Metal–Organic Frameworks Derived from Zero‐Valent Metal Substrates: Mechanisms of Formation and Modulation of Properties

Abstract

The replicative construction of metal-organic frameworks (MOFs) templated with solvent-insoluble solid substrates is of marked importance, as it allows for the assembly of 2D and 3D macro-and mesoscopic architectures with properties that are challenging to attain by the conventional solution-based synthesis approach. This work reports an in situ and direct construction of MOFs from zero-valent metal substrates via a green hydrothermal oxidation-MOF construction chemistry without the use of any additional metal source, chemical reagents, or acidification of solvent, and elucidates the zero-valent metal derived formation mechanisms of MOFs and their structure modulation to 1D nanofibers (NFs), 2D film, and 3D core-shell microstructures. Through modulation of the competing surface oxidationdissolution and MOF crystallization kinetics, Al@MIL-53 core-shell microstructures and MIL-53 (Al) NFs are obtained that exhibit unique morphologies and marked properties superior to the conventional MIL-53 (Al) powders. The generality of zero-valent metal-templated synthesis of MOFs is demonstrated with formation of MIL-53 (Al), HKUST-1, and ZIF-7 polycrystalline films on Al, Cu, and Zn metal meshes, elucidating the significance of the approach utilizing solid metal substrate that can be easily processed into various shapes, architectures, and compositions.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201808466.salts (e.g., nitrates, chlorides) with high ionic solubilities in polar solvents are prevailingly used, and the resulting MOFs are collected in the forms of crystalline powders in sizes ranging from several to a few hundred micrometers. [3] In an alternative synthesis route, the fabrication of MOFs into mesoscopic or hierarchical structures by the self-template preparative method has been reported in recent years. [4] MOF microcrystals assembled into 2D and 3D organized structures can provide significant benefits in many applications that cannot be achieved through conventional methods. The studies reported in the literature are mostly based on pseudomorphic replication or templated growth using solvent-insoluble preshaped solid metal oxides (Al 2 O 3 , [5] ZnO, [6] CuO [7] ) or hydroxides (Cu(OH) 2 , [8] Zn-Al layered double hydroxide, [9] Zn-Cu hydroxy double salts [10] ). In this heterogeneous method, there is a dual function of solid metal oxides as architecture-directing templates and metal sources of MOFs, since the nucleation of MOFs occurs locally on the parent solids rather than on the entire synthesis solution. The dissolution of metal ions from metal oxide substrates can be sluggish, limiting the overall MOFs growth kinetics due to the generally high lattice energies of metal oxides (4-6 MJ mol −1 ). [11] Nevertheless, this approach continues to attract attention with significant impact as it can provide various advantages including (i) environmental benefits by minimizing anion waste and the use of problematic solvents, (ii) facile enginee...

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