Literature Survey and Fundamental Aspects.
Metal-Organic Frameworks (MOFs)Metal-organic frameworks (MOFs), are crystalline compounds with extortionate porosity and they have recently attracted paramount interest for use in many prominent applications like gas storage, separation, drug delivery, sensing, and catalysis 1. They are additionally alluded to porous coordination polymers (PCPs). MOFs can be habituated in many different applications because of their distinctive tunable structures, extortionate porosities and purposeful sites that makes them unique 2. These extraordinary materials are formed by way of the self-assembly of metal-ions bridged by utilizing organic ligands to shape crystalline solids.
Different metal ions and organic linkers can be habituated to synthesize MOFs and this makes them tunable for different purposes. These materials are versatile, and they have attracted many researchers to design the ideal porous materials for each unique applications 3. Other reasons for this attraction in MOFs research is their ability to amalgamate the benefits of each homogeneous and heterogeneous catalysis. MOFs have great capabilities to be utilized in catalysis 4.
Zeolitic Imidazolate Frameworks (ZIFs)
Zeolitic Imidazolate Frameworks (ZIFs) are incipient emerging subordinate class of metal organic frameworks (MOFs), which 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 because of their unique and highly desired properties that they possess like high surface area, high crystallinity, great chemical robustness, thermal stabilities and abundant functionalities 6. ZIFs hold immense promise in many different applications and fields like in gas storage and separation, Biomedicine, chemical sensing, catalysis and water treatment 7. During synthesis of ZIFs, the type of imidazolate (linker) and the solvent that is utilized have a great influence on the structure that is obtained. The imidazolate linker can additionally be functionalized to achieve ZIFs with desired functionality for a concrete applications 8. Amongst the studied ZIFs, ZIF-8 is one of the most studied ZIFs because it is facile to synthesize and it is reproducible and it is composed of Zn2+ ions and 2-methylimidazolate (MeIm) which results in the Sodalite topology (SOD) 9.
Figure SEQ Figure * ARABIC 1: Coordination isoreticular relation between M-IM-M and Si-O-Si (top), and the Sodalite topology crystal structure of ZIF-8 9.
ZIF-67 is some other type of MOF that is synthesized from Cobalt (II) ion and 2-methylimidazole. It reveals a similar crystal shape and (SOD) topology as ZIF-8 but includes an exceptional metallic ion (Co) 4.
Figure SEQ Figure * ARABIC 2: Chemical and Sodalite topology structure of ZIF-67 10
ZIF materials have been historically synthesized with the aid of hydrothermal or solvothermal synthesis routines with varying reaction temperatures, respectively 11. For the synthesis of ZIFs, quite a few parameters has to be taken into account. The received shape is structured on the response stipulations used during the synthesis, like reagents, solvent and temperature. The desired coordination of the metal-ions used throughout the synthesis in aggregate with the linkers decide the acquired geometry of ZIFs. Solvents have a large in?uence on the ZIFs synthesis. The solvent can act as ligand for the duration of the synthesis, coordinating to the metal-ion main to a structure. The pores of the ZIFs are continually ?lled with solvent at some point of the synthesis, which act as space-?lling molecule 12.
Temperature can a?ect the synthesis of ZIFs in di?erent ways. The solubility of the organic linker relies upon on the temperature used all through the synthesis, higher temperatures results in a greater solubility of the linker. Temperature can additionally provide the electricity required for the reaction to occur and can make bigger the response rate. Di?erent synthesis temperatures can result in di?erent constructions for MOFs 13.
Historically, ZIFs are synthesised from organic solvents 11. The riding force for this technique is the reaction temperature. The solvothermal method is the most used approach for ZIFs syntheses and this would possibly be due to the simplicity of the method 14. The method is shown in Figure 3. Usually the reagents are ?rst dissolved, the options are blended and positioned in a closed vessel. The vessel is placed in an oven at increased temperature, which doesn’t have to be above the boiling pressure of the solvent, for 24-48 h. During this time the MOFs are fashioned as solid crystals via self-assembly which can be accumulated through ?ltration. The product is washed countless times to remove any reagent left and ?nally dried in an oven to take away the solvent and evacuate the pores 14,15.
