyl-p-phenylenediamine, otherwise, SP-COF-ED is a thermodynamically stable product. In addition, it was found that the crystal form of SP-COF-ED disappeared after immersion in water, while DP-COF-ED had good hydrolytic stability. The authors believe that the higher local distribution density of imine bond and methyl substituent in DP-COF-ED structure should be the main reason.
图4 反应溶剂对COF拓扑结构的影响[70]

Fig. 4 Effect of reaction solvent on the topological structure of COF[70]. Copyright 2010, Chinese Chemical Society

4.2 In-layer coplanar effect of two-dimensional COFs

Chen et al. Synthesized dibenzo [G, p] fused dinaphthalene (DBC) with better planarity by selective dehydrocyclization of tetraphenylethylene (4PE), and condensed the functionalized DBC with 4,4 ′ -biphenyldicarboxaldehyde to obtain high-crystallinity double-pore DBC-2P COF[71]. DBC-2P showed excellent thermal and chemical stability, and the crystal form and pore structure remained unchanged even after stirring in 12 M HCl, 4 M NaOH and boiling water for 7 days, while 4PE-2P collapsed after stirring for 2 H. Both powder XRD and theoretical calculation confirm that the planar structure of DBC monomer enhances the interlayer interaction of DBC-2P and narrows the interlayer spacing.
The formation of hydrogen bonds in the layer is also beneficial to the coplanar and layer-by-layer stacking. In 2013, Banerjee et al. Reacted 5,10,15,20-tetrakis (4-aminophenyl) -21H, 23H-porphyrin (Tph) with 2,5-dihydroxyterephthalaldehyde (Dha) or 2,5-dimethoxyterephthalaldehyde (Dma) via Schiff base to obtain two imine COFs: DhaTph and DmaTph[72]. Among them, DhaTph can maintain crystallinity after soaking in 3 M HCl and deionized water for one week, while methoxy-substituted DmaTph has relatively low crystallinity, porosity and chemical stability. The imine bond in DhaTph forms a hydrogen bond interaction (O — H … N = C) with the adjacent hydroxyl group, which improves the hydrolytic stability of the basic C = N group in acidic environment or aqueous solution, maintains the trans conformation of the imine bond and the benzene ring in the same plane, thus enhancing the rigidity of the layer structure and the stacking order of the layers. Subsequently, Jiang et al. Prepared a series of metalloporphyrin-centered two-dimensional COFs with similar structures, which also had intralayer hydrogen bonding on the "edge" of the framework structure, and regulated the number of hydrogen bonds (Fig. 5)[73]. It was found that with the increase of hydroxyl monomer content in the structure, the C = N absorption vibration frequency shifted to a lower wavenumber and decreased from 1622 cm-1 to 1612 cm-1, indicating that the imine bond formed an effective hydrogen bond with the hydroxyl group on the aldehyde monomer. With the increase of hydrogen bonding ratio, the crystallinity and porosity of COFs increase significantly, and the photochemical activity also increases. The authors emphasize that hydrogen bonding causes COFs to adopt a more regular planar conformation within the layer, triggering an extended π electron cloud delocalization, thereby enhancing the interlayer π-π stacking interaction.
图5 层内氢键增强共平面效应[73]

Fig. 5 The coplanarity of COF layers enhanced by intralayer hydrogen bond[73]

Vaidhyanathan et al. Constructed benzimidazole heterocycle-linked IISERP-COF3 by condensing Tp and 3,3-diaminobenzidine at room temperature[36]. An intramolecular hydrogen bond is for between that nitrogen atom of the imidazole heterocyclic ring and the adjacent phloroglucinol hydroxyl group, which is beneficial to the layer-by-layer overlap arrangement of the benzene ring, so that a highly ordered columnar π-π stacking can be maintain in the structure.
In 2018, Loh et al. Synthesized a two-dimensional hydrazone-linked antiparallel stacking structure COF by introducing an ether aliphatic side chain at the ortho-position of the hydrazide monomer, which has a complex hydrogen bond network, specifically divided into intralayer hydrogen bonds and interlayer hydrogen bonds[74]. The intralayer hydrogen bond is a six-membered ring formed by the hydrogen (N — H) on the hydrazide and the oxygen on the side chain. The interlayer hydrogen bond is more complex, one of which is formed by the hydrogen on the hydrazide and the oxygen (C = O) on the adjacent hydrazide in the layer, and the other can be formed by a hydrogen (C — H) at the second carbon on the side chain close to the oxygen atom and the oxygen (C = O) of the hydrazine in the adjacent layer. Through the tight antiparallel packing and hydrogen bond network, the rotation of intramolecular bonds is effectively suppressed, and then the fluorescence is induced. Zhao, on the other hand, synthesized similar antiparallel-stacked hydrazone-linked COF-DB and COF-DT via the condensation of 2,5-dimethoxyterephthalhydrazide (DMTHA) with 1,3,5-tris (2-formylpyridin-5-yl) benzene (BTTPA) and TATTA, respectively[75]. The reaction of hydrazide monomer without methoxyl group with BTTPA and TATTA failed to obtain crystalline materials, highlighting the important role of hydrogen bonding in the orderly assembly of materials.
Peng et al. Investigated how intramolecular hydrogen bond activation or passivation affects the cis-trans isomerization of the connecting unit through the condensation reaction of TPE-centered tetraamine monomer and 2,3-dihydroxy/dimethoxy p-phthalaldehyde, and then obtained two kinds of highly crystalline two-dimensional COFs with different topologies and porosities, star double-pore TPE-COF-OH and quadrilateral single-pore TPE-COF-OMe (Fig. 6)[76]. In addition, the N, O bidentate coordination site in TPE-COF-OH with cis-structure provides great convenience for post-synthesis modification, such as the TPE-COF-BF2 derivative obtained by boron complexation, which has unique optical properties, and its fluorescence intensity is 8 times that of TPE-COF-OH under the same conditions, which means fluorescence "turn-on" and obvious "aggregation fluorescence effect". The authors attribute this to the enhanced rigidity of the COF framework caused by boron complexation and the complete suppression of electron transport from the imine unit to the framework.
图6 TPE-COF-OH和TPE-COF-OMe结构及合成路线[76]

