Covalent Organic Frameworks with Ionic Liquid-Moieties (ILCOFs): Structures, Synthesis, and CO2 Conversion

CO2, an acidic gas, is usually emitted from the combustion of fossil fuels and leads to the formation of acid rain and greenhouse effects. CO2 can be used to produce kinds of value-added chemicals from a viewpoint based on carbon capture, utilization, and storage (CCUS). With the combination of unique structures and properties of ionic liquids (ILs) and covalent organic frameworks (COFs), covalent organic frameworks with ionic liquid-moieties (ILCOFs) have been developed as a kind of novel and efficient sorbent, catalyst, and electrolyte since 2016. In this critical review, we first focus on the structures and synthesis of different kinds of ILCOFs materials, including ILCOFs with IL moieties located on the main linkers, on the nodes, and on the side chains. We then discuss the ILCOFs for CO2 capture and conversion, including the reduction and cycloaddition of CO2. Finally, future directions and prospects for ILCOFs are outlined. This review is beneficial for academic researchers in obtaining an overall understanding of ILCOFs and their application of CO2 conversion. This work will open a door to develop novel ILCOFs materials for the capture, separation, and utilization of other typical acid, basic, or neutral gases such as SO2, H2S, NOx, NH3, and so on.


Introduction
Emitted from the combustion of fossil fuels in power plants, a large amount of CO 2 in the atmosphere leads to the greenhouse effect and global warming. It is reported by the World Meteorological Organization (WMO) in the "State of the Global Climate 2021" that the global mean temperature in 2021 was around 1.11 ± 0.13 • C above the 1850-1900 pre-industrial average, and the concentration of CO 2 reached 413.2 ± 0.2 ppm (2020) [1]. This results in harm to human life and the social economy. Carbon capture, utilization, and storage (CCUS) is a way of reducing carbon emissions involving CO 2 capture from high-emission sources and the air, CO 2 transportation from sources to sinks, and the reuse or permanent storage of the captured CO 2 [2]. Kinds of CCUS technologies continue to be developed. Due to its high reactivity with CO 2 , monoethanolamine (MEA) has been used in industrial processes to chemically capture CO 2 for many years [3]. However, this method also has the problems of volatility and serious equipment corrosion.
Ionic Liquids (ILs) are a kind of organic compound; they are composed of organic cations and organic or inorganic anions [4,5]. Typical cations include imidazolium, pyridinium, quaternary ammonium, quaternary phosphonium, etc., while typical anions include acidic anions such as halogen anions ([X] -), tetrafluoroborate ([BF 4 ] -), hexafluorophosphate ([PF 6 ] -), and bis(trifluoromethanesulfonyl)imide ([TFSI] -) and basic anions such as aprotic heterocyclic anions ([AHA] -) and phenolate anions. It is known that ILs are always liquid at room temperature or below 100 • C [6]. ILs have received much more attention because of their unique properties such as low vapor pressure, high chemical stability, wide liquid temperature range, and tunable structure-properties; they are applied as solvents and catalysts in many fields, such as the energy and environment [7][8][9], ILCOFs can be classified into three categories according to the locations of IL moieties. The IL moieties can be located on the main linkers, on the nodes, and on the side chains. The structures of different kinds of typical ILCOFs can be found in Figure 1. Because ILs are composed of anions and cations, cations covalently bonded to COFs with free anions are cationic ILCOFs (imidazolium ILCOFs, pyridinium ILCOFs, ammonium ILCOFs, and phosphonium ILCOFs), while anions covalently bonded to COFs with free cations are anionic ILCOFs (spiroborate ILCOFs, squaraine ILCOFs, and sulfonate ILCOFs). Zwitterionic ILCOFs are cations and anions both covalently bonded to COFs. The strategies for the synthesis of ILCOFs can be classified into two categories, including direct methods (pre-synthesis) and indirect methods (post-synthesis) based on the structures of starting materials ( Figure 2). As porosity is one of the important properties of COFs, the pore could also be adjusted through tuning the cations and anions of ILs, compared with IL-free COFs, which are only tuned by selecting desired nodes and linkers. Large IL or increased amounts of IL in COFs result in decreased pore sizes.

