Metal-organic frameworks

Metal-organic frameworks (MOFs), a subclass of porous coordination polymers (PCPs), are crystalline compounds with high porosity and have found uses in gas storage, separation, molecular shuttling, drug delivery, sensing, and catalysis 1. MOFs can be utilized in many different applications because of their distinctive tunable structures, high porosities and binding sites that makes them unique 2. These extraordinary crystalline solids are formed by way of the self-assembly of metal-ions that are bridged by organic ligands. MOFs can be synthesized from a wide variety of metal ions and organic linkers. This enable researchers to design the ideal porous materials for each unique application 3. MOFs can amalgamate the benefits of both homogeneous and heterogeneous catalysis Zeolitic Imidazolate Frameworks (ZIFs), a subfamily of MOFs, have a zeolite-like structure and properties. ZIF materials are mainly composed of transition metal ions like Zn2+ and Co2+ and imidazolate linkers 5. These materials have received sizeable attention due to their unique and highly desired properties, like a very high surface area, high crystallinity, great chemical robustness, high thermal stability and variable functionality 6. ZIFs hold immense promise in many different applications and fields like gas storage and separation, biomedicine, chemical sensing, catalysis and water treatment 7. During synthesis of ZIFs, the type of imidazolate (linker) and the solvent utilized have a great influence on the structure obtained. The imidazolate linker can additionally be functionalized to achieve ZIFs with desired functionality for concrete applications 8. ZIF-8 is one of the most studied ZIFs due to its ease of synthesis and reproducibility. It is composed of Zn2+ ions and 2-methylimidazolate (MeIm) linkers which results in the Sodalite topology (SOD) 9. ZIF-67 is synthesized from Cobalt (II) ions and 2-methylimidazole. It has a similar crystal shape and (SOD) topology as ZIF-8 but with Co as metal centers ZIF materials have been historically synthesized by hydrothermal or solvothermal synthesis routines with varying reaction temperatures 12. For the synthesis of ZIFs, the following are important: reagents, solvent and temperature. The coordination sphere of the metal-ions, together with the linkers will determine the geometry of ZIFs. Solvents have a large influence on the ZIFs synthesis since it can also coordinates to the metal ion. After the synthesis, the pores of the ZIFs are ?lled with solvent, which act as space-?lling molecules 13. Since the solubility of the organic linker relies upon on the synthesis temperature, higher temperatures generally result in a greater solubility. Although temperature influences the reaction rate, different synthesis temperatures may result in different topology for MOFs The solvothermal method is the most used approach for ZIF syntheses with high temperature as driving force, as shown in Figure 3 15. Usually the reagents are ?rst dissolved, and sealed in an autoclave. This is placed in an oven at a temperature, usually above the boiling point of the solvent, for 24-48 h. During this time the MOFs are formed as solid crystals via self-assembly, which can be isolated through ?ltration or centrifugation. The product is washed several times to remove any reagent left and ?nally dried in an oven to evaporate the solvent and evacuate the pores Pan et al 17 have been the ?rst to synthesize ZIF-8 via an aqueous route at room temperature, a Zn2+: MeIM molar ratio of 1:70, requiring a large excess of MeIM. To decrease the extra amount of linker needed, auxiliary deprotonation agents, such as triethylamine (TEA) can be used initiate the formation of ZIF-8 18. Zhou et al. synthesized ZIF-8, ZIF-67 and Zn/Co-ZIF at room temperature using the metal solution of (Zn(NO3)2.6H2O or Co(NO3)2.6H2O in methanol) with the solution of 2-methylimidazolate in methanol. Results showed that all samples consist of nano-sized crystals with a rhombic dodecahedron shape with a complete white powder (ZIF-8), light purple coloured powder (Zn/Co-ZIF) and intense purple powder (ZIF-67) 19. Sun et al. used a reverse micro-emulsion method to synthesize the ZIF-8 and ZIF-67 nanoparticles at room temperature with a mean particle size less than 5 nm. The particles showed much larger surface areas and micropores volume 4. Tanaka et al. showed that the particle diameter of ZIF-8 is inversely proportional to the MeIm/Zn ratio in aqueous solutions at room temperature (Table 1.1). By increasing the concentration of the linker (2-methylimidazole), the nucleation rate increases, leading to smaller particles 20.Tsai et al. showed that the size of ZIF-8 nanoparticles can be controlled by temperature during an isothermal benchtop reaction in methanol. As the temperature was increased from -15 to 60 °C, the average particle sizes decreased from 78 to 26 nm 21. Pan et al. successfully controlled the particle size of ZIF-8 particles utilizing surfactants as capping agents during synthesis. By increasing the wt % of surfactant cetyltrimethylammonium bromide (CTAB) from 0.0025 to 0.025, a decrease in particle size from 4000 nm to 110 nm was observed 17. In