Structural Characteristics and Environmental Applications of Covalent Organic Frameworks

Covalent organic frameworks (COFs) are emerging crystalline polymeric materials with highly ordered intrinsic and uniform pores. Their synthesis involves reticular chemistry, which offers the freedom of choosing building precursors from a large bank with distinct geometries and functionalities. The pore sizes of COFs, as well as their geometry and functionalities, can be predesigned, giving them an immense opportunity in various fields. In this mini-review, we will focus on the use of COFs in the removal of environmentally hazardous metal ions and chemicals through adsorption and separation. The review will introduce basic aspects of COFs and their advantages over other purification materials. Various fabrication strategies of COFs will be introduced in relation to the separation field. Finally, the challenges of COFs and their future perspectives in this field will be briefly outlined.


Introduction
Purification processes such as distillation, evaporation, concentration, and crystallization are important in basic research as well as playing an important role in industries. These processes, however, operate at the expense of environmental deterioration by consuming an enormous amount of energy, further promoting global warming. Industrial effluents also contain a large amount of chemical and bio-chemical hazardous ingredients polluting the already scarce freshwater resources. Moreover, many industries waste a large amount of precious chemical compounds and organic solvents due to the lack of economical separation/purification materials. Therefore, purification processes with low energy requirements may benefit the environment by saving energy on one hand and saving important capital costs on the other. In recent years, adsorption-(entrapment) and membrane (size exclusion)based purification have attracted immense research and industrial interest due to their low energy consumption as well as simple and environmentally friendly operation. Various amorphous materials such as hyper-cross-linked polymers (HCPs) [1,2], porous organic polymers (POPs) [3,4], conjugated microporous polymers (CMPs) [5][6][7], and activated carbon [8][9][10]; and crystalline materials such as metal-organic frameworks (MOFs) [11][12][13][14] and zeolites [15][16][17][18][19], have shown excellent preliminary separation performance. Covalent organic frameworks (MOFs) [11][12][13][14] and zeolites [15][16][17][18][19], have shown excellent preliminary separation performance. Covalent organic frameworks (COFs) are a class of crystalline framework material synthesized from purely organic building blocks. Their synthesis involves reticular chemistry, which gives immense freedom of pre-design. Their pore geometry, size, and functionalities can be pre-determined by choosing building units from a large bank. Yaghi and co-workers first reported COFs based on boroxine and boronate ester rings [20]. These COFs, however, are prone to deformation in the presence of even trace amounts of humidity rendering them unsuitable in aqueous conditions. Later, iminebased COFs were reported to have superior chemical and solvent stability. Banerjee and co-workers prepared β-ketoenamine COFs with exceptional stability at high temperatures and in extreme acidic and basic conditions [21]. Similarly, Thomas and co-workers reported triazine-based COFs prepared at 400 °C, exhibiting excellent thermo-chemical stabilities [22]. Some of the important applications and various types of COFs are shown in Scheme 1 and Figure 1, respectively. Among many important aspects, COFs offer pore post-functionalization due to their organic nature, for specific desirable applications such as gas storage [23][24][25], catalysis [26,27], electronic devices [11,28], electrode material for batteries [29][30][31][32], etc. The most attractive feature of COFs is their framework structure with uniform and extended porous channels, attracting the interest of scientists in the purification/separation field. Earth's environment is under threat from the increasing disposal of hazardous ingredients such as heavy metals and poisonous organic and bio-organic chemicals [33][34][35][36][37][38][39]. Due to their distinct features, COFs represent themselves as viable alternative materials to address these issues [40,41]. Suitable pore designs and functionalities can render COFs as adsorbents for trapping hazardous metals, organic and bio-pollutants, and greenhouse gases [39,[42][43][44][45][46][47]. As adsorbents, COF pores attract and trap pollutants based on their affinity towards functional COFs. COFs are also touted as excellent robust materials to fabricate separation membranes [48,49]. In the membrane form, pollutants are separated based on diffusion rates relying on the size, geometry, or charge of permeate and retentate. COFs have been reported to remove toxic components from both gases and liquids. Moreover, functionally decorated COFs can facilitate catalytic degradation of pollutants and convert them to clean energy. Our group has extensively worked on metal oxides and their composites for the application of photo-and electro-catalytic process [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67], but recently we have shifted our focus towards COFs due to their excellent characteristic properties. Although COFs were first reported in 2005, their environmental applications have only attracted interest very recently [68]. Since then, COFs have been extensively explored for various environmentally related applications. We believe that an up-to-date account is urgently required for scientists in this field. A review on a similar topic was published elsewhere in 2019 [69]. However, great progress has been made since then in this field and a review covering the new methods and strategies for the synthesis of COF materials towards environmental application is urgently needed to guide emerging scientists in this field.
co-workers prepared β-ketoenamine COFs with exceptional stability at high temperatures and in extreme acidic and basic conditions [21]. Similarly, Thomas and co-workers reported triazine-based COFs prepared at 400 °C, exhibiting excellent thermo-chemical stabilities [22]. Some of the important applications and various types of COFs are shown in Scheme 1 and Figure 1, respectively. Among many important aspects, COFs offer pore post-functionalization due to their organic nature, for specific desirable applications such as gas storage [23][24][25], catalysis [26,27], electronic devices [11,28], electrode material for batteries [29][30][31][32], etc. The most attractive feature of COFs is their framework structure extended porous channels, attracting the interest of scientists in the p tion field. Earth's environment is under threat from the increasing disp ingredients such as heavy metals and poisonous organic and bio-orga 39]. Due to their distinct features, COFs represent themselves as viabl rials to address these issues [40,41]. Suitable pore designs and functio COFs as adsorbents for trapping hazardous metals, organic and bio-pol house gases [39,[42][43][44][45][46][47]. As adsorbents, COF pores attract and trap p their affinity towards functional COFs. COFs are also touted as excelle to fabricate separation membranes [48,49]. In the membrane form, p rated based on diffusion rates relying on the size, geometry, or charg retentate. COFs have been reported to remove toxic components from uids. Moreover, functionally decorated COFs can facilitate catalytic de tants and convert them to clean energy. Our group has extensively wo ides and their composites for the application of photo-and electro-cat 67], but recently we have shifted our focus towards COFs due to their e istic properties. Although COFs were first reported in 2005, their envir tions have only attracted interest very recently [68]. Since then, COFs sively explored for various environmentally related applications. We to-date account is urgently required for scientists in this field. A review was published elsewhere in 2019 [69]. However, great progress has bee

