1 Introduction
1.1 Introduction of MOF Materials
Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are a class of porous materials with topological structures, which are synthesized by self-assembly of metal-containing nodes (secondary building units, SBUs) and organic ligands
[1,2]. Therefore, as an inorganic-organic hybrid nanoporous material, MOF itself is composed of two parts (such as Figure 1), which has significant diversity compared with traditional solid-state materials. It has the characteristics of high specific surface area (more than 7000 m
2/g), ultra-high porosity (up to about 90%), abundant active centers and adjustable structure
[3][4]. Therefore, MOF has a wide range of applications in adsorption, gas separation and storage, heterogeneous catalysis, photocatalysis, hydrogen evolution, oxygen evolution, CO
2 reduction, drug delivery and many other aspects
[5][6][7,8][9][10][11][12][13].
As can be seen from Fig. 1, the metal centers (SBUs) are nodes of the crystal lattice that are held in place by their directionality of binding to organic ligands (electrostatic attraction and coordinated metal-ligand interactions). The excellent catalytic performance of MOF is due to the existence of unsaturated coordination metal nodes in the structure. The unsaturated metal centers can be combined with DMF, water, ethanol and other solvent molecules to meet the coordination requirements, and then the solvent molecules can be removed by vacuum or heating to achieve the purpose of opening the metal sites. For example, in the thermogravimetric analysis of the paddle-wheel material MOF-11 prepared by Chen et al., it was found that all the water guests and ligands coordinated on the Cu atom could be removed at 260 ℃ to expose the unsaturated coordination sites, which showed excellent performance in gas selective adsorption and catalysis
[14].
However, not all MOF materials have the same rigid structure as MOF-11. For example, in the field of photocatalysis, some MOF materials lack thermal stability at high temperatures, especially above 250 ℃, which is mainly due to the lack of strong metal-ligand interaction in the MOF structure. For MOF materials, the type of organic ligand can greatly affect the structural stability of MOF when the same metal node is used. For example, the topological structure of Zr-MOF UiO-66 (with terephthalic acid as the ligand) is similar to that of UiO-67 (with 4,4 '-biphenyldicarboxylic acid as the ligand). The former shows excellent stability in aqueous solution, but the latter shows degradation after immersion in water due to longer organic ligands and larger pores, and the nodes are gradually transformed into monoclinic zirconia
[15,16][17].
Therefore, another factor limiting the application of MOF materials is that the MOF structure will collapse in aqueous solution or in contact with certain solvents, mainly because water and polar solvents can damage the MOF structure by solvating metal ions. The destruction in aqueous solution is generally divided into two steps: ligand displacement and hydrolysis. The destruction in acids and bases is generally caused by the protonation of organic ligands (such as carboxylic acids and nitrogen-containing MOF), which leads to the separation of metals, thus causing structural destruction
[18]. In the case of strong nucleophilic reagents such as hydroxide, amine and alkoxy, which can strongly coordinate with metal ions, these reagents are likely to destroy the MOF structure by competing with the coordinated metal bonds in the MOF structure
[2].
It can be seen that although some MOF material structures have sufficient stability (such as UiO-66), their functionality still needs to be improved. For example, in photocatalysis, some small molecular ligands (such as terephthalic acid) have no visible light response, although the introduction of —NH
2 can make the absorption red shift to achieve the purpose of broadening the light response range, but the introduction of new functional ligands (such as porphyrin, carbole and other conjugated systems) is an inevitable trend
[19]. The stability of new MOF materials must change after the introduction of macrocyclic ligands. However, MOF stability is a key prerequisite for its potential practical applications in storage, separation, catalysis, biomedicine, and many other fields. Therefore, ensuring the integrity of MOF structure in the process of use is an important problem to be solved before its actual use.
1.2 COF Material Introduction
Covalent organic framework (COF) materials are periodically arranged organic polymers with a certain degree of crystallinity and high porosity. The framework of COF is composed of light elements (B, C, N, O, Si, H, etc.) connected by strong covalent bonds (as shown in Figure 2), such as B — O, C — N, B — N, B — O — Si, — C = N —, etc. Therefore, COF materials have unique advantages such as low density, high crystallinity, adjustable pore structure, large specific surface area, and easy surface modification
[20,21]. Similarly, COFs have a wide range of applications in gas adsorption and storage, molecular separation, photocatalysis, hydrogen capture and storage, and so on
[22][23][24].
Two-dimensional (2D) or three-dimensional (3D) COF materials can be obtained by introducing building monomers with corresponding dimensions, so as to obtain channels or pores close to nanometer scale and have the characteristics of uniformity, tunability and the like
[25,26]. In addition, periodically ordered COFs π-conjugated systems can be constructed by introducing suitable molecules, which is beneficial to electron delocalization and thus enhances the charge transport properties of COF materials
[23]. This plays an important role in the catalytic performance of COF materials, especially photocatalysis, which can significantly reduce the recombination rate of electron-hole pairs. Therefore, compared with traditional catalytic materials, COF has stronger modifiability and wider application range.
However, COF materials also have some problems to be solved, for example, because COFs are mainly composed of light elements, their functions will be relatively simple. In addition, the insolubility and poor dispersibility of microcrystalline COFs powders in most solvents limit their processability
[20]. The relatively low density and weak interlayer molecular forces of COF materials can also lead to loose packing when preparing 2D COF films or thin films
[27].