Fig. 6 Structure and synthesis route of TPE-COF-OH and TPE-COF-OMe[76]. Copyright 2020, American Chemical Science

4.3 Layer-by-layer stacking force

The main driving force of layer-by-layer stacking in two-dimensional COFs is π-π stacking, which is beneficial to the ordered and tight stacking between layers by enhancing the conjugation of π electrons in layers, properly introducing interlayer hydrogen bonds, and donor-acceptor interactions.
π-π stacking force. The high delocalization of π electrons in the layer is beneficial to the enhancement of π-π interaction between layers, so COFs materials have high crystallinity, porosity and stability. Jiang Donglin's group successfully synthesized phenazine heterocycle-linked chemically stable two-dimensional CS-COF via 2,3,6,7,10,11-hexaaminotriphenyl and tert-butylpyrene-tetraketone polycondensation[5]. The COF has uninterrupted π electron conjugation extension in and between layers, can accommodate guest molecules and has high hole mobility, and has application prospect in photoswitches and photovoltaic cells.
Interlayer hydrogen bonds. In 2018, Banerjee et al. Synthesized six highly stable imine-linked COFs by condensing 1,3,5-trimethoxy-2,4,6-triformylbenzene (TpoMe) with different amine monomers[77]. The methoxy group can not only provide steric hindrance and hydrophobic environment for the imine bond to avoid the attack of H+ and H2O on the connecting bond, but also form interlayer hydrogen bonds C — H … N = C with the N atom in the imine bond in the adjacent layer to enhance the interlayer interaction. These as-synthesized COFs can maintain crystallinity for a long time in the environment of 18 M H2SO4, 12 M HCl, and 9 M NaOH. The group also synthesized a self-supporting large-size thin layer COF, TpoMe-DAQ, which also has interlayer hydrogen bonds, through the condensation reaction of TpoMe and 2,6-diaminoanthraquinone (DAQ) with redox activity[78]. It has good stability in acid and alkaline solutions, and can be used in supercapacitors with an area capacitance of 100000 and a cycle stability of more than 100000 times. In 2019, Yang et al. Condensed melamine with 1,4-diformylpiperazine to obtain two-dimensional PDC-MA-COF (Fig. 7)[79]. In the COF structure, there are two C — H … N interlayer hydrogen bonds between the piperazine rings of adjacent layers, which can play a role in "locking" the interlayer spacing and avoiding slippage between layers, thus maintaining the in-plane conformation and ordered pore structure of the COF, and reducing the total energy of the system.
图7 PDC-MA-COF层间氢键增强层层堆叠作用力[79]

Fig. 7 Interlayer hydrogen bond enhances the stacking force of PDC-MA-COF layers[79]. Copyright 2019, American Chemical Science