Evolution of COF Synthesis through Time
A brief evolution overview of various COF synthetic methods since their first report will be discussed here. The first COF was based on boroxine and boronate-ester linkages [20]. Although highly crystalline, these COFs are not suitable for an aqueous environment as the crystallinity and framework nature is destroyed even in the presence of a trace amount of water due to the electron-deficient boron. Later on, various other COFs based on imine [27,70], triazine [71], hydrazine [72], and keto-enol [29,73] linkages were reported, exhibiting excellent stability in organic, aqueous environments as well as harsh acidic and basic conditions. The synthesis of COFs intended for environmental applications generally follows bottom-up and top-down approaches. The former involves solvothermal [20], interfacial polymerization [74,75], in-situ growth [76], micro-wave assisted [77], and on-surface crystallization [78,79] methods, and the latter involves delamination of COF powders into mono/few-layer sheets for further applications [80]. COF pore geometry can be pre-determined by choosing suitable linker symmetry ( Figure 1) or modified through post-functionalization. Further elaboration of these synthetic and post-functionalization methods is beyond the scope of this review. Many other reviews have covered various aspects of COF reticular synthesis and properties in detail [81][82][83][84][85][86][87][88][89][90].

Important Aspects of COF Materials towards Cleaner Environment
Industries are polluting our environment in two ways: (i) disposal of hazardous chemicals in water reservoirs; (ii) and greenhouse gases into the air. Materials with advanced functionalities to trap these hazardous chemicals and gases are urgently required to address these issues [91]. COFs are materials that have intrinsic, uniform, ordered and tailorable pores along with high surface area, making them very attractive for trapping and separating these hazardous molecules. COFs with multi-functional pores have extended their applicability in environmental cleansing. Due to their high surface area and ordered porous channels, they can not only be used to trap/store gas molecules but can also function as separating media to remove unwanted chemicals from the waste solution. This application can reduce environmental pollution as well as help in the recovery of precious solvents to be re-used in industries, rendering the whole operation environmentally and economically friendly. Recent development has yielded COFs with extraordinary stability in harsh acidic and basic conditions, rendering them highly desirable in industrial purification and trapping applications [21,92]. The rational design of COFs needs careful consideration of many aspects for intended purification applications. In this section, we will discuss some important aspects of COFs such as structure, morphology, and charge of their pores, as well as their stability, which makes them ideal alternative materials for environmental applications.

Pore Structure
Trapping or separation processes involve distinct characters of pores, the most important being their size. The pore geometry and structure can be pre-designed by choosing monomers with appropriate symmetry, as shown in Figure 1. The size of environmental pollutants ranges from sub-nanometer (metals) to several nanometers (dyes) and even up to micrometer (bio-pollutants such as bacteria, etc.) [93,94]. Therefore, careful consideration should be given to designing COFs for specific purification applications. COFs with pore sizes ranging from 0.5-5 nm have been reported so far, making them highly desirable in size-dependent separation processes. Banerjee et al. exhibited control over pore size by cross-linking precursors of different lengths and obtained COFs with pores in the range of 1.4-2.6 nm. 1,3,5-trifromylphloroglucinol (TFP) was chosen as the aldehyde-bearing monomer, whereas four amine-bearing monomers with different lengths were chosen as linkers ( Figure 2) [74].
Similarly, Ditchel et al. efficiently reduced the pore size of COFs by choosing linkers with six methyl or ethyl groups directing into the pores of the framework [95]. Pore surface engineering or pore post-functionalization is another strategy to tune the pore structure of COFs. Jiang et al., for the first time, reported an interesting strategy to first synthesize COFs with azide functionalities [96]. The COFs were synthesized through the condensation reaction between azide-appended benzene diboronic acid (N3-BDBA) and benzene diboronic acid (BDBA) with hexahydroxytriphenylene (HHTP). The azide could be easily cross-linked with many moieties such as propyl acetate, -COOH, -NH 2 , -COOMe, -OH, and -C≡C through click chemistry. The pore size was controlled between 1.2 and 3 nm by employing this strategy. Moreover, introducing these functional groups through post-functionalization also rendered COFs with desirable wettability and charge surfaces.

