Preparation and Application of MOFs/ Hydrogel Composites

Chao Li; Yaoyu Qiao; Yuhong Li; Jing Wen; Naipu He; Baiyu Li

doi:10.7536/PC201062

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (11) : 1964-1971 DOI: 10.7536/PC201062

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Preparation and Application of MOFs/ Hydrogel Composites

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Abstract

Metal-organic frameworks(MOFs) have a large number of pore structures and active sites, which enable them to play a huge role in gas adsorption, catalysis, medical treatment and other fields. As a crystal powder, MOFs are brittleness, easy hydroxylation, difficulty in recycling and so on which limits its application. MOFs combining to flexible polymers, in particular combining to hydrogels greatly improve the flexibility, dispersibility in water, recyclability, and processability of composite materials, which broadens the application fields of MOFs. In the present paper, it is elaborated that the progress of the preparation of MOFs/hydrogel composites are based on three different methods including MOFs in situ growing from hydrogel, MOFs/hydrogel simultaneous generation, and hydrogel encapsulated MOFs. The characteristics of the above three preparation methods and their resulting product properties are summarized. Additionally, the applications of composite materials in biomedicine, catalysis, wastewater treatment, and gas adsorption are further summarized. Finally, preparation method improvement and future application of MOFs/ hydrogel composites are discussed and prospected.

Contents

1 Introduction

2 Preparation of MOFs/hydrogel composites

2.1 MOFs in situ growing from hydrogel

2.2 MOFs/hydrogel simultaneous generation

2.3 Hydrogel encapsulated MOFs

3 Application of MOFs/hydrogel composites

3.1 Biomedical applications

3.2 Catalysis

3.3 Wastewater treatment

3.4 CO₂adsorption

4 Conclusion and outlook

Graphical abstract

Keywords

MOFs / hydrogel / composite material / medical treatment / catalysis / wastewater treatment / gas adsorption

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Chao Li, Yaoyu Qiao, Yuhong Li, Jing Wen, Naipu He, Baiyu Li. Preparation and Application of MOFs/ Hydrogel Composites. Progress in Chemistry, 2021, 33(11): 1964-1971 DOI:10.7536/PC201062

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1 Introduction

Metal – organic frameworks (MOFs) are organic/inorganic hybrid materials formed by coordination bonds between organic ligands and metal ions or metal ion clusters^[1]. It has been widely used in gas storage, catalysis, molecular separation, drug delivery and other fields because of its high specific surface area, controllable pore size, functional pore channels, flexible chemical structure and other advantages^[2]^[3]^[4⇓~6]^[7]^[8,9]. However, due to the inherent properties of inorganic coordination materials, MOFs crystals tend to be brittle and can be decomposed into fine particles or fine powders, which can easily clog reactors and pipes in separation, catalysis and other industrial applications. When flushing with gas or liquid, severe material loss will inevitably occur^[10]. And because of the decomposition tendency of MOFs in water, the utilization efficiency of MOFs is not high^[11]. Therefore, the development of MOFs composite materials, which not only have the advantages of porous structure and large specific surface area of MOFs, but also have flexibility and are not easy to decompose, has attracted wide attention^[12,13].

Hydrogel is a kind of polymer network which is hydrophilic but insoluble in water. Hydrogels can swell rapidly in water but still maintain their shape and three-dimensional network structure, which is a kind of special "soft matter" with rapid development and integration of water absorption, water retention and slow release^[14,15]. The softness and stability in water of hydrogels are what MOFs materials lack. MOFs are easy to decompose in water due to the coordination between oxygen atoms in water molecules and metal ions. The stability of MOFs in aqueous phase is mainly affected by the bond energy between metal ions and organic ligands, framework structure and framework hydrophobic effect^[16]. During the formation of MOFs, the easily coordinated groups (such as —NH₂, — COOH and — OH) can coordinate with the metal ions of MOFs to occupy their active sites, and the formation of multi-coordinated structures further increases the steric hindrance. All these factors can increase the hydrolysis activation energy of MOFs, thus increasing the kinetic stability of MOFs in aqueous phase^[17]. According to this theory, the side groups (such as -NH₂, -OH, etc.) Of the polymer chains forming the three-dimensional network of the hydrogel are easy to coordinate with the metal ions constructing the MOFs during the combination of MOFs and the hydrogel, which is conducive to enhancing the stability of MOFs in the hydrogel system. In general, hydrogels have good biocompatibility, flexibility, water absorption and retention, mechanical properties and pore structure. The MOFs are distributed in the three-dimensional network structure of the hydrogel, so that the mechanical property and the specific surface area of the hydrogel are further improved; meanwhile, the defect that the MOFs are easy to decompose can be effectively improved, and the stability, the processability and the recyclability of the MOFs can be improved^[18]^[19]. Therefore, the composite of hydrogels and MOFs is one of the effective combination strategies of rigid materials and flexible materials, and this composite is expected to improve the performance of both materials, making it have potential application value in the fields of biomedicine, industrial catalysis, gas adsorption and water treatment.