this study a combination of these techniques were used in order to synthesize nZIF-8 and nZIF-67 with a narrow size distribution and an average particle size of ~50 nm.Postsynthetic modification (PSM) is a powerful synthetic approach to tuning materials for specific application. It can be done by exchange of the metal ions or ligands as well as anchoring ligands to the linker, without changing the structure of the material 22. PSM is performed by incorporation of a functionality group on the organic linkers or by changing the metal-ions. In some cases the direct synthesis of MOFs with desired functionality is hard to perform. This strategy is very necessary in development and enhancement of several hybrid solids. In many situations the direct synthesis of a MOFs with the desired functionality is challenging to perform 23. This can be due to low solubility and stability, or the geometry of the metal ion and linkers preventing the formation of the expected topology 24. Postsynthetic modification (PSM) is helps in designing the desired MOFs for one-of-a-kind applications.Post-synthetic exchange (PSE) is an approach of exchanging metal ions and/or the linkers in MOFs. This method may be used to alter MOFs under moderate conditions 25. PSE may be performed in two ways as shown in Fig. 3. It can either be a reaction of two solids exchanging their linkers/metal ions or a reaction between a solid and a solution of the linkers or metal ions Postsynthetic modification (PSM) is essentially accomplished after the synthesis of the framework. The principle of postsynthetic modification is presented in Fig. 4. Like any other processes, Postsynthetic modification has its own advantages and disadvantages. On the advantages part, PSM is a heterogeneous procedure which makes isolation regarding the production effortless via ?ltration. This prevents any problems with dissolving two di?erent chemicals, for example a polar and an apolar reagent. Moreover, PSM has an advantage of incorporating different functional groups in the framework of the MOFs, because it may be repeated multiple times, which supplies the opportunity to include di?erent useful functional groups within the framework leading to di?erent functionalities 26,27. For PSM to be a success, it has some requirements. The MOFs used for PSM must contain a metal ion or a functional group that can be modified and they must be stable under reaction conditions used for modification. The most used approach for postsynthetic modi?cation is carried out via putting the MOFs in a solution of the favoured linker or metal-ion, at accelerated temperatureMetal ions can be modi?ed in MOFs. This can be carried out via impregnation of metallic particles in the framework 28 or through changing the metal-ions 1. This is well known as metal-ion exchange (presented in Figure 4). The metal-ions in the framework are changed by way of other metal-ions to enhance the properties for different applications. The reaction is carried out by submerging the MOFs in a solution of a metal precursor. The specific metals that can be placed in the framework in?uences the stability of the framework, this framework stability follows the style of the Irving-Williams sequence for transition metal complexes which is presented below 30:
Ba2+ < Sr2+ < Ca2+ < Mg2+ < Mn2+ < Fe2+ < Co2+ < Ni2+ Zn2+
The process of metal-ion exchange is reversible, but the reverse reaction would possibly proceed slower as a result of weaker interactions of the metal-ion with the linker .The exchange of the metal-ions in MOFs can be partial or complete, and it is in?uenced by many parameters. The concentration of the metal-ion in solution in?uences the exchange rate. If the concentration is higher the exchange rate will increase because increasing the concentration of the metal-ion shifts the equilibrium to the exchange of the metal-ions 30. The stability of the framework must be taken into consideration once carrying out metal-ion exchange. The framework must be stable under conditions used for the exchange reaction, otherwise the framework can be destroyed Infrared spectroscopy (IR) can be employed to characterise the di?erent chemical bonds in a material and to identify the functional groups present. The comparison IR spectra of ZIF-8 with 2-methylimidazole (MeIM) and ZIF-67 with 2-methylimidazole (MeIM) is shown in Fig. 5 32. The spectra of 2-MeIM shows a strong and broadband elongating over the frequency range 3500 to 2500 cm-1 which is attributed to the N–H···N hydrogen bond between two 2-MeIM ligands and the N?H stretching vibration appears at 1843 cm-1, which completely disappears on the synthesized ZIF-8 and ZIF-67, indicating the deprotonation of the N–H groups of the 2-MeIM ligands upon coordination with metal ions. most of the absorption bands of ZIF-8 and ZIF-67 are akin to the vibrations of the 2-methylimidazole units, such as the peak at 1584 cm-1 , which is attributed to the C=N stretch mode, the bands at 1350–1500 cm-1 are attributed to the imidazole ring stretching. The vigorous bands at 1150 and 995 cm-1 is due to the C-N stretching of the imidazole units, weak peaks at 2930 cm-1 and 3138 cm-1 (assigned to aliphatic C-H and aromatic C-H.
