Introduction to metal-organic frameworks and their applications in chemical sensing
1.4 Synthesis protocols of MOFs
Several types of interactions exist during the formation of MOFs. Formation of coordination bonds between metal centers and ligands is the most common interaction. In addition, hydrogen bonds and other interactions like metal-metal bonds and π-π interactions are also found in the structures. Coordination bond is the strongest interaction among all of the interactions in MOFs. Structures with more coordination bonds show great stability. On the other hand, structures with more hydrogen bonds or other weak interactions suffer from structural stability.99
The last component present in framework of MOFs may be the guest molecules or template molecules, which can be removed later to leave open pore in MOF structures. As nature does not tend to form big void structures, MOFs are usually filled with guest molecules during synthesis. So, it is possible to add template molecules with specific sizes to modulate the pore size of MOFs. For example, organic amines can help to arrange the metal centers and ligands at appropriate distance during synthesis of MOF structures.100
1.4.1 Diffusion method
Diffusion method is a traditional method to form crystalline materials. Generally, two types of diffusion methods have been followed. In one method, layering is done by immiscible solvents: one is the precipitant solvent and the other one dissolves the product.
They are divided by a layer of solvent. The precipitant solvent is regarded as ‘anti-solvent’
as the reactant is insoluble in it and the liquid in which the dissolution of reactant takes place is regarded as ‘solvent’. At the interface, crystal growth occurs due to the gradual diffusion of the precipitant solvent into the separate layer. In another method, generally gradual diffusion of reactant occurs through dividing barrier. Sometimes, gels are applied as crystallization and diffusion media to reduce the diffusion rate and prevent the precipitation. The obtained single crystalline compound is characterized by X-ray diffraction technique.101, 102
1.4.2 Hydro/solvothermal synthesis
Originally, this method was first employed during synthesis of zeolites. It has also been set for the synthesis of MOFs.103 Generally, stainless steel autoclaves or sealed Pyrex tubes are used at high temperature and pressure containing mixture of linker, metal salt and solvent (Figure 1.8).104 In hydrothermal synthesis, only water is employed as the solvent. On the other hand, solvothermal synthesis includes various solvents other than water that are heated above the boiling point of the solvent under autogenous pressure.
The superheated solvents reduce the viscosity and enhance the diffusion of chemical species. Furthermore, at higher temperature, the dielectric constant decreases which help to solubilize the chemical species in the solvent.105 However, this method usually requires long reaction times (sometimes several hours up to several days) depending upon the factors including reaction solvent, reaction temperature, reagent concentrations, etc.106
Figure 1.8 Schematic diagram for MOF synthesis by hydrothermal method or solvent heating method. Reproduced with permission from ref. 107. Copyright 2018, MDPI.
1.4.3 Microwave method
Solvothermal synthesis usually requires long reaction times (days to weeks), bulky equipment and high energy consumption. To overcome these limitations, microwave- assisted synthesis of MOFs has been developed.108 Microwaves are normally produced by a magnetron which consists an oscillator converting high-voltage direct current into high- frequency radiation. In a typical laboratory instrument, waveguides transfer the generated energy from the magnetron to the sample chamber. Many solvent molecules, notably water, possess dielectric moments and rotate to align themselves with the alternating electric field of the microwaves. The heat generated due to the molecular movement and collision is dispersed in the molecules. The sample chamber is a Faraday cage which restricts the microwave radiation escaping into the environment. The main advantage of microwave synthesis is that energy is generated directly throughout the bulk of the material instead of by conduction from the external surface. Furthermore, microwave heating is almost instantaneous, occurs without heating the container and allows the use of temperatures above the boiling point of a used solvent. Microwave technique typically cannot produce single crystals but shape and size of particles can be finely controlled by this method.109, 110
1.4.4 Electrochemical method
Electrochemical method is predominantly applied for producing MOF powders on industrial scale.111 The benefits of this method compared to solvothermal synthesis are low reaction temperature and quick reaction time. In bulk scale, there is limitation in crystallization for the in-situ production of the metal ions near the support surface, which actually diminish the unfavorable accumulation of crystals during the synthesis of membrane.112 Furthermore, compared to solvothermal techniques, electrochemical synthesis method possesses more parameters for fine-tuning due to the simple adjustment of the voltage or imposing particular signals. Experimental set-up for electrochemical synthesis and coating of a ZIF (ZIF = zeolitic imidazolate framework) material is represented in Figure 1.9.113
Figure 1.9 Illustration of the experimental setup used to synthesis ZIF coatings. Two Co or Zn electrodes are immersed in a tributylmethylammonium methyl sulphate (TBMAMS) in N,N-dimethylformamide (DMF):H2O electrolyte solution containing an imidazolate linker. Heat is applied whilst deaerating the solution with gaseous N2 and 2.5 V applied potential difference generates the ZIF coating on the anode. Reproduced with permission from ref. 113 . Copyright 2015, Elsevier.