In this paper, the preparation methods and applications of metal-organic framework/hydrogel composites are systematically summarized, and the future development direction of the composites is discussed.

2 Preparation of Metal-Organic Framework/Hydrogel Composite

The preparation of MOFs/hydrogel composites can be divided into three types according to the synthesis process (Figure 1): (1) Hydrogel is prepared first, then the hydrogel is placed in the metal precursor solution, and then the organic linker is added to make MOFs grow on the hydrogel, that is, the in-situ generation method based on hydrogel MOFs (Figure 1A). (2) The MOFs/hydrogel composite was prepared by mixing the raw materials for the preparation of the hydrogel with the precursor solution for the preparation of MOFs, so that the MOFs and the hydrogel were generated simultaneously, i.e., the MOFs/hydrogel simultaneous generation method (fig. 1B). (3) MOFs were prepared first, and then the MOFs were placed in the precursor solution of the synthetic hydrogel to prepare the hydrogel composite material coated with MOFs, that is, the hydrogel coated MOFs method (Fig. 1C).

The combination of MOFs and hydrogel can be achieved by the three methods, and the properties of MOFs/hydrogel composites obtained by different preparation methods are different.

2.1 In situ formation of MOFs based on hydrogel

As shown in fig. 1A, most of the work using the in-situ growth method of MOFs on hydrogel to prepare MOFs/hydrogel composites will first make the hydrogel fix metal ions on the surface and pores through physical or chemical effects.This method is like building, hydrogel is used as the foundation, metal ions are covered first, and then organic linking agent is added to react with metal ions to grow MOFs. Here, the choice of hydrogels is very important. Hydrogels with excellent performance and stable immobilization of metal ions can often make the preparation work twice the result with half the effort. The current work mostly uses natural polymers such as gelatin, alginic acid, chitosan and so on as raw materials for the preparation of gel substrates, thus preparing MOFs/hydrogel composites with excellent performance.

Gelatin is a biodegradable substance, which has been widely used in the field of medicine, and its biological safety has been confirmed by its long-term clinical application^[20]. The presence of high levels of pyrrolidone in gelatin results in the formation of gels with excellent properties^[21]. Garai et al. selected gelatin as the hydrogel substrate, and they soaked the prepared gelatin hydrogel in Zn(NO₃)₂·6H₂O solution overnight for Zn²⁺ immobilization, and then put the gelatin hydrogel loaded with Zn²⁺ in ethanol solution of 2-methylimidazole overnight to prepare ZIF-8, thereby preparing ZIF-8/gelatin hydrogel composite material^[22].

Lu et al. Used the same method to prepare Zn-PAM composite hydrogel loaded with Zn²⁺ by reacting polyacrylamide (PAM) hydrogel with Zn²⁺, and then reacted the composite hydrogel with 2-methylimidazole solution to prepare ZIF-8/polyacrylamide hydrogel composite^[23].

Alginate is a natural biopolymer extracted from brown algae. It has been widely used in food, medicine, water treatment and other fields because of its non-toxic, biodegradable, good biocompatibility, low price and other advantages^[24,25]. It has high chelating affinity with multivalent metal ions (including alkaline earth metal ions, such as Ca²⁺ and Sr²⁺, etc.), and can form gels with good adsorption performance, so it is easy to prepare gels loaded with metal ions. Therefore, alginate is the most frequently used raw material for the preparation of MOFs/hydrogel composites^[26]. Taking advantage of this, Zhu et al. prepared Cu²⁺/ sodium alginate hydrogel beads by dropping the sodium alginate solution into the aqueous solution of copper nitrate trihydrate, and then put the hydrogel beads into the methanol solution of 2-methylimidazolium salt and sodium formate, and obtained the sodium alginate hydrogel/MOF composite material by washing and drying after growing at high temperature (Figure 2)^[27].