Compared with MOF materials, although the crystallinity of COF is slightly worse, the covalent bond connection between monomers in its structure helps to achieve the high chemical stability of its materials. For example, polyarylether-type COF materials (PAE-COF) show high chemical stability, which can still maintain crystallinity in harsh chemical environments such as boiling water, strong acid (HCl, H
2SO
4, HF), alkali (NaOH, MeONa), oxidizing medium (chromic acid solution) and reducing medium (LiAlH
4), which is stronger than some MOF materials with higher stability under the same treatment conditions, including UiO-66, ZIF-8, etc
[28].
In fact, the chemical stability of COFs is also related to its synthesis pathway. In the synthesis process, the framework structure is adjusted through the formation and extension of reversible covalent bonds and the continuous reformation of bonds, and finally the crystal thermodynamic product with the lowest free energy, COF, is obtained. Although some COFs have poor water stability, such as boric acid. However, the strategy of combining reversible and irreversible reactions can lead to crystalline COFs with high chemical stability, such as TpPa-1 (Figure 3). In the synthesis, the crystalline enolimine COF intermediate was obtained by reversible Schiff base reaction, after which the β-ketoamine-linked COF was obtained by irreversible proton tautomerization (only the covalent bond rearrangement did not affect the crystallinity). Because the second step is irreversible, TpPa-1 shows high chemical stability in both acidic (9 mol/L HCl) and alkaline (9 mol/L NaOH) media
[29].
As mentioned above, the function of pure COF materials is simpler than that of MOF materials because they are composed of light elements. However, COF materials have great potential in chemical stability, and COF materials with high chemical stability can be obtained through appropriate synthesis strategies and bonding methods. Therefore, enriching the functionality of COF is a problem that COF materials need to solve.
1.3 Introduction of MOF-COF Hybrid Materials
As mentioned above, although MOF and COF materials are crystalline porous materials with wide application prospects, they have inherent limitations, which limit the application performance of these two materials to a certain extent. In recent years, hybrid materials of MOF and composite materials of COF have attracted much attention
[20,30]. For example, Yan et al. Reported a core-shell structure of porous Fe
3O
4/COF nanosphere COF hybrid material Fe
3O
4@TpBD, which was synthesized by a monomer-mediated in situ growth strategy
[31]. In this method, —NH
2 was introduced on the surface of the prepared Fe
3O
4 nanospheres to form amino-functionalized nanospheres (Fe
3O
4-NH
2), and then 1,3,5-triformylphloroglucinol (Tp) was introduced on the surface of the Fe
3O
4-NH
2 nanospheres by Schiff base reaction, which realized the in situ growth of the COF shell, and the higher the concentration of COF monomer, the thicker the COF shell (15 – 65 nm). The Fe
3O
4/COF can effectively adsorb and remove typical endocrine disruptors (such as bisphenol A, bisphenol AF) in aqueous solution. For example, Wang et al. Reported a core-shell MOF @ MOF hybrid material NH
2-MIL-101(Al)@ZIF-8 for efficient adsorption and sensitive detection of Cu (II) in water, which was synthesized by an internal extended growth (IEG) mode under the control of polyvinylpyrrolidone (PVP)
[32]. The specific adsorption affinity of imidazole nitrogen in the framework of ZIF-8 for Cu (II) ions can be used to obtain a high adsorption trend (526.74 mg·g
-1), while the fluorescence properties of NH
2-MIL-101(Al) show a deviation related to the concentration of Cu (II), so the NH
2-MIL-101(Al)@ZIF-8 has excellent selectivity/sensitivity — the core-shell structure improves the interfacial preconcentration ability between individual MOFs, and the nanoflower structure accelerates the mass transfer process.
It can be seen that MOF and COF show stronger functions and properties than the original single MOF or COF after the corresponding hybridization. The inherent defects of MOF and COF themselves can be overcome by hybridization (core-shell hybridization as mentioned above). Therefore, among many hybrid strategies, MOF-COF hybrid materials are of great application potential and research value. In theory, MOF-COF hybrid materials can be designed to have both the excellent catalytic performance of MOF unsaturated coordination metal centers and the high chemical stability brought by COF covalent bonding. on the hybrid strategy, the hybridization of MOF with COF can be achieved by growing MOF on COF (MOF-on-COF) or growing COF on MOF (COF-on-MOF)
[33]. MOF-on-COF is generally used to improve the functionality of COF, and the MOF and COF used have good chemical stability. The hybridization of COF-on-MOF structure can protect the inner MOF by using the hydrophobic characteristics of COF to some extent. The covalent linkage of MOF and COF is generally achieved through the aldehyde-amine Schiff base reaction, the condensation reaction of amino and carboxyl, and the irreversible enol-ketone tautomerism. Because of the high strength of the covalent bond, MOF and COF can be firmly linked to achieve hybridization in this way
[34⇓~36]. For example, Zou et al. Reported that NH
2-MIL-125(Ti)/B-CTF-1(TBC) hybrid materials were synthesized by post-synthesis covalent modification strategy, mainly by modifying CTF-1 with 4-aminobenzoic acid to obtain COF with dense carboxyl functionalized surface, and then condensing —NH
2 in NH
2-MIL-125(Ti) with — COOH on the surface of COF to obtain MOF-modified CTF-1 hybrid materials, namely TBC
[35]. The MOF-COF hybrid material can be used as a medium for charge transfer due to the formation of an amide bond between the MOF and the COF, so that the separation efficiency of an electron-hole pair is remarkably improved. So the photocatalytic activity of MOF material NH
2-MIL-125(Ti) is promoted in the hybrid materials, and the experiment proves that TBC can be used as an excellent and stable photocatalytic system for hydrogen evolution under visible light. In addition, the MOF-COF hybrid framework can also be connected by coordination bonds, such as HN-Zn-imidazole bond, Mn — N bond, etc
[37,38]. Of course, the hybridization between MOF and COF can also be van der Waals force, hydrogen bond and π-π stacking, but the MOF-COF hybrid framework hybridized by these three forces often has poor structural stability
[39].