1.4.5 Mechanochemical method
Mechanochemistry is basically a solvent free process in which grinding of two reactants with a pestle and mortar can produce the desired compound.114 Ball mill process is actually a programmable mechanochemical process where no physical effort is required.
Hence, it allows more systematic studies of the process.115 The supplied kinetic energy has various effects on a crystalline solid including heating, reduction of particle size, local melting, phase changes to alternative polymorphs, formation of defects and dislocations in crystal lattices.116 Different methods are adopted in mechanochemical synthesis which involves neat grinding, grinding-annealing, liquid assisted grinding, kneading, etc. It has been observed that solvent-free reactions produce very pure products and reduce the formation of solvated species.117 Hence, this process has great importance for crystal engineers and solid-state chemists. Schematic presentation of mechanochemical synthesis of BIT-11 and BIT-11b (Ni-substituted ZIF-8 is denoted as BIT-11) is represented in Figure 1.10.118
Figure 1.10 Schematic representation of the mechanochemical synthesis of BIT-11 and BIT-11b. Reproduced with permission from ref. 118. Copyright 2014, Royal Society of Chemistry.
1.4.6 Sonochemistry method
In sonochemistry method, high-energy ultrasound is utilized to a reaction mixture to obtain the product.119 The dissolution of starting compounds can be enhanced by ultrasonication. Recently, the syntheses of organic compounds and nanomaterials are carried out through the application of sonochemistry.120 In the field of MOF science, the main reason to adopt sonochemical synthesis is that it is quick, energy-efficient, environment friendly, easy to handle and can be applied at ambient temperature.121 Special interest can be taken in future for scaling up of MOFs by this method.
1.4.7 Post-synthetic modification
The introduction of a particular functional group in MOFs for a special application is sometimes difficult due to the high sensitivity and reactivity of the additional functionality during the formation of MOFs. An efficient route to obtain MOFs having desired functionality is post-synthetic modification (PSM) .122 This method was originally suggested by Hoskins and Robson in 1990.50 Actually, in PSM, a MOF can be synthesized and modified in a heterogeneous manner after the formation of the solid lattice instead of synthesis of functionalized MOFs directly (Figure 1.11).123 PSM is advantageous compared to the pre-functionalization approach because of greater control over the types and number of functional groups that can be incorporated into the framework.
Furthermore, by using PSM, both the metal and organic species can be functionalized without affecting the overall stability of the framework.124 Hence, PSM is regarded as an excellent tool for preparing topologically identical, but functionally diverse frameworks.
Figure 1.11 A generic scheme for the post-synthetic modification (PSM) of MOFs.
Reproduced with permission from ref. 125. Copyright 2011, Royal Society of Chemistry.
Covalent modification is considered as the most popular route of PSM, in which functionalization of MOFs are performed for specific applications. PSM of amino group functionalized MOFs by aldehydes, anhydrides, isocyanates, acyl chlorides and alkyl bromides have been reported.126 Moreover, covalent modification of azide-functionalized MOFs through click reactions is also possible. The reactive azide groups in the MOF cavity undergo quantitative click reactions with alkynes to form various MOFs having tailored pore surfaces.127
Besides covalent modification, dative modification of MOFs through post- synthetic metalation was also developed. MOF with 2,2′-bipyridine (bpy) sites can be post- synthetically metalated with soft metal Cu2+ and Pd2+.128 The coordinatively unsaturated metal sites in MOFs can be modified by additional ligands. For instance, the vacant coordination sites in some Cr and Cu-based MOFs could be modified with alkane amines to improve CO2 uptake.129, 130 The coordinatively unsaturated Zr6-clusters can also bind with carboxylates through simple acid/base reactions. For example, the solvent-assisted ligand incorporation (SALI) method was developed for modifying the coordinatively unsaturated Zr-cluster of NU-1000 to enhance CO2 uptake.58
Post-synthetic ligand and metal ion exchange have been developed as efficient methods to gain new MOFs with various ligands and metal ions.131 Actually, the inert metal-ligand bonds in stable MOFs will significantly obstruct the process of the ligand and metal ion exchange. However, recent studies on the ligand and metal ion exchange of several stable MOFs prove that the robustness of such metal-ligand bonds was overestimated. Post-synthetic metal and ligand ion exchange processes were applied for several stable MOFs such as ZIFs, MIL series and UiO series.132