Hydrophobicity/Hydrophilicity of COFs
The morphology of COFs, such as their surface area and hydrophilic/hydrophobic nature, is very important for purification processes. COFs with a surface area of >2000 m 2 ·g −1 have already been reported [73,97]. Ordered porous channels along with such high surface area are highly desired for purification/trapping applications. Hydrophilic/hydrophobic properties of COFs are another important controllable aspect, which are exploited by merely choosing desired linkers or through post-functionalization. COFs with hydrophobic nature [98] will enhance their applications in organic media, whereas hydrophilic COFs [99] will work better in aqueous media. Zhang  pore surface functionalization and evaluated their application in harsh conditions [100]. They first synthesized the COF and the pores were grafted with fluoride. The contact angle was increased from 0 • to 150 • by varying fluoride grafting. The modified COF retained its crystallinity and hydrophobicity under extremely harsh conditions such as in boiling water and in solutions with pH ranging from 1 to 14. Similarly, Hu et al. synthesized a hydrophilic triazine-based COF ( Figure 3) and used it as a sensor for the detection of gallic acid (GA) and uric acid (UA) [101]. Similarly, Ditchel et al. efficiently reduced the pore size of COFs by choosing linkers with six methyl or ethyl groups directing into the pores of the framework [95]. Pore surface engineering or pore post-functionalization is another strategy to tune the pore structure of COFs. Jiang et al., for the first time, reported an interesting strategy to first synthesize COFs with azide functionalities [96]. The COFs were synthesized through the condensation reaction between azide-appended benzene diboronic acid (N3-BDBA) and benzene diboronic acid (BDBA) with hexahydroxytriphenylene (HHTP). The azide could be easily cross-linked with many moieties such as propyl acetate, -COOH, -NH2, -COOMe, -OH, and -C≡C through click chemistry. The pore size was controlled between 1.2 and 3 nm by employing this strategy. Moreover, introducing these functional groups through post-functionalization also rendered COFs with desirable wettability and charge surfaces.

Hydrophobicity/Hydrophilicity of COFs
The morphology of COFs, such as their surface area and hydrophilic/hydrophobic nature, is very important for purification processes. COFs with a surface area of >2000 m 2 ·g −1 have already been reported [73,97]. Ordered porous channels along with such high surface area are highly desired for purification/trapping applications. Hydrophilic/hydrophobic properties of COFs are another important controllable aspect, which are exploited by merely choosing desired linkers or through post-functionalization. COFs with hydrophobic nature [98] will enhance their applications in organic media, whereas hydrophilic The contact angle was increased from 0° to 150° by varying fluoride grafting. The modified COF retained its crystallinity and hydrophobicity under extremely harsh conditions such as in boiling water and in solutions with pH ranging from 1 to 14. Similarly, Hu et al. synthesized a hydrophilic triazine-based COF ( Figure 3) and used it as a sensor for the detection of gallic acid (GA) and uric acid (UA) [101].

Structural Stability
The first reported COFs based on boroxine and boronate ester linkages had poor stability in even a small amount of water. This phenomenon arises from the fact that the boron sites are electron deficient and can undergo nucleophilic reaction, resulting in structural degradation through hydrolysis. The effect is more severe as water is produced as a by-product during COF synthesis, which can facilitate the backward reaction and there-

Structural Stability
The first reported COFs based on boroxine and boronate ester linkages had poor stability in even a small amount of water. This phenomenon arises from the fact that the boron sites are electron deficient and can undergo nucleophilic reaction, resulting in structural degradation through hydrolysis. The effect is more severe as water is produced as a by-product during COF synthesis, which can facilitate the backward reaction and therefore severely affect their industrial applications. For environmental applications, COFs need to retain their ordered structure in practically harsh conditions. Linkers with strong covalent bonding to extend the framework, along with hydrogen bonding between interlayers, can overcome this shortcoming to some extent. Imine, azines, hydrazine, imides, and triazine-based COFs exhibit exceptional stability in harsh conditions because they are synthesized through acidic catalyst-based reversible reactions. Therefore, the backward reaction will be facilitated in an acidic environment but the COF should be stable in water as well as organic solvents [102]. Banerjee et al. explored a reversible/irreversible approach to improve COF stability one step further in solvents with pH ranging from 1 to 14 [21]. The same group also exhibited that increasing hydrogen bonding in the framework can also improve the stability of COFs [103]. They incorporated -OH functionalities adjacent to the -C=N bonds to introduce -OH-N=C hydrogen bonds, which ultimately safeguarded the imine nitrogen from hydrolysis in the presence of both acids and water. Similarly, other groups have improved COF stability through increasing intra-molecular hydrogen bonding to improve the COFs overall thermo-chemical stability [104,105].