According to the previous discussion, it can be understood that there are two main structures of MOF-COF hybrid materials: one is core-shell hybrid; The second is the layered hybrid structure. The main synthesis idea is to connect MOF and COF materials through strong covalent bonds to achieve the synthesis of hybrid materials. Compared with MOF-MOF hybridization, the research of MOF-COF materials still needs to be explored, and the synergistic effect of MOF and COF hybridization has important application value. In this paper, the development of MOF-COF hybrid materials in recent years is summarized, the main synthesis methods and application fields of MOF-COF hybrid materials are summarized, and the future development of MOF-COF hybrid materials is prospected.
2 Synthesis
The framework hybridization of MOF-COF mainly has two forms, one is MOF-on-COF and the other is COF-on-MOF. The main way of framework hybridization is to connect MOF and COF through covalent bonds to obtain hybrid materials with both advantages. The synthesis of MOF-COF hybrid materials will be introduced in two parts according to the structure type.
2.1 Synthesis of MOF-on-COF Structure
For MOF-on-COF materials, COF is used as the core or substrate for the growth of MOF. In 2016, Fu et al. First reported that MOF could be grown on COF to prepare composite membranes (Fig. 4)
[40]. First, a layer of polyaniline (PANI) was deposited on the porous SiO
2 disk to modify the SiO
2 disk, and then the polyaniline was condensed with terephthalaldehyde to form an imine bond, and the free aldehyde group was reacted with tetra- (4-benzylamino) -methane to obtain a continuous and uniformly distributed COF-300 layer. After that, terephthalic acid (a regulator of MOF ZIF-8) was used to react with the amino groups on the surface of COF-300 to form hydrogen bonds to promote the growth of the subsequent MOF. In the composite, the interlayer is composed of amorphous MOF and COF-300 nanocrystals, which can seal the space between the COF crystals. In 2017, Zhuang et al. Used benzoic acid (BA) functionalized COF to promote the growth of MOF on the surface, and synthesized ZIF-67/COF composite
[38]. Firstly, the Co(BA)
2/COF composite was obtained through the anchoring effect of BA on Co
2+ in BA-functionalized COF (BFC), and then 2-methylimidazole was added to synthesize ZIF-67/COF composite. Later, in 2018, Zhang et al. Successfully anchored NH
2-UiO-66(MOF) covalently to TpPa-1-COF (Tp: triformylphloroglucinol
[36]; Pa: p-phenylenediamine) surface synthesis yielded MOF/COF hybrid materials with high surface area and high crystallinity (Figure 5). TpPa-1-COF is a typical Schiff base COF structure. They successfully anchored MOF on the surface of COF by introducing different proportions of NH
2-UiO-66 into the TpPa-1-COF synthesis reaction system with excess Tp monomer and using aldehyde-amine Schiff base reaction. Finally, the hybrid materials were obtained in which the MOF was uniformly distributed on the flower-like COF surface through covalent bonds. In 2019, Zou et al. Reported that MOF was anchored on the COF layer by stepwise synthesis followed by covalent modification, which is the hybrid material NH
2-MIL-125(Ti)/B-CTF-1(TBC) introduced in the previous paper
[35].
In addition, recently Firoozi et al. Reported a Zn-MOF-5 and melamine-terephthalaldehyde-based 2D COF hybrid material MOF-5/COF (M5C)
[41]. The method mainly comprises the following steps of: carrying out -NH
2 functionalization on the synthesized MOF-5 by ethylenediamine to obtain a MOF-5-NH
2, and then adding melamine and terephthalaldehyde to synthesize the M5C. While Jiang et al. Reported the synthesis of a magnetic MOF-COF composite
[42]. The difference from the aforementioned Fe
3O
4@TpBD COF hybrid material structure is that Jiang et al. Controlled the growth of Zn-MOF (ZIF-8) on the surface of the COF layer of the magnetically modified COF material (Fe
3O
4@TAPB-COF). The synthesis method mainly comprises the following steps of: firstly synthesizing Fe
3O
4@TAPB-COF by using Fe
3O
4, TAPB (1,3,5-tris (4-aminophenyl) benzene) and TPA (terephthalaldehyde); then adding the prepared Fe
3O
4@TAPB-COF after mixing and stirring ZnCl
2, 2-methylimidazole, sodium formate and methanol for 2 H; and finally obtaining the Fe
3O
4@TAPB-COF@ZIF-8 composite material.
