Metal–Organic Frameworks for Separations

Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites.

The construction of thousands of well-defined, porous, metal–organic framework (MOF) structures, spanning a broad range of topologies and an even broader range of pore sizes and chemical functionalities, has fuelled the exploration of many applications. Accompanying this applied focus has been a recognition of the need to engender MOFs with mechanical, thermal and/or chemical stability. Chemical stability in acidic, basic and neutral aqueous solutions is important. Advances over recent years have made it possible to design MOFs that possess different combinations of mechanical, thermal and chemical stability. Here, we review these advances and the associated design principles and synthesis strategies. We focus on how these advances may render MOFs effective as heterogeneous catalysts, both in chemically harsh condensed phases and in thermally challenging conditions relevant to gas-phase reactions. Finally, we briefly discuss future directions of study for the production of highly stable MOFs. Metal–organic frameworks (MOFs) have shown promise in a broad range of applications, including catalysis. In this Review, the chemical, thermal and mechanical stabilities of MOFs, in particular with catalytic uses in mind, are discussed.

Chemical, thermal and mechanical stabilities of metal–organic frameworks

We report on a simple and straightforward method that enables the rapid room-temperature production of nanocrystals (finely tuned in size between ∼10 and 65 nm) and microcrystals (∼1 μm) of the prototypical microporous zeolitic imidazolate framework (ZIF) material ZIF-8. Control of crystal size is achieved in a novel approach by employing an excess of the bridging bidentate ligand and various simple auxiliary monodentate ligands with different chemical functionalities (carboxylate, N-heterocycle, alkylamine). The function of the monodentate ligands can be understood as a modulation of complex formation and deprotonation equilibria during crystal nucleation and growth. Using time-resolved static light scattering, the functioning of modulating ligands is monitored for the first time by in situ experiments, which offered significant insight into the crystal growth processes. Formation of nanocrystals is characterized by continuous, comparatively slow nucleation and fast crystal growth occurring on a time sca...

Controlling Zeolitic Imidazolate Framework Nano- and Microcrystal Formation: Insight into Crystal Growth by Time-Resolved In Situ Static Light Scattering

We report the structural evolution of zeolitic imidazolate framework-8 (ZIF-8) as a function of time at room temperature. We have identified the different stages of ZIF-8 formation (nucleation, crystallization, growth, and stationary periods) and elucidated its kinetics of transformation. We hypothesize that the observed semicrystalline-to-crystalline transformation may take place via solution- and solid-mediated mechanisms, as suggested by the observed phase transformation evolution and Avrami's kinetics, respectively. A fundamental understanding of ZIF-8 structural evolution as demonstrated in this study should facilitate the preparation of functional metal-organic framework phases with controlled crystal size and extent of crystallinity.

Structural Evolution of Zeolitic Imidazolate Framework-8

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Time‐Resolved In Situ Diffraction Study of the Solvothermal Crystallization of Some Prototypical Metal–Organic Frameworks

The structures of four iron(III) carboxylate materials with the MIL-53 structure and the guest molecules water, pyridine, or 2,6-dimethylpyridine (2,6-lutidine) have been solved and refined from high-resolution powder X-ray diffraction data. These materials have porous structures constructed from infinite chains of trans-corner-shared {FeO4(OH,F)2} octahedra cross-linked by benzene-dicarboxylate ligands. With pyridine, the unit cell is partially expanded with respect to the hydrated phase, with hydrogen bonds between N donors and OH framework atoms, whereas with lutidine, water is co-sorbed to give a fully expanded version of the MIL-53(Fe) structure in which water molecules bridge the N donors of the organic molecule and the framework OH groups by hydrogen bonding. Careful dehydration of MIL-53(Fe)[2,6-lutidine,H2O] provides a new material, MIL-53(Fe)[2,6-lutidine0.5], with an intermediate unit cell volume: in this case no host−guest hydrogen bonds are formed. Time-resolved in situ energy-dispersive X-ra...

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Kathryn M. Golden

Papers

Time‐Resolved In Situ Diffraction Study of the Solvothermal Crystallization of Some Prototypical Metal–Organic Frameworks

Selective Sorption of Organic Molecules by the Flexible Porous Hybrid Metal−Organic Framework MIL-53(Fe) Controlled by Various Host−Guest Interactions