Guanidinium-linked ILCOFs
A guanidinium-based ILCOF, BT-DGCl, was reported by Jansone-Popova et al. [64] and synthesized from benzene-1,3,5-triscarbaldehyde (BT) and diaminoguanidine hydrochloride (DG Cl ) ( Figure 5b). Their PXRD analysis revealed the low crystallinity of BT-DG Cl due to the presence of repulsive interactions between the positively charged guanidinium groups combined with the necessity to accommodate chloride counterions. These ILCOFs were used for the rapid and selective removal of toxic Cr(VI) oxoanions from water. Jia et al. [65] reported the magnetic Fe 3 O 4 @ BT-DG Cl for phosphopeptides capture.

Anions Are Located on the Main Linkers
There are two kinds of anionic ILCOFs with anions located on the main linkers, including the spiroborate anion and squaraine anion ( Figure 6).

Spiroborate-linked ILCOFs
Lee and Zhang et al. [66] synthesized two ICOFs with spiroborate linkage. ICOF-1 with [Me 2 NH 2 ] + was prepared from a macrocycle molecule with B(OMe) 3 and Me 2 NH. By using LiOH as the base instead of Me 2 NH, ICOF-2 was obtained. Feng et al. [67] reported a series of 3D anionic cyclodextrin (CD)-based COFs through the condensation of γ-CD and B(OMe) 3 . When the reaction was in the presence of LiOH under microwaveassisted solvothermal conditions, CD-COF-Li was obtained (Li + is the counterion). When the proton acceptor in the reaction was changed to dimethylamine (DMA) or piperazine (PPZ), CD-COF-DMA ([HDMA] + is the counterion) and CD-COF-PPZ ([H 2 PPZ] 2+ is the counterion) were obtained. It is obvious that CD-COF-DMA and CD-COF-PPZ are ILCOFs. Owing to the high porosity, flexible building blocks, and charged skeleton, CD-COFs show great potential in the fields of ion conduction and gas separation. Subsequently, Li and Zhang et al. [68] theoretically investigated the topology of spiroborate-linked ILCOFs.

Anions Are Located on the Main Linkers
There are two kinds of anionic ILCOFs with anions located on the main linkers, including the spiroborate anion and squaraine anion ( Figure 6). Spiroborate-linked ILCOFs Lee and Zhang et al. [66] synthesized two ICOFs with spiroborate linkage. ICOF-1 with [Me2NH2] + was prepared from a macrocycle molecule with B(OMe)3 and Me2NH. By using LiOH as the base instead of Me2NH, ICOF-2 was obtained. Feng et al. [67] reported a series of 3D anionic cyclodextrin (CD)-based COFs through the condensation of γ-CD and B(OMe)3. When the reaction was in the presence of LiOH under microwave-assisted solvothermal conditions, CD-COF-Li was obtained (Li + is the counterion). When the pro-
Different from three-component condensation, which should control the X value to retain the crystallinity and porosity, ammonium-grafted ILCOFs synthesized via twocomponent condensation were always obtained with crystallinity. Li and Liao et al. [92] reported a europium (Eu)-containing ILCOF (DhaTab-COF-EuIL) as a sensitive and selective acetone sensor. This ILCOF was microporous and crystalline and synthesized via a Schiff-base reaction between Dha and Tab, followed by an IL-modification (Williamson ether reaction) with AB and then an ion displacement with a Eu-based chelate anion. Yan et al. [93] prepared a kind of cationic ILCOF, DhaTab-S, via free-radical polymerization between a cationic surfactant, diallyldimethylammonium chloride (DMDAAC), and a vinyl-containing COF, DhaTab-V, which was prepared from Tab and vinyl-modified 2,5-dihydroxyterephthalaldehyde (Da-V). More recently, Liang and Qiu et al. [94] prepared a kind of ILCOF, Tp-BDOH-AB, through Williamson ether reactions between Tp-BDOH and AB, and Tp-BDOH COF was synthesized via the Schiff reaction of Tp and 3,3 -dihydroxybenzidine (BDOH). This ILCOF was studied for the efficient detection and adsorption of ReO 4 -/TcO 4 -. Jiang et al. [95] reported a series of quaternary ammonium (QA) functionalized nanoplate-like COF-QAs by the reaction of hydrazide building units with aldehyde units. Sui, Tian, and Chen et al. [96] reported a mesoporous cationic IL-COF (COF-NI) prepared by post-grafting the quaternary ammonium salt group into the pore channel of TPB-BPTP-COF. The one-pot post-synthesis was performed in dry DMF using CuI as the catalyst to react iodomethyltrimethylammonium iodide and NaN 3 with TPB-BPTP-COF. The structures and synthesis of typical ammonium-grafted ILCOFs can be found in Figure 11.