In recent years, it has been found that the powder XRD diffraction intensity of two-dimensional COFs decreases significantly after photocatalytic reaction, suggesting that the stacking order of layers perpendicular to the plane of the framework is weakened, which seriously affects the expansion of π conjugated systems within and between layers, and is not conducive to the generation, separation and transport of photogenerated charges. Guo, Xu and Takahashi et al. Have synthesized ketoenamine two-dimensional BT-COF with high crystallinity and benzothiadiazole (BT) unit by replacing acetic acid catalyst with pyrrolidine[80]. After annealing under reduced pressure, polyethylene glycol (PEG) was injected into the hexagonal channels of BT-COF with a diameter of 2. 4 nm to obtain PEG @ BT-COF. The long PEG chain is confined in the pore in a stretched state and is firmly bonded to the pore wall through hydrogen bonds, which enhances the interlayer π-π stacking force, thereby improving the stability of the framework structure. In the photocatalytic test, it was also found that PEG @ BT-COF had longer exciton lifetime and higher charge transport performance than BT-COF, and the framework structure of PEG @ BT-COF was still well maintained after six photocatalytic hydrogen evolution cycles of 48 H.
Donor-receptor type conjugated COF. Jiang et al. Used the strategy of self-compensating electron interaction between donor and acceptor to obtain five imine-linked COFs through the polycondensation of tetraamine monomer containing copper porphyrin center with terephthalaldehyde and 2,3,5,6-tetrafluoroterephthalaldehyde in different molar ratios[81]. It was found that the crystallinity and porosity of these COFs were strongly correlated with the proportion of phenyl or tetrafluorophenyl groups on the "edge" of the framework structure. The powder XRD diffraction peak of COF-TFPh50 with the molar ratio of phenyl or tetrafluorophenyl 1 ∶ 1 in the "edge" has the highest intensity, and the specific surface area (1389 m2/g) and porosity (1.11 cm3/g) are also the largest. Pawley refinement of the powder XRD pattern showed that the cell parameters a/B (25.22 Å) and C (3.8585 Å) of COF-TFPh50 were significantly smaller than those of COFs containing only phenyl or tetrafluorophenyl, indicating that the strong self-compensating π stacking effect makes the structure of COF-TFPh50 more compact.

4.4 Crystallization of amorphous polymer

The polymerization and crystallization of borate ester COFs are synchronous, while the synthesis of imine-linked COFs involves two processes: the initial rapid formation of amorphous polymer network and the subsequent error correction and gradual crystallization. Based on this, Guo et al. Rearranged the amorphous ketoenamine polymer into COF materials with crystal form and regular structure under the control of pyrrolidine[82]. Rosseinsky et al. Reacted triangular or tetrahedral acyl chloride monomers with cis-1,4-cyclohexanediamine monomers via a typical acyl chloride synthesis route to obtain two amorphous polyamide network structures[83]. Then the amide bond was changed into a reversible reaction under high temperature (250 ℃) and high pressure in a strong hydrolysis environment, and the amorphous polymer was crystallized after a long time treatment (3 ~ 7 d) to obtain two-dimensional CAF-1 and three-dimensional CAF-2 COFs, respectively.
Monomer exchange can also promote the transformation from disordered network to ordered framework structure, and even obtain more stable new bonding COFs materials. Zhu et al. Synthesized a linear dialdehyde and diamine monomer by condensation polymerization, and reacted with Tp under solvothermal reaction conditions, resulting in aldehyde monomer replacement and enol-keto-enamine conversion in the amorphous polymer.Compared with the COF-Tp obtained by direct polycondensation under the same conditions, the COF-Tp synthesized by monomer replacement has even higher crystallinity and up to 3. 4 times higher specific surface area[84]. Zeng and Zhao et al. Also used the monomer replacement strategy to successfully realize the conversion of amorphous polymers linked by imide bonds (or polyimide bonds) to COFs linked by polyimide bonds (or imide bonds)[85]. It is worth mentioning that in the conversion of amorphous polyimide polymer to imine bond COF, the addition of terephthalaldehyde can not break the stable imide bond for monomer replacement, while the two phenolic hydroxyl groups in 2,3-dihydroxy or 2,5-dihydroxy terephthalaldehyde can provide energetically favorable non-covalent bonding (such as hydrogen bonding) to promote monomer replacement and crystallization.
Jiang, Khan, and Wu et al. First prepared imine-linked TAPT-PA amorphous polymer films by heating a certain volume of a mixed solution of TAPT and terephthalaldehyde[86]. Then 2,5-dihydroxyterephthalaldehyde (DHTA) was added to the system, and the reaction was carried out under solvothermal conditions for 3 d to realize the in situ substitution of the original aldehyde monomer in the film by DHTA and the promotion of the crystallization process, and finally the AA-stacked TAPT-DHTA film with high crystallinity was successfully obtained, accompanied by the transition from non-porous (specific surface area 10.3 m2/g) structure to porous (448.3 m2/g) structure. The intralayer hydrogen bonds present in the TAPT-DHTA structure are essential for the COF to maintain the original film morphology. This strategy bridges the gap between amorphous polymer films and crystalline COF membranes, making it possible to fabricate COF membranes on a large scale.

5 Functionalization of COFs

The functionalization of COFs is of great significance for their application expansion and performance improvement, especially in the field of radionuclide separation and enrichment. At present, there are mainly two kinds of COFs functionalization strategies: "bottom-up" and "top-down"[8]. The "bottom-up" approach is to polymerize functional monomers or monomers containing functional side chains into the framework of COFs, which can endow the framework with functionality or change the chemical environment in the pore. In general, the "bottom-up" synthesis method can achieve the uniform distribution and quantitative adjustment of the functionalized side chains in the pore channels. For two-dimensional COFs, the functional units will also form parallel and independent columnar assemblies, which is conducive to energy transfer and carrier transport. For example, when the classical electron acceptor benzothiadiazole and its fluoro (chloro) derivative functional monomers are introduced, COF has a narrower band gap, which can effectively separate photoexcited electrons and holes, showing good photocatalytic performance[87]. Hydrazone-linked COF-IHEP1/2 were synthesized by condensation of phosphonate-functionalized monomer (2,5-bis [2- (diethoxyphosphoryl) ethoxy] -terephthaloyl hydrazide, TBBP) with mesitylaldehyde or 1,3,5-tris (p-formylphenyl) benzene, respectively (Fig. 8)[16]. Or condensed with Tp monomer to obtain ketoenamine-linked COF-IHEP10/11[88]. These COFs are all AA stacking structures, and the presence of phosphonate side chain groups makes the specific surface area decrease greatly, but they have ideal removal ability for U (VI) and tracer 239Pu(IV) at strong acidity.
图8 COF-IHEP1和COF-IHEP2结构及合成路线[16]