For the construction method of MOF-on-COF materials, Feng et al. Proposed a modular strategy for constructing hierarchical framework materials (as shown in Figure 6)
[43]. An integrated system is divided into smaller individual "modules" that can be interchanged, and these modules can be exchanged to obtain different structures without affecting other modules. Modular synthesis can be seen as a bottom-up convergent approach to generate target products with complex structures through sequence-defined reactions between different building blocks. As a validation of the concept, they performed hierarchical 2-module and 3-module MOF-on-COF structure controlled generation. For example, they used COF-303 as a module platform for the secondary growth of MOF-5, and found that there were imine fragments on the surface of COF and the defect sites inside COF, which would be partially decomposed during the secondary growth of MOF-5, releasing amines and aldehydes into the solution. The released fragments will further interfere with nucleation and regulate the growth of Zn-based MOF-5. Therefore, we can adjust the structure by regulating the proportion of the two. In addition, they also carried out the research of hierarchical 3 modules, designed and synthesized the (COF-303 @ PCN-160 (Zr)) @ MOF-5 composite with the relatively robust COF-303 @ PCN-160 (Zr) as the inner component and the relatively unstable MOF-5 as the outer component, as shown in Figure 7.
This method can be applied to the preparation of many COF @ MOF composites, but the strength order of covalent bonds and coordination bonds needs to be determined to ensure the integrity and stability of the core module in the synthesis process. For example, the design of (COF-303 @ PCN-160 (Zr)) @ MOF-5 follows the decreasing order of module bond strength: C = N > Zr-O > Zn-O.
2.2 COF-on-MOF structure synthesis
Similarly, MOF-COF hybrid frameworks for COF-on-MOF use MOF as a core or substrate to promote the surface growth of COF. Compared with MOF-on-COF hybrid materials, the types of COF-on-MOF hybrid materials studied in recent years are slightly more abundant, which may be due to the more stable structure of COF-on-MOF hybrid materials.
2.2.1 Schiff base reaction hybrid (C = N)
MOF is generally amino-functionalized, because the —NH
2 on MOF can condense with aldehyde monomers through Schiff base reaction, that is, to form C = N imine covalent bonds. COF seeds are then obtained on the surface of the MOF, followed by the use of additional monomers to promote the growth of COF on the MOF material
[44,45]. In 2018, Peng et al. Reported a MOF @ COF hybrid material NH
2-MIL-68@TPA-COF with hybrid core-shell structure connected by Schiff base reaction (as shown in Fig. 8)
[46]. They first prepared amino-functionalized NH
2-MIL-68, then aldehyde-functionalized the prepared NH
2-MIL-68 with Tris (4-formylphenyl) amine (TFPA) to obtain COF seeds on the surface of NH
2-MIL-68(CHO),MOF, and then added Tris (4-aminophenyl) amine (TAPA) to condense with TFPA on the surface of MOF to promote the growth of COF. In the same year, Sun et al. Reported the preparation of a metal (Pd) -doped core-shell TiATA @ LZU1 (Pd/TiATA @ LZU1)
[47]. The COF is grown on the surface of the MOF to form a core-shell hybrid structure by using a MOF NH
2-MIL-125(Ti)(TiATA) with an amino group to form an imine bond through a Schiff base reaction.
Recently, Lu et al. Prepared a series of NH
2-MIL-125@TAPB-PDA(PDA: (terephthalaldehyde) nanocomposites with different COF shell thicknesses
[48]. They first used low concentrations of organic monomers to form seed particles on the surface of amino-functionalized MOF, and then used high concentrations of monomers to connect the seeds on the surface of MOF through Schiff base reaction to form hyperbranched and cross-linked polyazomethine structures. Cheng et al. Reported a novel MOF @ COF filler with UiO-66-NH
2(MOF) encapsulated by TpPa-1 COF
[34]. They mixed activated UiO-66-NH
2 nanoparticles with Tp in THF under stirring, so that the amino group in UiO-66-NH
2 reacted with the aldehyde group in Tp by Schiff base reaction. Next, a THF solution of Pa was slowly added to the previous mixture to obtain MOF nanoparticles coated with an amorphous polyimide layer (MOF @ polyimide). Then they transferred MOF @ polyimine nanoparticles into mesitylene and 1,4-dioxane mixed solvent to obtain UiO-66-NH
2@TpPa-1 through irreversible enol-ketone tautomerization. Also hybridized by imine C = N bond via Schiff base reaction are NH
2-MIL-101(Fe)@NTU, UiO-66-NH
2@COF-TAPB-BTCA, MOF @ COF nanoenzyme (NMC
TP-TTA), Ti-MOF @ COFs, UiO @ COF, Co-MOF/COF, aza-MOFs @ COFs, UiO-66-NH
2@COF, Fe
3O
4@MOF@COF, etc
[49][50][51][52][53][54][55][56][57,58]. Among them, the synthesis of aza-MOFs @ COFs is to obtain the MOF @ COF structure through Schiff base reaction hybridization, and then use aza-Diels-Alder reaction for post-synthesis modification (as shown in Figure 9).