Anions Are Located on the Side Chains
ILCOFs with sulfonate anions located on the side chains were reported ( Figure 12). Luo et al. [98] reported an ammoniating COF for the extraction of uranium ions (UO 2 2+ ). They first prepared the SO 3  − units in the pore wall that could implement the coordination interaction toward uranyl. Qiu and Wang et al. [99] reported a kind of ILCOF, COF-HNU14, for CO 2 fixation. They first prepared the SO 3 Hanchored COF (TpPa-SO 3 H) from Tp and 2,5-diaminobenzenesulfonic acid (Pa-SO 3 H). TpPa-SO 3 H contained a large number of Bronsted acid sites and could implement the coordination interaction toward the basic IL 1-aminopropyl-3-methylimidazolium bromide ([APMIm][Br]) to form COF-HNU14. Ma et al. [100] reported three zwitterionic ILCOFs (XJCOF-1, XJCOF-2, XJCOF-3) containing equal numbers of anionic sulfonate and cationic ethidium groups. These ILCOFs were prepared from three sulfonate-containing anilines as anionic monomers and EB as cationic monomers through the Schiff reaction and the subsequent washing process to remove the hydrogen ion and the bromide ion.

Anions Are Located on the Side Chains
ILCOFs with sulfonate anions located on the side chains were reported (Figure Luo et al. [98] reported an ammoniating COF for the extraction of uranium ions (UO They first prepared the SO3H-anchored COF (COF-SO3H), and then the ion-exchange terial of [NH4][COF-SO3] was obtained by immerging COF-SO3H in NH3·H 2O. Such terial ([NH4][COF-SO3]) also contained abundant -SO3 − units in the pore wall that co implement the coordination interaction toward uranyl. Qiu and Wang et al. [99] repo a kind of ILCOF, COF-HNU14, for CO2 fixation. They first prepared the SO3H-ancho COF (TpPa-SO3H) from Tp and 2,5-diaminobenzenesulfonic acid (Pa-SO3H). TpPa-SO contained a large number of Bronsted acid sites and could implement the coordina interaction toward the basic IL 1-aminopropyl-3-methylimidazolium bromide ([ MIm][Br]) to form COF-HNU14. Ma et al. [100] reported three zwitterionic ILC (XJCOF-1, XJCOF-2, XJCOF-3) containing equal numbers of anionic sulfonate and catio ethidium groups. These ILCOFs were prepared from three sulfonate-containing anil as anionic monomers and EB as cationic monomers through the Schiff reaction and subsequent washing process to remove the hydrogen ion and the bromide ion.

CO 2 Conversion by ILCOFs
With the help of ILCOFs as sorbents and catalysts, CO 2 can be captured and converted into kinds of value-added chemicals through different reactions, such as the reduction of CO 2 with amine, the cycloaddition of CO 2 with propylene oxide, etc.

