Unlocking the pre-nucleation state as a route to materials discovery in MOFs

Metal-organic frameworks (MOFs) are extended porous materials with a wealth of compositional and structural tunability that makes them very attractive for numerous important applications, such as water harvesting, carbon capture, energy storage, and sensing. Their structures are exemplified by zeolitic imidazolate frameworks (ZIFs), which consist of tetrahedral metal nodes connected by functionalised imidazolate linkers to form topologies that are direct analogues of classical zeolites.

Despite the fact that several thousand MOFs are known, only 15 out of 239 known zeolite topologies have been reproduced as ZIFs. This suggests that there is a vast phase space of ZIFs that remains to be discovered, which is inaccessible to current synthetic methods. The challenge in developing a new discovery paradigm is to understand the atomic correlations during the materials formation process, thereby enabling the effects of different synthetic parameters to be predicted and utilised. However, despite significant interest, we know very little about the mechanisms that underpin MOF crystallisation and, therefore, we lack control over self-assembly and phase selection.

This research will build on recent advances in in situ measurements and understanding of the ZIF pre-nucleation state - the dynamically evolving mixture of complexes that exists in solution prior to and during crystallisation - to reveal the key intermediate species and structural relationships with ZIF products that determine phase selection. Specifically, the pre-nucleation state of selected ZIF formation reactions will be stabilised through careful analytical chemistry and characterised using high-resolution ex-situ techniques to identify the key intermediate species. A range of techniques will be used that spans several length scales of sensitivity, from X-ray total scattering and nuclear magnetic resonance spectroscopy (local order) to electrospray ionisation mass-spectrometry (ESI-MS; complexes and oligomers) and X-ray diffraction (long-range order), in order to generate a detailed picture of the pre-nucleation state. These data will then be used as a basis with which to interpret key in situ experiments: time-resolved ESI-MS, and synchrotron X-ray scattering, which will reveal how the intermediates evolve during ZIF crystallisation under normal reaction conditions. Computational calculations will identify the key interactions in the intermediates and the relative stabilities of different products, verifying the importance of competing pathways. Importantly, the in-situ experiments will generate kinetic data, from which the rates and activation energies of interconversion and crystallisation will be extracted to complete a detailed, quantitative model of ZIF formation.

The main output of this project will be a new mechanistic understanding of the ZIF pre-nucleation state and its effect on phase selection. It will reveal key intermediate species in two subsets of important ZIFs, namely those with the common sodalite (SOD) topology and those with the large channel gmelinite (GME) topology, which have potential uses in carbon capture, usage and storage. It will show how the evolution of pre-nucleation species leads to the formation of particular ZIF phases under different synthetic conditions. Collectively, these results will build a quantitative model of the ZIF crystallisation energy landscape by which their formation can be rationalised and ideal synthesis conditions can be predicted.

This research will show how intermediates can be targeted, stabilised and assembled to direct the structures of selected ZIFs, thereby opening up the possibility to discover a wealth of new phases via rational design. The methodology developed will be transferable to other MOF systems and crystalline materials, with far-reaching applications in solar cells, catalysis and energy storage. Thus, it will pave the way for a new generation of innovative products and technology.