Fig. 8 Structure and synthesis route of COF-IHEP1 and COF-IHEP2[16]. Copyright 2019, Chinese Chemical Society

However, through the "bottom-up" method, the design and synthesis of functionalized monomers are more difficult, and the COF crystal form is also difficult to maintain after the introduction of sterically hindered functionalized groups. Therefore, according to the pre-design, the "top-down" method of post-synthesis modification of COFs has become another important choice. In the method, on the premise of maintaining the crystal form of COFs, functional groups are modified to the pore wall or the interlayer of two-dimensional COFs by covalent bonds or other forces through reactive sites on the skeleton or side chains. Although the functional groups connected to the active site after modification can not be completely evenly distributed in the pore, they are more flexible and diverse in pore modification, and greatly expand the research ideas of functionalization. Azide-ethynyl, thiol-ene click reactions are widely used for post-synthetic modification of COFs, but the metal nanoparticles required as catalysts may contaminate the pore channels[6,7,14,89,90][65]. The ring-opening reaction of succinate has also been used for post-synthetic modification of COFs[91,92]. Jiang et al. Synthesized a series of COFs with different ratios of phenolic hydroxyl active sites through a three-component condensation method, and carried out a quantitative ring-opening reaction with succinic acid to obtain a carboxylic acid-functionalized side chain structure, while retaining the porosity and crystallinity of COFs, and the adsorption capacity of the material for CO2 was significantly improved[91]. Zhai et al. Polymerized 1-vinyl-3-ethylimidazolium bromide onto the pore wall of vinyl-containing TbDa-COF by radical grafting under γ-ray irradiation to obtain modified [C2vimBr]x%-TbDa-COF[93]. The results showed that the crystallinity and stability of COFs before and after irradiation were retained, and COFs showed good adsorption capacity for ReO 4 -. In 2016, Ma et al. Proposed a new method to functionalize the edge of COF directly as a reactive active site[94]. By microwave irradiation, conjugated C ≡ C was introduced into the skeleton as the edge of COF, and C ≡ C could be further functionalized by addition reaction.

