Analogous porous metal-organic frameworks: Synthesis, stability and application in adsorption

So far, a huge number of metal-organic frameworks (MOFs) have been synthesized and studied very widely for various applications like gas adsorption/storage, separation, catalysis, drug delivery, luminescence, magnetism, etc. Some of the MOFs are isomorphous, isostructural or isoreticular in topologies having nearly similar (analogous) framework structures. On the other hand, some of the MOFs also have very similar structures with different functional groups via direct synthesis or post-modification. In this highlight, MOFs having very similar structures will be classified into three categories: (1) analogous MOFs with different metallic components; (2) analogous MOFs with different linkers; (3) analogous MOFs with different functional groups. Moreover, various MOFs with very similar structures composed of different metallic, organic or functional groups will be compared especially with regard to their synthesis kinetics, chemical/thermal stability and their applications in the adsorption of hydrogen, acetylene, propylene, carbon dioxide and sulfur-containing compounds, and so on. The synthesis rate and chemical stability of analogous MOFs depend on the lability and inertness, respectively, of metal ions. On the other hand, thermal stability may be explained with the bond strength of metal-oxygen in common oxides. The thermal or chemical stability of analogous MOFs having extra functional groups depends on the functional groups tagged on the linkers; however, no comprehensive explanation is available. Adsorption depends strongly on the property of the metallic or organic moiety of analogous MOFs, and important parameters (size, binding strength, ionic character, density, redox ability, softness and acidity of the metal ions; length, polarity, and hydrophobicity of the linkers) for adsorption can be suggested. Based on the analysis of the reported results, it can be concluded that the metallic, organic or functional groups of analogous MOFs have dominant roles in the synthesis, stability and adsorption even though contradictory results were also reported. Understanding the effects of metallic, organic or functional moieties of very similar MOFs on the synthesis, stability and adsorption will lead to a new way to develop MOF materials that have various commercial applications.