This technique for preparing MOFs/hydrogel composites can also be combined with other techniques to prepare materials with more complex structures. Zhao et al. Also used the characteristic that sodium alginate can chelate with multivalent metal ions, selected sodium alginate as the raw material for preparing hydrogel, and combined with microfluidic technology, used Zn²⁺ or Cu²⁺ solution as the core.When the sodium alginate solution flows through the metal core, it will chelate with the metal core to form Cu²⁺/ alginate hydrogel hollow shell. After the MOFs precursor solution is injected into the hydrogel shell, MOFs grow in situ in the shell^[28]^[29,30]. Gracias et al. also selected alginate as the hydrogel precursor, combined with the 3D printing technology, used the precursor for preparing HKUST-1 and alginate as the ink for 3D printing, after curing, immersed in Cu(NO₃)₂ solution for crosslinking to prepare the hydrogel substrate, and at the same time, grew HKUST-1 in situ on the hydrogel substrate, as shown in Figure 3^[31].

2.2 Hydrogel/MOFs simultaneous formation method

Compare with that in-situ growth method of MOFs on a hydrogel matrix, the method is simpl, the raw materials for synthesizing the hydrogel and the precursor liquid for synthesizing the MOFs are adde into a pot to be stirred like porridge, and the growth and loading of the raw materials and the precursor liquid are carried out simultaneously to form an MOFs/hydrogel composite material which is wrapped with each other.

Shi et al also chose alginate as the raw material of hydrogel, but unlike Zhu et al mentioned above, they chose MOFs/hydrogel simultaneous generation method to prepare composite materials^[32]^[23]. Sodium alginate was dissolved in distilled water, and then the sodium alginate solution was dropped into the methanol solution of Co(NO₃)₂ and 2-methylimidazole salt, and the sodium alginate hydrogel/MOF composite material MA-M was obtained after stirring and drying. At the same time, the sodium alginate hydrogel/MOF composite AM-M was prepared by adding 2-methylimidazolium salt solution after the sodium alginate hydrogel was prepared by the method of first gelling and then loading MOFs. AM-M showed the same purple color as ZIF-67, proving that ZIF-67 grew on the alginate matrix, but MA-M obtained by the simultaneous preparation method of hydrogel and MOFs showed green color, indicating that Co²⁺ chelated with alginate and 2-methylimidazolium salt simultaneously to form a new MOFs structure.

Chitosan (CS), derived from chitin, is an ideal material for the preparation of hydrogels because of its wide source, low price, good biocompatibility, biodegradability and antibacterial properties^[33]. Chitosan contains a large number of primary amines (-NH₂), which can chelate with metal ions to achieve the purpose of loading metal ions, similar to sodium alginate, so it can also be used to prepare MOFs/hydrogel composites^[34]. Yao et al. Dissolved chitosan in acetic acid solution, added zinc nitrate hexahydrate to form Zn²⁺/ chitosan solution, mixed dimethylimidazolium salt solution with ammonium hydroxide solution, and finally mixed and stirred all the above mixed solutions to prepare ZIF-8/chitosan hydrogel composite material^[18].

2.3 Hydrogel coated MOFs method

The advantage of this method is that it can ensure that the synthesis of MOFs is not affected by the presence of hydrogel. MOFs/hydrogel composites wrapped with MOFs can be prepared by placing the prepared MOFs in the hydrogel precursor solution to synthesize hydrogel. However, because MOFs have been formed before the formation of composites, the interaction between MOFs and hydrogels is mainly physical, so the stability of products needs to be improved.

Poly (vinyl alcohol) (PVA) is a hydrophilic, biodegradable and biocompatible synthetic polymer, which has been widely used in the biomedical field and is an ideal material for the preparation of hydrogels^[35]. Wang et al. mixed and stirred the aqueous solution of Cu(NO₃)₂·3H₂O and the methanol solution of 1,3,5-benzenetricarboxylic acid to prepare HKUST-1, and then added HKUST-1 into the PVA-co-PE suspension to stir, and after repeated freezing treatment, prepared the PVA-co-PE/PVA nanofiber mixed hydrogel composite material wrapped with HKUST-1, as shown in Figure 4^[36]. Liu et al. Also chose PVA as the raw material to prepare the hydrogel. They suspended ZIF-8 nanoparticles in the aqueous phase and used surfactants to stabilize the silicone oil droplets in the PVA aqueous solution. Because the oil density was lower than that of water, the oil droplets floated on the aqueous phase. After the water evaporated, PVA solidified and embedded ZIF-8 nanoparticles, thus fixing the oil droplets^[37]. After removing the oil droplets, the dried porous MOFs @ PVA membranes were obtained. Subsequently, the porous PVA membranes were cross-linked with glutaraldehyde (GA) to prepare ZIF-8 @ PVA hydrogel films through the interaction between the hydroxyl groups in PVA and the aldehyde groups in GA.

