【Deep Coating Society Scientific and Technical Paper】——"Versatile" metal-organic framework material

"Versatile" metal-organic framework material

Zhao Chen Wang Chongchen

School of Environmental and Energy Engineering, Beijing University of Civil Engineering and Architecture

1.
Demystify metal-organic framework materials

Metal-organic framework (MOFs), also known as porous coordination polymers (PCPs), are a class of crystalline materials
with periodic network structures constructed by metal ions or metal clusters and multidentate organic ligands connected by coordination bond bridges 。 As early as the mid-90s of the 20th century, the first generation of MOFs materials has been combined by Kitagawa research in Japan [1], but at this time, the pore structure of MOFs materials also needs the support of guest molecules, and the pore size and stability are limited
.
Until 1999, Yaghi research at the University of California, Berkeley, combined to form MOF-5 (also known as IRMOF-1) with a three-dimensional open skeleton structure, which can be seen as self-assembling isolated secondary structural units (Zn4O) and organic ligands (terephthalic acid) [2].

It is worth mentioning that after removing guest molecules in the pore, MOF-5 can still maintain the integrity of
the skeleton.
Since the coordination of MOFs follows the theory of soft and hard acid-base, in 2006, the representative MAF-4 (also known as ZIF-8) material obtained by the team of Academician Chen Xiaoming of Sun Yat-sen University using 2-methylimidazole ligand (soft Lewis base) and metal Zn2+ ion (soft Lewis acid) assembly has the characteristics of high pore volume, high hydrophobicity, high thermal stability and chemical stability [3].

Subsequently, Yaghi's research group synthesized a variety of ZIF series zeolite materials using a variety of imidazole ligands with zinc ions or cobalt ions [4].

To date, most of the metal ions in the periodic table such as alkali metals, alkaline earth metals, transition metals, group IIIA metals, and rare earth metals can be used as nodal metal ions
for MOFs materials.
Organic molecules with rigid structures such as aromatic polycarboxylic acids and nitrogen-containing heterocyclics (pyridine, imidazole, pyrimidine, pyrazine, triazole, tetrazole) are often used as organic connecting units
for MOFs.

MOFs materials have undergone decades of development, and the currently synthesized MOFs materials have the characteristics of kinetic controllability, that is, changes in external conditions such as light, electricity and guest molecules will not cause irreversible changes to the pores of MOFs materials, which is representative Materials include IRMOF (isoreticular metal-organic framework), ZIF (zeolitic imidazolate framework), MIL (Materiel Institut Lavoisier), UiO (University of Oslo), HKUST (Hong Kong University of Science and Technology）、CAU（Christian Albrechts University）、PCN（Porous Coordination Network）、UTSA（University of Texas at San Antonio）、NOTT（University of Nottingham）、BUT（Beijing University of Technology), BUC (Beijing University of Civil Engineering and Architecture) and other series
.
At present, the MOFs methods that can be used for single crystal X-ray diffraction analysis mainly include solvent volatilization, diffusion, hydrothermal and solvothermal methods
.
In addition, the methods commonly used to prepare MOFs powder and membrane materials include ultrasonic method, microwave heating method, electrochemical synthesis method and mechanochemical synthesis method.

As of September 10, 2021, 40,719 academic papers can be retrieved in the Web of Science database with the theme of "metal-organic frameworks", and this number will definitely increase year by year, it can be said that MOFs are definitely a research hotspot in the fields of materials, chemistry and environment!

Figure 1 Large family of MOFs materials[5]

2.
Wonderful properties of metal-organic framework materials

As an organic-inorganic hybrid material, MOFs have both the characteristics of organic materials and inorganic materials, and have the advantages of rich structure, ultra-large specific surface area, morphology and size designability and chemical function diversity, so they have more outstanding advantages than common porous materials (inorganic porous zeolite, molecular sieve, activated carbon), mainly in the following aspects:

(1) Clear structure: MOFs are a new type of crystalline porous materials formed by metal clusters/ions and ligands through coordination self-assembly, so MOFs can determine their clear crystal structure by single crystal X-ray diffraction compared with some traditional materials, which provides direct evidence
for exploring the relevant chemical reaction mechanism/structure-activity relationship.

(2) Rich variety: Since the types of MOFs materials can be regulated by selecting metal ions and organic ligands under appropriate reaction conditions, countless organic ligands and a large number of metal ions can be combined to synthesize rich MOFs materials, and the addition of binary or even multivariate organic ligands or metal ions in the reaction system provides infinite possibilities
for the synthesis of MOFs 。 More than 80,000 MOFs can be retrieved at the Cambridge Crystallographic Data Center (CCDC) in the UK, and the theoretical number of
MOFs is unlimited.

(3) Designability and modifiability: As shown in Figure 2, different functional building units can be selected or designed to directly synthesize MOFs with different functions
.
Different classes of functional groups can also be introduced through post synthetic modification (PSM) to carry out targeted performance control and prepare MOFs materials
for the target use.

(4) Large specific surface area and porous structure: MOFs have a large specific surface area, pore volume and porosity, for example, NU-110 BET has a specific surface area of 7140 m2/g and a pore volume of 4.
40 cm3/g [6].

Moreover, the pore size is affected by the size and shape of the organic ligand and the coordination mode with metal ions, so the pore characteristics
can be changed by design.
At the same time, because the porous structure makes MOFs materials expose more active sites, they have shown considerable application prospects
in the fields of heterogeneous catalysis, gas storage and separation, molecular sensing, and optoelectronic materials.

(5) Biocompatibility: The metal ion center in MOFs materials can choose biocompatible Fe, Al, Ca, Mg and other elements, organic ligands in addition to common carboxylic acids and nitrogen-containing heterocyclic organic ligands, can choose peptide, adenine and other bioorganic molecules, the two in the appropriate reaction conditions combined to orientally construct biological MOFs materials (bio-MOF), which has been proved to be one of the ideal materials in the field of
biomedicine.

Figure 2 Schematic diagram of functionalized MOFs process prepared using direct self-assembly and post-modification methods

3.
"Versatile" metal-organic framework material

Compared with traditional organic polymers and inorganic porous materials, MOFs have a larger specific surface area and porosity, and have a variety of pore dimensions and topologies, making them attractive application prospects in gas storage and separation, water capture, pollutant adsorption, fluorescence sensing, catalysis, supercapacitors, medical fields, sterilization and algae removal, sample preparation and other fields, as shown
in Figure 3.