6 Application of COFs in separation and enrichment of radionuclides

With the increasing chemical and structural stability, COFs, as designable and functionalizable ordered and porous framework materials, have been reported to be used in the adsorption of radionuclides. This paper briefly summarizes the adsorption of several representative radionuclides on COFs[9].
6.1 UO 2 2 +
Uranium is one of the most important elements in the nuclear industry, which has radioactive and chemical toxicity. If all kinds of radioactive wastewater containing uranium are directly released into the natural environment, it will cause great harm. In these radioactive wastewater, uranium mainly exists in the form of hexavalent UO 2 2 +. The limited reserves of uranium resources in the earth's crust also threaten the sustainable development of nuclear energy. Seawater, on the other hand, contains about 4.5 billion tons of uranium, mainly in the form of uranyl carbonate complex anions (such as UO2(CO3 ) 3 4 -) at very low concentrations (3.3 ppb). It is usually necessary to introduce uranyl ion coordination groups into the structure of COFs, such as benzimidazole, sulfonic acid group, amidoxime (AO), phosphonic acid group, etc., to improve the adsorption performance of uranium, but most of the materials only perform well under nearly neutral conditions, and the protonation effect of functional groups will lead to the loss of adsorption capacity under the high acidity conditions of spent fuel reprocessing. In addition, tetravalent uranium is insoluble and easy to precipitate, so it can also be removed by reductive immobilization strategy.
Adsorption under near-neutral conditions. In 2015, Li et al. Introduced a coordination active group benzimidazole derivative (2- (2,4-dihydroxyphenyl) -benzimidazole (HBI)) into amide COF by post-synthetic modification to obtain COF-HBI[95]. The adsorption capacity of the material for uranium is 211 mg/G at pH = 4.5. However, at pH = 1, the adsorption tends to 0, which is attributed to the protonation of the imidazole nitrogen or hydroxyl group reducing the affinity for UO 2 2 +. In addition, the material has good irradiation stability, and its structure does not change significantly after 50 kGy γ-irradiation, and its adsorption performance for uranium in single uranyl ion or multi-ion competition system is basically unchanged after irradiation. Luo et al. Used the framework material TpPa-1 connected with ketoenamine units, which is stable in strong acid, strong alkali or wastewater, for the adsorption of uranium. Under the condition of pH = 6.0, the maximum adsorption capacity for uranium is 152 mg/G, and it shows excellent selectivity in the coexistence of various competing ions[96]. Luo et al. Synthesized a ketoenamine unit-linked COF modified with a sulfonic acid group (—SO3H) through a bottom-up strategy, which was soaked in concentrated ammonia to further obtain the ammoniated product [NH4]+[COF- SO 3 -][97]. The maximum adsorption capacity of [NH4]+[COF- SO 3 -] for uranium was as high as 851 mg/G at pH = 5.0,The author considers that the main interaction mechanism is the ion exchange between NH 4 + and UO 2 2 + and the coordination adsorption between — SO 3 - and UO 2 2 +. Even after 8 days of γ-irradiation (5 kGy), the adsorption performance is still stable, and the adsorption capacity of uranium in seawater extraction (10 ppb uranium, pH ~ 8) can still reach 17. 8 mg/G. Amidoxime (AO) is regarded as a star ligand for uranium extraction from seawater. Ma et al. Obtained COF-TpDb-AO by post-synthesis modification strategy and applied it to uranium extraction from seawater[98]. The COF has highly ordered one-dimensional pores, high specific surface area, and densely ordered AO functionalized groups, which have better adsorption capacity, kinetics, and affinity compared with amorphous POP-TpDa-AO, and can reduce various uranium-contaminated water samples from 1 ppm to less than 0.1 ppb in a few minutes, with an adsorption capacity of 127 mg/G for uranium in seawater.
Uranium adsorption under high acidity conditions. In 2019, hydrazone-linked phosphonate-functionalized two-dimensional COFs, microcyclic COF-IHEP1, and macrocyclic COF-IHEP2 were synthesized by our group through a "bottom-up" approach[16]. When COF-IHEP1 was treated in conventional organic solvents, boiling water or 3M HNO3, 1 M NaOH solution for 24 H, or even γ-irradiated at 200 kGy, the crystal structure of COF-IHEP1 was stable, and the good chemical and irradiation stability provided conditions for the adsorption of uranium and plutonium. Zeta potential characterization showed that the surface of COF-IHEP1 was negatively charged in the pH range of 1. 2 ~ 7.0, which was beneficial to the capture of uranium under acidic conditions. The saturated adsorption capacities of COF-IHEP1 and COF-IHEP2 for uranium at pH = 1.0 are 160 and 140 mg/G, respectively, and the adsorption of uranium by COF-IHEP1 in 1 M and 2 M HNO3 media is still as high as 112 and 70 mg/G. In addition, 90% of Pu (IV) was also removed under strong acid conditions (Fig. 9). Further synchrotron radiation EXAFS characterization and DFT theoretical calculations show that the strong coordination interaction between phosphonyl oxygen of phosphonate group and U (Ⅵ) and Pu (Ⅳ) is the main reason for the enrichment of nuclides. After that, methoxy-substituted or phosphonate-functionalized hydrazide monomers were polycondensed with Tp to obtain COF-JLU4 without phosphonate groups and COF-IHEP10 and COF-IHEP11 with 50% and 100% phosphonate groups, respectively. These COFs also have good chemical and irradiation stability[85]. The adsorption capacity of COF-JLU4, COF-IHEP10 and COF-IHEP11 for uranium are 102, 127 and 147 mg/G, respectively, at pH = 1. 0 under strong acid condition, which confirms the coordination between COFs framework and uranyl ion. Even in 1 M and 2 M HNO3, the uranium adsorption capacity of COF-IHEP11 is still as high as 92 and 82 mg/G, which can meet the requirements of spent fuel reprocessing.
图9 (a) COF-IHEP1 在不同酸性条件下对U(Ⅵ)的吸附;(b) COF-IHEP1/2 在不同酸性条件下对Pu(Ⅳ)的吸附[16]

Fig. 9 (a) Adsorption of U(Ⅵ) by COF-IHEP1 under different acidic conditions. (b) Adsorption of Pu(Ⅳ) by COF-IHEP1/2 under different acidic conditions[16]. Copyright 2019, Chinese Chemical Society