CO 2 Capture
CO 2 capture is an important topic in chemistry and a sustainable world. ILCOFs have a potential adsorption performance for CO 2 capture due to their porous structures and active sites. For example, Gao et al. [88] reported that an ILCOF could be used for CO 2 capture, and the capacity was 164.6 mg CO 2 per g IL at 0 • C and 1 bar. Dong et al. [81] synthesized an ILCOF with a highly selective adsorption for CO 2 over CH 4 , N 2 , and H 2 due to the relatively large porosity and the high density of imidazolium-based IL groups in ILCOF. Their results showed a CO 2 uptake amount of 106.04 cm 3 g −1 at 0 • C and 59.37 cm 3 g −1 at 25 • C under 1 bar, respectively. Only small uptake amounts of CH 4 (19.15 cm 3 g −1 at 0 • C, and 11.88 cm 3 g −1 at 25 • C), N 2 (7.29 cm 3 g −1 at 0 • C, and 5.24 cm 3 g −1 at 25 • C), and H 2 (1.36 cm 3 g −1 at 0 • C, and 0.78 cm 3 g −1 at 25 • C) were observed for ILCOF under the same conditions. Subsequently, Liu, Hua, and Wei et al. [51] showed that SJTU-COF-X (X=Br, Cl, AcO, CF 3 SO 3 ) enhanced CO 2 capture. Among them, the acetate anion containing ILCOF showed a CO 2 capacity of 171 mg g −1 at 0 • C and under 1 bar, which was increased to 1.7 times compared with that of the pristine COF.

Reduction of CO 2 with Amine
Amine could be formylated through the hierarchical reduction of CO 2 with amine as a substrate under phenylsilane (PhSiH 3 ) as a reductant, and different kinds of products (formamides, methylamines, and aminals) will be obtained through different pathways [101]. Gao et al. [88] showed that, with the amount of 5 mol% [Et 4 NBr] 50% -Py-COF as the catalyst, the N-methylformanilide with an isolated yield of 94% could be obtained through the formylation at 30 • C in DMF as the solvent, and the molar ratio of amine to PhSiH 3 was 1:2. Their results suggested that the [Et 4 NBr] 50% -Py-COF behaved as a bifunctional catalyst, which activated PhSiH 3 to react with CO 2 , yielding formoxysilane, and activated the amine through the hydrogen bond. As the carboxylate of betaine could activate CO 2 and enhance the reducibility of PhSiH 3 [102], The general mechanism showed that active hydride could be transferred from the hypervalent silicon species A to CO 2 to generate silyl formate B, which could be further reduced to generate silyl acetal D. B and D could continuously react with amines to achieve formamide C and aminal E, and E could be ultimately converted into the methylated product F. Furthermore, high CO 2 pressure resulted in product C, while low CO 2 pressure resulted in product E under certain temperatures ( Figure 13). Wang and Wang et al. [85] studied the formylation of various amines with CO 2 and PhSiH 3 using COF-HNU3 as an efficient catalyst. The mesoporosity and ordered open channels of COF-HNU3 contributed to the exposed active sites, favored the fast transportation of the substrates, and promoted the rapid conversion of the reactants.

Cycloaddition of CO 2 with Epoxides
The cycloaddition of CO 2 and epoxides to form value-added cyclic carbonates is a 100% atom-economical reaction and one of the efficient routes for CO 2 chemical fixation [103]. It is known that ILs or polymeric ILs with active hydrogen atoms or hydroxyl groups and halides or others will result in the efficient cycloaddition of CO 2 with epoxides [104][105][106][107][108][109][110][111]. The proposed mechanisms can be classified into three pathways, including the "epoxide activation" pathway, "CO 2 activation" pathway, and "epoxide & CO 2 simultaneous activation" pathway ( Figure 14).

Figure 13.
Hierarchical reduction of CO2 with amine and PhSiH3 to afford formamide, methylamine, and aminal. Reprinted with permission from ref. [89]. Copyright 2018 American Chemical Society.