Figure SEQ Figure * ARABIC 3: Conventional solvothermal synthesis of MOF structures 15
It is known that organic solvents are expensive, ?ammable and not environmental friendly. Lookup has been carried out to boost methods which use much less natural solvents. For this reason, synthesis techniques based on aqueous structures are preferable on account that they are environmental pleasant compared to techniques based totally on natural solvents. For ZIF-8 synthesis, Pan et al 16 have been the ?rst who observed a technique to synthesize ZIF-8 by usage of an aqueous device at room temperature. But they encountered problems with their synthesis; they used a Zn2+: HMeIm molar ratio of 1:70, which means a lot of HMeIm doesn’t react and is spilled by the use of this approach (to compare, the Zn2+: HMeIm molar ratio in ZIF-8 is 1:2). To decrease the extra amount of linker needed, auxiliary agents can be used to assist the formation of the needed product. Auxiliary agents, for example triethylamine (TEA) can help with the deprotonation of the imidazole and initiate the formation of ZIF-8 17
Figure SEQ Figure * ARABIC 4: Conventional solvothermal synthesis of MOF structures 15
Synthesis of ZIF nanoparticles with size control
Different methods on controlling ZIF particle size during synthesis have been reported previously. Tanaka et al. reported a method on controlling ZIF-8 particle size in aqueous solution at room temperature. Different 2-methylimidazole/zinc (MeIm/Zn) ratios ranging from 4 to 100 were utilized whilst keeping the water and zinc concentration constant 1. SEM analysis of the products from different MeIm/Zn ratio showed that the particle diameter was inversely proportional to MeIm/Zn ratio as reported in (Table 1.1). By increasing the concentration of the linker (2-methylimidazole), increases the concentration of free imidazolate ions which increases the nuclei for complex formation, leading to smaller particles.
Table 2.1: Different MeIm/Zn ratio with the resulted ZIF-8 particle diameter.
MeIm/Zn ratio Particle diameter (?m)
Tsai et al. reported synthesis of ZIF-8 nanoparticles with particle size control by temperature utilizing 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. Moreover, Pan et al. successfully controlled the particle size of ZIF-8 particles utilizing surfactants as capping agents to control the particle size during synthesis 2. By increasing the wt % of surfactant cetyltrimethylammonium bromide (CTAB) from 0.0025 to 0.025, resulted in a decrease in particle size from 4000 nm to 110 nm.
2 Y. Pan, D. Heryadi, F. Zhou, L. Zhou, G. Lestari, H.Su, Z. Lai, CrystEngComm, 2011, 13, 6937-6940.
S. Tanaka, K. Kida, M. Okita, Y. Ito, Y. Miyake, Chem. Let., 2012, 41, 1337-1339
Postsynthetic modification (PSM)
Postsynthetic modification (PSM) is a powerful synthetic approach in accordance in tuning materials purposefully after synthesis, both through changing the metal ions, ligands or including purposeful organizations to the linker barring essentially changing measurement and structural homes of materials 18. As an end result of their satisfactory and huge chemical variety, PSM is performed dense instances for Metal Organic Frameworks (MOFs), to boost diaphanous porous materials such as favored properties. This strategy is very necessary in development and enhancement of several hybrid solids and it is extensively applicable. This is accomplished through way of incorporation of a performance crew on the organic linkers or by means of altering the metal-ions. In many situations the direct synthesis concerning MOFs together with appreciated functionality is challenging to perform 19. This can bear many reasons like restrained solubility and stability, the appreciated geometry on the metal-ion and linkers or the preferred specific functional cluster will avert the formation of the detailed shape 20. Postsynthetic modification (PSM) is properly known to help in designing the desired MOFs for one-of-a-kind applications.
Postsynthetic exchange (PSE)
Post-synthetic exchange (PSE) is an effective approach of exchanging metal ions and/or the linkers in MOFs. PSE can create structural properties along a composition regarding metal ions and linkers that cannot be synthesised directly. This method has come to be an easy approach to alter MOFs below moderate conditions 21. This makes such viable way to perform PSE, concerning a wide extent of MOFs, regardless of their relatively low stability. PSE may be done within pair ways as proven in Fig. 3. It can remain a reaction of pair solids changing their linkers or their metal ions then a reactions among a solid and solution of the linker or metal ions 1.