MOFs/hydrogel composites can also be added with some materials to obtain more properties. For example, Cui et al. added carbonic anhydrase (CA) to the composites and prepared PVA/CS/CA @ ZIF-8 composites^[38]. Carbonic anhydrase is a metalloenzyme that catalyzes the hydration of carbon dioxide with bicarbonate and protons. This reaction plays an important role in many physiological and pathological processes^[39,40]. They first mixed CA with Zn(NO₃)₂·6H₂O and 2-methylimidazolium salt, the precursor of ZIF-8, to prepare ZIF-8 wrapped with CA by co-precipitation method, then added polyvinyl alcohol and chitosan solution to stir, and prepared PVA/CS/CA @ ZIF-8 composite materials by repeated freezing and thawing.

3 Study on the Application of MOFs/Hydrogel Composite

MOFs/hydrogel composites have the advantages of porosity, biocompatibility, uniform dispersion of active sites and easy recycling, which can greatly enhance the application of MOFs materials in different fields. At present, the related development is mainly used in biomedicine, catalysis, wastewater treatment and gas adsorption.

3.1 Biological Medicine

MOFs are widely used in the medical field because of their adjustable structure, large specific surface area and easy functionalization^[41]. The hydrogel has good biocompatibility and recyclability, and the composite material has obvious biocompatibility and recyclability after being combined with the MOFs, and is expected to improve the loading rate of the drug. Wounds in people with metabolic syndrome heal so slowly that treating them with traditional antibiotics can cause problems. The Centers for Disease Control predicts that by 2050, more people will die from antimicrobial bacteria than from all types of cancer combined^[42]. Therefore, the development of new wound dressings that do not rely on antibiotics has attracted much attention. MOFs materials can store and slowly release metal ions, such as zinc, copper and cobalt ions, which can promote wound healing within a certain concentration, but will produce toxicity beyond the limited concentration, so it is very important to find a sustained-release material that can control the release concentration and rate of copper ions^[43]. The combination of MOFs and hydrogel can not only effectively control the release of metal ions, but also accelerate wound healing^[24,33].

Zhao et al. immersed ultrafine fibers containing different numbers of MOFs in cell culture medium to detect their cytotoxicity, and found that the toxicity of microfibers to NIH 3T3 cells increased with the increase of metal ion concentration, which led to the destruction of cell morphology and the increase of the number of dead cells^[30]. However, when the concentration of metal ions was very low, the survival rate of NIH 3T3 cells increased slightly. Therefore, based on the controllable and sustainable release of MOFs, the microfibers can be used to improve cell survival. Subsequently, they explored its antibacterial activity and found that with the increase of copper ion concentration, the death rate of normal cells also increased. Through a series of optimizations, the concentration of copper ions suitable for biocompatibility and antibacterial use was set around 0.158 mmol·L^-1. Finally, the practical value of MOFs-loaded microfibers on wound healing was explored, and it was found that the wounds treated with MOFs-loaded microfibers almost healed after 9 days, while the wounds in the control group needed more time to heal, especially the copper and zinc composite MOF hydrogel significantly accelerated wound healing.

Liu et al. studied the inhibitory effect of MOFs @ PVA porous hydrogel films on bacterial adhesion, and found that the number of colonies of MOFs @ PVA porous hydrogel films was reduced by 39.1% compared with that of PVA^[37]. The results showed that the higher the concentration of ZIF-8 nanoparticles, the higher the inhibition rate of E. coli. However, the rapid release of zinc ions may be cytotoxic to cells, so it is necessary to control its concentration and release rate. Keeping the concentration of ZIF-8 in MOFs @ PVA film at 500μg·mL^-1 could ensure its bactericidal performance and biocompatibility, and the wound almost healed after 11 d, with a healing rate of 89. 6%. Meanwhile, the ZIF-8 @ PVA porous dressing can prevent infection, induce angiogenesis, and regenerate extracellular matrix.