Figure 3 Application areas of MOFs materials

(1) Gas storage and separation: Under the background of carbon neutrality, China's energy structure will gradually transition from fossil energy to clean energy
.
Hydrogen energy is a globally recognized clean energy, with the advantages of high calorific value and high conversion rate, and its development plays an indispensable role
in energy conservation and emission reduction, deep decarbonization, and improving utilization efficiency in the energy field.
However, the storage of hydrogen is one of
the main bottlenecks in hydrogen energy applications.
Due to their large specific surface area and adjustable pore structure, MOFs have received extensive attention
in the field of hydrogen storage.
Among them, materials such as MOF-5 and UiO-66 can achieve hydrogen
storage in low temperature environments.
The ultra-large specific surface area MOF-177 material (BET specific surface area of 4500 m2/g) reported by Yaghi research group stores up to 7.
5 wt% of hydrogen at 70 bar and 77 K [7].

To this end, the German chemical giant BASF has applied MOFs hydrogen storage materials to the fuel system of new energy vehicles (as shown in Figure 4) [8].

In addition to hydrogen, methane is also an ideal alternative fuel
.
Chen Banglin and Qian Guodong prepared three-dimensional porous MOFs containing Cu2+ and pyridyl aromatic tetracarboxylic acid ligands (named ZJU-5a), and their specific surface area and pore volume reached 2823 m2/g and 1.
074 cm3/g
, respectively.
Due to the open Cu2+ site, suitable pore structure and Lewis pyridine basic site in ZJU-5a, the adsorption capacity of methane can reach 224 cm3 (STP)/cm3 at 60 bar and 300 K [9].

By 2015, Eddaoudi's group had successfully prepared Al-soc-MOF-1 and were pleasantly surprised to find that its ability to store methane increased
as the temperature decreased.
In particular, under the conditions of 8 MPa and 258 K, the storage capacity of Al-soc-MOF-1 for methane reached 264 cm3 (STP)/cm3 and 0.
5 g/g, which for the first time met the dual requirements of the US Department of Energy for methane adsorption volume and mass [10].

In terms of gas separation, because some MOFs materials may have coordination unsaturated metal active centers during synthesis, they can chemically react with CO2, hydrocarbon compounds and volatile organic compounds to achieve gas separation
.
Generally speaking, the saturated adsorption capacity of CO2 by MOFs is positively correlated
with its specific surface area.
For example, MOF-210 has a specific surface area and pore volume of 6240 m2/g and 3.
60 cm3/g, respectively, and the adsorption capacity of CO2 at 50 bar and 298 K reaches 70.
6%[11].

At the same time, due to the different adsorption forces between CO2 and N2 and MOFs, the selective adsorption capacity
of CO2 can be improved by regulating the number of metal active centers, Lewis base sites and polar functional groups of MOFs.
In addition, the MOFs structure can rely on the different van der Waals forces of different alkane molecules, and the strong π electron interaction of olefin molecules for the separation
of alkanes or alkane/olefin mixtures.
However, Long's group also found that the coordination unsaturated Fe2+ center in Fe-MOF-74 has a strong interaction with ethylene molecules, and its separation efficiency for ethane and ethylene reaches 99.
0% and 99.
5%, respectively, at 1 bar and 45 oC [12].

。 In order to efficiently filter particles in the air, Wang Bo's research group at Beijing Institute of Technology used double-sided hot rolling processing to attach ZIF-8, ZIF-67 and Ni-ZIF-8 to the surface of plastic mesh, glass fiber fabric, metal mesh, non-woven fabric, melamine sponge and other substrates to prepare MOFs membrane materials [13].

The above membrane materials have good stability and excellent filtration effect
on airborne particles.
For example, the removal rates of PM2.
5 and PM10 by ZIF-8 membrane grown on the surface of melamine sponge foam reached 99.
5%±1.
7% and 99.
3±1.
2%,
respectively.
It is believed that high-performance MOFs powder and membrane materials will play a more important role in the field of air purification in the future!

Figure 4 BASF new energy heavy-duty truck equipped with MOFs energy storage materials[8]

(2) Water capture: Only 0.
0076% of freshwater resources can be directly used by humans, and most people in the world face the threat of
water scarcity.
It is estimated that the water content of water vapor and water droplets in the atmosphere has reached 13,000 trillion liters, which is about 10% of the total freshwater of all lakes, so the capture of water from the atmosphere is of great practical significance
to alleviate the human water crisis.
But capturing water molecules from the atmosphere under low humidity conditions (< 20%) is a technical challenge
.
To this end, in 2017, the Yaghi research group designed a conceptual water molecule trap in the air using MOF-801 as the core material (Figure 5) [14].

This technology uses low energy density (1 kW/m2) of sunlight as the energy source, and the adsorption capacity of MOF-801 to water vapor reaches 0.
25 kg/kg when the relative humidity is as low as 20%, which has strong research innovation and technology promotion.

In order to further improve the ability of MOFs materials to capture water molecules, in 2019, the research group of Wang Ruzhu of Shanghai Jiao Tong University encapsulated the superhygroscopic salt LiCl in MIL-101 (Cr) [15].

Porous MIL-101 (Cr) provides sufficient pore volume to store LiCl and water molecules and induce the growth
of nanoscale LiCl crystals through the confinement effect.
The results show that the water vapor adsorption capacity of LiCl@MIL-101 (Cr) at 30oC and 30% relative humidity is as high as 0.
77 kg/kg, which is much higher than the existing research results, which further promotes the development and application
of MOFs water capture technology.

Figure 5 Conceptual MOFs water trap[14]

(3) Pollutant adsorption: MOFs have an open pore structure and a large specific surface area, which can promote the diffusion of pollutants in MOFs in the water environment, and achieve efficient, rapid and highly selective adsorption
of target pollutants by regulating the coordination unsaturated metal sites, Lewis acid-base sites, organic functional groups, pore size, surface electrical properties, etc.
of MOFs 。 At present, the reported literature has proved that MOFs have excellent adsorption properties
for organic dyes, drugs and personal care products, pesticides and insecticides, heavy metal ions, radioactive ions, perfluorinated compounds, and oil substances.
The adsorption mechanism of MOFs skeleton and target target mainly includes electrostatic interaction, hydrogen bonding, acid-base interaction, coordination effect, hydrophobic interaction, π-π accumulation effect, ion exchange effect and pore size selectivity
.
In 2017, ZIF-67 prepared by electrochemical deposition by Wang Chongchen's research group of Beijing University of Civil Engineering and Architecture realized the adsorption and removal of 21 common organic dyes (16 anionic types, 4 cationic types and 1 neutral dye) in water [16].