In 2020, Qiu et al. Used the method of post-synthesis modification to amidoximate COFs containing cyano groups, such as sp2 carbon-conjugated TP-COF, TFPT-BTAN, and COF-PDAN, which showed excellent performance in uranyl ion enrichment[99⇓~101]. These amidoximated COFs are mainly complexed with uranyl ions through the AO groups on the pore walls in the one-dimensional channels. It was found that the adsorption of uranyl ion by AO was mainly attributed to O = U = O, and the FT-IR characterization showed that the O = U = O vibration peak appeared obviously after the adsorption of uranyl ion, and the XPS spectrum showed that the U 4F binding energy peak appeared obviously after the adsorption. Among them, the saturated adsorption energy of TFPT-BTAN-AO for uranyl ion is still as high as 128 mg/G at high acidity of 3 M HNO3. In addition, comparing TFPT-BTAN-AO with amorphous POP-TB-AO, it was found that TFPT-BTAN-AO had higher adsorption capacity and faster adsorption kinetics due to its regular pore structure and uniform and densely distributed pore sites on the pore wall. After soaking in 5 M HCl, 5 M NaOH, DMF and boiling water for 6 H and γ-irradiation for 50 kGy, the crystal form and structure of COF-PDAN-AO remain unchanged, and the adsorption performance of uranyl is basically not affected, so it can be used in the environment of high acidity and strong radiation during the reprocessing of spent fuel.
Physical adsorption. Compared with chemical adsorption, physical adsorption is hardly affected by acidity. Ma Lijian and Li Shoujian's group synthesized a variety of COFs materials based on size effect to remove uranyl ions under high acidity conditions[38]. In 2016, they used hexachlorocyclotriphosphazene and p-diphenylamine as raw materials to synthesize "stereo" ultramicroporous two-dimensional COF materials (MPCOF) by solvothermal method. Its crystallinity can be maintained after immersion in 1 M HNO3 for 5 H, which provides a basis for uranium removal at high acidity. Comparing the XPS patterns of MPCOF before and after the adsorption of uranyl ions, no change in electron binding energy was observed for the other three N atoms except for the intensity of the protonated N peak, indicating that there is no chemical interaction between MPCOF and UO 2 2 +. The authors believe that the smaller spindle-shaped pores in the ultramicroporous structure may restrict the movement of hydrated uranyl ions, but do not affect the movement of other smaller ions, thus achieving a highly selective enrichment of uranyl ions. The adsorption of uranyl ions in 1 M HNO3 solution is less affected by acidity, with an adsorption capacity of 57 mg/G and a selectivity of 64%. In 2019, the research group used ACOF, which can tautomerize between keto and enol forms, for the removal of uranyl and mercury ions[102]. The results show that C = O in the keto form has a stronger affinity for metal ions and can promote the transition from enol form to keto form in the adsorption process. At pH = 4.5, the adsorption capacity for uranyl ion can reach 169 mg/G. With the increase of acidity, the keto form is transformed into the enol form. Due to the size effect, the selectivity of ACOF for uranyl is enhanced, which can be as high as 96.2% at pH = 1.5. In 2020, the research group synthesized three more COFs with similar skeletons but different pore sizes[103]. Dp-COF has a double pore ring, and the size of the inner pore is slightly larger than that of uranyl hydrate, so it can adsorb uranium by size matching, and the adsorption capacity of uranyl in spent fuel simulant at pH = 1 can also reach 66. 3 mg/G.
Reduction fixation mechanism. In 2018, Ma et al. Prepared a hydrazone bond-linked COF membrane: Redox-COF1 by acetic acid-organic solvent interface method, and realized the reduction and fixation of U (Ⅵ) to U (Ⅳ) by making full use of the reduction activity of the ortho-phenolic hydroxyl group of aldehyde monomer[104]. The adsorption experiment shows that the adsorption capacity of Redox-COF1 material for uranium is 60 mg/G under the acidic condition of pH = 2; The adsorption selectivity of uranium in the multi-ion system is as high as 97%, which is significantly better than that of various adsorption materials reported before. This study shows that the solid phase extraction materials based on redox mechanism can alleviate the adverse effects of protonation to a great extent, which provides a new solution for the highly selective adsorption separation of materials at high acidity.
The photogenerated electrons produced in the photocatalytic process can also reduce U (Ⅵ), and the photocatalytic activity and photoelectric activity of COFs with highly conjugated π-electron system and semiconductor properties are expected to be greatly improved. In 2020, Qiu et al. Synthesized a naphthyl sp2- carbon COF with a high degree of planar conjugation, and the pendent cyano group was converted into an amidoxime functional group to obtain NDA-TN-AO[105]. The COF can be stable in 6 M HNO3, 3 M NaOH, saturated NaCl and other solutions for 24 H, and can also maintain the crystal form at the irradiation dose of 200 kGy. When applied to the extraction of uranium from seawater, NDA-TN-AO has excellent photocatalytic activity, which makes it have bacteriostatic and anti-biological scaling effects, mainly producing biological toxic active oxygen radicals. The adsorbed U (Ⅵ) was reduced to U (Ⅳ) by photogenerated electrons. The adsorption experiment showed that the adsorption capacity of uranium increased from 486 mg/G to 589 mg/G under simulated sunlight irradiation at pH = 5. The adsorption capacity of uranium in natural seawater is 6. 07 mg/G, which is 1. 33 times of that in the dark. In addition, due to the excellent chemical and structural stability of NDA-TN-AO, it was found that the uranium adsorption capacity decreased only slightly after six cycles with 0.1 M HNO3 as the desorbent.
Ma and Wang et al. Synthesized ketoenamine or imine-linked multifunctional COFs photocatalytic materials by a series of three-component condensation reactions of Tp/triazine center aldehyde monomer with 2,2 '-bipyridine-5,5' -diamine/cyano-modified aniline monomer, and the cyano group was converted into an amidoxime group by amidoximation treatment[106]. The results showed that amidoxime could enhance the hydrophilicity of COFs and adsorb UO 2 2 + with high selectivity, while triazine and bipyridine-Pd units synergistically promoted the photocatalytic reduction of adsorbed U (Ⅵ) and precipitated it as U(Ⅳ)O2. Among them, 4-Pd-AO COF can adsorb and reduce 4.62 mg/G uranium from natural seawater every day under visible light irradiation, which highlights its excellent performance. In addition, the 1O2 and superoxide radicals produced in the photocatalytic process also have antibacterial and anti-algal activities, thus inhibiting the formation of biofouling.
表1 COFs在放射性核素分离富集中的应用及其作用机理

