Tracking the formation of MOFs in solution

Summary by Francesca Firth

Figure 1: Schematic of stages in the formation of hcp UiO-66(Hf) in solvothermal conditions, through different cluster intermediates.

Metal-organic frameworks (MOFs) are of great interest for applications such as energy storage and carbon capture, particularly due to their outstanding chemical tunability. In particular, zirconium (Zr) and hafnium (Hf) MOFs are promising for real-world applications because they are very stable. Our previous work discovered that changing the synthesis conditions, including which solvent was used what temperature the reaction was carried out, changed the structure of the MOF that formed for the important UiO family of MOFs. In particular we found that we could introduce missing clusters or make MOFs with ‘double’ Hf₁₂ clusters, rather than classic UiO Hf₆ clusters. These new MOF structures have very different chemical and physical properties.

Figure 2: Tracking the formation of hcp UiO-66 using in situ XPDF measurements.

It would be really useful to understand how the synthesis conditions determine what kind of MOF forms, as this would allow us to rationally design conditions to produce a MOF with a particular, desired properties. One of the most important steps in MOF formation is the formation of the metal cluster (e.g. Hf₆ or Hf₁₂) from the precursor metal solution (in this case HfCl₄), as this determine what kind of MOF structure can form. Because this happens in solution, before the formation of a solid, crystalline MOF framework, it is very hard to track their formation using conventional crystallography. We used X-ray pair distribution function (XPDF) measurements at the Diamond Light Source to measure a series of Hf MOF reactions, as XPDF can see non-crystalline materials and molecules in solution, as well as crystalline MOFs. We were able to separate out the signal of the Hf clusters in solution, due to their high electron density.

We found that we could track the assembly of clusters from Hf₄, through Hf₆ and Hf₁₂ clusters into full, crystalline 3D MOF frameworks. We found that depending on whether acid was included in the reaction mixture, different clusters formed, and at different rates, and we also found that temperature greatly favours the formation of the larger Hf₁₂ clusters.

These insights, obtained using in situ XPDF measurements during reactions of both Hf precursor solutions and the full Hf MOF, advance our understanding of the growth of Hf metal-organic frameworks. In particular, we showed that the metal clusters are able to act as independent ‘secondary building units’, which form before the framework, and therefore direct the final framework structure. As we understand the formation of metal clusters better: we will be able to better design reaction conditions to select a particular cluster, and hence produce more specifically targeted MOFs: including perhaps new and unrealised MOFs with better catalytic and adsorption activity.