Pore Charge
Pore charge is a crucial factor in the separation or trapping of hazardous chemicals through electrostatic interactions. According to Donnan's theory, a negative chargeseparating barrier will repulse divalent anions whereas divalent cations will be attracted. Therefore, if the negatively charged separating barrier is an adsorption agent then the cations will be trapped inside the matrix, but if the barrier is a membrane then the positively charged ions will transport more efficiently leaving anions in the feed. The same phenomenon is applied to the oppositely charged separating barrier. So far, very little attention has been given to design-charged COFs. Ma and co-workers introduced cationic sites in the COF (EB-COF:Br) by reacting 1,3,5-triformylphloroglucnol and ethidium bromide (EB) (3,8-diamino-5-ethyl-6-phenylphenanthridinium bromide) [106]. Similarly, Oakey et al. prepared a negatively charged COF by introducing carboxyl functional groups in the COF's backbone and introduced them as fillers in the preparation of mixed matrix membranes [107]. In addition to the narrow size distribution of COF pores, the deprotonated -COOH − enhanced the rejection of bovine serum albumin (negatively charged protein) to as high as 81% at a COF loading of 0.8%. A similar approach was adopted to prepare a COF with modifiable carboxyl functional groups to prepare 12 COFs with variable aperture size and was self-assembled as continuous membranes [108]. It is expected that the synthesis of COFs based on charged pores for environmental application will attract more attention in coming years by incorporating functional groups such as hydroxyl, sulfonic acid, amine, etc.

Applications of COFs towards Environment
COFs are touted as ideal candidates for applications towards a clean environment. COFs can function as adsorbent/trapping materials for hazardous moieties. In addition, COFs can be used as a catalyst for green energy production, catalytic degradation of pollutants, as a sensor for gas detection, and as membrane-forming materials to separate solid as well as gaseous pollutants. The applications of COFs have been summarized in Table 1.

Adsorption
Thanks to their ordered porous framework structure and ease of functionalization, COFs can be used to adsorb many hazardous pollutants and toxic metals as well as adsorb hazardous gases. Several aspects of COFs such as their tunable architecture, uniform and tailorable pores, ease of incorporation of functional sites, low density, and large surface area make them appealing for adsorption applications.

Adsorption of Heavy Metals
The presence of heavy metals in water even in trace amounts poses a great threat to human health. The capture of these heavy metals is of crucial importance for a healthy environment. Hu et al. employed an interesting strategy to coat COFs with polydopamine (PDA) [109]. The COF@PDA showed an excellent adsorption affinity towards Cu(II) and the adsorption equilibrium was achieved within ten minutes with an adsorption capacity of 109.2 mg/g. Furthermore, the adsorbed Cu(II) was heated to obtain (COF@PDA@CuO) and used as a fire-retardant of epoxy resins. Tong et al. incorporated a thiol (-SH) group in the COF framework to adsorb Hg(II) from contaminated water [110]. In another work, three different covalent triazine framework-based COFs decorated with S ((MSCTF-1, MSCTF-2, and xSCTF-2) were synthesized and evaluated for Hg 2+ adsorption [111]. These COFs exhibited high adoption efficiency towards Hg 2+ (840.5 mg/g) thanks to the S in the backbone of COFs. Ma et al. employed a post-functionalization strategy to prepare a COF for mercury removal through adsorption [112]. They first synthesized the COF with vinyl functionalities that could be easily modified with S functionalities through a thiol-ene click reaction to obtain modified COF (COF-S-SH). The resulting COF exhibited unprecedented adsorption capacity for mercury. More importantly, COF-S-SH mercury concentration could be reduced from 5 ppm to 0.1 ppb, which is below the consumable limit of 2 ppb. They attributed such high adsorption to factors such as ordered porous structure and the existence of abundant chelating groups. Another serious threat to the environment is caused by radioactive metals such as effluents containing UO 2 2+ /Eu 3+ . Removal of these metals from the effluents needs urgent attention. Hu et al. proposed a composite material containing an MOF and a COF decorated with -NH 2 functionalities ((NH 2 -MIL-125(Ti)@TpPa-1) [113][114][115]. The composite material was able to remove both UO 2 2+ and Eu 3+ at fast rates of 536.73 mg/g and 593.97 mg/g respectively ( Figure 4).

Adsorption of Hazardous Chemicals and Gases
Zhao et al. synthesized a composite containing metal oxide, an MOF, and a COF (Fe 3 O 4 @TAPB-COF@ZIF-8) and applied it for the adsorption of bisphenol, an endocrinedisrupting chemical interfering with the human hormonal system [116]. The composite materials achieved detection of concentrations as low as 0.04 ng/mL in a wider range of 0.25-1000 ng/mL. Due to its stable organic framework structure, the composite material exhibited excellent repeatability and reproducibility. The same group also reported a type of COF (TAPT-DHTA) and employed it for the adsorption of phenoxy carboxylic acid (PCA) pesticides in plant food derivatives [117]. Effluents from textile industries containing organic dyes are posing one of the biggest challenges to our environment. Removal of these dyes also needs attention to safeguard the environment. Jamshidian et al. reported a composite material of MOF-COF (MIL-101-NH 2 @COF) to remove acid blue 9 dye, a type of carcinogenic organic chemical [114]. They obtained an adsorption capacity of 256 mg/g thanks to the enhanced affinity between the dye and the functionalities of composite materials. Greenhouse gases such as CO, CO 2 , and CH 4 pose yet another threat to the ozone layer and subsequently to the life on Earth. Porphyrin-based COFs were synthesized and evaluated as adsorbents for these greenhouse gases [115]. Due to the high surface area, sheet-like structure, and uniform pores, the metal-free porphyrin sheets exhibited excellent affinity towards these gases. In another report, triazine-based COFs were synthesized by trimerization reaction of 1,3-bis-, 1,3,5-tris-and 1,3,5,7-tetrakis(4-cyanophenyl)adamantane and employed for the adsorption of CO 2 [118]. Due to the stable structure and high BET surface area (1180 m 2 /g) with ideal pore size distribution (0.4-1 nm), an excellent CO 2 uptake of 58.1 cm 3 g −1 at 273 K was achieved. Similarly, a triazine-based COF was synthesized with a pore size in the range 0.52-0.54 nm [119]. The resulting COF (CTF-FUM-350) exhibited excellent adsorption for CO 2 while N 2 , a valuable gas, could be easily collected in the permeate.