Table 1 The application of COFs in the separation and enrichment of various radionuclides and involved adsorption mechanism

COFs Linkages Metal ions Functional group/Sorption
mechanism		 Absorption capacity (mg/g) /Conditions Recyclability ref
COF-HBI Amide U(VI) HBI 211 (pH 4.5) / 95
TpPa-1 β-ketoenamine U(VI) Chemical adsorption 152 (pH 6.0) 4 96
[NH4]+[COF- SO 3 -] β-ketoenamine U(VI) Ion-exchange/Coordination 851 (pH 5.0) / 97
COF-TpDb-AO β-ketoenamine U(VI) AO 394 (pH 6.0) / 98
IHEP1/11 Hydrazone U(VI) Phosphonate 160/147 (pH 1.0) 4 16,88
IHEP2/10/ COF-JLU4 Hydrazone U(VI) Phosphonate 140/127/102 (pH 1.0) / 16,88
TFPT-BTAN-AO C=C bond U(VI) AO 427 (pH 4.0) 6 99
MPCOF P—N bond U(VI) Physical adsorption 123 (pH 1.5) / 38
ACOF β-ketoenamine U(VI)
U(VI)		 Hydroquinone/Redox reaction
Size-matching effect		 169 (pH 4.5)
40 (pH 1.5)		 / 102
Redox-COF1 Hydrazone U(VI) Hydroquinone/Redox reaction 60 (pH 2) / 104
NDA-TN-AO C=C bond U(VI) AO/Photocatalytic reduction 589 (pH=5) 6 105
SIOC-COF-7 Imine I2 Physical adsorption (hollow microspheres) 4810 (75 ℃) 5 107
Meso-COF-3 Imine I2 Pores/Channels 4000 (75 ℃) / 108
BTT-TAPT-COF Imine I2 Electron donor atoms (N/S)/Chemical adsorption 2760 (78 ℃) 5 109
TJNU-201/202 Imine I2 Chemical/Physical adsorption 5625/4820 (150 ℃) / 110
SCU-COF-1 β-ketoenamine ReO 4 - Viologen-N+Cl-/Anion-exchange 367.6 (27 ℃) / 111
[C2vimBr]136%-
TbDa-COF		 Imine ReO 4 - C2vimBr-N+Br-/Anion-exchange 952 / 93

6.2 I2 vapor

Radioactive iodine vapor is one of the important pollutants in radioactive waste, which has strong mobility and easy migration, and has great harm to the ecological environment and human health. The main nuclides are 125I, 129I and 131I, among which 125I and 131I are relatively toxic. The enrichment and storage of gaseous iodine in nuclear power plant waste gas has become the most important issue in the treatment of radioactive waste gas all over the world.
For the physical retention of I2 vapor, porosity is an important factor to determine the separation performance, mainly including pore volume and pore diameter. In 2017, Zhao synthesized heteroporous SIOC-COF-7, which was used for iodine vapor adsorption for the first time[107]. The two different shapes of pores are beneficial to the diffusion of iodine, improve the mass transport and reduce the diffusion barrier, and can also accommodate iodine molecules. TEM characterization showed that the hollow microsphere structure of SIOC-COF-7 (the inner cavity of the microsphere and its porous spherical shell) also provided a large amount of storage space for volatile iodine, which made the material have a high adsorption capacity for iodine (4. 81 G/G), which was the highest value reported at that time. In 2019, Liu et al. Synthesized four COFs (Micro-COF-1/2 and Meso-COF-3/4) with different porosities for the adsorption of volatile iodine by adjusting the length of COF building monomer (Fig. 10)[108]. The adsorption experiments showed that the adsorption of iodine by COFs was affected by both pore volume and pore size: with the increase of pore size, the adsorption capacity of iodine by two kinds of microporous COFs (Micro-COF-1 and Micro-COF-2) increased. When the pore size increases to mesoporous, the adsorption capacity does not increase with the increase of pore size. The large pore volume provides a channel for iodine diffusion, but the small pore can provide adsorption interaction for iodine capture. The optimization results show that the COF with a pore volume of 0.84 cm3/g and a pore size of 4 nm has a maximum iodine adsorption capacity of 4.0 G/G at an ambient pressure of 75 ° C, and has a good cyclic adsorption performance. In 2021, Ma et al. Synthesized a series of flexible C-N linked FAL-COFs with high crystallinity, which have good elastic shape, self-adaptability and certain pore shrinkage effect[52]. More importantly, the flexible C — N bond helps to improve the adsorption capacity of guest molecules (such as nitrogen and gaseous iodine), and the saturated adsorption capacity is 27% and 22% higher than that of the traditional imine-bonded COFs, respectively.
图10 四种不同孔径COFs结构及其对I2蒸气的吸附[108]

