(Keynote) Gaining Control over the Morphology of Metal–Organic Frameworks

Abstract

Owing to their vast ratio of surface area to mass and volume, metal-organic frameworks (MOFs) revolutionized many applications that rely on chemical and physical interactions at surfaces. However, a great challenge today is to build MOF materials that translate the functionality at the molecular level effectively to the macroscopic world. To do this, effective transport of reagents to, from, and within MOF materials is critical. To overcome this barrier, porous materials will need to be structured with feature sizes from below one micrometre to the millimetre or centimetre scale. In fact, the challenge of morphology control over multiple length scales of MOF materials is overlooked by researchers so far but is the key to enable groundbreaking real-world applications.The synthesis of a MOF material always starts with the strategic selection of precursor materials. Besides an organic building block, an inorganic compound providing metal ions is necessary, which most commonly is a metal salt, as it easily dissolves. A relatively new and different approach is the use of metal oxides (MOs). Even though their use is challenging due to low solubility issues and slow and non-homogenous conversion, by carefully optimizing the experimental conditions the MO–to–MOF conversion can be carried out with preserving the morphology of the MO (Figure 1). Thus, the use of MO precursors, which offers a huge number of different morphologies that can be synthesized with high precision, potentially provides an easy and smart way for the precise synthesis of different MOF morphologies. This strategy opens the door to morphologies that cannot be achieved using reactions based on metal salt precursors or other approaches.This contribution focuses on (1) the study of different MOs as precursors for MOF formation, (2) giving a deep insight in the underlying chemistry, and (3) application of this knowledge to convert different complex 3D morphologies of MOs into MOFs. Our findings demonstrate that all MOs tested so far can be used as precursors for the synthesis of MOFs, although they not always preserve the parent shape. To illustrate the capacity of the SP approach and to uncover the full mechanistic details of the early stages of this process we explored the conversion of ZnO into the zeolitic imidazolate framework (ZIF) ZIF-8 in detail. ZIFs are a group of MOFs that are structurally very similar to the well-known zeolites. Zeolites are composed of [TO4] (T = Si, Al) tetrahedrons covalently joined by bridging oxygen atoms and ZIFs of [MN4] (M = transition metal ion) tetrahedrons with bridging five-membered imidazolate units with very similar bridging angles and framework topologies. Finally, we used the gained knowledge to convert different three dimensional 3D ZnO morphologies into ZIFs.Figure: Conversion of metal oxides (MOs) into metal—organic frameworks (MOFs) with the competing rates k1 (MO dissolution), k2 (diffusion), k3 (MOF nucleation), and k4 (MOF growth). Certain conditions of the reaction medium lead to MO dissolution, followed by diffusion of metal ions and organic linkers. Depending on the ratios of the different rates either homogeneous nucleation within the solution or heterogeneous nucleation on the MO-medium interface occurs (or a mixture of both). The former can be understood as a dissolution-crystallization (DC) mechanism, analogous to reactions using metal salt precursors, the latter as a shape-preservation (SP) mechanism. Figure 1