2.2.2 π-π stacking
Gao et al. Reported a core-shell MOF @ COF hybrid material (PCN-222-Co @ TpPa-1) constructed by strong π-π stacking interaction
[39]. They constructed a core-shell hybrid material PCN-222-Co @ TpPa-1 with strong π-π stacking by using the strong π-electron system of 2D COF with many benzene rings and planar structure characteristics and the aromaticity of MOF. First, they synthesized PCN-222-Co by ZrCl
4 and H
4TCPP-Co, then added Pa and used the interaction between amino group and carboxyl group to make Pa adsorbed on the surface of MOF, and made Pa uniformly distributed on the surface under the action of ultrasound, then added Tp to promote the growth of COF on the surface of MOF by Schiff base reaction, and finally synthesized PCN-222-Co @ TpPa-1. By comparing the UV-Vis absorption spectra, it was found that the main absorption peaks of PCN-222-Co and PCN-222-Co @ TpPa-1 were located at about 498 nm and 572 nm, respectively, with a large red shift of 74 nm, which indicated that there was a π-π stacking interaction between PCN-222 Co and TpPa-1 in PCN-222 Co @ TpPa-1.
2.2.3 Coordination induction
Sun et al. Explored a COF @ MOF hybrid material COF/Mn-MOF using metal Mn to coordinate with N in COF (Fig. 10)
[37]. Because of the strong coordination ability of Mn ion, its highest coordination number can reach 7. In the Mn-MOF structure, Mn is coordinated with six O (from BTC ligand) as the center, and the remaining one can be used to coordinate with the electron-rich N in COF. In addition, compared with Fe, Co and other elements, another reason for the realization of the coordination energy is the appropriate ionic radius of Mn ion. During the growth of Mn-MOF structures, the 2D layered COF structures tend to be dispersed and stacked on the surface of the spherical structure (perpendicular to the radial growth direction), and the connection between them is through Mn — N coordination bonds. In addition, the results of quantum mechanics calculation prove that the coordination of Mn — N can promote the effective recombination of COF and Mn-MOF, and tend to grow in the C direction perpendicular to the two-dimensional layered structure of COF.
In the synthesis method, 1,4-phenylenediamine and 1,3,5-tribenzaldehyde were used to prepare COF, and then the obtained COF was mixed with Mn(NO3)2·4H2O, and H3BTC was added to promote the formation of Mn-MOF, and finally the khaki COF/Mn-MOF hybrid material was obtained.
3 Application
Because MOF-COF hybrid materials have the advantages of both MOF and COF, they have excellent application properties in many fields, such as catalysis, gas adsorption and separation, energy storage materials, sensing, etc. (See Table 1).
3.1 Catalyze
3.1.1 Thermodynamic catalysis
Many substrates for heterogeneous catalytic reactions (such as styrene) are hydrophobic, so the wettability of catalyst materials to substrates plays a key role in catalytic performance. However, the surface and pore of some MOF materials are hydrophilic, such as NH
2-MIL-101(Fe), which makes it difficult for hydrophobic substrates to aggregate, thus limiting the application of MOF materials in heterogeneous catalysis. Cai et al. Prepared NH
2-MIL-101(Fe)@NTU core-shell hybrid catalyst by one-step synthesis, and used hydrophobic COF to achieve the modification of MOF
[49]. They found that the selectivity (84%) and conversion (32%) of the hybrid material for the target product benzaldehyde were higher than those of pure NH
2-MIL-101(Fe)(24% and 26%). In addition, the catalytic activity and selectivity of the core-shell hybrid catalyst did not change significantly after four consecutive cycles. The excellent catalytic effect is mainly due to the fact that the NH
2-MIL-101(Fe) provides a coordinated unsaturated catalytic center and the NTU-COF shell gathers the hydrophobic molecular substrate styrene, which promotes the free radical mechanism of direct synthesis of benzaldehyde from styrene.
Rahmati et al. Designed and synthesized a Co-MOF/COF hybrid material with both Lewis acidic center (Co
2+) and basic center (imine group in COF)
[54]. They found that Co-MOF/COF could efficiently catalyze the synthesis of benzylidenemalononitrile derivatives in the Knoevenagel condensation reaction, and could be used at least eight times without loss of catalytic activity. In addition, the catalyst has good catalytic effect under solvent-free conditions, and the amount of Co-MOF/COF catalyst is small (15 mg), and the reaction time is short (5 – 60 min).
Gao et al. Designed and synthesized a core-shell hybrid material PCN-222-Co @ TpPa-1 constructed by strong π-π stacking
[39]. The material has Lewis acid sites (Co (II) center and Zr (IV) cluster) and Brnsted base active sites (imine group in TpPa-1), and has high chemical stability and thermal stability. PCN-222-Co @ TpPa-1 catalyzed the one-pot dealdehyde-Knoevenagel condensation cascade reaction with high catalytic activity (up to 99.3% yield) and good recovery.
3.1.2 Photocatalysis
The photoresponse range, photogenerated carrier lifetime, and charge separation efficiency of the photocatalyst are important evaluation criteria. COF materials mainly rely on π-conjugated units to harvest light, and the D-A structure improves the charge transfer efficiency
[23]. MOF materials rely on organic ligands as "optical antennas" for light absorption, and metal nodes promote charge migration
[2]. MOF-COF hybrid materials can exert their respective advantages to produce synergistic effects, which is conducive to expanding light absorption and improving processes such as exciton generation and charge transfer.
For example, Peng et al. Combined photocatalytically active NH
2-MIL-68 with TPA-COF to obtain NH
2-MIL-68@TPA-COF composite
[46]. Rhodamine B was degraded by this MOF @ COF material under visible light (λ > 420 nm), and its photocatalytic degradation activity was found to be better than that of single NH
2-MIL-68. This is mainly because the NH
2-MIL-68@TPA-COF composite has a higher specific surface area and a smaller band gap, thus improving the photocatalytic activity.