Cycloaddition of CO2 with Epoxides
The cycloaddition of CO2 and epoxides to form value-added cyclic carbonates is a 100% atom-economical reaction and one of the efficient routes for CO2 chemical fixation [103]. It is known that ILs or polymeric ILs with active hydrogen atoms or hydroxyl groups and halides or others will result in the efficient cycloaddition of CO2 with epoxides [104][105][106][107][108][109][110][111]. The proposed mechanisms can be classified into three pathways, including the "epoxide activation" pathway, "CO2 activation" pathway, and "epoxide & CO2 simultaneous activation" pathway ( Figure 14). It was reported that the mechanism of the catalytic CO2 cycloaddition over ILCOFs was similar to that over ILs. Yao and Dong et al. [81] reported an IL-decorated COF, COF-IL, which could be used as a highly active catalyst for CO2 cycloaddition with epoxides under mild conditions (1 atm and ≤80 °C), without any co-catalyst assistance. They found that the epoxide was activated by COF-IL through the interaction of C2-H on the imidazolium with the oxygen on the epoxide. The results indicated that the catalytic performance Figure 13. Hierarchical reduction of CO 2 with amine and PhSiH 3 to afford formamide, methylamine, and aminal. Reprinted with permission from ref. [89]. Copyright 2018 American Chemical Society. Figure 13. Hierarchical reduction of CO2 with amine and PhSiH3 to afford formamide, methylamine, and aminal. Reprinted with permission from ref. [89]. Copyright 2018 American Chemical Society.

Cycloaddition of CO2 with Epoxides
The cycloaddition of CO2 and epoxides to form value-added cyclic carbonates is a 100% atom-economical reaction and one of the efficient routes for CO2 chemical fixation [103]. It is known that ILs or polymeric ILs with active hydrogen atoms or hydroxyl groups and halides or others will result in the efficient cycloaddition of CO2 with epoxides [104][105][106][107][108][109][110][111]. The proposed mechanisms can be classified into three pathways, including the "epoxide activation" pathway, "CO2 activation" pathway, and "epoxide & CO2 simultaneous activation" pathway ( Figure 14). It was reported that the mechanism of the catalytic CO2 cycloaddition over ILCOFs was similar to that over ILs. Yao and Dong et al. [81] reported an IL-decorated COF, COF-IL, which could be used as a highly active catalyst for CO2 cycloaddition with epoxides under mild conditions (1 atm and ≤80 °C), without any co-catalyst assistance. They found that the epoxide was activated by COF-IL through the interaction of C2-H on the imidazolium with the oxygen on the epoxide. The results indicated that the catalytic performance It was reported that the mechanism of the catalytic CO 2 cycloaddition over ILCOFs was similar to that over ILs. Yao and Dong et al. [81] reported an IL-decorated COF, COF-IL, which could be used as a highly active catalyst for CO 2 cycloaddition with epoxides under mild conditions (1 atm and ≤80 • C), without any co-catalyst assistance. They found that the epoxide was activated by COF-IL through the interaction of C2-H on the imidazolium with the oxygen on the epoxide. The results indicated that the catalytic performance of COF-IL exhibited a positive correlation, with increases in the reaction temperature, catalyst amount, and reaction time. Subsequently, as metalloporphyrin-containing materials usually possess visible-light-induced photothermal conversion behavior [112], the same authors also reported two quinoline-linked porphyrin-containing ILCOFs with or without metal coordination for catalytic CO 2 cycloaddition via visible-light-induced photothermal conversion [83]. Sun and Zhang et al. [61] reported four highly porous metalloporphyrinbased ILCOFs for highly efficient CO 2 cycloaddition. They showed that the metal sites contacted with the oxygen on the epoxide, resulting in the activation of the epoxide as well as the formation of an M-O bond. Wang et al. [85,99] synthesized two cationic ILCOFs (COF-HNU3 and COF-HNU4) and an anionic ILCOF (COF-HNU14) for the highly efficient catalysis of CO 2 cycloaddition with different epoxides under solvent-free and co-catalystfree conditions, owing to the excellent porosity and high density of the active sites of imidazolium salts within the nanoscopic channels of ILCOFs. In these IL-functionalized COFs, the turnover number (TON) of COF-HNU3 was as high as 495000. Yang, Qiao, and Han et al. [86] reported that imidazolium-based IL-decorated COFs with the [PW 12