It was found that the prepared ZIF-67 had preferential adsorption behavior for some organic dye molecules, so it was also used as a packing material for the solid phase extraction device to efficiently separate mixed dyes
.
In addition, the research group also fixed MOFs such as MIL-88A and BUC-17 to cotton fibers [17, 18] for the adsorption and removal of As3+, As5+, p-aminophenylarsenic acid (p-ASA) and
Roxarsine in the water environment.
Since the main adsorption mechanism is that the As element has a strong coordination effect with the unsaturated metal center in the MOFs material, the reaction system is less
disturbed by the coexisting substances.
At the same time, the bond between cotton fiber and MOFs material is very tight, so it effectively inhibits the loss of MOFs material in the adsorption process and prolongs the service life
of the material.
Particularly interesting is that in order to achieve green desorption of MOFs adsorbents and avoid the use of organic solvent cleaning or heating desorption, Wang Chongchen's research group used in situ ion exchange deposition method to prepare UiO-66-NH2/Ag3PO4 (UAP-X) photocontrolled adsorbents [19].

As shown in Figure 6, the presence of Ag+ in UAP-X in the dark undergoes a weak coordination reaction with the terminal site – NH2 in sulfamethoxazole with a maximum adsorption capacity of 200 mg/g
.
When the material is irradiated with visible light (> 420 nm), the Ag+ in AgPO4 will be reduced to Ag element by photogenerated electrons to achieve photocontrolled desorption
of sulfamethoxazole.
Experimental data show that UAP-120 can achieve 73% desorption efficiency
after 40 minutes of illumination.

MOFs are also used as adsorbents for the removal
of common cations (Hg2+, Pb2+, Cd2+, CO2+, Cu2+, etc.
) and anions (Cr2O72–, F–, PO43–, ClO4–) in aqueous environments.
Notably, radionuclide 99Tc is a long-lived fission product with a half-life of 2.
13 × 105 years, which poses a long-term radiological hazard
.
Under normal circumstances, 99Tc exists in the form of 99TcO4– anion with strong water solubility and high stability, which has a strong migration ability, and traditional nuclear waste treatment technology cannot effectively fix
99TcO4– .
Based on the above problems, the Wang Yinwo researcher group of Soochow University prepared the cationic MOFs material SCU-102 by self-assembling transition metal Ni2+ and tetradentate nitrogen neutral ligand[20].

Kinetic experiments show that SCU-102 can adsorb and remove 100% of 99TcO4– in water within 10 minutes, and the adsorption rate is significantly higher than that of traditional anionic resin materials
.
Moreover, when treating the actual contaminated groundwater, the adsorption selectivity of SCU-102 to 99TcO4– is not interfered by coexisting anions (SO42–, CO32–, Cl–, NO3–), and the partition coefficient is as high as 5.
6× 105 mL/g
.

Figure 6 Mechanism diagram of photocontrolled suction and desorption of UiO-66-NH2/Ag3PO4 complex[19]

(4) Fluorescence sensing: Qualitative and quantitative analysis of environmental pollutants and targets in biological fluids based on the detection of fluorescence changes is currently the most promising detection and analysis method
.
This method has the advantages
of easy operation, simple pre-treatment process and high efficiency.
Fluorescent MOFs (luminescent MOFs, LMOFs) not only have the porosity and modifiability of traditional MOFs materials, but also have specific fluorescence properties of organic ligands and metal center ions that make up their skeleton structure, which is a potential fluorescent sensing material
.
Specifically, the open pore structure of LMOFs, abundant Lewis acid-base sites, and unsaturated coordination metal sites can specifically bind the analyte of interest, thereby changing the light absorption and emission properties of LMOFs themselves, and ultimately realizing the detection
of LMOFs for specific substances 。 At present, LMOFs as fluorescent sensing materials mainly include four types, namely fluorescence enhanced (turn-on), fluorescence quenching (turn-off), first quenching and then enhanced (off-on) and ratiometric type, and the target detection targets involve ions, drugs and personal care products, persistent organic pollutants, explosives, biomarkers, etc
.
For example, the Yan Bing Research Group of Tongji University synthesized UiO-66-NH2-Eu fluorescent sensing materials using 2-aminoterephthalic acid, Zr4+ and Eu3+ as raw materials [21].

The material maintains fluorescence stability over a wide pH range (4.
0-10.
0), as shown in Figure 7, when water contains Cd2+ ions, it can significantly enhance the energy transfer efficiency of organic ligands to Eu3+ (also known as the "antenna effect"), thereby increasing the fluorescence emission intensity of the material and enabling sensing detection
of Cd2+.
At the same time, the reaction system undergoes a significant color change when irradiated with ultraviolet light, enabling quick and easy detection of Cd2+
in real water samples.
Compared with "turn-on" LMOFs, as early as 2009, the Li research group of Rutgers University of New Jersey used solvothermal method to synthesize Zn2(bpdc)2(bpee) with microporous structure (bpdc = 4,4-biphenyldimethyl ester, bpee = 1,2-bipyridineethylene).

The fluorescence of these LMOFs is quenched by 2,4-dinitrotoluene and 2,3-dimethyl-2,3-dinitrobutane, making it the first case of LMOFs to be used to detect explosive molecules [22].

Figure 7 UiO-66-NH2-Eu for fluorescence sensing to detect Cd2+ in water[21]

In addition to detecting sensing water or gaseous contaminants, LMOFs are also commonly used to analyze and detect biomarkers
.
For example, Yan Bing's research group applied the prepared Eu@Sc-MOFs materials to the detection of phenylacetaldehyde (PGA) in human serum and urine to assess the occupational risks of workers engaged in the production of glass fiber reinforced polyester [23].

