Metal-organic frameworks in separations: A review

It is possible to design high permeability and selective metal-organic frameworks (MOFs) with designable functionality. The large number of possible MOF structural permutations arise from the considerable number of possible metal nodes and the great variety of organic ligands used for these materials. Herein, we discuss the applications of MOFs in all manner of separations including gas adsorption/separations, membranes, gas chromatography (GC), liquid chromatography (LC), water harvesting and the computer/machine learning design of MOFs. Each application requires MOFs with specific structural motifs. Relevant properties include polarity, temperature stability, solvent stability, pore size, pore volume, surface area, etc. MOFs used for the adsorption and separation of gases can be quite different from those used in membrane technologies or as chromatographic stationary phases. In the area of chromatography, there are far more reports of GC separations than LC. Also, there has been considerable efforts at developing MOF chiral stationary phases. Hence additional chiral components are added to the MOF support, such as cyclodextrins and various amino acids. MOFs have been used as general adsorbents for both inorganic and organic molecules. A very unique MOF application involves water harvesting. It is shown that potable water can be made in arid environments by selectively adsorbing water vapor from air, even at low humidity. Such MOFs could have important analytical applications, as well. Finally, there is a new focus on automated design of MOFs with desired properties for specific tasks, using computational design and machine learning. This is briefly covered in the final section of this review.