MOF @ COF materials can also be doped to improve the photocatalytic performance. For example, Sun et al. Designed and synthesized a Pd-doped MOF @ COF composite Pd/TiATA @ LZU1
[47]. In this composite, the metal is the active center, the MOF core is the electron donor, and the COF shell is the mediator of electron transfer, forming a "donor-mediator-acceptor" synergistic system. The photocatalytic hydrogenation of olefins and the photocatalytic dehydrogenation of ammonia borane (NH
3BH
3,AB) were carried out under visible light, and the results showed that the hybrid material was very effective for both hydrogenation and dehydrogenation. It can be seen from the COF as a mediator that the covalent bond in the MOF-COF composite can promote the transfer of photogenerated electrons in the heterojunction. The hybrid material of NH
2-MIL-125(Ti)/B-CTF-1 synthesized by Zou et al. Using amide bond for hybridization, due to the presence of amide bond, the effective separation rate of charge generated by photoexcitation is obviously improved, and the MOF @ COF material can be used as an excellent and stable hydrogen evolution catalyst under visible light: the hydrogen production rate of 15 wt% NH
2-MIL-125(Ti)/B-CTF-1 is 360μmol·h
-1·g
-1, which is more than twice that of B-CTF-1
[35]. For another example, the hydrogen evolution rate of the NH
2-UiO-66/TpPa-1-COF(4∶6) hybrid material through C = N covalent connection reaches 23.41 mmol·g
-1·h
-1(TOF=402.36 h
-1), which is about 20 times higher than that of TpPa-1-COF
[36]. In addition, in terms of stability, NH
2-UiO-66/TpPa-1-COF(4∶6) showed excellent photocatalytic stability in 480 H for 20 consecutive cycles.
Lu et al. Prepared a series of NH
2-MIL-125@TAPB-PDA nanocomposites with different COF shell thickness and evaluated the photocatalytic performance of all samples by selective photocatalytic oxidation of benzyl alcohol to benzaldehyde
[48]. The benzaldehyde yield of MOF @ COF hybrid materials was much higher than that of the parent NH
2-MIL-125 and TAPB-PDA, which indicated that COF played an important role in improving the photocatalytic activity. The outer COF in the core-shell mainly plays the role of absorbing light, while the inner MOF is mainly a powerful catalyst for oxidizing reactants. Likewise, the MOF @ COF C = N bond plays an important role in the transport of photogenerated electrons and holes. It was found that when the COF content in the hybrid product reached NH
2-MIL-125@TAPB-PDA-3, the yield of benzaldehyde was the highest (94.7%), which was 2.5 times that of NH
2-MIL-125 and 15.5 times that of TAPB-PDA. In addition, the catalytic activity was still relatively high after five cycles under the same conditions (the conversion of benzyl alcohol was above 87.5% without by-products).
3.2 Adsorption and separation
3.2.1 Adsorption in solution
Both MOF and COF are porous materials and have adsorption capacity. MOF materials are unstable in aqueous solution and some organic solvents, and the framework structure is easy to collapse, which limits the application of MOF as an adsorption material in solution. COF materials can adsorb dyes by hydrogen bonding, electrostatic bonding and π-π stacking interaction
[59,60]. The MOF-COF hybrid framework material solves the instability of MOF in aqueous solution, and a good adsorption effect can be obtained by using the synergistic effect of the two.
Firoozi et al. Synthesized a MOF-5/COF composite M5C with high π-π stacking, hydrogen bonding and electrostatic interactions
[41]. They utilized COF to MOF via strong covalent bonding (C = N) to overcome the instability of MOF-5. The hybrid material can quickly and efficiently remove AO and RB from aqueous solution. At pH = 9. 5, the adsorption capacity of M5C adsorbent for AO and RB cationic dyes was 17. 95 mg/G and 16. 18 mg/G, respectively.
In addition, the Fe
3O
4@MOF@COF hybrid framework materials also have good adsorption properties. Li et al. Prepared a Fe
3O
4@MOF@COF magnetic composite for the detection and separation of trace Cu (II) in aqueous solution
[57]. Fe
3O
4@MOF@COF has the advantages of fast separation speed, high stability, high adsorption selectivity and large adsorption capacity. Because the composite material contains abundant nitrogen-containing groups, the lone pair electrons on N are beneficial to the adsorption of the Cu
2+ with high electronegativity, so that the composite material has good selectivity for the Cu
2+. Chen et al. Designed and synthesized a magnetic composite Fe
3O
4@MOF@COF for microextraction of sulfonamides (SAs) in food and environmental samples
[58]. The composite material has excellent adsorption performance for SA due to various interactions such as hydrogen bonding, π-π, hydrophobic interaction and the like, so the composite material can be used for SA analysis in actual samples. Similarly, Jiang et al. Reported a Fe
3O
4@TAPB-COF@ZIF-8 for the determination of trace bisphenols in energy drinks by solid phase extraction
[42]. The main principle is that there is a large delocalized π-π electron system in the composite and a benzene ring structure in the bisphenol, and the strong π-π stacking effect of the two leads to better adsorption performance. In addition, the hydrogen bond in bisphenols and the interaction between Lewis base-OH and Lewis acid center Zn in the composite are also beneficial to improve the adsorption effect.