The experimental results show that PGA molecules can effectively enhance the fluorescence intensity of Eu@Sc-MOFs materials at 615 nm, have good selectivity, and the minimum detection limit of PGA is 4.
16 ppb
.
More interestingly, the researchers developed a fluorescent test strip for quantitative analysis of PGA molecules, which, when combined with a smartphone app, found that as the PGA content increased, the color of the test strip changed from blue to red (as shown in Figure 8).

Overall, the sensing device is designed to be portable and easy to operate, making it ideal for on-site testing
of PGA molecules.
In addition, Chen Wenhua's research group at Southern Medical University applied a water-stable Cu-MOF material to the detection of HIV-1 and Ebola virus DNA and RNA, with detection limits as low as 196 pM and 73 pM [24].

The main principle is that the electrostatic interaction, π-π accumulation and hydrogen bonding between labeled carboxyfluorescein and Cu-MOF make the fluorescence quenching phenomenon
of the label.
Therefore, this analytical method provides a new avenue
for the early diagnosis of the virus.

Figure 8 Eu@Sc-MOFs detect PGA in the human body as "turn-on" fluorescent sensors[23]

(5) Catalysis: Compared with traditional inorganic catalytic materials, MOFs have been widely used in organic heterogeneous catalysis (oxidation reaction, reduction reaction), photocatalysis (hydrogen production, oxygen production, photocatalytic reduction of CO2 and toxic and high-valent metals, degradation of organic pollutants and organic synthesis), electrocatalysis (oxygen reduction, water decomposition and CO2 reduction reaction) and advanced oxidation (activation of hydrogen peroxide and persulfate) because of their advantages of crystal porosity, flexible modifiability and ultra-large specific surface area
。 For MOFs catalyzed oxidation and reduction reactions, they are mainly catalyzed by unsaturated metal sites, functional catalysis of organic ligands and encapsulated catalytically active guest molecules
.
For example, the research group of Dong Yubin of Shandong Normal University prepared a MOFs material containing coordination unsaturated CO2+ active sites, and the experimental results showed that CO2+ played a crucial role in the oxidation of cyclohexane, olefins and alcohols [25].

。 Fu Zhiyong's research team at South China University of Technology synthesized an aminofunctionalized MOF material using Zn2+, 2,4,6-tris(4-pyridinyl)-1,3,5-triazine and 2-aminoterephthalate as raw materials, and applied it to catalyze the reaction of benzaldehyde and Knoevenagel containing active methylene compounds [26]
.
Since the above two organic ligands are rich in Lewis base active sites (9 uncoordinated N atoms), the MOF material exhibits excellent catalytic activity
.
In addition, the adjustable pore structure of MOFs materials provides a way for the encapsulation of some functional guest molecules, especially metal nanoparticles, but the common method is to immerse MOFs materials in metal precursors and reduce them under the condition of external reducing agents (such as sodium borohydride), and the reaction conditions are relatively harsh
.
In 2020, the research group of Zhou Hongcai of Texas A&M University in the United States used tetrathiofauene tetraphenylcarboxylic acid and methylated tetrathiofafulene tetraphenylcarboxylic acid as organic ligands to synthesize stable Zr-MOFs based on Zr6 node
clusters.
More importantly, the material can reduce Pd2+ to Pd nanoparticles at room temperature without the need for additional reducing agents [27].

The experimental results confirmed that the prepared Pd@Zr-MOF had excellent selective catalytic activity
against benzyl alcohol with different substituents.

In recent years, MOFs have developed rapidly in the field of photocatalysis, and relevant studies have proved that MOFs can be applied to decomposition of water to hydrogen under light, photocatalytic reduction of CO2 and Cr(VI), and photocatalytic degradation of organic matter
.
Compared with traditional semiconductor materials, MOFs have the following advantages as photocatalysts [28]:

(1) Excellent adsorption performance
.
For photocatalytic reactions, the pre-adsorption effect of pollutants will significantly affect the efficiency
of subsequent photocatalytic reactions.
Since MOFs have diverse structures, huge specific surface area, pore volume and porosity, and their pore characteristics can be changed by design, the adsorption properties of MOFs materials can be adjusted according to demand.

(2) The structure of MOFs is designable
.
The light absorption properties of MOFs can be regulated by introducing some functional groups such as amino groups and porphyrin groups to achieve excellent photocatalytic activity in the visible or near-infrared light region.

(3) The porous structure of MOFs gives them more exposed active sites and catalytic target/product transport channels, which is conducive to the transfer and utilization of photogenerated charge, thereby relatively reducing the recombination of photogenerated hole-electron pairs, and comprehensively improving its utilization efficiency;

(4) The well-defined structural characteristics of MOFs make them an ideal model for studying the structure-activity relationship of photocatalysis;

(5) The structure of MOFs is diverse, and they are easy to compound with semiconductor materials or other materials with good conductivity to form heterojunctions, increase the photogenerated charge density, and promote the separation
of photogenerated carriers.
For example, in 2020, Deng Hexiang of Wuhan University and the Terasaki research group of ShanghaiTech University published an article in the international top journal Nature, using the multi-level pore structure characteristics of MOFs to prepare a new type of "molecular compartments" material in MOF [29].

That is, classical TiO2 photocatalytic materials were grown in MIL-101 (Cr) two size (29 and 34 Å) pore structures, respectively, so as to construct MOF/TiO2 composites with "molecular compartments" (as shown in Figure 9).

The results show that the existence of "molecular compartments" significantly improves the absorption capacity of light and the rate of carrier separation, and strengthens the synergistic effect
between TiO2 and metal clusters with catalytic properties.
Under 350 nm illumination, the quantum efficiency of CO2 reduction is as high as 11.
3%, which greatly improves the reaction efficiency
of existing MOFs-based materials to photocatalytic reduction of CO2.

Figure 9 MIL-101 (Cr) directional control of TiO2 growth location and quantity[29]

In the field of environmental remediation, in 2017, with the help of crystal engineering principles, Wang Chongchen's research group used vitamin H intermediate 1,3-dibenzylimidazol-2-keto-cis-4,5-dicarboxylic acid as the first ligand and 4,4'-bipyridine as the second ligand, and hydrothermal synthesis of a Zn-based MOF material that can maintain structural stability in a wide pH range (2-12), named BUC-21 [30].

