Metal Organic Frameworks (MOFs) are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. Describing and organizing the complex structures of MOFs could be a difficult and confusing task without a logical, unambiguous set of classifications. Recently, a system of nomenclature has been developed to fill this need. The inorganic sections of a MOF, or SBUs, can be described by topologies common to several structures. Each topology, also called a net, is assigned a symbol, consisting of three lower-case letters in bold. MOF-5, for example, has a pcu net. The database of net structures can be found at the Reticular Chemistry Structure Resource (rcsr.anu.edu.au).
Based on the combination of the building blocks, the length, the combination and the functionalization of the organic linker, a large variety of pore environments can be realized. Some interesting properties the MOFs exhibit include large surface areas, and relative ease of tuning and functionalizing. Furthermore, flexibility effects within the framework may be due to weaker bonds than those of zeolites. The unique properties observed in MOFs appear to have great potential mainly in applications related with gas storage and gas separations processes. For example, MOFs can be used to make a highly selective and permeable membrane to separate small gas molecules, particularly CO2 from CH4. This separation is necessary for natural gas purification and CO2 capture, it is also difficult due to the two molecules being very similar in size. However, certain MOFs are well able to separate the two gases.
Zeolitic imidazolate frameworks (ZIF) are one kind of metal-organic framework, which can also be used to reduce industrial emissions of carbon dioxide. One liter of ZIF crystals can store about 83 liters of CO2. The crystals are non-toxic and require little energy to create, making them an attractive possibility for carbon capture and storage. Further, the porous ZIF structures can be heated to high temperatures without decomposing and can be boiled in water or solvents for a week and remain stable, making them suitable for use in hot, energy-producing environments like power plants.
In order for these various MOF materials to be used in membranes (either as MOF films or as components in mixed matrix membranes), the crystal size is preferably less than one-micron. Since solvothermal synthesis typically produces crystals larger than 10 microns, other routes are needed to produce smaller crystals by increasing crystal nucleation while suppressing crystal growth.
It is also desirable to control the MOF morphology. In mixed matrix membrane applications, for example, anisotropic particles would lead to alignment of particles due to the flow fields associated with producing hollow fiber membranes. This situation can be avoided if more isotropic particles are used.
The three main techniques that have been applied to reduce MOF crystal size are sonication, microwave irradiation, and addition of a base. However, there is still room in the art for improved methods to synthesize MOF nanocrystals of uniform size.
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