3.2 Catalyze

MOFs materials have the advantages of high density, uniform dispersion of active sites and high pore structure, which can greatly promote the diffusion and transport of matrix and products. Therefore, MOFs materials are ideal catalysts^[44]. However, MOFs particles are very small and easy to adhere to the reactor wall, which makes it very difficult to recover the catalyst from the reaction system. Therefore, fixing MOFs on the hydrogel can greatly reduce the difficulty of recovery, thus playing a role in saving costs. At the same time, the hydrogel with porous structure reduces the possibility of MOFs aggregation and makes MOFs more evenly distributed in the hydrogel, thus improving the catalytic activity of MOFs.

Yan et al. Degraded cellulose under hydrothermal conditions and found that the total yield of formic acid, oxalic acid and acetic acid was only 35.11% without any catalyst^[45]. Using reduced graphene oxide hydrogel as catalyst, the total yield of the three acids was only 19. 07%, and no formic acid was formed. In addition, when pure ZIF-8 nanoparticles were used for catalysis, the total yield of the three acids was 58.63%, and the yield of formic acid was 48.52%, indicating that ZIF-8 was beneficial to the conversion of polymeric cellulose chains to formic acid. Finally, when ZIF-8/rGO hydrogel was used as catalyst, the yield of formic acid increased dramatically, and the total yield of three acids reached 99. 91%, indicating that 3D ZIF-8/rGO hydrogel could effectively promote the conversion of cellulose to formic acid.

3.3 Waste water treatment

At present, the problem of environmental pollution and resource shortage caused by improper treatment of wastewater has become an urgent problem to be solved. Organic dyes have been widely used in many industrial experimental fields, but they pose a serious threat to the environment and human health due to their toxicity, potential mutagenicity and carcinogenicity^[46]. MOFs are a kind of advanced porous crystalline materials composed of transition metal ions and organic binders. Their pore structure can effectively adsorb dyes. However, pure MOFs are unstable in water and difficult to recycle because of their single size and pore size. Therefore, the hydrogel with high porosity and environmental friendliness was selected to combine with MOFs to prepare MOFs/hydrogel composites with rich pore structure and environmental friendliness, which greatly improved the recyclability of MOFs while enhancing its adsorption capacity for dyes.

Zhu et al. Used the prepared MIL-100 (Fe) -alginate composite to remove dyes from contaminated water^[27]. After soaking for 5 d, the solution turned colorless, while the MIL-100 (Fe) -alginate composite turned deep red, which proved that the composite could effectively adsorb dyes in water. Wang et al. Immersed 0.02 G HKUST-1 and 0.1 G HKUST-1 @ PVA-co-PE/PVA mixed hydrogels (containing 0.02 G HKUST-1) in 40 mL dye solution to study the adsorption performance^[36]. It was found that the adsorption capacity of methylene blue MB by HKUST-1 and hybrid hydrogel was higher than that of the other two dyes, and the hybrid hydrogel was superior to HKUST-1 alone in decolorization. When the concentration of methylene blue solution was 600 mg·L^-1, the introduction of hydrogel increased the decolorization rate from 40.56% to 44.97%, and it was easier to operate and recover than using HKUST-1 alone. The results showed that the hybrid hydrogel could adsorb cationic dyes in water. Gracias et al. Put the 3D-printed HKUST-1/hydrogel composite into 10 mL aqueous solution containing 30 μM dye to test the dye adsorption performance, and found that when immersed in a single dye solution, HKUST-1 hydrogel had obvious adsorption on methylene blue (MB).96.3% of MB can be removed, which proves that the composite material can effectively adsorb the dye; meanwhile, the composite material can selectively absorb MB in a mixed solution of MB and rhodamine 6G (R6G), which proves that the composite material has the function of adsorbing the dye^[31].

Shi et al. tested the tetracycline adsorption of alginate hydrogel/MOFs composites prepared by different methods, and found that all the composites could adsorb tetracycline, but the maximum adsorption capacity of the hydrogel/MOFs composite MA-M directly prepared by the simultaneous preparation method of hydrogel and MOFs was 364.89 mg·g^-1.It is higher than the 302.32 mg·g^-1 of AM-M prepared by in-situ preparation method, and can reach the adsorption equilibrium faster, because the composite materials prepared by the simultaneous preparation method of hydrogel and MOFs have more active sites^[32]. The results show that the composite material can adsorb tetracycline in water well.