。 The MOF has a band gap value of 3.
4 eV, which can efficiently photocatalytically reduce Cr(VI) and organic dyes under ultraviolet light irradiation, and the catalytic activity is much greater than that of commercial P-25 titanium dioxide powder
.
At the same time, in order to overcome the problems of easy loss and poor recycling of MOFs powder materials in the photocatalytic process, Wang Chongchen's research group used the secondary crystal seeding method to grow UiO-66-NH2(Zr/Hf) in situ on the Al2O3 substrate to prepare MOFs membrane materials [31].

The experimental results show that the UiO-66-NH2(Zr) film can photocatalytically reduce 98% of Cr(VI) ions within 120 minutes under simulated sunlight irradiation, and the efficiency can still be maintained at 94% after 20 rounds of photocatalytic reaction, which has good application prospects
.

In addition, because MOFs contain transition metal elements (Fe, Co, Ni, Cu, etc.
) in their structure, they are also often used to activate hydrogen peroxide or persulfate to produce strong oxidizing free radicals (· OH and SO4-· etc.
), and then remove refractory organic pollutants
in water.
In 2021, Wang Chongchen's research group reported a composite composed of MIL-88A(Fe) and organic semiconductor material 3,4,9,10-peryltetracarbonyldiimide (PDINH), and applied it to chlorophosphate, a therapeutic drug for the enhanced removal of novel coronavirus (COVID-19) in water by photocatalytic-activated persulfate system [32].

。 The experimental results show that the band position between MIL-88A(Fe) and PDINH has good matching, and photogenerated electrons will migrate rapidly from PDINH to MIL-88A(Fe)
under the irradiation of low-power LED light source.
At the same time, the persulfate added to the reaction system acts as an electron trapping agent to generate SO4-· Isoactive radicals also inhibit the recombination
of photogenerated hole-electron pairs.
Combined with MIL-88A(Fe) good persulfate activation performance, the reaction system has efficient organic pollutant removal performance
.
As shown in Figure 10, the authors of this paper visualize the polluted natural environment, PDINH/MIL-88A(Fe), and persulfate as injured hearts, scalpels, and injections
, respectively.
It means that PDINH/MIL-88A(Fe) and persulfate synergistically and accurately act on the removal of chloroquine phosphate pollutants in water under the irradiation of sunlight, and finally achieve the purification effect of water!

Since MOFs have the characteristics of both homogeneous and heterogeneous catalysis, they also have a wide range of applications
in electrocatalysis.
For example, in order to solve the shortcomings of commercial precious metal catalysts (platinum, iridium, ruthenium) in cathode hydrogen evolution reaction applications, such as high cost and easy poisoning and inactivation, Li Dongsheng's research group of China Three Gorges University constructed AB@CTGU-5 composite materials with conductive cocatalyst acetylene black [33].

Experimental results show that the material has excellent electrocatalytic hydrogen evolution activity, that is, relatively positive starting point (18mV), low tafel slope (45mV/dec), high exchange current density (8.
6×10-4A/cm2) and high stability (96h).

In 2020, the Jiang Hailong Research Group of the University of Science and Technology of China used porphyryryl PCN-222 as a precursor to prepare Fe1–N–C, Co1–N–C and Ni1–N–C single-atom catalysts by modulating the types of metals embedded in the porphyrin ring [34].

Among them, Fe1–N–C electrocatalytic ammonia production efficiency was the best, at -0.
05 Vvs.
The ammonia production rate under RHE conditions was 1.
56×10-11 mol/(cm2·s), and the corresponding Faraday efficiency reached 4.
51%.

More importantly, this work demonstrates the advantages of MOFs as single-atom catalysts for porous materials, and provides new ideas
for the subsequent design and preparation of high-efficiency electrocatalytic materials.

Figure 10 PDINH/MIL-88A(Fe) photocatalytic activation of persulfate synergistic enhancement of degradation of chloroquine phosphate[32]

(6) Supercapacitor: MOFs have a large specific surface area, tunable aperture structure and capacitance redox center, making them suitable for
being supercapacitor electrode materials.
For example, the Sung-Hwan Han Research Group at Hanyang University in South Korea synthesized three Co-MOFs with average pore sizes of 2.
58, 13.
95, and 78.
96 nm using binary carboxylic acid ligands of different molecular lengths and Co2+ ions [35].

When the scanning speed is set to 10 mV/s, the maximum specific capacitance obtained by cyclic voltammetry testing is 131.
8, 147.
3 and 179.
2 F/g, respectively, and the above materials can still maintain their original crystalline phase structure
after 1000 cycles.
Beijing Institute of Technology's Wang Bo research group used electrochemical deposition to grow ZIF-67 and conductive polymer polyaniline (PANI) on carbon cloth [36].

Among them, PANI interweaves discrete ZIF-67 crystals in series, and at the same time serves as an electron transport channel between the inner surface and the outer circuit of ZIF-67, effectively enhancing the Faraday process
at the interface.
This study provides practical new ideas
for the subsequent development of all-solid-state flexible supercapacitors.
In 2020, the team of Academician Huang Wei of Nanjing University of Posts and Telecommunications used Langmuir-Schäfer membrane technology to prepare π-d conjugated Ni3 (HITP)2MOFs conductive films with high-density redox active sites, and used them as electrode materials to realize the construction of high-performance flexible transparent electrodes [37].

With an optical transmittance of 78.
4%, the chip resistance of the Ni3(HITP)2 electrode is 51.
3 Ω/sq
.
At a current density of 5 μA/cm2, the area capacitance of the Ni3(HITP)2 electrode is 1.
63 mF/cm2
.
More importantly, when the current density is increased to 50 times, the capacitance retention rate is 77.
4%.

This work provides new opportunities
for the subsequent development of flexible and high energy storage devices based on MOFs.
In addition, the energy storage efficiency
of supercapacitors can be improved by optimizing the reaction interface between the electrolyte and the electrode.
As shown in Figure 11, Yaghi's research group prepared MOFs into thin film nanocrystals and attached them to both sides of the diaphragm, and after soaking in an electrolyte solution, they formed coin-shaped supercapacitors [38].