Adsorption of Hazardous Chemicals and Gases
Zhao et al. synthesized a composite containing metal oxide, an MOF, and a COF (Fe3O4@TAPB-COF@ZIF-8) and applied it for the adsorption of bisphenol, an endocrinedisrupting chemical interfering with the human hormonal system [116]. The composite materials achieved detection of concentrations as low as 0.04 ng/mL in a wider range of 0.25-1000 ng/mL. Due to its stable organic framework structure, the composite material exhibited excellent repeatability and reproducibility. The same group also reported a type of COF (TAPT-DHTA) and employed it for the adsorption of phenoxy carboxylic acid (PCA) pesticides in plant food derivatives [117]. Effluents from textile industries containing organic dyes are posing one of the biggest challenges to our environment. Removal of these dyes also needs attention to safeguard the environment. Jamshidian et al. reported a composite material of MOF-COF (MIL-101-NH2@COF) to remove acid blue 9 dye, a type of carcinogenic organic chemical [114]. They obtained an adsorption capacity of 256 mg/g thanks to the enhanced affinity between the dye and the functionalities of composite materials. Greenhouse gases such as CO, CO2, and CH4 pose yet another threat to the ozone layer and subsequently to the life on Earth. Porphyrin-based COFs were synthesized and evaluated as adsorbents for these greenhouse gases [115]. Due to the high surface area, sheet-like structure, and uniform pores, the metal-free porphyrin sheets exhibited excellent affinity towards these gases. In another report, triazine-based COFs were synthesized by trimerization reaction of 1,3-bis-, 1,3,5-tris-and 1,3,5,7-tetrakis(4-cyanophenyl)adamantane and employed for the adsorption of CO2 [118]. Due to the stable structure and

COFs as Sensors for Chemicals
Alongside adsorption, detection and sensing of heavy metals is also an important area of research focus. Timely detection of these hazardous elements can prevent the pollution of the environment. Wang et al. prepared a thioether-functionalized COF for the detection of Hg 2+ . The π-conjugated framework functioned as a signal transducer, the densely packed and evenly distributed thioether as a receptor, and the uniform pores as mass transfer enhancers [68]. The resulting COF exhibited high sensitivity, selectivity, and visibility thanks to these combined factors. Recently, N, S-rich COF BTT-TZT was synthesized by Song et al. [120]. The COF showed excellent sensing of Hg 2+ in the range 0.54 nM-5.0 µM at a sensing limit of 0.18 nM. In the same report, they converted COF BTT-TZT (functioning as precursor and template) to hollow N, S-doped C@Pd nanorods to design electrochemical sensors for paracetamol. Similarly, Banerjee et al. synthesized imide-based COFs for the detection of hazardous nitrophenols [121]. The exfoliated COF nanosheets of TpBDH and TfpBDH showed excellent "turn-on" and "turn-off" effects in solid and dispersed states, respectively. Similarly, COFs have been reported for the detection of ammonia [122], nitroaromatic explosives [123][124][125], Cu 2+ [126], picric acid [127], gallic acid (GA) and uric acid (UA) [128], levodopa content in human urine and blood serum samples [129], etc.

COF as Pollutant Degradation Agent for Clean Environment
Catalytic degradation of pollutants is an attractive alternative to a clean environment. However, traditional catalysts face the challenges of low surface area, stability, and recyclability. Materials to address these issues are highly desired [130,131]. COFs are ideal candidates for the above-mentioned applications due to their ordered structure with high surface area and excellent thermo-chemical stability. Cai et al. reported a triazine functionalized COF (TTO-COF) based on olefin linkages, which exhibited excellent chemical stability under severe conditions (see Figure 5) [132]. 4-formyl phenyl) triazine and 2,4,6-trimethyl-1,3,5-triazine were used as precursors to synthesize the COF, which was stable at temperatures as high as 400 • C. More importantly, TTO-COF exhibited excellent photocatalytic degradation of organic dyes and organic pollutants. In another report, a series of COFs denoted as CuPor-Ph-COF were grown in situ on a g-C 3 N 4 support (CuPor-Ph-COF/g-C 3 N 4 ) (see Figure 5) [133]. Rhodamine B was chosen as a model pollutant for degradation. Due to the faster separation of photogenerated charges in COF pores, the composite CuPor-Ph-COF/g-C 3 N 4 exhibited higher photocatalytic activity compared to pure COF and g-C 3 N 4 . The Fenton reaction is an advanced oxidation process (AOP) for organic pollutant degradation that involves the generation of hydroxyl or peroxyl radicals from ferrous ion-activated hydrogen peroxide. Traditionally, the Fenton reaction is carried out through a homogenous catalyst that is not recyclable and makes the process expensive. Fang et al. synthesized a functionalized COF (JUC-521-Fe) and employed it as a heterogeneous catalyst to carry the Fenton reaction [134]. Rhodamine 6G, a highly toxic dye, was used as a model pollutant. JUC-521-Fe achieved a very high catalytic activity with a reaction rate constant of 2.8 × 10 −2 /min and more than 95% degradation efficiency in just 90 min. Similar approaches have been employed by other groups to degrade pollutants such as phenol [135], BPA [136], and organic dyes [137][138][139][140][141].