In addition, Garz Garzón-Tovar et al. Synthesized UiO-66-NH
2@COF-TAPB-BTCA composite materials by spray drying and two-step transformation from amorphous state (imine-based polymer) to crystalline state (imine-based COF), and found that micropores and mesopores were produced at the MOF/COF interface, the porosity was much higher than that of MOF and COF components, and the pores at the interface had good water absorption capacity
[50].
3.2.2 Gas adsorption and separation
The MOF-COF framework can also be used for gas adsorption and separation. MOF can be used as a filler for the preparation of MMMs (mixed matrix membranes), and can achieve good results
[61]. However, MOF materials still have shortcomings in selectivity and permeability. COF is a pure organic material with adjustable pore structure and high crystallinity, which plays an important role in the modification and optimization of MOF membrane materials.
Fu et al. Designed and synthesized a COF-300 @ ZIF-8 composite membrane material and used it for the separation of H
2/CO
2 mixed gas. Compared with the single MOF and COF membrane materials, the composite membrane material has higher separation selectivity, as shown in Fig. 11
[40]. The separation factors of [COF-300]-[Zn(bdc)
2(dabco)] and [COF-300]- [ZIF-8] composite membranes for the H
2/CO
2(1∶1) binary mixture were 12.6 and 13.5, respectively, which were much higher than those of COF-300, Zn
2(BDC)
2(dabco), and ZIF-8 membranes (6.0, 7.0, and 9.1). This is because the amorphous MOF at the interface of the composite can seal the space between the COF crystals, which is beneficial to improve the selectivity of the film. In addition, the interface layer allows the composite to maintain a permeability similar to that of the individual MOF and COF layers.
Cheng et al. Designed and synthesized a MOF @ COF hybrid filler UiO-66-NH
2@TpPa-1, and added it into PSf (polysulfone) to make MMMs membrane for the separation of CO
2/CH
4[34]. The pure organic COF layer has a high affinity for the polymer matrix, which enhances the compatibility of the polymer with the filler, thereby alleviating the agglomeration of the filler. This also prevents the formation of non-selective interfacial voids. In addition, the large pores (18.33 Å) of COF can prevent the clogging of MOF (5.11 Å) pores and form a rapid gas permeation channel within the membrane. MOF show affinity to CO
2, and its selective pore can allow rapid preferential penetration of CO
2, which can improve that CO
2 permeability and CO
2/CH
4 selectivity of MMMs simultaneously. The optimized MOF @ COF-based MMMs exhibited good CO
2 permeability (7.1 Barrer) and high CO
2/CH
4 selectivity (46.7). In addition, in terms of stability, MOF @ COF/PSf-5 MMMs still showed good and stable CO
2 permeability and CO
2/CH
4 selectivity in the continuous 100-h test.
3.3 Sensing
The MOF-COF hybrid framework also has applications in food safety, biosensing, and other fields. To avoid the abuse of antibiotics in biological systems, materials that can be sensitively detected at very low concentrations are needed. Liu et al. Designed and synthesized a novel nanostructured COF/MOF composite, Co-MOF @ TPN-COF, and used it as an electrochemical aptasensor to detect ampicillin (AMP) β-lactam antibiotics
[62]. The synthesized Co-MOF @ TPN-COF has the characteristics of both MOF and COF: nitrogen-rich functional groups, strong synergistic effect between triazine-rich ring and Co-MOF, π-π stacking and hydrogen bonding. This makes the Co-MOF @ TPN-COF-based aptasensor exhibit a very low detection limit (0.217 fg·mL
-1) in the AMP concentration range of 1.0 fg·mL
-1~2.0 ng·mL
-1, which is better than using Co-MOF and TPN-COF alone for AMP detection. In terms of stability, the aptasensor can still maintain about 15% of the initial value after a long period of storage for 15 days, which indicates that the aptasensor has good stability. Zhang et al. Designed and synthesized a core-shell composite UiO-66-NH
2@COF for electrochemical detection of ATP and antibiotics
[56]. The composite has high affinity between the phosphate group of the aptamer and the compact Zr (IV) site, and has strong π-π stacking interaction between the aptamer and the MOF @ COF, so the composite can significantly improve the electrochemical detection performance.
Wang et al. Constructed a multi-emission UiO-66 @ COF to realize ratiometric sensing and visible light detection of adenosine triphosphate (ATP) and phosphate ions
[53]. The Zr
4+ ion in UiO-66 has a high affinity for phosphate species, which improves the sensing selectivity. In the COF selection, phenylenediamine (PDA) -based COF and tetraaminotetraphenylacetylene (TPE) -based COF containing hydroxyl and alkyl groups were selected, respectively. The fluorescence performance was improved by hybridizing with UiO-66 to eliminate the aggregation quenching phenomenon (ACQ). The intramolecular hydrogen bond realizes the ESIPT (excited-state intramolecular proton transfer) structure and eliminates the intramolecular rotation of the strong multi-emission of the UiO @ COF composite. Otherwise, using the strong affinity of Zr
4+, the different structures of the two composites successfully achieved the discrimination of
and ATP. In addition, Yola et al. Synthesized a core-shell Ti-MOF @ COFS composite and used it as a signal amplification carrier in an immunosensor to detect GL-3 antigen
[52]. The MOF @ COF composite not only promotes the penetration of antibody protein through the pore, but also has a highly stable antigen-antibody interaction. The immunosensor has a high sensitivity with a linear range of 0.0001~20.0 ng·mL
-1 and a detection limit of 0.025 pg·mL
-1(25.0 fg·mL
-1).