Due to the symmetrical structure of the capacitor and the open pore structure of the electrode material MOFs, the migration rate of electrons between the electrolyte and the electrode is greatly increased, and the electrochemical performance
is better.
It is worth noting that MOFs can also be used as precursors for the preparation of porous carbon and metal oxides, and they are also high-performance supercapacitor materials
with potential applications.

Figure 11 Schematic diagram of the structure of a coin-type supercapacitor[38]

(7) Medical field: Under the premise of selecting metal ions and organic ligands with high biological safety, MOFs can be used in medical fields
such as tumor treatment (phototherapy, radiotherapy, microwave therapy and acoustic dynamic therapy) and drug delivery.
For example, in photodynamic therapy, tumor death is often achieved by stimulating photosensitizers to produce reactive oxygen species
.
However, the commonly used photosensitizers have poor water solubility, low payload, and poor
targeting of tumors.
More importantly, glutathione, which is overexpressed by tumor cells, can act as an antioxidant to resist the attack
of reactive oxygen species on tumor cells.
Therefore, the construction of functional materials that can achieve intelligent elimination of glutathione can significantly improve the effect of
photodynamic therapy.
In 2019, Zhang Xianzheng's research group at Wuhan University prepared a porphyryryryl MOF material sealed with Mn3+ by the "one-pot method" [39].

Among them, Mn3+ plays a dual role in quenching porphyrin ligand fluorescence and inhibiting the production of reactive oxygen species, making it an inert diagnostic and therapeutic material
.
As shown in Figure 12, when the MOF material is engulfed by tumor cells, the redox reaction between Mn3+ and intracellular glutathione causes the MOF material to be decomposed into Mn2+ and free porphyrin molecules, which not only achieves the consumption of the antioxidant glutathione, but also realizes Mn2+-based magnetic resonance imaging and porphyrin-based fluorescence imaging
.
At the same time, the controlled release of glutathione to porphyrin groups limits the concentration of active substances under light conditions to a certain extent, effectively avoids inflammation caused by spontaneous groups, and ultimately significantly improves the therapeutic efficiency
of photodynamic therapy for tumor cells.
In addition, due to their porosity and the presence of a large number of active sites on the surface, MOFs can be loaded and coupled to acoustic sensitive molecules to construct ultrasound-sensitive systems with good biocompatibility, which are further applied to the sonodynamic therapy of tumor cells
.
For example, Cai Lintao's research group at the Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences prepared a series of novel MOFs materials with Mn2+, Zn2+, and Ti4+ as metal centers and sound-sensitive tetramethylphenylporphyrin as ligands[40].

At the same time, using human blood proteins as the carrier, a sound-sensitive system
with the above MOFs materials and cores was constructed.
The experimental results show that the system still has acoustic energy excitation response characteristics
in muscle simulation tissues up to 10 cm thick.
With the assistance of low-power ultrasound, MOFs enriched inside the tumor can produce singlet oxygen in a directional manner, effectively inhibiting tumor growth and reducing damage to organs
.

Fig.
12 Schematic diagram of porphyryline MOFs applied to photodynamic therapy of tumors[39]

MOFs also serve as drug delivery systems
.
Traditional drug delivery systems are divided into organic drug delivery systems and inorganic drug delivery systems
.
The former generally has good biocompatibility, but the payload of the drug is low, and the control effect on the drug is poor; Although the latter has a higher drug load and can deliver drugs at the prescribed rate, it is less biocompatible and most metal nanoparticles cannot be biodegraded and have certain toxicity
.
MOFs, as a class of organic-inorganic hybrid materials, combine the advantages
of the above two drug delivery systems.
At the same time, the surface modification of MOFs materials can be carried out by using polyethylene glycol, silica, cyclodextrin, heparin and chitosan to improve the stability, targeting and biocompatibility of the drug loading system, further reduce the immune response, give the MOFs material "stealth" performance, improve the permeability to the cell membrane, and reduce side effects
.
For example, the Forgan research group at the University of Glasgow modified polyethylene glycol conjugation to the surface of UiO-66 [41].

The experimental results show that the presence of polyethylene glycol can significantly improve the stability of UiO-66 in phosphate when the pH value is 7.
4, and can also inhibit the "sudden release" behavior
of the drug.
At a pH value of 5.
5, the reaction system can achieve a slow-release effect
on the drug.
Chitosan is a typical biomaterial for improving oral drug absorption, and in 2017, the Hidalgo research group at the University of Versailles used chitosan for surface modification of MIL-100 (Fe) to achieve loading of ibuprofen drugs, and it is also the first MOFs-based oral drug carrier material reported [42].

Since the interaction mainly occurs between the hydroxyl group of chitosan and Fe3+ in MIL-100(Fe), the porosity and crystal structure
of MIL-100(Fe) are well preserved.
The coating effect of chitosan can effectively protect Fe3+ from enzymatic degradation
.
At the same time, the bioadhesion of chitosan can control the release of ibuprofen near the intestinal mucosa, effectively improving the permeability
of the drug to the cell membrane.
It is important to note that in topical drug delivery, it is important to develop a suitable and reliable platform for visualizing drug delivery due to the complexity of the intracellular environment
.
In 2021, the Shao Huawu research group of the Natural Products Research Center of the Chengdu Institute of Biology, Chinese Academy of Sciences synthesized MOF nanotubes with hollow structure for the first time using Zr4+ and tetrakis(4-carboxyphenyl) methane as raw materials (as shown in Figure 13) [43].

The material has strong fluorescence, biocompatibility, excellent loading capacity and pH-responsive release properties ("self-directed" effect).

Based on the above advantages, the MOF material realizes the visual delivery
of the anti-tumor drug doxorubicin.
In general, due to the strong modifiability of MOFs materials, it is believed that functionalized MOFs materials will play a more important role in targeted drug delivery, sustained release delivery and precision therapy in the future!

Fig.
13 Schematic diagram of MOF with vacuum structure and fluorescence properties and its "self-directed" delivery of antitumor drugs[43]

(8) Sterilization and algae removal: Antibiotics and other bactericidal drugs face the problem of bacterial drug resistance, and in order to achieve the effect of slow release, the active ingredients of the drug are often wrapped inside the capsule, so that such drugs cannot be applied to the sterilization
of external wounds 。 MOFs materials contain metal ions (such as Ag+, Zn2+, Co2+, Cu2+, etc.
) that play a decisive role in sterilization, and more importantly, the organic ligands that make up their structure have good biocompatibility, which is conducive to MOFs destroying the cell wall structure, thereby disrupting the ion balance inside bacterial cells, resulting in enzyme inactivation and cytoplasmic leakage, and finally achieving efficient and lasting sterilization
。 For example, the Aguado research group at the University of Alcalá used two Co-based MOFs materials (ZIF-67 and Co-SIM-1) for the inactivation of gram-negative bacteria (Pseudomonas malodori and Escherichia coli) [44].