COFs as Membrane-Forming Materials
Due to their tunable pore aperture and pore environment, COFs are touted as ideal candidates to be employed in separation membranes. The traditional separation membranes to separate pollutants either in the gaseous form or in the solid form from the aqueous or organic solvents usually lack regular porous channels, rendering them with low permeance performance. In this section, we will discuss different strategies to use COFs in the separation field towards environmental protection.

Mixed Matrix Membranes (MMMs)
Incorporation of COFs in the traditional polymeric materials to enhance their flux/permeance is perhaps the most explored approach. Polymeric membranes exhibit low flux due to their dense and irregularly porous structure. MMMs are prepared by add-

COFs as Membrane-Forming Materials
Due to their tunable pore aperture and pore environment, COFs are touted as ideal candidates to be employed in separation membranes. The traditional separation membranes to separate pollutants either in the gaseous form or in the solid form from the aqueous or organic solvents usually lack regular porous channels, rendering them with low permeance performance. In this section, we will discuss different strategies to use COFs in the separation field towards environmental protection. Incorporation of COFs in the traditional polymeric materials to enhance their flux/permeance is perhaps the most explored approach. Polymeric membranes exhibit low flux due to their dense and irregularly porous structure. MMMs are prepared by adding a filler material to make their structure loose and enhance the porosity. However, MMMs face the challenge of a trade-off between permeance and selectivity. Materials with regular porous structures and functionalities are highly desired to function as fillers [13]. COFs provide such opportunities. Dan et al. exfoliated COF (NUS-2 and NUS-3) into mono/few-layer nanosheets and embedded them in a poly(ether imide) (Ultem) or polybenzimidazole (PBI) matrix to fabricate mixed matrix membranes [142]. The resulting MMMs exhibited excellent trapping performance for CO 2 , an extremely alarming greenhouse gas. Using a similar approach, another highly selective MMM was fabricated by employing COF nanosheets as filler materials [143]. Due to the adsorption of gases in pores of COFs, a mixture of gases such as CO 2 /N 2 and CO 2 /CH 4 could be separated with a high degree of accuracy. Recently, Jiang et al. designed an innovative strategy of using COF hollow microspheres and incorporated them as fillers into Pebaxmatrix [144]. The COF microsphere surfaces were decorated with polyethylene glycol monomethyl ether (PEG) before their embedment in MMMs. COFs played a dual role in flux enhancement; the transport resistance was reduced due to the COFs' regular pores while the ethylene oxide groups on PEG reduced the pore size of COFs as well as enhancing their compatibility with the polymer matrix. As a result, an excellent CO 2 /CH 4 separation performance was achieved even surpassing the 2008 Robenson's upper bound. A more detailed review specifically on the CO 2 separation via COF is available elsewhere [145].
Besides hazardous gas separation, removal of pollutants from water as well as the transformation of unhealthy water into drinkable water (desalination) is another pressing issue that needs urgent attention. A cationic COF (TpEB) was employed as a filler during the fabrication of polyacrylonitrile (PAN) membranes. The charge on the COF enhanced the compatibility of the COF with the polymer matrix. As a result, a high rejection rate for BSA (an organic pollutant and fouling material) along with an excellent water permeance performance of 380 L/m 2 .h.bar [146] were obtained. Our group employed an interesting strategy to exploit the low-density nature of COFs [147]. A COF (COF TpHZ) was mixed as a filler in the polymeric matrix of poly(ether sulfone). Due to the low density of the COF compared to the polymer, they migrated to the upper surface during membrane formation to form a density-based gradient distribution. COFs endowed the membrane with enhanced hydrophilicity with a permeance of 2.48 kg/m 2 and a separation factor of 1430 for a water/ethanol mixture. The morphology of the filler material also has a distinct impact on the resulting MMMs. Gao et al. synthesized COF (TpPa-2) through microwave (MW) and mechanochemical (MC) methods [148]. They observed that compared to the MC method, the COF synthesized via the MW method exhibited a much higher surface area, good stability, small particle size, and lower agglomeration tendency. As a result, even a small amount of 0.2% COF as a filler improved the permeance manifolds while still keeping excellent selectivity, addressing the trade-off challenge. As mentioned earlier, the compatibility of filler material plays an important role in the resulting MMMs. Polymers that are organic in nature are more compatible with organic fillers compared to their inorganic counterparts. Xu et al. incorporated carboxylated COF (COF-COOH) as a filler in the preparation of MMMs [149]. The resulting MMMs showed enhanced hydrophilicity and negative charge rendering the MMMs a 4-fold higher permeance and excellent rejection rate for NaCl. Slowing the reaction rate of polyamide (PA) membrane synthesis can also enhance the denseness of the PA layer, thus enhancing the permeance. Gao et al. proposed that reducing the diffusion rate of an amine-bearing monomer from the aqueous to the organic phase can yield microdefects in the PA layer, which was proposed by Alan Turing many years ago (See Figure 6) [150]. They achieved this degree of slow diffusion by using polyvinyl alcohol as the filler material, which could slow down the diffusion of piperazine through hydrogen bonding interactions. Our group recently employed reactive COF nanosheets as fillers in the fabrication of MMMs [80]. The COF nanosheets played a dual role. In the aqueous phase, they interacted with piperazine through hydrogen bonding, therefore reducing its diffusion rate into an organic phase. During the interfacial polymerization stage, the COF nanosheets reacted with trimesoyl chloride (TMC) through their peripheral -NH 2 groups. Due to the synergistic effect, the resulting MMMs obtained an extraordinary 6.8-fold flux enhancement while keeping a steady salt rejection rate. Similarly, we reacted COF nanosheets with graphene oxide (GO) nanosheets for the first time to prepare a mixed nanosheet membrane assembled from the reacted two kinds of nanosheets [151]. GO nanosheets have mechanical stability but are non-porous, whereas COF nanosheets are porous. By reacting these two kinds of nanosheets and self-assembling them in membrane form, the water pathway was shortened by manifolds. Compared to a flux of ≈16 L/m 2 .h.bar for a pristine GO membrane, the GO-COF membranes exhibited 226 L/m 2 .h.bar permeance with similar dye rejection rates.