Humic acid is an important reducing organic matter in water, which is an important factor causing drinking water problems^[47]. Lu et al. Tested the adsorption capacity of ZIF-8 PAM hydrogel for organic pollutant humic acid (HA). HA was adsorbed rapidly at the beginning, and the adsorption slowed down after 6 H^[23]. It was found that the adsorption capacity of pure ZIF-8 powder on HA was 70.2 mg·g^-1 at room temperature, while the maximum adsorption capacity of ZIF-8 PAM hydrogel in HA solution with initial concentration of 175 mg·L^-1 reached 111.5±3.0 mg·g^-1, which was much larger than that of PAM hydrogel alone or ZIF-8 powder alone, proving that the composite material had good adsorption capacity for humic acid.

3.4 CO₂ adsorption

A large number of CO₂ emissions will lead to the greenhouse effect, which brings great harm to the earth's environment. Zeolite imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs), which are microporous crystalline materials with a tetrahedral network^[48]. The strong bonding between imidazolyl anions and metal cations results in the excellent thermal and chemical stability of ZIF. The pore structure of ZIFs makes it possible to adsorb CO₂ well^[49,50]. At present, there are many studies on the adsorption of CO₂ by ZIFs, but the adsorption capacity of pure ZIFs for CO₂ is still not large enough. In order to enhance the adsorption capacity of ZIFs for CO₂, some CO₂ response groups such as -NH₂ were introduced to modify ZIF-8^[51].

Chitosan is the only alkaline polysaccharide in nature, containing a large number of primary amine (—NH₂) groups, which can react with CO₂ to achieve the purpose of capturing. Therefore, the combination of chitosan and ZIFs can enhance the ability of ZIFs to capture CO₂^[52]. Yao et al. Selected chitosan as the raw material to prepare ZIF-8/chitosan hydrogel composites, and tested the CO₂ adsorption properties of pure ZIF-8 (FC0) and ZIF-8/chitosan hydrogel composites (FC2 and FC2-0).The adsorption capacity of pure ZIF-8 for CO₂ was about 0. 97 mmol/G, while the adsorption capacity of ZIF-8/chitosan hydrogel composite for CO₂ was 0. 56 mmol/G, and the corresponding adsorption capacity of pure ZIF-8 was 1. 21 mmol/g.The results proved that the composite could improve the adsorption ability of ZIF-8 for CO₂^[18].

Cui et al. Used CA @ ZIF-8 hydrogel membrane and PVA/CS/CA @ ZIF-8 hydrogel membrane to convert CO₂ into CaCO₃ to test the ability of PVA/CS/CA @ ZIF-8 hydrogel membrane to capture CO₂^[38]. It is found that the amount of calcium carbonate obtained by using CA @ ZIF-8 hydrogel membrane and PVA/CS/CA @ ZIF-8 hydrogel membrane is 20 times and 32. 6 times of that obtained by using free CA, respectively, which proves that the PVA/CS/CA @ ZIF-8 hydrogel membrane has better CO₂ capture ability than free CA and CA @ ZIF-8, and can capture CO₂ effectively.

4 Conclusion and prospect

Hydrogels have greatly improved the processability, stability and recyclability of MOFs, which has greatly expanded their application fields. At present, the three common methods of preparing MOFs/hydrogel composites have their own characteristics. The key of the in-situ formation of MOFs based on hydrogel is the interaction between metal ions and hydrogel, which makes the combination of the two more firm. At the same time, this method can make MOFs more evenly distributed in the network structure of the gel, but the disadvantage of using this method is that the loading of MOFs in the composite is low. Although the MOFs/hydrogel co-generation method and the hydrogel-coated MOFs method can improve the loading capacity of the composite materials on MOFs, the stability of the obtained products needs to be further improved.

Although MOFs/hydrogel composites have shown excellent properties, the combination mechanism and preparation process of MOFs and hydrogel are not yet mature, and the stability of MOFs and hydrogel combination and the loading of MOFs need to be improved. It is an urgent problem to explore the binding mechanism of MOFs and hydrogel and improve the preparation process of MOFs/hydrogel composites. At the same time, the packing structure and arrangement of MOFs in the hydrogel also need to be investigated. It is believed that the in-depth study on the in-situ formation of MOFs based on hydrogel will greatly expand its application scope by realizing the controllable stacking mode and loading capacity of MOFs.

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