3.4 Other
MOF-COF hybrid framework also has some applications in derivative materials, biological enzymes, energy storage and so on, but because MOF-COF hybrid framework needs to be further enriched, these aspects need to be further studied. Sun et al. Reported a COF/Mn-MOF composite through Mn-N coordination hybridization
[37]. Due to the activated Mn center, the benzene ring and the unique flower-like microsphere morphology of the hybrid material, the composite material has excellent lithium storage performance. As an anode material for lithium-ion batteries, the COF/Mn-MOF exhibits an extremely high reversible capacity of 1015 mAh·g
-1 after 650 cycles, which is attributed to the participation of the metal center (Mn
2+) and the C = C group on the benzene ring in lithium storage. In addition, its derivatives MnS @ NS-C also showed excellent lithium storage performance (up to more than 1000 mAh·g
-1 after 200 cycles). Peng et al. Synthesized an aza-MOFs @ COFs hybrid material by aza-Diels-Alder reaction and used it as a high-performance supercapacitor material
[55]. Synthesis of MOF @ COF as well as aza-Diels-Alder cycloaddition, resulting in the formation of aza-MOF @ COF hybrid electrodes with enhanced specific capacitance up to 20.35μF·cm
-2. The electrode has high durability, and the capacity retention rate is as high as 89.3% after 2000 cycles. In addition, the use of molecular spacers provides a high volumetric energy density of 1.16 F·cm
-3, and the improvement of electrochemical performance mainly comes from the post-synthesis modification.
Zhang et al. Designed and synthesized a MOF @ COF nanoenzyme and used it to enhance the inhibition of bacteria
[51]. They designed MOF @ COF NANase as an efficient peroxidase mimetic, with the metal node of MOF as the active center and the hierarchical nanocavity generated by the growth of covalent organic framework (COF) as the binding capsule, forming a customized pore microenvironment around the active site for enrichment and activation of substrate molecules for enhanced bacterial inhibition. Hybrid nanozymes with pseudopod-like morphology can tightly trap bacteria via multivalent topological interactions between "hairy" bacteria and spiky COF surfaces, and finally effectively kill bacteria via reactive oxygen species generated in situ. In addition, Zhuang et al. Used the synthesized ZIF-67/COF to prepare the derived highly dispersed Co
3O
4/N doped porous carbon (Co
3O
4/NPC), and the derivative has superior performance in oxygen evolution and application in lithium-ion batteries
[38].
4 Conclusion and prospect
In general, this paper summarizes the research progress of MOF-COF hybrid materials in recent years from three aspects: the types, synthesis and applications of MOF-COF hybrid materials.
There are generally two types of MOF-COF framework hybrids: one is core-shell structure, such as NH2-MIL-68@TPA-COF; The second is the layered hybrid structure, such as COF-300 @ ZIF-8. The hybridization between MOF and COF is mainly through covalent bonds, such as C = N, O = C — NH, etc. Or through metal coordination bonds, such as Mn-N, Zn-N, etc.; Or through hydrogen bonds, van der Waals forces, and π-π stacking, such as PCN-222-Co @ TpPa-1.
In comparison, the hybrid frameworks connected by covalent bonds and metal coordination bonds are more stable than those connected by hydrogen bonds, π-π stacking and van der Waals forces. Therefore, the current strategy is to functionalize the surface of MOF or COF with amino group, and then use Schiff base reaction to make COF or MOF grow on the surface to prepare composite materials. The C = N imine bond not only stabilizes the hybrid framework, but also facilitates charge transport, which greatly improves the catalytic performance of MOF-COF. Especially in photocatalytic applications, the presence of covalent bonds is beneficial to the generation of photogenerated charges and the effective separation of electron-hole pairs, reducing the recombination rate.
As the development of MOF-COF hybrid framework is still in the development stage, although it can cover some applications of MOF and COF materials, the performance of hybrid framework materials is generally better than that of hybrid MOF and COF. However, the application scope of MOF-COF hybrid framework is not wide enough. First, the structure of MOF-COF is less at present. Second, the hybrid strategy is not yet mature. At present, the main challenges of MOF-COF framework materials are as follows:
(1) that method is relatively single, and the synthesis mean is basically realized by amino functionalization;
(2) Molecular level crosslinking hybridization is very important to exert the synergistic effect between components, but there is only one example (such as COF/Mn-MOF), and this method needs further research and development;
(3) When designing MOF-COF framework composites, there are still many difficulties in achieving energy level matching to construct a suitable heterojunction.
The use of amino functionalization to achieve hybridization not only limits the synthesis of MOF-COF, but also makes the structure of MOF-COF basically convergent. Moreover, there are many kinds of MOF and COF, and it is very limited to use only MOF and COF with —NH2 to carry out hybrid research, and it is difficult to obtain new structures. In addition, on the theoretical side, the controlled conjugate growth mechanism of mixed-frame hybridization needs to be studied in depth.
Although the research of MOF-COF hybrid materials is still in its infancy and faces many challenges, as a new research direction of MOF and COF materials, the development of MOF-COF hybrid frameworks, especially the new stable structure of MOFCOF hybrid frameworks, has a wide range of applications and great potential value, which requires more attention and continuous research and exploration.
National Natural Science Foundation of China(22078241)
Fundamental Research Funds for the Central Universities.
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