3D MOFs to 2D sheets in 1 easy step

Figure: Conversion of hcp UiO-67 into nanosheets of hxl UiO-67

In the past decade or so, people have rediscovered the joys of two dimensions – particularly because of remarkable electronic properties of graphene and other related materials. Very thin sheets of nanoporous materials (like zeolites, for example) are extremely interesting because of their potential as catalysts or membranes for chemical separations. Their thinness lets molecules diffuse rapidly through them, but because they retain the small and uniform pores, they can still discriminate between different kinds of guest molecule.

Although chemists have created a myriad of different metal-organic frameworks (MOFs - materials made by connecting metal atoms or clusters with organic molecular linkers into a kind of molecular scaffolding), there isn’t yet the same variety in their two dimensional counterparts. This is partly because it has only been possible to make metal-organic nanosheets (MONs) from MOFs that don’t have strong bonds connecting the metal nodes in all three dimensions. These 2D MOFs are much rarer than the 3D materials.

In this paper, we describe a new 3D MOF (hcp UiO-67) which can be turned into 2D MONs by selectively breaking the bonds along one direction. We were able to make hcp UiO-67 by decreasing the organic linker-to-metal node ratio using acidic ‘modulators’ added to the reaction mixture. In most MOFs, this tends to introduce missing linkers into the framework structure, but in UiO-67 we found instead that the metal clusters condense forming a ‘double cluster’, which has a lower linker-to-metal ratio. Cluster condensation is quite common for purely inorganic framework materials, but is much rarer in MOFs. Deliberately inducing cluster condensation using acidic modulators might therefore be a useful route to create new kinds of framework.

Encouragingly, we also discovered that the transformation of 3D hcp UiO-67 into 2D hxl UiO-67 is reversible (in the right conditions). If the reassembly were to be carried out using organic molecular linkers with different chemistry (e.g. water-repelling or acidic groups), this assemble-diassemble-reassemble method might be able to be used to make new 3D MOFs which could not be made using normal synthetic methods.

We also explained why this selective bond-breaking happens in hcp UiO-67, using quantum chemical calculations to calculate the energetic cost for a removing a linker. We found that significantly more energy is needed to remove a linker from within the MOF plane rather than to remove one between them. In the ordinary fcu UiO-67, the higher symmetry means that all the linkers are equivalent, and so there can’t be any reason to take out one linkre rather than another. This means that unlike for hexagonal hcp UiO-67, where removing linkers creates 2D sheets, eliminating linkers from fcu UiO-67 will just cause the material to gradually fall apart. This understanding will help us design new 3D MOFs with similar reactivity.

In this paper we made use of the Diamond Light Source extensively, both to get high quality data from the microcrystalline materials we made (including to study local connectivity of the cluster) and also to study how hcp UiO-67 crystallises. We also used various different kinds of microscopy to study the sheets and establish their thicknesses, principally transmission electron microscopy.