The results showed that both MOFs could effectively inhibit the growth of bacteria, and when the dosage of the material increased to 5-10 mg/L, the logarithmic growth phase of the above two types of bacteria was effectively inhibited
.
In 2018, Wang Chongchen's research group synthesized two Ag-based MOFs materials BUC-51 and BUC-52 using 1,1-cyclopropanedicarboxylic acid and 1,1-cyclobutanedicarboxylic acid as the first ligand and 4,4'-bipyridine as the second ligand[45].

Because BUC-51 and BUC-52 can slowly release Ag+ to destroy the cell wall of Escherichia coli, resulting in cytoplasmic outflow and death, they have shown long-term killing of
Escherichia coli.
Subsequently, Wang Chongchen's research group synthesized another Ag-based MOF material BUC-16 using imidazole-4,5-dicarboxylic acid as an organic ligand [46].

It not only shows excellent bacteriostatic activity, but also effectively removes algae in water, such as microcystic algae, brittle algae, bicornella, flat fissuring, sponge, houttuynia, dimorphic gate algae, astrodisca, chlorella and Qiaowan spirulina
.
In addition to calculating the algae removal rate in the form of hemocytometer counts, the algae removal effect of BUC-16 was judged by using the foul-smelling substance β-cyclocitral released by algae as the research object
.
The experimental results show that the addition of BUC-16 can effectively reduce the release
of algae to β-cyclocitral.
On the 7th day of the experiment, all the selected microcystic algae were inactivated and no odorous gas was emitted
.
At the same time, the researchers also tested the cytotoxicity of BUC-16 in vitro with mouse embryonic fibroblasts (NIH/3T3) and found that it had good biocompatibility
.
In general, the composition and structure of MOFs will directly determine their antimicrobial rate, sustainability and bioavailability
.

In addition to relying on MOFs materials to release metal ions to achieve the goal of sterilization, MOFs materials can also generate active radicals through photocatalysis and advanced oxidation, which can also kill bacteria, fungi and even viruses
.
In 2019, the Wang Bo research group of Beijing Institute of Technology prepared ZIF-8 with ultra-high photocatalytic sterilization activity, which achieved efficient killing of E.
coli in water with a sterilization rate greater than 99.
9999% [47].

A variety of characterization and experimental methods were used to prove the superoxide radicals (· O2–) and hydrogen peroxide (H2O2) are the main active substances
.
At the same time, in order to achieve personal protection, the researchers also used the hot pressure method to prepare a mask
called "MOFilter".
As shown in Figure 14, by spraying microbial aerosols to simulate real use scenarios, it can be observed that no bacteria survive
on the three layers of MOFilter masks after 30 minutes of light.
Commercial N95 masks, on the other hand, have a large amount of live bacteria
remaining under the same conditions.
Under the background of the current normalization of the epidemic, the research results are of great application value
.
In addition, there are currently research teams that add MOFs as excipients to medical devices and surgical materials, solving a large number of practical needs!

Figure 14 Comparison of sterilization effect of MOFilter masks and commercial N95 masks[47]

(9) Sample preparation: Thanks to the super adsorption performance and modifiable characteristics of MOFs, they can have strong selectivity and extraction capacity for specific one or more targets, and then are widely used in sample preparation technology, including solid phase extraction (SPE), micro-solid phase extraction (μ-SPE), Magnetic solid phase extraction (MSPE) and solid phase microextraction, SPME, etc
。 In 2006, Yan Xiuping's research group of Nankai University used copper isonicotinate MOF (Cu(4-C5H4N-COO)2(H2O)4) as a solid phase extraction packing material for the first time, and realized the determination of trace polycyclic aromatic hydrocarbons in coal fly ash and water by flow injection online solid-phase extraction-high performance liquid chromatography, and the enrichment factor can reach 200 to 2337 [48].

。 The Mehdinia Research Group of K.
N.
Tush University of Technology, Iran, applied the successfully prepared methyl-modified MOF-5@ polyacrylonitrile composite (CH3-MOF-5@PAN) to the adsorption packing of solid-phase extraction columns [49].

CH3-MOF-5@PAN has a lower detection limit (0.
02 μg/L) and higher recovery (82.
8%-94.
8%)
for levonorgestrel and methyl acetate compared with commercially available C-18 fillers.
In order to overcome the problem of excessive use of MOFs by SPE technology, the Hian-Kee Lee research group at the National University of Singapore developed a μ-SPE technology based on ZIF-8 and applied it to the enrichment of polycyclic aromatic hydrocarbons in aqueous environments [50].

Because ZIF-8 is size-selective for polycyclic aromatic hydrocarbons of different molecular weights, and its structural Zn metal sites have strong interaction with electron-rich polycyclic aromatic hydrocarbons, ZIF-8 shows stronger extraction effect
than commercial C-18 and C-8 adsorbents.
In recent years, metal oxides such as iron oxides (Fe2O3, Fe3O4) and cobalt oxide (CoO) have unique magnetic characteristics and can be quickly recovered under the action of applied magnetic fields, so they are often used in MSPE technology in combination with MOFs materials
.
For example, the Hou Xiandeng Research Group of Sichuan University prepared magnetic Fe3O4@ZIF-8 composites by step-by-step assembly method, which can effectively enrich inorganic arsenic in urine [51].

The extraction process was detected at low concentrations of inorganic arsenic (> 3 ng/L) by coupled with hydride-atomic fluorescence spectrometry without interference
from the urine matrix.

Compared with SPE, μ-SPE and MSPE technologies, SPME is a technology
that integrates sampling, extraction, concentration and injection.
It can save 70% of the time of sample pretreatment, eliminates the use of organic solvents, and is particularly suitable for on-site sampling analysis and automation
.
For SPME technology, the fiber is at the heart of the technology, which determines the sensitivity, detection limit, confidence range and analysis range
of the analytical method.
At present, fiber coating materials are mainly polydimethylsiloxane (PDMS), polyacrylate (PA) and divinylbenzene (DVB).