Fig. 10 Structures of COFs with four different pore sizes and their adsorption for I2 vapor[108]. Copyright 2019, American Chemical Science

The extended π-conjugated structure and electron-rich heterocyclic (N/S) component in the structure of COFs can promote the rapid adsorption of iodine vapor. Cheng et al. Condensed S-containing aldehyde monomer benzo [6-B ′] terthiophene-2,5,8-trialdehyde with nitrogen-rich amine monomer 2,4,6-tris (4-aminophenyl) -1,3,5-triazine to obtain BTT-TAPT-COF, which can rapidly and reversibly adsorb iodine vapor with an adsorption capacity of 2.76 G/G, and can maintain more than 99% of the initial adsorption capacity after five adsorption cycles[109]. Zeng et al. Synthesized two imine-linked COFs from 1,3,5-trimethyl-2,4,6-tris (4-aminophenyl) -benzene with Tris (4-formylphenyl) amine and 1,3,5-tris (p-formylphenyl) benzene, respectively[110]. Among them, TJNU-201 has higher crystallinity than TJNU-202, and has higher specific surface area and porosity, and higher saturated adsorption capacity for iodine vapor under the same conditions (423 K) (TJNU-201 ∶ 5.625 G/G, TJNU-202 ∶ 4.820 G/G). The FT-IR spectra show that the C — H on the benzene ring has a red shift and the stretching vibration peak of the imine bond has a blue shift after the adsorption of I2, which confirms that there is electron transfer between the N atom in C = N and iodine, indicating that COFs enrich iodine vapor through chemical adsorption. Meanwhile, the chemisorption process was further confirmed by the characteristic peaks of I 3 - and I 5 - in the Raman spectra. In addition, part of iodine vapor was filled in the pores of COFs by physical adsorption, and the synergistic effect of physicochemical adsorption made both COFs show high adsorption capacity for iodine, and the adsorption performance was maintained after five cycles.
6.3 TcO 4 - / ReO 4 -
99Tc is one of the hazardous nuclides in the nuclear fuel cycle, which has a large inventory and a long half-life in the use process, and its main species is 99 TcO 4 - with high mobility and high oxidation valence.
Wang et al. Synthesized two-dimensional cation exchange SCU-COF-1 with ketoenamine as the linkage, which has extremely high acid stability (3 M HNO3,48 h) and irradiation stability (600 kGy)[111]. The material shows unprecedented performance in the adsorption of TcO 4 -, which can reach the adsorption equilibrium within 1 min, and shows good ion exchange selectivity for analogue ReO 4 - at higher temperature, such as the adsorption capacity of 702.4 mg/G after refluxing at 373 K for 24 H; Under the condition of NO 3 -: ReO 4 - = 100:1 (molar ratio), the removal rate of ReO 4 - was still as high as 60%. This is primarily because of the hydrophobicity of the SCU-COF-1 backbone, providing affinity for the relatively hydrophobic ReO 4 - with lower charge density. It is worth mentioning that SCU-COF-1still maintains a certain adsorption capacity and cyclic adsorption performance for TcO 4 - in simulated high acidity (3 M HNO3) spent fuel reprocessing solution and low-level radioactive waste, which has certain practicability. [93]. The results show that the crystallinity and stability of COFs are unchanged before and after irradiation. The adsorption capacity of [C2vimBr]136%-TbDa-COF for ReO 4 - was as high as 952 mg/G with high selectivity and fast adsorption kinetics. The high porosity and chemical stability of [C2vimBr]x%-TbDa-COF make it an ideal packing for dynamic separation column experiments, and the retention of ReO 4 -/ TcO 4 - on the column is more than 99.98% after four cycles, which is a new record of ReO 4 - adsorption removal rate.

7 Conclusion and prospect

Compared with conventional amorphous polymers, COFs have obvious advantages in π electron delocalization, high order and porous structure. With the development of COFs materials, the types of COFs bonds have been expanded, and the chemical and structural stability has been continuously improved, which makes it possible to design and synthesize novel functional adsorption materials for the separation and enrichment of specific radionuclide ions. In addition, from the chemical composition and structure of COFs, it can be seen that COFs have absolute advantages in the accurate judgment of adsorption mechanism and the determination of influencing factors, which is expected to guide the design and synthesis of high-performance solid adsorbents. At present, the application research of COFs in the field of radionuclide ion adsorption is still in its infancy, mostly staying at the level of functional group grafting, and the material design, fine regulation of function and in-depth discussion of mechanism are relatively lacking, which requires the joint efforts of researchers in various fields to expand the application at a higher level.

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