Stacking of COF Nanosheets in the Membrane Form
COFs are crystalline materials with layered structures assembled through pi-pi stacking. Several reports have highlighted the exfoliation or delamination of COFs into single or few layers [152,153]. Self-assembling these COF nanosheets (CONs) is yet another attractive approach towards fabricating COF membranes for environmental applications; yet very few reports exist on this topic. Jiang et al. reported an interesting strategy to exfoliate COFs into CONs and self-assemble them with cellulose nanofibers through a mixed dimensional assembly [154]. Due to the shielding effect, membranes with precise pore sizes in the range 0.45-1.0 nm were obtained. The resulting membrane exhibited a very high flux of 8.53 kg/m 2 .h.bar. The membranes could be used both for dehydration of n-

Method of Preparation Application Reference
Tp-PA1, TpPA-2 etc Liquid-liquid interfacial polymerization Dye separation [74] TFP-PDA Solid-vapor interfacial polymerization Dye separation [75] COF-LZU1 In-situ polymerization Dye separation [76] 4.4.2. Stacking of COF Nanosheets in the Membrane Form COFs are crystalline materials with layered structures assembled through pi-pi stacking. Several reports have highlighted the exfoliation or delamination of COFs into single or few layers [152,153]. Self-assembling these COF nanosheets (CONs) is yet another attractive approach towards fabricating COF membranes for environmental applications; yet very few reports exist on this topic. Jiang et al. reported an interesting strategy to exfoliate COFs into CONs and self-assemble them with cellulose nanofibers through a mixed dimensional assembly [154]. Due to the shielding effect, membranes with precise pore sizes in the range 0.45-1.0 nm were obtained. The resulting membrane exhibited a very high flux of 8.53 kg/m 2 .h.bar. The membranes could be used both for dehydration Energies 2021, 14, 2267 13 of 21 of n-butanol (separation factor of 3876) and desalination with 96.8% Na 2 SO 4 rejection. Li et al. explored the use of photo-isomerizable azobenzene as a driving agent for the exfoliation of a variety of COFs into CONs [155]. They obtained CONs with a larger lateral area compared to traditional exfoliated CONs due to the high compatibility between COF structure and azobenzene molecules. The membranes obtained from the resulting CONs exhibited very high permeance of 596 L/m 2 .h.bar with excellent retention of hazardous dye molecules. Similarly, Liu et al. exfoliated COF into CONs and self-assembled them into membrane [156]. Due to the ultralow solvent-interface interaction and plethora of porous channels, the membranes exhibited water permeance of 141.5 L/m 2 .h.bar and excellent molecular and ion separation. In another report, a cationic COF (EB-COF:Br) was synthesized through a bottom-up approach and then exfoliated into CONs [106]. These CONs were self-assembled with vacuum assistance into stacked COF membranes. Due to the high positive charge density on the membrane, they exhibited selective separation of hazardous dyes with different charges. These membranes obtained 98% rejection of anionic dyes.

