Metal organic frameworks (MOF) are crystalline compounds comprising metal clusters or ions coordinated to rigid organic molecules to form single, two, or three-dimensional porous structures. In certain cases, the pores are stable to eliminate guest molecules such as solvents and can be used for the storage of gases such as carbon dioxide and hydrogen.
MOFs originated from zeolites, which are porous aluminosilicate minerals that occur naturally but may also be produced synthetically.
A MOF includes two key components, a metal ion or a metal ion cluster and an organic molecule known as a linker. The organic units normally include one, two, three, or four ligands. The properties and structure of the MOF largely depend on the choice of the linker and the metal. For instance, the coordination preference of the metal impacts the shape and the size of pores by clearly defining how many ligands can bind to the metal and in which orientation.
Synthesis of MOFs
The synthesis of MOFs can be achieved in two ways:
- Hydrothermal methods
- Solvothermal methods
In simple terms, these techniques involve growing crystals from a hot solution. MOFs are normally assembled by the bridging of organic ligands that stay intact during the synthesis. For synthesizing zeolites, a range of templates or structure directing compounds are used, and templating is usually achieved using organic anions.
In the case of zeolites, these templates are removed by oxidation, but in MOFs, the templating of the frameworks is done using organic ligands and the SBU. MOFs that are needed for gas storage can be templated using metal-binding solvents such as N, N-diethylformamide, and water. When the solvent is removed, metal sites are exposed enabling binding of hydrogen at these sites.
Conventional synthesis is not as effective as a post-synthetic modification of MOFs, which opens up a totally new dimension of structural possibilities. For hydrogen storage applications, MOFs are needed that have modifications exposing metal sites. This can be done by conducting post-synthetic coordination of additional metal ions to sites present on the bridging ligands and adding and removing metal atoms to the metal site.
As MOF ligands tend to reversibly bind, the slow crystal growth enables re-dissolving of defects causing a material having near-equilibrium defect density and millimeter-scale crystals. Solvothermal synthesis is suitable for growing crystals that are suited for structure determination since crystals grow in a timeframe of hours to even several days.
Another evolving technique is known as a microwave-assisted solvothermal synthesis that can produce micron-scale crystals in a matter of second to minutes in quantities that are similar to those obtained by slow growth.
Researchers from Aldrich Materials Science have found a novel method for the design of MOFs under completely liquid-free conditions. Researchers believe that the high purity MOFs can be used as rare earth containing materials for detectors and sensors, magnetic or electronic materials. By this discovery, liquid-free techniques are extended to a novel class of 3D structured materials and can result in a range of products with unique products and suitable for applications that have not been known until now. In this liquid-free synthesis, MOF products of high purity are obtained that prevent contamination from liquid and solvent residues.
Shilun Qiu and the team from Jilin University in China report a universal and convenient method for the preparation of free-standing metal organic framework membranes with varied size and shape and thickness varying from hundreds of nanometers to hundreds of micrometers. This is an easy and convenient membrane fabrication method that can be applied to a range of other material compositions in order to obtain functional membranes having a diverse micropore structure, hence enabling the development of new functional nanodevices.
Hydrogen storage and MOFs
There is a growing interest in developing non-petroleum energy carriers for transportation applications. Hydrogen has a large energy content hence making it an attractive solution, and it also produces a clean exhaust product and can be obtained from a range of primary energy sources.
For hydrogen to be actually used for energy storage, it is important that dense storage methods are used, since the energy of uncompressed hydrogen is quite low. MOFs have become popular as materials suitable for hydrogen storage as they have high specific surface areas and chemically tunable structures. By adsorption to its surface, hydrogen molecules are stored in a MOF.
Additionally, MOFs do not have a 'dead' volume, meaning there is no storage capacity loss due to space blocking by non-accessible volume. The MOF storage capacity is restricted by the liquid-phase density of hydrogen due to the advantages offered by MOFs.
In order to realize the offered benefits of MOFs, hydrogen cannot be stored in MOFs at densities of more than its liquid phase density. The extent of gas absorption to a MOFs surface is based on the pressure and temperature of the gas.
It is feasible to use MOFs for hydrogen storage in automotive fuel cells that are required to operate efficiently at ambient pressure and temperature between 1 and 100 bar as these are safe for automotive applications.
Catalyst Applications
MOFs have great potential in a number of catalyst applications. Catalysts are used for the manufacture of most used chemicals worldwide. Since MOFs have a tunable porosity, increased surface area, and diversity in metal they are highly suitable to be used as catalysts.
MOFs enables easier recyclability and post-reaction separation than homogenous catalysts. In other cases, they also provide highly improved catalyst stability. Furthermore, they provide substrate size selectivity.
- Catalysis with MOF-encapsulated catalysts
- Catalysis with metal-free organic cavity modifiers
- MOFs for achiral catalysis
- Functional linkers in MOFs as catalytic sites
- Functional linkers can be also utilized as catalytic sites
- Entrapment of catalytically active noble metal nanoparticles in MOFs
- MOFs as reaction hosts with size selectivity
- Catalysis with metal ions or metal clusters
- Catalysis with functional struts
- MOFs for asymmetric catalysis
- Homochiral MOFs with interesting functionalities and reagent-accessible channels
- MOFs towards biomimetic design and photocatalysis
Further Applications
There are a number of other applications of MOFs that include the following:
- Gas separation – MOFs permit certain molecules to pass through their pores based on kinetic diameter and size. This is especially used for separating carbon dioxide.
- Gas purification – MOFs have strong chemisorption between its odor-generating, electron-rich molecules and the framework permitting passing of the gas through the MOF.
- Gas storage – MOFs can store carbon monoxide, carbon dioxide, oxygen, and methane because of their high adsorption enthalpies.
- Heterogeneous catalysis – Due to their size and shape selectivity and accessible bulk volume, MOFs are used for selectivity. Due to their porous structure, mass transport in the pores is enabled.
Sources and Further Reading
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