However, the above materials have poor selectivity for specific targets in the face of complex sample analysis, and MOFs materials have made certain progress
in the field of SPME due to their unique advantages.
As early as 2009, Yan Xiuping's research group of Nankai University grew MOF-199 film in situ on the surface of stainless steel fiber, which was applied to the enrichment and analysis of volatile benzene series [52]
.
MOF-199 fiber coating has a concentration factor of the target in the range of 19613 (benzene) to 110860 (xylene), which is much better than commercial PDMS/DVB fiber coating
.
The extraction enhancement mechanism was mainly caused by
the strong π complexation and π-π interaction between the Lewis acidic site of MOF-199, the organic ligand of 1,3,5-trimellitic acid and the benzene series.
With the advancement of follow-up research, the researchers introduced functional materials such as carbon-based materials, molecularly imprinted polymers and magnetic nanoparticles into MOFs to improve the stability and selectivity
of the extraction enrichment system.
For example, the Pan Daodong research group of Ningbo University grafted a polymeric aminophenylboronic acid molecular imprinted film on a Fe3O4@ZIF-8 support to prepare novel SPME coated fibers [53].

Compared with the traditional coating materials, the coating has better selectivity and recovery for the four estrogens in live fish and pork samples
due to the abundance of blot sites.
In 2021, the Pan Bingcai research group of Nanjing University achieved bifunctional modification of the material by grafting amino groups and fluoroalkyl groups on MIL-101 (Cr) (as shown in Figure 15, the material is named MIL-101-DETA-F) [54].

Furthermore, through the preparation and analysis method of SPME probe, the efficient enrichment and quantification
of trace and highly toxic per- and polyfluoroalkyl compounds in water were realized.
In combination with ultra-performance liquid chromatography-tandem mass spectrometer, the detection limit reached 0.
004-0.
12 ng/L, and the recovery rate was 76.
2%-108.
6%, which was significantly better than that of commercial PDMS/PVD coating materials
.
Ultra-selective adsorption is mainly achieved
by multiple forces such as size exclusion, hydrophobicity, electrostatic attraction, fluorine-fluorine interaction, hydrogen bonding and Lewis acid-base complexation.
This study provides new methods and ideas
for the analysis of trace per- and polyfluoroalkyl compounds in water.

Figure 15 Schematic diagram of MIL-101(Cr) dual-function modification technical route and preparation of SPME coated fiber[54]

4.
The foreseeable future

In summary, in the past two decades, MOFs materials have shown great application potential in many fields such as in vivo storage and separation, pollutant adsorption, fluorescence sensing, catalysis, supercapacitors, drug and biomolecular transport, sterilization and algae removal, and sample preparation, but there are still many topics to be further explored
.

(1) The synthesis of MOFs materials has factors such as low yield, high cost and harsh conditions, which limit their application
in engineering practice.
In the future, MOFs can
be prepared by macro mechanicalism and electrochemistry.

(2) At present, most MOFs materials have poor long-term stability under high temperature, water environment, acid or alkaline conditions, and in the future, we should focus on developing MOFs materials with super thermal stability, chemical stability and mechanical stability to prevent the dissolution of metal ions, organic ligands and the collapse
of the structure.

(3) Most of the existing MOFs are microporous materials (pore size< 2 nm), which will hinder and limit the mass transfer process of macromolecules inside the MOFs pores, thereby weakening the interaction
between target molecules and active sites.
Especially for heterogeneous catalytic reactions, the microporous structure will also affect the diffusion of catalytic products inside the material, resulting in catalyst poisoning and inactivation
.
Therefore, it is necessary to develop multi-stage composite porous MOFs materials
in the future.

(4) Most of the MOFs synthesized in the laboratory use hydrothermal method or solvothermal method, which requires high temperature and high pressure reaction conditions
.
Therefore, the experimenter cannot directly observe the transformation process of the matter, and the repeatability of material synthesis is not high, which is also called the "black box" process
.
In the future, it is necessary to develop visual synthesis equipment and assist computer simulation technology to transform the synthesis process of MOFs from "black box" to "gray box", or even "white box", so as to accurately control the microstructure and macroscopic functional characteristics of
MOFs materials.

(5) At present, the functions of new materials such as graphene, quantum dots, electro-optical ceramics, and corrosion-resistant alloys have been gradually developed, and appropriate methods can be used to compound MOFs with the above materials in the future, give full play to their respective advantages, and achieve the synergy of 1 + 1 > 2
.

(6) MOFs materials will enter the environment through various ways during preparation, use and disposal, so it is necessary for follow-up researchers to use the Life Cycle Assessment (LCA) method to analyze the resource and energy consumption, environmental impact assessment (climate change, eutrophication, ecotoxicity, etc.
) and environmental release (quality, concentration, existence form, etc.
) of
MOFs materials.
However, the lack of basic data leads to high uncertainty in the results of subsequent LCA evaluation, and it is urgent to establish and update the basic data
of LCA based on MOFs materials.

(7) At present, the concept of material genomics has been applied to the development of
covalent organic frameworks (COFs).
In the future, MOFs can be synthesized in a targeted manner by combining the new concept of "genetic structural unit" and the "reaction-like linkage assembly algorithm", which will ultimately make the development of MOFs more reliable and efficient
.

I believe that MOFs materials will bring more surprises to our production and life in the future, and some fields will even revolutionize changes, but this still needs a long way to go
.
Welcome friends who are curious about the unknown to join the research of MOFs materials, enrich the application of MOFs, and benefit mankind!

References omitted

About the author

Chen Zhao is currently a teacher at
the School of Environmental and Energy Engineering, Beijing University of Civil Engineering and Architecture.
His research interests include the design, synthesis and application
of MOFs-based materials in environmental remediation.
In recent years, he has published more than 10 papers as the first author in SCI journals such as Chemical Engineering Journal, Journal of Hazardous Materials, Chinese Journal of Catalysis, Environmental Pollution, Science of the Total Environment, etc.
One of the papers was selected as an ESI Highly Cited Paper
.
He serves as a young editor of SCI journals Chinese Chemical Letters
.