Pure COF as Active Separation Layer in Membranes
Membranes with a purely COF selective layer are an attractive goal, yet are very challenging to achieve. In fact, very few reports have so far exhibited successful membrane fabrication with COF as an active layer. The biggest challenge is the processability of COFs as the traditional COFs are synthesized as insoluble and unprocessable powders. Translating the porous structure of these powders into membrane form is a daunting challenge. Banerjee et al. reported an interfacial crystallization method to obtain various defect-free COF membranes [74]. To slow down the initial polymerization, they employed an interlayer containing water. The aldehyde monomer was dissolved in an organic solvent followed by the interlayer of water and then an aqueous solution containing a catalyst and an amine-bearing monomer. The reaction was kept in static conditions for 3-5 days to obtain highly crystalline COF membranes that exhibited an unprecedented high permeance both towards the water and organic solvents along with good dye rejection rates. This work still faced some challenges, such as the amine-bearing monomer also being soluble in the organic solvent. It led to a thicker membrane because the polymerization could not be limited to the interface. Besides, the liquid-liquid interface is very fragile and could be disrupted with even a gentle shaking. The membrane formation also took longer due to room temperature. Our group addressed those challenges by employing a solid-vapor interface to crystallize COF membranes as described in Figure 7 [75]. One monomer was grown on solid support. The second monomer was vaporized to react with the first monomer. Due to the static and stable solid-vapor interface, the reaction temperature could be increased up to 150 • C. Secondly, the reaction was confined at the interface. Therefore, highly crystalline and ultra-thin (120 nm) membranes were fabricated in just 9 h. Due to high crystallinity and low thickness; these membranes outperformed all other reported COF membranes by manifolds. Caro et al. employed an in-situ polymerization method to fabricate COF membranes directly on the alumina support [76]. The alumina support was first functionalized with APTES and a thin layer of aldehyde monomer was grown on it through in-situ polymerization. Later, the alumina support with aldehyde monomer was immersed in a mixed monomer solution to grow a thin layer of COF on top of the support. The composite membranes were used as such for nanofiltration purposes. They exhibited good permeance as well as dye rejection performance.

Conclusion and Perspectives
Reticular chemistry composed of COFs is the most advanced emerging materials science. Although the developments in COFs are still in the early stage, the salient feature of these materials, like the ease of functionalization with tailored pore windows and sizes, reflect their potential for certain applications. These characteristics suggest that these are an emerging class of material in a wide range of applications including photocatalysts, adsorbents for heavy metals and dyes, membranes for gas, and metal separation. A vast amount of the literature has been reviewed in this article. Emphasis has been given to highlighting the main characteristic features of COFs and their role in environmental remediation.
The structure, morphology, pores, and charge on COF pores were elaborated on for their role in environmental applications. Several COFs have been synthesized using various synthetic approaches comprising solvothermal methods. Although initially reported COFs were not stable, however, introducing hydrogen bonding and other functional groups has increased the stability of current COFs. The pore size of COFs can be predesigned by choosing specific sizes of monomers for particular applications. The pore size ranging from 0.5-5 nm has been reported so far making them highly desirable in size-

Conclusions and Perspectives
Reticular chemistry composed of COFs is the most advanced emerging materials science. Although the developments in COFs are still in the early stage, the salient feature of these materials, like the ease of functionalization with tailored pore windows and sizes, reflect their potential for certain applications. These characteristics suggest that these are an emerging class of material in a wide range of applications including photocatalysts, adsorbents for heavy metals and dyes, membranes for gas, and metal separation. A vast amount of the literature has been reviewed in this article. Emphasis has been given to highlighting the main characteristic features of COFs and their role in environmental remediation.
The structure, morphology, pores, and charge on COF pores were elaborated on for their role in environmental applications. Several COFs have been synthesized using various synthetic approaches comprising solvothermal methods. Although initially reported COFs were not stable, however, introducing hydrogen bonding and other functional groups has increased the stability of current COFs. The pore size of COFs can be pre-designed by choosing specific sizes of monomers for particular applications. The pore size ranging from 0.5-5 nm has been reported so far making them highly desirable in size-dependent separation processes. The pore charge is crucial when COFs are used as adsorbents or membranes for separation. The negatively charged pore separates the cations and vice versa.
Due to their pore and structural tunability, COFs have been extensively used as good adsorbents for hazardous materials. Heavy metals in water like Cu, Hg, Eu, organic dyes, and greenhouse gases have been removed effectively through COF chemistry. COFs have also shown excellent sensing for heavy metals with low detection limits in water and human urine and blood samples. COFs are ideal candidates for the photocatalytic degradation of organic dyes and pollutants with high stability and recyclability. Due to functionality and pore window size, COFs have been used in the preparation of defect-free membranes. MMMs have excellent separation performance for CO 2 separation from the air and natural gas, surpassing the upper bound curve. Besides, nanosheets of COFs have been fabricated on porous supports, which exhibited high selectivity. In the future, intense research is required for the application-oriented synthesis of COFs, such as COFs with large porosity for adsorption, active sites for photodegradation, and the ability of linkers to form defect-free films for separation membranes.
Despite such huge advancements in COF chemistry for environmental applications, several challenges can still be addressed.

1.
COFs with certain functionalities are still needed.

2.
Economically viable preparation methods are needed for commercialization.

3.
The crystallinity of the COF materials is still an issue that needs further incitement.

4.
Many COFs are not stable in humid conditions and this is a serious issue. 5. CO 2 separation in the presence of water and other acidic gases is not well explored. 6.
More structural performance of COFs need to be designed for environmental applications. 7.
The fragile nature of a pure nanosheet of COFs is challenging for industrial applications. Thus, flexible COF membranes would be an attractive approach. 8.
The environmental risk management data for COFs is still lacking, which needs to be addressed. Funding: This research received no external funding.