Scheme 1: Pair ways to perform PSE, solid-solid and solid-liquid 1.
Postsynthetic modification (PSM) is essentially accomplished after the synthesis of the framework. This edit that workable after impact resources with a specific shape and favored useful agencies optimising that for different applications. Other clear materials like zeolites, do not keep the handy chemical functionality then the bulk of open area in accordance with have deep possibilities because post-synthetic functionalisation. This makes vile porous transparent supplies less appropriate because of post-synthetic amendment in contrast according to MOFs 22. The precept 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 and again solely the metal-ion and/or organic linker has to be dissolved. This actually helps obviates any quandaries with dissolving two di?erent chemicals, for instance 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 likelihood to include di?erent useful functional groups within the framework leading to di?erent functionalities 23.
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 favored linker or metal-ion, at accelerated temperature. There will be an equilibrium between the linkers/metal-ions in the framework and in the solution, by using which they will trade except loss of crystallinity or porosity of the materials 1.
Metal ion exchange
Metal ions can withal be modi?ed in MOFs. This can be carried out via impregnation of metallic particles in the framework 24 or through changing the metal-ions 1. This is well known as metal-ion exchange (presented in Figure 5). The metal-ions in the framework are changed by way of other metal-ions to enhance the properties for different applications.
Figure SEQ Figure * ARABIC 5: Metal-ion exchange in ZIF-71 25.
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 steadiness of the framework, this framework steadiness follows the style of the Irving-Williams sequence for transition metal complexes which is presented below 26:
Ba2+ < Sr2+ < Ca2+ < Mg2+ < Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > 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 26. The steadiness of the framework must be taken into consideration once carrying out metal-ion exchange. The framework must be stable beneath conditions used for the exchange reaction, otherwise the framework can be destroyed 27.
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. 6 28. It was reported that 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, whilst the vibration 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 C-H stretch). A vigorous and broadband on 2-methylimidazole, elongating over the frequency range 2500 and 3000 cm-1 (attributes to the H?N···H hydrogen bridge), and the N?H stretch vibration appears again at 1843 cm-1, which is completely disappeared on the synthesized ZIF-8 and ZIF-67 29,30,31
Figure SEQ Figure * ARABIC 6: FTIR spectra of the as-prepared (a) ZIF-8 and (b) ZIF-67 film electrodes at different cycling states 28.
Powder X-Ray Di?raction
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 29,30,31,32,33. 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 25.
Figure SEQ Figure * ARABIC 7: PXRD patterns for ZIF-8 and ZIF-67 34
Accelerated Surface Area and Porosity Analysis (ASAP)
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 35. 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 35,36,37. 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 25.
Figure SEQ Figure * ARABIC 8: N2 adsorption–desorption isotherms of the as-prepared ZIF-8 and ZIF-67. 36
Electron Microscopy (SEM)
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) 36. The particles exhibited a hexagonal shape and a smooth surface as reported by Li et al 36, which is consistent with several reports. Sahin et al reported that ZIF-67 had polyhedral shape and agglomerated into larger particles 37. 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 38. The size and shape of ZIF particles in this case will depend on method and conditions of synthesis, respectively. Fei et al 25 found that before and after the metal exchange reaction in ZIF-8 material, the particles look similar.
Figure SEQ Figure * ARABIC 9: SEM images of ZIF-8 (a) and (b), ZIF-67 (c) and (f) at different magnifications 36.
Thermogravimetric Analysis (TGA)
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, synthesized in methanol by reverse micro-emulsion method 4. The TGA curve shows that ZIF-8a in micro-emulsion is thermally stable up to 500 °C, while ZIF-67a in micro-emulsion 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.
Figure SEQ Figure * ARABIC 10: TGA curves of (a) ZIF-8 and (b) ZIF-67 4.
Heterogeneous catalytic Applications of ZIFs
It has been reported recently that Metal Organic Frameworks (MOFs) have been utilized as photocatalysts, and they have shown remarkable photocatalytic activity 30. 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). 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 12. 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 30. [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 16.
Figure SEQ Figure * ARABIC 11: A proposed model of photocatalytic reaction mechanism of RDB on Fe-ZIF-8 catalyst under sun light illumination 30.
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