Conclusions and Outlook
With the combination of unique structures and properties of ionic liquids (ILs) and covalent organic frameworks (COFs), covalent organic frameworks with ionic liquidmoieties (ILCOFs) have been developed as a kind of novel and efficient sorbent, catalyst, and electrolyte since 2016. In this critical review, we first focus on the structures and synthesis of different kinds of ILCOFs materials, including ILCOFs with IL moieties located on the main linkers, on the nodes, and on the side chains. We then discuss the ILCOFs for CO 2 conversion, including the reduction and cycloaddition of CO 2 ( Figure 15). It is clear that the field of ILCOFs is still in its infancy.
authors also reported two quinoline-linked porphyrin-containing ILCOFs with or without metal coordination for catalytic CO2 cycloaddition via visible-light-induced photothermal conversion [83]. Sun and Zhang et al. [61] reported four highly porous metalloporphyrinbased ILCOFs for highly efficient CO2 cycloaddition. They showed that the metal sites contacted with the oxygen on the epoxide, resulting in the activation of the epoxide as well as the formation of an M-O bond. Wang et al. [85,99] synthesized two cationic ILCOFs (COF-HNU3 and COF-HNU4) and an anionic ILCOF (COF-HNU14) for the highly efficient catalysis of CO2 cycloaddition with different epoxides under solvent-free and cocatalyst-free conditions, owing to the excellent porosity and high density of the active sites of imidazolium salts within the nanoscopic channels of ILCOFs. In these IL-functionalized COFs, the turnover number (TON) of COF-HNU3 was as high as 495000. Yang, Qiao, and Han et al. [86] reported that imidazolium-based IL-decorated COFs with the [PW12O40] 3anion (POM@ImTD-COF) showed high catalytic activity for CO2 cycloaddition reaction under mild conditions (1 bar and 80 °C), with an IL as the co-catalyst ([N4444][Br]).

Conclusions and Outlook
With the combination of unique structures and properties of ionic liquids (ILs) and covalent organic frameworks (COFs), covalent organic frameworks with ionic liquid-moieties (ILCOFs) have been developed as a kind of novel and efficient sorbent, catalyst, and electrolyte since 2016. In this critical review, we first focus on the structures and synthesis of different kinds of ILCOFs materials, including ILCOFs with IL moieties located on the main linkers, on the nodes, and on the side chains. We then discuss the ILCOFs for CO2 conversion, including the reduction and cycloaddition of CO2 ( Figure 15). It is clear that the field of ILCOFs is still in its infancy. Several issues should be given more attention and need to be investigated further. For example, functionalized ILs strategies are useful for designing efficient CO2-philic IL-COFs. It is known that the functionalized ILs containing active sites on cations or anions are efficient for CO2 capture. The active sites are mainly negative-charged N atoms and O atoms on functional groups, such as amine groups, azolate anions, phenolate anions, and Several issues should be given more attention and need to be investigated further. For example, functionalized ILs strategies are useful for designing efficient CO 2 -philic ILCOFs. It is known that the functionalized ILs containing active sites on cations or anions are efficient for CO 2 capture. The active sites are mainly negative-charged N atoms and O atoms on functional groups, such as amine groups, azolate anions, phenolate anions, and imide anions [113,114]. Thus, it can be safely concluded that ILCOFs functionalized with these groups would obtain a high CO 2 capacity, which is good for CO 2 conversion. On the other side, with the high ionic conductivity and wide electrochemical window of ILs, the efficiency of the electrochemical conversion of CO 2 could be improved through tuning the structure of ILs [115]. Special structures with photochemical properties could be decorated in ILCOFs to improve the efficiency of the photochemical conversion of CO 2 [116,117]. Thus, more research is necessary on the development of ILCOFs and their applications for CO 2 conversion. This review article gives academic researchers an overall understanding of ILCOFs and opens a door to develop novel ILCOFs materials for CCUS and the utilization of other gases.