PXRD can be used to verify the topology of the studied materials. In Figure 7, the PXRD patterns of ZIF-8 and ZIF-67 materials are shown. The PXRD pattern of ZIF-8 and ZIF-67 particles are identical with respect to peak positions and the relative intensities yet well regarded top positions, including 002, 112, 013, and 222 are in agreement. The well-defined PXRD pattern shows that the ZIF-8 and ZIF-67 particles are highly crystalline, and all possess the same SOD (Sodalite) topology 33,34,35,36,37. Fei et al reported that the PXRD pattern of the exchanged ZIF-8(Zn/Mn) confirmed retention of the Sodalite topology upon metal exchange of ZIF-8 with Mn(II) metal ion The surface area, pore size and structure properties can be examined using the N2 adsorption-desorption isotherms which is usually measured at 77 K, utilizing the theory on Brunauer-Emmett-Teller (BET), respectively. Type I isotherms are of a microporous materials while type IV isotherms are of a mesoporous materials 32. In Figure 8, the ZIF-8 and ZIF-67 materials exhibits the type I adsorption isotherms showing an immensely high level of N2 adsorption at very low relative pressure, and this indicates a microporous nature of the ZIF-8 which is consistent with ZIF-67. The surface area of the ZIF materials are usually very high and they ranges from 1000 to 2000 m2.g-1 depending on the method and conditions used during syntheses 32,40,41. Fei et al reported that introduction of redox-active transition metal to the framework, reduces the surface area and pore size of the ZIF materials Scanning electron microscopy (SEM) can be used to get visual understanding of the synthesised particles. Figure 9 represents the high resolution SEM image which exhibit a narrow distribution for both ZIF-8 and ZIF-67 (~50 nm for ZIF-8, 80–300 nm for ZIF-67) 32. The particles exhibited a hexagonal shape and a smooth surface as reported by Li et al 32, which is consistent with several reports. Sahin et al reported that ZIF-67 had polyhedral shape and agglomerated into larger particles 40. The obtained average size of ZIF67 was 281 nm. Moreover, both ZIF-8 and ZIF-67 revealed a rhombic dodecahedral shape with sharp edges and smooth faces according to Saliba et al 41. The size and shape of ZIF particles in this case will depend on method and conditions of synthesis, respectively. Fei et al. found that before and after the metal exchange reaction in ZIF-8 material, the particles look similar Thermogravimetric analysis (TGA) can be used to measure the thermal stability of a material. The mass of a material is measured while heating with a certain ramp over time. The reported TGA in Fig. 10, was performed by Sun et al. on nano-sized ZIF-8 and ZIF-67 synthesized in methanol by reverse micro-emulsion method 4. The TGA curve shows that ZIF-8a is thermally stable up to 500 °C, while ZIF-67a is thermally stable up to 400 °C, which is consistent with the previous reports that the thermal stability of ZIF-67 is slightly lower than that of ZIF-8 39. All TGA curves shows approximately 10% weight-loss at low temperatures, which is often attributed to the removal of solvent. In addition, a sharp decrease in the weight was observed between 400 and 600 °C for both nZIF-8 and nZIF-67 due to the decomposition of organic frameworkIt has been reported recently that Metal Organic Frameworks (MOFs) have been utilized as photocatalysts, and they have shown remarkable photocatalytic activity 35. The photocatalysis by MOFs occurs when electrons are being transferred from the photo-excited organic ligands to metal clusters within MOFs (presented in Fig. 11). In addition, the photocatalytic activity of Zeolitic Imidazolate Framework-8 (ZIF-8) and its modified derivatives have been currently explored. ZIF-8 particles were utilized for photocatalytic degradation of methylene blue (MB) under UV irradiation. Remarkable photocatalytic activity for degradation methylene blue (MB) by ZIF-8 was observed by Jing et al. which was evidenced through the detection of hydroxyl radicals by way of a fluorescence approach 13. Thanh et al. utilized iron (II) doped ZIF-8 particles to degrade Remazol deep black B (RDB) textile dye under sunlight irradiation. The pristine or undoped ZIF-8 did not catalyse the degradation of RDB, while the doped (Fe-ZIF-8) exhibited a magnificent photocatalytic degradation of RDB under sunlight 35. [email protected] imidazole frameworks-8 photocatalyst was utilized for photocatalytic degradation of rhodamine B (RhB) under UV irradiation and they exhibited magnificent photocatalytic degradation and high photostability 18. Wang et al. used ZIF-67 materials cooperating with a ruthenium-based complex as photocatalyst for reduction of CO2, they exhibited a remarkable photocatalytic activity and stability due to the highest CO2 adsorption capability obtained

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